Jerry R. Mitchell

Recent studies of acetaminophen‐induced liver damage in animals indicate that acetaminophen is converted in the liver to a chemically reactive arylating agent that normally is detoxified by conjugation with glutathione. When the dose of acetaminophen is large enough to deplete hepatic glutathione, however, there is extensive arylation of hepatic macromolecules and cell death. This paper presents evidence that administration of glutathione‐like nucleophiles, such as cysteamine, protects mice from arylation of hepatic macromolecules, hepatic necrosis, and death caused by the reactive acetaminophen metabolite. Additional studies indicate that glutathione may serve a similar protective function in man as in other animals. Thus, logical treatment of patients overdosed with acetaminophen might be based on cysteamine or other nucleophiles.

The clinical spectrum of isoniazid-induced liver injury seems to be clinically, biochemically, and histologically indistinguishable from viral hepatitis, except that the injury occurs primarily in persons older than 35 years. A possible relation between susceptibility of patients to isoniazid liver injury and rapid metabolism (acetylation) of the drug has been found. Examination of isoniazid metabolites showed that patients with rapid acetylator phenotype hydrolyze much more isoniazid to isonicotinic acid and the free hydrazine moiety than do slow acetylators. The hydrazine moiety liberated from isoniazid is primarily acetylhydrazine, and studies in animals show this metabolite to be converted to a potent acylating agent that produces liver necrosis. It seems likely that formation of chemically reactive metabolites is also the biochemical event initiating isoniazid liver injury in man. Recognition of the seriousness of isoniazid hepatic injury, not readily accepted at first, has led to revisions in the uses of isoniazid prophylaxis.

N-Acetylcysteine is the drug of choice for the treatment of an acetaminophen overdose. It is thought to provide cysteine for glutathione synthesis and possibly to form an adduct directly with the toxic metabolite of acetaminophen, N-acetyl-p-benzoquinoneimine. However, these hypothese have not been tested in vivo, and other mechanisms of action such as reduction of the quinoneimine might be responsible for the clinical efficacy of N-acetylcysteine. After the administration to rats of acetaminophen (1 g/kg) intraduodenally (i.d.) and of [(35)S]-N-acetylcysteine (1.2 g/kg i.d.), the specific activity of the N-acetylcysteine adduct of acetaminophen (mercapturic acid) isolated from urine and assayed by high pressure liquid chromatography averaged 76+/-6% of the specific activity of the glutathione-acetaminophen adduct excreted in bile, indicating that virtually all N-acetylcysteine-acetaminophen originated from the metabolism of the glutathione-acetaminophen adduct rather than from a direct reaction with the toxic metabolite. N-Acetylcysteine promptly reversed the acetaminophen-induced depletion of glutathione by increasing glutathione synthesis from 0.54 to 2.69 mumol/g per h. Exogenous N-acetylcysteine did not increase the formation of the N-acetylcysteine and glutathione adducts of acetaminophen in fed rats. However, when rats were fasted before the administration of acetaminophen, thereby increasing the stress on the glutathione pool, exogenous N-acetylcysteine significantly increased the formation of the acetaminophen-glutathione adduct from 57 to 105 nmol/min per 100 g. Although the excretion of acetaminophen sulfate increased from 85+/-15 to 211+/-17 mumol/100 g per 24 h after N-acetylcysteine, kinetic simulations showed that increased sulfation does not significantly decrease formation of the toxic metabolite. Reduction of the benzoquinoneimine by N-acetylcysteine should result in the formation of N-acetylcysteine disulfides and glutathione disulfide via thiol-disulfide exchange. Acetaminophen alone depleted intracellular glutathione, and led to a progressive decrease in the biliary excretion of glutathione and glutathione disulfide. N-Acetylcysteine alone did not affect the biliary excretion of glutathione disulfide. However, when administered after acetaminophen. N-acetylcysteine produced a marked increase in the biliary excretion of glutathione disulfide from 1.2+/-0.3 nmol/min per 100 g in control animals to 5.7+/-0.8 nmol/min per 100 g. Animals treated with acetaminophen and N-acetylcysteine excreted 2.7+/-0.8 nmol/min per 100 g of N-acetylcysteine disulfides (measured by high performance liquid chromatography) compared to 0.4+/-0.1 nmol/min per 100 g in rats treated with N-acetylcysteine alone. In conclusion, exogenous N-acetylcysteine does not form significant amounts of conjugate with the reactive metabolite of acetaminophen in the rat in vivo but increases glutathione synthesis, thus providing more substrate for the detoxification of the reactive metabolite in the early phase of an acetaminophen intoxication when the critical reaction with vital macromolecules occurs.

It has long been known that many drugs can be converted in the body to various metabolites that evoke therapeutic and toxicologic responses. In most instances these metabolites are chemically inert and bring about their effects by combining reversibly with action sites in tissues. In some instances, however, drugs and other foreign compounds can be converted in the body to chemically reactive metabolites which either uncouple integrated biochemical processes in cells or combine cova­ lently with various tissue macromolecules, such as DNA, RNA, protein, and glyco­ gen. During the past several years it has become increasingly evident that chemically reactive metabolites mediate many different kinds of serious toxicity, including carcinogenesis, mutagenesis, cellular necrosis, hypersensitivity reactions, methemo­ globinemia, hemolytic anemia, blood dyscrasias, and fetotoxicities. This review is devoted mainly to the mechanisms by which various kinds of toxicities are mediated by chemically reactive metabolites of drugs and the factors that affect the severity of the toxicities. Since the pioneering work of the Millers in Wisconsin and of Magee and co­ workers in England, it has become increasingly evident that most if not all chemical carcinogens bring about their effects by combining covalently with DNA and other tissue macromolecules or by being transformed to chemically reactive metabolites that in turn combine covalently with tissue macromolecules (1-4). The many studies on the mechanism of formation of carcinogenic metabolites in the body have revealed that chemically inert substances can be converted to chemi­ cally reactive metabolites by a variety of different reactions (Figure I). For example,