Antiviral photodynamic therapy: Inactivation and inhibition of SARS-CoV-2 in vitro using methylene blue and Radachlorin.

Introduction: Recently, the COVID-19 pandemic has spread globally, necessitating the development of new methods for its prevention and treatment. The purpose of this study was to evaluate the antiviral activity of photodynamic therapy (PDT) against SARS-CoV-2 in vitro.

Methods: Vero E6 cells and SARS-CoV-2 isolated in Russia were used for PDT with methylene blue (MB) and Radachlorin. A continuous laser with wavelength λ = 662 nm in doses of 16 J/cm and 40 J/cm laser irradiation was used for PDT of a viral suspension and SARS-CoV-2-infected cells. The direct cytopathogenic effect of SARS-CoV-2 was evaluated via light microscopy to calculate the TCID in the samples and perform statistical analysis.

Results: Viral suspensions of SARS-CoV-2 that had a TCID greater than 10 were inactivated by PDT in the presence of MB and Radachlorin. Vero E6 cells were protected from 10 TCID of SARS-CoV-2 by PDT post infection. The range of protective concentrations was 1.0-10.0 μg/ml and 0.5-5.0 μg/ml for MB and Radachlorin, respectively. Additionally, it was found that MB and Radachlorin also possess significant antiviral activity even without PDT. The 50 % inhibitory concentration (IC) against 10 TCID of SARS-CoV-2 was found to be 0.22 and 0.33 μg/mL with the addition of MB and Radachlorin, respectively, to cells concomitantly with virus, whereas in the case of applying the photosensitizers at 3.5 h post infection, the IC was 0.6 and 2.0 μg/mL for MB and Radachlorin, respectively.

Conclusion: PDT shows high antiviral activity against SARS-CoV-2 when combined with MB and Radachlorin in vitro.