Inhibition of Cdk4/6 by PD 0332991 Results in Abrogation of Osteoclast Formation

Multiple myeloma (MM) is characterized by increased osteoclast activity resulting in bone destruction and development of lytic lesions. Despite overall survival achieved by new treatment modalities, new drugs are required to specifically and effectively inhibit bone destruction. PD 0332991 is a selective small molecule inhibitor of cyclin-dependent kinase (Cdk)4 and Cdk6 with oral bioavailability. We have demonstrated recently that inhibition of Cdk4/6 by PD 0332991 effectively controls MM tumor expansion in animal models and sensitizes chemoresistant MM tumor cells to bortezomib killing despite protection by bone marrow stromal cells. Currently clinical phase I/II trials are ongoing to test the efficacy of the combination of PD 0332991 and bortezomib.

Selective inhibition of Cdk4/6 by PD 0332991 leads to sustained G1 arrest but not apoptosis, and this appears to be cell type-specific. Consistent with these findings, PD 0332991 is well tolerated in MM patients. On this basis, we address the possibility that PD 0332991, alone and in combination with bortezomib, may inhibit osteoclastogenesis, by culturing non-adherent mononuclear bone marrow cells from MM patients cultured in the presence of M-CSF and RANKL for three weeks. Treatment of human osteoclast cultures with PD 0332991 for 3 weeks decreased osteoclast formation in a dose-dependent manner with an IC₅₀ 50 nM. The combination of PD 0332991 and bortezomib led to synergistic inhibition of osteoclast formation and completely abrogated osteoclastogenesis with low dose PD 0332991 (25 nM) and bortezomib (2 nM). Furthermore, we show in a time course study that treatment with PD 0332991 for the first week, but not the second or third week, was sufficient to inhibit osteoclast formation. PD 0332991 is the only known selective inhibitor of Cdk4 and Cdk6, which at concentration below 5 uM does not cross react with at least 38 kinases or induce apoptosis. Taken together, our data suggest that by inducing G1 arrest and inhibiting progenitor expansion, PD 0332991 is a powerful and selective inhibitor for osteoclastogenesis. We propose that targeting Cdk4/6 with PD 0332291 in combination therapy is a promising therapeutic strategy to improve bone integrity in MM.