Development of [18F]FEMPT, a high affinity PET tracer for 5-HT1AR.

Objectives The development of 5-HT_1AR agonist PET tracers for the past 2 decades has met with limited success. Through the structure activity relationship studies of arylpiperazine derivatives of 3,5-dioxo-(2H,4H)-1,2,4-triazine, we found 2-(4-(4-(7-(2-fluoroethoxy)naphthalen-1-yl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)-dione (FEMPT) as a high affinity 5-HT1AR ligand. This study describes the radiosynthesis and evaluation of [18F]FEMPT as a selective high affinity PET radiotracer for 5-HT1AR.

Methods The reference standard FEMPT and the corresponding radiolabeling precursor were synthesized from 4-methyl-2H-[1,2,4]triazine-3,5-dione. Radiosynthesis of [¹⁸F]FEMPT was achieved by reacting des-fluoroethoxy precursor with [¹⁸F]CH2CH2OTs. In vitro autoradiography studies of [¹⁸F]FEMPT were performed in postmortem human brain tissue sections. The specific binding of the [¹⁸F]FEMPT binding was determined by co-incubating the adjacent sections with the known 5-HT1AR ligand WAY100635.

Results The syntheses of FEMPT and its radiolabeling precursor proceeded in 4 steps from commercial 4-methyl-2H-[1,2,4]triazine-3,5-dione with 40% overall yield. FEMPT shows a Ki 0.1 nM to 5-HT1AR and an Emax 95% based on GTPgS binding assays of CHO cells expressing 5-HT1AR. FEMPT has no significant affinity to a variety of brain targets. The radiosynthesis of [¹⁸F]FEMPT was achieved in 25+5% yield at EOS. Autoradiography studies reveals higher binding of [18F]FEMPT in hippocampal CA1 and prefrontal cortex brain regions in comparison to cerebellum.

Conclusions FEMPT, a high affinity and selective 5-HT1AR ligand amenable for radiolabeling with [¹⁸F] isotope is identified, synthesized and radiolabeled. The details of radiosynthesis and in vitro and in vivo evaluations of the radioligand will be presented.

Research Support MH062185