Synthesis and in vitro biological evaluation of carbonyl group-containing inhibitors of vesicular acetylcholine transporter

To identify selective high-affinity inhibitors of the vesicular acetylcholine transporter (VAChT), we have interposed a carbonyl group between the phenyl and piperidyl groups of the prototypical VAChT ligand vesamicol and its more potent analogues benzovesamicol and 5-aminobenzovesamicol. Of 33 compounds synthesized and tested, 6 display very high affinity for VAChT (K _i, 0.25-0.66 nM) and greater than 500-fold selectivity for VAChT over σ₁ and σ₂ receptors. Twelve compounds have high affinity (K_i, 1.0-10 nM) and good selectivity for VAChT. Furthermore, 3 halogenated compounds, namely, trans-3-[4-(4-fluorobenzoyl) piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28b) (K_i = 2.7 nM, VAChT/sigma selectivity index = 70), trans-3-[4-(5-iodothienylcarbonyl) piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28h) (K_i = 0.66 nM, VAChT/sigma selectivity index = 294), and 5-amino-3-[4-(p-fluorobenzoyl) piperidinyl]-2-hydroxy-1,2,3,4,-tetrahydronaphthalene (30b) (K_i = 2.40 nM, VAChT/sigma selectivity index = 410) display moderate to high selectivity for VAChT. These three compounds can be synthesized with the corresponding radioisotopes so as to serve as PET/SPECT probes for imaging the VAChT in vivo.