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.

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.

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.

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.

Arvinas and Pfizer's drug vepdegestrant reduced the risk of tumor progression 40% compared to standard treatment for an ESR1-mutated population.

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.

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.

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