Studies involving an experimental, new drug reveal it helped stymie tumor growth.
Drugmaker Pfizer says palbociclib, when combined with letrozole, increased the time patients survived without tumors growing.
The drug is intended for postmenopausal women with certain kinds of tumors found in about 60 percent of patients.
Pfizer says it will release results of the study at the American Association for Cancer Research meeting in early April.
The company is now enrolling patients in a late-stage test of the drug.
Last April, the Food and Drug Administration designated it as a breakthrough therapy, helping speed up the review needed for approval and public release.