Indoleamine 2,3-dioxygenase as a novel therapeutic target for Huntington's disease

Huntington's disease (HD) is an autosomal dominant, neurodegenerative disorder. Despite the severe motor, psychiatric and cognitive symptoms and the great socioeconomic burden caused by the disease, available treatment is mainly symptomatic. The kynurenine pathway (KP) is the main metabolic route of tryptophan degradation, in the course of which several neuroactive compounds are generated. The imbalance of the neurotoxic and neuroprotectant metabolites can lead to excitotoxicity and overproduction of reactive oxygen species, which both contribute to the progression of HD. Indoleamine 2,3-dioxygenase 1 (IDO1) is a key enzyme of the KP that has various immune modulatory roles. Areas covered: Current knowledge of the involvement of KP in HD pathogenesis with a particular focus on IDO1. By reviewing the diverse roles of the enzyme in kynurenine production, immune modulation, and serotonin metabolism, we elucidate the factors that make this enzyme a therapeutic target. Expert opinion: Due to the complexity of HD and the various effects that IDO1 exerts, targeting this enzyme, while highly profitable, may be a great challenge. Through IDO1 activity, neurodegeneration, inflammatory processes and depressive symptoms, often related to HD, can be modulated. Ongoing trials of IDO1 inhibitors in other areas of medicine offer advantages for initiating approaches toward this enzyme as a therapeutic target.