MDR- and CYP3A4-Mediated Drug–Drug Interactions

Inhibition of P-glycoprotein Gene Expression and Function Enhances Triptolide-induced Hepatotoxicity in Mice

Triptolide (TP) is the major active principle of Tripterygium wilfordii Hook f. and very effective in treatment of autoimmune diseases. However, TP induced hepatotoxicity limited its clinical applications. Our previous study found that TP was a substrate of P-glycoprotein and its hepatobiliary clearance was markedly affected by P-gp modulation in sandwich-cultured rat hepatocytes. In this study, small interfering RNA (siRNA) and specific inhibitor tariquidar were used to investigate the impact of P-gp down regulation on TP-induced hepatotoxicity. The results showed that when the function of P-gp was inhibited by mdr1a-1 siRNA or tariquidar, the systemic and hepatic exposures of TP were significantly increased. The aggravated hepatotoxicity was evidenced with the remarkably lifted levels of serum biomarkers (ALT and AST) and pathological changes in liver. The other toxicological indicators (MDA, SOD and Bcl-2/Bax) were also significantly changed by P-gp inhibition. The data analysis showed that the increase of TP exposure in mice was quantitatively correlated to the enhanced hepatotoxicity, and the hepatic exposure was more relevant to the toxicity. P-gp mediated clearance played a significant role in TP detoxification. The risk of herb-drug interaction likely occurs when TP is concomitant with P-gp inhibitors or substrates in clinic.

HIV-1, HCV and alcohol in the CNS: potential interactions and effects on neuroinflammation

Approximately 25% of the HIV-1 positive population is also infected with HCV. The effects of alcohol on HIV-1 or HCV infection have been a research topic of interest due to the high prevalence of alcohol use in these infected patient populations. Although it has long been known that HIV-1 infects the brain, it has only been a little more than a decade since HCV infection of the CNS has been characterized. Both viruses are capable of infecting and replicating in microglia and increasing the expression of proinflammatory cytokines and chemokines, including IL-6 and IL-8. Investigations focusing on the effects of HIV-1, HCV or alcohol on neuroinflammation have demonstrated that these agents are capable of acting through overlapping signaling pathways, including MAPK signaling molecules. In addition, HIV-1, HCV and alcohol have been demonstrated to increase permeability of the blood-brain barrier. Patients infected with either HIV-1 or HCV, or those who use alcohol, exhibit metabolic abnormalities in the CNS that result in altered levels of n-acetyl aspartate, choline and creatine in various regions of the brain. Treatment of HIV/HCV co-infection in alcohol users is complicated by drug-drug interactions, as well as the effects of alcohol on drug metabolism. The drug-drug interactions between the antiretrovirals and the antivirals, as well as the effects of alcohol on drug metabolism, complicate existing models of CNS penetration, making it difficult to assess the efficacy of treatment on CNS infection.

Effect of mild-to-moderate smoking on viral load, cytokines, oxidative stress, and cytochrome P450 enzymes in HIV-infected individuals

Mild-to-moderate tobacco smoking is highly prevalent in HIV-infected individuals, and is known to exacerbate HIV pathogenesis. The objective of this study was to determine the specific effects of mild-to-moderate smoking on viral load, cytokine production, and oxidative stress and cytochrome P450 (CYP) pathways in HIV-infected individuals who have not yet received antiretroviral therapy (ART). Thirty-two human subjects were recruited and assigned to four different cohorts as follows: a) HIV negative non-smokers, b) HIV positive non-smokers, c) HIV negative mild-to-moderate smokers, and d) HIV positive mild-to-moderate smokers. Patients were recruited in Cameroon, Africa using strict selection criteria to exclude patients not yet eligible for ART and not receiving conventional or traditional medications. Those with active tuberculosis, hepatitis B or with a history of substance abuse were also excluded. Our results showed an increase in the viral load in the plasma of HIV positive patients who were mild-to-moderate smokers compared to individuals who did not smoke. Furthermore, although we did not observe significant changes in the levels of most pro-inflammatory cytokines, the cytokine IL-8 and MCP-1 showed a significant decrease in the plasma of HIV-infected patients and smokers compared with HIV negative non-smokers. Importantly, HIV-infected individuals and smokers showed a significant increase in oxidative stress compared with HIV negative non-smoker subjects in both plasma and monocytes. To examine the possible pathways involved in increased oxidative stress and viral load, we determined the mRNA levels of several antioxidant and cytochrome P450 enzymes in monocytes. The results showed that the levels of most antioxidants are unaltered, suggesting their inability to counter oxidative stress. While CYP2A6 was induced in smokers, CYP3A4 was induced in HIV and HIV positive smokers compared with HIV negative non-smokers. Overall, the findings suggest a possible association of oxidative stress and perhaps CYP pathway with smoking-mediated increased viral load in HIV positive individuals.

Macrocyclic lactones differ in interaction with recombinant P-glycoprotein 9 of the parasitic nematode Cylicocylus elongatus and ketoconazole in a yeast growth assay

Macrocyclic lactones (MLs) are widely used parasiticides against nematodes and arthropods, but resistance is frequently observed in parasitic nematodes of horses and livestock. Reports claiming resistance or decreased susceptibility in human nematodes are increasing. Since no target site directed ML resistance mechanisms have been identified, non-specific mechanisms were frequently implicated in ML resistance, including P-glycoproteins (Pgps, designated ABCB1 in vertebrates). Nematode genomes encode many different Pgps (e.g. 10 in the sheep parasite Haemonchus contortus). ML transport was shown for mammalian Pgps, Pgps on nematode egg shells, and very recently for Pgp-2 of H. contortus. Here, Pgp-9 from the equine parasite Cylicocyclus elongatus (Cyathostominae) was expressed in a Saccharomyces cerevisiae strain lacking seven endogenous efflux transporters. Pgp was detected on these yeasts by flow cytometry and chemiluminescence using the monoclonal antibody UIC2, which is specific for the active Pgp conformation. In a growth assay, Pgp-9 increased resistance to the fungicides ketoconazole, actinomycin D, valinomycin and daunorubicin, but not to the anthelmintic fungicide thiabendazole. Since no fungicidal activity has been described for MLs, their interaction with Pgp-9 was investigated in an assay involving two drugs: Yeasts were incubated with the highest ketoconazole concentration not affecting growth plus increasing concentrations of MLs to determine competition between or modulation of transport of both drugs. Already equimolar concentrations of ivermectin and eprinomectin inhibited growth, and at fourfold higher ML concentrations growth was virtually abolished. Selamectin and doramectin did not increase susceptibility to ketoconazole at all, although doramectin has been shown previously to strongly interact with human and canine Pgp. An intermediate interaction was observed for moxidectin. This was substantiated by increased binding of UIC2 antibodies in the presence of ivermectin, moxidectin, daunorubicin and ketoconazole but not selamectin. These results demonstrate direct effects of MLs on a recombinant nematode Pgp in an ML-specific manner.

Macrocyclic lactones (MLs) are widely used drugs against parasitic nematodes, but drug resistance is rapidly increasing in prevalence and spatial distribution in parasites of ruminants and horses, and is suspected in human nematodes after mass drug applications. Changes in expression levels or the amino acid sequences of P-glycoprotein (Pgp) transporters have frequently been implicated in ML resistance, but direct evidence for transport of MLs by nematode Pgps is still missing. Here, cloning of pgp-9 of the equine parasite Cylicocyclus elongatus and its functional recombinant expression in a Saccharomyces cerevisiae yeast strain deficient in seven endogenous ABC transporters is described. Expression decreased susceptibility to several fungicidal mammalian Pgp substrates including e.g. actinomycin D and ketoconazole, but had no influence on susceptibility to the benzimidazole thiabendazole, which is active against both, yeasts and nematodes. Addition of some MLs strongly increased ketoconazole susceptibility in yeasts expressing C. elongatus Pgp-9, while other MLs had no effect. These interactions are a strong hint that some MLs act as substrates or at least as inhibitors of Pgp-9 mediated drug transport.

Population pharmacokinetics and individualized dosage prediction of cyclosporine in allogeneic hematopoietic stem cell transplant patients

BACKGROUND

Cyclosporine (CsA), a potent immunosuppressive agent used to prevent rejection, is characterized by large individual variability. The purpose of this study was to explore the pharmacokinetic characteristics of CsA and establish a CsA population pharmacokinetic model that could be used for personalized therapy in allogeneic hematopoietic stem cell transplant (allo-HSCT) patients.

METHODS

Clinical data were obtained from 117 allo-HSCT patients. The data analysis was performed using NONMEM software. A first-order conditional estimation with interaction (FOCE-I) method within NONMEM was used to estimate the parameters. The covariates, including demographics, hematological indices, biochemical levels, concurrent drugs, and genetic polymorphisms of CYP3A4, CYP3A5, and ABCB1, were evaluated quantitatively. The stability of the final model was validated by a nonparametric bootstrap procedure.

RESULTS

A total of 1,571 observed concentrations were collected. A 1-compartment model with first-order absorption and elimination adequately described the pharmacokinetics of CsA. The typical values for clearance (CL), volume of distribution (V), and bioavailability were 29.6 L/hr, 605 L, and 0.619, respectively. The interindividual variability of these parameters was 20.4, 66.1, and 30.4%, respectively. The residual error was 31.4% and 23.7 ng/mL. The duration of CsA therapy, hematocrit, antifungal agent administration, triglycerides, and weight were identified as the main covariates that influenced CL, and hematocrit had a significant effect on V. The internal validation showed that the final model was stable and accurate.

CONCLUSIONS

This study established a population pharmacokinetic model of CsA in allo-HSCT patients that could provide the foundation for personalized use of CsA in the clinic.

Plasma and Intracellular Antiretroviral Concentrations in HIV-Infected Patients under Short Cycles of Antiretroviral Therapy

Study of plasma and intracellular concentrations of atazanavir, lopinavir, nevirapine, and efavirenz was conducted on 48 patients under short cycles of antiretroviral therapy. Intracellular concentrations (IC) were still measurable for all drugs after 85 h or 110 h drug intake despite the absence of drug in plasma for atazanavir and lopinavir. A linear relationship between plasma and intracellular efavirenz was observed. Further studies to fully understand the impact of IC in the intermittent antiviral treatment are required.

Hypoglycemia after antimicrobial drug prescription for older patients using sulfonylureas

IMPORTANCE

Certain antimicrobial drugs interact with sulfonylureas to increase the risk of hypoglycemia.

OBJECTIVE

To determine the risk of hypoglycemia and associated costs in older patients prescribed glipizide or glyburide who fill a prescription for an antimicrobial drug.

DESIGN, SETTING, AND PARTICIPANTS

This was a retrospective cohort study of Texas Medicare claims from 2006 to 2009 for patients 66 years or older who were prescribed glipizide or glyburide and who also filled a prescription for 1 of the 16 antimicrobials most commonly prescribed for this population.

METHODS

We assessed hypoglycemia events and associated Medicare costs in patients prescribed 1 of 7 antimicrobial agents thought to interact with sulfonylureas, using noninteracting antimicrobials as a comparison. We used a repeated measure logistic regression, controlling for age, sex, ethnicity, Medicaid eligibility, comorbidity, prior emergency department visits for hypoglycemia, prior hospitalizations for any cause, nursing home residence, and indication for the antimicrobial. We estimated odds of hypoglycemia, number needed to harm, deaths during hospitalization for hypoglycemia, and Medicare costs for hypoglycemia treatment.

MAIN OUTCOMES AND MEASURES

Any hospitalization or emergency department visit owing to hypoglycemia within 14 days of antimicrobial exposure.

RESULTS

In multivariable analyses controlling for patient characteristics and indication for antimicrobial drug use, clarithromycin (odds ratio [OR], 3.96 [95% CI, 2.42-6.49]), levofloxacin (OR, 2.60 [95% CI, 2.18-3.10]), sulfamethoxazole-trimethoprim (OR, 2.56 [95% CI, 2.12-3.10]), metronidazole (OR, 2.11 [95% CI, 1.28-3.47]), and ciprofloxacin (OR, 1.62 [95% CI, 1.33-1.97]) were associated with higher rates of hypoglycemia compared with a panel of noninteracting antimicrobials. The number needed to harm ranged from 71 for clarithromycin to 334 for ciprofloxacin. Patient factors associated with hypoglycemia included older age, female sex, black or Hispanic race/ethnicity, higher comorbidity, and prior hypoglycemic episode. In 2009, 28.3% of patients prescribed a sulfonylurea filled a prescription for 1 of these 5 antimicrobials, which were associated with 13.2% of all hypoglycemia events in patients taking sulfonylureas. The treatment of subsequent hypoglycemia adds $30.54 in additional Medicare costs to each prescription of 1 of those 5 antimicrobials given to patients taking sulfonylureas.

CONCLUSIONS AND RELEVANCE

Prescription of interacting antimicrobial drugs to patients on sulfonylureas is very common, and is associated with substantial morbidity and increased costs.

Acute rhabdomyolysis associated with coadministration of levofloxacin and simvastatin in a patient with normal renal function

We report a rare case of severe acute rhabdomyolysis in association with coadministration of levofloxacin and simvastatin in a patient with normal renal function. A 70-year-old Caucasian male was treated due to community acquired pneumonia with levofloxacin in a dosage of 500 mg once and then twice a day. On the 8th day of hospitalization the patient presented with acute severe rhabdomyolysis requiring an intensive care support. After discontinuation of levofloxacin and concomitant medication with simvastatin 80 mg/day, clinical and laboratory effects were totally reversible. Up to now, levofloxacin has been reported to induce rhabdomyolysis mainly in patients with impaired renal function, as the medication has a predominant renal elimination. In our case renal function remained normal during the severe clinical course. According to a recent case report rhabdomyolysis was observed due to interaction of simvastatin and ciprofloxacin. To our best knowledge this is the first case of interaction between simvastatin and levofloxacin to be reported. This case emphasizes the need of close monitoring of creatine kinase in patients under more than one potentially myotoxic medication especially when patients develop muscle weakness.

Determination of domperidone in human plasma using high performance liquid chromatography with fluorescence detection for clinical application

A simple and reliable method for the determination of domperidone in human plasma has been developed. Plasma samples (1mL) were pre-purified by a solid-phase extraction with Bond Elut(®) C18. The separation was achieved with XBridge™ C18 column (150mm×4.6mm i.d., 5μm) at 40°C. The mobile phase was a mixture of acetonitrile and 10mM ammonium acetate buffer (36:64, v/v), adjusted to pH 9.4 with 20% ammonium solution at a flow rate of 1.0mL/min. The peak was detected using fluorescence detector at excitation 282nm and emission 328nm. Retention times for domperidone and internal standard (propranolol) were 8.3min and 11.2min, respectively. The method showed a good linearity (r>0.999), precision (relative standard deviations <10.6%), and extraction recovery (85.7-99.7%) over a concentration of 1-100ng/mL. The lower limit of quantification (LLOQ) was 1.0ng/mL. This proposed method was successfully applied to a pharmacokinetic interaction study of domperidone in healthy Japanese volunteers.

Clinically relevant drug-drug interactions between antiretrovirals and antifungals

INTRODUCTION

Complete delineation of the HIV-1 life cycle has resulted in the development of several antiretroviral drugs. Twenty-five therapeutic agents belonging to five different classes are currently available for the treatment of HIV-1 infections. Advent of triple combination antiretroviral therapy has significantly lowered the mortality rate in HIV patients. However, fungal infections still represent major opportunistic diseases in immunocompromised patients worldwide.

AREAS COVERED

Antiretroviral drugs that target enzymes and/or proteins indispensable for viral replication are discussed in this article. Fungal infections, causative organisms, epidemiology and preferred treatment modalities are also outlined. Finally, observed/predicted drug-drug interactions between antiretrovirals and antifungals are summarized along with clinical recommendations.

EXPERT OPINION

Concomitant use of amphotericin B and tenofovir must be closely monitored for renal functioning. Due to relatively weak interactive potential with the CYP450 system, fluconazole is the preferred antifungal drug. High itraconazole doses (> 200 mg/day) are not advised in patients receiving booster protease inhibitor (PI) regimen. Posaconazole is contraindicated in combination with either efavirenz or fosamprenavir. Moreover, voriconazole is contraindicated with high-dose ritonavir-boosted PI. Echinocandins may aid in overcoming the limitations of existing antifungal therapy. An increasing number of documented or predicted drug-drug interactions and therapeutic drug monitoring may aid in the management of HIV-associated opportunistic fungal infections.

Phase 0 and phase III transport in various organs: combined concept of phases in xenobiotic transport and metabolism

The historical phasing concept of drug metabolism and elimination was introduced to comprise the two phases of metabolism: phase I metabolism for oxidations, reductions and hydrolyses, and phase II metabolism for synthesis. With this concept, biological membrane barriers obstructing the accessibility of metabolism sites in the cells for drugs were not considered. The concept of two phases was extended to a concept of four phases when drug transporters were detected that guided drugs and drug metabolites in and out of the cells. In particular, water soluble or charged drugs are virtually not able to overcome the phospholipid membrane barrier. Drug transporters belong to two main clusters of transporter families: the solute carrier (SLC) families and the ATP binding cassette (ABC) carriers. The ABC transporters comprise seven families with about 20 carriers involved in drug transport. All of them operate as pumps at the expense of ATP splitting. Embedded in the former phase concept, the term "phase III" was introduced by Ishikawa in 1992 for drug export by ABC efflux pumps. SLC comprise 52 families, from which many carriers are drug uptake transporters. Later on, this uptake process was referred to as the "phase 0 transport" of drugs. Transporters for xenobiotics in man and animal are most expressed in liver, but they are also present in extra-hepatic tissues such as in the kidney, the adrenal gland and lung. This review deals with the function of drug carriers in various organs and their impact on drug metabolism and elimination.

Expression of selected cytochrome P450 isoforms and of cooperating enzymes in colorectal tissues in selected pathological conditions

The current interest in CYP expression in the colon results from its uniqueness as a target organ for cancer. To date, the CYP expression profiles in the colon have not yet been subject of comprehensive research. In this study, we investigated 40 patients with Crohn's disease, 40 with ulcerative colitis, and 40 healthy subjects as a control group. Colon tissues were fixed, dehydrated, cleared in xylene and embedded in paraffin. Sections were prepared from paraffin blocks for immunohistochemical staining with specific antibodies. We used antibodies to the human CYP1A1, CYP2B6, CYP2C9, CYP2E1 and CYP3A4 isoforms, as well as antibodies to the human glycoprotein P, glutathione-S transferase and antibody to the UDP-glucuronosyltransferase. The sections were stained immunohistochemically and examined using light microscopy. Cellular localization was determined, and computer image analysis was used. In all cases with Crohn's disease, the proteins studied showed at least a twofold expression. Ulcerative colitis showed a much weaker influence regarding the expression of the proteins studied but in case of CYP2C9 and UDP-glucuronosyltransferase, a decrease of expression was observed.

Binary and ternary combinations of anti-HIV protease inhibitors: effect on gene expression and functional activity of CYP3A4 and efflux transporters

BACKGROUND

The purpose of this study is to identify the effect of binary and ternary combinations of anti-HIV protease inhibitors (PIs) on the expression of metabolizing enzyme (CYP3A4) and efflux transporters [multidrug resistance-associated protein 2 (MRP2), P-glycoprotein (P-gp) and breast cancer resistant protein (BCRP)] in a model intestinal cell line (LS-180).

METHODS

LS-180 cells were treated with various combinations of PIs (amprenavir, indinavir, saquinavir and lopinavir), and the mRNA expression levels of metabolizing enzyme and efflux transporters were measured using quantitative reverse transcription polymerase chain reaction. The alteration of gene expression was further correlated to the expression of nuclear hormone receptor PXR. Uptake of fluorescent and radioactive substrates was carried out to study the functional activity of these proteins. Cytotoxicity and adenosine triphosphate (ATP) assays were carried out to measure stress responses.

RESULTS

Binary and ternary combinations of PIs appeared to modulate the expression of CYP3A4, MRP2, P-gp and BCRP in a considerable manner. Unlike the individual PIs, their binary combinations showed much greater induction of metabolizing enzyme and efflux proteins. However, such pronounced induction was not observed in the presence of ternary combinations. The observed trend of altered mRNA expression was found to correlate well with the change in expression levels of PXR. The gene expression was found to correlate with activity assays. Lack of cytotoxicity and ATP activity was observed in the treatment samples, suggesting that these alterations in expression levels were probably not stress responses.

CONCLUSIONS

In the present study, we demonstrated that combinations of drugs can have serious consequences toward the treatment of HIV infection by altering their bioavailability and disposition.

MDR1 gene polymorphisms are associated with glucocorticoid-induced avascular necrosis of the femoral head in a Chinese population

P-glycoprotein (P-gp) is present in various tissue cells, required for the pumping of lipophilic drugs (including glucocorticoids) out of cells. We hypothesized that polymorphisms in the P-gp encoding gene (multidrug-resistant transporter-1 [MDR1]) are related to individual differences in glucocorticoid sensitivity and the development of glucocorticoid-induced avascular necrosis of the femoral head (GANFH). In this case-control study, we genotyped three known single-nucleotide polymorphisms (SNPs: C1236T, G2677T/A, and C3435T) within the MDR1 gene in 662 Chinese subjects. Statistically significant differences between GANFH patients and either healthy controls or glucocorticoid-resistant patients (non-GANFH) were found for the T allele or TT genotype of C3435T. The haplotype TTT, composed of these three SNPs, exhibited a significant association with the disease. No associations were identified between C1236T or G2677T/A and GANFH. Our results suggest that the C3435T polymorphism of the MDR1 gene is associated with susceptibility to GANFH in a Chinese population.

In vitro moxifloxacin drug interaction with chemotherapeutics: implications for retinoblastoma management

Retinoblastoma (RB) is a common malignant intraocular tumor primarily affecting children. Multidrug resistance (MDR) proteins (P-gp and MRPs) mediated chemoresistance have been considered as a major cause of treatment failure in treatment of RB. Ocular cells have shown good tolerability against moxifloxacin (MFX). Hence, the aim of present study was to investigate the effect of moxifloxacin on the functionality of MDR proteins. Furthermore, we have also examined an interaction of MFX with anticancer agents (Topotecan, etoposide and vinblastine) for RB treatment. For interaction of MFX with efflux transporter, model cell lines transfected with the efflux transporters (MDCK-MDR1 and MDCK-MRP2) were used to perform uptake and bi-directional transport experiments. Modulation of anticancer induced cell cytotoxicity, pro-inflammatory cytokines (IL-6 and IL-8) release and caspase-3 enzyme activity in presence of MFX was also evaluated. Result indicates that MFX is a substrate of both MDR1 and MRP2 efflux transporters. Furthermore elevation of anticancer uptake and bi-directional transport, reduction in IC50 cytotoxic value and modulation of antiproliferative and cytokines release in presence of MFX by anticancer agents was observed. Our results demonstrate that MFX may not only modulate the permeability of anticancer agents at efflux sites but it may also potentiate antiproliferative activity of anticancer agents in retinoblastoma cells. This study may be further extended to explore in vivo outcome of this finding.

CYP3A5*3 and C3435T MDR1 polymorphisms in prognostication of drug-resistant epilepsy in children and adolescents

Drug-resistant epilepsies still remain one of the most profound problems of contemporary epileptology. Several mechanisms of drug resistance are possible; among them, genetic factors have a prominent place. Much importance is attached to genes, which encode enzymes that metabolize antiepileptic drugs CYP 3A, which belong to the family of cytochromes P450 and the genome of multidrug resistance, such as multidrug resistance 1 (MDR1) that expresses P-glycoprotein (P-gp), a drug transporter protein. The aim of the study was to assess the relation between polymorphism of gene CYP3A5 and polymorphism C3435T of MDR1 gene with the occurrence of focal, drug-resistant epilepsy in children and youths up to 18 years of age. The study comprised 85 patients, and their age range was from 33 months to 18 years of age, suffering from epilepsy, partly responding well to treatment, partly drug resistant. The polymorphism of both genes has been analysed using the PCR-RFLP method. The study failed to corroborate association between polymorphism CYP3A5∗3 and C3435T polymorphism in MDR1 gene and pharmacoresistant epilepsy. The results of our research do not confirm the prognostic value of the polymorphisms examined in the prognostication of drug resistance in epilepsies.

Functional impact of ABCB1 variants on interactions between P-glycoprotein and methadone

Methadone is a widely used substitution therapy for opioid addiction. Large inter-individual variability has been observed in methadone maintenance dosages and P-glycoprotein (P-gp) was considered to be one of the major contributors. To investigate the mechanism of P-gp’s interaction with methadone, as well as the effect of genetic variants on the interaction, Flp-In™-293 cells stably transfected with various genotypes of human P-gp were established in the present study. The RNA and protein expression levels of human P-gp were confirmed by real-time quantitative RT-PCR and western blot, respectively. Utilizing rhodamine 123 efflux assay and calcein-AM uptake study, methadone was demonstrated to be an inhibitor of wild-type human P-gp via non-competitive kinetic (IC₅₀ = 2.17±0.10 µM), while the variant-type human P-gp, P-gp with 1236T-2677T-3435T genotype and P-gp with 1236T-2677A-3435T genotype, showed less inhibition potency (IC₅₀ = 2.97±0.09 µM and 4.43±1.10 µM, respectively) via uncompetitive kinetics. Methadone also stimulated P-gp ATPase and inhibited verapamil-stimulated P-gp ATPase activity under therapeutic concentrations. These results may provide a possible explanation for higher methadone dosage requirements in patients carrying variant-type of P-gp and revealed the possible drug-drug interactions in patients who receive concomitant drugs which are also P-gp substrates.

Berberine analogue IMB-Y53 improves glucose-lowering efficacy by averting cellular efflux especially P-glycoprotein efflux

OBJECTIVE

Cellular efflux transporters, especially P-glycoprotein (P-gp), impel berberine (BBR) out of cells, and therefore reduce bioavailability of the compound. This study was designed to overcome efflux of BBR using P-gp as a target.

MATERIALS/METHODS

Molecular docking study was done to identify BBR analogues that were with low affinity to P-gp. Flow cytometry was used to determine cellular efflux of chemicals. Pharmacokinetic study was performed in Wistar rats, following oral administration of the study compounds. The efficacies of chemicals on glucose homeostasis were determined both in cultured cells and diabetic KK-Ay and db/db mice.

RESULTS

In the molecular docking study, we found that among BBR analogues pseudo-berberine (IMB-Y53) has low affinity to P-gp. IMB-Y53 was retained in Caco-2, HL-7702 and C2C12 cells for a significantly longer period of time than BBR did. P-gp inhibitor tetrandrine (Tet) abolished the efflux of BBR at different extent depending on the expression level of P-gp; however, Tet had no impact on IMB-Y53 efflux. BBR increased P-gp expression dose-dependently in intestinal and liver cells; IMB-Y53 also up-regulated P-gp but at a much lower level as compared with BBR. Administered at equal dose in rats, the maximum plasma concentration (C(max)) and area under concentration-time curve (AUC(0-24)) of IMB-Y53 were 1.61 and 2.27-fold of those of BBR, respectively, indicating an improved bioavailability. IMB-Y53 stimulated glucose utility in cultured cells with a degree similar to that of BBR, but exhibited enhanced glucose-lowering efficacy in KK-Ay and db/db diabetic mice.

CONCLUSIONS

These results suggest that overcoming cellular efflux especially P-gp's function improves bioavailability and hypoglycemic effect of BBR.

Pharmacogenomics of Cytochrome P450 3A4: Recent Progress Toward the "Missing Heritability" Problem

CYP3A4 is the most important drug metabolizing enzyme in adult humans because of its prominent expression in liver and gut and because of its broad substrate specificity, which includes drugs from most therapeutic categories and many endogenous substances. Expression and function of CYP3A4 vary extensively both intra- and interindividually thus contributing to unpredictable drug response and toxicity. A multitude of environmental, genetic, and physiological factors are known to influence CYP3A4 expression and activity. Among the best predictable sources of variation are drug–drug interactions, which are either caused by pregnane X-receptor (PXR), constitutive androstane receptor (CAR) mediated gene induction, or by inhibition through coadministered drugs or other chemicals, including also plant and food ingredients. Among physiological and pathophysiological factors are hormonal status, age, and gender, the latter of which was shown to result in higher levels in females compared to males, as well as inflammatory processes that downregulate CYP3A4 transcription. Despite the influence of these non-genetic factors, the genetic influence on CYP3A4 activity was estimated in previous twin studies and using information on repeated drug administration to account for 66% up to 88% of the interindividual variation. Although many single nucleotide polymorphisms (SNPs) within the CYP3A locus have been identified, genetic association studies have so far failed to explain a major part of the phenotypic variability. The term “missing heritability” has been used to denominate the gap between expected and known genetic contribution, e.g., for complex diseases, and is also used here in analogy. In this review we summarize CYP3A4 pharmacogenetics/genomics from the early inheritance estimations up to the most recent genetic and clinical studies, including new findings about SNPs in CYP3A4 (*22) and other genes (P450 oxidoreductase (POR), peroxisome proliferator-activated receptor alpha (PPARA)) with possible contribution to CYP3A4 variable expression.

Warning against co-administration of 3,4-methylenedioxymethamphetamine (MDMA) with methamphetamine from the perspective of pharmacokinetic and pharmacodynamic evaluations in rat brain

3,4-Methylenedioxymethamphetamine (MDMA) and methamphetamine often cause serious adverse effects (e.g., rhabdomyolysis, and cardiac disease) following hyperthermia triggered by release of brain monoamines such as dopamine and serotonin. Therefore, evaluation of brain monoamine concentrations is useful to predict these drugs' risks in human. This study aimed to evaluate risks of co-administration of MDMA and methamphetamine, both of which are abused frequently in Japan, based on drug distribution and monoamine level in the rat brain. Rats were allocated to three groups: (1) sole MDMA administration (12 or 25 mg/kg, intraperitoneally), (2) sole methamphetamine administration (10 mg/kg, intraperitoneally) and (3) co-administration of MDMA (12 mg/kg, intraperitoneally) and methamphetamine (10 mg/kg, intraperitoneally). We monitored pharmacokinetic and pharmacodynamic variables for drugs and monoamines in the rat brain. Area under the curve for concentration vs. time until 600 min from drug administration (AUC₀₋₆₀₀) increased from 348.0 to 689.8 μgmin/L for MDMA and from 29.9 to 243.4 μMmin for dopamine in response to co-administration of methamphetamine and MDMA compared to sole MDMA (12 mg/kg) administration. After sole methamphetamine or that with MDMA administration, AUC₀₋₆₀₀ of methamphetamine were 401.8 and 671.1 μgmin/L, and AUC₀₋₆₀₀ of dopamine were 159.9 and 243.4 μMmin. In conclusion, the brain had greater exposure to MDMA, methamphetamine and dopamine after co-administration of MDMA and methamphetamine than when these two drugs were given alone. This suggests co-administration of MDMA with methamphetamine confers greater risk than sole administration, and that adverse events of MDMA ingestion may increase when methamphetamine is co-administered.

Evaluation of animal models for intestinal first-pass metabolism of drug candidates to be metabolized by CYP3A enzymes via in vivo and in vitro oxidation of midazolam and triazolam

1. To search an appropriate evaluation methodology for the intestinal first-pass metabolism of new drug candidates, grapefruit juice (GFJ)- and vehicle (tap water)-pretreated mice or rats were orally administered midazolam (MDZ) or triazolam (TRZ), and blood levels of the parent compounds and their metabolites were measured by liquid chromatography/MS/MS. A significant effect of GFJ to elevate the blood levels was observed only for TRZ in mice. 2. In vitro experiments using mouse, rat and human intestinal and hepatic microsomal fractions demonstrated that GFJ suppressed the intestinal microsomal oxidation of MDZ and especially TRZ. Substrate inhibition by MDZ caused reduction in 1'-hydroxylation but not 4-hydroxylation in both intestinal and hepatic microsomal fractions. The kinetic profiles of MDZ oxidation and the substrate inhibition in mouse intestinal and hepatic microsomal fractions were very similar to those in human microsomes but were different from those in rat microsomes. Furthermore, MDZ caused mechanism-based inactivation of cytochrome P450 3A-dependent TRZ 1'-hydroxylation in mouse, rat and human intestinal microsomes with similar potencies. 3. These results are useful information in the analysis of data obtained in mouse and rat for the evaluation of first-pass effects of drug candidates to be metabolized by CYP3A enzymes.

Alcohol consumption effect on antiretroviral therapy and HIV-1 pathogenesis: role of cytochrome P450 isozymes

INTRODUCTION

Alcohol consumption, which is highly prevalent in HIV-infected individuals, poses serious concerns in terms of rate of acquisition of HIV-1 infection, HIV-1 replication, response to highly active antiretroviral therapy (HAART) and AIDS/neuroAIDS progression. However, little is known about the mechanistic pathways by which alcohol exerts these effects, especially with respect to HIV-1 replication and the patient's response to HAART.

AREAS COVERED

In this review, the authors discuss the effects of alcohol consumption on HIV-1 pathogenesis and its effect on HAART. They also describe the role of cytochrome P450 2E1 (CYP2E1) in alcohol-mediated oxidative stress and toxicity, and the role of CYP3A4 in the metabolism of drugs used in HAART (i.e., protease inhibitors (PI) and non-nucleoside reverse transcriptase inhibitors (NNRTI)). Based on the most recent findings the authors discuss the role of CYP2E1 in alcohol-mediated oxidative stress in monocytes/macrophages and astrocytes, as well as the role of CYP3A4 in alcohol-PI interactions leading to altered metabolism of PI in these cells.

EXPERT OPINION

The authors propose that alcohol and PI/NNRTI interact synergistically in monocytes/macrophages and astrocytes through the CYP pathway leading to an increase in oxidative stress and a decrease in response to HAART. Ultimately, this exacerbates HIV-1 pathogenesis and neuroAIDS.

Enantioselective uptake of fexofenadine by Caco-2 cells as model intestinal epithelial cells

OBJECTIVES

Fexofenadine contains a chiral carbon in its chemical structure and is orally administered as a racemic mixture. This study evaluated the selective uptake of fexofenadine enantiomers by Caco-2 cells as a model of intestinal epithelial cells.

METHODS

R(+)-fexofenadine or S(-)-fexofenadine was applied to Caco-2 cells, followed by incubation. After incubation, the amounts of fexofenadine enantiomers in cells were determined. The kinetic parameters for the uptake of fexofenadine enantiomers by Caco-2 cells were estimated using the Michaelis-Menten equation.

KEY FINDINGS

The transporter-mediated uptake rate of R(+)-fexofenadine was 1.7-fold higher than that of S(-)-fexofenadine. The difference in transporter-mediated R(+)-fexofenadine and S(-)-fexofenadine uptake was completely diminished under ATP-depleted conditions and in the presence of organic anion transporter peptide (OATP) inhibitors. Also, a Dixon plot showed that each fexofenadine enantiomer was competitively inhibited by the other enantiomer. The ratio of R(+)-fexofenadine uptake to S(-)-fexofenadine uptake in the case of a racemic mixture was higher than that in the case of a single enantiomer.

CONCLUSION

This study suggested that the selective absorption of fexofenadine enantiomers by intestinal epithelial cells might have been due to the selective uptake mediated by OATPs and that the difference in intestinal absorption was enhanced with a racemic mixture.

Optimization of perfusate pH to improve microdialysis recovery of lipophilic compounds

INTRODUCTION

Microdialysis (MD) allows sampling of compounds in-vivo from tissues' interstitial fluid. However, molecules insoluble at physiological pH have usually extremely low recovery. The addition of albumin to the perfusate or the use of isotonic lipoemulsion improves recovery of these molecules although it requires a cleaning step before HPLC analysis. This study investigates the possibility of improving the MD recovery of compounds insoluble at physiological pH but soluble at a different pH. The probe is perfused with an isotonic solution adjusted to pH values at which the compound has maximum solubility. Ketoconazole (KTC), clotrimazole (CLT) and tretinoin (TTN) were selected as model drugs because they are almost insoluble at pH 7.4 but soluble at pH 4 for KTC and CTL; and at pH 9 for TTN.

METHODS

Linear microdialysis probes were used to collect KTC, CLT or TTN from a standard solution of the compounds. Probes were perfused with 0.01 M pH 7.4 isotonic buffer solution (1) without or (2) with 5% Bovine Serum Albumin (BSA); or (3) with 20% isotonic lipoemulsion; or (4) with 0.01 M pH 4 isotonic buffer solution for KTC and CLT or 0.01 M pH 9 isotonic buffer solution for TTN. The method was then tested in-vivo, in rabbit skin, to assess the skin tolerance to the non-physiological perfusates and to monitor KTC and TTN delivery from commercial cream products.

RESULTS

In-vitro, the optimized-pH perfusate increased MD recovery significantly (P<0.001): 6.9 (KTC), 8.3 (CLT), and 2.0 (TTN) times compared to the physiological pH and 1.4 and 1.2 compared to the BSA and lipoemulsion respectively. No evidence of irritation or edema was observed in-vivo. However, KTC and TTN were not detected in-vivo with any of the modified perfusate tested.

DISCUSSION

These findings show that the optimized-pH perfusate effectively increases the in-vitro microdialysis recovery of KTC, CLT and TTN and that it is well tolerated in-vivo. However, the compounds tested (KTC and TTN) could not be detected in-vivo.

Clinically significant psychotropic drug-drug interactions in the primary care setting

In recent years, the growing numbers of patients seeking care for a wide range of psychiatric illnesses in the primary care setting has resulted in an increase in the number of psychotropic medications prescribed. Along with the increased utilization of psychotropic medications, considerable variability is noted in the prescribing patterns of primary care providers and psychiatrists. Because psychiatric patients also suffer from a number of additional medical comorbidities, the increased utilization of psychotropic medications presents an elevated risk of clinically significant drug interactions in these patients. While life-threatening drug interactions are rare, clinically significant drug interactions impacting drug response or appearance of serious adverse drug reactions have been documented and can impact long-term outcomes. Additionally, the impact of genetic variability on the psychotropic drug's pharmacodynamics and/or pharmacokinetics may further complicate drug therapy. Increased awareness of clinically relevant psychotropic drug interactions can aid clinicians to achieve optimal therapeutic outcomes in patients in the primary care setting.

An investigation of the possible interaction between the use of Vitamin C and highly active antiretroviral therapy (HAART) adherence and effectiveness in treated HIV+ women

OBJECTIVES

Our goal in this study was to examine how Vitamin C interacts with antiretroviral therapy in individuals with HIV. We specifically evaluated how Vitamin C impacts highly active antiretroviral therapy (HAART) adherence and HAART effectiveness as adjudicated by HIV viral loads and CD4 cell counts. Women served as their own controls, comparing periods of Vitamin C usage with periods of non-usage.

DESIGN

An intra-individual, cross-sectional comparative study 'nested' in the WIHS observational cohort study.

SUBJECTS

Women in the Women's Interagency HIV Study (WIHS).

OUTCOME MEASURES

Adherence, CD4 count and viral load.

RESULTS

Our study population was drawn from 2813 HIV+ participants who contributed 44,588 visits in WIHS from October, 1994 to April, 2009. Among them, there were 1122 Vitamin C users with 4954 total visits where use was reported. In the multivariate model adjusting for age, education, race, income, drug use, Vitamin C use order and depression score, there was a 44% increase in the odds of ≥ 95% HAART adherence among participants during their period of Vitamin C use compared to when they were not using Vitamin C (OR=1.44; 95% CI=1.1-1.9; P-value=0.0179). There was an association with Vitamin C usage and CD4 counts on viral loads.

CONCLUSION

Vitamin C usage appears to be associated with improved adherence. Future Vitamin C studies should target specific HAART drugs, and prospective clinical outcomes.

Activation of the human nuclear xenobiotic receptor PXR by the reverse transcriptase-targeted anti-HIV drug PNU-142721

The human pregnane X receptor (PXR) is a member of the nuclear receptor superfamily of ligand-regulated transcription factors. PXR responds to a structurally diverse variety of endogenous and xenobiotic compounds, and coordinates the expression of genes central to the metabolism and excretion of potentially harmful chemicals, including human therapeutics. The reverse transcriptase inhibitor PNU-142721 has been designed to treat human immunodeficiency virus (HIV) infection. Although this compound has anti-HIV activity, it was established using cell-based assays that PNU-142721 is an efficacious PXR agonist. We present here the 2.8 Å resolution crystal structure of the human PXR ligand-binding domain in complex with PNU-142721. PXR employs one hydrogen bond and fourteen van der Waals contacts to interact with the ligand, but allows two loops adjacent to the ligand-binding pocket to remain disordered in the structure. These observations highlight the role structural flexibility plays in PXR's promiscuous responses to xenobiotics. The crystal structure also explains why PNU-173575, a thiomethyl metabolite of PNU-142721, exhibits enhanced PXR activation relative to the unmodified compound and why PNU-142721 can also activate rat PXR. Taken together, the results presented here elucidate the structural basis for PXR activation by PNU-142721 and related chemicals.

Differential effect of P-gp and MRP2 on cellular translocation of gemifloxacin

Fluoroquinolones are broad spectrum antibiotics widely indicated in the treatment of both human and animal diseases. The primary objective of this study was to assess short and long term affinities of gemifloxacin towards efflux transporters (P-gp, MRP2) and nuclear hormone receptor (PXR). Uptake and dose dependent inhibition studies were performed with [(14)C] erythromycin (0.25 μCi/ml) on MDCKII-MDR1 and MDCKII-MRP2 cells. Cellular accumulation of calcein-AM was further determined to confirm the affinity of gemifloxacin towards P-gp and MRP2. Transport studies were conducted to determine bi-directional permeability and to assess efflux ratio of gemifloxacin. LS-180 cells were treated with three different concentrations of gemifloxacin for 72 h and real-time PCR analysis was performed to study the quantitative gene expression levels of PXR, MDR1 and MRP2. Further, [(14)C] erythromycin uptake was also performed on LS-180 treated cells to better delineate the functional activity of efflux transporters. Results from our study suggest that gemifloxacin may be a substrate of both the efflux transporters studied. This compound inhibited both P-gp and MRP2 mediated efflux of [(14)C] erythromycin in a dose dependent manner with IC(50) values of 123 ± 2 μM and 16 ± 2 μM, respectively. The efflux ratio of [(14)C] erythromycin lowered from 3.56 to 1.63 on MDCKII-MDR1 cells and 4.93 to 1.26 on MDCKII-MRP2 cells. This significant reduction in efflux ratio further confirmed the substrate specificity of gemifloxacin towards P-gp and MRP2. Long term exposure significantly induced the expression of PXR (18 fold), MDR1 (6 fold) and MRP2 (6 fold). A decrease (20%) in [(14)C] erythromycin uptake further confirmed the elevated functional activity of P-gp and MRP2. In conclusion, our studies demonstrated that gemifloxacin is effluxed by both P-gp and MRP2. Long term exposure induced their gene expression and functional activity. This substrate specificity of gemifloxacin towards these efflux transporters may be one of the major factors accounting for low oral bioavailability (71%). Better understanding of these mechanistic interactions may aid in the development of newer strategies to achieve adequate therapeutic levels and higher bioavailability.

Influence of drug transport proteins on the pharmacokinetics and drug interactions of HIV protease inhibitors

Protease inhibitors, a class of antiretroviral agents frequently used in the treatment of HIV infection, interact with numerous transport proteins resulting in clinically significant drug-drug interactions (DDIs). This review focuses on the proteins that transport protease inhibitors and directly influence the pharmacokinetics of these drugs, as well as the transport proteins that are inhibited or induced by protease inhibitors. Clinically relevant DDIs involving drug transporters and protease inhibitors, either as "victim" drugs or as "perpetrator" drugs, and the pharmacokinetic consequences of such interactions are highlighted. A summary of transporter-mediated processes underlying the toxicity of protease inhibitors is provided. Finally, the effect of HIV infection or co-infection on drug transport proteins, and the implications for protease inhibitor pharmacokinetics is discussed. Transport proteins significantly influence the pharmacokinetics, efficacy and toxicity profiles of this important class of drugs.

Differential effects of ethanol on spectral binding and inhibition of cytochrome P450 3A4 with eight protease inhibitors antiretroviral drugs

BACKGROUND

Cytochrome P450 3A4 (CYP3A4) is the most abundant CYP enzyme in the liver, which metabolizes approximately 50% of the marketed drugs including antiretroviral agents. CYP3A4 induction by ethanol and its impact on drug metabolism and toxicity is known. However, CYP3A4-ethanol physical interaction and its impact on drug binding, inhibition, or metabolism is not known, except that we have recently shown that ethanol facilitates the binding of a protease inhibitor (PI), nelfinavir, with CYP3A4. The current study was designed to examine the effect of ethanol on spectral binding and inhibition of CYP3A4 with all currently used PIs that differ in physicochemical properties.

METHODS

We performed type I and type II spectral binding with CYP3A4 at 0 and 20 mM ethanol and varying PIs' concentrations. We also performed CYP3A4 inhibition using 7-benzyloxy-4-trifluoromethylcoumarin substrate and NADPH at varying concentrations of PIs and ethanol.

RESULTS

Atazanavir, lopinavir, saquinavir, and tipranavir showed type I spectral binding, whereas indinavir and ritonavir showed type II. However, amprenavir and darunavir did not show spectral binding with CYP3A4. Ethanol at 20 mM decreased the maximum spectral change (δA(max)) with type I lopinavir and saquinavir, but it did not alter δA(max) with other PIs. Ethanol did not alter spectral binding affinity (K(D)) and inhibition constant (IC(50)) of type I PIs. However, ethanol significantly decreased the IC(50) of type II PIs, indinavir and ritonavir, and markedly increased the IC(50) of amprenavir and darunavir.

CONCLUSIONS

Overall, our results suggest that ethanol differentially alters the binding and inhibition of CYP3A4 with the PIs that have different physicochemical properties. This study has clinical relevance because alcohol has been shown to alter the response to antiretroviral drugs, including PIs, in HIV-1-infected individuals.

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

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

The use of 13C-erythromycin as an in vivo probe to evaluate CYP3A-mediated drug interactions in rats

(14)C-erythromycin breath test has been utilized to evaluate the extent of CYP3A activity in vivo. However, its radioactivity sometimes impedes its clinical application. In this study, we employed erythromycin labeled with (13)C ((13)C-EM), a nonradioactive stable isotope, as an in vivo probe of breath test to evaluate CYP3A-mediated drug interactions in rats. A physiologically based pharmacokinetic (PBPK) model to describe (13)CO(2) exhalation altered by drug interactions was newly constructed. Rats received an intravenous or oral administration of (13)C-EM with or without a CYP3A inhibitor or inducer, that is, ketoconazole (KCZ) or dexamethasone (DEX), respectively. Breath samples were taken at designated times, measured with an infrared spectrophotometer, and the Δ(13) CO(2) value (‰) in each sample was obtained. The C(max) and AUC(0-t) of Δ(13) CO(2) were significantly decreased with KCZ and increased with DEX. The PBPK model in this study successfully described the (13)CO(2) exhalation after (13)C-EM administration in the absence and presence of drug interactions. In conclusion, this study proposed a simple and rapid in vivo methodology to utilize (13)C-EM for the quantitative analysis of CYP3A inhibition and induction. This method using small animals may be useful in early drug development processes.

Cellular localization and functional significance of CYP3A4 in the human epileptic brain

PURPOSE

Compelling evidence supports the presence of P450 enzymes (CYPs) in the central nervous system (CNS). However, little information is available on the localization and function of CYPs in the drug-resistant epileptic brain. We have evaluated the pattern of expression of the specific enzyme CYP3A4 and studied its co-localization with MDR1. We also determined whether an association exists between CYP3A4 expression and cell survival.

METHODS

Brain specimens were obtained from eight patients undergoing resection to relieve drug-resistant seizures or to remove a cavernous angioma. Each specimen was partitioned for either immunostaining or primary culture of human endothelial cells and astrocytes. Immunostaining was performed using anti-CYP3A4, MDR1, GFAP, or NeuN antibodies. High performance liquid chromatography-ultraviolet (HPLC-UV) analysis was used to quantify carbamazepine (CBZ) metabolism by these cells. CYP3A4 expression was correlated to DAPI) condensation, a marker of cell viability. Human embryonic kidney (HEK) cells were transfected with 4',6-diamidino-2-phenylindole (CYP3A4 to further evaluate the link between CYP3A4 levels, CBZ metabolism, and cell viability.

KEY FINDINGS

CYP3A4 was expressed by blood-brain barrier (BBB) endothelial cells and by the majority of neurons (75 ± 10%). Fluorescent immunostaining showed coexpression of CYP3A4 and MDR1 in endothelial cells and neurons. CYP3A4 expression inversely correlated with DAPI nuclear condensation. CYP3A4 overexpression in HEK cells conferred resistance to cytotoxic levels of carbamazepine. CYP3A4 levels positively correlated with the amount of CBZ metabolized.

SIGNIFICANCE

CYP3A4 brain expression is not only associated with drug metabolism but may also represent a cytoprotective mechanism. Coexpression of CYP3A4 and MDR1 may be involved in cell survival in the diseased brain.

Stereoselective interactions of warfarin enantiomers with the pregnane X nuclear receptor in gene regulation of major drug-metabolizing cytochrome P450 enzymes

BACKGROUND

Warfarin, an antagonist of vitamin K, is an oral coumarin anticoagulant widely used to control and prevent thromboembolic disorders. Warfarin is clinically available as a racemic mixture of R- and S-warfarin. The S-enantiomer has three to five times greater anticoagulation potency than its optical congener. Recently, vitamin K₂ function has been proposed via the pregnane X receptor (PXR) in osteocytes. PXR acts as a xenobiotic sensor that controls expression of many genes involved in drug/xenobiotic metabolic clearance.

OBJECTIVE

The aim was to examine whether enantiomers of warfarin stereoselectively interact with PXR to up-regulate main drug/xenobiotic-metabolizing enzymes of the cytochrome P450 superfamily.

METHODS

Interactions of warfarin enantiomers with PXR were tested by gene reporter assays and time-resolved fluorescence resonance energy transfer technology (TR-FRET) ligand binding assay. Up-regulation of PXR-target gene mRNAs by warfarin enantiomers was studied using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) in primary human hepatocytes.

RESULTS

We found that R-warfarin interacts with the PXR nuclear receptor. Consistently, R-warfarin significantly induced CYP3A4 and CYP2C9 mRNAs in cultures of primary human hepatocytes or in LS174T intestinal cells. On the other hand, S-warfarin is a less potent inducer of PXR-target genes in human hepatocytes and activates PXR only at supraphysiological concentrations. In addition, we showed that racemic 10- and 4'-hydroxywarfarins are also highly potent PXR ligands and inducers of CYP3A4 and CYP2C9 mRNA in human hepatocytes.

CONCLUSION

We showed that R-warfarin can significantly up-regulate major drug-metabolizing enzymes CYP3A4 and CYP2C9 in the liver and thus may cause drug-drug interactions (DDI) with co-administered drugs. The results warrant reconsideration of racemic warfarin usage in clinics.

Effect of ethanol on spectral binding, inhibition, and activity of CYP3A4 with an antiretroviral drug nelfinavir

Cytochrome P450 3A4 (CYP3A4) is the most abundant CYP enzyme in the liver and metabolizes approximately 50% of the drugs, including antiretrovirals. Although CYP3A4 induction by ethanol and impact of CYP3A4 on drug metabolism and toxicity is known, CYP3A4-ethanol physical interaction and its impact on drug binding, inhibition, or metabolism is not known. Therefore, we studied the effect of ethanol on binding and inhibition of CYP3A4 with a representative protease inhibitor, nelfinavir, followed by the effect of alcohol on nelfinavir metabolism. Our initial results showed that methanol, ethanol, isopropanol, isobutanol, and isoamyl alcohol bind in the active site of CYP3A4 and exhibit type I spectra. Among these alcohol compounds, ethanol showed the lowest K(D) (5.9±0.34mM), suggesting its strong binding affinity with CYP3A4. Ethanol (20mM) decreased the K(D) of nelfinavir by >5-fold (0.041±0.007 vs. 0.227±0.038μM). Similarly, 20mM ethanol decreased the IC(50) of nelfinavir by >3-fold (2.6±0.5 vs. 8.3±3.1μM). These results suggest that ethanol facilitates binding of nelfinavir with CYP3A4. Furthermore, we performed nelfinavir metabolism using LCMS. Although ethanol did not alter k(cat), it decreased the K(m) of nelfinavir, suggesting a decrease in catalytic efficiency (k(cat)/K(m)). This is an important finding because alcoholism is prevalent in HIV-1-infected persons and alcohol is shown to decrease the response to antiretroviral therapy.