Abstract A028: Non-genetic resistance to multi-selective RAS(ON) inhibitors in NRAS-mutant melanoma

Abstract Background: Selective inhibitors that target multiple active RAS isoforms (multi-RASi) have recently emerged as a promising therapeutic strategy for NRAS-mutant melanoma, which currently lacks effective targeted therapies. However, the rapid development of drug resistance remains a major barrier to their clinical efficacy. While approximately half of patients exhibit non-genetic resistance to multi-RASi, the molecular mechanisms driving this adaptation remain poorly defined. Methods: We used in vitro and in vivo models of NRAS-mutant melanoma to investigate adaptive responses to multi-RAS(ON) inhibition. Resistance-associated signaling dynamics were quantified by live-cell reporters, immunoblotting, and proteomics. Epigenetic modulation was tested using histone deacetylase (HDAC) inhibitors, and pharmacologic interventions were evaluated in patient-derived and syngeneic tumor models. Results: We identify a non-genetic mechanism of resistance that arises through an AND-gate interaction between mutant NRAS dosage and receptor tyrosine kinase (RTK) activation. Both NRAS overexpression and RTK activation are required to reinstate ERK signaling and sustain tumor regrowth; disruption of either node suppresses resistant proliferation. RTK activation broadly reactivates RAS signaling, whereas elevated NRAS levels determine the amplitude of ERK rebound. Transient HDAC inhibition reverses NRAS overexpression and restores multi-RASi sensitivity, revealing an epigenetically maintained resistant state. In parallel, vertical suppression of the MAPK pathway through combined MEK inhibition effectively prevents ERK reactivation and tumor progression. Conclusions: Our findings define an adaptive signaling circuit in which NRAS dosage and RTK activation cooperate to drive non-genetic resistance to RAS inhibition. These results highlight the therapeutic potential of combining epigenetic or vertical MAPK blockade to overcome resistance in NRAS-mutant melanoma. Citation Format: Sungsoo Kim, Ha-Ram Park, Minah Kim, Hee Won Yang. Non-genetic resistance to multi-selective RAS(ON) inhibitors in NRAS-mutant melanoma [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(5_Suppl_1):Abstract nr A028.