Abstract 6033: RA123, a new GLS1 allosteric inhibitor demonstrates in vitro and in vivo activity in multiple myeloma models

Abstract The conditional essential amino acid glutamine is utilized by tumors to sustain bioenergetic requirements in a nutrient-poor microenvironment. Based on this specific tumor need, efforts have been made to target the glutamine metabolism using glutaminase inhibitors. In this study, we present in vitro and in vivo studies demonstrating activities and target engagement of a new specific GLS1 allosteric inhibitor (RA123). In vitro, RA123 was tested in imid-resistant, CD38 low RPMI8226 multiple myeloma cell line using the ATP Cell titer Glo assay over a 72h treatment period. In vivo, RA123 was tested in mice xenografted with RPMI8226 multiple myeloma cell line. Compound was given at three different under a BID (twice a day) regimen. Tumor growth under treatment was monitored by caliper measurement for 20 days. A terminal PK/PD study on the same model was setup measuring impact of the drug on the glutamine and glutamate tumor content post 24 hours after the last administration. Over the same period, PK measurements were planned to document drug exposure in the plasma and in the tumor. Finally, in a stand-alone study, target engagement was documented using imaging modality. [18F]FSPG PET radiotracer was used to visualize and quantify impact of RA123 on tumor (RPMI8226) glutamate pool. RA123 was given at three doses under a BID regimen. Treatment was administered for 3 days, and PET signal was recorded at baseline and at end of day 3. In vitro study demonstrated that RA123 was able to induce cell killing of RPMI8226 after 72 hours of treatment. These data translated in vivo by a dose-dependent effect on cell growth. In a terminal PK/PD study, RA123 confirmed a dose-dependent impact on glutamine and glutamate tumor levels which correlated with both plasma and tumor exposure. Further confirmation of target engagement was observed with reduced PET uptake post- RA123 treatment at the three doses tested. In conclusion, this study demonstrated that RA123, a new specific GLS1 allosteric inhibitor was able to impact multiple myeloma tumor cell growth both in vitro and in vivo. This effect was associated with reduced glutamate pools and accumulation of glutamine levels within the tumor cells. Citation Format: Christophe Henry, Dimitri Gorge-Bernat, Pascal Pannier, Isabelle Meaux, Jane Cheng, Fangxian Sun, Olivier Pasquier, Philippe Lienard, Erwan Jouannot, Thierry Gouyon, Geneviève Estenne-Bouhtou, Bailin Zhang, Bérangère Thiers, Laurent Debussche, David Machnik. RA123, a new GLS1 allosteric inhibitor demonstrates in vitro and in vivo activity in multiple myeloma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6033.