Targeting Ligand-Induced Binding Sites on GPIIb/IIIa via Single-Chain Antibody Allows Effective Anticoagulation Without Bleeding Time Prolongation

A humanized single-chain variable fragment antibody against beta3 integrin in Escherichia coli

Patients with HIV-1 immune-related thrombocytopenia (HIV-1-ITP) have a unique antibody (Ab) against platelet GPIIIa49-66, which is capable of inducing oxidative platelet fragmentation in the absence of complement activation. By screening a human phage antibody library with the GPIIIa49-66 peptide as bait, we have developed several humanized phage Abs, which act similarly to the parental Ab. However, the presence of a stop codon in the heavy chain of the obtained phage clones limits their expression in soluble recombinant form. To circumvent this problem, we mutated the stop codon inside clone 11 that exhibits the highest binding activity to platelet GPIIIa49-66, resulting in a soluble scFv format (named A11) in Escherichia coli Rosseta. In in vitro binding assay, A11 exhibited similar binding specificity to parental Ab at various concentrations. Moreover, A11 is able to induce oxidative platelet fragmentation by preferentially binding to activated versus resting platelets. These findings provide a proof-of-principle for the development of a novel approach to inhibit arterial thrombosis by generating a selective scFv for the lysis of platelet-rich thrombi.

Dissolution of pre-existing platelet thrombus by synergistic administration of low concentrations of bifunctional antibodies against β3 integrin

Most antithrombotic approaches target prevention rather than the more clinically relevant issue of resolution of an existing thrombus. In this study, we describe a novel and effective therapeutic strategy for ex vivo clearance of pre-existing platelet thrombus by the combination of two bifunctional platelet GPIIIa49-66 ligands that target different parts of the arterial thrombus. We produced an additional GPIIIa49-66 agent (named APAC), which homes to activated platelets. Like our previously described SLK (which targets newly deposited fibrin strands surrounding the platelet thrombus), APAC destroys platelet aggregates ex vivo in an identical fashion with 85% destruction of platelet aggregates at 2 hours. The combined application of APAC and SLK demonstrated a ~2 fold greater platelet thrombus dissolution than either agent alone at a low concentration (0.025 µM). Platelet-rich clot lysis experiments demonstrated the time required for 50% platelet-rich fibrin clot lysis (T(50%)) by APAC (95 ± 6.1 min) or SLK (145 ± 7.1 min) was much longer than that by combined APAC + SLK (65 ± 7.6 min) at the final concentration of 0.025 µM (APAC + SLK vs APAC, p<0.05; APAC + SLK vs SLK, p<0.01). Thus these low concentrations of a combination of both agents are likely to be more effective and less toxic when used therapeutically in vivo.

Enzymatic single-chain antibody tagging: a universal approach to targeted molecular imaging and cell homing in cardiovascular disease

RATIONALE

Antibody-targeted delivery of imaging agents can enhance the sensitivity and accuracy of current imaging techniques. Similarly, homing of effector cells to disease sites increases the efficacy of regenerative cell therapy while reducing the number of cells required. Currently, targeting can be achieved via chemical conjugation to specific antibodies, which typically results in the loss of antibody functionality and in severe cell damage. An ideal conjugation technique should ensure retention of antigen-binding activity and functionality of the targeted biological component.

OBJECTIVE

To develop a biochemically robust, highly reproducible, and site-specific coupling method using the Staphylococcus aureus sortase A enzyme for the conjugation of a single-chain antibody (scFv) to nanoparticles and cells for molecular imaging and cell homing in cardiovascular diseases. This scFv specifically binds to activated platelets, which play a pivotal role in thrombosis, atherosclerosis, and inflammation.

METHODS AND RESULTS

The conjugation procedure involves chemical and enzyme-mediated coupling steps. The scFv was successfully conjugated to iron oxide particles (contrast agents for magnetic resonance imaging) and to model cells. Conjugation efficiency ranged between 50% and 70%, and bioactivity of the scFv after coupling was preserved. The targeting of scFv-coupled cells and nanoparticles to activated platelets was strong and specific as demonstrated in in vitro static adhesion assays, in a flow chamber system, in mouse intravital microscopy, and in in vivo magnetic resonance imaging of mouse carotid arteries.

CONCLUSIONS

This unique biotechnological approach provides a versatile and broadly applicable tool for procuring targeted regenerative cell therapy and targeted molecular imaging in cardiovascular and inflammatory diseases and beyond.

An activation-specific platelet inhibitor that can be turned on/off by medically used hypothermia

OBJECTIVE

Therapeutic hypothermia is successfully used, for example, in cardiac surgery to protect organs from ischemia. Cardiosurgical procedures, especially in combination with extracorporeal circulation, and hypothermia itself are potentially prothrombotic. Despite the obvious need, the long half-life of antiplatelet drugs and thus the risk of postoperative bleedings have restricted their use in cardiac surgery. We describe here the design and testing of a unique recombinant hypothermia-controlled antiplatelet fusion protein with the aim of providing increased safety of hypothermia, as well as cardiac surgery.

METHODS AND RESULTS

An elastin-mimetic polypeptide was fused to an activation-specific glycoprotein (GP) IIb/IIIa-blocking single-chain antibody. In silico modeling illustrated the sterical hindrance of a β-spiral conformation of elastin-mimetic polypeptide preventing the single-chain antibody from inhibiting GPIIb/IIIa at 37°C. Circular dichroism spectra demonstrated reverse temperature transition, and flow cytometry showed binding to and blocking of GPIIb/IIIa at hypothermic body temperature (≤32°C) but not at normal body temperature. In vivo thrombosis in mice was selectively inhibited at hypothermia but not at 37°C.

CONCLUSIONS

This is the first description of a broadly applicable pharmacological strategy by which the activity of a potential drug can be controlled by temperature. In particular, this drug steerability may provide substantial benefits for antiplatelet therapy.

Activated platelets in carotid artery thrombosis in mice can be selectively targeted with a radiolabeled single-chain antibody

Background

Activated platelets can be found on the surface of inflamed, rupture-prone and ruptured plaques as well as in intravascular thrombosis. They are key players in thrombosis and atherosclerosis. In this study we describe the construction of a radiolabeled single-chain antibody targeting the LIBS-epitope of activated platelets to selectively depict platelet activation and wall-adherent non-occlusive thrombosis in a mouse model with nuclear imaging using in vitro and ex vivo autoradiography as well as small animal SPECT-CT for in vivo analysis.

Methodology/Principal Findings

LIBS as well as an unspecific control single-chain antibody were labeled with ¹¹¹Indium (¹¹¹In) via bifunctional DTPA ( = ¹¹¹In-LIBS/¹¹¹In-control). Autoradiography after incubation with ¹¹¹In-LIBS on activated platelets in vitro (mean 3866±28 DLU/mm², 4010±630 DLU/mm² and 4520±293 DLU/mm²) produced a significantly higher ligand uptake compared to ¹¹¹In-control (2101±76 DLU/mm², 1181±96 DLU/mm² and 1866±246 DLU/mm²) indicating a specific binding to activated platelets; P<0.05. Applying these findings to an ex vivo mouse model of carotid artery thrombosis revealed a significant increase in ligand uptake after injection of ¹¹¹In-LIBS in the presence of small thrombi compared to the non-injured side, as confirmed by histology (49630±10650 DLU/mm² vs. 17390±7470 DLU/mm²; P<0.05). These findings could also be reproduced in vivo. SPECT-CT analysis of the injured carotid artery with ¹¹¹In-LIBS resulted in a significant increase of the target-to-background ratio compared to ¹¹¹In-control (1.99±0.36 vs. 1.1±0.24; P<0.01).

Conclusions/Significance

Nuclear imaging with ¹¹¹In-LIBS allows the detection of platelet activation in vitro and ex vivo with high sensitivity. Using SPECT-CT, wall-adherent activated platelets in carotid arteries could be depicted in vivo. These results encourage further studies elucidating the role of activated platelets in plaque pathology and atherosclerosis and might be of interest for further developments towards clinical application.

EGFP-EGF1 protein-conjugated PEG-PLA nanoparticles for tissue factor targeted drug delivery

In a strategy for anti-thrombotic therapy, we have expressed EGFP-EGF1 fusion protein, in which EGF1 can bind with tissue factor (TF). EGFP has previously been widely used as a fluorescent protein marker. EGFP-EGF1 protein was thiolated and conjugated to the malemide covering on the pegylated nanoparticles (NP) to form the EGFP-EGF1-NP. The EGFP-EGF1-NP was characterized in terms of morphology, size and zeta potential. In vitro cell viability experiment confirmed that the biodegradable EGFP-EGF1-NP was safe. To evaluate the delivering ability of EGFP-EGF1-NP, a fluorochrome dye, Dir, was incorporated into the nanoparticle, and the loading capacity and release property of the particle were examined. In vitro results showed that the binding ability of EGFP-EGF1-NP with TF-expressing cells was significantly stronger than that of non-conjugated NP. In vivo multispectral fluorescent imaging demonstrated that EGFP-EGF1-NP had high specificity and sensitivity in targeting thrombi. Our study demonstrated that EGFP-EGF1-NP is a promising TF-targeting drug delivery system for thrombolytic treatment.

Chapter 4 - Applications of nanotechnology in molecular imaging of the brain

Rapid advances in the field of nanotechnology promise revolutionary improvements in the diagnosis and therapy of neuroinflammatory disorders. An array of iron oxide nano- and microparticle agents have been developed for in vivo molecular magnetic resonance imaging (mMRI) of cerebrovascular endothelial targets, such as vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and the glycoprotein receptor GP IIb/IIIa expressed on activated platelets. Molecular markers of glioma cells, such as matrix metalloproteinase-2 (MMP-2), and markers for brain tumor angiogenesis, such as alpha (v) beta (3) integrin (alpha(v)beta(3)), have also been successfully targeted using nanoparticle imaging probes. This chapter provides an overview of targeted, iron oxide nano- and microparticles that have been applied for in vivo mMRI of the brain in experimental models of multiple sclerosis (MS), brain ischemia, cerebral malaria (CM), brain cancer, and Alzheimer's disease. The potential of targeted nanoparticle agents for application in clinical imaging is also discussed, including multimodal and therapeutic approaches.

An approach to molecular imaging of atherosclerosis, thrombosis, and vascular inflammation using microparticles of iron oxide

The rapidly evolving field of molecular imaging promises important advances in the diagnosis, characterization and pharmacological treatment of vascular disease. Magnetic resonance imaging (MRI) provides a modality that is well suited to vascular imaging as it can provide anatomical, structural and functional data on the arterial wall. Its capabilities are further enhanced by the use of a range of increasingly sophisticated contrast agents that target specific molecules, cells and biological processes. This article will discuss one such approach, using microparticles of iron oxide (MPIO).

MPIO have been shown to create highly conspicuous contrast effects on T₂^*-weighted MR images. We have developed a range of novel ligand-conjugated MPIO for molecular MRI of endothelial adhesion molecules, such as vascular cell adhesion molecule-1 (VCAM-1) and P-selectin expressed in vascular inflammation, as well as activated platelet thrombosis. This review discusses the application of ligand-targeted MPIO for in vivo molecular MRI in a diverse range of vascular disease models including acute vascular inflammation, atherosclerosis, thrombosis, ischemia-reperfusion injury and ischemic stroke. The exceptionally conspicuous contrast effects of ligand-conjugated MPIO provide a versatile and sensitive tool for quantitative vascular molecular imaging that could refine diagnosis and measure response to treatment. The potential for clinical translation of this new class of molecular contrast agent for clinical imaging of vascular syndromes is discussed.

Tick-derived Kunitz-type inhibitors as antihemostatic factors

Endogenous Kunitz-type inhibitors target a large number of serine proteinases, including coagulation factors VIIa and Xa, but not thrombin. By contrast, several two-domain Kunitz inhibitors of this major procoagulant proteinase have been isolated from both soft ticks (e.g., ornithodorin from Ornithodoros moubata) and hard ticks (e.g., boophilin from Rhipicephalus (Boophilus) microplus). Surprisingly, these anticoagulants do not follow the canonical mechanism of proteinase inhibition. Instead, their N-terminal residues bind across the thrombin active-site cleft, while C-terminal modules interact with the basic exosite I. The reactive-site loop of boophilin remains fully accessible in its complex with thrombin, and might interact with FXa according to the standard mechanism. A conceptually similar inhibition mechanism is employed by a related inhibitor of the TF-FVIIa complex isolated from Ixodes scapularis, ixolaris. Significant variations to the Kunitz fold are encountered in several of these factors, and are particularly evident in the single-domain FXa inhibitor, O. moubata TAP, and in soft tick-derived platelet antiaggregants (e.g., O. moubata disagregin). Altogether, these antihemostatic factors illustrate the divergence between hard and soft ticks. The unsurpassed versatility of tick-derived Kunitz inhibitors establishes them as valuable tools for biochemical investigations, but also as lead compounds for the development of novel antithrombotics.

Targeting atherosclerosis by using modular, multifunctional micelles

Subtle clotting that occurs on the luminal surface of atherosclerotic plaques presents a novel target for nanoparticle-based diagnostics and therapeutics. We have developed modular multifunctional micelles that contain a targeting element, a fluorophore, and, when desired, a drug component in the same particle. Targeting atherosclerotic plaques in ApoE-null mice fed a high-fat diet was accomplished with the pentapeptide cysteine-arginine-glutamic acid-lysine-alanine, which binds to clotted plasma proteins. The fluorescent micelles bind to the entire surface of the plaque, and notably, concentrate at the shoulders of the plaque, a location that is prone to rupture. We also show that the targeted micelles deliver an increased concentration of the anticoagulant drug hirulog to the plaque compared with untargeted micelles.

Magnetic Resonance Imaging Contrast Agent Targeted Toward Activated Platelets Allows In Vivo Detection of Thrombosis and Monitoring of Thrombolysis

Background— Platelets are the key to thrombus formation and play a role in the development of atherosclerosis. Noninvasive imaging of activated platelets would be of great clinical interest. Here, we evaluate the ability of a magnetic resonance imaging (MRI) contrast agent consisting of microparticles of iron oxide (MPIOs) and a single-chain antibody targeting ligand-induced binding sites (LIBS) on activated glycoprotein IIb/IIIa to image carotid artery thrombi and atherosclerotic plaques.

Methods and Results— Anti-LIBS antibody or control antibody was conjugated to 1-μm MPIOs (LIBS MPIO/control MPIO). Nonocclusive mural thrombi were induced in mice with 6% ferric chloride. MRI (at 9.4 T) was performed once before and repeatedly in 12-minute-long sequences after LIBS MPIO/control MPIO injection. After 36 minutes, a significant signal void, corresponding to MPIO accumulation, was observed with LIBS MPIOs but not control MPIOs ( P <0.05). After thrombolysis, in LIBS MPIO-injected mice, the signal void subsided, indicating successful thrombolysis. On histology, the MPIO content of the thrombus, as well as thrombus size, correlated significantly with LIBS MPIO-induced signal void (both P <0.01). After ex vivo incubation of symptomatic human carotid plaques, MRI and histology confirmed binding to areas of platelet adhesion/aggregation for LIBS MPIOs but not for control MPIOs.

Conclusions— LIBS MPIOs allow in vivo MRI of activated platelets with excellent contrast properties and monitoring of thrombolytic therapy. Furthermore, activated platelets were detected on the surface of symptomatic human carotid plaques by ex vivo MRI. This approach represents a novel noninvasive technique allowing the detection and quantification of platelet-containing thrombi.

Visualization of activated platelets by targeted magnetic resonance imaging utilizing conformation-specific antibodies against glycoprotein IIb/IIIa

Ruptured atherosclerotic plaques, lined with activated platelets, constitute an attractive target for magnetic resonance imaging (MRI). This study evaluated whether microparticles of iron oxide (MPIO) targeting ligand-induced binding sites (LIBS) on the activated conformation of glycoprotein IIb/IIIa could be used to image platelets. MPIO (size: 1 μm) were conjugated to anti-LIBS or control single-chain antibody. Following guidewire injury to mouse femoral artery, platelet adhesion was present after 24 h. Mice were perfused with anti-LIBS-MPIO (or control MPIO) via the left ventricle and 11.7-tesla MRI was performed on femoral arteries ex vivo. A 3D gradient echo sequence attained an isotropic resolution of 25 μm. MPIO binding, quantified by MRI, was 4-fold higher with anti-LIBS-MPIO in comparison to control MPIO (p < 0.01). In histological sections, low signal zones on MRI and MPIO correlated strongly (R² = 0.72; p < 0.001), indicating accurate MR quantification. In conclusion, anti-LIBS-MPIO bind to activated platelets in mouse arteries, providing a basis for the use of function-specific single-chain antibody-MPIO conjugates for molecular MRI, and represent the first molecular imaging of a conformational change in a surface receptor. This presents an opportunity to specifically image activated platelets involved in acute atherothrombosis with MRI.

A contrast agent recognizing activated platelets reveals murine cerebral malaria pathology undetectable by conventional MRI

Human and murine cerebral malaria are associated with elevated levels of cytokines in the brain and adherence of platelets to the microvasculature. Here we demonstrated that the accumulation of platelets in the brain microvasculature can be detected with MRI, using what we believe to be a novel contrast agent, at a time when the pathology is undetectable by conventional MRI. Ligand-induced binding sites (LIBS) on activated platelet glycoprotein IIb/IIIa receptors were detected in the brains of malaria-infected mice 6 days after inoculation with Plasmodium berghei using microparticles of iron oxide (MPIOs) conjugated to a single-chain antibody specific for the LIBS (LIBS-MPIO). No binding of the LIBS-MPIO contrast agent was detected in uninfected animals. A combination of LIBS-MPIO MRI, confocal microscopy, and transmission electron microscopy revealed that the proinflammatory cytokine TNF-alpha, but not IL-1beta or lymphotoxin-alpha (LT-alpha), induced adherence of platelets to cerebrovascular endothelium. Peak platelet adhesion was found 12 h after TNF-alpha injection and was readily detected with LIBS-MPIO contrast-enhanced MRI. Temporal studies revealed that the level of MPIO-induced contrast was proportional to the number of platelets bound. Thus, the LIBS-MPIO contrast agent enabled noninvasive detection of otherwise undetectable cerebral pathology by in vivo MRI before the appearance of clinical disease, highlighting the potential of targeted contrast agents for diagnostic, mechanistic, and therapeutic studies.

Salivating for knowledge: potential pharmacological agents in tick saliva

Joppe Hovius and colleagues review anticoagulant and immunosuppressive proteins present in tick saliva, and discuss how immunologically targeting such molecules could prevent transmission of tick-borne pathogens.