Screening and Optimization of Novel Low Bee-Toxicity Phenylacetohydrazone Compounds Based on Insect nAChR Selectivity

Abstract

Increasing numbers of neonicotinoids have recently been banned by the European Union (EU) and the United States of America (USA) due to their potential high risk to pollinating bees. In this work, a phenylacetohydrazone compound VS-04 was found via scaffold hopping to exert good mortality rate against soybean aphids based on a double combinational strategy of virtual screening and bioassay technique. The molecular interaction mode to honeybee nicotinic acetylcholine receptor (nAChR) for the newly found compound VS-04 obviously differed from that of the known imidacloprid. The newly synthesized phenylacetohydrazone compounds 3c and 3e caused soybean aphids to have a moderate mortality rate of approximately 70% at a concentration of 500 μg/mL. Importantly, the bee-toxicity study confirmed that compounds 3c and 3e showed much lower acute contact toxicity with LD50 value (30.35 and 124.4 μg/bee) of 3~4 orders of magnitude than that of imidacloprid (0.019 μg/bee). The hydrazone substructure of compound 3c was found to be a L-shape low-energy conformation confirmed by a crystal structure of its analog 3m and inversely point toward α subunit of honeybee receptor via a molecular interaction study, indicating its potential low bee-toxicity. Compounds 4f and 4g (optimized derivatives of 3e) indeed showed better insecticidal activity against soybean aphids with moderate LC50 values of 83.42 and 66.44μg/mL, respectively, compared with 3e (147.30 μg/mL). This work facilitates the discovery of suitable and ecofriendly candidates as alternatives to neonicotinoids based on different insect nAChR structure.