Near-infrared fluorescent read-out probe for ultra-sensitive imaging of leucine aminopeptidase in vitro and in vivo

Leucine aminopeptidase (LAP), a kind of proteolytic enzyme and an indicator of many kinds of cancer. Developing effective methods for tracking endogenous LAP activity is crucial for LAP-related cancer diagnosis and treatment. In this work, we report a new near-infrared fluorescent probe: CY-P, which is capable of highly effective monitoring of endogenous LAP activity in vitro and in vivo. CY-P was constructed by incorporating a dipeptide (Cys-Leu: Cysteine-Leucine) into a water-soluble NIR-emitting fluorophore (CY–OH: a semi-cyanine scaffold). Upon exposure to LAP, the amide bond in Cys-Leu will be specifically cleaved by LAP, and subsequent an intramolecular cyclization to release the unmasked phenol-based semi-cyanine. In this process, a dramatic fluorescence enhancement at 701 nm was observed, the evaluation of LAP activity is established in terms of the relationship between fluorescence light-up efficiency and LAP activity. In our experiments, CY-P features high feedback towards LAP with various advantages, such as low detection limit (4.9 × 10−5 U mL−1), fast-response (∼9 min), good biocompatibility, and NIR emission. As expected, the probe exhibits low cytotoxicity and excellent cell membrane permeability, which is successfully used to monitor endogenous LAP activity in living cancer cells and zebrafish models. Therefore, we anticipate that this newly designed fast LAP detection platform will provide an alternative method for the studies of related diseases.