A next-generation treatment for breast cancer from Pfizer Inc. and Arvinas Inc. posted disappointing results in a final-stage study, a potential setback for a drug Pfizer once called a blockbuster in the making.

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

The drug in question, called vepdegestrant, is an oral PROteolysis TArgeting Chimera (PROTAC) ER degrader. The phase 3 study assessed vepdegestrant against AstraZeneca’s hormone med Faslodex in 624 patients with ER+/HER2- advanced or metastatic breast cancer who had already tried CDK 4/6 inhibitors and endocrine therapy.

Vepdegestrant did not improve progression-free survival broadly but saw improvements in one specific patient cohort. Arvinas’ stock took a 43% tumble on the news, and analysts are downcast on the drug’s prospects.

Wedbush became the latest brokerage to downgrade Arvinas (NASDAQ:ARVN) on Wednesday after the company posted underwhelming late-stage trial data for vepdegestrant, an experimental breast cancer therapy it co-develops with Pfizer (NYSE:PFE).