title: Easy and efficient (111)indium labeling of long-term stored DTPA conjugated protein.
abstract: The labelling efficiency of long-term stored DTPA-conjugates has not been reported previously even though DTPA has been in extensive use as metal chelator in the development of radiopharmaceuticals and contrast agents. DTPA is often used as a bifunctional chelating agent conjugated to tumor targeting vehicles such as monoclonal antibodies and receptor directed peptides. The purpose of this study was to monitor the labelling efficiency of a DTPA-conjugate on long-term storage using 111In-chloride at two different temperatures and incubation times for the In-labelling.
  
  
  METHODS
  Cyclic-diethylene-triamine-pentaacetic acid (cDTAP) was conjugated to a polyclonal immunoglobulin-G (IgG) in borate buffer, pH 8.2 at +4?C for 4 hours. Then the DTPA-conjugate was dialyzed against 50 mmol/l sodium citrate buffer saline, pH 6.0 and stored at -80° C in aliquots of 1 mg/0.5 ml. The DTPA-conjugate was labeled with 111In-chloride in citrate buffer, pH 6. The labelling reaction was incubated at room temperature (RT) for 30 min and at +4?C for 90 min. Determination of labelling efficiency was performed using ITLC and an instant chromatography scanner equipped with a NaI crystal. The labelling efficiency of the DTPA-conjugate was monitored every third month for 12 months.
  
  
  RESULTS
  The median labelling efficiencies varied between 92 and 96% during the whole period. The two combinations of incubation times and temperatures (30 min at RT and 90 min at +4°C) had no affect on labelling efficiency of the DTPA-conjugate, stored for 12 months.
  
  
  CONCLUSIONS
  Our study shows that 111In-labelling can easily be performed within 30 min at RT for thermo-stable proteins like polyclonal, DTPA-conjugated IgG stored long-term at -80°C with a high 111In-labelling efficiency.
authors.affiliation: Department of Biomedical Sciences, University of Copenhagen, Denmark.
authors.forename: Amarnadh
authors.initials: A
authors.lastname: Nalla
authors.forename: Inge
authors.initials: I
authors.lastname: Buch
authors.forename: Birger
authors.initials: B
authors.lastname: Hesse
mesh_terms: D002614:Chelating Agents / Q000737:chemistry*; D004356:Drug Storage; D007074:Immunoglobulin G / Q000737:chemistry / Q000378:metabolism; D007204:Indium / Q000737:chemistry*; D007553:Isotope Labeling / Q000379:methods*; D004369:Pentetic Acid / Q000737:chemistry*; D019275:Radiopharmaceuticals / Q000737:chemistry*; D013696:Temperature
chemical_list: D002614:Chelating Agents; D007074:Immunoglobulin G; D019275:Radiopharmaceuticals; D007204:Indium; C020758:indium trichloride; D004369:Pentetic Acid