In vitro activity: PD 0332991 has little effect on other protein kinases including EGFR, FGFR, PGFR, IR. PD 0332991 is a non-ATP competitive inhibitor of Cdk4. PD 0332991 inhibits MDA-MB-435 breast carcinoma cells with IC50 of 66 nM, which is due to reduced Rb phosphorylation at Ser780. PD 0332991 inhibits thymidine incorporation into the DNA of Rb-positive human breast, colon, and lung carcinomas as well as human leukemias, with IC50 values ranging from 0.04-0.17 μM. PD 0332991 shows no activity in Rb-negative cells. PD 0332991 causes an accumulation of cells in G1 in MDA-MB-453 breast and Colo-205 carcinoma cells. PD 0332991 also shows activity in 5T33MM myeloma cells (immunocompetent model) and sensitizes the cells to killing by bortezomib. PD 0332991 inhibits luminal ER-positive as well as HER2-amplified breast cancer cell lines including MDA-MB-175, ZR-75-30, CAMA-1, MDA-MB-134, HCC-202 and UACC-893. PD 0332991 enhances the activity of tamoxifen and trastuzumab in these cell lines. PD 0332991 enhances the sensitivity of tamoxifen in the MCF7 tamoxifen-resistant cells. A recent study shows that PD 0332991 could suppress malignant rhabdoid tumor (MRT) cell lines including MP-MRT-AN, KP-MRT-RY, G401, KP-MRT-NS and the sensitivity of the MRT cell lines to PD 0332991 is inversely correlated with expression of p16.

Kinase Assay: A stock solution of PD0332991 is prepared in DMSO. CDK assays 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 (GST·RB-Cterm). The total volume in each well 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) or 12 μM ATP (for CDK2-cyclin E, CDK2-cyclin A, and CDC2-cyclin B) containing 0.25 μCi of [γ-32P]ATP, 20 ng of enzyme, 1 μg of GST·RB-Cterm, and PD 0332991 (0.001-0.1μM). All components except the [γ-32P]ATP are added to the wells, and the plate is placed on a plate mixer for 2 min. The reaction is started by adding the [γ-32P]ATP and the plate is incubated at 25 °C for 15 min. The reaction is terminated by addition of 0.1 mL of 20% trichloroacetic acid and the plate is kept at 4 °C for at least 1 hour to allow the substrate to precipitate. The wells are then washed 5 times with 0.2 mL of 10% trichloroacetic acid and radioactive incorporation is determined with a β plate counter.

Cell Assay: Cells (Tumor cell lines including MDA-MB-435, ZR-75-1, T-47D, MCF-7, H1299, Colo-205, MDA-MB-468, H2009, CRRF-CEM and K562) are seeded at 2 × 104 per well in a 96-well plate and incubated overnight. PD 0332991 (0.01-1 μM) is added to the wells and incubated at 37 °C for another 24 hours. [14C]Thymidine (0.1 μCi) is added to each well and incorporation of the radiolabel is allowed to proceed for 72 hours. Incorporated radioactivity is determined with a β plate counter.

Mol Cancer Ther. 2004 Nov;3(11):1427-38.