Evaluation of a GPIIb/IIIa antagonist YM337 in a primate model of middle cerebral artery thrombosis

An Updated Review on Glycoprotein IIb/IIIa Inhibitors as Antiplatelet Agents: Basic and Clinical Perspectives

The glycoprotein (GP) IIb/IIIa receptor is found integrin present in platelet aggregations. GP IIb/IIIa antagonists interfere with platelet cross-linking and platelet-derived thrombus formation through the competition with fibrinogen and von Willebrand factor. Currently, three parenteral GP IIb/IIIa competitors (tirofiban, eptifibatide, and abciximab) are approved for clinical use in patients affected by percutaneous coronary interventions (PCI) in the location of acute coronary syndrome (ACS). GP IIb/IIIa antagonists have their mechanism of action in platelet aggregation prevention, distal thromboembolism, and thrombus formation, whereas the initial platelet binding to damage vascular areas is preserved. This work is aimed to provide a comprehensive review of the significance of GP IIb/IIIa inhibitors as a sort of antiplatelet agent. Their mechanism of action is based on factors that affect their efficacy. On the other hand, drugs that inhibit GP IIb/IIIa already approved by the FDA were reviewed in detail. Results from major clinical trials and regulatory practices and guidelines to deal with GP IIb/IIIa inhibitors were deeply investigated. The cardiovascular pathology and neuro-interventional surgical application of GP IIb/IIIa inhibitors as a class of antiplatelet agents were developed in detail. The therapeutic risk/benefit balance of currently available GP IIb/IIa receptor antagonists is not yet well elucidated in patients with ACS who are not clinically evaluated regularly for early cardiovascular revascularization. On the other hand, in patients who have benefited from PCI, the antiplatelet therapy intensification by the addition of a GP IIb/IIIa receptor antagonist (intravenously) may be an appropriate therapeutic strategy in reducing the occurrence of risks of thrombotic complications related to the intervention. Development of GP IIb/IIIa inhibitors with oral administration has the potential to include short-term antiplatelet benefits compared with intravenous GP IIb/IIIa inhibitors for long-term secondary preventive therapy in cardiovascular disease. But studies showed that long-term oral administration of GP IIb/IIIa receptor inhibitors has been ineffective in preventing ischemic events. Paradoxically, they have been linked to a high risk of side effects by producing prothrombotic and pro-inflammatory events.

Prasugrel reduces ischaemic infarct volume and ameliorates neurological deficits in a non-human primate model of middle cerebral artery thrombosis

Several clinical trials have demonstrated the benefits of thienopyridine monotherapy in ischaemic stroke patients. Non-human primate models of ischaemic stroke have been used for various antithrombotic agents; however, to the best of our knowledge, there is no data on the effects of P2Y12 antagonists in models, such as the thrombotic middle cerebral artery occlusion (MCAO) monkey model. Accordingly, it remains unclear what level of inhibition of platelet aggregation (IPA) is required for optimal treatment of ischaemic stroke. In the present study, we investigated the effects of prasugrel, a third-generation thienopyridine antiplatelet drug, on platelet aggregation, thrombus formation and cerebral infarct volume in a non-human primate model. Daily oral administration of prasugrel resulted in significant and stable platelet inhibitory effects on Day 3, with IPA values ranging from 31% to 36% at 0.3mg/kg/day and from 44% to 50% at 1mg/kg/day. These IPA levels encompassed values observed in clinical trials of clopidogrel, and were thus selected for further study. In the thrombotic MCAO model, prasugrel increased MCA patency in a dose-dependent manner and significantly reduced ischaemic infarct volume by approximately 70% at 0.3mg/kg/day and 90% at 1mg/kg/day without increasing haemorrhagic infarction. Prasugrel also significantly reduced neurological deficit scores by 60% at 0.3mg/kg/day and 80% at 1mg/kg/day. In conclusion, prasugrel treatment resulted in effective reduction of ischaemic infarction and an associated improvement in neurological function without increasing haemorrhagic infarction. These data suggest that prasugrel monotherapy would be effective for the prevention of thrombotic stroke.

A more consistent intraluminal rhesus monkey model of ischemic stroke

Endovascular surgery is advantageous in experimentally induced ischemic stroke because it causes fewer cranial traumatic lesions than invasive surgery and can closely mimic the pathophysiology in stroke patients. However, the outcomes are highly variable, which limits the accuracy of evaluations of ischemic stroke studies. In this study, eight healthy adult rhesus monkeys were randomized into two groups with four monkeys in each group: middle cerebral artery occlusion at origin segment (M1) and middle cerebral artery occlusion at M2 segment. The blood flow in the middle cerebral artery was blocked completely for 2 hours using the endovascular microcoil placement technique (1 mm × 10 cm) (undetachable), to establish a model of cerebral ischemia. The microcoil was withdrawn and the middle cerebral artery blood flow was restored. A reversible middle cerebral artery occlusion model was identified by hematoxylin-eosin staining, digital subtraction angiography, magnetic resonance angiography, magnetic resonance imaging, and neurological evaluation. The results showed that the middle cerebral artery occlusion model was successfully established in eight adult healthy rhesus monkeys, and ischemic lesions were apparent in the brain tissue of rhesus monkeys at 24 hours after occlusion. The rhesus monkeys had symptoms of neurological deficits. Compared with the M1 occlusion group, the M2 occlusion group had lower infarction volume and higher neurological scores. These experimental findings indicate that reversible middle cerebral artery occlusion can be produced with the endovascular microcoil technique in rhesus monkeys. The M2 occluded model had less infarction and less neurological impairment, which offers the potential for application in the field of brain injury research.

The application of tetrahydroisoquinoline-3-carbonyl-TARGD(F)F as an anti-thrombotic agent having dual mechanisms of action

Platelet surface glycoproteins P-selectin and GPIIb/IIIa are implicated in the formation of platelet-fibrin-leukocyte thrombus and platelet-fibrin-platelet thrombus, respectively. In the current study, taking N-(3S-tetrahydroisoquinoline-3-carbonyl)-Thr-Ala-Arg-Gly-Asp-(Phe)-Phe (IQCA-TAFF) as a model compound, the molecular modeling, synthesis, and an evaluation system for a novel anti-thrombotic agent were investigated. The synthesis of IQCA-TAFF was achieved by coupling 3S-tetrahydro-isoquinoline-3-carboxylic acid (IQCA) and Thr-Ala-Arg-Gly-Asp(Phe)-Phe (TAFF). The molecular modeling indicated that IQCA-TAFF was able to occupy the active site pocket of P-selectin with its IQCA moiety and to block GPIIb/IIIa fibrinogen-binding sites with its TAFF moiety, respectively. These are consistent with the dual inhibition of the expressions of P-selectin and GPIIb/IIIa, and with the in vitro anti-platelet aggregation activity of IQCA-TAFF. Besides, the dual suppression of P-selectin and GPIIb/IIIa leads to significant in vivo efficacy of IQCA-TAFF, 500-fold higher than those of IQCA and TAFF, respectively. Transmission electron microscopy (TEM) images indicated that in water, IQCA-TAFF concentration-dependently formed nano-globes. The molecular modeling, in vitro bioassay, in vivo bioassay, action mechanism investigation, and nano-image visualization together constitute a model system to characterize the anti-thrombotic agent capable of simultaneously inhibiting P-selectin and GPIIb/IIIa mediated thrombosis.

Thrombotic stroke in the anesthetized monkey (Macaca mulatta): characterization by MRI--a pilot study

BACKGROUND

The lack of a relevant stroke model in large nonhuman primates hinders the development of innovative diagnostic/therapeutic approaches concerned with this cerebrovascular disease. Our objective was to develop a novel and clinically relevant model of embolic stroke in the anesthetized monkey that incorporates readily available clinical imaging techniques and that would allow the possibility of drug delivery including strategies of reperfusion.

METHODS

Thrombin was injected into the lumen of the middle cerebral artery (MCA) in 12 anesthetized (sevoflurane) male rhesus macaques (Macaca mulatta). Sequential MRI studies (including angiography, FLAIR, PWI, DWI, and gadolinium-enhanced T1W imaging) were performed in a 3T clinical MRI. Physiological and biochemical parameters were monitored throughout the investigations.

RESULTS

Once standardized, the surgical procedure induced transient occlusion of the middle cerebral artery in all operated animals. All animals studied showed spontaneous reperfusion, which occurred some time between 2 h and 7 days post-ictus. Eighty percent of the studied animals showed diffusion/perfusion mismatch. The ischemic lesions at 24 h spared both superficial and profound territories of the MCA. Some animals presented hemorrhagic transformation at 7 days post-ictus.

CONCLUSION

In this study, we developed a pre-clinically relevant model of embolic stroke in the anesthetized nonhuman primate.

Characterization of a novel thrombotic middle cerebral artery occlusion model in monkeys that exhibits progressive hypoperfusion and robust cortical infarction

In an attempt to establish a thrombotic middle cerebral artery (MCA) occlusion model using cynomolgus monkeys, we measured the blood flow in the main MCA tract and cerebral cortex, brain damage, and neurological deficits, and compared them with those of mechanical MCA occlusion model. Thrombotic occlusion was induced photochemically by green light application on the MCA following rose bengal treatment; mechanical occlusion was induced by MCA clipping for 3h. Patency of the main MCA tract showed two patterns in the thrombotic model: permanent occlusion or cyclical flow reduction (CFR). Regional cerebral blood flow (rCBF) decreased during occlusion followed by post-ischemic hyperperfusion in the clipping model, whereas rCBF reduction expanded time-dependently in the thrombotic occlusion model. Brain infarction and neurological scores in the thrombotic occlusion model were significantly larger than those in the clipping occlusion model. In histological assessment, microthrombi containing myeloperoxidase- and fibrinogen-positive cells were observed in the cortex following the thrombotic but not clipping occlusion. These results collectively suggest that this thrombotic MCA occlusion model, because it shows impairment of cerebral microcirculation, could provide a vital platform for understanding progressive ischemia as well as for evaluating potential therapeutic drugs.

Emergency Administration of Abciximab for Treatment of Patients With Acute Ischemic Stroke

Background and Purpose— Because of its success in treatment of acute cardiac ischemia, there is interest in the use of abciximab for treating patients with acute ischemic stroke. A previous dose-escalation study determined that abciximab could be given safely in a regimen of 0.25 mg/kg intravenous bolus followed by a 12-hour infusion at 0.125 μg/kg per minute (maximum 10 μg/min). This study was performed to obtain more information about the safety and potential efficacy of abciximab in patients with stroke.

Methods— An international randomized, double-blind, placebo-controlled phase 2 trial enrolled 400 patients within 6 hours of onset of ischemic stroke. The primary safety outcome was the rate of symptomatic hemorrhage that occurred during the first 5 days after stroke. The primary efficacy measure was the distribution of outcomes at 3 months after stroke using the modified Rankin Scale (mRS) based on an ordinal regression model of outcomes, adjusting for baseline severity of stroke, age, and interval from stroke.

Results— Symptomatic intracranial hemorrhage within 5 days was diagnosed in 7 of 195 (3.6%) patients treated with abciximab and 2 of 199 (1%) patients given placebo (odds ratio [OR], 3.7; P =0.09; 95% confidence interval [CI], 0.7 to 25.9). Asymptomatic hemorrhagic transformation was detected by brain imaging in 24 patients administered abciximab and 33 patients receiving placebo (OR, 0.74; P =0.25; 95% CI, 0.4 to 1.3). Treatment with abciximab showed a nonsignificant shift in favorable outcomes as measured by mRS scores at 3 months (OR, 1.20; P =0.33; 95% CI, 0.84 to 1.70).

Conclusions— Intravenously administered abciximab can be given to patients with a reasonable degree of safety. The trial also suggests that abciximab could improve outcomes at 3 months after stroke. A larger randomized, double-blind, placebo-controlled trial is necessary to test the efficacy of abciximab.

FK419, a nonpeptide platelet glycoprotein IIb/IIIa antagonist, ameliorates brain infarction associated with thrombotic focal cerebral ischemia in monkeys: comparison with tissue plasminogen activator

The binding of platelet glycoprotein (GP) IIb/IIIa to fibrinogen is the final common pathway in platelet aggregation, a process known to play a key role in the pathogenesis of ischemic brain damage. We compared the effects of FK419, a novel nonpeptide GPIIb/IIIa antagonist, with recombinant tissue plasminogen activator (rt-PA) on middle cerebral artery (MCA) patency and ischemic brain damage in a thrombotic stroke model in squirrel monkeys. FK419 not only inhibited in vitro platelet aggregation (IC50: 88 nmol/L), but also showed disaggregatory activity to aggregated platelet (EC50: 286 nmol/L). FK419 dose-dependently reduced the time to first reperfusion and total occlusion time of MCA blood flow when administered immediately after the termination of photoirradiation. FK419 reduced cerebral infarction and ameliorated neurologic deficits with similar dose-dependency. Although rt-PA reduced the time to first reperfusion, total occlusion time, and cerebral infarction, it did not significantly ameliorate neurologic deficits and induced petechial intracerebral hemorrhages. These results indicate: (1) FK419 restored cerebral blood flow after thrombotic occlusion of MCA, (2) FK419 reduced ischemic brain injury by its thrombolytic actions in a non-human primate stroke model, and (3) FK419 has superior antithrombotic efficacy and is safer than rt-PA.

FK419, a novel nonpeptide GPIIb/IIIa antagonist, restores microvascular patency and improves outcome in the guinea-pig middle cerebral artery thrombotic occlusion model: comparison with tirofiban

The antithrombotic efficacy of FK419, a novel nonpeptide platelet glycoprotein IIb/IIIa antagonist, was compared with tirofiban in guinea-pigs. FK419 and tirofiban similarly inhibited platelet aggregation in vitro (IC50 values: 0.43+/-0.076 and 0.41+/-0.053 micromol/L) and dispersed aggregated platelets (EC50 values: 2.3+/-0.88 and 2.0+/-0.81 micromol/L). FK419 inhibited retention of platelets and neutrophils in a collagen-coated bead column with greater potency than tirofiban (IC50 values of 0.90+/-0.133 and 2.4+/-0.21 micromol/L for platelet retention and 0.32+/-0.078 and 0.57+/-0.180 micromol/L for neutrophil retention). When FK419 or tirofiban were administered after photochemically induced middle cerebral artery (MCA) occlusion in guinea-pigs, they dose-dependently improved MCA patency. FK419 reduced neurological deficits and ischemic brain damage in a dose-dependent fashion, whereas tirofiban did not. Reduced regional cerebral blood flow in the striatum gradually returned to the preoccluded level with FK419 treatment; however, no restoration was observed with tirofiban even though the MCA was recanalized. These results indicate that FK419 ameliorates ischemic brain damage by not only lysing the obstructive thrombus in MCA but also preventing or restoring microcirculation deficits after occlusion/reperfusion, suggesting that FK419 would be an attractive intervention for the treatment of ischemic stroke patients.

Restoration of middle cerebral artery thrombosis by novel glycoprotein IIb/IIIa antagonist FK419 in guinea pig

We compared the antithrombotic efficacy of FK419 [(S)-2-acetylamino-3-[(R)-[1-[3-(piperidin-4-yl)propionyl]piperidin-3-ylcarbonyl]amino] propionic acid trihydrate], a novel nonpeptide glycoprotein IIb/IIIa antagonist, with recombinant tissue plasminogen activator (rt-PA) and other antithrombotic agents (aspirin, ozagrel, argatroban and heparin). FK419 not only inhibited ADP- and collagen-induced guinea pig platelet aggregation, but also induced disaggregation for ADP-induced aggregated platelets in vitro. In the photochemically induced middle cerebral artery thrombosis model in guinea pigs, FK419 dose-dependently shortened the time to first reperfusion and the total middle cerebral artery occlusion time and reduced ischemic brain damage and ameliorated neurological deficits measured 24 h after middle cerebral artery occlusion. Rt-PA similarly improved the middle cerebral artery patency, brain damage and neurological deficits. Neither aspirin, ozagrel, argatroban nor heparin restored the middle cerebral artery blood flow and improved the brain damage or neurological deficits. These results demonstrated that novel glycoprotein IIb/IIIa antagonist FK419 could disperse thrombus and ameliorated ischemic brain damage, suggesting that FK419 would be an attractive intervention for stroke patients.

Noncardiac applications of glycoprotein IIb/IIIa inhibitors

Platelet activation and aggregation have become increasingly recognized as the primary processes involved in the cascade that leads to thrombus formation in atherosclerotic vascular disease. Glycoprotein IIb/IIIa receptor inhibitors (GPI) favorably impact thrombus formation and distal embolization by inhibiting the final common pathway of platelet aggregation. Glycoprotein IIb/IIIa inhibitors have been used effectively in a wide variety of clinical scenarios including unstable angina, non-ST segment elevation myocardial infarction, ST segment elevation myocardial infarction, and low and high risk percutaneous coronary interventions with and without intracoronary stenting, however there is limited data regarding the use of these potent antiplatelet agents in the setting of extracardiac vascular disease. This article will review the non-cardiac applications of glycoprotein IIb/IIIa inhibitors in the setting of acute ischemic stroke, carotid and vertebral angioplasty and stenting, acute critical limb ischemia, and percutaneous interventions in peripheral arterial occlusive disease.

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.

Tacrolimus, a potential neuroprotective agent, ameliorates ischemic brain damage and neurologic deficits after focal cerebral ischemia in nonhuman primates

Tacrolimus (FK506), an immunosuppressive drug, is known to have potent neuroprotective activity and attenuate cerebral infarction in experimental models of stroke. Here we assess the neuroprotective efficacy of tacrolimus in a nonhuman primate model of stroke, photochemically induced thrombotic occlusion of the middle cerebral artery (MCA) in cynomolgus monkeys. In the first experiment, tacrolimus (0.01, 0.032, or 0.1 mg/kg) was intravenously administered immediately after MCA occlusion, and neurologic deficits and cerebral infarction volumes were assessed 24 hours after the ischemic insult. Tacrolimus dose-dependently reduced neurologic deficits and infarction volume in the cerebral cortex, with statistically significant amelioration of neurologic deficits at 0.032 and 0.1 mg/kg and significant reduction of infarction at 0.1 mg/kg. In the second experiment, the long-term efficacy of tacrolimus on neurologic deficits and cerebral infarction was assessed. Vehicle-treated monkeys exhibited persistent and severe deficits in motor and sensory function for up to 28 days. A single intravenous bolus injection of tacrolimus (0.1 or 0.2 mg/kg) produced long-lasting amelioration of neurologic deficits and significant reduction of infarction volume. In conclusion, we have provided compelling evidence that a single dose of tacrolimus not only reduces brain infarction but also ameliorates long-term neurologic deficits in a nonhuman primate model of stroke, strengthening the view that tacrolimus might be beneficial in treating stroke patients.

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.

Comparative studies of a humanized anti-glycoprotein IIb/IIIa monoclonal antibody, YM337, and abciximab on in vitro antiplatelet effect and binding properties

The effects of YM337, the Fab fragment of a humanized anti-glycoprotein IIb/IIIa (GPIIb/IIIa) monoclonal antibody C4G1, on in vitro platelet function and binding properties were compared with those of abciximab, the Fab fragment of the human/murine chimeric anti-GPIIb/IIIa monoclonal antibody 7E3. Both agents completely inhibited platelet aggregation caused by all agonists tested except ristocetin. Further, both inhibited human platelet adhesion to von Willebrand factor, fibrinogen, fibronectin and subendothelial matrix with similar potency. Fibrinogen binding to washed platelets was dose-dependently inhibited by both agents. In binding assay using 125I-YM337 and 125I-abciximab, Kd values determined with platelet-rich plasma were 6.74 +/- 0.56 nM for YM337 and 6.65 +/- 1.45 nM for abciximab, and the number of binding sites were 42,700 +/- 3,000 for YM337 and 76,000 +/- 5,400 for abciximab. GPIIb/IIIa was precipitated from the solubilized fraction of platelets by both agents. In contrast, integrin alphavbeta3 was precipitated from the solubilized fraction of human umbilical vein endothelial cells by abciximab but not by YM337. Fibrinogen binding to purified GPIIb/IIIa was dose-dependently inhibited by both agents. In contrast, vitronectin binding to purified integrin alphavbeta3 was dose-dependently inhibited by abciximab but not by YM337, supporting the idea that abciximab reacts to integrin alphavbeta3. Therefore, YM337 was suggested to bind to a different epitope of GPIIb/IIIa from abciximab. These results suggest that YM337 specifically acts on platelet GPIIb/IIIa receptors and has similar inhibitory properties on platelet aggregation and platelet adhesion to abciximab.

Vertebrobasilar thrombolysis with intravenous tirofiban: case report

The use of thrombolytic agents in the setting of established cerebral infarction is limited by concerns for hemorrhagic transformation. Novel thrombolytic approaches, which have received minimal consideration, may be associated with lower risks of hemorrhage. We illustrate vertebrobasilar thrombolysis with intravenous tirofiban, a selective platelet glycoprotein IIb/IIIa receptor antagonist, and discuss the potential thrombolytic properties of this class of antithrombotics.

Abciximab for the treatment of acute distal embolization associated with internal carotid artery angioplasty

Carotid artery angioplasty and stent placement (CAS) can be complicated by procedure-related distal embolization and thrombus formation, potentially resulting in neurological sequelae. Patient A had CAS of left internal carotid artery and had loss of vision in the left eye with no flow in the left ophthalmic artery. Patient B had CAS of right internal carotid artery and developed left sided hemiparesis with total occlusion of the right middle cerebral artery. A selective intra-arterial injection of 5 mg of abciximab locally followed by a bolus of 5 mg of abciximab intravenously resulted in complete resolution of the filling defect on repeat angiography after 10 min in both patients.

Differential antiplatelet effects of various glycoprotein IIb-IIIa antagonists

The blockade of platelet glycoprotein IIb-IIIa (GPIIb-IIIa) was recently introduced as a new antiplatelet strategy. At present, various GPIIb-IIIa inhibitors are available to treat patients with acute coronary syndrome or when undergoing percutaneous coronary interventions. The current study systematically evaluates the antiplatelet effects of GPIIb-IIIa inhibitors in clinical use. Using conformation-dependent monoclonal antibodies [ligand-induced binding sites (LIBS-1), PMI-1] and flow cytometry, we showed that the GPIIb-IIIa antagonists abciximab, integrelin, lamifiban, and tirofiban, but not EMD 122347 or YM 337, induced LIBS activity of platelet GPIIb-IIIa. The LIBS activity of GPIIb-IIIa antagonists correlates with a proaggregatory response of fixed platelets pretreated with GPIIb-IIIa antagonists (intrinsic activity). All tested GPIIb-IIIa antagonists completely inhibit concentration-dependent ADP (20 micromol/l)-induced aggregation. In contrast, substantial TRAP (25 micromol/l)-induced platelet aggregation still occurs even at high inhibitor concentrations of the tested GPIIb-IIIa antagonists. In addition, we show that GPIIb-IIIa antagonists are poor inhibitors of platelet release reaction (ATP and P-selectin secretion) especially when strong agonists such as TRAP are used to activate platelets. Inhibition of platelet procoagulant activity (thrombin generation) by GPIIb-IIIa antagonists is dependent on the type and concentration of antagonists and on the strength of stimulus (thrombin, tissue factor) used to induce platelet-dependent thrombin generation. The present data show that significant pharmacological differences exist between GPIIb-IIIa antagonists that may have consequences for antithrombotic strategies and for future drug development.

Microvascular consequences of thrombosis in small venules: an in vivo microscopic study using a novel model in the ear of the hairless mouse

Little is known of the development of chronic microvascular alterations following small vessel thrombosis, which is probably due to the lack of appropriate experimental models. Herein we report the first results on thrombosis-associated long-term changes of microvascular permeability and vessel tortuosity and diameter and blood perfusion using the ear of the hairless mouse, and intravital fluorescence microscopy. Thrombosis was induced photochemically in small venules (diameter: 75 to 100 microm) using light/dye exposure (fluorescein isothiocyanate-dextran 150,000), and the microcirculation compromised by the blockade of blood drainage was analyzed before and 30 minutes after induction of thrombosis as well as repeatedly over a 28-day observation period. Thrombus formation resulted in a marked increase (p<0.05) of microvascular permeability (0.85+/-0.11) when compared with baseline values (0.46+/-0.04). Permeability remained elevated (p<0.05) at days 1 (0.67+/-0.07), 3 (0.58+/-0.02), and 7 (0.60+/-0.06), but returned to normal after 28 days (0.43+/-0.03). Tortuosity, diameter, and red blood cell velocity of venular segments, located upstream of thrombus formation, were found unchanged during the entire 28-day observation period. This was probably due to the fact that blood flow from the thrombosis-affected tissue was frequently drained into nonaffected tissue via preexisting "through-fare" channels, serving as venulo-venular collaterals. In accordance, in 10 to 20% of these venular segments the direction of blood perfusion was found changed, while those changes were only rarely observed in venular vessel segments of the nonthrombotic contralateral ears. We conclude that thrombosis in small cutaneous venules is primarily characterized by an increased vascular permeability, reflecting an inflammatory response, similar to what is known from thrombophlebitis in patients. The model presented herein may be a versatile tool to study pathogenesis of chronic microcirculatory derangements in microthrombosis and their prevention by novel therapeutic strategies.

ME3277, a GPIIb/IIIa antagonist reduces cerebral infarction without enhancing intracranial hemorrhage in photothrombotic occlusion of rabbit middle cerebral artery

GPIIb/IIIa antagonists are expected to have a beneficial effect on acute cerebral infarction, however, the occurrence of intracranial hemorrhage has not been as widely investigated. A rabbit focal thrombotic occlusion model of the middle cerebral artery was established by creating a photochemical reaction between green light and Rose Bengal. Hemorrhagic transformation was common in the area of cerebral infarction. Using this model, the effect of a GPIIb/IIIa antagonist, ME3277 (low dose, (L); 0.15 mg/kg + 0.125 mg/kg x h, middle dose, (M); 0.3 mg/kg + 0.25 mg/kg x h and high dose, (H); 0.6 mg/kg + 0.5 mg/kg x h), aspirin (20 mg/kg) and sodium ozagrel (thromboxane A2 synthase inhibitor, 1 mg/kg + 2 mg/ kg x h) were evaluated. Drugs were intravenously administrated 30 minutes after the photochemical reaction for 24 hours. Aspirin inhibited the ex vivo platelet aggregation induced by arachidonic acid and collagen but not by adenosine diphosphate (ADP), while sodium ozagrel only inhibited the arachidonic acid-induced aggregation. ME3277 dose-dependently inhibited the platelet aggregation induced by all the inducers (approximately 60% in L, 80% in M, and 90% in H). At 24 hours of middle cerebral artery (MCA) occlusion, infarct volume was significantly reduced by aspirin and each dose of ME3277. These agents improved neurologic deficits, with ME3277 being more potent than aspirin. Sodium ozagrel did not alter the infarct volume nor neurologic deficits. No drug was found to worsen hemorrhage volume despite increasing bleeding time (2-3 fold) in the skin. In this model, the occluded artery was spontaneously recanalized and re-thrombosed frequently. One mechanism by which antiplatelet agents reduced infarct volume was inhibition of rethrombosis of the MCA. These results suggest that treatment with a GPIIb/IIIa antagonist is a useful intervention for acute cerebral infarction prolonging dose bleeding time to 3 times the basal value.

Potential role of abciximab in ischemic cerebrovascular disease

Abciximab is an intravenously administered antiplatelet agent that could be used alone or in conjunction with thrombolytic therapy to treat patients with acute ischemic stroke. However, experience with medication for treatment of patients with stroke is limited. Symptomatic intracranial bleeding is the most common serious complication of therapies that aim at restoring or improving blood flow to the brain after stroke and it is the most likely potential serious complication of the use of abciximab in this setting. Preliminary data suggest that abciximab is not associated with a prohibitively high rate of symptomatic intracranial hemorrhage. Further research is needed to determine the safety and potential efficacy of abciximab for treatment of ischemic stroke.

Abciximab in acute ischemic stroke. A randomized, double-blind, placebo-controlled, dose-escalation study

BACKGROUND AND PURPOSE

Abciximab is a potent parenterally administered platelet glycoprotein IIb/IIIa antagonist. Because this agent has been shown to improve outcomes in coronary artery disease, there is interest to evaluate whether it could improve cerebral perfusion and outcomes after ischemic stroke. This study was designed to evaluate the safety of abciximab in acute ischemic stroke and to obtain pilot efficacy data.

METHODS

We conducted a randomized, double-blind, placebo-controlled, dose-escalation trial. Seventy-four eligible and consenting patients presenting within 24 hours after ischemic stroke onset at 38 study sites were randomly allocated to receive either an escalating dose of abciximab (54 patients) or placebo (20 patients) in a ratio of 3:1. We studied 4 escalating doses of abciximab. Patients underwent a scheduled follow-up head CT scan 24 to 36 hours after the completion of study agent administration to monitor for bleeding complications and were evaluated through 3 months.

RESULTS

There were no cases of major intracranial hemorrhage. Asymptomatic parenchymal hemorrhages were detected on post-study agent CT in 4 of 54 abciximab patients (7%) and in 1 of 20 placebo patients (5%). Six additional abciximab patients had asymptomatic hemorrhagic lesions detected by unscheduled brain imaging during their follow-up period. Nine of 11 patients with asymptomatic hemorrhage had a baseline National Institutes of Health Stroke Scale score >14. At 3 months, there was a trend toward a higher rate of minimal residual disability (Barthel Index > or =95 or modified Rankin scale < or =1) among abciximab patients compared with those who received placebo.

CONCLUSIONS

Abciximab appears to be safe when administered up to 24 hours after stroke onset, and it might improve functional outcome.

Successful cerebral artery stent placement for total occlusion of the vertebrobasilar artery in a patient suffering from acute stroke. Case report

A 64-year-old man suffering from crescendo brainstem symptoms due to acute total occlusion of the vertebrobasilar artery was successfully treated by cerebral artery stent placement. The total occlusion of a long segment of the vertebrobasilar artery was completely recanalized by implanting two flexible, balloon-expandable coronary stents. The patient's clinical outcome 30 days later was favorable. No complications occurred during or after the procedure. This therapeutic option may prove to be a useful means to revascularize an acute total occlusion of the vertebrobasilar artery.