A drug used to treat hormone-receptor positive (HR+), HER2 negative (HER2-) breast cancer that is advanced or has spread to other parts of the body. It is used with fulvestrant in women whose disease has gotten worse after treatment with hormone therapy. It is used with letrozole in postmenopausal women who have not been treated with hormone therapy. It is also being studied in the treatment of other types of cancer. Palbociclib blocks certain proteins, which may help keep cancer cells from growing. It is a type of cyclin-dependent kinase inhibitor. Also called Ibrance.