Cilostazol is an anti-platelet drug that reversibly inhibits phosphodiesterase III (PDE-III), which is ubiquitously expressed in platelets and various tissues. PDE-III converts cyclic adenosine monophosphate (cAMP) to 5'-AMP and up-regulates the intracellular concentration of cAMP, a potent inhibitor of platelet aggregation. Unlike other anti-platelet drugs, cilostazol is unique because patients receiving this drug do not have a significantly prolonged bleeding time, but the reasons for this difference are still unknown. In this study, we have examined how cilostazol inhibits platelet thrombus formation using anti-coagulated normal whole blood in which the platelets were labeled with a fluorescent dye in comparison with the anti-GPIIb/IIIa agent, tirofiban. We used an in vitro assay to examine mural platelet thrombus growth on a collagen surface under a high-shear rate flow in the absence of ADAMTS13 activity. These experimental conditions mimic the blood flow in patients with thrombotic thrombocytopenic purpura. Using this model, we clearly determined that cilostazol down-regulates the height of mural platelet thrombi formed on a collagen surface in a dose-dependent manner, without affecting the surface coverage. The concentration of cilostazol used in this study was relatively high (60-120 mu M) compared to clinically relevant concentrations (1-3 mu M), which may be due to the in vivo synergistic effects of PDE-III present in other tissues aside from platelets. Cilostazol does not affect the initial formation of platelet thrombi, but does inhibit the height of thrombi. These results showed a sharp contrast to tirofiban, and address why cilostazol does not significantly prolong bleeding time, despite its strong anti-platelet activity. (C) 2012 Elsevier B.V. All rights reserved.