title: (99m)Tc-rituximab radiolabelled by photo-activation: a new non-Hodgkin's lymphoma imaging agent.
abstract: OBJECTIVE
  Rituximab was the first chimeric monoclonal antibody to be approved for treatment of indolent B-cell non-Hodgkin's lymphoma (NHL). It is directed against the CD20 antigen, which is expressed by 95% of B-cell NHLs. The aim of this study was to explore the possibility of radiolabelling rituximab with (99m)Tc for use as an imaging agent in NHL for early detection, staging, remission assessment, monitoring for metastatic spread and tumour recurrence, and assessment of CD20 expression prior to (radio)immunotherapy.
  
  
  METHODS
  Rituximab was purified from Mabthera solution (Roche), photo-activated at 302 nm by UV irradiation and radiolabelled with (99m)Tc. The effectiveness of the labelling method was evaluated by determination of the number of free thiol groups per photoreduced antibody, radiochemical purity and in vitro stability of (99m)Tc-rituximab.
  
  
  RESULTS
  On average, 4.4 free thiol groups per photoreduced antibody were determined. Radiolabelling yields greater than 95% were routinely observed after storage of the photo-activated antibody at -80 degrees C for 195 days. The direct binding assay showed preserved ability of (99m)Tc-rituximab to bind to CD20, with an average immunoreactive fraction of 93.3%. The internalisation rate was proven to be low, with only 5.3% of bound (99m)Tc-rituximab being internalised over 4 h at 37 degrees C.
  
  
  CONCLUSIONS
  Our results demonstrate that (99m)Tc-rituximab of high radiochemical purity and with preserved binding affinity for the antigen can be prepared by photoreduction and that the method shows good reproducibility. (99m)Tc-rituximab will be further explored as an imaging agent applicable in NHL for the purposes mentioned above.
authors.affiliation: Department for Nuclear Medicine, University Medical Centre Ljubljana, Zaloska 7, 1525 Ljubljana. Slovenia. tanja.stopar@kclj.si
authors.forename: T Gmeiner
authors.initials: TG
authors.lastname: Stopar
authors.forename: I
authors.initials: I
authors.lastname: Mlinaric-Rascan
authors.forename: J
authors.initials: J
authors.lastname: Fettich
authors.forename: S
authors.initials: S
authors.lastname: Hojker
authors.forename: S J
authors.initials: SJ
authors.lastname: Mather
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000378:metabolism* / Q000528:radiation effects; D058846:Antibodies, Monoclonal, Murine-Derived; D045744:Cell Line, Tumor; D004355:Drug Stability; D005240:Feasibility Studies; D006801:Humans; D016393:Lymphoma, B-Cell / Q000000981:diagnostic imaging* / Q000378:metabolism*; D015609:Organotechnetium Compounds / Q000737:chemistry / Q000378:metabolism* / Q000528:radiation effects; D010777:Photochemistry / Q000379:methods; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000528:radiation effects; D015203:Reproducibility of Results; D000069283:Rituximab; D012680:Sensitivity and Specificity; D013194:Staining and Labeling / Q000379:methods; D014466:Ultraviolet Rays
chemical_list: D000911:Antibodies, Monoclonal; D058846:Antibodies, Monoclonal, Murine-Derived; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; C509712:technetium 99m rituximab; D000069283:Rituximab