IDO1 and major depressive disorder: Thus, in the purpose of the current study is to systematically review and meta-analyze TRP, CAAs, and the activity of TRYCAT pathway as reflected by KYN/TRP ratio (IDO enzyme index), KA/KYN (kynurenine Aminotransferase, KAT enzyme index), neurotoxicity indices and solitary levels of TRYCATs in MDD/BD patients with melancholic or psychotic features and suicidal behaviors.