Vol. 282, Issue 2, 1011-1019, 1997

I. NGD 94-1: Identification of a Novel, High-Affinity Antagonist at the Human Dopamine D₄ Receptor

John F. Tallman, Renee J. Primus, Robbin Brodbeck, Linda Cornfield¹, Robin Meade, Kristine Woodruff², Phillip Ross³, Andrew Thurkauf and Dorothy W. Gallager

Neurogen Corporation, Branford, Connecticut

NGD 94-1 was evaluated for selectivity and in vitro functional activity at the recombinant human D_4.2 receptor stably expressed in Chinese hamster ovary cells. NGD 94-1 showed high affinity for the cloned human D_4.2 receptor (K_i = 3.6 ± 0.6 nM) and had greater than 600-fold selectivity for the D_4.2 receptor subtype compared with a wide variety of monoamine or other neurotransmitter receptor or modulatory sites except for 5-HT_1A and 5-HT₃ receptors, in which NGD 94-1 was approximately 50- and 200-fold selective, respectively, for the D_4.2 receptor. In measures of in vitro functional activity, NGD 94-1 showed an antagonist profile at the cloned human D_4.2 receptor subtype. NGD 94-1 completely reversed the decrease in forskolin-stimulated cAMP levels produced by the dopamine receptor full agonist quinpirole. Furthermore, NGD 94-1 produced a complete reversal of GTP³⁵S binding induced by quinpirole, but was unable on its own to affect GTP³⁵S binding. These data suggest that NGD 94-1 functions as an antagonist rather than a full or partial agonist at the human D_4.2 receptor. In addition, NGD 94-1 binding affinity at the D_4.2 receptor subtype was unaffected by G-protein activation by GTP, consistent with the binding affinity seen for other antagonists at the D₄ receptor. The binding of tritiated NGD 94-1 was saturable and of high affinity at cloned human D_4.2 receptors. Furthermore, the binding of [³H]NGD 94-1 to cloned human D_4.2 receptors expressed in Chinese hamster ovary cells displayed a pharmacological profile similar to that observed with the nonselective dopamine receptor ligand [³H]YM 09151-2. Saturation and pharmacological analyses of [³H]NGD 94-1 binding at cloned human D_4.2, D_4.4 and D_4.7 receptor variants showed no difference between the three variants. NGD 94-1 is a novel, high-affinity, D₄ receptor-selective antagonist. The clinical use of this subtype-specific compound should permit direct evaluation of the role of D₄ receptors in psychiatric disorders.