¹⁸F-Labeled Octanoates as Potential Agents for Cerebral Fatty Acid Studies. The Influence of 4-Substitution and the Fluorine Position on Biodistribution

In order to understand the structural features that might lead to an in vivo radiotracer for studying cerebral fatty acid metabolism, 8-[¹⁸F]fluorooctanoate derivatives with methyl or gem-dimethyl branching at the C4 position have been prepared. 3-[¹⁸F]Fluoro- and 4-[¹⁸F]fluorooctanoic acid have also been synthesized for studying the influence of the fluorine position on the in vivo behavior of ¹⁸F-Iabeled octanoic acid analogs. Radiochemical synthesis was achieved by the nucleophilic displacement of a tosylate or mesylate precursor with [¹⁸F]fluoride ion. Tissue distribution studies in rats showed low cerebral uptakes of these ¹⁸F-labeled fatty acid analogs with poor brain-to-blood ratios. 3-[¹⁸F]Fluorooctanoic acid showed considerable defluorination, evident as a high bone activity level. The initial uptake of activity in the brain after injection of ethyl 8-[¹⁸F]fluoro-4-methyloctanoate and 4-[¹⁸F]-fluorooctanoic acid remained virtually unchanged over an extended time period, similar to that previously observed for the unbranched analogs, ethyl 8-[¹⁸F]fluorooctanoate and its free acid. In contrast, the 4-gem-dimethyl branched analog was rapidly and preferentially taken up by the liver. It was shown in the metabolite analysis that labeled metabolites produced from 8-[¹⁸F]fluoro-4-methyloctanoic acid were found in blood, and that they could enter the brain to a significant degree. Thus, the present studies showed that radioactivity retention in the brain in the case of the 4-methyl branched analog was mainly attributable to its radioactive metabolites.