In Vitro Development of Local Antiviral Formulations with Potent Virucidal Activity Against SARS-CoV-2 and Influenza Viruses

<b>Background/Object:</b> This study investigates the in vitro antiviral potential of D-limonene (DLM), monolaurin (ML), and cetylpyridinium chloride (CPC) in formulations targeting SARS-CoV-2 and influenza viruses. The aim was to develop oral and nasal formulations with optimized concentrations of these active ingredients to evaluate their efficacy, safety, and stability. <b>Methods:</b> Oral (formulation D) and nasal (formulation E) products were developed using specific concentrations of DLM (0.2-0.3% <i>w</i>/<i>w</i>), ML (0.1-0.2% <i>w</i>/<i>w</i>), and CPC (0.05-0.075% <i>w</i>/<i>w</i>). In vitro virucidal activity assays were conducted to assess the antiviral efficacy of the formulations against SARS-CoV-2 and influenza viruses. Stability testing was also performed under various storage conditions. <b>Results:</b> Formulation D (0.3% <i>w</i>/<i>w</i> DLM, 0.2% <i>w</i>/<i>w</i> ML, 0.05% <i>w</i>/<i>w</i> CPC, and 1.5% <i>w</i>/<i>w</i> Cremophor RH40) demonstrated a 3.875 ± 0.1021 log reduction and 99.99 ± 0.0032% efficacy against SARS-CoV-2 within 120 s. Formulation E (0.2% <i>w</i>/<i>w</i> DLM, 0.05% <i>w</i>/<i>w</i> CPC, and 0.75% <i>w</i>/<i>w</i> Cremophor RH40) showed a 2.9063 ± 0.1197 log reduction and 99.87 ± 0.0369% efficacy against SARS-CoV-2. Both formulations achieved >99.99% efficacy and log reductions exceeding 4.000 against various influenza strains. Stability testing confirmed optimal performance at 4 °C with no microbial contamination. <b>Conclusions:</b> The findings suggest that both formulations exhibit broad-spectrum antiviral activity against SARS-CoV-2 and influenza viruses in vitro. These results support their potential for further clinical evaluations and therapeutic applications, particularly in oral and nasal spray formulations.