title: Molecular imaging of death receptor 5 occupancy and saturation kinetics in vivo by humanized monoclonal antibody CS-1008.
abstract: OBJECTIVE
  CS-1008 (tigatuzumab; phase I/II), an antihuman death receptor 5 (DR5) agonist, induces apoptosis and has cytotoxic activity against human cancer cell lines. This study reports on the preclinical validation of (111)In-labeled anti-DR5 humanized antibody CS-1008 as a diagnostic tool to study the DR5 occupancy in patients with cancer and establish dose ranges for receptor saturation kinetics in vivo.
  
  
  METHODS
  CS-1008 was radiolabeled and characterized for DR5 binding and labeling efficiency on TRAIL-sensitive DR5-positive colorectal cancer cells (COLO 205 and WiDr). Pharmacokinetic and biodistribution studies were conducted in BALB/c nu/nu mice bearing COLO 205, WiDr, or DR5-negative CT26 colon tumors. Planar gamma camera imaging and computerized tomography (CT) images were obtained to study receptor occupancy in vivo.
  
  
  RESULTS
  Scatchard analysis showed high and specific binding affinity (Kd, 1.05 ± 0.12 nmol/L) of (111)In-labeled CS-1008. (111)In-labeled CS-1008 was specifically taken up in mice bearing COLO 205 and WiDr tumors with prolonged tumor retention (26.25 ± 2.85%ID/g vs. 12.20 ± 2.24 at 168 hours post injection; n = 5, SD), and uptake correlated both with DR5 expression on tumor cells and antitumor activity. DR5 saturation was shown in vivo via both biodistribution studies and planar gamma camera imaging/CT imaging of (111)In-labeled CS-1008. Saturation of DR5 corresponded to maximal in vivo antitumor efficacy.
  
  
  CONCLUSIONS
  Imaging of DR5 receptor occupancy in vivo correlates with tumor concentration and in vivo efficacy, and is a novel molecular imaging technique that can be used to determine receptor occupancy and effective dose levels of DR5 agonist antibodies in the clinic.
authors.affiliation: Authors' Affiliations: Tumour Targeting Laboratory and Oncogenic Transcription Laboratory, Ludwig Institute for Cancer Research; Department of Nuclear Medicine and Centre for PET, Austin Health, Melbourne, Australia; Radioimmune & Inorganic Chemistry Section, Radiation Oncology Branch, Cancer Research Centre, Bethesda, Maryland; Clinical Development-Oncology, Daiichi Sankyo Pharmaceutical Development, Edison, New Jersey; and Global Project Management Department, Daiichi Sankyo Co. Ltd., Tokyo, Japan.
authors.forename: Ingrid J G
authors.initials: IJ
authors.lastname: Burvenich
authors.forename: Fook T
authors.initials: FT
authors.lastname: Lee
authors.forename: Glenn A
authors.initials: GA
authors.lastname: Cartwright
authors.forename: Graeme J
authors.initials: GJ
authors.lastname: O'Keefe
authors.forename: Dahna
authors.initials: D
authors.lastname: Makris
authors.forename: Diana
authors.initials: D
authors.lastname: Cao
authors.forename: Sylvia
authors.initials: S
authors.lastname: Gong
authors.forename: Anderly C
authors.initials: AC
authors.lastname: Chueh
authors.forename: John M
authors.initials: JM
authors.lastname: Mariadason
authors.forename: Martin W
authors.initials: MW
authors.lastname: Brechbiel
authors.forename: Robert A
authors.initials: RA
authors.lastname: Beckman
authors.forename: Kosaku
authors.initials: K
authors.lastname: Fujiwara
authors.forename: Reinhard
authors.initials: R
authors.lastname: von Roemeling
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Scott
mesh_terms: D000818:Animals; D061067:Antibodies, Monoclonal, Humanized / Q000378:metabolism* / Q000493:pharmacokinetics; D045744:Cell Line, Tumor; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D007205:Indium Radioisotopes; D007553:Isotope Labeling; D007700:Kinetics; D051379:Mice; D057054:Molecular Imaging; D011485:Protein Binding; D011877:Radionuclide Imaging; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D061067:Antibodies, Monoclonal, Humanized; D007205:Indium Radioisotopes; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; C552861:tigatuzumab