The herpes simplex virus thymidine kinase (HSV-TK) converts ganciclovir (GCV) into a toxic product and allows selective elimination of TK⁺ cells in vitro and in vivo. It is currently being used in clinical gene therapy trials as a therapeutic gene or as a safety marker. We have analyzed the susceptibility of different tumor cell lines to the TK/GCV-mediated “suicide” effect. Therefore, tumor cells TSA, J558L, EB, and ESB and, as a control, NIH-3T3 cells were infected with a retrovirus containing a hygromycin/TK fusion gene. All cell lines were sensitive to GCV in vitro; however, the concentration of GCV and the time needed to eliminate tumor cells completely considerably varied between different tumor cell lines. TSA-TK cells were completely eliminated within 10 days in 1 μg/ml GCV, whereas ESB-TK cells required 22 days in 10 μg/ml GCV. When two cell lines were examined, the differing sensitivity to GCV in vitro correlated with the ability to eradicate TK⁺ tumors in vivo. TSA-TK tumors could be eliminated in almost all animals by systemic GCV administration, whereas ESB-TK tumors were completely resistant. Different sensitivity to GCV was not due to different TK expression levels because the cells were similarly resistant to hygromycin, and Western blot analysis with an anti-TK antiserum revealed similar protein amounts in TSA/TK and ESB-TK cells. Together, the results demonstrate that tumor cells are highly different concerning the susceptibility to the TK/GCV effect, which, however, may be tested for in vitro. Recently, a number of clinical studies have been published assessing the usefulness of retroviral vectors containing the thymidine kinase (TK) gene for the elimination of transfected (TK⁺) tumor cells using the drug ganciclovir (GCV). Here, we could demonstrate differences in susceptibility to TK-mediated tumor killing by GCV: Following retroviral transfection with the TK gene, two tumor cell lines, although producing the same amount of intracellular TK, could not be equally efficiently eliminated by GCV both in vitro and in vivo.

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