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Efficacy of Adenovirus-mediated CD/5-FC and HSV-1 Thymidine Kinase/Ganciclovir Suicide Gene Therapies Concomitant with p53 Gene Therapy¹

Departments of Radiation Oncology and Molecular Biology, Henry Ford Health System, Detroit, Michigan 48202-3450

Recent evidence has suggested that tumor cells having a wild-type p53 status are more sensitive to chemotherapeutic agents and radiation than cells that lack functional p53. The heightened sensitivity of wild-type p53 cells is thought to be attributable to their propensity to undergo p53-mediated apoptosis after insult. Given that suicide gene therapy is essentially tumor-targeted chemotherapy, we examined the hypothesis that coexpression of wild-type p53 could enhance the efficacy of adenovirus-mediated suicide gene therapy. Human Hep3B and SK-OV-3 cells, which are null for p53, were infected with a pair of replication-deficient adenoviruses that expressed a cytosine deaminase/herpes simplex virus thymidine kinase (CD/HSV-1 TK) fusion gene without (fusion gene nonreplicative adenovirus, FGNR) or with (FGNRp53) the wild-type human p53 gene. The sensitivity of cells to the CD/5-fluorocytosine (CD/5-FC) and HSV-1 TK/ganciclovir (GCV) enzyme/prodrug systems was determined in vitro and in vivo. Coexpression of p53 did not enhance the cytotoxicity of either the CD/5-FC or HSV-1 TK/GCV system in vitro. The failure to observe an effect of p53 could not be explained on the basis of insufficient or transient p53 expression, because FGNRp53-infected cells growth arrested in G₁, induced Bax, and underwent apoptosis at an increased rate after prodrug treatment, particularly when the adenovirus E1A protein was present. Intratumoral injection of FGNRp53 concomitant with single or double prodrug therapy resulted in a tumor growth delay that was equal to or less than that observed with the FGNR virus. Our results indicate that coexpression of p53 may not necessarily improve the efficacy of adenovirus-mediated CD/5-FC and HSV-1 TK/GCV suicide gene therapies in vivo.

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