Discovery of a phosphatidylserine-recognizing peptide and its utility in molecular imaging of tumour apoptosis

A fluorogenic cyclic peptide for imaging and quantification of drug-induced apoptosis

Programmed cell death or apoptosis is a central biological process that is dysregulated in many diseases, including inflammatory conditions and cancer. The detection and quantification of apoptotic cells in vivo is hampered by the need for fixatives or washing steps for non-fluorogenic reagents, and by the low levels of free calcium in diseased tissues that restrict the use of annexins. In this manuscript, we report the rational design of a highly stable fluorogenic peptide (termed Apo-15) that selectively stains apoptotic cells in vitro and in vivo in a calcium-independent manner and under wash-free conditions. Furthermore, using a combination of chemical and biophysical methods, we identify phosphatidylserine as a molecular target of Apo-15. We demonstrate that Apo-15 can be used for the quantification and imaging of drug-induced apoptosis in preclinical mouse models, thus creating opportunities for assessing the in vivo efficacy of anti-inflammatory and anti-cancer therapeutics.

Programmed cell death or apoptosis is an essential biological process that is impaired in some diseases and can be used to assess the effectiveness of drugs. Here the authors design Apo-15 as a fluorogenic peptide for the detection and real-time imaging of apoptotic cells.

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.

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.

PSP1, a Phosphatidylserine-Recognizing Peptide, Is Useful for Visualizing Radiation-Induced Apoptosis in Colorectal Cancer In Vitro and In Vivo

Accurate and timely visualization of apoptotic status in response to radiation is necessary for deciding whether to continue radiation or change to another mode of treatment. This is especially critical in patients with colorectal cancer, which requires a delicate combination of surgery, radiation, and chemotherapy in order to achieve optimal outcome. In this study, we investigated the potential of phosphatidylserine-recognizing peptide 1 (PSP1) as an apoptosis-targeting probe, which identifies phosphatidylserine on cell surfaces. We first screened colon cancer cell lines for their sensitivity to radiation and selected two cell lines: HCT116 and HT29. Cell binding assay using fluorescence-activated cell sorting and optical imaging showed that HCT116 cells had better binding to PSP1 than HT29 cells. Thus, mouse xenograft model using HCT116 cells was generated and was topically irradiated with either single or fractionated dose of radiation followed by systemic administration of PSP1 for subsequent molecular optical imaging. We confirmed that the PSP1 probe was selectively bound to apoptosis-induced tumor in a radiation dose-dependent manner. We also observed that fractionated radiation regimen, which is recently being used in clinical situation, was more effective in inducing tumor apoptosis than corresponding single-dose radiation treatment. We then evaluated the correlation between tumor targeting of PSP1 and suppression effect of tumor development and found that tumor volume and fluorescence intensity were correlated before (correlation coefficient r² = 0.534) and after (r² = 0.848) radiation therapy. Our study shows that PSP1 peptide is an efficient index probe for deciding "go or no-go" for radiation therapy in colorectal cancer.

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.

High-Throughput Approaches to the Development of Molecular Imaging Agents

Molecular imaging allows for the visualization of changes at the cellular level in diseases such as cancer. A successful molecular imaging agent must rely on disease-selective targets and ligands that specifically interact with those targets. Unfortunately, the translation of novel target-specific ligands into the clinic has been frustratingly slow with limitations including the complex design and screening approaches for ligand identification, as well as their subsequent optimization into useful imaging agents. This review focuses on combinatorial library approaches towards addressing these two challenges, with particular focus on phage display and one-bead one-compound (OBOC) libraries. Both of these peptide-based techniques have proven successful in identifying new ligands for cancer-specific targets and some of the success stories will be highlighted. New developments in screening methodology and sequencing technology have pushed the bounds of phage display and OBOC even further, allowing for even faster and more robust discovery of novel ligands. The combination of multiple high-throughput technologies will not only allow for more accurate identification, but also faster affinity maturation, while overall streamlining the process of translating novel ligands into clinical imaging agents.

Tumor-targeting peptides from combinatorial libraries

Cancer is one of the major and leading causes of death worldwide. Two of the greatest challenges in fighting cancer are early detection and effective treatments with no or minimum side effects. Widespread use of targeted therapies and molecular imaging in clinics requires high affinity, tumor-specific agents as effective targeting vehicles to deliver therapeutics and imaging probes to the primary or metastatic tumor sites. Combinatorial libraries such as phage-display and one-bead one-compound (OBOC) peptide libraries are powerful approaches in discovering tumor-targeting peptides. This review gives an overview of different combinatorial library technologies that have been used for the discovery of tumor-targeting peptides. Examples of tumor-targeting peptides identified from each combinatorial library method will be discussed. Published tumor-targeting peptide ligands and their applications will also be summarized by the combinatorial library methods and their corresponding binding receptors.

Evaluation of Phosphatidylserine-Specific Peptide-Conjugated Liposomes Using a Model System of Malaria-Infected Erythrocytes

Malaria is one of the most prevalent parasitic diseases and is most widespread in tropical regions. The malarial parasite grows and reproduces in erythrocytes during its life cycle, resulting in programmed erythrocyte death, termed eryptosis. Lipid scrambling, which occurs following the exposure of anionic lipids such as phosphatidylserine (PS) on the outer surface of erythrocytes, is a characteristic physical change that occurs early during eryptosis. Here, we prepared "PS specific peptide (PSP)"-conjugated liposomes (PSP-liposomes) and investigated whether PSP-liposomes hold promise as a novel strategy for actively targeting eryptosis. Eryptosis was induced by exposing red blood cells (RBCs) to ionomycin, a known calcium ionophore. When PSP liposomes were mixed with either RBCs or RBCs undergoing eryptosis (E-RBCs), the amount of PSP-liposome bound to E-RBCs was much higher than the amount bound to RBCs. However, the amount of PSP-liposome bound to E-RBCs was significantly inhibited by the presence of annexin V protein, which binds specifically to PS. These results suggest that PSP-liposomes could be an effective drug nanocarrier for treating E-RBCs and malaria-infected erythrocytes.

Identification of the minimum pharmacophore of lipid-phosphatidylserine (PS) binding peptide-peptoid hybrid PPS1D1

We previously reported a unique peptide-peptoid hybrid, PPS1 that specifically recognizes lipid-phosphatidylserine (PS) and a few other negatively charged phospholipids, but not neutral phospholipids, on the cell membrane. The dimeric version of PPS1, i.e., PPS1D1 triggers strong cancer cell cytotoxicity and has been validated in lung cancer models both in vitro and in vivo. Given that PS and other negatively charged phospholipids are abundant in almost all tumor microenvironments, PPS1D1 is an attractive drug lead that can be developed into a globally applicable anti-cancer agent. Therefore, it is extremely important to identify the minimum pharmacophore of PPS1D1. In this study, we have synthesized alanine/sarcosine derivatives as well as truncated derivatives of PPS1D1. We performed ELISA-like competitive binding assay to evaluate the PS-recognition potential and standard MTS cell viability assay on HCC4017 lung cancer cells to validate the cell cytotoxicity effects of these derivatives. Our studies indicate that positively charged residues at the second and third positions, as well as four hydrophobic residues at the fifth through eighth positions, are imperative for the binding and activity of PPS1D1. Methionine at the first position was not essential, whereas the positively charged Nlys at the fourth position was minimally needed, as two derivatives that were synthesized replacing this residue were almost as active as PPS1D1.

Synthesis, Radiolabeling, and Biological Evaluation of Peptide LIKKPF Functionalized with HYNIC as Apoptosis Imaging Agent

A noninvasive method of detecting exposure of phosphatidylserine (PS) on the external surface of the plasma membrane such as nuclear imaging could assist the diagnosis and therapy of apoptosis related pathologies. The most studied imaging agent for apoptosis is Annexin V so far. Because of limitations of Annexin V other agents have been introduced such as small peptides and molecules. Radiopeptides that have affinity and bind to PS are good candidates for noninvasive imaging of apoptosis. The LIKKPF, introduced by Burtea et al, with nanomolar affinity for PS, was used as templete. The biological properties of LIKKPF radiolabeled with Tc-99 m was assessed in-vitro using apoptotic Jurkat cells and in-vivo using mouse model of liver apoptosis. The radiolabeled LIKKPF with (99m)Tc was stable in human serum at 37˚C for at least 2 h. Results showed that the radiolabeled LIKKPF has less affinity to PS compare to original phage peptide, but high enough for specific binding to apoptotic cells in-vitro and in-vivo. It is concluded that the less affinity of radiolabeled LIKKPF might be attributed to hydrophobicity of peptide. The future peptides should be more hydrophobic compare to LIKKPF.

Advantages of the phosphatidylserine-recognizing peptide PSP1 for molecular imaging of tumor apoptosis compared with annexin V

A number of peptide-based indicators have been identified and reported as potential apoptosis probes, offering great promise for early assessment of therapeutic efficacy in several types of cancer. Direct comparison of the newly developed probes with previously used ones would be an important step in assessing possible applications. Here, we compared the newly identified peptide-based phosphatidylserine (PS) indicator PSP1 (CLSYYPSYC) with annexin V, a common probe for molecular imaging of apoptotic cells, with respect to PS binding kinetics, apoptotic cell-targeting ability, and the efficacy of homing to apoptotic tumor cells in a mouse model after treatment with the anticancer agent camptothecin. Our results indicate that PSP1 efficiently targeted apoptotic cells and generated apoptosis/tumor-specific signals after cancer treatment in the animal model, whereas a similar dose of annexin V showed weak signals. The formation of a stable complex of PSP1 with PS might be one reason for the efficient in vivo targeting. We suggest that PSP1 has potential advantages for in vivo apoptotic cell imaging and could serve as a platform for the development of de novo peptide-based probes for apoptosis.

NMR chemical shift study of the interaction of selected peptides with liposomal and micellar models of apoptotic cells

The interaction between two peptides previously selected by phage display to target apoptotic cells and phospholipidic models of these cells (liposomes or micelles made of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and/or 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS, phosphatidylserine analog) was studied by the simple analysis of the changes induced on the proton NMR chemical shifts of the peptides. Our approach which does not need healthy and/or apoptotic cells for assessing the affinity of different peptides is fast and efficient and requires small amounts of peptide to determine the association constant, the interacting protons, and the number of interaction sites. The micellar model gave more reliable results than the liposomal one. The preferential interaction of the peptide with DPPS was evidenced by the change of the chemical shifts of specific amino acids of the peptides. Our micellar model is thus well suited to mimic apoptotic cells.

Advances in imaging probes and optical microendoscopic imaging techniques for early in vivo cancer assessment

A new chapter in the history of medical diagnosis happened when the first X-ray technology was invented in the late 1800s. Since then, many non-invasive and minimally invasive imaging techniques have been invented for clinical diagnosis to research in cellular biology, drug discovery, and disease monitoring. These imaging modalities have leveraged the benefits of significant advances in computer, electronics, and information technology and, more recently, targeted molecular imaging. The development of targeted contrast agents such as fluorescent and nanoparticle probes coupled with optical imaging techniques has made it possible to selectively view specific biological events and processes in both in vivo and ex vivo systems with great sensitivity and selectivity. Thus, the combination of targeted molecular imaging probes and optical imaging techniques have become a mainstay in modern medicinal and biological research. Many promising results have demonstrated great potentials to translate to clinical applications. In this review, we describe a discussion of employing imaging probes and optical microendoscopic imaging techniques for cancer diagnosis.

Self-assembled glycol chitosan nanoparticles for disease-specific theranostics

Hydrophobically modified glycol chitosan (hGC) conjugates spontaneously form self-assembled nanoparticles (NPs) in aqueous conditions, and glycol chitosan NPs (CNPs) have been extensively studied for the past few decades. For disease-specific theranostics, CNPs could be simply modified with imaging agents, and the hydrophobic domains of hGC are available for encapsulation of various drugs. Based on the excellent physiochemical and biological properties, CNPs have been investigated for multimodal imaging and target specific drug delivery. In particular, a recent application of CNPs has shown great potential as an efficient theranostic system because the CNPs could be utilized for a disease-specific theranostic delivery system of different imaging agents and therapeutics, simultaneously. Furthermore, various therapeutic agents including chemo-drugs, nucleotides, peptides, and photodynamic chemicals could be simply encapsulated into the CNPs through hydrophobic or charge-charge interactions. Under in vivo conditions, the encapsulated imaging agents and therapeutic drugs have been successfully delivered to targeted diseases. In this article, the overall research progress on CNPs is reviewed from early works. The current challenges of CNPs to overcome in theranostics are also discussed, and continuous studies would provide more opportunities for early diagnosis of diseases and personalized medicine.

Optical molecular imaging for diagnosing intestinal diseases

Real-time visualization of the molecular signature of cells can be achieved with advanced targeted imaging techniques using molecular probes and fluorescence endoscopy. This molecular optical imaging in gastrointestinal endoscopy is promising for improving the detection of neoplastic lesions, their characterization for patient stratification, and the assessment of their response to molecular targeted therapy and radiotherapy. In inflammatory bowel disease, this method can be used to detect dysplasia in the presence of background inflammation and to visualize inflammatory molecular targets for assessing disease severity and prognosis. Several preclinical and clinical trials have applied this method in endoscopy; however, this field has just started to evolve. Hence, many problems have yet to be solved to enable the clinical application of this novel method.

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.

Recent advances in the molecular imaging of programmed cell death: part I--pathophysiology and radiotracers

In humans, apoptosis (programmed cell death) is the most common form of cell death after necrosis. Apoptosis is a series of genetically preprogrammed biochemical and morphologic energy-requiring events that, after a specific external or internal stimulus, results in the physiologic disappearance of a cell via its self-disintegration and packaging of its contents into membrane vesicles called apoptotic bodies. Apoptotic bodies can readily be ingested, with their nutrients and even organelles recycled by neighboring cells or phagocytes without local inflammation. In contrast, necrosis is characterized by the primary loss of plasma membrane integrity and the uncontrolled release of a cell's contents, often causing local inflammation, tissue damage, and scarring. Alternate forms of cell death also exist, associated with specific molecular mechanisms involving enzymes, organelles, genes, external stimuli, or blockade of normal cell proliferation. In this review we will briefly outline the molecular mechanisms of apoptosis that can be imaged with radiotracers now under development.

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.

Phage display--a powerful technique for immunotherapy: 1. Introduction and potential of therapeutic applications

One of the most effective molecular diversity techniques is phage display. This technology is based on a direct linkage between phage phenotype and its encapsulated genotype, which leads to presentation of molecule libraries on the phage surface. Phage display is utilized in studying protein-ligand interactions, receptor binding sites and in improving or modifying the affinity of proteins for their binding partners. Generating monoclonal antibodies and improving their affinity, cloning antibodies from unstable hybridoma cells and identifying epitopes, mimotopes and functional or accessible sites from antigens are also important advantages of this technology. Techniques originating from phage display have been applied to transfusion medicine, neurological disorders, mapping vascular addresses and tissue homing of peptides. Phages have been applicable to immunization therapies, which may lead to development of new tools used for treating autoimmune and cancer diseases. This review describes the phage display technology and presents the recent advancements in therapeutic applications of phage display.

Imaging inflammation: molecular strategies to visualize key components of the inflammatory cascade, from initiation to resolution

Dysregulation of inflammation is central to the pathogenesis of innumerable human diseases. Understanding and tracking the critical events in inflammation are crucial for disease monitoring and pharmacological drug discovery and development. Recent progress in molecular imaging has provided novel insights into spatial associations, molecular events and temporal sequelae in the inflammatory process. While remaining a burgeoning field in pre-clinical research, increasing application in man affords researchers the opportunity to study disease pathogenesis in humans in situ thereby revolutionizing conventional understanding of pathophysiology and potential therapeutic targets. This review provides a description of commonly used molecular imaging modalities, including optical, radionuclide and magnetic resonance imaging, and details key advances and translational opportunities in imaging inflammation from initiation to resolution.

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.

Multimodal assessment of early tumor response to chemotherapy: comparison between diffusion-weighted MRI, 1H-MR spectroscopy of choline and USPIO particles targeted at cell death

The aim of this study was to determine the value of different magnetic resonance (MR) protocols to assess early tumor response to chemotherapy. We used a murine tumor model (TLT) presenting different degrees of response to three different cytotoxic agents. As shown in survival curves, cyclophosphamide (CP) was the most efficient drug followed by 5-fluorouracil (5-FU), whereas the etoposide treatment had little impact on TLT tumors. Three different MR protocols were used at 9.4 Tesla 24 h post-treatment: diffusion-weighted (DW)-MRI, choline measurement by (1) H MRS, and contrast-enhanced MRI using ultrasmall iron oxide nanoparticles (USPIO) targeted at phosphatidylserine. Accumulation of contrast agent in apoptotic tumors was monitored by T(2) -weighted images and quantified by EPR spectroscopy. Necrosis and apoptosis were assessed by histology. Large variations were observed in the measurement of choline peak areas and could not be directly correlated to tumor response. Although the targeted USPIO particles were able to significantly differentiate between the efficiency of each cytotoxic agent and best correlated with survival endpoint, they present the main disadvantage of non-specific tumor accumulation, which could be problematic when transferring the method to the clinic. DW-MRI presents a better compromise by combining longitudinal studies with a high dynamic range; however, DW-MRI was unable to show any significant effect for 5-FU. This study illustrates the need for multimodal imaging in assessing tumor response to treatment to compensate for individual limitations.

Synthesis of ApoSense compound [18F]2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoic acid ([18F]NST732) by nucleophilic ring opening of an aziridine precursor

INTRODUCTION

The small molecule 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoic acid (NST732) is a member of the ApoSense family of compounds, capable of selective targeting, binding and accumulation within cells undergoing apoptotic cell death. It has application in molecular imaging and blood clotting particularly for monitoring antiapoptotic drug treatments. We are investigating a fluorine-18-radiolabeled analog of this compound for positron emission tomography studies.

METHODS

We prepared the tosylate precursor methyl 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(tosyloxymethyl)butanoate (4) to synthesize fluorine-18-labeled NST732. Fluorination reaction of the tosylate precursor in 1:1 acetonitrile:dimethylsulfoxide with tetrabutyl ammonium fluoride proceeds through an aziridine intermediate (4A) to afford two regioisomers: 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-fluorobutanoate (5) and methyl 2-(5-(dimethylamino)naphthalene-1-sulfonamido)-2-(fluoromethyl)butanoate (6). Acid hydrolysis of the fluoromethylbutanoate (6) isomer produced NST732. As the fluorination reaction of the tosylate precursor proceeds through an aziridine intermediate (4A) and the fluorination conceivably could be done directly on the aziridine, we have separately prepared an aziridine precursor (4A). Fluorine-18 labeling of the aziridine precursor (4A) was performed with [(18)F]tetrabutyl ammonium fluoride to afford the same two regioisomers (5 and 6). The [18F]2-((5-dimethylamino)naphthalene-1-sulfonamido)methyl)-2-fluorobutanoic acid (NST732) was then obtained by the hydrolysis of corresponding [18F]-labeled ester (6) with 6 N hydrochloric acid.

RESULTS

Two regioisomers obtained from the fluorination reaction of aziridine were easily separated by high-performance liquid chromatography. The total radiochemical yield was 15%±3% (uncorrected, n=18) from the aziridine precursor in a 70-min synthesis time with a radiochemical purity>99%.

CONCLUSION

Fluorine-18-labeled ApoSense compound [18F]NST732 is prepared in moderate yield by direct fluorination of an aziridine precursor.

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.

Multimodality molecular imaging of apoptosis in oncology

OBJECTIVE

The purposes of this review are to describe the signaling pathways of and the cellular changes that occur with apoptosis and other forms of cell death, summarize tracers and modalities used for imaging of apoptosis, delineate the relation between apoptosis and inhibition of protein translation, and describe spectroscopic technologies that entail high-frequency ultrasound and infrared and midinfrared light in characterizing the intracellular events of apoptosis.

CONCLUSION

Apoptosis is a highly orchestrated set of biochemical and morphologic cellular events. These events present many potential targets for the imaging of apoptosis in vivo. Imaging of apoptosis can facilitate early assessment of anticancer treatment before tumor shrinkage, which may increase the effectiveness of delivery of chemotherapy and radiation therapy and speed drug development.

Radiotracers for noninvasive molecular imaging of tumor cell death

The need to monitor cancer therapy-induced cellular and tissue changes using noninvasive imaging techniques continues to stimulate both basic and clinical research. Monitoring changes in cellular proliferative capacity that occur after treatment with radiation and/or chemotherapy has the potential to provide longitudinal information on the cellular dynamics of tumors before, during, and after therapeutic intervention. Cells can lose their reproductive potential through one of several mechanisms, including apoptosis and autophagy (which are forms of programmed cell death), premature senescence, or necrosis. When a tumor responds to therapy, current imaging methods do not provide information about the exact mechanism of cell death executed. We are now beginning to develop the molecular imaging tools that will enable us to noninvasively image cell death mechanisms both in experimental models and in the clinical cancer environment. Studies with these imaging tools will contribute to a better understanding of therapeutic responses and assist in the design and evaluation of more effective treatments. This review examines the state-of-the-art in the use of (radio)tracers for the purpose of imaging mechanisms of tumor cell inactivation (cell death) in animal models and in clinical trials.

In vivo imaging of apoptosis in oncology: an update

In this review, data on noninvasive imaging of apoptosis in oncology are reviewed. Imaging data available are presented in order of occurrence in time of enzymatic and morphologic events occurring during apoptosis. Available studies suggest that various radiopharmaceutical probes bear great potential for apoptosis imaging by means of positron emission tomography and single-photon emission computed tomography (SPECT). However, for several of these probes, thorough toxicologic studies are required before they can be applied in clinical studies. Both preclinical and clinical studies support the notion that 99mTc-hydrazinonicotinamide-annexin A5 and SPECT allow for noninvasive, repetitive, quantitative apoptosis imaging and for assessing tumor response as early as 24 hours following treatment instigation. Bioluminescence imaging and near-infrared fluorescence imaging have shown great potential in small-animal imaging, but their usefulness for in vivo imaging in humans is limited to structures superficially located in the human body. Although preclinical tumor-based data using high-frequency-ultrasonography (US) are promising, whether or not US will become a routinely clinically useful tool in the assessment of therapy response in oncology remains to be proven. The potential of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) for imaging late apoptotic processes is currently unclear. Neither 31P MRS nor 1H MRS signals seems to be a unique identifier for apoptosis. Although MRI-measured apparent diffusion coefficients are altered in response to therapies that induce apoptosis, they are also altered by nonapoptotic cell death, including necrosis and mitotic catastrophe. In the future, rapid progress in the field of apoptosis imaging in oncology is expected.

Phage display and molecular imaging: expanding fields of vision in living subjects

In vivo molecular imaging enables non-invasive visualization of biological processes within living subjects, and holds great promise for diagnosis and monitoring of disease. The ability to create new agents that bind to molecular targets and deliver imaging probes to desired locations in the body is critically important to further advance this field. To address this need, phage display, an established technology for the discovery and development of novel binding agents, is increasingly becoming a key component of many molecular imaging research programs. This review discusses the expanding role played by phage display in the field of molecular imaging with a focus on in vivo applications. Furthermore, new methodological advances in phage display that can be directly applied to the discovery and development of molecular imaging agents are described. Various phage library selection strategies are summarized and compared, including selections against purified target, intact cells, and ex vivo tissue, plus in vivo homing strategies. An outline of the process for converting polypeptides obtained from phage display library selections into successful in vivo imaging agents is provided, including strategies to optimize in vivo performance. Additionally, the use of phage particles as imaging agents is also described. In the latter part of the review, a survey of phage-derived in vivo imaging agents is presented, and important recent examples are highlighted. Other imaging applications are also discussed, such as the development of peptide tags for site-specific protein labeling and the use of phage as delivery agents for reporter genes. The review concludes with a discussion of how phage display technology will continue to impact both basic science and clinical applications in the field of molecular imaging.

In vivo optical imaging of acute cell death using a near-infrared fluorescent zinc-dipicolylamine probe

Cell death is a fundamental biological process that is present in numerous disease pathologies. Fluorescent probes that detect cell death have been developed for a myriad of research applications ranging from microscopy to in vivo imaging. Here we describe a synthetic near-infrared (NIR) conjugate of zinc(II)-dipicolylamine (Zn²+-DPA) for in vivo imaging of cell death. Chemically induced in vivo models of myopathy were established using an ionphore, ethanol, or ketamine as cytotoxins. The Zn²+-DPA fluorescent probe or corresponding control was subsequently injected, and whole animal fluorescence imaging demonstrated probe uptake at the site of muscle damage, which was confirmed by ex vivo and histological analyses. Further, a comparative study with a NIR fluorescent conjugate Annexin V showed less intense uptake at the site of muscle damage and high accumulation in the bladder. The results indicate that the fluorescent Zn²+-DPA conjugate is an effective probe for in vivo cell death detection and in some cases may be an appropriate alternative to fluorescent Annexin V conjugates.

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.

Noninvasive imaging of cell death using an Hsp90 ligand

Cell death plays a central role in normal physiology and in disease. Common to apoptotic and necrotic cell death is the eventual loss of plasma membrane integrity. We have produced a small organoarsenical compound, 4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid, that rapidly accumulates in the cytosol of dying cells coincident with loss of plasma membrane integrity. The compound is retained in the cytosol predominantly by covalent reaction with the 90 kDa heat shock protein (Hsp90), the most abundant molecular chaperone of the eukaryotic cytoplasm. The organoarsenical was tagged with either optical or radioisotope reporting groups to image cell death in cultured cells and in murine tumors ex vivo and in situ. Tumor cell death in mice was noninvasively imaged by SPECT/CT using an (111)In-tagged compound. This versatile compound should enable the imaging of cell death in most experimental settings.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Apoptosis imaging: beyond annexin V

Induction of apoptosis is the primary mechanism through which most chemotherapies cause tumor cell death. Early assessment of tumor response is required to manage patients in terms of quality of life versus intensive chemotherapy. Although imaging with radiolabeled annexin V has been intensively investigated, it is still not sufficiently mature for clinical application. This article will summarize various alternative imaging techniques for visualization of phosphatidylserine externalization, activity of caspases, and mitochondrial membrane potential. Such imaging studies will promote the identification of novel molecular targets and the development of highly specific apoptosis-detecting imaging probes with potential clinical applications. It is highly possible that quantitative imaging of apoptosis will greatly improve clinical decision making in apoptosis-related diseases.

In vivo imaging of tumor apoptosis using histone H1-targeting peptide

In vivo imaging of apoptosis could allow monitoring of tumor response to cancer treatments such as chemotherapy. Using phage display, we identified the CQRPPR peptide, named ApoPep-1(Apoptosis-targeting Peptide-1), that was able to home to apoptotic and necrotic cells in tumor tissue. ApoPep-1 also bound to apoptotic and necrotic cells in culture, while only little binding to live cells was observed. Its binding to apoptotic cells was not dependent on calcium ion and not competed by annexin V. The receptor for ApoPep-1 was identified to be histone H1 that was exposed on the surface of apoptotic cells. In necrotic cells, ApoPep-1 entered the cells and bound to histone H1 in the nucleus. The imaging signals produced during monitoring of tumor apoptosis in response to chemotherapy was enhanced by the homing of a fluorescent dye- or radioisotope-labeled ApoPep-1 to tumor treated with anti-cancer drugs, whereas its uptake of the liver and lung was minimal. These results suggest that ApoPep-1 holds great promise as a probe for in vivo imaging of apoptosis, while histone H1 is a unique molecular signature for this purpose.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

The conserved histidine in epidermal growth factor-like domains of stabilin-2 modulates pH-dependent recognition of phosphatidylserine in apoptotic cells

Clearance of apoptotic cells is involved in the resolution of inflammation, and this mechanism is controlled by the regulation of pro- and anti-inflammatory cytokine production during the ingestion of apoptotic cells. Inflamed areas show extracellular acidity, and low pH stimulates cellular functions of immune cells. However, little is known about the influence of extracellular acidic pH on the function of phagocytic cells. In this study, we showed that stabilin-2-mediated phagocytosis is activated in low pH media (pH 6.8) and examined the molecular mechanisms underlying this pH-dependent enhancement of phagocytic activity. Stabilin-2, which is expressed in human monocyte derived macrophages (HMDM), is a phosphatidylserine (PS) receptor that mediates phagocytosis of apoptotic cells, and releases the anti-inflammatory cytokine, TGF-beta. The PS binding activity of stabilin-2 is enhanced in low pH, and a conserved histidine(1403) in close proximity to the PS binding loop is critical for pH-dependent activity. We propose that protonation of His(1403) may rearrange the PS binding loop to enhance binding affinity in low pH, indicating that acidic pH might act as a danger signal to stimulate stabilin-2-mediated phagocytosis to resolve inflammation. Considering that phosphatidylserine is an important target molecule for apoptotic cells in the acidic microenvironment of inflammation and tumors, our results also have implications for pH sensitive targeting of apoptotic cells.

Efficient identification of tubby-binding proteins by an improved system of T7 phage display

Mutation in the tubby gene causes adult-onset obesity, progressive retinal, and cochlear degeneration with unknown mechanism. In contrast, mutations in tubby-like protein 1 (Tulp1), whose C-terminus is highly homologous to tubby, only lead to retinal degeneration. We speculate that their diverse N-terminus may define their distinct disease profile. To elucidate the binding partners of tubby, we used tubby N-terminus (tubby-N) as bait to identify unknown binding proteins with open-reading-frame (ORF) phage display. T7 phage display was engineered with three improvements: high-quality ORF phage display cDNA library, specific phage elution by protease cleavage, and dual phage display for sensitive high throughput screening. The new system is capable of identifying unknown bait-binding proteins in as fast as approximately 4-7 days. While phage display with conventional cDNA libraries identifies high percentage of out-of-frame unnatural short peptides, all 28 tubby-N-binding clones identified by ORF phage display were ORFs. They encode 16 proteins, including 8 nuclear proteins. Fourteen proteins were analyzed by yeast two-hybrid assay and protein pull-down assay with ten of them independently verified. Comparative binding analyses revealed several proteins binding to both tubby and Tulp1 as well as one tubby-specific binding protein. These data suggest that tubby-N is capable of interacting with multiple nuclear and cytoplasmic protein binding partners. These results demonstrated that the newly-engineered ORF phage display is a powerful technology to identify unknown protein-protein interactions.