Mono- and Disubstituted-3,8-diazabicyclo[3.2.1]octane Derivatives as Analgesics Structurally Related to Epibatidine:  Synthesis, Activity, and Modeling

A series of 3,8-diazabicyclo[3.2.1]octanes substituted either at the 3 position (compounds <BO>1</BO>) or at the 8 position (compounds <BO>2</BO>) by a chlorinated heteroaryl ring were synthesized, as potential analogues of the potent natural analgesic epibatidine. When tested in the hot plate assay, the majority of the compounds showed significant effects, the most interesting being the 3-(6-chloro-3-pyridazinyl)-3,8-diazabicyclo[3.2.1]octane (<BO>1a</BO>). At a subcutaneous dose of 1 mg/kg, <BO>1a</BO> induced a significant increase in the pain threshold, its action lasting for about 45 min. <BO>1a</BO> also demonstrated good protection at a dose of 5 mg/kg in the mouse abdominal constriction test, while at 20 mg/kg it completely prevented the constrictions in the animals. Administration of naloxone (1 mg/kg ip) did not antagonize its antinociception while mecamylamine (2 mg/kg ip) did, thus suggesting the involvement of the nicotinic system in its action. Binding studies confirmed high affinity for the α<INF>4</INF>β<INF>2</INF> nAChR subtype (K<INF>i</INF> = 4.1 ± 0.21 nM). nAChR functional activity studies on three different cell lines showed that <BO>1a</BO> was devoid of any activity at the neuromuscular junction. Finally, due to the analogy in their pharmacological profile with that of epibatidine, compounds were compared from a structural and conformational point of view through theoretical calculations and high-field ¹H NMR spectroscopy. Results indicate that all of them present one conformation similar to that of epibatidine.