Anti-tumor drug candidate 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole induces single-strand breaks and DNA-protein cross-links in sensitive MCF-7 breast cancer cells

Purpose: The fluorinated benzothiazole analogue 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203, NSC 703786) exhibits selective and potent anticancer activity, and its lysylamide prodrug (Phortress, NSC 710305) recently entered Phase I clinical trials in the United Kingdom. Only cancer cells sensitive to the anti-proliferative effects of 5F 203 deplete this drug candidate from nutrient media. 5F 203 induces cell cycle arrest, cytochrome P450 1A1 (CYP 1A1) mRNA and protein expression, and is metabolized into reactive electrophilic species that can covalently bind to DNA and form adducts in sensitive (i.e., MCF-7) but not in resistant (i.e., MDA-MB-435) breast cancer cells. Methods: In this present study, we investigated additional anticancer effects of 5F 203 in MCF-7 cells. In addition, we sought to determine if cells deficient in the xeroderma pigmentosum D gene, a gene critical in DNA repair, would show greater sensitivity to the cytotoxic effects of 5F 203 than those complemented with XPD. Results: Alkaline Elution revealed that 5F 203 induced single-strand breaks and DNA-protein cross-links in sensitive MCF-7 cells. In contrast, we detected no double-strand breaks or protein-associated strand breaks typically associated with topoisomerase I (top1) or topoisomerase II (top2) inhibition. In addition, 5F 203 was unable to trap top1- or top2-DNA cleavage complexes in MCF-7 cells. 5F 203 induced cell cycle arrest in MCF-7 cells following DNA damage after brief exposures. Cells deficient in the nucleotide excision repair xeroderma pigmentosum group D (XPD) gene displayed sensitivity to 5F 203 while cells complemented with XPD displayed resistance to 5F 203. Conclusion: These data suggest that the anti-cancer activity of 5F 203 depends upon targets other than top1 or top2 and on the ability of this benzothiazole to form single-strand breaks and DNA-protein cross-links in cancer cells.

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