INHIBITION OF SPHINGOSINE KINASE 2 RESENSITISES BORTEZOMIB-RESISTANT MULTIPLE MYELOMA

Abstract

Multiple myeloma, a haematological malignancy of the plasma cell, is still largely incurable. The introduction of proteasome inhibitors such as bortezomib has significantly improved patient survival, however bortezomib resistance is still a major hurdle which requires new therapeutic options. One such option may be inhibitors to sphingolipid metabolism. Sphingolipids are a family of lipids which can act as signalling molecules, influencing cellular pathways such as apoptosis. The enzyme sphingosine kinase is an especially attractive target in cancer, as it converts pro-apoptotic ceramide and sphingosine to pro-survival sphingosine-1-phosphate. We have previously shown that inhibition of sphingosine kinase 2 with K145 causes synergistic cell death when combined with bortezomib in bortezomib-naive myeloma cells, and thus we decided to test this combination in the bortezomib-resistant setting. To do so, bortezomib-resistant myeloma cell lines were generated and analysed. A 5TGM1 cell line generated to be resistant to bortezomib was also found to be resistant to carfilzomib, demonstrating cross-resistance that has also been seen in the clinic. Notably, these cells were found to possess a clinically-relevant mutation in the bortezomib-binding site of proteasomal subunit PSMB5. The SK2 inhibitor K145, when combined with either bortezomib or carfilzomib, caused synergistic cell death in the bortezomib-resistant cell lines tested. This was preceded by a significant increase in ER stress markers, as measured by Western blot, suggesting that this cell death may be induced by enhanced ER stress. When tested in vivo in a highly aggressive, bortezomib-resistant myeloma model, the combination of bortezomib and K145 gave a significant decrease in disease burden and increase in survival, suggesting this combination may be effective in bortezomib-resistant patients.