Genetic contribution to variable human CYP3A-mediated metabolism

Midazolam and cyclosporin a metabolism in transgenic mice with liver-specific expression of human CYP3A4

Cytochrome P450 3A4 (CYP3A4) is a major determinant of the metabolism of many drugs, including important anticancer drugs, with sometimes profound impact on therapeutic efficacy and toxic side effects. To study in vivo CYP3A(4) functions, we have generated and characterized transgenic mice with functional expression of human CYP3A4 cDNA in the liver. Two transgenic lines displayed substantial, physiologically relevant and stable CYP3A4 levels in liver and moderate levels in kidney, but not in small intestine. The mice did not display obvious physiological abnormalities. The CYP3A4 substrate drugs midazolam and cyclosporin A were used to test functional activity of CYP3A4 in liver. The area under the plasma concentration versus time curve (AUC) of intravenously administered midazolam (30 mg/kg) was 2.2-fold decreased in the transgenic mice compared with wild-type (5.45 +/- 0.21 versus 11.7 +/- 0.46 microg . h ml(-1); P < 0.01), and early formation of the primary metabolite 1-hydroxymidazolam was about 2-fold increased, demonstrating the functionality of CYP3A4 in the liver. Similarly, following intravenous administration of cyclosporin A (20 mg/kg), CYP3A4 transgenic mice displayed a reduced plasma AUC compared with wild-type (24.3 +/- 0.66 versus 35.8 +/- 0.53 microg . h ml(-) (1); P < 0.01). Thus, midazolam and cyclosporin A, compounds with markedly different clearance rates and half-lives, both demonstrated clearly accelerated kinetics in the CYP3A4 transgenic mice. We expect that this CYP3A4 transgenic model will provide a useful tool to study the impact of CYP3A4 on drug levels, especially when combined with other transgenic and knockout strains.

Evaluation of first-pass cytochrome P4503A (CYP3A) and P-glycoprotein activities using alfentanil and fexofenadine in combination

Cytochrome P4503A (CYP3A) and P-glycoprotein (P-gp) are major determinants of oral bioavailability. Development of in vivo probe(s), for both CYP3A and P-gp, which could be administered in combination, is a current goal. Nevertheless, there is considerable overlap in CYP3A and P-gp substrate selectivities; there are few discrete probes. Alfentanil is a selective CYP3A probe but not a P-gp substrate. Fexofenadine is a P-gp probe but not a CYP3A substrate. This investigation tested the hypothesis that alfentanil and fexofenadine could be administered in combination to probe first-pass CYP3A and P-gp activities in humans. Two 3-way crossover studies were conducted in healthy volunteers. In the first protocol, subjects received oral alfentanil alone, fexofenadine alone, or fexofenadine 1 hour after alfentanil. In the second protocol, subjects abstained from citrus and apple products for 5 days and received fexofenadine alone, fexofenadine 1 hour after alfentanil, or alfentanil 4 hours after fexofenadine. An assay using solid-phase extraction and electrospray liquid chromatography/mass spectrometry was developed for the simultaneous quantification of plasma alfentanil and fexofenadine. In both protocols, alfentanil plasma concentrations and area under the concentration versus time curve (AUC) were unaffected by fexofenadine or meal composition. Fexofenadine given 1 hour after alfentanil and followed 1 hour later by a meal containing orange or apple juice had a somewhat lower AUC compared with fexofenadine alone (geometric mean ratio with and without the interacting drug = 0.73, 90% confidence interval [CI] = 0.59-1.04). Fexofenadine given 1 hour after alfentanil and followed 2 hours later by a meal not containing citrus or apple products had an AUC that was unchanged compared with fexofenadine alone (ratio = 0.91, 90% CI = 0.70-1.35). These results show that alfentanil disposition was not affected by fexofenadine. A dosing regimen was identified in which fexofenadine disposition was not affected by alfentanil. The timing and content of meals after fexofenadine had a significant effect on fexofenadine disposition. Alfentanil and fexofenadine in combination appear to be a useful probe for evaluating both first-pass CYP3A and P-gp activities in humans.

Genetic predictors of the maximum doses patients receive during clinical use of the anti-epileptic drugs carbamazepine and phenytoin

Phenytoin and carbamazepine are effective and inexpensive anti-epileptic drugs (AEDs). As with many AEDs, a broad range of doses is used, with the final "maintenance" dose normally determined by trial and error. Although many genes could influence response to these medicines, there are obvious candidates. Both drugs target the alpha-subunit of the sodium channel, encoded by the SCN family of genes. Phenytoin is principally metabolized by CYP2C9, and both are probable substrates of the drug transporter P-glycoprotein. We therefore assessed whether variation in these genes associates with the clinical use of carbamazepine and phenytoin in cohorts of 425 and 281 patients, respectively. We report that a known functional polymorphism in CYP2C9 is highly associated with the maximum dose of phenytoin (P = 0.0066). We also show that an intronic polymorphism in the SCN1A gene shows significant association with maximum doses in regular usage of both carbamazepine and phenytoin (P = 0.0051 and P = 0.014, respectively). This polymorphism disrupts the consensus sequence of the 5' splice donor site of a highly conserved alternative exon (5N), and it significantly affects the proportions of the alternative transcripts in individuals with a history of epilepsy. These results provide evidence of a drug target polymorphism associated with the clinical use of AEDs and set the stage for a prospective evaluation of how pharmacogenetic diagnostics can be used to improve dosing decisions in the use of phenytoin and carbamazepine. Although the case made here is compelling, our results cannot be considered definitive or ready for clinical application until they are confirmed by independent replication.

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.

Cytochrome P450 humanised mice

Humans are exposed to countless foreign compounds, typically referred to as xenobiotics. These can include clinically used drugs, environmental pollutants, food additives, pesticides, herbicides and even natural plant compounds. Xenobiotics are metabolised primarily in the liver, but also in the gut and other organs, to derivatives that are more easily eliminated from the body. In some cases, however, a compound is converted to an electrophile that can cause cell toxicity and transformation leading to cancer. Among the most important xenobiotic-metabolising enzymes are the cytochromes P450 (P450s). These enzymes represent a superfamily of multiple forms that exhibit marked species differences in their expression and catalytic activities. To predict how humans will metabolise xenobiotics, including drugs, human liver extracts and recombinant P450s have been used. New humanised mouse models are being developed which will be of great value in the study of drug metabolism, pharmacokinetics and pharmacodynamics in vivo, and in carrying out human risk assessment of xenobiotics. Humanised mice expressing CYP2D6 and CYP3A4, two major drug-metabolising P450s, have revealed the feasibility of this approach.

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.

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.

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.

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.

Association Between the CYP3A5 Genotype and Blood Pressure

We tested the hypothesis that the presence of a CYP3A5 * 1 allele is associated with increases in blood pressure in 2 studies of subjects with a total of 683 participants. The first study involving 271 subjects was part of a longitudinal study conducted at Indiana University Medical Center that consisted of 2 phases. The first phase studied the relationship of salt sensitivity with blood pressure, whereas the second phase, conducted ≈26 years later, studied the relationship between blood pressure, carbohydrate intolerance, and vascular compliance in the same subjects. The second study was a cross-sectional evaluation of 412 normotensive and hypertensive subjects conducted at the University of California San Diego. The second study (Mantel–Haenszel χ 2 test; P =0.05) showed that a greater proportion of black participants with poor blood pressure control had CYP3A5 * 1 /* 1 genotype. Evaluation of the untreated blood pressure from phase 1 of the first study showed that the blacks with CYP3A5 * 3 /* 3 (146±35 mm Hg) had a higher systolic blood pressure than those with the * 1 /* 3 (119±14.1 mm Hg; P =0.0006) and * 1 /* 1 (125±17.4 mm Hg; P =0.009) genotypes. For blacks in study 2, the CYP3A5 * 1 allele was more common in hypertensives (Fisher exact test; P =0.025) than normotensives. In whites there was no association between CYP3A5 genotype and blood pressure in either study. We conclude that although untreated blood pressure may be higher in blacks with the CYP3A5 * 3 /* 3 genotype, the CYP3A5 * 1 allele may be associated with hypertension that is more refractory to treatment in this ethnic group.

Tumoral drug metabolism: overview and its implications for cancer therapy

Drug-metabolizing enzymes (DME) in tumors are capable of biotransforming a variety of xenobiotics, including antineoplastics, resulting in either their activation or detoxification. Many studies have reported the presence of DME in tumors; however, heterogeneous detection methodology and patient cohorts have not generated consistent, firm data. Nevertheless, various gene therapy approaches and oral prodrugs have been devised, taking advantage of tumoral DME. With the need to target and individualize anticancer therapies, tumoral processes such as drug metabolism must be considered as both a potential mechanism of resistance to therapy and a potential means of achieving optimal therapy. This review discusses cytotoxic drug metabolism by tumors, through addressing the classes of the individual DME, their relevant substrates, and their distribution in specific malignancies. The limitations of preclinical models relative to the clinical setting and lack of data on the changes of DME with disease progression and host response will be discussed. The therapeutic implications of tumoral drug metabolism will be addressed-in particular, the role of DME in predicting therapeutic response, the activation of prodrugs, and the potential for modulation of their activity for gain are considered, with relevant clinical examples. The contribution of tumoral drug metabolism to cancer therapy can only be truly ascertained through large-scale prospective studies and supported by new technologies for tumor sampling and genetic analysis such as microarrays. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to improve drug efficacy and individualize therapy.

Metabolism-based drug-drug interactions: what determines individual variability in cytochrome P450 induction?

Individual variability in cytochrome P450 (P450) induction comprises an important component contributing to the difficulties in assessing and predicting metabolism-based drug-drug interactions in humans. In this article, we outline the major factors responsible for the individual variability in P450 induction, including variable transporter activity and metabolism of inducers in vivo, genetic variations of P450 genes and their regulatory regions, genetic variations of receptors and regulatory proteins required for induction, and different physiological and environmental elements. With a better understanding of the major determinants in P450 induction and a profile of the phenotypes of these determinants in each individual, it is believed that the individual variability in induction-mediated drug-drug interactions can be adequately evaluated.

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.

A phase I/II study of infusional vinblastine with the P-glycoprotein antagonist valspodar (PSC 833) in renal cell carcinoma

PURPOSE

P-glycoprotein (Pgp) inhibitors have been under clinical evaluation for drug resistance reversal for over a decade. Valspodar (PSC 833) inhibits Pgp-mediated efflux but delays drug clearance, requiring reduction of anticancer drug dosage. We designed an infusional schedule for valspodar and vinblastine to mimic infusional vinblastine alone. The study was designed to determine the maximally tolerated dose of vinblastine, while attempting to understand the pharmacokinetic interactions between vinblastine and valspodar and to determine the response rate in patients with metastatic renal cell cancer.

PATIENTS AND METHODS

Thirty-nine patients received continuous infusion valspodar and vinblastine. Vinblastine was administered for 3 days to compensate for the expected delay in clearance and the required dose reduction. Valspodar was administered initially at a dose of 10 mg/kg/d; the dose of vinblastine varied.

RESULTS

The maximum-tolerated dose of vinblastine was 1.3 mg/m(2)/d. As suggested previously, serum valspodar concentrations exceeded those needed for Pgp inhibition. Consequently, the dose of valspodar was reduced to 5 mg/kg, allowing a vinblastine dose of 2.1 mg/m(2)/d to be administered. Pharmacodynamic studies demonstrated continued inhibition of Pgp at lower valspodar doses by functional assay in Pgp-expressing CD56+ cells and by (99m)Tc-sestamibi imaging. A 15-fold range in cytochrome p450 activity was observed, as measured by midazolam clearance. No major responses were observed.

CONCLUSIONS

These results suggest that the pharmacokinetic impact of cytochrome P450 inhibition by valspodar can be reduced although not eliminated, while preserving Pgp inhibition, thus separating the pharmacokinetic and pharmacodynamic activities of valspodar.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Immunosuppressant Drug Monitoring: Is the Laboratory Meeting Clinical Expectations?

OBJECTIVE:

To review the literature relating to immunosuppressant drug measurement as performed in therapeutic drug monitoring laboratories associated with transplantation centers and consider whether the assay methods widely used for patient dosage management achieve acceptable quality criteria in the context of other sources of variability with these drugs.

DATA SOURCES:

Articles used were accessed primarily through MEDLINE, as well as references cited in related publications. Searches were restricted to organ transplantation in humans.

STUDY SELECTION AND DATA EXTRACTION:

Emphasis was placed on the literature relating to the quality of immunosuppressant drug assays, their limitations, and evidence of clinical benefit in dosage individualization.

DATA SYNTHESIS:

There is a dilemma evident between the quality of the analytical services offered by some diagnostic immunoassay manufacturers and the ability of a significant number of clinical laboratories globally to select only appropriate assay methods.

CONCLUSIONS:

In many cases, clinical laboratories fail to meet the reasonable clinical expectations required for interpretation of immunosuppressant drug assay results as an adjunct to optimal dosage individualization and patient care.

Pharmacokinetic Considerations Relating to Tacrolimus Dosing in the Elderly

Relaxation of the upper age limits for solid organ transplantation coupled with improvements in post-transplant survival have resulted in greater numbers of elderly patients receiving immunosuppressant drugs such as tacrolimus. Tacrolimus is a potent agent with a narrow therapeutic window and large inter- and intraindividual pharmacokinetic variability. Numerous physiological changes occur with aging that could potentially affect the pharmacokinetics of tacrolimus and, hence, patient dosage requirements. Tacrolimus is primarily metabolised by cytochrome P450 (CYP) 3A enzymes in the gut wall and liver. It is also a substrate for P-glycoprotein, which counter-transports diffused tacrolimus out of intestinal cells and back into the gut lumen. Age-associated alterations in CYP 3A and P-glycoprotein expression and/or activity, along with liver mass and body composition changes, would be expected to affect the pharmacokinetics of tacrolimus in the elderly. However, interindividual variation in these processes may mask any changes caused by aging. More investigation is needed into the impact aging has on CYP and P-glycoprotein activity and expression. No single-dose, intense blood-sampling study has specifically compared the pharmacokinetics of tacrolimus across different patient age groups. However, five population pharmacokinetic studies, one in kidney, one in bone marrow and three in liver transplant recipients, have investigated age as a co-variate. None found a significant influence for age on tacrolimus bioavailability, volume of distribution or clearance. The number of elderly patients included in each study, however, was not documented and may have been only small. It is likely that inter- and intraindividual pharmacokinetic variability associated with tacrolimus increase in elderly populations. In addition to pharmacokinetic differences, donor organ viability, multiple co-morbidity, polypharmacy and immunological changes need to be considered when using tacrolimus in the elderly. Aging is associated with decreased immunoresponsiveness, a slower body repair process and increased drug adverse effects. Elderly liver and kidney transplant recipients are more likely to develop new-onset diabetes mellitus than younger patients. Elderly transplant recipients exhibit higher mortality from infectious and cardiovascular causes than younger patients but may be less likely to develop acute rejection. Elderly kidney recipients have a higher potential for chronic allograft nephropathy, and a single rejection episode can be more devastating. There is a paucity of information on optimal tacrolimus dosage and target trough concentration in the elderly. The therapeutic window for tacrolimus concentrations may be narrower. Further integrated pharmacokinetic-pharmacodynamic studies of tacrolimus are required. It would appear reasonable, based on current knowledge, to commence tacrolimus at similar doses as those used in younger patients. Maintenance dose requirements over the longer term may be lower in the elderly, but the increased variability in kinetics and the variety of factors that impact on dosage suggest that patient care needs to be based around more frequent monitoring in this age group.

Regulation of CYP3A genes in the human respiratory tract

The CYP3A gene cluster consists of four members, CYP3A4, CYP3A5, CYP3A7 and CYP3A43. Especially the CYP3A4 and CYP3A5 enzymes play a significant role in the metabolism of numerous exogenous (drugs, pollutants, procarcinogens) and endogenous (steroids, bile acids) compounds. CYP3A5 protein is present in the liver and some extrahepatic tissues, such as the gut wall, kidney, adrenal gland, prostate and many cell types in the lung. In the lung, the highest amounts of CYP3A5 protein are present in bronchial and alveolar epithelial cells, bronchial glands and alveolar macrophages. The same cells types have little or no CYP3A4 expression. Cigarette smoking markedly represses CYP3A5 content in alveolar macrophages. CYP3A5 is upregulated by glucocorticoids via the glucocorticoid receptor (GR) in lung adenocarcinoma derived A549 cells. Tissue selective distribution of CYP3A4 is controlled by tissue enriched transcription factors, such as hepatic nuclear factor 4alpha (HNF4alpha), and ligand dependent nuclear receptors, most notably pregnane X receptor (PXR) and constitutive androstane receptor (CAR). The selective expression of CYP3A5 over CYP3A4 in specific lung cells is likely to be the sum of the effects of tissue-specific upregulating and downregulating transcription factors in these cells. Since the CYP3A4/5 enzymes mediate the metabolism of many exogenous and endogenous compounds with direct relevance to pulmonary physiology and pathology, the functions of these enzymes and factors controlling them should be elucidated in much more detail.

Mechanisms of drug resistance in cancer chemotherapy

The management of cancer involves procedures, which include surgery, radiotherapy and chemotherapy. Development of chemoresistance is a persistent problem during the treatment of local and disseminated disease. A plethora of cytotoxic drugs that selectively, but not exclusively, target actively proliferating cells include such diverse groups as DNA alkylating agents, antimetabolites, intercalating agents and mitotic inhibitors. Resistance constitutes a lack of response to drug-induced tumour growth inhibition; it may be inherent in a subpopulation of heterogeneous cancer cells or be acquired as a cellular response to drug exposure. Resistance varies. Although regulatory approval may require efficacy in as few as 20% of trial cohorts, a drug may subsequently be used in unselected patients displaying resistance to the treatment. Principal mechanisms may include altered membrane transport involving the P-glycoprotein product of the multidrug resistance (MDR) gene as well as other associated proteins, altered target enzyme (e.g. mutated topoisomerase II), decreased drug activation, increased drug degradation due to altered expression of drug-metabolising enzymes, drug inactivation due to conjugation with increased glutathione, subcellular redistribution, drug interaction, enhanced DNA repair and failure to apoptose as a result of mutated cell cycle proteins such as p53. Attempts to overcome resistance mainly involve the use of combination drug therapy using different classes of drugs with minimally overlapping toxicities to allow maximal dosages and with narrowest cycle intervals, necessary for bone marrow recovery. Adjuvant therapy with P-glycoprotein inhibitors and, in specific instances, the use of growth factor and protein kinase C inhibitors are newer experimental approaches that may also prove effective in abrogating or delaying onset of resistance. Gene knockout using antisense molecules may be another effective way of blocking drug resistance genes. Conversely, drug resistance may also be used to good purpose by transplanting retrovirally transformed CD34 cells expressing the MDR gene to protect the bone marrow during high-dose chemotherapy.

Lack of Interaction of Milnacipran with the Cytochrome P450 Isoenzymes Frequently Involved in the Metabolism of Antidepressants

OBJECTIVE

To compare the pharmacokinetics of milnacipran in extensive metabolisers (EMs) and poor metabolisers (PMs) of sparteine and mephenytoin, and to assess the influence of multiple administrations of milnacipran on the activity of cytochrome P450 (CYP) isoenzymes through its own metabolism and through various probes, namely CYP2D6 (sparteine/dextromethorphan), CYP2C19 (mephenytoin), CYP1A2 (caffeine) and CYP3A4 (endogenous 6-beta-hydroxy-cortisol excretion).

METHODS

Twenty-five healthy subjects, 12 EMs for both sparteine/dextromethorphan and mephenytoin, nine EMs for mephenytoin and PMs for sparteine/dextromethorphan (PM(2D6)) and four PMs for mephenytoin and EMs for sparteine/dextromethorphan (PM(2C19)) were administered milnacipran as a single 50 mg capsule on day 1 followed by a 50 mg capsule twice daily for 7 days. The pharmacokinetics of milnacipran and its oxidative metabolites were assessed after the first dose (day 1) and after multiple administration (day 8), and were compared for differences between CYP2D6 and CYP2C19 PMs and EMs. Metabolic tests were performed before (day -2), during (days 1 and 8) and after (day 20) milnacipran administration.

RESULTS

Milnacipran steady state was rapidly achieved. Metabolism was limited: approximately 50% unchanged drug, 30% as glucuronide and 20% as oxidative metabolite (mainly F2800 the N-dealkyl metabolite). Milnacipran administration to PM2D6 and PM2C19 subjects did not increase parent drug exposure or decrease metabolite exposure. Milnacipran oxidative metabolism is not mediated through CYP2D6 or CYP2C19 polymorphic pathways nor does it significantly interact with CYP1A2, CYP2C19, CYP2D6 or CYP3A4 activities.

CONCLUSION

Limited reciprocal pharmacokinetic interaction between milnacipran and CYP isoenzymes would confer flexibility in the therapeutic use of the drug when combined with antidepressants. Drug-drug interaction risk would be low, even if the combined treatments were likely to inhibit CYP2D6 and CYP2C19 isoenzyme activities.

Effect of CYP2D6*10 on the Pharmacokinetics of R- and S-Carvedilol in Healthy Japanese Volunteers

This study was performed to investigate the effect of CYP2D6*10 on the pharmacokinetics of R- and S-carvedilol in healthy Japanese volunteers. Five or 10 mg of carvedilol was orally administered to 23 subjects (22-44 years old), and blood samples were taken at 2 and 6 h after dosing. We determined the polymorphic alleles of CYP2D6 in each subject. The whole blood concentration of R- and S-carvedilol was measured by an HPLC method. The pharmacokinetic parameters in individual subjects were estimated by the Bayesian method using the nonlinear mixed effects model (NONMEM) program. The mean values of oral clearance for R- and S-carvedilol were estimated to be 1.01 and 2.15 l/h/kg, respectively. The oral clearance was highly correlated with the apparent volume of distribution among the subjects, suggesting that the interindividual difference in bioavailability was largely responsible for the pharmacokinetic variability of carvedilol. The oral clearance and also volume of distribution of both enantiomers were significantly lower in the subjects with the CYP2D6*10 allele than with the CYP2D6*1/*1 or *1/*2 genotype. These results suggested that the systemic and/or pre-systemic metabolism of R- and S-carvedilol in the liver is significantly decreased in Japanese with the CYP2D6*10 allele.

Cytochrome P450 3A polymorphisms and immunosuppressive drugs

With the use of powerful immunosuppressive drugs, organ transplantation has become the treatment of choice for many cases of end-stage chronic organ failure. The calcineurin inhibitors, cyclosporine and tacrolimus, which are the backbone of current immunosuppressive regimens, may be difficult to use because of the large interindividual variability of their pharmacokinetic characteristics and a narrow therapeutic index. Since cytochrome P450 (CYP) 3A4 and CYP3A5 are both involved in their metabolism, the consequences of the polymorphism of these enzymes were studied. It has been recently shown that the CYP3A5*3 polymorphism is associated with both the pharmacokinetics and pharmacodynamic consequences of tacrolimus. The association between the CYP3A4 and CYP3A5 polymorphisms and cyclosporine pharmacokinetics is more questionable. It is important to test these initial results prospectively to improve the individualized use of these drugs.

Role of Cytochrome P450 Activity in the Fate of Anticancer Agents and in Drug Resistance

Although activity of cytochrome P450 isoenzymes (CYPs) plays a major role in the fate of anticancer agents in patients, there are relatively few clinical studies that evaluate drug metabolism with therapeutic outcome. Nevertheless, many clinical reports in various non-oncology fields have shown the dramatic importance of CYP activity in therapeutic efficacy, safety and interindividual variability of drug pharmacokinetics. Moreover, variability of drug metabolism in the liver as well as in cancer cells must also be considered as a potential factor mediating cancer resistance. This review underlines the role of drug metabolism mediated by CYPs in pharmacokinetic variability, drug resistance and safety. As examples, biotransformation pathways of tamoxifen, paclitaxel and imatinib are reviewed. This review emphasises the key role of therapeutic drug monitoring as a complementary tool of investigation to in vitro data. For instance, pharmacokinetic data of anticancer agents have not often been published within subpopulations of patients who show ultra-rapid, extensive or poor metabolism (e.g. due to CYP2D6 and CYP2C19 genotypes). Besides kinetic variability in the systemic circulation, induction of CYP activity may participate in creating drug resistance by speeding up the cancer agent degradation specifically in the target cells. For one cancer agent, various mechanisms of resistance are usually identified within different cell clones. This review also tries to emphasise that drug resistance mediated by CYP activity in cancer cells should be taken into consideration to a greater degree. The unequivocal identification of the metabolising enzymes involved in clinical conditions will eventually allow improvement and individualisation of anticancer agent therapy, i.e. drug dosage and selection. In addition, a more complete understanding of the metabolism of anticancer agents will assist in the prediction of drug-drug interactions, as anticancer agent combinations are becoming more prevalent.

Factors Affecting Cytochrome P-450 3A Activity in Cancer Patients

PURPOSE

The purpose is to identify the demographic, physiologic, and inheritable factors that influence CYP3A activity in cancer patients.

EXPERIMENTAL DESIGN

A total of 134 patients (62 females; age range, 26 to 83 years) underwent the erythromycin breath test as a phenotyping probe of CYP3A. Genomic DNA was screened for six variants of suspected functional relevance in CYP3A4 (CYP3A4*1B, CYP3A4*6, CYP3A4*17, and CYP3A4*18) and CYP3A5 (CYP3A5*3C and CYP3A5*6).

RESULTS

CYP3A activity (AUC(0-40 min)) varied up to 14-fold in this population. No variants in the CYP3A4 and CYP3A5 genes were a significant predictor of CYP3A activity (P > 0.2954). CYP3A activity was reduced by approximately 50% in patients with concurrent elevations in liver transaminases and alkaline phosphatase or elevated total bilirubin (P < 0.001). In a multivariate analysis, CYP3A activity was not significantly influenced by age, sex, and body size measures (P > 0.05), but liver function combined with the concentration of the acute-phase reactant, alpha-1 acid glycoprotein, explained approximately 18% of overall variation in CYP3A activity (P < 0.001).

CONCLUSIONS

These data suggest that baseline demographic, physiologic, and chosen genetic polymorphisms have a minor impact on phenotypic CYP3A activity in patients with cancer. Consideration of additional factors, including the inflammation marker C-reactive protein, as well as concomitant use of other drugs, food constituents, and complementary and alternative medicine with inhibitory and inducible effects on CYP3A, is needed to reduce variation in CYP3A and treatment outcome to anticancer therapy.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

CYP3A variation and the evolution of salt-sensitivity variants

Members of the cytochrome P450 3A subfamily catalyze the metabolism of endogenous substrates, environmental carcinogens, and clinically important exogenous compounds, such as prescription drugs and therapeutic agents. In particular, the CYP3A4 and CYP3A5 genes play an especially important role in pharmacogenetics, since they metabolize >50% of the drugs on the market. However, known genetic variants at these two loci are not sufficient to account for the observed phenotypic variability in drug response. We used a comparative genomics approach to identify conserved coding and noncoding regions at these genes and resequenced them in three ethnically diverse human populations. We show that remarkable interpopulation differences exist with regard to frequency spectrum and haplotype structure. The non-African samples are characterized by a marked excess of rare variants and the presence of a homogeneous group of long-range haplotypes at high frequency. The CYP3A5*1/*3 polymorphism, which is likely to influence salt and water retention and risk for salt-sensitive hypertension, was genotyped in >1,000 individuals from 52 worldwide population samples. The results reveal an unusual geographic pattern whereby the CYP3A5*3 frequency shows extreme variation across human populations and is significantly correlated with distance from the equator. Furthermore, we show that an unlinked variant, AGT M235T, previously implicated in hypertension and pre-eclampsia, exhibits a similar geographic distribution and is significantly correlated in frequency with CYP3A5*1/*3. Taken together, these results suggest that variants that influence salt homeostasis were the targets of a shared selective pressure that resulted from an environmental variable correlated with latitude.

Association of CYP3A4 genotype with detection of Vγ/Jβ trans-rearrangements in the peripheral blood leukocytes of pediatric cancer patients undergoing chemotherapy for ALL

Cancer patients receiving chemotherapy are exposed to high doses of cytotoxic and genotoxic drugs which, in some cases, can lead to treatment related leukemia. Since this only occurs in a minority of patients, however, it is possible some individuals are predisposed due to genetic polymorphisms in genes for enzymes that mediate drug metabolism. To address this possibility we measured the genotoxicity of chemotherapeutic agents in patients receiving treatment for ALL by the frequency of the Vgamma/Jbeta trans-rearrangement in their peripheral blood leukocytes and compared this with CYP3A4 genotype. CYP3A4 is the most abundant of the cytochrome P450 (CYP) enzyme in the liver and intestine which contains a common -392A>G substitution in the promoter region (CYP3A4*1B allele). We found a significant increase in the frequency of rearrangements during chemotherapy only in patients homozygous for the wild type CYP3A4*1A allele. This provides a direct link between CYP3A4 genotype and susceptibility to drug genotoxicity thus strengthening the possibility that predisposition to treatment related leukemia may be measurable by simple genetic testing.

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

BACKGROUND

A body-weight-based dose of tacrolimus often results in marked individual diversity of blood drug concentration. Tacrolimus is a substrate for cytochrome P450 (CYP) 3A5 and p-glycoprotein encoded by CYP3A5 and MDR1 (ABCB1), respectively, having multiple single nucleotide polymorphisms. In this study, we genotyped CYP3A5 A6986G, MDR1 G2677(A/T), and C3435T polymorphisms and investigated the association between these polymorphisms and the pharmacokinetics of tacrolimus in renal transplant recipients.

METHODS

Thirty consecutive recipients were enrolled in this study. The pharmacokinetics of tacrolimus was analyzed on day 28 after transplant, when the daily dose was adjusted to the target trough level of 10-15 ng/mL. The polymerase chain reaction-restriction fragment length polymorphism and direct sequence method were used for genotyping the CYP3A5 and MDR1 polymorphisms, respectively.

RESULTS

The single tacrolimus dose per body weight was significantly higher in CYP3A5 *1 carriers than CYP3A5 *3/*3 carriers (0.143+/-0.050 vs. 0.078+/-0.031 mg/kg, P<0.001). The dose-adjusted trough level and the area under the concentration-time curve (AUC0-12) were significantly lower in CYP3A5 *1 carriers than CYP3A5 *3/*3 carriers (0.040+/-0.014 vs. 0.057+/-0.024 ng/mL/mg/kg, P=0.015 and 0.583+/-0.162 vs. 0.899+/-0.319 ng.hr/mL/mg/kg, P=0.004), respectively. The MDR1 polymorphism was not associated with any pharmacokinetic parameters.

CONCLUSIONS

Kidney transplant recipients with the CYP3A5 *1 allele required a higher daily tacrolimus dose compared with those with the CYP3A5 *3/*3 genotype to maintain both the target trough level and AUC0-12, suggesting that this polymorphism is useful for determining the appropriate dose of tacrolimus.

Clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplantation

The aim of this review is to analyse critically the recent literature on the clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplant recipients. Dosage and target concentration recommendations for tacrolimus vary from centre to centre, and large pharmacokinetic variability makes it difficult to predict what concentration will be achieved with a particular dose or dosage change. Therapeutic ranges have not been based on statistical approaches. The majority of pharmacokinetic studies have involved intense blood sampling in small homogeneous groups in the immediate post-transplant period. Most have used nonspecific immunoassays and provide little information on pharmacokinetic variability. Demographic investigations seeking correlations between pharmacokinetic parameters and patient factors have generally looked at one covariate at a time and have involved small patient numbers. Factors reported to influence the pharmacokinetics of tacrolimus include the patient group studied, hepatic dysfunction, hepatitis C status, time after transplantation, patient age, donor liver characteristics, recipient race, haematocrit and albumin concentrations, diurnal rhythm, food administration, corticosteroid dosage, diarrhoea and cytochrome P450 (CYP) isoenzyme and P-glycoprotein expression. Population analyses are adding to our understanding of the pharmacokinetics of tacrolimus, but such investigations are still in their infancy. A significant proportion of model variability remains unexplained. Population modelling and Bayesian forecasting may be improved if CYP isoenzymes and/or P-glycoprotein expression could be considered as covariates. Reports have been conflicting as to whether low tacrolimus trough concentrations are related to rejection. Several studies have demonstrated a correlation between high trough concentrations and toxicity, particularly nephrotoxicity. The best predictor of pharmacological effect may be drug concentrations in the transplanted organ itself. Researchers have started to question current reliance on trough measurement during therapeutic drug monitoring, with instances of toxicity and rejection occurring when trough concentrations are within 'acceptable' ranges. The correlation between blood concentration and drug exposure can be improved by use of non-trough timepoints. However, controversy exists as to whether this will provide any great benefit, given the added complexity in monitoring. Investigators are now attempting to quantify the pharmacological effects of tacrolimus on immune cells through assays that measure in vivo calcineurin inhibition and markers of immunosuppression such as cytokine concentration. To date, no studies have correlated pharmacodynamic marker assay results with immunosuppressive efficacy, as determined by allograft outcome, or investigated the relationship between calcineurin inhibition and drug adverse effects. Little is known about the magnitude of the pharmacodynamic variability of tacrolimus.

Stereo- and regioselectivity account for the diversity of dehydroepiandrosterone (DHEA) metabolites produced by liver microsomal cytochromes P450

The purpose of this study was to quantify the oxidative metabolism of dehydroepiandrosterone (3beta-hydroxy-androst-5-ene-17-one; DHEA) by liver microsomal fractions from various species and identify the cytochrome P450 (P450) enzymes responsible for production of individual hydroxylated DHEA metabolites. A gas chromatography-mass spectrometry method was developed for identification and quantification of DHEA metabolites. 7alpha-Hydroxy-DHEA was the major oxidative metabolite formed by rat (4.6 nmol/min/mg), hamster (7.4 nmol/min/mg), and pig (0.70 nmol/min/mg) liver microsomal fractions. 16alpha-Hydroxy-DHEA was the next most prevalent metabolite formed by rat (2.6 nmol/min/mg), hamster (0.26 nmol/min/mg), and pig (0.16 nmol/min/mg). Several unidentified metabolites were formed by hamster liver microsomes, and androstenedione was produced only by pig microsomes. Liver microsomal fractions from one human demonstrated that DHEA was oxidatively metabolized at a total rate of 7.8 nmol/min/mg, forming 7alpha-hydroxy-DHEA, 16alpha-hydroxy-DHEA, and a previously unidentified hydroxylated metabolite, 7beta-hydroxy-DHEA. Other human microsomal fractions exhibited much lower rates of metabolism, but with similar metabolite profiles. Recombinant P450s were used to identify the cytochrome P450s responsible for DHEA metabolism in the rat and human. CYP3A4 and CYP3A5 were the cytochromes P450 responsible for production of 7alpha-hydroxy-DHEA, 7beta-hydroxy-DHEA, and 16alpha-hydroxy-DHEA in adult liver microsomes, whereas the fetal/neonatal form CYP3A7 produced 16alpha-hydroxy and 7beta-hydroxy-DHEA. CYP3A23 uniquely formed 7alpha-hydroxy-DHEA, whereas other P450s, CYP2B1, CYP2C11, and CYP2D1, were responsible for 16alpha-hydroxy-DHEA metabolite production in rat liver microsomal fractions. These results indicate that the stereo- and regioselectivity of hydroxylation by different P450s account for the diverse DHEA metabolites formed among various species.

Functional characterization of four naturally occurring variants of human pregnane X receptor (PXR): one variant causes dramatic loss of both DNA binding activity and the transactivation of the CYP3A4 promoter/enhancer region

Metabolism of administered drugs is determined by expression and activity of many drug-metabolizing enzymes, such as the cytochrome P450 (P450s) family members. Pregnane X receptor (PXR) is a master transcriptional regulator of many drug/xenobiotic-metabolizing enzymes, including P450s and drug transporters. In this study, we describe the functional analysis of four naturally occurring human PXR (hPXR) variants (R98C, R148Q, R381W, and I403V) that we have recently identified. By a reporter gene assay using the CYP3A4 promoter/enhancer reporter in COS-7 or HepG2 cells, it was found that the R98C variant failed to transactivate the CYP3A4 reporter. The R381W and I403V variants also showed varying degrees of reduction in transactivation, depending on the dose of PXR activators, rifampicin, clotrimazole, and paclitaxel. The transcriptional activities of the R148Q variant were not significantly different from that of the wild-type hPXR. The electrophoretic mobility shift assay revealed that only the R98C variant lacked DNA binding. Furthermore, the cellular localization of the hPXR proteins was analyzed. All four variants as well as the wild-type hPXR localized exclusively to the nucleus, regardless of the presence or absence of rifampicin. These data suggest that the R98C, R381W, and I403V hPXR variants, especially R98C, may influence the expression of drug-metabolizing enzymes and transporters, which are transactivated by PXR.

Effects of growth hormone deficiency and rhGH replacement therapy on the 6beta-hydroxycortisol/free cortisol ratio, a marker of CYP3A activity, in growth hormone-deficient children

OBJECTIVE

To determine the effect of both growth hormone deficiency (GHD) and rhGH replacement therapy on CYP3A activity as well as the potential influence of gender.

METHODS

The sample consisted of 35 GHD children (16 males and 19 females), aged 2.9-13.1 years, and a control group of 35 healthy children matched for age and sex. The urinary ratio 6beta-hydroxycortisol/free cortisol was used as a marker of CYP3A activity. In patients, urine samples were collected at two times, prior to starting rhGH replacement treatment and 30 days after beginning therapy.

RESULTS

A significantly higher metabolic activity in GHD children was observed in relation to controls ( P=0.0001) without sex differences. Paired comparisons demonstrated a sexually dimorphic effect of rhGH therapy on the CYP3A activity. While boys displayed a significant decrease ( P=0.003), girls showed no significantly different values of CYP3A marker ( P>0.05). Unpaired comparison between controls and GHD children after therapy demonstrated absence of significant differences in boys ( P>0.05) and a strikingly higher activity in girls ( P=0.0001).

CONCLUSIONS

The data suggests that: (a) GHD in children increases CYP3A activity in a non-sex-dependent manner, (b) rhGH treatment for 30 days to GHD children results in a sexually dimorphic effect on CYP3A activity, with a significant decrease in males toward normalization in relation to controls and non-significant changes in females. The results of this study may have important clinical implications for GHD children, since changes in CYP3A activity importantly affect the metabolism of both steroid hormones and CYP3A substrate drugs.

UTILITY OF RECOMBINANT ENZYME KINETICS IN PREDICTION OF HUMAN CLEARANCE: IMPACT OF VARIABILITY, CYP3A5, AND CYP2C19 ON CYP3A4 PROBE SUBSTRATES

A systematic kinetic analysis of the metabolism of five benzodiazepines (low to high clearance compounds) was performed in CYP3A4, CYP3A5, and CYP2C19 baculovirus-expressed recombinant systems. The data obtained in the expression systems were scaled and compared with human liver microsomal predicted clearance and observed in vivo values, using either cytochrome P450 relative activity factors (RAFs) or the relative abundance approach. Interindividual variability, both in content (CYP3A4, CYP3A5) and activity (CYP3A4, CYP3A5, and CYP2C19), were incorporated in the clearance prediction by bootstrap analysis. These resampling Monte Carlo-based simulations were performed to justify any distribution assumptions in the generated range of the predicted clearance due to a limited sample size. This approach allowed extrapolation of the recombinant clearance data to specific population groups and investigation of the role of "minor" forms like CYP3A5 and CYP2C19 in comparison to the most prolific CYP3A4. The use of quinidine 3-hydroxylation and alprazolam 1'-hydroxylation as RAF markers for CYP3A4 and CYP3A5 activity, respectively, and the incorporation of variability improved the clearance prediction of the selected benzodiazepines (apart from flunitrazepam) to within 2-fold of the in vivo value. Clearance estimates from the immunoquantified protein levels were approximately 8-fold lower in comparison to the RAF approach. The differences observed in the benzodiazepine metabolite pathway ratios between CYP3A4 and CYP3A5, particularly for 1'- to 4-hydroxymidazolam and alprazolam, provided a useful measure of interindividual differences within the CYP3A family.

Fenproporex N-dealkylation to amphetamine—enantioselective in vitro studies in human liver microsomes as well as enantioselective in vivo studies in Wistar and Dark Agouti rats

Fenproporex (FP) is known to be N-dealkylated to R(-)-amphetamine (AM) and S(+)-amphetamine. Involvement of the polymorphic cytochrome P450 (CYP) isoform CYP2D6 in metabolism of such amphetamine precursors is discussed controversially in literature. In this study, the human hepatic CYPs involved in FP dealkylation were identified using recombinant CYPs and human liver microsomes (HLM). These studies revealed that not only CYP2D6 but also CYP1A2, CYP2B6 and CYP3A4 catalyzed this metabolic reaction for both enantiomers with slight preference for the S(+)-enantiomer. Formation of amphetamine was not significantly changed by quinidine and was not different in poor metabolizer HLM compared to pooled HLM. As in vivo experiments, blood levels of R(-)-amphetamine and S(+)-amphetamine formed after administration of FP were determined in female Dark Agouti rats (fDA), a model of the human CYP2D6 poor metabolizer phenotype (PM), male Dark Agouti rats (mDA), an intermediate model, and in male Wistar rats (WI), a model of the human CYP2D6 extensive metabolizer phenotype. Analysis of the plasma samples showed that fDA exhibited significantly higher plasma levels of both amphetamine enantiomers compared to those of WI. Corresponding plasma levels in mDA were between those in fDA and WI. Furthermore, pretreatment of WI with the CYP2D inhibitor quinine resulted in significantly higher amphetamine plasma levels, which did not significantly differ from those in fDA. The in vivo studies suggested that CYP2D6 is not crucial to the N-dealkylation but to another metabolic step, most probably to the ring hydroxylation. Further studies are necessary for elucidating the role of CYP2D6 in FP hydroxylation.

Implications of cytochrome P450 genetic polymorphisms on the toxicity of antitumor agents

In the treatment of cancer, a narrow therapeutic window generally exists between toxicity and suboptimal therapy. In addition, interindividual variation in drug metabolism seriously complicates therapy. Genetic polymorphisms in phase 1 and phase 2 enzymes are present in the population and may explain part of the observed interindividual variation in drug pharmacokinetics. For the cytochrome P450 superfamily, information on variant alleles encoding enzymes with decreased activity is rapidly on the increase. The potential of applying pharmacogenetic screening before therapy in the treatment of cancer seems to be greatest for CYP2B6 (cyclophosphamide treatment), CYP2C8 (paclitaxel therapy), and CYP3A5; however, the drugs of interest still need to be identified for this latter enzyme.

Hair analysis differentiates chronic from acute carbamazepine intoxication

This is a report of a 12-year-old epileptic child undergoing chronic treatment with carbamazepine who was found comatose. He was considered to have acute severe drug toxicity. Measurement of carbamazepine concentration in the patient's hair segments together with the carbamazepine blood levels were both important in determining the chronic nature of the patient's intoxication.

Herbal remedies in the United States: potential adverse interactions with anticancer agents

PURPOSE

Interest in the use of herbal products has grown dramatically in the Western world. Recent estimates suggest an overall prevalence for herbal preparation use of 13% to 63% among cancer patients. With the narrow therapeutic range associated with most anticancer drugs, there is an increasing need for understanding possible adverse drug interactions in medical oncology.

METHODS

In this article, a literature overview is provided of known or suspected interactions of the 15 best-selling herbs in the United States with conventional allopathic therapies for cancer.

RESULTS

Herbs with the potential to significantly modulate the activity of drug-metabolizing enzymes (notably cytochrome p450 isozymes) and/or the drug transporter P-glycoprotein include garlic (Allium sativum), ginkgo (Ginkgo biloba), echinacea (Echinacea purpurea), ginseng (Panax ginseng), St John' s wort (Hypericum perforatum), and kava (Piper methysticum). All of these products participate in potential pharmacokinetic interactions with anticancer drugs.

CONCLUSION

It is suggested that health care professionals and consumers should be aware of the potential for adverse interactions with these herbs, question their patients on their use of them, especially among patients whose disease is not responding to treatments as expected, and urge patients to avoid herbs that could confound their cancer care.

The CYP3A4*1B allele increases risk for small cell lung cancer: effect of gender and smoking dose

CYP3A isozymes are involved in tobacco carcinogen- and steroid-metabolism, and are expressed in human lung tissue showing interindividual variation in expression and activity. The CYP3A4*1B allele has been associated with a two-fold higher promoter activity and with high-grade prostate cancers. The very frequent intron 3 polymorphism in the CYP3A5 gene (CYP3A5*3) results in decreased CYP3A5 protein levels. A case-control study was conducted in 801 Caucasian lung cancer patients that included 330 adenocarcinomas, 260 squamous cell carcinomas, 171 small cell lung cancers (SCLC) and 432 Caucasian hospital-based controls. CYP3A-genotyping was performed by capillary polymerase chain reaction followed by fluorescence-based melting curve analysis. A significantly increased SCLC risk for CYP3A4*1B allele carriers [odds ratio (OR) 2.25, 95% confidence interval (CI) 1.11-4.55, P=0.02] was found. After dividing cases and controls by gender, an increased lung cancer risk for CYP3A4*1B carriers (OR 3.04, 95% CI 0.94-9.90, P=0.06) for women but not for men (OR 1.00, 95% CI 0.56-1.81) was revealed. Heavier smoking men (> or =20 pack-years) with the CYP3A4*1B allele had a significant OR for lung cancer of 3.42 (95% CI 1.65-7.14, P=0.001) compared to *1A/*1A carriers with lower tobacco exposure (<20 pack-years). For women, the respective OR was 8.00 (95% CI 2.12-30.30, P=0.005). Genotype frequencies were generally in Hardy-Weinberg equilibrium, except for CYP3A5 where a greater than expected number of CYP3A5*1 homozygotes was observed among cases (P=0.006). In addition, we observed linkage disequilibrium of CYP3A4 and CYP3A5 (P<0.00001), but a non-significantly increased lung cancer risk was only found for homozygous CYP3A5*1 allele carriers (OR 5.24, 95% CI 0.85-102.28, P=0.14) but not for heterozygotes. To confirm our observation that the CYP3A4*1B allele increases SCLC risk and modifies the smoking-related lung cancer risk in a gender-specific manner, further studies, including CYP3A haplotype analysis, will be necessary.

CYP3A4 and MDR1 alleles in a Portuguese population

Polymorphisms in cytochrome P450 CYP3A4 and multidrug resistance (MDR) 1 genes coding for the important drug-metabolising CYP3A4 and the ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp) are poorly documented in the Portuguese population. In this study we have determined the frequencies of CYP3A4 and MDR1 alleles in Portuguese Caucasians. Both genes were simultaneously analysed as these genes are known to be frequently co-induced and their products to show a pronounced overlap of substrates. CYP3A4 A-392G (CYP3A4*1B), T673C (CYP3A4*2) and MDR1 T-129C, G2677T and C3435T single nucleotide polymorphisms (SNPs) were analysed in 100 individuals from the southern region of the country. We observed a frequency of 4.0% for CYP3A4*1B, not significantly different from that reported on other Caucasian European populations. CYP3A4*2 was found at an allele frequency of 4.5%, constituting the first report of the presence of this allele outside the Finnish population. Significant differences were found concerning the MDR1 C3435T SNP frequency (64.5%) compared with other European populations, while no differences were found concerning G2677T (47.5%) or T-129C (5%) SNPs. Linkage between the C3435T and G2677T SNPs was observed, although not as evidently as documented in other Caucasian populations. No preferential associations were detected between CYP3A4 and MDR1 alleles.

Genetic variability in CYP3A5 and its possible consequences

The cytochrome P450 3A (CYP3A) subfamily members are the most abundant and important drug-metabolizing enzymes in humans, and wide interindividual variability in CYP3A expression and function is present. CYP3A4 alone cannot fully explain the observed constitutive variability because its genetic variants are relatively uncommon and have limited functional significance, whereas CYP3A5 expression in humans is highly variable and may be contributory. However, it is difficult to delineate the relative contribution of CYP3A4 and CYP3A5, and to differentiate their effects on drug metabolism as their protein structure, function and substrates are so similar. By contrast, molecular biology methods provide the ability to identify CYP3A4 and CYP3A5 genotypes with certainty. This review collates currently available data on CYP3A5 polymorphisms, provides information on the population frequency of each genetic variant in major ethnic groups, and describes in vitro and in vivo studies that have attempted to identify genotype-phenotype associations.

Induction of CYP3A by 2,3-Oxidosqualene:Lanosterol Cyclase Inhibitors Is Mediated by an Endogenous Squalene Metabolite in Primary Cultured Rat Hepatocytes

The effects of inhibitors of 2,3-oxidosqualene:lanosterol cyclase (cyclase) on cytochrome P450 expression were investigated in primary cultures of rat hepatocytes. Treatment of hepatocyte cultures for 24 h with either of the inhibitors [4'-(6-allyl-methyl-amino-hexyloxy)-2'-fluoro-phenyl]-(4-bromophenyl)-methanone fumarate (Ro 48-8071) or trans-N-(4-chlorobenzoyl)-N-methyl-(4-dimethylaminomethylphenyl)-cyclohexylamine (BIBX 79) selectively increased CYP3A mRNA and immunoreactive protein contents, with maximal accumulations occurring at 3 x 10(-5) M Ro 48-8071 and 10(-4) M BIBX 79. The abilities of Ro 48-8071, BIBX 79, and 3beta-(2-diethylaminoethoxy)androst-5-en-17-one.HCl (U18666A) to induce murine CYP3A were abolished in hepatocyte cultures prepared from pregnane X receptor (PXR)-null mice, and cotransfection of primary cultured rat hepatocytes with a dominant-negative PXR prevented cyclase inhibitor-inducible luciferase expression from a PXR-responsive reporter plasmid. Cyclase inhibitor-mediated CYP3A mRNA induction was eliminated when primary cultured rat hepatocytes were cotreated with any of the following agents that inhibit steps upstream of cyclase in the cholesterol biosynthetic pathway: squalestatin 1 (squalene synthase inhibitor), (E)N-ethyl-N-(6,6-dimethyl-2-hepten-4-ynyl)-3-[(3,3'-bithiophen-5-yl)methoxy]benzenemethanamine (NB-598, squalene monooxygenase inhibitor), or pravastatin (HMG-CoA reductase inhibitor). Ro 48-8071-inducible CYP3A mRNA expression was restored when pravastatin-treated cultures were incubated with medium containing mevalonate. The concentration-dependence of Ro 48-8071-mediated CYP3A mRNA induction corresponded to the cellular contents of metabolically labeled squalene 2,3-oxide and squalene 2,3:22,23-dioxide, but not 24(S),25-epoxycholesterol. These results indicate that cyclase inhibitors are capable of inducing CYP3A expression in primary cultured rat and mouse hepatocytes and that the effect is mediated as a consequence of cyclase blockade through the evoked accumulation of one or more squalene metabolites that activate the PXR.

Pharmacogenetic determinants of outcome in acute lymphoblastic leukaemia

Present day paediatric co-operative group acute lymphoblastic leukaemia (ALL) protocols cure approximately 80% of patients, a result achieved largely through the use of risk-stratified therapies that employ multiple chemotherapy agents. These risk-based therapies utilize host and leukaemia traits to select the most appropriate therapy. However, these risk-stratified approaches predict therapy response imperfectly and an important fraction of patients experience relapse or therapy-related toxicity. Pharmacogenetics, the study of genetic variations in drug-processing genes and individual responses to drugs, may enable the improved identification of patients at higher risk for either disease relapse or chemotherapy-associated side effects. While the impact of genetic variation in the thiopurine-S-methyltransferase gene on ALL treatment outcome and toxicity has been extensively studied, the role of other polymorphisms remains less well known. This review summarizes current research on the impact of genetic variation in drug-processing genes in paediatric ALL and reviews important methodological and statistical issues presently challenging the field of pharmacogenetics.

QUANTITATIVE CONTRIBUTION OF CYP2D6 AND CYP3A TO OXYCODONE METABOLISM IN HUMAN LIVER AND INTESTINAL MICROSOMES

Oxycodone undergoes N-demethylation to noroxycodone and O-demethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K(m) of 600 +/- 119 microM and a V(max) that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K(m) of 130 +/- 33 microM and a V(max) that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 microl/min/mg versus 1.5 microl/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.

Identification of a novel polymorphic enhancer of the human CYP3A4 gene

CYP3A4, the most abundant form of cytochrome P450 in the human adult liver, shows wide interindividual variation in its activity. This variability is thought to be caused largely by transcriptional and genetic factors, yet the underlying mechanisms are poorly understood. The purpose of this study was to clarify the mechanisms controlling the CYP3A4 gene transcription and to search for genetic polymorphisms in the 5'-flanking region of the CYP3A4 gene. Transient transfection of human hepatoma HepG2 cells and of normal human hepatocytes with a series of CYP3A4 promoter-luciferase reporter plasmids revealed that a region from -11.4 to -10.5 kilobases, designated the constitutive liver enhancer module of CYP3A4 (CLEM4), was important for the constitutive activation of the CYP3A4 gene. Gel shift assay using nuclear extracts prepared from HepG2 cells showed that HNF-1alpha, HNF-4alpha, USF1, and AP-1 interacted with CLEM4. Furthermore, the introduction of mutations into their binding sites demonstrated that essentially all sites were required for the maximal enhancer activity. Screening for genetic polymorphisms within CLEM4 in genomic DNA from French persons, we identified the novel variant, TGT insertion between -11,129 and -11,128 (-11,129_-11,128insTGT), whose allele frequency was 3.1%. The -11,129_-11,128insTGT resulted in the disruption of USF1 binding and a 36% reduction of the enhancer activity. These results suggest that CLEM4 is a constitutive enhancer of the CYP3A4 gene in the liver and that -11,129_-11,128insTGT may at least partly contribute to the interindividual variability of CYP3A4 expression.