Measurement of blood concentrations of FK506 (tacrolimus) and its metabolites in seven liver graft patients after the first dose by h.p.l.c.‐MS and microparticle enzyme immunoassay (MEIA).

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Abstract
1. Blood and urine concentrations of the macrolide immunosuppressant FK506 and its metabolites were measured in seven orthotopic liver transplant patients after the first oral dose of FK506 (0.04 +/‐ 0.02 mg kg‐1) used as primary immunosuppressant. A specific h.p.l.c.‐MS assay was used, allowing the measurement of parent drug and eight metabolites. Results were compared with those obtained using a microparticle enzyme immunoassay (MEIA). 2. Blood drug concentrations were described by an open two compartment model with first‐order absorption giving the following mean data: tmax: 1.9 (h), Cmax: 17.4 (microgram l‐1), AUC: 328.1 (microgram l‐1 h), t1/2,1: 0.74 (h). The terminal elimination half‐life was estimated at about 26 h using the h.p.l.c.‐MS assay. 3. The metabolites found in blood were demethyl‐ FK506 and demethyl‐hydroxy‐FK506, while in urine FK506 and eight of its metabolites were detected.