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

Multi-modality imaging for assessment of the microcirculation in peripheral artery disease: Bench to clinical practice

Peripheral artery disease (PAD) is a highly prevalent disorder with a high risk of mortality and amputation despite the introduction of novel medical and procedural treatments. Microvascular disease (MVD) is common among patients with PAD, and despite the established role as a predictor of amputations and mortality, MVD is not routinely assessed as part of current standard practice. Recent pre-clinical and clinical perfusion and molecular imaging studies have confirmed the important role of MVD in the pathogenesis and outcomes of PAD. The recent advancements in the imaging of the peripheral microcirculation could lead to a better understanding of the pathophysiology of PAD, and result in improved risk stratification, and our evaluation of response to therapies. In this review, we will discuss the current understanding of the anatomy and physiology of peripheral microcirculation, and the role of imaging for assessment of perfusion in PAD, and the latest advancements in molecular imaging. By highlighting the latest advancements in multi-modality imaging of the peripheral microcirculation, we aim to underscore the most promising imaging approaches and highlight potential research opportunities, with the goal of translating these approaches for improved and personalized management of PAD in the future.

Vulnerable Plaque Imaging

Atherosclerotic plaques progress as a result of inflammation. Both invasive and noninvasive imaging techniques have been developed to identify and characterize plaque as vulnerable (more likely to rupture and cause a clinical event). Imaging techniques to identify vulnerable include identifying vessels with focal subendothelial collections of I) inflammatory cells; II) lipid/ fatty acid; III) local regions of hypoxia; IV) local expression of angiogenesis factors; V) local expression of protease; VI) intravascular foci of thrombus; hemorrhage (most often seen in the aftermath of a clinical event); VII) apoptosis and VIII) microcalcification. This review provides an overview of atherosclerotic plaque progression and tracers which can visualize specific molecules associated with vulnerability.

A Fibrin Site-Specific Nanoprobe for Imaging Fibrin-Rich Thrombi and Preventing Thrombus Formation in Venous Vessels

Venous thromboembolism (VTE) is a prevalent public health issue worldwide. Before treatment, spatiotemporally accurate thrombus detection is essential. However, with the currently available imaging technologies, this is challenging. Herein, the development of a novel fibrin-specific nanoprobe (NP) based on the conjugation of poly(lactic-co-glycolic acid) with the pentapeptide Cys-Arg-Glu-Lys-Ala (CREKA) for selective and semiquantitative imaging in vivo is presented. By integrating Fe₃ O₄ and NIR fluorochrome (IR780), the NP can function as a highly sensitive sensor for the direct analysis of thrombi in vivo. The fibrin-specific NP distinguishes fibrin-rich thrombi from collagen-rich or erythrocyte-rich thrombi, which can be beneficial for future individually tailored therapeutic strategy. Furthermore, loading NPs with the ketotifen fumarate results in mast cell degranulation inhibition, and hence, NPs can prevent thrombosis without the risk of excessive bleeding. Thus, the use of fibrin-specific NPs may serve as a safe alternative approach for the detection and prevention of VTEs in susceptible populations in the future.

Detection and Characterization of Thrombosis in Humans Using Fibrin-Targeted Positron Emission Tomography and Magnetic Resonance

OBJECTIVES

The authors present a novel technique to detect and characterize LAA thrombus in humans using combined positron emission tomography (PET)/cardiac magnetic resonance (CMR) of a fibrin-binding radiotracer, [⁶⁴Cu]FBP8.

BACKGROUND

The detection of thrombus in the left atrial appendage (LAA) is vital in the prevention of stroke and is currently performed using transesophageal echocardiography (TEE).

METHODS

The metabolism and pharmacokinetics of [⁶⁴Cu]FBP8 were studied in 8 healthy volunteers. Patients with atrial fibrillation and recent TEEs of the LAA (positive n = 12, negative n = 12) were injected with [⁶⁴Cu]FBP8 and imaged with PET/CMR, including mapping the longitudinal magnetic relaxation time (T₁) in the LAA.

RESULTS

[⁶⁴Cu]FBP8 was stable to metabolism and was rapidly eliminated. The maximum standardized uptake value (SUV_Max) in the LAA was significantly higher in the TEE-positive than TEE-negative subjects (median of 4.0 [interquartile range (IQR): 3.0-6.0] vs 2.3 [IQR: 2.1-2.5]; P < 0.001), with an area under the receiver-operating characteristic curve of 0.97. An SUV_Max threshold of 2.6 provided a sensitivity of 100% and specificity of 84%. The minimum T₁ (T_1Min) in the LAA was 970 ms (IQR: 780-1,080 ms) vs 1,380 ms (IQR: 1,120-1,620 ms) (TEE positive vs TEE negative; P < 0.05), with some overlap between the groups. Logistic regression using SUV_Max and T_1Min allowed all TEE-positive and TEE-negative subjects to be classified with 100% accuracy.

CONCLUSIONS

PET/CMR of [⁶⁴Cu]FBP8 is able to detect acute as well as older platelet-poor thrombi with excellent accuracy. Furthermore, the integrated PET/CMR approach provides useful information on the biological properties of thrombus such as fibrin and methemoglobin content. (Imaging of LAA Thrombosis; NCT03830320).

An unmet clinical need: The history of thrombus imaging

Robust thrombus imaging is an unresolved clinical unmet need dating back to the mid 1970s. While early molecular imaging approaches began with nuclear SPECT imaging, contrast agents for virtually all biomedical imaging modalities have been demonstrated in vivo with unique strengths and common weaknesses. Two primary molecular imaging targets have been pursued for thrombus imaging: platelets and fibrin. Some common issues noted over 40 years ago persist today. Acute thrombus is readily imaged with all probes and modalities, but aged thrombus remains a challenge. Similarly, anti-coagulation continues to interfere with and often negate thrombus imaging efficacy, but heparin is clinically required in patients suspected of pulmonary embolism, deep venous thrombosis or coronary ruptured plaque prior to confirmatory diagnostic studies have been executed and interpreted. These fundamental issues can be overcome, but an innovative departure from the prior approaches will be needed.

Theranostic radioiodine-labelled melanin nanoparticles inspired by clinical brachytherapy seeds

Radioiodine is widely used in nuclear medicine, mainly serving as a tracer and therapeutic agent, and benefits from its various radioactive isotopes of iodine including I-123, I-124, I-125, I-131 and so on. Melanin is a natural material widely dispersed in the human skin, hair and eyes. The excellent biocompatibility and multifunctional abilities of melanin make it a perfect carrier for biomedical applications. Here, we fabricated theranostic radioiodine-labelled melanin nanoparticles (MNPs) through a novel Ag-I two-step method. The Ag-I labelling method for MNP radioiodine-labelling has advantages including a faster labelling time, higher labelling yield, and higher stability than the chloramine-T oxidation method reported previously. The obtained MNP-Ag-¹³¹I can be used for both single-photon emission computed tomography and Cherenkov radiation imaging. The β-rays of ¹³¹I also make it a good candidate as a cancer cell killer. The theranostic properties of this nanoparticle were also proved in a xenograft tumor model in vivo. In summary, this study provides a new concept for radioiodine labelling nanoparticles, which can be further investigated in various imaging and radiotherapy applications with different radioactive isotopes of iodine.

In Vivo Molecular Characterization of Abdominal Aortic Aneurysms Using Fibrin-Specific Magnetic Resonance Imaging

BACKGROUND

The incidence of abdominal aortic aneurysms (AAAs) will significantly increase during the next decade. Novel biomarkers, besides diameter, are needed for a better characterization of aneurysms and the estimation of the risk of rupture. Fibrin is a key protein in the formation of focal hematoma associated with the dissection of the aortic wall and the development of larger thrombi during the progression of AAAs. This study evaluated the potential of a fibrin-specific magnetic resonance (MR) probe for the in vivo characterization of the different stages of AAAs.

METHODS AND RESULTS

AAAs spontaneously developed in ApoE-/- mice following the infusion of angiotensin-II (Ang-II, 1 μg/kg-1·per minute). An established fibrin-specific molecular MR probe (EP2104R, 10 μmol/kg-1) was administered after 1 to 4 weeks following Ang-II infusion (n=8 per group). All imaging experiments were performed on a clinical 3T Achieva MR system with a microscopy coil (Philips Healthcare, Netherlands). The development of AAA-associated fibrin-rich hematoma and thrombi was assessed. The high signal generated by the fibrin probe enabled high-resolution MR imaging for an accurate assessment and quantification of the relative fibrin composition of focal hematoma and thrombi. Contrast-to-noise-ratios (CNRs) and R1-relaxation rates following the administration of the fibrin probe were in good agreement with ex vivo immunohistomorphometry (R2=0.83 and 0.85) and gadolinium concentrations determined by inductively coupled plasma mass spectroscopy (R2=0.78 and 0.72).

CONCLUSIONS

The fibrin-specific molecular MR probe allowed the delineation and quantification of changes in fibrin content in early and advanced AAAs. Fibrin MRI could provide a novel in vivo biomarker to improve the risk stratification of patients with aortic aneurysms.

Diagnosis of LVAD Thrombus using a High-Avidity Fibrin-Specific 99mTc Probe

Treatment of advanced heart failure with implantable LVADs is increasing, driven by profound unmet patient need despite potential serious complications: bleeding, infection, and thrombus. The experimental objective was to develop a sensitive imaging approach to assess early thrombus accumulation in LVADs under operational high flow and high shear rates.

Methods: A monomeric bifunctional ligand with a fibrin-specific peptide, a short spacer, and ^99mTc chelating amino acid sequence (F1A) was developed and compared to its tetrameric PEG analogue (F4A).

Results: ^99mTc attenuation by LVAD titanium (1 mm) was 23%. ^99mTc-F1A affinity to fibrin was K_d ~10 µM, whereas, the bound ^99mTc-F4A probe was not displaced by F1A (120,000:1). Human plasma interfered with ^99mTc-F1A binding to fibrin clot (p<0.05) in vitro, whereas, ^99mTc-F4A targeting was unaffected. The pharmacokinetic half-life of ^99mTc-F4A was 28% faster (124±41 min) than ^99mTc-F1A (176±26 min) with both being bioeliminated through the urinary system with negligible liver or spleen biodistribution. In mice with carotid thrombus, ^99mTc-F4A binding to the injured carotid was much greater (16.3±3.3 %ID/g, p=0.01) than that measured with an irrelevant negative control, ^99mTc-I4A (3.4±1.6 %ID/g). In an LVAD mock flow-loop (1:1, PBS:human plasma:heparin) operating at maximal flow rate, ^99mTc-F4A bound well to phantom clots in 2 min (p<0.05), whereas ^99mTc-F1A had negligible targeting. Excised LVADs from patients undergoing pump exchange or heart transplant were rewired, studied in the mock flow loop, and found to have spatially variable fibrin accumulations in the inlet and outlet cannulas and bearings.

Conclusions:^99mTc-F4A is a high-avidity prototype probe for characterizing thrombus in LVADs that is anticipated to help optimize anticoagulation, reduce thromboembolic events, and minimize pump exchange.

Evaluation of spectral photon counting computed tomography K-edge imaging for determination of gold nanoparticle biodistribution in vivo

Spectral photon counting computed tomography (SPCCT) is an emerging medical imaging technology. SPCCT scanners record the energy of incident photons, which allows specific detection of contrast agents due to measurement of their characteristic X-ray attenuation profiles. This approach is known as K-edge imaging. Nanoparticles formed from elements such as gold, bismuth or ytterbium have been reported as potential contrast agents for SPCCT imaging. Furthermore, gold nanoparticles have many applications in medicine, such as adjuvants for radiotherapy and photothermal ablation. In particular, longitudinal imaging of the biodistribution of nanoparticles would be highly attractive for their clinical translation. We therefore studied the capabilities of a novel SPCCT scanner to quantify the biodistribution of gold nanoparticles in vivo. PEGylated gold nanoparticles were used. Phantom imaging showed that concentrations measured on gold images correlated well with known concentrations (slope = 0.94, intercept = 0.18, RMSE = 0.18, R² = 0.99). The SPCCT system allowed repetitive and quick acquisitions in vivo, and follow-up of changes in the AuNP biodistribution over time. Measurements performed on gold images correlated with the inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements in the organs of interest (slope = 0.77, intercept = 0.47, RMSE = 0.72, R² = 0.93). TEM results were in agreement with the imaging and ICP-OES in that much higher concentrations of AuNPs were observed in the liver, spleen, bone marrow and lymph nodes (mainly in macrophages). In conclusion, we found that SPCCT can be used for repetitive and non-invasive determination of the biodistribution of gold nanoparticles in vivo.

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.

Multicolor spectral photon-counting computed tomography: in vivo dual contrast imaging with a high count rate scanner

A new prototype spectral photon-counting computed tomography (SPCCT) based on a modified clinical CT system has been developed. SPCCT analysis of the energy composition of the transmitted x-ray spectrum potentially allows simultaneous dual contrast agent imaging, however, this has not yet been demonstrated with such a system. We investigated the feasibility of using this system to distinguish gold nanoparticles (AuNP) and an iodinated contrast agent. The contrast agents and calcium phosphate were imaged in phantoms. Conventional CT, gold K-edge, iodine and water images were produced and demonstrated accurate discrimination and quantification of gold and iodine concentrations in a phantom containing mixtures of the contrast agents. In vivo experiments were performed using New Zealand White rabbits at several times points after injections of AuNP and iodinated contrast agents. We found that the contrast material maps clearly differentiated the distributions of gold and iodine in the tissues allowing quantification of the contrast agents’ concentrations, which matched their expected pharmacokinetics. Furthermore, rapid, repetitive scanning was done, which allowed measurement of contrast agent kinetics with high temporal resolution. In conclusion, a clinical scale, high count rate SPCCT system is able to discriminate gold and iodine contrast media in different organs in vivo.

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.

Development of silica-encapsulated silver nanoparticles as contrast agents intended for dual-energy mammography

OBJECTIVE

Dual-energy (DE) mammography has recently entered the clinic. Previous theoretical and phantom studies demonstrated that silver provides greater contrast than iodine for this technique. Our objective was to characterize and evaluate in vivo a prototype silver contrast agent ultimately intended for DE mammography.

METHODS

The prototype silver contrast agent was synthesized using a three-step process: synthesis of a silver core, silica encapsulation and PEG coating. The nanoparticles were then injected into mice to determine their accumulation in various organs, blood half-life and dual-energy contrast. All animal procedures were approved by the institutional animal care and use committee.

RESULTS

The final diameter of the nanoparticles was measured to be 102 (±9) nm. The particles were removed from the vascular circulation with a half-life of 15 min, and accumulated in macrophage-rich organs such as the liver, spleen and lymph nodes. Dual-energy subtraction techniques increased the signal difference-to-noise ratio of the particles by as much as a factor of 15.2 compared to the single-energy images. These nanoparticles produced no adverse effects in mice.

CONCLUSION

Silver nanoparticles are an effective contrast agent for dual-energy x-ray imaging. With further design improvements, silver nanoparticles may prove valuable in breast cancer screening and diagnosis.

KEY POINTS

• Silver has potential as a contrast agent for DE mammography. • Silica-coated silver nanoparticles are biocompatible and suited for in vivo use. • Silver nanoparticles produce strong contrast in vivo using DE mammography imaging systems.

Gold silver alloy nanoparticles (GSAN): an imaging probe for breast cancer screening with dual-energy mammography or computed tomography

Earlier detection of breast cancer reduces mortality from this disease. As a result, the development of better screening techniques is a topic of intense interest. Contrast-enhanced dual-energy mammography (DEM) is a novel technique that has improved sensitivity for cancer detection. However, the development of contrast agents for this technique is in its infancy. We herein report gold-silver alloy nanoparticles (GSAN) that have potent DEM contrast properties and improved biocompatibility. GSAN formulations containing a range of gold : silver ratios and capped with m-PEG were synthesized and characterized using various analytical methods. DEM and computed tomography (CT) phantom imaging showed that GSAN produced robust contrast that was comparable to silver alone. Cell viability, reactive oxygen species generation and DNA damage results revealed that the formulations with 30% or higher gold content are cytocompatible to Hep G2 and J774A.1 cells. In vivo imaging was performed in mice with and without breast tumors. The results showed that GSAN produce strong DEM and CT contrast and accumulated in tumors. Furthermore, both in vivo imaging and ex vivo analysis indicated the excretion of GSAN via both urine and feces. In summary, GSAN produce strong DEM and CT contrast, and has potential for both blood pool imaging and for breast cancer screening.

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.