Specific inhibition of ADP-induced platelet aggregation by clopidogrel in vitro

1 The thienopyridine clopidogrel is a specific inhibitor of ADP-induced platelet aggregation ex vivo. No direct effects of clopidogrel (less than or equal to 100 mu M) on platelet aggregation in vitro have been described so far. 2 Possible in vitro antiaggregatory effects (turbidimetry) of clopidogrel were studied in human platelet-rich plasma and in washed platelets. 3 Incubation of platelet-rich plasma with clopidogrel (less than or equal to 100 mu M) for up to 8 h did not result in any inhibition of ADP (6 mu M)-induced platelet aggregation. 4 Incubation of washed platelets with clopidogrel resulted in a time- (maximum effects after 30 min) and concentration-dependent (IC50 1.9 +/- 0.3 mu M) inhibition of ADP (6 mu M)-induced platelet aggregation. Clopidogrel (30 mu M) did not inhibit collagen (2.5 mu g ml(-1))-, U46619 (1 mu M)- or thrombin (0.1 u ml(-1))-induced platelet aggregation. The inhibition of ADP-induced aggregation by clopidogrel (30 mu M) was insurmountable indicating a non-equilibrium antagonism of ADP actions. The R enantiomer SR 25989 C (30 mu M) was significantly less active than clopidogrel (30 mu M) in inhibiting platelet aggregation (32 +/- 5 % vs 70 +/- 1 % inhibition, P < 0.05, n = 5). 5 In washed platelets, clopidogrel (less than or equal to 30 mu M) did not significantly reverse the inhibition of prostaglandin E-1 (1 mu M)-induced platelet cyclic AMP formation by ADP (6 mu M). 6 The antiaggregatory effects of clopidogrel were unchanged when the compound was removed from the platelet suspension. However, platelet inhibition by clopidogrel was completely abolished when albumin (350 mg ml(-1)) was present in the test buffer. 7 It is concluded that clopidogrel specifically inhibits ADP-induced aggregation of washed platelets in vitro without hepatic bioactivation. Inhibition of ADP-induced platelet aggregation by clopidogrel in vitro occurs in the absence of measurable effects on the reversal of PGE(1)-stimulated cyclic AMP by ADP.