Off-target activation of GCN2 by BRAF V600 inhibitors attenuates melanoma outgrowth

Clinical efficacy of small-molecule inhibitors can sometimes be attributed to multiple drug effects acting in concert. Herein, we define an off-target effect in which several clinical BRAF V600 inhibitors, including the widely used dabrafenib and encorafenib, interact directly with GCN2 to activate the Integrated Stress Response and ATF4. Blocking this off-target effect by co-drugging with a GCN2 inhibitor in A375 melanoma cells causes enhancement rather than suppression of cancer cell outgrowth, suggesting that the off-target activation of GCN2 is detrimental to these cells. This result is mirrored in PC9 lung cancer cells treated with erlotinib, an EGFR inhibitor, that shares the same off-target activation of GCN2. Using an in silico kinase inhibitor screen, we identified dozens of FDA-approved drugs that appear to share this off-target activation of GCN2 and ATF4. We further describe the activation of GCN2 signaling in melanoma patient-derived xenograft models treated with dabrafenib and trametinib, and observe a significant decrease in disease-free survival in a cohort of human melanoma patients with altered GCN2 pathway components. Finally, we describe the benefits of combining the next-generation “paradox breaker” RAF inhibitor naporafenib with a GCN2 activator, HC-7366, in A375 melanoma cells. Thus, GCN2 is emerging as a promising cotreatment candidate in melanoma and potentially beyond.