Willow bark extracts are authorized for rheumatic complaints and headache. As they contain a large number of polar constituents, the salicylates being the basis of their standardization, relevance of these for the mode of action is a matter of interest, especially with regard to the use of aqueous willow bark extracts as STW 33-I in therapy.
Therefore a literature review was conducted, focussed on the mode of action of willow bark extracts and fractions thereof.
This review revealed a considerable number of pharmacological studies [1], including in vitro- and in vivo studies in different pharmacological models [2], supporting the assumption that especially the polar fractions of the extract contribute to the anti-inflammatory action. Salicylic acid derivatives and their metabolites [3], together with different other polyphenols, were identified as relevant for the mechanisms of action, as was also confirmed by gene expression analyses [4].
Accordingly, by studies on the mode of action of an aqueous willow bark extract, STW 33-I, it could be shown, that the action is based on more than just tradition, given on the broad available evidence on willow bark and the salicylates, and can be well explained on a molecular level.
[1] Nahrstedt A. Wien Med Wochenschr. 2007;157(13-14):348-51.
[2] Khayyal MT et al., Arzneimittelforschung. 2005;55(11):677-87.
[3] Knuth S. et al. Planta Med. 2013;79(16):1489-94
[4] Ulrich-Merzenich G et al. Phytomedicine. 2012;19(10):930-9.