title: Factors affecting the endothelial retention of targeted microbubbles: influence of microbubble shell design and cell surface projection of the endothelial target molecule.
abstract: BACKGROUND
  In biologic systems, the arrest of circulating cells is mediated by adhesion molecules projecting their active binding domain above the cell surface to enhance bond formation and tether strength. Similarly, molecular spacers are used for ligands on particle-based molecular imaging agents. The aim of this study was to evaluate the influence of tether length for targeting ligands on ultrasound molecular imaging agents.
  
  
  METHODS
  Microbubbles bearing biotin at the end of variable-length polyethylene glycol spacer arms (MB(2000) and MB(3400)) were prepared. To assess in vivo attachment efficiency to endothelial counterligands that vary in their distance from the endothelial cell surface, contrast-enhanced ultrasound (CEU) molecular imaging of tumor necrosis factor-α-induced P-selectin (long distance) or intercellular adhesion molecule-2 (short distance) was performed with each agent in murine hind limbs. To assess the influence of the glycocalyx on microbubble attachment, CEU molecular imaging of intercellular adhesion molecule-2 was performed after degradation of the glycocalyx.
  
  
  RESULTS
  CEU molecular imaging targeted to P-selectin showed signal enhancement above control agent for MB(2000) and MB(3400), the degree of which was significantly higher for MB(3400) compared with MB(2000). CEU molecular imaging targeted to intercellular adhesion molecule-2 showed low overall signal for all agents and signal enhancement above control for MB(3400) only. Glycocalyx degradation increased signal for MB(3400) and MB(2000).
  
  
  CONCLUSIONS
  Microbubble targeting to endothelial ligands is influenced by (1) the tether length of the ligand, (2) the degree to which the endothelial target is projected from the cell surface, and (3) the status of the glycocalyx. These considerations are important for designing targeted imaging probes and understanding potential obstacles to molecular imaging.
authors.affiliation: Department of Biomedicine, University Hospital Basel, Petersgraben 4, Basel, Switzerland.
authors.forename: Elham
authors.initials: E
authors.lastname: Khanicheh
authors.forename: Martina
authors.initials: M
authors.lastname: Mitterhuber
authors.forename: Katharina
authors.initials: K
authors.lastname: Kinslechner
authors.forename: Lifen
authors.initials: L
authors.lastname: Xu
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Lindner
authors.forename: Beat A
authors.initials: BA
authors.lastname: Kaufmann
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism; D015703:Antigens, CD / Q000378:metabolism*; D002448:Cell Adhesion; D015815:Cell Adhesion Molecules / Q000378:metabolism*; D003287:Contrast Media / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000000981:diagnostic imaging* / Q000378:metabolism*; D004867:Equipment Design; D019276:Glycocalyx; D007089:Image Enhancement; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D045423:Microbubbles; D023421:Models, Animal; D019007:P-Selectin / Q000378:metabolism*; D014463:Ultrasonography / Q000379:methods
chemical_list: D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D015815:Cell Adhesion Molecules; D003287:Contrast Media; C480027:ICAM-2 protein, mouse; D019007:P-Selectin