Synthesis of [¹¹C]palmitic acid for PET imaging using a single molecular sieve 13X cartridge for reagent trapping, radiolabeling and selective purification

Introduction: Radiolabeled fatty acids are valuable metabolic tracers for PET imaging. Carbon-11 is widely used in clinical PET studies due to the prevalence of facile techniques enabling the incorporation of [¹¹C]CO₂ and [¹¹C]CH₃ into molecules and a short half-life (20.4min) that translates into low patient dose. However, the short half-life considerably limits the time for radiosynthesis. Furthermore, the majority of the syntheses of [¹¹C]palmitic acid in common use employ high starting [¹¹C]CO₂ activities and/or expensive equipment. Methods: [¹¹C]CO₂ was trapped with greater than 99.99% efficiency by a three stage cartridge packed with molecular sieve 13X, 100-120mesh. The labeling of n-pentadecylmagnesium bromide took place in 5min in the cartridge, and the [¹¹C]palmitic acid product was selectively eluted in ethanol following alkaline and acidic washes of the column. Results: The system reliably produced more than 925MBq (25mCi) of [¹¹C]palmitic acid suitable for human use from 7.4GBq (200mCi) of [¹¹C]CO₂ in 8min from end-of-bombardment. Conclusions: We have exploited the properties of the inexpensive molecular sieve 13X to develop a miniature, disposable and leak tight "gas capture" system for the rapid labeling and purification of [¹¹C]fatty acids in good yield and >99% radiochemical purity. The rapidity of the synthesis and purification allows small [¹¹C]CO₂ starting activities to be used, and with no requirement for expensive synthesis equipment or facilities, the system can be implemented in any radiopharmaceutical center.