By Deena Beasley
April 6 (Reuters) - Pfizer Inc's experimental breast cancer drug in a clinical trial nearly doubled the amount of time patients lived without their disease getting worse, but overall survival was not yet shown to be statistically significant, researchers said.
The Phase 2 study, which involved women with the most common form of breast cancer, found that those treated with hormone drug letrozole plus Pfizer's palbociclib lived for an average of 20.2 months before their cancer progressed, compared with 10.2 months for patients given letrozole alone.
The U.S. Food and Drug Administration has granted "breakthrough" status for palbociclib. Pfizer is still discussing a regulatory pathway for the drug and has not decided whether to seek accelerated approval based on Phase 2 trial results, said Mace Rothenberg, chief medical officer for Pfizer's oncology unit.
Palbociclib is viewed as one of the company's most important experimental drugs that some analysts believe could eventually claim annual sales of more than $5 billion, if approved by regulators.
The trial tested the pill, which targets proteins involved in cell division, in post-menopausal women with locally advanced or newly diagnosed breast cancer that had spread to other parts of the body.
The women had cancer that was both estrogen receptor positive - meaning tumors grow in response to estrogen - and HER2-negative, meaning that the HER2 protein is not causing the cancer. Such patients make up about 60 percent of advanced breast cancer cases.
The initial data showed overall survival of 37.5 months for the combination treatment, compared with 33.3 months for patients given just letrozole, an estrogen blocker sold by Novartis AG under the brand name Femara.
Researchers said that because only about 30 patients in each arm of the 165-patient trial had died it was still too early to define the drug's impact on survival.
"The curves are starting to separate ... It hasn't reached statistical significance, but patients are still being followed," said Dr. Richard Finn, associate professor of medicine at the University of California, Los Angeles, and a lead author of the study.
Side effects seen in the trial, including low blood cell counts and fatigue, were manageable, he said in a telephone interview ahead of the study's presentation on Sunday in San Diego at a meeting of the American Association for Cancer Research.
Hormonal agents, like Femara, have extended survival for women with estrogen-positive, HER2 negative breast cancer, but there have been no big advances in treatment for nearly two decades, said Dr Judy Garber, a breast cancer specialist at Boston's Dana-Farber Cancer Institute and AACR representative not involved in the palbociclib trial.
"This is garden variety breast cancer," she said. "When it recurs, all we really have are other hormonal agents ... This is the first new drug to really show promise."
Pfizer is conducting Phase 3 trials in breast cancer patients as well as earlier-stage trials in other types of cancer.
Companies trying to develop treatments similar to palbociclib include Novartis and Eli Lilly & Co, which presented Phase 1 data at the AACR meeting.
The small trial found that Lilly's drug, LY2835219, used on its own, shrank tumors in 25 percent of women with metastatic estrogen-positive breast cancer, and stabilized the cancer in 55 percent of the same group of women. (Reporting by Deena Beasley; Editing by Anthony Barker)