Instability of rhein-9-anthrone as a problem in pharmacological and analytical use

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 degrees 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.