3,4,6-Tri-O-acetyl-2-deoxy-2-[18F]fluoroglucopyranosyl phenylthiosulfonate: a thiol-reactive agent for the chemoselective 18F-glycosylation of peptides

Modeling of Biological Activity of PEO-Coated Titanium Implants with Conjugates of Cyclic RGD Peptide with Amino Acid Bisphosphonates

Titanium is considered to be the most essential metal in the field of implantology. The main factors determining metal biocompatibility, among others, include the morphology and chemical composition of the titanium surface. Therefore, the aim of this work was to develop approaches to control the biological activity of the titanium surface by creating coatings that combine both an inorganic phase with a given morphology and organic molecules containing an integrin-selective peptide that regulate cell adhesion and proliferation. As such, we synthesized new c(RGDfC) derivatives of amino acid bisphosphonates (four examples) with different bisphosphonate anchors and maleimide linkers. These molecules were deposited on a highly developed porous surface obtained via the plasma electrolytic oxidation (PEO) of coarse-grained and nanostructured titanium. In vitro studies demonstrated the increase in the viability degree of mesenchymal stem cells and fibroblasts on the surface of coarse-grained or nanostructured titanium modified with PEO and a c(RGDfC) derivative of ε-aminocaproic acid bisphophonate with an SMCC linker. As a result, the use of conjugates of amino acid bisphosphonates with a cyclic RGD peptide for the modification of PEO-coated titanium opens the ways for the effective control of the biological activity of the metal implant surface.

Evaluation of Polysaccharide-Peptide Conjugates Containing the RGD Motif for Potential Use in Muscle Tissue Regeneration

New scaffold materials composed of biodegradable components are of great interest in regenerative medicine. These materials should be: stable, nontoxic, and biodegrade slowly and steadily, allowing the stable release of biodegradable and biologically active substances. We analyzed peptide-polysaccharide conjugates derived from peptides containing RGD motif (H-RGDS-OH (1), H-GRGDS-NH₂ (2), and cyclo(RGDfC) (3)) and polysaccharides as scaffolds to select the most appropriate biomaterials for application in regenerative medicine. Based on the results of MTT and Ki-67 assays, we can state that the conjugates containing calcium alginate and the ternary nonwoven material were the most supportive of muscle tissue regeneration. Scanning electron microscopy imaging and light microscopy studies with hematoxylin-eosin staining showed that C2C12 cells were able to interact with the tested peptide-polysaccharide conjugates. The release factor (Q) varied depending on both the peptide and the structure of the polysaccharide matrix. LDH, Alamarblue^®, Ki-67, and cell cycle assays indicated that peptides 1 and 2 were characterized by the best biological properties. Conjugates containing chitosan and the ternary polysaccharide nonwoven with peptide 1 exhibited very high antibacterial activity against Staphylococcus aureus and Klebsiella pneumoniae. Overall, the results of the study suggested that polysaccharide conjugates with peptides 1 and 2 can be potentially used in regenerative medicine.

Novel [18F]-labeled thiol for the labeling of Dha- or maleimide-containing biomolecules

Background

Prosthetic approach for the radiolabeling of biologics with fluorine-18 is a robust strategy and has been employed for many years. It requires fast, biocompatible and selective reactions suited to these fragile molecules. Michael addition of a nucleophilic thiol moiety on α,β-unsaturated carbonyl entities is an interesting compromise between simplicity of preparation of the prosthetic reagent and control of the selectivity of the addition. The α,β-unsaturated carbonyl entity of the biologic can easily be generated by addition of a maleimide function using adequate heterobifunctional linkers or generated by selective modification of a cysteine residue leading to a dehydroalanine moiety. We report here the design, synthesis and radiosynthesis of a new fluoropyridine-based thiol [¹⁸F]FPySH and its conjugation via Michael addition on model dehydroalanine- or maleimide-containing biologics.

Results

The preparation of cold reference and labeling precursor of [¹⁸F]FPySH was achieved and its radiosynthesis was fully automated, enabling production of the thiol prosthetic group with a 7 ± 2.1% radiochemical yield after two steps. The conjugation of [¹⁸F]FPySH to two model Dha-containing molecules was then carried out in reducing conditions, yielding the corresponding adducts in 30–45 min reaction time. Furthermore, [¹⁸F]FPySH was employed to radiolabel the maleimide-modified c(RGDfK) peptide, affording the radiofluorinated analogue in 15 min.

Conclusion

We have developed an original [¹⁸F]-labeled thiol for site-selective conjugation and radiolabeling of Dha or maleimide-containing biomolecules of interest. Labeling of three model compounds was successfully carried out and gave the expected radiofluorinated adducts in less than 45 min, thus compatible with fluorine-18 half-life.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00160-5.

Sweetening Pharmaceutical Radiochemistry by 18F-Fluoroglycosylation: Recent Progress and Future Prospects

In the field of ¹⁸F-chemistry for the development of radiopharmaceuticals for positron emission tomography (PET), various labeling strategies by the use of prosthetic groups have been implemented, including chemoselective ¹⁸F-labeling of biomolecules. Among those, chemoselective ¹⁸F-fluoroglycosylation methods focus on the sweetening of pharmaceutical radiochemistry by offering a highly valuable tool for the synthesis of ¹⁸F-glycoconjugates with suitable in vivo properties for PET imaging studies. A previous review covered the various ¹⁸F-fluoroglycosylation methods that were developed and applied as of 2014 (Maschauer and Prante, BioMed. Res. Int. 2014, 214748). This paper is an updated review, providing the recent progress in ¹⁸F-fluoroglycosylation reactions and the preclinical application of ¹⁸F-glycoconjugates, including small molecules, peptides, and high-molecular-weight proteins.

Microfluidic-assisted preparation of RGD-decorated nanoparticles: exploring integrin-facilitated uptake in cancer cell lines

This study is about fine tuning the targeting capacity of peptide-decorated nanoparticles to discriminate between cells that express different integrin make-ups. Using microfluidic-assisted nanoprecipitation, we have prepared poly(lactic acid-co-glycolic acid) (PLGA) nanoparticles with a PEGylated surface decorated with two different arginine-glycine-aspartic acid (RGD) peptides: one is cyclic (RGDFC) and has specific affinity towards αvβ3 integrin heterodimers; the other is linear (RGDSP) and is reported to bind equally αvβ3 and α5β1. We have then evaluated the nanoparticle internalization in two cell lines with a markedly different integrin fingerprint: ovarian carcinoma A2780 (almost no αvβ3, moderate in α5β1) and glioma U87MG (very high in αvβ3, moderate/high in α5β1). As expected, particles with cyclic RGD were heavily internalized by U87MG (proportional to the peptide content and abrogated by anti-αvβ3) but not by A2780 (same as PEGylated particles). The linear peptide, on the other hand, did not differentiate between the cell lines, and the uptake increase vs. control particles was never higher than 50%, indicating a possible low and unselective affinity for various integrins. The strong preference of U87MG for cyclic (vs. linear) peptide-decorated nanoparticles was shown in 2D culture and further demonstrated in spheroids. Our results demonstrate that targeting specific integrin make-ups is possible and may open the way to more precise treatment, but more efforts need to be devoted to a better understanding of the relation between RGD structure and their integrin-binding capacity.

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

One-pot synthesis of glycosyl phenylthiosulfonates from sulfinate, S and glycosyl bromides

Glycosyl phenylthiosulfonates are reagents which are valuable for the S-glycosylation decoration of organic compounds and proteins. Here, one-pot multiple-component synthesis of glycosyl phenylthiosulfonates from sulfinate, sulfur powder and glycosyl bromides is reported. The reactions afford glycosyl phenylthiosulfonates in good yields under mild conditions. Further application and exploration of glycosyl phenylthiosulfonates are still on underway in our group.

Photoactivatable Reaction for Covalent Nanoscale Patterning of Multiple Proteins

This article describes a photochemical approach for independently patterning multiple proteins to an inert substrate, particularly for studies of cell adhesion. A photoactivatable chloropyrimidine ligand was employed for covalent immobilization of SnapTag fusion proteins on self-assembled monolayers of alkanethiolates on gold. A two-step procedure was used: first, patterned UV illumination of the surface activated protein capture ligands, and second, incubation with a SnapTag fusion protein bound to the surface in illuminated regions. Two different fluorescent proteins were patterned in registry with features of 400 nm in size over a 1 mm2 area. An example is given wherein an anti-carcinoembryonic antigen (anti-CEA) scFv antibody was patterned to direct the selective attachment of a human cancer cell line that express the CEA antigen. This method enables the preparation of surfaces with control over the density and activity of independently patterned proteins.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

A Facile N-Mercaptoethoxyglycinamide (MEGA) Linker Approach to Peptide Thioesterification and Cyclization

The C-terminal electrophilic activation of peptides by α-thioesterification requires strongly acidic conditions or complex chemical manipulations, which ultimately limit functional group compatibility and broad utility. Herein, we report a readily accessible N-mercaptoethoxyglycinamide (MEGA) solid-phase linker for the facile synthesis of latent peptide α-thioesters. Incubating peptide-MEGA sequences with 2-mercaptoethanesulfonic acid at mildly acidic pH yielded α-thioesters that were directly used in NCL without purification. The MEGA linker yielded robust access to thioesters ranging in length from 4 to 35 amino acids, and greatly simplified the synthesis of cyclic peptides. Finally, the high utility of MEGA was demonstrated by the one-pot synthesis of a functional analog of the Sunflower Trypsin Inhibitor 1.

A Comprehensive Evaluation of the Activity and Selectivity Profile of Ligands for RGD-binding Integrins

Integrins, a diverse class of heterodimeric cell surface receptors, are key regulators of cell structure and behaviour, affecting cell morphology, proliferation, survival and differentiation. Consequently, mutations in specific integrins, or their deregulated expression, are associated with a variety of diseases. In the last decades, many integrin-specific ligands have been developed and used for modulation of integrin function in medical as well as biophysical studies. The IC₅₀-values reported for these ligands strongly vary and are measured using different cell-based and cell-free systems. A systematic comparison of these values is of high importance for selecting the optimal ligands for given applications. In this study, we evaluate a wide range of ligands for their binding affinity towards the RGD-binding integrins αvβ3, αvβ5, αvβ6, αvβ8, α5β1, αIIbβ3, using homogenous ELISA-like solid phase binding assay.

β-Configured clickable [18F]FDGs as novel 18F-fluoroglycosylation tools for PET

We have developed a chemoselective ¹⁸F-fluoroglycosylation method for PET imaging that employ β-configured [¹⁸F]FDGs as prosthetic groups for ¹⁸F-labeling using CuAAC click chemistry.

A reactivity-based [18F]FDG probe for in vivo formaldehyde imaging using positron emission tomography

We present an aza-Cope-based reactivity probe for imaging formaldehyde in vivo using positron emission tomography.

Formaldehyde (FA) is a reactive carbonyl species (RCS) that plays a broad spectrum of roles in epigenetics, toxicology, and progression of diseases ranging from cancer to diabetes to neurodegeneration, motivating the development of translatable technologies for FA imaging. Here we report formaldehyde-caged-[¹⁸F]fluorodeoxyglucose-1 ([¹⁸F]FAC-FDG-1), an aza-Cope-based reactivity probe for in vivo FA imaging using positron emission tomography (PET). [¹⁸F]FAC-FDG-1 reacts selectively with FA over potentially competing analytes to generate [¹⁸F]FDG, allowing its FA-dependent uptake and retention in cell culture as well as in animal models. The relative uptake of [¹⁸F]FAC-FDG-1 was evaluated using FA-treated PC3 prostate cancer and U87-MG glioblastoma cells demonstrating a dose-dependent response to exogenously added FA. Moreover, [¹⁸F]FAC-FDG-1 is capable of FA detection in vivo using a PC3 tumor xenograft model. In addition to providing a unique tool for monitoring FA in living animals, these data establish a general approach for translatable detection of FA and other reactive biological analytes in vivo by exploiting the widely-available clinical [¹⁸F]FDG tracer as a masked aldehyde that can be caged by analyte-responsive triggers.

Caged [(18)F]FDG Glycosylamines for Imaging Acidic Tumor Microenvironments Using Positron Emission Tomography

Solid tumors are hypoxic with altered metabolism, resulting in secretion of acids into the extracellular matrix and lower relative pH, a feature associated with local invasion and metastasis. Therapeutic and diagnostic agents responsive to this microenvironment may improve tumor-specific delivery. Therefore, we pursued a general strategy whereby caged small-molecule drugs or imaging agents liberate their parent compounds in regions of low interstitial pH. In this manuscript, we present a new acid-labile prodrug method based on the glycosylamine linkage, and its application to a class of positron emission tomography (PET) imaging tracers, termed [(18)F]FDG amines. [(18)F]FDG amines operate via a proposed two-step mechanism, in which an acid-labile precursor decomposes to form the common radiotracer 2-deoxy-2-[(18)F]fluoro-d-glucose, which is subsequently accumulated by glucose avid cells. The rate of decomposition of [(18)F]FDG amines is tunable in a systematic fashion, tracking the pKa of the parent amine. In vivo, a 4-phenylbenzylamine [(18)F]FDG amine congener showed greater relative accumulation in tumors over benign tissue, which could be attenuated upon tumor alkalinization using previously validated models, including sodium bicarbonate treatment, or overexpression of carbonic anhydrase. This new class of PET tracer represents a viable approach for imaging acidic interstitial pH with potential for clinical translation.

PET imaging of bacterial infections with fluorine-18-labeled maltohexaose

A positron emission tomography (PET) tracer composed of (18)F-labeled maltohexaose (MH(18)F) can image bacteria in vivo with a sensitivity and specificity that are orders of magnitude higher than those of fluorodeoxyglucose ((18)FDG). MH(18)F can detect early-stage infections composed of as few as 10(5) E. coli colony-forming units (CFUs), and can identify drug resistance in bacteria in vivo. MH(18)F has the potential to improve the diagnosis of bacterial infections given its unique combination of high specificity and sensitivity for bacteria.

H-CRRETAWAC-OH, a lead structure for the development of radiotracer targeting integrin α5β1?

Imaging of angiogenic processes is of great interest in preclinical research as well as in clinical settings. The most commonly addressed target structure for imaging angiogenesis is the integrin α(v)β(3). Here we describe the synthesis and evaluation of [(18)F]FProp-Cys(*)-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys(*)-OH, a radiolabelled peptide designed to selectively target the integrin α(5)β(1). Conjugation of 4-nitrophenyl-(RS)-2-[(18)F]fluoropropionate provided [(18)F]FProp-Cys(*)-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys(*)-OH in high radiochemical purity (>95%) and a radiochemical yield of approx. 55%. In vitro evaluation showed α(5)β(1) binding affinity in the nanomolar range, whereas affinity to α(v)β(3) and α(IIb)β(3) was >50 μM. Cell uptake studies using human melanoma M21 (α(v)β(3)-positive and α(5)β(1)-negative), human melanoma M21-L (α(v)β(3)-negative and α(5)β(1)-negative), and human prostate carcinoma DU145 (α(v)β(3)-negative and α(5)β(1)-positive) confirmed receptor-specific binding. The radiotracer was stable in human serum and showed low protein binding. Biodistribution studies showed tumour uptake ranging from 2.5 to 3.5% ID/g between 30 and 120 min post-injection. However, blocking studies and studies using mice bearing α(5)β(1)-negative M21 tumours did not confirm receptor-specific uptake of [(18)F]FProp-Cys(*)-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys(*)-OH, although this radiopeptide revealed high affinity and substantial selectivity to α(5)β(1) in vitro. Further experiments are needed to study the in vivo metabolism of this peptide and to develop improved radiopeptide candidates suitable for PET imaging of α(5)β(1) expression in vivo.

Sweetening pharmaceutical radiochemistry by (18)f-fluoroglycosylation: a short review

At the time when the highly efficient [(18)F]FDG synthesis was discovered by the use of the effective precursor 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulfonyl- β -D-mannopyranose (mannose triflate) for nucleophilic (18)F-substitution, the field of PET in nuclear medicine experienced a long-term boom. Thirty years later, various strategies for chemoselective (18)F-labeling of biomolecules have been developed, trying to keep up with the emerging field of radiopharmaceutical sciences. Among the new radiochemical strategies, chemoselective (18)F-fluoroglycosylation methods aim at the sweetening of pharmaceutical radiochemistry by providing a powerful and highly valuable tool for the design of (18)F-glycoconjugates with suitable in vivo properties for PET imaging studies. This paper provides a short review (reflecting the literature not older than 8 years) on the different (18)F-fluoroglycosylation reactions that have been applied to the development of various (18)F-glycoconjugate tracers, including not only peptides, but also nonpeptidic tracers and high-molecular-weight proteins.

IsoDGR-tagged albumin: a new αvβ3 selective carrier for nanodrug delivery to tumors

A new cyclic peptide containing the isoDGR motif that, after coupling to albumin, selectively binds αvβ3, an integrin overexpressed in the tumor vasculature. IsoDGR-tagged albumin binds tumor vessels and can be exploited as a carrier for the preparation of tumor vasculature-selective nanomedicines, such as gold nanoparticles (Au) carrying tumor necrosis factor α (TNF), a potent vascular damaging agent.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Direct radiolabelling of proteins at cysteine using [18F]-fluorosugars

A strategy for the site-specific attachment of 2-deoxy-2-fluorosugars to cysteine and dehydroalanine tagged proteins is reported. When combined with thionation of fluorosugars, such as the widely available (18)F probe 2-deoxy-2-[(18)F]fluoroglucose ([(18)F]FDG), this methodology allows fast and direct access to site-specific [(18)F]FDG-labelled proteins.

New F-18 prosthetic group via oxime coupling

A novel fluorine-18 prosthetic ligand, 5-(1,3-dioxolan-2-yl)-2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridine [(18)F]2, has been synthesized. The prosthetic ligand is formed in high radiochemical yield (rcy = 71 ± 2%, n = 3) with excellent radiochemical purity (rcp = 99 ± 1%, n = 3) in a short reaction time (10 min). [(18)F]2 is a small, neutral, organic complex, easily synthesized in four steps from a readily available starting material. It can be anchored onto a target molecule containing an aminooxy functional group under acidic conditions by way of an oxime bond. We report herein two examples [(18)F]23 and [(18)F]24, potential imaging agents for β-amyloid plaques, which were labeled with this prosthetic group. This approach could be used for labeling proteins and peptides containing an aminooxy group. Biodistribution in male ICR mice for both oxime labeled complexes [(18)F]23 and [(18)F]24 were compared to that of the known β-amyloid plaque indicator, [(18)F]-AV-45, florbetapir 1. Oximes [(18)F]23 and [(18)F]24 are larger in size and therefore should reduce the blood-brain barrier (BBB) penetration. The brain uptake for oxime [(18)F]23 appeared to be reduced, but still retained some capability to cross the BBB. Oxime [(18)F]24 showed promising results after 2 min post injection (0.48% dose/gram); however, the uptake increased after 30 min post injection (0.92% dose/gram) suggesting an in vivo decomposition/metabolism of compound [(18)F]24. We have demonstrated a general protocol for the fluoride-18 labeling with a new prosthetic ligand [(18)F]2 that is tolerant toward several functional groups and is formed via chemoselective oxime coupling.

Multimerization of cRGD peptides by click chemistry: synthetic strategies, chemical limitations, and influence on biological properties

Integrin α(ν)β(3) is overexpressed on endothelial cells of growing vessels as well as on several tumor types, and so integrin-binding radiolabeled cyclic RGD pentapeptides have attracted increasing interest for in vivo imaging of α(ν)β(3) integrin expression by positron emission tomography (PET). Of the cRGD derivatives available for imaging applications, systems comprising multiple cRGD moieties have recently been shown to exhibit highly favorable properties in relation to monomers. To assess the synthetic limits of the cRGD-multimerization approach and thus the maximum multimer size achievable by using different efficient conjugation reactions, we prepared a variety of multimers that were further investigated in vitro with regard to their avidities to integrin α(ν)β(3.) The synthesized peptide multimers containing increasing numbers of cRGD moieties on PAMAM dendrimer scaffolds were prepared by different click chemistry coupling strategies. A cRGD hexadecimer was the largest construct that could be synthesized under optimized reaction conditions, thus identifying the current synthetic limitations for cRGD multimerization. The obtained multimeric systems were conjugated to a new DOTA-based chelator developed for the derivatization of sterically demanding structures and successfully labeled with (68)Ga for a potential in vivo application. The evaluated multimers showed very high avidities-increasing with the number of cRGD moieties-in in vitro studies on immobilized α(ν)β(3) integrin and U87MG cells, of up to 131- and 124-fold, respectively, relative to the underivatized monomer.

In vitro and in vivo characterization of novel 18F-labeled bombesin analogues for targeting GRPR-positive tumors

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a number of human tumors and has been targeted with radiolabeled bombesin analogues for the diagnosis and therapy of these cancers. Seven bombesin analogues containing various linkers and peptide sequences were designed, synthesized, radiolabeled with (18)F, and characterized in vitro and in vivo as potential PET imaging agents. Binding studies displayed nanomolar binding affinities toward human GRPR for all synthesized bombesin analogues. Two high-affinity peptide candidates 6b (K(i) = 0.7 nM) and 7b (K(i) = 0.1 nM) were chosen for further in vivo evaluation. Both tracers revealed specific uptake in GRPR-expressing PC-3 tumors and the pancreas. Compared to [(18)F]6b, compound [(18)F]7b was characterized by superior tumor uptake, higher specificity of tracer uptake, and more favorable tumor-to-nontarget ratios. In vivo PET imaging allowed for the visualization of PC-3 tumor in nude mice suggesting that [(18)F]7b is a promising PET tracer candidate for the diagnosis of GRPR-positive tumors in humans.

Positron emission tomography tracers for imaging angiogenesis

Position emission tomography imaging of angiogenesis may provide non-invasive insights into the corresponding molecular processes and may be applied for individualized treatment planning of antiangiogenic therapies. At the moment, most strategies are focusing on the development of radiolabelled proteins and antibody formats targeting VEGF and its receptor or the ED-B domain of a fibronectin isoform as well as radiolabelled matrix metalloproteinase inhibitors or alpha(v)beta(3) integrin antagonists. Great efforts are being made to develop suitable tracers for different target structures. All of the major strategies focusing on the development of radiolabelled compounds for use with positron emission tomography are summarized in this review. However, because the most intensive work is concentrated on the development of radiolabelled RGD peptides for imaging alpha(v)beta(3) expression, which has successfully made its way from bench to bedside, these developments are especially emphasized.

(18)F-Fluoroglucosylation of peptides, exemplified on cyclo(RGDfK)

PURPOSE

Oxime formation between an aminooxy-functionalized peptide and an (18)F-labelled aldehyde has recently been introduced as a powerful method for the rapid one-step chemoselective synthesis of radiofluorinated peptides.

MATERIALS AND METHODS

Here, the potential of using routinely produced and thus readily available [(18)F]fluorodeoxyglucose ([(18)F]FDG) as the aldehydic prosthetic group was investigated using an aminooxyacetyl-conjugated cyclic RGD peptide (cyclo(RGDfK(Aoa-(Boc)) as a model peptide.

RESULTS

The use of [(18)F]FDG from routine production ([(18)F]FDGTUM) containing an excess of D: -glucose did not allow the radiosynthesis of [(18)F]FDG-RGD in activities >37 MBq in reasonable yield, rendering the direct use of clinical grade [(18)F]FDG for the routine clinical synthesis of (18)F-labelled peptides impossible. Using no-carrier-added (n.c.a.) [(18)F]FDG obtained via HPLC separation of [(18)F]FDGTUM from excess glucose, however, afforded [(18)F]FDG-RGD in yields of 56-93% (decay corrected) and activities up to 37 MBq. Suitable reaction conditions were 20 min at 120 degrees C and pH 2.5, and a peptide concentration of 5 mM. In a preliminary in vivo biodistribution study in M21 melanoma-bearing nude mice, [(18)F]FDG-RGD showed increased tumour accumulation compared to the "gold standard" [(18)F]galacto-RGD (2.18 vs 1.49 %iD/g, respectively, at 120 min after injection), but also slightly increased uptake in non-target organs, leading to comparable tumour/organ ratios for both compounds.

CONCLUSION

These data demonstrate that chemoselective (18)F-labelling of aminooxy-functionalized peptides using n.c.a. [(18)F]FDG represents a radiofluorination/glycosylation strategy that allows preparation of (18)F-labelled peptides in high yield with suitable pharmacokinetics. As soon as the necessary n.c.a. preparation of [(18)F]FDG prior to reaction with the Aoa-peptide can be implemented in a fully automated [(18)F]FDG-synthesis, [(18)F]fluoroglucosylation of peptides may represent a promising alternative to currently used chemoselective one-step (18)F-labelling protocols.

A series of 2-O-trifluoromethylsulfonyl-D-mannopyranosides as precursors for concomitant 18F-labeling and glycosylation by click chemistry

A series of 'clickable' mannopyranosides bearing a triflate leaving group at C-2 position were synthesized and tested for their potential as (18)F-labeling precursors. 3,4,6-Tri-O-acetyl-2-O-trifluoromethanesulfonyl-beta-D-mannopyranosyl azide (2beta) was the most convenient precursor for a site-specific and reliable click chemistry-based three-step, two-pot concomitant (18)F-labeling and glycosylation of an alkyne-functionalized amino acid derivative.

Site-specific (18)F-labeling of the protein hormone leptin using a general two-step ligation procedure

The protein hormone leptin acts to regulate body fat and energy expenditure. Resistance to this hormone is implicated in human obesity and its pathophysiological consequences. In order to gain insight into the mechanism of leptin resistance, an (18)F-labeled derivative was developed to study the biodistribution of the hormone using positron emission tomography (PET). A two-step, site specific ligation approach was developed for this purpose, in which an aminooxy-reactive group was incorporated at the C-terminus of leptin using expressed protein ligation (EPL), which was subsequently derivatized with [ (18)F]fluorobenzaldehyde using an aniline-accelerated radiochemical oximation reaction. The modified hormone was shown to be biologically active in vitro and in vivo, and it was applied to PET imaging in ob/ ob mice. These protocols will allow for the routine production of site-specifically (18)F radiolabeled leptin, as well as other proteins, for use in PET imaging in systems from mouse to man.

Synthesis and application of [18F]FDG-maleimidehexyloxime ([18F]FDG-MHO): a [18F]FDG-based prosthetic group for the chemoselective 18F-labeling of peptides and proteins

2-[(18)F]Fluoro-2-deoxy-D-glucose ([(18)F]FDG) as the most important PET radiotracer is available in almost every PET center. However, there are only very few examples using [(18)F]FDG as a building block for the synthesis of (18)F-labeled compounds. The present study describes the use of [(18)F]FDG as a building block for the synthesis of (18)F-labeled peptides and proteins. [(18)F]FDG was converted into [(18)F]FDG-maleimidehexyloxime ([(18)F]FDG-MHO), a novel [(18)F]FDG-based prosthetic group for the mild and thiol group-specific (18)F labeling of peptides and proteins. The reaction was performed at 100 degrees C for 15 min in a sealed vial containing [(18)F]FDG and N-(6-aminoxy-hexyl)maleimide in 80% ethanol. [(18)F]FDG-MHO was obtained in 45-69% radiochemical yield (based upon [(18)F]FDG) after HPLC purification in a total synthesis time of 45 min. Chemoselecetive conjugation of [(18)F]FDG-MHO to thiol groups was investigated by the reaction with the tripeptide glutathione (GSH) and the single cysteine containing protein annexin A5 (anxA5). Radiolabeled annexin A5 ([(18)F]FDG-MHO-anxA5) was obtained in 43-58% radiochemical yield (based upon [(18)F]FDG-MHO, n = 6), and [(18)F]FDG-MHO-anxA5 was used for a pilot small animal PET study to assess in vivo biodistribution and kinetics in a HT-29 murine xenograft model.

Preparation of a promising angiogenesis PET imaging agent: 68Ga-labeled c(RGDyK)-isothiocyanatobenzyl-1,4,7-triazacyclononane-1,4,7-triacetic acid and feasibility studies in mice

Arg-Gly-Asp (RGD) derivatives have been labeled with various radioisotopes for the imaging of angiogenesis in ischemic tissue, in which alpha(v)beta(3) integrin plays an important role. In this study, cyclic Arg-Gly-Asp-D-Tyr-Lys [c(RGDyK)] was conjugated with 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (SCN-Bz-NOTA) and then labeled with (68)Ga. The labeled RGD so produced was subjected to an in vitro binding assay and in vivo biodistribution and PET studies.

METHODS

A mixture of SCN-Bz-NOTA (660 nmol) and c(RGDyK) (600 nmol) in 0.1 M sodium carbonate buffer (pH 9.5) was allowed to react for 20 h at room temperature in the dark for thiourea bond formation. The conjugate obtained was purified by semipreparative high-performance liquid chromatography (HPLC). The purified c(RGDyK)-SCN-Bz-NOTA (NOTA-RGD) was then labeled with (68)Ga from a (68)Ge/(68)Ga generator and purified by semipreparative HPLC. A competitive binding assay for c(RGDyK) and NOTA-RGD was performed with (125)I-c(RGDyK) as a radioligand and alpha(v)beta(3) integrin-coated plates as a solid phase. (68)Ga-NOTA-RGD (0.222 MBq/100 microL) was injected, through a tail vein, into mice with hind limb ischemia and into mice bearing human colon cancer SNU-C4 xenografts. Biodistribution and imaging studies were performed at 1 and 2 h after injection.

RESULTS

The labeling of NOTA-RGD with (68)Ga was straightforward. The K(i) values of c(RGDyK) and NOTA-RGD were 1.3 and 1.9 nM, respectively. In the biodistribution study, the mean +/- SD uptake of (68)Ga-NOTA-RGD by ischemic muscles was 1.6+/-0.2 percentage injected dose per gram (%ID/g); this uptake was significantly blocked by cold c(RGDyK) to 0.6+/-0.3 %ID/g (P<0.01). Tumor uptake was 5.1+/-1.0 %ID/g, and the tumor-to-blood ratio was 10.3+/-4.8. Small-animal PET revealed rapid excretion through the urine and high levels of tumor and kidney uptake.

CONCLUSION

Stable (68)Ga-NOTA-RGD was obtained in a straightforward manner at a high yield and showed a high affinity for alpha(v)beta(3) integrin, specific uptake by angiogenic muscles, a high level of uptake by tumors, and rapid renal excretion. (68)Ga-NOTA-RGD was found to be a promising radioligand for the imaging of angiogenesis.