Optimal antagonism of GPIIb/IIIa favors platelet adhesion by inhibiting thrombus growth. An ex vivo capillary perfusion chamber study in the guinea pig

Platelet accumulation in an endothelium-coated elastic vein valve model of deep vein thrombosis is mediated by GPIbα-VWF interaction

Deep vein thrombosis is a life-threatening disease that takes millions of people's lives worldwide. Given both technical and ethical issues of using animals in research, it is necessary to develop an appropriate in vitro model that would recapitulate the conditions of venous thrombus development. We present here a novel microfluidics vein-on-a-chip with moving valve leaflets to mimic the hydrodynamics in a vein, and Human Umbilical Vein Endothelial Cell (HUVEC) monolayer. A pulsatile flow pattern, typical for veins, was used in the experiments. Unstimulated human platelets, reconstituted with the whole blood, accumulated at the luminal side of the leaflet tips proportionally to the leaflet flexibility. Platelet activation by thrombin induced robust platelet accrual at the leaflet tips. Inhibition of glycoprotein (GP) IIb-IIIa did not decrease but, paradoxically, slightly increased platelet accumulation. In contrast, blockade of the interaction between platelet GPIbα and A1 domain of von Willebrand factor completely abolished platelet deposition. Stimulation of the endothelium with histamine, a known secretagogue of Weibel-Palade bodies, promoted platelet accrual at the basal side of the leaflets, where human thrombi are usually observed. Thus, platelet deposition depends on the leaflet flexibility, and accumulation of activated platelets at the valve leaflets is mediated by GPIbα-VWF interaction.

Comparison of antithrombotic efficacy between edoxaban, a direct factor Xa inhibitor, and fondaparinux, an indirect factor Xa inhibitor under low and high shear rates

Edoxaban is an oral, direct factor Xa (FXa) inhibitor under late-phase clinical development. This study compared the antithrombotic efficacy of edoxaban with that of an indirect FXa inhibitor, fondaparinux, in in vivo venous and arterial thrombosis models and in ex vivo perfusion chamber thrombosis model under low and high shear rates in rats. Venous and arterial thrombi were induced by platinum wire insertion into the inferior vena cava and by application of FeCl3 to the carotid artery, respectively. The perfusion chamber thrombus was formed by blood perfusion into a collagen-coated capillary at 150 s-1 (low shear rate) and 1,600 s-1 (high shear rate). Effective doses of edoxaban that reduced thrombus formation by 50% (ED50) in venous and arterial thrombosis models were 0.076 and 0.093 mg/kg/h, respectively. In contrast, ED50 of fondaparinux in the arterial thrombosis model (>10 mg/kg/h) was markedly higher compared to ED50 in the venous thrombosis model (0.021 mg/kg/h). In the perfusion chamber thrombosis model, the ratio of ED50 under high shear rate (1.13 mg/kg/h) to that under low shear rate (0.63 mg/kg/h) for edoxaban was 1.9, whereas that for fondaparinux was more than 66. While the efficacy of fondaparinux markedly decreased in arterial thrombosis and in a high-shear state, edoxaban exerted consistent antithrombotic effects regardless of flow conditions. These results suggest that shear rate is a key factor in different antithrombotic effects between edoxaban and fondaparinux.

The effect of elinogrel on high platelet reactivity during dual antiplatelet therapy and the relation to CYP2C19*2 genotype: first experience in patients

To study the effect of a new direct acting reversible P2Y(12) inhibitor, elinogrel (PRT060128), and the relation to cytochrome P450 (CYP) polymorphisms in patients with high platelet reactivity (HPR) on standard dual antiplatelet therapy.

METHODS AND RESULTS

We studied the pharmacodynamic and pharmacokinetic effects of a single 60-mg oral dose of elinogrel in 20 of 45 previously stented stable patients with HPR. We also genotyped for CYP2C19*2,3,5,17 and CYP3A5*3. Platelet reactivity fell within 4 h of dosing, the earliest time point evaluated as measured by the following assays: maximum 5 and 10 microM ADP LTA (P < 0.001 for both vs. predosing); maximum 20 microM ADP LTA (P < 0.05); VerifyNow (P < 0.001); thrombelastography (P < 0.05); VASP phosphorylation (P < 0.01); and perfusion chamber assay (P < 0.05); this was reversible within 24 h in these same assays (P = ns vs. predosing for all assays). CYP2C19*2 was present in 44% of all patients but was more frequent in HPR patients (77% vs. 16%, P = 0.0004).

CONCLUSIONS

HPR is reversibly overcome by a single 60-mg oral dose of elinogrel, a drug now being investigated in a phase 2 trial. CYP2C19*2 was associated with HPR during conventional dual antiplatelet therapy.

In vivo veritas: Thrombosis mechanisms in animal models

Experimental models have enhanced our understanding of atherothrombosis pathophysiology and have played a major role in the search for adequate therapeutic interventions. Various animal models have been developed to simulate thrombosis and to study in vivo parameters related to hemodynamics and rheology that lead to thrombogenesis. Although no model completely mimics the human condition, much can be learned from existing models about specific biologic processes in disease causation and therapeutic intervention. In general, large animals such as pigs and monkeys have been better suited to study atherosclerosis and arterial and venous thrombosis than smaller species such as rats, rabbits, and dogs. On the other hand, mouse models of arterial and venous thrombosis have attracted increasing interest over the past two decades, owing to direct availability of a growing number of genetically modified mice, improved technical feasibility, standardization of new models of local thrombosis, and low maintenance costs. To simulate rupture of an atherosclerotic plaque, models of arterial thrombosis often involve vascular injury, which can be achieved by several means. There is no animal model that is sufficiently tall, that can mimic the ability of humans to walk upright, and that possesses the calf muscle pump that plays an important role in human venous hemodynamics. A number of spontaneous or genetically engineered animals with overexpression or deletion of various elements in the coagulation, platelet, and fibrinolysis pathways are now available. These animal models can replicate important aspects of thrombosis in humans, and provide a valuable resource in the development of novel concepts of disease mechanisms in human patients.

Platelets undergo phosphorylation of Syk at Y525/526 and Y352 in response to pathophysiological shear stress

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

Biomarkers and Bioassays for Cardiovascular Diseases: Present and Future

Stratification of cardiac patients arriving at the emergency department is now being made according to the levels of acute cardiac biomarkers (i.e. cardiac troponin (cTn) or creatine kinase myocardial band (CK-MB)). Ongoing efforts are undertaken in an attempt to identify and validate additional cardiac biomarkers, for example, interleukin-6, soluble CD40L, and C-reactive protein, in order to further risk stratify patients with acute coronary syndrome. Several studies have also now shown an association of platelet transcriptome and genomic single nucleotide polymorphisms with myocardial infarction by using advanced genomic tools. A number of markers, such as myeloid-related protein 14 (MRP-14), cyclooxygenase-1 (COX-1), 5-lipoxygenase activating protein (FLAP), leukotriene A4 hydrolase (LTA4H) and myocyte enhancing factor 2A (MEF2A), have been linked to acute coronary syndromes, including myocardial infarction. In the future, these novel markers may pave the way toward personalized disease-prevention programs based on a person's genomic, thrombotic and cardiovascular profiles. Current and future biomarkers and bioassays for identifying at-risk patients will be discussed in this review.

Wine constituents inhibit thrombosis but not atherogenesis in C57BL/6 apolipoprotein E-deficient mice

Regular and moderate wine consumption is one of the explanations suggested for the lower incidence of cardiovascular events in France compared with other industrialized countries. We evaluated whether alcohol alone or combined with red wine polyphenols reduced plaque size and/or attenuated thrombotic reactivity at the site of advanced atherosclerotic lesions. Red wine extract, or purified (+)-catechin with alcohol, or alcohol alone, was added for 12 weeks to the drinking water of apoE-deficient (apoE−/−) C57B/ mice and wild-type counterparts. In the apoE−/−mice, all alcohol-containing mixtures were associated with a larger size of aortic atherosclerotic lesions. On the other hand, red wine extract and (+)-catechin significantly inhibited blood thrombotic reactivity (P<0·05) as assessed in a cylindrical perfusion chamber model of experimental thrombosis: area reductions in cross-sectional surface of theex vivothrombus were 64% and 63%, respectively. In the wild-type mice, red wine extract and (+)-catechin tended to reduce thrombogenicity, which was on the whole less marked than in the apoE−/−mice. These findings suggest that a moderate and regular consumption of red wine may protect against clinical cardiovascular events, mainly by attenuating the thrombogenic response rather than by reducing the development of atherosclerotic lesions. This antithrombogenic effect may include normalization of the abnormally high thrombogenic responsiveness in apoE−/−mice as well as a direct antithrombotic effect.

Breast adenocarcinoma cell adhesion to the vascular subendothelium in whole blood and under flow conditions: effects of alphavbeta3 and alphaIIbbeta3 antagonists

Tumour cell adhesion to vascular extracellular matrix (ECM), an important step of metastatic progression, is promoted by platelets. The aim of our study was to investigate, in whole blood under venous and arterial shear conditions, the respective role of tumour cell alphavbeta3 and platelet alphaIIbbeta3 integrins in MDA-MB-231 breast adenocarcinoma cell adhesion to human umbilical vein endothelial cell ECM. For that purpose, blood containing MDA-MB-231 cells was incubated with non-peptide antagonists specific for platelet alphaIIbbeta3 (lamifiban) or tumour cell alphavbeta3 (SB-273005). At 300 s(-1), each antagonist used alone did not modify tumour cell adhesion, whereas, at 1500 s(-1), tumour cell adhesion was decreased by 25% in presence of lamifiban indicating a role of platelet alphaIIbbeta3 at higher shear rate. However, a combination of SB-273005 and lamifiban, or c7E3 Fab (a potent inhibitor of both alphaIIbbeta3 and alphavbeta3) inhibited tumour cell adhesion by 40-45%, at either shear rate applied, indicating a cooperation between these two integrins in MDA-MB-231 cell adhesion to ECM, as well as the participation of other adhesive receptors on tumour cells and/or platelets. Thus, efficient anti-metastatic therapy should target multiple receptors on tumour cells and platelets.

Induction of heme oxygenase-1 expression inhibits platelet-dependent thrombosis

Heme oxygenase-1 (HO-1) plays a key role in protecting tissue from oxidative stress. Although some studies implicate HO-1 in modulating thrombosis after vascular injury, the impact of HO-1 on the rate of clot formation in vivo is poorly defined. This study examined the potential function of HO-1 in regulating platelet-dependent arterial thrombosis. Platelet-rich thrombi were induced in C57BL/6J mice by applying 10% ferric chloride to the exposed carotid artery. Mean occlusion time of wild-type mice (n = 10) was 14.6 +/- 1.0 min versus 12.9 +/- 0.6 min for HO-1-/- mice (n = 11, p = 0.17). However, after challenge with hemin, mean occlusion time was significantly longer in wild-type mice (16.3 +/- 1.2 min, n = 15) than HO-1-/- mice (12.0 +/- 1.0 min, n = 9; p = 0.021). Hemin administration induced an approximately twofold increase in oxidative stress, measured as plasma thiobarbituric acid reactive substances. Immunohistochemical analysis revealed that hemin induced a robust increase in HO-1 expression within the carotid arterial wall. Ex vivo blood clotting within a collagen-coated perfusion chamber was studied to determine whether the accelerated thrombosis observed in HO-1-/- mice was contributed to by effects on the blood itself. Under basal conditions, mean clot formation during perfusion of blood over collagen did not differ between wild-type mice and HO-1-/- mice. However, after hemin challenge, mean clot formation was significantly increased in HO-1-/- mice compared with wild-type controls. These results suggest that, under basal conditions, HO-1 does not exert a significant effect on platelet-dependent clot formation in vivo. However, under conditions that stimulate HO-1 production, platelet-dependent thrombus formation is inhibited by HO-1. Enhanced HO-1 expression in response to oxidative stress may represent an adaptive response mechanism to down-regulate platelet activation under prothrombotic conditions.

alphaIIbbeta3 antagonism vs. antiadhesive treatment to prevent platelet interactions with vascular subendothelium

Platelets adhering to blood vessels promote coagulation and inflammation, and release growth factors that trigger smooth muscle cell activation. We have therefore studied the pharmacological modification of platelet deposition quantitatively by comparing adhesion of flowing platelets to various subendothelial ligands in the absence or presence of an antialpha(IIb)beta(3) antagonist with the effects of antiadhesive treatment consisting of von Willebrand factor (VWF) and fibronectin neutralization or of the combined inhibition of platelet adhesion and aggregation. In vitro, perfusion of anticoagulated human blood over calf skin collagen reiterated that alpha(IIb)beta(3) antagonism prevents platelet aggregation, but not adhesion per se: single platelets strongly bound to collagen at wall shear rates of both 1300 and 2700 s(-1), largely VWF-independent. When perfused over a human umbilical vein endothelial cell-derived extracellular matrix, single alpha(IIb)beta(3)-antagonized platelets primarily adhered to matrix-bound VWF when perfused at 2700 s(-1), but at 1300 s(-1) they also adhered significantly to fibronectin. During perfusion of anticoagulated rabbit blood over de-endothelialized rabbit aorta at a wall shear rate of 1100 s(-1), alpha(IIb)beta(3) antagonism even increased the absolute numbers of adhering platelets and VWF neutralization redirected alpha(IIb)beta(3)-antagonized platelets towards other vascular ligands. Finally, in vivo, following photochemically induced blood vessel injury in mice, alpha(IIb)beta(3) antagonism inhibited platelet-rich thrombus formation, but platelet adhesion was only significantly inhibited when associated with fibronectin neutralization. In conclusion, antiadhesive platelet treatment more potently interferes with platelet deposition on injured blood vessels than alpha(IIb)beta(3) antagonism, but abrogating platelet adhesion can only be achieved by carefully selected antiplatelet drug combinations.

Activity of a synthetic hexadecasaccharide (SanOrg123781A) in a pig model of arterial thrombosis

The activity of SanOrg123781A, a new synthetic antithrombotic drug inhibiting both factor Xa and thrombin through antithrombin (AT), was compared to that of unfractionated heparin (UFH) and of the synthetic pentasaccharide (fondaparinux, SP) in an ex vivo arterial thrombosis model in the pig. Six groups of four pigs were administered intravenously with SanOrg123781A (1, 3, 10 and 30 nmol kg(-1)), UFH (30 nmol kg(-1)) or SP (30 nmol kg(-1)). In this arterial model in which platelet thrombus was formed on a thrombogenic surface under a constant high shear rate, UFH and SP had moderate antithrombotic effects while SanOrg123781A exhibited a strong, dose-dependent inhibitory activity on platelet adhesion and platelet thrombus formation. In contrast to UFH, SanOrg123781A did not modify the activated partial thromboplastin time (aPTT) even at 30 nmol kg(-1), but strongly inhibited thrombin generation. At the same dose, despite a lower antithrombotic activity than SanOrg123781A, UFH significantly affected all the coagulation parameters. Taken together, these results show that SanOrg123781A, due to its potent and selective antifactor Xa and antifactor IIa activities is a promising new antithrombotic agent even in arterial setting.

P2Y12 regulates platelet adhesion/activation, thrombus growth, and thrombus stability in injured arteries

The critical role for ADP in arterial thrombogenesis was established by the clinical success of P2Y12 antagonists, currently used at doses that block 40-50% of the P2Y12 on platelets. This study was designed to determine the role of P2Y12 in platelet thrombosis and how its complete absence affects the thrombotic process. P2Y12-null mice were generated by a gene-targeting strategy. Using an in vivo mesenteric artery injury model and real-time continuous analysis of the thrombotic process, we observed that the time for appearance of first thrombus was delayed and that only small, unstable thrombi formed in P2Y12-/- mice without reaching occlusive size, in the absence of aspirin. Platelet adhesion to vWF was impaired in P2Y12-/- platelets. While adhesion to fibrinogen and collagen appeared normal, the platelets in thrombi from P2Y12-/- mice on collagen were less dense and less activated than their WT counterparts. P2Y12-/- platelet activation was also reduced in response to ADP or a PAR-4-activating peptide. Thus, P2Y12 is involved in several key steps of thrombosis: platelet adhesion/activation, thrombus growth, and stability. The data suggest that more aggressive strategies of P2Y12 antagonism will be antithrombotic without the requirement of aspirin cotherapy and may provide benefits even to the aspirin-nonresponder population.

Reduced thrombus cohesion in an ex vivo human model of arterial thrombosis induced by clopidogrel treatment: kinetics of the effect and influence of single and double loading-dose regimens

OBJECTIVE

To compare the effects of clopidogrel on ex vivo thrombogenesis with those on ADP-dependent platelet aggregation, and to compare single and double loading-dose regimens.

METHODS AND RESULTS

Step 1: Volunteers (n=12) received clopidogrel 75 mg/day for 8 days. ADP-induced platelet aggregation was measured in platelet-rich plasma (PRP). Thrombogenesis was measured in an ex vivo model. Clopidogrel produced rapid platelet inhibition, increasing up to day 5. Maximal intensity of platelet aggregation correlated with density of platelet thrombus, surface of collagen covered by platelets and thrombus cross-sectional surface (p<0.001). Step 2: On day 1, volunteers (n=60) randomly received clopidogrel 75 mg, a single 300-mg loading dose or two 300-mg loading doses separated by a 12-h interval. On day 2, all volunteers received clopidogrel 75 mg. Both loading dose regimens enhanced platelet inhibition at all time points (p<0.03 vs. clopidogrel 75 mg). After 3 h, the antiplatelet effect of a loading dose was substantial, and the mean decrease in dense thrombus surface was greater in the loading-dose groups than in the 75 mg group (p=0.041 for the single loading dose). Ex vivo, there were no significant differences between loading-dose groups.

CONCLUSIONS

Clopidogrel reduces arterial thrombus cohesion by an effect that correlates with inhibition of ADP-induced platelet aggregation. A single 300-mg loading dose provides a rapid onset of such an antithrombotic effect, which was more significant at 24 h with the double loading dose.

Dissociation between fibrinogen and fibrin interaction with platelets in patients with different subtypes of Glanzmann's thrombasthenia: studies in an ex vivo perfusion chamber model

To explore the possible role of a residual or variant alphaIIbbeta3 integrin (alphaIIbbeta3) in thrombogenesis, we used a new ex vivo perfusion chamber model to examine blood from patients with different subtypes of Glanzmann's thrombasthenia (GT). Non-anticoagulated blood was perfused through capillaries coated with type III collagen for 4.5 min (shear rate: 1600/s). Platelet deposition was quantified as platelet adhesion and mean thrombus size volume; fibrin and von Willebrand Factor (VWF) were specifically revealed by immunohistochemistry. In two patients with variant and in one patient with type II GT, platelet adhesion was maximal and we observed an unexpected formation of thrombi that were smaller than normal in size. These thrombi were surrounded by a thick meshwork that displayed a strong staining for fibrin and VWF. In two patients with heterozygous GT, platelet adhesion and thrombogenesis were normal. In two patients with type I GT, there was no thrombus formation, although platelet adhesion was also maximal. These data suggest the existence of a substitute pathway for thrombogenesis mediated by fibrin and possibly alphaIIbbeta3 (alphaIIbbeta3 at a reduced level, as in type II, and/or abnormal) as this fibrin network was not observed in type I GT with no alphaIIbbeta3. These interactions might facilitate haemostasis and even lead to thrombosis under certain favourable conditions. Furthermore, these data might have pharmacological relevance to the development of anti-alphaIIbbeta3 antithrombotic agents.

CD40L stabilizes arterial thrombi by a beta3 integrin--dependent mechanism

CD40L, a member of the tumor necrosis factor family of ligands, plays a major role in immune responses via its receptor, CD40. Recently, CD40L has been detected on the surfaces of activated platelets and shown to activate endothelium. Here we further addressed the function of platelet CD40L. We show that absence of CD40L affects the stability of arterial thrombi and delays arterial occlusion in vivo. Infusion of recombinant soluble (rs)CD40L restored normal thrombosis, whereas rsCD40L lacking the KGD integrin-recognition sequence did not. CD40-deficient mice exhibited normal thrombogenesis. rsCD40L specifically bound to purified integrin alphaIIbbeta3 and to activated platelets in a beta3-dependent manner and induced platelet spreading. In addition, rsCD40L promoted the aggregation of either human or mouse platelets under high shear rates. Thus, CD40L appears to be an alphaIIbbeta3 ligand, a platelet agonist, and necessary for stability of arterial thrombi.

Clopidogrel-induced qualitative changes in thrombus formation correlate with stent patency in injured pig cervical arteries

Thienopyridines (ticlopidine or clopidogrel) alone or in combination with aspirin are now the reference antiplatelet therapy after stent implantation. To better understand the high efficacy and low risk of bleeding with these agents, we tested clopidogrel alone or with aspirin in an acute ex vivo flow chamber model and in a subacute in vivo arterial thrombosis model. Clopidogrel induced a dose-dependent increase in bleeding time (BT), inhibited ADP-induced platelet aggregation and in the flow chamber reduced thrombus size, and changed thrombus structure to broad-based structure composed of nondegranulated loosely attached platelets contrasting with the tight clumps of degranulated platelets seen without clopidogrel. The in vivo model involved angioplasty and stenting at the site of a preinduced arterial lesion and thrombosis in pig carotid arteries. Clopidogrel alone or with aspirin (but not aspirin alone) decreased the number of stented vessels occluded for more than 24 h and conversely reduced the number of occluding thrombus. At 96 h after stenting, 100% and 90% of the arteries were patent with clopidogrel/aspirin and clopidogrel alone, respectively (vs. 67% and 44% with aspirin and saline, respectively). Clopidogrel destabilizes thrombus without complete abolishment of platelet reactivity.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

Pro-coagulant state resulting from high levels of soluble P-selectin in blood

The plasma concentration of soluble adhesion receptors is increased under pathological circumstances, but their function remains enigmatic. Soluble P-selectin (sP-sel) is shed from activated platelets and endothelial cells. Mice genetically engineered to express P-selectin without the cytoplasmic tail (DeltaCT) constitutively show a 3- to 4-fold increase of sP-sel in plasma. We observed that the DeltaCT mice formed fibrin very readily. In an ex vivo perfusion chamber, there was more fibrin deposited at the site of platelet thrombus formation than in wild type (WT), whereas no fibrin deposits were detected using P-selectin-deficient blood during the same interval. Similarly, in vivo, the hemorrhage produced by local Shwartzman reaction was smaller in the DeltaCT mice than in WT. In contrast, we previously showed hemorrhage to be more prominent in P-selectin knock-out mice. Infusion of mouse P-sel-Ig chimera produced the same protective effect in WT mice as seen in the DeltaCT mice, indicating that the effect was due to increased levels of sP-sel. Mice infused with P-sel-Ig showed significantly more fibrin deposited on the luminal face of the injured vessels than control mice. Plasma from DeltaCT mice or mice infused with P-sel-Ig contained higher concentration of pro-coagulant microparticles and clotted one minute faster than WT. This pro-coagulant phenotype of DeltaCT mice could be reversed by a 4-day treatment with PSGL-Ig, a P-selectin inhibitor. We propose that sP-sel should no longer be considered only as a marker of inflammation or platelet activation, but also as a direct inducer of pro-coagulant activity associated with vascular and thrombotic diseases.

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.

Superiority of platelet integrin GPIIb-IIIa receptor antagonist over aspirin in preventing cyclic flow reductions in the guinea pig middle cerebral artery

We established a photothrombotic occlusion model in the guinea pig middle cerebral artery. In this model, the middle cerebral artery was recanalized within 10 to 20 min after thrombotic occlusion, with subsequent cyclic flow reductions. Cyclic flow reductions in the middle cerebral artery are expected to manage cerebral infarction by modulating arterial patency. Therefore, we evaluated the effect of several antiplatelet agents on the frequency of cyclic flow reductions and subsequent cerebral infarction using this model. A platelet integrin GPIIb-IIIa receptor antagonist, ME3277 (sodium hydrogen [4-[(4,5,6,7-tetrahydrothieno[3,2c]pyridin-2-yl) carbonylamino] acetyl-o-phenylene] dioxydiacetate, 0.3, 1 and 3 mg/kg i.v.) dose-dependently inhibited the ex vivo platelet aggregation induced by ADP (5 microM), collagen (0.8 and 20 microg/ml) and arachidonic acid (100 microM). While the same doses of ME3277 reduced the frequency of the cyclic flow reductions and increased the total patency time of the middle cerebral artery, time to thrombotic occlusion was prolonged only at the highest dose, 3 mg/kg. ME3277 (0.3-3 mg/kg) significantly reduced the infarct volume and improved the neurological deficit at 24 h. In contrast, aspirin (30 mg/kg) did not affect these variables in spite of complete inhibition of platelet aggregation induced by arachidonic acid and collagen (0.8 microg/ml). A thromboxane A2 synthetase inhibitor, sodium ozagrel, significantly increased the total patency time and reduced the infarct volume at 30 mg/kg. Inhibition of prostaglandin I2 generation could explain the effectiveness of sodium ozagrel but not aspirin in this model. These results suggest that platelet integrin GPIIb-IIIa receptor antagonists are more beneficial than aspirin for the treatment of cerebral thrombosis.

Guinea pig blood: a model for the pharmacologic modulation of the GPIb/IX-vWF axis

Antithrombotic activity of two recombinant GPIb-binding fragments of vWF, RG12986 (residues 445-733), and VCL (residues 504-728), were assessed in an ex vivo capillary perfusion chamber exposing human type III collagen to native nonanticoagulated guinea pig blood. Platelet adhesion and thrombus formation were evaluated by computer assisted morphometry for two shear rates (650 and 1800 s-1) and for two perfusion times (1.5 and 4 min). At 1800 s-1 and 4 min of perfusion, platelet adhesion decreased from 63 +/- 7% for control, to 46 +/- 4% for 20 mg/kg RG12986, and to 29 +/- 5% for 4 mg/kg VCL, and the mean thrombus height dropped from 40 +/- 8 microns to 24 +/- 3 microns and 7.5 +/- 1 microns, respectively. The two doses did not change bleeding time values. Our results suggest that guinea pig blood and the circular perfusion chamber represent a good model for the evaluation of limited amount of GPIb/IX-vWF axis inhibitors.