In vitro activity: PD 0332991 exhibits absolute selectivity for CDK4/6 with little or no activity against other CDKs. PD 0332991 is effective at reducing Rb phosphorylation at Ser780 and Ser795 in MDA-MB-435 breast carcinoma cells with IC50 of 66 nM and 63 nM, respectively. PD 0332991 is a potent inhibitor of cell growth and suppresses DNA replication by preventing cells from entering S phase. PD 0332991 inhibits thymidine incorporation into the DNA of Rb-positive human breast (such as MDA-MB-435, MCF-7), colon (H1299), and lung carcinomas (Colo-205) as well as human leukemias (CRRF-CEM and K562), with IC50 values ranging from 0.04-0.17 μM. PD 0332991 significant increases the percentage of MDA-MB-453 in G1 period. PD 0332991 inhibits phosphorylation of Rb in cycling CD138+ primary bone marrow myeloma cells, nontransformed primary B cells, MM1.S and CAG HMCLs cells line with IC50 of<0.1 μM, 0.05 μM, and 60-70 nM, respectively. PD 0332991 treatment also induces G1 arrest of CD138+ primary bone marrow myeloma and nontransformed primary B cells. PD 0332991 induces G1 arrest in MM1.S with IC50 of ~0.05 μM. PD 0332991 preferentially inhibits proliferation of luminal estrogen receptor-positive (including HER2-positive) human breast cancer cell lines. PD 0332991 increases gene expression of pRb and cyclin D1 and decreases gene expression of CDKN2A (p16) in most sensitive lines. PD 0332991 enhances sensitivity to tamoxifen in cell lines with conditioned resistance to ER blockade.

Kinase Assay: CDK assays for IC50 determinations and kinetic evaluation are performed in 96-well filter plates. All CDK-cyclin kinase complexes are expressed in insect cells through baculovirus infection and purified. The substrate for the assays is a fragment (amino acids 792–928) of pRb fused to GST (GSToRB-Cterm). The total reaction volume is 0.1 mL containing a final concentration of 20 mM Tris-HCl, pH 7.4, 50 mM NaCl, 1 mM dithiothreitol, 10 mM MgCl2 25 μM ATP (for CDK4-cyclin D1, CDK6-cyclin D2, and CDK6-cyclin D3) containing 0.25 μCi of [γ-32P]ATP, 20 ng of enzyme, 1 μg of GSToRB-Cterm, and appropriate dilutions of inhibitor. All components except the [γ-32P]ATP are added to the wells, and placed on a plate mixer for 2 min. The reaction is started by adding the [γ-32P]ATP, and incubated at 25℃ for 15 min. The reaction is terminated by addition of 0.1 mL of 20% trichloroacetic acid, and the plate is kept at 4 ℃ for at least 1 hr to allow the substrate to precipitate. The wells are then washed five times with 0.2 mL of 10% trichloroacetic acid, and radioactive incorporation is determined with a β plate counter.

Cell Assay: Cells (Human breast cancer cells MDA-MB-435) are seeded in duplicate at 5,000 to 10,000 cells per well in 24-well plates. The day after plating, different concentrations of PD 0332991 are added. Control wells without drug are also seeded. At the end of incubation, cells are trypsinizated and placed in Isotone solution and counted immediately using a Coulter Z2 particle counter.

Mol Cancer Ther. 2004 Nov;3(11):1427-38; Cancer Res. 2006 Aug 1;66(15):7661-7; Breast Cancer Res. 2009;11(5):R77. doi: 10.1186/bcr2419.