Abstract and Introduction
Background: Tacrolimus (TAC), acting as a calcineurin inhibitor, is an immunosuppressant widely used after kidney transplantation. TAC requires blood concentration monitoring due to large interindividual variability in its pharmacokinetics and a narrow therapeutic index. Since genetic factors are considered responsible for a part of the observed pharmacokinetic variability, hereby SNPs within the CYP3A4, CYP3A5 and ABCB1 genes in kidney transplant patients of Polish Caucasian origin were investigated.
Patients & methods: A total of 241 patients treated with TAC through the first year after kidney transplantation were genotyped for the presence of common SNPs: rs776746:A>G (CYP3A5*3), rs35599367:C>T (CYP3A4*22), rs2740574:A>G (CYP3A4*1B) and rs1045642:C>T (ABCB1 3435C>T) using TaqMan® assays.
Results:CYP3A5 expressers received significantly higher weight-adjusted TAC doses, and were characterized by markedly lower C0 and dose adjusted C0 values in the course of treatment. CYP3A4*1B was significantly associated with TAC pharmacokinetics in univariate analysis. Impact of the CYP3A4*22 allele was significant only at particular time points, that is, 3 months after transplantation, with marginal significance 6 months after transplantation. The ABCB1 genotype did not influence TAC pharmacokinetics. Multivariate analysis of all the studied loci demonstrated that only the CYP3A5*1 (starting from month 1) and CYP3A4*22 alleles (at 3 and 6 months) were independent predictors of TAC dose-adjusted C0.
Conclusion: Our results confirm the impact of the CYP3A4*22 allele on TAC pharmacokinetics, as a second significant genetic factor (in addition to the CYP3A5*1 allele) influencing TAC dose-adjusted blood concentrations in kidney transplant recipients.
Tacrolimus (TAC), acting as a calcineurin inhibitor, is used as an immunosuppressant after kidney transplantation in order to prevent acute organ rejection. A combination of TAC, an adjuvant agent (i.e., mycophenolate mofetil) and corticosteroids is currently the most common immunosuppressive regimen. However, TAC is characterized by a narrow therapeutic index and large interindividual variability in its pharmacokinetics, thus requiring blood concentration monitoring and dose adjustment in order to achieve therapeutic efficacy and avoid adverse effects, which are the most challenging during the first weeks after transplantation. For this time period, data from therapeutic drug monitoring are still limited, which leads to the most pronounced differences in drug concentration among patients, which might involve an increased risk for graft rejection. Hence, identification of factors contributing to variability in TAC pharmacokinetics can result in individualization of therapeutic strategy, increasing its safety and efficacy.
Among different variables, genetic factors are considered to be responsible, at least in part, for the observed differences. TAC is metabolized by the CYP450 3A family, which in adults includes CYP3A4 and CYP3A5 members, expressed both in liver and intestine. It is now generally accepted that much of the interindividual variability in TAC pharmacokinetics can be explained by the presence of a nonfunctional splicing-defect in the CYP3A5*3 allele, determined by a rs776746:G>A SNP, and patients characterized by the CYP3A5*3/*3 genotype (lacking CYP3A5 activity, denominated as CYP3A5 nonexpressers) require less TAC to reach target concentrations compared with carriers of the CYP3A5*1 allele (CYP3A5 expressers). However, the CYP3A5*1 allele is present only in up to 10% of Caucasian subjects. In the case of CYP3A4, no common lack-of-function alleles were described. The rs2740574(G) allele, which encodes a variant form of CYP3A4 (known as CYP3A4*1B), was identified in a gene promoter, and could potentially influence transcriptional activity. However, different studies performed during the last 20 years provided contradictory results. It is possible that linkage disequilibrium (LD) between that variant and CYP3A5*1 underlies the impact of CYP3A4*1B on TAC dosage reported in some populations. Recently, a novel CYP3A4 SNP (CYP3A4*22) was reported as significantly affecting TAC pharmacokinetics in kidney transplant recipients. Although it is an intronic variant, it was presented to significantly influence CYP3A4 mRNA levels and enzyme activity in the human liver. Another genetic factor considered to affect TAC dosage is a polymorphism of the ABCB1 gene (formerly designated as MDR1), coding for P-gp. It is known that calcineurin inhibitors: TAC and cyclosporin A are transported by P-gp, hence changes in P-gp activity might result in altered bioavailability of both drugs. Although the results of pharmacogenetic studies on ABCB1 polymorphisms were often contradictory and population-specific, a recent meta-analysis published by Li et al. demonstrated that SNP 3435C>T (rs1045642) could influence TAC pharmacokinetic parameters at different post-transplant times. Conversely, a meta-analysis published by Terrazino et al. on TAC-treated renal transplant recipients did not reveal consistent evidence of an effect of the ABCB1 3435C>T variant on TAC dose-adjusted concentrations. However, the authors found a modest effect of the SNP, that was limited to the first month after renal transplantation. Despite it being a silent polymorphism, not affecting the amino acid sequence, it was originally linked to twofold reduction of P-gp duodenal expression and increased concentration of its substrate, digoxin, in a study by Hoffmeyer et al.. It was initially suggested that the 3435C>T polymorphism might only be a marker of other SNPs in strong linkage (i.e., 2677G>T/A, and 1236C>T), forming common haplotypes. However, significant associations were more frequently identified in the case of 3435C>T than the latter SNPs. More recently it has been suggested that the 3435T allele, previously associated with reduced P-gp activity, changes the common codon of isoleucine for a rare one, which may result in altered protein folding, and consequently, influences transporter specificity. Hence, in the current study we decided to investigate the 3435C>T SNP exclusively and not other common ABCB1 polymorphisms.
As replication is very important in reaching a final consensus of genetic studies, hereby we investigated SNPs in the CYP3A4, CYP3A5 and ABCB1 genes in relation to TAC pharmacokinetics in kidney transplant patients of Polish–Caucasian origin.