Cytochrome P-450 and monooxygenase activity in hepatic microsomes from N-phenylimidazole-treated rats

Induction of cytochromes P450 2B and 2E1 in rat liver by isomeric picoline N-oxides

Pyridine derivatives are widely used solvents and precursors for the synthesis of chemicals of industrial importance. Oxidized metabolites have been implicated in the observed toxicity of pyridines and are known to induce drug-metabolizing enzymes in rat liver. In this study the three isomeric picoline (methylpyridine) N-oxides, as major oxidized metabolites of 2-, 3- and 4-picoline, were evaluated as inducers of cytochrome P450 (CYP) enzymes in rat liver. After a single dose of 100 mg/kg 24 h before sacrifice the 3- and 4-isomers were effective inducers of microsomal substrate oxidations associated with the phenobarbital-inducible CYPs 2B; upregulation of CYP2B protein was confirmed by immunoblotting. In contrast, the 2-isomer did not increase CYP2B protein or activity in rat liver but CYP2E1 protein expression was upregulated by the isomers to 160-200% of control. The three chemicals increased aniline 4-hydroxylation activity in rat liver, which is consistent with induction of CYPs 2B or 2E1 and 4-nitrophenol 2-hydroxylation activity was increased in microsomal fractions from 3- and 4-picoline N-oxide-treated rats. The activities of several other CYPs were also determined and CYP1A-dependent 7-ethylresorufin O-deethylation was increased (to approximately 6- and 2-fold of control) by the 3- and 4-isomer, respectively, whereas the activity of CYP3A-mediated androstenedione 6beta-hydroxylation was decreased by the agents--most notably by the 2-isomer. During NADPH-supported oxidation of CCl4, lipid peroxidation was increased in microsomes from 3- and 4-picoline N-oxide-pretreated rats and was modulated in vitro by the CYP2B inhibitor orphenadrine, but not by the CYP2E1 inhibitor 4-methylpyrazole. These findings establish that particular isomers of picoline N-oxide rapidly upregulate CYP2B or, to a lesser extent, CYP2E1 and implicate CYP2B in the enhanced lipid peroxidation observed in microsomes from rats treated with 3- and 4-picoline N-oxides. Such induction process may contribute to the hepatotoxicity of pyridines by enhancing the capacity for microsomal lipid peroxidation.

1-Benzylimidazole induces rat hepatic microsomal epoxide hydrolase with the elevation of its mRNA levels

The effects of 1-benzylimidazole on the expression of microsomal epoxide hydrolase (mEH) gene were examined in rat. Immunoblot analyses showed that hepatic microsomes isolated from rat treated with 1-benzylimidazole at 25, 50 or 100 mg/kg/day for 3 or 6 days exhibited 2-3-fold increases in mEH levels, relative to control microsomes. Northern and slot blot analysis revealed that 1-benzylimidazole is effective in increasing hepatic mEH mRNA levels at 24 h post-treatment (i.e. 4-5-fold). Hepatic mEH mRNA levels were elevated from 4-4- to 8-fold following 1-benzylimidazole administration in a time-dependent manner. These results demonstrated that 1-benzylimidazole induces mEH protein in rat and that the induction is primarily associated with accumulation of mEH mRNA levels.

Induction of hepatic microsomal cytochrome P450 and drug-metabolizing enzymes by 4-benzylpyridine and its structurally related compounds in rats. Dose- and sex-related differential induction of cytochrome P450 species

We examined the abilities of 4-, 3- and 2-benzylpyridine and 4-tert-butylpyridine to induce hepatic microsomal cytochrome P450 and drug-metabolizing enzymes in male and female rats in order to define the effects of pyridine-containing compounds on drug metabolism. 4-Benzylpyridine (0.4 mmol/kg, for 2 consecutive days) induced total cytochrome P450 to about three times that of the controls at 24 hr, and its inducing effect was sustained for 120 hr after the treatment in male and female rats. 4-Benzylpyridine was a more potent inducer of cytochrome P450 than 3- and 2-benzylpyridine, which induced the cytochrome to 71.4 and 43.9%, respectively, of that produced by the 4-substituted isomer. 4-tert-Butylpyridine also induced cytochrome P450. Immunoblot analysis revealed that a single treatment of male rats with 4-benzylpyridine at doses ranging from 0.05 to 0.80 mmol/kg induced cytochrome P450b/e and caused a maximum increase in the level of the isozyme at the 0.2 mmol/kg dose. 4-Benzylpyridine at doses from 0.40 to 0.80 mmol/kg also induced cytochrome P450c/d in male rats. In female rats, 4-benzylpyridine induced cytochrome P450b at doses ranging from 0.1 to 0.80 mmol/kg and produced a maximum increase in the level of this isozyme at 0.40 to 0.60 mmol/kg. Induction of cytochrome P450c/d by 4-benzylpyridine in female rats was observed at a dose of 0.20 mmol/kg, and the magnitude of the induction of the isozyme was increased in a dose-dependent manner. Both 3- and 2-benzylpyridine induced cytochrome P450b/e and/or c/d depending on the increase of total cytochrome P450 without changing the induction patterns of the isozymes. 4-tert-Butylpyridine induced cytochrome P450b at doses ranging from 0.20 to 0.60 mmol/kg and slightly induced P450c/d at doses ranging from 0.10 to 0.40 mmol/kg in male rats. These results and our previous report (Matsuura et al., Biochem Pharmacol 41: 1949-1956, 1991) clearly show that the pyridine compounds having lipophilic groups at the 4- or 3-position of the ring could be inducers of cytochrome P450. The present results also revealed that 4-benzylpyridine shows dose- and sex-related differences in the induction of cytochrome P450b/e and c/d in rats.

Structure-activity relationships in the induction of hepatic microsomal cytochrome P450 by clotrimazole and its structurally related compounds in rats

We investigated the structure-activity relationship in the induction of hepatic microsomal cytochrome P450 by clotrimazole and its structurally related compounds. For this purpose, we synthesized various compounds structurally analogous to clotrimazole and injected them into rats at a fixed dose of 0.2 mmol/kg. We found that the chlorine atom in clotrimazole was not necessary for the induction of cytochrome P450. The imidazole moiety of clotrimazole, however, was a very important structural component for the induction of cytochrome P450; triazole, but not pyrrole, could be substituted for this moiety. 1-Tritylimidazole, 1-diphenylmethylimidazole and 1-benzylimidazole were also found to be inducers of cytochrome P450, but, to a somewhat lesser extent, with a decreasing number of substituted phenyl groups. Thus, 1-benzylimidazole was a minimum structurally active unit for inducing cytochrome P450. In addition, 4-diphenylmethylpyridine and 4-benzylpyridine also induced cytochrome P450 to extents similar to those induced by the corresponding imidazole derivatives, but 4-benzylpiperidine lacked this effect. When the methylene unit of clotrimazole-related compounds was introduced by a hydroxy or amino group instead of imidazole, there was a less extensive increase in cytochrome P450 content. This inducing effect was lost completely by the lack of an imidazole moiety and imidazole itself. 1-Phenylethylimidazole and 1-benzylimidazole induced cytochrome P450 to a similar extent. All of these findings suggest that 1-substituted heteroaromatic compounds having two or more nitrogen atoms are likely to be required for inducing cytochrome P450. Immunoblot analysis revealed that clotrimazole and other various inducers found in this study increased cytochrome P450b/e content. These results could provide information on the effects of drugs and chemicals on cytochrome P450 induction.

The effects of propiconazole on hepatic xenobiotic biotransformation in the rat

Propiconazole, a foliar fungicide used for agricultural purposes was studied for its effects on the hepatic xenobiotic biotransformation in the rat. Rats were given an intraperitoneal injection of 0.1, 1, 10 or 100 mg/kg in corn oil for seven consecutive days. Induction was seen for cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, aldrin epoxidase, aminopyrine N-demethylase and microsomal expoxide hydrolase activities. Aniline p-hydroxylase and cytosolic glutathione S-transferase activities were unchanged. All responses occurred at only 100 mg/kg, except for that of aminopyrine N-demethylase which also occurred at the 10 mg/kg dose. SDS polyacrylamide gel electrophoresis showed increased staining of a protein band of molecular weight 54,000 corresponding to cytochrome P-450b and/or P-450d. Collectively these results suggest that cytochromes P-450b and P-450d have been induced after exposure of rats to propiconazole.

Induction of the rat hepatic microsomal mixed-function oxidases by 3 imidazole-containing antifungal agents: selectivity for the cytochrome P-450IIB and P-450III families of cytochromes P-450

Administration of the imidazole antifungal agents ketoconazole, miconazole and clotrimazole gave rise to increases in the microsomal cytochrome P-450 levels and the NADPH-dependent reduction of cytochrome c. Clotrimazole, and to a much lesser extent miconazole and ketoconazole, stimulated the dealkylation of pentoxyresorufin. All 3 agents gave rise to small, but significant increases in the O-deethylation of ethoxycoumarin and ethoxyresorufin. The antifungal-induced O-deethylation of ethoxycoumarin was much more sensitive to inhibition by metyrapone rather than by alpha-naphthoflavone. The binding of metyrapone to reduced microsomes was enhanced by treatment of animals with the 3 antifungal agents, clotrimazole being clearly the most potent. Immunoquantitation of cytochrome P-450 proteins using an ELISA procedure and employing anti-cytochrome P-450c (P-450IA1, P-448 low spin) and P-450b (P-450IIB1) antisera revealed that clotrimazole and miconazole, but not ketoconazole, induced the levels of phenobarbital-induced cytochromes P-450, while none of the antifungal agents increased the levels of cytochrome of P-448 proteins. Similar results were obtained using Western blots employing the above antibodies. On SDS-polyacrylamide gel electrophoresis microsomes derived from animals pretreated with clotrimazole showed intensification of a band at 51 kDa which was identified by Western blotting as the PCN-inducible form of cytochrome P-450 (cytochrome P-450p, P-450III family). Similar, but less pronounced intensification was seen with microsomes from animals pretreated with miconazole and ketoconazole. Furthermore, microsomes from clotrimazole- and ketoconazole-treated animals interacted with erythromycin to yield type I spectra. It is concluded that the imidazole-containing agents clotrimazole and miconazole, and to a much lesser extent ketoconazole, are potent inducers of the rat hepatic microsomal mixed-function oxidases, displaying selectivity towards the P-450IIB (phenobarbital-inducible) and P-450III (PCN-inducible) families of cytochrome P-450 proteins.

High magnitude hepatic cytochrome P-450 induction by an N-substituted imidazole antimycotic, clotrimazole

A 4-fold induction of hepatic microsomal cytochrome P-450 following 3 days of treatment of rats with clotrimazole (75 mg/kg), a potent monooxygenase inhibitor, greatly exceeded that evident from similar phenobarbital and dexamethasone treatment. The clotrimazole-induced microsomes exhibited a pattern of monooxygenase activities similar to that seen in microsomes from both phenobarbital- and dexamethasone-treated animals. Precautions were necessary to determine both monooxygenase activities and the full amount of cytochrome P-450 present in microsomes because of interference by residual clotrimazole in the microsomes.

Induction of hepatic oxidative and conjugative drug metabolism in the hamster by N-substituted imidazoles

Three antimycotic N-substituted imidazoles, clotrimazole, tioconazole and miconazole, were able to induce hepatic microsomal cytochrome P-450 and monooxygenase reactions in both male and female hamsters to an extent similar to that seen with phenobarbital treatment. Imidazole treatment did not alter the cytochrome P-450 concentration, and ketoconazole treatment decreased it. Cytosolic sulfo- and glutathione transferases were not significantly altered by any imidazole. Induction of microsomal morphine glucuronosyltransferase activity by each compound generally paralleled the effect on cytochrome P-450 in females but induction was not evident in males. Clotrimazole treatment, in contrast to phenobarbital treatment, also caused a large induction of l-naphthol glucuronosyltransferase in females. The potential for antimycotic imidazoles to alter the hepatotoxicity of compounds will require consideration of the inductive changes in both Phase I and Phase II drug metabolizing enzymes in addition to their known inhibitory effects on Phase I oxidations. The inductive effects differ for each imidazole, and in the hamster model, depend upon the sex of the animal.

Evidence for induction of hepatic microsomal cytochrome P-450 by cimetidine: binding and kinetic studies

The interaction of cimetidine with liver microsomes has been examined by spectral and equilibrium partition studies. First, difference spectroscopy has been used to evaluate the proportion of cytochrome P-450 in rat liver microsomes that exhibits an affinity for cimetidine in the pharmacologically relevant, low micromolar range of drug concentration. The value of 0.45 so obtained has confirmed that a substantial proportion of rat liver cytochrome P-450 has a high binding affinity for this drug. Second, a study of the binding of cimetidine to human liver microsomes by difference spectroscopy and partition equilibrium has detected a similar interaction, thus providing direct support for the postulate that the clinically observed impairment of oxidative drug metabolism may be due in part to inhibition of cytochrome P-450 monooxygenase by cimetidine. Hepatic microsomes from cimetidine-pretreated rats have been shown to exhibit elevated cytochrome P-450 specific content but a decreased proportion of sites with high affinity for the drug; this finding has been shown not to be the consequence of cimetidine-mediated, time-dependent, irreversible monooxygenase inhibition. Although cimetidine pretreatment caused enhanced specific activity of 7-ethoxyresorufin O-dealkylation, the specific activities for O-dealkylation of 7-ethoxycoumarin and 4-nitroanisole were decreased, as were those for the N-dealkylation of morphine, ethylmorphine, aminopyrine, and dimethylnitrosamine. Since cimetidine pretreatment was shown to cause no change in the Michaelis constants for oxidation of morphine or 7-ethoxyresorufin, it is argued that these results provide strong presumptive evidence for changes in the relative abundance of isoenzymes catalyzing these various oxidations. Thus, a dual role of cimetidine, acting both as inhibitor and inducer of the cytochrome P-450 system, is proposed to account for the impaired oxidative metabolism of some drugs that occurs during coadministration with this H2-receptor antagonist.