Compensatory pathways in tryptophan metabolism and immune regulation following IDO inhibition.

Indoleamine 2,3-dioxygenase 1 (IDO1) is an intracellular heme-dependent enzyme that catalyzes the initial oxidation of L-tryptophan into the kynurenine pathway, thereby linking tryptophan metabolism to immune regulation. IDO1 expression is mainly induced by interferon-γ during immune activation and inflammation, and its activity helps control immune responses through tryptophan depletion and the generation of immunomodulatory metabolites. Because of its central role in immunometabolism, IDO1 has been widely investigated as a therapeutic target, and several strategies have been developed to inhibit its enzymatic activity or reduce its functional contribution to immune suppression. However, increasing evidence indicates that IDO1 inhibition may not completely interrupt tryptophan catabolism or downstream immunoregulatory signaling. Instead, IDO1 blockade can be accompanied by compensatory mechanisms that preserve metabolic and immune regulatory outputs. These mechanisms may include activity or upregulation of alternative tryptophan-catabolizing enzymes such as TDO2, IDO2, and IL4I1; metabolic diversion toward serotonin and melatonin pathways; microbiota-derived indole production; and cell-type-specific metabolic rewiring. Therefore, understanding these adaptive responses is essential for interpreting the limited efficacy of IDO1-targeted approaches and for improving therapeutic strategies directed at tryptophan metabolism. This review discusses the main compensatory pathways that may emerge under IDO1 inhibition and their relevance to developing more effective immunometabolic interventions.