Prothrombotic effects of diclofenac on arteriolar platelet activation and thrombosis in vivo

Background: Diclofenac, like selective cyclooxygenase‐2 inhibitors, which alter vascular levels of platelet active prostaglandins, has been reported to increase rates of acute myocardial infarction. Objective: The study was performed to investigate, in an animal model of arterial thrombosis in vivo, whether diclofenac differentially influences platelet activation and thrombosis in vessels under non‐stimulated conditions or during acute systemic inflammation, such as induced by tumor necrosis factor‐α (TNF‐α). Methods: Platelet–vessel wall interaction (PVWI), firm platelet adhesion and arterial thrombosis following vessel injury were analyzed by intravital microscopy in arterioles of hamsters in the dorsal skinfold chamber model. Prostacyclin [prostaglandin I2 (PGI2)] and thromboxane A2 (TxA2) metabolites were measured. In vitro, endothelial adhesion molecule expression in cultured human microvascular endothelial cells was analyzed. Results: Under non‐stimulated conditions, diclofenac (1 mg kg−1) enhanced PVWI, which was not mediated by increased adhesion molecule expression, but by decreased systemic PGI2 levels. Following ferric chloride‐induced endothelial injury, diclofenac accelerated thrombotic vessel occlusion time, an effect that was reversed by the stable PGI2 analog iloprost. TNF‐α, through induction of endothelial adhesion molecule expression, also enhanced PVWI, firm adhesion, and arterial thrombosis, but simultaneous treatment with TNF‐α and diclofenac did not have an additive effect. Conclusions: By decreasing levels of PGI2 without, at the same time, altering prothrombotic TxA2 levels, diclofenac can exert prothrombotic effects. However, this is not the case when an inflammatory situation is created by TNF‐α treatment. These data may explain the enhanced risk of acute myocardial infarction observed in patients taking diclofenac.