PDXP and major depressive disorder: However, in line with reports in other medical conditions (see Ueland et al., 2017 for review), our exploratory analyses found that PLP concentrations were negatively associated with IL-6 and CRP concentrations at baseline in patients with depression, while, interestingly, the change in nicotinamide (vitamin B3) following a course of treatment with ECT was positively associated with the change in TNF-α.