title: Platelets take up the monoclonal antibody bevacizumab.
abstract: OBJECTIVE
  One of the key factors that promotes angiogenesis is vascular endothelial growth factor (VEGF). Platelets are the main source of VEGF in blood and contribute to angiogenesis by release of growth factors, including VEGF, from their alpha-granules on activation. The monoclonal antibody bevacizumab blocks VEGF in the blood of patients within hours after administration. Platelets are known to endocytose plasma proteins including immunoglobulins. We tested the hypothesis that platelets take up bevacizumab.
  
  
  METHODS
  Fluorescence-activated cell sorting analysis, immunofluorescence imaging, and Western blotting were used to study uptake and release of bevacizumab by platelets in vitro and in vivo. The angiogenic activity of platelets preincubated with bevacizumab was studied in endothelial proliferation assays. Finally, we determined whether treatment with bevacizumab neutralizes VEGF in platelets from cancer patients.
  
  
  RESULTS
  We found that platelets are able to take up bevacizumab. Activation of platelets preincubated with bevacizumab resulted in release of the antibody and release of VEGF neutralized by bevacizumab. Immunofluorescence microscopy revealed that FITC-labeled bevacizumab and P-selectin colocalize, indicating alpha-granule localization. In addition, bevacizumab uptake inhibited platelet-induced human endothelial cell proliferation. In in vivo rabbit experiments, FITC-labeled bevacizumab was present in platelets after 2 h and up to 2 weeks following i.v. administration. Finally, we found that platelets take up bevacizumab in patients receiving bevacizumab treatment. Within 8 h after bevacizumab administration, platelet VEGF was almost completely neutralized due to this uptake.
  
  
  CONCLUSIONS
  These studies show that bevacizumab is taken up by platelets and may explain its clinical effect on wound healing and tumor growth.
authors.affiliation: The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Johns Hopkins Medical Institutions, Baltimore, Maryland, USA. h.m.w.verheul@umcutrecht.nl
authors.forename: Henk M W
authors.initials: HM
authors.lastname: Verheul
authors.forename: Martijn P J
authors.initials: MP
authors.lastname: Lolkema
authors.forename: David Z
authors.initials: DZ
authors.lastname: Qian
authors.forename: Yvonne H A
authors.initials: YH
authors.lastname: Hilkes
authors.forename: Eleni
authors.initials: E
authors.lastname: Liapi
authors.forename: Jan-Willem N
authors.initials: JW
authors.lastname: Akkerman
authors.forename: Roberto
authors.initials: R
authors.lastname: Pili
authors.forename: Emile E
authors.initials: EE
authors.lastname: Voest
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000032:analysis / Q000378:metabolism*; D061067:Antibodies, Monoclonal, Humanized; D000068258:Bevacizumab; D001792:Blood Platelets / Q000737:chemistry / Q000378:metabolism*; D005434:Flow Cytometry; D006801:Humans; D018919:Neovascularization, Physiologic; D021381:Protein Transport; D011817:Rabbits; D042461:Vascular Endothelial Growth Factor A / Q000032:analysis / Q000037:antagonists & inhibitors
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D042461:Vascular Endothelial Growth Factor A; D000068258:Bevacizumab