[Optimization of cepharanthine solubility and evaluation of its inhibitory activity against coronaviruses]

Cepharanthine (CEP) is a natural bisbenzylisoquinoline alkaloid with broad-spectrum antiviral activity. However, the extremely poor water solubility severely limits the pharmaceutical development and in vivo application of CEP. This study aimed to identify safe and effective solubilizing systems to enhance the solubility of CEP and systematically assess how these changes affect the inhibitory activity of CEP against coronaviruses. The solubility of CEP in various natural acidic media, including Rosa roxburghii juice, vinegar, and apple cider vinegar, as well as in solubilizing systems such as vitamin C and glycyrrhizic acid, was systematically investigated. The GX_P2V coronavirus infection model was used to measure the antiviral activity (EC₅₀), cytotoxicity, and selectivity index (SI) of CEP under different solubilization conditions. Additionally, hydrochloride, pyruvate, and mesylate salts of CEP were prepared through acid-base neutralization and tested for their in vitro antiviral activity. The results showed that the solubility of CEP was significantly improved under acidic conditions with pH 2‒4. A mixture of vitamin C and CEP at a mass ratio of 4:1 formed a clear solution and demonstrated enhanced antiviral activity (EC₅₀=0.85 μmol/L, SI=17.01) compared with CEP alone. Although glycyrrhizic acid increased the solubility of CEP in 30% ethanol, it exhibited poor water stability and did not significantly enhance the antiviral activity. Among all tested systems, CEP hydrochloride showed the best antiviral selectivity (EC₅₀=1.05 μmol/L, SI>47). In conclusion, both the vitamin C-based formulation and CEP hydrochloride significantly improved the solubility and antiviral activity of CEP, highlighting its potential for further pharmaceutical development and clinical application.