title: Enhancement of the in vivo antitumor activity of clofarabine by 1-beta-D-[4-thio-arabinofuranosyl]-cytosine.
abstract: OBJECTIVE
  Clofarabine increases the activation of 1-beta-D-arabinofuranosyl cytosine (araC) in tumor cells, and combination of these two drugs has been shown to result in good clinical activity against various hematologic malignancies. 1-beta-D-[4-thio-arabinofuranosyl] cytosine (T-araC) is a new cytosine analog that has exhibited excellent activity against a broad spectrum of human solid tumors and leukemia/lymphoma xenografts in mice and is currently being evaluated in patients as a new drug for the treatment of cancer. Since T-araC has a vastly superior preclinical efficacy profile in comparison to araC, we have initiated studies to determine the potential value of clofarabine/T-araC combination therapy.
  
  
  METHODS
  In vitro studies have been conducted to determine the effect of clofarabine on the metabolism of T-araC, and in vivo studies have been conducted to determine the effect of the clofarabine/T-araC combination on five human tumor xenografts in mice.
  
  
  RESULTS
  Initial studies with various tumor cells in culture indicated that a 2-h incubation with clofarabine enhanced the metabolism of T-araC 24 h after its removal by threefold in three tumor cell types (HCT-116 colon, K562 leukemia, and RL lymphoma) and by 1.5-fold in two other tumor cell types (MDA-MB-435 breast (melanoma), and HL-60 leukemia). Pretreatment with clofarabine resulted in a slight decrease in metabolism of T-araC in RPMI-8226 myeloma cells (65% of control) and inhibited metabolism of T-araC in CCRF-CEM leukemia cells by 90%. In vivo combination studies were conducted with various human tumor xenografts to determine whether or not the modulations observed in vitro were reflective of the in vivo situation. Clofarabine and T-araC were administered on alternate days for five treatments each (q2dx5) with the administration of T-araC 24 h after each clofarabine treatment. Combination treatment of HCT-116, K562, HL-60, or RL tumors with clofarabine and T-araC resulted in dramatically superior anti-tumor activity than treatment with either agent alone, whereas this combination resulted in antagonism in CCRF-CEM tumors. The in vivo antitumor activity of clofarabine plus T-araC against HCT-116 tumors was much better than the activity seen with clofarabine plus araC.
  
  
  CONCLUSIONS
  These studies provide a rationale for clinical trials using this combination in the treatment of acute leukemias as well as solid tumors and suggest that this combination would exhibit greater antitumor activity than that of clofarabine plus araC.
authors.affiliation: Southern Research Institute, 2000 Ninth Avenue South, Birmingham, AL 35205, USA. parker@southernresearch.org
authors.forename: William B
authors.initials: WB
authors.lastname: Parker
authors.forename: Sue C
authors.initials: SC
authors.lastname: Shaddix
authors.forename: Karen S
authors.initials: KS
authors.lastname: Gilbert
authors.forename: Rodney V
authors.initials: RV
authors.lastname: Shepherd
authors.forename: William R
authors.initials: WR
authors.lastname: Waud
mesh_terms: D000227:Adenine Nucleotides / Q000627:therapeutic use*; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001087:Arabinonucleosides / Q000627:therapeutic use*; D000077866:Clofarabine; D004357:Drug Synergism; D004359:Drug Therapy, Combination; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D016513:Mice, SCID; D009374:Neoplasms, Experimental / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D014407:Tumor Cells, Cultured; D023041:Xenograft Model Antitumor Assays
chemical_list: D000227:Adenine Nucleotides; D000970:Antineoplastic Agents; D001087:Arabinonucleosides; D000077866:Clofarabine; C411255:4'-thio-arabinofuranosylcytosine