Pharmacological Actions of a Novel, High-Affinity, and Selective Human Dopamine D 3 Receptor Antagonist, SB-277011-A

SB-277011-A { trans - N -[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolininecarboxamide}, is a brain-penetrant, high-affinity, and selective dopamine D 3 receptor antagonist. Radioligand-binding experiments in Chinese hamster ovary (CHO) cells transfected with human dopamine D 3 or D 2 long (hD 3 , hD 2 ) receptors showed SB-277011-A to have high affinity for the hD 3 receptor (p K i = 7.95) with 100-fold selectivity over the hD 2 receptor and over 66 other receptors, enzymes, and ion channels. Similar radioligand-binding data for SB-277011-A were obtained from CHO cells transfected with rat dopamine D 3 or D 2 . In the microphysiometer functional assay, SB-277011-A antagonized quinpirole-induced increases in acidification in CHO cells overexpressing the hD 3 receptor (p K b = 8.3) and was 80-fold selective over hD 2 receptors. Central nervous system penetration studies showed that SB-277011-A readily entered the brain. In in vivo microdialysis studies, SB-277011-A (2.8 mg/kg p.o.) reversed the quinelorane-induced reduction of dopamine efflux in the nucleus accumbens but not striatum, a regional selectivity consistent with the distribution of the dopamine D 3 receptor in rat brain. SB-277011-A (2–42.3 mg/kg p.o.) did not affect spontaneous locomotion, or stimulant-induced hyperlocomotion. SB-277011-A (4.1–42.2 mg/kg p.o.) did not reverse prepulse inhibition deficits in apomorphine- or quinpirole-treated rats, but did significantly reverse the prepulse inhibition deficit in isolation-reared rats at a dose of 3 mg/kg p.o. SB-277011-A (2.5–78.8 mg/kg p.o.) was noncataleptogenic and did not raise plasma prolactin levels. Thus, dopamine D 3 receptor blockade produces few of the behavioral effects characteristic of nonselective dopamine receptor antagonists. The effect of SB-277011-A on isolation-induced prepulse inhibition deficit suggests that blockade of dopamine D 3 receptors may benefit the treatment of schizophrenia.