18 F-click labeling and preclinical evaluation of a new 18 F-folate for PET imaging

BACKGROUND The folate receptor (FR) is a well-established target for tumor imaging and therapy. To date, only a few 18 F-folate conjugates via 18 F-prosthetic group labeling for positron emission tomography (PET) imaging have been developed. To some extent, they all lack the optimal balance between efficient radiochemistry and favorable in vivo characteristics. METHODS A new clickable olate precursor was synthesized by regioselective coupling of folic acid to 11-azido-3,6,9-trioxaundecan-1-amine at the γ-position of the glutamic acid residue. The non-radioactive reference compound was synthesized via copper-catalyzed azide-alkyne cycloaddition of 3-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)prop-1-yne and γ-(11-azido-3,6,9-trioxaundecanyl)folic acid amide. The radiosynthesis was accomplished in two steps: at first a 18 F-fluorination of 2-(2-(2-(prop-2-yn-1-yloxy)ethoxy)ethoxy)ethyl-4-methylbenzenesulfonate, followed by a 18 F-click reaction with the γ-azido folate. The in vitro, ex vivo, and in vivo behaviors of the new 18 F-folate were investigated using FR-positive human KB cells in displacement assays and microPET studies using KB tumor-bearing mice. RESULTS The new 18 F-folate with oligoethylene spacers showed reduced lipophilicity in respect to the previously developed 18 F-click folate with alkyl spacers and excellent affinity (Ki = 1.6 nM) to the FR. Combining the highly efficient 18 F-click chemistry and a polar oligoethylene-based 18 F-prosthetic group facilitated these results. The overall radiochemical yield of the isolated and formulated product averages 8.7%. In vivo PET imaging in KB tumor-bearing mice showed a tumor uptake of 3.4% ID/g tissue, which could be reduced by FR blockade with native folic acid. Although the new 18 F-oligoethyleneglycole (OEG)-folate showed reduced hepatobiliary excretion over time, a distinct unspecific abdominal background was still observed. CONCLUSIONS A new 18 F-folate was developed, being available in very high radiochemical yields via a fast and convenient two-step radiosynthesis. The new 18 F-OEG-folate showed good in vivo behavior and lines up with several recently evaluated 18 F-labeled folates.