Background: Tacrolimus is metabolized predominantly to 13-O-demethyltacrolimus in the liver and intestine by cytochrome P450 3A (CYP3A). Patients with high concentrations of CYP3A5, a CYP3A isoenzyme polymorphically produced in these organs, require higher doses of tacrolimus, but the exact mechanism of this association is unknown.
Methods: cDNA-expressed CYP3A enzymes and a bank of human liver microsomes with known CYP3A4 and CYP3A5 content were used to investigate the contribution of CYP3A5 to the metabolism of tacrolimus to 13-O-demethyltacrolimus as quantified by liquid chromatography-tandem mass spectrometry.
Results: Demethylation of tacrolimus to 13-O-demethyltacrolimus was the predominant clearance reaction. Calculated K(m) and V(max) values for CYP3A4, CYP3A5, and CYP3A7 cDNA-expressed microsomes were 1.5 micromol/L and 0.72 pmol x (pmol P450)(-1) x min(-1), 1.4 micromol/L and 1.1 pmol x (pmol P450)(-1) x min(-1), and 6 micromol/L and 0.084 pmol x (pmol P450)(-1) x min(-1), respectively. Recombinant CYP3A5 metabolized tacrolimus with a catalytic efficiency (V(max)/K(m)) that was 64% higher than that of CYP3A4. The contribution of CYP3A5 to 13-O-demethylation of tacrolimus in human liver microsomes varied from 1.5% to 40% (median, 18.8%). There was an inverse association between the contribution of CYP3A5 to 13-O-demethylation and the amount of 3A4 protein (r = 0.90; P <0.0001). Mean 13-O-demethylation clearances in CYP3A5 high and low expressers, estimated by the parallel-tube liver model, were 8.6 and 3.57 mL x min(-1) x (kg of body weight)(-1), respectively (P = 0.0088).
Conclusions: CYP3A5 affects metabolism of tacrolimus, thus explaining the association between CYP3A5 genotype and tacrolimus dosage. The importance of CYP3A5 status for tacrolimus clearance is also dependent on the concomitant CYP3A4 activity.