Arvinas, Inc. and Pfizer Inc. announced updated clinical data from a Phase 1b combination cohort evaluating vepdegestrant, an investigational oral PROteolysis TArgeting Chimera (PROTAC®) estrogen receptor (ER) degrader, in combination with palbociclib (IBRANCE®). After six months of additional follow-up, these data are consistent with data presented at the San Antonio Breast Cancer Symposium (SABCS) in December 2023, and show that vepdegestrant plus palbociclib continue to demonstrate encouraging clinical activity in heavily pre-treated patients with a median of four lines of prior therapy with locally advanced or metastatic ER positive (ER+)/human epidermal growth factor 2 (HER2) negative (ER+/HER2-) breast cancer. These updated data were presented at the 2024 European Society for Medical Oncology (ESMO) Breast Cancer Annual Congress.

Vepdegestrant is an investigational PROTAC ER degrader designed to harness the body?s natural protein disposal system to specifically target and degrade the estrogen receptor. Vepdegestrant is being co-developed by Arvinas and Pfizer and is being evaluated as a monotherapy in the second-line setting in the ongoing Phase 3 VERITAC-2 trial and in the first-line setting in combination with palbociclib in the ongoing study lead-in cohort of the Phase 3 VERITAC-3 trial. The Phase 1b cohort of the ARV-471-mBC-101 study (NCT04072952) is designed to assess the safety, tolerability, and anti-tumor activity of vepdegestrant in combination with palbociclib among 46 patients with heavily pre-treated locally advanced or metastatic ER+/HER2- breast cancer.

Patients in the study received a median of four prior therapies (median of three in the metastatic setting); 87% were previously treated with a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor; 80% were previously treated with fulvestrant; and 78% were previously treated with chemotherapy, including 48% in the metastatic setting. Patients were treated once daily with oral doses of vepdegestrant at 180 mg (n=2), the recommended Phase 3 dose (RP3D) of 200 mg (n=21), 400 mg (n=3) or 500 mg (n=20), plus 125 mg of palbociclib given orally once daily for 21 days, followed by seven days off treatment in 28-day cycles. Initial data were presented at SABCS 2023 based on a data cutoff of June 6, 2023.

After six months of additional follow-up with a data cutoff of December 18, 2023, updated data from the study continue to demonstrate an encouraging clinical benefit rate, objective response rate and progression-free survival, and a consistent safety profile as previously reported at SABCS 2023. Data presented at the 2024 ESMO Breast Cancer Annual Congress: Clinical Benefit Rate (CBR): CBR, defined as the rate of confirmed complete response, partial response, or stable disease =24 weeks across all dose levels (n = 46) was 63% (95% CI: 47.5 - 76.8), with a CBR of 72% in patients with mutant ESR1 (n=29; 95% CI: 52.8 - 87.3) and a CBR of 53% in patients with wild-type ESR1 (n=15; 95% CI: 26.6 ? 78.7).

CBR in patients dosed at the RP3D of 200 mg (n=21) was 67% (95% CI: 43.0 - 85.4) with a CBR of 79% in patients with mutant ESR1 (n=14; 95% CI: 49.2 - 95.3) and a CBR of 43% in patients with wild-type ESR1 (n=7; 95% CI: 9.9 - 81.6). Objective Response Rate (ORR) and Duration of Response (DOR): The ORR in evaluable patients with measurable disease at baseline (n=31) was 42% (95% CI: 24.5 - 60.9) with a median DOR in 13 responders of 14.6 months (95% CI: 9.5 ? not reached).

At the RP3D of 200 mg (n=15), the ORR was 53% (95% CI: 25.6 ? 78.7). ORR in patients with mutant ESR1 (n=17): 47% (95% CI: 23.0 - 72.2).

ORR at the RP3D of 200 mg (n=10): 60% (95% CI: 26.2 - 87.8). ORR in patients with wild-type ESR1 (n=12): 42% (95% CI: 15.2 - 72.3). ORR at the RP3D of 200 mg (n=5): 40% (95% CI: 5.3 - 85.3).

Progression-free Survival (PFS): Median PFS (mPFS) based on 27 (59%) events across all dose levels was 11.2 months (95% CI: 8.2 ? 16.5) with a mPFS of 13.7 months (95% CI: 8.2 - NR) in patients with ESR1 mutation (n=29) and mPFS of 11.1 months (95% CI: 2.8 - 19.3) in patients with wild-type ESR1 (n=15). mPFS in patients dosed at the RP3D of 200 mg (n=21) based on 12 events (57%) was 13.9 months (95% CI: 8.1 - NR) with a mPFS of 13.9 months (95% CI: 8.1 - NR) in patients with ESR1 mutation (n=14) and mPFS of 11.2 months (95% CI: 1.8 - NR) in patients with wild-type ESR1 (n=7).

Circulating Tumor DNA (ctDNA): Exploratory ctDNA analyses found marked reduction (median change, -98.9%) in tumor fraction after one treatment cycle (all dose groups) regardless of ESR1 mutant status and robust on-treatment decreases in mutant ESR1 ctDNA levels sustained through cycle 7 (evaluated in patients in 200 mg dose cohort), as presented in the poster session. Safety Profile: The safety profile of vepdegestrant plus palbociclib was consistent with what was previously reported with Grade 3/4 treatment-related adverse events (TRAEs) =10% of neutropenia (91%) and decreased white blood cell count (15%); no grade 5 TRAEs or febrile neutropenia were reported. The majority of Grade 4 neutropenia events occurred in the first cycle of treatment and occurrences of Grade 3/4 neutropenia decreased following palbociclib dose reductions as described in the prescribing label.

The safety profile of vepdegestrant in combination with palbociclib was otherwise consistent with the profile of palbociclib and what has been observed in other clinical trials for vepdegestrant. Three of 46 patients discontinued palbociclib due to neutropenia including one out of 21 patients treated with the RP3D of vepdegestrant (200 mg) plus palbociclib 125 mg.