Preclinical Characterization of a Next-Generation Brain Permeable, Paradox Breaker BRAF Inhibitor

Purpose: Disease progression in BRAF V600E/K positive mel-anomas to approved BRAF/MEK inhibitor therapies is associated with the development of resistance mediated by RAF dimer induc-ing mechanisms. Moreover, progressing disease after BRAFi/MEKi frequently involves brain metastasis. Here we present the develop-ment of a novel BRAF inhibitor (Compound Ia) designed to address the limitations of available BRAFi/MEKi. Experimental Design: The novel, brain penetrant, paradox breaker BRAFi is comprehensively characterized in vitro, ex vivo, and in several preclinical in vivo models of melanoma mimicking peripheral disease, brain metastatic disease, and acquired resistance to first-generation BRAFi. Results: Compound Ia manifested elevated potency and selec-tivity, which triggered cytotoxic activity restricted to BRAF-mutated models and did not induce RAF paradoxical activation. In comparison to approved BRAFi at clinical relevant doses, this novel agent showed a substantially improved activity in a number of diverse BRAF V600E models. In addition, as a single agent, it outperformed a currently approved BRAFi/MEKi com-bination in a model of acquired resistance to clinically available BRAFi. Compound Ia presents high central nervous system (CNS) penetration and triggered evident superiority over approved BRAFi in a macro-metastatic and in a disseminated micro-metastatic brain model. Potent inhibition of MAPK by Compound Ia was also demonstrated in patient-derived tumor samples. Conclusions: The novel BRAFi demonstrates preclinically the potential to outperform available targeted therapies for the treatment of BRAF-mutant tumors, thus supporting its clinical investigation.