Glutaminase inhibitor CB-839 synergizes with carfilzomib in resistant multiple myeloma cells

// Ravyn M. Thompson 1 , Dominik Dytfeld 2 , Leticia Reyes 1 , Reeder M. Robinson 1 , Brittany Smith 1 , Yefim Manevich 1 , Andrzej Jakubowiak 3 , Mieczyslaw Komarnicki 2 , Anna Przybylowicz-Chalecka 2 , Tomasz Szczepaniak 2 , Amit K. Mitra 5 , Brian G. Van Ness 5 , Magdalena Luczak 4 , Nathan G. Dolloff 1, * 1 Department of Cell and Molecular Pharmacology & Experimental Therapeutics, Medical University of South Carolina, Charleston, SC, USA 2 Karol Marcinkowski University of Medical Sciences, Poznan, Poland 3 University of Chicago, Chicago, IL, USA 4 Institute of Bioorganic Chemistry, Polish Academy of Sciences, Poznan, Poland 5 University of Minnesota, Minneapolis, MN, USA * These authors contributed equally to this work Correspondence to: Nathan G. Dolloff, email: dolloffn@musc.edu Keywords: multiple myeloma, proteasome inhibitor, glutaminase, carfilzomib Received: December 08, 2016 Accepted: March 01, 2017 Published: March 16, 2017 ABSTRACT Curative responses in the treatment of multiple myeloma (MM) are limited by the emergence of therapeutic resistance. To address this problem, we set out to identify druggable mechanisms that convey resistance to proteasome inhibitors (PIs; e.g., bortezomib), which are cornerstone agents in the treatment of MM. In isogenic pairs of PI sensitive and resistant cells, we observed stark differences in cellular bioenergetics between the divergent phenotypes. PI resistant cells exhibited increased mitochondrial respiration driven by glutamine as the principle fuel source. To target glutamine-induced respiration in PI resistant cells, we utilized the glutaminase-1 inhibitor, CB-839. CB-839 inhibited mitochondrial respiration and was more cytotoxic in PI resistant cells as a single agent. Furthermore, we found that CB-839 synergistically enhanced the activity of multiple PIs with the most dramatic synergy being observed with carfilzomib (Crflz), which was confirmed in a panel of genetically diverse PI sensitive and resistant MM cells. Mechanistically, CB-839 enhanced Crflz-induced ER stress and apoptosis, characterized by a robust induction of ATF4 and CHOP and the activation of caspases. Our findings suggest that the acquisition of PI resistance involves adaptations in cellular bioenergetics, supporting the combination of CB-839 with Crflz for the treatment of refractory MM.