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

Abstract

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 sigma(1) and sigma(2) 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.

Figure 1

Representative compound structures for VAChT.

Scheme 1

^a Reagents and conditions: (a) aryl Grignard reagents/toluene, reflux; (b) 70% of sulfuric acid, 160 – 170°C; (c) Ethanol/Na₂CO₃, refluxed.

Scheme 2

^aReagents and conditions:(a) (i) BzCl, Et₃N; (ii) NaOH, aq. EtOH; (b) (i) Oxalyl chloride, CH₂Cl₂; (ii) arene, AlCl₃, CS₂, reflux; (c) 6N HCl, heat; (d) EtOH, Et₃N, reflux; e: NaOH, aq. EtOH.

Scheme 3

^aReagents and conditions: (a) di-tert-butyldicarbonate, CHCl₃, NaCl, H₂O, reflux; (b) NaOH (aq.), EtOH; (c) N-hydroxysuccinimide, DCC, CH₂Cl₂; (d) 4-substituted piperidine, CH₃CN, reflux; (e) HCl(g), EtOAc, 0°C; (f) trans-2-hydroxy-3-bromo-1,2,3,4-tetrahydronaphthalene, EtOH, Et₃N, reflux; (g) EtOH, Et₃N, reflux; h: NaOH, aq. EtOH.