In vivo molecular imaging of myocardial angiogenesis using the alpha(v)beta3 integrin-targeted tracer 99mTc-RAFT-RGD

Assessing Doxorubicin-Induced Cardiomyopathy by 99mTc-3PRGD2 Scintigraphy Targeting Integrin αvβ3 in a Rat Model

The present study evaluated interstitial alterations in doxorubicin-induced cardiomyopathy using a radiolabeled RGD peptide ^99mTc-3PRGD2 specific for integrin αvβ3 that targets myofibroblasts.Cardiomyopathy was induced in 20 Sprague-Dawley rats by intraperitoneal doxorubicin injections (2.5 mg/kg/week) for up to six weeks. ^99mTc-3PRGD2 scintigraphy was performed in control rats (n = 6) at baseline and three, six, and nine weeks after first doxorubicin administration (n = 6, 6, and 5 for each time point). For another three rats of 6-week modeling, cold c(RGDyK) was co-injected with ^99mTc-3PRGD2 to evaluate specific radiotracer binding. Semi-quantitative parameters were acquired to compare radiotracer uptake among all groups. The biodistribution of ^99mTc-3PRGD2 was evaluated by a γ-counter after scintigraphy. Haematoxylin and eosin, and Masson's staining were used to evaluate myocardial injury and fibrosis, while western blotting and immunofluorescence co-localization were used to analyze integrin αvβ3 expression in the myocardium.The ^99mTc-3PRGD2 half-life in the cardiac region (Heartt _1/2) of the 9-week model and heart radioactivity percentage (%Heart_20 min, %Heart_40 min and %Heart_60 min) of the 6 and 9-week models were significantly increased compared to the control. Heart-to-background ratio (HBR_20 min, HBR_40 min and HBR_60 min) increase began in the third week, continued until the sixth week, and was reversed in the ninth week, which paralleled the changing trend of cardiac integrin αvβ3 expression. The myocardial biodistribution of ^99mTc-3PRGD2 was significantly correlated with integrin β3 expression.The ^99mTc-3PRGD2 scintigraphy allows for non-invasive visualization of interstitial alterations during doxorubicin-induced cardiomyopathy.

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.

Protective effect of Gelofusine against cRGD-siRNA-induced nephrotoxicity in mice

Based on successful targeting to the αvβ3 integrin of cyclic arginine–glycine–aspartic acid (cRGD), cRGD-conjugated small interfering RNA (siRNA) exhibits tumor targeting and has become a new treatment strategy for solid tumors. However, the nephrotoxicity caused by its renal retention limits its clinical application. Here, we evaluated the protective effect of Gelofusine against cRGD-conjugated siRNA-induced nephrotoxicity in mice. Male Kunming mice (six per group) were either co-injected with Gelofusine and cRGD-siRNA or injected with cRGD-siRNA alone. After administration of these treatments five times, creatinine and blood urea nitrogen (BUN) levels were determined. Hematoxylin–eosin staining (HE staining) and transferase dUTP nick end labeling (TUNEL) analysis were used to compare the difference in renal damage between the groups. Additionally, fluorescence imaging was used to observe the distribution of cRGD-siRNA in vivo. The group co-injected with Gelofusine and cRGD-siRNA displayed lower creatinine and BUN levels than the cRGD-siRNA-alone group and showed less renal damage upon HE staining and TUNEL analysis. Gelofusine decreased the retention time and accelerated the elimination of cRGD-siRNA from the organs, as observed in the fluorescence images. These data indicate that Gelofusine significantly increased the excretion of cRGD-conjugated siRNA and reduced the associated renal damage.

Positron emission tomography of cerebral angiogenesis and TSPO expression in a mouse model of chronic hypoxia

The present study aimed to examine whether positron emission tomography (PET) could evaluate cerebral angiogenesis. Mice were housed in a hypoxic chamber with 8-9% oxygen for 4, 7, and 14 days, and the angiogenic responses were evaluated with a radiotracer, ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄, which targeted α_Vβ₃ integrin and was imaged with PET. The PET imaging results showed little uptake during all of the hypoxic periods. Immunofluorescence staining of the β₃ integrin, CD61, revealed weak expression, while the microvessel density assessed by CD31 staining increased with the hypoxic duration. These observations suggest that the increased vascular density originated from other types of vascular remodeling, unlike angiogenic sprouting. We then searched for any signs of vascular remodeling that could be detected using PET. PET imaging of ¹¹C-PK11195, a marker of the 18-kDa translocator protein (TSPO), revealed a transient increase at day 4 of hypoxia. Because the immunofluorescence of glial markers showed unchanged staining over the early phase of hypoxia, the observed upregulation of TSPO expression probably originated from non-glial cells (e.g. vascular cells). In conclusion, a transient increase in TSPO probe uptake was detected with PET at only the early phase of hypoxia, which indicates an early sign of vascular remodeling induced by hypoxia.

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.

Molecular imaging of cardiac remodelling after myocardial infarction

Myocardial infarction and subsequent heart failure is a major health burden associated with significant mortality and morbidity in western societies. The ability of cardiac tissue to recover after myocardial infarction is affected by numerous complex cellular and molecular pathways. Unbalance or failure of these pathways can lead to adverse remodelling of the heart and poor prognosis. Current clinical cardiac imaging modalities assess anatomy, perfusion, function, and viability of the myocardium, yet do not offer any insight into the specific molecular pathways involved in the repair process. Novel imaging techniques allow visualisation of these molecular processes and may have significant diagnostic and prognostic values, which could aid clinical management. Single photon-emission tomography, positron-emission tomography, and magnetic resonance imaging are used to visualise various aspects of these molecular processes. Imaging probes are usually attached to radioisotopes or paramagnetic nanoparticles to specifically target biological processes such as: apoptosis, necrosis, inflammation, angiogenesis, and scar formation. Although the results from preclinical studies are promising, translating this work to a clinical environment in a valuable and cost-effective way is extremely challenging. Extensive evaluation evidence of diagnostic and prognostic values in multi-centre clinical trials is still required.

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.

Molecular Imaging of Angiogenesis in Cardiac Regeneration

PURPOSE OF REVIEW

Myocardial infarction (MI) leading to heart failure displays an important cause of death worldwide. Adequate restoration of blood flow to prevent this transition is a crucial factor to improve long-term morbidity and mortality. Novel regenerative therapies have been thoroughly investigated within the past decades.

RECENT FINDINGS

Increased angiogenesis in infarcted myocardium has shown beneficial effects on the prognosis of MI; therefore, the proangiogenic capacity of currently tested treatments is of specific interest. Molecular imaging to visualize formation of new blood vessels in vivo displays a promising option to monitor proangiogenic effects of regenerative substances.

SUMMARY

Based on encouraging results in preclinical models, molecular angiogenesis imaging has recently been applied in a small set of patients. This article reviews recent literature on noninvasive in vivo molecular imaging of angiogenesis after MI as an integral part of cardiac regeneration.

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.

Identification of Angiogenesis Rich-Viable Myocardium using RGD Dimer based SPECT after Myocardial Infarction

Cardiac healing after myocardial ischemia is a complex biological process. Advances in understanding of wound healing response have paved the way for clinical testing of novel molecular imaging to improve clinical outcomes. A key factor for assessing myocardial viability after ischemic injury is the evaluation of angiogenesis accompanying increased expression of integrin αvβ3. Here, we describe the capability of an αvβ3 integrin-targeting SPECT agent, (99m)Tc-IDA-D-[c(RGDfK)]2, for identification of ischemic but viable myocardium, i.e., hibernating myocardium which is crucial to predict functional recovery after revascularization, the standard care of cardiovascular medicine. In vivo SPECT imaging of rat models with transient coronary occlusion showed significantly high uptake of (99m)Tc-IDA-D-[c(RGDfK)]2 in the ischemic region. Comparative measurements with (201)Tl SPECT and (18)F-FDG PET, then, proved that such prominent uptake of (99m)Tc-IDA-D-[c(RGDfK)]2 exactly matched the hallmark of hibernation, i.e., the perfusion-metabolism mismatch pattern. The uptake of (99m)Tc-IDA-D-[c(RGDfK)]2 was non-inferior to that of (18)F-FDG, confirmed by time-course variation analysis. Immunohistochemical characterization revealed that an intense signal of (99m)Tc-IDA-D-[c(RGDfK)]2 corresponded to the vibrant angiogenic events with elevated expression of αvβ3 integrin. Together, these results establish that (99m)Tc-IDA-D-[c(RGDfK)]2 SPECT can serve as a sensitive clinical measure for myocardial salvage to identify the patients who might benefit most from revascularization.

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.

(99)mTc-3PRGD2 scintimammography in palpable and nonpalpable breast lesions

The aim of this study was to explore the diagnostic performance of 99mTc-3(poly-(ethylene glycol),PEG)4-RGD2 (99mTc-3PRGD2) scintimammography (SMM) in patients with either palpable or nonpalpable breast lesions and compare SMM to mammography to assess the possible incremental value of SMM in breast cancer detection. We also investigated the αvβ3 expression in malignant and benign breast lesions. Ninety-four patients with 110 lesions were included in this study. Mammograms were evaluated according to the Breast Imaging Reporting and Data System (BI-RADS) by a specialized imaging radiologist. Prone SMM was performed 1 hour after injection of 99mTc-3PRGD2. Scintigraphic images were interpreted independently by two experienced nuclear medicine physicians using a three-point system, and the kappa value was calculated to determine the interreader agreement. The McNemar test was used to compare SMM and mammography with respect to sensitivity, specificity, and accuracy. Diagnostic values for breast cancer detection were evaluated for each lesion. Immunohistochemistry was performed to evaluate integrin αvβ3 expression. Histopathology revealed 46 malignant lesions and 64 benign lesions. The overall sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of SMM were 83%, 73%, 77%, 69%, and 85%, respectively. The kappa value between the two reviewers was 0.63. The diagnostic values of SMM were higher than those of mammography in evaluating overall breast lesions. A sensitivity of 91% was achieved when SMM and mammography results were combined with 60% of all false-negative mammography findings classified as true-positive results by SMM. Integrin αvβ3 expression was positively identified using SMM imaging. SMM is a promising tool to avoid unnecessary biopsies when used in addition to mammography and can be used to image αvβ3 expression in breast cancer with good image quality.

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.

PET imaging of cardiac wound healing using a novel [68Ga]-labeled NGR probe in rat myocardial infarction

PURPOSE

Peptides containing the asparagine-glycine-arginine (NGR) motif bind to aminopeptidase N (CD13), which is expressed on inflammatory cells, endothelial cells, and fibroblasts. It is unclear whether radiolabeled NGR-containing tracers could be used for in vivo imaging of the early wound-healing phase after myocardial infarction (MI) using positron emission tomography (PET).

PROCEDURES

Uptake of novel tracer [(68)Ga]NGR was assessed together with [(68)Ga]arginine-glycine-aspartic acid ([(68)Ga]RGD) and 2-deoxy-2-[(18) F]fluoro-D-glucose after myocardial ischemia/reperfusion (MI/R) injury using μ-PET and autoradiography, and relative expressions of CD13 and integrin β3 were assessed in fibroblasts, inflammatory cells, and endothelial cells by immunohistochemistry.

RESULTS

In the infarcted myocardium, uptake of [(68)Ga]NGR was maximal from days 3 to 7 after MI/R, and correlated with fibroblast and inflammatory cell infiltration as well as [(68)Ga]RGD uptake.

CONCLUSIONS

[(68)Ga]NGR allows noninvasive and sequential determination of CD13 expression in fibroblasts and inflammatory cells by PET. This will facilitate monitoring of CD13 in the individual wound healing processes, allowing patient-specific therapies to improve outcome after MI.

Imaging integrin αvβ3 on blood vessels with 111In-RGD2 in head and neck tumor xenografts

Arginine-glycine-aspartic acid (RGD)-based imaging tracers allow specific imaging of integrin αvβ3, a protein overexpressed during angiogenesis, leading to the possibility that it might serve as a tool to stratify patients for antiangiogenic treatment. However, these tracers have generally been characterized in xenograft models in which integrin αvβ3 was constitutively expressed by the tumor cells themselves. In the studies presented here, the use of (111)In-RGD2 as a tracer to image only integrin αvβ3 expression on blood vessels in the tumor was determined using tumor xenografts in which tumor cells were integrin αvβ3-negative.

METHODS

DOTA-E-[c(RGDfK)]2 was radiolabeled with (111)In ((111)In-RGD2), and biodistribution studies were performed in squamous cell carcinoma of the head and neck (HNSCC) xenograft mouse models to determine the optimal peptide dose to image angiogenesis. Next, biodistribution and imaging studies were performed at the optimal peptide dose in 3 HNSCC mouse models, FaDu, SCCNij3, and SCCNij202. Immunohistochemical analysis of tumor vascular and cell surface expression of integrin αvβ3 and correlation analysis of vascular integrin αvβ3 and autoradiography were completed.

RESULTS

All 3 HNSCC xenografts expressed integrin αvβ3 on the vessels only. The optimal peptide dose of (111)In-RGD2 was 1 μg or less for specific integrin αvβ3-mediated uptake of the tracer. SPECT/CT imaging showed clear uptake of the tracer in the periphery of the tumors, corresponding with well-vascularized areas of the tumor. Within the tumor, (111)In-RGD2 autoradiography coincided with vascular integrin αvβ3 expression, as determined immunohistochemically. Integrin αvβ3-mediated uptake was also detected in nontumor tissues, which, through immunohistochemical analysis, proved positive for integrin αvβ3.

CONCLUSION

(111)In-RGD2 allows the visualization of integrin αvβ3 in xenograft models in which integrin αvβ3 is expressed only on the neovasculature, such as in the HNSCC tumors. Thus, (111)In-RGD2 allows specific visualization of angiogenesis in tumor models lacking constitutive tumoral integrin αvβ3 expression but may be less useful for this purpose in many tumors in which tumor cells express integrin αvβ3.

Co-liposomes comprising a lipidated multivalent RGD-peptide and a cationic gemini cholesterol induce selective gene transfection in αvβ3 and αvβ5 integrin receptor-rich cancer cells

Palmitoylated-RGD4 mediated gene transfer and cell targeting using a cationic gemini cholesterol based liposome.

Molecular imaging of myocardial infarction

Myocardial infarction (MI), and subsequent heart failure, remains a major healthcare problem in the western and developing world and leads to substantial morbidity and mortality. After MI, the ability of the myocardium to recover is closely associated with a complex immune response that often leads to adverse remodeling of the ventricle, and poor prognosis. Currently used clinical imaging modalities allow the assessment of anatomy, perfusion, function, and viability but do not provide insights into specific biological processes. In contrast, novel non-invasive imaging methods, using targeted imaging agents, allow imaging of the molecular processes underlying the post-MI immune cell response, and subsequent remodeling. Therefore, this may have significant diagnostic, prognostic, and therapeutic value, and may help to improve our understanding of post-infarct remodeling, in vivo. Imaging modalities such as magnetic resonance imaging, single-photon emission computed tomography, and positron emission tomography have been used in concert with radiolabelled and (super) paramagnetic probes to image each phase of the immune response. These probes, which target apoptosis, necrosis, neutrophils, monocytes, enzymes, angiogenesis, extracellular matrix, and scar formation have been assessed and validated pre-clinically. Translating this work to the bedside in a cost-effective, clinically beneficial manner remains a significant challenge. This article reviews these new imaging techniques as well as the corresponding pathophysiology.

Imaging of bronchioloalveolar carcinoma in the mice with the αⅤβ3 integrin-targeted tracer (99m)Tc-RGD-4CK

Receptor-based imaging agents have shown improved specificity and sensitivity of cancer diagnosis by targeting the specific features of cancer. Here we reported the (99m)Tc-labeling of a cyclic polypeptide RGD-4CK and the characterization of this agent in vitro and in bronchioloalveolar carcinoma (BAC) xenograft model. The αⅤβ3 integrin receptor binding affinity of (99m)Tc-RGD-4CK was determined in BAC. The cancer targeting properties of (99m)Tc-RGD-4CK were determined in NCI-H358 xenografted nude mice. Moreover, the BAC uptake of (99m)Tc-RGD-4CK was blocked with nonradiolabeled RGD-4CK in xenografts. The competitive assay showed that (99m)Tc-RGD-4CK exhibited high specificity to BAC cell line NCI-H358. Biodistribution studies indicated that (99m)Tc-RGD-4CK exhibited high tumor uptake (4.12 ± 1.21% injected dose/g 120 minutes after injection) and prolonged tumor retention (2.08 ± 0.33% injected dose/g 240 minutes after injection) in NCI-H358 xenografted nude mice. Moreover, (99m)Tc-RGD-4CK produced a good tumor-to-lung ratio (2.38) because of low lung activity accumulation 120 minutes postinjection. BAC on the flank of xenografted mice was clearly visualized by single photon emission computed tomography/computed tomography imaging using (99m)Tc-RGD-4CK. In conclusion, this study provides evidence that (99m)Tc-RGD-4CK is a promising agent for noninvasive determination of αⅤβ3 integrin status and therapy monitoring in BAC.

Comparison of cyclic RGD peptides for αvβ3 integrin detection in a rat model of myocardial infarction

Background

Expression of α_vβ₃ integrin is increased after myocardial infarction as part of the repair process. Increased expression of α_vβ₃ has been shown by molecular imaging with ¹⁸F-galacto-RGD in a rat model. The ⁶⁸Ga-labelled RGD compounds ⁶⁸Ga-NODAGA-RGD and ⁶⁸Ga-TRAP(RGD)₃ have high specificity and affinity, and may therefore serve as alternatives of ¹⁸F-galacto-RGD for integrin imaging.

Methods

Left coronary artery ligation was performed in rats. After 1 week, rats were imaged with [¹³N]NH₃, followed by ¹⁸F-galacto-RGD, ⁶⁸Ga-NODAGA-RGD or ⁶⁸Ga-TRAP(RGD)₃ using a dedicated animal PET/CT device. Rats were killed, and the activity in tissues was measured by gamma counting. The heart was sectioned for autoradiography and histology. Immunohistochemistry was performed on consecutive sections using CD31 for the endothelial cells and CD61 for β₃ expression (as part of the α_vβ₃ receptor).

Results

In vivo imaging showed focal RGD uptake in the hypoperfused area of infarcted myocardium as defined with [¹³N]NH₃ scan. In autoradiography images, augmented uptake of all RGD tracers was observed within the infarct area as verified by the HE staining. The tracer uptake ratios (infarct vs. remote) were 4.7 ± 0.8 for ¹⁸F-galacto-RGD, 5.2 ± 0.8 for ⁶⁸Ga-NODAGA-RGD, and 4.1 ± 0.7 for ⁶⁸Ga-TRAP(RGD)₃. The ⁶⁸Ga-NODAGA-RGD ratio was higher compared to ⁶⁸Ga-TRAP(RGD)₃ (p = 0.04), but neither of the ⁶⁸Ga tracers differed from ¹⁸F-galacto-RGD (p > 0.05). The area of augmented ⁶⁸Ga-RGD uptake was associated with β₃ integrin expression (CD61).

Conclusion

⁶⁸Ga-NODAGA-RGD and ⁶⁸Ga-TRAP(RGD)₃ uptake was equally increased in the infarct area at 1 week post infarction as ¹⁸F-galacto-RGD. These results show the potential of ⁶⁸Ga-labelled RGD peptides to monitor integrin expression as a part of myocardial repair and angiogenesis after ischaemic injury in vivo.

Advances in molecular imaging of atherosclerosis and myocardial infarction: shedding new light on in vivo cardiovascular biology

Molecular imaging of the cardiovascular system heavily relies on the development of new imaging probes and technologies to facilitate visualization of biological processes underlying or preceding disease. Molecular imaging is a highly active research discipline that has seen tremendous growth over the past decade. It has broadened our understanding of oncologic, neurologic, and cardiovascular diseases by providing new insights into the in vivo biology of disease progression and therapeutic interventions. As it allows for the longitudinal evaluation of biological processes, it is ideally suited for monitoring treatment response. In this review, we will concentrate on the major accomplishments and advances in the field of molecular imaging of atherosclerosis and myocardial infarction with a special focus on magnetic resonance imaging.

Reduction of renal uptake of 111In-DOTA-labeled and A700-labeled RAFT-RGD during integrin αvβ3 targeting using single photon emission computed tomography and optical imaging

Integrin α(v)β(3) expression is upregulated during tumor growth and invasion in newly formed endothelial cells in tumor neovasculature and in some tumor cells. A tetrameric RGD-based peptide, regioselectively addressable functionalized template-(cyclo-[RGDfK])4 (RAFT-RGD), specifically targets integrin α(v)β(3) in vitro and in vivo. When labeled with indium-111, the RAFT-RGD is partially reabsorbed and trapped in the kidneys, limiting its use for further internal targeted radiotherapy and imaging investigations. We studied the effect of Gelofusine on RAFT-RGD renal retention in tumor-bearing mice. Mice were imaged using single photon emission computed tomography and optical imaging 1 and 24 h following tracer injection. Distribution of RAFT-RGD was further investigated by tissue removal and direct counting of the tracer. Kidney sections were analyzed by confocal microscopy. Gelofusine significantly induced a >50% reduction of the renal reabsorption of (111)In-DOTA-RAFT-RGD and A700-RAFT-RGD, without affecting tumor uptake. Injection of Gelofusine significantly reduced the renal retention of labeled RAFT-RGD, while increasing the tumor over healthy tissue ratio. These results will lead to the development of future therapeutic approaches.

PET imaging of angiogenesis after myocardial infarction/reperfusion using a one-step labeled integrin-targeted tracer 18F-AlF-NOTA-PRGD2

PURPOSE

The α(v)β(3) integrin represents a potential target for noninvasive imaging of angiogenesis. The purpose of this study was to evaluate a novel one-step labeled integrin α(v)β(3)-targeting positron emission tomography (PET) probe, (18)F-AlF-NOTA-PRGD2, for angiogenesis imaging in a myocardial infarction/reperfusion (MI/R) animal model.

METHODS

Male Sprague-Dawley rats underwent 45-min transient left coronary artery occlusion followed by reperfusion. The myocardial infarction was confirmed by ECG, (18)F-fluorodeoxyglucose (FDG) imaging, and cardiac ultrasound. In vivo PET imaging was used to determine myocardial uptake of (18)F-AlF-NOTA-PRGD2 at different time points following reperfusion. The control peptide RAD was labeled with a similar procedure and used to confirm the specificity. Ex vivo autoradiographic analysis and CD31/CD61 double immunofluorescence staining were performed to validate the PET results.

RESULTS

Myocardial origin of the (18)F-AlF-NOTA-PRGD2 accumulation was confirmed by (18)F-FDG and autoradiography. PET imaging demonstrated increased focal accumulation of (18)F-AlF-NOTA-PRGD2 in the infarcted area which started at day 3 (0.28 ± 0.03%ID/g, p < 0.05) and peaked between 1 and 3 weeks (0.59 ± 0.16 and 0.55 ± 0.13%ID/g, respectively). The focal accumulation decreased but still kept at a higher level than the sham group after 4 months of reperfusion (0.31 ± 0.01%ID/g, p < 0.05). Pretreatment with unlabeled arginine-glycine-aspartic acid (RGD) peptide significantly decreased tracer uptake, indicating integrin specificity of this tracer. At 1 week after MI/R, uptake of the control tracer (18)F-AlF-NOTA-RAD that does not bind to integrin, in the infarcted area, was only 0.21 ± 0.01%ID/g. Autoradiographic imaging showed the same trend of uptake in the myocardial infarction area. The time course of focal tracer uptake was consistent with the pattern of vascular density and integrin β(3) expression as measured by CD31 and CD61 immunostaining analysis.

CONCLUSION

PET imaging using one-step labeled (18)F-AlF-NOTA-PRGD2 allows noninvasive visualization of ischemia/reperfusion-induced myocardial angiogenesis longitudinally. The favorable in vivo kinetics and easy production method of this integrin-targeted PET tracer facilitates its future clinical translation for lesion evaluation and therapy response monitoring in patients with occlusive cardiovascular diseases.

PET imaging of aortic atherosclerosis: Is combined imaging of plaque anatomy and function an amaranthine quest or conceivable reality?

Traditionally, blood vessels have been studied using contrast luminography to determine the site, extent and severity of luminal compromise by atherosclerotic deposits. Similar anatomical data can now be acquired non-invasively using ultrasound, computed tomography or magnetic resonance imaging. Plaque stability is an important determinant of subsequent vascular events and currently functional data on the stability of plaque is less well provided by these methods. The search for non-invasive techniques to image combined plaque anatomy and function has been pursued with visionary anticipation. This expectation may soon be realised as imaging with radionuclide-labelled atheroma-targeted contrast agents has demonstrated that plaque functional characteristics can now be shown. Increasingly positron emission tomography/computed tomography (PET/CT) imaging with (18)F fluorodexoyglucose (FDG) and other radionuclides is being used to determine culprit plaques in complex clinically scenarios. Clinically, this information may prove extremely valuable in the assessment of stable and unstable patients and its use in prime time medical practice is eagerly awaited. We will discuss the current clinical applications of functional atheroma imaging in the aorta and highlight the promising preclinical data on novel image biomarkers of plaque instability. If clinical science is able to successfully translate these advances in vascular imaging from the bench to the bedside, a new paradigm will be achieved in cardiovascular diagnostics.

18F-FPPRGD2 and 18F-FDG PET of response to Abraxane therapy

Abraxane (nanoparticle albumin-bound paclitaxel) is an anticancer drug approved by the Food and Drug Administration. However, the mechanism of action of Abraxane is complex, and no established biomarker is available to accurately monitor its treatment outcomes. The aim of this study was to investigate whether the integrin-specific PET tracer 18F-FPPRGD2 (investigational new drug 104150) can be used to monitor early response of tumors to Abraxane therapy.

METHODS

Orthotopic MDA-MB-435 breast cancer mice were treated with Abraxane (25 mg/kg every other day, 3 doses) or phosphate-buffered saline. Tumor volume was monitored by caliper measurement. PET scans were obtained before and at different times after the start of treatment (days 0, 3, 7, 14, and 21) using 18F-FPPRGD2 and 18F-FDG. The tumoricidal effect was also assessed ex vivo by immunohistochemistry.

RESULTS

Abraxane treatment inhibited the tumor growth, and a significant difference in tumor volume could be seen at day 5 after the initiation of treatment. The tumor uptake of 18F-FPPRGD2 in the Abraxane-treated group was significantly lower on days 3 and 7 than at baseline but returned to the baseline level at days 14 and 21, indicative of relapse of the tumors after the treatment was halted. Immunohistologic staining confirmed that the change of 18F-FPPRGD2 uptake correlated with the variation of integrin level in the tumor vasculature induced by Abraxane treatment. No significant change of tumor (rather than vascular) integrin expression was observed throughout the study. No significant decrease of 18F-FDG uptake was found between the treated and the control tumors on days 3, 14, and 21, although an increase in 18F-FDG tumor uptake of treated mice, as compared with the control mice, was found on day 7. The increase of 18F-FDG on day 7 was related to the inflammatory response during therapy.

CONCLUSION

Abraxane-mediated downregulation of integrin αvβ3 expression on tumor endothelial cells can be quantitatively visualized by PET. The change of integrin expression precedes that of tumor size. Consequently, 18F-FPPRGD2 PET is superior to 18F-FDG PET in monitoring early response to treatment, favoring its potential clinical translation.

Application of click-click chemistry to the synthesis of new multivalent RGD conjugates

New multivalent RGD-containing macromolecules were designed by exploiting two orthogonal chemoselective ligations. They were next applied to a competitive cell adhesion assay and used for the non invasive optical imaging of tumour in small animals.

Molecular Imaging of Healing After Myocardial Infarction

The progression from acute myocardial infarction (MI) to heart failure continues to be a major cause of morbidity and mortality. Potential new therapies for improved infarct healing such as stem cells, gene therapy, and tissue engineering are being investigated. Noninvasive imaging plays a central role in the evaluation of MI and infarct healing, both clinically and in preclinical research. Traditionally, imaging has been used to assess cardiac structure, function, perfusion, and viability. However, new imaging methods can be used to assess biological processes at the cellular and molecular level. We review molecular imaging techniques for evaluating the biology of infarct healing and repair. Specifically, we cover recent advances in imaging the various phases of MI and infarct healing such as apoptosis, inflammation, angiogenesis, extracellular matrix deposition, and scar formation. Significant progress has been made in preclinical molecular imaging, and future challenges include translation of these methods to clinical practice.