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

Individualizing Therapy in Patients With Chronic Kidney Disease

Patients with chronic kidney diseases have multiple clinical abnormalities that may affect disposition of drugs, including alterations in glomerular filtration rate, excretion of plasma proteins, reductions in serum albumin, and reductions in drug metabolizing enzyme activity. Inflammation may also influence the previous factors. Concomitant drug therapies can lead to drug— drug interactions that may affect the pharmacokinetics of administered drugs. Pharmacogenomics has begun to be evaluated for effects of genotype and haplotype of drug metabolizing enzymes and transporters on drug disposition. Because of the multiple potential etiologies for alterations in drug disposition in patients with chronic kidney diseases, they require appropriate evaluation for implementation of individualized strategies in therapies to enhance efficacy and reduce toxicities. This review will highlight the disease- and patient-specific variables that are targets for patient-centered approaches to therapeutic interventions. The field of pharmacogenomics will be reviewed with reference to common therapies for transplantation and glomerular diseases.

The contributions of cytochromes P450 3A4 and 3A5 to the metabolism of the phosphodiesterase type 5 inhibitors sildenafil, udenafil, and vardenafil

The role of the genetically polymorphic CYP3A5 in the metabolism of CYP3A substrates is unclear. We investigated the contributions of the CYP3A4 and CYP3A5 isoforms to the metabolism of the phosphodiesterase type 5 inhibitors (PDE5Is) sildenafil, udenafil, and vardenafil. In vitro incubation studies of sildenafil N-demethylation, udenafil N-dealkylation, and vardenafil N-deethylation were conducted using recombinant CYP3A enzymes and 15 human liver microsome (HLM) preparations with predetermined CYP3A5 genotypes. Recombinant CYP3A4 and CYP3A5 both produced N-desalkyl metabolites of sildenafil, udenafil, and vardenafil. The catalytic efficiency (Cl(int) = V(max)/apparent K(m)) of the rCYP3A5 isoform for vardenafil N-deethylation was about 3.2-fold that of rCYP3A4, whereas the intrinsic clearance rates for N-dealkylation of both sildenafil and udenafil were similar between rCYP3A5 and rCYP3A4. The metabolite formation activity was higher in HLMs heterozygous for the CYP3A5*3 allele (n = 9) than in HLMs homozygous for CYP3A5*3 (n = 6). These findings suggest that CYP3A5 and CYP3A4 play a significant role in the metabolism of PDE5Is. The genetic polymorphism of CYP3A5 may contribute to interindividual variability in the disposition of PDE5Is, especially vardenafil. Further in vivo studies are needed to confirm the effects of CYP3A5 genotypes on the pharmacokinetics of PDE5Is.

Concordant gene expression in leukemia cells and normal leukocytes is associated with germline cis-SNPs

The degree to which gene expression covaries between different primary tissues within an individual is not well defined. We hypothesized that expression that is concordant across tissues is more likely influenced by genetic variability than gene expression which is discordant between tissues. We quantified expression of 11,873 genes in paired samples of primary leukemia cells and normal leukocytes from 92 patients with acute lymphoblastic leukemia (ALL). Genetic variation at >500,000 single nucleotide polymorphisms (SNPs) was also assessed. The expression of only 176/11,783 (1.5%) genes was correlated (p<0.008, FDR = 25%) in the two tissue types, but expression of a high proportion (20 of these 176 genes) was significantly related to cis-SNP genotypes (adjusted p<0.05). In an independent set of 134 patients with ALL, 14 of these 20 genes were validated as having expression related to cis-SNPs, as were 9 of 20 genes in a second validation set of HapMap cell lines. Genes whose expression was concordant among tissue types were more likely to be associated with germline cis-SNPs than genes with discordant expression in these tissues; genes affected were involved in housekeeping functions (GSTM2, GAPDH and NCOR1) and purine metabolism.

CYP3A5 genotype is not associated with a higher risk of acute rejection in tacrolimus-treated renal transplant recipients

OBJECTIVE

Patients expressing the tacrolimus-metabolizing enzyme, cytochrome P450 (CYP) 3A5, require more tacrolimus to reach target concentrations. We studied the influence of the CYP3A5(*)3 allele, which results in the absence of CYP3A5 protein, on tacrolimus dose and exposure, as well as the incidence of biopsy-proven acute rejection (BPAR) after renal transplantation.

METHODS

A total of 136 patients participating in a prospective, randomized-controlled clinical trial with the primary aim of comparing the efficacy of a fixed-dose versus a concentration-controlled mycophenolate mofetil immunosuppressive regimen, were genotyped for CYP3A5(*)3. The patients described herein, participated in a pharmacogenetic substudy and were all treated with mycophenolate mofetil, corticosteroids and tacrolimus. Tacrolimus predose concentrations (C(0)) were measured on day 3 and 10, and month 1, 3, 6 and 12.

RESULTS

Compared with CYP3A5(*)3/(*)3 individuals (n=110), patients carrying at least one CYP3A5(*)1 (wild-type) allele (CYP3A5 expressers; n=26) had a lower tacrolimus C(0) on day 3 only (16.6 versus 12.3 ng/ml, respectively), whereas dose-corrected tacrolimus C(0) were significantly lower in the latter group at all time points. After day 3, the overall daily tacrolimus dose was 68% higher in CYP3A5 expressers (P<0.001). The incidence of BPAR was comparable between CYP3A5 expressers and nonexpressers (8 versus 16%, respectively; P=0.36).

CONCLUSION

We conclude that patients expressing CYP3A5 need more tacrolimus to reach target concentrations and have a lower tacrolimus exposure shortly after transplantation. This delay in reaching target concentrations, however, did not result in an increased incidence of early BPAR and therefore, genotyping for CYP3A5 is unlikely to improve short-term transplantation outcome.

Association of genetic polymorphisms in SLCO1B3 and ABCC2 with docetaxel-induced leukopenia

Despite long-term clinical experience with docetaxel, unpredictable severe adverse reactions remain an important determinant for limiting the use of the drug. To identify a genetic factor(s) determining the risk of docetaxel-induced leukopenia/neutropenia, we selected subjects who received docetaxel chemotherapy from samples recruited at BioBank Japan, and conducted a case-control association study. We genotyped 84 patients, 28 patients with grade 3 or 4 leukopenia/neutropenia, and 56 with no toxicity (patients with grade 1 or 2 were excluded), for a total of 79 single nucleotide polymorphisms (SNPs) in seven genes possibly involved in the metabolism or transport of this drug: CYP3A4, CYP3A5, ABCB1, ABCC2, SLCO1B3, NR1I2, and NR1I3. Since one SNP in ABCB1, four SNPs in ABCC2, four SNPs in SLCO1B3, and one SNP in NR1I2 showed a possible association with the grade 3 leukopenia/neutropenia (P-value of <0.05), we further examined these 10 SNPs using 29 additionally obtained patients, 11 patients with grade 3/4 leukopenia/neutropenia, and 18 with no toxicity. The combined analysis indicated a significant association of rs12762549 in ABCC2 (P = 0.00022) and rs11045585 in SLCO1B3 (P = 0.00017) with docetaxel-induced leukopenia/neutropenia. When patients were classified into three groups by the scoring system based on the genotypes of these two SNPs, patients with a score of 1 or 2 were shown to have a significantly higher risk of docetaxel-induced leukopenia/neutropenia as compared to those with a score of 0 (P = 0.0000057; odds ratio [OR], 7.00; 95% CI [confidence interval], 2.95-16.59). This prediction system correctly classified 69.2% of severe leukopenia/neutropenia and 75.7% of non-leukopenia/neutropenia into the respective categories, indicating that SNPs in ABCC2 and SLCO1B3 may predict the risk of leukopenia/neutropenia induced by docetaxel chemotherapy.

Pharmacogenetics of immunosuppressive drugs: prospect of individual therapy for transplant patients

The immunosuppressive drugs used in solid-organ transplantation are potent and toxic agents with narrow therapeutic ranges. Underdosing is associated with immunological rejection of the transplanted organ, whereas overdosing results in infections, malignancy and direct toxicity to a number of organs. Pharmacokinetic heterogeneity makes initial dose determination difficult, as there is a poor correlation between dose and blood concentration. Therapeutic drug monitoring is available but the pharmacokinetic–pharmacodynamic association is imperfect and it does not help in achieving target blood concentrations during the critical early 2–3 days after transplantation. Genetic polymorphisms in drug targets, drug-metabolizing enzymes and drug efflux pumps have been identified as potential targets for developing a pharmacogenetic strategy to individualize initial drug choice and dose. To date, use of the CYP3A5 genotype to predict the appropriate initial dose of tacrolimus is the most promising option for individualization of drug therapy in organ transplantation.

Contribution of the N-glucuronidation pathway to the overall in vitro metabolic clearance of midazolam in humans

Midazolam (MDZ) is one of the most commonly used in vivo and in vitro CYP3A4 probe substrates for drug-drug interactions (DDI) studies. The major metabolic pathway of MDZ in humans consists of the CYP3A4-mediated 1'-hydroxylation followed by urinary excretion as 1'-O-glucuronide derivative. In the present study, following incubation of MDZ with human liver microsomes supplemented with UDP-glucuronic acid, two major high-performance liquid chromatography (HPLC) peaks were isolated. HPLC and liquid chromatography/tandem mass spectrometry analyses identified these two metabolites as quaternary direct N-glucuronides of MDZ, thus revealing an additional metabolic pathway for MDZ. (1)H NMR spectrometry studies were performed showing that these two glucuronides were beta-N-glucuronides, which could be considered as two different conformers of the same molecule. According to molecular modeling experiments, the two glucuronide derivatives could be involved in atropoisomerism equilibrium. The formation of MDZ N-glucuronide exhibited moderate intersubject variability (at most 4.5-fold difference, n = 10). Among the recombinant human UDP glucuronosyltransferase (UGT) isoforms tested, only isoform UGT1A4 catalyzed the N-glucuronidation of MDZ fitting a Michaelis-Menten model. K(m) and V(max) values were 29.9 +/- 2.4 microM and 659.6 +/- 19.0 pmol/min/mg protein, respectively. The N-glucuronide derivative was found in human hepatocytes incubated under control conditions but also in the presence of the well known CYP3A4 inhibitor, ketoconazole. In the context of the in vitro study of CYP3A4-mediated DDI using MDZ and ketoconazole, direct MDZ N-glucuronidation may partly compensate the decrease in MDZ metabolic clearance caused by the addition of the inhibitor, thus potentially leading to underestimation, at least in vitro, of the extent of DDI.

NOD2/CARD15 gene variants are linked to failure of antibiotic treatment in perianal fistulating Crohn's disease

OBJECTIVES

The Crohn's disease (CD) susceptibility gene, nucleotide-binding oligomerizetion domain 2 (NOD2)/caspase recruitment domain 15 (CARD15), is linked to the innate immune response associated with altered epithelial bacterial defense. Its relevance in antibiotic therapy of perianal fistulating CD remains elusive. The aim of the study was to explore systematically the association between NOD2/CARD15 variants and clinical response of perianal fistulas in patients using antibiotic therapy.

METHODS

Fifty-two patients (median age 36 yr) with draining perianal fistulas were treated with ciprofloxacin (N = 49) or metronidazole (N = 3) for a median duration of 7 wk. Complete response was defined as the absence of any draining fistula despite gentle finger compression. Genotyping for NOD2/CARD15 variants and human beta (beta)-defensin 2 (HBD-2) copies was performed by 5' nuclease assays (Applied Biosystems, Foster City, CA). The examiners and laboratory investigators were blinded. The Fisher exact test and Wilcoxon signed rank test were used for statistical analysis.

RESULTS

Ciprofloxacin was discontinued in one patient due to diarrhea after 2 wk. Complete fistula response was observed in 13 of 39 patients with NOD2/CARD15 wild-type (33.3%) compared with none in patients carrying NOD2/CARD15 variants (0%, P= 0.02). The median number of HBD-2 gene copies between responders and partial/nonresponders was similar.

CONCLUSIONS

The study result suggests a substantial contribution of NOD2/CARD15 to the antibiotic treatment outcome of perianal fistulating CD. NOD2/CARD15 variants may predispose to an altered intestinal microflora in perianal fistulas that is less responsive to antibiotic treatment.

Association of four DNA polymorphisms with acute rejection after kidney transplantation

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

Cytochrome P450 3A polymorphisms and immunosuppressive drugs: an update

Among the immunosuppressive drugs currently used in solid-organ transplantation, the calcineurin inhibitors cyclosporine and tacrolimus, and the mammalian target of rapamycin inhibitors sirolimus and everolimus, may be difficult to use because of large interindividual variability in their pharmacokinetic characteristics and a narrow therapeutic index. The promise of pharmacogenetics and pharmacogenomics is to elucidate the inherited basis of differences between individual responses to drugs, in order to identify the right drug and dose for each patient. As cytochrome P450 (CYP)3A4 and CYP3A5 are both involved in the metabolism of these drugs, the consequences of the polymorphism of these genes have been studied. It has been recently shown that the CYP3A5*3 polymorphism is associated with pharmacokinetics of tacrolimus and sirolimus. The association between the CYP3A4 and CYP3A5 polymorphisms and cyclosporine pharmacokinetics is more questionable. It is now of utmost importance to prospectively test these initial results to improve the individualized use of these drugs.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSION

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

The androgen receptor CAG repeat length polymorphism associates with prostate volume in Finnish men with benign prostatic hyperplasia

The development of benign prostatic hyperplasia requires the presence of testicular androgens during prostate development, puberty, and ageing. We thus examined the association of three polymorphisms, namely, CYP3A5 6986A>G, CYP19A1 1531C>T, and androgen receptor (AR) gene CAG repeat length, which have previously been linked to the androgen pathway and with clinical characteristics of benign prostatic hyperplasia. Tissue samples from 262 consecutive prostate operations were used for genotyping. Prostate volumes and prostate-specific antigen values were collected from patient records. Linear regression analysis was performed to study the polymorphisms in an age-adjusted model. We did not find any association between the CYP3A5 6986A>G polymorphism and clinical characteristics of benign prostatic hyperplasia. Further, the previously published CYP19A1 1531C>T polymorphism association with an enlarged prostate could not be confirmed with this material. However, we detected an association between short AR gene CAG repeat length and a small prostate volume, which confirms a previous finding in the Finnish population. The data presented suggest a negligible role for the CYP3A5 6986A>G polymorphism in benign prostate enlargement in the Finnish population. However, the results presented do provide further evidence for potentially different genetic mechanisms behind benign prostatic hyperplasia in Finnish and other Caucasian populations. This is based on the conflicting results for AR gene CAG repeat length associations with benign prostatic hyperplasia found in published works.

Association study of ABCB1 and CYP3A5 gene polymorphisms with sirolimus trough concentration and dose requirements in Chinese renal transplant recipients

The objective of this study was to investigate the possible association of the ABCB1 gene C3435 T polymorphism and the CYP3A5 gene A6986G polymorphism with sirolimus (SRL) trough concentration and dose requirements in Chinese stable renal transplant recipients. Blood samples were collected from 105 healthy volunteers and 50 renal transplant patients, whose polymorphisms of the ABCB1 and CYP3A5 genes were determined by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP). Plasma concentrations of SRL were determined with HPLC. The allele frequencies of the ABCB1 mutation in Chinese healthy volunteers and renal transplant recipients were 51.0% and 44.0% (p>0.05), while the allele frequencies of the CYP3A5 mutation were 72.9% and 71.0% (p>0.05). The SRL concentration/dose ratio (C/D) in patients with CYP3A5 (*)3/(*)3 were significantly higher than that of those with (*)1 allele (p<0.05). However, no significant differences were observed between C/D and ABCB1 SNPs (p>0.05). These results confirm that when treated with a SRL-based therapy and low-dose steroids, patients carrying the CYP3A5(*)1 allele required significantly more SRL to achieve adequate blood trough concentrations. In patients with SRL-based therapy, genotyping of the CYP3A5 genes may help to optimize the SRL management in renal transplant recipients.

Knockout of cytochrome P450 3A yields new mouse models for understanding xenobiotic metabolism

Cytochrome P450 3A (CYP3A) enzymes constitute an important detoxification system that contributes to primary metabolism of more than half of all prescribed medications. To investigate the physiological and pharmacological roles of CYP3A, we generated Cyp3a-knockout (Cyp3a-/-) mice lacking all functional Cyp3a genes. Cyp3a-/- mice were viable, fertile, and without marked physiological abnormalities. However, these mice exhibited severely impaired detoxification capacity when exposed to the chemotherapeutic agent docetaxel, displaying higher exposure levels in response to both oral and intravenous administration. These mice also demonstrated increased sensitivity to docetaxel toxicity, suggesting a primary role for Cyp3a in xenobiotic detoxification. To determine the relative importance of intestinal versus hepatic Cyp3a in first-pass metabolism, we generated transgenic Cyp3a-/- mice expressing human CYP3A4 in either the intestine or the liver. Expression of CYP3A4 in the intestine dramatically decreased absorption of docetaxel into the bloodstream, while hepatic expression aided systemic docetaxel clearance. These results suggest that CYP3A expression determines impairment of drug absorption and efficient systemic clearance in a tissue-specific manner. The genetic models used in this study provide powerful tools to further study CYP3A-mediated xenobiotic metabolism, as well as interactions between CYP3A and other detoxification systems.

Effect of race on vincristine-associated neurotoxicity in pediatric acute lymphoblastic leukemia patients

BACKGROUND

This report examines the association between race and vincristine-associated neurotoxicity in pediatric patients with precursor B cell acute lymphoblastic leukemia (preB ALL). Given that in vitro vincristine is metabolized more efficiently by cytochrome P450 (CYP) 3A5 than by CYP3A4 and that 70% African-Americans (vs. 20% of Caucasians) express CYP3A5, one may hypothesize that African-Americans metabolize vincristine more efficiently resulting in lower vincristine exposure and associated-toxicity.

PROCEDURE

A retrospective analysis of vincristine-related side effects in pediatric patients treated for preB ALL was performed. Data were compared between Caucasians (n = 92) and African-Americans (n = 21) to examine the relationship between race and vincristine-associated neurotoxicity thus using race as a surrogate for CYP3A5 genotype. Race, age, gender, disease subtype, highest grade of vincristine-associated neurotoxicity (per NIH Common Terminology Criteria for Adverse Events version 3.0), number of omitted and reduced vincristine doses, cumulative vincristine dose, and disease outcome were captured.

RESULTS

34.8% of Caucasians experienced symptoms consistent with vincristine-related neurotoxicity compared to 4.8% of African-Americans (P = 0.007). The average grade of neurotoxicity for Caucasians was 2.72 versus grade 1 neurotoxicity in the African-American (P < 0.0001). Four percent of total doses administered to Caucasian patients were reduced due to vincristine-related neurotoxicity compared to 0.1% given to African-Americans (P < 0.0001). 1.2% of all protocol-indicated doses for Caucasians were held due to severe vincristine-associated toxicity compared to 0.1% of doses for African-Americans (P < 0.01).

CONCLUSIONS

The data support the hypothesis pharmacogenetic polymorphisms in CYP3A5 expression contribute to variability in vincristine metabolism and neurotoxicity.

Pharmacogenetics of cyclosporine in children suggests an age-dependent influence of ABCB1 polymorphisms

OBJECTIVE

To evaluate whether variations in the ABCB1, ABCC2, SLCO1B1, CYP3A4, CYP3A5, or NR1I2 genes are associated with the pharmacokinetics of cyclosporine in pediatric renal transplant candidates, and whether the effects of these variants are related to age.

METHODS

A total of 104 pediatric patients (aged 0.36-16.3 years) were genotyped for 17 putatively functionally significant sequence variations in the ABCB1, SLCO1B1, ABCC2, CYP3A4, CYP3A5, and NR1I2 genes. The patients had undergone a pharmacokinetic study with intravenous and oral ciclosporine (INN, cyclosporin) before renal transplantation.

RESULTS

In the whole population, the mean+/-SD cyclosporine oral bioavailability was 0.38+/-0.09, volume of distribution was 2.3+/-0.54 l/kg, and systemic clearance normalized by allometric body weight was 0.88+/-0.16 l/h/kg3/4. The prehepatic extraction ratio was 0.51+/-0.13, and the hepatic extraction ratio was 0.24+/-0.04, the former explaining 95% of the variability in oral bioavailability (P<0.0001). In children older than 8 years, the pre-hepatic extraction was 0.64+/-0.09 in those with the ABCB1 c.2677GG genotype, 0.52+/-0.11 in those with the c.2677GT genotype, and 0.41+/-0.03 in those with the c.2677TT genotype (P=0.021, r2=0.334), leading to corresponding differences in oral bioavailability (0.28+/-0.07, 0.36+/-0.07, and 0.44+/-0.04, respectively; P=0.012, r2=0.372). Similar associations were observed with the ABCB1 c.1236C>T variant and the related haplotype c.1199G-c.1236C-c.2677G-c.3435C (P<0.05). The estimated oral dose requirement and clearance of cyclosporine remained largely unexplained by the genetic variations.

CONCLUSIONS

Although these data suggest an age-related effect of ABCB1 polymorphism on oral bioavailability, further studies are required on the predictive value of genotyping for individualization of cyclosporine dosing in children.

Frequencies of CYP3A5 genotypes and haplotypes in a Korean population

BACKGROUND AND OBJECTIVE

CYP3A, the drug-metabolizing enzyme is an important factor in the pharmacokinetics of many drugs. Polymorphism of the CYP3A5 gene is known to influence the functionality of the CYP3A5 enzymes. The full extent of CYP3A5 genetic polymorphism was analysed in a Korean population.

METHODS

Specific polymerase chain reaction-restriction fragment length polymorphism tests for CYP 3AP1 through CYP3A5*7 or direct sequencing were used to identify reported CYP3A5 variant alleles, using 194 unrelated samples.

RESULTS AND DISCUSSION

The most frequent single nucleotide polymorphism (SNP) was 6986A>G (CYP3A5*3). The next most frequent SNP was 31611C>T. Haplotype analysis using detected SNPs revealed that the most frequent haplotype was *3A (frequency: 0.724), followed by *1E (frequency: 0.211), *3C (frequency: 0.034) and *1A (frequency: 0.023). We did not find CYP3AP1*3, CYP3A5*6, or *7 in this Korean sample.

CONCLUSION

A large proportion of Koreans may have relatively low levels of metabolically active CYP3A5 protein and therefore may be at risk of high levels of drugs metabolized by this enzyme, after administration of conventional doses.

CYP3A5*3 and CYP3A4*1B allele distribution and genotype combinations: differences between Spaniards and Central Americans

The aim of this study was to detect genotypic differences between three populations of healthy volunteers from Northern Spain (204 subjects), Nicaragua (120 subjects), and El Salvador (112 subjects) regarding CYP3A4*1B and CYP3A5*3 polymorphisms. No significant differences were found by comparing allelic frequencies between the two Central American populations. The CYP3A5*3 allele frequency was significantly different (P < 0.01) between Central Americans (76%) and Spaniards (91%). By contrast, CYP3A4*1B allele was more prevalent among Central Americans (12.5%) than among North Spaniards (4%) (P < 0.01). Analysis of CYP3A4-3A5 genotype combinations revealed that individuals carrying CYP3A4*1B/CYP3A5*1 were more represented in Central Americans (16.9%) than in Spaniards (5.4%), suggesting a marked linkage disequilibrium. These data are compatible with a higher CYP3A enzyme activity in Central Americans as opposed to Spaniards and other white groups, which could imply differences in dose requirements for drugs metabolized by CYP3A and should be considered in allele-disease association studies.

The ontogeny of drug metabolism enzymes and implications for adverse drug events

Profound changes in drug metabolizing enzyme (DME) expression occurs during development that impacts the risk of adverse drug events in the fetus and child. A review of our current knowledge suggests individual hepatic DME ontogeny can be categorized into one of three groups. Some enzymes, e.g., CYP3A7, are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation, but are silenced or expressed at low levels within one to two years after birth. SULT1A1 is an example of the second group of DME. These enzymes are expressed at relatively constant levels throughout gestation and minimal changes are observed postnatally. ADH1C is typical of the third DME group that are not expressed or are expressed at low levels in the fetus, usually during the second or third trimester. Substantial increases in enzyme levels are observed within the first one to two years after birth. Combined with our knowledge of other physiological factors during early life stages, knowledge regarding DME ontogeny has permitted the development of robust physiological based pharmacokinetic models and an improved capability to predict drug disposition in pediatric patients. This review will provide an overview of DME developmental expression patterns and discuss some implications of the data with regards to drug therapy. Common themes emerging from our current knowledge also will be discussed. Finally, the review will highlight gaps in knowledge that will be important to advance this field.

Pharmacogenomic and pharmacokinetic determinants of erlotinib toxicity

PURPOSE

To assess the pharmacogenomic and pharmacokinetic determinants of skin rash and diarrhea, the two primary dose-limiting toxicities of the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor erlotinib.

PATIENTS AND METHODS

A prospective clinical study of 80 patients with non-small-cell lung cancer, head and neck cancer, and ovarian cancer was performed. Detailed pharmacokinetics and toxicity of erlotinib were assessed. Polymorphic loci in EGFR, ABCG2, CYP3A4, and CYP3A5 were genotyped, and their effects on pharmacokinetics and toxicities were evaluated.

RESULTS

A novel diplotype of two polymorphic loci in the ABCG2 promoter involving -15622C/T and 1143C/T was identified, with alleles conferring lower ABCG2 levels associated with higher erlotinib pharmacokinetic parameters, including area under the curve (P = .019) and maximum concentration (P = .006). Variability in skin rash was best explained by a multivariate logistic regression model incorporating the trough erlotinib plasma concentration (P = .034) and the EGFR intron 1 polymorphism (P = .044). Variability in diarrhea was associated with the two linked polymorphisms in the EGFR promoter (P < .01), but not with erlotinib concentration.

CONCLUSION

Although exploratory in nature, this combined pharmacogenomic and pharmacokinetic model helps to define and differentiate the primary determinants of skin and gastrointestinal toxicity of erlotinib. The findings may be of use both in designing trials targeting a particular severity of rash and in considering dose and schedule modifications in patients experiencing dose-limiting toxicities of erlotinib or similarly targeted agents. Further studies of the relationship between germline polymorphisms in EGFR and the toxicity and efficacy of EGFR inhibitors are warranted.

TotalPlex gene amplification using bulging primers for pharmacogenetic analysis of acute lymphoblastic leukemia

Genetic polymorphism among patients with acute lymphoblastic leukemia (ALL) is an important factor in the effectiveness and toxicity of anti-leukemic drugs. Genotyping of various polymorphisms that impact the outcome of anti-leukemic drug therapy (pharmacogenetics) presents an attractive approach for developing individualized therapy. We developed an easy and accurate method of analyzing multiple genes using a small amount of DNA, which we termed TotalPlex amplification. We used 16 pairs of specific bulging specific primers (SBS primers) for simultaneous amplification of 16 loci in a single PCR tube. Sixteen single nucleotide polymorphisms (SNPs) (CYP3A4*1B A>G, CYP3A5*3 G>A, GSTP1 313 A>G, GSTM1 deletion, GSTT1 deletion, MDR1 exon 21 G>T/A, MDR1 exon 26 C>T, MTHFR 677 C>T, MTHFR 1298 A>C, NR3C1 1088 A>G, RFC 80 G>A, TPMT 238 G>C, TPMT 460 G>A, TPMT 719 A>G, VDR intron 8 G>A, VDR FokI T>C) that have been implicated in the pharmacogenetics of ALL therapy were analyzed by TotalPlex amplification and SNP genotyping. We successfully amplified specific gene fragments using 16 pairs of primers in one PCR reaction tube with minimal spurious amplification products using TotalPlex amplification coupled to a multiplexed bead array detection system. The genotypes of 16 loci from 34 different genomic DNA (gDNA) samples derived using the TotalPlex system were consistent with the results of several standard genotyping methods, including automatic sequencing, PCR restriction fragment length polymorphism (RFLP) analysis, PCR, and allele-specific PCR (AS-PCR). Thus, the TotalPlex system represents a useful method of amplification that can improve the time, cost, and sample size required for high-throughput pharmacogenetic analysis of SNPs.

Advances in adjuvant endocrine therapy for postmenopausal women

Hormone receptor-positive cancers are the most common tumor subtype among postmenopausal women with breast cancer. Despite substantial improvements in disease-free survival and overall survival with tamoxifen and chemotherapy, recurrences still occur, and may ultimately lead to death from breast cancer. Importantly, disease recurrence includes both early and late events, with over half of all recurrences detected more than 5 years from initial breast cancer diagnosis. In recent years, a number of large, randomized trials have evaluated the role of the aromatase inhibitors (AIs) in postmenopausal women with hormone receptor-positive breast cancer. These studies have tested one of three approaches: (1) an upfront AI, (2) a sequential approach after 2-3 years of tamoxifen, and (3) extended endocrine therapy beyond 5 years. Results of these studies have challenged the previous standard of a 5-year course of tamoxifen alone. While the AIs have become a standard component of treatment for most postmenopausal women, many questions remain as to how best tailor endocrine treatment to individual patients. In addition, despite the gains achieved with the AIs, many recurrences are not prevented, and novel strategies are urgently needed, particularly for those women at high risk of recurrence. In this article, we review the efficacy and toxicity data from the available trials of endocrine therapy in the postmenopausal setting. We outline controversies in choosing the optimal endocrine approach, and we discuss selected ongoing studies. Finally, we highlight future research directions, such as the need to understand host and tumor heterogeneity.

Integrated pharmacogenetic prediction of irinotecan pharmacokinetics and toxicity in patients with advanced non-small cell lung cancer

To define an integrated pharmacogenetic model for predicting irinotecan pharmacokinetic (PK) and severe toxicity, we evaluated multivariate analysis using 15 polymorphisms within seven genes with putative influence on metabolism and transport of irinotecan. A total of 107 NSCLC patients treated with irinotecan were evaluated for PK and genotyped for the UGT1A1*6, UGT1A1*28, UGT1A9*22, ABCB11236C>T, 2677G>T/A, 3435C>T, ABCC2-24C>T, 1249G>A, 3972C>T, ABCG234G>A, 421C>A, and SLCO1B1 -11187G>A, 388A>G, and 521T>C, and CYP3A5*3 polymorphisms. Multivariate linear and logistic regression analyses including genotypes and clinicopathologic factors were performed. SN-38 AUC was significantly correlated with ANCs (r=-0.3, p=0.009) and grade 4 neutropenia (p=0.01). The UGT1A1*6/*6, UGT1A9*1/*1 or *1/*22, and SLCO1B1 521TC or CC genotypes, and female-gender were predictive for higher AUC(SN-38) in multivariate analysis. Among them, SLCO1B1 521TC or CC and UGT1A1*6/*6 genotypes were independently predictive for grade 4 neutropenia in multivariate analysis (OR=3.8 and 7.4, respectively). Although no significant association was observed between PK parameters and grade 3 diarrhea, UGT1A9*1/*1, ABCC23972CC, and ABCG234GA or AA genotypes were independently predictive for grade 3 diarrhea in multivariate analysis (OR=6.3, 5.6, and 5.1, respectively). Patient selection based on integrated pharmacogenetic model would be helpful for predicting irinotecan-PK and severe toxicities in NSCLC patients.

Pharmacology, pharmacokinetics and pharmacogenomics of paclitaxel

Paclitaxel is widely used in many cancers including ovarian, breast, lung, head and neck and primary unknown. Paclitaxel is extensively metabolized by cytochrome P450s and excreted in bile. The cytochromes involved include 2C8 and 3A4. This is a review of the pharmacokinetics, pharmacodynamics, drug interactions, metabolism and pharmacogenomics of paclitaxel.

Effect of an antiretroviral regimen containing ritonavir boosted lopinavir on intestinal and hepatic CYP3A, CYP2D6 and P-glycoprotein in HIV-infected patients

This study aimed to quantify the inhibition of cytochrome P450 (CYP3A), CYP2D6, and P-glycoprotein in human immunodeficiency virus (HIV)-infected patients receiving an antiretroviral therapy (ART) containing ritonavir boosted lopinavir, and to identify factors influencing ritonavir and lopinavir pharmacokinetics. We measured activities of CYP3A, CYP2D6, and P-glycoprotein in 28 patients before and during ART using a cocktail phenotyping approach. Activities, demographics, and genetic polymorphisms in CYP3A, CYP2D6, and P-glycoprotein were tested as covariates. Oral midazolam clearance (overall CYP3A activity) decreased to 0.19-fold (90% confidence interval (CI), 0.15-0.23), hepatic midazolam clearance and intestinal midazolam availability changed to 0.24-fold (0.20-0.29) and 1.12-fold (1.00-1.26), respectively. In CYP2D6 extensive metabolizers, the plasma ratio AUC(dextromethorphan)/AUC(dextrorphan) increased to 2.92-fold (2.31-3.69). Digoxin area under the curve (AUC)(0-12) (P-glycoprotein activity) increased to 1.81-fold (1.56-2.09). Covariates had no major influence on lopinavir and ritonavir pharmacokinetics. In conclusion, CYP3A, CYP2D6, and P-glycoprotein are profoundly inhibited in patients receiving ritonavir boosted lopinavir. The covariates investigated are not useful for a priori dose selection.

CYP3A5 genotype is associated with longer patient survival after kidney transplantation and long-term treatment with cyclosporine

The CYP3A5*1 allele has been linked to high expression of CYP3A5 and metabolism of cyclosporine. We evaluated the role of CYP3A5*1 for long-term survival in renal transplant patients in a cohort of 399 patients who underwent cadaveric or living donor kidney allograft transplantation. All patients were treated with a similar cyclosporine-based immunosuppressive maintenance therapy protocol. The mean duration of follow-up was 8.6+/-3.7 years. In univariate survival analysis, the presence of the CYP3A5*1 allele in recipients significantly increased patient survival P=0.028 (log-rank), resulting in a hazard ratio (HR) of 0.52 (95% CI=0.29-0.94). When the presence of the CYP3A5*1 allele was included in multivariate Cox regression analyses accounting for major risk factors for patient death, CYP3A5*1 still conferred a protective effect. Further, haplotype analysis at the CYP3A5 locus confirmed that CYP3A5*1 might indeed be responsible for this survival benefit.

The influence of CYP3A5 expression on the extent of hepatic CYP3A inhibition is substrate-dependent: an in vitro-in vivo evaluation

Despite several studies suggesting that CYP3A5 expression can influence the extent of hepatic CYP3A-mediated inhibition, a systematic in vitro-in vivo evaluation of this potential clinically important issue has not been reported. Using representative probes from two distinct CYP3A substrate subgroups (midazolam, erythromycin), the inhibitory potency of fluconazole was evaluated in pooled human liver microsomes (HLM) with a low or high specific CYP3A5 content, in recombinant CYP3A enzymes (rCYP3A), and in healthy volunteers lacking or carrying the CYP3A5(*)1 allele. Fluconazole was a slightly more potent inhibitor of CYP3A activity in CYP3A5-HLM than in CYP3A5+ HLM with midazolam (K(i) of 15 and 25 microM, respectively) but not with erythromycin (IC(50) of 70 and 54 microM, respectively). In comparison, fluconazole was a much more potent inhibitor of rCYP3A4 than rCYP3A5 with both midazolam (K(i) of 7.7 and 54 microM, respectively) and erythromycin (IC(50) of 100 and 350 microM, respectively). As predicted from HLM, with i.v. midazolam, the average (+/- S.D.) in vivo K(i) (K(i,iv)) was significantly higher in CYP3A5(*)1 carriers (24 +/- 17 and 17 +/- 8 microM for homozygous and heterozygous groups, respectively) than in noncarriers (13 +/- 6 microM) (p = 0.02). With the erythromycin breath test, the average K(i,iv) was not different between homozygous CYP3A5(*)1 carriers (30 +/- 12 microM) and noncarriers (58 +/- 53 microM). In conclusion, the effect of CYP3A5 on hepatic CYP3A-mediated inhibitory drug-drug interactions is substrate-dependent, and HLM, rather than rCYP3A, are the preferred in vitro system for predicting these interactions in vivo.

Pharmacogenomics in gastrointestinal disorders

It is anticipated that unraveling the human genome will have a direct impact on the management of specific diseases. Variations or mutations in genes involved in drug metabolism or disease pathophysiology in gastroenterology and hepatology are expected to have effect on response to therapy. The spectrum of diseases is vast. Thus, we focus this review on clinical pharmacogenetics of inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, liver transplantation, and colon cancer. Although only a few genotyping tests are used regularly in clinical practice, we anticipate that in the future there will be more routine use of many of the tests described in this review.

Comparison of basal gene expression and induction of CYP3As in HepG2 and human fetal liver cells

Human fetal liver (HFL) cell culture was initiated from a pool of six normal human liver tissues. The proliferation and viability of HFL cells were evaluated using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide assay, and the cells increased by more than 100-fold by culture for 15 d. The levels of expression of albumin (ALB), hepatocyte nuclear factor 4alpha, hepatocyte growth factor, CYP3A4, CYP3A5, and CYP3A7 mRNAs in HFL cells increased with culture period, while that of alpha-fetoprotein (AFP) mRNA decreased gradually. In HepG2 cells, however, the expression levels of ALB and AFP mRNAs were not changed, and the levels of expression of CYP3A4, CYP3A5, and CYP3A7 mRNAs decreased gradually. The mRNA expression of major CYP isoforms including CYP3As, i.e., CYP1A2, CYP2A6, CYP2B6, CYP2C (2C9 and 2C19), CYP2D6, and CYP2E1, could be detected in HepG2 cells. With the exception of CYP1A2, all of the CYP mRNAs expressed in HepG2 cells were detected in HFL cells. In HFL cells, CYP3A4 and CYP3A7 mRNA expression levels were markedly up-regulated by dexamethasone (DEX), but not by rifampicin (RIF). CYP3A5 mRNA expression was increased to a level 3-fold greater than control by DEX. On the other hand, CYP3A4, CYP3A5, and CYP3A7 mRNA expression levels in HepG2 cells were increased from 2- to 3-fold by treatment with DEX and RIF. Pregnane X receptor mRNA was expressed in HepG2 cells, but not HFL cells. These results indicate that the character of HFL cells with regard to CYP expression was different from that of HepG2 cells.

1199G>A and 2677G>T/A polymorphisms of ABCB1 independently affect tacrolimus concentration in hepatic tissue after liver transplantation

OBJECTIVE

Tacrolimus is an immunosuppressive drug widely used in hepatic transplantation to avoid graft rejection. Its pharmacokinetics is characterized by a large interindividual variability requiring the use of therapeutic drug monitoring in daily clinical practice. Some genetic polymorphisms in biotransformation enzymes or transporter proteins, such as CYP3A5 and P-glycoprotein (ABCB1), in donors and/or recipients, appear as important determinants of the Tac blood pharmacokinetics. A recent study has shown that Tac hepatic tissue concentrations vary greatly among patients and are well correlated with graft outcome. The aim of our study was to investigate the effect of genetic polymorphisms in biotransformation enzymes (CYP3A5 and CYP3A7) or in their regulatory protein pregnane X receptor as well as in transporter proteins (ABCB1 and OATP-C) on Tac pharmacokinetics in liver transplant patients and more specifically on Tac hepatic concentrations.

METHODS

One hundred and fifty liver donors were genotyped for 13 different polymorphisms. Tac blood and hepatic concentrations were compared according to hepatic genotypes.

RESULTS AND CONCLUSION

We confirmed that Tac dose requirement (on the basis of blood therapeutic drug monitoring) was higher among patients expressing hepatic CYP3A5 (at least one CYP3A5*1 allele) compared with patients who did not (CYP3A5*3/*3). Hepatic expression of CYP3A5, however, did not seem to influence Tac hepatic concentrations. In contrast, ABCB1 genetic polymorphisms significantly influenced Tac hepatic concentrations, whereas their impact on blood concentrations seemed negligible. Among these ABCB1 polymorphisms, the 1199G>A and 2677G>T/A single nucleotide polymorphisms seemed to reduce the activity of P-gp on Tac. As Tac hepatic concentrations have been significantly related to the graft outcome, it might be interesting, in the future, to genotype donors for ABCB1 polymorphisms to better individualize the Tac immunosuppressive therapy in hepatic transplantation.

Influence of garlic (Allium sativum) on the pharmacokinetics of docetaxel

PURPOSE

The herbal supplement garlic (Allium sativum) is commonly used by cancer patients. Preclinical studies have shown that allicin, a major component of garlic, may affect cytochrome P450 3A4 (CYP3A4) activity. This study examines the influence of garlic supplementation on the pharmacokinetics of docetaxel, a CYP3A4 substrate.

EXPERIMENTAL DESIGN

Women with metastatic breast cancer were treated with docetaxel (30 mg/m(2)) given weekly for 3 of 4 weeks. Three days after the initial dose of docetaxel, patients received 600 mg of garlic twice daily for 12 consecutive days. Docetaxel pharmacokinetics were assessed during the first three administrations.

RESULTS

In 10 evaluable patients, the mean baseline clearance of docetaxel was 30.8 L/h/m(2) [95% confidence intervals (95% CI), 16.7-44.9]. Coadministration of garlic reduced mean clearance of docetaxel to 23.7 L/h/m(2) (95% CI, 15.5-31.8) and 20.0 L/h/m(2) (95% CI, 13.3-26.7) on days 8 and 15, respectively (P = 0.17). Additional pharmacokinetic variables of docetaxel, including peak concentration (P = 0.79), area under the curve (P = 0.36), volume of distribution (P = 0.84), and half-life (P = 0.36), were also not statistically significantly different. The mean area under the curve ratio between day 15 and day 1 was 3.74 in three individuals with the CYP3A5*1A/*1A genotype (all African American) compared with 1.02 in six individuals with the CYP3A5*3C/*3C genotype (all Caucasian).

CONCLUSIONS

This study indicates that garlic does not significantly affect the disposition of docetaxel. However, it cannot be excluded that garlic decreases the clearance of docetaxel in patients carrying a CYP3A5*1A allele.

Sex-dependent genetic markers of CYP3A4 expression and activity in human liver microsomes

OBJECTIVE

To find genetic markers of the individual cytochrome P450 (CYP)3A expression.

METHODS

A large collection of liver samples phenotyped for CYP3A expression and activity was genotyped for CYP3A variants. Data were analyzed for associations between CYP3A phenotypes and genotypes, and for evidence of recent selection.

RESULTS

We report associations between the hepatic CYP3A4 protein expression level, as well as its enzymatic activity, measured as verapamil N-dealkylation, and genetic polymorphisms from two regions within the CYP3A gene cluster. One region is defined by several variants, mostly located within CYP3A7, the other by a single nucleotide polymorphism in intron 7 of CYP3A4. The effects of these single nucleotide polymorphisms are sex-dependent. For example, female carriers of T alleles of the single nucleotide polymorphism rs4646437C>T in CYP3A4 intron 7 have, respectively, 5.1-fold and 2.7-fold higher expression and activity compared with male T-carriers, but only 2.2-fold and 1.4-fold higher expression and activity compared with males of genotype CC. A regression analysis indicates that the impact of these single nucleotide polymorphisms in men goes beyond the previously reported sex effect. The rs4646437C undergoes positive selection in Caucasians, as evidenced by its relative extended haplotype homozygosity value located within the uppermost percentile of a genome-wide test set of haplotypes in the same 5% frequency bin.

CONCLUSIONS

Our findings reconcile the apparent contradiction between the evidence for the influence of the individual genetic makeup on CYP3A4 expression and activity suggested by clinical studies, and the failure to identify the responsible gene variants.

Expression of CYP3A isoforms and P-glycoprotein in human stomach, jejunum and ileum

1. CYP3A isoforms metabolise a diverse array of clinically important drugs and P-glycoprotein (P-gp), a transmembrane efflux pump, can extrude a wide variety of drugs from the cell. It has been suggested that the function of CYP3A4 is complementary to that of P-gp along the gastrointestinal (GI) tract, together forming a coordinated intestinal barrier against xenobiotics. Therefore, the expression of CYP3A4, CYP3A5, CYP3A7 and ABCB1 (P-gp) genes were quantified in five normal samples from the human stomach, seven from the jejunum and eight from the ileum by real-time reverse transcription-polymerase chain reaction and western blot analysis. 2. In the tissues examined, measurable mRNA expression of CYP3A was found in almost all samples from the stomach, jejunum and ileum. The rank order for CYP3A mRNA expression was CYP3A4 > CYP3A5 > CYP3A7 in the GI tract studied, whereas median mRNA CYP3A4 expression was highest in the small intestine and lowest in the stomach. Expression of ABCB1 mRNA was found in almost all samples and the median mRNA expression level was comparable in the jejunum and ileum, but lower in the stomach. Our data also show a significant correlation between all mRNA transcripts studied and a wide interindividual variation. 3. At the protein level, CYP3A4 was detected in all stomach and small intestine samples, the levels being substantially higher in the small intestine than in the stomach. P-Glycoprotein was detected in all GI samples, but no statistically significant difference was found along the GI tract considered. 4. Collectively, these results demonstrate that CYP3A4 is the main CYP3A expressed in the GI tract investigated, an extensive interindividual variability in the expression of the different CYP3A isoforms in all tissues examined and P-gp apoprotein levels similar in the stomach, jejunum and ileum.

Impact of CYP3A4 haplotypes on irinotecan pharmacokinetics in Japanese cancer patients

BACKGROUND AND PURPOSE

Cytochrome P450 3A4 (CYP3A4) converts an anticancer prodrug, irinotecan, to inactive metabolites such as APC. However, the contribution of CYP3A4 genetic polymorphisms to irinotecan pharmacokinetics (PK) and pharmacodynamics (PD) is not fully elucidated. In paclitaxel-administered cancer patients, an association of CYP3A4*16B harboring the low activity allele *16 [554C > G (Thr185Ser)] has been shown with altered metabolite/paclitaxel area under the plasma concentration-time curve (AUC) ratios, suggesting a possible impact of *16B on the PK of other drugs. In this study, the effects of CYP3A4 haplotypes including *16B on irinotecan PK/PD were investigated in irinotecan-administered patients.

METHODS

The CYP3A4 genotypes for 177 Japanese cancer patients who received irinotecan were defined in terms of 4 major haplotypes, i.e., *1A (wild type), *1G (IVS10 + 12G > A), *16B [554C > G (Thr185Ser) and IVS10 + 12G > A], and *18B [878T > C (Leu293Pro) and IVS10 + 12G > A]. Associations of CYP3A4 genotypes with irinotecan PK and severe toxicities (grade 3 diarrhea and grade 3 or 4 neutropenia) were investigated.

RESULTS

Area under the concentration-time curve ratios of APC/irinotecan, an in vivo parameter for CYP3A4 activity, were significantly higher in females than in males. The male patients with *16B showed significantly decreased AUC ratios (APC/irinotecan) with 50% of the median value of the non-*16B male patients (no *16B-bearing female patients in this study), whereas no significant alteration in the AUC ratios was observed in the patients with *18B. A slight trend toward increasing AUC ratios (20%) was detected in both male and female patients bearing *1G. Multivariate analysis confirmed contributions of CYP3A4*16B (coefficient +/- SE = -0.18 +/- 0.077, P = 0.021) and *1G (0.047 +/- 0.021, P = 0.029) to the AUC ratio. However, no significant association was observed between the CYP3A4 genotypes and total clearance of irinotecan or toxicities (severe diarrhea and neutropenia).

CONCLUSION

This study suggested that CYP3A4*16B was associated with decreased metabolism of irinotecan to APC. However, the clinical impact of CYP3A4 genotypes on total clearance and irinotecan toxicities was not significant.

The CYP3A4*18 allele, the most frequent coding variant in asian populations, does not significantly affect the midazolam disposition in heterozygous individuals

The objective of this study was to identify CYP3A4 variants in Koreans and to characterize their functional consequences in vitro and in vivo. Four single nucleotide polymorphisms were identified in 50 Koreans by direct DNA sequencing. In an additional genotyping using 248 subjects, CYP3A4(*)18 was confirmed as the most frequent coding variant in Koreans at 1.7%, and its frequency was similar to that of Asians, suggesting that CYP3A4(*)18 would be the highest coding variant in Asians. The recombinant CYP3A4.18 protein prepared in baculovirus expression system showed 67.4% lower Vmax and 1.8-fold higher K(m) for midazolam 1'-hydroxylation compared with the wild type. The mean values of Cmax and area under the concentration curve (AUC) in the CYP3A4(*)1/(*)18 and CYP3A5(*)1/(*)3 subjects (n = 8) were 63% and 32% higher than in CYP3A4(*)1/(*)1 and CYP3A5(*)1/(*)3 carriers (n = 8), respectively. Although the in vitro assay exhibited a significant reduction of the enzyme activity for midazolam, the in vivo differences associated with the CYP3A4(*)1/(*)18 tend to be low (P < 0.07 in Cmax and P < 0.09 in AUC). In summary, the heterozygous CYP3A4(*)1/(*)18 does not appear to cause a significant change of midazolam disposition in vivo; however, the clinical relevance of CYP3A4(*)18/(*)18 remains to be evaluated.

Genetic polymorphism of cytochrome P450s and P-glycoprotein in the Finnish population

The objective of this study was to investigate the genetic polymorphism of selected cytochrome P450 (CYP) enzymes and ABCB1 (encoding P-glycoprotein) of central importance with regard to the disposition of clinically used drugs in the Finnish population and to compare the results to pre-existing data from Caucasian populations. A random sample of 449 Finns was studied. Single nucleotide polymorphisms (SNPs) were genotyped using blood-derived genomic DNA and 5'-nuclease assays. We found that the allele frequencies of CYP1A2 SNP g.-163C>A, CYP2C8*3, CYP2C9*2, CYP2C9*3 and CYP2C19*2 were similar to those seen in other Caucasian populations. However, the allelic frequency of the variant ABCB1 SNP c.3435C>T allele was lower than previously reported. The frequency of the homozygous CYP3A5*1 expression was significantly higher than expected based on Hardy-Weinberg calculations (observed n = 8 vs. expected n = 3, P = 0.01). Other genotype frequencies corresponded to the expected values. The strong linkage between the CYP2C8*3 and the CYP2C9*2 alleles was confirmed in this study and the number of individuals with the rare haplotype CYP2C8*3*3/CYP2C9*2*2 was higher than expected. We conclude that the frequency of mutated CYP alleles in Finns were in agreement with earlier findings in Caucasian populations, but a lower frequency of the ABCB1 variant allele 3435T corresponding to that reported in Asian populations was found. The higher than expected frequency of the CYP3A5*1*1 genotype and the CYP2C8*3*3/CYP2C9*2*2 haplotype may influence the response to treatment with drugs metabolized by these enzymes.

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

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

Effects of genetic polymorphism of cytochrome P450 enzymes on the pharmacokinetics of benzodiazepines

Pharmacogenetic studies have shown that several cytochrome P450 (CYP) enzymes exhibit genetic polymorphisms. Several benzodiazepines (BZPs) are metabolized predominantly or partly by polymorphic CYP2C19 and CYP3A4/5. The pharmacokinetics of diazepam, etizolam, quazepam and desmethylclobazam have been shown to be affected by CYP2C19 polymorphism. The CYP3A5 polymorphism has been reported to affect the pharmacokinetics of alprazolam, but its effect on midazolam kinetics has been inconclusive. For etizolam and desmethylclobazam, some data suggest that CYP2C19 deficiency leads to side-effects or toxicity. For the remaining BZPs the clinical significance of the observed pharmacokinetic changes remains unclear. Further studies on the effects of genetic polymorphisms of CYP enzymes on the pharmacokinetics and pharmacodynamics of BZPs are necessary to guide treatment individualization and optimization.