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Metabolism of tacrolimus (FK 506) in rat liver microsomes. Effect of rifampin and dexamethasone.

The in vitro metabolism of tacrolimus (TAC, FK 506) was investigated in the liver microsomes prepared from normal rats as well as rats treated with dexamethasone (DEX) and rifampin (RIF). The rate of tacrolimus metabolism was similar in control and RIF treated rat liver microsomes, whereas it significantly increased in microsomes obtained from dexamethasone treated rats. Seven different possible metabolites were identified in the microsomal preparations from rats treated with rifampin or dexamethasone whereas the microsomes from the control rats failed to produce the mono-demethylated and monohydroxylated metabolite of TAC (TAC+2, m/z = 805.5). There was an apparent difference in the amount of individual metabolites formed in different groups. This indicates quantitative differences in the induction of cytochrome P450 3A, an enzyme sub family known to be primarily responsible for tacrolimus metabolism. Lack of induction of tacrolimus metabolism by rifampin can be attributed to the lack of effect of rifampin in inducing cytochrome P450 3A in rats.