Pfizer Inc.’s next-generation treatment for breast cancer posted mixed results in a final-stage study, a potential setback for a drug the company once called a blockbuster in the making.

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.

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.

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.