Gene-directed enzyme prodrug therapy (GDEPT) consists in targeted delivery to tumor cells of a suicide gene
responsible for in situ conversion of a prodrug into cytotoxic metabolites. One of the major limitations of this strategy in
clinical application was the poor prodrug activation capacity of suicide gene. We built a highly efficient suicide gene capable
of bioactivating the prodrug cyclophosphamide (CPA) by fusing a CYP2B6 triple mutant with NADPH cytochrome
P450 reductase (CYP2B6TM-RED). Expression of this fusion gene via a recombinant lentivirus (LV) vector converted resistant
human (A549) and murine (TC1) pulmonary cell lines into CPA-susceptible cell lines. We tested the efficiency of
our GDEPT strategy in C57Bl/6 immunocompetent mice, using TC1 cells expressing the HPV-16 E6/E7 oncoproteins. In
mice bearing tumors composed only of TC1-CYP2B6TM-RED cells, four CPA injections (140 mg/Kg once a week)
completely eradicated the tumors for more than two months. Tumors having only 25% of TC1-CYP2B6TM-RED cells
were also completely eradicated by five CPA injections, demonstrating a major in vivo bystander effect. Moreover, surviving
mice were rechallenged with parental TC1 cells. The tumors regressed spontaneously 7 days after cell inoculation
or grew more slowly than in control naive mice due to a strong immune response mediated by anti-E7CD8+T cells. These
data suggest that combining the CYPB6TM-RED gene with CPA may hold promise as a highly effective treatment for
solid tumors in humans.
Keywords: Cytochrome P450 2B6, lentivirus, cyclophosphamide, lung cancer.
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