MDR1 haplotypes do not affect the steady-state pharmacokinetics of cyclosporine in renal transplant patients

Combinations of common SNPs of the transporter gene ABCB1 influence apparent bioavailability, but not renal elimination of oral digoxin

Effects of different genotypes on the pharmacokinetics of probe substrates may support their use as phenotyping agents for the activity of the respective enzyme or transporter. Digoxin is recommended as a probe substrate to assess the activity of the transporter P-glycoprotein (P-gp) in humans. Current studies on the individual effects of three commonly investigated single nucleotide polymorphisms (SNPs) of the ABCB1 gene encoding P-gp (C1236T, G2677T/A, and C3435T) on digoxin pharmacokinetics are inconclusive. Since SNPs are in incomplete linkage disequilibrium, considering combinations of these SNPs might be necessary to assess the role of polymorphisms in digoxin pharmacokinetics accurately. In this study, the relationship between SNP combinations and digoxin pharmacokinetics was explored via a population pharmacokinetic approach in 40 volunteers who received oral doses of 0.5 mg digoxin. Concerning the SNPs 1236/2677/3435, the following combinations were evaluated: CGC, CGT, and TTT. Carriers of CGC/CGT and TTT/TTT had 35% higher apparent bioavailability compared to the reference group CGC/CGC, while no difference was seen in CGC/TTT carriers. No significant effect on renal clearance was observed. The population pharmacokinetic model supports the use of oral digoxin as a phenotyping substrate of intestinal P-gp, but not to assess renal P-gp activity.

Combined Influence of Genetic Polymorphism and DNA Methylation on ABCB1 Expression and Function in Healthy Chinese Males

BACKGROUND AND OBJECTIVES

It is well known that the expression and function of ATP-binding cassette transporter B1 (ABCB1) show high interindividual variability, but the reasons have not yet been fully elucidated. In this study, combined influence of genetic polymorphism and DNA methylation on ABCB1 mRNA expression and digoxin pharmacokinetics in healthy Chinese males was analyzed.

METHODS

A total of 93 subjects who were homozygous for the ABCB1 1236-2677-3435 TTT or CGC haplotype were enrolled in this study. DNA methylation status of the ABCB1 promoter and ABCB1 mRNA expression level in exfoliated intestinal epithelial cells were analyzed using bisulfite sequencing PCR and real-time PCR. The pharmacokinetics of digoxin in subjects were investigated after administration of a single oral dose of digoxin 0.5 mg.

RESULTS

The DNA methylation levels of ABCB1 promoter showed no significant difference between TTT/TTT and CGC/CGC carriers (P = 0.54). Subjects with TTT/TTT haplotype pair and high methylation status (TTT/TTT-HM) showed a significantly lower ABCB1 mRNA level compared to other subjects. Compared with TTT/TTT-HM subgroup, the area under the plasma concentration-time curve from time zero to 72 h (AUC_0-72) of digoxin was decreased by 26.9 %, the maximum plasma concentration (C _max) was decreased by 25 % and the apparent oral clearance (CL/F) was increased by 21.2 % in CGC/CGC-LM subgroup. The values of time to maximum concentration (t _max) and terminal elimination half-life (t _1/2) showed no significant difference.

CONCLUSIONS

Both genetic polymorphism and DNA methylation variation should be taken into consideration to explain the interindividual variability in ABCB1 expression and function more clearly.

Clinical Pharmacogenetics of the Major Adenosine Triphosphate−Binding Cassette and Solute Carrier Drug Transporters

Interindividual variability in drug response is a significant problem in clinical practice, and it is likely that genetic variation among the drug transport genes are major contributors to such variability. Numerous genetic alterations affecting the members of the adenosine triphosphate-binding cassette (ABC) and solute carrier (SLC) families of transporters have been identified. Considerable data exist regarding how mutations in the ABCB1 gene that encodes p-glycoprotein impact drug disposition and response in vivo, but many study reports are conflicting on both the direction of any effect as well as the significance of any alteration. Many possible reasons for such discrepant study results have been identified, and efforts to improve the quality of such pharmacogenetic clinical association studies are ongoing. For most other clinically important transporters relatively, little clinical data exist regarding the significance of known genetic variants despite in vitro evidence of altered function for many of these transporters. What clinical data do exist suggest that certain mutations in ABCG2 and SLCO1B1 may be of importance clinically. Until the current uncertainties regarding the importance of genetic variants in drug transporter genes are clarified, the clinical application of existing pharmacogenetic data should be done with caution.

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.

Diltiazem augments the influence of MDR1 genotype status on cyclosporine concentration in Chinese patients with renal transplantation

AIM

Co-administration of diltiazem can reduce the dosage of cyclosporine (CsA) in patients with renal transplantation. In this study, we investigated how diltiazem altered the relationship between MDR1 genetic polymorphisms and CsA concentration in Chinese patients with renal transplantation.

METHODS

A total of 126 renal transplant patients were enrolled. All the patients received CsA (2-4 mg·kg(-1)·d(-1)), and diltiazem (90 mg/d) was co-administered to 76 patients. MDR1-C1236T, G2677T/A, and C3435T polymorphisms were genotyped. The whole blood concentration was measured using the FPIA method, and the adjusted trough concentrations were compared among the groups with different genotypes.

RESULTS

In all patients, MDR1-C1236T did not influence the adjusted CsA trough concentration. With regard to MDR1-3435, the adjusted CsA trough concentration was significantly higher in TT carriers than in CC and CT carriers when diltiazam was co-administered (58.83±13.95 versus 46.14±7.55 and 45.18±12.35 ng/mL per mg/kg, P=0.011), and the differences were not observed in patients without diltiazam co-administered. With regard to MDR1-2677, the adjusted CsA trough concentration was significantly higher in TT carriers than in GG and GT carriers when diltiazam was co-administered (61.31±12.93 versus 52.25±7.83 and 39.70±7.26 ng/mL per mg/kg, P=0.0001). The differences were also observed in patients without diltiazam co-administered (43.27±5.95 versus 35.22±7.55 and 29.54±5.35 ng/mL per mg/kg, P=0.001). The adjusted CsA trough blood concentration was significantly higher in haplotype T-T-T and haplotype T-T-C carriers than in non-carriers, regardless of diltiazem co-administered.

CONCLUSION

MDR1 variants influence the adjusted CsA trough concentration in Chinese patients with renal transplant, and the influence more prominent when diltiazem is co-administered.

The Effect of ABCB1 C3435T Polymorphism on Cyclosporine Dose Requirements in Kidney Transplant Recipients: A Meta-Analysis

Cyclosporine A (CsA) is a substrate of the multi-drug efflux pump P-glycoprotein (P-gp) encoded by ABCB1. Among the various single nucleotide polymorphisms (SNPs) of ABCB1, C3435T has been extensively investigated to determine the relationship with the pharmacokinetics of CsA. However, the results are controversial. This meta-analysis was designed to evaluate the influence of C3435T SNP on the dose-adjusted trough (C0 /D) and peak (Cmax /D) concentrations of CsA. Based on a literature search of four authoritative databases, 13 studies since 2001 concerning 1293 kidney transplant recipients were included. The results indicated a significant difference of C0 /D and Cmax /D between 3435CC and 3435TT genotype carriers (weighted mean difference (WMD) of C0 /D: 4.18 (ng ml(-1))/(mg kg(-1)), 95% CIs: 1.00-7.37, p = 0.01; WMD of Cmax /D: 20.85 (ng ml(-1))/(mg kg(-1)), 95% CIs: 2.25-39.46, p = 0.03). Subgroup analysis by ethnicity demonstrated that C0 /D was lower in Asian CC versus TT genotype carriers (WMD = 10.32 (ng ml(-1))/(mg kg(-1)), 95% CIs: 4.78-15.85, p = 0.0003) but did not vary by genotype for Caucasian recipients. Moreover, significant variation of C0 /D was found at 1 week and 1-3 months after transplantation between CC and TT genotype carriers. Therefore, this meta-analysis showed a correlation between ABCB1 C3435T polymorphism and the dose-adjusted concentration of CsA. Patients with 3435CC genotype will require a higher dose of CsA to achieve target therapeutic concentrations when compared with 3435TT carriers after kidney transplantation, especially in the Asian population and especially during the early and middle time periods after transplantation.

Association of ABCB1 gene polymorphisms and haplotypes with therapeutic efficacy of glucocorticoids in Chinese patients with immune thrombocytopenia

Resistance to glucocorticoids (GCs) remains a tricky problem complicating the therapy of ITP. Recently, ATP binding cassette gene B1 gene (ABCB1) was reported to be correlated with susceptibility and therapeutic efficacy of autoimmune diseases through P-glycoprotein (Pgp). We investigated three single nucleotide polymorphisms (SNPs) of ABCB1 and their haplotypes by PCR-RFLP (restriction fragment length polymorphism) method in 471 ITP patients and 383 healthy controls, patients were further assigned into GCs-responsive and -non-responsive group according to the therapeutic effects of GCs. We observed a remarkable difference in genotypes of G2677T/A between GCs-responsive and non-responsive group, but not between patients and controls. A frequently expression of T/A allele within G2677T/A was recorded in GCs-responsive group. Furthermore, we found that some haplotypes (CGC, CTC/CAC, CTT/CAT, TGC, TGT, TTC/TAC and TTT/TAT, in the order of position 1236-2677-3435) were presented significantly differences between non-responsive and responsive group. No difference of C1236T and C3435T polymorphisms was observed between ITP and controls, and between the GCs-responsive and -non-responsive group. Our findings suggest that ABCB1 polymorphisms, as well as haplotypes derived from C1235T, G2677T/A and C3435T, are associated with inter-individual differences of GCs treatment in ITP.

Sequential Analysis of Tacrolimus Dosing in Adult Lung Transplant Patients WithABCB1Haplotypes

The genetic polymorphisms in the ABCB1 gene, which encodes for the membrane pump, P-glycoprotein, have been previously demonstrated to have an association with tacrolimus dosing in organ transplant patients. This study associated the haplotype and genotype for ABCB1 G2677T and C3435T variants with a sequential analysis of tacrolimus blood level (ng/mL) per mg/day dosage ([L/D]) administered to 91 adult lung transplant patients at 1, 3, 6, 9, and 12 months after transplantation. Haplotype 22 carriers had a significantly higher tacrolimus [L/D] value in comparison with nonhaplotype 22 carriers (P = .04) only at 1 month after transplant. Sequential analysis demonstrated that ABCB1 genotypes 00 and 01 had low tacrolimus [L/D] values at 1 and 3 months, but these values increased substantially at 6, 9, and 12 months after transplantation. This was not true of the other genotypes with the exception of genotypes 10 and 21, which had small numbers of patients but had consistently low tacrolimus [L/D]. Haplotype analysis also suggested that the homozygous for ABCB1 2677 variant allele had more of an impact on tacrolimus [L/D] in haplotype analysis than that of ABCB1 3435. In conclusion, sequential analysis of tacrolimus [L/D] with haplotypes can explain previous clinical observations of changes in tacrolimus dosage over time but suggests that this effect is limited to individual patient haplotypes. Sequential analysis of drug dosing and haplotypes relationships can provide important information about the induction or inhibition of drug-drug and disease-drug interactions among specific haplotypes.

Using genetic and clinical factors to predict tacrolimus dose in renal transplant recipients

AIMS

Tacrolimus has a narrow therapeutic window and shows significant interindividual difference in dose requirement. In this study we aim to first identify genetic factors that impact tacrolimus dose using a candidate gene association approach, and then generate a personalized algorithm combining identified genetic and clinical factors to predict individualized tacrolimus dose.

MATERIALS & METHODS

We screened 768 SNPs in 15 candidate genes in metabolism, transport and calcineurin inhibition pathways of tacrolimus, for association with tacrolimus dose in a discovery cohort of 96 patients.

RESULTS

Four polymorphisms in CYP3A5 and one polymorphism in CYP3A4 were identified to be significantly associated with tacrolimus stable dose (p < 8.46 × 10(-5)). The same SNPs were identified when dose-normalized trough tacrolimus concentration was analyzed. The CYP3A5*1 allele was associated with significantly higher stable dose, bigger dose increase, higher risk of being underdosed and lower incidence of post-transplant hyperlipidemia. ABCB1 polymorphisms were not associated with stable dose. No significant difference was found between CYP3A5 expressers and nonexpressers in incidence of acute rejection and time to first rejection. Age, ethnicity and CYP3A inhibitor use could predict 30% of tacrolimus dosing variability. Adding the identified genetic polymorphisms to the algorithm increased the predictability to 58%. In two validation cohorts of 77 and 64 patients, the algorithm containing both genetic and clinical factors produced correlation coefficients of 0.63 and 0.42, respectively. This algorithm gave a prediction of the stable doses closer to the actual doses when compared with another algorithm based only on the CYP3A5 genotype.

CONCLUSION

CYP3A5 genotype is the most significant genetic factor that impacts tacrolimus dose among the genes studied. This study generated the first pharmacogenomics model that predicts tacrolimus stable dose based on age, ethnicity, genotype and comedication use. Our results highlight the importance of incorporating both genetic and clinical, demographic factors into dose prediction.

Pharmacogenetics in immunosuppressants: impact on dose requirement of calcineurin inhibitors in renal and liver pediatric transplant recipients

PURPOSE OF REVIEW

Calcineurin inhibitors (CNI) are the mainstay immunosuppressive therapy in pediatric solid organ transplantation. These drugs have narrow therapeutic window, and continuous therapeutic drug monitoring is required to keep blood levels within the therapeutic range. Personalization of immunosuppressive therapy according to the genetic profile may provide a way to optimize drug dosing from the first day of transplantation. In this review, we will highlight the recent pharmacogenetic studies of CNIs in pediatric renal and liver transplantation.

RECENT FINDINGS

CNIs are metabolized by CYP3A4 and CYP3A5. In the intestine, the absorption of these drugs is limited by the P-glycoprotein efflux transporter. Most of the pediatric studies showed an association between CYP3A5 genetic variation and CNI dosing. Carriers of the wild-type allele (CYP3A5*1) required higher doses of CNIs as compared with individuals homozygous to the variant CYP3A5*3 allele. CYP3A4 and ABCB1 (encoding P-glycoprotein) genetic variations did not show an association with CNI dosing.

SUMMARY

The pharmacogenetics of CNIs has been widely investigated in adults, little is known about this field in the pediatric groups. Prospective studies are needed to elucidate the effect of genetic variations on CNI drug dosing and to investigate their impact on short and long-term clinical outcome.

Pharmacogenetics of psoriasis

Psoriasis is an inflammatory hyperproliferative skin disorder with a strong genetic predisposition. While many effective modalities are currently available for treating psoriasis, response to therapy is quite variable among patients. The genetic component underlying the response to pharmacotherapy in psoriasis is slowly beginning to emerge and represents a specialized field of genetics referred to as pharmacogenetics. The identification of genetic variants has the potential to improve the management of patient care by identifying which patients should avoid a specific drug and which patients should be administered a modified dose. A suitable approach in implementing such a strategy could potentially reduce medical costs and improve success of drug therapy. This article summarizes the clinical aspects of psoriasis, its genetic susceptibility and highlights the current landscape of genetic targets for psoriasis pharmacotherapy.

Do drug transporter (ABCB1) SNPs influence cyclosporine and tacrolimus dose requirements and renal allograft outcome in the posttransplantation period?

Polymorphisms in the drug transporter gene (ABCB1) may play a significant role in individualizing cyclosporine (CsA) and tacrolimus (Tac) dosage and subsequently the allograft outcome in renal transplant recipients. In total, 225 recipients on CsA and 75 on Tac-based immunosuppression regimen were recruited, and 6 common polymorphic sites in the ABCB1 gene were analyzed for association with dose-adjusted levels of CsA/Tac. Furthermore, association of ABCB1 single-nucleotide polymorphisms (SNPs) with allograft outcome was examined. GG and CC genotype patients at ABCB1 2677G>T and ABCB1 3435C>T were associated with lower dose-adjusted levels of CsA and Tac at 1 month (P = .057, P = .034), 3 months (P = .001, P = .015), and 6 months (P = .043) posttransplantation. Wild-type patients at 1236C>T (log P = .025) and 2677G>T (log P = .002) in CsA and 2677G>T (log P = .008) and 3435C>T (log P = .015) in Tac therapy patients demonstrated lower mean time to allograft rejection. No influence of ABCB1 haplotypes on CsA/Tac dose-adjusted levels was observed. Wild-type patients at ABCB1 2677G>T and 3435C>T were associated with lower dose-adjusted levels and thereby were at increased risk of allograft rejection because of under-immunosuppression in the early part of posttransplantation. Thus, genetic evaluation may be helpful to identify patients at risk for allograft rejection and also to individualize immunosuppressant dosing.

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.

Influence of ABCB1 gene polymorphisms on the pharmacokinetics of verapamil among healthy Chinese Han ethnic subjects

AIMS

To assess the association between polymorphisms of the ABCB1 gene and the pharmacokinetics of verapamil among healthy Chinese Han ethnic subjects.

METHODS

Based on polymorphisms of the ABCB1 gene at positions 2677 and 3435, 24 healthy male participants were divided into three groups: 2677GG/3435CC (n = 6), 2677GT/3435CT (n = 12) and 2677TT/3435TT (n = 6). Each subject had received a single oral dose of verapamil (80 mg) under fasting conditions. Multiple blood samples were collected over 24 h, and plasma concentrations of verapamil were determined by HPLC. Pharmacokinetic characteristics were compared between the different genotypic groups.

RESULTS

The pharmacokinetics parameters of verapamil differed significantly among the three genotypic groups. AUC(last) was significantly lower among individuals with the 2677TT/3435TT (159.5 +/- 79.0 ng ml(-1) h) and 2677GT/3435CT (189.3 +/- 73.1 ng ml(-1) h) genotypes than those with the 2677GG/3435CC genotype (303.1 +/- 83.7 ng ml(-1) h) (P= 0.004 and P= 0.008, respectively). However, the CL/F value was higher among subjects with the 2677TT/3435TT (523.0 +/- 173.7 l h(-1)) genotype than those with the 2677GT/3435CT (452.2 +/- 188.6 l h(-1)) or 2677GG/3435CC (265.4 +/- 72.8 l h(-1)) genotypes. A significant difference was also found between the latter two groups (P= 0.034). In addition, the C(max) tended to be higher among subjects with the 2677GG/3435CC genotype than those with the 2677GT/3435CT or 2677TT/3435TT genotypes (42.2 +/- 3.9 vs 32.2 +/- 16.2 vs 38.1 +/- 13.7 ng ml(-1)).

CONCLUSIONS

Our study showed for the first time that verapamil pharmacokinetics may be influenced by particular genetic polymorphisms of the ABCB1 gene among healthy Chinese Han ethnic subjects. An individualized dosage regimen design incorporating such information may improve the efficacy of the drug whilst reducing adverse reactions.

Pharmacogenetics of calcineurin inhibitors in renal transplantation

Cyclosporine A and tacrolimus (Tac) are inmunosuppresive drugs with a narrow therapeutic range. Underdosing is associated with organ rejection, whereas overdosing could result in toxicity. Therapeutic drug monitoring at different postdose times is necessary to maintain the blood concentrations within a target window. These calcineurin inhibitors are characterized by a broad interindividual pharmacokinetics variability, which makes the determination of the initial dose difficult. In a patient receiving a dose, the amount of the drug that is measured in the blood determines its bioavailability, which depends on the absorption, biotransformation, and elimination of the drug. These processes are primarily controlled by efflux pumps and enzymes of the cytochrome P (CYP) 450 family. DNA variants at the genes encoding these proteins contribute to the interindividual heterogeneity for calcineurin inhibitors metabolism. Cyclosporine A and Tac are metabolized by CYP3A4 and CYP3A5, and several single nucleotide polymorphisms in the two genes have been associated with differences in drug clearance. Carriers of the CYP3A5 wild-type allele have a higher CYP3A5 expression compared with individuals who are homozygous for a common DNA variant that affects gene splicing (CYP3A5*3). For renal transplant recipients receiving Tac, homozygotes for this nonexpression allele would exhibit significantly lower Tac clearance and may require a lower dose to remain within the blood target concentration compared with CYP3A5 expressors. To date, this CYP3A5 variant is the only reported genetic factor to predict the appropiate starting dosage of Tac, avoiding overdosing and improving the outcome of renal transplantation.

Overview of pharmacogenetics

Pharmacogenetics is the study of relationships between genetic variation and inter-individual differences with respect to drug response. As the field has matured over the past 15 years, a remarkable diversity of pathways, variation types, and mechanisms have been found to be relevant pharmacogenetic factors. Today, pharmacogenetics is becoming more important in pharmacology for target validation, lead optimization, and understanding of idiosyncratic toxicity. This unit provides an overview of the history of pharmacogenetics and current research applications in drug discovery, as well as a discussion of research quality issues relevant for human subjects research in the pharmacogenetics laboratory.

CYP3A7, CYP3A5, CYP3A4, and ABCB1 genetic polymorphisms, cyclosporine concentration, and dose requirement in transplant recipients

Cyclosporine is a substrate of cytochrome P450 (CYP) 3A and of the transporter ABCB1, for which polymorphisms have been described. In particular, CYP3A5 *3/*3 genotype results in the absence of CYP3A5 activity, whereas CYP3A7 *1/*1C genotype results in high CYP3A7 expression in adults. Log-transformed dose-adjusted cyclosporine trough concentration and daily dose per weight were compared 1, 3, 6, and 12 months after transplantation between CYP3A and ABCB1 genotypes in 73 renal (n = 64) or lung (n = 9) transplant recipients. CYP3A5 expressors (*1/*3 genotype; n = 8-10) presented significantly lower dose-adjusted cyclosporine trough concentrations (P < 0.05) and required significantly higher daily doses per weight (P < 0.01) than the nonexpressors (*3/*3 genotype; n = 55-59) 1, 3, 6, and 12 months after transplantation. In addition, 7 days after transplantation, more CYP3A5 expressors had uncorrected trough cyclosporine concentration below the target concentration of 200 ng/mL than the nonexpressors (odds ratio = 7.2; 95% confidence interval = 1.4-37.3; P = 0.009). CYP3A4 rs4646437C>T influenced cyclosporine kinetics, the T carriers requiring higher cyclosporine dose. CYP3A7*1C carriers required a 1.4-fold to 1.6-fold higher cyclosporine daily dose during the first year after transplantation (P < 0.05). In conclusion, CYP3A4, CYP3A5, and CYP3A7 polymorphisms affect cyclosporine metabolism, and therefore, their genotyping could be useful, in association with therapeutic drug monitoring, to prospectively optimize cyclosporine prescription in transplant recipients. The administration of a CYP3A genotype-dependent cyclosporine starting dose should therefore be tested prospectively in a randomized controlled clinical trial to assess whether it leads to an improvement of the patients outcome after transplantation, with adequate immunosuppression and decreased toxicity.

Effect of genetic polymorphisms of CYP3A5 and MDR1 on cyclosporine concentration during the early stage after renal transplantation in Chinese patients co-treated with diltiazem

OBJECTIVE

The aim of this study was to assess the influence of the cytochrome (CYP450)3A5 and multidrug resistance (MDR1) gene polymorphisms on cyclosporine A (CsA) trough concentration during the early stage after renal transplantation in Chinese patients co-treated with diltiazem.

METHODS

CYP3A5*3 (A6986G) and MDR1 C1236T, G2677T/A and C3435T polymorphisms were determined by PCR followed by restriction fragment length polymorphism (RFLP) analysis. A total of 112 Chinese renal transplant patients were enrolled in the study. The whole blood trough concentration was measured at 7 days after transplantation, and the dose-adjusted trough levels were compared among the different genotypes.

RESULTS

The dose-adjusted trough levels of CsA were significantly higher in MDR1 2677TT carriers than in GG plus GT carriers (59.5 +/- 15.9 vs. 34.5 +/- 9.4 vs. 43.2 +/- 13.6 ng/mL per mg per kg; P < 0.0001). In patients who were co-treated with diltiazem, compared with carriers of haplotype T-T-C, the carriers of haplotype C-G-C and haplotype T-G-T had significantly lower dose-adjusted trough blood concentrations of CsA than the non-carrier group (P = 0.002, P = 0.000 and P = 0.000, respectively). However, no evidence was found that there was a relationship between the CYP3A5*3, MDR1 C1236T and MDR1 C3435T polymorphisms and CsA dose-adjusted trough concentrations.

CONCLUSION

This study demonstrated that the G2677T/A single nucleotide polymorphisms in MDR1 and MDR1 haplotypes C-G-C, T-G-T and T-T-C are associated with the CsA concentration during the very early post-transplant period in Chinese renal transplant patients co-treated with diltiazem. These polymorphisms may be useful for determining the appropriate initial dose of CsA after renal transplantation.

Meta-analysis of the effect of MDR1 C3435T polymorphism on cyclosporine pharmacokinetics

The published data revealed conflicting results of the polymorphism of MDR1 exon 26 SNP C3435T on the pharmacokinetics of cyclosporine; thus, the aim was to conduct a meta-analysis of significant magnitude to investigate the influence of SNP C3435T on the pharmacokinetics of cyclosporine. A literature search was conducted to locate the relevant papers by using the PubMed electronic source from 1997 and onwards. The pharmacokinetic parameters, including AUC(0-4), AUC(0-12), AUC(0-inf), C(max), CL/F and trough concentration (C(0)), were extracted and a meta-analysis was performed by using Stata version 9.1. A total of 14 papers concerning 1036 individuals were included in the meta-analysis. The overall results showed no major influence of SNP C3435T on the pharmacokinetic parameters, including AUC(0-4), AUC(0-inf), CL/F, C(max) and C(0), although AUC(0-12) was lower in subjects with CC genotype. A subanalysis by ethnic population showed that C(0) was lower in Caucasian individuals harbouring CC genotype. In conclusion, our meta-analysis of available studies has thus far failed to demonstrate a definitive correlation between the SNP C3435T in MDR1 gene and alterations in P-glycoprotein function that can result in altered pharmacokinetics of cyclosporine, although it was indicated in this meta-analysis that the carrier of CC genotype of the SNP C3435T of MDR1 had lower cyclosporine exposure presented as AUC(0-12) than those with at least one T allele. There seems to be ethnic differences in the relationship between the SNP C3435T of MDR1 and cyclosporine pharmacokinetics.

Genetic polymorphisms and the fate of the transplanted organ

There has been an abundance of publications describing genetic variability in molecules affecting innate and adaptive immunity, pharmacogenetics, and other nonimmunological factors like the renin-angiotensin aldosterone system, coagulation, and fibrosis markers. Studies indicated some associations between polymorphisms in these candidate genes with outcomes in organ transplantation and underlined a potential role of genetic variability in transplantation. To be clinically applicable, large prospective studies must be performed to better define the potential benefits of genotyping on these genetic markers and clinical outcomes. The purposes of this review are to summarize recent data describing associations of polymorphisms in both immunological and nonimmunological molecules with transplant outcomes, with a particular emphasis on renal transplantation, and discuss limitations and clinical implications.

Association of MDR1, CYP3A4*18B, and CYP3A5*3 polymorphisms with cyclosporine pharmacokinetics in Chinese renal transplant recipients

OBJECTIVE

The objective of this study was to retrospectively evaluate the effects of MDR1, CYP3A4*18B, and CYP3A5*3 genetic polymorphisms on cyclosporine A (CsA) pharmacokinetics in Chinese renal transplant patients during the first month after transplantation.

METHODS

A total of 103 renal transplant recipients receiving CsA were genotyped for MDR1 (C1236T, G2677T/A, and C3435T), CYP3A4*18B, and CYP3A5*3. The predose and 2-h postdose concentrations of CsA (C(0) and C(2), respectively) were determined by fluorescence polarization immunoassay, and their relationships with corresponding genotypes and haplotypes were investigated.

RESULTS

Patients with a CYP3A4*1/*1 genotype were found to have a higher dose-adjusted concentration compared with those with CYP3A4*18B/*18B, as follows: for C(2), 19.3% (P = 0.008) during days 8-15, 35.2% (P = 0.008) during days 16-30, and for C(0), 39.7% (P = 0.012) during days 16-30. The dose-adjusted C(0) was higher in patients with MDR1 1236CC compared with those with 1236TT in the first month postoperation. The dose-adjusted C(0) in patients with the CYP3A5*3/*3 genotype was 25.5% and 30.7% higher than those with the wild-type genotype during days 8-15 (P = 0.011) and days 16-30 (P = 0.015), respectively. Haplotype analysis revealed that the dose-adjusted C(0) was higher in the first month following surgery in carriers of haplotype MDR1 CAC than in noncarriers. Polymorphisms of MDR1 and CYP3A5*3 did not affect dose-adjusted C(2.)

CONCLUSION

The data suggests that the CYP3A4*18B genotype affects CsA pharmacokinetics during the first month following surgery in Chinese renal transplant recipients. Patients with CYP3A4*18B alleles may require higher doses of CsA to reach the target levels. Large prospective studies may be needed to further explore the impact of MDR1 and CYP3A5*3 polymorphisms on CsA pharmacokinetics in renal transplant recipients.

Association of MDR1, CYP3A4*18B, and CYP3A5*3 polymorphisms with cyclosporine pharmacokinetics in Chinese renal transplant recipients

OBJECTIVE

The objective of this study was to retrospectively evaluate the effects of MDR1, CYP3A4*18B, and CYP3A5*3 genetic polymorphisms on cyclosporine A (CsA) pharmacokinetics in Chinese renal transplant patients during the first month after transplantation.

METHODS

A total of 103 renal transplant recipients receiving CsA were genotyped for MDR1 (C1236T, G2677T/A, and C3435T), CYP3A4*18B, and CYP3A5*3. The predose and 2-h postdose concentrations of CsA (C(0) and C(2), respectively) were determined by fluorescence polarization immunoassay, and their relationships with corresponding genotypes and haplotypes were investigated.

RESULTS

Patients with a CYP3A4*1/*1 genotype were found to have a higher dose-adjusted concentration compared with those with CYP3A4*18B/*18B, as follows: for C(2), 19.3% (P = 0.008) during days 8-15, 35.2% (P = 0.008) during days 16-30, and for C(0), 39.7% (P = 0.012) during days 16-30. The dose-adjusted C(0) was higher in patients with MDR1 1236CC compared with those with 1236TT in the first month postoperation. The dose-adjusted C(0) in patients with the CYP3A5*3/*3 genotype was 25.5% and 30.7% higher than those with the wild-type genotype during days 8-15 (P = 0.011) and days 16-30 (P = 0.015), respectively. Haplotype analysis revealed that the dose-adjusted C(0) was higher in the first month following surgery in carriers of haplotype MDR1 CAC than in noncarriers. Polymorphisms of MDR1 and CYP3A5*3 did not affect dose-adjusted C(2.)

CONCLUSION

The data suggests that the CYP3A4*18B genotype affects CsA pharmacokinetics during the first month following surgery in Chinese renal transplant recipients. Patients with CYP3A4*18B alleles may require higher doses of CsA to reach the target levels. Large prospective studies may be needed to further explore the impact of MDR1 and CYP3A5*3 polymorphisms on CsA pharmacokinetics in renal transplant recipients.

Biotransformation enzymes and drug transporters pharmacogenetics in relation to immunosuppressive drugs: impact on pharmacokinetics and clinical outcome

Immunosuppressive drugs commonly used after organ transplantation to prevent acute rejection including tacrolimus, cyclosporine, sirolimus, and mycophenolic acid are characterized by a narrow therapeutic index and broad interindividual variability in their pharmacokinetics. Adequate immunosuppression aims to reach an optimal benefit-risk ratio. Therapeutic drug monitoring represents a crucial step in routine practice to maintain blood concentrations within the target window, because the bioavailability of these drugs depends on their absorption, distribution, biotransformation, and elimination. Single nucleotide polymorphisms (SNPs) in genes encoding biotransformation enzymes (CYP3A) and drug transporters (ABCB1) have opened up a promising way for the selection of individual dosages. The relationship of these SNPs with immunosuppressive drug pharmacokinetics was extensively studied after kidney, liver, heart, and lung transplantations. Patient susceptibility to nephrotoxicity in the long term was also reported in relation to some SNPs, which could allow effective assessment of individual risk and selection of treatment according to patient parameters. Further studies are needed to provide evidence that a genetic analysis combined with therapeutic drug monitoring has the potential to optimize drug use after transplantation.

Association of four DNA polymorphisms with acute rejection after kidney transplantation

Renal transplant outcomes exhibit large inter-individual variability, possibly on account of genetic variation in immune-response mediators and genes influencing the pharmacodynamics/pharmacokinetics of immunosuppressants. We examined 21 polymorphisms from 10 genes in 237 de novo renal transplant recipients participating in an open-label, multicenter study [Cyclosporine Avoidance Eliminates Serious Adverse Renal-toxicity (CAESAR)] investigating renal function and biopsy-proven acute rejection (BPAR) with different cyclosporine A regimens and mycophenolate mofetil. Genes were selected for their immune response and pharmacodynamic/pharmacokinetic relevance and were tested for association with BPAR. Four polymorphisms were significantly associated with BPAR. The ABCB1 2677T allele tripled the odds of developing BPAR (OR: 3.16, 95% CI [1.50-6.67]; P=0.003), as did the presence of at least one IMPDH2 3757C allele (OR: 3.39, 95% CI [1.42-8.09]; P=0.006). BPAR was almost fivefold more likely in patients homozygous for IL-10 -592A (OR: 4.71, 95% CI [1.52-14.55]; P=0.007) and twice as likely in patients with at least one A allele of TNF-alpha G-308A (OR: 2.18, 95% CI [1.08-4.41]; P=0.029). There were no statistically significant interactions between polymorphisms, or the different treatment regimens. Variation in genes of immune response and pharmacodynamic/pharmacokinetic relevance may be important in understanding acute rejection after renal transplant.

Pharmacogenetics as a tool for optimising drug therapy in solid-organ transplantation

Existing immunosuppressive therapies used for solid-organ transplantation have narrow therapeutic indices, whereby underdosing is associated with acute immunological rejection of the transplanted organ and overdosing is associated with infections and malignancy, as well as organ-specific toxicities. There is significant inter-individual variation in the pharmacokinetics and pharmacodynamics of these drugs, an issue that has been addressed, in part, by therapeutic drug monitoring. Genetic polymorphisms in drug metabolising enzymes, drug efflux pumps and drug targets which may underly this heterogeneity have been identified and may provide a tool to guide prescribing. There are a number of associations between genotype and pharmacology, but as of now, only thiopurine-S-methyltransferase and cytochrome P450 3A5 have a sufficiently large influence to have potential in guiding therapy. Recent studies have also identified that donor genotype may play a significant role in immunosuppressive drug pharmacokinetics and pharmacodynamics.

Polymorphisms of Multidrug Resistance Gene (MDR1) and Cyclosporine Absorption in De Novo Renal Transplant Patients

BACKGROUND

Several single nucleotide polymorphisms (SNPs) in the multidrug resistance (MDR1) gene may play a role in the interindividual variation of cyclosporine A (CsA) absorption in renal transplant patients.

METHODS

An analysis of CsA absorption measured by the dose- and weight-adjusted 4 hr area under the time-concentration curve, AUC(0-4)/mg doseCsA/kg, was conducted on day 3 after transplantation, in 69 de novo renal transplant patients who were genotyped for MDR1 SNPs. Follow-up pharmacogenomic analysis at 1 month posttransplant was performed utilizing dose- and weight-adjusted 2-hour postdose CsA concentration (C2).

RESULTS

AUC(0-4)/mg doseCsA/kg was significantly higher (P=0.024) in (C/C)3435 individuals than in a grouped population of (C/T)3435 and (T/T)3435 patients on postoperative day 3. G2677T variants were not significantly correlated with CsA absorption (P=0.084). The number of C3435-G2677 haplotypes was the best predictor of CsA exposure. At 1 month posttransplant, no correlation was seen between MDR1 SNPs and CsA exposure. The frequency of wild-type variants for C3435T and G2677T were 61% and 77.6%, respectively. SNPs at G2677T and C3435T loci were found to be in linkage disequilibrium.

CONCLUSIONS

MDR1 polymorphisms are associated with differences in CsA exposure only in the first posttransplant week.

Effects of genetic polymorphisms on the pharmacokinetics of calcineurin inhibitors

PURPOSE

The effects of genetic polymorphisms on the pharmacokinetics of calcineurin inhibitors were examined.

SUMMARY

The bioavailability and metabolism of cyclosporine and tacrolimus are primarily controlled by efflux pumps and members of the cytochrome P-450 (CYP) isoenzyme system found in the liver and gastrointestinal tract. The number and severity of adverse effects from these drugs are related to the overall exposure, measured by length of therapy and blood drug concentration. One contributing factor to the inconsistent pharmacokinetics of calcineurin inhibitors may be variable expression of functional CYP3A4, CYP3A5, and P-glycoprotein (PGP) efflux pumps, which may be the result of single-nucleotide polymorphisms found on the genes encoding for CYP3A4, CYP3A5, and PGP. CYP3A5*3 and CYP3A5*6 are the most common polymorphisms of CYP3A5. Using genetic markers to adjust initial doses of cyclosporine or tacrolimus may prove difficult, considering the variety of polymorphism known to affect CYP3A4, CYP3A5, and the multidrug resistance-1 (MDR1) gene (the gene that codes for PGP). Studies have found that carriers of CYP3A5*1 consistently have higher clearance rates of tacrolimus than do CYP3A5*3 homozygotes. The influences of CYP3A5 alleles on cyclosporine metabolism and the MDR1 C3435T polymorphism on tacrolimus metabolism remain controversial.

CONCLUSION

For renal transplant recipients receiving tacrolimus as an immunosuppressant, practitioners can expect CYP3A5*1 carriers to have a tacrolimus clearance 25-45% greater than that of CYP3A5*3 homozygotes, with proportional dosing needs to maintain adequate immunosuppression. Since inadequate immunosuppression is linked to graft rejection, evaluation of CYP3A5 polymorphisms may be helpful in determining an appropriate starting dosage, rapidly achieving adequate immunosuppression, and ultimately improving the outcome of renal transplantation.

Multi-drug resistance gene-1 (MDR-1) haplotypes and the CYP3A5*1 genotype have no influence on ciclosporin dose requirements as assessed by C0 or C2 measurements

The intestinal efflux pump P-glycoprotein (P-gp), the product of the multi-drug resistance-1 (MDR-1) gene, significantly influences the pharmacokinetics of several drugs. Ciclosporin is a substrate for P-gp and is metabolized by cytochrome P450 (CYP) 3A enzymes. P-gp activity is affected by several known single nucleotide polymorphisms (SNPs) and haplotypes. MDR-1 genotypes of SNPs C1236T, G2677T/A and C3435T, as well as haplotypes C-G-C and T-T-T and CYP3A5*1 genotype (predictive of CYP3A5 expression), were related to ciclosporin blood concentrations measured at both 0 and 2 h after drug dosing in 197 stable renal transplant patients. Significant differences (of a magnitude unlikely to be relevant clinically) in dose-normalized blood ciclosporin concentrations were found only between MDR-1 genotypes of the C1236T SNP and between haplotype groups C-G-C and T-T-T in patients that were expressers of CYP3A5. MDR-1 SNPs and haplotypes and also CYP3A5*1 genotype, do not appear to have a major influence on ciclosporin pharmacokinetics.

Role of pharmacogenetics of ATP-binding cassette transporters in the pharmacokinetics of drugs

Interindividual differences of drug response are an important cause of treatment failures and adverse drug reactions. The identification of polymorphisms explaining distinct phenotypes of drug metabolizing enzymes contributed in part to the understanding of individual variations of drug plasma levels. However, bioavailability also depends on a major extent from the expression and activity of drug transport across biomembranes. In particular efflux transporters of the ATP-binding cassette (ABC) family such as ABCB1 (P-glycoprotein, P-gp), the ABCC (multidrug resistance-related protein, MRP) family and ABCG2 (breast cancer resistance protein, BCRP) have been identified as major determinants of chemoresistance in tumor cells. They are expressed in the apical membranes of many barrier tissue such as the intestine, liver, blood-brain barrier, kidney, placenta, testis and in lymphocytes, thus contributing to plasma, liquor, but also intracellular drug disposition. Since expression and function exhibit a broad variability, it was hypothesized that hereditary variances in the genes of membrane transporters could explain at least in part interindividual differences of pharmacokinetics and clinical outcome of a variety of drugs. This review focuses on the functional significance of single nucleotide polymorphisms (SNP) of ABCB1, ABCC1, ABCC2, and ABCG2 in in vitro systems, in vivo tissues and drug disposition, as well as on the clinical outcome of major indications.

MDR1 C3435T polymorphisms correlate with cyclosporine levels in de novo renal recipients

BACKGROUND

Single nucleotide polymorphisms (SNPs) in the multidrug resistance (MDR1) gene correlate with the intestinal function of P-glycoprotein (PGP). PGP serves as a hydrophobic export pump that extrudes cyclosporine (CsA) across the luminal membrane thus preventing CsA absorption. These genetic variants may predict CsA exposure levels in the early posttransplantation period.

METHODS

CsA absorption profiles were established in 75 renal transplant patients using total daily dose and body weight adjusted 4-hour area under the time-concentration curve, AUC(0-4)/mg dose/kg body weight, on posttransplant day 3. These patients were subsequently genotyped for C3435T and G2677T polymorphisms using real-time polymerase chain reaction. An analysis was conducted to assess the independent impact of C3435T and G2677T SNPs on CsA bioavailability.

RESULTS

C3435T polymorphisms were found to be an independent predictor of CsA AUC(0-4)/mg dose/kg levels on postoperative day 3. An inverse correlation was found between the number of T alleles and AUC values such that every T allele was associated with an approximate 15% decrement in AUC(0-4)/mg dose/kg (P = .034). A similar nonsignificant trend was observed for G2677T polymorphisms.

CONCLUSIONS

MDR1 SNPs are correlated with CsA exposure in the early post-transplant period. Polymorphisms, in conjunction with other criteria, may become a useful tool to optimize initial drug dosing in renal transplantation.

Multidrug resistance gene-1 polymorphisms and resistance to cyclosporine A in patients with steroid resistant ulcerative colitis

BACKGROUND

Cyclosporine A (CsA) is inconstantly effective in inducing remission in acute attacks of ulcerative colitis (UC) not responding to steroids. This study aimed to establish whether multidrug resistance gene (MDR)1 polymorphisms would be associated with CsA failure.

PATIENTS AND METHODS

The distribution of the different genotypes of single nucleotide polymorphisms (SNP) G2677T/A and C3435T of MDR1 exons 21 and 26, respectively, was studied in 154 patients (mean age, 44 yr) who had received CsA to treat severe attacks of steroid resistant UC in 11 centers in France and Belgium. Patients were classified as CsA failure (n = 50) when they needed colectomy within 30 days after CsA initiation. The SNPs were detected by use of a 5' nuclease allelic discrimination assay.

RESULTS

There was a significant association between the G2677T/A polymorphism distribution (exon 21) and the risk for CsA failure (P = 0.0001). The TT genotype of exon 21 was significantly associated with the risk compared with the two other genotypes (odds ratio, 3.77; 95% confidence interval, 1.42-9.97, P = 0.007). There was no significant association between the genotype C3435T distribution (exon 26) and the risk of CsA failure (P = 0.23).

CONCLUSION

The TT genotype of exon 21 MDR1 polymorphisms is associated with a higher risk of CsA failure in patients with steroid resistant UC. Further studies should be performed to establish whether other treatments could be more efficient to avoid surgery in this subset of patients.

Reduced methylprednisolone clearance causing prolonged pharmacodynamics in a healthy subject was not associated with CYP3A5*3 allele or a change in diet composition

The influence of diet and genetics was investigated in a healthy white person who had distinctly low methylprednisolone clearance. Pharmacokinetic and pharmacodynamic parameter values were similar on 2 occasions during the consumption of a low-carbohydrate diet and a Weight Watchers diet, indicating that the decreased clearance was unlikely attributable to a change in diet composition. Although the subject was found to be homozygous for CYP3A5*3, genetic findings were not significant for a number of other CYP3A4 and CYP3A5 allelic variants. Because of the high prevalence of CYP3A5*3/*3 in whites and because 5 of 7 white control subjects are also homozygous for CYP3A5*3, this genotype cannot fully explain the reduced metabolism of the drug. Other genetic or contributing factors might have been involved. New polymerase chain reaction-based genotyping methods for functionally defective CYP3A5*6, *8, *9, and *10 alleles were developed in this study. These assays will be useful for CYP3A5 genotype analysis in future clinical studies.

Prediction of systemic exposure to cyclosporine in Japanese pediatric patients

The monitoring of the blood concentration at 2 h (C(2)) after the oral administration of a cyclosporine (CsA) microemulsion was reconfirmed to be useful for the prediction of systemic exposure, the area under the blood concentration-time curve from 0 to 4 h (AUC(0-4)), in a group of Japanese patients, consisting of 33 children aged 5-15 years and 19 young adults aged 16-27 years, with a greater correlation for C(2) (r = 0.927) than the trough concentration (r = 0.488). The dose-normalized AUC(0-4) was independent of gender or indications for CsA, while it depended on body size, i.e., the age (P = 0.065) and total body weight (P = 0.026). MDR1 C3435T had a weak, but insignificant effect (P = 0.072); it was about 22-31% lower in the patients with TT(3435). Co-administration of a steroid and further treatment with nifedipine had a more intensive effect (P = 0.018); co-administration resulted in a 51% increase in the dose-normalized AUC(0-4). A strong effect was also observed for the serum total cholesterol level (P = 0.001). Collectively, the discrepancies in the results on MDR1 C3435T among investigators might be due to variability in the age/total body weight, co-administration drugs or serum lipid level.

The pharmacogenetics of calcineurin inhibitors: one step closer toward individualized immunosuppression?

The immunosuppressive drugs cyclosporin (CsA) and tacrolimus (Tac) are widely used to prevent acute rejection following solid-organ transplantation. However, the clinical use of these agents is complicated by their many side effects, a narrow therapeutic index and highly variable pharmacokinetics. The variability in CsA and Tac disposition has been attributed to interindividual differences in the expression of the metabolizing enzymes cytochrome P450 (CYP) 3A4 and 3A5, and in the expression of the drug transporter P-glycoprotein (encoded by the ABCB1 gene, formerly known as the multidrug resistance 1 gene). Variation in the expression of these genes could in turn be explained by several recently-identified single nucleotide polymorphisms (SNPs). Determination of these SNPs in (future) transplant recipients has the potential to identify individuals who are at risk of under-immunosuppression or the development of adverse drug reactions. Ultimately, genotyping for CYP3A and ABCB1 may lead to further individualization of immunosuppressive drug therapy for the transplanted patient.

Implications of genetic polymorphisms in drug transporters for pharmacotherapy

Drug transporters are increasingly recognized as a key determinant of drug disposition and response. It is now widely appreciated that expression of the ATP-dependent efflux transporter, MDR1 (ABCB1, P-glycoprotein), in organs such as the gastrointestinal tract, liver and kidney significantly alters the extent of drug absorption and excretion. Moreover, expression of MDR1 at the level of the blood-brain barrier limits the entry of many drugs into the central nervous system. Given such an important role of MDR1 in the drug disposition process, it is not surprising to see increasing focus on the role of single nucleotide polymorphisms (SNPs) in this transporter as a potential determinant of interindividual variability in drug disposition and pharmacological response. However, drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. A growing list of membrane-spanning proteins involved in the in- or outward transport of a large variety of drugs has been recognized and characterized over the past few years in almost all tissues, including organic anion and cation transporters (OAT, OCT, solute carrier family SLC22A), organic anion transport proteins (OATP, solute carrier family SLCO, formerly SLC21A), and MRPs (ABCCs), other members of the ATP-binding cassette family. We are just beginning to appreciate their role for drug delivery and disposition and the contribution of genetic polymorphisms in these transport proteins to interindividual variability in the efficacy and safety for pharmacotherapy. This review summarizes the consequences of inherited differences in drug transport for pharmacotherapy. With the main focus on ABCB1, an update of recent advances is given and clinically relevant examples are used to illustrate how heritable differential drug transport can help to explain individual variability in drug response. The pharmacogenetics of other transporters is briefly introduced.

MDR1 genotype-related pharmacokinetics: fact or fiction?

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

Does pharmacogenetics have the potential to allow the individualisation of immunosuppressive drug dosing in organ transplantation?

The immunosuppressive drugs used in organ transplantation have a narrow therapeutic index, with rejection occurring as a consequence of underdosing and infection, malignancy and a number of drug-specific side effects with excessive dosing. Significant heterogeneity in the dose of drug required to achieve therapeutic blood concentrations adds to the complexity of the problem, which has been partly resolved by therapeutic drug monitoring. Single nucleotide polymorphisms have been identified in genes encoding metabolic enzymes, drug efflux pumps and drug targets for most of the drugs in widespread use. A pharmacogenetic approach to immunosuppressive drug prescribing remains to be tested. Based on current evidence, the most promising strategy would be use of the cytochrome P450 3A5 expressor genotype to guide initial dosing with tacrolimus.

Bayesian estimation of cyclosporin exposure for routine therapeutic drug monitoring in kidney transplant patients

AIMS

AUC-based monitoring of cyclosporin A (CsA) is useful to optimize dose adaptation in difficult cases. We developed a population pharmacokinetic model to describe dose-exposure relationships for CsA in renal transplant patients and applied it to the Bayesian estimation of AUCs using three blood concentrations.

METHODS

A total of 84 renal graft recipients treated with CsA microemulsion were included in this study. Population pharmacokinetic analysis was conducted using NONMEM. A two-compartment model with zero-order absorption and a lag time best described the data. Bayesian estimation was based on CsA blood concentrations measured before dosing and 1 h and 2 h post dose. Predictive performance was evaluated using a cross-validation approach. Estimated AUCs were compared with AUCs calculated by the trapezoidal method. The Bayesian approach was also applied to an independent group of eight patients exhibiting unusual pharmacokinetic profiles.

RESULTS

Mean population pharmacokinetic parameters were apparent clearance 30 l h(-1), apparent volume of distribution 79.8 l, duration of absorption 52 min, absorption lag time 7 min. No significant relationships were found between any of the pharmacokinetic parameters and individual characteristics. A good correlation was obtained between Bayesian-estimated and experimental AUCs, with a mean prediction error of 2.8% (95% CI [-0.6, 6.2]) and an accuracy of 13.1% (95% CI [7.5, 17.2]). A good correlation was also obtained in the eight patients with unusual pharmacokinetic profiles (r(2) = 0.96, P < 0.01).

CONCLUSIONS

Our Bayesian approach enabled a good estimation of CsA exposure in a population of patients with variable pharmacokinetic profiles, showing its usefulness for routine AUC-based therapeutic drug monitoring.

Potential role of drug transporters in the pathogenesis of medically intractable epilepsy

The pathogenesis underlying pharmacoresistance in epilepsy is unclear. One of the candidate mechanisms that has attracted growing interest is the limitation of antiepileptic drug (AED) access to the seizure focus by a range of efflux transporters, the prototype of which is P-glycoprotein (P-gp). P-gp is encoded by the multidrug resistance (MDR1 or ABCB1) gene. Predominantly expressed in organs with excretory functions and at blood-tissue barriers, P-gp is thought to act as a physiologic defense by extruding xenobiotics from mammalian cells and affording protection of sensitive organs. The high level of P-gp in the cerebrovascular endothelium is believed to contribute to the functionality of the blood-brain barrier. Overexpression of P-gp causes multidrug resistance in certain cancers. It has been hypothesized that overexpression of P-gp and other efflux transporters in the cerebrovascular endothelium, in the region of the epileptic focus, also may lead to drug resistance in epilepsy. This hypothesis is supported by the findings of elevated expression of efflux transporters in epileptic foci in patients with drug-resistant epilepsy, induction of expression by seizures in animal models, and experimental evidence that some commonly used AEDs are substrates. Conflicting reports suggest a possible association between variants of the MDR1 gene and medical intractability in epilepsy. Further studies to delineate the exact role, if any, of P-gp and other efflux transporters in drug-resistant epilepsy are warranted.

Clinical pharmacogenetics of immunosuppressive drugs in organ transplantation

Organ transplantation has become an important additional option for patients with organ failure. Immunosuppressive drugs showing a very narrow therapeutic window have to be administered. Different transporters and metabolic pathways are responsible for absorption and metabolism of these drugs; for instance, the P-glycoprotein (P-gp) pump regulates the absorption of a drug, and its metabolism is catalyzed by cytochrome P450s (CYPs). As the phenotypes of P-gp or the CYPs are predetermined by their genotypes, genetic testing prior to drug therapy may help to predict the drug doses required. This review describes polymorphisms of the genes coding for P-gp and CYPs, and focuses on the compounds cyclosporin and tacrolimus. It is hoped that this information might help to judge the value of pharmacogenetic testing prior to immunosuppressive therapy in solid organ transplantation.