Pharmacogenetics of immunosuppressants: progress, pitfalls and promises

Humanized Mouse Model as a Novel Approach in the Assessment of Human Allogeneic Responses in Organ Transplantation

The outcome of organ transplantation is largely dictated by selection of a well-matched donor, which results in less chance of graft rejection. An allogeneic immune response is the main immunological barrier for successful organ transplantation. Donor and recipient human leukocyte antigen (HLA) mismatching diminishes outcomes after solid organ transplantation. The current evaluation of HLA incompatibility does not provide information on the immunogenicity of individual HLA mismatches and impact of non-HLA-related alloantigens, especially in vivo. Here we demonstrate a new method for analysis of alloimmune responsiveness between donor and recipient in vivo by introducing a humanized mouse model. Using molecular, cellular, and genomic analyses, we demonstrated that a recipient's personalized humanized mouse provided the most sensitive assessment of allogeneic responsiveness to potential donors. In our study, HLA typing provided a better recipient-donor match for one donor among two related donors. In contrast, assessment of an allogeneic response by mixed lymphocyte reaction (MLR) was indistinguishable between these donors. We determined that, in the recipient's humanized mouse model, the donor selected by HLA typing induced the strongest allogeneic response with markedly increased allograft rejection markers, including activated cytotoxic Granzyme B-expressing CD8+ T cells. Moreover, the same donor induced stronger upregulation of genes involved in the allograft rejection pathway as determined by transcriptome analysis of isolated human CD45+cells. Thus, the humanized mouse model determined the lowest degree of recipient-donor alloimmune response, allowing for better selection of donor and minimized immunological risk of allograft rejection in organ transplantation. In addition, this approach could be used to evaluate the level of alloresponse in allogeneic cell-based therapies that include cell products derived from pluripotent embryonic stem cells or adult stem cells, both undifferentiated and differentiated, all of which will produce allogeneic immune responses.

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.

CYP and SXR gene polymorphisms influence in opposite ways acute rejection rate in pediatric patients with renal transplant

Background

We evaluated the role of CYP3A5, ABCB1 and SXR gene polymorphisms in the occurrence of acute kidney rejection in a cohort of pediatric renal transplant recipients.

Methods

Forty-nine patients were genotyped for CYP3A5, ABCB1 and SXR polymorphisms and evaluated with tacrolimus through levels in a retrospective monocenter study.

Results

Patients with the A allele of CYP3A5 treated with tacrolimus had a higher risk of acute rejection than those without the A allele, while patients carrying the homozygous GG variant for SXR A7635GG did not show any episode of acute rejection.

Conclusion

Genetic analysis of polymorphisms implicated in drug metabolism and tacrolimus trough levels may help to forecast the risk of acute rejection and individualize drug dosage in children undergoing renal 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.

Personalized treatment in heart transplantation

PURPOSE OF REVIEW

We are entering the era of personalized medicine, in which pharmacogenomics and biomarker-based assays can be used to tailor diagnostic tests and drug therapies to individual patients. This new approach to patient-specific care offers the potential to maximize the efficacy of available medical treatments while reducing the incidence of adverse side effects. Here, we present approaches to personalize the care of heart transplant recipients.

RECENT FINDINGS

Four strategies for personalized posttransplant care are described, including use of pharmacogenomic data to individualize the use of immunosuppressive drugs, immune monitoring to prevent acute rejection while reducing the long-term consequences of over immunosuppression, noninvasive surveillance for acute rejection, and targeted prophylaxis against opportunistic infections.

SUMMARY

The long-term survival of heart transplant recipients is limited by side effects of immunosuppressive drugs, including infectious complications, renal dysfunction, and malignancy. We discuss strategies to maximize the benefits of immunosuppressive and prophylactic therapies while minimizing their long-term toxicities.

Mycophenolate mofetil-related leukopenia in children and young adults following kidney transplantation: Influence of genes and drugs

MMF is commonly prescribed following kidney transplantation, yet its use is complicated by leukopenia. Understanding the genetics mediating this risk will help clinicians administer MMF safely. We evaluated 284 patients under 21 years of age for incidence and time course of MMF-related leukopenia and performed a candidate gene association study comparing the frequency of 26 SNPs between cases with MMF-related leukopenia and controls. We matched cases by induction, steroid duration, race, center, and age. We also evaluated the impact of induction and SNPs on time to leukopenia in all cases. Sixty-eight (24%) patients had MMF-related leukopenia, of which 59 consented for genotyping and 38 were matched with controls. Among matched pairs, no SNPs were associated with leukopenia. With non-depleting induction, UGT2B7-900A>G (rs7438135) was associated with increased risk of MMF-related leukopenia (P = .038). Time to leukopenia did not differ between patients by induction agent, but 2 SNPs (rs2228075, rs2278294) in IMPDH1 were associated with increased time to leukopenia. MMF-related leukopenia is common after transplantation. UGT2B7 may influence leukopenia risk especially in patients without lymphocyte-depleting induction. IMPDH1 may influence time course of leukopenia after transplant.

Influence of CYP3A4 and CYP3A5 polymorphisms on tacrolimus and sirolimus exposure in stable kidney transplant recipients

BACKGROUND

Polymorphisms in genes encoding for drug-metabolizing enzymes and drug transporters are among multiple factors that modulate the pharmacokinetic variability of tacrolimus (TAC) and sirolimus (SRL). This study aimed to evaluate the influence of single nucleotide polymorphisms (SNPs) on TAC and SRL dose-adjusted concentrations (C0/D) in stable kidney transplant recipients.

METHODS

This is an exploratory and prospective study, which includes 46 stable kidney transplant recipients. These patients were monitored from the 3rd to the 24th month after transplantation. The SRL group consisted of 25 patients receiving TAC, prednisone (PRED), and mycophenolate sodium (MPS), which were converted from TAC to SRL at 3rd month after transplantation. The TAC group consisted of 21 patients who underwent treatment with TAC, PRED, and MPS. Both groups were genotyped for CYP3A4 rs2242480 (g.20230G>A), CYP3A5 rs15524 (g.31611C>T), CYP2C8 rs10509681 (c.1196A>G) and ABCB1 rs1045642 (c.3435C>T), rs1128503 (c.1236C>T), and rs2032582 (c.2677G>T/A) polymorphisms.

RESULTS

In the TAC group, CYP3A4 rs2242480 A allele carriers were associated with lower TAC C0/D. For CYP3A5 rs15524 SNP, C0/D was higher among patients carrying TT genotype when compared with CT and CC genotype carriers in the SRL and, more consistently, in the TAC groups. For ABCB1 rs1045642 SNP, TT genotype was associated with reduced SRL C0/D, but only at month 15.

CONCLUSIONS

CYP3A4 rs2242480 and CYP3A5 rs15524 SNPs resulted in significant changes in SRL and TAC C0/D at different times after transplantation.

Precision monitoring of immunotherapies in solid organ and hematopoietic stem cell transplantation

Pharmacological immunotherapies are a key component of post-transplant therapy in solid-organ and hematopoietic stem cell transplantation. In current clinical practice, immunotherapies largely follow a one-size fits all approach, leaving a large portion of transplant recipients either over- or under-immunosuppressed, and consequently at risk of infections or immune-mediated complications. Our goal here is to review recent and rapid advances in precision and genomic medicine approaches to monitoring of post-transplant immunotherapies. We will discuss recent advances in precision measurements of pharmacological immunosuppression, measurements of the plasma and gut microbiome, strategies to monitor for allograft injury and post-transplant malignancies via circulating cell-free DNA, and comprehensive measurements of the B and T cell immune cell repertoire.

Opinions, hopes and concerns regarding pharmacogenomics: a comparison of healthy individuals, heart failure patients and heart transplant recipients

It is not yet known whether healthy individuals and patients with a chronic disease have similar attitudes towards pharmacogenomics. Thus we conducted a survey of 175 healthy volunteers, 175 heart failure (HF) patients and 100 heart transplant recipients to compare their opinions on this subject. Most participants (>90%) stated that they would accept pharmacogenomic testing and expressed high hopes regarding its potential applications. Overall, interest for pharmacogenomics was shared equally among the three groups. In contrast, after adjusting for age, gender, education and income, healthy individuals were more likely to voice concerns about potential employment (P=0.008 vs HF, odds ratio (OR)=2.93, confidence interval (CI)=1.33-6.47; P=0.010 vs Transplant, OR=2.46, CI=1.24-4.90) and insurance discrimination (P=0.001 vs HF, OR=5.58, CI=2.01-15.48; P<0.001 vs Transplant, OR=4.98, CI=2.03-12.21) and were possibly more worried by confidentiality issues. These findings highlight the need for strict legislation and proper educational strategies directed at the general population to facilitate the clinical implementation of pharmacogenomics.

ABCB1 haplotype influences the sirolimus dose requirements in Chinese renal transplant recipients

Sirolimus, an immunosuppressive drug used to prevent organ rejection after renal transplantation, has a narrow therapeutic index and a large inter-individual variability of pharmacokinetics. The aim of this study was to analyse the dose-normalized trough blood concentrations (C0 /D ratio) of sirolimus in patients with different genotypes and attempt to investigate the possible associations between ABCB1/CYP3A5 genotypes and sirolimus dose requirements in Chinese renal transplant recipients. Blood samples were collected from 85 Chinese renal transplant recipients who were treated with sirolimus for at least 3 months and polymorphisms of the ABCB1 and CYP3A5 were determined by the SNaPShot multiplex assay. The blood concentrations of sirolimus were determined with HPLC. A significant allele-dependent effect was observed between the CYP3A5*3 polymorphism and the C0 /D ratio of sirolimus. The patients bearing at least one CYP3A5*1 allele had a lower sirolimus C0/D ratio compared with those with a homozygous CYP3A5*3 genotype (p < 0.05). No significant differences of sirolimus C0/D ratios were observed among various ABCB1 1236C>T, 2677G>T/A and 3435C>T genotype groups. However, haplotype analysis including ABCB1 1236C>T, 2677G>T/A and 3435C>T SNPs showed that the mean sirolimus C0/D of subjects carrying the CGC/CGC diplotype was about 30% lower compared with those carrying the CGC/TTT or TTT/TTT diplotype, whether or not they expressed the CYP3A5 (p < 0.05). These results demonstrated that the haplotype of ABCB1 might be a better index for the prediction of sirolimus blood concentration than single SNPs. Genotyping of ABCB1 and CYP3A5 might help to optimize individualized sirolimus treatments for Chinese renal transplant recipients.

Interaction of immunosuppressive drugs with human organic anion transporter (OAT) 1 and OAT3, and multidrug resistance-associated protein (MRP) 2 and MRP4

Renal proximal tubule transporters can play a key role in excretion, pharmacokinetic interactions, and toxicity of immunosuppressant drugs. Basolateral organic anion transporters (OATs) and apical multidrug resistance-associated proteins (MRPs) contribute to the active tubular uptake and urinary efflux of these drugs, respectively. We studied the interaction of 12 immunosuppressants with OAT1- and OAT3-mediated [(3)H]-methotrexate (MTX) uptake in cells, and adenosine triphosphate-dependent [(3)H]-MTX transport in membrane vesicles isolated from human embryonic kidney 293 cells overexpressing human MRP2 and MRP4. Our results show that at a clinically relevant concentration of 10 μM, mycophenolic acid inhibited both OAT1- and OAT3-mediated [(3)H]-MTX uptake. Cytarabine, vinblastine, vincristine, hydrocortisone, and mitoxantrone inhibited only OAT1, whereas tacrolimus, azathioprine, dexamethasone, cyclosporine, and 6-mercaptopurine had no effect on both transporters. Cyclophosphamide stimulated OAT1, but did not affect OAT3. With regard to the apical efflux transporters, mycophenolic acid, cyclophosphamide, hydrocortisone, and tacrolimus inhibited MRP2 and MRP4, whereas mitoxantrone and dexamethasone stimulated [(3)H]-MTX transport by both transporters. Cyclosporine, vincristine, and vinblastine inhibited MRP2 only, whereas 6-mercaptopurine inhibited MRP4 transport activity only. Cytarabine and azathioprine had no effect on either transporter. In conclusion, we charted comprehensively the differences in inhibitory action of various immunosuppressive agents against the 4 key renal anion transporters, and we provide evidence that immunosuppressant drugs can modulate OAT1-, OAT3-, MRP2-, and MRP4-mediated transport of MTX to different extents. The data provide a better understanding of renal mechanisms underlying drug-drug interactions and nephrotoxicity concerning combination regimens with these compounds in the clinic.

The role of genetics in drug dosing

Renal transplantation is the optimal form of renal replacement therapy (RRT) for the majority of patients. Both short- and long-term graft rejection are well recognized complications following transplantation, and optimal immunosuppression is often difficult to achieve. Pharmacodynamics (PD) and pharmacokinetics (PK) are hard to predict in all patients, and best practice involves the use of standard dosing based on weight and therapeutic drug monitoring (TDM). Pharmacogenetics (PG) is the use of genetic screening to predict metabolic responses to different immunosuppressive drugs and enables more accurate predictions of PD and PK to be made. This has the potential to improve graft outcome by reducing both short- and long-term graft rejection.

Genetic and genomic approaches to the detection of heart transplant rejection

Since Christiaan Barnard performed the first heart transplant in 1967, over 100,000 heart transplants have been performed worldwide. As was true then, rejection remains the major threat to the function and survival of the allograft. The development of the endomyocardial biopsy as a means to monitor for rejection has allowed heart transplantation to thrive as a therapy for patients with end-stage heart disease. The need for a noninvasive method of rejection surveillance led to the development of the first genetic test for allograft rejection, the AlloMap^®. In this article, after presenting the pathological and clinical features of cardiac allograft rejection, the authors discuss the development and application of gene-expression testing for the detection of cardiac allograft rejection. We then explore emerging 'omic' approaches that will be the rejection detection methods of the future.

Development of population PK model with enterohepatic circulation for mycophenolic acid in patients with childhood-onset systemic lupus erythematosus

AIM

This study aimed to develop a population pharmacokinetic (PK) enterohepatic recycling model for MPA in patients with childhood-onset systemic lupus erythematosus (cSLE).

METHODS

MPA concentration-time data were from outpatients on stable oral mycophenolate mofetil (MMF) and collected under fasting conditions, with standardized meals (1 and 4 h post-dose). Sampling times were pre-dose, 20, 40 min, 1, 1.5, 2, 3, 4, 6 and 9 h, post dose. The population PK analysis simultaneously modelled MPA and 7-O-MPA-β-glucuronide (MPAG) concentrations using nonlinear mixed effect modelling.

RESULTS

PK analysis included 186 MPA and MPAG concentrations (mg l(-1)) from 19 patients. cSLE patients, age range 10-28 years, median 16.5 years were included. Mean ± SD disease duration was 3.8 ± 3.7 years. The final PK model included a gallbladder compartment for enterohepatic recycling and bile release time related to meal times, with first order absorption and single series of transit compartments. The PK estimates for MPA were CL(1) /F 25.3 l h(-1), V(3) /F 20.9 l, V(4) /F 234 l and CL(2) /F 19.8 l h(-1).

CONCLUSION

The final model fitted the complex processes of absorption and enterohepatic circulation (EHC) in those treated with MMF for cSLE and could be applied in Bayesian dose optimization algorithms.

Gender differences in pharmacokinetics of tacrolimus and their clinical significance in kidney transplant recipients

BACKGROUND

The possible influence of gender on tacrolimus disposition and response in kidney transplant recipients is an issue of medical importance.

OBJECTIVE

The aim of this study was to detect interpatient pharmacokinetic variability of tacrolimus due to patients' gender and to assess the predictability of individual tacrolimus concentrations using abbreviated AUC measurements. The secondary objective was to find the best sampling time to predict the exposure of tacrolimus in kidney transplant recipients.

METHODS

Gender-related first oral dose tacrolimus pharmacokinetics studies were conducted in 20 Serbian kidney transplant recipients (10 men/10 women) on quaternary immunosuppressive therapy. The first tacrolimus oral dose (0.05 mg/kg) was given on day 5 post-transplant. Blood concentrations were measured by microparticle enzyme immunoassay method. Associations between each sampling time point of concentrations and 12 hours after the administration AUC (AUC(0-12)) were evaluated by Pearson correlation coefficients. Abbreviated sampling equations were derived by multiple stepwise regression analyses.

RESULTS

AUC(0-12) showed remarkable interindividual variations after the first tacrolimus oral dose. There were significantly lower values of AUC in women than men (P < 0.05). The most important time point influencing AUC(0-12) was the concentration of tacrolimus measured 2 hours after administration(C(2)) in women, whereas in men the most important time points were the concentrations at 1 (C(1)), 4 (C(4)), and 12 (C(12)) hours as an abbreviated AUC.

CONCLUSION

Our results show significant differences between men and women. C(2) seems to be indicator of total body exposure to tacrolimus in the early period after kidney transplant in women. The three-point sampling method seems to be a good indicator of abbreviated AUC for a tacrolimus monitoring strategy in men.

Impact of food and herbal medication on calcineurin inhibitor dose in renal transplant patients: a cross-sectional study

The incidence and severity of interactions of herbal products with calcineurin inhibitor (CNI) metabolism in renal transplant recipients have not been systematically investigated. These patients have a high rate of herbal product consumption, including products interfering with CNI metabolism. The study aimed at identifying an impact of herbs and foods on CNI metabolism in a cohort of renal transplant recipients by conducting dietary interviews (1) in patients with very low and high CNI maintenance dose requirements and (2) by retrospective analysis of unexplained marked deviations from CNI baseline trough levels. Of 73 renal transplant recipients, 59 were treated with a CNI-based immunosuppressive regimen. Seven patients with an exceptionally high or low CNI dose were interviewed. Five of these seven patients had not consumed any plant product with known influence on CNI metabolism. In one patient chicory-coffee and bitter chocolate had been suspected as contributing to high CNI dose requirement, but the dose could not be lowered after discontinuation of these foods. Participating nephrologists reported three as yet unexplained temporary deviations from baseline CNI trough levels, of which two could be linked to newly started consumption of high volumes of herbal teas and the other to St. John's wort. Consumption of herbal products within the study cohort had no detectable impact on maintenance doses of CNI. However, herbal products, and specifically teas when consumed by the liter, could be linked to temporary strong deviations from CNI trough levels. The study demonstrates that as yet unnoticed herbal interactions with CNI can be detected by detailed dietary analysis, but that the overall impact on maintenance doses of CNI appears to be low.

Effect of CYP3A and ABCB1 single nucleotide polymorphisms on the pharmacokinetics and pharmacodynamics of calcineurin inhibitors: Part II

The calcineurin inhibitors ciclosporin (cyclosporine) and tacrolimus are immunosuppressant drugs used for the prevention of organ rejection following transplantation. Both agents are metabolic substrates for cytochrome P450 (CYP) 3A enzymes - in particular, CYP3A4 and CYP3A5 - and are transported out of cells via P-glycoprotein (ABCB1). Several single nucleotide polymorphisms (SNPs) have been identified in the genes encoding for CYP3A4, CYP3A5 and P-glycoprotein, including CYP3A4 -392A>G (rs2740574), CYP3A5 6986A>G (rs776746), ABCB1 3435C>T (rs1045642), ABCB1 1236C>T (rs1128503) and ABCB1 2677G>T/A (rs2032582). The aim of this review is to provide the clinician with an extensive overview of the recent literature on the known effects of these SNPs on the pharmacodynamics of ciclosporin and tacrolimus in solid-organ transplant recipients. Literature searches were performed and all relevant primary research articles were critiqued and summarized. There is no evidence that the CYP3A4 -392A>G SNP has an effect on the pharmacodynamics of either ciclosporin or tacrolimus; however, studies have been limited. For patients prescribed ciclosporin, the CYP3A5 6986A>G SNP may influence long-term survival, possibly because of a different metabolite pattern over time. This SNP has no clear association with acute rejection during ciclosporin therapy. Despite a strong association between the CYP3A5 6986A>G SNP and tacrolimus pharmacokinetics, there is no consistent evidence of organ rejection as a result of genotype-related under-immunosuppression. This is likely to be explained by the practice of performing tacrolimus dose adjustments in the early phase after transplantation. The effect of the CYP3A5 6986A>G SNP on ciclosporin- and tacrolimus-related nephrotoxicity and development of hypertension is unclear. Similarly, the ABCB1 SNPs exert no clear influence on either ciclosporin or tacrolimus pharmacodynamics, with studies showing conflicting results in regard to the main parameters of acute rejection and nephrotoxicity. In kidney transplant patients, consideration of the donor kidney genotype rather than the recipient genotype may be more important when assessing development of nephrotoxicity. Studies with low patient numbers may account for many inconsistent results to date. The majority of studies have only evaluated the effects of individual SNPs; however, multiple polymorphisms may interact to produce a combined effect. Further haplotype analyses are likely to be useful, particularly ones that consider both donor and recipient genotype. The effects of polymorphisms associated with the pregnane X receptor, organic anion transporting polypeptides, calcineurin inhibitor target sites and immune response pathways need to be further investigated. A large standardized clinical trial is now required to evaluate the relationship between the pharmacokinetics and pharmacodynamics of CYP3A5-mediated tacrolimus metabolism, particularly in regard to the outcomes of acute rejection and nephrotoxicity. It is not yet clear whether pharmacogenetic profiling of calcineurin inhibitors will be a useful clinical tool for personalizing immunosuppressant therapy.

Effect of CYP3A and ABCB1 single nucleotide polymorphisms on the pharmacokinetics and pharmacodynamics of calcineurin inhibitors: Part I

The calcineurin inhibitors ciclosporin (cyclosporine) and tacrolimus are immunosuppressant drugs used for the prevention of organ rejection following transplantation. Both agents are metabolic substrates for cytochrome P450 (CYP) 3A enzymes--in particular, CYP3A4 and CYP3A5--and are transported out of cells via P-glycoprotein (ABCB1). Several single nucleotide polymorphisms (SNPs) have been identified in the genes encoding for CYP3A4, CYP3A5 and P-glycoprotein, including CYP3A4 -392A>G (rs2740574), CYP3A5 6986A>G (rs776746), ABCB1 3435C>T (rs1045642), ABCB1 1236C>T (rs1128503) and ABCB1 2677G>T/A (rs2032582). The aim of this review is to provide the clinician with an extensive overview of the recent literature on the known effects of these SNPs on the pharmacokinetics of ciclosporin and tacrolimus in solid-organ transplant recipients. Literature searches were performed, and all relevant primary research articles were critiqued and summarized. Influence of the CYP3A4 -392A>G SNP on the pharmacokinetics of either ciclosporin or tacrolimus appears limited. Variability in CYP3A4 expression due to environmental factors is likely to be more important than patient genotype. Influence of the CYP3A5 6986A>G SNP on the pharmacokinetics of ciclosporin is also uncertain and likely to be small. CYP3A4 may play a more dominant role than CYP3A5 in the metabolism of ciclosporin. The CYP3A5 6986A>G SNP has a well established influence on the pharmacokinetics of tacrolimus. Several studies in kidney, heart and liver transplant recipients have reported an approximate halving of tacrolimus dose-adjusted trough concentrations and doubling of tacrolimus dose requirements in heterozygous or homozygous carriers of a CYP3A5*1 wild-type allele compared with homozygous carriers of a CYP3A5*3 variant allele. Carriers of a CYP3A5*1 allele take a longer time to reach target blood tacrolimus concentrations. Influence of ABCB1 3435C>T, 1236C>T and 2677G>T/A SNPs on the pharmacokinetics of ciclosporin and tacrolimus remains uncertain, with inconsistent results. Genetic linkage between the three variant genotypes suggests that the pharmacokinetic effects are complex and not related to any one ABCB1 SNP. It is likely that these polymorphisms exert a small but combined effect, which is additive to the effects of the CYP3A5 6986A>G SNP. In liver transplant patients, recipient and donor liver genotypes may act together in determining overall drug disposition, hence the importance of assessing both. Studies with low patient numbers may account for many inconsistent results to date. Meta-analyses of the current data should help resolve some discrepancies. The majority of studies have only evaluated the effects of individual SNPs; however, multiple polymorphisms may interact to produce a combined effect. Further haplotype analyses are likely to be useful. It is not yet clear whether pharmacogenetic profiling of calcineurin inhibitors will be a useful clinical tool for personalizing immunosuppressant therapy.

Immunosuppression armamentarium in 2010: mechanistic and clinical considerations

Effective immunosuppression is the key to successful organ transplantation, with success being defined as minimal rejection risk with concomitant minimal drug toxicities. Despite the general recognition of this fact, a paucity of appropriate clinical trials in children has contributed to lack of standardization of clinical management regimens, resulting in an extensive diversity of favored approaches. Nonetheless, although consensus has not been reached on the ideal approach to immunosuppression in pediatric transplantation, new drug therapies have contributed to a continuing improvement in graft and patient survival. Future clinical research must focus on diminishing the extensive burden of toxicities of these therapeutic agents in children.

New insights into the pharmacokinetics and pharmacodynamics of the calcineurin inhibitors and mycophenolic acid: possible consequences for therapeutic drug monitoring in solid organ transplantation

Although therapeutic drug monitoring (TDM) of immunosuppressive drugs has been an integral part of routine clinical practice in solid organ transplantation for many years, ongoing research in the field of immunosuppressive drug metabolism, pharmacokinetics, pharmacogenetics, pharmacodynamics, and clinical TDM keeps yielding new insights that might have future clinical implications. In this review, the authors will highlight some of these new insights for the calcineurin inhibitors (CNIs) cyclosporine and tacrolimus and the antimetabolite mycophenolic acid (MPA) and will discuss the possible consequences. For CNIs, important relevant lessons for TDM can be learned from the results of 2 recently published large CNI minimization trials. Furthermore, because acute rejection and drug-related adverse events do occur despite routine application of CNI TDM, alternative approaches to better predict the dose-concentration-response relationship in the individual patient are being explored. Monitoring of CNI concentrations in lymphocytes and other tissues, determination of CNI metabolites, and CNI pharmacogenetics and pharmacodynamics are in their infancy but have the potential to become useful additions to conventional CNI TDM. Although MPA is usually administered at a fixed dose, there is a rationale for MPA TDM, and this is substantiated by the increasing knowledge of the many nongenetic and genetic factors contributing to the interindividual and intraindividual variability in MPA pharmacokinetics. However, recent, large, randomized clinical trials investigating the clinical utility of MPA TDM have reported conflicting data. Therefore, alternative pharmacokinetic (ie, MPA free fraction and metabolites) and pharmacodynamic approaches to better predict drug efficacy and toxicity are being explored. Finally, for MPA and tacrolimus, novel formulations have become available. For MPA, the differences in pharmacokinetic behavior between the old and the novel formulation will have implications for TDM, whereas for tacrolimus, this probably will not to be the case.

Use of personalized medicine in the selection of patients for renal transplantation: views of Quebec transplant physicians and referring nephrologists

AIM

To explore the views of physicians on the use of personalized medicine tools to develop a new method for selecting potential recipients of a renal allograft.

METHODS

A total of 22 semidirected interviews, using clinical case studies.

RESULTS

According to the participants, this method has several possible applications within renal transplantation (individualizing immunosuppressive therapy, help with decision making, and possibly with the selection of patients). It could be more effective than the method presently used. The method must be validated scientifically, and must also involve clinical judgment.

CONCLUSION

The use of personalized medicine within transplantation must be in the best interests of the patient. An ethical reflection is necessary in order to focus on the possibility of patients being excluded, as well as on the resolution of the equity/efficacy dilemma. Empirical research has shown itself to be essential for ascertaining the views of the clinicians who will be working with the tools provided by personalized medicine.

Advances in translational transplant immunology

Recent developments in basic and translational immunology open new exciting perspectives for inducing transplantation tolerance in the clinic. Induction of tolerance, defined as permanent acceptance of the transplant in the absence of continuous immunosuppression, is an achievable goal. However, a number of hurdles still need to be overcome before immunosuppressive drugs can be safely withdrawn in solid organ transplant recipients. Additional strategies for improving long-term outcomes were examined, including best methods for apply using various biomarkers in the clinical setting to improve the diagnosis and management of ongoing renal damage. Detection of a potential or existing immune response to tissue grafts is an important first step in improving the survival of heart, liver, and kidney transplant patients.

ABCB1 genotypes predict cyclosporine-related adverse events and kidney allograft outcome

Cyclosporine A (CsA) is a substrate of P-glycoprotein, an efflux transporter encoded by the ABCB1 gene. Compared with carriers of the wild-type gene, carriers of T allelic variants in exons 21 or 26 have reduced P-glycoprotein activity and, secondarily, increased intracellular concentration of CsA; therefore, carriers of T variants might be at increased risk for CsA-related adverse events. We evaluated the associations between ABCB1 genotypes (in exons 12, 21, and 26) and CsA-related outcomes in 147 renal transplant recipients who were receiving CsA-based immunosuppression and were included in the Mycophenolate Steroids Sparing study. During a median of 65.5 mo follow-up, carriers of T allelic variants in exons 21 or 26 had a three-fold risk for delayed graft function (DGF), a trend to slower recovery of renal function and lower GFR at study end, and significantly higher incidences of new-onset diabetes and cytomegalovirus reactivation compared with carriers of the wild-type genotype. T variants in both exons 21 and 26 were independently associated with 3.8- and 3.5-fold higher risk for DGF, respectively (P = 0.022 and P = 0.034). The incidence of acute rejection and the mean CsA dose and blood levels were comparable in genotype groups. In conclusion, renal transplant recipients with T allelic variants in ABCB1 exons 21 or 26 are at increased risk for CsA-related adverse events. Genetic evaluation may help to identify patients at risk and to modulate CsA therapy to optimize graft and patient outcomes.

Effect of cytokine and pharmacogenomic genetic polymorphisms in transplantation

Consolidating the information that we have on pharmacogenetics and on cytokine genetics to produce patient-oriented individualized drug regimens is an important challenge in transplantation medicine. Using a multi-variant approach based on genetic profile and other relevant clinical factors a score system may be developed to predict the severity of rejection, infection, or other complications associated with transplantation. The ultimate goal of these studies is to improve patient outcome through individualized drug regimens.

Can immune monitoring help to minimize immunosuppression in kidney transplantation?

The serious side-effects and complications related to the life-long use of immunosuppressors in transplantation have fuelled research into their possible minimization or even complete elimination. The field of transplantation is therefore tentatively moving from a phase of empiric immunosuppression towards individualized therapy. This process is highly dependent on the development of immune monitoring tests to detect an individual 'level of risk'. Immune monitoring is a way of measuring functional and molecular correlates of immune reactivity to provide clinically useful information for therapeutic decision-making. The technological breakthroughs over the last decade provide firm grounds for the achievement of this goal. Large, multicentric and prospective studies in the near future are now crucial if these tests are to achieve the necessary approval from the regulatory authorities and promptly enter the clinic for routine use.