Structural-based virtual screening and in vitro assays for small molecules inhibiting the feline coronavirus 3CL protease as a surrogate platform for coronaviruses

Feline infectious peritonitis (FIP) which is caused by feline infectious peritonitis virus (FIPV), a variant of feline coronavirus (FCoV), is a member of family Coronaviridae, together with severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2. So far, neither effective vaccines nor approved antiviral therapeutics are currently available for the treatment of FIPV infection. Both human and animal CoVs shares similar functional proteins, particularly the 3CL protease (3CL(pro)), which plays the pivotal role on viral replication. We investigated the potential drug-liked compounds and their inhibitory interaction on the 3CL(pro) active sites of CoVs by the structural-bases virtual screening. Fluorescence resonance energy transfer (FRET) assay revealed that three out of twenty-eight compounds could hamper FIPV 3CL(pro) activities with IC50 of 3.57 +/- 0.36 mu M to 25.90 +/- 1.40 mu M, and Ki values of 2.04 +/- 0.08 to 15.21 +/- 1.76 mu M, respectively. Evaluation of antiviral activity using cell-based assay showed that NSC629301 and NSC71097 could strongly inhibit the cytopathic effect and also reduced replication of FIPV in CRFK cells in all examined conditions with the low range of EC50 (6.11 +/- 1.90 to 7.75 +/- 0.48 mu M and 1.99 +/- 0.30 to 4.03 +/- 0.60 mu M, respectively), less than those of ribavirin and lopinavir. Analysis of FIPV 3CL(pro)-ligand interaction demonstrated that the selected compounds reacted to the crucial residues (His41 and Cys144) of catalytic dyad. Our investigations provide a fundamental knowledge for the further development of antiviral agents and increase the number of anti-CoV agent pools for feline coronavirus and other related CoVs.