Nucleoside transport inhibition and platelet aggregation in human blood: R75231 and its enantiomers, draflazine and R88016.

In this study, we determined whether R75231, (+/-)-2-(aminocarbonyl)-N-(4-amino-2,6-dichlorophenyl)-4-[5,5-bis( 4-fluoro- phenyl)pentyl]-1-piperazineacetamide, and its two enantiomers, all nucleoside transport inhibitors, could play a role as anti-aggregatory agents. First, we determined the binding characteristics of [3H]nitrobenzylthioinosine, also a nucleoside transport inhibitor, on intact human erythrocytes. The Kd value was 0.27 +/- 0.04 nM and the Bmax 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 ((+)-R75231), to displace [3H]nitrobenzylthioinosine. R75231 had an IC50 value of 2.2 +/- 0.3 nM. R88021 was twice as potent as R75231 and R88016 was 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 alone, 10 microM, had no effect on ADP-induced platelet aggregation. However, in the presence of 1 microM R75231, 10 microM of adenosine inhibited the aggregatory response completely. Dose-response curves indicated that the IC50 values of draflazine and R88016 were approximately 0.5 microM and 10 microM, respectively. R75231 and its enantiomers are valuable research tools to assess the role of the nucleoside transporter. Moreover, R75231 and draflazine (R88021) may prove to be useful as anti-aggregatory agents.