Radiolabelling of isopeptide N epsilon-(gamma-glutamyl)-L-lysine by conjugation with N-succinimidyl-4-[18F]fluorobenzoate

One-Step Synthesis of [18F]Aromatic Electrophile Prosthetic Groups via Organic Photoredox Catalysis

To avoid the harsh conditions that are oftentimes adopted in direct radiofluorination reactions, conjugation of bioactive ligands with 18F-labeled prosthetic groups has become an important strategy to construct novel PET agents under mild conditions when the ligands are structurally sensitive. Prosthetic groups with [18F]fluoroarene motifs are especially appealing because of their stability in physiological environments. However, their preparation can be intricate, often requiring multistep radiosynthesis with functional group conversions to prevent the decomposition of unprotected reactive prosthetic groups during the harsh radiofluorination. Here, we report a general and simple method to generate a variety of highly reactive 18F-labeled electrophiles via one-step organophotoredox-mediated radiofluorination. The method benefits from high step-economy, reaction efficiency, functional group tolerance, and easily accessible precursors. The obtained prosthetic groups have been successfully applied in PET agent construction and subsequent imaging studies, thereby demonstrating the feasibility of this synthetic method in promoting imaging and biomedical research.

Development of brain-penetrable antibody radioligands for in vivo PET imaging of amyloid-β and tau

Introduction

Alzheimer's disease (AD) is characterized by the misfolding and aggregation of two major proteins: amyloid-beta (Aβ) and tau. Antibody-based PET radioligands are desirable due to their high specificity and affinity; however, antibody uptake in the brain is limited by the blood-brain barrier (BBB). Previously, we demonstrated that antibody transport across the BBB can be facilitated through interaction with the transferrin receptor (TfR), and the bispecific antibody-based PET ligands were capable of detecting Aβ aggregates via ex vivo imaging. Since tau accumulation in the brain is more closely correlated with neuronal death and cognition, we report here our strategies to prepare four F-18-labeled specifically engineered bispecific antibody probes for the selective detection of tau and Aβ aggregates to evaluate their feasibility and specificity, particularly for in vivo PET imaging.

Methods

We first created and evaluated (via both in vitro and ex vivo studies) four specifically engineered bispecific antibodies, by fusion of single-chain variable fragments (scFv) of a TfR antibody with either a full-size IgG antibody of Aβ or tau or with their respective scFv. Using [18F]SFB as the prosthetic group, all four 18F-labeled bispecific antibody probes were then prepared by conjugation of antibody and [18F]SFB in acetonitrile/0.1 M borate buffer solution (final pH ~ 8.5) with an incubation of 20 min at room temperature, followed by purification on a PD MiniTrap G-25 size exclusion gravity column.

Results

Based on both in vitro and ex vivo evaluation, the bispecific antibodies displayed much higher brain concentrations than the unmodified antibody, supporting our subsequent F18-radiolabeling. [18F]SFB was produced in high yields in 60 min (decay-corrected radiochemical yield (RCY) 46.7 ± 5.4) with radiochemical purities of >95%, confirmed by analytical high performance liquid chromatography (HPLC) and radio-TLC. Conjugation of [18F]SFB and bispecific antibodies showed a 65%-83% conversion efficiency with radiochemical purities of 95%-99% by radio-TLC.

Conclusions

We successfully labeled four novel and specifically engineered bispecific antibodies with [18F]SFB under mild conditions with a high RCY and purities. This study provides strategies to create brain-penetrable F-18 radiolabeled antibody probes for the selective detection of tau and Aβ aggregates in the brain of transgenic AD mice via in vivo PET imaging.

Protease-activated receptor 2 (PAR2)-targeting peptide derivatives for positron emission tomography (PET) imaging

The proteolytically-activated G protein-coupled receptor (GPCR) protease-activated receptor 2 (PAR2), is implicated in various cancers and inflammatory diseases. Synthetic ligands and in vitro imaging probes targeting this receptor have been developed with low nanomolar affinity, however, no in vivo imaging probes exist for PAR2. Here, we report the strategic design, synthesis, and biological evaluation of a series of novel 4-fluorobenzoylated PAR2-targeting peptides derived from 2f-LIGRLO-NH₂ (2f-LI-) and Isox-Cha-Chg-Xaa-NH₂ (Isox-) peptide families, where the 4-fluorobenzoyl moiety acts as the ¹⁹F-standard of an ¹⁸F-labeled probe for potential use in in vivo imaging. We found that several of the 4-fluorobenzoylated peptides from the 2f-LI-family exhibited PAR2 selectivity with moderate potency (EC₅₀ = 151-252 nM), whereas several from the Isox-family exhibited PAR2 selectivity with high potency (EC₅₀ = 13-42 nM). Our lead candidate, Isox-Cha-Chg-Ala-Arg-Dpr(4FB)-NH₂ (EC₅₀ = 13 nM), was successfully synthesized with fluorine-18 with a radiochemical yield of 37%, radiochemical purity of >98%, molar activity of 20 GBq/μmol, and an end of synthesis time of 125 min. Biodistribution studies and preliminary PET imaging of the tracer in mice showed predominantly renal clearance. This ¹⁸F-labeled tracer is the first reported PAR2 imaging agent with potential for use in vivo. Future work will explore the use of this tracer in cancer xenografts and inflammation models involving upregulation of PAR2 expression.

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.

Room Temperature Al18 F Labeling of 2-Aminomethylpiperidine-Based Chelators for PET Imaging

Positron emission tomography (PET) is a non-invasive molecular imaging technology that is constantly expanding, with a high demand for specific antibody-derived imaging probes. The use of tracers based on temperature-sensitive molecules (i. e. Fab, svFab, nanobodies) is increasing and has led us to design a class of chelators based on the structure of 2-aminomethylpiperidine (AMP) with acetic and/or hydroxybenzyl pendant arms (2-AMPTA, NHB-2-AMPDA, and 2-AMPDA-HB), which were investigated as such for {Al¹⁸ F}²⁺ -core chelation efficiency. All the compounds were characterized by HPLC-MS analysis and NMR spectroscopy. The AlF-18 labeling reactions were performed under various conditions (pH/temperature), and the radiolabeled chelates were purified and characterized by radio-TLC and radio-HPLC. The stability of labeled chelates was investigated up to 240 min in human serum (HS), EDTA 5 mM, PBS and 0.9 % NaCl solutions. The in vivo stability of [Al¹⁸ F(2-AMPDA-HB)]^- was assessed in healthy nude mice (n=6). Radiochemical yields between 55 % and 81 % were obtained at pH 5 and room temperature. High stability in HS was measured for [Al¹⁸ F(2-AMPDA-HB)]^- , with 90 % of F-18 complexed after 120 min. High stability in vivo, rapid hepatobiliary and renal excretion, with low accumulation of free F-18 in bones were measured. Thus, this new Al¹⁸ F-chelator may have a great impact on immuno-PET radiopharmacy, by facilitating the development of new fluorine-18-labeled heat-sensitive biomolecules.

On-demand radiosynthesis of N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) on an electrowetting-on-dielectric microfluidic chip for 18F-labeling of protein

An all-electronic, droplet-based batch microfluidic device, operated using the electrowetting on dielectric (EWOD) mechanism was developed for on-demand synthesis of N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB), the most commonly used ¹⁸F-prosthetic group for biomolecule labeling. In order to facilitate the development of peptides, and proteins as new diagnostic and therapeutic agents, we have diversified the compact EWOD microfluidic platform to perform the three-step radiosynthesis of [¹⁸F]SFB starting from the no carrier added [¹⁸F]fluoride ion. In this report, we established an optimal microliter droplet reaction condition to obtain reliable yields and synthesized [¹⁸F]SFB with sufficient radioactivity for subsequent conjugation to the anti-PSCA cys-diabody (A2cDb) and for small animal imaging. The three-step, one-pot radiosynthesis of [¹⁸F]SFB radiochemistry was adapted to a batch microfluidic platform with a reaction droplet sandwiched between two parallel plates of an EWOD chip, and optimized. Specifically, the ratio of precursor to base, droplet volume, reagent concentration, reaction time, and evaporation time were found be to be critical parameters. [¹⁸F]SFB was successfully synthesized on the EWOD chip in 39 ± 7% (n = 4) radiochemical yield in a total synthesis time of ∼120 min ([¹⁸F]fluoride activation, [¹⁸F]fluorination, hydrolysis, and coupling reaction, HPLC purification, drying and reformulation). The reformulation and stabilization step for [¹⁸F]SFB was important to obtain a high protein labeling efficiency of 33.1 ± 12.5% (n = 3). A small-animal immunoPET pilot study demonstrated that the [¹⁸F]SFB-PSCA diabody conjugate showed specific uptake in the PSCA-positive human prostate cancer xenograft. The successful development of a compact footprint of the EWOD radiosynthesizer has the potential to empower biologists to produce PET probes of interest themselves in a standard laboratory.

An all-electronic, droplet-based batch microfluidic device, operated using the electrowetting on dielectric (EWOD) mechanism was developed for on-demand synthesis of acommonly used ¹⁸F-prosthetic group for biomolecule labeling.

Peptidomimetic growth hormone secretagogue derivatives for positron emission tomography imaging of the ghrelin receptor

The ghrelin receptor is a seven-transmembrane (7-TM) receptor known to have an increased level of expression in human carcinoma and heart failure. Recent work has focused on the synthesis of positron emission tomography (PET) probes designed to target and image this receptor for disease diagnosis and staging. However, these probes have been restricted to small-molecule quinalizonones and peptide derivatives of the endogenous ligand ghrelin. We describe the design, synthesis and biological evaluation of a series of 4-fluorobenzoylated growth hormone secretagogues (GHSs) derived from peptidic (GHRP-1, GHPR-2 and GHRP-6) and peptidomimetic (G-7039, [1-Nal⁴]G-7039 and ipamorelin) families in order to test locations for the insertion of fluorine-18 for PET imaging. The peptidomimetic G-7039 was found to be the most suitable for ¹⁸F-radiolabelling as its non-radioactive 4-fluorobenzoylated analogue ([1-Nal⁴,Lys⁵(4-FB)]G-7039), had both a high binding affinity (IC₅₀ = 69 nM) and promising in vitro efficacy (EC₅₀ = 1.1 nM). Prosthetic group radiolabelling of the precursor compound [1-Nal⁴]G-7039 using N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) delivered the PET probe [1-Nal⁴,Lys⁵(4-[¹⁸F]-FB)]G-7039 in an average decay-corrected radiochemical yield of 48%, a radio-purity ≥ 99% and an average molar activity of >34 GBq/μmol. This compound could be investigated as a PET probe for the detection of diseases that are characterised by overexpression of the ghrelin receptor.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Fast indirect fluorine-18 labeling of protein/peptide using the useful 6-fluoronicotinic acid-2,3,5,6-tetrafluorophenyl prosthetic group: A method comparable to direct fluorination

Fluorine-18 labeling of biomolecules is mostly performed by an indirect labeling method using a prosthetic group. Fluorine-18 labeled 6-fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester is a useful prosthetic group to radiolabel a protein. Recently, we reported an improved preparation of this prosthetic group. To test the conjugation efficiency of the labeled ester prepared by this method, we have performed conjugation reactions with a peptide, a protein, and a small molecule. Prostate-specific membrane antigen targeting small molecule [18 F]DCFPyL, αvβ3 integrin receptors targeting peptide [18 F]c(RGDfK) and [18 F]albumin were prepared in good radiochemical yields. The conjugation reactions were completed at 40°C to 50°C in 10 minutes. The overall radiochemical yield was 25% to 43% in 30 to 45 minutes.

Continuous-Flow Synthesis of N-Succinimidyl 4-[18F]fluorobenzoate Using a Single Microfluidic Chip

In the field of positron emission tomography (PET) radiochemistry, compact microreactors provide reliable and reproducible synthesis methods that reduce the use of expensive precursors for radiolabeling and make effective use of the limited space in a hot cell. To develop more compact microreactors for radiosynthesis of ¹⁸F-labeled compounds required for the multistep procedure, we attempted radiosynthesis of N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) via a three-step procedure using a microreactor. We examined individual steps for [¹⁸F]SFB using a batch reactor and microreactor and developed a new continuous-flow synthetic method with a single microfluidic chip to achieve rapid and efficient radiosynthesis of [¹⁸F]SFB. In the synthesis of [¹⁸F]SFB using this continuous-flow method, the three-step reaction was successfully completed within 6.5 min and the radiochemical yield was 64 ± 2% (n = 5). In addition, it was shown that the quality of [¹⁸F]SFB synthesized on this method was equal to that synthesized by conventional methods using a batch reactor in the radiolabeling of bovine serum albumin with [¹⁸F]SFB.

Targeting lysyl oxidase for molecular imaging in breast cancer

INTRODUCTION

Lysyl oxidase (LOX; ExPASy ENZYME entry: EC 1.4.3.13) and members of the LOX-like family, LOXL1-LOXL4, are copper-dependent enzymes that can modify proteins of the extracellular matrix. Expression of LOX is elevated in many human cancers, including breast cancer. LOX expression correlates with the level of tissue hypoxia, and it is known to play a critical role in breast cancer metastasis. The goal of the present study was to target LOX with (1) molecular probe fluorescent labeling to visualize LOX in vitro and (2) a radiolabeled peptide to target LOX in vivo in three different preclinical models of breast cancer.

METHODS

Gene expression of all five members of the LOX family was analyzed at the transcript level via microarray analysis using tissue biopsy samples from 176 patients with breast cancer. An oligopeptide sequence (GGGDPKGGGGG) was selected as a substrate-based, LOX-targeting structure. The peptide was labeled with fluorescein isothiocyanate (FITC) for confocal microscopy experiments with the murine breast cancer cell line EMT-6. In vivo molecular imaging experiments were performed using a C-terminal amidated peptide, GGGDPKGGGGG, labeled with a short-lived positron emitter, fluorine-18 ((18)F), for positron emission tomography (PET) in three different breast cancer models: EMT6, MCF-7 and MDA-MB-231. The PET experiments were carried out in the presence or absence of β-aminopropionitrile (BAPN), an irreversible inhibitor of LOX.

RESULTS

Immunostaining experiments using a LOX-specific antibody on EMT-6 cells cultured under hypoxic conditions confirmed the elevation of LOX expression in these cells. An FITC-labeled oligopeptide, FITC-Ava-GGGDPKGGGGG-NH2, was found to be localized in different cellular compartments under these conditions. After injection of [(18)F]fluorobenzoate-GGGDPKGGGGG-NH2, radioactivity uptake was visible in all three breast cancer models in vivo. Tumor uptake was reduced by predosing the animals with 2 mg of BAPN 4 h or 24 h before injection of the radiotracer.

CONCLUSIONS

The present data support further investigation into the development of LOX-binding radiolabeled peptides as molecular probes for molecular imaging of LOX expression in cancer.

Synthesis of fluorine-18 radio-labeled serum albumins for PET blood pool imaging

We sought to develop a practical, reproducible and clinically translatable method of radiolabeling serum albumins with fluorine-18 for use as a PET blood pool imaging agent in animals and man. Fluorine-18 radiolabeled fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester, [(18)F]F-Py-TFP was prepared first by the reaction of its quaternary ammonium triflate precursor with [(18)F]tetrabutylammonium fluoride ([(18)F]TBAF) according to a previously published method for peptides, with minor modifications. The incubation of [(18)F]F-Py-TFP with rat serum albumin (RSA) in phosphate buffer (pH9) for 15 min at 37-40 °C produced fluorine-18-radiolabeled RSA and the product was purified using a mini-PD MiniTrap G-25 column. The overall radiochemical yield of the reaction was 18-35% (n=30, uncorrected) in a 90-min synthesis. This procedure, repeated with human serum albumin (HSA), yielded similar results. Fluorine-18-radiolabeled RSA demonstrated prolonged blood retention (biological half-life of 4.8 hours) in healthy awake rats. The distribution of major organ radioactivity remained relatively unchanged during the 4 hour observation periods either by direct tissue counting or by dynamic PET whole-body imaging except for a gradual accumulation of labeled metabolic products in the bladder. This manual method for synthesizing radiolabeled serum albumins uses fluorine-18, a widely available PET radionuclide, and natural protein available in both pure and recombinant forms which could be scaled up for widespread clinical applications. These preclinical biodistribution and PET imaging results indicate that [(18)F]RSA is an effective blood pool imaging agent in rats and might, as [(18)F]HSA, prove similarly useful as a clinical imaging agent.

18F-Labeled Peptides: The Future Is Bright

Radiolabeled peptides have been the subject of intense research efforts for targeted diagnostic imaging and radiotherapy over the last 20 years. Peptides offer several advantages for receptor imaging and targeted radiotherapy. The low molecular weight of peptides allows for rapid clearance from the blood and non-target tissue, which results in favorable target-to-non-target ratios. Moreover, peptides usually display good tissue penetration and they are generally non-immunogenic. A major drawback is their potential low metabolic stability. The majority of currently used radiolabeled peptides for targeted molecular imaging and therapy of cancer is labeled with various radiometals like 99mTc, 68Ga, and 177Lu. However, over the last decade an increasing number of 18F-labeled peptides have been reported. Despite of obvious advantages of 18F like its ease of production in large quantities at high specific activity, the low β+ energy (0.64 MeV) and the favorable half-life (109.8 min), 18F-labeling of peptides remains a special challenge. The first part of this review will provide a brief overview on chemical strategies for peptide labeling with 18F. A second part will discuss recent technological advances for 18F-labeling of peptides with special focus on microfluidic technology, automation, and kit-like preparation of 18F-labeled peptides.

A dual inhibitor of matrix metalloproteinases and a disintegrin and metalloproteinases, [¹⁸F]FB-ML5, as a molecular probe for non-invasive MMP/ADAM-targeted imaging

BACKGROUND

Numerous clinical studies have shown a correlation between increased matrix metalloproteinase (MMP)/a disintegrin and metalloproteinase (ADAM) activity and poor outcome of cancer. Various MMP inhibitors (MMPIs) have been developed for therapeutic purposes in oncology. In addition, molecular imaging of MMP/ADAM levels in vivo would allow the diagnosis of tumors. We selected the dual inhibitor of MMPs and ADAMs, ML5, which is a hydroxamate-based inhibitor with affinities for many MMPs and ADAMs. ML5 was radiolabelled with (18)F and the newly obtained radiolabelled inhibitor was evaluated in vitro and in vivo.

MATERIALS AND METHODS

ML5 was radiolabelled by direct acylation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) for PET (positron emission tomography). The resulting radiotracer [(18)F]FB-ML5 was evaluated in vitro in human bronchial epithelium 16HBE cells and breast cancer MCF-7 cells. The non-radioactive probe FB-ML5 and native ML5 were tested in a fluorogenic inhibition assay against MMP-2, -9, -12 and ADAM-17. The in vivo kinetics of [(18)F]FB-ML5 were examined in a HT1080 tumor-bearing mouse model. Specificity of probe binding was examined by co-injection of 0 or 2.5mg/kg ML5.

RESULTS

ML5 and FB-ML5 showed high affinity for MMP-2, -9, -12 and ADAM-17; indeed IC50 values were respectively 7.4 ± 2.0, 19.5 ± 2.8, 2.0 ± 0.2 and 5.7 ± 2.2 nM and 12.5 ± 3.1, 31.5 ± 13.7, 138.0 ± 10.9 and 24.7 ± 2.8 nM. Radiochemical yield of HPLC-purified [(18)F]FB-ML5 was 13-16% (corrected for decay). Cellular binding of [(18)F]FB-ML5 was reduced by 36.6% and 27.5% in MCF-7 and 16 HBE cells, respectively, after co-incubation with 10 μM of ML5. In microPET scans, HT1080 tumors exhibited a low and homogeneous uptake of the tracer. Tumors of mice injected with [(18)F]FB-ML5 showed a SUVmean of 0.145 ± 0.064 (n=6) which decreased to 0.041 ± 0.027 (n=6) after target blocking (p<0.05). Ex vivo biodistribution showed a rapid excretion through the kidneys and the liver. Metabolite assays indicated that the parent tracer represented 23.2 ± 7.3% (n=2) of total radioactivity in plasma, at 90 min post injection (p.i.).

CONCLUSION

The nanomolar affinity MMP/ADAM inhibitor ML5 was successfully labelled with (18)F. [(18)F]FB-ML5 demonstrated rather low binding in ADAM-17 overexpressing cell lines. [(18)F]FB-ML5 uptake showed significant reduction in the HT1080 tumor in vivo after co-injection of ML5. [(18)F]FB-ML5 may be suitable for the visualization/quantification of diseases overexpressing simultaneously MMPs and ADAMs.

Improved and optimized one-pot method for N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) synthesis using microwaves

N-Succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) is a potential prosthetic agent for novel tracer development in positron emission tomography (PET). Previously, we reported a microwave-assisted one-pot synthesis of [(18)F]SFB with high efficacy. Herein, we reveal an improved and optimized approach based on this former model for producing [(18)F]SFB. With optimized approaches, the entire protocol can be completed within 25min, and [(18)F]SFB is generated in satisfactory quality for direct use without further purification via high-performance liquid chromatography.

Neither azeotropic drying, nor base nor other additives: a minimalist approach to 18F-labeling

A novel radiofluorination procedure using only precursor and [¹⁸F]fluoride without the need for azeotropic drying, base and other ingredients was developed.

Single-step syntheses of no-carrier-added functionalized [18F]fluoroarenes as labeling synthons from diaryliodonium salts

Radiotracers labelled with short-lived fluorine-18 (t(1/2) = 109.7 min) are keenly sought for biomedical imaging with positron emission tomography (PET). The radiotracers are mostly required at high specific radioactivities, necessitating their radiosyntheses from cyclotron-produced no-carrier-added [(18)F]fluoride ion. PET radiotracers encompass wide structural diversity and molecular weight. Hence, diverse (18)F-labeling methodology is needed to accomplish the required radiosyntheses in a simple and rapid manner. A useful strategy is to introduce nucleophilic [(18)F]fluoride ion first into a labeling synthon that may then be applied to label the target radiotracer. Here, we show that various functionalized [(18)F]fluoroarenes may be rapidly synthesized as labeling synthons through single-step reactions of appropriate diaryliodonium salts with [(18)F]fluoride ion. Decay-corrected radiochemical yields (RCYs) varied with position of functional group, choice of electron-rich aryl ring in the diaryliodonium salt, and choice of anion. Under best conditions, (18)F-labeled fluorobenzaldehydes, fluorobenzyl halides, fluorobenzoic acid esters and fluorophenyl ketones were obtained selectively in 40-73%, 20-55%, 46-89% and 81-98% RCYs, respectively. This versatile straightforward methodology will enhance the scope for producing structurally complex, yet useful, PET radiotracers.

Crystal structure of N-succinimidyl-4-fluorobenzoate, C11H8FNO4

C11H8FNO4, monoclinic, P21/n (no. 14), a = 9.177(3) Å, b = 5.472(2) Å, c = 22.414(7) Å, β = 95.264(4)°, V = 1120.7 Å3, Z = 4, Rgt(F) = 0.0493, wRref(F2) = 0.1195, T = 203 K.

Synthesis of [(18)F]SiFB: a prosthetic group for direct protein radiolabeling for application in positron emission tomography

N-Succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB) is a highly reactive prosthetic group for radiolabeling of proteins for use in positron emission tomography (PET). It is similar to N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB), the 'gold-standard' prosthetic group for protein (18)F-labeling, but can be synthesized using a much shorter and technically easier procedure. A recently reported simple procedure to obtain anhydrous (18)F- by avoiding time-consuming azeotropic drying is applied with a slight modification to prevent basic hydrolysis of the active N-hydroxysuccinimide (NHS) ester moiety of [(18)F]SiFB. The labeling of [(18)F]SiFB is performed by a fast (18)F-(19)F isotopic exchange (IE) reaction at room temperature (20-25 °C) within 30 min. [(18)F]SiFB is purified using a C18 cartridge instead of HPLC, further decreasing the overall time required for protein labeling. High specific activities > 18.5 GBq μmol(-1) (> 500 Ci mmol(-1)) can be obtained. Finally, incubation of [(18)F]SiFB with the desired protein in an aqueous solution at pH 9, followed by HPLC purification, provides the final solution of the labeled protein ready for in vivo applications.

Efficient method for site-specific 18F-labeling of biomolecules using the rapid condensation reaction between 2-cyanobenzothiazole and cysteine

An efficient method based on a rapid condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine has been developed for (18)F-labeling of N-terminal cysteine-bearing peptides and proteins. An (18)F-labeled dimeric cRGD ([(18)F]CBTRGD(2)) has been synthesized with an excellent radiochemical yield (92% based on radio-HPLC conversion, 80% decay-corrected, and isolated yield) and radiochemical purity (>99%) under mild conditions using (18)F-CBT, and shown good in vivo tumor targeting efficiency for PET imaging. The labeling strategy was also applied to the site-specific (18)F-labeling of a protein, Renilla lucifierase (RLuc8) with a cysteine residue at its N-terminus. The protein labeling was achieved with 12% of decay-corrected radiochemical yield and more than 99% radiochemical purity. This strategy should provide a general approach for efficient and site-specific (18)F-labeling of various peptides and proteins for in vivo molecular imaging applications.

Oxalic acid supported Si-18F-radiofluorination: one-step radiosynthesis of N-succinimidyl 3-(di-tert-butyl[18F]fluorosilyl)benzoate ([18F]SiFB) for protein labeling

N-Succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB), a novel synthon for one-step labeling of proteins, was synthesized via a simple (18)F-(19)F isotopic exchange. A new labeling technique that circumvents the cleavage of the highly reactive active ester moiety under regular basic (18)F-labeling conditions was established. In order to synthesize high radioactivity amounts of [(18)F]SiFB, it was crucial to partially neutralize the potassium oxalate/hydroxide that was used to elute (18)F(-) from the QMA cartridge with oxalic acid to prevent decomposition of the active ester moiety. Purification of [(18)F]SiFB was performed by simple solid-phase extraction, which avoided time-consuming HPLC and yielded high specific activities of at least 525 Ci/mmol and radiochemical yields of 40-56%. In addition to conventional azeotropic drying of (18)F(-) in the presence of [K(+)⊂2.2.2.]C(2)O(4), a strong anion-exchange (SAX) cartridge was used to prepare anhydrous (18)F(-) for nucleophilic radio-fluorination omitting the vacuum assisted drying of (18)F(-). Using a lyophilized mixture of [K(+)⊂2.2.2.]OH resolubilized in acetonitrile, the (18)F(-) was eluted from the SAX cartridge and used directly for the [(18)F]SiFB synthesis. [(18)F]SiFB was applied to the labeling of various proteins in likeness to the most commonly used labeling synthon in protein labeling, N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). Rat serum albumin (RSA), apo-transferrin, a β-cell-specific single chain antibody, and erythropoietin were successfully labeled with [(18)F]SiFB in good radiochemical yields between 19% and 36%. [(18)F]SiFB- and [(18)F]SFB-derivatized RSA were directly compared as blood pool imaging agents in healthy rats using small animal positron emission tomography. Both compounds demonstrated identical biodistributions in healthy rats, accurately visualizing the blood pool with PET.

In vivo positron emission tomography imaging of protease activity by generation of a hydrophobic product from a noninhibitory protease substrate

PURPOSE

To develop an imaging technology for protease activities in patients that could help in prognosis prediction and in design of personalized, protease-based inhibitors and prodrugs for targeted therapy.

EXPERIMENTAL DESIGN

Polyethylene glycol (PEG) was covalently attached to the N-terminus of a hydrophilic peptide substrate (GPLGVR) for matrix metalloproteinase (MMP) to increase hydrophilicity. PEG-peptide was then linked to a hydrophobic tetramethylrhodamine (TMR) domain and labeled with (18)F to form a PEG-peptide-(18)F-TMR probe. Specific cleavage of the probe by MMP2 was tested in vitro by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF). In vivo imaging of MMP2-expressing tumors was evaluated by micro-PET.

RESULTS

The hydrophobic TMR fragment (948 Da) was specifically generated by MMP2 enzymes and MMP-expressing HT1080 cells but not control MCF-7 cells. MMP-expressing HT1080 cells and tumors selectively accumulated the hydrolyzed, hydrophobic TMR fragment at sites of protease activity. Importantly, we found that (18)F-labeled probe ((18)F-TMR) preferentially localized in HT1080 tumors but not control MCF-7 tumors as shown by micro-PET. Uptake of the probe in HT1080 tumors was 18.4 ± 1.9-fold greater than in the MCF-7 tumors 30 minutes after injection. These results suggest that the PEG-peptide-(18)F-TMR probe displays high selectivity for imaging MMP activity.

CONCLUSIONS

This strategy successfully images MMP expression in vivo and may be extended to other proteases to predict patient prognosis and to design personalized, protease-based inhibitors and prodrug-targeted therapies.

Microwave-assisted one-pot synthesis of N-succinimidyl-4[ ¹⁸F]fluorobenzoate ([¹⁸F]SFB)

Biomolecules, including peptides¹⁻⁹, proteins¹⁰⁻¹¹, and antibodies and their engineered fragments¹²⁻¹⁴, are gaining importance as both potential therapeutics and molecular imaging agents. Notably, when labeled with positron-emitting radioisotopes (e.g., Cu-64, Ga-68, or F-18), they can be used as probes for targeted imaging of many physiological and pathological processes.¹⁵⁻¹⁸ Therefore, significant effort has devoted to the synthesis and exploration of ¹⁸F-labeled biomolecules. Although there are elegant examples of the direct ¹⁸F-labeling of peptides,¹⁹⁻²² the harsh reaction conditions (i.e., organic solvent, extreme pH, high temperature) associated with direct radiofluorination are usually incompatible with fragile protein samples. To date, therefore, the incorporation of radiolabeled prosthetic groups into biomolecules remains the method of choice.²³(,)²⁴ N-Succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB),²⁵⁻³⁷ a Bolton-Hunter type reagent that reacts with the primary amino groups of biomolecules, is a very versatile prosthetic group for the ¹⁸F-labeling of a wide spectrum of biological entities, in terms of its evident in vivo stability and high radiolabeling yield. After labeling with [¹⁸F]SFB, the resulting [F]fluorobenzoylated biomolecules could be explored as potential PET tracers for in vivo imaging studies.¹ Most [¹⁸F]SFB radiosyntheses described in the current literatures require two or even three reactors and multiple purifications by using either solid phase extraction (SPE) or high-performance liquid chromatography (HPLC). Such lengthy processes hamper its routine production and widespread applications in the radiolabeling of biomolecules. Although several module-assisted [¹⁸F]SFB syntheses have been reported²⁹⁻³²,⁴¹⁻⁴² they are mainly based on complicated and lengthy procedures using costly commercially-available radiochemistry boxes (Table 1). Therefore, further simplification of the radiosynthesis of [¹⁸F]SFB using a low-cost setup would be very beneficial for its adaption to an automated process. Herein, we report a concise preparation of [¹⁸F]SFB, based on a simplified one-pot microwave-assisted synthesis (Figure 1). Our approach does not require purification between steps or any aqueous reagents. In addition, microwave irradiation, which has been used in the syntheses of several PET tracers,³⁸⁻⁴¹ can gives higher RCYs and better selectivity than the corresponding thermal reactions or they provide similar yields in shorter reaction times.³⁸Most importantly, when labeling biomolecules, the time saved could be diverted to subsequent bioconjugation or PET imaging step. ²⁸(,)⁴³The novelty of our improved [¹⁸F]SFB synthesis is two-fold: (1) the anhydrous deprotection strategy requires no purification of intermediate(s) between each step and (2) the microwave-assisted radiochemical transformations enable the rapid, reliable production of [¹⁸F]SFB.

Radiolabeling of phosphatidylserine-binding peptides with prosthetic groups N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM) and N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

The widely used (18)F-prosthetic group N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB) and the recently developed N-[6-(4-[(18)F]fluorobenzylidene)aminooxyhexyl]maleimide ([(18)F]FBAM) were investigated for radiolabeling of two representative phosphatidylserine-binding peptides. The prosthetic groups were compared with respect to required reactions conditions for optimum labeling, radiolabeling yield and chemoselectivity. The N-terminus labeled product produced by reaction of [(18)F]SFB with binding peptide LIKKPF was produced in 18% radiochemical yield while no N-terminus labeled product could be isolated following [(18)F]SFB reaction with PDGLSR. When the peptides were modified by addition of a cysteine residue at the N-terminus they provided almost quantitative radiochemical yields with [(18)F]FBAM. Results indicate that for the peptides in this study, [(18)F]FBAM is a more useful prosthetic group compared to [(18)F]SFB due to its excellent chemoselectivity and high radiochemical yield.

Automated radiosynthesis of the thiol-reactive labeling agent N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide ([18F]FBAM)

The two-step radiosynthesis of N-[6-(4-[(18)F]fluorobenzylidene)aminooxyhexyl]maleimide ([(18)F]FBAM) was adapted to a remotely controlled synthesizer module. After optimization of reaction conditions as well as solid phase extraction based purification steps, the final [(18)F]FBAM was obtained in a decay-corrected radiochemical yield of 29±4% (related to [(18)F]fluoride, n=12) within a total synthesis time of 40 min. The radiochemical purity of [(18)F]FBAM was in the range 94-98%, the specific activity was determined with 13.4-17.2 GBq/μmol.

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.

Synthesis of hypoxia imaging agent 1-(5-deoxy-5-fluoro-α-D-arabinofuranosyl)-2-nitroimidazole using microfluidic technology

INTRODUCTION

Microfluidic technology allows fast reactions in a simple experimental setup, while using very low volumes and amounts of starting material. Consequently, microfluidic technology is an ideal tool for radiolabeling reactions involving short-lived positron emitters. Optimization of the complex array of different reaction conditions requires knowledge of the different reaction parameters linked to the microfluidic system as well as their influence on the radiochemical yields. 1-(5-Deoxy-5-fluoro-α-d-arabinofuranosyl)-2-nitroimidazole ([(18)F]FAZA) is a frequently used radiotracer for PET imaging of tumor hypoxia. The present study describes the radiosynthesis of [(18)F]FAZA by means of microfluidic technology and subsequent small animal PET imaging in EMT-6 tumor-bearing mice.

METHODS

Radiosyntheses were performed using the NanoTek Microfluidic Synthesis System (Advion BioSciences, Inc.). Optimal reaction conditions were studied through screening different reaction parameters like temperature, flow rate, residency time, concentration of the labeling precursor (1-(2,3-di-O-acetyl-5-O-tosyl-α-d-arabinofuranosyl)-2-nitroimidazole) and the applied volume ratio between the labeling precursor and [(18)F]fluoride.

RESULTS

Optimized reaction conditions at low radioactivity levels (1 to 50 MBq) afforded 63% (decay-corrected) of HPLC-purified [(18)F]FAZA within 25 min. Higher radioactivity levels (0.4 to 2.1 GBq) gave HPLC-purified [(18)F]FAZA in radiochemical yields of 40% (decay-corrected) within 60 min at a specific activity in the range of 70 to 150 GBq/μmol. Small animal PET studies in EMT-6 tumor-bearing mice showed radioactivity accumulation in the tumor (SUV(20min) 0.74 ± 0.08) resulting in an increasing tumor-to-muscle ratio over time.

CONCLUSIONS

Microfluidic technology is an ideal method for the rapid and efficient radiosynthesis of [(18)F]FAZA for preclinical radiopharmacological studies. Careful analysis of various reaction parameters is an important requirement for the understanding of the influence of different reaction parameters on the radiochemical yield using microfluidic technology. Exploration of microfluidic technology for the radiosynthesis of other PET radiotracers in clinically relevant radioactivity levels is currently in progress.

One step radiosynthesis of 6-[(18)F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester ([(18)F]F-Py-TFP): a new prosthetic group for efficient labeling of biomolecules with fluorine-18

The labeling of biomolecules for positron emission tomography (PET) with no-carrier-added fluorine-18 is almost exclusively accomplished using prosthetic groups in a two step procedure. The inherent complexity of the process renders full automation a challenge and leads to protracted synthesis times. Here we describe a new (18)F-labeled prosthetic group based on nicotinic acid tetrafluorophenyl ester. Reaction of [(18)F]fluoride at 40 degrees C with the trimethylammonium precursor afforded 6-[(18)F]fluoronicotinic acid tetrafluorophenyl ester ([(18)F]F-Py-TFP) directly in 60-70% yield. [(18)F]F-Py-TFP was conveniently purified by Sep-Pak cartridge prior to incubation with a peptide containing the RGD sequence. The desired conjugate was formed rapidly and in good yields. An in vitro receptor-binding assay for the integrin alpha(v)beta(3) was established to explore competition with peptide and peptidomimetic prepared from F-Py-TFP with (125)I-echistatin. The nonradioactive conjugates were found to possess high binding affinities with calculated K(i) values in the low nanomolar range.

Assessment of an 18F-labeled phosphoramidate peptidomimetic as a new prostate-specific membrane antigen-targeted imaging agent for prostate cancer

Prostate-specific membrane antigen (PSMA) is a transmembrane protein commonly found on the surface of late-stage and metastatic prostate cancer and a well-known imaging biomarker for staging and monitoring therapy. Although (111)In-labeled capropmab pendetide is the only approved agent available for PSMA imaging, its clinical use is limited because of its slow distribution and clearance that leads to challenging image interpretation. A small-molecule approach using radiolabeled urea-based PSMA inhibitors as imaging agents has shown promise for prostate cancer imaging. The motivation of this work is to explore phosphoramidates as a new class of potent PSMA inhibitors to develop more effective prostate cancer imaging agents with improved specificity and clearance properties.

METHODS

N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) was conjugated to S-2-((2-(S-4-amino-4-carboxybutanamido)-S-2-carboxyethoxy)hydroxyphosphorylamino)-pentanedioic acid (Phosphoramidate (1)), yielding S-2-((2-(S-4-(4-(18)F-fluorobenzamido)-4-carboxybutanamido)-S-2-carboxyethoxy)hydroxyphosphorylamino)-pentanedioic acid (3). In vivo studies were conducted in mice bearing either LNCaP (PSMA-positive) or PC-3 (PSMA-negative) tumors. PET images were acquired at 1 and 2 h with or without a preinjection of a nonradioactive version of the fluorophosphoramidate. Tissue distribution studies were performed at the end of the 2 h imaging sessions.

RESULTS

Phosphoramidate (1) and its fluorobenzamido conjugate (2) were potent inhibitors of PSMA (inhibitory concentration of 50% [IC(50)], 14 and 0.68 nM, respectively). PSMA-mediated tumor accumulation was noted in the LNCaP versus the PC-3 tumor xenografts. The LNCaP tumor uptake was also blocked by the administration of nonradioactive (2) prior to imaging studies. With the exception of the kidneys, tumor-to-tissue and tumor-to-blood ratios were greater than 5:1 at 2 h. The strong kidney uptake may be due to the known PSMA expression in the mouse kidney, because significant reduction (>6-fold) in kidney activity was seen in mice injected with (2).

CONCLUSION

(18)F-labeled phosphoramidate (3) is a representative of a new class of PSMA targeting peptidomimetic molecules that shows great promise as imaging agents for detecting PSMA+ prostate tumors.

In vivo positron emission tomography (PET) imaging with an alphavbeta6 specific peptide radiolabeled using 18F-"click" chemistry: evaluation and comparison with the corresponding 4-[18F]fluorobenzoyl- and 2-[18F]fluoropropionyl-peptides

Numerous radiolabeled peptides have been utilized for in vivo imaging of a variety of cell surface receptors. For applications in PET using [(18)F]fluorine, peptides are radiolabeled via a prosthetic group approach. We previously developed solution-phase (18)F-"click" radiolabeling and solid-phase radiolabeling using 4-[(18)F]fluorobenzoic and 2-[(18)F]fluoropropionic acids. Here we compare the three different radiolabeling approaches and report the effects on PET imaging and pharmacokinetics. The prosthetic groups did have an effect; metabolites with significantly different polarities were observed.

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.

Systematic comparison of two novel, thiol-reactive prosthetic groups for 18F labeling of peptides and proteins with the acylation agent succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

A systematic comparison of 4-[18F]fluorobenzaldehyde-O-(2-{2-[2-(pyrrol-2,5-dione-1-yl)ethoxy]-ethoxy}-ethyl)oxime ([18F]FBOM) and 4-[18F]fluorobenzaldehyde-O-[6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl]oxime ([18F]FBAM) as prosthetic groups for the mild and efficient 18F labeling of cysteine-containing peptides and proteins with the amine-group reactive acylation agent, succinimidyl-4-[18F]fluorobenzoate ([18F]SFB), is described. All three prosthetic groups were prepared in a remotely controlled synthesis module. Synthesis of [18F]FBOM and [18F]FBAM was accomplished via oxime formation through reaction of appropriate aminooxy-functionalized labeling precursors with 4-[18F]fluorobenzaldehyde. The obtained radiochemical yields were 19% ([18F]FBOM) and 29% ([18F]FBAM), respectively. Radiolabeling involving [18F]FBAM and [18F]FBOM was exemplified by the reaction with cysteine-containing tripeptide glutathione (GSH), a cysteine-containing dimeric neurotensin derivative, and human native low-density lipoprotein (nLDL) as model compounds. Radiolabeling with the acylation agent [18F]SFB was carried out using a dimeric neurotensin derivative and nLDL. Both thiol-group reactive prosthetic groups show significantly better labeling efficiencies for the peptides in comparison with the acylation agent [18F]SFB. The obtained results demonstrate that [18F]FBOM is especially suited for the labeling of hydrophilic cysteine-containing peptides, whereas [18F]FBAM shows superior labeling performance for higher molecular weight compounds as exemplified for nLDL apolipoprotein constituents. However, the acylation agent [18F]SFB is the preferred prosthetic group for labeling nLDL under physiological conditions.

Microwave-induced nucleophilic [18F]fluorination on aromatic rings: synthesis and effect of halogen on [18F]fluoride substitution of meta-halo (F, Cl, Br, I)-benzonitrile derivatives

The meta-halo-3-methylbenzonitrile derivatives (-F, -Cl, -Br, -I) were synthesized as model compounds to study reactivity towards aromatic nucleophilic substitution. A single-mode microwave system was incorporated into a commercial radiochemical synthetic module for (18)F labeling. Labeling yields of 64% for fluoro-, 13% for bromo- and 9% for chloro-precursors were achieved in DMSO in <3 min. The observed order of reactivity of the leaving groups toward aromatic nucleophilic substitution was F>>Br>Cl>>>I.

Synthesis of N-succinimidyl 4-[18F]fluorobenzoate, an agent for labeling proteins and peptides with 18F

This protocol describes the step-by-step procedure for the synthesis of N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB), an agent widely used for labeling proteins and peptides with the positron-emitting radionuclide 18F. The protocols for the synthesis of unlabeled SFB and the quaternary salt precursor 4-formyl-N,N,N-trimethyl benzenaminium trifluoromethane sulfonate also are described. For the [18F]SFB synthesis, the quaternary salt is first converted to 4-[18F]fluorobenzaldehyde. Oxidation of the latter provides 4-[18F]fluorobenzoic acid, which is converted to [18F]SFB by treatment with N,N-disuccinimidyl carbonate. Using this method, [18F]SFB can be synthesized in decay-corrected radiochemical yields of 30%-35% and a specific radioactivity of 11-12 GBq micromol(-1). The total synthesis and purification time required is about 80 min, starting from delivery of the [18F]fluoride. [18F]SFB remains an optimal reagent for labeling proteins and peptides with 18F because of good conjugation yields and metabolic stability.

Use of a peptide derived from foot-and-mouth disease virus for the noninvasive imaging of human cancer: generation and evaluation of 4-[18F]fluorobenzoyl A20FMDV2 for in vivo imaging of integrin alphavbeta6 expression with positron emission tomography

Expression of the epithelial-specific integrin alphavbeta6 is low or undetectable in most adult tissues but may be increased during wound healing and inflammation and is up-regulated dramatically by many different carcinomas, making alphavbeta6 a promising target for the in vivo detection of cancer using noninvasive imaging. In addition, alphavbeta6 is recognized as promoting invasion and correlates with aggressive behavior of human cancers and thus agents that recognize alphavbeta6 specifically in vivo will be an essential tool for the future management of alphavbeta6-positive cancers. Recently, we identified the peptide NAVPNLRGDLQVLAQKVART (A20FMDV2), derived from foot-and-mouth disease virus, as a potent inhibitor of alphavbeta6. Using flow cytometry and ELISA, we show that this peptide is highly selective, inhibiting alphavbeta6-ligand binding with a IC50 of 3 nmol/L, an activity 1,000-fold more selective for alphavbeta6 than for other RGD-directed integrins (alphavbeta3, alphavbeta5, and alpha5beta1). A20FMDV2 was radiolabeled on solid-phase using 4-[18F]fluorobenzoic acid, injected into mice bearing both alphavbeta6-negative and alphavbeta6-positive (DX3puro/DX3purobeta6 cell lines) xenografts and imaged using a small animal positron emission tomography (PET) scanner. Rapid uptake (<30 min) and selective retention (>5 h) of radioactivity in the alphavbeta6-positive versus the alphavbeta6-negative tumor, together with fast renal elimination of nonspecifically bound activity, resulted in specific imaging of the alphavbeta6-positive neoplasm. These data suggest that PET imaging of alphavbeta6-positive tumors is feasible and will provide an important new tool for early detection and improved management of many types of cancers.

Radiosynthesis of 18F-(R8,15,21, L17)-vasoactive intestinal peptide and preliminary evaluation in mice bearing C26 colorectal tumours

BACKGROUND

Radiolabelled vasoactive intestinal peptide (VIP) and its analogues have shown their potential as imaging agents for diagnosing tumours expressing VIP receptor. However, the fast proteolytic degradation in vivo has limited their clinical use.

AIM

To prepare the 18F-labelled (R8,15,21, L17)-VIP analogue in a convenient way and to evaluate its potential as an imaging agent for VIP receptor-positive tumours.

METHODS

Radiolabelled (R8,15,21, L17)-VIP was obtained by conjugation with N-succinimidyl 4-([18F]fluoromethyl) benzoate and purified by HPLC. Radiochemical purity and specific radioactivity were measured by analytical HPLC. In-vitro stability of the product was carried out in HSA solution and analysed by HPLC. Biodistribution study was carried out in mice bearing C26 colorectal tumours.

RESULTS

18F-(R8,15,21, L17)-VIP was obtained in greater than 99% radiochemical purity within 60 min in decay-for-corrected radiochemical yields of 21.8+/-4.7% (n=5) and a specific activity of 17.76 GBq x mumol(-1) at the end of synthesis (EOS). Results of in-vitro studies demonstrated a high stability in human serum albumin (HSA) solution. Biodistribution data showed a rapid blood clearance and specific binding towards receptor-positive tumours.

CONCLUSION

18F-(R8,15,21, L17)-VIP was prepared by a convenient method. Preliminary biodistribution results showed its potential for imaging tumours over-expressing VIP receptors and encouraged further investigation.