Abstract
1039
Objectives Radiolabeled amino acids such as [11C]methionine, 6-[18F]fluoro-L-dihydroxy phenylalanine ([18F]FDOPA), 3-methoxy-6-[18F]fluoro-L-dihydroxy phenylalanine ([18F]OMFD) and O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) are of great importance for the diagnosis of brain disorders since these amino acid transporter based radiotracers show advantage over [18F]FDG. By searching for new 18F-radiolabeled amino acids, to overcome some limitations of [18F]FDOPA, we have developed a new [18F]fluoroethylated DOPA, [18F]OFED.
Methods The radiosynthesis of [18F]OFED is based on a nucleophilic aliphatic substitution with [18F]fluoride and was performed in an automated synthesizer TracerlabFXN. Briefly the tosylated ethyl group of a triphenyl-di-BOC protected phenylalanine precursor was reacted with a [18F]KF/K222 complex in acetonitrile at 100°C. After deprotection with 2M HCl at 100°C, followed by isolation via semi-preparative HPLC (RP18, water, 1% ethanol) and sterile filtration, [18F]OFED was obtained as injectable solution. [18F]OFED was analyzed for radiochemical and chemical purity, pH-value as well as for residual solvents.
Results A starting activity of 6-17 GBq of [18F]fluoride provided 1.6-5.8 GBq of [18F]OFED within 60 min synthesis time, what corresponds to an overall RCY of 47% (n=6, d.c.). [18F]OFED was obtained with a radiochemical purity of 95-98% and the content of non-radioactive OFED was determined between 0.55 and 2.90 µg/ml resulting in a specific activity of 45-81 GBq/µmol. The pH-value (7.1-7.3) and the content of acetone and acetonitrile (3-14 µg/ml) were found to be in the pharmacologic permitted area.
Conclusions We have developed a new [18F]fluoroethylated DOPA derivative, [18F]OFED, for amino acid transporter based PET diagnostics, which can be synthesized in high yield, specific activity and good pharmaceutical quality.
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