title: Potential use of imatinib in Ewing's Sarcoma: evidence for in vitro and in vivo activity.
abstract: BACKGROUND
  Ewing's sarcoma cells express c-kit, a receptor tyrosine kinase, and its ligand, stem cell factor (SCF), creating a potential autocrine loop that may promote tumor survival. We thus examined whether the specific tyrosine kinase inhibitor imatinib mesylate (hereafter imatinib; formerly STI571) could inhibit the proliferation of Ewing's sarcoma cells in vitro and in vivo.
  
  
  METHODS
  The effect of imatinib on c-kit expression and phosphorylation in Ewing's sarcoma cells was examined by immunoblotting. The effect of imatinib on cell growth and apoptosis was examined with an MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay and with a morphologic test and Annexin V staining, respectively. The effect of imatinib oral therapy (every 12 hours for 5-7 days) on primary tumor growth was assessed in Ewing's sarcoma xenografts in SCID/bg mice (5 or 10 mice per group).
  
  
  RESULTS
  All Ewing's sarcoma cell lines tested were sensitive to imatinib-mediated apoptosis with a concentration inhibiting growth by 50% (IC50) of 10-12 micro M. Imatinib inhibited SCF-mediated c-kit phosphorylation (IC50 = 0.1-0.5 microM). In the xenograft model, imatinib treatment resulted in the regression or control of primary Ewing's sarcomas. After 6 days of treatment, the mean lower extremity volume including xenograft tumor was 3744 mm3 (95% confidence interval [CI] = 3050 to 4437 mm3), 1442 mm3 (95% CI = 931 to 1758 mm3), and 346 mm3 (95% CI = 131 to 622 mm3) in mice treated with carrier alone or with imatinib at 50 mg/kg or at 100 mg/kg, respectively.
  
  
  CONCLUSIONS
  Imatinib interferes with growth of all Ewing's sarcoma cell lines tested in vitro and in vivo. Targeted inhibition of tyrosine kinase-dependent autocrine loops, therefore, may be a viable therapeutic strategy for Ewing's sarcoma.
authors.affiliation: Pediatric Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA. merchanm@mail.nih.gov
authors.forename: Melinda S
authors.initials: MS
authors.lastname: Merchant
authors.forename: Chan-Wook
authors.initials: CW
authors.lastname: Woo
authors.forename: Crystal L
authors.initials: CL
authors.lastname: Mackall
authors.forename: Carol J
authors.initials: CJ
authors.lastname: Thiele
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D001549:Benzamides; D001859:Bone Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000276:immunology / Q000378:metabolism*; D004195:Disease Models, Animal; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D000068877:Imatinib Mesylate; D015151:Immunoblotting; D020128:Inhibitory Concentration 50; D010766:Phosphorylation / Q000187:drug effects; D010879:Piperazines / Q000494:pharmacology*; D011233:Precipitin Tests; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D019009:Proto-Oncogene Proteins c-kit / Q000187:drug effects* / Q000378:metabolism; D011743:Pyrimidines / Q000494:pharmacology*; D012512:Sarcoma, Ewing / Q000188:drug therapy* / Q000201:enzymology / Q000276:immunology / Q000378:metabolism*; D019089:Stem Cell Factor / Q000378:metabolism*; D013997:Time Factors; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D001549:Benzamides; D004791:Enzyme Inhibitors; D010879:Piperazines; D011743:Pyrimidines; D019089:Stem Cell Factor; D000068877:Imatinib Mesylate; D011505:Protein-Tyrosine Kinases; D019009:Proto-Oncogene Proteins c-kit