Plasma acetylsalicylate and salicylate and platelet cyclooxygenase activity following plain and enteric-coated aspirin.

Compressed and enteric-coated acetylsalicylate (ASA) tablets have been compared in normal healthy subjects. Plasma ASA and salicylate (SA) were measured by high pressure liquid chromatography (HPLC). Platelet cyclooxygenase activity in vitro was studied by a radiometric technique. Following ingestion of 650 mg of ASA in the form of compressed tablets, cyclooxygenase activity was inhibited 95% within 45 min. Enzyme activity was observed to increase within 8 h and reached 10% of control level by 24 h. The pattern suggests that only circulating platelets are affected by ASA ingestion. Following the administration of 650 mg of ASA as enteric-coated tablets comparable inhibition of cyclooxygenase activity was observed, although the effect was delayed, reflecting the delayed appearance of ASA in the plasma. Return to control levels followed a pattern similar to that observed with the compressed tablet.