title: Molecular imaging of cell death in tumors. Increasing annexin A5 size reduces contribution of phosphatidylserine-targeting function to tumor uptake.
abstract: OBJECTIVE
  Annexin A5 is a phosphatidylserine binding protein that binds dying cells in vivo. Annexin A5 is a potential molecular imaging agent to determine efficacy of anti-cancer therapy in patients. Its rapid clearance from circulation limits tumor uptake and, hence, its sensitivity. The aim of this study is to determine if non-invasive imaging of cell death in tumors will benefit from increasing circulation time of annexin A5 by increasing its size.
  
  
  METHODS
  Annexin A5 size was increased by complexation of biotinylated annexin A5 with Alexa-Fluor680-labeled streptavidin. The non-binding variant of annexin A5, M1234, was used as negative control. The HT29 colon carcinoma xenograft model in NMRI nude mice was used to measure tumor uptake in vivo. Tumor uptake of fluorescent annexin A5-variants was measured using non-invasive optical imaging.
  
  
  RESULTS
  The annexin A5-streptavidin complex (4 ∶ 1, moles:moles, Mw ∼ 200 kDa) binds phosphatidylserine-expressing membranes with a Hill-coefficient of 5.7 ± 0.5 for Ca2+-binding and an EC50 of 0.9 ± 0.1 mM Ca2+ (EC50 is the Ca2+ concentration required for half maximal binding)(annexin A5: Hill-coefficient 3.9 ± 0.2, EC50 1.5 ± 0.2 mM Ca2+). Circulation half-life of annexin A5-streptavidin is ± 21 minutes (circulation half-life of annexin A5 is ± 4 min.). Tumor uptake of annexin A5-streptavidin was higher and persisted longer than annexin A5-uptake but depended less on phosphatidylserine binding.
  
  
  CONCLUSIONS
  Increasing annexin A5 size prolongs circulation times and increases tumor uptake, but decreases contribution of PS-targeting to tumor uptake and abolishes power to report efficacy of therapy.
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
authors.forename: Lisette
authors.initials: L
authors.lastname: Ungethüm
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
authors.forename: Martijn
authors.initials: M
authors.lastname: Chatrou
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
authors.forename: Dennis
authors.initials: D
authors.lastname: Kusters
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
authors.forename: Leon
authors.initials: L
authors.lastname: Schurgers
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
authors.forename: Chris P
authors.initials: CP
authors.lastname: Reutelingsperger
mesh_terms: D000818:Animals; D017304:Annexin A5 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis; D001710:Biotin / Q000737:chemistry / Q000378:metabolism; D019073:HT29 Cells; D006207:Half-Life; D006801:Humans; D006834:Hydrazines / Q000737:chemistry; D008279:Magnetic Resonance Imaging; D051379:Mice; D008819:Mice, Nude; D018613:Microscopy, Confocal; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism* / Q000473:pathology; D010718:Phosphatidylserines / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011859:Radiography; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000493:pharmacokinetics; D019809:Streptavidin / Q000737:chemistry* / Q000378:metabolism; D014018:Tissue Distribution; D014183:Transplantation, Heterologous
chemical_list: C417561:Alexa 488 hydrazide; D017304:Annexin A5; D006834:Hydrazines; D010718:Phosphatidylserines; D011994:Recombinant Proteins; D001710:Biotin; D019809:Streptavidin