Treatment of paracetamol (acetaminophen) poisoning with N-acetylcysteine

Pharmacokinetics of drug overdose

Pharmacokinetics of drugs taken in overdose may differ from those observed following therapeutic doses. Differences are due both to dose-dependent changes and to effects of drugs or pathophysiological consequences of the overdose on kinetics. Dose-dependent changes in rate and extent of absorption, bioavailability (saturation of first-pass metabolism), distribution (saturation of protein binding sites) and metabolism are discussed. Gastrointestinal motility is affected both by specific drug actions, such as delayed gastric emptying by anticholinergic drugs, and by general nervous system depression caused by many drugs. Drug-induced circulatory insufficiency may retard tissue distribution and reduce clearance. Disturbances in blood and urine pH may alter distribution and clearance of weak acids and bases. Drug-induced renal or hepatic failure can significantly decrease clearance. Hypothermia is a common complication of drug overdose and might retard distribution and also reduce clearance. The data concerning pharmacokinetics during overdose are usually incomplete and difficult to interpret. Doses and times of ingestion are uncertain, duration of blood and urine sampling is often inadequate to distinguish absorption from distribution and elimination phases, active metabolites are not measured, protein binding is not determined and clinical features of patients not adequately described. We have, however, reviewed available data for salicylate, paracetamol (acetaminophen), barbiturates, ethchlorvynol, glutethimide, chloral hydrate, tricyclic antidepressants, lithium, phenytoin, ethanol, theophylline, digoxin, amphetamine and phencyclidine.

Direct protection against acetaminophen hepatotoxicity by propylthiouracil. In vivo and in vitro studies in rats and mice

Hepatotoxicity caused by acetaminophen can be prevented by enzyme-catalyzed conjugation of its reactive metabolite with glutathione (GSH). Since we have shown in previous studies that 6-N-propyl-2-thiouracil (PTU) can substitute for GSH as a substrate for the GSH S-transferases, we examined the possibility that PTU might also protect against acetaminophen hepatotoxicity by direct chemical interaction with the reactive metabolite of acetaminophen. In an in vitro system consisting of [(3)H]acetaminophen, liver microsomes from phenobarbital-pretreated rats, and an NADPH-generating system, we found that PTU had a dose-dependent additive effect with GSH on inhibition of acetaminophen covalent binding. PTU administration also resulted in a dose-dependent decrease in both GSH depletion and covalent binding in vivo in acetaminophen-treated mice. To examine the possible mechanisms by which PTU exerts its protective effect, we studied the action of PTU on both acetaminophen conjugation and metabolic activation. PTU had no effect upon acetaminophen pharmacokinetics in phenobarbital-pretreated rats, as examined by measuring acetaminophen concentration in bile, urine, and blood after an intraperitoneal dose, nor did it alter the total amount of polar conjugates formed. Microsomes from PTU-treated rats were unaltered in cytochrome P-450 concentrations and p-nitroanisole-O-demethylase, benzo-alpha-pyrene hydroxylase, and cytochrome c-reductase activities. Furthermore PTU did not decrease acetaminophen-GSH adduct formation in vitro, suggesting that there was no reduction in drug activation. However, in bile from [(35)S]PTU and [(3)H]acetaminophen treated rats, as well as in incubates of the two drugs with liver microsomes, a new (35)S- and (3)H-containing product could be identified. By both thin layer chromatography and high pressure liquid chromatography this new product, which co-eluted with [(3)H]acetaminophen, was separated from unreacted [(35)S]PTU. The formation of this product in vitro was a function of PTU concentration and reached a maximum of 0.06 mumol/min per mg protein at 0.5 mM PTU. In vivo, the total biliary excretion of this product over 4 h (116 nmol) equaled the net reduction in acetaminophen metabolite covalent binding in the liver of phenobarbital-pretreated rats (108 nmol). We conclude that PTU, independent of its antithyroid effect, diminishes hepatic macromolecular covalent binding of acetaminophen reactive metabolite both in vivo and in vitro, and it does so by detoxifying the reactive metabolite through direct chemical interaction in a manner similar to GSH. These observations may define the mechanism by which PTU is protective against liver injury caused by acetaminophen.

N-acetylcysteine-induced inhibition of gastric emptying: a mechanism affording protection to mice from the hepatotoxicity of concomitantly administered acetaminophen

Swiss Webster male mice, 22 +/- 3 g, killed 17-18 h following the concomitant oral administration of acetaminophen (350 mg/kg) and N-acetyl-cysteine (NAC, 100-500 mg/kg, treated) had statistically significant lower plasma transaminases (GOT and GPT) than control mice (acetaminophen + water). Possible mechanisms underlying this protective effect of NAC were examined. NAC (500 mg/kg) reduced [14C]acetaminophen-derived radioactivity in the blood and tissues but increased the percentage of the dose in the gastrointestinal tract. Depletion of hepatic sulphydryl compounds below 75% of the control value was prevented by NAC treatment, whereas urinary excretion of mercapturate and sulfate, metabolites derived from sulphydryls, were proportionally increased and excretion of unchanged drug was decreased by NAC. Absorption of acetaminophen from the small intestine was prevented by NAC and this was attributed to an inhibition in gastric emptying. Since all changes observed following NAC treatment could be attributed to inhibition of gastric emptying, it was considered the major mechanism responsible for affording in mice protection from acetaminophen-induced hepatocellular damage following concomitant oral administration.

N-Acetyl-DL-penicillamine and acetaminophen toxicity in mice

N-Acetyl-DL-penicillamine (IIIb), a structural analog of N-acetyl-L-cysteine (IIIa), did not protect mice from lethal doses of acetaminophen (I), whereas IIIa offered protection. This lack of efficacy of IIIb probably is due to the decreased nucleophilicity of its sulfhydryl group compared to that of IIIa, the probable involvement of cysteine in and conjugate addition to the reactive intermediates of I, and the absence of metabolic conversion of IIIb to inorganic sulfate.

Paracetamol

Paracetamol is an analgesic and antipyretic agent which was first marketed for use as a drug in the U.K. in 1956. It has since become popular with the medical profession and the general public as an alternative to aspirin.

Intravenous N-acetylcysteine for paracetamol poisoning

Intravenously administered N-acetylcysteine was used to treat 11 patients at risk of hepatic damage from paracetamol poisoning. Complete protection against liver failure was seen in all cases without any adverse effects. The efficacy and safety of intravenously administered N-acetylcysteine make it the treatment of choice in paracetamol poisoning.

Severe barbiturate and paracetamol overdose: the simultaneous removal of both poisons by haemoperfusion

Charcoal haemoperfusion was successful in the treatment of severe butobarbitone poisoning complicated by resistant hypotension. At the same time the rapid removal of paracetamol may have lessened the severity of the subsequent hepatic injury. The mechanism and management of shock associated with barbiturate poisoning, and the possible application of haemoperfusion in paracetamol poisoning are discussed.

Inappropriate methods for the emergency determination of plasma paracetamol

Methods for the estimation of plasma paracetamol which depend on acid hydrolysis to p-aminophenol without a prior extraction step also measure inactive metabolites which are present in high concentrations. The extent of the overestimate obtained with such methods was determined using 24 samples from patents after paracetamol overdosage. There was a positive error of between 40 and 700% compared with a high-performance liquid chromatography reference method which measured only unchanged paracetamol. These non-specific methods should not be used to determine the need for specific therapy in patients with paracetamol poisoning.

Is there excessive use of gastric lavage in the treatment of self-poisoning?

An attempt to identify those factors which influenced the decision to perform gastric lavage in 236 cases of deliberate self-poisoning seen over 6 months showed that 87% of patients seen within 4 hours of ingestion of the poison had a lavage, irrespective of the number of tablets and nature of drug taken. Overall, 77% had a gastric lavage. Most of the late lavages were carried out for salicylate ingestion. The changing pattern of drugs used for attempted selfpoisoning suggests that at least 50% of patients are being unnecessarily subjected to gastric lavage.

Drug-induced liver injury

In the usual course of events, the liver detoxifies hepatocytotoxic intermediates of drug metabolism. But when protective systems are overloaded by normally harmless drugs, the intermediates can cause massive, even fatal, hepatic necrosis.