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

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.

A Peptide-Based Fluorescent Sensor for Anionic Phospholipids

Anionic phospholipids are key cell signal mediators. The distribution of these lipids on the cell membrane and intracellular organelle membranes guides the recruitment of signaling proteins leading to the regulation of cellular processes. Hence, fluorescent sensors that can detect anionic phospholipids within living cells can provide a handle into revealing molecular mechanisms underlying lipid-mediated signal regulation. A major challenge in the detection of anionic phospholipids is related to the presence of these phospholipids mostly in the inner leaflet of the plasma membrane and in the membranes of intracellular organelles. Hence, cell-permeable sensors would provide an advantage by enabling the rapid detection and tracking of intracellular pools of anionic phospholipids. We have developed a peptide-based, cell-permeable, water-soluble, and ratiometric fluorescent sensor that entered cells within 15 min of incubation via the endosomal machinery and showed punctate labeling in the cytoplasm. The probe could also be introduced into living cells via lipofection, which allows bypassing of endosomal uptake, to image anionic phospholipids in the cell membrane. We validated the ability of the sensor toward detection of intracellular anionic phospholipids by colocalization studies with a fluorescently tagged lipid and a protein-based anionic phospholipid sensor. Further, the sensor could image the externalization of anionic phospholipids during programmed cell death, indicating the ability of the probe toward detection of both intra- and extracellular anionic phospholipids based on the biological context.

Radiolabeled Peptides for Molecular Imaging of Apoptosis

Apoptosis is a regulated cell death induced by extrinsic and intrinsic stimulants. Tracking of apoptosis provides an opportunity for the assessment of cardiovascular and neurodegenerative diseases as well as monitoring of cancer therapy at early stages. There are some key mediators in apoptosis cascade, which could be considered as specific targets for delivering imaging or therapeutic agents. The targeted radioisotope-based imaging agents are able to sensitively detect the physiological signal pathways which make them suitable for apoptosis imaging at a single-cell level. Radiopeptides take advantage of both the high sensitivity of nuclear imaging modalities and favorable features of peptide scaffolds. The aim of this study is to review the characteristics of those radiopeptides targeting apoptosis with different mechanisms.

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.

Imaging Cell Death: Focus on Early Evaluation of Tumor Response to Therapy

Cell death plays a prominent role in the treatment of cancer, because most anticancer therapies act by the induction of cell death including apoptosis, necrosis, and other pathways of cell death. Imaging cell death helps to identify treatment responders from nonresponders and thus enables patient-tailored therapy, which will increase the likelihood of treatment response and ultimately lead to improved patient survival. By taking advantage of molecular probes that specifically target the biomarkers/biochemical processes of cell death, cell death imaging can be successfully achieved. In recent years, with the increased understanding of the molecular mechanism of cell death, a variety of well-defined biomarkers/biochemical processes of cell death have been identified. By targeting these established cell death biomarkers/biochemical processes, a set of molecular imaging probes have been developed and evaluated for early monitoring treatment response in tumors. In this review, we mainly present the recent advances in identifying useful biomarkers/biochemical processes for both apoptosis and necrosis imaging and in developing molecular imaging probes targeting these biomarkers/biochemical processes, with a focus on their application in early evaluation of tumor response to therapy.

Phosphatidylserine targeting peptide-functionalized pH sensitive mixed micelles for enhanced anti-tumor drug delivery

In recent decades, targeted drug delivery systems (TDDS) have been widely used as an ideal method of improving therapeutic effects and reducing systemic side effects of chemotherapeutic agents. Historically, a handful of methods have been developed to further improve the targeting ability of delivery systems. Thus, in this study, two methods, taking advantage of tumor characteristics, were used for the creation of a multi-targeted delivery system. The first was the fabrication of pH-sensitive micelles, lending the ability to increase drug release by exploiting the acidic tumor environment. The second method was through utilization of the surface-exposed phosphatidylserine (PS) of tumors, which is normally found in the inner leaflet in healthy cells. Using PS as a target site, PS binding peptide (PSBP-6) was conjugated to pH-sensitive mixed micelles, (consisting of poly (ethylene glycol)-b-poly (D, L-lactide) (PEG-PDLLA) and poly (ethylene glycol)-b-poly (L-histidine) (PEG-PHIS)). After successful preparation of micelles, paclitaxel (PTX), a common chemotherapeutic agent, was selected to measure drug loading capacity and encapsulation efficiency, showing 7.9% and 83.5%, respectively. The in vitro release of PTX from mixed micelles at pH 5.0, 6.5, and 7.4 was 78.1, 56.8, and 51.4%, respectively, indicating acid-triggered drug release. The PSBP-6-modified, mixed micelles exhibited significantly enhanced in vitro cytotoxicity and demonstrated more efficient cellular uptake compared to unmodified mixed micelles in the HeLa cell line. Moreover, pharmacokinetic, in vivo biodistribution, and fluorescence imaging studies showed that PSBP-6-PEG-PDLLA/PEG-PHIS mixed micelles provide prolonged time in blood circulation and enhanced tumor accumulation. These results suggest that the use of PS as a novel targeting site is advantageous, and that these new multi-targeted mixed micelles show great potential for realization of broad prospects in the targeted treatment of tumors for chemotherapeutic delivery.

Potential-Resolved Electrochemiluminescence Nanoprobes for Visual Apoptosis Evaluation at Single-Cell Level

In this work, a potential-resolved electrochemiluminescence (ECL) method is developed and used for the apoptosis diagnosis at the single-cell level. The apoptosis of cells usually induces the decreasing expression of epidermal growth factor receptor (EGFR) and promotes phosphatidylserine (PS) eversion on the cell membrane. Here, Au@L012 and g-C₃N₄ as ECL probes are functionalized with epidermal growth factor (EGF) and peptide (PSBP) to recognize the EGFR and PS on the cell surface, respectively, showing two well-separated ECL signals during a potential scanning. Experimental results reveal that the relative ECL change of g-C₃N₄ and Au@L012 correlates with the degree of apoptosis, which provides an accurate way to investigate apoptosis without interference that solely changes EGFR or PS. With a homemade ECL microscopy, we simultaneously evaluate the EGFR and PS expression of abundant individual cells and, therefore, achieve the visualization analysis of the apoptosis rate for normal and cancer cell samples. This strategy contributes to visually studying tumor markers and pushing the application of ECL imaging for the disease diagnosis at the single-cell level.

A glucose/O2 fuel cell-based self-powered biosensor for probing a drug delivery model with self-diagnosis and self-evaluation

Extending the application of self-powered biosensors (SPB) into the drug delivery field is highly desirable. Herein, a robust glucose/O2 fuel cell-based biosensor is successfully integrated with a targeted drug delivery system to create a self-sustained and highly compact drug delivery model with self-diagnosis and self-evaluation (DDM-SDSE). The glucose/O2 fuel cell-based biosensor firstly performs its diagnostic function by detecting the biomarkers of cancer. The drug delivery system attached on the anode of the glucose/O2 fuel cell can be released during the diagnostic operation to guarantee the occurrence of a therapy process. Accompanied by the therapy process, the glucose/O2 fuel cell-based biosensor can also act as an evaluation component to dynamically monitor the therapy efficacy by analyzing drug-induced apoptotic cells. In addition, the use of an abiotic catalyst largely improves the stability of the glucose/O2 fuel cell without sacrificing the output performance, further ensuring long-time dynamic evaluation as well as highly sensitive diagnosis and evaluation in this DDM-SDSE. Therefore, the present study not only expands the application of SPBs but also offers a promising in vitro "diagnosis-therapy-evaluation" platform to acquire valuable information for clinical cancer therapy.

Avenues to molecular imaging of dying cells: Focus on cancer

Successful treatment of cancer patients requires balancing of the dose, timing, and type of therapeutic regimen. Detection of increased cell death may serve as a predictor of the eventual therapeutic success. Imaging of cell death may thus lead to early identification of treatment responders and nonresponders, and to “patient‐tailored therapy.” Cell death in organs and tissues of the human body can be visualized, using positron emission tomography or single‐photon emission computed tomography, although unsolved problems remain concerning target selection, tracer pharmacokinetics, target‐to‐nontarget ratio, and spatial and temporal resolution of the scans. Phosphatidylserine exposure by dying cells has been the most extensively studied imaging target. However, visualization of this process with radiolabeled Annexin A5 has not become routine in the clinical setting. Classification of death modes is no longer based only on cell morphology but also on biochemistry, and apoptosis is no longer found to be the preponderant mechanism of cell death after antitumor therapy, as was earlier believed. These conceptual changes have affected radiochemical efforts. Novel probes targeting changes in membrane permeability, cytoplasmic pH, mitochondrial membrane potential, or caspase activation have recently been explored. In this review, we discuss molecular changes in tumors which can be targeted to visualize cell death and we propose promising biomarkers for future exploration.

Peptide-Based Photoelectrochemical Cytosensor Using a Hollow-TiO2/EG/ZnIn2S4 Cosensitized Structure for Ultrasensitive Detection of Early Apoptotic Cells and Drug Evaluation

The ability to rapidly detect apoptotic cells and accurately evaluate therapeutic effects is significant in cancer research. To address this target, a biocompatible, ultrasensitive photoelectrochemical (PEC) cytosensing platform was developed based on electrochemically reduced graphene (EG)/ZnIn₂S₄ cosensitized TiO₂ coupled with specific recognition between apoptotic cells and phosphatidylserine-binding peptide (PSBP). In this strategy, the HL-60 cells were selected as a model and C005, nilotinib, and imatinib were selected as apoptosis inducers to show cytosensing performances. In particular, a TiO₂ photoactive substrate was designed as hollow spheres to enhance the PEC performance. Graphene was electrodeposited on the hollow TiO₂-modified electrode to accelerate electron transfer and increase conductivity, followed by in situ growth of ZnIn₂S₄ nanocrystals as photosensitizers via successive ionic layer adsorption and reaction method, forming a TiO₂/EG/ZnIn₂S₄ cosensitized structure that was used as a PEC matrix to immobilize PSBP for the recognition of early apoptotic cells. The detection of apoptotic cells was based on steric hindrance originating from apoptotic cell capture to induce an obvious decrease in the photocurrent signal. The ultrahigh sensitivity of the cytosensor resulted from enhanced PEC performance, bioactivity, and high binding affinity between PSBP and apoptotic cells. Compared with other assays, incorporate toxic elements were avoided, such as Cd, Ru, and Te, which ensured normal cell growth and are appropriate for cell analysis. The designed PEC cytosensor showed a low detection limit of apoptotic cells (as low as three cells), a wide linear range from 1 × 10³ to 5 × 10⁷ cells/mL, and an accurate evaluation of therapeutic effects. It also exhibited good specificity, reproducibility, and stability.

Identification of lipid-phosphatidylserine (PS) as the target of unbiasedly selected cancer specific peptide-peptoid hybrid PPS1

Phosphatidylserine (PS) is an anionic phospholipid maintained on the inner-leaflet of the cell membrane and is externalized in malignant cells. We previously launched a careful unbiased selection targeting biomolecules (e.g. protein, lipid or carbohydrate) distinct to cancer cells by exploiting HCC4017 lung cancer and HBEC30KT normal epithelial cells derived from the same patient, identifying HCC4017 specific peptide-peptoid hybrid PPS1. In this current study, we identified PS as the target of PPS1. We validated direct PPS1 binding to PS using ELISA-like assays, lipid dot blot and liposome based binding assays. In addition, PPS1 recognized other negatively charged and cancer specific lipids such as phosphatidic acid, phosphatidylinositol and phosphatidylglycerol. PPS1 did not bind to neutral lipids such as phosphatidylethanolamine found in cancer and phosphatidylcholine and sphingomyelin found in normal cells. Further we found that the dimeric version of PPS1 (PPS1D1) displayed strong cytotoxicity towards lung cancer cell lines that externalize PS, but not normal cells. PPS1D1 showed potent single agent anti-tumor activity and enhanced the efficacy of docetaxel in mice bearing H460 lung cancer xenografts. Since PS and anionic phospholipid externalization is common across many cancer types, PPS1 may be an alternative to overcome limitations of protein targeted agents.

Nanoparticle design considerations for molecular imaging of apoptosis: Diagnostic, prognostic, and therapeutic value

The present review analyzes various approaches for the design and synthesis of different nanoparticles for imaging and therapy. Nanoparticles for computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET) and optical imaging are discussed. The influence of nanoparticle size, shape, surface charge, composition, surface functionalization, active targeting and other factors on imaging and therapeutic efficacy is analyzed. Cyto- and genotoxicity of nanoparticles are also discussed. Special attention in the review is paid to the imaging of apoptotic tissues and cells in different diseases.

Imaging and therapeutic applications of zinc(ii)-dipicolylamine molecular probes for anionic biomembranes

This feature article describes the development of synthetic zinc(ii)-dipicolylamine (ZnDPA) receptors as selective targeting agents for anionic membranes in cell culture and living subjects. There is a strong connection between anionic cell surface charge and disease, and ZnDPA probes have been employed extensively for molecular imaging and targeted therapeutics. Fluorescence and nuclear imaging applications include detection of diseases such as cancer, neurodegeneration, arthritis, and microbial infection, and also quantification of cell death caused by therapy. Therapeutic applications include selective targeting of cytotoxic agents and drug delivery systems, photodynamic inactivation, and modulation of the immune system. The article concludes with a summary of expected future directions.

Live-cell imaging to detect phosphatidylserine externalization in brain endothelial cells exposed to ionizing radiation: implications for the treatment of brain arteriovenous malformations

OBJECT Stereotactic radiosurgery (SRS) is an established intervention for brain arteriovenous malformations (AVMs). The processes of AVM vessel occlusion after SRS are poorly understood. To improve SRS efficacy, it is important to understand the cellular response of blood vessels to radiation. The molecular changes on the surface of AVM endothelial cells after irradiation may also be used for vascular targeting. This study investigates radiation-induced externalization of phosphatidylserine (PS) on endothelial cells using live-cell imaging. METHODS An immortalized cell line generated from mouse brain endothelium, bEnd.3 cells, was cultured and irradiated at different radiation doses using a linear accelerator. PS externalization in the cells was subsequently visualized using polarity-sensitive indicator of viability and apoptosis (pSIVA)-IANBD, a polarity-sensitive probe. Live-cell imaging was used to monitor PS externalization in real time. The effects of radiation on the cell cycle of bEnd.3 cells were also examined by flow cytometry. RESULTS Ionizing radiation effects are dose dependent. Reduction in the cell proliferation rate was observed after exposure to 5 Gy radiation, whereas higher radiation doses (15 Gy and 25 Gy) totally inhibited proliferation. In comparison with cells treated with sham radiation, the irradiated cells showed distinct pseudopodial elongation with little or no spreading of the cell body. The percentages of pSIVA-positive cells were significantly higher (p = 0.04) 24 hours after treatment in the cultures that received 25- and 15-Gy doses of radiation. This effect was sustained until the end of the experiment (3 days). Radiation at 5 Gy did not induce significant PS externalization compared with the sham-radiation controls at any time points (p > 0.15). Flow cytometric analysis data indicate that irradiation induced growth arrest of bEnd.3 cells, with cells accumulating in the G2 phase of the cell cycle. CONCLUSIONS Ionizing radiation causes remarkable cellular changes in endothelial cells. Significant PS externalization is induced by radiation at doses of 15 Gy or higher, concomitant with a block in the cell cycle. Radiation-induced markers/targets may have high discriminating power to be harnessed in vascular targeting for AVM treatment.

Phenoxide-Bridged Zinc(II)-Bis(dipicolylamine) Probes for Molecular Imaging of Cell Death

Cell death is involved in many pathological conditions, and there is a need for clinical and preclinical imaging agents that can target and report cell death. One of the best known biomarkers of cell death is exposure of the anionic phospholipid phosphatidylserine (PS) on the surface of dead and dying cells. Synthetic zinc(II)-bis(dipicolylamine) (Zn2BDPA) coordination complexes are known to selectively recognize PS-rich membranes and act as cell death molecular imaging agents. However, there is a need to improve in vivo imaging performance by selectively increasing target affinity and decreasing off-target accumulation. This present study compared the cell death targeting ability of two new deep-red fluorescent probes containing phenoxide-bridged Zn2BDPA complexes. One probe was a bivalent version of the other and associated more strongly with PS-rich liposome membranes. However, the bivalent probe exhibited self-quenching on the membrane surface, so the monovalent version produced brighter micrographs of dead and dying cells in cell culture and also better fluorescence imaging contrast in two living animal models of cell death (rat implanted tumor with necrotic core and mouse thymus atrophy). An (111)In-labeled radiotracer version of the monovalent probe also exhibited selective cell death targeting ability in the mouse thymus atrophy model, with relatively high amounts detected in dead and dying tissue and low off-target accumulation in nonclearance organs. The in vivo biodistribution profile is the most favorable yet reported for a Zn2BDPA complex; thus, the monovalent phenoxide-bridged Zn2BDPA scaffold is a promising candidate for further development as a cell death imaging agent in living subjects.

Radiopharmacological evaluation of (18)F-labeled phosphatidylserine-binding peptides for molecular imaging of apoptosis

INTRODUCTION

Radiolabeled phosphatidylserine (PS)-binding peptides represent an innovative strategy for molecular imaging of apoptosis with positron emission tomography (PET). The goal of this study was the radiopharmacological evaluation of radiolabeled peptides for their binding to PS on apoptotic cancer cells, involving metabolic stability, cellular uptake, biodistribution, and dynamic PET imaging experiments.

METHODS

Binding of peptides LIKKPF, PGDLSR, FBz-LIKKPF, FBz-PGDLSR, FBAM-CLIKKPF and FBAM-CPGDLSR to PS was analyzed in a newly developed radiometric binding assay using (64)Cu-labeled wild-type annexin-V as radiotracer. Radiolabeling of most potent peptides with fluorine-18 was carried out with thiol-selective prosthetic group [(18)F]FBAM to give [(18)F]FBAM-CLIKKPF and [(18)F]FBAM-CPGDLSR. [(18)F]FBAM-labeled peptides were studied in camptothecin-induced apoptotic human T lymphocyte Jurkat cells, and in a murine EL4 tumor model of apoptosis using dynamic PET imaging and biodistribution.

RESULTS

Peptides LIKKPF and PGDLSR inhibited binding of (64)Cu-labeled annexin-V to immobilized PS in the millimolar range (IC50 10-15 mM) compared to annexin-V (45 nM). Introduction of FBAM prosthetic group slightly increased inhibitory potencies (FBAM-CLIKKPF: IC50 = 1 mM; FBAM-CPGDLSR: IC50 = 6 mM). Radiolabeling succeeded in good radiochemical yields of 50-54% using a chemoselective alkylation reaction of peptides CLIKKPF and CPGDLSR with [(18)F]FBAM. In vivo metabolic stability studies in mice revealed 40-60% of intact peptides at 5 min p.i. decreasing to 25% for [(18)F]FBAM-CLIKKPF and less than 5% for [(18)F]FBAM-CPGDLSR at 15 min p.i.. Cell binding of [(18)F]FBAM-CLIKKPF in drug-treated Jurkat cells was significantly higher compared to untreated cells, but this was not observed for [(18)F]FBAM-CPGDLSR. Dynamic PET imaging experiments showed that baseline uptake of [(18)F]FBAM-CLIKKPF in EL4 tumors was higher (SUV(5min) 0.46, SUV(60min) 0.13) compared to [(18)F]FBAM-CPGDLSR (SUV(5min) 0.16, SUV(60min) 0.10). Drug-treated EL4 tumors did not show an increased uptake for both [(18)F]FBAM-labeled peptides.

CONCLUSION

Although both (18)F-labeled peptides [(18)F]FBAM-CLIKKPF and [(18)F]FBAM-CPGDLSR showed higher binding to apoptotic Jurkat cells in vitro, their in vivo uptake profiles were not different in apoptotic EL4 tumors. This may explained by the relatively low potency of both compounds to compete with binding of (64)Cu-labeled annexin-V to PS. Overall the novel competitive radiometric PS-binding assay with (64)Cu-labeled annexin-V represents a versatile and very robust screening platform to analyze potential PS-binding compounds in vitro. Further studies will be necessary to evaluate alternative peptide structures toward their use as PET radiotracers imaging apoptosis in vivo.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Development of peptide-based radiotracers for imaging apoptosis in vivo remains a significant challenge.

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.

Library synthesis, screening, and discovery of modified Zinc(II)-Bis(dipicolylamine) probe for enhanced molecular imaging of cell death

Zinc(II)-bis(dipicolylamine) (Zn-BDPA) coordination complexes selectively target the surfaces of dead and dying mammalian cells, and they have promise as molecular probes for imaging cell death. A necessary step toward eventual clinical imaging applications is the development of next-generation Zn-BDPA complexes with enhanced affinity for the cell death membrane biomarker, phosphatidylserine (PS). This study employed an iterative cycle of library synthesis and screening, using a novel rapid equilibrium dialysis assay, to discover a modified Zn-BDPA structure with high and selective affinity for vesicles containing PS. The lead structure was converted into a deep-red fluorescent probe and its targeting and imaging performance was compared with an unmodified control Zn-BDPA probe. The evaluation process included a series of FRET-based vesicle titration studies, cell microscopy experiments, and rat tumor biodistribution measurements. In all cases, the modified probe exhibited comparatively higher affinity and selectivity for the target membranes of dead and dying cells. The results show that this next-generation deep-red fluorescent Zn-BDPA probe is well suited for preclinical molecular imaging of cell death in cell cultures and animal models. Furthermore, it should be possible to substitute the deep-red fluorophore with alternative reporter groups that enable clinically useful, deep-tissue imaging modalities, such as MRI and nuclear imaging.

Polymeric micelles for apoptosis-targeted optical imaging of cancer and intraoperative surgical guidance

In a two-step strategy, an intraperitoneal (IP) injection of poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL) micelles containing paclitaxel (PTX), cyclopamine (CYP), and gossypol (GSP) at 30, 30, and 30 mg/kg, respectively, debulked tumor tissues by 1.3-fold, based on loss of bioluminescence with <10% body weight change, and induced apoptosis in peritoneal tumors when used as neoadjuvant chemotherapy (NACT) in an ES-2-luc-bearing xenograft model for ovarian cancer. In a second step, a single intravenous (IV) injection of apoptosis-targeting GFNFRLKAGAKIRFGS-PEG-b-PCL micelles containing a near-infrared (NIR) fluorescence probe, DiR (1,1′-dioctadecyltetramethyl indotricarbocyanine iodide), resulted in increased peritoneal DiR accumulation in apoptosis-induced ES-2-luc tumor tissues (ex vivo) by 1.5-fold compared with DiR molecules delivered by methoxy PEG-b-PCL micelles (non-targeted) at 48 h after IV injection in a second step. As a result, a tandem of PEG-b-PCL micelles enabled high-resolution detection of ca. 1 mm diameter tumors, resulting in resection of approximately 90% of tumors, and a low peritoneal cancer index (PCI) of ca. 7. Thus, a tandem of PEG-b-PCL micelles used for NCAT and NIR fluorescence imaging of therapy-induced apoptosis for intraoperative surgical guidance may be a promising treatment strategy for metastatic ovarian cancer.

Illuminating the lipidome to advance biomedical research: peptide-based probes of membrane lipids

Systematic investigation of the lipidome will reveal new opportunities for disease diagnosis and intervention. However, lipidomic research has been hampered by the lack of molecular tools to track specific lipids of interest. Accumulating reports indicate lipid recognition can be achieved with properly constructed short peptides in addition to large proteins. This review summarizes the key developments of this area within the past decade. Select lantibiotic peptides present the best examples of low-molecular-weight probes of membrane lipids, displaying selectivity comparable to lipid-binding proteins. Designed peptides, through biomimetic approaches and combinational screening, have begun to demonstrate their potential for lipid tracking in cultured cells and even in living organisms. Biophysical characterization of these lipid-targeting peptides has revealed certain features critical for selective membrane binding, including preorganized scaffolds and the balance of polar and nonpolar interactions. The knowledge summarized herein should facilitate the development of molecular tools to target a variety of membrane lipids.

Optical imaging of cell death in traumatic brain injury using a heat shock protein-90 alkylator

Traumatic brain injury is a major public health concern and is characterised by both apoptotic and necrotic cell death in the lesion. Anatomical imaging is usually used to assess traumatic brain injuries and there is a need for imaging modalities that provide complementary cellular information. We sought to non-invasively image cell death in a mouse model of traumatic brain injury using a near-infrared fluorescent conjugate of a synthetic heat shock protein-90 alkylator, 4-(N-(S-glutathionylacetyl) amino) phenylarsonous acid (GSAO). GSAO labels both apoptotic and necrotic cells coincident with loss of plasma membrane integrity. The optical GSAO specifically labelled apoptotic and necrotic cells in culture and did not accumulate in healthy organs or tissues in the living mouse body. The conjugate is a very effective imager of cell death in brain lesions. The optical GSAO was detected by fluorescence intensity and GSAO bound to dying/dead cells was detected from prolongation of the fluorescence lifetime. An optimal signal-to-background ratio was achieved as early as 3 h after injection of the probe and the signal intensity positively correlated with both lesion size and probe concentration. This optical GSAO offers a convenient and robust means to non-invasively image apoptotic and necrotic cell death in brain and other lesions.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

Multicolor fluorescence imaging of traumatic brain injury in a cryolesion mouse model

Traumatic brain injury is characterized by initial tissue damage, which then can lead to secondary processes such as cell death and blood-brain-barrier disruption. Clinical and preclinical studies of traumatic brain injury typically employ anatomical imaging techniques and there is a need for new molecular imaging methods that provide complementary biochemical information. Here, we assess the ability of a targeted, near-infrared fluorescent probe, named PSS-794, to detect cell death in a brain cryolesion mouse model that replicates certain features of traumatic brain injury. In short, the model involves brief contact of a cold rod to the head of a living, anesthetized mouse. Using noninvasive whole-body fluorescence imaging, PSS-794 permitted visualization of the cryolesion in the living animal. Ex vivo imaging and histological analysis confirmed PSS-794 localization to site of brain cell death. The nontargeted, deep-red Tracer-653 was validated as a tracer dye for monitoring blood-brain-barrier disruption, and a binary mixture of PSS-794 and Tracer-653 was employed for multicolor imaging of cell death and blood-brain-barrier permeability in a single animal. The imaging data indicates that at 3 days after brain cryoinjury the amount of cell death had decreased significantly, but the integrity of the blood-brain-barrier was still impaired; at 7 days, the blood-brain-barrier was still three times more permeable than before cryoinjury.

Beyond annexin V: fluorescence response of cellular membranes to apoptosis

Dramatic changes in the structure of cell membranes on apoptosis allow easy, sensitive and non-destructive analysis of this process with the application of fluorescence methods. The strong plasma membrane asymmetry is present in living cells, and its loss on apoptosis is commonly detected with the probes interacting strongly and specifically with phosphatidylserine (PS). This phospholipid becomes exposed to the cell surface, and the application of annexin V labeled with fluorescent dye is presently the most popular tool for its detection. Several methods have been suggested recently that offer important advantages over annexin V assay with the ability to study apoptosis by spectroscopy of cell suspensions, flow cytometry and confocal or two-photon microscopy. The PS exposure marks the integrated changes in the outer leaflet of cell membrane that involve electrostatic potential and hydration, and the attempts are being made to provide direct probing of these changes. This review describes the basic mechanisms underlying the loss of membrane asymmetry during apoptosis and discusses, in comparison with the annexin V-binding assay, the novel fluorescence techniques of detecting apoptosis on cellular membrane level. In more detail we describe the detection method based on smart fluorescent dye F2N12S incorporated into outer leaflet of cell membrane and reporting on apoptotic cell transformation by easily detectable change of the spectral distribution of fluorescent emission. It can be adapted to any assay format.

In vivo targeting of cell death using a synthetic fluorescent molecular probe

A synthetic, near-infrared, fluorescent probe, named PSS-794 was assessed for its ability to detect cell death in two animal models. The molecular probe contains a zinc(II)-dipicolylamine (Zn(2+)-DPA) affinity ligand that selectively targets exposed phosphatidylserine on the surface of dead and dying cells. The first animal model used rats that were treated with dexamethasone to induce thymic atrophy. Ex vivo fluorescence imaging and histological analysis of excised organs showed thymus uptake of PSS-794 was four times higher than a control fluorophore that lacked the Zn(2+)-DPA affinity ligand. In addition, the presence of PSS-794 produced a delayed and higher build up of dead and dying cells in the rat thymus. The second animal model employed focal beam radiation to induce cell death in tumor-bearing rats. Whole-body and ex vivo imaging showed that the amount of PSS-794 in a radiation-treated tumor was almost twice that in a non-treated tumor. The results indicate that PSS-794 may be useful for preclinical optical detection of tumor cell death due to therapy.

Improved radiosynthesis of the apoptosis marker 18F-ICMT11 including biological evaluation

We improved the specific radioactivity of the apoptosis imaging isatin derivative (18)F-ICMT11. We then evaluated (18)F-ICMT11 in EL4 tumor-bearing mice 24h after treatment with etoposide/cyclophosphamide combination therapy. Dynamic PET imaging demonstrated increased uptake in the drug-treated (0.115±0.011 SUV) compared to the vehicle-treated EL4 tumors (0.083±0.008 SUV). This effect correlated to the observed increases in apoptotic index.

Transbilayer phospholipids molecular imaging

Nuclear medicine has become a key part of molecular imaging. In the present review article, we focus on the transbilayer phospholipids as exquisite targets for radiolabelled probes in molecular imaging. Asymmetry of phospholipid distribution is a characteristic of mammalian cell membranes. Phosphatidylcholine and sphyngomyelin cholinophospholipids are primarily located within the external leaflet of the cell membrane. Phosphatidylserine and phosphatidylethanolamine aminophospholipids, and also phosphatidylinositol are primarily located within the internal leaflet of the cell membrane. New radiolabelled tracers have been designed in preclinical and clinical research for PET-CT and SPECT-CT molecular imaging of transbilayer phospholipids.