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Department of Medicine, Committee on Cancer Biology and Cancer Research Center, University of Chicago, Chicago, Illinois 60637 [M. L. F., S. M. D., L. D. F., R. J. H., M. E. D.]; Laboratory of Comparative Carcinogenesis, National Cancer Institute at Frederick, Frederick, Maryland 21702-1201 [R. C. M.]; and University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania 15213 [E. G. Z., M. J. E.]

² To whom requests for reprints should be addressed, at 5841 South Maryland Avenue, Box MC2115, University of Chicago, Chicago, IL 60637. Phone: (773) 702-4441; Fax: (773) 702-0963; E-mail: edolan{at}medicine.bsd.uchicago.edu

O⁶-Benzylguanine (O⁶-BG), a potent inactivator of the DNA repair protein O⁶-alkylguanine-DNA alkyltransferase (AGT), is presently in clinical trials combined with alkylating agents that modify the O⁶ position of DNA guanine residues, i.e., 1,3-bis(2-chloroethyl)-1-nitrosourea and temozolomide. Previous work demonstrated that O⁶-BG also enhances the cytotoxicity of cyclophosphamide, ifosfamide, and nitrogen mustards in Chinese hamster ovary cells. We have extended this study to include other clinically relevant agents that form interstrand and intrastrand cross-links including cisplatin and carboplatin. Pretreatment of a series of head and neck tumor cell lines (i.e., SQ20b, JSQ3, SCC25, SCC35, and SCC61), Chinese hamster ovary cells, and HT29 human colon tumor cells with O⁶-BG (100 µM for 2 h before treatment and 2 h during treatment) resulted in a 2-fold decrease in the ED₅₀ of cisplatin and a concomitant increase in the percentage of cells undergoing apoptosis. The enhancement was independent of AGT activity. Similar enhancement was observed with carboplatin, but no enhancement was seen in AGT-deficient cell lines with radiation or temozolomide, demonstrating the dependence of the effect on bifunctional, cross-linking agents. Furthermore, levels of platinum on DNA after treatment with cisplatin increased 1.4-fold in SQ20b cells and 4.5-fold in JSQ3 cells immediately after treatment with O⁶-BG plus cisplatin and remained elevated for 48 h. Consistent with greater cytotoxicity and apoptosis is the 2-fold higher amount of DNA damage when cells are treated with O⁶-BG plus cisplatin compared with cisplatin alone. Modulation of cisplatin therapy with O⁶-BG might improve the prognosis of patients with head and neck, ovarian, testicular, or lung cancer who are treated with this drug.