The epidermal growth factor receptor (EGFr) is overexpressed in many human cancers making it an interesting target for cancer diagnostics. In this study, epidermal growth factor (EGF) was fluorescently labeled with either Cy5.5, a red dye, or IRDye® 800CW, a near-infrared dye (NIR). These conjugates were delivered intravenously at 1 nmol dye equivalence to mice with high EGFr expression (MDA-MB-468) or low EGFr expression (MDA-MB-435) human breast cancer xenografts. Targeting specificity was shown with the indiscriminate uptake of free dye and loss of specific uptake of EGF-dye conjugates following preadministration of C225, an antibody against the EGFr. The targeting of each EGF-dye conjugate was measured using the ratio of the signal in the tumor region to that of the background region. EGF-IRDye® 800CW had a higher tumor to background ratio (TBR) than EGF-Cy5.5 due to a reduction in the autofluorescence associated with NIR excitation. Both targeted dyes had higher TBRs than free dye or the C225 pre-blocked animals. Immunohistochemical staining for total EGFr and EGFr-phos showed that when EGF-dye conjugates were delivered to the xenografts, in the presence or absence of C225 blocking, EGFr expression was upregulated and the receptors were activated. Xenografts receiving free dye had a lower level of EGFr expression and showed less activated receptors. These findings relate the importance of determining the biological effects of the imaging conjugates on the disease as well as finding the optimal fluorescent dye to maximize target to background ratio.
[First AACR International Conference on Molecular Diagnostics in Cancer Therapeutic Development, Sep 12-15, 2006]