The histone deacetylase inhibitor PXD101 increases the efficacy of irinotecan in in vitro and in vivo colon cancer models

Purpose: Histone deacetylase inhibitors (HDACIs), such as PXD101 and suberoylanilide hydroxamic acid, inhibit proliferation and stimulate apoptosis of tumor cells. The enhanced effectiveness of chemotherapy or radiotherapy when combined with HDACIs has been observed in several cancers. In this study, we investigated the antitumor effect of PXD101 combined with irinotecan in colon cancer. Methods: HCT116 and HT29 colon cancer cells for cell viability assay were treated with PXD101 and/or SN-38, the active form of irinotecan. Antitumor effects of HCT116 and HT29 xenografts treated with these combinations were evaluated. [¹⁸F]FLT-PET was used to detect early responses to PXD101 and irinotecan in colon cancer. Results: PXD101 and SN38 possessed dose-dependent antiproliferative activity against HCT116 and HT29 cells and exerted a synergistic effect when used in combination. In xenografted mice, PXD101 in combination with irinotecan dramatically inhibited tumor growth without causing additive toxicity. Apoptotic effects on xenograft tumors were greater with combined treatment than with irinotecan alone. [ ¹⁸F]FLT-PET imaging revealed a 64% decrease in [¹⁸F]FLT uptake in tumors of HCT116 xenograft-bearing mice treated with a combination of PXD101 and irinotecan, indicating a decrease in thymidine kinase 1 (TK1) activity. These results were supported by Western blot analyses showing a decrease in tumor thymidine kinase 1 protein levels, suggesting that [ ¹⁸F]FLT-PET can be used to non-invasively detect early responses to these agents. Conclusions: These data show that PXD101 increases the cytotoxic activity of irinotecan in in vitro and in vivo colon cancer models and suggest these agent combinations should be explored in the treatment of colon cancer.