METABOLISM OF THE IMMUNOSUPPRESSANT TACROLIMUS IN THE SMALL-INTESTINE- CYTOCHROME-P450, DRUG-INTERACTIONS, AND INTERINDIVIDUAL VARIABILITY

The small intestinal metabolism of tacrolimus, which is used as an imm unosuppressant in transplantation medicine, was investigated in this s tudy. Tacrolimus was metabolized in vitro by isolated human, pig, and rat small intestinal microsomes. The metabolites generated were identi fied by HPLC/MS. Tacrolimus and its metabolites were quantified using HPLC or HPLC/MS. The cytochrome P450 (CYP) enzymes responsible for tac rolimus metabolism in small intestine were identified using; specific CYP antibodies and inhibitors. For characterization of the interindivi dual variability, microsomes were isolated from small intestinal sampl es of patients who had undergone resection for various reasons. In an in vitro model using pig small intestinal microsomes, 32 drugs were an alyzed for their interactions with tacrolimus metabolism. After incuba tion with human, rat, and pig small intestinal microsomes, the metabol ites 13-O-demethyl and 13,15-O-demethyl tacrolimus were identified. Th e metabolism of tacrolimus by human small intestine was inhibited by a nti-CYP3A, troleandomycin, and erythromycin, indicating that, as in th e liver, CYP3A enzymes are the major enzymes for tacrolimus metabolism in the human small intestine. Metabolism of tacrolimus by small intes tinal microsomes isolated from 14 different patients varied between 24 and 110 pmol/13-O-demethyl tacrolimus/min/mg microsomal protein, with a mean +/- SD of 54.2 +/- 29.2 pmol/min/mg. Of 32 drugs tested, 15 we re found to inhibit small intestinal tacrolimus metabolism: bromocrypt ine, corticosterone, cyclosporine, dexamethasone, ergotamine, erythrom ycin, ethinyl estradiol, josamycin, ketoconazole, nifedipine, omeprazo le, progesterone, rapamycin, troleandomycin, and verapamil. All of the se drugs inhibited tacrolimus metabolism by human liver microsomes as well. It is concluded that tacrolimus is metabolized by cytochrome CYP 3A enzymes in the small intestine. The rate of the CYP3A enzymatic act ivities varies about 5 times from patient to patient, and drugs that i nterfere with the in vitro metabolism of tacrolimus in the liver also inhibit its small intestinal metabolism.