Epacadostat Overcomes Cetuximab Resistance in Colorectal Cancer by Targeting IDO-Mediated Tryptophan Metabolism.

Primary or acquired mutations in RAS/RAF genes resulting in cetuximab resistance have limited its clinical application in colorectal cancer (CRC) patients. The mechanism of this resistance remains unclear. RNA sequencing from cetuximab-sensitive and -resistant specimens revealed an activation of the tryptophan pathway and elevation of IDO1 and IDO2 in cetuximab-resistant CRC patients. In vitro, in vivo, and clinical specimens confirmed the upregulation of IDO1and IDO2 and the Kyn/Trp after cetuximab treatment. Additionally, the IDO inhibitor, epacadostat, could effectively inhibit the migration and proliferation of cetuximab-resistant CRC cells while promoting apoptosis. Compared to epacadostat monotherapy, the combination of cetuximab and epacadostat showed a stronger synergistic anti-tumor effect. Furthermore, in vivo experiments confirmed that combination therapy effectively suppressed tumor growth. Mechanistically, KEGG pathway analysis revealed the activation of the IFN-γ pathway in cetuximab-resistant CRC tissues. Luciferase reporter assays confirmed the transcriptional activity of IDO1 following cetuximab treatment. Silencing IFN-γ then suppressed the upregulation induced by cetuximab. Moreover, we observed that the combination reduced the concentration of the tryptophan metabolite kynurenine, promoted the infiltration of CD8+ T lymphocytes, and enhanced the polarization of M1 macrophages within the tumor microenvironment, thereby exerting potent anti-tumor immune effects. Overall, our results confirm the remarkable therapeutic efficacy of combining cetuximab with epacadostat in cetuximab-resistant CRC. Our findings may provide a novel target for overcoming cetuximab resistance in CRC.