title: Sequence-dependent interaction of 5-fluorouracil and arabinosyl-5-azacytosine or 1-beta-D-arabinofuranosylcytosine.
abstract: We studied the cytotoxicity of arabinosyl-5-azacytosine (Ara-AC), a dCyd antagonist which inhibits DNA synthesis, in combination with 5-fluorouracil (FUra) in two human colon cancer cell lines, HCT 116 and SNU-C4. Clonogenic assays done following sequential or concurrent 24-hr exposures to Ara-AC and FUra showed that the sequence Ara-AC followed by FUra resulted in more than additive lethality in the HCT 116 cell lines and additive lethality in the SNU-C4 cells. In contrast, the reverse sequence, FUra followed by Ara-AC, was antagonistic in both cell lines. A similar interaction between FUra and 1-beta-D-arabinofuranosylcytosine (Ara-C) was evident in HCT 116 cells; at concentrations which individually diminished viability by 34 and 62%, respectively, the sequence Ara-C followed by FUra decreased viability by 97%. Pulse-labeling with [3H]dUrd showed profound inhibition of DNA synthesis by the sequence Ara-AC followed by FUra, with over 90% inhibition lasting for up to 48 hr following Ara-AC exposure. When FUra preceded Ara-AC, however, earlier recovery from inhibition of DNA synthesis occurred. FUra pretreatment did not appreciably alter the quantity or distribution of [3H]Ara-AC or [3H]Ara-C nucleotides after a 4- to 6-hr exposure. Pre-exposure to FUra decreased Ara-AC incorporation into DNA by 37 and 73% at 6 hr in HCT 116 and SNU-C4, respectively. FUra pretreatment also inhibited Ara-C incorporation into DNA by over 50% at 6 and 24 hr. The antagonism of Ara-AC and Ara-C cytotoxicity by FUra pretreatment can thus be explained by diminished incorporation of the dCyd analogs into DNA resulting from inhibition of DNA synthesis by FUra-induced dTTP and dCTP depletion. In contrast, when Ara-AC or Ara-C preceded FUra, their incorporation into DNA was not disturbed, and prolonged inhibition of DNA synthesis was observed.
authors.affiliation: Gastrointestinal Tumor Section, National Cancer Institute, Bethesda, MD 20892.
authors.forename: J L
authors.initials: JL
authors.lastname: Grem
authors.forename: C J
authors.initials: CJ
authors.lastname: Allegra
mesh_terms: D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use; D001374:Azacitidine / Q000008:administration & dosage / Q000494:pharmacology* / Q000031:analogs & derivatives; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line / Q000187:drug effects; D002467:Cell Nucleus / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D003110:Colonic Neoplasms / Q000188:drug therapy / Q000473:pathology*; D003561:Cytarabine / Q000008:administration & dosage / Q000494:pharmacology*; D003562:Cytidine / Q000037:antagonists & inhibitors; D004273:DNA, Neoplasm / Q000096:biosynthesis; D003845:Deoxycytosine Nucleotides / Q000378:metabolism; D004334:Drug Administration Schedule; D004347:Drug Interactions; D005472:Fluorouracil / Q000008:administration & dosage / Q000494:pharmacology*; D006801:Humans; D009696:Nucleic Acids / Q000378:metabolism; D013942:Thymine Nucleotides / Q000378:metabolism; D014410:Tumor Stem Cell Assay
chemical_list: D004273:DNA, Neoplasm; D003845:Deoxycytosine Nucleotides; D009696:Nucleic Acids; D013942:Thymine Nucleotides; D003561:Cytarabine; C024107:2'-deoxycytidine 5'-triphosphate; D003562:Cytidine; C022121:fazarabine; D001374:Azacitidine; C024157:thymidine 5'-triphosphate; D005472:Fluorouracil