title: Fludarabine-mediated circumvention of cytarabine resistance is associated with fludarabine triphosphate accumulation in cytarabine-resistant leukemic cells.
abstract: The combination of cytarabine (ara-C) with fludarabine is a common approach to treating resistant acute myeloid leukemia. Success depends on a fludarabine triphosphate (F-ara-ATP)-mediated increase in the active intracellular metabolite of ara-C, ara-C 5'-triphosphate (ara-CTP). Therapy-resistant leukemia may exhibit ara-C resistance, the mechanisms of which might induce cross-resistance to fludarabine with reduced F-ara-ATP formation. The present study evaluated the effect of combining ara-C and fludarabine on ara-C-resistant leukemic cells in vitro. Two variant cell lines (R1 and R2) were 8-fold and 10-fold more ara-C resistant, respectively, than the parental HL-60 cells. Reduced deoxycytidine kinase activity was demonstrated in R1 and R2 cells, and R2 cells also showed an increase in cytosolic 5'-nucleotidase II activity. Compared with HL-60 cells, R1 and R2 cells produced smaller amounts of ara-CTP. Both variants accumulated less F-ara-ATP than HL-60 cells and showed cross-resistance to fludarabine nucleoside (F-ara-A). R2 cells, however, accumulated much smaller amounts of F-ara-ATP and were more F-ara-A resistant than R1 cells. In HL-60 and R1 cells, F-ara-A pretreatment followed by ara-C incubation produced F-ara-ATP concentrations sufficient for augmenting ara-CTP production, thereby enhancing ara-C cytotoxicity. No potentiation was observed in R2 cells. Nucleotidase might preferentially degrade F-ara-A monophosphate over ara-C monophosphate, leading to reduced F-ara-ATP production and thereby compromising the F-ara-A-mediated potentiation of ara-C cytotoxicity in R2 cells. Thus, F-ara-A-mediated enhancement of ara-C cytotoxicity depended on F-ara-ATP accumulation in ara-C-resistant leukemic cells but ultimately was associated with the mechanism of ara-C resistance.
authors.affiliation: Department of Hematology and Oncology, University of Fukui, Fukui, Japan.
authors.forename: Shuji
authors.initials: S
authors.lastname: Yamamoto
authors.forename: Takahiro
authors.initials: T
authors.lastname: Yamauchi
authors.forename: Yasukazu
authors.initials: Y
authors.lastname: Kawai
authors.forename: Haruyuki
authors.initials: H
authors.lastname: Takemura
authors.forename: Shinji
authors.initials: S
authors.lastname: Kishi
authors.forename: Akira
authors.initials: A
authors.lastname: Yoshida
authors.forename: Yoshimasa
authors.initials: Y
authors.lastname: Urasaki
authors.forename: Hiromichi
authors.initials: H
authors.lastname: Iwasaki
authors.forename: Takanori
authors.initials: T
authors.lastname: Ueda
mesh_terms: D001085:Arabinofuranosylcytosine Triphosphate / Q000819:agonists / Q000378:metabolism / Q000494:pharmacology*; D003561:Cytarabine / Q000494:pharmacology*; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D004357:Drug Synergism; D018922:HL-60 Cells; D006801:Humans; D015470:Leukemia, Myeloid, Acute / Q000188:drug therapy* / Q000378:metabolism; D014740:Vidarabine / Q000819:agonists / Q000031:analogs & derivatives* / Q000494:pharmacology
chemical_list: D003561:Cytarabine; D001085:Arabinofuranosylcytosine Triphosphate; D014740:Vidarabine; C024352:fludarabine