Effect of alternate-day regular and enteric-coated aspirin on platelet aggregation, bleeding time, and thromboxane A2 levels in bleeding-time blood.

The effectiveness of low-dose aspirin for primary prevention of cardiovascular mortality is being assessed among the nearly 22,000 United States physicians currently participating in the Physicians' Health Study. Because of occasional reports of gastric irritation among study participants, two enteric-coated aspirin preparations were tested as possible alternatives to regular compressed aspirin for platelet inhibition. Thirty-three volunteers were assigned randomly to one of four treatment groups: regular aspirin (325 mg), placebo, and two enteric-coated aspirin preparations (325 mg). Pills were administered every other day, duplicating the regimen used in the Physicians' Health Study. Bleeding times, platelet aggregation, and thromboxane A2 levels produced by aggregating platelets in vitro, as well as in collected bleeding-time blood, were determined. Measurements were taken before and after a single dose as well as after seven alternate-day doses. Regular and enteric-coated aspirin preparations were equally efficacious in prolonging the bleeding time, inhibiting platelet aggregation, and suppressing thromboxane A2 production. There was virtually complete suppression of thromboxane A2 production (over 99 percent), by platelets in vitro and in collected bleeding-time blood. The levels were still profoundly reduced (89 percent) 48 hours after the last dose. Enteric-coated aspirin may provide an alternative to regular aspirin in a low-dose regimen designed to inhibit platelet activity.