Structural identification of methyl protodioscin metabolites in rats’ urine and their antiproliferative activities against human tumor cell lines

Methyl protodioscin (MPD), a furostanol saponin, is a preclinical drug shown potent antiproliferative activities against most cell lines from leukemia and solid tumors. The metabolites of MPD in rats’ urine after single oral doses of 80 mg/kg were investigated in this research. Ten metabolites were isolated and purified by liquid–liquid extraction, open-column chromatography, medium-pressure liquid chromatography, and preparative high-performance liquid chromatography. The structural identification of the metabolites was carried out by high resolution mass spectra, NMR spectroscopic methods including 1H NMR, 13C NMR and 2D NMR, as well as chemical ways. The 10 metabolites were elucidated to be dioscin (M-1), pregna-5,16-dien-3β-ol-20-one-O-α-l-rhamnopyranosyl-(1 → 2)-[α-l-rhamnopyranosyl-(1 → 4)]-β-d-glucopyranoside (M-2), diosgenin (M-3), protobioside (M-4), methyl protobioside (M-5), 26-O-β-d-glucopyrannosyl(25R)-furan-5-ene-3β, 22α, 26-trihydroxy-3-O-α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranoside(M-6),26-O-β-d-glucopyranosyl(25R)-furan-5-ene-3β,26-dihydroxy-22-methoxy-3-O-α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranoside (M-7), prosapogenin A of dioscin (M-8), prosapogenin B of dioscin (M-9), and diosgenin-3-O-β-d-glucopyranoside (M-10), respectively. M-1 was the main urinary metabolite of MPD in rats. Some metabolites showed potent antiproliferative activities against HepG2, NCI-H460, MCF-7 and HeLa cell lines in vitro.