title: Site-specifically 11C-labeled Sel-tagged annexin A5 and a size-matched control for dynamic in vivo PET imaging of protein distribution in tissues prior to and after induced cell death.
abstract: BACKGROUND
  Radiolabeled annexin A5 (AnxA5) is widely used for detecting phosphatidylserine exposed on cell surfaces during apoptosis. We describe here a new method for labeling AnxA5 and a size-matched control protein with short-lived carbon-11, for probing the specificity of in vivo cell death monitoring using positron emission tomography (PET) imaging.
  
  
  METHODS
  AnxA5 and the control protein were recombinantly expressed with a C-terminal "Sel-tag", the tetrapeptide -Gly-Cys-Sec-Gly-COOH. The proteins were then labeled either fluorescently for in vitro corroborations of binding behaviors or with 11C for dynamic in vivo PET studies.
  
  
  RESULTS
  AnxA5 demonstrated retained calcium-dependent binding to apoptotic cells after the C-terminus modification. The control protein showed no functional binding. The 11C-ligands demonstrated similar in vivo pharmacokinetic behavior in healthy mice except for higher uptake in kidney and higher intact elimination to urine of AnxA5. After inducing hepatic apoptosis, however, the uptake of labeled AnxA5 in the targeted tissue increased compared to baseline levels while that of the control protein tended to decrease.
  
  
  CONCLUSIONS
  These data suggest that the combined use of these two tracers can facilitate differentiating specific AnxA5 binding and its changes caused by induced cell death from uptake due to non-specific permeability and retention effects at baseline or after therapy.
  
  
  CONCLUSIONS
  The Sel-tag enables rapid and mild reactions with electrophilic agents giving site-specifically labeled proteins for multi-probe analyses. The combined use of 11C-labeled AnxA5 and a size-matched control protein with dynamic PET can be useful for evaluating drug effects on target as well as off-target tissues.
authors.affiliation: Division of Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Qing
authors.initials: Q
authors.lastname: Cheng
authors.forename: Li
authors.initials: L
authors.lastname: Lu
authors.forename: Jonas
authors.initials: J
authors.lastname: Grafström
authors.forename: Maria Hägg
authors.initials: MH
authors.lastname: Olofsson
authors.forename: Jan-Olov
authors.initials: JO
authors.lastname: Thorell
authors.forename: Erik
authors.initials: E
authors.lastname: Samén
authors.forename: Katarina
authors.initials: K
authors.lastname: Johansson
authors.forename: Hanna-Stina
authors.initials: HS
authors.lastname: Ahlzén
authors.forename: Stig
authors.initials: S
authors.lastname: Linder
authors.forename: Elias S J
authors.initials: ES
authors.lastname: Arnér
authors.forename: Sharon
authors.initials: S
authors.lastname: Stone-Elander
mesh_terms: D000818:Animals; D017304:Annexin A5 / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism; D017209:Apoptosis; D001483:Base Sequence; D001682:Biological Availability; D002118:Calcium / Q000378:metabolism; D002250:Carbon Radioisotopes; D007553:Isotope Labeling / Q000379:methods*; D007668:Kidney / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000000981:diagnostic imaging / Q000378:metabolism* / Q000473:pathology; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D049268:Positron-Emission Tomography / Q000379:methods*; D011859:Radiography; D019275:Radiopharmaceuticals / Q000032:analysis* / Q000138:chemical synthesis; D013194:Staining and Labeling / Q000379:methods
chemical_list: D017304:Annexin A5; D002250:Carbon Radioisotopes; D019275:Radiopharmaceuticals; D002118:Calcium