Synthesis and radiofluorination of a novel monocarboxylate transporter 1 inhibitor for tumor imaging by PET

We developed a new ¹⁸F-labeled radioligand for in vivo imaging of MCT1-overexpressing brain tumors by PET. A new fluorinated analogue of α-cyano-4-hydroxycinnamic acid (RM231) was synthesized from m-anisidine via alkylation, ortho formylation and Knoevenagel condensation in 50% overall yield. Its MCT1 inhibition activity was evaluated via [¹⁴C]lactate uptake assay on rat brain endothelial 4 cells. The mesylated precursor was similarly prepared in 52% overall yield. Radiosynthesis of [¹⁸F]RM231 was achieved by a two-step reaction, starting with the radiofluorination using [¹⁸F]-K₂CO₃-K222 complex followed by protective group removal via hydrolysis under optimized reaction conditions. RM231 showed relatively high MCT1 inhibition activity (IC50 = 12 nM). The radiolabeled intermediate was obtained by an optimized procedure (acetonitrile, 5.5 mg of K222, 0.7 mg of K₂CO₃, 12-15 GBq of K¹⁸F, 100 ^◦C, 8 min) with 44-50% yield determined by radio-HPLC analysis (N=3, non-isolated). The final product was obtained by hydrolysis with TFA in dry dichloromethane at room temperature for 10 minutes with 29% yield (radio-HPLC, non-isolated).[¹⁸F]RM231 could be obtained after separation using semi-preparative HPLC (RP C18 column; 30% ACN, 20 mM NH₂₄CO₂ H). Currently, attempts are made to stabilize and formulate the final product appropriately for biological investigation. The newly developed MCT1 radioligand is anticipated to be a useful agent for imaging of the tumors with PET. Accordingly, animal studies on the new radiotracer are currently under investigation.