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Title: Fluorine-18 radiolabelling and in vitro / in vivo metabolism of [18F]D4-PBR111
Authors: Wyatt, NA
Safavi-Naeini, M
Wotherspoon, ATL
Arthur, A
Nguyen, A
Parmar, A
Hamze, H
Day, CM
Zahra, D
Matesic, L
Davis, E
Rahardjo, GL
Yepuri, NR
Shepherd, R
Murphy, RB
Pham, TQ
Nguyen, VH
Callaghan, PD
Holden, PJ
Grégoire, MC
Darwish, TA
Fraser, BH
Keywords: Fluorine 18
Tracer techniques
Issue Date: 26-May-2019
Publisher: John Wiley & Sons, Inc
Citation: Wyatt, N. A., Safavi-Naeini, M., Wotherspoon, A., Arthur, A., Nguyen, A. P., Parmar, A., Hamze, H., Day, C. M., Zahra, D., Matesic, L., Davis, E., Rahardjo, G. L., Yepuri, N. R., Shepherd, R. K., Murphy, R. B., Pham, T. Q., Nguyen, V. H., Callaghan, P. D., Holden, P. J., Grégoire, M.-C., Darwish, T.A. & Fraser, B. H. (2019). Fluorine-18 radiolabelling and in vitro / in vivo metabolism of [18F]D4-PBR111. Paper presented at the 23rd International Symposium on Radiopharmaceutical Sciences (ISRS 2019), Beijing, China, 26 to 31 May, 2019.In Journal of Labelled Compounds & Pharmaceuticals 62(S1), S123-S588. doi:10.1002/jlcr.3725
Abstract: Objectives The 18 kDa Translocator Protein (TSPO) is a receptor protein located in the outer mitochondrial membrane. TSPO is a bio-marker for inflammation associated with numerous diseases including cancer, multiple sclerosis, Parkinson's and Alzheimer's diseases, stroke, Huntington's disease, and HIV encephalitis. Consequently, there is significant interest in radiolabelled TSPO ligands as new radiotracers. This includes [18F]PBR111 which shows potential for imaging neuroinflammation but suffers from significant de‐fluorination in vivo (rats). This leads to non‐specific bone uptake and low signal‐to-noise ratios in vivo, leading to lower quality PET images. To address these problems, a deuterated 2nd generation radiotracer has been synthesised and its metabolic stability compared to regular [18F]PBR111. Methods The synthesis of [18F]D4‐PBR111 radiolabelling precursor was achieved following an adaptation of our previously published method.5 The radio‐synthesis of [18F]PBR111 and [18F]D4‐PBR111 was performed on a Synthra synthesis module by nucleophilic substitution of PBR111 or D4‐PBR111 tosylate precursor with [18F]fluoride based on adaptations of conditions previously described. In vitro metabolism was evaluated by incubating [18F]PBR111 or [18F]D4‐PBR111 with either rat or human liver microsomes, NADPH generating solution and 0.1M potassium phosphate buffer pH 7.4 (PBS) at 37°C. Supernatant was collected at various time points over a 60 min period. The supernatant was then analysed via radio‐HPLC to determine the metabolite components. A control sample was used for each assay containing all the components of the assay apart from the NADPH generating solution to rule out any breakdown of the radiotracer not caused by the microsomal process. In vivo PET and metabolite studies consisted of male Sprague Dawley Rats (n = 16) being injected with either 100 MBq [18F]PBR111 or [18F] D4‐PBR111 and a PET acquisition was performed for a 60 min scan. During this process blood samples were taken from the rats at various time points up to 60 min via the femoral artery. Plasma was separated from whole blood and the percentage of the metabolite components and the in‐tact tracer was analysed by radio‐HPLC and Solid Phase Extraction (SPE) as previously described. Results In vitro assay results showed the presence of 7 visible [18F] radio‐metabolite peaks and were given a numerical value based on their polarity on the HPLC chromatogram. The results showed that rat microsomes metabolised both [18F]PBR111 and [18F]D4‐PBR111 much faster than the human microsomes. In vivo PET imaging in rats showed a 42% reduction of the median [18F]D4‐PBR111 (Figure 1, A) uptake in bone (vertebrae) compared to non‐deuterated [18F] PBR111 (Figure 1B). This data supports the hypothesis that the introduction of deuterium has significantly reduced the defluorination of the PBR111 radiotracer. Conclusions A deuterated radiotracer [18F]D4‐PBR111 was developed and evaluated in vivo in rats, demonstrating that it is more resistant to metabolic breakdown compared to non‐deuterated [18F]PBR111. Careful choice of the site of deuteration resulted in a decreased rate of defluorination, and a notable increase in the median uptake of the radiotracer in regions with high TSPO expression. Rat and human liver microsomal assays were an effective screening tool for predicting the potential metabolic differences between the deuterated and non‐deuterated analogues. Our results provide further evidence of the benefit that deuterium can have, not only, in stabilisation but also in altering the metabolic profile of a radiotracer. Further studies are now underway to evaluate [18F]D4‐PBR111 vs [18F]PBR111 in animal disease models. © 2019 The Authors
Description: Volume 62, Supplement 1 of the Journal of Labelled Compounds & Pharmaceuticals is comprised of the Abstracts from ISRS 2019.
ISSN: 1099-1344
Appears in Collections:Conference Publications

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