ABT-594 [(R)-5-(2-AZETIDINYLMETHOXY)-2-CHLOROPYRIDINE] - A NOVEL, ORALLY EFFECTIVE ANALGESIC ACTING VIA NEURONAL NICOTINIC ACETYLCHOLINE-RECEPTORS - I - IN-VITRO CHARACTERIZATION

The discovery of (+/-)-epibatidine, a naturally occurring neuronal nic otinic acetylcholine receptor (nAChR) agonist with antinociceptive act ivity 200-fold more potent than that of morphine, has renewed interest in the potential role of nAChRs in pain processing. However, (+/-)-ep ibatidine has significant side-effect liabilities associated with pote nt activity at the ganglionic and neuromuscular junction nAChR subtype s which limit its potential as a clinical entity. ABT-594 [(R)-5-(2-az etidinylmethoxy)-2-chloropyridine] is a novel, potent cholinergic nACh R ligand with analgesic properties (see accompanying paper by Bannon e t al., 1998b) 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 [H- 3](-)-cytisine to alpha 4 beta 2 neuronal nAChRs (Ki 37 pM, rat brain; K-i = 55 pM, transfected human receptor). At the alpha 1 beta 1 delta gamma neuromuscular nAChR labeled by [I-125]cr-bungarotoxin (alpha-Bt x), ABT-594 has a K-i value of 10,000 nM resulting in a greater than 1 80,000-fold selectivity of the compound for the neuronal alpha 4 beta 2 nAChR. In contrast, (+/-)-epibatidine has Ki values of 70 pM and 2.7 nM at the (alpha 4 beta 2 and alpha 1 beta 1 delta gamma nAChRs, resp ectively, giving a selectivity of only 38-fold. The S-enantiomer of AB T-594, A-98593 has activity at the neuronal alpha 4 beta 2 nAChR ident ical with ABT-594 (K-i = 34-39 pM), which demonstrates a lack of stere ospecific binding similar to that reported previously for (+/-)-epibat idine. A similar lack of stereoselectivity is seen at the human alpha 7 receptor. However, A-98593 is 3-fold more potent at the neuromuscula r nAChR (K-i = 3420 nM) and the brain alpha-Btx-sensitive nAChR (K-i = 4620 nM) than ABT-594. ABT-594 has weak affinity in binding assays fo r 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 transporte r binding sites. Functionally, ABT-594 is an agonist. At the transfect ed human alpha 4 beta 2 neuronal nAChR (K177 cells), with increased Rb -86(+) efflux as a measure of cation efflux, ABT-594 had an EC,, value of 140 nM with an intrinsic activity (IA) compared with (-)-nicotine of 130%; at the nAChR subtype expressed in IMR-32 cells (sympathetic g anglion-like), an EC50 of 340 nM (IA = 126%); at the F11 dorsal root g anglion cell line (sensory ganglion-like), an EC,, of 1220 nM (IA-71%) ; and via direct measurement of ion currents, an EC50 value of 56,000 nM (IA = 83%) at the human alpha 7 homo-oligimeric nAChR produced in o ocytes. A-98593 is 2-to 3-fold more potent and displays approximately 50% greater intrinsic activity than ABT-594 in all four functional ass ays. 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-5 94 (30 mu M) inhibits the release of calcitonin gene-related peptide f rom C-fibers terminating in the dorsal horn of the spinal cord, an eff ect mediated via nAChRs. Pharmacologically, ABT-594 has an in vitro pr ofile distinct from that of the prototypic nicotinic analgesic (+/-)-e pibatidine, with the potential for substantially reduced side-effect l iability and, as such, represents a potentially novel therapeutic appr oach to pain management.