Gail Lynn Shaw Wright

1000 Background: Pretreated HER2+ MBC lacks a defined standard of care, although T is commonly used. M has similar HER2 binding and antiproliferative effects as T. By contrast, M’s Fc region is engineered to increase affinity for both alleles of the activating Fc receptor (FcR), CD16A, and decrease affinity for the inhibitory FcR, CD32B. The low affinity CD16A-158F allele (~85% of population) has been associated with diminished clinical response to T. In a Phase 1 trial, M demonstrated acceptable safety, anti-tumor activity, and evidence of HER2-specific antibody and T-cell responses. Methods: SOPHIA (NCT02492711), a randomized, open-label P3 trial, enrolled pts with HER2+ MBC after pertuzumab and 1–3 lines of prior Tx for MBC. Pts were randomized 1:1 to M (15 mg/kg IV q3w + C) or T (6 [8 for loading dose] mg/kg IV q3w + C), stratified by met sites (≤2, > 2), lines of Tx for met disease (≤2, > 2), and C choice (standard dose capecitabine, eribulin, gemcitabine, or vinorelbine). Primary endpoints are central blinded PFS and OS, assessed sequentially using the stratified log-rank test. Objective response rate (ORR) was a secondary endpoint. 257 PFS events were required to provide 90% power to show PFS superiority at 2-sided α = 0.05. Results: Intent-to-treat analysis (536 pts: M 266; T 270) occurred after 265 PFS events. M prolonged PFS over T (median 5.8 vs 4.9 mo, hazard ratio [HR], 0.76; 95% CI, 0.59–0.98; P= 0.033). Treatment effects were more pronounced in pts with CD16A genotypes containing a 158F allele (median PFS 6.9 vs 5.1 mo, HR, 0.68; 95% CI, 0.52–0.90; P= 0.005). In 524 pts with baseline measurable disease (M 262; T 262), ORR was higher with M (22%; 95% CI, 17.3-27.7%) vs T (16%; 95% CI 11.8-21.0%). Safety profiles were comparable in 529 pts who received study therapy. Grade ≥3 AEs and serious AEs occurred in 138 (52%) and 39 (15%) vs 128 (48%) and 46 (17%) pts on M vs T, respectively. PFS data cutoff: 10/10/18. Conclusions: In combination with chemotherapy in pretreated HER2+ MBC, M improves PFS over T with comparable safety. CD16A genotyping suggests a differential benefit in patients with a 158F allele. OS data are maturing. Clinical trial information: NCT02492711.

1018 Background: H3B-6545, a selective, small molecule covalent antagonist of ERα demonstrated preclinical and preliminary clinical activity against ER+ breast cancer (Hamilton EP, SABCS, 2020). This study evaluated the activity and tolerability of H3B-6545 in patients (pts) with metastatic ER+, HER2-, breast cancer refractory to endocrine therapy. Methods: Patients received H3B-6545 once daily at the recommended phase II dose of 450 mg. The primary objective of the phase II is to estimate the objective response rate (ORR), progression-free survival (PFS), clinical benefit rate (CBR) and secondary objectives include safety. Results: 83 pts were treated with 450 mg in the phase II part of the trial. Additionally, 11 pts were treated with 450 mg in the phase I part of the trial and are included in this analysis. Median age was 62 years (range: 38 to 87 years), 81% had liver and/or lung metastases, and the median number of prior therapies for metastatic disease was 3 (range: 1 to 8). Prior CDK4/6 inhibitors, aromatase inhibitors, fulvestrant, and chemotherapy were received by 85%, 80%, 72%, and 50% of the pts, respectively. 58 pts (62%) had detectable ESR1 mutations in liquid biopsies, including 10 (11%) and 19 pts (20%) who had clonal Y537S and clonal D538G mutation, respectively. As of January 29, 2021, grade (gr) 2 or higher adverse events (AE) reported in ≥10% were anemia (19%), fatigue (16%), nausea (17%), and diarrhea (12%). Laboratory gr 2 or higher abnormalities reported in ≥10% pts were creatinine clearance decrease (38%), hemoglobin decrease (37%), bilirubin increase (12%), ALT increase (14%), AST increase (13%), and creatinine increase (11%). AE of gr 1 sinus bradycardia (asymptomatic) was reported in 34% and gr 2 (symptomatic, no intervention needed) was reported in 5%. Gr 2 and 3 QTcF prolongation were reported in 2 and 3 pts, respectively. There were no treatment-related deaths. Efficacy estimates are presented in the table below. Responses were observed in heavily pretreated pts, pts with visceral metastases and in pts who received prior fulvestrant, CDK4/6 inhibitor, and/or chemotherapy in the metastatic setting. Conclusions: H3B-6545 has a manageable safety profile and demonstrated single-agent anti-tumor activity in heavily pretreated ER+, HER2- mBC patients. Clinical activity was observed in pts with ESR1 mutations. Clinical trial information: NCT03250676 .[Table: see text]

Abstract Background: Addition of CDK 4/6 inhibitors to endocrine therapy has provided significant improvements in outcomes for patients (pts) with metastatic breast cancer (mBC). However, acquired resistance to front-line therapy remains a challenge, and response to late lines of therapy is poor. H3B-6545 is a selective, orally available, small molecule covalent antagonist of the estrogen receptor (ERα) H3B-6545 binds covalently to a cysteine residue at position 530 of both wild-type and the constitutively active mutant ERα proteins, including Y537S. H3B-6545 demonstrated significant antitumor activity in multiple PDX breast cancer models, including those with mutated ESR1 (the gene encoding ERα).Methods: This is a multicenter phase I/II trial. The primary objective of the phase I is to determine the maximum tolerated dose and recommended phase 2 dose (RP2D) in pretreated pts with ER+, HER2- mBC. Secondary objectives include safety and antitumor activity. The primary objective of the phase II is to estimate the objective response rate (ORR) and secondary objectives include clinical benefit rate (CBR), progression-free survival (PFS) and safety. The trial was designed to exclude a lower limit of ORR of 5% at one-sided level of significance of 0.05 and a power of 90%. Results: Between August 2017 and February 2020, 130 pts were enrolled; 47 in the Phase I part and 83 in the Phase II part of the trial. A total of 105 pts, including 73 patients on 450 mg, were response-evaluable. The phase I evaluated once daily doses from 100 to 600 mg. No dose-limiting toxicities (DLT) were observed at doses up to 450 mg and 2 DLTs were observed in 2 (grade 3 fatigue and grade 3 drug eruption) of 7 pts on the 600 mg cohort. Consequently, the dose of 450 mg was selected as the RP2D. Median age was 62 years (range: 31 to 87 years), 82% had liver and/or lung metastases, and the median number of prior therapies for metastatic disease was 3 (range: 1 to 10), with 41% of the pts receiving ≥ 4 prior therapies in the metastatic setting. Prior CDK4/6 inhibitors, fulvestrant, and chemotherapy were received by 87%, 71%, and 54% of the pts, respectively. 75 pts (58%) had detectable ESR1 mutations. As of May 31, 2020, grade 2 or higher adverse events reported in ≥10% were anemia (20%), fatigue (16%), nausea (14%), diarrhea (11%) and AST increase (11%). Three cases of grade 4 AE were reported (serum bilirubin, urinary tract obstruction, and hyponatremia), all considered related to disease progression. Grade 1 sinus bradycardia (asymptomatic) was reported in 35% and grade 2 (symptomatic, no intervention needed) was reported in 4%. Grade 2 and 3. QTcF prolongation were reported in 2 and 3 patients, respectively. There were no treatment-related deaths. In the response-evaluable group, 13 confirmed partial responses (PR, 12%. 90% confidence limits: 7.5%-19%), including 11 PRs (15%, 90% confidence limits: 8.7%-23.7) on 450 mg dose, were observed, thus achieving the primary objective of the trial. Stable disease (SD) and clinical benefit rates (≥23 weeks) were 45% and 33% respectively at 450 mg and 46% and 34%, respectively on all doses. Responses were observed in heavily pretreated pts, pts with visceral metastases and in pts who received prior fulvestrant, prior CDK4/6 inhibitor, and/or prior chemotherapy, in the metastatic setting. Three PRs (25%) and 4 SDs were observed in 12 pts in whom clonal ESR1 Y537S was the main ERα driver. Median PFS, in all pts and in pts with clonal ESR1 Y537S was 3.7 months and 7.3 months, respectively. Conclusions: H3B-6545 has a manageable safety profile and demonstrated single-agent anti-tumor activity in heavily pretreated ER+, HER2- mBC patients including those with a constitutively active clonal ESR1 Y537S mutation. Citation Format: Erika P Hamilton, Judy S Wang, Timothy Pluard, Stephen Johnston, Aki Aki Morikawa, Claire E Dees, Robert H Jones, Barbara Haley, Anne Armstrong, Adam L Cohen, Pamela Munster, Gail Wright, Fadi Kayali, Manav Korpal, Lihua Yu, Lisa Cantagallo, Benoit Destenaves, Zhaojie Zhang, Lei Gao, Marc J Pipas, Tarek Sahmouud, Antonio Gualberto, Dejan Juric. Phase I/II trial of H3B-6545, a novel selective estrogen receptor covalent antagonist (SERCA), in estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD8-06.