The safety of clopidogrel.

IMPORTANCE OF THE FIELD: Clopidogrel is an antiplatelet prodrug widely used in acute coronary syndromes and after intravascular stent placement. Acute or chronic use can cause bleeding, a major adverse effect, which can lead to drug discontinuation or noncompliance with therapy. AREAS COVERED IN THIS REVIEW: The mechanism of action of clopidogrel, safety of different loading doses of clopidogrel, its use as a solitary antiplatelet agent and concomitant use with other antiplatelet agents such as aspirin and warfarin are discussed. Thrombotic thrombocytopenic purpura, a rare but important adverse event, has also been reviewed. Literature searches including randomized controlled trials were conducted in Medline. WHAT THE READER WILL GAIN: Clinicians will be able to understand the safety profile of using different doses of clopidogrel and the incidence of bleeding when used alone or in combination with other antiplatelet agents. TAKE HOME MESSAGE: Compared to aspirin, clopidogrel when taken alone causes less severe bleeding and less intracranial hemorrhage. Higher loading dose of clopidogrel is associated with increased bleeding. When combined with other antiplatelet agents, risk of bleeding increases, but like any other drug, the risks have to be weighed against potential benefits.

Platelet activation patterns in platelet size sub-populations: differential responses to aspirin in vitro.

Circulating platelets are heterogeneous in size and structure. Whether this translates into differences in platelet function and efficacy of antiplatelet therapy is unclear. Hence, we decided to investigate the activation patterns among different platelet populations differentiated by size, and to compare the inhibitory effects of aspirin in these populations. Circulating platelets from 9 healthy volunteers were separated by size and stratified into the largest and smallest quintiles. Platelets were stimulated with 75 µM arachidonic acid (AA), 10 µM ADP or 25 µM TRAP. Alpha-granule protein secretion and expression (P-selectin, VWF, fibrinogen), surface-protein activation (activated integrin αIIbß3) were assessed. Platelet thromboxane B(2) (TxB(2)) synthesis following AA stimulation was measured in vitro before and after incubation with 265 µM aspirin. Reticulated (juvenile) platelets were assessed using thiazole orange staining. A greater number of large platelets in the largest quintile were reticulated compared with the smallest quintile (6.1 ± 2.8% vs. 1.2 ± 1.5% respectively, p < 0.001). Larger platelets also synthesized more TxB(2) than small platelets both before (1348 ± 276 pg/mL vs. 1023 ± 214 pg/mL, respectively, p = 0.01) and after aspirin (1029 ± 190 pg/mL vs. 851 ± 159 pg/mL, respectively, p = 0.03). After stimulation with each agonist, a greater proportion of large platelets bound fibrinogen, VWF, P-selectin and activated integrin αIIbß3 than small platelets both in the presence and in the absence of in vitro aspirin. In an in vitro setting, large platelets appear to be more active than small platelets and continue to be more active even after in vitro aspirin. Platelets exhibit heterogeneity in size and structure. Whether this translates into platelet function and efficacy of antiplatelet therapy is unclear. We evaluated platelet functional properties and the effects of aspirin on separated platelet subpopulations in an in vitro setting. Platelets were sorted into the largest and smallest size quintiles using flow cytometry forward scatter. Alpha-granule protein release, dense granule content, surface protein activation and thromboxane synthesis were significantly greater in large platelets compared with small platelets, before and after stimulation with arachidonic acid, ADP and TRAP. Even after incubation with aspirin, large platelets continued to be more active than small platelets. In conclusion, large platelets are more active than small platelets and aspirin fails to eliminate these differential activation properties.