Structure vs. activity in the sulfonylurea-mediated disulfiram-ethanol reaction

Metabolic activation of n-butyraldoxime by rat liver microsomal cytochrome P450. A requirement for the inhibition of aldehyde dehydrogenase

n-Butyraldoxime (n-BO) is known to cause a disulfiram/ethanol-like reaction in humans, a manifestation of the inhibition of hepatic aldehyde dehydrogenase (AIDH). As with a number of other in vivo inhibitors of AIDH, n-BO does not inhibit purified AIDH in vitro, suggesting that a metabolite of n-BO is the actual inhibitor of this enzyme. In re-examination of the effect of n-BO on blood acetaldehyde levels following ethanol in the Sprague-Dawley rat, we found that pretreatment with substrates and/or inhibitors of cytochrome P450 blocked the n-BO-induced rise in blood acetaldehyde in the following order of decreasing potency: 1-benzylimidazole (0.1 mmol/kg) > 3-amino-1,2,4-triazole (1.0 g/kg) > ethanol (3.0 g/kg) > phenobarbital (0.1% in the drinking water, 7 days) > SKF-525A (40 mg/kg). Rat liver microsomes were shown to catalyze the conversion of n-BO to an active metabolite that inhibited yeast AIDH. This reaction was dependent on NADPH and molecular oxygen and was inhibited by CO and 1-benzylimidazole. Hydroxylamine, postulated by others to be a metabolite of n-BO, inhibited AIDH via a catalase-mediated reaction and not through an NADPH-supported microsome-catalyzed reaction. Using GLC-mass spectrometry, 1-nitrobutane (an N-oxidation product) and butyronitrile (a dehydration product) were identified as metabolites from microsomal incubations of n-BO. However, neither of these metabolic products inhibited AIDH directly or in the presence of liver microsomes and NADPH. We conclude that another NADPH-dependent, cytochrome P450-catalyzed metabolic product of n-BO is responsible for the inhibition of AIDH by n-BO.

Failure of glutathione and cysteine prodrugs to block the chlorpropamide-induced inhibition of aldehyde dehydrogenase in vivo

Augmentation of cellular L-cysteine or glutathione (GSH) levels in vivo by the administration of prodrugs of L-cysteine or GSH, viz. 2(R,S)-methylthiazolidine-4(R)-carboxylic acid (MTCA), 2(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)thiazolidine-4(R)-car boxylic acid (RibCys) and GSH monoethyl ester (GSH-OEt), did not block the inhibition of aldehyde dehydrogenase (AlDH) by chlorpropamide (CP) or N1-ethylchlorpropamide (N1-EtCP), as shown by their inability to protect AlDH and thereby prevent the elevation of blood acetaldehyde (AcH) in ethanol-treated rats. Since the formation of an alkylcarbamoylating species by conjugation of n-propylisocyanate, a potential metabolite of CP or N1-EtCP, with GSH or L-cysteine is possible, intervention by GSH or cysteine may not produce a detoxified product. Evaluation of the two products that could theoretically be produced in vivo, viz. S-(n-propylcarbamoyl)-L-cysteine and S-(n-propylcarbamoyl)-GSH, indicated that these compounds inhibit rather than spare AlDH in rats. Indeed, the latter were as effective as N1-EtCP, a direct acting inhibitor of AlDH, and all three were better inhibitors of AlDH in vivo than CP itself. Thus, formation of S-conjugates of the active CP metabolite produced in vivo may not be a detoxication process, but may in fact represent redistribution of a transportable form of this highly reactive metabolite.

A nonhypoglycemic chlorpropamide analog that inhibits aldehyde dehydrogenase

Chlorpropamide (CP), a sulfonylurea-type oral hypoglycemic agent, is known to provoke a flushing reaction reminiscent of the disulfiram-ethanol reaction in certain individuals. This is manifested in rodents by an increase in blood acetaldehyde levels after ethanol administration. When the sulfonamide N1-nitrogen of CP was substituted with an ethyl group, the product, N1-ethylchlorpropamide, was found to be three times as active as CP in raising ethanol-derived blood acetaldehyde. However, whereas CP lowered fasting blood glucose in rats measured over 6 h, N1-ethylchlorpropamide was devoid of hypoglycemic activity, suggesting that the latter might be potentially useful as an alcohol deterrent agent.

The effect of cephalosporin antibiotics on alcohol metabolism: a review

A review is made of the pharmacological, biochemical and chemical aspects of the unpleasant 'Antabuse-like' reaction that may be induced in drinkers of alcohol by pre-treatment with certain beta-lactam antibiotics with a 1-methyltetrazole-5-thiol sidechain (such as moxalactam, cefamandole and cefoperazone). The symptoms are due to abnormally elevated blood acetaldehyde levels consequent upon the inactivation of hepatic aldehyde dehydrogenase. There is very little direct effect of the antibiotics on this enzyme and therefore it is concluded that a reactive metabolite of the antibiotics' essential sidechain is responsible for the reaction. A likely candidate for this active species is either the symmetrical disulphide 5,5'-dithiobis(1-methyltetrazole) formed by oxidation of 1-methyltetrazole-5-thiol, or the related mixed disulphide, methyl 5-(1-methyltetrazolyl) disulphide. The first of these is a potent inactivator of cytoplasmic aldehyde dehydrogenase only, the second affects both cytoplasmic and mitochondrial isoenzymes. 1-Methyltetrazole-5-thiol or derivatives have the potential to be used therapeutically as 'anti-alcohol' compounds in the same way as disulfiram (Antabuse) or calcium cyanamide.

Effect of tolbutamide and chlorpropamide on acetaldehyde metabolism in two inbred strains of mice

The mechanisms by which chlorpropamide and tolbutamide disrupt acetaldehyde metabolism were studied in C57BL and DBA mice. Acute po administration of varying doses of tolbutamide or chlorpropamide 2.5 hr before a 3.0 g/kg ip dose of ethanol to C57BL and DBA mice resulted in significant elevations of blood acetaldehyde when measured 2.5 hr after ethanol dosing. Dose-response analysis revealed a significant (p less than .05) difference in ED50 values for the elevated blood acetaldehyde response to tolbutamide in DBA (60 mg/kg) and C57BL (100 mg/kg) mice. The ED50 value for potentiation by chlorpropamide of blood acetaldehyde concentration was similar (23 to 32 mg/kg) in both inbred strains. At higher doses of chlorpropamide, DBA mice displayed elevations of blood acetaldehyde nearly threefold greater than those measured in C57BL mice treated identically. Measurements of aldehyde dehydrogenase (ALDH) in hepatic subcellular fractions, obtained from both inbred strains treated with 100 mg/kg tolbutamide or chlorpropamide prior to a 3.0 g/kg dose of ethanol, revealed a 50 to 80% inhibition of the low-Km ALDH present in mitochondria. Chlorpropamide and tolbutamide did not inhibit ALDH in vitro, suggesting that metabolites of these hypoglycemic agents may be responsible for the genotypic-dependent alterations in in vivo acetaldehyde oxidation.