[Inhibition of cytochromes P-450. Biochemical mechanisms and possible therapeutic consequences. I--Inactivation by formation of reversible complexes]

Inactivation of cytochrome P-450 dependent monoxygenases, a major family of hepatic oxidative drug-metabolizing enzymes, has been an area of intensive pharmacological and toxicological investigation. Several classes of drugs are known to be inducers or inhibitors of P-450 isoenzymes. The reversible inhibition of the oxidative microsomal metabolism could be directly related to the ability of the drug to bind cytochrome P-450 as shown by the formation of an inhibitory complex. Such effects result in many drug-drug interactions, affect drug disposition and drug action and could interact with the metabolism of endogenous steroids. This review is concerned primarily with a mechanistic interpretation of the reversible interactions between several classes of therapeutic agents and various P-450 isozymes. The effects of structural modifications on the inhibitory activity were also described.

Influence of muramyl dipeptide on established experimental arthritis in rats

The effects of MDP, a potent inducer of cytokines, were studied in four batches of Wistar Furth rats with established experimental arthritis. Arthritic rats were given a daily sc injection of 10, 100, 200 or 400 micrograms MDP respectively. Muramyl dipeptide increased the severity of clinical events in a dose-dependent manner, with the exception of the 10 micrograms dose which was ineffective. The levels of anti-collagen antibodies were not however significantly enhanced by MDP. Radiological lesions and histological changes were maximal at high dosage regimens. Paradoxically, the acute phase reactive alpha 1 glycoprotein was little affected by MDP treatment.

[Variation in the urinary excretion of 6-beta-hydroxycortisol in humans after administration of the new isoquinoline derivative, PK-11195 (52028 RP)]

Among in vivo tests to assess liver drug metabolizing enzyme induction, urinary 6-beta-hydroxycortisol (6-beta-OHF), plasma gamma-glutamyltransferase (GGT) and urinary D-glucaric acid are most frequently proposed. 6-beta-OHF is the most abundant unconjugated metabolite of cortisol in human urine. We measured its elimination during a clinical trial in 16 human healthy volunteers (men and women), these persons being treated by a new isoquinoleine derivative, 52028 RP (PK-11195). This drug is an antagonist of peripheral type benzodiazepine binding sites. Urinary excretion of 6-beta-OHF increased significantly (3.5 fold, p less than 0.01) on the 5th day of treatment (400 mg/day, orally) and remained increased as long as the treatment was continued (15 days). Control values were again observed 5 days after stopping the treatment. Plasma gamma-glutamyltransferase activity and D-glucaric acid urinary elimination are increased more than 2 fold. The data demonstrated that 6-beta-OHF in the most sensitive among the three tests performed to detect a drug metabolism induction, during this clinical trial.

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.

[Changes of plasma and tissue gamma-glutamyltransferase under the influence of drugs]

Measurement of gamma-glutamyltransferase (GGT) activity in plasma is widely used in clinical biology (in order to detect hepatic diseases or to monitor treatment for alcoholism), and also in pharmacology (since this test is the only plasmatic marker for hepatic induction in human). However, the correct interpretation of a high plasmatic activity should take into account the various analytical factors which can affect results, as well as the physiological parameters known to modify this activity. It also requires its comparison to defined reference values. Several mechanisms may be involved in the increase of plasmatic activity as an index of hepatic induction, such as an increase in the protein synthesis, a release of the enzyme from the membrane or a modification in the biliary flux.

Study of the in vitro bioactivation of albendazole in human liver microsomes and hepatoma cell lines

The metabolism of albendazole (ABZ), a benzimidazole anthelminthic, was studied in either microsomal preparations of human liver biopsies or cultured human hepatoma cell lines. Metabolites were analyzed by HPLC. Our data show that microsomes from human biopsies and two human cell lines, HepG2 and Hep3B, oxidize the drug to the sulfoxide very efficiently, whereas the third cell line tested, SK-HEP-1, does not. Both cytochrome P-450 dependent monooxygenases and flavin-containing monooxygenases appear to be involved in human ABZ metabolism. Using the cell line displaying the highest ABZ-metabolizing activity, HepG2, the cytotoxic and the inducing effects of the parent drug ABZ and of two primary metabolites, the sulfoxide and the sulfone were studied. These three chemicals provoked a rise in mitotic index resulting from cell division blockage at the prophase or at the metaphase (ABZ metabolites) stage, and ABZ was more cytotoxic than its metabolites. With regard to enzyme-inducing effects, our data clearly demonstrate that the sulfoxide and, to a lesser degree, the sulfone are potent inducers of some drug metabolizing enzymes (i.e., cytochrome P-488 dependent monooxygenases and UDP glucuronyltransferase), whereas ABZ fails to increase and even slightly decreases these enzymatic activities. In conclusion, the HepG2 human hepatoma cell line appears to be suitable for the study of many parameters of metabolism and action of ABZ and other structurally related compounds in humans.

Albendazole sulfonation by rat liver cytochrome P-450c

The metabolism of albendazole (ABZ) was studied in perfused livers from control and ABZ-treated rats (10.6 mg/kg, per os, each day for 10 days). In the perfusion fluid, the concentration of ABZ-sulfoxide (SO-ABZ) remained unchanged in treated, as compared to control animals, whereas ABZ-sulfone (SO2-ABZ) was increased in treated animals. In bile, only SO-ABZ was present. The transformation kinetics of SO-ABZ to SO2-ABZ in microsomes from rats treated with ABZ, 3-methylcholanthrene, Aroclor and isosafrole were biphasic. This suggests that enzyme activity was a consequence of two enzyme systems, one characterized by low affinity and high capacity, the other by high affinity and low capacity, the latter could be induced by 3-methylcholanthrene, ABZ, Aroclor and isosafrole. Cytochrome P-450c was induced potently in vivo by ABZ as proven by increased monooxygenase (7-ethoxyresorufin and 7-ethoxycoumarin-O-deethylase) activities and by Elisa test (a 5-fold increase in hemoprotein concentration was observed). Purified and reconstituted cytochrome P-450c from 3-methylcholanthrene or ABZ-treated rat liver were able to produce SO2-ABZ (2.01 and 1.70 nmol/mg/15 min, respectively, whereas cytochrome P-450b produced 10 times less SO2-ABZ). Immunological assays, as well as activity measurements showed a relationship between cytochrome P-450c-3-methylcholanthrene and cytochrome P-450c-ABZ. We conclude that induction of cytochrome P-450c by ABZ is the probable explanation for the enhanced formation of SO2-ABZ in vivo.

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.

Induction of plasma and tissue enzymes by drugs: significance in toxicological studies

1. Observations of drug metabolism enzyme induction in animals during early toxicological trials, and phase I or II of clinical trials, are of importance for the development of a new chemical of pharmacological interest. 2. Induction can be detected by determination of enzyme activities or enzyme proteins in tissues (subcellular fractions or cells in culture). 3. Indirect methods for checking induction can involve determinations of endogenous (i.e. glucaric acid, 6-beta-hydroxycortisol) or xenobiotic (antipyrine) metabolites which are produced by the inducible enzyme systems. 4. Recent progress in the knowledge of biochemical mechanisms of induction and the large number of identified isoenzymes in each enzyme 'family' have complicated the interpretation in pharmaco-toxicological studies. 5. This paper describes the present state of the art concerning the relationships between the isoenzymes and the major classes of inducers concerning cytochromes P-450, UDP-glucuronosyltransferases, gamma-glutamyltransferase and epoxide hydrolases.

Sulphoxidation of albendazole by the FAD-containing and cytochrome P-450 dependent mono-oxygenases from pig liver microsomes

1. Two distinct microsomal pathways involved in the metabolism of albendazole (ABZ) to albendazole-sulphoxide (SO.ABZ) by pig liver microsomes have been identified and quantified. 2. The binding of ABZ to microsomal cytochrome P-450 (Type I spectrum, Ks = 25.5 microM), the decrease of the rate of sulphoxidation by antibody against NADPH cytochrome c reductase, and the use of purified cytochrome P-450 A demonstrated the contribution of a cytochrome P-450-dependent mono-oxygenase to the metabolism of ABZ. 3. The involvement of FAD-containing mono-oxygenase (FMO) was shown by thermal pretreatment of microsomes, n-octylamine activation of the reaction, and by using purified pig liver FMO. 4. From Km and Vmax values, it would appear that the relative contributions of the two systems depend on the concentration of ABZ.

Induction of hepatic cytochrome P-450c-dependent monooxygenase activities by dantrolene in rat

The effect of dantrolene sodium, a skeletal muscle relaxant, on drug metabolizing enzymes has been investigated after treatment of rats with a dose of 200 mg/kg for five days. We observed an induction of cytochrome P-450c and epoxide hydrolase in immunoassays and activities. An enhancement of the UDP-glucuronosyltransferase (GT1) activity was observed. We also reported a decrease of both liver cytochrome P-450 content and microsomal cytochrome P-450b dependent N-demethylation activities. On the other hand, the binding of dantrolene on microsomal cytochrome P-450 produced a type I difference spectrum, these data were obtained with liver microsomal cytochrome P-450c induced by 3-methylcholanthrene.

Interactions of perhexiline maleate and arylalkylamine derivatives with cytochrome P-450 and in-vitro hydroxylation

In order to find a substitute for perhexiline maleate, an antiangina drug, for which several side effects due to poor hydroxylation have been reported, an arylalkylamine series with antianginal properties has been synthesized. The aim of the present work was to select a more rapidly hydroxylated compound than perhexiline maleate in this series. Two criteria have been retained. The binding of the molecules to liver microsomal cytochrome P-450 and their rate of hydroxylation were both studied in vitro using phenobarbital induced rat liver microsomes. Incubation with cofactors and extraction procedures have been tested on one of the molecules of the series taken as example: N-2-dicyclohexyl-2-phenethylamine. All of the molecules tested in the series substrates were type I substrates; nevertheless, no correlation was found between binding on cytochrome P-450 and oxidative metabolism of the drugs. Two of the studied molecules were more easily hydroxylated than the others and than perhexiline maleate: (N-2-dicyclohexyl-2-phenethylamine) and (II) (N-cyclohexyl-2-diphenylethylamine) with the following respective kinetics: Apparent Vmax: 0.073 and 0.32 units, apparent Km: 6.9 X 10(-5) M and 22.2 X 10(-5) M.

Immobilization of pig liver microsomes. Stability of cytochrome P-450-dependent monooxygenase activities

Microsomes from pig liver were covalently coupled to Sepharose activated by CNBr and to Sephadex activated by 1,1'-carbonyldiimidazole. Microsomes were also entrapped inside Ca-alginate and kappa-carrageenan gels. The concentration of immobilized cytochrome P-450 was determined by CO-difference spectra. The activity of the monooxygenase system was demonstrated by the N-demethylation of aminopyrine, the O-demethylation of p-nitroanisole, and the hydroxylation of perhexiline maleate. Upon immobilization, a 30-40% and a 60-70% decrease in Vappmax for the O- and N-demethylations were respectively observed. The Vappmax values for the hydroxylation of perhexiline maleate were essentially the same for the different immobilized forms and for the freely suspended microsomal cytochrome P-450. Under storage at 4 degrees C, microsomes entrapped inside kappa-carrageenan and Ca-alginate were less stable than the free microsomes, whereas immobilization on CNBr-activated Sepharose improved the stability of the hepatic microsomal monooxygenase system at the same temperature. These types of immobilized microsomes have the advantage of being easily recovered and reused for other assays. Finally, microsomes entrapped inside kappa-carrageenan or Ca-alginate can be used to follow up the continuous metabolization of p-nitroanisole for several hours in a stirred-batch reactor.

Comparison of cytochrome P-450 content and activities in liver microsomes of seven animal species, including man

The cytochrome P-450 content found in human livers obtained post mortem was between 0.21-0.42 nmol/mg protein. The kinetic parameters of the mono-oxygenase activities--Km and Vmax--were determined in liver microsomes for N-demethylation (aminopyrine, benzphetamine, ethylmorphine), O-demethylation (4-nitroanisole), O-deethylation (7-ethoxycoumarin) and hydroxylation (benzo[a]pyrene), in an attempt to establish an inter-species comparison between man and the six animal species studied. The four substrates studied (aminopyrine, benzphetamine, ethylmorphine, benzo[a]pyrene) were shown to be less active in humans than the male rat, which is the most commonly used model. However, other animal species, such as the female Sprague-Dawley rat and the pig, are much more similar to man. From a procedural point of view, the optimal substrate concentrations vary from one experimental species to another. Due to the apparent Km observed, for example, the activities of the guinea-pig require a higher substrate concentration.

Heterogeneity of hepatic microsomal UDP-glucuronosyltransferase(s) activities: comparison between human and mammalian species activities

The first comparative profiles of UDP-glucuronosyltransferase(s) (UDPGT) activities obtained under standard conditions in vitro in mammals (man, rat [Wistar and Gunn], mouse, monkey [Papio papio and Cynomolgus], pig, guinea pig, rabbit, dog) are presented for 16 aglycones. A decreasing scale of these activities was obtained from planar to bulky molecules. The scale was identical for each of the mammals studied, including man. Statistical analysis of the results revealed a division of the aglycones into three groups, one being correlated with the molecular form called GT1 the two others with the GT2 form. The profile of activities in the Gunn rat revealed very weak activity towards planar molecules (GT1). These results provide evidence that under standard conditions, human UDPGT activities are comparable to those from other animals.

Differential regulation of two microsomal epoxide hydrolases in hyperplastic nodules from rat liver

Two microsomal epoxide hydrolases of the rat liver were found to be differentially regulated in hyperplastic nodules. Whilst the activity for substrates of the well-known microsomal epoxide hydrolase with a broad substrate specificity (EHb), benzo[a]pyrene 4,5-oxide and androstene oxide (16 alpha, 17 alpha-epoxyandrosten-3-one), was greatly (approximately 5-fold) increased in the nodule microsomes and moderately (approximately 2-fold) increased in the surrounding tissue, that for the substrate of the novel microsomal epoxide hydrolase, cholesterol 5 alpha,6 alpha-oxide (EHch) remained unchanged. Since both enzymes convert endogenous steroid epoxides but with distinct structural features, this differential regulation may indicate a role of endogenous steroid epoxide(s) of a defined structure during hepatocarcinogenesis. Alternatively, this differential regulation may serve as a marker during hepatocarcinogenesis.

Assessment of the Mulder and Van Doorn kinetic procedure and rapid centrifugal analysis of UDP-glucuronosyltransferase activities

The optimal experimental conditions of the enzyme assay described by Mulder and Van Doorn (1975, Biochem J. 151, 131-140) for the measurement of UDP-glucuronosyltransferase activities were tested towards structurally different aglycones. This assessment of this assay revealed that addition of Triton X-100 as enzyme activator was necessary because of its apparent inhibitory effects on interfering reactions. Under these conditions, accordance of the data with results published in the literature was obtained. We present for the first time an UDP-glucuronosyltransferase assay adapted on a fast analyser centrifuge which allows a rapid and sensitive measurement of enzyme activity that is very useful for kinetic constant determination, without consuming a large volume of reagents.

Inhibition studies of microsomal UDP-glucuronosyltransferase activities by furosemide and salicylamide

Contrarily to cytochrome P-450, a few inhibitors of UDP-glucuronosyltransferase have been described. We verified the nature of the in vitro inhibition due to furosemide, using 4 different aglycones (morphine, p-nitrophenol, borneol and eugenol) presumably belong may to different clusters of UDP-glucuronosyltransferase activities. The variations of these corresponding kinetic parameters (Km, Vmax, specific activities) must correspond to different inhibition mechanisms of furosemide, for example different site(s) of fixation in the area of the active site of UDPGT. Because these variations were not as classically described, we checked the impact of furosemide pretreatment on in vitro levels of different UDPGT activities. We compared these result, with another inhibitor (salicylamide). The apparent induction due to the both molecules enforced the hypothesis of a complexe inhibition mechanism.

Effect of pretreatment with hydroxylated xenobiotics on the activities of rat liver UDP-glucuronosyl-transferases

Six hydroxylated substrates were examined as potential inducers of UDP-glucuronosyltransferases towards their conjugation and the conjugation of other 'model' aglycones with glucuronic acid. Time-dependence (4, 14 and 28 days) and dose-dependence of treatments (from 25 mg/kg to 1 g/kg) were examined for some of these compounds. Monoterpenoid alcohols (borneol and terpineol) did not enhance the glucuronidation of the ten substrates tested. p-Hydroxybiphenyl gives typical 'substrate-induction' towards its own conjugation. Eugenol, a previously described inducer of UDP-glucuronosyltransferase activities in mouse, and 4-methylumbelliferone, give a general enhancement of all activities tested, especially those of the '3-methylcholanthrene-inducible' group of aglycones. p-Nitrophenol at low dose (2.5 mg/kg) gives a limited 'polycyclic aromatic hydrocarbon'-like increase.

Studies of UDP-glucuronosyltransferase activity toward eugenol, using a gas chromatographic method of measurement

A method for the assay of uridine diphosphate (UDP)-glucuronosyltransferase activities toward some phenolic compounds and monoterpenoid alcohols is described. The method is based on the disappearance of the free substrate after incubation with microsomes and UDP-glucuronate. This disappearance is recorded using a gas chromatographic process. This method has been used, for example, to characterize the glucuronidation process of eugenol (4-allyl-2-methoxyphenol). The method could be extended to other substrates. Analytical conditions are given for some of them, especially monoterpenoid alcohols since the studies of their conjugations are a growing field of interest in evaluation of heterogeneity of UDP-glucuronosyltransferase. The method could also be used with other biological materials including cell suspension and crude liver biopsies.

Effect of a dietary protein deficiency on the development of hepatic drug-metabolizing enzymes in young rats

The effect of protein deprivation on the activities of hepatic drug-metabolizing enzymes was studied in young rats whose mothers had previously been on a protein-restricted diet during pregnancy. Dietary protein deficiency (5% lactic casein as the protein source) lowered the amount of cytochrome P-450 and the activities of epoxide hydrolase and UDP-glucuronosyltransferase (UDPGT) (l-borneol as the substrate) by about 45, 63, and 48%, for the first 8 weeks, respectively. Interestingly, UDPGT estimated with p-nitrophenol as the substrate was far less affected than that estimated from l-borneol glucuronidation. This finding provides further evidence of the heterogeneity of UDPGT. Restoration of a balanced diet for 15 days following protein deprivation quickly restored cytochrome P-450 and enzyme activities to control values. Our experiments showed that the development of drug-metabolizing enzymes was changed more by the diet in young rats than in older rats. This could affect the toxicity of drugs that are normally metabolized by these pathways.

Induction of group-1 and group-2 UDP-glucuronosyltransferase in microsomes from the livers of C57 Bl/6 mice

In a study of multiple forms of liver microsomal UDPglucuronosyltransferase (EC 2.4.1.17) with different substrate specificities, enzyme induction was examined in inducible C57 Bl/6 mice pretreated with phenobarbital, 3-methylcholanthrene, Aroclor 1254, safrole, beta-naphthoflavone, and pregnenolone-16-alpha-carbonitrile. Phenobarbital and 3-methylcholanthrene were much more potent inducers than the other four compounds. 3-methylcholanthrene stimulated the glucuronidation of 4-nitrophenol, 4-bromophenol, and 4-chlorophenol ("group-1' substrates) more than phenobarbital, whereas phenobarbital stimulated the glucuronidation of 4-hydroxyphenobarbital, menthol, eugenol, 4-hydroxybiphenyl, morphine, 1-borneol, 4-hydroxyphenytoin, and 1,2,3,4-tetrahydro-1-naphthol ("group-2' substrates). This pattern is analogous to that previously observed in rat-liver microsomes.