AbstractThere are currently relatively few small-molecule antiviral drugs that are either approved or emergency approved for use against SARS-CoV-2. One of these is remdesivir, which was originally repurposed from its use against Ebola and functions by causing early RNA chain termination. We used this as justification to evaluate three molecules we had previously identified computationally with antiviral activity against Ebola and Marburg. Out of these we previously identified pyronaridine, which inhibited the SARS-CoV-2 replication in A549-ACE2 cells. Herein, the in vivo efficacy of pyronaridine has now been assessed in a K18-hACE transgenic mouse model of COVID-19. Pyronaridine treatment demonstrated a statistically significant reduction of viral load in the lungs of SARS CoV-2 infected mice. Furthermore, the pyronaridine treated group reduced lung pathology, which was also associated with significant reduction in the levels of pro-inflammatory cytokines/chemokine and cell infiltration. Notably, pyronaridine inhibited the viral PLpro activity in vitro (IC50 of 1.8 µM) without any effect on Mpro, indicating a possible molecular mechanism involved in its ability to inhibit SARS-CoV-2 replication. Interestingly, pyronaridine also selectively inhibits the host kinase CAMK1 (IC50 of 2.4 µM). We have also generated several pyronaridine analogs to assist in understanding the structure activity relationship for PLpro inhibition. Our results indicate that pyronaridine is a potential therapeutic candidate for COVID-19.One sentence summaryThere is currently intense interest in discovering small molecules with direct antiviral activity against the severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2). Pyronaridine, an antiviral drug with in vitro activity against Ebola, Marburg and SARS-CoV-2 has now statistically significantly reduced the viral load in mice along with IL-6, TNF-α, and IFN-β ultimately demonstrating a protective effect against lung damage by infection to provide a new potential treatment for testing clinically.