Background The procoagulant effect of anionic phospholipid may play a major role in the development of arterial thrombosis.

Methods and Results Annexin V, a calcium-dependent anionic-phospholipid–binding protein, was expressed and isolated from Escherichia coli and its antithrombotic effect examined in a rabbit carotid artery thrombosis model. A partially occlusive thrombus was formed in the left carotid artery by application of electric current to produce an ≈50% occlusion of the lumen. After the current was discontinued, flow ceased completely within 42±12 minutes (n=6) because of continuing platelet/fibrin thrombus formation. When annexin V was given at doses of 2.8 to 16.6 μg · kg⁻¹ · min⁻¹ for a period of 180 minutes, starting at the time the current was stopped, there was a dose-dependent inhibition of thrombus formation. At a dose of 5.6 μg · kg⁻¹ · min⁻¹, blood flow remained patent throughout the infusion and for an additional 60 minutes after the infusion was stopped. In addition, there was a decrease in thrombus weight (16±7.4 versus 2.0±1.0 g), ¹²⁵I-fibrin deposition (≈45% reduction, P<.001), and ¹¹¹In-labeled platelet accumulation (≈43% reduction, P<.001). Prior mixing of annexin V with phosphatidylserine micelles abolished the antithrombotic effect of annexin V, whereas mixing with phosphatidylcholine micelles had no effect. The antithrombotic effect of annexin V was not associated with bleeding tendency, as judged by the amount of blood absorbed in a gauze pad placed in a surgical incision extending to the muscle tissue and by the standard template bleeding time.

Conclusions These observations support a potentially important role for anionic phospholipid exposure in platelets in arterial thrombosis, and inhibition of this activity could be a novel target for therapy in coronary thrombosis and stroke and after angioplasty.