Synergistic Anti-Myeloma Activity of a Proteasome Inhibitor Marizomib and IMiD® Immunomodulatory Drug Pomalidomide

Background and Rationale: Proteasome inhibitor bortezomib is an effective therapy for the treatment of relapsed and refractory multiple myeloma (RRMM); however, prolonged treatment can be associated with toxicity, peripheral neuropathy and drug resistance. Our earlier studies showed that a novel proteasome inhibitor marizomib is distinct from bortezomib in its chemical structure, mechanisms of action, and effects on proteasomal activities (Chauhan et al., Cancer Cell 2005, 8:407-419). We also showed that marizomib triggers synergistic anti-MM activity in combination with lenalidomide (Chauhan et al., Blood 2010, 115:834-45). Pomalidomide, like lenalidomide, is an analogue of thalidomide with potent immunomodulatory activity, and has been approved by FDA for treatment of RRMM patients who have received at least two prior therapies including lenalidomide and bortezomib and showed disease progression on or within 60 days of completion of the last therapy. Approval of treatment is based on progression-free survival. Here we utilized in vitro and in vivo models of MM to examine the anti-MM activity of combined marizomib and pomalidomide.

Materials and Methods:MM celllines, patient tumor cells, and peripheral blood mononuclear cells (PBMCs) from normal healthy donors were utilized to assess the anti-MM activity of marizomib and pomalidomide. Cell viability, apoptosis, and migration assays were performed using WST/MTT, Annexin V staining, and Transwell Inserts, respectively. Synergistic/additive anti-MM activity was analyzed by isobologram analysisusing “CalcuSyn” software program. Proteasome activity was measured, as previously described (Chauhan et al., Cancer Cell 2005, 8:407-419). In vitro angiogenesis was assessed using matrigel capillary-like tube structure formation assays. MM.1S-tumor-bearing mice were treated with vehicle control, marizomib, pomalidomide or marizomib plus pomalidomide at the indicated doses for 21 days on a twice-weekly schedule for marizomib and 4 consecutive days weekly for pomalidomide. Statistical significance was determined using a Student’s t test. Pomalidomide was purchased from Selleck chemicals, USA; and marizomib was obtained from Triphase Inc., USA.

Results: MM cell lines (MM.1S, MM.1R, INA-6, RPMI-8226, Dox-40, U266, LR5, ANBL6.WT, and ANBL6.BR) and primary patient MM cells were pretreated with DMSO control or with pomalidomide for 24h; marizomib was then added for an additional 24h, followed by assessment of cell viability. A significant decrease in viability of all cell lines and patient cells was observed in response to treatment with combined low doses of marizomib and pomalidomide, compared with either agent alone. Isobologram analysis confirmed the synergistic anti-MM activity of these agents (CI < 1.0). Tumor cells from 5 of 7 patients were obtained from patients whose disease was progressing while on bortezomib, dexamethasone, and lenalidomide therapies. Moreover, the cytotoxicity of combination therapy was observed in MM cell lines sensitive and resistant to conventional (dex, doxorubicin, melphalan) and novel (bortezomib) therapies. No significant decrease in viability of PBMCs from normal healthy donors was observed in response to treatment with combined low doses of marizomib and pomalidomide, suggesting selective anti-MM activity and a favorable therapeutic index for this combination regimen. Furthermore, marizomib plus pomalidomide inhibits proliferation of MM cells even in the presence of BM stromal cells. Mechanistic studies showed that marizomib plus pomalidomide-induced apoptosis was associated with: 1) activation of caspase-8, caspase-9, caspase-3, and PARP; 2) downregulation of Cereblon, IRF4, c-Myc, and Mcl-1; and 3) enhanced inhibition of chymotrypsin-like, caspase-like and trypsin-like proteasome activities versus single agent alone. Furthermore, combined low doses of marizomib and pomalidomide blocked migration of MM cells and angiogenesis. In vivo studies using a subcutaneous human MM xenograft models show that combined low doses of marizomib and pomalidomide are well tolerated, inhibit tumor growth, and prolong survival.

Conclusion: Our preclinical studies in MM disease models support a clinical trial of combined marizomib and pomalidomide to improve outcome in patients with relapsed and refractory MM.