Palbociclib, also known as PD0332991, is an orally available pyridopyrimidine-derived cyclin-dependent kinase inhibitor with potential antineoplastic activity. The compound was developed by Pfizer under the internal code PF-00080665 before advancing through clinical evaluation for breast cancer. The isethionate salt form is registered separately from the free base and hydrochloride forms, with the free base at CAS 571190-30-2 and the isethionate at CAS 827022-33-3. The isethionate salt was selected for clinical development primarily for solubility and formulation reasons rather than altered pharmacology.
Application of PD 0332991 isethionate
Palbociclib isethionate has potent anti-proliferative activity and induces cell cycle arrest in cancer cells, which makes it useful in research on HR-positive, HER2-negative breast cancer and hepatocellular carcinoma. Researchers use it specifically to probe G1-to-S phase transition control, since the compound's mechanism depends on an intact, functional retinoblastoma protein pathway – a dependency that itself serves as a biomarker for predicting tumor sensitivity in experimental models.
In Vitro
Palbociclib isethionate is a highly selective inhibitor of CDK4 and CDK6, with IC50 values of 11 nM and 16 nM respectively in cell-free assays, and shows no activity against CDK1, CDK2, CDK5, EGFR, FGFR, PDGFR, or InsR. In cell line proliferation assays, IC50 values for PD-0332991 ranged from 25.0 nM to 700 nM, with the compound inducing G0/G1 cell-cycle arrest, late apoptosis, and blockade of Rb phosphorylation. palbociclib isethionate CAS 827022-33-3 sensitivity correlated with loss of p16 and p15 tumor suppressors and with E2F1 expression status across tested cell lines – a pattern that has informed biomarker-driven patient selection strategies in later clinical work.
In Vivo
Oral administration of PD 0332991 to mice bearing Colo-205 human colon carcinoma produced marked tumor regression, with therapeutic doses eliminating phospho-Rb and the proliferative marker Ki-67 in tumor tissue. Down-regulation of genes under E2F transcriptional control accompanied tumor response, indicating that CDK4/6 inhibition alone is sufficient to drive tumor regression and reduce tumor burden in sensitive models.
Biochemical and Physiological Actions
PD 0332991 isethionate selectively inhibits CDK4/cyclin D1 kinase activity, blocking retinoblastoma protein phosphorylation; this prevents Rb-positive tumor cells from progressing into S phase and arrests them in G1, suppressing DNA replication and tumor cell proliferation. Because hypophosphorylated Rb binds and sequesters E2F transcription factors, the downstream effect cascades into reduced expression of S-phase genes required for DNA synthesis – the molecular basis for the durable cell-cycle arrest seen across multiple tumor types.
Features and Benefits of PD 0332991 isethionate
The compound's principal advantage as a research tool is its narrow target profile: selective CDK4/6 inhibition without measurable activity against a panel of 36 additional kinases gives experiments a clean pharmacological readout, free from confounding off-target effects. Combined with its oral bioavailability and well-characterized Rb-dependent mechanism, the compound remains a benchmark reference for studies of cell-cycle regulation and CDK4/6-targeted therapeutic strategies.
