Improving tumor uptake and pharmacokinetics of (64)Cu-labeled cyclic RGD peptide dimers with Gly(3) and PEG(4) linkers

NGR-Based Radiopharmaceuticals for Angiogenesis Imaging: A Preclinical Review

Angiogenesis plays a crucial role in tumour progression and metastatic spread; therefore, the development of specific vectors targeting angiogenesis has attracted the attention of several researchers. Since angiogenesis-associated aminopeptidase N (APN/CD13) is highly expressed on the surface of activated endothelial cells of new blood vessels and a wide range of tumour cells, it holds great promise for imaging and therapy in the field of cancer medicine. The selective binding capability of asparagine-glycine-arginine (NGR) motif containing molecules to APN/CD13 makes radiolabelled NGR peptides promising radiopharmaceuticals for the non-invasive, real-time imaging of APN/CD13 overexpressing malignancies at the molecular level. Preclinical small animal model systems are major keystones for the evaluation of the in vivo imaging behaviour of radiolabelled NGR derivatives. Based on existing literature data, several positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radioisotopes have been applied so far for the labelling of tumour vasculature homing NGR sequences such as Gallium-68 (68Ga), Copper-64 (64Cu), Technetium-99m (99mTc), Lutetium-177 (177Lu), Rhenium-188 (188Re), or Bismuth-213 (213Bi). Herein, a comprehensive overview is provided of the recent preclinical experiences with radiolabelled imaging probes targeting angiogenesis.

Homodimer 99mTc-HYNIC-E(SSSLTVPWY)2 peptide improved HER2-overexpressed tumor targeting and imaging

We hypothesized that a novel design of the LTVPWY (LY) peptide might exhibit a great potential for improving binding affinity and targeting HER2-overexpressed tumors. Hence, new dimer construction of ^99mTc-labeled LY [^99mTc-HYNIC-E(SSSLTVPWY)₂] (^99mTc-DLY) was introduced. Afterward, a head-to-head comparison of in vitro and in vivo experiments was performed between ^99mTc-DLY and ^99mTc-HYNIC-SSSLTVPWY as the monomer analog. The blocking dosage of trastuzumab reduced the uptake of the dimer about 20% more efficiently than the monomer in the SKOV-3 cell line. A twofold increase in competitive binding affinity and biological half-life was observed for ^99mTc-DLY. The ovarian-tumor-bearing mice were detected with high contrast where the tumor-to-muscle ratio of ^99mTc-DLY was notably increased about 40% using a gamma camera. The biodistribution experiment revealed an approximately 10% enhancement in tumor/blood, tumor/muscle, and tumor/bone ratios for the dimer. More rapid blood clearance was another achievement of the homodimer design. Overall, ^99mTc-DLY successfully affected the pharmacokinetics and consequently the visualization of HER2-overexpressing tumors.

Current understandings and clinical translation of nanomedicines for breast cancer therapy

Breast cancer is one of the most frequently diagnosed cancers that is threatening women's life. Current clinical treatment regimens for breast cancer often involve neoadjuvant and adjuvant systemic therapies, which somewhat are associated with unfavorable features. Also, the heterogeneous nature of breast cancers requires precision medicine that cannot be fulfilled by a single type of systemically administered drug. Taking advantage of the nanocarriers, nanomedicines emerge as promising therapeutic agents for breast cancer that could resolve the defects of drugs and achieve precise drug delivery to almost all sites of primary and metastatic breast tumors (e.g. tumor vasculature, tumor stroma components, breast cancer cells, and some immune cells). Seven nanomedicines as represented by Doxil® have been approved for breast cancer clinical treatment so far. More nanomedicines including both non-targeting and active targeting nanomedicines are being evaluated in the clinical trials. However, we have to realize that the translation of nanomedicines, particularly the active targeting nanomedicines is not as successful as people have expected. This review provides a comprehensive landscape of the nanomedicines for breast cancer treatment, from laboratory investigations to clinical applications. We also highlight the key advances in the understanding of the biological fate and the targeting strategies of breast cancer nanomedicine and the implications to clinical translation.

Effect of Peptide Receptor Radionuclide Therapy in Combination with Temozolomide against Tumor Angiogenesis in a Glioblastoma Model

Cell adhesion receptor integrin α_vβ₃ is a promising biomarker for developing tumor-angiogenesis targeted theranostics. In this study, we aimed to examine the therapeutic potential of peptide receptor radionuclide therapy (PRRT) with ¹⁸⁸Re-IDA-D-[c(RGDfK)]₂ (11.1 MBq). The results showed that the tumor volume was significantly decreased by 81% compared with the vehicle-treated group in U87-MG xenografts. The quantitative in vivo anti-angiogenic responses of PRRT were obtained using ^99mTc-IDA-D-[c(RGDfK)]₂ SPECT and corresponded to the measured tumor volume. PRRT combined with temozolomide (TMZ) resulted in a 93% reduction in tumor volume, which was markedly greater than that of each agent used individually. In addition, histopathological characterization showed that PRRT combined with TMZ was superior to PRRT or TMZ alone, even when TMZ was used at half dose. Overall, our results indicated that integrin-targeted PRRT and TMZ combined therapy might be a new medical tool for the effective treatment of glioblastoma.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

Molecular Imaging of Angiogenesis in Oncology: Current Preclinical and Clinical Status

Angiogenesis is an active process, regulating new vessel growth, and is crucial for the survival and growth of tumours next to other complex factors in the tumour microenvironment. We present possible molecular imaging approaches for tumour vascularisation and vitality, focusing on radiopharmaceuticals (tracers). Molecular imaging in general has become an integrated part of cancer therapy, by bringing relevant insights on tumour angiogenic status. After a structured PubMed search, the resulting publication list was screened for oncology related publications in animals and humans, disregarding any cardiovascular findings. The tracers identified can be subdivided into direct targeting of angiogenesis (i.e., vascular endothelial growth factor, laminin, and fibronectin) and indirect targeting (i.e., glucose metabolism, hypoxia, and matrix metallo-proteases, PSMA). Presenting pre-clinical and clinical data of most tracers proposed in the literature, the indirect targeting agents are not 1:1 correlated with angiogenesis factors but do have a strong prognostic power in a clinical setting, while direct targeting agents show most potential and specificity for assessing tumour vascularisation and vitality. Within the direct agents, the combination of multiple targeting tracers into one agent (multimers) seems most promising. This review demonstrates the present clinical applicability of indirect agents, but also the need for more extensive research in the field of direct targeting of angiogenesis in oncology. Although there is currently no direct tracer that can be singled out, the RGD tracer family seems to show the highest potential therefore we expect one of them to enter the clinical routine.

PET Diagnostic Molecules Utilizing Multimeric Cyclic RGD Peptide Analogs for Imaging Integrin αvβ3 Receptors

Multimeric ligands consisting of multiple pharmacophores connected to a single backbone have been widely investigated for diagnostic and therapeutic applications. In this review, we summarize recent developments regarding multimeric radioligands targeting integrin α_vβ₃ receptors on cancer cells for molecular imaging and diagnostic applications using positron emission tomography (PET). Integrin α_vβ₃ receptors are glycoproteins expressed on the cell surface, which have a significant role in tumor angiogenesis. They act as receptors for several extracellular matrix proteins exposing the tripeptide sequence arginine-glycine-aspartic (RGD). Cyclic RDG peptidic ligands c(RGD) have been developed for integrin α_vβ₃ tumor-targeting positron emission tomography (PET) diagnosis. Several c(RGD) pharmacophores, connected with the linker and conjugated to a chelator or precursor for radiolabeling with different PET radionuclides (¹⁸F, ⁶⁴Cu, and ⁶⁸Ga), have resulted in multimeric ligands superior to c(RGD) monomers. The binding avidity, pharmacodynamic, and PET imaging properties of these multimeric c(RGD) radioligands, in relation to their structural characteristics are analyzed and discussed. Furthermore, specific examples from preclinical studies and clinical investigations are included.

A simple and robust method for radiochemical separation of no-carrier-added 64Cu produced in a research reactor for radiopharmaceutical preparation

Copper-64 is an excellent theranostic radiometal that is gaining renewed attention of the clinical community in the recent times. In order to meet the increasing demand of this radiometal, we have demonstrated the viability of its production via ⁶⁴Zn (n,p) ⁶⁴Cu reaction in a nuclear reactor. A semi-automated radiochemical separation module based on selective extraction of ⁶⁴Cu as dithizonate complex was developed. The maximum available activity at the end of irradiation was ~ 700 MBq. The overall yield of ⁶⁴Cu after the separation process was >85% and it could be obtained with ~12 GBq/μg specific activity, >99.9% radionuclidic purity and >98% radiochemical purity. The separated ⁶⁴Cu could be utilized for preparation of a wide variety of radiopharmaceuticals.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

An atom-economical design of PET tracer for imaging αvβ3 integrin via utilizing the three-in-one function of 64Copper

In this study, α_vβ₃ integrin in U87 tumor cells was imaged with a ⁶⁴Cu-peptidic probe, in which the linear peptide GHRGDHG is used as a pre-ligand, while ⁶⁴Cu bears three functional roles that include generation of the PET signal, coordination with two GH moieties of the pre-ligand, and cyclizing the linear pre-ligand into an active cyclic-RGD form (termed as ⁶⁴Cu-Cyclo-RGD) for α_vβ₃ integrin.

Synthesis and characterization of 64Cu- and Cy5.5-labeled tetraiodothyroacetic acid derivatives for tumor angiogenesis imaging

It was previously reported that tetraiodothyroacetic acid (tetrac) inhibits angiogenesis by binding to the cell surface receptor for thyroid hormone on integrin α_Vβ₃. Therefore, we synthesized and evaluated two ⁶⁴Cu-labeled tetrac derivatives and a Cy5.5-labeled tetrac derivative for tumor angiogenesis imaging. Tetrac was structurally modified to conjugate with 1,4,7,10-tetraazacyclododecane-N,N',N″,N″'-tetraacetic acid (DOTA) via its hydroxy or carboxylic acid end, and the resulting DOTA-conjugated tetrac derivatives were then labeled with ⁶⁴Cu. Tetrac was also conjugated with Cy5.5 via its carboxylic acid end. All three tetrac derivatives (1-3) exhibited greater inhibitory activity than tetrac against endothelial cell tube formation. The U87MG cell binding of [⁶⁴Cu]2 showed a time-dependent increase over 24 h and it was inhibited by 38% at 4 h in the presence of tetrac, indicating specificity of [⁶⁴Cu]2 to the thyroid hormone receptor site on integrin α_Vβ₃. Positron emission tomography (PET) images of U87MG tumor-bearing mice injected with [⁶⁴Cu]1 and [⁶⁴Cu]2 revealed that high radioactivity accumulated in the tumors, and that the tumor uptake and tumor-to-nontarget uptake ratio were higher in small tumors than in large tumors. In addition, the Cy5.5-labeled tetrac derivative (3) displayed a strong near-infrared (NIR) signal in the tumors. Taken together, these results suggest that these ligands hold promise as imaging agents for visualization of tumor angiogenesis.

The Influence of Different Spacers on Biological Profile of Peptide Radiopharmaceuticals for Diagnosis and Therapy of Human Cancers

BACKGROUND

Cancer is the leading cause of death worldwide. Early detection can reduce the disadvantageous effects of diseases and the mortality in cancer. Nuclear medicine is a powerful tool that has the ability to diagnose malignancy without harming normal tissues. In recent years, radiolabeled peptides have been investigated as potent agents for cancer detection. Therefore, it is necessary to modify radiopeptides in order to achieve more effective agents.

OBJECTIVE

This review describes modifications in the structure of radioconjugates with spacers who have improved the specificity and sensitivity of the peptides that are used in oncologic diagnosis and therapy.

METHODS

To improve the biological activity, researchers have conjugated these peptide analogs to different spacers and bifunctional chelators. Many spacers of different kinds, such as hydrocarbon chain, amino acid sequence, and poly (ethyleneglycol) were introduced in order to modify the pharmacokinetic properties of these biomolecules.

RESULTS

Different spacers have been applied to develop radiolabeled peptides as potential tracers in nuclear medicine. Spacers with different charge and hydrophilicity affect the characteristics of peptide conjugate. For example, the complex with uncharged and hydrophobic spacers leads to increased liver uptake, while the composition with positively charged spacers results in high kidney retention. Therefore, the pharmacokinetics of radio complexes correlates to the structure and total charge of the conjugates.

CONCLUSION

Radio imaging technology has been successfully applied to detect a tumor in the earliest stage. For this purpose, the assessment of useful agents to diagnose the lesion is necessary. Developing peptide radiopharmaceuticals using spacers can improve in vitro and in vivo behavior of radiotracers leading to better noninvasive detection and monitoring of tumor growth.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Cyclic Peptides: Promising Scaffolds for Biopharmaceuticals

To date, small molecules and macromolecules, including antibodies, have been the most pursued substances in drug screening and development efforts. Despite numerous favorable features as a drug, these molecules still have limitations and are not complementary in many regards. Recently, peptide-based chemical structures that lie between these two categories in terms of both structural and functional properties have gained increasing attention as potential alternatives. In particular, peptides in a circular form provide a promising scaffold for the development of a novel drug class owing to their adjustable and expandable ability to bind a wide range of target molecules. In this review, we discuss recent progress in methodologies for peptide cyclization and screening and use of bioactive cyclic peptides in various applications.

Biodistribution and Internal Radiation Dosimetry of 99mTc-IDA-D-[c(RGDfK)]2 (BIK-505), a Novel SPECT Radiotracer for the Imaging of Integrin αvβ3 Expression

Background: Integrin α_vβ₃ is a molecular marker for the estimation of tumor angiogenesis. ^99mTc-IDA-D-[c(RGDfK)]₂ (also known as BIK-505) is a recently developed radiotracer for single-photon emission computed tomography, with good affinity for integrin α_vβ₃. In this study, the authors investigated the whole-body distribution and internal radiation dosimetry of ^99mTc-IDA-D-[c(RGDfK)]₂ in elderly human participants.

Materials and Methods: Six healthy volunteers underwent whole-body simultaneous anterior and posterior scans, preceded by transmission scans using cobalt-57 flood source, with a dual head gamma camera system, at 0, 1, 2, 4, 8, and 24 h postinjection of ^99mTc-IDA-D-[c(RGDfK)]₂ (injected radioactivity [mean ± SD] = 388.7 ± 29.3 MBq). Anterior and posterior images were geometrically averaged and attenuation corrected to delineate the regions of interest in the liver, gallbladder, kidneys, urinary bladder, spleen, brain, and large intestine. Radiation dose for each organ and the effective doses (EDs) were estimated using OLINDA/EXM 1.1 software.

Results: High radiation doses of renal and biliary excretion tracks such as the urinary bladder wall, upper large intestine, kidneys, liver, and gallbladder wall (19.15 ± 6.84, 19.28 ± 4.78, 15.67 ± 0.90, 9.13 ± 1.71, and 9.09 ± 2.03 μGy/MBq, respectively) were observed. The ED and effective dose equivalent were 5.08 ± 0.53 and 7.11 ± 0.58 μSv/MBq, respectively.

Conclusions: Dosimetry results were comparable to other radiolabeled peptides and were considered safe and efficient for clinical usage.

Synthesis and photophysical studies of a multivalent photoreactive RuII-calix[4]arene complex bearing RGD-containing cyclopentapeptides

Photoactive ruthenium-based complexes are actively studied for their biological applications as potential theragnostic agents against cancer. One major issue of these inorganic complexes is to penetrate inside cells in order to fulfil their function, either sensing the internal cell environment or exert a photocytotoxic activity. The use of lipophilic ligands allows the corresponding ruthenium complexes to passively diffuse inside cells but limits their structural and photophysical properties. Moreover, this strategy does not provide any cell selectivity. This limitation is also faced by complexes anchored on cell-penetrating peptides. In order to provide a selective cell targeting, we developed a multivalent system composed of a photoreactive ruthenium(II) complex tethered to a calix[4]arene platform bearing multiple RGD-containing cyclopentapeptides. Extensive photophysical and photochemical characterizations of this Ru(II)–calixarene conjugate as well as the study of its photoreactivity in the presence of guanosine monophosphate have been achieved. The results show that the ruthenium complex should be able to perform efficiently its photoinduced cytotoxic activity, once incorporated into targeted cancer cells thanks to the multivalent platform.

PET imaging of Hsp90 expression in pancreatic cancer using a new 64Cu-labeled dimeric Sansalvamide A decapeptide

Heat shock protein 90 (Hsp90) plays a vital role in the progress of malignant disease and elevated Hsp90 expression has been reported in pancreatic cancer. In this study, we radiolabeled a dimeric Sansalvamide A derivative (Di-San A1) with ⁶⁴Cu, and evaluated the feasibility of using ⁶⁴Cu-Di-San A1 for PET imaging of Hsp90 expression in a mouse model of pancreatic cancer. A macrocyclic chelator NOTA (1,4,7-triazacyclononane-1,4,7-trisacetic acid) was conjugated to Di-San A1. ⁶⁴Cu-Di-San A1 was successfully prepared in a radiochemical yield > 97% with a radiochemical purity > 98%. ⁶⁴Cu-Di-San A1 is stable in PBS and mouse serum with > 92% of parent probe intact after 4 h incubation. The cell binding and uptake revealed that ⁶⁴Cu-Di-San A1 binds to Hsp90-positive PL45 pancreatic cancer cells, and the binding can be effectively blocked by an Hsp90 inhibitor (17AAG). For microPET study, ⁶⁴Cu-Di-San A1 shows good in vivo performance in terms of tumor uptake in nude mice bearing PL45 tumors. The Hsp90-specific tumor activity accumulation of ⁶⁴Cu-Di-San A1 was further demonstrated by significant reduction of PL45 tumor uptake with a pre-injected blocking dose of 17AAG. The ex vivo PET imaging and biodistribution results were consistent with the quantitative analysis of PET imaging, demonstrating good tumor-to-muscle ratio (5.35 ± 0.46) of ⁶⁴Cu-Di-San A1 at 4 h post-injection in PL45 tumor mouse xenografts. ⁶⁴Cu-Di-San A1 allows PET imaging of Hsp90 expression in PL45 tumors, which may provide a non-invasive method to quantitatively characterize Hsp90 expression in pancreatic cancer.

Evaluation of a dimeric-cRGD peptide for targeted PET-CT imaging of peripheral angiogenesis in diabetic mice

The α V  β3 integrin plays an important role in many physiological functions and pathological disorders. α V  β3 is minimally expressed in normal quiescent endothelial cells, but significantly upregulated during neovascularization. In this study, we evaluated a 64Cu-labeled dimeric cRGD tracer targeted at α V  β3 integrin and report its applicability to assess peripheral angiogenesis in diabetes mellitus (DM). We established a murine model of type-1 DM characterized by elevated glucose, glycated serum protein (GSP), and glycated hemoglobin A1c (HbA1c). We demonstrated that our imaging probe is specific to α V  β3 integrin under both normo- and hyperglycemic conditions. We found that the analysis of in vivo PET-CT images correlated well with gamma well counting (GWC). Both GWC and PET-CT imaging demonstrated increased uptake of 64Cu-NOTA-PEG4-cRGD2 in the ischemic hindlimb in contrast to non-ischemic control. GWC of the distal ischemic tissue from DM mice showed significantly lower probe accumulation than in non-DM mice. The immunofluorescence staining of the ischemic tissues showed a 3-fold reduction in CD31 and 4-fold reduction in the α V  β3 expression in DM vs. non-DM animals. In conclusion, we successfully demonstrated that diabetes-associated reductions in peripheral angiogenesis can be non-invasively detected with PET-CT imaging using targeted dimeric-cRGD probe.

64Cu-Labeled multifunctional dendrimers for targeted tumor PET imaging

We report the use of multifunctional folic acid (FA)-modified dendrimers as a platform to radiolabel with 64Cu for PET imaging of folate receptor (FR)-expressing tumors. In this study, amine-terminated generation 5 (G5) poly(amidoamine) dendrimers were sequentially modified with fluorescein isothiocyanate (FI), FA, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), followed by acetylation of the remaining dendrimer terminal amines. The as-formed multifunctional DOTA-FA-FI-G5·NHAc dendrimers were then radiolabeled with 64Cu via the DOTA chelation. We show that the FA modification renders the dendrimers with targeting specificity to cancer cells overexpressing FR in vitro. Importantly, the radiolabeled 64Cu-DOTA-FA-FI-G5·NHAc dendrimers can be used as a nanoprobe for specific targeting of FR-overexpressing cancer cells in vitro and targeted microPET imaging of the FR-expressing xenografted tumor model in vivo. The developed 64Cu-labeled multifunctional dendrimeric nanoprobe may hold great promise to be used for targeted PET imaging of different types of FR-expressing cancer.

Synthesis, Chemical Characterization and Multiscale Biological Evaluation of a Dimeric-cRGD Peptide for Targeted Imaging of α V β 3 Integrin Activity

Cyclic peptides containing the Arg-Gly-Asp (RGD) sequence have been shown to specifically bind the angiogenesis biomarker α_Vβ₃ integrin. We report the synthesis, chemical characterization, and biological evaluation of two novel dimeric cyclic RGD-based molecular probes for the targeted imaging of α_Vβ₃ activity (a radiolabeled version, ⁶⁴Cu-NOTA-PEG₄-cRGD₂, for PET imaging, and a fluorescent version, FITC-PEG₄-cRGD₂, for in vitro work). We investigated the performance of this probe at the receptor, cell, organ, and whole-body levels, including its use to detect diabetes associated impairment of ischemia-induced myocardial angiogenesis. Both versions of the probe were found to be stable, demonstrated fast receptor association constants, and showed high specificity for α_Vβ₃ in HUVECs (K_d ~ 35 nM). Dynamic PET-CT imaging indicated rapid blood clearance via kidney filtration, and accumulation within α_Vβ₃-positive infarcted myocardium. ⁶⁴Cu-NOTA-PEG₄-cRGD₂ demonstrated a favorable biodistribution, slow washout, and excellent performance with respect to the quality of the PET-CT images obtained. Importantly, the ratio of probe uptake in infarcted heart tissue compared to normal tissue was significantly higher in non-diabetic rats than in diabetic ones. Overall, our probes are promising agents for non-invasive quantitative imaging of α_Vβ₃ expression, both in vitro and in vivo.

Developing Targeted Hybrid Imaging Probes by Chelator Scaffolding

Positron emission tomography (PET) as well as optical imaging (OI) with peptide receptor targeting probes have proven their value for oncological applications but also show restrictions depending on the clinical field of interest. Therefore, the combination of both methods, particularly in a single molecule, could improve versatility in clinical routine. This proof of principle study aims to show that a chelator, Fusarinine C (FSC), can be utilized as scaffold for novel dimeric dual-modality imaging agents. Two targeting vectors (a minigastrin analogue (MG11) targeting cholecystokinin-2 receptor overexpression (CCK2R) or integrin α_Vβ₃ targeting cyclic pentapeptides (RGD)) and a near-infrared fluorophore (Sulfo-Cyanine7) were conjugated to FSC. The probes were efficiently labeled with gallium-68 and in vitro experiments including determination of logD, stability, protein binding, cell binding, internalization, and biodistribution studies as well as in vivo micro-PET/CT and optical imaging in U-87MG α_Vβ₃- and A431-CCK2R expressing tumor xenografted mice were carried out. Novel bioconjugates showed high receptor affinity and highly specific targeting properties at both receptors. Ex vivo biodistribution and micro-PET/CT imaging studies revealed specific tumor uptake accompanied by slow blood clearance and retention in nontargeted tissues (spleen, liver, and kidneys) leading to visualization of tumors at early (30 to 120 min p.i.). Excellent contrast in corresponding optical imaging studies was achieved especially at delayed time points (24 to 72 h p.i.). Our findings show the proof of principle of chelator scaffolding for hybrid imaging agents and demonstrate FSC being a suitable bifunctional chelator for this approach. Improvements to fine-tune pharmacokinetics are needed to translate this into a clinical setting.

Enhancing PET Signal at Target Tissue in Vivo: Dendritic and Multimeric Tris(hydroxypyridinone) Conjugates for Molecular Imaging of αvβ3 Integrin Expression with Gallium-68

Tris(hydroxypyridinone) chelators conjugated to peptides can rapidly complex the positron-emitting isotope gallium-68 (68Ga) under mild conditions, and the resulting radiotracers can delineate peptide receptor expression at sites of diseased tissue in vivo. We have synthesized a dendritic bifunctional chelator containing nine 1,6-dimethyl-3-hydroxypyridin-4-one groups (SCN-HP9) that can coordinate up to three Ga3+ ions. This derivative has been conjugated to a trimeric peptide (RGD3) containing three peptide groups that target the αvβ3 integrin receptor. The resulting dendritic compound, HP9-RGD3, can be radiolabeled in 97% radiochemical yield at a 3-fold higher specific activity than its homologues HP3-RGD and HP3-RGD3 that contain only a single metal binding site. PET scanning and biodistribution studies show that [68Ga(HP9-RGD3)] demonstrates higher receptor-mediated tumor uptake in animals bearing U87MG tumors that overexpress αvβ3 integrin than [68Ga(HP3-RGD)] and [68Ga(HP3-RGD3)]. However, concomitant nontarget organ retention of [68Ga(HP9-RGD3)] results in low tumor to nontarget organ contrast in PET images. On the other hand, the trimeric peptide homologue containing a single tris(hydroxypyridinone) chelator, [68Ga(HP3-RGD3)], clears nontarget organs and exhibits receptor-mediated uptake in mice bearing tumors and in mice with induced rheumatoid arthritis. PET imaging with [68Ga(HP3-RGD3)] enables clear delineation of αvβ3 integrin receptor expression in vivo.

Peptide-based imaging agents for cancer detection

Selective receptor-targeting peptide based agents have attracted considerable attention in molecular imaging of tumor cells that overexpress corresponding peptide receptors due to their unique properties such as rapid clearance from circulation as well as high affinities and specificities for their targets. The rapid growth of chemistry modification techniques has enabled the design and development of various peptide-based imaging agents with enhanced metabolic stability, favorable pharmacokinetics, improved binding affinity and selectivity, better imaging ability as well as biosafety. Among them, many radiolabeled peptides have already been translated into the clinic with impressive diagnostic accuracy and sensitivity. This review summarizes the current status in the development of peptide-based imaging agents with an emphasis on the consideration of probe design including the identification of suitable peptides, the chemical modification of probes and the criteria for clinical translation. Specific examples in clinical trials have been provided as well with respect to their diagnostic capability compared with other FDA approved imaging agents.

Improved Pharmacokinetics Following PEGylation and Dimerization of a c(RGD-ACH-K) Conjugate Used for Tumor Positron Emission Tomography Imaging

Improving the in vivo pharmacokinetics (PK) of positron emission tomography (PET) radiotracers is of critical importance to tumor diagnosis and therapy. In the case of peptide-based radiotracers, the modification and addition of a linker or spacer functional group often offer faster in vivo pharmacokinetic behavior. In this study, the authors introduced two new PEGlyated dimeric c(RGD-ACH-K) conjugates, in which an aminocyclohexane carboxylic acid (ACH) is inserted into the ring chain of the cyclic RGD peptides, with a common bifunctional chelator (DOTA or NOTA) used for labeling with radiometals (including ⁶⁸Ga and ⁶⁴Cu). The addition of polyethylene glycol (PEG) and dimerization of c(RGD-ACH-K) affected the PK of the renal system and the tumor-targeting ability, relative to unmodified molecule. As a result, both ⁶⁴Cu-DOTA-E[c(RGD-ACH-K)]₂ (complex 1) and ⁶⁴Cu-NOTA-E[c(RGD-ACH-K)]₂ (complex 2) exhibited specific tumor-targeting properties relative to tumor-blocking control group, most likely resulting from improved in vivo tumor imaging. The in vivo tumor-to-blood ratio of the ⁶⁴Cu(NOTA) complex shows better PET imaging than that of the ⁶⁴Cu(DOTA) complex, which should lead to improved dosimetry and increased suitability for noninvasive monitoring of tumor growth or tumor-targeted radionuclide therapy.

New Glucocyclic RGD Dimers for Positron Emission Tomography Imaging of Tumor Integrin Receptors

Most studies of radiolabeled arginine-glycine-aspartic acid (RGD) peptides have shown in vitro affinity for integrin ανβ3, allowing for the targeting of receptor-positive tumors in vivo. However, major differences have been found in the pharmacokinetic profiles of different radiolabeled RGD peptide analogs. The purposes of this study were to prepare (64)Cu-DOTA-gluco-E[c(RGDfK)]2 (R8), (64)Cu-NOTA-gluco-E[c(RGDfK)]2 (R9), and (64)Cu-NODAGA-gluco-E[c(RGDfK)]2 (R10) and compare their pharmacokinetics and tumor imaging properties using small-animal positron emission tomography (PET). All three compounds were produced with high specific activity within 10 minutes. The IC50 values were similar for all the substances, and their affinities were greater than that of c(RGDyK). R8, R9, and R10 were stable for 24 hours in human and mouse serums and showed high uptake in U87MG tumors with high tumor-to-blood ratios. Compared to the control, a cyclic RGD peptide dimer without glucosamine, R10, showed low uptake in the liver. Because of their good imaging qualities and improved pharmacokinetics, (64)Cu-labeled dimer RGD conjugates (R8, R9, and R10) may have potential applications as PET radiotracers. R9 (NOTA) with highly in vivo stability consequentially showed an improved PET tumor uptake than R8 (DOTA) or R10 (NODAGA).

Radiolabeled cyclic RGD peptides as radiotracers for tumor imaging

The integrin family comprises 24 transmembrane receptors, each a heterodimeric combination of one of 18α and one of 8β subunits. Their main function is to integrate the cell adhesion and interaction with the extracellular microenvironment with the intracellular signaling and cytoskeletal rearrangement through transmitting signals across the cell membrane upon ligand binding. Integrin α_vβ₃ is a receptor for the extracellular matrix proteins containing arginine–glycine–aspartic (RGD) tripeptide sequence. The α_vβ₃ is generally expressed in low levels on the epithelial cells and mature endothelial cells, but it is highly expressed in many solid tumors. The α_vβ₃ levels correlate well with the potential for tumor metastasis and aggressiveness, which make it an important biological target for development of antiangiogenic drugs, and molecular imaging probes for early tumor diagnosis. Over the last decade, many radiolabeled cyclic RGD peptides have been evaluated as radiotracers for imaging tumors by SPECT or PET. Even though they are called “α_vβ₃-targeted” radiotracers, the radiolabeled cyclic RGD peptides are also able to bind α_vβ₅, α₅β₁, α₆β₄, α₄β₁, and α_vβ₆ integrins, which may help enhance their tumor uptake due to the “increased receptor population.” This article will use the multimeric cyclic RGD peptides as examples to illustrate basic principles for development of integrin-targeted radiotracers and focus on different approaches to maximize their tumor uptake and T/B ratios. It will also discuss important assays for pre-clinical evaluations of the integrin-targeted radiotracers, and their potential applications as molecular imaging tools for noninvasive monitoring of tumor metastasis and early detection of the tumor response to antiangiogenic therapy.

Comparison of biological properties of 99mTc-labeled cyclic RGD Peptide trimer and dimer useful as SPECT radiotracers for tumor imaging

INTRODUCTION

This study sought to evaluate a ^99mTc-labeled trimeric cyclic RGD peptide (^99mTc-4P-RGD₃) as the new radiotracer for tumor imaging. The objective was to compare its biological properties with those of ^99mTc-3P-RGD₂ in the same animal model.

METHODS

HYNIC-4P-RGD₃ was prepared by reacting 4P-RGD₃ with excess HYNIC-OSu in the presence of diisopropylethylamine. ^99mTc-4P-RGD₃ was prepared using a kit formulation, and evaluated for its tumor-targeting capability and biodistribution properties in the BALB/c nude mice with U87MG human glioma xenografts. Planar and SPECT imaging studies were performed in athymic nude mice with U87MG glioma xenografts. For comparison purpose, ^99mTc-3P-RGD₂ (a α_vβ₃-targeted radiotracer currently under clinical evaluation for tumor imaging in cancer patients) was also evaluated in the same animal models. Blocking experiments were used to demonstrate the α_vβ₃ specificity of ^99mTc-4P-RGD₃.

RESULTS

^99mTc-4P-RGD₃ was prepared with >95% RCP and high specific activity (~200GBq/μmol). ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂ shared almost identical tumor uptake and similar biodistribution properties. ^99mTc-4P-RGD₃ had higher uptake than ^99mTc-3P-RGD₂ in the intestines and kidneys; but it showed better metabolic stability. The U87MG tumors were clearly visualized by SPECT with excellent contrast with ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂.

CONCLUSION

Increasing peptide multiplicity from 3P-RGD₂ to 4P-RGD₃ offers no advantages with respect to the tumor-targeting capability. ^99mTc-4P-RGD₃ is as good a SPECT radiotracer as ^99mTc-3P-RGD₂ for imaging α_vβ₃-positive tumors.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Evaluation of two novel ⁶⁴Cu-labeled RGD peptide radiotracers for enhanced PET imaging of tumor integrin αvβ₃

PURPOSE

Our goal was to demonstrate that suitably derivatized monomeric RGD peptide-based PET tracers, targeting integrin αvβ3, may offer advantages in image contrast, time for imaging, and low uptake in nontarget tissues.

METHODS

Two cyclic RGDfK derivatives, (PEG)2-c(RGDfK) and PEG4-SAA4-c(RGDfK), were constructed and conjugated to NOTA for (64)Cu labeling. Their integrin αvβ3-binding properties were determined via a competitive cell binding assay. Mice bearing U87MG tumors were intravenously injected with each of the (64)Cu-labeled peptides, and PET scans were acquired during the first 30 min, and 2 and 4 h after injection. Blocking and ex vivo biodistribution studies were carried out to validate the PET data and confirm the specificity of the tracers.

RESULTS

The IC50 values of NOTA-(PEG)2-c(RGDfK) and NOTA-PEG4-SAA4-c(RGDfK) were 444 ± 41 nM and 288 ± 66 nM, respectively. Dynamic PET data of (64)Cu-NOTA-(PEG)2-c(RGDfK) and (64)Cu-NOTA-PEG4-SAA4-c(RGDfK) showed similar circulation t 1/2 and peak tumor uptake of about 4 %ID/g for both tracers. Due to its marked hydrophilicity, (64)Cu-NOTA-PEG4-SAA4-c(RGDfK) provided faster clearance from tumor and normal tissues yet maintained excellent tumor-to-background ratios. Static PET scans at later time-points corroborated the enhanced excretion of the tracer, especially from abdominal organs. Ex vivo biodistribution and receptor blocking studies confirmed the accuracy of the PET data and the integrin αvβ3-specificity of the peptides.

CONCLUSION

Our two novel RGD-based radiotracers with optimized pharmacokinetic properties allowed fast, high-contrast PET imaging of tumor-associated integrin αvβ3. These tracers may facilitate the imaging of abdominal malignancies, normally precluded by high background uptake.

(68)Ga-labeled 3PRGD2 for dual PET and Cerenkov luminescence imaging of orthotopic human glioblastoma

β-Emitters can produce Cerenkov radiation that is detectable by Cerenkov luminescence imaging (CLI), allowing the combination of PET and CLI with one radiotracer for both tumor diagnosis and visual guidance during surgery. Recently, the clinical feasibility of CLI with the established therapeutic reagent Na(131)I and the PET tracer (18)F-FDG was demonstrated. (68)Ga possesses a higher Cerenkov light output than (18)F and (131)I, which would result in higher sensitivity for CLI and improve the outcome of CLI in clinical applications. However, the research on (68)Ga-based tumor-specific tracers for CLI is limited. In this study, we examined the use of (68)Ga-radiolabeled DOTA-3PRGD2 ((68)Ga-3PRGD2) for dual PET and CLI of orthotopic U87MG human glioblastoma. For this purpose, the Cerenkov efficiencies of (68)Ga and (18)F were measured with the IVIS Spectrum system (PerkinElmer, USA). The CLI signal intensity of (68)Ga was 15 times stronger than that of (18)F. PET and CLI of (68)Ga-3PRGD2 were performed in U87MG human glioblastoma xenografts. Both PET and CLI revealed a remarkable accumulation of (68)Ga-3PRGD2 in the U87MG human glioblastoma xenografts at 1 h p.i. with an extremely low background in the brain when compared with (18)F-FDG. Furthermore, (68)Ga-3PRGD2 was used for dual PET and CLI of orthotopic human glioblastoma. The orthotopic human glioblastoma was clearly visualized by both imaging modalities. In addition, the biodistribution of (68)Ga-3PRGD2 was assessed in normal mice to estimate the radiation dosimetry. The whole-body effective dose is 20.1 ± 3.3 μSv/MBq, which is equal to 3.7 mSv per whole-body PET scan with a 5 mCi injection dose. Thus, (68)Ga-3PRGD2 involves less radiation exposure in patients when compared with (18)F-FDG (7.0 mSv). The use of (68)Ga-3PRGD2 in dual PET and CLI shows great promise for tumor diagnosis and image-guided surgery.

Peptide receptor radionuclide therapy: an overview

Peptide receptor radionuclide therapy (PRRT) is a site-directed targeted therapeutic strategy that specifically uses radiolabeled peptides as biological targeting vectors designed to deliver cytotoxic levels of radiation dose to cancer cells, which overexpress specific receptors. Interest in PRRT has steadily grown because of the advantages of targeting cellular receptors in vivo with high sensitivity as well as specificity and treatment at the molecular level. Recent advances in molecular biology have not only stimulated advances in PRRT in a sustainable manner but have also pushed the field significantly forward to several unexplored possibilities. Recent decades have witnessed unprecedented endeavors for developing radiolabeled receptor-binding somatostatin analogs for the treatment of neuroendocrine tumors, which have played an important role in the evolution of PRRT and paved the way for the development of other receptor-targeting peptides. Several peptides targeting a variety of receptors have been identified, demonstrating their potential to catalyze breakthroughs in PRRT. In this review, the authors discuss several of these peptides and their analogs with regard to their applications and potential in radionuclide therapy. The advancement in the availability of combinatorial peptide libraries for peptide designing and screening provides the capability of regulating immunogenicity and chemical manipulability. Moreover, the availability of a wide range of bifunctional chelating agents opens up the scope of convenient radiolabeling. For these reasons, it would be possible to envision a future where the scope of PRRT can be tailored for patient-specific application. While PRRT lies at the interface between many disciplines, this technology is inextricably linked to the availability of the therapeutic radionuclides of required quality and activity levels and hence their production is also reviewed.

Synthesis and evaluation of the targeted binding of RGD-containing PEGylated-PEI/DNA polyplex micelles as radiotracers for a tumor-targeting imaging probe

Polyplex micelles with pEGFP and RGD-modified poly(ethylene glycol)-grafted polyethylenimine (E[c(RGDyK)]₂-PEG-g-PEI) and were labeled with ^99mTc for the in vivo study as proficient probes for molecular imaging.

Comparison of three dimeric 18F-AlF-NOTA-RGD tracers

PURPOSE

RGD peptide-based radiotracers are well established as integrin αvβ3 imaging probes to evaluate tumor angiogenesis or tissue remodeling after ischemia or infarction. In order to optimize the labeling process and pharmacokinetics of the imaging probes, we synthesized three dimeric RGD peptides with or without PEGylation and performed in vivo screening.

PROCEDURES

Radiolabeling was achieved through the reaction of F-18 aluminum-fluoride complex with the cyclic chelator, 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). Three imaging probes were synthesized as (18)F-AlF-NOTA-E[c(RGDfK)]2, (18)F-AlF-NOTA-PEG4-E[c(RGDfK)]2, and (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2. The receptor binding affinity was determined by competitive cell binding assay, and the stability was evaluated by mouse serum incubation. Tumor uptake and whole body distribution of the three tracers were compared through direct tissue sampling and PET quantification of U87MG tumor-bearing mice.

RESULTS

All three compounds remained intact after 120 min incubation with mouse serum. They all had a rapid and relatively high tracer uptake in U87MG tumors with good target-to-background ratios. Compared with the other two tracers, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 had the highest tumor uptake and the lowest accumulation in the liver. The integrin receptor specificity was confirmed by co-injection of unlabeled dimeric RGD peptide.

CONCLUSION

The rapid one-step radiolabeling strategy by the complexation of (18)F-aluminum fluoride with NOTA-peptide conjugates was successfully applied to synthesize three dimeric RGD peptides. Among the three probes developed, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 with relatively low liver uptake and high tumor accumulation appears to be a promising candidate for further translational research.

Introduction of an 8-aminooctanoic acid linker enhances uptake of 99mTc-labeled lactam bridge-cyclized α-MSH peptide in melanoma

The purpose of this study was to examine the effects of amino acid, hydrocarbon, and polyethylene glycol (PEG) linkers on the melanoma targeting and imaging properties of (99m)Tc-labeled lactam bridge-cyclized HYNIC-linker-Nle-CycMSHhex (hydrazinonicotinamide-linker-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2) peptides.

METHODS

Four novel peptides (HYNIC-GGGNle-CycMSHhex, HYNIC-GSGNle-CycMSHhex, HYNIC-PEG2Nle-CycMSHhex, and HYNIC-AocNle-CycMSHhex) were designed and synthesized. The melanocortin-1 receptor binding affinities of the peptides were determined in B16/F1 melanoma cells. The biodistribution of (99m)Tc(ethylenediaminediacetic acid [EDDA])-HYNIC-GGGNle-CycMSHhex, (99m)Tc(EDDA)-HYNIC-GSGNle-CycMSHhex, (99m)Tc(EDDA)-HYNIC-PEG2Nle-CycMSHhex, and (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were determined in B16/F1 melanoma-bearing C57 mice at 2 h after injection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were further examined because of its high melanoma uptake.

RESULTS

The inhibitory concentrations of 50% (IC50) for HYNIC-GGGNle-CycMSHhex, HYNIC-GSGNle-CycMSHhex, HYNIC-PEG2Nle-CycMSHhex, and HYNIC-AocNle-CycMSHhex were 0.7 ± 0.1, 0.8 ± 0.09, 0.4 ± 0.08, and 0.3 ± 0.06 nM, respectively, in B16/F1 melanoma cells. Among these four (99m)Tc-labeled peptides, (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex displayed the highest melanoma uptake (22.3 ± 1.72 percentage injected dose/g) at 2 h after injection. (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex exhibited high tumor-to-normal-organ uptake ratios except for the kidneys. The tumor-to-kidney uptake ratios of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex were 3.29, 3.63, and 6.78 at 2, 4, and 24 h, respectively, after injection. The melanoma lesions were clearly visualized by SPECT/CT using (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex as an imaging probe at 2 h after injection.

CONCLUSION

High melanoma uptake and fast urinary clearance of (99m)Tc(EDDA)-HYNIC-AocNle-CycMSHhex highlighted its potential for metastatic melanoma detection in the future.

FITC-conjugated cyclic RGD peptides as fluorescent probes for staining integrin αvβ3/αvβ5 in tumor tissues

This study sought to evaluate FITC-conjugated cyclic RGD peptides (FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂) as fluorescent probes for in vitro assays of integrin α_vβ₃/α_vβ₅ expression in tumor tissues. FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂ were prepared, and their integrin α_vβ₃/α_vβ₅ binding affinity was determined using the displacement assay against ¹²⁵I-echistatin bound to U87MG glioma cells. IC₅₀ values of FITC-Galacto-RGD₂, FITC-3P-RGD₂, and FITC-RGD₂ were calculated to be 28 ± 8, 32 ± 7, and 89 ± 17 nM, respectively. The integrin α_vβ₃/α_vβ₅ binding affinity followed a general trend: FITC-Galacto-RGD₂ ∼ FITC-3P-RGD₂ > FITC-RGD₂. The xenografted tumor-bearing models were established by subcutaneous injection of 5 × 10⁶ tumor cells into shoulder flank (U87MG, A549, HT29, and PC-3) or mammary fat pad (MDA-MB-435) of each athymic nude mouse. Three to six weeks after inoculation, the tumor size was 0.1–0.3 g. Tumors were harvested for integrin α_vβ₃/α_vβ₅ staining, as well as hematoxylin and eosin (H&E) staining. Six human carcinoma tissues (colon cancer, pancreatic cancer, lung adenocarcinoma, squamous cell lung cancer, gastric cancer, and esophageal cancer) were obtained from recently diagnosed cancer patients. Human carcinoma slides were deparaffinized in xylene, rehydrated with ethanol, and then used for integrin α_vβ₃/α_vβ₅ staining, as well as H&E staining. It was found that the tumor staining procedures with FITC-conjugated cyclic RGD peptides were much simpler than those with the fluorescence-labeled integrin α_vβ₃ antibodies. Since FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂ were able to co-localize with the fluorescence-labeled integrin β₃ antibody, their tumor localization and tumor cell binding are integrin α_vβ₃-specific. Quantification of the fluorescent intensity in five xenografted tumors (U87MG, MDA-MB-435, A549, HT29, and PC-3) and six human carcinoma tissues revealed an excellent linear relationship between the relative integrin α_vβ₃/α_vβ₅ expression levels determined with FITC-Galacto-RGD₂ and those obtained with the fluorescence-labeled anti-human integrin β₃ antibody. There was also an excellent linear relationship between the tumor uptake (%ID/g) of ^99mTc-3P-RGD₂ (an integrin α_vβ₃/α_vβ₅-targeted radiotracer) and the relative integrin α_vβ₃/α_vβ₅ expression levels from the quantification of fluorescent intensity in the tumor tissues stained with FITC-Galacto-RGD₂. These results suggest that FITC-conjugated cyclic RGD peptides might be useful to correlate the in vitro findings with the in vivo imaging data from an integrin α_vβ₃/α_vβ₅-targeted radiotracer. The results from this study clearly showed that the FITC-conjugated cyclic RGD peptides (particularly FITC-3P-RGD₂ and FITC-Galacto-RGD₂) are useful fluorescent probes for assaying relative integrin α_vβ₃/α_vβ₅ expression levels in tumor tissues.

Comparison of biological properties of (111)In-labeled dimeric cyclic RGD peptides

INTRODUCTION

In this study two (111)In-labeled dimeric cyclic RGD peptides, (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2), were evaluated as radiotracers for breast tumor imaging. The objective was to evaluate the impact of SAA, PEG2 and 1,2,3-triazole linkers as compare to PEG4 on the tumor uptake and excretion kinetics of (111)In radiotracers.

METHODS

DOTA-Galacto-RGD2 was prepared by conjugation of Galacto-RGD2 with DOTA-OSu in the presence of diisopropylethylamine. Its integrin αvβ3 binding affinity was determined using a whole-cell displacement assay against (125)I-echistatin bound to U87MG glioma cells, and was compared with those of c(RGDfK), DOTA-3P-RGD2 and DOTA-3P-RGK2 (a nonsense peptide conjugate with "scrambled" RGK sequences). (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) were prepared and evaluated for their tumor-targeting capability and biodistribution properties in athymic nude mice bearing MDA-MB-435 breast tumor xenografts. Planar imaging studies were performed to demonstrate the utility of (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) for breast tumor imaging.

RESULTS

IC50 values of DOTA-Galacto-RGD2, DOTA-3P-RGD2, and DOTA-3P-RGK2 were calculated to be 27±2, 29±4, 596±48nM, respectively. The tumor uptake values of (111)In(DOTA-Galacto-RGD2) (6.79±0.98, 6.56±0.56, 4.17±0.61 and 1.09±0.13 %ID/g at 1, 4, 24 and 72hours p.i., respectively) were almost identical to those of (111)In(DOTA-3P-RGD2) (6.17±1.65, 5.94±0.84, 3.40±0.50 and 0.99±0.20 %ID/g, respectively). (111)In(DOTA-Galacto-RGD2) had a faster clearance from blood and muscle than (111)In(DOTA-3P-RGD2), leading to higher tumor/blood and tumor/muscle ratios. (111)In(DOTA-3P-RGD2) had lower liver uptake and better tumor/liver ratios than (111)In(DOTA-Galacto-RGD2). The tumor uptake of (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) was both integrin αvβ3 and RGD-specific. Imaging data suggest that (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) are useful as radiotracers for imaging integrin αvβ3-positive breast tumors.

CONCLUSION

The results from this study suggest that replacing PEG4 linkers between two RGD moieties with a pair of SAA, PEG2 and 1,2,3-triazole groups has little impact on integrin αvβ3 binding affinity and tumor uptake of (111)In-labeled dimeric cyclic RGD peptides. Despite the subtle differences in their excretion kinetics from noncancerous tissues, (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) are useful radiotracers for imaging integrin αvβ3-positive breast tumors.

Characterization and evaluation of DOTA-conjugated Bombesin/RGD-antagonists for prostate cancer tumor imaging and therapy

INTRODUCTION

Here we present the metallation, characterization, in vivo and in vitro evaluations of dual-targeting, peptide-based radiopharmaceuticals with utility for imaging and potentially treating prostate tumors by virtue of their ability to target the αVβ3 integrin or the gastrin releasing peptide receptor (GRPr).

METHODS

[RGD-Glu-6Ahx-RM2] (RGD: Arg-Gly-Asp; Glu: glutamic acid; 6-Ahx: 6-amino hexanoic acid; RM2: (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2)) was conjugated to a DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) bifunctional chelator (BFCA) purified via reversed-phase high-performance liquid chromatography (RP-HPLC), characterized by electrospray ionization-mass spectrometry (ESI-MS), and radiolabeled with (111)In or (177)Lu. Natural-metallated compounds were assessed for binding affinity for the αVβ3 integrin or GRPr in human glioblastoma U87-MG and prostate PC-3 cell lines and stability prior to in vivo evaluation in normal CF-1 mice and SCID mice xenografted with PC-3 cells.

RESULTS

Competitive displacement binding assays with PC-3 and U87-MG cells revealed high to moderate binding affinity for the GRPr or the αVβ3 integrin (IC50 range of 5.39±1.37 nM to 9.26±0.00 nM in PC-3 cells, and a range of 255±47 nM to 321±85 nM in U87-MG cells). Biodistribution studies indicated high tumor uptake in PC-3 tumor-bearing mice (average of 7.40±0.53% ID/g at 1h post-intravenous injection) and prolonged retention of tracer (mean of 4.41±0.91% ID/g at 24h post-intravenous injection). Blocking assays corroborated the specificity of radioconjugates for each target. Micro-single photon emission computed tomography (microSPECT) confirmed favorable radiouptake profiles in xenografted mice at 20h post-injection.

CONCLUSIONS

[RGD-Glu-[(111)In-DO3A]-6-Ahx-RM2] and [RGD-Glu-[(177)Lu- DO3A]-6-Ahx-RM2] show favorable pharmacokinetic and radiouptake profiles, meriting continued evaluation for molecular imaging in murine U87-MG/PC-3 xenograft models and radiotherapy studies with (177)Lu and (90)Y conjugates.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

These heterovalent, peptide-targeting ligands perform comparably with many mono- and multivalent conjugates with the potential benefit of increased sensitivity for detecting cancer cells exhibiting differential expression of target receptors.

Impact of multiple negative charges on blood clearance and biodistribution characteristics of 99mTc-labeled dimeric cyclic RGD peptides

This study sought to evaluate the impact of multiple negative charges on blood clearance kinetics and biodistribution properties of ^99mTc-labeled RGD peptide dimers. Bioconjugates HYNIC-P6G-RGD₂ and HYNIC-P6D-RGD₂ were prepared by reacting P6G-RGD₂ and P6D-RGD₂, respectively, with excess HYNIC-OSu in the presence of diisopropylethylamine. Their IC₅₀ values were determined to be 31 ± 5 and 41 ± 6 nM, respectively, against ¹²⁵I-echistatin bound to U87MG glioma cells in a whole-cell displacement assay. Complexes [^99mTc(HYNIC-P6G-RGD₂)(tricine)(TPPTS)] (^99mTc-P6G-RGD₂) and [^99mTc(HYNIC-P6D-RGD₂)(tricine)(TPPTS)] (^99mTc-P6D-RGD₂) were prepared in high radiochemical purity (RCP > 95%) and specific activity (37–110 GBq/μmol). They were evaluated in athymic nude mice bearing U87MG glioma xenografts for their biodistribution. The most significant difference between ^99mTc-P6D-RGD₂ and ^99mTc-P6G-RGD₂ was their blood radioactivity levels and tumor uptake. The initial blood radioactivity level for ^99mTc-P6D-RGD₂ (4.71 ± 1.00%ID/g) was ∼5× higher than that of ^99mTc-P6G-RGD₂ (0.88 ± 0.05%ID/g), but this difference disappeared at 60 min p.i. ^99mTc-P6D-RGD₂ had much lower tumor uptake (2.20–3.11%ID/g) than ^99mTc-P6G-RGD₂ (7.82–9.27%ID/g) over a 2 h period. Since HYNIC-P6D-RGD₂ and HYNIC-P6G-RGD₂ shared a similar integrin α_vβ₃ binding affinity (41 ± 6 nM versus 31 ± 5 nM), the difference in their blood activity and tumor uptake is most likely related to the nine negative charges and high protein binding of ^99mTc-P6D-RGD₂. Despite its low uptake in U87MG tumors, the tumor uptake of ^99mTc-P6D-RGD₂ was integrin α_vβ₃-specific. SPECT/CT studies were performed using ^99mTc-P6G-RGD₂ in athymic nude mice bearing U87MG glioma and MDA-MB-231 breast cancer xenografts. The SPECT/CT data demonstrated the tumor-targeting capability of ^99mTc-P6G-RGD₂, and its tumor uptake depends on the integrin α_vβ₃ expression levels on tumor cells and neovasculature. It was concluded that the multiple negative charges have a significant impact on the blood clearance kinetics and tumor uptake of ^99mTc-labeled dimeric cyclic RGD peptides.

MicroPET imaging of CD13 expression using a (64)Cu-labeled dimeric NGR peptide based on sarcophagine cage

CD13 receptor as a tumor vasculature biomarker has attracted great attention in cancer research. Through phage display screening, NGR-containing peptides have been characterized as specific ligands binding to CD13 receptor. In this study, a (64)Cu-labeled dimeric NGR peptide based on sarcophagine cage was synthesized and evaluated for micropositron emission tomography (PET) imaging of CD13 expression in vivo. Macrocyclic chelating agent (sarcophagine cage) was conjugated with two azide moieties, followed by mixing with an alkyne-containing NGR peptide to rapidly provide the Sar-NGR2 peptide by click chemistry. Radiolabeling of Sar-NGR2 with (64)Cu was achieved in >90% decay-corrected yield with radiochemical purity of >99%. The cell uptake study showed that (64)Cu-Sar-NGR2 binds to CD13-positive HT-1080 cells, but not to CD13-negative MCF-7 cells. MicroPET imaging results revealed that (64)Cu-Sar-NGR2 exhibits good tumor uptake in CD13-positive HT-1080 xenografts and significantly lower tumor uptake in CD13-negative MCF-7 xenografts. The CD13-specific binding of (64)Cu-Sar-NGR2 was further verified by significant reduction of tumor uptake in HT-1080 tumor xenografts with coinjection of a nonradiolabeled NGR peptide. The biodistribution results demonstrated good tumor/muscle ratio (8.28 ± 0.37) of (64)Cu-Sar-NGR2 at 24 h pi in HT-1080 tumor xenografts, which is in agreement with the quantitative analysis of microPET imaging. In conclusion, sarcophagine cage has been successfully applied to the construction of a (64)Cu-labeled dimeric NGR-containing peptide. In vitro and in vivo studies demonstrated that (64)Cu-Sar-NGR2 is a promising PET probe for imaging CD13 expression in living mice.

(44)Sc: an attractive isotope for peptide-based PET imaging

The overexpression of integrin α_vβ₃ has been linked to tumor aggressiveness and metastasis in several cancer types. Because of its high affinity, peptides containing the arginine–glycine–aspartic acid (RGD) motif have been proven valuable vectors for noninvasive imaging of integrin α_vβ₃ expression and for targeted radionuclide therapy. In this study, we aim to develop a ⁴⁴Sc-labeled RGD-based peptide for in vivo positron emission tomography (PET) imaging of integrin α_vβ₃ expression in a preclinical cancer model. High quality ⁴⁴Sc (t_1/2, 3.97 h; β⁺ branching ratio, 94.3%) was produced inexpensively in a cyclotron, via proton irradiation of natural Ca metal targets, and separated by extraction chromatography. A dimeric cyclic-RGD peptide, (cRGD)₂, was conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and radiolabeled with ⁴⁴Sc in high yield (>90%) and specific activity (7.4 MBq/nmol). Serial PET imaging of mice bearing U87MG tumor xenografts showed elevated ⁴⁴Sc-DOTA-(cRGD)₂ uptake in the tumor tissue of 3.93 ± 1.19, 3.07 ± 1.17, and 3.00 ± 1.25 %ID/g at 0.5, 2, and 4 h postinjection, respectively (n = 3), which were validated by ex vivo biodistribution experiments. The integrin α_vβ₃ specificity of the tracer was corroborated, both in vitro and in vivo, by competitive cell binding and receptor blocking assays. These results parallel previously reported studies showing similar tumor targeting and pharmacokinetic profiles for dimeric cRGD peptides labeled with ⁶⁴Cu or ⁶⁸Ga. Our findings, together with the advantageous radionuclidic properties of ⁴⁴Sc, capitalize on the relevance of this isotope as an attractive alternative isotope to more established radiometals for small molecule-based PET imaging, and as imaging surrogate of ⁴⁷Sc in theranostic applications.