LAUSR

Background Tacrolimus, a drug for prevention of rejection after kidney transplantation, has a narrow therapeutic window and is metabolized by the cytochrome P540 3A (CYP3A) system. Tacrolimus exposure increases after steroid tapering in many patients. The pregnane X receptor (PXR)-a mediator for CYP3A-has a steroid receptor and might regulate CYP3A5 activity depending on single nucleotide polymorphisms (SNPs) of CYP3A5 or PXR. This may contribute to differences in tacrolimus exposure after steroid tapering. Methods In a cohort of renal transplant recipients, the influence of CYP3A5 and PXR SNPs (A7635G, C8055T and C25385T) on the dose-normalized Tacrolimus trough concentration (DnC0) and their potential interaction with each other after steroid taper were analysed by linear regression. Eligible were all 83 outpatient renal transplant patients on tacrolimus and steroids in a pharmacokinetic steady state at least 6 weeks after transplantation and whose blood was available for genetic analysis. Results Compared with the CYP3A5*1/*3 genotype, the CYP3A5*3/*3 SNP showed a significantly stronger increase in DnC0 after steroid taper (+0.29 µg/L/mg; P = 0.002). Of the tested PXR SNPs, PXR G7635G individuals had a significantly stronger increase in DnC0 (compared with A7635A, +0.31 µg/L/mg; P = 0.02), with a weaker increase in A7635G heterozygotes (+0.17 µg/L/mg; P = 0.124). There was neither interaction nor association between CYP3A5 and PXR SNPs. Conclusions The magnitude of the DnC0 increase due to steroid taper after renal transplantation is related to CYP3A5 SNPs. Independently, the PXR G7635G SNP is related to this increase, proving the role of PXR in tacrolimus metabolism.