India Pharma Outlook Team | Wednesday, 18 March 2026
Pfizer Inc. today reported positive topline findings from its Phase 2 FOURLIGHT-1 trial, marking a key milestone in the atirmociclib breast cancer study for patients with HR-positive HER2-negative metastatic breast cancer.
The study tested atirmociclib in combination with fulvestrant against fulvestrant alone or everolimus plus exemestane in patients previously treated with CDK4/6 inhibitors.
The trial achieved its primary endpoint, showing a statistically significant improvement in progression-free survival in breast cancer, with a hazard ratio of 0.60 and strong p-value significance. Results were consistent across all subgroups, including patients with different prior treatment durations, menopausal status, and presence of visceral disease. Notably, over 90 percent of patients started treatment within three months of prior CDK4/6 therapy, reflecting a real-world setting.
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“These results are especially encouraging given that the FOURLIGHT-1 study enrolled patients whose disease had progressed soon after prior CDK4/6 inhibitor therapy, a difficult-to-treat population,” said Jeff Legos, chief oncology officer, Pfizer.
He added, “The strength of these data reinforces our confidence that atirmociclib may meaningfully differentiate from the CDK4/6 inhibitor class, the standard-of-care backbone in HR-positive breast cancer, with the potential for improved efficacy and tolerability. We are continuing to accelerate development of this next-generation cell cycle inhibitor in earlier lines of therapy where it may offer even greater benefit for patients.”
The atirmociclib breast cancer study also showed a manageable safety profile, with only 6.4 percent discontinuation due to adverse events and no new safety concerns. Overall survival data remains immature, with about 20 percent of events reported.
Pfizer is advancing a Phase 3 trial and exploring earlier treatment use, aiming to strengthen outcomes in advanced breast cancer treatment and expand options for patients with limited therapies.
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