Evaluation of annexin V as a platelet-directed thrombus targeting agent

Targeting phosphatidylserine for radionuclide-based molecular imaging of apoptosis

One major characteristic of programmed cell death (apoptosis) results in the increased expression of phosphatidylserine (PS) on the outer membrane of dying cells. Consequently, PS represents an excellent target for non-invasive imaging of apoptosis by single-photon emission computed tomography (SPECT) and positron emission tomography (PET). Annexin V is a 36 kDa protein which binds with high affinity to PS in the presence of Ca²⁺ ions. This makes radiolabeled annexins valuable apoptosis imaging agents for clinical and biomedical research applications for monitoring apoptosis in vivo. However, the use of radiolabeled annexin V for in vivo imaging of cell death has been met with a variety of challenges which have prevented its translation into the clinic. These difficulties include: complicated and time-consuming radiolabeling procedures, sub-optimal biodistribution, inadequate pharmacokinetics leading to poor tumour-to-blood contrast ratios, reliance upon Ca²⁺ concentrations in vivo, low tumor tissue penetration, and an incomplete understanding of what constitutes the best imaging protocol following induction of apoptosis. Therefore, new concepts and improved strategies for the development of PS-binding radiotracers are needed. Radiolabeled PS-binding peptides and various Zn(II) complexes as phosphate chemosensors offer an innovative strategy for radionuclide-based molecular imaging of apoptosis with PET and SPECT. Radiolabeled peptides and Zn(II) complexes provide several advantages over annexin V including better pharmacokinetics due to their smaller size, better availability, simpler synthesis and radiolabeling strategies as well as facilitated tissue penetration due to their smaller size and faster blood clearance profile allowing for optimized image contrast. In addition, peptides can be structurally modified to improve metabolic stability along with other pharmacokinetic and pharmacodynamic properties. The present review will summarize the current status of radiolabeled annexins, peptides and Zn(II) complexes developed as radiotracers for imaging apoptosis through targeting PS utilizing PET and SPECT imaging.

An αIIbβ3- and phosphatidylserine (PS)-binding recombinant fusion protein promotes PS-dependent anticoagulation and integrin-dependent antithrombosis

Blood platelets are required for normal wound healing, but they are also involved in thrombotic diseases, which are usually managed with anticoagulant drugs. Here, using genetic engineering, we coupled the disintegrin protein echistatin, which specifically binds to the platelet integrin α_IIbβ₃ receptor, to annexin V, which binds platelet membrane-associated phosphatidylserine (PS), to create the bifunctional antithrombotic molecule recombinant echistatin-annexin V fusion protein (r-EchAV). Lipid binding and plasma coagulation studies revealed that r-EchAV dose-dependently binds PS and delays plasma clotting time. Moreover, r-EchAV inhibited ADP-induced platelet aggregation in a dose-dependent manner and exhibited potent antiplatelet aggregation effects. r-EchAV significantly prolonged activated partial thromboplastin time, suggesting that it primarily affects the in vivo coagulation pathway. Flow cytometry results indicated that r-EchAV could effectively bind to the platelet α_IIbβ₃ receptor, indicating that r-EchAV retains echistatin's receptor-recognition region. In vivo experiments in mice disclosed that r-EchAV significantly prolongs bleeding time, indicating a significant anticoagulant effect in vivo resulting from the joint binding of r-EchAV to both PS and the α_IIbβ₃ receptor. We also report optimization of the r-EchAV production steps and its purification for high purity and yield. Our findings indicate that r-EchAV retains the active structural regions of echistatin and annexin V and that the whole molecule exhibits multitarget-binding ability arising from the dual functions of echistatin and annexin V. Therefore, r-EchAV represents a new class of anticoagulant that specifically targets the anionic membrane-associated coagulation enzyme complexes at thrombogenesis sites and may be a potentially useful antithrombotic agent.

Glucocorticoids reduce chemotherapeutic effectiveness on OSCC cells via glucose-dependent mechanisms

Synthetic corticosteroids are routinely administered during the treatment of several diseases, including malignancies. However, recent evidence suggests that corticosteroids may have tumor-promoting effects, particularly in epithelial neoplasms. Our aim was to assess the role of the recently characterized cancer-associated glucocorticoid (GC) system in the resistance to chemotherapy of oral malignant keratinocytes. Human malignant oral keratinocyte cell lines H314/H357/H400/BICR16/BICR56 were tested with: two chemotherapeutic agents, doxorubicin (DOXO) and 5-fluorouracil (5-FU), as well as hydrocortisone (HC), adrenocorticotropic hormone (ACTH), 5-pregnen-3-beta-ol-20-one-16-alfa-carbonitrile (PCN), and two glucose uptake inhibitors, Fasentin and WZB. Both DOXO and 5-FU induced apoptosis in a dose-dependent and time-dependent manner. HC administration (100 nM) reduced the effectiveness of both chemotherapeutic agents to a variable extent in all 5 oral squamous cell carcinoma cell lines. ACTH also reduced the effectiveness of DOXO on 2 cell lines tested (H357 and BICR56). The glucose uptake inhibitors Fasentin and WZB were able to partially block the increased resistance to the cytotoxic drugs induced by HC. In summary, we have demonstrated, for the first time, the importance of cortisol on oral cancer cells ability to proliferate and combat the effectiveness of chemotherapeutic agents. This effect appears to be glucose dependent.

Thrombin@Fe3O4 nanoparticles for use as a hemostatic agent in internal bleeding

Bleeding remains one of the main causes of premature mortality at present, with internal bleeding being the most dangerous case. In this paper, magnetic hemostatic nanoparticles are shown for the first time to assist in minimally invasive treatment of internal bleeding, implying the introduction directly into the circulatory system followed by localization in the bleeding zone due to the application of an external magnetic field. Nanoparticles were produced by entrapping human thrombin (THR) into a sol-gel derived magnetite matrix followed by grinding to sizes below 200 nm and subsequent colloidization. Prepared colloids show protrombotic activity and cause plasma coagulation in in vitro experiments. We also show here using a model blood vessel that the THR@ferria composite does not cause systematic thrombosis due to low activity, but being concentrated by an external magnetic field with simultaneous fibrinogen injection accelerates local hemostasis and stops the bleeding. For instance, a model vessel system with circulating blood at the puncture of the vessel wall and the application of a permanent magnetic field yielded a hemostasis time by a factor of 6.5 shorter than that observed for the control sample. Biocompatibility of composites was tested on HELF and HeLa cells and revealed no toxic effects.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

Postchemotherapy and tumor-selective targeting with the La-specific DAB4 monoclonal antibody relates to apoptotic cell clearance

Early identification of tumor responses to treatment is crucial for devising more effective and safer cancer treatments. No widely applicable, noninvasive method currently exists for specifically detecting tumor cell death after cytotoxic treatment and thus for predicting treatment outcomes.

METHODS

We have further characterized the targeting of the murine monoclonal antibody DAB4 specifically to dead tumor cells in vitro, in vivo, and in clinical samples. We found that sustained DAB4 binding to treated cells was closely associated with markers of intrinsic apoptosis and DNA double-strand break formation. In a competition binding assay, DAB4 bound EL4 murine thymic lymphoma cells in preference to the normal counterpart of murine thymocytes. Defective in vivo clearance of apoptotic cells augmented in vivo accumulation of DAB4 in tumors particularly after chemotherapy but was unchanged in normal tissues. Tumor targeting of DAB4 was selective for syngeneic murine tumors and for human tumor xenografts of prostate cancer (PC-3) and pancreatic cancer (Panc-1) before and more so after chemotherapy. Furthermore, DAB4 was shown to bind to dead primary acute lymphoblastic leukemic blasts cultured with cytotoxic drugs and dead epithelial cancer cells isolated from peripheral blood of small cell lung carcinoma patients given chemotherapy.

CONCLUSION

Collectively, these results further demonstrate the selectivity of DAB4 for chemotherapy-induced dead tumor cells. This postchemotherapy selectivity is related to a relative increase in the availability of DAB4-binding targets in tumor tissue rather than in normal tissues. The in vitro findings were translated in vivo to human xenograft models and to ex vivo analyses of clinical samples, providing further evidence of the potential of DAB4 as a marker of tumor cell death after DNA-damaging cytotoxic treatment that could be harnessed as a predictive marker of treatment responses.

Multiplexed in vivo fluorescence optical imaging of the therapeutic efficacy of photodynamic therapy

In our study we wanted to elucidate a time frame for in vivo optical imaging of the therapeutic efficacy of photodynamic therapy (PDT) by using a multiplexed imaging approach for detecting apoptosis and vascularization. The internalization of the photosensitizer Foslip(®) into tongue-squamous epithelium carcinoma cells (CAL-27) was examined in vitro and in vivo. For detecting apoptosis, annexin V was covalently coupled to the near-infrared dye DY-734 and the spectroscopic properties and binding affinity to apoptotic CAL-27 cells were elucidated. CAL-27 tumor bearing mice were treated with PDT and injected 2 days and 2 weeks thereafter with DY-734-annexin V. PDT-induced changes in tumor vascularization were detected with the contrast agent IRDye(®) 800CW RGD up to 3 weeks after PDT. A perinuclear enrichment of Foslip(®) could be seen in vitro which was reflected in an accumulation in CAL-27 tumors in vivo. The DY-734-annexin V (coupling efficiency 30-50%) revealed a high binding affinity to apoptotic compared to non-apoptotic cells (17.2% vs. 1.2%) with a KD-value of 20 nm. After PDT-treatment, the probe showed a significantly higher (p <0.05) contrast in tumors at 2 days compared to 2 weeks after therapy (2-8 h post injection). A reduction of the vascularization could be detected after PDT especially in the central tumor areas. To detect the therapeutic efficacy of PDT, a multiplexed imaging approach is necessary. A detection of apoptotic cells is possible just shortly after therapy, whereas at later time points the efficacy can be verified by investigating the vascularization.

Longitudinal PET imaging of doxorubicin-induced cell death with 18F-Annexin V

PURPOSE

This study aims to apply longitudinal positron emission tomography (PET) imaging with (18)F-Annexin V to visualize and evaluate cell death induced by doxorubicin in a human head and neck squamous cell cancer UM-SCC-22B tumor xenograft model.

PROCEDURES

In vitro toxicity of doxorubicin to UM-SCC-22B cells was determined by a colorimetric assay. Recombinant human Annexin V protein was expressed and purified. The protein was labeled with fluorescein isothiocyanate for fluorescence staining and (18)F for PET imaging. Established UM-SCC-22B tumors in nude mice were treated with two doses of doxorubicin (10 mg/kg each dose) with 1 day interval. Longitudinal (18)F-Annexin V PET was performed at 6 h, 24 h, 3 days, and 7 days after the treatment started. Following PET imaging, direct tissue biodistribution study was performed to confirm the accuracy of PET quantification.

RESULTS

Two doses of doxorubicin effectively inhibited the growth of UM-SCC-22B tumors by inducing cell death including apoptosis. The cell death was clearly visualized by (18)F-Annexin V PET. The peak tumor uptake, which was observed at day 3 after treatment started, was significantly higher than that in the untreated tumors (1.56 ± 0.23 vs. 0.89 ± 0.31%ID/g, p < 0.05). Moreover, the tumor uptake could be blocked by co-injection of excess amount of unlabeled Annexin V protein. At day 7 after treatment, the tumor uptake of (18)F-Annexin had returned to baseline level.

CONCLUSIONS

(18)F-Annexin V PET imaging is sensitive enough to allow visualization of doxorubicin-induced cell death in UM-SCC-22B xenograft model. The longitudinal imaging with (18)F-Annexin will be helpful to monitor early response to chemotherapeutic anti-cancer drugs.

Single-Photon Emission Computed Tomographic Imaging of the Early Time Course of Therapy-Induced Cell Death Using Technetium 99m Tricarbonyl His-Annexin A5 in a Colorectal Cancer Xenograft Model

As apoptosis occurs over an interval of time after administration of apoptosis-inducing therapy in tumors, the changes in technetium 99m (99mTc)-tricarbonyl (CO)3 His-annexin A5 (His-ann A5) accumulation over time were examined. Colo205-bearing mice were divided into six treatment groups: (1) control, (2) 5-fluorouracil (5-FU; 250 mg/kg), (3) irinotecan (100 mg/kg), (4) oxaliplatin (30 mg/kg), (5) bevacizumab (5 mg/kg), and (6) panitumumab (6 mg/kg). 99mTc-(CO)3 His-ann A5 was injected 4, 8, 12, 24, and 48 hours posttreatment, and micro–single-photon emission computed tomography was performed. Immunostaining of caspase-3 (apoptosis), survivin (antiapoptosis), and LC3-II (autophagy marker) was also performed. Different dynamics of 99mTc-(CO)3 His-ann A5 uptake were observed in this colorectal cancer xenograft model, in response to a single dose of three different chemotherapeutics (5-FU, irinotecan, and oxaliplatin). Bevacizumab-treated mice showed no increased uptake of the radiotracer, and a peak of 99mTc-(CO)3 His-ann A5 uptake in panitumumab-treated mice was observed 24 hours posttreatment, as confirmed by caspase-3 immunostaining. For irinotecan-, oxaliplatin-, and bevacizumab-treated tumors, a significant correlation was established between the radiotracer uptake and caspase-3 immunostaining ( r = .8, p < .05; r = .9, p < .001; r = .9, p < .001, respectively). For 5-FU- and panitumumabtreated mice, the correlation coefficients were r = .7 ( p = .18) and r = .7 ( p = .19), respectively. Optimal timing of annexin A5 imaging after the start of different treatments in the Colo205 model was determined.

Visualization of cell death in mice with focal cerebral ischemia using fluorescent annexin A5, propidium iodide, and TUNEL staining

To monitor stroke-induced brain damage and assess neuroprotective therapies, specific imaging of cell death after cerebral ischemia in a noninvasive manner is highly desirable. Annexin A5 has been suggested as a marker for imaging cell death under various disease conditions including stroke. In this study, C57BL6/N mice received middle cerebral artery occlusion (MCAO) and were injected intravenously with either active or inactive Cy5.5-annexin A5 48 hours after reperfusion. Some mice also received propidium iodide (PI), a cell integrity marker. Only in mice receiving active Cy5.5-annexin A5 were fluorescence intensities significantly higher over the hemisphere ipsilateral to MCAO than on the contralateral side. This was detected noninvasively and ex vivo 4 and 8 hours after injection. The majority of cells positive for fluorescent annexin A5 were also positive for PI and fragmented DNA as detected by terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling (TUNEL) staining. This study demonstrates the high specificity of annexin A5 for visualization of cell death in a mouse model of stroke. To our knowledge, this is the first study to compare the distribution of injected active and inactive annexin A5, PI, and TUNEL staining. It provides important information on the experimental and potential clinical applications of annexin A5-based imaging agents in stroke.

Molecular SPECT Imaging: An Overview

Molecular imaging has witnessed a tremendous change over the last decade. Growing interest and emphasis are placed on this specialized technology represented by developing new scanners, pharmaceutical drugs, diagnostic agents, new therapeutic regimens, and ultimately, significant improvement of patient health care. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) have their signature on paving the way to molecular diagnostics and personalized medicine. The former will be the topic of the current paper where the authors address the current position of the molecular SPECT imaging among other imaging techniques, describing strengths and weaknesses, differences between SPECT and PET, and focusing on different SPECT designs and detection systems. Radiopharmaceutical compounds of clinical as well-preclinical interest have also been reviewed. Moreover, the last section covers several application, of μSPECT imaging in many areas of disease detection and diagnosis.

Emerging diagnostic and therapeutic molecular imaging applications in vascular disease

Assessment of vascular disease has evolved from mere indirect and direct measurements of luminal stenosis to sophisticated imaging methods to depict millimeter structural changes of the vasculature. In the near future, the emergence of multimodal molecular imaging strategies may enable robust therapeutic and diagnostic ('theragnostic') approaches to vascular diseases that comprehensively consider structural, functional, biological and genomic characteristics of the disease in individualized risk assessment, early diagnosis and delivery of targeted interventions.This review presents a summary of recent preclinical and clinical developments in molecular imaging and theragnostic applications covering diverse atherosclerosis events such as endothelial activation, macrophage inflammatory activity, plaque neovascularization and arterial thrombosis. The main focus is on molecular targets designed for imaging platforms commonly used in clinical medicine including magnetic resonance, computed tomography and positron emission tomography. A special emphasis is given to vascular ultrasound applications, considering the important role this imaging platform plays in the clinical and research practice of the vascular medicine specialty.

In vitro and in vivo evaluation of [99mTc]-labeled tricarbonyl His-annexin A5 as an imaging agent for the detection of phosphatidylserine-expressing cells

INTRODUCTION

Apoptosis is one of the mechanisms behind successful chemotherapy and radiation treatment. Radiolabeled annexin A5 has been demonstrated to be a successful tool in the detection of apoptosis following chemotherapy in vivo.

METHODS

His-tagged annexin A5 was labeled with [(99m)Tc]-tricarbonyl and evaluated as apoptosis imaging radiotracer in vitro and in vivo. The binding of the radiotracer was evaluated in Colo205 cells stimulated with 5-FU (1 mM) for 4 and 24 h, and confirmed by flow cytometry. Biodistribution and dosimetric studies were performed in healthy nude mice (n=5) via planar scintigraphy. [(99m)Tc]-(CO)(3) His-annexin A5 was also evaluated for in vivo imaging of spontaneous apoptosis in Colo205-bearing mice (n=12).

RESULTS

The labeling procedure yielded a compound with 95-99% radiochemical purity and good in vitro stability. In vitro binding experiments indicated that the radiotracer retained its PS-binding activity. [(99m)Tc]-(CO)(3) His-annexin A5 rapidly cleared from the blood and predominantly accumulated in the kidneys. Absorbed dose (per organ) was found to be 116 ± 64 μGy/MBq for the kidneys and 10.38 ± 0.50 μGy/MBq for the liver. The effective dose was 7.00 ± 0.28 μSv/MBq. Spontaneous apoptosis in Colo205-bearing mice was visualised by [(99m)Tc]-(CO)(3) His-annexin A5 SPECT and correlated well with caspase-3 immunostaining (R=0.867, P<.01).

CONCLUSION

[(99m)Tc]-(CO)(3) His-annexin A5 may be a useful novel radioligand for the in vivo detection of cell death associated with PS expression. A simple, noninvasive way of detecting apoptosis in vivo could have many applications including a better understanding of the extent and timing of apoptosis in response to cancer therapies and assessment of early tumor response.

Translational imaging: imaging of apoptosis

Since its original description in 1972, apoptosis or programmed cell death has been recognized as the major pathway by which the body precisely regulates the number and type of its cells as part of normal embryogenesis, development, and homeostasis. Later it was found that apoptosis was also involved in the pathogenesis of a number of human diseases, cell immunity, and the action of cytotoxotic drugs and radiation therapy in cancer treatment. As such, the imaging of apoptosis with noninvasive techniques such as with radiotracers, including annexin V and lipid proton magnetic resonance spectroscopy, may have a wide range of clinical utility in both the diagnosis and monitoring therapy of a wide range of human disorders. In this chapter we review the basic biochemical and morphologic features of apoptosis and the methods developed thus far to image this complex process in humans.

Radionuclide Imaging of Apoptosis in Malignancies: Promise and Pitfalls of Tc-Hynic-rh-Annexin V Imaging

Radionuclide detection of apoptosis with of ^99mTc-Hynic-rh-Annexin V scintigraphy is an effective tool for in vivo visualisation and monitoring of apoptosis in various malignant tumour. Early therapy-induced increase of the tumour tracer uptake correlates with favourable outcome, whereas stable or decreased uptake correlates with stable disease or tumour progression. Therefore sequential ^99mTc-Hynic-rh-Annexin V scintigraphy could be used to predict therapy outcome on a patient-to-patient basis within 48 hours after the start of treatment. However, moderate tumour-to-background ratio and therapy-induced changes in normal tissues could confound image analysis. To assure accurate interpretation of Annexin V scans, the awareness of the biophysiological and biochemical properties contributing to the tracer distribution is essential. In with manuscript we discuss the patterns of Annexin V tumour uptake and illustrate the most frequent pitfalls associated with Annexin V imaging in correlation with CT and MRI imaging.

Technetium 99m–Labeled Annexin V Scintigraphy of Platelet Activation in Vegetations of Experimental Endocarditis

Background— The pathophysiology of infective endocarditis involves a pathogen/host tissue interaction, leading to formation of infected thrombotic vegetations. Annexin V is a ligand of phosphatidylserines exposed by activated platelets and apoptotic cells. Because vegetations are platelet-fibrin clots in which platelet proaggregant activity is enhanced by bacterial colonization, we investigated the ability of annexin V labeled with technetium Tc 99m ( 99m Tc-ANX) to provide functional imaging of these vegetations in experimental models of infective endocarditis. This ability was assessed in rabbits and rats because of the different interest of these 2 species in preclinical analysis.

Methods and Results— Nonbacterial thrombotic endocarditis was induced with the use of a catheter left indwelling through the aortic or tricuspid valve, and animals were injected with either a bacterial inoculum or saline. Scintigraphic investigations were performed 5 days later and showed a higher 99m Tc-ANX uptake by vegetations in infected versus noninfected animals (ratio, 1.3 for in vivo acquisitions and 2 for autoradiography; P <0.0001 for all), whereas no significant uptake was present in controls. Right-sided endocarditis was associated with pulmonary uptake foci corresponding to emboli. Histological analysis of vegetations showed a specific uptake of 99m Tc-ANX at the interface between circulating blood and vegetation. In parallel, underlying myocardial tissue showed myocyte apoptosis and mucoid degeneration, without extracellular matrix degradation at this stage.

Conclusions— 99m Tc-ANX is suitable for functional imaging of platelet-fibrin vegetations in endocarditis, as well as embolic events. 99m Tc-ANX uptake reflects mainly platelet activation in the luminal layer of vegetations. This uptake is enhanced by bacterial colonization.

In vivo dynamic imaging of myocardial cell death using 99mTc-labeled C2A domain of synaptotagmin I in a rat model of ischemia and reperfusion

OBJECTIVES

This study was designed to investigate the capability of a small-animal SPECT imager, FastSPECT II, for dynamic rat heart imaging and to characterize the in vivo kinetic properties of 99mTc-C2A-glutathione-s-transferase (GST), a molecular probe targeting apoptosis and necrosis, in detecting cell death in ischemic-reperfused rat hearts.

METHODS

C2A-GST was radiolabeled with 99mTc via 2-iminothiolane thiolation. Myocardial ischemia-reperfusion was induced by 30-min ligation of the left coronary artery followed by 120-min reperfusion in seven rats. FastSPECT II cardiac images of 99mTc-C2A-GST in list-mode acquisition were recorded for 2 h using FastSPECT II.

RESULTS

Tomographic images showed a focal radioactive accumulation (hot spot) in the lateral and anterior walls of the left ventricle. The hot spot was initially visualized 10 min after injection and persisted on the 2-h images. Quantitative analysis demonstrated that the hot-spot radioactivity increased significantly within 30 min postinjection and experienced no washout up to the end of the 2-h study. The ratio of the hot spot/viable myocardium was 4.52+/-0.24, and infarct-to-lung ratio was 8.22+/-0.63 at 2 h postinjection. The uptake of 99mTc-C2A-GST in the infarcted myocardium was confirmed by triphenyl tetrazolium chloride staining and autoradiography analysis.

CONCLUSIONS

FastSPECT II allows quantitative dynamic imaging and functional determination of radiotracer kinetics in rat hearts. An in vivo kinetic profile of 99mTc-C2A-GST in the ischemic-reperfused rat heart model was characterized successfully. The pattern of accelerated 99mTc-C2A-GST uptake in the ischemic area at risk after reperfusion may be useful in detecting and quantifying ongoing myocardial cell loss induced by ischemia-reperfusion.

99m Tc-Annexin-V Functional Imaging of Luminal Thrombus Activity in Abdominal Aortic Aneurysms

Background— The mural thrombus of abdominal aortic aneurysms (AAA) is involved in aneurysm progression via several interdependent biological processes including platelet activation. 99m Tc-annexin V (ANX) is a scintigraphic tracer that binds to phosphatidylserine exposed on activated platelets and apoptotic cells. Here, we evaluated the potential of ANX imaging to assess mural thrombus biological activity in an experimental AAA model. The clinical applicability was further tested ex vivo on human samples of excised AAA thrombi.

Methods and Results— Experimental AAA was created by infusing elastase into infrarenal abdominal aorta in 17 rats, and 6 sham-operated rats were used as controls. Abdominal ANX scintigraphy was performed 2 weeks later followed by quantitative autoradiography and histological studies. Among the 13 rats which developed AAA, 11 displayed intense ANX uptake within AAA by scintigraphy. ANX uptake in the aneurysms on planar and single-photon emission computed tomography (SPECT) imaging was higher than that observed in infrarenal aorta of sham-operated controls (target/background ratio: 5.7±0.9 versus 1.33±0.21; P <0.005 for SPECT). Aneurysm-to-background activity ratios obtained by scintigraphy correlated with ANX activity in corresponding autoradiograms ( R =0.69; P <0.02). This activity was located in the thrombus area where activated platelets and polymorphonuclear leukocytes accumulated. Similar patterns were also found in all of the 7 human AAA thrombi harvested during surgery.

Conclusions— ANX imaging may assess mural thrombus renewal activity linked to permanent flowing blood interface.

Preparation, in vitro and in vivo evaluation of 99mTc-Annexin B1: A novel radioligand for apoptosis imaging

To develop a radiopharmaceutical for apoptosis imaging, Annexin B1, a new Ca2+-dependent phosphatidylserine (PS)-binding protein, was directly radiolabeled with (99m)Tc. This procedure yields up to 96% of radiochemical purity and higher radiolabeling efficiency. The preparation has been found to be sufficiently stable in vitro. Binding assay with human activated platelets indicated that (99m)Tc-Annexin B1 retained its PS binding activity. Biodistribution in mice revealed that (99m)Tc-Annexin B1 rapidly cleared from the blood and predominantly accumulated in the kidney. The increase in hepatic uptake in anti-Fas antibody treated mice correlated to histologic evidence of fulminant hepatic apoptosis. These data suggest that (99m)Tc-Annexin B1 can be used as a novel radiotracer to detect apoptosis in vivo.

Fusion proteins comprising annexin V and Kunitz protease inhibitors are highly potent thrombogenic site-directed anticoagulants

The anionic phospholipid, phosphatidyl-L-serine (PS), is sequestered in the inner layer of the plasma membrane in normal cells. Upon injury, activation, and apoptosis, PS becomes exposed on the surfaces of cells and sheds microparticles, which are procoagulant. Coagulation is initiated by formation of a tissue factor/factor VIIa complex on PS-exposed membranes and propagated through the assembly of intrinsic tenase (factor VIIIa/factor IXa), prothrombinase (factor Va/factor Xa), and factor XIa complexes on PS-exposed activated platelets. We constructed a novel series of recombinant anticoagulant fusion proteins by linking annexin V (ANV), a PS-binding protein, to the Kunitz-type protease inhibitor (KPI) domain of tick anticoagulant protein, an aprotinin mutant (6L15), amyloid beta-protein precursor, or tissue factor pathway inhibitor. The resulting ANV-KPI fusion proteins were 6- to 86-fold more active than recombinant tissue factor pathway inhibitor and tick anticoagulant protein in an in vitro tissue factor-initiated clotting assay. The in vivo antithrombotic activities of the most active constructs were 3- to 10-fold higher than that of ANV in a mouse arterial thrombosis model. ANV-KPI fusion proteins represent a new class of anticoagulants that specifically target the anionic membrane-associated coagulation enzyme complexes present at sites of thrombogenesis and are potentially useful as antithrombotic agents.

Functional and clinical repercussions of myocyte apoptosis in the multifaceted damage by ischemia/reperfusion injury: old and new concepts after 10 years of contributions

Ten years ago, the first finding of apoptotic cell death on the 'crime scene' of cardiac ischemia/reperfusion injury profoundly dismayed the scientific community. This observation jarred with the deeply rooted conviction that cardiac myocytes stoically 'break, but do not bend' in the fight against ischemia, instead of spontaneously accepting a peaceful demise for the greater good. Ten years later, a number of studies not only proved right the coexistence of necrosis and apoptosis on the ischemic battle field, but also implicated myocyte apoptosis in the pathogenesis of all the shapes and shades that cardiac ischemic injury can take on.

Optimization of expression of an Annexin V-Hirudin chimeric protein in Escherichia coli

A human Annexin V-Hirudin chimeric protein, Annexin V-Hirudin C, was expressed in Escherichia coli. A broad range of parameters such as plasmid stability during propogation and expression, expression capacity stability, the culture media, the growth time before induction and the induction duration were examined and optimized. Recombinant Annexin V-Hirudin C was purified from the cell lysate supernatants by ethanol precipitation, DEAE-cellulose chromatography and Sephadex G-75 chromatography, and the purified protein showed dose-dependent thrombin inhibitory activity. The overall production of purified Annexin V-Hirudin C protein is 10 mg/l/OD600.

Apoptosis-detecting radioligands: current state of the art and future perspectives

This review provides a critical and thorough overview of the radiopharmaceutical development and in vivo evaluation of all apoptosis-detecting radioligands that have emerged so far, along with their possible applications in nuclear medicine. The following SPECT and PET radioligands are discussed: all forms of halogenated Annexin V (i.e. (123)I-labelled, (124)I-labelled, (125)I-labelled, (18)F-labelled), (99m)Tc/(94m)Tc-labelled Annexin V derivatives using different chelators and co-ligands (i.e. BTAP, Hynic, iminothiolane, MAG(3), EDDA, EC, tricarbonyl, SDH) or direct (99m)Tc-labelling, (99m)Tc-labelled Annexin V mutants and (99m)Tc/(18)F-radiopeptide constructs (i.e. AFIM molecules), (111)In-DTPA-PEG-Annexin V, (11)C-Annexin V and (64)Cu-, (67)Ga- and (68)Ga-DOTA-Annexin V. In addition, the potential role and clinical relevance of anti-PS monoclonal antibodies and other alternative apoptosis markers are reviewed, including: anti-Annexin V monoclonal antibodies, radiolabelled caspase inhibitors and substrates and mitochondrial membrane permeability targeting radioligands. Nevertheless, major emphasis is placed on the group of Annexin V-based radioligands, in particular (99m)Tc-Hynic-Annexin V, since this molecule is by far the most extensively investigated and best-characterised apoptosis marker at present. Furthermore, the newly emerging imaging modalities for in vivo detection of programmed cell death, such as MRI, MRS, optical, bioluminescent and ultrasound imaging, are briefly described. Finally, some future perspectives are presented with the aim of promoting the development of potential new strategies in pursuit of the ideal cell death-detecting radioligand.

Comparison between human pharmacokinetics and imaging properties of two conjugation methods for 99mTc-annexin A5

Annexin A5 (AnxA5) is a protein with high affinity for phosphatidyl serine, a phospholipid exposed on the cell surface during apoptosis. This phenomenon has been used for determination of cell death after myocardial infarction. To evaluate the potential of (99m)Tc-AnxA5 for in vivo scintigraphy of apoptotic cells, the pharmacokinetics and imaging properties of two radiopharmaceuticals, (99m)Tc-(n-1-imino-4-mercaptobutyl)-AnxA5 (I-AnxA5) and (99m)Tc-(4,5-bis(thioacetamido)pentanoyl)-AnxA5 (B-AnxA5), were studied. I-AnxA5 was administered intravenously to seven patients and one healthy volunteer, and B-AnxA5 was administered to 12 patients. All patients in the pharmacokinetic study had myocardial disease. Additionally, imaging was performed in a patient with acute myocardial infarction, as well as in three patients with different malignancies. The plasma concentration, excretion and biodistribution of (99m)Tc-AnxA5 were measured, as well as levels of AnxA5 antigen. The kinetic data of both radiopharmaceuticals in plasma fitted a two-compartment model. Both preparations had similar half-lives, but a different distribution over the two compartments. Plasma levels of AnxA5 antigen showed a broad variation. Both radiopharmaceuticals accumulated in the kidney, liver and gut. B-AnxA5 was excreted significantly faster than I-AnxA5. Both compounds can be used for imaging of the head/neck region, the thorax and the extremities. B-AnxA5 has a faster clearance and a lower radiation dose. Imaging of apoptosis in the abdomen will be difficult with both radiopharmaceuticals, and especially with B-AnxA5 because of its faster appearance in the gut.

Non-invasive in vivo imaging of myocardial apoptosis and necrosis

Myocardial necrosis plays an important role in the pathogenesis of various cardiovascular disorders and can result from different myocardial insults. Its non-invasive identification and localisation therefore may help in the diagnosis of these disorders, as well as in prognosis and assessment of treatment response. Apoptosis, or programmed cell death, is important in the spectrum of myocardial damage since it is gradually becoming more apparent that cell death may begin as apoptosis and not as necrosis. First attempts to directly visualise the area of myocardial necrosis were based on recognition of myocardial infarction with "hot spot imaging agents" in patients with chest pain. Since then, the study of myocardial necrosis with gamma imaging agents has gone beyond the detection of myocardial infarction, and attempts have been made to diagnose other cardiovascular disorders associated with cardiac cell death such as heart transplant rejection, myocarditis, cardiotoxicity and cardiomyopathies. Traditionally, two hot spot imaging agents have been used for the detection of myocardial necrosis, (99m)Tc-pyrophosphate and (111)In-antimyosin. In addition, preliminary studies have demonstrated promising results with (99m)Tc-glucarate. Recently, (99m)Tc-annexin V has been successfully used for non-invasive gamma imaging of apoptosis after acute myocardial infarction, acute myocardial ischaemia, acute cardiac allograft rejection and malignant intracardiac tumours. This review article focusses on the characteristics of these different myocardial necrotic and apoptotic markers and compares their role in the assessment of myocardial damage.

Noninvasive strategies to image cardiovascular apoptosis

Apoptosis consists of a complex set of biochemical events initiated by an array of different stimuli and enzymatic pathways. There is a set of common morphologic and biochemical features of apoptosis that could be exploited as hot or cold targets to image cardiovascular apoptosis. First, the authors review the potential array of targets that can be used to identify apoptosis. Then, the authors examine the history and current status of radiolabeled annexin V, the agent currently used to image apoptosis.

Expression and purification of recombinant human annexin V in Escherichia coli

Human annexin V cDNA was cloned into plasmid pET19b and fused to a ten consecutive histidine tag at N-terminal. When expressed in E. coli BL21(DE3) LysS, the recombinant His10-annexin V accumulated in soluble form in the cytoplasm. By two-step chromatography, i.e., metal chelate affinity chromatography and anion exchange chromatography, recombinant His10-annexin V was purified to homogeneity on silver-stained SDS-PAGE gel. Recombinant annexin V, 7.4 mg, was obtained from a 1 litre flask culture.

A porcine model of carotid artery thrombosis for thrombolytic therapy and angioplasty: application of PTFE graft-induced stenosis

PURPOSE

To develop a porcine carotid artery thrombosis model for the evaluation of thrombolytic therapy and adjunctive angioplasty procedures.

METHODS

Bilateral carotid thrombosis was induced in 16 pigs using endothelial crush injury followed by external polytetrafluoroethylene (PTFE, 5 x 2 cm2) wrap placement to create segmental carotid stenosis. Light microscopy was used to examine thrombus composition. Selective carotid catheterization was performed via a femoral approach. Two hours following carotid artery occlusion, a urokinase (250,000 IU) and heparin (1000 U) solution was pulse-sprayed in 1 carotid artery while the contralateral vessel received the control saline vehicle. The efficacy of thrombolytic therapy was assessed using carotid arteriography and intravascular ultrasound. The feasibility and technical efficacy of balloon angioplasty within the carotid stenosis model were also evaluated.

RESULTS

Carotid artery occlusion occurred in 30 +/- 6 minutes following endothelial injury plus PTFE wrap placement. Histological examination of carotid arteries showed endothelial irregularity with fibrin-rich and platelet-rich thrombus. Urokinase was effective in recanalizing all occluded arteries (100%), while the control saline vehicle showed no effective thrombolysis (p < 0.001). Angioplasty was successful in restoring normal diameter in all arteries (100%).

CONCLUSIONS

This carotid artery thrombosis model, which incorporates intimal injury with segmental stenosis, is simple to create and reproducible. It provides not only a model for the evaluation of thrombolytic therapy but also a practical training tool for adjunctive endovascular interventions.

Apoptosis: the importance of nuclear medicine

Apoptosis is a genetically controlled, energy-dependent process which removes unwanted cells from the body. Because of its orderly progression, apoptosis is also known as programmed cell death or cell suicide. Once initiated, apoptosis is characterized by a series of biochemical and morphological changes involving the cytoplasm, nucleus and cell membrane. Cytoplasmic changes include cytoskeletal disruption, cytoplasmic shrinkage and condensation; prominent changes in the nucleus include peripheral chromatin clumping and inter-nucleosomal DNA cleavage (DNA ladder formation); and membrane changes include the expression of phosphatidylserine on the outer surface of the cell membrane and blebbing (resulting in the formation of cell membrane-bound vesicles or apoptotic bodies). These events allow the cell to digest and package itself into membrane-bound packets containing autodigested cytoplasm and DNA, which can then be easily absorbed by adjacent cells or phagocytes. An endogenous human protein, annexin V (molecular weight approximately 35,000), has an affinity of about 10(-9) M for phosphatidylserine exposed on the surface of apoptotic cells. Annexin V can be labelled with radionuclides such as iodine or technetium, or positron emitting agents. Experimental studies in cells confirm that fluorescence and 99Tc(m)-labelled annexin have comparable affinity for apoptotic cells. In vivo studies with 99Tc(m)-labelled annexin confirm that radiolabelled annexin V can be used to image apoptotic cells/tissues in vivo. In this article, we review experimental data using annexin V imaging and discuss its possible future use to identify apoptosis in vivo.

Current status of radionuclide tracer imaging of thrombi and atheroma

The imaging of thrombi and atherosclerotic plaques has great potential for decision making in the management of patients with all types of disease within the circulatory system. This importance is owing to the developments showing that areas of moderate stenosis with underlying atheroma are physiologically reactive and capable of causing reversible clinical symptoms that can progress to irreversible end-organ damage if not effectively treated. Identifying and quantifying areas of smaller vulnerable plaque and areas of acute thrombosis will assist in identification of patients at risk and help determine when and how to treat these patients. Initial efforts in this area used nonspecific constituents of thrombi and atheroma that were radiolabeled using long-lived isotopes, which had high background activity that required imaging over 48 to 72 hours. Newer approaches have focused on the use of small antibody fragments or small peptides, so-called molecular recognition units, that specifically target antigens present only in areas of thrombosis or active atherogenesis. These compounds are labeled Technetium-99 m (99mTc) and provide excellent images. Efforts to image thrombi have been directed at the IIB/IIIA receptor, which is present in low concentration on the cell membrane of circulating quiescent platelets, but on stimulation and active thrombosis, more than 80,000 potential binding sites per platelet appear. One such peptide has been clinically approved for imaging of deep vein thrombophlebitis. Parallel efforts are being made for imaging areas of active atherogenesis by targeting smooth muscle cells and other constituents unique for vulnerable plaques. Efforts in developing these modalities are important to expand the applications to new areas in nuclear cardiology.

Annexin V Delays Apoptosis While Exerting an External Constraint Preventing the Release of CD4+ and PrPc+ Membrane Particles in a Human T Lymphocyte Model

Phosphatidylserine exposure in the exoplasmic leaflet of the plasma membrane is one of the early hallmarks of cells undergoing apoptosis. The shedding of membrane particles carrying Ags testifying to their tissue origin is another characteristic feature. Annexin V, a protein of as yet unknown specific physiologic function, presents a high Ca2+-dependent affinity for phosphatidylserine and forms two-dimensional arrays at the membrane surface. In this study, we report the delaying action of annexin V on apoptosis in the CEM human T cell line expressing CD4 and the normal cellular prion protein (PrPc), two Ags of particular relevance to cell degeneration and with different attachments to the membrane. The effect of annexin V was additive to that of z-Val-Ala-Asp-fluoromethyl ketone, a potent caspase inhibitor. Annexin V significantly reduced the degree of proteolytic activation of caspase-3, and totally blocked the release of CD4+ and PrPc+ membrane particles. z-Val-Ala-Asp-fluoromethyl ketone was a more powerful antagonist of caspase-3 processing, but prevented the shedding of CD4+ vesicles only partially and had no effect on that of PrPc+ ones. These results suggest that an external membrane constraint, such as that exerted by annexin V, has important consequences on the course of programmed cell death and on the dissemination of particular Ags. In vivo, annexin V had a significant protective effect against spleen weight loss in mice treated by an alkylating agent previously shown to induce lymphocyte apoptosis.

In vivo detection and imaging of phosphatidylserine expression during programmed cell death

One of the earliest events in programmed cell death is the externalization of phosphatidylserine, a membrane phospholipid normally restricted to the inner leaflet of the lipid bilayer. Annexin V, an endogenous human protein with a high affinity for membrane bound phosphatidylserine, can be used in vitro to detect apoptosis before other well described morphologic or nuclear changes associated with programmed cell death. We tested the ability of exogenously administered radiolabeled annexin V to concentrate at sites of apoptotic cell death in vivo. After derivatization with hydrazinonicotinamide, annexin V was radiolabeled with technetium 99m. In vivo localization of technetium 99m hydrazinonicotinamide-annexin V was tested in three models: fuminant hepatic apoptosis induced by anti-Fas antibody injection in BALB/c mice; acute rejection in ACI rats with transplanted heterotopic PVG cardiac allografts; and cyclophosphamide treatment of transplanted 38C13 murine B cell lymphomas. External radionuclide imaging showed a two- to sixfold increase in the uptake of radiolabeled annexin V at sites of apoptosis in all three models. Immunohistochemical staining of cardiac allografts for exogenously administered annexin V revealed intense staining of numerous myocytes at the periphery of mononuclear infiltrates of which only a few demonstrated positive apoptotic nuclei by the terminal deoxynucleotidyltransferase-mediated UTP end labeling method. These results suggest that radiolabeled annexin V can be used in vivo as a noninvasive means to detect and serially image tissues and organs undergoing programmed cell death.

Renal artery occlusion model in dogs for the evaluation of thrombolytic agents

The purpose of this study was to develop a model of renal artery occlusion and to investigate the effects of various thrombolytic agents on an acute occlusion of the renal artery with respect to ischemic tolerance of renal parenchyma. In order to do this, a thrombosis model in dogs (n = 36) was established and a total of 72 dorsal renal arteries occluded using autologous clot material. For the in vitro preparing of a clot, autologous blood (20 mL) was withdrawn and 100 U thrombin immediately added. Then 1 mL of the clot material was injected into the dorsal branch of the exposed renal artery. The dogs were divided into 8 groups (2 control groups, 6 therapy groups with local and systemic thrombolytic therapy). Thrombolysis was performed using urokinase, single-chain urokinase, and recombinant tissue-plasminogen activator. In all cases the clot preparation technique allowed complete and stable occlusion of the renal arteries. Local and systemic application of the thrombolytic agents, however, resulted in complete recanalization of the clot material in all study groups. Recombinant tissue-plasminogen activator turned out to be the most effective agent in terms of recanalization time. The technique described allowed effective and reproducible artery occlusion for in vivo experimental work to study comparatively thrombolytic agents with respect to fibrin specificity, lytic efficacy, and side effects.

Secretory production of recombinant urokinase-type plasminogen activator-annexin V chimeras in Pichia pastoris

To produce a thrombi-targeting plasminogen activator, we expressed a fused gene that contains a modified pre-sequence of Mucor pussilus rennin (MPR) followed by a chimeric gene of single-chain urokinase-type plasminogen activator (scu-PA)::annexin V (AV). The fused gene was ligated into an integrative vector, under the control of the alcohol oxidase 1 (AOX1) promoter (p), and transformed into Pichia pastoris. Transformants were monitored for the secretion of fibrinolytic activity. The highest expressing clone, HB225, secreted as much as 600 international units (IU) of fibrinolytic activity per ml of culture medium under optimal conditions. It contained three tandem copies of the full-size vector disruptively integrated into the AOX1 sequence. Western blot analysis revealed that the secreted chimera was highly susceptible to proteolysis. Addition of excess amino acids (aa) to the culture medium minimized the degree of proteolysis. Two major species of chimera, 85 and 65 kDa, were then isolated from the culture medium. The former was the intact form consisting of a single-chain and showing full enzyme activity after activation by plasmin. The latter was an enzymatically processed form consisting of two chains held by a disulfide bond, having full enzyme activity without activation. Both chimeras exhibited calcium-dependent phospholipid (PL)-binding affinities similar to the parent AV.

Prourokinase-annexin V chimeras. Construction, expression, and characterization of recombinant proteins

Annexin V is a human protein that binds with high affinity to the abundant phosphatidylserine molecules exposed on activated platelets and accumulates selectively in thrombi after intravenous administration in animal models of arterial thrombosis. We designed two chimeras that use annexin V as a means to target thrombolytic agents to platelet-containing thrombi: prourokinase (1-411)-annexin V (1-320); and prourokinase (144-411)-annexin V (1-320) (amino acid numbers of parent proteins given in parentheses). Chimeras were produced by cytoplasmic expression in Escherichia coli, refolded, and purified in single-chain form. Both chimeras had the same specific activity as annexin V in binding to cell membranes containing exposed phosphatidylserine. After activation with plasmin, both chimeras had specific amidolytic activity similar to that of urokinase. Both chimeras activated plasminogen in vitro with kinetic parameters similar to those for urokinase, and both showed full activity compared to urokinase in an assay of clot lysis in vitro. This study shows the feasibility of producing chimeric plasminogen activators in which annexin V provides the thrombus-targeting component; although not yet tested in vivo, such chimeras may have advantages over antibody-based targeting agents.