Structural features of azidopyridinyl neonicotinoid probes conferring high affinity and selectivity for mammalian α4β2 and Drosophila nicotinic receptors

Abstract

The higher toxicity of neonicotinoid insecticides such as N-(6-chloropyridin-3-ylmethyl)-2-nitroiminoimidazolidine (imidacloprid) to insects than mammals is due in large part to target site specificity at the corresponding nicotinic acetylcholine receptors (nAChRs). We propose that neonicotinoids with a protonated N-unsubstituted imine or equivalent substituent recognize the anionic subsite of the mammalian α4β2 nAChR whereas the negatively charged (δ-) tip of the neonicotinoid, insecticides interacts with a putative cationic subsite of the insect nAChR. This hypothesis can be tested by using two photoaffinity probes that differ only in the N-unsubstituted imine vs negatively charged (δ-) tip. Synthesis methodology was developed for compounds combining three moieties: pyridin-3-ylmethyl or 6-chloropyridin-3-ylmethyl and their 4- and 5-azido analogues; imidazolidine, 4-imidazoline or 4-thiazoline; and N-unsubstituted imine, nitroimine, cyanoimine, or nitromethylene. Structure-activity studies compared displacement of [3H] nicotine binding in mammalian α4β2 nAChR and [3H]imidacloprid binding in Drosophila nAChR. Preferred compounds are N-(5-azido-6-chloropyridin-3-ylmethyl) with 2-iminothiazoline for α4β2 (Ki = 0.47 nM) and with 2-nitroiminothiazoline or 2-nitromethyleneimidazolidine for Drosophila (Ki = 0.72-3.9 nM).