Synthesis and pharmacological characterization of 1-phenyl-, 4-phenyl-, and 1-benzyl-1, 2, 3, 4-tetrahydroisoquinolines as dopamine receptor ligands

A series of 1-phenyl-, 4-phenyl-, and 1-benzyl-1, 2, 3, 4-tetrahydroisoquinolines have been prepared as ring-contracted analogues of the prototypical D₁dopamine receptor antagonist SCH23390 [(R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2, 3, 4, 5-tetrahydro-1H-3-benzazepine]. The affinity and selectivity of these isoquinolines for receptors was determined by three biochemical endpoints in membrane homogenates prepared from rat corpus striatum: the potency to compete for [³H]SCH23390 binding sites; the potency to compete for [³H]spiperone (a D₂receptor ligand) binding sites; and effects on dopamine-stimulated adenylate cyclase. Competitive binding measurements at D₁sites showed SCH23390 to possess the highest affinity, followed by 1-phenyl > 1-benzyl > 4-phenyl for the isoquinolines. These results were highly correlated with the ability of the test compounds to antagonize dopamine-stimulated adenylate cyclase (r = 0.98). None of the compounds alone stimulated cAMP formation at concentrations of 10 nM to 100 μM. D₂competition binding showed the 1-benzyl derivative to possess the highest affinity, followed by 4-phenyl > SCH23390 > 1-phenyl. The tertiary 1-phenyl derivative was more potent than the secondary 1-phenyl analogue in all assays. Interestingly, resolution and single-crystal X-ray analysis of the tertiary N-methyl-1-phenyltetrahydroisoquinoline showed the most active enantiomer to possess the S absolute configuration, in contrast to the benzazepine (R)-SCH23390.