Abstract B027: Functional genomics studies identify determinants of response vs. resistance to pharmacological inhibitors of RAS in multiple myeloma

Abstract Up to 50% of multiple myeloma (MM) patients harbor K/NRAS mutations. Pharmacologic inhibitors targeting mutant KRAS have clinical activity in solid tumors. Here, we pre-emptively studied the molecular determinants of response vs. resistance to RAS inhibition in preclinical MM models. We used mutant-selective inhibitors of KRASG12C (MRTX-1257) and KRASG12D (MRTX-1133) ; the broader spectrum mutant-KRAS inhibitor BI-2865; or the tricomplex pan-RAS inhibitor RMC-6236. Human MM lines responded to these inhibitors at clinically relevant concentrations comparable to solid tumor patients. RMC-6236 was active against multiple K/NRAS mutant and wildtype MM lines with upstream Ras-activating lesions. Treatment of NSG mice with MRTX-1257 or MRTX-1133 was active against diffuse lesions of XG7 or KP6 cells, respectively, but tumor escape ensued eventually. We harvested the tumor cells at full-blown relapse and continued treatment with the same inhibitors in vitro. Cells from inhibitor- and vehicle-treated mice responded similarly in vitro. DNA barcoding-based clonal tracking showed no significant enrichment of barcodes in inhibitor-treated vs. DMSO-treated control MM cells. Co-culture of human MM cell lines with bone marrow stromal cells attenuated their response to G12C inhibitors, whereas the genetically engineered mouse MM cells VQmyc (NRASQ61R) responded to RMC6236, though less so in the presence of IL6. We conducted 36 genome-scale CRISPR knockout (KO) and activation screens in 10 MM lines revealing that genetic perturbations influencing response to RAS inhibition are heterogeneous and cell line-specific. KO and activation screens yielded complementary results per inhibitor-cell line pair. For example, resistance was recurrently conferred by CRISPRa of upstream surface receptors, KRAS itself, or positive regulators of RAS-MAPK; or CRISPR KO of negative regulators of RAS-MAPK, NFκB or PI3K/Akt signaling; the stress sensor KEAP1, etc. We identified shared gene perturbations across all 3 classes of RAS inhibitors and others with distinct roles (e. g. PPIA for RMC6236). Many core hits from our CRISPR studies are common with similar publicly available K-Ras inhibitor screens in solid tumors, while others hits are selective to MM. Notably, with Ras inhibitor treatment, top differential expressed genes, including some known regulators of Ras pathway, are not among the top hits of CRISPR studies in the same cell lines, underscoring the value of systematic CRISPR-based functional genomics. Non-genomic adaptations and diverse genomic perturbations influence responsiveness of MM cells to RAS inhibitors, with heterogeneous “resistomes” across different cell lines. This systematic functional genomic characterization will hopefully inform personalized uses of Ras inhibitors in future clinical studies Citation Format: Arnold Bolomsky, Torsten Steinbrunn, Omar Al-Odat, Ricardo de Matos Simoes, Ryosuke Shirasaki, Oliver Bohorquez, Olga Dashevsky, Eleni Anastasakou, James G. Christensen, Andrew Aguirre, Ryan Young, Constantine S. Mitsiades. Functional genomics studies identify determinants of response vs. resistance to pharmacological inhibitors of RAS in multiple myeloma abstract. In: Proceedings of the AACR Special Conference in Cancer Research: RAS Oncogenesis and Therapeutics; 2026 Mar 5-8; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Res 2026;86 (5Suppl₁): Abstract nr B027.