Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression

Identification and characterization of novel sequence variations in the cytochrome P4502D6 (CYP2D6) gene in African Americans

Cytochrome P4502D6 (CYP2D6) genotyping reliably predicts poor metabolizer phenotype in Caucasians, but is less accurate in African Americans. To evaluate discordance we have observed in phenotype to genotype correlation studies, select African American subjects were chosen for complete resequencing of the CYP2D6 gene including 4.2 kb of the CYP2D7-2D6 intergenic region. Comparisons were made to a CYP2D6(*)1 reference sequence revealing novel SNPs in the upstream, coding and intervening sequences. These sequence variations, defining four functional alleles (CYP2D6(*)41B, (*)45A and B and (*)46), were characterized for their ability to influence splice site strength, transcription level or catalytic protein activity. Furthermore, their frequency was determined in a population of 251 African Americans. A -692(TGTG) deletion (CYP2D6(*)45B) did not significantly decrease gene expression, nor could any other upstream SNP explain a genotype-discordant case. CYP2D6(*)45 and (*)46 have a combined frequency of 4% and can be identified by a common SNP. Carriers are predicted to exhibit an extensive or intermediate CYP2D6 phenotype.

Variability of CYP3A7 expression in human fetal liver

Fetal liver CYP3A7 plays an important role in placental estriol synthesis during pregnancy, yet little is known concerning the extent or consequences of variability in expression. The purpose of this investigation was to characterize the variability in CYP3A7 expression using several phenotypic measures in a panel of 54 fetal livers ranging in age from 76 days to 32 weeks gestation. CYP3A7 mRNA expression was measured using quantitative polymerase chain reaction, whereas immunoreactive CYP3A7 was determined using an affinity-purified antipeptide antibody. Variability in catalytic activity was evaluated using testosterone and dehydroepiandrosterone (DHEA) as substrates. Across the entire panel, CYP3A7 was the most abundant CYP3A mRNA species present and varied 634-fold from 151 to 95,700 transcripts/ng total RNA, corrected for 18S ribosomal RNA. CYP3A4 expression was minimal based on mRNA expression (1000-fold lower than CYP3A7) and the ratio of testosterone 2alpha- (T2alphaH) to 6beta- (T6betaH) hydroxylation. T2alphaH and T6betaH were highly correlated (r(2) = 0.859), and the correlation increased (r(2) = 0.974) in livers with CYP3A5*3/*3 genotypes implying that the same enzyme (CYP3A7) generated both products. Overall, T2alphaH and DHEA16alphaH activities varied 175- and 250-fold, respectively. A subset of five samples had extremely low mRNA, protein, and catalytic activity, possibly due to pathology affecting fetal viability (anencephaly, porencephaly). In the remaining samples, T2alphaH activity varied 6.7-fold (358 +/- 142, range 97 to 643 pmol/min/mg) and DHEA16alphaH activity varied 6.2-fold (8.07 +/- 2.87, range 2.41 to 14.9 nmol/min/mg). Observed variability in CYP3A7 activity was not related to CYP3A7*2, and alternative regulatory mechanisms require further investigation.

Cytochrome P450 gene-based drug prescribing and factors impacting translation into routine clinical practice

Pharmacogenetics represents a rapidly advancing, competitive field of investigation. Due to the potential for clinically recognizable interactions between a set of old polymorphic genes and a relatively new environmental insult (drugs), many human geneticists believe that variability in the drug-metabolizing enzyme systems will soon translate into clinical practice across entire populations. Despite this, the field has not yet received widespread clinical acceptance. This article will review the common cytochrome P450 gene polymorphisms and discuss the factors that may facilitate (or attenuate) their translation into clinical practice.

A Novel Polymorphic Cytochrome P450 Formed by Splicing of CYP3A7 and the Pseudogene CYP3AP1

The cytochrome P450 3A7 (CYP3A7) is the most abundant CYP in human liver during fetal development and first months of postnatal age, playing an important role in the metabolism of endogenous hormones, drugs, differentiation factors, and potentially toxic and teratogenic substrates. Here we describe and characterize a novel enzyme, CYP3A7.1L, encompassing the CYP3A7.1 protein with the last four carboxyl-terminal amino acids replaced by a unique sequence of 36 amino acids, generated by splicing of CYP3A7 with CYP3AP1 RNA. The corresponding CYP3A7-3AP1 mRNA had a significant expression in liver, kidney, and gastrointestinal tract, and its presence was found to be tissue-specific and dependent on the developmental stage. Heterologous expression in yeast revealed that CYP3A7.1L was a functional enzyme with a specific activity similar to that of CYP3A7.1 and, in some conditions, a different hydroxylation specificity than CYP3A7.1 using dehydroepiandrosterone as a substrate. CYP3A7.1L was found to be polymorphic due to a mutation at position -6 of the first splicing site of CYP3AP1 (CYP3A7_39256T-->A), which abrogates the pseudogene splicing. This polymorphism had pronounced interethnic differences and was in linkage disequilibrium with other functional polymorphisms described in the CYP3A locus: CYP3A7*2 and CYP3A5*1. Therefore, the resulting CYP3A haplotypes express different sets of enzymes within the population. In conclusion, a novel mechanism, consisting of the splicing of the pseudogene CYP3AP1 to CYP3A7, causes the formation of the novel CYP3A7.1L having a different tissue distribution and functional properties than the parent CYP3A7 enzyme, with possible developmental, physiological, and toxicological consequences.

Genetic diversity and new therapeutic concepts

The differences in medicinal drug responses among individuals had been known for quite some time. Some patients exhibit a life-threatening adverse reaction while others fail to show an expected therapeutic effect. Intermediate responses between the above two extreme cases are also known. In fact, it has been recently reported that approximately 100,000 deaths and more than 2 million hospitalizations annually in the United States are due to properly prescribed medications. This interindividual variability could be due in part to genetically determined characteristics of target genes or drug metabolizing enzymes. This has now been substantiated by a variety of studies. We know that "one size fits all" is not correct. Therefore, the application of pharmacogenetic concepts to clinical practice is an excellent goal in the postgenomic era. The successful completion of the human genome project provided necessary molecular tools, such as high-throughput SNP genotyping, HapMap, and microarray, that can be applied to develop proper therapeutic options for individuals. Recently, there have been considerable scientific, corporate, and policy interest in pharmacotherapy. However, identification of causal variations in a target gene is only a starting point, and the progress in this rapidly developing field is slower than expected. One major drawback could be due to the multigene determinant of drug response that requires a genome-wide screening. Additionally, application of pharmacogenetic knowledge into clinical practice requires a high level of accuracy, precision (risk/benefit ratio), and strict regulations. This is because the pharmacogenetic approach raises several ethical, moral, and legal questions. It is also necessary that both health professionals and the general public must be urgently educated. Despite these limitations, translation of pharmacogenomic data into clinical practice would certainly provide better opportunities to increase the safety and efficacy of medicine in the future.

Identification of the cytochrome P450 enzymes involved in the metabolism of domperidone

The objective was to identify the major cytochrome P450 enzyme(s) involved in the metabolism of domperidone. Experiments were performed using human liver microsomes (HLMs), recombinant human cytochrome P450 enzymes, cytochrome P450 chemical inhibitors and monoclonal antibodies directed against cytochrome P450 enzymes. Four metabolites were identified from incubations performed with HLMs and excellent correlations were observed between the formation of domperidone hydroxylated metabolites (M1, M3 and M4), N-desalkylated domperidone metabolite (M2) and enzymatic markers of CYP3A4/5 (r2 = 0.9427, 0.951, 0.9497 and 0.8304, respectively). Ketoconazole (1 microM) decreased the formation rate of M1, M2, M3 and M4 by 83, 78, 75 and 88%, respectively, whereas the effect of other inhibitors (quinidine, furafylline and sulfaphenazole) was minimal. Important decreases in the formation rate of M1 (68%), M2 (64%) and M3 (54%) were observed with anti-CYP3A4 antibodies. Formation of M1, M2 and M3 in HLMs exhibited Michaelis-Menten kinetics (Km: 166, 248 and 36 microM, respectively). Similar Km values were observed for M1, M2 and M3 when incubations were performed with recombinant human CYP3A4 (Km: 107, 273 and 34 microM, respectively). The data suggest that CYP3As are the major enzymes involved in the metabolism of domperidone.

CYP3A5 genotypes and risk of oesophageal cancer in two South African populations

CYP3A5 is the major cytochrome P450 enzyme in the oesophagus and metabolises many potentially carcinogenic compounds. The frequencies of CYP3A5 allelic variants, CYP3A5*2, *3, *6 and *7 which code for enzymes with severely decreased activities were compared between 241 oesophageal cancer patients and 272 controls in Black and Mixed Ancestry South Africans. A significantly higher frequency of CYP3A5*3 was observed in the controls compared to patients amongst the Mixed Ancestry group (P=0.025). Individuals homozygous for defective CYP3A5 had reduced risk of developing oesophageal cancer (P=0.032).

CYP3A5 gene polymorphism and risk of prostate cancer in a Japanese population

The CYP3A5 gene (CYP3A5) encodes the cytochrome P450 3A5, which catalyzes the 6beta-hydroxylation of testosterone. We explored association between the CYP3A5 A6986G polymorphism and a risk of prostate cancer in 260 prostate cancer patients, 199 BPH patients and 212 male controls. The CYP3A5 gene polymorphism did not influence significantly a risk of developing of prostate cancer in general. However, compared with males with the GG genotype, those with the AA genotype had a 0.23-fold decreased risk of developing low-grade prostate cancer (P=0.023), and a 0.31-fold decreased risk of developing localized (stages A-C) prostate cancer (P=0.044). The CYP3A5 A6986G polymorphism may be specifically associated with a decreased risk of low-grade or early stage prostate cancer.

Polymorphisms of drug-metabolizing enzymes and risk of childhood acute lymphoblastic leukemia

The involvement of phase I and II enzymes is well documented in the metabolism of a wide range of drugs and xenobiotics. Single-nucleotide polymorphisms (SNPs) of these enzymes are also known to alter their protein expression and function. Moreover, genetic susceptibility and environmental exposure have been proposed to be an etiology of cancer. We hypothesized that polymorphisms of these enzymes might affect the risk of childhood acute lymphoblastic leukemia (ALL). CYP 1A1, CYP 3A4*1B, CYP 3A5*3, CYP 3A5*6, GSTM1, and GSTT1 polymorphisms were genotyped by using PCR-RFLP in 107 children with ALL and 320 healthy controls. Allele and genotype frequencies of each of the SNPs were compared between two groups. It was found that the allele frequencies of CYP 1A1*1, *2A, *2B, and *4 were not different between cases and controls. CYP 3A4*1B allele frequency was only 0.8% and 0.9% in ALL and controls, respectively. CYP 3A5*1/*1, *1/*3, and *3/*3 genotype frequencies showed no statistically significant difference between patients and controls. CYP 3A5*6 was not detected in our population. The GSTM1 null genotype was significantly increased in children with ALL (OR 1.7; 95% CI, 1.0, 2.7). In contrast, the GSTT1 null genotype did not show this effect. Our data thus demonstrate that the GSTM1 null genotype might increase the risk of childhood ALL in a Thai population.

CYP3A5 genetic polymorphisms in different ethnic populations

Cyp3A5 activity varies within any given ethnic population, suggesting that genetic variation within the Cyp3A5 gene may be the most important contributor to interindividual and interracial differences in Cyp3A-dependent drug clearance and response. The full extent of Cyp3A5 polymorphism in a white and an indigenous African population was analyzed using DNA direct sequencing procedures. The presence of 10 and 12 single nucleotide polymorphisms was detected in the white and African samples, respectively. Thirteen novel mutations occurring at low frequencies were identified in these populations. Significant differences were observed in the distribution of Cyp3A5*3, Cyp3A5*6, and Cyp3A5*7 alleles among white and African populations. The frequency of Cyp3A5*3 allele in white Canadians (approximately 93%) is higher than in Zimbabweans (77.6%) (p < 0.001). In contrast, Cyp3A5*6 and Cyp3A5*7 alleles are relatively frequent in African subjects (10-22%) but absent in white subjects (p < 0.001). These differences may reflect evolutionary pressures generated by environmental factors in geographically distinct regions. However, the genetic polymorphism of Cyp3A5 alone does not explain the interindividual differences in Cyp3A-mediated metabolism.

Contribution of CYP3A5 to hepatic and renal ifosfamide N-dechloroethylation

Ifosfamide nephrotoxicity is attributed to the formation of a toxic metabolite, chloroacetaldehyde, via N-dechloroethylation, a reaction that is purportedly catalyzed by CYP3A and CYP2B6. Because allelic variants of CYP3A5 are associated with polymorphic expression of microsomal CYP3A5 in human liver and kidneys, we hypothesized that ifosfamide N-dechloroethylation depends on CYP3A5 genotype. We compared ifosfamide N-dechloroethylation activity in cDNA-expressed CYP3A4 and CYP3A5. Ifosfamide N-dechloroethylation was also assessed in liver (N = 20) and kidney (N = 21) microsomes from human donors with different CYP3A5 genotypes. Ifosfamide N-dechloroethylation was catalyzed by recombinant CYP3A5 at a rate comparable with recombinant CYP3A4. In human liver microsomes matched for CYP3A4 protein content, N-dechloroethylation was more than 2-fold higher in that from donors carrying CYP3A5*1 allele that express CYP3A5 relative to that from donors homozygous for the mutant CYP3A5*3. Correlation analysis revealed that ifosfamide N-dechloroethylation was significantly associated with CYP3A4 and CYP3A5 protein concentration but not with age, sex, or CYP2B6 protein concentration. In hepatic microsomes not expressing CYP3A5 protein, ifosfamide N-dechloroethylation was inhibited 53 to 61% and 0 to 3% by monoclonal antibodies specific for CYP3A4/5 or CYP2B6, respectively. Ifosfamide N-dechloroethylation was not detected in renal microsomes obtained from CYP3A5*3/*3 donors. In contrast, it was readily measurable in microsomes isolated from four kidneys of CYP3A5*1 carriers, which was almost completely inhibited by the CYP3A inhibitor ketoconazole. CYP2B6 protein could not be detected in this panel of human renal microsomes. In conclusion, CYP3A5*1 genotype is associated with higher rates of ifosfamide N-dechloroethylation in human liver and kidneys.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Contribution of CYP3A5 to the in vitro hepatic clearance of tacrolimus

BACKGROUND

Tacrolimus is metabolized predominantly to 13-O-demethyltacrolimus in the liver and intestine by cytochrome P450 3A (CYP3A). Patients with high concentrations of CYP3A5, a CYP3A isoenzyme polymorphically produced in these organs, require higher doses of tacrolimus, but the exact mechanism of this association is unknown.

METHODS

cDNA-expressed CYP3A enzymes and a bank of human liver microsomes with known CYP3A4 and CYP3A5 content were used to investigate the contribution of CYP3A5 to the metabolism of tacrolimus to 13-O-demethyltacrolimus as quantified by liquid chromatography-tandem mass spectrometry.

RESULTS

Demethylation of tacrolimus to 13-O-demethyltacrolimus was the predominant clearance reaction. Calculated K(m) and V(max) values for CYP3A4, CYP3A5, and CYP3A7 cDNA-expressed microsomes were 1.5 micromol/L and 0.72 pmol x (pmol P450)(-1) x min(-1), 1.4 micromol/L and 1.1 pmol x (pmol P450)(-1) x min(-1), and 6 micromol/L and 0.084 pmol x (pmol P450)(-1) x min(-1), respectively. Recombinant CYP3A5 metabolized tacrolimus with a catalytic efficiency (V(max)/K(m)) that was 64% higher than that of CYP3A4. The contribution of CYP3A5 to 13-O-demethylation of tacrolimus in human liver microsomes varied from 1.5% to 40% (median, 18.8%). There was an inverse association between the contribution of CYP3A5 to 13-O-demethylation and the amount of 3A4 protein (r = 0.90; P <0.0001). Mean 13-O-demethylation clearances in CYP3A5 high and low expressers, estimated by the parallel-tube liver model, were 8.6 and 3.57 mL x min(-1) x (kg of body weight)(-1), respectively (P = 0.0088).

CONCLUSIONS

CYP3A5 affects metabolism of tacrolimus, thus explaining the association between CYP3A5 genotype and tacrolimus dosage. The importance of CYP3A5 status for tacrolimus clearance is also dependent on the concomitant CYP3A4 activity.

Inter-individual variation of several cytochrome P450 2D6 splice variants in human liver

To examine the possibility that inter-individual differences in splicing partially explain the observed differences in CYP2D6 activity, we amplified its full-length cDNA in 96 human liver RNA samples and discovered five splice variants: intron 5 retention, intron 6 retention, intron 5 and intron 6 double retention, exon 3 skipping, and partial intron 1 retention. All of the CYP2D6 splice variants we identified are probably nonfunctional transcripts. Substantial inter-individual variation in the proportions of the CYP2D6 transcript represented by splice variants, measured by real-time PCR, suggests that the presence of these splice variants contributes to the population variation in CYP2D6 activity. Relatively high levels of intron 6 retention were not correlated with the newly discovered single nucleotide polymorphism 2988G > A in intron 6 (CYP2D6*41) but did correlate with the more common CYP2D6*34 allele. Our study prompts further investigations to explore the effect of these splice variants on drug metabolism.

Hepatocellular carcinoma and polymorphisms in carcinogen-metabolizing and DNA repair enzymes in a population with aflatoxin exposure and hepatitis B virus endemicity

High rates of hepatocellular carcinoma (HCC) in The Gambia, West Africa, are primarily due to a high prevalence of chronic hepatitis B virus infection and heavy aflatoxin exposure via groundnut consumption. We investigated genetic polymorphisms in carcinogen-metabolizing (GSTM1, GSTT1, HYL1*2) and DNA repair (XRCC1) enzymes in a hospital-based case-control study. Incident HCC cases (n = 216) were compared with frequency-matched controls (n = 408) with no clinically apparent liver disease. Although the prevalence of variant genotypes was generally low, in multivariable analysis (adjusting for demographic factors, hepatitis B virus, hepatitis C virus, and TP53 status), the GSTM1-null genotype [odds ratio (OR), 2.45; 95% confidence interval (95% CI), 1.21-4.95] and the heterozygote XRCC1-399 AG genotype (OR, 3.18; 95% CI, 1.35-7.51) were significantly associated with HCC. A weak association of the HYL1*2 polymorphism with HCC was observed but did not reach statistical significance. GSTT1 was not associated with HCC. The risk for HCC with null GSTM1 was most prominent among those with the highest groundnut consumption (OR, 4.67; 95% CI, 1.45-15.1) and was not evident among those with less than the mean groundnut intake (OR, 0.64; 95% CI, 0.20-2.02). Among participants who had all three suspected aflatoxin-related high-risk genotypes [GSTM1 null, HLY1*2 (HY/HH), and XRCC1 (AG/GG)], a significant 15-fold increased risk of HCC was observed albeit with imprecise estimates (OR, 14.7; 95% CI, 1.27-169). Our findings suggest that genetic modulation of carcinogen metabolism and DNA repair can alter susceptibility to HCC and that these effects may be modified by environmental factors.

Effects of NO-1886 (Ibrolipim), a lipoprotein lipase-promoting agent, on gene induction of cytochrome P450s, carboxylesterases, and sulfotransferases in primary cultures of human hepatocytes

In the present study, the effects on expression of cytochrome P450 (CYP1A1, CYP1A2, CYP3A4 and CYP3A5), carboxylesterase (CES1 and CES2) and sulfotransferase (CHST1, CHST3, CHST4, CST, SULT2A1 and TPST2) mRNA in primary cultures of cryopreserved human hepatocytes were evaluated after exposure to NO-1886 (diethyl 4-[(4-bromo-2-cyanophenyl) carbamoyl] benzylphosphonate) for 48 hr at 2, 10, and 50 microM. Analysis was performed by RT-PCR in the presence of TaqMan probe. CYP1A1 and CYP1A2 mRNA levels after exposure to 50 microM omeprazole (positive control for CYP1As) were increased by 162 (p<0.001) and 37 times (p<0.001), respectively, compared with untreated controls. However, these mRNA levels were increased by 2 times or less after exposure to NO-1886. CYP3A4 and CYP3A5 mRNA levels after exposure to 50 microM rifampicin (positive control for CYP3As) were significantly increased by 5.8 (p<0.01) and 2.0 times (p<0.01), respectively, compared with untreated controls. The CYP3A4 mRNA level after exposure to 10 microM NO-1886 was increased by 1.3 times (p<0.05). Further, the CYP3A4 mRNA level after exposure to 50 microM NO-1886 was significantly increased by 3.6 times (p<0.001). However, the CYP3A5 mRNA level after exposure to 50 microM NO-1886 was not significantly increased. CES1 and CES2 mRNA levels after exposure to 50 microM NO-1886 were significantly increased by 1.4 (p<0.05) and 2.6 times (p<0.01), respectively, compared with untreated controls. CHST1, CST and SULT2A1 mRNA levels after exposure to 50 microM NO-1886 were significantly increased by 3.8 (p<0.001), 1.8 (p<0.01) and 4.4 times (p<0.01), respectively. CHST3, CHST4 and TPST2 mRNA levels after exposure to 50 microM NO-1886 were not significantly increased. This in vitro technique using primary cultured human hepatocytes is expected to be very useful for the preclinical evaluation of the induction of drug-metabolizing enzymes in humans.

CYP3A4 and CYP3A5 genotyping by Pyrosequencing

Background

Human cytochrome P450 3A enzymes, particularly CYP3A4 and CYP3A5, play an important role in drug metabolism. CYP3A expression exhibits substantial interindividual variation, much of which may result from genetic variation. This study describes Pyrosequencing assays for key SNPs in CYP3A4 (CYP3A4*1B, CYP3A4*2, and CYP3A4*3) and CYP3A5 (CYP3A5*3C and CYP3A5*6).

Methods

Genotyping of 95 healthy European and 95 healthy African volunteers was performed using Pyrosequencing. Linkage disequilibrium, haplotype inference, Hardy-Weinberg equilibrium, and tag SNPs were also determined for these samples.

Results

CYP3A4*1B allele frequencies were 4% in Europeans and 82% in Africans. The CYP3A4*2 allele was found in neither population sample. CYP3A4*3 had an allele frequency of 2% in Europeans and 0% in Africans. The frequency of CYP3A5*3C was 94% in Europeans and 12% in Africans. No CYP3A5*6 variants were found in the European samples, but this allele had a frequency of 16% in the African samples. Allele frequencies and haplotypes show interethnic variation, highlighting the need to analyze clinically relevant SNPs and haplotypes in a variety of ethnic groups.

Conclusion

Pyrosequencing is a versatile technique that could improve the efficiency of SNP analysis for pharmacogenomic research with the ultimate goal of pre-screening patients for individual therapy selection.

Screening of 12 SNPs of CYP3A4 in a Chinese population using oligonucleotide microarray

Human cytochrome P450 3A4 (CYP34A) plays an important role in the metabolism of many endo- and xenomaterials. It also exhibits a substantial interindividual variation in enzymatic activity. It has been shown that the mutant alleles of CYP3A4 encoding inactive/decreased enzymes are largely caused by single nucleotide polymorphisms (SNPs) in the gene sequence. In the present study, with the goal of detecting the known SNPs of CYP3A4, an oligonucleotide microarray was created. A genotyping standard for this microarray was also established using constructed plasmids as standard templates. The 12 SNPs of CYP3A4 in 387 Chinese DNA samples were screened using this oligonucleotide microarray. Three heterozygous subjects of CYP3A4*/*4, 5 heterozygous subjects of CYP3A4*1/*5, 4 heterozygous subjects of CPY3A4*1/6, and 6 heterozygous subjects of CYP3A4*1/*18 were found. The genotyping results of the 18 heterozygous subjects and 12 wild-type subjects were validated by direct sequencing.

Differential mechanism-based inhibition of CYP3A4 and CYP3A5 by verapamil

The genetic basis for polymorphic expression of CYP3A5 has been recently identified, but the significance of CYP3A5 expression is unclear. The purpose of this study is to quantify the capability of verapamil, a mechanism-based inhibitor of CYP3A, and its metabolites to inhibit the activities of CYP3A4 and CYP3A5, and to determine whether CYP3A5 expression in human liver microsomes alters the inhibitory potency of verapamil. Testosterone 6beta-hydroxylation or midazolam 1'-hydroxylation was used to quantify CYP3A activity. The possibility that verapamil and its metabolites form metabolic-intermediate complex (MIC) with CYP3A was assessed using dual beam spectrophotometry. Verapamil and N-desalkylverapamil (D617) were found to have little inhibitory effect on cDNA-expressed CYP3A5 activity and did not form a MIC with cDNA-expressed CYP3A5 as indicated by the appearance of the characteristic peak at 455 nm. At 50 microM, norverapamil showed time-dependent inhibition of CYP3A5 (30%), but to a much lesser extent compared with that of CYP3A4 (80%). The estimated values of the inactivation parameters k(inact) and K(I) of norverapamil were 4.53 microM and 0.07 min(-1) for cDNA-expressed CYP3A5, and 10.3 microM and 0.30 min(-1) for cDNA-expressed CYP3A4. Human liver microsomes that expressed CYP3A5 were less inhibited by both verapamil and norverapamil. The inactivation efficiency of verapamil and norverapamil was 30 times and 45 times lower, respectively, for CYP3A5-expressing microsomes compared with CYP3A5-non-expressing microsomes. These findings indicate that the presence of variable CYP3A5/CYP3A4 expression in the liver may contribute to the interindividual variability associated with verapamil-mediated drug interactions.

Tailored Therapy of Colorectal Cancer

Colorectal cancer is a leading cause of cancer mortality and morbidity. Despite the recent results of systemic chemotherapy, more than 40% of patients with advanced cancer still do not achieve substantial benefits with cytotoxic agents. Therefore, new tailored strategies are warranted to improve the probability of disease control. The rationale for this approach is based on the identification of the in vivo interactions among patient's characteristics, disease physiopathology, and drug pharmacodynamics and pharmacokinetics. Genomic and proteomic technologies may clarify the mechanisms involved in cancer growth and progression in each single patient and in the molecular basis of interindividual differences of anticancer drugs in terms of efficacy and toxicity. Despite the recent encouraging data, the clinical use of targeted therapy is hampered by several questions, such as: optimal biologic dose and schedule, lack of predictive surrogate biomarkers, and modalities of combination with chemotherapy/radiotherapy. Further efforts are needed to improve the reliability of genomic and proteomic technologies, in particular regarding their standardization and widespread availability. These unsolved issues make tailored therapy an open challenge.

A flexible array format for large-scale, rapid blood group DNA typing

BACKGROUND

Typing for blood group antigens is currently performed by hemagglutination. The necessary reagents are becoming costly and limited in availability, and the methods are labor-intensive. The purpose of this study was to determine the feasibility of the use of large-scale DNA analysis in a microarray as a substitute for blood group typing.

STUDY DESIGN AND METHODS

DNA, extracted from blood samples that had been phenotyped for some of the red blood cell antigens, was analyzed for selected blood group alleles by bead array (BeadChip, (BioArray Solutions Ltd., Warren, NJ) Illumina) [corrected] and by manual polymerase chain reaction (PCR)-based assays. Selected alleles were identified by enzyme-mediated elongation of probes, which were on color-encoded beads assembled into arrays on silicon chips. The performance of a prototype BeadChip (BioArray Solutions Ltd., Warren, NJ) [corrected] (BLOOD-1) containing single-nucleotide polymorphisms (SNPs) for FYA/B, FY-GATA, DOA/B, COA/B, LWA/B, DIA/B, and SC1/SC2 was verified with DNA from serologically characterized donors. It was then used to analyze more than 400 samples of partially defined phenotype. Samples from Chinese, Ashkenazi, and Thai donors (total n = 227) were tested with BLOOD-1. An expanded BeadChip (BioArray Solutions Ltd., Warren, NJ) [corrected] with a total of 18 SNPs (36 alleles; SNPs in BLOOD-1 and M/N, S/s, Lu(a)/Lu(b), K/k, FY265[for the Fy(X) polymorphism], Jk(a)/Jk(b), DO323[for Hy], DO350[for Jo(a)], and HgbS) was then evaluated with a subset of previously tested samples from Chinese, Ashkenazi, and New York blood donors (127) and an additional set of samples from Israeli donors (total n = 188).

RESULTS

Results obtained by BeadChip (BioArray Solutions Ltd., Warren, NJ) [corrected] analysis were concordant with those obtained with the manual PCR-restriction fragment length polymorphism, allele-specific PCR, and hemagglutination assays. The frequencies of the alleles in the samples from different ethnic panels were within the expected ranges; however, two new DO alleles were discovered.

CONCLUSION

It has been shown that microarray technology can be used to type DNA and detect new alleles in donor cohorts.

CYP3A5*3 and CYP3A4*18 single nucleotide polymorphisms in a Chinese population

BACKGROUND

Human cytochrome P450 3A evolved to catalyze the metabolism of numerous common therapy drugs and endogenous molecules. Members of the CYP3A are the majority expressed in human liver and intestine, and there are marked interindividual differences in their protein expression and activity. The activity of CYP3A enzyme in Chinese is highly variable, exceeding 14-fold, and contributes greatly to variation in oral bioavailability and systemic clearance of CYP3A substrates. The genetic factors play an important role in the interindividual variability in CYP3A activity. Detection of CYP3A5 and CYP3A4 variant alleles and knowledge about their allelic frequency in specific ethnic groups are important to lead to individualized drug dosing and improved therapeutics.

METHODS

We determined the allelic frequency of the CYP3A5*3 and CYP3A4*18 in a group of 302 Chinese subjects by using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assays.

RESULTS

In the group of 302 unrelated individuals, the frequency of the CYP3A5*3 and CYP3A4*18 variant allele in Chinese population were 0.778 (95% CI: 0.754, 0.802) and 0.01 (95% CI: 0, 0.02), respectively.

CONCLUSIONS

We developed a simple assay for the detection of the CYP3A4*18 allele and showed that in a Chinese population, CYP3A4*18 and CYP3A5*3 allelic frequencies are similar to that reported previously in Chinese resident in Taiwan. The frequency of the CYP3A5*3 allele in Chinese population is similar to the Japanese but lower than Caucasians. Meanwhile, our findings suggest that an approximate 62% of the Chinese population carrying CYP3A5*3/*3 genotype may appear not to express CYP3A5 protein.

The influence of age and sex on the clearance of cytochrome P450 3A substrates

Cytochrome P450s (CYPs) are an important family of enzymes in the metabolism of many therapeutic agents and endogenous metabolic reactions. The CYP3A subfamily is especially prominent in these metabolic activities. This review article focuses on how the factors of age and sex may influence the in vivo activity of human CYP3A. The functional activity of CYP3A varies based on issues such as interaction with one or more substrates and between individuals and/or localisation. For CYP3A substrates, intrinsic clearance is the component of total clearance that is contributed by the enzymes. Depending on the route of administration and the contribution of hepatic blood flow to overall clearance, sensitivities to changes in CYP3A activities may differ. Additionally, age may influence the hepatic blood flow and, in turn, affect CYP3A activity. A review of the literature regarding age influences on the clearance of CYP3A substrates does suggest that age can affect the clearance of certain CYP3A substrates.CYP3A is responsible for a large number of endogenous metabolic reactions involving steroid hormones, and enzyme activity has been reported to be induced and/or inhibited in the presence of some sex steroids. Based on published studies for most CYP3A substrates, sex does not appear to influence clearance; however, with certain substrates significant sex-related differences are found. In such cases, women primarily have higher clearance than men.

Alprazolam as a probe for CYP3A using a single blood sample: pharmacokinetics of parent drug, and of alpha- and 4-hydroxy metabolites in healthy subjects

OBJECTIVES

(1) To determine the pharmacokinetic parameters of alprazolam and its two metabolites in plasma from healthy volunteers; (2) to identify a suitable single time point to take a plasma sample for CYP3A phenotyping.

METHODS

Twelve healthy Swedish volunteers received a single oral dose of 1 mg alprazolam. Blood samples were collected before drug intake and frequently up to 72 h thereafter. A liquid-chromatography/mass-spectrometry (LC/MS) method was used for the quantification of alprazolam, and 4- and alpha-hydroxyalprazolam.

RESULTS

The interindividual variation in the area under the concentration-time curve (AUC) was two, three and fourfold for alprazolam, 4-hydroxyalprazolam and alpha-hydroxyalprazolam, respectively. Plasma concentration ratios collected between 1 h and 48 h for both alprazolam/4-hydroxyalprazolam and alprazolam/alpha-hydroxyalprazolam correlated significantly to the corresponding AUC0-infinity ratios.

CONCLUSIONS

The metabolic ratios of alprazolam to respective metabolite in a single plasma sample at 3-24 h are suggested to reflect the alprazolam 4- and alpha-hydroxylation activities. In future, it will be important to study these activities in populations where CYP3A5, in addition to CYP3A4, is expressed at a high frequency and to clarify the relative importance of the two enzymatic pathways for in vivo clearance of alprazolam.

Ontogeny of drug metabolizing enzymes in the neonate

Fetal exposure to xenobiotics is modulated to a considerable degree by the metabolic capabilities of the mother and the placenta. However, once liberated from the uterine environment the neonate is instantly exposed to a wide array of new macromolecules in the form of byproducts of cellular metabolism, dietary constituents, environmental toxins and pharmacologic agents. The rapid and efficient biotransformation of these compounds by Phase I and Phase II drug-metabolizing enzymes is an essential process if the infant is to avoid the accumulation of reactive compounds that could produce cellular injury or tissue dysfunction. Genetic polymorphisms and environmental factors are known to contribute dramatically to individual variation in the activity of drug-metabolizing enzymes. More recently, it has become apparent that programmed, developmental, regulatory events occur - independent of genotype - which further add to individual variation in drug metabolism. An appreciation of the impact of ontogeny on the expression and functional activity of the major drug-metabolizing enzymes enables the practicing clinician to predict the ultimate consequence of drug administration in the neonate to help guide optimal drug therapy.

Recombinant CYP3A4*17 is defective in metabolizing the hypertensive drug nifedipine, and the CYP3A4*17 allele may occur on the same chromosome as CYP3A5*3, representing a new putative defective CYP3A haplotype

Genetic variation in CYP3A activity may influence the rate of the metabolism and elimination of CYP3A substrates in humans. We previously reported four new CYP3A4 coding variants in three different racial groups. In the present study, we examined metabolism of nifedipine by the recombinant forms of these allelic variants. Metabolism of nifedipine by the L293P (CYP3A4*18), M445T (CYP3A4*3), and P467S (CYP3A4*19) allelic variants was not significantly different from wild-type CYP3A4*1. However, F189S (CYP3A4*17) exhibited a >99% decrease in both V(max) and CL(max) of nifedipine compared with CYP3A4*1. Of 72 racially diverse individuals, CYP3A4*17 was identified in 1 of 24 Caucasian samples [1:5 Eastern European (Adygei ethnic group)]. Genotyping of an extended set of 276 genomic DNAs of Caucasians (100 from the Coriell Repository and an additional 176 from the United States) for CYP3A4*17 detected no additional individuals containing the CYP3A4*17 allele. However, additional genotyping of four more Adygei samples available from Coriell detected an additional individual carrying the CYP3A4*17 allele. New specific polymerase chain reaction-restriction fragment length polymorphism genotyping procedures were developed for the major splice variant of CYP3A5 (CYP3A5*3) and CYP3A4*17. Genotyping revealed that the two individuals carrying CYP3A4*17 were either homozygous or heterozygous for the more frequent CYP3A5*3 allele, suggesting that the two alleles may exist on the same chromosome as a new putative CYP3A poor metabolizer haplotype. We predict that individuals who are homozygous for defective alleles of both of these genes would metabolize CYP3A substrates poorly. The new genetic tests will be useful in future clinical studies to investigate genotype/phenotype associations.

NADPH-dependent metabolism of 17beta-estradiol and estrone to polar and nonpolar metabolites by human tissues and cytochrome P450 isoforms

The endogenous estrogens, 17beta-estradiol (E(2)) and estrone (E(1)), undergo extensive metabolism in animals and humans, and a large number of their hydroxylated and keto metabolites have been identified in biological samples. The formation of most of the oxidative estrogen metabolites is catalyzed by cytochrome P450 (CYP) enzymes. Precise knowledge of the CYP-mediated formation of these metabolites, particularly those with unique biological activities (e.g., 4-hydroxy-E(2), 16alpha-hydroxy-E(1), 15alpha-hydroxy-E(2), 16-epiestriol, and 2-methoxyestradiol) in human liver and extrahepatic target tissues and cells, would add significantly to our understanding of the diverse biological functions that are associated with endogenous estrogens. In this article, we review recent results on the NADPH-dependent metabolism of endogenous estrogens to polar (hydroxylated and keto) metabolites as well as to nonpolar metabolites by human tissues and recombinant human CYP isoforms. The available data show that a large number of polar and nonpolar metabolites of E(2) and E(1) are formed by human tissues, and a variety of human CYP isoforms are involved in the NADPH-dependent formation of polar as well as nonpolar estrogen metabolites. These enzymes have varying degrees of catalytic activity and distinct regioselectivity.

Pharmacogenomics and acquired long QT syndrome

During the past decade pharmaceutical companies have been faced with the withdrawal of some of their marketed drugs because of rare, yet lethal, postmarketing reports associated with ventricular arrhythmias. The implicated drugs include antiarrhythmics, but also non-cardiac drugs, such as histamine blockers, antipsychotics, and antibiotics. These undesired effects involve prolongation of the QT interval, which may lead to characteristic ventricular tachyarrhythmias, known as torsades de pointes. These clinical symptoms of the acquired long QT syndrome (LQTS) are also found in an inherited form of the disease, called congenital LQTS. Nowadays, a number of environmental (non-genetic) and genetic risk factors for acquired LQTS have been described. Non-genetic factors include female gender, hypokalemia, and other heart diseases. The knowledge of genetic risk factors is emerging rapidly. During the last decade, mutations in several genes encoding ion channels have been shown to cause congenital LQTS. In acquired LQTS, a number of ‘silent’ mutation carriers in these LQTS genes have been identified, and functional polymorphisms in the same genes have been found that are associated with an increased vulnerability for the disease. Furthermore, there is also evidence that interindividual differences in drug metabolism, caused by functional polymorphisms in drug-metabolizing enzyme genes, may be a risk factor for acquired LQTS, especially if multiple drugs are involved. This review evaluates the current knowledge on these risk factors for acquired LQTS, with an emphasis on the genetic risk factors. It also assesses the potential to develop pharmacogenetic tests that will enable clinicians and pharmaceutical companies to identify at an early stage patients or individuals in the general population who are at risk of acquired LQTS.

The grapefruit juice effect is not limited to cytochrome P450 (P450) 3A4: evidence for bergamottin-dependent inactivation, heme destruction, and covalent binding to protein in P450s 2B6 and 3A5

Bergamottin (BG), a component of grapefruit juice, is a mechanism-based inactivator of cytochromes P450 (P450) 2B6 and 3A5 in the reconstituted system. The inactivation of both P450s was NADPH-dependent and irreversible. The kinetic constants for the inactivation of the 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of P450 2B6 were: K(I), 5 microM; k(inact) 0.09 min(-1); and t1/2, 8 min. The kinetic constants obtained for the inactivation of the testosterone 6beta-hydroxylation activity of P450 3A5 were: K(I), 20 microM; k(inact) 0.045 min(-1); and t1/2, 15 min. Incubations of P450s 2B6 and 3A5 with 20 microM BG at 37 degrees C for 20 min resulted in an approximately 60% loss in the catalytic activity that was accompanied by a significant loss in intact heme and a similar decrease in the reduced CO difference spectrum. The extrapolated partition ratios for BG with P450s 2B6 and 3A5 were approximately 2 and approximately 20, respectively. Liquid chromatography-mass spectroscopy analysis of the BG-inactivated samples showed that the mass of the inactivated apoprotein had increased by approximately 388 Da for both P450 2B6 and P450 3A5. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [14C]BG was irreversibly bound to the apoprotein in the BG-inactivated samples. The stoichiometry of binding was approximately 0.5 mol BG metabolite/mol of each P450 inactivated. High-pressure liquid chromatography analysis of the metabolites of BG showed that P450 2B6 generated two major metabolites, whereas P450 3A5 generated three additional metabolites. Two of metabolites were identified as 6',7'-dihydroxybergamottin and bergaptol.

CYP isoforms involved in the metabolism of clarithromycin in vitro: comparison between the identification from disappearance rate and that from formation rate of metabolites

To clarify whether CYP2C19 is involved in the overall metabolism of clarithromycin (CAM) or not, in vitro studies using human liver microsomes and recombinant CYPs were performed by an approach based on the disappearance rate of parent compound from the incubation mixture. In addition, the results of disappearance rate were compared with those obtained from the formation rates of the major metabolites of CAM, 14-(R)-hydroxy-CAM and N-demethyl-CAM. The intrinsic clearance (CL(int)) values determined from the disappearance of CAM in nine different human liver microsomes were highly correlated with the testosterone 6beta-hydroxylation activity (r=0.957, p<0.001). The CL(int) of CAM was markedly reduced by selective inhibitors of CYP3A4 (ketoconazole and troleandomycin) and by polyclonal antibodies raised against CYP3A4/5 in human liver microsomes. Among the 11 isoforms of recombinant human CYP, only CYP3A4 revealed the metabolic activity for the disappearance of CAM. These results were fairly consistent with those obtained from the conventional approach based on the formation of major metabolites of CAM. Comparison of the kinetic parameters estimated from the disappearance rate of CAM and the formation rates of 14-(R)-hydroxy-CAM and N-demethyl-CAM indicates that N-demethylation and 14-(R)-hydroxylation account for 65% of CL(int) derived from the disappearance of CAM in human liver microsomes. The findings suggest that CYP3A4 plays a predominant role in the overall metabolic clearance of CAM as well as in the formation of 14-(R)-hydroxy-CAM and N-demethyl-CAM. CYP2C19 does not appear to be involved in the overall metabolism of CAM at least in human liver microsomes. A combination of the disappearance rate of a parent compound and the formation rate of metabolites appears to be a useful approach for estimating the percentage contribution of the formation of metabolites to the overall metabolic clearance of a parent compound in vitro.

CYP3A5*1-carrying graft liver reduces the concentration/oral dose ratio of tacrolimus in recipients of living-donor liver transplantation

OBJECTIVES

Tacrolimus is widely used for immunosuppressive therapy after organ transplantation, but its pharmacokinetics shows such great interindividual variation that control of its blood concentration is difficult. We have previously reported that an intestinal P-glycoprotein (MDR1) contributes to this variation as an absorptive barrier, but the role of hepatic metabolism is not clear.

METHODS

In this study, we have evaluated the genotypes of MDR1 and cytochrome P450 (CYP) 3A in donor and recipient, and the influence of polymorphisms on mRNA expression and the tacrolimus concentration/dose (C/D) ratio in recipients of living-donor liver transplantation (LDLT).

RESULTS

The expression level of MDR1 and tacrolimus C/D ratio were not affected by either MDR1 C3435T or G2677T/A. The CYP3A4*1B genotype was not detected, but the CYP3A5*3 genotype had an allelic frequency of 76.3%. The mRNA level of CYP3A5 was significantly reduced by the *3/*3 genotype, and the tacrolimus C/D ratio was decreased in recipients engrafted with partial liver carrying CYP3A5*1/*1 genotype. An analysis of the combination of intestinal MDR1 level and liver CYP3A5 genotype revealed that the tacrolimus C/D ratio was lower in the group with higher MDR1 levels regardless of CYP3A5 genotype during postoperative week 1.

CONCLUSIONS

These results indicate that in recipients of LDLT, the pharmacokinetics of tacrolimus is influenced by flux via P-glycoprotein in the intestine during the first week; after that, it is mostly the hepatic metabolism that contributes to the excretion of tacrolimus, and carriers of the CYP3A5*1/*1 genotype require a high dose of tacrolimus to achieve the target concentration.

ABCB1 genotype of the donor but not of the recipient is a major risk factor for cyclosporine-related nephrotoxicity after renal transplantation

Cyclosporine (CsA) nephrotoxicity is a severe complication in organ transplantation because it leads to impaired renal function and chronic allograft nephropathy, which is a major predictor of graft loss. Animal models and in vivo studies indicate that the transmembrane efflux pump P-glycoprotein contributes substantially to CsA nephrotoxicity. It was hypothesized that the TT genotype at the ABCB1 3435C-->T polymorphism, which is associated with decreased expression of P-glycoprotein in renal tissue, is a risk factor for developing CsA nephrotoxicity. In a case-control study, 18 of 97 patients developed CsA nephrotoxicity and showed complete recovery of renal function in all cases when switched to a calcineurin inhibitor-free regimen. Both recipients and donors were genotyped for ABCB1 polymorphisms at the positions 3435C-->T and 2677G-->T/A. For controlling for population stratification, two additional polymorphisms, CYP2D6*4 and CYP3A5*3, with intermediate allelic frequencies were studied. The P-glycoprotein low expressor genotype 3435TT only of renal organ donors but not of the recipients was overrepresented in patients with CsA nephrotoxicity as compared with patients without toxicity (chi2 = 10.5; P = 0.005). CsA dosage, trough levels, and the concentration per dose ratio were not different between the patient groups. In a multivariate model that included several other nongenetic covariates, only the donor's ABCB1 3435TT genotype was strongly associated with CsA nephrotoxicity (odds ratio, 13.4; 95% confidence interval, 1.2 to 148; P = 0.034). A dominant role of the donor's ABCB1 genotype was identified for development of CsA nephrotoxicity. This suggests that P-glycoprotein is an important factor in CsA nephrotoxicity.

Phase I trial of 17-allylamino-17-demethoxygeldanamycin in patients with advanced cancer

PURPOSE

We determined the maximum-tolerated dose (MTD) and the dose-limiting toxicities (DLT) of 17-allylamino-17-demethoxygeldanamycin (17-AAG) when infused on days 1, 8, and 15 of a 28-day cycle in advanced solid tumor patients. We also characterized the pharmacokinetics of 17-AAG, its effect on chaperone and client proteins, and whether cytochrome P450 (CYP) 3A5 and NAD(P)H:quinone oxidoreductase 1 (NQO1) polymorphisms affected 17-AAG disposition or toxicity.

PATIENTS AND METHODS

An accelerated titration design was used. Biomarkers were measured in peripheral-blood mononuclear cells (PBMCs) at baseline and on days 1 and 15, and pharmacokinetic analysis was performed on day 1 of cycle 1. CYP3A5*3 and NQO1*2 genotypes were determined and correlated with pharmacokinetics and toxicity.

RESULTS

Twenty-one patients received 52 courses at 11 dose levels. DLTs at 431 mg/m(2) were grade 3 bilirubin (n = 1), AST (n = 1), anemia (n = 1), nausea (n = 1), vomiting (n = 1), and myalgias (n = 1). No tumor responses were seen. 17-AAG consistently increased heat shock protein (Hsp) 70 levels in PBMCs. At the MTD, the clearance and half-life (t(1/2)) of 17-AAG were 11.6 L/h/m(2) and 4.15 hours, respectively; whereas the active metabolite 17-aminogeldanamycin had a t(1/2) of 7.63 hours. The CYP3A5*3 and NQO1*2 polymorphisms were not associated with 17-AAG toxicity. The CYP3A5*3 polymorphism was associated with higher 17-AAG clearance.

CONCLUSION

The MTD of weekly 17-AAG is 308 mg/m(2). 17-AAG induced Hsp70 in PBMCs, indicating that Hsp90 has been affected. Further evaluation of 17-AAG is ongoing using a twice-weekly regimen, and this schedule of 17-AAG is being tested in combination with chemotherapy.

The effect of variable CYP3A5 expression on cyclosporine dosing, blood pressure and long-term graft survival in renal transplant patients

OBJECTIVE

Cyclosporine is extensively metabolized by cytochrome-P450 3A (CYP3A) enzymes in the liver and intestine including the CYP3A5 isoenzyme. CYP3A5 is also expressed in the kidney and has been implicated in blood pressure regulation. Appreciable expression of CYP3A5 occurs in carriers of the CYP3A5*1 allele, while the CYP3A5*3 allele is associated with low expression. We tested whether the presence of the CYP3A5*1 allele in renal transplant recipients and in donor kidneys influences cyclosporine dose requirements, blood pressure and long-term graft survival in renal transplant patients during chronic treatment with a cyclosporine-based immunosuppressive regimen.

METHODS

We studied 399 Caucasian patients from our single-center registry with stable graft function for more than 10 weeks after transplantation. The genotypes for CYP3A5*1/*3 were determined by a TaqMan PCR method. Cyclosporine dose requirements, blood pressure and graft survival were analyzed in relation to the presence or absence of the CYP3A5*1 allele in recipients and donor kidneys.

RESULTS

The CYP3A5*1 allele was found in 15.5% of the recipients and in 11.8% of the donor kidneys. The recipient CYP3A5*1 allele had no effect on cyclosporine dose and blood concentrations at trough with and without dose-adjustment. Blood pressure, number of antihypertensive compounds used for treatment and graft survival evaluated by Kaplan-Meier curves and Cox regression analysis were also not affected by the CYP3A5*1 allele either in recipients or donor kidneys.

CONCLUSIONS

Cyclosporine dose requirements, blood pressure and long-term renal graft survival are not influenced by the CYP3A5*1 allele in Caucasian patients.

Increased levels of aflatoxin-albumin adducts are associated with CYP3A5 polymorphisms in The Gambia, West Africa

OBJECTIVES

Major risk factors for hepatocellular carcinoma (HCC) are hepatitis viruses and exposure to aflatoxins, including aflatoxin B1 (AFB1). The mutagenic effect of AFB1 results from hepatic bioactivation to AFB1-exo-8,9-epoxide. This is in part catalysed by CYP3A5, an enzyme expressed polymorphically. We investigated the role of CYP3A5 polymorphisms in the formation of AFB1-exo-8,9-epoxide in The Gambia, a population exposed to high aflatoxin levels.

METHODS

Common CYP3A5 polymorphisms were identified in an African-American population. Subsequently, 288 Gambian subjects were genotyped and CYP3A5 activity predicted using haplotypes of the three variant loci (CYP3A5*3, *6 and *7) associated with decreases in protein expression. CYP3A5 expression was then compared to aflatoxin-albumin (AF-alb) adduct, a biomarker of AFB1 bioactivation; data were also analysed in relation to expression of other aflatoxin-metabolizing enzymes.

RESULTS

CYP3A5 haplotypes reflecting high CYP3A5 protein expression were associated with increased AF-alb. Compared to individuals with predicted low expression those predicted to express CYP3A5 from one allele displayed 16.1% higher AF-alb (95% CI: -2.5, 38.2, P = 0.093) and homozygous expressers displayed 23.2% higher AF-alb levels (95% CI: -0.01, 52.0, P = 0.051). The effect of the CYP3A5 polymorphism was strongest in individuals with low CYP3A4 activity with a 70.1% increase in AF-alb (95% CI: 11.8, 158.7, P < 0.05) in high compared to low expressers. A similar effect was observed for individuals with null alleles of GSTM1, which conjugates the AFB1-exo-8,9-epoxide to reduced glutathione.

CONCLUSIONS

The CYP3A5 polymorphism is associated with increased levels of the mutagenic AFB1-exo-8,9-epoxide, particularly in individuals with low CYP3A4, and this may modulate individual risk of HCC.

An MDR1-3435 variant is associated with higher plasma nelfinavir levels and more rapid virologic response in HIV-1 infected children

OBJECTIVE

The multidrug-resistance transporter gene (MDR1) encoding for P-glycoprotein (P-gp) and genes encoding for isoenzymes of cytochrome P450 (CYP) have an important role in transport and metabolism of antiretroviral agents. This research examined the impact of single nucleotide polymorphisms (SNP) of MDR1 and CYP genes on nelfinavir and efavirenz pharmacokinetics and the response to highly active antiretroviral therapy (HAART) in HIV-1 infected children.

METHODS

Seventy-one HIV-1-infected children from PACTG 382 receiving nelfinavir, efavirenz and one or two nucleoside reverse transcriptase inhibitors had genomic DNA from PBMC evaluated for MDR1 and CYP SNP by real-time PCR. Plasma drug concentrations, CD4 lymphocyte counts and HIV-1 RNA were measured during HAART.

RESULTS

The frequencies of C/C, C/T and T/T genotypes in the MDR1-3435-C-->T polymorphisms were 44% (n = 31), 46% (n = 33) and 10% (n = 7), respectively. Ninety-one percent of children with the C/T genotype reached plasma HIV-1 RNA < 400 copies/ml by week 8 compared to 59% of children with the C/C genotype (P = 0.01). Children with the C/T genotypes had higher 8 h postdose concentration (P = 0.02) and lower clearance rate (P = 0.04) for nelfinavir compared to those with the C/C genotype. The seven children with the T/T genotype had nelfinavir pharmacokinetics and virologic response similar to those with the C/C genotype. No compensatory polymorphisms were observed between MDR1 and CYP genotypes.

CONCLUSIONS

HIV-1 infected children with the MDR1-3435-C/T genotype had more rapid virologic responses to HAART at week 8 with higher plasma nelfinavir concentrations compared to those with the C/C genotype. These findings suggest that P-gp may play an important role in the pharmacokinetics and virologic response to HAART containing nelfinavir.

Polymorphisms in cytochrome P4503A5 (CYP3A5) may be associated with race and tumor characteristics, but not metabolism and side effects of tamoxifen in breast cancer patients

Tamoxifen (TAM) is widely used for treatment and prevention of breast cancer. TAM is metabolized by cytochrome P450 (CYP450) enzymes, including CYP3A5. Although two genetic polymorphisms in CYP3A5 are known (CYP3A5*3 and CYP3A5*6), the effects of these polymorphisms on TAM metabolism, TAM side effects, and tumor characteristics are unknown. Thus, this work tested the hypothesis that CYP3A5 polymorphisms are associated with differential TAM levels, TAM side effects, and tumor characteristics in breast cancer patients. Postmenopausal women with breast cancer (n=98) were recruited from a single cancer center. Polymorphic status was established using polymerase chain reactions (PCR). The associations between polymorphic status, race, TAM levels, side effects, and tumor characteristics were assessed using t-tests and logistic regression models. The data indicate that 40.7% of the breast cancer patients had the CYP3A5*3 polymorphism, and 9.1% had the CYP3A5*6 polymorphism. In addition, Caucasian women were 26 times more likely to carry the CYP3A5*3 polymorphism than African American (AA) women, whereas AA women were nine times more likely to carry the CYP3A5*6 polymorphism than Caucasian women. No significant differences were seen in TAM or TAM metabolite levels or TAM side effects by polymorphic status. There was a significant difference, however, in mean tumor size in women with the CYP3A5*6 polymorphism (3.6+/-0.98 cm) compared to those without the polymorphism (2.0+/-0.18 cm) (P<0.02). Taken together, these data suggest that racial differences in CYP3A5 polymorphisms exist although the polymorphisms do not appear to be associated with levels of TAM metabolites and side effects.

Metabolism of alfentanil by cytochrome p4503a (cyp3a) enzymes

The synthetic opioid alfentanil is an analgesic and an in vivo probe for hepatic and first-pass CYP3A activity. Alfentanil is a particularly useful CYP3A probe because pupil diameter change is a surrogate for plasma concentrations, thereby affording noninvasive assessment of CYP3A. Alfentanil undergoes extensive CYP3A4 metabolism via two major pathways, forming noralfentanil and N-phenylpropionamide. This investigation evaluated alfentanil metabolism in vitro to noralfentanil and N-phenylpropionamide, by expressed CYP3A5 and CYP3A7 in addition to CYP3A4, with and without coexpressed or exogenous cytochrome b(5). Effects of the CYP3A inhibitors troleandomycin and ketoconazole were also determined. Rates of noralfentanil and N-phenylpropionamide formation by CYP3A4 and 3A5 in the absence of b(5) were generally equivalent, although the metabolite formation ratio differed, whereas those by CYP3A7 were substantially less. CYP3A4 and 3A5 were equipotently inhibited by troleandomycin, whereas ketoconazole was an order of magnitude more potent toward CYP3A4. Cytochrome b(5) qualitatively and quantitatively altered alfentanil metabolism, with b(5) coexpression having a greater effect than exogenous addition. Addition or coexpression of b(5) markedly stimulated the formation of both metabolites and changed the formation of noralfentanil but not N-phenylpropionamide from apparent single-site to multisite Michaelis-Menten kinetics. These results demonstrate that alfentanil is a substrate for CYP3A5 in addition to CYP3A4, and the effects of the CYP3A inhibitors troleandomycin and ketoconazole are CYP3A enzyme-selective. Alfentanil is one of the few CYP3A substrates that is metabolized in vitro as avidly by both CYP3A4 and 3A5. Polymorphic CYP3A5 expression may contribute to inter-individual variability in alfentanil metabolism.

Consequences of genetic polymorphisms for sirolimus requirements after renal transplant in patients on primary sirolimus therapy

Sirolimus (SRL) is a substrate for cytochromes P-450 3A and P-glycoprotein, the product of the MDR1 gene. We postulated that single nucleotide polymorphisms (SNPs) of these genes could be associated with inter-individual variations in SRL requirements. We then evaluated in 149 renal transplant recipients the effect of polymorphisms CYP3A4*1/*1B, CYP3A5*1/*3 and MDR1 SNPs in exon 12, 21 and 26 on SRL concentration/dose (C/D) ratio 3 months after sirolimus introduction. SRL C/D ratio was significantly higher in patients treated with calcineurin inhibitors. The CYP3A4*1B and CYP3A5*1 alleles were present in 17% and 21% of patients, respectively. When treated with a SRL-based therapy and low-dose steroids, patients carrying the CYP3A4*1B or the CYP3A5*1 alleles required significantly more SRL to achieve adequate blood trough concentrations (p < 0.01 and p < 0.02, respectively). None of the MDR1 SNPs was associated with the SRL concentration/dose ratio. These findings suggest that the variations in SRL requirements are secondary to both genetic and non-genetic factors including pharmacokinetic interactions. In patients with SRL-based therapy, genotyping of the CYP3As genes may help to optimize the SRL management in transplant recipients.

HMG-CoA reductase inhibitor pharmacogenomics: overview and implications for practice

HMG-CoA reductase inhibitors (statins) are widely prescribed and recommended as first-line therapy for most patients with hypercholesterolemia or established coronary heart disease. However, there is interpatient variability in lipid-lowering response to statins that is not explained by initial cholesterol levels and inadequate dosing alone. Genetic polymorphisms may contribute. This review discusses the potential contribution of polymorphisms in genes encoding proteins involved in drug metabolism and transport, cholesterol biosynthesis, lipid metabolism and others to lipid responses to statins.

Inhibition of Human Intestinal Wall Metabolism by Macrolide Antibiotics: Effect of Clarithromycin on Cytochrome P450 3A4/5 Activity and Expression*

BACKGROUND

Clarithromycin increases both hepatic and intestinal availability of the selective cytochrome P450 (CYP) 3A probe midazolam. This study was designed to identify determinants of variability in the extent of intestinal wall CYP3A inhibition by clarithromycin, such as CYP3A5 genotype, and the mechanism of inhibition.

METHODS

Ten healthy volunteers received 500 mg oral clarithromycin twice a day for 7 days. Before and after administration of clarithromycin, small-bowel mucosal biopsy specimens were obtained endoscopically. Intestinal CYP3A activity was determined from the rate of 1'-hydroxymidazolam and 4-hydroxymidazolam formation by incubation of small-bowel homogenate with midazolam (25 micromol/L) and NADPH for 5 minutes. Intestinal CYP3A4 and CYP3A5 messenger ribonucleic acid was quantified by real-time reverse transcriptase-polymerase chain reaction. Intestinal CYP3A4 and CYP3A5 protein concentrations were determined by immunoblotting. Serum and homogenate concentrations of midazolam, clarithromycin, and metabolites were determined by liquid chromatography-mass spectrometry. CYP3A5 genotype was determined by real-time polymerase chain reaction.

RESULTS

The formation of 1'-hydroxymidazolam (1.36 +/- 0.46 pmol . min(-1) . mg(-1) at baseline versus 0.35 +/- 0.16 pmol . min(-1) . mg(-1) after administration) and 4-hydroxymidazolam (0.39 +/- 0.12 pmol . min(-1) . mg(-1) at baseline versus 0.12 +/- 0.05 pmol . min(-1) . mg(-1) after administration) was significantly (P < .001) reduced after clarithromycin administration. Clarithromycin administration did not result in a significant change in intestinal CYP3A4 and CYP3A5 messenger ribonucleic acid and protein expression. All subjects had detectable serum clarithromycin concentrations after 7 days of clarithromycin (3.71 +/- 2.43 micromol/L). The mean concentration of clarithromycin in the intestinal biopsy homogenate was 1.2 +/- 0.7 nmol/L (range, 0.42-2.39 nmol/L). Compared with CYP3A5 nonexpressers, subjects with at least 1 CYP3A5*1 allele (CYP3A5 expressers) had greater inhibition of intestinal CYP3A activity after treatment with clarithromycin. There was a strong linear relationship between the decrease in intestinal CYP3A activity and baseline catalytic activity (R(2) = 0.9).

CONCLUSION

Baseline intestinal activity of CYP3A4 was a key determinant of variability of the inhibitory effect of clarithromycin among individuals. CYP3A5*1 alleles were associated with greater baseline intestinal CYP3A activity and, therefore, greater extent of inhibition. The primary in vivo mechanism was not rapidly reversible competitive or irreversible inhibition but was likely formation of metabolic intermediate complexes.

The impact of pharmacogenomic factors on acute persistent rejection in adult lung transplant patients

Persistent rejection in the face of treatment and multiple episodes of rejection are associated with the development of chronic rejection and graft loss in solid organ transplantation. The factors that create an environment for rejection that persists in the face of treatment are as yet not understood. The objective of this study was to evaluate the risk factors, including human multidrug resistance gene (MDR1), cytochrome P4503A5 (CYP3A5) and cytokine gene polymorphisms, associated with acute persistent rejection (APR) in lung transplant patients. One hundred and twenty-five adult lung transplant patients were studied. MDR1 G2677T, C3435T and CYP3A5 polymorphisms were assessed by direct sequencing of the polymorphic region in patient DNA. Cytokine genotyping for five cytokines was performed using the polymerase chain reaction-sequence specific primers (PCR-SSP) technique. Multivariate regression analysis was used to identify the predictors of acute persistent rejection. The dependent variable was the presence or absence of acute persistent rejection based on lung biopsies during the first postoperative year. The independent variables were MDR1 G2677T and C3435T, CYP4503A5 and cytokine polymorphisms, survival status, age, gender, survival days and HLA mismatches. The MDR1 C3435T polymorphism and age were independently associated with acute persistent rejection (p = 0.025, odds ratio = 0.29, 95% CI 0.1-0.86 and p = 0.016, odds ratio = 0.94, 95% CI 0.89-0.98, respectively). For the MDR1 C3435T polymorphism, 72% of patients with the C allele had acute persistent rejection in comparison to 52% for TT patients (p = 0.04). For age, a significant difference was found between the nonrejection group and the rejection group (mean+/-S.D. 52.1+/-11.2 vs. 44.4+/-12.3, p = 0.01). This is the first report of the association of a drug disposition genotype with drug-resistant acute rejection in organ transplant patients. The major predictor of acute persistent rejection in the first postoperative year for lung transplant patients was the MDR1 C3435T genotype. This association could be due to drug resistance, altered drug disposition or other immunologic effects associated with P-glycoprotein (P-gp) function. Future prospective treatment algorithms should be developed that will incorporate the knowledge of gene polymorphisms into treatment regimens to improve the outcome following lung transplantation.

Genetic polymorphisms of cytochrome P450 among patients with Balkan endemic nephropathy (BEN)

OBJECTIVES

The concept of multifactorial etiology of BEN anticipates that a combination of polymorphic gene variants and various environmental factors causes an increased risk for the disease. CYP enzymes play a key role in the metabolic activation of environmental chemicals and toxins. CYP3A enzymes are particularly relevant for xenobiotic metabolism because of their broad substrate specificity and abundant expression in the human liver, intestine, and kidney. Previous phenotyping analysis on CYP2D6 enzyme activity in BEN patients proposed a modifying effect of CYP2D6 gene variants on BEN risk, but it was not approved with molecular-genetic methods. The aim of the current case-control study was to compare the frequency of CYP2D6 and CYP3A5 polymorphisms, as well as one CYP3A4 promoter variant in BEN patients and controls in order to investigate a possible association between individual genetic variations in these genes and susceptibility to BEN.

DESIGN AND METHODS

Ninety-six nonrelated Bulgarian BEN patients from endemic villages in the Vratza district and 112 healthy Bulgarians from nonendemic areas (controls) were genotyped. Identification of alleles was done by allele-specific PCR or by rapid-cycle amplification on the LightCycler, followed by sequence-specific detection.

RESULTS

The UM, PM, and EM + IM genotype frequencies of CYP2D6 did not differ significantly between the two groups (P > 0.05). The CYP3A4*1B allele was only found in the heterozygous form, with allelic frequencies of 5.21% in the patients and 2.23% in the healthy individuals (P = 0.11). The CYP3A5*1 allele was more prevalent in BEN patients with a frequency of 9.38% compared to 5.36% in the controls and was associated with a higher risk for BEN (OR 2.41, 95% CI 1.09-5.33) (P = 0.02).

CONCLUSIONS

Our results demonstrate that the CYP3A5*1 allele, previously reported as a marker for CYP3A5 expression in human kidney, is associated with increased risk for BEN, while CYP3A4*1B and CYP2D6 genotypes do not significantly modify the risk for the disease.

CYP3A4, CYP3A5, and CYP3A43 genotypes and haplotypes in the etiology and severity of prostate cancer

The CYP3A genes reside on chromosome 7q21 in a multigene cluster. The enzyme products of CYP3A4 and CYP3A43 are involved in testosterone metabolism. CYP3A4 and CYP3A5 have been associated previously with prostate cancer occurrence and severity. To comprehensively examine the effects of these genes on prostate cancer occurrence and severity, we studied 622 incident prostate cancer cases and 396 controls. Substantial and race-specific linkage disequilibrium was observed between CYP3A4 and CYP3A5 in both races but not between other pairs of loci. We found no association of CYP3A5 genotypes with prostate cancer or disease severity. CYP3A43*3 was associated with family history-positive prostate cancer (age- and race-adjusted odds ratio = 5.86, 95% confidence interval, 1.10-31.16). CYP3A4*1B was associated inversely with the probability of having prostate cancer in Caucasians (age-adjusted odds ratio = 0.54, 95% confidence interval, 0.32-0.94). We also observed significant interactions among these loci associated with prostate cancer occurrence and severity. There were statistically significant differences in haplotype frequencies involving these three genes in high-stage cases (P < 0.05) compared with controls. The observation that CYP3A4 and CYP3A43 were associated with prostate cancer, are not in linkage equilibrium, and are both involved in testosterone metabolism, suggest that both CYP3A4*1B and CYP3A43*3 may influence the probability of having prostate cancer and disease severity.

Lipid-lowering response to statins is affected by CYP3A5 polymorphism

Individuals expressing the polymorphic CYP3A5 enzyme might show a more than average efficiency in the metabolism of lovastatin, simvastatin and atorvastatin. We studied whether the expression of CYP3A5 is associated with an impaired lipid-lowering response to statins in 69 Caucasian patients. Lovastatin, simvastatin and atorvastatin were significantly less effective in CYP3A5 expressors than in non-expressors. The mean serum total cholesterol concentration at 1 year was 23% higher (P = 0.0014) and the mean serum low-density lipoprotein cholesterol concentration was 24% higher (P = 0.036) in subjects possessing the CYP3A5*1 allele (CYP3A5 expressors, n = 7) than in subjects homozygous for the CYP3A5*3 allele (non-expressors, n = 39). The mean percentage reduction in serum total cholesterol from baseline was significantly smaller in CYP3A5 expressors than in non-expressors (17% versus 31%, P = 0.026). No association between hypolipidemic efficacy and CYP3A5 polymorphism was observed among 23 subjects taking statins that are not dependent on CYP3A5 (fluvastatin, pravastatin). These findings suggest that CYP3A5 may be a genetic determinant of interindividual differences in response to certain statins.

Pharmacogenomics and stomach cancer

In subgroups of gastric cancer patients, chemotherapy treatments carry a high risk of toxicity without any clear evidence of antitumor activity. Individualization of therapy is required to treat each patient with the optimal drug and dose. Genetic polymorphisms are the hereditary determinants for interindividual variations of drug effect and the genetic approach represents a new tool to design a tailored therapy. This review focuses on the relevance of the host polymorphisms involved in metabolism, cellular transport and interaction with molecular targets of the drugs used in gastric cancer in conventional or innovative chemotherapy regimens. Pharmacogenetic studies based on a single gene or multi-gene approach (pharmacogenomics) are promising to identify gastric cancer patients at risk for adverse toxicity, but larger and controlled studies are needed to justify changes in the chemotherapeutic strategies.

Tacrolimus Pharmacogenetics: The CYP3A5*1 Allele Predicts Low Dose-Normalized Tacrolimus Blood Concentrations in Whites and South Asians

Previously, we demonstrated that the dose-normalized tacrolimus blood concentration after renal transplantation was associated with a single nucleotide polymorphism (SNP) in the CYP3AP1 gene, probably through linkage with an SNP in the CYP3A5 gene. Individuals with at least one CYP3A5*1 allele synthesize CYP3A5 and CYP3A5*3/*3 homozygotes do not. We now present results with direct typing of the CYP3A5 genotype for this group of 180 kidney-only transplant recipients from a single center. South Asian and white patients with at least one CYP3A5*1 allele achieved twofold lower dose-normalized tacrolimus blood concentrations compared with CYP3A5*3/*3 homozygotes, confirming our previous findings for the CYP3AP1 SNP. There was a significant delay in achieving target blood concentrations in those with at least one CYP3A5*1 allele. Determination of the CYP3A5*1/*3 genotype could be used to predict the tacrolimus dose requirement and, given incomplete linkage, would be better than determination of the CYP3AP1 genotype.

Pharmacokinetic principles of immunosuppressive drugs

A basic tenet of clinical pharmacology is that the pharmacologic activity of an exogenously administered agent is related to the free drug concentration available at its receptor or ligand-binding site. The discipline of pharmacokinetics can be defined as the study of the processes that lead to the availability of an agent to its site of action. In this review we will discuss basic principles of pharmacokinetics that govern the absorption, distribution, metabolism, elimination and binding of immunosuppressive drugs commonly utilized in whole organ transplantation. In a discipline such as organ transplantation, where the agents utilized carry significant toxicity and where failure of efficacy can have dire consequences, knowledge of the pharmacokinetics of the agents utilized has become a basic skill for all transplant professionals. In this review we describe some of the underlying principles that govern the disposition of the agents commonly utilized in solid organ transplantation. In addition, we hope this review will help in understanding some of the basic drug interactions encountered in transplant practice.

CYP3A5 polymorphism and the ethnic differences in cyclosporine pharmacokinetics in healthy subjects

To determine the relationship between CYP3A5 polymorphism and cyclosporine pharmacokinetic parameters among healthy volunteers, an oral cyclosporine (CsA) pharmacokinetic study was performed in 16 healthy subjects. Blood CsA concentrations were measured by high-performance liquid chromatography. Concentration-versus-time data were analyzed by a noncompartmental method using WinNonLin, and the blood samples were genotyped for the CYP3A5 using the polymerase chain reaction and pyrosequencing. CsA pharmacokinetic parameters were dichotomized and compared using the 1-way ANOVA test according to the CYP3A5*3C genotype. There were 6 homozygous A/A (wild type), 6 homozygous G/G (variant), and 4 heterozygous A/G genotypes for CYP3A5*3 C in these 16 healthy volunteers. All whites were G/G group, and all African Americans except 1 were either A/A or A/G group. The mean AUC (ng x h/mL) of CsA for the 3 genotype groups were 4962 +/- 1074 (A/A), 6677 +/- 1153 (G/G), and 5416 +/- 1817 (A/G), (A/A versus G/G, P = 0.03), and the mean CL/F (mL/min/kg) were 15.6 +/- 3.1 (A/A), 12.0 +/- 2.3 (G/G), and 14.7 +/- 5.9 (A/G), (A/A versus G/G, P = 0.04). None of the other parameters were significantly different among the 3 genotypes. In conclusion, the CYP3A5*3C polymorphism appears to affect AUC and CL/F of oral CsA significantly in healthy subjects, which may partly explain some of the differences of pharmacokinetics in CsA between African Americans and whites.