Abstract 3248: Circulating tumor DNA analysis of patients with BRAF-mutated advanced unresectable solid tumors treated with plixorafenib (FORE8394/PLX8394) in phase 1/2a study

Abstract Plixorafenib is a selective, potent BRAFi that inhibits class 1, 2, and fusion BRAF mutations. Plixorafenib evades paradoxical MAPK activation by disrupting BRAF dimerization and reduces toxicities and resistance that were observed in early generation BRAFi. Circulating tumor DNA (ctDNA) has proven its predictive or prognostic value for patients (pts) on treatment. We present here the analysis of longitudinal ctDNA from plixorafenib-treated pts, an exploratory endpoint from this Phase 1/2a study (NCT02428712). A total of 211 plasma samples from 71 pts, 65 with BRAF alterations (BRAFmut not required in dose escalation study) were tested using PredicineCARE, which detects SNVs, INDELs, CNVs and fusions in ctDNA down to 0.05% allele frequency. Samples were obtained prior to treatment, at subsequent cycles, and at disease progression. The ctDNA concordance with tissue-based NGS data at baseline (BL) was 83.3% for V600E, 72.7% for class 2 SNVs, and 33.3% for BRAF fusions. Most discordance in V600E was found in papillary thyroid carcinoma pts. V600E variant allele frequency (VAF) decreased from C1D1 to C2D1 in 17/20 pts. Decreases were greater in responders. BL V600E VAF was higher in non-responders. In 6 pts with available paired tumor biopsies, decreases in pERK validated ctDNA results after treatment. Class 2 and fusion VAFs also declined after treatment, supporting differentiated activity from early generation BRAFi. In longitudinal samples, changes in ctDNA correspond to tumor size across tumor types, suggesting that ctDNA may be a surrogate marker for monitoring disease. Genomic profiles at disease progression were compared to BL to assess new mutations and pathways between subjects in response group (CR+PR+SD) and non-response (PD) group (non-CRC due to intrinsic resistance). No new mutations in MAPK pathway genes were observed, supporting the dimer breaker property of plixorafenib. In ovarian cancer pts, BRAF VAF dominated over other mutations and matched tumor volume, suggesting that BRAF VAFs drove disease outcome. In non-responders, heterogeneity of genomic profile, co-occurrent RAS, MAPK-associated or NF1 mutations were seen, including melanoma pts who all received prior MAPKi therapies. In CRC, multiple driver mutations were found at BL associated with receptor tyrosine kinase activation, confirming resistance pathways and supporting clinical utility of combination. Despite genomic heterogeneity in V600+ melanoma and CRC, early decreases in BRAF VAF were observed. ctDNA accurately detects BRAF mutations identified in tumor biopsy and longitudinal ctDNA levels is a potential surrogate marker for monitoring disease. Response to plixorafenib was observed through declines in ctDNA BRAF VAF and pERK. Furthermore, the “paradox breaking” property of plixorafenib was confirmed, as no new MAPK mutations emerged after treatment. Citation Format: Rona Yaeger, Eric J. Sherman, Macarena de la Fuente, Frank Q. Tsai, Nicholas Butowski, Carl E. Allen, Natraj R. Ammakkanavar, Philip Lammers, Jessica C. Jang, Michael Paz, Sam Wang, Stacie Peacock Shepherd, Ping Jiang, Jordi Rodon. Circulating tumor DNA analysis of patients with BRAF-mutated advanced unresectable solid tumors treated with plixorafenib (FORE8394/PLX8394) in phase 1/2a study [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 3248.