Introduction: Anti-EGFR (epidermal growth factor receptor) antibody based treatment strategies have been successfully implemented in head and neck squamous cell carcinoma (HNSCC). Unfortunately, predicting a therapeutic response remains a challenge and is far from reliable. Although significant effort has been invested in understanding EGFR-mediated changes in cell signaling with treatment and its efficacy, the delivery and histological localization in (peri-)tumoral compartments of antibody-based therapeutics in human tumors is poorly understood nor made visible. In this first in human study of systemically administered fluorescently labeled cetuximab, we sought to localize antibody delivery to tumor compartments by optical molecular imaging (i.e. denominated as in vivo fluorescence immunohistochemistry) and correlates with biological markers thought to influence antibody delivery to tumors.

Materials and Methods: Specimens were collected from nine consented patients enrolled in a clinical trial evaluating the safety and tumor-specificity of systemically injected cetuximab-IRDye800 in patients with HNSCC. The distribution of intratumoral anti-EGFR antibody cetuximab-IRDye800 fluorescence as a correlate of antibody distribution in surgical resection specimens was assessed and correlated with histopathological analysis and tumor biological characteristics.

Results: Cetuximab-IRDye800 localized specifically to tumor cells; significantly higher mean fluorescent intensities (MFIs) were measured within the tumor compartment compared with the stromal compartment or normal epithelium (P<0.001). Fluorescence intensity correlated significantly with EGFR density (P<0.001), after excluding keratinizing areas in HNSCC, which demonstrated discordant regions with high EGFR expression and low fluorescence. Consistent with some of the commonly known toxicities related to cetuximab therapy, we found strong fluorescence intensity in sebaceous, submandibular and sublingual glands.

Conclusions: We were able for the first time in humans to evaluate (peri-)tumoral localization of an anti-EGFR fluorescently labeled antibody by optical molecular imaging. Clearly, in vivo fluorescence immunohistochemistry with fluorescently labeled antibodies correlating morphological characteristics to levels of antibody delivery, may improve treatment paradigms based on understanding intratumoral antibody delivery.