Selective molecular imaging of viable cancer cells with pH-activatable fluorescence probes

Abstract

A goal of cancer research is to develop specific and sensitive tumor-imaging techniques for early detection while minimizing background signals from nontarget, 'normal' tissues. The authors have designed a 'pH-activatable' probe, consisting of a targeted macromolecule (monoclonal antibody) and a fluorescence probe, which is activated after internalization in the lysosomes of targeted cancer cells. The utility of this approach for imaging HER2-positive lung cancer cells in mice is shown. A long-term goal of cancer diagnosis is to develop tumor-imaging techniques that have sufficient specificity and sensitivity. To achieve this goal, minimizing the background signal originating from nontarget tissues is crucial. Here we achieve highly specific in vivo cancer visualization by using a newly designed targeted 'activatable' fluorescent imaging probe. This agent is activated after cellular internalization by sensing the pH change in the lysosome. Novel acidic pH–activatable probes based on the boron-dipyrromethene fluorophore were synthesized and then conjugated to a cancer-targeting monoclonal antibody. As proof of concept, ex vivo and in vivo imaging of human epidermal growth factor receptor type 2–positive lung cancer cells in mice was performed. The probe was highly specific for tumors with minimal background signal. Furthermore, because the acidic pH in lysosomes is maintained by the energy-consuming proton pump, only viable cancer cells were successfully visualized. The design concept can be widely adapted to cancer-specific, cell surface–targeting molecules that result in cellular internalization.