Nicotine abuse leads to approximately 400,000 deaths a year in the US. In spite of this, it is estimated that 30,000 people start smoking each day. Both the psychological and physiological effects of tobacco smoke are attributed to nicotine. Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central and peripheral nervous systems including several regions of the brain. Two major classes of nAChRs, alpha4beta2 and alpha7, have been identified in rat and human brains. The possibility exists that specific subtypes mediate specific functions, especially as this relates to nicotine addiction. Thus, the availability of a variety of ligands that bind with high affinity and selectivity for each subtype are needed. It is also desirable to have both agonists and antagonists since the role of nAChRs in addiction is not known. Epibatidine is a nicotinic agonist whose biological effects appear to be mediated by alpha4beta2 nAChRs. Methyllycaconitine (MLA) is the most potent nonpeptide competitiVe antagonists at alpha7 neuronal nAChRs thus far reported. The high potency of epibatidine for alpha4beta2 nAChRs and the potency and selectivity of MLA toward the brain alpha7 receptor makes these agents very useful lead compounds for the development of new ligands for studying these nicotinic subtypes. The overall objective of the research proposed in this application is to conduct SAR studies to develop novel epibatidine and MLA analogs for probing alpha4beta2 and alpha7 nicotinic receptors. Pharmacological profiles of compounds identified as potent and/or selective for one of the subtypes will be examined in animals and could lead to drugs for treatment of nicotine dependence, pain, and other neurological disorders. [125I]lodo-MLA and [123I]iodo-MLA will be prepared and characterized as a potential new radioligand and SPECT ligand, respectively, for studying the alpha7 nicotinic receptor. Significant results from new preliminary studies in this revised application include: (1) A new, high yield, convenient synthesis of 7-(tert-butoxycarbonyl)- 7- azabicyclo [2.2.1]hept-2-ene (C4b), a key intermediate used in the synthesis of epibatidine analogs, was developed. (2) 2-exo-[5 '-(2'-Chloro-3'-phenylpyridinyl)-7- azabicyclo -[2.2.1]heptane (C23) was synthesized and was shown to possess high affinity for the nAChR suggesting that the addition of groups to the 3-position of the pyridine ring can be used to probe receptor-selectivity and pharmacological activity. (3) An epibatidine analog possessing modification in the norbornane part of the structure was designed and synthesized. (4) (CH3)3SnMLA, the precursor needed for the synthesis of [125I]iodo-MLA as well as[123I]iodo-MLA was synthesized. (5) The potency of the epibatidine analogs prepared at RTI in activating expressed alpha4beta2 receptors was shown by studies at MCV to correlate well with the K-app binding affinities.
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