Structural Analysis of Inhibitor Binding to the Feline Enteric Coronavirus (FECV) Main Protease

Coronaviruses include various strains that reside in natural animal reservoirs, with zoonotic transmission posing risks to both domesticated animals and human health. Recent efforts to address coronavirus infections have focused on developing inhibitors targeting the main protease (M^pro), some of which exhibit potential broad-spectrum efficacy. This study presents crystal structures of four clinically relevant inhibitors—GC376, PF-00835231, nirmatrelvir, and ibuzatrelvir—bound to M^pro from the feline coronavirus strain FECV-UU23. Structural analysis identified distinct FECV-specific features within the active site where these inhibitors bind and revealed S4 loop as a susceptible structural region essential for the enhanced binding of inhibitors in UU23 M^pro. We therefore propose to incorporate sterically constrained, functionally tailored heterocyclic moieties at the P3 site of known inhibitors which can optimally engage Q187, P188, and S189 residues of the S4 loop. The findings presented enhance understanding of inhibitor specificity and reinforce the promise of these inhibitor scaffolds for developing antivirals against feline coronavirus strains, with possible applications in broad-spectrum coronavirus therapy.