Abstract PS15-03: Vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, plus palbociclib in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer: phase 1b cohort

Abstract Background: Vepdegestrant (ARV-471) is an oral PROTAC ER degrader. In a phase 1/2 study (NCT04072952), vepdegestrant monotherapy had a favorable safety profile and encouraging clinical activity, and showed robust ER degradation. The phase 1b cohort of this study is evaluating vepdegestrant in combination with the cyclin-dependent kinase (CDK)4/6 inhibitor palbociclib (palbo). In xenograft models, vepdegestrant plus palbo showed substantially greater tumor growth inhibition vs fulvestrant plus palbo, supporting investigation in patients with breast cancer. Methods: Eligible patients for the phase 1b combination cohort had ER+/HER2- locally advanced/metastatic breast cancer and had received ≥1 prior endocrine therapy and ≤2 chemotherapy regimens for advanced disease; prior CDK4/6 inhibitor treatment was permitted. Vepdegestrant was given orally once daily (QD) continuously at doses of 180 mg (n=2), 200 mg (n=21), 400 mg (n=3), or 500 mg (n=20); palbo 125 mg was given orally QD for 21 days followed by 7 days off treatment in 28-day cycles. The primary endpoints were dose-limiting toxicities (DLTs) in the first cycle and safety (adverse events [AEs] and laboratory abnormalities). Enrollment in these 4 dose levels is complete; we report data as of June 6, 2023. Results: Across 46 patients in the phase 1b cohort, 45 (97.8%) patients were female with a median age of 62.0 y (range: 29–78). Patients had received a median of 4 prior therapies (range: 1–11) in any setting, including 87.0% prior CDK4/6 inhibitors (78.3% prior palbo), 80.4% prior fulvestrant, and 76.1% prior chemotherapy (45.7% in the metastatic setting). There were no DLTs. Treatment-emergent AEs (TEAEs) leading to dose reductions or discontinuation of vepdegestrant occurred in 5 and 4 patients, respectively. TEAEs leading to dose reductions or discontinuation of palbo occurred in 34 and 8 patients, respectively. Grade 3/4 treatment-related AEs (TRAEs) to either vepdegestrant or palbo in ≥10% of patients were neutropenia (89.1%), decreased white blood cell count (15.2%), and decreased platelet count (10.9%); there were no grade 5 TRAEs and no patients had febrile neutropenia. The clinical benefit rate (rate of confirmed complete response, partial response, or stable disease ≥24 wks) in patients treated with vepdegestrant plus palbo was 63.0% (95% CI: 47.5–76.8). The objective response rate in evaluable patients with measurable disease at baseline (n=31) was 41.9% (95% CI: 24.5–60.9). Pharmacokinetics (PK) showed dose-dependent exposure for vepdegestrant, consistent with data for vepdegestrant administered as monotherapy; palbo exposure was similar across dose levels of vepdegestrant and modestly higher compared with historical palbo PK data. Circulating tumor DNA levels were evaluated in patients who received vepdegestrant 200 mg QD plus palbo and demonstrated substantial and sustained decreases in ESR1 mutant allele fraction across multiple treatment cycles. Conclusions: The combination of vepdegestrant plus palbo showed promising clinical activity in heavily pretreated patients with ER+/HER2- advanced breast cancer who had received extensive prior treatment. The safety profile of vepdegestrant plus palbo was generally consistent with the known safety profiles of the 2 agents except for an increased occurrence of grade 3/4 neutropenia, which was readily managed with laboratory monitoring and dose reductions of palbo. There is an ongoing study lead-in to the global VERITAC-3 study (NCT05909397) that is evaluating 2 doses of palbo (100 mg and 75 mg) in combination with vepdegestrant 200 mg QD to determine the recommended phase 3 combination to compare with letrozole plus palbo as first-line treatment for ER+/HER2- advanced breast cancer. Citation Format: Erika Hamilton, Rinath Jeselsohn, Sara Hurvitz, Dejan Juric, Hyo Han, Melinda Telli, George Zahrah, Rita Nanda, Yuanyuan Zhang, Weiwei Tan, Elizabeth Duperret, Eric Zhi, Cecile Mather, Anne Schott. Vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, plus palbociclib in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer: phase 1b cohort [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS15-03.