Arvinas (ARVN) and Pfizer (PFE) said Tuesday that a phase 3 clinical trial of vepdegestrant monotherapy as an investigational treatment for advanced or metastatic breast cancer met its primary endpoint in the estrogen receptor 1-mutant population, but did not reach statistical significance in progression-free survival improvement in the intent-to-treat population.

The study is evaluating vepdegestrant versus fulvestrant in adults with estrogen receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer.

The companies said vepdegestrant showed a statistically significant and clinically meaningful improvement in progression-free survival compared with fulvestrant in patients whose tumors harbor ESR1 mutations, with the results exceeding the pre-specified target hazard ratio of 0.60.

The trial continues to assess overall survival as a secondary endpoint, the companies said.

Shares of Arvinas were down 43% in recent premarket activity while Pfizer stock was marginally lower.