Epigenetic therapy in gastrointestinal cancer: The right combination

Epigenetics is a relatively recent field of molecular biology that has arisen over the past 25 years. Cancer is now understood to be a disease of widespread epigenetic dysregulation that interacts extensively with underlying genetic mutations. The development of drugs targeting these processes has rapidly progressed; with several drugs already FDA approved as first-line therapy in hematological malignancies. Gastrointestinal (GI) cancers possess high degrees of epigenetic dysregulation, exemplified by subtypes such as CpG island methylator phenotype (CIMP), and the potential benefit of epigenetic therapy in these cancers is evident. The application of epigenetic drugs in solid tumors, including GI cancers, is just emerging, with increased understanding of the cancer epigenome. In this review, we provide a brief overview of cancer epigenetics and the epigenetic targets of therapy including deoxyribonucleic acid (DNA) methylation, histone modifications, and chromatin remodeling. We discuss the epigenetic drugs currently in use, with a focus on DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors, and explain the pharmacokinetic and mechanistic challenges in their application. We present the strategies employed in incorporating these drugs into the treatment of GI cancers, and explain the concept of the cancer stem cell in epigenetic reprogramming and reversal of chemo resistance. We discuss the most promising combination strategies in GI cancers including: (1) epigenetic sensitization to radiotherapy, (2) epigenetic sensitization to cytotoxic chemotherapy, and (3) epigenetic immune modulation and priming for immune therapy. Finally, we present preclinical and clinical trial data employing these strategies thus far in various GI cancers including colorectal, esophageal, gastric, and pancreatic cancer.