Relationships of Selected CYP and ABCB1 Single Nucleotide Polymorphisms and Patient Characteristics with the Pharmacokinetics of Twice Daily Tacrolimus and Advagraf® in Renal Transplant Recipients
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Tacrolimus is one of the most potent immunosuppressive agents. It has almost replaced ciclosporin (CsA) as the drug of first choice for the prevention of graft rejection after kidney transplantation (Bouamar et al, 2013). Tacrolimus was initially available as a preparation requiring twice daily administration: Prograf®. A prolonged release preparation, Advagraf®, has become available recently with proven efficacy and safety following once daily dosing. Tacrolimus has a narrow therapeutic window and its high pharmacokinetic variability renders dose selection challenging. Therefore, therapeutic drug monitoring (TDM) is used routinely to direct tacrolimus dosing. To some extent, this variability is influenced by genetic factors. Enzymes in the cytochrome P4503A family (CYP3A) and the drug transporter P-glycoprotein (P-gp) play important roles in the absorption and metabolism of tacrolimus (MacPhee et al, 2002). The influence of the CYP3A5*3 and ABCB1 3435C>T genotypes on the pharmacokinetics of immediate release tacrolimus; Prograf® is well-defined. However, it is unclear for prolonged release tacrolimus; Advagraf®. Recently identified polymorphisms CYP3A4*22 and P450 Oxidoreductase (POR*28) were reported to have additional effects on tacrolimus pharmacokinetics and dose requirement (Jonge et al., 2011; Elens et al., 2013). Recently, 4β-hydroxycholesterol (4β-OHC) has been shown to be an endogenous marker of P450 3A activity in clinical practice (Diczfalusy et al., 2011). Prednisolone is a known inducer of both CYP3A and P-gp. The role of CYP3A4*22 and POR*28 in prednisolone metabolism is unknown. An inverse correlation between corticosteroid daily dose and tacrolimus exposure was demonstrated in renal transplant recipients (Anglicheau et al., 2003a). Achieving therapeutic trough concentration is of vital importance during the period immediately after transplantation. Therefore, the identification of parameters predictive of the optimal tacrolimus dosage would be a great clinical asset in the determination of adequate tacrolimus administration. Furthermore, high within-patient variability (WPV) in tacrolimus exposure is considered as a risk factor for allograft loss and late acute rejection (Wu, et al. 2011). The causes of this variability are not completely understood. The studies outlined in this thesis were carried out on stable renal transplant patients treated with twice daily tacrolimus (Prograf®or Adoport®) and were switched to the same total daily dose of Advagraf®. 24 hours pharmacokinetic profiles were performed before and two weeks after the change. In order to exclude the use of prednisolone as a confounding factor, only patients on not more than 5 mg prednisolone daily were included. The within-patient variability (WPV) was calculated based on the dose-normalized tacrolimus trough blood concentrations (C0). Analysis of C0 was also made during periods of stable tacrolimus doses. This study was designed to assess the influence of genetic polymorphisms CYP3A5*3 and ABCB1 3435C >T on tacrolimus pharmacokinetics of immediate- and prolonged- release tacrolimus formulations and their correlation with tacrolimus dosing in 64 stable renal transplant recipients. Genotyping at CYP3A4*22 and POR*28 loci in this study was undertaken to ascertain any influence of these genes on the pharmacokinetics of twice and once daily tacrolimus formulations. The influence of switching stable renal transplant patients to once daily tacrolimus formulation (Advagraf®) on WPV was investigated. In a secondary exploratory study to investigate the potential utility of 4β-OHC as a CYP3A biomarker in informing tacrolimus dosing, 4β-OHC concentrations in plasma samples was measured and the relationship between 4β-OHC, CYP3A5*3 genotype and tacrolimus exposure was examined. As another secondary exploratory study, prednisolone plasma concentrations were measured to explore the relationship between the above mentioned genetic polymorphisms and prednisolone exposure and its effect on tacrolimus dose. A significantly lower tacrolimus exposure was observed in CYP3A5 expressers compared with CYP3A5 non-expressers for both formulations. In contrast to CYP3A5*3 genotype, ABCB1 3435C>T gene had a minor influence on tacrolimus exposure irrespective of tacrolimus formulation. When combined, tacrolimus pharmacokinetics and dose requirements were significantly correlated with the combined-genotype grouping. The CYP3A4*22 CT genotype was associated with significantly greater tacrolimus exposure (AUC0-24, Cmax) compared with the CYP3A4*22 CC genotype. POR*28 CT/TT genotype was associated with significantly lower tacrolimus exposure compared with the POR*28 CC genotype in CYP3A5 non-expressing subjects.Switching from immediate to prolonged release tacrolimus formulations in kidney transplant patients was associated with a significantly lower tacrolimus trough concentration (C0), but had no influence on WPV. CYP3A5 genotype had no impact on WPV. Plasma concentration of 4β-OHC was greater in CYP3A5 expressers. The 4β-OHC/C ratio was significantly correlated with tacrolimus exposure and dose requirement. Prednisolone exposure was not influenced by CYP3A5*3, CYP3A4*22, ABCB1 3435C>T or POR*28 genotype. Our results indicate that CYP3A5*3, ABCB1 3435C>T and CYP3A4*22 polymorphisms are important determinants of tacrolimus disposition and may explain part of the clinically observed high between-individual variability in tacrolimus pharmacokinetics. POR*28 is associated with tacrolimus dose requirement in CYP3A5 non-expressers. Thus, genotyping at these loci before renal transplantation may provide important information about the optimal initial dose of tacrolimus. Pharmacogenetic dosing strategies based on these genotypes are likely to be equally applicable to prescribing the once daily tacrolimus formulation, Advagraf®, as to twice daily formulations. Moreover, switching from immediate to prolonged release tacrolimus formulations had no influence on WPV. 4β-OHC/C ratio may be a useful biomarker for tacrolimus dosing in renal transplanted patients. Genotyping at CYP3A5*3, CYP3A4*22, POR*28 and ABCB1 3435C>T loci is unlikely to allow individualization of prednisolone dose.
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