Abstract 2999: Cytotoxicity with hydroxamic acid HDACis in combination with antimetabolites is highly sequence dependent in solid tumor cells
Authors: Flanagan Sheryl A. , Ackroyd Jeffrey J. , Kramer Aaron , Feigin Jennifer , Shewach Donna S. 
Published: July 2016

Abstract
Used alone or in combination with other chemotherapeutics, the antimetabolites 2’, 2’-difluoro-2’-deoxycytidine (gemcitabine, dFdCyd), and 5-fluorouracil (5-FU)/fluorodeoxyuridine (FdUrd) comprise the mainstay of treatment for gastrointestinal malignancies, such as pancreatic and colorectal cancer. Gemcitabine with cisplatin is also first-line therapy for non-small cell lung cancer (NSCLC). While these treatments can prolong survival, many patients still relapse and there is need for improvement to current therapies. Cytotoxicity elicited by dFdCyd and 5-FU/FdUrd is influenced by drug metabolism/activation, DNA repair proteins, cell cycle progression, and apoptosis. Histone deacetylase inhibitors (HDACi) target histone deacetylases, which regulate gene expression through deacetylation of lysine residues on histones and some non-histone proteins. HDACis have been shown to increase expression of p21 and increase apoptosis which could enhance cytotoxicity with antimetabolites. The success of HDACis, including vorinostat and panobinostat, in the treatment of hematologic malignancies has encouraged their use in solid tumors. However, results from clinical trials in patients treated with HDACis alone or with other drugs, including antimetabolites have been mixed and the mechanism(s) underlying successful therapy with HDACis is largely unknown. In the present studies, we used solid tumor cell lines that represent tumors commonly treated with antimetabolites; A549 (NSCLC) and PANC1 (pancreatic cancer) cells, and HT29 (colon cancer) cells to investigate the effect of an HDACi with either dFdCyd or FdUrd, respectively, and we measured clonogenic survival. The data demonstrate that cytotoxicity with an HDACi and an antimetabolite is highly sequence dependent; vorinostat added concurrently with dFdCyd or FdUrd or prior to either antimetabolite resulted in antagonistic cytotoxicity, whereas the addition of the antimetabolite followed by HDACi was additive/synergistic with dFdCyd. Cytotoxicity with panobinostat and antimetabolites was also sequence dependent. Cell cycle distribution studies revealed the accumulation of cells in G1 and G2/M following HDACi addition (≥90% of cells), thus rendering S-phase specific dFdCyd less cytotoxic upon its subsequent addition. Similarly, concurrent addition of HDACi and dFdCyd produced less S-phase accumulation than observed with dFdCyd alone. These data demonstrate that administration of the HDACis and antimetabolites must be sequenced appropriately so that cell cycle effects complement rather than antagonize drug mechanisms.
Citation Format: Sheryl A. Flanagan, Jeffrey J. Ackroyd, Aaron Kramer, Jennifer Feigin, Donna S. Shewach. Cytotoxicity with hydroxamic acid HDACis in combination with antimetabolites is highly sequence dependent in solid tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2999.