The kynurenine pathway in major depressive disorder, bipolar disorder, and schizophrenia: a systematic review and meta-analysis of cerebrospinal fluid studies.

INTRODUCTION

The kynurenine pathway has been attracting attention as a relevant pathway in schizophrenia (SZ), bipolar disorder (BD), and major depressive disorder (MDD). We conducted a systematic review and meta-analysis of the kynurenine pathway metabolites from cerebrospinal fluid (CSF) samples in SZ, BD, and MDD.

METHODS

PubMed and Scopus databases were systematically searched to identify peer-reviewed case-control studies until April 2022 that assessed kynurenine metabolites, namely, tryptophan (TRP), kynurenine (KYN), kynurenic acid (KA), quinolinic acid (QA), and 3- hydroxykynurenine (3-HK) in SZ, BD, or MDD subjects compared with healthy controls (HC). The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. The random effects model method was selected when comparing the standardized mean differences (SMD) between two groups.

RESULTS

There were 23 articles that met the inclusion criteria (k=8, k=8, k=11, for SZ, BD, and MDD, respectively). In SZ, KA levels were increased [SMD=2.64, confidence interval (CI) =1.16 to 4.13, p=0.0005, I2=96%, k=6, n=384 subjects]. TRP (k=5) and KYN (k=4) did not differ significantly. In BD, TRP levels (k=7) did not differ significantly. The level of KA was increased in MDD (k=2), but the small number of studies made not possible for statistical significance evaluation. Finally, in MDD, although some studies tended to have an increased level of KYN in those with remission versus decreased levels in those with current depression, no significant difference was found in any of the kynurenine metabolite levels. Similarly, there was an increased level of QA (k=2) but the number of studies (k= 2) was small.

CONCLUSION

KA, which has possibly neuroprotective effects, is increased in SZ. QA, which has neurotoxic effects, may be increased in MDD. There were no alterations in BD. There may be alterations in this pathway based on population characteristics and mood states. Future studies should explore the utility of these metabolites as biomarkers.