Activatable near-infrared fluorescent probe for in vivo imaging of fibroblast activation protein-alpha.
Fibroblast activation protein-alpha (FAPα) is a cell surface glycoprotein which is selectively expressed by tumor-associated fibroblasts in malignant tumors but rarely on normal tissues. FAPα 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 study on FAPα inhibitors has led to the discovery of many FAPα-targeting inhibitors including a drug candidate in a phase II clinical trial, the development of imaging probes to monitor the expression and activity of FAPα in vivo has largely lagged behind. Herein, we report an activatable near-infrared (NIR) fluorescent probe (ANP(FAP)) for in vivo optical imaging of FAPα. 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 FAPα cleavage. Because of the efficient fluorescence resonance energy transfer (FRET) between Cy5.5 and QSY21 in ANP(FAP), high contrast on the NIR fluorescence signal can be achieved after the cleavage of the peptide sequence by FAPα both in vitro and in vivo. In vitro assay on ANP(FAP) indicated the specificity of the probe to FAPα. 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 FAPα expression, while much lower signal and tumor contrast were observed in the C6 tumor without FAPα 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 FAPα expressing tumors.
Molecular Imaging Program at Stanford (MIPS), Department of Radiology, and Bio-X Program, Canary Center at Stanford for Cancer Early Detection, Stanford University, Stanford, CA 94305-5344, USA.
Jinbo
J
Li
Kai
K
Chen
Hongguang
H
Liu
Kai
K
Cheng
Meng
M
Yang
Jiping
J
Zhang
Jonathan D
JD
Cheng
Yan
Y
Zhang
Zhen
Z
Cheng
D000595:Amino Acid Sequence; D000818:Animals; D002232:Carbocyanines / Q000737:chemistry; D045744:Cell Line, Tumor; D015195:Drug Design; D010450:Endopeptidases; D005260:Female; D005456:Fluorescent Dyes / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D018093:Gelatinases / Q000737:chemistry / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D007259:Infrared Rays; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D057054:Molecular Imaging / Q000379:methods*; D061848:Optical Imaging; D012697:Serine Endopeptidases / Q000737:chemistry / Q000378:metabolism*; D013379:Substrate Specificity; D000099167:Fibroblast Activation Protein Alpha
D002232:Carbocyanines; D010450:Endopeptidases; D005456:Fluorescent Dyes; D018093:Gelatinases; D008565:Membrane Proteins; D012697:Serine Endopeptidases; D000099167:Fibroblast Activation Protein Alpha; C098793:CY5.5 cyanine dye