AbstractIn this study, we first demonstrated that loratadine (LOR), a promising world widely used oral anti‐histamine, effectively inhibits growth of tumors derived from human colon cancer cells (COLO 205) in an in vivo setting. In vitro study demonstrated that the anti‐tumor effects of LOR in COLO 205 cells were mediated by causing G2/M phase cell growth cycle arrest and caspase 9‐mediated apoptosis. Cell‐cycle arrest induced by LOR (75 µM, 24 h) was associated with a significant decrease in protein levels of cyclin B1, cell division cycle (Cdc) 25B, and Cdc25C, leading to accumulation of Tyr‐15‐phosphorylated Cdc2 (inactive form). Interestingly, LOR (75 µM, for 4 h) treatment also resulted in a rapid and sustained phosphorylation of Cdc25C at Ser‐216, leading to its translocation from the nucleus to the cytoplasm because of increased binding with 14‐3‐3. We further demonstrated that the LOR‐induced Cdc25C (Ser‐216) phosphorylation was blocked in the presence of checkpoint kinase 1 (Chk1) specific inhibitor (SB‐218078). The cells treated with LOR in the presence of Chk1 specific inhibitor (SB‐218078) were then released from G2/M arrest into apoptosis. These results implied that Chk1‐mediated phosphorylation of Cdc25C plays a major role in response to LOR‐mediated G2/M arrest. Although the Chk1‐mediated cell growth arrest in response to DNA damage is well documented, our results presented in this study was the first report to describe the Chk1‐mediated G2/M cell‐cycle arrest by the histamine H1 antagonist, LOR. © 2006 Wiley‐Liss, Inc.