Abstract 5374: Synergistic tumor cell killing with ganciclovir and vorinostat is due to inhibition of homologous recombination
Authors: Ladd Brendon , Shewach Donna S. 
Published: June 2012

Abstract
The indiscriminate nature of traditional cancer chemotherapy, which targets all dividing cells, has initiated a search for more selective approaches such as suicide gene therapy. With this strategy, only the cells containing the suicide gene are capable of activating a prodrug to a toxic metabolite, thus conferring selectivity for tumor cells while sparing normally dividing tissues. One of the most commonly used suicide gene therapy strategies transfers the cDNA for herpes simplex virus thymidine kinase (HSV-TK) into tumor cells followed by treatment with the antiviral drug ganciclovir (GCV). GCV is phosphorylated initially by HSV-TK, and after further phosphorylation by cellular kinases the active triphosphate is incorporated into the DNA of dividing cells. Recently, we have demonstrated a role for homologous recombination (HR) repair in promoting survival after treatment of GCV in HSV-TK containing cells. Based on these findings, we hypothesize that pharmacologic inhibition of HR will result in synergistic cell kill when combined with GCV. Previous reports have demonstrated that inhibition of lysine deacetylases (KDACs) can inhibit HR. KDACs can also increase the expression of the cell receptor utilized by adenoviruses for infection, thus making this class of drugs a potentially beneficial addition to gene therapy with HSV-TK/GCV. Therefore we evaluated the effect of the KDAC inhibitor Vorinostat (SAHA) on HR and cytotoxicity with HSV-TK/GCV.
The ability of SAHA to inhibit HR was evaluated in a cell based recombination reporter assay. SAHA (1 to 10µM) produced increasing inhibition (51 to 85%) of HR. Western blot analysis demonstrated that these concentrations of SAHA produced a dose-dependent decrease in Rad51, an essential protein for HR. HR can be visualized in intact cells by the formation of Rad51 foci at sites of DNA damage. In U251tk cells (stably expressing HSV-TK), GCV produced a time-dependent increase in Rad51 foci, demonstrating a role for HR in repair of GCV-induced DNA damage. At concentrations that inhibited HR and decreased Rad51 expression, SAHA completely blocked Rad51 foci formation with GCV treatment. Together, these data demonstrate that SAHA inhibits HR in response to GCV. We then evaluated cytotoxicity with the combination of GCV and SAHA and observed a synergistic increase in cell kill. To further assess the role of HR in the synergistic cytotoxicity with SAHA and GCV, we evaluated the drug combination in HR proficient and deficient CHO cells stably expressing HSV-TK. The results demonstrated that the combination of SAHA and GCV produced synergistic cell kill in HR proficient cells but only additive cell kill in HR deficient cells. Collectively, these data demonstrate that inhibition of HR by SAHA can account, at least in part, for the synergistic cytotoxicity with GCV. These data suggest that the combination of SAHA and HSV-TK/GCV merits further evaluation in vivo.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5374. doi:10.1158/1538-7445.AM2011-5374