Technetium-99m HYNIC-annexin V: a potential radiopharmaceutical for the in-vivo detection of apoptosis

Synthesis, biodistribution and effects of farnesyltransferase inhibitor therapy on tumour uptake in mice of 99mTc labelled epidermal growth factor

OBJECTIVE

The goal of this study was to develop a 99mTc labelled human epidermal growth factor (hEGF) for the in-vivo prediction of cancer cell response to farnesyltransferase inhibitor (FTI) therapy. This is based on the observation that internalization of EGF receptors is inhibited by FTIs.

METHODS

We describe the radiolabelling of 99mTc-hEGF using the hydrazinonicotinamide (HYNIC) linker. Binding characteristics of 99mTc-HYNIC-hEGF to the EGF receptor are explored using an in-vitro binding assay. Biodistribution data of the compound in mice and tumour uptake in LoVo tumour bearing athymic mice before and after farnesyltransferase inhibitor therapy are presented.

RESULTS

No colloid formation was observed. Binding parameters and LoVo tumour uptake of 99mTc-HYNIC-hEGF did not differ significantly from directly labelled 123I-hEGF values. However, the biodistribution data of the 99mTc-HYNIC-hEGF showed higher uptake in liver and intestines and decreased stomach uptake compared to its 123I analogue. Eight hours after farnesyltransferase inhibitor therapy with R115777, LoVo tumour uptake of 99mTc-HYNIC-hEGF decreased significantly, as shown using planar gamma scintigraphy (the ratio tumour vs. thigh dropped from 2.54+/-0.83 to 0.99+/-0.18). These data confirm the results obtained using 123I-hEGF.

CONCLUSION

These data suggest that 99mTc-HYNIC-hEGF is a promising and selective new radiotracer for in-vivo monitoring of the EGF receptor with SPECT. Moreover, 99mTc-HYNIC-hEGF is a possible tool for early therapy response prediction of farnesyltransferase inhibitors.

Synthesis and in vivo evaluation of [11C]PJ34, a potential radiotracer for imaging the role of PARP-1 in necrosis

Poly(ADP-ribose) polymerase-1 (PARP-1) is an abundant nuclear enzyme of eukaryotic cells that has been implicated in response to DNA injury. PARP-1 detects single-strand DNA breaks induced by a variety of genotoxic insults. A hyperactivation of PARP-1 is believed to play a critical role in tissues undergoing cellular death by necrosis. Therefore, a radiotracer that could image PARP-1 levels with PET could provide a useful tool in measuring necrosis in a variety of pathological conditions. The phenanthridinone derivative, 2-(dimethylamino)-N-(5,6-dihydro-6-oxophenanthridin-2-yl)acetamide (PJ34), has a high affinity for PARP-1 (IC(50) = 20 nM) and is a suitable lead compound for PET radiotracer development. The synthesis of [(11)C]PJ34 was accomplished by base-catalyzed reaction of the corresponding des-methyl precursor, N-(5,6-dihydro-6-oxophenanthridin-2-yl)-2-(methylamino)acetamide with [(11)C]methyl iodide in DMF. The radiolabeling yield was 60% and the specific activity was approximately 2000 mCi/micromol (decay corrected to E.O.B.). The total radiosynthesis time was approximately 50 min. Preliminary in vivo biodistribution studies in a rodent model of diabetes indicate that [(11)C]PJ34 displays a high uptake in tissues where PARP-1 is hyperactivated. These data indicate that [(11)C]PJ34 may be a useful radiotracer for imaging tissues undergoing cellular death via necrosis.

Vulnerable plaque: Definition, detection, treatment, and future implications

Atherosclerosis continues to account for significant morbidity and mortality in most of the world. The major proportion of atherosclerosis mortality is related to atherosclerotic coronary artery disease, yet there still is not an optimal method for making the diagnosis of vulnerable plaque in vivo. The search for such an undefined method, along with studies on amelioration of currently available technology, gains special significance when the association between the qualitative definition of lesions in an individual and cardiovascular risks are considered. We, therefore, start by defining the critical lesion of coronary atherosclerosis and review the advantages and potential for clinical use of various methods to detect the vulnerable plaque and comment on possible future implications in this field.

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.

Magneto/optical annexin V, a multimodal protein

Multimodal proteins, or proteins labeled with both fluorescent and magnetic reporter groups, can be used in a wide range of applications including FACS or fluorescence microscopy, MRI and or near-infrared based optical imaging, or to fractionate cells by magnetic cell sorting. A problem with multimodal proteins, however, is the need to maximize bioactivity, often achieved by minimizing the number of modification points of the protein, while attaching fluorescent and magnetic labels. Here we describe the synthesis of a magneto/optical form of annexin V, achieved by reacting the amino-CLIO nanoparticle with Cy5.5 and SPDP, to produce a fluorescent, sulfhydryl reactive nanoparticle. A single reactive sulfhydryl group was added to annexin V by reaction with SATA that preserved the protein's ability to bind apoptotic Jurkat T cells. Reacting SATAylated annexin V with an SPDP activated nanoparticle yielded Anx-CLIO-Cy5.5, a magneto/optical form of annexin V. The binding of Anx-CLIO-Cy5.5 was specific for apoptotic Jurkat T cells and had an EC(50) of 3.66 nM. This was comparable to the strength of the interaction of unmodified annexin V with apoptotic cells, measured as the displacement of FITC-annexin by annexin V (2.4 nM). Our conjugation strategy preserves the strength of the interaction between annexin V and apoptotic cells, while yielding a probe, Anx-CLIO-Cy5.5, that is readily detectable by standard MR imaging or NIRF optical methods.

Absence of cardiotoxicity of the experimental cytotoxic drug cyclopentenyl cytosine (CPEC) in rats

The experimental anticancer drug cyclopentenyl cytosine (CPEC) was associated with cardiotoxicity in a phase I study. The aim of the present study was twofold; first we investigated whether the observed effects could be reproduced in in-vitro and in-vivo rat models. Second, we intended to investigate the underlying mechanism of the possible cardiotoxicity of CPEC. Effects on frequency and contractility were studied on the isolated atria of 18 male Wistar rats. Atria were incubated with 0.1 mmol L(-1) (n = 6) or 1 mmol L(-1) (n = 6) CPEC for 1.5 h and compared with control atria (incubation with buffer solution, n = 6). The cardiac apoptosis-inducing potential was studied in-vivo on 66 rats by 99mTc-Annexin V scintigraphy, followed by postmortem determination of radioactivity in tissues, histological confirmation with the TUNEL assay (late-phase apoptosis), and immunohistochemical staining for cleaved caspase 3 and cytochrome C (early-phase apoptosis). Serum levels of the necrotic cardiomyopathy marker troponin T were also determined. No effect on heart frequency was found in the isolated atria after CPEC treatment. A trend towards a decrease of contraction force was observed. However, the differences were not statistically significant. 99mTc-Annexin V scintigraphy showed no increase in cardiac uptake ratio upon CPEC treatment in the in-vivo rat model, which was confirmed by determination of radioactivity in heart versus blood ratios. At each section a few individual isolated late apoptotic cells (< 5) could be identified by the TUNEL assay in the highest CPEC dose group (90 mg kg(-1)) but not in controls or in rats treated with 60 mg kg(-1) CPEC. Staining for the early apoptosis markers cleaved caspase 3 and cytochrome C did not reveal any significant differences between treated and control rats. Cardiac troponin T levels were not increased after CPEC treatment. CPEC does not affect heart frequency or contraction force in our cardiotoxicity models. Moreover, we did not find an indication of CPEC-induced apoptosis in heart tissue.

Nuclear medicine in the era of genomics and proteomics: lessons from annexin V

In the past decade, genomics and proteomics have begun to develop many new targets for potential diagnostic and therapeutic agents. Among the life sciences, nuclear medicine is also deeply involved in the field of clinical investigation. Experience with radiolabeled annexin V highlights the many steps required to translate a good basic-science concept into the clinical setting. This model also emphasizes the value of synergy between basic and medical specialties in developing and optimizing a clinically useful product initially derived from basic investigation.

Optimal modification of annexin V with fluorescent dyes

The many uses of chemically modified annexin Vs necessitate an understanding of the optimal degree of modification and modification sites of the protein. When reacted with the N-hydroxysuccinimide ester of Cy5.5, annexin V with one modification per mole of protein retained its affinity for phosphatidylserine of apoptotic cells, whereas modification with two dyes per mole of protein caused a complete loss of activity. A tryptic digest LC/MS method was used to identify the modification sites as either of two closely spaced lysine residues, in position 286 or 290. The crystal structure indicated the location of these lysines was distal to the phosphatidylserine binding sites on annexin V. These results can be used to develop active or inactive fluorescent control annexin V proteins and to suggest strategies for attaining higher levels of modification with retention of bioactivity.

Assessment of Therapeutic Tumor Response Using 99mTc-Ethylenedicysteine-Glucosamine

PURPOSE

The aim of this study was to evaluate 99mTc-ethylenedicysteine-glucosamine (EC-DG) for the assessment of tumor growth.

METHOD

To evaluate whether 99mTc-EC-DG is involved in cell nuclei activity, in vitro thymidine incorporation, and cell-cycle assays of EC-DG were conducted using lung and breast cancer cells. Biodistribution of 99mTc-EC-DG in lung tumor-bearing mice (0.5-4 hours, 1 Ci/mouse, i. v.) was used to estimate the radiation-absorbed dose. Autoradiograms of 99mTc-EC-DG and 18F-FDG were compared in nude mice bearing uterine sarcoma. Rabbits inoculated with VX-2 cells were imaged with 99mTc-EC-DG and 99mTc-EC. For therapeutic assessment studies, scintigraphic imaging studies with 99mTc-EC-DG in mammary tumor-bearing rats were conducted at various days after treatment with paclitaxel and cisplatin. The imaging findings were correlated immunohistochemical assays (mRNA expression, apoptosis, and cell-cycle changes in tumor), and flow cytometry analysis was performed.

RESULTS

In vitro cellular uptake assays indicated that cell nuclei activity could be assessed by 99mTc-EC-DG. Scintigraphy and autoradiograms in animal models demonstrated that the tumor could be clearly visualized by 99mTc-EC-DG. The efficacy of paclitaxel and cisplatin treatment in rodent models could be assessed using tumor/muscle ratios. Immunohistochemical staining indicated a reduced expression of bFGF and an increased apoptosis and cell-cycle changes after paclitaxel and cisplatin treatment.

CONCLUSION

99mTc-EC-DG is involved in cell nuclei activity and could assess the therapeutic tumor response.

In vivo imaging of radiation-induced apoptosis in follicular lymphoma patients

PURPOSE

To evaluate (99m)Tc-Annexin-V (TAV) scintigraphy in monitoring radiation-induced apoptotic cell death in follicular lymphoma (FL) patients.

PATIENTS AND METHODS

Eleven FL patients (7 female and 4 male; median age, 58 years; range, 42-80 years) with recurrent disease underwent TAV imaging before and 24 hours after the last fraction of the 2 x 2 Gy involved field radiotherapy regimen. Fine-needle aspiration cytology was performed on 5 consecutive days to determine the optimal time window for apoptosis detection and to confirm the apoptotic nature of the response. The TAV scintigraphy (total body studies and SPECT of the irradiated sites) was performed 4 hours after the administration of the radiopharmaceutical. Tumor uptake was scored in a semiquantitative manner as absent (-) weak (+/-), present (+), or intense (++) with corresponding categories for the cytologic slides. Response evaluation was performed after 1 week and 4 weeks both in terms of completeness and speed of remission.

RESULTS

Baseline TAV uptake was absent in 6 and weak in 5 patients. Sequential cytology indicated that the optimal time period for apoptosis assessment was between 24 and 48 hours after the last fraction of the 2 x 2 Gy regimen. Baseline cytology was concordant with baseline TAV in all patients. Apoptotic feature appearance (nuclear chromatin condensation, margination and apoptotic body formation) after low-dose irradiation matched the irradiation response in all patients. In all but 1 patient the posttreatment TAV uptake matched the posttreatment cytology. In these 10 patients the cytology and TAV results correlated with the type and onset of the clinical response.

CONCLUSION

Tumor (99m)Tc-Annexin-V uptake can be increased after 2 x 2 Gy involved field radiotherapy. This increase was concordant with the appearance of apoptotic morphology as determined by cytology, and correlated with the clinical outcome. Apoptotic cell death can be observed on Day 4 of this regimen and if so predicts a complete remission within 1 week.

Suppression of prostate carcinogenesis by dietary supplementation of celecoxib in transgenic adenocarcinoma of the mouse prostate model

Epidemiological studies and clinical observations suggest that nonsteroidal anti-inflammatory drugs and certain selective cyclooxygenase (COX)-2 inhibitors may reduce the relative risk of clinically evident prostate cancer. This prompted us to investigate the chemopreventive potential of celecoxib, a selective COX-2 inhibitor, against prostate carcinogenesis in a transgenic adenocarcinoma of the mouse prostate (TRAMP) model. Similar to prostate cancer in humans, prostate malignancies in TRAMP mice progress from precursor intraepithelial lesions, to invasive carcinoma that metastasizes to lymph nodes, liver, lungs, and occasionally to bone. The basal enzyme activity and protein expression of COX-2 is significantly higher (>4-fold) in the dorsolateral prostate of TRAMP mice up to 24 weeks of age compared with their nontransgenic littermates. Eight-week-old TRAMP mice were randomly divided and fed either control diet (AIN 76A) or a custom prepared AIN 76A diet containing 1500-ppm celecoxib ad libitum for 24 weeks, a dosage that would compare with the normal recommended dose for the treatment of human disease. Studies from two independent experiments, each consisting of 10 mice on test, showed that the cumulative incidence of prostate cancer development at 32 weeks of age in animals fed with AIN 76A diet was 100% (20 of 20) as observed by tumor palpation, whereas 65% (13 of 20), 35% (7 of 20), and 20% (4 of 20) of the animals exhibited distant site metastases to lymph nodes, lungs, and liver. Celecoxib supplementation to TRAMP mice from 8-32 weeks of age exhibited significant reduction in tumor development (5 of 20) with no signs of metastasis. Celecoxib feeding resulted in a significant decrease in prostate (56%; P < 0.0003) and genitourinary weight (48%; P < 0.008). Sequential magnetic resonance imaging analysis of celecoxib-fed mice documented lower prostate volume compared with the AIN 76A-fed group. Histopathological examination of celecoxib-fed animals showed reduced proliferation, and down-modulation of COX-2 and prostaglandin E2 levels in the dorsolateral prostate and plasma, respectively. These results correlated with retention of antimetastasis markers, viz E-cadherin, and alpha- and beta-catenin, along with a significant decrease in vascular endothelial growth factor protein expression. Celecoxib supplementation also resulted in enhanced in vivo apoptosis in the prostate as monitored by several techniques including a recently perfected technique of 99mTc-labeled annexin V in live animals followed by phosphor imaging. One striking observation in an additional study was that celecoxib feeding to mice with established tumors (16 weeks of age) significantly improved their overall survival (P = 0.014), compared with AIN 76A-fed group. Our findings suggest that celecoxib may be useful in chemoprevention of prostate cancer.

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.

The imaging of apoptosis with the radiolabeled annexin V: optimal timing for clinical feasibility

In recent years, the imaging of drug-induced apoptosis has become one of the centers of interest in experimental and clinical research. In particular, the accurate monitoring of chemosensitivity as well as the early prediction of chemoresistance in response to various pro-apoptotic interventions are critical requirements for the best management of oncology patients. The use of technetium [(99m)Tc]-labeled annexin V on animal and human models of cancers provides a proof of principle for the feasibility of a non-invasive, in vivo detection of an apoptotic signal and then for the early assessment of tumor response in the course of chemotherapy. Although promising, however, the initial clinical data point out on the technical limitations that are still to be resolved in terms of tumor-to-background ratio and optimal timing for the imaging of apoptosis. In the present review article, we report the results of animal studies aimed to the evaluation of apoptotic peaks following chemotherapy. In the light of these basic research works, we analyze the profiles of radiolabeled annexin V uptake over time as observed in clinical trials. We then discuss possible new imaging strategies designed to optimize the visualization of apoptotic changes within tumor tissues using the [(99m)Tc]-labeled annexin V. We also suggest longer lived forms of radiolabeled annexin V designed to better understand the temporal patterns of apoptotic tumor response, which in turn, may help to capture the best time-window for the imaging of cell death.

Optimization of the preparation of 99mTc-labeled Hynic-derivatized Annexin V for human use

Hydrazino nicotinate (Hynic) is one of the most attractive bifunctional agents designed for the labeling of proteins with (99m)Tc. Recently, a (99m)Tc-labeled Hynic-Annexin V derivative has been described and successfully evaluated in animal models of apoptosis. Prior to a phase I human study, the preparation of (99m)Tc-Hynic-Annexin V has been optimized. The influence of the Hynic-load of Annexin V, amount of protein, nature and amount of reducing agent, activity and co-ligand on the labeling yield were evaluated using ITLC and size-exclusion FPLC. Optimal labeling yields were obtained when 60-90 microgram Hynic-Annexin V was labeled with up to 1.11 GBq (30 mCi) (99m)TcO(4)-using 10-20 microgram SnCl(2).2H(2)O as reducing agent and 1.5 mg tricine as the co-ligand. Biodistribution in normal mice was comparable to literature data.

Intravascular modalities for detection of vulnerable plaque: current status

Progress in the diagnosis, treatment, and prevention of atherosclerotic coronary artery disease is dependent on a greater understanding of the mechanisms of coronary plaque progression. Autopsy studies have characterized a subgroup of high-risk, or vulnerable, plaques that result in acute coronary syndromes or sudden cardiac death. These angiographically modest plaques share certain pathologic characteristics: a thin, fibrous cap, lipid-rich core, and macrophage activity. Diagnostic techniques for vulnerable-plaque detection, including serologic markers and noninvasive and invasive techniques, are needed. Recent advances in intravascular imaging have significantly improved the ability to detect high-risk, or vulnerable, plaque in vivo by using various features of plaque vulnerability as methods of identification. The characteristic anatomy of a thin, fibrous cap overlying a lipid pool has promoted high-resolution imaging, such as intravascular ultrasound, optical coherence tomography, and intracoronary magnetic resonance. The lipid-rich core is identifiable by angioscopically detected color changes on the plaque surface or by its unique absorption of energy, or "Raman shift," of its cholesterol core, driving coronary spectroscopy. Finally, temperature heterogeneity arising at foci of plaque inflammation has prompted the development of intracoronary thermography. In this review, we will discuss these techniques, their relative advantages and limitations, and their potential clinical application.

Optical imaging of apoptosis as a biomarker of tumor response to chemotherapy

A rapid and accurate assessment of the antitumor efficacy of new therapeutic drugs could speed up drug discovery and improve clinical decision making. Based on the hypothesis that most effective antitumor agents induce apoptosis, we developed a near-infrared fluorescent (NIRF) annexin V to be used for optical sensing of tumor environments. To demonstrate probe specificity, we developed both an active (i.e., apoptosis-recognizing) and an inactive form of annexin V with very similar properties (to account for nonspecific tumor accumulation), and tested the agents in nude mice each bearing a cyclophosphamide (CPA) chemosensitive (LLC) and a chemoresistant LLC (CR-LLC). After injection with active annexin V, the tumor-annexin V ratio (TAR; tumor NIRF/background NIRF) for untreated mice was 1.22+/-0.34 for LLC and 1.43+/-0.53 for CR-LLC (n=4). The LLC of CPA-treated mice had significant elevations of TAR (2.56+/-0.29, P=.001, n=4), but only a moderate increase was obtained for the CR-LLC (TAR=1.89+/-0.19, P=.183). The in vivo measurements correlated well with terminal deoxyribosyl transferase-mediated dUTP nick end labeling indexes. When inactive Cy-annexin V was used, with or without CPA treatment and in both CCL and CR-CCL tumors, tumor NIRF values ranged from 0.91 to 1.17 (i.e., tumor were equal to background). We conclude that active Cy-annexin V and surface reflectance fluorescence imaging provide a nonradioactive, semiquantitative method of determining chemosensitivity in LLC xenografts. The method maybe used to image pharmacologic responses in other animal models and, potentially, may permit the clinical imaging of apoptosis with noninvasive or minimally invasive instrumentation.

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.

In vivo monitoring of apoptosis

The biochemical and physiological processes involved in apoptosis were described from the perspective of detection by standard, clinical, noninvasive imaging modalities. The difficulties of monitoring apoptosis in vivo were discussed. Magnetic resonance imaging (MRI) approaches used to study apoptosis were surveyed. The cell shrinkage associated with apoptosis can be detected due to changes in tissue water T(2) and T(1)rho relaxation times and apparent diffusion coefficient (ADC). Magnetic resonance spectroscopy (MRS) approaches used to study apoptosis in vivo have largely centered on the formation of cytoplasmic lipid bodies, detected by 1H MRS, and metabolic/bioenergetic changes detected by 31P and 13C MRS. The most successful approach to in vivo mapping of apoptosis uses the high specific binding of annexin V or synaptotagmin I to phosphatidylserine (PS) that appears on the extracellular plasma membrane of cells during apoptosis. Technetium-99m (99mTc)-radiolabeling of the annexin V and superparamagnetic iron oxide (SPIO) labeling of the C2 domain of synaptotagmin I allow good in vivo apoptosis detection by gamma camera imaging and MRI, respectively.

Imaging with 99mTc ECDG targeted at the multifunctional glucose transport system: feasibility study with rodents

PURPOSE

To evaluate the feasibility of technetium 99m ((99m)Tc) ethylenedicysteine-deoxyglucose (ECDG) imaging in tumor-bearing rodents.

MATERIALS AND METHODS

ECDG was synthesized by means of reacting ethylenedicysteine with glucosamine, with carbodiimide as the coupling agent. Hexokinase assays were performed at an ultraviolet wavelength of 340 nm. To determine whether blood glucose level could be altered, ECDG or glucosamine was injected into six rats. In a separate study, ECDG followed by insulin was administered to three rats. To determine biodistribution, lung tumor cells were intramuscularly injected into the hind legs of 18 nude mice. The animals were then injected with (99m)Tc ECDG or fluorine 18 ((18)F) fluorodeoxyglucose (FDG) (0.037-0.074 MBq per mouse). Radioactivity was measured in tissue excised from the animals. Scintigraphy was performed in three groups: in group 1 to demonstrate that different-sized tumors could be imaged after (99m)Tc ECDG administration, in group 2 to ascertain whether tumor uptake of (99m)Tc ECDG was perfusion related, and in group 3 to demonstrate that tumor uptake of (99m)Tc ECDG occurred by means of a glucose-mediated process.

RESULTS

ECDG was positive for phosphorylation at hexokinase assay. Blood glucose level increased with ECDG injection and decreased with insulin administration. Tumor-to-brain tissue and tumor-to-muscle tissue ratios of (99m)Tc ECDG uptake were higher than those of (18)F FDG uptake. Scintigraphic results demonstrated the feasibility of (99m)Tc ECDG imaging.

CONCLUSION

There are similarities between (99m)Tc ECDG uptake and (18)F FDG uptake in tumors, and study findings supported the potential use of (99m)Tc ECDG as a functional imaging agent.

Vascular-targeted molecular imaging using functionalized polymerized vesicles

In this review we will discuss the use of multivalent polymerized vesicles (PVs) combined with magnetic resonance imaging (MRI) and gamma scintigraphy to image expression of vascular molecular receptors in vivo. Specifically, we will present our data on the use of this technology to design imaging agents toward specific vascular receptors in both a mouse and rabbit tumor model and in the EAE mouse, a model for human multiple sclerosis (MS). Examples will be shown where the in vivo specificity of the targeted molecular imaging agents was validated in the animal models. Since the PVs are designed to carry either contrast or therapeutic agents or both, we can potentially use vascular-targeted imaging for selecting patients and guiding vascular-targeted therapies in these patients. Using this combined vascular-targeted imaging and therapy approach, personalized treatment can potentially be delivered, maximizing efficacy and minimizing side effects.

Developments in whole-body molecular imaging of live subjects

New molecular imaging technologies are being developed specifically for imaging animal models of human disease. Positron emission tomography (PET) in particular allows in vivo biochemistry to be studied with a high degree of sensitivity and specificity, and provides direct in vivo information on molecular and cellular pathways that underlie disease mechanisms and therapeutics. However, clinical PET systems have inadequate resolution for imaging small animals. Thus, specialized high-resolution PET hardware and software are now being developed.

In vivo imaging of acute cardiac rejection in human patients using (99m)technetium labeled annexin V

Annexin V binds phosphatidylserine moieties on apoptotic cells. This study reports the initial experience at Stanford University Medical Center with 99mTc-labeled annexin V imaging as a noninvasive measure of apoptosis in acute cardiac rejection. Ten cardiac transplant patients had 99mTc Annexin V imaging and endomyocardial biopsy (EMB) performed within 24 h. No complications related to 99mTc annexin V administration occurred. Eight patients had ISHLT grade of acute rejection of 1A or less. Five patients had two or more areas of uptake noted in the right ventricle on imaging studies. Two of these patients had positive biopsies: one patient had grade 2 rejection with two focal uptake areas and another had grade 3A rejection with three foci. An additional five patients had either one or zero hot spot areas and corresponding negative EMBs. 99mTc-annexin V appears to be well tolerated and may identify patients with acute cardiac rejection.

A review of the causes of central pontine myelinosis: yet another apoptotic illness?

One of the well recognized stimuli for central pontine myelinosis (CPM) is the rapid correction of chronic hyponatraemia. Conventionally this has been perceived to lead to pontine glial cell swelling through osmosis and eventually to cell death. However, although a purely osmotic argument has been central to any patho-physiological understanding of CPM, there are deficiencies in this approach that do not account for why certain individuals develop CPM with relatively mild osmotic insults. Here we review the varying aetiologies of CPM and propose a novel hypothesis for CPM causation by suggesting that individuals predisposed to CPM have inadequate energy provision as well as other factors that result in a pro-apoptotic drive, which renders them susceptible to brain injury from diverse causes. In CPM, the precipitant of brain injury appears to be osmotic stress. Furthermore, this model suggests a number of therapeutic interventions that may prevent or at least mitigate the consequences of CPM.

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.

99mTc annexin V imaging of neonatal hypoxic brain injury

BACKGROUND AND PURPOSE

Delayed cell loss in neonates after cerebral hypoxic-ischemic injury (HII) is believed to be a major cause of cerebral palsy. In this study, we used radiolabeled annexin V, a marker of delayed cell loss (apoptosis), to image neonatal rabbits suffering from HII.

METHODS

Twenty-two neonatal New Zealand White rabbits had ligation of the right common carotid artery with reduction of inspired oxygen concentration to induce HII. Experimental animals (n=17) were exposed to hypoxia until an ipsilateral hemispheric decrease in the average diffusion coefficient occurred. After reversal of hypoxia and normalization of average diffusion coefficient values, experimental animals were injected with (99m)Tc annexin V. Radionuclide images were recorded 2 hours later.

RESULTS

Experimental animals showed no MR evidence of blood-brain barrier breakdown or perfusion abnormalities after hypoxia. Annexin images demonstrated multifocal brain uptake in both hemispheres of experimental but not control animals. Histology of the brains from experimental animals demonstrated scattered pyknotic cortical and hippocampal neurons with cytoplasmic vacuolization of glial cells without evidence of apoptotic nuclei by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining. Double staining with markers of cell type and exogenous annexin V revealed that annexin V was localized in the cytoplasm of scattered neurons and astrocytes in experimental and, less commonly, control brains in the presence of an intact blood-brain barrier.

CONCLUSIONS

Apoptosis may develop after HII even in brains that appear normal on diffusion-weighted and perfusion MR. These data suggest a role of radiolabeled annexin V screening of neonates at risk for the development of cerebral palsy.