Bioavailability of different preparations of paracetamol

Detection of paracetamol binding to albumin in blood serum using 2D-IR spectroscopy

Two-Dimensional Infrared (2D-IR) spectroscopy is used to detect binding of paracetamol with proteins in blood serum. Quantitative peak patterns are observed indicating structural changes of the albumins' secondary structure when paracetamol bound.

Tolerance and pharmacokinetics of propacetamol, a paracetamol formulation for intravenous use

In 12 healthy volunteers, paracetamol pharmacokinetics were compared following administration of 1 g propacetamol HCl given intravenously over a 15-min period and 500 mg paracetamol given orally. Mean +/- SD total AUC (microgram/ml.h) following the iv formulation was significantly (P < 0.01) greater than following oral paracetamol (25.53 +/- 4.27 vs 21.04 +/- 4.49) corresponding to a mean oral bioavailability of paracetamol of 82.2 +/- 9.4%. Between 1 and 2 h after administration, paracetamol plasma concentrations became very similar following both formulations. In another study, 2 g propacetamol HCl was given both as a 15-min infusion and as a 2-min bolus injection to six healthy volunteers. Contrary to mild to moderate local discomfort experienced during the 2-min bolus injection, the 15-min infusion was well tolerated without any complaints reported.

The effect of metoclopramide and prochlorperazine on the absorption of effervescent paracetamol in migraine

Antiemetics modify gastric emptying, a rate-limiting step in drug absorption. The absorption of effervescent paracetamol in water solution was studied in three groups of 10 female patients during acute migraine attacks. Paracetamol was preceded 30 min earlier by a rectal dose of metoclopramide, prochlorperazine maleate, or placebo. Each patient was retested with paracetamol when headache-free. Migraine attacks delayed slightly the absorption of paracetamol solution. Prior administration of rectal prochlorperazine had a minor delaying effect on paracetamol absorption. The peak concentration, the time to reach the peak, and the area under the time-concentration curve from 0 to 6 h of paracetamol were similar with the three treatments.

Clinical pharmacokinetics of paracetamol

In therapeutic doses paracetamol is a safe analgesic, but in overdosage it can cause severe hepatic necrosis. Following oral administration it is rapidly absorbed from the gastrointestinal tract, its systemic bioavailability being dose-dependent and ranging from 70 to 90%. Its rate of oral absorption is predominantly dependent on the rate of gastric emptying, being delayed by food, propantheline, pethidine and diamorphine and enhanced by metoclopramide. Paracetamol is also well absorbed from the rectum. It distributes rapidly and evenly throughout most tissues and fluids and has a volume of distribution of approximately 0.9L/kg. 10 to 20% of the drug is bound to red blood cells. Paracetamol is extensively metabolised (predominantly in the liver), the major metabolites being the sulphate and glucuronide conjugates. A minor fraction of drug is converted to a highly reactive alkylating metabolite which is inactivated with reduced glutathione and excreted in the urine as cysteine and mercapturic acid conjugates. Large doses of paracetamol (overdoses) cause acute hepatic necrosis as a result of depletion of glutathione and of binding of the excess reactive metabolite to vital cell constituents. This damage can be prevented by the early administration of sulfhydryl compounds such as methionine and N-acetylcysteine. In healthy subjects 85 to 95% of a therapeutic dose is excreted in the urine within 24 hours with about 4, 55, 30, 4 and 4% appearing as unchanged paracetamol and its glucuronide, sulphate, mercapturic acid and cysteine conjugates, respectively. The plasma half-life in such subjects ranges from 1.9 to 2.5 hours and the total body clearance from 4.5 to 5.5 ml/kg/min. Age has little effect on the plasma half-life, which is shortened in patients taking anticonvulsants. The plasma half-life is usually normal in patients with mild chronic liver disease, but its prolonged in those with decompensated liver disease.