AQ-RA 741

AQ-RA 741 is a potent and selective M2 antagonist, with high affinity for cardiac M2 sites (pKi = 8.30) [1].

The M2 muscarinic receptor subtype is involved in the regulation of heart rate, mediating muscarinic receptor-dependent movement, antinociceptive responses and temperature control [2].

In radioligand binding studies, the affinity of AQ-RA 741 for cardiac M2 sites, cortical M1 sites and grandular M3 sites are of pKi values of 8.30, 7.70 and 6.82, respectively. That means AQ-RA 741 showed high affinity for cardiac M2 sites, compared to that for cortical M1 sites and grandular M3 sites. Functional studies showed that AQ-RA 741 is a competitive antagonist. It has a 60 to 87-fold higher affinity to bind cardiac muscarinic receptors than to bind muscarinic receptors in tracheal, intestinal or bladder smooth muscle [1].

M2 selectivity of AQ-RA 741 was also confirmed by in vivo experiments. In rats, guinea-pigs and cats, vagally or agonist-induced bradycardia (?log ID50 = 7.24–7.53 i.v.) were preferentially inhibited by AQ-RA 741. The ratio range of observed potencies between effects mediated by cardiac and other muscarinic receptor was between 9- and greater than 100-fold. These results concluded that AQ-RA 741 is of remarkable in vivo selectivity as a potent and selective M2 antagonist [1].

References:
[1].  Doods H, Entzeroth M and Mayer N. Cardioselectivity of AQ-RA 741, a novel tricyclic antimuscarinic drug. Eur J Pharmacol, 1991, 192(1):147-52.
[2].  Gomeza J, Shannon H, Kostenis E, et al. Pronounced pharmacologic deficits in M2 muscarinic acetylcholine receptor knockout mice. Proc Natl Acad Sci U S A, 1999, 96(4):1692-7.

Cardioselectivity of AQ-RA 741, a novel tricyclic antimuscarinic drug.[Pubmed:2040358]

Eur J Pharmacol. 1991 Jan 3;192(1):147-52.

The interaction of the AF-DX 116 analogue, AQ-RA 741 (11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11- dihydro-6H-pyrido[2,3-b] [1,4]benzodiazepin-6-one), with muscarinic receptors, in vitro and in vivo, was examined. In radioligand binding studies, AQ-RA 741 showed high affinity for cardiac M2 sites (pKi = 8.30), intermediate affinity for cortical M1 sites (pKi = 7.70) and low affinity for glandular M3 sites (pKi = 6.82). Functional studies showed AQ-RA 741 to be a competitive antagonist and to have a 60 to 87-fold higher affinity for cardiac muscarinic receptors than for muscarinic receptors in intestinal, tracheal or bladder smooth muscle. In vivo experiments confirmed the M2 selectivity of AQ-RA 741. In rats, cats and guinea-pigs AQ-RA 741 preferentially inhibited the vagally or agonist-induced bradycardia (-log ID50 = 7.24-7.53 i.v.). The ratio of potencies observed between effects mediated by cardiac and other muscarinic receptor ranged between 9- and more than 100-fold. The results show that AQ-RA 741 is a potent and selective M2 antagonist with remarkable in vivo selectivity.

Pharmacological profile of selective muscarinic receptor antagonists on guinea-pig ileal smooth muscle.[Pubmed:8200421]

Eur J Pharmacol. 1994 Mar 3;253(3):275-81.

The present study examined the effects of a series of tricyclic muscarinic receptor antagonists on muscarinic receptors present in the guinea-pig ileum, both in vitro and in vivo. The selectivity profiles of these antagonists and that of atropine were determined by their affinity for cortical muscarinic M1, cardiac M2 and submandibular M3 receptors and for m4 receptors expressed in CHO cells. The compounds pirenzepine, UH-AH 37, AQ-RA 391 and AQ-RA 618 possessed high affinity (pKi 7.94-8.22) for muscarinic M1 receptors. Pirenzepine exhibited the most pronounced muscarinic M1 selectivity. AF-DX 384 and AQ-RA 741 possessed an approximately 10-fold higher affinity for the cardiac muscarinic M2 receptor than AF-DX 116. However, both compounds also exhibited high affinity for muscarinic m4 receptors. High affinity for muscarinic M3 and m4 receptors was observed for UH-AH 37, AQ-RA 391 and AQ-RA 681. The antagonists were then tested for their interaction with the muscarinic receptors which are responsible for the methacholine-induced contraction of longitudinal muscle in vitro, circular muscle in vivo and muscarinic receptors which mediate the distension-evoked ascending reflex contraction of circular muscle in vitro. Compounds showing high affinity for muscarinic M3 receptors (e.g. AQ-RA 618) were the most potent antagonists in the functional experiments. Comparison of the binding displacement data with the functional results indicates that the effects of methacholine on the longitudinal and circular muscle of the guinea-pig ileum were predominantly mediated by muscarinic M3-type receptors. In contrast, the correlation between muscarinic M2 receptor affinity and antagonism of muscarinic receptors in the ileum was very weak.

Antagonist binding profiles of five cloned human muscarinic receptor subtypes.[Pubmed:1994002]

J Pharmacol Exp Ther. 1991 Feb;256(2):727-33.

A variety of muscarinic antagonists are currently used as tools to pharmacologically subclassify muscarinic receptors into M1, M2 and M3 subtypes. In the present study, we have determined the affinity profiles of several of these antagonists at five cloned human muscarinic receptors (m1-m5) stably expressed in Chinese hamster ovary cells (CHO-K1). At all five receptors, the (R)-enantiomers of trihexyphenidyl and hexbutinol displayed considerably higher affinities (up to 525-fold) than their corresponding (S)-isomers. The stereoselectivity ratios [inhibition constant(S)/inhibition constant(R)] for both pairs of enantiomers were lowest at m2 receptors, suggesting that less stringent configurational demands are made by this receptor subtype. The "M1-selective" antagonist (R)-trihexyphenidyl displayed high affinities for m1 and m4 receptors. The "M2-selective" antagonists himbacine, (+-)-5,11-dihydro-11- ([(2-[(dipropylamino)methyl]-1- piperidinyl)ethyl)amino]carbonyl)-6H-pyrido(2,3-b)(1,4)benzodiazepine-6- one (AF-DX 384), 11-[4-[4-(diethylamino)butyl]-1-piperidinyl)acetyl)-5,11- dihydro-6H-pyrido(2,3-b) (1,4)benzodiazepine-6-one (AQ-RA 741) and (+)-(11-[2-[(diethylamino) methyl]-1-piperidinyl)acetyl)-5,11-di-hydro-6H-pyrido(2,3-b)(1,4) benzodiazepine-6-one [AF-DX 250; the (+)-enantiomer of AF-DX 116] exhibited high affinities for m2 and m4, intermediate affinities for m1 and m3 and low affinities for m5 receptors. This selectivity profile was most prominent for AQ-RA 741, which displayed 195- and 129-fold higher affinities for m2 and m4 receptors than for m5 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)