Tacrolimus (FK506) metabolite patterns in blood from liver and kidney transplant patients

The metabolite patterns of tacrolimus in blood were evaluated in 41 kidney and liver graft recipients. Trough concentrations of tacrolimus and its metabolites were measured by HPLC-mass spectrometry and microparticle enzyme immunoassay in parallel. A statistically significant correlation between results of both assays was observed for kidney and liver transplant patients (r = 0.77, P < 0.001 and r = 0.71, P < 0.001, respectively). The main metabolites in blood were demethyl, demethylhydroxy, didemethyl, didemethylhydroxy, and hydroxy tacrolimus. These metabolites added up to 42% (range 0-145%) of the tacrolimus concentration in liver transplant patients and to 44.8% (range 16-152%) in kidney transplant patients. During episodes of impaired lifer function, concentrations of tacrolimus and its metabolites were increased compared with normal liver function, indicating accumulation of metabolites, in particular second-generation metabolites such as didemethyl and didemethylhydroxy tacrolimus. Stepwise regression analysis including tacrolimus, its metabolites, and liver function parameters suggested a model including serum activities of gamma-glutamyltransferase, alkaline phosphatase, and alanine aminotransferase as predictors for increased concentrations of demethyl tacrolimus, didemethyl tacrolimus, and the parent drug.