Antiviral activity of a crude extract of Thalassia testudinum Banks ex König against the human enteroviruses echovirus 9 and coxsackievirus A16

Context: Echovirus 9 (E9) and coxsackievirus A16 (CVA16) are etiological agents of aseptic meningitis and hand, foot, and mouth disease, respectively. Currently, there are no approved antiviral drugs against these viruses. The marine plant Thalassia testudinum represents a potential source of bioactive compounds for the treatment of viral diseases with clinical and epidemiological importance. Aims: To evaluate the in vitro antiviral activity of a hydroalcoholic extract of the marine plant T. testudinum against clinical isolates of echovirus 9 and coxsackievirus A16. Methods: The cytotoxicity of the extract was evaluated in Vero and RD cells via the MTT method. The antiviral activity (EC50) was determined by the inhibition of cytopathic effects. Extracellular virucidal activity and reduction in viral yield were determined via a viral titration assay. The viral genome was quantified after manual extraction and amplified using semi-nested RT-PCR. Results: The extract was non-cytotoxic to Vero or RD cells (CC50 = 3840.96 ± 116.12 µg/mL and 3822 ± 220.5 µg/mL, respectively) and inhibited the virus replication (CE50 = 512.82 ± 22.32 µg/mL and 1367.1 ± 58.66 µg/mL, respectively), verified by the selective index values of 7.48 and 2.79 against echovirus 9 and coxsackievirus A16, respectively. The extract showed extracellular virucidal activity against E9 and reduced the infective titer by more than two logarithms. The CVA16 genome concentration was reduced when the cells were treated with increasing concentrations of the extract, although no significant differences were detected. Conclusions: Thalassia testudinum crude hydroalcoholic extract inhibited the replication of the human enteroviruses E9 and CVA16. These results provide preliminary evidence of the potential of this plant as a source of antiviral compounds.