Improved safety through tamoxifen-regulated induction of cytotoxic genes delivered by Ad vectors for cancer gene therapy

The transfer of pro-apoptotic genes to tumors is one of the most promising strategies for anticancer gene therapy. However, the use of potentially tox ic genes, such as tumor suppressor genes or apoptotic genes, needs controll able transgene activation. To achieve regulation of the transgene at a desi red time, we developed an adenovirus (Ad) vector in which the apoptotic act ivity of the target gene has been made 4-OHT-dependent by fusion to the lig and binding-domain of the estrogen receptor (ER). For evaluation of the sys tem in human tumor cells, we used the E2F1 gene which encodes a transcripti on factor that triggers massive apoptosis in several human cancers. AdER-E2 F1 expressed high levels of transgene over at least I week. Upon activation of E2F1 by the ligand 4-hydroxy-tamoxifen (4-OHT) the ER-E2F1 fusion prote in correctly translocated from the cytosol to the nucleus, transactivated E 2F-dependent promoters, and rapidly induced substantial E2F1-related toxici ty. Finally, experiments in nude mice showed tightly regulated tumor growth suppression in vivo. Taken together our system represents a powerful appro ach for tight regulation and rapid induction of cytotoxicity as the major c riteria for safe gene delivery.