title: Pretargeted Immuno-PET of Pancreatic Cancer: Overcoming Circulating Antigen and Internalized Antibody to Reduce Radiation Doses.
abstract: 5B1 is a fully human, monoclonal antibody that has shown promise for the PET imaging of cancers expressing carbohydrate antigen 19.9 (CA19.9)--a carbohydrate prevalent in cells with aberrant glycosylation and an established effector of metastasis. The long physiologic half-life of the antibody and interference from circulating CA19.9 may increase the time required to generate quality images as well as the risk of radiation exposure to healthy tissues during repeated PET imaging. Pretargeting methodologies are an effective approach to expeditiously acquire PET images, but in this case, the pretargeting approach is complicated by the internalization of 5B1 by CA19.9-expressing cells. We sought to adapt and optimize a pretargeting strategy that exploits the bioorthogonal reaction between transcyclooctene (TCO) and tetrazine (Tz) to overcome these complications.
  
  
  METHODS
  5B1 was modified with TCO, and a novel NOTA-PEG7-Tz radioligand was synthesized with the goal of improving on a previously reported analog. BxPC3 and Capan-2 cells were evaluated for their ability to internalize anti-CA19.9 antibodies using a fluorometric assay, and xenografts of the same lines were used for in vivo studies. The pretargeting approach was optimized, and the 2 radioligands were compared using biodistribution and PET imaging in murine models of pancreatic cancer.
  
  
  RESULTS
  BxPC3 and Capan-2 cells were shown to rapidly internalize anti-CA19.9 monoclonal antibodies, including 5B1. (64)Cu-NOTA-PEG7-Tz showed improved in vivo pharmacokinetics relative to (64)Cu-NOTA-Tz using 5B1-TCO as the targeting vector. PET imaging and biodistribution studies showed that injecting the radioligand 72 h after the administration of 5B1-TCO resulted in the best uptake (8.2 ± 1.7 percentage injected dose per gram at 20 h after injection) and tumor-to-background activity concentration ratios. Dosimetry calculations revealed that the pretargeting system produced a greater than 25-fold reduction in total body radiation exposure relative to (89)Zr-desferrioxamine-5B1. PET/CT imaging in an orthotopic Capan-2 xenograft model--which secretes large amounts of CA19.9 and more rapidly internalizes anti-CA19.9 antibodies--showed that this approach is viable even in the difficult circumstances presented by a circulating antigen and internalized targeting vector.
  
  
  CONCLUSIONS
  The 5B1-TCO and (64)Cu-NOTA-PEG7-Tz system evaluated in these studies can delineate CA19.9-positive xenografts in murine models of pancreatic cancer despite the challenges posed by the combination of circulating antigen and internalization of the 5B1-TCO.
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, New York, New York Program in Molecular Pharmacology, Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Jacob L
authors.initials: JL
authors.lastname: Houghton
authors.affiliation: Department of Chemistry, Hunter College and the Graduate Center of the City University of New York, New York, New York; and.
authors.forename: Brian M
authors.initials: BM
authors.lastname: Zeglis
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, New York, New York Program in Molecular Pharmacology, Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Dalya
authors.initials: D
authors.lastname: Abdel-Atti
authors.affiliation: MabVax Therapeutics, San Diego, California.
authors.forename: Ritsuko
authors.initials: R
authors.lastname: Sawada
authors.affiliation: MabVax Therapeutics, San Diego, California.
authors.forename: Wolfgang W
authors.initials: WW
authors.lastname: Scholz
authors.affiliation: Department of Radiology, Memorial Sloan Kettering Cancer Center, New York, New York Program in Molecular Pharmacology, Memorial Sloan Kettering Cancer Center, New York, New York lewisj2@mskcc.org.
authors.forename: Jason S
authors.initials: JS
authors.lastname: Lewis
mesh_terms: D000818:Animals; D000912:Antibodies, Neoplasm; D000951:Antigens, Neoplasm; D045744:Cell Line, Tumor; D003301:Copper Radioisotopes; D003676:Deferoxamine / Q000138:chemical synthesis / Q000493:pharmacokinetics; D006573:Heterocyclic Compounds, 1-Ring / Q000138:chemical synthesis / Q000493:pharmacokinetics; D006801:Humans; D051379:Mice; D009368:Neoplasm Transplantation; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging*; D049268:Positron-Emission Tomography / Q000379:methods*; D011829:Radiation Dosage; D011868:Radioisotopes; D019275:Radiopharmaceuticals / Q000138:chemical synthesis; D014018:Tissue Distribution; D015040:Zirconium
chemical_list: D000912:Antibodies, Neoplasm; D000951:Antigens, Neoplasm; D003301:Copper Radioisotopes; D006573:Heterocyclic Compounds, 1-Ring; D011868:Radioisotopes; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D015040:Zirconium; D003676:Deferoxamine
keywords: CA19.9; PET imaging; pancreatic cancer; pretargeting