An experimental breast cancer treatment co-developed by Pfizer and Arvinas significantly improved the time some patients lived without the disease worsening, but failed to achieve statistically significant results in others in a late-stage study.

Pfizer Inc.’s next-generation treatment for breast cancer posted mixed results in a final-stage study, a potential setback for a drug the company once called a blockbuster in the making.

Pfizer (NYSE:PFE) and its partner Arvinas (NASDAQ:ARVN) for the experimental breast cancer drug vepdegestrant announced mixed initial results from a Phase 3 trial for the protein degrader on Tuesday.

Pfizer and Arvinas' experimental breast cancer treatment failed to delay the progression of the disease in a broader group of patients in a late-stage trial, sending the Connecticut-based biotech's shares down 36% before the bell.

Arvinas stock lost half its value Tuesday after the biotech company reported mixed results from its Pfizer-partnered breast cancer drug.