Preclinical PET imaging of EGFR levels: pairing a targeting with a non-targeting Sel-tagged Affibody-based tracer to estimate the specific uptake

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 Z EGFR:2377 and its size-matched non-binding control Z Taq:3638 were recombinantly fused with a C-terminal selenocysteine-containing Sel-tag (Z EGFR:2377 -ST and Z Taq: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 Z EGFR:2377 -ST-DyLight488. [Methyl- 11 C]-labeled Z EGFR:2377 -ST-CH 3 and Z Taq:3638 -ST-CH 3 showed similar distributions in vivo, except for notably higher concentrations of the former in particularly the liver and the blood. [Methyl- 11 C]-Z EGFR:2377 -ST-CH 3 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 Z EGFR: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.