Efficacy of Hybrid Tetrahydrobenzo[d]thiazole Based Aryl Piperazines D-264 and D-301 at D 2 and D 3 Receptors

In elucidating the role of pharmacodynamic efficacy at D 3 receptors in therapeutic effectiveness of dopamine receptor agonists, the influence of study system must be understood. Here two compounds with D 3 over D 2 selectivity developed in our earlier work, D-264 and D-301, are compared in dopamine receptor-mediated G-protein activation in striatal regions of wild-type and D 2 receptor knockout mice and in CHO cells expressing D 2 or D 3 receptors. In caudate-putamen of D 2 knockout mice, D-301 was ~3-fold more efficacious than D-264 in activating G-proteins as assessed by [ 35 S]GTPγS binding; in nucleus accumbens, D-301 stimulated G-protein activation whereas D-264 did not. In contrast, the two ligands exerted similar efficacy in both regions of wild-type mice, suggesting both ligands activate D 2 receptors with similar efficacy. In D 2 and D 3 receptor-expressing CHO cells, D-264 and D-301 appeared to act in the [ 35 S]GTPγS assay as full agonists because they produced maximal stimulation equal to dopamine. Competition for [ 3 H]spiperone binding was then performed to determine K i /EC 50 ratios as an index of receptor reserve for each ligand. Action of D-301, but not D-264, showed receptor reserve in D 3 but not in D 2 receptor-expressing cells, whereas dopamine showed receptor reserve in both cell lines. Gα o 1 is highly expressed in brain and is important in D 2 -like receptor-G protein coupling. Transfection of Gα o 1 in D 3 - but not D 2 -expressing CHO cells led to receptor reserve for D-264 without altering receptor expression levels. D-301 and dopamine exhibited receptor reserve in D 3 -expressing cells both with and without transfection of Gα o 1. Altogether, these results indicate that D-301 has greater intrinsic efficacy to activate D 3 receptors than D-264, whereas the two compounds act on D 2 receptors with similar intrinsic efficacy. These findings also suggest caution in interpreting E max values from functional assays in receptor-transfected cell models without accounting for receptor reserve.