George J. Traiger

Relative hepatotoxicity of substituted thiobenzamides and thiobenzamide-S-oxides in the rat☆

The effect of substituents on the relative hepatotoxicity of various derivatives of thiobenzamide (TB) and thiobenzamide-S-oxide (TBSO) has been investigated in the rat. For five para-substituted thiobenzamides (p-XC6H4CSNH2; X = CH3O, CH3, H, Cl, CF3) hepatotoxicity estimated by plasma glutamic pyruvic transaminase (GPT) or plasma bilirubin responses followed a strict Hammett-type dependence on the electronic properties of the substituent, toxicity increasing dramatically with increasing electron donation (ϱ = −3.4 −1.4, respectively). This observation is consistent with the possible involvement of a critical S-oxidative biotransformation in the production of hepatic injury by these compounds. p-Hydroxythiobenzamide was not hepatotoxic at the highest dose studied, possibly because routes of biotransformation other than S-oxidation (e.g., conjugation) supervened. 2,6-Disubstituted thiobenzamides bearing either electron-donating or electron-withdrawing substituents also failed to produce hepatic injury. This was suggested to result from the steric effect of the substituents forcing the thioamide group out of conjugation with the aromatic ring. TBSO was significantly more hepatotoxic than TB, as indicated by earlier and/or larger increases in plasma GPT, plasma bilirubin, and hepatic triglycerides. The hepatotoxicity of TBSO was partially blocked by SKF-525A or N-octylimidazole, but was not affected by pretreatment of the rats with phenobarbital. For para-substituted TBSO derivatives (p-XC6H4C(SO)NH2; X = CH3, H, Cl) the slope of the Hammett correlation for the plasma GPT response was essentially identical to that for the parent TB derivatives. In contrast, the plasma bilirubin response to TBSO was not altered by substituents. The above findings show that the hepatotoxicity of TB derivatives depends strongly on chemical factors in a manner consistent with S-oxidation as a critical step leading to liver injury. However neither the toxic responses to TB nor those to TBSO are uniformly affected in a simple direct manner by induction or inhibition of cytochrome P-450 enzymes. This may indicate that other enzyme systems, oxidative, reductive, or both, may also play a role in governing the toxic responses to TB and TBSO.

Toxic effects of methylcyclopentadienyl manganese tricarbonyl (MMT) in rats: Role of metabolism

Abstract The toxicity of methylcyclopentadienyl manganese tricarbonyl (MMT) has been investigated in relation to its in vivo biotransformation in the rat. The LD 50 dose of MMT was found to be 50 mg/kg for oral administration and 23 mg/kg for ip administration. Death appeared to be caused by severe pulmonary hemorrhagic edema. Histological studies of MMT-treated animals revealed pathologic changes in lungs, liver, and kidney. Phenobarbital pretreatment protected rats from the lethal effects of 2.5 times the LD 50 dose of MMT, it shifted the site of tissue injury from the lungs to the liver, and it caused a doubling of the rate of biliary excretion of MMT metabolites. The possibility is discussed that MMT per se is toxic without bioactivation, and that the protective effect of phenobarbital pretreatment is due to a first-pass effect preventing toxic concentrations of orally administered MMT from reaching the systemic circulation.

Pharmacokinetics of 2-butanol and its metabolites in the rat.

A pharmacokinetic model is presented to describe the biotransformation of 2-butanol (2-OL) and its metabolites (2-butanone, 3-hydroxy-2-butanone, and 2,3-butanediol) using in vivo experimental blood concentrations. A flow limited model is developed to simulate 2-OL, 2-butanone (2-ONE), 3-hydroxy-2-butanone (3H-2B), and 2,3-butanediol (2,3-BD) blood concentrations in rats after oral administration of 2-OL. Assuming the only important site of 2-OL biotransformation is the liver, the tissues included are the liver and a volume of distribution, essentially body water in the case of 2-OL and its metabolites. A distribution coefficient is found to be necessary to describe the low concentration of 3H-2B in blood after administration of 2-OL. The need for this coefficient may be due to partitioning, binding, or altered transport rates from the liver. Inhibition of 2-ONE metabolism to 3H-2B by 2-OL has been included to explain a time delay in the appearance of 3H-2B after administration of 2-OL. Subsequent experimental verification confirms the mixed function oxidase inhibitory properties of 2-OL. The model is able to simulate blood concentrations and elimination of all four compounds after the oral administration of 2-OL. Additionally, the model also simulates the results obtained after i.v. administration of 3H-2B and 2,3-BD.

Substituent effects on the hepatotoxicity of thiobenzamide derivatives in the rat.

Abstract Thiobenzamide has been found to produce dose-dependent increases in three biochemical indexes of hepatic injury in rats, plasma bilirubin, plasma glutamic pyruvic transaminase, and hepatic triglyceride content. The toxicity of thiobenzamide was greatly altered by the addition of para substituents to the ring. Thus by all three biochemical indexes p -methoxythiobenzamide was markedly more toxic than thiobenzamide, and conversely p -chlorothiobenzamide markedly less so. Benzonitrile, p -anisonitrile, and p -chlorobenzonitrile, which are anticipated metabolites of the thioamides, were found to have no significant hepatotoxicity. However, after phenobarbital pretreatment, thiobenzamide caused a fourfold rise in transaminase activity, and SKF 525-A produced a modest decrease in transaminase activity as well as significant decreases in plasma bilirubin and hepatic triglyceride content. Thus, as with other toxic thione-containing compounds such as thiourea and thioacetamide, metabolic activation appears to be an important aspect of the toxicity of thiobenzamide.

Toxicity-distribution relationships among 3-alkylfurans in mouse liver and kidney

The present study was designed to extend previous observations regarding toxicity of furans and related compounds to liver and kidney. It was desired to test a series of homologous 3- alkylfurans , where changes in lipophilic character might be related to changes in toxicity. Additionally, it was desired to measure distribution of toxins to the target organs to ascertain whether organ selectivity might be determined by the concentrations attained in the target organs by the toxins. A synthesis for 3- ethylfuran and 3- pentylfuran was devised, and the toxicity of these, in addition to 3- methylfuran and furan itself, to mouse liver and kidney at 2.6 mmol/kg was determined. 2- Furamide and 2- ethylfuran were used as examples of substances known to be toxic to liver and kidney, respectively. 3- Methylthiophene was also included to determine whether results with furans extend to the closely related thiophenes . Histopathological examination of both organs was done, and quantitative estimates of liver toxicity were obtained from plasma levels of glutamate-pyruvate transaminase. Renal urine concentrating ability and plasma urea nitrogen levels were useful as quantitative indices of nephrotoxicity. It was found that both 3-ethyl and 3- pentylfuran exhibited pronounced toxicity to the kidney, and that both also caused moderate liver damage. Furan caused serious damage to the liver and produced somewhat lesser effects on the kidney. Equimolar doses of 3- methylfuran did not significantly damage either organ. Among the 3-alkyl furans, there is an impression that the more volatile compounds damage lung (3- methylfuran is reported to be a potent lung toxin), with liver and kidney toxicity increasing with molecular weight, and that compounds found in higher concentration produce greater damage in liver and kidney. However, among compounds other than alkyl furans, there is no obvious correlation between toxicity and organ concentration of toxin.

Potentiation of CCl4 of hepatotoxicity in rats by a metabolite of 2-butanone: 2,3-butanediol

The role of ketaone metabolism in 2-butanone-induced potentiaion of carbon tetrachloride (CCl4) hepatotoxicity was studied in rats. The blood concentrations of 2-butanol, 3-hydroxy-2-butanone and 2,3-butanediol detected 4 h after dosing were 3.2 mg/100 ml, 2.4 mg/100 ml and 8.6 mg/100 ml, respectively. Eighteen hours after 2-butanone, the concentration of 2,3-butanediol rose to 25.6 mg/100 ml, while the concentrations of 2-butanol and 3-hydroxy-2-butanone declined to 0.6 mg/100 ml and 1.4 mg/100 ml, respectively. A 16-h pretreatment with either 2-butanone (2.1 ml/kg, p.o.) or 2,3-butanediol (2.12 ml/kg, p.o.) markedly enhanced the hepatotoxic response to CCl4 (0.1 ml/kg, i.p.), as measured by serum glutamic pyruvic transaminase activity and hepatic triglyceride content. In vivo, limited formation of 3-hydroxy-2-butanone occurred after this dose of 2,3-butanediol. These data suggest that the production of 3-hydroxy-2-butanone and 2,3-butanediol via 2-butanone metabolism may participate in the augmented necrogenic effect of CCl4 seen after pretreatment with 2-butanone.

Effect of substituents on arene oxide-mediated liver toxicity among substituted bromobenzenes.

Abstract A series of seven ortho-substituted bromobenzenes was evaluated for their potential to produce liver injury in rats, as reflected in histologically observed centrolobular necrosis and in increases in serum glutamate-pyruvate transaminase. Absorption characteristics of the compounds following intraperitoneal dosing with solutions in sesame oil were estimated by determining the liver concentration of each compound after 4 hr. Twenty-four-hour toxicity values for the compounds were similar for both indexes of liver damage (histology and SGPT activity) and indicated that bromobenzenes bearing electron-withdrawing substituents at the ortho position are very potent liver toxicants. Bromobenzenes with electron-releasing substituents are less toxic, and those bearing substituents with less pronounced electronic character elicited intermediate toxic effects.

Pneumotoxic effects of thiobenzamide derivatives.

Primary thiomides such as thiobenzamide (TB) are well known hepatotoxins in the rat. Among para-substituted TB derivatives relative hepatotoxicity varies in accordance with the electronic properties of the parasubstituent. In contrast, several N-substituted TBs have been found to be potent lung toxins in rats and mice. For N-methylthiobenzamide (NMTB) the LD50 was found to be 0.315 (95% confidence interval (CI) 0.228-0.436) mmol/kg in the rat and 0.224 (95% CI 0.191--0.264) mmol/kg in the mouse. The N-mono-substituted TBs produce alveolar and perivascular pulmonary edema, together with massive pleural effusions (hydrothorax). In this regard their toxicity resembles qualitatively that of the arylthioureas. Furthermore, pretreatment of rats with sub-lethal doses of NMTB was found to protect them against subsequent challenge with supra-lethal doses. N,N-Dimethylthiobenzamide (DMTB) also causes lung injury in the rat, but only at much higher doses than with the N-mono-substituted TBs. The similarity in toxic responses elicited by the N-mono-substituted TBs and the arylthioureas is paralleled by similarities in their chemical structures and their metabolic disposition which involves (among other things) S-oxygenation by the microsomal flavin-containing monooxygenase (EC 1.14.13.8). Thus, a possible role for S-oxidized metabolites in the lung toxicity of these compounds must be considered.

Nonciliated bronchiolar epithelial (Clara) cell necrosis induced by organometallic carbonyl compounds

The administration of transition metal organometallic compounds such as manganese, chromium, and iron carbonyls by the i.p. route, and nickel by inhalation (mice) or intravenously (rats), resulted in selective necrosis of the nonciliated bronchiolar epithelial (Clara) cells and variable pulmonary parenchymal damage in BALB/c mice and Fischer-derived rats within 24 h of administration. The pulmonary toxicity of methylcyclopentadienyl manganese tricarbonyl (MMT), a representative of this group of compounds, was enhanced by pretreatment with piperonyl butoxide (PB), an inhibitor of the mixed-function oxidase system. This finding suggests that Clara cell necrosis can result from direct toxicity and that the specificity of toxic agents for Clara cells may not be related solely to the presence of the mixed-function oxidase system.

The effect of nephrotoxic furans on urinary N-acetyl-glucosaminidase levels in mice

The effects of several potentially nephrotoxic furans on urinary levels of N-acetylglucosaminidase (NAG) were examined to determine whether this test might serve as a useful quantitative index of nephrotoxicity for this series of compounds. Whereas others have found that nephrotoxins such as sodium salicylate and biphenyl cause increases in urinary NAG, we observed that these furans, which were shown to be nephrotoxic by histological procedures, caused significant decreases in urinary levels of the enzyme. This effect was dose-related in the one case examined.