CYP3A7, CYP3A5, CYP3A4, and ABCB1 genetic polymorphisms, cyclosporine concentration, and dose requirement in transplant recipients. Ther Drug Monit. 2008;30:689-699. [PMID: 18978522 DOI: 10.1097/FTD.0b013e31818a2a60]

Frequencies and roles of CYP3A5, CYP3A4 and ABCB1 single nucleotide polymorphisms in Italian teenagers after kidney transplantation

The main genes involved in the pharmacokinetics of immunosuppressive drugs are those encoding cytochrome P450 (CYP) family enzymes and multidrug resistance 1 (ABCB1). In this study, 87 Italian teenagers with transplanted kidneys (mean age 11.6 ± 4.8 years) receiving calcineurin inhibitors (CNIs) were genotyped for the single nucleotide polymorphisms (SNPs) CYP3A5*1/3 and CYP3A4*1B for CYP3A, and C1236T, G2677T/A, C3435T and IVS21+49 for ABCB1, and retrospectively evaluated for the influence of the screened SNPs on CNI blood level at different post-transplantation times. The CYP3A5*1 allele was present in 7% of the patients, and the CYP3A4*1B allele was present in 3% of patients. The ABCB1 C1236T, G2677T/A and C3435T SNPs C, G and T occurred frequently (55%, 53% and 54%, respectively). The frequency of the T allele of IVS21+49 was 86%. The frequency of SNPs in both genes was comparable with that reported in other European Caucasian populations but different from that found in Asians or Afro-Americans. None of the cyclosporine (CsA) pharmacokinetic parameters were associated with the CYP3A5 genetic polymorphism, whereas the presence of the A allele in some patients was responsible for the required administration of a significantly increased dose of tacrolimus (Tac) that was necessary to reach therapeutic target levels. None of the Tac pharmacokinetic parameters were associated with ABCB1 SNPs, but ABCB1 SNPs had early effects on the CsA exposure index and dose requirements. In conclusion, because SNPs of the CYP3A and ABCB1 genes may be associated with CNI pharmacokinetic parameters and exposure indices, pre-transplant genetic screening should be considered in order to avoid immunosuppressant-related adverse events.

Cytochrome P450 and ABCB1 genetics: association with quetiapine and norquetiapine plasma and cerebrospinal fluid concentrations and with clinical response in patients suffering from schizophrenia. A pilot study

Variability in response to atypical antipsychotic drugs is due to genetic and environmental factors. Cytochrome P450 (CYP) isoforms are implicated in the metabolism of drugs, while the P-glycoprotein transporter (P-gp), encoded by the ABCB1 gene, may influence both the blood and brain drug concentrations. This study aimed to identify the possible associations of CYP and ABCB1 genetic polymorphisms with quetiapine and norquetiapine plasma and cerebrospinal fluid (CSF) concentrations and with response to treatment. Twenty-two patients with schizophrenia receiving 600 mg of quetiapine daily were genotyped for four CYP isoforms and ABCB1 polymorphisms. Quetiapine and norquetiapine peak plasma and CSF concentrations were measured after 4 weeks of treatment. Stepwise multiple regression analysis revealed that ABCB1 3435C > T (rs1045642), 2677G > T (rs2032582) and 1236C > T (rs1128503) polymorphisms predicted plasma quetiapine concentrations, explaining 41% of the variability (p = 0.001). Furthermore, the ABCB1 polymorphisms predicted 48% (p = 0.024) of the variability of the Δ PANSS total score, with the non-carriers of the 3435TT showing higher changes in the score. These results suggest that ABCB1 genetic polymorphisms may be a predictive marker of quetiapine treatment in schizophrenia.

An update on ABCB1 pharmacogenetics: insights from a 3D model into the location and evolutionary conservation of residues corresponding to SNPs associated with drug pharmacokinetics

The human ABCB1 protein, (P-glycoprotein or MDR1) is a membrane-bound glycoprotein that harnesses the energy of ATP hydrolysis to drive the unidirectional transport of substrates from the cytoplasm to the extracellular space. As a large range of therapeutic agents are known substrates of ABCB1 protein, its role in the onset of multidrug resistance has been the focus of much research. This role has been of particular interest in the field of pharmacogenomics where genetic variation within the ABCB1 gene, particularly in the form of single nucleotide polymorphisms (SNPs), is believed to contribute to inter-individual variation in ABCB1 function and drug response. In this review we provide an update on the influence of coding region SNPs within the ABCB1 gene on drug pharmacokinetics. By utilizing the crystal structure of the mouse ABCB1 homolog (Abcb1a), which is 87% homologous to the human sequence, we accompany this discussion with a graphical representation of residue location for amino acids corresponding to human ABCB1 coding region SNPs. Also, an assessment of residue conservation, which is calculated following multiple sequence alignment of 11 confirmed sequences of ABCB1 homologs, is presented and discussed. Superimposing a 'heat map' of residue homology to the Abcb1a crystal structure has permitted additional insights into both the conservation of individual residues and the conservation of their immediate surroundings. Such graphical representation of residue location and conservation supplements this update of ABCB1 pharmacogenetics to help clarify the often confounding reports on the influence of ABCB1 polymorphisms on drug pharmacokinetics and response.

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

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

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.

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.

The missing linkage: what pharmacogenetic associations are left to find in CYP3A?

IMPORTANCE OF THE FIELD

An enormous amount of drugs and endogenous substrates are metabolized by the enzymes encoded in the CYP3A gene cluster, making variation at this locus of utmost importance in the field of pharmacogenetics. However, the identification of genetic variation that contributes to the wide phenotypic variability at this locus has been elusive. While dozens of studies have investigated the effects of coding variants, none have found the definitive answer to what variant or variants explain the distribution of enzyme activity and clinical effects seen with the drug metabolized by these genes.

AREAS COVERED IN THIS REVIEW

This review highlights the recent pharmacogenetic work at the CYP3A locus, in particular studies on known functional variants in CYP3A4 and CYP3A5. In addition, common pharmacogenetic strategies as well as considerations specific to the CYP3A locus are discussed.

WHAT THE READER WILL GAIN

The reader will gain a greater understanding of the complexities involved in studying the CYP3A locus, population differences that may affect pharmacogenetic studies at this locus and the importance of variation that affect gene regulation.

TAKE HOME MESSAGE

More innovative and comprehensive methods to assay this region are needed, with particular attention paid to the role of gene regulation and non-coding sequence.

Genetic polymorphism of metabolic enzymes P450 (CYP) as a susceptibility factor for drug response, toxicity, and cancer risk

The polymorphic P450 (CYP) enzyme superfamily is the most important system involved in the biotransformation of many endogenous and exogenous substances including drugs, toxins, and carcinogens. Genotyping for CYP polymorphisms provides important genetic information that help to understand the effects of xenobiotics on human body. For drug metabolism, the most important polymorphisms are those of the genes coding for CYP2C9, CYP2C19, CYP2D6, and CYP3A4/5, which can result in therapeutic failure or severe adverse reactions. Genes coding for CYP1A1, CYP1A2, CYP1B1, and CYP2E1 are among the most responsible for the biotransformation of chemicals, especially for the metabolic activation of pre-carcinogens. There is evidence of association between gene polymorphism and cancer susceptibility. Pathways of carcinogen metabolism are complex, and are mediated by activities of multiple genes, while single genes have a limited impact on cancer risk. Multigenic approach in addition to environmental determinants in large sample studies is crucial for a reliable evaluation of any moderate gene effect. This article brings a review of current knowledge on the relations between the polymorphisms of some CYPs and drug activity/toxicity and cancer risk.

Genetic polymorphism of CYP3A5 in Indian chronic myeloid leukemia patients

CYP3A5 is an important genetic contributor to inter-individual differences in CYP3A-dependent clinically important drugs of metabolism and also of various endogenous compounds and environmental contaminants. The CYP3A5*3 allele results in a truncated protein with loss of CYP3A5 expression and CYP3A5*6 is associated with lower CYP3A5 catalytic activity. The polymorphism analysis was performed by PCR-RFLP and some representative cases by direct sequencing. Our case control study involved 183 consecutive North Indian CML patients in chronic phase of disease and 208 geographically and racially matched healthy controls. PCR-RFLP was carried out to determine the frequency of CYP3A5*3 and CYP3A5*6 genotypes. The relationship between these allelic variants and risk of CML was assessed by means of odds ratio (OR) with 95% confidence limits calculated by logistic regression. The frequencies of CYP3A5*1/*1, CYP3A5*1/*3, and CYP3A5*3/*3 genotypes in CML and controls were examined, and the quantitative comparison of the frequency distributions between CML versus control were performed, showing no significant differences among these comparison pairs (P = 0.88, 0.65, and 0.80, respectively). However, we did not find the CYP3A5*6 allele in any of the controls and leukemia patients. It is concluded that there is no association of this polymorphism with the risk of chronic myeloid leukemia.

ABCB1 and cytochrome P450 polymorphisms: clinical pharmacogenetics of clozapine

To examine the genetic factors influencing clozapine kinetics in vivo, 75 patients treated with clozapine were genotyped for CYPs and ABCB1 polymorphisms and phenotyped for CYP1A2 and CYP3A activity. CYP1A2 activity and dose-corrected trough steady-state plasma concentrations of clozapine correlated significantly (r = -0.61; P = 1 x 10), with no influence of the CYP1A2*1F genotype (P = 0.38). CYP2C19 poor metabolizers (*2/*2 genotype) had 2.3-fold higher (P = 0.036) clozapine concentrations than the extensive metabolizers (non-*2/*2). In patients comedicated with fluvoxamine, a strong CYP1A2 inhibitor, clozapine and norclozapine concentrations correlate with CYP3A activity (r = 0.44, P = 0.075; r = 0.63, P = 0.007, respectively). Carriers of the ABCB1 3435TT genotype had a 1.6-fold higher clozapine plasma concentrations than noncarriers (P = 0.046). In conclusion, this study has shown for the first time a significant in vivo role of CYP2C19 and the P-gp transporter in the pharmacokinetics of clozapine. CYP1A2 is the main CYP isoform involved in clozapine metabolism, with CYP2C19 contributing moderately, and CYP3A4 contributing only in patients with reduced CYP1A2 activity. In addition, ABCB1, but not CYP2B6, CYP2C9, CYP2D6, CYP3A5, nor CYP3A7 polymorphisms, influence clozapine pharmacokinetics.