Antitumor activity of the selective pan-RAF inhibitor TAK-632 in BRAF inhibitor-resistant melanoma

The mitogen-activated protein kinase (MAPK) pathway is particularly important for the survival and proliferation of melanoma cells. Somatic mutations in BRaf and NRAS are frequently observed in melanoma. Recently, the BRaf inhibitors vemurafenib and dabrafenib have emerged as promising agents for the treatment of melanoma patients with BRAF-activating mutations. However, as BRaf inhibitors induce Raf paradoxical activation via Raf dimerization in BRaf wild-type cells, rapid emergence of acquired resistance and secondary skin tumors as well as presence of few effective treatment options for melanoma bearing wild-type BRaf (including NRAS-mutant melanoma) are clinical concerns. Here, we demonstrate that the selective pan-RAF inhibitor TAK-632 suppresses Raf activity in BRaf wild-type cells with minimal Raf paradoxical activation. Our analysis using RNAi and TAK-632 in preclinical models reveals that the MAPK pathway of NRAS-mutated melanoma cells is highly dependent on Raf. We also show that TAK-632 induces Raf dimerization but inhibits the kinase activity of the Raf dimer, probably because of its slow dissociation from Raf. As a result, TAK-632 demonstrates potent antiproliferative effects both on NRAS-mutated melanoma cells and BRAF-mutated melanoma cells with acquired resistance to BRaf inhibitors through NRAS mutation or BRaf truncation. Furthermore, we demonstrate that the combination of TAK-632 and the MAPK kinase (MEK) inhibitor TAK-733 exhibits synergistic antiproliferative effects on these cells. Our findings characterize the unique features of TAK-632 as a pan-RAF inhibitor and provide rationale for its further investigation in NRAS-mutated melanoma and a subset of BRAF-mutated melanomas refractory to BRaf inhibitors.