Fresh findings from the phase 3 postMONARCH study provided additional evidence regarding the tolerability and efficacy of combining abemaciclib with fulvestrant for patients with HR-positive, HER2-negative advanced breast cancer following disease progression on endocrine therapy and a CDK4/6 inhibitor.
While the initial deployment of CDK4/6 inhibitors alongside endocrine therapy has enhanced survival outcomes for individuals dealing with HR-positive, HER2-negative advanced breast cancer, “disease progression occurs in almost all patients,” wrote Kevin Kalinsky, MD, from Winship Cancer Institute at Emory University in Atlanta, Georgia, and colleagues.
A pressing clinical demand exists for safe and effective therapeutic options that can succeed initial regimens of CDK4/6 inhibitors combined with endocrine therapy while postponing the necessity for antibody-drug conjugates or chemotherapy. Abemaciclib is an oral CDK4/6 inhibitor with greater selectivity for CDK4 than CDK6, allowing continuous dosing.
Previous trials have shown that abemaciclib plus endocrine therapy improved progression-free survival and overall survival for patients with advanced breast cancer.
Phase 3 Trial Design
The postMONARCH investigation functioned as an international, double-blind, randomized, placebo-controlled phase 3 study that enrolled 368 participants diagnosed with HR-positive, HER2-negative advanced breast cancer whose disease had advanced despite prior CDK4/6 inhibition and endocrine therapy. Patients were randomly assigned to receive either abemaciclib plus fulvestrant or placebo plus fulvestrant.
Earlier evaluations from postMONARCH showed a statistically significant enhancement in progression-free survival for the abemaciclib and fulvestrant combination (median PFS, 6.0 months; 6-month PFS rate, 50%) relative to the placebo and fulvestrant group (median PFS, 5.3 months; 6-month PFS rate, 37%).
The research investigators now present findings from the ultimate overall survival assessment, which had been pre-established to occur upon reaching 200 overall survival events utilizing a hazard ratio threshold of 0.753.
Abemaciclib-Fulvestrant Shows Numerical Trend Toward Longer Survival
The median follow-up was duration was approximately 37 months. Among participants, the median overall survival reached 32.1 months in the cohort given abemaciclib alongside fulvestrant compared to 24.6 months in the control arm receiving placebo plus fulvestrant (HR, 0.80; P=0.11).
Even though statistical significance was not met, an observable numerical tendency favored the abemaciclib-fulvestrant regimen, demonstrating a 20% decrease in mortality risk. Correspondingly, median overall survival calculations appeared numerically elevated for abemaciclib-fuvestrant at 32.1 months when weighed against the 24.6 months observed in the placebo-fulvestrant group.
The difference in OS was consistent across subgroups.
Treatment Considerations
Dr. Kalinsky and colleagues determined, “The totality of evidence supports the consideration of abemaciclib-based treatment [for] patients with hormone receptor-positive, HER2-negative ABC [advanced breast cancer] after disease progression on CDK4/6i [inhibitor] plus ET [endocrine therapy].”
Clinical Trial Questions
What was the primary trial design for postMONARCH?
The evaluation took the form of an international, double-blind, randomized, placebo-controlled phase 3 study encompassing 368 subjects suffering from HR-positive, HER2-negative advanced breast cancer who experienced disease progression after undergoing CDK4/6 inhibition combined with endocrine therapy.
How many patients received abemaciclib in the study?
A total of 182 patients were randomly assigned to receive abemaciclib plus fulvestrant, while 186 patients received placebo plus fulvestrant.
What was the final overall survival duration in the treatment arm?
The median overall survival was 32.1 months in the abemaciclib-fulvestrant arm compared to 24.6 months in the placebo-fulvestrant arm, with a median follow-up duration of approximately 37 months.
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