Targeting Cdk4/6 in Combination Therapy Overcomes Proteasome Inhibitor Resistance in Multiple Myeloma through Synergistic Mitochondria Depolarization.

Deregulation of Cdk4 or Cdk6 is central to the loss of cell cycle control in cancer. In myeloma, over-expression of Cdk4-cyclin D1 or Cdk6-cyclin D2 predisposes primary bone marrow (BM) myeloma cells to proliferation in vivo¹. Conversely, silencing Cdk4/6 by its physiologic inhibitor, p18INK4c, is required for G1 cell cycle arrest during the generation of normal plasma cells². Targeting Cdk4/6, therefore, is critical for the control of disease progression and drug resistance in myeloma, but no Cdk4/6-specific inhibitor was available. We have now targeted Cdk4/6 in combination therapy in myeloma using PD 0332991 (Pfizer), a novel orally bioactive, small molecule Cdk4/6-specific inhibitor. We demonstrate that as a single agent, PD 0332991 rapidly inhibits Cdk4/6 (IC50 ∼ 60 nM) and induces exclusive G1 cell cycle arrest in primary human myeloma cells in the presence of BM stromal cells. Non-invasive whole body imaging reveals that PD 0332991 nearly completely prevents tumor growth in a rapidly disseminated xenograft human myeloma model without overt toxicity³. Because PD 0332991 is reversible and does not induce apoptosis when used as a specific Cdk4/6 inhibitor, we have further developed two strategies to target Cdk4/6 in combination therapy. In the first, the release of PD 0332991-induced G1 arrest leads to synchronous S phase reentry and preferential killing of cycling myeloma cells by cytotoxic agents. In the second, prolonging PD 0332991-induced G1 arrest markedly sensitizes primary myeloma cells to killing by cytotoxic agents including the proteasome inhibitors bortezomib and NPI-0052, as well as the steroid dexamethasone. The Cdk4/6-based combination therapy rapidly reduces tumor load in animal models. More importantly, it overcomes drug resistance in primary myeloma cells isolated from refractory relapse patients despite the protection from BM stromal cells. Synergistic killing of myeloma cells lies in the synergistic induction of mitochondria depolarization. Based on this study and the favorable outcome of a PD 0332991 Phase I clinical trial, a Phase I/II PD 0332991-bortezomib clinical trial for myeloma is underway. Targeting Cdk4/6 by PD 0332991 in combination with a selective cytotoxic agent, therefore, represents the first promising cell cycle-based therapy in myeloma, and possibly other cancers.