Molecular imaging of therapeutic response to EGF receptor blockade in colorectal cancer

Objectives We have evaluated non-invasive molecular imaging methods as correlative biomarkers of therapeutic efficacy of cetuximab in preclinical mouse models of colorectal cancer (CRC). Molecular imaging was validated at genomic, proteomic, and immunohistochemical levels to quantify epidermal growth factor (EGF) binding, apoptosis and proliferation.

Methods Optical imaging probes targeting EGFR expression and apoptosis were synthesized and evaluated in vitro and in vivo. Proliferation was assessed by [¹⁸F]-FLT PET. Assessment of inhibition of EGFR signaling by cetuximab was accomplished by concomitant imaging of NIR800-EGF, NIR700-Annexin-V, and [¹⁸F]-FLT in human CRC cell line xenografts. Imaging results were validated by q-RT-PCR, microarray, western blot, and immunohistochemical (IHC) analysis of tumor tissues immediately following in vivo imaging.

Results NIR800-EGF accumulation in tumors reflected relative EGFR expression and EGFR occupancy by cetuximab. NIR700-Annexin-V accumulation correlated with cetuximab-induced apoptosis as assessed by immunohistochemical staining of caspase 3. No significant difference in tumor proliferation was noted between treated and untreated animals by [¹⁸F]-FLT PET or Ki67 IHC.

Conclusions These imaging approaches may prove useful for serial, non-invasive monitoring of the biological effects of EGFR inhibition in preclinical studies. It is anticipated that these assays can be readily adapted for clinical use.

Research Support GI SPORE (P50 95103), MMHCC (U01 084239), K25 CA127349, U24 CA126588, Burroughs Wellcome Fund