Molecular MRI of Thrombosis

Fibrinogen aptamer functionalized gold-coated iron-oxide nanoparticles for targeted imaging of thrombi

Targeting of molecular constituents of thrombi with aptamer functionalized core-shell nanoparticles (CSN) allowed for high resolution clot delineation in T2-weighted magnetic resonance imaging. Meanwhile, the gold-coating demonstrated sufficient contrast capabilities in computed tomography (1697 HU μM^-1). This targeted CSN formulation could allow for precise imaging of blood clots at low nanomolar concentrations.

Imaging High-Risk Atherothrombosis Using a Novel Fibrin-Binding Positron Emission Tomography Probe

BACKGROUND AND PURPOSE

High-risk atherosclerosis is an underlying cause of cardiovascular events, yet identifying the specific patient population at immediate risk is still challenging. Here, we used a rabbit model of atherosclerotic plaque rupture and human carotid endarterectomy specimens to describe the potential of molecular fibrin imaging as a tool to identify thrombotic plaques.

METHODS

Atherosclerotic plaques in rabbits were induced using a high-cholesterol diet and aortic balloon injury (N=13). Pharmacological triggering was used in a group of rabbits (n=9) to induce plaque disruption. Animals were grouped into thrombotic and nonthrombotic plaque groups based on gross pathology (gold standard). All animals were injected with a novel fibrin-specific probe ⁶⁸Ga-CM246 followed by positron emission tomography (PET)/magnetic resonance imaging 90 minutes later. ⁶⁸Ga-CM246 was quantified on the PET images using tissue-to-background (back muscle) ratios and standardized uptake value.

RESULTS

Both tissue-to-background (back muscle) ratios and standardized uptake value were significantly higher in the thrombotic versus nonthrombotic group (P<0.05). Ex vivo PET and autoradiography of the abdominal aorta correlated positively with in vivo PET measurements. Plaque disruption identified by ⁶⁸Ga-CM246 PET agreed with gross pathology assessment (85%). In ex vivo surgical specimens obtained from patients undergoing elective carotid endarterectomy (N=12), ⁶⁸Ga-CM246 showed significantly higher binding to carotid plaques compared to a D-cysteine nonbinding control probe.

CONCLUSIONS

We demonstrated that molecular fibrin PET imaging using ⁶⁸Ga-CM246 could be a useful tool to diagnose experimental and clinical atherothrombosis. Based on our initial results using human carotid plaque specimens, in vivo molecular imaging studies are warranted to test ⁶⁸Ga-CM246 PET as a tool to stratify risk in atherosclerotic patients.

Thrombus Imaging Using 3D Printed Middle Cerebral Artery Model and Preclinical Imaging Techniques: Application to Thrombus Targeting and Thrombolytic Studies

Diseases with the highest burden for society such as stroke, myocardial infarction, pulmonary embolism, and others are due to blood clots. Preclinical and clinical techniques to study blood clots are important tools for translational research of new diagnostic and therapeutic modalities that target blood clots. In this study, we employed a three-dimensional (3D) printed middle cerebral artery model to image clots under flow conditions using preclinical imaging techniques including fluorescent whole-body imaging, magnetic resonance imaging (MRI), and computed X-ray microtomography (microCT). Both liposome-based, fibrin-targeted, and non-targeted contrast agents were proven to provide a sufficient signal for clot imaging within the model under flow conditions. The application of the model for clot targeting studies and thrombolytic studies using preclinical imaging techniques is shown here. For the first time, a novel method of thrombus labeling utilizing barium sulphate (Micropaque^®) is presented here as an example of successfully employed contrast agents for in vitro experiments evaluating the time-course of thrombolysis and thus the efficacy of a thrombolytic drug, recombinant tissue plasminogen activator (rtPA). Finally, the proof-of-concept of in vivo clot imaging in a middle cerebral artery occlusion (MCAO) rat model using barium sulphate-labelled clots is presented, confirming the great potential of such an approach to make experiments comparable between in vitro and in vivo models, finally leading to a reduction in animals needed.

Biomimetic fibrin-targeted and H2O2-responsive nanocarriers for thrombus therapy

Thrombosis is a principle cause of various life-threatening cardiovascular diseases. However, current antithrombotic treatments using drugs only offer limited efficacy due to short half-life, low targeting ability to the thrombus site, and unexpected bleeding complications. Taking into account of the biological characteristics of thrombus including upregulation of hydrogen peroxide (H₂O₂) and abundance of fibrin, we engineered a H₂O₂-responsive nanocarrier for thrombus-targeting delivery of an antithrombotic agent (i.e., tirofiban). The nanocarrier was composed of a drug-conjugated dextran nanocore and a red blood cell (RBC) membrane shell, and its surface was functionalized with a fibrin-targeting peptide, CREKA. Tirofiban was conjugated to dextran through a H₂O₂-cleavable phenylboronic ester linkage. The fibrin-targeting RBC membrane-cloaked dextran-tirofiban conjugate nanoparticles (i.e., T-RBC-DTC NPs) can scavenge H₂O₂ and provide controlled release of tirofiban to achieve site-specific antithrombotic effects. In RAW 264.7 cells and HUVECs, the T-RBC-DTC NPs effectively scavenged H₂O₂ and protected cells from H₂O₂-induced cytotoxicity. In the ferric chloride-induced carotid thrombosis mouse model, the T-RBC-DTC NPs efficiently accumulated at the injured carotid artery and exhibited significantly enhanced antithrombotic activity compared to free drug. The T-RBC-DTC NPs also exhibited good biocompatibility according to histology analysis. Overall, our results indicated that this bioengineered nanocarrier offers a promising therapeutic strategy for thrombotic disorders.

Thrombo-tag, an in vivo formed nanotracer for the detection of thrombi in mice by fast pre-targeted molecular imaging

Radioisotope-labelled nanoparticles permit novel applications in molecular imaging, while recent developments in imaging have enabled direct visualization of biological processes. While this holds true for pathological processes that are stable in time, such as cancer, imaging approaches are limited for phenomena that take place in the range of minutes, such as thrombotic events. Here, we take advantage of bioorthogonal chemistry to demonstrate the concept of nanoparticle-based fast pre-targeted imaging. Using a newly designed nanoparticle that targets platelets we show the applicability of this approach developing thrombo-tag, an in vivo produced nanoparticle that labels thrombi. We show that thrombo-tag allows specific labelling of platelets that accumulate in the injured pulmonary vasculature, or that aggregate in brains of mice suffering thrombotic processes. The fast kinetics and high specificity features of thrombo-tag may critically expand the application of molecular imaging to the most prevalent and debilitating diseases in the clinics.

Diagnostic Accuracy of High-Resolution Black-Blood MRI in the Evaluation of Intracranial Large-Vessel Arterial Occlusions

BACKGROUND AND PURPOSE

3D high-resolution black-blood MRI or MR vessel wall imaging allows evaluation of the intracranial arterial wall and extraluminal pathology. We investigated the diagnostic accuracy and reliability of black-blood MRI for the intraluminal detection of large-vessel arterial occlusions.

MATERIALS AND METHODS

We retrospectively identified patients with intracranial arterial occlusions, confirmed by CTA or DSA, who also underwent 3D black-blood MRI with nonenhanced and contrast-enhanced T1 sampling perfection with application-optimized contrasts by using different flip angle evolution (T1 SPACE) sequences. Black-blood MRI findings were evaluated by 2 independent and blinded neuroradiologists. Large-vessel intracranial arterial segments were graded on a 3-point scale (grades 0-2) for intraluminal baseline T1 hyperintensity and contrast enhancement. Vessel segments were considered positive for arterial occlusion if focal weak (grade 1) or strong (grade 2) T1-hyperintense signal and/or enhancement replaced the normal intraluminal black-blood signal.

RESULTS

Thirty-one patients with 38 intracranial arterial occlusions were studied. The median time interval between black-blood MRI and CTA/DSA reference standard studies was 2 days (range, 0-20 days). Interobserver agreement was good for T1 hyperintensity (κ = 0.63) and excellent for contrast enhancement (κ = 0.89). High sensitivity (100%) and specificity (99.8%) for intracranial arterial occlusion diagnosis was observed with either intraluminal T1 hyperintensity or contrast-enhancement imaging criteria on black-blood MRI. Strong grade 2 intraluminal enhancement was maintained in >80% of occlusions irrespective of location or chronicity. Relatively increased strong grade 2 intraluminal T1 hyperintensity was noted in chronic/incidental versus acute/subacute occlusions (45.5% versus 12.5%, P = .04).

CONCLUSIONS

Black-blood MRI with or without contrast has high diagnostic accuracy and reliability in evaluating intracranial large-vessel arterial occlusions with near-equivalency to DSA and CTA.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

Activatable magnetic resonance nanosensor as a potential imaging agent for detecting and discriminating thrombosis

The early detection and accurate characterization of life-threatening diseases such as cardiovascular disease and cancer are critical to the design of treatment. Knowing whether or not a thrombus in a blood vessel is new (fresh) or old (constituted) is very important for physicians to decide a treatment protocol. We have designed smart MRI nano-sensors that can detect, sense and report the stage or progression of cardiovascular diseases such as thrombosis. The nanosensors were functionalized with fibrin-binding peptide to specifically target thrombus and were also labelled with fluorescent dye to enable optical imaging. We have demonstrated that our nanosensors were able to switch between the T1 and T2 signal depending on thrombus age or the presence or absence of thrombin at the thrombus site. The developed nanosensors appeared to be non-toxic when tested with Chinese Hamster Ovarian cells within the tested concentrations. The working principle demonstrated in this study can be applied to many other diseases such as cancer.

Effect of Model Thrombus Volume and Elastic Modulus on Magnetomotive Ultrasound Signal Under Pulsatile Flow

Direct ultrasonic imaging of arterial and venous thrombi could aid in diagnosis and treatment planning by providing rapid and cost-effective measurements of thrombus volume and elastic modulus. Toward this end, it was demonstrated that open-air magnetomotive ultrasound (MMUS) provides specific contrast to superparamagnetic iron oxide-labeled model thrombi embedded in gelatin-based blood vessel-mimicking flow phantoms. MMUS was performed on model thrombi in the presence of pulsatile flow that mimics cardiac-induced motion found in real vasculature. The MMUS signal and contrast-to-noise ratio (CNR) were measured across a range of physiologically relevant thrombus volumes and elastic moduli. Model thrombus volumes as small as 0.5 ml were shown to be detectable (CNR > 1) over the entire range of elastic moduli tested (3.5-40 kPa). It was also found that MMUS signal and CNR are increased with increasing thrombus volume ( ) and decreasing elastic modulus ( ), while variations in pulsatile flow rate had little effect. These findings demonstrate that MMUS has promise as a direct in vivo thrombosis imaging modality for quantifying thrombus volume and stiffness.

Peptide-based fibrin-targeting probes for thrombus imaging

The development of new methods to image the onset and progression of thrombosis is an unmet need. Non-invasive molecular imaging techniques targeting specific key structures involved in the formation of thrombosis have demonstrated the ability to detect thrombus in different disease state models and in patients. Due to its high concentration in the thrombus and its essential role in thrombus formation, the detection of fibrin is an attractive strategy for identification of thrombosis. Herein we provide an overview of recent and selected fibrin-targeted probes for molecular imaging of thrombosis by magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), and optical techniques. Emphasis is placed on work that our lab has explored over the last 15 years that has resulted in the progression of the fibrin-binding PET probe [64Cu]FBP8 from preclinical studies into human trials.

Multisite Thrombus Imaging and Fibrin Content Estimation With a Single Whole-Body PET Scan in Rats

OBJECTIVE

Thrombosis is a leading cause of morbidity and mortality worldwide. Current diagnostic strategies rely on imaging modalities that are specific for distinct vascular territories, but a thrombus-specific whole-body imaging approach is still missing. Moreover, imaging techniques to assess thrombus composition are underdeveloped, although therapeutic strategies may benefit from such technology. Therefore, our goal was to test whether positron emission tomography (PET) with the fibrin-binding probe (64)Cu-FBP8 allows multisite thrombus detection and fibrin content estimation.

APPROACH AND RESULTS

Thrombosis was induced in Sprague-Dawley rats (n=32) by ferric chloride application on both carotid artery and femoral vein. (64)Cu-FBP8-PET/CT imaging was performed 1, 3, or 7 days after thrombosis to detect thrombus location and to evaluate age-dependent changes in target uptake. Ex vivo biodistribution, autoradiography, and histopathology were performed to validate imaging results. Arterial and venous thrombi were localized on fused PET/CT images with high accuracy (97.6%; 95% confidence interval, 92-100). A single whole-body PET/MR imaging session was sufficient to reveal the location of both arterial and venous thrombi after (64)Cu-FBP8 administration. PET imaging showed that probe uptake was greater in younger clots than in older ones for both arterial and venous thrombosis (P<0.0001). Quantitative histopathology revealed an age-dependent reduction of thrombus fibrin content (P<0.001), consistent with PET results. Biodistribution and autoradiography further confirmed the imaging findings.

CONCLUSIONS

We demonstrated that (64)Cu-FBP8-PET is a feasible approach for whole-body thrombus detection and that molecular imaging of fibrin can provide, noninvasively, insight into clot composition.

Multimodal Molecular Imaging Reveals High Target Uptake and Specificity of 111In- and 68Ga-Labeled Fibrin-Binding Probes for Thrombus Detection in Rats

We recently showed the high target specificity and favorable imaging properties of 64Cu and Al18F PET probes for noninvasive imaging of thrombosis. Here, our aim was to evaluate new derivatives labeled with either with 68Ga, 111In, or 99mTc as thrombus imaging agents for PET and SPECT. In this study, the feasibility and potential of these probes for thrombus imaging was assessed in detail in 2 animal models of arterial thrombosis. The specificity of the probes was further evaluated using a triple-isotope approach with multimodal SPECT/PET/CT imaging.

METHODS

Radiotracers were synthesized using a known fibrin-binding peptide conjugated to 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid monoamide (DOTA-MA), or a diethylenetriamine ligand (DETA-propanoic acid [PA]), followed by labeling with 68Ga (FBP14, 68Ga-NODAGA), 111In (FBP15, 111In-DOTA-MA), or 99mTc (FBP16, 99mTc(CO)3-DETA-PA), respectively. PET or SPECT imaging, biodistribution, pharmacokinetics, and metabolic stability were evaluated in rat models of mural and occlusive carotid artery thrombosis. In vivo target specificity was evaluated by comparing the distribution of the SPECT and PET probes with preformed 125I-labeled thrombi and with a nonbinding control probe using SPECT/PET/CT imaging.

RESULTS

All 3 radiotracers showed affinity similar to soluble fibrin fragment DD(E) (inhibition constant=0.53-0.83 μM). After the kidneys, the highest uptake of 68Ga-FBP14 and 111In-FBP15 was in the thrombus (1.0±0.2 percentage injected dose per gram), with low off-target accumulation. Both radiotracers underwent fast systemic elimination (half-life, 8-15 min) through the kidneys, which led to highly conspicuous thrombi on PET and SPECT images. 99mTc-FBP16 displayed low target uptake and distribution consistent with aggregation or degradation. Triple-isotope imaging experiments showed that both 68Ga-FBP14 and 111In-FBP15, but not the nonbinding derivative 64Cu-D-Cys-FBP8, detected the location of the 125I-labeled thrombus, confirming high target specificity.

CONCLUSION

68Ga-FBP14 and 111In-FBP15 have high fibrin affinity and thrombus specificity and represent useful PET and SPECT probes for thrombus detection.

Radiation Dosimetry of the Fibrin-Binding Probe ⁶⁴Cu-FBP8 and Its Feasibility for PET Imaging of Deep Vein Thrombosis and Pulmonary Embolism in Rats

The diagnosis of deep venous thromboembolic disease is still challenging despite the progress of current thrombus imaging modalities and new diagnostic algorithms. We recently reported the high target uptake and thrombus imaging efficacy of the novel fibrin-specific PET probe (64)Cu-FBP8. Here, we tested the feasibility of (64)Cu-FBP8 PET to detect source thrombi and culprit emboli after deep vein thrombosis and pulmonary embolism (DVT-PE). To support clinical translation of (64)Cu-FBP8, we performed a human dosimetry estimation using time-dependent biodistribution in rats.

METHODS

Sprague-Dawley rats (n = 7) underwent ferric chloride application on the femoral vein to trigger thrombosis. Pulmonary embolism was induced 30 min or 2 d after DVT by intrajugular injection of a preformed blood clot labeled with (125)I-fibrinogen. PET imaging was performed to detect the clots, and SPECT was used to confirm in vivo the location of the pulmonary emboli. Ex vivo γ counting and histopathology were used to validate the imaging findings. Detailed biodistribution was performed in healthy rats (n = 30) at different time points after (64)Cu-FBP8 administration to estimate human radiation dosimetry. Longitudinal whole-body PET/MR imaging (n = 2) was performed after (64)Cu-FBP8 administration to further assess radioactivity clearance.

RESULTS

(64)Cu-FBP8 PET imaging detected the location of lung emboli and venous thrombi after DVT-PE, revealing significant differences in uptake between target and background tissues (P < 0.001). In vivo SPECT imaging and ex vivo γ counting confirmed the location of the lung emboli. PET quantification of the venous thrombi revealed that probe uptake was greater in younger clots than in older ones, a result confirmed by ex vivo analyses (P < 0.001). Histopathology revealed an age-dependent reduction of thrombus fibrin content (P = 0.006), further supporting the imaging findings. Biodistribution and whole-body PET/MR imaging showed a rapid, primarily renal, body clearance of (64)Cu-FBP8. The effective dose was 0.021 mSv/MBq for males and 0.027 mSv/MBq for females, supporting the feasibility of using (64)Cu-FBP8 in human trials.

CONCLUSION

We showed that (64)Cu-FBP8 PET is a feasible approach to image DVT-PE and that radiogenic adverse health effects should not limit the clinical translation of (64)Cu-FBP8.

MR imaging probes: design and applications

This perspective outlines strategies towards the development of MR imaging probes that our lab has explored over the last 15 years. Namely, we discuss methods to enhance the signal generating capacity of MR probes and how to achieve tissue specificity through protein targeting or probe activation within the tissue microenvironment.

Platelets: still a therapeutical target for haemostatic disorders

Platelets are cytoplasmatic fragments from bone marrow megakaryocytes present in blood. In this work, we review the basis of platelet mechanisms, their participation in syndromes and in arterial thrombosis, and their potential as a target for designing new antithrombotic agents. The option of new biotechnological sources is also explored.

Effect of Chelate Type and Radioisotope on the Imaging Efficacy of 4 Fibrin-Specific PET Probes

Thrombus formation plays a major role in cardiovascular diseases, but noninvasive thrombus imaging is still challenging. Fibrin is a major component of both arterial and venous thrombi and represents an ideal candidate for imaging of thrombosis. Recently, we showed that (64)Cu-DOTA-labeled PET probes based on fibrin-specific peptides are suitable for thrombus imaging in vivo; however, the metabolic stability of these probes was limited. Here, we describe 4 new probes using either (64)Cu or aluminum fluoride (Al(18)F) chelated to 2 NOTA derivatives.

METHODS

Probes were synthesized using a known fibrin-specific peptide conjugated to either NODAGA (FBP8, FBP10) or NOTA-monoamide (FBP9, FBP11) as chelators, followed by labeling with (64)Cu (FBP8 and FBP9) or Al(18)F (FBP10 and FBP11). PET imaging efficacy, pharmacokinetics, biodistribution, and metabolic stability were assessed in a rat model of arterial thrombosis.

RESULTS

All probes had similar nanomolar affinity (435-760 nM) for the soluble fibrin fragment DD(E). PET imaging allowed clear visualization of thrombus by all probes, with a 5-fold or higher thrombus-to-background ratio. Compared with the previous DOTA derivative, the new (64)Cu probes FBP8 and FBP9 showed substantially improved metabolic stability (>85% intact in blood at 4 h after injection), resulting in high uptake at the target site (0.5-0.8 percentage injected dose per gram) that persisted over 5 h, producing increasingly greater target-to-background ratios. The thrombus uptake was 5- to 20-fold higher than the uptake in the contralateral artery, blood, muscle, lungs, bone, spleen, large intestine, and heart at 2 h after injection and 10- to 40-fold higher at 5 h. The Al(18)F derivatives FBP10 and FBP11 were less stable, in particular the NODAGA conjugate (FBP10, <30% intact in blood at 4 h after injection), which showed high bone uptake and low thrombus-to-background ratios that decreased over time. The high thrombus-to-contralateral ratios for all probes were confirmed by ex vivo biodistribution and autoradiography. The uptake in the liver (<0.5 percentage injected dose per gram), kidneys, and blood were similar for all tracers, and they all showed predominant renal clearance.

CONCLUSION

FBP8, FBP9, and FBP11 showed excellent metabolic stability and high thrombus-to-background ratios and represent promising candidates for imaging of thrombosis in vivo.

In vivo molecular imaging of thrombosis and thrombolysis using a fibrin-binding positron emission tomographic probe

BACKGROUND

Fibrin is a major component of arterial and venous thrombi and represents an ideal candidate for molecular imaging of thrombosis. Here, we describe imaging properties and target uptake of a new fibrin-specific positron emission tomographic probe for thrombus detection and therapy monitoring in 2 rat thrombosis models.

METHODS AND RESULTS

The fibrin-binding probe FBP7 was synthesized by conjugation of a known short cyclic peptide to a cross-bridged chelator (CB-TE2A), followed by labeling with copper-64. Adult male Wistar rats (n=26) underwent either carotid crush injury (mural thrombosis model) or embolic stroke (occlusive thrombosis model) followed by recombinant tissue-type plasminogen activator treatment (10 mg/kg, IV). FBP7 detected thrombus location in both animal models with a high positron emission tomographic target-to-background ratio that increased over time (>5-fold at 30-90 minutes, >15-fold at 240-285 minutes). In the carotid crush injury animals, biodistribution analysis confirmed high probe uptake in the thrombotic artery (≈0.5%ID/g; >5-fold greater than blood and other tissues of the head and thorax). Similar results were obtained from ex vivo autoradiography of the ipsilateral versus contralateral carotid arteries. In embolic stroke animals, positron emission tomographic-computed tomographic imaging localized the clot in the internal carotid/middle cerebral artery segment of all rats. Time-dependent reduction of activity at the level of the thrombus was detected in recombinant tissue-type plasminogen activator-treated rats but not in vehicle-injected animals. Brain autoradiography confirmed clot dissolution in recombinant tissue-type plasminogen activator-treated animals, but enduring high thrombus activity in control rats.

CONCLUSIONS

We demonstrated that FBP7 is suitable for molecular imaging of thrombosis and thrombolysis in vivo and represents a promising candidate for bench-to-bedside translation.

Construction and evaluation of Fe₃O₄-based PLGA nanoparticles carrying rtPA used in the detection of thrombosis and in targeted thrombolysis

Thrombotic disease is extremely harmful to human health, but early detection and treatment can help improve prognoses and reduce mortality. To date, few studies have used MR molecular imaging in the early detection of thrombi and in the dynamic monitoring of the thrombolytic efficiency. In this article, we construct Fe3O4-based poly(lactic-co-glycolic acid) (PLGA) nanoparticles to use in the detection of thrombi and in targeted thrombolysis using MRI monitoring. Cyclic arginine-glycine-aspartic peptide (cRGD) was grafted onto the chitosan (CS) surface to synthesize a CS-cRGD film using carbodiimide-mediated amide bond formation. A double emulsion solvent evaporation method (water in oil in water [W/O/W]) was used to construct Fe3O4-based PLGA nanoparticles carrying recombinant tissue plasminogen activator (rtPA) (Fe3O4-PLGA-rtPA/CS-cRGD). Fe3O4-PLGA, Fe3O4-PLGA-rtPA, and Fe3O4-PLGA-rtPA/CS nanoparticles were constructed using the same W/O/W method. The results showed that the Fe3O4-based nanoparticles were constructed successfully and have a regular shape, a relatively uniform size, a high carrier rate of Fe3O4 and encapsulation efficiency of rtPA, and a relatively high activity of released rtPA. Transmission electron microscope (TEM) images revealed that the iron oxide particles were relatively uniformly distributed in the nano-spherical shell. The Fe3O4-based nanoparticles could be imaged using a clinical MRI scanner, and there were no significant differences in the transverse relaxation rate (R2*) or in the signal-to-noise ratio (SNR) values between the Fe3O4-based nanoparticles and an Fe3O4 solution with the same concentration of Fe3O4. In vitro and in vivo experiments confirmed that the Fe3O4-PLGA-rtPA/CS-cRGD nanoparticles specifically accumulated on the edge of the thrombus and that they had a significant effect on the thrombolysis compared with the Fe3O4-PLGA, Fe3O4-PLGA-rtPA, and Fe3O4-PLGA-rtPA/CS nanoparticles and with free rtPA solution. These results suggest the potential of the Fe3O4-PLGA-rtPA/CS-cRGD nanoparticles as a dual-function tool in the early detection of a thrombus and in the dynamic monitoring of the thrombolytic efficiency using MRI.

Preparation and characterization of gadolinium-loaded PLGA particles surface modified with RGDS for the detection of thrombus

Thrombotic disease is a leading cause of death and disability worldwide. The development of magnetic resonance molecular imaging provides potential promise for early disease diagnosis. In this study, we explore the preparation and characterization of gadolinium (Gd)-loaded poly (lactic-co-glycolic acid) (PLGA) particles surface modified with the Arg-Gly-Asp-Ser (RGDS) peptide for the detection of thrombus. PLGA was employed as the carrier-delivery system, and a double emulsion solvent-evaporation method (water in oil in water) was used to prepare PLGA particles encapsulating the magnetic resonance contrast agent Gd diethylenetriaminepentaacetic acid (DTPA). To synthesize the Gd-PLGA/chitosan (CS)-RGDS particles, carbodiimide-mediated amide bond formation was used to graft the RGDS peptide to CS to form a CS-RGDS film that coated the surface of the PLGA particles. Blank PLGA, Gd-PLGA, and Gd-PLGA/CS particles were fabricated using the same water in oil in water method. Our results indicated that the RGDS peptide successfully coated the surface of the Gd-PLGA/CS-RGDS particles. The particles had a regular shape, smooth surface, relatively uniform size, and did not aggregate. The high electron density of the Gd-loaded particles and a translucent film around the particles coated with the CS and CS-RGDS films could be observed by transmission electron microscopy. In vitro experiments demonstrated that the Gd-PLGA/CS-RGDS particles could target thrombi and could be imaged using a clinical magnetic resonance scanner. Compared with the Gd-DTPA solution, the longitudinal relaxation time of the Gd-loaded particles was slightly longer, and as the Gd-load concentration increased, the longitudinal relaxation time values decreased. These results suggest the potential of the Gd-PLGA/CS-RGDS particles for the sensitive and specific detection of thrombus at the molecular level.

Contrast-enhanced imaging of SPIO-labeled platelets using magnetomotive ultrasound

The ability to image platelets in vivo can provide insight into blood clotting processes and coagulopathies, and aid in identifying sites of vascular endothelial damage related to trauma or cardiovascular disease. Toward this end, we have developed a magnetomotive ultrasound (MMUS) system that provides contrast-enhanced imaging of superparamagnetic iron oxide (SPIO) labeled platelets via magnetically-induced vibration. Platelets are a promising platform for functional imaging contrast because they readily take up SPIOs and are easily harvested from blood. Here we report a novel MMUS system that accommodates an arbitrarily thick sample while maintaining portability. We employed a frequency- and phase-locked motion detection algorithm based on bandpass filtering of the differential RF phase, which allows for the detection of sub-resolution vibration amplitudes on the order of several nanometers. We then demonstrated MMUS in homogenous tissue phantoms at SPIO concentrations as low as 0.09 mg ml(-1) Fe (p < 0.0001, n = 6, t-test). Finally, we showed that our system is capable of three-dimensional imaging of a 185 µL simulated clot containing SPIO-platelets. This highlights the potential utility for non-invasive imaging of platelet-rich clots, which would constitute a fundamental advance in technology for the study of hemostasis and detection of clinically relevant thrombi.

Fibrin-targeted PET probes for the detection of thrombi

There is an ongoing effort to develop better methods for noninvasive detection and characterization of thrombi. Here we describe the synthesis and evaluation of three new fibrin-targeted positron emission tomography (PET) probes (FBP1, FBP2, FBP3). Three fibrin-specific peptides were conjugated as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-monoamides at the C- and N-termini and chelated with (64)CuCl2. Probes were prepared with a specific activity ranging from 10 to 130 μCi/nmol. Both the peptides and the probes exhibited nanomolar dissociation constants (Kd) for the soluble fibrin fragment DD(E), although the Cu-DOTA derivatization resulted in a 2-3 fold loss in affinity relative to the parent peptide. Biodistribution and imaging studies were performed in a rat model of carotid artery thrombosis. For FBP1 and FBP2 at 120 min post injection, the vessel containing the thrombus showed the highest concentration of radioactivity after the excretory organs, that is, the liver and kidneys. This was confirmed ex vivo by autoradiography, which showed >4-fold activity in the thrombus-containing artery compared to the contralateral artery. FBP3 showed much lower thrombus uptake, and the difference was traced to greater metabolism of this probe. Hybrid MR-PET imaging with FBP1 or FBP2 confirmed that these probes were effective for the detection of an arterial thrombus in this rat model. A thrombus was visible on PET images as a region of high activity that corresponded to a region of arterial occlusion identified by simultaneous MR angiography. FBP1 and FBP2 represent promising new probes for the molecular imaging of thrombi.

Molecular imaging of fibrin deposition in deep vein thrombosis using fibrin-targeted near-infrared fluorescence

OBJECTIVES

The goal of this study was to develop and validate a new fibrin-targeted imaging agent that enables high-resolution near-infrared fluorescence (NIRF) imaging of deep vein thrombosis (DVT).

BACKGROUND

NIRF imaging of fibrin could enable highly sensitive and noninvasive molecular imaging of thrombosis syndromes in vivo.

METHODS

A fibrin-targeted peptide was conjugated to a near-infrared fluorophore Cy7, termed FTP11-Cy7. The NIRF peptide is based on a fibrin-specific imaging agent that has completed Phase II clinical magnetic resonance imaging trials. In vitro binding of FTP11-Cy7 to human plasma clots was assessed by using fluorescence reflectance imaging. Next, FTP11-Cy7 was intravenously injected in mice with femoral DVT induced by topical 7.5% ferric chloride treatment. Intravital fluorescence microscopy and noninvasive fluorescence molecular tomography-computed tomography were performed in 32 mice with DVT, followed by histological analyses.

RESULTS

In vitro human clot-binding analyses showed a 6-fold higher NIRF clot target-to-background ratio (TBR) of FTP11-Cy7 than free Cy7 (6.3 ± 0.34 vs. 1.2 ± 0.03; p < 0.0001). The thrombus TBR of acute and subacute femoral DVT with FTP11-Cy7 obtained by using intravital fluorescence microscopy was >400% higher than control free Cy7. Binding of FTP11-Cy7 to thrombi was blocked by a 100-fold excess of unlabeled competitor peptide both in vitro and in vivo (p < 0.001 for each). Histological analyses confirmed that FTP11-Cy7 specifically accumulated in thrombi. Noninvasive fluorescence molecular tomography-computed tomography imaging of fibrin in jugular DVT demonstrated strong NIRF signal in thrombi compared with sham-operated jugular veins (mean TBR 3.5 ± 0.7 vs. 1.5 ± 0.3; p < 0.05).

CONCLUSIONS

The fibrin-targeted NIRF agent FTP11-Cy7 was shown to avidly and specifically bind human and murine thrombi, and enable sensitive, multimodal intravital and noninvasive NIRF molecular imaging detection of acute and subacute murine DVT in vivo.