Synthesis, Radiolabeling and Biological Evaluation of a Novel Trace Amine-Associated Receptor 1 (TAAR1) PET radioligand

Objectives: Trace amine-associated receptor 1 (TAAR1) activation ultimately results in the attenuation of monoaminergic signaling. Accordingly, TAAR1 has emerged as a valuable target for the modulation of dopaminergic and adrenergic signal transduction. Negative TAAR1 modulation constitutes an innovative approach to tackle hypo-dopaminergic disorders such as Parkinson’s disease. Nonetheless, the development of TAAR1 antagonists has proven challenging due to the lack of selectivity for the vast majority of reported structures. We envisioned that a suitable TAAR1 PET radioligand would facilitate the development of selective TAAR1 antagonists. As such, the aim of this study was to develop a suitable TAAR1 PET radioligand that allows non-invasive receptor visualization in vivo.Methods: The synthesis of target compound 4 was performed in two steps, starting from commercially available pyridine carboxylic acid derivative 1. These steps included a modified Buchwald-Hartwig cross-coupling and subsequent amide formation (Figure 1A). To obtain precursor 7, intermediate 2 was reacted with 3-aminophenolmethoxymethyl ether, followed by acidic ether hydrolysis in refluxing methanol. Our lead molecule [11C]4 ([11C]TAAR1-1911) were labeled by 11C-methylation. Ex vivo biodistribution was performed and dynamic PET was acquired under baseline and blockade conditions. At 30 min post injection, metabolite studies were performed to determine the percentage of intact [11C]TAAR1-1911 in the brain and the plasma.Results: Target compound 4 was obtained in an overall yield of 55% over two steps. The initial amination reaction was accomplished at high temperatures, however, without the use of a metal catalyst, affording key intermediate 2 in 77% yield. The subsequent esterifications with 3-aminophenolmethyl ether yielded target compound 4. Similarly, carboxylic acid derivative 2 was reacted with amine 5 to afford amide 6, which was directly used in the subsequent step. The latter involved an acidic hydrolysis of the alcohol protecting group and afforded phenolic precursor 7 in a yield of 59% over two steps (Figure 1A). Although the radiosynthesis proved to be challenging with Cs2/K2CO3, the use of NaOH instead led to a significant increase of RCY (Figure 1B), along with molar activities >37 GBq/µmol and excellent radiochemical purities. Employing the shake-flask method, a logD of 3.4 ± 0.2 was determined, which is within the recommended range for CNS-targeted probes. Ex vivo biodistribution studies at different time points indicated a rather low brain uptake, along with a fast washout. Highest accumulation of radioactivity was observed in the liver, kidneys and the intestine (Figure 1C), suggesting multiple major clearance pathways. Indeed, no specific binding was observed in vivo during blockade experiments. The latter can be explained by the fast metabolism of [11C]TAAR1-1911, as evidenced by the low fractions of intact parent tracer in the brain and plasma at 30 min post injection (Figure 1D).Conclusion: We report the successful synthesis and radiolabeling of a carbon-11 bearing nicotinamide derivative with high affinity for TAAR1. Notwithstanding the encouraging in vitro results, the further development of [11C]TAAR1-1911 was hampered by metabolic instability in vivo. Future studies will aim at introducing structural modifications that would improve the metabolic stability, thus allowing specific and selective TAAR1-targeted in vivo imaging.eSR