Fibrinogen receptor antagonists

Platelet glycoprotein IIb/IIIa antagonists. What are the relevant issues concerning their pharmacology and clinical use?

During the last decade, intensive efforts have been made to evaluate the role of the platelet glycoprotein (GP) IIb/IIIa complex in platelet-mediated thrombus formation. Significant efforts have also been made to design potent antagonists of this "final common pathway" of platelet aggregation to be used as novel therapeutic strategies to treat acute coronary syndromes. Although several different GP IIb/IIIa antagonists have convincingly demonstrated the usefulness of this platelet-directed therapeutic strategy, a number of lingering unsolved and sometimes misunderstood issues concerning the pharmacology and optimal clinical usefulness of these agents remain to be explored. This article reviews these issues, which include antagonist affinity, reversibility, and receptor specificity. Other issues are related to the effects of GP IIb/IIIa receptor availability, neoepitopes induced by antagonist binding with the potential to mediate thrombocytopenia, optimal methods of platelet monitoring and, perhaps ultimately, the potential therapeutic index of the oral class of GP IIb/IIIa antagonists. All of these specific issues are likely to be illuminated in the next several years, which will greatly determine the breadth of this therapeutic class.

Comparison of the antiplatelet effect of YM337 and abciximab in rhesus monkeys

We directly compared the effects of YM337, the Fab fragment of the humanized monoclonal antibody C4G1, on platelet aggregation and template bleeding time with those of abciximab, the Fab fragment of the human/murine chimeric monoclonal antibody 7E3, in rhesus monkeys. The duration of inhibition of platelet aggregation by abciximab after i.v. bolus injection was much longer than that by YM337. Although YM337 significantly prolonged template bleeding time at 5 min after i.v. bolus injection, this action recovered within 1 h after injection. In contrast, although abciximab also prolonged template bleeding time, the duration of this effect was sustained. In a dose-escalating continuous infusion study, we evaluated the relationship between inhibition of platelet aggregation and prolongation of template bleeding time. Platelet aggregation was inhibited by over 80% by both agents at 3 microg/kg per min, and template bleeding time was prolonged to about 30 min at 30 microg/kg per min for YM337 and 10 microg/kg per min for abciximab. Interestingly, plasma concentrations between inhibition of platelet aggregation and prolongation of template bleeding time did not overlap with YM337, but did overlap with abciximab. These results suggest that YM337 allows easier control of antiplatelet activity with less effect on bleeding time than abciximab, and has a wider therapeutic window than abciximab.

Orally active fibrinogen receptor antagonists. 2. Amidoximes as prodrugs of amidines

The potent and selective GP IIb-IIIa antagonist lamifiban (1, Ro 44-9883) is currently in clinical development as an injectable antithrombotic agent for treating and preventing acute coronary syndromes. However, for secondary prevention of thrombotic occlusions, orally active inhibitors are needed. By means of a prodrug strategy, the modest oral absorption of 1 in mice was improved by a factor of 9. In addition, these studies demonstrated that an amidoxime group can serve as a prodrug functionality for an amidino group. Application of this principle to the structurally related amidino carboxylate 13 led to the amidoxime ester 18 which was absorbed approximately 20 times better, after oral administration to mice, than 13. Due to the modification of the amidino group as well as of the carboxylate group, 18 completely lost its ability to interact with purified platelet GP IIb-IIIa. After oral administration of 18 to rats, dogs, and rhesus monkeys, the bioavailability of the active derivative 13 was 26 +/- 5, 25 +/- 6, and 33 +/- 6%, respectively, and the elimination half-life was 4.1 +/- 1.7, 11.4 +/- 1.1, and 5.1 +/- 1.4 h, respectively. On the basis of these properties, the orally active 18 (Ro 48-3657), a double prodrug of the potent and selective non-peptide GP IIb-IIIa antagonist 13 (Ro 44-3888), was selected as clinical candidate for evaluation as a prophylactic agent in patients at high risk for arterial thrombosis.