(A) Platelet thrombus formation occurs in a stepwise process. In the first phase, circulating platelets adhere to activated endothelium or exposed subendothelial matrix proteins exposed as a consequence of vascular injury. Platelet adhesion is an activating event that results in conversion of the integrin αIIbβ3 from a resting to an active conformation, in which it is competent to bind plasma fibrinogen. In the second phase, circulating platelets bind to immobilized fibrinogen and are thereby recruited into the growing thrombus. In the third phase of the process, platelets recruited to the thrombus by binding to immobilized fibrinogen are activated by soluble agonists released by activated, adherent platelets that form the initial platelet monolayer. Of particular importance is the platelet dense (δ) granule constituent, ADP, which binds to two GPCRs, including the Gαq-coupled P2Y₁ receptor and the Gαi-coupled P2Y₁₂ receptor. GPCR-mediated activation of platelets bound to immobilized fibrinogen initiates the third or perpetuation phase of the thrombus formation process because it results in activation of αIIbβ3, binding of plasma fibrinogen, and recruitment of additional platelets into the growing thrombus. (B) Targeting of soluble CD39 to activated platelets interferes with the perpetuation phase, but not the initiation or accumulation phases, of thrombus formation. Targ-CD39 represents a fusion protein composed of soluble CD39 fused to a single-chain Fv fragment (scFv) of an antibody that is specific for the active conformation of the platelet-specific integrin αIIbβ3. Targeting of CD39 to activated αIIbβ3 ensures that hydrolysis of ADP is delayed until after the first layer of platelets has already been activated and allowed to recruit the second layer of platelets. Consequently, Targ-CD39 interferes only with ADP-mediated activation of recruited platelets, which affects only the perpetuation phase of thrombus formation.