Rapacuronium bromide is an allosteric modulator of muscarinic acetylcholine receptor (mAChR).
Rapacuronium binds to all muscarinic receptor subtypes at physiologically relevant concentrations and displays micromolar affinity and slight selectivity towards M2 receptor. Rapacuronium exhibits complex effects on the kinetics of ACh binding and subsequent receptor activation estimated from stimulation of [35S]GTPγS binding. Rapacuronium alone concentration dependently lowers [35S]GTPγS binding to membranes with a maximal effect of approximately 25% at odd-numbered subtypes and 15% at even-numbered subtypes, with EC50 ranging from 28 μM at M2 receptors to 76 μM at M3 receptors. While the EC50 values of Rapacuronium in inhibiting [35S]GTPγS binding at individual subtypes correlated with affinities measured in binding experiments with [3H]ACh (R2 = 0.76) they are lower (4- to 12-fold) at all subtypes. Measurements of ACh-stimulated [35S]GTPγS binding in the presence of 0.1, 1 and 10 μM Rapacuronium shows differential effects of Rapacuronium on receptor activation by an orthosteric agonist at individual receptor subtypes. At even-numbered subtypes 1 μM and 10 μM Rapacuronium significantly increases ACh EC50, with lowering of EMAX at 10 μM Rapacuronium. At this subtype 0.1 and 1 μM Rapacuronium causes a significant 2-fold decrease in ACh EC50 and approximately 60% and 35% increase in EMAX, respectively. Rapacuronium at 10 μM increases ACh EC50 by about 3-fold without a significant change in EMAX. Rapacuronium (0.1 - 10 μM) has no effect on ACh efficacy at the M1 and M5 subtypes but decreases the EC50 of ACh in stimulating [35S]GTPγS binding by 1.5- and 4-fold, respectively, at concentrations of 0.1 and 1 μM. However, this effect is not evident at 10 μM Rapacuronium[1].
Time course of the neuromuscular effects of Rapacuronium following the administration of the 2×ED90 doses to rats and guinea-pigs with ED90 of 5953±199 and 187±16 µg/kg in rat and guinea pig, respectively[2].
[1]. Jakubík J, et al. Divergence of allosteric effects of Rapacuronium on binding and function of muscarinic receptors. BMC Pharmacol. 2009 Dec 28;9:15. [2]. Vizi ES, et al. A new short-acting non-depolarizing muscle relaxant (SZ1677) without cardiovascular side-effects. Acta Anaesthesiol Scand. 2003 Mar;47(3):291-300.