Abnormal drug metabolism after barbiturate and paracetamol overdose

Antidote removal during haemodialysis for massive acetaminophen overdose

CONTEXT

Haemodialysis is sometimes used for patients with massive acetaminophen overdose when signs of "mitochondrial paralysis" (lactic acidosis, altered mental status, hypothermia and hyperglycaemia) are present. The role of haemodialysis is debated, in part because the evidence base is weak and the endogenous clearance of acetaminophen is high. There is also concern because the antidote acetylcysteine is also dialyzable. We prospectively measured serum acetylcysteine concentrations during haemodialysis in three such cases.

CASE DETAILS

Three adults each presented comatose and acidemic 10 to ~18 h after ingesting > 1000mg/kg of acetaminophen. Two were hypothermic and hyperglycaemic. Serum lactate concentrations ranged from 7 mM to 12.5 mM. All three were intubated, and initial acetaminophen concentrations were as high as 5980 μM (900 μg/mL). An intravenous loading dose of 150 mg/kg acetylcysteine was initiated between 10.8 and ~18 h post ingestion, and additional doses were empirically administered during haemodialysis to compensate for possible antidote removal. A single run of 3-4 h of haemodialysis removed 10-20 g of acetaminophen (48-80% of remaining body burden), reduced serum acetaminophen concentrations by 56-84% (total clearance 3.4-7.8 mL/kg/min), accelerated native acetaminophen clearance (mean elimination half-life 580 min pre-dialysis, 120 min during and 340 min post-dialysis) and corrected acidemia. Extraction ratios of acetylcysteine across the dialysis circuit ranged from 73% to 87% (dialysance 3.0 to 5.3 mL/kg/min). All three patients recovered fully, and none developed coagulopathy or other signs of liver failure.

DISCUSSION

When massive acetaminophen ingestion is accompanied by coma and lactic acidosis, emergency haemodialysis can result in rapid biochemical improvement. As expected, haemodialysis more than doubles the clearance of both acetaminophen and acetylcysteine. Because acetylcysteine dosing is largely empirical, we recommend doubling the dose during haemodialysis, with an additional half-load when dialysis exceeds 6 h.

Gastric emptying in rats with acetaminophen-induced hepatitis

The objective of this work was to study the gastric emptying (GE) of liquids in fasted and sucrose-fed rats with toxic hepatitis induced by acetaminophen. The GE of three test meals (saline, glucose and mayonnaise) was evaluated in Wistar rats. For each meal, the animals were divided into two groups (N = 24 each). Group I was fed a sucrose diet throughout the experiment (66 h) while group II was fasted. Forty-two hours after the start of the experiment, each group was divided into two subgroups (N = 12 each). Subgroup A received a placebo and subgroup B was given acetaminophen (1 g/kg). Twenty-four hours later, the GE of the three test meals was assessed and blood samples were collected to measure the serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and acetaminophen. In group IB, the mean AST and ALT values were 515 and 263 IU/l, respectively, while for group IIB they were 4014 and 2472 IU/l, respectively. The mean serum acetaminophen levels were higher in group IIB (120 micrograms/ml) than in group IB (87 micrograms/ml). The gastric retention values were significantly higher in group IIB than in group IIA for all three test meals: saline, 51 vs 35%; glucose, 52 vs 38% and mayonnaise, 51 vs 29% (median values). The correlation between gastric retention and AST levels was significant (P < 0.05) for group IIB for the three test meals: r = 0.73, 0.67 and 0.68 for saline, glucose and mayonnaise, respectively. We conclude that GE is altered in rats with hepatic lesions induced by acetaminophen, and that these alterations may be related to the liver cell necrosis caused by the drug.

Time-dependent pharmacokinetics of high dose thiopental infusion in intensive care patients

PURPOSE

In patients with severe head injuries receiving long-term infusion for reducing intracranial pressure, a decline in concentrations was apparent following attainment of an initial steady state. This could be explained by an increased rate of elimination. An adequate modeling of the plasma disposition curves was used to demonstrate clearly the metabolic induction.

METHODS

The concentration-time data of 17 patients were fit by a one compartment pharmacokinetic model in which the decline of plasma concentration during infusion was due to an increase in the clearance rate of thiopental following a latency period. This time-dependent clearance model provided estimates of initial and final clearance rates.

RESULTS

This study demonstrated that large interindividual variations were observed during the course of the thiopental time-dependent pharmacokinetics. Depending on the patient, one or two steps of induction occurred. The mean initial and final clearance rates were 1.22 +/- 0.82 mL/min/kg and 10.5 +/- 23 mL/min/kg. The latency period for the first induction averaged 69 +/- 56 h. For 6 subjects, the rate of thiopental metabolism continued to change with time and there was a second step of induction.

CONCLUSIONS

Induction of thiopental metabolism occur within therapeutic ranges, but it was not established that attainment of individual limits in dosing rate, total dose, or treatment duration occur in the process. Thus, monitoring is needed for achievement of a target plasma concentration.

Paracetamol (acetaminophen) poisoning

Paracetamol poisoning caused by intentional overdose remains a common cause of morbidity. In this article the mechanism of toxicity and the clinical effects and treatment of poisoning, including specific antidotal therapy, are reviewed. Areas for further research directed at reducing morbidity and mortality from paracetamol poisoning are considered.

Disposition of antipyrine in patients with extensive metastatic liver disease

In the present study the effect of metastatic liver disease on hepatic drug metabolism has been examined by studying the pharmacokinetics of antipyrine and the urinary excretion of antipyrine and its three major metabolites (4-hydroxyantipyrine, norantipyrine, and 3-hydroxymethylantipyrine) in 12 patients with extensive metastatic liver disease, and in 12 matched healthy controls. In the patients total liver volume, the volume of the liver parenchyma, and the volume of the liver metastases were determined by computed tomography. The volume of liver metastases always exceeded 35% of the total liver volume. There were no significant differences between the patients and controls in plasma half-life, plasma clearance, or apparent volume of distribution of antipyrine. The cumulative urinary excretion of antipyrine and its three major metabolites was significantly lower in patients [44 (18) %] than in controls [71 (8) %]. The excretion of antipyrine itself was unchanged and the decrease in cumulative excretion was due to reduced excretion of the three metabolites. The results show that the activity of the hepatic mixed function oxidases was not impaired even in patients with extensive metastatic liver disease. This may be because liver metastases do not cause a corresponding reduction in the volume of normal hepatic parenchyma. The decreased urinary excretion of the three major metabolites of antipyrine, which are mainly glucuronidated, may have been due to an alteration in the process of conjugation.

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)

Time course of phenobarbital and cimetidine mediated changes in hepatic drug metabolism

Four healthy subjects were investigated weekly for 14 weeks by the antipyrine one sample saliva test, the 48-h urinary excretion of major antipyrine metabolites and the 2-h aminopyrine breath test before, during and after stimulation and inhibition of drug metabolism with phenobarbital and cimetidine, respectively. The phenobarbital-induced enhancement of antipyrine clearance (1.33-2.03 times) and of the aminopyrine breath test (0.94-1.19 times) occurred one week after beginning drug administration and persisted for 10 days after its cessation. The cimetidine-related inhibition of antipyrine clearance (0.62-0.85 times) and of the aminopyrine breath test (0.52-0.93 times) was observed 24 h after beginning cimetidine administration and subsided within two days after the last dose. During enhancement and inhibition the clearance of antipyrine to 3-hydroxymethyl-, 4-hydroxy- and norantipyrine varied as the total antipyrine clearance. The intraindividual variation in antipyrine clearance was 6-8%, and the corresponding variation in urinary excretion of antipyrine metabolites was 10-20%. It is concluded that the influence of phenobarbital and cimetidine on hepatic microsomal enzyme activity can be monitored simply by measurement of the blood concentration of the drug. Whether this simple relationship applies to other microsomally mediated drug interactions requires further evaluation.

Massive intoxication with acetaminophen and propoxyphene: unexpected survival and unusual pharmacokinetics of acetaminophen

A 28-year-old woman ingested an estimated 58 g acetaminophen and 9 g propoxyphene 20 h before hospitalization. Her serum acetaminophen concentration at 22 h was 485 micrograms/mL and declined with an unusually long half-life of 14 h. Hemodialysis for 4 h (started at 36 h) reduced the acetaminophen concentration from 250 to 32 micrograms/mL. The patient's complete recovery was remarkable because of the large amounts of drugs ingested, the delayed treatment, and prior exposure to enzyme inducers (known to increase acetaminophen hepatotoxicity). Administration of N-acetylcysteine prevented inorganic sulfate depletion usually caused by acetaminophen and may have increased the formation of acetaminophen sulfate. Some patients eliminate large overdoses of acetaminophen very slowly. Measures to enhance the elimination of this drug and its toxic metabolite by these individuals may be useful even when diagnosis or hospitalization is delayed.

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.

Acetominophen toxicity. Report of two cases

Both chronic low-dose use within accepted therapeutic limits and acute overdosage of acetominophen with as little as 7 Gm or 150 mg./kg. can result in irreversible centrilobular hepatic necrosis. Prompt and proper treatment with N-acetyl-cysteine in strict adherence to accepted protocol will prevent potentially fatal results. Clinicians who prescribe acetominophen must likewise be cognizant of other potential side effects, such as methemoglobinemia, hemolysis, and prothrombin suppression.

Studies on the mechanism of paracetamol-induced protection against paracetamol hepatotoxicity

In rats, 3 days treatment with paracetamol (1 oral dose of 1 g/kg daily) produced a complete protection against the hepatotoxic actions of a further dose of paracetamol as documented by determination of serum enzyme activities (glutamic-oxaloacetic transaminase, (GOT), glutamic-pyruvic transaminase (GPT), sorbitol dehydrogenase (SDH), bromsulphthalein retention and histological investigations. Subacute paracetamol treatment decreased liver glutathione levels by 46%, liver microsomal cytochrome P-450 content by 23%, hepatic hydroxylation of aniline by 29% and hepatic demethylation of aminopyrine by 46%. It afforded also some protection against the hepatotoxic actions of carbon tetrachloride, bromobenzene and thioacetamide, but did not influence the antiphlogistic activity of paracetamol (carrageenan paw edema test). Plasma and liver concentrations of free paracetamol after oral administration of 1 g/kg paracetamol were somewhat higher in the subacutely paracetamol-pretreated rats than in the non-pretreated control animals whereas no differences in the concentrations of conjugated paracetamol were found between the 2 groups. Pretreatment with paracetamol did not influence the urinary excretion of free paracetamol but caused some shift in the urinary excretion of paracetamol conjugates: pretreated rats excreted 23% less of the paracetamol glucuronide and sulfate and 33% more of the paracetamol mercapturate than the control animals. A depression of the microsomal mixed-function oxidase activity is presumed to be the main cause of the paracetamol-induced protection against paracetamol hepatotoxicity.

Enzyme induction following a single dose of amobarbital in dogs

The elimination of amobarbital in dogs was investigated by injecting various doses of amobarbital into a given animal. At low doses (3 mg/kg) serum levels declined in a first-order fashion. Superficially, at high doses (20 mg/kg) the relationship between serum concentration and time could be quantitatively characterized by simple one-compartment saturable kinetics. Indeed, qualitatively, saturation of the amobarbital-metabolizing enzymes was indicated by a shallower initial slope of the semilogarithmic concentration--time profile at the high than at the low dose. However, in addition, an acute enzyme induction phenomenon was observed which was indicated by a shorter terminal half-life of amobarbital at the high dose than after the low dose and also by a shortening in antipyrine half-life.

A comparative study of antipyrine pharmacokinetics in saliva and plasma using a colourimetric method of antipyrine analysis

1. Half-life, apparent volume of distribution and metabolic clearance rate for antipyrine elimination were reliably estimated from either plasma or saliva samples. 2. Pharmacokinetic analysis of antipyrine from saliva utilizing a simple and sensitive colourimetric technique provided a convenient method for assessing the activity of hepatic microsomal drug-metabolizing enzymes.

Interactions of CNS drugs--hypnotics and sedatives

The physicochemical properties of the barbiturates and a brief review of microsomal enzyme induction introduces this literature review of the interactions of sedative and hypnotic drugs. Food has been shown to delay absorption of barbiturates; barbiturates, in turn, may interfere with the absorption of griseofulvin, dicumarol, and folic acid. Barbiturate-produced enzyme induction may result in interactions with the oral anticoagulants, the anticonvulsants, vitamin D, bilirubin, digitoxin, doxycycline, and perhaps other drugs. The problem of additive central nervous system depression and the relative lack of documented evidence is considered. Finally, the interactions of nonbarbiturate hypnotics, such as glutethimide, chloral hydrate, and others are reviewed.

Serum enzyme activities and hepatic triglyceride levels in acute and subacute acetaminophen-treated rats

The dose- and time-related hepatotoxic effects of acetaminophen were investigated in rats using biochemical parameters as indices of hepatotoxicity supplemented by the histopathological examination of the livers. The acute or subacute (twice daily for 7 days) administration of 0.25 g/kg acetaminophen did not produce any noticeable hepatocellular damage. On the other hand, dose-dependent elevations in serum enzyme glutamic-oxaloacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT) and sorbitol dehydrogenase (SDH) activities and hepatic triglyceride (TG) levels were observed following the administration of single doses of 0.5 and lg/kg acetaminophen. Maximal hepatic damage occurred 12-18 h after acute dosing, while the hepatic function returned to control levels by 48-72 h. In contrast with the acutely treated rats, the serum enzyme activities and the hepatic TG levels remained unchanged following 7-day treatment with 0.5 or 1 g/kg acetaminophen. Also, histopathologically the degree of acetaminophen-induced hepatic necrosis was found to be far less extensive in rats given 0.5 and 1 g/kg acetaminophen twice daily for up to one week, as compared with the animals sacrificed 18 h after administering single equivalent doses of this drug. The results suggest that the liver function is reversibly impaired following acetaminophen overdosage, and that the intensity of acetaminophen-induced hepatotoxicity becomes less severe after repeated exposure to this hepatotoxin.

Simple analgesics

A number of drugs are available that act fairly specifically as "mild" analgesics, although this description by no means implies that their clinical effectiveness is limited to the relief of slight pain and trivial disability. They are effective by mouth and their action is mediated peripherally. Among the possible mechanisms of action, the inhibition of prostaglandin synthesis is currently regarded as most likely to be relevant. Some centrally acting drugs of the narcotic analgesic type, such as codeine and dextropropoxyphene are effective orally; they are usable in the same way as other mild analgesics and may be preferable for some types of pain. Many problems arise in the assessment and comparison of mild analgesics, both experimentally and clinically. Subjective assessments may be made on a pain scale by the patient himself, or by a trained observer. Individual variations are all-important, and the limitations of controlled trials need to be remembered. Alternative drugs and mixtures have little advantage over aspirin, but specific drug tolerance, in the long term, varies from patient to patient. Gastric irritation is most likely to occur with aspirin in the presence of chronic dyspepsia or acute precipitating causes such as alchoholic gastritis. Allergy also occurs in some susceptible individuals. The risk of renal damage with phenacetin is increasingly appreciated, and the possibility of hepatic damage from paracetamol is now recognised. Other side-effects and interactions are summarized in the review, and some notes are given on therapeutic and non-therapeutic use.