BACKGROUND:: The P450 oxidoreductase (POR)*28 variant allele has been associated with altered cytochrome P450 3A enzyme activities. Both CYP3A5 and CYP3A4 are involved in the metabolism of calcineurin inhibitors and recent data show that POR*28 may explain part of the variability observed in tacrolimus (Tac) pharmacokinetics. The aim of this study was to investigate the impact of the POR*28 allele on Tac and cyclosporine A (CsA) immunosuppressive therapies. METHODS:: Kidney transplant recipients receiving either Tac (n = 184) or CsA (n = 174), participating in a prospective multicenter trial, were genotyped for POR*28, CYP3A4*22, and CYP3A5*3. RESULTS:: CYP3A5 expressers that were carriers of at least 1 POR*28 allele had a 16.9% decrease in dose-adjusted predose concentrations when compared CYP3A5 expressers that carried the POR*1/*1 genotype (P = 0.03), indicating an increased CYP3A5 activity for POR*28 carriers. In CYP3A5, nonexpressers carrying 2 POR*28 alleles, a 24.1% (confidence interval95% =-39.4% to-4.9%; P = 0.02) decrease in dose-adjusted predose concentrations was observed for Tac, suggesting higher CYP3A4 activity. For CsA, POR*28/*28 patients not expressing CYP3A5 and not carrying the CYP3A4*22 decrease-of-function allele showed 15% lower CsA dose-adjusted predose concentrations (P = 0.01), indicating also increased CYP3A4 activity. In both cohorts (ie, Tac and CsA), the POR*28 allele was neither associated with the incidence of delayed graft function nor with biopsy-proven acute rejection. These results were further confirmed in 2 independent cohorts. CONCLUSIONS:: Our results show that the POR*28 allele is associated with increased in vivo CYP3A5 activity for Tac in CYP3A5 expressers, whereas POR*28 homozygosity was associated with a significant higher CYP3A4 activity in CYP3A5 nonexpressers for both Tac and CsA.

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Therapeutic Drug Monitoring
Department of Clinical Chemistry