A Novel Antitumor Antibiotic, KW‐2189 Is Activated by Carboxyl Esterase and Induces DNA Strand Breaks in Human Small Cell Lung Cancer Cells

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Abstract
KW‐2189 has been selected as a lead compound for clinical trial among duocarmycin derivatives with structural similarity to CC‐1065, a cyclopropylpyrroloindole. The purpose of this study was to examine the DNA‐binding potency and the mechanisms of cytotoxicity of KW‐2189. In order to analyze DNA‐binding activity of KW‐2189, plasmid pBR322 was treated with KW‐2189 with or without pretreatment with carboxyl esterase, which we demonstrated to be an activating enzyme, and the products were examined by agarose gel electrophoresis and restriction enzyme analysis. Cytotoxic activity was examined by exposing a human small cell lung cancer cell line, NCI‐H69 to KW‐2189 with or without carboxyl esterase. Alkaline elution was performed to examine whether KW‐2189 induces DNA strand breaks. DNA treated with KW‐2189 and carboxyl esterase migrated faster than KW‐2189‐treated DNA, which migrated at the same rate as untreated DNA. In addition DNA treated with esterase‐activated KW‐2189 was protected from digestion by some restriction enzymes. KW‐2189 showed concentration‐ and time‐dependent growth inhibitory effect with IC50 values (drug concentration required for 50% growth inhibition) of 58 nM (96 h) to 1900 nM (1 h) in H69 cells. The IC50 values of 4‐h exposure of H69 to KW‐2189 with 0, 26, 130, 650 mU/ml carboxyl esterase were 460, 120, 30, and 7 nM, respectively. Time‐dependent enhancement of cytotoxicity by carboxyl esterase was also observed. KW‐2189 induced DNA strand breaks in H69 cells in a concentration‐dependent manner around the IC50 value. We conclude that 1) KW‐2189 is activated by carboxyl esterase to its active form(s), 2) activated KW‐2189 has a stronger DNA‐binding activity and cytotoxicity than KW‐2189, 3) DNA cleavage is one of the major mechanisms of KW‐2189‐mediated cytotoxicity.