Abstract GS01-12: MONARCH 3: Final overall survival results of abemaciclib plus a nonsteroidal aromatase inhibitor as first-line therapy for HR+, HER2- advanced breast cancer

Abstract Background: Abemaciclib is approved both for high-risk early breast cancer as well as advanced breast cancer (ABC) in the first- and second-line setting. In MONARCH 2, the addition of abemaciclib to fulvestrant significantly improved both progression-free survival (PFS) and overall survival (OS) in patients with HR+, HER2- ABC with disease progression on prior endocrine therapy (ET). In MONARCH 3, the addition of abemaciclib to a nonsteroidal aromatase inhibitor (NSAI) significantly improved PFS (HR, 0.540; 95% CI, 0.418-0.698; p=0.000002) as initial therapy in HR+, HER2- ABC. At the last interim OS analysis (~252 events, at 5.8 years follow-up [FU]), a numerically favorable median OS difference (12.6 months) was observed (HR, 0.754; 95% CI, 0.584-0.974; p=0.0301, non-significant [NS]). Here, we present the final OS analysis of MONARCH 3 (NCT02246621). Methods: MONARCH 3 is a randomized, double-blind, Phase 3 study of abemaciclib + NSAI (anastrozole or letrozole) vs placebo + NSAI in postmenopausal women with HR+, HER2- ABC without prior systemic therapy in the advanced setting. OS was a gated secondary endpoint and chemotherapy-free survival (CFS) an exploratory endpoint. Final OS analysis was planned after approximately 315 OS events had occurred in the intent-to-treat (ITT) population with a split alpha between the ITT population and the subgroup with visceral disease (sVD) according to a prespecified graphical testing scheme. Kaplan-Meier method and stratified Cox proportional hazards models were used for time-to-event analyses. All reported p-values are two-sided. Results A total of 493 women were randomized 2:1 to receive abemaciclib + NSAI (n=328) or placebo + NSAI (n=165). After a median FU of 8.1 years, 7% of patients were still receiving treatment in the abemaciclib arm vs 3% in the placebo arm. In the ITT population, 314 OS events were observed (198 deaths among 328 patients [60%] in the abemaciclib arm and 116 among 165 [70%] in the placebo arm; HR, 0.804; 95% CI, 0.637-1.015; p=0.0664, NS). Median OS was 66.8 months in the abemaciclib arm and 53.7 months in the placebo arm, a numerical difference of 13.1 months in the ITT population. In the sVD, 178 events were observed (113 deaths among 173 patients [65%] in the abemaciclib arm and 65 among 90 [72%] in the placebo arm; HR, 0.758; 95% CI, 0.558-1.030; p=0.0757, NS). Median OS was 63.7 months in the abemaciclib arm and 48.8 months in the placebo arm, a numerical difference of 14.9 months in the sVD. Consistent OS differences were observed across prespecified subgroups. PFS benefit was sustained (median 29.0 vs 14.8 months; HR, 0.535; 95% CI, 0.429-0.668; nominal p<0.0001) with substantial difference in 6-year PFS rates (23.3% vs 4.3% for abemaciclib vs placebo). CFS was also improved with abemaciclib vs placebo (median 46.7 vs 30.6 months; HR, 0.693; 95% CI, 0.557-0.863; nominal p=0.0010). No new safety signals were observed with longer term use. Conclusion In patients with HR+, HER2- ABC, abemaciclib in combination with a NSAI resulted in numerically longer OS compared to NSAI alone; however, statistical significance was not reached after a median FU of 8.1 years. The clinically meaningful improvement in median OS (>13 months) combined with the sustained significant improvement in median PFS (>14 months) and substantial extension in median CFS (>16 months) continue to support the use of abemaciclib in combination with NSAI as first-line therapy in ABC. Citation Format: Matthew Goetz, Masakazu Toi, Jens Huober, Joo Hyuk Sohn, Olivier Trédan, Inhae Park, Mario Campone, Shin-Cheh Chen, Luis Manual Manso, Shani Paluch-Shimon, Orit C. Freedman, Valerie Andre, Abhijoy Saha, Gertjan van Hal, Ashwin Shahir, Hiroji Iwata, Stephen RD Johnston, Joyce O'Shaughnessy, Xavier Pivot, Sara Tolaney, Sara Hurvitz, Antonio Llombart. MONARCH 3: Final overall survival results of abemaciclib plus a nonsteroidal aromatase inhibitor as first-line therapy for HR+, HER2- 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 GS01-12.

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