title: Evaluation of the anti-HER2 C6.5 diabody as a PET radiotracer to monitor HER2 status and predict response to trastuzumab treatment.
abstract: OBJECTIVE
  The rapid tumor targeting and pharmacokinetic properties of engineered antibodies make them potentially suitable for use in imaging strategies to predict and monitor response to targeted therapies. This study aims to evaluate C6.5 diabody (C6.5 db), a noncovalent anti-HER2 single-chain Fv dimer, as a radiotracer for predicting response to HER2-targeted therapies such as trastuzumab.
  
  
  METHODS
  Immunodeficient mice bearing established HER2-positive tumor xenografts were injected with radioiodinated C6.5 db and imaged by PET/CT. Radiotracer biodistribution was quantified by biopsied tumor and normal tissues. Potential competition between trastuzumab and C6.5 db was examined in vitro by flow cytometry and coimmunoprecipitations.
  
  
  RESULTS
  Biodistribution analysis of mice bearing xenografts with varying HER2 density revealed that the tumor uptake of (125)I-C6.5 db correlates with HER2 tumor density. In vitro competition experiments suggest that the C6.5 db targets an epitope on HER2 that is distinct from that bound by trastuzumab. Treatment of mice affected with SK-OV-3 tumor with trastuzumab for 3 days caused a 42% (P = 0.002) decrease in tumor uptake of (125)I-C6.5 db. This is consistent with a dramatic decrease in the tumor PET signal of (124)I-C6.5 db after trastuzumab treatment. Furthermore, mice affected with BT-474 tumor showed an approximately 60% decrease (P = 0.0026) in C6.5 db uptake after 6 days of trastuzumab treatment. Immunohistochemistry of excised xenograft sections and in vitro flow cytometry revealed that the decreased C6.5 db uptake on trastuzumab treatment is not associated with HER2 downregulation.
  
  
  CONCLUSIONS
  These studies suggest that (124)I-C6.5 db-based imaging can be used to evaluate HER2 levels as a predictor of response to HER2-directed therapies.
authors.affiliation: Developmental Therapeutics Program, Department of Diagnostic Imaging, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
authors.forename: Smitha
authors.initials: S
authors.lastname: Reddy
authors.forename: Calvin C
authors.initials: CC
authors.lastname: Shaller
authors.forename: Mohan
authors.initials: M
authors.lastname: Doss
authors.forename: Irina
authors.initials: I
authors.lastname: Shchaveleva
authors.forename: James D
authors.initials: JD
authors.lastname: Marks
authors.forename: Jian Q
authors.initials: JQ
authors.lastname: Yu
authors.forename: Matthew K
authors.initials: MK
authors.lastname: Robinson
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000494:pharmacology; D001706:Biopsy; D045744:Cell Line, Tumor; D015536:Down-Regulation; D018734:Genes, erbB-2; D057809:HEK293 Cells; D006801:Humans; D007128:Immunoglobulin Fragments / Q000737:chemistry; D051379:Mice; D016513:Mice, SCID; D009368:Neoplasm Transplantation; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000494:pharmacology; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000737:chemistry* / Q000276:immunology; D057127:Single-Chain Antibodies / Q000737:chemistry*; D014057:Tomography, X-Ray Computed / Q000379:methods; D000068878:Trastuzumab
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D007128:Immunoglobulin Fragments; D019275:Radiopharmaceuticals; D018719:Erb-b2 Receptor Tyrosine Kinases; D057127:Single-Chain Antibodies; D000068878:Trastuzumab