Non-beta-oxidizable omega-[18F]fluoro long chain fatty acid analogs show cytochrome P-450-mediated defluorination: implications for the design of PET tracers of myocardial fatty acid utilization.

The nature of the in vivo defluorination of non-beta-oxidazable no-carrier-added omega-[18F]fluoro long chain fatty acid (LCFA) analogs was studied with the aim of developing PET tracers of LCFA utilization. Extensive defluorination of 15-[18F]fluoro-3-thia-pentadecanoic acid (FTPA) in mouse was evidenced by radioactivity uptake by bone. [18F]Fluoride in the blood was verified analytically. Incubations of FTPA in rat-liver homogenates and subcellular fractions thereof showed a strong defluorination process in microsomes which was O2- and NADPH-dependent. In contrast, defluorination of FTPA was relatively slow in Langendorff perfused rat heart. High bone uptake in mouse was also observed with 14-[18F]fluoro-13,13-dimethyl-3-thia-tetradecanoic acid, where gem-dimethyl substitution precludes direct elimination of H18F. These data indicate that the defluorination of non-beta-oxidizable omega-[18F]fluoro LCFA analogs is primarily governed by cytochrome P-450-mediated omega-oxidation. Therefore, labeling at the (omega-3) carbon was proposed to provide a more stabile 18F-label. Defluorination of the (omega-3)-labeled 13 (R,S)-[18F]fluoro-3-thia-hexadecanoic acid was lower than that of FTPA in mouse and was independent of O2 and NADPH in vitro. Thus, (omega-3) labeling with 18F is preferable to omega labeling of non-beta-oxidizable LCFA analogs.