Targeting CDK4/CDK6 Impairs Osteoclast Progenitor Pool Expansion and Blocks Osteolytic Lesion Development in Multiple Myeloma.

Multiple myeloma (MM) is characterized by increased osteoclast activity resulting in bone destruction and development of lytic lesions. PD0332991 is a selective small molecule inhibitor of cyclin-dependent kinase (CDK)4 and CDK6 with oral bioavailability. Recently we demonstrated that inhibition of CDK4/CDK6 by PD0332991 effectively controls MM tumor expansion in animal models and sensitizes MM for cytotoxic killing (Baughn et al, Cancer Res. 2006; Menu et al, Cancer Res. 2008; Huang et al, unpublished). Currently clinical phase I/II trials are ongoing to test the efficacy of the combination of PD0332991 and bortezomib. In vivo data further indicate that PD0332991 preferentially targets tumor cells and rapidly cycling bone marrow cells. This led us to investigate the possibility that PD0332991 may also inhibit osteoclastogenesis via restricting progenitor cell expansion and MM-induced bone destruction. PD0332991 significantly (p<0.01) decreased the number of lytic lesions by 81%, in addition to reducing tumor burden in the bone marrow of immunocompetent 5T2MM murine model. In a dose-dependent manner, PD0332991 inhibited osteoclastogenesis and the fusion of osteoclasts in human (IC₅₀ <50 nM) marrow cultures in vitro. Importantly, treatment with PD0332991 for the first week, but not the second or third week, was sufficient to inhibit osteoclast formation. These data suggest that PD0332991 acts preferentially on the early stage of OCL development. This was confirmed by a reduction of osteoclast precursor colonies (CFU-M, CFU-GM) under PD0332991 treatment, due to inhibition of DNA synthesis and diminished expansion of the osteoclast progenitor pool. The basis for the inhibition of osteoclast precursor proliferation was G1 cell cycle arrest following inhibition of CDK4/CDK6-specific phosphorylation of Rb by PD0332991, but not cell death, as evidenced by the intact cell morphology and absence of caspase activation. The combination of PD0332991 and bortezomib synergistically abrogated human osteoclast formation. Further, our in vivo and in vitro data showed that PD0332991 has no effects on osteoblastogenesis or genes inducing osteoblast development including Bsp, Ocn, and Runx2.

Collectively, our data suggest that by inducing G1 arrest in osteoclast precursors and inhibiting the osteoclast progenitor pool expansion, PD0332991 is a powerful and selective treatment for MM-induced osteolytic bone lesions. We propose that targeting CDK4/CD6 with PD0332291 in combination therapy is a promising therapeutic strategy to both suppress tumor expansion and improve bone integrity in MM.

Roodman:Novartis: Consultancy, Research Funding, Speakers Bureau; Amgen: Consultancy; Celgene: Consultancy; Acceleron: Consultancy. Lentzsch:Celgene: Consultancy, Speakers Bureau; Pfizer: Consultancy.