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Pharmacology: FC tablet: Paracetamol may act by inhibiting prostaglandin synthesis in the CNS and through a peripheral action by blocking pain-impulse generation. The peripheral action may also be due to inhibition of the synthesis of prostaglandin or to inhibit the synthesis or action of other substances, which sensitize pain receptors to mechanical or chemical stimulation. Paracetamol probably produces antipyresis by acting centrally on the hypothalamic heat-regulating center to produce peripheral vasodilation resulting in increased blood flow through the skin, sweating and heat loss. The central action probably involves inhibition of prostaglandin synthesis in the hypothalamus.
Pharmacodynamics: Infusion soln: The precise mechanism of the analgesic and antipyretic properties of paracetamol has yet to be established; it may involve central and peripheral actions.
IFIMOL IV provides onset of pain relief within 5 to 10 minutes after the start of administration. The peak analgesic effect is obtained in 1 hour and the duration of this effect is usually 4 to 6 hours.
IFIMOL IV reduces fever within 30 minutes after the start of administration with a duration of the antipyretic effect of at least 6 hours.
Pharmacokinetics: FC tablet: Paracetamol is rapidly and completely absorbed from the gastrointestinal tract.
Approximately 85% of a 1 g dose is recovered from the urine in 24 hours, about 3% is excreted unchanged, the balance being conjugated principally to the glucuronide or sulfate. Peak plasma concentrations of the free and conjugated drug are achieved ½ to 1 Hour after administration. The plasma half life of the unchanged drug is about 2 hours.
Infusion soln: Adults: Absorption: Paracetamol pharmacokinetics is linear up to 2 g after single administration and after repeated administration during 24 hours.
The bioavailability of paracetamol following infusion of 500 mg and 1 g of IFIMOL IV is similar to that observed following infusion of 1 g and 2 g propacetamol (corresponding to 500 mg and 1 g paracetamol respectively). The maximal plasma concentration (Cmax) of paracetamol observed at the end of 15-minutes intravenous infusion of 500 mg and 1 g of IFIMOL IV is about 15 μg/ml and 30 μg/ml respectively.
Distribution: The volume of distribution of paracetamol is approximately 1 L/kg.
Paracetamol is not extensively bound to plasma proteins.
Following infusion of 1 g paracetamol, significant concentrations of paracetamol (about 1.5 μg/ml) were observed in the Cerebro Spinal Fluid as and from the 20th minute following infusion.
Metabolism: Paracetamol is metabolised mainly in the liver following two major hepatic pathways: glucuronic acid conjugation and sulphuric acid conjugation. The latter route is rapidly saturable at doses that exceed the therapeutic doses. A small fraction (less than 4%) is metabolised by cytochrome P450 to a reactive intermediate (N·acetyl benzoquinone imine) which, under normal conditions of use, is rapidly detoxified by reduced glutathione and eliminated in the urine after conjugation with cysteine and mercapturic acid. However, during massive overdosing, the quantity of this toxic metabolite is increased.
Elimination: The metabolites of paracetamol are mainly excreted in the urine. 90% of the dose administered is excreted in 24 hours, mainly as glucuronide (60-80%) and sulphate (20-30%) conjugates. Less than 5% is eliminated unchanged. Plasma half-life is 2.7 hours and total body clearance is 18 L/h.
Neonates, infants and children: The pharmacokinetic parameters of paracetamol observed in infants and children are similar to those observed in adults, except for the plasma half-life that is slightly shorter (1.5 to 2 h) than in adults. In neonates, the plasma half-life Is longer than in infants i.e. around 3.5 hours. Neonates, infants and children up to 10 years excrete significantly less glucuronide and more sulphate conjugates than adults.
Table 1. Age related pharmacokinetic values (standardized clearance, *CLstd/Foral (l.h-1 70kg-1), are presented as follows. (See Table 1.)
Click on icon to see table/diagram/imageSpecial populations: Renal Insufficiency: In cases of severe renal impairment (creatinine clearance 10-30 mL/min), the elimination of paracetamol is slightly delayed, the elimination half-life ranging from 2 to 5.3 hours.
For the glucuronide and sulphate conjugates, the elimination rate is 3 times slower in subjects with severe renal impairment than in healthy subjects. Therefore, it is recommended, when giving paracetamol to patients with severe renal impairment (creatinine clearance ≤30 mL/min), to increase the minimum interval between each administration to 6 hours (see Dosage & Administration).
Elderly subjects: The pharmacokinetics and the metabolism of paracetamol are not modified in elderly subjects. No dose adjustment is required in this population.
