An adverse tumor-protective effect of IDO1 inhibition

Indoleamine 2, 3-dioxygenase 1 (IDO1) is a metabolic enzyme catalyzing the conversion of the essential amino acid tryptophan to kynurenine. It is induced by IFNg and its expression is linked to poor prognosis across various cancer types. T cells are particularly sensitive to IDO1-dependent tryptophan deprivation in the tumor microenvironment, leading to tumor immune resistance. Therefore, IDO1 inhibitors, such as epacadostat, have been developed, aiming to restore T cell antitumor activity. However, a recent phase III trial assessing the clinical benefit of epacadostat with the PD-1 antibody pembrolizumab in melanoma failed. This prompted us to study the role of IDO1, and the effect of its inhibition, on tumor cells that are under IFNg treatment and T cell attack. We showed that IDO1-mediated tryptophan depletion contributes to the anti-tumor effect of CD8 T cells. Thus, while epacadostat expectedly replenished tryptophan -the desired effect-, this led to adverse protection of melanoma cells from T cell-derived IFNg. RNA sequencing and ribosome profiling of (patient-derived) melanoma cell lines revealed that IFNg caused general protein translation shut down, which was reversed by IDO1 inhibition. Impaired translation was accompanied by an amino acid deprivation-dependent stress response driving ATF4high and MITFlow transcriptomic signatures, which was also observed in melanoma patient tumors. Downregulation of MITF in tumors and PDX- derived cell lines upon treatment was strongly predictive of response to response, to checkpoint blockade-treatment and IFNg, respectively. Conversely, MITF restoration in cultured tumor cells caused T cell resistance. These results highlight the critical role of tryptophan in the melanoma response to T cell-derived IFNg, uncovering an unexpected negative consequence of IDO1 inhibition. 16 low-passage (less than 30) Patient-derived cell lines were plated at day 0 and treated with recombinant human IFNg (Preprotech) at 10ng/mL at day 1 and harvested after 24 hours (day2).