Co-administration of an undisclosed novel IDO1 inhibitor decreased production of KP metabolites kynurenine, QUIN, KYNA and 3-HK but did not reverse sickness behaviours, suggesting that KP induction occurs as a result of CD40-TNF activation but pathway metabolites are not necessary for the CD40-TNF-α-induced initiation of sickness and depression-like behaviours [217]. Here, TNF is linked to major depressive disorder.