In an attempt to assess the structural requirements of hexahydro-sila-difenidol for potency and selectivity, a series of analogues modified in the amino group and the phenyl ring were investigated for their affinity to muscarinic M 1 -(rabbit vas deferens), Mr (guinea-pig atria) and Mr (guinea-pig ileum) receptors. All compounds were competitive antagonists in the three tissues. Their affinities to the three muscarinic receptor subtypes differed by more than two orders of magnitude and the observed receptor selectivities were not associated with high affinity. The pyrrolidino and hexamethyleneimino analogues, compounds substituted in the phenylring with a methoxy group or a chlorine atom as weil as p-fluoro-hexahydro-difenidol displayed the same affinity profile as the parent compound, hexahydro-sila-difenidol: M 1 = M 3 > M 2 • A different selectivity patternwas observed for p-fluoro-hexahydro-sila-difenidol: M 3 > M 1 > M 2 • This compound exhibited its highest affinity for M 3 -receptors in guinea-pig ileum (pA 2 = 7.84), intermediate affinity for M 1 -receptors in rabbit vas deferens (pA 2 = 6.68) and lowest affinity for the Mrreceptors in guinea-pig atria (pA 2 = 6.01). This receptor selectivity profile of p-fluoro-hexahydro-sila-difenidol was confirmed in ganglia (M 1 ), atria (M 2 ) and ileum (M 3 ) of the rat. Furthermore, dose ratios obtained with either pirenzepine (Mt) or hexahydrosila-difenidol (M 2 and M 3 ) and the p-fluoro analogue used in combination suggested that the antagonism was additive, implying mutual competition with a single population of muscarinic receptor subtypes. These results indicate that p-fluoro-hexahydro-sila-difenidol represents a valuable tool for characterization of muscarinic receptor subtypes.

1 The affinities of the (R)-and (S)-enantiomers of hexahydro-difenidol (1) and its acetylenic analogues hexbutinol (2), hexbutinol methiodide (3) and p-fluoro-hexbutinol (4) (stereochemical purity > 99.8%) for muscarinic receptors in rabbit vas deferens (M,), guinea-pig atria (M2) and guinea-pig ileum (M3) were measured by dose-ratio experiments.2 The (R)-enantiomers consistently showed higher affinities than the (S)isomers. The stereoselectivity ratios [(R)/(S)] were greatest with the enantiomers of I (vas deferens: 550; ileum: 191; atria: 17) and least with those of the p-Fluoro-analogue 4 (vas deferens: 34; ileum: 8.5; atria: 1.7). 3 The enantiomeric potency ratios for compounds 1-4 were highest in rabbit vas deferens, intermediate in guinea-pig ileum and much less in guinea-pig atria. Thus, these ratios may serve as a predictor of muscarinic receptor subtype identity. showed a novel receptor selectivity profile with preference for M3 receptors: M3 > M2 > M1. 5 These results do not conform to Pfeiffer's rule that activity differences between enantiomers are greater with more potent compounds. (S)p-

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