Clopidogrel is an effective new
antiplatelet agent useful for the treatment of ischemic cerebrovascular, cardiac, and
peripheral arterial disease. However, the mechanism of
clopidogrel action is not well understood, although it is known to inhibit
ADP-evoked platelet aggregation. In the current study, the effect of
clopidogrel on recently identified human platelet
ADP receptors and their signaling pathways was investigated by using platelets from
clopidogrel-treated subjects, 6 healthy volunteers (2 females and 4 males) who received 75 mg of
clopidogrel daily for 7 days. Blood was taken and various platelet receptor signaling pathways were analyzed before treatment, after 7 days of medication, and 4 weeks
after treatment had ceased. Platelet tests included the analysis of aggregation, rapid
calcium influx,
calcium mobilization from intracellular stores,
adenylyl cyclase, and phosphorylation of
vasodilator-stimulated phosphoprotein (VASP). The data indicate that
clopidogrel does not affect those platelet
ADP receptors coupled to
cation influx (P2X1
ADP receptors) or
calcium mobilization (P2Y1
ADP receptors). In contrast,
clopidogrel treatment specifically impairs the
ADP receptor coupled to G(i)/
adenylyl cyclase (P2Y(AC)
ADP receptors).
Clopidogrel abolishes the inhibitory P2Y(AC) receptor-mediated
ADP effects on
prostaglandin E(1)-stimulated, cAMP-dependent phosphorylation of VASP without affecting
epinephrine,
thrombin, and
thromboxane signaling. VASP phosphorylation is known to be closely correlated with the inhibition of platelet and
fibrinogen receptor (
glycoprotein IIb/IIIa) activation. Therefore, inhibition of the platelet P2Y(AC)
ADP receptor and its intracellular signaling, including decreased VASP phosphorylation, is suggested as a molecular mechanism of
clopidogrel action.