title: The interaction between two radiosensitizers: 5-iododeoxyuridine and caffeine.
abstract: 5-Iododeoxyuridine (IUdR) and caffeine are recognized as potential radiosensitizers with different mechanisms of interaction with ionizing radiation (IR). To assess the interaction of these two types of radiosensitizers, we compared treatment responses to these drugs alone and in combination with IR in two p53-proficient and p53-deficient pairs of human colon cancer cell lines (HCT116 versus HCT116 p53-/- and RKO versus RKO E6). Based on clonogenic survival, the three single agents (IR, IUdR, and caffeine) as well as IUdR or caffeine combined with IR are less or equally effective in p53-deficient human tumor cells compared with p53-proficient tumor cells. However, using both radiosensitizers, a significantly greater radiosensitization was found in p53-deficient human tumor cells. To better understand the interaction of these two radiosensitizers, additional studies on DNA repair and cell cycle regulation were done. We found that caffeine enhanced IUdR-DNA incorporation and IUdR-mediated radiosensitization by partially inhibiting repair (removal) of IUdR in DNA. The repair of IR-induced DNA double-strand breaks was also inhibited by caffeine. However, these effects of caffeine on IUdR-mediated radiosensitization were not found in p53-proficient cells. Cell cycle analyses also showed a greater abrogation of IR-induced S- and G2-phase arrests by caffeine in p53-deficient cells, particularly when combined with IUdR. Collectively, these data provide the mechanistic bases for combining these two radiosensitizers to enhance tumor cytotoxicity. This differential dual mode of radiosensitization by combining IUdR and caffeine-like drugs (e.g., UCN-01) in p53-deficient human tumors may lead to a greater therapeutic gain.
authors.affiliation: Department of Radiation Oncology, Case Western Reserve University School of Medicine and University Hospitals of Cleveland/Ireland Cancer Center, Cleveland, Ohio 44106-6068, USA.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Seo
authors.forename: Tao
authors.initials: T
authors.lastname: Yan
authors.forename: Jane E
authors.initials: JE
authors.lastname: Schupp
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Yamane
authors.forename: Tomas
authors.initials: T
authors.lastname: Radivoyevitch
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Kinsella
mesh_terms: D020137:Base Pair Mismatch; D002110:Caffeine / Q000494:pharmacology*; D004260:DNA Repair; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D045325:HCT116 Cells; D006801:Humans; D007065:Idoxuridine / Q000494:pharmacology*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D016159:Tumor Suppressor Protein p53 / Q000172:deficiency / Q000502:physiology
chemical_list: D011838:Radiation-Sensitizing Agents; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; D002110:Caffeine; D007065:Idoxuridine