Enantioselective activity of usnic acid: a comprehensive review and future perspectives

This review is focused on the comparison of the biological and pharmacological activities of usnic acid enantiomers. Most of the available data refer to (+)-usnic acid, while the left-handed isomer has been less often significantly studied. Special attention was paid to the experiments comparing both (+)- and (−)-usnic acid at the same time, the results of which indicated interesting differences, however no tendency as to which enantiomer was more potent could be observed. Nevertheless, more studies, especially on (−)-usnic acid, are needed to give a final explanation for the similarities and differences between both usnic acid enantiomers. These should be especially directed to steric structure–activity relationship of the enantiomers, tested under the same experimental conditions, which may help to explain the possible mechanisms of their actions.