Pharmacological Characterization of the Muscarinic Agonist (3R,4r)-3-(3-hexylsulfanyl-pyrazin-2-yloxy)-1-aza-bicyclo[2.2.1]heptane (WAY-132983) in in Vitro and in Vivo Models of Chronic Pain

Here, we have investigated the in vitro pharmacology of a muscarinic agonist, (3R,4R)-3-(3-hexylsulfanyl-pyrazin-2-yloxy)-1-aza-bicyclo[2.2.1]heptane (WAY-132983), and we demonstrated its activity in several models of pain. WAY-132983 had a similar affinity for the five muscarinic receptors (9.4–29.0 nM); however, in calcium mobilization studies it demonstrated moderate selectivity for M1 (IC50 = 6.6 nM; Emax = 65% of 10 μM carbachol-stimulation) over the M3 (IC50 = 23 nM; Emax = 41%) and M5 receptors (IC50 = 300 nM; Emax = 18%). WAY-132983 also activated the M4 receptor, fully inhibiting forskolin-induced increase in cAMP levels (IC50 = 10.5 nM); at the M2 receptor its potency was reduced by 5-fold (IC50 = 49.8 nM). In vivo, WAY-132983 demonstrated good systemic bioavailability and high brain penetration (>20-fold over plasma levels). In addition, WAY-1329823 produced potent and efficacious antihyperalgesic and antiallodynic effects in rodent models of chemical irritant, chronic inflammatory, neuropathic, and incisional pain. It is noteworthy that efficacy in these models was observed at doses that did not produce analgesia or ataxia. Furthermore, a series of antagonist studies demonstrated that the in vivo activity of WAY-132983 is mediated through activation of muscarinic receptors primarily through the M4 receptor. The data presented herein suggest that muscarinic agonists, such as WAY-132983, may have a broad therapeutic efficacy for the treatment of pain.