Preclinical Activity of the Type II RAF Inhibitor Tovorafenib in Tumor Models Harboring Either a BRAF Fusion or an NF1 Loss-of-Function Mutation.

Genomic alterations and dysregulation of the MAPK pathway have been described in many different types of cancers. BRAFV600 mutations and BRAF fusions, found in both pediatric and adult cancers, are oncogenic drivers that drive constitutive activation of the RAF pathway. Neurofibromin 1 (NF1) loss-of-function (LOF) mutations, which occur in many cancer types, result in decreased neurofibromin GAP function, thus activating RAS. Tovorafenib is an oral selective, central nervous system–penetrant, type II RAF inhibitor which inhibits RAF monomers and dimers. In this study, the impact of tovorafenib alone or in combination with MEK inhibitor, pimasertib, was explored in adult or pediatric tumor models harboring BRAF fusions or NF1-LOF mutations. Tovorafenib resulted in tumor regression in an AGK::BRAF fusion melanoma patient-derived xenograft model in vivo, while exhibiting little antitumor activity in NF1-LOF tumor models. In vitro anti-proliferative activity of tovorafenib and pathway modulation was evaluated in NF1-LOF tumor cell lines. Little anti-proliferative activity was observed in the NF1-LOF tumor cell lines treated with tovorafenib. In NF1-LOF tumor cells treated with tovorafenib, an increase in phosphorylated ERK was observed at low concentrations, with inhibition of phosphorylated ERK at higher concentrations. When tovorafenib was combined with pimasertib in vitro, synergy was observed in an NF1-LOF embryonal rhabdomyosarcoma patient-derived xenograft model ex vivo and an NF1-LOF Malignant peripheral nerve sheath tumors (MPNST) cell line in vitro, suggesting that vertical pathway inhibition is needed in the NF1-LOF mutant setting. Tovorafenib demonstrated efficacy in BRAF fusion but not in NF1-LOF mutant tumor models. Vertical pathway inhibition by combining type II RAF plus MEK inhibitors may have clinical relevance in NF1-LOF mutant tumors.