Abstract. Reactive oxygen species (ROS) are generated in the cell through multiple mechanisms. Intracellular ROS are rapidly detoxified by various enzymatic and non-enzymatic mechanisms; however, disruption of the oxidant-antioxidant balance causes oxidative stress and elicits cell damage. The oxidative stress induced by chemotherapy is known to cause side effects in patients with cancer. However, few studies have examined whether anticancer drugs induce oxidative stress in cancer cells. Furthermore, the precise mechanism by which anticancer drugs induce the generation of ROS remains unclear. In the present study, to investigate whether anticancer drugs induce oxidative stress, DLD-1 human colorectal cancer cells were treated with 20 different anticancer drugs and then stained with CellROX ® ROS detection reagent. Furthermore, an oxygen radical absorbance capacity assay in the presence of copper was performed to estimate the oxidative activities of the anticancer drugs in the absence of cells. The data of the present study using assay methods in the presence and absence of cells suggest that nimustine, actinomycin D, doxorubicin, mitomycin C, mitoxantrone, carmofur, gemcitabine, mercaptopurine, camptothecin, paclitaxel, vinblastine, and vinorelbine are able to induce oxidative stress. IntroductionReactive oxygen species (ROS) include oxygen molecules (O 2 ), superoxide anion radicals (O 2 -), hydroxyl free radicals (HO -) and hydrogen peroxide (H 2 O 2 ). ROS are generated as a result of single-or multi-electron reductions of oxygen by cellular enzymes or in the mitochondrial respiratory pathway (1-5). Although an increase in the level of intracellular ROS leads to oxidative stress and DNA damage, the effects of ROS are normally balanced by antioxidants, such as reduced glutathione (GSH), ascorbic acid, and ureic acid (6). Disruption of the oxidant-antioxidant balance through alterations to cellular homeostasis or defective repair of ROS-induced damage is involved in the pathogenesis of several diseases (7). In particular, it can be the primary trigger and/or mediator of carcinogenesis by contributing to the initiation of cellular malignancy and the progression of cancer (8-10).Furthermore, it is known that anticancer drugs induce oxidative stress in patients with cancer being treated with chemotherapy. Elevated levels of oxidants in the circulation have been reported in patients with cancer following administration of epirubicin (11,12). Epirubicin and doxorubicin possess an anthracycline skeleton, and generate ROS that lead to DNA damage and subsequently antitumor activity (13,14). Vinblastine and vinorelbine belong to the class of vinca alkaloids. It has been demonstrated that vinorelbine depletes intracellular GSH and increases intracellular ROS production (15). However, few studies have investigated the association between oxidative stress and the effects of anticancer drugs.Available methods for measuring oxidative stress inducibility include direct measurement of intracellular ROS, indirect measurement of...