BYSTANDER EFFECTS OF DIFFERENT ENZYME-PRODRUG SYSTEMS FOR CANCER GENE-THERAPY DEPEND ON DIFFERENT PATHWAYS FOR INTERCELLULAR TRANSFER OF TOXIC METABOLITES, A FACTOR THAT WILL GOVERN CLINICAL CHOICE OF APPROPRIATE REGIMES

Transfer of suicide genes into tumor cells renders them sensitive to c ytotoxic effects of specific prodrugs. We show here that both the herp es simplex virus thymidine kinase/ganciclovir (tk/GCV) and thymidine p hophorylase/5'-deoxy-5-fluorouridine (tp/DFUR) suicide gene systems ca n induce cell death in tumor cells. Additionally in mixed cultures of cells with and without the suicide gene, death occurred in both cell t ypes, indicative of a bystander effect. We demonstrate, in human and r odent cell lines, that the tk/GCV bystander effect requires gap juncti onal intercellular communication (GJIC). Where cultures lack GJIC, no bystander effect was observed. In communicating cultures, no correlati on between level of GJIC and bystander effect was seen and this was du e to high levels of tk activity. Additionally, we demonstrate that tra nsfer of toxic metabolites from tk(+) to tk(-) cells occurs within 2 h r of GCV application and, as no apoptosis could be detected until afte r this time, apoptosis is the result, not the cause, of the tk/GCV bys tander effect. In the tp/DFUR system, a medium-mediated bystander effe ct, independent of GJIC and apoptosis, was observed. We demonstrated t hat combining tk/GCV and tp/DFUR suicide gene systems in culture was m ore effective than either therapy alone.