Activatable Near-Infrared Fluorescent Probe for In Vivo Imaging of Fibroblast Activation Protein-alpha

摘要

Fibroblast activation protein-alpha (FAPa) is a cell surface glycoprotein which is selectively expressed by tumor-associated fibroblasts in malignant tumors but rarely on normal tissues. FAPa has also been reported to promote tumor growth and invasion and therefore has been of increasing interest as a promising target for designing tumor-targeted drugs and imaging agents. Although medicinal studyon FAPa inhibitors has led to the discovery of many FAPa-targeting inhibitors including a drug candidate in a phase II clinical trial, the development of imaging probes to monitor the expression and activity of FAPa in vivo has largely lagged behind. Herein, we report an activatable near-infrared (NIR) fluorescent probe (ANP_(FAP)) for in vivo optical imaging of FAPa. The ANP_(FAP) consists of a NIR dye (Cy5.5) and a quencher dye (QSY21) which are linked together by a short peptide sequence (KGPGPNQC) specific for FAPa cleavage. Because of the efficient fluorescence resonance energy transfer (FRET) between CyS.5 arid QSY21 in ANP_(FAP), high contrast on the NIR fluorescence signal can be achieved after the cleavage of the peptide sequence by FAPa both in vitro and in vivo. In vitro assay on ANP_(FAP) indicated the specificity of the probe to FAPa. The in vivo optical imaging using ANP_(FAP) showed fast tumor uptake as well as high tumor to background contrast on U87MG tumor models with FAPa expression, while much lower signal and tumor contrast were observed in the C6 tumor without FAPa expression, demonstrating the in vivo targeting specificity of the ANP_(FAP). Ex vivo imaging also demonstrated ANP_(FAP) had high tumor uptake at 4 h post injection. Collectively, these results indicated that ANP_(FAP) could serve as a useful NIR optical probe for early detection of FAPa expressing tumors.