Pfizer Reports Positive Phase 2 FOURLIGHT-1 Trial Results For Atirmociclib in HR+/HER2- Metastatic Breast Cancer

Pfizer Inc. announced positive topline findings from the Phase 2 FOURLIGHT-1 trial evaluating atirmociclib in combination with fulvestrant for patients with hormone receptor–positive, HER2-negative advanced or metastatic breast cancer previously treated with CDK4/6 inhibitor–based therapy. The randomised study compared the combination against fulvestrant alone and everolimus plus exemestane. It met its primary endpoint, showing a statistically significant and clinically meaningful improvement in progression-free survival, with a hazard ratio of 0.60 (95% CI: 0.440–0.825; p=0.0007).

Progression-free survival benefits were consistent across all prespecified patient subgroups. These included differences in performance status, menopausal status, presence of visceral metastases, duration of prior CDK4/6 inhibitor therapy (less than or greater than 12 months), and the specific CDK4/6 inhibitor previously used. Notably, more than 90% of participants began atirmociclib within three months of completing prior CDK4/6 treatment, reflecting real-world treatment sequencing.

Overall survival, a secondary endpoint, remains immature, with only about one-fifth of participants experiencing events at the time of analysis. FOURLIGHT-1 represents the first randomized Phase 2 study of atirmociclib in HR-positive metastatic breast cancer. The investigational therapy is being developed as a potential first-in-class CDK4 inhibitor.

“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. “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 combination regimen demonstrated a manageable and generally well-tolerated safety profile. Treatment-emergent adverse events led to discontinuation in 6.4% of patients receiving atirmociclib. Safety findings were consistent with earlier studies, and no new safety concerns emerged. Detailed results are planned for presentation at an upcoming scientific meeting.

These results reinforce Pfizer’s development strategy to advance atirmociclib into earlier lines of treatment, where endocrine-based regimens may provide longer disease control. A Phase 3 registrational trial in the first-line metastatic setting is currently underway, and findings from a Phase 2 neoadjuvant study in early breast cancer are expected to be presented at a future conference.