Pfizer Inc. announced Monday that its experimental breast cancer drug met its primary endpoint during a mid-stage trial, maintaining the drugmaker's lead "in the race for a new treatment standard for the disease," Reuters reported.

During the Phase 2 trial, the oral drug, palbociclib, significantly improved patient survival rates without their condition worsening. Analysts believe the treatment could have annual sales of more than $5 billion, Reuters reported.

The trial suggests "the potential for palbociclib to transform the standard of care for post-menopausal women with ER+ and HER2- advanced breast cancer," Mace Rothenberg, Pfizer Oncology's chief medical officer, said in a statement. "This is encouraging information for these women, who represent approximately 60 percent of the advanced breast cancer population."

The trial compared the effects of a combination treatment of palbociclib and letrozole with letrozole alone in post-menopausal women.

Pfizer has started two Phase 3 studies of palbociclib in patients with advanced and/or metastatic breast cancer.

The drug works by blocking two enzymes, cyclin dependent kinases (CDK) 4 and 6, which enable tumor cell growth.

The U.S. Food and Drug Administration has granted palbociclib "breakthrough" status in April based on initial data from the trial, indicating that the drug offers "substantial improvement over existing treatments for serious life-threatening disease," Reuters reported.

Drug companies working to develop similar treatments include Novartis AG and Eli Lilly & Co., Reuters reported.

Breast cancer killed more than 500,000 women worldwide in 2011, according to the World Health Organization.