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.

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.

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 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.

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.