Abstract PO3-05-08: Updated results from VERITAC evaluating vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer

Background: Vepdegestrant (ARV-471) is an oral PROTAC ER degrader with activity toward wild-type and mutant ER. The phase 2 expansion (VERITAC) of a phase 1/2 study (NCT04072952) tested 2 vepdegestrant doses (200 mg once daily [QD] and 500 mg QD) in heavily pretreated patients with ER+/HER2- advanced breast cancer. Vepdegestrant 200 mg QD was selected as the phase 3 monotherapy dose based on comparable efficacy and favorable tolerability vs 500 mg QD and robust ER degradation (data cutoff: Jun 6, 2022). We present updated data for the vepdegestrant 200 mg QD cohort after 12 additional months of follow-up. Methods: Vepdegestrant was administered to patients with ER+/HER2- locally advanced/metastatic breast cancer who had received ≥1 prior endocrine therapy for ≥6 months, ≥1 cyclin-dependent kinase (CDK)4/6 inhibitor, and ≤1 chemotherapy regimen. The primary endpoint was clinical benefit rate (CBR; rate of confirmed complete response, partial response, or stable disease ≥24 weeks). Results: As of Jun 6, 2023, 35 patients received vepdegestrant 200 mg QD; 34 (97.1%) were female and median age was 63 y (range: 42–79). Patients had received a median of 4 prior regimens (range: 1–9) in all settings and 3 prior regimens (range: 0–7) in the metastatic setting; 100% had prior CDK4/6 inhibitors, 88.6% had prior aromatase inhibitors, 74.3% had prior fulvestrant, and 74.3% had prior chemotherapy (45.7% in metastatic setting). CBR with vepdegestrant 200 mg QD was 37.1% (95% CI: 21.5–55.1) in all evaluable patients (n=35) and the objective response rate was 8.3% (95% CI: 1.0–27.0) among 24 patients with measurable disease at baseline. Median progression-free survival in all evaluable patients was 3.5 months (95% CI: 1.8–8.2). As of the data cutoff date, 14 patients in the 200 mg QD cohort received treatment for ≥24 weeks (4 for ≥48 weeks) with 1 patient ongoing. No patients required a dose reduction due to a treatment-emergent adverse event (TEAE); 2 (5.7%) patients had a TEAE that led to vepdegestrant discontinuation (grade 3 QT prolongation and grade 3 anemia). The most common treatment-related adverse events (≥10%) were fatigue (43%), hot flush (20%), nausea (17%), arthralgia (11%), increased aspartate aminotransferase (11%), and increased blood alkaline phosphatase (11%); all were grade 1/2. Substantial on-treatment decreases in mutant ESR1 circulating tumor DNA levels were observed with vepdegestrant 200 mg QD and sustained for multiple treatment cycles. Conclusions: With 12 months of additional follow-up from the first data report, durable clinical activity with vepdegestrant 200 mg QD was seen in heavily pretreated patients with ER+/HER2- advanced breast cancer, in addition to sustained reduction in circulating mutant ESR1 tumor DNA levels. Vepdegestrant 200 mg QD continued to show a favorable safety profile. The ongoing global, randomized phase 3 VERITAC-2 study (NCT05654623) is evaluating vepdegestrant 200 mg QD vs intramuscular fulvestrant in patients with ER+/HER2- advanced breast cancer after prior combination CDK4/6 inhibitor therapy and endocrine therapy. Citation Format: Sara Hurvitz, Anne Schott, Cynthia Ma, Rita Nanda, George Zahrah, Natasha Hunter, Antoinette Tan, Melinda Telli, Jesus Anampa Mesias, Rinath Jeselsohn, Pamela Munster, Eric Zhi, Elizabeth Duperret, Cecile Mather, Erika Hamilton, Hyo Han. Updated results from VERITAC evaluating vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer [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 PO3-05-08.