- Palbociclib is a drug that may stop cancer cells from growing. Palbociclib blocks activity of two closely related enzymes (proteins that help chemical reactions in the body occur), called Cyclin D Kinases 4 and 6 (CDK 4/6). These proteins are part of a pathway, or a sequence of steps which is known to regulate cell growth. Laboratory testing has suggested palbociclib may stop the growth of hormone receptor positive breast cancer.