Chemosensitivity to the indoloquinone EO9 is correlated with DT-diaphorase activity and its gene expression

EO9, a new bioreductive indoloquinone alkylating agent, requires activation by a two-electron reduction, which can be catalysed by the NAD(P)H:quinone oxidoreductase DT-diaphorase (DTD) (EC 1.6.99.2). Seven human and four murine tumor cell lines from different histological origins were evaluated for their DTD enzyme activity (evaluated using dichlorophenolindophenol and EO9 as substrates), DTD gene expression and chemosensitivity to EO9. In general the cell lines could be divided into two groups: leukemic cells which were relatively resistant to EO9 (IC50 > or = 0.5 microM) and had no measurable DTD activity, and solid tumor cells, which were more sensitive to the drug (IC50 < 0.06 nM) and contained a high DTD activity (> 90 nmol/min/mg). The expression of the DTD gene was measured by semiquantitative PCR in the human cell lines and an excellent correlation between gene expression and enzyme activity was observed (r2 = 0.94). A higher DTD gene expression also correlated with higher chemosensitivity to EO9. Protection of chemosensitivity to EO9 by dicoumarol, a strong and specific inhibitor of DTD activity, was dependent on duration of exposure and concentration of dicoumarol. Inhibition was best observed by short exposure to dicoumarol and EO9 together, demonstrating that bioactivation of EO9 by DTD is essential. In conclusion, DTD activity and expression appear to predict sensitivity to EO9 in a variety of cell lines. Evaluation of activity or expression in patients' tumor samples might predict the response to EO9.