Organic synthesis, radiofluorination and in vitro evaluation of two novel fluorenones targeting the alpha 7 nicotinic acetylcholine receptor (α7 nAChR)

Objectives: Derivatives of the antiviral drug tilorone have recently been discovered as novel α7 nAChR ligands. Based on a tricyclic heteroaromatic unit linked to one or two conformationally rigidified cyclic amines, highly affine compounds were obtained [1]. [18F]DBT10 [2] and [18F]ASEM [3] (Fig.) were developed as promising α7 nAChR PET tracers. This study investigates bioisosteric sulfur-free analogues of DBT10 and its ortho-isomer obtained by replacement of the functional group SO2 by CO.
Methods: The p-fluoro and o-fluoro derivatives 1 and 2 and the corresponding nitro precursors were prepared in three and five steps from the appropriate nitro-fluoren-9-ones. Affinities towards human α7, α4β2, and α3β4 nAChRs were determined. Radiosynthesis of [18F]1 and [18F]2 via nucleophilic aromatic radiofluorination was optimised and finally performed using an automated module. In vitro autoradiography of [18F]1 on pig brain slices was performed.
Results: The fluorenones 1 and 2 were prepared in 25% yield. Both bind with high affinity and selectivity towards nAChRs (1: Ki = 1.18 nM, 1500 nM, and 46.0 nM; 2: Ki = 1.12 nM, 1796 nM, and 33.2 nM for α7, α4β2, and α3β4 nAChR, respectively). Highest labeling efficiencies (≈ 80%) were obtained in DMF under microwave assisted heating, yielding [18F]1 and [18F]2 with radiochemical purities of ≥ 97% and molar activities of 30 ± 6 GBq/μmol and 44 ± 3 GBq/μmol, respectively. Binding of [18F]1 on pig brain slices was markedly reduced by α7 nAChR-specific ligands.
Conclusions: New highly affine α7 nAChR ligands were synthesized based on the tilorone scaffold by replacement of the SO2 by a CO group. The high nitro-to-[18F]fluoro conversion obtained for the fluorenones suggests a comparable electron withdrawing effect of the two functionalities. Further studies will investigate the potential of [18F]1 and [18F]2 as PET imaging agents.
References [1] M. R. Schrimpf, K. B. Sippy, C. A. Briggs, et al. Bioorg. Med. Chem. Lett. 2012, 22, 1633-1638. [2] R. Teodoro, M. Scheunemann, W. Deuther-Conrad, et al. Molecules 2015, 20, 18387-18421. [3] A. G. Horti, Y. Gao, H. Kuwabara, et al. J. Nucl. Med. 2014, 55, 672-677.