Ex Vivo Breast Cancer Cell Purging by Adenovirus-Mediated Cytosine Deaminase Gene Transfer and Short-Term Incubation with 5-Fluorocytosine Completely Prevents Tumor Growth after Transplantation

Abstract

Peripheral blood progenitor harvests of breast cancer patients are contaminated with tumor cells, suggesting a potential role for these cells in the relapse after high-dose chemotherapy. Whereas physical purging methods do not eliminate contaminating tumor cells completely, pharmacological purging, although highly efficient, is hampered by a strong nonspecific toxicity toward hematopoietic progenitor cells. Taking advantage of the high efficiency of adenovirus-mediated gene transfer to epithelial cells, we selectively loaded breast cancer cells in vitro with a cytotoxic drug by gene transfer of the prodrug-converting enzyme cytosine deaminase (AdCMV.CD) and 5-fluorocytosine (5-FC). Despite the low dose of vector administered, limited exposure to 5-FC, and transplantation only of viable tumor cells into SCID mice, all animals that received cells treated in vitro with AdCMV.CD plus 5-FC were completely free of tumor development. These data show that the selective loading of tumor cells with AdCMV.CD/5-FC might be useful for purging of autografts.

Overview summary

A replication-deficient adenovirus vector was evaluated to transfer the cytosine deaminase gene into contaminating breast cancer cells within autologous stem cell grafts. Expression of the gene in target cells confers sensitivity toward the nontoxic prodrug 5-fluorocytosine (5-FC) by converting it to the toxic drug 5-fluorouracil (5-FU). The aim of generating a selective transfer into micrometastatic breast cancer cells, while sparing hematopoietic progenitors, is achieved by taking advantage of the high transfer efficiency into breast cancer cells. Consequently, the already low-dose Ad vector and 5-FC administration leads to tumor cell death in vitro, suggesting that tumor cells can be eliminated from stem cell autografts by this approach. Using a SCID mouse transplantation model, in vivo experiments demonstrated that the prevention of tumor development in vivo is specific for the pretreatment of breast cancer cells with AdCMV.CD and 5-FC.