title: Preclinical PET imaging of EGFR levels: pairing a targeting with a non-targeting Sel-tagged Affibody-based tracer to estimate the specific uptake.
abstract: BACKGROUND
  Though overexpression of epidermal growth factor receptor (EGFR) in several forms of cancer is considered to be an important prognostic biomarker related to poor prognosis, clear correlations between biomarker assays and patient management have been difficult to establish. Here, we utilize a targeting directly followed by a non-targeting tracer-based positron emission tomography (PET) method to examine some of the aspects of determining specific EGFR binding in tumors.
  
  
  METHODS
  The EGFR-binding Affibody molecule ZEGFR:2377 and its size-matched non-binding control ZTaq:3638 were recombinantly fused with a C-terminal selenocysteine-containing Sel-tag (ZEGFR:2377-ST and ZTaq:3638-ST). The proteins were site-specifically labeled with DyLight488 for flow cytometry and ex vivo tissue analyses or with (11)C for in vivo PET studies. Kinetic scans with the (11)C-labeled proteins were performed in healthy mice and in mice bearing xenografts from human FaDu (squamous cell carcinoma) and A431 (epidermoid carcinoma) cell lines. Changes in tracer uptake in A431 xenografts over time were also monitored, followed by ex vivo proximity ligation assays (PLA) of EGFR expressions.
  
  
  RESULTS
  Flow cytometry and ex vivo tissue analyses confirmed EGFR targeting by ZEGFR:2377-ST-DyLight488. [Methyl-(11)C]-labeled ZEGFR:2377-ST-CH3 and ZTaq:3638-ST-CH3 showed similar distributions in vivo, except for notably higher concentrations of the former in particularly the liver and the blood. [Methyl-(11)C]-ZEGFR:2377-ST-CH3 successfully visualized FaDu and A431 xenografts with moderate and high EGFR expression levels, respectively. However, in FaDu tumors, the non-specific uptake was large and sometimes equally large, illustrating the importance of proper controls. In the A431 group observed longitudinally, non-specific uptake remained at same level over the observation period. Specific uptake increased with tumor size, but changes varied widely over time in individual tumors. Total (membranous and cytoplasmic) EGFR in excised sections increased with tumor growth. There was no positive correlation between total EGFR and specific tracer uptake, which, since ZEGFR:2377 binds extracellularly and is slowly internalized, indicates a discordance between available membranous and total EGFR expression levels.
  
  
  CONCLUSIONS
  Same-day in vivo dual tracer imaging enabled by the Sel-tag technology and (11)C-labeling provides a method to non-invasively monitor membrane-localized EGFR as well as factors affecting non-specific uptake of the PET ligand.
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Qing
authors.initials: Q
authors.lastname: Cheng
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Helena
authors.initials: H
authors.lastname: Wållberg
authors.affiliation: Department of Clinical Neurosciences, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Jonas
authors.initials: J
authors.lastname: Grafström
authors.affiliation: Department of Clinical Neurosciences, Karolinska Institutet, Stockholm, Sweden.|Karolinska Experimental Research and Imaging Center, Department of Comparative Medicine, Karolinska University Hospital, Stockholm, Sweden.
authors.forename: Li
authors.initials: L
authors.lastname: Lu
authors.affiliation: Department of Clinical Neurosciences, Karolinska Institutet, Stockholm, Sweden.|Neuroradiology Department, R3:00, Karolinska University Hospital, SE-17176, Stockholm, Sweden.
authors.forename: Jan-Olov
authors.initials: JO
authors.lastname: Thorell
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Maria
authors.initials: M
authors.lastname: Hägg Olofsson
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Stig
authors.initials: S
authors.lastname: Linder
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Katarina
authors.initials: K
authors.lastname: Johansson
authors.affiliation: Department of Clinical Neurosciences, Karolinska Institutet, Stockholm, Sweden.|Neuroradiology Department, R3:00, Karolinska University Hospital, SE-17176, Stockholm, Sweden.
authors.forename: Tetyana
authors.initials: T
authors.lastname: Tegnebratt
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Elias S J
authors.initials: ES
authors.lastname: Arnér
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden. sharon.stone-elander@ki.se.|Department of Clinical Neurosciences, Karolinska Institutet, Stockholm, Sweden. sharon.stone-elander@ki.se.|Neuroradiology Department, R3:00, Karolinska University Hospital, SE-17176, Stockholm, Sweden. sharon.stone-elander@ki.se.
authors.forename: Sharon
authors.identifier: http://orcid.org/0000-0002-7990-1498
authors.initials: S
authors.lastname: Stone-Elander
authors.affiliation: Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Hanna-Stina Martinsson
authors.initials: HS
authors.lastname: Ahlzén
authors.affiliation: Division of Protein Technology, School of Biotechnology, Royal Institute of Technology, Stockholm, Sweden.
authors.forename: Stefan
authors.initials: S
authors.lastname: Ståhl
keywords: Affibody molecule; Dual tracer imaging; Epidermal growth factor receptor; Positron emission tomography; Sel-tag