Pfizer’s Next-Gen CDK4 Inhibitor Shows Phase 2 Breast Cancer Benefit

Pfizer Inc. has reported positive topline results from the Phase 2 FOURLIGHT-1 trial (NCT06105632) evaluating the investigational CDK4 inhibitor atirmociclib in combination with fulvestrant for patients with hormone receptor (HR)-positive, HER2-negative advanced or metastatic breast cancer whose disease progressed following prior CDK4/6 inhibitor therapy.

The randomized study met its primary endpoint, demonstrating a statistically significant and clinically meaningful improvement in progression-free survival (PFS) compared with control treatment arms consisting of fulvestrant alone or everolimus plus exemestane. Investigator-assessed results showed a hazard ratio of 0.60 (95% CI: 0.440–0.825; p=0.0007), indicating a 40% reduction in the risk of disease progression or death.

The PFS benefit was consistent across all prespecified patient subgroups, including menopausal status, performance status, presence of visceral metastases, duration of prior CDK4/6 inhibitor treatment, and the type of prior CDK4/6 therapy received (with >90% of patients initiating atirmociclib within three months of their last CDK4/6 regimen).

Overall survival (OS), a key secondary endpoint, remains immature, with only around 20% of events recorded at the time of analysis. The data represent the first randomized Phase 2 evidence supporting atirmociclib, an investigational and potentially first-in-class selective CDK4 inhibitor, in HR-positive metastatic breast cancer.

According to Jeff Legos, Chief Oncology Officer at Pfizer, the findings are particularly encouraging because the study population included patients whose disease progressed soon after 1L CDK4/6 inhibitor treatment, a setting where therapeutic options are limited. He noted that the results reinforce confidence that atirmociclib could differentiate from existing CDK4/6 inhibitors by potentially delivering improved efficacy and tolerability.

The safety profile observed in FOURLIGHT-1 was manageable and consistent with earlier studies, with only 6.4% of patients discontinuing treatment due to adverse events and no new safety signals identified. Detailed trial results are expected to be presented at a future medical meeting.

The FOURLIGHT-1 trial enrolled 264 patients across 14 countries with HR-positive, HER2-negative advanced or metastatic breast cancer previously treated with CDK4/6 inhibitor-based therapy. Beyond PFS, the study is evaluating secondary endpoints including overall survival, objective response rate, duration of response, and clinical benefit rate.

Based on these findings, Pfizer is advancing atirmociclib into earlier treatment settings. A Phase 3 registrational study in the first-line metastatic setting is currently ongoing, and additional Phase 2 neoadjuvant data in early breast cancer are expected to be shared at an upcoming scientific meeting.

Reference

Pfizer Announces Positive Topline Phase 2 Results for Next-Generation CDK4 Inhibitor, Atirmociclib, in Second-Line Metastatic Breast Cancer, 17 March 2026, Pfizer Announces Positive Topline Phase 2 Results for Next-Generation CDK4 Inhibitor, Atirmociclib, in Second-Line Metastatic Breast Cancer | Pfizer

A Study to Learn About the Study Medicine Called PF-07220060 in Combination With Fulvestrant in People With HR-positive, HER2-negative Advanced or Metastatic Breast Cancer Who Progressed After a Prior Line of Treatment, ClinicalTrials.gov ID NCT06105632, https://clinicaltrials.gov/study/NCT06105632