IDO1 and neoplasm: Although in vitro studies have shown EPA, a highly selective IDO1 inhibitor with moderate oral bioactivity, results in increased activity of cytotoxic T lymphocyte51,52, in vivo studies (phase III clinical trials) have failed to show significant response to treatment after application of EPA or its superiority to pembrolizumab, which could be because of the limited capacity for IDO1 inhibition at the tumor site53,54.