Radiosynthesis and pre-clinical evaluation of [¹⁸F]fluoro-[1,2-²H₄]choline☆

Abstract 

Introduction

Choline radiotracers are widely used for clinical PET diagnosis in oncology. [¹¹C]Choline finds particular utility in the imaging of brain and prostate tumor metabolic status, where 2-[¹⁸F]fluoro-2-deoxy-d-glucose (‘FDG’) shows high background uptake. More recently we have extended the clinical utility of [¹¹C]choline to breast cancer where radiotracer uptake correlates with tumor aggressiveness (grade). In the present study, a new choline analog, [¹⁸F]fluoro-[1,2-²H₄]choline, was synthesized and evaluated as a potential PET imaging probe.

Methods

[¹⁸F]Fluorocholine, [¹⁸F]fluoro-[1-²H₂]choline and [¹⁸F]fluoro-[1,2-²H₄]choline were synthesized by alkylation of the relevant precursor with [¹⁸F]fluorobromomethane or [¹⁸F]fluoromethyl tosylate. Radiosynthesis of [¹⁸F]fluoromethyl tosylate required extensive modification of the existing method. [¹⁸F]Fluorocholine and [¹⁸F]fluoro-[1,2-²H₄]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, [¹⁸F]fluoro-[1-²H₂]choline, were then evaluated in vivo in a time-course biodistribution study in HCT-116 tumor-bearing mice.

Results

Alkylation with [¹⁸F]fluoromethyl tosylate proved to be the most reliable radiosynthetic route. Stability models indicate that [¹⁸F]fluoro-[1,2-²H₄]choline possesses increased chemical and enzymatic (choline oxidase) oxidative stability relative to [¹⁸F]fluorocholine. The distribution of the three radiotracers, [¹⁸F]fluorocholine, [¹⁸F]fluoro-[1-²H₂]choline and [¹⁸F]fluoro-[1,2-²H₄]choline, showed a similar uptake profile in most organs. Crucially, tumor uptake of [¹⁸F]fluoro-[1,2-²H₄]choline was significantly increased at late time points compared to [¹⁸F]fluorocholine and [¹⁸F]fluoro-[1-²H₂]choline.

Conclusions

Stability analysis and biodistribution suggest that [¹⁸F]fluoro-[1,2-²H₄]choline warrants further in vivo investigation as a PET probe of choline metabolism.

Keywords: Choline, Fluorocholine, Choline kinase, Fluorine-18, Isotope effect, Quantum tunneling