The aim of this study was to investigate the antiproliferative effects of C-1311 (Symadex), a member of the imidazoacridinone family, in human colorectal cancer cells. In the in vitro screen, C-1311 led to the most prominent growth inhibition in HT29, HCT116, and COLO205 cell lines when compared to oxaliplatin, CPT-11, DFUR, 5-FU and capecitabine. The GI 50 values for C-1311 ranged from 0.12 to 0.83 μM and the TGI concentrations (resulting in total growth inhibition) were 6- to 13-fold lower than those of other agents. In the hollow fiber assay In Vivo, C-1311 caused 77% growth inhibition of HT29 in the intraperitoneal site as compared to paclitaxel (17% growth inhibition). In the subcutaneous site, C-1311 produced 57% growth inhibition while paclitaxel showed no cell growth inhibition effects. This unique cytotoxicity profile of C-1311 warrants further investigation and supports its clinical development in colon cancer patients. Symadex (C-1311) is currently in phase 2 clinical trials.