The evaluation of novel natural products as inhibitors of human glutathione transferase P1-1

Glutathione transferase P1-1 is over expressed in some cancer cells and contributes to detoxification of anticancer drugs, leading to drug-resistant tumors. The inhibition of human recombinant GSTP1-1 by natural plant products was investigated using 10 compounds isolated from plants indigenous to Southern and Central Africa. Monochlorobimane and 1-chloro-2,4-dinitrobenzene were used to determine GST activity. Each test compound was screened at 33 and 100 mu M. Isofuranonapthoquinone (1) (from Bulbine frutescens) showed 68% inhibition at 33 mu M, and sesquiterpene lactone (2) (from Dicoma anomala) showed 75% inhibition at 33 mu M. The IC50 value of 1 was 6.8 mu M. The mode of inhibition was mixed, partial (G site) and noncompetitive (H site) with K-i values of 8.8 and 0.21 mu M, respectively. Sesquiterpene 2 did not inhibit the CDNB reaction. Therefore, isofuranonapthoquinone 1 needs further investigations in vivo because of its potent inhibition of GSTP1-1 in vitro.