Lack of Bioequivalence Among Low-dose, Enteric-coated Aspirin Preparations

Low-dose aspirin (75 mg or 81 mg) is considered to be the lowest effective dose for cardiovascular protection; however, the use of enteric preparations has created a source of variability in bioavailability. As part of regulatory requirements, we carried out bioequivalence tests for two 75 mg enteric-coated aspirin preparations (Caprin and Protek) using Nu-Seals 75 mg aspirin as the comparator. The primary endpoint was serum thromboxane levels after 14 days of treatment. Protek failed to meet bioequivalence, as it was significantly less effective than Nu-Seals. In contrast, Caprin was not bioequivalent with Nu-Seals but as it was more effective it was granted approval. However, 75 mg plain aspirin was found to be more effective than Nu-Seals at inhibiting serum thromboxane production. Thus, there is significant variation in the ability of low-dose aspirin preparations to inhibit serum thromboxane production.