Inter-individual variations of the effect of low dose aspirin regime on platelet cyclooxygenase activity.

Thirteen healthy men (age range 24-59 years) received three single doses (30, 75, and 150 mg/day) of aspirin for seven days, followed by a wash-out period of three weeks, in a randomized order. The arachidonic acid metabolite 12-L-5,8,10-heptadecatrienoic acid (12-HHT) was taken as a measure of platelet cyclooxygenase activity. There was a large inter-individual variation in 12-HHT production prior to and during aspirin treatment. After one week of treatment the mean reduction was 69, 72 and 83% for the doses 30, 75 and 150 mg/day respectively. When the degree of cyclooxygenase inhibition was expressed per microgram aspirin administered per kg bw, a positive correlation was established to the activity before medication. It was found that doses exceeding 1500 micrograms per kg bw is required to achieve a predictable reduction in cyclooxygenase activity. Thus, by determining the pre-treatment cyclooxygenase activity in an individual it should be possible to adjust the enzyme activity to any desired level below 40% of its initial value. 150 mg aspirin/day for one week had a stimulating effect on the platelet basal production of 12-HHT when measured three weeks after the cessation of treatment. This rebound phenomenon was also observed up to six weeks after a single dose of 600 or 1200 mg of aspirin.