Discovery and mutasynthetic optimization of ansatrienin B: A new broad-spectrum inhibitor against RNA viruses.

RNA viruses, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), flaviviruses, and alphaviruses, represent a major source of emerging human infectious diseases. They pose a persistent threat to public health; however, few therapeutic options are available for severe infections. Through a natural product screening campaign, we identified ansatrienin B as a broad-spectrum inhibitor of multiple RNA viruses, including SARS-CoV-2, flaviviruses (e.g., YFV, WNV, DENV), and alphaviruses (e.g., CHIKV). Time-of-drug-addition assays indicated that ansatrienin B acts at both the early (entry) and intermediate (replication) stages of the viral life cycle. Surface plasmon resonance (SPR) and molecular docking studies validated a direct interaction between ansatrienin B and the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 and WNV. Combined RNA pull-down and RdRp enzymatic activity assays (in gel, solution, and cellular forms) further demonstrated that ansatrienin B disrupts both the binding of RdRp to viral RNA and its enzymatic activity. In vivo, ansatrienin B showed significant efficacy in mouse models infected with SARS-CoV-2 or WNV infection. To facilitate screening and elucidate the structure-activity relationship (SAR), we generated a focused ansatrienin library via a mutasynthetic approach. Supplementation of four 3,5-AHBA analogs into a ΔmycB1-B4 mutant strain of Streptomyces flaveolus yielded 30 novel ansatrienin derivatives. Evaluation of anti-SARS-CoV-2 activity identified four analogs with enhanced potency, enabling the establishment of a preliminary SAR. Collectively, these findings establish ansatrienin B as a novel inhibitor targeting RdRp and provide a foundation for the development alternative broad-spectrum antiviral agents.