mTOR inhibition enhances the antitumor efficacy of RAF dimer-MEK blockade by inhibiting the ATF4-MTHFD2 pathway

RAF monomer inhibitors are clinically approved for the treatment of BRAF ^V600 -mutant melanoma in combination with a MEK inhibitor, but ineffective in other melanoma subtypes. Moreover, RAF dimer inhibitors, such as belvarafenib, when combined with MEK inhibitors (cobimetinib) have promising but limited efficacy in non- BRAF -mutant melanomas. Here, we report that the mTOR inhibitor sapanisertib improves the efficacy of combined belvarafenib and cobimetinib therapy in NRAS , NF1 , and KIT -mutant melanomas. Mechanistically, sapanisertib combined with belvarafenib and cobimetinib suppressed ATF4 expression and its target gene MTHFD2 while inducing DNA damage, revealing a previously underappreciated role of the ATF4-MTHFD2 axis in DNA damage repair and drug response. Human and murine models resistant to combined belvarafenib and cobimetinib exhibited elevated levels of ATF4 and MTHFD2 and were sensitive to sapanisertib. This study provides novel treatment opportunities for patients with non- BRAF -mutant melanomas, or those who relapse following belvarafenib and cobimetinib combination therapy.