In vitro characterization of a γ-secretase radiotracer in mammalian brain

Abstract

Inhibition of γ-secretase is a potential therapeutic target for Alzheimer's disease (AD). The present studies have characterized the in vitro properties of a radiolabeled small molecule γ-secretase inhibitor, [ 3H]compound D (Yan et al., 2004, J. Neurosci.24, 2942-2952) in mammalian brain. [3H]Compound D was shown to bind with nanomolar affinity (Kd = 0.32-1.5 nM) to a single population of saturable sites in rat, rhesus and human brain cortex homogenates, the density of binding sites ranging from 4 to 7 nM across the species. Competition studies with a structurally diverse group of γ-secretase inhibitors with a wide range of binding affinities showed that the binding affinities of these compounds correlated well with their ability to inhibit γ-secretase in vitro. Autoradiographic studies showed that the specific binding of [ 3H]compound D was widely distributed throughout adult rat, rhesus and normal human brain. There did not appear to be any difference in distribution of [3H]compound D specific binding sites in AD cortex compared with control human cortex as measured using tissue section autoradiography, nor any correlation between γ-secretase binding and plaque burden as measured immunohistochemically. [3H]compound D is a useful tool to probe the expression and pharmacology of γ-secretase in mammalian brain. © 2005 Merck & Co Inc.