Pharmacological Characterisation of the Enantiomers of BM‐5, a Muscarinic Partial Agonist with Opposed Enantioselectivity between Affinity and Efficacy*

Abstract

The interaction of (R) and (S) enantiomers of the chiral oxotremorine analogue BM-5 with muscarinic acetylcholine receptors was studied in vitro using radioligand binding and isolated tissue preparations. The in vivo effects of (R)-BM-5 were also studied in anaesthetised cat. No receptor or tissue selectivity was found for either enantiomer in radioligand binding studies in cells expressing human muscarinic receptors (M 1 -M 5 ) or in guinea pig tissues. The affinity of (R)-BM-5 was about 40 times, or 15-60 times higher than that of (S)-BM-5 in recombinant cells or in guinea pig tissues, respectively. Both enantiomers induced contraction of the guinea pig isolated urinary bladder and ileum. (R)-BM-5 was more potent than (S)-BM-5 in bladder (EC 50 590 and 3500 nM, respectively) and in ileum (EC 50 39 and 2600 nM, respectively). The maximal agonist effect was lower for (R)-BM-5 than for (S)-BM-5 in bladder (2.7% and 6.6%, respectively) and in ileum (32% and 48%, respectively). Contractions were completely inhibited by atropine (1 mM). In vivo, (R)-BM-5 induced bladder contraction and salivation after intravenous administration in the anaesthetised cat (ED 50 4.1 and 6.2 mg kg ª1 , respectively). In conclusion, (R)-and (S)-BM-5 act as partial muscarinic agonists in the isolated bladder and ileum. (R)-BM-5 was the more potent enantiomer but had a lower maximal agonist effect giving an opposed enantioselectivity for affinity and efficacy. (R)-BM-5 showed agonist activity in vivo, confirming in vitro findings. From affinity and efficacy data it can be concluded that the effects of racemic BM-5 are mediated by the (R)-enantiomer.BM-5 (N-methyl-N-(1-methyl-4-pyrrolidino-2-butynyl)acetamide) is an oxotremorine analogue ) that contains a stereogenic centre. Hence, BM-5 exists as two optical isomers: (R)-and (S)-BM-5. A method for the preparation of the enantiomers was first described by Dahlbom et al. (1982) and was later improved by Nilsson et al. (1992).Racemic BM-5 has been shown to be both a muscarinic agonist and antagonist in vivo (Casamenti et al. 1986;Engströ m et al. 1987;Ringdahl et al. 1987; Sethy et al. 1988). Studies in vitro on isolated tissues showed racemic BM-5 to be a partial agonist in the ileum and an antagonist in the urinary bladder (Ringdahl 1986). Overall, the selectivity of racemic BM-5 between muscarinic receptors is very low, as demonstrated with radioligand binding studies in different tissues in rat (Engströ m et al. 1987) and in cell lines expressing different receptor subtypes (Baumgold & Drobnick 1989).It has been shown that (R)-BM-5 is the most potent en- antiomer both in vitro and in vivo .(R)-and (S)-BM-5 were partial agonists in the isolated guinea pig ileum (Ringdahl 1984). The maximal agonist effect was slightly higher for (S)-BM-5 than for (R)-BM-5. The latter, but not (S)-BM-5, significantly increased the basal release of acetylcholine in rat striatum in vivo , an effect mediated by muscarinic antagonists. Both enantiomers of BM-5 acted as muscarinic agonists in rat cerebr...