NUCLEOSIDE TRANSPORT INHIBITION AND PLATELET-AGGREGATION IN HUMAN BLOOD - R75231 AND ITS ENANTIOMERS, DRAFLAZINE AND R88016

In this study, we determined whether R75231, bis(4-fluoro-phenyl)penty l]-1-piperazineacetamide, and its two enantiomers, all nucleoside tran sport inhibitors, could play a role as anti-aggregatory agents. First, we determined the binding characteristics of [H-3]nitrobenzylthioinos ine, also a nucleoside transport inhibitor, on intact human erythrocyt es. The K-d value was 0.27 +/- 0.04 nM and the B-max was 23.5 +/- 5.1 pmol/10(9) erythrocytes. Second, we studied the ability of R75231 and its enantiomers R88021 ((-)-R75231, or draflazine) and R88016 ((+)-R75 231), to displace [H-3]nitrobenylthioninosine. R75231 had an IC50 valu e of 2.2 +/- 0.3 nM. R88021 was twice as potent as R75231 and R88016 w as approximately 20-fold less potent than R75231. Finally, the ability of these nucleoside transport inhibitors to enhance anti-aggregatory effects of adenosine was examined in whole human blood. Adenosine alon e, 10 mu M, had no effect on ADP-induced platelet aggregation. However , in the presence of 1 mu M R75231, 10 mu M of adenosine inhibited the aggregatory response completely. Dose-response curves indicated that the IC50 values of draflazine and R88016 were approximately 0.5 mu M a nd 10 mu M, respectively. R75231 and its enantiomers are valuable rese arch tools to assess the role of the nucleoside transporter. Moreover, R75231 and draflazine (R88021) may prove to be useful as anti-aggrega tory agents.