Palbociclib (PD-0332991) is an orally available pyridopyrimidine-derived cyclin-dependent kinase (CDK) inhibitor with potential antineoplastic activity. Palbociclib is a highly specific inhibitor of cyclindependent kinase 4 (Cdk4) (IC50, 0.011 μmol/L) and Cdk6 (IC50, 0.016 μmol/L), having no activity against a panel of 36 additional protein kinases.