Synthesis of [18F]fluoroacetaldehyde. Application to [18F]fluoroethylation of benzylamine under reductive alkylation conditions

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.

Development & automation of a novel [(18)F]F prosthetic group, 2-[(18)F]-fluoro-3-pyridinecarboxaldehyde, and its application to an amino(oxy)-functionalised Aβ peptide

2-[(18)F]-Fluoro-3-pyridinecarboxaldehyde ([(18)F]FPCA) is a novel, water-soluble prosthetic group. It's radiochemistry has been developed and fully-automated for application in chemoselective radiolabelling of amino(oxy)-derivatised RI-OR2-TAT peptide, (Aoa-k)-RI-OR2-TAT, using a GE TRACERlab FX-FN. RI-OR2-TAT is a brain-penetrant, retro-inverso peptide that binds to amyloid species associated with Alzheimer's Disease. Radiolabelled (Aoa-k)-RI-OR2-TAT was reproducibly synthesised and the product of the reaction with FPCA has been fully characterised. In-vivo biodistribution of [(18)F]RI-OR2-TAT has been measured in Wistar rats.

Automation of [(18) F]fluoroacetaldehyde synthesis: application to a recombinant human interleukin-1 receptor antagonist (rhIL-1RA)

[(18) F]Fluoroacetaldehyde is a biocompatible prosthetic group that has been implemented pre-clinically using a semi-automated remotely controlled system. Automation of radiosyntheses permits use of higher levels of [(18) F]fluoride whilst minimising radiochemist exposure and enhancing reproducibility. In order to achieve full-automation of [(18) F]fluoroacetaldehyde peptide radiolabelling, a customised GE Tracerlab FX-FN with fully programmed automated synthesis was developed. The automated synthesis of [(18) F]fluoroacetaldehyde is carried out using a commercially available precursor, with reproducible yields of 26% ± 3 (decay-corrected, n = 10) within 45 min. Fully automated radiolabelling of a protein, recombinant human interleukin-1 receptor antagonist (rhIL-1RA), with [(18) F]fluoroacetaldehyde was achieved within 2 h. Radiolabelling efficiency of rhIL-1RA with [(18) F]fluoroacetaldehyde was confirmed using HPLC and reached 20% ± 10 (n = 5). Overall RCY of [(18) F]rhIL-1RA was 5% ± 2 (decay-corrected, n = 5) within 2 h starting from 35 to 40 GBq of [(18) F]fluoride. Specific activity measurements of 8.11-13.5 GBq/µmol were attained (n = 5), a near three-fold improvement of those achieved using the semi-automated approach. The strategy can be applied to radiolabelling a range of peptides and proteins with [(18) F]fluoroacetaldehyde analogous to other aldehyde-bearing prosthetic groups, yet automation of the method provides reproducibility thereby aiding translation to Good Manufacturing Practice manufacture and the transformation from pre-clinical to clinical production.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

Biodistribution, pharmacokinetics and metabolism of interleukin-1 receptor antagonist (IL-1RA) using [¹⁸F]-IL1RA and PET imaging in rats

BACKGROUND AND PURPOSE

Positron emission tomography (PET) has the potential to improve our understanding of the preclinical pharmacokinetics and metabolism of therapeutic agents, and is easily translated to clinical studies in humans. However, studies involving proteins radiolabelled with clinically relevant PET isotopes are currently limited. Here we illustrate the potential of PET imaging in a preclinical study of the biodistribution and metabolism of ¹⁸F-labelled IL-1 receptor antagonist ([¹⁸F]IL-1RA) using a novel [¹⁸F]-radiolabelling technique.

EXPERIMENTAL APPROACH

IL-1RA was radiolabelled by reductive amination on lysine moieties with [¹⁸F]fluoroacetaldehyde. Sprague-Dawley rats were injected intravenously with [¹⁸F]IL-1RA and imaged with a PET camera for 2 h. For the study of IL-1RA metabolites by ex vivoγ-counting of samples, rats were killed 20 min, 1 h or 2 h after injection of [¹⁸F]IL-1RA.

KEY RESULTS

[¹⁸F]IL-1RA distribution into the major organs of interest was as follows: kidneys >> liver > lungs >> brain. In lungs and liver, [¹⁸F]IL-1RA uptake peaked within 1 min post-injection then decreased rapidly to reach a plateau from 10 min post-injection. In the brain, the uptake exhibited slower pharmacokinetics with a smaller post-injection peak and a plateau from 6 min onward. IL-1RA was rapidly metabolized and these metabolites represented ∼40% of total activity in plasma and ∼80% in urine, 20 min after injection. CONCLUSIONS AND IMPLICATIONS Preclinical PET imaging is a feasible method of assessing the biodistribution of new biological compounds of therapeutic interest rapidly. The biodistribution of [¹⁸F]IL-1RA reported here is in agreement with an earlier study suggesting low uptake in the normal brain, with rapid metabolism and excretion via the kidneys.

Radiolabeling with fluorine-18 of a protein, interleukin-1 receptor antagonist

IL-1RA is a naturally occurring antagonist of the pro-inflammatory cytokine interleukin-1 (IL-1) with high therapeutic promise, but its pharmacokinetic remains poorly documented. In this report, we describe the radiolabeling of recombinant human interleukin-1 receptor antagonist (rhIL-1RA) with fluorine-18 to allow pharmacokinetic studies by positron emission tomography (PET). rhIL-1RA was labeled randomly by reductive alkylation of free amino groups (the epsilon-amino group of lysine residues or amino-terminal residues) using [(18)F]fluoroacetaldehyde under mild reaction conditions. Radiosyntheses used a remotely controlled experimental rig within 100min and the radiochemical yield was in the range 7.1-24.2% (decay corrected, based on seventeen syntheses). We showed that the produced [(18)F]fluoroethyl-rhIL-1ra retained binding specificity by conducting an assay on rat brain sections, allowing its pharmakokinetic study using PET.