The Effects of Two Synthetic Glycoprotein II b/III a Antagonists, Ro 43-8857 and L-700,462, on Platelet Aggregation and Bleeding in Guinea-Pigs and Dogs: Evidence that Ro 43-8857 Is Orally Active

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Abstract
In vitro platelet aggregation studies in whole blood were used to define the species-specificity profile of two synthetic GP-IIb/IIIa antagonists, Ro 43-8857 and L-700,462. Aggregation of rhesus monkey platelets was inhibited with a similar potency to human platelets, whereas both compounds were poor antagonists in mini-pig, rabbit or hamster blood. Compared to human platelets, Ro 43-8857 was 2-3-fold less active as an inhibitor of dog and guinea-pig platelet aggregation, whereas L-700,462 was, respectively, 4- and 14-fold less active in these species. In vivo investigations with these two compounds were performed in anesthetized guinea-pigs and conscious dogs, with bleeding times measured on small mesenteric arteries or on the inner jowl respectively. Ex vivo ADP-induced whole blood platelet aggregation was completely inhibited in guinea-pigs by Ro 43-8857 following intravenous administration of 0.1 mg/kg and intraduodenal administration of 3 mg/kg, with a duration of action exceeding 5 hours. Mesenteric bleeding times were prolonged by Ro 43-8857 only at doses causing supra-maximal inhibition of aggregation, suggesting these two effects could be partially dissociated. L-700,462 (3 mg/kg i. v.) was shorter acting than Ro 43-8857 in guinea-pigs (duration ~1 hour) and the antiaggregatory effect was accompanied by mesenteric bleeding time prolongations. In conscious dogs, ex vivo aggregation was inhibited to —80% by Ro 43-8857 (0.3 mg/kg i. v. or 10 mg/kg p. o.) and L-700,462 (1 mg/kg i.v.). However, bleeding time prolongations accompanied these anti-aggregatory effects with both compounds. In conclusion, we have shown clear differences between two synthetic GP-IIb/IIIa antagonists, both in terms of their species-specificity in vitro and in terms of their in vivo profile, and in particular the propensity to promote bleeding from mesenteric arteries in guinea-pigs. However, the ability of Ro 43-8857 to discriminate between anti-aggregatory and bleeding effects was not evident when the bleeding time measurements were performed on the dog jowl. This suggests that the species and/or vessels on which the bleeding time is performed, is also an important consideration when characterizing and comparing antiplatelet compounds, even with drugs acting via the same mechanism. These results are relevant for the future design of in vivo animal experiments to characterize this new class of compounds and in the interpretation of the data obtained to the clinical situation. The animal models described here are well suited for comparative studies of different GP-IIb/IIIa antagonists, providing information on in vivo potency, duration of action and effect-bioavailability following different routes of administration. Orally active GP-IIb/IIIa antagonists have not previously been described in the literature. The long duration of action and oral activity shown by Ro 43-8857 suggests a potential use of such compounds in arterial thrombotic disorders requiring chronic therapy.