Abstract 3935: Durable inhibition of ERK phosphorylation is required for tumor regression in novel human and mouse models of NF1-inactivated melanoma

Abstract Inactivating mutations in Neurofibromin 1 (NF1), a RAS-GTPase, have been identified in a significant subset of melanomas, both concurrent with and exclusive of oncogenic BRAF and RAS mutations. However, FDA-approved therapies targeting the MAPK pathway have been largely inactive in this molecular cohort. To address the underlying biology, we first leveraged sequencing data from 1, 912 melanoma tumor/normal pairs generated as part of the MSK-IMPACT sequencing initiative and identified oncogenic mutations in TP53 as the most significant event common to NF1-inactivated melanoma, with oncogenic BRAF mutations frequently co-occurrent. Given the lack of models for studying NF1 loss, we next generated a cohort of genetically engineered mice harboring conditional alleles for Nf1fl/fl, Trp53fl/fl, and/or oncogenic BRAF V600E (BrafCA) and the transgene Tyr::CreERT2, restricting induction to melanocytes. While melanocyte-specific, homozygous knockout of Nf1 induced hyperpigmentation, it was insufficient for melanomagenesis. Combined Nf1/Trp53 knockout induced tumor formation with an incidence of 55.56%; tumor onset and hyperpigmentation were hastened upon the addition of oncogenic BRAF V600E. Other genetic cohorts induced tumors with variable penetrance and latency (BrafCA/Nf1, BrafCA/Trp53). All tumors displayed features of human melanoma including pigmented melanocytic and amelanotic spindle-shaped neoplasms, with most staining positive for S100. To facilitate preclinical and functional studies, we generated 22 mouse tumor-derived, syngeneic cell lines. As anticipated, loss of Nf1 conditioned the response to treatment with a BRAF monomer inhibitor (vemurafenib), while Nf1-inactivated cells retained sensitivity to MEK inhibition (trametinib) or the combination. Combined inhibition of BRAF/SHP2 inhibited ERK phosphorylation and cell proliferation in BrafCA/Nf1/Trp53-mutant cells; however, inhibition of ERK signaling was transient. Combination treatments that resulted in more durable inhibition of ERK, including the triple combination of RAF/MEK/SHP2 inhibitors, achieved tumor regression in mice with established BrafCA/Nf1/Trp53-null tumors as well as in a genetically similar human PDX of melanoma (SK-MEL-1273A). Overall, we find that addition of SHP2 to MEK or RAF inhibitors resulted in more durable MAPK pathway inhibition and greater tumor regression in mice bearing NF1-null tumors. The testing of additional combinations of novel agents that selectively target the MAPK pathway and other parallel pathways contributing to resistance and survival are ongoing. Citation Format: Olvania-Danyca Hilaire, Alexis M. Jones, Moriah H. Nissan, Sebastien Monette, Jordan Eichholz, Sandra Misale, Isabell Schulze, Naresh Vasani, Cailian Liu, Xia Yang, Elisa De Stanchina, Nikolaus Schultz, Michael F. Berger, Neal Rosen, Taha Merghoub, David B. Solit, Aphrothiti J. Hanrahan. Durable inhibition of ERK phosphorylation is required for tumor regression in novel human and mouse models of NF1-inactivated melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3935.