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Disparity Between In Vivo EGFR Expression and ⁸⁹Zr-Labeled Cetuximab Uptake Assessed with PET

¹ Department of Radiation Oncology (MAASTRO), GROW—School for Oncology and Developmental Biology, Maastricht University, Maastricht, The Netherlands; ² Department of Nuclear Medicine, University Hospital Gasthuisberg and KU Leuven, Leuven, Belgium; ³ Departments of Nuclear Medicine and PET Research and of Otolaryngology/Head and Neck Surgery, VU University Medical Centre, Amsterdam, The Netherlands; and ⁴ Ontario Cancer Institute/Princess Margaret Hospital, Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada

Correspondence: For correspondence or reprints contact: Hugo Aerts, Department of Radiation Oncology (MAASTRO), Maastricht University, Dr. Tanslaan 12, Maastricht, The Netherlands 6221JE. E-mail: hugo.aerts{at}maastro.nl

The epidermal growth factor receptor (EGFR) is highly expressed in a significant number of human malignancies, and its expression is associated with tumor aggressiveness and overall treatment resistance. The monoclonal antibody cetuximab is increasingly used in clinical settings as a treatment modality in combination with more conventional therapies, such as radio- and chemotherapy. Currently, little is known about tumor-specific uptake and overall pharmacokinetics. Noninvasive quantification of cetuximab uptake could provide important diagnostic information for patient selection and therapy evaluation. To this end, we have developed and validated a novel probe using cetuximab labeled with the long-lived positron emitter ⁸⁹Zr for PET imaging. Methods: Tumor cell lines with varying EGFR expression levels were used for in vivo tumor imaging experiments. PET with ⁸⁹Zr-labeled cetuximab (3.75 ± 0.14 MBq) was performed on tumor-bearing NMRI-nu mice at multiple time points after injection (ranging from 1 to 120 h) and quantified by drawing regions of interest on selected tissues. Uptake was compared by biodistribution -counting, and ex vivo EGFR expression levels were quantified using Western blot analysis. Results: Uptake of ⁸⁹Zr-labeled cetuximab was demonstrated in the EGFR-positive tumors. However, the EGFR levels measured in vivo did not correlate with the relative signal obtained by PET. Tumor-to-blood ratios were significantly higher in the cell lines with intermediate (compared with the high) EGFR expression starting from 24 h after injection. Normal tissue uptake was unaffected by the different tumor types. Ex vivo -counting experiments confirmed the observed in vivo PET results. A similar disparity was found between ⁸⁹Zr-labeled cetuximab tumor uptake and in vivo EGFR expression levels as demonstrated by Western blotting. Conclusion: The ⁸⁹Zr-labeled cetuximab imaging probe is a promising tool for noninvasive evaluation of cetuximab uptake. Our results demonstrate a disparity between in vivo EGFR expression levels and cetuximab uptake. In a general sense, the results indicate a disparity between antibody uptake and expression levels of a biologic target in a tumor, suggesting that additional pharmacokinetic or pharmacodynamic mechanisms influence tumor delivery of this therapy. These additional mechanisms may explain why receptor expression levels alone are not sufficient to predict patient response.

Key Words: zirconium-89 • cetuximab • EGFR • PET imaging • disparity

^* Contributed equally to this work.

COPYRIGHT © 2009 by the Society of Nuclear Medicine, Inc.

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