20S proteasome inhibitory activity of flavonoids isolated from Spatholobus suberectus

The promising biological role of the ubiquitin proteasome pathway (UPP) in cancer therapy has recently emerged. Inhibition of proteasome has been proposed as a new therapeutic target for treatment of cancer. Bioassay-guided fractionation of a MeOH extract of the stems of Spatholobus suberectus resulted in seven compounds, liquiritigenin (1), isoliquiritigenin (2), genistein (3), daidzein (4), medicarpin (5), 7-hydroxyflavanone (6) and formononetin (7), which were evaluated for the first time for their inhibitory effect on 20S proteasome. Among the isolated compounds, 2, 3 and 6 exhibited inhibitory activities on human 20S proteasome with IC(50) values of 4.88 ± 1.5, 9.26 ± 1.2 and 5.21 ± 1.5 µm, respectively.