Acetaminophen pharmacokinetics after overdose

A generalisation of T-optimality for discriminating between competing models with an application to pharmacokinetic studies

The T-optimality criterion is used in optimal design to derive designs for model selection. To set up the method, it is required that one of the models is considered to be true. We term this local T-optimality. In this work, we propose a generalisation of T-optimality (termed robust T-optimality) that relaxes the requirement that one of the candidate models is set as true. We then show an application to a nonlinear mixed effects model with two candidate non-nested models and combine robust T-optimality with robust D-optimality. Optimal design under local T-optimality was found to provide adequate power when the a priori assumed true model was the true model but poor power if the a priori assumed true model was not the true model. The robust T-optimality method provided adequate power irrespective of which model was true. The robust T-optimality method appears to have useful properties for nonlinear models, where both the parameter values and model structure are required to be known a priori, and the most likely model that would be applied to any new experiment is not known with certainty.

A review of the hepatotoxicity of paracetamol at therapeutic or near-therapeutic dose levels, with particular reference to alcohol abusers

The number of published reports associating hepatotoxicity with paracetamol ingestion at therapeutic or near-therapeutic dose levels is small but is, nevertheless, suggestive of a relationship. There is however, mounting evidence that certain groups of patients, such as alcohol-dependent people, patients receiving enzyme-inducing drugs (particularly anti-convulsant and anti-tuberculosis medications) as well as those with certain infectious diseases, are rendered more susceptible to paracetamol-induced hepatotoxicity. Seventy-four case reports where therapeutic or near-therapeutic doses of paracetamol resulted in hepatic injury are reviewed and factors and mechanisms which might explain this apparently increased vulnerability to damage are discussed.

Paracetamol: new vistas of an old drug

Paracetamol (acetaminophen) is one of the most popular and widely used drugs for the treatment of pain and fever. It occupies a unique position among analgesic drugs. Unlike NSAIDs it is almost unanimously considered to have no antiinflammatory activity and does not produce gastrointestinal damage or untoward cardiorenal effects. Unlike opiates it is almost ineffective in intense pain and has no depressant effect on respiration. Although paracetamol has been used clinically for more than a century, its mode of action has been a mystery until about one year ago, when two independent groups (Zygmunt and colleagues and Bertolini and colleagues) produced experimental data unequivocally demonstrating that the analgesic effect of paracetamol is due to the indirect activation of cannabinoid CB(1) receptors. In brain and spinal cord, paracetamol, following deacetylation to its primary amine (p-aminophenol), is conjugated with arachidonic acid to form N-arachidonoylphenolamine, a compound already known (AM404) as an endogenous cannabinoid. The involved enzyme is fatty acid amide hydrolase. N-arachidonoylphenolamine is an agonist at TRPV1 receptors and an inhibitor of cellular anandamide uptake, which leads to increased levels of endogenous cannabinoids; moreover, it inhibits cyclooxygenases in the brain, albeit at concentrations that are probably not attainable with analgesic doses of paracetamol. CB(1) receptor antagonist, at a dose level that completely prevents the analgesic activity of a selective CB(1) receptor agonist, completely prevents the analgesic activity of paracetamol. Thus, paracetamol acts as a pro-drug, the active one being a cannabinoid. These findings finally explain the mechanism of action of paracetamol and the peculiarity of its effects, including the behavioral ones. Curiously, just when the first CB(1) agonists are being introduced for pain treatment, it comes out that an indirect cannabino-mimetic had been extensively used (and sometimes overused) for more than a century.

Alcohol exposure and paracetamol-induced hepatotoxicity

Most instances of hepatotoxicity due to paracetamol in the United Kingdom and Australia are the result of large overdoses of the drug taken with suicidal or parasuicidal intent. In contrast, serious hepatotoxicity at recommended or near-recommended doses for therapeutic purposes has been reported, mainly from the United States and in association with chronic alcohol use, leading to the widely held belief that chronic alcoholics are predisposed to paracetamol-related toxicity at relatively low doses. Yet the effects of alcohol on paracetamol metabolism are complex. Studies performed in both experimental animals and humans indicate that chronic alcohol use leads to a short-term, two- to threefold increase in hepatic content of cytochrome P4502E1, the major isoform responsible for the generation of the toxic metabolite from paracetamol, although increased oxidative metabolism of paracetamol at recommended doses has not been demonstrated clinically. A reduced hepatic content of glutathione, required to detoxify the reactive metabolite, has been documented in chronic alcoholics, due probably to associated fasting and malnutrition, providing a metabolic basis for any possible predisposition of this group to hepatotoxicity at relatively low paracetamol doses. Simultaneous alcohol and paracetamol ingestion reduces oxidative metabolism of paracetamol in both rodents and humans, predominantly as a consequence of depletion in cytosol of free NADPH. The possibilities that chronic alcohol use may predispose to paracetamol-related hepatotoxicity and that alcohol taken with paracetamol may protect against it, based on these metabolic observations, are examined in this review.

Treatment of advanced malignancies with high-dose acetaminophen and N-acetylcysteine rescue

High-dose acetaminophen (HDAC) produces hepatocellular necrosis and cytotoxic changes in other tissues that express mixed-function-oxidase (MFO) activity. N-acetylcysteine (NAC), administered within 8 hr of HDAC exposure, replenishes reduced glutathione and prevents these effects. Numerous cell culture and animal studies have demonstrated that NAC may differentially protect normal cells compared with malignant cells from the toxic effects of chemotherapeutic agents and radiation. It was therefore proposed that HDAC with NAC rescue may be effective in malignancies that express MFO activity. To test this hypothesis, a phase I trial of HDAC with NAC rescue was conducted on 19 patients with advanced cancer. HDAC was escalated from 6 to 20 g/m2 PO using a standard IV NAC rescue regimen. A total of 78 treatments were administered. Moderate fatigue, anorexia, and weight loss were the main toxicities observed. Transient grade 3 liver toxicity was noted following 1 treatment. Alopecia and renal and hematological toxicities were not observed. Responses after 4 courses administered weekly were as follows: response in at least 1 site-8 (partial 3, improved 3, mixed 2); stable disease-3; progressive disease-3; inevaluable-5. In conclusion, HDAC was tolerated with moderate fatigue, anorexia, and weight loss but few other effects using a standard IV NAC rescue regimen. A maximum tolerated dose was not reached at 20 g/m2. A 3/19 (15.8%) partial response rate was observed.

Acetaminophen and its toxicity

Acetaminophen (APAP) is considered one of the safest of all minor analgesics, but when taken in large doses (greater than 10 g) toxicity occurs. Severely poisoned patients experience hepatic and/or renal failure. The major metabolic pathway of APAP is formation of glucuronide and sulfate conjugates. A minor pathway is formation of a reactive metabolite that conjugates with glutathione (GSH). When GSH is depleted, the reactive metabolite causes necrosis of hepatic and other tissues. Treatment of APAP toxicity involves supplying alternate sulfhydryl donors or inhibiting oxidative formation of the reactive metabolite. Estimation of plasma APAP levels is necessary for effective treatment.