Effect of platelet antigen polymorphism on platelet inhibition by aspirin, clopidogrel, or their combination

OBJECTIVES We studied the modifier effect of platelet antigen polymorphism (Pl(A2)) on platelet inhibition by acetylsalicylic acid (ASA, i.e., aspirin), clopidogrel, or their combination in patients with coronary heart disease. BACKGROUND Clopidogrel, when administered with ASA, was shown to significantly improve the outcome of patients with acute coronary syndromes compared with patients receiving only ASA. We have shown previously that the effect of ASA on platelets is modified by the glycoprotein IIIa single nucleotide polymorphism Pl(A2). Hence, an important pharmacogenetic question remains whether the antiplatelet effect of clopidogrel is uniform for all patients or, like acetylsalicylic acid, more selective. METHODS Thirty Pl(A1/A1) and 30 Pl(A1/A2) patients were assigned randomly to ASA 325 mg/day, clopidogrel 75 mg/day, or both. After 10 days, platelet function was studied. RESULTS Clopidogrel provided stronger platelet inhibition than ASA with adenosine diphosphate as the agonist, and combination therapy resulted in greater inhibition than either inhibitor used alone (p < 0.0001). The use of ASA resulted in greater inhibition compared with clopidogrel with epinephrine (p < 0.0001) and collagen as agonists (p < 0.0001). With collagen as the agonist, platelets from Pl(A1/A2) donors were markedly and significantly less inhibited by ASA (p = 0.005). In contrast, with clopidogrel, no significant difference could be detected between inhibition of Pl(A1/A1) and Pl(A1/A2) platelets. CONCLUSIONS The combination of ASA and clopidogrel appears, superior to either agent alone in inhibiting platelet function. Pl(A2) functions as an important modifier for platelet responsiveness to ASA but not to clopidogrel. These findings could have significant impact on the future design of pharmacogenetic antithrombotic strategies for patients with coronary heart disease.