title: Anti-tumor innate immunity activated by intermittent metronomic cyclophosphamide treatment of 9L brain tumor xenografts is preserved by anti-angiogenic drugs that spare VEGF receptor 2.
abstract: BACKGROUND
  Metronomic cyclophosphamide given on an intermittent, 6-day repeating schedule, but not on an exposure dose-equivalent daily schedule, activates an anti-tumor innate immune response that leads to major regression of large implanted gliomas, without anti-angiogenesis.
  
  
  METHODS
  Mice bearing implanted 9L gliomas were used to investigate the effects of this 6-day repeating, immunogenic cyclophosphamide schedule on myeloid-derived suppressor cells, which are pro-angiogenic and can inhibit anti-tumor immunity, and to elucidate the mechanism whereby the innate immune cell-dependent tumor regression response to metronomic cyclophosphamide treatment is blocked by several anti-angiogenic receptor tyrosine kinase inhibitors.
  
  
  RESULTS
  Intermittent metronomic cyclophosphamide scheduling strongly increased glioma-associated CD11b+ immune cells but not CD11b+Gr1+ myeloid-derived suppressor cells, while bone marrow and spleen reservoirs of the suppressor cells were decreased. The inhibition of immune cell recruitment and tumor regression by anti-angiogenic receptor tyrosine kinase inhibitors, previously observed in several brain tumor models, was recapitulated in the 9L tumor model with the VEGFR2-specific inhibitory monoclonal antibody DC101 (p < 0.01), implicating VEGFR2 signaling as an essential step in metronomic cyclophosphamide-stimulated immune cell recruitment. In contrast, sorafenib, a multi-receptor tyrosine kinase inhibitor with comparatively weak VEGF receptor phosphorylation inhibitory activity, was strongly anti-angiogenic but did not block metronomic cyclophosphamide-induced innate immunity or tumor regression (p > 0.05).
  
  
  CONCLUSIONS
  The interference by receptor tyrosine kinase inhibitors in the immunogenic actions of intermittent metronomic chemotherapy is not a consequence of anti-angiogenesis per se, as demonstrated in an implanted 9L tumor model. Furthermore, this undesirable interaction with tyrosine kinase inhibitors can be avoided by using anti-angiogenic drugs that spare the VEGFR2 pathway.
authors: Joshua C
authors: JC
authors: Doloff
authors: Chong-Sheng
authors: CS
authors: Chen
authors: Division of Cell and Molecular Biology, Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA. djw@bu.edu.
authors: David J
authors: DJ
authors: Waxman
mesh_terms: D059250:Administration, Metronomic; D020533:Angiogenesis Inhibitors / Q000008:administration & dosage; D000818:Animals; D001932:Brain Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D003520:Cyclophosphamide / Q000008:administration & dosage*; D006801:Humans; D007113:Immunity, Innate / Q000187:drug effects*; D051379:Mice; D040301:Vascular Endothelial Growth Factor Receptor-2 / Q000096:biosynthesis*; D023041:Xenograft Model Antitumor Assays
chemical_list: D020533:Angiogenesis Inhibitors; D003520:Cyclophosphamide; D040301:Vascular Endothelial Growth Factor Receptor-2