The use of replication-competent viral vectors is a promising new approach for delivery directly to cancer cells of genes that can serve as the blueprint for therapeutic proteins to kill the cells from within. We have previously shown that replication-competent retrovirus (RCR) vectors, which are capable of tumor-selective spread from cell to cell, has greatly improved therapeutic effects compared to conventional replication-defective vectors previously used. These vectors can efficiently and selectively deliver marker genes and therapeutic genes to cancer cells throughout an entire tumor mass.

Gene therapy with RCR vectors thus has the potential for highly efficient delivery of suicide genes, which will allow high doses of non-toxic pro-drugs to be converted locally to cytotoxic drugs only within the infected cancer cells, without causing side effects to uninfected normal cells. Since conventional chemotherapy faces the problem of drug resistance due to the limited doses that can be applied systemically, we hypothesized that the combination of intracellular suicide gene therapy and systemic chemotherapy should be more effective than either therapy alone. Here we used the human breast cancer cell line MDA-MB 231 as a model system to examine the cytotoxicity of doxorubicin chemotherapy alone vs. gene therapy alone, in comparison with both treatments combined. Gene therapy was performed using RCR vectors carrying the yeast cytosine deaminase (CD) suicide gene, which encodes an enzyme that converts the non-toxic pro-drug 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU), a classic chemotherapy agent. 5-FC was administered at doses of 0 mM, 0.125 mM, 0.5 mM, and 2 mM. Doxorubicin chemotherapy was administered at doses of 0 µM, 0.001 µM, 0.01 µM, 0.1 µM, and 1µM. The IC50 of Doxorubicin on MDA-MB 231 cells was 0.1µM. Combination therapies were tested at different RCR transduction ratios (0, 20, 50, 100%).

Our results showed that cytotoxicity to breast cancer cell line MDA-MB 231 was dose-dependent for both RCR-CD + 5-FC and Doxorubicin, either individually or in combination. However, low dose Doxorubicin treatment, which had almost no cytotoxicity itself, resulted in almost complete cell killing when combined with RCR-CD + 5-FC treatment when MDA-MB 231 cells were transduced at levels as low as 20%. Therefore, we have demonstrated that combining both suicide gene therapy and chemotherapy can kill breast cancer cells with greater efficiency than either treatment alone. Combining these two treatment strategies thus has the potential to provide the same potency as either treatment alone at higher dosages, while reducing the frequency or severity of side effects.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA