Imaging Delivery Research Articles

Real-time and non-invasive optical imaging of tumor-targeting glycol chitosan nanoparticles in various tumor models

Recently, various nanoparticle systems have been developed for tumor-targeted delivery of imaging agents or drugs. However, large amount of them still have insufficient tumor accumulation and this limits their further clinical applications. Moreover, the in vivo characteristics of nanoparticles have been largely unknown, because there are few proper technologies to achieve the direct and non-invasive characterization of nanoparticles in live animals. In this paper, we determined the key factors of nanoparticles for in vivo tumor-targeting using our glycol chitosan nanoparticles (CNPs) which have proved their tumor-targeting ability in many previous papers. For this study, CNPs were labeled with near-infrared fluorescence (NIRF) dye, Cy5.5 for in vivo analysis by non-invasive optical imaging techniques. With these Cy5.5-CNPs, the factors such as in vitro/in vivo stability, deformability, and rapid uptake into target tumor cells and their effects on in vivo tumor-targeting were evaluated in various tumor-bearing mice models. In flank tumor models, Cy5.5-CNPs were selectively localized in tumor tissue than other organs, and the real-time intravascular tracking of CNPs proved the enhanced permeation and retention (EPR) effect of nanoparticles in tumor vasculature. Importantly, tumor-targeting CNPs showed an excellent tumor-specificity in brain tumors, liver tumors, and metastasis tumor models, indicating their great potential in both cancer imaging and therapy.

Protein Cage Nanoparticles Bearing the LyP-1 Peptide for Enhanced Imaging of Macrophage-Rich Vascular Lesions

Cage-like protein nanoparticles are promising platforms for cell- and tissue-specific targeted delivery of imaging and therapeutic agents. Here, we have successfully modified the 12 nm small heat shock protein from Methanococcus jannaschii (MjHsp) to detect atherosclerotic plaque lesions in a mouse model system. As macrophages are centrally involved in the initiation and progression of atherosclerosis, targeted imaging of macrophages is valuable to assess the biologic status of the blood vessel wall. LyP-1, a nine residue peptide, has been shown to target tumor-associated macrophages. Thus, LyP-1 was genetically incorporated onto the exterior surface of MjHsp, while a fluorescent molecule (Cy5.5) was conjugated on the interior cavity. This bioengineered protein cage, LyP-Hsp, exhibited enhanced affinity to macrophage in vitro. Furthermore, in vivo injection of LyP-Hsp allowed visualization of macrophage-rich murine carotid lesions by in situ and ex vivo fluorescence imaging. These results demonstrate the potential of LyP-1-conjugated protein cages as nanoscale platforms for delivery of imaging agents for the diagnosis of atherosclerosis.

A Novel Peptide Probe for Imaging and Targeted Delivery of Liposomal Doxorubicin to Lung Tumor

Targeted delivery of imaging agents and therapeutics to tumors would provide early detection and increased therapeutic efficacy against cancer. Here we have screened a phage-displayed peptide library to identify peptides that selectively bind to lung tumor cells. Evaluation of individual phage clones after screening revealed that a phage clone displaying the CSNIDARAC peptide bound to H460 lung tumor cells at higher extent than other phage clones. The synthetic CSNIDARAC peptide strongly bound to H460 cells and was efficiently internalized into the cells, while little binding of a control peptide was seen. It also preferentially bound to other lung tumor cell lines as compared to cells of different tumor types. In vivo imaging of lung tumor was achieved by homing of fluorescence dye-labeled CSNIDARAC peptide to the tumor after intravenous injection into mice. Ex vivo imaging and microscopic analysis of isolated organs further demonstrated the targeting of CSNIDARAC peptide to tumor. The CSNIDARAC peptide-targeted and doxorubicin-loaded liposomes inhibited the tumor growth more efficiently than untargeted liposomes or free doxorubicin. In vivo imaging of fluorescence dye-labeled liposomes demonstrated selective homing of the CSNIDARAC-liposomes to tumor. In the same context, higher levels of doxorubicin and apoptosis in tumor tissue were observed when treated with the targeted liposomes than untargeted liposomes or free doxorubicin. These results suggest that the CSNIDARAC peptide is a promising targeting probe that is able to direct imaging agents and therapeutics to lung tumor.

Human ferritin cages for imaging vascular macrophages

Atherosclerosis is a leading cause of death worldwide. Macrophages are key components of vascular inflammation, which contributes to the development and complications of atherosclerosis. Ferritin, an iron storage and transport protein, has been found to accumulate in macrophages in human atherosclerotic plaques. We hypothesized that ferritin could serve as an intrinsic nano-platform to target delivery of imaging agents to vascular macrophages to detect high-risk atherosclerotic plaques. Here we show that engineered human ferritin protein cages, either conjugated to the fluorescent Cy5.5 molecule or encapsulating a magnetite nanoparticle, are taken up in vivo by macrophages in murine atherosclerotic carotid arteries and can be imaged by fluorescence and magnetic resonance imaging. These results indicate that human ferritin can serve as a nanoparticle platform to image vascular inflammation in vivo.

Development of a liposomal delivery system for temperature-triggered release of a tumor targeting agent, Ln(III)-DOTA-phenylboronate

Liposomes, capable of temperature-triggered content release at the site of interest, can be of great importance for imaging and therapy of tumors. The delivery of imaging agents or therapeutics can be improved by application of liposomes with a gel-to-liquid phase-transition temperature suitable for mild hyperthermia (41-43°C), and by prolonging their circulation time by incorporation of lipids containing polyethyleneglycol moieties. Still, the rapid wash out of the delivered material from the tumor tissue is a major obstacle for both imaging and therapy. In this study, we developed an optimized temperature sensitive liposomal system to be used with mild hyperthermia: highly stable at physiological temperature and with a sharp transition of the bilayer at 41.5°C, with subsequent rapid release of entrapped compounds such as calcein or tumor cell-targeting contrast agents. Intravital microscopy on calcein/rhodamine containing liposomes was applied to demonstrate the applicability of this system in vivo. The calcein loaded liposomes were injected iv into nude mice with a human BLM melanoma tumor implanted in a dorsal skin-fold window chamber. Arrival of the liposomes at the tumor site and content release after temperature increase were monitored. The results demonstrated not only accumulation of the liposomes at the tumor site, but also a massive release of calcein after increase of the temperature to 41°C. The versatility of the thermosensitive liposomes was further demonstrated by encapsulation of a tumor cell-targeting DOTA-phenylboronate conjugate and its release at elevated temperatures. The DOTA ligand in this system is able to chelate a variety of metals suitable for both diagnostic and therapeutic applications, whereas the phenylboronate function is able to target specifically to tumor cells through a covalent binding with sialic acid moieties over-expressed on their surface upon heat-triggered release from the liposomal carrier.

Tumour-targeting properties of antibodies specific to MMP-1A, MMP-2 and MMP-3

Matrix metalloproteinases (MMPs), a group of more than 20 zinc-containing endopeptidases, are upregulated in many diseases, but several attempts to use radiolabelled MMP inhibitors for imaging tumours have proved unsuccessful in mouse models, possibly due to the limited specificity of these agents or their unfavourable pharmacokinetic profiles. In principle, radiolabelled monoclonal antibodies could be considered for the selective targeting and imaging of individual MMPs. We cloned, produced and characterized high-affinity monoclonal antibodies specific to murine MMP-1A, MMP-2 and MMP-3 in SIP (small immunoprotein) miniantibody format using biochemical and immunochemical methods. We also performed comparative biodistribution analysis of their tumour-targeting properties at three time points (3 h, 24 h, 48 h) in mice bearing subcutaneous F9 tumours using radioiodinated protein preparations. The clinical stage L19 antibody, specific to the alternatively spliced EDB domain of fibronectin, was used as reference tumour-targeting agent for in vivo studies. All anti-MMP antibodies and SIP(L19) strongly stained sections of F9 tumours when assessed by immunofluorescence methods. In biodistribution experiments, SIP(SP3), specific to MMP-3, selectively accumulated at the tumour site 24 and 48 h after intravenous injection, but was rapidly cleared from other organs. By contrast, SIP(SP1) and SIP(SP2), specific to MMP-1A and MMP-2, showed no preferential accumulation at the tumour site. Antibodies specific to MMP-3 may serve as vehicles for the efficient and selective delivery of imaging agents or therapeutic molecules to sites of disease.

A Novel Biotechnological Approach for Targeted Regenerative Cell Therapy and Molecular Imaging of Atherothrombosis

Targeted delivery increases the efficacy of drugs and regenerative cell therapy while reducing the dose required along with negative side effects. Similarly, targeted delivery of imaging agents will localise and retain them at the disease sites, enhancing the sensitivity and accuracy of current imaging techniques. Here we report a novel method employing Staphylococcus aureus sortase A enzyme for the conjugation of a single-chain antibody (scFv), anti-GPIIb/IIIa-scFv to nanoparticles and cells for imaging and cell delivery in cardiovascular disease. This scFv specifically binds to activated platelets, which play a pivotal role in atherosclerosis, thrombosis and inflammation. The scFv was successfully conjugated to magnetic particles of iron oxide, which are powerful contrast agents for MR imaging and also to model cells (CHO). The conjugation efficiency was between 50–70% and the bioactivity of the scFv after coupling was preserved. The targeting of scFv-coupled CHO cells and nanoparticles to activated platelets was strong as demonstrated in in vitro static adhesion assay, under shear stress in a flow chamber system and in intravital microscopy experiments. This unique biotechnological approach combining biological and chemical coupling provides a highly useful tool for regenerative cell therapy (e.g. stem cells) as well as targeted molecular imaging in cardiovascular disease and beyond. View Large Image Figure Viewer Download Hi-res image

Top-down particle fabrication: control of size and shape for diagnostic imaging and drug delivery.

This review discusses rational design of particles for use as therapeutic vectors and diagnostic imaging agent carriers. The emerging importance of both particle size and shape is considered, and the adaptation and modification of soft lithography methods to produce nanoparticles are highlighted. To this end, studies utilizing particles made via a process called Particle Replication In Non-wetting Templates are discussed. In addition, insights gained into therapeutic cargo and imaging agent delivery from related types of polymer-based carriers are considered.

Steroid Receptors as Molecular Targets for Cancer Diagnosis and Therapy

The targeted delivery of chemotherapeutics is essential to minimizing the side effects by sparing the normal tissues by exposure to cytotoxic agents. The various receptors are overexpressed on the surface of the tumor tissues as compared to the normal tissues because their requirement for various essential components is high for the growth and development of tumor tissues. These overexpressed receptors on the tumor surface have been investigated for selective delivery of imaging, cytotoxic, or other therapeutic agents. Hormonal receptors viz. peptide and steroid receptors (SRs), which are present over the tumor surface, are also used for tumor targeting of various molecules for therapeutic use or imaging purpose. The key role of SRs is in the transactivation step of protein synthesis and to maintain the homeostasis in normal and diseased states. These receptors are overactivated in different diseases, including tumorigenesis. Hence, these SRs may act as a potential target for selective delivery of different therapeutic or cytotoxic agents. The selective delivery of these agents may be a better treatment strategy for endocrine cancer because it results in cytosolic and nuclear delivery of cytotoxic agents as well. The objective of the present review is to explore the importance of sex SRs as a vital target in endocrine cancer therapy. Potential strategies investigated for the delivery of imaging agents, chemotherapeutics, and toxins via steroidal ligands have been discussed. The importance of gene therapy in different diseases, including endocrine cancer, is well defined; thus, some effective gene delivery approaches investigated in endocrine cancer via steroidal ligands have also been discussed.

Folate Receptor Saturation, Recycling, and Release Kinetics in Activated Macrophages

Activated macrophages over-express a receptor for the vitamin, folic acid. Conjugation of folic acid to low molecular weight drugs, genes, liposomes, nanoparticles, and imaging agents does not significantly compromise the vitamin's affinity for its receptor, thus facilitating both therapeutic and imaging agents to be targeted as folate conjugates to activated macrophages. In these studies the in vivo kinetics and binding characteristics of the folate receptor on macrophages has been characterized with the objective of optimizing targeted drug delivery. Our results show that saturation of the receptor in inflamed tissues can be reached at a dose of 100 nmol/kg. Furthermore, the rapid recycling characteristics of the receptor (every 10–20 minutes) suggests that frequent dosing will allow for maximal uptake of folate conjugates by activated macrophages accumulated in inflamed tissues. Finally, the short circulation half-life (30 minutes) of the water-soluble folate conjugate used in this study minimizes the possibility of nonspecific uptake by receptor-negative tissues. The kinetic observations made in this study can be utilized to optimize the therapeutic efficacy and imaging agent delivery of folate conjugates to activated macrophages in vivo.

Quantum Dots Bearing Lectin-Functionalized Nanoparticles as a Platform for In Vivo Brain Imaging

Delivery of imaging agents to the brain is highly important for the diagnosis and treatment of central nervous system (CNS) diseases, as well as the elucidation of their pathophysiology. Quantum dots (QDs) provide a novel probe with unique physical, chemical, and optical properties, and become a promising tool for in vivo molecular and cellular imaging. However, their poor stability and low blood-brain barrier permeability severely limit their ability to enter into and act on their target sites in the CNS following parenteral administration. Here, we developed a QDs-based imaging platform for brain imaging by incorporating QDs into the core of poly(ethylene glycol)-poly(lactic acid) nanoparticles, which was then functionalized with wheat germ agglutinin and delivered into the brain via nasal application. The resulting nanoparticles, with high payload capacity, are water-soluble, stable, and showed excellent and safe brain targeting and imaging properties. With PEG functional terminal groups available on the nanoparticles surface, this nanoprobe allows for conjugation of various biological ligands, holding considerable potential for the development of specific imaging agents for various CNS diseases.

Rhodamine-loaded poly(lactic-co-glycolic acid) nanoparticles for investigation of in vitro interactions with breast cancer cells

Nanoparticle-based drug delivery systems are considered promising for the delivery of imaging agents and drugs for the detection and treatment of illnesses, including cancer. Investigation of nanoparticle interactions with the diseased cells can lead to better designs. In this work, poly(lactic-co-glycolic acid) nanoparticles loaded with rhodamine 6G were prepared by nanoprecipitation with high encapsulation efficiency. In vitro release studies demonstrated that rhodamine escaped from the nanoparticles at a very slow rate at physiological pH, thus making it ideal for imaging studies. At acidic pH this agent was released quickly, suggesting charge interactions between the polymer and rhodamine. Microscopy and flow cytometry studies show higher uptake in MDA-MB-231 breast cancer cells when exposed to rhodamine-loaded nanoparticles than to rhodamine in solution.

Signal ampflication using nanoplatform cluster formation

Multifunctional nanoplatforms are being developed for targeted delivery of imaging and therapeutic agents. The nanoscale size of these platforms allows a large carrying capacity, which translates to a high ratio of targeted agent for each ligand–receptor interaction. Here we used a simple in vitro system to show that the targeting ratio can be further amplified by controlled cluster formation using multiple cycles of coupling reactions. As the receptor sites become more sparse the relationship between the amplification of the signal from an imaging agent and the reaction cycles approaches a cubic relationship, which is the theoretical limit for this relationship. These results demonstrate that this approach has the potential to significantly enhance the capacity of multifunctional platforms to be used for targeted delivery of imaging agents or drugs.

The Extra-domain A of Fibronectin Is a Vascular Marker of Solid Tumors and Metastases

One of the most promising new avenues for the development of more selective and efficacious cancer therapies relies on the antibody-mediated targeted delivery of bioactive agents (e.g., cytokines) to the tumor environment. The identification of quantitative differences in the expression of accessible vascular proteins in metastatic lesions and host organs facilitate the development of antibody-based strategies, which should be highly efficient and selective, considering the fact that an over-exuberant neovasculature is a characteristic feature of aggressive cancers, and that tumor blood vessels are readily accessible for i.v. administered therapeutic agents. Metastasis is the main cause of death in cancer. The availability of metastasis-specific antigens accessible from the bloodstream will allow a selective delivery of therapeutic agents to metastatic lesions using antibodies as vehicles. Using a combination of vascular biotinylation of 129Sv mice bearing F9 liver metastases and mass spectrometry, we have identified 435 accessible proteins in metastasis and host organ specimens, of which 117 were exclusively detected in metastases. In particular, we found that the alternatively spliced extra-domain A (EDA) of fibronectin is strongly expressed in the neovasculature of liver metastases, while being undetectable in most normal organs. A human antibody to EDA was used to show EDA expression in the neovasculature of metastases and primary tumors of human cancer patients and to target mouse liver metastases and subcutaneous tumors in vivo. Human antibody fragments specific to the EDA domain of fibronectin promise to serve as general vehicles for the efficient and selective delivery of imaging agents or therapeutic molecules to metastatic sites.

Folate-mediated tumor cell uptake of quantum dots entrapped in lipid nanoparticles

Quantum dots (QDs) are fluorescent semiconductor nanocrystals with superior optical properties compared to organic dyes currently undergoing rapid development for biological applications, particularly in fluorescence imaging. The folate receptor, overexpressed in a broad spectrum of malignant tumors, is an attractive target for selective delivery of imaging agents to tumor cells. This study examines nanoparticles containing QDs entrapped in a lipid shell, and post-loaded with a folate-lipid conjugate for targeting to mouse and human tumor cells expressing the folate receptor. Hydrophobic QDs were mixed with 1,2 dipalmitoyl-sn-glycero-3 phosphocholine and methoxy–polyethylene–glycol–distearoyl–phosphatidyl–ethanolamine (mPEG-DSPE) generating a nanoparticle referred to as lipodot, with a mean diameter size of ∼ 100 nm. Folate-derivatized PEG-DSPE was post-loaded into the lipodots at 0.5% lipid molar concentration. Mouse J6456 lymphoma cells (J6456-FR) and human head and neck KB cancer cells (KB-FR), up-regulated for their folate receptors, were incubated with folate-targeted and non-targeted lipodots in vitro. Using fluorescence microscopy, it was found that only folate-targeted lipodots were taken up by tumor cells. Confocal depth scanning showed substantial internalization. Confirming the specificity of folate-targeted lipodots, binding and internalization were inhibited by free folate, and no uptake was found in a folate-receptor negative cell line. Selective binding and uptake of folate-targeted lipodots by J6456-FR cells was also observed in vivo after intra-peritoneal injection in mice bearing ascitic J6456-FR tumors based on FACS analysis and confocal imaging of harvested cells from the peritoneal cavity. Folate-targeted lipodots represent an attractive approach for tumor cell labeling both in vitro and in vivo.

Antinucleosome Antibody-Modified Liposomes and Lipid-Core Micelles for Tumor-Targeted Delivery of Therapeutic and Diagnostic Agents

Liposomes and lipid-core micelles prepared of polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugates have been modified with nucleosome-specific monoclonal antinuclear autoantibody (ANA) 2C5 (mAb 2C5) specifically recognizing a broad variety of cancer cells through the cancer cell surface-bound nucleosomes. mAb 2C5 preserves its specific properties upon the binding with the lipid-based pharmaceutical nanocarriers, and 2C5-modified immunoliposomes and immunomicelles demonstrate an enhanced binding with tumor cells both in vitro and in vivo. We have investigated the delivery of therapeutic and diagnostic agents with such tumor-targeted immunoliposomes and immunomicelles to various tumors in vivo and in vitro. Both lipid-based nanocarriers provided enhanced tumor delivery of imaging agents (111In) and antitumor drugs (doxorubicin and photodynamic therapy agents) to tumor cells under different experimental settings. Pharmaceutical lipid-based nanoparticular carriers modified with mAb 2C5 could represent universal systems for tumor-specific delivery of various soluble and insoluble pharmaceuticals.

Vitamin B12 Transport Proteins: Crystallographic Analysis of β‐axial Ligand Substitutions in Cobalamin Bound to Transcobalamin

Cobalamin (Cbl, vitamin B12) is an essential micronutrient that is synthesized only by bacteria. Mammals have developed a complex system for internalization of this vitamin from the diet. Three binding proteins (haptocorrin, intrinsic factor, transcobalamin (TC)) and several specific cell surface receptors are involved in the process of intestinal absorption, plasma transport and cellular uptake. The recent literature on the binding proteins is briefly reviewed. A structural study is presented addressing a unique feature of TC among the three proteins, i.e., the displacement of the weak Co(III)-ligand H2O at the upper (or beta) axial side of H2O-Cbl by a histidine side chain. We have investigated crystallographically the beta-ligand exchange on Cbl bound to TC by crystallization of bovine holo-TC in the presence of either cyanide or sulfite. The resulting electron density maps show that the histidine side chain has been displaced by an exogenous ligand CN(-) or SO(3)(-2)to a lower extent than expected based on their higher affinity for Co and excess concentration with respect to histidine. This may reflect either reduced affinities of CN(-) and SO(3)(-2)or the advantageous binding of the protein-integrated His-residue when competing for the beta-site of Cbl bound to TC. The loop hosting the histidine residue appears more flexible after disruption of the coordination bond His-Cbl but no other differences are observed in the overall structure of holo-TC. These structural results are discussed in relation to a possible physiological role of histidine substitution for H2O and regarding the role of beta-conjugated Cbl-analogues recently proposed for targeted delivery of imaging agents.

Enhanced accumulation of long-circulating liposomes modified with the nucleosome-specific monoclonal antibody 2C5 in various tumours in mice: gamma-imaging studies

To further improve tumour targeting and delivery of imaging agents by long-circulating liposomes via the coupling of the anti-cancer monoclonal antibody 2C5 with nucleosome-restricted activity, which can recognize the surface of various tumours but not normal cells and can specifically target pharmaceutical carriers to tumour cells in vitro and in vivo. The 2C5 antibody was attached to the surface of long-circulating PEG-liposomes (LCL) by the post-insertion technique after antibody modification with a single-terminus activated PEG-lipid derivative to yield nucleosome-specific tumour-targeted liposomes. Tumour cell binding of the targeted liposomes was verified both by fluorescence microscopy and by flow cytometry in several cell lines using fluorescently labelled liposomes. 111In-radiolabelled liposomal formulations (prepared using membrane-anchored chelating groups) were used to examine in vivo biodistribution and tumour accumulation of liposomes by direct gamma scintigraphy. The 2C5 antibody-modified LCL demonstrated a three- to eightfold increase in in vitro specific cell binding to various cancer cell lines of diverse origin. 111In-labelled tumour-targeted liposomes demonstrated prolonged circulation and doubled tumour accumulation compared with that of control formulations. Whole-body gamma scintigraphic imaging of mice implanted with different tumours revealed markedly faster (6 h post injection for 2C5-LCL vs 24 h for non-specific analogues) and superior in vivo tumour visualization with 111In-2C5-LCL than with the 2C5-free formulations in tested tumour models. The 2C5 antibody-modified LCL effectively and specifically accumulate in various tumours and can serve as delivery vehicles for imaging agents, allowing for fast and efficient tumour visualization.

Melanoma and Lymphocyte Cell-Specific Targeting Incorporated into a Heat Shock Protein Cage Architecture

Protein cages, including viral capsids, ferritins, and heat shock proteins (Hsps), can serve as nanocontainers for biomedical applications. They are genetically and chemically malleable platforms, with potential as therapeutic and imaging agent delivery systems. Here, both genetic and chemical strategies were used to impart cell-specific targeting to the Hsp cage from Methanococcus jannaschii. A tumor vasculature targeting peptide was incorporated onto the exterior surface of the Hsp cage. This protein cage bound to alpha(v)beta(3) integrin-expressing cells. Cellular tropism was also imparted by conjugating anti-CD4 antibodies to the exterior of Hsp cages. These Ab-Hsp cage conjugates specifically bound to CD4(+) cells. Protein cages have the potential to simultaneously incorporate multiple functionalities, including cell-specific targeting, imaging, and therapeutic agent delivery. We demonstrate the simultaneous incorporation of two functionalities, imaging and cell-specific targeting, onto the Hsp protein cage.

Breaching Biological Barriers: Protein Translocation Domains as Tools for Molecular Imaging and Therapy

The lipid bilayer of a cell presents a significant barrier for the delivery of many molecular imaging reagents into cells at target sites in the body. Protein translocation domains (PTDs) are peptides that breach this barrier. Conjugation of PTDs to imaging agents can be utilized to facilitate the delivery of these agents through the cell wall, and in some cases, into the cell nucleus, and have potential for in vitro and in vivo applications. PTD imaging conjugates have included small molecules, peptides, proteins, DNA, metal chelates, and magnetic nanoparticles. The full potential of the use of PTDs in novel in vivo molecular probes is currently under investigation. Cells have been labeled in culture using magnetic nanoparticles derivatized with a PTD and monitored in vivo to assess trafficking patterns relative to cells expressing a target antigen. In vivo imaging of PTD-mediated gene transfer to cells of the skin has been demonstrated in living animals. Here we review several natural and synthetic PTDs that have evolved in the quest for easier translocation across biological barriers and the application of these peptide domains to in vivo delivery of imaging agents.