The addition of abemaciclib to fulvestrant led to a significant improvement in progression-free survival (PFS) versus fulvestrant alone in patients with advanced HR⁺/HER2^–negative breast cancer who had disease progression on a CDK4/6 inhibitor and endocrine therapy, according to data from the phase 3 postMONARCH study.

The combination of a CDK4/6 inhibitor and endocrine therapy is a first-line treatment option for patients with advanced, HR⁺/HER2^– breast cancer. While disease progression occurs in nearly all patients, the optimal treatment for patients who experience progression on a CDK4/6 inhibitor plus endocrine therapy remains uncertain.

Phase 3 postMONARCH trial (NCT05169567) explored the effectiveness of fulvestrant plus abemaciclib versus placebo in patients with advanced HR+/HER2- breast cancer who progressed on first-line treatment with a CDK4/6 inhibitor plus endocrine therapy. The results were presented by Dr. Kevin Kalinsky, MD, MS, from Emory University, in Georgia¹.

In postMONARCH, 368 patients who showed disease progression on first-line therapy with any CDK4/6 inhibitor plus endocrine were 1:1 randomly assigned to treatment with abemaciclib plus fulvestrant or placebo plus fulvestrant. The primary endpoint was investigator-assessed PFS, and secondary endpoints included PFS by blinded independent central review (BICR).

Primary analysis showed significantly improved median PFS in patients treated with abemaciclib plus fulvestrant versus fulvestrant alone: 6.0 (5.6-8.6) months versus 5.3 (3.7-5.6) months (HR 0.73 [0.57-0.95]; P=0.02). In other words, adding abemaciclib led to a 27% reduction in the risk for developing a PFS event. The Benefit of abemaciclib was observed in patients with no visceral metastases (HR 0.53 [0.34-0.8]).

Data from the secondary analysis showed that abemaciclib improved PFS over placebo when assessed by BICR, as well. The median PFS with abemaciclib/fulvestrant was 12.9 months (95% CI: 9.5-not reached) versus 5.6 months (95% CI: 3.9-.7.7) with fulvestrant alone (HR 0.55 [0.39-0.77]; P=0.0004). In other words, adding abemaciclib led to a 45% reduction in the risk for developing a PFS event. The 6- month PFS rates in the abemaciclib and placebo arms were 68% and 45%, respectively. Safety data observed in the study were consistent with what has previously been reported with abemaciclib

“Abemaciclib plus fulvestrant offers a targeted therapy option after disease progression on a CDK4/6 inhibitor for patients with HR⁺/HER2^– advanced breast cancer,” concluded Dr. Kalinsky.

Medical writing support was provided by Marten Dooper, PhD.
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