title: Indoleamine 2,3-dioxygenase: is it an immune suppressor?
abstract: This article covers what is currently known about the role of the enzyme indoleamine 2,3-dioxygenase (IDO) in cancer-related immunosuppression and the clinical research on IDO inhibitors. A PUBMED search was performed using the terms IDO, indoleamine 2,3-dioxygenase, 1-MT. IDO is an inducible enzyme that catalyzes the rate-limiting first step in tryptophan catabolism. This enzyme is overexpressed in response to IFNgamma in a variety of different malignancies. IDO causes immunosuppression through breakdown of tryptophan in the tumor microenvironment and tumor-draining lymph nodes. The depletion of tryptophan and toxic catabolites renders effector T cells inactive and dendritic cells immunosuppressive. Preclinical data suggest that IDO inhibition can delay tumor growth, enhance dendritic cell vaccines, and synergize with chemotherapy through immune-mediated mechanisms. The lead IDO inhibitor, d-1-methyl-tryptophan (d-1-MT), was selected for phase I trials and seems to have immune modulating activity. Subsequently, another isoform of IDO, IDO2, was discovered and found to be the target of d-1-MT. Multiple single-nucleotide polymorphisms in IDO2 affecting its catalytic activity may serve as a pharmacogenetic predictive biomarker for d-1-MT. The IDO pathway is an important mechanism of tumor-related immunosuppression and blocking it could improve cancer immunotherapy outcomes. Clinical development of d-1-MT and other IDO inhibitors as systemic immunomodulators to be combined with other immune modulators, vaccines, and chemotherapy are ongoing.
authors.affiliation: Women's Oncology Department, Tampa, FL, USA. hatem.soliman@moffitt.org
authors.forename: Hatem
authors.initials: H
authors.lastname: Soliman
authors.forename: Melanie
authors.initials: M
authors.lastname: Mediavilla-Varela
authors.forename: Scott
authors.initials: S
authors.lastname: Antonia
mesh_terms: D000818:Animals; D006801:Humans; D007108:Immune Tolerance / Q000276:immunology; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000276:immunology*; D009369:Neoplasms / Q000201:enzymology* / Q000276:immunology*
chemical_list: D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase