Selective uptake of radiolabeled annexin V on acute porcine left atrial thrombi

Imaging of cell death in malignancy: Targeting pathways or phenotypes?

Cell death is fundamental in health and disease and resisting cell death is a hallmark of cancer. Treatment of malignancy aims to cause cancer cell death, however current clinical imaging of treatment response does not specifically image cancer cell death but assesses this indirectly either by changes in tumor size (using x-ray computed tomography) or metabolic activity (using 2-[18F]fluoro-2-deoxy-glucose positron emission tomography). The ability to directly image tumor cell death soon after commencement of therapy would enable personalised response adapted approaches to cancer treatment that is presently not possible with current imaging, which is in many circumstances neither sufficiently accurate nor timely. Several cell death pathways have now been identified and characterised that present multiple potential targets for imaging cell death including externalisation of phosphatidylserine and phosphatidylethanolamine, caspase activation and La autoantigen redistribution. However, targeting one specific cell death pathway carries the risk of not detecting cell death by other pathways and it is now understood that cancer treatment induces cell death by different and sometimes multiple pathways. An alternative approach is targeting the cell death phenotype that is "agnostic" of the death pathway. Cell death phenotypes that have been targeted for cell death imaging include loss of plasma membrane integrity and dissipation of the mitochondrial membrane potential. Targeting the cell death phenotype may have the advantage of being a more sensitive and generalisable approach to cancer cell death imaging. This review describes and summarises the approaches and radiopharmaceuticals investigated for imaging cell death by targeting cell death pathways or cell death phenotype.

Interest and Limits of [18F]ML-10 PET Imaging for Early Detection of Response to Conventional Chemotherapy

One of the current challenges in oncology is to develop imaging tools to early detect the response to conventional chemotherapy and adjust treatment strategies when necessary. Several studies evaluating PET imaging with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) as a predictive tool of therapeutic response highlighted its insufficient specificity and sensitivity. The [¹⁸F]FDG uptake reflects only tumor metabolic activity and not treatment-induced cell death, which seems to be relevant for therapeutic evaluation. Therefore, to evaluate this parameter in vivo, several cell death radiotracers have been developed in the last years. However, few of them have reached the clinical trials. This systematic review focuses on the use of [¹⁸F]ML-10 (2-(5-[¹⁸F]fluoropentyl)-2-methylmalonic acid) as radiotracer of apoptosis and especially as a measure of tumor response to treatment. A comprehensive literature review concerning the preclinical and clinical investigations conducted with [¹⁸F]ML-10 was performed. The abilities and applications of this radiotracer as well as its clinical relevance and limitations were discussed. Most studies highlighted a good ability of the radiotracer to target apoptotic cells. However, the increase in apoptosis during treatment did not correlate with the radiotracer tumoral uptake, even using more advanced image analysis (voxel-based analysis). [¹⁸F]ML-10 PET imaging does not meet current clinical expectations for early detection of the therapeutic response to conventional chemotherapy. This review has pointed out the challenges of applying various apoptosis imaging strategies in clinical trials, the current methodologies available for image analysis and the future of molecular imaging to assess this therapeutic response.

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.

SPECT and PET radiopharmaceuticals for molecular imaging of apoptosis: from bench to clinic

Owing to the central role of apoptosis in many human diseases and the wide-spread application of apoptosis-based therapeutics, molecular imaging of apoptosis in clinical practice is of great interest for clinicians, and holds great promises. Based on the well-defined biochemical changes for apoptosis, a rich assortment of probes and approaches have been developed for molecular imaging of apoptosis with various imaging modalities. Among these imaging techniques, nuclear imaging (including single photon emission computed tomography and positron emission tomography) remains the premier clinical method owing to their high specificity and sensitivity. Therefore, the corresponding radiopharmaceuticals have been a major focus, and some of them like ^99mTc-Annexin V, ¹⁸F-ML-10, ¹⁸F-CP18, and ¹⁸F-ICMT-11 are currently under clinical investigations in Phase I/II or Phase II/III clinical trials on a wide scope of diseases. In this review, we summarize these radiopharmaceuticals that have been widely used in clinical trials and elaborate them in terms of radiosynthesis, pharmacokinetics and dosimetry, and their applications in different clinical stages. We also explore the unique features required to qualify a desirable radiopharmaceutical for imaging apoptosis in clinical practice. Particularly, a perspective of the impact of these clinical efforts, namely, apoptosis imaging as predictive and prognostic markers, early-response indicators and surrogate endpoints, is also the highlight of this review.

Molecular imaging of apoptosis: from micro to macro

Apoptosis, or programmed cell death, is involved in numerous human conditions including neurodegenerative diseases, ischemic damage, autoimmune disorders and many types of cancer, and is often confused with other types of cell death. Therefore strategies that enable visualized detection of apoptosis would be of enormous benefit in the clinic for diagnosis, patient management, and development of new therapies. In recent years, improved understanding of the apoptotic machinery and progress in imaging modalities have provided opportunities for researchers to formulate microscopic and macroscopic imaging strategies based on well-defined molecular markers and/or physiological features. Correspondingly, a large collection of apoptosis imaging probes and approaches have been documented in preclinical and clinical studies. In this review, we mainly discuss microscopic imaging assays and macroscopic imaging probes, ranging in complexity from simple attachments of reporter moieties to proteins that interact with apoptotic biomarkers, to rationally designed probes that target biochemical changes. Their clinical translation will also be our focus.

Radiolabeled apoptosis imaging agents for early detection of response to therapy

Since apoptosis plays an important role in maintaining homeostasis and is associated with responses to therapy, molecular imaging of apoptotic cells could be useful for early detection of therapeutic effects, particularly in oncology. Radiolabeled annexin V compounds are the hallmark in apoptosis imaging in vivo. These compounds are reviewed from the genesis of apoptosis (cell death) imaging agents up to recent years. They have some disadvantages, including slow clearance and immunogenicity, because they are protein-based imaging agents. For this reason, several studies have been conducted in recent years to develop low molecule apoptosis imaging agents. In this review, radiolabeled phosphatidylserine targeted peptides, radiolabeled bis(zinc(II)-dipicolylamine) complex, radiolabeled 5-fluoropentyl-2-methyl-malonic acid (ML-10), caspase-3 activity imaging agents, radiolabeled duramycin, and radiolabeled phosphonium cation are reviewed as promising low-molecular-weight apoptosis imaging agents.

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.

Annexin V and anti-Annexin V antibodies: two interesting aspects in acute myocardial infarction

Background

Myocardial infarction is the combined result of environmental factors and personal predispositions. Prothrombotic factors might play an important role in this phenomenon. Annexin V (ANV) is a calcium-dependent glycoprotein widely present in various tissues exerting a potent anticoagulant effect in vitro by reducing plaque adhesion and aggregation. Anti-annexin V antibodies (aANVAs) are detected in various diseases like rheumatoid arthritis, systemic lupus erythematosus and anti-phospholipid antibody syndrome. The study of ANV in Acute Myocardial Infarction (AMI) might shed light on hypercoagulability mechanisms in the pathogenesis of acute coronary syndromes. This study was conducted to investigate the association of plasma ANV, aANVAs and anti-cardiolipin antibodies (aCLAs) with AMI.

Methods

This study recruited 45 patients with the diagnosis of AMI according to WHO criteria in their first 24 hours of admission. 36 matched individuals were studied as the control group with normal coronary artery angiography. Plasma levels of ANV, aANVAs and aCLAs were determined by enzyme-linked immunosorbent assay and the results were compared.

Results

Plasma ANV levels in the patients with AMI on admission were significantly lower than those in the control group (p = 0.002). Positive test for aANVAs were found to be present in a significant number of our patients (p = 0.004). The studied groups were similar in their rate of patients with positive aCLAs tests. ANV, aANVAs and aCLAs were not correlated with hypertension, diabetes mellitus, hyperlipidemia, sex, age and smoking.

Conclusion

Our findings suggest that low plasma ANV levels along with positive aANVAs tests in patients with AMI are indicative of hypercoagulable state that is not related to the traditional cardiovascular risk factors.

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.

Staphylokinase-annexin XI chimera exhibited efficient in vitro thrombolytic activities

Annexins (ANXs) are a family of calcium dependent phospholipid binding proteins. Phospholipids such as phosphatidylserine are rapidly exposed on the surfaces of injured endothelial cells, activated platelets, and apoptotic cells in a large number of disorders. In this study, annexin V and XI (ANXV and ANXXI) were individually fused to the C-terminal of staphylokinase (SAK), a fibrin-selective thrombolytic protein, to form chimeras for evaluation of their in-vitro thrombolytic activities. The two chimeras were found to have plasminogen activation activity of comparable efficiency. When the chimeras were challenged under higher concentrations of plasmin for 1 h, hydrolysis of them into moieties was not seen on SDS-PAGE. In two thrombolytic assays, SAK-ANXXI was found to resolve both platelet rich plasma (PRP) clots and platelet poor plasma (PPP) clots with an efficiency similar to that of SAK. However, SAK-ANXV showed significantly reduced efficiency. With regard to anticoagulation ability, SAK-ANXXI was also found to have a stronger effect on dose-dependent extension of clotting time among the four tested proteins. The unique long N-terminal tail of ANXXI, composed of 202 residues, in contrast to the 16 residues of ANXV, probably served successfully to dispatch two moieties to function properly in a complicated microenvironment. Hence, a new option other than the most committed ANXV for the ANX based chimera without elaboration of linker construction is presented.

Non-invasive imaging of atherosclerotic plaque macrophage in a rabbit model with F-18 FDG PET: a histopathological correlation

Background

Coronary atherosclerosis and its thrombotic complications are the major cause of mortality and morbidity throughout the industrialized world. Thrombosis on disrupted atherosclerotic plaques plays a key role in the onset of acute coronary syndromes. Macrophages density is one of the most critical compositions of plaque in both plaque vulnerability and thrombogenicity upon rupture. It has been shown that macrophages have a high uptake of ¹⁸F-FDG (FDG). We studied the correlation of FDG uptake with histopathological macrophage accumulation in atherosclerotic plaques in a rabbit model.

Methods

Atherosclerosis was induced in rabbits (n = 6) by a combination of atherogenic diet and balloon denudation of the aorta. PET imaging was performed at baseline and 2 months after atherogenic diet and coregistered with magnetic resonance (MR) imaging. Normal (n = 3) rabbits served as controls. FDG uptake by the thoracic aorta was expressed as concentration (μCi/ml) and the ratio of aortic uptake-to-blood radioactivity. FDG uptake and RAM-11 antibody positive areas were analyzed in descending aorta.

Results

Atherosclerotic aortas showed significantly higher uptake of FDG than normal aortas. The correlation of aortic FDG uptake with macrophage areas assessed by histopathology was statistically significant although it was not high (r = 0.48, p < 0.0001). When uptake was expressed as the ratio of aortic uptake-to-blood activity, it correlated better (r = 0.80, p < 0.0001) with the macrophage areas, due to the correction for residual blood FDG activity.

Conclusion

PET FDG activity correlated with macrophage content within aortic atherosclerosis. This imaging approach might serve as a useful non-invasive imaging technique and potentially permit monitoring of relative changes in inflammation within the atherosclerotic lesion.

Functional comparison of annexin V analogues labeled indirectly and directly with iodine-124

We are interested in imaging cell death in vivo using annexin V radiolabeled with (124)I. In this study, [(124)I]4IB-annexin V and [(124)I]4IB-ovalbumin were made using [(124)I]N-hydroxysuccinimidyl-4-iodobenzoate prepared by iododestannylation of N-hydroxysuccinimidyl-4-(tributylstannyl)benzoate. [(124)I]4IB-annexin V binds to phosphatidylserine-coated microtiter plates and apoptotic Jurkat cells and accumulates in hepatic apoptotic lesions in mice treated with anti-Fas antibody, while [(124)I]4IB-ovalbumin does not. In comparison with (124)I-annexin V, [(124)I]4IB-annexin V has a higher rate of binding to phosphatidylserine in vitro, a higher kidney and urine uptake, a lower thyroid and stomach content uptake, greater plasma stability and a lower rate of plasma clearance. Binding of radioactivity to apoptotic cells relative to normal cells in vitro and in vivo appears to be lower for [(124)I]4IB-annexin V than for (124)I-annexin V.

MBP-annexin V radiolabeled directly with iodine-124 can be used to image apoptosis in vivo using PET

A noninvasive method of measuring programmed cell death in the tumors of cancer patients using positron-emission tomography (PET) would provide valuable information regarding their response to therapeutic intervention. Our strategy is to radiolabel annexin V, a protein that binds to phosphatidylserine moieties that are translocated to the external leaflet of plasma membranes during apoptosis. We developed a phosphatidylserine-ELISA capable of distinguishing wild type from point mutant annexin V that is known to have a lower phosphatidylserine binding affinity. A maltose-binding protein/annexin V chimera was synthesized and purified with high yield using amylose resin. We showed that it bound to phosphatidylserine in the ELISA as well as to that exposed on apoptotic Jurkat cells; therefore, it was used in the development of a method for radiolabeling annexin V using iodine radionuclides. MBP-annexin V retained its phosphatidylserine binding properties on direct iodination, but at high levels of oxidizing agents (iodogen and chloramine T), its specificity for phosphatidylserine was compromised. (124)I-MBP-annexin V was successfully used to image Fas-mediated hepatic cell death in BDF-1 mice using PET. In conclusion, we have shown that MBP-annexin V and the phosphatidylserine ELISA are useful tools for the development of methods for radiolabeling annexin V for PET imaging.

Vulnerable Plaque Imaging

The concept of vulnerable plaque is well established with increasing evidence from clinical and basic research. The paradigm has shifted from focusing exclusively on the hemodynamic effects of plaque (ie, resulting lumenal stenosis alone as a predictor of stroke risk) to assessment of the structure and composition of plaque (eg, denuded endothelium with inflammatory elements as a nidus for platelet-fibrin clumping). It is increasingly evident that methods to detect and characterize vulnerable plaque must be developed and optimized. Although MR imaging, CT, and ultrasound provide data regarding single lesions, future investigations relying heavily on nuclear medicine techniques may offer functional assessment of the entire cardiovascular system.

Preparation of F-18 labeled annexin V: a potential PET radiopharmaceutical for imaging cell death

The clinical response to antitumor therapy is measured using imaging, such as CT or MRI, 6-12 weeks following chemotherapy treatment. The images at that time reflect both tumor cell death and new growth. Therefore, the amount of tumor cell death caused by chemotherapy cannot be efficiently quantified with current imaging modalities. A quantitative measurement of tumor cell death immediately following chemotherapy is needed to help validate both new agents and to optimize administration of existing therapies. Annexin V is a 36kD protein that binds to exposed phosphatidylserine (PS) on dying cells. In order to synthesize a probe that can detect cell death in vivo, the positron emitter F-18 was conjugated to annexin V via the compound N- succinimidyl-4-[18F]fluorobenzoate, [18F]SFB. The decay corrected radiochemical yield of F-18 labeled annexin V from 18F fluoride was 17.6 +/- 5.6% (n = 4) in three hours. The stepwise radiochemical yield of the conjugation step with annexin V was as high as 70% when a protein concentration of 5 mg/ml was used. Cancer cells treated with the chemotherapeutic agent, etoposide, showed an 88% increase in the binding of F-18 labeled annexin V compared to untreated cells. We conclude that [18F] labeled annexin V can be readily prepared by the conjugation of annexin V with [18F]SFB and that the positron-emitting compound is biologically active in detecting apoptosis.

The therapeutic potential of novel anticoagulants

Conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin. These agents display liabilities prompting the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins(LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and, more recently, approval for their use in the USA. Paralleling the development of LMWHs has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors exert their effects independent of either antithrombin III (ATIII) or heparin cofactor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases)direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of the senatural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant peptide-5 (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness, and sometimes relative ineffectiveness,in arterial and venous thromboembolic disorders has been demonstrated. They are being tested for their potential as new antithrombotic agents that act via direct enzyme inhibition. Thus,the clinician should in future be able to target different thrombotic conditions with proven, specific anticoagulant interventions.

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.

Measurement of the affinity and cooperativity of annexin V–membrane binding under conditions of low membrane occupancy

We developed a method for measuring the binding affinity of annexin V for phospholipid vesicles and cells at very low levels of membrane occupancy. The annexin V-117 mutant was labeled with fluorescein iodoacetamide on its single N-terminal cysteine residue; binding to phospholipid vesicles containing phosphatidylserine (PS) and 2% rhodamine-phosphatidylethanolamine was measured by fluorescence quenching due to resonance energy transfer; binding to cells with exposed PS was measured by fluorometry after elution of bound protein. The equilibrium constant was calculated as a function of the midpoint of the calcium titration curve, the Hill coefficient, and the concentration of membrane binding sites. Calcium titrations at very low ratios of protein to membrane revealed Hill coefficients of approximately 8 for both vesicles and cells, far higher than previously measured, but as the protein-membrane ratio was increased above 3% of maximum membrane occupancy, the value of the Hill coefficient progressively decreased to a limiting value of about 2. High Hill coefficients were also observed for measurements performed at different ionic strengths and with membrane PS content varied over the range from 20 to 50%. This method allows the accurate determination of the affinity and cooperativity of annexin V-membrane binding and will be useful for the evaluation of modified annexin V derivatives intended for diagnostic and therapeutic applications.

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.

Detection of Acute Atrial Thrombus in a Porcine Model With Atrial Fibrillation With Tc 99m-Apcitide

RATIONALE AND OBJECTIVES

Technetium-99m apcitide (AcuTect) is a peptide with high affinity and specificity for glycoprotein IIb/IIIa receptors on platelets and is currently approved for the diagnosis of deep venous thrombosis. This study evaluates the use of Tc-99m apcitide for detecting intracardiac thrombus in an animal model with atrial fibrillation.

METHODS

Thrombogenic material (0.23+/- 0.03 g) was implanted within the left atrium of 5 swine with induced atrial fibrillation. Scintigraphy was performed with a small field of view gamma camera (minimum of 400000 counts) 1 hour after implantation at 10, 60, and 120 minutes after the injection of the Tc-99m apcitide. Animals were then euthanized and a postmortem examination performed to confirm thrombus formation.

RESULTS

: In all animals, thrombi and microthrombi were confirmed within the left atrial appendage. The average wet weight of the thrombus was 1.4 +/- 0.2 g. Tc-99m apcitide detected left atrial thrombus in all animals.

CONCLUSIONS

This study suggests that AcuTect may prove useful for detecting intracardiac thrombus in future clinical studies in man.

Biodistribution and dosimetry study of 123I-rh-annexin V in mice and humans

This study reports on the optimization of the labelling procedure of clinical grade 123I-rh-annexin V and on the investigation of the biodistribution and dosimetry of 123I-rh-annexin V, a tracer proposed for the study of apoptosis in mice and humans. Research grade 123I-rh-annexin V was prepared as described previously, whereas clinical grade 123I-rh-annexin V was prepared according to a modified IodoGen method. NMRI mice, 3-4 weeks of age, received research grade 123I-rh-annexin V (74.0+/-3.7 kBq/mouse) by intravenous (i.v.) injection and killed at preset time points. Afterwards, the collected organs, blood, urine and faeces were counted for radioactivity and determined as %ID/g tissue or %ID over time. Secondly, six volunteers with normal liver and kidney function underwent whole-body scans up to 21 h after i.v. injection of clinical grade 123I-rh-annexin V (345+/-38 MBq). Time-activity curves were generated for the organs of interest, e.g., thyroid, heart, liver, kidneys and whole body, by fitting the organ specific geometric mean counts, obtained from region of interest analysis of acquired images in humans. The MIRD formulation was applied to calculate the absorbed radiation doses for various organs. Clinical grade 123I-rh-annexin V was obtained in radiochemical yields of 87.0+/-6.5% and radiochemical purities >98%. In mice, research grade 123I-rh-annexin V accumulated primarily in liver, kidney, stomach and lung tissue, limiting its usefulness for imaging of ongoing apoptosis in the abdominal and thoracic region. Clearance was predominantly urinary. In humans, acquired images with the clinical grade radioligand showed low lung uptake, resulting in good imaging conditions for the thoracic region. On the other hand, delayed imaging of the abdominal region was impeded due to extensive bowel activity. The highest absorbed doses were received by the thyroid, the kidneys, the heart wall, the liver and bone surfaces. The average effective dose of 123I-rh-annexin V was estimated to be 0.02 mSv.MBq-1. The amount of 123I-rh-annexin V required for in vivo imaging, results in an acceptable effective dose to the patient.

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.

Biological imaging for the diagnosis of inflammatory conditions

Radiopharmaceuticals used for in vivo imaging of inflammatory conditions can be conveniently classified into six categories according to the different phases in which the inflammatory process develops. The trigger of an inflammatory process is a pathogenic insult (phase I) that causes activation of endothelial cells (phase II); there is then an increase of vascular permeability followed by tissue oedema (phase III). Phase IV is characterised by infiltration of polymorphonuclear cells, and a self-limiting regulatory process called apoptosis is observed (phase V). If the inflammatory process persists, late chronic inflammation takes place (phase VI). In some pathological conditions, such as organ-specific autoimmune diseases, chronic inflammation is present early in the disease. The aim of nuclear medicine in the field of inflammation/infection is to develop noninvasive tools for the in vivo detection of specific cells and tissues. This would allow early diagnosis of initial pathophysiological changes that are undetectable by clinical examination or by other diagnostic tools, and could also be used to evaluate the state of activity of the disease during therapy. These potential applications are of great interest in clinical practice. In this review, we describe the various approaches that have been developed in the last 25 years of experience. Recent advances in the diagnosis of inflammatory processes have led to the development of specific radiopharmaceuticals that are intended to allow specific stage-related diagnosis.

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.

18F FDG uptake in the large arteries: a correlation study with the atherogenic risk factors

It has been reported that there is a high correlation between fluorodeoxyglucose (FDG) uptake in the aorta and macrophage content of atherosclerotic lesions in an experimental rabbit model. We evaluated the frequency of FDG uptake in the large arteries in relation to the atherogenic risk factors. We also investigated whether FDG uptake of the large arteries is related to clinically known coronary artery disease. The presence of FDG uptake was assessed in the abdominal aorta (AA), iliac (IA), and proximal femoral arteries (FAs) in 156 patients. Medical history of the atherogenic risk factors (age, cigarette smoking, hypertension, diabetes, high cholesterol, and obesity) and coronary artery disease (CAD) was identified for each patient. The frequency of vascular FDG uptake was compared between the patients without risk factors (Group I, 23 patients) and those with at least 1 risk factor (Group II, 133 patients). The correlation of each risk factor and known CAD with arterial FDG uptake was also assessed in the 3 different arteries. There was a significant difference in the frequency of FDG uptake between the 2 groups for the FA (22% vs 70%) and IA (30% vs 54%), but not for the AA (35% vs 53%). Among all risk factors, age was the most significant and consistent factor correlating with FDG uptake in all 3 arteries. Hypercholesterolemia also correlated consistently with FDG uptake in all 3 arteries. The correlation between the remaining risk factors and arterial FDG uptake was rather artery specific than consistent throughout all 3 arteries. A higher frequency of FDG uptake in the FA was seen in patients with CAD compared with those without CAD. Not all risk factors correlated with FDG uptake in different arteries. Among the risk factors, age and hypercholesterolemia most consistently correlated with FDG uptake in the AA, and the IA and proximal FAs. The positive correlation of arterial FDG uptake with the atherogenic risk factors suggested a promising role for FDG-PET imaging in the diagnosis of atherosclerosis and follow-up after treatment intervention.

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.