Discovery of Novel Quinazolinone-2-carbohydrazide Derivatives as Effective Succinate Dehydrogenase Inhibitors and Biosafety Assessment on Rice and Zebrafish

To develop potent and safe antifungal agents in agriculture, a total of 48 novel quinazolinone-2-carbohydrazide derivatives were designed and synthesized based on the pharmacophore hybridization method. The bioassay results demonstrated that many compounds exhibited powerful and broad-spectrum inhibition activities <i>in vitro</i> against the tested fungi. For instance, compounds <b>E23</b> and <b>F23</b> possessed EC₅₀ (half-maximal effective concentration) values of 0.41 and 0.47 μg/mL against <i>Rhizoctonia solani</i>, comparable to that of the commercial fungicide Boscalid (EC₅₀ = 0.49 μg/mL). Additionally, compound <b>E23</b> also demonstrated pronounced antifungal effects against <i>Verticillium dahliae</i>, <i>Alternaria solani</i>, and <i>Colletotrichum gloeosporioides</i> with EC₅₀ values of 0.27, 1.15, and 0.27 μg/mL, respectively. <i>In vivo</i> assays on rice plants revealed that this compound at 200 μg/mL exhibited nearly equipotent curative and protective effects against <i>R. solani</i>, as compared with the positive controls Boscalid and Carbendazim. Moreover, compound <b>E23</b> had an obvious inhibition activity against fungal succinate dehydrogenase (SDH) from <i>R. solani</i> with the half-maximal inhibition concentration (IC₅₀) of 11.76 μM, and the interactions between compound <b>E23</b> and SDH enzyme were further confirmed by molecular docking studies. Furthermore, the presence of compound <b>E23</b> also triggered obvious morphological changes of fungal mycelia and increased the conductivity and permeability of fungal cell membranes. Interestingly, the built three-dimensional quantitative structure-activity relationship (3D-QSAR) models showed that the electrostatic effect played a more important role in maintaining anti-<i>R. solani</i> effects of target compounds than other effects. Finally, the biosafety assessment of compound <b>E23</b> demonstrated its safety toward rice and zebrafish.