ABSTRACT
Clinacanthus nutans Lindau (C. nutans) is a well-known medicinal plant used in Thai folklore medicine for treatment of herpes simplex virus (HSV) infection. To study the inhibitory activity of crude ethyl acetate extract of C. nutans against HSV type 1 strain F (HSV-1F), crude ethyl acetate extract of C. nutans was tested in vitro on the Vero cells by using plaque reduction assay. The results showed that crude extract did not exert cytotoxic effects against Vero cells at the concentration of 19 \μg/ml or below and the CC₅₀ was 76 \μg/ml. The viruses were mixed with C. nutans crude extract at the subtoxic concentration of 19 \μg/ml and adsorbed on Vero cells. The infected Vero cells were then incubated in medium containing with crude extract. The results revealed that C. nutans crude extract completely (100%) inhibited the HSV-1F plaque formation and the IC₅₀ was 7.6 \μg/ml. To study the possible mode of antiviral action of crude extract, the experiment was conducted separately on the pre-or post-infection stage. In the pre-infection stage, crude extract produced 97% plaque reduction in a time-dependent manner, whereas it did not exert the inhibitory effect in the post-infection stages. This study suggested that the antiviral mode of action of crude ethyl acetate extract of C. nutans was effective greatly at the pre-infection stage, which may be due to the virucidal activity, inhibition of the viral attachment or the penetration. Therefore, there is still a clear need to further identify the possible mechanism of antiviral infection at the pre-infection stage of C. nutans crude extract at both the molecular and cellular levels.