Impact of CYP3A5 and MDR1(ABCB1) C3435T polymorphisms on the pharmacokinetics of tacrolimus in renal transplant recipients

Monitoring peripheral blood CD4+ adenosine triphosphate activity after living donor liver transplantation: impact of combination assays of immune function and CYP3A5 genotype

BACKGROUND

The immune response after living donor liver transplantation (LDLT) was evaluated, with specific attention focused on the inter-relationship among clinical conditions, pharmacokinetics of tacrolimus, and cytochrome P450 3A5 (CYP3A5) genotypes.

METHODS

Forty 40 adult patients who underwent LDLT in the period 2002-2009 were enrolled in the study. Peripheral blood was collected from these patients between June 2009 and December 2009 and analyzed for CD4+ adenosine triphosphate (ATP) activity (ImmuKnow assay), tacrolimus concentration, and CYP3A5 genotype. Based on the results of the ImmuKnow assay (i.e., strength of immune response), each patient was categorized into one of the three established zones of immune response: Group A (low response, n = 13), Group B (moderate response, n = 24), and Group C (strong response, n = 3).

RESULTS

There was no correlation between the concentration of tacrolimus and ATP levels in the blood. The patients in Group A required much higher tacrolimus doses to maintain the blood concentration, as evidenced by the tacrolimus concentration/dose (C/D) ratios in Group A being significantly lower than those in Group B. Almost half of the patients in Group A suffered from infectious complications. The C/D ratios were significantly lower in the six patients with the CYP3A5 1 allele than in the 14 patients with the CYP3A5 3 allele; one-half of the patients with the CYP3A5 1 allele belonged to Group A.

CONCLUSIONS

Our results demonstrate that the immunKnow assay of immune function is an excellent tool for monitoring immune response, especially in the patients with CYP3A5 1 allele (expressors).

Developmental pharmacogenetics of immunosuppressants in pediatric organ transplantation

Cyclosporine, tacrolimus, sirolimus, and mycophenolate mofetil are the primary immunosuppressants used on pediatric organ transplantation. Therapeutic drug monitoring is used in daily practice, because their clinical use is hampered by a narrow therapeutic index and large variability. Tailoring immunosuppressive therapy to the individual patient to optimize efficacy and minimize toxicity is therefore essential. Because research in pharmacogenetics already identified polymorphisms impacting their pharmacokinetic parameters in adults, developmental pharmacogenetics of immunosuppressants holds promises for optimizing dosage regimens and improving clinical outcome in children. In this review, we focus on the impact of age and pharmacogenetics on these immunosuppressants in children undergoing organ transplantation.

Pharmacogenetic testing and therapeutic drug monitoring are complementary tools for optimal individualization of drug therapy

Genetic factors contribute to the phenotype of drug response, but the translation of pharmacogenetic outcomes into drug discovery, drug development or clinical practice has proved to be surprisingly disappointing. Despite significant progress in pharmacogenetic research, only a few drugs, such as cetuximab, dasatinib, maraviroc and trastuzumab, require a pharmacogenetic test before being prescribed. There are several gaps that limit the application of pharmacogenetics based upon the complex nature of the drug response itself. First, pharmacogenetic tests could be more clinically applicable if they included a comprehensive survey of variation in the human genome and took into account the multigenic nature of many phenotypes of drug disposition and response. Unfortunately, much of the existing research in this area has been hampered by limitations in study designs and the nonoptimal selection of gene variants. Secondly, although responses to drugs can be influenced by the environment, only fragmentary information is currently available on how the interplay between genetics and environment affects drug response. Third, the use of a pharmacogenetic test as a standard of care for drug therapy has to overcome significant scientific, economic, commercial, political and educational barriers, among others, in order for clinically useful information to be effectively communicated to practitioners and patients. Meanwhile, the lack of efficacy is in this process is quite as costly as drug toxicity, especially for very expensive drugs, and there is a widespread need for clinically and commercially robust pharmacogenetic testing to be applied. In this complex scenario, therapeutic drug monitoring of parent drugs and/or metabolites, alone or combined with available pharmacogenetic tests, may be an alternative or complementary approach when attempts are made to individualize dosing regimen, maximize drug efficacy and enhance drug safety with certain drugs and populations (e.g. antidepressants in older people).

Pharmacogenetics of immunosuppressant polymorphism of CYP3A5 in renal transplant recipients

The tacrolimus is metabolized primarily by CYP3A5, a member of the single nucleotide polymorphism family. It shows cytochrome P450 (SNP) in intron 3, which consists of a change of base, G for A, producing a stop codon. The result is a nonfunctional protein (allele *3). Allele *1 is the wild type. The patients that show the allelic variant *3 in homozygosis (G/G) are slow metabolizers of the immunosuppressant, increasing its concentration in blood. In contrast, heterozygote A/G alleles *1/*3 are intermediate metabolizers, whereas those of allele *1 in homozygosis (A/A) are normal metabolizers. The aim of this study was to determine CYP 3A5 polymorphism among adult renal transplant recipients and the general Argentinean population. We analyzed 21 recipients and 36 healthy controls. All subjects gave written informed consent approved by the local committee. To determine the polymorphism, we extracted DNA from peripheral blood and used polymerase chain reaction (PCR) to amplify intron 3 of the CYP 3A5. The presence of variant was confirmed by direct sequencing. Among the controls the CYP3A5 genotype *3/*3 (G/G) was detected in 32 individuals, 4 showed *1/*3 (A/G), and none had *1/*1 (A/A); among the recipients, the results were as follows: 18, 2, and 1, respectively. The frequencies of polymorphism in both groups were similar, although they differed from those published for other populations. These results are the basis for the development of a pharmacogenomic program applied to organ transplantation. The genetic polymorphisms can determine responses to drugs. The molecular diagnosis must be transferred to clinical practice so as to guide selection of medicine and drug doses to be optimal for each individual.

The occurrence of diarrhea not related to the pharmacokinetics of MPA and its metabolites in liver transplant patients

PURPOSE

Mycophenolate mofetil (MMF) is a pro-drug that is hydrolyzed to release mycophenolic acid (MPA). Subsequently MPA is extensively metabolized to phenyl mycophenolic acid glucuronide (MPAG) and MPA acyl glucuronide (AcMPAG). It was presumed that the closest association is between plasma AcMPAG concentrations and the incidence of diarrhea. This study aimed to investigate the correlation between pharmacokinetics of MPA, MPAG, and AcMPAG and diarrhea in liver transplant recipients on MMF with tacrolimus.

METHODS

Sixty-seven patients receiving liver transplantation were included. The pharmacokinetics of MPA and its metabolites were monitored repeatedly in the early stage (within 2 weeks) and in the late stage after transplant. The plasma concentrations of MPA, MPAG, and AcMPAG were determined by the HPLC method.

RESULTS

Twenty-two patients (32.8%) suffered from episodes of diarrhea. Compared with the data from the early stage, AUC(0-12h) of MPA, MPAG, and AcMPAG increased significantly in both groups in the later stage. AUC(0-12h) of MPA, MPAG, and AcMPAG were not different significantly between the group with diarrhea and the group without diarrhea, either in the early stage or in the late stage (P > 0.05).

CONCLUSION

These results suggest that systemic exposures to MPA and its metabolites are not associated with the incidence of diarrhea in liver transplant recipients.

Influence of ABCB1 genetic polymorphisms on the pharmacokinetics of levosulpiride in healthy subjects

The purposes of this study were to clarify the involvement of P-glycoprotein in the absorption of levosulpiride in knockout mice that lack the Abcb1a/ 1b gene, and to evaluate the relationship between genetic polymorphisms in ABCB1 (exon 12, 21 and 26) and levosulpiride disposition in healthy subjects. The plasma and brain samples were obtained after oral administration (10 microg/g) of levosulpiride to abcb1a/1b(-/-) and wild-type mice (n=3 approximately 6 at each time point). The average brain-to-plasma concentration ratio and blood-brain barrier partitioning of levosulpiride were 2.3- and 2.0-fold higher in Abcb1a/1b(-/-) mice than in wild-type mice, respectively. A total of 58 healthy Korean volunteers receiving a single oral dose of 25 mg levosulpiride participated in this study. The subjects were evaluated for polymorphisms of the ABCB1 exon 12 C1236T, exon 21 G2677A/T (Ala893Ser/Thr) and exon 26 C3435T using polymerase chain reaction restriction fragment length polymorphism. The PK parameters (AUC(0-4h), AUC(0-infinity) and C(max.)) of ABCB1 2677TT and 3435TT subjects were significantly higher than those of subjects with at least one wild-type allele (P<0.05). The results indicate that levosulpiride is a P-glycoprotein substrate in vivo, which is supported by the effects of SNPs 2677G>A/T in exon 21 and 3435C>T in exon 26 of ABCB1 on levosulpiride disposition.

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.

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.

No impact of age on dose-adjusted pharmacokinetics of tacrolimus, mycophenolic acid and prednisolone 1 month after renal transplantation

PURPOSE

The purpose of the study was to assess the impact of age on the pharmacokinetics of immunosuppressive drugs.

METHODS

One hundred and ten renal transplant recipients, including 12 elderly patients over 60 years of age, 57 middle-aged patients between 40 and 59 years and 41 young adult patients 20 to 39 years of age were studied. To evaluate dose-adjusted pharmacokinetics and cytochrome P450 (CYP) 3A5 pharmacogenetics, the concentrations of tacrolimus, mycophenolic acid (MPA), MPA glucuronide (MPAG) and prednisolone were measured at 1 month post-transplantation.

RESULTS

There were no differences in dose (D) and body weight (BW)-adjusted pharmacokinetic parameters of tacrolimus among the three groups. D/BW-adjusted C(max), C(0) and AUC(0-12) values of tacrolimus were significantly greater in patients with the CYP3A5*3/*3 genotype than in those with the CYP3A5*1 allele in young and middle-aged patients as previously reported, but not in the elderly. There were no significant differences in the D-adjusted pharmacokinetics of prednisolone and MPA among the three groups.

CONCLUSION

The aging process itself may have a small effect on the pharmacokinetics of tacrolimus, MPA, or prednisolone. However, a larger number of subjects need to be studied to confirm the impact of age on the CYP3A5 pharmacogenetics of tacrolimus in the elderly.

Effect of CYP3A5 genotype on renal allograft recipients treated with tacrolimus

OBJECTIVE

Tacrolimus concentrations are associated with CYP3A5 genotype. The purpose of this study was to evaluate the outcomes and drug concentrations/doses among a posttransplant population with various CYP3A5 genotypes within 12 months.

METHODS

Sixty seven kidney recipients receiving immunosuppression with tacrolimus + mycophenolate mofetil + prednisolone were grouped according to their CYP3A5 genotypes (*1/*1; *1/*3; *3/*3). The initial dose of tacrolimus (0.15 mg/kg/d) was adjusted according to achieve a target therapeutic window. All patients underwent a protocol biopsy at 1 month posttransplantation. We assayed serum creatinine and tacrolimus blood trough concentrations to calculate the concentration per dosage during follow-up. We also investigated the incidence of acute rejection episodes and the nephrotoxicity of tacrolimus according to the renal biopsy.

RESULTS

There was no significant difference among serum creatinine concentrations. Tracrolimus blood concentrations showed a significant difference at day 7 and 1 month with no significant difference at 3, 6, or 12 months among the three groups. The CYP3A5*3/*3 group showed the largest concentration per dosage (C/D) and CYP3A5*1/*1, the smallest C/D. There was a significant difference among the three groups. The occurrence of an acute rejection episode within 3 months showed a significant difference among the three groups but not from 3 to 12 months after transplantation. Nephrotoxicity was greatest among the CYP3A5*3/*3 group.

CONCLUSION

CYP3A5 influenced the blood concentrations of tacrolimus. Our study suggested to choose the initial dosage according to the CYP3A5 genotype to obtain a better outcome and reduce the incidences of acute rejection episodes and nephrotoxicity.

CYP3A polymorphisms and immunosuppressive drugs in solid-organ transplantation

Most immunosuppressive drugs have a narrow therapeutic index and large interpatient variabilities in their pharmacokinetic and pharmacodynamic profiles. Identification of functional single-nucleotide polymorphisms in genes encoding for drug metabolizing enzymes has great potential to improve the drug efficacy and safety profiles, since these genetic factors may be important biomarkers for individualization of immunosuppressive therapy. This article summarizes current knowledge regarding the impact of CYP3A polymorphisms on immunosuppressive drug pharmacokinetics. Many retrospective studies have shown a clear relationship between CYP3A5*1/*3 polymorphism and tacrolimus pharmacokinetics, while the influence of CYP3A5*1/*3 or CYP3A4*/*1B on ciclosporin and sirolimus exposure are still questionable. CYP3A polymorphisms may partially contribute to the clinical variability of the enzyme-mediated drug interactions. Drug-drug interactions may also influence the phenotypic consequence of CYP3A polymorphisms. Population pharmacodynamic/kinetic/genomic modeling was proposed as an emerging and promising approach to quantitatively explore the contribution of genetic polymorphisms to the large interpatient variability in the pharmacokinetic and pharmacodynamic profiles of immunosuppressive drugs. Prospective, randomized studies in large patient populations are needed to further clarify the genetic effects of CYP3A on immunosuppressive drug exposure and response.

Tacrolimus concentrations in relation to CYP3A and ABCB1 polymorphisms among solid organ transplant recipients in Korea

BACKGROUND

Cytochrome P450 3A (CYP3A) and the drug transporter P-glycoprotein (P-gp) affect the bioavailability of tacrolimus, the most commonly used immunosuppressive agent in organ transplant recipients. We have determined the genotypic frequencies of the CYP3A and ATP-binding cassette sub-family B member 1 (ABCB1) genes, which encode the CYP3A and P-gp proteins, respectively, in Korean organ transplant recipients and donors, and have assessed the influence of CYP3A and ABCB1 polymorphisms on tacrolimus concentrations.

METHODS

Using chip-based MALDI-TOF mass spectrometry, 506 solid organ transplant recipients and 62 corresponding of liver transplant donors were genotyped for CYP3A4*6, CYP3A4*18, CYP3A5*3, CYP3A5P1*3, ABCB1 c.2677G>A/T, and ABCB1 c.3435C>T alleles, and their steady-state blood concentrations of tacrolimus were measured.

RESULTS

Frequencies of variant alleles among the transplant recipients were CYP3A5*3 76.8%, CYP3A5P1*3 75.9%, ABCB1 c.2677A/T 52.8%, ABCB1 c.3435T 36.9%, CYP3A4*18 1.9%, and CYP3A4*6 0.3%. The CYP3A5P1*3 allele was strongly linked to the CYP3A5*3 allele (r=0.816). Patients with the CYP3A5*3 and CYP3A5P1*3 alleles showed higher blood tacrolimus concentrations per adjusted dose ratio than did patients with wild-type alleles, among both liver transplant donors and renal transplant recipients.

CONCLUSION

The CYP3A5 genotype of the liver is considered to show the most important association with tacrolimus concentrations. Ultimately, genotyping for CYP3A5 may help optimal individualization of immunosuppressive drug therapy for patients undergoing solid organ transplantation.

Influence of CYP3A5 genetic polymorphism on tacrolimus daily dose requirements and acute rejection in renal graft recipients

Tacrolimus is a widely used immunosuppressive drug in organ transplantation. Its oral bioavailability varies greatly between individuals, and it is a substrate of cytochrome P450 3A (CYP3A) and P-glycoprotein. Our objective was to determine the influence of CYP3A5 and ABCB1 genetic polymorphisms on tacrolimus daily requirements and on transplantation outcome. One hundred and thirty-six renal graft recipients treated with tacrolimus were genotyped for CYP3A5 (6986A>G), ABCB1 exon26 (3435C>T) and exon21 (2677G>T/A) single nucleotide polymorphisms. Genotypes were correlated to tacrolimus daily dose at 1-week, 1-, 6- and 12-month post-transplantation and with transplantation outcome. At 1-month post-transplantation, tacrolimus daily dose was higher for patients with CYP3A5*1/*1 genotype compared to CYP3A5*3/*3 genotype (0.26 +/- 0.03 versus 0.16 +/- 0.01 mg/kg/day, respectively, P < 0.0001). Similar results were obtained at 6- and 12-month post-transplantation. Furthermore, CYP3A5*1 homozygotes were associated with increased risk of acute rejection episodes compared to patients with CYP3A5*1/*3 and CYP3A5*3/*3 genotypes (38% versus 10% and 9%, respectively, P = 0.01). CYP3A5 genetic polymorphism was not associated with tacrolimus-related nephrotoxicity. ABCB1 polymorphisms were not related with transplantation outcome. CYP3A5 genetic polymorphism appeared in our study to affect tacrolimus daily dose requirements and transplantation outcome. Screening for this single nucleotide polymorphism before the transplantation might be helpful for the selection of adequate initial daily dose and to achieve the desired immunosuppression.

CYP3A5 *1 allele associated with tacrolimus trough concentrations but not subclinical acute rejection or chronic allograft nephropathy in Japanese renal transplant recipients

PURPOSE

We assessed reported associations of CYP3A5 *1 allele with a delay in achieving target tacrolimus concentrations, and occurrence of biopsy-confirmed subclinical acute rejection (SAR) and chronic allograft nephropathy (CAN) in Japanese subjects.

METHODS

Forty-one renal allograft recipients were studied. The targeted tacrolimus trough concentrations were 20-25 ng/mL up to 2 weeks post-transplantation, 10-15 ng/mL up to 6 weeks, and 5-10 ng/mL thereafter. At 1 month and 1 year post-transplantation, allograft biopsies were performed.

RESULTS

The CYP3A5 *1/*1 + *1/*3 (expresser) and *3/*3 (nonexpresser) alleles were detected in 19 and 22 patients, respectively. Although the mean trough concentrations were lower in CYP3A5 expressers than nonexpressers for the first 3 weeks, no difference in frequency of SAR among CYP3A5 genotypes was found. The mean trough concentrations were lower from 8 to 12 months post-transplantation, and the frequency of CAN was lower in CYP3A5 expressers.

CONCLUSIONS

In contrast to the previous reports, the CYP3A5 *1 allele was not associated with the frequency of SAR or CAN, suggesting that further studies of different immunosuppressive strategies using tacrolimus are needed to confirm the adequate dosing and concentration of tacrolimus for each CYP3A5 genotype.

Genetic prediction of renal transplant outcome

PURPOSE OF REVIEW

In the present study, we reviewed recent publications regarding potential genetic predictors of transplant outcome, including acute rejection, metabolism of immunosuppressive medications, long-term transplant outcome, and posttransplant complications.

RECENT FINDINGS

Recent studies are based on the candidate gene analysis; specifically, cytokine genes and innate immune response molecules present popular targets for studies of acute rejection and long-term outcome. Pharmacogenomic studies are mostly focused on the genes of drug targets or the corresponding enzymes metabolizing the drug. Posttransplant complications (i.e. infections, diabetes, and malignancies) are associated with a variety of genes of different pathways. Most of the studies are based on just recipient, but not donor, genotype analysis. Positive results are balanced by reports of no association when evaluating the same genetic polymorphisms. Some of the reports might be affected by insufficient study design, including small sample size, lack of adjustment for potential confounders, and multiple comparisons.

SUMMARY

The field remains controversial because of differences in populations, study design, and statistical methods. Studies are based on candidate gene analysis; however, genome-wide association studies are lacking. Future research should be directed at better designed studies, larger sample size, evaluating both recipients and donors, and implementation of genome-wide association studies.

Structure, function and regulation of P-glycoprotein and its clinical relevance in drug disposition

1. P-glycoprotein (P-gp/MDR1), one of the most clinically important transmembrane transporters in humans, is encoded by the ABCB1/MDR1 gene. Recent insights into the structural features of P-gp/MDR1 enable a re-evaluation of the biochemical evidence on the binding and transport of drugs by P-gp/MDR1. 2. P-gp/MDR1 is found in various human tissues in addition to being expressed in tumours cells. It is located on the apical surface of intestinal epithelial cells, bile canaliculi, renal tubular cells, and placenta and the luminal surface of capillary endothelial cells in the brain and testes. 3. P-gp/MDR1 confers a multi-drug resistance (MDR) phenotype to cancer cells that have developed resistance to chemotherapy drugs. P-gp/MDR1 activity is also of great clinical importance in non-cancer-related drug therapy due to its wide-ranging effects on the absorption and excretion of a variety of drugs. 4. P-gp/MDR1 excretes xenobiotics such as cytotoxic compounds into the gastrointestinal tract, bile and urine. It also participates in the function of the blood-brain barrier. 5. One of the most interesting characteristics of P-gp/MDR1 is that its many substrates vary greatly in their structure and functionality, ranging from small molecules such as organic cations, carbohydrates, amino acids and some antibiotics to macromolecules such as polysaccharides and proteins. 6. Quite a number of single nucleotide polymorphisms have been found for the MDR1 gene. These single nucleotide polymorphisms are associated with altered oral bioavailability of P-gp/MDR1 substrates, drug resistance, and a susceptibility to some human diseases. 7. Altered P-gp/MDR1 activity due to induction and/or inhibition can cause drug-drug interactions with altered drug pharmacokinetics and response. 8. Further studies are warranted to explore the physiological function and pharmacological role of P-gp/MDR1.

Influence of CYP3A5, ABCB1 and NR1I2 polymorphisms on prednisolone pharmacokinetics in renal transplant recipients

The objective of this study was to evaluate whether genetic polymorphisms of CYP3A5 (A6986G, CYP3A5*3), ABCB1 (C1236T, G2677T/A, C3435T) and NR1I2 (A7635G) significantly impact the pharmacokinetics of prednisolone in renal transplant recipients. Ninety-five recipients were given repeated doses of triple therapy immunosuppression consisting of prednisolone, tacrolimus and mycophenolate mofetil. Twenty-eight days after renal transplantation, plasma prednisolone concentrations were measured by high-performance liquid chromatography. Comparisons of the CYP3A5 and ABCB1 genotypes revealed no significant differences in the prednisolone pharmacokinetics. The mean prednisolone C(max) for recipients (n=14) having both the ABCB1 3435CC genotype and the CYP3A5*3/*3 genotype was significantly higher than those (n=11) having both ABCB1 3435TT and CYP3A5*3/*3 genotypes (180ng/mL versus 129ng/mL, P=0.0392). The plasma concentrations of prednisolone in recipients having both ABCB1 3435CC and CYP3A5*3/*3 genotypes tended to be higher than those having both ABCB1 3435TT and CYP3A5*3/*3 genotypes. The mean AUC(0-24) and C(max) values for prednisolone in recipients having the NR1I2 7635G allele (AG: n=45, GG: n=32) were significantly lower than in patients having the 7635AA allele (n=18) (7635GG versus 7635AA, P=0.0308 for AUC(0-24), P=0.0382 for C(max) of prednisolone). In conclusion, NR1I2 (A7635G) rather than CYP3A5 or ABCB1 allelic variants affected patient variability of plasma prednisolone concentration. Recipients carrying the NR1I2 7635G allele seemed to possess higher metabolic activity for prednisolone in the intestine, greatly reducing its maximal plasma concentration.

Lack of tacrolimus circadian pharmacokinetics and CYP3A5 pharmacogenetics in the early and maintenance stages in Japanese renal transplant recipients

AIMS

We investigated whether tacrolimus pharmacokinetics shows circadian variation and the influence of the CYP3A5 A6986G polymorphism on the pharmacokinetics in both the early and maintenance stages after renal transplantation.

METHODS

Tacrolimus was administered twice daily at specified times (09.00 and 21.00 h) throughout the pre- and post-transplant period according to the trough-targeting strategy. Fifty recipients with stable graft function were studied on day 28 and beyond 1-year post transplantation. Whole blood samples were collected prior to and 1, 2, 3, 4, 6, 9 and 12 h after both the morning and evening doses during hospitalization.

RESULTS

Tacrolimus pharmacokinetics did not show circadian variation in either the early or maintenance stage [AUC(0-12) 197.1 (95% confidence interval 182.9, 212.3) in daytime vs. 203.6 ng h ml(-1) (189.8, 217.4) in the night-time at day 28, 102.0 (92.1, 111.9) vs. 107.7 (97.9, 117.5) at 1 year, respectively]. In CYP3A5 *1 allele carriers (CYP3A5 expressers), body weight-adjusted oral clearance was markedly decreased from the early stage to the maintenance stage [0.622 (0.534, 0.709) to 0.369 l h(-1) kg(-1) (0.314, 0425)] compared with a smaller decrease [0.368 (0.305, 0.430) to 0.305 (0.217, 0.393)] in CYP3A5 non-expressers; however, the CYP3A5 genetic variation did not influence tacrolimus chronopharmacokinetics.

CONCLUSION

Equivalent daytime and night-time tacrolimus pharmacokinetics were achieved during both the early and maintenance stages with our specified-time administration strategy. The CYP3A5 polymorphism may be associated with the time-dependent changes in the oral clearance of tacrolimus, suggesting that genotyping of this polymorphism is useful for determining the appropriate dose of tacrolimus in both the early and maintenance stages after renal transplantation.

Tacrolimus pharmacokinetics and pharmacogenetics: influence of adenosine triphosphate-binding cassette B1 (ABCB1) and cytochrome (CYP) 3A polymorphisms

Tacrolimus, an immunosuppressant used after organ transplantation, has a narrow therapeutic range and its pharmacokinetic variability complicates its daily dose assessment. P-glycoprotein (P-gp), encoded by the adenosine triphosphate-binding cassette B1 (ABCB1) and the cytochrome (CYP) 3A4 and 3A5 enzymes appears to play a role in the tacrolimus metabolism. In the present study, two different renal transplant recipient groups were used to examine the influence of ABCB1 and CYP3A polymorphisms on the daily tacrolimus dose and several pharmacokinetic parameters. In total 63 Caucasian renal transplant recipients divided into 26 early [median (range) of the days since transplantation - 16 (3-74)] and 37 late [median (range) of the days since transplantation - 1465 (453-4128)] post-transplant recipients were genotyped for ABCB1 and CYP3A polymorphisms. The pharmacokinetic parameters of tacrolimus were determined for all renal transplant recipients and correlated with their corresponding genotypes. A significant difference in allele frequencies of the CYP3A4*1B (P = 0.028) and CYP3A5*1 (P = 0.022) alleles was observed between the early and late post-transplant recipient groups. Significantly higher dose-normalized trough levels (dnC(0)), dose-normalized area under the curve (dnAUC(0-12)), and dose-normalized maximum concentration (dnC(max)) were observed for carriers of the CYP3A5*3 variant allele in both renal transplant patient groups. Except for the daily tacrolimus dose (P = 0.025) no significant differences were observed for carriers of the CYP3A4*1B variant allele. Neither the individual ABCB1 polymorphisms nor the ABCB1 haplotypes were associated with any pharmacokinetic parameter. We noticed that patients carrying a CYP3A5*1 allele require a twofold higher tacrolimus dose compared with homozygous carriers of the CYP3A5*3 variant allele to maintain the target dnAUC(0-12). Therefore, genotyping for the CYP3A5*3 variant allele can contribute to a better and more individualized immunosuppressive therapy in transplant patients.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

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 tumor necrosis factor-alpha, interleukin-10, cytochrome P450 3A5 and ABCB1 in Chinese liver transplant patients treated with immunosuppressant tacrolimus

BACKGROUND

Cytokine production in the host immune response after transplantation may contribute to the variable CYP3A-dependent drug disposition. We investigated the effect of TNF-alpha, IL-10, CYP3A5 and ABCB1 polymorphisms on immunosuppressant tacrolimus pharmacokinetics in liver transplant patients.

METHODS

Genetic polymorphisms in TNF-alpha, IL-10, CYP3A5 and ABCB1 were studied in 70 liver transplant recipients and 70 donors. Tacrolimus dosage and blood concentration were investigated at 1, 2 and 3 weeks after transplantation.

RESULTS

The IL-10 G-1082A polymorphism in recipients was significantly associated with tacrolimus concentration/dose (C/D) ratios (IL-10-1082GG < GA < AA) within the first 3weeks posttransplantation (P < 0.05). Recipients with the capacity for low IL-10 production (-1082AA) carrying CYP3A5 non-expressor (CYP3A5()3/()3) liver had the highest C/D ratios (mean: 172.4, 161.7, 160.3 for 1, 2 and 3 weeks posttransplantation, respectively), whereas recipients with intermediate or high production of IL-10 (-1082GA or GG) engrafted with CYP3A5 expressor (CYP3A5()1 carrier) liver were found to have the lowest ratios (96.4, 78.0 and 75.4, respectively, P < 0.01).

CONCLUSIONS

The IL-10 G-1082A and CYP3A5()3 polymorphisms may influence the interindividual variability of tacrolimus pharmacokinetics in Chinese liver transplant patients. This finding provided a new interpretation for the variable immunosuprressant disposition after transplantation.

Influence of rabeprazole and lansoprazole on the pharmacokinetics of tacrolimus in relation to CYP2C19, CYP3A5 and MDR1 polymorphisms in renal transplant recipients

The objective of this study was to evaluate whether genetic polymorphisms of CYP2C19, CYP3A5 and MDR1 significantly impact the interaction between tacrolimus and rabeprazole or lansoprazole. Seventy-three recipients were randomly assigned after renal transplantation to receive repeated doses of tacrolimus for 28 days with a regimen of either 20 mg of rabeprazole or 30 mg of lansoprazole. Blood concentrations of tacrolimus were measured by microparticle enzyme immunoassay. The mean daily dose and the dose-adjusted area under the plasma concentration-time curves from 0 to 12 h (AUC(0-12)) of tacrolimus coadministered with rabeprazole or lansoprazole were the lowest and highest, respectively, in CYP2C19 poor metabolizers (PMs) having the CYP3A5*3/*3 genotype (0.084 and 0.112 mg/kg/day and 1.269 and 1.033 ng.h/ml/mg/kg, respectively). On the other hand, the mean dose-adjusted AUC(0-12) of tacrolimus coadministered with rabeprazole or lansoprazole were the highest in CYP2C19 PMs having the MDR13435CC+CT genotype, but not significantly. The present study indicates that there are significant interactions between tacrolimus and rabeprazole or lansoprazole in CYP2C19 PM renal transplant recipients bearing the CYP3A5*3/*3 genotypes. For recipients having these genetic polymorphisms, lower dosages of tacrolimus are required to achieve the target therapeutic index.

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.

Influence of Cytochrome P450 (CYP)??3A5 Polymorphisms on the Pharmacokinetics of Lansoprazole Enantiomers in CYP2C19 Extensive Metaboliser Renal Transplant Recipients

BACKGROUND AND OBJECTIVE

Lansoprazole is extensively metabolised by cytochrome P450 (CYP) 2C19 and CYP3A4. The purpose of this study was to evaluate the effects of CYP3A5 polymorphism (A6986G) on the pharmacokinetics of lansoprazole enantiomers in renal transplant recipients who are CYP2C19 extensive metabolisers (EMs).

METHODS

Among 40 Japanese CYP2C19 EMs, 20 had the CYP3A5*1 allele (*1/*1 in two subjects and *1/*3 in 18 subjects) and 20 had the CYP3A5*3/*3 genotype. After repeated oral doses of racemic lansoprazole 30mg once daily for 28 days, plasma concentrations of lansoprazole enantiomers were determined using high performance liquid chromatography.

RESULTS

The mean area under the plasma concentration-time curves from 0 to infinity (AUC(infinity)) of (R)- and (S)-lansoprazole in recipients with the CYP3A5*1 allele were 3145 and 384 ng * h/mL, respectively, compared with 4218 and 587 ng * h/mL in recipients with the CYP3A5*3/*3 genotype. The AUC(infinity) and the maximum plasma concentration of (R)- and (S)-lansoprazole in subjects with the CYP3A5*3/*3 genotype were greater than subjects with CYP3A5*1/*1 + *1/*3 alleles. The mean R/S ratio for AUC of lansoprazole in each CYP3A5 genotype group was the same (12.6).

CONCLUSION

Our findings show that CYP3A5 genotype is not an important determinant of enantioselective disposition of lansoprazole. Based on our results and those of previous studies, the enantioselective disposition of lansoprazole appears to be primarily influenced by enantioselective metabolism by CYP2C19 rather than by CYP3A.

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

It is currently not clear whether the concentration-time curves of the immunosuppressants differ with respect to the CYP3A5, MDR1, or MRP2 genotype in dose-adapted stable kidney transplant patients. Dose/trough concentration ratios were obtained in 134 tacrolimus and 20 sirolimus-treated patients, and plasma concentration-time profiles were obtained from 16 (tacrolimus) and 10 (sirolimus) patients. Genotyping was carried out for CYP3A5 6986A>G; ABCB1 2677G>T/A, 3435C>T and ABCC2 -24C>T; 1249G>A; 3972C>T. Dose/trough concentration ratios were 0.67+/-0.3 and 1.36+/-0.73 x 10(3) l (P<0.00001) for tacrolimus and 0.42+/-0.17 and 0.84+/-0.46 x 10(3) l (P=0.18) for sirolimus in CYP3A5 non-expressors and expressors. The unadjusted tacrolimus area under curve (AUC)(0-12) was 106.8+/-17.5 ng/ml x h compared with 133.3+/-42.2 ng/ml x h (P=0.37) without affecting serum creatinine. Mean unadjusted AUC(0-24) of sirolimus did not differ significantly either. Therefore, CYP3A5 expressor status and not transporter variants is a main determinant of oral clearance, particularly for tacrolimus. Dose adaptation according to trough levels, however, appears to be sufficient to maintain similar concentration-time profiles.

Cyp3A4, Cyp3A5, and MDR-1 genetic influences on tacrolimus pharmacokinetics in renal transplant recipients

OBJECTIVE

The immunosuppressive drug tacrolimus requires strict therapeutic monitoring due to its narrow therapeutic index and great inter-individual variability. Cytochrome P450 3A4 (Cyp3A4) and Cyp3A5 are the most important contributors to tacrolimus metabolism while the P-glycoprotein pump (MDR-1) modulates its bioavailability. The objective was to investigate the association between Cyp3A4, Cyp3A5, and MDR-1 polymorphisms and tacrolimus pharmacokinetics in the early period after renal transplantation.

METHODS

Forty-four renal transplant recipients were genotyped for 8 Cyp3A4, 7 Cyp3A5, and 5 MDR-1 genetic variants affecting the proteins' expression and/or function. Dose-adjusted tacrolimus though levels were determined during the first week after transplantation and correlated with corresponding genotype.

RESULTS

We found no correlation between Cyp3A4 polymorphism and tacrolimus pharmacokinetics. Patients who do not carry both Cyp3A5*3 alleles achieved lower mean dose-adjusted tacrolimus blood concentrations (p<0.001) and needed a longer time to reach the target concentration (10-12 ng/ml; p<0.001) compared to Cyp3A5*3 homozygotes. Patients with less than three copies of MDR-1 (T-129C, C3435T and G2677T) polymorphisms, associated with reduced expression of P-glycoprotein, had also lower dose-adjusted tacrolimus blood concentrations compared to patients having equal to or greater than three copies of MDR-1 genetic variants (P=0.003). There was no difference in the rate of biopsy-confirmed acute rejection among groups during the first 3 months after transplantation.

CONCLUSION

The complete absence of Cyp3A5*3 allele and the accumulation of less than three copies of MDR-1 (T-129C, C3435T and G2677T) polymorphisms are associated with lower tacrolimus blood levels identifying these genotypes as markers for patients requiring higher tacrolimus doses.

ABCB1 genotype and PGP expression, function and therapeutic drug response: a critical review and recommendations for future research

The product of the ABCB1 gene, P-glycoprotein (PGP), is a transmembrane active efflux pump for a variety of drugs. It is a putative mechanism of multidrug resistance in a range of diseases. It is postulated that ABCB1 polymorphisms contribute to variability in PGP function, and that therefore multidrug resistance is, at least in part, genetically determined. However, studies of ABCB1 genotype or haplotype and PGP expression, activity or drug response have produced inconsistent results. This critical review of ABCB1 genotype and PGP function, including mRNA expression, PGP-substrate drug pharmacokinetics and drug response, highlights methodological limitations of existing studies, including inadequate power, potential confounding by co-morbidity and co-medication, multiple testing, poor definition of disease phenotype and outcomes, and analysis of multiple drugs that might not be PGP substrates. We have produced recommendations for future research that will aid clarification of the association between ABCB1 genotypes and factors related to PGP activity.

Influence of different allelic variants of the CYP3A and ABCB1 genes on the tacrolimus pharmacokinetic profile of Chinese renal transplant recipients

Tacrolimus has a narrow therapeutic window and a wide interindividual variation in its pharmacokinetics. The cytochrome P450 3A (CYP3A) and the ATP-binding cassette B1 (ABCB1) genes play an important role in the tacrolimus disposition. Therefore, the aim of this study was to evaluate whether CYP3A and ABCB1 polymorphisms are associated with the area under the time concentration curve (AUC0-12) calculated using a two time point sample strategy. The CYP3A and ABCB1 genotypes were determined by real-time polymerase chain reaction (RT-PCR) fluorescence resonance energy transfer (FRET) assays in 103 Chinese renal transplant recipients and consequently related to their dose-normalized (dn)AUC0-12. A significant allele-dependent effect (Kruskal-Wallis; p < 0.001) was observed between the CYP3A5*3 polymorphism and the dnAUC0-12. Multiple regression analysis showed that the CYP3A5*3 polymorphism is the most significant independent variable and explained 35% of the dose requirement variability in relation to tacrolimus use. Regarding the ABCB1 G2677T/A and C3435T polymorphisms, a trend was observed between the different genotypes and the dnAUC0-12. In conclusion, the CYP3A5*3 polymorphism may be an important factor in determining the dose requirement for tacrolimus and genotyping can help determine the initial daily dose required by individual patients for adequate immunosuppression.

The effect of MDR1 (ABCB1) polymorphism on the pharmacokinetic of tacrolimus in Turkish renal transplant recipients

There is marked interindividual variability in trough blood levels of tacrolimus (TRL) following standard dosing. TRL is a substrate for P-glycoprotein (P-gp), the product of the multidrug resistance-1 (MDR1)(ABCB1) gene. P-gp acts as a membrane efflux pump, which affects TRL absorption from the gut. Some of the single nucleotide polymorphisms (SNP) of ABCB1 gene are associated with pharmacokinetic characteristics of TRL. The objective of this study was to determine the role of ABCB1 C3435T polymorphism on TRL dose requirements, trough values and dose-adjusted trough TRL concentrations among Turkish renal transplant recipients. Renal transplant recipients receiving TRL (n=92) were genotyped for ABCB1. TRL daily doses, trough concentrations, dose-adjusted trough concentrations, demographic features, and clinical data were obtained at 1, 6, and 12 months after renal transplantation. The frequency of the ABCB1 3435 CC genotype was 30.4%, whereas 47.8% of patients were 3435 CT and 21.7% of patients were 3435 TT. TRL daily doses were significantly lower among patients with the 3435 TT genotype at months 1 and 6. At 6 and 12 months after transplantation patients who were homozygous for the ABCB1 3435 CC showed significantly lower dose-adjusted trough TRL concentrations compared with subjects of 3435 TT and CT genotypes. Knowledge of ABCB1 genotype may be useful to adjust the optimal dose of TRL in transplant patients, thereby rapidly achieving target concentrations.

Tacrolimus dose requirement in relation to donor and recipient ABCB1 and CYP3A5 gene polymorphisms in Chinese liver transplant patients

The aim of this study was to investigate whether the heterogeneity in tacrolimus dose requirement is associated with ABCB1 and CYP3A5 gene polymorphisms in Chinese liver transplant patients during the first month after transplantation. ABCB1 and CYP3A5 genotyping was performed using the polymerase chain reaction restriction sites polymorphism-based procedure in Chinese liver transplant recipients (n = 50) and their corresponding donors (n = 50). Tacrolimus whole-blood trough concentrations were measured by immunoassays on the IMx analyzers (Abbott Diagnostics Laboratories, Abbott-Park, IL). Doses required to achieve target blood concentrations and dose-adjusted trough concentrations (concentration/dose [C/D] ratios) were compared among patients according to allelic status of ABCB1 and CYP3A5. The ABCB1 3435CC was observed in 23 subjects (23%), whereas 64 (64%) carried 3435CT and 13 (13%) carried 3435TT. The CYP3A5*1/*1 was observed in 13 subjects (13%), 50 (50%) carried *1/*3, and 37 (37%) carried*3/*3. The tacrolimus C/D ratios were obviously lower in recipients carrying ABCB1 3435CC genotype. For CYP3A5, recipients who received organs from CYP3A5*3/*3 donors had higher C/D ratios. But the donors' ABCB1 and recipients' CYP3A5 genotype did not affect the recipients' pharmacokinetics. Analysis of the combination of recipients' ABCB1 and donors' CYP3A5 genotypes revealed that the tacrolimus C/D ratios were significantly lower in the ABCB1 3435CC-carrying recipients, regardless of donors' CYP3A5 genotype. In conclusion, our finding suggests that the recipients' ABCB1 and donors' CYP3A5 genotype affect the tacrolimus dose requirements. ABCB1 C3435T polymorphism is a major determinant of tacrolimus trough concentration in Chinese liver transplant recipients, and recipients with 3435CC genotype will require higher dose of tacrolimus.

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.

Impact of ABCB1 (MDR1) haplotypes on tacrolimus dosing in adult lung transplant patients who are CYP3A5 *3/*3 non-expressors

BACKGROUND

The influence of ABCB1 (MDR1) polymorphisms on tacrolimus dosing has been questioned in previous studies with contradictory findings, possibly due to the association between CYP3A5 polymorphisms and tacrolimus dosing. The objective of this study was to assess the effect of ABCB1 haplotypes from 3 distinct polymorphic sites on the tacrolimus level/dose [L/D] in lung transplant patients limited to CYP3A5 *3/*3 non-expressors.

METHOD

A total of 91 lung transplant patients treated primarily with tacrolimus and prednisone were enrolled, and clinical information on drug dosing and blood levels was collected. The [L/D] was calculated for patients receiving tacrolimus at 1, 3, 6, 9 and 12 months post transplant. ABCB1 polymorphisms at C1236T, G2677T, and C3435T were assessed by PCR amplification and DNA sequencing. Haplotypes were estimated by Arlequin ver.2.00. Haplotype effects on tacrolimus [L/D] were assessed by two-way ANOVA.

RESULTS

Of the 10 haplotypes, CGC, TTT and CGT accounted for 44.1%, 40.7% and 7.6% of the total haplotypes, respectively. The tacrolimus [L/D] value in the CGC-CGC patients was significantly lower than in patients with CGC-TTT and TTT-TTT genotypes at the first month (mean [L/D]=1.45 versus 3.10 and 3.97 ngxmL(-)(1) /mg/day), and throughout the first post transplant year.

CONCLUSION

This study demonstrates that ABCB1 haplotypes derived from three common polymorphisms are associated with tacrolimus dosing in lung transplant patients when eliminating the confounder CYP3A5 genotype.