Palbociclib is a member of the class of pyridopyrimidines that is 2-{[5-(piperazin-1-yl)pyridin-2-yl]amino}pyrido[2,3-d]pyrimidin-7-one bearing additional methyl, acetyl and cyclopentyl substituents at positions 5, 6 and 8 respectively. It is used in combination with letrozole for the treatment of metastatic breast cancer. It has a role as an EC 2.7.11.22 (cyclin-dependent kinase) inhibitor and an antineoplastic agent. It is a pyridopyrimidine, an aminopyridine, a secondary amino compound, a member of piperidines, an aromatic ketone, a member of cyclopentanes and a tertiary amino compound.