Paradoxical Thrombotic Effects of Aspirin: Experimental Study on 1000 Animals

Aspirin administration decreases the risk of vascular ischemic problems. However, aspirin withdrawal may temporarily increase this risk. Previous studies reported that high dilutions of aspirin might cause a pro-thrombotic effect. This paper studies the effect of the lower end of the aspirin dose-response curve, its possible mechanism and clinical implications. Protocol: Wistar rats were distributed into 100 groups of 10 rats each. Aspirin was injected at 100 mg/kg, 1 mg/kg and at several different aspirin dilutions along with cyclooxygenase (COX) 1 (SC-560), COX 2 (NS-398) or both selective inhibitors simultaneously using a laser-induced thrombosis model. Results: The higher doses of aspirin decreased thrombosis. An opposite trend was observed with the lowest doses. SC-560 produced an anti-thrombotic effect antagonized by the highest aspirin dilutions. NS-398 created a pro-thrombotic effect that was antagonized by aspirin at higher doses. Simultaneous inhibition of COX 1 and 2 produced changes similar to COX 1 inhibition. Conclusion: COX 2 inhibition induced a pro-thrombotic effect that was antagonized by aspirin at 1 mg/kg or 100 mg/kg. The administration of the lowest aspirin doses induced a pro-thrombotic effect stronger than the antithrombotic effect of COX 1 selective inhibition. The mechanism of this last pro-thrombotic effect is induced by residual aspirin and is independent of COX 1 inhibition. This study may explain the cause of the paradoxical thrombo-embolic complications observed after aspirin discontinuation, an effect of residual aspirin rather than a rebound effect, and highlights the importance of low doses of substances as a barely studied source of side-effects.