Design, synthesis, and insecticidal potency of novel 3-methyl-pyrazole derivatives against Culex pipiens larvae

Mosquito-borne illnesses remain a serious public health concern worldwide, with the growing resistance of vectors such as Culex pipiens reducing the efficacy of standard insecticides. In pursuit of alternative control strategies, a new library of 3-methyl-pyrazole derivatives was rationally designed, synthesized through multistep reactions, and comprehensively characterized using IR, NMR, and mass spectrometry. Larvicidal screening of all nineteen synthesized 2-methyl-pyrazole derivatives, conducted according to WHO protocols, revealed several highly active candidates. Among these, derivatives 7 and 12 exhibited the most potent activity, with LC₅₀ values of 0.16 µg/mL and 0.30 µg/mL, respectively - far surpassing the performance of the reference insecticide chlorpyrifos (LC₅₀ = 52.90 µg/mL). Complementary in silico analyses, including molecular docking and molecular dynamics (MD) simulations, were employed to elucidate the mode of action of the synthesized compounds. Docking studies were performed for all nineteen 3-methyl-pyrazole derivatives against two principal neural targets: acetylcholinesterase (AChE), the enzyme responsible for acetylcholine degradation, and the nicotinic acetylcholine receptor (nAChR), a major ligand-gated ion channel involved in synaptic transmission. To further examine binding stability, MD simulations were conducted only for the two most active derivatives (7 and 12) in complex with AChE, using chlorpyrifos as a reference ligand. The simulations confirmed that the active compounds formed stable and persistent interactions within the catalytic site, consistent with a neurotoxic mechanism involving inhibition of AChE. Structure-activity relationship (SAR) analysis further identified functional groups contributing to potency. Overall, this preliminary investigation establishes a computational and experimental foundation for future optimization, toxicological validation, and eventual field application of next-generation larvicides designed to mitigate resistance and control mosquito-borne diseases.