Metabolic activation of N-hydroxy-2-aminofluorene and N-hydroxy-2-acetylaminofluorene by monomorphic N-acetyltransferase (NAT1) and polymorphic N-acetyltransferase (NAT2) in colon cytosols of Syrian hamsters congenic at the NAT2 locus

Acetylator genotype is regulated at the polymorphic acetyltransferase (NAT2) gene locus in humans and other mammals such as Syrian hamsters. Human slow acetylator phenotypes have been associated with increased incidences of urinary bladder cancers, whereas rapid acetylators have been associated with increased incidences of colorectal cancers. The genetic predisposition of rapid acetylators to colorectal cancers suggests localized metabolic activation of arylamine carcinogen metabolites by polymorphic N-acetyltransferase (NAT2) in colon tissues. We tested this hypothesis in Bio. 82.73/H Syrian hamster lines which are congenic at the NAT2 gene locus. Congenic Bio. 82.73/H Syrian hamsters expressed acetylator genotype-dependent N-acetyltransferase activity in colon cytosols toward arylamine carcinogens such as 2-aminofluorene and 4-aminobiphenyl. Partial purification of the hamster colon cytosol by anion exchange chromatography identified two N-acetyltransferase isozymes analogous to those previously described in liver and urinary bladder. One of the isozymes (NAT2) exhibited acetylator genotype-dependent expression for the N-acetylation of each arylamine tested: p-aminophenol; 2-aminofluorene; 4-aminobiphenyl; 3,2'-dimethyl-4-aminobiphenyl; and 2-amino-dipyrido[1,2-a:3',2'd]imidazole as well as for the metabolic activation (via O-acetylation) of N-hydroxy-2-aminofluorene to form DNA adducts. Although NAT2 catalyzed the metabolic activation of N-hydroxy-2-acetyl-aminofluorene to DNA adducts, the rates were lower, were paraoxon-sensitive, and did not reflect acetylator genotype. A second isozyme (NAT1) also catalyzed the N-acetylation of each arylamine as well as the metabolic activation of N-hydroxy-2-aminofluorene and N-hydroxy-2-acetylaminofluorene to DNA adducts at rates that were independent of acetylator genotype. Metabolic activation of N-hydroxy-2-aminofluorene catalyzed by both NAT1 and NAT2 was resistant to 100 microM paraoxon, an inhibitor of microsomal deacetylases. Metabolic activation of N-hydroxy-2-acetylaminofluorene by NAT1 and NAT2 was partially sensitive to 100 microM paraoxon. Michaelis-Menten kinetic constants were determined for the colon NAT1 and NAT2 isozymes and compared to previous determinations for liver NAT1 and NAT2. For each of the arylamines tested, both apparent Km and apparent Vmax were higher for NAT2 than NAT1. In rapid acetylator hamster colon, NAT2/NAT1 activity ratios were 18 and 13 for the N-acetylation of 2-aminofluorene and 4-aminobiphenyl and 28 for the O-acetylation of N-hydroxy-2-aminofluorene. These results strongly support the role of the polymorphic NAT2 gene locus in the local metabolic activation of N-hydroxyarylamine carcinogens in colon and provide mechanistic support for human epidemiological studies suggesting a predisposition of rapid acetylators to colorectal cancer.

Evidence for peroxidase-mediated metabolism of cyclophosphamide

Lung and bladder injury are side-effects of chemotherapy with cyclophosphamide (CP). Although the mixed-function oxidases are known to metabolize CP to reactive species, inhibitors of these enzymes have no significant effect on therapeutic activity or lung toxicity in vivo. In contrast, inhibitors of prostaglandin H synthase (PHS) can significantly decrease CP-induced lung injury. The current studies examined whether peroxidases can bioactivate CP in vitro. A mixture of 14C (ring) and 3H (side chain)-labeled CP (0.25-2 mM) was incubated with fresh lung and liver microsomes, horseradish peroxidase, or purified PHS using H2O2 or arachidonate as cosubstrates. All systems showed irreversible binding of CP-derived radioactivity to protein, increases in oxygen consumption, and generation of polar metabolites, including acrolein. Arachidonate-catalyzed binding of CP-derived radioactivity to fresh lung or liver microsomes, as well as H2O2 or arachidonate-supported generation of polar metabolites by purified PHS, was maximal by 2 min. In contrast, NADPH-catalyzed microsomal binding was linear for 30 min. This is consistent with the rapid self-inactivation of PHS in vitro. Arachidonate-catalyzed binding of CP-derived radioactivity to pulmonary microsomes was greater than liver, whereas the converse was true with NADPH. Incubation of 0.5 to 4 mM CP with PHS and arachidonate resulted in concentration-dependent increases in prostaglandin E2 synthesis. One mM H2O2 generated more polar metabolites from CP than 0.3 mM arachidonate when incubated with 100 units of purified PHS (10 +/- 2 and 4 +/- 1.5 nmol in 2 min, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic activation of phosphoinositide hydrolysis induced in brain slices by norepinephrine and the excitatory amino acid agonist, trans-ACPD

Norepinephrine and trans-1-aminocyclopentyl-1,3-dicarboxylic acid (ACPD) each individually stimulated hydrolysis of phosphoinositides and when tested in combination caused a stimulation that was 50-90% greater than additive in hippocampal and cortical slices of the rat but not in striatal slices. This synergistic augmentation of hydrolysis of phosphoinositide was evident with all stimulatory concentrations of norepinephrine and of ACPD up to 1 mM, at which point ACPD was inhibitory. A time-course study revealed no lag in the synergistic interaction and no down-regulation through 60 min of incubation of the augmented response to the combined agonists. The synergistic reaction was mediated by alpha 1-adrenergic receptors and by metabotropic excitatory amino acid receptors. Increased intracellular calcium, but not activation of protein kinase C, may play a role in mediating the synergistic interaction. Thus, a unique synergistic stimulatory interaction was found between two receptors coupled with phosphoinositide metabolism, a finding which also supports the suggestion that these two systems are co-localized in some cells.

Role of bradykinin in microbial infection: enhancement of septicemia by microbial proteases and kinin

Data presented herein will show that bradykinin, microbial proteases which activate the kinin generating cascade, and kininase inhibitors can enhance septicemia by approximately 10 to 100 fold in mice infected intraperitoneally (i.p.) with a strain of bacteria, Pseudomonas aeruginosa 621, which does not usually produce a kinin generating protease. Bacterial spreading was evaluated either in the blood or in the spleen by colony formation on agar plates. Using the P. aeruginosa kaguma strain which produces a large amount of proteases, further experiments were carried out. Results showed that two different protease inhibitors (ovomacroglobulin and a synthetic peptide inhibitor against pseudomonal elastase) as well as a kinin antagonist suppressed bacterial dissemination to 1/10-1/100 of control. Similar results were observed in experiments using Vibrio vulnificus. These data support the hypothesis that microbial proteases and especially bradykinin is responsible for facilitation of microbial dissemination in vivo.

Biotransformation of tri-substituted methoxyamphetamines by Cunninghamella echinulata

1. Four trimethoxyamphetamine analogues were incubated with the filamentous fungus Cunninghamella echinulata. 2. 2,4,5-Trimethoxyamphetamine and 2,5-dimethoxy-4-ethoxyamphetamine were poorly metabolized by C. echinulata ATCC 9244 and C. echinulata var. elegans ATCC 9245. 2,5-Dimethoxy-4-(n)-propoxyamphetamine was mainly metabolized through N-acetylation and O-dealkylation with minor amounts of several aliphatic hydroxylation metabolites formed. 2,5-Dimethoxy-4-methylthioamphetamine was extensively metabolized to the corresponding sulphoxide. 3. 2,5-Dimethoxy-4-methylthioamphetamine metabolism was inhibited by ethanol and quinidine. Sparteine did not inhibit the formation of the sulphoxide and may have shunted the substrate through alternate metabolic pathways. 4. Incubation conditions can affect the rate and extent of fungal biotransformation of 2,5-dimethoxy-4-methylthioamphetamine, and influence dextrose utilization, ammonia formation and pH.

Influence of long-term ethanol treatment on in vitro biotransformation of benzo(a)pyrene in microsomes of the liver, lung and small intestine from male and female rats

The influence of long-term ethanol exposure of rats on the microsomal biotransformation of benzo(a)pyrene [B(a)P] was studied. Male and female Wistar rats received an increasing amount of ethanol in their drinking water: percentages rose to 15% (w/v) in 3 weeks. The ethanol content was kept at a concentration of 15% for another 3 weeks. Livers, lungs and intestinal epithelial cells of the rats were then isolated and microsomal fractions prepared. In all organs, the metabolite most formed was 3-hydroxy-B(a)P. In the liver, males showed significantly higher B(a)P hydroxylase activity than females. On the basis of experiments using monoclonal antibodies, a significant part of the B(a)P biotransformation in male rat liver microsomes can be attributed to the male specific P4502C11. In the lung and intestine, there were no significant differences between the sexes. In the liver, ethanol treatment significantly decreased the microsomal formation of phenolic metabolites. In microsomes of intestinal epithelial cells, ethanol treatment enhanced the formation of phenols and diols. In conclusion, ethanol consumption by rats in moderate amounts leads to an alteration in the microsomal biotransformation of B(a)P. Effects are most prominent in the liver, where the formation of phenols is significantly decreased.

[The effect of phytin, benzonal and their combination administered prophylactically on the pharmacodynamics of drugs metabolized in the liver in hypokinesia]

Hexenal, meprobamate, amidopyrine and ethylmorphine produced a significantly marked effect in animals under hypokinesia as compared with normal rats. When phytin, benzonal and their combination were used for preventive purposes, impaired pharmacodynamics of the tested drugs metabolizing in the liver disappeared. The investigations demonstrated that the preventive use of phytin in combination with benzonal is the most optimal in correcting the impairments of drug pharmacodynamics in hypokinesia.

Mutagenic activity of p-toluenesulfonhydrazide

Toluenesulfonhydrazide (TSH) is a high volume production chemical for which there is relatively little toxicological data. In this study, the mutagenic activity of TSH was determined in the Salmonella/mammalian microsome assay and the in vitro chromosomal aberration assay using Chinese hamster ovary cells. TSH induced gene mutations both with and without metabolic activation in the Salmonella/mammalian microsome assay but that it did not induce chromosomal aberrations in Chinese hamster ovary cells. The results of this study indicate that TSH is an in vitro mutagen and should be assessed for in vivo mutagenicity.

Inhibition of phenytoin bioactivation and teratogenicity by dietary n-3 fatty acids in mice

Evidence suggests that the teratogenicity of the anticonvulsant drug phenytoin (DPH) can result from its bioactivation via embryonic prostaglandin synthase and/or maternal cytochromes P450. This study examined whether DPH bioactivation and teratogenicity could be reduced by dietary n-3 fatty acids. Female CD-1 mice were fed diets containing 2 wt% safflower oil and 10 wt% of either hydrogenated coconut oil, safflower oil, or a cod liver oil/linseed oil mixture (CLO/LO) for three weeks prior to impregnation and throughout gestation. DPH (55 or 65 mg/kg) was administered via intraperitoneal injections to pregnant mice at 0900 on gestational days 12 and 13, and on day 19 fetuses were given teratologic assessments. A similar dietary study evaluated in vivo covalent binding of radiolabeled DPH administered on day 12, and dams were killed 24 h later. A reduction in DPH-induced cleft palates and a decrease in DPH covalent binding to embryonic protein was observed in the CLO/LO group. Feeding CLO/LO enhanced incorporation of n-3 fatty acids into embryos and inhibited embryonic prostaglandin synthase activity. No differences in maternal hepatic cytochromes P450 activities were observed among dietary treatments. These data indicate that dietary n-3 fatty acids could reduce DPH teratogenicity via inhibition of embryonic prostaglandin synthase bioactivation of DPH.

Methimazole-mediated modulation of netobimin biotransformation in sheep: a pharmacokinetic assessment

The effects of modulation of liver microsomal sulphoxidation on the disposition kinetics of netobimin (NTB) metabolites were investigated in sheep. A zwitterion suspension of NTB was given orally at 7.5 mg/kg to sheep either alone (control treatment) or co-administered with methimazole (MTZ) orally (NTB + MTZ oral treatment) or intra-muscularly (NTB + MTZ i.m.) at 3 mg/kg. Blood samples were taken serially over a 72 h period and plasma was analysed by HPLC for NTB and its major metabolites, i.e. albendazole (ABZ), albendazole sulphoxide (ABZSO) and albendazole sulphone (ABZSO2). Only trace amounts of NTB parent drug and ABZ were detected in the earliest samples after either treatment. There were significant modifications to the disposition kinetics of ABZSO in the presence of MTZ. ABZSO elimination half-life increased from 7.27 h (control treatment) to 14.57 h (NTB + MTZ oral) and to 11.39 h (NTB + MTZ i.m.). ABZSO AUCs were significantly higher (P less than 0.05) for the NTB + MTZ oral treatment (+55%) and for the NTB + MTZ i.m. treatment (+61%), compared with the NTB alone treatment. The mean residence times for ABZSO were 12.66 +/- 0.68 h (control treatment), 18.85 +/- 2.35 h (NTB + MTZ oral) and 17.02 +/- 0.90 h (NTB + MTZ i.m.). There were no major changes in the overall pharmacokinetics of ABZSO2 for the concomitant MTZ treatments. However, delayed appearance of this metabolite in the plasma resulted in longer ABZSO2 lag times and a delayed Tmax for treatments with MTZ.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone pulmonary metabolism and tumorigenicity in mice by analogues of the investigational chemotherapeutic drug 4-ipomeanol

4-Ipomeanol (IPO) is an investigational chemotherapeutic drug with specific toxicity toward the lung. It is metabolically activated to reactive intermediates by cytochrome P450 enzymes present in Clara cells. 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a highly carcinogenic tobacco-specific nitrosamine with organo-specificity for the lung. Like IPO, which it resembles structurally, it is metabolically activated by cytochrome P450 enzymes of rat Clara cells. We synthesized nontoxic analogues of IPO and tested their activities as inhibitors of the metabolism and tumorigenicity of NNK. The IPO analogues synthesized were 4-hydroxy-1-phenyl-1-pentanone (HPP), 7-hydroxy-1-phenyl-1-octanone (HPO), 4-hydroxy-1-(2-thienyl)-1-pentanone (HTP), and 4-hydroxy-1-(3-pyridyl)-1-pentanone (HPYP). When added to A/J mouse lung microsomal incubations, all compounds significantly inhibited the oxidative pathways of NNK metabolism--alpha-hydroxylation and pyridine N-oxidation--to varying extents. Inhibition of carbonyl reduction of NNK was generally less effective. Inhibition of alpha-hydroxylation by IPO, HPP, and HTP was more pronounced in incubations with lung microsomes than with liver microsomes. None of the IPO analogues showed significant toxicity when given to A/J mice at a dose of 25 mumol; IPO itself was lethal at this dose. HPP and HPO, at doses of 25 mumol, significantly inhibited lung tumor multiplicity in mice treated with NNK; the other analogues and IPO itself were ineffective. The results of this study provide new leads for development of inhibitors of NNK metabolism and chemical probes for the active site of P450 enzymes in Clara cells.

Effects of antibacterial agents on in vitro ovine ruminal biotransformation of the hepatotoxic pyrrolizidine alkaloid jacobine

Ingestion of pyrrolizidine alkaloids, naturally occurring plant toxins, causes illness and death in a number of animal species. Senecio jacobaea pyrrolizidine alkaloids cause significant economic losses due to livestock poisoning, particularly in the Pacific Northwest. Some sheep are resistant to pyrrolizidine alkaloid poisoning, because ovine ruminal biotransformation detoxifies free pyrrolizidine alkaloids in digesta. Antibacterial agents modify ruminal fermentation. Pretreatment with antibacterial agents may account for some animal variability in resistance to pyrrolizidine alkaloid toxicosis, and antibacterial agents can also be used for characterizing ruminal pyrrolizidine alkaloid-biotransforming microflora. The objective of this study was to evaluate the effects of antibacterial agents on biotransformation of a predominant S. jacobaea pyrrolizidine alkaloid, jacobine, in ovine ruminal contents. Ovine ruminal jacobine biotransformation was tested in vitro with 20 independent antibacterial agents. Low amounts of rifampin and erythromycin prevented jacobine biotransformation. Chlortetracycline, lasalocid, monensin, penicillin G, and tetracycline were slightly less effective at inhibiting jacobine biotransformation. Bacitracin, crystal violet, kanamycin, and neomycin were moderately inhibitory against jacobine biotransformation. Brilliant green, chloramphenicol, gramicidin, nalidixic acid, polymyxin B SO4, sodium azide, streptomycin, sulfisoxazole, and vancomycin had little to no effect on jacobine biotransformation. The antibiotics that were most effective at inhibiting biotransformation were those that are active against gram-positive bacteria. Therefore, gram-positive bacteria are most likely critical members of the jacobine-biotransforming consortia.

Some nutritional requirements of a mixed culture transforming Reichstein's compound S into prednisolone

Reichstein's compound S was successfully converted to prednisolone in a single-step fermentation using a mixed culture of Curvularia lunata and Mycobacterium smegmatis. Introducing additional medium at the time of bacterial inoculation and increasing the M. smegmatis inoculum to 8% were necessary for maximal dehydrogenation of cortisol to prednisolone (86%). However, beef extract, corn-steep solids, and malt extract were inhibitory to the dehydrogenase activity and stimulatory to hydroxylase. Of the vitamins tested, nicotinic acid and riboflavin at 0.2 and 1.13 mg/L, respectively, resulted in maximum transformation of Reichstein's compound S (100%) and optimized prednisolone yields (92%) in the mixed culture. The trace elements present in the medium were sufficient for maximal transformation, and there was no need for an exogenous supply. Addition of ATP, sodium acetate, and NAD inhibited the dehydrogenation reaction.

[Ligand spectrum of hemoglobin activity of methemoglobin-reductase and hemolytic resistance of erythrocytes during chronic exposure to nitrates]

It is found that nitrite-ions formed as a result of biotransformation during long term feeding of calves with sodium and potassium nitrates induce changes in some biochemical parameters of blood, including HS-glutathione content in erythrocytes, acid hemolytic resistance of erythrocytes, activity of NAD-dependent methemoglobin-reductase, correlation of ligand forms of hemoglobin and its total content. It is supposed that the observed changes are of an adaptational character and, as a whole, provide for the optimization of both quantitative and qualitative composition of population of erythroid cells at the expense of erythropoiesis intensification.

Effect of p-xylene inhalation on the bioactivation of bromobenzene in rat lung and liver

It is unclear whether the pneumotoxicity observed with bromobenzene (BB) in phenobarbital-induced rats is related to BB bioactivation in lung, liver or both. To help differentiate pulmonary from hepatic bioactivation, BB was administered alone and in combination with p-xylene, which inhibits pulmonary but induces hepatic cytochromes P450. Exposure to p-xylene alone (3400 ppm for 4 hr) produced no changes in bronchoalveolar lavage fluid (BALF) measurements (gamma-glutamyl transpeptidase, lactate dehydrogenase, protein, white blood cell count) or serum sorbitol dehydrogenase. p-Xylene increased hepatic microsomal benzyloxy- (BROD), pentoxy- (PROD), and ethoxy- (EROD) resorfuin O-dealkylase activities but decreased pulmonary microsomal BROD and PROD. Immunoblot analysis revealed an induction of hepatic but not pulmonary microsomal P450IIB apoprotein. When rats were exposed to p-xylene (2800 ppm) or room air for 4 hr, treated 12 hr later with BB (0.5 ml/kg, ip) or corn oil, and killed after 12 hr, p-xylene increased hepatic P450IIB (27-fold) concomittant with a similar increase in BROD activity. p-Xylene also increased hepatic P450IA apoprotein (3.4-fold) with a complimentary increase in EROD activity. p-Xylene potentiated BB-induced hepatotoxicity. In pulmonary microsomes p-xylene and BB each produced similar decreases in both EROD and BROD activities. The combination of p-xylene and BB had an additive effect on pulmonary P450IA1 reduction. BALF analysis and histopathology revealed no pneumotoxicity with any treatment. p-Xylene potentiation of BB-induced hepatotoxicity without pneumotoxicity suggests that the liver does not produce metabolites of BB which are directly involved in pulmonary damage.

Effects of valproate on xenobiotic biotransformation in rat liver. In vivo and in vitro experiments

Male Wistar rats were in vivo exposed for 2 weeks to 100 micrograms/ml sodium valproate by subcutaneous implantation of osmotic pumps and hepatocytes were isolated. As an in vitro model co-cultures of rat hepatocytes with epithelial cells were daily treated with valproate (25, 50, 100, 200 micrograms/ml) for 2 weeks. In both models the cytochrome P-450 content and the enzymatic activities of 7-ethoxycoumarin O-deethylase, aldrin epoxidase and glutathione S-transferase were determined in valproate-treated hepatocytes, in controls and in phenobarbital-induced cells. It appeared that in both systems the cytochrome P-450 content and the 7-ethoxycoumarin O-deethylase activity increased significantly after valproate treatment. On the other hand, the activities of aldrin epoxidase and glutathione S-transferase decreased. A cDNA probe, encoding rat P450IIB2 was used to determine whether mRNAs encoding the P450IIB subfamily were induced by valproate. It became clear that the inducing effect of valproate was even more pronounced in vitro than in vivo.

Participation of rat liver cytochrome P450 2E1 in the activation of N-nitrosodimethylamine and N-nitrosodiethylamine to products genotoxic in an acetyltransferase-overexpressing Salmonella typhimurium strain (NM2009)

The possible roles of cytochrome P450 (P450) enzymes in the metabolic activation of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) by rat liver microsomes have been examined in a system containing the bacterial tester strain Salmonella typhimurium NM2009, a newly developed strain showing high O-acetyltransfer activities. The DNA-damaging activity could be determined by measuring expression of the umu gene in a plasmid containing the fused umuC-lacZ gene construct in the bacteria. The following lines of evidence support the view that both NDMA and NDEA are principally oxidized to reactive products by P450 2E1 in rat liver microsomes. First, NDMA and NDEA were activated by rat liver microsomes in a protein- and substrate-dependent manner and the former chemical was more active than the latter; both activities were induced in rats treated with P450 2E1 inducers such as ethanol, acetone and isoniazid and by starvation. Second, activation of NDMA and NDEA were both inhibited significantly by antibodies raised against rat P450 2E1 and by P450 2E1 inhibitors such as diethyldithiocarbamate and 4-methylpyrazole in rat liver microsomes. Finally, in reconstituted monooxygenase systems containing purified rat P450 enzymes, P450 2E1 gave the highest rates of the activation of both NDMA and NDEA; the addition of rabbit cytochrome b5 to the system caused about a 1.5-fold increase in both reactions. In separate experiments we also found that N-nitrosomethylacethoxymethylamine, a compound that reacts with DNA after ester cleavage, is more genotoxic in S.typhimurium NM2009 than in S.typhimurium NM2000, a strain that is defective in O-acetyltransferase activity. Part of the pathway involved in the activation of nitrosamines is suggested to be acetylation of alkyldiazohydroxides formed by P450 or acetylesterase, because the genotoxic activity of N-nitrosomethylacethoxymethylamine in S.typhimurium NM2009 could be inhibited by the O-acetyltransferase inhibitor pentachlorophenol. These results indicate that NDMA and NDEA are oxidized to gentoxoic products by rat liver microsomes and that a P450 2E1 enzyme plays a major role in the activation of these two potent carcinogens. The activation pathway of N-nitrosodialkylamines through acetylation by O-acetyltransferase has been proposed. This simple bacterial system for measuring genotoxicity should facilitate studies on the activation of N-nitroso alkylamines.

Direct activation of purified Go-type GTP binding protein by tricyclic antidepressants

A growing body of evidence suggests that tricyclic antidepressant agents (TCAs) interact with GTP binding proteins (G proteins). We have investigated if TCAs directly alter the function of the purified Go protein which is specifically expressed in neuronal tissue. Several TCAs markedly enhanced the GTPase activity of Go protein in a pertussis toxin-susceptible manner, whereas MAO-inhibitor and anxiolytic agent did not. This enhancing effect of TCAs on Go function may be due to an increase in the GDP-GTP exchange reaction occurring on Go. Thus, it is very likely that TCAs can modify various signal transduction by directly interacting with G proteins in brain cells.

Morphine produces a multiphasic effect on the release of substance P from rat trigeminal nucleus slices by activating different opioid receptor subtypes

Morphine (MOR) produces a concentration-dependent multiphasic effect (inhibitions and facilitations) on K(+)-evoked substance P (SP) release from rat trigeminal nucleus slices. In this study, we tested the action of selective opioid receptor antagonists on this multiphasic effect of MOR. 1 nM MOR produced an inhibition of K(+)-evoked release of SP that was affected only by the selective mu 1-opioid receptor antagonist naloxonazine (1 nM). MOR at 100 nM elicited an increase in SP release which was abolished selectively by the mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA; 20 nM) and attenuated by the delta-opioid receptor antagonist, ICI 174,864 (0.3 microM). 3 microM MOR produced an inhibition of SP release that was reversed only by ICI 174,864 (0.3 microM). MOR at even higher concentrations (30 microM) produced an enhancement of SP release that was reversed selectively by 3 nM n-binaltorphimine (n-BNI; 3 nM), a kappa-opioid receptor antagonist. In slices pretreated with 20 nM beta-FNA and in the presence of 0.3 microM ICI 174,864 (mu- and delta-opioid receptor blockade), both 100 nM and 3 microM MOR elicited a strong facilitation of K(+)-evoked SP release which was sensitive to 3 nM n-BNI. Thus, the increase in SP release produced by 100 nM may be mediated by the simultaneous stimulation of beta-FNA-sensitive mu- and excitatory delta-opioid receptors whereas the facilitation of SP release induced by 30 microM MOR could be due to the activation of kappa-opioid receptors. 1 nM and 3 microM MOR may inhibit SP release by stimulating naloxonazine-sensitive mu 1- and inhibitory delta-opioid receptors, respectively.

Mefloquine metabolism by human liver microsomes. Effect of other antimalarial drugs

A number of drugs have been studied for their effect on the metabolism of the antimalarial drug mefloquine by human liver microsomes (N = 6) in vitro. The only metabolite generated was identified as carboxymefloquine by co-chromatography with the authentic standard. Ketoconazole caused marked inhibition of carboxymefloquine formation with IC50 and Ki values of 7.5 and 11.2 microM, respectively. The inhibition of ketoconazole, a known inhibitor of cytochrome P450 isozymes, and the dependency of metabolite formation on the presence of NADPH indicated that cytochrome P450 isozyme(s) catalysed metabolite production. Of compounds actually or likely to be coadministered with mefloquine to malaria patients only primaquine and quinine produced marked inhibition (IC50, 17.5 and 122 microM; Ki, 8.6 and 28.5 microM, respectively). However, despite these in vitro data with primaquine, clinical studies have failed to show any significant effect of single dose primaquine on the pharmacokinetics of mefloquine. With quinine, because peak plasma concentrations are very close to the Ki value, there is likely to be inhibition of mefloquine metabolism in patients receiving both drugs. Sulfadoxine, artemether, artesunate and tetracycline did not significantly inhibit carboxymefloquine formation.

Structure-toxicity relationship of ethylene glycol ethers

The ultimate purpose of the present study was to evaluate correlations between acute in vivo and in vitro toxicity and log P (P is n-octanol-water partition coefficient). The in vitro toxicity to cloned cells (neuroblastoma N18TG-2 and glioma C6) in culture (ED50) and the in vivo toxicity to mice (LD50) of ethylene glycol ethers were studied in terms of the structure-activity relationship. The test ethers showed a wide range of ED50 values in both cells. LD50 was determined under two conditions: LD50-cont. was estimated in mice pretreated with olive oil and LD50-CCl4 in CCl4-pretreated mice. Multiple regression analyses revealed a significant correlation between log 1/LD50 and log P as follows: log (1/LD50-cont.) = -0.120 (log P)2+0.487log P-1.182, and log (1/LD50-CCl4) = -0.128 (log P)2+0.566log P-1.157. There was no significant correlation either between ED50 and LD50 or between ED50 for N18TG-2 and ED50 for C6. The results suggest that metabolic activation might not occur during acute toxicity from the ethers, and that hydrophobicity, expressed as log P, plays an important role in acute toxicity.

[H2 antagonists as inhibitors of cytochrome P-450 in rat liver: in vitro and in vivo effects]

Cytochrome P-50 is a well known participant in the metabolism of xenobiotics as well as an activator or inactivator of hepatotoxic substances and carcinogenic agents. H2 antagonists, cimetidine, famotidine and ranitidine were used to inhibit cytochrome P-450 in rat liver. After 200 mg cimetidine, 85% inhibition of cytochrome P-450 in vitro and 50% in vivo were demonstrated through demethylation of aminopyrine. Inhibition was further confirmed by differential absorption spectra (Type II). The percentage inhibition obtained with famotidine or ranitidine were lower than those obtained with cimetidine. Inhibition of the microsomal oxidative system by cimetidine could lead to decreased production of superoxide radicals and protection against damage induced by toxic agents activated in the liver.