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Background CDK4/6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) have a well-established role in the management of hormone receptor-positive (HR+)/HER2- metastatic breast cancer (MBC). The benefit of continuing CDK4/6i beyond progression in combination with a different ET has not been confirmed. Preclinical data suggest synergy between CDK4/6i and PD-L1 inhibition. The PACE trial prospectively evaluates whether continuation of the CKD4/6i palbociclib beyond progression on prior CDK4/6i and aromatase inhibitor (AI), with a change in ET to fulvestrant, improves outcomes beyond change to fulvestrant alone, as well as explores the activity of the palbociclib, fulvestrant, and avelumab triplet. Methods PACE is a multicenter randomized open-label investigator-initiated phase II trial, open at 11 U.S. sites. Eligible patients (pts) had HR+/HER2- evaluable MBC with prior progression on AI and any CDK4/6i after > 6 months (mo) of therapy in the MBC setting, or during/within 12 mo in the adjuvant setting, with no more than 1 prior line of chemotherapy for MBC. Pts were randomized 1:2:1 to fulvestrant alone (F); fulvestrant and palbociclib (F+P); or fulvestrant, palbociclib, avelumab (F+P+A), with tumor assessments every 8 weeks. Blood for circulating tumor DNA (ctDNA) analysis was collected at baseline, at times of tumor assessments, and at progression. The primary objective was to evaluate progression-free survival (PFS) with F+P vs F; secondary objectives included PFS with F+P+A vs F, objective response rate (ORR) in all arms, and safety. A sample size of 220 patients was planned to provide 80% power to detect an improvement in PFS with HR 0.6154 with F+P vs F (6.5 vs 4 mo; α(1)=0.05). Results A total of 220 pts were randomized from 9/2017-2/2022 (F: n=55, F+P: n=111, F+P+A: n=54); median age 57 years (range 25-83), 85% non-Hispanic (7.7% non-Hispanic black), 8.6% Hispanic, 6.4% unknown. 40% had de novo MBC, 60% had visceral disease, and 14% bone-only disease. 16% had 1 prior line of chemotherapy for MBC, 90% had received prior palbociclib, 4.5% ribociclib, 4.1% abemaciclib, 1.4% palbociclib and ribociclib. Pts entered the trial after a median 19 mo of prior CDK4/6i plus AI (interquartile range 12-31 mo). A total of 10 (5%) pts received protocol therapy as first line ET for MBC, 169 (77%) as second line, and 41 (17%) as beyond second line. 88% entered the trial directly after progression on CDK4/6i. After a median follow-up of 24 mo, 18 pts remained on protocol treatment. PFS was not improved with F+P vs F (median 4.6 vs 4.8 mo; HR=1.11, 90% CI 0.79-1.55; 2-sided p=0.62). Median PFS was 8.1 mo with F+P+A (HR=0.75 vs F, 90% CI 0.50-1.12; 2-sided p=0.23). ORR was 7.3% (90%CI 1.5-13.0) with F, 9.0% F+P (4.5-13.5%) and 13.0% F+P+A (5.4-20.5%). No new safety signals have been observed. Analysis of ctDNA panel sequencing encompassing 70 genes from 184 baseline samples, including correlation with known and hypothesized resistance genes, will be presented. Conclusions For ER+/HER2- breast cancer, combining palbociclib with fulvestrant beyond progression on prior CDK4/6i and AI did not significantly improve PFS compared with using fulvestrant alone. The observed longer PFS when a PD-L1 inhibitor was added to fulvestrant plus palbociclib is an intriguing signal in this ER+ population. Translational studies of blood and tumor tissue are ongoing and will be presented. Citation Format: Erica L. Mayer, Yue Ren, Nikhil Wagle, Reshma Mahtani, Cynthia Ma, Angela DeMichele, Massimo Cristofanilli, Jane Meisel, Kathy D. Miller, Trevor Jolly, Elizabeth Riley, Rubina Qamar, Priyanka Sharma, Sonya Reid, Natalie Sinclair, Meredith Faggen, Caroline Block, Naomi Ko, Ann Partridge, Wendy Y. Chen, Michelle K. DeMeo, Victoria Attaya, Amanda Okpoebo, Yuan Liu, Eric Gauthier, Harold Burstein, Meredith Regan, Sara Tolaney. GS3-06 Palbociclib After CDK4/6i and Endocrine Therapy (PACE): A Randomized Phase II Study of Fulvestrant, Palbociclib, and Avelumab for Endocrine Pre-treated ER+/HER2- Metastatic 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 GS3-06.

Background: CDK4/6 inhibitors (CDK4/6i) paired with endocrine therapy (ET) are considered first-line (1L) therapy for patients (pts) with HR+ HER2- advanced breast cancer (aBC). A minority of pts will demonstrate primary resistance to CDK4/6i, as characterized by early progression. Thymidine kinase 1 (TK1) is a cell-cycle regulated enzyme downstream of CDK4/6 and involved in nucleotide metabolism during DNA synthesis. Prior studies have shown TK1 may serve as a biomarker of response to CDK4/6i, with early TK1 activity (TK1a) suppression after initiation of CDK 4/6i therapy associated with improved PFS. Lack of TK1a suppression may be associated with primary resistance to CDK4/6i. In this study, we aim to analyze response to subsequent lines of therapy and overall survival (OS) of pts with early progression on 1L CDK4/6i. Methods: Pts with HR+ HER2- aBC from a phase II trial of an alternative schedule of palbociclib (palbo alt dosing trial NCT 3007979) and from a retrospective palbociclib study were included in this analysis. Pts in the palbo alt dosing trial underwent baseline and C1D15 TK1a analysis after initiation on CDK4/6i. C1D15 TK1a suppression was defined at TK1a < 30 Du/L. Pts in the retrospective palbociclib study included pts receiving palbo as part of their standard of care 1L therapy for HR+ HER2- aBC at Washington University in Saint Louis from 2016 to 2021. Clinical information, including treatment start and stop dates on each of the next-line therapies, were collected from the electronic medical record. PFS was estimated by the treatment duration on a specified treatment regimen. Early progression on CDK4/6i was defined as PFS < 6 mo. Best response was defined as next line of therapy with the numerically longest PFS. OS was defined as time to death from the initiation of CDK4/6i. Results: Of the 54 pts enrolled on the palbo alt dosing trial, 51 pts were evaluable for clinical benefit and 46 pts were evaluable for TK1a suppression rate at C1D15. 7 pts (15.2%) were found without TK1a suppression at C1D15. This lack of TK1a suppression on palbo was associated with a significantly shorter PFS (median PFS=3.1 mo) compared to not reached in pts with TK1a suppression at C1D15. We conducted clinical analysis on N=26 pts who exhibited early progression on CDK4/6i which included 10 pts from the palbo alt dosing trial and 16 from the retrospective study. The average subsequent line of therapies in this cohort was 3, with the most common second line (2L) therapy being chemotherapy (N=17, 65.4%) and ET (N=8, 30.8%). The median PFS for pts receiving 2L chemotherapy and ET was 4.09 mo and 3.64 mo, respectively. 10 pts received both chemotherapy and ET with 7 (70.0%) achieving best response with chemotherapy compared to 3 pts (30.0%) who achieved best response with ET. The median OS for the cohort was 14.6 mo. Conclusions: Early progression on CDK4/6i is associated with a particularly poor prognosis. In our cohort, the median OS was far below the expected median OS for pts receiving 1L palbo as reported in the PALOMA-2 trial (14.6 mo vs 53.9 mo). Early progression on CDK4/6i is associated with more aggressive disease which may respond more favorably to chemotherapy, as demonstrated by best response to therapy. Further prospective studies are warranted to explore this treatment approach. Citation Format: Katherine K. Clifton, Shana N. Thomas, Jingqin Luo, Jing Xi, Nusayba A. Bagegni, Foluso O. Ademuyiwa, Rama Suresh, Ashley Frith, Andrew A. Davis, Ron Bose, Katherine Weilbaecher, Whitney L. Hensing, Timothy Pluard, Massimo Cristofanilli, Hyo S. Han, Adam M. Brufsky, Kevin Kalinsky, Shom Goel, Seth A. Wander, Lindsay L. Peterson, Cynthia Ma. PD13-09 Clinical outcomes of patients with HR+ HER2- advanced breast cancer with early progression on CDK4/6 inhibitors [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 PD13-09.

Background: HER2 activating mutations occur in 2-5% of metastatic breast cancer (MBC) patients. These mutations cluster in the kinase domains and at amino acids 309-310 in the extracellular domain. The MutHER, SUMMIT, and PlasmaMATCH clinical trials have shown neratinib monotherapy or neratinib plus fulvestrant combination produce clinical benefit in 28% to 46% in HER2-mutated MBC patients, but median progression-free survival was only 3.6 to 5.4 months. In order to improve the knowledge and outcomes for patients with HER2-mutated MBC, we compared the mutational landscape and gene expression of HER2-mutated MBC patients to HER2-amplified and HER2-wild type MBC patients. Methods: De-identified data from a cohort of stage 4 breast cancer patients (n=1,834) sequenced with the Tempus xT (DNA-seq of 595-648 genes, whole exome-capture RNA-seq) solid tumor assay was retrospectively analyzed. The most recent sample of the patient was used for analyses. Patients were stratified by HER2 mutational status: HER2-wild type (HER2-wt), HER2-amplifications (HER2-amps), or HER2-mutants (HER2-muts). Additionally, a sub-analysis was conducted among HER2-mutants to compare kinase domain mutations to other HER2 mutations. Patient demographic characteristics were compared between groups along with the prevalence of individual gene alterations (pathogenic/likely pathogenic short variants and copy number alterations), adjusted for false-discovery. Results: Within the cohort, 62 (3.4%) patients harbored HER2-muts tumors and 125 (6.8%) patients had HER2-amps tumors. Three patients, whose tumors had both HER2 mutation and amplification, were classified among the HER2-muts. Relative to the HER2-wt cohort (median = 53 yo), HER2-muts patients were older (median = 55 yo) while HER2-amps patients were younger (median = 49 yo) (P< 0.001). Significant differences were observed in genomic alterations co-occurring with HER2-muts compared to HER2-wt and HER2-amps, including CDK12 (4.8% vs 0.2% vs 74%), CDH1 (44% vs 11% vs 4.8%), ESR1 (4.8% vs 21% vs 8.8%), TOP2A (3.2% vs 0.5% vs 14%), and ERBB3 (11% vs 0.9% vs 0.8%). Of note, the majority of CDK12 mutations are amplifications (96%). Median HER2 mRNA log10 gene expression differed among the three cohorts (HER2-muts (3.79), HER2-wt (3.56), HER2-amps (4.54); P< 0.001). Among HER2-muts patients, 46 (74.2%) patients had HER2 kinase domain mutations (HER2-kinase). Relative to other HER2-muts patients, HER2-kinase patients displayed a lower prevalence of CDH1 mutations (35% vs 69%; P = 0.018). No significant differences were noted in ERBB3 co-mutations, however all 7 were missense variants (5 of which are p.E928G variants) and occurred among HER2-kinase patients. Conclusions: Real-world data show increased HER2 mRNA expression in both HER2-mut and HER2-amps MBC patients. Co-occurring genomic alterations were different among all three groups. Notably, ERBB3 and CDH1 alterations co-occurred commonly in HER2-mut MBC patients, while ESR1 alterations co-occurred in only 4.8%. All ERBB3 co-mutations occurred with HER2 kinase domain mutations, while CDH1 co-mutations were less prevalent in the HER2-kinase group. Understanding the frequency and spectrum of HER2 mutations in MBC, as well as co-mutations, will help to guide combination treatment strategies and improve the clinical benefit of targeted therapy in HER2-mutated MBC. Citation Format: Whitney L. Hensing, Shana N. Thomas, Sherif El-Refai, Elizabeth Mauer, Cynthia Ma, Ron Bose. Gene expression and mutation profiles in HER2-mutated metastatic 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 P2-23-10.

Background: KEYNOTE-522 was a randomized, double-blind, placebo-controlled phase 3 trial which resulted in the FDA approval of pembrolizumab with neoadjuvant chemotherapy for patients (pts) with newly diagnosed, high-risk, early-stage triple negative breast cancer (TNBC). Given the improvement in pathological complete response (pCR) and event-free survival rates, this regimen has emerged as standard-of-care (SOC) therapy. Adverse events in pts on this treatment regimen in clinical practice is unknown and understanding the real-world toxicity of this regimen is critical. Methods: In this IRB approved retrospective, single-center study we examined pts with early-stage TNBC who received planned treatment per KEYNOTE-522 per SOC from 2021-present. This regimen includes a year of pembrolizumab combined with 4 cycles of neoadjuvant carboplatin/paclitaxel followed by 4 cycles of doxorubicin/cyclophosphamide. Number and length of treatment delays, treatment related toxicities of all grades, and pCR rate were collected from the electronic medical record. Results: Of the 87 identified pts, 2 were excluded due to locally recurrent or metastatic disease and 6 did not receive immunotherapy due to concerns for toxicity or patient preference. Of the 79 pts who initiated treatment with chemotherapy and immunotherapy, median age of the cohort was 52 (27-77). 9 pts had a BRCA1 mutation and 1 pt had a BRCA2 mutation. 41 (51.9%) had T1-2 disease and 38 (48.1%) had T3-4 disease. 37 (46.8%) pts had N0 disease and 42 (53.2%) had N1-3 disease. 15 pts had baseline comorbidities, including heart disease, kidney disease, type II DM, and/or peripheral neuropathy. 68 pts (86.1%) had baseline ECOG 0, 9 (11.4%) had ECOG 1, and 2 (2.5%) had ECOG 2. At the time of analysis, 70 pts (88.6%) were receiving active treatment, of which 47 (67.1%) had completed ≥50% of the planned neoadjuvant therapy. Of pts completing ≥50% of planned neoadjuvant therapy and pts off therapy (N=56), 31 (55.4%) had 1 or more hospitalizations and 23 (41.1%) had 1 or more emergency room visits. 30 pts had treatment delays (53.6%) and 21 pts (37.5%) had dose reductions. Rates of adverse events are presented in Table 1. Of the 79 analyzed pts, 35 have undergone surgery. pCR rate was 45.7% (N=16). 8 (22.9%) pts had RCB-I, 4 (11.4%) pts had RCB-II, 3 (8.6%) pts had RCB-III, and 4 (11.4%) pts had residual disease without RCB calculation. Updated analysis will be included at time of presentation. Conclusions: In this single-center retrospective study of pts receiving chemoimmunotherapy for TNBC, we found higher rates of grade 3 toxicity, including nausea, fatigue, neutropenia, diarrhea, peripheral neuropathy, and hypothyroidism, and lower pCR rate than was reported in the KEYNOTE-522 trial. This may reflect a more heterogeneous population of pts treated in routine clinical practice who are typically less fit than pts on clinical trials. Additionally, pts in this study had higher T stages (48.1% with T3-4 disease vs 26.0% in trial) and node positive disease (53.7% with N1-3 disease vs 48.8% in trial). Limitations include immaturity of data and small sample size; however, these data warrant validation through longer-term follow-up and multicenter validation. Adverse Events in pts receiving Keynote-522 regimen as SOC and on clinical trial Citation Format: Mara Hofherr, Jennifer Hedgecorth, Foluso O. Ademuyiwa, Lindsay L. Peterson, Nusayba A. Bagegni, Rama Suresh, Ashley Frith, Ron Bose, Katherine Weilbaecher, Cynthia Ma, Andrew A. Davis, Katherine K. Clifton. Real-world analysis of adverse events of patients with triple negative breast cancer receiving therapy per KEYNOTE-522 [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 P3-06-06.

Background: Triple negative breast cancer (TNBC), defined by negative hormone receptor (HR) status and the lack of HER2 gene amplification, is a significant clinical challenge because of its aggressive clinical course and association with a poor prognosis. Recent studies demonstrated that approximately a third of TNBC has low expression of HER2, so called HER2-low TNBC, which could be targeted by the new generation HER2-directed antibody-drug conjugate (ADC). Trastuzumab deruxtecan (also known as T-Dxd, DS-8201a) is a novel HER2-directed ADC with a topoisomerase 1 inhibitor payload and a tetrapeptide-based cleavable linker, which has been FDA approved for patients with metastatic HER2-positive, or amplified breast cancer. T-Dxd has also shown to improve survival outcomes when compared to physicians’ choice chemotherapy in patients with metastatic HR-negative/HER2-low breast cancer in the Phase 3 DESTINY-Breast04 trial. However, resistance is invariable. We hypothesize that small molecule inhibitors against DNA damage response (DDR) pathways could potentiate the DNA damaging effect of T-Dxd and improve its anti-tumor activity in HER2-low TNBC. Method: We conducted preclinical experiments to assess the anti-tumor activity of T-Dxd and each of the DDR pathway inhibitors, including the ATR inhibitor AZD6738, the ATM inhibitor AZD1390, and the PARP inhibitor olaparib, as well as the Wee 1 inhibitor AZD1775, either alone or in combination, compared to the vehicle treatment, in a panel of HER2-low TNBC PDX models in vivo, as well as mechanistic studies using HER2-low TNBC cell lines in vitro. HER2-low was defined as 1+ or 2+ on HER2 immunohistochemistry. Results: Four HER2-low TNBC PDX models, including WHIM2 (TP53 mutated, basal-like), WHIM6 (TP53 WT, basal-like), WHIM12 (TP53, PIK3CA, and PTEN mutated, claudin-low), and WHIM30 (BRCA1 and TP53 mutated, basal-like), as well as a panel of 4 TNBC cell lines with variable HER2 expression were included in the study. We demonstrated that the addition of DDR pathway inhibitors to T-Dxd led to further tumor shrinkage and delayed tumor progression in HER2-low TNBC PDX models in vivo and induced synergistic anti-tumor effects in vitro. Enhanced DNA damaging effect and apoptosis induction were observed with combination therapies in vitro. Biomarker analysis on xenograft tumors harvested 3 days following treatment with either Vehicle, T-Dxd, olaparib, or T-Dxd plus olaparib in WHIM6, was also performed, which demonstrated that the addition of olaparib enhanced DNA damage (increased pH2AX), and promoted apoptosis (increased cleaved PARP). Further mechanistic studies in HER2-low TNBC cell lines demonstrated the formation of DNA topoisomerase 1 covalent complexes (TopoIcc) following treatment with T-Dxd. Interestingly, while the formation of Topolcc was reduced with the addition of either ATR or ATM inhibitor, no reduction was observed with the addition of olaparib. Additionally, we observed that combination therapies reduced cell migration in vitro. The combination of olaparib with T-Dxd, in particular, reduced mammosphere formation and Epithelial Mesenchymal Transition (EMT) modulation markers, including SNAIL, SLUG and/or TWIST. Conclusion: Our study demonstrated that the combination of T-Dxd and DDR pathway inhibitors induced additive or synergistic anti-tumor effect in HER2-low TNBC. Our data from the studies of cell lines and PDX models provided preclinical rationale for the combination of T-Dxd and DDR pathway inhibitors in patients with HER2-low TNBC. The olaparib combination showed unique mechanistic characteristics compared with other DDR inhibitors, which are being further examined in additional HER2-low TNBC models. Citation Format: Adrian Gonzalez-Gonzalez, Zhanfang Guo, Alice Meroni, Emily Cybulla, Jeremy Hoog, Alessandro Vindigni, Cynthia Ma. Preclinical study of trastuzumab deruxtecan (T-Dxd; DS-8201a) in combination with DNA damage response pathway inhibitors in HER2-low/Hormone receptor negative breast cancer patient-derived xenograft models [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 P2-17-03.

Background: CDK 4/6i have altered the therapeutic landscape of HR+, HER2- MBC, improving progression free and overall survival (PFS and OS) compared to endocrine therapy (ET) alone. Despite durable responses to CDK 4/6i in a large majority of patients, treatment response monitoring in this population has historically included numerous serial blood-based and imaging studies at frequent time points. There is a growing global interest in utilizing novel non-invasive biomarker-driven disease monitoring assessments to improve patient outcomes and reduce health care costs. Thymidine kinase 1 (TK1), a key cell-cycle regulated enzyme important for nucleotide metabolism during DNA synthesis, is regulated by the E2F pathway, downstream of CDK 4/6. Studies have shown that DiviTum® TK1 activity (TKa) may serve as both a prognostic and predictive biomarker of CDK 4/6i treatment response (McCartney et al, Clin Canc Res, 2020; Malorni et al, Eur J Cancer, 2022; Bagegni et al, Breast Cancer Res, 2017). Early TKa suppression within 2 weeks (wk) post CDK 4/6i therapy initiation is associated with improved PFS, suggesting a subgroup of patients who may be able to de-escalate imaging frequency. Elevated TKa at baseline and post CDK 4/6i may identify patients with CDK 4/6i-resistant disease and disease progression (PD) requiring early therapy modification. TK IMPACT is a prospective, single-arm trial designed to assess the impact of incorporation of DiviTum® TKa on a physician’s decision regarding subsequent timing of routine disease monitoring modalities in patients with advanced HR+, HER2- MBC receiving ET plus CDK 4/6i (NCT04968964). Methods: Blood sample collections will be analyzed using DiviTum® TKa at baseline (bl), wk 2, 4, 6, 8, and Q 4 wks thereafter beginning at wk 8 during the first 24-wk time period of study enrollment (+/- 3 days); followed by Q 12 wks thereafter, until PD or 36 months, whichever occurs first. Optional repeat TKa within 2-4 wks (+/-3 days) is permitted in case of rising TKa. Research blood (bl, wk 2, 12, 24, 48, and PD) and optional archival tumor tissue collection at diagnosis and PD will be obtained for correlatives. The investigator will record intended imaging modalities and timing prior to receipt of TKa, followed by documentation of any changes in imaging testing interval after receipt of TKa. Key eligibility criteria include postmenopausal women age ≥18 years with HR+, HER2- MBC, to initiate (Cohort 1) or are currently receiving (≤24 months, Cohort 2) any FDA approved first line ET plus CDK 4/6i with a life expectancy > 6 months. The primary endpoint is any physician-reported intended change in imaging testing interval post TKa by study cohort, within the first 48-wk period of study participation. Key secondary endpoints are concordance rate between TKa values and progression status at first on-study imaging and longitudinal TKa dynamics. Key exploratory endpoints include plasma and tumor tissue-based biomarkers of CDK 4/6i response and resistance. A total of 40 patients will be enrolled (n=20/Cohort). The expected change rate is 20% with a 95% Wilson confidence interval of 0.105~0.248 across all patients and if within each cohort, with a 95% Wilson confidence interval of 0.081~0.416 for N=20. N=40 allows the lower limit of the 95% CI > 10% and that of the N=20 in Cohort 1 to be ~10%, indicating some clinically meaningful influence of TKa progression on patient management. The study is open to accrual and has presently enrolled 5 patients. Citation Format: Nusayba A. Bagegni, Isabella Grigsby, Leslie Nehring, Jingqin Luo, Jennifer Powers Carson, David W. Gibson, Meghan Horvath, Katherine K. Clifton, Foluso O. Ademuyiwa, Rama Suresh, Ashley Frith, Andrew A. Davis, Lindsay L. Peterson, Ron Bose, Amy Williams, Mattias Bergqvist, Cynthia Ma. TK IMPACT: Treatment Monitoring of Hormone Receptor Positive (HR+), HER2 Negative (HER2-) Metastatic Breast Cancer (MBC) Patients Receiving CDK 4/6 Inhibitors (CDK4/6i) with DiviTum® Thymidine Kinase 1 Activity [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 OT3-11-01.

Late-stage relapse also known as late recurrence breast cancer is a metastatic disease that occurs more than five years after initial adjuvant endocrine therapy treatment. The 5-year relative survival rate is less than 30% and there are limited treatment options for patients with late-stage relapse because of poor performance status. We hypothesize that long non-coding RNA (lncRNA) protein interactions regulate genes to promote late-stage relapse and these interactions can be targeted using RNA therapeutics. To address this, we used RNA-sequencing to identify LINC00355 to be up-regulated in 24 late-stage relapse samples compared to a unique cohort of 72 primary breast cancer samples. Analysis of 480 primary breast cancer samples from The Cancer Genome Atlas (TCGA) also shows decrease expression of LINC00355 compared to late-stage relapse. To better assess LINC00355 cell type expression, we analyzed 26 publicly available primary breast cancer single cell sequencing data samples and detected 0.90% or less of cells expressing LINC00355, which was restricted to cancer epithelial cells. Due to cellular localization playing an important role in lncRNA function, we isolated nuclear and cytoplasmic lysates and orthogonally validated with RNAscope in situ hybridization (RNA-ISH) to show that LINC00355 is located in the nucleus of MCF7 long-term estrogen deprived (MCF7 LTED) cells. LTED cells are deprived of estrogen for longer than three years meant to recapitulate the acquired resistance to aromatase inhibitors. Next, we determined that LINC00355 binds to MENIN protein to regulate p27KIP expression to promote proliferation with UV crosslinking immunoprecipitation and chromatin immunoprecipitation qPCR of LINC00355 overexpression and mutant constructs. Furthermore, RNA-ISH assays validated high nuclear expression of LINC00355 and RNA-ISH protein co-detection assays showed co-localization of LINC00355-MENIN interaction in a panel of late-stage relapse breast cancer patient tissues compared to primary tissues. Lastly to determine clinical significance, we treated MCF7 LTED cells with two locked nucleic acid antisense oligos (ASOs) targeting LINC00355 to observe a significant decrease in proliferation, cell viability, and invasion. Overall, this is the first study to show the importance of lncRNA-protein interactions and therapeutically target lncRNAs with ASOs in late-stage relapse breast cancer. Moving forward, we intend to further understand how LINC00355 contributes to the progression of late-stage relapse with the intent of creating novel cancer diagnostics and therapies. Citation Format: DeAnna Wells, Kyla Gelev, Prasanth Thunuguntla, Reyka Jayasinghe, Li Ding, Jieya Shao, Cynthia Ma, Jessica Silva-Fisher. Nuclear long non-coding RNA LINC00355 expression and treatment using antisense oligonucleotides in Estrogen Receptor positive late-stage relapse 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 P2-22-01.

Background: Neoadjuvant immune checkpoint inhibition (ICI) in combination with chemotherapy is approved for patients with high-risk, early-stage triple-negative breast cancer (TNBC) based on improved outcomes in the KEYNOTE-522 trial. However, some patients have primary resistant disease and do not achieve a pathological complete response (pCR), while others experience significant toxicity. Tumor-associated macrophages (TAMs) are a potential resistance mechanism for ICIs and are dependent on colony-stimulating factor 1 receptor (CSF1R). Therefore, we examined the addition of cabiralizumab, a CSF1R inhibitor, to neoadjuvant paclitaxel, carboplatin, and nivolumab to assess the safety, tolerability, and changes in the tumor microenvironment (TME) in patients with early-stage TNBC. Methods: This is a phase Ib/II, single-institution, randomized controlled clinical trial (NCT04331067) in patients with newly diagnosed Stage II-III TNBC. The primary endpoints include: (1) to determine the safety of a 12-week neoadjuvant regimen of paclitaxel (80 mg/m2 IV q week) + carboplatin (AUC5 IV q3 weeks) + nivolumab (240 mg IV q2 weeks) with or without cabiralizumab (4 mg/kg IV q2 weeks) and (2) to evaluate the effect of cabiralizumab on TAMs and changes in tumor infiltrating lymphocytes (TILs) in the TME between baseline and an on-treatment biopsy after 4 weeks of therapy. Adjuvant treatment is per investigator’s choice. Secondary objectives include evaluation of pCR rate and recurrence-free survival. Paired tissue and bone marrow biopsies are collected for evaluation of the TME and disseminated tumor cells, respectively. The study was designed to enroll 50 patients, including a 12-patient safety lead-in cohort. Here, we report the planned interim analysis of the safety lead-in cohort. Results: Between December 2020 and May 2022, we enrolled 12 patients to the safety lead-in, including 6 patients in each arm. 5 of 12 patients (41.7%) enrolled are underrepresented minorities, including 4 Black patients and 1 Hispanic patient. 2 of 6 patients in the nivolumab arm experienced grade 3 severe toxicity, including 1 patient who developed sepsis and 1 who developed peripheral neuropathy. 3 of 6 patients in the nivolumab + cabiralizumab arm developed grade 3 severe toxicity including 2 patients who experienced myositis and 1 patient who developed periorbital edema. Of the first 10 patients enrolled, 5 had a pCR (2 pCR in cabiralizumab arm, 3 pCR in non-cabiralizumab arm) and 3 had non-pCR (1 RCB-1 and 1 RCB-3 in cabiralizumab arm, 1 RCB-1 in non-cabiralizumab arm). 2 patients came off study prior to surgery (1 due to toxicity and 1 due to missing study visits). Data from the final 2 patients still on treatment will be available at the time of presentation. Discussion: Full safety, pathologic, and clinical response data in the safety lead-in cohort for patients with early-stage TNBC receiving neoadjuvant chemotherapy + nivolumab with or without cabiralizumab, will be presented. Citation Format: Andrew A. Davis, Leonel Hernandez-Aya, Jingqin Luo, Mateusz Opyrchal, Foluso O. Ademuyiwa, Nusayba A. Bagegni, Katherine K. Clifton, Jill Anderson, Trish Hammerschmidt, Leslie Nehring, David DeNardo, Mark Watson, Rebecca Aft, Cynthia Ma, Katherine Weilbaecher. Phase Ib/II study to evaluate safety and tolerability of cabiralizumab in combination with nivolumab and neoadjuvant chemotherapy in patients with localized triple-negative 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 P3-06-07.

Introduction: Estrogen receptor (ER) loss occurs in about 20% of recurrent breast cancers (BC) and is associated with unresponsiveness to endocrine therapy (ET) and poor prognosis. Prior studies evaluating ER-loss included predominately patients with invasive ductal carcinoma (IDC), and therefore the impact of ER-loss in invasive lobular carcinoma (ILC) is unknown. In this retrospective analysis, using real-world data, we aimed to determine the prevalence and clinical significance of ER-loss in ILC. Methods: Advanced BC were molecularly profiled at Caris Life Sciences (Phoenix, AZ) with NextGen Sequencing of DNA (592-gene panel or whole-exome sequencing), RNA (whole transcriptome sequencing, WTS) and immunohistochemistry (IHC) of select markers. A large real-world evidence (RWE) database combining Caris’ molecular data with clinical information obtained from insurance claims data (CODEai) was interrogated and overall survival (OS) was calculated from time of tissue collection to last patient contact. A tumor was considered to have ER-loss if therapies approved only for ER-positive BC were prescribed prior to obtaining a negative ER IHC result. OS was compared using Kaplan-Meier estimates for defined patient cohorts; significance was determined as p values < 0.05. For molecular analyses, Fisher-Exact or Chi-Square tests were used to determine p values. Correction for multiple comparisons was performed using Benjamini-Hochberg to calculate q values. Results: The RWE database included 24,824 patients with advanced BC. At the time of tissue collection for molecular profiling, 6,786 advanced BC patients had been previously treated with ET (with or without mTOR or CDK4/6 inhibitors), of whom 1,338 had data available on histologic classification and ER IHC. The final analytical cohort included 263 patients with ILC and 1,075 with IDC. ER-loss was identified in 11.4% of ILC (n=30/263) and 19.6% (n=210/1075) of IDC (p=0.0017). In ILC, ER-loss was associated with significantly worse OS (HR: 1.75, 95%CI: 1.10-2.79, p=0.016) compared with no ER-loss. In the cohort of patients with ER-loss, patients with ILC had significantly worse OS compared with IDC (HR=2.03, 95% CI: 1.267-3.251, p=0.003). Further, when 1,016 tumors with ER-loss (regardless of histology) were stratified by the median OS (mOS=11mo), positive PD-L1 expression (34% vs. 22%, p=0.04, q=0.22), HER2 IHC positivity (16% vs. 7.8%, p=0.003, q=0.08) and HER2 amplification (16% vs. 4.7%, p=0.0006, q=0.04) were enriched in patients with longer mOS; while amplification of TEFB (0.38% vs. 2.6%, p=0.047, q=0.23) and MYB (0.38% vs. 2.6%, p=0.047, q=0.23) were enriched in patients with shorter mOS. WTS identified 197 differentially expressed genes, the majority of which were enriched in patients with longer mOS (q< 0.05). Conclusions: In this large real-word dataset, ER-loss likely occurred in 11.4% of ILC and was associated with worse OS compared to both patients with IDC and ER-loss and ILC without ER-loss. Our analysis had several limitations; notably, our definition of ER-loss was based on prior treatment, we could not distinguish between de novo or recurrent metastatic disease and time of tissue collection was not standardized during the course of treatment. Thus, additional studies are needed to confirm these findings. However, this study does suggest that ER-loss occurs in a subset of patients with ILC and has poor prognostic implications. Citation Format: Whitney L. Hensing, Joanne Xiu, W. Michael Korn, Stephanie L. Graff, Irene Kang, Evanthia T. Roussos Torres, Arielle L. Heeke, Andrew A. Davis, Nusayba A. Bagegni, Katherine K. Clifton, Ron Bose, Cynthia Ma, Foluso O. Ademuyiwa. Prevalence and prognosis of ER-loss in advanced invasive lobular carcinoma [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 P3-05-08.

Purpose: To evaluate prediction of response and event-free survival (EFS) following neoadjuvant endocrine therapy by SET2,3 index of nonproliferation gene expression related to estrogen and progesterone receptors adjusted for baseline prognosis. Experimental Design: A correlative study was conducted of SET2,3 measured from gene expression profiles of diagnostic tumor (Agilent microarrays) in 379 women with cStage II–III breast cancer from the American College of Surgeons Oncology Group Z1031 neoadjuvant aromatase inhibitor trial SET2,3 was dichotomized using the previously published cutoff. Fisher exact test was used to assess the association between SET2,3 and low proliferation at week 2–4 [Ki67 ≤ 10% or complete cell-cycle arrest (CCCA; Ki67 ≤ 2.7%)] and PEPI-0 rate in cohort B, and the association between SET2,3 and ypStage 0/I in all patients. Cox models were used to assess EFS with respect to SET2,3 excluding cohort B patients who switched to chemotherapy. Results: Patients with high SET2,3 had higher rate of pharmacodynamic response than patients with low SET2,3 (Ki67 ≤ 10% in 88.2% vs. 56.9%, P < 0.0001; CCCA in 50.0% vs. 26.2%, P = 0.0054), but rate of ypStage 0/I (24.0% vs. 20.4%, P = 0.4580) or PEPI = 0 (28.4% vs. 20.6%, P = 0.3419) was not different. Patients with high SET2,3 had longer EFS than patients with low SET2,3 (HR, 0.52, 95% confidence interval: 0.34–0.80; P = 0.0026). Conclusions: This exploratory analysis of Z1031 data demonstrated a higher rate of pharmacodynamic suppression of proliferation and longer EFS in high SET2,3 disease relative to low SET2,3 disease. The ypStage 0/I rate and PEPI = 0 rate were similar with respect to SET2,3.

PURPOSE CALGB 40603 ( NCT00861705 ), a 2 × 2 randomized phase II trial, demonstrated that adding carboplatin or bevacizumab to weekly paclitaxel (wP) followed by doxorubicin and cyclophosphamide significantly increased the pathologic complete response (pCR) rate in stage II-III triple-negative breast cancer. We now report long-term outcomes (LTOs) and correlative science end points. PATIENTS AND METHODS The Kaplan-Meier method was used to estimate LTOs in 443 patients who initiated study treatment. Log-rank tests and Cox proportional hazards models evaluated the impact of clinical characteristics, pathologic response, calculated residual cancer burden (RCB) in patients with residual disease (RD), treatment assignment, and dose delivery during wP on LTOs, including event-free survival (EFS). Genomic predictors of treatment response and outcomes were assessed on pretreatment tumor samples by mRNA sequencing. RESULTS Among baseline characteristics, only the clinical stage was associated with LTOs. At a median follow-up of 7.9 years, LTOs were not significantly improved with either carboplatin or bevacizumab, overall or in patients with basal-like subtype cancers by genomic analysis. Patients with pCR (n = 205, 46.3%) had significantly higher 5-year EFS (85.5% v 56.6%, log-rank P < .0001) and overall survival (87.9% v 63.4%, P < .0001) rates compared with patients with RD, even those with RCB class I. Among clinical and genomic features, evidence of immune activation, including tumor-infiltrating lymphocytes and low B-cell receptor evenness, was associated with pCR and improved EFS. CONCLUSION Despite higher pCR rates, neither carboplatin nor bevacizumab appeared to improve LTOs although the study was not powered to assess these secondary end points. pCR was associated with superior LTOs even when compared with minimal RD. Markers of immune activation in pretreatment tumor biopsies were independently associated with higher pCR rates and improved survival.

Supplementary Tables

Purpose: Mammaglobin-A (MAM-A) is overexpressed in 40% to 80% of primary breast cancers. We initiated a phase I clinical trial of a MAM-A DNA vaccine to evaluate its safety and biologic efficacy.Experimental Design: Patients with breast cancer with stable metastatic disease were eligible for enrollment. Safety was monitored with clinical and laboratory assessments. The CD8 T-cell response was measured by ELISPOT, flow cytometry, and cytotoxicity assays. Progression-free survival (PFS) was described using the Kaplan–Meier product limit estimator.Results: Fourteen subjects have been treated with the MAM-A DNA vaccine and no significant adverse events have been observed. Eight of 14 subjects were HLA-A2+, and the CD8 T-cell response to vaccination was studied in detail. Flow cytometry demonstrated a significant increase in the frequency of MAM-A–specific CD8 T cells after vaccination (0.9% ± 0.5% vs. 3.8% ± 1.2%; P < 0.001), and ELISPOT analysis demonstrated an increase in the number of MAM-A–specific IFNγ-secreting T cells (41 ± 32 vs. 215 ± 67 spm; P < 0.001). Although this study was not powered to evaluate progression-free survival (PFS), preliminary evidence suggests that subjects treated with the MAM-A DNA vaccine had improved PFS compared with subjects who met all eligibility criteria, were enrolled in the trial, but were not vaccinated because of HLA phenotype.Conclusion: The MAM-A DNA vaccine is safe, capable of eliciting MAM-A–specific CD8 T-cell responses, and preliminary evidence suggests improved PFS. Additional studies are required to define the potential of the MAM-A DNA vaccine for breast cancer prevention and/or therapy. Clin Cancer Res; 20(23); 5964–75. ©2014 AACR.

Supplementary Figure 1: Expression of MAM-A and HLA-A2 in breast cancer cell lines. Supplementary Figure 2: MAM-A DNA vaccination increases breast cancer-induced TNF-α production by MAM-A-specific CD8 T cells. Supplementary Figure 3: MAM-A DNA vaccination increases NKG2D expression in MAM-A-specific CD8 T cells. Supplementary Figure 4: MAM-A DNA vaccination increases DAP10 adapter protein expression in MAM-A-specific CD8 T cells. Supplementary Figure 5: MAM-A DNA vaccination increases perforin expression in MAM-A-specific CD8 T cells. Supplementary Figure 6: Therapy received by the patients (screen failed and vaccinated).

Supplementary Table from Evaluation of Sensitivity to Endocrine Therapy Index (SET2,3) for Response to Neoadjuvant Endocrine Therapy and Longer-Term Breast Cancer Patient Outcomes (Alliance Z1031)

Raw ctDNA data

Purpose:Standard-of-care treatment for metastatic hormone receptor–positive (HR+), HER2-negative (HER2−) breast cancer includes endocrine therapy (ET) combined with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i). Optimal treatment after progression on CDK4/6i is unknown. The TRINITI-1 trial investigated ribociclib, a CDK4/6i that has recently demonstrated significant overall survival benefit in two phase III trials, in combination with everolimus and exemestane in patients with HR+, HER2− advanced breast cancer (ABC) after progression on a CDK4/6i.Patients and Methods:This multicenter, open-label, single-arm, phase I/II study included patients with locally advanced/metastatic HR+/HER2− breast cancer. The primary endpoint was clinical benefit rate (CBR) at week 24 among patients with ET-refractory disease with progression on a CDK4/6i. Other endpoints included safety and biomarker analysis.Results:Of 104 patients enrolled (phases I and II), 96 had prior CDK4/6i. Recommended phase II doses (all once daily days 1–28 of 28-day cycle) were ribociclib 300 mg, everolimus 2.5 mg, and exemestane 25 mg (group 1) and ribociclib 200 mg, everolimus 5 mg, and exemestane 25 mg (group 2). CBR among 95 efficacy-evaluable patients (phases I and II) at week 24 was 41.1% (95% confidence interval, 31.1–51.6), which met the primary endpoint (predetermined threshold: 10%). Common adverse events included neutropenia (69.2%) and stomatitis (40.4%). No new safety signals were observed; no grade 3/4 QTc prolongation was reported.Conclusions:Preliminary TRINITI-1 safety and efficacy results support further investigation of CDK4/6 blockade and targeting of the PI3K/AKT/mTOR signaling pathway in patients with ET-refractory HR+/HER2− ABC after progression on a CDK4/6i.

Purpose:To evaluate prediction of response and event-free survival (EFS) following neoadjuvant endocrine therapy by SET2,3 index of nonproliferation gene expression related to estrogen and progesterone receptors adjusted for baseline prognosis. Experimental Design:A correlative study was conducted of SET2,3 measured from gene expression profiles of diagnostic tumor (Agilent microarrays) in 379 women with cStage II–III breast cancer from the American College of Surgeons Oncology Group Z1031 neoadjuvant aromatase inhibitor trial SET2,3 was dichotomized using the previously published cutoff. Fisher exact test was used to assess the association between SET2,3 and low proliferation at week 2–4 [Ki67 ≤ 10% or complete cell-cycle arrest (CCCA; Ki67 ≤ 2.7%)] and PEPI-0 rate in cohort B, and the association between SET2,3 and ypStage 0/I in all patients. Cox models were used to assess EFS with respect to SET2,3 excluding cohort B patients who switched to chemotherapy.Results:Patients with high SET2,3 had higher rate of pharmacodynamic response than patients with low SET2,3 (Ki67 ≤ 10% in 88.2% vs. 56.9%, P < 0.0001; CCCA in 50.0% vs. 26.2%, P = 0.0054), but rate of ypStage 0/I (24.0% vs. 20.4%, P = 0.4580) or PEPI = 0 (28.4% vs. 20.6%, P = 0.3419) was not different. Patients with high SET2,3 had longer EFS than patients with low SET2,3 (HR, 0.52, 95% confidence interval: 0.34–0.80; P = 0.0026).Conclusions:This exploratory analysis of Z1031 data demonstrated a higher rate of pharmacodynamic suppression of proliferation and longer EFS in high SET2,3 disease relative to low SET2,3 disease. The ypStage 0/I rate and PEPI = 0 rate were similar with respect to SET2,3.

Supplemental Figure 1. ER, PI3K/mTOR, and CDK4/6 Pathways in HR+ Breast Cancer Supplemental Figure 2. Median Ctrough From Cycle 1 Day 15 in Cohorts A and B. Supplemental Figure 3. Time to Progression or Time on Study Treatment Supplemental Figure 4. Baseline ctDNA Mutations and Time to Disease Progression or Death

Background: Cyclin-dependent kinase 4/6 inhibitors (CKD4/6i) have demonstrated clinical utility extending progression-free survival (PFS) and overall survival (OS) for advanced hormone receptor positive and HER2 negative (HR+/HER2-) breast cancer patients. The efficacy in early-stage breast cancer (eBC) is unclear, with conflicting results from adjuvant CDK4/6i trials on invasive disease-free survival. Thus, there is a critical need to identify biomarkers of response (BoR) to determine which, if any, eBC patients could benefit from this treatment. This BoR could also stratify advanced BC patients for likelihood to respond to CDK4/6i. Metabolism is influenced by both genome and environment, and changes in the metabolome can be correlated with drug responsiveness. Thus, metabolite BoRs may serve to identify eBC patients for which CDK4/6i would offer a therapeutic benefit.Methods: Plasma and serum samples from 50 early-stage ER+/HER2- breast cancer patients, treated with neoadjuvant CDK4/6 inhibitor palbociclib (palbo) and aromatase inhibitor (AI) anastrozole on NeoPalAna trial (ClinicalTrials.gov identifier NCT01723774), were collected from treatment-naïve patients (BL) and 3 consecutive time points: anastrozole,1 mg daily for 4 weeks (C1D1), anastrozole plus palbo,125 mg daily, for 15 days (C1D15), and for 4-5 months before surgery (SURG). Metabolites were extracted from all samples via methanol and chloroform precipitation and quantified using an unbiased, non-destructive, nuclear magnetic resonance (NMR)-based profiling platform (Olaris®, Inc., Waltham, MA). Statistical analysis and machine learning was used to identify differential metabolites and generate predictive models. A separate validation set of samples was collected from a subset of patients (N=6) who received an additional cycle of palbo treatment prior to surgery to assess model accuracy. Results: Non-parametric differential expression analysis of BL/C1D1, BL/C1D15, and C1D1/C1D15 identified 53 ,97, and 90 differential NMR resonances in plasma (p2.7%). Analysis of the responder (R) and non-responder (NR) groups identified that 13 plasma and 14 serum resonances (21 unique resonances and 6 overlapping) were differentially expressed (p Citation Format: Chen Dong, Shana Thomas, Chandrashekhar Honrao, Leonardo O. Rodrigues, Nathalie Tessier, Bo Zhang, Souzan Sanati, Kiran Vij, Brenda J. Ernst, Karen S. Anderson, Mateusz Opyrchal, Foluso Ademuyiwa, Lindsay L. Peterson, Matthew P. Goetz, Donald Northfelt, Elizabeth O'Day, Cynthia Ma. Identifying a metabolite signature that correlates with tumor proliferation in early-stage breast cancer patients treated with CDK4/6 inhibitors from matched plasma and serum samples [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 P5-13-20.

Background: ESR1 hotspot mutations (HS) (i.e. 380, 536, 537, and 538) are important drivers of resistance to aromatase inhibitors, but the differential impact of genomic variants (HS vs non-HS) on response to endocrine therapies (ET) under clinical development, such as novel oral Selective Estrogen Receptor Degraders and Modulators (SERDs and SERMs), is not known. The aim of the study was to evaluate the impact of non-HS ESR1 mutations on the pharmacodynamics of SERDs and SERMs as an additional ET resistance mechanism. Materials and Methods: The study analyzed a multi-institutional cohort of 1008 patients with hormone receptor positive metastatic breast cancer characterized by circulating tumor DNA (ctDNA). Pathway classification was defined based on previous work (i.e. RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, p53, Cell Cycle, Notch, PI3K). Single nucleotide variations (SNVs) were annotated through OncoKB; co-occurrence was tested by Fisher’s exact test. A structure-based computational strategy was used to create 3D-models of ESR1 mutants and predict changes in binding affinity (dAff) across approved and experimental drugs. A positive dAff reflects a lower affinity of the drug for mutant ESR1 compared with wild type and thus a potential for a reduced response. Results: Among the total 680 detected ESR1 mutations, 633 were missense, and 631 were gain-of-function. The most frequent mutations were in codon 537 (N=305), followed by 538 (N=224). No significant MAF differences were observed across ESR1 variants (P=0.0829). The L391F mutation resulted in an increased binding affinity for Lasofoxifene (LAS) (dAff -0.34), Giredestrant (GIR) (dAff -0.18), Elacestrant (ELA) (dAff -0.08) and Amcenestrant (AMC) (dAff -0.41), while a decreased binding affinity was observed for 4OH-Tamoxifen (TAM) (dAff 0.01), Imlunestrant (IML) (dAff 0.15), Fulvestrant (FUL) (dAff 0.43), and Camizestrant (CAM) (dAff 0.02). V392F decreased binding affinity for TAM (dAff 0.05), LAS (dAff 0.13), IML (dAff 0.11), GIR (dAff 0.11), FUL (dAff 0.04), CAM (dAff 0.05), AMC (dAff 0.06) but not for ELA (dAff -0.01). F404L decreased binding affinity for FUL (dAff 0.07), ELA (dAff 0.73), and CAM (dAff 0.26), while it increased binding affinity for TAM (dAff -0.27), LAS (dAff -0.02), IML (dAff -0.05), GIR (dAff -0.69), and AMC (dAff -2.01). G415E increased binding affinity for LAS, (dAff -0.15) GIR (dAff -0.02) and ELA (dAff -0.08), while it decreased binding affinity for TAM (dAff 0.11), IML (dAff 0.09), FUL (dAff 0.29), CAM (dAff 0.19) and AMC (dAff 0.10). Mutations in codon 537 did not affect dAff for TAM, GIR, and ELA; a significant decrease in binding affinity was observed for FUL and AMC, whereas it was increased for LAS. Mutational co-occurrence was tested between ESR1 mutations in FUL docking sites and oncogenic pathways. Significant associations were observed for cell cycle SNVs (P=0.047), Notch SNVs (P=0.020), and ER SNVs (P< 0.001). Within these pathways, significant single-gene associations were observed for FBXW7 SNVs (P=0.020), ESR1 SNVs (P< 0.001), and GATA3 SNVs (P= 0.016). Given the highly significant co-occurrence of non-HS with other ESR1 mutations, combined models were examined. The Y537/F404 combination resulted in decreased binding affinity for FUL and increased binding affinity for LAS, while L536/F404 decreased binding affinity for TAM and increased binding affinity for IML, ELA, and AMC. Notably, L540/F404 restored the FUL-ESR1 interaction resulting in an increased binding affinity (dAff -2.1). Conclusions: The study suggests that genomic variability in drug targets detectable through ctDNA may modulate therapeutic response. Preclinical models are under development to investigate the combined endocrine resistance mechanism suggested by the significant co-occurrence between ESR1 mutations in SERDs/SERMs docking sites and ESR1 hotspot mutations and provide valuable additional insights for drug development and future treatment algorithms. Citation Format: Lorenzo Gerratana, Rossana Roncato, Mattia Sturlese, Andrew A. Davis, Marko Velimirovic, Carolina REDUZZI, Katherine K. Clifton, Whitney L. Hensing, Ami N. Shah, Charles S. Dai, Paolo D’Amico, Arielle J. Medford, Alessandra Franzoni, Linda Cucciniello, Firas Wehbe, Seth A. Wander, Barbara Belletti, William Gradishar, Amir Behdad, Giuseppe Damante, Cynthia Ma, Fabio Puglisi, Aditya Bardia, Massimo Cristofanilli. PD10-01 Impact of ESR1 mutations on Selective Estrogen Receptor Degraders and Modulators: an integrated liquid-biopsy and pharmacodynamics approach. [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 PD10-01.

Introduction The mammalian target of rapamycin (mTOR) is a downstream mediator in the phosphatidylinositol 3-kinase/Akt signaling pathway, and plays a central role in cell proliferation, growth, differentiation, migration, and survival. Temsirolimus (CCI-779), a selective inhibitor of the mTOR, is an ester analog of rapamycin (sirolimus) with improved aqueous solubility and pharmacokinetic (PK) properties. Preclinical studies have confirmed additive and synergistic antitumor activity in cancer cell lines (breast, prostate cancer) with combinations of taxanes and mTOR inhibitors. We conducted a phase I open-label, dose-escalation study to determine the maximal tolerated dose (MTD) of docetaxel in combination with temsirolimus in patients with refractory solid tumors. Patients and methods Eligible patients had a diagnosis of a refractory solid malignancy, measurable disease, and adequate organ function. Patients were sequentially enrolled in 4 dose level intravenous combinations of docetaxel and temsirolimus. Temsirolimus was administered weekly with docetaxel administered every 3 weeks. Laboratory data for tumor markers and radiologic imaging were conducted prestudy and then after every 2 cycles of the treatment. Radiologic response was assessed by Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Blood samples for PK and pharmacodynamic analysis were planned to be drawn at MTD. Apart from the traditional 3+3 design, we also implemented Bayesian Optimal Interval design which uses isotonic regression method to select MTD. We proceeded with isotonic regression analysis by using 20% dose-limiting toxicity (DLT) rate as target. Results Twenty-six patients were treated in this study in 4 cohorts and dose levels. Fourteen males and 12 females were enrolled with a median age of 50 years (range of 27 to 72 y) and median Eastern Cooperative Oncology Group performance score of 1. Tumor histologies included pancreas (6), colon (5), rectum (3), gallbladder (2), non-small cell lung (2), endometrium (1), neuroendocrine (1), esophagus (1), stomach (1), pharynx (1), small intestine (1), and duodenum (1). Stable disease was observed in 2/4 (50%), 3/7 (43%), 4/10 (40%), and 3/5 (60%) patients in cohorts 1, 2, 3, and 4, respectively. Dose escalation in cohorts 2, 3, and 4 was complicated by DLTs such as grade 4 neutropenia and grade 3 diarrhea and an inability for patients to tolerate treatments during and beyond cycle 1 without dose reductions. Therefore, we could not determine an MTD or recommended phase II dose using the traditional 3+3 study analysis. Blood samples for PK and pharmacodynamic analysis were not collected since MTD was not determined. By using 20% DLT rate closest to the target, isotonic regression analysis showed identical estimated DLT rates in dose -1 (docetaxel 50 mg/m2 and temsirolimus 15 mg/m2) and dose level 1 (docetaxel 60mg/m2 and temsirolimus 15 mg/m2). Conclusions Dose escalation of docetaxel and temsirolimus was limited by severe myelosuppressive toxicity in this phase I study. Most of the DLTs occurred after cycle 1 of therapy hence, we were unable to determine MTD or collect blood samples for PK and pharmacodynamic analysis. Our trial did not meet its objectives due to significant DLTs with this chemotherapy combination. Although our novel use of Bayesian Optimal Interval design using isotonic regression method to select MTD showed identical estimated DLT rates in dose levels 1 and -1, clinically our patients were not able to complete 2 cycles of this regimen without dose reductions due to myelosuppressive toxicity in either of these dose levels, and hence, escaped clinical validity. This combination regimen should not be studied further at the dose levels and schedules tested in our study.

Aim The aim was to explore how nurse mentors experience the assessment of nursing students in clinical placements at hospitals and in municipal health care. Design The study is qualitative with an explorative and descriptive design. Methods Based on an interview guide, we conducted 19 individual qualitative interviews and four focus group interviews with nurse mentors from various levels and fields of nursing education at a Norwegian university. Results Feedback in and on action was an integrated part of the formative assessment. In the summative assessment, where the university lecturer also participates, the nurse mentors perceived their role as passive. A disturbing finding was that divergent views on the student's competence sometimes occurred in these situations, thus challenging the credibility of the student assessment. Perceptions of nursing values and concerns embedded in nursing practice as collective criteria appear to have an impact on the mentors’ assessment of the nursing students.

A hub and spoke model offers an effective and efficient approach to providing informed guidance to those who need it. The National Center for Ethics in Health Care (NCEHC) at the Veterans Health Administration, Department of Veterans Affairs, is the largest known hub and spoke healthcare ethics delivery model. In this article, we describe ways NCEHC's hub and spoke configuration succeeded during the COVID-19 pandemic, as well as limitations of the model and possible improvements to inform adoption at other healthcare systems.

Estrogen receptor (ER) positive breast cancer is a heterogeneous group of diseases. While the major goal of neoadjuvant treatment in patients with locally advanced disease is to downstage the cancer to facilitate resectability and breast-conserving surgery, there is increasing evidence that response in the neoadjuvant setting has prognostic implications in predicting long term relapse outcomes. However, questions remain as to strategies that optimize neoadjuvant treatment selection of endocrine therapy versus chemotherapy, and whether adjuvant therapy could be tailored based on treatment response in the neoadjuvant setting. In this lecture, we will discuss evolving data regarding pathologic and molecular characteristics of ER positive breast cancer that assist with treatment selection in the neoadjuvant setting and the potential role of pathologic and biomarker response to neoadjuvant endocrine therapy in tailoring adjuvant chemotherapy decisions. Citation Format: Cynthia Ma. Hormone receptor 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 ED11-1.

Background: Imlunestrant is a novel, orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties that result in sustained inhibition of estrogen receptor (ER)-dependent gene transcription and cell growth. Preclinically, imlunestrant has favorable efficacy and pharmacokinetic (PK) properties, including antitumor activity in ESR1-mutant models, along with enhanced efficacy when combined with abemaciclib. In dose escalation (Phase 1a) and dose expansion (Phase 1b) in the EMBER study, imlunestrant monotherapy was well tolerated with favorable safety, PK and encouraging antitumor activity in heavily pre-treated ER+, HER2- advanced breast cancer (aBC) patients (Jhaveri, ASCO 2022); imlunestrant recommended phase 2 dose (RP2D) was determined as 400mg QD. Here, we present the phase 1b dose expansion of imlunestrant with abemaciclib ± aromatase inhibitor (AI) in EMBER (NCT04188548). Methods: Phase 1b enrolled patients with ER-positive (ER+), HER2-negative (HER2-) aBC [shown prior endocrine therapy (ET) sensitivity or untreated de novo aBC; ≤1 prior therapies for aBC but must not have received a prior CDK4/6 inhibitor]. Patients were randomized, based on menopausal status and presence of visceral metastases, to receive imlunestrant + abemaciclib OR imlunestrant + abemaciclib + AI. Men and premenopausal women received a concomitant GnRH agonist. Serial plasma samples were obtained for PK and ctDNA analysis. Key endpoints included safety and tolerability, PK, objective response rate (ORR) per RECIST v1.1 (ORR: complete response [CR] or partial response [PR]) in patients with measurable disease), and clinical benefit rate (CBR: CR or PR, or stable disease ≥24 weeks) in patients enrolled ≥24 weeks prior to data cut. Results: As of 26 May 2022, 85 patients have received imlunestrant [n=80 at 400 mg (RP2D); n=5 at 800 mg] in combination with abemaciclib (150mg twice daily) ± AI. Forty-eight (56%) patients had visceral disease and 9% had at least 1 ESR1 mutation detected in ctDNA at baseline. Patients were predominantly (75%) ET pre-treated, 51% with an AI; and 8% and 5%, respectively, had received prior chemotherapy or fulvestrant, for aBC. The most common treatment-emergent adverse events were diarrhea (87%), nausea (58%), fatigue (45%), neutropenia (39%) and abdominal pain (34%). The majority of treatment-related AEs (TRAEs) were Grade 1 or 2, with Grade ≥3 TRAEs occurring in 36% of patients. Most common TRAEs at RP2D (400mg) were diarrhea (81%), nausea (45%), fatigue (33%) and neutropenia (35%). No patient discontinued treatment due to an AE. Dose reductions were required of both imlunestrant and abemaciclib in 6 (7%) patients and of either imlunestrant in 3 (4%) or abemaciclib in 22 (26%) patients. Preliminary efficacy is presented in Table 1. Conclusion: Imlunestrant in combination with abemaciclib ± AI showed acceptable safety and tolerability, comparable to the MONARCH 2 trial of fulvestrant + abemaciclib, along with evidence of clinical activity in ER+, HER2- aBC patients. These data suggest no additive toxicity of imlunestrant when administered in combination with abemaciclib, along with comparable clinical benefit to that observed in MONARCH 2. Further data will be presented at the meeting. The phase 3, EMBER-3 study is ongoing; evaluating imlunestrant, investigator’s choice ET, and imlunestrant + abemaciclib in ET pre-treated ER+, HER2- aBC patients (NCT04975308). Table 1. Preliminary efficacy in combination therapies in EMBER Citation Format: Komal Jhaveri, Hwei-Chung Wang, Cynthia Ma, Elgene Lim, Jessica J. Tao, Luis Manso, Jean-Yves Pierga, Ritesh Parajuli, Yolanda Jerez Gilarranz, Yen-Shen Lu, Muralidhar Beeram, Tim Larson, Ajay Dhakal, Roohi Ismail-Khan, Claudia Karacsonyi, Shanshan Cao, Cynthia Osborne, Shawn T. Estrem, Bastien Nguyen, Yujia Li, Eunice Yuen. PD13-12 Imlunestrant, an oral selective estrogen receptor degrader, in combination with abemaciclib with or without an aromatase inhibitor, in estrogen receptor-positive advanced breast cancer: Results from the phase 1a/b EMBER 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 PD13-12.

Background: In the SUMMIT trial, original cohorts of patients with HR+, HER2-negative (locally assessed), HER2-mutant MBC received N alone or N+F, with promising clinical response rates but abbreviated duration. Clinical progression coincided with emergence of additional HER2 mutations and/or amplification of the mutant allele [Smyth et al. Cancer Discov 2020;10:198–213]. Addition of T to the combination was postulated to prolong response; the combination of N+F+T in heavily pretreated patients with HR+, HER2-mutant MBC who had received CDK4/6 inhibitors (n=51) yielded a confirmed overall response rate (ORR) of 35.3%, median duration of response (DOR) of 14.3 months, clinical benefit rate (CBR) of 47.1%, and median progression-free survival (PFS) of 8.2 months [Jhaveri et al. J Clin Oncol 2022;40:1028]. Seven of these 51 patients were part of a randomized (1:1:1) cohort who received N+F+T, F+T, or F alone. Patients randomized to F+T or F could crossover to N+F+T upon progression. No patients responded to F or F+T; however, one of four patients who crossed over to N+F+T upon progression on F+T responded to the triplet, as did two of six who crossed over upon progression on F. We undertook ctDNA sequencing in patients who responded to N+F+T upfront and after crossover. Methods: Patients with HR+, HER2-negative MBC with activating HER2 mutation(s) and prior CDK4/6i received N+F+T (oral N 240 mg/d with loperamide prophylaxis, im F 500 mg d1, d15, and d29 of cycle 1 then q4w, iv T 8 mg/kg initially then 6 mg/kg q3w) or F+T or F alone. Efficacy endpoints included investigator-assessed ORR and CBR (RECIST v1.1), DOR, and best overall response. ctDNA was collected at baseline, every third cycle during treatment, and at the end of treatment and analyzed by next-generation sequencing. Samples were analyzed using the TEMPUS xF assay. Results: Sequencing results from ctDNA analysis are pending at the time of this abstract submission. Baseline HER2 mutations and co-alterations will be reported and compared with those found in tissue samples. Genomic spectrum and variant allele frequencies in samples taken at baseline, on treatment, and at the end of treatment from patients who experienced complete or partial response to N+F+T and then progressed (n=25) will be sequenced and mechanisms of acquired resistance will be posited. ctDNA genomic profiles of serial time points from patients randomized to F or F+T before and after crossover to N+F+T (n=10) will also be evaluated. Conclusions: Similarities and differences between the mechanisms of acquired resistance to N+F+T, and those previously reported to be associated with progression on N or N+F, will be discussed. Citation Format: Cynthia Ma, James Waisman, Adam M. Brufsky, Eddy S. Yang, Hans Wildiers, John P. Crown, Sarina A. Piha-Paul, Jennifer M. Suga, José Ángel García-Sáenz, Valentina Gambardella, Angel Guerrero, Salomon Stemmer, Ron Bose, Tonya Novara-Demgen, Daniel DiPrimeo, Lisa D. Eli, Komal Jhaveri. Genomic analysis of circulating tumor DNA (ctDNA) from patients with HR+, HER2-mutant metastatic breast cancer (MBC) enrolled in SUMMIT: mechanisms of acquired resistance to neratinib + fulvestrant + trastuzumab (N+F+T) [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 PD17-01.

Background: ARV-471 is a selective, orally administered PROteolysis TArgeting Chimera (PROTAC®) protein degrader that targets wild-type and mutant ER. ARV-471 is being evaluated in patients with ER+/HER2- locally advanced or metastatic breast cancer in a first-in-human phase 1/2 study (NCT04072952). In the phase 1 dose escalation, ARV-471 monotherapy (dose range: 30–700 mg total daily dose) showed a manageable safety profile in patients who had previously received endocrine therapy and a cyclin-dependent kinase (CDK) 4/6 inhibitor. The clinical benefit rate (CBR; rate of confirmed complete or partial response or stable disease ≥24 weeks) was 40% (95% CI: 26–56) in 47 evaluable patients. The phase 2 expansion portion of the study (VERITAC) evaluated 2 doses of ARV-471.Methods: In VERITAC, ARV-471 monotherapy was administered at doses of 200 mg once daily (QD) or 500 mg QD to patients with ER+/HER2- locally advanced/metastatic breast cancer who had received ≥1 prior endocrine therapy for ≥6 months, ≥1 CDK4/6 inhibitor, and ≤1 chemotherapy regimen. The primary endpoint of CBR was evaluated in patients enrolled ≥24 weeks prior to the data cutoff. Results: As of June 6, 2022, 71 patients received ARV-471 (200 mg QD [n=35]; 500 mg QD [n=36]) in VERITAC. Across all treated patients, 69 (97.2%) were female and median age was 60 y (range: 41–86). Patients had received a median of 4 prior regimens in all settings (range: 1–10); 100% had prior CDK4/6 inhibitors, 78.9% had prior fulvestrant, and 73.2% had prior chemotherapy. ARV-471 was well tolerated at both doses, with most treatment-related adverse events (TRAEs) grade 1/2; the most common TRAEs were fatigue and nausea (Table). In all, 3 patients (1 in the 200 mg QD cohort and 2 in the 500 mg QD cohort) discontinued ARV-471 due to treatment-emergent adverse events (TEAEs); 3 patients had ARV-471 dose reductions due to TEAEs (all from 500 mg QD to 400 mg QD). CBR was 37.1% (95% CI: 21–55) in 35 evaluable patients treated at 200 mg QD and 38.9% (95% CI: 23–57) in 36 evaluable patients treated at 500 mg QD. CBR in evaluable patients with mutant ESR1 in the 200 mg QD (n=19) and 500 mg QD (n=22) cohorts was 47.4% (95% CI: 24–71) and 54.5% (95% CI: 32–76), respectively. Conclusions: In the phase 2 VERITAC expansion cohorts of patients with ER+/HER2- locally advanced/metastatic breast cancer and prior CDK4/6 inhibitor treatment, ARV-471 monotherapy showed evidence of clinical activity based on CBR, which was further enhanced in the subgroup with ESR1 mutations. The manageable AE profile observed in the phase 1 portion of the study was maintained during cohort expansion at doses of 200 mg QD and 500 mg QD. Additional analyses are ongoing.Table. TRAEs reported in ≥10% of patients overall aNo grade 3/4 TRAE occurred in >1 patient. AST=aspartate aminotransferase Citation Format: Anne F. Schott, Sara Hurvitz, Cynthia Ma, Erika Hamilton, Rita Nanda, George Zahrah, Natasha Hunter, Antoinette R. Tan, Melinda Telli, Jesus Anampa Mesias, Rinath Jeselsohn, Pamela Munster, Haolan Lu, Richard Gedrich, Cecile Mather, Janaki Parameswaran, Hyo S. Han. GS3-03 ARV-471, a PROTAC® estrogen receptor (ER) degrader in advanced ER-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer: phase 2 expansion (VERITAC) of a phase 1/2 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 GS3-03.

BACKGROUND: Immune checkpoint inhibitors in combination with chemotherapy have demonstrated an improvement of pathologic complete response (pCR) in patients with HR-HER2- and MammaPrint (MP) High Risk, HR+HER2- tumors in the I-SPY2 TRIAL. However, not all patients benefit from immune checkpoint blockade and these new agents come with additional financial burden and significant long-lasting side effects such as adrenal insufficiency. Thus, it is imperative to better understand who benefits. Response Predictive Subtypes (RPS) were developed in the I-SPY2 TRIAL using pre-treatment expression data from 987 MP High Risk patients; 39% of HR+HER2- tumors and 63% of HR-HER2- tumors were identified as immune sensitive. In I-SPY2.2, RPS tumor classification uses ImPrint, a 53-gene signature that has been independently validated to predict the likelihood of a pCR with PD1-PDL1 immune checkpoint inhibitors with high sensitivity and specificity. Using a real-world dataset of 10,000 patients enrolled in the FLEX trial, we identified immune sensitive (ImPrint+) patients within immunohistochemistry (IHC) subtypes and within MP and BluePrint (BP) subgroups. METHODS: FLEX (NCT03053193) is an ongoing registry trial with 97 sites open in the United States and 2 international sites. Patients enrolled in FLEX have early-stage breast cancer and receive standard of care MP testing with or without BP molecular subtyping and consent to clinically annotated full genome data collection. MP is a 70-gene risk of distant recurrence signature that classifies patients as Low Risk or High Risk. MP High Risk can be further stratified into High 1 and High 2, which have demonstrated differences in chemosensitivity and pCR rates in the I-SPY2 TRIAL (NCT01042379). BP, an 80-gene molecular subtyping signature, categorizes patients’ tumors as Luminal-, HER2- or Basal-Type. RESULTS: Of the 10,021 patients, 9.1% of the FLEX patient population are ImPrint+ and are predicted to have a meaningful pCR rate with immune checkpoint inhibitors. Younger (≤ 50 years) or pre/peri-menopausal patients, patients with larger or node-positive tumors, and patients of Black or Latin race/ethnicity independently had a higher likelihood of having ImPrint+ tumors (Table 1). ImPrint+ tumors were identified in all clinical subtypes by IHC. There is a higher likelihood of ImPrint+ tumors being MP High 2 or BP Basal-Type tumors. Within BP Basal tumors, 74.7% of HR+ and 66.0% of HR- tumors were ImPrint+. CONCLUSIONS: The focus of immune therapy trials has been on patients with HR-HER2-, MP High Risk patients. Indeed, most patients who are predicted to benefit have MP High 2 or BP Basal-Type tumors, including some HR+ patients, which is consistent with I-SPY2 results. Importantly, this large real-world dataset enables the identification of populations who may benefit from immune therapy outside of traditional clinical trial populations and supports the testing of checkpoint inhibitors in the immune-positive subtype. Younger women and patients of Black or Latin race/ethnicity who typically have more aggressive tumors also have higher proportions of ImPrint+ tumors. Thus, it is critical that these populations be included in clinical trials. This first look at immune sensitivity in over 10,000 FLEX patients with ImPrint generates preliminary data and hypotheses that will be explored in future FLEX substudies, including an analysis of lobular cancers and long-term outcomes in ImPrint+ patients across all races and ages. Table 1. Clinical characteristics of ImPrint+ and ImPrint- tumors. Citation Format: Adam M. Brufsky, Midas Kuilman, Rita Mukhtar, Denise M. Wolf, Christina Yau, Joyce O’Shaughnessy, Cathy Graham, Vijayakrishna K. Gadi, Pat Whitworth, Alexander Hindenburg, Ian Grady, Gordon Srkalovic, Kent Hoskins, Ajay Dhakal, Cynthia Ma, Natasha Hunter, Jennifer Crozier, Blanche Mavromatis, Lorenza Mittempergher, Christine Finn, Shraddha Modh, Erin B. Yoder, Patricia Dauer, Andrea Menicucci, Bas van der Baan, William Audeh, Laura J. Esserman. ImPrint immune signature in 10,000 early-stage breast cancer patients from the real-world FLEX database [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 PD9-08.

Background: CDK4/6 inhibitors (CDK4/6i) are standard first-line (1L) regimens for HR+/HER2- advanced breast cancer (aBC). Recent data from the randomized phase II MAINTAIN trial reported a PFS benefit for patients (pts) who received a new endocrine therapy plus ribociclib (ribo) versus new endocrine therapy alone) following progression on CDK4/6i as compared to pts who received endocrine therapy alone. However, second-line (2L) treatment patterns and patient outcomes following 1L CDK4/6i are relatively undescribed. Here we describe real-world 2L treatment patterns following 1L CDK4/6i and associated clinical outcomes from a large clinical genomics database. Methods: Real-world evidence (RWE) was sourced from the GuardantINFORM (Guardant Health) database which comprises aggregated commercial payer health claims and de-identified records from over 207,000 pts with comprehensive circulating tumor DNA (ctDNA) results via Guardant360 (G360) from 2014 to 2021. Pts with HR+/HER2- aBC with >1 claim of CDK4/6i and >1 claim for treatment after the index G360 test were included. Real-world time to treatment discontinuation (rwTTD) and real-world time to next treatment (rwTTNT) were assessed in months as proxies for progression-free survival. Real-world overall survival (rwOS) was also reported in months. Results: 2,795 pts met criteria for inclusion; 2,361 (84.5%) were treated with 1L palbociclib (palbo), 271 (9.7%) with 1L abemaciclib (abema) and 163 (5.8%) with 1L ribo. Chemotherapy (chemo,35.5%) and endocrine-only therapy (32.8%) were the most common 2L therapy regardless of the 1L CDK4/6i agent (Table 1). Other 2L agents included endocrine backbone change (14.7%) or CDK4/6i change (7.7%). Endocrine backbone changes were observed more frequently (15.6%) in pts receiving 1L palbo while CDK4/6i changes were more frequent in pts receiving abema (14.0%) or ribo (22.0%). Pts treated with 2L CDK4/6i had improved rwTTNT, rwTTD and rwOS compared to 2L chemo regardless of 1L agent [rwTTNT: 10.2 (95% CI: 7.2-11.7) vs. 7.2 (6.5-8.1); rwTTD: 6.8 (95% CI: 5.8-8.5) vs. 4.3 (95% CI:3.9-4.7); rwOS: NR (95% CI: 40.0-NR) vs. 34.8 (95% CI: 31.3-37.2)]; improvement in rwTTNT, rwTTD and rwOS were also observed for pts with 2L endocrine backbone changes compared to chemo [rwTTNT: 8.5 (95% CI: 7.2-9.6) vs. 7.2 (6.5-8.1); rwTTD: 6.9 (95% CI: 5.8-7.9) vs. 4.3 (95% CI:3.9-4.7); rwOS: 63.4 (95% CI: 51.2-NR) vs. 34.8 (95% CI: 31.3-37.2)]. Pts treated with 2L alpelisib had the shortest rwOS regardless of 1L CDK4/6i agent used [any 1L: NR (95% CI: 23.6-NR)]. Conclusions: A variety of 2L regimens following 1L CDK4/6i were observed, with an improvement in rwTTNT, rwTTD and rwOS in pts receiving 2L CDK4/6i or 2L endocrine backbone change only relative to 2L chemo. These data are hypothesis generating, and the observed improvement may be secondary to therapy choice versus pts who received 2L chemo having more aggressive disease. Larger randomized trials are ongoing to study sequencing and efficacy of 2L treatments following 1L CDK4/6i. Table 1. Distribution of 2L therapies by 1L CDK4/6i agent. Citation Format: Katherine K. Clifton, Seth A. Wander, Cynthia Ma, Andrew A. Davis, Caroline Weipert, Nicole Zhang, Leslie Bucheit. Real-world second-line treatment patterns and associated clinical outcomes for 2795 patients with advanced HR+ HER2- breast cancer treated with first-line CDK4/6 inhibitors [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 P4-01-18.

Background Metastatic breast cancer (MBC) remains incurable despite significant treatment advances. Coordinating care for patients with MBC can be challenging given the various treatment options, available clinical trials, and frequent need for ancillary services. To optimize MBC care, we designed a project for adapting and developing an academic and community practice collaborative care model for MBC care (Project ADAPT), based on the Ending Metastatic Breast Cancer for Everyone (EMBRACE) program developed at Dana Farber Cancer Institute. Objective We aim to describe the implementation science–based study design and innovative components of Project ADAPT. Methods Project ADAPT uses the Dynamic Adaptation Process informed by the Exploration, Preparation, Implementation, Sustainment framework. Washington University School of Medicine (WUSM) partnered with 3 community hospitals in the St. Louis region covering rural and urban settings. The exploration and preparation phases provide patient and provider feedback on current referral practices to finalize the approach for the implementation phase. At the implementation phase, we will enroll patients with MBC at these 3 community sites to evaluate potential collaborative care at WUSM and assess the impact of this collaborative care model on referral satisfaction and acceptability for patients with MBC and their providers. Patients may then return to their community site for care or continue to receive part of their care at WUSM. We are incorporating virtual and digital health strategies to improve MBC care coordination in order to minimize patient burden. Results The exploration phase is ongoing. As of August 2021, we have recruited 21 patient and provider participants to complete surveys of the current collaborative care process at WUSM. Using a 2-tailed paired t test, 44 patients (including 10 patients from the exploration phase) and 32 oncologists are required to detect an effect size of 0.5 with 80% power at a level of significance of .05. Throughout this phase and in preparation for the implementation phase, we have iteratively updated and refined our surveys for the implementation phase based on testing of our data collection instruments. Our partner sites are in various stages of the single institutional review board (IRB) approval process. We have ongoing engagement with all partner sites, which has helped solidify our participant recruitment strategies and design patient-friendly recruitment materials. In addition, we have included a patient advocate on the research team. Members of the research team have launched a single IRB Support Network at WUSM to create a repository of the single IRB procedures in order to streamline the partner site onboarding process and facilitate enhanced collaboration across institutions. Conclusions With this robust model, we expect that patients with MBC will receive optimal care regardless of geographical location and the model will improve patient and provider experiences when navigating the health system. International Registered Report Identifier (IRRID) DERR1-10.2196/35736

BACKGROUND Metastatic breast cancer (MBC) remains incurable despite significant treatment advances. Coordinating care for patients with MBC can be challenging given the various treatment options, available clinical trials, and frequent need for ancillary services. To optimize MBC care, we designed a project for adapting and developing an academic and community practice collaborative care model for MBC care (Project ADAPT), based on the Ending Metastatic Breast Cancer for Everyone (EMBRACE) program developed at Dana Farber Cancer Institute. OBJECTIVE We aim to describe the implementation science–based study design and innovative components of Project ADAPT. METHODS Project ADAPT uses the Dynamic Adaptation Process informed by the Exploration, Preparation, Implementation, Sustainment framework. Washington University School of Medicine (WUSM) partnered with 3 community hospitals in the St. Louis region covering rural and urban settings. The exploration and preparation phases provide patient and provider feedback on current referral practices to finalize the approach for the implementation phase. At the implementation phase, we will enroll patients with MBC at these 3 community sites to evaluate potential collaborative care at WUSM and assess the impact of this collaborative care model on referral satisfaction and acceptability for patients with MBC and their providers. Patients may then return to their community site for care or continue to receive part of their care at WUSM. We are incorporating virtual and digital health strategies to improve MBC care coordination in order to minimize patient burden. RESULTS The exploration phase is ongoing. As of August 2021, we have recruited 21 patient and provider participants to complete surveys of the current collaborative care process at WUSM. Using a 2-tailed paired t test, 44 patients (including 10 patients from the exploration phase) and 32 oncologists are required to detect an effect size of 0.5 with 80% power at a level of significance of .05. Throughout this phase and in preparation for the implementation phase, we have iteratively updated and refined our surveys for the implementation phase based on testing of our data collection instruments. Our partner sites are in various stages of the single institutional review board (IRB) approval process. We have ongoing engagement with all partner sites, which has helped solidify our participant recruitment strategies and design patient-friendly recruitment materials. In addition, we have included a patient advocate on the research team. Members of the research team have launched a single IRB Support Network at WUSM to create a repository of the single IRB procedures in order to streamline the partner site onboarding process and facilitate enhanced collaboration across institutions. CONCLUSIONS With this robust model, we expect that patients with MBC will receive optimal care regardless of geographical location and the model will improve patient and provider experiences when navigating the health system. INTERNATIONAL REGISTERED REPORT DERR1-10.2196/35736

Combining cyclin-dependent kinase (CDK) inhibitors with endocrine therapy improves outcomes for metastatic estrogen receptor positive (ER+) breast cancer patients but its value in earlier stage patients is unclear. We examined evolutionary trajectories of early-stage breast cancer tumors, using single cell RNA sequencing (scRNAseq) of serial biopsies from the FELINE clinical trial (#NCT02712723) of endocrine therapy (letrozole) alone or combined with the CDK inhibitor ribociclib. Despite differences in subclonal diversity evolution across patients and treatments, common resistance phenotypes emerged. Resistant tumors treated with combination therapy showed accelerated loss of estrogen signaling with convergent up-regulation of JNK signaling through growth factor receptors. In contrast, cancer cells maintaining estrogen signaling during mono- or combination therapy showed potentiation of CDK4/6 activation and ERK upregulation through ERBB4 signaling. These results indicate that combination therapy in early-stage ER+ breast cancer leads to emergence of resistance through a shift from estrogen to alternative growth signal-mediated proliferation.

BACKGROUND Breast cancer is one of the most common cancers associated with brain metastases (BM). Up to 50% of patients with metastatic triple-negative breast cancer (TNBC) develop BM which portends a poor prognosis, with a median survival of 4.4-7.3 months. There is a lack of effective systemic therapy. Irinotecan is a topoisomerase-1 inhibitor with a response rate of 5-23% in advanced breast cancer. Nal-IRI is an intravenous liposomal formulation of irinotecan, with greater efficacy in tumor growth inhibition and the ability to cross the blood brain barrier (BBB) than irinotecan, resulting in a prolonged survival in preclinical TNBC BM models. Additionally, Nal-IRI has demonstrated promising anti-tumor activity in patients with TNBC- BM in a phase-I trial (NCT01770353). Pembrolizumab is a humanized anti-PD-1 monoclonal antibody which has shown efficacy in TNBC. The purpose of this study is to evaluate whether the combination of Nal-IRI and pembrolizumab can provide a synergic effect to control the CNS disease and prolong survival in TNBC with progressive BM. METHODS This is a phase II, single arm trial with a safety lead-in to evaluate the efficacy of nal-IRI (50-70mg/m2 IV Q2W) in combination with pembrolizumab (400mg IV Q6W). Simon’s 2-stage design is used, with 18 patients in the 1st stage and additional 24 at 2nd stage for a total of 42. The first 6 patients will serve as a safety lead-in. Key eligibilities include: histologically/cytologically confirmed TNBC with new or progressive BM; prior immunotherapy is allowed but not prior nal-IRI/irinotecan; prior sacituzumab-govitecan is permitted if disease stable for ≥16-week while on therapy and ≥24-week washout prior to starting trial; measurable disease; and ≤4 prior lines of therapy in the metastatic setting. The primary endpoint is CNS disease control rate (DCR) at 6 months using RANO-BM criteria. Secondary endpoints include CNS and non-CNS ORR, PFS and OS. (ClinicalTrials.gov: NCT05255666)

Despite the breast cancer microenvironment being recognized as a critical participant in tumor progression and therapeutic response, current technologies for tumor cell-to-cell interaction analyses are limited to qualitative and descriptive histological methods. Here, we utilized multiplex immunofluorescence (MxIF), RNA sequencing (RNA-seq), and microarray gene expression to quantitatively assess, using the BostonGene automated pipeline, the immune microenvironment composition of clinical stage 2 or 3 primary estrogen receptor-positive (ER+) HER2-negative (HER-) breast tumors (n = 29) from patients treated with neoadjuvant palbociclib in combination with anastrozole. Microenvironment characteristics of pretreatment tumor biopsies were associated with molecular subtype, cytokine signatures, and Ki67 response. MxIF analysis (21 markers, 2-13 ROIs per slide) of the tumor immune composition revealed that while the cellular composition was similar for each tumor region of the same patient, significant cellular heterogeneity was observed in tumor biopsies from different patients. The spatial heterogeneity was found in the distribution of both immune infiltrate cells and stromal components. T cells, CD68+ and CD163+ macrophages, and B cells were the predominant cell populations in the stromal component. Further MxIF analysis showed an increase in Ki67+ tumor cells in nonresponders (NR) versus responders (R). The number of immune cells was similar between Luminal A and B breast cancer molecular subtypes. A lower number of Ki67+ tumor cells was found in Luminal A compared to Luminal B tumors, as expected. MxIF analysis of cell-to-cell interactions revealed that distinct groups of cellular neighborhoods were significantly increased in NR compared with R: the immune inflammation cluster with a high density of CD4 T cells, Tregs, and CD8 T cells in the stromal component (p = 0.03); CD163+ macrophage-enriched cluster (p < 0.001); and tertiary lymphoid structures cluster (p < 0.001). Transcriptomic analysis showed that while CXCL1, CCL17, CCL13, CXCL9, and CCL5 expression correlated with the high density of CD4 and CD8 T cells in the stromal component cluster, VEGFA, IL1B, CXCL16, CCL3, and CCL7 were increased in the CD163+ and CD68+ macrophage-enriched clusters, indicating that differential cytokine expression is associated with the tumor architecture.Comprehensively characterizing, via MxIF, the immune microenvironment of ER+ HER2- breast cancer provided resolution of cellular architecture and elucidated spatial relationships among the immune and stromal cells of the tumor. Quantitatively assessing breast tumor cell-to-cell interactions has the potential to identify imaging markers of response, ultimately leading to the development of effective therapy options. Citation Format: Maria Bruttan, Souzan Sanati, Anna Belozerova, Ilia Galkin, Akshaya Ramachandran, Pavel Ovcharov, Grigory Perelman, Viktor Svekolkin, Ekaterina Postovalova, Alexander Bagaev, Krystle Nomie, Shana Thomas, Jeremy Hoog, Ravshan Ataullakhanov, James Hsieh, Cynthia Ma. Tumor microenvironment heterogeneity identifies potential biomarkers of response in ER-positive breast cancers treated with palbociclib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6151.