Mechanism of action of paracetamol protective agents in mice in vivo

Effects of N-acetylcysteine and dithiothreitol on glutathione and protein thiol replenishment during acetaminophen-induced toxicity in isolated mouse hepatocytes

Isolated mouse hepatocytes were incubated with 1.0 mM acetaminophen (AA) for 1.5 h to initiate glutathione (GSH) and protein thiol (PSH) depletion and cell injury. Cells were subsequently washed to remove non-covalently bound AA and resuspended in medium containing N-acetylcysteine (NAC, 2.0 mM) or dithiothreitol (DTT, 1.5 mM). The effects of these agents on the replenishment of GSH and total PSH content were related to the development of cytotoxicity. When cells exposed to AA were resuspended in medium containing NAC or DTT, both agents replenished GSH and total PSH content to levels observed in untreated cells but only DTT was able to attenuate cytotoxicity. Addition of the GSH synthesis inhibitor, buthionine sulfoximine (BSO, 1.0 mM, 1.5 h), to cells in incubation medium containing AA, enhanced GSH and total PSH depletion and potentiated cytotoxicity. Resuspension of these cells in medium containing NAC did not alter the potentiating effects of BSO; GSH and PSH levels were not replenished and no cytoprotective effects were observed. However, when cells exposed to AA and BSO were resuspended in medium containing DTT, PSH content was replenished but GSH levels were not restored. In addition, DTT was able to delay the development of cytotoxicity. It appears that DTT, unlike NAC, has a GSH-independent mechanism of PSH replenishment. These observations suggest that while replenishment of GSH and total PSH content does not result in cytoprotection, the regeneration of critical PSH by DTT may play an important role in the maintenance of proper cell structure and/or function.

Hepatoprotective effects of lobenzarit disodium on acetaminophen-induced liver damage in mice

We have studied the effects of the immunomodulator drug lobenzarit in the model of acute hepatotoxicity induced by a high oral dose (600 mg/kg) of acetaminophen in mice. Lobenzarit at doses of 25, 50 and 100 mg/kg i.p. decreased significantly the activity of alanine aminotransferase in serum, which was increased by acetaminophen alone, and increased the concentration of reduced glutathione in mice liver, which is depleted by acetaminophen. Lobenzarit also reduced liver damage induced by acetaminophen in mice, which was observed by electron microscopy. The hepatoprotective effects of lobenzarit were dose-dependent and they were produced when lobenzarit was administered 30 min before acetaminophen or 2 and 4 h after it. It is concluded that lobenzarit exerts some effects which resemble those of an antidote of acetaminophen such as N-acetylcysteine.

Effect of pregnenolone-16 alpha-carbonitrile and dexamethasone on acetaminophen-induced hepatotoxicity in mice

Recently, we demonstrated that a microsomal enzyme inducer with a steroidal structure, pregnenolone-16 alpha-carbonitrile (PCN), markedly decreased the hepatotoxicity of acetaminophen (AA) in hamsters. Therefore, it was of interest to determine if PCN, as well as another steroid microsomal enzyme inducer, dexamethasone (DEX), would decrease the toxicity of AA in mice, another species sensitive to AA hepatotoxicity. Mice were pretreated with PCN or DEX (100 and 75 mg/kg, ip, for 4 days, respectively) and were given AA (300-500 mg/kg, ip). Twenty-four hours after AA administration, liver injury was assessed by measuring serum activities of sorbitol dehydrogenase and alanine aminotransferase and by histopathological examination. Neither PCN nor DEX protected markedly against AA hepatotoxicity in mice; PCN tended to decrease AA-induced hepatotoxicity, whereas DEX was found to enhance AA-induced hepatotoxicity and it produced some hepatotoxicity itself. DEX decreased the glutathione concentration (36%) in liver and increased the biliary excretion of AA-GSH, which reflects the activation of AA, whereas PCN produced neither effect. Thus, whereas PCN has been shown to markedly decrease the hepatotoxicity of AA in hamsters, apparently by decreasing the isoform of P450 responsible for activating AA to N-acetyl-p-benzoquinoneimine, this does not occur in mice after induction with either PCN or DEX. In contrast, DEX enhances AA hepatotoxicity apparently by decreasing liver GSH levels and increasing the activation of AA to a cytotoxic metabolite.

S-adenosylmethionine protects against acetaminophen hepatotoxicity in two mouse models

Because S-adenosylmethionine promotes synthesis of hepatic glutathione in chronic liver disease and is well tolerated in man, we investigated its use as an antidote to acetaminophen hepatotoxicity in two mouse models. In C57Bl6 mice, deaths were abolished by S-adenosylmethionine given within 1 hr of 3.3 mmol/kg body wt acetaminophen (0 of 32 vs. 13 of 49, p less than 0.005) and reduced if given 2 to 5 hours after acetaminophen administration (4 of 42 vs. 13 of 49, p less than 0.01). Mixed disulfate/tosylate salt of S-adenosylmethionine abolished mortality in C3H mice given 2 mmol/kg body wt acetaminophen (0 of 24 vs. 4 of 18; p less than 0.05). In both mouse models, S-adenosylmethionine reduced depletion of plasma (median = 20.8 mumol/L vs. 14.6 mumol/L) and liver glutathione (198% vs. 100%; p less than 0.05), liver damage and release of AST after acetaminophen administration. Pretreatment with buthionine sulfoximine, which inhibits glutathione synthesis, abolished the beneficial effect of S-adenosylmethionine on survival and plasma glutathione level. S-adenosylmethionine reduces acetaminophen hepatotoxicity by metabolism of the active moiety to glutathione. This benefit may last as long as 5 hr after acetaminophen ingestion.

Intravenous acetylcysteine in paracetamol induced fulminant hepatic failure: a prospective controlled trial

OBJECTIVE

To see whether intravenous acetylcysteine would improve outcome in patients with fulminant hepatic failure after paracetamol overdose.

DESIGN

A prospective randomised controlled study.

SETTING

The Institute of Liver Studies, King's College Hospital, London.

PATIENTS

50 consecutive patients (21 male) aged 16-60 with fulminant hepatic failure after paracetamol overdose who had not previously received acetylcysteine.

INTERVENTIONS

Conventional intensive liver care plus either acetylcysteine (25 patients) in the same dose regimen as used early after a paracetamol overdose, except that the infusion was continued until recovery from encephalopathy or death, or an equivalent volume of 5% dextrose (25 patients).

MAIN OUTCOME MEASURES

Survival; incidence of cerebral oedema, renal failure, and hypotension requiring inotropic support; liver function as assessed by prolongation of the prothrombin time; and degree of encephalopathy.

RESULTS

The rate of survival was significantly higher in the acetylcysteine treated group than in the controls (48% (12/25 patients) v 20% (5/25); p = 0.037, 95% confidence interval for difference in proportions surviving 3% to 53%). Acetylcysteine treated patients had a lower incidence of cerebral oedema (40% (10/25) v 68% (17/25); p = 0.047, 95% confidence interval for difference in incidence 2% to 54%), and fewer developed hypotension requiring inotropic support (48% (12/25) v 80% (20/25); p = 0.018, 95% confidence interval 7% to 57%). Rates of deterioration and recovery of liver function, however, were similar in the two groups. No adverse reactions to acetylcysteine were seen.

CONCLUSIONS

Acetylcysteine is safe and effective in fulminant hepatic failure after paracetamol overdose.

Drug antioxidant effects. A basis for drug selection?

A free radical is any species capable of independent existence that contains one or more unpaired electrons. Free radical reactions have been implicated in the pathology of more than 50 human diseases. Radicals and other reactive oxygen species are formed constantly in the human body, both by deliberate synthesis (e.g. by activated phagocytes) and by chemical side-reactions. They are removed by enzymic and nonenzymic antioxidant defence systems. Oxidative stress, occurring when antioxidant defences are inadequate, can damage lipids, proteins, carbohydrates and DNA. A few clinical conditions are caused by oxidative stress, but more often the stress results from the disease. Sometimes it then makes a significant contribution to the disease pathology, and sometimes it does not. Several antioxidants are available for therapeutic use. They include molecules naturally present in the body [superoxide dismutase (SOD), alpha-tocopherol, glutathione and its precursors, ascorbic acid, adenosine, lactoferrin and carotenoids] as well as synthetic antioxidants [such as thiols, ebselen (PZ51), xanthine oxidase inhibitors, inhibitors of phagocyte function, iron ion chelators and probucol]. The therapeutic efficacy of SOD, alpha-tocopherol and ascorbic acid in the treatment of human disease is generally unimpressive to date although dietary deficiencies of the last two molecules should certainly be avoided. Xanthine oxidase inhibitors may be of limited relevance as antioxidants for human use. Exciting preliminary results with probucol (antiatherosclerosis), ebselen (anti-inflammatory), and iron ion chelators (in thalassaemia, leukaemia, malaria, stroke, traumatic brain injury and haemorrhagic shock) need to be confirmed by controlled clinical trials. Clinical testing of N-acetylcysteine in HIV-1-positive subjects may also be merited. A few drugs already in clinical use may have some antioxidant properties, but this ability is not widespread and drug-derived radicals may occasionally cause significant damage.

Acetaminophen overdose: a 48-hour intravenous N-acetylcysteine treatment protocol

STUDY OBJECTIVE

To determine the safety and efficacy of a 48-hour IV N-acetylcysteine (IV NAC) treatment protocol for acute acetaminophen overdose.

DESIGN

Nonrandomized trial open to all eligible patients.

SETTING

Multicenter; hospitals included moderate- and high-volume private, university, and municipal hospitals in urban and suburban settings.

TYPE OF PARTICIPANTS

Two hundred twenty-three patients were entered. Of these, 179 met inclusion criteria: acute acetaminophen overdose, plasma acetaminophen concentration above the treatment nomogram line, treatment with IV NAC according to the protocol, and sufficient data to determine outcome.

INTERVENTIONS

IV NAC treatment consisted of a loading dose of 140 mg/kg followed by 12 doses of 70 mg/kg every four hours.

MEASUREMENTS AND MAIN RESULTS

Patients were grouped for analysis according to risk group based on the initial plasma acetaminophen concentration. Hepatotoxicity (aspartate aminotransferase or alanine aminotransferase of more than 1,000 IU/L) developed in 10% (five of 50) of patients at "probable risk" when IV NAC was started within ten hours of acetaminophen ingestion and in 27.1% (23 of 85) when therapy was begun after ten to 24 hours. Among "high-risk" patients first treated 16 to 24 hours after overdose, hepatotoxicity occurred in 57.9% (11 of 19). There were two deaths (two of 179, 1.1%). Adverse reactions resulting from NAC occurred in 32 of 223 cases (14.3%), consisting in 29 of 32 patients (91% of reactions) of transient, patchy, skin erythema or mild urticaria during the loading dose that did not require discontinuation of therapy.

CONCLUSION

This 48-hour IV NAC protocol is safe and effective antidotal therapy for acetaminophen overdose. Based on available data, it is equal to 72-hour oral and 20-hour IV treatment protocols when started early and superior to the 20-hour IV regimen when treatment is delayed. Further study will be required to determine its relative efficacy in the high-risk patient treated very late.

Use of N-acetylcysteine in clinical toxicology

The major use of N-acetylcysteine in clinical toxicology is in the treatment of acetaminophen (paracetamol) overdosage. The hepatorenal toxicity of acetaminophen is mediated by a reactive metabolite normally detoxified by reduced glutathione. If glutathione is depleted, covalent binding to macromolecules and/or oxidation of thiol enzymes can lead to cell death. Oral or intravenous N-acetylcysteine or oral D,L-methionine mitigates acetaminophen-induced hepatorenal damage if given within 10 hours, but becomes less effective thereafter. In vivo, N-acetylcysteine forms L-cysteine, cystine, L-methionine, glutathione, and mixed disulfides; L-methionine also forms cysteine, thus giving rise to glutathione and other products. Oral therapy with N-acetylcysteine or methionine for acetaminophen poisoning is contraindicated in the presence of coma or vomiting, or if activated charcoal has been given by mouth. Nausea, vomiting, and diarrhea may also occur as a result of oral N-acetylcysteine administration. Anaphylactoid reactions including angioedema, bronchospasm, flushing, hypotension, nausea/vomiting, rash, tachycardia, and respiratory distress may occur 15-60 minutes into N-acetylcysteine infusion (20 hours intravenous regimen) in up to 10% of patients. Following accidental intravenous overdosage, the adverse reactions of N-acetylcysteine are similar but more severe; fatalities have occurred. A reduction in the loading dose of N-acetylcysteine may reduce the risk of adverse reactions while maintaining efficacy. Administration of N-acetylcysteine for a longer period might provide enhanced protection for patients in whom acetaminophen absorption or elimination is delayed. N-acetylcysteine may also have a role in the treatment of toxicity from carbon tetrachloride, chloroform, 1,2-dichloropropane, and other compounds. The possible use of N-acetylcysteine and other agents in the prevention of the neuropsychiatric sequelae of acute carbon monoxide poisoning is an important area for future research.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy

Glutathione, which is synthesized within cells, is a component of a pathway that uses NADPH to provide cells with their reducing milieu. This is essential for (a) maintenance of the thiols of proteins (and other compounds) and of antioxidants (e.g. ascorbate, alpha-tocopherol), (b) reduction of ribonucleotides to form the deoxyribonucleotide precursors of DNA, and (c) protection against oxidative damage, free radical damage, and other types of toxicity. Glutathione interacts with a wide variety of drugs. Despite its many and varied cellular functions, it is possible to achieve therapeutically useful modulations of glutathione metabolism. This article emphasizes an approach in which the synthesis of glutathione is selectively inhibited in vivo leading to glutathione deficiency. This is achieved through use of transition-state inactivators of gamma-glutamylcysteine synthetase, the enzyme that catalyzes the first and rate-limiting step of glutathione synthesis. The effects of marked glutathione deficiency, thus produced in the absence of applied stress, include cellular damage associated with severe mitochondrial degeneration in a number of tissues. Such glutathione deficiency is not prevented or reversed by giving glutathione. The cellular utilization of GSH involves its extracellular degradation, uptake of products, and intracellular synthesis of GSH. This is a normal pathway by which cysteine moieties are taken up by cells. Glutathione deficiency induced by inhibition of its synthesis may be prevented or reversed by administration of glutathione esters which, in contrast to glutathione, are readily transported into cells and hydrolyzed to form glutathione intracellularly. Research derived from this model has led to several potentially useful therapeutic approaches, one of which is currently in clinical trial. Thus, certain tumors, including those that exhibit resistance to several drugs and to radiation, are sensitized to these modalities by selective inhibition of glutathione synthesis. An alternative interpretation is suggested which is based on the concept that some resistant tumors have high capacity for glutathione synthesis and that such increased capacity may be as significant or more significant in promoting the resistance of some tumors than the cellular levels of glutathione. Therapeutic approaches are proposed in which normal cells may be selectively protected against toxic antitumor agents and radiation by cysteine- and glutathione-delivery compounds. Current studies suggest that research on other modulations of glutathione metabolism and transport would be of interest.

Role of cysteine and taurine in regulating glutathione synthesis by periportal and perivenous hepatocytes

The uptake and metabolism of 35S-labelled sulphur amino acids were compared in periportal (PP) and perivenous (PV) rat hepatocytes, isolated by digitonin/collagenase perfusion, to identify the factors underlying the previously observed [Kera, Penttilä & Lindros, Biochem. J. (1988) 254, 411-417] higher rate of GSH replenishment in PP cells. The buthionine sulphoximine-inhibitable synthesis of GSH was faster in PP than in PV hepatocytes with both cysteine (6.1 versus 5.0 mumol/h per g of cells) and methionine (4.5 versus 3.3 mumol/h per g) as well as with endogenous precursors and L-2-oxo-4-thiazolidinecarboxylate as substrates. However, the uptake of cysteine by PP cells was slower than by PV cells (8.6 versus 10.3 mumol/h per g of cells), whereas methionine was taken up at similar rates. The activity of gamma-glutamylcysteine synthetase (GCS) was slightly higher in digitonin lysates from the PP than from the PV zone. Production of sulphate, the major catabolite of [35S]cysteine sulphur, as well as incorporation of the label into protein occurred at similar rates in PP and PV cells. Taurine, on the other hand, was produced from [35S]cysteine much faster by PV than by PP cells (0.7 versus 0.1 mumol/h per g of cells). Accordingly, the taurine content of PV hepatocytes tended to be higher and to increase faster during incubation with methionine. These results imply that metabolism of taurine is highly zonated within the acinus. They also suggest that both the slightly lower GCS activity and the fast metabolism of cysteine to taurine limit the capacity of PV hepatocytes to synthesize GSH.

Improved outcome of paracetamol-induced fulminant hepatic failure by late administration of acetylcysteine

The influence of acetylcysteine, administered at presentation to hospital, on the subsequent clinical course of 100 patients who developed paracetamol-induced fulminant hepatic failure was analysed retrospectively. Mortality was 37% in patients who received acetylcysteine 10-36 h after the overdose, compared with 58% in patients not given the antidote. In patients given acetylcysteine, progression to grade III/IV coma was significantly less common than in those who did not receive the antidote (51% vs 75%), although the median peak prothrombin time was similar for both groups. Whether the beneficial effect is related to replenishment of glutathione stores or a consequence of another hepatic protective mechanism of acetylcysteine requires further study.

Evidence for a direct role of intracellular calcium in paracetamol toxicity

There is considerable evidence that an increase in cytosolic Ca2+ is involved in the cytotoxicity of a variety of agents. However, the direct demonstration of such involvement has proved difficult. In the present study, loading of freshly isolated hamster hepatocytes with the Ca2+ specific chelator Quin 2 (2-[(2-bis[carboxymethyl]amino-5-methyl-phenoxy)methyl]-6-methoxy-8- bis-[carboxymethyl]amino-quinoline) provided significant protection against the loss of viability caused by paracetamol. This was evident both when the cells were co-incubated with Quin 2-AM and paracetamol, and when the cells were incubated with Quin 2-AM after prior exposure to paracetamol and its complete removal from the hepatocytes. These observations provide direct evidence that an increase in intracellular Ca2+ is the cause of cell death in hepatocytes exposed to paracetamol. Further, the fact that Quin 2 is protective even after some time suggests that, for alterations of cytosolic Ca2+ to be detrimental, they must be sustained. The effects of Quin 2 on plasma membrane blebbing of paracetamol-exposed hepatocytes were less pronounced than on cell viability. This is in contrast to the effects of the direct-acting thiol-reducing reducing agent, dithiothreitol, which was equally effective in preventing blebbing and loss of viability. It is concluded that alterations of cytosolic Ca2+ are less directly linked to plasma membrane blebbing than to loss of cell viability.

Acute acrylonitrile toxicity: studies on the mechanism of the antidotal effect of D- and L-cysteine and their N-acetyl derivatives in the rat

Thiol-containing antidotes for acute acrylonitrile (AN) toxicity may exert their action by chemically reacting with AN, by replacing critical sulfhydryl groups cyanoethylated by AN, and by detoxifying cyanide produced from AN metabolism. We have evaluated the ability of the optical isomers of cysteine and N-acetylcysteine to act as antidotes against AN toxicity in order to assess the relative importance of each of these three antidotal mechanisms. The toxicity of AN was determined in male Sprague-Dawley rats and compared to the toxicity determined after treatment with 2 mmol/kg of thiol antidote by computing a protective index (median lethal dose with antidote/median lethal dose without antidote). The protective indices of L-cysteine, D-cysteine, N-acetyl-L-cysteine, and N-acetyl-D-cysteine were 2.03, 1.97, 1.76, and 1.25, respectively. Measurements of urinary mercapturates, derived from the non-oxidative pathway of AN metabolism, indicated that none of the antidotes was able to significantly increase the excretion of these metabolites. Blood cyanide generated from the oxidative metabolism of AN and butyronitrile was also determined. All of the antidotes, except N-acetyl-D-cysteine, lowered blood cyanide levels. A comparison of these results with the predicted relative abilities of the enantiomers to participate in each of the three antidotal mechanisms leads to the conclusion that, under these experimental conditions, the best correlation exists with the cyanide detoxification mechanism.

Sulfur-containing amino acids that increase renal glutathione protect the kidney against papillary necrosis induced by 2-bromoethylamine

Papillary necrosis was observed in the kidneys of rats, 72 h after receiving a single injection of bromoethylamine (BEA). This effect was associated with renal glutathione (GSH) depletion 1 h after the administration of BEA. Stimulation of renal GSH synthesis by pretreatment of the animals either with glutamine + glycine + cystine or N-acetyl-L-cysteine was attempted. Low doses of these precursors administered previously to BEA, respectively, decreased or abolished the GSH depletion. Nevertheless, both pretreatments failed to modify the magnitude of renal papillary necrosis. High doses of these precursors did not modify the BEA-induced GSH depletion, but they significantly increased GSH levels 24 h after BEA administration. At this time, although a smaller intensity of renal papillary necrosis was observed with the amino acid mixture pretreatment, N-acetyl-L-cysteine pretreated rats showed no papillary necrosis. It is suggested that the observed protective effects against BEA-induced renal papillary injury may be ascribed in some measure, to a mechanism independent of GSH.

Selective protein arylation by acetaminophen and 2,6-dimethylacetaminophen in cultured hepatocytes from phenobarbital-induced and uninduced mice. Relationship to cytotoxicity

To evaluate the mechanistic importance of covalent binding in acetaminophen (APAP)-induced hepatotoxicity, we compared the effects of 2,6-dimethylacetaminophen (2,6-DMA) to those of APAP in primary cultures of mouse hepatocytes. Immunochemical analysis of electrophoretically separated proteins has shown that the majority of covalent binding after a cytotoxic dose of APAP occurs on two major bands of 44 and 58 kD (Bartolone et al., Biochem Pharmacol 36: 1193-1196, 1987). At equimolar concentrations, 2,6-DMA bound proteins only 15% as extensively as did APAP and was not cytotoxic in hepatocytes from uninduced mice. However, when the hepatocytes were obtained from phenobarbital-induced mice, APAP administration resulted in increased protein arylation and a more rapid onset of cytotoxicity. Furthermore, in the cells from phenobarbital-induced mice, 2,6-DMA not only resulted in increased binding but also in overt cytotoxicity. Since our affinity-purified anti-APAP antibody did not cross-react with 2,6-DMA, a new antibody specific for 2,6-DMA was prepared and, after affinity purification, was used to detect 2,6-DMA protein adducts by Western blotting. Results indicated that, in hepatocytes from both phenobarbital-induced and non-induced mice, the binding of 2,6-DMA was also highly selective with the most prominent target being the 58 kD cytosolic protein. However, by contrast to APAP, only minimal binding to the 44 kD protein was detected after 2,6-DMA treatment. Although several additional protein adducts were increased in treated cells from phenobarbital-induced mice, the 58 kD protein was clearly the most prominently arylated target associated with both APAP and 2,6-DMA cytotoxicity. These data suggest that both the specificity of covalent binding as well as the extent of binding to the major targets may play an important role in the ensuing toxicity.

Intrahepatic delivery of glutathione by conjugation to dextran

Glutathione was covalently attached to dextran (T-40) by the CNBr activation method. The compound obtained was a water-soluble powder containing 10 (w/w%) glutathione, which was gradually released from the conjugate in aqueous media. Mice depleted of glutathione by treatment with buthionine sulfoximine, a potent inhibitor of gamma-glutamylcysteine synthetase, exhibited a significant increase in hepatic glutathione level after intravenous injection of the conjugate. In mice given a lethal dose of acetaminophen, the survival rate increased progressively with coadministration of the conjugate, whereas little improvement was found when free glutathione was given. The conjugate maintained the serum transaminase activities at lower level after acetaminophen administration. These findings suggest that the dextran conjugate of glutathione is transported into hepatic cells and is intracellularly hydrolyzed to free form, which protects mice from hepatotoxicity due to acetaminophen.

Mutagenicity studies on paracetamol in human volunteers. II. Unscheduled DNA synthesis and micronucleus test

The possible genotoxic effect of paracetamol (PC) was studied in a group of 11 healthy volunteers. PC was administered in the form of tablets 3 x 1000 mg in the course of 8 h. Blood samples and buccal mucosa cells were taken 0, 24, 72 and 168 h after the first administration of the drug. Each blood sample was used for the termination of the unscheduled DNA synthesis (UDS) in peripheral lymphocytes and ascorbemia in plasma. Buccal mucosa cells were analysed for micronuclei. After PC administration the level of UDS induced by MNNG was decreased to T/C = 4.11 +/- 0.56 after 24 h vs. T/C = 5.02 +/- 0.47 (p less than 0.01) at 0 h. The frequency of micronucleated cells in the buccal mucosa was increased after 72 h to 0.38 +/- 0.07% vs. 0.19 +/- 0.06% (p less than 0.01) before PC administration. If PC was administered simultaneously with ascorbic acid (AA), also in a dose of 3 X 1000 mg, a decreased level of UDS was observed after 24, 72 and 168 h and the increased number of micronuclei was qualitatively the same as the PC alone: 0.38 +/- 0.09% after 72 h vs. 0.20 +/- 0.05% at 0 h AA did not decrease the genotoxic effect of PC, but prolonged the influence of PC on UDS.

Effect of oral glutathione on hepatic glutathione levels in rats and mice

Administration of oral glutathione (GSH) increases hepatic GSH levels in fasted rats, in mice treated with GSH depletors such as diethyl maleate and in mice treated with high doses of paracetamol. An increase in hepatic GSH levels after administration of oral GSH does not occur in animals treated with buthionine sulphoximine, an inhibitor of GSH synthesis. Administration of oral GSH leads to an increase in the concentration of L-cysteine, a precursor of GSH, in portal blood plasma. Oral administration of L-methionine produced a significant decrease of hepatic ATP in fasted rats, but not in fed rats. Administration of N-acetylcysteine or GSH did not affect the hepatic ATP levels. The results show that the oral intake of GSH is a safe and efficient form of administration of its constituent amino acids in cases when GSH synthesis is required to replete hepatic GSH levels.

The role of heme oxygenase and aryl hydrocarbon hydroxylase in the protection by cysteamine from acetaminophen hepatotoxicity

Administration of cysteamine to rats depressed hepatic aryl hydrocarbon hydroxylase (AHH) activity, cytochrome P-450, and total heme at 24 hr. Total heme remained decreased at 48 hr when all other parameters returned to control values. A significant 5-fold increase in heme oxygenase activity occurred in rat liver 5 hr after treatment, when AHH activity and total heme were unchanged. Histological examination of liver biopsies from rats treated with cysteamine revealed normal hepatic architecture. The observed effects of cysteamine on hepatic drug-metabolizing enzymes in vivo were not due to cysteamine-induced hepatotoxicity. Our results indicate that cysteamine increases heme oxygenase activity in rat liver, with a subsequent decrease in total heme, AHH activity, and cytochrome P-450 content. The depression of P-450 by cysteamine is likely to be an important mechanism for its protection in acetaminophen overdose. The protection studies illustrate this mechanism. Centrilobular hepatic necrosis and elevation in transaminase activity following a toxic dose of acetaminophen were prevented by treatment with cysteamine. The hepatoprotective effect of cysteamine was evident when acetaminophen was administered 24 hr after cysteamine but did not occur when acetaminophen was administered 5 hr after cysteamine or simultaneously. All groups of rats receiving cysteamine showed decreased mortality compared to the group receiving acetaminophen alone.

The antioxidant action of N-acetylcysteine: its reaction with hydrogen peroxide, hydroxyl radical, superoxide, and hypochlorous acid

N-acetylcysteine has been widely used as an antioxidant in vivo and in vitro. Its reaction with four oxidant species has therefore been examined. N-acetylcysteine is a powerful scavenger of hypochlorous acid (H--OCl); low concentrations are able to protect alpha 1-antiproteinase against inactivation by HOCl. N-acetylcysteine also reacts with hydroxyl radical with a rate constant of 1.36 X 10(10) M-1s-1, as determined by pulse radiolysis. It also reacts slowly with H2O2, but no reaction of N-acetylcysteine with superoxide (O2-) could be detected within the limits of our assay procedures.

Efficacy of oral N-acetylcysteine in the treatment of acetaminophen overdose. Analysis of the national multicenter study (1976 to 1985)

During the investigational use of oral N-acetylcysteine as an antidote for poisoning with acetaminophen, 11,195 cases of suspected acetaminophen overdose were reported. We describe the outcomes of 2540 patients with acetaminophen ingestions treated with a loading dose of 140 mg of oral N-acetylcysteine per kilogram of body weight, followed four hours later by 70 mg per kilogram given every four hours for an additional 17 doses. Patients were categorized for analysis on the basis of initial plasma acetaminophen concentrations and the interval between ingestion and treatment. Hepatotoxicity developed in 6.1 percent of patients at probable risk when N-acetylcysteine was started within 10 hours of acetaminophen ingestion and in 26.4 percent of such patients when therapy was begun 10 to 24 hours after ingestion. Among patients at high risk who were treated 16 to 24 hours after an acetaminophen overdose, hepatotoxicity developed in 41 percent--a rate lower than that among historical controls. When given within eight hours of acetaminophen ingestion, N-acetylcysteine was protective regardless of the initial plasma acetaminophen concentration. There was no difference in outcome whether N-acetylcysteine was started zero to four or four to eight hours after ingestion, but efficacy decreased with further delay. There were 11 deaths among the 2540 patients (0.43 percent); in the nine fatal cases in which aminotransferase was measured before treatment, values were elevated before N-acetylcysteine was started. No deaths were clearly caused by acetaminophen among patients in whom N-acetylcysteine therapy was begun within 16 hours. We conclude that N-acetylcysteine treatment should be started within eight hours of an acetaminophen overdose, but that treatment is still indicated at least as late as 24 hours after ingestion. On the basis of available data, the 72-hour regimen of oral N-acetylcysteine is as effective as the 20-hour intravenous regimen described previously, and it may be superior when treatment is delayed.

Antidotal effectiveness of N-acetylcysteine in reversing acetaminophen-induced hepatotoxicity. Enhancement of the proteolysis of arylated proteins

The post-arylative mechanisms by which N-acetylcysteine (NAC) reduces the severity of the hepatotoxicity induced by acetaminophen (APAP) were investigated in primary cultures of mouse hepatocytes. When administered at selected times immediately following removal of medium containing 10 mM APAP, 2.0 mM NAC was shown to restore glutathione levels through 16 hr of APAP pretreatment and to minimize the leakage of glutamate-oxaloacetate transaminase resulting from the first 8 hr of drug exposure. This temporal difference defined a critical period in which cells were responsive to NAC and permitted the investigation of potential post-arylative mechanisms of the antidote. In the absence of NAC during the recovery period, the cellular loss of covalently-bound APAP could be accounted for by the appearance of arylated proteins in the medium without any apparent degradation of APAP-bound proteins. By contrast, when NAC was present during the recovery period, there was a decrease in intracellular protein-bound APAP which could not be accounted for by that detected in the medium. Since during the recovery period the low residual intracellular concentration of APAP could not contribute significantly to any additional covalent binding in this system, NAC could not merely be acting as a nucleophilic trap for the reactive electrophile. Furthermore, NAC is not likely to dissociate covalently bound APAP from proteins. Hence, the overall decrease in covalent binding observed in cultures previously exposed to APAP for up to 8 hr must have arisen from an NAC-dependent enhancement of the degradation of the arylated proteins. However, after a more prolonged exposure to APAP, the ineffectiveness of NAC may have resulted from APAP-induced irreparable damage to the intracellular proteolytic system. These data suggest that the post-arylative efficacy of NAC may reside in the ability of the antidote to restore the functional capacity of the proteolytic system to rid the cells of arylated proteins.

Prevention of acetaminophen-induced hepatotoxicity by dimethyl sulfoxide

Dimethyl sulfoxide (DMSO) has previously been shown to protect against acetaminophen (APAP)-induced hepatotoxicity, but the mechanism of this effect was not clear. Treatment of mice with 1 mg/kg DMSO 4 h before 250 mg/kg APAP resulted in significantly less hepatotoxicity than with APAP alone, as measured by serum glutamic pyruvic transaminase (SGPT) content 24 h after APAP. Protection was also evident when 1 ml/kg DMSO was given 4, but not 8 h after 250 mg/kg APAP. The APAP-induced depletion of liver glutathione was prevented in mice pretreated with DMSO, although DMSO alone had no effect on liver glutathione levels. The hepatic concentration of cytochrome P-450 (P450) 4 h after treatment of mice with 1 ml/kg DMSO, was significantly decreased compared to saline-treated animals. However, while this DMSO pretreatment significantly decreased the activity of cytochrome P-450-linked aminopyrine-N-demethylase, it increased the activity of aniline hydroxylase. Covalent binding of [14C]APAP to hepatic protein in vivo was significantly decreased in mice pretreated with DMSO. Covalent binding of [14C]APAP to hepatic microsomal protein in vitro was not significantly altered after in vivo treatment with DMSO. However, the presence of DMSO in the in vitro incubation mixture significantly decreased covalent binding of [14C]APAP in a dose-dependent manner compared to microsomal fractions from untreated, saline-treated or DMSO pretreated animals. These data suggest that the DMSO-induced alterations in cytochrome P-450 content and activity may not be the cause of the observed protective action of this chemical. The ability to competitively inhibit APAP bioactivation or to directly scavenge free radicals produced during APAP metabolism, including the activated species which covalently binds to protein, may account for the hepatoprotection afforded by DMSO.

Dissociation of covalent binding from the oxidative effects of acetaminophen. Studies using dimethylated acetaminophen derivatives

The cytotoxic effects of 10 mM acetaminophen (APAP) in primary cultures of non-induced mouse hepatocytes are accompanied by depletion of intracellular glutathione (GSH), arylation of protein, and loss of protein sulfhydryl (PSH) groups. Investigation of the stoichiometry of the covalent binding and PSH loss after APAP exposure demonstrated a greater loss in PSH than could be accounted for by covalent binding to proteins and suggests that APAP exhibits both oxidative and arylative actions in cell culture. Subcellular fractionation revealed that the PSH oxidation induced by APAP was greatest in the microsomal fraction. Exposure of the hepatocytes to 10 mM 3,5-dimethyl-acetaminophen (3,5-DMA) or 2,6-dimethyl-acetaminophen (2,6-DMA) permitted dissociation of the oxidative and arylative properties of APAP. Even though treatment of cultured hepatocytes with 3,5-DMA did not result in covalent binding, there was a more rapid depletion of intracellular GSH, oxidation of PSH, and cytotoxicity compared to APAP. This investigation also provides the first evidence that the cytotoxic effects of both APAP and 3,5-DMA are accompanied by the formation of protein aggregates of high molecular weight that are not disulfide linked. The aggregates probably reflect the oxidative properties of these drugs and may be a mediator of their toxic effects. By contrast, 2,6-DMA, which did bind to cellular proteins and deplete GSH, did not lead to PSH loss, protein aggregation, or cytotoxicity. Since PSH oxidation and protein aggregation correlated well with cytotoxicity, these data suggest that the oxidative component of APAP and 3,5-DMA can play a significant role in eliciting cellular damage in cultured hepatocytes.

In vivo studies on toxic effects of concurrent administration of paracetamol and its N-acetyl-DL-methionine ester (SUR 2647 combination)

1. Single p.o. doses of paracetamol 400 and 800 mg/kg or SUR 2647 combination (free paracetamol + paracetamol-N-acetyl-DL-methionate, paracetamol/methionine ratio 2:1) equivalent to paracetamol 400 and 800 mg/kg were given to Bom:NMRI mice. Vehicle treated (1% w/v aqueous methylcellulose) mice were established as a control group. 2. All treatment groups irrespective of medication caused an initial GSH depletion. However, SUR 2647 combination 400 mg/kg caused a much earlier hepatic GSH recovery than paracetamol 400 mg/kg. SUR 2647 combination 800 mg/kg caused a higher hepatic GSH level than paracetamol 800 mg/kg. 3. There was no significant difference in the plasma ALAT level after SUR 2647 combination 400 or 800 mg/kg and the control group. Paracetamol 400 and 800 mg/kg caused significant plasma ALAT elevations compared to the control group. 4. The addition of N-acetyl-DL-methionine esterified to paracetamol, as in the SUR 2647 combination, enhances the hepatic GSH synthesizing capacity in Bom:NMRI mice after experimental overdosage and offers protection of hepatic cell integrity as assessed by plasma ALAT level compared to paracetamol alone.

Effects of N-acetylcysteine on fetal development and on phenytoin teratogenicity in mice

The teratogenicity of phenytoin may result from its enzymatic bioactivation to a reactive intermediate, which interacts irreversibly with fetal tissues. Since glutathione (GSH) is involved in the detoxification of many reactive intermediates, N-acetylcysteine (NAC), a glutathione precursor, was evaluated for its effects on murine fetal development and phenytoin teratogenicity. NAC, 100 to 275 mg/kg, was given intraperitoneally (ip) or per os (po), or as 266 to 410 mg/kg in the drinking water, at various times before or after phenytoin, 65 to 75 mg/kg ip, on gestational days 12 and 13. Dams were killed on gestational day 19, fetal resorptions were noted, and fetuses were examined for anomalies. Significant reductions in phenytoin-induced fetal weight loss and cleft palates were observed when NAC was given by gavage 6 hours after phenytoin or in the drinking water with the lower dose of phenytoin. NAC administered in the drinking water also reduced the incidence of resorptions produced by the higher dose of phenytoin and enhanced postpartum survival in fetuses exposed to 65 or 75 mg/kg phenytoin (P less than .05). Conversely, the incidence of resorptions increased when NAC was given by gavage at other times before or after phenytoin, by single or repetitive ip injections, or in high concentrations in the drinking water (P less than .05). When given with the higher dose of phenytoin, NAC administered via the drinking water significantly increased the incidence of phenytoin-induced cleft palates and fetal weight loss (P less than .05). Similar results were obtained with a single ip injection of NAC and a lower dose of phenytoin. Thus, when given orally, NAC can partially reduce phenytoin teratogenicity and embryopathy. However, altering the route of NAC administration, or increasing the dose of phenytoin and/or NAC, enhanced phenytoin embryotoxicity, and NAC alone at higher doses had embryopathic effects.

Effect of sulphydryl drugs on paracetamol-induced hepatotoxicity in mice

It has been shown that the major in vivo biotransformation of thiol-containing drugs such as captopril (CP) and penicillamine (PA) involve mixed disulphide formation with endogenous thiols derived from cysteine. At high doses, both drugs produced a dose-dependent depletion of glutathione (GSH) and may perturb GSH and related GSH-enzymes. In this study the possible interactions of these drugs with paracetamol, which produce hepatotoxicity after GSH depletion, were investigated. Following co-administration of CP (50-250 mg/kg) or PA (43-257 mg/kg) with paracetamol (300 mg/kg), the hepatotoxic effect produced by paracetamol was diminished. The protective effect was comparable to that produced by N-acetylcysteine (500 mg/kg) and L-cysteine (500 mg/kg). However, pre-treatment with buthionine sulfoximine (BSO), a specific inhibitor of GSH synthesis, abolished the protective effects of CP, N-acetylcysteine and L-cysteine while the protective effect of PA was unaffected. This suggests that, although both CP and PA may act as alternative sulphydryl nucleophiles to GSH to prevent arylation of essential cellular macromolecules by the reactive metabolite of paracetamol, the underlying mechanisms of these drug interactions may be distinctly different.

Nutritional status, glutathione levels, and ototoxicity of loop diuretics and aminoglycoside antibiotics

Chinchillas deprived of food for 48 h prior to the administration of a combined dose of ethacrynic acid (10 mg/kg) and kanamycin (100 mg/kg) suffered a profound hearing loss. Fed animals did not demonstrate any hearing loss at the same dose levels. Drug metabolism may be the common pathway by which ototoxic agents interact, by a mechanism which is common to both the cochlea and the kidney. Glutathione (GSH) is a tripeptide which is involved in several pathways in the detoxification of active oxygen and reactive species formed during xenobiotic metabolism. The enhanced auditory dysfunction was paralleled by one-third decline in hepatic glutathione levels in the food-deprived animals. Manipulation of endogenous GSH levels may mitigate the toxicities of many of these drugs, which otherwise limit their clinical usefulness. These results also indicate that nutritional status may have important clinical implications during drug therapy.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Selective effects of N-acetylcysteine stereoisomers on hepatic glutathione and plasma sulfate in mice

Administration of 1200 mg/kg N-acetyl-L-acetyl-L-cysteine to mice apparently increased the rate of glutathione synthesis resulting in prolonged elevation of the hepatic glutathione pool. At 3 hr, peak glutathione concentrations nearly doubled. The fraction of glutathione in the oxidized state remained at normally low values over this period. The L isomer also significantly increased plasma and urinary concentrations of inorganic sulfate. Plasma concentrations nearly doubled at their peak and urinary excretion over 24 hr rose some threefold above control. Consistent with the known stereoselectivity of many biological processes, the unnatural D isomer of N-acetylcysteine failed to increase hepatic glutathione. Liver concentrations remained similar to control suggesting that the D isomer was unable to increase the rate of glutathione synthesis. The D isomer further differed from its L enantiomer in failing to increase the plasma concentration and the urinary excretion of inorganic sulfate. Congruent with these observations, much more N-acetyl-D-cysteine (47% of dose) was recovered unchanged in 24 hr urine than N-acetyl-L-cysteine (6.1% of dose). These findings are of toxicologic interest because they identify the N-acetylcysteine stereoisomers as a pair of agents that may be useful in separating biologic effects of sulfhydryls that are related primarily to their physical properties (i.e. reduction, radical scavenging) from those that accrue from their ability to increase glutathione and sulfate availability in vivo.

Comparison of the protective effects of N-acetylcysteine, 2-mercaptopropionylglycine and dithiothreitol against acetaminophen toxicity in mouse hepatocytes

The effects of N-acetylcysteine (NAC), 2-mercaptopropionylglycine (MPG) and dithiothreitol (DTT) on the metabolism and toxicity of acetaminophen (APAP) were examined in isolated mouse hepatocytes maintained in primary culture on collagen-coated dishes. Both NAC and MPG increased the formation of the glutathione and sulfate conjugates of APAP and decreased the covalent binding of the APAP reactive metabolite to cellular protein. DTT did not increase APAP metabolism but did decrease covalent binding. NAC, MPG and DTT decreased plasma membrane damage, as measured by leakage of lactate dehydrogenase from hepatocytes, during a 4-h incubation in 5.0 mM APAP. NAC, MPG and DTT also reduced the APAP-induced fall in glutathione levels in these cells. In other experiments, hepatocytes were exposed to 5.0 mM APAP for 1 h and then incubated during a post-exposure period in APAP-free medium. Damage increased during this post-exposure incubation. Addition of DTT, but not NAC or MPG, after APAP exposure protected the hepatocytes from plasma membrane damage during the post-exposure period. These results indicate that NAC and MPG exert their protective effects by their action on the reactive metabolite of APAP. As well as its effect in reducing the formation of the reactive metabolite, DTT has a potent protective effect against the toxic processes initiated by the APAP reactive metabolite.

Efficacy of paracetamol-esterified methionine versus cysteine or methionine on paracetamol-induced hepatic GSH depletion and plasma ALAT level in mice

The effect of paracetamol-N-acetyl-DL-methionate (PAM) in preventing paracetamol-induced hepatic glutathione (GSH) depletion and hepatic cell damage assessed by plasma ALAT level, was compared to those of concomitantly administered paracetamol and N-acetyl-L-cysteine (NAC) or N-acetyl-DL-methionine (NAM) and paracetamol 400 mg/kg (P) alone. PAM, NAM and NAC reduced hepatic GSH depletion compared to P. The concomitant administration of GSH precursors in either form apparently maintained hepatic cell integrity as evaluated by plasma ALAT compared to predose and 16 hr control measurements. No statistically significant difference between PAM, NAM and NAC was observed. In group P a statistically significant, but transitory, rise in plasma ALAT level following dosage was seen. NAC was more effective than PAM and NAM in the prevention of GSH depletion 1 hr after dosing but was less effective in promoting de novo GSH synthesis towards 16 hr. There was no statistically significant difference between PAM and NAM with respect to effect on GSH depletion or hepatic cell integrity. PAM and NAM increased the GSH level significantly above control level 16 hr after dosing. PAM is rapidly cleaved to paracetamol and methionine following dosage as shown by the observed plasma paracetamol level. PAM compares favourably in hepatoprophylactic effect, to concomitant administration of equimolar doses of free N-acetyl-DL-methionine added to the paracetamol formulation.

Effects of ethanol ingestion on the hepatotoxicity and metabolism of paracetamol in mice

The influence of ethanol on paracetamol-induced liver damage was studied in mice and related to changes in microsomal monooxygenases and plasma paracetamol metabolites in the same group of animals. Paracetamol (400 mg/kg body wt, p.o.) was administered alone or simultaneously with ethanol (3 g/kg, p.o.) to mice fed either a chow diet or pretreated for 4 weeks with a liquid diet containing ethanol (20% of energy). Acute ethanol administration protected against paracetamol hepatotoxicity, but this protection was complete only in mice not fed ethanol previously. Acute ethanol administration also appeared to reduce paracetamol monooxygenation in vivo, but ethanol (50 mM) added to microsomal incubations in vitro had no significant effect on paracetamol activation and covalent binding. The chronic ingestion of ethanol in the diet increased paracetamol-related liver damage, but there appeared to be no induction of paracetamol monooxygenation in these animals. We are unable to confirm current concepts that the potentiation of paracetamol hepatotoxicity by chronic ethanol ingestion and its reduction by acute ethanol administration result solely from contrasting effects of ethanol on cytochrome P-450, and alternative explanations are proposed.