Development of novel analogs of the monocarboxylate transporter ligand FACH and biological validation of one potential radiotracer for positron emission tomography (PET) imaging

Monocarboxylate transporters 1-4 (MCT1-4) are involved in several metabolism-related diseases, especially cancer, providing the chance to be considered as relevant targets for diagnosis and therapy. [¹⁸F]FACH was recently developed and showed very promising preclinical results as a potential positron emission tomography (PET) radiotracer for imaging of MCTs. Given that [¹⁸F]FACH did not show high blood-brain barrier permeability, the current work is aimed to investigate whether more lipophilic analogs of FACH could improve brain uptake for imaging of gliomas, while retaining binding to MCTs. The 2-fluoropyridinyl-substituted analogs 1 and 2 were synthesized and their MCT1 inhibition was estimated by [¹⁴C]lactate uptake assay on rat brain endothelial-4 (RBE4) cells. While compounds 1 and 2 showed lower MCT1 inhibitory potencies than FACH (IC₅₀ = 11 nM) by factors of 11 and 25, respectively, 1 (IC₅₀ = 118 nM) could still be a suitable PET candidate. Therefore, 1 was selected for radiosynthesis of [¹⁸F]1 and subsequent biological evaluation for imaging of the MCT expression in mouse brain. Regarding lipophilicity, the experimental log D_7.4 result for [¹⁸F]1 agrees pretty well with its predicted value. In vivo and in vitro studies revealed high uptake of the new radiotracer in kidney and other peripheral MCT-expressing organs together with significant reduction by using specific MCT1 inhibitor α-cyano-4-hydroxycinnamic acid. Despite a higher lipophilicity of [¹⁸F]1 compared to [¹⁸F]FACH, the in vivo brain uptake of [¹⁸F]1 was in a similar range, which is reflected by calculated BBB permeabilities as well through similar transport rates by MCTs on RBE4 cells. Further investigation is needed to clarify the MCT-mediated transport mechanism of these radiotracers in brain.