Phase 1b study of H3B-6545 in combination with palbociclib in women with metastatic estrogen receptor–positive (ER+), human epidermal growth factor receptor 2 (HER2)-negative breast cancer

e13025 Background: H3B-6545, a highly Selective ERα Covalent Antagonist (SERCA), inactivates both wild-type and mutant ERα by targeting cysteine 530 and enforcing a unique antagonist conformation. At the dose of 450 mg daily, H3B-6545 has a manageable safety profile and demonstrated preliminary single-agent antitumor activity in heavily pretreated ER+, HER2- mBC patients (Hamilton et al, San Antonio Breast Cancer Symposium, 2020). Methods: The study evaluates the safety, pharmacokinetics (PK), and efficacy of H3B-6545 in combination with palbociclib in patients with ER+, HER2- metastatic breast cancer (MBC). The escalation phase enrolls patients with 2 or more prior therapies in the metastatic setting. Up to one prior chemotherapy and up to one prior CDK4/6 inhibitor were allowed. Results: As of January 31, 2021, 10 patients were enrolled; 7 in Cohort 1 (H3B-6545 300 mg QD and palbociclib 100 mg QD) and 3 in Cohort 2 (H3B-6545 300 mg QD and palbociclib 125 mg QD). One patient in Cohort 1 was not evaluable for dose limiting toxicity (DLT) assessment and no DLT was observed in the 6 evaluable patients. One patient discontinued study treatment because of progression and no patients discontinued study treatment due to adverse events (AE). Grade 3 or 4 neutropenia and thrombocytopenia were observed in 4 patients and 1 patient, respectively. One patient had grade 3 hypercalcemia, generalized muscle weakness, hypophosphatemia, fall, and anemia and one patient had grade 3 lipase increase. Four patients had grade 1 bradycardia or sinus bradycardia (asymptomatic) 1 patient had grade 2 sinus bradycardia (symptomatic, no intervention required). Preliminary PK analysis suggested no clinically relevant drug-drug interactions between H3B-6545 and palbociclib, to be confirmed with data from additional cohorts. Recruitment is currently ongoing in Cohort 2. Updated results will be presented. Conclusions: H3B-6545, in combination with palbociclib, was well-tolerated. Clinical trial information: NCT04288089.