Intracellular glutathione in the peripheral blood cells of HIV-infected patients: failure to show a deficiency

OBJECTIVE

To determine whether HIV-infected patients have a deficiency of intracellular glutathione (GSH) in peripheral blood mononuclear cells (PBMC) and erythrocytes.

DESIGN

Initial experiments determining the stability of intracellular GSH preceded the measurement of GSH levels in 33 HIV-positive patients and 40 control subjects within 1 h of isolation of their blood cells. In addition, the susceptibility of erythrocytes to dapsone hydroxylamine-induced methaemoglobinaemia was evaluated.

METHODS

GSH levels were determined by an high-performance liquid chromatography method utilizing a fluorescent probe, monobromobimane. The bimane-GSH adduct formed in PBMC was also characterized by mass spectrometry. Methaemoglobin formation on exposure to dapsone hydroxylamine was determined spectrophotometrically.

RESULTS

GSH levels remained stable for only 1 h after cell isolation, thereafter showing a decrease of 20 and 60% at 4 and 24H, respectively, There was no difference in the GSH levels in PBMC and erythrocytes of the HIV-positive patients compared with controls. The GSH levels were not related to the disease stage or to CD4+ cell counts. There was no difference in GSH levels in PBMC taken from trimethoprim-sulphamethoxazole-hypersensitive and non-hypersensitive patients. Methaemoglobinaemia on exposure of erythrocytes to dapsone hydroxylamine was concentration-dependent, but there was no significant difference between patients and controls.

CONCLUSION

In contrast to previous studies, no deficiency of intracellular GSH in the PBMC and erythrocytes of HIV-infected patients was found. The discrepancy between studies may be methodological reflecting the instability of GSH, which requires prompt sample analysis.

Dexamethasone metabolism by human liver in vitro. Metabolite identification and inhibition of 6-hydroxylation

The metabolism of the synthetic glucocorticoid dexamethasone in human liver microsomal incubations has been studied. Metabolites were analyzed by radiometric high-performance liquid chromatography and were identified by liquid-chromatography-mass spectrometry; in addition, the major metabolite 6beta-hydroxydexamethasone was identified by cochromatography with a chemically synthesized standard. A total of 17 human livers were used in this study and the following metabolites were identified: 6beta-hydroxydexamethasone, 6 alpha-hydroxydexamethasone, 6-hydroxy-9 alpha-fluoro-androsta-1,4-diene-11 beta-hydroxy-16 alpha-methyl-3,17-dione (6-hydroxy-9 alpha-F-A) and 9 alpha-fluoro-androsta-1,4-diene-11 beta-hydroxy-16 alpha-methyl-3,17-dione (9 alpha-F-A). Dexamethasone underwent side-chain cleavage to form 9 alpha-F-A. This metabolite was then a substrate for 6-hydroxylation. There was considerable interindividual variability in metabolic profiles. Mean (+/-S.D.) K(m) values for 6 beta- and 6 alpha-hydroxydexamethasone formation were 23.2 +/- 3.8 and 25.6 +/- 1.6 microM (n = 4), respectively. The corresponding V max values were 14.3 +/- 9.9 and 4.6 +/- 3.1 pmol x min(-1) mg protein (-1). Ketoconazole (3 microM) completely inhibited 6 alpha- and 6 beta-hydroxylation, indicating that formation of both metabolites was catalyzed by CYP3A4. This was confirmed in studies of correlations between the rate of metabolite formation and the relative expression of CYP3A4: r = 0.74 for 6 beta-hydroxydexamethasone, P = .003; r = 0.70 for 6 alpha-hydroxydexamethasone, P = .006. In addition to ketoconazole, both ellipticine and gestodene caused marked inhibition of 6-hydroxylation. Ellipticine is clearly not a selective CYP1A inhibitor as has been stated previously. However, furafylline (CYP1A inhibitor), tolbutamide (CYP2C substrate), and sulfaphenazole (CYP2C inhibitor) were essentially noninhibitory. The relatively simple metabolic profile of dexamethasone compared to other steroids may point to this being a potentially useful in vivo probe for CYP3A4 in humans.

Bioactivation of carbamazepine in the rat in vivo. Evidence for the formation of reactive arene oxide(s)

The metabolism of carbamazepine (CBZ) and its major metabolite in humans, carbamazepine 10,11-epoxide (CBZ-E), was examined in the rat in vivo. Particular emphasis was placed on the identification of dihydrohydroxythio adducts, which are detoxication products of reactive arene oxide intermediates. Anesthetized and cannulated male Wistar rats were administered [3H]CBZ (25 micrograms.kg-1 or 25 mg.kg-1) or [3H]CBZ-E (25 micrograms.kg-1 or 25 mg.kg-1) intravenously and bile and urine collected for 5 hr. Less than 8% of drug was excreted in the urine for each dosing regimen. Biliary excretion accounted for 73.7 +/- 6.2 and 41.8 +/- 6.2% (mean +/- SD, N = 4) of administered CBZ (25 micrograms.kg-1 and 25 mg.kg-1, respectively) and 47.6 (N = 2) and 28.1 +/- 6.0% of administered CBZ-E (25 micrograms.kg-1 and 25 mg.kg-1, respectively). The major route of metabolism of both CBZ and CBZ-E was N-glucuronidation. In rats given CBZ (25 mg.kg-1), the N-glucuronide of the parent compound accounted for 12.6 +/- 2.6% of the dose, whereas CBZ-E N-glucuronide accounted for 12.3 +/- 3.8% of the dose. At the lower dose of 25 micrograms.kg-1, these accounted for 18.6 +/- 3.0 and 36.7 +/- 4.7% of the dose, respectively. Similarly, for rats given CBZ-E (25 micrograms.kg-1), the N-glucuronide of the parent compound was the major metabolite, accounting for 19.1 +/- 4.5% of the dose. O-glucuronides were relatively minor metabolites of both drugs. Glutathione adducts were identified in the bile of both groups of animals. Although these adducts were relatively minor metabolites of CBZ-E (1.8% of the dose), they were more substantial products of the metabolism of CBZ. Three isometric glutathionyl dihydrohydroxy-CBZ adducts were identified by LC/MS. They collectively accounted for 5.8 +/- 0.9% of the dose. In conclusion, we have provided evidence, in rats, for the generation of a reactive arene oxide species from CBZ. If not adequately detoxified, via conjugation with glutathione, this has the potential to initiate cellular damage. In humans, a similar mechanism may be involved in CBZ-associated hypersensitivity.

4-Hydroxyhexenal is a potent inducer of the mitochondrial permeability transition

Mitochondria undergo at least two types of structural alteration in response to various physiological and pathophysiological stimuli. One type is nonreversible and is associated with mitochondrial lysis. The second is reversible and appears to be associated with calcium-mediated activation of a specific inner mitochondrial membrane channel. The mechanisms underlying the induction of this second alteration, termed a mitochondrial permeability transition (PT), have been the subject of a great deal of recent research. Using rat liver mitochondria, our data demonstrate that calcium-mediated PT induction can be affected by the lipid peroxidation byproducts 4-hydroxynonenal and 4-hydroxyhexenal (HHE). 4-Hydroxynonenal appears inactive at concentrations <1 micromole but displays both stimulatory and inhibitory effects as part of a biphasic dose response between approximately 1 and 200 micromole. In contrast, HHE consistently enhances calcium-mediated induction of the PT, even at femtomolar concentrations. The exquisite specificity and sensitivity of HHE led to further studies to examine the nature of this induction. Studies showing that HHE-mediated induction could be prevented by cyclosporin A confirmed PT involvement. Further studies showed that induction was dependent on both calcium and electron transport chain function. Pretreatment of the HHE with glutathione also prevented PT induction, but simultaneous addition of the thiol reagents dithiothreitol or glutathione, which often prevents PT induction, was ineffective, attesting to the effectiveness of HHE as an inducer. Together, these data provide a possible mechanistic explanation for the previously observed effects of lipid peroxidation on PT induction.

Pathogenesis of plaque variants of porcine reproductive and respiratory syndrome virus in pregnant sows

OBJECTIVE

To determine whether porcine reproductive and respiratory syndrome virus plaque variants vary in their pathogenicity in causing late-term reproductive failure.

DESIGN

Four groups of 2 sows each at 86 days of gestation were inoculated intranasally with PRRSV small (MN-Hs) and large (MN-HI) plaque variants, field isolate, and cell culture medium, respectively. In addition, 2 sows each at 86 days of gestation were inoculated intranasally or IM with MN-Hs virus.

ANIMALS

14 pregnant sows.

PROCEDURE

Inoculated sows were allowed to deliver at term, and each litter was examined for clinical abnormality and presence of virus.

RESULTS

Two sows infected with MN-Hs virus delivered 14 live and 5 dead pigs, whereas 2 sows infected with MN-HI virus delivered 0 live and 25 dead pigs. Two sows inoculated with a field isolate (MN-W) delivered 10 live and 20 dead pigs. Two control sows had 26 normal fetuses at slaughter at 107 days of gestation. Virus was isolated from 16 (66.7%) of 24 liveborn pigs, 9 (64.3%) of 14 stillborn pigs, and 3 (12.0%) of 25 mummified fetuses of the 6 infected sows. Subsequently, 4 MN-Hs-infected sows delivered 40 live and 11 dead pigs.

CONCLUSIONS

Marked difference in the pathogenicity in pregnant sows between porcine reproductive and respiratory syndrome virus strains was documented. The MN-Hs virus is considered to be of low pathogenicity, but the other viruses are highly pathogenic for late-term pregnant sows.

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)