Potential effect of a fluorine position in the aromatic molecule (CP-118,954) on the neuropharmacological property and PET imaging quantification for AChE expression

Objectives: The position of the aromatic fluorine substitution (ortho, meta, and para) can influence the disposition and biological activity of a receptor-binding ligand. In an effort to verify the biological effect of a fluorine position in AChE inhibitor, we performed comparative PET imaging and microdialysis studies with different fluorine substituted ligands at three position of benzyl moiety in CP-118,954 (1).

Methods: Three different fluorine substituted CP-118,954 analogs (2-4) were prepared according to the literature. For PET imaging studies, three 18F-labeled ligands also were synthesized by using two radiolabeling steps: nucleophilic aromatic substitution followed by reductive alkylation. PET imaging, microdialysis, and stability studies of three ligands were performed in a healthy SD rat after i.v. injection.

Results: Three radiolabeled CP-118,953 analogs ([¹⁸F]2-4) have been efficiently synthesized in 11-40% of radiochemical yield (n.d.c) with >99% of radiochemical purity, 270-420 GBq/mol of specific activity. Among three ligands, in vitro AChE binding affinity of the meta-fluorine substituted CP-118,954 analog (3, IC₅₀ = 1.4 nM) has a similar to that of CP-118,954 (1, IC₅₀ = 1.2 nM). The comparative experiments also established that 3 along with [¹⁸F]3 induced highest ACh concentration (>10 ng/mL) as well as radioactivity in the striatum of rat brain an AChE inhibitor. In in vivo stability studies, [¹⁸F]3 remained stable (>98%) in brain homogenates until 1 hr after i.v. injection.

Conclusion: These data support that [¹⁸F]3 is a promising AChE PET imaging ligand for assessment of cholinergic activity in the brain and a representative example for verifying the biological change of receptor-binding ligand disposition which was achieved by fixing the meta-position of fluorine atom in the aromatic molecule. Research Support: