The Combination of Nirmatrelvir and Ivermectin Exerts Strongly Synergistic Antiviral and Anti-Inflammatory Effects Against Murine Coronavirus Infection of Macrophages

The emergence of SARS-CoV-2 variants and antiviral resistance highlights the need for improved therapeutic strategies against COVID-19 and other coronavirus infections. By pairing direct-acting antivirals with repurposed host-modulating drugs, combination therapy approaches may enhance antiviral efficacy while mitigating inflammation. In this study, we evaluated the effects of combining Nirmatrelvir (a SARS-CoV-2 main protease inhibitor) with Ivermectin, Azithromycin or Doxycycline—using a murine hepatitis virus (MHV) infection model of RAW264.7 macrophages. Checkerboard assays demonstrated that the Nirmatrelvir–Ivermectin combination exhibited strongly synergistic antiviral activity. This combination achieved potent inhibition of live virus titer of at least 5 to 6 log10 and reduction in viral RNA load of 3 log10, at drug concentrations much lower than the respective monotherapies. The Nirmatrelvir–Ivermectin combination treatment affected the coronavirus replication cycle at the same time-point of 8 h as Nirmatrelvir monotherapy. Moreover, multiplex cytokine protein profiling revealed that Nirmatrelvir–Ivermectin markedly suppressed key pro-inflammatory cytokines and chemokines associated with the cytokine storm, including IL-6, TNF-α, IL-1β, and MCP-1. Conversely, the combinations of Nirmatrelvir with Azithromycin or Doxycycline exhibited only additive or mildly additive effects, with alterations in certain cytokine levels. These findings support the potential of Nirmatrelvir–Ivermectin as a promising novel combination therapy with both antiviral and anti-inflammatory benefits, warranting further in vivo validation and clinical investigation.