Apoptosis: the importance of nuclear medicine

The Influence of Proteins on Fate and Biological Role of Circulating DNA

Circulating DNA has already proven itself as a valuable tool in translational medicine. However, one of the overlooked areas of circulating DNA research is its association with different proteins, despite considerable evidence that this association might impact DNA’s fate in circulation and its biological role. In this review, we attempt to shed light on current ideas about circulating DNA origins and forms of circulation, known biological effects, and the clinical potential of circulating tumor deoxyribonucleoprotein complexes.

Long-circulating XTEN864-annexin A5 fusion protein for phosphatidylserine-related therapeutic applications

Annexin A5 (anxA5) is a marker for apoptosis, but has also therapeutic potential in cardiovascular diseases, cancer, and, due to apoptotic mimicry, against dangerous viruses, which is limited by the short blood circulation. An 864-amino-acid XTEN polypeptide was fused to anxA5. XTEN864-anxA5 was expressed in Escherichia coli and purified using XTEN as tag. XTEN864-anxA5 was coupled with DTPA and indium-111. After intravenous or subcutaneous injection of ¹¹¹In-XTEN864-anxA5, mouse blood samples were collected for blood half-life determination and organ samples for biodistribution using a gamma counter. XTEN864-anxA5 was labeled with 6S-IDCC to confirm binding to apoptotic cells using flow cytometry. To demonstrate targeting of atherosclerotic plaques, XTEN864-anxA5 was labeled with MeCAT(Ho) and administered intravenously to atherosclerotic ApoE^−/− mice. MeCAT(Ho)-XTEN864-anxA5 was detected together with MeCAT(Tm)-MAC-2 macrophage antibodies by imaging mass cytometry (CyTOF) of aortic root sections. The ability of anxA5 to bind apoptotic cells was not affected by XTEN864. The blood half-life of XTEN864-anxA5 was 13 h in mice after IV injection, markedly longer than the 7-min half-life of anxA5. 96 h after injection, highest amounts of XTEN864-anxA5 were found in liver, spleen, and kidney. XTEN864-anxA5 was found to target the adventitia adjacent to atherosclerotic plaques. XTEN864-anxA5 is a long-circulating fusion protein that can be efficiently produced in E. coli and potentially circulates in humans for several days, making it a promising therapeutic drug.

Design, Synthesis, Radiolabeling, and Biologic Evaluation of Three 18F-FDG-Radiolabeled Targeting Peptides for the Imaging of Apoptosis

Background: Early detection of apoptosis is very important for therapy and follow-up treatment in various pathologic conditions. Annexin V interacts strongly and specifically with phosphatidylserine, specific biomarkers of apoptosis with some limitations. Small peptides are suitable alternatives to annexin V. A reliable and noninvasive in vivo technique for the detection of apoptosis is in great demand. Based on our previous studies, three new peptide analogs of LIKKPF (Leu-Ile-Lys-Lys-Pro-Phe) as apoptosis imaging agents were developed. Materials and Methods: Aoa-LIKKP-Cl-F, Aoe-LIKKP-Pyr-F, and Aoe-LIKKP-Nap-F were synthesized, functionalized with aminooxy, and radiolabeled with ¹⁸F-FDG. Their biologic properties were evaluated in vitro using apoptotic Jurkat cells. ¹⁸F-FDG-Aoe-LIKKP-Pyr-F peptide was injected into normal and apoptotic mice models for biodistribution and in vivo positron emission tomography/computed tomography imaging studies. Results: ¹⁸F-FDG-Aoe-LIKKP-Pyr-F peptide showed higher affinity for apoptotic cells. The localization of peptide in apoptotic liver mice was confirmed in biodistribution and imaging studies. Conclusion: The results showed that Aoe-LIKKP-Pyr-F peptide is an auspicious agent for molecular imaging of apoptosis.

Real-time imaging of retinal cell apoptosis by confocal scanning laser ophthalmoscopy

Retinal cell apoptosis occurs in many eye conditions, including glaucoma, diabetic retinopathy and Alzheimer's disease. Real-time detection of retinal cell apoptosis has potential clinical value in early disease detection, as well as evaluating disease progression and treatment efficacy. Here, we describe our novel imaging technology DARC (Detection of Apoptosing Retinal Cells), which can be used to visualize single retinal neurons undergoing apoptosis in real time, by using fluorescently labeled Annexin A5 and confocal scanning laser ophthalmoscopy (cSLO ). Clinical trials of DARC in glaucoma patients are due to start shortly, but in this chapter, we describe this technique in experimental animal models.

Generation of blood circulating DNA: the sources, peculiarities of circulation and structure

Extracellular nucleic acids (exNA) were described in blood of both healthy and illness people as early as in 1948, but staied overlooked until middle 60-th. Starting from the beginning of new millennium and mainly in the last 5 years exNA are intensively studied. Main attention is directed to investigation of exNA as the source of diagnostic material whereas the mechanisms of their generation, as well as mechanisms to providing long-term circulation of exNA in the bloodstream are not established unambiguously. According to some authors, the main source of circulating nucleic acids in blood are the processes of apoptosis and necrosis, while others refer to the possible nucleic acid secretion by healthy and tumor cells. Circulating DNA were found to be stable in the blood for a long time, escaping from the action of DNA hydrolyzing enzymes and are apparently packed in different supramolecular complexes. This review presents the opinions of various authors and evidence in favor of all the theories describingappearance of extracellular DNA, the features of the circulation and structure of the extracellular DNA and factors affecting the time of DNA circulation in blood

An in vivo molecular imaging probe (18)F-Annexin B1 for apoptosis detection by PET/CT: preparation and preliminary evaluation

There is an increasing need to develop non-invasive molecular imaging strategies for visualizing and quantifying apoptosis status of diseases (especially for cancer) for diagnosis and monitoring treatment response. Since externalization of phosphatidylserine (PS) is one of the early molecular events during apoptosis, Annexin B1 (AnxB1), a member of Annexins family with high affinity toward the head group of PS, could be a potential positron emission tomography (PET) imaging probe for imaging cell death process after labeled by positron-emitting nuclides, such as (18)F. In the present study, we investigated a novel PET probe, (18)F-labeled Annexin B1 ((18)F-AnxB1), for apoptosis imaging. (18)F-AnxB1 was prepared reliably by conjugating AnxB1 with a (18)F-tag, N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB), in a radiolabeling yield of about 20 % within 40 min. The in vitro binding of (18)F-AnxB1 with apoptotic cells induced by anti-Fas antibody showed twofold increase compared to those without treatment, confirmed by flow cytometric analysis with AnxV-FITC/PI staining. Stability tests demonstrated (18)F-AnxB1 was rather stable in vitro and in vivo without degradation. The serial (18)F-AnxB1 PET/CT scans in healthy rats outlined its biodistribution and pharmacokinetics, indicating a rapid renal clearance and predominant accumulation into kidney and bladder at 2 h p.i. (18)F-AnxB1 PET/CT imaging was successfully applied to visualize in vivo apoptosis sites in tumor induced by chemotherapy and in kidney simulated by ischemia-reperfusion injury. The high-contrast images were obtained at 2 h p.i. to delineate apoptotic tumor. Apoptotic region could be still identified by (18)F-AnxB1 PET 4 h p.i., despite the high probe retention in kidneys. In summary, we have developed (18)F-AnxB1 as a PS-specific PET probe for the apoptosis detection and quantification which could have broad applications from disease diagnosis to treatment monitoring, especially in the cases of cancer.

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.

99mTc-HYNIC-Annexin A5 in Oncology: Evaluating Efficacy of Anti-Cancer Therapies

Evaluation of efficacy of anti-cancer therapy is currently performed by anatomical imaging (e.g., MRI, CT). Structural changes, if present, become apparent 1-2 months after start of therapy. Cancer patients thus bear the risk to receive an ineffective treatment, whilst clinical trials take a long time to prove therapy response. Both patient and pharmaceutical industry could therefore profit from an early assessment of efficacy of therapy. Diagnostic methods providing information on a functional level, rather than a structural, could present the solution. Recent technological advances in molecular imaging enable in vivo imaging of biological processes. Since most anti-cancer therapies combat tumors by inducing apoptosis, imaging of apoptosis could offer an early assessment of efficacy of therapy. This review focuses on principles of and clinical experience with molecular imaging of apoptosis using Annexin A5, a widely accepted marker for apoptosis detection in vitro and in vivo in animal models. 99mTc-HYNIC-Annexin A5 in combination with SPECT has been probed in clinical studies to assess efficacy of chemo- and radiotherapy within 1-4 days after start of therapy. Annexin A5-based functional imaging of apoptosis shows promise to offer a personalized medicine approach, now primarily used in genome-based medicine, applicable to all cancer patients.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

A universally applicable 68Ga-labeling technique for proteins

Although protein-based PET imaging agents are projected to become important tracer molecules in the future, the labeling of complex biomolecules with PET radionuclides is inexpedient and, most of the time, challenging.

METHODS

Here we present a straightforward labeling chemistry to attach the versatile radionuclide (68)Ga to proteins. Introducing the (68)Ga chelating agent NODA-GA-T (2,2'-(7-(1-carboxy-4-(2-mercaptoethylamino)-4-oxobutyl)-1,4,7-triazonane-1,4-diyl)diacetic acid) by reaction with proteins chemically processed with sulfo-SMCC (4-(N-maleimidomethyl)cyclohexane-1-carboxylic acid 3-sulfo-N-hydroxysuccinimide ester sodium salt) results in labeling precursors, enabling a simple and rapid kit-labeling procedure that requires no workup of the radiolabeled proteins. Various (68)Ga- proteins were labeled using this method, and the radiochemical yields and specific activities of the labeled proteins were determined. To show that the radiotracers are applicable for in vivo studies, proof-of-concept small-animal PET images were acquired in healthy rats using (68)Ga rat serum albumin for blood-pool imaging and (68)Ga-annexin V for apoptosis imaging in mice with a left ventricular myocardial infarction.

RESULTS

The proteins could be modified, yielding 1.2-1.7 (68)Ga-labeling sites per protein molecule. All investigated proteins could be labeled in high radiochemical yields of 95% or more in less than 10 min in 1 step, using acetate-buffered medium (pH 3.5-4.0) at room temperature without any further purification. The labeled proteins displayed specific activities of 20-45 GBq/μmol (540-1,200 Ci/mmol). In the proof-of-concept in vivo studies, (68)Ga rat serum albumin and (68)Ga-annexin V were successfully used for in vivo imaging. Both radiotracers showed a favorable biodistribution in the animal models, thus demonstrating the usefulness of the developed approach for the kit (68)Ga labeling of proteins.

CONCLUSION

The preprocessing of proteins proceeds in high chemical yields and with high protein recovery rates after purification. These precursors can be stored for several months at -20°C without degradation, and (68)Ga labeling can be performed in a 1-step kit-labeling reaction in high radiochemical yields. Two of the derivatized model proteins were successfully used in proof-of-concept in vivo imaging studies to prove the applicability of this kit (68)Ga-labeling technique.

Radiolabeled annexin V imaging: a useful technique for determining apoptosis in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) that involves myelin, oligodendrocytes and axons and culminates in consecutive neuronal death and progressive neurologic disability. Based on magnetic resonance imaging (MRI), neuroaxonal loss in MS results in brain atrophy and has a strong correlation with neurological disability. The newer MR imaging tools seem to be sensitive biomarkers for measuring the pathogenetic processes associated with disease activity and progression. However, they are unable to detect apoptosis in neurodegenerative diseases. Annexin V has a high affinity for phosphatidylserine (PS) that presents on the outer surface of the plasma membrane early on during the onset of apoptosis. Radiolabeled annexin V imaging may reveal the initiation and degree of neuronal apoptosis. We propose that radiolabeled annexin V imaging is a useful modality in determining apoptosis in MS and can assess and monitor the effectiveness of neuroprotective and immunomodulatory therapies on the clinical course of MS.

Peptide-based imaging agents targeting phosphatidylserine for the detection of apoptosis

A 14-residue phosphatidylserine (PS)-binding peptide FNFRLKAGQKIRFG (PSBP-0) was scanned with Ala. In addition, a radiometal chelator (SAAC) was introduced at selected sites of the lead peptides. Substitution of the Gln(6) residue in PSBP-0 with Ala resulted in a significant increase in binding affinity to PS as determined by surface plasmon resonance sensorgrams. The binding affinity of the resulting peptide FNFRLKAGAKIRFG (PSBP-6, molecular mass = 1623 Da) to PS (K(d) ∼ 100 nM) increased 10-fold as compared to PSBP-0 (K(d) ∼ 1.38 μM). Introduction of SAAC-Re to the N terminus of PSBP-6 further increased the binding affinity of the resulting peptide SAAC(Re)-PSBP-6 (K(d) ∼ 26 nM). SAAC(Re)-PSBP-6 shows specific binding to apoptotic cells in cell-based assays. Biodistribution studies showed significantly higher uptake of SAAC((99 m)Tc)-PSBP-6 in B16/F10 melanoma treated with poly(L-glutamic acid)-paclitaxel than untreated tumors (4.06 ± 0.55% ID/g vs 1.61 ± 0.33% ID/g, P = 0.00011). SAAC((99 m)Tc)-PSBP-6 is a promising probe for noninvasive imaging of apoptotic cells.

PET imaging of apoptosis with 64Cu-labeled streptavidin following pretargeting of phosphatidylserine with biotinylated annexin-V

PURPOSE

In vivo detection of apoptosis is a diagnostic tool with potential clinical applications in cardiology and oncology. Radiolabeled annexin-V (anxV) is an ideal probe for in vivo apoptosis detection owing to its strong affinity for phosphatidylserine (PS), the molecular flag on the surface of apoptotic cells. Most clinical studies performed to visualize apoptosis have used (99m)Tc-anxV; however, its poor distribution profile often compromises image quality. In this study, tumor apoptosis after therapy was visualized by positron emission tomography (PET) using (64)Cu-labeled streptavidin (SAv), following pre-targeting of apoptotic cells with biotinylated anxV.

METHODS

Apoptosis was induced in tumor-bearing mice by photodynamic therapy (PDT) using phthalocyanine dyes as photosensitizers, and red light. After PDT, mice were injected i.v. with biotinylated anxV, followed 2 h later by an avidin chase, and after another 2 h with (64)Cu-DOTA-biotin-SAv. PET images were subsequently recorded up to 13 h after PDT.

RESULTS

PET images delineated apoptosis in treated tumors as early as 30 min after (64)Cu-DOTA-biotin-SAv administration, with tumor-to-background ratios reaching a maximum at 3 h post-injection, i.e., 7 h post-PDT. Omitting the administration of biotinylated anxV or the avidin chase failed to provide a clear PET image, confirming that all three steps are essential for adequate visualization of apoptosis. Furthermore, differences in action mechanisms between photosensitizers that target tumor cells directly or via initial vascular stasis were clearly recognized through differences in tracer uptake patterns detecting early or delayed apoptosis.

CONCLUSION

This study demonstrates the efficacy of a three-step (64)Cu pretargeting procedure for PET imaging of apoptosis. Our data also confirm the usefulness of small animal PET to evaluate cancer treatment protocols.

Anatomical and Functional Imaging of Tumors in Animal Models: Focus on Pheochromocytoma

This review focuses on anatomical (computed tomography, magnetic resonance imaging) and functional (positron emission tomography) imaging methods for tumor localization and identification of experimentally induced tumors in animal models, especially pheochromocytoma. Although anatomical imaging can precisely locate primary and metastatic tumors, functional imaging has high specificity for some tumors, especially those of endocrine origin. This is due to the fact that endocrine tumor cells take up hormone precursors, express hormone receptors and transporters, and synthesize, store, and release hormones. These characteristic properties of endocrine tumors enable investigators to create highly specific radiopharmaceuticals, particularly for positron emission tomography. For example, localization of pheochromocytoma involves [18F]-6F-dopamine. It is a highly specific radiopharmaceutical since it uses the norepinephrine transporter system expressed in most pheochromocytoma cells. Here we review both anatomical and functional imaging methods that are used conjointly in order to localize and identify specific characteristics of tumors.

Positron Emission Tomography Imaging of Small Animals in Anticancer Drug Development

Positron emission tomography (PET) imaging of small animals enables researchers to bridge the gap between in vitro science and in vivo human studies. The imaging paradigm can be established and refined in animals before implementation in humans and image data related to ex vivo assays of biological activity. Small animal PET (saPET) imaging enables assessment of baseline focal pathophysiology, pharmacokinetics, biological target modulation and the efficacy of novel drugs. The potential and challenge of this technology as applied to anticancer drug development is discussed here.

Functional imaging of endocrine tumors: role of positron emission tomography

This article provides an update on functional imaging approaches for diagnostic localization of endocrine tumors, with emphasis on positron emission tomography (PET). [18F]Fluorodeoxyglucose PET scanning is now a widely accepted imaging approach in clinical oncology. Benefits include improved patient outcome facilitated by staging and monitoring of disease and better treatment planning. [18F]Fluorodeoxyglucose PET is also useful in some endocrine tumors, particularly in recurrent or metastatic thyroid cancer where the degree of accumulation of the radionuclide has prognostic value. However, this imaging approach does not take full advantage of the unique characteristics of endocrine tumors. Endocrine tumor cells take up hormone precursors, express receptors and transporters, and synthesize, store, and release hormones. These characteristics offer highly specific targets for PET. Radiopharmaceuticals developed for such approaches include 6-[18F]fluorodopamine, and [11C]hydroxyephedrine for localization of pheochromocytomas, [11C]5-hydroxytryptophan and [11C]L-dihydroxyphenylalanine for carcinoid tumors, and [11C]metomidate for adrenocortical tumors. These functional imaging approaches are not meant to supplant conventional imaging modalities but should be used conjointly to better identify specific characteristics of endocrine tumors. This represents a relatively new and evolving approach to imaging that promises to answer specific questions about the behavior and growth of endocrine tumors, their malignant potential, and responsiveness to different treatment modalities.

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.

Multi-modality Imaging Identifies Key Times for Annexin V Imaging as an Early Predictor of Therapeutic Outcome

Radiolabeled annexin V may provide an early indication of the success or failure of anticancer therapy on a patient-by-patient basis as an in vivo marker of tumor cell killing. An important question that remains is when, after initiation of treatment, should annexin V imaging be performed. To address this issue, we obtained simultaneous in vivo measurements of tumor burden and uptake of radiolabeled annexin V in the syngeneic orthotopic murine BCL1 lymphoma model using in vivo bioluminescence imaging (BLI) and small animal single-photon emission computed tomography (SPECT). BCL1 cells labeled for fluorescence and bioluminescence assays (BCL1-gfp/luc) were injected into mice at a dose that leads to progressive disease within two to three weeks. Tumor response was followed by BLI and SPECT before and after treatment with a single dose of 10 mg/kg doxorubicin. Biodistribution analyses revealed a biphasic increase of annexin V uptake within the tumor-bearing tissues of mice. An early peak occurring before actual tumor cells loss was observed between 1 and 5 hr after treatment, and a second longer sustained rise from 9 to 24 hr after therapy, which heralds the onset of tumor cell loss as confirmed by BLI. Multimodality imaging revealed the temporal patterns of tumor cell loss and annexin V uptake revealing a better understanding of the timing of radiolabeled annexin V uptake for its development as a marker of therapeutic efficacy.

Improved Radiolabeling of PEGylated Protein: PEGylated Annexin V for Noninvasive Imaging of Tumor Apoptosis

We have developed a one-step procedure to introduce both polyethylene glycol (PEG) and the metal chelator diethylenetriaminepentaacetic acid (DTPA) to proteins through a heterofunctional PEG precursor. The PEG precursor contains DTPA at one end and an amine-reactive isothiocyanate (SCN-) functional group at the other end. It was obtained as lyophilized powder and could be stored at 4 degrees C for several months. Protein conjugation was achieved by simply mixing the proteins and the PEG precursor SCN-PEG-DTPA in an aqueous solution. As exemplified by the PEGylation and radiolabeling of annexin V, the resulting conjugate 111In-DTPA-PEG-annexin V showed selective binding to apoptotic cells in vitro and increased blood half-life in vivo. The PEGylated, radiolabeled annexin V may be useful in the noninvasive imaging of apoptosis.

Iodine-124 labelled annexin-V as a potential radiotracer to study apoptosis using positron emission tomography

Annexin-V is a calcium-dependent protein that binds with high affinity to phosphaditylserine exposed during apoptosis. The aim of this study was to radiolabel annexin-V with iodine-124 for use as a potential probe of apoptosis by positron emission tomography. Annexin-V was radioiodinated directly using the cyclotron-produced positron emitter iodine-124 by the chloramine-T (CAT) method and indirectly by the pre-labelled reagent N-succinimidyl 3-[124I]iodobenzoate ([124I]m-SIB). Some reaction parameters of the CAT method such as reaction time and pH were optimised to give radiochemical yields of 22.3 +/- 2.6%(n = 3, gel-filtration). After incubation with [124I]m-SIB, radiolabelled annexin-V was obtained in 14% and 25% yield by FPLC and gel-filtration, respectively. The radiochemical purities from direct and indirect labelling were 97.7 +/- 1.0%(n = 3) and 96.7 +/- 2.1%(n = 3), respectively. The new radiotracers could be stored for up to four days without significant de-iodination. The biological activity of radiolabelled annexin-V was tested in control and camptothecin-treated (i.e. apoptotic) human leukaemic HL60 cells. A significantly higher (21%) binding in treated cells was observed with [125I]m-SIB-annexin-V. The binding of [125I]m-SIB labelled annexin-V to camptothecin treated cells was blocked (68%) by a 100-fold excess of unlabelled annexin-V.

Detection of apoptotic frequency in Chinese hamster ovary (CHO-K1) cells after gamma-irradiation using both neutral Comet assay andterminal desoxynucleotidyl transferase (TdT) assay

OBJECTIVES

Two assay methods, namely the neutral Comet assay andterminal desoxynucleotidyl transferase (TdT) assay, were carried out for comparison to investigate the capability of using the neutral Comet assay as an alternative for detection of apoptosis.

MATERIALS AND METHODS

Chinese hamster ovary-K1 (CHO-K1) cells were exposed to gamma-rays with different doses and then the frequencies of apoptotic cells were determined at given points of time using the neutral Comet assay andTdT assay.

RESULTS

Apoptotic frequency of CHO-K1 cells after gamma-irradiation is dependent on both time after irradiation and radiation dose using either the neutral Comet assay orTdT assay. There are differences between the data obtained using the neutral Comet assay andTdT assay (p<0.01, Student's t-test).

CONCLUSIONS

The neutral Comet assay appears to be an appropriate tool for detection of radiationinduced apoptosis at the early stage of the process. Compared to the other methods such as theTdT assay, the neutral Comet assay is a rapid, simple and economical method for detection of apoptosis.

Modulation of the multidrug resistance of glioma by glutathione levels depletion--interaction with Tc-99M-Sestamibi and Tc-99M-Tetrofosmin

We have investigated the effect of glutathione (GSH) depletion on the chemosensitivity of human malignant glioma cell lines: G111, G5 and G152. All the cell lines showed a multidrug resistant (MDR) phenotype associated with MRP1 expression, high intracellular levels of GSH, and depolarized plasma membranes. Tc-99M-Sestamibi (MIBI) and Tc-99M-Tetrofosmin (Tfos) were used for monitoring the MDR mechanisms. Modulation of GSH content was performed with butoxysulfoximide (BSO) pre-treatment alone or in combination with GSH ethyl ester. MIBI and Tfos accumulation in the cells was inversely correlated to the GSH content, a higher accumulation was found after BSO pre-treatment and addition of GSH ethyl ester reversed this process. BSO could therefore play a role as a chemosensitizing drug and thus help to overcome MDR. However, higher accumulation of MIBI and Tfos was observed even in the sensitive cells suggesting another effect of BSO on the cell physiological characteristics. No sign of apoptosis has been found indicating a possible direct effect on the plasma membrane fluidity and permeability. MIBI and Tfos don't follow the expected behavior of a MDR probe in the glioma cells and given the particular morpho-physiological characteristics of these types of tumors, Tfos could be rather used as a marker of the tumor growth and proliferation.