Palbociclib exhibits absolute selectivity for CDK4/6 with little or no activity against other CDKs. Palbociclib is effective at reducing Rb phosphorylation at Ser and Ser in MDA-MB-435 breast carcinoma cells with IC50 of 66 nM and 63 nM, respectively. Palbociclib is a potent inhibitor of cell growth and suppresses DNA replication by preventing cells from entering S phase. Palbociclib 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. Palbociclib significant increases the percentage of MDA-MB-453 in G1 period. Palbociclib 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. Palbociclib treatment also induces G1 arrest of CD138+ primary bone marrow myeloma and nontransformed primary B cells. Palbociclib induces G1 arrest in MM1.S with IC50 of appr 0.05 μM. Palbociclib preferentially inhibits proliferation of luminal estrogen receptor-positive (including HER2-positive) human breast cancer cell lines. Palbociclib increases gene expression of pRb and cyclin D1 and decreases gene expression of CDKN2A (p16) in most sensitive lines. Palbociclib enhances sensitivity to ICI 47699 in cell lines with conditioned resistance to ER blockade.