Radiosynthesis and pre-clinical evaluation of [(18)F]fluoro-[1,2-(2)H(4)]choline

Introduction: Choline radiotracers are widely used for clinical PET diagnosis in oncology. [(11)C]Choline finds particular utility in the imaging of brain and prostate tumor metabolic status, where 2-[(18)F]fluoro-2-deoxy-D-glucose ('FDG') shows high background uptake. More recently we have extended the clinical utility of [(11)C]choline to breast cancer where radiotracer uptake correlates with tumor aggressiveness (grade). In the present study, a new choline analog, [(18)F]fluoro-[1,2-(2)H(4)]choline, was synthesized and evaluated as a potential PET imaging probe.

Methods: [(18)F]Fluorocholine, [(18)F]fluoro-[1-(2)H(2)]choline and [(18)F]fluoro-[1,2-(2)H(4)]choline were synthesized by alkylation of the relevant precursor with [(18)F]fluorobromomethane or [(18)F]fluoromethyl tosylate. Radiosynthesis of [(18)F]fluoromethyl tosylate required extensive modification of the existing method. [(18)F]Fluorocholine and [(18)F]fluoro-[1,2-(2)H(4)]choline were then subjected to in vitro oxidative stability analysis in a chemical oxidation model using potassium permanganate and an enzymatic model using choline oxidase. The two radiotracers, together with the corresponding di-deuterated compound, [(18)F]fluoro-[1-(2)H(2)]choline, were then evaluated in vivo in a time-course biodistribution study in HCT-116 tumor-bearing mice.

Results: Alkylation with [(18)F]fluoromethyl tosylate proved to be the most reliable radiosynthetic route. Stability models indicate that [(18)F]fluoro-[1,2-(2)H(4)]choline possesses increased chemical and enzymatic (choline oxidase) oxidative stability relative to [(18)F]fluorocholine. The distribution of the three radiotracers, [(18)F]fluorocholine, [(18)F]fluoro-[1-(2)H(2)]choline and [(18)F]fluoro-[1,2-(2)H(4)]choline, showed a similar uptake profile in most organs. Crucially, tumor uptake of [(18)F]fluoro-[1,2-(2)H(4)]choline was significantly increased at late time points compared to [(18)F]fluorocholine and [(18)F]fluoro-[1-(2)H(2)]choline.

Conclusions: Stability analysis and biodistribution suggest that [(18)F]fluoro-[1,2-(2)H(4)]choline warrants further in vivo investigation as a PET probe of choline metabolism.