Molecular imaging reveals trastuzumab-induced epidermal growth factor receptor downregulation in vivo

Previous in vitro studies demonstrated that treating tumors expressing both epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 with trastuzumab resulted in increased EGFR homodimerization and subsequent rapid downregulation of EGFR. We investigated whether molecular imaging using near-infrared fluorescence (NIRF) imaging and PET probes could sensitively detect trastuzumab-induced EGFR downregulation in vivo. The F(ab')2 antibody fragment PaniF(ab')2 was generated by digesting the anti-EGFR monoclonal antibody panitumumab. PaniF(ab')2 was labeled with either a NIRF dye or (68)Ga, and optical imaging and small-animal PET imaging of Dye-PaniF(ab')2 and (68)Ga-PaniF(ab')2, respectively, were performed in HT-29 tumor-bearing nude mice treated with trastuzumab or untreated control. Longitudinal NIRF imaging studies revealed significantly reduced tumor uptake of Dye-PaniF(ab')2 on days 5 and 7 in trastuzumab-treated HT-29 tumors, compared with control. Western blotting confirmed the downregulation of EGFR after treatment with trastuzumab. Small-animal PET on day 5 after trastuzumab treatment also demonstrated decreased (68)Ga-PaniF(ab')2 uptake in trastuzumab-treated HT-29 tumors. The tumor uptake value of (68)Ga-PaniF(ab')2 obtained from PET imaging had an excellent linear correlation with the uptake value measured using biodistribution. The downregulation of EGFR induced by trastuzumab treatment could be detected noninvasively using optical and PET imaging. This molecular imaging strategy could provide a dynamic readout of changes in the tumor signaling and may facilitate the noninvasive monitoring of the early tumor response to drug treatment.