Some 6-arylthio( aryloxy and alkylthio )-3-( acetamidomethyl , benzamidomethyl, methoxy and unsubstituted )-2-arylimidazo[1,2-b] pyridazines have been prepared and examined for their ability to displace [3H]diazepam from rat brain membranes. The most active compound was 3-acetamidomethyl-2-(3',4'-methylenedioxyphenyl)-6-phenylthioimidazo[1,2-b] pyridazine with IC50 4.4 nM. The 3-acylaminomethyl-6-(2- and 3-methoxyphenylthio)-2-phenylimidazo[1,2-b] pyridazines proved less active than their 6-phenylthio analogues, and larger substituents at the 2- and 6-positions markedly decreased binding. Significant differences in binding ability have been observed between 3-acylaminomethyl-2-aryl-6-phenylthioimidazo[1,2-b] pyridazines and the corresponding imidazo [1,2-a]pyridines.
2-Aryl-3-methoxy-6-(pyridinylmethylthio and pyridinylmethylamino)imidazo[1,2-b]pyridazines have been prepared and examined as ligands for benzodiazepine receptors. Most were highly effective in displacing [3H]diazepam from central benzodiazepine receptors present in rat brain membranes but showed little capacity for its displacement from mitochondrial (peripheral-type) benzodiazepine receptors present in rat kidney membranes. For example, 3-methoxy-2-(3′,4′-methylenedioxyphenyl)-6-(pyridin-2′′-ylmethylthio)imidazo[1,2-b]pyridazine had an IC50 value of 1 · 7 nM for central receptors but gave only 39% displacement at 1000 nM for mitochondrial receptors. Of all the compounds described in this series of papers, 3-methoxy-6-(2′-methoxybenzylamino)-2-(3′′,4′′-methylenedioxyphenyl)imidazo[1,2-b]pyridazine was the most active in displacing [3H]diazepam from central receptors (IC50 0·3 nM), and it had a low affinity for mitochondrial receptors (40% displacement of [3H]diazepam at 1000 nM).