Peptides and peptide hormones for molecular imaging and disease diagnosis

Targeted PDT agent eradicates TrkC expressing tumors via photodynamic therapy (PDT)

This contribution features a small molecule that binds TrkC (tropomyosin receptor kinase C) receptor that tends to be overexpressed in metastatic breast cancer cells but not in other breast cancer cells. A sensitizer for ¹O₂ production conjugated to this structure gives 1-PDT for photodynamic therapy. Isomeric 2-PDT does not bind TrkC and was used as a control throughout; similarly, TrkC– cancer cells were used to calibrate enhanced killing of TrkC+ cells. Ex vivo, 1- and 2-PDT where only cytotoxic when illuminated, and 1-PDT, gave higher cell death for TrkC+ breast cancer cells. A 1 h administration-to-illumination delay gave optimal TrkC+/TrkC–-photocytotoxicity, and distribution studies showed the same delay was appropriate in vivo. In Balb/c mice, a maximum tolerated dose of 20 mg/kg was determined for 1-PDT. 1- and 2-PDT (single, 2 or 10 mg/kg doses and one illumination, throughout) had similar effects on implanted TrkC– tumors, and like those of 2-PDT on TrkC+ tumors. In contrast, 1-PDT caused dramatic TrkC+ tumor volume reduction (96% from initial) relative to the TrkC– tumors or 2-PDT in TrkC+ models. Moreover, 71% of the mice treated with 10 mg/kg 1-PDT (n = 7) showed full tumor remission and survived until 90 days with no metastasis to key organs.

Relationship between apoptosis imaging and radioiodine therapy in tumor cells with different sodium iodide symporter gene expression

The therapeutic efficacy of radioiodine (¹³¹I) therapy has been reported to be variable among cancer patients and even between metastatic regions in the same patients. Because the expression level of sodium iodide symporter (NIS) cannot reflect the efficacy of therapy, other strategies are required to predict the precise therapeutic effect of ¹³¹I therapy. In this research, we investigated the correlation between iodine (I) uptake, apoptosis imaging, and therapeutic efficacy. Two HT29 cell lines, cytomegalovirus (CMV)-NIS (or NIS+++) and TERT-NIS (or NIS+), were established by retroviral transfection. I uptake was estimated by I-uptake assay and gamma camera imaging. Apoptosis was evaluated by confocal microscopy and a Maestro fluorescence imaging system (CRi Inc., Woburn, MA) using ApoFlamma (BioACTs, Seoul, Korea), a fluorescent dye-conjugated apoptosis-targeting peptide 1 (ApoPep-1). Therapeutic efficacy was determined by tumor size. The CMV-NIS showed higher I uptake and ApoFlamma signals than TERT-NIS. In xenograft models, CMV-NIS also showed high 99m technetium signals and ApoFlamma signals. Tumor reduction had a stronger correlation with apoptosis imaging signals than with gamma camera imaging signals, which reflect I uptake. Higher NIS-expressing tumors showed increased apoptosis and I uptake, resulting in a significant tumor reduction. Moreover, tumor reduction showed a strong correlation with ApoFlamma imaging compared to I-uptake imaging.

Atomic layer deposition of L-alanine polypeptide

L-Alanine polypeptide thin films were synthesized via atomic layer deposition (ALD). Instead of using an amino acid monomer as the precursor, an L-alanine amino acid derivatized with a protecting group was used to prevent self-polymerization, increase the vapor pressure, and allow linear cycle-by-cycle growth emblematic of ALD. The successful deposition of a conformal polypeptide film has been confirmed by FTIR, TEM, and Mass Spectrometry, and the ALD process has been extended to polyvaline.

[Application of target peptide in siRNA delivery for the research of lung cancer therapy]

肺癌被认为是全球发病率最高的一种恶性肿瘤，其对化疗药物不敏感且易产生耐药性，因此增强肺癌药物治疗效果是近年来研究的热点。siRNA是一种小RNA分子，可以沉默与之互补的目标mRNA，是一种基因治疗手段。靶向肽是一类小分子多肽，它可以与siRNA联合使用，利用其与肿瘤表面物质特异性结合发挥靶向作用。联合靶向肽的特异性和siRNA的治疗作用，增加siRNA在靶点位置的聚集，增强沉默效果，可以提高肺癌对药物的敏感性并降低耐药作用，进而增强肺癌治疗效果，为肺癌的靶向治疗提供新的方向和策略。本文将对靶向肽在递送siRNA进行肺癌治疗研究中的应用作一简要综述。

Facile synthesis of functional gadolinium-doped CdTe quantum dots for tumor-targeted fluorescence and magnetic resonance dual-modality imaging

Magnetic quantum dots (MQDs) are an important class of agents for fluorescence (FL)/magnetic resonance (MR) dual-modal imaging due to their excellent optical and magnetic properties. However, functional MQDs prepared by a simple room-temperature route as FL/MR dual-modal imaging probes are lacking. Herein, we report the fabrication of Gd-doped CdTe quantum dots (Gd:CdTe QDs) as an agent for FL/MR dual-modality imaging. The as-designed QDs with an ultrasmall particle size are synthesized by a facile one-pot aqueous synthesis approach at room temperature. They emit strong fluorescence at 640 nm with a quantum yield of 37% in water, and they have a high longitudinal relaxation rate (r₁) value of 3.27 mM^-1 s^-1. With the further conjugation of folic acid, the Gd:CdTe QDs can successfully label live HepG2 cells for targeted cellular imaging and present no evidence of cellular toxicity up to the concentration of 0.5 mg mL^-1. They have been employed as a suitable contrast agent successfully for tumor-targeted FL/MR dual-modal imaging in a mouse model.

PEGylated Prussian blue nanocubes as a theranostic agent for simultaneous cancer imaging and photothermal therapy

Theranostic agents with both imaging and therapeutic functions have attracted enormous interests in cancer diagnosis and treatment in recent years. In this work, we develop a novel theranostic agent based on Prussian blue nanocubes (PB NCs), a clinically approved agent with strong near-infrared (NIR) absorbance and intrinsic paramagnetic property, for in vivo bimodal imaging-guided photothermal therapy. After being coated with polyethylene glycol (PEG), the obtained PB-PEG NCs are highly stable in various physiological solutions. In vivo T1-weighted magnetic resonance (MR) and photoacoustic tomography (PAT) bimodal imaging uncover that PB-PEG NCs after intravenous (i.v.) injection show high uptake in the tumor. Utilizing the strong and super stable NIR absorbance of PB, in vivo cancer treatment is then conducted upon i.v. injection of PB-PEG NCs followed by NIR laser irradiation of the tumors, achieving excellent therapeutic efficacy in a mouse tumor model. Comprehensive blood tests and careful histological examinations reveal no apparent toxicity of PB-PEG NCs to mice at our tested dose, which is two-fold of the imaging/therapy dose, within two months. Our work highlights the great promise of Prussian blue with well engineered surface coating as a multifunctional nanoprobe for imaging-guided cancer therapy.

18F-glutathione conjugate as a PET tracer for imaging tumors that overexpress L-PGDS enzyme

Lipocalin-type prostaglandin D synthase (L-PGDS) has been correlated with the progression of neurological disorders. The present study aimed at evaluating the imaging potency of a glutathione conjugate of fluorine-18-labeled fluorobutyl ethacrynic amide ([¹⁸F]FBuEA-GS) for brain tumors. Preparation of [¹⁸F]FBuEA-GS has been modified from the -4-tosylate derivative via radiofluorination in 5% radiochemical yield. The mixture of nonradioactive FBuEA-GS derived from a parallel preparation has be resolved to two isomers in a ratio of 9∶1 using analytic chiral reversed phase high performance liquid chromatography (RP-HPLC). The two fluorine-18-labeled isomers purified through nonchiral semipreparative RP-HPLC as a mixture were studied by assessing the binding affinity toward L-PGDS through a gel filtration HPLC, by analyzing radiotracer accumulation in C6 glioma cells, and by evaluating the imaging of radiotracer in a C6 glioma rat with positron emission tomography. The inhibition percentage of the production of PGD₂ from PGH₂ at the presence of 200 µM of FBuEA-GS and 4-Dibenzo[a,d]cyclohepten-5-ylidene-1-[4-(2H-tetrazol-5-yl)butyl]piperidine (AT-56) were 74.1±4.8% and 97.6±16.0%, respectively. [¹⁸F]FBuEA-GS bound L-PGDS (16.3–21.7%) but not the isoform, microsomal prostaglandin E synthase 1. No binding to GST-alpha and GST-pi was observed. The binding strength between [¹⁸F]FBuEA-GS and L-PGDS has been evaluated using analytic gel filtration HPLC at the presence of various concentrations of the cold competitor FBuEA-GS. The contrasted images indicated that the radiotracer accumulation in tumor lesions is probably related to the overexpression of L-PGDS.

Design and evaluation of radiolabeled tracers for tumor imaging

The growing understanding of tumor biology and the identification of tumor-specific genetic and molecular alterations, such as the overexpression of membrane receptors and other proteins, allows for personalization of patient management using targeted therapies. However, this puts stringent demands on the diagnostic tools used to identify patients who are likely to respond to a particular treatment. Radionuclide molecular imaging is a promising noninvasive method to visualize and characterize the expression of such targets. A number of different proteins, from full-length antibodies and their derivatives to small scaffold proteins and peptide receptor-ligands, have been applied to molecular imaging, each demonstrating strengths and weaknesses. Here, we discuss the concept of molecular targeting and, in particular, molecular imaging of cancer-associated targets. Additionally, we describe important biotechnological considerations and desired features when designing and developing tracers for radionuclide molecular imaging.

Peptide-based cancer therapy: opportunity and challenge

Cancer is one of the leading causes of death worldwide. Conventional cancer therapies mainly focus on mass cell killing without high specificity and often cause severe side effects and toxicities. Peptides are a novel class of anticancer agents that could specifically target cancer cells with lower toxicity to normal tissues, which will offer new opportunities for cancer prevention and treatment. Anticancer peptides face several therapeutic challenges. In this review, we present the sources and mechanisms of anticancer peptides and further discuss modification strategies to improve the anticancer effects of bioactive peptides.

177Lu-labeled RGD-BBN heterodimeric peptide for targeting prostate carcinoma

INTRODUCTION

Radiolabeled Arg-Gly-Asp (RGD) and bombesin (BBN) heterodimers have been investigated for dual targeting of tumor integrin αvβ3 receptors and gastrin-releasing peptide receptors. The goal of this study was to evaluate the potential use of a Lu-labeled RGD-BBN heterodimer for targeted prostate cancer therapy.

MATERIALS AND METHODS

A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated RGD-BBN peptide (DO3A-RGD-BBN) was radiolabeled with Lu and purified by high-performance liquid chromatography. The in-vivo biodistribution study of Lu-DO3A-RGD-BBN was carried out in mice bearing human prostate cancer PC3 xenografts. The receptor-targeting specificity of the radiolabeled peptide was assayed by injecting the tracer with the unlabeled RGD-BBN peptide. Radiation absorbed doses in adult male patients, based on biodistribution data from mice, were also calculated.

RESULTS

DO3A-RGD-BBN peptides were successfully labeled with Lu, and high radiochemical purity (>95%) could be achieved after high-performance liquid chromatography purification. In human PC3 xenograft-bearing mice, the tumor accumulation of Lu-DO3A-RGD-BBN was 5.88±1.12, 2.77±0.30, 2.04±0.19, and 1.18±0.19%ID/g at 0.5, 2, 24, and 48 h, respectively. With rapid clearance from normal tissues, the radiolabeled probe displayed high tumor-to-blood and tumor-to-muscle ratios. On calculating the radiation absorbed doses for Lu-DO3A-RGD-BBN, we found that the prostate tumor and the pancreas were the organs receiving the highest radiation absorbed doses.

CONCLUSION

Dual integrin αvβ3 and GPRP-targeted agent Lu-DO3A-RGD-BBN shows excellent prostate cancer-targeting ability, and it is worthy of further evaluation for prostate cancer-targeted therapy.

Recent advances in nanoparticle-based nuclear imaging of cancers

Nuclear imaging techniques that include positron emission tomography (PET) and single-photon computed tomography have found great success in the clinic because of their inherent high sensitivity. Radionuclide imaging is the most popular form of imaging to be used for molecular imaging in oncology. While many types of molecules have been used for radionuclide-based molecular imaging, there has been a great interest in developing newer nanomaterials for use in clinic, especially for cancer diagnosis and treatment. Nanomaterials have unique physical properties which allow them to be used as imaging probes to locate and identify cancerous lesions. Over the past decade, a great number of nanoparticles have been developed for radionuclide imaging of cancer. This chapter reviews the different kinds of nanomaterials, both organic and inorganic, which are currently being researched for as potential agents for nuclear imaging of variety of cancers. Several radiolabeled multifunctional nanocarriers have been extremely successful for the detection of cancer in preclinical models. So far, significant progress has been achieved in nanoparticle structure design, in vitro/in vivo trafficking, and in vivo fate mapping by using PET. There is a great need for the development of newer nanoparticles, which improve active targeting and quantify new biomarkers for early disease detection and possible prevention of cancer.

IMPROVED SYNTHESIS OF 10-(2-ALKYLAMINO-2-OXOETHYL)-1,4,7,10-TETRAAZACYCLODODECANE-1,4,7-TRIACETIC ACID DERIVATIVES BEARING ACID-SENSITIVE LINKERS

Alkylation of the hydrobromide salts of 1,4,7-tris(methoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane and 1,4,7-tris(ethoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane with appropriate α-bromoacetamides, followed by hydrolysis, provides convenient access to 10-(2-alkylamino-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid derivatives that contain acid-sensitive functional groups. The utility of the method is demonstrated by improved syntheses of two known DOTA monoamides bearing acid-sensitive ω-tritylthio alkyl chains in much higher yields based on cyclen as the starting material.

Biodistribution of (99m)tc tricarbonyl glycine oligomers

^99mTc tricarbonyl glycine monomers, trimers, and pentamers were synthesized and evaluated for their radiolabeling and in vivo distribution characteristics. We synthesized a ^99mTc-tricarbonyl precursor with a low oxidation state (I). ^99mTc(CO)₃(H₂O)₃⁺ was then made to react with monomeric and oligomeric glycine for the development of bifunctional chelating sequences for biomolecules. Labeling yields of ^99mTc-tricarbonyl glycine monomers and oligomers were checked by high-performance liquid chromatography. The labeling yields of ^99mTc-tricarbonyl glycine and glycine oligomers were more than 95%. We evaluated the characteristics of ^99mTc-tricarbonyl glycine oligomers by carrying out a lipophilicity test and an imaging study. The octanol-water partition coefficient of ^99mTc tricarbonyl glycine oligomers indicated hydrophilic properties. Single-photon emission computed tomography imaging of ^99mTc-tricarbonyl glycine oligomers showed rapid renal excretion through the kidneys with a low uptake in the liver, especially of ^99mTc tricarbonyl triglycine. Furthermore, we verified that the addition of triglycine to prototype biomolecules (AGRGDS and RRPYIL) results in the improvement of radiolabeling yield. From these results, we conclude that triglycine has good characteristics for use as a bifunctional chelating sequence for a ^99mTc-tricarbonyl- based biomolecular imaging probe.

Creating diversity by site-selective peptide modification: a customizable unit affords amino acids with high optical purity

The development of peptide libraries by site-selective modification of a few parent peptides would save valuable time and materials in discovery processes, but still is a difficult synthetic challenge. Herein natural hydroxyproline is introduced as a "convertible" unit for the production of a variety of optically pure amino acids, including expensive N-alkyl amino acids, and to achieve the mild, efficient, and site-selective modification of peptides.

Enhanced aqueous Suzuki-Miyaura coupling allows site-specific polypeptide 18F-labeling

The excesses of reagents used in protein chemistry are often incompatible with the reduced or even inverse stoichiometries used for efficient radiolabeling. Analysis and screening of aqueous Pd(0) ligand systems has revealed the importance of a guanidine core and the discovery of 1,1-dimethylguanidine as an enhanced ligand for aqueous Suzuki–Miyaura cross-coupling. This novel Pd catalyst system has now allowed the labeling of small molecules, peptides, and proteins with the fluorine-18 prosthetic [¹⁸F]4-fluorophenylboronic acid. These findings now enable site-specific protein ¹⁸F-labeling under biologically compatible conditions using a metal-triggered reaction.

Carbon-11 radiolabeling of an oligopeptide containing tryptophan hydrochloride via a Pictet-Spengler reaction using carbon-11 formaldehyde

A procedure for the synthesis of a(11)C-labeled oligopeptide containing [1-(11)C]1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid ([1-(11)C]Tpi) from the corresponding Trp•HCl-containing peptides has been developed involving a Pictet-Spengler reaction with [(11) C]formaldehyde. The synthesis of [1-(11)C]Tpi from Trp and [(11)C]formaldehyde was examined as a model reaction with the aim of developing a facile and effective method for the labeling of peptides with carbon-11. The Pictet-Spengler reaction of Trp and [(11)C]formaldehyde in acidic media (TsOH or HCl) afforded the desired [1-(11)C]Tpi in a moderate radiochemical yield. Herein, the application of a Pictet-Spengler reaction to an aqueous solution of Trp•HCl gave the desired product with a radiochemical yield of 45.2%. The RGD peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was then selected as a substrate for the labeling reaction with [(11)C]formaldehyde. The radiolabeling of a Trp•HCl-containing RGD peptide using the Pictet-Spengler reaction was successful. Furthermore, the remote-controlled synthesis of a [1-(11)C]Tpi-containing RGD peptide was attempted by using an automatic production system to generate [(11)C]CH3 I. The radiochemical yield of the [1-(11) C]Tpi-containing RGD at the end of synthesis (EOS) was 5.9 ± 1.9% (n = 4), for a total synthesis time of about 35 min. The specific activity was 85.7 ± 9.4 GBq/µmol at the EOS.

Kit-like 18F-labeling of RGD-19F-arytrifluroborate in high yield and at extraordinarily high specific activity with preliminary in vivo tumor imaging

INTRODUCTION

Positron Emission Tomography (PET) is a rapidly expanding, cutting edge technology for preclinical evaluation, cancer diagnosis and staging, and patient management. A one-step aqueous (18)F-labeling method, which can be applied to peptides to provide functional in vivo images, has been a long-standing challenge in PET imaging. Over the past few years, we have sought a rapid and mild radiolabeling method based on the aqueous radiosynthesis of in vivo stable aryltrifluoroborate (ArBF(3)(-)) conjugates. Recent access to production levels of (18)F-Fluoride led to a fluorescent-(18)F-ArBF(3)(-) at unprecedentedly high specific activities of 15Ci/μmol. However, extending this method to labeling peptides as imaging agents has not been explored.

METHODS

In order to extend these results to a peptide of clinical interest in the context of production-level radiosynthesis, we applied this new technology for labeling RGD, measured its specific activity by standard curve analysis, and carried out a preliminary evaluation of its imaging properties.

RESULTS

RGD was labeled in excellent radiochemical yields at exceptionally high specific activity (~14Ci/μmol) (n = 3). Preliminary tumor-specific images corroborated by ex vivo biodistribution data with blocking controls show statistically significant albeit relatively low tumor uptake along with reasonably high tumor:blood ratios (n = 3).

CONCLUSIONS

Isotope exchange on a clinically useful (18)F-ArBF(3)(-) radiotracer leads to excellent radiochemical yields and exceptionally high specific activities while the anionic nature of the aryltrifluoroborate prosthetic results in very rapid clearance. Since rapid clearance of the radioactive tracer is generally desirable for tracer development, these results suggest new directions for varying linker arm composition to slightly retard clearance rather than enhancing it.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This work is the first to use production levels of (18)F-activity to directly label RGD at specific activities that are an order of magnitude higher than most reports and thereby increases the distribution window for radiotracer production and delivery.

RGD-modified apoferritin nanoparticles for efficient drug delivery to tumors

Ferritin (FRT) is a major iron storage protein found in humans and most living organisms. Each ferritin is composed of 24 subunits, which self-assemble to form a cage-like nanostructure. FRT nanocages can be genetically modified to present a peptide sequence on the surface. Recently, we demonstrated that Cys-Asp-Cys-Arg-Gly-Asp-Cys-Phe-Cys (RGD4C)-modified ferritin can efficiently home to tumors through RGD-integrin αvβ3 interaction. Though promising, studies on evaluating surface modified ferritin nanocages as drug delivery vehicles have seldom been reported. Herein, we showed that after being precomplexed with Cu(II), doxorubicin can be loaded onto RGD modified apoferritin nanocages with high efficiency (up to 73.49 wt %). When studied on U87MG subcutaneous tumor models, these doxorubicin-loaded ferritin nanocages showed a longer circulation half-life, higher tumor uptake, better tumor growth inhibition, and less cardiotoxicity than free doxorubicin. Such a technology might be extended to load a broad range of therapeutics and holds great potential in clinical translation.

Nanoparticles for Improving Cancer Diagnosis

Despite the progress in developing new therapeutic modalities, cancer remains one of the leading diseases causing human mortality. This is mainly attributed to the inability to diagnose tumors in their early stage. By the time the tumor is confirmed, the cancer may have already metastasized, thereby making therapies challenging or even impossible. It is therefore crucial to develop new or to improve existing diagnostic tools to enable diagnosis of cancer in its early or even pre-syndrome stage. The emergence of nanotechnology has provided such a possibility. Unique physical and physiochemical properties allow nanoparticles to be utilized as tags with excellent sensitivity. When coupled with the appropriate targeting molecules, nanoparticle-based probes can interact with a biological system and sense biological changes on the molecular level with unprecedented accuracy. In the past several years, much progress has been made in applying nanotechnology to clinical imaging and diagnostics, and interdisciplinary efforts have made an impact on clinical cancer management. This article aims to review the progress in this exciting area with emphases on the preparation and engineering techniques that have been developed to assemble "smart" nanoprobes.

Polyaspartic acid coated iron oxide nanoprobes for PET/MRI imaging

Iron oxide nanoparticles, due to their exceptional magnetic property, biocompatibility, and biodegradability, have long been studied as contrast agents for magnetic resonance imaging (Xie et al., Curr Med Chem 16(10):1278-1294, 2009; Xie et al., Adv Drug deliv Rev 62(11):1064-1079, 2010). While previous applications mostly target reticuloendothelial system (RES) organs such as liver and lymph nodes, recent efforts have been made to impart targeting peptides or antibodies onto particle surface to enable site-specific targeting after systemic administration (Xie et al., Adv Drug Deliv Rev 62(11):1064-1079, 2010; Cai and Chen, Small 3(11):1840-1854, 2007; Corot et al., Adv Drug Deliv Rev 58 (14):1471-1504, 2006; Xie et al., Acc Chem Res 44(10):883-892). Moreover, other imaging functionalities can be loaded onto nanoparticles to achieve multimodality imaging probes (Cai and Chen, Small 3(11):1840-1854, 2007; Lee et al., J Nucl Med Soc Nucl Med 49(8):1371-1379, 2008). In this protocol, we describe the procedure of constructing an iron oxide nanoparticle (IONP)-based probe with high affinity towards integrin αvβ3 for positron emission tomography (PET) and magnetic resonance imaging (MRI) dual modality imaging. The related characterizations and validation experiments, including particle concentration determination, Prussian blue staining, animal model preparation, and in vivo PET/MRI imaging will also be discussed.

Progress in molecular imaging in endoscopy and endomicroscopy for cancer imaging

Imaging is an essential tool for effective cancer management. Endoscopes are important medical instruments for performing in vivo imaging in hollow organs. Early detection of cancer can be achieved with surveillance using endoscopy, and has been shown to reduce mortality and to improve outcomes. Recently, great advancements have been made in endoscopic instruments, including new developments in optical designs, light sources, optical fibers, miniature scanners, and multimodal systems, allowing for improved resolution, greater tissue penetration, and multispectral imaging. In addition, progress has been made in the development of highly-specific optical probes, allowing for improved specificity for molecular targets. Integration of these new endoscopic instruments with molecular probes provides a unique opportunity for significantly improving patient outcomes and has potential to further improve early detection, image guided therapy, targeted therapy, and personalized medicine. This work summarizes current and evolving endoscopic technologies, and provides an overview of various promising optical molecular probes.

Biological evaluation of an ornithine-modified (99m)Tc-labeled RGD peptide as an angiogenesis imaging agent

INTRODUCTION

Radiolabeled RGD peptides that specifically target integrin α(ν)β(3) have great potential in early tumor detection through noninvasive monitoring of tumor angiogenesis. Based on previous findings of our group on radiopeptides containing positively charged aminoacids, we developed a new cyclic cRGDfK derivative, c(RGDfK)-(Orn)(3)-CGG. This new peptide availing the polar linker (Orn)(3) and the (99m)Tc-chelating moiety CGG (Cys-Gly-Gly) is appropriately designed for (99m)Tc-labeling, as well as consequent conjugation onto nanoparticles.

METHODS

A tumor imaging agent, c(RGDfK)-(Orn)(3)-[CGG-(99m)Tc], is evaluated with regard to its radiochemical, radiobiological and imaging characteristics.

RESULTS

The complex c(RGDfK)-(Orn)(3)-[CGG-(99m)Tc] was obtained in high radiochemical yield (>98%) and was stable in vitro and ex vivo. It presented identical to the respective, fully analytically characterized (185/187)Re complex retention time in RP-HPLC. In contrary to other RGD derivatives, we showed that the new radiopeptide exhibits kidney uptake and urine excretion due to the ornithine linker. High tumor uptake (3.87±0.48% ID/g at 60 min p.i.) was observed and was maintained relatively high even at 24 h p.i. (1.83±0.05 % ID/g), thus providing well-defined scintigraphic imaging. Accumulation in other organs was negligible. Blocking experiments indicated target specificity for integrin receptors in U87MG glioblastoma cells.

CONCLUSION

Due to its relatively high tumor uptake, renal elimination and negligible abdominal localization, the new (99m)Tc-RGD peptide is considered promising in the field of imaging α(ν)β(3)-positive tumors. However, the preparation of multifunctional SPECT/MRI contrast agents (RGD-conjugated nanoparticles) for dual modality imaging of integrin expressing tumors should be further investigated.

Integrin-targeted trifunctional probe for cancer cells: a "seeing and counting" approach

We report the design and synthesis of a trifunctional probe for seeing and counting cancer cells using both fluorescence imaging (FI) and inductively coupled plasma mass spectrometry (ICPMS) for the first time. It consisted of a guiding cyclic RGD peptide unit to catch cancer cells via targeting the α(v)β(3) integrin overexpressed on their surface, a 5-amino-fluorescein moiety for FI using confocal laser scanning microscopy (CLSM) as well as a 2-aminoethyl-monoamide-DOTA group for loading stable europium ion and subsequent ICPMS quantification of the cancer cells without the use of radioactive isotopes. In addition to FI, the LOD (3σ) of the α(v)β(3) integrin was down to 69.2-309.4 amol per cell depending on the type of the α(v)β(3)-positive cancer cells when using ICPMS and those of the cancer cell number reached 17-75. This probe developed enables us not only to see but also to count the α(v)β(3)-positive cancer cells ultrasensitively, paving a new way for early diagnosis of cancer.

Real-time monitoring of caspase cascade activation in living cells

We introduce a simple, versatile and robust one-step technique that enables real-time imaging of multiple intracellular caspase activities in living cells without the need for complicated synthetic protocols. Conventional fluorogenic probes or recently reported activatable probes have been designed to target various proteases but are limited to extracellular molecules. Only a few have been applied to image intracellular proteases in living cells because most of these probes have limited cell-permeability. Our platform does not need complicated synthetic processes; instead it involves a straightforward peptide synthesis and a simple mixing step with a commercial transfection agent. The transfection agent efficiently delivered the highly quenched fluorogenic probes, comprised of distinctive pairs of dyes and quenchers, to the initiator caspase-8 and the effector caspase-3 in MDA-MB-435 cells, allowing dual-imaging of the activities of both caspases during the apoptotic process induced by TNF-related apoptosis induced ligand (TRAIL). With the combination of multiple fluorogenic probes, this simple platform can be applied to multiplexed imaging of selected intracellular proteases to study apoptotic processes in pathologies or for cell-based high throughput screening systems for drug discovery.

Extra- and intracellular imaging of human matrix metalloprotease 11 (hMMP-11) with a cell-penetrating FRET substrate

Matrix metalloprotease 11 (MMP-11), a protease associated with invasion and aggressiveness of cancerous tissue, was postulated as a prognostic marker for pancreatic, breast, and colon cancer patients. Expression analysis, however, did not reveal localization and regulation of this protease. Thus, cellular tools for the visualization of MMP-11 are highly desirable to monitor presence and activity and to elucidate the functional role of MMP-11. Therefore, fluorescein-Dabcyl-labeled Foerster resonance energy transfer (FRET) substrates were developed. The design focused on enhanced peptide binding to human MMP-11, employing an unusual amino acid for the specificity pocket P1'. The addition of several arginines resulted in a cell-permeable FRET substrate SM-P124 (Ac-GRRRK(Dabcyl)-GGAANC(MeOBn)RMGG-fluorescein). In vitro evaluation of SM-P124 with human MMP-11 showed a 25-fold increase of affinity (k(cat)/K(m) = 9.16 × 10(3) m(-1) s(-1), K(m) = 8 μm) compared with previously published substrates. Incubation of pancreatic adenocarcinoma cell line MIA PaCa-2 and mamma adenocarcinoma cell line MCF-7 with the substrate SM-P124 (5 μm) indicated intra- and extracellular MMP-11 activity. A negative control cell line (Jurkat) showed no fluorescent signal either intra- or extracellularly. Negative control FRET substrate SM-P123 produced only insignificant extracellular fluorescence without any intracellular fluorescence. SM-P124 therefore enabled intra- and extracellular tracking of MMP-11-overexpressing cancers such as pancreatic and breast adenocarcinoma and might contribute to the understanding of the activation pathways leading to MMP-11-mediated invasive processes.

Moving theranostics trom bench to bedside in an interdisciplinary research team

The Laboratory of Molecular Imaging and Nanomedicine of the National Institute of Biomedical Imaging and Bioengineering at the National Institutes of Health has been led by Xiaoyuan Chen since 2009. The Laboratory of Molecular Imaging and Nanomedicine contains three interdisciplinary, versatile groups: the Theranostic Nanomedicine; PET/Optical Imaging Probe; and Biological Molecular Imaging Sections, consisting of chemists, engineers, biologists and clinicians working together to transfer biomedical technology from bench to bedside. To find out more about our laboratory and meet our group, please visit us at www.nibib.nih.gov/Research/Intramural/xchen.

Inorganic chemistry in nuclear imaging and radiotherapy: current and future directions

Radiometals play an important role in diagnostic and therapeutic radiopharmaceuticals. This field of radiochemistry is multidisciplinary, involving radiometal production, separation of the radiometal from its target, chelate design for complexing the radiometal in a biologically stable environment, specific targeting of the radiometal to its in vivo site, and nuclear imaging and/or radiotherapy applications of the resultant radiopharmaceutical. The critical importance of inorganic chemistry in the design and application of radiometal-containing imaging and therapy agents is described from a historical perspective to future directions.

F-18 labeling protocol of peptides based on chemically orthogonal strain-promoted cycloaddition under physiologically friendly reaction conditions

We introduce the high-throughput synthesis of various (18)F-labeled peptide tracers by a straightforward (18)F-labeling protocol based on a chemo-orthogonal strain-promoted alkyne azide cycloaddition (SPAAC) using aza-dibenzocyclootyne-substituted peptides as precursors with (18)F-azide synthon to develop peptide based positron emission tomography (PET) molecular imaging probes. The SPAAC reaction and subsequent chemo-orthogonal purification reaction with azide resin proceeded quickly and selectively under physiologically friendly reaction conditions (i.e., toxic chemical reagents-free, aqueous medium, room temperature, and pH ≈7), and provided four (18)F-labeled tumor targetable bioactive peptides such as cyclic Arg-Gly-Asp (cRGD) peptide, bombesin (BBN), c-Met binding peptide (cMBP), and apoptosis targeting peptide (ApoPep) in high radiochemical yields as direct injectable solutions without any HPLC purification and/or formulation processes. In vitro binding assay and in vivo PET molecular imaging study using the (18)F-labeled cRGD peptide also demonstrated a successful application of our (18)F-labeling protocol.

Rationale for the use of radiolabelled peptides in diagnosis and therapy

Nuclear medicine techniques are becoming more important in imaging oncological and infectious diseases. For metabolic imaging of these diseases, antibody and peptide imaging are currently used. In recent years peptide imaging has become important, therefore the rationale for the use of peptide imaging is described in this article. Criteria for a successful peptide tracer are a high target specificity, a high binding affinity, a long metabolic stability and a high target-to-background ratio. Tracer internalization is also beneficial. For oncological imaging, many tracers are available, most originating from regulatory peptides, but penetrating peptides are also being developed. Peptides for imaging inflammatory and infectious diseases include regulatory peptides, antimicrobial peptides and others. In conclusion, for the imaging of oncological, imflammatory and infectious diseases, many promising peptides are being developed. The ideal peptide probe is characterized by rapid and specific target localization and binding with a high tumour-to-background ratio.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Imaging agents for the chemokine receptor 4 (CXCR4)

The interaction between the chemokine receptor 4 (CXCR4) and stromal cell-derived factor-1 (SDF-1, also known as CXCL12) is a natural regulatory process in the human body. However, CXCR4 over-expression is also found in diseases such as cancer, where it plays a role in, among others, the metastatic spread. For this reason it is an interesting biomarker for the field of diagnostic oncology, and therefore, it is gaining increasing interest for applications in molecular imaging. Especially "small-molecule" imaging agents based on T140, FC131 and AMD3100 have been extensively studied. SDF-1, antibodies, pepducins and bioluminescence have also been used to visualize CXCR4. In this critical review reported CXCR4 targeting imaging agents are described based on their affinity, specificity and biodistribution. The level wherein CXCR4 is up-regulated in cancer patients and its relation to the different cell lines and animal models used to evaluate the efficacy of the imaging agents is also discussed (221 references).

Multifunctional imaging probe based on gadofulleride nanoplatform

A FAR over-expressed tumor targeting multifunctional imaging probe has been fabricated based on gadofulleride nanoplatform. The combination of highly efficient MRI contrast enhancement and sensitive fluorescence imaging along with the preferential uptake toward FAR tumor cells suggest that the obtained multifunctional imaging probe possesses complementary capabilities for anatomical resolution and detection sensitivity.