Antiviral compounds from Icelandic lichens

Although one third of prescription drugs are derived from natural sources [1], lichens have only been investigated to a limited extent from a pharmacological perspective. The aim of the study was to investigate whether antiviral compounds could be found in Icelandic lichens, to isolate active compounds in a purified form and elucidate their chemical structure and to confirm antiviral activity of isolated compounds and compare it with that of marketed antiviral drugs.

Extracts were made from ten lichen species and screened for antiviral activity in vitro against three different viruses by using the plaque reduction assay (PRA). Two compounds exhibiting potent antiviral activity against respiratory syncytial (RS) virus were isolated and purified and their activity confirmed using both the PRA method and ELISA. The compounds were the depsidone salazinic acid from Parmelia saxatilis (L.) Ach. and the benzyl depside alectorialic acid from Alectoria nigricans (Ach.) Nyl. The activity of both lichen compounds was more potent than that of the marketed drug ribavirin, which is used to treat serious respiratory conditions resulting from RS infection. The IC₅₀ value for salazinic acid as determined by ELISA was 11.9µg/mL, for alectorialic acid 17.0µg/mL and for ribavirin 22.9µg/mL. The lichen compounds were no cytotoxic at antiviral concentrations. Activity against herpes viruses I and II was less potent than activity against RS virus.

Acknowledgements: Icelandic Council of Science, University of Iceland Research Fund, The Icelandic Research Fund for Graduate Students.

Reference: 1. Kinghorn, A.D. (2001), J. Pharm. Pharmacol. 53: 135–148.