CYP3A5 genotype markedly influences the pharmacokinetics of tacrolimus and sirolimus in kidney transplant recipients

Pharmacogenetics of Calcineurin inhibitors in kidney transplant recipients: the African gap. A narrative review

Calcineurin inhibitors (CNIs) are the mainstay of immunosuppression in kidney transplantation. Interpatient variability in the disposition of calcineurin inhibitors is a well-researched phenomenon and has a well-established genetic contribution. There is great diversity in the makeup of African genomes, but very little is known about the pharmacogenetics of CNIs and transplant outcomes. This review focuses on genetic variants of calcineurin inhibitors' metabolizing enzymes (CYP3A4, CYP3A5), related molecules (POR, PPARA) and membrane transporters involved in the metabolism of calcineurin inhibitors. Given the genetic diversity across the African continent, it is imperative to generate pharmacogenetic data, especially in the era of personalized medicine and emphasizes the need for studies specific to African populations. The study of allelic variants in populations where they have greater frequencies will help answer questions regarding their impact. We aim to fill the knowledge gaps by reviewing existing research and highlighting areas where African research can contribute.

A descriptive study of the single-nucleotide polymorphisms known to affect the Tacrolimus trough concentration per dose, among a population of kidney failure patients in a tertiary hospital in Ghana

BACKGROUND

The burden of chronic kidney disease (CKD) and kidney failure in Ghana is on the ascendency, with the prevalence of CKD estimated at 13.3%. Patients with CKD who progress to kidney failure require life sustaining kidney replacement therapy (KRT) which is almost exclusively available in Ghana as haemodialysis. Kidney transplantation is considered the best KRT option for patients with irreversible kidney failure due to its relative cost efficiency as well as its superiority in terms of survival and quality of life. However, because transplants may trigger an immune response with potential organ rejection, immunosuppressants such as tacrolimus dosing are required.

OBJECTIVE

This study sought to determine single nucleotide polymorphisms in CYP3A5, CYP3A4 and MDR1 genes that affect the pharmacokinetics of Tacrolimus in a population of Ghanaian patients with kidney failure.

METHOD

This cross-sectional study comprised of 82 kidney failure patients undergoing maintenance haemodialysis at the Renal and Dialysis unit of Korle-Bu Teaching Hospital (KBTH). Clinical and demographic data were collected and genomic DNA isolated. Samples were genotyped for specific SNPs using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP).

RESULTS

Participants, 58/82 (70.73%) harbored the wildtype CYP3A5*1/*1 AA genotype, 20/82 (24.39%) carried the heterozygous CYP3A5*1/*3 AG genotype, and 4/82 (4.88%) had the homozygous mutant CYP3A5*3/*3 GG genotype. Also, 6/82 (7.32%) carried the wildtype AA genotype, 11/82 (13.41%) had the heterozygous AG genotype, and 65/82 (79.27%) harbored the homozygous mutant GG genotype of CYP3A4*1B (-290 A>G). For MDR1_Ex21 (2677 G>T), 81/82 (98.78%) carried the wildtype GG genotype, while 1/82 (1.22%) had the heterozygous GT genotype. For MDR1_Ex26 (3435 C>T), 63/82 (76.83%) had the wildtype CC genotype, while 18/82 (21.95%) carried the heterozygous CT genotype, and 1/82 (1.22%) harbored the mutant TT genotype.

CONCLUSION

SNPs in CYP3A4, CYP3A5, and MDR1 genes in a population of Ghanaian kidney failure patients were described. The varying SNPs of the featured genes suggest the need to consider the genetic status of Ghanaians kidney failure patients prior to transplantation and tacrolimus therapy.

Pharmacogenomic Analysis of CYP3A5*3 and Tacrolimus Trough Concentrations in Vietnamese Renal Transplant Outcomes

Purpose

CYP3A5 polymorphisms have been associated with variations in the pharmacokinetics of tacrolimus (Tac) in kidney transplant patients. Our study aims to quantify how the CYP3A5 genotype influences tacrolimus trough concentrations (C0) in a Vietnamese outpatient population by selecting an appropriate population pharmacokinetic model of Tac for our patients.

Patients and Methods

The external dataset was obtained prospectively from 54 data of adult kidney transplant recipients treated at the 103 Military Hospital. All published Tac population pharmacokinetic models were systematically screened from PubMed and Scopus databases and were selected based on our patient's available characteristics. Mean absolute prediction error (MAPE), mean prediction error, and goodness-of-fit plots were used to identify the appropriate model for finding the formula that identifies the influence of CYP3A5 genotype on the pharmacokinetic data of Vietnamese patients.

Results

The model of Zhu et al had a good predictive ability with MAPE of 19.29%. The influence of CYP3A5 genotype on tacrolimus clearance was expressed by the following formulas: CL/F=27 , 2 × [ ( WT/70 ) 0 , 75 ] × [ ( HCT/0 , 35 ) -0 , 501 ] × [ ( POD/180 ) 0 , 0306 ] × CYP3A5 ( L/h ) . The simulation result showed that Tac C0 was significantly higher in patients not expressing CYP3A5 (p< 0.001).

Conclusion

The incorporation of the CYP3A5 phenotype into Zhu's structural model has significantly enhanced our ability to predict Tacrolimus trough levels in the Vietnamese population. This study's results underscore the valuable role of CYP3A5 phenotype in optimizing the forecast of Tac concentrations, offering a promising avenue to assist health-care practitioners in their clinical decision-making and ultimately advance patient care outcomes.

The impact of CYP3A4 and CYP3A5 genetic variations on tacrolimus treatment of living-donor Egyptian kidney transplanted patients

BACKGROUND

Tacrolimus (TAC) is the mainstay of immunosuppressive regimen for kidney transplantations. Its clinical use is complex due to high inter-individual variations which can be partially attributed to genetic variations at the metabolizing enzymes CYP3A4 and CYP3A5. Two single nucleotide polymorphisms (SNPs), CYP3A4*22 and CYP3A5*3, have been reported as important causes of differences in pharmacokinetics that can affect efficacy and/or toxicity of TAC.

OBJECTIVE

Investigating the effect of CYP3A4*22 and CYP3A5*3 SNPs individually and in combination on the TAC concentration in Egyptian renal recipients.

METHODS

Overall, 72 Egyptian kidney transplant recipients were genotyped for CYP3A4*22 G>A and CYP3A5*3 T>C. According to the functional defect associated with CYP3A variants, patients were clustered into: poor (PM) and non-poor metabolizers (Non-PM). The impact on dose adjusted through TAC concentrations (C0) and daily doses at different time points after transplantation was evaluated.

RESULTS

Cyp3A4*1/*22 and PM groups require significantly lower dose of TAC (mg/kg) at different time points with significantly higher concentration/dose (C0/D) ratio at day 10 in comparison to Cyp3A4*1/*1 and Non-PM groups respectively. However, CyP3A5*3 heterozygous individuals did not show any significant difference in comparison to CyP3A5*1/*3 individuals. By comparing between PM and Non-PM, the PM group had a significantly lower rate of recipients not reaching target C0 at day 14.

CONCLUSION

This is the first study on Egyptian population to investigate the impact of CYP3A4*22 and CYP3A5*3 SNPs individually and in combination on the TAC concentration. This study and future multicenter studies can contribute to the individualization of TAC dosing in Egyptian patients.

Influence of genetic variants on the pharmacokinetics and pharmacodynamics of sirolimus: a systematic review

Sirolimus is an antiproliferative and immunosuppressive compound inhibiting the mTOR pathway, which is often activated in congenital low-flow vascular malformations. Studies have demonstrated the efficacy of sirolimus for this disease. Studies in kidney transplant patients suggest that genetic variants can influence these pharmacokinetic parameters. Therefore, a systematic literature search was performed to gain insight into pharmacogenetic studies with sirolimus. Most studies investigated CYP3A4 and CYP3A5, with inconsistent results. No pharmacogenetic studies focusing on sirolimus have been performed for low-flow vascular malformations. We analyzed two common variants of CYP3A4 and CYP3A5 (CYP3A4*22 and CYP3A5*3, respectively) in patients (n = 59) with congenital low-flow vascular malformations treated with sirolimus. No association with treatment outcome was identified in this small cohort of patients.

Three Layers of Personalized Medicine in the Use of Sirolimus and Its Derivatives for the Treatment of Cancer

Rapamycin and its derivatives are mTOR inhibitors which are FDA-approved for use as immunosuppressants and chemotherapeutic agents. These agents are currently approved to treat renal cell carcinomas, soft tissue sarcomas, and other rare tumors. As tumor treatment paradigms are moving away from organ-based drug selection and moving towards tumor characteristics for individualized treatment it is important to identify as many properties as possible that impact the efficacy of the rapalogues. A review of the current literature was conducted to identify enzymes involved in the metabolism of Sirolimus, Everolimus, Ridaforolimus, and Temsirolimus along with characteristics of tumors that predict the efficacy of these agents. This review also sought to establish whether the genetic characteristics of the patient might influence the activity of the rapalogues or lead to side effects from these agents. Current evidence suggests that tumors with mutations in the mTOR signal transduction pathway are sensitive to rapalogue treatment; the rapalogues are metabolized by cytochromes such as CYP3A4, CYP3A5, and CYP2C8 and transported by ABC transporters that are known to vary in activity in individuals; and that tumors can express these transporters and detoxifying enzymes. This results in three levels of genetic analysis that could impact the effectiveness of the mTOR inhibitors.

Pharmacokinetic Evaluation of Tacrolimus in Chinese Adult Patients during the Early Stages Post-Lung Transplantation

BACKGROUND

Although tacrolimus has been widely used in patients undergoing lung transplantation, few studies have reported the pharmacokinetics of tacrolimus in Chinese patients after lung transplantation. Thus, we aimed to investigate the pharmacokinetics and influential factors in this patient cohort in the early stage after lung transplantation.

METHODS

We enrolled 14 adult lung transplant recipients who were treated with tacrolimus and then intensively collected blood samples within a 12-h dosing interval. The pharmacokinetic parameters of tacrolimus were calculated using non-compartmental analysis, and the influence of pathophysiological characteristics and CYP3A5*3 and CYP3A4*1G genotypes on the pharmacokinetics of tacrolimus was assessed. Using linear regression analysis, we investigated the correlation between tacrolimus concentration at different sampling points and measured the area under the time-concentration curve (AUC_0-12h).

RESULTS

Geometric mean of apparent clearance (CL/F) was 18.13 ± 1.65 L/h in non-CYP3A5*3/*3 carriers, five times higher than that in CYP3A5*3/*3 carriers (p < 0.001). Furthermore, the tacrolimus concentration 4 h after administration had the strongest correlation with AUC_0-12h (R² = 0.979).

CONCLUSION

The pharmacokinetics of tacrolimus varied largely between patients during the early stage post-transplantation, which could be partially explained by CYP3A5*3 genetic polymorphisms.

Precision sirolimus dosing in children: The potential for model-informed dosing and novel drug monitoring

The mTOR inhibitor sirolimus is prescribed to treat children with varying diseases, ranging from vascular anomalies to sporadic lymphangioleiomyomatosis to transplantation (solid organ or hematopoietic cell). Precision dosing of sirolimus using therapeutic drug monitoring (TDM) of sirolimus concentrations in whole blood drawn at the trough (before the next dose) time-point is the current standard of care. For sirolimus, trough concentrations are only modestly correlated with the area under the curve, with R 2 values ranging from 0.52 to 0.84. Thus, it should not be surprising, even with the use of sirolimus TDM, that patients treated with sirolimus have variable pharmacokinetics, toxicity, and effectiveness. Model-informed precision dosing (MIPD) will be beneficial and should be implemented. The data do not suggest dried blood spots point-of-care sampling of sirolimus concentrations for precision dosing of sirolimus. Future research on precision dosing of sirolimus should focus on pharmacogenomic and pharmacometabolomic tools to predict sirolimus pharmacokinetics and wearables for point-of-care quantitation and MIPD.

Impact of single-nucleotide polymorphisms on tacrolimus pharmacokinetics in liver transplant patients after switching to once-daily dosing

BACKGROUND

The effects of multidrug resistance-1 (MDR1), ABCC2, and P450 oxidoreductase (POR)*28 gene polymorphisms on tacrolimus metabolism following a switch to once-daily dosing have not been elucidated. We investigated the effects of recipient and donor CYP3A5, MDR1, ABCC2, and POR*28 polymorphisms on tacrolimus pharmacokinetics following a switch to once-daily tacrolimus dosing.

METHODS

Eighty-seven liver transplant recipients who were switched from twice- to once-daily tacrolimus dosing following living-donor liver transplantation and 81 matched donors were genotyped for CYP3A5, MDR1 (1236C>T, 2677G>T/A, and 3435C>T), ABCC2 (-24C>T, 1249G>A, and 3972C>T), and POR*28. Tacrolimus dose-adjusted trough levels (C0/dose) before and after the switch were determined and calculated based on past medical records. Recipients were divided into two groups, one group constituted of 38 patients with a C0/dose decrease of less than 30% following conversion (group 1) and the other constituted of 49 patients with a C0/dose decrease of ≥ 30% (group 2).

RESULTS

CYP3A5 *1/*3 and *3/*3 were more frequent in recipients in group 1 (60.5% vs. 36.8%), while CYP3A5 *1/*1 was more frequent in group 2 (59.2% vs. 32.7%) (p = 0.016). The proportions of donor ABCC2 1249G>A genotypes AA and AG were higher in group 2 than in group 1 (20.4% vs. 5.3%; p = 0.042).

CONCLUSION

Recipient CYP3A5 polymorphism and donor ABCC2 1249G>A polymorphism affected tacrolimus pharmacokinetics following the switch to once-daily dosing. Dose adjustment to maintain therapeutic tacrolimus levels following the switch to once-daily dosing should be considered based on polymorphisms in both the recipient and donor.

Population Pharmacokinetic Analysis and Dosing Optimization of Sirolimus in Children With Tuberous Sclerosis Complex

Sirolimus is confirmed to be effective in the treatment of tuberous sclerosis complex (TSC) and related disorders. The study aims to establish a population pharmacokinetic model of oral sirolimus for children with TSC and provide an evidence-based approach for individualization of sirolimus dosing in the pediatric population. A total of 64 children were recruited in this multicenter, retrospective pharmacokinetic study. Whole-blood concentrations of sirolimus, demographic, and clinical information were collected and analyzed using a nonlinear mixed-effects population modeling method. The final model was internally and externally validated. Then Monte Carlo simulations were performed to evaluate and optimize the dosing regimens. In addition, the efficacy and safety of sirolimus therapy was assessed retrospectively in patients with epilepsy or cardiac rhabdomyomas associated with TSC. Finally, the sirolimus pharmacokinetic profile was described by a 1-compartment model with first-order absorption and elimination along with body weight and total daily dose as significant covariates. The typical population parameter estimates of apparent volume of distribution and apparent clearance were 69.48 L and 2.79 L/h, respectively. Simulations demonstrated that dosage regimens stratified by body surface area may be more appropriate for children with TSC. These findings could be used to inform individualized dosing strategies of sirolimus for pediatric patients with TSC.

Machine learning-based method for tacrolimus dose predictions in Chinese kidney transplant perioperative patients

WHAT IS KNOWN AND OBJECTIVES

Tacrolimus (TAC), a first-line immunosuppressant in solid-organ transplant, has a narrow therapeutic window and large inter-individual variability, which affects its use in clinical practice. Successful predictions using machine learning algorithms have been reported in several fields. However, a comparison of 10 machine learning model-based TAC pharmacogenetic and pharmacokinetic dosing algorithms for kidney transplant perioperative patients of Chinese descent has not been reported. The objective of this study was to screen and establish an appropriate machine learning method to predict the individualized dosages of TAC for perioperative kidney transplant patients.

METHODS

The records of 2551 patients were collected from three transplant centres, 80% of which were randomly selected as a 'derivation cohort' to develop the dose prediction algorithm, while the remaining 20% constituted a 'validation cohort' to validate the final algorithm selected. Important features were screened according to our previously established population pharmacokinetic model of tacrolimus. The performances of the algorithms were evaluated and compared using R-squared and the mean percentage in the remaining 20% of patients.

RESULTS AND DISCUSSION

This study identified several factors influencing TAC dosage, including CYP3A5 rs776746, CYP3A4 rs4646437, haematocrit, Wuzhi capsules, TAC daily dose, age, height, weight, post-operative time, nifedipine and the medication history of the patient. According to our results, among the 10 machine learning models, the extra trees regressor (ETR) algorithm showed the best performance in the training set (R-squared: 1, mean percentage within 20%: 100%) and test set (R-squared: 0.85, mean percentage within 20%: 92.77%) of the derivation cohort. The ETR model successfully predicted the ideal TAC dosage in 97.73% of patients, especially in the intermediate dosage range (>5 mg/day to <8 mg/day), whereby the ideal TAC dosage could be successfully predicted in 99% of the patients.

WHAT IS NEW AND CONCLUSION

The results indicated that the ETR algorithm, which was chosen to establish the dose prediction model, performed better than the other nine machine learning models. This study is the first to establish ETR algorithms to predict TAC dosage. This study will further promote the individualized medication of TAC in kidney transplant patients in the future, which has great significance in ensuring the safety and effectiveness of drug use.

Early Prediction of Tacrolimus-Induced Tubular Toxicity in Pediatric Refractory Nephrotic Syndrome Using Machine Learning

Background and Aims: Tacrolimus(TAC)-induced nephrotoxicity, which has a large individual variation, may lead to treatment failure or even the end-stage renal disease. However, there is still a lack of effective models for the early prediction of TAC-induced nephrotoxicity, especially in nephrotic syndrome(NS). We aimed to develop and validate a predictive model of TAC-induced tubular toxicity in children with NS using machine learning based on comprehensive clinical and genetic variables. Materials and Methods: A retrospective cohort of 218 children with NS admitted between June 2013 and December 2018 was used to establish the models, and 11 children were prospectively enrolled for external validation. We screened 47 clinical features and 244 genetic variables. The changes in urine N- acetyl- β-D- glucosaminidase(NAG) levels before and after administration was used as an indicator of renal tubular toxicity. Results: Five machine learning algorithms, including extreme gradient boosting (XGBoost), gradient boosting decision tree (GBDT), extremely random trees (ET), random forest (RF), and logistic regression (LR) were used for model generation and validation. Four genetic variables, including TRPC6 rs3824934_GG, HSD11B1 rs846910_AG, MAP2K6 rs17823202_GG, and SCARB2 rs6823680_CC were incorporated into the final model. The XGBoost model has the best performance: sensitivity 75%, specificity 77.8%, accuracy 77.3%, and AUC 78.9%. Conclusion: A pre-administration model with good performance for predicting TAC-induced nephrotoxicity in NS was developed and validated using machine learning based on genetic factors. Physicians can estimate the possibility of nephrotoxicity in NS patients using this simple and accurate model to optimize treatment regimen before administration or to intervene in time after administration to avoid kidney damage.

Significance of Ethnic Factors in Immunosuppressive Therapy Management After Organ Transplantation

Clinical outcomes after organ transplantation have greatly improved in the past 2 decades with the discovery and development of immunosuppressive drugs such as calcineurin inhibitors, antiproliferative agents, and mammalian target of rapamycin inhibitors. However, individualized dosage regimens have not yet been fully established for these drugs except for therapeutic drug monitoring-based dosage modification because of extensive interindividual variations in immunosuppressive drug pharmacokinetics. The variations in immunosuppressive drug pharmacokinetics are attributed to interindividual variations in the functional activity of cytochrome P450 enzymes, UDP-glucuronosyltransferases, and ATP-binding cassette subfamily B member 1 (known as P-glycoprotein or multidrug resistance 1) in the liver and small intestine. Some genetic variations have been found to be involved to at least some degree in pharmacokinetic variations in post-transplant immunosuppressive therapy. It is well known that the frequencies and effect size of minor alleles vary greatly between different races. Thus, ethnic considerations might provide useful information for optimizing individualized immunosuppressive therapy after organ transplantation. Here, we review ethnic factors affecting the pharmacokinetics of immunosuppressive drugs requiring therapeutic drug monitoring, including tacrolimus, cyclosporine, mycophenolate mofetil, sirolimus, and everolimus.

Evaluating the Impact of CYP3A5 Genotype on Post-Transplant Healthcare Resource Utilization in Pediatric Renal and Heart Transplant Recipients Receiving Tacrolimus

Purpose

CYP3A5 genotype is a significant contributor to inter-individual tacrolimus exposure and may impact the time required to achieve therapeutic concentrations and number of tacrolimus dose adjustments in transplant patients. Increased modifications to tacrolimus therapy may indicate a higher burden on healthcare resources. The purpose of this study was to evaluate whether CYP3A5 genotype was predictive of healthcare resource utilization in pediatric renal and heart transplant recipients.

Patients and Methods

Patients <18 years of age with a renal or heart transplant between 6/1/2014–12/31/2018 and tacrolimus-based immunosuppression were included. Secondary use samples were obtained for CYP3A5 genotyping. Clinical data was retrospectively collected from the electronic medical record. Healthcare resource utilization measures included the number of dose changes, number of tacrolimus concentrations, length of stay, number of clinical encounters, and total charges within the first year post-transplant. Rejection and donor-specific antibody (DSA) formation within the first year were also collected. The impact of CYP3A5 genotype was evaluated via univariate analysis for the first year and multivariable analysis at 30, 90, 180, 270, and 365 days post-transplant.

Results

Eighty-five subjects were included, 48 renal transplant recipients and 37 heart transplant recipients. CYP3A5 genotype was not associated with any outcomes in renal transplant, however, a CYP3A5 expresser phenotype was a predictor of more dose changes, more tacrolimus concentrations, longer length of stay, and higher total charges in heart transplant recipients. CYP3A5 genotype was not associated with rejection or DSA formation. Age and induction therapy were associated with higher total charges.

Conclusion

CYP3A5 genotype may predict healthcare resource utilization in the first year post-transplant, although this may be mitigated by differences in tacrolimus management. Future studies should evaluate the impact of genotype-guided dosing strategies for tacrolimus on healthcare utilization resources.

Clinical Impact of the Adaptation of Initial Tacrolimus Dosing to the CYP3A5 Genotype After Kidney Transplantation: Systematic Review and Meta-Analysis of Randomized Controlled Trials

OBJECTIVE

The aim of this systematic review and meta-analysis was to compare the clinical outcomes between genotype-guided and conventional tacrolimus doses in kidney transplantation patients.

MATERIALS AND METHODS

We performed a comprehensive literature search of the PubMed, EMBASE, and Cochrane databases from the date of inception to 26 February 2020. References of the retrieved articles were also reviewed and any further relevant studies were included. The search terms included 'tacrolimus', 'cytochrome P-450 CYP3A', 'polymorphism, genetic', 'genomics', 'genome', 'genotype', 'genes', 'alleles', and 'pharmacogenetics'.

RESULTS

Our study showed that the genotype-guided group included an increased proportion of patients with tacrolimus concentrations in the therapeutic range at steady state (risk ratio [RR] 1.40, 95% confidence interval [CI] 1.14-1.72, p = 0.001; high quality), with a trend for achieving therapeutic concentrations earlier compared with those in the conventional group. However, there was no statistical difference in the incidence of delayed graft function (RR 1.98, 95% CI 0.92-1.76, p = 0.12; moderate quality), incidence of acute rejection (RR 1.00, 95% CI 0.64-1.55, p = 1.00; moderate quality), incidence of graft survival censored for death (RR 1.02, 95% CI 0.98-1.06, p = 0.37; moderate quality), and incidence of adverse effects (AEs).

CONCLUSIONS

Although the genotype-guided group had a higher proportion of patients within the targeted concentration and less median time to achieve the therapeutic range, the clinical endpoints, including delayed graft function, acute rejection, graft survival censored for death, and AEs were similar in both groups. All in all, evidence suggested there was no utility in pharmacogenetics for tacrolimus based on the cytochrome P450 (CYP) 3A5 genotype. Studies with Chinese and African American populations are needed due to the frequency of genetic polymorphisms of CYP3A5. Furthermore, a dosing algorithm that includes demographic and clinical factors plus multiple genetic variants should be added for consideration, and may optimize early tacrolimus exposure.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

The Genetic Polymorphism of CYP3A4 rs2242480 is Associated with Sirolimus Trough Concentrations Among Adult Renal Transplant Recipients

BACKGROUND

The large interindividual variability in the genetic polymorphisms of sirolimus (SIR)- metabolizing enzymes, transporters, and receptors can lead to qualitatively and quantitatively distinct therapeutic responses.

OBJECTIVE

We examined the impact of numerous candidate single-nucleotide polymorphisms (SNPs) involved in the trough concentration of SIR-based immunosuppressant regimen.

METHODS

This is a retrospective, long-term cohort study involving 69 renal allograft recipients. Total DNA was isolated from recipient blood samples and trough SIR concentrations were measured by microparticle enzyme immunoassay. Genome sequence reading was targeted based on next-generation sequencing. The association of tagger SNPs to SIR trough concentrations with non-genetic covariate adjusting was analyzed using logistic regression.

RESULTS

A total of 300 SNPs were genotyped in the recipient DNA samples using target sequencing analysis. Only the SNP of CYP3A4 (Ch7: 99361466 C>T, rs2242480) had a significantly higher association with SIR trough concentration as compared to the other 36 tagger SNPs. The mean trough SIR concentration of patients in the CYP3A4 rs2242480-CC group was more significant compared to that of the CYP3A4 rs2242480-TC and TT group, respectively 533.3; 157.4 and 142.5 (ng/ml)/mg/kg, P<0.0001. After adjusting the SNPs, there was no significant association between clinical factors such as age, follow-up period, the incidence of delayed graft function, immunosuppression protocol, and sex with SIR trough concentration.

CONCLUSION

These findings indicated a significant association of polymorphism in the CYP3A4 (Ch7: 99361466 C>T, rs2242480) with SIR trough concentration after 1-year administration in patients who have undergone kidney transplantation.

The effect of ABCB1 polymorphism on sirolimus in renal transplant recipients: a meta-analysis

Background

Sirolimus (SRL) is an immunosuppressive drug and substrate of the P-glycoprotein (P-GP) encoded by ABCB1. The relationship between ABCB1 polymorphism and the pharmacokinetics of SRL in different studies were conflicting in renal transplant recipients. Thus, this meta-analysis aims to investigate the influence of ABCB1 C3435T, C1236T, and G2677T/A polymorphisms on the dose-adjusted trough level (C/D) of SRL in renal transplant recipients.

Methods

PubMed, Embase, and the Cochrane Library were searched for relevant studies. The quality of each eligible study was assessed according to Newcastle-Ottawa Scale. The STATA 15.0 was adopted to perform the meta-analysis. The fixed-effects model was used for pooled results with low heterogeneity (I² ≤50%); otherwise, the random-effects model was used.

Results

A total of 6 studies were included in the meta-analysis. Results of pooled analysis showed no significant association of SRL C/D ratio with ABCB1 C3435T polymorphism. The subgroup analysis based on different ethnic groups and different time-points after SRL initiation in renal transplant recipients were also conducted. No significant association was observed in these subgroups. Significant associations were showed between ABCB1 C1236T polymorphism and the C/D ratio of SRL in the homozygous model (TT vs. CC; WMD: −45.54; 95% CI: −75.15, −15.94; P=0.003), and also in subgroup of Caucasian (TT vs. CC; WMD: −46.57; 95% CI: −91.90, −1.25; P=0.044 and TT vs. CC + CT; WMD: −52.10; 95% CI: −95.38, −8.82; P=0.018). Significant differences were found in association between the ABCB1 G2677T/A polymorphism and the C/D ratio of SRL, including the homozygous model (TT vs. GG; WMD: −76.47; 95% CI: −126.37, −26.58; P= 0.003), the heterozygous model (GT vs. GG,WMD: 178.62; 95% CI: 125.03, 232.22; P= 0.000), the dominant model (GT + TT vs. GG; WMD: 82.23; 95% CI: 36.28, 128.17; P=0.000), the recessive model (TT vs. GG + GT; WMD: −179.38; 95% CI: −283.33, −75.42; P=0.001), and the over-dominant model (GT vs. GG + TT; WMD: 199.44; 95% CI: 84.84, 314.05; P=0.001).

Conclusions

No significant association exists between ABCB1 C3435T polymorphism and the C/D ratio of SRL in renal transplant recipients. To achieve target therapeutic concentrations, ABCB1 C1236T homozygous mutant TT genotype will require a higher dose of sirolimus than wild type GG, especially in Caucasian renal transplant recipients. ABCB1 G2677T/A TT genotype will also need a higher dose of sirolimus genotype. Genotyping of ABCB1 might help to improve the individualization of SRL for renal transplant recipients. Further studies are expected to provide high-quality evidence.

Genome-Wide Association Study of Tacrolimus Pharmacokinetics Identifies Novel Single Nucleotide Polymorphisms in the Convalescence and Stabilization Periods of Post-transplant Liver Function

After liver transplantation, the liver function of a patient is gradually restored over a period of time that can be divided into a convalescence period (CP) and a stabilizing period (SP). The plasma concentration of tacrolimus, an immunosuppressant commonly used to prevent organ rejection, varies as a result of variations in its metabolism. The effects of genetic and clinical factors on the plasma concentration of tacrolimus appear to differ in the CP and SP. To establish a model explaining the variation in tacrolimus trough concentration between individuals in the CP and SP, we conducted a retrospective, single-center, discovery study of 115 pairs of patients (115 donors and 115 matched recipients) who had undergone liver transplantation. Donors and recipients were genotyped by a genome-wide association study (GWAS) using an exome chip. Novel exons were identified that influenced tacrolimus trough concentrations and were verified with bootstrap analysis. In donors, two single-nucleotide polymorphisms showed an effect on the CP (rs1927321, rs1057192) and four showed an effect on the SP (rs776746, rs2667662, rs7980521, rs4903096); in recipients, two single-nucleotide polymorphisms showed an effect in the SP (rs7828796, rs776746). Genetic factors played a crucial role in tacrolimus metabolism, accounting for 44.8% in the SP, which was higher than previously reported. In addition, we found that CYP3A5, which is known to affect the metabolism of tacrolimus, only influenced tacrolimus pharmacokinetics in the SP.

The Effects of CYP3A5 Genetic Polymorphisms on Serum Tacrolimus Dose-Adjusted Concentrations and Long-Term Prognosis in Chinese Heart Transplantation Recipients

BACKGROUND AND OBJECTIVES

Effective management of immunosuppressants is extemely important to improve prognosis of heart transplant recipients. We aim to investigate the effects of cytochrome P450 (CYP) 3A5 (rs776746) single nucleotide polymorphisms (SNPs) on serum tacrolimus concentrations/doses (C/Ds, ng/mL per mg/kg) and long-term prognosis in Chinese heart transplant recipients.

METHODS

We detected the CYP3A5 SNPs of 203 consecutive Chinese heart transplant recipients between August 2005 and July 2012, and 55 of them who received tacrolimus-based immunosuppressive therapy were enrolled in this study. The tacrolimus C/Ds at 1, 3, 6, 12, 24 and 36 months after transplantation were routinely calculated. X-ray-guided endomyocardial biopsies (EMBs) were performed at 1, 3 and 6 months after heart transplantion to evaluate acute rejection degrees. All participants were then followed up annually until May 2018. The designed primary endpoint was all-cause mortality.

RESULTS

In 55 heart transplant recipients (43 males and 12 females), CYP3A5 non-expressors (CYP3A5*3/*3, n = 40) had significantly higher tacrolimus C/Ds than expressors (CYP3A5*1/*3, n = 15) at all time points (P < 0.001). Chi-squared test showed no significant differences in EMB-proven acute rejections between the two groups within 6 months after heart transplantion. The median follow-up period was 94.7 months, and eight patients died. Kaplan-Meier analysis showed CYP3A5 expressors tend to have higher mortality than non-expressors (20% vs 12.5%, log-rank: P = 0.314).

CONCLUSIONS

CYP3A5 SNPs affect tacrolimus pharmacokinetics in Chinese heart transplant recipients, and non-expressors have higher tacrolimus C/Ds. In addition, expressors tend to have a worse long-term prognosis than non-expressors.

The Impact of Pharmacogenomics in Personalized Medicine

Recent advances in Pharmacogenomics have made it possible to understand the reasons behind the different response of a drug. Discovery of genetic variants and its association with the varying response of drug provide the basis for recommending a drug and its dose to an individual patient. Genetic makeup-based prescription, design, and implementation of therapy not only improve the outcome of treatments but also reduce the risk of toxicity and other adverse effects. A better understanding of individual variations and their effect on drug response, metabolism excretion, and toxicity will replace the trial-and-error approach of treatment. Evidence of the clinical utility of pharmacogenetics testing is only available for a few medications, and FDA labels only require pharmacogenetics testing for a small number of drugs. Although there is a great promise, there are not many examples where Pharmacogenomics impacts clinical utility. Some genetic variants related to different diseases have been reported, and many have not been studied yet. The information related to the outcome of treatment with a particular drug and a genetic variant can be used to release a warning/label for the use of that drug. There are many limitations in the way of implementing the goal of personalized medicine. Future advances in the field of genomics, diagnosis approaches, data analysis, clinical decision-making, and sustainable business model for personalization of therapy can speed up the individualization of therapy based on genetic makeup.

Pharmacokinetics, Pharmacodynamics and Pharmacogenetics of Tacrolimus in Kidney Transplantation

Background:

Tacrolimus (Tac, or FK506), a calcineurin inhibitor (CNI), is the first-line immu-nosuppressant which consists of the footstone as immunosuppressive regimens in kidney transplantation. However, the drug toxicity and the significant differences of pharmacokinetics (PK) and pharmacodynam-ics (PD) among individuals are hidden troubles for clinical application. Recently, emerging evidences of Tac pharmacogenetics (PG) regarding drug absorption, metabolism, disposition, excretion and response are discovered for better understanding of this drug.

Method:

We reviewed the published articles regarding the Tac PG and its effects on PK and PD in kidney transplantation. In addition, we summarized information on polygenic algorithms.

Results:

The polymorphism of genes encoding metabolic enzymes and transporters related to Tac were largely investigated, but the results were inconsistent. In addition to CYP3A4, CYP3A5 and P-gp (also known as ABCB1), single nucleotide polymorphisms (SNPs) might also affect the PK and PD parameters of Tac.

Conclusion:

The correlation between Tac PK, PD and PG is very complex. Although many factors need to be verified, it is envisaged that thorough understanding of PG may assist clinicians to predict the optimal starting dosage, help adjust the maintenance regimen, as well as identify high risk patients for adverse ef-fects or drug inefficacy

Clinical Pharmacogenomics: Applications in Nephrology

Pharmacogenomics is a tool for practitioners to provide precision pharmacotherapy using genomics. All providers are likely to encounter genomic data in practice with the expectation that they are able to successfully apply it to patient care. Pharmacogenomics tests for genetic variations in genes that are responsible for drug metabolism, transport, and targets of drug action. Variations can increase the risk for drug toxicity or poor efficacy. Pharmacogenomics can, therefore, be used to help select the best medication or aid in dosing. Nephrologists routinely treat cardiovascular disease and manage patients after kidney transplantation, two situations for which there are several high-evidence clinical recommendations for commonly used anticoagulants, antiplatelets, statins, and transplant medications. Successful use of pharmacogenomics in practice requires that providers are familiar with how to access and use pharmacogenomics resources. Similarly, clinical decision making related to whether to use existing data, whether to order testing, and if data should be used in practice is needed to deliver precision medicine. Pharmacogenomics is applicable to virtually every medical specialty, and nephrologists are well positioned to be implementation leaders.

Risk factors associated with Clostridium difficile infection in kidney transplant recipients

BACKGROUND

Solid organ transplant recipients are especially vulnerable to Clostridium difficile infection (CDI) due to cumulative risk factors including increased exposure to healthcare settings, persistent immunosuppression, and higher rates of antimicrobial exposure. We aimed to identify risk factors associated with CDI development in kidney transplant recipients including implications of immunosuppressive therapies and acid-suppressing agents.

METHODS

This was a single-center, non-interventional, retrospective case-control study of adult subjects between June 1, 2009 and June 30, 2013. During this time, 728 patients underwent kidney transplantation. Overall, 22 developed CDI (cases) and were matched 1:3 with 66 controls. Cases and controls were also matched for induction agent, kidney allograft type (living or deceased), and time from transplant to CDI result (±60 days).

RESULTS

The majority of subjects received a deceased donor kidney (77.3%) and basiliximab induction therapy (86.4%). The overall CDI incidence was 3%. Factors independently associated with CDI were average tacrolimus trough (AOR = 1.25, 95% CI = 1.00-1.56, P = .048) and antibiotic exposure for urinary tract infections (UTI) (AOR = 4.17, 95% CI = 1.12-15.54, P = .034). Proton pump inhibitor use was not associated with CDI (OR = 0.81, 95% CI = 0.29-2.29, P = .691).

CONCLUSION

Maintaining a clinically appropriate tacrolimus trough and judicious antibiotic use and selection for UTI treatment could potentially reduce CDI in the kidney transplant population.

Artificial neural network model for predicting the bioavailability of tacrolimus in patients with renal transplantation

The objective of the current study was to explore the role of ABCB1 and CYP3A5 genetic polymorphisms in predicting the bioavailability of tacrolimus and the risk for post-transplant diabetes. Artificial neural network (ANN) and logistic regression (LR) models were used to predict the bioavailability of tacrolimus and risk for post-transplant diabetes, respectively. The five-fold cross-validation of ANN model showed good correlation with the experimental data of bioavailability (r2 = 0.93-0.96). Younger age, male gender, optimal body mass index were shown to exhibit lower bioavailability of tacrolimus. ABCB1 1236 C>T and 2677G>T/A showed inverse association while CYP3A5*3 showed a positive association with the bioavailability of tacrolimus. Gender bias was observed in the association with ABCB1 3435 C>T polymorphism. CYP3A5*3 was shown to interact synergistically in increasing the bioavailability in combination with ABCB1 1236 TT or 2677GG genotypes. LR model showed an independent association of ABCB1 2677 G>T/A with post transplant diabetes (OR: 4.83, 95% CI: 1.22-19.03). Multifactor dimensionality reduction analysis (MDR) revealed that synergistic interactions between CYP3A5*3 and ABCB1 2677 G>T/A as the determinants of risk for post-transplant diabetes. To conclude, the ANN and MDR models explore both individual and synergistic effects of variables in modulating the bioavailability of tacrolimus and risk for post-transplant diabetes.

A New CYP3A5*3 and CYP3A4*22 Cluster Influencing Tacrolimus Target Concentrations: A Population Approach

BACKGROUND

Single nucleotide polymorphisms (SNPs) in the CYP3A5 and CYP3A4 genes have been reported to be an important cause of variability in the pharmacokinetics of tacrolimus in renal transplant patients. The aim of this study was to merge all of the new genetic information available with tacrolimus pharmacokinetics to generate a more robust population model with data from renal transplant recipients.

METHODS

Tacrolimus exposure data from 304 renal transplant recipients were collected throughout the first year after transplantation and were simultaneously analyzed with a population pharmacokinetic approach using NONMEM^® version 7.2.

RESULTS

The tacrolimus whole-blood concentration versus time data were best described by a two-open-compartment model with inter-occasion variability assigned to plasma clearance. The following factors led to the final model, which significantly decreased the minimum objective function value (p < 0.001): a new genotype cluster variable combining the CYP3A5*3 and CYP3A4*22 SNPs defined as extensive, intermediate, and poor metabolizers; the standardization of tacrolimus whole blood concentrations to a hematocrit value of 45%; and age included as patients <63 years versus patients ≥63 years. External validation confirmed the prediction ability of the model with median bias and precision values of 1.17 ng/mL (95% confidence interval [CI] -3.68 to 4.50) and 1.64 ng/mL (95% CI 0.11-5.50), respectively. Simulations showed that, for a given age and hematocrit at the same fixed dose, extensive metabolizers required the highest doses followed by intermediate metabolizers and then poor metabolizers.

CONCLUSIONS

Tacrolimus disposition in renal transplant recipients was described using a new population pharmacokinetic model that included the CYP3A5*3 and CYP3A4*22 genotype, age, and hematocrit.

The Pharmacogenetics of Immune-Modulating Therapy

Immunosuppressive drugs are a prerequisite in organ transplantation to prevent rejection and are also widely used in inflammatory diseases such as inflammatory bowel disease (IBD) or also in some hematologic malignancies-depending on the mode of action. For thiopurine analogs the polymorphic thiopurine S-methyltransferase (TPMT) was early detected to be associated with thiopurine-induced leukopenia; recent studies identified also NUDT15 to be related to this severe side effect. For drugs like methotrexate and mycophenolate mofetil a number of ADME genes like UDP-glucuronosyltransferases (UGTs) and ABC efflux transporters were investigated, however, with partly contradicting results. For calcineurin inhibitors like cyclosporine and in particular tacrolimus however, cytochrome P450 3A4 and 3A5 variants were found to significantly affect the pharmacokinetics. Genetic variants in genes encoding relevant pharmacodynamic proteins, however, lacked compelling evidence to affect the clinical outcome. This chapter reviews the current evidence on the association of pharmacogenetic traits to dose finding and clinical outcome of small-molecule immunosuppressants. Moreover this chapter critically summarizes suitability to apply pharmacogenetics in clinical practice in order to optimize immunosuppressant therapy.

Influence of ABCC2, CYP2C8, and CYP2J2 Polymorphisms on Tacrolimus and Mycophenolate Sodium-Based Treatment in Brazilian Kidney Transplant Recipients

STUDY OBJECTIVE

To investigate the influence of single nucleotide polymorphisms (SNPs) in genes encoding metabolizing enzymes (CYP2C8, CYP2J2, and UGT2B7) and transporters (ABCC2 and ABCG2) on dose and dose-adjusted trough blood concentrations (C:D ratio), clinical outcomes, and occurrence of adverse events of tacrolimus and mycophenolate sodium in Brazilian kidney transplant recipients.

DESIGN

Pharmacogenetic analysis of patients enrolled in a previously published study.

PATIENTS

One hundred forty-eight adult kidney transplant recipients treated with tacrolimus, enteric-coated mycophenolate sodium, and prednisone for 90 days posttransplantation.

MEASUREMENTS AND MAIN RESULTS

ABCC2 c.-24C>T and c.3972C>T, ABCG2 c.421C>A, CYP2C8*3, CYP2J2 c.-76G>T, and UGT2B7 c.372A>G SNPs were determined by real-time polymerase chain reaction. The CYP3A5*3C SNP data were used to eliminate the confounding effect of this variant on the results. ABCC2 c.3972T allele carriers showed higher tacrolimus C:D values than did carriers of the c.3972CC genotype. The CYP2C8*3 variant was also associated with slightly higher tacrolimus C:D values and higher estimated glomerular filtration rate but only in CYP3A5-nonexpressing patients (CYP3A5*3C/*3C carriers). None of the SNPs were associated with mycophenolate sodium dose or episodes of biopsy-confirmed acute rejection or delayed graft function. The CYP2J2 c.-76T allele was associated with increased risk for treatment-induced nausea and/or vomiting (OR: 5.30, 95% confidence interval 1.49-18.79, p<0.05).

CONCLUSION

The ABCC2 c.3972C >T polymorphism affected tacrolimus C:D in Brazilian kidney transplant recipients. Further, CYP2C8*3 and CYP2J2 c.-76G>T SNPs influenced the renal function of these patients and the occurrence of adverse events during treatment with tacrolimus and mycophenolate sodium.

Influence of genetic polymorphisms of CYP3A5 and ABCB1 on sirolimus pharmacokinetics, patient and graft survival and other clinical outcomes in renal transplant

BACKGROUND

In transplant patients receiving de novo anticalcineurin-free sirolimus (SRL)-based immunosuppression, we determined the influence of cytochrome P450 3A5 (CYP3A5) and ATP-binding cassette, sub-family B (MDR/TAP), member (ABCB1) genotypes on SRL blood levels and medium-term relevant clinical outcomes, in order to improve effectiveness of immunosuppression strategies when anti-mammalian target of rapamycin (anti-mTOR) inhibitor is indicated for clinical reasons.

METHODS

Forty-eight renal transplant recipients (suffered 48% diabetes mellitus, 91% hypertension, and 47% dyslipidemia) were genotyped for CYP3A5 (6986A>G) and ABCB1 (3435C>T) polymorphisms by polymerase chain reaction-restriction fragment length polymorphism. Sirolimus blood levels were determined using microparticle enzyme immunoassay technique. Relationships between genotypes and pharmacokinetics, graft function, and patient-graft survival were determined by univariate analysis.

RESULTS

CYP3A5*1/*3 showed lower SRL levels than CYP3A5*3/*3 (4.13±1.54 vs. 8.49±4.18 ng/mL; p=0.003) and level/dose ratio (LDR) (92.74±37.47 vs. 178.62±116.45; p=0.019) in early post-transplant period. In ABCB1 polymorphisms, CT genotypes showed higher SRL levels than CC and TT (8.93±2.22 vs. 7.28±2.47 vs. 7.35±1.15 ng/mL; p=0.038) in the late period; LDR in CC and CT were 171.29±36.24 vs. 335.66±138.71 (p=0.003), despite receiving lower doses (p=0.018). Acute rejection rate was 14% vs. 42% for *3/*3 and 14% (TT), 48% (CT), and 31% (CC). Median patient survival was 45 months, significantly lower than that of *3/*3 patients (69 months). Death-censored graft survival during 5-year follow-up was similar for both CYP3A5 genotypes and significantly lower in TT than CT and CC groups, without survival differences.

CONCLUSIONS

CYP3A5 and ABCB1 polymorphisms influenced SRL levels; preliminary data suggest this may affect patient and graft survival. Genotyping renal transplant patients could help select candidates for SRL (genotype*3/*3 for CYP3A5 and CT for ABCB1), when anti-mTOR immunosuppression is indicated.

Low heritability in pharmacokinetics of talinolol: a pharmacogenetic twin study on the heritability of the pharmacokinetics of talinolol, a putative probe drug of MDR1 and other membrane transporters

BACKGROUND

Efflux transporters like MDR1 and MRP2 may modulate the pharmacokinetics of about 50 % of all drugs. It is currently unknown how much of the variation in the activities of important drug membrane transporters like MDR1 or MRP2 is determined by genetic or by environmental factors. In this study we assessed the heritability of the pharmacokinetics of talinolol as a putative probe drug for MDR1 and possibly other membrane transporters.

METHODS

Talinolol pharmacokinetics were investigated in a repeated dose study in 42 monozygotic and 13 same-sex dizygotic twin pairs. The oral clearance of talinolol was predefined as the primary parameter. Heritability was analyzed by structural equation modeling and by within- and between-subject variance and talinolol clearance was correlated with polymorphisms in MDR1, MRP2, BCRP, MDR5, OATP1B1, and OCT1.

RESULTS

Talinolol clearance varied approximately ninefold in the studied sample of healthy volunteers. The correlation of clearances between siblings was not significantly different for the monozygotic and dizygotic pairs. All data analyses consistently showed that variation of talinolol pharmacokinetics was mainly determined by environmental effects. Structural equation modeling attributed 53.5 % of the variation of oral clearance to common environmental effects influencing both siblings to the same extent and 46.5 % to unique environmental effects randomly affecting individual subjects. Talinolol pharmacokinetics were significantly dependent on sex, body mass index, total protein consumption, and vegetable consumption.

CONCLUSIONS

The twin study revealed that environmental factors explained much more of the variation in pharmacokinetics of talinolol than genetic factors.

TRIAL REGISTRATION

European clinical trials database number: EUDRA-CT 2008-006223-31. Registered 26 September 2008. ClinicalTrials.gov number: NCT01845194 .

Effect of ABCB1 diplotype on tacrolimus disposition in renal recipients depends on CYP3A5 and CYP3A4 genotype

The relevance of most genetic polymorphisms beyond CYP3A5*1 on tacrolimus disposition remains unclear. We constructed a predictive mixed model for tacrolimus dose-corrected trough concentration (C₀/dose) at months 3, 12 and 24 after transplantation in a retrospective cohort of 766 predominantly Causasian adult renal recipients (n=2042 trough concentrations). All patients were genotyped for 32 single-nucleotide polymorphisms with a proven or possible relevance to tacrolimus disposition based on the previous studies. Of these, ABCB1, ABCC2, OATP1B1, COMT, FMO, PPARA and APOA5 were analyzed as (functional) diplotype groups. Predictors of C₀/dose were CYP3A5*1, hematocrit, age, CYP3A4*22, use of concomitant CYP3A4 inhibitor or inducer, ALT, estimated glomerular filtration rate, tacrolimus formulation (once vs twice daily), ABCB1 diplotype and time after transplantation. The effect of ABCB1 diplotype was small but strongly accentuated in CYP3A4*22 carriers and non-existent in CYP3A5 expressors. ABCC2 diplotype had a limited effect on C₀/dose that was only statistically significant in CYP3A5 non-expressors.

Which Genetic Determinants Should be Considered for Tacrolimus Dose Optimization in Kidney Transplantation? A Combined Analysis of Genes Affecting the CYP3A Locus

BACKGROUND

Tacrolimus is established as immunosuppressant after kidney transplantation. Polymorphism of the cytochrome P450 3A5 (CYP3A5) gene contributes significantly to tacrolimus dose requirements. Recently, CYP3A4*22 was reported to additionally affect tacrolimus pharmacokinetics (PK). In addition, there are further polymorphic genes, possibly influencing CYP3A activity [pregnane x receptor NR1I2, P450 oxidoreductase (POR), and peroxisome proliferator-activator receptor alpha (PPARA)]. We aimed to investigate combined effects of these gene variants on tacrolimus maintenance dose and PK in patients with stable kidney transplantation of 2 study centers.

METHODS

A total of 223 white patients (German cohort, 136; Danish cohort, 87) was included and genotyped for CYP3A5 (rs776746), CYP3A4 (rs35599367), NR1I2 (rs2276707), POR (rs1057868), and PPARA (rs4253728). Dosage and trough concentration/dose ratios were considered separately. A subset was investigated for comprehensive PK parameters.

RESULTS

Tacrolimus dose, trough concentration, and trough concentration/dose ratio did not differ between the German and Danish cohort. CYP3A5*3 and CYP3A4*22 contributed to dose requirements only in the German and in the total cohort. Homozygous carriers of both variants required 4.8 ± 3.1 mg, whereas carriers of the wild types required 165% higher mean tacrolimus doses (12.5 ± 7.7 mg, P = 1.4 × 10). The PK investigation revealed only nonsignificant impact of CYP3A4 genotypes on AUC12h in CYP3A5 nonexpressers (P = 0.079, power = 57%). For the entire sample, the final multiple linear regression model for trough concentration/dose ratio included CYP3A5, CYP3A4, and age. It explained 18.3% of the interindividual variability of tacrolimus trough concentration/dose ratios (P = 8.8 × 10).

CONCLUSIONS

Therapeutic drug monitoring remains essential in clinical care of patients with kidney transplantation. Genotyping of CYP3A5 and CYP3A4, however, could facilitate rapid dose finding to adapt the appropriate immunosuppressant dose, whereas other genetic factors had only little or no effect.

Sirolimus and Everolimus Pathway: Reviewing Candidate Genes Influencing Their Intracellular Effects

Sirolimus (SRL) and everolimus (EVR) are mammalian targets of rapamycin inhibitors (mTOR-I) largely employed in renal transplantation and oncology as immunosuppressive/antiproliferative agents. SRL was the first mTOR-I produced by the bacterium Streptomyces hygroscopicus and approved for several medical purposes. EVR, derived from SRL, contains a 2-hydroxy-ethyl chain in the 40th position that makes the drug more hydrophilic than SRL and increases oral bioavailability. Their main mechanism of action is the inhibition of the mTOR complex 1 and the regulation of factors involved in a several crucial cellular functions including: protein synthesis, regulation of angiogenesis, lipid biosynthesis, mitochondrial biogenesis and function, cell cycle, and autophagy. Most of the proteins/enzymes belonging to the aforementioned biological processes are encoded by numerous and tightly regulated genes. However, at the moment, the polygenic influence on SRL/EVR cellular effects is still not completely defined, and its comprehension represents a key challenge for researchers. Therefore, to obtain a complete picture of the cellular network connected to SRL/EVR, we decided to review major evidences available in the literature regarding the genetic influence on mTOR-I biology/pharmacology and to build, for the first time, a useful and specific “SRL/EVR genes-focused pathway”, possibly employable as a starting point for future in-depth research projects.

Pharmacogenetic aspects of the use of tacrolimus in renal transplantation: recent developments and ethnic considerations

INTRODUCTION

Tacrolimus (Tac) is effective in preventing acute rejection but has considerable toxicity and inter-individual variability in pharmacokinetics and pharmacodynamics. Part of this is explained by polymorphisms in genes encoding Tac-metabolizing enzymes and transporters. A better understanding of Tac pharmacokinetics and pharmacodynamics may help to minimize different outcomes amongst transplant recipients by personalizing immunosuppression.

AREAS COVERED

The pharmacogenetic contribution of Tac metabolism will be examined, with a focus on recent discoveries, new developments and ethnic considerations.

EXPERT OPINION

The strongest and most consistent association in pharmacogenetics is between the CYP3A5 genotype and Tac dose requirement, with CYP3A5 expressers having a ~ 40-50% higher dose requirement compared to non-expressers. Two recent randomized-controlled clinical trials using CYP3A5 genotype, however, did not show a decrease in acute rejections nor reduced toxicity. CYP3A4*22, CYP3A4*26, and POR*28 are also associated with Tac dose requirements and may be included to provide the expected improvement of Tac therapy. Studies focusing on the intracellular drug concentrations and on calcineurin inhibitor-induced nephrotoxicity also seem promising. For all studies, however, the ethnic prevalence of genotypes should be taken into account, as this may significantly impact the effect of pre-emptive genotyping.

Prediction of tacrolimus metabolism and dosage requirements based on CYP3A4 phenotype and CYP3A5(*)3 genotype in Chinese renal transplant recipients

AIM

To examine how the endogenous CYP3A4 phenotype and CYP3A5(*)3 genotype of Chinese renal transplant recipients influenced the dose-corrected trough concentration (C0/D) and weight-corrected daily dose (D/W) of tacrolimus.

METHODS

A total of 101 medically stable kidney transplant recipients were enrolled, and their blood and urine samples were gathered. The endogenous CYP3A4 phenotype was assessed by the ratio of 6β-hydroxycortisol and 6β-hydroxycortisone to cortisol and cortisone in urine. CYP3A5(*)3 genotype was determined using PCR-RELP.

RESULTS

In overall renal transplant recipients, a multiple regression analysis including the endogenous CYP3A4 phenotype, CYP3A5(*)3 genotype and post-operative period accounted for 60.1% of the variability in C0/D ratio; a regression equation consisting of the endogenous CYP3A4 phenotype, post-operative period, body mass index, CYP3A5(*)3 genotype, gender, total bilirubin and age explained 61.0% of the variability in D/W ratio. In CYP3A5(*)3/(*)3 subjects, a combination of the endogenous CYP3A4 phenotype, post-operative period and age was responsible for 65.3% of the variability in C0/D ratio; a predictive equation including the endogenous CYP3A4 phenotype, post-operative period, body mass index, gender and age explained 61.2% of the variability in the D/W ratio. Base on desired target range of tacrolimus trough concentrations, individual daily dosage regimen was calculated, and all the observed daily doses were within the predicted range.

CONCLUSION

This study provides the equations to predict tacrolimus metabolism and dosage requirements based on the endogenous CYP3A4 phenotype, CYP3A5(*)3 genotype and other non-genetic variables.

Impact of Genetic Polymorphisms of ABCB1 (MDR1, P-Glycoprotein) on Drug Disposition and Potential Clinical Implications: Update of the Literature

ATP-binding cassette transporter B1 (ABCB1; P-glycoprotein; multidrug resistance protein 1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types. Numerous structurally unrelated compounds, including drugs and environmental toxins, have been identified as substrates. ABCB1 limits the absorption of xenobiotics from the gut lumen, protects sensitive tissues (e.g. the brain, fetus and testes) from xenobiotics and is involved in biliary and renal secretion of its substrates. In recent years, a large number of polymorphisms of the ABCB1 [ATP-binding cassette, sub-family B (MDR/TAP), member 1] gene have been described. The variants 1236C>T (rs1128503, p.G412G), 2677G>T/A (rs2032582, p.A893S/T) and 3435C>T (rs1045642, p.I1145I) occur at high allele frequencies and create a common haplotype; therefore, they have been most widely studied. This review provides an overview of clinical studies published between 2002 and March 2015. In summary, the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance. Future research activities are warranted, considering a deep-sequencing approach, as well as well-designed clinical studies with appropriate sample sizes to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes.

The impact of CYP3A5*3 polymorphism on sirolimus pharmacokinetics: insights from predictions with a physiologically-based pharmacokinetic model

AIMS

Sirolimus is an mTOR inhibitor metabolized by CYP3A4 and CYP3A5. Reported effects of CYP3A5 polymorphisms on sirolimus pharmacokinetics (PK) have shown unexplained discrepancies across studies. We quantitatively assessed the effect of CYP3A5*3 status on sirolimus PK by in vitro assessment and simulation using a physiologically-based PK (PBPK) model. In addition, we explored designs for an adequately powered pharmacogenetic association study.

METHOD

In vitro metabolism studies were conducted to confirm individual CYP contribution to sirolimus metabolism. PK profiles were simulated in CYP3A5 expressers and non-expressers with a PBPK model. The pre-dose concentration predictions were used as the outcome parameter to estimate the required sample size for a pharmacogenetic association study.

RESULTS

Sirolimus metabolism was inhibited by over 90% by ketoconazole, a CYP3A specific inhibitor. The PBPK model developed based on CL(int) of recombinant CYP3A4, CYP3A5 and CYP2C8 predicted a small CYP3A5*3 effect on simulated sirolimus PK profiles. A subsequent power analysis based on these findings indicated that at least 80 subjects in an enrichment design, 40 CYP3A5 expressers and 40 non-expressers, would be required to detect a significant difference in the predicted trough concentrations at 1 month of therapy (P < 0.05, 80% power).

CONCLUSIONS

This study suggests that CYP3A5 contribution to sirolimus metabolism is much smaller than that of CYP3A4. Observed discrepancies across studies could be explained as the result of inadequate sample size. PBPK model simulations allowed mechanism-based evaluation of the effects of CYP3A5 genotype on sirolimus PK and provided preliminary data for the design of a future prospective study.

CYP3A5 and ABCB1 genotype influence tacrolimus and sirolimus pharmacokinetics in renal transplant recipients

CYP3A5 and ABCB1 polymorphisms have been shown to influence tacrolimus blood concentrations and dose requirements, but the conclusion in the current reports were inconformity. Sirolimus are also metabolized by CYP3A subfamily and are substrates of the P-gp. The aim was to determine whether these polymorphisms affect tacrolimus (TAC) and sirolimus (SRL) trough concentrations and dose requirements after renal transplantation. 153 renal transplant recipients were enrolled into this study, 112 were treated with TAC-based regimen, Another 43 recipients received SRL-based regimen. The recipients’ mean follow-up time was 20 mo (range 15–27 mo). All renal transplant recipients were all in a stable stage. The trough concentration and daily dose of TAC and SRL were gained from each recipient. All recipients were genotyped for CYP3A5 (6986A>G), CYP3A4 intron 6 (CYP3A4*22), CYP3A4*18, ABCB1 exon 26 (3435C>T), exon 12 (1236C>T) and 2677G>T/A SNPs by HRM analysis (high-resolution melting curve analysis). The TAC and SRL concentration/dose ratio (C/D) in recipients with CYP3A5 (*)3/(*)3 were significantly higher than that of those with (*)1 allele (P < 0.05). However, there was no significant correlation between adjusted TAC and SRL trough concentrations or dose requirements with CYP3A4 and ABCB1 SNPs genetic polymorphisms. In recipients with TAC-based or SRL-based therapy, the CYP3A5 genes (6986A>G) can influence the TAC and SRL pharmacokinetics in renal transplant recipients.

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

Personalization of the immunosuppressive treatment in renal transplant recipients: the great challenge in "omics" medicine

Renal transplantation represents the most favorable treatment for patients with advanced renal failure and it is followed, in most cases, by a significant enhancement in patients’ quality of life. Significant improvements in one-year renal allograft and patients’ survival rates have been achieved over the last 10 years primarily as a result of newer immunosuppressive regimens. Despite these notable achievements in the short-term outcome, long-term graft function and survival rates remain less than optimal. Death with a functioning graft and chronic allograft dysfunction result in an annual rate of 3%–5%. In this context, drug toxicity and long-term chronic adverse effects of immunosuppressive medications have a pivotal role. Unfortunately, at the moment, except for the evaluation of trough drug levels, no clinically useful tools are available to correctly manage immunosuppressive therapy. The proper use of these drugs could potentiate therapeutic effects minimizing adverse drug reactions. For this purpose, in the future, “omics” techniques could represent powerful tools that may be employed in clinical practice to routinely aid the personalization of drug treatment according to each patient’s genetic makeup. However, it is unquestionable that additional studies and technological advances are needed to standardize and simplify these methodologies.

Clinical implementation of pharmacogenetics in kidney transplantation: calcineurin inhibitors in the starting blocks

Pharmacogenetics has generated many expectations for its potential to individualize therapy proactively and improve medical care. However, despite the huge amount of reported genetic associations with either pharmacokinetics or pharmacodynamics of drugs, the translation into patient care is still slow. In fact, strong evidence for a substantial clinical benefit of pharmacogenetic testing is still limited, with a few exceptions. In kidney transplantation, established pharmacogenetic discoveries are being investigated for application in the clinic to improve efficacy and to limit toxicity associated with the use of immunosuppressive drugs, especially the frequently used calcineurin inhibitors (CNIs) tacrolimus and ciclosporin. The purpose of the present review is to picture the current status of CNI pharmacogenetics and to discuss the most promising leads that have been followed so far.