Arvinas shares tumble as breast cancer drug with Pfizer shows mixed results

By Mariam Sunny

March 11 (Reuters) - Pfizer PFE.N and Arvinas' ARVN.O experimental treatment failed to delay progression of breast cancer in patients with most common type of the disease in a late-stage trial, sending the smaller biotech's shares tumbling 53% to an all-time low.

The oral drug, vepdegestrant, failed to show benefit in a larger set of patients, but helped to extend progression-free survival in a subgroup of participants with a specific gene mutation, the companies said on Tuesday.

They were testing the drug against AstraZeneca's AZN.L Faslodex in patients whose cancer progressed despite prior treatment, including hormone-based therapy.

The data means that use of the drug, expected to become a blockbuster if approved, could be sharply limited to those patients with the gene mutation.

The benefit in the subgroup was anticipated, making the failure in the larger set of patients a "more meaningful let down", BMO Capital analyst Evan Seigerman said.

However, the population still represents a "sizable opportunity", Seigerman said. He expects peak global sales of $2.9 billion in this population alone.

Vepdegestrant belongs to a novel class of drugs called PROTAC ER degraders, which are designed to harness the body's natural protein disposal system to specifically target and degrade proteins that spur tumor growth.

Breast cancer accounts for about one-third of all new female cancers each year in the U.S., according to the American Cancer Society.

The late-stage study enrolled 624 previously treated patients with a type of breast cancer that accounts for nearly 70% of all such cancers.

Approved treatments for this type of advanced breast cancer include Eli Lilly's LLY.N Verzenio, Pfizer's PFE.N Ibrance and Novartis' NOVN.S Kisqali.

The companies said they would share detailed data with health authorities to potentially support regulatory filings, as well as at a medical conference later this year.

Shares of Arvinas slumped 53% to $8.24, while Pfizer fell nearly 2% to $26.12.