Cytochrome P-448 and the activation of toxic chemicals and carcinogens

The Safety Evaluation of Drugs and Chemicals by the Use of Computer Optimised Molecular Parametric Analysis of Chemical Toxicity (COMPACT)

The historical development of the safety evaluation of drugs and chemicals is critically reviewed, and failures of the present approach using experimental animals are examined. The roles of the cytochromes P450 in the detoxication of drugs and chemicals, and in their activation to mutagens, carcinogens and neoantigens, are described, and the importance of the selective induction of the cytochromes P450 in the manifestation of chemical toxicity/carcinogenicity is highlighted. The computer graphic procedure of COMPACT, which relates chemical structures to metabolism by individual cytochromes P450, and hence to their potential toxicity/carcinogenicity, is described, and the advantages and disadvantages of this method of safety evaluation, which does not use experimental animals, are considered.

Potential risk of acute hepatotoxicity of kodo poisoning due to exposure to cyclopiazonic acid

Kodo millet (Paspalum scrobiculatum L.) is a staple food of some sections of people of North India. Consumption of Kodo millet is often found to cause intoxication and poisoning. The grains are frequently infested with Aspergillus tamarii Kita, which produced substantial amount of a mycotoxin, cyclopiazonic acid (CPA). Investigations were carried out to evaluate the hepatotoxic/preneoplastic changes in rat liver following single and multiple dose administration of CPA. Results showed a marked increase in the activity of glutamate pyruvate transaminase (GPT) and glutamate oxaloacetate transaminase (GOT) following CPA exposures, suggesting acute hepatotoxicity. Significant increase was also observed in gamma glutamyl transpeptidase (GGT) activity following CPA exposures, indicating preneoplastic changes in the liver. The results reveal that Kodo poisoning might cause acute hepatotoxicity in men and animals. The findings thus suggest that the consumption of contaminated Kodo millet is a serious health hazard due to exposure to CPA produced by Aspergillus tamarii associated with the millet.

Effect of environmental pollutants on hepatocellular function in rats: 3-methylcholanthrene and Aroclor-1254

Environmental pollutants, Aroclor-1254 (PCB) and 3-methylcholanthrene (MC), were employed in this study to investigate some aspects of the induction of hepatic drug metabolism in rats. PCB and MC treatments increased 7-ethoxyresorufin and 7-ethoxycoumarin O-deethylase activities related to cytochrome P-448. Cytochrome P-450 reductase activity was increased by PCB while no effect was observed by MC treatment. Pretreatment with PCB resulted in approximately 50% increase in the phospholipid content of the microsomes whereas MC caused no change. Liver microsomal cholesterol content was decreased while triglycerides were increased by PCB. The ratio between saturated and unsaturated fatty acids (saturation index) decreased in the total microsomes and phospholipids with PCB treatment, whereas MC did not alter the ratio, except that the major effect of MC was observed in the acyl derivatives of microsomal phosphatidylethanolamine. It is proposed that the uniaxial rotation and mobility of hemoproteins may be restricted by an increase in the saturation index of the membrane, while a decreased index may facilitate contact with reductases for electron transfer by enhanced membrane fluidity. The decreased saturation index after treatment with MC may play a role in carcinogenicity by triggering induction of free radicals.

Liver fibrosis in guinea pigs experimentally induced by combined copper and aflatoxin application

Aflatoxin B1 alone (0.05 mg resp. 0.037 mg/kg/d), copper alone (6.6 mg/kg/d or 200 mg/l drinking water) or a combination of both was administrated orally for 6 months to young guinea pigs from the first/second day of life. In the copper group there were no pathomorphological changes. For the aflatoxin B1 group liver damage was established. In the combined group liver injury was more frequent and more severe compared to the aflatoxin B1 group. Compared with the copper group biliary copper excretion was diminished and the kidney copper content was elevated in the Afl. B1 + Cu group. While copper concentrations in bile and kidney correlated with other parameters, notably the pathological lesions of the liver, no such correlation was found for liver copper. Therefore in this experiment the degree of Cu accumulation was not decisive for the liver lesions. The livers' capacity for excreting Cu by bile seems to be a much more important factor. Histologically only the livers of the combined group exhibited degeneration, atrophy and steatosis of liver cells, and a fibrosis more or less pronounced. For childhood cirrhosis (ICC and ICT), a combined etiology--a liver damaging agent plus elevated alimentary copper--is a plausible hypothesis.

Experimental induction of liver fibrosis in young guinea pigs by combined application of copper sulphate and aflatoxin B1

Aflatoxin B1 alone (0.05 mg resp. 0.037 mg/kg/d), copper alone (6.6 mg/kg/d or 200 mg/l drinking water) or a combination of both was administered orally for 6 months to young guinea pigs from the first/second day of life. In the copper group there were no pathomorphological changes. For the aflatoxin B1 group, liver damage was established. In the combined group, liver injury was more frequent and more severe compared to the aflatoxin B1 group and biliary copper excretion was diminished compared with the copper group. Histologically, only the livers of this group exhibited degeneration, atrophy and steatosis of liver cells, inflammatory processes and a more or less prominent fibrosis. For childhood cirrhosis (ICC and ICT) a combined etiology--a liver damaging agent plus elevated alimentary copper--is a plausible hypothesis.

Vitamins as antimutagens: advantages and some possible mechanisms of antimutagenic action

Dietary natural inhibitors of mutagenesis and carcinogenesis are of particular importance because they may be useful for human cancer prevention and do not have undesirable xenobiotic effects on living organisms. As was shown in numerous experiments, many endogenous substances, usually obtained in food or synthesized by cells, possess some inhibitory activity towards natural or man-made environmental mutagens which often induce increased frequency of cancer. Among such substances are vitamins, thiol compounds, porphyrin derivatives, polyphenols and others, the antigenotoxicity of which is well established in various genetic tests. Probably a number of these compounds are included in the defense systems of organisms protecting them from harmful exogenous influences continuously affecting genetic material and other components of cells. Some vitamins show protective effects; for example, E, A and C vitamins are active against well-known mutagens both in vitro and in vivo. Genetic properties of other vitamins have been insufficiently explored, but positive results were obtained for a number of them suggesting the desirability of further studies in this field. Synergism of some vitamins activity, both with other vitamins and non-vitamin substances, is of particular interest because clarifying some of their mechanisms of action could be important for understanding the functions of our defense systems.

Effects of ethylene glycol on the ultrastructure of hepatocytes

The ultrastructure of rat hepatocytes after acute experimental ethylene glycol poisoning was examined. On the 1st, 5th and 14th days after poisoning the material from the centrolobular zone (zone III) was collected. Proliferation and enlargement of SER at the early period of poisoning and evidence of mitochondrial damage both at the early and late time after ethylene glycol intoxication were found. In the liver extensive capillary deposits surrounded by membranes were seen, filled with flocculent material of middle electron density. The results show destruction of the cytoplasmic organelles, especially mitochondria, on the 1st and 5th days after ethylene glycol intoxication, and symptoms of damage removing together with regeneration on the 15th day of the experiment.

Early hepatic changes induced by nuarimol in rats

It is commonly believed that in short-term tests hepatic cytochrome P-450b inducers stimulate liver enlargement and mitogenesis in the absence of overt hepatotoxic effects. In this investigation male Wistar rats received naurimol (an organochlorine pesticide) in one, three and five oral doses of 31.5, 62.5 and 125 mg kg-body wt. day-, whereupon the effects on liver were determined. The early effects were dose-dependent increases in p-nitroanisole metabolism, hepatocyte proliferation (DNA synthesis and mitotic activity) and liver weight. Five administrations of the lowest does (31.5 mg kg-1 body wt. day-1) did not change liver weight, despite increased p-nitroanisole metabolism and hepatocyte proliferation. In contrast to p-nitroanisole metabolism and hepatomegaly, proliferation was only transient and disappeared even when treatment continued. The increase in binuclear hepatocytes and signs of necrosis suggested that the hepatomitogenic effect of nuarimol reflected a regenerative response, which may simulate the proliferation caused by partial hepatectomy.

Hepa-1 enzyme induction assay as an in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo

The Hepa-1 enzyme induction assay (assay of the induction of CYP1A1 catalytic activities in the Hepa-1 mouse hepatoma cell line by various compounds or mixtures) was evaluated as an in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo. C57BL/6J mice were fed for three weeks four selected commercially available diets (one semisynthetic and three standard natural ingredient diets) exhibiting different enzyme-inducing effects in the Hepa-1 assay. beta-Naphthoflavone mixed in a semisynthetic diet (33 and 330 mg/kg of diet) was used as a model inducer. CYP1A1-dependent enzyme activities (aryl hydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase) were measured in the small intestinal mucosa and liver. There was good agreement between the induction of CYP1A1 in vitro and in vivo: the rank order of the enzyme activities elicited by the diets was the same in the mice as in the Hepa-1 cells. The standard diets were less effective inducers than beta-naphthoflavone in the Hepa-1 cells and in the mice, especially in the small intestinal mucosa. The Hepa-1 enzyme induction assay thus seems to be a mechanistically sound, reliable and sensitive in vitro indicator of the CYP1A1-inducing potencies of laboratory rodent diets in vivo.

The modulatory effects of tryptamine and tyramine on the S9-mediated mutagenesis of IQ and MeIQ in Salmonella strain TA98

The S9-mediated mutagenesis of IQ and MeIQ in Salmonella strain TA98 was modulated by introduction to the assay of tryptamine or tyramine. Both biogenic amines inhibited or enhanced the mutagenic response as a function of amine concentration, strain of rat used as the S9 source, and the IQ-type mutagen tested. Enhancement of IQ mutagenesis by tryptamine (10-80 microM) was observed in the presence of S9 preparations derived from Aroclor 1254-pretreated Fischer rats; the enhancing effect ceased at tryptamine concentrations > 160 microM. When Sprague-Dawley-S9 or Wistar-S9 were used for activation, the enhancement of IQ mutagenesis by tryptamine shifted to inhibition at tryptamine concentrations > 40 microM, with Sprague-Dawley-S9, and > 20 microM, with Wistar-S9. By contrast, MeIQ-mutagenesis was enhanced by tryptamine (10-160 microM), regardless of the rat strain used as S9 source. Tyramine was a weaker enhancer of MeIQ mutagenesis than was tryptamine and, unlike tryptamine, its inhibitory effects on IQ mutagenesis were observed only with Wistar-S9. Tryptamine (10-80 microM) inhibited cytochromes P450IA1 and P450IA2 activities, monitored by the O-deethylation of ethoxyresorufin and Glu-P-1 mutagenesis in TA98, respectively. These data suggest that the effects of biogenic amines on IQ and MeIQ bioactivation are complex. Furthermore, this study demonstrates that tryptamine and tyramine act both as enhancers (comutagens) and as inhibitors (antimutagens) of IQ and MeIQ mutagenesis, depending on the testing conditions.

Effect of rociverine on P450-dependent monooxygenases and its N-deethylation metabolism in rat liver microsomes

Rociverine [2-(diethylamino)-1-methylethyl cis-1-hydroxy [bicyclohexyl]-2-carboxylate] citrate (ROC) is an antispasmodic agent therapeutically active in humans at doses of 0.5-1 mg/kg. This study investigated the effect of acute administration of the drug on hepatic microsomal cytochrome P450 (P450)-catalysed drug metabolism. Only high doses (> or = 100 mg/kg) of ROC were able to induce in rats the hepatic microsomal pentoxyresorufin O-depenthylase (PROD) and 16 beta-testosterone hydroxylase activities both associated with P4502B1/2 and the erythromycin N-dimethylase (ErD) and 2 beta-testosterone hydroxylase activities both dependent on P4503A1/2. However, at 100 and 200 mg/kg of ROC, the 16 beta-testosterone hydroxylase and PROD were the most induced activities, suggesting that P4502B1/2 are the isoforms most sensitive to ROC induction. Accordingly, ROC treatment enhanced, in a dose-dependent manner, the amount of P4502B1/2 and 3A1/2 in microsomes as assayed by western blotting. The northern blot analysis of ROC-treated rat liver showed that the P4502B1/2 induction appears to be regulated at the mRNA level as in the induction by phenobarbital (PB). The oxidative metabolism of ROC with hepatic microsomes from control or PB- and ROC-induced rats resulted in a N-deethyl ROC derivative (major metabolite) and an unknown minor ROC derivative. The kinetic parameters for the N-deethylation of ROC were studied with purified P4502B1 and with microsomes from control or rats treated with various inducers (phenobarbital, ethanol, beta-naphthoflavone, dexamethasone and rociverine). It was found that phenobarbital-, dexamethasone- and rociverine-induced microsomes deethylated ROC with a Vmax about five times higher than that (0.9 nmol/min/mg protein) of control microsomes, although with a similar affinity (Km approximately 0.3 mM). In a reconstituted system, the purified P4502B1 metabolized ROC with a high deethylation rate (22 nmol/min/nmol P450). Moreover, the ROC deethylation was inhibited by compounds such as hexobarbital, metyrapone and triacetyloleandomicin, selective inhibitors for P4502B and/or P4503A enzymes. On the other hand ROC, when added in vitro, inhibited the 16 beta- and 2 beta-testosterone hydroxylases and the PROD and ErD activities. Taken together, these results indicate that the ROC-inducible P4502B and P4503A are involved in ROC deethylation. In conclusion, it has been demonstrated that ROC is a weak phenobarbital-like inducer of P450, probably able at high and reiterated doses to alter its own metabolism, at least in the rat liver.

Interlaboratory comparison of microsomal ethoxyresorufin and pentoxyresorufin O-dealkylation determinations: standardization of assay conditions

Assay conditions and results of cytochrome P-450 dependent 7-ethoxyresorufin (ER) and 7-pentoxyresorufin (PR) O-dealkylation (OD) by rat liver microsomes were compared by four laboratories in the Netherlands. Microsomal mixtures were prepared from control, 3-methylcholanthrene and phenobarbital pretreated animals, resulting in different levels of cytochrome P-450 isozymes. EROD and PROD activities were determined in each laboratory according to their own protocols. Considerable variability was found both between and within laboratories. Further studies demonstrated that protocol differences are important factors causing this interlaboratory variation. Main factors of influence were buffer type, batch of resorufin used for calibration, substrate solvent and substrate concentration. Based on the results obtained, standardized protocols for optimized measurement of microsomal EROD and PROD activities were developed. Additional experiments demonstrated that the use of these standardized protocols reduced intralaboratory variation in both the EROD and the PROD assay, whereas it also reduced the interlaboratory variability for the PROD determinations. The interlaboratory variation for measurement of microsomal EROD activities was only reduced for the laboratories using a Cobas-Bio analyzer. The results of the present study demonstrate clearly that data obtained with EROD and PROD activity measurements are highly sensitive to factors frequently varying from one laboratory to another. In addition, they demonstrate the necessity to be careful with absolute values presented in the literature for these activities, unless well characterized assay conditions are applied.

Influence of beta-myrcene on sister-chromatid exchanges induced by mutagens in V79 and HTC cells

The influence of beta-myrcene (MC) on sister-chromatid exchanges (SCE) in V79 cells induced by 4 S9 mix-activated indirect mutagens was studied. The mutagens used were cyclophosphamide (CP), benzo[a]pyrene (BP), aflatoxin B1 (AFB) and 9,10-dimethyl-1,2-benz[a]anthracene (DMBA). MC effectively inhibited SCEs induced by CP and AFB in a dose-dependent manner, but it had no effect on SCE induction by BP and DMBA. MC also reduced CP-induced SCE frequencies in a hepatic tumor cell line (HTC). These cells are metabolically competent and activate CP into its biologically active metabolites. Our results support the suggestion that MC modulates the genotoxicity of indirect-acting mutagens by inhibiting certain forms of the cytochrome P-450 enzymes required for activation of premutagens like CP and AFB.

Nutritional requirements for detoxication of environmental chemicals

The biological defence systems against oxygen radical toxicity and chemical toxicity, and their component enzymes, are described, and the nutritional requirements for biological defence against chemical and oxygen toxicity, including calories, protein, lipids and lipotropes, vitamins and minerals, are reviewed in the context of their contribution to the mechanisms of detoxication. Modulation of the cytochromes P-450, and hence toxicity, by dietary components are considered; the P450I family, induced by food pyrolysis mutagens, and the P450IIE family, induced by alcohol and fasting, contribute substantially to chemical toxicity and carcinogenicity. It is concluded that: (i) the detoxication system of terrestrial fauna has evolved over greater than 300 million years to protect animals from dietary plant toxins; (ii) protection against chemical and oxygen toxicity requires all categories of nutrients; and (iii) the rôle of food and nutrition in detoxication is essential to survival.

Coumarin-induced changes in delta-aminolaevulinic acid synthase and cytochrome P-450 in chick embryo liver

Coumarin occurs naturally in the diet and inhibits several cytochrome P-450 enzymes in laboratory animals. The effect of coumarin was examined on haem biosynthesis and cytochrome P-450 activities in the 18-day-old chick embryo liver in ovo. At 40 and 50 mumol/embryo coumarin increased delta-aminolaevulinic acid synthase, porphyrins, cytochrome P-450, benzphetamine N-demethylase and benzo[a]pyrene hydroxylase. At 10 mumol/embryo coumarin decreased aniline 4-hydroxylase, and at both 10 and 50 mumol/embryo it decreased 7-ethoxyresorufin O-deethylase, coumarin 7-hydroxylase and nitrosodimethylamine N-demethylase. 7-Hydroxycoumarin and 5, 7-methoxycoumarin at 40 mumol/embryo had none of these effects. Coumarin (5-500 microM) added to liver microsomes inhibited aniline hydroxylase by 45%, but not nitrosodimethylamine N-demethylase, and inhibited 7-ethoxyresorufin O-deethylase in microsomes from 3-methylcholanthrene-treated embryos by 15 and 100% at coumarin concentrations of 250 and 500 microM, respectively. Coumarin 7-hydroxylase activity in chick embryo liver was comparable with that reported for human liver and greater than in the rat. The data indicate that coumarin can both increase and decrease cytochrome P-450 activities in chick embryo liver and can induce haem biosynthesis. Because the chick embryo liver hydroxylates coumarin at position 7 in a manner similar to humans, it may be a more suitable model than the rat for studying some of the metabolic effects of coumarin.

Different inhibition and induction profiles of hepatic drug metabolism in rats and dogs by two structurally related pyridyl diazinone cardiotonic agents

ICI 153,110 and ICI 170,777, two pyridyl diazinone cardiotonic agents, produced a different profile of effects on hepatic microsomal mixed function oxidase enzymes following multiple oral dosing to rats and dogs; these differences may be related to the molecular dimensions of the two molecules. ICI 153,110 significantly increased levels of total P450, ethoxycoumarin O-deethylase and ethoxyresorufin O-deethylase in rat microsomes, indicating an induction profile (P448) similar to that of beta-naphthoflavone. This was supported by gel electrophoresis (SDS-PAGE) of microsomal proteins; a similar type of induction was observed in dog microsomes. In contrast, ICI 170,777 produced no changes indicating enzyme induction in either rat or dog. Instead, ICI 170,777 appeared to inhibit specifically the activity of aldrin epoxidase in the rat. Inhibitory activity was also indicated in the rat by prolongation of pentobarbitone sleeping time following single oral doses of either ICI 153,110 or ICI 170,777. The time-course of this effect appeared to correlate more closely with the profile of circulating metabolites, although both parent compounds were found to produce type II spectral changes on interaction with control rat microsomes. The molecular dimensions (area/depth2) of the compounds supported the finding that only ICI 153,110 should interact with or induce P448 isozymes.

A retrospective study of the molecular toxicology of benoxaprofen

The molecular and electronic structural characteristics of the hepatotoxic and phototoxic anti-rheumatic drug, benoxaprofen, indicate that it falls in the interface between the area of parametric space associated with substrates of cytochrome P450I and that associated with substrates of other cytochromes P450, combining fairly planar molecular geometry (area/depth2 = 2.5) with relatively low activation energy (delta E = E(LEMO) - E(HOMO) = 12.0). Benoxaprofen may therefore be a substrate for cytochrome P450I so that, like many other P450I substrates, it may be oxygenated to a reactive intermediate, thereby causing hepatotoxicity. Benoxaprofen also has a molecular structure closely similar to that of clofibrate and may thus be a possible substrate for cytochrome P450IV and result in hepatic peroxisomal proliferation. The structural similarity of benoxaprofen with the furocoumarin, psoralen, is associated with its known phototoxicity. QSAR analysis of the acute toxicities and anti-inflammatory activities of 16 analogues of benoxaprofen has been undertaken to identify a drug candidate likely to have similar anti-inflammatory activity to benoxaprofen but with lower toxicity.

Effect of different doses of 3-methylcholanthrene on the localization of the 3-methylcholanthrene-inducible isoenzymes of cytochrome P450 within the centrilobular and periportal zones of the rat liver

Immunohistochemical staining techniques were used to investigate the localization of the 3-methylcholanthrene inducible isoenzymes (P450 IA1 and IA2) in the rat liver. The rats were induced with different doses of 3-methylcholanthrene, ranging from 2.5 to 25 mg/kg body weight. A heterogeneous induction pattern was observed with induction doses of 2.5; 5; 7.5 and 10 mg/kg body weight with the highest concentration of the isoenzymes around the central vein. With a dose of 25 mg/kg body weight, a homogeneous pattern was found. Induction with a dose of 15 mg/kg body weight resulted in an intermediate situation.

The importance of pharmacokinetic and receptor studies in drug safety evaluation

The importance of pharmacokinetic and receptor studies in the preclinical and clinical safety evaluation of candidate drugs is reviewed with reference to a number of recently developed drugs. Different aspects of the relationships between pathways of metabolism, pharmacokinetics, receptor interactions, and drug toxicity are illustrated. The failure of animal toxicity studies to predict drug toxicity in humans, due to species differences in metabolism and pharmacokinetics, is illustrated by reference to the anti-inflammatory antiviral terpenoid carbenoxolone, the antiasthmatic candidate drug FPL 52757, and the cardiotonic drug amrinone. The false prediction of adverse effects in man from toxicity manifested in experimental animals, due to species differences in pharmacokinetics or receptor activities, is exemplified with reference to the antiepileptic valproic acid, the hypolipidemic drug ciprofibrate, the antipeptic ulcer drug, omeprazole, and the progestogen lynestrenol. Finally, the importance of adequate, repeat-dose, clinical pharmacokinetic studies in patients as distinct from healthy volunteers to evaluate any effect of the disease state, in the elderly and the young to examine the effects of age, and in sufficiently large populations to detect genetic anomalies and idiosyncrasies is illustrated by reference to the anti-rheumatoid drug benoxaprofen, the antiangina drug perhexiline, and the diuretic tienilic acid.

Chronic toxicity and oncogenicity of picloram in Fischer 344 rats

The chronic toxicity and oncogenicity of the herbicide picloram was studied in male and female Fischer 344 rats administered 0, 20, 60, or 200 mg/kg.d technical-grade picloram via their feed for 2 yr. A comprehensive set of in-life and clinical pathology parameters was measured and an extensive list of tissues was examined grossly and by light microscopy from control and treatment groups of animals. The primary treatment-related effect observed in the study was hepatocellular swelling and altered tinctorial properties in the central regions of the liver lobules of both sexes of rats ingesting 60 or 200 mg/kg.d picloram. Males were more affected than females. Increases in liver weights accompanied these changes in both sexes of rats ingesting the high dose level of picloram. All other histopathologic lesions observed were typical of those that normally occur in aged Fischer 344 rats. There were no treatment-related increases in the incidence of any particular tumor type or in total tumors. No treatment-related effects were observed in rats ingesting 20 mg/kg.d of the test material.

Induction of the P-450 I family of proteins by polycyclic aromatic hydrocarbons: possible relationship to their carcinogenicity

The hypothesis has been put forward that mutagenic polycyclic aromatic hydrocarbons which induce the P-450 I family of cytochromes, the major enzyme system responsible for their activation, are likely to be carcinogenic. In order to test this hypothesis, rats have been pretreated with a number of polycyclic aromatic hydrocarbons of different mutagenic and carcinogenic potency and hepatic P-450 I activity was monitored using chemical probes such as the O-deethylation of ethoxyresorufin and metabolic activation of Glu-P-1 to mutagens, and immunologically employing polyclonal antibodies against purified rat P-450 I A1. All compounds studied enhanced P-450 I activity and induced P-450 I apoproteins but the extent of induction was very markedly different. The results are discussed with reference to the mutagenicity of these chemicals in the Ames test and their carcinogenicity in the classical mouse skin model. A relationship appears to exist between carcinogenicity of polycyclic aromatic hydrocarbons and their ability to induce hepatic P-450 I activity.

Effects of mutagenic and non-mutagenic aniline derivatives on rat liver drug-metabolizing enzymes

1. The effect of 4,4'-methylene bis(2-chloroaniline) (MOCA), 4,4'-methylene dianiline (MDA) and 4,4'-sulphonyldianiline (Dapsone) in vivo on xenobiotic biotransformation in male rat liver was studied. 2. Treatment with MOCA or MDA but not Dapsone caused a dose-dependent increase in ethoxyresorufin O-deethylase activity and a concomitant decrease in aldrin epoxidase activity in male rats. 3. Treatment with MOCA or MDA resulted in dose-dependent increases in ethoxycoumarin O-deethylation and epoxide hydrolation, while only MOCA induced cytosolic glutathione S-transferase activity. 4. Treatment with Dapsone resulted in no changes in xenobiotic biotransformation except for the induction of aniline hydroxylation. 5. The results are consistent with the contention that there is a relationship between carcinogenic chemicals and particular alterations in the activities of biotransformation enzymes.

Induction of liver microsomal cytochrome P-450 isozymes by 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide

1. 1-(2-Chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (RP 52028), an antagonist of peripheral type benzodiazepine binding sites, a potential anticonvulsant, has been shown to have an inducing effect on drug-metabolizing enzymes. 2. RP 52028 was administered orally at 20-800 mg/kg for 1 week, and enzymic activities were determined using a panel of substrates. Western blot analyses were performed using several specific polyclonal and monoclonal antibodies directed against cytochrome P-450 isozymes. 3. RP 52028 appears to be an inducer of cytochrome P-450 II B1 (P-450b) and related enzymic activities; i.e. benzphetamine, ethylmorphine and aminopyrine demethylation.

Effect of phenytoin, carbamazepine, and valproic acid on caffeine metabolism

Three groups of non-smoking epileptic patients without liver disease receiving antiepileptic monotherapy have been compared with 10 healthy non-smoking volunteers. Group 1 received phenytoin (n = 10), Group 2 carbamazepine (n = 10) and Group 3 valproic acid (n = 6). Cytochrome P-450 activity was monitored by measuring urinary 6-beta-hydroxycortisol output and systemic antipyrine clearance. Both, 6-beta-hydroxycortisol output and antipyrine clearance were significantly enhanced in patients on phenytoin and carbamazepine, but not in those on valproic acid. On the other hand, phenytoin alone increased the clearance of caffeine from 1.5 (controls) to 3.6 ml.min-1.kg-1, and reduced its half life from 4.8 to 2.4 h. Carbamazepine and valproic acid had no effect on caffeine metabolism. The results are in keeping with the well known heterogeneity of the hepatic monooxygenase system, as phenytoin and carbamazepine induce different panels of cytochrome P-450 isoenzymes. Phenytoin treatment may impair the validity of the caffeine liver function test.

Preferential induction of the rat hepatic P450 I proteins by the food carcinogen 2-amino-3-methyl-imidazo[4,5-f]quinoline

1. Administration of the food carcinogen, 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) to rats gave rise to significant dose-dependent increases in the microsomal O-deethylations of ethoxycoumarin and ethoxyresorufin but had no effect on the O-dealkylation of pentoxyresorufin and the NADPH-dependent reduction of cytochrome c, and decreased the N-demethylation of dimethylnitrosamine. Microsomal cytochrome b5 and total cytochrome P-450 levels decreased following the administration of the carcinogen. 2. Hepatic microsomal preparations from IQ-treated animals were much more efficient than control in activating the premutagen 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole to mutagenic intermediates in the Ames test. 3. Immunoquantification of two of the major families of cytochrome P-450, namely P450 I and P450 II B, using ELISA techniques showed that treatment with IQ induced the apoprotein levels of the P450 I family but not of P450 II B. 4. Immunoblot analysis employing polyclonal antibodies against P450 I revealed that IQ induced both isoenzymes of this family, namely P450 I A1 and A2. 5. It is concluded that IQ is an inducer of the rat hepatic monooxygenases, selectively inducing the P450 I family as predicted by a computer-graphic analysis of its dimensions which showed that it is a large, essentially planar, molecule.

Metabolic activation of chemical carcinogens by hepatic preparations from streptozotocin-treated rats

The effect of insulin-dependent diabetes on the hepatic microsomal activation of chemical carcinogens to mutagenic intermediates in the Ames test was investigated in rats pretreated with streptozotocin. In order to discern between the effects of streptozotocin itself and that of the resulting diabetes, groups of streptozotocin-treated rats received either nicotinamide simultaneously with the diabetogenic agent to prevent the onset of diabetes or daily treatment with insulin in order to antagonise the effects of diabetes. The activation of two nitrosamines, nitrosopiperidine and nitrosopyrrolidine was markedly increased following treatment of the animals with streptozotocin, the effect being preventable by nicotinamide and effectively antagonised by insulin. A similar increase in mutagenic response was also seen when Glu-P-1, a carcinogen generated during the cooking of proteinaceous food, was employed as the mutagen. In contrast, the diabetic rats were less efficient than control animals in activating the aromatic amine 2-aminofluorene to mutagenic intermediates. Concomitant administration of nicotinamide with streptozotocin prevented the decrease in mutagenicity, and daily treatment of diabetic rats with insulin partially restored mutagenic response to control levels. Streptozotocin-induced diabetes had no effect on the mutagenicity of 4-aminobiphenyl and the two polycyclic aromatic hydrocarbons, benzo(a)pyrene and 3-methylcholanthrene. The present findings clearly illustrate that diabetes modulates the metabolic activation of carcinogenic chemicals, the effect being dependent on the nature of the carcinogen.

Prediction of chemical carcinogenicity from molecular and electronic structures: a comparison of MINDO/3 and CNDO/2 molecular orbital methods

Molecular orbital methods have been used to calculate the electronic structures of a number of chemical carcinogens and non-carcinogens using the MINDO/3 (Modified Intermediate Neglect of Differential Overlap, version 3) and CNDO/2 (Complete Neglect of Differential Overlap) procedures; results from the two methods show good agreement. Spatial conformations of the same series of chemicals have also been determined. High affinity for cytochromes P-448 (P450 I) and high potential for chemical carcinogenicity/toxicity show good correlation with molecular planarity (high values of area/depth2) and low values of the difference between the frontier orbital energies (delta E) of the chemical. This computer graphic procedure identifies substrates of the cytochromes P-448 and is highly suited for the rapid screening of new chemicals for potential mutagenicity, carcinogenicity and certain other forms of toxicity. The implications of these findings to the mechanism of chemical carcinogenicity are discussed.

Metabolic activation of carcinogens and toxic chemicals

1. The spatial parameters and electronic structures of 100 exogenous and endogenous chemicals have been determined by computer graphics, from which their oxidative metabolism by the cytochrome P-448 (activation) or the other families of cytochromes P-450 (generally detoxication) have been predicted. 2. The spatial parameters of these chemicals primarily determine the family of cytochrome P-450 by which the chemicals are metabolized and the electronic structures primarily determine their ease of oxidative metabolism. 3. The role of oxidative metabolism of xenobiotics by the cytochromes P-448, and their binding to the cytosolic Ah receptor, are considered in relationship to the mechanisms of chemical toxicity, mutagenicity, carcinogenicity, and co-carcinogenicity. 4. The mechanisms of chemical toxicity and carcinogenesis are considered in respect of activation through cytochrome P-448-mediated, conformationally-hindered oxygenation to reactive intermediates which, unlike most cytochrome P-450-oxygenated metabolites, are not acceptable substrates for conjugation and detoxication and therefore react with essential intracellular macromolecules. 5. The computer graphic method of determining the molecular conformations and electronic structures of molecules is a rapid, scientifically-based procedure for evaluation of the potential toxicity, mutagenicity and carcinogenicity of chemicals.

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.

Induction of xenobiotic biotransformation by the insecticide chlordimeform, a metabolite 4-chloro-o-toluidine and a structurally related chemical o-toluidine

Chlordimeform, 4-chloro-o-toluidine and o-toluidine have all been found to have carcinogenic properties. Due to an empirical link between such properties and alteration of some biotransformation enzymes, the abilities of these three chemicals to affect cytochrome P-450 mediated biotransformation, epoxide hydrolase and glutathione S-transferase have been examined. Chlordimeform had no effect on the cytochrome P-450 content, aniline p-hydroxylase or glutathione S-transferase activities, but induced ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase and epoxide hydrolase activities and decreased aldrin epoxidase and aminopyrine N-demethylase activities. The metabolite 4-chloro-o-toluidine increased cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, glutathione S-transferase and epoxide hydrolase activities. o-Toluidine induced cytochrome P-450, ethoxyresorufin-O-deethylase, ethoxycoumarin-O-deethylase, and aldrin epoxidase activities. Ethoxy-resorufin-O-deethylase activity was induced approximately eight times by chlordimeform and 18 times by 4-chloro-o-toluidine and o-toluidine. Induction was seen at 50 mg/kg with chlordimeform and at 10 mg/kg with the other treatments. Chlordimeform increased the 7 alpha and 16 alpha androstenedione hydroxylase pathways. 4-Chloro-o-toluidine increased the 7 alpha, 16 beta and 16 alpha hydroxylase pathways, while o-toluidine increased the 7 alpha, 6 beta, 16 beta and 16 alpha hydroxylase pathways. All three chemicals marginally decreased the testosterone pathways. SDS-PAGE of rat microsomes revealed an increase in a protein band of MW c54,000 for the chlordimeform and 4-chloro-o-toluidine treated groups. Taken together with the increase in ethoxyresorufin-O-deethylase activity these observations are consistent with the induction of hepatic isozyme P-450d. Thus each chemical has been shown to induce various pathways of biotransformation with increases in the P-450c and P-450d specific substrate ethoxyresorufin-O-deethylase being a consistent finding.

Anthraflavic acid inhibits the mutagenicity of the food mutagen IQ: mechanism of action

The ability of anthraflavic acid to inhibit the mutagenicity of IQ was investigated using the Ames test and employing hepatic activation systems from Aroclor 1254-pretreated rats. Incorporation of anthraflavic acid into the S9 mix caused a concentration-dependent decrease in the mutagenicity of IQ. A similar effect was seen when microsomes only were employed as activation systems. Cytosol, as we have previously demonstrated, potentiated the microsome-mediated mutagenicity of IQ and this potentiation was also inhibited by anthraflavic acid. In contrast, anthraflavic acid had no effect on the mutagenicity of the direct-acting microsome-generated metabolites of IQ. It is concluded that anthraflavic acid is a potent inhibitor of IQ mutagenicity by virtue of its ability to inhibit both its microsomal and cytosolic activation pathways.

Alteration of liver microsomal monooxygenases and substrate competition with aniline hydroxylase from rats chronically fed low-fat and high-fat-containing alcohol diets

Male Sprague-Dawley rats fed ethanol (EtOH) 36% of total calories for four weeks in a liquid diet containing either 34% (HF) or 12% (LF) of calories as fat were studied with respect to induction of microsomal monooxygenases (MFO) and substrate competition with EtOH-inducible aniline hydroxylase. The specific activity and turnover of aniline hydroxylase were induced to similar extents by HF-EtOH and LF-EtOH diets. Whereas, both LF-EtOH and HF-EtOH caused a decrease in the turnover of arylhydrocarbon (benzo[a]pyrene) hydroxylase (AHH) and aldrin epoxidase compared to pair-fed (PF) controls, LF-EtOH but not HF-EtOH increased the turnover of ethoxycoumarin and ethoxyresorufin O-deethylase (ECOD and EROD). The increase in ECOD and EROD and the decrease in AHH by EtOH is contrary to the parallel induction of these activities by 3-methylcholanthrene (3-MC) and Aroclor 1254 (Aroclor). Benzo(a)pyrene (BaP) stimulated aniline hydroxylase in the HF-EtOH and PF systems, whereas with LF diet, stimulation was seen only in the EtOH group. Ethoxycoumarin (EC) inhibited aniline hydroxylase by microsomes from EtOH- and pyrazole-treated rats, whereas it stimulated aniline hydroxylase by control microsomes, suggesting that the EC effects were associated with EtOH-inducible cytochrome P-450. Ethoxyresorufin (ER) inhibited aniline hydroxylase in EtOH and PF groups, thus the differential effects of EC were not nonspecific O-deethylase effects. The effects of EtOH feeding on ECOD, EROD, and AHH (ie, substrates for 3-MC-inducible cytochrome P-450) displayed a greater differential between the experimental and control group with the LF- than with the HF-containing diet. The findings suggest that the alteration of certain MFO activities by chronic EtOH ingestion can be modified by the content of dietary fat. Moreover, the competition dynamics of MFO substrates toward EtOH-inducible aniline hydroxylase are altered by EtOH feeding and, in turn, modified by dietary fat.

Inducing effect of albendazole on rat liver drug-metabolizing enzymes and metabolite pharmacokinetics

Albendazole (ABZ), methyl (5-(propylthio)-1H-benzimidazol-2-yl)carbamate, is a broad spectrum anthelmintic drug. S-oxidation to the sulfoxide (SO-ABZ) and the sulfone (SO2-ABZ) are the first steps of its bioconversion. SO-ABZ is pharmacologically active and embryotoxic in rats. In the present study, rat liver microsomal drug-metabolizing enzymes were assayed after 10 days oral administration with 40 mumol ABZ/kg per day. The activities of 4-nitroanisole O-demethylase, benzo[a]pyrene hydroxylase, 7-ethoxycoumarin O-deethylase, and 7-ethoxyresorufin O-deethylase increased 6-, 7-, 8-, and 30-fold, respectively. By immunoblotting an increase in cytochrome P-448 was observed. UDP-glucuronosyltransferase (GT) type 1 activities (1-naphthol, 7-hydroxycoumarin, 4-nitrophenol, and 4-methylumbelliferone) were significantly higher than in control microsomes (3- to 4-fold), while GT type 2 activities and bilirubin-GT remained unchanged. Microsomal epoxide hydrolase (benzo[a]pyrene oxide) increased 2-fold. Microsomal gamma-glutamyltransferase activity was unchanged. The in vivo SO-ABZ plasma level was decreased when the SO2-ABZ plasma level was increased. In vitro sulfoxidation and sulfonation were, however, unchanged. Although a range of imidazole derivatives, including benzimidazole itself, were commonly reported as inhibitors of monooxygenase activities, ABZ behaved as an inducer of cytochrome P-448, GT1, and epoxide hydrolase.

The effect of selenium on the hepatic drug metabolism and inducibility in rat

1. Rats were fed either a normal or selenium-deficient diet for 4 weeks. The subgroup on selenium deficient diet had selenium supplementation as 3 ppm Se in the drinking water. Benzo(a)pyrene was given intraperitoneally as an inducer. 2. Se deficiency decreased glutathione peroxidase and cytochrome c-reductase activities while other activities were unchanged as compared to normal diet. 3. Selenium deficiency was a prerequisite for the induction of glutathione peroxidase, S-reductase and S-transferase enzymes. 4. Benzo(a)pyrene increased hepatic microsomal cytochrome P-450 content in rats on normal and selenium supplemented diet but not in the selenium deficient group. 5. The 7-ethoxyresorufin and 7-ethoxycoumarin deethylase, aryl hydrocarbon hydroxylase and cytochrome c-reductase activities were increased by benzo(a)pyrene in all the dietary groups. 6. The UDP-glucuronosyltransferase activity was also increased by benzo(a)pyrene in all the experimental groups and this was true with p-nitrophenol and phenolphthalein as aglycons.

Anthraflavic acid is a potent and specific inhibitor of cytochrome P-448 activity

Consideration of the computer-optimised dimensions of anthraflavic acid indicates that it is essentially a planar molecule with a large area/depth ratio, that would preferentially interact with the polycyclic aromatic hydrocarbon-induced family of cytochrome P-450 proteins (cytochromes P-448). Anthraflavic acid was a potent inhibitor of the O-deethylations of ethoxycoumarin and ethoxyresorufin, both catalysed primarily by cytochromes P-448, in Arochlor-1254-induced hepatic microsomes. Similarly anthraflavic acid markedly inhibited the mutagenicity of 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-I) in the Ames test. In contrast, it has no effect on the dealkylation of pentoxyresorufin, a reaction catalysed primarily by the phenobarbital-induced cytochromes P-450, and NADPH-dependent reduction of cytochrome c. It is concluded that anthraflavic acid is a potent and specific inhibitor of cytochrome P-448 activity.

Interactions of imidazole antifungal agents with purified cytochrome P-450 proteins

The imidazole N-substituted antifungal agents ketoconazole, miconazole and clotrimazole have been shown to be potent inhibitors of oxidative metabolism by both a phenobarbital-induced cytochrome P-450 (P-450b) and a 3-methylcholanthrene-induced cytochrome P-448-protein (P-450c) in reconstituted systems. All three compounds inhibited the cytochrome P-450b-dependent 7-pentoxyresorufin-O-dealkylase and the cytochrome P-450c-dependent 7-ethoxyresorufin-O-deethylase activities. When 7-benzyloxyresorufin and 7-ethoxycoumarin were employed as substrates with both cytochrome preparations, all three antifungal compounds exhibited selective inhibition of the cytochrome P-450b preparation; ketoconazole was always the weakest inhibitor. The three antifungal agents were also shown to elicit a type II difference spectral interaction with both isoenzymes, the magnitude of the spectral interaction being greater with the cytochrome P-450b preparation.

Molecular orbital calculations and quantitative structure-activity relationships for some polyaromatic hydrocarbons

1. Correlations between biological activity and electronic structure for 7 polyaromatic hydrocarbons are reported. 2. Molecular orbital (MO) calculations by the MINDO/3 method indicate that the hydrophobic parameter, log P (the logarithm of the octanol/water partition coefficient) exhibits a parallelism with total electrophilic superdelocalizability, and a similar relationship is shown with protein binding. 3. Inhibition of dimethylnitrosamine (DMN) demethylase activity is linearly related to total nucleophilic superdelocalizability, and other correlations are shown with mutagenicity and benzo[a]pyrene hydroxylase (AHH) activity.

Spectral and kinetic studies of the interaction of imidazole anti-fungal agents with microsomal cytochromes P-450

1. The imidazole antifungal agents, ketoconazole, miconazole and clotrimazole have been shown to be potent inhibitors of the phenobarbital-induced cytochromes P-450 and the 3-methylcholanthrene-induced cytochromes P-448-dependent rat hepatic microsomal mixed-function oxidases. 2. All three drugs were more potent inhibitors of the phenobarbital-induced O-deethylation of ethoxycoumarin than of the 3-methylcholanthrene-induced activity indicating selective inhibition of the phenobarbital-induced cytochromes P-450. In both types of microsomes ketoconazole was always the weakest inhibitor. 3. All three compounds elicited type II spectral interactions with both types of microsomes, and had similar Ks values. Miconazole and clotrimazole, and to a lesser extent ketoconazole, also interacted with the substrate binding sites of both phenobarbital-induced cytochromes P-450 and to a lesser extent with the 3-methylcholanthrene-induced cytochrome P-448. 4. It is concluded that at least part of the inhibitory effect of these antifungal agents may reflect competitive inhibition at the substrate binding site.

Effects of picloram on xenobiotic biotransformation in rat liver

1. The effect of picloram on model xenobiotic substrate biotransformation in vivo was studied in female and male rat liver. 2. Treatment with picloram had little effect on epoxide hydratase and glutathione S-transferase activity, but caused a dose-dependent increase in ethoxyresorufin-O-deethylase activity and a concomitant decrease in aldrin epoxidase activity in male rats. 3. Treatment of male rats with equivalent doses of 2-acetylaminofluorene, 2-amino-anthracene and picloram induced ethoxyresorufin-O-deethylase activity to the same degree. 4. Treatment of female rats with picloram resulted in dose-dependent increases in ethoxyresorufin and ethoxycoumarin-O-deethylation without decreasing aldrin epoxidase activity. 5. Picloram binds to liver microsomal preparations from rats pretreated with phenobarbitone and/or 3-methylcholanthrene, giving a type I spectrum. 6. The results indicate that picloram is a 3-methylcholanthrene-type inducer, and the implications are discussed.