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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. In a phase 1 study, imlunestrant monotherapy showed favourable safety, pharmacokinetics (PK) and preliminary efficacy in heavily pre-treated ER-positive (ER+) advanced breast cancer patients (Jhaveri ASCO 2022). Here, we present pharmacodynamic (PD) data from the preoperative window of opportunity (WOO) study (EMBER-2, NCT04647487), evaluating the biological activity of imlunestrant monotherapy in ER+, HER2-negative (HER2-) early breast cancer (EBC). Methods: Post-menopausal women with stage I–III operable ER+ (>50%) or Allred score >5, HER2- untreated EBC ≥1 cm in diameter were randomized 1:1 to imlunestrant 400 mg once daily (QD) or imlunestrant 800 mg QD for 15 days (treatment window of -2 to +7 days) up to the surgery date. Pre- and on-treatment tumor samples were compared for changes in PD biomarkers. Primary study objective was change in ER expression (measured by IHC and quantified by H-score). Secondary objectives were change in progesterone receptor (PR) expression (measured by IHC and quantified by H-score) and Ki-67 (measured by IHC and expressed by percentage positive scoring) along with evaluation of safety and tolerability. Results: From Apr 28, 2021, to Mar 11, 2022, 58 patients were enrolled of which 54 were biomarker-evaluable for ER expression (400 mg: n = 28; 800 mg: n = 26). Patient demographics and tumor characteristics for all enrolled patients were similar across cohorts, with a median age of 64 years (50-83), 72% invasive ductal carcinoma (IDC), 28% invasive lobular carcinoma (ILC), 59% stage I, 36% stage II and 5% stage III disease. 91% of the patients had a compliance rate higher than 80%. Among biomarker evaluable patients, relative reduction in PD biomarkers after a median of 15 days (range 13 to 23 days) of treatment are presented in Table 1. There was no significant difference in PD biomarker modulation noted between the two imlunestrant doses (400 mg vs 800 mg) or based on tumor histology (IDC, ILC). Imlunestrant was well tolerated. There were no discontinuations due to adverse events (AEs). Treatment-related AEs (TRAEs) were mainly grade 1, most commonly: fatigue (10%), diarrhea (9%), hot flushes (7%), and nausea (5%). There were no TRAEs of diarrhea and nausea observed at the 400 mg dose. No grade 3 or higher TRAEs were reported. Conclusion: Imlunestrant demonstrated evidence of target engagement along with consistent biological activity across all evaluated dose levels and was well tolerated in an EBC population, further supporting continued adjuvant development in the ongoing EMBER-4 study. Additional biomarker analyses for the EMBER-2 study are also planned. Table 1. Relative reduction in PD biomarkers from Baseline to Day 15 Citation Format: Patrick Neven, Nicole Stahl, Maria Vidal, Miguel Martín, Nadia Harbeck, Peter A. Kaufman, Francois-Clement Bidard, Peter A. Fasching, Philippe Aftimos, Erika Hamilton, Stacey Carter, Peter Schmid, Duncan Wheatley, Manali Bhave, Kelly K. Hunt, Swati A. Kulkarni, Roohi Ismail-Khan, Claudia Karacsonyi, Shawn T. Estrem, Umut Ozbek, Bastien Nguyen, Eva Ciruelos. A preoperative window-of-opportunity study of imlunestrant in estrogen receptor-positive, HER2-negative early breast cancer: Results from the EMBER-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 P6-10-06.

Background: Targeting constitutively active mutant kinases with selective small molecule inhibitors is a key therapeutic pillar of precision oncology. Phosphatidylinositol-4,5bisphosphate-3 kinase, catalytic subunit alpha (PIK3CA) mutations leading to oncogenic activation of PI3Kα represent the largest opportunity for this approach in solid tumors. However, there is no selective inhibitor that targets mutant PI3Kα in the clinic. Toxicity related to non-selective inhibition of WT PI3Kα (hyperglycemia) and other PI3K isoforms limits the tolerability, dosing and efficacy of the orthosteric inhibitor, alpelisib, the only approved solid tumor PI3K inhibitor. RLY-2608, a novel oral allosteric PI3Kα inhibitor, is uniquely designed to overcome these limitations via mutant- and isoform-selective PI3Kα inhibition for greater target coverage, improved tolerability and antitumor activity. We initiated a first-in-human (FIH), study to evaluate the clinical activity of RLY-2608 as a single agent in advanced solid tumor patients (pts) with PI3KCA mutations and in combination with fulvestrant in pts with PIK3CA mutant, HR+, HER2- metastatic breast cancer (MBC). Methods: This is a global, multi-center, dose escalation/expansion study (NCT05216432) of RLY2608 as a single agent in adults who have advanced solid tumors and are refractory, intolerant, or declined standard therapy and RLY-2608 in combination with fulvestrant in previously treated pts with HR+/HER2- MBC. Eligibility criteria include presence of PI3KCA mutation (blood or tumor) per local assessment, ECOG performance status 0-1, measurable or evaluable disease per RECIST 1.1 and no prior PI3K inhibitor (except combination group 2). RLY-2608 is administered on a continuous schedule with 4-week cycles. Adverse events (AEs) per CTCAE v5, PK, biomarkers (mutant ctDNAs and insulin pathway markers) and anti-tumor activity are assessed serially. Dose escalation employs a Bayesian Optimal Interval design to identify MTD and RP2D. Following dose escalation, pts will be treated with RLY-2608 at the MTD/RP2D in a monotherapy dose expansion with 5 groups (N=75, 15 each): 1. Clear cell ovarian carcinoma 2. Head and neck squamous cell carcinoma 3. Cervical cancer 4. Other solid tumors 5. PI3KCA double mutations. In addition, two expansion cohorts will enroll patients with HR+/HER2- MBC treated with RLY-2608 and fulvestrant combination (N = 30, 15 each): 1. No prior PI3K therapy 2. Intolerant to PI3K inhibitors. The primary endpoints are MTD/RP2D and AE profile for single agent and combination; key secondary endpoints are PI3KCA genotype in blood and tumor, PK, biomarkers, and overall response rate. US enrollment began December 2021 and ex-USA startup is under way. Citation Format: Andreas Varkaris, Erika Hamilton, Jason Henry, Alexander I. Spira, Alison M. Schram, Julia E. McGuinness, Gege Tan, Xiaoyan Li, Tamieka Hunter, Ramin Samadani, Alison Timm, Djuro Karanovic, Vivek Subbiah, Cesar A. Perez. First-in-human global multi-center study of RLY-2608, a pan mutant and isoform selective PI3Kα inhibitor, as a single agent in advanced solid tumor patients and in combination with fulvestrant in patients with advanced 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 OT3-22-01.

Background: Datopotamab deruxtecan (Dato-DXd) is an antibody-drug conjugate (ADC) consisting of a humanized anti-TROP2 IgG1 monoclonal antibody covalently linked to a highly potent topoisomerase I (Topo I) inhibitor payload via a stable, tumor-selective, tetrapeptide-based cleavable linker. Dato-DXd demonstrated compelling single-agent antitumor activity in heavily pretreated patients (pts) with metastatic triple-negative BC (Krop, SABCS 2021). This is the first report of results from the TROPION-PanTumor01 study in pts with unresectable or metastatic hormone receptor–positive (HR+)/human epidermal growth factor receptor 2–negative (HER2−; including HER2-low and HER2-zero) BC. Methods: TROPION-PanTumor01 (NCT03401385) is a phase 1, multicenter, open-label, 2-part dose-escalation/expansion study evaluating Dato-DXd in previously treated pts with solid tumors. Based on previous clinical findings and exposure-response results from pts with NSCLC, Dato-DXd 6 mg/kg IV Q3W is being evaluated in pts with unresectable or metastatic HR+/HER2− BC that progressed on standard therapies. The primary objectives were safety and tolerability. Tumor responses, including ORR (complete response [CR] + partial response [PR]) and DCR (CR + PR + stable disease [SD]), were assessed per RECIST version 1.1 by blinded independent central review. Results: As of the April 29, 2022, data cutoff, 41 pts had received Dato-DXd (median follow-up, 10.9 mo [range, 7-13]); 9 pts were ongoing. The primary cause of treatment discontinuation was disease progression (63%; progressive disease [PD] or clinical progression). Median age was 57 y (range, 33-75); 54% had de novo metastatic disease. Pts were heavily pretreated (Table) with a median of 5 (range, 3-10) prior regimens in the advanced setting; 95% had prior CDK4/6i (adjuvant/metastatic). Median time from initial treatment for metastatic disease to the first dose of Dato-DXd was 42.7 mo (range, 10.2-131.1). Treatment-emergent adverse events (TEAEs; all cause) were observed in 98% (any grade) and 41% (grade ≥3) of pts. Most common TEAEs (any grade, grade ≥3) were stomatitis (80%, 10%), nausea (56%, 0%), fatigue (46%, 2%), and alopecia (37%, 0%). Serious TEAEs were observed in 6 pts (15%); 1 pt died due to dyspnea, which was not considered to be treatment related. Dose reductions occurred in 5 pts due to stomatitis (n=3), fatigue (n=2), keratitis (n=1), and decreased appetite (n=1) (>1 AE per pt); 14 pts had treatment delayed due to stomatitis (n=8), retinopathy (n=1), dysphagia (n=1), fatigue (n=1), malaise (n=1), COVID-19 (n=1), cellulitis (n=1), urinary tract infection (n=1), decreased lymphocyte count (n=1), and nasal congestion (n=1; >1 AE per pt). Three pts discontinued treatment due to keratitis (n=1) and pneumonitis (n=2); 1 case of pneumonitis was adjudicated as grade 2 drug-related interstitial lung disease. The ORR was 29% (11 confirmed PRs; 1 pending confirmation), the DCR was 85% (35/41), and the clinical benefit rate (CR + PR + SD ≥6 mo) was 41% (17/41). Conclusions: Dato-DXd demonstrated a manageable safety profile and encouraging antitumor activity, with high disease control in heavily pretreated pts, the majority having received prior CDK4/6i. Based on these findings, the TROPION-Breast01 (NCT05104866) randomized phase 3 study comparing 2L+ Dato-DXd vs investigator’s choice chemotherapy is currently enrolling pts with HR+/HER2− BC. Prior Therapies in the Adjuvant or Metastatic Setting Citation Format: Funda Meric-Bernstam, Ian Krop, Dejan Juric, Takahiro Kogawa, Erika Hamilton, Alexander I. Spira, Toru Mukohara, Takuya Tsunoda, Senthil Damodaran, Jonathan Greenberg, Wen Gu, Fumiaki Kobayashi, Hong Zebger-Gong, Yui Kawasaki, Rie Wong, Aditya Bardia. PD13-08 Phase 1 TROPION-PanTumor01 Study Evaluating Datopotamab Deruxtecan (Dato-DXd) in Unresectable or Metastatic Hormone Receptor–Positive/HER2–Negative Breast Cancer (BC) [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 PD13-08.

Background: Datopotamab deruxtecan (Dato-DXd) is an antibody-drug conjugate (ADC) consisting of a humanized anti-TROP2 IgG1 mAb covalently linked to a highly potent topoisomerase I (Topo I) inhibitor payload via a stable, tumor selective, tetrapeptide-based cleavable linker. Dato-DXd has previously shown encouraging activity in heavily pretreated patients (pts) with metastatic TNBC (Krop, SABCS 2021). Here we report updated results from the TROPION-PanTumor01 study in pts with advanced/metastatic TNBC. Methods: TROPION-PanTumor01 (NCT03401385) is a phase 1, multicenter, open-label, 2-part dose-escalation/expansion study evaluating Dato-DXd in previously treated pts with solid tumors. Based on previous clinical and exposure-response results from pts with NSCLC, Dato-DXd 6 mg/kg IV Q3W is being evaluated in pts with advanced TNBC that relapsed/progressed on standard therapies; 2 pts received 8 mg/kg prior to selection of 6 mg/kg. The primary objectives were safety and tolerability. Tumor responses, including objective response rate (ORR; complete response [CR] + partial response [PR]) and disease control rate (DCR; CR + PR + stable disease [SD]), were assessed per RECIST v1.1 by blinded independent central review. Results: As of April 29, 2022, 44 pts received Dato-DXd (median follow-up, 16.6 mo [range, 13-22]) at the time of data cutoff. The primary cause of treatment discontinuation was disease progression (86%; PD or clinical progression), and 4 pts are still receiving therapy. Median age was 53 y (range, 32-82); 32% had de novo metastatic disease. Pts were heavily pretreated with a median of 3 (range, 1-10) prior regimens in the metastatic setting. Prior treatments included taxanes (91%), anthracyclines (75%), capecitabine (61%), platinum (52%), immunotherapy (43%), Topo I inhibitor–based ADC therapy (32%), and PARPi (18%). Treatment-emergent adverse events (TEAEs; all cause) occurred in 100% (any grade) and 50% (grade ≥3) of pts, respectively. Most common TEAEs (any grade, grade ≥3) were stomatitis (73%, 11%), nausea (66%, 2%), vomiting (39%, 5%), fatigue (34%, 7%), and alopecia (36%, 0%). One pt had grade 3 decreased neutrophil count; no cases of interstitial lung disease (ILD) or grade ≥3 diarrhea were observed. Serious TEAEs were reported in 9 pts (20%); no deaths associated with adverse events (AEs) were observed. Dose reductions occurred in 8 pts (18%) due to stomatitis (n=3), fatigue (n=2), dry eye (n=1), retinal exudates (n=1), and dysgeusia (n=1); 12 pts (27%) delayed treatment due to stomatitis (n=5), dry eye (n=1), keratitis (n=1), blurred vision (n=1), fatigue (n=1), bronchitis (n=1), skin infection (n=1), musculoskeletal chest pain (n=1), dysgeusia (n=1), chronic obstructive pulmonary disease (n=1), and dyspnea (n=1; >1 AE per pt). One pt (2%) discontinued treatment due to grade 1 pneumonitis (which was centrally adjudicated as not ILD). ORR in all pts was 32% (1 CR, 13 PRs), DCR was 80% (35/44), and clinical benefit rate (CR + PR + SD ≥6 mo) was 34% (15/44). Median duration of response was not yet reached; median progression-free survival (mPFS) was 4.3 mo (95% CI, 3.0-7.3), and median overall survival (mOS) was 12.9 mo (95% CI, 10.1-14.7). In the subset of pts without prior Topo I inhibitor–based ADC therapy and with measurable disease at baseline, ORR was 44% (12/27). Among all pts without prior Topo I inhibitor–based ADC therapy (n=30), mPFS was 7.3 mo (95% CI, 3.0-NE), and mOS was 14.3 mo (95% CI, 10.5-NE). Conclusions: Dato-DXd continues to demonstrate encouraging and durable antitumor activity, along with a manageable safety profile, in heavily pretreated pts with metastatic TNBC. Based on these findings, the phase 3 randomized TROPION-Breast02 (NCT05374512) trial comparing Dato-DXd vs chemotherapy as 1L therapy for pts with metastatic TNBC is currently underway. Citation Format: Aditya Bardia, Ian Krop, Funda Meric-Bernstam, Anthony W. Tolcher, Toru Mukohara, Aaron Lisberg, Toshio Shimizu, Erika Hamilton, Alexander I. Spira, Kyriakos P. Papadopoulos, Jonathan Greenberg, Wen Gu, Fumiaki Kobayashi, Hong Zebger-Gong, Yui Kawasaki, Rie Wong, Takahiro Kogawa. Datopotamab Deruxtecan (Dato-DXd) in Advanced Triple-Negative Breast Cancer (TNBC): Updated Results From the Phase 1 TROPION-PanTumor01 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 P6-10-03.

Background: The current first-line (1L) standard of care (SOC) for human epidermal growth factor receptor 2-positive (HER2+) metastatic breast cancer (MBC) is trastuzumab (T) plus pertuzumab (P) and a taxane. Despite advances in 1L SOC, most patients (pts) progress during maintenance therapy with T+P. Tucatinib is a tyrosine kinase inhibitor (TKI) approved in combination with T and capecitabine for adults with HER2+ MBC, with and without brain metastases (BM). In HER2CLIMB, the addition of tucatinib significantly prolonged progression-free survival (PFS) and overall survival (OS) in pts with HER2+ MBC and was well tolerated. Adding tucatinib also reduced the risk of disease progression or death in pts with untreated and/or active BM (Murthy et al. 2020, Curigliano et al. 2021). HER2CLIMB-05 investigates whether adding tucatinib to 1L SOC as maintenance therapy will extend PFS while maintaining quality of life (QOL). Methods: HER2CLIMB-05 (NCT05132582) is a phase 3, randomized, double-blind study evaluating tucatinib plus T+P as maintenance therapy for HER2+ MBC. Approximately 650 pts will be enrolled. Eligible pts will have advanced HER2+ disease, no progression on 4–8 cycles of prior 1L SOC, ECOG Performance Status of 0 or 1, and no or asymptomatic BM. Exclusion criteria include prior treatment with anti-HER2 and/or anti-epidermal growth factor receptor TKI (prior SOC for early BC is permitted) or inability to undergo contrast magnetic resonance imaging of the brain. Pts will be randomized 1:1 to receive either tucatinib or placebo twice daily, with T+P once every 21 days. Pts with HR+ disease may receive endocrine therapy. The primary endpoint is investigator-assessed PFS. Secondary endpoints include OS (key endpoint), time to deterioration of health-related QOL, central nervous system PFS, safety, and pharmacokinetic (PK) parameters. PFS and OS will be compared using a 2-sided stratified log-rank test between treatment groups. Time-to-event endpoints will be summarized using the Kaplan–Meier method. PK and safety data will be summarized using descriptive statistics. Enrollment is ongoing in the US, with additional sites planned. Citation Format: Erika Hamilton, Junji Tsurutani, Giuseppe Curigliano, Miguel Martín, Ciara C. O’Sullivan, Joo Hyuk Sohn, Konstantinos Tryfonidis, Libero Santarpia, Shan Yang, Véronique Diéras. Phase 3 study of tucatinib or placebo in combination with trastuzumab and pertuzumab as maintenance therapy for HER2+ metastatic breast cancer (HER2CLIMB-05, trial in progress) [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 OT2-16-03.

MT-5111 is a 55kD engineered toxin body targeting HER2 in solid tumors that binds to an epitope distinct from trastuzumab and pertuzumab, offering potential combination strategies with other HER2-targeting agents. MT-5111 works by internalizing, self-routing through intracellular compartments to the cytosol, and inducing potent cell-kill via the enzymatic and permanent inactivation of ribosomes. This is a phase 1 study in adults with advanced HER2+ solid tumors. MT-5111 is dosed weekly IV over 30 min in every 21-day cycle until disease progression, unacceptable toxicity, death, or withdrawn consent. The study has dose escalation (Part A) cohorts enrolling patients (pts) with any HER2+ cancer (CA) and expansion (Part B) cohorts for HER2+ breast cancer (BC), Gastric or Gastroesophageal junction adenocarcinoma (GEA), or other HER2+ solid CA. As of 30 June 2022, 42 total pts had enrolled (36 in Part A on 0.5-23 µg/kg/dose, 6 pts with BC in Part B1 on 10 µg/kg/dose). Median age 65 years, 28 (66.78%) pts were female, median of 4 prior systemic and 2 prior HER2-targeting treatment (tx). 17 pts with BC, 6 with biliary CA, 9 with GEA, and 10 with other solid CA have enrolled. Of the 17 BC pts, 15 received ≥ 10 µg/kg/dose. Tx emergent adverse events (TEAEs) have been reported in 40 (95%) pts, and tx-related AEs (TRAE) occurred in 23 (55%) pts. No pt experienced G4-G5 TRAE. G1 troponin elevations were noted in 5 pts without clinical signs of cardiotoxicity (1 pt 6.75 µg/kg, 2 pts 10 µg/kg, 1 pt 17 µg/kg, 1 pt 23 µg/kg). Reversible G1/G2 infusion-related reactions were reported in 2 pts. Tx-related G1-G3 rash was observed in 5 pts (4 pts ≥ 10 µg/kg/dose); maculopapular in 2 pts, acneiform in 1 pt and associated with pruritus in 3 pts. The G3 rash developed one wk after first dose of 23 µg/kg, was declared a DLT, improved with systemic steroid therapy and the pt continued tx at the same dose without recurrence. Best overall response per RECIST thus far is stable disease (SD) in 17 pts, non-CR/non-PD in 1 pt, and progressive disease (PD) in 14 pts. 1 pt had non-CR/non-PD for 30 wks (1 μg/kg, BC); 1 pt had SD for 24wks (10 μg/kg, pancreatic); 1 pt is on tx with SD through 8 cycles (10 μg/kg, BC). Of the 10 BC pts who received ≥ 10 μg/kg/dose, the best response was 5 SD. The mean serum concentration of MT-5111 has increased in a dose-proportional manner starting at 6.75µg/kg/dose (Table 1). The soluble HER2 (sHER2) levels at end of tx were higher compared to baseline in cohorts that received ≤ 4.5µg/kg/dose, but similar or lower in cohorts that received ≥ 6.75µg/kg/dose. Higher MT-5111 doses have been well tolerated and may saturate circulating sHER2, leading to more predictable serum concentrations and tumor penetration. The Cmax in humans at doses ≥6.75 µg/kg/dose is above the in vitro IC50 for high HER2+ cell lines (0.029nM) and at 17 µg/kg/dose, above the IC50 for moderate HER2+ cells (1.6nM). In conclusion, the dose proportionate increase in serum concentration with levels above the in vitro IC50 and the leveling off/reduction of sHER2 indicate exposure to MT-5111 is at clinically therapeutic levels. Skin toxicity at higher doses may indicate on-target effect as observed in other EGFR-targeted therapies where it is associated with clinical response and a better prognosis. sHER2 biomarker data is expected for all cohorts with PK correlation and 23µg/kg safety and efficacy data. PK profile of MT-5111, C1D1 values Citation Format: Meena Okera, Brian A. Van Tine, Joleen M. Hubbard, Minal Barve, Erika Hamilton, Monica M. Mita, Frances Valdes-Albini, Daniel Ahn, Admasu Mamuye, Joshua Pelham, Amy Yuet, Diana Yurewicz, Yanning Liu, Andres Machado Sandri, William J. Edenfield, Aki Morikawa, William Gradishar, Rajiv Kumar, Zev A. Wainberg. A phase 1 study of the novel immunotoxin MT-5111 in patients with HER2+ tumors: interim 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 OT2-11-01.

Background An improvement in progression-free survival was shown with trastuzumab deruxtecan versus trastuzumab emtansine in patients with HER2-positive metastatic breast cancer in the progression-free survival interim analysis of the DESTINY-Breast03 trial. The aim of DESTINY-Breast03 was to compare the efficacy and safety of trastuzumab deruxtecan versus trastuzumab emtansine. Methods This open-label, randomised, multicentre, phase 3 trial was done in 169 study centres in North America, Asia, Europe, Australia, and South America. Eligible patients were aged 18 or older, had HER2-positive unresectable or metastatic breast cancer previously treated with trastuzumab and a taxane, had an Eastern Cooperative Oncology Group performance status 0-1, and at least one measurable lesion per Response Evaluation Criteria in Solid Tumours version 1.1. Patients were randomly assigned (1:1) to receive trastuzumab deruxtecan 5 center dot 4 mg/kg or trastuzumab emtansine 3 center dot 6 mg/kg, both administered by intravenous infusion every 3 weeks. Randomisation was stratified by hormone receptor status, previous treatment with pertuzumab, and history of visceral disease, and was managed through an interactive web-based system. Within each stratum, balanced block randomisation was used with a block size of four. Patients and investigators were not masked to the treatment received. The primary endpoint was progression-free survival by blinded independent central review. The key secondary endpoint was overall survival and this prespecified second overall survival interim analysis reports updated overall survival, efficacy, and safety results. Efficacy analyses were performed using the full analysis set. Safety analyses included all randomly assigned patients who received at least one dose of study treatment. This study is registered with ClinicalTrials.gov, NCT03529110. Findings Between July 20, 2018, and June 23, 2020, 699 patients were screened for eligibility, 524 of whom were enrolled and randomly assigned to receive trastuzumab deruxtecan (n=261) or trastuzumab emtansine (n=263). Median duration of study follow-up was 28 center dot 4 months (IQR 22 center dot 1-32 center dot 9) with trastuzumab deruxtecan and 26 center dot 5 months (14 center dot 5-31 center dot 3) with trastuzumab emtansine. Median progression-free survival by blinded independent central review was 28 center dot 8 months (95% CI 22 center dot 4-37 center dot 9) with trastuzumab deruxtecan and 6 center dot 8 months (5 center dot 6-8 center dot 2) with trastuzumab emtansine (hazard ratio [HR] 0 center dot 33 [95% CI 0 center dot 26-0 center dot 43]; nominal p

HER2-targeted therapies have substantially improved outcomes for patients with HER2-positive breast and gastric or gastro-oesophageal junction cancers. Several other cancers exhibit HER2 expression or amplification, suggesting that HER2-targeted agents can have broader therapeutic impact. Zanidatamab is a humanised, bispecific monoclonal antibody directed against two non-overlapping domains of HER2. The aim of this study was to evaluate the safety and anti-tumour activity of zanidatamab across a range of solid tumours with HER2 expression or amplification.This first-in-human, multicentre, phase 1, dose-escalation and expansion trial included patients aged 18 years and older, with a life expectancy of at least 3 months, with an Eastern Cooperative Oncology Group performance status of 0 or 1, and locally advanced or metastatic, HER2-expressing or HER2-amplified solid tumours of any kind who had received all available approved therapies. The primary objectives of part 1 were to identify the maximum tolerated dose, optimal biological dose, or recommended dose of zanidatamab; all patients were included in the primary analyses. Part 1 followed a 3 + 3 dose-escalation design, including different intravenous doses (from 5 mg/kg to 30 mg/kg) and intervals (every 1, 2, or 3 weeks). The primary objective of part 2 was to evaluate the safety and tolerability of zanidatamab monotherapy in solid tumours. This trial is registered with ClinicalTrials.gov (NCT02892123), and parts 1 and 2 of the trial are complete. Part 3 of the study evaluates the use of zanidatamab in combination with chemotherapy and is ongoing.Recruitment took place between Sept 1, 2016, and March 13, 2021. In Part 1 (n=46), no dose-limiting toxicities were detected and the maximum tolerated dose was not reached. The recommended dose for part 2 (n=22 for biliary tract cancer; n=28 for colorectal cancer; and n=36 for other HER2-expressing or HER2-amplified cancers excluding breast or gastro-oesophageal cancers; total n=86) was 20 mg/kg every 2 weeks. The most frequent treatment-related adverse events in part 1 of the study were diarrhoea (24 [52%] of 46 patients; all grade 1-2) and infusion reactions (20 [43%] of 46 patients; all grade 1-2). The most frequent treatment-related adverse events in part 2 of the study were diarrhoea (37 [43%] of 86 patients; all grade 1-2 except for one patient) and infusion reactions (29 [34%] of 86 patients; all grade 1-2). A total of six grade 3 treatment-related adverse events were reported in four (3%) of 132 patients. In part 2, 31 (37%; 95% CI 27·0-48·7) of 83 evaluable patients had a confirmed objective response. There were no treatment-related deaths.These results support that HER2 is an actionable target in various cancer histologies, including biliary tract cancer and colorectal cancer. Evaluation of zanidatamab continues in ongoing studies.Zymeworks.

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: Breast cancer (BC) is the most commonly diagnosed cancer and one of the leading causes of cancer death in women. Despite significant therapeutic advances, the majority of patients with unresectable or recurrent/metastatic disease eventually develop resistance to available standard of care (SOC) therapies. B7-H4 is a poor prognostic factor and is overexpressed in several cancers including endometrial, ovarian, and breast. As a member of the CD28/B7 family of cell surface proteins, it promotes tumorigenesis by suppressing anti-tumor immunity. XMT-1660 is a B7-H4-targeted Dolasynthen antibody drug conjugate with a precise, optimized drug-to-antibody ratio and a DolaLock microtubule inhibitor payload with controlled bystander effect. In the preclinical setting, XMT-1660 has demonstrated anti-tumor activity in TNBC and ER+/HER2- patient-derived xenograft mouse models, which included tumors from heavily pre-treated patients (Collins et al, AACR 2022). Increased anti-tumor activity tended to be more frequent in models with higher B7-H4 expression, providing rationale for a Ph1 clinical trial. Methods: The Ph1 trial includes a first-in-human open-label dose escalation (DES) portion followed by dose expansion (EXP) evaluating XMT-1660 in patients with BC, EC, and OC following progression on SOC as applicable (i.e., CDK4/6i + ET; platinum-based chemotherapy). In the DES, Bayesian Optimal Interval (BOIN) design will be used to determine the MTD. Patients will receive XMT-1660 IV Q3 weeks. Primary endpoints in DES are to assess safety and determine a recommended phase 2 dose (RP2D) and assessment of preliminary efficacy as a secondary endpoint. In the EXP portion, cohorts enrolling TNBC, ER+/HER2- BC, EC/OC are planned and additional patients may be enrolled based on emerging data. The primary endpoint of EXP is to assess safety and tolerability, overall response rate, disease control rate, and duration of response. Secondary endpoints include pharmacokinetic analysis and antidrug antibodies. Patients are not selected by B7-H4 status, but baseline tumors samples are collected for retrospective tumor tissue evaluation. The trial is currently enrolling patients. NCT05377996 Citation Format: Erika Hamilton, Arvind Chaudhry, Alexander I. Spira, Sylvia Adams, Nour Abuhadra, Antonio Giordano, Ritesh Parajuli, Hyo S. Han, Amy Weise, Aubri Marchesani, Kate Josephs, Chu Ri Shin, Kevin Kalinsky. XMT-1660: A Phase 1b trial of a B7-H4 targeted Antibody Drug Conjugate (ADC) in Breast, Endometrial, and Ovarian 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 OT1-03-01.

Purpose Resistance to endocrine therapy poses a major clinical challenge for patients with hormone receptor-positive (HR +), human epidermal growth factor receptor 2-negative (HER2–) metastatic breast cancer (MBC). We present the preplanned 24-month final overall survival (OS) results, alongside updated progression-free survival (PFS), and objective response rate (ORR) results. Methods nextMONARCH is an open-label, controlled, randomized, Phase 2 study of abemaciclib alone or in combination with tamoxifen in women with endocrine-refractory HR + , HER2– MBC previously treated with chemotherapy. Patients were randomized 1:1:1 to: abemaciclib 150 mg and tamoxifen 20 mg (A + T), abemaciclib 150 mg (A-150), or abemaciclib 200 mg and prophylactic loperamide (A-200). OS was the main prespecified secondary endpoint. PFS, ORR, and safety at 24 months were compared to previously reported primary analysis results. Results Of the 234 patients enrolled, 12 were receiving study treatment at data cutoff (28Jun2019). Median follow-up was 27.2 months. Median OS was 24.2 months in the A + T arm, 20.8 months in A-150, and 17.0 months in A-200 (A + T versus A-200: HR 0.62; 95%CI [0.40, 0.97], P = 0.03 and A-150 versus A-200: HR 0.96; 95%CI [0.64, 1.44], P = 0.83). PFS and ORR results at 24 months were consistent with the primary analysis. The safety profile corresponded with previous reports. Conclusion The addition of tamoxifen to abemaciclib demonstrated greater OS benefit than monotherapy. This study confirmed the single-agent activity of abemaciclib in heavily pretreated women with endocrine-refractory HR + , HER2– MBC, as well as the previously reported primary PFS and ORR results, with no new safety signals observed. Trial Registration ClinicalTrials.gov Identifier: NCT02747004.

This single-arm pilot study (NCT03329937) evaluated neoadjuvant niraparib antitumor activity and safety in patients with localized HER2-negative, BRCA-mutated breast cancer. Twenty-one patients received niraparib 200 mg once daily in 28-day cycles. After 2 cycles, tumor response (≥30% reduction from baseline) by MRI was 90.5% and 40.0% (6 of 15) of patients who received only niraparib (2–6 cycles) had pathological complete response; no new safety signals were identified. High niraparib intratumoral concentration was observed.

It has been suggested that the benefit of adjuvant chemotherapy (CT) in premenopausal women with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 negative (HER2−) early breast cancer may be related, at least in part, to CT-induced ovarian function suppression (OFS) in this subgroup of patients. Although this hypothesis has not been directly tested in large randomized clinical trials, the observations from prospective studies have been remarkably consistent in showing a late benefit of CT among the subgroup of patients who benefit (ie, women who were close to menopause). The hypothesis has important clinical implications, as it may be possible to spare the associated adverse effects of adjuvant CT in a select group of women with early breast cancer, in favor of optimizing OFS and endocrine therapy (ET), without compromising clinical outcomes. Such an approach has the added benefit of preserving the key quality of life outcomes in premenopausal women, particularly by preventing the irreversible loss of ovarian function that may result from CT use. For this reason, we convened an international panel of clinical experts in breast cancer treatment to discuss the key aspects of the available data in this area, as well as the potential clinical implications for patients. This article summarizes the results of these discussions and presents the consensus opinion of the panel regarding optimizing the use of OFS for premenopausal women with HR+, HER2− early breast cancer.

Antibody-drug conjugates (ADCs) embody a simple, but elegant, vision for cancer therapy—the delivery of a potent cytotoxic agent to tumor cells with minimal damage to normal cells—so-called smart chemo. Although there were significant challenges in achieving this milestone culminating in the first Food and Drug Administration approval in 2000, subsequent advancements in technology have led to rapid drug development with regulatory approvals for ADCs targeting a variety of tumor types. The most successful application for solid tumors has been in breast cancer, with ADCs becoming the standard of care across traditional human epidermal growth factor receptor 2 (HER2)+, hormone receptor+ (HR+) and triple-negative disease subtypes. Moreover, the improved features and gains in potency with the development of ADCs have expanded the treatment-eligible population to those with low/heterogeneous expression of the target antigen on the tumor with trastuzumab deruxtecan or in the case of sacituzumab govitecan, agnostic to target expression. Despite their antibody-directed homing, these novel agents come with their share of toxicities obligating appropriate patient selection and vigilant monitoring while on treatment. As more ADCs are included in the treatment armamentarium, mechanisms of resistance need to be studied and understood for optimal sequencing. Modifying the payload to use immune-stimulating agents or combination therapies with immunotherapy and other effective targeted therapies may further extend the utility of these agents in the treatment of solid tumors.

PDF file - 797K, Figure S1. Barrier phenotype of human brain microvascular endothelial cells and astrocytes in monocultures and co-cultures. Figure S2. alpha/beta-crystallin does not affect cell viability of TNBC cells in standard monolayer culture. Figure S3. alpha/beta-crystallin overexpression increases liver metastases in an orthotopic model of TNBC. Figure S4. Silencing alpha/beta-crystallin inhibits liver metastases in an orthotopic model of TNBC. Figure S5. alpha/beta-crystallin does not affect proliferation or apoptosis in primary mammary tumors and brain metastatic lesions determined at autopsy. Table S1. Patient characteristics Table S2. alpha/beta-crystallin expression in paired tumors. Table S3. Association of alpha/beta-crystallin expression with breast cancer subtype. Table S4. Incidence of metastases in mice in orthotopic models of TNBC .

XLS file - 16K, Pulmonary and pleural effusions, pneumonitis.

Supplementary Figures: Supplementary Figure 1. Summary of duration on AZD9496 therapy, previous therapies, and circulating biomarkers status at baseline for each patient ranked by ascending dose; Supplementary Figure 2. Tumor pharmacodynamics. Supplementary Tables: Supplementary Table 1. Study Plan showing time-points of collection of biosamples for exploratory research; Supplementary Table 2. ddPCR primers and probes;Supplementary Table 3. Antibodies and conditions for IHC analysis Antibodies for IHC analysis; Supplementary Table 4. Patients with {greater than or equal to}1 CTC/7.5 mL WB within 120 days of fulvestrant wash-out period prior C1D1.

SUPPLEMENTARY APPENDIX TABLES

Purpose:Common resistance mechanisms to endocrine therapy (ET) in estrogen receptor (ER)–positive metastatic breast cancers include, among others, ER loss and acquired activating mutations in the ligand-binding domain of the ER gene (ESR1LBDm). ESR1 mutational mediated resistance may be overcome by selective ER degraders (SERD). During the first-in-human study of oral SERD AZD9496, early changes in circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) were explored as potential noninvasive tools, alongside paired tumor biopsies, to assess pharmacodynamics and early efficacy.Experimental Design:CTC were enumerated/phenotyped for ER and Ki67 using CellSearch in serial blood draws. ctDNA was assessed for the most common ESR1LBDm by droplet digital PCR (BioRad).Results:Before starting AZD9496, 11 of 43 (25%) patients had ≥5 CTC/7.5 mL whole blood (WB), none of whom underwent reduction to +, two of whom had CTC-ER+ reduction. CTC-Ki67 status did not change appreciably. Patients with ≥5 CTC/7.5 mL WB before treatment had worse progression-free survival (PFS) than patients with P = 0.0003). Fourteen of 45 (31%) patients had ESR1LBDm+ ctDNA at baseline, five of whom had ≥2 unique mutations. Baseline ESR1LBDm status was not prognostic. Patients with persistently elevated CTC and/or ESR1LBDm+ ctDNA at C1D15 had worse PFS than patients who did not (P = 0.0007).Conclusions:Elevated CTC at baseline was a strong prognostic factor in this cohort. Early on-treatment changes were observed in CTC-ER+ and ESR1LBDm+ ctDNA, but not in overall CTC number. Integrating multiple biomarkers in prospective trials may improve outcome prediction and ET resistance mechanisms' identification over a single biomarker.

Purpose:Resistance to treatment with endocrine therapy in patients with HR+, HER2− advanced breast cancer (ABC) is common and dual inhibition of CDK4/6 and PI3K pathways may delay the development of resistance. This phase Ib trial evaluates the safety and tolerability of triple and double regimens containing the CDK4/6 inhibitor ribociclib.Patients and Methods:In this open-label, multicenter, phase Ib study, 70 postmenopausal women with HR+, HER2− ABC were enrolled into one of four treatment combinations: ribociclib (once daily, 3 weeks on, 1 week off) plus fulvestrant; ribociclib (continuous dosing) plus fulvestrant; ribociclib plus alpelisib plus fulvestrant; or ribociclib plus buparlisib plus fulvestrant.Results:The recommended phase II dose (RP2D) of ribociclib was confirmed to be 600 mg (3 weeks on, 1 week off) and 400 mg (continuous dosing) plus fulvestrant 500 mg. For the triple combination with buparlisib, the RP2D was ribociclib 400 mg plus buparlisib 30 mg plus fulvestrant 500 mg. Enrollment for the triple combinations was stopped due to unexpected toxicity. No RP2D was determined for the alpelisib combination. The safety profiles of the ribociclib plus fulvestrant combinations were consistent with those in previous studies. There was no marked difference in ribociclib exposure in the presence of triple-combination partners. The highest overall response rate was seen in the buparlisib triple combination (25.0%; 95% confidence interval, 9.8–46.7).Conclusions:Ribociclib plus fulvestrant demonstrated safety in the treatment of patients with HR+, HER2− ABC. Triple combinations with alpelisib or buparlisib plus fulvestrant are not recommended for phase II investigation.See related commentary by Clark et al., p. 371

Purpose: A phase II study of dasatinib, an inhibitor of multiple oncogenic tyrosine kinases including Src, was conducted to evaluate 16-week progression-free rate and tolerability in patients with previously treated metastatic breast cancer (MBC). Real-time assessment of potential tissue biomarkers of Src inhibition was used to optimize dosing.Experimental Design: Eligibility criteria required that patients have measurable MBC, biopsiable tumor, and unlimited prior therapies. For the analysis of change in protein biomarkers of Src inhibition, focal adhesion kinase, paxillin, and p-Src, patients underwent metastatic biopsies at baseline and 4 weeks. Patients who tolerated the starting dose of dasatinib (50 or 70 mg orally twice daily) for the first 28-day cycle, and displayed suboptimal Src inhibition, were escalated to a higher dose (70 or 100 mg).Results: The trial was closed early with 31 patients because of a statistical boundary that required at least 4 (13%) patients without disease progression to continue accrual. These 31 patients had a median of 2 prior lines of chemotherapy for MBC. The most notable toxicity was pleural effusions in 16 patients (52%). Twenty patients had evaluable metastatic biopsies. None of the tumors showed the predefined optimal level of Src inhibition at week 4.Conclusions: Single-agent dasatinib did not exhibit significant antitumor activity in patients with heavily pretreated MBC. There were no clinically meaningful decreases before and after dasatinib exposure between exploratory tissue biomarkers of Src inhibition which may be attributable to challenges in defining biomarker endpoints for multitargeted tyrosine kinase inhibitors. Clin Cancer Res; 17(18); 6061–70. ©2011 AACR.

Purpose: Basal-like breast tumors are typically (ER/PR/HER2) triple-negative and are associated with a high incidence of brain metastases and poor clinical outcomes. The molecular chaperone αB-crystallin is predominantly expressed in triple-negative breast cancer (TNBC) and contributes to an aggressive tumor phenotype in preclinical models. We investigated the potential role of αB-crystallin in brain metastasis in TNBCs.Experimental Design: αB-crystallin expression in primary breast carcinomas and brain metastases was analyzed by immunohistochemistry among patients with breast cancer with brain metastases. αB-crystallin was overexpressed or silenced in two different TNBC cell lines. The effects on cell adhesion to human brain microvascular endothelial cells (HBMEC) or extracellular matrix proteins, transendothelial migration, and transmigration across a HBMEC/astrocyte coculture blood–brain barrier (BBB) model were examined. In addition, the effects of overexpressing or silencing αB-crystallin on brain metastasis in vivo were investigated using orthotopic TNBC models.Results: In a cohort of women with breast cancer brain metastasis, αB-crystallin expression in primary breast carcinomas was associated with poor overall survival and poor survival after brain metastasis, even among patients with TNBC. Stable overexpression of αB-crystallin in TNBC cells enhanced adhesion to HBMECs, transendothelial migration, and BBB transmigration in vitro, whereas silencing αB-crystallin inhibited these events. αB-crystallin promoted adhesion of TNBC cells to HBMECs, at least in part, through an α3β1 integrin–dependent mechanism. αB-crystallin overexpression promoted brain metastasis, whereas silencing αB-crystallin inhibited brain metastasis in orthotopic TNBC models.Conclusion: αB-crystallin is a novel regulator of brain metastasis in TNBC and represents a potential biomarker and drug target for this aggressive disease. Clin Cancer Res; 20(1); 56–67. ©2013 AACR.

Background: SERENA-1 (NCT03616587) is a Phase 1, multi-part, open-label study of camizestrant in women with ER+/HER2− advanced breast cancer. Parts A/B and C/D (escalation/expansion) examined camizestrant as monotherapy and in combination with palbociclib respectively and have been presented previously.1,2 Here we present data from parts G/H which examined camizestrant in combination with abemaciclib. Methods: The primary objective was to determine the safety and tolerability of camizestrant 75 mg once daily (QD) in combination with abemaciclib 150 mg twice daily (BID). Secondary objectives included investigation of anti-tumor response and pharmacokinetics (PK). Participants were previously treated women of any menopausal status (pre-menopausal women received this combination alongside ongoing ovarian function suppressors). Prior treatment with ≤2 lines of chemotherapy in the advanced setting was permitted. There was no limit on the number of lines of prior endocrine treatment in the advanced setting; previous treatment with CDK4/6 inhibitors (CDK4/6i) and fulvestrant was permitted. Results: As of 1st June 2022, 24 patients had received camizestrant in combination with abemaciclib with a median 7.7 month follow up. Tolerability of the combination of camizestrant and abemaciclib was consistent with that of each drug individually. No patient required camizestrant dose reduction. All camizestrant-related heart rate decreases were Grade 1 (asymptomatic). PK data for camizestrant in combination with abemaciclib were consistent with camizestrant as monotherapy and published abemaciclib steady-state PK data, indicating no clinically relevant drug-drug interaction. In these heavily pre-treated patients (46% prior chemotherapy, 75% prior CDK4/6i, 54% prior fulvestrant; all in the advanced disease setting) and of whom 67% had visceral metastases, the objective response rate was 5/19 (26.3%), the clinical benefit rate at 24 weeks was 16/24 (66.7%) and the median progression-free survival had not been reached, with 8/24 patients experiencing a progression event. These data support the use of camizestrant 75 mg QD combined with the approved abemaciclib dose. Conclusions: Camizestrant 75 mg QD in combination with abemaciclib 150 mg BID was well tolerated with encouraging clinical activity. The inclusion of this regimen in the ongoing Phase 3, SERENA-6 trial 3, of camizestrant combined with CDK4/6i versus an aromatase inhibitor, will further clarify the role of this combination in the treatment of patients with ER+/HER2− advanced breast cancer with tumors expressing ESR1 mutations. References 1. Baird R, Oliveira M, Ciruelos Gil EM, et al. SABCS 2020 Virtual Meeting. Abstract PS11-05. 2. Oliveira M, Hamilton EP, Incorvati J, et al. J Clin Oncol 40, 2022 (suppl 16; abstr 1032). 3. SERENA-6 trial. Available at https://clinicaltrials.gov/ct2/show/NCT04964934 We acknowledge Helen Heffron, PhD, from InterComm International who provided medical writing support funded by AstraZeneca. Citation Format: Nicholas Turner, Christos Vaklavas, Emiliano Calvo, Javier Garcia-Corbacho, Jason Incorvati, Manuel Ruiz Borrego, Chris Twelves, Anne Armstrong, Begoña Bermejo, Erika Hamilton, Mafalda Oliveira, Eva Ciruelos, Peter Kabos, Manish R Patel, Maria Borrell, Howard Burris, Bruno de Paula, Alejandro Falcon, Cristina Hernando, Irene Moreno, Ciara S. O’Brien, Elena Shagisultanova, Ivan Victoria Ruiz, Judy S. Wang, Mei Wei, Tim Brier, Danielle Carroll, Carmela Ciardullo, Lisa Gibbons, itziar irurzun-Arana, Tony Jack, bistra kirova, Teresa Klinowska, Justin Lindemann, Julie Maidment, Alastair Mathewson, Rhiannon Maudsley, Robert McEwen, Christopher Morrow, Andy Sykes, Richard D. Baird. SERENA-1: Updated analyses from a Phase 1 study of the next generation oral selective estrogen receptor degrader camizestrant (AZD9833) combined with abemaciclib, in women with ER-positive, HER2-negative advanced 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 P3-07-28.

Trastuzumab emtansine is the current standard treatment for patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer whose disease progresses after treatment with a combination of anti-HER2 antibodies and a taxane.We conducted a phase 3, multicenter, open-label, randomized trial to compare the efficacy and safety of trastuzumab deruxtecan (a HER2 antibody-drug conjugate) with those of trastuzumab emtansine in patients with HER2-positive metastatic breast cancer previously treated with trastuzumab and a taxane. The primary end point was progression-free survival (as determined by blinded independent central review); secondary end points included overall survival, objective response, and safety.Among 524 randomly assigned patients, the percentage of those who were alive without disease progression at 12 months was 75.8% (95% confidence interval [CI], 69.8 to 80.7) with trastuzumab deruxtecan and 34.1% (95% CI, 27.7 to 40.5) with trastuzumab emtansine (hazard ratio for progression or death from any cause, 0.28; 95% CI, 0.22 to 0.37; P0.001). The percentage of patients who were alive at 12 months was 94.1% (95% CI, 90.3 to 96.4) with trastuzumab deruxtecan and 85.9% (95% CI, 80.9 to 89.7) with trastuzumab emtansine (hazard ratio for death, 0.55; 95% CI, 0.36 to 0.86; prespecified significance boundary not reached). An overall response (a complete or partial response) occurred in 79.7% (95% CI, 74.3 to 84.4) of the patients who received trastuzumab deruxtecan and in 34.2% (95% CI, 28.5 to 40.3) of those who received trastuzumab emtansine. The incidence of drug-related adverse events of any grade was 98.1% with trastuzumab deruxtecan and 86.6% with trastuzumab emtansine, and the incidence of drug-related adverse events of grade 3 or 4 was 45.1% and 39.8%, respectively. Adjudicated drug-related interstitial lung disease or pneumonitis occurred in 10.5% of the patients in the trastuzumab deruxtecan group and in 1.9% of those in the trastuzumab emtansine group; none of these events were of grade 4 or 5.Among patients with HER2-positive metastatic breast cancer previously treated with trastuzumab and a taxane, the risk of disease progression or death was lower among those who received trastuzumab deruxtecan than among those who received trastuzumab emtansine. Treatment with trastuzumab deruxtecan was associated with interstitial lung disease and pneumonitis. (Funded by Daiichi Sankyo and AstraZeneca; DESTINY-Breast03 ClinicalTrials.gov number, NCT03529110.).

Background: More than two-thirds of advanced breast cancer (ABC) is hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-). Endocrine therapy (ET) is the core treatment for patients with HR+/HER2- ABC, most commonly an aromatase inhibitor (AI), tamoxifen, or the selective estrogen receptor degrader (SERD), fulvestrant. ET is increasingly also combined with a cyclin-dependent kinase (CDK) 4/6 inhibitor, and this has become the first-line standard of care for HR+/HER2- ABC in many countries. However, despite the fact that combination therapy prolongs life compared with ET alone, most patients eventually develop therapy resistance and HR+/HER2- ABC is considered incurable. Overcoming therapy resistance is a major challenge, partially because it is associated with several different mechanisms. The phosphoinositide 3-kinase/protein kinase/phosphatase and tensin homologue (PI3K/AKT/PTEN) pathway regulates cell growth, proliferation and survival. Increased PI3K/AKT/PTEN pathway activity has been observed as one of the key mechanisms driving resistance to the combination therapy of ET and CDK4/6 inhibitors. Capivasertib is a potent selective inhibitor of all three AKT isoforms that suppresses PI3K/AKT/PTEN pathway activity. Capivasertib has demonstrated synergistic anticancer activity with standard ET agents in in vitro and in vivo experimental models. Moreover, the Phase II FAKTION trial found that patients with AI-resistant HR+/HER2- ABC had longer progression-free survival (PFS) when capivasertib was added to fulvestrant therapy, compared with fulvestrant alone. CAPItello-292 is a Phase Ib/III trial examining the safety, tolerability and clinical benefit of adding AKT inhibition (with capivasertib) to combined CDK4/6 inhibition (with palbociclib) and ET (with fulvestrant) for patients with ET-resistant HR+/HER2- ABC. Trial design: CAPItello-292 (NCT04862663) is enrolling patients with HR+/HER2- locally advanced or metastatic breast cancer whose disease progressed or recurred while receiving ET or within 12 months of completing an ET (neo)adjuvant regimen. The Phase Ib trial will identify the recommended Phase III dose (RP3D) for combined capivasertib, palbociclib and fulvestrant therapy. Up to 72 patients will be enrolled for the Phase Ib part. The initial regimen will be capivasertib (320 mg twice daily; 4 days on, 3 days off through each 28-day cycle), palbociclib (125 mg once daily for 21 days of each 28-day cycle) and fulvestrant (500 mg once every 28 days, plus a loading dose on day 15 of the first cycle). In the Phase III part, approximately 628 patients will be randomized 1:1 to receive either the RP3D of the triplet or placebo plus palbociclib and fulvestrant until disease progression, unacceptable toxicity or withdrawal of consent. The Phase III primary endpoint is PFS. Secondary endpoints include safety, tolerability and overall survival, as well as PFS of patients whose tumors have PIK3CA/AKT1/PTEN alterations. The first site for the study was activated in mid-May 2021. As of July 2021, two patients have been dosed in the first cohort. Funding and acknowledgements: CAPItello-292 is funded and overseen by AstraZeneca. AstraZeneca funded medical writing support provided by Rose Goodchild, PhD, of Oxford PharmaGenesis, Oxford, UK. This abstract was previously presented at the 2021 European Society for Medical Oncology (ESMO) Annual Meeting, 3019TiP, Erika Hamilton et al. - Reused with permission. Citation Format: Hope S Rugo, Gaia Schiavon, Lynda M Grinsted, Elza C De Bruin, Maria Teresa Catanese, Erika Hamilton. CAPItello-292: A phase Ib/III study of capivasertib, palbociclib and fulvestrant, versus placebo, palbociclib and fulvestrant, for endocrine therapy-resistant HR+/HER2− 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 OT2-14-01.

Background: Tucatinib (TUC), approved in multiple regions for HER2+ metastatic breast cancer, is a highly selective HER2-directed tyrosine kinase inhibitor with minimal EGFR inhibition. TUC is being developed as a novel therapy for patients with HER2+ metastatic breast cancer, colorectal cancer, and gastric cancer. In xenograft models of HER2+ and HER2-mutated tumors, dual targeting of HER2 with TUC and trastuzumab showed superior activity to either agent alone. Somatic HER2 mutations occur in approximately 3% of breast cancers, mostly in patients with hormone receptor-positive disease and the lobular sub-type. HER2 mutations lead to enhanced tyrosine kinase activity and tumorigenesis in preclinical models and have been postulated as a mechanism of endocrine therapy resistance. The SGNTUC-019 basket study (NCT04579380) is evaluating TUC in combination with trastuzumab in patients with HER2+ or HER2-mutated solid tumors, including a cohort of patients with locally advanced unresectable or metastatic breast cancer that is HER2-mutated and not overexpressed/amplified. Trial Design: SGNTUC-019 is a multi-cohort, open-label, international Phase 2 study evaluating patients with previously treated solid tumors displaying HER2 overexpression/amplification or activating mutations. Eligible breast cancer patients must have HER2-mutated, locally advanced unresectable or metastatic disease with progression on or after ≥1 prior line of therapy (chemotherapy, endocrine or targeted therapy) in the locally advanced or metastatic disease setting. Additionally, an ECOG Performance Status of ≤1; adequate hepatic, hematological, renal, and cardiac functions; and no previous HER2-directed therapy are required for enrollment. For eligibility, HER2 mutations can be demonstrated in a previous or on-study next-generation sequencing (NGS) assay of circulating tumor DNA or a previous tissue NGS assay. The breast cancer cohort will enroll 30 response-evaluable patients with HER2-mutated disease. Patients with HER2+ (overexpression/amplification) breast cancer will not be enrolled. The primary objective in each cohort is antitumor activity. The primary endpoint is a confirmed objective response rate, and secondary endpoints are the disease control rate, duration of response, progression-free survival, and overall survival. Safety and efficacy endpoints will be summarized descriptively. For response rates, the 2-sided exact interval using the Clopper-Pearson method will be calculated. Patients will receive TUC 300 mg orally twice a day and trastuzumab 8 mg/kg intravenously on Cycle 1 Day 1 and 6 mg/kg every 21 days from Cycle 2 Day 1. Hormone receptor-positive HER2-mutated breast cancer patients will also receive fulvestrant 500 mg intramuscularly every 4 weeks and on Cycle 1 Day 15. Disease assessments per RECIST v1.1 are every 6 weeks for 24 weeks, then every 12 weeks. Patients in the breast cancer cohort will undergo baseline brain magnetic resonance imaging, and those patients with brain metastases may be eligible for the study. Quality of life will be evaluated using EQ-5D-5L every 2 cycles. Enrollment began in December 2020 and is ongoing globally. Citation Format: Alicia Okines, Paula R. Pohlmann, Jorge Ramos, Luke Walker, Erika Hamilton. SGNTUC-019: Phase 2 basket study of tucatinib and trastuzumab in previously treated solid tumors with HER2 alterations: HER2-mutated breast cancer cohort (ongoing clinical trial) [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 OT1-15-01.

Background Tucatinib is an oral, reversible, small-molecule tyrosine kinase inhibitor highly selective for HER2 with minimal inhibition of the human epidermal growth factor receptor. Tucatinib is approved in the US for use in combination with trastuzumab and capecitabine in patients with HER2+ metastatic breast cancer, with and without brain metastases, who have received 1 or more prior anti-HER2-based regimens in the metastatic setting. Trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate (ADC) comprising a HER2-directed monoclonal antibody conjugated to a topoisomerase I inhibitor payload, is approved in the US for patients with HER2+ metastatic breast cancer who have received 2 or more prior anti-HER2-based regimens in the metastatic setting. In HER2+ breast cancer xenograft models, tucatinib increased the antitumor activity of a HER2-directed ADC comprising a HER2-directed monoclonal antibody conjugated with 8 exatecan moieties (T-Ex) when compared to T-Ex alone (Kulukian, et al. (2019). Abstract P1-18-09. Proceedings of San Antonio Breast Cancer Symposium, December 10-14, 2019). While significant advances have been made in the treatment of patients with HER2+ breast cancer, treatment of metastatic disease remains a clinical challenge for which options are limited, particularly in patients with brain metastases. The HER2CLIMB-04 study is evaluating the efficacy and safety of tucatinib in combination with T-DXd in patients with HER2+ metastatic breast cancer following progression on 2 or more HER2-directed regimens in the metastatic setting. Trial Design HER2CLIMB-04 (NCT04539938) is a single-arm, open-label, multicenter, phase 2 study of tucatinib in combination with T-DXd in previously treated patients aged ≥18 years with unresectable, locally advanced, or metastatic HER2+ breast cancer. Patients with brain metastases, including active brain metastases, may be enrolled. A safety lead-in portion of the study will enroll 10 patients who will be followed for at least 1 cycle. If tucatinib combined with T-DXd has a manageable safety profile, the trial will enroll approximately 60 response-evaluable patients (including the 10 patients from the safety lead-in), approximately evenly distributed between patients with and without brain metastases. The primary endpoint is confirmed objective response rate (cORR) by investigator assessment per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1. Secondary endpoints are progression-free survival (PFS), duration of response (DoR), disease control rate (DCR) by investigator assessment per RECIST 1.1, OS, and safety. Exploratory endpoints will include cORR, PFS, DCR, and DoR by independent central review per RECIST 1.1, pharmacokinetic analyses, biomarker analyses, and changes in patient-reported outcomes using the European Quality of Life 5-Dimension 5-Level instrument. Efficacy and safety will be summarized with descriptive endpoints to describe continuous variables. For response rates, the 2-sided exact confidence interval using the Clopper-Pearson method will be calculated. Enrollment began in late 2020 at approximately 30 study sites in the US. Citation Format: Erika Hamilton, Lisa Carey, Jorge Ramos, Yiyi Chen, Ian Krop. HER2CLIMB-04: phase 2 trial of tucatinib + trastuzumab deruxtecan in patients with HER2+ locally advanced or metastatic breast cancer with and without brain metastases (trial in progress) [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 OT1-13-01.

Background: Probody® therapeutic candidates (Pb-Tx) are masked antibodies designed to be conditionally activated in the tumor microenvironment by tumor-associated proteases. This allows Pb-Tx to address previously undruggable targets that are highly expressed in both tumor and normal tissues. Praluzatamab ravtansine is a masked antibody-drug conjugate targeting CD166 with a DM4 payload. A phase 1 monotherapy study showed safety and durable clinical activity (clinical benefit rate at 24 weeks [CBR24] 28%) in patients (pts) with metastatic HR-positive/HER2 non–amplified breast cancer (mHR+/HER2− BC) or metastatic triple-negative breast cancer (mTNBC). Pacmilimab, a masked PD-L1 inhibitor, demonstrated monotherapy activity in pts with TNBC with an acceptable tolerability profile.Trial design: This phase 2, open-label study with 3 parallel arms (~40 evaluable/arm) will evaluate praluzatamab ravtansine monotherapy (7 mg/kg Q3W) in pts with mHR+/HER2− BC or mTNBC, and praluzatamab ravtansine (7 mg/kg Q3W) combined with pacmilimab (1200 mg Q3W) in pts with mTNBC. Adult pts with an ECOG PS 0–1, measurable disease, and willingness to receive ocular prophylaxis for DM4-related toxicities will be enrolled. Key eligibility criteria for the mHR+/HER2− BC cohort include 2–4 prior regimens in the metastatic setting (excluding single-agent endocrine therapy). No CD166 screening required for HR+/HER2− BC but mTNBC must be CD166+. Pts with mTNBC must have received 1–3 prior metastatic regimens. For mTNBC pts who receive the doublet, key exclusion criteria include known PDL1 negative tumor, history of autoimmune disease, and progression within 120 days of 1st dose of an immuno-oncology agent. Pts with corneal disorders will be excluded. Primary endpoint is overall response rate (ORR) assessed by an independent radiology committee per RECIST v1.1. Secondary endpoints include ORR by investigator, duration of response, CBR16 & 24, progression-free survival, and overall survival. This study will also evaluate safety and tolerability, pharmacokinetics, and antidrug antibodies with praluzatamab ravtansine as monotherapy and in combination with pacmilimab. This trial is enrolling (NCT04596150). Citation Format: Kathy Miller, Leisha A. Emens, Sara M. Tolaney, Sara A. Hurvitz, Erika Hamilton, Virginia Paton, Alison Hannah, Valentina Boni. Trial in progress: Phase 2, open-label study to evaluate the safety and efficacy of praluzatamab ravtansine in metastatic HER2 non-amplified breast cancer as monotherapy and combination with pacmilimab [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 OT1-12-02.

Background: Survival advantages with neoadjuvant chemotherapy (NACT) have been observed in patients (pts) with locally advanced breast cancer (BC) who achieve pathological complete response (pCR). Predicting risk of early recurrence in pts without pCR remains difficult. Circulating tumor DNA (ctDNA) may be a biomarker of neoadjuvant therapy response and recurrence. This study applied tumor genotyping of peripheral blood in pts with HER2− tumor BRCA-mutated (tBRCAm) BC treated with neoadjuvant niraparib. The relationship between ctDNA and tumor response was also characterized, with a focus on circulating TP53 mutant allelic frequency (MAF). Methods: Blood was collected from 21 pts enrolled in the study (NCT03329937) evaluating neoadjuvant niraparib in pts with localized HER2− tBRCAm BC. Pts received niraparib 200 mg once daily for two 28-day cycles. After 2 cycles, pts underwent surgery or received up to 6 cycles of niraparib and/or NACT prior to surgery. Blood samples were collected at screening, Cycle 1 Day 28 (C1D28), Cycle 2 Day 28 (C2D28), and pre-surgery. Ultrasound (USG) was done after each cycle and MRI after Cycle 2. Next-generation sequencing of cell-free plasma DNA was carried out using a proprietary target capture and analysis (Paweletz, Clin Cancer Res 2016). Results: ctDNA and TP53 mutations (TP53m) were detected at screening in 10/21 pts (47.6%). Across all 21 pts there was a correlation between TP53 MAF and baseline tumor stage (T) (mean [standard deviation], T1 0 [0], T2 0.05 [0.09], and T3 0.21 [0.12]; P=0.005). Linear regression modelling showed a positive correlation between tumor volume and baseline TP53 MAF (R2=0.206, P=0.0386). Baseline TP53 MAF also appeared to be associated with advanced disease stage (Table 1). Depletion of detectable TP53m from baseline was evident by C1D28 and persisted to pre-surgery (Table 2). This corresponded with decreases in tumor volume at C1D28 and further decreases at C2D28. Of 11 pts analyzed, 6 pts with >90% tumor volume decreases by MRI or USG at C2D28 had sustained TP53m depletion. Three pts had pCR; of these, 2 had sustained TP53m depletion (data unavailable for third pCR pt). Two pts with less robust tumor responses had increased pre-surgery TP53 MAF. Conclusions: We report the potential utility of ctDNA as a predictive biomarker for neoadjuvant niraparib response in pts with HER2− tBRCAm BC. Although sample size limited analysis of genes other than TP53, baseline ctDNA strongly correlated with tumor volume, and TP53 MAF was associated with tumor volume and trended towards association with disease stage. Longitudinal analysis of TP53 MAF suggested a correlation with niraparib response. Further study of ctDNA dynamics during NACT and correlation with pCR and long-term clinical outcomes is warranted. Funding: GlaxoSmithKline (GSK) study 213355; NCT03329937. Medical writing support was provided by Fishawack Indicia, part of Fishawack Health, funded by GSK. Table 1.Baseline TP53 MAF association with disease stage and subtypeTP53 MAFDisease Stage (I–III)Disease SubtypeStage IStage IIStage IIIHR+TNBC(n=8)(n=10)(n=3)(n=6)(n=15)Mean (SD)0.01 (0.02)0.07 (0.12)0.10 (0.05)0.02 (0.02)0.06 (0.10)HR, hormone receptor; MAF, mutant allelic frequency; SD, standard deviation; TNBC, triple-negative breast cancer. Table 2.TP53 MAF correlation with tumor volume decrease by ultrasound and MRIOutcomeScreeningCycle 1 Day 28 Cycle 2 Day 28 Pre-Surgeryn11997TP53 MAF, mean, (SD)0.09 (0.11)000.01 (0.02)n11112Percent change in tumor volume from baseline by MRI, mean (SD)-N/A*−72.6 (22.5)N/A*Percent change in tumor volume from baseline by ultrasound, mean (SD)-−60.8 (24.0)−80.6 (21.8)−95.9 (4.3)*MRI was only performed at Cycle 2 Day 28. MAF, mutant allelic frequency; MRI, magnetic resonance imaging; N/A, not applicable; SD, standard deviation. Citation Format: Ming Shan, Laura Spring, Minetta C Liu, Erika Hamilton, Hanna Irie, Cesar A Santa-Maria, Steven J Isakoff, James Reeves, Leif W Ellisen, Lee P Lim, Kavita Garg, Caterina Bertucci, Bin Feng, Hailei Zhang, Kaiming Sun, Julie R Graham, Erin Hofstatter, Hyo Han. Relationship between circulating tumor DNA and response to neoadjuvant niraparib in HER2-negative, BRCA-mutated 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 P5-13-21.

Background: Resistance to current endocrine therapies remains a clinical challenge. Fulvestrant has established estrogen receptor (ER) degradation as a critical therapeutic strategy characterized by approximately 50% ER degradation, a clinical benefit rate (CBR) of < 20% in the post CDK 4/6 setting, and a suboptimal intramuscular route of administration. ARV-471 is a selective, orally bioavailable PROteolysis-TArgeting Chimera (PROTAC®) small molecule that induces degradation of wildtype and mutant ER. ARV-471 demonstrates superior ER degradation and antitumor activity compared to fulvestrant in endocrine sensitive and resistant xenograft models. Methods: This is a multi-center, first-in-human, open label study to assess the safety, pharmacokinetics, pharmacodynamics, and preliminary antitumor activity of ARV-471 administered orally in patients with ER+/HER2- advanced/metastatic breast cancer. The phase 1 monotherapy dose escalation used a 3+3 design with backfill to assess ARV-471 in pre-/post-menopausal women. Premenopausal women had to be on ovarian suppression. Patients were required to have received at least one prior CDK 4/6 inhibitor and at least 2 prior endocrine therapies. Up to 3 prior lines of chemotherapy were allowed. Results: As of June 6th, 2021, 50 patients were treated in the monotherapy escalation at total daily doses of 30mg (n=3), 60mg (n=3), 120mg (n=7), 180/200mg (n=11), 360mg (n=15), 500mg (n=8), and maximum administered dose of 700mg (n=3). A maximum tolerated dose was not reached and no dose limiting toxicities (DLTs) were observed. The most common (≥ 10%) treatment related adverse events (TRAEs) were nausea (24%), fatigue (12%), and vomiting (10%) that were predominantly grade 1 in severity. Two patients experienced grade 3 adverse events (AEs) that were potentially related to ARV-471: 1 patient treated at 180mg had a transient headache lasting 1 day, and 1 patient treated at 360mg had a venous thromboembolism (VTE) a few days after a minor procedure. There were no AEs > grade 3 potentially related to ARV-471. All AEs were manageable with only one patient discontinuing ARV-471 due to a TRAE (grade 3 VTE). ARV-471 demonstrated a dose-related increase in plasma exposure up to 500mg, with doses of 60mg daily and above resulting in steady-state Cmax and AUC24 that exceeded the exposure associated with tumor regression in preclinical models. Analysis of 12 paired biopsies from patients treated at 30 to 360mg daily demonstrated up to 90% ER degradation in tumors expressing WT or mutant ER. Of 34 patients who were evaluable for clinical benefit (confirmed complete response, partial response, or stable disease ≥ 24 weeks) the CBR was 41%. As of the data cutoff date, 6 of the 34 patients were continuing to receive study treatment, including 2 patients who had been on treatment for over 16 months. Two confirmed partial responses were observed among the 28 patients with baseline measurable disease and at least 1 on-treatment tumor assessment. Conclusion: ARV-471 was well tolerated with no DLTs at total daily doses up to 700mg. ARV-471 demonstrated robust ER degradation in paired biopsy samples and encouraging clinical activity (41% CBR) in patients who received prior CDK 4/6 inhibitors. ARV-471 is now being evaluated in the VERITAC Phase 2 monotherapy expansion at 200 mg and 500 mg once daily. Citation Format: Erika Hamilton, Linda Vahdat, Hyo S Han, Jennifer Ranciato, Richard Gedrich, Chi F Keung, Deborah Chirnomas, Sara Hurvitz. First-in-human safety and activity of ARV-471, a novel PROTAC® estrogen receptor degrader, in ER+/HER2- locally advanced or metastatic 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 PD13-08.