The five-year survival rate for patients with metastatic colorectal cancer is less than 15%, necessitating an urgent need to develop novel therapeutic strategies for this disease. Loss of differentiation is associated with worse overall survival in colorectal cancer, suggesting that strategies to re-induce differentiation may provide clinical benefit. Differentiation therapy is effective in the treatment of acute promyelocytic leukemia but whether this approach can be efficacious in treating colorectal cancer is unknown. Inhibitors of the MAPK signaling pathway including the MEK inhibitor Trametinib can induce markers of differentiation in colorectal cancer cells. Trametinib also induces expression of Cdx-2, a known driver of colonic cell differentiation. Similarly, histone deacetylase inhibitors (HDACi) induce markers of differentiation in multiple tumor cell lines, including colon cancer cells. The aim of this study was to assess the potential therapeutic benefit of combining MEK (Trametinib) and HDAC (Panobinostat) inhibitors to further promote differentiation in colorectal cancer cells, and to elucidate the mechanistic basis for this effect. Combination treatment of HT29 and T84 cells with Trametinib and Panobinostat significantly enhanced mRNA and protein expression of the differentiation markers Cadherin 17 (CDH17) and Keratin 20 (KRT20), compared to either agent alone. Trametinib also induced expression of Cdx-2, an effect that was enhanced upon combination treatment with Panobinostat. Notably, Cdx-2 knockdown attenuated CDH17 and KRT20 induction by the combination, establishing a direct role for Cdx-2 in differentiation induction by the Trametinib/Panobinostat combination. Concomitant with inducing differentiation, the drug combination also invoked more apoptosis than either agent alone. To validate these findings in vivo, HT29 cells grown as xenografts were treated with Trametinib and Panobinostat alone and in combination. Mice treated with the combination had significantly smaller tumours than mice treated with vehicle or either agent alone. Consistent with the in vitro findings, immunostaining of the tumour xenografts demonstrated a strong increase in CDH17, KRT20 and Cdx2 expression upon treatment with the drug combination. Collectively, this study highlights a drug combination strategy to re-induce differentiation for the treatment of colorectal cancers.

Citation Format: Laura J. Jenkins, Ian Y. Luk, Janson W. Tse, Jennifer Mooi, Amardeep S. Dhillon, John M. Mariadason. Combining MEK and HDAC inhibitors as a therapeutic strategy to promote differentiation of colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3089. doi:10.1158/1538-7445.AM2017-3089