Effects of oxaprozin alone or in combination with aspirin on hemostasis and plasma protein binding

Oxaprozin, a new nonsteroidal antiinflammatory agent, was studied alone and in combination with aspirin for its effects on hemostasis and protein binding in 10 healthy adults. When both oxaprozin and aspirin were given separately for seven days and in combination for five days, both drugs prolonged bleeding time and inhibited collagen- and epinephrine-induced platelet aggregation to a similar degree. The effects of the combination of oxaprozin and aspirin were not additive. The data from the protein binding study showed that oxaprozin was more than 99 per cent bound to plasma proteins. Aspirin displaced oxaprozin from its binding sites. As a result, the rate of plasma clearance of oxaprozin significantly increased from 20 to 26 ml/min/kg (P less than 0.05), and the plasma half-life decreased from 45 to 40 hours. Platelet count and the humoral clotting mechanism were not affected by either drug alone or in combination. There was no clinical evidence of bleeding. One subject who received oxaprozin for 12 days and, in addition, aspirin for the last five days developed a rash that subsided after both drugs were discontinued; one subject treated with aspirin experienced tinnitus. These data suggest that oxaprozin, like aspirin and other nonsteroidal antiinflammatory drugs, should be used with caution when administered to patients who have suffered trauma, who undergo surgery, or who have known defects in hemostasis.