Experimental breast cancer drugs from Pfizer and Eli Lilly slowed or halted growth of advanced tumors in studies, potentially adding a new treatment option for patients and top-selling therapies for the companies.

Pfizer’s palbociclib doubled to 20.2 months the time in which patients’ advanced breast cancer didn’t progress, the drugmaker said on Sunday.

In an unrelated study, Lilly’s drug, bemaciclib, aided 61 percent of patients with metastatic, hormone-sensitive breast cancer, meaning the size of their tumors shrank by 30 percent or didn’t increase for 24 weeks.

Doctors and investors have been awaiting results for the medicines, part of a class called cyclin-dependent kinase 4/6 inhibitors. The therapies offer a new treatment avenue for about 80 percent of patients whose breast tumors are hormone receptor-positive.

“We took a new drug and asked how it works in a specific set of breast cancers based on science,” said Richard Finn, an associate professor at the University of California at Los Angeles’ Jonsson Comprehensive Cancer Center, and co-principal investigator of Pfizer’s study. “This is really the type of benefit we’re aiming for, and it’s the type of benefit that results from a rational development program.”

Breast cancer is the most common form of cancer in women, killing an estimated 40,000 a year, according to the National Cancer Institute.

Hormone receptor-positive forms of the disease are typically treated by blocking hormone production in the body. When that doesn’t work, the tumor cells can begin to grow and spread, requiring chemotherapy to kill them.

Patients “don’t often do well with chemotherapy,” said Judy Garber, an oncologist at the Dana-Farber Cancer Institute in Boston and a past president of the American Association for Cancer Research. “You often feel like you’re given more and more toxicity and less and less activity.”

The CDK drugs are a promising option, she said.

The results of both studies were released on Sunday at the American Association for Cancer Research meeting in San Diego.