title: Time and sequence dependence of hydroxyurea in combination with gemcitabine in human KB cells.
abstract: Gemcitabine (Gem) is a deoxycytidine analogue whose active metabolite, dFdCTP, blocks DNA elongation and has a cytotoxic effect. Hydroxyurea (HU) is an S-phase specific inhibitor of ribonucleotide reductase (RR) with a broad spectrum of antitumor effects. We report here that low-dose HU enhanced the activity of Gem in a time- and sequence-dependent manner. Exposure of human oropharyngeal carcinoma KB cells to HU followed by the addition of Gem at various times significantly enhanced cytotoxicity when compared to controls. The greatest enhancement of cytotoxicity occurred when Gem was added 8 hours after HU. By treating KB cells with radiolabeled-Gem following HU treatment, we further confirmed that the incorporation of dFdCTP into DNA increased 6-fold over control reactions under these conditions. The mechanism of the time- and sequence-dependent enhancement is associated with a decrease in hRRM2 RNA, protein, and activity between 4 and 8 hours. The subsequent depletion of dNTP pools allows for increased incorporation of dFdCTP into cells arrested in S-phase, resulting in higher levels of cytotoxicity than either treatment alone.
authors.affiliation: Department of Medical Oncology and Therapeutics Research, City of Hope National Medical Center, Duarte, CA 91010, USA.
authors.forename: Bingsen
authors.initials: B
authors.lastname: Zhou
authors.forename: Shu
authors.initials: S
authors.lastname: Mi
authors.forename: Xueli
authors.initials: X
authors.lastname: Mo
authors.forename: Jennifer
authors.initials: J
authors.lastname: Shih
authors.forename: Jeffery
authors.initials: J
authors.lastname: Tsai
authors.forename: Edward
authors.initials: E
authors.lastname: Hu
authors.forename: Margaret
authors.initials: M
authors.lastname: Hsu
authors.forename: Kristie
authors.initials: K
authors.lastname: Kay
authors.forename: Yun
authors.initials: Y
authors.lastname: Yen
mesh_terms: D000971:Antineoplastic Combined Chemotherapy Protocols / Q000008:administration & dosage*; D015152:Blotting, Northern; D015153:Blotting, Western; D002453:Cell Cycle / Q000187:drug effects; D003565:Cytidine Diphosphate / Q000378:metabolism; D004273:DNA, Neoplasm / Q000378:metabolism; D003841:Deoxycytidine / Q000008:administration & dosage* / Q000031:analogs & derivatives*; D003854:Deoxyribonucleotides / Q000378:metabolism; D004334:Drug Administration Schedule; D004357:Drug Synergism; D006801:Humans; D006918:Hydroxyurea / Q000008:administration & dosage*; D007624:KB Cells; D012262:Ribonucleoside Diphosphate Reductase / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D000093542:Gemcitabine
chemical_list: D004273:DNA, Neoplasm; D003854:Deoxyribonucleotides; D003841:Deoxycytidine; D003565:Cytidine Diphosphate; C093713:ribonucleotide reductase M2; D012262:Ribonucleoside Diphosphate Reductase; D006918:Hydroxyurea; D000093542:Gemcitabine