Vol. 285, Issue 2, 777-786, May 1998

ABT-594 [(R)-5-(2-Azetidinylmethoxy)-2-Chloropyridine]: A Novel, Orally Effective Analgesic Acting via Neuronal Nicotinic Acetylcholine Receptors: I. In Vitro Characterization

Diana L. Donnelly-Roberts, Pamela S. Puttfarcken, Theresa A. Kuntzweiler, Clark A. Briggs, David J. Anderson, Jeffrey E. Campbell, Marietta Piattoni-Kaplan, David G. Mckenna, James T. Wasicak, Mark W. Holladay, Michael Williams and Stephen P. Arneric

Neurological and Urological Diseases Research, Pharmaceutical Products Division, Abbott Laboratories, Abbott Park, Illinois and CNS Research, The DuPont Merck Pharmaceutical Company, Wilmington, Delaware (S.P.A.)

The discovery of (±)-epibatidine, a naturally occurring neuronal nicotinic acetylcholine receptor (nAChR) agonist with antinociceptive activity 200-fold more potent than that of morphine, has renewed interest in the potential role of nAChRs in pain processing. However, (±)-epibatidine has significant side-effect liabilities associated with potent activity at the ganglionic and neuromuscular junction nAChR subtypes which limit its potential as a clinical entity. ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine] is a novel, potent cholinergic nAChR ligand with analgesic properties (see accompanying paper by ) that shows preferential selectivity for neuronal nAChRs and a consequently improved in vivo side-effect profile compared with (±)-epibatidine. ABT-594 is a potent inhibitor of the binding of [³H]()-cytisine to 42 neuronal nAChRs (K_i = 37 pM, rat brain; K_i = 55 pM, transfected human receptor). At the 11 neuromuscular nAChR labeled by [¹²⁵I]-bungarotoxin (-Btx), ABT-594 has a K_i value of 10,000 nM resulting in a greater than 180,000-fold selectivity of the compound for the neuronal 42 nAChR. In contrast, (±)-epibatidine has K_i values of 70 pM and 2.7 nM at the 42 and 11 nAChRs, respectively, giving a selectivity of only 38-fold. The S-enantiomer of ABT-594, A-98593 has activity at the neuronal 42 nAChR identical with ABT-594 (K_i = 34-39 pM), which demonstrates a lack of stereospecific binding similar to that reported previously for (±)-epibatidine. A similar lack of stereoselectivity is seen at the human 7 receptor. However, A-98593 is 3-fold more potent at the neuromuscular nAChR (K_i = 3420 nM) and the brain -Btx-sensitive nAChR (K_i = 4620 nM) than ABT-594. ABT-594 has weak affinity in binding assays for adrenoreceptor subtypes alpha-1B (K_i = 890 nM), alpha-2B (K_i = 597 nM) and alpha-2C (K_i = 342 nM), and it has negligible affinity (K_i > 1000 nM) for approximately 70 other receptors, enzyme and transporter binding sites. Functionally, ABT-594 is an agonist. At the transfected human 42 neuronal nAChR (K177 cells), with increased ⁸⁶Rb⁺ efflux as a measure of cation efflux, ABT-594 had an EC₅₀ value of 140 nM with an intrinsic activity (IA) compared with ()-nicotine of 130%; at the nAChR subtype expressed in IMR-32 cells (sympathetic ganglion-like), an EC₅₀ of 340 nM (IA = 126%); at the F11 dorsal root ganglion cell line (sensory ganglion-like), an EC₅₀ of 1220 nM (IA = 71%); and via direct measurement of ion currents, an EC₅₀ value of 56,000 nM (IA = 83%) at the human 7 homo-oligimeric nAChR produced in oocytes. A-98593 is 2- to 3-fold more potent and displays approximately 50% greater intrinsic activity than ABT-594 in all four functional assays. In terms of potency, ABT-594 is 8- to 64-fold less active than (±)-epibatidine and also has less IA in these functional assays. ABT-594 (30 µM) inhibits the release of calcitonin gene-related peptide from C-fibers terminating in the dorsal horn of the spinal cord, an effect mediated via nAChRs. Pharmacologically, ABT-594 has an in vitro profile distinct from that of the prototypic nicotinic analgesic (±)-epibatidine, with the potential for substantially reduced side-effect liability and, as such, represents a potentially novel therapeutic approach to pain management.