The foundation for the standard of care for the treatment of advanced colon cancer remains the fluoropyrimidine 5-Fluorouracil (5-FU). 5-FU induces cytotoxicy through multiple pathways, including incorporation of toxic intermediates into DNA and RNA. However, the primary mechanism of action of 5-FU is through inhibition of the enzyme thymidylate synthase (TS), an essential enzyme in DNA synthesis. Expression of TS represents a well-characterized resistance mechanism. Patients bearing tumors that overexpress TS are less likely to respond to 5-FU based chemotherapy. Currently there are no effective chemotherapeutic regimens available to treat advanced colon cancer patients demonstrating TS-mediated resistance. Therefore, drugs that decrease the expression of TS are of continuous clinical interest.

Histone deacetylase inhibitors (HDACi) are a class of novel compounds that have generated a significant amount of interest due to their ability to selectively target cancer cells. This has led to the recent approval suberoylanilide hydroxamic acid (SAHA) for the treatment of cutaneous T-cell lymphoma (CTCL). HDACi have been shown to alter global acetylation patterns in the cell and lead to increased acetylation of both histone and non-histone proteins. Alteration in global acetylation patterns leads to cell cycle arrest, increased apoptosis, and increased differentiation within tumor cells. Previous microarray studies in breast and bladder cancer cell lines have shown that TS gene expression is significantly downregulated by multiple HDACi. We have extended these studies into colon cancer cell lines demonstrating that TS protein and mRNA are significantly downregulated following treatment with a panel of HDACi. Downregulation of TS was shown to be independent of p53, p21, and HDAC2 through the use of cell lines with mutations or absence of these proteins. These observations were extended in vivo whereas TS was shown to be downregulated in HCT116 mouse xenografts by the clinically relevant HDACi SAHA and LBH589. Synergistic interactions on growth inhibition of SAHA with the fluoropyrimidines 5-FU and fluorodeoxyuridine (FUdR) were observed in HCT116 and HT29 cells. Furthermore, depletion of thymidylate pool levels, a mechanistic endpoint of TS inhibition, were enhanced by the combination of SAHA with 5-FU or FUdR in colon cancer cell lines. These results demonstrate that HDACi cause decreases in TS gene expression, and lead to synergistic interactions with fluoropyrimidines. This data suggests a role for combination therapy of HDACi with 5-FU based chemotherapeutic regimens as a strategy to overcome TS-mediated resistance.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA