As human amniotic fluid-derived stem cells (hAFSCs) are capable of multiple lineage differentiation, extensive self-renewal and tumor targeting, they may be valuable for clinical anticancer therapies. In this study, we used hAFSCs as vehicles for targeted delivery of therapeutic suicide genes to breast cancer cells. hAFSCs were engineered to produce AF2.CD-TK cells in order to express two suicide genes encoding bacterial cytosine deaminase (CD) and herpes simplex virus thymidine kinase (HSV-TK) that convert non-toxic prodrugs, 5-fluorocytosine (5-FC) and mono-phosphorylate ganciclovir (GCV-MP), into cytotoxic metabolites, 5-fluorouracil (5-FU) and triphosphate ganciclovir (GCV-TP), respectively. In cell viability test in vitro, AF2.CD-TK cells inhibited the growth of MDA-MB-231 human breast cancer cells in the presence of the 5-FC or GCV prodrugs, or a combination of these two reagents. When the mixture of 5-FC and GCV was treated together, an additive cytotoxic effect was observed in the cell viability.In animal experiments using female BALB/c nude mouse xenografts, which developed by injecting MDA-MB-231 cells, treatment with AF2.CD-TK cells in the presence of 5-FC and GCV significantly reduced tumor volume and weight to the same extent seen in the mice treated with 5-FU. Histopathological and fluorescent staining assays further showed that AF2.CD-TK cells were located exactly at the site of tumor formation. Furthermore, breast tissues treated with AF2.CD-TK cells and two prodrugs maintained their normal structures (for example, the epidermis and reticular layers) while breast tissue structures in 5-FU-treated mice were almost destroyed by the potent cytotoxicity of the drug. Taken together, these results indicate that AF2.CD-TK cells can serve as excellent vehicles in a novel therapeutic cell-based gene-directed prodrug system to selectively target breast malignancies. INTRODUCTIONBreast cancer is one of the most common malignancies and the leading cause of cancer-related death in women worldwide. 1 Chemotherapy has been the main treatment for highly metastatic breast cancer. Although various chemotherapeutic agents are currently available, their applications are limited because of undesirable side effects, substantial toxicities to normal tissues and high-dose requirements resulting from drug resistance. In this study, we present a novel, low-toxic and effective chemotherapeutic strategy for treating breast cancer using genetically engineered stem cells to deliver suicide genes.Suicide genes are viral or bacterial enzymes that can convert non-toxic prodrugs into toxic metabolites that cause tumor cell death when introduced into tumor sites. Among a number of suicide gene/prodrug combinations, cytosine deaminase (CD)/ 5-fluorocytosine (5-FC) and herpes simplex virus thymidine kinase (HSV-TK)/ganciclovir (GCV) have been most extensively studied. The product of the CD gene converts a far less toxic prodrug, 5-FC, into its cytotoxic form, 5-FU, a nucleotide analog that induces cell death by inhibiting thymidyla...