Noninvasive Imaging of Angiogenesis With a 99m Tc-Labeled Peptide Targeted at α v β 3 Integrin After Murine Hindlimb Ischemia

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.

In Vivo Imaging of Acute Hindlimb Ischaemia in Rat Model: A Pre-Clinical PET Study

BACKGROUND

To better understand ischaemia-related molecular alterations, temporal changes in angiogenic Aminopeptidase N (APN/CD13) expression and glucose metabolism were assessed with PET using a rat model of peripheral arterial disease (PAD).

METHODS

The mechanical occlusion of the base of the left hindlimb triggered using a tourniquet was applied to establish the ischaemia/reperfusion injury model in Fischer-344 rats. 2-[18F]FDG and [68Ga]Ga-NOTA-c(NGR) PET imaging performed 1, 3, 5, 7, and 10 days post-ischaemia induction was followed by Western blotting and immunohistochemical staining for APN/CD13 in ischaemic and control muscle tissue extracts.

RESULTS

Due to a cellular adaptation to hypoxia, a gradual increase in [68Ga]Ga-NOTA-c(NGR) and 2-[18F]FDG uptake was observed from post-intervention day 1 to 7 in the ischaemic hindlimbs, which was followed by a drop on day 10. Conforming pronounced angiogenic recovery, the NGR accretion of the ischaemic extremities differed significantly from the controls 5, 7, and 10 days after ischaemia induction (p ≤ 0.05), which correlated with the Western blot and immunohistochemical results. No remarkable radioactivity was depicted between the normally perfused hindlimbs of either the ischaemic or the control groups.

CONCLUSIONS

The PET-based longitudinal assessment of angiogenesis-associated APN/CD13 expression and glucose metabolism during ischaemia may continue to broaden our knowledge on the pathophysiology of PAD.

Safety, biodistribution and radiation dosimetry of the Arg-Gly-Asp peptide 99m Tc-maraciclatide in healthy volunteers

BACKGROUND

99m Tc-Maraciclatide is a radiolabelled RGD (Arg-Gly-Asp) peptide that binds with high affinity to α v β 3 and α v β 5 integrins, common receptors upregulated in disease states involving angiogenesis and inflammation. As such, it holds promise as a novel diagnostic imaging agent for a range of pathological conditions. The present study provides the safety, biodistribution and radiation dosimetry of 99m Tc-maraciclatide in healthy volunteers.

METHODS

A phase 1, randomised, placebo-controlled study assessed the safety, biodistribution and radiation dosimetry of 99m Tc-maraciclatide in healthy volunteers. Participants were randomised into three groups receiving 99m Tc-maraciclatide and three chemical amounts of maraciclatide in an escalating dose protocol. Eight participants in each group received the required amount of maraciclatide via intravenous injection, with the remaining two receiving a placebo. Biodistribution was assessed by acquiring scintigraphic images at time points up to 24 h after a bolus injection of 99m Tc-maraciclatide. 99m Tc-maraciclatide activity in plasma and urine was measured up to 7 days post-administration.

RESULTS

99m Tc-maraciclatide was safe and well tolerated, with no serious adverse events reported. Initial uptakes of 99m Tc were highest in the gastrointestinal tract (20%), liver (15%), and lungs (9%). Similarly, the regions with the highest normalised cumulated activities were the contents of the urinary bladder and voided urine (3.4 ± 0.4 MBq*h/MBq), the combined walls of the small intestine and upper and lower large intestine (0.9 ± 0.2 MBq*h/MBq), liver (0.8 ± 0.2 MBq*h/MBq), lung (0.4 ± 0.1 MBq*h/MBq). The main route of 99m Tc excretion was renal (55%), with a systemic urinary clearance of approximately 6.7 ml/min/kg. The pharmacokinetic analysis gave a mean apparent terminal elimination half-life of the unlabelled molecular maraciclatide of approximately 1 h, independent of dose. The mean ED per unit injected activity was 7.8 ± 0.8 µSv/MBq.

CONCLUSION

99m Tc-maraciclatide is a safe radiopharmaceutical formulation with a dosimetry profile similar to other 99m Tc-based imaging agents.

Nano-radiopharmaceuticals as therapeutic agents

In recent years, there has been an increased interest in exploring the potential synergy between nanotechnology and nuclear medicine. The application of radioactive isotopes, commonly referred to as radiopharmaceuticals, is recognized in nuclear medicine for diagnosing and treating various diseases. Unlike conventional pharmaceutical agents, radiopharmaceuticals are designed to work without any pharmacological impact on the body. Nevertheless, the radiation dosage employed in radiopharmaceuticals is often sufficiently high to elicit adverse effects associated with radiation exposure. Exploiting their capacity for selective accumulation on specific organ targets, radiopharmaceuticals have utility in treating diverse disorders. The incorporation of nanosystems may additionally augment the targeting capability of radiopharmaceuticals, leveraging their distinct pharmacokinetic characteristics. Conversely, radionuclides could be used in research to assess nanosystems pharmacologically. However, more investigation is needed to verify the safety and effectiveness of radiopharmaceutical applications mediated by nanosystems. The use of nano-radiopharmaceuticals as therapeutic agents to treat various illnesses and disorders is majorly covered in this review. The targeted approach to cancer therapy and various types of nanotools for nano-radiopharmaceutical delivery, is also covered in this article.

Positron emission tomography imaging of lung cancer: An overview of alternative positron emission tomography tracers beyond F18 fluorodeoxyglucose

Lung cancer has been the leading cause of cancer-related mortality in China in recent decades. Positron emission tomography-computer tomography (PET/CT) has been established in the diagnosis of lung cancer. ¹⁸F-FDG is the most widely used PET tracer in foci diagnosis, tumor staging, treatment planning, and prognosis assessment by monitoring abnormally exuberant glucose metabolism in tumors. However, with the increasing knowledge on tumor heterogeneity and biological characteristics in lung cancer, a variety of novel radiotracers beyond ¹⁸F-FDG for PET imaging have been developed. For example, PET tracers that target cellular proliferation, amino acid metabolism and transportation, tumor hypoxia, angiogenesis, pulmonary NETs and other targets, such as tyrosine kinases and cancer-associated fibroblasts, have been reported, evaluated in animal models or under clinical investigations in recent years and play increasing roles in lung cancer diagnosis. Thus, we perform a comprehensive literature review of the radiopharmaceuticals and recent progress in PET tracers for the study of lung cancer biological characteristics beyond glucose metabolism.

Multi-Scale Imaging of Vascular Pathologies in Cardiovascular Disease

Cardiovascular disease entails systemic changes in the vasculature. The endothelial cells lining the blood vessels are crucial in the pathogenesis of cardiovascular disease. Healthy endothelial cells direct the blood flow to tissues as vasodilators and act as the systemic interface between the blood and tissues, supplying nutrients for vital organs, and regulating the smooth traffic of leukocytes into tissues. In cardiovascular diseases, when inflammation is sensed, endothelial cells adjust to the local or systemic inflammatory state. As the inflamed vasculature adjusts, changes in the endothelial cells lead to endothelial dysfunction, altered blood flow and permeability, expression of adhesion molecules, vessel wall inflammation, thrombosis, angiogenic processes, and extracellular matrix production at the endothelial cell level. Preclinical multi-scale imaging of these endothelial changes using optical, acoustic, nuclear, MRI, and multimodal techniques has progressed, due to technical advances and enhanced biological understanding on the interaction between immune and endothelial cells. While this review highlights biological processes that are related to changes in the cardiac vasculature during cardiovascular diseases, it also summarizes state-of-the-art vascular imaging techniques. The advantages and disadvantages of the different imaging techniques are highlighted, as well as their principles, methodologies, and preclinical and clinical applications with potential future directions. These multi-scale approaches of vascular imaging carry great potential to further expand our understanding of basic vascular biology, to enable early diagnosis of vascular changes and to provide sensitive diagnostic imaging techniques in the management of cardiovascular disease.

Radioactive polymeric nanoparticles for biomedical application

Nowadays, emerging radiolabeled nanosystems are revolutionizing medicine in terms of diagnostics, treatment, and theranostics. These radionuclides include polymeric nanoparticles (NPs), liposomal carriers, dendrimers, magnetic iron oxide NPs, silica NPs, carbon nanotubes, and inorganic metal-based nanoformulations. Between these nano-platforms, polymeric NPs have gained attention in the biomedical field due to their excellent properties, such as their surface to mass ratio, quantum properties, biodegradability, low toxicity, and ability to absorb and carry other molecules. In addition, NPs are capable of carrying high payloads of radionuclides which can be used for diagnostic, treatment, and theranostics depending on the radioactive material linked. The radiolabeling process of nanoparticles can be performed by direct or indirect labeling process. In both cases, the most appropriate must be selected in order to keep the targeting properties as preserved as possible. In addition, radionuclide therapy has the advantage of delivering a highly concentrated absorbed dose to the targeted tissue while sparing the surrounding healthy tissues. Said another way, radioactive polymeric NPs represent a promising prospect in the treatment and diagnostics of cardiovascular diseases such as cardiac ischemia, infectious diseases such as tuberculosis, and other type of cancer cells or tumors.

[68Ga]-NODAGA-RGD Positron Emission Tomography (PET) for Assessment of Post Myocardial Infarction Angiogenesis as a Predictor for Left Ventricular Remodeling in Mice after Cardiac Stem Cell Therapy

Angiogenesis plays a central role in the healing process following acute myocardial infarction. The PET tracer [⁶⁸Ga]-NODAGA-RGD, which is a ligand for the α_vβ₃ integrin, has been investigated for imaging angiogenesis in the process of healing myocardium in both animal and clinical studies. It’s value as a prognostic marker of functional outcome remains unclear. Therefore, the aim of this work was to establish [⁶⁸Ga]-NODAGA-RGD for imaging angiogenesis in the murine infarct model and evaluate the tracer as a predictor for cardiac remodeling in the context of cardiac stem cell therapy. [⁶⁸Ga]-NODAGA-RGD PET performed seven days after left anterior descending coronary artery (LAD) occlusion in 129S6 mice showed intense tracer accumulation within the infarct region. The specificity was shown in a sub-group of animals by application of the competitive inhibitor cilengitide prior to tracer injection in a subgroup of animals. Myocardial infarction (MI) significantly reduced cardiac function and resulted in pronounced left ventricular remodeling after three weeks, as measured by cardiac MRI in a separate group. Cardiac induced cells (CiC) that were derived from mESC injected intramyocardially in the therapy group significantly improved left ventricular ejection fraction (LVEF). Surprisingly, CiC transplantation resulted in significantly lower tracer accumulation seven days after MI induction. Accordingly, we successfully established the PET tracer [⁶⁸Ga]-NODAGA-RGD for the assessment of α_vβ₃ integrin expression in the healing process after MI in the mouse model. Yet, our results indicate that the mere extent of angiogenesis following MI does not serve as a sufficient prognostic marker for functional outcome.

VEGF receptor targeted imaging of angiogenic response to limb ischemia in diabetic vs. non-diabetic Yucatan minipigs

Background

New therapies to treat diabetic peripheral artery disease (PAD) require target-specific non-invasive imaging modalities to follow efficacy. As a translational study, we performed targeted imaging of receptors for vascular endothelial growth factor (VEGF) in response to anterior femoral artery occlusion (FAO) in Yucatan minipigs and compare the normal response to response in diabetic Yucatan minipigs.

Methods

Eleven Yucatan minipigs, 6 non-diabetic (non-D) and 5 purpose bred diabetic (D) (Sinclair, Auxvasse MO), underwent intravascular total occlusion of the anterior femoral artery (FA). At days 1 and 28, pigs underwent SPECT/CT ²⁰¹Tl hindlimb perfusion imaging and at day 7 were injected with [^99mTc]DOTA-PEG-scVEGF (scV/Tc) tracer targeting VEGF receptor, and underwent biopsies of the hindlimb muscles for gamma counting and histology, followed by imaging. One day after the final scan, pigs underwent contrast angiography of the lower extremities. Counts from scans were converted to percentage injected activity (%IA).

Results

Perfusion was lower in the occluded hindlimb compared to non-occluded on day 1 in both the D and non-D pigs. At day 7, scV/Tc count ratio of counts from ROIs drawn in proximal gastrocnemius muscle for the occluded over non-occluded limb was significantly higher in non-D vs. D pigs (1.32 ± 0.06 vs. 1.04 ± 0.13, P = 0.02) reflecting higher level of angiogenesis. Perfusion increased between days 1 and 28 in the muscles in the occluded limb for the non-diabetic pigs while the diabetic pig showed no increase (+ 0.13 ± 0.08 %IA vs. − 0.13 ± 0.11, P = 0.003). The anterior FA showed poor contrast filling beyond occluder and qualitatively fewer bridging collaterals compared to non-D pigs at 28 days.

Conclusion

VEGF receptor targeted imaging showed the effects of diabetes to suppress angiogenesis in response to occlusion of the anterior femoral artery of purpose bred diabetic Yucatan minipigs and indicates potential applicability as a marker to follow efficacy of novel therapies to improve blood flow by stimulating angiogenesis in diabetic PAD.

Imaging the Landmarks of Vascular Recovery

Background: Peripheral arterial disease (PAD) is a major worldwide health concern. Since the late 1990s therapeutic angiogenesis has been investigated as an alternative to traditional PAD treatments. Although positive preclinical results abound in the literature, the outcomes of human clinical trials have been discouraging. Among the challenges the field has faced has been a lack of standardization of the timings and measures used to validate new treatment approaches.

Methods: In order to study the spatiotemporal dynamics of both perfusion and neovascularization in mice subjected to surgically-induced hindlimb ischemia (n= 30), we employed three label-free imaging modalities (a novel high-sensitivity ultrasonic Power Doppler methodology, laser speckle contrast, and photoacoustic imaging), as well as a tandem of radio-labeled molecular probes, ^99mTc-NC100692 and ^99mTc-BRU-5921 respectively, designed to detect two key modulators of angiogenic activity, α_Vβ₃ and HIF-1α , via scintigraphic imaging.

Results: The multimodal imaging strategy reveals a set of “landmarks”—key physiological and molecular events in the healing process—that can serve as a standardized framework for describing the impact of emerging PAD treatments. These landmarks span the entire process of neovascularization, beginning with the rapid decreases in perfusion and oxygenation associated with ligation surgery, extending through pro-angiogenic changes in gene expression driven by the master regulator HIF-1α , and ultimately leading to complete functional revascularization of the affected tissues.

Conclusions: This study represents an important step in the development of multimodal non-invasive imaging strategies for vascular research; the combined results offer more insight than can be gleaned through any of the individual imaging methods alone. Researchers adopting similar imaging strategies and will be better able to describe changes in the onset, duration, and strength of each of the landmarks of vascular recovery, yielding greater biological insight, and enabling more comprehensive cross-study comparisons. Perhaps most important, this study paves the road for more efficient translation of PAD research; emerging experimental treatments can be more effectively assessed and refined at the preclinical stage, ultimately leading to better next-generation therapies.

Simultaneous photoacoustic imaging of intravascular and tissue oxygenation

Hypoxia, a low tissue oxygenation condition caused by insufficient oxygen supply, leads to potentially irreversible tissue damage, such as brain infarction during stroke. Intravascular oxygenation has long been used by photoacoustic imaging, among other imaging modalities, to study hypoxia. However, intravascular oxygenation describes only the oxygen supply via microcirculation, which does not directly reflect the amount of free oxygen available for metabolism in the interstitial fluid. Therefore, to fully understand hypoxia, it is highly desirable to monitor blood oxygenation as well as tissue oxygenation during the same biological process. In this work, by combining high-resolution photoacoustic microscopy (PAM) and a novel bioreducible N-oxide-based hypoxia-sensitive probe HyP-650, we have demonstrated simultaneous imaging of intravascular oxygenation and tissue hypoxia. We have established detailed chemical, optical, and photoacoustic properties of HyP-650 for hypoxic activation in vitro and in living cells. We have also performed PAM on hindlimb ischemia models and tumor-bearing mice to study the correlation between intravascular oxygenation and tissue oxygenation at various hypoxic levels. We expect that Hyp-650 enhanced photoacoustic imaging will find a variety of applications in brain and cancer research.

Noninvasive In Vivo Quantification of Adeno-Associated Virus Serotype 9-Mediated Expression of the Sodium/Iodide Symporter Under Hindlimb Ischemia and Neuraminidase Desialylation in Skeletal Muscle Using Single-Photon Emission Computed Tomography/Computed Tomography

BACKGROUND

We propose micro single-photon emission computed tomography/computed tomography imaging of the hNIS (human sodium/iodide symporter) to noninvasively quantify adeno-associated virus 9 (AAV9)-mediated gene expression in a murine model of peripheral artery disease.

METHODS

AAV9-hNIS (2×10¹¹ viral genome particles) was injected into nonischemic or ischemic gastrocnemius muscles of C57Bl/6J mice following unilateral hindlimb ischemia ± the α-sialidase NA (neuraminidase). Control nonischemic limbs were injected with phosphate buffered saline or remained noninjected. Twelve mice underwent micro single-photon emission computed tomography/computed tomography imaging after serial injection of pertechnetate (^99mTcO₄^-), a NIS substrate, up to 28 days after AAV9-hNIS injection. Twenty four animals were euthanized at selected times over 1 month for ex vivo validation. Forty-two animals were imaged with ^99mTcO₄^- ± the selective NIS inhibitor perchlorate on day 10, to ascertain specificity of radiotracer uptake. Tissue was harvested for ex vivo validation. A modified version of the U-Net deep learning algorithm was used for image quantification.

RESULTS

As quantitated by standardized uptake value, there was a gradual temporal increase in ^99mTcO₄^- uptake in muscles treated with AAV9-hNIS. Hindlimb ischemia, NA, and hindlimb ischemia plus NA increased the magnitude of ^99mTcO₄^- uptake by 4- to 5-fold compared with nonischemic muscle treated with only AAV9-hNIS. Perchlorate treatment significantly reduced ^99mTcO₄^- uptake in AAV9-hNIS-treated muscles, demonstrating uptake specificity. The imaging results correlated well with ex vivo well counting (r²=0.9375; P<0.0001) and immunoblot analysis of NIS protein (r²=0.65; P<0.0001).

CONCLUSIONS

Micro single-photon emission computed tomography/computed tomography imaging of hNIS-mediated ^99mTcO₄^- uptake allows for accurate in vivo quantification of AAV9-driven gene expression, which increases under ischemic conditions or neuraminidase desialylation in skeletal muscle.

Imaging VEGF Receptors and αvβ3 Integrins in a Mouse Hindlimb Ischemia Model of Peripheral Arterial Disease

PURPOSE

To compare targeted imaging of vascular endothelial growth factor (VEGF) receptors vs. α_vβ₃ integrins in a mouse hindlimb ischemia model of peripheral artery disease.

PROCEDURES

Male wild-type (WT) C57BL/6 mice (8- to 10-week old) (n = 24) underwent left femoral artery ligation. The right leg served as control. Five days later, mice were injected with either VEGF receptor targeting [^99mTc]DOTA-PEG-scVEGF ([^99mTc]scV) (n = 8) or with α_vβ₃-targeting tracer [^99mTc]HYNIC-cycloRGD ([^99mTc]RGD) (n = 8) and underwent single photon emission computed tomography (SPECT) x-ray computed tomography imaging. To assess non-specific [^99mTc]scV uptake, six additional mice received a mixture of [^99mTc]scV and 30-fold excess of targeting protein, scVEGF. Tracer uptake as %ID was measured using volumetric regions encompassing the hindlimb muscles and as %ID/g from harvested limb muscles. Double and triple immunofluorescent analysis on tissue sections established localization of α_vβ₃, VEGFR-1, VEGFR-2, as well as certain cell lineage markers.

RESULTS

Tracer uptake, as %ID/g, was higher in ligated limbs of mice injected with [^99mTc]scV compared to ligated hindlimbs in mice injected with [^99mTc]RGD (p = 0.02). The ratio of tracer uptake for ligated/control hindlimb was borderline higher for [^99mTc]scV than for [^99mTc]RGD (p = 0.06). Immunofluorescent analysis showed higher prevalence of VEGFR-1, VEGFR-2, and α_vβ₃, in damaged vs. undamaged hindlimb tissue, but with little co-localization of these markers. Double immunofluorescent staining showed partial co-localization of VEGFR-1, VEGFR-2, and α_vβ_3, with endothelial cell marker FVIII, but not with CD31. Immunostaining for VEGFR-1 and VEGFR-2 additionally co-localized with lineage markers for endothelial progenitor cell and monocytes/macrophages, with a more diverse pattern of co-localization for VEGFR-2.

CONCLUSION

In a mouse hindlimb ischemia model of peripheral artery disease, [^99mTc]scV SPECT tracer-targeting VEGF receptors showed a more robust signal than [^99mTc]RGD tracer-targeting α_vβ₃. Immunofluorescent analysis suggests that uptake of [^99mTc]scV and [^99mTc]RGD in damaged tissue is due to non-overlapping cell populations and reflects different dynamic processes and that enhanced uptake of [^99mTc]scV may be due to the presence of VEGF receptors on additional cell types.

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.

Early prediction of revascularisation by angiomotin-targeting positron emission tomography

This study aimed to develop a PET imaging agent of angiomotin (AMOT) expression, a potential biomarker of functional tissue regeneration in post-ischaemic conditions.

Methods: Hindlimb ischaemia was induced by ligature and resection of the right femoral artery in mice, and clinical score and limb perfusion were evaluated up to 30 days after surgery. AMOT expression was evaluated by histology and Western blot analysis. NODAGA-conjugates of AMOT ligand, sCD146, were designed, synthesised and radiolabelled with gallium-68. ⁶⁸Ga-sCD146 microPET/CT imaging was performed from day 1 to day 30 after ischaemia. ⁶⁸Ga-sCD146 specificity for AMOT was evaluated by autoradiography.

Results: Immunohistochemistry showed a significant endothelial overexpression of AMOT from day 5 up to day 10 in the ischaemic hindlimb. ⁶⁸Ga-sCD146 PET signal intensity correlated significantly with AMOT immunohistochemistry evaluation. ⁶⁸Ga-sCD146 PET imaging showed a significant uptake in the ischaemic hindlimb from day 2 to day 15, peaking on day 5 (ipsi/contralateral ratio = 2.4 ± 1.3, P = 0.0005) and significantly decreased after pharmacological blocking (62.57 ± 11% decrease in PET signal P = 0.032). Finally, we observed a significant correlation between day 5 ⁶⁸Ga-sCD146 PET signal intensity and clinical recovery (day 28) or hindlimb perfusion recovery (day 30).

Conclusions: This work reports for the first time an early and sustained increase in AMOT expression after hindlimb ischaemia in mice. We therefore developed an AMOT-targeting imaging agent, ⁶⁸Ga-sCD146, and showed its specific uptake up to 21 days after ischaemic hindlimb using microPET imaging. Correlation of early post-ischaemic PET signal with both delayed perfusion recovery and clinical outcome allows us to postulate that ⁶⁸Ga-sCD146 represents a promising radiotracer for tissue angiogenesis assessment.

ImmunoPET of CD146 in a Murine Hindlimb Ischemia Model

Peripheral arterial disease (PAD) consists of a persistent obstruction of lower-extremity arteries further from the aortic bifurcation attributable to atherosclerosis. PAD is correlated with an elevated risk of morbidity and mortality as well as of deterioration of the quality of life with claudication and chronic leg ischemia being the most frequent complications. Therapeutic angiogenesis is a promising therapeutic strategy that aims to restore the blood flow to the ischemic limb. In this context, assessing the efficacy of pro-angiogenic treatment using a reliable noninvasive imaging technique would greatly benefit the implementation of this therapeutic approach. Herein, we describe the angiogenesis and perfusion recovery characteristics of a mouse model of PAD via in vivo positron emission tomography (PET) imaging of CD146 expression. For that, ischemia was generated by ligation and excision of the right femoral artery of Balb/C mice and confirmed through laser Doppler imaging. The angiogenic process, induced by ischemia, was noninvasively monitored and quantified through PET imaging of CD146 expression in the injured leg using a ⁶⁴Cu-labeled anti-CD146 monoclonal antibody, ⁶⁴Cu-NOTA-YY146, at post-operative days 3, 10, and 17. The CD146-specific character of ⁶⁴Cu-NOTA-YY146 was verified via a blocking study performed in another cohort at day 10 after surgery. Tracer uptake was correlated with in situ CD146 expression by histological analysis. PET scan results indicated that ⁶⁴Cu-NOTA-YY146 uptake in the injured leg was significantly higher, with the highest uptake with a value of 14.1 ± 2.0 %ID/g at post-operative day 3, compared to the normal contralateral hindlimb, at all time points (maximum uptake of 2.2 ± 0.2 %ID/g). The pre-injection of a blocking dose resulted in a significantly lower tracer uptake in the ischemic hindlimb on day 10 after surgery, confirming tracer specificity. CD146/CD31 immunofluorescent co-staining showed an excellent correlation between the high uptake of the tracer with in situ CD146 expression levels and a marked co-localization of CD146 and CD31 signals. In conclusion, persistent and CD146-specific tracer accumulation in the ischemic hindlimb was observed, confirming the feasibility of ⁶⁴Cu-NOTA-YY146 to be used as an imaging agent to monitor the progression of angiogenesis and recovery in future PAD research.

Imaging αvβ3 integrin expression in skeletal metastases with 99mTc-maraciclatide single-photon emission computed tomography: detection and therapy response assessment

PURPOSE

Osteoclast activity is an important factor in the pathogenesis of skeletal metastases and is a potential therapeutic target. This study aimed to determine if selective uptake of 99mTc-maraciclatide, a radiopharmaceutical targeting αvβ3 integrin, occurs in prostate cancer (PCa) bone metastases and to observe the changes following systemic therapy.

METHODS

The study group comprised 17 men with bone-predominant metastatic PCa who underwent whole-body planar and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging with 99mTc-maraciclatide before (n = 17) and 12 weeks after (n = 11) starting treatment with abiraterone. Tumour to normal bone (T:N) ratios, tumour to muscle (T:M) ratios and CT Hounsfield units (HU) were measured in up to five target metastases in each subject. An oncologist blinded to study scans assessed clinical responses up to 24 weeks using conventional criteria.

RESULTS

Before treatment, metastases showed specific 99mTc-maraciclatide accumulation (mean planar T:N and T:M ratios 1.43 and 3.06; SPECT T:N and T:M ratios 3.1 and 5.19, respectively). Baseline sclerotic lesions (389-740 HU) showed lower T:M ratios (4.22 vs. 7.04, p = 0.02) than less sclerotic/lytic lesions (46-381 HU). Patients with progressive disease (PD; n = 5) showed increased planar T:N and T:M ratios (0.29 and 12.1%, respectively) and SPECT T:N and T:M ratios (11.9 and 20.2%, respectively). Patients without progression showed decreased planar T:N and T:M ratios (0.27 and -8.0%, p = 1.0 and 0.044, respectively) and SPECT T:N and T:M ratios (-21.9, and -27.2%, p = 0.3 and 0.036, respectively). The percentage change in CT HU was inversely correlated with the percentage change in SPECT T:M ratios (r = -0.59, p = 0.006).

CONCLUSIONS

99mTc-maraciclatide accumulates in PCa bone metastases in keeping with increased αvβ3 integrin expression. Greater activity in metastases with lower CT density suggests that uptake is related to osteoclast activity. Changes in planar and SPECT T:M ratios after 12 weeks of treatment differed between patients with and without PD and 99mTc-maraciclatide imaging may be a potential method for assessing early response.

Evaluation of a dimeric-cRGD peptide for targeted PET-CT imaging of peripheral angiogenesis in diabetic mice

The α V  β3 integrin plays an important role in many physiological functions and pathological disorders. α V  β3 is minimally expressed in normal quiescent endothelial cells, but significantly upregulated during neovascularization. In this study, we evaluated a 64Cu-labeled dimeric cRGD tracer targeted at α V  β3 integrin and report its applicability to assess peripheral angiogenesis in diabetes mellitus (DM). We established a murine model of type-1 DM characterized by elevated glucose, glycated serum protein (GSP), and glycated hemoglobin A1c (HbA1c). We demonstrated that our imaging probe is specific to α V  β3 integrin under both normo- and hyperglycemic conditions. We found that the analysis of in vivo PET-CT images correlated well with gamma well counting (GWC). Both GWC and PET-CT imaging demonstrated increased uptake of 64Cu-NOTA-PEG4-cRGD2 in the ischemic hindlimb in contrast to non-ischemic control. GWC of the distal ischemic tissue from DM mice showed significantly lower probe accumulation than in non-DM mice. The immunofluorescence staining of the ischemic tissues showed a 3-fold reduction in CD31 and 4-fold reduction in the α V  β3 expression in DM vs. non-DM animals. In conclusion, we successfully demonstrated that diabetes-associated reductions in peripheral angiogenesis can be non-invasively detected with PET-CT imaging using targeted dimeric-cRGD probe.

A bioreducible N-oxide-based probe for photoacoustic imaging of hypoxia

Hypoxia occurs when limited oxygen supply impairs physiological functions and is a pathological hallmark of many diseases including cancer and ischemia. Thus, detection of hypoxia can guide treatment planning and serve as a predictor of patient prognosis. Unfortunately, current methods suffer from invasiveness, poor resolution and low specificity. To address these limitations, we present Hypoxia Probe 1 (HyP-1), a hypoxia-responsive agent for photoacoustic imaging. This emerging modality converts safe, non-ionizing light to ultrasound waves, enabling acquisition of high-resolution 3D images in deep tissue. HyP-1 features an N-oxide trigger that is reduced in the absence of oxygen by heme proteins such as CYP450 enzymes. Reduction of HyP-1 produces a spectrally distinct product, facilitating identification via photoacoustic imaging. HyP-1 exhibits selectivity for hypoxic activation in vitro, in living cells, and in multiple disease models in vivo. HyP-1 is also compatible with NIR fluorescence imaging, establishing its versatility as a multimodal imaging agent.

Hypoxia is a hallmark of many diseases including cancer and ischemia, and detection can be invasive and of low resolution and specificity. Here the authors show a hypoxia probe that converts non-ionizing light to ultrasound, which enables the acquisition of high-resolution 3D images in deep tissue.

Expanding Acquisition and Clutter Filter Dimensions for Improved Perfusion Sensitivity

A method is explored for increasing the sensitivity of power-Doppler imaging without contrast enhancement. We acquire 1-10 s of echo signals and arrange it into a 3-D spatiotemporal data array. An eigenfilter developed to preserve all three dimensions of the array yields power estimates for blood flow and perfusion that are well separated from tissue clutter. This method is applied at high frequency (24-MHz pulses) to a murine model of an ischemic hindlimb. We demonstrate enhancements to tissue perfusion maps in normal and ischemic tissues. The method can be applied to data from any ultrasonic instrument that provides beamformed RF echo data.

Multimodal Assessment of Mesenchymal Stem Cell Therapy for Diabetic Vascular Complications

Peripheral arterial disease (PAD) is a debilitating complication of diabetes mellitus (DM) that leads to thousands of injuries, amputations, and deaths each year. The use of mesenchymal stem cells (MSCs) as a regenerative therapy holds the promise of regrowing injured vasculature, helping DM patients live healthier and longer lives. We report the use of muscle-derived MSCs to treat surgically-induced hindlimb ischemia in a mouse model of type 1 diabetes (DM-1). We serially evaluate several facets of the recovery process, including α_Vβ₃-integrin expression (a marker of angiogenesis), blood perfusion, and muscle function. We also perform microarray transcriptomics experiments to characterize the gene expression states of the MSC-treated is- chemic tissues, and compare the results with those of non-ischemic tissues, as well as ischemic tissues from a saline-treated control group.

The results show a multifaceted impact of mMSCs on hindlimb ischemia. We determined that the angiogenic activity one week after mMSC treatment was enhanced by approximately 80% relative to the saline group, which resulted in relative increases in blood perfusion and muscle strength of approximately 42% and 1.7-fold, respectively. At the transcriptomics level, we found that several classes of genes were affected by mMSC treatment. The mMSCs appeared to enhance both pro-angiogenic and metabolic genes, while suppressing anti-angiogenic genes and certain genes involved in the inflammatory response. All told, mMSC treatment appears to exert far-reaching effects on the microenvironment of ischemic tissue, enabling faster and more complete recovery from vascular occlusion.

Synthesis, Chemical Characterization and Multiscale Biological Evaluation of a Dimeric-cRGD Peptide for Targeted Imaging of α V β 3 Integrin Activity

Cyclic peptides containing the Arg-Gly-Asp (RGD) sequence have been shown to specifically bind the angiogenesis biomarker α_Vβ₃ integrin. We report the synthesis, chemical characterization, and biological evaluation of two novel dimeric cyclic RGD-based molecular probes for the targeted imaging of α_Vβ₃ activity (a radiolabeled version, ⁶⁴Cu-NOTA-PEG₄-cRGD₂, for PET imaging, and a fluorescent version, FITC-PEG₄-cRGD₂, for in vitro work). We investigated the performance of this probe at the receptor, cell, organ, and whole-body levels, including its use to detect diabetes associated impairment of ischemia-induced myocardial angiogenesis. Both versions of the probe were found to be stable, demonstrated fast receptor association constants, and showed high specificity for α_Vβ₃ in HUVECs (K_d ~ 35 nM). Dynamic PET-CT imaging indicated rapid blood clearance via kidney filtration, and accumulation within α_Vβ₃-positive infarcted myocardium. ⁶⁴Cu-NOTA-PEG₄-cRGD₂ demonstrated a favorable biodistribution, slow washout, and excellent performance with respect to the quality of the PET-CT images obtained. Importantly, the ratio of probe uptake in infarcted heart tissue compared to normal tissue was significantly higher in non-diabetic rats than in diabetic ones. Overall, our probes are promising agents for non-invasive quantitative imaging of α_Vβ₃ expression, both in vitro and in vivo.

Comparison of Tc-99m maraciclatide and Tc-99m sestamibi molecular breast imaging in patients with suspected breast cancer

Background

Molecular breast imaging (MBI) performed with ^99mTc sestamibi has been shown to be a valuable technique for the detection of breast cancer. Alternative radiotracers such as ^99mTc maraciclatide may offer improved uptake in breast lesions. The purpose of this study was to compare relative performance of ^99mTc sestamibi and ^99mTc maraciclatide in patients with suspected breast cancer, using a high-resolution dedicated gamma camera for MBI. Women with breast lesions suspicious for malignancy were recruited to undergo two MBI examinations—one with ^99mTc sestamibi and one with ^99mTc maraciclatide. A radiologist interpreted MBI studies in a randomized, blinded fashion to assign an assessment score (1–5) and measured lesion size. Lesion-to-background (L/B) ratio was measured with region-of-interest analysis.

Results

Among 39 analyzable patients, 21 malignant tumors were identified in 21 patients. Eighteen of 21 tumors (86%) were seen on ^99mTc sestamibi MBI and 19 of 21 (90%) were seen on ^99mTc maraciclatide MBI (p = 1). Tumor extent measured with both radiopharmaceuticals correlated strongly with pathologic size (^99mTc sestamibi, r = 0.84; ^99mTc maraciclatide, r = 0.81). The L/B ratio in detected breast cancers was similar for the two radiopharmaceuticals: 1.55 ± 0.36 (mean ± S.D.) for ^99mTc sestamibi and 1.62 ± 0.37 (mean ± S.D.) for ^99mTc maraciclatide (p = 0.53). No correlation was found between the L/B ratio and molecular subtype for ^99mTc sestamibi (r_s = 0.12, p = 0.63) or ^99mTc maraciclatide (r_s = −0.12, p = 0.64). Of 20 benign lesions, 10 (50%) were seen on ^99mTc sestamibi and 9 of 20 (45%) were seen on ^99mTc maraciclatide images (p = 0.1). The average L/B ratio for benign lesions was 1.34 ±0.40 (mean ±S.D.) for ^99mTc sestamibi and 1.41 ±0.52 (mean ±S.D.) for ^99mTc maraciclatide (p = 0.75). Overall diagnostic performance was similar for both radiopharmaceuticals. AUC from ROC analysis was 0.83 for ^99mTc sestamibi and 0.87 for ^99mTc maraciclatide (p = 0.64).

Conclusions

^99mTc maraciclatide offered comparable lesion uptake to ^99mTc sestamibi, in both malignant and benign lesions. There was good correlation between lesion extent and uptake measured from both radiopharmaceuticals. ^99mTc maraciclatide offered a marginal (but not significant) improvement in sensitivity over ^99mTc sestamibi. Our findings did not support an association between the uptake of either radiopharmaceutical and tumor molecular subtype.

Trial registration

ClinicalTrials.gov, NCT00888589

A novel Tc-99m and fluorescence labeled peptide as a multimodal imaging agent for targeting angiogenesis in a murine hindlimb ischemia model

The serine-aspartic acid-valine (SDV) peptide binds specifically to integrin αvβ3. We developed a Tc-99m and TAMRA labeled peptide, Tc-99m SDV-ECG-K-TAMRA for multimodal imaging of angiogenesis. Tc-99m SDV-ECG-K-TAMRA was prepared in high yield (>96%) and showed low cytotoxicity. Tc-99m tetrofosmin images 1 week after operation, revealed significantly decreased perfusion of the ischemic hindlimb, and the perfusion recovered gradually for 4 weeks. In contrast, Tc-99m SDV-ECG-K-TAMRA uptake was maximal 1 week after the operation (ischemic-to-non-ischemic uptake ratio =5.03±1.01) and decreased gradually. The ischemic-to-non-ischemic ratio of Tc-99m SDV-ECG-K-TAMRA and Tc-99m tetrofosmin was strongly negatively correlated (r =-0.94). A postmortem analysis revealed increased angiogenesis markers and uptake of Tc-99m SDV-ECG-K-TAMRA by ischemic tissue. Our in vivo and in vitro studies revealed substantial uptake of Tc-99m SDV-ECG-K-TAMRA by ischemic tissue. Tc-99m SDV-ECG-K-TAMRA could be a good candidate dual-modality imaging agent to assess angiogenesis.

SPECT and PET imaging of angiogenesis and arteriogenesis in pre-clinical models of myocardial ischemia and peripheral vascular disease

Purpose

The extent of neovascularization determines the clinical outcome of coronary artery disease and other occlusive cardiovascular disorders. Monitoring of neovascularization is therefore highly important. This review article will elaborately discuss preclinical studies aimed at validating new nuclear angiogenesis and arteriogenesis tracers. Additionally, we will briefly address possible obstacles that should be considered when designing an arteriogenesis radiotracer.

Methods

A structured medline search was the base of this review, which gives an overview on different radiopharmaceuticals that have been evaluated in preclinical models.

Results

Neovascularization is a collective term used to indicate different processes such as angiogenesis and arteriogenesis. However, while it is assumed that sensitive detection through nuclear imaging will facilitate translation of successful therapeutic interventions in preclinical models to the bedside, we still lack specific tracers for neovascularization imaging. Most nuclear imaging research to date has focused on angiogenesis, leaving nuclear arteriogenesis imaging largely overlooked.

Conclusion

Although angiogenesis is the process which is best understood, there is no scarcity in theoretical targets for arteriogenesis imaging.

Molecular Imaging of Angiogenesis and Vascular Remodeling in Cardiovascular Pathology

Angiogenesis and vascular remodeling are involved in a wide array of cardiovascular diseases, from myocardial ischemia and peripheral arterial disease, to atherosclerosis and aortic aneurysm. Molecular imaging techniques to detect and quantify key molecular and cellular players in angiogenesis and vascular remodeling (e.g., vascular endothelial growth factor and its receptors, αvβ3 integrin, and matrix metalloproteinases) can advance vascular biology research and serve as clinical tools for early diagnosis, risk stratification, and selection of patients who would benefit most from therapeutic interventions. To target these key mediators, a number of molecular imaging techniques have been developed and evaluated in animal models of angiogenesis and vascular remodeling. This review of the state of the art molecular imaging of angiogenesis and vascular (and valvular) remodeling, will focus mostly on nuclear imaging techniques (positron emission tomography and single photon emission tomography) that offer high potential for clinical translation.

SPECT Imaging of 2-D and 3-D Distributed Sources with Near-Field Coded Aperture Collimation: Computer Simulation and Real Data Validation

The imaging of distributed sources with near-field coded aperture (CA) remains extremely challenging and is broadly considered unsuitable for single-photon emission computerized tomography (SPECT). This study proposes a novel CA SPECT reconstruction approach and evaluates the feasibilities of imaging and reconstructing distributed hot sources and cold lesions using near-field CA collimation and iterative image reconstruction. Computer simulations were designed to compare CA and pinhole collimations in two-dimensional radionuclide imaging. Digital phantoms were created and CA images of the phantoms were reconstructed using maximum likelihood expectation maximization (MLEM). Errors and the contrast-to-noise ratio (CNR) were calculated and image resolution was evaluated. An ex vivo rat heart with myocardial infarction was imaged using a micro-SPECT system equipped with a custom-made CA module and a commercial 5-pinhole collimator. Rat CA images were reconstructed via the three-dimensional (3-D) MLEM algorithm developed for CA SPECT with and without correction for a large projection angle, and 5-pinhole images were reconstructed using the commercial software provided by the SPECT system. Phantom images of CA were markedly improved in terms of image quality, quantitative root-mean-squared error, and CNR, as compared to pinhole images. CA and pinhole images yielded similar image resolution, while CA collimation resulted in fewer noise artifacts. CA and pinhole images of the rat heart were well reconstructed and the myocardial perfusion defects could be clearly discerned from 3-D CA and 5-pinhole SPECT images, whereas 5-pinhole SPECT images suffered from severe noise artifacts. Image contrast of CA SPECT was further improved after correction for the large projection angle used in the rat heart imaging. The computer simulations and small-animal imaging study presented herein indicate that the proposed 3-D CA SPECT imaging and reconstruction approaches worked reasonably well, demonstrating the feasibilities of achieving high sensitivity and high resolution SPECT using near-field CA collimation.

αVβ3 integrin-targeted microSPECT/CT imaging of inflamed atherosclerotic plaques in mice

Background

α_Vβ₃-integrin is expressed by activated endothelial cells and macrophages in atherosclerotic plaques and may represent a valuable marker of high-risk plaques. We evaluated ^99mTc-maraciclatide, an integrin-specific tracer, for imaging vascular inflammation in atherosclerotic lesions in mice.

Methods

Apolipoprotein E-negative (ApoE^−/−) mice on a Western diet (n = 10) and normally fed adult C57BL/6 control mice (n = 4) were injected with ^99mTc-maraciclatide (51.8 ± 3.7 MBq). A blocking peptide was infused in three ApoE^−/− mice; this condition served as another control. After 90 min, the animals were imaged via single-photon emission computed tomography (SPECT). While maintained in the same position, the mice were transferred to computed tomography (CT) to obtain contrast-enhanced images of the aortic arch. Images from both modalities were fused, and signal was quantified in the aortic arch and in the vena cava for subtraction of blood-pool activity. The aorta was carefully dissected after imaging for gamma counting, autoradiography, and histology.

Results

Tracer uptake was significantly higher in ApoE^−/− mice than in both groups of control mice (1.56 ± 0.33 vs. 0.82 ± 0.24 vs. 0.98 ± 0.11, respectively; P = 0.006). Furthermore, higher tracer activity was detected via gamma counting in the aorta of hypercholesterolemic mice than in both groups of control mice (1.52 ± 0.43 vs. 0.78 ± 0.19 vs. 0.47 ± 0.31 ^99mTc-maraciclatide %ID/g, respectively; P = 0.018). Autoradiography showed significantly higher tracer uptake in the atherosclerotic aorta than in the control aorta (P = 0.026). Finally, in the atherosclerotic aorta, immunostaining indicated that the integrin signal came predominantly from macrophages and was correlated with the macrophage CD68 immunomarker (r = 0.73).

Conclusions

^99mTc-maraciclatide allows in vivo detection of inflamed atherosclerotic plaques in mice and may represent a non-invasive approach for identifying high-risk plaques in patients.

Novel Applications of Radionuclide Imaging in Peripheral Vascular Disease

Peripheral vascular disease (PVD) is a progressive atherosclerotic disease that leads to stenosis or occlusion of blood vessels supplying the lower extremities. Current diagnostic imaging techniques commonly focus on evaluation of anatomy or blood flow at the macrovascular level and do not permit assessment of the underlying pathophysiology associated with disease progression or treatment response. Molecular imaging with radionuclide-based approaches can offer novel insight into PVD by providing noninvasive assessment of biological processes such as angiogenesis and atherosclerosis. This article discusses emerging radionuclide-based imaging approaches that have potential clinical applications in the evaluation of PVD progression and treatment.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Dimeric [(68)Ga]DOTA-RGD peptide targeting αvβ 3 integrin reveals extracellular matrix alterations after myocardial infarction

PURPOSE

We evaluated a dimeric RGD-peptide, [(68)Ga]DOTA-E-[c(RGDfK)]2, for positron emission tomography (PET) imaging of myocardial integrin expression associated with extracellular matrix remodeling after myocardial infarction (MI) in rat.

PROCEDURES

Male Sprague-Dawley rats were studied at 7 days and 4 weeks after MI induced by permanent ligation of the left coronary artery and compared with sham-operated controls.

RESULTS

In vivo imaging revealed higher tracer uptake in the infarcted area than in the remote non-infarcted myocardium of the same rats both at 7 days (MI/remote ratio, 2.25 ± 0.24) and 4 weeks (MI/remote ratio, 2.13 ± 0.37) post-MI. Compared with sham-operated rats, tracer uptake was higher also in the remote, non-infarcted myocardium of MI rats both at 7 days and 4 weeks where it coincided with an increased interstitial fibrosis. Standardized uptake values correlated well with the results of tracer kinetic modeling. Autoradiography confirmed the imaging results showing 5.1 times higher tracer uptake in the infarcted than remote area. Tracer uptake correlated with the amount of β3 integrin subunits in the infarcted area.

CONCLUSIONS

Our results show that integrin-targeting [(68)Ga]DOTA-E-[c(RGDfK)]2 is a potential tracer for monitoring of myocardial extracellular matrix remodeling after MI using PET.

Treatment effect with anti-RAGE F(ab')2 antibody improves hind limb angiogenesis and blood flow in Type 1 diabetic mice with left femoral artery ligation

We investigated treatment with a receptor for advanced glycation endproduct (RAGE) blocking antibody on angiogenic response to hind limb ischemia in diabetic mice. Streptozotocin treated C57BL/6 mice received either murine monoclonal anti-RAGE F(ab')2 intraperitoneally (n=10) or saline (n=9) for 9 weeks. Diabetic plus 10 non-diabetic C57BL/6 mice underwent left femoral artery ligation and 5 days later angiogenesis imaging with (99m)Tc-Arg-Gly-Asp (RGD) nanoSPECT/CT. Twenty-four days later, hind limb blood flow was measured with ultrasound, the mice were euthanized, and tissue was taken for immunohistochemistry. The angiogenic imaging signal in ischemic limbs was higher in RAGE-ab treated versus saline treated mice at day 5 (3.1±1.4 vs 1.68±0.35, p=0.02) and blood flow was higher at day 24 (1.49±0.5 vs 0.61±0.39, p=0.04). Immunohistochemistry of ischemic muscles showed greater capillary density in the RAGE-ab treated group versus the vehicle-treated group (p<0.001) (NS from non-diabetic mice). In conclusion, treatment with anti-RAGE F(ab')2 in diabetic mice improves neovascularization in the ischemic leg.

The role of molecular imaging in the evaluation of myocardial and peripheral angiogenesis

Angiogenesis, or the formation of new microvasculature, is a physiological process that may occur in the setting of chronic tissue ischemia and can play an important role in improving tissue perfusion and blood flow following myocardial infarction or in the presence of peripheral vascular disease (PVD). Molecular imaging of angiogenesis within the cardiovascular system is a developing field of study. Targeted imaging of angiogenesis has the potential for non-invasive assessment of the underlying molecular signaling events associated with the angiogenic process and, when applied in conjunction with physiological perfusion imaging, may be utilized to predict and evaluate clinical outcomes in the setting of ischemic heart disease or PVD. This review discusses the developing radiotracer-based imaging techniques and technology currently in use that possess potential for clinical translation, with specific focus on PET and SPECT imaging of myocardial and peripheral angiogenesis.

Evaluation of ανβ3-mediated tumor expression with a 99mTc-labeled ornithine-modified RGD derivative during glioblastoma growth in vivo

In this study, a novel way of distinguishing the intrinsic relationship between ανβ3 integrin targeting and detection of tumor growth by using a radiolabeled tracer based on a cyclic Arg-Gly-Asp (RGD) peptide was provided. The potential of the in vivo scintigraphic imaging of the developing vasculature from the early stage of tumor growth was evaluated. Alongside with the scintigraphic images, biodistribution studies were performed at distinct time points to validate this noninvasive imaging approach. The ability to noninvasively assess the tumor growth of ανβ3 integrin-positive glioblastoma tumors provides a method to better understand tumor angiogenesis in vivo and allows for a direct assessment of anti-integrin treatment efficacy.

MEK inhibitors, novel anti-adhesive molecules, reduce sickle red blood cell adhesion in vitro and in vivo, and vasoocclusion in vivo

In sickle cell disease, sickle erythrocyte (SSRBC) interacts with endothelial cells, leukocytes, and platelets, and activates coagulation and inflammation, promoting vessel obstruction, which leads to serious life-threatening complications, including acute painful crises and irreversible damage to multiple organs. The mitogen-activated protein kinase, ERK1/2, is abnormally activated in SSRBCs. However, the therapeutic potential of SSRBC ERK1/2 inactivation has never been investigated. I tested four different inhibitors of MEK1/2 (MEK), the kinase that activates ERK1/2, in a model of human SSRBC adhesion to TNFα-activated endothelial cells (ECs). SSRBC MEK inhibition abrogated adhesion to non-activated and TNFα-activated ECs to levels below baseline SSRBC adhesion to non-activated ECs in vitro. SSRBC MEK inhibition also prevented SSRBCs from activating naïve neutrophils to adhere to endothelium. To determine the effect of MEK inhibitors on SSRBC adherence in vivo, sham-treated or MEK inhibitor-treated SSRBCs were infused to nude mice previously treated with TNFα. Sham-treated SSRBCs displayed marked adhesion and occlusion of enflamed vessels, both small and large. However, SSRBC treatment with MEK inhibitors ex vivo showed poor SSRBC adhesion to enflamed vessels with no visible vasoocclusion in vivo. In addition, MEK inhibitor treatment of SSRBCs reduced SSRBC organ trapping and increased the number of SSRBCs circulating in bloodstream. Thus, these data suggest that SSRBC ERK1/2 plays potentially a critical role in sickle pathogenesis, and that MEK inhibitors may represent a valuable intervention for acute sickle cell crises.

Image-guided pro-angiogenic therapy in diabetic stroke mouse models using a multi-modal nanoprobe

Purpose: The efficacy of pro-angiogenic therapy is difficult to evaluate with current diagnostic modalities. The objectives were to develop a non-invasive imaging strategy to define the temporal characteristics of angiogenesis and to evaluate the response to pro-angiogenic therapy in diabetic stroke mouse models.

Methods: A home-made α_νβ₃ integrin-targeted multi-modal nanoprobe was intravenously injected into mouse models at set time points after photothrombotic stroke. Magnetic resonance imaging (MRI) and near-infrared fluorescence (NIRF) imaging were carried out at 24 h post-injection. Bone marrow-derived endothelial progenitor cells (EPCs) were infused into the mouse models of ischemic stroke to stimulate angiogenesis.

Results: The peak signal intensity in the ischemic-angiogenic area of diabetic and wild-type mouse models was achieved on day 10, with significantly lower signal enhancement observed in the diabetic models. Although the signal intensity was significantly higher after EPC treatment in both models, the enhancement was less pronounced in the diabetic animals compared with the wild-type controls. Histological analysis revealed that the microvessel density and expression of β₃ integrin were correlated with the signal intensity assessed with MRI and NIRF imaging.

Conclusions: The non-invasive imaging method could be used for early and accurate evaluation of the response to pro-angiogenic therapy in diabetic stroke models.

Ceramide-activated phosphatase mediates fatty acid-induced endothelial VEGF resistance and impaired angiogenesis

Endothelial dysfunction, including endothelial hyporesponsiveness to prototypical angiogenic growth factors and eNOS agonists, underlies vascular pathology in many dysmetabolic states. We investigated effects of a saturated free fatty acid, palmitic acid (PA), on endothelial cell responses to VEGF. PA-pretreated endothelial cells had markedly diminished Akt, eNOS, and ERK activation responses to VEGF, despite normal VEGFR2 phosphorylation. PA inhibited VEGF-induced angiogenic cord formation in Matrigel, and PA-treated endothelial cells accumulated early species (C16) ceramide. The serine palmitoyltransferase inhibitor myriocin reversed these defects. Protein phosphatase 2A (PP2A) became more eNOS-associated in PA-treated cells; the PP2A inhibitor okadaic acid reversed PA-induced signaling defects. Mice fed a diet high in saturated fat for 2 to 3 weeks had impaired i) aortic Akt and eNOS phosphorylation to infused VEGF, ii) ear angiogenic responses to intradermal adenoviral-VEGF injection, and iii) vascular flow recovery to hindlimb ischemia as indicated by laser Doppler and αVβ3 SPECT imaging. High-fat feeding did not impair VEGF-induced signaling or angiogenic responses in mice with reduced serine palmitoyltransferase expression. Thus, de novo ceramide synthesis is required for these detrimental PA effects. The findings demonstrate an endothelial VEGF resistance mechanism conferred by PA, which comprises ceramide-induced, PP2A-mediated dephosphorylation of critical activation sites on enzymes central to vascular homeostasis and angiogenesis. This study defines potential molecular targets for preservation of endothelial function in metabolic syndrome.

Characterization of the arterial anatomy of the murine hindlimb: functional role in the design and understanding of ischemia models

RATIONALE

Appropriate ischemia models are required for successful studies of therapeutic angiogenesis. While collateral routes are known to be present within the innate vasculature, there are no reports describing the detailed vascular anatomy of the murine hindlimb. In addition, differences in the descriptions of anatomical names and locations in the literature impede understanding of the circulation and the design of hindlimb ischemia models. To understand better the collateral circulation in the whole hindlimb, clarification of all the feeding arteries of the hindlimb is required.

OBJECTIVE

The aim of this study is to reveal the detailed arterial anatomy and collateral routes in murine hindlimb to enable the appropriate design of therapeutic angiogenesis studies and to facilitate understanding of the circulation in ischemia models.

METHODS AND RESULTS

Arterial anatomy in the murine hindlimb was investigated by contrast-enhanced X-ray imaging and surgical dissection. The observed anatomy is shown in photographic images and in a schema. Previously unnoticed but relatively large arteries were observed in deep, cranial and lateral parts of the thigh. The data indicates that there are three collateral routes through the medial thigh, quadriceps femoris, and the biceps femoris muscles. Furthermore, anatomical variations were found at the origins of the three feeding arteries.

CONCLUSIONS

The detailed arterial anatomy of murine hindlimb and collateral routes deduced from the anatomy are described. Limitations on designs of ischemia models in view of anatomical variations are proposed. These observations will contribute to the development of animal studies of therapeutic angiogenesis using murine hindlimb ischemia models.

Radiotracer imaging of peripheral vascular disease

Peripheral vascular disease (PVD) is an atherosclerotic disease affecting the lower extremities, resulting in skeletal muscle ischemia, intermittent claudication, and, in more severe stages of disease, limb amputation and death. The evaluation of therapy in this patient population can be challenging, as the standard clinical indices are insensitive to assessment of regional alterations in skeletal muscle physiology. Radiotracer imaging of the lower extremities with techniques such as PET and SPECT can provide a noninvasive quantitative technique for the evaluation of the pathophysiology associated with PVD and may complement clinical indices and other imaging approaches. This review discusses the progress in radiotracer-based evaluation of PVD and highlights recent advancements in molecular imaging with potential for clinical application.

Specific uptake of 99mTc-NC100692, an αvβ3-targeted imaging probe, in subcutaneous and orthotopic tumors

INTRODUCTION

The αvβ3 integrin, which is expressed by angiogenic epithelium and some tumor cells, is an attractive target for the development of both imaging agents and therapeutics. While optimal implementation of αvβ3-targeted therapeutics will require a priori identification of the presence of the target, the clinical evaluation of these compounds has typically not included parallel studies with αvβ3-targeted diagnostics. This is at least partly due to the relatively limited availability of PET radiopharmaceuticals in comparison to those labeled with (99m)Tc. In an effort to begin to address this limitation, we evaluated the tumor uptake of (99m)Tc-NC100692, a cyclic RGD peptide that binds to αvβ3 with ~1-nM affinity, in an αvβ3-positive tumor model as well as its in vivo specificity.

METHODS

MicroSPECT imaging was used to assess the ability of cilengitide, a therapeutic with high affinity for αvβ3, to block and displace (99m)Tc-NC100692 in an orthotopic U87 glioma tumor. The specificity of (99m)Tc-NC100692 was quantitatively evaluated in mice bearing subcutaneous U87MG tumors, by comparison of the biodistribution of (99m)Tc-NC100692 with that of the non-specific structural analogue (99m)Tc-AH-111744 and by blocking uptake of (99m)Tc-NC100692 with excess unlabeled NC100692.

RESULTS

MicroSPECT imaging studies demonstrated that uptake of (99m)Tc-NC100692 in the intracranial tumor model was both blocked and displaced by the αvβ3-targeted therapeutic cilengitide. Biodistribution studies provided quantitative confirmation of these imaging results. Tumor uptake of (99m)Tc-NC100692 at 1h post-injection was 2.8 ± 0.7% ID/g compared to 0.38 ± 0.1% ID/g for (99m)Tc-AH-111744 (p < 0.001). Blocking (99m)Tc-NC100692 uptake by pre-injecting the mice with excess unlabeled NC100692 reduced tumor uptake by approximately five-fold, to 0.68 ± 0.3% ID/g (p = 0.01).

CONCLUSION

These results confirm that (99m)Tc-NC100692 does, in fact, target the αvβ3 integrin and may, therefore, be useful in identifying patients prior to anti-αvβ3 therapy as well as monitoring the response of these patients to therapy.

PET/SPECT imaging of hindlimb ischemia: focusing on angiogenesis and blood flow

Peripheral artery disease (PAD) is a result of the atherosclerotic narrowing of blood vessels to the extremities, and the subsequent tissue ischemia can lead to the up-regulation of angiogenic growth factors and formation of new vessels as a recovery mechanism. Such formation of new vessels can be evaluated with various non-invasive molecular imaging techniques, where serial images from the same subjects can be obtained to allow the documentation of disease progression and therapeutic response. The most commonly used animal model for preclinical studies of PAD is the murine hindlimb ischemia model, and a number of radiotracers have been investigated for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging of PAD. In this review article, we summarize the PET/SPECT tracers that have been tested in the murine hindlimb ischemia model as well as those used clinically to assess the extremity blood flow.

Radiolabelled RGD peptides for imaging and therapy

Imaging of angiogenesis has become increasingly important with the rising use of targeted antiangiogenic therapies like bevacizumab (Avastin). Non-invasive assessment of angiogenic activity is in this respect interesting, e.g. for response assessment of such targeted antiangiogenic therapies. One promising approach of angiogenesis imaging is imaging of specific molecular markers of the angiogenic cascade like the integrin α(v)β(3). For molecular imaging of integrin expression, the use of radiolabelled peptides is still the only approach that has been successfully translated into the clinic. In this review we will summarize the current data on imaging of α(v)β(3) expression using radiolabelled RGD peptides with a focus on tracers already in clinical use. A perspective will be presented on the future clinical use of radiolabelled RGD peptides including an outlook on potential applications for radionuclide therapy.

Endothelial-derived neuregulin is an important mediator of ischaemia-induced angiogenesis and arteriogenesis

AIMS

Neuregulins (NRG) are growth factors that are synthesized by endothelial cells (ECs) and bind to erbB receptors. We have shown previously that NRG is proangiogenic in vitro, and that NRG/erbB signalling is important for autocrine endothelial angiogenic signalling in vitro. However, the role of NRG in the angiogenic response to ischaemia is unknown. We hypothesized that endothelial NRG is required for ischaemia-induced angiogenesis in vivo and that exogenous administration of NRG will enhance angiogenic responses after ischaemic insult.

METHODS AND RESULTS

An endothelial-selective inducible NRG knockout mouse was created and subjected to femoral artery ligation. Endothelial NRG deletion significantly decreased blood flow recovery (by 40%, P < 0.05), capillary density, α(v)β(3) integrin activation, and arteriogenesis after ischaemic injury. Isolated ECs from knockout mice demonstrated significantly impaired cord formation in vitro, suggesting that NRG signalling performs an important cell autonomous function. Recombinant human NRG (rNRG) has not only reversed the angiogenic defect in knockout mice but also accelerated blood flow recovery in wild-type mice.

CONCLUSION

Endothelial production of NRG is required for angiogenesis and arteriogenesis induced by ischaemic injury. Furthermore, exogenous administration of rNRG can enhance this process, suggesting a potential role for NRG in vascular disease.

New application of optical agent to image angiogenesis in hindlimb ischemia

Optical agents targeting α(v)β₃ are potential tools to image the angiogenic response to limb ischemia. The left (L) femoral artery was ligated in 17 mice and sham surgery performed on the contralateral right (R) hindlimb. Seven days later, IntegriSense (2 nmol) was injected into 11 mice and 6 were probe controls. Six hours later, mice underwent optical imaging. Ratios of photon flux in the L/R limbs were calculated. Tissue was stained for α(v) , CD31, and lectin. The signal was increased in the ischemic limbs compared to contralateral legs and ratio of photon flux in L/R limb averaged 2.37. Control probe showed no hindlimb signal. IntegriSense colocalized with CD31 by dual fluorescent staining. Ratios for L/R hindlimbs correlated with quantitative lectin staining (r = 0.88, p = 0.003). Optical imaging can identify and quantify angiogenic response to hindlimb ischemia.