Synthesis and in silico studies of new thiophene-isoquinolinone hybrids as potential larvicides against Culex pipiens.

Mosquito-borne diseases remain a significant global public health challenge. This challenge is further exacerbated by the growing resistance of disease-vector species such as Culex pipiens to conventional insecticides. In this study, we present the design, synthesis, and biological assessment of a new series of thiophene-isoquinolinone hybrids as potential larvicides. Among the synthesized compounds, derivatives 5f, 6, and 7 showed significant larvicidal effectiveness against Culex pipiens larvae, with LC₅₀ values of 0.3, 0.1, and 1.85 µg/mL, respectively. Notably, all twelve thiophene-isoquinolinone derivatives were much more toxic than the reference organophosphate insecticide chlorpyrifos (LC₅₀ = 293.8 µg/mL), demonstrating the strength of these chemical structures. Interestingly, the synthetic intermediate compound 1a, a thiophene-based half-ester, exhibited the highest activity (LC₅₀ = 0.004 µg/mL), outperforming all final derivatives despite not being fully optimized. Mechanistic bioassays showed consistent neurotoxic symptoms that suggest a disruption of cholinergic function. Molecular docking and molecular dynamics simulations supported this observation, revealing strong and specific interactions with both acetylcholinesterase (AChE) and nicotinic acetylcholine receptors (nAChRs), which points to a possible dual-target mechanism. Density Functional Theory (DFT) calculations further confirmed the favorable electronic properties and reactivity of the active compounds. The structural variety within this series, along with consistently high potency, may lower the risk of cross-resistance and aid resistance management strategies through compound rotation or combination. Overall, these findings highlight thiophene-isoquinolinone hybrids as a promising option for developing next-generation larvicides that target neurophysiological pathways in insect vectors.