title: Synergistic effect of combination topotecan and chronomodulated radiation therapy on xenografted human nasopharyngeal carcinoma.
abstract: OBJECTIVE
  To investigate the in vivo chronomodulated radiosensitizing effect of topotecan (TPT) on human nasopharyngeal carcinoma (NPC) and its possible mechanisms.
  
  
  METHODS
  Xenografted BALB/c (nu/nu) NPC mice were synchronized with an alternation of 12 hours of light from 0 to 12 hours after light onset (HALO) and 12 hours of darkness to establish a unified biological rhythm. Chronomodulated radiosensitization of TPT was investigated by analysis of tumor regrowth delay (TGD), pimonidazole hydrochloride, histone H2AX phosphorylation, (γ-H2AX) topoisomerase I (Top I), cell cycle, and apoptosis after treatment with (1) TPT (10 mg/kg) alone; (2) radiation therapy alone (RT); and (3) TPT+RT at 3, 9, 15, and 21 HALO. The tumor-loaded mice without any treatment were used as controls.
  
  
  RESULTS
  The TPT+RT combination was more effective than TPT or RT as single agents. The TPT+RT combination at 15 HALO was best (TGD = 58.0 ± 3.6 days), and TPT+RT at 3 HALO was worst (TGD = 35.0 ± 1.5 days) among the 4 TPT+RT groups (P<.05). Immunohistochemistry analysis revealed a significantly increased histone H2AX phosphorylation expression and decreased pimonidazole hydrochloride expression in the TPT+RT group at the same time point. The results suggested that the level of tumor hypoxia and DNA damage varied in a time-dependent manner. The expression of Top I in the TPT+RT group was also significantly different from the control tumors at 15 HALO (P<.05). Cell apoptosis index was increased and the proportion of cells in S phase was decreased (P<.05) with the highest value in 15 HALO and the lowest in 3 HALO.
  
  
  CONCLUSIONS
  This study suggested that TPT combined with chronoradiotherapy could enhance the radiosensitivity of xenografted NPC. The TPT+RT group at 15 HALO had the best therapeutic effect. The chronomodulated radiosensitization mechanisms of TPT might be related to circadian rhythm of tumor hypoxia, cell cycle redistribution, DNA damage, and expression of Top I.
authors.affiliation: Department of Oncology, Affiliated Hospital of Luzhou Medical College, Luzhou, China.
authors.forename: YanLing
authors.initials: Y
authors.lastname: Zhang
authors.forename: Xin
authors.initials: X
authors.lastname: Chen
authors.forename: PeiRong
authors.initials: P
authors.lastname: Ren
authors.forename: Zhou
authors.initials: Z
authors.lastname: Su
authors.forename: HongYing
authors.initials: H
authors.lastname: Cao
authors.forename: Jie
authors.initials: J
authors.lastname: Zhou
authors.forename: XiaoYan
authors.initials: X
authors.lastname: Zou
authors.forename: ShaoZhi
authors.initials: S
authors.lastname: Fu
authors.forename: Sheng
authors.initials: S
authors.lastname: Lin
authors.forename: Juan
authors.initials: J
authors.lastname: Fan
authors.forename: Bo
authors.initials: B
authors.lastname: Yang
authors.forename: XiaoYang
authors.initials: X
authors.lastname: Sun
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhou
authors.forename: Yue
authors.initials: Y
authors.lastname: Chen
authors.forename: LingLin
authors.initials: L
authors.lastname: Yang
authors.forename: JingBo
authors.initials: J
authors.lastname: Wu
mesh_terms: D000818:Animals; D002277:Carcinoma; D015687:Cell Hypoxia; D019454:Chronotherapy / Q000379:methods*; D003131:Combined Modality Therapy / Q000379:methods; D004249:DNA Damage; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D006657:Histones / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D000077274:Nasopharyngeal Carcinoma; D009303:Nasopharyngeal Neoplasms / Q000378:metabolism / Q000517:prevention & control / Q000532:radiotherapy*; D009364:Neoplasm Recurrence, Local / Q000517:prevention & control; D009593:Nitroimidazoles / Q000378:metabolism; D010766:Phosphorylation; D011836:Radiation Tolerance / Q000187:drug effects*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D013997:Time Factors; D059004:Topoisomerase I Inhibitors / Q000494:pharmacology*; D019772:Topotecan / Q000494:pharmacology*; D014183:Transplantation, Heterologous
chemical_list: C468783:H2AX protein, human; D006657:Histones; D009593:Nitroimidazoles; D011838:Radiation-Sensitizing Agents; D059004:Topoisomerase I Inhibitors; C033815:pimonidazole; D019772:Topotecan; D004264:DNA Topoisomerases, Type I