Sensitisation of human carcinoma cells to the prodrug CB1954 by adenovirus vector-mediated expression of E. coli nitroreductase

The enzyme nitroreductase from E. coli can reduce the weak, monofunctional alkylating agent 5-(aziridin-1-yl)-2, 4-dinitrobenzamide (CB1954) to a potent cytotoxic species that generates interstrand crosslinks in DNA. Nitroreductase therefore has potential as a "suicide enzyme" for cancer gene therapy, as cells that express nitroreductase become selectively sensitive to the prodrug CB1954. We have incorporated a nitroreductase expression cassette into a replication-defective adenovirus vector (Ad-CMV-ntr), which allowed efficient gene transfer to SK-OV-3 or IGROV-1 ovarian carcinoma cells. Nitroreductase levels increased in line with multiplicity of infection, and this was reflected in increasing sensitisation of the cells to CB1954, reaching an optimum (approx. 2, 000-fold sensitisation) with 25-50 p.f.u. per cell. Similar Ad-CMV-ntr-dependent sensitisation to CB1954 was seen in 3 of 6 low-passage primary ovarian tumour lines. Cells grown at low-serum concentration to inhibit proliferation remained equally susceptible to the Ad-CMV-ntr-dependent cytotoxicity of CB1954, indicating a distinct advantage over retroviral gene delivery and other popular enzyme-prodrug systems for human tumours with a low rate of cell proliferation. Additionally, cisplatin-resistant cells were sensitised towards CB1954 by Ad-CMV-ntr as efficiently as the parental cells, indicating that the system could be effective in patients with cisplatin-resistant tumours. In a murine xenograft model for disseminated peritoneal carcinomatosis with ascites, treatment of nude mice bearing intraperitoneal SUIT2 tumours with Ad-CMV-ntr and CB1954 almost doubled the median survival from 14 to 26 days (p < 0.0001).