CDK4/6 and SHP2 mediate BRAF/MEK inhibitor resistance in Class 2 and 3 BRAF mutant cancers

Class 2 and 3 non-V600E BRAF mutations are oncogenic drivers in many cancer types. Currently, there are no established targeted therapies with proven efficacy for cancers with non-V600E BRAF mutations. We developed the investigator-initiated, Phase II BEAVER clinical trial (NCT03839342) to evaluate the efficacy of BRAF and MEK inhibitors in patients with non-V600E BRAF mutations. The best objective response rate was 14% (3/21). By analyzing genomic data from patient tumors, circulating tumor DNA (ctDNA), patient-derived xenograft (PDX) models generated from enrolled patients, and Class 2 & 3 BRAF mutant cell lines, we discovered MAPK-dependent and independent mechanisms of resistance to BRAF/MEK inhibition. These mechanisms included the acquisition of new mutations in NRAS, MAP2K1, RAF1, and RB in ctDNA at the time of disease progression. CDK4/6 and SHP2 were identified as mediators of intrinsic resistance to BRAF/MEK inhibition in Class 2 & 3 BRAF mutant tumors. Therapeutic strategies combining CDK4/6 or SHP2 inhibitors with BRAF/MEK inhibitors were more effective than BRAF/MEK inhibitors alone in these cancers.