Biopharm Drug Dispos. 1992 May; 13(4): 243-53.

In vitro reduction of rhein anthraquinone to rhein anthrone by rat cecal microflora and some intestinal bacterial strains.

de Witte P, Van Hoestenberghe A, Eyssen H.

Laboratory of Pharmaceutical Biology and Phytopharmacology, Katholieke Universiteit Leuven, Belgium.

After in vitro incubation of cecal content from CVL or gnotobiotic rats with rhein anthraquinone (1 mg g-1) for 18 h at 37 degrees, the anthraquinone was converted to rhein anthrone for 23.5 (SD +/- 3.4) per cent and 19.4 (+/- 4.7) per cent, respectively. Liquid cultures of some strictly anaerobic fecal bacteria of man and mouse incubated with rhein anthraquinone (62.5 micrograms ml-1) for 48 h at 37 degrees, revealed only small amounts of reduced substance. Although the underlying mechanisms of the differences in both conditions remain unclear, the experimental data accentuate the need for using cecal content when exploring the in vitro metabolism of anthranoids by the intestinal microflora. The conversion was chemically proven by derivatization of the anthrone with 4-nitroso-N-N-dimethylaniline in pyridine, followed by hydrolysis and identification of the released anthranoid by mass spectrometry. The in vitro reduction capacity of the cecal content of CVL rats was drastically decreased by oral administration to rats of different combinations of neomycin sulfate, metronidazole, bacitracin and Na-penicillin G. In these cases the conversion ranked between 0.2 per cent and 5.2 per cent the first day after administration. It is concluded that rhein anthraquinone in cecal content is reduced to the highly reactive and labile rhein anthrone, which accounts for the in vivo disappearance of dihydroxy-anthranoid equivalents in routine analysis after oral administration of anthraquinones to experimental animals.