Abstract
Combination antiviral therapy offers a rational strategy to enhance treatment efficacy and limit resistance in RNA virus infections; however, synergistic interactions must be demonstrated quantitatively rather than inferred solely from drug mechanisms. Favi-piravir, a purine analogue that induces lethal mutagenesis, and Ribavirin, a nucleoside analogue that depletes intracellular GTP pools and promotes error-prone replication, both exhibit broad activity against RNA viruses, including coronaviruses. Despite their mechanistic complementarity, the pharmacodynamic interaction between these two agents has not been rigorously defined for SARS-CoV-2. In this study, we evaluated the antiviral effects of Favipiravir and Ribavirin alone and in constant-ratio combinations using SARS-CoV-2-infected Vero E6 cells. Median-effect modelling and Chou–Talalay analysis were applied to determine the Combination Index (CI) and Dose-Reduction Index (DRI) across multiple fractional effect levels. Favipiravir and Ribavirin each demonstrated dose-dependent inhibition of viral replication, enabling accurate derivation of median-effect parameters. The 1:1 combination produced consistent synergy (CI < 1) across Fa₅₀–Fa₉₀, accompanied by favourable dose-reduction values for both drugs. The 1:2 ratio showed mild synergy, while the 2:1 ratio shifted toward additive or weakly antagonistic effects, highlighting the ratiodependent nature of the interaction. Isobolo-gram analysis supported these observations, demonstrating deviation of experimental points below the line of additivity for synergistic combinations. These findings reveal a previously uncharacterized synergistic relationship between Favipiravir and Ribavirin against SARS-CoV-2 and provide quantitative guidance for the potential optimization of dual-nucleoside antiviral regimens. Further in vivo and pharmacokinetic studies are warranted to assess clinical relevance.