Abstract LB-269: IDH2 inhibition enhances proteasome inhibitor responsiveness in hematological malignancies
Authors: Bergaggio Elisa , Riganti Chiara , Garaffo Giulia , Mereu Elisabetta , Vitale Nicoletta , Bandini Cecilia , Pellegrino Elisa , Omedè Paola , Todoerti Katia , Audrito Valentina , Rossi Antonio , Bertoni Francesco , Deaglio Silvia , Neri Antonino , Palumbo Antonio , Piva Roberto 
Published: August 2018

Abstract
The introduction of proteasome inhibitors (PIs) into the clinic has transformed the treatment of patients affected by multiple myeloma (MM) and mantle-cell lymphoma (MCL) establishing new standards of care. Despite these improvements, patients continuously relapse or are intrinsically resistant to PIs.
Here, to identify druggable targets that synergize with PIs, we carried out a functional screening in MM cell lines using a short hairpin RNA library targeting 152 cancer driver genes, highly representative of all signaling pathways. The Isocitrate Dehydrogenase 2 (IDH2) gene was identified as a top candidate, showing a synthetic lethal activity with the PI Carfilzomib (CFZ). IDH2 is a NADP(+) dependent mitochondrial enzyme which catalyzes the oxidative decarboxylation of isocitrate to 2-oxoglutarate in the tricarboxylic acid (TCA) cycle.
We demonstrated that combinations of the pharmacological IDH2 inhibitor AGI-6780 with FDA approved PIs significantly increased apoptotic cell death in ten MM cell lines, both sensitive and resistant to PIs. Combined treatments triggered synergistic cytotoxicity also in four MCL and in two Burkitt's lymphoma cell lines. Importantly, CFZ/AGI-6780 treatment increased death of primary CD138-positive cells from nine MM patients and exhibited a favorable cytotoxicity profile towards peripheral blood mononucleated cells and bone marrow-derived stromal cells.
Mechanistically, CFZ/AGI-6780 combination significantly decreased TCA cycle activity and ATP levels, as a consequence of enhanced IDH2 enzymatic inhibition. In contrast, only a slight increase of mitochondrial reactive oxygen species (ROS) was observed. CFZ treatment reduced the expression of nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme required for IDH2 activation through the NAD(+)-dependent deacetylase SIRT3. Consistently, combination of CFZ with either NAMPT (FK866) or SIRT3 (AGK7) inhibitors impaired IDH2 activity and increased MM cell death, thus phenocopying CFZ/AGI-6780 effects and putting the proteasome in a direct link with IDH2 inhibition. Finally, inducible IDH2 knock-down enhanced the therapeutic efficacy of CFZ in a subcutaneous xenograft model of MM, resulting in inhibition of tumor progression and extended survival. In conclusion, our data demonstrate that IDH2 inhibition increases 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.
Citation Format: Elisa Bergaggio, Chiara Riganti, Giulia Garaffo, Elisabetta Mereu, Nicoletta Vitale, Cecilia Bandini, Elisa Pellegrino, Paola Omedè, Katia Todoerti, Valentina Audrito, Antonio Rossi, Francesco Bertoni, Silvia Deaglio, Antonino Neri, Antonio Palumbo, Roberto Piva. IDH2 inhibition enhances proteasome inhibitor responsiveness in hematological malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-269.