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 (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.

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.

Pfizer paid a whopping $1 billion upfront for rights to vepdegestrant (formerly ARV-4710 in 2021 – consisting of $650 million in cash and a $350 million equity investment – with another $1.4 billion in backloaded milestone payments.

Trial results show vepdegestrant, a protein-degrading drug Arvinas is developing with Pfizer, improved on fulvestrant in a subgroup of breast cancer patients, but not the overall study population.

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.

The drug, vepdegestrant, failed to meaningfully delay cancer progression for all patients enrolled in the study but met its goals for a subset who have a specific genetic mutation

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).

Pharma giant Pfizer and Arvinas’s human epidermal growth factor receptor 2 (HER2) negative breast cancer therapy, vepdegestrant, has seen mixed results after it was only able to extend progression-free survival (PFS) in certain patients in a Phase III trial.

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