The histone deacetylase inhibitor PXD101 synergises with 5-fluorouracil to inhibit colon cancer cell growth in vitro and in vivo

Abstract Purpose Histone deacetylase inhibitors (HDACi) inhibit the growth of cancer cells, and combinations of HDACi with established chemotherapeutics can lead to synergistic effects. We have investigated effects of PXD101 (HDACi in phase II clinical trials) in combination with 5-fluorouracil, on tumour cell proliferation and apoptosis both in vitro and in vivo. Experimental design HCT116 cells were studied using proliferation and clonogenic assays. Synergistic inhibition of proliferation and clonogenicity was determined by incubation with PXD101 and 5-fluorouracil, and analysis using CalcuSyn™ software. The effect of combining PXD101 and 5-fluorouracil on apoptosis was examined in vitro using PARP-cleavage and TUNEL. Finally, the effectiveness of combining PXD101 and 5-fluorouracil in vivo was tested using both HT-29 and HCT116 xenograft models. Results Synergistic inhibition of proliferation and clonogenicity was obtained when HCT116 cells were incubated with PXD101 and 5-fluorouracil. 5-fluorouracil combined with PXD101 also increased DNA fragmentation and PARP cleavage in HCT116 cells. Incubation with PXD101 down regulated thymidylate synthase expression in HCT116 cells. In vivo studies, using mouse HT29 and HCT116 xenograft models, showed improved reductions in tumour volume compared to single compound, when PXD101 and 5-fluorouracil were combined. Conclusions PXD101 and 5-fluorouracil synergistically combine in their anti-tumour effects against colon cancer cells in vitro and show enhanced activity when combined in vivo. Based on the results presented herein, a rationale for the use of PXD101 and 5-fluorouracil in combination in the clinic has been demonstrated.