The position of fluorine in CP-118,954 affects AChE inhibition potency and PET imaging quantification for AChE expression in the rat brain
The in vitro inhibition potency against acetylcholinesterase (AChE) of fluorinated derivatives of CP-118,954 (1) has been shown to depend upon the position of aromatic fluorine (F) substitution on the N-benzyl moiety. Indeed, the meta-F-substituted compound 3 (IC50 = 1.4 nM) shows similar potency with the parent compound 1 (IC50 = 1.2 nM), whereas the ortho-F derivative 2 (IC50 = 3.2 nM) and para-F derivative 4 (IC50 = 10.8 nM) were found to be less potent AChE inhibitors. A comparative in vivo microdialysis study in rats showed that 3 has the strongest effect on the neuropharmacological properties as AChE inhibitor. For PET imaging studies, a radiolabeled ligand ([18F]3) was synthesized through nucleophilic aromatic substitution reaction of diaryliodonium salt-based aldehyde precursor followed by reductive alkylation in a two-step radiolabeling procedure with 11.5 ± 1.2% (n = 24, non-decay corrected) radiochemical yield and over 99% radiochemical purity. In a comparative PET imaging study of the three 18F-containing derivatives of CP-118,954 ([18F]2–4), [18F]3 showed the highest radioactivity in the AChE-rich region of normal rat brain which visually reflected the in vitro AChE-binding affinity of 3. These findings support [18F]3 as a promising AChE-targeted PET imaging ligand for the assessment of cholinergic activity into the brain, providing also insights into the AChE ligand disposition, which depends upon the position of the aromatic fluorine in the benzyl moiety.