N-Dealkylation of chlorimipramine and chlorpromazine by rat liver microsomal cytochrome P450 isoenzymes

Metabolism and pharmacokinetics of pharmaceuticals in cats (Felix sylvestris catus) and implications for the risk assessment of feed additives and contaminants

In animal health risk assessment, hazard characterisation of feed additives has been often using the default uncertainty factor (UF) of 100 to translate a no-observed-adverse-effect level in test species (rat, mouse, dog, rabbit) to a 'safe' level of chronic exposure in farm and companion animal species. Historically, both 10-fold factors have been further divided to include chemical-specific data in both dimensions when available. For cats (Felis Sylvestris catus), an extra default UF of 5 is applied due to the species' deficiency in particularly glucuronidation and glycine conjugation. This paper aims to assess the scientific basis and validity of the UF for inter-species differences in kinetics (4.0) and the extra UF applied for cats through a comparison of kinetic parameters between rats and cats for 30 substrates of phase I and phase II metabolism. When the parent compound undergoes glucuronidation the default factor of 4.0 is exceeded, with exceptions for zidovudine and S-carprofen. Compounds that were mainly renally excreted did not exceed the 4.0-fold default. Mixed results were obtained for chemicals which are metabolised by CYP3A in rats. When chemicals were administered intravenously the 4.0-fold default was not exceeded with the exception of clomipramine, lidocaine and alfentanil. The differences seen after oral administration might be due to differences in first-pass metabolism and bioavailability. Further work is needed to further characterise phase I, phase II enzymes and transporters in cats to support the development of databases and in silico models to support hazard characterisation of chemicals particularly for feed additives.

Pharmacokinetic alterations in poloxamer 407-induced hyperlipidemic rats

1. Plasma lipid profile abnormalities in hyperlipidemia can potentially alter the pharmacokinetics of a drug in a complex manner. To evaluate these pharmacokinetic alterations in hyperlipidemia and to determine the underlying mechanism(s), poloxamer 407-induced hyperlipidemic rats (HL rats), a well-established animal model of hyperlipidemia have been used. 2. In this review, we summarize findings on the pathophysiological and gene expression changes in drug-metabolizing enzymes and transporters in HL rats. We discuss pharmacokinetic changes in drugs metabolized primarily via hepatic cytochrome P450 (CYPs) in terms of alterations in hepatic intrinsic clearance (CL^'_int), free fraction in plasma (f_u) and hepatic blood flow rate (Q_H), depending on the hepatic excretion ratio, as well as drugs eliminated primarily by mechanisms other than hepatic CYPs. 3. For lipoprotein-bound drugs, increased binding to lipoproteins resulted in lower f_u values and volumes of distribution, with some exceptions. Generally, slower non-renal clearance (or total body clearance) of drugs that are substrates of hepatic CYP3A and CYP2C is well explained by the following factors: alterations in CL^'_int (due to down-regulation of hepatic CYPs), decreased f_u and/or possible decreased Q_H. 4. These consistent findings across studies in HL rats suggest more studies are needed at the clinical level for optimal pharmacotherapies for hyperlipidemia.

Pharmacokinetics and interactions of headache medications, part I: introduction, pharmacokinetics, metabolism and acute treatments

Recent progress in the treatment of primary headaches has made available specific, effective and safe medications for these disorders, which are widely spread among the general population. One of the negative consequences of this undoubtedly positive progress is the risk of drug-drug interactions. This review is the first in a two-part series on pharmacokinetic drug-drug interactions of headache medications. Part I addresses acute treatments. Part II focuses on prophylactic treatments. The overall aim of this series is to increase the awareness of physicians, either primary care providers or specialists, regarding this topic. Pharmacokinetic drug-drug interactions of major severity involving acute medications are a minority among those reported in literature. The main drug combinations to avoid are: i) NSAIDs plus drugs with a narrow therapeutic range (i.e., digoxin, methotrexate, etc.); ii) sumatriptan, rizatriptan or zolmitriptan plus monoamine oxidase inhibitors; iii) substrates and inhibitors of CYP2D6 (i.e., chlorpromazine, metoclopramide, etc.) and -3A4 (i.e., ergot derivatives, eletriptan, etc.), as well as other substrates or inhibitors of the same CYP isoenzymes. The risk of having clinically significant pharmacokinetic drug-drug interactions seems to be limited in patients with low frequency headaches, but could be higher in chronic headache sufferers with medication overuse.

Contribution of human cytochrome p-450 isoforms to the metabolism of the simplest phenothiazine neuroleptic promazine

1. The aim of the present study was to identify human cytochrome p-450 isoforms (CYPs) involved in 5-sulphoxidation and N-demethylation of the simplest phenothiazine neuroleptic promazine in human liver. 2. The experiments were performed in the following in vitro models: (A). a study of promazine metabolism in liver microsomes-(a). correlations between the rate of promazine metabolism and the level and activity of CYPs; (b). the effect of specific inhibitors on the rate of promazine metabolism (inhibitors: CYP1A2-furafylline, CYP2D6-quinidine, CYP2A6+CYP2E1-diethyldithiocarbamic acid, CYP2C9-sulfaphenazole, CYP2C19-ticlopidine, CYP3A4-ketoconazole); (B). promazine biotransformation by cDNA-expressed human CYPs (Supersomes 1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2E1, 3A4); (C). promazine metabolism in a primary culture of human hepatocytes treated with specific inducers (rifampicin-CYP3A4, CYP2B6 and CYP2C inducer, 2,3,7,8-tetrachlordibenzeno-p-dioxin (TCDD)-CYP1A1/1A2 inducer). 3. In human liver microsomes, the formation of promazine 5-sulphoxide and N-desmethylpromazine was significantly correlated with the level of CYP1A2 and ethoxyresorufin O-deethylase and acetanilide 4-hydroxylase activities, as well as with the level of CYP3A4 and cyclosporin A oxidase activity. Moreover, the formation of N-desmethylpromazine was correlated well with S-mephenytoin 4'-hydroxylation. 4. Furafylline (a CYP1A2 inhibitor) and ketoconazole (a CYP3A4 inhibitor) significantly decreased the rate of promazine 5-sulphoxidation, while furafylline and ticlopidine (a CYP2C19 inhibitor) significantly decreased the rate of promazine N-demethylation in human liver microsomes. 5. The cDNA-expressed human CYPs generated different amounts of promazine metabolites, but the rates of CYP isoforms to catalyse promazine metabolism at therapeutic concentration (10 microM) was as follows: 1A1>2B6>1A2>2C9>3A4>2E1>2A6>2D6>2C19 for 5-sulphoxidation and 2C19>2B6>1A1>1A2>2D6>3A4>2C9>2E1>2A6 for N-demethylation. The highest intrinsic clearance (V(max)/K(m)) was found for CYP1A subfamily, CYP3A4 and CYP2B6 in the case of 5- sulphoxidation, and for CYP2C19, CYP1A subfamily and CYP2B6 in the case of N-demethylation. 6. In a primary culture of human hepatocytes, TCDD (a CYP1A subfamily inducer), as well as rifampicin (mainly a CYP3A4 inducer) induced the formation of promazine 5-sulphoxide and N-desmethylpromazine. 7. Regarding the relative expression of various CYPs in human liver, the obtained results indicate that CYP1A2 and CYP3A4 are the main isoforms responsible for 5-sulphoxidation, while CYP1A2 and CYP2C19 are the basic isoforms that catalyse N-demethylation of promazine in human liver. Of the other isoforms studied, CYP2C9 and CYP3A4 contribute to a lesser degree to promazine 5-sulphoxidation and N-demethylation, respectively. The role of CYP2A6, CYP2B6, CYP2D6 and CYP2E1 in the investigated metabolic pathways of promazine seems negligible.

Human term placental lipoxygenase-mediated N-demethylation of phenothiazines and insecticides in the presence of linoleic acid

This study investigated the hypothesis that human term placental lipoxygenase (HTPLO) and soybean lipoxygenase (SLO) are capable of mediating N-demethylation of selected phenothiazines and insecticides in the presence of linoleic acid (LA). In addition to being LA dependent, the N-demethylation reaction mediated by HTPLO and SLO was limited by incubation time, pH of the medium, concentration of the enzyme and the substrate. Using Nash reagent to monitor formaldehyde production, the specific activity for LA-dependent N-demethylation of chlorpromazine, a model phenothiazine, was determined to be 1.7+/-0.3 nmoles/min/mg HTPLO. Besides chlorpromazine, N-demethylation of promazine, promethazine and trimeprazine was also observed. The insecticide, aminocarb, displayed a specific activity of 2.2+/-0.3 nmoles/min/mg HTPLO for N-demethylation. Other insecticides, namely chlordimeform, dicrotophos and zectran, were oxidized in a similar manner. As compared with HTPLO, the rates of N-demethylation of phenothiazines and insecticides mediated by SLO were higher. Classical inhibitors of lipoxygenase, as well as antioxidants and free radical scavengers, caused a dose-dependent reduction in the production of formaldehyde from chlorpromazine and aminocarb by HTPLO. These results clearly demonstrate the ability of polyunsaturated free fatty acids to support N-demethylation of xenobiotics via the lipoxygenase pathway.