title: Tumor pretargeting: role of avidin/streptavidin on monoclonal antibody internalization.
abstract: Radioimmunodetection of tumor can be improved by introducing a two-step system in which radiolabeled streptavidin is administrated after the injection of a biotinylated monoclonal antibody (MAb) (two-step) or radiolabeled biotin is injected after biotinylated MAb and avidin (three-step). The anti-carcinoembryonic antigen (CEA) MAb FO23C5 has been recently exploited in a three-step protocol based on the avidin-biotin system. The anti-folate receptor (FR) MAb MOv18 has proven suitable for radioimmunodetection of ovarian cancer using directly radiolabeled MAb or in a two-step method. In this study, we analyzed the suitability of MOv18 in a three-step protocol in ovarian carcinoma patients and the internalization events after formation of the MOv18-avidin complex.
  
  
  METHODS
  Selected patients with documented metastatic lesions were enrolled in a three-step radioimaging analysis with biotinylated MOv18 and FO23C5, avidin and (111)In-labeled biotin. Two-step internalization experiments were conducted in vitro with MOv18 and MOv19 MAbs on the FR-overexpressing IGROV1 cell line and with the anti-CEA MAb FO23C5 on the LS174T cell line. Cells were incubated sequentially with biotinylated MAb and 125I-labeled streptavidin or with 125I-biotinylated MAb and cold streptavidin.
  
  
  RESULTS
  In the in vivo study, SPECT revealed the majority of metastatic lesions in patients injected with biotinylated MOv18; however, the tumor-to-background ratio was relatively low. In the in vitro study, a consistent internalization was induced by antigen-biotinylated MAb-streptavidin complex formation at the cell surface in both antigenic systems analyzed. However, the extent of internalization was lower in the CEA model.
  
  
  CONCLUSIONS
  The internalization ability of avidin suggests its potential clinical application for delivering toxic agents in a two-step approach (biotinylated MAb + avidin conjugate). The suitability of a given MAb for three-step clinical applications (biotinylated MAb + avidin + biotin) should be previously investigated by using appropriate in vitro experiments.
authors.affiliation: Oncologia Sperimentale E, Istituto Nazionale Tumori, Milan, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Casalini
authors.forename: E
authors.initials: E
authors.lastname: Luison
authors.forename: S
authors.initials: S
authors.lastname: Ménard
authors.forename: M I
authors.initials: MI
authors.lastname: Colnaghi
authors.forename: G
authors.initials: G
authors.lastname: Paganelli
authors.forename: S
authors.initials: S
authors.lastname: Canevari
mesh_terms: D000911:Antibodies, Monoclonal; D001360:Avidin / Q000008:administration & dosage; D001426:Bacterial Proteins / Q000008:administration & dosage; D001710:Biotin; D002272:Carcinoembryonic Antigen / Q000096:biosynthesis; D002352:Carrier Proteins / Q000276:immunology; D003110:Colonic Neoplasms / Q000276:immunology; D005260:Female; D058973:Folate Receptors, GPI-Anchored; D006801:Humans; D007205:Indium Radioisotopes; D007457:Iodine Radioisotopes; D009362:Neoplasm Metastasis; D010051:Ovarian Neoplasms / Q000473:pathology; D016719:Radioimmunodetection; D011956:Receptors, Cell Surface / Q000276:immunology; D019809:Streptavidin; D015899:Tomography, Emission-Computed, Single-Photon; D014407:Tumor Cells, Cultured
chemical_list: D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins; D002272:Carcinoembryonic Antigen; D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D007205:Indium Radioisotopes; D007457:Iodine Radioisotopes; D011956:Receptors, Cell Surface; D001360:Avidin; D001710:Biotin; D019809:Streptavidin