Inhibition of ligand induced promoter occupancy in vivo by a dominant negative RXR

BACKGROUND

Retinoid X receptors (RXRs) heterodimerize with other nuclear hormone receptors and control ligand mediated transcription. To address how RXRs function as heterodimers, we investigated activities of truncated RXR alpha and RXR beta that lack approximately 20 conserved C-terminal amino acids.

RESULTS

The truncated RXRs formed heterodimers and bound to respective DNA elements in vitro. By transient reporter assays we found that these RXRs act as dominant negative receptors and inhibit ligand dependent transcription by the retinoic acid receptor (RAR) and vitamin D receptor. P19 embryonal carcinoma cells stably expressing the truncated RXR beta (termed delta C2) were deficient in activating the endogenous RAR beta gene and an RA responsive reporter. To study the dominant negative activity of delta C2 further, genomic footprinting analysis was performed for the RAR beta2 promoter. In control P19 clones, the RA responsive element (RARE) and other elements in the promoter were protected after RA treatment. However, in delta C2 clones RA-induced protection was markedly inhibited at all elements.

CONCLUSIONS

These results indicate that the C-terminal region of RXR is required for full RARE occupancy in vivo, a RA dependent process that leads to the recruitment of other factors to the promoter and the subsequent transcriptional activation. Thus, RXRs play an integral role in ligand dependent transcription.

The metabolic formation of reactive intermediates from clozapine, a drug associated with agranulocytosis in man

Clozapine, a dibenzodiazepine antipsychotic, is associated with a 0.8% incidence of agranulocytosis. This clinically restrictive toxicity has been attributed to its chemically reactive metabolites. The generation of such metabolites--assessed via covalent binding and formation of thioether adducts--was investigated using human, rat and mouse liver microsomes and human neutrophils and bone marrow cells. In every instance, one major glutathione adduct of clozapine--C-6 glutathionyl clozapine--was formed in the presence of added glutathione. Adduct formation by the neutrophils and myeloid cells was dependent on cell activation by phorbol myristate acetate. Small fractions of drug underwent covalent binding to microsomes (1-6.8%) and to protein coincubated with neutrophils (0.47%) and myeloid cells (0.21%). Clozapine did not deplete intracellular glutathione in activated neutrophils. Clozapine was also metabolized in vivo to glutathione conjugates in rats and mice, the conjugates eliminated in bile over a 3-hr period representing 38% and 33% of the dose, respectively. In addition to the principal clozapine adduct found in vitro, the C-8 glutathionyl derivative of deschloroclozapine was excreted by both species. It is concluded that clozapine undergoes bioactivation in several tissues and considerable bioactivation in vivo. The reactive metabolites generated by neutrophils and myeloid cells may play an important role in the metabolic causation of clozapine-induced agranuiocytosis.

N-Hydroxylation of dapsone by multiple enzymes of cytochrome P450: implications for inhibition of haemotoxicity

1. The adverse reactions associated with the administration of dapsone are believed to be caused by metabolism to its hydroxylamine. Previous reports suggest that CYP3A4 is responsible for this biotransformation [1]. 2. Data presented in this paper illustrate the involvement of more than one cytochrome P450 enzyme in dapsone hydroxylamine formation using human liver microsomes. Eadie-Hofstee plots demonstrated bi-phasic kinetics in several livers. No correlation could be established between hydroxylamine formation and CYP3A concentrations in six human livers (r = -0.47; P = 0.34). 3. Studies with low molecular weight inhibitors illustrate the importance of CYP2C9 and CYP3A in dapsone N-hydroxylation. 4. Differential sensitivity of dapsone N-hydroxylation to selective CYP inhibitors indicated that the contribution of individual CYP enzymes varies between livers. Selective inhibition ranged from 6.8 to 44.1% by 5 microM ketoconazole, and from 24.0 to 68.4% by 100 microM sulphaphenazole. The extent of inhibition, by either ketoconazole or sulphaphenazole was dependent on the CYP3A content of the liver. 5. The levels of expression of these cytochrome P450 enzymes may be an important determinant of individual susceptibility to the toxic effects of dapsone, and may influence the ability of an enzyme inhibitor to block dapsone toxicity in vivo. Because of the inability to produce complete inhibition, selective CYP inhibitors are unlikely to offer any clinical advantage over cimetidine in decreasing dapsone hydroxylamine formation in vivo.

Oocyst production and immunogenicity of Cryptosporidium muris (strain MCR) in mice

Three-week-old ICR SPF mice were orally inoculated with one of 5 doses ranging from 2 x 10(2) to 2 x 10(6) oocysts of Cryptosporidium muris (strain MCR) per mouse. Oocyst inoculation was directly proportional to the amount of oocysts shed and was inversely proportional to the period required for peak oocyst production and to the prepatent period. Peak oocyst production occurred between fifteen and thirty-one days with a patent period from 61 to 64 days. Three days after all mice stopped shedding oocysts, they were orally challenged with a single dose of 2 x 10(6) oocysts of the same species. Marked seroconversion for IgG antibody accompanied recovery from mice inoculated with 5 x 10(5) oocysts. Mice administered with carrageenan excreted a small number of oocysts for 49.0 days on the average after challenge inoculation (ACI) and control mice for 14.2 days in a dose-independent fashion. Just before challenge infection, phagocytic activity of peritoneal macrophages (M phi) and the number of peripheral M phi were dramatically decreased. Mild challenge infection implies that the immunogenicity of C. muris (strain MCR) is very strong, despite M phi blocker carrageenan administration.

Genetic polymorphism of cytochrome P4502E1 and risk of alcoholic liver disease in Caucasians

Genetic factors may be of importance in determining inter-individual susceptibility to alcoholic liver disease (ALD). Among the candidate genes which have been considered to be important are those which code for enzymes involved in alcohol metabolism. Cytochrome P4502E1 (CYP2E1) metabolizes alcohol to acetaldehyde and the hydroxyethyl radical, and is also inducible by alcohol. A Rsa I restriction fragment length polymorphism (RFLP) in the 5'-flanking region of the CYP2E1 gene has been identified by other investigators, studies showing that the mutant allele (termed c2) shows greater transcriptional activity, higher protein levels and increased activity compared with the wild-type allele (c1). We have used PCR-RFLP analysis to determine whether the frequency of these alleles differed in 95 Caucasian patients with ALD compared with 205 control subjects (comprising 58 alcoholics with no liver disease, 47 patients with non-alcoholic liver disease and 100 healthy volunteers). In controls, the frequency (0.024) of the c2 allele was similar to that previously reported in other Caucasian populations. The c2 allele frequency in patients with ALD (0.1), however, was significantly (p = 0.0003; odds ratio (OR) 4.5, 95% CI 1.9-10.9) higher than in control subjects. The findings indicate that Caucasians carrying the Rsa I c2 allele of the CYP2E1 gene may be at higher risk of developing ALD if they abuse alcohol.

Genetic analysis of microsomal epoxide hydrolase in patients with carbamazepine hypersensitivity

Carbamazepine therapy is occasionally complicated by hypersensitivity reactions, the mechanism of which is poorly understood. It has been suggested that affected individuals may have a genetically-determined defect of microsomal epoxide hydrolase. The aim of this study was to determine whether a single genetic mutation or pattern of mutations could be used to predict individual susceptibility to carbamazepine-hypersensitivity. DNA was isolated from 10 carbamazepine-hypersensitive patients and 10 healthy volunteers. The patients had developed various forms of toxicity with carbamazepine, including toxic epidermal necrolysis, Stevens-Johnson syndrome, hepatitis and pneumonitis. The technique of polymerase chain reaction single-strand conformation polymorphism analysis (PCR-SSCP) was used to screen for mutations in all nine exons of the microsomal epoxide hydrolase gene. Any new mutations detected by this method were characterised by direct sequencing of the DNA. In addition, in the most severely affected patient, we sequenced all nine exons of the gene. There was a higher frequency of mutations in the hypersensitive group when compared with the controls, but there was no consistent mutation (or pattern of mutations) in the microsomal epoxide hydrolase gene which was common to the hypersensitive group. DNA sequencing of all nine exons of the microsomal epoxide hydrolase gene from the most severely affected patient showed the sequence to be "wild-type," when compared to the previously published sequences. The results of this study suggest that a single mutation within the coding region of the microsomal epoxide hydrolase gene cannot be the sole determinant of the predisposition to carbamazepine hypersensitivity.

Kinetic parameters of lymphocyte microsomal epoxide hydrolase in carbamazepine hypersensitive patients. Assessment by radiometric HPLC

Idiosyncratic hypersensitivity reactions with carbamazepine have been postulated to be due to a deficiency of microsomal epoxide hydrolase (HYL1), although this is based on indirect evidence. Using 3H-cis stilbene oxide (0.5 Ci/mmol) as a substrate, we have developed a radiometric HPLC assay sensitive enough to measure the kinetic parameters of HYL1 in lymphocytes. The intra-assay coefficient of variation was 8%. Enzyme activity has been measured in lymphocytes from six carbamazepine hypersensitive patients, six patients on carbamazepine without any adverse effects, and twelve drug-naive healthy volunteers. No significant difference was observed in three kinetic parameters of the enzyme among these three groups. The values for Km, Vmax, and intrinsic clearance ranged from 6.1-89.9 microM, 3.0-23.2 pmoles diol formed/min/mg protein, and 0.147-0.493 microliter/min/mg protein. There was no difference in enzyme activity between patients currently on carbamazepine and healthy volunteers, indicating a lack of induction of lymphocyte HYL1 by carbamazepine. Co-incubation of lymphocytes with 1,1,1-trichloropropene oxide, an inhibitor of hepatic HYL1, resulted in an 82% inhibition of activity, similar to that observed with the hepatic enzyme. The healthy volunteers were genotyped as being either GSTM1 positive (n = 6) or GSTM1 negative (n = 6). This did not affect the kinetic parameters of lymphocyte microsomal epoxide hydrolase. Our results suggest that there is normal HYL1 activity in lymphocytes of hypersensitive patients using cis-stilbene oxide as a substrate.

Detection of autoantibodies directed against human hepatic endoplasmic reticulum in sera from patients with halothane-associated hepatitis

1. Previous studies have demonstrated the presence of antibodies to trifluoroacetylated hepatic proteins (TFA-proteins) in sera from patients with the severe form of halothane-associated hepatitis (halothane hepatitis). The TFA-proteins are produced via cytochrome P450-mediated metabolism of halothane to the reactive species TFA-chloride. 2. To investigate the presence of autoantibodies (which recognize various non-TFA-modified human hepatic polypeptides) in patients with halothane hepatitis immunoblotting experiments were performed using microsomal fractions prepared freshly from livers of five different (halothane-free) tissue donors. Blots were developed using 15 well-characterised sera from patients with halothane hepatitis. 3. Autoantibodies to human hepatic polypeptides were detected in most, but not all, of the patients' sera. The pattern of antibody reactivity varied markedly between sera. Although no common pattern of antibody recognition was observed, polypeptides of molecular mass between 60 and 80 kDa were the predominant targets. A similar protein recognition pattern was seen when each positive serum was tested against the five individual human liver samples. 4. Such autoantibodies were not detected in sera from 16 normal human blood donors, but were detected in three of six sera from patients exposed to halothane without developing hepatitis. 5. The autoantibodies are thought to arise in patients exposed to halothane as a consequence of a halothane-induced immune response to chemically-modified proteins. Such antibodies could contribute to the complex pathological processes involved in halothane hepatitis.

An investigation of the interaction between halofantrine, CYP2D6 and CYP3A4: studies with human liver microsomes and heterologous enzyme expression systems

1. We have assessed the interaction of the antimalarial halofantrine with cytochrome P450 (CYP) enzymes in vitro, with the use of microsomes from human liver and recombinant cell lines. 2. Rac-halofantrine was a potent inhibitor (IC50 = 1.06 microM, Ki = 4.3 microM) of the 1-hydroxylation of bufuralol, a marker for CYP2D6 activity. Of a group of structurally related antimalarials tested, only quinidine (IC50 = 0.04 microM) was more potent. 3. Microsomes prepared from recombinant CYP2D6 and CYP3A4 cell lines were shown to catalyse halofantrine N-debutylation. 4. The metabolism of halofantrine to its N-desbutyl metabolite by human liver microsomes showed no correlation with CYP2D6 genotypic or phenotypic status and there was no consistent inhibition by quinidine. 5. The rate of halofantrine metabolism showed a significant correlation with both CYP3A4 protein levels (r = 0.88, P = 0.01) and the rate of felodipine metabolism (r = 0.86, P = 0.013), a marker substrate for CYP3A4 activity. Inhibition studies showed that ketoconazole is a potent inhibitor of halofantrine metabolism (IC50 = 1.57 microM). 6. In conclusion, we have demonstrated that halofantrine is a potent inhibitor of CYP2D6 in vitro and can also be metabolised by the enzyme. However, in human liver microsomes it appears to be metabolised largely by CYP3A4.

The bioactivation of amodiaquine by human polymorphonuclear leucocytes in vitro: chemical mechanisms and the effects of fluorine substitution

Amodiaquine, a 4-aminoquinoline antimalarial, has been associated with hepatitis and agranulocytosis in humans. Drug hypersensitivity reactions, especially agranulocytosis, have been attributed to reactive intermediates generated by the oxidants discharged from stimulated polymorphonuclear leucocytes (PMN). The metabolism of amodiaquine to both stable and chemically reactive metabolites by human PMN has been investigated in vitro. Incubation of [14C]-amodiaquine with PMN resulted in irreversible binding of radiolabel to protein and depletion of intracellular reduced glutathione, which were enhanced by phorbol myristate acetate (PMA), a PMN activator. Two metabolites were identified: the C-5' glutathione adduct of amodiaquine, derived from both endogenous and exogenous glutathione, and 4-amino-7-chloroquinoline, which was presumed to be formed by hydrolysis of amodiaquine quinoneimine. Desethylamodiaquine, the major plasma metabolite of amodiaquine in humans, also underwent bioactivation to a chemically reactive species in the presence of PMA-stimulated PMN. Substitution of the 4'-hydroxyl group in amodiaquine with fluorine significantly reduced irreversible binding to protein and abolished depletion of intracellular glutathione in the presence of PMA. These findings indicate that the bioactivation of amodiaquine by PMN is associated with the formation of a quinoneimine intermediate. Such a reactive metabolite, if produced in PMN or bone marrow in vivo, may be responsible for the drug's myelotoxicity.

Determination of human hepatic cytochrome P4501A2 activity in vitro use of tacrine as an isoenzyme-specific probe

Oxidative metabolism of the cognition activator tacrine (1,2,3,4-tetrahydro-9-aminoacridine) is thought to be catalyzed by cytochrome P4501A2 (CYP1A2). In this study, the use of tacrine as a specific substrate to measure CYP1A2 activity in vitro was investigated. Tacrine metabolism was assessed in 16 human liver microsomal samples. Initially, the percentage conversion of tacrine to stable metabolites (i.e. 1-, 2-, 4-, and 7-hydroxytacrine) at a single time point was correlated with levels of CYP1A2 apoprotein. Apoprotein was detected by immunoquantification using a monospecific CYP1A2 antipeptide antibody. Significant correlations were seen between CYP1A2 content and the degree of 1-hydroxylation (r = 0.81, p < 0.001), 7-hydroxylation (r = 0.70, p < 0.001), and metabolism to all stable products (r = 0.82, p < 0.001). The major metabolite formed in all livers was 1-hydroxytacrine. The conversion of tacrine to this metabolite was examined in more detail. The rate of formation varied from 19.2 pmol min-1 mg-1 to 101.0 pmol min-1 mg-1. There was a significant correlation (r = 0.84, p < 0.001) between the rate of formation and CYP1A2 levels. Tacrine metabolism was also compared with the rate of formation of 3-methylxanthine, from theophylline, a reaction previously shown to be catalyzed by CYP1A2. Significant correlations were found between 3-methylxanthine formation and all quantified tacrine metabolites. The rate of 3-methylxanthine generation also correlated with CYP1A2 apoprotein levels. It is concluded, therefore, that tacrine is a valuable probe for the determination of human hepatic CYP1A2 activity in vitro.

Vesicular stomatitis virus infection induces a nuclear DNA-binding factor specific for the interferon-stimulated response element

Vesicular stomatitis virus (VSV) has a broad host range. It replicates in the cytoplasm and causes rapid cytopathic effects. We show that following VSV infection, a nuclear factor that binds to a select set of interferon-stimulated responsive elements (ISRE) is induced in many cell types. This factor, tentatively called VSV-induced binding protein (VIBP), was estimated to have an approximate molecular mass of 50 kDa and was distinct from known members of the interferon regulatory factor family, that are known to bind to the ISRE. Induction of VIBP required tyrosine kinase activity but did not require cellular transcription. Treatment of cells with cycloheximide, which inhibits translation, only partially inhibited induction of VIBP. However, type I interferons and staurosporine, both of which inhibit VSV transcription, inhibited VIBP induction. Moreover, a double-stranded RNA analog, poly(I)-poly(C) also induced a DNA-binding activity very similar to that of VIBP. These results indicate that a preexisting cellular protein is activated upon VSV infection and that this activation requires primary viral transcripts. The functional activity of VIBP was analyzed in cells stably transfected with a herpesvirus thymidine kinase-luciferase reporter gene that is under control of the ISRE. While activity of the control promoter without ISRE was strongly inhibited following VSV infection (as a result of virus-mediated transcriptional shutdown of the host cell), the inhibition was reversed by the ISRE-containing promoter, albeit partially, which suggests that VSV infection differentially affects transcription of host genes. Although VIBP was induced in all other cells tested, it was not induced in embryonal carcinoma cells after VSV infection, suggesting developmental regulation of VIBP inducibility.

Evaluation of the generation of genotoxic and cytotoxic metabolites of benzo[a]pyrene, aflatoxin B1, naphthalene and tamoxifen using human liver microsomes and human lymphocytes

1. The ability of model stable epoxides and metabolites generated by human liver microsomes from benzo[a]pyrene, aflatoxin B1, naphthalene and tamoxifen to produce cytotoxicity and genotoxicity in human peripheral lymphocytes has been investigated. 2. The stable epoxides 1,1,1 trichloropropene-2,3-oxide (100 microM) and trans stilbene oxide (100 microM) as well as metabolites generated from aflatoxin B1 (30 microM) and naphthalene (100 microM) by an extracellular metabolising system were toxic to isolated resting mononuclear leucocytes (MNLs), whereas glycidol (100 microM), benzo[a]pyrene (100 microM) and tamoxifen (50 microM) were not. 3. The stable epoxides 1,1,1 trichloropropene-2,3-oxide (100 microM) and trans stilbene oxide (100 microM) but not glycidol (100 microM) were toxic to dividing lymphocytes only after a 72-h exposure. Tamoxifen (30 microM), aflatoxin B1 (30 microM) and their metabolites were also toxic to dividing lymphocytes. Benzo[a]pyrene (100 microM) and naphthalene (100 microM) were not toxic either in the absence or presence of the extracellular metabolising system. 4. Benzo[a]pyrene (100 microM) and aflatoxin B1 (30 microM) were directly genotoxic to lymphocytes, this genotoxicity was significantly enhanced by the presence of the extracellular metabolising system. This indicates that both intracellular and extracellular bioactivation of these two compounds can produce genotoxicity. In contrast, naphthalene and tamoxifen were non-genotoxic.

Clinical pharmacokinetics of tacrine

Tacrine is currently the only treatment approved for use in Alzheimer's disease. There is, however, considerable debate over its effectiveness due to conflicting clinical trial results. Most investigators agree, nevertheless, that a definite sub-population of patients do benefit from therapy with tacrine. Tacrine is associated with large pharmacokinetic interindividual variation within both patient and control groups. This is thought to influence both the efficacy and incidence of symptomatic adverse effects in individual patients. Following oral administration of tacrine the drug is rapidly and well absorbed with peak plasma concentrations (Cmax) achieved within 0.5 to 3 hours (after a single dose of 20 to 50mg). Tacrine appears to have a wide tissue distribution, which is reflected by its large volume of distribution. High concentrations of the drug were found in the kidney, liver, adrenal gland and brain tissue in animal models. It has a low bioavailability following oral intake, thought to result from extensive first-pass metabolism. Bioavailability can be increased upon rectal administration. The drug is rapidly and extensively metabolised in humans. In vitro metabolism studies have demonstrated the importance of cytochrome P450 (CYP1A2) in the biotransformation of tacrine to 1-, 2-, 4- and 7-hydroxylated metabolites. In humans, mono- and dihydroxylated tacrine and glucuronide conjugates were identified in the urine, which was the primary route of excretion. The elimination half-life of tacrine was short, 1.5 to 2.5 hours after single oral and intravenous doses and 2.9 to 3.6 hours after multiple oral doses. At low doses (10mg) of tacrine, the pharmacokinetic profile was nonlinear and the oral bioavailability of the drug was disproportionately low in comparison to higher doses of tacrine (20mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione S-transferase mu genotype (GSTM1*0) in Alzheimer's patients with tacrine transaminitis

1. Tacrine (1,2,3,4-tetrahydro-9-aminoacridine) which is used in Alzheimer's disease, causes elevation of liver transaminases ('tacrine transaminitis') in 40-50% of patients. This may be related to the formation of a chemically reactive metabolite from tacrine, which can be detoxified in vitro by glutathione. 2. Glutathione-S-transferase mu (GSTM1), a detoxication enzyme, is polymorphically expressed being absent in about 50% of patients. Its role in the detoxication of the reactive metabolite of tacrine is not known. 3. The frequency of the enzyme deficiency (GSTM1*0) has been investigated in patients with tacrine transaminitis using polymerase chain reaction (PCR) to determine whether the GSTM1 status can be used as an absolute predictive factor for susceptibility to tacrine transaminitis. 4. The frequency of the GSTM1*0 genotype in patients with tacrine transaminitis (n = 33; 45.5%) was not significantly different from that in patients treated with tacrine without liver dysfunction (n = 37; 43%), and when compared with all the controls used in the study (n = 167; 56%). 5. The frequency of the GSTM1*0 genotype in patients with Alzheimer's disease (n = 79; 46%) was not significantly different from that in healthy volunteers (n = 121; 59.5%). 6. Our results indicate that the GSTM1 status cannot be used clinically to predict individual susceptibility to tacrine transaminitis, and that patients with the GSTM1*0 genotype are unlikely to have an increased risk of tacrine-induced liver damage. Furthermore, the GSTM1 status was not associated with Alzheimer's disease.

The effect of chemical substitution on the metabolic activation, metabolic detoxication, and pharmacological activity of amodiaquine in the mouse

The adverse reactions associated with the antimalarial amodiaquine (AQ), agranulocytosis and hepatotoxicity, have been attributed to the bioactivation of the drug to a quinone imine metabolite. Therefore the effect of chemical modification on the metabolism of AQ was studied, with particular reference to the prevention of bioactivation and the introduction of glucuronidation. Glutathione conjugates of AQ and desethylAQ were eliminated in bile after intraportal administration of [3H]AQ (54 mumol/kg, 20 microCi/kg) to anesthetized male CD1 mice. Thioether conjugates excreted into bile over 3 h accounted for 28% of the administered dose. Fluorine substitution at the C-4 position of AQ blocked bioactivation, as measured by formation of thioether conjugates, and resulted in a 5-fold decrease in biliary excretion of radiolabeled dose: ca 6% versus ca 29%. Additional substitution of a primary alcohol function into one of the ethyl moieties introduced glucuronidation as a pathway of elimination, with 10% of the dose being excreted in bile as an O-glucuronide of the parent compound over a 3-h period; excretion of total radioactivity in bile increased 2.5-fold. These substitutions resulted in a 2-fold greater excretion of radiolabel into urine: 41% and 39% for DFAQ and HDFAQ, respectively, versus 23% for AQ. Novel carboxylic acid and N-oxide metabolites of the fluorinated analogues were identified. AQ and the two fluorinated analogues had similar activity against Plasmodium berghei in mice. These results demonstrate that the metabolism of AQ can be diverted from extensive bioactivation to direct detoxication by simple chemical substitutions that do not impair pharmacological activity.

Metabolism and bioactivation of clozapine by human liver in vitro

The metabolism of clozapine by human liver has been investigated in vitro. Irreversible protein-binding and conjunction with model nucleophiles have been used as markers for bioactivation of clozapine, while stable metabolite formation has been assessed using radiometric HPLC. In all nine liver microsomal preparations investigated, clozapine was extensively metabolized to the stable products desmethylclozapine (range 19%-27.2%), N-oxide (1.5-20.5%) and three polar metabolites (0-20.8%), and was bioactivated to a protein-reactive metabolite (0.6-2.1%). The CYP2D6 genotype did not influence the capacity of the livers to form these metabolites. All metabolic pathways were inhibited by ketoconazole, indicating the involvement of the cytochrome P450 enzymes. Isozyme-selective inhibitor studies demonstrated that whereas demethylation was performed by CYP1A2, N-oxidation and chemically reactive metabolite formation were dependent upon multiple forms of P450. The N-oxide was readily reduced back to clozapine in the presence of NADPH, this conversion being inhibited by ascorbic acid. Glutathione (1 mM) decreased covalent binding by 70%. The amount of putative adduct formed in the presence of glutathione (13.4 +/- 0.9%) was much greater than the covalent binding (mean 1.1 +/- 0.2%). The bioactivation of clozapine was, like the N-oxidation of clozapine, a reversible process. In summary, our results indicate clozapine undergoes extensive metabolism by human liver to both stable and chemically reactive metabolites, the formation of which is catalyzed by the cytochrome P450 enzymes. The role of the reactive metabolite, which may be a free radical, in the pathogenesis of clozapine agranulocytosis and hepatotoxicity requires further study.

Oocyst production and immunogenicity of Cryptosporidium baileyi in chickens and mallards

Two-day-old chickens and mallards were orally inoculated with one of 5 doses varying from 2 x 10(2) to 2 x 10(6) of C. baileyi oocysts per individual. Generally, the more oocysts inoculated were, the longer the patent periods were, and the more oocysts shedding were. Meanwhile increasing the inoculative dose, the prepatent periods were shortened except that mallards inoculated with 2 x 10(2) and 2 x 10(3) oocysts failed to produce the oocysts. The more parasites involving oocysts appeared from the chicken in comparison to the mallard. In the chickens challenged with a single dose of 2 x 10(6) oocysts, a small number of oocysts were detected from feces on days 4-14 after challenge infection (ACI) in all of carrageenan administered groups and in the control groups inoculated with 2 x 10(2) and 2 x 10(3) oocysts. In the mallards, a few oocysts were also recognized on days 5-15 ACI in all of carrageenan treated groups and in the control groups inoculated with 2 x 10(2), 2 x 10(3) and 2 x 10(4) oocysts. Just prior to challenge infection, phagocytic activity of peritoneal macrophages (Mø) and the number of peripheral Mø in both birds were significantly decreased in the carrageenan treated groups as compared to the control groups. Mild challenge infection in both birds denoted that the immunogenicity of C. baileyi to the birds was very strong, despite Mø blocker carrageenan administration.

Cimetidine improves the therapeutic/toxic ratio of dapsone in patients on chronic dapsone therapy

We have previously shown that cimetidine, given concurrently for 2 weeks to patients on chronic dapsone therapy, reduced methaemoglobinaemia by inhibiting the formation of the toxic hydroxylamine metabolite of dapsone. The aim of the present study was to examine the effect of this combination on the benefit/toxic ratio of dapsone over a longer period. Eight patients (six dermatitis herpetiformis, one linear IgA disease, one folliculitis decalvans) on long-term dapsone 50-100 mg daily, took cimetidine 1.6 g daily concurrently for 3 months. At 3-weekly intervals, a clinical assessment was made, plasma dapsone and methaemoglobin were measured, and parameters of oxidative haemolysis were monitored. The dapsone level rose from 2298 +/- 849 ng/ml (mean +/- SD) at baseline to 3006 +/- 1131 ng/ml at week 3 of cimetidine (P < 0.01). This rise in plasma dapsone was sustained during cimetidine administration, falling to 2446 +/- 954 ng/ml when cimetidine was stopped (P < 0.02). The methaemoglobin fell from 5.5 +/- 2.2% (mean +/- SD) at baseline to 3.9 +/- 1.1% at week 3 (P < 0.01), and remained low until week 12, when there was a return to baseline values (P < 0.01). The haemoglobin did not change from the baseline of 12.7 +/- 0.3 g/dl (mean +/- SD), and other parameters of haemolysis were unaltered. There was a fall in the visual analogue score for headache (P < 0.05), but this was not associated with any deterioration in control of the skin disorders. Hence, long-term concurrent cimetidine results in increased plasma dapsone levels without increased haemolysis, and is accompanied by reduced methaemoglobinaemia for more than 2 months.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of hepatic metabolism in the bioactivation and detoxication of amodiaquine

1. The hepatic metabolism of the antimalarial drug amodiaquine was investigated in order to gain further insight into the postulated metabolic causation of the hepatotoxicity, which restricts the use of the drug. After intraportal (i.p.) administration (54 mumol/kg) to the anaesthetized rat, the drug was excreted in bile (23 +/- 3% dose over 5 h; mean +/- SD, n = 6) primarily as thioether conjugates. 2. After i.p. administration, 20% of the dose was excreted into urine over 24 h as parent compound and products of N-dealkylation and oxidative deamination. Desethylamodiaquine accumulated in liver, but was not a substrate for bioactivation as measured by biliary elimination of a glutathione adduct. 3. Prior administration of ketoconazole, an inhibitor of P450, reduced biliary excretion by 50% and effected a corresponding decrease in the amount of drug irreversibly bound to liver proteins. This indicated a role for P450 in the bioactivation of amodiaquine to a reactive metabolite that conjugates with glutathione and protein. 4. De-ethylation and irreversible binding were observed in vitro using male rat liver microsomes, and were again inhibited by ketoconazole. However, no such binding was observed with human (six individuals) hepatic microsomes despite extensive turnover of amodiaquine to desethylamodiaquine. 5. Amodiaquine quinoneimine underwent rapid reduction in the presence of either human or rat liver microsomes. Therefore in vitro studies may underestimate the bioactivation of amodiaquine in vivo. These data indicate that the extent of protein adduct formation in the liver will depend on the relative rates of oxidation of amodiaquine and reduction of its quinoneimine. This in turn may be a predisposing factor in the idiosyncratic hepatotoxicity associated with amodiaquine. 6. Substitution of a fluorine for the phenolic hydroxyl group in amodiaquine blocked bioactivation of the drug in vivo. Insertion of an N-hydroxyethyl function enabled partial clearance of amodiaquine and its deshydroxyfluoro analogue via O-glucuronidation and altered the balance between phase I oxidation and direct phase II conjugation of amodiaquine.