Published online before print July 16, 2009, doi: 10.1161/ATVBAHA.109.193086
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© 2009 American Heart Association, Inc.
Integrative Physiology/Experimental Medicine |
Hybrid In Vivo FMT-CT Imaging of Protease Activity in Atherosclerosis With Customized Nanosensors
From the Center for Systems Biology (M.N., PW., G.T., P.P., B.M., M.J.P., F.K.S., R.W.), Massachusetts General Hospital and Harvard Medical School, Boston; the Center for Molecular Imaging Research (M.N., P.W., P.P., E.A., M.J.P., F.K.S., R.W.), Massachusetts General Hospital and Harvard Medical School, Charlestown; and VisEn Medical Inc (K.G., M.R.), Bedford, Mass.
Correspondence to Ralph Weissleder, MGH-CSB, CPZN-5206, 185 Cambridge Street, Boston, MA 02124. E-mail rweissleder{at}mgh.harvard.edu
Objective— Proteases are emerging biomarkers of inflammatory diseases. In atherosclerosis, these enzymes are often secreted by inflammatory macrophages, digest the extracellular matrix of the fibrous cap, and destabilize atheromata. Protease function can be monitored with protease activatable imaging probes and quantitated in vivo by fluorescence molecular tomography (FMT). To address 2 major constraints currently associated with imaging of murine atherosclerosis (lack of highly sensitive probes and absence of anatomic information), we compared protease sensors (PS) of variable size and pharmacokinetics and coregistered FMT datasets with computed tomography (FMT-CT).
Methods and Results— Coregistration of FMT and CT was achieved with a multimodal imaging cartridge containing fiducial markers detectable by both modalities. A high-resolution CT angiography protocol accurately localized fluorescence to the aortic root of atherosclerotic apoE–/– mice. To identify suitable sensors, we first modeled signal kinetics in-silico and then compared 3 probes with oligo-L-lysine cleavage sequences: PS-5, 5 nm in diameter containing 2 fluorochromes, PS-25, a 25-nm version with an elongated lysine chain and PS-40, a polymeric nanoparticle. Serial FMT-CT showed fastest kinetics for PS-5 but, surprisingly, highest fluorescence in lesions of the aortic root for PS-40. PS-40 robustly reported therapeutic effects of atorvastatin, corroborated by ex vivo imaging and qPCR for the model protease cathepsin B.
Conclusions— FMT-CT is a robust and observer-independent tool for noninvasive assessment of inflammatory murine atherosclerosis. Reporter-containing nanomaterials may have unique advantages over small molecule agents for in vivo imaging.
To address constraints currently associated with imaging of murine atherosclerosis (lack of highly sensitive probes and absence of anatomic information), we investigated variable protease sensors and coregistered FMT data with computed tomography. This work describes the evolution of FMT-CT into a robust and observer-independent tool for imaging of murine atherosclerosis.
Key Words: FMT-CT • molecular imaging • atherosclerosis • protease activity • inflammation
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