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(Circulation. 2007;115:2292-2298.)
© 2007 American Heart Association, Inc.

Imaging

Optical Visualization of Cathepsin K Activity in Atherosclerosis With a Novel, Protease-Activatable Fluorescence Sensor

Farouc A. Jaffer, MD, PhD^*; Dong-Eog Kim, MD, PhD^*; Luisa Quinti, PhD; Ching-Hsuan Tung, PhD; Elena Aikawa, MD, PhD; Ashvin N. Pande, MD; Rainer H. Kohler, PhD; Guo-Ping Shi, DSc; Peter Libby, MD; Ralph Weissleder, MD, PhD

From the Center for Molecular Imaging Research (F.A.J., D.K., L.Q., C.T., E.A., A.N.P., R.H.K., R.W.) and Cardiology Division (F.A.J.), Massachusetts General Hospital, Boston, and Donald W. Reynolds Cardiovascular Clinical Research Center, Harvard Medical School, Boston, Mass (F.A.J., A.N.P., P.L., R.W.). Dr Kim is currently at the Department of Neurology, Dongguk University International Hospital, Goyang, South Korea. Drs Pande, Shi, and Libby currently are at the Cardiovascular Division, Brigham and Women’s Hospital, Boston, Mass.

Correspondence to Farouc Jaffer or Ralph Weissleder, MGH-CMIR, 149 13th St, Room 5406, Boston, MA 02129. E-mail fjaffer{at}partners.org or weissleder@helix.mgh.harvard.edu

Received September 12, 2006; accepted February 26, 2007.

Background— Cathepsin K (CatK), a potent elastinolytic and collagenolytic cysteine protease, likely participates in the evolution and destabilization of atherosclerotic plaques. To assess better the biology of CatK activity in vivo, we developed a novel near-infrared fluorescence (NIRF) probe for imaging of CatK and evaluated it in mouse and human atherosclerosis.

Methods and Results— The NIRF imaging agent consists of the CatK peptide substrate GHPGGPQGKC-NH₂ linked to an activatable fluorogenic polymer. In vitro, CatK produced a 2- to 14-fold activation of the agent over other cysteine and matrix metalloproteinases (P<0.0001), as well as a >8-fold activation over a control imaging agent (P<0.001). Optical imaging of atheroma revealed >100% NIRF signal increases in apolipoprotein E^–/– mice in vivo (n=13; P<0.05, CatK imaging agent versus control agent) and in human carotid endarterectomy specimens ex vivo (n=14; P<0.05). Fluorescence microscopy of plaque sections demonstrated that enzymatically active CatK (positive NIRF signal) localized primarily in the vicinity of CatK-positive macrophages. Augmented NIRF signal (reflecting CatK activity) colocalized with disrupted elastin fibers within the media underlying plaques.

Conclusions— Use of this novel protease-activatable NIRF agent for optical imaging in vivo demonstrated preferential localization of enzymatically active CatK to macrophages, consistent with their known greater elastinolytic capabilities compared with smooth muscle cells. Augmented CatK proteolysis in atheromata further links CatK to vascular remodeling and plaque vulnerability.

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