Myocardial uptake of 99mTc-annexin-V and 111In-antimyosin-antibodies after ischemia-reperfusion in rats

In-vivo monitoring of erythropoietin treatment after myocardial infarction in mice with [⁶⁸Ga]Annexin A5 and [¹⁸F]FDG PET

BACKGROUND

Several studies substantiate the cardioprotective effects of erythropoietin (EPO). Our goal was to quantify the effects of EPO treatment on the early expression of the apoptosis marker phosphatidylserine as well as on the left ventricular volumes and function by means of small animal PET.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in C57BL/6 mice. Animals were assigned to saline or EPO groups and underwent Annexin PET (day 2) and gated FDG PET (days 6 and 30). Annexin uptake was significantly higher in the infarction than in remote myocardium, with no differences between treatment groups. Infarct size showed a slight decrease in the EPO group and a slight increase in the controls, which did not reach statistical significance. Follow-up analyses revealed a significant increase of end-diastolic and end-systolic volumes in the EPO group, in which a stable left ventricular ejection fraction (LVEF) was maintained.

CONCLUSION

We find that deleterious effects of EPO can outweigh cardioprotective effects. The present EPO treatment did not significantly reduce apoptosis after MI, but seemingly provoked significant myocardial dilation while maintaining a stable LVEF. Molecular mechanisms of EPO treatment may need further elucidation to optimize therapy regimens.

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.

Nanomedicine in cardiovascular therapy: recent advancements

Cardiovascular disease (CVD) is comprised of a group of disorders affecting the heart and blood vessels of the human body and is one of the leading causes of death worldwide. Current therapy for CVD is limited to the treatment of already established disease, and it includes pharmacological and/or surgical procedures, such as percutaneous coronary intervention with stenting and coronary artery bypass grafting. However, lots of complications have been raised with these modalities of treatment, including systemic toxicity with medication, stent thrombosis with percutaneous coronary intervention and nonsurgical candidate patients for coronary artery bypass grafting. Nanomedicine has emerged as a potential strategy in dealing with these obstacles. Applications of nanotechnology in medicine are already underway and offer tremendous promise. This review explores the recent developments of nanotechnology in the field of CVD and gives an insight into its potential for diagnostics and therapeutics applications. The authors also explore the characteristics of the widely used biocompatible nanomaterials for this purpose and evaluate their opportunities and challenges for developing novel nanobiotechnological tools with high efficacy for biomedical applications, such as radiological imaging, vascular implants, gene therapy, myocardial infarction and targeted delivery systems.

Imaging Myocardial Ischemia and Reperfusion Injury via Cy5.5-Annexin V

AIM

The aim of this article is to present the results of an imaging study of myocardial apoptosis induced by ischemia/reperfusion injury.

METHODS

Twenty nude mice were randomly divided into an experimental group (10 mice) and control group (10 mice). In the experimental group, myocardial apoptosis was induced by ligation of the left anterior descending coronary artery (LAD) for 30 min. This was followed by reperfusion for 90 min. In the control group, the heart was exposed for the same length of time as in the experimental group. Cy5.5-annexin V (25 μg) was injected into both sets of mice after the onset of reperfusion. At 90 min post-injection, the mice were imaged. The region of interest (ROI) was obtained, and the fluorescence intensity of the ROI was quantified. The animals were sacrificed, and myocardial apoptosis was assayed by TUNEL assay.

RESULTS

Fluorescence intensity in the ischemia/reperfusion hearts was significantly higher than that in the control group (P < 0.05). In the TUNEL assay, more apoptotic cells were observed in the experimental group than in the control group, correlating with imaging results.

CONCLUSION

Fluorescence imaging of Cy5.5-annexin V in a mouse model of myocardial ischemia/reperfusion can be used in vivo as a noninvasive means of detecting ischemia/reperfusion-induced apoptotic cells in the heart.

Therapeutic potential of fucoidan in myocardial ischemia

Fucoidan, a sulfated polysaccharide extracted from brown seaweed, is a candidate for the treatment of ischemic diseases. The aim of this study was to measure the therapeutic potential of fucoidan in a rat model of myocardial ischemia-reperfusion injury. Forty rats were submitted to myocardial ischemia-reperfusion injury by transient occlusion of the left coronary artery. Rats were then randomized into 2 groups: fucoidan (5 mg/kg, intramuscularly; n = 20) or control (saline intramuscularly; n = 20) was administered 1 hour before injury and daily thereafter for 1 month. At 1 month, plasma levels of stromal cell-derived factor-1α (SDF-1α) were assessed by enzyme-linked immunosorbent assay kit. Hearts were evaluated by histoimmunochemistry. Fucoidan induced significant antifibrotic effects, reducing the infarct scar size by almost 30% on Sirius red-stained sections (9.45% ± 4.27% vs. 13% ± 5.67% in controls; P = 0.03). Vascular density in the fucoidan group (α-actin, RECA-1, or lectin BS1 stained) was increased by 40% (2.18 ± 0.79 mm vs. 1.49 ± 0.42 mm in controls ×200; P = 0.001). Plasma SDF-1α at 1 month was not significantly different between the 2 groups. However, increased immunostaining density of SDF-1α and vascular endothelial growth factor in fibrotic ischemic tissues was observed in fucoidan-treated animals versus controls. In conclusion, fucoidan enhanced tissue repair in myocardial ischemia-reperfusion by promoting revascularization (in situ vascular endothelial growth factor and SDF-1α overexpression) and limiting fibrosis. Consequently, fucoidan may be useful for myocardial ischemic patients.

Radiolabeled fucoidan as a p-selectin targeting agent for in vivo imaging of platelet-rich thrombus and endothelial activation

P-selectin expression is involved in the pathophysiology of biologically active arterial thrombus and endothelial activation after a transient ischemic event. Fucoidan is a polysaccharidic ligand of P-selectin, with a nanomolar affinity. In the present study, we propose a new approach of P-selectin molecular imaging based on radiolabeled fucoidan.

METHODS

Two kinds of experimental models were selected to evaluate the ability of radiolabeled fucoidan to detect P-selectin expression: platelet-rich arterial thrombi (vegetations of infective endocarditis and arterial mural thrombus) and myocardial ischemia-reperfusion. These 2 settings were chosen because they were clinically relevant, and both were associated with an important overexpression of platelet and endothelial P-selectin, respectively.

RESULTS

(99m)Tc-fucoidan SPECT was able to detect the presence of platelet-rich arterial thrombi in all animals, with a median target-to-background ratio of 5.2 in vegetations of endocarditis and 3.6 in mural aneurysmal thrombus, and to detect a persistent endothelial activation at 2 h after reperfusion. In this latter model, the magnitude of the signal was correlated with the extent of myocardium that underwent transient ischemia. The sensitivity of selectivity of the uptake and retention of (99m)Tc-fucoidan in both settings was excellent.

CONCLUSION

This study supports (99m)Tc-fucoidan as a relevant imaging agent for in vivo detection of biologic activities associated with P-selectin overexpression, such as arterial thrombus and ischemic memory. Given the reported wide availability at a low cost, and its low toxicity, fucoidan seems to overcome some of the limitations of previous P-selectin-targeted imaging agents.

Molecular MRI of acute necrosis with a novel DNA-binding gadolinium chelate: kinetics of cell death and clearance in infarcted myocardium

BACKGROUND

Current techniques to image cell death in the myocardium are largely nonspecific. We report the use of a novel DNA-binding gadolinium chelate (Gd-TO) to specifically detect the exposed DNA in acutely necrotic (ruptured) cells in vivo.

METHODS AND RESULTS

In vivo MRI was performed in 20 mice with myocardial infarction (MI). The mice were injected with Gd-TO or Gd-DTPA at varying time points after MI. MRI was performed 2 hours after probe injection, to avoid nonspecific signal from the late gadolinium enhancement effect. Cell rupture (Gd-TO uptake) was present within 2 hours of infarction but peaked 9 to 18 hours after the onset of injury. A significant increase in the longitudinal relaxation rate (R(1)) in the infarct was seen in mice injected with Gd-TO within 48 hours of MI, but not in those injected more than 72 hours after MI (R(1)=1.24±0.08 and 0.92±0.03 s(-1), respectively, P<0.001). Gd-DTPA, unlike Gd-TO, washed completely out of acute infarcts within 2 hours of injection (P<0.001). The binding of Gd-TO to exposed DNA in acute infarcts was confirmed with fluorescence microscopy.

CONCLUSIONS

Gd-TO specifically binds to acutely necrotic cells and can be used to image the mechanism and chronicity of cell death in injured myocardium. Cell rupture in acute MI begins early but peaks many hours after the onset of injury. The ruptured cells are efficiently cleared by the immune system and are no longer present in the myocardium 72 hours after injury.

Imaging of apoptosis in the heart with nanoparticle technology

Apoptosis plays an important role in the loss of cardiomyocytes in both ischemic injury and heart failure. Pioneering work with single photon emission computed tomography imaging of (99)Tc-annexin showed that cell death in the heart could be imaged in vivo. Over the last 5 years a significant amount of experience with annexin-labeled magnetic nanoparticles, principally AnxCLIO-Cy5.5, has also been gained. Here, we review the experience with AnxCLIO-Cy5.5 in the heart and compare this experience to that of earlier studies with (99)Tc-annexin. The imaging of apoptosis with AnxCLIO-Cy5.5 provides valuable insights not only into molecular imaging in the heart but, more broadly, into the use of nanoparticle technology for molecular imaging in general.

Synthesis and preliminary evaluation of radiolabeled bis(zinc(II)-dipicolylamine) coordination complexes as cell death imaging agents

The aim of this study was the development of (⁹⁹m)Tc labeled bis(zinc(II)-dipicolylamine) (Zn²⁺-DPA) coordination complexes, and the in vivo evaluation of their usefulness as radiotracers for the detection of cell death. DPA ligand 1 was labeled with (⁹⁹m)Tc via the (⁹⁹m)Tc-tricarbonyl core ([(⁹⁹m)Tc(CO)₃-1]³⁺) or via HYNIC ((⁹⁹m)Tc-HYNIC-1) in good radiochemical yields. Highest in vitro stabilities were demonstrated for [(⁹⁹m)Tc(CO)₃-1]³⁺. A mouse model of hepatic apoptosis (anti-Fas mAb) was used to demonstrate binding to apoptotic cells. (⁹⁹m)Tc-HYNIC-1 showed the best targeting of apoptotic hepatic tissue with a 2.2 times higher liver uptake in anti-Fas treated mice as compared to healthy animals. A rat model of ischemia-reperfusion injury was used to further explore the ability of the (⁹⁹m)Tc-labeled Zn²⁺-DPA coordination complexes to target cell death. Selective accumulation could be detected for both tracers in the area at risk, correlating with histological proof of cell death. Area at risk to normal tissue uptake ratios were 3.82 for [(⁹⁹m)Tc(CO)₃-1]³⁺ and 5.45 for (⁹⁹m)Tc-HYNIC-1.

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.

Advances in radionuclide molecular imaging in myocardial biology

Molecular imaging is a new and evolving field that employs a targeted approach to noninvasively assess biologic processes in vivo. By assessing key elements in specific cellular processes prior to irreversible end-organ damage, molecular tools will allow for earlier detection and intervention, improving management and outcomes associated with cardiovascular diseases. The goal of those working to expand this field is not just to provide diagnostic and prognostic information, but rather to guide an individual's pharmacological, cell-based, or genetic therapeutic regimen. This article will review molecular imaging tools in the context of our current understanding of biological processes of the myocardium, including angiogenesis, ventricular remodeling, inflammation, and apoptosis. The focus will be on radiotracer-based molecular imaging modalities with an emphasis on clinical application. Though this field is still in its infancy and may not be fully ready for widespread use, molecular imaging of myocardial biology has begun to show promise of clinical utility in acute and chronic ischemia, acute myocardial infarction, congestive heart failure, as well as in more global inflammatory and immune-mediated responses in the heart-like myocarditis and allogeneic cardiac transplant rejection. With continued research and development, molecular imaging promises to be an important tool for the optimization of cardiovascular care.

Apoptotic abscess imaging with 99mTc-HYNIC-rh-Annexin-V

Abscess formation causes systemic and localized up-regulation of neutrophil [polymorphonuclear leukocytes (PMNs)] signaling pathways. In the abscess, following bacterial ingestion or PMN activation by inflammatory mediators, PMN apoptosis is elevated and leads to the externalization of phosphatidylserine. Annexin-V (AnxV) has been shown to have high affinity to externalized phosphatidylserine. We hypothesized that (99m)Tc-AnxV will target high densities of apoptotic PMNs and image abscesses. AnxV, conjugated with hydrazinenicaotinamide (HYNIC), was labeled with reduced (99m)TcO(4)(-) and its purity was determined by instant thin-layer chromatography. Apoptosis was induced in isolated human PMNs by incubation in 2% saline for 17 and 22 h at 37 degrees C. PMNs were then incubated with (99m)Tc-HYNIC-AnxV and associated (99m)Tc was determined. Abscesses were induced in mice by intramuscular injection of bacteria or turpentine. Following intravenous administration of (99m)Tc-HYNIC-AnxV, mice were imaged and tissue distribution studied at 4 and 24 h. Radiochemical purity of (99m)Tc-HYNIC-AnxV was 84.9+/-8.11%. At 17 h, (99m)Tc-HYNIC-AnxV bound to apoptotic PMNs was 71.6+/-0.01% and 48.6+/-0.01% for experimental and control cells, respectively (P=.002). At 22 h, experimental cells retained 74.9+/-0.02% and control cells retained 47.2+/-0.02% (P=.005). (99m)Tc-HYNIC-AnxV associated with bacterial abscesses was 1.25+/-0.09 and 3.75+/-0.83 percent injected dose per gram (%ID/g) at 4 and 24 h compared to turpentine abscesses which was 1.02+/-0.16 and 0.72+/-0.17 %ID/g at 4 (P<or=.05) and 24 h (P<or=.01). (99m)Tc-HYNIC-AnxV represents a minimally invasive and promising agent to image and potentially distinguish between infectious and inflammatory abscesses.

Molecular MRI of cardiomyocyte apoptosis with simultaneous delayed-enhancement MRI distinguishes apoptotic and necrotic myocytes in vivo: potential for midmyocardial salvage in acute ischemia

BACKGROUND

A novel dual-contrast molecular MRI technique to image both cardiomyocyte apoptosis and necrosis in vivo within 4 to 6 hours of ischemia is presented. The technique uses the annexin-based nanoparticle AnxCLIO-Cy5.5 (apoptosis) and simultaneous delayed-enhancement imaging with a novel gadolinium chelate, Gd-DTPA-NBD (necrosis).

METHODS AND RESULTS

Mice with transient coronary ligation were injected intravenously at the onset of reperfusion with AnxCLIO-Cy5.5 (n=7) or the control probe Inact_CLIO-Cy5.5 (n=6). T2*-weighted MR images (9.4 T) were acquired within 4 to 6 hours of reperfusion. The contrast-to-noise ratio between injured and uninjured myocardium was measured. The mice were then injected with Gd-DTPA-NBD, and delayed-enhancement imaging was performed within 10 to 30 minutes. Uptake of AnxCLIO-Cy5.5 was most prominent in the midmyocardium and was significantly greater than that of Inact_CLIO-Cy5.5 (contrast-to-noise ratio, 8.82+/-1.5 versus 3.78+/-1.1; P<0.05). Only 21+/-3% of the myocardium with accumulation of AnxCLIO-Cy5.5 showed delayed-enhancement of Gd-DTPA-NBD. Wall thickening was significantly reduced in segments with delayed enhancement and/or transmural accumulation of AnxCLIO-Cy5.5 (P<0.001). Fluorescence microscopy of AnxCLIO-Cy5.5 and immunohistochemistry of Gd-DTPA-NBD confirmed the presence of large numbers of apoptotic but potentially viable cardiomyocytes (AnxCLIO-Cy5.5 positive, Gd-DTPA-NBD negative) in the midmyocardium.

CONCLUSIONS

A novel technique to image cardiomyocyte apoptosis and necrosis in vivo within 4 to 6 hours of injury is presented and reveals large areas of apoptotic but viable myocardium in the midmyocardium. Strategies to salvage the numerous apoptotic but potentially viable cardiomyocytes in the midmyocardium in acute ischemia should be investigated.