Palbociclib (PD-0332991) HCl is a highly selective inhibitor of CDK4/6 with IC50 of 11 nM/16 nM respectively. It shows no activity against CDK1/2/5, EGFR, FGFR, PDGFR, InsR, etc. 

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.