Showing papers in "Current Drug Metabolism in 2009"

Tyrosine kinase inhibitors - a review on pharmacology, metabolism and side effects.

Abstract: Tyrosine kinase inhibitors (TKI) are effective in the targeted treatment of various malignancies. Imatinib was the first to be introduced into clinical oncology, and it was followed by drugs such as gefitinib, erlotinib, sorafenib, sunitinib, and dasatinib. Although they share the same mechanism of action, namely competitive ATP inhibition at the catalytic binding site of tyrosine kinase, they differ from each other in the spectrum of targeted kinases, their pharmacokinetics as well as substance-specific adverse effects. With variations from drug to drug, tyrosine kinase inhibitors cause skin toxicity, including folliculitis, in more than 50% of patients. Among the tyrosine kinase inhibitors that are commercially available as yet, the agents that target EGFR, erlotinib and gefitinib, display the broadest spectrum of adverse effects on skin and hair, including folliculitis, paronychia, facial hair growth, facial erythema, and varying forms of frontal alopecia. In contrast, folliculitis is not common during administration of sorafenib and sunitinib, which target VEGFR, PDGFR, FLT3, and others, whereas both agents have been associated with subungual splinter hemorrhages. Periorbital edema is a common adverse effect of imatinib. Besides the haematological side effects of most of TKIs like anemia, thrombopenia and neutropenia, the most common extra-heamatologic adverse effects are edema, nausea, hypothyroidism, vomiting and diarrhea. Regarding possible long term effects, recently cardiac toxicity with congestive heart failure is under debate in patients receiving imatinib and sunitinib therapy; however, this observation was probably relate to patients selection, although, TKIs overall appear to be a very well tolerated drug class.

Zebrafish: A Complete Animal Model for In Vivo Drug Discovery and Development

Abstract: In last few years, the use of zebrafish (Danio rerio) in scientific research is growing very rapidly. Initially, it was a popular as a model of vertebrate development because zebrafish embryos are transparent and also develop rapidly. Presently, the research using zebrafish is expanding into other areas such as pharmacology, clinical research as a diseases model and interestingly in drug discovery. The use of zebrafish in pharmaceutical research and discovery and drug development is mainly screening of lead compounds, target identification, target validation, morpholino oligonucleotide screens, assay development for drug discovery, physiology based drug discovery, quantitative structure-activity relationship (QSAR) and structure – activity relationships (SAR) study and drug toxicity study. In this paper, we have described properly all the areas of dug discovery where zebrafish is used as a tool. We are hopeful that the use of these techniques or methods will make the zebrafish a prominent model in drug discovery and development research in the forthcoming years.

The Impact of Probiotic on Gut Health

Abstract: The gastrointestinal tract (GIT) microbiota plays an important role in host health due to its involvement in nutritional, immunologic and physiological functions. Microbial imbalances have been associated with enhanced risk of specific diseases. This observation has allowed the introduction of microorganisms as probiotics which are microbes with demonstrated health benefits in humans when ingested in foods. The mechanisms of action include the inhibition of pathogen growth by competition for nutritional sources and adhesion sites, secretion of antimicrobial substances, toxin inactivation. Consequently, the primary clinical interest in the application of probiotics has been in the prevention and treatment of gastrointestinal infections and antibiotic-associated diarrhea diseases. The well-characterized immunomodulatory potential of specific probiotic strains, beyond the effect on the composition of the microbiota, has been be used as innovative tools to alleviate intestinal inflammation, normalize gut mucosal dysfunction, and down-regulate hypersensitivity reactions. Clinical efficacy of specific probiotic strains has been demonstrated in, rotaviruss diarrhea, antibiotic associated diarrhea, irritable bowel syndrome and food allergies. Further, recent clinical and nutritional studies have uncovered the function of specific strains in energy metabolism and thereby have opened up new angles on their exploitation. However, as these processes are highly specific, it is important to characterize the properties of specific probiotic strains an in order to select the best strains or strain combinations for the target in question. Advances have prompted increased the interest of researchers and industry and new applications and targets are being discovered.

Therapeutic nanoparticles to combat cancer drug resistance.

Abstract: This review focuses on the application of drug-loaded nanoparticles (NPs), also called therapeutic NPs, to combat cancer chemoresistance. Many cancer patients have encouraging response to first line chemotherapies but end up with cancer progression or cancer recurrence that requires further treatment. Response to subsequent chemotherapies with various agents usually drops significantly due to formidable cancer chemoresistance. A number of mechanisms have been postulated to account for cancer chemoresistance or poor response to chemotherapy. The best studied mechanism of resistance is mediated through the alteration in the drug efflux proteins responsible for the removal of many commonly used anticancer drugs. Therapeutic NPs have emerged as an innovative and promising alternative of the conventional small molecule chemotherapies to combat cancer drug resistance and have shown enhanced therapeutic efficacy and reduced adverse side effects as compared to their small molecule counterparts. Here the possible mechanisms of therapeutic NPs to combat cancer chemoresistance are reviewed, including prolonging drug systemic circulation lifetime, targeted drug delivery, stimuli-responsive drug release, endocytic uptake of drugs and co-delivering chemo-sensitizing agents. We also call attention to the current challenges and needs of developing therapeutic NPs to combat cancer drug resistance.

Potential of Flavonoids as Anti-inflammatory Agents: Modulation of Pro- Inflammatory Gene Expression and Signal Transduction Pathways

Abstract: Flavonoids are a large class of naturally occurring compounds widely present in fruits, vegetables, and beverages derived from plants. Reports have suggested that these compounds might be useful for the prevention of a number of diseases, partly due to their antiinflammatory properties. It has been demonstrated that flavonoids are able to inhibit expression of isoforms of inducible nitric oxide synthase, ciclooxygenase and lipooxygenase, which are responsible for the production of a great amount of nitric oxide, prostanoids and leukotrienes, as well as other mediators of the inflammatory process such as cytokines, chemokines or adhesion molecules. Modulation of the cascade of molecular events leading to the over-expression of those mediators include inhibition of transcription factors such as nuclear factor kappa B, activator protein 1, signal transducers and activators of transcription, CCAAT/enhancer binding protein and others. Effects on the binding capacity of transcription factors may be regulated through the inhibition of protein kinases involved in signal transduction, such as mitogen activated protein kinases. Although the numerous studies published with in vitro approaches allow identifying molecular mechanisms of flavonoid effects, the limited bioavailability of these molecules makes necessary validation in humans. Whatever the case, the data available make clear the potential utility of dietary flavonoids or new flavonoid-based agents for the possible treatment of inflammatory diseases. The present review summarizes recent research data focusing on the modulation of the expression of different inflammatory mediators by flavonoids and the effects on cell signaling pathways responsible for their anti-inflammatory activity.

Bioavailability of flavonoids: a review of their membrane transport and the function of bilitranslocase in animal and plant organisms.

Abstract: Fruits and vegetables are rich in flavonoids, and ample epidemiological data show that diets rich in fruits and vegetables confer protection against cardiovascular, neurodegenerative and inflammatory diseases, and cancer. However, flavonoid bioavailability is reportedly very low in mammals and the molecular mechanisms of their action are still poorly known. This review focuses on membrane transport of flavonoids, a critical determinant of their bioavailability. Cellular influx and efflux transporters are reviewed for their involvement in the absorption of flavonoids from the gastro-intestinal tract and their subsequent tissue distribution. A focus on the mammalian bilirubin transporter bilitranslocase (TCDB 2.A.65.1.1) provides further insight into flavonoid bioavailability and its relationship with plasma bilirubin (an endogenous antioxidant). The general function of bilitranslocase as a flavonoid membrane transporter is further demonstrated by the occurrence of a plant homologue in organs (petals, berries) where flavonoid biosynthesis is most active. Bilitranslocase appears associated with sub-cellular membrane compartments and operates as a flavonoid membrane transporter.

Amphotericin B and its new derivatives - mode of action.

Abstract: Amphotericin B (AmB) is a well known antifungal and antiprotozoal antibiotic used in the clinic for several decades. Clinical applications of AmB, however, are limited by its nephrotoxicity and many other acute side effects which are not acceptable by patients when their life is not threaten. In order to improve the therapeutic index of this drug, lipid formulations have been introduced and many efforts have been made to obtain less toxic AmB derivatives by chemical modifications of the parent drug. This review presents concise knowledge about this fascinating compound and a critical review of the data published within last few years about the mechanism of action of this antibiotic. In particular, in the present work we discuss: i) structure and properties of AmB and its recently synthesized new derivatives; ii) antifungal and antileishmanial activity and toxicity of these compounds; and iii) mode of action of AmB and its derivatives at cellular and molecular levels, with particular attention paid to interactions of AmB and different components of cellular membranes.

Resveratrol: A Natural Polyphenol with Multiple Chemopreventive Properties (Review)

Abstract: Resveratrol, a naturally occurring polyphenol, shows pleiotropic health beneficial effects, including anti-oxidant, anti-inflammatory, anti-aging, cardioprotective and neuroprotective activities. Due to the several protective effects and since this compound is widely distributed in the plant kingdom, resveratrol can be envisaged as a chemo-preventive/curative agent introduced almost daily with the diet. Currently, a number of preclinical findings suggest resveratrol as a promising nature's weapon for cancer prevention and treatment. A remarkable progress in elucidating the molecular mechanisms underlying anti-cancer properties of resveratrol has been achieved in the last years. Concerning the resveratrol mechanism of action as a protective (vs. normal cells and tissues) and toxic (vs. cancer cells) compound, many studies focus on its antioxidant capacity as well as on its ability to trigger and favor the apoptotic cascade in malignant cells. However, a generalized mechanism of action able to explain this dual effect of resveratrol has not yet been clearly established. In addition to these important functions, resveratrol is reported to exhibit several other biological/biochemical protective effects on heart, circulation, brain and age-related diseases which are summarized in this Review.

Non-Digestible Food Ingredients, Colonic Microbiota and the Impact on Gut Health and Immunity: A Role for Metabolomics

Abstract: Increasing health issues related to immune and gut function such as inflammatory disorders, resistance to infections and metabolic syndrome demand modern analytical approaches to accelerate nutritional research aimed at health promotion and disease prevention. Gut microbial-human mutualism endows the host 'superorganism' with a fitness advantage including nutritional, immune and intestinal health aspects. The gut microbiome enlarges our genome and enhances our metabolic potential. Dietary modulation can significantly alter the microbiota community and metabolic activity, and consequently impacts on nutrient bioavailability and host metabolism. Although in an early stage, microbial metabolites generated during colonic fermentation of food stuffs may have beneficial or deleterious effects on intestinal health and immunity, as summarized in this review. However, current evidence is largely based on in vitro and animal studies while substantiation in humans is lacking. The challenge to establish coherent links between the bioconversion of non-digestible food ingredients, their bioavailability and their downstream effects on the host metabolism may be achieved by metabolomics. In this review, metabolomics studies focusing on microbe-host mutualism have demonstrated that metabolomics is capable of detecting and tracking diverse microbial metabolites from different non-digestible food ingredients, of discriminating between phenotypes with different inherent microbiota and of potentially diagnosing infection and gastrointestinal diseases. Integrative approaches such as the combined analysis of the metabolome in different biofluids together with other -omics technologies will cover exogenous and endogenous effects and hence show promise to generate novel hypotheses for innovative functional foods impacting gut health and immunity.

Role of NF-κB in the regulation of cytochrome P450 enzymes

Abstract: Nuclear factor kappa B (NF-kappaB) is an important transcription factor that regulates a wide spectrum of genes including cytochrome P450 (CYP), the most important family of drug metabolizing enzymes. Therefore, in this review, we addressed the potential role of NF-kappaB in CYP regulation. We proposed three mechanisms by which NF-kappaB can regulate CYP expression and activity. First, NF-kappaB can directly regulate the expression of CYP1A1, CYP2B1/2, CYP2C11, CYP2D5, CYP2E1, CYP3A7, and CYP27B1 through binding to the promoter region of these genes. Second, NF-kappaB indirectly regulates the transcription of CYP genes through mutual repression with some nuclear receptors that are involved in CYP regulation such as AhR, CAR, GR, PXR, RXR, PPAR, FXR, and LXR. Finally, NF-kappaB can regulate CYP activity at post-transcriptional level by inducing heme oxygenase or by affecting the CYP protein stability. In addition, increased inflammatory mediators, oxidative stress, and subsequent NF-kappaB activation have been demonstrated in many conditions such as inflammatory bowel diseases, rheumatoid arthritis, psychological stress, diabetes, aging, cancer, renal diseases, and congestive heart failure. Meanwhile, there is a significant alteration of CYP expression and activity in these diseases. Therefore, we propose that NF-kappaB could be one of the links between inflammation, oxidative stress, and CYP regulation in these diseases. In conclusion, NF-kappaB plays a crucial role in the regulation of CYP through several mechanisms and this role can explain the altered CYP regulation in many conditions.

Substrate specificity, regulation, and polymorphism of human cytochrome P450 2B6

Abstract: CYP2B6 is mainly expressed in the liver that has been thought historically to play an insignificant role in human drug metabolism. However, increased interest in this enzyme has been stimulated by the discovery of polymorphic and ethnic differences in CYP2B6 expression, identification of additional substrates for CYP2B6, and evidence for co-regulation with CYP3A4. This paper updates our knowledge about the structure, function, regulation and polymorphism of CYP2B6. CYP2B6 can metabolise approximately 8% of clinically used drugs (n > 60), including cyclophosphamide, ifosfamide, tamoxifen, ketamine, artemisinin, nevirapine, efavirenz, bupropion, sibutramine, and propofol. CYP2B6 is one of the CYP enzymes that bioactivate several procarcinogens and toxicants. This enzyme also metabolizes arachidonic acid, lauric acid, 17beta-estradiol, estrone, ethinylestradiol, and testosterone. Typical substrates of CYP2B6 are non-planar molecules, neutral or weakly basic, highly lipophilic with one or two hydrogen-bond acceptors. The crystal structure of CYP2B6 has not been resolved, while several pharmacophore and homology models of human CYP2B6 have been reported. Human CYP2B6 is closely regulated by constitutive androstane receptor (CAR/NR1I3) which can activate CYP2B6 expression upon ligand binding. Pregnane X receptor and glucocorticoid receptor also play a role in the regulation of CYP2B6. Induction of CYP2B6 may partially explain some clinical drug interactions observed. For example, coadministered carbamazepine decreases the systemic exposure of bupropion. There is a wide interindividual variability in the expression and activity of CYP2B6. Such a large variability is probably due to effects of genetic polymorphisms and exposure to drugs that are inducers or inhibitors of CYP2B6. To date, at least 28 allelic variants and some subvariants of CYP2B6 (*1B through *29) have been described and some of them have been shown to have important functional impact on drug clearance and drug response. For example, the efavirenz plasma levels in African-American subjects with the CYP2B6 homozygous 516T/T genotype are approximately 3-fold higher than individuals carrying the homozygous G/G genotype. The CYP2B6 516T/T genotype is associated with 1.7-fold greater plasma levels of nevirapine in HIV-infected patients. Smokers with the 1459C>T (R487C) variant of CYP2B6 may be more vulnerable to abstinence symptoms and relapse following treatment with bupropion as a smoking cessation agent. Further studies in the structure, function, regulation and polymorphism of CYP2B6 are warranted.

The effect of proton pump inhibitors on the human microbiota.

Abstract: Proton pump inhibitors (PPIs) are commonly used to treat acid-related diseases, most notably gastroesophageal reflux disease. PPIs are designed to shut down the gastric proton pump (H+/K+-ATPase) of parietal cells, thereby raising the pH of the stomach. While effective, a number of side effects have been associated with PPI use. Naturally occurring bacteria, some of which are acid-producing and contain ATPase enzymes, have also been found within the stomach, upper gastrointestinal tract, and oral cavity. Likewise, a number of fungi are known to inhabit the human body; some of these fungi contain H+-ATPase enzymes. Recent literature has suggested that PPIs may be inadvertently affecting these bacteria and fungi in two different ways: 1) PPIs may directly target the proton pumps of the bacteria and fungi, and/or 2) PPIs may indirectly affect the microenvironment of the flora via changes in pH. These unintended interactions are exasperated by the systemic distribution of PPIs throughout the body and may potentially lead to some of the side effects observed with PPI use. Herein we summarize what is currently known about the interactions between the PPIs and the natural human microbiota.

The Transcriptional Regulation of the Human CYP2C Genes

Abstract: In humans, four members of the CYP2C subfamily (CYP2C8, CYP2C9, CYP2C18, and CYP2C19) metabolize more than 20% of all therapeutic drugs as well as a number of endogenous compounds. The CYP2C enzymes are found predominantly in the liver, where they comprise approximately 20% of the total cytochrome P450. A variety of xenobiotics such as phenobarbital, rifampicin, and hyperforin have been shown to induce the transcriptional expression of CYP2C genes in primary human hepatocytes and to increase the metabolism of CYP2C substrates in vivo in man. This induction can result in drug-drug interactions, drug tolerance, and therapeutic failure. Several drug-activated nuclear receptors including CAR, PXR, VDR, and GR recognize drug responsive elements within the 5' flanking promoter region of CYP2C genes to mediate the transcriptional upregulation of these genes in response to xenobiotics and steroids. Other nuclear receptors and transcriptional factors including HNF4alpha, HNF3gamma, C/EBPalpha and more recently RORs, have been reported to regulate the constitutive expression of CYP2C genes in liver. The maximum transcriptional induction of CYP2C genes appears to be achieved through a coordinative cross-talk between drug responsive nuclear receptors, hepatic factors, and coactivators. The transcriptional regulatory mechanisms of the expression of CYP2C genes in extrahepatic tissues has received less study, but these may be altered by perturbations from pathological conditions such as ischemia as well as some of the receptors mentioned above.

Pharmacokinetics of biotech drugs: peptides, proteins and monoclonal antibodies.

Abstract: With the advances in recombinant DNA biotechnology, molecular biology and immunology, the number of biotech drugs, including peptides, proteins and monoclonal antibodies, available for clinical use has dramatically increased in recent years. Although pharmacokinetic principles are equally applicable to the large molecule drugs and conventional small molecule drugs, the underlying mechanisms for the processes of absorption, distribution, metabolism and excretion (ADME) of large molecule drugs are often very different from that of small molecule drugs. Therefore, a good understanding of the ADME processes of large molecule drugs is essential in support of the development of therapeutic biologics. The purpose of this article is to review the current knowledge of the ADME processes that govern the pharmacokinetics of biotech drugs. The challenges encountered by orally administered peptide and protein drugs, and the nature of lymphatic absorption after subcutaneous administration will be discussed. In addition, molecular mechanisms of biodistribution, metabolism and renal excretion of biotech drugs will also be discussed. Finally, approaches used for prediction of human pharmacokinetics of protein drugs will be briefly discussed.

Genetic polymorphism of the human cytochrome P450 2C9 gene and its clinical significance

Abstract: Human cytochrome P450 2C9 (CYP2C9) accounts for approximately 20% of total hepatic CYP content and metabolizes approximately 15% clinically used drugs including S-warfarin, tolbutamide, phenytoin, losartan, diclofenac, and celecoxib. To date, there are at least 33 variants of CYP2C9 (*1B through to *34) being identified. CYP2C9*2 and CYP2C9*3 differ from the wild-type CYP2C9*1 by a single point mutation: CYP2C9*2 is characterised by a 430C>T exchange in exon 3 resulting in an Arg144Cys amino acid substitution, whereas CYP2C9*3 shows an exchange of 1075A>C in exon 7 causing an Ile359Leu substitution in the catalytic site of the enzyme. CYP2C9*2 is frequent among Caucasians with approximately 1% of the population being homozygous carriers and 22% heterozygous. The corresponding figures for the CYP2C9*3 allele are 0.4% and 15%, respectively. Worldwide, a number of other variants have also to be considered. The CYP2C9 polymorphisms are relevant for the efficacy and adverse effects of numerous nonsteroidal anti-inflammatory agents, sulfonylurea antidiabetic drugs and, most critically, oral anticoagulants belonging to the class of vitamin K epoxide reductase inhibitors. Numerous clinical studies have shown that the CYP2C9 polymorphism should be considered in warfarin therapy and practical algorithms how to consider it in therapy are available. These studies have highlighted the importance of the CYP2C9*2 and *3 alleles. Warfarin has served as a practical example of how pharmacogenetics can be utilized to achieve maximum efficacy and minimum toxicity. Polymorphisms in CYP2C9 have the potential to affect the toxicity of CYP2C9 drugs with somewhat lower therapeutic indices such as warfarin, phenytoin, and certain antidiabetic drugs. CYP2C9 is one of the clinically significant drug metabolising enzymes that demonstrates genetic variants with significant phenotype and clinical outcomes. Genetic testing of CYP2C9 is expected to have a role in predicting drug clearance and implementing individualized pharmacotherapy. Prospective clinical studies with large samples are required to establish gene-dose and gene-effect relatiohsips for CYP2C9.

Structure, Function, Regulation and Polymorphism of Human Cytochrome P450 2A6

Abstract: The CYP2A6 gene spans a region of approximately 6 kb pairs consisting of 9 exons and has been mapped to the long arm of chromosome 19 (between 19q12 and 19q13.2). The CYP2A6 protein has 494 amino acids and is an important hepatic Phase I enzyme that metabolizes approximately 3% of therapeutic drugs (n > 30; e.g. valproic acid, pilocarpine, tegafur, fadrozole, ifosfamide, cyclophosphamide, nicotine, tamoxifen, promazine, propofol, and cisapride), environmental toxicants (e.g. gasoline additives), and many procarcinogens such as nitrosamines and aflatoxin B(1). This enzyme also participates in the biotransformation of several endogenous compounds such as retinoid acids and steroids. Because CYP2A6 is responsible for 70-80% of the initial metabolism of nicotine, CYP2A6 has been proposed to be a novel target for smoking cessation. Site-directed mutagenesis and homology modeling studies have identified a number of amino acids (e.g. F300, A301, S208, S369, and L370) that play a role in substrate recognition and binding. CYP2A6 shows a crystal structure with a compact, hydrophobic active site with Asn297 serving as one hydrogen bond donor and orienting substrates for regio-selective oxidation. CYP2A6 contains the second smallest active site cavity among the human CYPs with known structures. The regulation mechanism of CYP2A6 expression is not fully understood, but available data suggest that several nuclear receptors including constitutive androstane receptor, pregnane X receptor and glucocorticoid receptor are involved in its regulation. Pilocarpine and tranylcypromine are commonly used as selective competitive inhibitors of CYP2A6. Selegiline, methoxsalen, (R)-(+) menthofuran and decursinol angelate are mechanism-based inhibitors of CYP2A6. Both in vitro and in vivo studies have demonstrated a wide (20- to >100-fold) interindividual variation in CYP2A6 expression and activity, which is due primarily to genetic polymorphisms in the CYP2A6 gene, but CYP2A6 activity is also modified by certain drugs and pathological and environmental factors. To date, more than 36 variant alleles (*1B through *37) of the CYP2A6 gene have been identified. There have been 278 SNPs found in the CYP2A6 upstream sequence, 8 introns and 9 exons in NCBI dbSNP. Polymorphism of CYP2A6 has been associated with smoking behavior, drug clearance and lung cancer risk. Further studies are warranted to explore the role of CYP2A6 in clinical practice, drug development and toxicology.

Variation of Drug Kinetics in Pregnancy

Abstract: Significant changes in the physiological and biotransformation processes that govern pharmacokinetics occur during pregnancy. Consequently, the disposition of many medications is altered in gestation and the efficacy and toxicity of drugs used by pregnant women can be difficult to predict or can lead to serious side effects. Gastrointestinal absorption and bioavailability of drugs vary due to changes in gastric secretion and small intestine motility. Various pregnancy-related hemodynamic changes such as an increase in cardiac output, blood volume, the volume of distribution (Vd), renal perfusion and glomerular filtration may affect drug disposition and elimination, and can cause increase or decrease in the terminal elimination half-life of drugs. Changes in maternal drug biotransformation activity also contribute to alterations in pharmacokinetics of drugs taken in pregnancy. Therefore, pregnant women may require different dosing regimens or their adjustment than both men and non-pregnant women. In addition, the prenatal pharmacotherapy is unique due to the presence of feto-placental unit. Considerations regarding transplacental pharmacokinetics and safety for the developing fetus are thus essential aspects of medication in pregnancy. The aim of this review is to summarize major physiological and biotransformation changes associated with pregnancy that affect pharmacokinetics in pregnant women. In addition, we point out the most important examples of altered kinetics of drugs administered in pregnancy with mechanistic explanation of the phenomena based on maternal adaptation in pregnancy.

Chemotherapy-induced modifications to gastrointestinal microflora: evidence and implications of change.

Abstract: Mucositis is a common side effect of chemotherapy which remains poorly understood. Despite advances in the understanding of oral and small intestinal mucositis over recent years, large intestinal mucositis, including diarrhoea, has not been well defined and the underlying mechanisms of the condition are yet to be established. The majority of the literature available concerning large intestinal mucositis is based on clinical observations, with very little basic research existing. However, from the little research conducted, it is likely that the intestinal microflora play a role in the development of chemotherapy-induced mucositis. This review will explore the potentially important relationship between intestinal microflora and the subsequent development of chemotherapy-induced mucositis.

The heme oxygenase/biliverdin reductase pathway in drug research and development.

Abstract: The heme oxygenase/biliverdin reductase (HO/BVR) axis catalyzes the degradation of heme, but this system and its by-products, carbon monoxide (CO) and bilirubin, have also been shown to exert cytoprotective effects by activating pro-survival pathways and scavenging free radicals. Naturally occurring substances that upregulate the inducible isoform of HO (HO-1) have therefore been proposed as potential new drugs for the treatment of free radical-induced disease. A number of existing drugs have also been shown to regulate the HO/BVR system, and this capacity is considered an additional mechanism for their therapeutic activity. However, upregulation of the HO/BVR axis is not always beneficial for cells: the heme depletion and accumulation of CO and bilirubin it causes are potentially toxic. Therefore, new pharmacological modulators of HO/BVR activity must act in a dose-dependent manner. This would allow dose titration to achieve a desired pharmacologic effect without producing toxicity. Unfortunately, this goal is more complicated than it seems because toxicity has to be defined in terms of each of the main products of heme metabolism. Furthermore, sensitivity to the therapeutic/toxic effects of these products is likely to be tissue- or cell-type specific. The solution may lie in the use of novel drug-delivery systems that allow targeted delivery of low doses of the HO/BVR modulator to selected tissues.

Transcriptional regulation of cytochrome p450 genes by the nuclear receptor hepatocyte nuclear factor 4-alpha.

Abstract: Hepatocyte nuclear factor 4-alpha (HNF4alpha, NR2A1) is a nuclear receptor (NR) required for liver development and for controlling the expression of many hepatic-specific genes associated with important metabolic pathways. Many studies have also identified HNF4alpha as a direct transactivator of numerous xenobiotic-metabolizing cytochrome P450 (CYP) genes, suggesting that this factor is a global regulator which supports CYP transcription in the liver. Moreover, HNF4alpha expression displays a significant variability in human liver which may account for a proportion of the inter-individual variability in the expression of drug-metabolism genes and the clearance rate of a wide variety of prescribed drugs. In the last few years, a number of complex interactions and cross-talks between HNF4alpha and other transcription factors and coregulators have also surfaced, and the impact on CYP gene expression has been demonstrated. Thus, it is now clear that HNF4alpha modulates CYP expression in the liver by interacting with the xenosensor receptors (PXR and CAR), the glucocorticoid receptor (GR), the feeding-fasting cycle target PGC-1alpha, the sexual-dimorphism factor Stat5b, and other liver-enriched factors, such as C/EBPs. In addition to regulating drug elimination pathways, HNF4alpha also triggers pleiotropic effects on cholesterol and fatty acid metabolism, glucose homeostasis and inflammation. As a whole, current evidence indicates that HNF4alpha is a central regulator in the network of NRs that integrates drug-metabolism not only with the liver intermediate metabolism, but also with a number of patho-physiological conditions where the CYP expression is altered. The purpose of this review is to summarize and discuss these studies and their conclusions, with particular emphasis on the role of HNF4alpha in the regulation of drug-metabolizing CYP genes in the human liver.

Human CYP2C8: Structure, Substrate Specificity, Inhibitor Selectivity, Inducers and Polymorphisms

Abstract: Human CYP2C8 is a key member of the CYP2C family and metabolizes more than 60 clinical drugs. A number of active site residues in CYP2C8 have been identified based on homology modeling and site-directed mutagenesis studies. In the structure of CYP2C8, the large active site cavity exhibits a trifurcated topology that approximates a T or Y shape, which is consistent with the finding that CYP2C8 can efficiently oxidize relatively large substrates such as paclitaxel and cerivastatin. The active site cavity of CYP2C8 contains at least 48 amino acid residues and many of them are important for substrate binding. The structures of CYP2C8 in complex with distinct ligands have revealed that the enzyme can bind divergent substrates and inhibitors without extensive conformational changes. CYP2C8 is a major catalyst in the metabolism of paclitaxel, amodiaquine, troglitazone, amiodarone, verapamil and ibuprofen, with a secondary role in the biotransformation of cerivastatin and fluvastatin. CYP2C8 also metabolises endogenous compounds such as retinoids and arachidonic acid. Many drugs are inhibitors of CYP2C8 and inhibition of this enzyme may result in clinical drug interactions. The pregnane X receptor, constitutive androstane receptor, and glucocorticoid receptor are likely to involve the regulation of CYP2C8. A number of genetic mutations in the CYP2C8 gene have been identified in humans and some of them have functional impact on the clearance of drugs. Further studies are needed to delineate the role of CYP2C8 in drug development and clinical practice.

Polymorphic drug metabolism in anaesthesia.

Abstract: A substantial part of the interindividual variability in response to drugs and xenobiotics is related to genetically-determined impairment in drug metabolism Several drug-metabolising enzymes are polymorphic in humans and often polymorphisms are strongly related to altered drug biodisposition and to the risk of developing adverse effects Drugs used in general anaesthesia undergo polymorphic metabolism Among these, halothane is metabolized by cytochrome P450 (CYP) 2E1 and, to a lesser extent, by CYP3A4 and CYP2A6 CYP2E1 also plays a key role in the metabolism of isoflurane, sevoflurane, enflurane and desflurane CYP2B6, CYP3A4 and CYP2C9 play a relevant role in the metabolism of ketamine The enzymes involved in the metabolism of thiopental and etomidate remains to be elucidated Propofol is metabolized mainly by glucuronidation by uridine diphosphate- glucuronosyltransferases (UGTs) and by hydroxylation by CYP2B6 and CYP2C enzymes The enzymes SULT1A1 and NQO1 participate in later steps in propofol metabolism All the above-mentioned anaesthetic-metabolising enzymes are polymorphic in man The present review analyzes the importance of enzymes in the metabolism of anaesthetics and common polymorphisms related to the biotransformation of general anaesthetics and it raises hypotheses on genetic and non-genetics factors related to altered response to anaesthetics that require further investigation Based on functional relevance and allele frequencies, we identify the most promising targets for the clinical use of pharmacogenomic techniques in anaesthesia to prevent altered pharmacokinetics or adverse drug effects

Pharmacokinetics and pharmacodynamics of endoperoxide antimalarials.

Abstract: There are several clinically useful endoperoxides, mainly artemisinin derivatives available in market for the treatment of malaria These are highly potent drugs, with fastest parasite reduction ratio, broadest parasite stage specificity and effectiveness against all species of plasmodium in human Endoperoxides are crystalline compounds having poor aqueous solubility Several theories have been proposed for their mechanism of action, but the understanding is still incomplete The major limitation of this class of compounds is the short half-life, requiring frequent administration, leading to noncompliance and recrudescence Therefore, WHO recommends their use in combination with long acting antimalarial drugs (Artemisinin based combination therapy, ACT) to manage drug resistance, recrudescence, and non compliance Endoperoxide compounds bind selectively to malaria-infected red blood cells and moderately to human plasma proteins Artemisinin derivatives are converted primarily to the bioactive metabolite dihydroartemisinin after parenteral, oral or rectal administration The rate of conversion is lowest for artelinic acid and highest for the water-soluble artesunate Such conversion occurs largely in the liver by CYP enzymes Oral bioavailability in animals ranges between 19 to 35% Based on their liphophilicity, they tend to cross the blood-brain barrier, causing neurotoxicity in animal models Efforts have been made to understand and develop pharmacokinetic-pharmacodynamic (PK-PD) correlation and identify PK-PD indices of endoperoxides In the absence of the above, the selection of doses in ACTs has been empirical There are several reports on clinical pharmacokinetic interactions of endoperoxides and their long acting partner drugs but as on date no clinically significant interaction has been reported This review is an update on physicochemical, pharmacokinetic and pharmacodynamic properties of the endoperoxide antimalarials

The gut and intestinal bacteria in chronic heart failure.

Abstract: Chronic heart failure (CHF) is now recognized as a multisystem disorder with increased sympathetic tone, hormonal derangements, an anabolic/catabolic imbalance, endothelial dysfunction, and systemic low-grade inflammation affecting various organ systems. Pro-inflammatory cytokines appear to play important roles in that context. There is increasing evidence for the gut to have a pathophysiological role for both chronic inflammation and malnutrition in CHF. Indeed, disturbed intestinal microcirculation and barrier function in CHF seem to trigger cytokine generation, thereby contributing to further impairment in cardiac function. On the other hand, myocardial dysfunction can induce microcirculatory injuries leading to a disruption in the intestinal barrier. This amplifies the inflammatory response. Furthermore, alterations of specific absorption functions of the intestinal mucosa in CHF may aggravate symptoms of cachexia. The increased number of adherent bacteria seen in patients with CHF and elevated systemic levels of anti-lipopolysaccharide immunoglobulin A underscore this fact. Therefore, the gut poses an interesting target for therapeutic interventions in patients with CHF.

Pharmacogenomics in drug induced liver injury.

Abstract: Drug-induced liver injury (DILI) is a severe adverse effect. The majority of DILI cases are idiosyncratic and several mechanisms have been postulated to explain why some subjects develop DILI with drugs that are safe for the majority of individuals. Major mechanisms proposed for DILI are based on the production of reactive metabolites, immune-mediated hepatotoxicity, a "danger signal" hypothesis and/or alterations in mitochondrial function. These mechanisms are compatible with the hypothesis for genetic variability in drug metabolism or bioactivation and are a major determinant for DILI. In this review we summarize present knowledge on underlying mechanisms, and clinical expression as well as genetic and non-genetic factors that modulate the risk of developing DILI. With regard to DILI pharmacogenomics, we summarize current evidence on the role of polymorphisms in genes coding for the drug-metabolizing enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, CYP3A5, NAT2, GSTM1, GSTT1, UGT1A1, UGT1A3, UGT1A9 and UGT2B7. Conclusive evidence for association with DILI risk has been obtained for non-mutated CYP2E1, slow NAT2 and slow GSTM1 genotypes. For the rest of the genes additional pharmacogenomics and toxicogenomics studies are required. We identify potential sources of heterogeneity in studies carried out so far as well as new genetic targets which require further investigation.

Towards a quantitative framework for the prediction of DDIs arising from cytochrome P450 induction

Abstract: Although CYP induction is not generally considered to be as clinically relevant as CYP inhibition, there are important examples where induction has caused both therapeutic failure, due to insufficient exposure to parent drug, and toxicity, mediated by increased formation of reactive metabolites. Furthermore, while there has been considerable progress in the extrapolation of in vitro data to predict the in vivo consequences of enzyme inhibition, less attention has been given to the quantitative impact of enzyme induction as a mechanism of drug-drug interaction (DDI) and as a component of compound selection and early drug development. We discuss current approaches in the context of a mechanistic framework for the prediction of the extent and time-course of enzyme induction in vivo based on in vitro experimentation. Factors influencing the extent of DDI due to CYP induction are summarised, and areas deficient in information that would allow more accurate prediction within target populations are highlighted.

Insights into the Structure, Function, and Regulation of Human Cytochrome P450 1A2

Abstract: CYP1A2 is one of the major CYPs in human liver ( approximately 13%) and metabolises a variety of clinically important drugs, such as clozapine, lidocaine, theophylline, tacrine, and leflunomide. CYP1A2 is one of the major enzymes that bioactivate a number of procarcinogens and thus induction of CYP1A2 may increase the carcinogenicity of these compounds. This enzyme also metabolizes several important endogenous compounds including steroids, retinols, melatonin, uroporphyrinogen and arachidonic acid. In the recently published crystal structure of CYP1A2 in complex with alpha-naphthoflavone, its compact active site is closed without clear solvent or substrate access channels. Not surprisingly, CYP1A2 has a relatively small volume of the active site cavity of 375 A(3), which is 44.2% larger than that of CYP2A6 (260 A(3)), but much smaller than that of CYP3A4 (1385 A(3)) and 2C8 (1438 A(3)). Generally, CYP1A2 substrates contain planar ring that can fit the narrow and planar active site of the enzyme. Like many of other CYPs, CYP1A2 is subject to induction and inhibition by a number of compounds. Similar to CYP1A1 and 1B1, CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR), a ligand-activated transcription factor and a basic helix-loop-helix protein belonging to the Per-Arnt-Sim family of transcription factors. Knockout of Cyp1a2 in mice has provided a very useful tool for the functional investigation of this gene. Further studies are needed to explore the clinical and toxicological significance of CYP1A2.

Role of phase II drug metabolizing enzymes in cancer chemoprevention.

Abstract: Chemical insults, such as environmental or occupational carcinogenic agents, play a major role in the pathogenesis of many cancers. Many carcinogens exert genotoxic and cytotoxic effects via bioactivation into electrophilic species, a process catalyzed primarily by phase I drug metabolizing enzymes, typically cytochrome P450s. These reactive intermediates can induce DNA and RNA damage, and formation of protein adducts. The reactive species are often detoxified by phase II drug metabolizing enzymes, such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases (UGTs), sulfotransferase (ST) and N-acetyltransferase (NAT). Phase II enzymes classically conjugate these hydrophobic intermediates to a water-soluble group, thus masking their reactive nature, and allowing subsequent excretion. Therefore, strategies that modulate the levels of phase II enzymes by either pharmacological or nutritional means can lead to enhanced elimination of reactive species. Agents that preferentially activate phase II over phase I enzymes can be beneficial as chemopreventives. Compounds, such as isothiocyanates and dithiolthiones have been shown to act as transcriptional activators of phase II enzymes. A consensus enhancer element, known as antioxidant response element (ARE), in the regulatory domains of many phase II genes and an ARE-binding transcription factor nuclear factor E2-related factor 2 (Nrf2) have been implicated in the action of many chemopreventive agents. In this review, we will discuss the mechanisms of regulation of phase II enzymes, including the signal transduction events elicited by chemopreventive agents. We will also summarize the data available for these agents in preclinical models of tumorigenesis. Some chemopreventive agents have progressed to various stages of clinical trials, e.g. biomarker studies in healthy volunteers or in susceptible populations. These clinical data will be reviewed. Finally, we will provide a commentary on implementation of discovery and development programs for novel chemopreventive agents that are based on rational drug design, with lead optimization towards a safe and efficacious regimen in man.

Expression, Function and Regulation of Mouse Cytochrome P450 Enzymes: Comparison With Human Cytochrome P450 Enzymes

Abstract: The present review focuses on the expression, function and regulation of mouse cytochrome P450 (Cyp) enzymes. Information compiled for mouse Cyp enzymes is compared with data collected for human CYP enzymes. To date, approximately 40 pairs of orthologous mouse-human CYP genes have been identified that encode enzymes performing similar metabolic functions. Recent knowledge concerning the tissue expression of mouse Cyp enzymes from families 1 to 51 is summarized. The catalytic activities of microsomal, mitochondrial and recombinant mouse Cyp enzymes are discussed and their involvement in the metabolism of exogenous and endogenous compounds is highlighted. The role of nuclear receptors, such as the aryl hydrocarbon receptor, constitutive androstane receptor and pregnane X receptor, in regulating the expression of mouse Cyp enzymes is examined. Targeted disruption of selected Cyp genes has generated numerous Cyp null mouse lines used to decipher the role of Cyp enzymes in metabolic, toxicological and biological processes. In conclusion, the laboratory mouse is an indispensable model for exploring human CYP-mediated activities.

Gender differences in p-glycoprotein expression and function: effects on drug disposition and outcome

Abstract: Gender differences in drug concentrations, drug response and toxicity have been attributed to various distinct yet interrelated physiological and molecular mechanisms. Drug transporters and metabolising enzymes play an important role in the xenobiotic cascade and are important regulators of drug disposition at the molecular level. The proposal of a dynamic interplay between drug metabolism and efflux has positioned drug transporters as important mediators of gender disparity in respect to drug disposition and therapeutic response. In examining the effects of gender on drug disposition and response we will specifically direct our focus on the role of the predominant drug transporter, P-glycoprotein. This review focuses on the role of the P-glycoprotein as a molecular mediator of gender differences in both drug exposure and response. Differences in transporter expression and function will be discussed together with the molecular basis for the observed difference in drug exposure between the sexes. Gender differences affecting transporter expression and function at the effect compartment and the effect of this on drug response will also be discussed.