Pfizer shares jumped nearly 3 percent following the news.
The breakthrough designation is meant to speed up development and review of experimental treatments seen as big advances.
The drug, palbociclib, is being evaluated as an initial treatment for a subgroup that includes about 60 percent of postmenopausal women whose breast cancer is locally advanced or has spread elsewhere in the body. Their tumors are fueled by the hormone estrogen.
Pfizer is currently running a late-stage study of palbociclib, comparing its effects in combination with letrozole with the effects of letrozole alone. Letrozole, sold under the brand Femara, blocks production of estrogen.