18F-Labeled indole-based analogs as highly selective radioligands for imaging sigma-2 receptors in the brain

Abstract

We have designed and synthesized a series of indole-based σ2 receptor ligands containing 5,6-dimethoxyisoindoline or 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline as pharmacophore. In vitro competition binding assays showed that all ten ligands possessed low nanomolar affinity (Ki = 1.79–5.23 nM) for σ2 receptors and high subtype selectivity (Ki (σ2)/Ki (σ1) = 56–708). Moreover, they showed high selectivity for σ2 receptor over the vesicular acetylcholine transporter (>1000-fold). The corresponding radiotracers [18F]16 and [18F]21 were prepared by an efficient one-pot, two-step reaction sequence with a home-made automated synthesis module, with 10–15% radiochemical yield and radiochemical purity of >99%. Both radiotracers showed high brain uptake and σ2 receptor binding specificity in mice.