Synthesis of a Novel [125I]Neonicotinoid Photoaffinity Probe for theDrosophilaNicotinic Acetylcholine Receptor

The insect nicotinic acetylcholine receptor (nAChR) is the target for the major insecticide imidacloprid (IMI) and for the first candidate photoaffinity probe described here. Addition to 1-[(6-chloro-3-pyridinyl) methyl]-4,5-dihydro-2-nitromethylene-1H-imidazolidine (the nitromethylene analog of the nitroimine IMI) of formaldehyde and any one of several primary amines is known to give hexahydro-8-nitroimidazo[1,2-c]pyrimidine derivatives. These imidazopyrimidines with a wide range of N-substituents were found to inhibit [3H]IMI binding to the Drosophila or Musca nAChR by 50% (IC50) at 0.7-38 nM. Esterification of the N-(2-hydroxyethyl) derivative with 2-azido-5-(trimethylstannyl)-benzoic acid and then iododestannylation using Na125I and chloramine-T provide the candidate photoaffinity probe 6-[2-(2-azido-5-[125I]iodobenzoyl) ethyl]-1-[(6-chloro-3-pyridinyl)methyl]-1,2,3,5,6,7-hexahydro-8 -nitroimidazo[1,2-c]pyrimidine. This compound (unlabeled) has an IC50 of 8 nM for [3H]-IMI binding in Drosophila head membranes, and the 125I-labeled photoaffinity probe labels only a 66 kDa protein(s) at a specific site inhibited by (-)-nicotine, consistent with the insecticide-binding subunit of the nAChR.