Induction of multidrug resistance-1 and cytochrome P450 mRNAs in human mononuclear cells by rifampin

From Endogenous Compounds as Biomarkers to Plasma-Derived Nanovesicles as Liquid Biopsy; Has the Golden Age of Translational Pharmacokinetics-Absorption, Distribution, Metabolism, Excretion-Drug-Drug Interaction Science Finally Arrived?

It is now established that a drug's pharmacokinetics (PK) absorption, distribution, metabolism, excretion (ADME) and drug-drug interaction (DDI) profile can be modulated by age, disease, and genotype. In order to facilitate subject phenotyping and clinical DDI assessment, therefore, various endogenous compounds (in plasma and urine) have been pursued as drug-metabolizing enzyme and transporter biomarkers. Compared with biomarkers, however, the topic of circulating extracellular vesicles as "liquid biopsy" has received little attention within the ADME community; most organs secrete nanovesicles (e.g., exosomes) into the blood that contain luminal "cargo" derived from the originating organ (proteins, messenger RNA, and microRNA). As such, ADME profiling of plasma exosomes could be leveraged to better define genotype-phenotype relationships and the study of ontogeny, disease, and complex DDIs. If methods to support the isolation of tissue-derived plasma exosomes are successfully developed and validated, it is envisioned that they will be used jointly with genotyping, biomarkers, and modeling tools to greatly progress translational PK-ADME-DDI science.

The correlation between the expression of genes involved in drug metabolism and the blood level of tacrolimus in liver transplant receipts

Immunosuppressive medications, such as tacrolimus and mycophenolate mofetil, are commonly used for reducing the risk of organ rejection in receipts of allogeneic organ transplant. The optimal dosages of these drugs are required for preventing rejection and avoiding toxicity to receipts. This study aimed to identify the correlation between the expression profiling of genes involved in drug metabolism and the blood level of tacrolimus in liver transplant receipts. Sixty-four liver transplant receipts were enrolled in this retrospective study. Receipts were divided into low (2–5.9 ng/ml) and high (6–15 ng/ml) tacrolimus groups. Clinical assessment showed that the blood level of tacrolimus was inversely correlated with the liver function evaluated by blood levels of total bilirubin and creatinine. Compared to the high tacrolimus group, expression levels of six cytochrome P450 enzymes, CYP1A1, CYP2B6, CYP3A5, CYP4A11, CYP19A1, and CYP17A1 were significantly higher in the low tacrolimus group. The expression levels of these genes were negatively correlated with the tacrolimus blood level. Enzyme assays showed that CYP3A5 and CYP17A1 exerted direct metabolic effects on tacrolimus and mycophenolate mofetil, respectively. These results support clinical application of this expression profiling of genes in drug metabolism for selection of immunosuppressive medications and optimal dosages for organ transplant receipts.

Coadministration of Rifampin Significantly Reduces Odanacatib Concentrations in Healthy Subjects

This open-label 2-period study assessed the effect of multiple-dose administration of rifampin, a strong cytochrome P450 3A (CYP3A) and P-glycoprotein inducer, on the pharmacokinetics of odanacatib, a cathepsin K inhibitor. In period 1, 12 healthy male subjects (mean age, 30 years) received a single dose of odanacatib 50 mg on day 1, followed by a 28-day washout. In period 2, subjects received rifampin 600 mg/day for 28 days; odanacatib 50 mg was coadministered on day 14. Blood samples for odanacatib pharmacokinetics were collected at predose and on day 1 of period 1 and day 14 of period 2. Coadministration of odanacatib and rifampin significantly reduced odanacatib exposure. The odanacatib AUC_0-∞ geometric mean ratio (90% confidence interval) of odanacatib + rifampin/odanacatib alone was 0.13 (0.11-0.16). The harmonic mean ± jackknife standard deviation apparent terminal half-life (t_½ ) was 71.6 ± 10.2 hours for odanacatib alone and 16.0 ± 3.4 hours for odanacatib + rifampin, indicating greater odanacatib clearance following coadministration with rifampin. Samples were collected in period 2 during rifampin dosing (days 1, 14, and 28) and after rifampin discontinuation (days 35, 42, and 56) to evaluate the ratio of plasma 4β-hydroxycholesterol to total serum cholesterol as a CYP3A4 induction biomarker; the ratio increased ∼5-fold over 28 days of daily dosing with 600 mg rifampin, demonstrating sensitivity to CYP3A4 induction.

Verapamil and rifampin effect on p-glycoprotein expression in hepatocellular carcinoma

Background:

High expression of p-glycoprotein (P-gp) has been associated with a poor prognosis in patients with hepatocellular carcinoma (HCC). It is likely that P-gp overexpression is responsible for multidrug resistance in HCC.

Objectives:

The aim of this study was to elucidate the effect of potent carcinogen nitrosamine with and without verapamil and rifampin drugs on P-gp expression at the mRNA level in HCC.

Materials and Methods:

Four groups of rats (n = 5) were selected with different treatments and one group as control. mRNA concentration changes were monitored using quantitative PCR (QPCR).

Results:

A significant difference was found between verapamil treated group and the control regarding the mRNA level. The mdr1a mRNA was significantly decreased in the verapamil group (P ≤ 0.001). Rifampin administrated group had a decreased level of the mdr1a mRNA compared to the control group (P ≤ 0.006). No significant changes were observed in HCC induced rats regarding the mdr1a mRNA level when treated with verapamil and rifampin. An enhanced expression of the mdr1a gene was found In the HCC induced animals when treated with drugs.

Conclusions:

Verapamil and rifampin were found specific and effective against P-gp expression in HCC. In conclusion, treatment efficacy of most anticancer drugs is increased in combination with verapamil and rifampin against most advanced HCC.

Psychotropic drug-drug interactions involving P-glycoprotein

Multidrug resistance P-glycoprotein (P-gp; also known as MDR1 and ABCB1) is expressed in the luminal membrane of the small intestine and blood-brain barrier, and the apical membranes of excretory cells such as hepatocytes and kidney proximal tubule epithelia. P-gp regulates the absorption and elimination of a wide range of compounds, such as digoxin, paclitaxel, HIV protease inhibitors and psychotropic drugs. Its substrate specificity is as broad as that of cytochrome P450 (CYP) 3A4, which encompasses up to 50 % of the currently marketed drugs. There has been considerable interest in variations in the ABCB1 gene as predictors of the pharmacokinetics and/or treatment outcomes of several drug classes, including antidepressants and antipsychotics. Moreover, P-gp-mediated transport activity is saturable, and is subject to modulation by inhibition and induction, which can affect the pharmacokinetics, efficacy or safety of P-gp substrates. In addition, many of the P-gp substrates overlap with CYP3A4 substrates, and several psychotropic drugs that are P-gp substrates are also CYP3A4 substrates. Therefore, psychotropic drugs that are P-gp substrates may cause a drug interaction when P-gp inhibitors and inducers are coadministered, or when psychotropic drugs or other medicines that are P-gp substrates are added to a prescription. Hence, it is clinically important to accumulate data about drug interactions through studies on P-gp, in addition to CYP3A4, to assist in the selection of appropriate psychotropic medications and in avoiding inappropriate combinations of therapeutic agents. There is currently insufficient information available on the psychotropic drug interactions related to P-gp, and therefore we summarize the recent clinical data in this review.

Expression and inducibility of cytochrome P450s (CYP1A1, 2B6, 2E1, 3A4) in human cord blood CD34(+) stem cell-derived differentiating neuronal cells

The status of xenobiotic metabolism in developing human brain cells is not known. The reason is nonavailability of developing human fetal brain. We investigate the applicability of the plasticity potential of human umbilical cord blood stem cells for the purpose. Characterized hematopoietic stem cells are converted into neuronal subtypes in eight days. The expression and substrate-specific catalytic activity of the cytochrome P450s (CYPs) CYP1A1 and 3A4 increased gradually till day 8 of differentiation, whereas CYP2B6 and CYP2E1 showed highest expression and activity at day 4. There was no significant increase in the expression of CYP regulators, namely, aryl hydrocarbon receptor (AHR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), and glutathione-S-transferase (GSTP1-1) during differentiation. Differentiating cells showed significant induction in the expression of CYP1A1, 2B6, 2E1, 3A4, AHR, CAR, PXR, and GSTP1-1 when exposed to rifampin, a known universal inducer of CYPs. The xenobiotic-metabolizing capabilities of these differentiating cells were confirmed by exposing them to the organophosphate pesticide monocrotophos (MCP), a known developmental neurotoxicant, in the presence and absence of a universal inhibitor of CYPs-cimetidine. Early-differentiating cells (day 2) were found to be more vulnerable to xenobiotics than mature well-differentiated cells. For the first time, we report significant expression and catalytic activity of selected CYPs in human cord blood hematopoietic stem cell-derived neuronal cells at various stages of maturity. We also confirm significant induction in the expression and catalytic activity of selected CYPs in human cord blood stem cell-derived differentiating neuronal cells exposed to known CYP inducers and MCP.

Beta-Amyloid Downregulates MDR1-P-Glycoprotein (Abcb1) Expression at the Blood-Brain Barrier in Mice

Neurovascular dysfunction is an important component of Alzheimer's disease, leading to reduced clearance across the blood-brain barrier and accumulation of neurotoxic β-amyloid (Aβ) peptides in the brain. It has been shown that the ABC transport protein P-glycoprotein (P-gp, ABCB1) is involved in the export of Aβ from the brain into the blood. To determine whether Aβ influences the expression of key Aβ transporters, we studied the effects of 1-day subcutaneous Aβ1-40 and Aβ1-42 administration via Alzet mini-osmotic pumps on P-gp, BCRP, LRP1, and RAGE expression in the brain of 90-day-old male FVB mice. Our results demonstrate significantly reduced P-gp, LRP1, and RAGE mRNA expression in mice treated with Aβ1-42 compared to controls, while BCRP expression was not affected. The expression of the four proteins was unchanged in mice treated with Aβ1-40 or reverse-sequence peptides. These findings indicate that, in addition to the age-related decrease of P-gp expression, Aβ1-42 itself downregulates the expression of P-gp and other Aβ-transporters, which could exacerbate the intracerebral accumulation of Aβ and thereby accelerate neurodegeneration in Alzheimer's disease and cerebral β-amyloid angiopathy.

Functional analysis of genetic variations in the 5'-flanking region of the human MDR1 gene

P-glycoprotein (P-gp), the product of the MDR1 gene, shows large interindividual variations in expression, which leads to differences in the pharmacokinetics of the substrate drugs. The functions of single nucleotide polymorphisms located in the nuclear receptor-responsive element of the 5'-flanking region in the human MDR1 gene were analyzed in order to clarify the mechanism underlying the interindividual variation in P-gp expression. Electrophoretic mobility shift assays revealed that the -7833C>T substitution in the nuclear receptor-responsive region of MDR1 decreases the binding affinities of four nuclear receptors to their responsive elements: vitamin D receptor (VDR), thyroid hormone receptor (TR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR). A reporter gene assay revealed that the C-to-T substitution at -7833 also reduces the transcriptional activation of MDR1 by VDR, TRβ, CAR, and PXR. However, another SNP (-1211T>C substitution), which results in the formation of a xenobiotic responsive element-like sequence and a hypoxia responsive element-like sequence, failed to affect the aryl hydrocarbon receptor-dependent and hypoxia-induced transcriptional activation of MDR1. Although the frequency of the -7833C>T substitution in MDR1 is relatively low, the SNP is crucial because it may alter the pharmacokinetics of P-gp substrates in a small subset of the population.

Impact of drug transporters on cellular resistance towards saquinavir and darunavir

OBJECTIVES

Highly active antiretroviral therapy is complicated by drug-drug interactions and the development of viral resistance. Drug interactions involve transporters that may critically affect the pharmacokinetics of many antiretroviral drugs and contribute to the formation of functional sanctuary sites. We therefore investigated the effect of saquinavir and darunavir on drug transporter expression and functional consequences for cellular resistance towards these compounds.

METHODS

Induction of transporters was investigated in LS180 cells over a period of 4 weeks by means of RT-PCR, and for some transporters also at the protein and functional levels. Cellular resistance was measured by growth inhibition assays.

RESULTS

Incubation with 10 µM darunavir for 1 week significantly increased mRNA expression of P-glycoprotein (P-gp/MDR1/ABCB1) 3.8-fold and of organic anion-transporting polypeptide 2B1 (SLCO2B1) 1.9-fold. In contrast, 10 µM saquinavir significantly increased mRNA expression of P-gp 5.7-fold, multidrug resistance-associated protein 1 (MRP1/ABCC1) 2.3-fold, MRP2/ABCC2 4.5-fold, MRP3/ABCC3 2.0-fold, MRP4/ABCC4 1.8-fold, MRP5/ABCC5 3.8-fold, breast cancer resistance protein (BCRP/ABCG2) 4.1-fold, SLCO1B1 4.6-fold, SLCO2B1 1.8-fold and SLCO3A1 1.8-fold. P-gp induction was also confirmed at the protein and functional levels. Induction by darunavir caused an increase in cellular resistance towards this compound, as measured in growth inhibition assays; however, saquinavir treatment did not cause reduced sensitivity of cells, indicating unchanged intracellular concentration. Hence, induction by darunavir increased drug efflux and might therefore lead to a suboptimal intracellular concentration of darunavir.

CONCLUSIONS

The study revealed substantial induction of several drug transporters by saquinavir and darunavir, possibly leading to decreased efficacy of antiretrovirals and drugs used to treat co-morbidity.

Lack of P-glycoprotein induction by rifampicin and phenobarbital in human lymphocytes

The efficacy of drugs acting within lymphocytes depends on their intracellular concentrations, which could be modulated by membrane efflux transporters including P-glycoprotein (P-gp), encoded by the MDR1 gene. In particular, P-gp induction may compromise the efficacy of its substrates. Rifampicin and phenobarbital have been shown to induce P-gp in hepatic and intestinal cells through the activation of the nuclear receptors PXR and CAR. However, controversial data exist in human lymphocytes. We investigated the effect of these drugs on P-gp activity and expression in lymphocytes in vitro and ex vivo. CCRF-CEM cells and peripheral blood mononuclear cells (PBMCs) from healthy volunteers were incubated in the presence of rifampicin, phenobarbital, or without any drug. P-gp activity was measured by flow cytometry using DiOC(6) efflux. MDR1, PXR and CAR mRNA expression were measured by quantitative RT-PCR. Neither P-gp activity nor MDR1 mRNA expression were modified by rifampicin or phenobarbital both in CCRF-CEM cells and PBMCs. Moreover, P-gp protein expression at the membrane was neither detectable nor induced. The very weak PXR and CAR mRNA expression levels in these cells could partly explain these results. Therefore, P-gp induction by rifampicin and phenobarbital may play a negligible role in drug interactions occurring within lymphocytes.

Detection of ABCC2, CYP2B6 and CYP3A4 in human peripheral blood mononuclear cells using flow cytometry

ABCC2 has a wide tissue distribution and can mediate the efflux of a number of therapeutic compounds from cells and contribute to potential treatment failure. Its diverse expression and ability to efflux a number of substrates imply a number of physiological and pharmacological roles. CYP2B6 and CYP3A4 are responsible for the metabolism of a number of therapeutic compounds. Reports on the expression of these proteins in various cells and tissues have been contradictory mainly due to differences in experimental approach and cell type studied. With the advances in commercially available antibodies we describe here a simplified technique for the detection of ABCC2, CYP2B6 and CYP3A4 in human peripheral blood mononuclear cells (PBMC) by flow cytometry. Results are expressed as mean increase in fluorescence compared to isotypically matched controls. Using these assays we confirmed the expression of these proteins in human PBMC. These methods are rapid and reproducible and have potential use for both in vitro and clinical applications.

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.

A pharmacogenetic strategy for immunosuppression based on the CYP3A5 genotype

Pharmacogenetics has the potential to complement therapeutic drug monitoring in achieving target blood concentrations of the immunosuppressive drugs during the critical early period after transplantation when adequate drug exposure is essential to prevent rejection. The most attractive candidate for a pharmacogenetic strategy is tacrolimus dosing based on the CYP3A5 genotype.

Induction of P-glycoprotein in lymphocytes by carbamazepine and rifampicin: the role of nuclear hormone response elements

AIMS

Carbamazepine (CBZ) is an inducer of cytochrome P450 enzymes, which have been implicated in many drug interactions. However, for immunosuppressant and anti-HIV drugs, whose main site of action is the lymphocyte, induction of P-glycoprotein (Pgp) may also be important. In this study, we have investigated whether CBZ acts as an inducer of Pgp in lymphocytes.

METHODS

Pgp expression was assessed by flow cytometry and real-time reverse transcriptase-polymerase chain reaction using lymphocytes from four healthy subjects after incubation with therapeutic concentrations of CBZ, using rifampicin as a positive control. Binding to DR-4 elements in the MDR1 promoter was assessed by electrophoretic mobility shift assay (EMSA) and a luciferase-reporter construct.

RESULTS

CBZ increased MDR1 mRNA expression at 6 h by 3.7-fold [95% confidence interval (CI) 0, 7.6) when compared with controls. CBZ increased lymphocyte Pgp expression at 72 h by 7.6-fold (95% CI 2.1, 13.2) over control values. EMSA revealed a 2.1-fold (95% CI 1.5, 2.7) increased binding to the DR-4 element of CBZ when compared with control values. Activation of the DR-4 element was confirmed using reporter constructs. Rifampicin also had similar effects in all experiments.

CONCLUSIONS

Carbamazepine induces Pgp in a manner comparable to rifampicin, by increasing binding to the DR4 element. This has implications for interactions involving drugs whose site of action is the lymphocyte.

Evidence for cytochrome P450 3A expression and catalytic activity in rat blood lymphocytes

Freshly isolated peripheral blood lymphocytes from control rats were found to catalyze the N-demethylation of erythromycin, known to be mediated by cytochrome P450 3A (CYP3A) isoenzymes in rat liver. Pretreatment of rats with dexamethasone (100 mg/kgx3 days, i.p.), a CYP3A inducer, resulted in 3-4-fold increase in the activity of erythromycin demethylase (EMD) in freshly isolated peripheral blood lymphocytes. This increase in the enzyme activity was found to be associated with an increase in the rate of the reaction and affinity of the substrate towards the enzyme. Significant inhibition of the EMD activity on in vitro addition of ketoconazole, a specific CYP3A inhibitor in liver and polyclonal antibody raised against rat liver CYP3A have suggested that EMD activity in blood lymphocytes is catalyzed primarily by CYP3A isoenzymes. Further, immunoblot analysis with polyclonal antibody raised against rat liver CYP3A revealed significant immunoreactivity, co-migrating with the liver isoenzyme, indicating constitutive expression of CYP3A in blood lymphocytes. Pretreatment with dexamethasone was found to significantly increase the expression of CYP3A protein in freshly isolated rat blood lymphocytes, as observed with liver. Likewise, significant CYP3A mRNA detected in control rat blood lymphocytes has further demonstrated constitutive expression of CYP3A isoenzymes in blood lymphocytes. Furthermore, several fold increase in CYP3A mRNA expression following pretreatment with dexamethasone showed similarities in the regulation of CYP3A isoenzymes in rat blood lymphocytes with the liver enzyme. The data suggest that the blood lymphocytes can be used to monitor tissue expression of CYP3A isoenzymes and validate the suitability of lymphocytes as surrogates of CYP status in less accessible target tissues.

Significance of the minor cytochrome P450 3A isoforms

Cytochrome P450 (CYP) 3A4 is responsible for most CYP3A-mediated drug metabolism but the minor isoforms CYP3A5, CYP3A7 and CYP3A43 also contribute. CYP3A5 is the best studied of the minor CYP3A isoforms. It is well established that only approximately 20% of livers express CYP3A5. The most common reason for the absence of expression is a splice site mutation. The frequency of variant alleles shows interethnic differences, with the wild-type CYP3A5*1 allele more common in Africans than Caucasians and Asians. In individuals who express CYP3A5, the percentage contributed to total hepatic CYP3A by this isoform is still unclear, with estimates ranging from 17% to 50%. CYP3A5 is also expressed in a range of extrahepatic tissues. Only limited information is available on the regulation of CYP3A5 expression but it appears to be inducible via the glucocorticoid receptor, pregnane X receptor and constitutive androstane receptor-beta, as for CYP3A4. Although information on the substrate specificity of CYP3A5 is limited compared with CYP3A4, there have been a number of recent pharmacokinetic studies on a small range of substrates in individuals of known genotype to investigate the contribution of CYP3A5. In the case of midazolam, ciclosporin, nifedipine and docetaxel, clearance by individuals with a CYP3A5-expressing genotype did not differ from that for nonexpressors, but in the case of tacrolimus, eight independent studies have demonstrated faster clearance by those carrying one or two CYP3A5*1 alleles. This may reflect faster turnover of tacrolimus by CYP3A5 than the other substrates. CYP3A5 genotype may affect cancer susceptibility. Certain combined CYP3A4/CYP3A5 haplotypes show differential susceptibility to prostate cancer and there is a nonsignificant increase in the risk of small-cell lung cancer for a CYP3A5*1/*1 genotype. Females positive for CYP3A5*1 appear to reach puberty earlier, which may affect breast cancer risk. CYP3A5*1 homozygotes may have higher systolic blood pressure.CYP3A7 is predominantly expressed in fetal liver but is also found in some adult livers and extrahepatically. The molecular basis for expression in adult liver relates to upstream polymorphisms, which appear to increase homology to CYP3A4 and make regulation of expression more similar. CYP3A7 has a specific role in hydroxylation of retinoic acid and 16alpha-hydroxylation of steroids, and is therefore of relevance both to normal development and carcinogenesis.CYP3A43 is the most recently discovered CYP3A isoform. In addition to a low level of expression in liver, it is expressed in prostate and testis. Its substrate specificity is currently unclear. Polymorphisms predicting absence of active enzyme have been identified.

Vincristine transcriptional regulation of efflux drug transporters in carcinoma cell lines

The increased expression of drug transporters following cancer chemotherapy contributes to resistance. This may reflect transcriptional up-regulation and/or clonal selection. We quantified the expression of mRNA for ABCB1 (mdr1), ABCC1 (mrp1), ABCC2 (mrp2) and ABCC3 (mrp3) to evaluate the potential contribution of induction. ABCB1, ABCC1-3 mRNAs were quantified by real time RT-PCR and normalized to GAPDH. We used intestinal cells that express high pregnane X receptor (LS174T), low pregnane X receptor (Caco-2) and lung cells (A549) that express glucocorticoid receptor and low pregnane X receptor. Rifampin (10 microM) caused significant induction of ABCB1 (595+/-263%, p<0.05) in LS174T cells but induction was absent in Caco-2 or A549 cells. ABCC1 was not induced in any cell at 24, 48 and 72 h following rifampin treatment. In contrast, vincristine (10 nM and 100 nM), a ligand for ABCB1 and ABCC1-3 and a potential PXR/CAR ligand, induced ABCC2 and ABCC3 expression in LS174T cells at 48 h (372+/-87% and 303+/-42%, respectively, p<0.05). A similar induction of ABCC2 and ABCC3 genes was also seen with 10 nM VCR in A549 cells following 48 h treatment. In summary, there may be a significant contribution of transcriptional activation to multi-drug resistance. However, this is cell selective and is not necessarily dependent on PXR mediated effects.

MDR1 genotype-related pharmacokinetics: fact or fiction?

Multidrug resistant transporter MDR1/P-glycoprotein, the gene product of MDR1, is a glycosylated membrane protein of 170 kDa, belonging to the ATP-binding cassette superfamily of membrane transporters. A number of various types of structurally unrelated drugs are substrates for MDR1, and MDR1 and other transporters are recognized as an important class of proteins for regulating pharmacokinetics. The first investigation of the effects of MDR1 genotypes on pharmacotherapy was reported in 2000; a silent single nucleotide polymorphism (SNP), C3435T in exon 26, was found to be associated with the duodenal expression of MDR1, and thereby the plasma concentration of digoxin after oral administration. In the last 5 years, clinical studies have been conducted around the world on the association of MDR1 genotype with MDR1 expression and function in tissues, and with the pharmacokinetics and pharmacodynamics of drugs; however, there are still discrepancies in the results on C3435T. In 1995, a novel concept to predict in vivo oral pharmacokinetic performance from data on in vivo permeability and in vitro solubility has been proposed, and this Biopharmaceutical Classification System strongly suggested that the effects of intestinal MDR1 on the intestinal absorption of substrates is minimal in the case of commercially available oral drugs, and therefore MDR1 genotypes are little associated with the pharmacokinetics after oral administration. This review summarizes the latest reports for the future individualization of pharmacotherapy based on MDR1 genotyping, and attempts to explain discrepancies.

Roles of rifampicin in drug-drug interactions: underlying molecular mechanisms involving the nuclear pregnane X receptor

Rifampicin, an important drug in the treatment of tuberculosis, is used extensively despite its broad effects on drug-drug interactions, creating serious problems. The clinical importance of such interactions includes autoinduction leading to suboptimal or failed treatment. The concomitantly administered effects of rifampicin on other drugs can result in their altered metabolism or transportation that are metabolised by cytochromes P450 or transported by p-glycoprotein in the gastrointestinal tract and liver. This review paper summarises recent findings with emphases on the molecular mechanisms used to explain these broad drug-drug interactions. In general, rifampicin can act on a pattern: rifampicin activates the nuclear pregnane X receptor that in turn affects cytochromes P450, glucuronosyltransferases and p-glycoprotein activities. This pattern of action may explain many of the rifampicin inducing drug-drug interactions. However, effects through other mechanisms have also been reported and these make any explanation of such drug-drug interactions more complex.

Interactions between the dopamine agonist, bromocriptine and the efflux protein, P-glycoprotein at the blood–brain barrier in the mouse

Bromocriptin (BCT) is a dopaminergic receptor agonist, poorly transported through the blood-brain barrier (BBB) and responsible for central side effects. Interactions between BCT and the efflux protein, P-glycoprotein (Pgp), have been described in vitro but nothing is known in vivo nor at the BBB level. At the BBB, in vivo, we investigated BCT as (i) a Pgp substrate by comparing the brain uptake in CF1 mdr1a(-/-) and mdr1a(+/+) mice with or without inhibitors of Pgp (valspodar, elacridar); (ii) a Pgp inducer by looking at the effect of repeated doses of BCT on cerebral uptake of digoxin and comparing it to the effect of dexamethasone and rifampicin; (iii) a Pgp inhibitor by determining the effect of a single dose of BCT on cerebral uptake of digoxin and comparing it to the effect of valspodar. CF1 mdr1a(-/-) mice showed much higher brain uptake of BCT than CF1 mdr1a(+/+) mice and brain uptake of BCT was higher in CF1 mdr1a(+/+) mice pre-treated with valspodar or elacridar indicating that BCT is a Pgp substrate at the BBB level. Brain uptake of digoxin was not modified in CF1 mdr1a(+/+) mice pre-treated with a single dose or repeated doses of BCT, indicating that BCT is neither a Pgp inductor nor a Pgp inhibitor at the BBB in the chosen experimental setting. In vivo, at the mouse BBB level and in our experimental conditions, bromocriptin is a Pgp substrate but is not a Pgp modulator.

Cytogenetic damage in lymphocytes of patients undergoing therapy for small cell lung cancer and ovarian carcinoma

The level of cytogenetic damage in peripheral blood lymphocytes of patients undergoing chemotherapy has been analyzed incisively 20 years ago. The results showed that the highest level of cytogenetic damage was observed at the end of therapy. In recent years, the doses of anticancer drugs were intensified thanks to the discovery of colony stimulating factors. Therefore, it was interesting to analyze the kinetics of micronuclei formation in lymphocytes of patients undergoing modern chemotherapy. The frequencies of micronuclei were measured in lymphocytes of 6 patients with small cell lung cancer treated with a combination of cisplatin and etoposide and 7 patients with ovarian carcinoma treated with a combination of taxol and cisplatin. 3 patients with lung cancer received radiotherapy in addition to chemotherapy. Micronuclei were analyzed in lymphocytes collected before the start of therapy and 1 day before each following cycle of chemotherapy. The micronucleus frequencies were compared with the kinetics of leukocyte counts. The micronucleus frequencies showed an interindividual variability. On average, the frequencies of micronuclei increased during the first half of therapy and declined thereafter, reaching, in some patients with ovarian carcinoma, values below the pre-treatment level. Leukocyte counts decreased strongly at the beginning of therapy with an upward trend at the end. We suggest that the decline of micronuclei was due to repopulation of lymphocytes and acquired drug resistance.

Expression of the drug transporters MDR1/ABCB1, MRP1/ABCC1, MRP2/ABCC2, BCRP/ABCG2, and PXR in peripheral blood mononuclear cells and their relationship with the expression in intestine and liver

ATP binding cassette (ABC)-transporters like P-glycoprotein (multidrug resistance (MDR)1/ABCB1), the multidrug resistance associated proteins 1 and 2 (MRP1/ABCC1 and MRP2/ABCC2), and the breast cancer resistance protein (BCRP/ABCG2) have a large impact on the pharmacokinetics of numerous drugs and may also modulate the effectiveness of drug therapy. Prediction of a patient's susceptibility to xenobiotics and individualization of drug therapy would become possible, if a simple test were available for an easy screening of transporter expression. This study quantified the mRNA expression of the four ABC-transporters and of the pregnane X receptor (PXR), a key regulator in drug metabolism and efflux, in peripheral blood mononuclear cells (PBMCs), and corresponding liver or small intestine samples of humans by real-time reverse transcription-polymerase chain reaction (RT-PCR). The results obtained prove the absence of a correlation between the expression of four major ABC-transporters in PBMCs and in the intestine or liver. For all transporters (except MRP1/ABCC1 in the intestine), mRNA amount of the ABC-transporters was positively correlated with PXR expression in PBMCs and intestine. In conclusion, the study suggests that basal expression levels of the transporters are directly influenced by PXR expression in liver and PBMCs and demonstrates that PBMCs do not qualify as surrogate tissue for the expression of the four ABC-transporters in small intestine and liver. However, the transporter status in PBMCs remains important for drugs, whose primary site of therapeutic action is the lymphocyte and which are known substrates of the transporters.

Cytochrome P450 mRNA expression in peripheral blood lymphocytes as a predictor of enzyme induction

OBJECTIVE

Previous reports have supported the concept that messenger ribonucleic acid (mRNA) concentrations for cytochrome P450 (CYP) enzymes in peripheral blood mononuclear cells may be predictive of systemic enzyme activity. We investigated whether changes in mRNA expression for CYP1A2,CYP2C19, CYP2D6 and CYP3A4 in peripheral blood lymphocytes (PBLs) may serve as surrogate markers for changes in CYP enzyme activity following the administration of rifampin.

METHODS

On day 1 and day 9 of the study, 12 healthy volunteers were administered caffeine 100 mg, debrisoquine 10 mg and omeprazole 40 mg orally, along with midazolam 0.025 mg/kg intravenously. Blood samples and urine were collected for 8 h after drug administration. The subjects took rifampin 300 mg (n = 6) or 600 mg (n = 6) daily on days 2-8. Total RNA was isolated from PBLs on day 1 and day 9, and mRNA expression for the CYP enzymes and hGAPDH were determined by means of quantitative, real-time, reverse-transcriptase polymerase chain reaction. CYP1A2 activity was estimated by calculating the plasma paraxanthine to caffeine AUC ratio (caffeine metabolic ratio; CMR), CYP2C19 activity by the 2-h omeprazole hydroxylation index (HI), CYP2D6 activity by the urinary debrisoquine recovery ratio (DBRR) and CYP3A4 activity by midazolam clearance.

RESULTS

Median midazolam clearance (0.362 to 0.740 l/kg/h), omeprazole HI (0.752 to 0.214), CMR (0.365 to 0.450) and DBRR (0.406 to 0.479) all changed significantly following rifampin, consistent with the expected enzyme induction. CYP1A2,CYP2D6 and CYP3A4 mRNA content were measurable in all samples. CYP2C19 mRNA was inconsistently detectable. There were no significant correlations between changes in enzyme activity and mRNA expression by Spearman's rank order correlation.

CONCLUSION

The results do not support the use of mRNA expression assays for CYP1A2, CYP2C19, CYP2D6 and CYP3A4 enzymes in PBLs as surrogates for quantifying changes in systemic enzyme activity in the setting of enzyme induction.

Anti-inflammatory activity of ansamycins

Inflammation represents a complex biologic and biochemical process involving cells of the immune system and a plethora of biologic mediators in response to mechanical, chemical or infectious injuries. When mobilization of effector cells and molecules becomes excessive, the beneficial aspect of this response--to limit damage and promote healing, can be overriden, resulting in host-cell and tissue dysfunction. Based on the hypothesis that chronic infections underly some inflammatory diseases, antibacterial therapy has long been assessed in various inflammatory settings. Recently, the anti-inflammatory activity of some antibacterial agents has also been suspected. Of these duel-action drugs, ansamycins represent an interesting family. Although their therapeutic use is restricted to potentially infectious inflammatory diseases, many experimental data suggest that these drugs also possess direct inhibitory activity on some crucial proinflammatory effectors. To date, the potent antimycobacterial activity of the therapeutically useful ansamycins precludes their widespread use in inflammatory diseases. However, biosynthetic manipulation remains an attractive route for the generation of pharmacologically useful analogs.

Second-Generation Antihistamines

Antihistamines are useful medications for the treatment of a variety of allergic disorders. Second-generation antihistamines avidly and selectively bind to peripheral histamine H1 receptors and, consequently, provide gratifying relief of histamine-mediated symptoms in a majority of atopic patients. This tight receptor specificity additionally leads to few effects on other neuronal or hormonal systems, with the result that adverse effects associated with these medications, with the exception of noticeable sedation in about 10% of cetirizine-treated patients, resemble those of placebo overall. Similarly, serious adverse drug reactions and interactions are uncommon with these medicines. Therapeutic interchange to one of the available second-generation antihistamines is a reasonable approach to limiting an institutional formulary, and adoption of such a policy has proven capable of creating substantial cost savings. Differences in overall efficacy and safety between available second-generation antihistamines, when administered in equivalent dosages, are not large. However, among the antihistamines presently available, fexofenadine may offer the best overall balance of effectiveness and safety, and this agent is an appropriate selection for initial or switch therapy for most patients with mild or moderate allergic symptoms. Cetirizine is the most potent antihistamine available and has been subjected to more clinical study than any other. This agent is appropriate for patients proven unresponsive to other antihistamines and for those with the most severe symptoms who might benefit from antihistamine treatment of the highest potency that can be dose-titrated up to maximal intensity.

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.

Active transport of imatinib into and out of cells: implications for drug resistance

Imatinib is a tyrosine kinase inhibitor that is effective in the treatment of chronic myeloid leukemia (CML). Not all patients achieve cytogenetic response. Some patients even lose the initial cytogenetic response. In this study, we investigated the active cellular transport of imatinib to gain a better understanding of the possible mechanisms of imatinib resistance. We used the leukemic cell line CCRFCEM and its drug-resistant subline VBL(100) to measure the uptake of carbon 14 ((14)C)-labeled imatinib. Imatinib uptake was temperature dependent, indicative of an active uptake process. Additionally, incubations with transport inhibitors showed that verapamil, amantadine, and procainamide, inhibitors of the human organic cation transporter 1 (hOCT1), significantly decreased imatinib uptake into CEM cells, whereas the inhibition of hOCT2 or hOCT3 had no effect, indicating that influx into the cells is an active process likely to be mediated by hOCT1. Studies using transfected MDCK cell lines revealed an active efflux component attributable to MDR1 (ABCB1). Both hOCT1 and MDR1 were expressed in CML primary cells and cell lines. The results indicate that active transport processes mediate the influx and efflux of imatinib. Differential expression of influx (hOCT1) and efflux (MDR1) transporters may be a critical determinant of intracellular drug levels and, hence, resistance to imatinib.

Expression of Pregnane-X-Receptor Transcript in Peripheral Blood Mononuclear Cells and Correlation with Mdr1 Mrna

P-glycoprotein (P-gp) limits bioavailability and accumulation of HIV protease inhibitors (PIs). PIs are ligands for the pregnane-X-receptor (PXR), which regulates P-gp expression. This occurs when ligands activate the receptor, initiating binding to response elements in the MDR1 promoter. PXR also activates cytochrome P4503A4 (CYP3A4) and a correlation between hepatic PXR and CYP3A4 mRNA has been reported. We have examined the relationship between MDR1 and PXR mRNA in peripheral blood cells and demonstrate a significant correlation in 18 volunteers (R2=0.4; P<0.005). PXR was approximately 250-fold lower in peripheral blood mononuclear cells than in liver (1.6 ±1.2 vs 450 ±298; n=6; P<0.01).

Cytochrome p450 gene expression levels in peripheral blood mononuclear cells in comparison with the liver

Cytochromes p450 (CYPs) compose a superfamily of similar proteins involved in detoxification and elimination, as well as activation of a wide variety of compounds. Most CYP family members are localized in the liver. In order to assess whether peripheral blood leukocytes (PBL) are available as a surrogate for the determination of CYP gene expression levels in the liver, we compared CYP gene expression levels in PBL with those in liver tissues from patients with hepatocellular carcinoma (HCC). We measured CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 2F1, 2J2, 3A4, 3A5, 3A7, 4A11, 4B1 and CYP27 gene expressions in PBL and in the liver by real-time reverse-transcription (RT)-PCR. We could detect expression of CYP1A1, 1A2, P1B1, 2A6, 2B6 and 2E1 genes in PBL and all the genes except for CYP2F1 in the liver. Although gene expression levels within each subfamily were closely correlated within PBL and within the liver, a clear correlation of gene expression levels between PBL and liver tissues was found only for CYP4B1. Although inter-individual variation of the expression level of each CYP gene was wide, the induced level was proportional to the basal expression level. Therefore, monitoring of CYP gene expression levels in PBL, especially those of CYP4B1, could be available as a biomarker for monitoring of exposure to environmental pollutants and assessing the associated risk. Compared with non-tumor tissue, HCC tissues tended to show overexpression of multiple CYP genes, indicating that individualized selection and more effective administration of chemotherapeutic agents could perhaps be based on the pattern of CYP overexpression.

Carvedilol: a new candidate for reversal of MDR1/P-glycoprotein-mediated multidrug resistance

In 1983, carvedilol [1-[carbazolyl-(4)-oxy]-3-[(2-methoxyphenoxyethyl)amino]-2-propanol] was designed and developed as a beta-adrenoceptor antagonist with vasodilating activity for efficacious and safe treatment of hypertension and coronary artery disease. Carvedilol belongs to the 'third generation' of beta-adrenoceptor antagonists and shows selectivity for the beta1- rather than beta2-adrenoceptor. Carvedilol is also an alpha1-blocking agents, with around 2- to 3-fold more selectivity for beta1- than alpha1-adrenoceptors. This degree of alpha1-blockade is responsible for the moderate vasodilator properties of carvedilol, being different from other beta-adrenoceptor antagonists. In addition, carvedilol is a potent antioxidant, with a 10-fold greater activity than vitamin E. Some carvedilol metabolites found in human plasma also exhibit antioxidative activity approximately 50- to 100-fold greater than carvedilol and other antioxidants. These unique properties of carvedilol, i.e. adrenergic (beta1, beta2 and alpha1) blockade and antioxidative activity, may be important in preventing progressive deterioration of left ventricular dysfunction and chronic heart failure. Recently, carvedilol has been demonstrated to reverse multidrug resistance (MDR) to anticancer drugs in tumor cells in vitro and its reversal effects were comparable with verapamil, which has been used in the first clinical trial for the reversal of MDR. This review introduces the reversal activity and usefulness against MDR, as well as an overview of the pharmacological and pharmacokinetic properties, of carvedilol.

Whole-blood cultures from renal-transplant patients stimulated ex vivo show that the effects of cyclosporine on lymphocyte proliferation are related to P-glycoprotein expression

BACKGROUND

Cyclosporine (CsA) is a substrate for the MDR-1 gene product P-glycoprotein (P-gp). CsA efficacy may be modulated by lymphocyte P-gp expression levels. In this study, CsA inhibition of lymphocyte proliferation in whole-blood cultures ex vivo has been related to (1) lymphocyte P-gp expression and (2) the C3435T polymorphism in the MDR-1 gene, which has been reported to alter P-gp function.

METHODS

In 30 renal-transplant recipients taking CsA monotherapy, P-gp expression was measured by flow cytometry. Whole-blood samples were stimulated with purified protein derivative (PPD) and phytohemagglutinin (PHA). CsA resistance ex vivo was defined as less than 10% reduction in proliferation with either PPD or PHA at 2 hours compared with 0 hours.

RESULTS

CsA resistance was associated with greater P-gp expression using either PPD (median expression, resistant 1.89 vs. sensitive 0.96, P =0.02) or PHA (1.66 vs. 0.96, respectively, P =0.02). Whole-blood CsA levels in resistant and sensitive patients were similar. The C3435T polymorphism did not affect inhibition of proliferation by CsA (P >0.05 for all between genotype group comparisons).

CONCLUSIONS

Our results indicate that lymphocyte P-gp expression determines the degree of inhibition of proliferation by CsA ex vivo; whether this also affects CsA effectiveness in vivo and therefore graft survival requires further study.

The effects of protease inhibitors and nonnucleoside reverse transcriptase inhibitors on p-glycoprotein expression in peripheral blood mononuclear cells in vitro

Several antiretroviral compounds have been shown to be substrates for the efflux protein P-glycoprotein (P-gp) although few studies have investigated the effects of drug on expression of this protein. Here, an in vitro system has been adopted to investigate the effects of protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors (NNRTIs) on P-gp expression in peripheral blood mononuclear cells (PBMCs). PBMCs isolated from healthy volunteers were incubated with 10 or 100 microM PI (saquinavir, ritonavir, lopinavir, indinavir, nelfinavir, amprenavir) or 10 microM NNRTI (efavirenz, nevirapine) for 72 hours. Surface P-gp expression was measured by flow cytometry and compared with vehicle-incubated controls. Toxicity was assessed by MTT assay and the effects of each compound were compared between individuals with differing genotypes at position 3435 of exon 26 of MDR1, which was assigned by restriction fragment length polymorphism. Significant increases in median P-gp expression were observed following incubation with 10 microM nelfinavir (10.2 versus 6.7% P-gp-positive cells) and efavirenz (10.0 versus 6.7% P-gp-positive cells). No significant differences in induction were observed between genotypes (CC, CT, TT). Following incubation with 100 microM PI, significant upregulation of P-gp occurred except with amprenavir. However, nelfinavir, ritonavir, and lopinavir caused marked toxicity, indicating that at higher concentrations, the increase in P-gp may be at least partially related to a stress response. These results indicate the potential of some PIs and NNRTIs to induce P-gp expression in PBMCs in vitro.

Quantitative determination of MDR1 mRNA expression in peripheral blood lymphocytes: a possible role of genetic polymorphisms in the MDR1 gene

BACKGROUND

P-glycoprotein is a transmembrane efflux pump that extrudes a wide variety of drugs, thereby reducing their intracellular access. In humans, P-glycoprotein is encoded by the MDR1 gene. Recently, several single nucleotide polymorphisms in the MDR1 gene were identified. Moreover, it was postulated that, in addition to the full-length P-glycoprotein, a 'mini' P-glycoprotein was also present in lymphocytes.

MATERIALS AND METHODS

We investigated the effect of the genetic polymorphisms G2677T and C3435T in the MDR1 gene on MDR1 mRNA expression in FACS-sorted peripheral blood CD4+, CD8+, CD19+, and CD56+ cells. MDR1 mRNA expression was determined in 45 healthy individuals using a real-time quantitative RT-PCR.

RESULTS

We detected the highest expression of MDR1 mRNA in CD56+ cells, followed by CD8+ > CD4+ > CD19+ cells. However, genetic polymorphisms of the MDR1 gene failed to affect (P > 0.05) MDR1 mRNA levels in the peripheral blood lymphocytes. Furthermore, the transcript levels for the MDR1 N-terminal half were almost two-fold lower than that of the MDR1 C-terminal half in all cell populations investigated (P < 0.0001).

CONCLUSIONS

An almost two-fold difference in MDR1 C- and N-terminal half expressions supports the presence of mini-P-glycoprotein, an alternatively spliced form of the full-length molecule, in peripheral blood lymphocytes.

Hepatic cytochrome P450 2E1 activity in nondiabetic patients with nonalcoholic steatohepatitis

Cytochrome P450 2E1 (CYP2E1) plays an important role in the pathogenesis of nonalcoholic steatohepatitis (NASH) in animal models, but its role in the pathogenesis of human NASH is unclear. Therefore, we measured hepatic CYP2E1 activity and its correlates in a cohort of nondiabetic patients with NASH (NDN) and controls to explore its role in the pathogenesis of human NASH. Hepatic CYP2E1 activity was assessed using the oral clearance (CL(PO)) of chlorzoxazone (CHZ) in 20 NDN and 17 age, gender, and body mass index (BMI)-matched controls. The relationship between hepatic CYP2E1 activity and demographic and anthropometric variables; fasting levels of insulin, glucose, lipids, and beta-OH butyrate; insulin resistance; and nocturnal hypoxemia was assessed. Furthermore, expression of CYP2E1 in the peripheral lymphocytes was assessed using reverse transcription-polymerase chain reaction (RT-PCR). The CL(PO) of CHZ was significantly (P =.03) greater in NDN (41 +/- 12 L/h) compared with controls (33 +/- 16 L/h). Lymphocyte CYP2E1 messenger RNA was significantly higher in NDN compared with controls (11.5 x 10(3) +/- 10 x 10(3) vs. 2.6 x 10(3) +/- 1.2 x 10(3) molecules/microg total RNA, respectively, P <.001). On univariate analysis, BMI, respiratory quotient, high-density lipoprotein, triglycerides, insulin, insulin resistance, hypoxemia, and beta-OH butyrate significantly correlated with hepatic CYP2E1 activity. However, on stepwise regression analysis, only nocturnal hypoxemia (r = 0.50, P =.009) and beta-OH butyrate (r = 0.37, P =.04) were independent predictors of hepatic CYP2E1 activity. In conclusion, hepatic CYP2E1 activity and lymphocyte CYP2E1 expression are enhanced in NDN. The significant correlations noted between CYP2E1 and hypoxemia and beta-OH butyrate suggest that these factors play a role in increased CYP2E1 activity that is seen in patients with NASH.

Diabetes mellitus increases the in vivo activity of cytochrome P450 2E1 in humans

AIMS

Cytochrome P450 2E1 (CYP2E1) is thought to activate a number of protoxins, and has been implicated in the development of liver disease. Increased hepatic expression of CYP2E1 occurs in rat models of diabetes but it is unclear whether human diabetics display a similar up-regulation. This study was designed to test the hypothesis that human diabetics experience enhanced CYP2E1 expression.

METHODS

The pharmacokinetics of a single dose of chlorzoxazone (500 mg), used as an index of hepatic CYP2E1 activity, was determined in healthy subjects (n = 10), volunteers with Type I (n = 13), and Type II (n = 8) diabetes mellitus. Chlorzoxazone and 6-hydroxychlorzoxazone in serum and urine were analysed by high-performance liquid chromatography. The expression of CYP2E1 mRNA in peripheral blood mononuclear cells was quantified by reverse transcriptase-polymerase chain reaction.

RESULTS

The mean +/- s.d. (90% confidence interval of the difference) chlorzoxazone area under the plasma concentration-time curve was significantly (P </= 0.05) reduced in obese Type II diabetics (15.7 +/- 11.3 micro g h ml-1; 9, 22) compared with healthy subjects (43.5 +/- 16.9 micro g h ml-1; 16, 40) and Type I diabetics (32.8 +/- 9.2 micro g h ml-1; 9, 25). There was a significant two-fold increase in the oral clearance of chlorzoxazone in obese Type II diabetics compared with healthy volunteers and Type I diabetics. The protein binding of chlorzoxazone was not significantly different between the three groups. In contrast, Type 1 diabetics and healthy volunteers demonstrated no difference in the oral clearance of chlorzoxazone. The urinary recovery of 6-hydroxychlorzoxazone as a percentage of the administered dose was not different between healthy, Type I and obese Type II diabetics. The elimination half-life of chlorzoxazone did not differ between the three groups. CYP2E1 mRNA was significantly elevated in Type I and obese Type II diabetics compared with healthy volunteers. The oral clearance of chlorzoxazone, elimination half-life, Tmax, and Cmax were not significantly influenced by weight, body mass index, serum glucose, serum cholesterol, or glycosylated haemoglobin.

CONCLUSIONS

There was a marked increase in hepatic CYP2E1 activity in obese Type II diabetics as assessed by chlorzoxazone disposition. Increased expression of CYP2E1 mRNA in peripheral blood mononuclear cells was found in both types of diabetes mellitus. Adverse hepatic events associated with Type II diabetes may be in part a result of enhanced CYP2E1 expression and activity.

Interindividual variability and tissue-specificity in the expression of cytochrome P450 3A mRNA

The elucidation of the individual contributions of the four CYP3A genes to the overall CYP3A activity has been hampered by similarities in their sequence and function. We investigated the expression of CYP3A mRNA species in the liver and in various other tissues using gene-specific TaqMan probes. CYP3A4 transcripts were the most abundant CYP3A mRNA in each of the 63 white European livers tested and accounted on average for 95% of the combined CYP3A mRNA pool. CYP3A5 and CYP3A7 each contributed on average 2%, whereas CYP3A43 contributed 0.3% transcripts to this pool. Fourteen percent of livers exhibited an increased share of CYP3A5 transcripts (range 4-20%). These livers were either heterozygous for the marker of the CYP3A5 polymorphism, the CYP3A5*1A allele, or expressed very low levels of CYP3A4 mRNA. The CYP3A7 expression was bimodal, and it was increased in 15% livers. CYP3A4 was the dominant CYP3A in the intestine, followed by CYP3A5. CYP3A5 and CYP3A7, but not CYP3A4, were also expressed in the adrenal gland and in the prostate, whereas only CYP3A5 was detected in the kidney. These three tissues were shown to express much lower levels of pregnane X receptor mRNA than the intestine, indicating possibly a different mode of regulation of CYP3A expression. Expression of CYP3A genes was undetectable in peripheral blood lymphocytes. In summary, these assays and results should aid in our efforts to further dissect the regulation and the physiological and pharmacological significance of CYP3A isozymes.