Up-Regulation of Anthranilic Acid Formation and Pro-Cognitive Effect of Indoleamine 2,3-Dioxygenase Inhibition

Inhibition of indoleamine 2,3-dioxygenase (IDO) is a promising therapeutic strategy for cognitive impairment in Alzheimer's disease (AD). The pro-cognitive effect is often attributed to restoring glycolysis by preventing tryptophan (Trp) conversion to kynurenine (Kyn). However, this overlooks the metabolic fate of Trp when IDO is blocked. Since IDO and tryptophan 2,3-dioxygenase (TDO) compete for the same substrate, inhibiting IDO may shunt Trp toward TDO, potentially increasing Kyn and its downstream catabolites. This commentary explores the hypothesis that the upregulation of anthranilic acid (AA), a Kyn catabolite, contributes to the cognitive benefits of IDO inhibition. Recent evidence shows elevated AA in animal models of AD and individuals with mild cognitive impairment and preclinical AD, where it may serve as an early biomarker. Notably, these elevations and the pro-cognitive effects of AA-modulating compounds like sodium benzoate exhibit sex-specificity, being more prominent in females. The mechanism may involve AA's dual action on G-protein coupled receptors: antagonism of GPR17 promotes myelination, while agonism of GPR109A may protect myelin from degradation. Preserving myelin integrity is critical, as demyelination is an early event in AD pathogenesis. We propose that AA upregulation is not merely a biomarker but part of a compensatory defense mechanism. Therefore, the pro-cognitive effect of IDO inhibition may be partly mediated by the subsequent shunting of Trp toward TDO and production of the myelin-preserving metabolite, AA. This reframes the therapeutic goal from reducing neurotoxic kynurenines to leveraging the protective potential of the entire pathway.