EXP3179 inhibits collagen-dependent platelet activation via glycoprotein receptor-VI independent of AT1-receptor antagonism - Potential impact on atherothrombosis

Objective - Thrombus formation after atherosclerotic plaque rupture critically involves the platelet collagen receptor glycoprotein ( GP) VI. We investigated the impact of EXP3179, an active metabolite of the angiotensin II type 1 (AT(1))-receptor antagonist Losartan (LOS) on GPVI-dependent platelet activation. Methods and Results - EXP3179 and LOS but not EXP3174 - the major AT(1)-receptor blocking metabolite of LOS dose-dependently inhibited collagen-I ( P < 0.01) and GPVI-dependent platelet aggregation ( P < 0.01) analyzed by optical aggregometry. Platelet activation was further determined by flow cytometry measuring the expression of platelet PAC-1, an epitope of the activated fibrinogen-receptor complex. EXP3179 and LOS inhibited collagen-I ( P < 0.01) and GPVI-dependent PAC-1 expression ( P < 0.01). EXP3179 and LOS but not EXP3174 decreased the adhesion of GPVI-receptor expressing Chinese hamster ovarian cells on collagen-I under arterial shear conditions determined by flow chamber analysis ( P < 0.01 and P < 0.05). EXP3179 also reduced human atherosclerotic plaque material-induced platelet aggregation ( P < 0.01) in vitro and murine platelet adhesion after acute vessel injury in vivo as determined by intravital microscopy ( P < 0.01). Conclusion - EXP3179 acts as a specific inhibitor of the platelet collagen receptor GPVI independent of AT(1)-receptor antagonism. Further investigations may clarify its individual potential as a novel pharmacological approach to specifically inhibit atherothrombotic events by GPVI-receptor blockade.