UR-3216: a manageable oral GPIIb/IIIa antagonist

UR-3216, a prodrug, is a novel, selective, and orally active platelet surface glycoprotein (GPIIb/IIIa) receptor antagonist. The most important property of UR-3216 is the very tight binding of its active metabolite to platelets (Ki for resting platelets is < 1 nM). UR-2992, the active form of UR-3216, binds to platelets for a long period of time, while the unbound drug is rapidly cleared. Therefore, after an initial loading dose of 0.1 mg/kg, only once daily repeated low maintenance doses of UR-3216 (< 0.05 mg/kg p.o.) are required. This regimen maintains a high level of inhibition of platelet aggregation and, due to a small peak-to-trough ratio, severe bleeding is avoided. The therapy with UR-3216 is easy to manage, because it has low peak-to-trough ratio and high efficacy (> 80% inhibition of platelet aggregation). In addition, UR-3216 does not produce excessive bleeding or thrombocytopenia and does not interact with abciximab. UR-3216 is excreted mostly in bile, so that it will not accumulate in patients with chronic renal dysfunction. UR-2316 has the following abciximab-like features: (a) its half-lives for residence on platelets, inhibition of platelets aggregation and bleeding time prolongation are 60 to 80 h, 24, and 2 h, respectively; (b) its receptor binding occupancy is similar to that of abciximab (Mab1 is inhibited and Mab2 is unaltered). In conclusion, UR-3216 is a promising, orally active GPIIb/IIIa antagonist for the treatment of cardiovascular diseases.     

Oral GPIIb/IIIa antagonists: what went wrong?

GPIIb/IIIa receptor antagonists block fibrinogen binding to platelets and as a result inhibit platelet aggregation. They are very potent inhibitors due to the critical role fibrinogen binding plays in platelet aggregation. When given intravenously these drugs have been shown to be very effective as adjuvant therapy in percutaneous coronary intervention and in acute coronary syndromes. However, despite being as potent as their intravenous counterparts, all of the oral inhibitors showed no benefit or even increased mortality in clinical trials. There are a number of reasons for their failure. The target was different, chronic treatment to prevent thrombotic events as opposed to short-term treatment to prevent acute events and as a result, different dosing regimens were used. The acute use aims for a high level of inhibition (80-90%) while the chronic use produced lower levels of inhibition. Many of the oral inhibitors had low bioavailability that led to a large peak-trough difference. Most GPIIb/IIIa antagonists have the ability to activate platelets through a GPIIb/IIIa-mediated process. This is known as partial agonism. In the presence of high drug levels, such as during an infusion this is not a problem, however combined with the low trough levels with oral inhibitors this can lead to an increase in platelet aggregation. Other problems include drug-induced conformational changes in GPIIb/IIIa (ligand-regulated binding sites) and possible pharmacogenomics effects in the response to the drugs, in particular the Pl(A) polymorphism in GPIIb/IIIa. By addressing these issues it is possible for a new generation of oral GPIIb/IIIa antagonist to be developed.     

Platelet GPIIb/IIIa binding characteristics of small molecule RGD mimetic: distinct binding profile for Roxifiban

A number of non-peptide orally active RGD mimetic prodrug such as Orbofiban, Sibrafiban, SR121566, Roxifiban and others entered into the clinical evaluation stage. Some of these agents were terminated and some are still in clinical trials. The present study examined the platelet GPIIb/IIIa binding profiles for the active form of Roxifiban, Sibrafiban, SR121566 and Orbofiban using 3H-Roxifiban active form (XV459), 3H-DMP728, 125I-Echistatin, and 125I-Fibrinogen. Either DMP728, Orbofiban, Sibrafiban, SR121566 or Roxifiban active form as well as other RGD mimetic bind to the same binding site(s) on human platelets as evident from the competitive inhibition of binding of each other to human platelet. Additionally, Roxifiban active form competed with FITC labeled GPIIb/IIIa antagonist cyclic RGD peptidomimetic (XL086) as demonstrated using confocal microscopy technique. Roxifiban active form (XV459) demonstrated the highest potency in inhibiting 3H-XV459, 3H-DMP728, 125I-Echistatin, and 125I-Fibrinogen binding to human platelets as compared to the others. Structure activity relationship within the isoxazoline Roxifiban series showed that substituent at the alpha-carbon next to the carboxy terminal represents an exosite for the affinity binding to human platelets leading to slow platelet dissociation rate. These data indicated a distinct binding profile for Roxifiban (high affinity to both activated and resting platelets associated with a relatively slow K(off)) as compared to others. These differences might determine the pharmacodynamics and pharmackokinetics of the different GPIIb/IIIa antagonists.     

Enhancement of fibrinolysis by gel-filtered platelets and its quenching by cytochalasin B and GPIIb/IIIa antagonists

The effects of gel-filtered platelets on euglobulin clot lysis time (ECLT) were analyzed to elucidate the possible role of platelets in thrombolysis. Gel-filtered platelet-supplemented ECLT (plt-ECLT) was significantly shorter than ECLT without platelets (regular ECLT). Abciximab, anti-glycoprotein IIb/IIIa (GPIIb/IIIa) antibody, and cytochalasin B nullified the enhancement of ECLT by platelets, and increased plt-ECLT beyond regular ECLT. When gel-filtered platelets were used after disruption, ECLT was not shortened but rather became longer than regular ECLT, probably due to natural fibrinolysis inhibitors released from platelets. Therefore, for platelets to enhance fibrinolysis, intact cell structure and cytoskeletal reorganization after thrombin stimulation is required. Various GPIIb/IIIa antagonists prolonged plt-ECLT. The concentrations of GPIIb/IIIa antagonists required to prolong plt-ECLT, were varied. Interestingly, the effects of these antagonists were independent of their ability to inhibit thrombin-induced platelet aggregation, but dependent on their ability to induce clot retraction. T-250, a GPIIb/IIIa antagonist, had the smallest effect on plt-ECLT. These drugs do not affect regular ECLT or tissue plasminogen activator (tPA)-catalyzed Glu-plasminogen activation in the presence of thrombinactivated platelets. Although their overall effect on thrombolysis is inhibitory, platelets could promote fibrinolysis through a GPIIb/IIIa-dependent mechanism.     

Potent, orally active GPIIb/IIIa antagonists containing a nipecotic acid subunit. Structure-activity studies leading to the discovery of RWJ-53308

Although intravenously administered antiplatelet fibrinogen receptor (GPIIb/IIIa) antagonists have become established in the acute-care clinical setting for the prevention of thrombosis, orally administered drugs for chronic use are still under development. Herein, we present details from our exploration of structure-activity surrounding the prototype fibrinogen receptor antagonist RWJ-50042 (racemate of 1), which was derived from a unique approach involving the gamma-chain of fibrinogen (Hoekstra et al. J. Med. Chem. 1995, 38, 1582). Our analogue studies culminated in the discovery of RWJ-53308 (2), a potent, orally active GPIIb/IIIa antagonist. To progress from RWJ-50042 to a suitable candidate for clinical development, we conducted a series of optimization cycles that employed solid-phase parallel synthesis for the rapid, efficient preparation of nearly 250 analogues, which were assayed for fibrinogen receptor affinity and inhibition of platelet aggregation induced by four different activators. This strategy produced several promising analogues for advanced study, including 3-(3,4-methylenedioxybenzene)-beta-amino acid analogue 3 (significant improved in vivo potency) and 3-(3-pyridyl)-beta-amino acid 2 (significantly improved potency, oral absorption, and duration of action). In dogs, 2 displayed significant ex vivo antiplatelet activity on oral administration at 1.0 mg/kg, 16% systemic oral bioavailability, minimal metabolic transformation, and an excellent safety profile. Additionally, 2 was found to be efficacious in three in vivo thrombosis models: canine arteriovenous (AV) shunt (0.01-0.1 mg/kg, iv), guinea pig photoactivation-induced injury (0.3-3 mg/kg, iv), and guinea pig ferric chloride-induced injury (0.3-1 mg/kg, iv). On the basis of its noteworthy preclinical data, RWJ-53308 (2) was selected for clinical evaluation.     

Clinical trials with glycoprotein IIb/IIIa antagonists - No benefit without bleeding?

As the glycoprotein GPIIb/IIIa receptor is the final common pathway in platelet aggregation, antagonists of this receptor cause a profound inhibition of aggregation induced by any agonist. The short-term efficacy and safety of GPIIb/IIIa antagonists in patients undergoing coronary angioplasty was demonstrated with murine 7E3 Fab, but this antibody was immunogenic. Abciximab is a chimeric human-mouse monoclonal antibody that is less immunogenic. The first major trial with a GPIIb/IIIa antagonist was the EPIC trial with abciximab, which showed that abciximab reduced the ischemic complications of coronary balloon angioplasty and atherectomy in high-risk patients, but increased the risk of bleeding. Subsequent studies showed that using less concurrent heparin reduced bleeding. Abciximab also reduced the rate of revascularization. Further studies have shown that the benefits of abciximab extended to all patients undergoing angioplasty (EPILOG), including patients with unstable angina (CAPTURE) and acute myocardial infarction (RAPPORT). Clinical trials with eptifibatide and tirofiban have failed to demonstrate benefit, at the doses used, in angioplasty. Abciximab and eptifibatide, but not oral xemilofiban, improve the safety of the coronary stenting procedure. Short-term intravenous treatment with lamifiban, eptifibatide or tirofiban is beneficial in acute coronary syndromes (unstable angina, non-Q wave myocardial infarction). Orally active GPIIb/IIIa antagonists are being developed for use in acute coronary syndromes and myocardial infarction. However, no benefit has been shown with lefradafiban in acute coronary syndromes and sibrafiban and orbofiban are harmful. Eptifibatide, lamifiban and abciximab improve coronary patency in myocardial infarction, and long- term trials of GPIIb/IIIa antagonists are being conducted in acute myocardial infarction. Abciximab can cause thrombocytopenia, and all the GPIIb/ IIIa antagonists increase the incidence of bleeding, but there is no excess of intracranial hemorrhage. (c) 2001 Prous Science. All rights reserved.     

Characterization of the anti‐platelet actions of FK419, a novel non‐peptide antagonist of platelet GPIIb/IIIa

The anti‐platelet properties of FK419 ((S)‐2‐acetylamino‐3‐[(R)‐[1‐[3‐(piperidin‐4‐yl)propionyl]piperidin‐3‐ylcarbonyl] amino]propionic acid), a novel non‐peptide GPIIb/IIIa antagonist, were compared in a variety of experimental settings, both in vitro and in vivo, with other GPIIb/IIIa antagonists including xemilofiban, lamifiban, tirofiban, and FK633. Receptor binding studies suggested that FK419 had potent GPIIb/IIIa antagonistic activity that is comparable with those of reference antagonists. FK419 effectively inhibited human platelet aggregation, regardless of agonist stimuli (IC₅₀ = 35–170 nM). FK419 demonstrated in vitro species‐dependent anti‐platelet activity, with higher potency in human than in dog, guinea pig, or rat tissue, and dose‐dependently inhibited ex vivo platelet aggregation in dogs and guinea pigs. In contrast to other antagonists, FK419 minimally affected template bleeding time at doses that completely inhibited platelet aggregation in canines. These results demonstrate that FK419 is a novel, potent, and selective GPIIb/IIIa antagonist that safely inhibits platelet aggregation in vivo, suggesting that it may be a promising anti‐platelet agent for thrombotic diseases. Drug Dev. Res. 61:233–241, 2004. © 2004 Wiley‐Liss, Inc.     

Chemical Approaches to Improve the Oral Bioavailability of Peptidergic Molecules

This review discusses both tools and strategies that may be employed as approaches towards the pursuit of orally active compounds from peptidergic molecules. Besides providing a review of these subjects, this paper provides an example of how these were utilized in a research programme at SmithKline Beecham involving the development of orally active GPIIb/IIIa antagonists. The tools for studying oral drug absorption in-vitro include variants of the Ussing chamber which utilize either intestinal tissues or cultured epithelial cells that permit the measurement of intestinal permeability. Example absorption studies that are described are mannitol, cephalexin, the growth hormone-releasing peptide SK&F 110679 and two GPIIb/ IIIa antagonist peptides SK&F 106760 and SK&F 107260. With the exception of cephalexin, these compounds cross the intestine by passive paracellular diffusion. Cephalexin, on the other hand, crosses the intestine via the oligopeptide transporter. Structure-transport studies are reviewed for this transporter. The tools for studying oral drug absorption in-vivo involve animals bearing in-dwelling intestinal or portal vein catheters. A study of the segmental absorption of SK&F 106760 is provided. The review concludes with two chemical strategies that may be taken towards the enhancement of oral bioavailability of peptidergic molecules. The first strategy involves the chemical modification of peptides which enhance intestinal permeability, specifically the modification of amide bonds. The second strategy involves the design of compounds bearing nonpeptide templates, which are more amenable to the discovery of compounds with oral activity, from peptide pharmacophore models. An example is given regarding the discovery of SB 208651, a potent orally active GPIIb/IIIa antagonist, designed from the peptides SK&F 106760 and SK&F 107260.     

Anti-platelet effects of GPIIb/IIIa and P-selectin antagonism,platelet activation,and binding to neutrophils

Platelet activation with GPIIb/IIIa binding to fibrinogen, aggregation and interaction with leukocytes constitute the principal mediator of thrombosis. Although the clinical benefits of GPIIb/IIIa antagonists have been documented, the relationship between their anti-platelet properties, platelet activation and binding to leukocytes is still debated. We investigated the effects of abciximab, tirofiban, roxifiban, and an anti-P-selectin blocking monoclonal antibody (Mab) on isolated human platelet aggregation using optical aggregometer, and on platelet P-selectin and GPIIb/IIIa expression, and platelet-neutrophil binding using flow cytometry. Thrombin at 0.025 U/ml induced maximal platelet aggregation (76.3 +/- 2.6%), P-selectin expression (88.5 +/- 4%), GPIIb/IIIa activation (PAC-1 binding, 86.2 +/- 8.9%) and platelet-neutrophil binding (58.0 +/- 6.4%). The GPIIb/IIIa antagonists inhibited in a concentration-dependent manner platelet aggregation (IC50 of 100 nM for abciximab and tirofiban and 50 nM for roxifiban) and PAC-1 binding, without any effect on P-selectin. None of these agents affected significantly platelet-neutrophil binding, whereas an anti-P-selectin Mab abolished this binding and amplified the effect of abciximab on platelet aggregation. These results indicate that the effects of these GPIIb/IIIa antagonists on platelet aggregation are not related to inhibition of platelet activation, as P-selectin levels and platelet-neutrophil binding remained unaffected, and highlight the participation of P-selectin with GPIIb/IIIa in platelet aggregation.     

Intravenous glycoprotein IIb/IIIa antagonists: their benefits, problems and future developments

The intravenous Glycoprotein (GP) IIb/IIIa antagonists are potent antiplatelet agents that are particularly effective in patients who undergo percutaneous coronary intervention (PCI). Questions remain about their benefit in the setting of primary PCI, as well as in patients with acute coronary syndromes who do not undergo PCI. The dosing of these drugs is critical to their efficacy and for some agents may not yet be optimized. Differences in the level of platelet inhibition achieved with previous and current dosing strategies of these agents are discussed. In addition, the pharmacology of GPIIb/IIIa antagonists is more complex than initially appreciated. These drugs appear to have partial agonist properties and as a result may be prothrombotic at lower doses. Recent evidence also suggests that at least some of the GPIIb/IIIa antagonists may have anti-inflammatory as well as anti-thrombotic activity. Future research should clarify these issues. Because of the observed inter-individual variation in the response to GPIIb/IIIa antagonists, future trials of these agents should also look at individual tailoring of the dose to an optimum level of platelet inhibition. No definite clinical predictors of this inter-individual variation have been identified, but the Pl(A) polymorphism in GPIIIa appears to be associated with an adverse response to treatment with the oral GPIIb/IIIa antagonists in particular.     

Orally active isoxazoline glycoprotein IIb/IIIa antagonists with extended duration of action

Modification of the alpha-carbamate substituent of isoxazoline GPIIb/IIIa (alphaIIb beta3) antagonist DMP 754 (7) led to a series of alpha-sulfonamide and alpha-sulfamide diaminopropionate isoxazolinylacetamides which were found to be potent inhibitors of in vitro platelet aggregation. Aryl- and heteroaryl-alpha-sulfonamide groups, in conjunction with (5R)-isoxazoline (2S)-diaminopropionate stereochemistry, were found to impart a pronounced duration of antiplatelet effect in dogs, potentially due to high affinity for unactivated platelets. Isoxazolylsulfonamide 34b (DMP 802), a highly selective GPIIb/IIIa antagonist, demonstrated a prolonged duration of action after iv and po dosing and high affinity for resting and activated platelets. The prolonged antiplatelet profile of DMP 802 in dogs and the high affinity of DMP 802 for human platelets may be predictive of clinical utility as a once-daily antiplatelet agent.     

Antiplatelet therapies: platelet GPIIb/IIIa antagonists and beyond

Cardiovascular and cerebrovascular diseases continue to be leading causes of death throughout the world. Blood platelets play a pivotal role not only in haemostasis but also in the pathogenesis of thrombosis and atherosclerosis, platelet aggregation being an essential step in the formation of either an effective haemostatic plug or an intravascular thrombus. The benefits of various antiplatelet therapies ranging from aspirin, ticlopidine, Clopidogrel, and intravenous platelet GPIIb/IIIa antagonists in various thromboembolic disorders are well documented. Despite of the success of intravenous acute GPIIb/IIIa blockade when given in conjunction with heparin, chronic oral GPIIb/IIIa antagonists with or without aspirin failed in various cardiovascular settings. This review highlights the role of the various antiplatelet therapies in thrombotic disorders as well as future directions.     

Prevention of platelet glycoprotein IIb/IIIa activation by 3,4-methylenedioxy-beta-nitrostyrene, a novel tyrosine kinase inhibitor

Binding fibrinogen to activated glycoprotein (GP)IIb/IIIa is the final common pathway of platelet aggregation and has become a successful target for antiplatelet therapy. In the present study, we found that a small chemical compound, 3,4-methyl-enedioxy-beta-nitrostyrene (MNS), exhibited potent and broad-spectrum inhibitory effects on human platelet aggregation caused by various stimulators. Moreover, addition of MNS to human platelets that had been aggregated by ADP caused a rapid disaggregation. We demonstrated that the antiaggregatory activity of MNS is due to inhibition of GPIIb/IIIa activation by measuring the binding amount of PAC-1 in platelets. In contrast, MNS is not a direct antagonist of GPIIb/IIIa, because MNS did not affect fibrinogen binding to fixed ADP-stimulated platelets. By investigating how MNS inhibits GPIIb/IIIa activation, we found that MNS potently inhibited the activity of tyrosine kinases (Src and Syk) and prevented protein tyrosine phosphorylation and cytoskeletal association of GPIIb/IIIa and talin, but it had no direct effects on protein kinase C, Ca2+ mobilization, Ca2+-dependent enzymes (myosin light chain kinase and calpain), and arachidonic acid metabolism, and it did not affect the cellular levels of cyclic nucleotides. Therefore, MNS represents a new class of tyrosine kinase inhibitor that potently prevents GPIIb/IIIa activation and platelet aggregation without directly affecting other signaling pathways required for platelet activation. Because MNS inhibits GPIIb/IIIa functions in a manner different from GPIIb/IIIa antagonists, this feature may provide a new strategy for treatment of platelet-dependent thrombosis.     

Pharmacodynamics and pharmacokinetics of YM128, a GPIIb/IIIa antagonist prodrug

We examined the biochemical properties of YM‐57029 ({4‐[4‐(4‐Carbamimidoylphenyl)‐3‐oxopiperazin‐1‐yl]piperidino}acetic acid monohydrochloride trihydrate) and the pharmacodynamics and pharmacokinetics of its prodrug, YM128 (Ethyl (Z)‐(4‐{4‐[4‐(N²‐hydroxycarbamimidoyl)phenyl]‐3‐oxopiperazin‐1‐yl}piperidino)acetate), an orally‐active glycoprotein IIb/IIIa (GPIIb/IIIa) antagonist. YM‐57029 strongly inhibited aggregation of human platelets induced by various agonists, with IC50 values ranging from 3.6 to 51 nM. YM‐57029 specifically inhibited fibrinogen binding to purified GPIIb/IIIa about 1,000‐fold more potently than Arg‐Gly‐Asp‐Ser (RGDS). Moreover, YM‐57029 effectively inhibited an Arg‐Gly‐Asp (RGD) peptide binding to platelets, suggesting that YM‐57029 competed with the RGD sequence of ligand. YM‐57029 or YM128 dose‐dependently inhibited ex vivo platelet aggregation after iv bolus injection or oral administration to beagle dogs and cynomolgus monkeys. However, YM128 exerted more potent and prolonged inhibitory effects on platelet aggregation than YM‐57029 after oral administration to cynomolgus monkeys. Furthermore, YM‐57029 prolonged template bleeding time at a dose that inhibited ex vivo platelet aggregation during cumulative iv infusion to cynomolgus monkeys. Metabolic and pharmacokinetic studies showed that YM128 effectively converted into YM‐57029 in liver microsomes from humans as well as dogs and monkeys, and that bioavailabilities of YM128 in dogs and monkeys were 32.3 and 22.2%, respectively. These results suggest that YM128, a prodrug of YM‐57029, may be a valuable GPIIb/IIIa antagonist with good bioavailability in humans. Drug Dev. Res. 55:149–161, 2002. © 2002 Wiley‐Liss, Inc.     

Abciximab,eptifibatide, and tirofiban exhibit dose-dependent potencies to dissolve platelet aggregates

Platelet GPIIb/IIIa antagonists are not only used to prevent platelet aggregation, but also in combination with thrombolytic agents for the treatment of coronary thrombi. Recent data indicate a potential of abciximab alone to dissolve thrombi in vivo. We investigated the potential of abciximab, eptifibatide, and tirofiban to dissolve platelet aggregates in vitro. Adenosine diphosphate (ADP)-induced platelet aggregation could be reversed in a concentration-dependent manner by all three GPIIb/IIIa antagonists when added after the aggregation curve reached half-maximal aggregation. The concentrations chosen are comparable with in vivo plasma concentrations in clinical applications. Disaggregation reached a maximum degree of 72.4% using 0.5 microg/ml tirofiban, 91.5% using 3.75 microg/ml eptifibatide, and 48.4% using 50 microg/ml abciximab (P < 0.05, respectively). A potential fibrinolytic activity of the GPIIb/IIIa antagonists was ruled out by preincubation with aprotinin or by a plasma clot assay. A stable model Chinese hamster ovary (CHO) cell line expressing the activated form of GPIIb/IIIa was used to confirm the disaggregation capacity of GPIIb/IIIa antagonists found in platelets. Not only abciximab, but also eptifibatide and tirofiban have the potential to disaggregate newly formed platelet clusters in vitro. Because enzyme-dependent fibrinolysis does not appear to be involved, competitive removal of fibrinogen by the receptor antagonists is the most likely mechanism.     

Targeting the platelet integrin GPIIb/IIIa

The platelet integrin GPIIb/IIIa plays an essential role in thrombus formation through interactions with adhesive ligands and has emerged as a primary target for the development of anti-thrombotic agents. Receptor activation is under strict control, with activators, inhibitors, and signalling mechanisms controlling its conformation. Structural biology research has produced high-resolution images defining the ligand binding site at the atomic level. Successful blockade of this ligand binding has validated GPIIb/IIIa as a therapeutic target in cardiovascular medicine. GPIIb/IIIa inhibitors were the first rationally designed anti-platelet agents and have been used effectively in a wide variety of clinical scenarios including unstable angina, myocardial infarction, and high risk percutaneous coronary interventions with and without intracoronary stenting. Three inhibitors (abciximab, eptifibatide, and tirofiban) are currently licensed for human use. Surprisingly, oral GPIIb/IIIa antagonists have not been successful and there is an unmet need for effective anti-GPIIb/IIIa drugs that cause less bleeding problems and that can be orally applied. Here we review our current knowledge about GPIIb/IIIa structure, signalling pathways and receptor function, the benefits and limitations of current GPIIb/IIIa blockers and we take a look forward how the lessons learned from the mixture of success and failure of GPIIb/IIIa blocker development can be transformed in new and better GPIIb/IIIa blockers.     

Metabolism-directed optimisation of antithrombotics: the prodrug principle

Thromboembolic disorders are the major cause of mortality and morbidity in Western societies. Coagulation enzymes, such as thrombin, factor Xa and a tissue factor/factor VIIa complex, together with platelet GPIIb/IIIa receptors, are the focal point of attention in pharmaceutical research aimed at finding new antithrombotic agents. However, finding orally active drugs for these particular molecular targets has proved to be anything but straightforward. Thrombin, factor Xa, tissue factor/factor VIIa and platelet GPIIb/IIIa receptors display a preference for molecules containing highly basic arginine and/or acidic aspartate moieties, which are, however, associated with poor bioavailability after oral application. Different approaches have been taken to achieve favourable absorption, metabolism, distribution and clearance, without compromising the antithrombotic activity of the compounds. This review highlights the use of the prodrug principle in optimising antithrombotic agents.     

Recent advances in antiplatelet agents

Platelet aggregation plays a key role in the pathogenesis of thromboembolic diseases such as myocardial infarction, stroke, unstable angina and peripheral artery disease. Until recently, aspirin was the only antiplatelet agent available to prevent or treat these events. Over the past several years, there has been a substantial expansion in the antiplatelet armamentarium as well as in the understanding of the clinical importance of antiplatelet therapy in limiting the complications of thrombosis. Aspirin was one of the first agents to be adopted and it remains as the standard therapy with the higher amount of available clinical information. Following aspirin, ADP receptor antagonists like ticlopidine and clopidogrel as well as phosphodiesterase inhibitors dipyridamole and cilostazol have been introduced. Glycoprotein (GP) IIb/IIIa receptor antagonists like eptifibatide, tirofiban and abciximab are the newer antiplatelet agents which act at the end of the common pathway of platelet aggregation. Although results of clinical studies with the first oral GPIIb/IIIa antagonists were disappointing, agents of the new generation might expand the potential application of GPIIb/IIIa targeted therapy. This review will highlight recent advances in the development of aspirin, phosphodiesterase inhibitors, ADP receptor antagonists and the platelet glycoprotein IIb/IIIa inhibitors. The emphasis of this paper has been placed on the chemical aspects of these agents.     

Effects of TRA-418, a novel TP-receptor antagonist, and IP-receptor agonist, on human platelet activation and aggregation

[4-[2-(1,1-Diphenylethylsulfanyl)-ethyl]-3,4-dihydro-2H-benzo[1,4]oxazin-8-yloxy]-acetic acid N-Methyl-d-glucamine salt (TRA-418) has both thromboxane A2 (TP)-receptor antagonist and prostacyclin (IP)-receptor agonist properties. The present study examined the advantageous effects of TRA-418 based on the dual activities, over an agent having either activity alone and also the difference in the effects of TRA-418 and a glycoprotein alphaIIb/beta3 integrin (GPIIb/IIIa) inhibitor. TRA-418 inhibited platelet GPIIb/IIIa activation as well as P-selectin expression induced by adenosine 5'-diphosphate, thrombin receptor agonist peptide 1-6 (Ser-Phe-Leu-Leu-Arg-Asn-NH2), and U-46619 in the presence of epinephrine (U-46619+ epinephrine). TRA-418 also inhibited platelet aggregation induced by those platelet-stimulants in Ca2+ chelating anticoagulant, citrate and in nonchelating anticoagulant, d-phenylalanyl-l-prolyl-l-arginyl-chloromethyl ketone (PPACK). The TP-receptor antagonist SQ-29548 inhibited only U-46619+epinephrine-induced GPIIb/IIIa activation, P-selectin expression, and platelet aggregation. The IP-receptor agonist beraprost sodium inhibited platelet activation. Beraprost also inhibited platelet aggregation induced by platelet stimulants we tested in citrate and in PPACK. The GPIIb/IIIa inhibitor abciximab blocked GPIIb/IIIa activation and platelet aggregation. However, abciximab showed slight inhibitory effects on P-selectin expression. TRA-418 is more advantageous as an antiplatelet agent than TP-receptor antagonists or IP-receptor agonists separately used. TRA-418 showed a different inhibitory profile from abciximab in the effects on P-selectin expression.     

Glycoprotein IIb/IIIa receptor antagonists in non-ST elevation acute coronary syndromes and percutaneous revascularisation: a review of trial reports.

Acute coronary syndromes and percutaneous coronary interventions share a common pathophysiological mechanism of intimal disruption and platelet aggregation. Glycoprotein (GP) IIb/IIIa receptor antagonists, which interrupt the final common pathway of platelet activation and aggregation, have been shown to have clear clinical benefit as acute therapy. Treatment of 1000 patients with parenteral formulations prevents at least 1 death, 20 deaths or myocardial infarctions (MIs), and 30 deaths, MIs or revascularisation procedures over a 30-day period. These benefits are sustained at 6 months. Clinical trials of oral formulations are underway. The challenges of dose, haemorrhage and thrombocytopenia must be surmounted before oral antagonists can be incorporated into clinical practice. Despite enrolment of thousands of patients in randomised trials of GPIIb/IIIa antagonists, much additional information is needed to refine the use of this therapy in practice. Application of this drug class will advance a new therapeutic standard for ischaemic heart disease.