Antiplatelet efficacy and specificity of DMP728, a novel platelet GPIIb/IIIa receptor antagonist

Advanced targeted nanomedicines for vulnerable atherosclerosis plaque imaging and their potential clinical implications

Atherosclerosis plaques caused by cerebrovascular and coronary artery disease have been the leading cause of death and morbidity worldwide. Precise assessment of the degree of atherosclerotic plaque is critical for predicting the risk of atherosclerosis plaques and monitoring postinterventional outcomes. However, traditional imaging techniques to predict cardiocerebrovascular events mainly depend on quantifying the percentage reduction in luminal diameter, which would immensely underestimate non-stenotic high-risk plaque. Identifying the degree of atherosclerosis plaques still remains highly limited. vNanomedicine-based imaging techniques present unique advantages over conventional techniques due to the superior properties intrinsic to nanoscope, which possess enormous potential for characterization and detection of the features of atherosclerosis plaque vulnerability. Here, we review recent advancements in the development of targeted nanomedicine-based approaches and their applications to atherosclerosis plaque imaging and risk stratification. Finally, the challenges and opportunities regarding the future development and clinical translation of the targeted nanomedicine in related fields are discussed.

The choice of targets and ligands for site-specific delivery of nanomedicine to atherosclerosis

As nanotechnologies advance into clinical medicine, novel methods for applying nanomedicine to cardiovascular diseases are emerging. Extensive research has been undertaken to unlock the complex pathogenesis of atherosclerosis. However, this complexity presents challenges to develop effective imaging and therapeutic modalities for early diagnosis and acute intervention. The choice of ligand-receptor system vastly influences the effectiveness of nanomedicine. This review collates current ligand-receptor systems used in targeting functionalized nanoparticles for diagnosis and treatment of atherosclerosis. Our focus is on the binding affinity and selectivity of ligand-receptor systems, as well as the relative abundance of targets throughout the development and progression of atherosclerosis. Antibody-based targeting systems are currently the most commonly researched due to their high binding affinities when compared with other ligands, such as antibody fragments, peptides, and other small molecules. However, antibodies tend to be immunogenic due to their size. Engineering antibody fragments can address this issue but will compromise their binding affinity. Peptides are promising ligands due to their synthetic flexibility and low production costs. Alongside the aforementioned binding affinity of ligands, the choice of target and its abundance throughout distinct stages of atherosclerosis and thrombosis is relevant to the intended purpose of the nanomedicine. Further studies to investigate the components of atherosclerotic plaques are required as their cellular and molecular profile shifts over time.

Adhesion molecules: potential therapeutic and diagnostic implications

The role of cell adhesion molecules (CAMs) and extracellular matrix (ECM) proteins in various pathological processes, including angiogenesis, thrombosis, inflammation, apoptosis, cell migration, and proliferation is well documented. These processes can lead to both acute and chronic disease states such as ocular diseases, metastasis, unstable angina, myocardial infarction, stroke, osteoporosis, a wide range of inflammatory diseases, vascular remodeling, and neurodegenerative disorders. A key success in this field was identification of the role of platelet glycoprotein (GP)IIb/IIIa in the prevention and diagnosis of various thromboembolic disorders. The use of soluble adhesion molecules as potential diagnostic markers for acute and chronic leukocyte, platelet, and endothelial cell insult is becoming increasingly common. The development of various therapeutic and diagnostic candidates based on the key role of CAMs, with special emphasis on integrins in various diseases, as well as the structure-function aspects of cell adhesion and signaling of the different CAMs and ECM are highlighted.

Cell adhesion molecules: potential therapeutic & diagnostic implications

The role of cell adhesion molecules (CAM) and extracellular matrix proteins (ECM) in various pathological processes including angiogenesis, thrombosis, apoptosis, cell migration & proliferation are well documented. These processes can lead to both acute and chronic disease states such as ocular diseases, metastasis, unstable angina, myocardial infarction, stroke, osteoporosis, a wide range of inflammatory diseases, vascular remodeling, and neurodegenerative disorders. A key success in this field is evident from the potential role of the platelet GPIIb/IIIa integrin in the prevention and diagnosis of various thromboembolic disorders. Additionally, the use of soluble adhesion molecules as potential diagnostic markers for acute and chronic leukocyte, platelet, and endothelial cellular insult are increasingly utilized. The development of various therapeutic and diagnostic candidates based on the key role of CAM, with special emphasis on integrins in various diseases as well as the structure-function aspects of cell adhesion and signaling of the different CAM and ECM are highlighted.

Substituted benzocyloheptenes as potent and selective alpha(v) integrin antagonists

A novel series of potent and specific alpha(v) integrin antagonists has been obtained by aminoalkyl substitutions on benzocyloheptene acetic acids as a rigid GD bioisostere. The preferred compounds 1-2, 1-3 and 1-8, showed nano- to subnanomolar IC(50) values on alpha(v)beta(3) and alpha(v)beta(5) integrins, with favorable pharmacokinetics.

Synthesis and pharmacology of modified amidine isoxazoline glycoprotein IIb/IIIa receptor antagonists

Selective antagonism of the platelet GPIIb/IIIa receptor represents an attractive mechanism for the prevention and treatment of a number of thrombotic disease states. The antiplatelet activity of the oral GPIIb/IIIa receptor antagonists DMP 754 and DMP 802 have been disclosed. In this paper, the synthesis and biological evaluation of a series of potent N-substituted benzamidine isoxazolines are explored. The effect of benzamidine substitution on the duration of antiplatelet efficacy in dog is presented.

Ring constrained analogues of beta-alanine-containing GPIIb/IIIa receptor antagonists

A series of ring constrained analogues of the GPIIb/IIIa receptor antagonist XR299 (1) was investigated as potential inhibitors of glycoprotein IIb/IIIa, a platelet receptor that plays a key role in platelet aggregation and platelet adhesion. Ring size was found to have a large effect on in vitro potency. Selected compounds showed good in vitro activity, a preference for binding to activated platelets, and modest duration of action when dosed i.v. as a racemate in a canine model.

Effect of glycoprotein IIb/IIIa inhibitors on CD62p expression, platelet aggregates, and microparticles in vitro

Flow cytometry can detect platelet activation (CD62p), aggregate formation, microparticle formation, and glycoprotein IIb/IIIa (GP IIb/IIIa) receptor occupancy in one sample at the level of single particles. We studied the effect of GP IIb/IIIa inhibitors on platelet activation with flow cytometry in vitro. Citrated whole blood was incubated with increasing concentrations of three different GP IIb/IIIa inhibitors (c7E3, DMP728, XJ757), then thrombin or adenosine diphosphate (ADP) was added, and after 1 minute the sample was fixed. Samples with thrombin but without c7E3 had a decrease in platelet count, from a mean of 260,000 platelets/microl to 56,000 platelets/microL, and aggregates increased. Samples with concentrations of c7E3 that resulted in 80% or more receptor blockade had no decrease in platelet count, and no aggregates were formed, but the number of CD62p-positive single platelets increased from 1200 to 7400 platelets/microL. The two other inhibitors (DMP 725, XJ757) or ADP instead of thrombin gave similar results. Microparticle formation did not change with platelet activation in the presence of a GP IIb/IIIa inhibitor. With small inhibitor doses resulting in <80% receptor blockade, the number of aggregates did not change or was even higher than that in samples without inhibitor. GP IIb/IIIa inhibitors do prevent aggregate formation but they do not prevent activation of platelets. With GP IIb/IIIa inhibition, more activated single platelets remain in the blood. One may expect an increasing number of circulating, activated platelets with the use of GP IIb/IIIa inhibitors.

Antiplatelet therapies: from aspirin to GPIIb/IIIa-receptor antagonists and beyond

This review discusses recent advances in antiplatelet therapies, comparative analysis between the antiplatelet/ antithrombotic efficacy of various antiplatelet strategies and that of platelet glycoprotein GPIIb/IIIa-receptor antagonists, issues in the development of chronic anti-GPIIb/IIIa-receptor therapy and potential adjunct strategies using GPIIb/IIIa-receptor antagonists. Acute coronary syndromes are secondary to unstable angina, ST-segment elevation, and acute myocardial infarction. These involve the rupture of a vulnerable atherosclerotic plaque, leading to platelet adhesion, activation and aggregation at the site of rupture. Several studies suggest that complex or ulcerated plaques, which might promote further thrombotic events, can persist for more than one month after the acute event. These data suggest the potential added benefit of chronic oral therapy with antiplatelet drugs beyond the well-documented benefit of acute intravenous use of various GPIIb/IIIa-receptor antagonists. However the efficacy-safety ratio or the risk-benefit ratio for chronic oral antiplatelet therapy needs to be defined. Both aspirin and clopidogrel are available for chronic oral use. By contrast, there are tremendous challenges ahead with the oral GPIIb/IIIa-receptor antagonists because of their lack of expected benefit over aspirin. However, much still remains to be defined with regard to the optimization of current and future antiplatelet therapies or their optimized combinations.

Intravenous and oral antithrombotic efficacy of the novel platelet GPIIb/IIIa antagonist roxifiban (DMP754) and its free acid form, XV459

Currently used antiplatelet drugs, including aspirin, ticlopidine, and others, are effective against certain but not all of the many endogenous platelet activators. Because of their limited efficacy, a significant number of serious thromboembolic complications still occur, highlighting the need for a more effective therapy. DMP754 (roxifiban), a prodrug of XV459, is a recently discovered, potent antiplatelet agent with high affinity and specificity for platelet GPIIb/IIIa receptors that blocks platelet aggregate formation regardless of the agonist (IC(50)=0.030 to 0.05 micromol/L) or anticoagulant used for blood collection. DMP754 rapidly converts to its active free-acid form, XV459, which has a comparable high affinity for both resting and activated platelets (K(d)=1 to 2 nmol/L) and a relatively slow rate of dissociation from resting platelets. The present study was undertaken to determine intravenous and oral antithrombotic efficacies of DMP754 and XV459 and to compare them with those of other antiplatelet and anticoagulant agents in canine models of arterial thrombosis. In these models, thrombosis was induced either electrolytically (200-microA anodal current) in the carotid artery or mechanically by external clamping of the femoral artery along with stenosis, which resulted in either total occlusive thrombus formation or cyclic flow reduction, respectively. DMP754 and XV459 were given either intravenously (0.1 mg/kg bolus) or orally (0.1 to 0.4 mg/kg). Additionally, the antithrombotic efficacies of DMP754, aspirin, heparin, and ticlopidine in the canine carotid artery electrolytic injury model were compared. DMP754 demonstrated oral bioavailability of 20.8% in dogs after administration at different doses and prevented cyclic flow reduction (ED(90-100)=<0.1 mg/kg IV or PO). Additionally, both DMP754 and XV459 (0.1 mg/kg IV or 0.3 to 0.4 mg/kg PO) demonstrated maximal antithrombotic efficacy in preventing electrically induced carotid and coronary artery thrombosis and significant antithrombotic efficacy (P<0.001) at relatively low doses in different settings of arterial thrombosis in the canine model. DMP754 resulted in a significant reduction in thrombus mass and sustained arterial blood flow with 100% prevention of occlusive and nonocclusive thrombosis. In contrast, administration of aspirin (10 mg/kg PO for 2 days), heparin (10 IU/kg IV bolus followed by 90 IU/kg IV infusion over 3 hours), or ticlopidine (300 mg/kg PO for 3 days) before initiation of arterial thrombosis did not reduce the incidence of electrolytic injury-induced occlusive arterial thrombosis. These studies demonstrated a distinct antithrombotic efficacy of DMP754 as compared with existing strategies and suggest potential intravenous and oral antithrombotic uses of DMP754 in the prevention and treatment of thromboembolic disorders.

Disposition and exposure of the fibrinogen receptor antagonist XV459 on alphaIIBbeta3 binding sites in the guinea pig

The disposition of XV459, a potent, selective GP IIb/IIIa antagonist, has been examined following intravenous administration of XP280, the benzenesulphonate salt, and 3H-SA202, the trifluroacetic acid salt, to male guinea pigs. A liquid chromatography-mass spectrometry (LC-MS) method was developed and validated for XV459 quantitation in guinea pig plasma with an LLOQ of 0.1 ng/mL. Intravenous infusions (30 min) of XP280 at doses of 0.5 and 2.0 microg/kg were administered to guinea pigs which were sequentially sacrificed at 0.5, 1, 1.5, 4, 8, 12, 24, 48 and 72 h postinitiation of infusion. Maximum total (unbound and GP IIb/IIIa displaced) XV459 plasma concentration of approximately 3.5 microg/mL was obtained at the 2.0 microg/kg dose. Pooling individual concentration-time data yielded a systemic clearance of 1.42 mL/min/kg, Vss of 0.24 L/kg, and a terminal half-life of 2.8 h in the guinea pig at the 0.5 microg/kg dose. The 2.0 microg/kg dose yielded XV459 exposure that was less than proportional to the previous dose. Similar behaviour has been observed in human trials. Cumulative (up to 72 h) urinary and faecal recovery of total radioactivity was 66.4 and 11.2%, respectively. The time course of spleen, marrow and whole blood radioactivity profiles was similar, suggesting that XV459 was not preferentially sequestered on non-plasma GP IIb/IIIa binding sites. Tissue to blood ratios of 20.7 and 8.3 for the spleen and bone marrow, respectively, indicate that increased (relative to blood) exposure was evident for sites containing the GP IIb/IIIa receptor. In vitro studies confirmed the similarity of XV459 binding to both resting and activated platelets in the guinea pig and humans. Given the comparability of dissociation rate constants and IC50s based on in vitro platelet aggregation, human dosimetry estimates should assume similar partitioning of radiolabelled XV459 as in the guinea pig. These results suggest that the guinea pig may indeed be an appropriate animal model for pharmacokinetic and distribution studies with DMP754; in conjunction with recent pharmacological findings with GP IIb/IIIa antagonists, our results suggest that the guinea pig may be the rodent species of choice for preclinical studies with some other GP IIb/IIIa antagonists.

Isoxazoline GPIIb/IIIa antagonists bearing a phosphoramidate

Isoxazolinylacetamides bearing a phosphoramidate group alpha- to the carboxylate moiety (3) were prepared and evaluated for in vitro antiplatelet efficacy. They were found to bind GPIIb/IIIa with high affinity and were potent antagonists of ADP mediated platelet aggregation.

Platelet glycoprotein IIb/IIIa receptor antagonists derived from isoxazolidines

A series of isoxazolidines has been synthesized as mimetics of the RGD sequence and was evaluated as antagonists of the platelet glycoprotein IIb/IIIa receptor. These compounds were shown to be highly potent GPIIb/IIIa antagonists, exhibiting submicromolar potencies.

Synthesis and evaluation of RGD peptidomimetics aimed at surface bioderivatization of polymer substrates

Several RGD peptidomimetics have been prepared, in a convergent way, from the common ortho-aminotyrosine template (O-substituted with an anchorage-arm or a methyl group, and alpha N-substituted with a fluorine tag for XPS analysis), and various omega-aminoacid derivatives. The most flexible compounds have shown a biological activity similar to that of the peptide reference (RGDS) in the platelet aggregation test. The compound 16a could be fitted (by modelisation) with DMP 728 and c(RGDfV), two cyclic peptides that are good ligands of integrins. The compound 16b has been covalently fixed on the surface of a poly(ethylene terephthalate) membrane used as support for mammalian cell cultivation.

Inhibition of platelet-mediated clot retraction by integrin antagonists

The effects of the platelet alphaIIb beta3 integrin (GPIIb/IIIa) antagonists XV459 (non-peptide), c7E3 (Fab monoclonal antibody) and DMP728 (cyclic peptide) as well as the alpha(v)beta3 integrin antagonists, LM609 (monoclonal antibody) and XT199 (non-peptide) on clotting and platelet-mediated clot retraction were examined. While 30 nM of XV459 had no significant effect on the kinetics of coagulation, platelet-mediated clot retraction was nearly fully inhibited at this concentration (Relative Retraction Rate = 0.09). XV459 resulted in a concentration related-response curve. Other experiments demonstrated that platelet aggregation was maximally inhibited at XV459 concentrations ranging from 30-50 nM. Similarly, c7E3 demonstrated comparable inhibitory efficacy in inhibiting either clot retraction or platelet aggregation. In contrast, DMP728, an equally potent anti-aggregatory agent with an IC50 of 20-50 nM in inhibiting platelet aggregation induced by various agonists, was found to be a less potent inhibitor of platelet-mediated clot retraction with a half-maximal inhibition of clot retraction at approximately 0.7 microM, and maximum effects at concentrations of 10 microM. The alpha(v)beta3 integrin antagonists, LM609 or XT199 were without any significant effects on either platelet-mediated clot retraction or platelet aggregation. In conclusion, these data suggest a differential efficacy among different GPIIb/IIIa antagonists in inhibiting platelet-mediated clot retraction in spite of the equivalent anti-aggregatory potency. Additionally, the alpha(v)beta3 integrin antagonists do not affect platelet-mediated clot retraction or aggregation. Further studies with the previously described alphaIIb beta3 integrin antagonists as well as others revealed a distinct correlation between the Kd to resting and activated platelets and the efficacy in inhibiting platelet-mediated clot retraction.

Dissociation between the anti-aggregatory & anti-secretory effects of platelet integrin alpha IIb beta 3 (GPIIb/IIIa) antagonists, c7E3 and DMP728

The effects of alpha IIb beta 3 antagonists on the blockade of fibrinogen binding to platelet alpha IIb beta 3 are well documented, however, little is known about their effects on platelet secretion. We compare here the effect of two potent alpha IIb beta 3 antagonists, c7E3 and DMP728, on platelet secretion. Using human platelet-rich plasma, P-selectin expression was measured by flow cytometry and type 1 plasminogen activator inhibitor (PAI-1) secretion as well as beta-thromboglobulin (beta-TG) were determined by ELISA. At various concentrations of the antagonists that inhibited 80-95% of platelet aggregation, neither had any effect on P-selectin expression. In contrast, thrombin-stimulated PAI-1 secretion is only inhibited by c7E3, 49.6% at 3.5 mumol/L (p < 0.05), but not at any other maximally effective anti-aggregatory concentrations of c7E3 or DMP728. Furthermore, a lack of any significant effects on platelet granular secretion of beta-TG induced by either thrombin or ADP was demonstrated with DMP728, c7E3 or LM609. Two protein kinase inhibitors, staurosporine and herbimycin, blocked both ADP and thrombin-induced P-selectin expression at 10 mumol/L, but not PAI-1 secretion. Taken together this suggests that: (1) the mechanism of platelet granular secretion is independent of the integrin alpha IIb beta 3 and (2) the subcellular locations of PAI-1, beta-TG and P-selectin or the signaling mechanisms that regulate their secretion might be different. Although there is no direct effect of platelet alpha IIb beta 3 antagonists on platelet secretion of PAI-1, beta-TG and P-selectin, the present data demonstrates that reduction of platelet number by alpha IIb beta 3 antagonists, via the reduction in thrombus size, might be an alternate mechanism for reduced platelet secretion. In conclusion, a discoupling between the anti-aggregatory and the anti-secretory effects of alpha IIb beta 3 antagonists has been demonstrated.

Role of platelet adhesion in homeostasis and immunopathology

Various molecules expressed on the surface of platelets have been shown to mediate the protective or deleterious role of these cells in immuno-inflammatory mechanisms. Increasing evidence points to the involvement of the cell adhesion molecules, gpIIb-IIIa, P-selectin, CD31, LFA-1, and CD36 in the interaction between platelets and endothelial cells as well as other cell types. The possible role of these molecules in the ability of platelets to support endothelium and to protect against tumour necrosis factor mediated cytolysis or parasitic invasion are reviewed. The involvement of platelets as effectors of tissue damage in cerebral malaria, lipopolysaccharide induced pathology, and pulmonary fibrosis is also discussed. This has then been extended to include the intercellular mechanisms underpinning their pathogenic role in metastasis, transplant rejection, stroke, brain hypoxia, and related conditions. A better understanding of the complex regulation and hierarchical organisation of these various platelet adhesion molecules may prove useful in the development of new approaches to the treatment of such diseases.

Prevention of carotid artery thrombosis by oral platelet GPIIb/IIIa antagonist in dogs

BACKGROUND AND PURPOSE

Current antithrombotic therapy in acute ischemic stroke and myocardial infarction in which a combination of antiplatelet agents (aspirin) and anticoagulants (heparin) was used led to partial reduction of acute thrombotic complications. Recent advances in antiplatelet research led to the discovery of the platelet glycoprotein IIb/IIIa complex (GPIIb/IIIa), the final common pathway for platelet aggregation. The present study was undertaken to determine the oral antithrombotic efficacy of a potent and specific platelet GPIIb/ IIIa antagonist, DMP728, in an electrically induced carotid artery thrombosis model in dogs. Based on the powerful antiplatelet efficacy of this mechanism in inhibiting all agonist-induced platelet aggregation as well as in inhibiting platelet procoagulant activity (thrombin generation and hence fibrin formation), an orally active antagonist for this integrin receptor might have potential benefits in stroke.

METHODS

Anesthetized dogs were instrumented for monitoring of arterial blood pressure, heart rate, and carotid artery flow velocity. Animals were treated with saline or DMP728 (0.1 to 1.0 mg/kg PO). Thrombus formation (platelet-rich aggregate with fibrous coating and a few erythrocytes) by anodal electrolytic stimulation (300 microA) to the intimal surface of the right carotid artery was initiated 120 minutes after oral DMP728 administration and continued for 180 minutes. Whole blood cell counts, ex vivo platelet aggregation, and template bleeding time were determined at different time points throughout the study.

RESULTS

DMP728 administered at 0.1 to 1.0 mg/kg PO exhibited dose-dependent antithrombotic efficacy in this model. DMP728 was shown to be significantly effective in inhibiting ex vivo platelet aggregation and in inhibiting thrombosis at 0.3 to 1.0 mg/kg PO. The antiplatelet, antithrombotic effects of DMP728 were demonstrated without any significant changes in the different hemodynamic or coagulation parameters. These data demonstrated the oral antithrombotic efficacy of DMP728 in dogs.

CONCLUSIONS

Platelet GPIIb/IIIa blockade with an orally active antagonist was shown to be safe and effective in the prevention of carotid artery occlusive thrombosis.

Design, synthesis, and in vitro activities of benzamide-core glycoprotein IIb/IIIa antagonists: 2,3-diaminopropionic acid derivatives as surrogates of aspartic acid

In an effort to discover novel nonpeptide glycoprotein IIb/IIIa (GPIIb/IIIa, alpha IIb/beta 3) inhibitors, we investigated RGD mimetics featuring a 3-substituted benzoic acid as the core, benzamidine as the basic moiety, and a series of beta- and alpha-substituted beta-alanine derivatives as aspartic acid surrogates. It was found that the use of beta-methyl beta-alanine slightly improved the anti-aggregant potency in human platelet-rich plasma over the unsubstituted beta-alanine compound, while beta-substitution with a trifluoromethyl group resulted in considerable loss in activity. Significant enhancement (up to 100-fold) in potency was obtained when the beta-alanine was replaced with N2-substituted 1-2,3-diaminopropionic acid derivatives. Among the three types of alpha-substituents (carbamate, amide, and sulfonamide) investigated, no apparent preference was observed with respect to in vitro potency. However, alkyl groups were more favorable than arylalkyl groups (Cbz) in the carbamate analogues. We also investigated piperidine, piperazine, and N-formamidinopiperidine as replacements for the benzamidine moiety. The former two replacements led to a drop in potency while the latter replacement resulted in maintenance of activity as compared with the corresponding benzamidine analogue.

Role of platelets in restenosis after percutaneous coronary revascularization

The role of platelets in the process of restenosis after percutaneous coronary intervention is not fully understood. After vascular injury there is extensive platelet activation, adhesion, aggregation and secretion. Through the liberation of growth factors, such as platelet-derived growth factor, and surface expression of cell adhesion molecules, such as the glycoprotein IIb/IIIa integrin, platelets appear to be a pivotal mediator of the vascular injury response. Experimental models have demonstrated that profound, prolonged thrombocytopenia, or blockade of the IIb/IIIa receptor, may reduce neointimal hyperplasia after arterial balloon injury. However, multiple clinical trials testing conventional or new platelet agents have not yielded any salutary effects. The recent finding that abciximab, a monoclonal antibody fragment directed against IIb/IIIa, reduced clinical restenosis after coronary angioplasty by 26% in patients raises questions about the mechanism of benefit. The alpha v beta 3 vitronectin receptor is responsible for binding endothelial cells to platelets, and it also has a key role in modulating smooth muscle cell migration. It is possible that the antibody fragment exerts its effect on restenosis by means of alpha v beta 3, because abciximab fully cross-reacts to this integrin owing to the shared beta 3 subunit. To date, the other platelet glycoprotein IIb/IIIa inhibitors, including Integrelin, Tirofiban, Lamifiban and Xemilofiban, are specific in binding to this particular integrin. Considerable further study is necessary to unravel the effects of platelets on the restenosis process.

Oral antiplatelet, antithrombotic efficacy of DMP 728, a novel platelet GPIIb/IIIa antagonist

BACKGROUND

Currently used antiplatelet drugs, including aspirin and ticlopidine, are effective against certain but not all of the many endogenous platelet activators. Because of their limited efficacy, a significant number of serious thromboembolic complications still occur, highlighting the need for a more effective therapy. DMP 728 has been characterized as a potent and specific platelet glycoprotein IIb/IIIa complex (GPIIb/IIIa) antagonist. The goals of the present study were to determine the oral antiplatelet and antithrombotic efficacies of DMP 728 in various arterial thrombosis models in dogs.

METHODS AND RESULTS

In conscious and anesthetized mongrel dogs, DMP 728 at 0.02 to 1.0 mg/kg PO in gelatin capsules produced dose-dependent antiplatelet effects in inhibiting ex vivo platelet aggregation induced by ADP and prolonging template bleeding time. DMP 728 effects on bleeding time prolongation could be reversed more rapidly than those on platelet aggregation inhibition. A maximal antiplatelet effect for DMP 728 was demonstrated at 1.0 mg/kg PO. DMP 728 demonstrated dose-dependent oral antiplatelet effects with an absolute oral bioavailability of 8% to 12% in dogs. Additionally, the antithrombotic efficacy of DMP 728 was examined after intravenous and oral administration at different doses in various models of arterial thrombosis. In the coronary artery Folts' model in dogs, DMP 728 demonstrated maximal antithrombotic efficacy at 0.01 mg/kg IV and < 0.6 mg/kg PO. Additionally, DMP 728 at 0.1 and 1.0 mg/kg IV or PO demonstrated 60% to 100% prevention of primary thrombosis (P < .01) in an electrolytically induced carotid artery thrombosis model in dogs.

CONCLUSIONS

These data suggest that DMP 728, a low-molecular-weight GPIIb/IIIa receptor antagonist, may have therapeutic potential as an oral antithrombotic agent in coronary and carotid artery thromboembolic disorders.

Calcium, calmodulin and protein kinase C dependence of platelet shape change

Platelet shape change (PSC) represents the initial phase of platelet activation and is normally investigated in ethylene diamine tetraacetic acid (EDTA) containing platelet rich plasma (PRP); EDTA is a potent chelator of calcium and therefore reduces ionized calcium to negligible levels. It is therefore assumed that it is a process independent of calcium. To test the hypothesis that PSC may be dependent upon intracellular calcium, we examined the effect of 8-(N,N-Diethylamino) octyl 3,4,5-Trimethoxybenzoate hydrochloride (TMB-8), an inhibitor of intercellular calcium mobilization on PSC. It produced a dose dependent inhibition of PSC. We then examined whether PSC was dependent upon calmodulin and protein kinase C, a calcium dependent enzyme which is cardinal to platelet aggregation. Both calmidazolium, a specific inhibitor of calmodulin, and H-9, a specific inhibitor of protein kinase C, produced dose dependent inhibition of PSC. Finally, we investigated whether GP IIb/IIIa receptor which binds fibrinogen was involved in PSC; DMP 728 [(cyclic [D-2-amino-butyryl-N2-methyl-L-arginyl-glycyl-L-aspartyl-3- (a min o-methyl-benzoic acid], methanesulfonic acid salt] a potent GP IIb/IIIa receptor antagonist was without any effect on PSC. We conclude that PSC is a calcium, calmodulin and protein kinase C dependent process like platelet aggregation but that it does not require extracellular calcium or the participation of platelet GP IIb/IIIa complex.

Platelet GPIIb/IIIa receptor occupancy studies using a novel fluoresceinated cyclic Arg-Gly-Asp peptide

DMP 728 is a potent and specific platelet GPIIb/IIIa antagonist. Like all GPIIb/IIIa antagonists, DMP 728 has a steep dose-response relationship in inhibiting platelet aggregation. In this study the relationships between receptor occupancy, platelet aggregation and bleeding time was determined in anesthetized dogs after intravenous infusion of DMP 728 (0.01 and 0.1 mg/kg/2h). Receptor occupancy was determined by flow cytometry using XL086, a novel fluorescent cyclic RGD peptide that binds to GPIIb/IIIa with high specificity and affinity (kd approximately 55 nM). Mean number of GPIIb/IIIa as determined by flow cytometric assay was approximately 53,8000 and 79,000 on unactivated and ADP-activated platelets respectively. After DMP 728 intravenous infusion, there was a dose- and time-dependent increase in receptor occupancy, inhibition of platelet aggregation and bleeding time. The two methods of receptor occupancy determination correlate with each other with an r2 = 0.78. The present data suggest that blockade of only 40-60% (approximately 40,000 receptors) of the total platelet GPIIb/IIIa was required to achieve > 90% inhibition of platelet aggregation and > 15 min bleeding time. Our results showed the potential clinical utility of this approach in the study of GPIIb/IIIa dose-response relationship.

Intravenous antiplatelet efficacy and safety of the platelet GPIIb/IIIa antagonist, DMP 728 in anesthetized dogs

DMP 728, cyclo (D-2-aminobutyrate-N-Methyl-L-Arginyl-Glycyl-L-Aspartyl- 3-amino-methyl-benzoic acid) methanesulfonate salt, is a novel antiplatelet agent with high affinity and specificity for human and canine platelet GPIIb/IIIa (alpha 2/beta 3) receptors. DMP 728 demonstrated a potent antiplatelet efficacy in inhibiting ADP-induced platelet aggregation in either human or canine PRP with an IC50 of 0.046 and 0.015 microM, respectively. The IC50 of DMP 728 in inhibiting human platelet aggregation in PRP ranged from 0.02-0.05 microM regardless of the agonist used or even their combinations. Additionally, DMP 728 displayed a much greater affinity in inhibiting 125I-fibrinogen binding to stimulated human platelets as compared to the linear peptide RGDS or fibrinogen. The present study was undertaken to examine the i.v. antiplatelet efficacy and safety of DMP 728 in anesthetized dogs. In anesthetized mongrel dogs, DMP 728 (0.001-1.0 mg/kg, i.v. bolus) produced a dose-dependent inhibition of ex vivo platelet aggregation induced by ADP. The onset of inhibition was immediate, and the duration of antiplatelet effects was dose-dependent. A maximal inhibition of platelet aggregation and a reversible prolongation of bleeding time at 0.01 mg/kg were shown. Additionally, the antiplatelet efficacy/safety of DMP 728 was examined after i.v. administration at different infusion rates ranging from 0.008 to 0.833 micrograms/kg/min for 2 hours. A minimal antiplatelet effect was observed at the 0.008 micrograms/kg/min for 2 hours, while a maximal inhibition of platelet aggregation along with a reversible prolongation of bleeding time was achieved at 45-60 min post-infusion of 0.08 micrograms/kg/min x 2 hours. Prolongation of bleeding time was significantly reduced upon the cessation of the infusion while maximal inhibition of platelet aggregation was maintained longer. At all of the above regimens, DMP 728 did not result in any significant effects on platelet counts. Furthermore, DMP 728 did not elicit any other platelet unrelated adverse effects over wide range of doses. These data suggest that DMP 728, a low molecular weight platelet GPIIb/IIIa receptor antagonist, is a potent and systemically active antiplatelet agent with reversible effects on bleeding time.

Determination of DMP 728, a IIb/IIIa receptor antagonist, in rat and dog plasma by high-performance liquid chromatography with fluorimetric detection

A specific and sensitive HPLC assay for the determination of DMP 728 in dog and rat plasma has been developed. The method involves solid-phase extraction of DMP 728 and the internal standard from plasma using a C2 column. The extracted compounds are derivatized with benzoin under alkaline conditions. Using a mixture of acetonitrile and 0.1 M potassium phosphate buffer (25:75, v/v, pH 7.4) as mobile phase, the derivatized products are separated on a Regis semipermeable surface C8 column and monitored fluorometrically using 325 nm and 425 nm as excitation and emission wavelengths, respectively. The assay is linear from 2.5 to 1000 ng/ml in dog plasma and from 5 to 1000 ng/ml in rat plasma. The limit of quantitation is 2.5 ng/ml using 0.5 ml of dog plasma and 5 ng/ml using 0.5 ml of rat plasma. The assay has been used in pharmacokinetic studies of DMP 728 in dogs and rats.