We investigated the effects of epigenetic modifiers such as DNA methyltransferase (DNMT) or histone deacetylase (HDAC) inhibitors on the cytotoxicity induced by 3 anticancer drugs (5-fluorouracil (5-FU), irinotecan (CPT-11) or its active form SN38, and oxaliplatin (L-OHP)) in human colorectal cancer (CRC) cells. Cytotoxicity in 4 CRC cell lines (HT29, SW480, SW48 and HCT116) was examined by colorimetric assay after drug treatment for 72 h. The effects of drug combinations were analyzed by an isobologram method. SW480 cells showed the lowest sensitivity to cytotoxicity induced by the anticancer drugs among the 4 CRC cell lines. In SW480 cells, DNMT inhibitors, such as decitabine (DAC), azacytidine and zebularine (Zeb), showed synergic effects on the cytotoxicity induced by anticancer drugs except for SN-38 plus Zeb, while HDAC inhibitors, trichostatin A, suberoylanilide hydroxamic acid and valproic acid, showed antagonistic effects. DAC showed the most potent synergic effects among the epigenetic modifiers studied. Thus, we examined whether the synergic effect of DAC is observed in other different CRC cell lines, HT29, SW48 and HCT116 cells. In all 4 CRC cell lines, the cytotoxicity of L-OHP was enhanced in a synergic manner by cotreatment with DAC. However, synergic effects of DAC with 5-FU or CPT-11 (SN-38) were not observed in 4 CRC cell lines.Key words DNA methyltransferase inhibitor; synergism; colorectal cancer; isobologram; histone deacetylase inhibitor Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the world.1) CRC treatment strategies are highly dependent on tumor stage. Metastatic CRC (stage IV) is treated with combined chemotherapy, including 5-fluorouracil (5-FU) plus either irinotecan (CPT-11) or oxaliplatin (L-OHP). Recently, it has been reported that combined chemotherapy plus biological agents (anti-angiogenic or antiepidermal growth factor receptor molecules) increased stage IV patients' median survival period.2) Despite many therapeutic options, an efficient drug combination remains to be found for CRC patients. In addition, tailored therapy for each CRC patient is still a challenge.It is well-established that epigenetic alterations such as DNA methylation and histone modifications play a crucial role in the initiation and progression of cancer, including CRC. Unlike genetic alterations, epigenetic alterations are reversible and can be targeted by epigenetic modifiers.3) Epigenetic modifiers such as DNA methyltransferase (DNMT) inhibitors and histone deacetylase (HDAC) inhibitors have been administered for several years and have been evaluated as a treatment approach for a variety of cancers in the United States. DNMT inhibitors, such as decitabine (5-aza-2′-deoxycytidine; DAC) and 5-azacytidine (AC), are being used for the treatment of myelodysplastic syndromes (MDS), but the efficacy is variable. 4,5) In addition, DAC is used usually for patients with acute myeloid leukemia who are not candidates for standard remission, including chemotherapy. 6) On the other h...