Abstract 3519: Histone deacetylase inhibitors upregulates thymidine phosphorylase gene and protein expression and synergize with capecitabine in breast cancer cells

Capecitabine is a pro-drug which was designed to take advantage of the increased levels of thymidine phosphorylase (TP), a key enzyme for its conversion to 5-florouracil (5-FU), observed in tumours as opposed to normal tissues, potentially allowing for selective toxicity. Capecitabine is a valuable substitute for bolus or infusion 5-FU either as monotherapy or in combination with other cytotoxic drugs in the treatment of several tumor types including breast cancer. We have recently demonstrated for the first time, that the histone deacetylase-inhibitor (HDAC-I) vorinostat induces synergistic antitumour effects in combination with capecitabine by up-regulating, in vitro and in vivo, in colorectal cancer cells but not in ex vivo treated peripheral blood lymphocytes, the mRNA and protein expression of TP. We have also demonstrated that TP knockdown by a specific siRNA significantly impairs the synergistic apoptotic cell death induced by vorinostat/capecitabine combination. In this study we demonstrated a time and dose-dependent induction of TP mRNA and protein expression by several HDAC-Is in different breast cancer cell lines such MCF-7, SKBR3, MDA-MB-231and MDA-MB-468 cells. Interestingly, this effect was shared by pan-HDAC-Is such as panobinostat (LBH589), trichostatin A (TSA) or vorinostat and by class I HDAC-Is such as valproic acid (VPA) and entinostat (MS275), but not by the specific inhibitor of HDAC-6 tubacin, indicating that targeting class I HDACs is crucial for TP induction. Moreover, the mechanism of TP induction is dependent on a transcriptional effect since preliminary data showed that HDAC-Is did not interfere with TP protein stability. On these bases we investigated potential antitumor interaction between capecitabine and either vorinostat or panobinostat or VPA, demonstrating synergistic antiproliferative effects in all breast cancer cells independently of p53, ER or Her2 status. Overall, this study suggests that HDAC-Is, by upregulating TP expression, may improve the therapeutic index of capecitabine representing an innovative antitumour strategy for the treatment of breast cancer that warrants further clinical evaluation. 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 3519. doi:10.1158/1538-7445.AM2011-3519