IDO1 and systemic sclerosis: To this end, the main aim of this study is to examine the sex-related differences in the KP in SSc patients considering the peripheral concentrations of TRP, KYNA, and KYN as well as kynurenine pathway enzyme activity indicators, indoleamine 2,3-dioxygenase (IDO) and tryptophan 2-3dioxygenase (TDO), represented by the ratio of KYN/TRP and kynurenine aminotransferases (KATs) calculated as the KYNA/KYN ratio and KYNA/TRP ratio (see Scheme 1), while controlling for age.