ADENOSINE-MEDIATED INHIBITION OF PLATELET-AGGREGATION BY ACADESINE - A NOVEL ANTITHROMBOTIC MECHANISM IN-VITRO AND IN-VIVO

Inhibition of platelet aggregation by acadesine was evaluated both in vitro and ex vivo in human whole blood using impedance aggregometry, a s well as in vivo in a canine model of platelet-dependent cyclic coron ary flow reductions. In vitro, incubation of acadesine in whole blood inhibited ADP-induced platelet aggregation by 50% at 240+/-60 mu M. In hibition of platelet aggregation was time dependent and was prevented by the adenosine kinase inhibitor, 5'-deoxy 5-iodotubercidin, which bl ocked conversion of acadesine to its 5'-monophosphate, ZMP, and by ade nosine deaminase. Acadesine elevated platelet cAMP in whole blood, whi ch was also prevented by adenosine deaminase. In contrast, acadesine h ad no effect on ADP-induced platelet aggregation or platelet cAMP leve ls in platelet-rich plasma, but inhibition of aggregation was restored when isolated erythrocytes were incubated with acadesine before recon stitution with platelet-rich plasma. Acadesine (100 mg/kg i.v.) admini stered to human subjects also inhibited platelet aggregation ex vivo i n whole blood. In the canine Felts model of platelet thrombosis, acade sine (0.5 mg/kg per min, i.v.) abolished coronary flow reductions, and this activity was prevented by pretreatment with the adenosine recept or antagonist, 8-sulphophenyltheophylline. These results demonstrate t hat acadesine exhibits antiplatelet activity in vitro, ex vivo, and in vivo through an adenosine-dependent mechanism. Moreover, the in vitro studies indicate that inhibition of platelet aggregation requires the presence of erythrocytes and metabolism of acadesine to acadesine mon ophosphate (ZMP).