Affibody-Based PET Imaging to Guide EGFR-Targeted Cancer Therapy in Head and Neck Squamous Cell Cancer Models

In head and neck squamous cell cancer, the human epidermal growth factor receptor 1 (EGFR) is the dominant signaling molecule among all members of the family. So far, cetuximab is the only approved anti-EGFR monoclonal antibody used for the treatment of head and neck squamous cell cancer, but despite the benefits of adding it to standard treatment regimens, attempts to define a predictive biomarker to stratify patients for cetuximab treatment have been unsuccessful. We hypothesized that imaging with EGFR-specific radioligands may facilitate noninvasive measurement of EGFR expression across the entire tumor burden and allow for dynamic monitoring of cetuximab-mediated changes in receptor expression. Methods: EGFR-specific Affibody molecule (Z_EGFR:03115) was radiolabeled with ⁸⁹Zr and ¹⁸F. The radioligands were characterized in vitro and in mice bearing subcutaneous tumors with varying levels of EGFR expression. The protein dose for imaging studies was assessed by injecting ⁸⁹Zr-deferoxamine-Z_EGFR:03115 (2.4-3.6 MBq, 2 microg) either together with or 30 min after increasing amounts of unlabeled Z_EGFR:03115 (1, 5, 10, 15, and 20 microg). PET images were acquired at 3, 24, and 48 h after injection, and the image quantification data were correlated with the biodistribution results. The EGFR expression and biodistribution of the tracer were assessed ex vivo by immunohistochemistry, Western blot, and autoradiography. To downregulate the EGFR level, treatment with cetuximab was performed, and ¹⁸F-aluminium fluoride-NOTA-Z_EGFR:03115 (12 microg, 1.5-2 MBq/mouse) was used to monitor receptor changes. Results: In vivo studies demonstrated that coinjecting 10 microg of nonlabeled molecules with ⁸⁹Zr-deferoxamine-Z_EGFR:03115 allows for clear tumor visualization 3 h after injection. The radioconjugate tumor accumulation was EGFR-specific, and PET imaging data showed a clear differentiation between xenografts with varying EGFR expression levels. A strong correlation was observed between PET analysis, ex vivo estimates of tracer concentration, and receptor expression in tumor tissues. Additionally, ¹⁸F-aluminium fluoride-NOTA-Z_EGFR:03115 could measure receptor downregulation in response to EGFR inhibition. Conclusion: Z_EGFR:03115-based radioconjugates can assess different levels of EGFR level in vivo and measure receptor expression changes in response to cetuximab, indicating a potential for assessment of adequate treatment dosing with anti-EGFR antibodies.