Kynurenine pathway metabolites as biomarkers in major depressive disorder

Abstract

The kynurenine pathway (KP) is the main route of tryptophan (TRP) degradation and an important axis of neuroimmune communication. This narrative review summarises clinical and preclinical evidence supporting KP metabolites as candidate biomarkers in major depressive disorder (MDD), with emphasis on KP imbalance and NAD⁺ homeostasis. Inflammation-driven activation of indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase (TDO), and kynurenine-3-monooxygenase (KMO) can divert flux toward the neurotoxic branch, increasing quinolinic acid (QA) and 3-hydroxykynurenine (3-HK) and reducing kynurenic acid (KYNA) formation.