SPECT imaging of fibrin using fibrin-binding peptides

Engineered Molecular Therapeutics Targeting Fibrin and the Coagulation System: a Biophysical Perspective

The coagulation cascade represents a sophisticated and highly choreographed series of molecular events taking place in the blood with important clinical implications. One key player in coagulation is fibrinogen, a highly abundant soluble blood protein that is processed by thrombin proteases at wound sites, triggering self-assembly of an insoluble protein hydrogel known as a fibrin clot. By forming the key protein component of blood clots, fibrin acts as a structural biomaterial with biophysical properties well suited to its role inhibiting fluid flow and maintaining hemostasis. Based on its clinical importance, fibrin is being investigated as a potentially valuable molecular target in the development of coagulation therapies. In this topical review, we summarize our current understanding of the coagulation cascade from a molecular, structural and biophysical perspective. We highlight single-molecule studies on proteins involved in blood coagulation and report on the current state of the art in directed evolution and molecular engineering of fibrin-targeted proteins and polymers for modulating coagulation. This biophysical overview will help acclimatize newcomers to the field and catalyze interdisciplinary work in biomolecular engineering toward the development of new therapies targeting fibrin and the coagulation system.

Molecular Imaging of Arterial and Venous Thrombosis

Thrombosis contributes to one in four deaths worldwide and is the cause of a large proportion of mortality and morbidity. A reliable and rapid diagnosis of thrombosis will allow for immediate therapy, thereby providing significant benefits to patients. Molecular imaging is a fast-growing and captivating area of research, in both preclinical and clinical applications. Major advances have been achieved by improvements in three central areas of molecular imaging: 1) Better markers for diseases, with increased sensitivity and selectivity; 2) Optimised contrast agents with improved signal to noise ratio; 3) Progress in scanner technologies with higher sensitivity and resolution. Clinically available imaging modalities used for molecular imaging include, magnetic resonance imaging (MRI), X-ray computed tomography (CT), ultrasound, as well as nuclear imaging, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT). In the preclinical imaging field, optical (fluorescence and bioluminescent) molecular imaging has provided new mechanistic insights in the pathology of thromboembolic diseases. Overall, the advances in molecular imaging, driven by the collaboration of various scientific disciplines, have substantially contributed to an improved understanding of thrombotic disease, and raises the exciting prospect of earlier diagnosis and individualised therapy for cardiovascular diseases. As such, these advances hold significant promise to be translated to clinical practice and ultimately to reduce mortality and morbidity in patients with thromboembolic diseases.

Docking, synthesis, in-vitro evaluation, and optimization of reaction conditions for direct radiolabeling of CGPRPPC with technetium-99m through the GAGG sequence

OBJECTIVE

With respect to the reported promising results of cyclic peptide CGPRPPC in early detection of thrombotic lesions, we developed a practical approach for technetium-99m labeling of this peptide using the Glycine-Alanine-Glycine-Glycine (GAGG) sequence as a chelating moiety.

MATERIALS AND METHODS

The peptide conjugated to GAGG was prepared using the solid-phase method. The optimization of radiolabeling conditions was performed on the basis of such variables as incubation time, reaction temperature, pH, and concentration of peptide and stannous chloride. Moreover, the stability and fibrin-binding affinity of the radiolabeled peptide were measured. The peptide-fibrin interactions were analyzed by docking studies using HEX and Auto dock 4.2. Softwares.

RESULTS

The amounts of synthesized peptide and stannous chloride required for optimal radiolabeling through GAGG were 10 µmol/l and 5 µg, respectively. The best radiochemical purity% (>93%) was achieved at pH 7-8 within 15 min and a reaction temperature of 37°C. On the basis of in silico and in-vitro results, the GAGG-conjugated CGPRPPC peptide showed better binding affinity versus the HYNIC-conjugated one.

CONCLUSION

We could radiolabel the fibrin-targeting peptide with high radiochemical purity% and stability during a short incubation period without a boiling step. Compared with the HYNIC-conjugated peptide, a higher binding affinity was found. Therefore, the GAGG chelating moiety possesses a considerable potentiality in Technetium 99m labeling of peptides while CGPRPPC maintains its binding properties to thrombotic lesions.

[18F]FDG-Labeled CGPRPPC Peptide Serving as a Small Thrombotic Lesions Probe, Including a Comparison with [99mTc]-Labeled Form

Purpose: Fibrin is a perfect target for specific imaging of all types of thrombotic lesions. Cyclic peptides were introduced as the best scaffolds out of the different types of probes for thrombi detection. This study was conducted to label previously synthesized peptide-targeting fibrin with [¹⁸F]FDG and its in vitro and in vivo assessments. Materials and Methods: CGPRPPC peptide functionalized with 6-hydrazinonicotinamide and Eei-NHS was synthesized and cyclized using air oxidation method. The cyclic sequences were labeled with [¹⁸F]FDG at 85°C within 30 min. The stability studies were performed in human plasma. Fibrin-binding and platelet aggregation tests were performed in vitro. Biodistribution and scintigraphy imaging in normal mice and carotid thrombotic rat model were considered as in vivo studies. Results: Radiolabeled peptides show a good stability in human plasma and also high-affinity binding for human fibrin. Platelet aggregation test confirmed specific binding of radiopeptides to fibrin. A key problem with the authors' previous research was inability to detect small-vessel thrombi. The results of positron emission tomography/computed tomography scanning show high specific uptake of [¹⁸F]FDG-labeled CGPRPPC in small-sized thrombosis. Conclusion: The experiment revealed that radiolabeling of cyclic peptide (CGPRPPC) with [¹⁸F]FDG enables us to detect small thrombotic lesions in small animal models with high resolution.

Nuclear imaging of thrombosis in small animal

Thromboembolic disorders are a major cause of morbidity and mortality worldwide. The progress in noninvasive imaging techniques has led to the development of radionuclide imaging based on SPECT and PET approaches to observe molecular and cellular processes that may underlie the onset and progression of disease. The advantages of using normal and genetically modified small animal research have spurred the development of dedicated small animal imaging systems. Animal models of venous and arterial thrombosis are largely used and have improved our understanding of the etiology and pathogenesis of thrombosis. Here, we review the literature regarding nuclear imaging of thrombosis in mice and rats.

Engineering of plasminogen activators for targeting to thrombus and heightening thrombolytic efficacy

Thrombotic occlusion of the coronary artery, which triggers acute myocardial infarction, is one of the major causes of death in the USA. Currently, arterial occlusions are treated with intravenous plasminogen activators (PAs), which dissolve the clot by activating plasminogen. However, PAs indiscriminately generate plasmin, which depletes critical clotting factors (fibrinogen, factor V, and factor VIII), precipitates a lytic state in the blood, and produces bleeding complications in a large patient population. PAs have been extensively investigated to achieve thrombus specificity, to attenuate the bleeding risk, and to widen their clinical applications. In this review, we discuss various strategies that have been pursued since the beginning of thrombolytic therapy. We review the biotechnological approaches that have been used to develop mutant and chimeric PAs for thrombus selectivity, including the use of specific antibodies for targeting thrombi. We discuss particulate carrier-based systems and triggered-release concepts. We propose new hypotheses and strategies to spur future studies in this research arena. Overall, we describe the approaches and accomplishments in the development of patient-friendly and workable delivery systems for thrombolytic drugs.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

Evaluation of iron oxide nanoparticle micelles for magnetic particle imaging (MPI) of thrombosis

Magnetic particle imaging (MPI) is an emerging medical imaging modality that directly visualizes magnetic particles in a hot-spot like fashion. We recently developed an iron oxide nanoparticle-micelle (ION-Micelle) platform that allows highly sensitive MPI. The goal of this study was to assess the potential of the ION-Micelles for MPI-based detection of thrombi. To this aim, an in vivo carotid artery thrombosis mouse model was employed and ex vivo magnetic particle spectrometer (MPS) measurements of the carotid arteries were performed. In addition, we studied the effect of functionalization of the ION-Micelle nanoplatform with fibrin-binding peptides (FibPeps) with respect to nanoparticle thrombus uptake and hence thrombus detection. In vivo quantitative MR imaging pre- and post-ION-Micelle injection was performed as reference for visualization of ION-micelle uptake. ION-Micelles significantly decreased T₂ values in the thrombi with respect to pre-injection T₂ values (p < 0.01) and significantly increased ex vivo MPS thrombus signal with respect to the noninjured, contralateral carotid (p < 0.01). Functionalization of the ION-Micelles with the FibPep peptides did not result in an increased MPS thrombus signal with respect to the non-fibrin binding ION-Micelles. The lack of a significant increased thrombus uptake for the FibPep-ION-Micelles indicates that (non-fibrin-specific) entrapment of nanoparticles in the mesh-like thrombi is the key contributor to thrombus nanoparticle uptake. Therefore, (nontargeted) ION-Micelles might be of value for noninvasive MPI-based diagnosis, characterization and treatment monitoring of thrombosis.

The evolution of fibrin-specific targeting strategies

Fibrin-specific targeting capabilities have been highly sought for over 50 years due to their implications for bio-molecule delivery, diagnostics, and regenerative medicine. Yet only recently has our full knowledge of fibrin's complex polymerization dynamics and biological interactions begun to be fully exploited in pursuit of this goal. This highlight will discuss the range of rapidly changing strategies for specifically targeting fibrin over the precursor fibrinogen and the advantages and disadvantages of these approaches for various applications.

Targeted thrombolysis strategies for neuroprotective effect

Stroke is usually treated by systemic thrombolytic therapy if the patient presents within an appropriate time window. There is also widespread interest in the development of thrombolytic agents that can be used in cases of delayed presentation. Current agents that can be used in cases of delayed presentation of nerve damage by thrombus. Current systemic thrombolytic therapy is associated with adverse effects such as fibrinogenolysis and bleeding. In an attempt to increase the efficacy, safety, and specificity of thrombolytic therapy, a number of targeted thrombolytic agents have been studied in recent years. This review focuses on the concepts underlying targeted thrombolytic therapy and describes recent drug developments in this field.

Imaging of cerebrovascular pathology in animal models of Alzheimer's disease

In Alzheimer's disease (AD), vascular pathology may interact with neurodegeneration and thus aggravate cognitive decline. As the relationship between these two processes is poorly understood, research has been increasingly focused on understanding the link between cerebrovascular alterations and AD. This has at last been spurred by the engineering of transgenic animals, which display pathological features of AD and develop cerebral amyloid angiopathy to various degrees. Transgenic models are versatile for investigating the role of amyloid deposition and vascular dysfunction, and for evaluating novel therapeutic concepts. In addition, research has benefited from the development of novel imaging techniques, which are capable of characterizing vascular pathology in vivo. They provide vascular structural read-outs and have the ability to assess the functional consequences of vascular dysfunction as well as to visualize and monitor the molecular processes underlying these pathological alterations. This article focusses on recent in vivo small animal imaging studies addressing vascular aspects related to AD. With the technical advances of imaging modalities such as magnetic resonance, nuclear and microscopic imaging, molecular, functional and structural information related to vascular pathology can now be visualized in vivo in small rodents. Imaging vascular and parenchymal amyloid-β (Aβ) deposition as well as Aβ transport pathways have been shown to be useful to characterize their dynamics and to elucidate their role in the development of cerebral amyloid angiopathy and AD. Structural and functional imaging read-outs have been employed to describe the deleterious affects of Aβ on vessel morphology, hemodynamics and vascular integrity. More recent imaging studies have also addressed how inflammatory processes partake in the pathogenesis of the disease. Moreover, imaging can be pivotal in the search for novel therapies targeting the vasculature.

Near-infrared fluorescence imaging of murine atherosclerosis using an oxidized low density lipoprotein-targeted fluorochrome

The aim of this study was to explore the feasibility of detecting plaques using an NIR797 fluorochrome-labeled, anti-oxLDL antibody (anti-oxLDL-NIR797) and near-infrared fluorescence (NIRF) imaging in a murine model of atherosclerosis. Anti-mouse oxLDL polyclonal antibodies were conjugated to NIR797 dyes to synthesis oxLDL-targeted NIRF probe. In situ and ex vivo NIRF imaging of the high-cholesterol diet-induced atherosclerotic lesions of apoE-/- mice (baseline) as well as ex vivo NIRF imaging in the progression and regression group (without or with atorvastatin treatment for another 8 weeks) were performed 24 h after an intravenous injection of 1 mg/kg of anti-oxLDL-NIR797, while phosphate-buffered saline (PBS) was used for the controls. The plaque areas were investigated using Oil Red O (ORO) staining. Aortas isolated from the apoE-/- mice 24 h post-injection exhibited a selective, strong, heterogeneous NIRF signal enhancement in the aortic root, arch, and bifurcation, whereas the PBS and competitive inhibition groups had limited NIRF signal changes (p < 0.05). There was a significant correlation between ORO staining and NIRF in the atherosclerotic aortas that received anti-oxLDL-NIR797. Immunofluorescence studies confirmed the colocalization of the oxLDL/macrophages and NIR797 fluorochromes. Furthermore, the atherosclerotic lesions of atorvastatin-treated mice showed reduced anti-oxLDL-NIR797 uptake and oxLDL expression. These results indicate that NIRF plaque imaging is feasible with an oxLDL-targeted NIRF probe. Thus, oxLDL-based molecular imaging of atherosclerotic plaques is feasible and may provide important methods for characterizing vulnerable plaques and monitoring the response to therapeutic interventions for atherosclerosis.