Pharmacology of hexahydro-difenidol, hexahydro-sila-difenidol and related selective muscarinic antagonists.

A series of hexahydro-difenidol (HHD) and hexahydro-sila-difenidol (HHSiD) analogues modified in the amino group, the phenyl ring and in the alkylene chain were investigated for their binding and functional properties at muscarinic M1, M2 and M3 receptors. Novel muscarinic receptor antagonists were obtained which exhibited different receptor selectivity profiles from the parent compounds HHD and HHSiD (M1 congruent to M3 greater than M2), e.g. HHD and HHSiD methiodides, M1 greater than M2 congruent to M3; p-fluoro-HHSiD, M3 greater than M1 greater than M2; trans-hexbutenol, M1 greater than M3 greater than M2; and (s)-p-fluoro-hexbutinol, M3 greater than M2 congruent to M1. Stereoselectivity ratios [(R)/(S)] for the enantiomers of HHD, hexbutinol and p-fluoro-hexbutinol were highest at M1, intermediate at M3 and lowest at M2 receptors.

Stereoselective inhibition of muscarinic receptor subtypes by the enantiomers of hexahydro-difenidol and acetylenic analogues.

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 greater than 99.8%) for muscarinic receptors in rabbit vas deferens (M1), 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 1 (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. 4. (S)-p-Fluoro-hexbutinol [(S)-4] showed a novel receptor selectivity profile with preference for M3 receptors: M3 greater than M2 greater than or equal to M1. 5. These results do not conform to Pfeiffer's rule that activity differences between enantiomers are greater with more potent compounds.

Affinity profiles of hexahydro-sila-difenidol analogues at muscarinic receptor subtypes.

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 M1-(rabbit vas deferens), M2- (guinea-pig atria) and M3- (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 phenyl ring with a methoxy group or a chlorine atom as well as p-fluoro-hexahydro-difenidol displayed the same affinity profile as the parent compound, hexahydro-sila-difenidol: M1 approximately M3 greater than M2. A different selectivity pattern was observed for p-fluoro-hexahydro-sila-difenidol: M3 greater than M1 greater than M2. This compound exhibited its highest affinity for M3-receptors in guinea-pig ileum (pA2 = 7.84), intermediate affinity for M1-receptors in rabbit vas deferens (pA2 = 6.68) and lowest affinity for the M2-receptors in guinea-pig atria (pA2 = 6.01). This receptor selectivity profile of p-fluoro-hexahydro-sila-difenidol was confirmed in ganglia (M1), atria (M2) and ileum (M3) of the rat. Furthermore, dose ratios obtained with either pirenzepine (M1) or hexahydrosila-difenidol (M2 and M3) 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.