Enhanced ganciclovir killing and bystander effect of human tumor cells transduced with a retroviral vector carrying a herpes simplex virus thymidine kinase gene mutant

Gene transfer of the herpes simplex virus thymidine kinase (TK) gene associ ated with ganciclovir (GCV) treatment can lead to death of TK-expressing ce lls, and of neighboring TK- cells because of the bystander effect. Thus, a small proportion of TK+ cells in a tumor can lead to its complete regressio n after GCV treatment. However, a lack of efficacy of gene transfer into tu mors associated with low GCV sensitivity and poor bystander effect of human cancer cells currently limit the clinical use of this suicide gene therapy approach. To increase the potency of suicide gene therapy, we have tested the GCV sensitivity and the bystander effect of TK mutants that have been p reviously described. After retroviral transduction of the TK mutants into h uman tumor cells of various origins, we have found a strong killing effect of GCV with cells expressing the mutants TK30 or TKF161C. The GCV sensitivi ty of several human tumor cell types expressing TK30 was 9- to 500-fold hig her than cells containing wild-type TK. Furthermore, TK30-expressing cells were able to kill bystander cells much more efficiently than TK-expressing cells. Thus, TK30 mutant is a promising candidate for suicide gene therapy clinical trials.