Suicide gene therapy for premalignant disease: A new strategy for the treatment of intraepithelial neoplasia

The potential of gene therapy to treat premalignant disease or recurrent cancer has not been investigated. The goal of the present investigation was to explore the efficacy of pro-drug-mediated, suicide gene therapy as a strategy to treat incipient neoplasia in stratified squamous epithelium. To test this strategy, a tissue model of premalignancy was generated by mixing normal human keratinocytes (NHK) that express the bacterial cytosine deaminase gene (CD) with premalignant keratinocytes which have been genetically marked with the bacterial gene for β-galactosidase (II-4-β-gal) in skin-like organotypic cultures. Preliminary studies in monolayer cultures demonstrated that CD-transduced NHK (NHK/CD) efficiently expressed the transgene and deaminated the pro-drug 5-fluorocytosine (5FC) to the toxic product 5-fluorouracil (5FU). The capacity of NHK/CD to kill II-4-β-gal cells through bystander effect was assayed in both submerged culture and in the organotypic model of premalignancy. In submerged cultures, it was found that CD-mediated killing of II-4-β-gal cells did not require cell-cell contact and that the LD₅₀ of 5FC for efficient bystander killing of 11-4-β-gal was 0.5 mM. When this concentration of pro-drug was used in organotypic cultures, a significant number of dysplastic II-4-β-gal cells were eliminated from the tissue. Bystander killing of II-4-β-gal cells was related to the number of NHK/CD present. These findings demonstrated that potentially malignant keratinocytes could be eliminated from a dysplastic tissue through activation of pro-drug and killing of adjacent cells through the bystander effect. By establishing and in vitro model to eliminate premalignant cells using suicide gene therapy, these studies provide a new approach for the treatment of incipient cancer as it develops, thereby preventing invasive disease.