Radiosynthesis and first evaluation in mice of [¹⁸F]NS14490 for molecular imaging of α₇ nicotinic acetylcholine receptors

[¹⁸F]NS14490, a new potential radiotracer for neuroimaging of α₇ nicotinic acetylcholine receptors (α₇ nAChRs), was synthesized and evaluated in vitro and in vivo. Radioligand binding studies using [³H]methyllycaconitine and NS14490 as competitor showed a good target affinity (K_iα7= 2.5 nM) and a high selectivity towards other nAChR. Radiosynthesis of [¹⁸F]NS14490 was performed by two different labelling procedures: a two-step synthesis using a prosthetic group, which led to 7% labelling yield, and the convenient direct nucleophilic substitution of the corresponding tosylate precursor, which resulted in 70% labelling yield. After optimisation of the isolation, purification and formulation process, biodistribution studies were performed in CD-1 mice. The brain uptake of [¹⁸F]NS14490 was comparably low (0.16% ID g^-1 wet weight at 5 min p.i.). The radiotracer showed a high metabolic stability in plasma and brain. Also, the target specificity was proven by pre-administration of a highly affine α₇ ligand providing a rationale basis for further in vivo evaluation.