Evaluation of the activity of traditional Greek medicinal plants against herpes simplex virus

Herpes simplex viruses (HSV) are important human pathogens causing a variety of diseases ranging in severity from mild to severe, and in certain cases, they can even become life threatening, especially in the case of highly susceptible adults. Following a primary infection HSV rests latent mainly in trigeminal ganglia and persists for the life-time of the host with periodic reactivations. The widespread use of nucleoside-based drugs, such as aciclovir (ACV), for the treatment of HSV infections has led to the emergence of drug-resistant HSV mutants. Therefore, the discovery of novel anti-HSV drugs, preferably targeting processes different than those affected by nucleoside analogues, i.e. virus DNA replication, deserves great efforts.

Interestingly, it was found that traditional medicinal plants are promising sources for new anti-HSV drugs. The present study focuses on the evaluation of extracts derived from four Greek plants, i.e. Artemisia absinthium, Echium italicum, Melissa officinalis and Onosma elengatissima, either for their virucidal activity or their abilities to inhibit HSV-1 propagation. Air-dried, powdered aerial parts from the above mentioned plants were extracted at room temperature with a series of solvents of increasing polarity, i.e. petroleum ether, CH₂Cl₂, MeOH, 1:1 mixture of MeOH-H₂O and H₂O. The dried extracts were dissolved in DMSO or in sterile deionised and distilled water and tested both for cytotoxicity as well as for their antiviral action. MTT asays indicated that none of the above extracts, except the E. italicum aqueous extract, displayed significant cytotoxicity at concentrations up to 41 mcg/ml. Plaque reduction assays indicated that extracts from all four plants displayed anti-HSV activity. The CH₂Cl₂ and MeOH extracts were found to possess mainly virucidal activity whereas the more polar extracts displayed both virucidal activity and inhibition of the virus entry to the host cells in vitro. Specifically, the aqueous extract from A. absinthium displayed very potent inhibition of the HSV-1 attachment, without showing any virucidal or cytotoxic effects.