Development and radiosynthesis of the first 18F-labeled inhibitor of monocarboxylate transporters (MCTs)

Monocarboxylate transporters 1 and 4 (MCT1 and MCT4) are involved in tumor development and progression. Their expression levels are related to clinical disease prognosis. Accordingly, both MCTs are promising drug targets for treatment of a variety of human cancers. The non-invasive imaging of these MCTs in cancers is regarded to be advantageous for assessing MCT-mediated effects on chemotherapy and radiosensitization using specific MCT inhibitors. Herein, we describe a method for the radiosynthesis of [18F]FACH ((E)-2-cyano-3-{4-[(3-[18F]fluoropropyl)(propyl) amino]-2-methoxyphenyl}acrylic acid), as a novel radiolabeled MCT1/4 inhibitor for imaging with PET. A fluorinated analog of α-cyano-4-hydroxycinnamic acid (FACH) was synthesized and the inhibition of MCT1 and MCT4 was measured via an [14C]lactate uptake assay. Radiolabeling was performed via a two-step protocol comprising the radiosynthesis of the intermediate (E)/(Z)-[18F]tert-Bu-FACH (tert-butyl (E)/(Z)-2-cyano-3-{4-[(3-[18F]fluoropropyl)(propyl)amino]-2-methoxyphenyl}acrylate) followed by deprotection of the tert-butyl group. The radiofluorination was successfully implemented using either K[18F]F-K2.2.2-carbonate or [18F]TBAF. The final deprotected product [18F]FACH was only obtained when [18F]tert-Bu-FACH was formed by the latter procedure. After optimization of the deprotection reaction, [18F]FACH was obtained in high radiochemical yields (39.6 ± 8.3%, EOB) and radiochemical purity (>98%).