Phenobarbital induction does not potentiate hepatotoxicity but accelerates liver cell necrosis from acetaminophen overdose in the rat

Long-Term Anticonvulsant Therapy Worsens Outcome in Paracetamol-Induced Fulminant Hepatic Failure

1 Paracetamol hepatotoxicity has been found to be potentiated by anticonvulsant drugs in animal experiments; isolated case reports in humans sugest that long-term anticonvulsant therapy may also adversely influence outcome following overdose.

2 We compared the clinical course, after paracetamol overdose, of 18 patients on long-term anticonvulsant therapy with corresponding features in two published series of paracetamol-induced fulminant hepatic failure from this unit: 297 patients seen between 1973 and 1985 and a further 99 between October 1986 and April 1988.

3 Mortality in those patients who were taking anticonvulsants, but who did not receive N-acetylcysteine, was higher than in either of these series (93.3% vs 64.6% and vs 57.9%, P< 0.025). Although not statistically significant, there were also trends towards more severe coma (grade 3 or 4: 93.3% vs 75.4%, 1986-88), acidosis (pH less than 7.30: 40% vs 22.6%, 1973-85) and coagulopathy (prothrombin time greater than 100 s: 53.3% vs 33.7%, 1973-85). In the small number of patients given N-acetylcysteine, mortality was similar to that in the 1986-88 series (1/3 vs 15/42).

4 We conclude that chronic use of anticonvulsants enhances clinical features of paracetamol toxicity and discuss possible mechanisms by which this could be mediated.

Liver hypertrophy: a review of adaptive (adverse and non-adverse) changes--conclusions from the 3rd International ESTP Expert Workshop

Preclinical toxicity studies have demonstrated that exposure of laboratory animals to liver enzyme inducers during preclinical safety assessment results in a signature of toxicological changes characterized by an increase in liver weight, hepatocellular hypertrophy, cell proliferation, and, frequently in long-term (life-time) studies, hepatocarcinogenesis. Recent advances over the last decade have revealed that for many xenobiotics, these changes may be induced through a common mechanism of action involving activation of the nuclear hormone receptors CAR, PXR, or PPARα. The generation of genetically engineered mice that express altered versions of these nuclear hormone receptors, together with other avenues of investigation, have now demonstrated that sensitivity to many of these effects is rodent-specific. These data are consistent with the available epidemiological and empirical human evidence and lend support to the scientific opinion that these changes have little relevance to man. The ESTP therefore convened an international panel of experts to debate the evidence in order to more clearly define for toxicologic pathologists what is considered adverse in the context of hepatocellular hypertrophy. The results of this workshop concluded that hepatomegaly as a consequence of hepatocellular hypertrophy without histologic or clinical pathology alterations indicative of liver toxicity was considered an adaptive and a non-adverse reaction. This conclusion should normally be reached by an integrative weight of evidence approach.

Does cytochrome P450 liver isoenzyme induction increase the risk of liver toxicity after paracetamol overdose?

Paracetamol (acetaminophen, N-acetyl-p-aminophenol, 4-hydroxyacetanilide) is the most common cause of acute liver failure in developed countries. There are a number of factors which potentially impact on the risk of an individual developing hepatotoxicity following an acute paracetamol overdose. These include the dose of paracetamol ingested, time to presentation, decreased liver glutathione, and induction of cytochrome P450 (CYP) isoenzymes responsible for the metabolism of paracetamol to its toxic metabolite N-acetyl-p-benzoquinoneimine (NAPQI). In this paper, we review the currently published literature to determine whether induction of relevant CYP isoenzymes is a risk factor for hepatotoxicity in patients with acute paracetamol overdose. Animal and human in vitro studies have shown that the CYP isoenzyme responsible for the majority of human biotransformation of paracetamol to NAPQI is CYP2E1 at both therapeutic and toxic doses of paracetamol. Current UK treatment guidelines suggest that patients who use a number of drugs therapeutically should be treated as “high-risk” after paracetamol overdose. However, based on our review of the available literature, it appears that the only drugs for which there is evidence of the potential for an increased risk of hepatotoxicity associated with paracetamol overdose are phenobarbital, primidone, isoniazid, and perhaps St John’s wort. There is no evidence that other drugs often quoted as increasing risk, such as carbamazepine, phenytoin, primidone, rifampicin, rifabutin, efavirenz, or nevirapine, should be considered risk factors for hepatotoxicity in patients presenting with acute paracetamol overdose.

A novel model of acetaminophen-induced acute hepatic failure in rabbits

BACKGROUND

Few reliable and reproducible animal models of acute hepatic failure exist or conform to the criteria proposed by Terblanche and Hickman (Dig. Dis. Sci. 36: 770, 1991). In this prospective randomized study we describe the selective induction of CYP450 enzymes, depletion of glutathione, and hepatotoxic insult using acetaminophen in the development and characterization of a novel rabbit model of acute hepatic failure.

MATERIALS AND METHODS

Male New Zealand white rabbits weighing 3-5 kg were used. After preliminary dose ranging experiments, two groups of New Zealand white (n = 8 in each group) rabbits had CYP450 induction with phenobarbitone (40 mg/kg ip for 5 days) or with 20-methylcholanthrene (80 mg/kg ip). The glutathione synthetase inhibitor buthionine sulfoxime (2 mmol/kg iv) was then administered prior to acetaminophen administration (500 mg/kg sc). Clinical observations were recorded and arterial blood was sampled over 72 h.

RESULTS

Grade I-III encephalopathy occurred at 5-12, 12-25, and 28-56 h, respectively, in animals pretreated with 20-methylcholanthrene, but not in the phenobarbitone pretreated group. Mortality was 75% in the 20-methylcholanthrene group compared to 0% in the phenobarbitone group. Blood lactate (P < 0.05), prothrombin time (P < 0.005), aspartate transaminase (P < 0.005), and creatinine (P < 0.05) were higher in the 20-methylcholanthrene group compared to the phenobarbitone group. Histological changes were marked in the 20-methylcholanthrene group with massive coagulative hepatic necrosis compared to minimal histological damage in the phenobarbitone group.

CONCLUSION

The induction with 20-methylcholanthrene, glutathione depletion with buthionine sulfoxime, and subcutaneous administration of acetaminophen have led to the development of an animal model that parallels clinical, biochemical, and histological features of human hepatic failure.

Management of paracetamol overdose: current controversies

Paracetamol (acetaminophen) is one of the most frequently used analgesics, and is the most commonly used substance in self-poisoning in the US and UK. Paracetamol toxicity is manifested primarily in the liver. Treatment with N-acetyl-cysteine (NAC), if started within 10 hours from ingestion, can prevent hepatic damage in most cases. Pharmacokinetic data relating plasma paracetamol concentration to time after ingestion have been used to generate a 'probable hepatoxicity line' to predict which cases of paracetamol overdose will result in hepatotoxicity and should be treated with NAC. However, later studies use a 25% lower line as their 'possible hepatotoxicity line'. Although adopting the original line may save considerable resources, further studies are needed to determine whether such an approach is safe. On the basis of the metabolism of paracetamol, several risk factors for paracetamol toxicity have been proposed. These risk factors include long term alcohol (ethanol) ingestion, fasting and treatment with drugs that induce the cytochrome P450 2E1 enzyme system. Although some studies have suggested that these risk factors may be associated with worse prognosis, the data are inconclusive. However, until further evidence is available, we suggest that the lower line should be used when risk factors are present. In Canada and the UK, the intravenous regimen for NAC is used almost exclusively; in the US, an oral regimen is used. Both regimens have been shown to be effective. There is no large scale study with direct comparison between these 2 therapeutic protocols and controversy still exists as to which regimen is superior. During the last few years there has been an increase in the number of reports of liver failure associated with prolonged paracetamol administration for therapeutic reasons. The true incidence of this phenomenon is not known. We suggest testing liver enzyme levels if a child has received more than 75 mg/kg/day of paracetamol for more than 24 hours during febrile illness, and to treat with NAC when transaminase levels are elevated. Paracetamol overdose during pregnancy should be treated with either oral or intravenous NAC according to the regular protocols in order to prevent maternal, and potentially fetal, toxicity. Unless severe maternal toxicity develops, paracetamol overdose does not appear to increase the risk for adverse pregnancy outcome.

Chronic acetaminophen overdosing in children: risk assessment and management

Acetaminophen is currently the pediatric analgesic and antipyretic of choice. Although children appear to tolerate single, high-dose ingestions well, the literature is replete with reports of significant morbidity and mortality after repeated supra-therapeutic dosing. Proposed risk factors for injury with chronic use include age, total dose, duration, presence of intercurrent febrile illness, starvation, co-administration of cytochrome P450-inducing drugs, underlying hepatic disease, and unique genetic makeup. Evaluation of these children should include serum acetaminophen concentration, prothrombin time, and serum bilirubin and transaminase concentrations. The Rumack-Mathew nomogram should not be used to estimate the risk of hepatotoxicity in cases of chronic ingestion. Based on history, clinical examination, and laboratory findings, patients may be placed in three categories: those without hepatic injury and with no residual acetaminophen to be metabolized, those without injury but with some acetaminophen to be metabolized, and those with hepatotoxicity. Those without injury and no residual acetaminophen need not be treated or followed. Patients with hepatotoxicity or potential for hepatotoxicity based on residual acetaminophen should be treated with N-acetylcysteine. Most importantly, because so many parents are unaware of the potential risk of inappropriate dosing, education is the key to preventing future cases.

Paracetamol metabolism in patients with ulcerative colitis

AIMS

The capacity for sulphation of phenols appears to be impaired in the colonic mucosa of patients with ulcerative colitis. The aim of the present study was to investigate the systemic capacity for sulphation of phenols in patients with ulcerative colitis assessed by the metabolic clearances of paracetamol to the sulphate, glucuronide and glutathione derived metabolites.

METHODS

Ten patients with ulcerative colitis and 10 control subjects received a single oral dose of paracetamol (1 g). Venous blood samples were collected frequently for pharmacokinetic determinations (one compartment model). Urine was collected for 24 h. Plasma samples were analysed for parent drug and urine samples for parent drug and metabolites by h.p.l.c. Partial metabolic clearances were calculated as the fractional urinary recovery of each conjugate multiplied by the apparent oral clearance of paracetamol.

RESULTS

The apparent oral clearance of paracetamol and the partial clearances of its metabolites were not significantly different between the two study groups. Median value and the corresponding 25th and 75th percentiles for the clearance of the sulphate metabolites were 93.6 (82.5-138.8) ml kg(-1)h(-1) and 77.4 (75.5-99.1), patients with ulcerative colitis and control subjects, respectively.

CONCLUSIONS

These results do not indicate a general impairment of the systemic capacity for sulphation of paracetamol in patients with ulcerative colitis.

The influence of disulfiram on acetaminophen metabolism in man

1. Acetaminophen clearance and its partial clearance to its major metabolites has been determined before and after 5 days treatment with the anti-alcohol abuse agent disulfiram (200 mg daily). The study was conducted in 10 subjects, five without liver disease and five with alcoholic cirrhosis of the liver. Acetaminophen was given i.v. at a dose of 500 mg. Plasma samples were obtained up to 8 h after injection and urine collected for 24 h. 2. Across all subjects acetaminophen plasma clearance was reduced from 0.249 +/- 0.061 to 0.217 +/- 0.066 l/min after disulfiram treatment (mean +/- SD, P less than 0.05). Thus no change in acetaminophen dosage would be required in patients treated with disulfiram. 3. The partial clearance of acetaminophen to its glucuronide, sulphate and glutathione derivatives (i.e. cysteine and N-acetyl cysteine) was not significantly changed by disulfiram treatment. Thus it seems unlikely that the previously observed protective effects of disulfiram against acetaminophen-induced hepatotoxicity in animals due to inhibition of metabolism will be seen in man.

CCl4 cirrhosis in rats: irreversible histological changes and differentiated functional impairment

Cirrhosis of the rat liver was induced by a 12 week individualized CCl4/phenobarbital treatment. After treatment, all surviving animals (81%) showed cirrhosis of the liver. The cirrhosis induced was irreversible when evaluated 24 weeks after cessation of treatment. Quantitative liver function measurements were reduced in a differentiated manner. Ranked according to the most pronounced changes they are: capacity of urea-N synthesis (CUNS), galactose elimination capacity (GEC) and antipyrine clearance (APC). Hepatic glutathione concentrations were only slightly decreased after the CCl4 treatment. It is possible to produce a high incidence of irreversible cirrhosis with differentiated functional impairment in the rat.

Pharmacokinetics and pharmacodynamics of oxazepam and metabolism of paracetamol in severe hypothyroidism

1. The effect of severe hypothyroidism on the pharmacokinetics and pharmacodynamics of oxazepam 15 mg given orally (n = 10) and the metabolism of paracetamol 750 mg given intravenously (n = 8) was investigated before and after treatment with levothyroxine. 2. The median total and unbound clearance of oxazepam increased significantly during the study period from 0.78 ml min-1 kg-1 (0.40-1.25) to 1.22 ml min-1 kg-1 (0.66-1.94) and from 9.3 ml min-1 kg-1 (5.2-14.2) to 15.9 ml min-1 kg-1 (7.8-21.8), respectively (P less than 0.01). 3. The elimination half-life of oxazepam was prolonged by hypothyroidism to a median (range) value of 9.3 h (5.4-21.9) compared with 7.5 h (4.8-10.5) in the euthyroid state (P less than 0.05). 4. Hypothyroidism did not affect the protein binding of oxazepam; median values of the free percentage being 8.2% as compared with 7.7% when euthyroid. 5. The median (range) clearance of paracetamol under hypothyroid conditions was 3.12 ml min-1 kg-1 (1.64-4.40) and 4.70 ml min-1 kg-1 (3.18-5.70) following replacement therapy (P less than 0.01). This increase was associated with a comparable increase in the partial clearance to the glucuronide metabolite: 1.86 ml min-1 kg-1 to 2.70 ml min-1 kg-1. 6. Hypothyroidism was associated with decreased performance in a finger tapping test that was exacerbated by oxazepam. When the patients were euthyroid oxazepam did not produce any effect.

Long-term administration of toxic doses of paracetamol (acetaminophen) to rats

The effect of dosing paracetamol, 4.25 g/kg BW, twice weekly for 18 weeks was assessed in female Wistar rats 24 h after the last dose. Hepatic function, estimated as the prothrombin index, was more depressed in rats given one paracetamol dose than in chronically treated rats. Cytochrome P-450 and protein concentrations in liver homogenate and microsomes were higher in chronically treated rats. Urinary excretion of paracetamol glucuronide and mercapturate was higher and paracetamol sulfate unchanged after the chronic treatment. Hepatic glutathione was identically depleted after one dose and chronic paracetamol treatment. Histological examination of livers from chronically treated animals showed varying degrees of centrilobular necrosis. We conclude that long-term treatment with paracetamol in toxic doses leads to partial maintenance of the well-known protective effect after a few toxic doses. Signs of chronic toxicity consisted of weight loss, progressing to death. We suggest this chronic toxicity to be due to methionine/cysteine deficiency since urinary excretion of sulfur-containing paracetamol metabolites closely corresponds to calculated dietary intake of sulfur-containing amino acids.

Disulfiram prevents acetaminophen hepatotoxicity in rats

Hepatic necrosis due to an oral acetaminophen overdose (4.25 g/kg b.wt.) was prevented by pretreatment with disulfiram 100 mg/kg, given for 3 weeks or as a single dose. Twenty-four hours after acetaminophen the impairment of hepatic function, measured as prothrombin index, and the depletion of hepatic glutathione were prevented. Hepatic cytochrome P-450 levels were unchanged but cytochrome P-450 mediated p-nitroanisole demethylation was reduced by disulfiram pretreatment. Disulfiram pretreatment reduced 24 hour urinary excretion of acetaminophen-mercapturate and- cysteine while excretion of -sulfate and -glucuronide was unchanged. After 72 hours acetaminophen induced hepatic necrosis were prevented. Identical observations were made in animals pretreated with disulfiram for 3 weeks. Five hours after acetaminophen overdose its irreversible binding to hepatic proteins was not changed. After 24 hours, however, it was increased in animals pretreated with a single disulfiram dose and unchanged in animals pretreated for 3 weeks. The protective mechanism of disulfiram after acetaminophen overdose is not mediated via a change in overall irreversible binding of acetaminophen to hepatic protein.

Therapeutic doses of codeine have no effect on acetaminophen clearance or metabolism

In nine healthy volunteers, the clearance and metabolism of acetaminophen 1000 mg i.v. was evaluated with and without two concomitant oral doses of codeine in order to investigate a possible interaction. Plasma acetaminophen was followed for 720 min and urine was collected for 24 h after each dose for determination of metabolites. When codeine was coadministered, the average total clearance of acetaminophen and its clearance by glucuronidation, sulphation and mercapturate formation were 0.58 to 1.12-times the control values. It is concluded that therapeutic doses of codeine do not influence the clearance or metabolism of acetaminophen.

Single dose oxazepam has no effect on acetaminophen clearance or metabolism

The metabolism of acetaminophen and oxazepam in humans is mainly dependent on the microsomal capacity for glucuronide conjugation. The clearance of acetaminophen and the formation of metabolites were evaluated in 7 patients before and during concomitant administration of oxazepam 30 mg. The subjects received a single 500 mg dose of acetaminophen i.v. and concentrations in plasma were measured for 360 minutes and in urine for 24 h in order to estimate the production of metabolites. The single therapeutic dose of oxazepam had no effect on the clearance of acetaminophen or on formation of its metabolites.

Acetaminophen metabolism by the perfused rat liver twelve hours after acetaminophen overdose

The effect of a toxic dose of acetaminophen on the hepatic conjugations of acetaminophen was studied in single pass perfused livers from rats given acetaminophen overdose 12 hr prior to perfusion and from control rats. Four different acetaminophen concentrations (0.1-6 mmol/1) were used in each perfusion. Glucuronidation of acetaminophen was increased and sulfation of acetaminophen occurred at an unchanged rate in acetaminophen damaged livers as compared to control livers. Hepatic glutathione concentrations declined to about 0.4 mumol/g liver during perfusion, possibly due to excretion of glutathione to perfusion medium, but in spite of this the formation of glutathione conjugates was increased with acetaminophen concentrations increasing up to about 5 mmol. We conclude that decreased sulfation, glucuronidation and glutathione conjugation in the liver is not present in the early development of acetaminophen-induced hepatic damage.