Viral delivery of P450 reductase recapitulates the ability of constitutive overexpression of reductase enzymes to potentiate the activity of mitomycin C in human breast cancer xenografts

Indolequinones such as mitomycin C (MMC) require enzy- matic bioreduction to yield cytotoxic moieties. An attractive approach to overcome the potential variability in reductive bioactivation between tumors is to exploit specific enzyme-bioreductive drug combinations in an enzyme-directed gene therapy (GDEPT) approach. To this end, human breast cancer cell lines (T47D, MDA468, and MDA231) that overexpress either DT-diaphorase (DTD) or NADPH:cytochrome P450 reductase (P450R) have been developed. Cytotoxicity of MMC was evalu- ated in the panel of cell lines following aerobic or anoxic exposure in vitro. DTD and/or P450R overexpression sensitized cells to MMC in air with no further increase in the cytotoxicity of MMC under anoxia. The most profound effect was seen in the MDA468 cells, where a 27-fold increase in potency was observed for MMC in the DTD-overexpressing cell line. The MMC sensitization achieved through DTD and P450R overexpression in MDA468 cells was maintained in vivo. Xenografts established from the clonal lines exhibited significant tumor control following MMC treatment (treated/control [T/C] 17% and 51% for DTD and P450R xenografts, respectively) that was not seen in wild-type tumors (T/C 102%). Delivery of a clinically relevant adenoviral vector
encoding P450R to MDA468 wild-type tumors yielded comparable P450R activity to that seen in the P450R clonal xenografts and resulted in greater MMC sensitiza- tion (T/C 46%). The model systems developed will facilitate the identification of novel indolequinone agents that are targeted toward a specific enzyme for bioacti- vation and are consequently of potential use in a GDEPT approach.