OBJECTIVE The objective of this study was to explore the effect of CDA-2, a selective inhibitor of abnormal methylation enzymes in cancer cells, on the therapeutic efficacy of cytotoxic chemotherapy.METHODS Advanced cancer patients, all of whom had previously undergone chemotherapy, were randomly divided into 2 groups, one receiving chemotherapy only as the control group, and the other receiving CDA-2 in addition to chemotherapy as the combination group. The therapeutic efficacies and the toxic manifestations of the 2 groups were compared based on the WHO criteria.RESULTS Of 454 cancer patients enrolled in phase Ⅲ clinical trials of CDA-2, 80, 188, and 186 were breast cancer,NSCLC, and primary hepatoma patients, respectively.Among them 378 patients completed treatments according to the protocols. The results showed that the overall effective rate of the combination group was 2.6 fold that of the control group, 4.8 fold in the case of breast cancer, 2.3 fold in the case of primary hepatoma, and 2.2 fold in the case of NSCLC. Surprisingly, the combination therapy appeared to work better for stage Ⅳ than stage Ⅲ patients. CDA-2 did not contribute additional toxicity. On the contrary, it reduced toxic manifestations of chemotherapy, particularly regarding white blood cells, nausea and vomiting.CONCLUSION Modulation of abnormal methylation enzymes by CDA-2 is definitely helpful to supplement chemotherapy. It significantly increased the therapeutic efficacy and reduced the toxic manifestation of cytotoxic chemotherapy on breast cancer and NSCLC.