Highly potent anti-human GPVI monoclonal antibodies derived from GPVI knockout mouse immunization

Progress toward a Glycoprotein VI Modulator for the Treatment of Thrombosis

Pathogenic thrombus formation accounts for the etiology of many serious conditions including myocardial infarction, stroke, deep vein thrombosis, and pulmonary embolism. Despite the development of numerous anticoagulants and antiplatelet agents, the mortality rate associated with these diseases remains high. In recent years, however, significant epidemiological evidence and clinical models have emerged to suggest that modulation of the glycoprotein VI (GPVI) platelet receptor could be harnessed as a novel antiplatelet strategy. As such, many peptidic agents have been described in the past decade, while more recent efforts have focused on the development of small molecule modulators. Herein the rationale for targeting GPVI is summarized and the published GPVI modulators are reviewed, with particular focus on small molecules. A qualitative pharmacophore hypothesis for small molecule ligands at GPVI is also presented.

Progress in the development of antiplatelet agents: Focus on the targeted molecular pathway from bench to clinic

Antiplatelet drugs serve as a first-line antithrombotic therapy for the management of acute ischemic events and the prevention of secondary complications in vascular diseases. Numerous antiplatelet therapies have been developed; however, currently available agents are still associated with inadequate efficacy, risk of bleeding, and variability in individual response. Understanding the mechanisms of platelet involvement in thrombosis and the clinical development process of antiplatelet agents is critical for the discovery of novel agents. The functions of platelets in thrombosis are regulated by two major mechanisms: the interaction between surface receptors and their ligands, and the downstream intracellular signaling pathways. Recently, most of the progress made in antiplatelet drug development has been achieved with P2Y receptor antagonists. Additionally, the usage of GP IIb/IIIa receptor antagonists has decreased, because it is associated with a higher risk of bleeding and thrombocytopenia. Agents targeting other platelet surface receptors such as PARs, TP receptor, EP3 receptor, GPIb-IX-V receptor, P-selectin, as well as intracellular signaling factors, such as PI3Kβ, have been evaluated in an attempt to develop the next generation of antiplatelet drugs, reduce or eliminate interpatient variability of drug efficacy and significantly lower the risk of drug-induced bleeding. The aim of this review is to describe the pathways of platelet activation in thrombosis, and summarize the development process of antiplatelet agents, as well as the preclinical and clinical evaluations performed on these agents.

Biochemical and immunological characterization of a novel monoclonal antibody against mouse leukotriene B4 receptor 1

Leukotriene B₄ (LTB₄) receptor 1 (BLT1) is a G protein-coupled receptor expressed in various leukocyte subsets; however, the precise expression of mouse BLT1 (mBLT1) has not been reported because a mBLT1 monoclonal antibody (mAb) has not been available. In this study, we present the successful establishment of a hybridoma cell line (clone 7A8) that produces a high-affinity mAb for mBLT1 by direct immunization of BLT1-deficient mice with mBLT1-overexpressing cells. The specificity of clone 7A8 was confirmed using mBLT1-overexpressing cells and mouse peripheral blood leukocytes that endogenously express BLT1. Clone 7A8 did not cross-react with human BLT1 or other G protein-coupled receptors, including human chemokine (C-X-C motif) receptor 4. The 7A8 mAb binds to the second extracellular loop of mBLT1 and did not affect LTB₄ binding or intracellular calcium mobilization by LTB₄. The 7A8 mAb positively stained Gr-1-positive granulocytes, CD11b-positive granulocytes/monocytes, F4/80-positive monocytes, CCR2-high and CCR2-low monocyte subsets in the peripheral blood and a CD4-positive T cell subset, Th1 cells differentiated in vitro from naïve CD4-positive T cells. This mAb was able to detect Gr-1-positive granulocytes and monocytes in the spleens of naïve mice by immunohistochemistry. Finally, intraperitoneal administration of 7A8 mAb depleted granulocytes and monocytes in the peripheral blood. We have therefore succeeded in generating a high-affinity anti-mBLT1 mAb that is useful for analyzing mBLT1 expression in vitro and in vivo.

Design, development and characterization of ACT017, a humanized Fab that blocks platelet's glycoprotein VI function without causing bleeding risks

Glycoprotein VI is a platelet-specific collagen receptor critical for in vivo formation of arterial thrombosis. It is also considered as an attractive target for the development of anti-thrombotic drugs because blocking glycoprotein (GP)VI inhibits platelet aggregation without inducing detrimental effects on physiologic hemostasis.

Here, we present data on the identification, in vitro and ex vivo pharmacology of a humanized Fab fragment designated as ACT017. ACT017 was selected out of 15 humanized variants based upon structural and functional properties. It was produced under GMP-like conditions followed by detailed physico-chemical analysis and functional characterization indicating high antigen-binding specificity and affinity. In addition, we demonstrate, in a dose-escalation study, that ACT017 has a high capacity to specifically inhibit collagen-induced platelet aggregation ex vivo after injection to the macaque without inducing thrombocytopenia, GPVI depletion or bleeding side effects as is the case for conventional anti-platelets. Therefore, ACT017 is a promising therapeutic candidate for the development of a new generation of safe and efficient anti-thrombotic drugs.

Clinical significance of receptor shedding-platelet GPVI as an emerging diagnostic and therapeutic tool

Platelet membrane bedecked with a wide array of receptors offers a platform to regulate platelet responsiveness, thrombotic propensity, inflammatory disposition, and immune reactivity under diverse pathophysiological conditions. Ectopic proteolytic cleavage of such receptors irreversibly inactivates receptor-mediated intracellular signaling governing cellular functions, further releases soluble fragments into circulation which might modulate functions of target cells. Glycoprotein VI-(GPVI) is a membrane glycoprotein expressed in platelets and megakaryocytes. Platelet GPVI surface expression is enhanced following acute ischemic events like myocardial infarction and cerebral stroke, serves as an imminent diagnostic tool independent of markers of tissue necrosis, and is associated with poor prognosis. Platelets undergo GPVI shedding and thereby contribute to soluble plasma levels of sGPVI, with distinct diagnostic and prognostic attributes. This review summarizes the functional significance and mechanistic basis whereby GPVI surface availability is up- or downregulated on platelets and the impact of GPVI in diagnostic, prognostic, and therapeutic strategies in diseases where platelets play a regulatory role. Further, we also highlight how novel non-invasive platelet-based diagnostic and therapeutic strategies have evolved utilizing GPVI for lesion-directed antithrombotic therapy or to counteract atherosclerotic disposition to ameliorate care of patients particularly in the context of cardio-cerebro-vascular medicine.

Beyond antiplatelets: The role of glycoprotein VI in ischemic stroke

Background

Platelets are essential to physiological hemostasis or pathological thrombus formation. Current antiplatelet agents inhibit platelet aggregation but leave patients at risk of systemic side-effects such as hemorrhage. Newer therapeutic strategies could involve targeting this cascade earlier during platelet adhesion or activation via inhibitory effects on specific glycoproteins, the thrombogenic collagen receptors found on the platelet surface.

Aims

Glycoprotein VI (GPVI) is increasingly being recognized as the main platelet-collagen receptor involved in arterial thrombosis. This review summarizes the crucial role GPVI plays in ischemic stroke as well as the current strategies used to attempt to inhibit its activity.

Summary of review

In this review, we discuss the normal hemostatic process, and the role GPVI plays at sites of atherosclerotic plaque rupture. We discuss how the unique structure of GPVI allows for its interaction with collagen and creates downstream signaling that leads to thrombus formation. We summarize the current strategies used to inhibit GPVI activity and how this could translate to a clinically viable entity that may compete with current antiplatelet therapy.

Conclusion

From animal models, it is clear that GPVI inhibition leads to an abolished platelet response to collagen and reduced platelet aggregation, culminating in smaller arterial thrombi. There is now an increasing body of evidence that these findings can be translated into the development of a bleeding free pharmacological entity specific to sites of plaque rupture in humans.

A 2D-DIGE-based proteomic analysis reveals differences in the platelet releasate composition when comparing thrombin and collagen stimulations

Upon stimulation, platelets release a high number of proteins (the releasate). There are clear indications that these proteins are involved in the pathogenesis of several diseases, such as atherosclerosis. In the present study we compared the platelet releasate following platelet activation with two major endogenous agonists: thrombin and collagen. Proteome analysis was based on 2D-DIGE and LC-MS/MS. Firstly, we showed the primary role of thrombin and collagen receptors in platelet secretion by these agonists; moreover, we demonstrated that GPVI is the primary responsible for collagen-induced platelet activation/aggregation. Proteomic analysis allowed the detection of 122 protein spots differentially regulated between both conditions. After excluding fibrinogen spots, down-regulated in the releasate of thrombin-activated platelets, 84 differences remained. From those, we successfully identified 42, corresponding to 37 open-reading frames. Many of the differences identified correspond to post-translational modifications, primarily, proteolysis induced by thrombin. Among others, we show vitamin K-dependent protein S, an anticoagulant plasma protein, is up-regulated in thrombin samples. Our results could have pathological implications given that platelets might be playing a differential role in various diseases and biological processes through the secretion of different subsets of granule proteins and microvesicles following a predominant activation of certain receptors.

Implications of glycoprotein VI for theranostics

Glycoprotein VI (GPVI), a membrane glycoprotein solely expressed in platelets and megakaryocytes, plays a critical role in thrombus formation due to collagen/GPVI-mediated platelet activation and adhesion. Recent studies have shown that surface expression of GPVI on circulating platelets is enhanced in acute cardiovascular diseases such as myocardial infarction and ischaemic stroke. Increased GPVI levels are associated with poor clinical outcome and are an early indicator for imminent myocardial infarction in patients with chest pain. The soluble form of the dimeric GPVI fusion protein (sGPVI-Fc) binds with high affinity to collagen and atherosclerotic plaque tissue. Non-invasive imaging studies with radiolabelled sGPVI-Fc show specific binding activity to vascular lesions in vivo. Further, sGPVI-Fc has been developed as a new therapeutic platelet-based strategy for lesion-directed antithrombotic therapy. This review summarises the potential of GPVI for diagnostic and therapeutic options based on novel non-invasive molecular imaging modalities to ameliorate care of patients with cardiovascular diseases.

The future of glycoprotein VI as an antithrombotic target

The treatment of acute coronary syndromes has been considerably improved in recent years with the introduction of highly efficient antiplatelet drugs. However, there are still significant limitations: the recurrence of adverse vascular events remains a problem, and the improvement in efficacy is counterbalanced by an increased risk of bleeding, which is of particular importance in patients at risk of stroke. One of the most attractive targets for the development of new molecules with potential antithrombotic activity is platelet glycoprotein (GP)VI, because its blockade appears to ideally combine efficacy and safety. This review summarizes current knowledge on GPVI regarding its structure, its function, and its role in physiologic hemostasis and thrombosis. Strategies for inhibiting GPVI are presented, and evidence of the antithrombotic efficacy and safety of GPVI antagonists is provided.

Current concepts of platelet activation: possibilities for therapeutic modulation of heterotypic vs. homotypic aggregation

Thrombogenic and inflammatory activity are two distinct aspects of platelet biology, which are sustained by the ability of activated platelets to interact with each other (homotypic aggregation) and to adhere to circulating leucocytes (heterotypic aggregation). These two events are regulated by distinct biomolecular mechanisms that are selectively activated in different pathophysiological settings. They can occur simultaneously, for example, as part of a pro-thrombotic/pro-inflammatory response induced by vascular damage, or independently, as in certain clinical conditions in which abnormal heterotypic aggregation has been observed in the absence of intravascular thrombosis. Current antiplatelet drugs have been developed to target specific molecular signalling pathways mainly implicated in thrombus formation, and their ever increasing clinical use has resulted in clear benefits in the treatment and prevention of arterial thrombotic events. However, the efficacy of currently available antiplatelet drugs remains suboptimal, most likely because their therapeutic action is limited to only few of the signalling pathways involved in platelet homotypic aggregation. In this context, modulation of heterotypic aggregation, which is believed to contribute importantly to acute thrombotic events, as well to the pathophysiology of atherosclerosis itself, may offer benefits over and above the classical antiplatelet approach. This review will focus on the distinct biomolecular pathways that, following platelet activation, underlie homotypic and heterotypic aggregation, aiming potentially to identify novel therapeutic targets.

Design and reshaping of an scFv directed against human platelet glycoprotein VI with diagnostic potential

Blood platelets play a key role in physiological hemostasis and in thrombosis. As a consequence, platelet functional analysis is widely used in the diagnosis of hemorrhagic disorders as well as in the evaluation of thrombosis risks and of the efficacy of antithrombotics. Glycoprotein (GP) VI is a platelet-specific collagen-signaling receptor. Clinical studies suggest that increased GPVI expression is associated with a risk of arterial thrombosis. Conversely, GPVI deficiencies have been identified in patients with defective platelet responses to collagen. Currently, there is no standard test available for measuring GPVI expression, essentially because antibodies usually cross-link GPVI upon binding, leading to platelet activation and consecutive changes in GPVI expression. Here, we designed a recombinant monovalent antibody fragment (scFv) derived from an anti-GPVI monoclonal IgG, 3J24, with the characteristics required to analyze GPVI expression. Guided by in silico modeling and V-KAPPA chain analysis, a Protein L (PpL) recognition pattern was engineered in the scFv, making possible its purification and detection using PpL conjugates. The PpL affinity-purified scFv is functional. It retains GPVI-binding specificity and allows detection of platelet surface-expressed GPVI without inducing platelet activation. In conclusion, the reshaped scFv may be very useful in the development of diagnostic approaches.

An acquired inhibitor to the GPVI platelet collagen receptor in a patient with lupus nephritis

BACKGROUND

GPVI is a major platelet collagen signaling receptor. In rare cases of immune thrombocytopenic purpura (ITP), autoantibodies to GPVI result in receptor shedding.

OBJECTIVES

To investigate a possible pathogenic role of plasma anti-GPVI antibody located in a woman with lupus nephritis.

METHODS

Measured were (i) platelet aggregation to collagen and convulxin, (ii) platelet GPVI expression (flow cytometry and western blotting), (iii) plasma soluble GPVI (sGPVI, dual antibody ELISA), and (iv) plasma anti-GPVI antibody (ELISA using recombinant sGPVI).

RESULTS

In 2006 and early 2007, the patient had a normal platelet count but a virtual absence of platelet aggregation to collagen and convulxin. Her platelets responded normally to other agonists including cross-linking ITAM-dependent FcgammaRIIA by monoclonal antibody, IV.3. Flow cytometry and western blotting showed a platelet deficiency of GPVI. Plasma sGPVI levels were undetectable whereas ELISA confirmed the presence of anti-GPVI antibody. Sequencing revealed a normal GPVI cDNA structure. The patient's plasma and the isolated IgG3 fraction activated and induced GPVI shedding from normal platelets. A deteriorating clinical condition led to increasingly strict immunosuppressive therapy. This was globally associated with a fall in plasma anti-GPVI titres, the restoration of platelet GPVI and the convulxin response, and the loss of her nephrotic syndrome.

CONCLUSIONS

Our results show that this patient acquired a potent anti-GPVI IgG3 antibody with loss of GPVI and collagen-related platelet function. Further studies are required to determine whether anti-GPVI antibodies occur in other lupus patients with nephritis.

Presence of platelet-associated anti-glycoprotein (GP)VI autoantibodies and restoration of GPVI expression in patients with GPVI deficiency

BACKGROUND

Glycoprotein (GP)VI deficiency is a rare platelet disorder with a mild bleeding tendency. However, its pathophysiology remains unclear.

OBJECTIVES

We characterized a novel GPVI-deficient patient with immune thrombocytopenic purpura and searched for the presence of anti-GPVI autoantibodies in this and another patient with GPVI deficiency.

METHODS AND RESULTS

A 12-year-old Japanese girl (case 1) with moderate thrombocytopenia and mild bleeding showed selectively impaired collagen-induced platelet aggregation. Flow cytometric analysis indicated that the patient had a defect in the expression of GPVI-FcRgamma. An eluate of her platelet-associated IgG contained anti-alpha(IIb)beta3 autoantibodies. Moreover, using GPVI-FcRgamma-transfected cells, we unexpectedly identified anti-GPVI antibodies against the soluble ectodomain of GPVI in the eluate, despite the patient's GPVI deficiency. In contrast, anti-GPVI antibodies were not detectable in her plasma. In another case of GPVI deficiency (case 2) without detectable plasma anti-GPVI antibodies, we again detected platelet-associated anti-GPVI antibodies. In a 2-year follow-up of case 1, the platelet count increased to within the normal range and the bleeding tendency improved. Interestingly, GPVI was again expressed on her platelets, in association with a decrease in the relative amount of anti-GPVI antibodies.

CONCLUSIONS

This is the first demonstration of platelet-associated anti-GPVI antibodies in GPVI-deficient subjects, in one case with spontaneous restoration of GPVI expression. These results strongly suggest an autoimmune mechanism in GPVI deficiency.

Design and humanization of a murine scFv that blocks human platelet glycoprotein VI in vitro

Platelet adhesion and aggregation at the site of vascular injury is essential for hemostasis, but can also lead to arterial occlusion in thrombotic disorders. Glycoprotein (GP) VI is the major platelet membrane receptor that interacts directly with collagen, the most thrombogenic compound in the blood vessels. GPVI could therefore be a major therapeutic target. Fab fragments of the anti-GPVI murine monoclonal IgG 9O12 have previously been shown to completely block collagen-induced platelet aggregation, to inhibit the procoagulant activity of collagen-stimulated platelets, and to prevent thrombus formation under arterial flow conditions without significantly prolonging the bleeding time. Here, we engineered recombinant scFvs that preserve the functional properties of 9O12, and could constitute building blocks for designing new compounds with potentially therapeutic antithrombotic properties. First, the 9O12 variable domains were cloned, sequenced, and expressed as a recombinant murine scFv, which was fully characterized. This scFv preserved all the characteristics that make 9O12 Fab potentially useful for therapeutic applications, including its high affinity for GPVI, ability to inhibit platelet adhesion, and aggregation with collagen under arterial flow conditions. A humanized version of this scFv was also designed after complementarity-determining region grafting and structural refinements using homology-based modeling. The final product was produced in recombinant bacteria. It retained GPVI-binding specificity and high affinity, which are the main parameters usually impaired by humanization procedures. This is a simple, efficient and straightforward method that could also be used for humanizing other antibodies.

Compromised ITAM-based platelet receptor function in a patient with immune thrombocytopenic purpura

BACKGROUND

Receptors on platelets that contain immunoreceptor tyrosine-based activation motifs (ITAMs) include collagen receptor glycoprotein (GP) VI, and FcgammaRIIa, a low affinity receptor for immunoglobulin (Ig) G.

OBJECTIVES

We examined the function of GPVI and FcgammaRIIa in a patient diagnosed with immune thrombocytopenic purpura (ITP) who had unexplained pathological bruising despite normalization of the platelet count with treatment.

METHODS AND RESULTS

Patient platelets aggregated normally in response to ADP, arachadonic acid and epinephrine, but not to GPVI agonists, collagen or collagen-related peptide, or to FcgammaRII-activating monoclonal antibody (mAb) 8.26, suggesting ITAM receptor dysfunction. Plasma contained an anti-GPVI antibody by MAIPA and aggregated normal platelets. Aggregating activity was partially (approximately 60%) blocked by FcgammaRIIa-blocking antibody, IV.3, and completely blocked by soluble GPVI ectodomain. Full-length GPVI on the patient platelet surface was reduced to approximately 10% of normal levels, and a approximately 10-kDa GPVI cytoplasmic tail remnant and cleaved FcgammaRIIa were detectable by western blot, indicating platelet receptor proteolysis. Plasma from the patient contained approximately 150 ng mL(-1) soluble GPVI by ELISA (normal plasma, approximately 15 ng mL(-1)) and IgG purified from patient plasma caused FcgammaRIIa-mediated, EDTA-sensitive cleavage of both GPVI and FcgammaRIIa on normal platelets.

CONCLUSIONS

In ITP patients, platelet autoantibodies can curtail platelet receptor function. Platelet ITAM receptor dysfunction may contribute to the increased bleeding phenotype observed in some patients with ITP.

Inhibition of collagen-induced platelet aggregation by anopheline antiplatelet protein, a saliva protein from a malaria vector mosquito

During blood feeding, mosquitoes inject saliva containing a mixture of molecules that inactivate or inhibit various components of the hemostatic response to the bite injury as well as the inflammatory reactions produced by the bite, to facilitate the ingestion of blood. However, the molecular functions of the individual saliva components remain largely unknown. Here, we describe anopheline antiplatelet protein (AAPP) isolated from the saliva of Anopheles stephensi, a human malaria vector mosquito. AAPP exhibited a strong and specific inhibitory activity toward collagen-induced platelet aggregation. The inhibitory mechanism involves direct binding of AAPP to collagen, which blocks platelet adhesion to collagen and inhibits the subsequent increase in intracellular Ca(2+) concentration ([Ca(2+)]i). The binding of AAPP to collagen effectively blocked platelet adhesion via glycoprotein VI (GPVI) and integrin alpha(2)beta(1). Cell adhesion assay showed that AAPP inhibited the binding of GPVI to collagen type I and III without direct effect on GPVI. Moreover, intravenously administered recombinant AAPP strongly inhibited collagen-induced platelet aggregation ex vivo in rats. In summary, AAPP is a malaria vector mosquito-derived specific antagonist of receptors that mediate the adhesion of platelets to collagen. Our study may provide important insights for elucidating the effects of mosquito blood feeding against host hemostasis.

The Fab Fragment of a Novel Anti-GPVI Monoclonal Antibody, OM4, Reduces In Vivo Thrombosis Without Bleeding Risk in Rats

Background— Inhibition of GPVI has been proposed as a useful antithrombotic strategy; however, in vivo proof-of-concept animal studies targeting GPVI are lacking. We evaluated a novel anti-human GPVI monoclonal antibody OM4 Fab in rats.

Methods and Results— OM4 Fab specifically inhibited collagen-induced aggregation of rat platelets in vitro with an IC 50 of 20 to 30 μg/mL but not ADP and AA-induced platelet aggregation. After intravenous administration of OM4 Fab, a rapid inhibition of ex vivo platelet aggregation was observed with a gradual recovery within 60 to 90 minutes which corresponded to the decline in OM4 Fab plasma concentration and time-dependent decrease in platelet-bound OM4 Fab. In contrast to previous reports in mice, intravenous OM4 Fab did not deplete platelet GPVI. Injection of OM4 IgG caused acute thrombocytopenia. In a modified Folts model of cyclic flow reduction in rat carotid artery, the number of complete occlusions was significantly reduced by intravenous administration of OM4 Fab (20 mg/kg) before or after mechanical injury to the vessel, without prolongation of bleeding time.

Conclusion— Fab fragment of the monoclonal antibody OM4 effectively inhibits collagen induced platelet aggregation in vitro and ex vivo, and in vivo thrombosis in rats without prolonging bleeding time. Antibodies against GPVI may have therapeutic potential, inhibiting thrombosis without prolonging bleeding time.