In Vitro Evaluation of Repurposed Antivirals and Their Combinations Against Respiratory Syncytial Virus and Human Parainfluenza Virus Type 2: Evidence of Ribavirin–Oseltamivir Synergy

ABSTRACT

Treating respiratory viral infections remains challenging, necessitating exploration of novel therapeutic approaches. Alternative strategies, such as drug repurposing or combinations, are essential for treating them. This study evaluated the in vitro activities of the antivirals ribavirin, favipiravir, and oseltamivir, and the mucolytic drugs N‐acetylcysteine (NAC) and erdosteine, alone or in combination, against Human Parainfluenza Virus (HPIV) and Respiratory Syncytial Virus (RSV). HPIV‐2 and RSV were cultured in VERO and HEp‐2 cell lines, respectively. Antiviral efficacy was assessed by calculating IC ₅₀ values using MTT and plaque reduction assays. For combination studies, synergy scores were analyzed using the SynergyFinder program. We investigated potential interactions underlying the synergistic activities using an in silico approach, that combined virtual screening and molecular docking. The IC ₅₀ values of ribavirin, favipiravir, and oseltamivir against HPIV‐2 and RSV ranged from 4.46 to 14.1 µg/mL, and the results from both methods were highly similar. All three antivirals' cytotoxicity was well above their C‐max, and SIs were 12.43‐36.69 for HPIV‐2 and 15.88–38.12 for RSV. The ribavirin‐oseltamivir combination synergy scores of 13.35 for HPIV‐2 and 17.98 for RSV indicate clear synergism, while other combinations were additive. Docking analysis identified potential molecular interactions that may contribute to the antiviral activity of the ribavirin–oseltamivir combination against RSV and HPIV‐2. Ribavirin, favipiravir, and oseltamivir showed good activity against HPIV‐2 and RSV, and the ribavirin‐oseltamivir combination exhibited strong synergistic interactions, suggesting promising therapeutic potential.