Pharmacological characterization of a novel muscarinic partial agonist, YM796, in transfected cells expressing the m1 or m2 muscarinic receptor gene

To investigate the pharmacological effect of a novel compound YM796, we performed radioligand binding experiments and correlative biochemical experiments using the transfected murine fibroblast B82 cells which expressed the m1 and m2 muscarinic receptor genes (cloned cell lines designated as LK3-3 and M2LKB2-2, respectively). [³H](-)methyl-3-quinuclidinyl benzilate ([³H](-)MQNB) binding in these transfected cell lines was inhibited by different optical isomers of YM796 and other muscarinic drugs, atropine, pirenzepine, AF-DX 116, as well as selected agonists. (-)YM796, (+)YM796 and (±)YM796 inhibited [³H](-)MQNB binding in LK3-3 cells with K_i values of 16.4 μM, 30.1 μM and 21.8 μM and in M2LKB2-2 cells with K_i values of 52.0 μM, 108 μM and 77.1 μM, respectively. From functional assays we found the two isomers, (-)YM796 and (+)YM796 had different intrinsic activities for the M₁ and M₂ muscarinic receptors. (-)YM796 revealed agonistic activity: stimulation of [³H]IP₁ accumulation in LK3-3 cells with an EC₅₀ value of 26.5 μM, which was less efficacious (the E_max value was 5.6 times basal) than carbachol, a full agonist (the E_max value was 17.2 times basal). Interestingly, (-)YM796 did not show significant inhibition of cAMP formation in M2LKB2-2 cells except at extremely high concentrations (> 1mM). (+)YM796 exhibited no significant efficacy for the M₁ and M₂ muscarinic receptors. These results suggest that (-)YM796 represents a muscarinic partial agonist with functional selectivity for the M₁ muscarinic receptors whereas (+)YM796 shows no efficacy for either M₁ or M₂ muscarinic receptors in these transfected cells.