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It's About Time: Scheduling Alters Effect of Histone Deacetylase Inhibitors on Camptothecin-Treated Cells

¹ Graduate Center for Toxicology and ² Department of Microbiology, Immunology and Molecular Genetics, University of Kentucky, Lexington, Kentucky

Requests for reprints: Stephen G. Zimmer, Department of Microbiology, Immunology and Molecular Genetics, L.P. Markey Cancer Center, University of Kentucky, Room 309, Combs Research Building, 800 Rose Street, Lexington, KY 40536. Phone: 859-323-5753; Fax: 859-257-9608; E-mail: szimm1{at}uky.edu.

Chemotherapeutic treatment with combinations of drugs is front-line therapy for many types of cancer. Combining drugs which target different signaling pathways often lessens adverse side effects while increasing the efficacy of treatment and reducing patient morbidity. A defined scheduling protocol is described by which histone deacetylase inhibitors (HDIs) facilitate the cytotoxic effectiveness of the topoisomerase I inhibitor camptothecin in the killing of tumor cells. Breast and lung cancer cell lines were treated with camptothecin and sodium butyrate (NaB) or suberoylanilide hydroxamic acid on the day of, the day before, or the day after camptothecin addition. Depending on the time of addition, NaB-treated cells displayed a spectrum of responses from protection to sensitization, indicating the critical nature of timing in the use of HDIs. The IC₈₀ (72-hour assay) dose of 100 nmol/L camptothecin could be lowered to 15 nmol/L camptothecin while maintaining or surpassing cell killing of the single agent if combined with an HDI added 24 to 48 hours after camptothecin. Experiments determined that cells arrested in G₂-M by camptothecin were most sensitive to subsequent HDI addition. Western blot analysis indicated that in camptothecin-arrested cells, NaB decreases cyclin B levels, as well as the levels of the antiapoptotic proteins XIAP and survivin. These findings suggest that reducing the levels of these critical antiapoptotic factors may increase the efficacy of topoisomerase I inhibitors in the clinical setting if given in a sequence that does not prevent or inhibit tumor cell progression through the S phase.

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