Development of (18)F-fluoroethylcholine for cancer imaging with PET: synthesis, biochemistry, and prostate cancer imaging

Abstract

The effectiveness of (11)C-choline PET in detecting various cancers, including prostate cancer, is well established. This study was aimed at developing an (18)F-substituted choline analog, (18)F-fluoroethylcholine (FECh), as a tracer of cancer detection.

Methods: No-carrier-added (18)F-FECh was synthesized by 2-step reactions: First, tetrabutylammonium (TBA) (18)F-fluoride was reacted with 1,2-bis(tosyloxy)ethane to yield 2-(18)F-fluoroethyl tosylate; and second, 2-(18)F-fluoroethyl tosylate was reacted with N,N-dimethylethanolamine to yield (18)F-FECh, which was then purified by chromatography. An automated apparatus was constructed for preparation of the (18)F-FECh injection solution. In vitro experiments were performed to examine the uptake of (18)F-FECh in Ehrlich ascites tumor cells, and the metabolites were analyzed by solvent extraction followed by various kinds of chromatography. Clinical studies of (18)F-FECh PET were performed on patients with untreated primary prostate cancer as follows: A dynamic (18)F-FECh PET study was performed on 1 patient and static PET studies were performed on 16 patients, and the data were compared with those of (11)C-choline PET on the same patients.

Results: (18)F-FECh was prepared in high yield and purity. The performance of the automated apparatus was excellent. The in vitro experiment revealed that (18)F-FECh was incorporated into tumor cells by active transport, then phosphorylated (yielding phosphoryl-(18)F-FECh) in the cells, and finally integrated into phospholipids. The clinical PET studies showed marked uptake of (18)F-FECh in prostate cancer. A dynamic PET study on 1 patient revealed that the blood level of (18)F-FECh decreased rapidly (in 1 min), the prostate cancer level became almost maximal in a short period (1.5 min) and it remained constant for a long time (60 min), and the urinary radioactivity became prominent after a short time lag (5 min). Static PET studies conducted under bladder irrigation showed no difference between (18)F-FECh uptake and (11)C-choline uptake in prostate cancer. However, (18)F-FECh gave a slightly higher spatial resolution of the image, which was attributed to the shorter positron range of (18)F.

Conclusion: The synthesis of (18)F-FECh was easy and reliable. (18)F-FECh PET was very effective in detecting prostate cancer in patients. The chemical trap, consisting of active transport of (18)F-FECh and formation of phosphoryl-(18)F-FECh, seemed to be involved in the uptake mechanism of (18)F-FECh in tumors.