Inter-Laboratory Comparison of Photometric and LC Assay Methods for Hydroxyanthracene Glycosides Determination in Senna Herbal Drugs and Dry Extracts

Herbal preparations of senna leaves or fruits are commonly used as laxatives. The European Medicines Agency (EMA) sets a daily dose of 10-30 mg for the hydroxyanthracene glycosides (HAG) determined by spectrophotometry. A recent revision of the European Pharmacopoeia monographs on senna replaced the photometric assay with HPLC. The new method offers improved reproducibility and specificity but results in lower content, while the dose recommendation in the EMA monograph still refers to the photometric determination. The systematic difference between the photometric and the HPLC method has been studied in a collaborative inter-laboratory trial by seven participating quality control laboratories. The goal was to compare precision and reproducibility and ideally to derive a conversion factor. The HAG determination by spectrophotometry according to the method from Ph. Eur. Edition 8.3. was compared to the HPLC assay according to the new method for senna leaves or pods (Ph. Eur. Ed. 10.1). The photometric method showed a low inter-laboratory reproducibility of about ± 28% (RSD) but an intra-laboratory precision of around 2%, while the HPLC method showed an inter-laboratory precision of around ± 10% and an intra-laboratory precision of 1.5%. Assay values obtained by HPLC were about 90% of the values of the photometric method, but this relation seems to be sample dependent. Because of its dependency on the raw material, a simple conversion factor between the two methods is not yet satisfying but an additional method-specific dose recommendation for the EMA monographs is proposed by the authors.