[H]4-(Dimethylamino)-N-(4-(4-(2- Methoxyphenyl)Piperazin-1-yl)- Butyl)Benzamide: A Selective Radioligand for Dopamine D3 receptors. II. Quantitative Analysis of Dopamine D3 and D2 Receptor Density Ratio in the Caudate-Putamen

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, displays high affinity and moderate selectivity for dopamine D3 receptors versus dopamine D2 receptors (Chu et al. [2005] Bioorg Med Chem 13:77–87). In this study, WC-10 was radiolabeled with tritium (specific activity 5 80 Ci/mmol), and quantitative autoradiography studies were conducted using rhesus monkey and Sprague-Dawley rat brain sections. Kd values for the binding of [ H]WC-10 to D3 receptors obtained from quantitative autoradiography with rhesus monkey and rat brain sections are in agreement with Kd values obtained from cloned human and rat receptors (Xu et al. [2009] Synapse 63:717-728). The D2 selective antagonist [ H]raclopride binds with 11-fold higher affinity to human HEK D2L (Kd 5 1.6 nM) than HEK D3 (Kd 5 18 nM) receptors; [H]raclopride binds to rat Sf9 rD2L receptors with a Kd of 6.79 nM, a value that is 4-fold lower than binding to human HEK D2L receptors and 2.5-fold higher than binding to rat Sf9 rD3 receptors. In vitro quantitative autoradiography studies with [H]WC-10 and [H]raclopride were conducted on adult rat and rhesus monkey brain sections. A mathematical model for calculating the absolute densities of dopamine D2 and D3 receptors based on the in vitro receptor binding data of [H]WC-10 and [H]raclopride was developed. Synapse 64:449– 459, 2010. VC 2010 Wiley-Liss, Inc.