Abstract 598: HSK42360: A potent, brain permeable, BRAF paradox breaker for the treatment of BRAF-driven cancers

Abstract Mutations in the BRAF gene are among the most common identified in human cancers approximately 7% of all cancers evaluated, including 70% of melanomas, 50% of thyroid-like cancers, 10% to 20% of colorectal cancers, 13% of serous ovarian cancers, 3-7% of non-small-cell lung cancers, and most recently, hairy cell leukemia. Approved BRAF inhibitors (or +MEKi) against BRAFV600E/K improved initial clinical efficacy. However intrinsic and acquired resistance often arise through BRAF paradoxical activation, limiting durable responses to current BRAF inhibitors. In addition, Approved BRAFi responds poorly to patients with BRAF mutant gliomas or brain metastases, mainly due to limited brain permeability. More effective therapeutic options are therefore needed. To address this clinical challenge, we developed a potent, brain-penetrating paradox breaker HSK42360, as a promising therapy for patients with BRAFV600E mutation, progression or brain metastases following treatment with approved BRAFi or BRAFi/MEKi therapies. HSK42360 is a potent BRAFV600E inhibitor with IC50 value of 5 nM at enzymatic level. Further characterization showed that HSK42360 did not induce homodimer or heterodimer of CRAF with CRAF or BRAF and thereby blocked reactivation of the MAPK pathway in HCT116 cells, whereas Dabrafenib significantly induced RAF dimerization and MAPK reactivation. HSK42360 exhibited significant anti-proliferation activity against multiple tumor cell lines with BRAFV600E, including A375 (melanoma), DU4475 (breast cancer), and COLO205 (colon cancer), but was highly selective for BRAF wild-type cells. HSK42360 was administered orally once daily and showed broad-spectrum antitumor activity in the COLO205 and A375 CDX models. Immunohistochemistry showed a close correlation between the inhibition of tumor growth and ERK1/2 phosphorylation in A375 CDX tumor tissues. To explored the impact of HSK42360 in a brain micro-metastasis model aimed at mimicking early CNS metastatic spread, tumor progression was monitored through BLI (full name) imaging, which revealed that HSK42360 was highly brain penetrant with Kp, uu greater than 1 and triggered potent antitumor activity and prolonged survival in brain metastatic models of melanoma. We next tested the hypothesis that HSK42360 could retain activity in models presenting RAF dimer-driven resistance. Surprisingly, HSK42360 demonstrated promising antitumor effects in the first gen-resistant NRASmA375 (NRASQ61K, BRAFV600E) CDX model, supporting its use as a follow-on therapy for current BRAFi resistance. Collectively, these preclinical studies validated that HSK42360 is a novel brain-permeable BRAF paradox breaker that exhibits outstanding antitumor activity in brain metastasis and RAF dimer-driven resistance. Citation Format: Shidong Gao, Yangguang Li, Ju Wang, Pingming Tang, Zongjun Shi, Yao Li, Pangke Yan. HSK42360: A potent, brain permeable, BRAF paradox breaker for the treatment of BRAF-driven cancers abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 598.