(Reuters) -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 oral drug, vepdegestrant, however, helped improve progression-free survival in a subgroup of patients with a genetic form of the most common type of breast cancer.

The companies were testing the drug against AstraZeneca's Faslodex in patients whose cancer progressed despite receiving hormone-based therapy before.

The late-stage study enrolled 624 previously treated patients with breast cancer, classified as estrogen-receptor positive, human epidermal growth factor receptor 2-negative (ER+/HER2-), which accounts for nearly 70% of all breast cancers.

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

Vepdegestrant was generally well tolerated and its safety profile was consistent with what has been observed in previous studies, the companies said.

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

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

(Reporting by Mariam Sunny in Bengaluru; Editing by Shilpi Majumdar)