Synthesis, resolution, and in vitro evaluation of three vesicular acetylcholine transporter ligands and evaluation of the lead fluorine-18 radioligand in a nonhuman primate

Abstract

The vesicular acetylcholine transporter (VAChT) is a reliable biomarker for assessing cholinergic dysfunction associated with dementia. We recently reported three new potent and selective carbon-11 labeled VAChT radiotracers. Herein, we report the resolution with a Chiralcel OD column of three additional fluorine containing VAChT ligands in which a fluoroethoxy or fluoroethylamino moiety was substituted for the methoxy group. An in vitro competitive binding assay showed that (−)-7 had high potency for VAChT (K_i-VAChT = 0.31 ± 0.03 nM) and excellent selectivity for VAChT versus σ receptors (K_i-σ1 = 1870 ± 250 nM, K_i-σ2 = 5480 ± 140 nM). Three different radiolabeling approaches were explored; the radiosynthesis of (−)-[¹⁸F]7 was successfully accomplished via a stepwise two-pot, three-step method with moderate yield (11 ± 2%) and high radiochemical purity (>98%). PET imaging studies in a nonhuman primate indicated that (−)-[¹⁸F]7 rapidly entered the brain and accumulated in the VAChT-enriched striatum. The uptake of (−)-[¹⁸F]7 in the target striatal area peaked at 10 min and displayed improved clearance kinetics compared to the VAChT tracer [¹⁸F]VAT, which has been approved by the Food and Drug Administration (FDA) for first-in-man studies. These studies justify further investigation of (−)-[¹⁸F]7 and exploration of the structure–activity relationships of these fluoroethoxy and fluoroethylamino analogs.