Introduction:
Non-aspirin NSAIDs attenuate the antiplatelet action of aspirin leading to High on Aspirin Platelet Reactivity (HAPR). However, detailed features of this interaction are still unresolved. Both competitive as well as non-competitive interaction at the cyclooxygenase-1 (COX-1) enzyme have been proposed.
Hypothesis:
We examined whether competitive enzyme kinetics describe COX-1 inhibition by aspirin and HAPR and if they are compatible with results obtained experimentally.
Methods:
We have developed an enzyme kinetic model of the aspirin/NSAID interaction at COX-1 using mathematical modelling and have varied enzyme kinetic parameters to predict the occurrence and the extent of HAPR. Subsequently, the predictions obtained were experimentally verified with selected NSAIDs using light transmission aggregometry and immunoassay for platelet thromboxane (TX) synthesis.
Results:
Enzyme kinetic modelling predicted that only low- and medium-affinity NSAIDs (Ki ≥1 μM), but not high-affinity NSAIDs, interfere with COX-1 inhibition by aspirin. Furthermore, the interaction was predicted to be most extensive at high concentrations of arachidonic acid (1mM). Experimental results confirmed the model by showing that low or medium-affinity COX-1 inhibitors piroxicam, dipyrone and celecoxib (each n=5-10) largely prevented the inhibition of aggregation and platelet TX synthesis in vitro by 30 μmol/L aspirin whereas the high-affinity COX-1 inhibitor SC560 (n=6) did not interfere with aspirin at all. In accordance to the mathematical model, aspirin/NSAID interaction was more pronounced in presence of high (1mmol/L) versus low (0.3mmol/L) concentration of arachidonic acid. Hence, the kinetic model was validated experimentally.
Conclusion:
Both enzyme kinetic modelling and experimental results suggest competitive antagonism of NSAID and aspirin at COX-1 as a cause for HAPR, possibly resulting in cardiovascular events. Critical parameters that influence this interaction are drug concentrations and NSAID affinity. Interestingly, the concentration of arachidonic acid also influences the NSAID/aspirin interaction, suggesting that “fatty acid tone” and lipid metabolism may influence this unfavourable drug/drug interaction.
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