Abemaciclib Plus Fulvestrant in Advanced Breast Cancer After Progression on CDK4/6 Inhibition: Results From the Phase III postMONARCH Trial

PURPOSE Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy (ET) are the standard first-line treatment for hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2–) advanced breast cancer (ABC); however, disease progression occurs in almost all patients and additional treatment options are needed. Herein, we report outcomes of the postMONARCH trial investigating a switch in ET with/without CDK4/6 inhibition with abemaciclib after disease progression on CDK4/6i. METHODS This double-blind, randomized phase III study enrolled patients with disease progression on previous CDK4/6i plus aromatase inhibitor as initial therapy for advanced disease or recurrence on/after adjuvant CDK4/6i + ET. Patients were randomly assigned (1:1) to abemaciclib + fulvestrant or placebo + fulvestrant. The primary end point was investigator-assessed progression-free survival (PFS). Secondary end points included PFS by blinded independent central review, objective response rate (ORR), and safety. RESULTS This study randomly assigned 368 patients (abemaciclib + fulvestrant, n = 182 placebo + fulvestrant, n = 186). At the primary analysis (258 events), the hazard ratio (HR) was 0.73 (95% CI, 0.57 to 0.95; nominal P = .017), with median PFS 6.0 (95% CI, 5.6 to 8.6) versus 5.3 (95% CI, 3.7 to 5.6) months and 6-month PFS rates of 50% and 37% in the abemaciclib + fulvestrant and placebo + fulvestrant arms, respectively. These results were supported by BICR-assessed PFS (HR, 0.55 95% CI, 0.39 to 0.77; nominal P < .001). A consistent treatment effect was seen across major clinical and genomic subgroups, including with/without ESR1 or PIK3CA mutations. Among patients with measurable disease, investigator-assessed ORR was improved with abemaciclib + fulvestrant versus placebo + fulvestrant (17% v 7%; nominal P = .015). No new safety signals were observed, with findings consistent with the known safety profile of abemaciclib. CONCLUSION Abemaciclib + fulvestrant significantly improved PFS after disease progression on previous CDK4/6i + ET in patients with HR+, HER2– ABC, offering an additional targeted therapy option for these patients.