1. The role of endogenous ADP in platelet aggregation in vivo remains unclear due to the lack of suitable P2T-antagonist probes. This paper describes the potency, selectivity and specificity of the novel P 2T-purinoceptor antagonist, FPL 67085 (2-propylthio-D-beta,gamma-dichloromethylene ATP) both in vitro and in the anaesthetized rat in vivo. 2. FPL 67085 (3-30 nM) produced concentration-dependent rightward displacement of the concentration-effect (E/[A]) curve for ADP-induced aggregation of human washed platelets with no effect on ADP-independent aggregation at < or = 10 microM. 3. Logistic fitting of ADP E/[A] data indicated that the antagonist effect of FPL 67085 did not consistently accord with simple competition: in some preparations depression of the asymptote was seen. Schild analysis of data combined from all preparations, regardless of the antagonist profile observed, gave an apparent pKB of 8.9 (slope parameter 0.90). 4. The potency of FPL 67085 was unaffected by the P1-purinoceptor antagonist, 8-sulphophenyltheophylline, was similar (IC50 0.6-3.8 nM) in human and rat washed platelets or whole blood and, in rat blood, did not change following 2-30 min incubation at 37 degrees C. 5. FPL 67085 was a weak (pA50 approximately 4.2) partial agonist in tissues containing P2X- or P2Y-purinoceptors, indicating some 30,000 fold selectivity for the P2T-subtype. 6. In anaesthetized rats, intravenous infusion of FPL 67085 produced rapidly-reversible, dose-related inhibition of ADP-induced platelet aggregation measured ex vivo (ID50 1.3 micrograms kg-1 min-1) with no significant effect on haemodynamics or circulating cell counts. 7. Thus, FPL 67085 is a potent, specific and selective inhibitor of ADP-induced platelet aggregation both in vitro and in vivo. As such, it represents a novel pharmacological tool to define the role of endogenous ADP in thrombosis and the potential of P2T-purinoceptor antagonists as a novel class of infusible anti-thrombotic agents for acute use in man.
