Efficacy of Hybrid Tetrahydrobenzodthiazole Based Aryl Piperazines D-264 and D-301 at D-2 and D-3 Receptors

摘要

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 similar to 3-fold more efficacious than D-264 in activating G-proteins as assessed by S-35GTP gamma S 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 S-35GTP gamma S assay as full agonists because they produced maximal stimulation equal to dopamine. Competition for H-3spiperone binding was then performed to determine K-i/EC50 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. G alpha(o)1 is highly expressed in brain and is important in D-2-like receptor-G protein coupling. Transfection of G alpha(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 G alpha(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.