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 [(11)C]CO2 and [(11)C]CH3 into molecules and a short half-life (20.4 min) that translates into low patient dose. However, the short half-life considerably limits the time for radiosynthesis. Furthermore, the majority of the syntheses of [(11)C]palmitic acid in common use employ high starting [(11)C]CO2 activities and/or expensive equipment.

METHODS: [(11)C]CO2 was trapped with greater than 99.99% efficiency by a three stage cartridge packed with molecular sieve 13X, 100-120 mesh. The labeling of n-pentadecylmagnesium bromide took place in 5 min in the cartridge, and the [(11)C]palmitic acid product was selectively eluted in ethanol following alkaline and acidic washes of the column.

RESULTS: The system reliably produced more than 925 MBq (25 mCi) of [(11)C]palmitic acid suitable for human use from 7.4 GBq (200 mCi) of [(11)C]CO2 in 8 min 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 [(11)C]fatty acids in good yield and >99% radiochemical purity. The rapidity of the synthesis and purification allows small [(11)C]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.