Differential blockade of muscarinic receptor subtypes by polymethylene tetraamines. Novel class of selective antagonists of cardiac M-2 muscarinic receptors

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Abstract
Several N,N''-bis[6-[(2-methoxybenzyl)amino]hexyl]-1,.omega.-alkanediamine tetrahydrochlorides (1-7) were synthesized and evaluated for their blocking activity on muscarinic receptors in guinea pig atria and rat ileum and bladder. The results were compared with those obtained for the classical nonselective muscarinic antagonist atropine. It was discovered that optimum activity is associated with an eight-carbon chain (compound 4) in guinea pig atria whereas, in both rat ileum and bladder, the 12-carbon analogue 7 had the highest activity. In addition, polymethylene tetraamines 1-6 displayed high selectivity toward guinea pig atria muscarinic receptors. The discriminatory power of 1-6 was not shared by 7. All the tetraamines were shown to be competitive antagonists of muscarinic receptors. N,N''-Bis[(6-[2-methoxybenzyl)amino]hexyl]-1,8-octanediamine (4) was the most potent and selective toward muscarinic receptors in atria, with a pA2 value of 8.13 and a selectivity ratio (atria vs. ileum or bladder) of ca. 270. At a concentration of 10 .mu.M, tetraamine 4 did not affect histamine and 5-hydroxytryptamine receptors of guinea pig ileum or .alpha.-adrenoreceptors of guinea pig atria whereas it inhibited postsynapic .alpha.-adrenoreceptors of rat vas deferens with a -log K value of 5.23 and nicotinic receptors of frog rectus abdominis with an IC50 value of 0.23 .mu.M. It is concluded that 4 is a novel, powerful, and selective tool in the characterization of muscarinic receptor subtypes.