Pibrozelesin (KW-2189) is a semisynthetic water-soluble derivative of the antineoplastic antibiotic duocarmycin B2. Activated by carboxyl esterase, pibrozelesin alkylates DNA by binding to adenine-thymine (A-T)-rich sequences in the minor groove of DNA, thereby inhibiting DNA replication and inducing apoptosis. KW-2189 induced DNA strand breaks in H69 cells in a concentration-dependent manner. DNA cleavage is one of the major mechanisms of KW-2189-mediated cytotoxicity. KW-2189 showed evidence of anti-tumor activity in hepatocellular carcinoma (HCC). However, because of significant and prolonged hematologic toxicity, when given as a single dose every 6 weeks, further development of this drug in HCC is not possible. Pibrozelesin had been in phase II clinical trial for the treatment of advanced malignant melanoma and advanced renal cell carcinoma. No activity in metastatic renal cell carcinoma was demonstrated and the lack of significant antitumor activity in advanced malignant melanoma treatment was shown.