Cytochrome P450-mediated metabolism in the human gut wall

Incorporating population variability and susceptible subpopulations into dosimetry for high-throughput toxicity testing

Momentum is growing worldwide to use in vitro high-throughput screening (HTS) to evaluate human health effects of chemicals. However, the integration of dosimetry into HTS assays and incorporation of population variability will be essential before its application in a risk assessment context. Previously, we employed in vitro hepatic metabolic clearance and plasma protein binding data with in vitro in vivo extrapolation (IVIVE) modeling to estimate oral equivalent doses, or daily oral chemical doses required to achieve steady-state blood concentrations (Css) equivalent to media concentrations having a defined effect in an in vitro HTS assay. In this study, hepatic clearance rates of selected ToxCast chemicals were measured in vitro for 13 cytochrome P450 and five uridine 5'-diphospho-glucuronysyltransferase isozymes using recombinantly expressed enzymes. The isozyme-specific clearance rates were then incorporated into an IVIVE model that captures known differences in isozyme expression across several life stages and ethnic populations. Comparison of the median Css for a healthy population against the median or the upper 95th percentile for more sensitive populations revealed differences of 1.3- to 4.3-fold or 3.1- to 13.1-fold, respectively. Such values may be used to derive chemical-specific human toxicokinetic adjustment factors. The IVIVE model was also used to estimate subpopulation-specific oral equivalent doses that were directly compared with subpopulation-specific exposure estimates. This study successfully combines isozyme and physiologic differences to quantitate subpopulation pharmacokinetic variability. Incorporation of these values with dosimetry and in vitro bioactivities provides a viable approach that could be employed within a high-throughput risk assessment framework.

Pharmacokinetic and pharmacodynamic interactions between almorexant, a dual orexin receptor antagonist, and desipramine

Almorexant is a dual orexin receptor antagonist (DORA) with sleep-enabling effects in humans. Insomnia is often associated with mental health problems, including depression. Hence, potential interactions with antidepressants deserve attention. Desipramine was selected as a model drug because it is mainly metabolized by CYP2D6, which is inhibited by almorexant in vitro. A single-center, randomized, placebo-controlled, two-way crossover study in 20 healthy male subjects was conducted to evaluate the pharmacokinetic and pharmacodynamic interactions between almorexant and desipramine. Almorexant 200mg or matching placebo (double-blind) was administered orally once daily in the morning for 10 days, and a single oral dose of 50mg desipramine (open-label) was administered on Day 5. Almorexant increased the exposure to desipramine 3.7-fold, suggesting that almorexant is a moderate inhibitor of desipramine metabolism through inhibition of CYP2D6. Conversely, desipramine showed no relevant effects on the pharmacokinetics of almorexant. Pharmacodynamic evaluations indicated that almorexant alone reduced visuomotor coordination, postural stability, and alertness, and slightly increased calmness. Desipramine induced a reduction in subjective alertness and an increase in pupil/iris ratio. Despite the increase in exposure to desipramine, almorexant and desipramine in combination showed the same pharmacodynamic profile as almorexant alone, except for prolonging reduced alertness and preventing the miotic effect of almorexant. Co-administration also prolonged the mydriatic effect of desipramine. Overall, repeated administration of almorexant alone or with single-dose desipramine was well tolerated. The lack of a relevant interaction with antidepressants, if confirmed for other DORAs, would be a key feature for a safer class of hypnotics.

Perspective and potential of oral lipid-based delivery to optimize pharmacological therapies against cardiovascular diseases

Cardiovascular diseases (CVDs) remain the major cause of morbidity and mortality globally. Despite the large number of cardiovascular drugs available for pharmacological therapies, factors limiting the efficient oral use are identified, including low water solubility, pre-systemic metabolism, food intake effects and short half-life. Numerous in vivo proof-of-concepts studies are presented to highlight the viability of lipid-based delivery to optimize the oral delivery of cardiovascular drugs. In particular, the key performance enhancement roles of oral lipid-based drug delivery systems (LBDDSs) are identified, which include i) improving the oral bioavailability, ii) sustaining/controlling drug release, iii) improving drug stability, iv) reducing food intake effect, v) targeting to injured sites, and vi) potential for combination therapy. Mechanisms involved in achieving these features, range of applicability, and limits of available systems are detailed. Future research and development efforts to address these issues are discussed, which is of significant value in directing future research work in fostering translation of lipid-based formulations into clinical applications to reduce the prevalence of CVDs.

Oral co-administration of elacridar and ritonavir enhances plasma levels of oral paclitaxel and docetaxel without affecting relative brain accumulation

Background:

The intestinal uptake of the taxanes paclitaxel and docetaxel is seriously hampered by drug efflux through P-glycoprotein (P-gp) and drug metabolism via cytochrome P450 (CYP) 3A. The resulting low oral bioavailability can be boosted by co-administration of P-gp or CYP3A4 inhibitors.

Methods:

Paclitaxel or docetaxel (10 mg/kg) was administered to CYP3A4-humanised mice after administration of the P-gp inhibitor elacridar (25 mg kg⁻¹) and the CYP3A inhibitor ritonavir (12.5 mg kg⁻¹). Plasma and brain concentrations of the taxanes were measured.

Results:

Oral co-administration of the taxanes with elacridar increased plasma concentrations of paclitaxel (10.7-fold, P<0.001) and docetaxel (four-fold, P<0.001). Co-administration with ritonavir resulted in 2.5-fold (paclitaxel, P<0.001) and 7.3-fold (docetaxel, P<0.001) increases in plasma concentrations. Co-administration with both inhibitors simultaneously resulted in further increased plasma concentrations of paclitaxel (31.9-fold, P<0.001) and docetaxel (37.4-fold, P<0.001). Although boosting of orally applied taxanes with elacridar and ritonavir potentially increases brain accumulation of taxanes, we found that only brain concentrations, but not brain-to-plasma ratios, were increased after co-administration with both inhibitors.

Conclusions:

The oral availability of taxanes can be enhanced by co-administration with oral elacridar and ritonavir, without increasing the brain penetration of the taxanes.

Gut-on-a-Chip microenvironment induces human intestinal cells to undergo villus differentiation

Existing in vitro models of human intestinal function commonly rely on use of established epithelial cell lines, such as Caco-2 cells, which form polarized epithelial monolayers but fail to mimic more complex intestinal functions that are required for drug development and disease research. We show here that a microfluidic 'Gut-on-a-Chip' technology that exposes cultured cells to physiological peristalsis-like motions and liquid flow can be used to induce human Caco-2 cells to spontaneously undergo robust morphogenesis of three-dimensional (3D) intestinal villi. The cells of that line these villus structures are linked by tight junctions, and covered by brush borders and mucus. They also reconstitute basal proliferative crypts that populate the villi along the crypt-villus axis, and form four different types of differentiated epithelial cells (absorptive, mucus-secretory, enteroendocrine, and Paneth) that take characteristic positions similar to those observed in living human small intestine. Formation of these intestinal villi also results in exposure of increased intestinal surface area that mimics the absorptive efficiency of human intestine, as well enhanced cytochrome P450 3A4 isoform-based drug metabolizing activity compared to conventional Caco-2 cell monolayers cultured in a static Transwell system. The ability of the human Gut-on-a-Chip to recapitulate the 3D structures, differentiated cell types, and multiple physiological functions of normal human intestinal villi may provide a powerful alternative in vitro model for studies on intestinal physiology and digestive diseases, as well as drug development.

In vitro inhibition of human CYP1A2, CYP2D6, and CYP3A4 by six herbs commonly used in pregnancy

Black elderberry, cranberry, fennel, ginger, horsetail, and raspberry leaf, herbs frequently used in pregnancy, were investigated for their in vitro CYP1A2, 2D6, and 3A4 inhibitory potential. Aqueous or ethanolic extracts were made from commercially available herbal products, and incubations were performed with recombinant cDNA-expressed human CYP enzymes in the presence of positive inhibitory controls. Metabolite formation was determined by validated LCMS/MS or HPLC methodologies. IC50 inhibition constants were estimated from CYP activity inhibition plots using non-linear regression. The most potent inhibition was shown for fennel towards CYP2D6 and 3A4 with respective IC50 constants of 23 ± 2 and 40 ± 4 µg/ml, horsetail towards CYP1A2 with an IC50 constant of 27 ± 1 µg/ml, and raspberry leaf towards CYP1A2, 2D6, and 3A4 with IC50 constants of 44 ± 2, 47 ± 8, and 81 ± 11 µg/ml, respectively. Based on the recommended dosing of the different commercial herbal products, clinically relevant systemic CYP inhibitions could be possible for fennel, horsetail, and raspberry leaf. In addition, fennel and raspberry leaf might cause a clinically relevant inhibition of intestinal CYP3A4. The in vivo inhibitory potential of these herbs towards specific CYP enzymes should be further investigated.

Cytochrome P450 2J2: distribution, function, regulation, genetic polymorphisms and clinical significance

Cytochrome P450 2J2 (CYP2J2) is an enzyme mainly found in human extrahepatic tissues, with predominant expression in the cardiovascular systems and lower levels in the intestine, kidney, lung, pancreas, brain, liver, etc. During the past 15 years, CYP2J2 has attracted much attention for its epoxygenase activity in arachidonic acid (AA) metabolism. It converts AA to four epoxyeicosatrienoic acids (EETs) that have various biological effects, especially in the cardiovascular systems. In recent publications, CYP2J2 is shown highly expressed in various human tumor cells, and its EET metabolites are demonstrated to implicate in the pathologic development of human cancers. CYP2J2 is also a human CYP that involved in phase I xenobiotics metabolism. Antihistamine drugs and many other compounds were identified as the substrates of CYP2J2, and studies have demonstrated that these substrates have a broad structural diversity. CYP2J2 is found not readily induced by known P450 inducers; however, its expression could be regulated in some pathological conditions, might through the activator protein-1(AP-1), the AP-1-like element and microRNA let-7b. Several genetic mutations in the CYP2J2 gene have been identified in humans, and some of them have been shown to have potential associations with some diseases. With the increasing awareness of its roles in cancer disease and drug metabolism, studies about CYP2J2 are still going on, and various inhibitors of CYP2J2 have been determined. Further studies are needed to delineate the roles of CYP2J2 in disease pathology, drug development and clinical practice.

The gut as a sensory organ

The gastrointestinal tract presents the largest and most vulnerable surface to the outside world. Simultaneously, it must be accessible and permeable to nutrients and must defend against pathogens and potentially injurious chemicals. Integrated responses to these challenges require the gut to sense its environment, which it does through a range of detection systems for specific chemical entities, pathogenic organisms and their products (including toxins), as well as physicochemical properties of its contents. Sensory information is then communicated to four major effector systems: the enteroendocrine hormonal signalling system; the innervation of the gut, both intrinsic and extrinsic; the gut immune system; and the local tissue defence system. Extensive endocrine-neuro-immune-organ-defence interactions are demonstrable, but under-investigated. A major challenge is to develop a comprehensive understanding of the integrated responses of the gut to the sensory information it receives. A major therapeutic opportunity exists to develop agents that target the receptors facing the gut lumen.

Utility of in vitro systems and preclinical data for the prediction of human intestinal first-pass metabolism during drug discovery and preclinical development

A growing awareness of the risks associated with extensive intestinal metabolism has triggered an interest in developing robust methods for its quantitative assessment. This study explored the utility of intestinal S9 fractions, human liver microsomes, and recombinant cytochromes P450 to quantify CYP3A-mediated intestinal extraction in humans for a selection of marketed drugs that are predominantly metabolized by CYP3A4. A simple competing rates model is used to estimate the fraction of drug escaping gut wall metabolism (fg) from in vitro intrinsic clearance in humans. The fg values extrapolated from the three in vitro systems used in this study, together with literature-derived fg from human intestinal microsomes, were validated against fg extracted from human in vivo pharmacokinetic (PK) profiles using a generic whole-body physiologically-based pharmacokinetic (PBPK) model. The utility of the rat as a model for human CYP3A-mediated intestinal metabolism was also evaluated. Human fg from PBPK compares well with that from the grapefruit juice method, justifying its use for the evaluation of human in vitro systems. Predictive performance of all human in vitro systems was comparable [root mean square error (RMSE) = 0.22-0.27; n = 10]. Rat fg derived from in vivo PK profiles using PBPK has the lowest RMSE (0.19; n = 11) for the prediction of human fg for the selected compounds, most of which have a fraction absorbed close to 1. On the basis of these evaluations, the combined use of fg from human in vitro systems and rats is recommended for the estimation of CYP3A4-mediated intestinal metabolism in lead optimization and preclinical development phases.

CYP2R1 is a major, but not exclusive, contributor to 25-hydroxyvitamin D production in vivo

Bioactivation of vitamin D consists of two sequential hydroxylation steps to produce 1α,25-dihydroxyvitamin D3. It is clear that the second or 1α-hydroxylation step is carried out by a single enzyme, 25-hydroxyvitamin D 1α-hydroxylase CYP27B1. However, it is not certain what enzyme or enzymes are responsible for the initial 25-hydroxylation. An excellent case has been made for vitamin D 25-hydroxylase CYP2R1, but this hypothesis has not yet been tested. We have now produced Cyp2r1 (-/-) mice. These mice had greater than 50% reduction in serum 25-hydroxyvitamin D3. Curiously, the 1α,25-dihydroxyvitamin D3 level in the serum remained unchanged. These mice presented no health issues. A double knockout of Cyp2r1 and Cyp27a1 maintained a similar circulating level of 25-hydroxyvitamin D3 and 1α,25-dihydroxyvitamin D3. Our results support the idea that the CYP2R1 is the major enzyme responsible for 25-hydroxylation of vitamin D, but clearly a second, as-yet unknown, enzyme is another contributor to this important step in vitamin D activation.

Physiologically based pharmacokinetic models for the optimization of antiretroviral therapy: recent progress and future perspective

Anti-HIV therapy is characterized by the chronic administration of antiretrovirals (ARVs), and consequently, several problems can arise during the management of HIV-positive patients. ARV disposition can be simulated by combining system data describing a population of patients and in vitro drug data through physiologically based pharmacokinetic (PBPK) models, which mathematically describe absorption, distribution, metabolism and elimination. PBPK modeling can find application in the investigation of clinically relevant scenarios, while providing the opportunity for a better understanding of the mechanisms regulating drug distribution. In this review, we have analyzed the most recent applications of PBPK models for ARVs and highlighted some of the most interesting areas of use, such as drug–drug interaction, pharmacogenetics, factors regulating absorption and tissue penetration, as well as therapy optimization in special populations. The application of the PBPK modeling approach might not be limited to the investigation of hypothetical clinical issues, but could be used to inform future prospective clinical trials.

An in vitro metabolic system of gut flora and the metabolism of ginsenoside Rg3 and cholic acid

For orally administered drugs, the metabolism of a drug by the gut flora plays an important role in the bioavailability, activation and disposition of the drug in vivo. However, no in vitro system is currently available to evaluate the metabolism of a drug by the gut flora before the drug is absorbed into the body. This paper presents an in vitro metabolic system in an anaerobic environment that could be used to evaluate the metabolism of an endogenous compound, cholic acid, and a xenobiotic compound, ginsenoside Rg3. We showed that the proliferation of the anaerobic bacteria of the gut content of hamsters produced a similar composition of gut flora in a culture medium for yeast to that in vivo. Incubation of ginsenoside Rg3 and cholic acid in the anaerobic in vitro system efficiently produced the metabolites Rh2 and deoxycholic acid, respectively, similar to those seen in the gut content in vivo. In comparison with in vivo analysis, this anaerobic in vitro metabolic system is convenient, reproducible, economic and animal saving, and can easily be applied to assess the transformation and disposition of a drug before it enters into the circulatory system.

Considering CYP1A2 phenotype and genotype for optimizing the dose of olanzapine in the management of schizophrenia

INTRODUCTION

Schizophrenia, a mental disorder, is a debilitating condition which typically strikes young people in their early 20's. Antipsychotic medications are widely prescribed for the treatment of schizophrenia however a balancing act is necessary to provide the correct dose to each patient. It is suggested that a large number of patients discontinue antipsychotic pharmacotherapy because the treatments provided do not always reduce the positive symptoms of the disease, while many have adverse effects on the patients. This implies that neither the incorrect drug nor the optimal dosage for that patient is achieved.

AREAS COVERED

The current review investigates variability in response to olanzapine with a specific focus on the common intrinsic and extrinsic factors that influence both olanzapine and CYP1A2 activity. Furthermore, the authors discuss the utilization of phenotyping and genotyping of CYP1A2 and their potential utility in clinical practice for olanzapine dosing regimens. The authors also consider the potential of pharmacometrics compared to pharmacogenomics as a tool to personalize medicine.

EXPERT OPINION

Careful consideration must be given to the impact of a genetic variant on the disposition of a drug prior to implementing genetic 'tests' to determine response. CYP1A2 phenotypic assessment can yield important information regarding the disposition of olanzapine; however, it relies on the accuracy of the metric and the minimal impact of other metabolic pathways. The application of pharmacometrics provides an effective method to establish covariates that significantly influence olanzapine disposition which can incorporate phenotype and/or genotype.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Presystemic metabolism of AZ'0908, a novel mPGES-1 inhibitor: an in vitro and in vivo cross-species comparison

AZ'0908 is a novel microsomal prostaglandin E synthase-1 inhibitor intended for oral administration. Pharmacokinetic experiments in rats showed that bioavailability was much lower than anticipated and increased following pretreatment with the nonspecific cytochrome P450 (CYP) inhibitor 1-aminobenzotriazole, presumably by inhibition of intestinal metabolism. Stability experiments in rat liver and intestinal fractions revealed that the intrinsic clearance (Cl(int)) was much higher in intestinal than in liver microsomes. Caco2 experiments showed that AZ'0908 was a substrate for breast cancer resistance protein. Permeability was generally high and the efflux component was saturable predicting good absorption. The Cl(int) values in human intestinal microsome and S9 fractions were low. A correlation occurred between in vitro intestinal metabolism and in vivo intestinal loss in rats and dogs. Enzyme identification experiments showed that human CYP2J2 was involved in the oxidation of AZ'0908. In rats, the major metabolic enzyme was not identified. However, rat CYP2J2 analogs were not investigated. Intestinal metabolism appeared to be a major occurrence, explaining intestinal loss of AZ'0908 in the rats. In view of good overall permeability, low in vitro intestinal turnover, and relative low intestinal abundance of CYP2J2, we predict that intestinal metabolism of AZ'0908 in human does not exert a major issue.

Carbon monoxide-bound red blood cells protect red blood cell transfusion-induced hepatic cytochrome P450 impairment in hemorrhagic-shock rats

Red blood cell (RBC) transfusions for massive hemorrhage induce systemic ischemic-reperfusion and influence the disposition and pharmacological activity of drugs as a result of a reduction in the level of expression and activity of cytochrome P450s (P450). It was reported that, when organ-preserving solutions are exposed to carbon monoxide (CO), the treatment was effective in suppressing the postreperfusion reduction in renal P450 levels in cases of kidney transplantation. Therefore, we hypothesized that transfusions with RBC that contain bound CO (CO-RBC) would protect the hepatic level of rat P450 during a massive hemorrhage, compared with plasma expanders and RBC resuscitation. To achieve this, we created 40% hemorrhagic-shock model rats, followed by resuscitation, with use of recombinant human serum albumin, RBCs, and CO-RBCs. At 1 hour after resuscitation, the expressions of hepatic P450 isoforms (1A2, 2C11, 2E1, and 3A2) were significantly decreased in the RBC resuscitation group, compared with the sham group. Such alterations in hepatic P450 significantly resulted in an increase in the plasma concentrations of substrate drugs (caffeine [1A2], tolbutamide [2C11], chlorzoxazone [2E1], and midazolam [3A2]) for each P450 isoform, and thus, the hypnotic action of midazolam could be significantly prolonged. Of interest, the reductions in hepatic P450 activity observed in the RBC group were significantly suppressed by CO-RBC resuscitation, and consequently, the pharmacokinetics of substrate drugs and the pharmacological action of midazolam remained at levels similar to those under sham conditions. These results indicate that CO-RBC resuscitation has considerable potential in terms of achieving safe and useful drug therapy during massive hemorrhages.

High-fat diet alters gene expression in the liver and colon: links to increased development of aberrant crypt foci

BACKGROUND

Obesity is associated with an increased risk of colon cancer. High-fat diets that lead to obesity may be a contributing factor, but the mechanisms are unknown.

AIMS

This study examines susceptibility to azoxymethane (AOM)-induced precancerous lesions in mice in response to consumption of either a low or a high-fat diet and associated molecular changes in the liver and colon.

METHODS

Gene markers of xenobiotic metabolism, leptin-regulated inflammatory cytokines and proliferation were assessed in liver and colon in response to high-fat feeding to determine links with increased sensitivity to AOM.

RESULTS

High-fat feeding increased development of AOM-induced precancerous lesions and was associated with increased CYP2E1 gene expression in the liver, but not the colon. Leptin receptors and the colon stem cell marker (Lgr5) were down-regulated in the proximal colon, with a corresponding up-regulation of the inflammatory cytokine (IL6) in response to high-fat feeding. Notably in the distal colon, where aberrant crypt foci develop in response to AOM, the proliferative stem cell marker, Lgr5, was significantly up-regulated with high-fat feeding.

CONCLUSIONS

The current study provides evidence that high-fat diets can alter regulation of molecular markers of xenobiotic metabolism that may expose the colon to carcinogens, in parallel with activation of β-catenin-regulated targets regulating colon epithelial cells. High-fat diets associated with obesity may alter multiple molecular factors that act synergistically to increase the risk of colon cancer associated with obesity.

Constitutive expression and phenobarbital modulation of drug metabolizing enzymes and related nuclear receptors in cattle liver and extra-hepatic tissues

In humans and rodents, phenobarbital (PB) induces hepatic and extra-hepatic drug metabolizing enzymes (DMEs) through the activation of specific nuclear receptors (NRs). In contrast, few data about PB transcriptional effects in veterinary species are available. The constitutive expression and modulation of PB-responsive NR and DME genes, following an oral PB challenge, were investigated in cattle liver and extra-hepatic tissues (duodenum, kidney, lung, testis, adrenal and muscle). Likewise to humans and rodents, target genes were expressed to a lower extent compared to the liver with few exceptions. Phenobarbital significantly affected hepatic CYP2B22, 2C31, 2C87, 3A and UDP-glucuronosyltransferase 1A1-like, glutathione S-transferase A1-like and sulfotransferase 1A1-like (SULT1A1-like) mRNAs and apoprotein amounts; in extra-hepatic tissues, only duodenum showed a significant down-regulation of SULT1A1-like gene and apoprotein. Nuclear receptor mRNAs were never affected by PB. Presented data are the first evidence about the constitutive expression of foremost DME and NR genes in cattle extra-hepatic tissues, and the data obtained following a PB challenge are suggestive of species-differences in drug metabolism; altogether, these information are of value for the extrapolation of pharmacotoxicological data among species, the characterization of drug-drug interactions as well as the animal and consumer's risk caused by harmful residues formation.

Non-coplanar polychlorinated biphenyls (PCBs) are direct agonists for the human pregnane-X receptor and constitutive androstane receptor, and activate target gene expression in a tissue-specific manner

The polychlorinated biphenyl group possesses high environmental persistence, leading to bioaccumulation and a number of adverse effects in mammals. Whilst coplanar PCBs elicit their toxic effects through agonism of the aryl hydrocarbon receptor; however, non-coplanar PCBs are not ligands for AhR, but may be ligands for members of the nuclear receptor family of proteins. To better understand the biological actions of non-coplanar PCBs, we have undertaken a systematic analysis of their ability to activate PXR and CAR-mediated effects. Cells were exposed to a range of non-coplanar PCBs (99, 138, 153, 180 and 194), or the coplanar PCB77: Direct activation of PXR and CAR was measured using a mammalian receptor activation assay in human liver cells, with rifampicin and CITCO used as positive controls ligands for PXR and CAR, respectively; activation of target gene expression was examined using reporter gene plasmids for CYP3A4 and MDR1 transfected into liver, intestine and lung cell lines. Several of the non-coplanar PCBs directly activated PXR and CAR, whilst the coplanar PCB77 did not. Non-coplanar PCBs were also able to activate PXR/CAR target gene expression in a substitution- and tissue-specific manner. Non-coplanar PCBs act as direct activators for the nuclear receptors PXR and CAR, and are able to elicit transcriptional activation of target genes in a substitution- and tissue-dependent manner. Chronic activation of PXR/CAR is linked to adverse effects and must be included in any risk assessment of PCBs.

Subepithelial trypsin induces enteric nerve-mediated anion secretion by activating proteinase-activated receptor 1 in the mouse cecum

Serine proteases are versatile signaling molecules and often exert this function by activating the proteinase-activated receptors (PAR(1)-PAR(4)). Our previous study on the mouse cecum has shown that the PAR(1)-activating peptide (AP) and PAR(2)-AP both induced electrogenic anion secretion. This secretion mediated by PAR(1) probably occurred by activating the receptor on the submucosal secretomotor neurons, while PAR(2)-mediated anion secretion probably occurred by activating the receptor on the epithelial cells. This present study was aimed at using trypsin to further elucidate the roles of serine proteases and PARs in regulating intestinal anion secretion. A mucosal-submucosal sheet of the mouse cecum was mounted in Ussing chambers, and the short-circuit current (I(sc)) was measured. Trypsin added to the serosal side increased I(sc) with an ED(50) value of approximately 100 nM. This I(sc) increase was suppressed by removing Cl(-) from the bathing solution. The I(sc) increase induced by 100 nM trypsin was substantially suppressed by tetrodotoxin, and partially inhibited by an NK(1) receptor antagonist, by a muscarinic Ach-receptor antagonist, and by 5-hydroxytryptamine-3 (5-HT(3)) and 5-HT(4) receptor antagonists. The I(sc) increase induced by trypsin was partially suppressed when the tissue had been pretreated with PAR(1)-AP, but not by a pretreatment with PAR(2)-AP. These results suggest that the serine protease, trypsin, induced anion secretion by activating the enteric secretomotor nerves. This response was initiated in part by activating PAR(1) on the enteric nerves. Serine proteases and PARs are likely to be responsible for the diarrhea occurring under intestinal inflammatory conditions.

Mass spectrometry-based quantification of CYP enzymes to establish in vitro/in vivo scaling factors for intestinal and hepatic metabolism in beagle dog

PURPOSE

Physiologically based models, when verified in pre-clinical species, optimally predict human pharmacokinetics. However, modeling of intestinal metabolism has been a gap. To establish in vitro/in vivo scaling factors for metabolism, the expression and activity of CYP enzymes were characterized in the intestine and liver of beagle dog.

METHODS

Microsomal protein abundance in dog tissues was determined using testosterone-6β-hydroxylation and 7-hydroxycoumarin-glucuronidation as markers for microsomal protein recovery. Expressions of 7 CYP enzymes were estimated based on quantification of proteotypic tryptic peptides using multiple reaction monitoring mass spectrometry. CYP3A12 and CYP2B11 activity was evaluated using selective marker reactions.

RESULTS

The geometric mean of total microsomal protein was 51 mg/g in liver and 13 mg/cm in intestine, without significant differences between intestinal segments. CYP3A12, followed by CYP2B11, were the most abundant CYP enzymes in intestine. Abundance and activity were higher in liver than intestine and declined from small intestine to colon.

CONCLUSIONS

CYP expression in dog liver and intestine was characterized, providing a basis for in vitro/in vivo scaling of intestinal and hepatic metabolism.

Polycyclic aromatic hydrocarbons activate CYP3A4 gene transcription through human pregnane X receptor

Aryl hydrocarbon receptor (AhR) activators have been shown to induce members of the cytochrome P450 (P450) 1 family. Here we demonstrate that the AhR activators induce CYP3A4 through human pregnane X receptor (PXR). AhR activators, polycyclic aromatic hydrocarbons (PAHs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increased CYP3A4 reporter activity and CYP3A4 mRNA expression in HepG2 cells. The CYP3A4 reporter activity was also increased by treatment with cigarette tar. The increased CYP3A4 reporter activity was clearly knocked down by the introduction of human PXR-small interfering RNA, but not by that of human AhR-small interfering RNA. The CYP3A4 reporter activity enhanced by overexpression of human PXR was further increased by treatment with PAHs and TCDD as well as by treatment with rifampicin. These results suggest that PAHs contained in cigarette smoke induce CYP3A4 in human liver.

The pharmacogenetics of antimalaria artemisinin combination therapy

INTRODUCTION

Plasmodium falciparum malaria is one of the world's most lethal infectious diseases, commanding millions of drug administrations per year. The pharmacogenetics of these drugs is poorly known, although its application can be pivotal for the optimized management of this disease.

AREAS COVERED

The main components of artemisinin combination therapy (ACT), the worldwide main antimalarial strategy, are metabolized by the polymorphic CYP3A4 (mefloquine, artemether, lumefantrine), CYP2C8 (amodiaquine), CYP2A6 (artesunate) and CYP1A1/2 (amodiaquine/desethylamodiaquine), with dihydroartemisinin being acted by Phase II UDP-glucuronosyltransferases. The worldwide adoption of ACT is leading to a large number of antimalarial treatments. Simultaneously, the feared development of parasite drug resistance might drive dosing increases. In these scenarios of increased drug exposure, pharmacogenetics can be a key tool supporting evidence-based medicine aiming for the longest possible useful lifespan of this important chemotherapy.

EXPERT OPINION

Translation in this moment is not operationally possible at an individual level, but large population studies are achievable for: i) the development of robust pharmacogenetics markers; and ii) the parallel development of a pharmacogenetic cartography of malaria settings. Advances in the understanding of antimalarial pharmacogenetics are urgent in order to protect the exposed populations, enhance the effectiveness of ACT and, consequently, contributing for the long aimed elimination of the disease.

Quantitative prediction of cytochrome P450 (CYP) 2D6-mediated drug interactions

BACKGROUND AND OBJECTIVES

An approach was recently proposed for quantitative predictions of cytochrome P450 (CYP) 3A4-mediated drug-drug interactions. This approach relies solely on in vivo data. It is based on two characteristic parameters: the contribution ratio (CR; i.e. the fraction of victim drug clearance due to metabolism by a specific CYP) and the inhibition ratio (IR) of the inhibitor. Knowledge of these parameters allows forecasting of the ratio between the area under the plasma concentration-time curve (AUC) of the victim drug when the inhibitor is co-administered and the AUC of the victim drug administered alone. The goals of our study were to extend this method to CYP2D6-mediated interactions, to validate it, and to forecast the magnitude of a large number of interactions that have not been studied so far.

METHODS

A three-step approach was pursued. First, initial estimates of CRs and IRs were obtained by several methods, using data from the literature. Second, an external validation of these initial estimates was carried out, by comparing the predicted AUC ratios with the observed values. Third, refined estimates of CRs and IRs were obtained by orthogonal regression in a Bayesian framework.

RESULTS

Thirty-nine AUC ratios were available for external validation. The mean prediction error of the ratios was 0.31, while the mean prediction absolute error was 1.14. Seventy AUC ratios were available for the global analysis. Final estimates of CRs and IRs were obtained for 39 substrates and 11 inhibitors, respectively. The mean prediction error of the AUC ratios was 0.04, while the mean prediction absolute error was 0.51.

CONCLUSIONS

Predictive distributions for 615 possible interactions were obtained, giving detailed information on some drugs or inhibitors that have been poorly studied so far, such as metoclopramide, bupropion and terbinafine.

mRNA expression profiles of P450 3A enzymes in the liver and small intestine of the domestic pig

The cytochrome P450 (P450) 3A family is considered to be the most important and abundantly expressed P450 subfamily in mammals. The mRNA expression levels of four P450 3A enzymes in porcine liver and small intestine were investigated using real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The expression of P450 3A mRNAs (P450 3A pool) was higher in the liver than that in the small intestine. In the small intestine, the P450 3A mRNAs were gradually decreased from the duodenum to the ileum. P450 3A29 and P450 3A22 were predominantly expressed both in liver and small intestine tissues with larger ratios in the P450 3A pool than the other P450 3A enzymes. These results demonstrate that P450 3A29 and P450 3A22 probably serve as the major P450 3A contributors for both the hepatic and intestinal P450 3A pool. This work provides a deeper comprehension of the contribution of P450 3A enzymes to xenobiotic metabolism in pigs.

Small molecules as friends and foes of the immune system

Every organism is in contact with numerous small molecules (<1000 Da). Chemicals may cause or trigger adverse health effects, including diseases of the immune system. They may also be exploited as drugs. In this review, we look at the interaction between small molecules and the immune system. We discuss the hapten and pharmacological interaction concepts of chemical interaction to trigger T cells and how chemicals can participate in cellular signaling pathways. As a sensor of small molecules, the arylhydrocarbon receptor controls expression of many xenobiotic metabolizing enzymes, including those in the immunological barrier organs; the skin and gut. The relevance of the arylhydrocarbon receptor in the dynamic interaction of the immune system with the chemical environment is therefore discussed.

Intestinal enzymatic metabolism of drugs

Objectives

The intestinal stability of perorally administered drugs has so far been determined using simulated intestinal fluid containing porcine pancreatin (SIF/P), as human gastrointestinal fluids are in most cases not available. In this study the metabolism of six low molecular mass drugs in SIF/P was compared with that in freshly collected porcine intestinal juice and on excised porcine intestinal mucosa.

Methods

The drugs used were oseltamivir, atazanavir, diloxanide, diltiazem, cephalothin and cefoxitin. Metabolism studies were carried out by incubating each drug in the in-vitro models and by analysing the percentage of unmodified remaining drug at fixed time points.

Key findings

Three drugs showed higher degradation on porcine mucosa compared with that in SIF/P and for five compounds a significantly higher metabolism in collected porcine intestinal juice versus SIF/P was observed. Metabolism of diloxanide furoate in collected intestinal juice, for example, was 40-fold higher compared with SIF/P. Moreover, the involvement of different metabolic pathways in porcine mucosa and intestinal juice was observed for cephalothin, being metabolized to desacetylcephalothin and thienyl-acetylglycine, whereas these metabolites were not found in SIF/P. In addition, diltiazem solution (0.25% m/v) was found to be significantly degraded in intestinal juice whereas its metabolism in SIF/P was negligible.

Conclusions

These findings demonstrated that the use of SIF/P for evaluation of presystemic drug metabolism could be highly misleading. Incubation of drugs in freshly collected porcine intestinal juice will likely lead to the improvement of the mimicry of body conditions to evaluate presystemic drug metabolism.

The effect of hepatocyte growth factor on gene transcription during intestinal adaptation

PURPOSE

Previously, we investigated the physiologic effects of hepatocyte growth factor (HGF) on intestinal adaptation using a massive small bowel resection (MSBR) rat model. To correlate these altered physiologic changes with gene alterations, we used microarray technology at 7, 14, and 21 days after MSBR.

METHODS

Forty-five adult female rats were divided into 3 groups and underwent 70% MSBR, MSBR + HGF (intravenous 150 μg/kg per day), or sham operation (control). Five animals per group were killed at each time point. Ileal mucosa was harvested and RNA extracted. Rat Gene Chips and Expression Console software (Affymetrix, Santa Clara, CA) were used. Statistical analysis was done by analysis of variance using Partek Genomics Suite (Partek, Inc, St Louis, MO). Results were significant if fold change was more than 2 or less than -2, with P < .05.

RESULTS

Compared with the control group, MSBR group had significant increases in up-regulated and down-regulated genes. The MSBR-HGF group had further increases in up-regulated and down-regulated genes compared with the MSBR group. At 7 days, 6 cellular hypertrophy families had 30 genes up-regulated, and HGF up-regulated an additional 14 genes. At 21 days, 5 hyperplasia gene families had 32 up-regulated genes. Hepatocyte growth factor up-regulated an additional 16 genes.

CONCLUSIONS

Microarray analysis of intestinal adaptation identified an early emphasis on hypertrophy and later emphasis on hyperplasia. This is the first demonstration that the effect of HGF on intestinal adaptation is recruitment of more genes rather than an increase in the fold change of already up-regulated genes.

Ambient tonicity and intestinal cytochrome CYP3A

IMPORTANCE OF THE FIELD

Recently-discovered tonicity-dependence of human CYP3A expression in vitro may be a novel mechanism of CYP3A regulation in the intestinal epithelia, which exists in a dynamic osmotic environment influenced by food intake.

AREAS COVERED IN THIS REVIEW

A combination of focused and comprehensive literature searches to identify any relevant reports using Medline (from 1950 to 7 November 2009) through the OVID system.

WHAT THE READER WILL GAIN

An update on current knowledge on osmotic environment in the gastrointestinal (GI) tract and its impact on intestinal CYP3A expression and function with special emphasis on the tonicity-sensitive transcription factor nuclear factor of activated T cells 5 (NFAT5).

TAKE HOME MESSAGE

In vitro hypertonicity of ambient osmotic environment in cultured human cells increases expression of CYP3A through transcriptional enhancement by osmosensitive NFAT5. Although post-prandial osmolality in the GI lumen in vivo is substantially increased, NFAT5 activation has not been reported. Similarly, high-salt diet increases intestinal CYP3A function in humans, but it is not known whether these changes are mediated directly by NFAT5.

Validation of a differential in situ perfusion method with mesenteric blood sampling in rats for intestinal drug interaction profiling

The present study explored the feasibility of a differential setup for the in situ perfusion technique with mesenteric cannulation in rats to assess drug interactions at the level of intestinal absorption. In contrast to the classic, parallel in situ perfusion setup, the differential approach aims to identify intestinal drug interactions in individual animals by exposing the perfused segment to a sequence of multiple conditions. First, the setup was validated by assessing the interaction between the P-glycoprotein (P-gp) inhibitor verapamil and the transport probes atenolol (paracellular transport), propranolol (transcellular) and talinolol (P-gp mediated efflux). While transport of atenolol and propranolol remained constant for the total perfusion time (2 h), a verapamil-induced increase in talinolol transport was observed within individual rats (between 3.2- and 5.2-fold). In comparison with the parallel setup, the differential in situ perfusion approach enhances the power to detect drug interactions with compounds that exhibit strong subject-dependent permeability. This was demonstrated by identifying an interaction between amprenavir and ketoconazole (P-gp and CYP3A inhibitor) in five out of seven rats (permeability increase between 1.9- and 4.2-fold), despite high inter-individual differences in intrinsic permeability for amprenavir. In combination with an increased throughput (up to 300%) and a reduced animal use (up to 50%), the enhanced power of the differential approach improves the utility of the biorelevant in situ perfusion technique with mesenteric blood sampling to elucidate the intestinal interaction profile of drugs and drug candidates.

Azole antimycotics and drug interactions in the perioperative period

PURPOSE OF REVIEW

A beneficial effect of antifungal prophylaxis on the prevention of invasive fungal infections has increased the use of azole antimycotics in intensive care and during the perioperative period. At the same time more severe illnesses are treated and multiple drug therapies are employed. Thus, the potential for severe drug-drug interactions has increased. Previous studies have shown that azole antimycotics increase the risk of many clinically significant drug interactions with potentially hazardous consequences.

RECENT FINDINGS

A recent pharmacoepidemiological study has found a more than five-fold incidence ratio in the adjusted rate of sudden death from cardiac diseases among those patients who were given simultaneously inhibitors of cytochrome P450 (CYP) enzymes and their substrates. Although new triazole antifungals are well tolerated, they still cause significant inhibition of CYP enzymes.

SUMMARY

This review focuses on azole antimycotics and anesthetic drugs being used during the perioperative period and discusses the possible clinically significant drug-drug interactions. Azole antimycotics are amongst the strongest inhibitors of CYP-mediated drug metabolism. Anesthesiologists must be aware of the interaction potential of azole antimycotics to be able to adjust their perioperative strategies according to the patient's condition and concomitant medication.

Theoretical considerations on quantitative prediction of drug-drug interactions

The prediction of drug-drug interactions (DDIs) associated with change in clearance for metabolism is reviewed, particularly focusing on pharmacokinetic theories for prediction based on in vitro and in vivo observation. First, there is discussion about how quantitative determination of the contribution of major clearance pathways is fundamental for the accurate prediction of DDIs. Secondly, the concentrations of causative drugs at sites of interactions are discussed. Although DDIs have been predicted from in vitro pharmacokinetic parameters based on predicted hepatic unbound concentrations of inhibitors and inducers, there are noticeable discrepancies between predicted and observed magnitudes of these DDIs. To solve these issues, a method for the prediction of unbound hepatic concentration is proposed based on theoretical considerations. Finally, a pharmacokinetic model to describe the intestinal first pass metabolism is considered, particularly focusing on the importance of the Q(gut) model. Although this Q(gut) model was proposed as an empirical model, theoretical considerations suggest that the model is regarded as a physiologically-based pharmacokinetic model that can predict significance of intestinal DDIs. Theoretical considerations proposed in the present article may be helpful for future analysis of DDIs.