Near-infrared optical imaging of protease activity for tumor detection.
OBJECTIVE
To build and test an optical imaging system that is sensitive to near-infrared fluorescent molecular probes activated by specific enzymes in tumor tissues in mice.


METHODS
The imaging system consisted of a source that delivered 610-650-nm excitation light within a lighttight chamber, a 700-nm longpass filter for selecting near-infrared fluorescence emission photons from tissues, and a charge-coupled device (CCD) for recording images. The molecular probe was a biocompatible autoquenched near-infrared fluorescent compound that was activated by tumor-associated proteases for cathepsins B and H. Imaging experiments were performed 0-72 hours after intravenous injection of the probe in nude mice that bore human breast carcinoma (BT-20).


RESULTS
The imaging system had a maximal spatial resolution of 60 microns, with a field of view of 14 cm2. The detection threshold of the nonquenched near-infrared fluorescent dye was subpicomolar in the imaging phantom experiments. In tissue, 250 pmol of fluorochrome was easily detected during the 10-second image acquisition. After intravenous injection of the probe into the tumor-bearing animals, tumors as small as 1 mm became detectable because of tumor-associated enzymatic activation of the quenched compound.


CONCLUSIONS
Tumor proteases can be used as molecular targets, allowing visualization of millimeter-sized tumors. The development of this technology, probe design, and optical imaging systems hold promise for molecular imaging, cancer detection, and evaluation of treatment.
Mahmood
U
U
Department of Radiology, Massachusetts General Hospital, Boston 02125, USA.
Tung
C H
CH
Bogdanov
A
A
Weissleder
R
R
D000230:Adenocarcinoma / Q000175:diagnosis; D000818:Animals; D002401:Cathepsin B / Q000032:analysis; D056655:Cathepsin H; D002403:Cathepsins / Q000032:analysis; D003546:Cysteine Endopeptidases / Q000032:analysis; D010450:Endopeptidases / Q000032:analysis*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008325:Mammary Neoplasms, Experimental / Q000175:diagnosis; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D009369:Neoplasms / Q000175:diagnosis*; D019047:Phantoms, Imaging; D019265:Spectroscopy, Near-Infrared / Q000295:instrumentation*; D014407:Tumor Cells, Cultured
D002403:Cathepsins; D010450:Endopeptidases; D003546:Cysteine Endopeptidases; D002401:Cathepsin B; C534126:CTSH protein, human; D056655:Cathepsin H; C534127:Ctsh protein, mouse