Retrovirally TransducedEscherichia coli gptGenes Combine Selectability with Chemosensitivity Capable of Mediating Tumor Eradication

Abstract

“Suicide genes” encoding exceptional sensitivity to chemotherapeutic agents can potentially improve the selectivity of cancer therapy. As a component of a retroviral gene therapy vector, a suicide gene might also improve the safety of gene therapy by permitting the subsequent ablation of transduced cells exhibiting neoplastic or other aberrant behavior. An extra gene, however, cannot easily be added to vectors already carrying a therapeutic gene and a selectable drug resistance marker without compromising gene expression. To circumvent this obstacle, we have investigated a retrovirally transduced Escherichia coli gpt gene on the basis of evidence that it might subserve a dual sensitivity/resistance function. A gpt vector was used to transduce the gene into murine K3T3 sarcoma cells. In vitro, gpt-positive K3T3 clones could be selected on the basis of resistance to a regimen containing mycophenolic acid and xanthine; the same clones were 18 to 86 times as sensitive to 6-thioxanthine (6TX) as their gpt-negative counterparts. In mice, systemic 6TX therapy induced durable regressions in 19/20 gpt-positive K3T3 sarcomas without affecting gpt-negative tumors. These results indicate that selectability and in vivo chemosensitivity can be expressed in the same cell population from a single retrovirally transduced gene and imply the additional possibility of fusing the gpt gene with a therapeutic gene to create vectors expressing three important functions from a single gene. Chemosensitivity genes can potentially improve cancer chemotherapy and increase the safety of retroviral gene therapy by conferring a suicide potential on recipient cells, but the inverse relationship between gene number and gene expression is an obstacle to their use in vectors already bearing therapeutic and drug resistance genes. This study, using a gpt vector to transduce the gene into murine sarcoma cells, demonstrated that gpt-positive cells, selected in vitro by virtue of resistance to a mycophenolic acid/xanthine combination, are significantly more sensitive to 6-thioxanthine than gpt-negative cells and that tumors originating from these cells in vivo can often be eradicated by 6-thioxanthine therapy. The results suggest the potential utility of the gpt gene for both cancer gene therapy and as a means of adding a suicide function to a drug resistance function in gene therapy vectors without adding an extra gene.