The major chemical constituents of Hp have been characterised and described in a number of published studies from different groups [10-12]. In their recent investigations, Clarkson and colleagues used an HPLC-SPE (solid-phase extraction)-NMR tandem methodology to confirm that iridoid glycosides and glycosides of the phenylethonoid acteoside are the major constituents of ethanol extracts of Harpago-phytum . They also showed, using the same technology, that two commercial preparations of Hp contained a similar spectrum of constituents as their laboratory extract. So it can be assumed that some, or all, of these main chemical constituents of ethanol extracts of Hp are responsible for the medicinal and pharmacological effects of Hp. In their analysis of a water extract of Hp, Boje and colleagues  found glycosides of harpagoside, and of acteoside (and acteoside analogues), in similar amounts and to be the major ingredients.
Extracts of Hp are characterised by the presence of iridoid glycosides, and especially harpagoside, which was first identified in Hp extracts back in the 1960s. Acteoside and other related polyphenolic glycosides were first isolated from Hp by Berger et al.  and represent a major constituent of Hp extracts. Clarkson and colleagues  were able to determine that the ethanol extract contained a high proportion (about 13%) of the original dried plant material and that the glycosides of acteoside represented more than 30% of the extracted material. Therefore, this group of chemicals appears to represent a major fraction of Hp extracts in addition to the characteristic iridoid glycosides. Acteoside, shown to exert anti-inflammatory effects, is also known as verbascoside and is present as aglycoside or glycoside in other herbal drug extracts (see further discussion below).
The HPLC-SPE-NMR methodology was described by Clarkson and colleagues  to be a paradigm shift in the analysis of biologically derived extracts in the sense that it allows novel natural products to be identified with improved rapidity and sensitivity. For example, using this methodology, the researchers identified novel chemical entities in the aqueous ethanol Hp extract. These novel natural products were chicane-type tricyclic diterpenes and are of interest in drug discovery as novel chemical entities. However, for specialists in herbal medicine who are interested in the pharmacological mode of action of the herbal drugs such as Hp, they are of no interest given their presence in the extract in such small quantities.
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