Stereoselective recognition of the enantiomers of phenglutarimide and of six related compounds by four muscarinic receptor subtypes

1 We have compared the binding properties of the enantiomers of phenglutarimide (1) and of six related compounds to M(1) receptors in NB-OK-1 cells, M(2) receptors in rat heart, M(3) receptors in rat pancreas and the M(4) receptors of rat striatum, with their functional (antimuscarinic) properties in rabbit vas deferens (M(1)/M(4)-like), guinea-pig atria (M(2)) and guinea-pig ileum (M(3)) receptors. The binding properties of the enantiomers of three of the compounds were also measured on cloned human m1-m4 receptors expressed by CHO cells, using [H-3]-N-methylscopolamine ([H-3]-NMS) as radioligand. 2 The high affinity enantiomers behaved as competitive antagonists in binding and pharmacological studies. (S)-phenglutarimide (pK(i)-M(i) = 9.0/0.3) and (R)-thienglutarimide (pK(i)-M(i) = 8.6/9.2) recognized selectively the native M(1) > M(4) > M(3) > M(2) receptors in tissues as well as the respective cloned receptors. 3 The pA(2) values at the inhibitory heteroreceptors in the rabbit vas deferens, and at the guinea-pig atria and ileum for the seven more potent enantiomers were compatible with the previous classification of these receptors as M(1)/M(4)-like, M(2) and M(3), respectively. 4 Replacement of the phenyl by a thienyl ring or of the diethylamino by a piperidino group in the phenylglutarimide molecule did not affect markedly the potencies of the high affinity enantiomer. In contrast, replacement of the phenyl by a cyclohexyl ring decreased 20 fold the active enantiomers potency. Methylation of the piperidine-2,6-dione nitrogen also reduced markedly the eutomers' affinities, more on the M(1) than on the other subtypes. 5 The selectivity profiles (recognition of four receptor subtypes) of six of the seven less active enantiomers were different from the corresponding more active enantiomers selectivity profiles, suggesting that the preparations used in this study were pure. However, we cannot not exclude the hypothesis that the batch of (S)-thienglutarimide used in this study was contaminated by less than 0.02% of the eutomer. 6 In contrast with the eutomer binding site, replacement of the phenyl ring by a thienyl or cyclohexyl ring did not affect binding of the low affinity enantiomers to the muscarinic receptor or the [H-3]-NMS-receptor complex. The replacement of the diethylamino group by a piperidine ring, and N-methylation of the piperidine-2.6-dione moiety increased slightly these enantiomers' potencies. 7 The muscarinic receptors were extremely stereoselective, and had up to 20 000 fold lower affinity for the less active enantiomers. However, the stereochemical requirements of the muscarinic receptor sybtypes were different for the enantiomers of compounds 1-7, being most stringent at M(1) receptors. 8 The weaker enantiomers behaved as competitive antagonists in pharmacological studies, at least in the concentration-range investigated.