08:01 AM EDT, 03/17/2026 (MT Newswires) -- Pfizer ( PFE ) said Tuesday that its experimental breast cancer drug atirmociclib has met the primary endpoint in a mid-stage study of patients with hormone receptor-positive, HER2-negative advanced or metastatic breast cancer who had previously received CDK4/6 inhibitor treatment.

In the trial, patients who received atirmociclib with fulvestrant had their cancer under control for longer than those who received other treatment options, the company said.

Pfizer ( PFE ) said the study enrolled 264 patients across 14 countries. The company said the drug was generally well tolerated, with more than 6% of patients stopping treatment due to side effects and no new safety concerns reported.

Pfizer ( PFE ) said overall survival, a secondary endpoint, is still immature. The company said it is continuing to accelerate development of the drug.