Current Drug Metabolism (v.12, #6)

Xenobiotic Metabolizing Cytochrome P450 in Pig, a Promising Animal Model by Emanuela Puccinelli, Pier Giovanni Gervasi, Vincenzo Longo (507-525).
The pig has been used as an important animal model for human studies because of its similarity in size, physiology and diseasedevelopment. However, in contrast to the extensive data available on the cytochrome P450 (CYP) system for humans and rodents, thedata related to pig are limited because of, among others, the presence of intra-species differences (domestic pigs and minipigs). Theknowledge of the CYP superfamily in a given experimental animal is crucial for pharmacological and toxicological tests in developingdrugs and for understanding the metabolic pathways of toxicants and carcinogens. In addition, information on the CYP system in pigs isimportant since it plays a dominant role in the metabolism of veterinary drugs, whose residues remain in the porcine tissues which arefood for humans. The aim of the present review is to examine - in the liver and extrahepatic tissues of pig - our current knowledge of the xenobioticmetabolizingCYPs belonging to families 1-4, in terms of drug metabolism, substrate specificity, inhibition, gene expression and receptor-driven regulation, in comparison with human data. It is hoped, furthermore, that this review may stimulate research on the porcinedrug-metabolizing enzymes in order to evaluate the hypothesis whereby pig data may better reflect human drug metabolism and toxicitythan those obtained from the traditional non-rodent models.

The Roles of Cytochrome P450 in Ischemic Heart Disease by Motohiko Sato, Utako Yokoyama, Takayuki Fujita, Satoshi Okumura, Yoshihioro Ishikawa (526-532).
Cytochrome P450 (CYP) represents a large family of enzymes that catalyze the oxidation of endogenous and exogenous compounds.The functions of CYP enzymes in the metabolism of xenobiotics have well been established in the liver. However, some CYPenzymes are highly expressed in the heart and catalyze arachidonic acid oxidation to a variety of eicosanoids, which attenuates ischemiareperfusioninjury of the heart. CYP-mediated cardioprotection is associated with activation of multiple pathways such as sarcolemmaland mitochondrial potassium channels, p42/p44 MAPK and PI3K-AKT signaling in cells. CYP enzymes also represent a significantsource of reactive oxygen species (ROS) that may target cellular homeostatic mechanisms and mitochondria. CYP isoforms expressed inthe heart are critical for generation of epoxyeicosatrienoic acids (EETs) and ROS. It has been demonstrated that CYP2J2 generates cardioprotectiveEETs, whereas another isozyme in the heart, CYP2C, generates EETs as well as detrimental ROS. Genetic polymorphismsof CYP2C or CYP2J2 have a pathologic impact on coronary artery diseases. Cardiac CYP enzymes can be involved in drug metabolismwithin the heart and influence pharmacologic efficacy. Metabolism mediated by CYP enzymes influences the survival of cardiomyocytesduring ischemia, which is critical for treatment of human ischemic heart disease. In this review, we summarize current knowledge of thisenzyme family and discuss the roles of CYP in ischemia-reperfusion injury of the heart.

Molecular Modeling Used to Evaluate CYP2C9-Dependent Metabolism: Homology Modeling, Molecular Dynamics and Docking Simulations by Jessica E. Mendieta-Wejebe, Jose Correa-Basurto, Erika M. Garcia-Segovia, Gisela Ceballos-Cancino, Martha C. Rosales-Hernandez (533-548).
Cytochrome P450 (CYP) 2C9 is the principal isoform of the CYP2C subfamily in the human liver and is involved in the oxidationof several endogenous and xenobiotic compounds, including many therapeutic drugs. The metabolism of drugs by CYP2C9 canyield either safe or toxic products, which may be related to the recognition and binding modes of the substrates to this isoform. These interactionscan be studied using in silico methods such as quantum chemistry, molecular dynamics and docking simulations, which canalso be useful for predicting the structure of metabolites. In these types of studies, the ligand and the protein must be tridimensional models;thus, the protein can be built by homology modeling or retrieved from the Protein Data Bank. Therefore, the current review emphasizesthe importance of using in silico methods to predict the metabolism of CYP2C9 because these computational tools have allowed thedescription of the principal characteristics of the active site of this isoform at the molecular level and the chemical properties of itsligands.

Stereoselectivity of Human Cytochrome P450 in Metabolic and Inhibitory Activities by Toshiro Niwa, Norie Murayama, Hiroshi Yamazaki (549-569).
This review focuses on stereoselectivity of human cytochrome P450 (P450 or CYP) in the area of metabolism and inhibition.A meta-analysis was performed based on the reported values regarding (1) values of the Michaelis-Menten constant (Km), maximal velocity(Vmax), and intrinsic clearance (Vmax/Km) for 45 metabolic reactions of 19 substrates and (2) inhibition constants (Ki) for 6 inhibitors.The median (R)/(S)-enantiomer ratios of the Km, Vmax, and Vmax/Km values for CYP1A2, CYP2B6, CYP2C19, CYP2D6, and CYP3A4were in the range of 0.80-1.53, whereas the median ratios of Vmax, and Vmax/Km values for CYP2C9 were 0.43 and 0.60, respectively. Inaddition, the parameters for metabolic reactions (25-80%) of (R)-enantiomers of these P450s were comparable to those of (S)-enantiomers (the ratios were between 0.5 and 2), whereas 45-69% of Vmax and Vmax/Km values for the (R)-enantiomer in CYP2C9 wereless than half of those for the (S)-enantiomer, although the kinetic behavior of the stereoselectivity depended on the metabolic reaction.There is a limited number of reports regarding stereoselective inhibition and induction in vitro. The present information gives insight intothe contribution of stereoselectivity to metabolism mediated by P450s and the risk of adverse drug-drug interactions due to stereoselectivity.

Different antidepressant drugs are currently used for the treatment of depression in cancer patients, such as second-generationantidepressants and, recently, the extracts of Hypericum perforatum. These agents are susceptible to metabolically-based drug interactionswith anticancer drugs. The aim of the present article is to provide an updated review of clinically relevant metabolic drug interactionsbetween selected anticancer drugs and antidepressants, focusing on selective serotonin reuptake inhibitors (SSRIs) and Hypericumextract. SSRIs can cause pharmacokinetic interactions through their in vitro ability to inhibit one or more cytochrome P450 isoenzymes(CYPs). SSRIs differ in their potential for metabolic drug interactions with anticancer drugs. Fluoxetine and paroxetine are potent inhibitorsof CYP2D6 and administration of these SSRIs reduces the clinical benefit of an anticancer drug, such as tamoxifen, by decreasingthe formation of active metabolites of this drug. Women with breast cancer who receive paroxetine in combination with tamoxifen are atincreased risk for death. Other SSRIs, including citalopram, escitalopram, are weak or negligible inhibitors of CYP2D6 and are less likelyto interact with anticancer drugs, while sertraline causes significant inhibition of this isoform only at high doses. Hypericum extract, byinducing both the CYP3A4 and the P-glycoprotein (P-gp), can reduce the plasma concentrations of different antineoplastic agents such asimatinib, irinotecan and docetaxel, thus reducing the clinical efficacy of these drugs. Although these interactions are often predictable, theuse of fluoxetine, paroxetine and Hypericum extract should be avoided in cancer patients.

Recent regulatory guidance suggests that drug metabolites identified in human plasma should be present at equal or greaterlevels in at least one of the animal species used in safety assessments (MIST). Often synthetic standards for the metabolites do not exist,thus this has introduced multiple challenges regarding the quantitative comparison of metabolites between human and animals. Variousbioanalytical approaches are described to evaluate the exposure of metabolites in animal vs. human. A simple LC/MS/MS peak area ratiocomparison approach is the most facile and applicable approach to make a first assessment of whether metabolite exposures in animalsexceed that in humans. In most cases, this measurement is sufficient to demonstrate that an animal toxicology study of the parent drughas covered the safety of the human metabolites. Methods whereby quantitation of metabolites can be done in the absence of chemicallysynthesized authentic standards are also described. Only in rare cases, where an actual exposure measurement of a metabolite is needed,will a validated or qualified method requiring a synthetic standard be needed. The rigor of the bioanalysis is increased accordingly basedon the results of animal:human ratio measurements. This data driven bioanalysis strategy to address MIST issues within standard drugdevelopment processes is described.

Can Dietary Antioxidants Reduce the Incidence of Brain Tumors? by S.A. Sheweita, B.Y. Sheikh (587-593).
The incidence of brain tumor and other types of cancer are markedly increased during the last few decades. There are manyetiological and environmental factors involved in the initiation of different types of cancers including brain tumors. Mutations in tumorsuppressor gene p53 and its expression are associated with shorter survival and higher mortality rate of patients with brain tumors. Anotherfactor, N-nitrosamines have received much attention as a potential risk factor for brain tumor. These compounds are potent carcinogensand occur widely in the environment, and also can be formed endogenously in the stomach from the interaction of ingested nitrate ornitrite with secondary amines. Free radicals are another etiological factor of brain tumor and are removed by cellular antioxidants in thehuman body. Brain tissue is vulnerable to the damaging effects of free radicals as a result of low antioxidant levels. Interestingly, there isan inverse correlation between the total antioxidant levels and oxidative DNA damage in transitional meningioma compared with normalbrain tissues. Also, an inverse relationship between antioxidant levels and grades of malignancy has been found after histopathologicalexamination of brain tumors. Moreover, high intake of vitamin E is correlated with greater survival for all patients diagnosed as Grade IIImalignant glioma. Dietary supplementation with antioxidants [e.g. vitamins C & E] was found to reduce the incidence of brain tumors inchildren whose mothers took these vitamins throughout pregnancy. On the other hand, decreases in antioxidant levels were correlatedwith the severity of malignancy of brain tumors, and also with accumulation of considerable amounts of oxidative stress products includingfree radicals which damage this tissue. The mechanisms of protection of these antioxidants against brain tumors might be due to inhibitionof the nitrosation process, decreasing of tumor necrotic factor, scavenging of free radicals, inhibition of telomerase activity whichfacilitates telomere attrition. It is concluded that administration of antioxidants could reduce the incidence of brain tumors and probablyother types of cancer.

Utility of Measuring Serum Concentrations of Anti-TNF Agents and Anti-Drug Antibodies in Inflammatory Bowel Disease by Ivan Guerra, Maria Chaparro, Fernando Bermejo, Javier P. Gisbert (594-598).
Tumor necrosis factor alpha (TNF..) is a cytokine with a critical role in the pathogenesis of some chronic inflammatory diseases,such as inflammatory bowel diseases. Anti-TNF agents, which neutralize the biological activity of TNF.., are widely used amongthe different therapeutic options for the treatment of patients with inflammatory bowel diseases. These drugs are very useful in clinicalpractice, but some patients experience lack and loss of response during the treatment. Drug serum concentration, antibodies against anti-TNF agents, clearance of the drug, formation of immune complexes, a more severe disease and probably other unknown factors can influencethe treatment’s efficacy. Nowadays, the management of patients with lack or loss of response is empirical. The measurement ofdrug concentrations and antibodies against anti-TNF agents might be useful for improving the selection of patients that will benefit fromthe maintenance treatment. In clinical practice, these methods may help us decide which strategy will be used in cases of loss of response:treatment intensification, shortening the infusion interval, increasing the dose, switching to another anti-TNF agent or to a drug with anothermechanism of action. The optimal strategy in the future may be comprised of an early detection of loss of response to the treatmentby assessing clinical symptoms and finding evidence of activity of the disease on endoscopic or radiological examinations when necessary,as well as a better management of anti-TNF treatment aided by measuring the serum concentration of the drug and antibodiesagainst the drug.