Inhibition Of Hepatic Cytochrome P450 Research Articles

Eriodictyol, Not Its Glucuronide Metabolites, Attenuates Acetaminophen-Induced Hepatotoxicity.

Acetaminophen (APAP) is one of the most commonly used oral analgesics and antipyretics, but hepatotoxicity including liver failure may occur after overdose. The therapeutic options for treating APAP hepatotoxicity are limited. Eriodictyol, a dietary flavonoid with anti-inflammatory and antioxidant properties, was used here to determine its protective effects against APAP-induced hepatotoxicity in mice. Various administration routes and pharmacokinetics-pharmacodynamics (PK-PD) analyses were used to determine these effects. Protective effects were observed in intravenously and intraperitoneally but not in intragastrically administered eriodictyol. LC-MS/MS analysis revealed two monoglucuronide metabolites of eriodictyol in liver and intestine microsomes. Recombinant human uridine-5'-diphospho -glucuronosyltransferase (UGT) isoforms and chemical inhibition studies demonstrated that UGT1As (mainly UGT1A1, UGT1A9, UGT1A10) and UGT2B7 were likely the main contributors to eriodictyol glucuronidation. Intragastric administration of eriodictyol, which displayed lower parent and higher metabolite concentrations in the plasma, did not elicit protective effects against APAP hepatotoxicity, when compared to the intraperitoneal injection of eriodictyol. The relative bioavailability of eriodictyol was increased to 216.84% with the coadministration of glycyrrhetinic acid (GA), an inhibitor of UGT1As. Intragastric administration of eriodictyol in combination with GA also induced protective effects against APAP hepatotoxicity. Furthermore, intragastric administration of eriodictyol attenuated APAP hepatotoxicity in heterozygous Ugt1 (Ugt1+/-) mice but not in its wild-type littermates. Thus, UGT1A-mediated metabolic inactivation reduced the protective effect of eriodictyol. Eriodictyol attenuated APAP hepatotoxicity via inhibition of hepatic cytochrome P450 (cyp) 2e1 and cyp3a11 activities; reserve of glutathione (GSH) by improvement of glutathione peroxidase (GSH-Px), glutathione reductase (GR), and glutathione S-transferase (GST) activities; elevation of superoxide dismutase (SOD) activity; and reduction of malondialdehyde (MDA) level. Our findings indicate that parenterally administered eriodictyol may be used to treat APAP-induced hepatotoxicity, and its efficacy can be enhanced by UGT1As down-regulation.

Assessment of inhibition of porcine hepatic cytochrome P450 enzymes by 48 commercial drugs

Drug interactions due to inhibition of hepatic cytochrome P450 (CYP450) enzymes are not well understood in veterinary medicine. Forty-eight commercial porcine medicines were selected to evaluate their potential inhibition on porcine hepatic CYP450 enzymes at their commercial doses and administration routes. Those drugs were first assessed through a single point inhibitory assay at 3 µM in porcine liver microsomes for six specific CYP450 metabolisms (phenacetin o-deethylation, coumarin 7-hydroxylation, tolbutamide 4-hydroxylation, bufuralol 1-hydroxylation, chlorozoxazone 6-hydroxylation and midazolam 1'-hydroxylation). When the inhibition was > 10% in the single point inhibitory assay, IC50 values (inhibitory concentrations that decrease biotransformation of selected substrate by 50%) were determined. Overall, 17 drugs showed in vitro inhibition on one or more porcine hepatic CYP450 metabolisms with different IC50 values. The potential in vivo porcine hepatic CYP450 inhibition by those drugs was assessed by combining the in vitro data and in vivo Cmax (maximum plasma concentrations from pharmacokinetic studies of the porcine medicines at their commercial doses and administration routes). Three drugs showed high potential inhibition to one or two porcine hepatic CYP450 isoforms at their commercial doses and administration routes, while seven drugs had medium risk and seven had low risk of such in vivo inhibition. These data are useful to prevent potential drug interactions in veterinary medical practice.

Critique of trials in allergic bronchopulmonary aspergillosis and fungal allergy.

Treatment of allergic bronchopulmonary aspergillosis (ABPA) emerged from uncontrolled published series of patients on prolonged oral glucocorticosteroids. Oral steroids remain the mainstay of treatment, but dose regime and duration have never been standardized. Oral steroids are problematic due to frequency of relapse, lack of steroid action on airway fungal burden and toxicities. Prospective randomized double-blind placebo-controlled trials of inhaled corticosteroids (n=1) or oral itraconazole (n=2) in patients with asthma and ABPA have shown efficacy for the latter drug only. Itraconazole is an effective steroid-sparing agent with anti-inflammatory aspects. Use of inhaled budesonide with itraconazole can lead to adrenal suppression due to inhibition of hepatic cytochrome P450. Despite combined use of oral steroids and itraconazole in ABPA, relapses and steroid dependence or toxicity have led to examination of alternative agents in uncontrolled case reports and series. Reports of nebulized amphotericin B in several patients with cystic fibrosis (CF) and ABPA suggest a potential benefit. Voriconazole has also been used with some success but also some toxicity. Both itraconazole and voriconazole have also been used in some CF-ABPA patients as monotherapy with mixed results. A recent report described treatment of refractory CF-ABPA with monthly high-dose intravenous (IV) methylprednisolone, also with mixed results. No controlled trials of voriconazole, inhaled amphotericin, or IV pulse steroids have been published. Future studies should focus on controlled trials of antifungal and immunomodulatory agents since conventional steroid therapy remains problematic.

Pyrimidinylimidazole inhibitors of CSBP/P38 kinase demonstrating decreased inhibition of hepatic cytochrome P450 enzymes

Pyrimidine analogs of the pyridinylimidazole class of CSBP/p38 kinase inhibitors were prepared in an effort to reduce the potent inhibition of hepatic cytochrome P450 observed for the pyridinyl compounds. The substitution of pyrimidin-4-yl, 2-methoxypyrimidin-4-yl, or 2-methylaminopyrimidin-4-yl for pyridin-4-yl effectively dissociates CSBP/p38 kinase from P450 inhibition for this series and furthermore achieves an increase in oral activity.

In vitro inhibition of hepatic cytochrome P450 and enzyme activity by butyltin compounds in marine mammals

The present study attempted to examine the in-vitro inhibition of hepatic microsomal P450 content and activity by butyltins in marine mammals and discussed on their possible effects in animals in the wild. Decreases in P450 content and the activities of ethoxyresorufin O-deethylase (EROD, catalyzed by CYPIA subfamily) and penthoxyresorufin O-depentylase (PROD, catalyzed by CYP2B subfamily) by tributyltin (TBT) were observed in in-vitro experiments using hepatic microsomes of a pinniped and a cetacean. Among P450 family, EROD activity is more sensitive to TBT than P450 content and PROD activity, indicating a specific mode of action of TBT on different P450 forms. On the other hand, dibutyltin and monobutyltin have no inhibitory effect on EROD activity at concentrations less than 0.5 m m, indicating that the inhibition of enzyme activity in hepatic microsome of marine mammal is mainly by TBT. TBT concentrations that affect P450 contents and activities are above 10 times higher than the values found in the liver of various marine mammals.

Inhibition and metabolite complexation of rat hepatic microsomal cytochrome p450 by tricyclic antidepressants

Administration of imipramine (IMIP) and other tricyclic antidepressants to humans and experimental animals has been associated with inhibition of hepatic cytochrome P450 (P450)-mediated drug oxidation. This study investigated the capacity of several structurally related tricyclic antidepressants to inhibit musomal P450 activity in vitro. It was found that IMIP, desipramine (DES), amitriptyline (AMIT) and nortriptyline (NOR) were poor inhibitors of P450 activity unless they were preincubated with musomes and NADPH prior to transfer to flasks containing substrate. Thus, subsequent experiments characterized the time-dependent intensification of inhibition produced by the drugs. Preincubation of the N-methylaminoalkyi agents DES and NOR (200 μM) with NADPH-supplemented musomes for 30 min led to an approximate 30% decrease in spectrally apparent P450 content; the N, N-dimethylaminoalkyl drugs IMIP and AMIT did not significantly decrease apparent P450 content. Analysis of optical difference spectra of musomes during NADPH-mediated metabolism of these drugs revealed a prominent increase in absorbance at 454 nm with DES and NOR but not IMIP or AMIT. Monospecific antibodies to the male-specific P450 2C11 and, to a lesser extent, P450 3A2 were effective in preventing the formation of the DES metabolite 454 nm-Soret peak. In addition, the 454 nm absorbance was not produced by the incubation of DES with NADPH-fortified hepatic musomes from adult female or immature male rats. Studies with the steroid substrate testosterone, which undergoes P450-specific positional hydroxylation, indicated that P450 2C11-mediated 2α- and 16α-hydroxylation were most susceptible to the time-dependent intensification of inhibition produced by DES (8.5 and 7.0 min preincubation required for loss of 50% activity, respectively) and NOR (4.0 and 4.0 min for loss of 50% of both activities). The 6β- (P450 3A2) and 7α-hydroxylase (P450 2A1) pathways were somewhat less susceptible to inhibition than 2α- and 16α-hydroxylation. These findings suggest that DES and NOR form a metabolite intermediate (MI)-complex, characterized by a Soret region absorbance maximum near 454 nm in the optical difference spectrum, with musomal P450 in male rat liver in vitro. Studies with the steroid substrate testosterone as well as immunoinhibition experiments are consistent with the proposition that this MI complex forms principally with the male-specific enzymes P450 2C11 and 3A2. Although a human orthologue of P450 2C11 has not yet been identified, P450s of the 3A subfamily are quantitatively important enzymes in human liver. MI complexation of such enzymes could be a feasible underlying mechanism for certain clinically important drug interactions involving tricyclic antidepressants.

Protective effect of glutathione on the in vitro inhibition of hepatic cytochrome P-450 by captan.

The in vitro effect of various concentrations of captan on hepatic microsomal cytochrome P-450 from pehnobarbital-pretreated rats was studied. The I-50 value, namely the concentration of the inhibitor necessary to produce 50% loss of cytochrome P-450 was determined from theplotted inhibition curve. The presence of ethylenediaminetetraacetic acid (EDTA) in microsomal incubations prior to the addition of captan failed to prevent the loss of cytochrome P-450 by captan. In contrast, reduced glutathione (0.5 mM) added to microsomal incubations before captan (0.1 mM) afforded almost complete protection of cytochrome P-450 from captan inhibition. These data indicate that the inhibitory effect of captan on vitally important drug-metabolizing enzyme system, of which cytochrome P-450 is a major component, can be prevented by prior presence of reduced glutathione (GSH) but not of EDTA.