Bystander cytotoxicity in human medullary thyroid carcinoma cells mediated by fusion yeast cytosine deaminase and 5-fluorocytosine

In our work, we have evaluated efficiency of gene-directed enzyme/prodrug therapy (GDEPT) based on combination of fusion yeast cytosine deaminase (yCD) and 5-fluorocytosine (5FC) on model human medullary thyroid carcinoma (MTC) cell line TT. We determined the efficiency of this GDEPT approach in suicide and bystander cytotoxicity induction. We have shown significant bystander effect in vitro and 5FC administration resulted in potent antitumor effect in vivo. Furthermore, we have unraveled high efficiency of cell-mediated GDEPT, when human mesenchymal stromal cells (MSC) were used as delivery vehicles in direct cocultures in vitro. Nevertheless, effector MSC exhibited inhibitory effect on TT cell proliferation and abrogated TT xenotransplant growth in vivo. We suggest that yCD/5FC combination represents another experimental treatment modality to be tested in MTC and our data further support the exploration of MSC antitumor potential for future use in metastatic MTC therapy.