Positron Emission Tomography and Optical Imaging to Monitor Bioorthogonal Diels-Alder Click Chemistry of Trastuzumab with a Porphyrin

Click chemistry to allow in vivo conjugation of a fluorophore porphyrin (Por)-tetrazine (Tz) with the human epidermal growth factor receptor 2 (HER2)-targeting trastuzumab conjugated with trans-cyclooctene (TCO) is described here. In vitro experiments confirmed successful click reactions between Por-Tz and trastuzumab-TCO and validated preserved trastuzumab immunoreactivity (no significant change in HER2 binding, p > 0.05). Positron emission tomography (PET) of [⁸⁹Zr]Zr-DFO-trastuzumab-TCO demonstrated 17.1 ± 2.9% injected dose per gram of tumor at 48 h postinjection. Optical imaging showed an ∼10-fold increase in the click group for Por-Tz when compared with Por-Tz alone. This preclinical data demonstrate a pretargeted approach for dual PET and optical imaging of HER2-expressing tumors.