Effect of exposure of rabbit hepatocytes to sulfur-containing anthelmintics (oxfendazole and fenbendazole) on cytochrome P4501A1 expression

A pharmacology-based comparison of the activity of albendazole and flubendazole against Echinococcus granulosus metacestode in sheep

Cyst echinococcosis (CE) is a zoonotic disease caused by the larval stage of the Echinococcus granulosus helminth parasite. The work reported here aimed to compare the efficacy of albendazole (ABZ) and flubendazole (FLBZ) against CE in naturally infected sheep. Additionally, their comparative pharmacokinetic behaviour and the assessment of serum liver enzymes activities were studied. Twelve (12) naturally infected sheep were allocated to the following experimental groups: unmedicated control group, FLBZ-treated and ABZ-treated. Treatments were orally performed every 48 h, over 55 days at dose rate of 10 (FLBZ) and 8.5 (ABZ) mg/kg (equimolar dose rates). The efficacy of the drug treatments was based on protoscoleces' vitality/viability. The kinetic disposition assessment included the Initial and Final Kinetic Studies which implicated the collection of blood samples after both the first and the last drug administration. Blood samples were processed to measure drug concentrations by HPLC. The protoscoleces' vitality observed in the untreated control group (98%) was significantly reduced in the presence of both ABZ and FLBZ. 90% of mice inoculated with protoscoleces in the control group developed hydatid cysts in their peritoneal cavity (viability study). However, only 25% (FLBZ) and 33% (ABZ) of mice inoculated with protoscoleces recovered from treated sheep, developed hydatid cysts in their abdominal cavity. Reduced FLBZ (R-FLBZ) was the main metabolite recovered in the bloodstream after oral administration of FLBZ to sheep. Low plasma concentrations of FLBZ parent drug were measured up to 48 h post-administration. ABZ was not detected in plasma at any time post-treatment, being its metabolites ABZ sulphoxide (ABZSO) and ABZ sulphone (ABZSO₂) recovered in plasma. Hepatotoxicity due to the continued treatment with either ABZ or FLBZ was not observed. A 3-fold increase ethoxyresorufin O-deethylase activity, a cytochrome P450 1A (CYP1A)-dependent enzyme reaction, was observed in liver microsomes obtained from sheep receiving ABZ, compared to those of the unmedicated and FLBZ-treated animals. In conclusion, FLBZ is an available anthelmintic which may be developed into an effective and safe drug for the human CE treatment. Despite the low plasma concentrations measured by FLBZ/R-FLBZ, an important reduction in protoscoleces' vitality was observed in cysts located in sheep liver. Modern pharmaceutical technology may help to greatly improve FLBZ systemic exposure improving its efficacy against CE.

Liver tumor promoting effect of omeprazole in rats and its possible mechanism of action

Omeprazole (OPZ), a proton pump inhibitor, is a cytochrome P450 (CYP) 1A1/2 inducer. Some CYP1A inducers are known to have liver tumor promoting effects in rats and the ability to enhance oxidative stress. In this study, we performed a two-stage liver carcinogenesis bioassay in rats to examine the tumor promoting effect of OPZ (Experiment 1) and to clarify a possible mechanism of action (Experiment 2). In Experiment 1, male F344 rats were subjected to a two-third partial hepatectomy, and treated with 0, 138 or 276 mg/kg OPZ by oral gavage once a day for six weeks after an intraperitoneal injection of N-diethylnitrosamine (DEN). Liver weights significantly increased in the DEN+OPZ groups, and the number and area of glutathione S-transferase placental form (GST-P) positive foci significantly increased in the DEN+276 mg/kg OPZ group. In Experiment 2, the same experiment as Experiment 1 was performed, but the dosage of OPZ was 0 or 276 mg/kg. The number and area of GST-P positive foci as well as liver weights significantly increased in the DEN+276 mg/kg OPZ group. The number of proliferative cell nuclear antigen (PCNA)-positive cells also significantly increased in the same group. Real-time RT-PCR showed that the expression of AhR battery genes including Cyp1a1, Cyp1a2, Ugt1a6 and Nqo1, and Nrf2 battery genes including Gpx2, Yc2, Akr7a3, Aldh1a1 Me1 and Ggt1 were significantly upregulated in this group. However, the production of microsomal reactive oxygen species (ROS) and formation of thiobarbituric acid-reactive substances (TBARS) decreased, and 8-hydroxydeoxyguanosine (8-OHdG) content remained unchanged in this group. These results indicate that OPZ, CYP1A inducer, is a liver tumor promoter in rats, but oxidative stress is not involved in the liver tumor promoting effect of OPZ.

The effects of flubendazole and mebendazole on cytochromes P4501A in pheasant hepatocytes

Many benzimidazoles are known inducers of cytochromes P4501A (CYP1A) in laboratory animals and cell lines. As flubendazole and mebendazole are benzimidazole anthelmintics often used in a pheasant, in the present study an effect of these drugs in primary cultures of pheasant (Phasianus colchicus) hepatocytes was investigated. After 48 h incubation of the hepatocytes with the benzimidazoles (0.2-5 microM), CYP1A activities -- ethoxyresorufin O-deethylation (EROD) and methoxyresorufin O-demethylation (MROD) activities were measured and the CYP1A protein levels were determined by Western blotting. None of the tested benzimidazoles influenced the CYP1A protein content. No pharmacologically significant enhancement of CYP1A after exposure of the hepatocytes to flubendazole and mebendazole was found. Inhibition of the EROD/MROD activities caused by both tested substances was observed only at the highest concentration (5 microM). From a point of view of CYP1A induction or inhibition, the treatment of pheasants by both anthelmintics tested seems to be safe. Our study demonstrates the inter-species differences in CYP1A inducibility and the importance of induction/inhibition studies on target animals.

Modulation of porcine biotransformation enzymes by anthelmintic therapy with fenbendazole and flubendazole

Fenbendazole (FEN) and flubendazole (FLU) are benzimidazole anthelmintics often used in pig management for the control of nematodoses. The in vivo study presented here was designed to test the influence of FLU and FEN on cytochrome P4501A and other cytochrome P450 (CYP) isoforms, UDP-glucuronosyl transferase and several carbonyl reducing enzymes. The results indicated that FEN (in a single therapeutic dose as well as in repeated therapeutic doses) caused significant induction of pig CYP1A, while FLU did not show an inductive effect towards this isoform. Some of the other hepatic and intestinal biotransformation enzymes that were assayed were moderately influenced by FEN or FLU. Strong CYP1A induction following FEN therapy in pigs may negatively affect the efficacy and pharmacokinetics of FEN itself or other simultaneously or consecutively administered drugs. From the perspective of biotransformation enzyme modulation, FLU would appear to be a more convenient anthelmintic therapy of pigs than FEN.

Effect of thiabendazole on some rat hepatic xenobiotic metabolising enzymes

The effect of thiabendazole (TB) on some rat hepatic xenobiotic metabolising enzymes has been investigated. Male Sprague-Dawley rats were fed control diet or diets containing 102-5188 ppm TB for 28 days. As a positive control for induction of hepatic xenobiotic metabolism, rats were also fed diets containing 1457 and 10,155 ppm butylated hydroxytoluene (BHT). Treatment with TB and BHT resulted in dose-dependent increases in relative liver weight. TB was found to be a mixed inducer of cytochrome P450 (CYP) forms in the CYP1A and CYP2B subfamilies. The administration of high doses of TB resulted in the induction of 7-ethoxyresorufin O-deethylase and 7-pentoxyresorufin O-depentylase activities, CYP1A1, CYP1A2, CYP2B1 and CYP2B1/2 mRNA levels and CYP1A2 and CYP2B1/2 apoprotein levels. In contrast, BHT was a CYP2B form inducer, increasing 7-pentoxyresorufin O-depentylase activity, CYP2B1 and CYP2B1/2 mRNA levels and CYP2B1/2 apoprotein levels. Both TB and BHT induced GSH S-transferase activities towards a range of substrates. In addition, TB and BHT markedly induced GSTP1 mRNA levels, but had only a small effect on GSTT1 mRNA levels. In summary, these results demonstrate that TB induces both phase I and II xenobiotic metabolising enzymes in rat liver.

The effects of fenbendazole, flubendazole and mebendazole on activities of hepatic cytochromes P450 in pig

Fenbendazole (FBZ), flubendazole (FLBZ) and mebendazole (MBZ) are benzimidazole anthelmintics widely used in veterinary medicine. The effects of these drugs on cytochromes P450 (CYP) were investigated in primary cultures of swine (Sus scrofa f. domestica) hepatocytes. After 48-h incubation of hepatocytes with benzimidazoles (0.1-2.5 microm), ethoxyresorufin O-deethylation (EROD), benzoxyresorufin O-dearylation (BROD), testosterone hydroxylase (6beta-TOH) and testosterone oxidase (17-TO) activities were measured and the CYP1A and 3A protein levels were determined by Western blotting. FBZ produced a significant, concentration-dependent increase of CYP1A activity (EROD) and protein level. No enhancement of CYP1A was observed after exposure to FLBZ and MBZ. All benzimidazoles tested did not cause any induction of CYP3A (BROD, 6beta-TOH, 17-TO activities and protein content). On the other hand, MBZ produced a significant, concentration-dependent decrease of CYP3A (BROD, 6beta-TOH and 17-TO) activities. Pharmacological and toxicological consequences of CYP1A induction and CYP3A inhibition should be taken into account in treatment of pigs with FBZ and MBZ.

Benzimidazole drugs and modulation of biotransformation enzymes

Benzimidazole drugs (e.g., anthelmintics albendazole, fenbendazole, oxfenbendazole, thiabendazole, mebendazole; inhibitors of proton pump omeprazole, lansoprasole, pantoprasole) represent substances used in both human and veterinary medicine; however, from the point of view of induction and inhibition of biotransformation enzymes, research has been carried out mainly due to the initiative of human pharmacologists. The purpose of the present review is to inform about inductive and inhibitive effects of benzimidazole drugs in man, animals and cell cultures. Pharmacological and toxicological consequences of modulation of biotransformation enzymes are discussed and the significance of studies in the field of modulation of biotransformation enzymes in food-producing animals is explained. Since the modulating effect of benzimidazoles strongly varies depending on structure of the individual substances, the particular attention is paid to structure-modulation relationships.

Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals

The induction of expression of genes for xenobiotic metabolizing enzymes in response to chemical insult is an adaptive response found in most organisms. In vertebrates, the AhR is one of several chemical/ligand-dependent intracellular receptors that can stimulate gene transcription in response to xenobiotics. The ability of the AhR to bind and be activated by a range of structurally divergent chemicals suggests that the AhR contains a rather promiscuous ligand binding site. In addition to synthetic and environmental chemicals, numerous naturally occurring dietary and endogenous AhR ligands have also been identified. In this review, we describe evidence for the structural promiscuity of AhR ligand binding and discuss the current state of knowledge with regards to the activation of the AhR signaling pathway by naturally occurring exogenous and endogenous ligands.

The effects of benzimidazole anthelmintics on P4501A in rat hepatocytes and HepG2 cells

Benzimidazole anthelmintics including albendazole, fenbendazole, and mebendazole are widely used in veterinary medicine. The effects of these benzimidazoles on cytochrome P4501A were investigated in primary cultures of rat hepatocytes and in the HepG2 cell line. After incubation of rat hepatocytes and HepG2 for 24-, 48-, and 72-h cells with drugs at various concentrations (0.1-50 microM), the enzyme activities associated with P4501A1/2 (7-ethoxyresorufin O-deethylation and 7-methoxyresorufin O-demethylation) were measured. The P4501A1/2 protein levels in both model systems were determined by Western blotting. Although all benzimidazoles provoked a significant increase of P4501A1/2 protein levels and P4501A activities, large differences in the induction response were found which was dependent on drug structure, concentration, and model system used. Based on the results, relationships between induction potency and structure of drug were demonstrated, as well as differences between the in vitro systems used. Therefore, pharmacological and toxicological consequences of cytochrome P4501A induction by benzimidazole drugs should be taken into account in veterinary therapy.

Time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes

In the present study, time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes, a species of economic importance in Mediterranean countries, were investigated. Cross-bred rabbits were anesthetised and their livers perfused in situ by a two-step collagenase technique; cells suspensions were filtered, seeded in collagen-coated dishes and cultivated at 37 degrees C in a controlled atmosphere for 24 and 72 h. Cytochrome P450 and b(5) contents as well as the catalytic activity of some P450-dependent monooxygenases were measured in subcellular fractions obtained by differential ultracentrifugation; microsomal proteins were also subjected to immunoblotting, using antibodies to rat P4501A, 2B, 2E1 and 3A isoforms. The activity of some microsomal hydrolytic enzymes was also determined. As regards conjugative enzymes, glutathione content and activities of glutathione S-transferase, uridindiphosphoglucuronosyl-transferase, acetyl-transferase and 1,2-epoxibuthane glutathione transferase were assayed. An overall reduction of the catalytic activity was observed 72 h after plating, reaching in certain instances the level of statistical significance. On the whole, our data confirm those previously reported with hepatocytes obtained from other species; however, the evidence that DMEs were still measurable after 72 h supports the usefulness of this in vitro method for drug metabolism studies in the rabbit as well.

Effects of prochloraz and nonylphenol diethoxylate on hepatic biotransformation enzymes in trout: a comparative in vitro/in vivo-assessment using cultured hepatocytes

The suitability of cultured rainbow trout hepatocytes as a model system for the assessment of xenobiotic effects on hepatic biotransformation enzymes in fish was examined. Two model water pollutants, the imidazole fungicide prochloraz and the alkylphenolic compound nonylphenol diethoxylate (NP2EO), were investigated in a comparative in vitro/in vivo approach. Biotransformation enzymes were measured in cultured rainbow trout hepatocytes following exposure to xenobiotics in vitro, or in the liver of juvenile rainbow trout (Oncorhynchus mykiss) exposed in vivo. The patterns of biochemical responses to the model pollutants were generally similar between in vitro and in vivo investigations. Levels of cytochrome P4501A (CYP1A) protein and the catalytic activity of the CYP1A-dependent enzyme 7-ethoxyresorufin-O-deethylase (EROD) were induced in vitro after 24 h of exposure to 1.0 microM prochloraz. In vitro, higher prochloraz concentrations induced only the levels of CYP1A above control levels, but not EROD activity. In vivo exposure of juvenile trout to 0.27 microM prochloraz resulted in an induction of CYP1A and EROD after 7 and 14 days, while 0.027 microM prochloraz had no effects. In vitro, the 6beta- and 16beta-hydroxylation of testosterone was significantly decreased by 1.0-3.0 microM prochloraz, while in vivo these variables were significantly inhibited after exposure to 0.27 microM prochloraz for 7 and 14 days. NP2EO did not affect EROD activity in vitro. In vivo, EROD activity and CYP1A remained unchanged following 7 days of exposure to 0.32 or 1.30 microM NP2EO. NP2EO (15-50 microM) inhibited the 16beta-hydroxylation and glucuronidation of testosterone in vitro. In vivo, 7 days of exposure to 0.32 or 1.30 microM NP2EO resulted in a significant inhibition of the 6beta- and 16beta-hydroxylation of testosterone. The good qualitative correspondence between in vitro and in vivo results indicates that studies using trout hepatocytes allow the identification of biochemical targets of xenobiotic effects in fish liver. However, more research is needed before quantitative predictions, e.g. of effective concentrations, can be made from in vitro investigations.

Structural and mechanistic aspects of transcriptional induction of cytochrome P450 1A1 by benzimidazole derivatives in rat hepatoma H4IIE cells

The effect of several structurally different benzimidazole compounds on CYP1A1 expression at the transcriptional, mRNA and protein levels was investigated in the rat hepatoma H4IIE cell line. Omeprazole, thiabendazole, carbendazim, 2-mercaptobenzimidazole and 2-mercapto-5-methoxybenzimidazole caused a dose-dependent increase in CYP1A1 protein levels that reached maximum effect at 250 microm, as measured by Western blot. In addition, hydroxyomeprazole, 2-aminobenzimidazole and 2-mercapto-5-nitro-benzimidazole caused a notable increase in CYP1A1 protein expression, whereas 5-O-desmethylomeprazole, 2-hydroxybenzimidazole, 2-benzimidazole propionic acid and 5-benzimidazole carboxylic acid were ineffective. Thus, benzimidazole substituted with a thiol or an amino group in the 2-position were active inducers. Northern blot analysis confirmed an extensive increase of CYP1A1 mRNA induced by omeprazole and 2-mercapto-5-methoxybenzimidazole which was 32% and 49% of maximal induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) respectively, whereas thiabendazole and carbendazim showed approximately 15% increase as compared to TCDD. Transient transfection of H4IIE cells, with a XRE-pGL3 reporter gene construct revealed a 2.3-4.3-fold induction by carbendazim, thiabendazole, and 2-mercapto-5-methoxybenzimidazole as compared to a 3.3- and 23-fold induction by omeprazole and TCDD, respectively. Thus, these data indicate that the benzimidazoles utilize the aryl hydrocarbon receptor-arnt-XRE-mediated signal-transduction pathway for induction of the CYP1A1 gene.

Cytochrome 1A1 induction by primaquine in human hepatocytes and HepG2 cells: absence of binding to the aryl hydrocarbon receptor

Malaria remains the most prevalent infectious disease of tropical and subtropical areas of the world. It represents a crucial problem in public health care, affecting 750 million people annually, of whom at least two million die. Various antimalarials currently used were studied for their capability to induce expression of the cytochrome P450 1A1 (CYP1A1) gene, an enzyme that plays an important role in the activation of xenobiotics to genotoxic derivatives. Studies on human hepatocytes and HepG2 cell lines showed that primaquine was capable of dose dependently increasing both the ethoxyresorufin-O-deethylase activity and CYP1A1 mRNAs, suggesting a transcriptional activation of this gene. Moreover, alpha-naphthoflavone, a partial aryl hydrocarbon receptor (AhR) antagonist, and 8-methoxypsoralen, which interferes with the binding of activated AhR to the xenobiotic responsive element, were shown to suppress CYP1A1 induction when added to the cultures. However, neither primaquine nor its metabolites were able to displace [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin from AhR in competitive binding studies using 9S-enriched fractions of human cytosol. These data, together with the induction of CYP1A1 promoter-directed chloramphenicol acetyl transferase gene expression, suggest that CYP1A1 induction involves the participation of the AhR but not a direct primaquine-receptor interaction. This supports the notion that an alternative ligand-independent mechanism has to be considered. Given the pharmaco-toxicological significance of CYP1A1 induction, these findings may have important implications in the treatment of malaria with primaquine and new analogs.

Carbaryl, a carbamate insecticide, is a ligand for the hepatic Ah (dioxin) receptor

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates many of the biological and toxicological actions of a variety of hydrophobic natural and synthetic chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin). Induction of CYP1A1 gene expression is one such response that is known to be regulated by the AhR complex. It was recently reported (Ledirac et al., Toxicol. Appl. Pharmacol. 144, 177-182, 1997) that, although carbaryl, a carbamate insecticide, can induce AhR-dependent expression of CYP1A1, it was not an AhR ligand. Since this apparent ligand-independent activation of the AhR is difficult to reconcile given what is known about the mechanism of AhR action, we have examined the ability of carbaryl to stimulate the AhR signaling pathway. Not only was dioxin responsive element-driven luciferase gene expression induced by carbaryl in stably transfected mouse, rat, guinea pig, and human cells, gel retardation analysis revealed that carbaryl stimulated AhR transformation and DNA binding in vitro and in cells in culture. Dose-response experiments revealed that carbaryl was 300,000-fold less potent that the prototypical inducer, TCDD, in both inducing luciferase gene expression and stimulating AhR transformation and DNA binding in vitro, suggesting that carbaryl itself was the inducing agent. The identification of carbaryl as an AhR ligand was demonstrated by its ability to competitively inhibit [3H]-TCDD to the guinea pig hepatic cytosolic AhR. Our results confirm that carbaryl is both a weak AhR ligand and inducer of AhR-dependent gene expression and argue against its proposed ligand-independent mechanism of AhR activation.

Ah receptor-dependent CYP1A induction by two carotenoids, canthaxanthin and beta-apo-8'-carotenal, with no affinity for the TCDD binding site

The assays of several phase I and phase II xenobiotic-metabolizing enzyme activities, as well as CYP1A immunoblot analysis, were performed in liver microsomes and cytosol of male C57BL/6 mice (Ah receptor-responsive), of male DBA/2 mice (Ah receptor-low responsive) and of female Ah receptor gene knockout mice that were fed diets containing 300 mg/kg of a nonprovitamin A carotenoid, canthaxanthin, or a provitamin A carotenoid, beta-apo-8'-carotenal for 14 days, or which were injected i.p. with 3-methylcholanthrene. Previous studies have shown that some carotenoids, such as canthaxanthin and beta-apo-8'-carotenal, are strong inducers of liver CYP1A1 and 1A2 when given to rats. In this work, only canthaxanthin induced both CYP1A1 and 1A2 in C57BL/6 mice, whereas beta-apo-8'-carotenal induced only CYP1A2 in this strain. Neither of the two carotenoids modified CYP1A1/2 protein contents or enzyme activities in Ah receptor-low responsive DBA/2 or in Ah receptor gene knockout mice. Cytosol prepared from C57BL/6 mice liver tissue was incubated with [3H] 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence of canthaxanthin or beta-apo-8'-carotenal and analyzed by sucrose density gradient sedimentation: neither of the carotenoids, even when present in large excess, competed with TCDD for the TCDD binding site of the cytosolic Ah receptor of C57BL/6 mice. In brief, the carotenoids canthaxanthin or beta-apo-8'-carotenal induced Cyp1a genes in mice through an Ah receptor-dependent pathway, but did not bind to the Ah receptor.