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Purpose: The role of post-mastectomy radiation therapy (PMRT) in breast cancer patients after neoadjuvant chemotherapy (NAC) is highly controversial. This study aimed to evaluate the impact of PMRT according to pathologic nodal status. Methods and Materials: We retrospectively reviewed 682 patients with clinical stage II-III breast cancer who underwent NAC and mastectomy from 2013 to 2017. Of total, 596 (87.4%) received PMRT and 86 (12.6%) did not. We investigated the relationship amongst locoregional recurrence-free survival (LRRFS), disease-free survival (DFS), overall survival (OS), and various prognostic factors. Subgroup analyses to identify the patients who may benefit from PMRT were also performed. Results: The median follow-up duration was 67 months. In ypN+ patients (n = 368, 51.2%), PMRT showed significant benefit for LRRFS, DFS, and OS (all p < 0.001). Based on multivariate analysis, histologic grade (HG) III (hazard ratio [HR] = 3.67, p = 0.002), lymphovascular invasion (LVI) (HR = 2.38, p = 0.045), and ypN2-3 (HR = 2.37, p = 0.02) were identified as significant risk factors for poor LRRFS. In ypN1 patients with more than two factors among (i) luminal subtype (ii) HG I-II and (iii) absence of LVI, PMRT showed no significant difference in LRRFS (p = 0.18). In ypN0 patients (n = 351, 48.8%), PMRT was not significantly associated with LRRFS, DFS, and OS. However, PMRT showed better LRRFS in triple-negative breast cancer (TNBC) subtype (p = 0.04). Conclusion: PMRT had a high impact on treatment outcomes in patients with residual lymph nodes following NAC and mastectomy. Among the ypN0 patients, PMRT may be beneficial only in the TNBC subtype. Citation Format: Dowook Kim, Jin Ho Kim, In Ah Kim, Ji Hyun Chang, Kyung Hwan Shin. Role of postmastectomy radiation therapy in breast cancer patients according to pathologic nodal status after neoadjuvant chemotherapy [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-10-07.

Background: Combination chemotherapy (CT) remains a standard of care in advanced breast cancer (ABC) with aggressive disease features (rapidly progressing or highly symptomatic disease, including life-threatening visceral crisis). To date, no data have been published on a head-to-head comparison of CDK4/6 inhibitor (CDK4/6i) + endocrine therapy (ET) vs combination CT in this patient (pt) population. RIGHT Choice, a randomized, open-label, multi-national, Phase II trial, investigated the efficacy and safety of first-line ribociclib (RIB) + ET vs combination CT in pre/perimenopausal pts with HR+/HER2− ABC with aggressive disease (where combination CT is clinically indicated by physician’s judgment). Here we report the results of the primary endpoint of progression-free survival (PFS) and key secondary endpoints from this study. Methods: Pre/perimenopausal pts with HR+/HER2− ABC (>10% estrogen receptor–positive [ER+]) and no prior systemic therapy for ABC were randomized 1:1 to receive either RIB (600 mg daily, 3 weeks on/1 week off) with letrozole/anastrozole + goserelin or investigator’s choice of CT (docetaxel + capecitabine, paclitaxel + gemcitabine, or capecitabine + vinorelbine). Randomization was stratified by presence of liver metastases and whether the disease-free interval (duration from date of complete tumor resection for primary breast cancer lesion to the date of documented disease recurrence) was less than two years. Pts included in the trial had ABC not amenable to curative therapy for which combination CT was clinically indicated by physician’s judgment (i.e., symptomatic visceral metastases, rapid progression of disease or impending visceral compromise, or markedly symptomatic non-visceral disease). Median PFS (mPFS) and median time to treatment failure (mTTF) were evaluated by Kaplan-Meier methods. Overall response rate (ORR) was also analyzed, with quality of life and biomarker analyses planned. RIGHT Choice is registered at ClinicalTrials.gov (NCT03839823). Results: A total of 222 pts (112 RIB + ET; 110 CT) were enrolled from Feb 2019 to Nov 2021. Pts with symptomatic visceral metastases (n=150; 67.6%), rapid disease progression (n=41; 18.5%), and markedly symptomatic non-visceral metastases (n=31; 14.0%) were included. Overall, 116 pts (52.3%) had visceral crisis based on guideline definitions. A majority of pts (n=190; 85.6%) had tumors that were ≥50% ER+. At data cutoff (Apr 12, 2022), median follow-up was 24.1 mo; 45.5% and 23.6% of pts remained on treatment in the RIB + ET arm and combination CT arm, respectively. The primary endpoint was met, with a statistically significant PFS benefit of ≈1 year for RIB + ET vs combination CT (mPFS, 24.0 vs 12.3 mo; hazard ratio, 0.54; 95% CI, 0.36-0.79; P=.0007). OS data were immature at data cutoff. The mTTF was ≈10 mo longer for RIB + ET vs CT (18.6 vs 8.5 mo; hazard ratio, 0.45; 95% CI, 0.32-0.63). The ORR was similar for RIB + ET vs CT (65.2% vs 60.0%). No new safety signals were observed in pts on RIB. Lower rates of treatment-related serious adverse events (AEs; 1.8% vs 8.0%) and lower rates of discontinuation due to treatment-related AEs (7.1% vs 23.0%) were seen with RIB + ET vs CT, respectively. AEs observed with combination CT were consistent with the published data. Conclusions: This analysis demonstrated a statistically significant and clinically meaningful PFS benefit with RIB + ET over combination CT in the first-line pre/perimenopausal pt population with aggressive HR+/HER2− ABC disease. This is the first study comparing a CDK4/6i + ET vs combination CT and demonstrating the superiority of RIB + ET in aggressive HR+/HER2− ABC. This evidence supports RIB+ ET use as a preferred option for this pt population. Citation Format: Yen-Shen Lu, Eznal Izwadi Bin Mohd Mahidin, Hamdy Azim, Yeşim ERALP, Yoon-Sim Yap, Seock-Ah Im, Julie Rihani, James Bowles, Teresa Delgar Alfaro, Jiwen Wu, Melissa Gao, Khemaies Slimane, Nagi El Saghir. Primary results from the randomized Phase II RIGHT Choice trial of premenopausal patients with aggressive HR+/HER2− advanced breast cancer treated with ribociclib + endocrine therapy vs physician’s choice combination chemotherapy [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS1-10.

The brain is a common target organ for breast cancer metastasis, and the risk of brain metastasis is usually high for patients with HER2-positive breast cancer. While T-DXd showed positive results in HER2-positive breast cancer brain metastasis (BCBM), its activity and blood-brain barrier (BBB) permeability in active BCBM need to be further validated (TUXEDO-1 included 15 patients with active brain metastasis). Tucatinib plus Trastuzumab and Capecitabine triplet showed efficacy in patients with brain metastasis, but Tucatinib alone has poor BBB penetration. There is still an unmet medical need for active HER2-positive BCBM. VRN101099 is a highly selective kinase inhibitor of HER2 (Eurofins scanMAX Kinase Profiling, S-score (35) of 0.01). VRN101099 has single- to double-digit nanomolar (IC50) cellular potency in HER2-dependent cancer cells, and Ba/F3 cells expressing HER2 wild type or mutations with selectivity over wildtype EGFR. VRN101099 inhibited proliferation of BT474 with 3.6 nM IC50 but HaCaT with 829.2 nM IC50. VRN101099 binds HER2 kinase by forming a covalent bond to the Cys805 and its irreversible inhibition resulted in a longer target resident time than Tucatinib, confirmed by in vitro washout experiments. Robust in vivo activity of VRN101099 was observed in HER2-positive BT474 xenograft models. Moreover, once daily oral administration of VRN101099 significantly regressed intracranial BT474 tumor, demonstrating greater efficacy than twice daily oral administration of Tucatinib. These results were well explained by the superior brain to plasma exposure of VRN101099 to Tucatinib. Also, VRN101099 showed high exposure in the target organ, the fat pad, which potentiated better clinical translation. These anti-tumor efficacies are correlated with pharmacodynamic responses, as confirmed by decreased HER2, AKT, and ERK phosphorylation. In summary, VRN101099 is a brain penetrant, orally bioavailable, irreversible, and highly selective inhibitor of HER2 with therapeutic potential in HER2-positive BCBM. These data support the clinical development of VRN101099 in HER2-driven cancers. Citation Format: Yikyung Ko, Jihye yoo, Hong-ryul Jung, Hyerim Lim, YeongDeok Lee, Se Hyuk Kim, Dong Guk Shin, Serin Cho, Myung hoe Heo, Haelee Kim, Ha Yeon Cho, Ah Reum Han, Eunhwa Ko, Hwan Geun Choi, Deakwon Kim, Sunghwan Kim. VRN101099, Brain Permeable HER2 Kinase Inhibitor, Shows the Anti-tumor Activity in Preclinical Models of HER2-positive Cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-11-03.

Background: Early biologic response to endocrine therapy, such as changes in Ki67 labeling index (LI), has been suggested to predict long-term outcomes in hormone sensitive breast cancer. The addition of a CDK4/6 inhibitor to endocrine therapy has been shown to augment biological response in breast cancer. Pre-operative Endocrine Prognostic Index (PEPI) scores, generated based on post-treatment Ki67 LI, have been shown to predict patient outcomes. EndoPredict® is a multigene assay that predicts the risk of distant recurrence in patients with operable estrogen receptor (ER)-positive HER2-negative breast cancer. This study was conducted to evaluate the efficacy of the neoadjuvant endocrine therapy plus palbociclib versus neoadjuvant endocrine therapy plus placebo. Patients and Methods: This is a phase III randomized, double-blind study of neoadjuvant hormonal therapy plus palbociclib versus neoadjuvant hormonal therapy plus placebo in untreated pre/peri- and post-menopausal women with operable, hormone receptor-positive (ER and/or progesterone receptor), HER2-negative breast cancer. The other major inclusion criteria included tumor size ≥ 15mm, T1c-3N0-1, Ki67 LI ≥14% by central assessment, and no previous history of radiotherapy or systemic therapy for breast cancer. Patients were randomly assigned 1:1 to receive 16 weeks of hormonal therapy plus palbociclib or hormonal therapy plus placebo. Hormonal therapy consisted of letrozole for post-menopausal patients and tamoxifen plus LH-RH agonist for pre/peri-menopausal patients. The co-primary endpoints included PEPI score and EPclin Risk Score, a score combining EndoPredict® molecular score with clinical factors. These scores were sequentially analyzed on a modified intent-to-treat basis according to the gatekeeping procedure: if statistical significance was detected on the PEPI score, the statistical significance of EPclin Risk Score would be assessed. The sample size was 100 patients in each arm, which was calculated with < 5% type I error rate (two sided) and 80% power. Results: Between 16 July 2019 – 7 July 2021, 141 eligible patients were randomized from 25 participating institutes in Japan, Korea, Taiwan, Hong Kong and Australia. One hundred twenty-six patients completed the treatment duration and surgical samples were collected to evaluate endpoints. All randomized patients were evaluable for safety assessment. Randomization was well-balanced in terms of age, menopausal status and cancer stage. The proportion of patients who had a low, moderate, or high PEPI score was 15.2%, 50.0% and 34.8% in the hormonal therapy plus palbociclib arm and 13.3%, 55.0% and 31.7% in the hormonal therapy plus placebo arm, respectively. There was no statistically significant difference in PEPI score between two arms (one-sided p-value=0.563). The proportion of patients who had a high risk EPclin Risk Score seemed lower in the palbociclib arm than in the placebo arm (62.1% vs 68.3%) although hypothesis testing was not performed on EPclin Risk Score because statistical significance was not detected on the PEPI score. No new safety signals were found in the study. Permanent discontinuation from the study in association with adverse events was reported for 7 (9.7%) patients in the hormonal therapy plus palbociclib arm and for 0 patients in the hormonal therapy plus placebo arm. Conclusions: The addition of palbociclib to neoadjuvant hormonal therapy did not improve efficacy measured by PEPI score. In palbociclib arm, the rate of patients who had a high risk EPclin Risk Score after treatment was lower than in placebo arm. Translational researches are ongoing to analyze molecular changes by treatments. The role of chemotherapy after neoadjuvant therapy is under investigation. Clinical trial identification: NCT03969121 Funding: Pfizer Inc. Citation Format: Takayuki Ueno, Louis W.C. Chow, Wonshik Han, Chiun Sheng Huang, G Bruce Mann, Satoshi Morita, Hironori Haga, Elham Fakhrejahani, Takayuki Kobayashi, Kenichi Inoue, Mariko Tokiwa, Hirofumi Suwa, Tomoyuki Aruga, Sachiko Minamiguchi, Yosuke Yamada, Yuko Tanabe, Masahiro Takada, Toshinari Yamashita, Hiroji Iwata, Chi-Feng Chung, Sachiko Takahara, Eriko Tokunaga, Shigeru Imoto, Eun Sook Lee, Yasuaki Sagara, Jee Hyun Kim, Richard H DeBoer, Hyun-Ah Kim, Hung Wen Lai, Ming-Feng Hou, Michelle White, Yoshiko Umeyama. Neoadjuvant hormonal therapy plus palbociclib versus hormonal therapy plus placebo in women with operable, hormone sensitive and HER2-negative primary breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-09-01.

Human Epidermal growth factor Receptor 2 (HER2) is overexpressed in 20% to 30% of breast cancers and is associated with poor prognosis. Patients with advanced HER2-type breast cancer are treated with HER2-directed tyrosine kinase inhibitors as well as antibodies, but many patients finally acquire therapeutic resistance. Therefore, it is important to develop new treatment strategies that circumvent HER2 positive resistant breast cancer. Among the suggested mechanisms of resistance to HER2-targeted therapeutics, tumor hypoxia and relevant biological factors are known as important players. In this study, we aimed to explore the role of tumor hypoxia in the development of resistance to HER2-targeted therapeutics, and whether the role of an immunological therapy targeting this could contribute to overcoming resistance. To this end, we established lapatinib-resistant HER2 positive breast cancer cells in vitro (SKBR3, BT-474 for human and MMC for murine cells). First, we examined if lapatinib-sensitivity is different according to oxygen concentration using lapatinib-resistant cell lines (LR-SKBT3, LR-BT-474, LR-MMC) compared with parent cell lines. Resistant cells were resistant both in normoxic and hypoxic conditions, but parent cells were highly sensitive in both of the oxygen concentration. In the Western blot analysis, HIF1α and c-MET were overexpressed in the resistant cells compared with parent cell lines in both normoxic and hypoxic condition. Interestingly, treatment with lapatinib had reduced HIF1α and c-MET expression at normoxia in parent cell lines but not in resistant cell lines. Under the hypoxic conditions, the difference in expression of HIF1α and c-MET was more pronounced between parent cell lines and resistant cell lines treated with lapatinib. These results indicate that the expression of HIF1α and c-MET was involved in lapatinib-resistant. In a previous study, we have identified epitopes from HIF1α/c-MET and demonstrated effect of delayed bone metastasis in triple negative breast cancer model when used as peptide vaccine. Herein, we evaluated whether the immunization using HIF1α/c-MET peptide vaccine is effective in inhibition of lapatinib-resistant HER2 positive breast cancer growth in vivo. MMTVneu-transgenic mouse model was used and 6- 8 weeks old mice were injected with LR-MMC subcutaneously. Immunization using the HIF1α/c-MET peptide vaccine significantly inhibited growth of LR-MMC cells in MMTVneu-transgenic mice compared with controls. HIF1α/c-MET specific IFN-γ-secreting T cell responses in the splenocytes were significantly higher in the immunized group than controls. Immunohistochemical staining of T cells revealed that expression of tumor HIF1α and c-MET was significantly decreased, while the number of CD8+ T cell was increased in the tumors from immunized animals compared with control tumors. In conclusion, our study demonstrated that the vaccine targeting HIF1α/c-MET might be a promising cancer immunotherapy in lapatinib-resistant HER2 positive breast cancer. In vivo study to evaluate efficacy of combination therapy (lapatinib and HIF1α/c-MET vaccine) is ongoing. Citation Format: Ji Min Lee, Jin Hwa Hong, Soon Young Lim, Ju Won Kim, Ah Reum Lim, Myung Han Hyun, Kyong Hwa Park. Vaccination against HIF1α/c-MET peptides overcome lapatinib resistance and inhibits tumor growth by immunomodulating in HER2 positive breast cancer. [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-20-04.

Background: Breast cancer with low HER2 expression (HER2-low) is of high clinical relevance because of new therapeutic options with antibody-drug conjugates. We have recently shown in a large cohort from neoadjuvant clinical trials that HER2-low breast cancer has different molecular characteristics as well as different clinical outcomes compared to HER2-zero. Considering the positive correlation between HER2-low expression and hormone receptor positivity observed consistently in many investigations, we have extended our analysis to HR+ tumors from the post-neoadjuvant PenelopeB trial. In PenelopeB, patients with HR+ breast cancer and residual disease after neoadjuvant chemotherapy (NACT) were randomized to post-neoadjuvant palbociclib versus placebo in addition to endocrine therapy. We evaluated the molecular phenotype and clinical outcomes of HER2-low compared to HER2-zero patients. Methods: A total of 1250 patients were randomized, HER2 status was available for 1151 tumors from pretherapeutic core biopsy, determined mainly by local pathology, and from 1213 tumors from the post-NACT sample, determined as part of central pathology. For 1119 patients a paired HER2-status was both available. HER2-zero was defined as IHC0 and HER2-low-positive was defined as IHC1+ or IHC2+/ISH-. Gene expression analysis of 2549 genes using the HTG oncology biomarker panel was performed in 620 pretherapeutic biopsies and 780 post-NACT residual tumor samples, with 539 paired gene expression samples. Breast cancer subtypes were determined using the AIMS approach. Results: In pretherapeutic biopsies, 695 tumors (60%) were HER2-low and 457 (40%) were HER2-zero. A HER2-low status in the biopsy was significantly linked to improved iDFS (HR 0.76 (0.60-0.96; p=0.02). In residual tumors, 632 tumors (60%) were HER2-low and 581 (40%) were HER2-zero, without any prognostic impact of HER2 low status. In addition, a shift of HER2-low-status comparing core biopsy and residual tumor was observed in 415 (37%) of 1119 tumors. 161 (14%) had a shift from HER2-zero to HER2-low and 254 (23%) shifted from HER2-low to HER2-zero. A shift from HER2-zero to HER2-low in the post-NACT samples was significantly linked to reduced iDFS (HR 1.43 [95%CI 1.01-2.01]), p=0.04), compared to HER2-low group, while a shift from HER2-low to HER2-zero was associated with better iDFS compared to HER2-zero group, although not statistically significant (p=0.17). We did not observe a significant correlation of HER2-low status and AIMS molecular subtypes. In particular, the HER2-enriched (HER2E) subtype was assigned to only 4.3% of HER2-zero and 3.1% of HER2-low tumors. Significant iDFS differences were observed for HER2-low-status in combination with AIMS subtypes (lumB/basal/HER2E vs. lumA/normL; overall p-value < 0.0001) for both pretherapeutic biopsies and residual tumor. Patients with post-NACT HER2-low tumors had an improved survival in the subgroups of aggressive AIMS subtypes (lumB/basal/HER2E), but not in the less aggressive AIMs subtypes (lumA/normL), with a positive test for interaction (p=0.02). For the pre-NACT samples a similar, but non-significant trend was observed. We evaluated a total of 620 core biopsies for differences in gene expression comparing HER2-low and HER2-zero tumors. A total of 417 genes were statistically significantly different, but in a hierarchical clustering there was no clear separation of HER2-low and HER2-zero tumors. Conclusions: In the PenelopeB cohort of HR+ tumors, a HER2-low status in pretherapeutic core biopsies is related to improved disease-free survival, especially for those tumors that have a more aggressive intrinsic subtype. A shift of HER2-low status was observed before and after chemotherapy, indicating an adaptation of the pathway activity to therapy-induced stress, which might become relevant for future diagnostic and therapeutic approaches. Citation Format: Carsten Denkert, Miguel Martín, Michael Untch, Hervé R. Bonnefoi, Erik S. Knudsen, Seock-Ah Im, Angela DeMichele, Agnieszka Witkiewicz, Laura Van ’t Veer, Sung-Bae Kim, Harry D. Bear, Nicole McCarthy, Karen Gelmon, Frederik Marmé, José Ángel García-Sáenz, Nicholas Turner, Federico Rojo, Martin Filipits, Lesley-Ann Martin, Peter A. Fasching, Christian Schem, Catherine M. Kelly, Toralf Reimer, Masakazu Toi, Hope Rugo, Michael Gnant, Andreas Makris, Yuan Liu, Karsten Weber, Sivaramakrishna Rachakonda, Sibylle Loibl. HER2-06 Outcome analysis of HER2-zero or HER2-low hormone receptor-positive (HR+) breast cancer patients - characterization of the molecular phenotype in combination with molecular subtyping [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr HER2-06.

Background: Dual HER2 blockade with trastuzumab and pertuzumab combined with docetaxel (DHP) is the standard first-line treatment option for patients with HER2-positive metastatic breast cancer. However, HER2 positivity alone often fails to predict treatment outcome of HER2 targeted therapy. The magnitude of HER2 immunohistochemical staining (IHC) and HER2/CEP17 ratio from in situ hybridization (ISH) is a well-known predictive biomarker for HER2 targeted therapy. In the neo-adjuvant setting, patients with HER2 IHC 3+ have higher pathologic complete response (pCR) rate compared to HER2 IHC< 3+ with HER2 ISH positive. In addition, higher HER2/CEP17 ratio is associated with higher pCR rate in HER2-positive breast cancer patients treated with dual HER2 blockade regimens. However, there is no data whether there is an association between HER2/CEP17 ratio and treatment outcome of DHP regimen in patients with advanced of metastatic breast cancer especially among those with HER2 IHC 3+. Methods: We performed a retrospective cohort study with patients with locally advanced or metastatic HER2-positive breast cancer who were treated with first-line palliative DHP regimen between August 2008 and January 2021 at Seoul National University Hospital. In the clinical setting, HER2 IHC 3+ is defined as HER2 positive and no further ISH testing is required. Additional ISH was performed in patients with HER2 IHC 3+ without archival HER2 ISH results. The association between HER2/CEP17 ratio and treatment outcome was assessed. Results: In total, 165 patients were included with a median follow-up duration of 28.0 months. Among the 165 patients, 35 patients had archival ISH result and additional ISH was performed in 53 patients. The correlation between HER2/CEP17 ratio and treatment outcome was assessed in 88 patients. Cox proportional hazard analysis revealed that HER2/CEP17 ratio is correlated with PFS (HR 0.23, 95% CI 0.11-0.49, p < 0.001). When dichotomized by the median HER2/CEP17 ratio of 4.17, patients with higher HER2/CEP17 ratio had significantly longer PFS (37.5 vs. 17.4 months, p = 0.003) and numerically higher ORR (54.5% vs. 34.1%, p = 0.085). Multivariate analysis revealed that HER2/CEP17 ratio is an independent prognostic factor for PFS (HR 0.72, p = 0.001). Of 88 patients with ISH results, 25 had HER2 IHC 1+ or 2+ and 63 had HER2 IHC 3+. HER2/CEP17 ratio was associated with PFS in both HER2 IHC 1+/2+ patients (HR 0.12, 95% CI 0.02-0.88, p = 0.037) and HER2 IHC 3+ patients (HR 0.18, 95% CI 0.07-0.49, p = 0.001). Patients with higher HER2/CEP17 ratio had longer PFS in both HER2 IHC 1+/2+ patients (28.6 vs. 12.9 months, p = 0.003) and HER2 IHC 3+ patients (Not reached vs. 18.3 months, p = 0.005) when dichotomized by the median HER2/CEP17 of 2.95 and 4.75, respectively. Conclusion: This is the first study to report that higher HER2/CEP17 ratio is associated with longer PFS in HER2-positive advanced breast cancer patients treated with palliative first-line DHP. The strength of this study is that we identified prognostic role of ISH even in patients with HER2 IHC 3+. It would be helpful to perform ISH not only in patients whose HER2 IHC is ambiguous, but also in patients with HER2 IHC 3+ to make better prediction of treatment outcome. Table 1. Patient characteristics. Citation Format: Jeongmin Seo, Jiwon Koh, Dae-Won Lee, Han Suk Ryu, Kyung-Hun Lee, Tae-Yong Kim, Seock-Ah Im. High HER2/CEP17 ratio is associated with better treatment outcomes in advanced HER2-positive breast cancer treated with pertuzumab, trastuzumab, and docetaxel regardless of HER2 2+ or 3+ results [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-04-01.

Background: Breast cancer (BC) with low-level HER2 expression (HER2-low) is defined by an immunohistochemistry (IHC) score of 1+ or 2+ without HER2 gene amplification or excess HER2 gene copy number, as measured by in situ hybridization (ISH). This represents approximately half of patients with BC overall (estimated as 55% for hormone-receptor positive [HR+] BC and 38% for triple-negative breast cancer [TNBC]; Scott, ASCO, 2021). Some data suggest that patients with HER2-low BC may respond differently to treatment than those whose BC has no HER2 expression (HER2-0). In this post hoc unplanned analysis, we analyzed data from two pivotal phase 3 studies (Studies 305 and 301) comparing eribulin with other chemotherapeutic agents (treatment of physician’s choice and capecitabine, respectively [“control”]) in patients with both HER2-low and HER2-0 MBC. Methods: Patients with MBC, 2–5 (Study 305) or < 2 (Study 301) prior lines of chemotherapy for advanced/metastatic disease, and who had received an anthracycline and a taxane, were analyzed. HER2-expression status was determined by IHC and/or ISH assays. Median progression-free survival (PFS) and overall survival (OS) were calculated using the Kaplan–Meier method adjusted by study; comparisons of PFS and OS between treatment groups were performed using stratified (by prior capecitabine use, geographic region, and study) log-rank tests. Hazard ratios were estimated by a stratified Cox model. For each study, median PFS and OS were also calculated for HR+ and TNBC subgroups. Results: Baseline characteristics were generally balanced between treatment groups among patients with HER2-low (n=427) and HER2-0 (n=824) BC. Patients with HER2-low or HER2-0 BC showed trends toward benefit with eribulin treatment. In patients with HER2-low and HER2-0 BC, median OS was longer with eribulin vs control (15.1 vs 12.0 months and 15.2 vs 12.5 months, respectively); median PFS by independent imaging review (IIR) was also longer with eribulin vs control (4.0 vs 3.1 months and 3.9 vs 3.1 months, respectively). Objective response rate (ORR) by IIR was also higher with eribulin vs control in patients with HER2-low and HER2-0 BC (13.7% vs 9.2% and 10.2% vs 7.4%, respectively). In a separate analysis, median OS was longer with eribulin vs capecitabine in patients with TNBC and HER2-low and HER2-0 (15.4 vs 10.3 months and 14.4 vs 8.9 months, respectively). Conclusions: In this post hoc analysis, treatment with eribulin demonstrated trends toward improved OS, PFS, and ORR compared with chemotherapy controls in patients with HER2-low or HER2-0 MBC. Funding source: This trial was sponsored by Eisai Inc., Nutley, NJ, USA. Medical writing support was provided by Oxford PharmaGenesis Inc., Newtown, PA, USA, and was funded by Eisai Inc., Nutley, NJ, USA. Citation Format: Chris Twelves, Peter A. Kaufman, Ahmad Awada, Seock-Ah Im, Bagrat Lalayan, Ran Xie, Linda T. Vahdat, Javier Cortés. Efficacy of eribulin mesylate in HER2-low and HER2-0 metastatic breast cancer (MBC): Results from an analysis of two phase 3 studies [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-03-02.

Background: Mucinous carcinoma is the third most common histological subtype of breast cancer after ductal and lobular carcinomas, accounting for approximately 3% of invasive breast cancers. Although considered a favourable subtype with de-escalation of treatment recommended in the National Comprehensive Cancer Network guidelines, recurrence can occur and supporting data is limited. We thus examined prognostic factors of pure mucinous breast cancer (PMBC) in an international multicentre cohort study. Methods: Patients diagnosed between January 2000 to December 2015 with hormone receptor-positive HER2-negative stage I to III PMBC, invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) at 6 centers in Singapore, Taiwan, Korea and Japan were evaluated. Cox proportional hazards regression analyses were used to compare relapse-free survival (RFS), distant relapse-free survival (DRFS) and overall survival (OS) by histological subtypes, and to identify prognostic factors for PMBC. Results: Of 23,105 women eligible for analysis, 20,684 had IDC, 1,475 had ILC and 946 had PMBC. The median follow-up was 6.6 years; 5-year RFS, DRFS and OS for PMBC were 94.6%, 96.5% and 98.1% respectively. On multivariable cox regression analyses, PMBC demonstrated superior RFS (hazard ratio [HR] = 0.70, 95% CI: 0.56 - 0.88), DRFS (HR = 0.69, 95% CI: 0.53 - 0.89) and OS (HR = 0.70, 95% CI: 0.52 - 0.93) compared to IDC, while ILC had comparable survival outcomes as IDC. When restricted to only PMBC, significant independent prognostic factors for RFS included ethnicity (vs Chinese, “Others” [non-Chinese/Japanese/Korean, mainly Malay and Indian]: HR = 2.62, 95% CI 1.23 – 5.57), older age (vs < 40 years, >70 years: HR = 3.53, 95% CI 1.67 – 7.46), tumor size (vs T1, T3-4: HR = 2.79, 95% CI 1.45 – 5.37), positive lymph nodes (HR = 2.04, 95% CI: 1.10 – 3.77), use of radiotherapy (HR = 0.54, 95% CI 0.33 – 0.91) and endocrine therapy (HR = 0.31, 95% CI 0.12 – 0.77). On further analysis, the inferior RFS, DRFS and OS in older patients (>70 years) were driven largely by non-breast cancer deaths rather than relapses. Use of endocrine therapy was also associated with superior DRFS (HR = 0.26, 95% CI 0.09 – 0.73) but not OS. In a subgroup analysis, use of chemotherapy was associated with improved DRFS (HR = 0.25, 95% CI 0.08 – 0.82) and OS (HR = 0.07, 95% CI 0.01 – 0.37) with a trend in RFS (HR = 0.41, 95% CI 0.14 – 1.24) for lymph node-positive PMBC; no differences in outcomes were observed for the lymph node-negative subgroup. Conclusions: Larger tumor size, lymph node positivity and ethnicity were significant factors for RFS in PMBC. Use of endocrine therapy was associated with superior RFS and DRFS, while chemotherapy was associated with better DRFS and OS for lymph-node positive PMBC. Citation Format: Ryan Tan, Whee Sze Ong, Kyung-Hun Lee, Seri Park, Jabed Iqbal, Yeon Hee Park, Jeong-Eon Lee, Jong Han Yu, Ching-Hung Lin, Yen-Shen Lu, Makiko Ono, Takayuki Ueno, Yoichi Naito, Tatsuya Onishi, Geok hoon Lim, Su-Ming Tan, Han-Byoel Lee, Jiwon Koh, Han Suk Ryu, Wonshik Han, Veronique Kiak Mien Tan, Fuh-Yong Wong, Seock-Ah Im, Puay Hoon Tan, Yoon-Sim Yap. Prognostic factors in non-metastatic hormone receptor-positive HER2-negative mucinous breast cancer: an international multicentre cohort study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-05-36.

Background: Chemotherapy is the main treatment in patients with pre-treated endocrine-resistant HR+/HER2– metastatic breast cancer, but has limited efficacy and substantial toxicities. The antibody-drug conjugate Dato-DXd consists of a humanized IgG1 mAb targeting TROP2 attached via a stable cleavable linker to a topoisomerase I (TopI) inhibitor payload. Heavily pre-treated patients with metastatic triple-negative breast cancer in the TROPION-PanTumor01 (NCT03401385) study of Dato-DXd showed a manageable safety profile and highly encouraging objective response rates (ORR by blinded independent central review [BICR]: 34% in all patients; 52% in patients treatment-naïve to TopI inhibitor-based therapies). The metastatic HR+/HER2– breast cancer cohort of TROPION-PanTumor01 has completed enrollment (n=41); data are currently maturing. Trial design: TROPION-Breast01 (NCT05104866) is an ongoing, global, phase 3, open-label, randomized trial evaluating efficacy and safety of Dato-DXd vs investigators’ choice of chemotherapy (ICC) in patients with inoperable or metastatic HR+/HER2– breast cancer. Patients (n≈700) are randomized 1:1 to Dato-DXd 6 mg/kg IV Q3W or ICC (eribulin, capecitabine, vinorelbine, or gemcitabine) until progression. Adults with an ECOG performance status of 0–1, who experienced progression on or are unsuitable for endocrine therapy, and received 1–2 prior lines of standard-of-care chemotherapy in the inoperable or metastatic setting are eligible. Monotherapy treatment with inhibitors of mTOR, PD-[L]1, CDK4/6 and PARP do not count as prior chemotherapy lines. Patients must have ≥1 measurable lesion per RECIST 1.1 and an archival or fresh formalin-fixed and paraffin-embedded tumor sample. Clinically inactive brain metastases are permitted. Dual primary endpoints are progression-free survival (PFS) by BICR, and overall survival. Secondary endpoints include PFS per investigator, ORR, disease control rate, patient-reported outcomes, and Dato-DXd pharmacokinetics and immunogenicity. Exploratory endpoints include TROP2 expression and exposure–efficacy relationship. Patients are stratified by number of prior chemotherapy lines, prior CDK4/6 inhibitor use, and region. At the time of writing 236 patients have been enrolled across 19 countries. Citation Format: Aditya Bardia, Kevin Kalinsky, Junji Tsurutani, Erika Hamilton, Joo Hyuk Sohn, Kyong Hwa Park, Yeon Hee Park, Seock-Ah Im, Keun Seok Lee, Daisy Dastur, Vincent Haddad, Sabrina Khan, Binghe Xu, Barbara Pistilli, Hope Rugo. Datopotamab deruxtecan (Dato-DXd), a TROP2 antibody-drug conjugate, vs investigators’ choice of chemotherapy in previously-treated, inoperable or metastatic HR+/HER2– breast cancer: TROPION-Breast01 [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-03-04.

Background: Obesity, the modern ‘epidemic’, has shared correlation with fatty liver disease and breast cancer. However, previous studies on the relation between fatty liver and breast cancer have shown conflicting results on the impact of fatty liver on the survival and recurrence of breast cancer patients. And there was no attempt to find out the effect of liver fibrosis, which is the consequence of fatty liver disease, on female breast cancer patients. So we attempted to investigate the prognostic value of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis(NASH)-related liver fibrosis in patients with breast cancer undergoing surgery, using noninvasive tools like liver-to-spleen attenuation(L/S) ratio and Fibrosis-4(FIB-4) score, respectively. Methods: A total of 933 patients diagnosed with primary invasive breast cancer and receiving surgery at the university-affiliated referral center between April 2006 and December 2019 were included. After excluding patients who had significant alcohol consumption and hepatitis viral infection,838 patients were divided into two groups according to the L/S ratio of 1 measured by the preoperative low-dose computed tomography: 91 patients(10.9%) with a L/S ratio< 1 vs 747 patients(89.1%) with a L/S ratio≥1. They were also divided into two groups based on the FIB-4 score of 2.67: 804 patients (95.9%) with a FIB-4 score< 2.67 vs 34 patients (4.1%) with a FIB-4 score≥2.67. The Cox proportional hazards model was used to calculate the hazard ratio (HR) and the 95% confidence interval (CI). Results: Patients with NAFLD were older, had higher BMI, and had a higher proportion of mastectomy and hyper-transaminasemia. They showed worse overall, disease-free, and regional recurrence-free survivals compared to those without NAFLD (p=0.008, 0.043, and 0.017, respectively), but no significant differences in local recurrence-free, systemic recurrence-free, and contralateral breast cancer-free survivals. The survival outcome of breast cancer did not show any relationship with NASH-related liver fibrosis (overall survival; p=0.061, disease-free survival; p=0.557). NAFLD was a significant risk factor for mortality in multivariable analysis (HR, 2.077; 95% CI, 1.052–4.102; p=0.035). After stratifying for subtypes of breast cancer, the L/S ratio remained a significant predictor of overall, disease-free, local recurrence-free, and regional recurrence-free survivals in only the hormone receptor-positive/HER2−negative subtype (p=0.007, 0.005, 0.009, and < 0.001, respectively). Conclusion: NAFLD is significantly associated with decreased overall survival, disease-free survival and increased regional recurrence in patients with breast cancer especially the hormone receptor-positive/HER2−negative subtype. NASH-related fibrosis was not associated with survival. Therefore, NAFLD should be assessed in the preoperative setting for predicting long-term prognoses of breast cancer patients Citation Format: Hyunsu Yeoh, Siwon Jang, Jong-Ho Cheun, Jin Ah Kwon, Myoung Seok Lee, Bumjo Oh, In Sil Choi, Sohee Oh, Jongjin Kim, Jeong Hwan Park, Won Kim, Ki-Tae Hwang. Prognostic impact of non-alcoholic fatty liver disease and non-alcoholic steatohepatitis-related liver fibrosis on postoperative long-term outcomes of breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-05-06.

Background: We analyzed the treatment outcome of breast cancer patients with brain metastases (BM) in Korea to identify the prognostic factors and the role of whole brain radiation therapy (WBRT). Methods: Seven hundred thirty patients of breast cancer with BM treated at 17 institutions in Korea from 2000 to 2014 were analyzed. The median follow-up duration was 12 months. The analysis consisted of three cohorts: in cohort A, a total of 730 patients were included; in cohort B, 538 patients with available follow-up imaging after initial brain-directed treatment; and in cohort C, 54 patients receiving salvage WBRT due to recurrent BM after initial Stereotactic radiosurgery or WBRT. Overall survival (OS) was calculated from BM diagnosis in cohort A or from the last day of salvage WBRT in cohort C. Results: Median OS of cohort A was 15 months. In multivariate analysis, histologic grade 3, extracranial metastasis, number of BM >4, hormone receptor (HR) or HER2 negativity, and shorter time interval to diagnosis of BM were associated with inferior OS. Among 538 patients in cohort B, 201 showed subsequent development of new BM at a median of 11 months after stereotactic radiosurgery or WBRT for the management of initial BM (at 1 year, HR+/HER2- 51.9%, HER2+ 44.0%, and TNBC 69.6%, respectively; p=0.008). Upfront WBRT reduced subsequent development of new BM, which showed the significant difference among molecular subtypes (HR+/HER2-, 42% reduction at 1 year, p4, and involvement of both tentoria increased subsequent development of new BM. Anti-HER2 therapy for HER2+ patients and upfront WBRT significantly reduced risk of new BM. In cohort C, upfront WBRT prolonged the salvage WBRT-free duration (median 6.9 vs. 8.7 months, p=0.058). Median OS was 6.8 months after salvage WBRT. Longer interval to salvage WBRT, controlled primary tumor, high dose of salvage WBRT (BED10 >37.5 Gy), and systemic treatment after salvage WBRT showed better OS. Uncontrolled extracranial systemic disease and salvage WBRT due to local progression without distant intracranial failure showed worse OS. Conclusions: The rates of new BM showed the significant differences among molecular subtypes. Upfront WBRT decreased subsequent development of new BM and this effect was dependent on the molecular subtype as well. Anti-HER2 therapy for HER2+ patients significantly decreased the subsequent development of new BM. On salvage WBRT setting, the patients having high dose of salvage WBRT, stable extracranial systemic disease and subsequent systemic therapy showed better OS. Citation Format: Jae Sik Kim, Kyubo Kim, Wonguen Jung, Kyung Hwan Shin, Seock-Ah Im, Hee-Jun Kim, Yong Bae Kim, Jee Suk Chang, Jee Hyun Kim, Doo Ho Choi, Yeon Hee Park, Dae Yong Kim, Tae Hyun Kim, Byung Ock Choi, Sea-Won Lee, Suzy Kim, Jeanny Kwon, Ki Mun Kang, Woong-Ki Chung, Kyung Su Kim, Won Sup Yoon, Jin Hee Kim, Jihye Cha, Yoon Kyeong Oh, In Ah Kim. Multicenter study for brain metastasis from breast cancer in Korea: The significance of molecular subtype (KROG 1612) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-21-01.

Background: Chemotherapy, together with immune checkpoint inhibitors, improves outcomes vs chemotherapy alone for patients (pts) with metastatic (m)TNBC PD-L1+ disease. Most of these pts progress within a year. In a previous study, D (anti-PD-L1) combined with chemotherapy enhanced antitumor immune responses in early TNBC (Loibl. Ann Oncol 2019). In TNBC, activation of the PI3K/AKT/PTEN pathway and high CD73 expression are common. BEGONIA is an ongoing 2-part, multicenter, multi-arm, open-label platform study, evaluating safety and efficacy of D or D+P combined with novel therapies as first-line treatment for mTNBC (NCT03742102). Preliminary results from 2 arms were presented at ASCO 2021 (Abstract #1023). Here, we report results from Arm 1 D+P, Arm 2 D+P+C, and Arm 5 D+P+O. C is an oral, selective, ATP-competitive catalytic inhibitor of all 3 AKT isoforms, and O is a mAb targeting CD73. Methods: Eligible pts had untreated, unresectable, locally advanced or metastatic TNBC. In Arms 1 and 5, pts received D 1500 mg IV Q4W + P 90 mg/m2 IV day (d)1, d8, d15 of every cycle. Pts in Arm 5 also received O 3000 mg IV on d1 and d15 for the first 2 cycles, then Q4W. In Arm 2, pts received D 1500 mg IV Q4W + P 80/90 mg/m2 IV in 4-week cycles (d1, d8, d15, 1 week off) + C 400 mg BID in 4-week cycles (d2-5 × 3 weeks, 1 week off). Primary objectives were safety and tolerability. Secondary endpoints included objective response rate (ORR) and duration of response. Tumors were assessed Q8W per RECIST v1.1. The first 6 pts treated in Arms 2 and 5 were evaluated for dose-limiting toxicities (DLTs), with additional pts enrolled if treatment was tolerated. PD-L1 expression was assessed retrospectively. Previously presented data from Arm 1 D+P, are included for reference (Schmid. ASCO 2021, #1023). Results: In Arm 2 (data cutoff Mar 2021), 30 pts received D+P+C (15 P[80], 15 P[90]; total 13 ongoing); 2 pts (6.7%) discontinued all treatment due to AEs. The rates of dose delays were 13 pts (43%) for D and 15 (50%) for P; dose interruptions were 1 (3%) for D, 12 (40%) for P, 15 (50%) for C; dose reductions were 12 (40%) for P and 14 (47%) for C. Treatment-related (tr)SAEs and G3/4 trAEs were experienced by 7 (23%) and 22 (73%) pts. In Arm 5 (data cutoff Sep 2020), 33 pts received D+P+O (14 ongoing); no pts discontinued due to AEs. The rates of dose delays were 13 pts (39%) for D, 10 (30%) for P, 10 (30%) for O; dose interruptions were 2 (6%) for D, 10 (30%) for P, 3 (9%) for O; and dose reductions were 12 (36%) for P. trSAEs and G3/4 trAEs were experienced by 1 (3%) and 5 (15%) pts. In both arms, there were no DLTs or deaths due to AEs. The Table presents follow-up time and efficacy outcomes for Arms 1, 2, and 5. Responses were observed regardless of PD-L1 expression. The potential value of mutations in the PI3K pathway and CD73 expression as predictive biomarkers will be discussed for Arms 2 and 5, respectively. Updated data for Arm 1 will be presented. Conclusions: The safety profiles of triplet combinations in Arms 2 and 5 were consistent with the individual agents; however, in Arm 2, there was a relatively high rate of G3/4 trAEs but a low discontinuation rate for AEs. Although BEGONIA was not designed to compare activity across arms and numbers were small, the ORR of each triplet therapy was numerically similar to D+P. Biomarker analysis may elucidate pts that benefit from the combination of C or O with D+P. Funding: AstraZeneca Table. Efficacy outcomes in Arms 1, 2, and 5 of BEGONIAArm 1Arm 2Arm 5D+P N=23D+P(80)+C n=15D+P(90)+C n=15All D+P+C N=30D+P+O N=33Duration of follow-up at data cutoff, months, median (range)16.6 (8.5-19.8)6.7 (2-9)16.8 (6-21)8.2 (2-21)8.6 (4.1-14.6)Confirmed ORR, n (%)13 (56.5)8 (53.3)8 (53.3)16 (53.3)15 (45.5)95% CI34.5-76.826.6-78.726.6-78.7NC28.1-63.3CR, n10111PR12871514SD (Unconfirmed PR)7 (3)6 (2)4 (2)10 (4)13 (4)PD31345Percentage with ongoing response at data cutoff53.8%75.0%25.0%50.0%66.7%Arm 1 data cutoff was Sep 2020. C, capivasertib; CI, confidence interval; CR, complete response; D, durvalumab; NC, not calculable; O, oleclumab; ORR, objective response rate; P, paclitaxel; PD, progressive disease; PR, partial response; SD, stable disease. Citation Format: Peter Schmid, Zbigniew Nowecki, Seock-Ah Im, Wei-Pang Chung, Simon Lord, Anne Armstrong, Cynthia X Ma, Robert Huisden, Ross Stewart, Rakesh Kumar, Gaia Schiavon, Hannah Dry, Ana Nunes, Kyung Hae Jung, Yeon Hee Park. BEGONIA: Phase 1b/2 study of durvalumab (D) combinations in locally advanced/metastatic triple-negative breast cancer (TNBC): Results from Arm 1 D + paclitaxel (P), Arm 2 D+P + capivasertib (C), and Arm 5 D+P + oleclumab (O) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD10-03.

Purpose: T-DM1, an antibody-drug conjugate, has significant antitumor activity in patients with HER2-positive metastatic breast cancer (mBC) patients who had progressed after trastuzumab-based chemotherapy. This study was conducted to investigate the clinical practice and factors related with outcomes of T-DM1 use for HER2-positive mBC patients in the nation-wide real-world setting. Method: This complete enumeration study included the patients with HER2-positive mBC who received T-DM1 as palliative therapy from August 2017 to December 2018 under the registry of Health Insurance Review & Assessment Service in Korea. Safety and outcomes of T-DM1 including overall response rate (ORR), progression-free survival (PFS), and overall survival were evaluated. Factors significant in univariate analysis were analyzed in multivariate model. Result: From the sixty institutions, a total of 824 patients were enrolled. Mean age was 58 years-old, 818 patients (99.3%) were female and 516 patients (62.6%) had relapsed after curative treatment. About 40% patients received T-DM1 as first or second line treatment, 21.5% received it as third line and 37.3% as fourth or over line. During a median follow-up of 16.8 months, the ORR was 32.8%, median PFS was 7.2 months and median OS was not reached. In multivariate analysis, clinical factors associated with the lower PFS were age (< 65 year-old, hazard ratio[HR] 1.53, 95% confidence interval[CI]: 1.214-1.919, p < 0.001), poor ECOG performance status (PS ≥ 2, HR 1.98, 95% CI: 1.493-2.626, p < 0.001), previous pertuzumab use (HR 1.40, 95% CI: 1.118-1.742, p = 0.003) and previous lapatinib use (HR 1.29, 95% CI: 1.047-1.586, p =0.017). The common grade 3-4 adverse events were thrombocytopenia (13.0%), neutropenia (2.9%), and elevation of liver enzyme (2.5%). Hypokalemia (≤ 3.0 mmol/L) and any-grade bleeding event such as epistaxis and gum-bleeding occurred in 25 (3.1%) and 94 patients (11.4%), respectively. Conclusion: This is the first, nationwide, real-world data about T-DM1 use of the HER2-positive metastatic breast cancer patients in Korea. The efficacy and toxicity profile of T-DM1 in the real-world practice were comparable with those of randomized trials. Patients’ factors and previous anti-HER2 therapy could predict the outcomes of T-DM1. Further studies to reveal the subtypes of good responders to T-DM1 and the sequence of anti-HER2 therapy are warranted. Citation Format: Sun Kyung Baek, Jae-Ho Jeong, Yeon-Hee Park, Hee Kyung Ahn, Min Hwan Kim, In Hae Park, Young Ju Suh, Dae-Won Lee, Sung Hoon Sim, Jee Hyun Kim, Hyun-Jeong Shim, Yeesoo Chae, Su-Jin Koh, Hyorak Lee, Jieun Lee, Jae-Ho Byun, Youngmi Seol, Eun Mi Lee, Jin Seok Ahn, Kyung-Hae Jung, Seock-Ah Im, Keun Seok Lee, Joohyuk Sohn, Kyoung Eun Lee. A nationwide real-world study for evaluation of efficacy and safety of T-DM1 in patients with HER2-positive locally-advanced unresectable or metastatic breast cancer in Korea (KCSG BR19-15) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-18-32.

Introduction Addition of ovarian function suppression to conventional endocrine therapy alone in premenopausal women provides a survival benefit with moderate to high risk hormone-receptor positive breast cancer, especially who received chemotherapy Prediction of menstruation recovery after chemotherapy is important for deciding subsequent endocrine treatment and addressing fertility issues. Methods In the adding OFS after chemotherapy trial (ASTRRA), patients who resumed ovarian function up to 2 years after chemotherapy were randomized to receive either 5 years of tamoxifen or adding 2 years of OFS with tamoxifen. With these 1383 patients, we developed a model that predicts when patients recover menstruation within 3 years after chemotherapy using several variables including age, BMI, chemotherapy regimen and duration, serum E2 and FSH level. Results A total of 1017 patients data were used to develop prediction model and 366 patients data were used for external validation. In development group, 546 (53.6%) patients resumed menstruation during follow up period of 5 years. In multivariable analysis, younger age and AC based regimen without taxane were strong predictive factor for menstruation recovery. However predictive value of chemotherapy regimen was not constant over time. Therefore, we conducted another model with patients (n= 624) who did not recover menstruation within one year. In this patient group, predictive factors for menstruation recovery was age and serum E2 level at 6 months after chemotherapy. We also conducted a simplified scoring system to estimate change of recovery by using risk factors mentioned above. Conclusion Younger age is an important persisting factor predicting menstrual recovery after chemotherapy. Although chemotherapy regimen predicts shor-term menstrual recovery, over time, patient factors have more predictive influence on recovery. Recovery of serum E2 level would be important to predict subsequent menstruation recovery. Citation Format: Young Joo Lee, Woo C Noh, Seok J Nam, Byeong-Woo Park, Eun S Lee, Seock A Im, Yong S Jung, Jung H Yoon, Sung S Kang, Kyong H Park, Soo-Jung Lee, Min H Lee, Joon Jeong, Sung Y Kim, Hyun-Ah Kim, Se-Hwan Han, Wonshik Han, Min H Hur, Seonok Kim, Sei-hyun Ahn, Hee J Kim. Prediction of menstruation recovery timing in premenopausal breast cancer patients taking tamoxifen after chemotherapy: An ASTRRA substudy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-10-07.

Background: SB3 (trastuzumab-dttb) is a biosimilar approved globally based on its similarity with reference trastuzumab (TRZ) demonstrated by thorough comparability exercises in analytical, biological, and clinical studies. In a randomized, double-blind, multicenter Phase 3 study of 875 patients with HER2-positive early or locally advanced breast cancer in the neoadjuvant setting, equivalent efficacy, similar safety, pharmacokinetics, and immunogenicity between SB3 and TRZ were shown. However, when quality attributes of TRZ were examined, downward drifts in antibody-dependent cell-mediated cytotoxicity activities (ADCC) were observed in the TRZ lots with expiry dates ranging from Aug 2018 to Dec 2019. Some of these lots of the reference product were found to be used in the Phase 3 study. After completing the Phase 3 study, patients from select countries were included in a follow-up observational study to monitor cardiac safety and survival. Here, we report the final survival results, including post-hoc subgroup analysis based on ADCC status, at a median follow-up of 68 months. Methods: During the follow-up observational study, the protocol was amended to include additional patients who originally were enrolled in the Phase 3 study but had not been followed in the observational study, in order to collect a larger sample of survival data. For these additional patients, medical records from the last assessment in the Phase 3 study through the date of enrollment in the follow-up study were collected retrospectively. As post-hoc analysis, patients in the TRZ arm were stratified into two subgroups: patients who received during neoadjuvant treatment at least one vial of TRZ with downward drift in ADCC as “Drifted TRZ”, and the others as “Non-drifted TRZ”. Event-free survival (EFS) and overall survival (OS) were assessed. Results: Of 875 patients randomized in the Phase 3 study, 538 patients (SB3, N=267; TRZ, N=271) were enrolled in the follow-up observational study: 367 patients were initially enrolled in the follow-up study, and 171 patients were additionally enrolled following the protocol amendment. The median follow-up duration was 68 months from randomization in the Phase 3 study. 54 events (20.2%) in the SB3 arm, and 67 events (24.7%) in the TRZ arm were reported (HR 0.84 [0.58, 1.20], p=0.335). 22 deaths (8.2%) and 38 deaths (14%) were reported in SB3 and TRZ arms, respectively (HR 0.61 [0.36, 1.05], p=0.073). In post-hoc analysis, of 271 patients in TRZ arm, 107 patients were grouped as “Non-drifted TRZ”, and 164 patients as “Drifted TRZ”. 19 events (17.8%) in the Non-drifted TRZ group and 48 (29.3%) events in the Drifted TRZ group occurred (HR 2.57 [1.28, 5.14], p=0.008). 9 deaths (8.4%) in the Non-drifted TRZ group and 29 deaths (17.7%) in the Drifted TRZ group were reported (HR 3.87 [1.37, 10.93], p=0.011). No difference was observed between SB3 arm and Non-drifted TRZ group in terms of EFS (HR 1.28 [0.73, 2.22], p=0.391) and OS (HR 0.99 [0.42, 2.31], p=0.975). Conclusions: Comparable long-term efficacy results in EFS and OS were shown at 68 months of follow-up, further supporting biosimilarity of SB3 to the reference product. Currently, these follow-up results represent the longest monitoring data of patients treated with a trastuzumab biosimilar for HER2-positive early or locally advanced breast cancer. Citation Format: Xavier Pivot, Mark D Pegram, Javier Cortes, Diana Lüftner, Gary H Lyman, Giuseppe Curigliano, Igor M Bondarenko, Mikhail Dvorkin, Jin Hee Ahn, Seock-Ah Im, Maria Litwiniuk, Yaroslav V Shparyk, Gwo Fuang Ho, Nikolay V Kislov, Marek Wojtukiewicz, Tomasz Sarosiek, Yee Soo Chae, Jin Seok Ahn, Hyerin Jang, Sujung Kim, Jiwon Lee, Soo Young Lee, Ye Chan Yoon. Final survival analysis of a phase 3 study comparing SB3 (trastuzumab biosimilar) and reference trastuzumab in HER2-positive early or locally advanced breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-04.

Background: In high-risk hormone-receptor (HR)+/HER2- BC patients germline (g) mutations can be found in approximately 14% in BRCA1/2 and in BRCA1/2 and other BC predisposition genes in 20% (Pohl-Rescigno E, et al. JAMA Oncol 2020). In metastatic BC CDK4/6 inhibitors may have greater activity in patients with a BRCA mutation detected in ctDNA (André F, et al. J Clin Oncol 2020). The PENELOPE-B trial did not to show an improved invasive disease-free survival (iDFS) by adding palbociclib to ET in high-risk HR+/HER2- BC (Loibl S, et al. J Clin Oncol 2021). Methods: Blood samples from 898 of 1250 PENELOPE-B patients were available. 445 patients were sampled following a case-cohort design (220 cases defined as patients with any event during follow-up and 225 randomly selected patients without any event [non-cases]) and analyzed for germline variants in BRCA1/2 and 16 non-BRCA1/2 cancer predisposition genes (ATM, BARD1, BRIP1, CDH1, CHEK2, FANCM, MRE11A, NBN, PALB2, PTEN, RAD50, RAD51C, RAD51D, STK11, TP53, XRCC2) by targeted next generation sequencing (NGS). The primary definition of mutational status was the prevalence of a pathogenic mutation (mt) in one or more analyzed BC predisposition genes. Statistical analyses for time-to-event endpoints (iDFS, distant disease-free survival [DDFS], and overall survival [OS]) were based on inverse probability weighting: weighted Cox proportional hazard models and Kaplan-Meier estimates were used. Results: 442 of 445 patients (placebo arm: 104 cases and 105 non-cases; palbociclib arm: 114 cases and 119 non-cases) were successfully analyzed for mutational status. A total of 42 (9.5%) patients (placebo arm: 9.1%; palbociclib arm: 9.9%) carried any mutation. 15 (3.4%) patients had a gBRCA1/2 mt (one of whom carried a gATM mt and one a gCHEK2 mt in addition to gBRCA2 mt) and 29 (6.6%) had mutations in one of the other BC predisposition genes (n=8 CHEK2, n=7 PALB2, n=5 ATM, n=2 RAD50, n=1 for BARD1, FANCM, MRE11A, RAD51C, RAD51D, TP53 and n=1 both RAD51D and BRIP1). The mutational status with respect to all genes analyzed showed no significant correlation to clinical baseline variables. With regard to gBRCA1 and gBRCA2 genes only, the mutational status significantly correlated with age but not with other clinical variables: all 15 (100%) gBRCA mt carriers were younger than 50 years compared to 238 (56%) wildtype (wt) patients (p=0.002). The iDFS rate after 3 years was 80.9% in patients with any mutation and 79.5% in patients without. Mutational status (mt vs. wt) based on all genes analyzed was not prognostic (iDFS: hazard ratio 1.015, 95%CI 0.558-1.784; DDFS: 0.970, 95%CI 0.521-1.758; OS: 0.768, 95%CI 0.274-1.615). Neither the mutated patients had a benefit from palbociclib treatment (palbociclib vs placebo; iDFS: hazard ratio 0.766, 95%CI 0.263-3.022; DDFS: 0.897, 95%CI 0.275-3.489; OS: 0.666, 95%CI 0.063-5.671) nor the wt patients (iDFS: hazard ratio 0.918, 95%CI 0.650-1.303; DDFS: 0.966, 95%CI 0.679-1.393; OS: 0.901, 95%CI 0.573-1.433); interaction tests for treatment arm/mutational status for all time-to-event endpoints were not statistically significant. Analysis in the subgroups of patients by gBRCA1/2 showed similar results but had less statistical power. Conclusions: In this case-cohort analysis of 442 patients enrolled in the PENELOPE-B trial, the detection of BC predisposition genes was lower than expected with 10%. This is probably due to the low rate of gBRCA1/2 carriers (3.4%), which could be influenced by the selection criteria of the trial. Patients with gBRCA1/2 or other BC disposition genes had a comparable outcome to non-carriers in the PENELOPE-B trial. Citation Format: Sibylle Loibl, Jan Hauke, Karen Gelmon, Frederik Marmé, Corinna Ernst, Miguel Martin, Michael Untch, Hervé Bonnefoi, Erik Knudsen, Seock-Ah Im, Angela DeMichele, Laura Van’t Veer, Sung-Bae Kim, Harry Bear, Nicole McCarthy, Nicholas Turner, Agnieszka Witkiewicz, Federico Rojo, Peter A Fasching, José A García-Sáenz, Catherine M Kelly, Toralf Reimer, Masakazu Toi, Hope S Rugo, Carsten Denkert, Michael Gnant, Andreas Makris, Yuan Liu, Olga Valota, Bärbel Felder, Karsten Weber, Valentina Nekljudova, Eric Hahnen. Germline BRCA1/2 and other predisposition genes in high-risk early-stage HR+/HER2- breast cancer (BC) patients treated with endocrine therapy (ET) with or without palbociclib: A secondary analysis from the PENELOPE-B study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-13-36.

Purpose: Exposure to particulate matter (PM) air pollution has been associated with adverse effect on respiratory disease, but no study has investigated its impact on radiation-induced pneumonitis (RIP) in patients with breast cancer who received adjuvant radiation therapy (RT). Materials and Methods: We conducted a retrospective review of 2,762 breast cancer patients who received postoperative RT between 2017 and 2020 in a single institution. Particulate matter data including PM2.5 and PM10 were retrieved from the open dataset in the ‘Gyeonggi Data Dream’. We used the average, median and maximum values of the PM2.5 and PM10 measured during daytime when a patient visited to the hospital for RT. We excluded the patients when missing PM data exceeded 50% of the total treatment period (N=26). Finally, altogether 2,736 patients were analyzed. Results: Overall incidence rate of RIP was 1.74%. There were no significant differences in average value of PM2.5 and number of RT fractions between RIP (+) and RIP (-) groups, but marginal differences were found in RT techniques (3-dimentional conformal RT vs. intensity modulated RT, P=0.053) and proportion of PM2.5 value ≥ 35 (μg/m3) (P=0.053) between the two groups. After adjusting for age, RT technique, regional irradiation, fractionation and boost, the average value of PM2.5 was significantly associated with a higher risk of RIP (Odds ratio [OR] 1.05, 95% confidential incidence [CI] 1.00-1.10, P=0.047) when patients received ≥ 20 fractions of RT. Especially, PM2.5 ≥ 35 (μg/m3) showed significant higher risk of RIP (OR 4.32, 95% CI 1.34-13.87, P=0.014) in patients with ≥ 20 fractions of RT after adjustment of aforementioned covariates. Other PM data (median, maximum value of PM2.5 and all PM10 values) were not related to the RIP regardless of the number of RT fractions. Conclusion: This is the first study to reveal the association between PM2.5 and RIP in breast cancer patients who received 20 fractions or more of postoperative RT. Citation Format: Dong-Yun Kim, In Ah Kim, Bum-Sup Jang. The impact of PM2.5 on the radiation-induced pneumonitis in patients with breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-28.

Background PARP inhibitors have proven efficacy in breast cancer patients with germline BRCA1 or BRCA2 mutations (gBRCAmt) but also have potential to be effective in cancers with defects in homologous recombination (HR) DNA repair. Moreover, synergy between PARP inhibition and immune checkpoint blockade is expected based on increased mutation burden, neoantigen expression, and immunogenic cell death. Triple negative breast cancer (TNBC) or low estrogen receptor (ER)-positive breast cancers may have diverse defects in HR repair. Methods This is a window of opportunity, serial biopsy trial of olaparib and durvalumab before standard neoadjuvant chemotherapy for TNBC or low ER+ breast cancer (NCT03594396). Patients had clinical stage II/III TNBC or low ER+ HER2- breast cancers where ER was expressed in 10% or lower in tumor cells. Olaparib 300mg bid was given for 4 weeks without rest. One dose of durvalumab 1500mg was given on day 15. Study tumor biopsy and blood sample were taken before the study treatment, after 2 weeks of olaparib before durvalumab, and 2 weeks after durvalumab at the completion of 4 weeks of olaparib. FDG-PET/MRI was taken at baseline, after 2 weeks, after 4 weeks of study treatment, and computed tomography (CT) scan at baseline and after 4 weeks. After the study treatment, standard neoadjuvant chemotherapy with 4 cycles of doxorubicin+cyclophosphamide followed by 4 cycles of docetaxel was given. Primary endpoint was the changes of tumor biology detected by serial tumor biopsy. Secondary endpoints were pathological complete response (pCR) rate, response rate, prediction of early metabolic response by functional HR status, and safety. Functional HR deficiency was assessed by counting RAD51 foci as a marker for HR repair in the baseline tumor tissues before and after 30Gy irradiation to induce DNA damage. Results Fifty-four female patients were enrolled (median age 40 years, range 24-68). ER was negative (TNBC) in 43 patients and low-positive in 11 patients. Clinical stage was II in 25 patients and III in 29 patients, and axillary lymph node involvement was in 47 patients. gBRCAmt was assessed in 53 patients and 16 (30.1%) harbored pathogenic mutations. Functional HR status as measured by RAD51 foci formation was deficient in 27 (50%) patients and proficient in 27 (50%) patients. Functional HR deficiency was related with early metabolic response by FDG-PET after 2 weeks of olaparib (response in 17/27 HR-deficient tumors [63.0%] vs. 7/27 HR-proficient tumors [25.9%]; p=.006). After 4 weeks of olaparib and durvalumab, HR-deficiency was still related to metabolic response (23/27 vs. 17/27, respectively; p=.062) but HR-proficient tumors also showed metabolic decline. Moreover, HR deficiency was also related with RECIST response measured by CT after 4 weeks (17/27 vs. 9/27; p=.029). As of June 2021, 40 patients completed the neoadjuvant treatment and surgery; among those, 30 achieved pCR (pCR rate 75%). Among 13 patients with gBRCAmt who underwent surgery, 11 achieved pCR (84.6%). Updated results on pCR will be presented. Conclusions Olaparib and durvalumab followed by standard neoadjuvant chemotherapy shows very high pCR rate in TNBC or low ER+ stage II/III breast cancer. Functional HR status as measured by RAD51 foci formation was predictive of early metabolic response to olaparib. Genomic and transcriptomic analyses are underway in the samples before, during, and after olaparib and durvalumab. Citation Format: Seock-Ah Im, Kyung-Hun Lee, Ahrum Min, Daewon Lee, Tae Yong Kim, Han Suk Ryu, Jiwon Koh, Gi-Jeong Cheon, Yoon-Jung Shin, Yujin Kim, Hyeong-Gon Moon, Wonshik Han, Han-Byoel Lee, Morgan Diolaiti, David Quigley, Alan Ashworth, Nariya Cho. Window of opportunity trial of neoadjuvant olaparib and durvalumab for triple negative or low ER-positive breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD15-08.

CDK4/6 inhibitors such as palbociclib in combination with endocrine therapy (ET) have remarkablyimproved the outcome of patients with ER+/HER2- metastatic breast cancer (MBC). However, manypatients are intrinsically resistant to CDK4/6i therapy, and those who respond eventually acquireresistance. Although high baseline CCNE1 expression and rare alterations in RB1 and FAT1 geneshave been shown to be associated with CDK4/6i resistance, the molecular mechanisms of CDK4/6iresistance are complex and remain poorly understood. To better understand and overcome CDK4/6iresistance, we performed multi-omics profiling of paired tumor biopsies from ER+/HER2- MBCpatients treated with palbociclib combined with ET. Tumor biopsies taken at pre-treatment, on-treatment, and progressive disease (PD) from 71 patients were profiled using whole-exomesequencing (WES), whole-transcriptome sequencing (RNA-Seq) and IHC analysis. Ourcomprehensive analysis identified several tumor intrinsic molecular markers associated with worsePFS, including the Luminal B subtype (p=0.012, HR=2.593), BRCA1/2 pathogenic mutation (p=0.012,HR=2.67) and mutation signatures linked to APOBEC enzymatic activity (p=0.002, HR=3.19).Conversely, the estrogen response signature (p=0.006, HR=0.43) was associated with favorableprognosis. Unsupervised analysis revealed a cluster of tumors enriched in homologousrecombination deficiency (HRD) linked genomic scars that was associated with poor prognosis(p=0.005, HR=2.49). Of note, these HRD-high tumors responded even more poorly to treatment whenco-occurring with TP53 somatic mutations. Integrative analysis further identified three poorprognosis clusters (IC2-4) enriched in Luminal B, proliferative and HRD features when compared tothe favorable prognosis cluster (IC1).Comparing baseline vs. PD samples, we observed a pattern of post-treatment enrichment for the poorprognosis markers. In addition, breast cancer-associated genes such as BRCA1/2, TP53 and PTENharbored a higher prevalence of genomic alterations including somatic mutation, amplification,. deletion and gene fusion at PD. Cell cycle gene expression and signatures also markedly increased atPD compared to baseline whereas estrogen response signatures decreased. Upon diseaseprogression, tumors had frequently switched to molecular subtypes with aggressive and estrogenindependent characteristics, demonstrating high plasticity in response to CDK4/6i and ET treatment.These patterns of acquired resistance were validated by IHC analysis of cyclins E1 and E2, Ki67 andpRb. To investigate the genomic alterations responsible for acquired resistance, we compared 21paired baseline and PD samples. We observed that PD-specific RB1 loss-of-function events occurredwith higher prevalence than previously reported, underscoring a major role of cell cycle de-regulation in conferring resistance to CDK4/6 inhibition. In this prospective longitudinal multi-omicsstudy, we identified novel candidate biomarkers that can be used to improve prediction of responseto CDK4/6i. In addition, we derived new insights into the molecular mechanisms of drug resistanceto palbociclib plus ET that will help guide therapeutic strategies and drug development inHR+/HER2− MBC. Citation Format: Zhengyan Kan, Seock-Ah Im, Kyunghee Park, Ji Wen, Kyung-Hun Lee, Yoon-La Choi, Won-Chul Lee, Ahrum Min, Vinicius Bonato, Seri Park, Sripad Ram, Dae-Won Lee, Ji-Yeon Kim, Su Kyeong Lee, Won-Woo Lee, Jisook Lee, Miso Kim, Scott L. Weinrich, Han Suk Ryu, Tae Yong Kim, Stephen Dann, Diane Fernandez, Jiwon Koh, Song Yi Park, Shibing Deng, Eric Powell, Rupesh Kanchi Ravi, Jadwiga Bienkowska, Paul A. Rejto, Woong-Yang Park, Yeon Hee Park. Serial genomic profiling reveals molecular mechanisms of breast cancer resistance to palbociclib [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD2-08.

Background: More than two thirds of patients with advanced breast cancer (ABC) have estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2−) tumors. In most countries, current standard-of-care first-line treatments include an aromatase inhibitor or fulvestrant, a selective ER degrader, combined with cyclin-dependent kinase 4/6 inhibitors. Concurrent use of luteinizing hormone-releasing hormone agonists is recommended for men and premenopausal women with ABC. Nevertheless, almost all ABCs eventually become resistant to endocrine therapy and the disease is incurable in these cases. New therapies are needed to combat endocrine therapy resistance, maintain patient health-related quality-of-life, and delay the need for chemotherapy. AZD9833 (camizestrant) is a highly potent, next-generation selective ER degrader and pure ER antagonist that has demonstrated anticancer properties across a range of preclinical models, including those with ER-activating mutations (Scott et al, AACR 2020). A Phase I dose-escalation and expansion study (SERENA-1) has demonstrated that AZD9833 is well tolerated and has a promising antitumor activity when administered alone or in combination with the cyclin-dependent kinase 4/6 inhibitor palbociclib (Baird et al, SABCS 2020). SERENA-4 (NCT04711252) is a randomized, multicenter, double-blind, Phase III trial to evaluate the safety and efficacy of AZD9833 in combination with palbociclib for patients with ER+/HER2− ABC who have not received systemic treatment in the advanced disease setting. Methods: SERENA-4 will enroll 1342 patients with de novo or recurrent ER+/HER2− ABC who have not previously received systemic treatment for their locoregionally recurrent or metastatic disease. Patients with recurrent disease must have received adjuvant aromatase inhibitor or tamoxifen therapy for at least 24 months without relapse. Patients will be randomized 1:1 to receive oral treatment with either (a) AZD9833 75 mg once daily, palbociclib 125 mg once daily for 21 days followed by 7 days off treatment and a placebo for anastrozole 1 mg once daily or (b) anastrozole 1 mg once daily, palbociclib (same as active arm) and a placebo for AZD9833 75 mg once daily. Men and premenopausal women will also receive a luteinizing hormone-releasing hormone agonist. The primary endpoint is progression-free survival (PFS; up to 5 years). Secondary endpoints include overall survival (up to 8 years), second PFS, time to chemotherapy, objective response rate, and changes in health-related quality-of-life measures. Enrollment began in January 2021. As of 02 July 2021, the number of open sites is 57 across 15 countries. Acknowledgments: We thank Julia Mawer, PhD, of Oxford PharmaGenesis, Oxford, UK, for providing medical writing support funded by AstraZeneca Funding: The SERENA-4 trial is funded and overseen by AstraZeneca. © 2021 American Society of Clinical Oncology, Inc. Reused with permission. This abstract was accepted and previously presented at the 2021 ASCO Annual Meeting. All rights reserved. Citation Format: Fabrice André, Seock-Ah Im, Patrick Neven, Richard D Baird, Johannes Ettl, Matthew P Goetz, Erika Hamilton, Hiroji Iwata, Zefei Jiang, Anil Abraham Joy, Vincent Haddad, Andrew Walding, Manuel Selvi Miralles, Cynthia Huang Bartlett, Antonio Llombart-Cussac. SERENA-4: A Phase III comparison of AZD9833 (camizestrant) plus palbociclib, versus anastrozole plus palbociclib, for patients with ER-positive/HER2-negative advanced breast cancer who have not previously received systemic treatment for advanced disease [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-11-06.

Background: Despite the availability of multiple HER2-targeted therapies, most patients (pts) with advanced HER2-positive BC experience disease progression, and an unmet medical need remains. Zanidatamab (zani) binds to HER2 across a range of expression levels and induces formation of receptor clusters, resulting in receptor internalization and downregulation that affect signal transduction. Zani potently activates antibody-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis, and complement-dependent cytotoxicity. In this ongoing phase 1 study (NCT02892123), data from BC pts treated with zani monotherapy have been previously reported. Here, we evaluated the safety and antitumor activity of zani in combination with chemo in HER2-positive BC pts. Methods: Pts with locally advanced and/or metastatic HER2-positive (immunohistochemistry [IHC] 3+ or IHC 2+/fluorescence in situ hybridization+) BC who received prior trastuzumab (tras), pertuzumab (pert), and T-DM1 were enrolled. Eligibility criteria included Eastern Cooperative Oncology Group performance status ≤1 and adequate organ function. Prior history of treated stable brain metastases was allowed. Zani (20 mg/kg Q2W or 30 mg/kg Q3W) was administered in combination with paclitaxel (pac), capecitabine (cap), or vinorelbine (vin). Zani + chemo was continued until disease progression or toxicity; if chemo was discontinued due to toxicity, zani treatment could be continued. Primary endpoints were safety and tolerability assessments; secondary efficacy endpoints included objective response rate (ORR) and disease control rate (DCR; complete response [CR] + partial response [PR] + stable disease [SD]) per RECIST v1.1, and progression-free survival (PFS). Clinical benefit rate (CBR) was defined as SD ≥24 weeks or best overall response of CR or PR. Results: As of 3 May 2021, 20 HER2-positive BC pts had enrolled (zani + pac, n=4; zani + cap, n=9; zani + vin, n=7). Ten pts were hormone receptor positive (data available for 19 pts), and 7 pts had a prior history of brain metastases. Median age was 53 years (range, 38-72), with median prior systemic therapies of 4.5 (range, 2-7) and median of 2 regimens in the metastatic setting. Pts received a median of 3 prior HER2 regimens; all pts received tras and T-DM1, 17 received pert, and 6 received tyrosine kinase inhibitors. In the 16 response evaluable (measurable disease and either post-baseline assessments or earlier death/progression) pts, confirmed ORR was 37.5% (with 1 additional response pending confirmation), CBR was 50%, and DCR was 81.3%. Median duration of response was not reached (range, 1.8+ to 16.8+ mo), with 4 of 6 confirmed responses ongoing. Among all pts (n=20), the most common (≥30% of pts) zani- and/or chemo-related adverse events (AEs) were diarrhea (65%), nausea (45%), peripheral neuropathy (35%), and fatigue (30%). One pt experienced grade 3 diarrhea leading to dose reduction of zani, and 1 pt discontinued zani due to AEs (abdominal pain and nausea, both grade ≤2). Four grade 4 AEs were observed in 4 (20%) pts: neutrophil count decreased (2 pts) and neutropenia (1 pt), all related to chemo, and hyponatremia (1 pt, not related to study treatment). Serious AEs were observed in 2 (10%) pts (pleural effusion, 1 pt; pneumonitis and upper respiratory tract infection, 1 pt), none related to zani. No grade 5 AEs were reported. Conclusions: Zani in combination with chemo is well tolerated, with encouraging and durable antitumor activity in heavily pretreated (including prior tras, pert, and T-DM1) pts with HER2-positive BC. These data support further investigation of zani as a novel therapeutic for these pts. The cap and vin cohorts continue to enroll, and additional cohorts are evaluating zani ± chemo in combination with tucatinib. Citation Format: Philippe L Bedard, Seock-Ah Im, Elena Elimova, Sun Young Rha, Rachel Goodwin, Cristiano Ferrario, Keun-Wook Lee, Diana Hanna, Funda Meric-Bernstam, Jose Mayordomo, Murali Beeram, Erika Hamilton, Jorge Chaves, Melody Cobleigh, Tony Mwatha, Joseph Woolery, Do-Youn Oh. Zanidatamab (ZW25), a HER2-targeted bispecific antibody, in combination with chemotherapy (chemo) for HER2-positive breast cancer (BC): Results from a phase 1 study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-07.

Background: The Phase III MONALEESA (ML)-2, -3, and -7 trials showed significant improvement in progression-free survival (PFS) and overall survival (OS) with ribociclib (RIB) + endocrine therapy (ET) over placebo (PBO) + ET in patients (pts) with HR+/HER2− advanced breast cancer (ABC); improvement in OS with cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) has been observed in some, but not all clinical trials. Gene expression analyses for each separate ML study were reported previously. Given the differences in CDK4 vs CDK6 inhibition between RIB and other CDK4/6i, we evaluated the association between cell cycle (CC)–related genes and outcomes based on pooled analysis of gene expression using tumor samples from the ML-2, -3, and -7 trials. Methods: Gene expression data were generated from pre-treatment archival tumor samples (primary, 73%; metastatic, 27%) with a customized NanoString nCounter panel (781 genes) including genes involved in CC, other signaling pathways, and breast cancer biology. Samples were pooled from 1139 pre- and postmenopausal pts with HR+/HER2− ABC across the 3 ML studies, which included pts on first- and second-line therapy. Data were categorized into training (80%) and test (20%) datasets. The training dataset was used to analyze each gene (modeled continuously) individually for an association with PFS, and genes with a gene × treatment (tx) interaction P value Results: This report focused on CC-related genes and signatures. Gene expression levels of CDKN2B and the expression ratio of CCND1/CDKN2A showed a predictive relationship with benefit from RIB in both training and test sets (Table). PFS benefit with RIB was consistent regardless of the CDK4/CDK6 expression ratio or level of expression of CCNE1, CDK2, RB1, combined CC-related genes, E2F gene signatures, RB gene signature, combined DNA-replication genes, or combined proliferation-related genes. A machine learning approach identified a clinico-genomic signature that was prognostic for PFS benefit with RIB. Selected variables included gene expression levels of FXBO5, PGR, RBBP8, and STC2 and several clinical features (tx arm, de novo disease, prior ET, and visceral disease). Pts with a low signature score had a longer mPFS vs pts with a high signature score, in the RIB (HR, 0.37; 95% CI, 0.22-0.62) and PBO (HR, 0.30; 95% CI, 0.15-0.59) arms. Conclusion: In the largest pooled analysis of the association of gene expression profile data with CDK4/6i tx response in pts with HR+/HER2− ABC, the PFS benefit with RIB + ET over ET alone was consistent irrespective of expression levels of most CC genes. Variation in magnitude of RIB benefit was observed, depending on CDKN2B expression levels, CCND1/CDKN2A expression ratio, and machine learning–derived signature scores. The clinico-genomic CDK4/6i signature requires validation in additional datasets. Table 1: Progression-Free Survival by Gene Expression Subgroup Citation Format: Aditya Bardia, Faye Su, Nadia Solovieff, Fabrice Andre, Carlos Arteaga, Patrick Neven, Yoon-Sim Yap, Yen-Shen Lu, Stephen K. Chia, Dennis Slamon, Seock-Ah Im, Arunava Chakravartty, Agnes Lteif, Tetiana Taran, Debu Tripathy. Pooled gene expression analysis and association with treatment response in patients with HR+/HER2− advanced breast cancer in the MONALEESA-2, -3, and -7 trials [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD17-08.

Background: Molecular plasticity of breast cancer is crucial for the development of therapy-resistant disease. In this investigation, we studied changes in molecular signatures between pretherapeutic (pre-Tx) and post-therapeutic (post-NACT) tumor samples from patients included in the PENELOPE-B (NCT01864746) trial. The phase III PENELOPE-B study did not show a significant benefit from palbociclib in women with centrally confirmed HR+, HER2- primary breast cancer without a pathological complete response after taxane-containing neoadjuvant chemotherapy (NACT) and at high-risk of relapse (CPS-EG score ≥3 or 2 and ypN+) (Loibl et al. JCO 2021). However, first translational investigations showed that a small number of patients with a luminal-B tumor subtype, based on absolute intrinsic molecular subtyping (AIMS, Paquet & Hallet, JNCI 2014) subtyping after NACT, had a numerical benefit from post-NACT palbociclib. We have therefore extended the analysis and included a cohort of paired pre-Tx and post-NACT samples. Methods: We investigated gene expression in pre-Tx (n=259) tumor tissue samples using the HTG EdgeSeq Oncology Biomarker Panel including 2549 genes (HTG Molecular Diagnostics Inc.); for the same patients the same panel on post-NACT residual tumor samples were available. The paired samples were selected based on a case-cohort approach. Based on 91 genes of this panel, the AIMS subtype was calculated. In addition, we performed exploratory biomarker analyses to identify genes and gene signatures with prognostic and predictive relevance. After completion of NACT, PENELOPE-B patients were randomized to palbociclib versus placebo in addition to standard endocrine therapy. Results: The prevalence of AIMS subtypes, in particular LumA vs LumB, changed in pre-Tx and post-NACT tumors. In the pre-Tx samples, 115 (44%) and 123 (47%) of tumors had LumA and LumB subtypes, respectively, as expected from a high-risk cohort. However, in the post-NACT samples, LumA tumors were predominant (n=183, 71%) over LumB (n=30; 12%). 78 (30%) and 6 (2%) tumors switched their subtype from LumB to LumA and LumA to LumB, respectively. For further analyses, we compared the groups of low proliferating (LumA and NormL) and high proliferating subtypes (LumB, BasalL and HER2E). In bivariable Cox regression analysis, the grouped pre-Tx and post-NACT AIMS subtypes were independent prognostic factors for iDFS: HR=1.85 (1.16-2.98, p=0.011) for pre-Tx LumB/BasalL/HER2E vs LumA/NormL and HR=2.18 (1.24-3.84, p=0.007) for post-NACT. Similar results were found when adjusted for prognostic clinical factors and for DDFS and OS endpoints although the pre-Tx subtype did not reach significance. These and further Cox models investigating interaction effects show that patients with tumors developing from high (pre-Tx) to low proliferation (post-NACT) had a higher iDFS risk compared to stable low proliferating tumors but a lower iDFS risk compared to stable high proliferating tumors. Neither in the pre-Tx LumB/BasalL/HER2E nor in the pre-Tx LumA/NormL subgroup a benefit from palbociclib was observed. Based on the results of the AIMS subtyping, we extended the exploratory analysis to identify genes that might be involved in the prognostic and predictive effects as well as genes driving the subtype switch. The analysis is ongoing and the relevant genes will be presented at the conference. Conclusions: Our findings show that the switch from high-risk molecular subtypes (in particular LumB) to low-risk subtypes (in particular LumA) is common in neoadjuvant therapy of luminal tumors. The adaptation of luminal high-risk tumors to therapy-induced stress is crucial for the clinical outcome and the results suggest that molecular defined tumor subtypes might not be as stable as originally thought. Citation Format: Carsten Denkert, Frederik Marmé, Miguel Martin, Michael Untch, Hervé Bonnefoi, Sung-Bae Kim, Harry Bear, Agnieszka Witkiewicz, Seock-Ah Im, Angela DeMichele, Laura Van’t Veer, Nicole McCarthy, Thorsten Stiewe, Karen A. Gelmon, José A. García-Sáenz, Catherine M. Kelly, Toralf Reimer, Erik Knudsen, Nicholas Turner, Federico Rojo, Peter A. Fasching, Julia Teply-Szymanski, Yuan Liu, Masakazu Toi, Hope S. Rugo, Michael Gnant, Andreas Makris, Bärbel Felder, Karsten Weber, Sibylle Loibl. Molecular plasticity of luminal breast cancer and response to CDK 4/6 inhibition - The biomarker program of the PENELOPE-B trial investigating post-neoadjuvant palbociclib [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD2-04.

Background Trastuzumab deruxtecan (T-DXd, DS-8201) is an antibody-drug conjugate with a HER2 antibody, tetrapeptide-based cleavable linker, and a novel topoisomerase I inhibitor payload. DESTINY-Breast01 (NCT03248492) is an open-label, international, multicenter, phase 2 study of T-DXd in patients with HER2 positive metastatic breast cancer (MBC) and supported regulatory approval in the US and Japan. Updated longer-term safety and efficacy results are presented here. Methods All patients were required to have MBC that progressed on or after T-DM1. 253 patients were enrolled and 184 received T-DXd 5.4 mg/kg, representing the primary analysis set. The primary endpoint was ORR. Additional endpoints included duration of response, PFS, and OS. Results Patients had received a median of 6 previous lines of treatment for MBC. In this updated data cutoff (8 June 2020) compared to the prior data cutoff (1 Aug 2019), median duration of follow-up has increased from 11.1 to 20.5 mo; 37 patients (20.1%) remain on treatment. Confirmed ORR was 61.4% (12 CRs) with a median duration of response of 20.8 mo; the disease control rate was 97.3% (95% CI, 93.8-99.1). The updated mPFS was 19.4 mo (95% CI, 14.1 mo-NE). Estimated OS was 85% (95% CI, 79%-90%) at 12 months and 74% (95% CI, 67%-80%) at 18 months. The preliminary mOS is 24.6 mo (estimated at 35% maturity with only 17 patients at risk at 24 months). The safety profile of T-DXd was similar as that previously reported; with an additional 9 mo follow-up, only 3 new cases of T-DXd-related interstitial lung disease (ILD) were reported. Results are summarized in the table below. Conclusion Consistent with prior results, T-DXd demonstrated high rates of durable responses in a heavily pretreated population of patients with MBC. From this single-arm, phase 2 study, the PFS and immature OS results are encouraging; these endpoints will be further evaluated in the ongoing randomized controlled studies of T-DXd. For patients who remained on treatment for this longer duration (double that of the previous report), the rate of discontinuation or ILD did not notably increase. Continued attention to pulmonary symptoms and careful monitoring is warranted. Updated Results for DESTINY-Breast011 Aug 2019 datacut8 Jun 2020 datacutPatients remaining on treatment, n/N (%)79/184 (42.9%)37/184 (20.1%)Median duration of follow-up11.1 months20.5 monthsORR60.9%61.4%CR6.0%6.5%PR54.9%54.9%SD36.4%35.9%Duration of response, median (95% CI)14.8 months (13.8-16.9)20.8 months (15.0-NE)PFS, median (95% CI)16.4 months (12.7-NE)19.4 months (14.1-NE)OSMedian (95% CI)NE (NE-NE)24.6 months (23.1-NE)Point estimate at 12 mo (95% CI)86.2% (79.8-90.7)85% (79-90)Point estimate at 18 mo (95% CI)–74% (67-80)SafetyPatients with a TEAE, n (%)183 (99.5%)183 (99.5%)Grade ≥3105 (57.1%)113 (61.4%)Associated with discontinuation28 (15.2%)34 (18.5%)Associated with death10 (5.4%)10 (5.4%)Drug-related ILD per ILD adjudication committeea25 (13.6%)28 (15.2%)Grade 5 drug-related ILD per ILD adjudication committee4 (2.2%)5 (2.7%)a1 grade 1 and 1 grade 3 event are pending adjudication and are not included. Citation Format: Shanu Modi, Cristina Saura, Toshinari Yamashita, Yeon Hee Park, Sung-Bae Kim, Kenji Tamura, Fabrice Andre, Hiroji Iwata, Yoshinori Ito, Junji Tsurutani, Joohyuk Sohn, Neelima Denduluri, Christophe Perrin, Kenjiro Aogi, Eriko Tokunaga, Seock-Ah Im, Keun Seok Lee, Sara A Hurvitz, Javier Cortes, Caleb Lee, Yali Liu, Lin Zhang, Javad Shahidi, Antoine Yver, Jose Baselga, Ian E Krop. Updated results from DESTINY-breast01, a phase 2 trial of trastuzumab deruxtecan (T-DXd ) in HER2 positive metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD3-06.

Background: Alterations in BRCA1/2 are associated with ~9% of all BCs. Ola (a poly ADP-ribose polymerase inhibitor [PARPi]) is approved for treating pts with HER2-negative metastatic BC with germline BRCA mutation (gBRCAm), demonstrating an improvement in progression-free survival (PFS). Ceralasertib (an ataxia telangiectasia and Rad3-related protein ATR inhibitor) targets DNA damage repair and cell cycle regulation. Preclinical studies show synergistic antitumor effects of ola+cer vs ola monotherapy supporting the clinical evaluation of this combination. Methods: Study 4 is a modular multicenter Phase I study of cer combinations (NCT02264678). Module 2 established the Recommended Phase 2 Dose of ola+cer as ola 300 mg bid daily + cer 160 mg qd D1-7 q 28d. We report the results of two expansion cohorts testing ola+cer in pts with triple negative breast cancer with no alterations in homologous recombination repair (HRR)-related genes (HRR wt group), and pts with advanced HER2- BRCAm BC (BRCAm group), with a data cut-off of 19th June 2019. Eligible pts had to have received 1-2 prior lines of chemotherapy for metastatic disease, and were PARPi naive. Patients may have enrolled directly based on a local BRCAm test result, but all pts submitted a tumor specimen for central confirmation of a deleterious (or suspected deleterious) germline or somatic BRCA1/2 mutation. The primary objective was to investigate the safety and tolerability of the combination; secondary objectives included assessment of objective response rate by RECIST 1.1. Results: Twenty-five pts were enrolled in the HRR wt group: median age 53 (31-75), ECOG PS 0/1 13 (52%)/12 (48%), median number of prior chemotherapy lines 2 of which 8 pts (32%) had received prior platinum. Thirty-seven pts enrolled in the BRCAm group: median age 51 (24-69), ECOG PS 0/1 20 (54%)/17 (46%), median number of prior chemotherapy lines 1 of which 19 pts (51%) had received prior platinum. The most frequent all causality Adverse Events (AEs) were nausea 43 (69%), anemia 36 (58%), diarrhea 20 (32%) and vomiting 20 (32%); CTCAE ≥ Grade 3 AEs included anemia 15 (24%), neutropenia 6 (10%) with few patients discontinuing treatment. In the HRR wt group, no responses were observed, 12 (48%) pts achieved a best response of stable disease and 13 (52%) had disease progression. The median PFS was 3.1 months (80%CI 2.0,3.9). In the BRCAm group, 13 (35%) pts achieved a confirmed response (including 1 complete response), 17 (46%) stable disease and 7 (19%) pts disease progression. The median PFS in pts with a centrally confirmed BRCAm was 11.5 months (80%CI 5.8-14.8, n=30), 7.7 months (80%CI 5.8-11.4, n=37) in all pts enrolled in the BRCAm group. Eleven of the 13 responders in the BRCAm group were on study treatment for a longer duration than their treatment immediately prior to enrollment. As of 18th June 20, preliminary data shows 10 pts are still ongoing with a treatment duration ranging 20 to 35 months. Conclusion: In pts with HER2- BRCAm breast cancer, clinical efficacy was observed with durable radiological response despite adverse prognostic features in this Phase I population. The observations in this study are currently being tested in a global multicenter open-label randomised Phase 2 study: VIOLETTE (NCT03330847). Citation Format: Emma Dean, Matthew G Krebs, Seock-Ah Im, Mario Campone, Sophie Postel-Vinay, Tobias Arkenau, Juanita Lopez, Wassim Abida, Duncan Jodrell, Keun-Wook Lee, SunYoung Rha, Irene Kang, Rebecca Roylance, Graeme Parr, Claire Smith, Andrew Goldwin, Elhan Sanay, Natasha Lukashchuk, Alan Lau, Andrew Pierce, Bienvenu Loembe. Ceralasertib (cer) in combination with olaparib (ola) in patients (pts) with advanced breast cancer (BC): Results of phase I expansion cohorts [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS11-18.

Background: Margetuximab (M) is an investigational Fc-engineered, anti-HER2 monoclonal antibody with the same epitope specificity as trastuzumab (T). Relative to T, Fc engineering of M increases binding affinity for the activating Fc receptor (FcR) CD16A and decreases affinity for the inhibitory FcR CD32B. The SOPHIA trial (NCT02492711) randomized pretreated HER2+ MBC patients to either M or T, each with chemotherapy (C). M+C improved progression-free survival (PFS) benefit over T+C. Infusion related reactions (IRR) have been observed after infusion of therapeutic proteins, typically during the first infusion, with a first-dose incidence of up to 40% for T (Herceptin Prescribing Information®). A retrospective analysis of 197 patients showed 16% IRRs on T, mostly after the first infusion; this rate was lower with (10%) compared to without (19%) premedication (Thompson, 2014). Here, we report IRR safety and tolerability data in the SOPHIA trial patients following HER2-targeted antibody therapy. Methods: The open-label Phase 3 SOPHIA trial enrolled patients with HER2+ MBC after at least 2 prior anti-HER2 therapies, including pertuzumab; 91% in both groups also received ado-trastuzumab emtansine. Randomization of 536 patients was 1:1 to M (15 mg/kg IV q3w) or T (6 [8 loading dose] mg/kg IV q3w), each with Investigator selected C (standard dose capecitabine, eribulin, gemcitabine, or vinorelbine). M was given as 120-minute infusions. T was given as a 90-minute infusion in Cycle 1, then a 30-minute infusion from Cycle 2 onward. Recommended elective premedication included acetaminophen, ibuprofen, diphenhydramine, ranitidine, dexamethasone, or equivalents. IRR safety analyses were conducted on 530 patients (264 M+C and 266 T+C) that received any study therapy. Results: A higher proportion of patients experienced IRRs on the M arm (35 [13.3%]) than on the T arm (9 [3.4%]). Most IRRs in both groups were severity Grade 1 or 2, occurred on Cycle 1 Day 1, and resolved within 24 hours. In patients receiving M, Grade 3 IRR occurred in 4 patients (1.5%), including 3 after vinorelbine and 1 after eribulin. Adverse events associated with Grade 3 IRRs included chills, fever, nausea, diarrhea, dyspnea, and/or hypertension. Two patients receiving M (0.8%) discontinued due to IRR, versus none on T. Of patients with IRRs, the most common symptoms in both treatment groups were chills (M: 17 [48.6%]; T: 5 [55.6%]) and fever (M: 13 [37.1%]; T: 2 [22.2%]). There was no observed hypotension in either group. In both groups, more than half of IRR events were addressed by dose interruption only. All IRRs all were medically manageable. IRR rates were higher in patients without premedication for both groups. Of 264 subjects receiving M, 218 (82.6%) received premedication and 46 (17.4%) did not; IRRs were observed in 28 (12.8%) of those receiving premedication and 7 (15.2%) of those not premedicated. All 4 patients on M with Grade 3 IRRs received premedication, 3 with steroids. Of 266 subjects receiving T, 173 (65%) received premedication and 93 (35%) did not; IRRs were observed in 5 (2.9%) of those receiving premedication and 4 (4.3%) of those not premedicated. IRR risk was unaffected by chemotherapy subgroup or CD16A genotype. Conclusions: In the SOPHIA trial, IRR events occurred in a greater proportion of patients on M than on T. Most were mild to moderate in severity, limited to Cycle 1, and resolved on the same day. Symptom patterns were similar between groups, and premedication did not eliminate the hazard of IRRs in either group. Severe IRRs and discontinuations due to IRRs were rare. IRRs on first infusion of M appear to resemble those observed following first infusion of T. Citation Format: Javier Cortes, Fatima Cardoso, Giuseppe Curigliano, William J Gradishar, Seock-Ah Im, Hope S Rugo, Shakeela W Bahadur, Alfredo Falcone, Serafin Morales Murillo, David A Riseberg, Antonino Musolino, Trevor M Feinstein, Miguel H Abreu, Young-Hyuck Im, Yelena Novik, Timothy Pluard, Lupe G Salazar, Rossana Berardi, Viorela Pop, Shengyan Hong, Kenneth Jacobs, Edwin Rock, Mark D Pegram. Infusion related reactions in the phase 3 SOPHIA trial of margetuximab + chemotherapy vs trastuzumab + chemotherapy in patients with pretreated HER2+ metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-24.

Background: NET is offered to postmenopausal patients (pts) with clinical stage 2/3 ER+/HER2- BC to promote breast-conserving surgery. Also limited surgical accessibility during the COVID19 pandemic has increased NET utility. Inability to identify ET-resistant disease at diagnosis risks disease progression (PD) and delays more effective treatments. Dowsett et al. recently demonstrated that baseline levels of ER, progesterone receptor (PR), Ki67 (>15% vs ≤15%), and Ki67 (>10% vs ≤10%) 2-4 weeks (wks) after starting NET may improve appropriate patient (pt) selection for NET (PMC7280290). The ER, PR and Ki67-based prediction model divides pts with primary ER+/HER2- BC into 3 groups for appropriateness for NET: (Group 1) NET is likely to be inappropriate (Allred ER 10% has been associated with worse outcome (PMC5455353), or (Group 3) NET is appropriate (ER 8 and PgR ≥6). The ALTERNATE trial (NCT01953588) randomized postmenopausal women with clinical stage II or III, ER+ (Allred score 6-8)/HER2- BC to receive anastrozole (ANA), fulvestrant (FUL), or ANA + FUL for 6 months, unless Ki67 was >10% on wk 4 or 12 biopsy, in which case pts were triaged to receive neoadjuvant chemotherapy (NCT) or surgery. As previously reported, the ET-sensitive disease (mPEPI 0 plus pCR) rates were similar across the treatment arms and overall 22% (286 of 1,299) pts had Ki67 >10% at wk 4 or 12. The ALTERNATE trial therefore provides a large independent data set to evaluate the NET appropriateness model. Results: Among 1,299 eligible pts randomized to receive 6 months of NET, 214 were excluded due to absent HR Allred score (n=41) or absence of pre-treatment and wk 4 Ki67 determinations (n=173). The proportions of the remaining 1,085 pts in Group 1, 2 and 3 were 1% (n=10), 43% (n= 468), and 56% (n=607), respectively. On-study Ki67 >10% prompting conversion from NET to NCT/Surgery occurred in: Group 1 90% (9 of 10), Group 2 30% (141 of 468), and Group 3 17% (104 of 607) (Table 1). Among the 1,075 pts in Groups 2 and 3, 260 (24%) pts had Ki67 ≤15% at baseline (BL), among whom only 14 (5.4%) had Ki67 >10% at wk 4, compared to 231 of the 815 (28.3%) who had BL Ki67 >15% and subsequent Ki67 >10% at wk 4. 2% of pts who remained on NET due to on-treatment Ki67 Conclusion: ALTERNATE trial data support a model whereby levels of ER, PR and Ki67 at diagnosis can be used for the identification of postmenopausal pts with primary ER+/HER2- BC who are appropriate for NET. When baseline ER Allred scores are >6 and Ki67 ≤15%, there is a low likelihood of ET-resistant disease. When BL Ki67 is >15%, ET sensitivity is variable, and on-treatment biopsy for Ki67 may assist in triaging regarding NET appropriateness, particularly given the extremely low local PD rates seen in ALTERNATE when on-treatment Ki67 was Table 1 Baseline levels of ER, PR, and Ki67 in Relation to Wk 4 Ki67 (N=1,085)BaselineWeek 4GroupNERAllred ScorePRAllred ScoreKi67Ki67 ≤10%N (%)Ki67 >10%N (%)1N=2615%1 (12.5%)7 (87.5%)2AN=647 or 815%148 (63%)87 (37%)2BN=466 or 7≥6≤15%42 (91.3%)4 (8.7%)51 (30.2)N=1236 or 7≥6>15%76 (61.8%)47 (38.2%)3N=1508≥6≤15%143 (95.3%)7 (4.7%)104 (17.1)N=4578≥6>15%360 (78.8%)97 (21.2%) Citation Format: Matthew J Ellis, Vera Suman, A. Marilyn Leitch, Souzan Sanati, Kiran Vij, Gary W Unzeitig, Jeremy Hoog, Mark Watson, Olwen Hahn, Joseph Guenther, Abigail Caudle, Erika Crouch, Horacio Maluf, Mitch Dowsett, Amy Tiersten, Monica Mita, Wajeeha Razaq, Tina J Hieken, Yang Wang, Travis Dockter, Jo Anne Zujewski, Anna Weiss, Clifford Hudis, Eric P Winer, Kelly Hunt, Ann H Partridge, Cynthia X Ma, Lisa A Carey. Validation of a predictive model for potential response to neoadjuvant endocrine therapy (NET) in postmenopausal women with clinical stage II or III estrogen receptor positive (ER+) and HER2 negative (HER2-) breast cancer (BC): An ALTERNATE trial analysis (Alliance A011106) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD2-10.

Background: Triple-negative breast cancer (TNBC) is associated with poor survival outcomes and treatment options are limited. These tumors lack therapeutic targets and become rapidly resistant to chemotherapy. The anti–PD-1 antibody pembrolizumab showed durable antitumor activity and manageable safety in patients with TNBC in the KEYNOTE-012, KEYNOTE-086, and KEYNOTE-119 studies. The combination of lenvatinib, an antiangiogenic multiple receptor tyrosine kinase inhibitor, with pembrolizumab has shown promising clinical outcomes in early-phase clinical trials across several cancer types. LEAP-005 (ClinicalTrials.gov, NCT03797326) is an ongoing study evaluating the efficacy and safety of lenvatinib combined with pembrolizumab in patients with previously treated advanced solid tumors. Here, we report the first results from the TNBC cohort of LEAP-005. Methods: This ongoing, multicohort, open-label, phase 2 study enrolled patients aged ≥18 y with previously treated, histologically or cytologically confirmed advanced TNBC. PD-L1 expression was assessed at a central laboratory using the PD-L1 IHC 22C3 pharmDx assay and measured using the combined positive score (CPS; number of PD-L1–positive tumor cells, lymphocytes, and macrophages divided by total number of tumor cells x 100). Patients received lenvatinib 20 mg once daily orally plus pembrolizumab 200 mg every 3 weeks intravenously for a maximum of 35 pembrolizumab doses, then lenvatinib alone until progressive disease or unacceptable toxicity. Primary endpoints were objective response rate (ORR) by blinded independent central review per RECIST version 1.1 and safety. Key secondary endpoints were disease control rate (DCR; defined as best overall response of complete response [CR], partial response [PR], or stable disease [SD] per RECIST v1.1), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Safety was monitored through 30 days after the last dose of study drug (90 days for serious AEs), with AEs graded using NCI CTCAE v4.0. Results: 31 patients have been enrolled in the TNBC cohort of LEAP-005. Median age was 56 y (range, 37 to 85), 58% had received ≥2 prior lines of therapy, and 26% had CPS ≥10 tumors. As of the April 10, 2020 data cutoff, median follow-up was 7 mo (range, 4 to 13). ORR was 29% (95% CI: 14–48), with 1 CR and 8 PRs. 9 pts had SD, and the DCR (CR + PR + SD) was 58% (95% CI: 39–76). 4 responses (1 CR and 3 PRs) were in patients with CPS ≥10 tumors (n=8) for an ORR of 50% (95% CI: 16–84), and 5 responses (all PRs) were in patients with CPS Citation Format: Hyun Cheol Chung, Esma Saada-Bouzid, Federico Longo Muñoz, Eduardo Yanez, Seock-Ah Im, Eduardo Castanon, Donna M. Graham, Javier Garcia-Corbacho, Juanita Lopez, Razi Ghori, Corina Dutcus, Alan Smith, Kevin Norwood, Carlos Gomez-Roca. Lenvatinib plus pembrolizumab for previously treated, advanced triple-negative breast cancer: Early results from the multicohort phase 2 LEAP-005 study [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS12-07.

Background: Young-PEARL study showed median progression-free survival (PFS) of 20·1 months in the palbociclib plus endocrine therapy group versus 14·4 months in the capecitabine group (hazard ratio 0·659 [95% CI 0·437-0·994], log-rank p=0·0235). We conducted exploratory biomarker analysis to predict the efficacy of the trial. Methods: This was a phase II trial that randomized 184 patients with HR+ MBC in premenopausal women to palbociclib plus exemestane with GNRH agonist (Arm A, n=79) versus capecitabine (Arm B, n=62). We performed targeted sequencing (CancerSCANTM) containing 375 cancer-related genes (141 patients) and whole transcriptome sequencing (165 patients) using baseline tumor samples to examine genomic alteration in relation to drug response in terms of PFS. Result: By research-based PAM50 subtyping, 73% of patients classified as Luminal (Luminal A and Luminal B), and showed better prognosis in all patients and Arm A (p Conclusions: The alteration of a few genes including Rb1 loss may be associated with resistance of palbociclib in HR-positive premenopausal population with MBC. Luminal type showed better prognosis, and BRCA2 pathogenic mutation showed worse prognosis regardless luminal/non-luminal type. ESR1 mutation was found in low population frequency because all patients didn’t received AI therapy. Further exploration of molecular variables is warranted to determine and validate biomarkers of efficacy. Clinical trial information: NCT02592746. Citation Format: Kyunghee Park, Gun Min Kim, Kyung Hae Jung, Seok Yun Kang, In Hae Park, Jee Hyun Kim, Hee Kyung Ahn, Woong-Yang Park, Seock-Ah Im, Yeon Hee Park. Exploratory biomarker analysis of Young-PEARL [palbociclib plus exemestane with GnRH agonist versus capecitabine in premenopausal women with HR (hormone receptor)-positive, HER2-negative metastatic breast cancer (MBC)] study [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-19.

RUNX3 regulates EMT during development lapatinib resistanceBackground: Lapatinib is a small molecular inhibitor of HER2 and EGFR tyrosine kinase, which is used for HER2 positive metastatic breast cancer patients. However, significant proportion of patients relapse due to acquired resistance. The epithelial to mesenchymal transition (EMT), which induces cell migration, invasion and cancer stem cell progression, was one of the mechanism that confers to drug resistance. We determined the correlation of lapatinib resistance and EMT, and identified a regulatory factor of EMT which associated with the development process of acquired lapatinib resistance.Methods: Acquired lapatinib resistant SK-BR-3 (SK-BR-3 LR) cells were established by continuously exposing to lapatinib for 7 months. The sensitivity of lapatinib was confirmed by MTT assay. Cell cycle progression was verified using flow cytometry analysis. Expression of signal transduction molecules were determined using quantitative PCR, western blotting and transcriptome data analysis. Cell migration and invasion ability were verified using wound healing assay and Boyden chamber assay. siRNA knock-down system were used for further analysis. Results: Lapatinib resistant (SK-BR-3 LR) cells showed aggressive morphology compared with parental cells. The growth rate and cell cycle progression were increased in SK-BR-3 LR cells. Expression of Snail, Vimentin, SOX2, Nanog, TGF-b1 and Smad proteins increased in SK-BR-3 LR cells. In SK-BR-3 LR cells, cell migration and invasion were significantly increased. Correlated with a promoter methylation of RUNX3, expression of RUNX3 was down-regulated in SK-BR-3 LR cells. In Runx3 knock-down cells TGF-b1, SOX2, smad molecules were up-regulated. Conclusion: SK-BR-3 LR cells showed aggressive phenotype. Expression of EMT markers, cancer stem cell markers, TGF-β pathway associated molecules and Smad proteins were increased in SK-BR-3 LR cells. Also, the increase of cell migration and invasion was observed in LR cells. RUNX3 affected to cell migration and invasion through regulation of EMT associated molecules. Therefore, RUNX3 might be a specified molecule, which partially contributes resistance to lapatinib through regulates EMT. Citation Format: So Hyeon Kim, Ahrum Min, Seongyeong Kim, Yoonjung Park, Yu Jin Kim, Seock-Ah Im. Runx3 regulates emt during development lapatinib resistance [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS16-28.

Background: Abnormal cell cycle progression is a characteristic of cancer, and targeting the cell cycle is a strategy for cancer treatment. TCGA reported that 7% and 12% of gastric cancers exhibit CCND1 or CCNE1 alterations, respectively. Besides, Cyclin D1 is overexpressed in 25% to 60% of invasive breast carcinomas, and gene amplification is observed in 10% to 30% of breast cancer cases. Furthermore, CDK4/6 and CDNK2A/B aberrations are frequently observed in gastric (39.6%) and breast (7.6%) cancers. The presence of such abnormalities in cell cycle-related molecules suggests that gastric and breast cancers are good candidates for treatment with cell cycle inhibitors. Palbociclib is a specific inhibitor of CDK4/6, a vital regulator of the G1 checkpoint, and has been approved by the FDA because it provided a significant benefit by extending PFS in a phase III trial for hormone-positive advanced breast cancer. However, the predictive marker of palbociclib is not determined. Even though CDKN2A loss has been considered as a sensitive marker of palbociclib, there is no preclinical evidence to support whether CDKN2A deficient cancer shows sensitivity to palbociclib, especially in gastric and breast cancer. Therefore, we investigated the effects of palbociclib on CDKN2A loss gastric and breast cancer cell lines as well as patient derived-xenograft (PDX) models. Methods: The cytotoxic assay, cell cycle analysis, and western blotting were conducted to determine the anti-tumor effect and action mechanisms of palbociclib on gastric and breast cancer cell lines. Moreover, modulation of CDKN2A expression was conducted by siRNA and plasmid overexpression. These in vitro data were validated in vivo model and gastric cancer PDX models which have CDKN2A loss as well. Results: There is a meaningful correlation between CDKN2A loss and palbociclib sensitivity among gastric and breast cancer cell lines. CDKN2A loss cells showed G1 cell cycle arrest by blocking Rb phosphorylation and inhibited proliferative cell signaling. Moreover, palbociclib promoted senescence rather than apoptosis. The depletion of CDKN2A expression using siRNA increased palbociclib sensitivity with G1 cell cycle arrest accompanied by senescence. In contrast, CDKN2A overexpression in sensitive cells showed insensitivity to palbociclib. The anti-tumor effects of palbociclib on CDKN2A loss breast cancer cells were validated in the xenograft model, and the two different gastric cancer PDX models have CDKN2A loss also showed a significant response to palbociclib as well. Conclusions: CDK4/6 inhibitor palbociclib showed an anti-tumor effect in vitro and in vivo xenograft model of CDKN2A loss gastric and breast cancer. Our results suggest that palbociclib has therapeutic potential for the treatment of not only breast cancer but also gastric cancer, not limited to a hormone-positive breast cancer type. Our results provide a rationale for the future clinical trials of palbociclib in the treatment of breast cancers. Citation Format: Ahrum Min, Yu Jin Kim, Miso Lee, Kyung-Hun Lee, Seock-Ah Im. CDKN2A loss can be a predictive marker of palbociclib in breast and gastric cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-40.

Background: Lapatinib is a small molecular inhibitor of HER2 and EGFR tyrosine kinases, which is approved for HER2-positive metastatic breast cancer as second line treatment. Significant proportion of patients experience disease progression due to acquired resistance. Activation of DNA damage repair (DDR) is one of the drug-resistance mechanism, however, the impact of DDR on sensitivity to lapatinib is unclear. Thus, lapatinib resistance mechanism was explored from the standpoint of DDR activation. Methods: Acquired lapatinib-resistant (LR) SK-BR-3 cell lines were generated by continuously exposing to lapatinib, starting with 30 nmol/L and incrementally increasing to 10 μmol/L over 7 months. MTT assay was used to confirm lapatinib sensitivity. Cell cycle progression was analyzed using BrdU assay. Replication fork speed and stalled fork were analyzed by DNA fiber assay. Expression of the molecules was examined using Western blotting, immunofluorescence assay and transcriptome data analysis. DNA strand breaks and repair efficacy were evaluated through alkaline comet assay. Results: G1/S phase transition was increased and early/late S phase population was increased in SK-BR-3 LR cells. Replication fork speed was accelerated and replication stress are elevated in SK-BR-3 LR cells. p-Chk1, Rad51, Rad51B, Rad51C and XRCC3 were upregulated in SK-BR-3 LR cells. After irradiation or treatment with ATR inhibitor, γ-H2AX was continuously increased in parental cells. In contrast, γ-H2AX did not change significantly in SK-BR-3 LR cells, and Chk1 was activated after irradiation or treatment with ATR inhibitor. Tail of comet disappeared at early time point in in SK-BR-3 LR cells after induction of DNA damage compared with parental cells. These results demonstrated that DDR was activated and DNA damage repair capacity was enhanced in LR cells. Conclusion: Replication stress is elevated in SK-BR-3 LR cells. Up-regulation of molecules involved in the homologous recombination repair pathway was observed in SK-BR-3 LR cells. Moreover, DNA repair capacity was increased in SK-BR-3 LR cells. These data suggested that activation of DNA damage repair pathway caused by replication stress contributes to lapatinib resistance. Citation Format: So Hyeon Kim, Ahrum Min, Seongyeong Kim, Yu Jin Kim, Sujin Ham, Hae Min Hwang, Minyoung Lee, Changyun Lee, Jinyong Kim, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Replication stress activates the DNA damage response and contributes to lapatinib resistance in HER2-positive SK-BR-3 cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB228.

Background: SHR-A1811 is an ADC comprised of a humanized anti-HER2 monoclonal antibody (trastuzumab), a cleavable linker, and a DNA topoisomerase I inhibitor payload. Here we assessed SHR-A1811 in HER2-expressing/mutated unresectable, advanced, or metastatic solid tumors. Methods: Pts were eligible if they had HER2 positive breast cancer (BC), HER2 positive gastric/GEJ carcinoma, HER2 low-expressing BC, HER2-expressing/mutated NSCLC, or other HER2-expressing/mutated solid tumors, and were refractory or intolerant to standard therapy. SHR-A1811 at doses of 1.0-8.0 mg/kg was given Q3W (IV). The primary endpoints were DLT, safety, and the RP2D. Results: From Sep 7, 2020 to Sep 28, 2022, 250 pts who had undergone a median of 3 prior treatment lines in the metastatic setting received at least one dose of SHR-A1811 in dose escalation, PK expansion, and indication expansion part. As of data cutoff on Sep 28, 2022, 1 pt experienced DLT. Treatment-related adverse events (TRAEs) were reported in 243 (97.2%) pts. Grade ≥3 TRAEs, serious TRAEs, and treatment-related deaths were reported in 131 (52.4%), 31 (12.4%), and 3 (1.2%) pts, respectively. Interstitial lung disease (AESI) was reported in 8 (3.2%) pts. Exposures of SHR-A1811, total antibody, and the payload were generally proportional to dose from 3.2 to 8.0 mg/kg. ORR was 61.6% (154/250, 95% CI 55.3-67.7) in all pts. Objective responses were observed in pts with HER2 positive BC (88/108, ORR 81.5%, 95% CI 72.9-88.3), HER2-low BC (43/77, ORR 55.8%, 95% CI 44.1-67.2), urothelial carcinoma (7/11), colorectal cancer (3/10), gastric/GEJ carcinoma (5/9), biliary tract cancer (5/8), NSCLC (1/3), endometrial cancer (1/2), and H&N cancer (1/1). Subgroup analyses of ORR are shown in Table 1. The 6-month PFS rate was 73.9% in all pts. Conclusions: SHR-A1811 was well-tolerated and showed promising antitumor activity in heavily pretreated advanced solid tumors. Table 1. Subgroup analyses of ORR No. of prior treatment lines in metastatic setting in all pts (N=250) HER2 positive BC (N=108) HER2-low BC (N=77) Other tumor types (N=65) ≤3 81.8% (45/55) 58.7% (27/46) 36.7% (18/49) >3 81.1% (43/53) 51.6% (16/31) 31.3% (5/16) Prior anti-HER2 therapies in pts with BC (N=185)* HER2 positive BC (N=108) HER2-low BC (N=77) All BC (N=185) Any 82.2% (88/107, 73.7-89.0) 68.8% (11/16, 41.3-89.0) 80.5% (99/123, 72.4-87.1) Trastuzumab 81.9% (86/105, 73.2-88.7) 75.0% (9/12, 42.8-94.5) 81.2% (95/117, 72.9-87.8) Pertuzumab 83.0% (39/47, 69.2-92.4) 100% (5/5, 47.8-100) 84.6% (44/52, 71.9-93.1) Pyrotinib 86.9% (53/61, 75.8-94.1) 71.4% (5/7, 29.0-96.3) 85.3% (58/68, 74.6-92.7) Lapatinib 80.0% (28/35, 63.1-91.6) 100% (1/1, 2.5-100) 80.6% (29/36, 64.0-91.8) T-DM1 82.4% (14/17, 56.6-96.2) 100% (3/3, 29.2-100) 85.0% (17/20, 62.1-96.8) Other HER2-ADC (except T-DM1)** 60.0% (9/15, 32.3-83.7) 50.0% (2/4, 6.8-93.2) 57.9% (11/19, 33.5-79.8) ORR in pts with tumor types other than BC (N=65) HER2 IHC3+ or IHC2+/ISH+ (N=36) HER2 IHC2+/ISH- or IHC1+ or unknown (N=29) All other tumor types (N=65) % (n/N) 38.9% (14/36) 31.0% (9/29) 35.4% (23/65) ORR was shown as % (n/N, 95% CI) or % (n/N). *ORR is calculated using the number of subjects previously treated with anti-HER2 cancer therapy in advanced/metastatic setting as denominator; 2-sided 95% CIs are estimated using Clopper-Pearson method. **Includes RC48-ADC, A166, DP303c, MRG002, ARX788, TAA013, DX126-262, PF-06804103, and BAT8001. Citation Format: Herui Yao, Min Yan, Zhongsheng Tong, Xinhong Wu, Min-Hee Ryu, Jee Hyun Kim, John Park, Yahua Zhong, Weiqing Han, Caigang Liu, Mark Voskoboynik, Qun Qin, Jian Zhang, Minal Barve, Ana Acuna-Villaorduna, Vinod Ganju, Seock-Ah Im, Changsheng Ye, Yongmei Yin, Amitesh C. Roy, Li-Yuan Bai, Yung-Chang Lin, Chia-Jui Yen, Hui Li, Ki Young Chung, Shanzhi Gu, Jun Qian, Yuee Teng, Yiding Chen, Yu Shen, Kaijing Zhao, Shangyi Rong, Xiaoyu Zhu, Erwei Song. Safety, tolerability, pharmacokinetics, and antitumor activity of SHR-A1811 in HER2-expressing/mutated advanced solid tumors: A global phase 1, multi-center, first-in-human study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT175.

Background AZD5305 is a selective PARP1 inhibitor (PARPi) generated for improved therapeutic index compared with non-selective PARPi. Measurement of PARylation inhibition has been used to determine the pharmacodynamics (PD) of PARPi in clinical trials, but the assays used were insensitive and non-specific. Here we report on a new, highly sensitive and specific PARylation assay as evaluated in the PETRA trial (NCT04644068). Methods PETRA is a Phase 1/2 clinical study investigating AZD5305 alone or in combination with other anticancer agents in patients (pts) with advanced solid tumors. Peripheral blood mononuclear cells (PBMC) collected at multiple timepoints during treatment (cycle [C] 0 day [D] 1 pre-dose, 1, 4, 24 and 48 hours [hrs] post-dose, and C1D1 pre-dose) were lysed, subjected to an ex-vivo PARylation reaction, and quantified with the novel MSD PARylation assay. Results PARylation data were available from 32 patients treated from 20 mg to 140 mg. AZD5305 significantly inhibited PARylation in PBMC with maximum PARylation inhibition occurring around time to plasma concentration maximum (Tmax, 1 to 4 hrs after dosing; Table). At longer timepoints, a majority of patients showed >90% PARylation inhibition up to 48 hrs, 72 hrs, and even 168 hrs. Inhibition >90% for up to 72 hrs was observed in 4/4 evaluable samples at 140 mg, while inhibition >90% for an extended period (up to 168 hrs) was observed in 3/4 samples at 20 mg and 1/2 samples at 60 mg. These data demonstrate that AZD5305 causes significant PARylation inhibition at doses ≥20 mg which is consistent with in vitro findings. Conclusions Significant and durable PARylation inhibition was demonstrated with the novel PARylation assay at all doses of AZD5305 in PBMC samples from the majority of pts tested. Table. Residual PARylation levels by dose in the PETRA study 20 mg 60 mg Time post-dose 1 hour 4 hours 1 hour 4 hours Patients with >90% PARylation inhibition, n (%) 9/15 (60.0) 10/15 (66.7) 11/13 (84.6) 10/13 (76.9) Median residual PARylation levels (residual PARylation), % 5.3 5.0 4.0 3.7 Citation Format: Benedetta Lombardi, Paola Marco-Casanova, Athanasios Karapetsas, Mya Hornett, Edit Lukacs, Ko Sugibayashi, Sabina Cosulich, Adam Dowson, Ganesh Moorthy, Jessica Brown, Timothy A. Yap, Alison M. Schram, Judith Balmana, Stephen J. Luen, Seock-Ah Im, Adam Sharp, Richard Baird, Elizabeth A. Harrington, Spiros Linardopoulos. A highly sensitive and specific PARylation assay confirms significant and durable target engagement by AZD5305 in patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT269.

Nonmelanoma skin cancer (NMSC) is the most common skin cancer burden on the U.S. population. Environmental exposure to chemical carcinogens is one of the major causes of NMSC initiation, promotion, and progression. Ursolic acid (UA) is a naturally abundant pentacyclic triterpenoid showing anticancer potentials against diverse cancers. In the current study, we developed a two-stage skin carcinogenesis model in SKH1 hairless mice by administering cancer-initiating agent benzo[a]pyrene (B[a]P) and promoting agent 12-O-tetra-decanoylphorbol-13-acetate to study the epigenetic, transcriptomic, and metabolic changes at different stages (5, 20, and 26 weeks) during the development of NMSC, and investigated how UA regulates B[a]P-mediated alterations for NMSC interception. We found that UA protects against B[a]P-induced tumorigenesis at different phases of NMSC. Epigenetic CpG methyl-seq showed UA abrogated B[a]P-mediated alterations in differentially methylated regions (DMRs) profiles. Transcriptomic RNA-seq data exhibited UA reversed the differentially expressed genes (DEGs) of several inflammatory genes, such as chemokine ligand 8 (Ccl8) and interleukin 17F (Il17f), and epigenetic genes, such as DNA-methyltransferase 3-like (Dnmt3l) and protein-l-isoaspartate O-methyltransferase domain-containing protein 1 (Pcmtd1) during different stages of NMSC. Association study between DEGs and DMRs showed that B[a]P promoted transcription of kallikrein-related peptidase 13 (Klk13) by promoter demethylation, while UA suppressed Klk13 expression through hypermethylation in the promoter during the initiation stage, indicating the early intervention of UA. Ingenuity pathway analysis further showed significant upregulation of macrophage-stimulating protein-recepteur d'origine nantais (MSP-RON) signaling pathway by B[a]P during the initiation stage while suppressed by UA treatment. The metabolomic study revealed UA modulated cancer-associated changes in metabolisms, including the TCA cycle and pyruvate metabolism/metabolites during the promotion phase, indicating UA plays a critical role in regulating B[a]P-regulated metabolic changes and intercepting NMSC progression. In summary, UA protects against the environmental carcinogen B[a]P-driven epigenetic, transcriptomic, and metabolic changes during the initiation, promotion, and progression of NMSC, potentially contributing to the anticancer effects of UA. (Supported by NIH R01 CA200129 to A.N.K) Citation Format: Md. Shahid Sarwar, Christina N. Ramirez, Hsiao-Chen Dina Kuo, Pochung Chou, Renyi Wu, Davit Sargsyan, Ahmad Shannar, Rebecca Mary Peter, Ran Yin, Yujue Wang, Xiaoyang Su, Ah-Ng Kong. Metabolic rewiring and epigenetic reprogramming by the environmental carcinogen benzo[a]pyrene in a two-stage skin carcinogenesis mouse model and cancer interception by triterpenoid ursolic acid. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5266.

Understanding tissue heterogeneity is critical for elucidating cell-cell interactions with important implications in immuno-oncology, inflammation, and neuroscience. Tissue heterogeneity poses immense challenges to understanding underlying molecular mechanisms using techniques such as qRT-PCR or bulk sequencing. While single-cell RNA sequencing can provide information about precise cellular composition of tissues, data analysis can be cumbersome and spatial context is lost. With single-cell spatial platforms such as RNAscope, target gene and protein expression can be visualized to characterize cell types and tissue neighborhoods. Here, we demonstrate a novel method for the simultaneous detection of RNA and protein using a modified co-detection assay. This novel co-detection assay enables visualization of a combination of up to 12 RNA and/or protein targets on the same sample. We used a set of antibodies targeting key immune and tumor cell markers- PD1, CD3, CD4, CD8, CD68, FOXP3 and KRT17, along with RNA biomarkers to interrogate the tumor microenvironment (TME) in human FFPE tumor samples. Using a combination of RNA and protein targets, we characterized different cell types such as T cells, macrophages, and tumor cells in the TME. We also developed a novel method for visualizing intercellular interactions between PD1 and PD-L1, offering insights into popular checkpoint mechanisms that are targeted for therapeutic intervention in cancer treatment. The assay offers a powerful technique for visualizing target RNA biomarkers in specific cell-types identified by cell-marker protein expression. This is a valuable tool for multi-omic analysis and accurate interrogation of complex tissues to obtain insights into novel biomarkers and therapeutic targets. Citation Format: Anushka Dikshit, Sayantani Basak, Vasudha Murlidhar, Ge-Ah Kim, Julia Yu, Sonali Deshpande, Li-Chong Wang, Maithreyan Srinivasan. Interrogating the tumor-immune landscape with a novel automated RNAscope™ assay for multiplexed detection of RNA and protein. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6771.

Lung cancer is the leading cause of cancer death worldwide. There is an unmet need for therapies to improve lung cancer outcomes without causing additional toxicities. Tax-interacting protein 1 (TIP1) is a radiation-inducible antigen that plays a role in lung cancer progression and resistance to therapy. TIP1 is a targetable cell surface protein. We developed human antibodies (Ab) that bind specifically to lung cancer. This study aimed to evaluate our lead anti-TIP1 human antibody targeting lung cancer in vitro and in vivo. Anti-TIP1 IgG1 Ab was developed by biopanning phage display human antibody library. The purified Ab was characterized by size-exclusion chromatography-HPLC (SEC-HPLC), mass spectrometry, ELISA, BIAcore and flow cytometry. Ab was labeled with 89Zr using desferrioxamine chelator. The surface binding of 89Zr-Ab was evaluated in the A549 and H460 lung cancer cells. Stability of the 89Zr-Ab was assessed in human serum. Mice bearing 3 patient-derived xenografts (PDX) lung tumors or A549 tumors were injected with 50 µCi of 89Zr-Ab. Small animal PET imaging and biodistribution were conducted over 7 days. Specific tumor uptake was evaluated in biodistribution studies on day 5 by injecting cold Ab before 89Zr-Ab. We obtained 95% pure anti-TIP1 Ab by SEC-HPLC. Mass spectrometry confirmed the intact mass and glycosylation pattern of the anti-TIP1 IgG1. The Ab is bound to recombinant TIP1 protein and cancer cell surface with high affinity. The radiochemical purity of the 89Zr-Ab was 99.9%, and the molar activity was 11.54 MBq/nmol with a radiolabeling yield of 91.4%. Both A549 and H460 cell lines demonstrated specific binding as >70% inhibition was observed when the cold Ab was added. The ratio of SUV tumor to SUV muscle increased from 5.8 at 2 days to 6.3 at 7 days in PDX. In biodistribution studies, we found significantly higher (p Citation Format: Abhay Kumar Singh, Calvin D. Lewis, Cristian AWV Boas, Philipp Diebolder, Prashant N. Jethva, Aaron Rhee, Jong Hee Song, Young Ah Goo, Shunqiang Li, Yongjian Liu, Buck Rogers, Vaishali Kapoor, Dennis E. Hallahan. Preclinical development of a 89Zr-labeled human antibody as a novel immuno-PET agent for noninvasive cancer detection and monitoring response to treatment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5035.

Background: PARP inhibitors have shown antitumor activities against solid tumors with HRD (homologous recombination deficiency). The definition of HRD and other potential biomarkers besides HRD should be further evaluated for PARP inhibitors. JPI-547 is a novel PARP inhibitor, simultaneously targeting tankyrase1/2, other members of the PARP family, that are involved in the Wnt/β-catenin pathway. Method: Antiproliferative effect of JPI-547 and a variety of PARP inhibitors (olaparib, veliparib, talazoparib, niraparib, and rucaparib) were determined by MTT assay or clonogenic assay in 9 human pancreatic ductal adenocarcinoma (PDAC) cell lines (Capan-1, HPAF-II, Capan-2, AsPC-1, SNU-410, SNU-213, SNU-324, MIA-PaCa2, and PANC-1). Transcriptome data and gene dependency score of the cell lines were obtained from the CCLE database and DepMap respectively. DNA damage was monitored by immunofluorescent imaging of γ-H2AX and DR-GFP assay determined the homologous recombination repair (HRR) efficiency. A xenograft tumor model was established to substantiate the in vivo antitumor effect of JPI-547. Results: JPI-547 more potently blocks Poly(ADP-ribosyl)ation than olaparib, and induces a strong antiproliferative effect on Capan-1, a cell line with BRCA2 del. JPI-547 leads to cell cycle arrest and induces enhanced apoptotic cell death than olaparib. JPI-547 inhibits tumor growth of Capan-1 in vivo, suggesting the potent antitumor activity of JPI-547 against PDAC with HRD. Cell lines harboring RNF43 LOF mutations (HPAF-II, AsPC-1, and Capan-2), intrinsically addicted to Wnt/β-catenin pathway, are more sensitive to JPI-547 than cells with RNF43 wild types. Interestingly, RNF43 mutations could not distinguish the sensitivity of other PARP inhibitors except JPI-547. CTNNB1 gene dependency score and β-catenin levels positively correlate with cellular sensitivity to JPI-547. JPI-547-induced DNA damage was alleviated in HR-proficient PDAC cells. DR-GFP assays confirm that JPI-547 does not directly alter the HRR efficiency of Wnt-addicted PDAC cells. Collectively, these data indicate that the vulnerabilities of Wnt-addicted PDAC cells to JPI-547 were irrelevant to HRD mimicking. Rather, JPI-547 stabilizes AXIN-2 in Wnt-addicted PDAC cells and downregulates the active form of β-catenin level in the nucleus, thereby disrupting the transcription of its target genes. Knockdown of β-catenin neutralized the antiproliferative effect of JPI-547, suggesting that inhibition of the β-catenin pathway is an important mode of action by JPI-547 in Wnt-addicted PDAC cells. Conclusion: JPI-547 shows promising, preclinical antitumor effects against PDAC cells with HRD or Wnt-addiction, providing a rationale for further biomarker-driven clinical development of JPI-547 for the treatment of patients with PDAC. Citation Format: Kyoung-Seok Oh, Ah-Rong Nam, Ju-Hee Bang, Yoojin Jeong, Sea Young Choo, Hyo Jung Kim, Su In Lee, Jae-Min Kim, Jeesun Yoon, Tae-Yong Kim, Banyoon Cheon, Hyunju Cha, John Kim, Do-Youn Oh. JPI-547, a dual inhibitor of PARP/Tankyrase, shows promising antitumor activity against pancreatic cancers with homologous recombination repair deficiency or Wnt-addiction. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4496.

Background: Medulloblastoma (MB) is the most common malignant pediatric brain tumor and the development of leptomeningeal dissemination (LM seeding) through the cerebrospinal fluid (CSF) circulation is a major cause of treatment failure. We have previously established the MB LM seeding mouse model. In this study, we aimed to determine molecules that can target seeding (metastatic) cells by analyzing their specific genetic and functional characteristics. Methods: Seeding and non-seeding cells were isolated and cultured, respectively, and microarray analysis was carried out. Proliferation and wound-healing assays were performed through the HCS system, and viability, migration and adhesion assays were accomplished. In addition, the therapeutic effect of the selected inhibitor was analyzed by region of interest (ROI) analysis of bioluminescence in vivo. Results: The gene expression and functional characteristics of the successfully obtained seed cells and non-seed cells were compared and analyzed. Differentially expressed genes (DEGs) analysis indicated that heat shock protein 70 (Hsp70) was in the highest rank in seeding cells, and pathway enrichment analysis revealed that the metabolism was the main pathway enriched in seeding cells. Proliferation of seeding cells was slower than that of non-seeding cells, but there was no significant difference in viability. Notably, the wound-healing of seeding cells was delayed, but the adhesion ability on collagen was higher. Among the six HSP70 inhibitors, VER155008 was efficiently sensitive to seeding cells, and suppression of expression of MRP proteins along with HSP70 was also confirmed. In addition, it was observed that the treatment of VER155008 reduced seeding through the spinal cord in vivo. Conclusion: Our study suggests that VER15008 can be a promising treatment option, targeting LM seeding of MB. Citation Format: Seung Ah Choi, Saehim Ha, Hee-Young Lee, Joo Whan Kim, Jin Chul Paeng, Ji Hoon Phi. Overcoming the leptomeningeal seeding of medulloblastoma by targeting HSP70 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2468.

Triple-negative breast cancer (TNBC) accounts for 20% of all breast cancer and has limited therapeutic option except for immunotherapy because ER, PR, and HER2 targeted for treatment are absent. It also tends to be more aggressive meaning more likely to metastasize faster to other organs such as lungs, liver, and bone, and its recurrence rate is high which leads to a poor prognosis. Immunotherapy is an option to treat TNBC such as programmed death ligand 1 (PD-L1) antibody. However, tumor cells use various immune evasion mechanisms in addition to PD-1/PD-L1 pathway to interrupt the effect of immunotherapy. Moreover, tumor microenvironment (TME) is one of the important factors for the effective anti-tumor immune responses from immunotherapy. Hypoxia is a typical event in TNBC. Hypoxia inducible factor 1-alpha (HIF1α) is a transcription factor that regulates angiogenesis by expressing VEGF, and in addition, upregulates the c-MET which increases the proliferation, migration, and survival of tumor cells. Moreover, HIF1α directly regulates the expression of CD47. CD47 is a transmembrane protein that binds to SIRPα of macrophages to inhibit the phagocytosis of macrophages. Therefore, the efficacy of immune checkpoint inhibitors decreases even if there is a higher degree of tumor-infiltrating lymphocytes (TILs). Combination therapy with immune checkpoint and tumor associated antigens (TAAs) derived peptide vaccine could inhibit metastasis. In present study, we found that HIF1α/c-MET peptide vaccination delayed metastasis in C3(1)Tag mouse. We also investigated whether combination therapy with immune checkpoint inhibitors and peptide vaccine inhibited metastasis. Next, we observed that combination strategy of HIF1α/c-MET peptide vaccine and treated anti-PD-L1 antibody or/and anti-CD47 antibody was more effective in systemically inoculated M6 cells growth in C3(1)Tag mice. Immunohistochemical analysis of the tumors showed that the protein expression of CD47, HIF1α, and c-MET were significantly decreased. On the other hand, CD4+ T cells, CD8+ T cells and M1 macrophages increased, but M2 macrophages decreased, resulting in delayed metastasis in combination therapy group compared to single therapy and control group. In addition, IFNγ ELISPOT showed that peptide vaccination significantly increased the HIF1α/c-MET specific IFNγ-secreting T cell response in splenocytes. We also assessed osteoclastogenesis in bones, which showed reduction of osteoclast in combination therapy group compared to single therapy and control group.Taken together, combination therapy with immune checkpoint inhibitor targeting PD-L1 and CD47 and peptide vaccine appears to be a promising strategy that inhibit metastasis by modulating tumor microenvironment in triple negative breast cancer. Citation Format: JinHwa Hong, Jimin Lee, Soon Young Lim, Ju Won Kim, Ah Reum Lim, Kyong Hwa Park. Combination therapy of immune checkpoint inhibitor and HIF1α/c-MET peptide vaccine suppresses metastasis by modulating tumor microenvironment in triple-negative breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5087.

Background: One of the major topics about colorectal cancer (CRC) is the core role of various immune cells against cancer cells. To comprehensively understand genetic basis of immune system underlying CRC progression, association of single-nucleotide polymorphisms (SNPs) and polygenic risk scores (PRS) of immune-related genes were investigated with colorectal cancer survival. Methods: CRC patients enrolled in the Seoul National University Hospital with prospective follow-up were included in the study. From blood-derived DNA from CRC patients, genome-wide SNPs were genotyped by using the Korea Biobank Array (KoreanChip). 2,729 immune-related genes were selected from Ensembl, Gene Ontology (GO), and KEGG database and 37,398 mapped SNPs were extracted. SNP or PRS-based Cox proportional hazard models were fit for events of overall survival (OS) and progression-free survival (PFS) estimating effect sizes with hazard ratios (HRs). PRS was calculated as effect size weighted sum of risk alleles of individual patients and categorized into tertiles. To investigate enriched pathways, protein-protein interaction networks analysis was conducted. Bonferroni-corrected p-value (1.3 × 10−6) was used as statistically significance threshold. Results: Among 960 CRC patients, 154 (16.0%) occurred events of death and 245 (25.5%) of progression during the follow-up (median=1,698 days, range=7-2,563 days). For OS, a statistically significant associations were mapped to ACTR3B, ST6GAL1, COTL1, PRKCZ, and CAMK1D genes with the strongest SNP rs14808985 (HR=3.07, P=1.87 × 10−7). For PFS, marginal association was mapped to MECOM gene with the strongest SNP rs16854234 (HR=1.48, P=6.13 × 10−6). In PRS analysis, the highest tertile group showed the prominently increased risk for OS (HR=25.0, P Conclusions: We identified novel predictive biomarker genetic variants in ACTR3B and MECOM genes associated with OS and PFS, respectively, among CRC patients. Additionally, we presented PRS as a useful biomarker for survival outcomes and suggested enriched biological pathways involved in CRC progression. These knowledges can promote the understanding of CRC survival and the development of immune-related therapeutic interventions for CRC patients in the future. Citation Format: Dabin Yun, Nan Song, Jin-Ah Sim, Min Jung Kim, Ji Won Park, Seung Yong Jeong, Aesun Shin. Novel predictive biomarker SNPs and polygenic risk scores of immune-related genes for colorectal cancer survival in Korea. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5234.

Background: Cyclin dependent kinase 9 (CDK9) regulates progression of RNA polymerase II through phosphorylation. Inhibition of CDK9 induces transcription-replication conflicts (T-R conflicts), leading to replication stress. AZD4573, a highly selective CDK9 inhibitor, showed anti-tumor effects in acute myeloid leukemia in vitro and in vivo. Recently it is being evaluated in phase 2 clinical trial (NCT04630756). However, the anti-tumor effect of AZD4573 is unknown in breast cancer. Thus, we investigated the anti-tumor effect of AZD4573 in breast cancer cells and explored the underlying mechanism. Methods: To evaluate the antitumor effect in vitro, MTT assays were preformed using increasing concentration of AZD4573 (doses range : 0-100 nM). The IC50 values were calculated using Sigma Plot software. Cell cycle analysis was performed using flow cytometry analysis. The expression of signal transduction molecules was determined using quantitative PCR, western blotting, and immunofluorescence. Apoptotic cell death was verified by annexin-V assay. DNA strand breaks and repair efficacy were evaluated through alkaline comet assay. siRNA knock-down system was used to confirm the action mechanism. Results: SK-BR-3, HCC70, HCC1937 and ZR-75-1 were sensitive to AZD4573 with IC50 ranging from 9.16-25.81 nM, whereas T47D and HCC1428 were less-sensitive (IC50 >100 nM), as assessed by MTT assay. After treatment of AZD4573, sensitive and less-sensitive cells showed increased T-R conflicts in 10 minutes. T-R conflicts were maintained in SK-BR-3, HCC70, HCC1937 and ZR-75-1 for 3 hours, but T-R conflicts disappeared immediately in T47D and HCC1428 cells. Moreover, expression of γ-H2AX, cleaved PARP and cleaved caspase-3 was increased, indicating the induction of apoptosis in sensitive cells. Contrary to these results, γ-H2AX induction and apoptosis were not observed in T47D and HCC1428 cells. siRNA knock-down of CDK9 in HCC1937 resulted in an increase of T-R conflicts, γ-H2AX, with induction of PARP cleavage and caspase-3 cleavage. Conclusion: AZD4573 induced T-R conflicts and subsequent DNA damage. Accumulation of DNA damage caused caspase-3-dependent apoptotic cell death in sensitive breast cancer cells. However, less-sensitive breast cancer cells resolved T-R conflicts immediately and maintained genomic stability. These data suggest that AZD4573 induced significant anti-tumor effects by apoptotic cell death through accumulation of DNA damage, following T-R conflicts. Citation Format: Minyoung Lee, So Hyeon Kim, Sujin Ham, Hae Min Hwang, Youlim Noh, Yu-Jin Kim, Ahrum Min, Jinyong Kim, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Cyclin dependent kinase 9 inhibitor AZD4573 induces cell death through DNA damage accumulation in breast cancer cells in vitro [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5978.

Childhood cancers are rare and clinically diverse. They are typically associated with few driver mutational events than adult cancers, constituting potential targets for patient-specific precision therapy approaches. Here, we evaluated the feasibility of using a patient-derived tumor cell (PDC) based drug screening system and integrated multi-omics data to achieve precision oncology for pediatric patients. We established a PDC library derived from a few passage-cultured tumor cells from surgically resected tumor specimens and conducted chemical screening, which composed of various target agents of major oncogenic pathways (e.g. receptor tyrosine kinase inhibitor, proteasome inhibitor and histone deacetylase etc.). Next Generation Sequencing was performed to characterize the genomic and transcriptomic traits of tumors. Overall, success of establishing PDCs of pediatric cancers was high (80.4%) and comparable to adult cancers. The amount of obtained tissue was an important factor for the success of PDC establishment; the estimated optimal weight for PDC establishment was small (1.14g) and it is noteworthy that a small amount of tumor sample is sufficient to identify the potential hit(s) of parental tumors. The platform provided therapeutic options to pediatric tumors regardless of actionable targets. Our PDC approach identified several potential gene-drug associations, including anti-PI3K/Akt agents for neuroblastomas and anti-SHH agents for sarcomas with EWSR1 fusion. Given that a considerable proportion of pediatric tumors still lack actionable targets with matched treatment options, PDC-based drug screening profiles can be an optimal therapeutic strategy for these cases. Collectively, our analysis confirmed the feasibility of PDC-based in vitro drug screening systems to guide the therapeutic strategy for pediatric patients. This approach will accelerate the preclinical research for pediatric tumors to understand their pathophysiology and investigate the potential therapeutic strategies to fulfill the future precision oncology. Citation Format: Gi Ju Lee, Seung-Won Choi, Seung Ah Choi, Hee-Jin Cho, Robyn Gartrell, Ji Won Lee, Joo Whan Kim, Nam-Gu Her, Raul Rabadan, Ki Woong Sung, Do-Hyun Nam, Seung-Ki Kim. Proof of principle for pharmacogenomic-guided precision oncology for pediatric malignancy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4486.

Background: HER2 (Human epithelial growth factor receptor 2)-targeting therapies have been approved for patients with HER2-positive breast and gastric cancer and use have been attempted in various solid tumor types, including biliary tract cancer. However, resistance mechanism remains as a major challenge of HER2-targeting therapies. YAP (Yes-associated protein) is a major downstream effector of Hippo pathway, and it plays an essential role in cancer cell proliferation, survival and differentiation. Moreover, YAP is emerging as a key player of resistance mechanism of cytotoxic and targeted drugs. Yap is also an important immunosuppressive molecule as it works as a negative regulator of T cell tumor infiltration. In this study, we intend to elucidate the role of YAP in mechanism of trastuzumab resistance and T cell immune response in HER2-positive cancer cells. Methods: We established four trastuzumab-resistant (HR) cell lines (N87HR, SNU216HR, SNU2670HR and SNU2773HR) from HER2-postive gastric and biliary tract cancer cell lines. To inhibit the function of YAP, siRNA and Verteporfin (YAP-TEAD inhibitor) were used. MTT assay, cell cycle analysis, western blot and migration assay were performed to analyze the antitumor effects through YAP downregulation. In order to evaluate immune-modulation by YAP, human PBMC co-culture was used and immune markers were analyzed by RT-PCR and flow cytometry. Mouse xenograft models were established using SNU2773 and SNU2773HR cells. Results: We confirmed that the expressions of pYAP and YAP were elevated in HR cells (SNU216HR, SNU2670HR and SNU2773HR) compared to parental cells. Reducing the expressed YAP in HR cells inhibited tumor cell growth and migration and induced apoptosis. Immune suppression markers such as PD-L1, CD155, and galectin-9 were effectively decreased, while CD80, a stimulation marker, was increased by verteporfin treatment. Also, when YAP was decreased, CCL5 and CXCL10, well known CD8+ T cell recruitment cytokines, were increased. In HR cells treated with siYAP and verteporfin, there was a trend of increasing CD4+ and CD8+ T cells when co-culture with PBMC. In vivo experiment data showed greater tumor growth inhibition effects with SNU2773HR than SNU2773 xenograft models when treated with verteporfin. Conclusion: The expression of YAP is elevated in trastuzumab-resistant (HR) cells and inhibition of YAP shows anti-tumor effects and activation of T cell responses. Collectively, our data suggests that the inhibition of YAP is one of many promising strategies to overcome trastuzumab resistance and T cell regulatory mechanisms in HER2-positive cancers. Citation Format: Ah Rong Nam, Jeesun Yoon, Kyoung-Seok Oh, Jae-Min Kim, Ju-Hee Bang, Yoojin Jeong, Sea Young Choo, Hyo Jung Kim, Su In Lee, Tae-Yong Kim, Do-Youn Oh. Targeting of YAP overcomes trastuzumab-resistance and promotes immune responses in HER2-positive cancers. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3872.

Background Germline mutations of breast cancer susceptibility gene BRCA1 and BRCA2 (gBRCA1/2) are associated with elevated risk of breast cancer in young women in Asia. BRCA1 and BRCA2 proteins contribute to genomic stability through homologous recombination (HR)-mediated double strand DNA break repair (DDR) in cooperation with other HR-related proteins. In this study, we analyzed the targeted sequencing data of the breast cancer patients with gBRCA1/2 mutations to investigate the landscape of HR-related gene mutations and their clinical implications. Materials and Methods Data of the breast cancer patients with pathogenic gBRCA1/2 mutations and qualified targeted next generation sequencing, SNUH FiRST cancer panel, were analyzed. Single nucleotide polymorphisms, small insertions and deletions were analyzed with functional annotations using ANNOVAR. HR-related genes were defined as ABL1, ATM, ATR, BARD1, BRCA1, BRCA2, CDKN1A, CDKN2A, CHEK1, CHEK2, FANCA, FANCD2, FANCG, FANCI, FANCL, KDR, MUTYH, PALB2, POLE, POLQ, RAD50, RAD51, RAD51D, RAD54L, and TP53. Mismatch-repair genes were MLH1, MSH2, and MSH6. Clinical data were analyzed with cox proportional hazard models and survival analyses. Results Fifty five Korean breast cancer patients with known gBRCA1/2 mutations and qualified targeted NGS data were analyzed. Ethnically distinct mutations in gBRCA1/2 genes were noted, with higher frequencies of Val1833Ser (14.8%), Glu1210Arg (11.1%), and Tyr130Ter (11.1%) in gBRCA1 and Arg2494Ter (25.0%) and Lys467Ter (14.3%) in gBRCA2. Considering subtypes, gBRCA1 mutations were associated with triple-negative breast cancers (TNBC), while gBRCA2 mutations were more likely hormone receptor-positive breast cancers. At least one missense mutation of homologous recombination (HR)-related genes were observed in 44 cases (80.0%). The most frequently co-mutated gene was TP53 (38.1%). In patients with gBRCA1/2 mutations, however, genetic variations of TP53 occurred in locations different from the known hotspots of those with sporadic breast cancers. The patients with both gBRCA1/2 and TP53 mutations were more likely to have TNBC, high Ki-67 values, and increased genetic mutations, especially of HR-related genes. Survival benefit was observed in the TP53 mutants of patients with gBRCA2 mutations, compared to those with TP53 wildtypes. Conclusion Our study showed distinct genetic landscape of breast cancer patients with gBRCA1 and gBRCA2 mutations in the Asian populations. Further studies on precision medicine are needed for tailored treatments of patients with genetic diversity among different ethnic groups. Citation Format: Jinyong Kim, Kyeonghun Jeong, Hyeji Jun, Kwangsoo Kim, Tae-Yong Kim, Dae-Won Lee, Go-un Woo, Songyi Park, Hanbaek Yi, Kyung-Hun Lee, Seock-Ah Im. Mutations of TP53 and genes related to homologous recombination repair in breast cancer with germline BRCA1/2 mutations [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-03-20.

Backgrounds Since OlympiAD study, National Comprehensive Cancer Network guideline recommends assessment of germline BRCA1/2 mutation in all patients with recurrent or metastatic breast cancer to identify candidates for PARP inhibitor therapy, which is not always possible in clinical practice due to limited resources for testing. Data on the prevalence of gBRCA mutation is still lacking, especially in patients with non-high risk for hereditary breast and ovarian cancer syndrome. In this study, we investigated prevalence of gBRCA mutation in unselected Korean patients with HER2-negative advanced BC in a prospective cohort and analyzed oncologic outcome. Methods Eligible patients were diagnosed with HER2-negative advanced BC and had initiated palliative systemic treatment. Peripheral blood was prospectively drawn from each patient and gBRCA mutation status was assessed by next generation sequencing using NGeneBio BRCAaccuTest®. In 100 patients, somatic mutations including BRCA1/2 from tumor tissue were investigated using targeted panel sequencing. To estimate the prevalence of gBRCA mutation with margin of error to be no more than ±4% at the 95% confidence interval in a population size of 20,000, 583 patients were to be enrolled. Results A total of 583 patients were enrolled between Oct 2019 and Mar 2022, and the prevalence of gBRCA mutation was analyzed in 570 patients, excluding ineligible patients. Median age was 54 years old (range 26-87) and 567 patients were female. 475 patients had HR+/HER2- BC and 94 patients had triple negative breast cancer (TNBC). The overall prevalence of gBRCA1/2 pathogenic mutation was 7.3% (42/570) in unselected patients. The prevalence of gBRCA1 mutation was 1.6%(9/570) overall, 0.8%(4/475) in HR+/HER2- BC, and 5.3%(5/94) in TNBC. The prevalence of gBRCA2 mutation was 5.8%(33/570) overall, 6.3%(30/475) in HR+/HER2- BC, 3.2%(3/94) in TNBC. Prevalence in low risk TNBC (>60 years at first BC diagnosis, no known family history of relevant cancer and unilateral breast cancer) was 10.5% (2/19, all 2 patients had gBRCA2 mutation). Prevalence in low risk HR+/HER2- (>40 years at first BC diagnosis, no known family history of relevant cancer and unilateral breast cancer) was 5.9% (18/307, 17 patients had gBRCA2 mutation). The overall prevalence of gBRCA1/2 pathogenic mutation in Korean patients with low risk HER2-negative advanced BC was 6.1%. The result of somatic mutation, treatment patterns and clinical outcome according to gBRCA1/2 mutation will be further analyzed. Conclusions The prevalence of gBRCA mutation among Korean patients with HER2-negative advanced BC classified as low risk (6.1%) in this study supports routine testing of gBRCA mutation in this population. Citation Format: Hee Kyung Ahn, Jee Hung Kim, Mirae Kim, Seri Park, Su-Jin Koh, Joo Hyuk Sohn, Myoung Joo Kang, Kyung Hae Jung, Kyoung Eun Lee, Jieun Lee, Sung Ae Koh, Yee Soo Chae, Jae Ho Byun, In Hae Park, Hee-Jun Kim, Jee Hyun Kim, Han Jo Kim, Joo Young Jung, Jung Lim Lee, Yoon Young Cho, Kyong Hwa Park, Ji-Yeon Kim, Seock-Ah Im, Yeon Hee Park. Prevalence of germline BRCA mutations in unselected Korean patients with HER2-negative breast cancer: A Prospective cohort study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-03-14.

Background: Trastuzumab deruxtecan (T-DXd) is approved in the United States and European Union for use in patients (pts) with HER2+ unresectable/metastatic breast cancer (mBC) after ≥1 prior anti–HER2 regimen(s). Approval was based on the randomized, multicenter, open-label, phase 3 DESTINY-Breast03 study (NCT03529110), in which T-DXd demonstrated statistically significant and clinically meaningful improvement in progression-free survival (PFS) compared with trastuzumab emtansine (T-DM1). At the primary interim analysis (data cutoff May 21, 2021), the risk of disease progression or death was reduced by 72% with T-DXd (P < 0.001; Cortes et al. N Engl J Med 2022). Overall survival (OS) data were immature for both treatment groups; although the prespecified cutoff for significance was not reached (NR), a trend toward benefit with T-DXd was observed. With further follow-up, we report results from the prespecified OS analysis of DESTINY-Breast03 (data cutoff July 25, 2022), including updated efficacy and safety. Methods: Pts with HER2+ mBC previously treated with trastuzumab and a taxane in either the metastatic setting or (neo)adjuvant setting with progression within 6 mo of therapy, who could have received pertuzumab, were randomly assigned 1:1 to receive T-DXd 5.4 mg/kg every 3 weeks (Q3W) or T-DM1 3.6 mg/kg Q3W until disease progression. The primary endpoint was PFS by blinded independent central review (BICR). The key secondary endpoint was OS (80% powered at 2-sided significance level of 5%); other secondary endpoints included objective response rate (ORR), duration of response (DoR), PFS based on investigator assessment, and safety. Results: 524 pts received either T-DXd (n = 261) or T-DM1 (n = 263). As of the updated data cutoff, median duration of study follow-up was 28.4 mo (range, 0.0-46.9 mo) for T-DXd and 26.5 mo (range, 0.0-45.0 mo) for T-DM1. Median treatment duration was 18.2 mo (range, 0.7-44.0 mo) for T DXd and 6.9 mo (range, 0.7-39.3 mo) for T-DM1. The risk of death was reduced by 36% (HR, 0.64; P = 0.0037) with T-DXd; median OS (mOS) was NR (95% CI, 40.5 mo-not evaluable [NE]), with 72 (27.6%) OS events, for T-DXd vs NR (95% CI, 34.0 mo-NE), with 97 (36.9%) OS events, for T-DM1. Landmark 12-mo OS rate was 94.1% (95% CI, 90.4-96.4) for T-DXd vs 86.0% (95% CI, 81.1-89.8) for T-DM1; 24-mo OS rate was 77.4% (95% CI, 71.7-82.1) for T-DXd vs 69.9% (95% CI, 63.7-75.2) for T-DM1. The P value for OS crossed the prespecified boundary (P = 0.013) and was statistically significant. mPFS by BICR was 28.8 mo (95% CI, 22.4-37.9 mo) with T-DXd, compared with 6.8 mo (95% CI, 5.6-8.2 mo) with T-DM1; HR, 0.33; nominal P < 0.000001. Key efficacy and safety results are shown in the table. Grade ≥3 treatment-emergent adverse events were experienced by 56.4% of T-DXd-treated pts and 51.7% of T DM1-treated pts. Drug-related interstitial lung disease/pneumonitis, as evaluated by an independent adjudication committee, was experienced by 39 pts (15.2%) in the T-DXd arm and 8 pts (3.1%) in the T DM1 arm; no adjudicated drug-related grade 4 or 5 events were observed in pts who received T-DXd. Conclusions: Updated results confirm the superiority of T-DXd compared with T-DM1 for pts with HER2+ mBC previously treated with an anti-HER2 therapy, with highly clinically meaningful and statistically significant benefit in OS and PFS and a manageable safety profile with longer treatment duration. Editorial Acknowledgment Under the guidance of authors, assistance in medical writing and editorial support was provided by Laura Halvorson, PhD, and Rachel Hood, PhD, of ApotheCom, and was funded by Daiichi Sankyo. Funding This study was funded by Daiichi Sankyo and AstraZeneca. Table. Summary of Efficacy Results for T-DXd and T-DM1 Citation Format: Sara Hurvitz, Roberto Hegg, Wei-Pang Chung, Seock-Ah Im, William Jacot, Vinod Ganju, Joanne Win Yang Chiu, Binghe Xu, Erika Hamilton, Srinivasan Madhusudan, Hiroji Iwata, Sevilay Altintas, Jan-Willem Henning, Giuseppe Curigliano, José Manuel Pérez-García, Anton Egorov, Yali Liu, Jillian Cathcart, Shahid Ashfaque, Javier Cortés. GS2-02 Trastuzumab deruxtecan versus trastuzumab emtansine in patients with HER2-positive metastatic breast cancer: Updated survival results of the randomized, phase 3 study DESTINY-Breast03 [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS2-02.

Background: The Penelope-B trial did not show improvement in invasive disease-free survival (iDFS) with the addition of palbociclib to endocrine therapy (ET) in patients with high-risk early breast cancer (BC) after neoadjuvant chemotherapy (NACT). Biomarkers may be able to identify subgroups of patients deriving benefit from Palbociclib and guide future studies. Estrogen-receptor (ER), progesterone-receptor (PgR) and Ki-67 might be helpful in identifying patients benefiting from palbociclib. Concordantly, tumors with elevated expression of Cyclin D1 and phosphorylated retinoblastoma protein (phospho-RB) may harbor more dependency on CDK4/6 and thus higher sensitivity to palbociclib. Methods: The percentage of positive ER and PgR cells and Ki-67 assessed in surgical specimens after NACT were combined to obtain the immunohistochemical score 3 (IHC3, Cuzick et al JCO 2011, low vs high based on the median IHC3 value). Cyclin D1 and phospho-RB Ser 807/811 immunoreactive (phospho-RB) scores were analyzed in residual tumors after NACT (range 0-12 each). Proportional hazard regression model was used to assess the predictive and prognostic value of IHC3 and treatment on iDFS. Subgroup analysis was performed according to BC intrinsic subtypes (luminal-A/normal-like, luminal-B/HER2-enriched/basal) and HER2-status (HER2 0, HER2 low). Cox/Fine-Gray regression was used to define the predictive and prognostic value of CyclinD1 (≤1, >1), phospho-RB (≤2, >2) as dichotomized and continuous variables on iDFS, distant DFS (DDFS), locoregional invasive recurrence-free interval (LRRFI) and overall survival (OS). Multivariate analyses (MVA) were adjusted for age (≤50 vs >50), Ki-67 (≤15 vs >15), region (non-Asian vs Asian), ypN (ypN0-1 vs ypN2-3), risk status (CPS-EG=2 ypN+ vs ≥3), cT (cT1-2 vs cT3-4), ypT (ypT0-2 vs ypT3-4), and grade (G1-2 vs G3). The MVA for IHC3 includes all the covariates above except Ki-67. p< 0.05 was defined as statistically significant. Results: Data for ER, PgR, Ki-67, HER2, Cyclin D1 and phospho-RB were available for 1250 patients. Overall, 98.9% of the patients had ER+ tumors, 75.0% PgR+, 52.2% had HER2 low, 25.5% Ki-67>15, 50% had IHC3 score higher than median, 93.9% had Cyclin D1 >1, 57.8% had phospho-RB >2. Patients with IHC3 score high had a worse iDFS compared to patients with IHC3 score low (MVA HR 2.28 95%CI (1.78-2.91), p< 0.0001). Patients with luminal-A/normal-like tumors and IHC3 low had an improved iDFS with the addition of palbociclib to ET (MVA HR 0.35 95%CI (0.14-0.90), test for interaction p=0.01). No difference was observed according to HER2 status. Cyclin D1>1 has no predictive value but is prognostic for better iDFS (MVA HR 0.62 95%CI (0.41-0.94), p=0.023), LRRFI (MVA HR 0.30 95%CI (0.15-0.63), p=0.001) and OS (MVA HR 0.50 95%CI (0.28-0.89), p=0.019). Similar results were obtained when Cyclin D1 was analysed as a continuous variable. Phospho-RB had neither predictive nor prognostic value. Phospho-RB highly correlates with Ki-67 (p< 0.001, Spearman correlation 0.248). Conclusions: Patients with high Cyclin D1 expression had a favorable prognosis independent of treatment arm, but patients with luminal-A/normal-like tumors and IHC3 low after NACT had an improved outcome when receiving palbociclib in addition to adjuvant ET. Theses exploratory studies suggest specific signatures/phenotypes could predict benefit from Palbociclib in high-risk early breast cancer. Citation Format: Erik S. Knudsen, Sivaramakrishna Rachakonda, Frederik Marmé, Miguel Martín, Michael Untch, Hervé R. Bonnefoi, Wolfgang D. Schmitt, Sung-Bae Kim, Harry D. Bear, Agnieszka Witkiewicz, Seock-Ah Im, Angela DeMichele, Laura Van’t Veer, Nicole McCarthy, Bruno V. Sinn, Karen Gelmon, José Ángel García-Sáenz, Catherine M. Kelly, Toralf Reimer, Nicholas Turner, Federico Rojo, Martin Filipits, Peter A. Fasching, Christian Schem, Lesley-Ann Martin, Yuan Liu, Masakazu Toi, Hope Rugo, Michael Gnant, Andreas Makris, Jenny Furlanetto, Karsten Weber, Carsten Denkert, Sibylle Loibl. Immunohistochemical markers and determinants of clinical response in the Penelope-B trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD17-06.