Abstract A073: Plixorafenib (PLX8394/FORE8394) treatment leads to sustained tumor regression in preclinical models of <i>BRAF</i> -mutated primary and brain metastatic melanoma

Abstract BRAF alterations are one of the most common oncogenic drivers in melanoma, occurring in 40-50% of melanoma patients, with V600E being the most common. While early generation BRAFi are available to treat V600E-mutated primary and brain metastatic melanoma, paradoxical MAPK activity induced by these BRAFi results in poor tolerability and low durable response, particularly in patients with CNS metastasis. Plixorafenib is a 1st in class BRAF paradox breaker that selectively inhibits V600 monomers and disrupts the RAF dimer interface, preventing paradoxical MAPK pathway activation and yielding activity against other BRAF alterations. In a Phase 1/2a study (NCT02012231), anti-tumor activity of plixorafenib was seen across multiple solid tumors, including BRAF-altered melanoma and primary CNS tumors (66.7% ORR, mDOR of 13.9 months). Thus, we hypothesized plixorafenib would be effective for patients with BRAFmut melanoma and CNS metastasis. Preclinical models of primary and brain metastatic melanoma were tested using V600E-mutated A375 cell line. In both subcutaneous and intracranial models, plixorafenib treatment at 30mg/kg inhibited tumor growth. Analysis of pERK expression in subcutaneous tumor at the end of treatment showed continued pERK suppression by plixorafenib, indicating sustained on-target effect. After 35 days of treatment in the CNS metastatic model (n=7/group), plixorafenib reduced tumor burden by 88.6% compared to vehicle group, with one achieving complete response. Plixorafenib was well tolerated, as no significant weight loss was observed throughout treatment. RNAseq from intracranial tumors at the end of treatment showed an increase in interferon and inflammatory responses due to plixorafenib. Circulating tumor DNA from patients in the Phase 1/2A study revealed baseline mutations in NF1 and PI3K pathway that corresponded to resistance to plixorafenib, but no emerging mutations in the MAPK pathway were found after treatment. To explore additional resistance pathways in preclinical models, transcriptomics in CNS tumors with sustained tumor growth inhibition and those refractory to treatment were compared, revealing an increase in the Hallmark gene sets, MYC targets, TGFβ Signaling, and TNFα Signaling in resistant tumors. In three resistant A375 cell lines generated in vitro, plixorafenib suppressed pERK activity in all three resistant clones despite no activity on cell growth. RNAseq performed in parental and resistant cells showed an upregulation in the E2F targets and p53 signaling, suggesting a dysregulation in cell cycling and DNA damage response may drive resistance. Altogether these data demonstrate sustained on-target suppression of the MAPK pathway throughout plixorafenib treatment. No emerging mutations within the MAPK pathway were found, and alternative oncogenic pathways were enriched, supporting a differentiated mechanism of resistance compared to early generation BRAFi. These data suggest that plixorafenib monotherapy would be clinically effective against both primary and CNS metastatic melanoma. Citation Format: Jessica C. Jang, Ping Jiang, Stacie Peacock Shepherd. Plixorafenib (PLX8394/FORE8394) treatment leads to sustained tumor regression in preclinical models of BRAF-mutated primary and brain metastatic melanoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A073.