[ 3 H]4‐(Dimethylamino)‐ N ‐[4‐(4‐(2‐methoxyphenyl)piperazin‐ 1‐yl)butyl]benzamide, a selective radioligand for dopamine D 3 receptors. I. In vitro characterization

4‐(Dimethylamino)‐ N ‐(4‐(4‐(2‐methoxyphenyl)piperazin‐1‐yl)butyl)benzamide ( WC‐10 ), a N ‐phenyl piperazine analog, has been shown to have high affinity and selectivity for dopamine D 3 receptors versus dopamine D 2 receptors (Chu et al. [2005] Bioorg Med Chem 13:77–87). In this study, WC‐10 was radiolabeled with tritium (specific activity = 80 Ci/mmol) and [ 3 H] WC‐10 binding to genetically cloned dopamine D 2L and D 3 receptors was evaluated in vitro. [ 3 H] WC‐10 binds with a 66‐fold higher affinity to human HEK D 3 than HEK D 2L receptors, with a dissociation constant ( K d ) of 1.2 nM at HEK D 3 receptors. However, [ 3 H] WC‐10 binds to rat Sf9 rD 3 receptors with a K d of 3.9 nM, a value that is 3‐fold lower than binding to human HEK D 3 receptors and 40‐fold value higher than binding to rat Sf9 rD 2L receptors. The K d values obtained from saturation binding experiments were consistent with the results determined from kinetic ( k on and k off ) studies. The pharmacologic profiles of a series of dopaminergic drugs for inhibiting the binding of [ 3 H] WC‐10 to D 3 receptors was in agreement with previously reported data. In vitro autoradiography studies of rat and monkey brains show that [ 3 H] WC‐10 labeled D 3 sites in the striatal region. Synapse 63:717–728, 2009. © 2009 Wiley‐Liss, Inc.