In vitro and in vivo characterisation of [3H]ANSTO-14 binding to the sigma 1 binding sites

N-(4-phenylbutyl)-3-hydroxy-4-azahexacyclo[5.4.1.0(2,6).0(3, 10).0(5,9) .0(8,11)]dodecane (ANSTO-14) showed the highest activity for the sigma 1 site (Ki = 9.4 nM) and 19-fold sigma 1/sigma 2 selectivity. The present study showed that [3H]ANSTO-14 binds to a single high-affinity site in guinea pig brain membranes with an equilibrium Ki of 8.0 +/- 0.3 nM, in good agreement with the kinetic studies (Kd = 13.3 +/- 5.4 nM, n = 4), and a Bmax of 3.199 +/- 105 fmol/mg protein (n = 4). The in vivo biodistribution of [3H]ANSTO-14 showed a high uptake in the diencephalon. Pretreatment of rats with sigma ligands including (+)-pentazocine (sigma 1), ANSTO-14 (sigma 1), and DTG (sigma 1 and sigma 2) did not significantly reduce radiotracer uptake in the brain, but did in the spleen. A labelled metabolite was found in the liver and brain. Due to its insensitivity to sigma ligands, the accumulation of [3H]ANSTO-14 in the brain indicates high nonspecific binding. Therefore, [3H]ANSTO-14 is a suitable ligand for labelling sigma 1 sites in vitro but is not suitable for brain imaging of sigma binding sites in vivo.