Metformin confers sensitization to syrosingopine in Multiple Myeloma cells by metabolic blockage and inhibition of protein synthesis.

Multiple Myeloma (MM) remains an incurable haematological malignancy despite substantial advances in therapy. The hypoxic bone marrow induces metabolic rewiring in MM cells contributing to survival and drug resistance. Therefore, targeting metabolic pathways may offer an alternative treatment option. In this study, we re-purpose two FDA-approved drugs, syrosingopine and metformin. Syrosingopine was used as a dual inhibitor of monocarboxylate transporter 1 and 4 (MCT1/4) and metformin as an inhibitor for oxidative phosphorylation (OXPHOS). Anti-tumour effects were evaluated for single agents and in combination therapy. Survival and expression data for MCT1/MCT4 were obtained from the TT2, Mulligan and MMRF-cohort. Cell death, viability and proliferation were measured using Annexin V/7-AAD, CellTiterGlo and BrdU, respectively. Metabolic effects were assessed using Seahorse Glycolytic Rate assays and LactateGlo assays. Differential protein expression was determined using western blotting and the SUnSET method was implemented to quantify protein synthesis. Finally, the syngeneic 5T33MMvv model was used for in vivo analysis. High level expression of MCT1 and MCT4 both correlated with a significantly lower overall survival of patients. Lactate production as well as MCT1/MCT4 expression were significantly upregulated in hypoxia, confirming the Warburg effect in MM. Dual inhibition of MCT1/4 with syrosingopine resulted in intracellular lactate accumulation and reduced cell viability and proliferation. However, only at higher doses (>10 μM), was syrosingopine able to induce cell death. By contrast, combination treatment of syrosingopine with metformin was highly cytotoxic for MM cell lines and primary patient samples and resulted in a suppression of both glycolysis and OXPHOS. Moreover, pathway analysis revealed an upregulation of the energy sensor p-AMPKα, and more downstream a reduction in protein synthesis. Finally, the combination treatment resulted in a significant reduction in tumour burden in vivo. This study proposes an alternative combination treatment for MM and provides insight in the intracellular effects. This article is protected by copyright. All rights reserved.