A series of 3-acylaminomethyl-6-(chloro, iodo and methyl)-2-(phenyl, 4′-t-butylphenyl, 4′-cyclohexyl- phenyl, biphenyl-4′-yl, 4′-chlorophenyl and 4′-iodophenyl)imidazo[1,2-b]pyridazines and imidazo[1,2- a]pyridines has been prepared and examined for interaction with central and mitochondrial (peripheral- type) benzodiazepine receptors. The imidazo[1,2-b]pyridazines were generally more selective for the mitochondrial receptors than the corresponding imidazo[1,2-a]pyridines. Of these compounds, 3- acetamidomethyl-2-(biphenyl-4′-yl)-6-chloroimidazo[1,2-b]pyridazine (9) proved to be the most selective in studies of the displacement of [3H]diazepam from peripheral-type and central benzodiazepine receptors (IC50 2·8 nM and 0% displacement at 1000 nM, respectively).
The syntheses of some ethyl 2-2′-aryl-6′-(chloro, iodo and methoxy)imidazo[1,2-b]pyridazin-3′-yl}-2-(acetoxy, acylamino, hydroxy and methoxy)acetates, 6-methyl-2-(p-tolyl)- and 6-chloro-2-(4′-chlorophenyl)-3-trimethylammoniomethylimidazo[1,2-b]pyridazine iodides (and reactions thereof), 2-benzoyl 6-substituted imidazo[1,2-b]pyridazines and 7-(methoxy, chloro and phenylthio)-2-phenylpyrimido-[1,2-b]pyridazin-5-ium-3-olates are reported. The ability of these compounds to displace [3H]diazepam from rat forebrain membrane [central benzodiazepine receptor (BZR)] and rat kidney membrane [mitochondrial (peripheral-type) benzodiazepine receptor (PBR)] has been examined. The most active compound was ethyl 2-6′-chloro-2′-(p-tolyl)imidazo[1,2-b]pyridazin-3′-yl}-2-hydroxyacetate with IC50 24 nM for displacement from the BZR and 91% displacement at 1000 nM from the PBR; the most selective for the PBR was 6-methyl-3-methylsulfonylmethyl-2-(p-tolyl)imidazo[1,2-b]pyridazine (PBR, IC50 92 nM; BZR, 15% inhibition of binding by [3H]diazepam at 1000 nM).