title: Micro-SPECT/CT with 111In-DTPA-pertuzumab sensitively detects trastuzumab-mediated HER2 downregulation and tumor response in athymic mice bearing MDA-MB-361 human breast cancer xenografts.
abstract: Pertuzumab is a HER2 dimerization inhibitor that binds to an epitope unique from that of trastuzumab. Our objective was to determine whether SPECT with (111)In-diethylenetriaminepentaacetic acid-pertuzumab ((111)In-DTPA-pertuzumab) could sensitively detect an early molecular response to trastuzumab manifested by HER2 downregulation and a later tumor response revealed by a decreased number of HER2-positive viable tumor cells.
  
  
  METHODS
  Changes in HER2 density in SKBr-3 and MDA-MB-361 BC cells exposed to trastuzumab (14 microg/mL) in vitro were measured by saturation binding assays using (111)In-DTPA-pertuzumab and by confocal immunofluorescence microscopy and flow cytometry with fluorescein isothiocyanate-labeled HER2/neu antibodies. Imaging of HER2 downregulation was studied in vivo in athymic mice with subcutaneous MDA-MB-361 tumors treated for 3 d with trastuzumab (4 mg/kg) or nonspecific human IgG (hIgG) or phosphate-buffered saline (PBS). Imaging of tumor response to trastuzumab was studied in mice bearing subcutaneous MDA-MB-361 xenografts treated with trastuzumab (4 mg/kg), followed by weekly doses of nonspecific hIgG or rituximab or PBS (2 mg/kg). Mice were imaged on a micro-SPECT/CT system at 72 h after injection of (111)In-DTPA-pertuzumab. Tumor and normal-tissue biodistribution was determined.
  
  
  RESULTS
  (111)In-DTPA-pertuzumab saturation binding to SKBr-3 and MDA-MB-361 cells was significantly decreased at 72 h after exposure in vitro to trastuzumab (14 microg/mL), compared with untreated controls (62% +/- 2%, P < 0.0001; 32% +/- 9%, P < 0.0002, respectively). After 3 d of trastuzumab, in vivo tumor uptake of (111)In-DTPA-pertuzumab decreased 2-fold in trastuzumab- versus PBS-treated mice (13.5 +/- 2.6 percentage injected dose per gram [%ID/g] vs. 28.5 +/- 9.1 %ID/g, respectively; P < 0.05). There was also a 2-fold decreased tumor uptake in trastuzumab- versus PBS-treated mice by image volume-of-interest analysis (P = 0.05), suggesting trastuzumab-mediated HER2 downregulation. After 3 wk of trastuzumab, tumor uptake of (111)In-DTPA-pertuzumab decreased 4.5-fold, compared with PBS-treated mice (7.6 +/- 0.4 vs. 34.6 +/- 9.9 %ID/g, respectively; P < 0.001); this decrease was associated with an almost-completed eradication of HER2-positive tumor cells determined immunohistochemically.
  
  
  CONCLUSIONS
  (111)In-DTPA-pertuzumab sensitively imaged HER2 downregulation after 3 d of treatment with trastuzumab and detected a reduction in viable HER2-positive tumor cells after 3 wk of therapy in MDA-MB-361 human breast cancer xenografts.
authors.affiliation: Department of Pharmaceutical Sciences, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Kristin
authors.initials: K
authors.lastname: McLarty
authors.forename: Bart
authors.initials: B
authors.lastname: Cornelissen
authors.forename: Zhongli
authors.initials: Z
authors.lastname: Cai
authors.forename: Deborah A
authors.initials: DA
authors.lastname: Scollard
authors.forename: Danny L
authors.initials: DL
authors.lastname: Costantini
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Done
authors.forename: Raymond M
authors.initials: RM
authors.lastname: Reilly
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000008:administration & dosage* / Q000493:pharmacokinetics*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000008:administration & dosage; D001943:Breast Neoplasms / Q000175:diagnosis / Q000188:drug therapy / Q000378:metabolism; D045744:Cell Line, Tumor; D015536:Down-Regulation / Q000187:drug effects; D005260:Female; D006801:Humans; D007205:Indium Radioisotopes / Q000493:pharmacokinetics; D051379:Mice; D008819:Mice, Nude; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods* / Q000662:veterinary; D014057:Tomography, X-Ray Computed / Q000379:methods* / Q000662:veterinary; D000068878:Trastuzumab; D016896:Treatment Outcome
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D007205:Indium Radioisotopes; D018719:Erb-b2 Receptor Tyrosine Kinases; D000068878:Trastuzumab; C485206:pertuzumab