N-hydroxybenzenecarboximidic acid derivatives: a new class of nitroxyl-generating prodrugs.

On the basis of the propensity of Piloty's acid to generate nitroxyl (HNO), we previously prepared a number of N,O-bisacylated Piloty's acid derivatives and showed that such prodrugs underwent a disproportionation reaction following ester hydrolysis to give an unstable intermediate that hydrolyzed to nitroxyl. To expand the versatility of this series, we desired some mixed N,O-diacylated Piloty's acid derivatives and devised a synthetic route to them. Such efforts led us, serendipitously, to a new series of heretofore unreported nitroxyl-generating compounds. Thus, benzohydroxamic acid was acylated on the hydroxylamino oxygen and the resulting product converted to its sodium salt. Treatment of this salt with arenesulfonyl chorides would be expected to give the mixed N,O-diacylated derivatives of Piloty's acid. However, the products obtained were the isomeric carboximidic acid derivatives whose structures were deduced from the IR and (13)C NMR spectral frequencies associated with the sp(2) carbons. The structures were verified by analysis of the X-ray crystal structure of a prototype compound of this series. When incubated with porcine liver esterase or mouse plasma, these N-acyloxy-O-arenesulfonylated benzenecarboximidic acid derivatives liberated HNO, measured as N(2)O, as well as the expected arenesulfinic acid and benzoic acid. Alkaline hydrolysis also produced N(2)O, but the major products were the arenesulfonic acid and benzohydroxamic acid. Thus, these N-hydroxybenzenecarboximidic acid derivatives represent a new series of nitroxyl prodrugs that require enzymatic bioactivation before nitroxyl can be liberated.

Diethylcarbamoylating/nitroxylating agents as dual action inhibitors of aldehyde dehydrogenase: a disulfiram-cyanamide merger.

Benzenesulfohydroxamic acid (Piloty's acid) was functionalized on the hydroxyl group with the N,N-diethylcarbamoyl group, and the hydroxylamine nitrogen was substituted with acetyl (1a), pivaloyl (1b), benzoyl (1c), and ethoxycarbonyl (1d) groups. Only compound 1d inhibited yeast aldehyde dehydrogenase (AlDH) in vitro (IC(50) 169 microM). When administered to rats, 1d significantly raised blood acetaldehyde levels following ethanol challenge, thus serving as a diethylcarbamoylating/nitroxylating, dual action inhibitor of AlDH in vivo. A more potent dual action agent was N-(N, N-diethylcarbamoyl)-O-methylbenzenesulfohydroxamic acid (5c), which was postulated to release diethylcarbamoylnitroxyl (9), a highly potent diethylcarbamoylating/nitroxylating agent, following metabolic O-demethylation in vivo. The dual action inhibition of AlDH exhibited by 1d, and especially 9, constitutes a merger of the mechanism of action of the alcohol deterrent agents, disulfiram and cyanamide.

Nitrosyl cyanide, a putative metabolic oxidation product of the alcohol-deterrent agent cyanamide.

When incubated with catalase/glucose-glucose oxidase, 13C-labeled cyanamide gave rise not only to 13C-labeled cyanide, but also to 13C-labeled CO2. Moreover, a time-dependent formation of nitrite was observed when cyanamide was oxidized in this system. These results suggested that the initial product of cyanamide oxidation, viz. N-hydroxycyanamide, was being further oxidized by catalase/H2O2 to nitrosyl cyanide (O = N-C = N). Theoretically, nitrosyl cyanide can hydrolyze to the four end-products detected in the oxidative metabolism of cyanamide in vitro, viz. nitroxyl, cyanide, nitrite, and CO2. Accordingly, both unlabeled and 13C-labeled nitrosyl cyanide were synthesized by the low temperature (-40 to -50 degrees) nitrosylation of K-(18-crown-6)cyanide with nitrosyl tetrafluoroborate. The product, a faint blue liquid at this temperature, was transferred as a gas to phosphate-buffered solution, pH 7.4, where it was solvolyzed. Analysis of the headspace by gas chromatography showed the presence of N2O, the dimerization/dehydration product of nitroxyl, while cyanide was detected in the aqueous solution, as measured colorimetrically. [13C]CO2 was analyzed by GC/MS. An oxidative biotransformation pathway for cyanamide that accounts for all the products detected and involving both N-hydroxycyanamide and nitrosyl cyanide as tandem intermediates is proposed.

Carbethoxylating agents as inhibitors of aldehyde dehydrogenase.

N,O-Dicarbethoxy-4-chlorobenzenesulfohydroxamate (1c) and O-carbethoxy-N-hydroxysaccharin (6), both potential carbethoxylating agents, inhibited yeast aldehyde dehydrogenase (AlDH) with IC50's of 24 and 56 microM, respectively. The esterase activity of the enzyme was commensurably inhibited. AlDH activity was only partially restored on incubation with mercaptoethanol (20 mM) for 1 h. On incubation with rat plasma, 1c liberated nitroxyl, a potent inhibitor of AlDH. Under the same conditions, nitroxyl generation from 6 was minimal, a result compatible with a previous observation that nitroxyl generation from N-hydroxysaccharin (7), the product of the hydrolysis of the carbethoxy group of 6, was minimal at physiological pH. Since chemical carbethoxylating agents represented by the O-carbethoxylated N-hydroxyphthalimide, 1-hydroxybenzotriazole, and N-hydroxysuccinimide (8, 9, and 10, respectively) likewise inhibited yeast AlDH, albeit with IC50's 1 order of magnitude higher, we postulate that 1c and 6 act as irreversible inhibitors of AlDH by carbethoxylating the active site of the enzyme.

Latent alkyl isocyanates as inhibitors of aldehyde dehydrogenase in vivo.

On the basis of our previous observation that N1-alkyl substituted chlorpropamide derivatives when administered to rats nonenzymatically eliminated n-propyl isocyanate, a known inhibitor of aldehyde dehydrogenase (AlDH), we have synthesized other latentiated n-propyl isocyanates as in vivo inhibitors of AlDH. N1-Allylchlorpropamide 3 was, as expected, a potent inhibitor of hepatic AlDH in rats, as indicated by the 4-fold increase in the levels of ethanol-derived blood acetaldehyde relative to that elicited by chlorpropamide itself. Closely following in activity in decreasing order were N3-(n-propylcarbamoyl)uracil (7),N-(n-propylcarbamoyl)saccharin (6), and the S-(n-propylcarbamoyl) derivative (9) of benzyl mercaptan. However, two hydantoin derivatives, 5 and 8, were totally inactive in inhibiting AlDH in vivo. A prodrug of N1-ethylchlorpropamide, viz., its N3-trifluoroacetyl derivative (4b), was a good in vivo inhibitor of AlDH, mimicking the activity of the parent N1-ethylchlorpropamide. These results suggest that latent alkyl isocyanates are inhibitors of AlDH, giving further support to the hypothesis that the inhibition of AlDH in vivo by the hypoglycemic agent chlorpropamide may be due to the release of n-propyl isocyanate following metabolic bioactivation.

Glutaramyl-beta-alanyl spacer group for haptenic coupling to proteins. Preparation of immunogens for antibody production against polychlorinated biphenyls.

By use of a glutaramyl-beta-alanyl spacer group, a hapten for the polychlorinated biphenyl, 2,2',4,4',5,5'-hexachlorobiphenyl (1), viz., 2-amino-2',4,4',5,5'-pentachlorobiphenyl (2), was successfully conjugated to carrier proteins to provide immunogens with high hapten/protein molar substitution ratios (MSR's). The procedure allows for the incorporation of beta-[3H]-alanine into the immunogen, thereby providing an accurate radiochemical method for the quantitative assessment of MSR. The use of the glutaramyl spacer group was prompted by the observation that the corresponding succinamyl group was subject to side reactions manifested by succinimide formation during the carboxyl activation step to an activated ester for subsequent coupling to proteins, thus severely compromising the coupling yields. The glutaramyl-beta-alanyl spacer group should be generally applicable for protein conjugation of any hapten with an amino functional group in the molecule.

Redistribution and enhanced urinary excretion of 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) in rats using HCB-specific IgG and Fab fragments.

Drug-specific antibody fragments can enhance the elimination of some drugs by redistributing drug from tissues into serum and allowing renal excretion of the drug-antibody complex. This approach could potentially be used to enhance the elimination of compounds such as polychlorinated biphenyls that have very long elimination half-lives. As a first step in testing this hypothesis, the effects of 2,2',4,4',5,5'-hexachlorobiphenyl (HCB)-specific antibodies and their corresponding Fab fragments on HCB disposition were studied in rats. Antibodies to HCB were produced in chickens, and the corresponding Fab fragments were produced by digestion with papain. To study antibody effects on HCB distribution, [14C]HCB (0.1 mg) was administered i.v. to rats. Two weeks later, after distribution to tissues was complete, anti-HCB IgG or control IgG was administered i.v. The serum radiolabel concentration 2 hr after IgG administration increased 185 +/- 64% in animals treated with specific antibody vs 51 +/- 19% in control animals (P < 0.001). The increase in serum radiolabel concentration was apparent within 30 min and maximal at 2 hr. To study effects on HCB excretion, anti-HCB or control Fab fragment was administered 2 weeks after [14C]HCB. Urinary HCB excretion over the next 24 hr, measured by gas chromatography, was 10-fold greater in the group treated with anti-HCB Fab (P < 0.01). These data demonstrate that anti-HCB IgG can redistribute HCB rapidly from tissues into serum and that anti-HCB Fab can enhance urinary HCB excretion. While the magnitude of these changes was small, the data suggest that increasing HCB excretion using drug-specific antibody fragments is feasible, and can serve as a model for enhancing the excretion of compounds that have very long elimination half-lives.

Effects of ethanol and inhibitors on the binding and metabolism of acetaminophen and N-acetyl-p-benzoquinone imine by hepatic microsomes from control and ethanol-treated rats.

Acetaminophen is metabolized by cytochrome P450 to N-acetyl-p-benzoquinone imine (NABQI). This metabolite reacts with critical cellular macromolecules to give toxicity. The administration of 10% ethanol in the drinking water to 100 g male rats for 6 weeks markedly increases the toxicity of acetaminophen. This increase was associated with a 71% increase in microsomal protein binding of acetaminophen [4.8 pmol/min/mg protein in control microsomes versus 8.2 pmol/min/mg protein in ethanol microsomes (P less than 0.01)] and a 131% increase in aniline hydroxylase [0.52 nmol/min/mg protein in control microsomes versus 1.20 nmol/min/mg protein in ethanol microsomes (P less than 0.001)]. On the other hand, cysteine conjugation of acetaminophen showed an increase of only 12% [2.8 nmol/min/mg protein in control microsomes versus 3.1 nmol/min/mg protein in ethanol microsomes (P less than 0.05)]. Ethylmorphine- and benzphetamine N-demethylases did not increase. In microsomes from both control and ethanol animals, imidazole (1 mM) inhibited the two N-demethylases, aniline hydroxylation and acetaminophen binding by 85-95% but inhibited the cysteine conjugation by only 50%. For control and ethanol animals, both 80% CO/20% O2 and SKF-525A (1 mM) totally inhibited cysteine conjugation but only inhibited the other activities by about 36-60%. KCN (1 mM) had no effect on any of the activities except protein binding (60-67% inhibition). Scavengers of reactive oxygen [mannitol (1 mM), dimethyl sulfoxide (1 mM), superoxide dismutase (15 micrograms/mL) and catalase (65 micrograms/mL)] had no effect on any of the reactions. Of all these treatments only CO/O2 decreased the protein binding and cysteine conjugation of NABQI in the presence of either NADP+ or NADPH. The data from the inhibitor studies and the effect of ethanol on acetaminophen and NABQI metabolism would suggest that protein binding and cysteine conjugation are catalyzed by different isozymes of cytochrome P450. Finally, the current results indicate that the increased toxicity of acetaminophen observed with ethanol more closely parallels the increase in protein binding activity rather than cysteine conjugation.

Prodrugs of L-cysteine as protective agents against acetaminophen-induced hepatotoxicity. 2-(Polyhydroxyalkyl)- and 2-(polyacetoxyalkyl)thiazolidine-4(R)-carboxylic acids.

Eight prodrugs of L-cysteine (1a-h) were synthesized by the condensation of the sulfhydryl amino acid with naturally occurring aldose monosaccharides containing three, five, and six carbon atoms. The resulting 2-(polyhydroxyalkyl)thiazolidine-4(R)-carboxylic acids (TCAs) are capable of releasing L-cysteine and the sugars by nonenzymatic ring opening and hydrolysis. Thus, when added to rat hepatocyte preparations in vitro, these TCAs (1.0 mM) raised cellular glutathione (GSH) levels 1.2-2.1-fold relative to controls. On the basis of this finding, the cysteine prodrugs were tested as protective agents against acetaminophen-induced hepatotoxicity in a mouse model. The TCA derived from D-ribose and L-cysteine (RibCys, 1d) showed the greatest therapeutic promise of the series, with a 100% (12/12) survival profile compared to 17% without treatment. However, the degree of stimulation of GSH production in rat hepatocytes by these prodrugs did not correlate with the extent of protection afforded in mice, suggesting that pharmacokinetic parameters must supervene in vivo. To evaluate the effect of increased lipid solubility, we prepared prodrugs 2a-c by using peracetylated aldehydic sugars in the condensation reaction. These compounds, however, displayed acute toxicity to mice, possibly due to liberation of the acetylated sugars themselves. Nevertheless, the efficacy of the unacetylated TCAs, and RibCys (1d) in particular, suggests that the prodrug approach for the delivery of L-cysteine to the liver represents a viable means of augmenting existing detoxication mechanisms in protecting cells against xenobiotic substances that are bioactivated to toxic, reactive metabolites.

Analysis of hepatic reduced glutathione, cysteine and homocysteine by cation-exchange high-performance liquid chromatography with electrochemical detection.

A high-performance liquid chromatographic method employing a mercury-based electrochemical detector and a cation-exchange column is described for the simultaneous measurement of reduced glutathione, cysteine, and homocysteine in liver homogenates. Sample preparation involves precipitation of protein with perchloric acid, removal of perchlorate by precipitation as its potassium salt and dilution with mobile phase. Mercaptoethylglycine is used as the internal standard. Using this procedure, the sum of the individual hepatic thiols agreed well with the total thiols determined with Ellman's reagent. Comparisons were made with (a) control rats, (b) rats depleted of hepatic thiols by pargyline pretreatment, and (c) rats administered L-cysteine.

2-Substituted thiazolidine-4(R)-carboxylic acids as prodrugs of L-cysteine. Protection of mice against acetaminophen hepatotoxicity.

A number of 2-alkyl- and 2-aryl-substituted thiazolidine-4(R)-carboxylic acids were evaluated for their protective effect against hepatotoxic deaths produced in mice by LD90 doses of acetaminophen. 2(RS)-Methyl-, 2(RS)-n-propyl-, and 2(RS)-n- pentylthiazolidine -4(R)-carboxylic acids (compounds 1b,d,e, respectively) were nearly equipotent in their protective effect based on the number of surviving animals at 48 h as well as by histological criteria. 2(RS)-Ethyl-, 2(RS)-phenyl-, and 2(RS)-(4-pyridyl)thiazolidine-4(R)-carboxylic acids (compounds 1c,f,g) were less protective. The enantiomer of 1b, viz., 2(RS)- methylthiazolidine -4(S)-carboxylic acid (2b), was totally ineffective in this regard. Thiazolidine-4(R)-carboxylic acid (1a), but not its enantiomer, 2a, was a good substrate for a solubilized preparation of rat liver mitochondrial proline oxidase [Km = 1.1 x 10(-4) M; Vmax = 5.4 mumol min-1 (mg of protein)-1]. Compound 1b was not a substrate for proline oxidase but dissociated to L-cysteine in this system. At physiological pH and temperature, the hydrogens on the methyl group of 1b underwent deuterium exchange with solvent D2O (k1 = 2.5 X 10(-5) s), suggesting that opening of the thiazolidine ring must have taken place. Indeed, 1b labeled with 14C in the 2 and methyl positions was rapidly metabolized by the rat to produce 14CO2, 80% of the dose being excreted in this form in the expired air after 24 h. It is suggested that these 2-substituted thiazolidine-4(R)-carboxylic acids are prodrugs of L-cysteine that liberate this sulfhydryl amino acid in vivo by nonenzymatic ring opening, followed by solvolysis.

2,5,5-Trimethylthiazolidine-4-carboxylic acid, a D(-)-penicillamine-directed pseudometabolite of ethanol. Detoxication mechanism for acetaldehyde.

A directed detoxication mechanism for acetaldehyde (AcH) is described wherein ethanol-derived AcH, circulating in the blood of rats given ethanol-1-14C and disulfiram or pargyline, was sequestered by condensation with administered D(-)-penicillamine (1). The product of this condensation, 2,5,5-trimethylthiazolidine-4-carboxylic acid (3), which was excreted in the urine without acetyl conjugation, was quantitatively determined by inverse isotope dilution measurements. Acetylation of the urine permitted the isolation of the corresponding N-acetyl derivative in crystalline form. The chirality of 3 was deduced by NMR analysis to be 72% 2S, 4S and 28% 2R, 4S. Although acetylation selectively acetylated the predominant isomer, this acetylated derivative was identical in all respects with a chemically synthesized product. This suggests that the in vivo condensation of AcH and 1 is not enzyme mediated.

A simple, rapid and semi-automated method for the quantitative determination of expired 14CO2 in metabolism studies with laboratory animals.

The automatic features of a commercial instrument which prepares 14CO2 for liquid scintillation counting have been adapted to assay the 14CO2 collected in aqueous NaOH from the expired air of experimental laboratory animals in metabolism experiments. The method involves the liberation of 14CO2 trapped as Na214CO3 with dilute H2SO4 and the entrainment of the 14CO2 with nitrogen into the sample handling system. Recoveries were 87.3+/-1.2% and precision+/-2.0%. The method was simple and rapid; 6-8 samples were processed in one hour.

Lowering of blood acetaldehyde levels-a possible approach to prevention of alcoholic cardiomyopathy.

Based on the assumption that circulating acetaldehyde (AcH) is cardiotoxic, D-penicillamine was administered to dogs given alcohol orally, or given AcH intravenously. Paralleling the increase in plamsa norepinephrine (NE) and epinephrine (E) induced by AcH infusion, hemodynamic measurements showed a positive inotropic response with increase in pulse, blood pressure, left ventricular contractility, and cardiac output. Infusion of D-penicillamine abruptly lowered circulating levels of AcH and catecholamines, which was accompanied by an appropriate hemodynamic response.