Photostability of sennosides in solution and their degradation products 1

The leaves and pods of Alexandrian and Tinnevelly senna were used for centuries as laxatives. Their main active constituents are the dianthron glucosides (sennoside A – D and A1) and anthraquinone glucosides [1]. The photochemical instability of sennoside A was published in 1980 [2]. The authors postulated a degradation to rhein. However, the degradation products were not well examined and only one solvent was used in the study. The aim was to test the photostability of sennosides A, B and C in different solvents, to detect the degradation products as well as to examine if and when the aglycones aloe-emodin and rhein are formed.

Sennosides were dissolved in different solvents and exposed to light (Fluora® growth lamps), samples were taken over a period of 14 days to 1 month and analysed by UHPLC-PDA/QDa.

After 18h of light exposure 50 – 60% of the sennosides degraded in a solution with methanol/0.1% sodium bicarbonate (70:30 v/v) into other sennosides and rhein-8-glucoside. The effect is less distinct in solution of 0.1%sodium bicarbonatewith a degradation of ˜10% and did not occur with storage at room temperature and 4 °C while protected from light. Also, further degradation was stopped without light exposure. A formation of the aglycones aloe-emodin or rhein was not measured at any time, in contrary, in a senna extract exposed to light rhein degraded as well.

Fig. 1: Chromatograms at 270nm of sennoside B in methanol/0.1% NaHCO₃ (70:30 v/v) before (upper) and after 14 days of light exposure (lower)

[1] Hänsel, R. & Sticher, O. Pharmakognosie – Phytopharmazie 9th edition. Springer Medizin Verlag Heidelberg; 2010

[2] Nabil Aboul-Enein M, Wasser G.M, El-Difrawy S.M. Fitoterapia 1980; 51: 157 – 159.