Pharmacological characterisation of the enantiomers of BM-5, a muscarinic partial agonist with opposed enantioselectivity between affinity and efficacy

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 μM). In vivo, (R)-BM-5 induced bladder contraction and salivation after intravenous administration in the anaesthetised cat (ED 50 4. 1 and 6.2 μg kg -1 , 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.