Evaluation of the disintegrin, triflavin, in a rat middle cerebral artery thrombosis model

Structure-Function Relationship of the Disintegrin Family: Sequence Signature and Integrin Interaction

Disintegrins are small cysteine-rich proteins found in a variety of snake venom. These proteins selectively modulate integrin function, heterodimeric receptors involved in cell-cell and cell-matrix interaction that are widely studied as therapeutic targets. Snake venom disintegrins emerged from the snake venom metalloproteinase and are classified according to the sequence size and number of disulfide bonds. Evolutive structure and function diversification of disintegrin family involves a stepwise decrease in the polypeptide chain, loss of cysteine residues, and selectivity. Since the structure elucidation of echistatin, the description of the structural properties of disintegrins has allowed the investigation of the mechanisms involved in integrin-cell-extracellular matrix interaction. This review provides an analysis of the structures of all family groups enabling the description of an expanded classification of the disintegrin family in seven groups. Each group presents a particular disulfide pattern and sequence signatures, facilitating the identification of new disintegrins. The classification was based on the disintegrin-like domain of the human metalloproteinase (ADAM-10). We also present the sequence and structural signatures important for disintegrin-integrin interaction, unveiling the relationship between the structure and function of these proteins.

Anti-platelet effect of cumanastatin 1, a disintegrin isolated from venom of South American Crotalus rattlesnake

Disintegrins have been previously described in the venom of several snake families inhibiting signal transduction, cell-cell interactions, and cell-matrix interactions and may have therapeutic potential in heart attacks, thrombotic diseases, and cancers. This investigation describes the first disintegrin isolated from South American Crotalus venom (Venezuelan rattlesnake Crotalus durissus cumanensis), which inhibits platelet adhesion to matrix proteins. C. d. cumanensis crude venom was first separated on a Sephadex G-100 column into 4 fractions (SI to SIV). Crude venom and SIII fraction significantly diminished platelet adhesion to fibrinogen (Fg) and to fibronectin (Fn). Anti-adhesive SIII fraction was further separated by DEAE-Sephacel followed by C-18 reverse phase high performance liquid chromatography (HPLC). The platelet anti-adhesive fraction obtained was designated as cumanastatin-1. This disintegrin has a mass of 7.442 kDa as determined by mass spectrometry (MALDI-TOF/TOF) and pI of 8.5. Cumanastatin-1 also inhibited ADP-induced platelet aggregation with an IC(50) of 158 nM. However, it did not significantly inhibit collagen and thrombin-induced platelet aggregation. Cumanastatin-1 considerably inhibited anti-alpha(IIb)beta(3) integrin binding to platelets in a dose-dependent manner; however, it did not present any effect on the alpha(5)beta(1) integrin or on P-selectin.

Antithrombotic Effects of FK419, a Novel Nonpeptide Platelet GPIIb/IIIa Antagonist, in a Guinea Pig Photochemically Induced Middle Cerebral Artery Thrombosis Model: Comparison with Ozagrel and Argatroban

Platelet activation and subsequent aggregation play a key role in the pathogenesis of ischemic brain damage. Recent studies revealed that enhanced platelet activation is also observed after ischemia, suggesting that secondary thrombus formation might participate in the development of cerebral infarction. The binding of platelet glycoprotein GPIIb/IIIa (integrin alpha(IIb)beta3) to fibrinogen is the final common pathway in platelet aggregation. Therefore, GPIIb/IIIa antagonists might be useful in acute ischemic stroke as well as in the secondary prevention of ischemic stroke. In the present study, we evaluated the effect of three compounds, FK419 ((S)-2-acetylamino-3-[(R)-[1-[3-(piperidin-4-yl) propionyl] piperidin-3-ylcarbonyl] amino] propionic acid trihydrate), a novel nonpeptide GPIIb/IIIa antagonist, ozagrel, a selective thromboxane A(2) synthase inhibitor, and argatroban, a thrombin inhibitor, on middle cerebral artery (MCA) patency and ischemic brain damage using photochemically induced MCA thrombosis model in guinea pigs. FK419, ozagrel, or argatroban was administered 5 min after the termination of photoirradiation. FK419 dose-dependently improved MCA patency by decreasing the total occlusion time, time to continuous reperfusion, and the number of cyclic flow reductions, at doses that inhibited ADP-induced platelet aggregation ex vivo. In contrast, ozagrel only improved total occlusion time, and argatroban showed no improvement in MCA patency. FK419 also reduced ischemic brain damage in a dose-dependent fashion, whereas ozagrel and argatroban did not. Finally, FK419 ameliorated neurological deficits, whereas ozagrel and argatroban did not. These results indicate that FK419, a GPIIb/IIIa antagonist, ameliorates ischemic brain damage by improving MCA patency after occlusion and that FK419 is a promising candidate for the treatment of acute ischemic stroke.

A novel disintegrin-like domain of a high molecular weight metalloprotease inhibits platelet aggregation

Disintegrin is one of the functionally distinct domains in high molecular weight metalloproteases from various snake venoms and generally has an Arg-Gly-Asp (RGD) sequence that is recognized by specific cell surface integrins. A cDNA encoding the disintegrin-like domain of a snake venom metalloprotease was cloned, expressed in Pichia pastoris, and molecular function of the recombinant protein was characterized. The cDNA sequence indicated that the disintegrin-like domain contains an Asp-Glu-Cys-Asp (DECD) sequence in place of the RGD motif. The expressed disintegrin-like protein was designated as halydin and it was able to inhibit human platelet aggregation in a dose-dependent manner. Unlike other typical RGD-disintegrins, the recombinant non-RGD disintegrin, halydin, inhibited platelet aggregation by suppressing platelet adhesion to collagen rather than by blocking fibrinogen binding to glycoprotein (GP) IIb-IIIa on the platelet surface. Experimental evidence suggests that halydin binds to integrin alpha2beta1 on the platelet surface.

Glicoprotein (GP) IIb/IIIa inhibitors for acute stroke treatment

The benefit of antiplatelet therapy remains unclear, although it does appear that aspirin monotherapy started within 48 hours of stroke onset may result in a modest clinical improvement. Glicoprotein (GP) IIb/IIIa antagonists are currently considered the most powerful specific inhibitors of platelet activation in acute thrombosis. Glicoprotein IIb/IIIa inhibitor therapy could merit a prominent role also in the initial management of patients with acute ischemic stroke. Abciximab may be promising in this setting and should be evaluated in further clinical trials.

Combining antiplatelet and thrombolytic therapies for stroke

Pharmacological therapy for acute nonhaemorrhagic stroke has become a reality over the last 5 years. Mechanistically, both thrombolytic (tissue plasminogen activator and urokinase) and antiplatelet (aspirin) monotherapy have demonstrated efficacy. However, unintended actions limit the extent of clinical improvement in each circumstance. For example, in addition to excess bleeding, tissue plasminogen activator therapy has been associated with complement activation, neuronal toxicity and laminin degradation, while aspirin may reduce nitric oxide synthase activity and cerebral blood flow. Attention is now directed toward improving the therapeutic index for each class of agents. Generally, while thrombolytic therapy is focused on developing agents with greater fibrin specificity and safety (that is, a reduction in intracranial haemorrhage rate), the development of antiplatelet agents is primarily focused on achieving greater potency. The latter is being investigated by combining agents with different mechanisms (aspirin and dipyridamole, aspirin and clopidogrel) as well as agents designed to block the glycoprotein IIb/IIIa receptor, the final common pathway for platelet aggregation. Thus, combination therapy using both thrombolytic and antiplatelet agents will further attempt to improve the therapeutic index by increasing potency and improving the safety profile. Anecdotal case studies support the merits of this approach and are consistent with the data reported for myocardial ischaemia and interventional strategies. It is anticipated that drug therapy directed at both thrombolytic and antiplatelet targets will ultimately result in a widened therapeutic window that will allow acute stroke therapy to be administrated to a much greater number of patients than is currently possible.

Dynamic platelet accumulation at the site of the occluded middle cerebral artery and in downstream microvessels is associated with loss of microvascular integrity after embolic middle cerebral artery occlusion

Information is lacking regarding dynamic platelet accumulation at the site of the occluded middle cerebral artery (MCA) and the relationship between platelet aggregation in downstream cerebral microvessels and loss of perfusion and vascular integrity of these microvessels. In the present study, we employed a model of embolic MCA occlusion in the rat to simultaneously measure temporal and spatial profiles of platelet accumulation at the site of the embolus occluding the MCA and within downstream cerebral microvessels. We also measured the integrity of microvessels and matrix metalloproteinase (MMP) activity in ischemic brain. Rats (n=36) were subjected to embolic MCA occlusion. Immunohistochemistry was used to detect microvascular integrity, plasminogen activator inhibitor 1 (PAI-1) and the deposition of fibrin. SDS-PAGE zymography was used to measure MMP2 and MMP9 activities. Accumulation of platelets and increases in PAI-1 immunoreactivity at the site of the embolus occluding the MCA were detected 1 h (n=7) and 4 h (n=7) after ischemia, respectively, and numbers of GPIIb/IIIa immunoreactive downstream cerebral microvessels increased significantly (209+/-59; n=7; P<0.05) 4 h after ischemia, suggesting dynamic platelet aggregation. A significant (n=7; P<0.01) diffuse loss of type IV collagen immunoreactivity in microvessels was temporally associated with platelet GPIIb/IIIa immunoreactivity within the vessels. Triple immunostaining revealed that microvessels containing platelet aggregates exhibited loss of type IV collagen immunoreactivity and both intra- and extra-vascular fibrin deposition, suggesting that intravascular platelet aggregation is associated with decreases in the integrity of the microvascular basal lamina and blood-brain barrier leakage. A significant increase (P<0.05) in MMP9 was detected at 4 h (n=3) and 24 h (n=3) after ischemia but levels of MMP2 were not significantly changed in ischemic brain. Our data suggest that dynamic platelet aggregation in ischemic brain may contribute to time-dependent resistance to fibrinolysis. In addition, platelet deposition and increased MMP9 coincided with degradation of type IV collagen and loss of vascular integrity. These data suggest an important role for post-occlusive distal platelet deposition in the pathophysiology of stroke.

Changes in local cerebral blood flow in photochemically induced thrombotic occlusion model in rats

We demonstrated earlier that in a photochemically induced thrombotic occlusion model, a reperfusion-like phenomenon may be involved in the progress of brain damage. Therefore, we now investigated changes in local cerebral blood flow in a photochemical model compared with changes in a thermocoagulated occlusion model, using autoradiography. At 5 min, and 3, 6 and 24 h after middle cerebral artery occlusion, local cerebral blood flow was measured by intravenous injection of 4-iodo[N-methyl-14C]antipyrine (20 mu Ci). In the ischemic core zone, the reduction in blood flow was similar in the two models. However, blood flow in the ischemic border zone in the photochemical model decreased transiently in the third hour after ischemia and then increased again, while the blood flow in a thermocoagulated model continued to decrease. Time courses of brain damage formation in both models were no different up to 24 h. These findings suggest that the transient reduction in cerebral blood flow in the third hour following ischemia may contribute to a reperfusion-like phenomenon in a photochemical model.

Interspecies comparison of the antiplatelet, antithrombotic, and hemorrhagic effects of SR 121566A, a novel nonpeptide GP IIb/IIIa antagonist

We report the results from an interspecies comparison of the antiplatelet, antithrombotic, and hemorrhagic actions of SR 121566A, a novel nonpeptide antiplatelet agent with high affinity and specificity for the GP IIb/IIIa complex. SR 121566A exhibited in vitro antiplatelet activity against adenosine diphosphate (ADP)-induced aggregation with a rank order of potency [humans = baboons = dogs] > marmosets > guinea pigs > rabbits. These in vitro findings were predictive for the ex vivo antiplatelet potency after i.v. administration of SR 121566A to dogs, guinea pigs, and rabbits [median effective dose (ED50) values, 0.02, 0.05, and 0.15 mg/kg]. The antiplatelet actions of SR 121566A translated into an acute antithrombotic effect in an arteriovenous shunt model after i.v. administration in dogs, guinea pigs, rabbits, and marmosets (ED50, 0.08, 0.10, 0.50, and 0.007 mg/kg). Hemorrhagic effects of SR 121566A were observed in guinea pigs and rabbits at doses that represented 2-3 times the antithrombotic ED50, whereas in marmosets, no bleeding was observed at the antithrombotic ED90. These results demonstrate that SR 121566A exhibits favorable actions in terms of antithrombotic potency and hemostatic safety in different animal species, suggesting that, in humans, SR 121566A will be a good candidate as an antithrombotic compound.

Antiplatelet therapy in acute cerebral ischemia

BACKGROUND

Improved recognition of stroke signs and symptoms has paralleled the development of pharmacological strategies that may be examined to reduce stroke mortality and morbidity. Presently, tissue plasminogen activator is the only therapy that significantly improves outcome in acute stroke, with no agent demonstrating a significant reduction in mortality.

SUMMARY OF REVIEW

Antiplatelet agents are a heterogenous class of drugs that have been successfully used for more than 2 decades in secondary stroke prevention. These agents include aspirin, with or without dipyridamole, and more recently, the adenosine antagonists ticlopidine and clopidogrel. However, studies of the use of antiplatelet agents within 48 hours of the ictus have examined only aspirin. Only 1 study, the Multicentre Acute Stroke Trial-Italy (MAST-I), entered patients within 6 hours of the ictus. These data suggest that an improvement in mortality may be related to the speed of administration. No significant adverse events were noted with early antiplatelet monotherapy. However, MAST-I did note a significant increase in early mortality in patients receiving aspirin plus streptokinase, a finding not adequately explained by an increase in the intracranial hemorrhage rate.

CONCLUSIONS

The use of antiplatelet therapy in acute stroke, clinical or experimental, has only recently received attention. It is likely that the use of antiplatelet agents for acute stroke therapy will be less restrictive than that currently seen for thrombolytics. Future studies should include an examination of those agents that have previously demonstrated efficacy in secondary stroke prevention, most notably, aspirin. The recognition that all platelet stimuli share a final common pathway that is dependent on the surface glycoprotein IIb/IIIa (fibrinogen) receptor has resulted in the development of various agents which block this receptor and are currently the focus for clinical trials. The role of nitric oxide in stroke therapy will depend on minimizing the hypotensive side effects of this agent. Stroke models are needed to provide preliminary data on the efficacy of antiplatelet therapy, especially as relates to the interaction of antiplatelet agents with thrombolytics.

Neuroprotective effects depend on the model of focal ischemia following middle cerebral artery occlusion

The purpose of the present study was to compare the characteristics of the photochemical-induced thrombotic occlusion model and the thermocoagulated occlusion model of the middle cerebral artery in rats. We evaluated the neuroprotective effects of a NMDA receptor antagonist, (+)-MK-801 (dizocilpine, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptan-5,10-imine), an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, YM90K (6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione monohydrochloride), a Ca2+ channel antagonist, S-312-d (S-(+)-methyl-4,7-dihydro-3-isobutyl-6-methyl-4-(3-nitrophenyl)-thieno[2 ,3-b]pyridine-5-carboxylate), the radical scavengers, MCI-186 (3-methyl-1-phenyl-2-pyrazolin-5-one) and EPC-K1 (L-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyl-tridecyl)-2H-1-be nzopyran-6yl-hydrogen phosphate] potassium salt), and a calcineurin inhibitor, FK506 (tacrolimus, Prograf). Although all tested agents in the present study attenuated the brain damage in the photochemical-induced thrombotic occlusion model, the radical scavengers did not attenuate the brain damage in the thermocoagulated occlusion model. The time course of brain damage and brain edema formation in the two models was examined. The time course of brain damage was not different in the two models, but the time course of brain edema was quite different. Brain edema formation in the photochemical-induced thrombotic occlusion model was significantly greater (P < 0.01) than that in the thermocoagulated occlusion model at all time point studied until 24 h after occlusion of the middle cerebral artery. The present study suggests that the photochemical-induced thrombotic occlusion model has characteristics of both permanent ischemia and ischemia-reperfusion.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.