ZIKV Inhibitors Based on Pyrazolo[3,4‑d]pyridazine-7-one Core: Rational Design, In Vitro Evaluation, and Theoretical Studies

The Zika virus (ZIKV) is believed to cause birth defects, and no anti-ZIKV drugs have been approved by medical organizations to date. Starting from antimicrobial lead compounds with a pyrazolo­[3,4-d]­pyridazine-7-one scaffold, we synthesized 16 derivatives and screened their ability to interfere with ZIKV infection utilizing a cell-based phenotypic assay. Of these, five compounds showed significant inhibition of ZIKV with a selective index value greater than 4.6. In particular, compound 9b showed the best anti-ZIKV activity with a selectivity index of 22.4 (half-maximal effective concentration = 25.6 μM and 50% cytotoxic concentration = 572.4 μM). Through the brine shrimp lethality bioassay, 9b, 10b, 12, 17a, and 19a showed median lethal dose values in a range of 87.2–100.3 μg/mL. Compound 9b was also targeted to the NS2B-NS3 protease of ZIKV using molecular docking protocols, in which it acted as a noncompetitive inhibitor and strongly bound to five key amino acids (His51, Asp75, Ser135, Ala132, Tyr161). Utilizing the pharmacophore model of 9b, the top 20 hits were identified as prospective inhibitors of NS2B-NS3 protease, and six of them were confirmed for their stability with the protease via redocking and molecular dynamics simulations.