Clinical pharmacokinetics of paracetamol

The effect of acetaminophen administration on its disposition and body stores of sulphate

This investigation was designed to investigate the effects of ingestion of multiple therapeutic doses of acetaminophen on the disposition of the drug and on the cosubstrate, sulfate. Nine healthy volunteers and nine outpatients receiving acetaminophen for chronic pain were involved in the study. Volunteers were given a single 650 mg oral dose of acetaminophen. One week later they were given 650 mg of acetaminophen every six hours for five doses. Patients were maintained on their normal treatment and dosage schedules (600 mg every 3 to 8 h) for the study. In healthy volunteers the half-life of acetaminophen after single and multiple dosing was not significantly different. However, the fraction of acetaminophen recovered in the urine as the sulfate conjugate was less and the glucuronide conjugate greater after multiple dosing than after a single of the drug. There was no difference in the percentage recovered as the parent compound between single and multiple dosing. Serum sulfate levels fluctuated over the 6-h period following administration of single and multiple doses of acetaminophen to volunteers. The mean serum sulfate concentration was less after administration of five sequential 650 mg doses of acetaminophen than after a single dose. The renal clearance of inorganic sulfate showed a corresponding decrease. Unexpectedly, patients on chronic acetaminophen therapy exhibited elevated serum sulfate levels (levels higher than the maximum sulfate concentration seen in volunteers).

Paracetamol and phenacetin

Since their synthesis in the late 1800s paracetamol (acetaminophen) and phenacetin have followed divergent pathways with regard to their popularity as mild analgesic/antipyretic drugs. Initially, paracetamol was discarded in favour of phenacetin because the latter drug was supposedly less toxic. Today the opposite is true, and paracetamol, along with aspirin, has become one of the two most popular 'over-the-counter' non-narcotic analgesic agents. This marked increase in the wide approval attained by paracetamol has been accompanied by the virtual commercial demise of phenacetin because of its role, albeit somewhat circumstantial, in causing analgesic nephropathy. Both paracetamol and phenacetin are effective mild analgesics, suitable for treating mild to moderate pain, and their actions are broadly comparable with those of aspirin and related salicylates, although they do not appear to possess significant anti-inflammatory activity. Since a major portion of a dose of phenacetin is rapidly metabolised to paracetamol, it seems possible that phenacetin owes some of its therapeutic activity to its main metabolite, paracetamol, whereas its most troublesome side effect (methaemoglobinaemia) is due to another metabolite, p-phenetidine. The mechanism of action of paracetamol is poorly defined, although it has been speculated that it may selectively inhibit prostaglandin production in the central nervous system, which would account for its analgesic/antipyretic properties. The lack of any significant influence on peripheral cyclooxygenase would explain the absence of anti-inflammatory activity. At therapeutic doses paracetamol is well tolerated and produces fewer side effects than aspirin. The most frequently reported adverse effect associated with paracetamol is hepatotoxicity, which occurs after acute overdosage (usually doses greater than 10 to 15g are needed) and, very rarely, during long term treatment with doses at the higher levels of the therapeutic range. Paracetamol damages the liver through the formation of a highly reactive metabolite which is normally inactivated by conjugation with glutathione. Overdoses of paracetamol exhaust glutathione stores, thus allowing the accumulation of this toxic metabolite which covalently binds with vital cell elements and can result in liver necrosis. Glutathione precursors (notably intravenous N-acetylcysteine) have proved remarkably successful in treating paracetamol overdose, as long as treatment is initiated within 10 hours.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Human pharmacokinetics of analgesics and methods for their determination in biological fluids

The main pharmacokinetic data of analgesics--biological half-lives, apparent volumes of distribution, total body clearances--obtained in humans, and their clinical relevance are summarized. Special emphasis has been given to the analytical methods used for the quantitative determination of these drugs in biological fluids.

Plasma protein binding of dipyrone metabolites in man

Four metabolites of dipyrone, 4-methylaminoantipyrine (MAA), 4-aminoantipyrine (AA), 4-formylaminoantipyrine (FAA) and 4-acetylaminoantipyrine (AAA) can be identified in human plasma after its oral administration. The plasma protein binding of the metabolites in samples from 20 healthy volunteers was determined by ultrafiltration. None of the metabolites were found to be extensively bound to plasma proteins. The binding of MAA and AA was relatively higher than of FAA and AAA, as expected from their chemical structure. The mean percentage plasma protein binding was 57.6% for MAA, 47.9 for AA, 17.8 for FAA and 14.2% for AAA. The correlation between the unbound concentration in plasma and the total concentrations of MAA, AA, FAA and AAA was linear. No association was evident between the total protein plasma concentration and the extent of binding. The possible therapeutic implications related to protein binding of several analgesic and non-steroidal anti-inflammatory drugs are discussed.

Effect of two antacids on the bioavailability of paracetamol

The effect of two antacids on the bioavailability of paracetamol has been investigated in 12 young healthy volunteers. Following a random cross over design, each subject swallowed, on three separate occasions, one weak apart, 500 mg paracetamol alone, or together with two different aluminium hydroxide, magnesium hydroxide preparations (Dimalan and Maalox). Plasma paracetamol levels were measured by HPLC. The bioavailability of paracetamol was not altered by either antacid, but they both delayed the time to peak plasma concentration (0.85 h; 1.43 h; 1.25 h, without antacid, with Dimalan and with Maalox respectively). The peak plasma concentration was not affected by concurrent antacid administration.

Effect of migraine attacks on paracetamol absorption

The absorption of effervescent paracetamol (1000 mg) was investigated in nine female patients during a migraine attack and in the same patients when headache free. Migraine attack decreased (P less than 0.05) the areas under the serum paracetamol concentration-time curves (AUC) of 0-2 h, 0-4 h and 0-6 h and the peak serum concentration. The severity of nausea correlated significantly with the decrease in the AUC values. Our results support findings of delayed gastric emptying in migraine attacks. Both a delay and an impairment of drug absorption may follow.

Comparison of a traditional paracetamol medication and a new paracetamol/paracetamol-methionine ester combination

The SUR 2647 combination is a sachet formulation containing free paracetamol and its N-acetyl-methionate ester (SUR 2647). In a randomized, single-blind, between-patient study the onset of analgesia, duration and efficacy of the SUR 2647 combination vs paracetamol was investigated in out-patients after oral surgery. One group (n = 27) received sachets of SUR 2647 combination 2 b.i.d. (equivalent to 2 g paracetamol X 2) on the day of operation, and one sachet b.i.d. (equivalent to 1 g paracetamol X 2) for the following two days. The other group (n = 26) received paracetamol tablets 2 q.i.d. on the day of operation (1 g X 4) and one tablet q.i.d. (0.5 g X 4) for the following two days. Several objective and subjective assessments, including pain score on a visual analogue scale, were recorded for comparison of the postoperative courses. Median onset of analgesia for both groups was less than or equal to 0.5 h. The duration after SUR 2647 combination was greater than or equal to 5.5 h as compared to greater than or equal to 2.5 h for paracetamol. Mean pain scores showed that the SUR 2647 combination regime reduced pain significantly more than the paracetamol regime from 0.5 to 3.0 h after initiation of medication. The mean pain scores did not show a significant difference during the remaining observation period. Mild to moderate drowsiness was reported in both treatment groups, but it was more common in subjects given SUR 2647 combination.

Antipyretic analgesic drugs as models for studies of drug disposition in old age

Alterations in drug metabolizing capacity associated with the aging process can be elucidated using analgesic-antipyretic agents as model compounds. Antipyrine serves to profile drug oxidizing capacity. Drug oxidation is significantly impaired in old age, but age-related changes are far greater in men than in women. Acetaminophen analogously serves to profile conjugating capacity, which is minimally influenced by age both in men and in women. Changes in the capacity to biotransform these model compounds are reasonably well predictive of parallel changes in the same person's ability to metabolize other drugs transformed by the same pathway.

Drug prescribing in renal failure: dosing guidelines for adults

The data base for rational guidelines to safe, efficacious drug prescribing in adults with renal insufficiency are presented in tabular form. Current medical literature was extensively surveyed to provide as much specific information as possible. When information is lacking, however, recommendations are based on pharmacokinetic variables in normal subjects. Nephrotoxicity, important adverse effects, and special considerations in renal patients are noted. Adjustments are suggested for hemodialysis and peritoneal dialysis when appropriate.

Pharmacokinetic study of the fate of acetaminophen and its conjugates in rats

Pharmacokinetic studies of the fate of acetaminophen and its major metabolites, acetaminophen sulfate (AS) and acetaminophen glucuronide (AG), were made in rats. The rates of conjugate formation were calculated by deconvolution. The Michaelis-Menten equation gave maximum velocity and Michaelis constant (Km) values of 4.92 mumol/min/kg and 109 microM for AS formation, and 2.76 mumol/min/kg and 915 microM for AG formation. However, AG formation showed approximately first-order behavior in the present dose range because of its large Km value. The disposition of acetaminophen could be described by a two-compartment model with simultaneous first-order and Michaelis-Menten type elimination kinetics for AS formation. Curve fitting of the data based on this model was successfully done for doses of up to 1058 mumol/kg, suggesting that sulfation proceeds without depletion of sulfate in the blood at least up to this dose. The disposition of AS could be described by a two-compartment model and was apparently dose-independent over an 8-fold dose range. Although a slight dose dependence in the elimination of AG was suggested over a 16-fold dose range, for the purpose of the present study, it was assumed that the disposition of AG is approximately linear. The excretion of AS in the bile was negligibly small, whereas a considerable amount of AG was excreted into the bile. The results following intraduodenal injection of AS or AG indicated that AS or AG was hydrolyzed by the microflora and the liberated acetaminophen was reabsorbed, confirming enterohepatic circulation of the conjugates. This was consistent with the urinary metabolite excretion patterns observed after acetaminophen injection in normal and bile fistula rats. Based on the kinetic parameters obtained, the plasma concentrations of AS and AG after acetaminophen injection were simulated, and a fairly good agreement was obtained between calculated and observed values at the dose of 264.6 mumol/kg. Although the urinary metabolite excretion pattern differs from that of humans, the kinetic parameters obtained for rats were similar to those for humans in some respects, suggesting that the rat might be useful as a model animal to predict human data.

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.

Close correlation of acetaminophen clearance with that of conjugated benzodiazepines but not oxidized benzodiazepines

The clearance of the antipyretic-analgesic drug acetaminophen, biotransformed in humans by glucuronide and sulfate conjugation, was evaluated in 32 healthy young and elderly volunteers. Subjects received a single 650-mg dose of acetaminophen, and multiple plasma concentrations measured over the next 12 h. Random subgroups of subjects also participated in studies of the oxidized benzodiazepines diazepam, desmethyldiazepam and alprazolam, and of the conjugated benzodiazepines lorazepam, oxazepam and temazepam. Acetaminophen clearance was not related to that of the oxidized benzodiazepines, but was highly correlated with clearance of lorazepam (r = 0.70, n = 11, p less than 0.02), oxazepam (r = 0.76, n = 14, p less than 0.005) and temazepam (r = 0.63, n = 16, p less than 0.01). Thus acetaminophen may serve as a probe or marker compound to evaluate drug conjugating capacity in humans.