Design and synthesis of 1,3,4-oxadiazole-tethered N-substituted isatin hybrids as SARS-CoV-2 main protease inhibitors: biological evaluation, kinetic investigation, and in silico studies

Abstract

The ongoing emergence of SARS-CoV-2 mutations underscores the urgent need for new antivirals that target key viral proteins. This study describes the design, synthesis, and evaluation of two series of 1,3,4-oxadiazole-tethered N-substituted isatin hybrids as inhibitors of the SARS-CoV-2 main protease (Mpro): 1,2,3-triazole-linked derivatives (9a–h) and pyrazole-linked derivatives (15a–d). Compounds 15a–c were identified as the most active derivatives in initial FRET-based screening. With an IC50 of 15.38 µM, 15c was the most effective inhibitor, as determined by subsequent enzymatic assays. 15a and 15b had IC50 values of 24.43 and 30.55 µM, respectively. The active chemicals inhibit Mprovia a noncompetitive/mixed mechanism (α = 0.41–0.44), at a KI value of 13.24 µM, according to enzyme kinetic studies. Additionally, the compounds exhibited favorable physicochemical and ADMET profiles and minimal cytotoxicity against normal IMR-90 cells. The stable predicted binding of the most active compound, 15c, within the Mpro active site was further supported by molecular docking and molecular dynamics simulations. Collectively, these findings identify 15c as the most active derivative in the present series and provide preliminary SAR insights that may guide further optimization of this scaffold as a potential class of SARS-CoV-2 Mpro inhibitors.