Synthesis of a carbon-11 labeled ligand targeting trace amine associated receptor 1

Objectives: Trace amine-associated receptor-1 (TAAR1), which are one of G protein-coupled receptors (GPCRs), are widely studied due to their involvement in the pathogenesis of neuropsychiatric disorders. TAAR1 specific compounds have been applied in the development of novel therapeutics for schizophrenia and Parkinson’s related psychoses [1]. The goal of this project was to develop a TAAR1-selective ligand labeled with carbon-11, and to perform preliminary in vivo evaluation for PET imaging.

Methods: Compound 3 is selected as the molecule of interest because of its high in vitro binding affinity to TAAR1. The cold compound was synthesized from 6-chloro-5-(trifluoromethyl)nicotinic acid over 2 steps, involving S_NAr replacement of CF₃ with tetrahydropyrrole and condensation with 3-methoxyaniline. Following the similar route, corresponding precursor 5 was obtained by additional deprotection of phenolic hydroxyl with 3M HCl (aq) [2]. Radiosynthesis of [¹¹C]3 was achieved using [¹¹C]CH₃I with K₂CO₃ in DMSO at 100 ^oC for 5 min. Dynamic PET imaging (60 min) were carried out in mice to validate the in vivo binding specificity.

Results: The non-radioactive compounds 3 and precursor 5 were obtained in total yield of 56% and 20%, respectively. Compound 3 was reported to possess strong in vitro binding affinity potency to TAAR1 in mice with K_i = 2 nM. The [¹¹C]3 was synthesized through ¹¹C-methylation of a phenolic precursor in an average of 10% (n = 3) decay-corrected radiochemical yield (RCY), relative to starting [¹¹C]CO₂ at the end of synthesis, with high radiochemical purity (>99%). No radiolysis of [¹¹C]3 was detected up to 90 min after formulation (5% EtOH in saline). Unfortunately, PET dynamic scan indicated peak SUV in mice brain is limited to 0.4 after i.v. 2 min. Conclusion: We have successfully radiolabeled a potent TAAR1-selective antagonist in high radiochemical yield and high molar activity. Further ADME optimization will be performed in order to improve brain permeability and guide the development of the next-generation TAAR1 PET ligands. References: [1] Cell Mol Neurobiol. 2019, Doi:10.1007/s10571-019-00772-7; [2] Bioorg. Med. Chem. Lett. 2011, 21(4), 1227-1231.