Nonsymmetrical Near-Infrared Rhodamines for Carboxylesterase-Activatable Tumor Imaging and Photodynamic Therapy

Fluorophores with large Stokes shifts, desired quantum yields, and tunable near-infrared (NIR) absorption/emission wavelengths are useful for developing activatable probes for bioimaging and biomedical applications. However, very few fluorophores possess these properties. Here, we engineer new nonsymmetrical NIR rhodamine-based fluorophores through the introduction of varying phenothiazine moieties as auxochromes for fluorogenic imaging and targeted photodynamic therapy. It is shown that rhodamine incorporated with phenoselenazine exhibits substantially red-shifted absorption/emission profiles, large Stokes shift, and efficient production of reactive oxygen species. We further engineer a new activatable probe for carboxylesterase (CE) by caging the secondary amine group with a furan substrate. It is demonstrated that the probe with a predominant spirolactone state is switched to the open zwitterion form upon CE-catalyzed hydrolysis, allowing specific imaging of CE activity and induction of apoptosis in CE-positive cells via PDT. The probe derived with a polyethylene glycol moiety is further developed for specific imaging and PDT-mediated ablation of tumors in vivo. This study highlights the great potential for developing nonsymmetrical NIR rhodamine-based fluorophores for theranostic applications.