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.

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.

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.

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).

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