Imaging of EGFR and EGFR tyrosine kinase overexpression in tumors by nuclear medicine modalities

Imaging of EGFR expression in murine xenografts using site-specifically labelled anti-EGFR 111In-DOTA-Z EGFR:2377 Affibody molecule: aspect of the injected tracer amount

INTRODUCTION

Overexpression of epidermal growth factor receptor (EGFR) is a prognostic and predictive biomarker in a number of malignant tumours. Radionuclide molecular imaging of EGFR expression in cancer could influence patient management. However, EGFR expression in normal tissues might complicate in vivo imaging. The aim of this study was to evaluate if optimization of the injected protein dose might improve imaging of EGFR expression in tumours using a novel EGFR-targeting protein, the DOTA-Z(EGFR:2377) Affibody molecule.

METHODS

An anti-EGFR Affibody molecule, Z(EGFR:2377), was labelled with (111)In via the DOTA chelator site-specifically conjugated to a C-terminal cysteine. The affinity of DOTA-Z(EGFR:2377) for murine and human EGFR was measured by surface plasmon resonance. The cellular processing of (111)In-DOTA-Z(EGFR:2377) was evaluated in vitro. The biodistribution of radiolabelled Affibody molecules injected in a broad range of injected Affibody protein doses was evaluated in mice bearing EGFR-expressing A431 xenografts.

RESULTS

Site-specific coupling of DOTA provided a uniform conjugate possessing equal affinity for human and murine EGFR. The internalization of (111)In-DOTA-Z(EGFR:2377) by A431 cells was slow. In vivo, the conjugate accumulated specifically in xenografts and in EGFR-expressing tissues. The curve representing the dependence of tumour uptake on the injected Affibody protein dose was bell-shaped. The highest specific radioactivity (lowest injected protein dose) provided a suboptimal tumour-to-blood ratio. The results of the biodistribution study were confirmed by gamma-camera imaging.

CONCLUSION

The (111)In-DOTA-Z(EGFR:2377) Affibody molecule is a promising tracer for radionuclide molecular imaging of EGFR expression in malignant tumours. Careful optimization of protein dose is required for high-contrast imaging of EGFR expression in vivo.

Molecular imaging in clinical trials

Imaging of biological processes using specific molecular probes allows exploration of the mechanism of action and efficacy for new therapies. This molecular imaging has made use of modalities including single photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), and optical techniques. Molecular imaging can be used to explore many of the hallmarks of cancer biology, including angiogenesis, proliferation, tissue invasion, evasion of apoptosis, and self-sufficiency in growth signals. Since many of these aspects of cancer biology are in turn the targets of novel therapies in development, molecular imaging techniques have great potential to inform trials of these new agents. The high cost of clinical drug development mandates the optimisation of early phase trial design to maximise the collection of evidence for efficacy and proof of mechanism, endpoints which have, in a number of examples, already been provided by molecular imaging. The variety provided by novel chemistry, and the availability of isotopes with varying physical properties, particularly suits PET imaging as a functional modality for application in clinical trials.

Strategies for molecular imaging of epidermal growth factor receptor tyrosine kinase in cancer

A wealth of research has focused on developing targeted cancer therapies by specifically inhibiting epidermal growth factor receptor tyrosine kinase (EGFR-TK). However, the outcome of most EGFR-TK-targeted drugs that were approved by the Food and Drug Administration or entered clinical trials has been only moderate. Enhancement of EGFR-targeted therapy hinges on a reliable in vivo quantitative molecular imaging method. Such a method would enable monitoring of receptor drug binding and receptor occupancy in vivo; determination of the duration of EGFR inhibition in vivo; and, potentially, identification of a primary or secondary mutation in EGFR leading to drug interaction or loss of EGFR recognition by the drug. This review analyzes the most recent strategies to visualize and quantify EGFR-TK in cancer by nuclear medicine imaging and describes future directions.