IDH2 inhibition enhances proteasome inhibitor responsiveness in hematological malignancies

Proteasome inhibitors (PIs) are extensively used for the therapy of multiple myeloma (MM) and mantle-cell lymphoma (MCL). However, patients continuously relapse or are intrinsically resistant to this class of drugs. Here, to identify targets that synergize with PIs, we carried out a functional screening in MM cell lines using a short hairpin RNA library against cancer driver genes. Isocitrate dehydrogenase 2 (IDH2) was identified as a top candidate, showing a synthetic lethal activity with the PI carfilzomib (CFZ). Combinations of FDA approved PIs with a pharmacological IDH2 inhibitor (AGI-6780) triggered synergistic cytotoxicity in MM, MCL, and Burkitt's lymphoma (BL) cell lines. CFZ/AGI-6780 treatment increased death of primary CD138+ cells from MM patients and exhibited a favorable cytotoxicity profile towards peripheral blood mononuclear cells and bone marrow-derived stromal cells. Mechanistically, CFZ/AGI-6780 combination significantly decreased tricarboxylic acid (TCA) cycle activity and ATP levels, as a consequence of enhanced IDH2 enzymatic inhibition. Specifically, CFZ treatment reduced the expression of nicotinamide phosphoribosyltransferase (NAMPT), thus limiting IDH2 activation through the NAD+-dependent deacetylase SIRT3. Consistently, combination of CFZ with either NAMPT or SIRT3 inhibitors impaired IDH2 activity and increased MM cell death. Finally, inducible IDH2 knockdown enhanced the therapeutic efficacy of CFZ in a subcutaneous xenograft model of MM, resulting in inhibition of tumor progression and extended survival. Taken together, these findings indicate that NAMPT/SIRT3/IDH2 pathway inhibition enhances the therapeutic efficacy of PIs, thus providing compelling evidence for treatments with lower and less toxic doses and broadening the application of PIs to other malignancies.