An orally available pyridopyrimidine-derived cyclin-dependent kinase (CDK) inhibitor with potential antineoplastic activity. PD-0332991 selectively inhibits cyclin-dependent kinases (particularly Cdk4/cyclin D1 kinase), which may inhibit retinoblastoma (Rb) protein phosphorylation; inhibition of Rb phosphorylation prevents Rb-positive tumor cells from entering the S phase of the cell cycle (arrest in the G1 phase), resulting in suppression of DNA replication and decreased tumor cell proliferation.