Instability of Rhein-9-Anthrone as a Problem in Pharmacological and Analytical Use

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Abstract
The stability of rhein-9-anthrone, one of the main and most active metabolites of the sennosides A+B, was studied under physiological conditions. A high-pressure liquid chromatographic method for the determination of rhein-9-anthrone and its oxidation products, rhein and sennidins A+B, was developed. Rhein-9-anthrone, dissolved in Tyrode buffer at pH 6.5 or 7.5 at a concentration of 10–4 mol/l, completely disappeared from a solution warmed to 37 °C within 30 min. More than 90% were transformed into rhein and sennidins A+B. Pharmacological experiments in rats showed that net water absorption in the small intestine was reversed to net secretion by rhein-9-anthrone, whereas rhein and sennidins in corresponding concentrations were ineffective. As at least rhein is known to stimulate secretion under different experimental conditions, all pharmacological results with rhein-9-anthrone must be interpreted with caution and should be checked whether they essentially differ from those of its main oxidation products.

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