Synergistic Enhancement of Conventional Anti-MM Drugs Efficacy with Plant Isothiocyanates: Therapeutic Implications

Multiple myeloma (MM) is a hematologic malignancy characterized by an abundant clonal proliferation of plasma cells in the bone marrow. Despite significant recent progress in the treatment of MM, this plasma cell dyscrasia remains incurable. Therefore, the pursuit of new investigational therapeutic agents, used alone or in combination is critically important to improve patient outcome.

In our previous studies, we introduced plant isothiocyanates (ITCs) as compounds with potent cytotoxicity against MM cell lines and primary MM samples. The effect of these compounds was associated with pleitotropic effects on signal transduction pathways regulating the apoptotic threshold of MM cells. This led us to hypothesize that ITCs could be rationally combined with other anti-MM agents and enhance their anti-tumor activity.

On the basis of previous studies in different cancer types, we examined whether the ITCs, suforaphane (SFN) and PEITC, showed inhibitory activity against histone deacetylases (HDAC) in MM. We found that ITCs, SFN and PEITC showed inhibitory activity against histone deacetylases (HDAC) using fluorometric HDAC assay. Therefore, we tested combination treatment of ITCs with the HDAC inhibitor vorinostat. This combination had potent synergistic effect against MM cells, and was associated with apoptotic cell death following G2/M cell cycle block and depletion of mitochondrial potential. Interestingly, synergistic inhibition of HDACs was not observed. In addition, combination had additive and synergistic effect on phosphorylation of c-Jun and p38MAPK, respectively. Because the BM microenvironment is considered a key regulator of MM cell biological behavior, we evaluated the impact of bone marrow stromal cells (BMSCs) on the combination treatment. Our co-culture system where MM cells were labeled with CFSE proliferation dye, and therefore easily distinguished from BMSCs, the interaction between these cell populations did not confer MM cells resistance to the combination treatment.

Next, we evaluated combinations of ITCs with established anti-MM agents. We observed that combination of ITCs with melphalan and dexamethasone led to synergistic induction of MM cells apoptosis, with activation of caspases -8 and -9, and cleavage of PARP. The synergistic effect of the ITC combination with melphalan was not abrogated by the MM cell co-culture with stroma, while dexamethasone synergized only with SFN. The combination of SFN and melphalan synergistically increased activation of p53 and p38MAPK at later time point than PEITC and melphalan. Dexamethasone alone activated NFkB and p38MAPK, but combination with ITCs decreased the phosphorylation of both signaling molecules.

Our study raises the intriguing possibility that dietary supplementation with ITCs during the treatment of MM patients could potentially modulate therapeutic responses. Our results specifically indicate that ITCs can enhance the activity of certain conventional and novel anti-MM agents, providing the preclinical framework for future clinical studies of combination regimens that include ITCs for the treatment of MM.

Richardson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees. Anderson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Mitsiades:Millennium: Consultancy, Honoraria; Novartis Pharmaceuticals: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria; Merck &Camp;o.: Consultancy, Honoraria; Kosan Pharmaceuticals: Consultancy, Honoraria; Pharmion: Consultancy, Honoraria; Centrocor: Consultancy, Honoraria; PharmaMar: Patents & Royalties; OSI Pharmaceuticals: Research Funding; Amgen Pharmaceuticals: Research Funding; AVEO Pharma: Research Funding; EMD Serono: Research Funding; Sunesis: Research Funding; Gloucester Pharmaceuticals: Research Funding.