An activatable fluorescent probe for fibroblast activation protein α and its bioimaging applications

Fibroblast activation protein α (FAP) is a member of the plasma membrane-bound serine protease family, which exhibits exopeptidase and endopeptidase activity in organisms. Recently, considerable studies have focused on the key role of FAP in various pathological processes, indicating that it may be a therapeutic target for treating human disorders and diseases. In the present study, a fluorescent probe for FAP was constructed by the introduction of a substrate recognition dipeptide that was rationally modified by tert-butoxycarbonyl group to the fluorophore DDAN. The probe DDAF, synthesized from DDAN, exhibited high selectivity and sensitivity for FAP, with a detection limit of 0.71 ng mL^-1. DDAF was successfully applied for the rapid screening of potent inhibitors of FAP, and a quinoline derivative was identified that exhibited a strong inhibitory effect on FAP. Subsequently, the in vitro visualization and monitoring of FAP in living cancer cells and tissues were performed using DDAF, indicating the widespread expression of FAP in a variety of tumors. These findings provide a promising tool for discovering potent therapeutic agents targeting FAP and for the real-time visualization and monitoring of FAP in tumors.