Aspirin Pharmacokinetics

Absorption of Aspirin

Aspirin is rapidly absorbed after oral administration, with some hydrolysis to salicylate before absorption. Absorption is delayed by the presence of food and is impaired in patients suffering from migraine attacks. Absorption is more rapid in patients with achlorhydria and also following administration of polysorbates and antacids.

Blood concentration

Single and multiple 100 mg doses of enteric-coated aspirin give systemic bioavailabilities of between 15% and 20% of that seen with immediate-release aspirin preparations. Cmax of aspirin for several enteric-coated preparations has been shown to be approximately 100 – 200 ng/ml with a half-life of approximately 1.7 hours. Plasma concentrations of salicylic add increase disproportionately with dose – a 325 mg dose having a half-life of 2-3 hours and higher doses showing lower plasma concentrations in the presence of an increased half-life due to a disproportionate increase in volume of distribution.

Distribution

Aspirin is found in the saliva, milk, plasma, and synovial fluid at concentrations less than blood and crosses the placenta.

Salicylate – extensive protein binding
Aspirin – protein binding to a small extent

Metabolism

In the blood, rapid hydrolysis to salicylic acid; glucuronic acid/ glycine conjugation to form glucuronides and salicyluronic acid؛ oxidation of a small proportion.

Excretion

Excreted in the urine mainly as salicyluronic acid. Salicylate is reabsorbed by renal tubules in acid urine, and alkaline diuresis will increase the rate of excretion; 85% of the dose is excreted as free salicylate.