The tryptophan catabolite or kynurenine pathway in a major depressive episode with melancholia, psychotic features and suicidal behaviors; a systematic review and meta-analysis

Background Major depressive disorder (MDD) and bipolar disorder (BD) with melancholia and psychotic features and suicidal behaviors are accompanied by activated immune-inflammatory and oxidative pathways which may stimulate indoleamine 2,3- dioxygenase (IDO), the first and rate-limiting enzyme of the tryptophan catabolite (TRYCAT) pathway resulting in increased tryptophan degradation and elevated tryptophan catabolites (TRYCTAs). Objective The purpose of the current study is to systematically review and meta-analyze levels of TRP, its competing amino-acids (CAAs) and TRYCATs in patients with severe affective disorders. Methods PubMed, Google Scholar and SciFinder were searched in the present study and we recruited 35 studies to examine 4,647 participants including 2,332 unipolar (MDD) and bipolar (BD) depressed patients and 2,315 healthy controls. Results Severe patients showed significant lower (p<0.0001) TRP (standardized mean difference, SMD=-0.517, 95% confidence interval, CI: -0.735; -0.299) and TRP/CAA (SMD= -0.617, CI: -0.957; -0.277) levels with moderate effect sizes, while no significant difference in CAAs were found. Kynurenine (KYN) levels were unaltered in severe MDD/BD phenotypes, while the KYN/TRP ratio showed a significant increase only in patients with psychotic features (SMD= 0.224, CI: 0.012; 0.436). Quinolinic acid (QA) was significantly increased (SMD= 0.358, CI: 0.015; 0.701) and kynurenic acid (KA) significantly decreased (SMD= -0.260, CI: -0.487; -0.034) in severe MDD/BD. Conclusion Patients with affective disorders with melancholic and psychotic features and suicidal behaviors show normal IDO enzyme activity but a lowered availability of plasma/serum TRP to the brain, which is probably due to other processes such as low albumin levels.