Molecular mechanism of ziresovir targeting the fusion glycoprotein of respiratory syncytial virus

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in infants and the elderly worldwide. Although prophylactic monoclonal antibodies and RSV vaccines are available for preventing severe RSV infection, unmet medical need remains for an effective antiviral agent to treat patients who do not benefit from these interventions. Ziresovir (formerly AK0529) is a potent, selective, and orally bioavailable RSV fusion inhibitor with proved antiviral efficacy and clinical benefits. To understand the molecular mechanism of action, we computationally modeled ziresovir with the RSV fusion (F) protein. Here, we present a cryo-EM structure of the RSV F protein-ziresovir complex, elucidating the molecular interactions underlying the drug binding, revealing ziresovir specifically binds to the central cavity within the metastable prefusion conformation of RSV F protein. Leveraging this structural insight, we engineered site-directed RSV mutants guided by both the cryo-EM binding model and drug-resistant RSV variants for fusion inhibitors identified in vitro, and demonstrated that these resistant viruses do not replicate as efficient as wild-type RSV and indicated a fitness cost for viral escape from drug treatment. Collectively, these findings unveil the structural mechanism of ziresovir-mediated viral inhibition, providing a framework for developing the next-generation RSV fusion inhibitors.