Pfizer (NYSE: PFE) reported Phase II results for atirmociclib in HR-positive, HER2-negative metastatic breast cancer, with the FOURLIGHT-1 trial meeting its primary endpoint by demonstrating a 40% reduction in the risk of disease progression or death compared to standard-of-care options. The data mark the first randomized Phase II readout for a selective CDK4 inhibitor in patients whose disease has progressed on prior CDK4/6 inhibitor therapy, a population with limited treatment options and no established targeted retreatment strategy.
FOURLIGHT-1 is an open-label, randomized, multicenter Phase II study that enrolled 264 patients across 14 countries with HR-positive, HER2-negative advanced or metastatic breast cancer who had progressed after CDK4/6 inhibitor-based treatment. Patients were randomized to atirmociclib plus fulvestrant versus physician's choice of fulvestrant alone or everolimus plus exemestane. The primary endpoint was investigator-assessed progression-free survival.
Atirmociclib plus fulvestrant reduced the risk of progression or death by 40% versus control, with a hazard ratio of 0.60 (95% CI: 0.440–0.825; p=0.0007). The benefit was consistent across prespecified subgroups, including patients with visceral disease, those with prior CDK4/6 inhibitor treatment lasting less than or more than 12 months, and regardless of which CDK4/6 inhibitor was previously received. More than 90% of enrolled patients initiated atirmociclib within three months of their last CDK4/6 inhibitor treatment, indicating a population with recent disease progression rather than a prolonged treatment-free interval. Overall survival data were not mature, with approximately 20% of participants having had an event at the time of analysis.
The safety profile was consistent with prior studies of atirmociclib. Discontinuation due to treatment-emergent adverse events occurred in 6.4% of patients on the atirmociclib arm, and no new safety signals were identified, according to the company. Detailed safety data were not disclosed and are expected at a future medical meeting.
Atirmociclib is an oral, selective inhibitor of cyclin-dependent kinase 4 (CDK4), a protein that drives the transition from G1 to S phase of the cell cycle. Unlike currently approved CDK4/6 inhibitors such as palbociclib, ribociclib, and abemaciclib, which inhibit both CDK4 and CDK6, atirmociclib targets CDK4 alone. The rationale for selective CDK4 inhibition centers on the hypothesis that sparing CDK6 may reduce hematologic toxicity, particularly neutropenia, which is the dose-limiting side effect of dual CDK4/6 inhibitors. Whether CDK4 selectivity translates into a clinically distinct efficacy or tolerability profile remains to be established in larger trials.