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 [ 18 F]FPySH and its conjugation via Michael addition on model dehydroalanine- or maleimide-containing biologics. Results The preparation of cold reference and labeling precursor of [ 18 F]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 [ 18 F]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, [ 18 F]FPySH was employed to radiolabel the maleimide-modified c(RGDfK) peptide, affording the radiofluorinated analogue in 15 min. Conclusion We have developed an original [ 18 F]-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.

Radiolabeling of peptides with fluorine-18 is hurdled by their chemical sensitivity and complicated processes. Original triflyl-pyridine intermediates afforded ammonium precursors that were radiolabeled at low temperature. From that study, a generic tag has been designed to allow a simple one-step/late-stage radiolabelling of peptides. The strategy has been transposed to an automated "on-resin" radiolabelling.

Methods for the radiolabeling of biologics with fluorine-18 have been of interest for several decades. A common approach consists in the preparation of a prosthetic reagent, a small molecule bearing a fluorine-18 that is conjugated with the macromolecule to an appropriate function. Click chemistry, and more particularly cycloadditions, is an interesting approach to radiolabel molecules thanks to mild reaction conditions, high yields, low by-products formation, and strong orthogonality. Moreover, the chemical functions involved in the cycloaddition reaction are stable in the drastic radiofluorination conditions, thus allowing a simple radiosynthetic route to prepare the prosthetic reagent. We report herein the radiosynthesis of 18 F-FPyZIDE, a pyridine-based azide-bearing prosthetic reagent. We exemplified its conjugation via copper-catalyzed cycloaddition (CuAAC) and strain-promoted cycloaddition (SPAAC) with several terminal alkyne or strained alkyne model compounds.

Heparan Sulfate (HS) mimetics are able to block crucial interactions of the components of the extracellular matrix in angiogenic processes and as such, represent a valuable class of original candidates for cancer therapy. Here we first report the synthesis and in vitro angiogenic inhibition properties of a conjugated, novel and rationally-designed octasaccharide-based HS mimetic. We also herein report its labeling with fluorine-18 and present the preliminary in vivo Positron Emission Tomography imaging data in rats. This constitutes one of the rare examples of labeling and in vivo evaluation of a synthetic, polysaccharide-based, macromolecule.

Imaging of TSPO 18kDa with PET is more and more considered as a relevant biomarker of inflammation in numerous diseases. Development of new radiotracers for TSPO 18kDa has seen acceleration in the last years and the challenge today is to make available large amounts of such a radiotracer in compliance with GMP standards for application in humans. We present in this technical note automated productions of [18F]DPA-714, [18F]PBR111 and [18F]FEDAA1106, three promising radiotracers for TSPO 18kDa imaging, using a TRACERlab FX-FN synthesizer. This note also includes the quality control data of the validation batches for the manufacturing qualification of clinical production of [18F]DPA-714.

There is growing interest in the use of radiolabelled peptides as receptor targeting agents for diagnostic imaging of various cancer types using positron emission tomography. In this work, 143-(2[F-18]fluoropyridin-3-yloxy)propyllpyrrole-2,5-dione ([F-18]FPyME) has been used for parallel fluorine-18 labelling of PEPHC1, a peptide selective towards the cancer-specific mutation of the epidermal growth factor receptor (EGFRvIII), and a number of truncated and mutated analogues. Conjugation of the peptides with [F-18]FPyME was achieved within 10 min in non-decay-corrected radiochemical yields of 30-50%. The high yield of the conjugation reaction combined with its short synthesis time allows the labelling of several peptides from a single batch of [F-18]FPyME.

Overexpression of the translocator protein, TSPO (18 kDa), formerly known as the peripheral benzodiazepine receptor, is a hallmark of activation of cells of monocytic lineage (microglia and macrophages) during neuroinflammation. Radiolabeling of TSPO ligands enables the detection of neuroinflammatory lesions by PET. Two new radioligands, 11C-labeled N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-yl]acetamide (DPA-713) and 18F-labeled N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-yl)acetamide (DPA-714), both belonging to the pyrazolopyrimidine class, were compared in vivo and in vitro using a rodent model of neuroinflammation. Methods: 11C-DPA-713 and 18F-DPA-714, as well as the classic radioligand 11C-labeled (R)-N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)isoquinoline-3-carboxamide (PK11195), were used in the same rat model, in which intrastriatal injection of (R,S)-α-amino-3-hydroxy-5-methyl-4-isoxazolopropionique gave rise to a strong neuroinflammatory response. Comparative endpoints included in vitro autoradiography and in vivo imaging on a dedicated small-animal PET scanner under identical conditions. Results: 11C-DPA-713 and 18F-DPA-714 could specifically localize the neuroinflammatory site with a similar signal-to-noise ratio in vitro. In vivo, 18F-DPA-714 performed better than 11C-DPA-713 and 11C-PK11195, with the highest ratio of ipsilateral to contralateral uptake and the highest binding potential. Conclusion: 18F-DPA-714 appears to be an attractive alternative to 11C-PK11195 because of its increased bioavailability in brain tissue and its reduced nonspecific binding. Moreover, its labeling with 18F, the preferred PET isotope for radiopharmaceutical chemistry, favors its dissemination and wide clinical use. 18F-DPA-714 will be further evaluated in longitudinal studies of neuroinflammatory conditions such as are encountered in stroke or neurodegenerative diseases.

1901 Objectives [11C]PBR28 is a recently reported compound displaying exceptional properties for the in vivo imaging of the peripheral benzodiazepine receptor (or TSPO 18 kDa) using PET (Briard et al., J Med Chem. 2008, 51:17-30 ; Fujita et al., Neuroimage2008, 39:1289-98 and Imaizumi et al., Neuroimage2008, 40:43-52). This meta/para-bi-substituted pyridine leaves open the option of fluorine introduction at an ortho position, and therefore offers an opportunity for labeling with the longer half-life positron-emitter fluorine-18, which is the subject of the work presented herein. Methods 6-Fluoro-PBR28 (N-(2-methoxybenzyl)-N-(6-fluoro-4-phenoxypyridinyl-3-yl)acetamide) and its 6-bromo analog were synthesized from commercially available 4-chloro-3-nitropyridine. Fluorine-18 labeling involves: (A) reaction of K[18F]F-Kryptofix®222 with 2-3 mg of the bromo derivative at 165°C for 5 min in DMSO, (B) C-8 PrepSep cartridge pre-purification, (C) semi-preparative HPLC purification and (D) SepPak®Plus-based formulation. Results 6-Fluoro-PBR28 and its bromo analog were both synthesized in six chemical steps, respectively in 16% and 19% overall yield. Ready-to-inject 6-[18F]fluoro-PBR28 (>95% radiochemically pure) was prepared using our Zymate-XP robotic system, in 90 minutes and 10% non-decay-corrected yield. SRA ranged from 74 to 111 GBq/micromole. Conclusions 6-Fluoro-PBR28 was labeled with fluorine-18 in one single step using a bromine-for-fluorine heteroaromatic substitution. Dynamic µPET studies are currently underway in our rodent model of neuroinflammation (unilaterally AMPA-induced striatum-lesioned rats).