A Systematic Review of the Effect of CYP3A5 Genotype on the Apparent Oral Clearance of Tacrolimus in Renal Transplant Recipients

Impact of CYP2D6*2A, CYP2D6*4 and CYP3A5*3 genetic polymorphisms on Bisoprolol peak concentration and clinical response in acute coronary syndrome patients

AIMS

Acute coronary syndrome (ACS) represents a major cause of death. Bisoprolol is commonly used in the management of ACS. This study aims to investigate the impact of CYP2D6*2A, CYP2D6*4 and CYP3A5*3 genetic polymorphisms on pharmacokinetics and clinical response of bisoprolol in ACS patients.

METHODS

This is an open-label cohort study that included 127 ACS patients and studied the effect of CYP3A5*3, CYP2D6*2A and CYP2D6*4 genotyping using real-time polymerase chain reaction on steady state bisoprolol plasma peak concentration analysed by high performance liquid chromatography-fluorescence detector.

RESULTS

Regarding CYP3A5*3, the mean peak bisoprolol concentration for CC, CT and TT genotypes were 4.25 ± 1.20, 3.93 ± 1.10 and 1.79 ± 0.69 ng/mL, respectively (P < .001). Higher systolic (126 ± 5.47 mmHg), diastolic blood pressure (82 ± 2.73 mmHg) and heart rate (97.80 ± 3.03 beats/min) were also observed in CYP3A5*3 TT carriers (P < .05). In CYP2D6*2A, the peak concentration of bisoprolol was lower in CC carriers (3.54 ± 1 ng/mL) compared to GG (4.38 ± 1.25 ng/mL) and GC carriers (4.07 ± 1.29 ng/mL, P = .019). In CYP2D6*4, the mean bisoprolol peak concentration in CC carriers was 3.98 ± 1.31 ng/mL, which was lower than T allele carriers (4.5 ± 0.8, P = .02). No differences in heart rate, systolic, diastolic blood pressure or bisoprolol dose were observed among CYP2D6*2A or CYP2D6*4 variants. Smokers exhibited lower bisoprolol peak concentration (3.96 ± 1.2 ng/mL) compared to nonsmokers (4.55 ± 1.34 ng/mL, P = .037).

CONCLUSION

There is an association between CYP3A5*3, CYP2D6*4, CYP2D6*2A variants and bisoprolol peak concentration, which may serve as a guide in the future in choosing the optimum dose of bisoprolol in ACS patients.

CYP3A5 Polymorphisms Leading to Tacrolimus Toxicity Following an Adult Renal Transplant

Tacrolimus is one of the calcineurin inhibitors used for maintaining immuno-suppression in thoracic and abdominal transplantations including heart, lung, liver, intestine, pancreas, and renal transplants. It has a narrow therapeutic window requiring therapeutic drug monitoring (TDM). Genetic polymorphism in the expression of cytochrome P3A5 enzyme plays a significant role in the bioavailability of tacrolimus in patients, leading to toxicity or rejection. In this case, we studied a renal transplant patient who received a standard dose of tacrolimus and experienced toxicity related to the poor expression of cytochrome P450 3A5 (CYP3A5), which required the withholding of tacrolimus and cutting the dose for several days with more frequent TDM. Similar cases have been reported before, yet there is no consensus on the appropriate dosage. The projected cost of additional TDM and hospitalization significantly exceeds the one-time cost of genetic CYP3A5 testing. In high-risk renal transplant recipients, pharmacogenetic testing must be considered to cut the time to limit TDM, prevent extended hospitalization, and reduce the total cost of transplantation.

The prevalence of pharmacogenetic variants of vitamin K epoxide reductase complex subunit 1 gene (rs9923231), cytochrome P450 family 2 subfamily C member 9 gene (rs1799853) and cytochrome P450 family 3 subfamily-A member-5 gene (rs776746) among 13 ethnic groups of Pakistan

BACKGROUND

Pharmacogenomics (PGx) plays a central role in the selection of targeted therapies that underpins precision-medicine. We investigated the prevalence of three important pharmacogenetic variants of VKORC1, CYP2C9, and CYP3A5 genes among Pakistani populations.

METHODS

A total of 1104 individuals were included representing thirteen major ethnicities. Samples were genotyped by using PCR-RFLP analysis. The allelic and genotypic frequencies of the three SNV's were calculated and were compared with the world's population data (ALFA, gnomAD, and 1000Genome, 1 K databases), using the chi-square test.

RESULTS

We found overall frequencies of functional-alleles of VKORC1 0.43, CYP2C9 0.94, and CYP3A5 0.14 in our population. Data showed a low prevalence of homozygous functional genotypes of VKORC1 (0.18; 0.0-0.45) and CYP3A5 (0.04; 0.0-0.22), and a high frequency of CYP2C9 (0.885; 0.80-1.0) across ethnicities. Genotyping distribution of VKCOR1 functional genotype was varied across ethnic groups such as 0.0-0.10 in Brahuis and Mohanas, Sindhis, Rajputs, and Gujjars populations, 0.11-0.20 in Makranis, Parsis, and Burusho populations, and 0.20-0.30 in Kalash, Kashmiris and Baloch populations. The highest VKORC1 (CC) was found in Pathans (0.45) and Hazaras (0.39) populations. Interestingly, we found a high prevalence of functional genotype CYP2C9 (rs1799853; C) and non-functional genotype of CYP3A5 (rs776746; T) across various ethnic groups of Pakistan.

CONCLUSION

Data regarding prevalence of clinically important pharmacogenomics SNVs could be useful in drug adjustment and avoiding adverse drug reactions in a specific ethnic population. This could help in moving current medical practices toward precision medicine in our part of the world.

Use of Pharmacogenetics to Optimize Immunosuppressant Therapy in Kidney-Transplanted Patients

Immunosuppressant drugs (ISDs) are routinely used in clinical practice to maintain organ transplant survival. However, these drugs are characterized by a restricted therapeutic index, a high inter- and intra-individual pharmacokinetic variability, and a series of severe adverse effects. In particular, genetic factors have been estimated to play a role in this variability because of polymorphisms regarding genes encoding for enzymes and transporters involved in the ISDs pharmacokinetic. Several studies showed important correlations between genetic polymorphisms and ISDs blood levels in transplanted patients; therefore, this review aims to summarize the pharmacogenetics of approved ISDs. We used PubMed database to search papers on pharmacogenetics of ISDs in adults or pediatric patients of any gender and ethnicity receiving immunosuppressive therapy after kidney transplantation. We utilized as search term: “cyclosporine or tacrolimus or mycophenolic acid or sirolimus or everolimus and polymorphism and transplant”. Our data showed that polymorphisms in CYP3A5, CYP3A4, ABCB1, and UGT1A9 genes could modify the pharmacokinetics of immunosuppressants, suggesting that patient genotyping could be a helpful strategy to select the ideal ISDs dose for each patient.

Association of CYP3A5 polymorphisms and parathyroid hormone with blood level of tacrolimus in patients with end-stage renal disease

Because tacrolimus is predominantly metabolized by CYP3A, the blood concentration/dose (C/D) ratio is affected by CYP3A5 polymorphism. Parathyroid hormone (PTH) expression increases in secondary hyperparathyroidism, which is frequently associated with end‐stage renal disease. Recently, PTH has been shown to downregulate CYP3A expression at mRNA level. In this study, we examined the influence of CYP3A5 polymorphism on and association of serum intact‐PTH (iPTH) level with blood tacrolimus concentration in patients with end‐stage renal disease just before kidney transplantation. Forty‐eight patients who satisfied the selection criteria were analyzed. Subjects were classified into two phenotype subgroups: CYP3A5 expressor (CYP3A5*1/*1 and *1/*3; n = 15) and CYP3A5 nonexpressor (CYP3A5*3/*3; n = 33). The blood tacrolimus C/D (per body weight) ratio was significantly lower in CYP3A5 expressors than that in CYP3A5 nonexpressors. A significant positive correlation was found between tacrolimus C/D and iPTH concentrations (r = 0.305, p = 0.035), and the correlation coefficient was higher after excluding 20 patients co‐administered CYP3A inhibitor or inducer (r = 0.428, p = 0.023). A multiple logistic regression analysis by stepwise selection identified CYP3A5 polymorphism and serum iPTH level as significant factors associated with tacrolimus C/D. These results may suggest the importance of dose design considering not only the CYP3A5 phenotype but also serum iPTH level when using tacrolimus in patients who undergo renal transplantation.

Impact of Clotrimazole Fungal Prophylaxis on Tacrolimus Exposure in Kidney Transplant Recipients: A Retrospective Study

Tacrolimus, an immunosuppressant prescribed to reduce the risk of organ rejection, is metabolized by cytochrome P450 and is a substrate for P-glycoprotein. Many medications affect tacrolimus concentrations, making it difficult to maintain exposure within its narrow therapeutic index. Clotrimazole troches, prescribed to posttransplant recipients immediately for the first 30 days for oral candidiasis prevention, are considered nonsystemic. However, data suggest a potential drug interaction, affecting tacrolimus exposure. To assess the magnitude of the effect of clotrimazole on tacrolimus trough levels, 97 kidney transplant recipients, on a stable dose of tacrolimus, were retrospectively evaluated. Tacrolimus trough concentrations were analyzed 7 and 14 days before and after discontinuation of clotrimazole. The median change in tacrolimus trough level was -1.3 ng/mL (confidence interval, -2.5, -1.0; P < .001) at day 7 and -2.8 ng/mL (confidence interval, -3.3, -1.6; P < .001) at day 14 after clotrimazole discontinuation, from a median baseline of 8.9 ng/mL. Overall, a reduction in tacrolimus level was observed in 60% of patients after discontinuation of clotrimazole. When assessing the effect of race and sex, no influence was found on the degree of change in tacrolimus level after clotrimazole discontinuation. In conclusion, clotrimazole exerts a significant interaction on tacrolimus where close monitoring of tacrolimus trough levels after discontinuation of clotrimazole is warranted.

Tacrolimus Exposure in Obese Patients: and A Case-Control Study in Kidney Transplantation

BACKGROUND

Tacrolimus pharmacokinetics in obese (Ob) patients has been poorly studied. In this article, the authors explored the impact of obesity on tacrolimus exposure in kidney transplant recipients (KTRs) and estimated a more suitable initial dosage in this population.

METHODS

A retrospective, observational, monocentric case-control study was performed in obese KTRs (BMI > 30 kg/m2) who received tacrolimus between 2013 and 2017 (initial dose: 0.15 mg/kg/d) (actual weight). Nonobese (Nob) controls (BMI <30 kg/m2) were matched for age and sex. Weekly centralized monitoring of tacrolimus trough levels was performed by liquid chromatography/mass spectrometry until the third month (M3). Target trough levels were set between 8 and 10 ng/mL. All patients received antilymphocyte globulin, corticosteroids, and mycophenolate mofetil.

RESULTS

Of the 541 KTRs, 28 tacrolimus-treated Ob patients were included and compared with 28 NOb-matched controls. With a mean of 22 assays/patient, tacrolimus trough levels were higher in Ob patients (mean 9.9 versus 8.7 ng/mL; P = 0.008); the weight-related dose of Tac was lower at M3 (mean 0.10 versus 0.13 mg/kg/d, P < 0.0001). The tacrolimus concentration to dose (C0/D) was higher in the Ob cohort [mean 116 versus 76 (ng/mL)/(mg/kg/d); P = 0.001]. In Ob patients, a mean decrease of -4.6 mg/d in the 3 months after tacrolimus initiation was required (versus -1.12 in NOb; P = 0.001) to remain within the therapeutic range. Obesity, high mycophenolate mofetil daily dose at M3, and CYP3A5 expression were independently associated with higher tacrolimus exposure. Four dose-adaptation strategies were simulated and compared with the study results.

CONCLUSIONS

An initial dose calculation based on either ideal or lean body weight may allow for faster achievement of tacrolimus trough level targets in Ob KTRs, who are at risk of overexposure when tacrolimus is initiated at 0.15 mg/kg/d. A prospective study is required to validate alternative dose calculation strategies in these patients.

Pharmacogenetics of common SNP affecting drug metabolizing enzymes: comparison of allele frequencies between European and Malaysian/Singaporean

Compared to Europe, data on genetic variation in genes transcribing drug metabolizing enzymes among Asian is limited due to ethnic diversity. Here we compare frequencies for clinically relevant single nucleotide polymorphism (SNP) commonly observed in drug metabolizing enzymes between European and Malaysian/Singaporean. Minor allele frequencies (MAF) for the indicated SNPs for European, South Asian and East Asian populations were obtained from the NCBI website (https://www.ncbi.nlm.nih.gov/snp). The SNP prevalence among Malaysian/Singaporean was characterized from gene association studies. Generally, some SNPs in CYP2D6 and CYP2C19 do not show good agreement between the two populations as to the MAF value obtained. CYP2D6*4 tends to be more common among European, whereas CYP2D6*10 is more common in Malays and Chinese among Singaporean. Regardless of different phenotype, MAF of CYP2D6*4 for Indians is similar to that seen by the European. Singaporeans show smaller MAF for CYP2C19*17 but higher CYP2C19*2 frequencies as opposed to European ones. Following growing attention to the contribution of CYP3A4/5, N-acetyltransferases (NAT2), thiopurine methyltransferase (TPMT) and uridine diphosphate glucuronosyltransferases (UGT)2B7 in predicting drug response across Europe, there are limited pharmacogenetics (PGx) studies examining the gene-drug interaction among Malaysian/Singaporean. To better understand the heterogeneity of the drug response, PGx studies for the abovementioned enzymes between ethnics in Malaysian/Singaporean should be identified.

Pharmacogenetics of common SNP affecting drug metabolizing enzymes: comparison of allele frequencies between European and Malaysian/Singaporean

Compared to Europe, data on genetic variation in genes transcribing drug metabolizing enzymes among Asian is limited due to ethnic diversity. Here we compare frequencies for clinically relevant single nucleotide polymorphism (SNP) commonly observed in drug metabolizing enzymes between European and Malaysian/Singaporean. Minor allele frequencies (MAF) for the indicated SNPs for European, South Asian and East Asian populations were obtained from the NCBI website (https://www.ncbi.nlm.nih.gov/snp). The SNP prevalence among Malaysian/Singaporean was characterized from gene association studies. Generally, some SNPs in CYP2D6 and CYP2C19 do not show good agreement between the two populations as to the MAF value obtained. CYP2D6*4 tends to be more common among European, whereas CYP2D6*10 is more common in Malays and Chinese among Singaporean. Regardless of different phenotype, MAF of CYP2D6*4 for Indians is similar to that seen by the European. Singaporeans show smaller MAF for CYP2C19*17 but higher CYP2C19*2 frequencies as opposed to European ones. Following growing attention to the contribution of CYP3A4/5, N-acetyltransferases (NAT2), thiopurine methyltransferase (TPMT) and uridine diphosphate glucuronosyltransferases (UGT)2B7 in predicting drug response across Europe, there are limited pharmacogenetics (PGx) studies examining the gene-drug interaction among Malaysian/Singaporean. To better understand the heterogeneity of the drug response, PGx studies for the abovementioned enzymes between ethnics in Malaysian/Singaporean should be identified.

Temporary decrease in tacrolimus clearance in cytochrome P450 3A5 non-expressors early after living donor kidney transplantation: Effect of interleukin 6-induced suppression of the cytochrome P450 3A gene

Tacrolimus is important for immunosuppression in kidney transplantation. In this historical cohort and in vitro study, we evaluated the changes in tacrolimus pharmacokinetics early after living donor kidney transplantation and the effects of interleukin (IL)-6 on cytochrome P450 3A4 (CYP3A4) and cytochrome P450 3A5 (CYP3A5) expression. In the historical cohort study, 22 patients who met the inclusion criteria were classified into CYP3A5 expressors and non-expressors (n = 16 and 6, respectively). The blood tacrolimus concentration per dose ratio (C/D) temporarily increased post-kidney transplantation on days 3-4 only in CYP3A5 non-expressors. The effects of IL-6 on CYP3A4 and CYP3A5 expression were also investigated in vitro using HepG2 and Caco-2 cells. IL-6 induced a significant concentration- and time-dependent decrease in CYP3A4 and CYP3A5 expression in both cells. The mean CYP3A4 expression level at 12 hours after IL-6 exposure (% of 0 hour) was 44.0 and 62.6 in HepG2 and Caco-2 cells, respectively, whereas the CYP3A5 expression level was 30.7 and 52.4, respectively. We hypothesize that CYP3A5 non-expressors might exhibit a temporary decrease in the oral clearance of tacrolimus via an increase in serum IL-6 concentrations early after kidney transplantation. These results may help develop strategies to improve kidney transplant outcome.

Sensitization in transplantation: Assessment of risk (STAR) 2019 Working Group Meeting Report

The purpose of the STAR 2019 Working Group was to build on findings from the initial STAR report to further clarify the expectations, limitations, perceptions, and utility of alloimmune assays that are currently in use or in development for risk assessment in the setting of organ transplantation. The goal was to determine the precision and clinical feasibility/utility of such assays in evaluating both memory and primary alloimmune risks. The process included a critical review of biologically driven, state-of-the-art, clinical diagnostics literature by experts in the field and an open public forum in a face-to-face meeting to promote broader engagement of the American Society of Transplantation and American Society of Histocompatibility and Immunogenetics membership. This report summarizes the literature review and the workshop discussions. Specifically, it highlights (1) available assays to evaluate the attributes of HLA antibodies and their utility both as clinical diagnostics and as research tools to evaluate the effector mechanisms driving rejection; (2) potential assays to assess the presence of alloimmune T and B cell memory; and (3) progress in the development of HLA molecular mismatch computational scores as a potential prognostic biomarker for primary alloimmunity and its application in research trial design.

Effect of CYP3A5 on the Once-Daily Tacrolimus Conversion in Stable Liver Transplant Patients

Cytochrome P450 (CYP) 3A5 polymorphism influences tacrolimus metabolism, but its effect on the drug pharmacokinetics in liver transplant recipients switched to once-daily extended-release formulation remains unknown. The aim of this study is to analyze the effect of CYP3A5 polymorphism on liver function after once-daily tacrolimus conversion in liver transplant patients. A prospective open-label study included 60 stable liver transplant recipients who underwent 1:1 conversion from twice-daily tacrolimus to once-daily tacrolimus. All participants were genotyped for CYP3A5 polymorphism. The study was registered at ClinicalTrials.gov (NCT 02882113). Twenty-eight patients were enrolled in the CYP3A5 expressor group and 32 in the non-expressor group. Although there was no statistical difference, incidence of liver dysfunction was higher in the expressor group than in the non-expressor group when converted to once-daily extended-release tacrolimus (p = 0.088). No biopsy-proven acute rejection, graft failure, and mortality were observed in either group. The decrease in dose-adjusted trough level (−42.9% vs. −26.1%) and dose/kg-adjusted trough level of tacrolimus (−40.0% vs. −23.7%) was significantly greater in the expressor group than in the non-expressors after the conversion. A pharmacokinetic analysis was performed in 10 patients and tacrolimus absorption in the non-expressor group was slower than in the expressor group. In line with this observation, the area under the curve for once-daily tacrolimus correlated with trough level (Cmin) in the non-expressors and peak concentration (Cmax) in the expressors. CYP3A5 genotyping in liver transplant recipients leads to prediction of pharmacokinetics after switching from a twice-daily regimen to a once-daily dosage form, which makes it possible to establish an appropriate dose of tacrolimus.

Body weight-based initial dosing of tacrolimus in renal transplantation: Is this an ideal approach?

BACKGROUND

Tacrolimus dosing immediately posttransplant is based on body weight. Recent studies have highlighted that the dosing of tacrolimus purely based on weight may not be appropriate, particularly in individuals who are obese.

OBJECTIVES

This study aimed to estimate the effect of body mass index (BMI) and the weight-based dosing on tacrolimus trough levels in recipients of renal transplants.

DESIGN AND PARTICIPANTS

This study was conducted on 400 of the 863 patients registered in the Salford, UK, renal transplant database between 2012 and 2019 who had complete and analysable datasets. Data were collected at baseline (first tacrolimus trough level after transplantation), after 1 month and 6 months posttransplantation. The cohort was split into three groups based on BMI (kg/m² ; Group 1 ≤ 25, Group 2 > 25-30 and Group 3 > 30) which were compared with respect to tacrolimus dose, plasma levels and concentration/dose (C/D) ratio at the three-time points.

RESULTS

Patients in the higher BMI group (Group 3) had significantly higher baseline tacrolimus trough levels despite receiving a lower initiation dose per kilogram of body weight. After 1 and 6-months posttransplant, the higher BMI group were receiving a significantly lower tacrolimus dose relative to their body weight, with a significant negative correlation between body weight and tacrolimus/kg body weight. There was no adverse relationship evident between tacrolimus dosing or concentration and graft function.

CONCLUSIONS

Our study showed that standard dosing of tacrolimus based on body weight in individuals who were obese did not adversely affect their tacrolimus concentrations or transplant function.

Use of biomarkers to improve immunosuppressive drug development and outcomes in renal organ transplantation: A meeting report

On September 27-28, 2018 the Food and Drug Administration (FDA) and the Critical Path Institute's Transplant Therapeutics Consortium convened a public workshop titled "Evidence-Based Treatment Decisions in Transplantation: The Right Dose & Regimen for the Right Patient/Individualized Treatment." The workshop facilitated cooperative engagement of transplant community stakeholders, including pharmaceutical industry, academic researchers, clinicians, patients, and regulators to discuss methods to advance the development of novel immunosuppressive drugs for use in solid organ transplantation. Day 1 focused on the utility of biomarkers in drug development_, with considerations for seeking regulatory endorsement for use in clinical trials. Biomarkers add value to drug development by improving patient selection criteria, safety monitoring, endpoint selection, and more. Regulatory endorsement through the FDA Biomarker Qualification Program encourages the use of biomarkers in drug development by instilling confidence and consistency in biomarker interpretation across trials. Public-private partnerships or consortia allow stakeholders to share expertise, resources, and data in pursuit of biomarker qualification. Biomarkers relevant to pretransplant risk assessment, early posttransplant care, and assessment of immune response, immunosuppressive drug efficacy, and graft function as discussed on day 1 of the workshop are described.

Association of Genetic Variants in CYP3A4, CYP3A5, CYP2C8, and CYP2C19 with Tacrolimus Pharmacokinetics in Renal Transplant Recipients

BACKGROUND

Our study aimed to investigate the pharmacogenetics of cytochrome P3A4 (CYP3A4), CYP3A5, CYP2C8, and CYP2C19 and their influence on TAC Pharmacokinetics (PKs) in short-term renal transplant recipients.

METHODS

A total of 105 renal transplant recipients were enrolled. Target Sequencing (TS) based on next-generation sequencing technology was used to detect all exons, exon/intron boundaries, and flanking regions of CYP3A4, CYP3A5, CYP2C8, and CYP2C19. After adjustment of Minor Allele Frequencies (MAF) and Hardy-Weinberg Equilibrium (HWE) analysis, tagger Single-nucleotide Polymorphisms (SNPs) and haplotypes were identified. Influence of tagger SNPs on TAC concentrations was analyzed.

RESULTS

A total of 94 SNPs were identified in TS analysis. Nine tagger SNPs were selected, and two SNPs (rs15524 and rs4646453) were noted to be significantly associated with TAC PKs in short-term post-transplant follow-up. Measurement time points of TAC, body mass index (BMI), usage of sirolimus, and incidence of Delayed Graft Function (DGF) were observed to be significantly associated with TAC PKs. Three haplotypes were identified, and rs15524-rs4646453 was found to remarkably contribute to TAC PKs. Recipients carrying H2/H2 (GG-AA) haplotype also showed significantly high weight- and dose-adjusted TAC concentrations in posttransplant periods of 7, 14, and 30 days and 3 and 6 months.

CONCLUSIONS

Two tagger SNPs, namely, rs15524 and rs4646453, are significantly related to the variability of TAC disposition, and TAC measurement time points, BMI, usage of sirolimus, and incidence of DGF contribute to this influence. Recipients carrying H2/H2 (GG-AA) haplotype in rs15524-rs4646453 may require a low dosage of TAC during 1-year follow-up posttransplant.

Caveolin-1 rs4730751 single-nucleotide polymorphism may not influence kidney transplant allograft survival

Caveolin-1 is a protein (encoded by the CAV1 gene) supposedly harboring a protective effect against fibrosis. CAV1 rs4730751 single nucleotide polymorphism (SNP) AA genotype was initially associated with lower graft survival compared to non-AA. However, subsequent studies could not find the same effect. CAV1 rs4730751 SNP was investigated on 918 kidney donors. Multivariate Cox-model analyses were performed to evaluate risk factors for graft loss. Longitudinal changes on long-term estimated glomerular filtration rate (eGFRs) were evaluated with a linear mixed model. Histopathological findings from protocolled biopsies after 3 months post transplantation were also analyzed. Donor CAV1 rs4730751 genotyping proportions were 7.1% for AA, 41.6% for AC and 51.3% for CC. The AA genotype, compared to non-AA, was not associated with lower graft survival censored or not for death (multivariate analysis: HR = 1.23 [0.74–2.05] and HR = 1.27 [0.84–1.92]). Linear mixed model on long-term eGFRs revealed also no significant difference according to the genotype, yet we observed a trend. AA genotype was also not associated with a higher degree of fibrosis index on protocolled kidney biopsies at 3 months. To conclude, donor CAV1 rs4730751 SNP may impact on kidney transplantation outcomes, but this study could not confirm this hypothesis.

Therapeutic concentration achievement and allograft survival comparing usage of conventional tacrolimus doses and CYP3A5 genotype-guided doses in renal transplantation patients

AIMS

Although cytochromeP450(CYP)3A5 gene polymorphism affects personalized tacrolimus doses, there is no consensus as to whether CYP3A5 genotypes should be determined to adjust the doses. The aims were to compare the therapeutic ranges and clinical outcomes between the conventional and genotype-guided tacrolimus doses.

METHODS

This randomized controlled study compared 63 cases of the conventional tacrolimus dose group (0.1 mg/kg/day) with 62 cases of the genotype-guided doses group of 0.125, 0.1 and 0.08 mg/kg for CYP3A5*1/*1, *1/*3, and *3/*3 genotypes for the initial 3 days of kidney transplantation. After day 3, dose adjustment occurred in both groups to achieve therapeutic concentrations.

RESULTS

The genotype-guided group had an increased proportion of patients with tacrolimus concentrations in the therapeutic range at the steady state on day 3 (40.3 vs 23.8%, P = .048). A lower proportion of over-therapeutic concentration patients was noted in the genotype-guided group in the CYP3A5*3/*3 genotype (9.7 vs 27%, P = .013). Unexpectedly, more delayed graft functions (DGFs) were in the genotype-guided group (41.9 vs 22.2%, P = .018) especially in the CYP3A5*1/*1 participants who might have had an aggravated DGF by a longer ischaemic time and higher serum donor creatinine levels than in the control group. There were no significant differences of glomerular filtration rates or graft or patient survivals over a median 37-month follow-up period.

CONCLUSIONS

Determination of the CYP3A5 genotype improved therapeutic range achievement. CYP3A5*1/*1 patients who have high risks of DGF should be closely monitored because of an increased risk of DGF and reduced glomerular filtration rate with high tacrolimus doses.

Once-daily vs twice-daily tacrolimus for de novo living kidney transplantation patients including ABO/HLA compatible and incompatible: A randomized trial

Tacrolimus (TAC) is available as a twice-daily capsule (TAC-BID), once-daily capsule (TAC-QD), and once-daily tablet. Recipients with ABO-incompatible/anti-human leukocyte antigen (HLA)-incompatible transplantation were excluded in previous trials and have thus not been evaluated. We conducted a 5-year trial to determine whether TAC-QD is noninferior to TAC-BID for transplant outcomes. Adults who underwent de novo living kidney transplantation were randomly assigned (62 TAC-QD; 63 TAC-BID). We did not exclude ABO-/HLA- incompatible transplantation. TAC was initiated 7 days preoperatively (0.10 mg/kg/d). Mycophenolate mofetil, methylprednisolone, and basiliximab were administered. The primary endpoint was graft failure (non-censored for death). We performed a noninferiority test. The noninferiority margin was 10% in risk difference. Five-year graft failure rates were 6.5% and 9.5% for TAC-QD and TAC-BID, respectively (noninferiority, P = 0.009). The estimated glomerular filtration rates were similar between the groups (noninferiority, P < 0.001). TAC-QD did not have point estimates of risk difference above the inferiority margin in any assessed endpoints. However, a tendency of interaction was observed between biopsy-proven acute rejection and the follow-up period. In a living kidney transplant population with 40% of patients with ABO/HLA incompatibility, the effect of TAC-QD was not appreciably worse on various clinical transplant outcomes than that of TAC-BID over 5 years.

Impact of POR and CYP3A5 Polymorphisms on Trough Concentration to Dose Ratio of Tacrolimus in the Early Post-operative Period Following Kidney Transplantation

BACKGROUND

Tacrolimus, a critical dose drug, is widely used in transplantation. Knowing the contribution of genetic factors, which significantly influence tacrolimus variability, is beneficial in the personalization of its starting dose. The significant impact of CYP3A5*3 polymorphisms on tacrolimus exposure has been reported. Conflicting results of the additional influence of POR*28 polymorphisms on tacrolimus pharmacokinetic interindividual variability have been observed among different populations. The objective of this study was to explore the interaction between POR*28 and CYP3A5*3 polymorphisms and their main effects on tacrolimus trough concentration to dose ratios on day 7 after kidney transplantation.

METHODS

Two hundred sixteen adult kidney transplant recipients participated in this retrospective study. All participants received a twice daily tacrolimus regimen. Blood samples and data were collected on day 7 after transplantation. A 2-way analysis of covariance was performed. Tested covariates were age, hemoglobin, serum albumin, and prednisolone dose.

RESULTS

A 2 × 2 analysis of covariance revealed that the interaction between CYP3A5 polymorphisms (CYP3A5 expresser and CYP3A5 nonexpresser) and POR polymorphisms (POR*28 carrier and POR*28 noncarrier) was not significant (F(1, 209) = 2.473, P = 0.117, (Equation is included in full-text article.)= 0.012). The predicted main effect of CYP3A5 and POR polymorphisms was significant (F(1, 209) = 105.565, P < 0.001, (Equation is included in full-text article.)= 0.336 and F(1, 209) = 4.007, P = 0.047, (Equation is included in full-text article.)= 0.019, respectively). Hemoglobin, age, and steroid dose influenced log C0/dose of tacrolimus (F(1, 209) = 20.612, P < 0.001, (Equation is included in full-text article.)= 0.090; F(1, 209) = 14.360, P < 0.001, (Equation is included in full-text article.)= 0.064; and F(1, 209) = 5.512, P = 0.020, (Equation is included in full-text article.)= 0.026, respectively).

CONCLUSIONS

After adjusting for the influences of hemoglobin, age, and prednisolone dose, significant impacts of the CYP3A5 and POR polymorphisms on tacrolimus exposure were found. The effect of POR*28 and CYP3A5*3 polymorphisms during the very early period after kidney transplantation is independent of each other.

CYP3A5*3 Genetic Polymorphism and Tacrolimus Concentration in Myanmar Renal Transplant Patients

BACKGROUND

Genetic polymorphism is an important factor that influences tacrolimus concentrations and has the potential to predict the optimal dosage of tacrolimus in personalized medicine. Tacrolimus, a drug of narrow therapeutic index, is used in renal transplant recipients as an immunosuppressant agent. It is a substrate of cytochrome P450 3A (CYP3A) and has highly variable pharmacokinetic parameters.

OBJECTIVE

The aim of this study was to identify the proportion of CYP3A5 gene polymorphism in Myanmar kidney transplant recipients and to determine the impact of CYP3A5 gene polymorphisms on tacrolimus level in CYP3A5 expressors and nonexpressors.

METHODS

This study included 41 adult Myanmar post-renal transplant patients. Tacrolimus trough blood levels were determined and CYP3A5 genotype analysis was conducted by using polymerase chain reaction amplification of target followed by detection by restriction fragment length polymorphism analysis.

RESULTS

The CYP3A5 nonexpressors and expressors were detected in 25 (60.97%) and 16 (39.02%) of the 41 renal transplant recipients, respectively. The tacrolimus concentration/dose ratio in the CYP3A5 expressor group was lower than in the CYP3A5 nonexpressor group (1.49 ± 0.69 vs 3.49 ± 3.08 [P = .003] at 1 month; and 1.54 ± 0.9 vs 7.88 ± 8.25 [P = .0001] at 3 months).

CONCLUSIONS

The findings of the present study revealed that more than one half of the study population were carrying the mutant allele CYP3A5*3(A6986G). CYP3A5 genetic polymorphism is one of the important factors in determining daily requirements for tacrolimus and in adjusting tacrolimus trough concentrations.

CYP3A5 polymorphisms in renal transplant recipients: influence on tacrolimus treatment

Tacrolimus is a commonly used immunosuppressant after kidney transplantation. It has a narrow therapeutic range and demonstrates wide interindividual variability in pharmacokinetics, leading to potential underimmunosuppression or toxicity. Genetic polymorphism in CYP3A5 enzyme expression contributes to differences in tacrolimus bioavailability between individuals. Individuals carrying one or more copies of the wild-type allele *1 express CYP3A5, which increases tacrolimus clearance. CYP3A5 expressers require 1.5 to 2-fold higher tacrolimus doses compared to usual dosing to achieve therapeutic blood concentrations. Individuals with homozygous *3/*3 genotype are CYP3A5 nonexpressers. CYP3A5 nonexpression is the most frequent phenotype in most ethnic populations, except blacks. Differences between CYP3A5 genotypes in tacrolimus disposition have not translated into differences in clinical outcomes, such as acute rejection and graft survival. Therefore, although genotype-based dosing may improve achievement of therapeutic drug concentrations with empiric dosing, its role in clinical practice is unclear. CYP3A5 genotype may predict differences in absorption of extended-release and immediate-release oral formulations of tacrolimus. Two studies found that CYP3A5 expressers require higher doses of tacrolimus in the extended-release formulation compared to immediate release. CYP3A5 genotype plays a role in determining the impact of interacting drugs, such as fluconazole, on tacrolimus pharmacokinetics. Evidence conflicts regarding the impact of CYP3A5 genotype on risk of nephrotoxicity associated with tacrolimus. Further study is required.

NFAT-regulated cytokine gene expression during tacrolimus therapy early after renal transplantation

AIMS

Despite pharmacokinetic monitoring of calcineurin inhibitors, the long-term outcome after transplantation (Tx) is still hampered by the side effects of these drugs. The aim of the present study was to characterize nuclear factor of activated T cells (NFAT)-regulated gene expression as a potential pharmacodynamic biomarker for further individualization of tacrolimus (Tac) therapy.

METHODS

In 29 renal allograft recipients, samples were drawn once pre-Tx, and before and 1.5 h after Tac dosing at approximately 1 week, 6 weeks and 1 year post-Tx. Tac concentrations were measured by immunoassay, while the expression of genes encoding NFAT-regulated cytokines [interleukin 2 (IL2), interferon gamma (IFNG), colony stimulating factor 2 (CSF2)] and cytochrome P450 3A5 (CYP3A5) genotyping were determined by real-time polymerase chain reaction.

RESULTS

The cytokine response after Tac dosing varied up to 46-fold between patients and changed significantly with time post-engraftment. Tac concentrations 1.5 h postdose (C_1.5 ) >15 μg l^-1 were associated with strong cytokine inhibition and residual gene expression (RGE) ≤10%, while lower Tac C_1.5 resulted in more variable responses (RGE 2.5-68.7%). Patients with ongoing subclinical acute rejection (n = 5) demonstrated limited cytokine inhibition (RGE 39.7-72.6%), while patients with polyoma virus viraemia (n = 3) had relatively strong inhibition of cytokines (RGE 2.5-32.5%). By contrast, there was no association between Tac exposure and rejection or viraemia.

CONCLUSIONS

The findings of our study support the potential of NFAT-regulated gene expression measurements as a pharmacodynamic tool for additional monitoring of Tac therapy, especially in the context of overimmunosuppression and viraemia.

Contrasting exome constancy and regulatory region variation in the gene encoding CYP3A4: an examination of the extent and potential implications

OBJECTIVE

CYP3A4 expression varies up to 100-fold among individuals, and, to date, genetic causes remain elusive. As a major drug-metabolizing enzyme, elucidation of such genetic causes would increase the potential for introducing personalized dose adjustment of therapies involving CYP3A4 drug substrates. The foetal CYP3A isoform, CYP3A7, is reported to be expressed in ∼10% of European adults and may thus contribute towards the metabolism of endogenous substances and CYP3A drug substrates. However, little is known about the distribution of the variant expressed in the adult.

METHODS

We resequenced the exons, flanking introns, regulatory elements and 3'UTR of CYP3A4 in five Ethiopian populations and incorporated data from the 1000 Genomes Project. Using bioinformatic analysis, we assessed likely consequences of observed CYP3A4 genomic variation. We also conducted the first extensive geographic survey of alleles associated with adult expression of CYP3A7 - that is, CYP3A7*1B and CYP3A7*1C.

RESULTS AND CONCLUSION

Ethiopia contained 60 CYP3A4 variants (26 novel) and more variants (>1%) than all non-African populations combined. No nonsynonymous mutation was found in the homozygous form or at more than 2.8% in any population. Seventy-nine per cent of haplotypes contained 3'UTR and/or regulatory region variation with striking pairwise population differentiation, highlighting the potential for interethnic variation in CYP3A4 expression. Conversely, coding region variation showed that significant interethnic variation is unlikely at the protein level. CYP3A7*1C was found at up to 17.5% in North African populations and in significant linkage disequilibrium with CYP3A5*3, indicating that adult expression of the foetal isoform is likely to be accompanied by reduced or null expression of CYP3A5.

External evaluation of published population pharmacokinetic models of tacrolimus in adult renal transplant recipients

AIM

Several tacrolimus population pharmacokinetic models in adult renal transplant recipients have been established to facilitate dose individualization. However, their applicability when extrapolated to other clinical centres is not clear. This study aimed to (1) evaluate model external predictability and (2) analyze potential influencing factors.

METHODS

Published models were screened from the literature and were evaluated using an external dataset with 52 patients (609 trough samples) collected by postoperative day 90 via methods that included (1) prediction-based prediction error (PE%), (2) simulation-based prediction- and variability-corrected visual predictive check (pvcVPC) and normalized prediction distribution error (NPDE) tests and (3) Bayesian forecasting to assess the influence of prior observations on model predictability. The factors influencing model predictability, particularly the impact of structural models, were evaluated.

RESULTS

Sixteen published models were evaluated. In prediction-based diagnostics, the PE% within ±30% was less than 50% in all models, indicating unsatisfactory predictability. In simulation-based diagnostics, both the pvcVPC and the NPDE indicated model misspecification. Bayesian forecasting improved model predictability significantly with prior 2-3 observations. The various factors influencing model extrapolation included bioassays, the covariates involved (CYP3A5*3 polymorphism, postoperative time and haematocrit) and whether non-linear kinetics were used.

CONCLUSIONS

The published models were unsatisfactory in prediction- and simulation-based diagnostics, thus inappropriate for direct extrapolation correspondingly. However Bayesian forecasting could improve the predictability considerably with priors. The incorporation of non-linear pharmacokinetics in modelling might be a promising approach to improving model predictability.

The role of CYP3A5 polymorphism and dose adjustments following conversion of twice-daily to once-daily tacrolimus in renal transplant recipients

Background

Tacrolimus is available as twice-daily Prograf® (Tac-BID) and the once-daily formulation, Advagraf® (Tac-OD). Although therapeutically equivalent, some transplant recipients require dose adjustments to achieve similar tacrolimus trough concentrations [Tac C₀] after conversion between formulations. Tacrolimus is primarily metabolized by cytochrome P450 3A5 (CYP3A5). We sought to determine whether genetic polymorphisms in the CYP3A5 enzyme; CYP3A5 *1/*1 and CYP3A5 *1/*3 (expressers) compared to CYP3A5 *3/*3 (non-expressers) could account for discrepancies in dose requirements following conversion from Tac-BID to Tac-OD.

Methods

A cohort of 60 renal transplant recipients (RTR) from our larger conversion study of 496 patients underwent additional testing for CY3A5 genetic polymorphisms. Analysis included demographics, tac dosing and [Tac C₀] pre- and post-conversion and dosing changes relative to CYP3A5 genotypes. CYP3A5 genetic polymorphisms were identified through analysis of genomic DNA.

Results

Conversion from tac bid to tac OD in this cohort required a mean (SD) dose increase from 3.1 (1.0) mg/day to 3.8 (1.3) mg/day (p = 0.007), to achieve similar [Tac C₀]. The *1/*3 expresser group required a greater percentage dose adjustment (56.7 %) in converting from Tac-BID to Tac-OD as compared to the *3/*3 non-expresser group (26.6 %). Similar findings were observed with the both expresser groups combined (*1/*1 &*1/*3). The expressers were significantly more highly represented in the East Asian cohort.

Conclusions

The CYP3A5 expresser polymorphism necessitates an increase in dosing upon conversion from Tac-BID to Tac-OD, with the expresser genotypes contributing significantly to this finding. Given the variability in frequency of CYP3A5 genotypes in various ethnic groups, future studies should account for both isoenzyme polymorphism and ethnicity in optimizing dosing requirements.

Trial registration

Clinical trials.gov identifier: NCT01884480

Improved Tacrolimus Target Concentration Achievement Using Computerized Dosing in Renal Transplant Recipients--A Prospective, Randomized Study

BACKGROUND

Early after renal transplantation, it is often challenging to achieve and maintain tacrolimus concentrations within the target range. Computerized dose individualization using population pharmacokinetic models may be helpful. The objective of this study was to prospectively evaluate the target concentration achievement of tacrolimus using computerized dosing compared with conventional dosing performed by experienced transplant physicians.

METHODS

A single-center, prospective study was conducted. Renal transplant recipients were randomized to receive either computerized or conventional tacrolimus dosing during the first 8 weeks after transplantation. The median proportion of tacrolimus trough concentrations within the target range was compared between the groups. Standard risk (target, 3-7 μg/L) and high-risk (8-12 μg/L) recipients were analyzed separately.

RESULTS

Eighty renal transplant recipients were randomized, and 78 were included in the analysis (computerized dosing (n = 39): 32 standard risk/7 high-risk, conventional dosing (n = 39): 35 standard risk/4 high-risk). A total of 1711 tacrolimus whole blood concentrations were evaluated. The proportion of concentrations per patient within the target range was significantly higher with computerized dosing than with conventional dosing, both in standard risk patients (medians, 90% [95% confidence interval {95% CI}, 84-95%] vs 78% [95% CI, 76-82%], respectively, P < 0.001) and in high-risk patients (medians, 77% [95% CI, 71-80%] vs 59% [95% CI, 40-74%], respectively, P = 0.04).

CONCLUSIONS

Computerized dose individualization improves target concentration achievement of tacrolimus after renal transplantation. The computer software is applicable as a clinical dosing tool to optimize tacrolimus exposure and may potentially improve long-term outcome.

Relationship between mRNA expression levels of CYP3A4, CYP3A5 and SXR in peripheral mononuclear blood cells and aging in young kidney transplant recipients under tacrolimus treatment

Aim: The activity of several key enzymes involved in the metabolism of many drugs is subject to change closely related to the age of patients. This possibility must also be considered in the case of tacrolimus, the most important calcineurins inhibitor, which is widely used in pediatric kidney transplantation. As well as in the liver and intestine, some of the enzymes involved in the metabolism of tacrolimus were also isolated in the peripheral blood mononuclear cells (PBMCs), where also appear to play an important regulatory action. Therefore, the influence of some external factors on the expression of specific mRNA can be determined noninvasively. Patients & methods: The correlation between the levels of mRNA specific for key enzymes SXR, CYP3A and ABCB1 involved in the metabolism of tacrolimus was evaluated in PBMCs obtained from a selected population of 29 young kidney transplant recipients. A possible correlation between the expression of these specific mRNAs and tacrolimus pharmacokinetics was also investigated. Results: The patients’ age and their blood concentrations of SXR mRNA were directly correlated with the expression of CYP3A4, CYP3A5 mRNAs, but not of ABCB1 mRNA in the PBMCs. tacrolimus-normalized daily dose was strongly correlated with patient's age and multivariable regression indicates the CYP3A4-specific mRNA as the sole independent variable influencing tacrolimus concentration-to-dose ratio. Conclusion: Aging and SXR mRNA significantly affect the expression of CYP3A4- and CYP3A5-specific mRNA as measured by their concentration in PBMC.

Original submitted 17 November 2014; Revision submitted 6 February 2015

Donor IL-18 rs5744247 polymorphism as a new biomarker of tacrolimus elimination in Chinese liver transplant patients during the early post-transplantation period: results from two cohort studies

Aim: This study evaluated the relationships between IL-18 polymorphisms and tacrolimus elimination in Chinese liver transplant patients. Patients & methods: Eighty-four liver transplant patients from Shanghai (training set) and 50 patients from Shandong (validating set) were inculded. IL-18 polymorphisms (rs5744247, rs7106524, rs549908, rs187238 and rs1946518) and CYP3A5 rs776746 were genotyped. Results: In training set, daily drug dose, total bilirubin, donor CYP3A5 rs776746 and IL-18 rs5744247 genotypes were screened to construct prediction model for tacrolimus elimination. This model was confirmed in validating set (p < 0.001). Donor IL-18 rs5744247 polymorphism was an independent predictor of tacrolimus elimination in the first week after transplantation in both training (p = 0.008) and validating cohorts (p = 0.033). Conclusion: Donor IL-18 rs5744247 polymorphism may influence on tacrolimus elimination.

Original submitted 16 July 2014; Revision submitted 12 November 2014

Association between interleukin-18 promoter variants and tacrolimus pharmacokinetics in Chinese renal transplant patients

PURPOSE

Interleukin 18 (IL-18) is a potent proinflammatory cytokine thought to down-regulate cytochrome P450 (CYP) enzyme activities. This study aimed to assess the potential influence of two functional single nucleotide polymorphisms (SNPs) in the IL-18 promoter region on the tacrolimus pharmacokinetics in Chinese renal transplant patients.

METHODS

We enrolled 96 renal allograft recipients receiving tacrolimus-based immunosuppressive regiments. Two functional SNPs in the IL-18 gene promoter region at the positions -137G/C (rs187283) and -607A/C (rs1946518) and one SNP (rs776746) of CYP3A5 were genotyped using a Mass ARRAY platform. Tacrolimus daily doses (mg/day) and trough tacrolimus concentration (ng/ml) were continuously recorded for 1 month after transplantation.

RESULTS

The tacrolimus C/D ratio was significantly associated with the IL-18 rs1946518 gene polymorphism in the first month after transplantation (P = 0.0225). We studied the influence of its polymorphism on tacrolimus C/D ratios in subjects with different CYP3A5 genotype backgrounds, and among patients with CYP3A5 expressers, the difference among the three genotypes was even more striking (P < 0.001). We did not find significant differences in tacrolimus C/D ratios between the IL-18 rs187238 genotypes, either nominally or according to the CYP3A5 genotype. In a simple linear regression model, age, hemoglobin (Hb), CYP3A5 gene polymorphisms, and IL-18 A-607C gene polymorphisms were associated with log-transformed tacrolimus C/D ratios (P < 0.05). In the final multiple linear regression model, CYP3A5 polymorphisms were the most important variant, accounting for 19.5 % of total variation involved in tacrolimus pharmacokinetics.

CONCLUSION

Our findings suggest that a combined analysis of CYP3A5 and IL-18 promoter polymorphisms may help clinicians develop individualized tacrolimus treatment, which is based on determining CYP3A5 genotype.

Reaction phenotyping: advances in the experimental strategies used to characterize the contribution of drug-metabolizing enzymes

During the process of drug discovery, the pharmaceutical industry is faced with numerous challenges. One challenge is the successful prediction of the major routes of human clearance of new medications. For compounds cleared by metabolism, accurate predictions help provide an early risk assessment of their potential to exhibit significant interpatient differences in pharmacokinetics via routes of metabolism catalyzed by functionally polymorphic enzymes and/or clinically significant metabolic drug-drug interactions. This review details the most recent and emerging in vitro strategies used by drug metabolism and pharmacokinetic scientists to better determine rates and routes of metabolic clearance and how to translate these parameters to estimate the amount these routes contribute to overall clearance, commonly referred to as fraction metabolized. The enzymes covered in this review include cytochrome P450s together with other enzymatic pathways whose involvement in metabolic clearance has become increasingly important as efforts to mitigate cytochrome P450 clearance are successful. Advances in the prediction of the fraction metabolized include newly developed methods to differentiate CYP3A4 from the polymorphic enzyme CYP3A5, scaling tools for UDP-glucuronosyltranferase, and estimation of fraction metabolized for substrates of aldehyde oxidase.

Pharmacogenetics and immunosuppressive drugs in solid organ transplantation

The transplantation literature includes numerous papers that report associations between polymorphisms in genes encoding metabolizing enzymes and drug transporters, and pharmacokinetic data on immunosuppressive drugs. Most of these studies are retrospective in design, and although a substantial number report significant associations, pharmacogenetic tests are hardly used in clinical practice. One of the reasons for this poor implementation is the current lack of evidence of improved clinical outcome with pharmacogenetic testing. Furthermore, with efficient therapeutic drug monitoring it is possible to rapidly correct for the effect of genotypic deviations on pharmacokinetics, thereby decreasing the utility of genotype-based dosing. The future of pharmacogenetics will be in treatment models in which patient characteristics are combined with data on polymorphisms in multiple genes. These models should focus on pharmacodynamic parameters, variations in the expression of drug transporter proteins, and predictors of toxicity. Such models will provide more information than the relatively small candidate gene studies performed so far. For implementation of these models into clinical practice, linkage of genotype data to medication prescription systems within electronic health records will be crucial.

Concomitant proton pump inhibitors with mycophenolate mofetil and the risk of rejection in kidney transplant recipients

BACKGROUND

Recent pharmacokinetic studies have demonstrated that proton pump inhibitors (PPI) reduce exposure of mycophenolic acid. However, the clinical significance of this drug-drug interaction on transplantation outcomes has not been determined.

METHODS

This was a retrospective cohort study in kidney transplant recipients who were prescribed rabbit antithymocyte globulin, calcineurin inhibitor, mycophenolate mofetil, and steroids. We evaluated the impact of PPI use on the 1-year rates of biopsy-proven acute rejection (BPAR).

RESULTS

Two hundred thirteen patients who were prescribed PPI were compared with 384 patients who were on standard acid-suppressive therapy with ranitidine. BPAR occurred in similar rates in both groups (15% vs. 12%; P=0.31). In a multivariable analysis, black race was associated with a higher risk of rejection (risk ratio [RR], 2.38; 95% confidence interval [CI], 1.41-4.03). While controlling for rejection risk factors, PPI exposure was associated with an increased risk of rejection in black patients (RR, 1.93; 95% CI, 1.18-3.16) but not in non-black patients (RR, 0.54; 95% CI, 0.19-1.49). At 1 year, BPAR type, BPAR grade, patient and graft survival, graft function, and time to BPAR were not associated with PPI exposure.

CONCLUSION

In this retrospective study, PPI use in the first transplant year was associated with an increased risk for BPAR in black patients but not in non-black patients. It is possible that a reduction in mycophenolic acid exposure contributed to the increased risk.

Progress in pharmacogenomics: bridging the gap from research to practice

Genetic information is increasingly used to optimize clinical treatment of patients, but obstacles remain to practical implementation as well as challenges to our understanding of genetic variation in drug response. These areas that particularly require research attention include gene-environment interactions, the consequences of genetic variation, and the impact of epigenetics on gene expression and function. In this issue of Clinical Pharmacology & Therapeutics focused on pharmacogenetics, we discuss some of the recent advances in understanding from a variety of viewpoints.

Genes and beans: pharmacogenomics of renal transplant

Advances in the management of patients after solid organ transplantation have led to dramatic decreases in rates of acute rejection, but long-term graft and patient survival have remained unchanged. Individualized therapy after transplant will ideally provide adequate immunosuppression while limiting the adverse effects of drug therapy that significantly impact graft survival. Therapeutic drug monitoring represents the best approximation of individualized drug therapy in transplant at this time; however, obtaining pharmacogenomic data in transplant patients has the potential to enhance our current practice. Polymorphisms of target genes that impact pharmacokinetics have been identified for most immunosuppressants, including tacrolimus, cyclosporine, mycophenolate, azathioprine and sirolimus. In the future, pre-emptive assessment of a patient's genetic profile may inform drug selection and provide information on specific doses that will improve efficacy and limit toxicity.

Genetic polymorphisms in warfarin and tacrolimus-related genes VKORC1, CYP2C9 and CYP3A5 in the Greek-Cypriot population

BACKGROUND

Two variants in the gene encoding the cytochrome P450 2C9 enzyme (CYP2C9) are considered the most significant genetic risk factors associated with bleeding after warfarin prescription. A variant in the vitamin K epoxide reductase (VKORC1) has been also associated by several studies with warfarin response. Another variant in the P450 3A5 enzyme (CYP3A5) gene is known to affect the metabolism of many drugs, including tacrolimus.

FINDINGS

We conducted a population genetic study in 148 unrelated healthy Greek-Cypriot volunteers (through PCR-RFLP assays), in order to determine the frequencies of the above pharmacogenetics variants and to compare allele frequencies with those in other major ethnic groups. The allele frequencies of CYP2C9*2, CYP2C9*3 and CYP3A5*3 were found to be 0.162, 0.112 and 0.943 respectively, whereas VKORC1 - 1639A was 0.534. The latter frequency differs significantly when compared with Caucasians, Asians and Africans (p < 0.001) and is still significant when compared with the geographically and culturally closely related to Greek-Cypriots, Hellenes of Greece (p = 0.01). Interestingly ~18% of our population are carriers of four or three risk alleles regarding warfarin sensitivity, therefore they have a high predisposition for bleeding after taking high or even normal warfarin doses.

CONCLUSIONS

Our data show no significant difference in the frequency of CYP2C9 and CYP3A5 allelic variants when compared to the Caucasian population, but differ significantly when compared with Africans and Asians (p < 0.001). Also, the frequency of variant VKORC1 - 1639A differs between Greek-Cypriots and every other population we compared. Finally, about 1/5 Greek-Cypriots carry three or four risk alleles and ~50% of them carry at least two independent risk alleles regarding warfarin sensitivity, a potentially high risk for over-anticoagulation.

Pharmacogenotyping of CYP3A5 in predicting dose-adjusted trough levels of tacrolimus among Malaysian kidney-transplant patients

Tacrolimus (FK506) is a calcineurin inhibitor with a narrow therapeutic index that exhibits large interindividual variation. Seventy-eight kidney transplant patients treated with tacrolimus were recruited to study the correlation of dose adjusted trough level (level/dose; L/D) of tacrolimus with CYP3A5 and ABCB1 genotypes, as well as the mRNA copy number of ABCB1 in blood. Patients were genotyped for ABCB1 (C1236T, G2677T/A, and C3435T) and CYP3A5 (G6986A), while ABCB1 mRNA transcript copy number was determined by absolute quantification (real-time PCR) in 46 patients. CYP3A5*3 genotypes were found to be a good predictor of tacrolimus L/D in kidney-transplant patients. Significantly higher L/D was observed among non-expressors (2.85, 95%: 2.05–3.70 (ng·mL–1)/(mg·kg–1)) as compared with the expressors (1.15, 95%: 0.95–1.80 (ng·mL–1)/(mg·kg–1)) of CYP3A5 (Mann–Whitney U test; P < 0.001). No correlation was observed between L/D and the ABCB1 genotypes. A significant inverse correlation of blood ABCB1 mRNA level with L/D was demonstrated (Spearman’s Rank Order correlation; P = 0.016, rs = –0.348). However, in multiple regression analysis, only CYP3A5*3 genotype groups were found to be significantly correlated with tacrolimus L/D (P < 0.001). These findings highlight the importance of CYP3A5*3 pharmacogenotyping among kidney-transplant patients treated with tacrolimus, and confirm the role of blood cell P-glycoprotein in influencing the L/D for tacrolimus.

Characterization of T-5 N-oxide formation as the first highly selective measure of CYP3A5 activity

Almost half of prescription medications are metabolized by cytochrome P450 3A4 and 3A5. CYP3A4 and 3A5 have significant substrate overlap, and there is currently no way to selectively monitor the activity of these two enzymes, which has led to the erroneous habit of attributing the cumulative activity to CYP3A4. While CYP3A4 expression is ubiquitous, CYP3A5 expression is polymorphic, with large individual differences in CYP3A5 expression level. The CYP3A5 genotype has been shown to alter the pharmacokinetics of drugs in clinical trials. We report the first tool compound capable of determining CYP3A5 activity in biologic samples containing both enzymes. Oxidation of T-5 by CYP3A5 yields an N-oxide metabolite that is over 100-fold selective over CYP3A4. Formation of T-5 N-oxide highly correlates with the CYP3A5 genotype and CYP3A5 expression levels in human liver microsomes and human hepatocytes.

CYP3A5 genotype had no impact on intrapatient variability of tacrolimus clearance in renal transplant recipients

BACKGROUND

The risk of long-term chronic allograft nephropathy and graft loss after kidney transplantation is increased in patients with a high intrapatient variability in the clearance of tacrolimus. CYP3A5 genotype has a significant influence on the oral bioavailability of tacrolimus and is a potential influence on variability of exposure.

METHODS

The study population consisted of 118 renal transplant recipients with stable renal function 12 months after transplant. The intrapatient variability of tacrolimus concentration was calculated. The patients were divided into low- and high-intraindividual variability groups using the median variability of tacrolimus clearance as the cutoff value.

RESULTS

No differences in baseline characteristics were observed between the expressers (n = 37) and nonexpressers (n = 81) except for ethnicity, which is in line with previous observations. Tacrolimus dose requirement was significantly higher in patients expressing CYP3A5, confirming earlier observations (P < 0.0001). However, intraindividual variability of tacrolimus clearance was not related to CYP3A5 genotype (P = 0.3331).

CONCLUSIONS

The intrapatient variability of tacrolimus clearance was not associated with CYP3A5 genotype in stable renal transplant recipients.

A taste of individualized medicine: physicians' reactions to automated genetic interpretations

The potential of pharmacogenomics is well documented, and functionality exploiting this knowledge is about to be introduced into electronic medical records. To explore physicians' reactions to automatic interpretations of genetic tests, we built a prototype with a simple interpretive algorithm. The algorithm was adapted to the needs of physicians handling immunosuppressive treatment during organ transplantation. Nine physicians were observed expressing their thoughts while using the prototype for two patient scenarios. The computer screen and audio were recorded, and the qualitative results triangulated with responses to a survey instrument. The physicians' reactions to the prototype were very positive; they clearly trusted the results and the theory behind them. The explanation of the algorithm was prominently placed in the user interface for transparency, although this design led to considerable confusion. Background information and references should be available, but considerably less prominent than the result and recommendation.

Effect of CYP3A5 expression on the inhibition of CYP3A-catalyzed drug metabolism: impact on modeling CYP3A-mediated drug-drug interactions

The purpose of this study was to determine the impact of CYP3A5 expression on inhibitory potency (Ki or IC50 values) of CYP3A inhibitors, using recombinant CYP3A4 and CYP3A5 (rCYP3A4 and rCYP3A5) and CYP3A5 genotyped human liver microsomes (HLMs). IC50 ratios between rCYP3A4 and rCYP3A5 (rCYP3A5/rCYP3A4) of ketoconazole (KTZ) and itraconazole (ITZ) were 8.5 and 8.8 for midazolam (MDZ), 4.7 and 9.1 for testosterone (TST), 1.3 and 2.8 for terfenadine, and 0.6 and 1.7 for vincristine, respectively, suggesting substrate- and inhibitor-dependent selectivity of the two azoles. Due to the difference in the IC50 values for CYP3A4 and CYP3A5, nonconcordant expression of CYP3A4 and CYP3A5 protein can significantly affect the observed magnitude of CYP3A-mediated drug-drug interactions in humans. Indeed, the IC50 values of KTZ and ITZ for CYP3A-catalyzed MDZ and TST metabolism were significantly higher in HLMs with CYP3A5*1/*1 and CYP3A5*1/*3 genotypes than in HLMs with the CYP3A5*3/*3 genotype, showing CYP3A5 expression-dependent IC50 values. Moreover, when IC50 values of KTZ and ITZ for different HLMs were kinetically simulated based on CYP3A5 expression level and enzyme-specific IC50 values, a good correlation between the simulated and the experimentally measured IC50 values was observed. Further simulation analysis revealed that both the Ki ratio (for inhibitors) and Vmax/Km ratio (for substrates) between CYP3A4 and CYP3A5 were major factors for CYP3A5 expression-dependent IC50 values. In conclusion, the present study demonstrates that CYP3A5 genotype and expression level have a significant impact on inhibitory potency for CYP3A-catalyzed drug metabolism, but that the magnitude of its effect is inhibitor-substrate pair specific.

From gut to kidney: transporting and metabolizing calcineurin-inhibitors in solid organ transplantation

Since their introduction circa 35 years ago, calcineurin-inhibitors (CNI) have become the cornerstone of immunosuppressive therapy in solid organ transplantation. However, CNI's possess a narrow therapeutic index with potential severe consequences of drug under- or overexposure. This demands a meticulous policy of Therapeutic Drug Monitoring (TDM) to optimize outcome. In clinical practice optimal dosing is difficult to achieve due to important inter- and intraindividual variation in CNI pharmacokinetics. A complex and often interdependent set of factors appears relevant in determining drug exposure. These include recipient characteristics such as age, race, body composition, organ function, and food intake, but also graft-related characteristics such as: size, donor-age, and time after transplantation can be important. Fundamental (in vitro) and clinical studies have pointed out the intrinsic relation between the aforementioned variables and the functional capacity of enzymes and transporters involved in CNI metabolism, primarily located in intestine, liver and kidney. Commonly occurring polymorphisms in genes responsible for CNI metabolism (CYP3A4, CYP3A5, CYP3A7, PXR, POR, ABCB1 (P-gp) and possibly UGT) are able to explain an important part of interindividual variability. In particular, a highly prevalent SNP in CYP3A5 has proven to be an important determinant of CNI dose requirements and drug-dose-interactions. In addition, a discrepancy in genotype between graft and receptor has to be taken into account. Furthermore, common phenomena in solid organ transplantation such as inflammation, ischemia- reperfusion injury, graft function, co-medication, altered food intake and intestinal motility can have a differential effect on the expression enzymes and transporters involved in CNI metabolism. Notwithstanding the built-up knowledge, predicting individual CNI pharmacokinetics and dose requirements on the basis of current clinical and experimental data remains a challenge.

Pharmacogenomics of drug-metabolizing enzymes: a recent update on clinical implications and endogenous effects

Interindividual differences in drug disposition are important causes for adverse drug reactions and lack of drug response. The majority of phase I and phase II drug-metabolizing enzymes (DMEs) are polymorphic and constitute essential factors for the outcome of drug therapy. Recently, both genome-wide association (GWA) studies with a focus on drug response, as well as more targeted studies of genes encoding DMEs have revealed in-depth information and provided additional information for variation in drug metabolism and drug response, resulting in increased knowledge that aids drug development and clinical practice. In addition, an increasing number of meta-analyses have been published based on several original and often conflicting pharmacogenetic studies. Here, we review data regarding the pharmacogenomics of DMEs, with particular emphasis on novelties. We conclude that recent studies have emphasized the importance of CYP2C19 polymorphism for the effects of clopidogrel, whereas the CYP2C9 polymorphism appears to have a role in anticoagulant treatment, although inferior to VKORC1. Furthermore, the analgesic and side effects of codeine in relation to CYP2D6 polymorphism are supported and the influence of CYP2D6 genotype on breast cancer recurrence during tamoxifen treatment appears relevant as based on three large studies. The influence of CYP2D6 polymorphism on the effect of antidepressants in a clinical setting is yet without any firm evidence, and the relation between CYP2D6 ultrarapid metabolizers and suicide behavior warrants further studies. There is evidence for the influence of CYP3A5 polymorphism on tacrolimus dose, although the influence on response is less studied. Recent large GWA studies support a link between CYP1A2 polymorphism and blood pressure as well as coffee consumption, and between CYP2A6 polymorphism and cigarette consumption, which in turn appears to influence the lung cancer incidence. Regarding phase II enzyme polymorphism, the anticancer treatment with mercaptopurines and irinotecan is still considered important in relation to the polymorphism of TPMT and UGT1A1, respectively. There is a need for further clarification of the clinical importance and use of all these findings, but the recent research in the field that encompasses larger studies and a whole genome perspective, improves the possibilities be able to make firm and cost-effective recommendations for drug treatment in the future.