CYP3A5*1/*3 genotype influences the blood concentration of tacrolimus in response to metabolic inhibition by ketoconazole

Objectives Ketoconazole retards metabolic degradation of tacrolimus through its effect on the cytochrome P-450 enzyme system and allows reduction in treatment costs. Enzyme activity is determined by a single nucleotide polymorphism (*1/*3) in the CYP3A5 gene. Methods We prospectively investigated the impact of this polymorphism on tacrolimus concentration in a cohort of 79 renal transplant recipients on ketoconazole. Genotyping was carried out by using polymerase chain reaction-restriction fragment length polymorphism technique. Dose-adjusted trough level (CO) was calculated at baseline and at 3, 7, 15, 30, and 60 days. Results The baseline C-0 was significantly lower in those with at least one *1 allele [44.95 +/- 14.12 vs. 63.43 +/- 14.72 (ng/ml)/(mg/kg/day), P<0.0001]. After starting ketoconazole in all genotypes, dose-normalized C-0 increased and the cost of therapy decreased. Compared with baseline, the magnitude of increase was 112% and 79% in those without and with *1 allele, respectively (P<0.001). The cost savings were 32% and 39% in mycophenolate mofetil-treated and 47% and 61% in azathioprine-treated patients who were with and without one *1 allele, respectively. Conclusion We show that the CYP3A5*1/*3 polymorphism is an important determinant of the response to inhibition of tacrolimus metabolism by ketoconazole, with a 30% greater inhibition in those lacking *1 allele. This finding will allow better dose adjustment and minimize exposure to subtherapeutic or toxic concentrations. Pharmacogenetics and Genomics 19:458-463 (C) 2009 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.