Julian Grigg

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

2-[18F]-Fluoro-3-pyridinecarboxaldehyde ([18F]FPCA) is a novel, water-soluble prosthetic group. Its 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 Alzheimers 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 [18F]RI-OR2-TAT has been measured in Wistar rats.

In vivo characterisation of a therapeutically relevant self-assembling 18F-labelled β-sheet forming peptide and its hydrogel using positron emission tomography

Positron emission tomography (PET) and fluorescence labelling have been used to assess the pharmacokinetics, biodistribution and eventual fate of a hydrogel‐forming nonapeptide, FEFKFEFKK (F9), in healthy mice, using 18F‐labelled and fluorescein isothiocyanate (FITC)‐labelled F9 analogues. F9 was site‐specifically radiolabelled with 2‐[18F]fluoro‐3‐pyridinecarboxaldehyde ([18F]FPCA) via oxime bond formation. [18F]FPCA‐F9 in vivo fate was evaluated both as a solution, following intravenous administration, and as a hydrogel when subcutaneously injected. The behaviour of FITC‐F9 hydrogel was assessed following subcutaneous injection. [18F]FPCA‐F9 demonstrated high plasma stability and primarily renal excretion; [18F]FPCA‐F9 when in solution and injected into the bloodstream displayed prompt bladder uptake (53.4 ± 16.6 SUV at 20 minutes postinjection) and rapid renal excretion, whereas [18F]FPCA‐F9 hydrogel, formed by co‐assembly of [18F]FPCA‐F9 monomer with unfunctionalised F9 peptide and injected subcutaneously, showed gradual bladder accumulation of hydrogel fragments (3.8 ± 0.4 SUV at 20 minutes postinjection), resulting in slower renal excretion. Gradual disaggregation of the F9 hydrogel from the site of injection was monitored using FITC‐F9 hydrogel in healthy mice (60 ± 3 over 96 hours), indicating a biological half‐life between 1 and 4 days. The in vivo characterisation of F9, both as a gel and a solution, highlights its potential as a biomaterial.

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

[18F]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 [18F]fluoride whilst minimising radiochemist exposure and enhancing reproducibility. In order to achieve full‐automation of [18F]fluoroacetaldehyde peptide radiolabelling, a customised GE Tracerlab FX‐FN with fully programmed automated synthesis was developed. The automated synthesis of [18F]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 [18F]fluoroacetaldehyde was achieved within 2 h. Radiolabelling efficiency of rhIL‐1RA with [18F]fluoroacetaldehyde was confirmed using HPLC and reached 20% ± 10 (n = 5). Overall RCY of [18F]rhIL‐1RA was 5% ± 2 (decay‐corrected, n = 5) within 2 h starting from 35 to 40 GBq of [18F]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 [18F]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.

Influence of transport line material on the molar activity of cyclotron produced [18F]fluoride

INTRODUCTION Production of fluorine-18-labeled radiopharmaceuticals is always associated with the varying levels of the same compound containing stable fluorine-19. In practice, this affects the molar activity (Am), defined as amount of radioactivity divided by the molar quantity (Bq/mol). We have focused on studying how the material of the transport tubing connecting the cyclotron target chamber to the synthesis device affects the concentration of fluoride in the water arriving to the reaction vessel and subsequently the Am of the fluorine-18 labeled radiopharmaceuticals produced. METHODS Batches of irradiated and non-irradiated water were analyzed for fluoride content after being transported via non-fluorinated (PEEK, PP) and fluorinated (PTFE, ETFE) tubing or using no tubing at all. Am for the [18F]fluoride was determined and compared with the Am of [18F]fluciclatide, synthesized from the same [18F]fluoride containing batches of water. RESULTS Significantly higher concentrations of fluoride were seen in irradiated water that was transported in fluorinated tubing compared to non-irradiated water transported in tubing of the same material. This elevation of fluoride concentration is presumably caused by the interaction of ionizing radiation with the fluorinated tubing used between the target chamber and hot cell. Likewise, a significant difference was seen for PEEK tubing (non-fluorinated). This could be due to the fact that fluorine containing compounds are used in the manufacture of PEEK. When using fluorinated tubing for transport of the irradiated water, the resulting fluciclatide concentrations were significantly higher compared to when using non-fluorinated tubing. No significant difference was seen between fluciclatide concentrations when PTFE or ETFE tubing was compared to each other. Using no tubing resulted in lowest fluciclatide concentration. CONCLUSIONS Fluorinated tubing is a source of stable fluoride, and Am can be increased by using non-fluorinated transport tubing. Of all the tubing materials studied PP is preferred.

A Review of Approaches to 18F Radiolabelling Affinity Peptide & Proteins

Affinity peptide and protein- (APP) based radiotracers are an increasingly popular class of radiotracer in positron emission tomography (PET), which was once dominated by the use of small molecule radiotracers. Radiolabelled monoclonal antibodies (mAbs) are important examples of APPs, yet a preference for smaller APPs, which exhibit fast pharmacokinetics and permit rapid PET aided diagnosis, has become apparent. 18 F exhibits favourable physical characteristics for APP radiolabelling and has been described as an ideal PET radionuclide. Notwithstanding, 18 F radiolabelling of APP is challenging, and this is echoed in the literature where a number of diverse approaches have been adopted. This review seeks to assess and compare the approaches taken to 18 F APP radiolabelling with the intention of highlighting trends within this expanding field. Generic themes have emerged in the literature, namely the use of mild radiolabelling conditions, a preference of site-specific methodologies with an impetus for short, automated procedures which produce high-yielding [18 F]APPs.