Treatment of paracetamol (acetaminophen) poisoning with N-acetylcysteine

Difficulties in using animal data to predict pharmacological response in man

Variation among species in their response to xenobiotic agents depends upon two sets of factors: pharmacokinetic and pharmacodynamic. Pharmacokinetic factors, of which the rate of metabolic transformation is the most important, determine the concentration of the agent in plasma and tissues as a function of time after it is administered, while pharmacodynamic factors define the tissue response to a given concentration. Both are subject to considerable uncertainty when dealing with a new compound, so that it is not in general possible to predict reliably the response of human subjects to a new compound solely from studies on experimental animals. This uncertainty can be reduced substantially by understanding its mechanisms of action. Greater accuracy in predicting the metabolism of foreign compounds in man is also likely to be achieved by studies of the species variation of cytochrome P450s, and by the use of human hepatocytes and other cell lines to study xenobiotic metabolism.

Acetaminophen hepatotoxicity: correspondence of selective protein arylation in human and mouse liver in vitro, in culture, and in vivo

Human and mouse liver were exposed to an APAP-activating system, in vitro. Subsequent immunochemical analysis of electrophoretically separated proteins with an affinity-purified anti-APAP antibody indicated that when a cytosolic fraction from human liver was incubated with APAP, an NADPH-regenerating system, and mouse microsomes selective APAP binding occurred predominantly to proteins of approximately 38, 58, and 130 kDa. To evaluate whether similar proteins are targeted in situ, primary cultures of human hepatocytes were treated with 10 mM APAP for 4 hr prior to immunochemical analysis. APAP binding was again detected in protein bands of approximately 38, 58, and 130 kDa. In addition, selective binding was also noted to other cytosolic protein bands, e.g., approximately 52 and 62 kDa. For mouse liver, the majority of the binding, in vitro or in culture, was to proteins of approximately 44 and 58 kDa with lesser binding to proteins of approximately 33 and 130 kDa among others. By contrast, at the times monitored, little covalent binding was detected in the 44-kDa region in the human liver experiments. Most noteworthy was the finding that when the protein arylation patterns on liver samples from a human APAP fatality were compared to those from a mouse given a hepatotoxic dose of APAP, the binding patterns were similar to those detected after the in vitro and the culture experiments with mouse and human livers. Furthermore, an immunohistochemical analysis revealed that as with the mouse, APAP covalent binding in the human liver exhibited a distinct zonal pattern consistent with centrilobular binding. That APAP arylation of the 58- and 130-kDa proteins was observed in livers from both mice and humans suggests that the mouse provides a valid model for studying the mechanistic importance of covalent binding. Elucidation of the identities and functions of the common targeted proteins may clarify their toxicological significance.

1,2-Dichloropropane (DCP) toxicity is correlated with DCP-induced glutathione (GSH) depletion and is modulated by factors affecting intracellular GSH

Acute 1,2-dichloropropane (DCP) poisoning in humans is relatively frequent in Italy, where DCP is widely diffused as a constituent of commercial solvents and dry cleaners. In this study we have investigated the effects of DCP on intracellular glutathione (GSH) content in main target tissues of male Wistar rats, i.e. liver, kidney and blood, in order to establish if a correlation between DCP-induced GSH depletion and tissue damage exists. Administration of DCP (2 ml/kg body weight orally) caused a dramatic loss of tissue GSH occurring 24 h after DCP intoxication, followed by a slow restoration approaching physiological levels after 96 h. GSH depletion was associated with a marked increase in serum GOT, GPT, 5'-nucleotidase, gamma-glutamyl transpeptidase, alkaline phosphatase, urea and creatinine, and a significant degree of hemolysis. When animals were pretreated with a GSH depleting agent, buthionine-sulfoximine (BSO) (0.5 g/kg body weight) i.p. 4 h before DCP intoxication, an increase of overall mortality was found, significantly different from the group of animals treated with DCP alone. On the contrary, the administration of a GSH precursor, N-acetylcysteine (NAC) i.p. (250 mg/kg body weight) 2 and 16 h after DCCP intoxication prevented the dramatic loss of cellular GSH and reduced the extent of injury in target tissues, as demonstrated by laboratory indices. Furthermore, statistical analysis of the data revealed a correlation between: (1) depletion of liver GSH and increase in serum GOT, GPT, 5'-nucleotidase, (2) depletion of kidney GSH and increase in serum urea and creatinine and (3) depletion of blood GSH and the occurrence of hemolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetaminophen hepatotoxicity: is there a role for prostaglandin synthesis?

The hepatotoxicity of acetaminophen (APAP) overdose depends on metabolic activation to a toxic reactive metabolite via hepatic mixed function oxidase. In vitro studies have indicated that APAP may also be cooxidized by prostaglandin H synthetase. The present experiments were designed to assess the possible contribution of hepatic prostaglandin synthesis to APAP toxicity. Adult fed male mice were overdosed with 400 mg APAP/kg. Liver toxicity was estimated by measurement of serum transaminases. Hypertonic xylitol or sodium chloride (2250 mOsm/l), administered intragastrically to stimulate prostaglandin synthesis, increased APAP toxicity. By contrast, the cyclooxygenase inhibiting drugs aspirin (at 25 mg/kg) and indomethacin (at 10 mg/kg) protected against APAP-induced toxicity. APAP kinetics were not affected by hypertonic xylitol or indomethacin, nor were hepatic glutathione levels in overdosed mice. Imidazole, a nonspecific thromboxane synthetase inhibitor, also protected overdosed mice. This drug prolonged hexobarbital sleeping time and prevented the depletion of hepatic glutathione that followed APAP intoxication. Thus, the data support the conclusion that APAP-induced hepatoxicity may be modulated not only by inhibition of cytochrome P450 mediated oxidation, but also by controlling hepatic cyclooxygenase activity.

Detection of N-acetylcysteine, cysteine and their disulfides in urine by liquid chromatography with a dual-electrode amperometric detector

A method for the determination of N-acetylcysteine, cysteine and their disulfides in urine is described. The thiols and disulfides are separated by reversed-phase ion-pair chromatography with octyl sodium sulfate as the ion-pairing reagent and detected with a dual-electrode amperometric detector using Au/Hg amalgam electrodes. Both the thiols and disulfides are detected with this system. In addition, dimers and mixed disulfides can be detected individually.

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.

Diagnosis and treatment of acute poisoning with volatile substances

1. The acute toxicity of many volatile compounds is similar, being more related to physical properties than to chemical structure. 2. Volatile substance abusers experiences euphoria and disinhibition but this may be followed by nausea and vomiting, dizziness, coughing and increased salivation; cardiac arrhythmias, convulsions, coma and death occur in severe cases. 3. Laboratory analysis of blood and urine samples collected up to 24 h post-exposure may be helpful if the diagnosis of volatile substance abuse is in doubt. 4. There is only a weak correlation between blood toluene and 1,1,1-trichloroethane concentrations and the clinical features of toxicity, possibly because of rapid initial tissue distribution and elimination. 5. Recovery normally occurs quickly once exposure has ceased but support for respiratory, renal or hepatic failure may be needed as well as treatment for cardiac arrhythmias. Therapy with intravenous acetylcysteine should be considered in cases of acute carbon tetrachloride poisoning.

Effect of N-acetylcysteine on plasma cysteine and glutathione following paracetamol administration

The effect of oral N-acetyl-L-cysteine (NAC) on plasma sulphhydryls has been studied in healthy volunteers. Following NAC 30 mg.kg-1, total NAC in plasma (i.e. free NAC and NAC as disulphides) reached a median peak concentration of 67 nmol.ml-1 within 45 to 60 min, and disappeared with an apparent half-life of 1.3 h. Only a fraction of total NAC (AUC 163 nmol.ml-1.h) was in the form of free NAC (AUC 12 nmol.ml-1.h, peak concentration 9 nmol.ml-1). Free cysteine was markedly increased (peak increment 49 nmol.ml-1; AUC 80 nmol.ml-1.h). Total cysteine and free and total glutathione in plasma were unchanged. Following the administration of 2 g paracetamol plasma cysteine and glutathione decreased (median decrement in AUC over 3 h was 5.1 nmol.ml-1.h and 3.8 nmol.ml-1.h, respectively). In contrast, the administration of 2 g NAC together with paracetamol resulted in an increase in the AUC of cysteine (+29.2 nmol.ml-1.h) and glutathione (+4.6 nmol.ml-1.h). The data show that NAC leads to a marked increase in circulating cysteine, in part by reacting with cystine and thereby forming mixed disulphides with cysteine and releasing free cysteine as shown in vitro. NAC had no effect on plasma glutathione in the absence of increased stress on the glutathione pools. However, NAC supports glutathione synthesis when the demand for glutathione is increased, as during the metabolism of paracetamol.

The disposition and kinetics of intravenous N-acetylcysteine in patients with paracetamol overdosage

Seventeen patients received standard treatment with intravenous N-acetylcysteine for 18 episodes of severe poisoning with paracetamol (acetaminophen). The dose of N-acetylcysteine was 150 mg/kg given in 15 min followed by 50 mg/kg in 4 h and 100 mg/kg over the next 16 h. Liver damage was absent or mild on 13 occasions (ALT greater than 500 mu/l) and severe on 5 (ALT less than 1000 mu/l). Total plasma N-acetylcysteine was estimated by HPLC. The mean maximum plasma concentration after the initial loading dose was 554 mg/l. Concentrations then fell rapidly and after 12 h a mean steady-state level of about 35 mg/l was maintained. When the infusion was discontinued N-acetylcysteine disappeared with a half-life of 5.7 h. The mean steady-state volume of distribution, AUC, mean residence time and total clearance were 536 ml/kg, 1748 mg.h.l-1, 2.91 h and 3.18 ml.min-1.kg-1. These values are generally consistent with those previously reported with much smaller doses and the disposition of N-acetylcysteine does not appear to be dose-dependent. The elimination of N-acetylcysteine was not impaired in the patients with severe liver damage, and the pharmacokinetic variables and plasma concentrations were similar in patients with and without hepatotoxicity. The dosage schedule for intravenous N-acetylcysteine should probably be modified since adverse reactions invariably occur early when plasma concentrations are at their highest, and liver damage was prevented just as effectively at the lowest as at the highest Cmax. High initial concentrations of N-acetylcysteine can be avoided with simple alternative regimens based on the kinetic data of this study.

Clinical toxicology--past, present and future

1. The alarming increase in the incidence of self-poisoning in Western countries in the 1950s prompted the establishment of the National Poisons Information Service in the UK and the designation of certain Regional Poisoning Treatment Centres. 2. The substances taken in acute poisoning episodes largely reflect the poisons available in the community and, in the UK at least, have changed with fashions in prescribing although psychotropic drugs and analgesics always predominate. 3. Intensive supportive care with repeat-dose oral activated charcoal and even haemoperfusion has been proved effective in acute poisoning with central nervous depressant drugs such as barbiturates even though these latter drugs are now rarely encountered in overdose. 4. Other advances in clinical toxicology include the introduction of the opiate antagonist naloxone, Fab antibody fragments for life-threatening digoxin overdosage and proven treatment for paracetamol poisoning. Analytical toxicology has also made a major contribution. 5. On the debit side, formal psychiatric assessment of patients after acute poisoning remains contentious, tricyclic antidepressants are still a major problem and there is no effective treatment for poisoning with paraquat or for paracetamol when presentation is delayed. 6. As to the future, although the 'epidemic' of serious acute poisoning of the 1960s and 70s appears to be past its peak, there will always be unusual and serious problems and the UK poisons information services must develop to make the best use of computer-based technology.

Pharmacokinetics and bioavailability of reduced and oxidized N-acetylcysteine

The pharmacokinetics and bioavailability of N-acetylcysteine (NAC) have been determined after its intravenous and oral administration to 6 healthy volunteers. According to a randomized cross-over design each subject received NAC 200 mg i.v. and 400 mg p.o., and blood samples were collected for 30 h. Reduced NAC had a volume of distribution (VSS) of 0.59 l.kg-1 and a plasma clearance of 0.84 l.h-1.kg-1. The terminal half-life after intravenous administration was 1.95 h. The oral bioavailability was 4.0%. Based on total NAC concentration, its volume of distribution (VSS) was 0.47 l.kg-1 and its plasma clearance was 0.11 l.h-1.kg-1. The terminal half-life was 5.58 h after intravenous administration and 6.25 h after oral administration. Oral bioavailability of total NAC was 9.1%.

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.

Dextropropoxyphene overdose. Epidemiology, clinical presentation and management

This paper comprehensively reviews the worldwide situation regarding acute overdosage of dextropropoxyphene (propoxyphene). The changing epidemiology of this type of poisoning over the last 20 years is described with discussion of concurrent trends and, in particular, the effects of different preventive measures adopted in various countries. The clinical pharmacology of dextropropoxyphene relevant to the clinical toxic effects resulting from acute overdosage is described, and the management is detailed. In particular, the importance of early diagnosis and treatment is stressed in view of the potentially lethal complications that may suddenly occur with this poisoning. Recommendations for the correct use of the specific narcotic antagonist, naloxone, are made, together with other intensive supportive measures. As dextropropoxyphene is frequently taken together with other toxic agents, the concomitant effects of alcohol and sedative drugs are described and the treatment of paracetamol (acetaminophen) in combination with dextropropoxyphene is emphasised. The most effective preventive measures for the future are suggested, but caution is advised regarding the prescription for 'at risk' patients of alternative analgesics, which may be no safer in overdosage.

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.

Enhanced acetaminophen toxicity associated with prior alcohol consumption in mice: prevention by N-acetylcysteine

It is well established that hepatotoxicity is associated with an overdose of acetaminophen and that this hepatotoxicity can be increased by prior alcohol exposure in either humans or animal models. N-Acetylcysteine (NAC) has been developed as a tool to prevent the hepatotoxicity associated with acetaminophen overdosing. The present investigation observed that prior acute and chronic ingestion of alcohol to mice resulted in enhanced toxicity following acetaminophen injection. This increased toxicity was prevented by treatment with NAC. These results suggest that NAC may be a useful tool for combatting the enhanced acetaminophen toxicity associated with alcohol ingestion.

Acetaminophen hepatotoxicity: studies on the mechanism of cysteamine protection

Inhibition of the cytochrome P-450-dependent formation of the acetaminophen-reactive metabolite was investigated as a possible mechanism for cysteamine protection against acetaminophen hepatotoxicity. Studies in isolated hamster hepatocytes indicated that cysteamine competitively inhibited the cytochrome P-450 enzyme system as represented by formation of the acetaminophen-glutathione conjugate. However, cysteamine was not a potent inhibitor of glutathione conjugate formation (Ki = 1.17 mM). Cysteamine also weakly inhibited the glucuronidation of acetaminophen (Ki = 2.44 mM). In vivo studies were in agreement with the results obtained in isolated hepatocytes; cysteamine moderately inhibited both glucuronidation and the cytochrome P-450-dependent formation of acetaminophen mercapturate. The overall elimination rate constant (beta) for acetaminophen was correspondingly decreased. Since cysteamine decreased both beta and the apparent rate constant for mercapturate formation (K'MA), the proportion of the dose of acetaminophen which is converted to the toxic metabolite (K'MA/beta) was not significantly decreased in the presence of cysteamine. Apparently, cysteamine does inhibit the cytochrome P-450-dependent formation of the acetaminophen-reactive metabolite, but this effect is not sufficient to explain antidotal protection.

One- and two-electron oxidation of reduced glutathione by peroxidases

The oxidation of glutathione by horseradish peroxidase or lactoperoxidase forms a thiyl free radical, as demonstrated with the spin-trapping ESR technique. Reactions of this thiyl free radical result in oxygen consumption, which is inhibited by the radical trap 5,5-dimethyl-1-pyrroline-N-oxide. In contrast to L-cysteine oxidation, glutathione oxidation is highly hydrogen peroxide-dependent. The oxidation of glutathione by glutathione peroxidase forms GSSG without forming a thiyl radical intermediate except in the presence of the thiyl radical-generating horseradish peroxidase.

Non-narcotic analgesics. Problems of overdosage

The first cases of fulminant hepatic failure due to paracetamol poisoning were reported in 1966, and in the United Kingdom this condition is now responsible for more cases of acute hepatic failure than any other cause. Adults account for the majority of serious and fatal cases of paracetamol poisoning and it is extremely rare for young children to ingest sufficient paracetamol to cause more than minimal liver damage. A single measurement of the plasma paracetamol concentration is an accurate predictor of liver damage provided that it is taken not earlier than 4 hours after ingestion of the overdose. Peak disturbance of liver function occurs 2 to 4 days after the overdose, often accompanied by mild jaundice, after which recovery is usually rapid and complete. In a few patients, fulminant hepatic failure, manifested by increasing jaundice and encephalopathy, may develop by the third to fifth day. Acute renal failure may complicate paracetamol poisoning, often in the context of severe liver damage. Renal failure, which is often non-oliguric, typically becomes apparent 24 to 72 hours after overdosage. The treatment of paracetamol intoxication should include gastric lavage, which has been shown to be of value for up to 6 hours after ingestion of a paracetamol overdose. Further general treatment may include parenteral fluid replacement and a prophylactic infusion of dextrose (5-10%) in patients at risk of hepatic failure. Specific protective agents in those patients at risk of paracetamol-induced liver damage include N-acetylcysteine and methionine which are most effective if given within 8 to 10 hours of ingestion of the overdose. Hepatic and renal failure should be managed conventionally. In recent years in the United Kingdom there has been a gradual decline in the number of hospital admissions and the number of deaths from aspirin poisoning. Salicylates in overdose directly stimulate the respiratory centre and so cause a respiratory alkalosis. Metabolic acidosis occurs in severe poisoning because of impairment of the oxidative metabolism of energy substrates. At very high salicylate concentrations respiratory depression may occur, possibly associated with neuroglycopenia, adding respiratory acidosis to the worsening metabolic acidosis. In addition to a mixed acid-base disturbance, hypokalaemia and hypoglycaemia may be present. Nausea and vomiting increase the fluid deficit. If dehydration is sufficiently severe, decreasing cardiac output may hasten development of lactic acidosis and acute renal failure.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of the protective action of n-acetylcysteine and methionine against paracetamol toxicity in the hamster

The mechanism of the protective action of methionine and N-acetylcysteine against the toxicity of paracetamol was investigated in vivo. N-acetylcysteine inhibited the O-deethylation of ethoxyresorufin (cytochrome P-448) while methionine enhanced the N-demethylation of benzphetamine (cytochrome P-450) and increased hepatic microsomal levels of cytochrome P-450. These observations indicate that N-acetylcysteine, but not methionine, could afford protection against paracetamol hepatotoxicity, at least partly, by inhibiting cytochrome P-448 activity and thus the generation of the reactive intermediate. However, previous studies demonstrating no decrease in the urinary excretion of glutathione conjugates of paracetamol (derived from the reactive intermediate) in animals treated with N-acetylcysteine suggest that this is unlikely to be the prevailing mechanism of action. Administration of a large dose of paracetamol, as expected, depleted glutathione levels and inhibited cytosolic glutathione transferase activity. Administration of either N-acetylcysteine or methionine 1 h after paracetamol prevented both effects. On the basis of the present work and previously published observations, it is concluded that the major mechanism of action of N-acetylcysteine and methionine in vivo is by acting as precursors of intracellular glutathione.

Concomitant use of activated charcoal and N-acetylcysteine

Activated charcoal is a safe, effective, inexpensive adjunct in the management of most toxic ingestions. It has the ability to adsorb a wide variety of drugs and chemicals, one of which is acetaminophen. N-acetylcysteine (NAC) is the specific antidote available for serious overdoses of acetaminophen. Current management of acetaminophen overdose, however, does not recommend the concomitant oral administration of these two useful agents because adsorption and inactivation of NAC by charcoal is believed to occur. Our study was designed to help evaluate the effect of activated charcoal on N-acetylcysteine absorption. Ten healthy male volunteers were each given in the first, or control, phase of the study an oral dose of 140 mg/kg NAC, and venous blood samples were obtained. In the second phase, after a washout period, each subject received 60 g activated charcoal orally followed immediately by 140 mg/kg NAC. NAC serum levels were measured using gas-liquid chromatography, and levels were compared with and without the concomitant administration of charcoal. Although only a small number of the subjects completed the study, the results showed that in both phases there were no significant differences in the peak NAC levels, the plasma half-life of NAC, or the calculated area under the curve. We recommend that NAC and activated charcoal not be used clinically until further studies are completed.

Plasma glutathione S-transferase measurements by radioimmunoassay: a sensitive index of hepatocellular damage in man

Plasma glutathione S-transferase (GST) basic and N/A2b concentrations have been measured by specific radioimmunoassay in serial samples taken from patients admitted following a paracetamol overdose. The activity of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were also measured. The sensitivities of the various measurements for detecting hepatocellular damage were compared. The measurement of either basic or N/A2b GST proved equally sensitive for detecting liver damage and both were superior to aminotransferase measurements. The abnormalities in GST were, on average, approximately 5- to 10-fold greater than the conventional aminotransferase measurements provided that correct timing of sampling was employed. The data presented suggest GST measurement is a sensitive non-invasive method for investigating acute drug-induced hepatotoxicity. The short plasma half-life of GST also allows early recognition of when active cellular damage has ceased.

Effect of antidotal N-acetylcysteine on the pharmacokinetics of acetaminophen in dogs

The effects of N-acetylcysteine (NAC) on the pharmacokinetic parameters of acetaminophen (AP) in adult female beagles were studied. Each of eight dogs received a single i.v. injection of 150 mg/kg of AP as a 5% solution in a vehicle of 40% aqueous propylene glycol at 0 h. Each of four AP-treated dogs (Group I) received an oral dose of 140 mg/kg NAC as a 20% aqueous solution at 0 h, and 70 mg/kg at 30 min and 1 h post-AP administration. Four dogs (Group II) served as controls and received isotonic saline orally. Mild signs of AP toxicosis seen in both groups within 2-3 h of AP administration including depression, weakness, recumbency and methaemoglobinaemia. Relative to Group II, treatment with NAC (Group I) enhanced the elimination of AP from the body as indicated by the decreased plasma half-life (t1/2 = 1.06 h for Group I v. 1.78 h for Group II) and a higher elimination rate constant (beta = 0.67/h for Group I v. 0.40/h for Group II). Changes in the area under plasma concentration curve data (AUC = 0.39 mg.h/ml for Group I v. 0.65 mg.h/ml for Group II) were associated with a 61% increase in total body clearance of AP in Group I. The apparent volume of drug distribution Vdarea was not affected.

Thiyl radicals--formation during peroxidase-catalyzed metabolism of acetaminophen in the presence of thiols

We confirm using EPR spectroscopy in conjunction with the spin probe 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidine (OXANOH) that horseradish peroxidase catalyzed metabolism of the analgesic acetaminophen occurs via a one electron mechanism. When either glutathione cysteine or N-acetylcysteine were included in the reaction the thiols reduced the acetaminophen-derived radicals to generate thiyl radicals which were trapped with the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and observed using EPR spectroscopy. Similarly, DMPO-thiyl radical adducts were observed during prostaglandin synthase catalyzed oxidation of acetaminophen in the presence of either glutathione or N-acetylcysteine. This is a mechanism of removal of reactive xenobiotic free radicals generated in metabolic systems but whether it represents a true detoxification reaction depends on the subsequent fate of the thiyl radicals generated.

Mechanism of action of paracetamol protective agents in mice in vivo

The mechanism of action of cysteine, methionine, N-acetylcysteine (NAC) and cysteamine in protecting against paracetamol (APAP) induced hepatotoxicity in male C3H mice in vivo has been investigated by, characterising the effect of the individual protective agents on the metabolism of an hepatotoxic dose of APAP, and determining the efficacy of the protective agents in animals treated with buthionine sulphoximine (BSO), a specific inhibitor of glutathione (GSH) synthesis. Co-administration of cysteine, methionine or NAC increased, while co-administration of cysteamine decreased, the proportion of GSH-derived conjugates of APAP excreted in the urine of mice administered APAP, 300 mg/kg. Pretreatment of animals with BSO abolished the protective effect of cysteine, methionine and NAC, whereas cysteamine still afforded protection against APAP after BSO treatment. In conjunction with other data, these results suggest the most likely mechanism for the protective effect of cysteine, methionine and NAC is by facilitating GSH synthesis, while the most likely mechanism for the protective effect of cysteamine is inhibition of cytochrome P-450 mediated formation of the reactive metabolite of APAP.

Adverse reactions to N-acetylcysteine

Clinical details of seven patients who suffered adverse reactions to N-acetylcysteine as Parvolex are documented. Skin testing was carried out to diluted Parvolex, and its individual components N-acetylcysteine and ethylenediaminetetra-acetate, in five reacting patients and five patients who had received Parvolex with no ill-effects. Weal responses to high concentrations (20 mg/ml) of acetylcysteine as Parvolex were significantly greater (p less than 0.02) in reactors. There were no other significant differences between the groups. In two patients who reacted, the effects of intradermal Parvolex could be inhibited by prior therapy with the antihistamine terfenadine. These results suggest a 'pseudo-allergic' rather than an immunological aetiology for adverse reactions to Parvolex.

Adverse reactions to acetylcysteine and effects of overdose

Since the introduction in 1979 of intravenous acetylcysteine (Parvolex) as an antidote for overdosage of paracetamol the National Poisons Information Service and the manufacturer have been notified of 38 adverse reactions that were anaphylactoid in nature and 19 accidental overdoses. The most common feature of the anaphylactoid reaction to normal dosage was rash; other features reported included angioedema, hypotension, and bronchospasm; all the patients recovered. The features associated with an overdose of acetylcysteine were similar but more severe; two patients died, but the extent to which the overdose of acetylcysteine may have been implicated was not clear in either case.

[N-Acetylcysteine as an antidote in accidental acrylonitrile poisoning]

Acute acrylonitrile intoxications are followed by loss of consciousness, convulsions, respiratory arrest, and may end fatally. Common cyanide antidotes have not proved to be effective. In previous animal experiments we could demonstrate that the cyanide antidotes show some protective effect only after oral acrylonitrile application. Only a minimal amount of inhaled acrylonitrile will be transformed into cyanide, in contrast to pharmacokinetics after oral intake. After acrylonitrile inhalation the toxic effect of the whole molecule ( cyanethylation ) is important, and sulfhydryl compounds show antidotal effects. Further animal experiments demonstrate the superior antidotal effects of N-acetyl-cysteine after acrylonitrile inhalation. Intravenous injection of N-acetyl-cysteine in high doses is recommended according to the treatment of paracetamol poisoning.

Selective inhibition of acetaminophen oxidation and toxicity by cimetidine and other histamine H2-receptor antagonists in vivo and in vitro in the rat and in man

Acetaminophen-induced hepatotoxicity results from hepatic enzymatic oxidation of acetaminophen to a toxic, electrophilic intermediate. Acetaminophen is ordinarily eliminated after conjugation with glucuronic acid and sulfate to nontoxic derivatives. Cimetidine has been shown to inhibit the hepatic oxidation of a number of drugs and to protect rats from acetaminophen-induced hepatic necrosis. The aim of this study was to define the mechanism by which cimetidine reduced acetaminophen-induced hepatic necrosis and to determine whether inhibition of formation of the reactive metabolite(s) of acetaminophen occurred also in man. In vivo cimetidine pretreatment decreased covalent binding of [3H]acetaminophen to the liver from 552 +/- 23.8 to 170 +/- 31.6 nmol/g protein 2 h after a toxic dose of acetaminophen in 3-methylcholanthrene pretreated rats (P less than 0.05). Cimetidine pretreatment also significantly reduced the rate of hepatic glutathione depletion. Both cimetidine and metiamide produced dose-dependent inhibition of acetaminophen oxidation in vitro, whereas inhibition by ranitidine and cimetidine sulfoxide was quantitatively less. Inhibition of acetaminophen oxidation by cimetidine and metiamide was primarily competitive with an inhibition constant (Ki) of 130 +/- 16 and 200 +/- 50 microM, respectively. By contrast, cimetidine inhibited acetaminophen glucuronidation minimally with a Ki of 1.39 +/- 0.23 mM. Similar results were obtained using human liver microsomes as a source of enzymes. In a dose-related fashion, cimetidine also reduced acetaminophen-induced toxicity to human lymphocytes when incubated with microsomes and NADPH. Pharmacokinetics of acetaminophen elimination were studied in normal volunteers with and without co-administration of cimetidine 300 mg every 6 h. In normal volunteers, cimetidine decreased the fractional clearance of the oxidized (potentially toxic) metabolites of acetaminophen more than the conjugated metabolites. This finding confirmed the hypothesis that cimetidine is a relatively selective inhibitor of the oxidation of acetaminophen to reactive metabolites in man as well as in animals. When considered together with the results of previous studies showing improved survival and decreased hepatoxicity in acetaminophen-poisoned animals, the present results provide a rational basis for assessing possible benefits of cimetidine treatment of acetaminophen overdoses in man.

Sex related differences in acetaminophen toxicity in the mouse

Acetaminophen LD50 was two fold lower in female than male mice, the greater sensitivity of female mice for acetaminophen was also reflected in the serum enzyme levels of glutamic oxaloacetic and pyruvic transaminases (SGOT/SGPT), where the enhancement of both enzymes was higher in female than male mice. We could also observe the L-cysteine protection against the toxic effect of acetaminophen. Our results demonstrate that the lethal action of acetaminophen administrated either P.O. or S.C., is sex related.

The effect of cysteine oxidation on isolated hepatocytes

Isolated hepatocytes incubated with 4mM-cysteine lose reduced glutathione, adenine nucleotides and intracellular enzymes, thus showing extensive membrane damage. The toxic effects of cysteine are enhanced by NH4Cl. Lactate, ethanol and unsaturated fatty acids afford significant protection against cysteine-induced cytoxicity. Addition of catalase to the incubation medium also protected against cysteine toxicity, indicating that H2O2 formed during the oxidation of cysteine is involved in the toxic effects observed. Under anaerobic conditions cysteine did not cause leakage of lactate dehydrogenase from cells, confirming that rapid autoxidation is an essential condition for development of the toxic effects of cysteine.

Mechanism of action of N-acetylcysteine in the protection against the hepatotoxicity of acetaminophen in rats in vivo

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.

Comparison of activated charcoal and ipecac syrup in prevention of drug absorption

The efficacy of activated charcoal and ipecac syrup in the prevention of drug absorption was studied in 6 healthy adult volunteers, using a randomized, cross-over design. Paracetamol 1000 mg, tetracycline 500 mg and aminophylline 350 mg were ingested on an empty stomach with 100 ml water. Then, after 5 or 30 min, the subjects ingested, either activated charcoal suspension (50 g charcoal), syrup of ipecac, or, only after 5 min, water 300 ml. Activated charcoal, given either after 5 or 30 min, significantly (p less than 0.01 or less 0.05) reduced the absorption of these 3 drugs measured, for example as AUC0-24 h. Syrup of ipecac caused emesis on each occasion, with a mean delay of 15 min. When ipecac was given 5 min after the drugs, its effect on absorption was significant, but when it was given after 30 min only the absorption of tetracycline was reduced. Activated charcoal was significantly (p less than 0.05) more effective than ipecac in reducing drug absorption when given at the same time points. In cases of acute intoxication, depending on the quality and quantity of the drugs ingested, the relative efficacy of charcoal and ipecac may be somewhat different from that observed in the present study. Despite its emetic action, however, ipecac syrup is not very effective in preventing drug absorption and, in general, activated charcoal should also be given after induced emesis or gastric lavage.

Drug-induced lipid peroxidation in mice--II. Protection against paracetamol-induced liver necrosis by intravenous liposomally entrapped glutathione

If injected intravenously 2 hr before the drug, a dose of more than 175 mg/kg body weight glutathione (0.57 mmol/kg) protected male mice from acute liver necrosis induced by intraperitoneal administration of 400 mg/kg (2.65 mmol/kg) paracetamol. Soluble glutathione yielded a limited, and liposomally entrapped glutathione an optimal dose-dependent protective effect against drug-induced lipid peroxidation (as measured by in vivo ethane exhalation) liver necrosis (assessed by serum transaminases) and hepatic glutathione depletion (determined post mortem). N-Acetylcysteine solution had no effect in this model.

Studies on paracetamol-induced lipid peroxidation

Post-mitochondrial supernatants isolated from livers of rats given a single large oral dose of paracetamol (800 mg/kg) showed rapid rates of lipid peroxidation when incubated in vitro. As a result of paracetamol administration the level of reduced glutathione (GSH) declined to approx. 20-25% of the peak physiological value. Addition of reduced GSH to the supernatant inhibited the peroxidation. Paracetamol-induced lipid peroxidation was inhibited in vitro by antioxidants (e.g. vitamin E) but was unaffected by superoxide dismutase and mannitol. N-acetyl cysteine and cysteamine inhibited lipid peroxidation in vitro in a cytosol-dependent manner in the absence of glutathione. Lipid peroxidation probably occurs simultaneously with the proposed covalent binding of the active metabolite of paracetamol. Since the former process is known to cause severe and extensive membrane damage, it may be a very important factor in paracetamol-induced liver necrosis.