LBA1001 Background: The combination of CDK4/6 inhibitors (CDK4/6i) + endocrine therapy (ET) is the standard first line treatment for HR+, HER2- advanced breast cancer (ABC). While disease progression occurs in nearly all patients (pts) with ABC, the optimal treatment for pts who experience progression on a CDK4/6i + ET remains uncertain. Real-world evidence suggests that use of abemaciclib after disease progression on a prior CDK4/6i prolongs progression-free survival (PFS) in ABC; however, Phase 2 trials with other CDK4/6i have generated mixed results. Here we present the primary outcome analysis for the Phase 3 postMONARCH trial (NCT05169567) of fulvestrant + abemaciclib or placebo in pts with HR+, HER2- ABC following disease progression on prior CDK4/6i + ET. Methods: postMONARCH was a global, double-blind, placebo-controlled study with pts randomized 1:1 to abemaciclib + fulvestrant or placebo + fulvestrant. Eligible pts had disease progression on a CDK4/6i + AI as initial therapy for ABC or relapse on/after a CDK4/6i + ET as adjuvant therapy for early breast cancer. No other prior treatment for ABC was permitted. Primary endpoint was investigator-assessed PFS; secondary endpoints included PFS by blinded independent central review (BICR), overall survival (OS), objective response rate (ORR), and safety. Assuming a hazard ratio (HR) of 0.7, the study had ~80% power to detect superiority for abemaciclib, with a cumulative 2-sided type I error of 0.05. Kaplan-Meier method was used to estimate PFS curves and treatment effect was estimated using a stratified Cox proportional hazard model. Results: A total of 368 pts were randomized to abemaciclib + fulvestrant (n = 182) or placebo + fulvestrant (n= 186). Most pts (99%) enrolled directly after CDK4/6i + ET as initial therapy for ABC. Prior CDK4/6i was 59% palbociclib, 33% ribociclib, and 8% abemaciclib. At interim analysis, the study reached the pre-specified criteria for significantly improved investigator-assessed PFS with abemaciclib + fulvestrant compared to placebo + fulvestrant (169 events, HR = 0.66; 95% CI 0.48 – 0.91; p = 0.01). At primary analysis (258 events), the HR was 0.73 (95% CI 0.57 – 0.95), with PFS rates at 6 months of 50% vs 37% for the abemaciclib and placebo arms, respectively. Consistent effect was seen across major clinical and genomic subgroups, including pts with baseline ESR1 or PIK3CA mutations. ORR was improved with abemaciclib compared to placebo (17% vs 7%, respectively, in pts with measurable disease). PFS according to BICR was also improved with HR = 0.55 (95% CI 0.39 - 0.77). OS remains immature (20.9% event rate). Safety was consistent with the known profile of abemaciclib. Conclusions: Abemaciclib + fulvestrant demonstrated statistically significant PFS improvement in pts with ABC progression on prior CDK4/6i-containing therapy. Clinical trial information: NCT05169567 .

Abemaciclib plus fulvestrant vs fulvestrant alone for HR+, HER2- advanced breast cancer following progression on a prior CDK4/6 inhibitor plus endocrine therapy: Primary outcome of the phase 3 postMONARCH trial.