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

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-HT 3 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 GTPγ 35 S binding induced by quinpirole, but was unable on its own to affect GTPγ 35 S 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 D 4 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 [ 3 H]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 [ 3 H]YM 09151–2. Saturation and pharmacological analyses of [ 3 H]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, D 4 receptor-selective antagonist. The clinical use of this subtype-specific compound should permit direct evaluation of the role of D 4 receptors in psychiatric disorders.