Search

Background Molecular imaging of thrombus within fissures of vulnerable atherosclerotic plaques requires sensitive detection of a robust thrombus-specific contrast agent. In this study, we report the development and characterization of a novel ligand-targeted paramagnetic molecular imaging agent with high avidity for fibrin and the potential to sensitively detect active vulnerable plaques. Methods and Results The nanoparticles were formulated with 2.5 to 50 mol% Gd-DTPA-BOA, which corresponds to >50 000 Gd 3+ atoms/particle. Paramagnetic nanoparticles were characterized in vitro and evaluated in vivo. In contradistinction to traditional blood-pool agents, T1 relaxation rate as a function of paramagnetic nanoparticle number was increased monotonically with Gd-DTPA concentration from 0.18 mL · s −1 · pmol −1 (10% Gd-DTPA nanoparticles) to 0.54 mL · s −1 · pmol −1 for the 40 mol% Gd-DTPA formulations. Fibrin clots targeted in vitro with paramagnetic nanoparticles presented a highly detectable, homogeneous T1-weighted contrast enhancement that improved with increasing gadolinium level (0, 2.5, and 20 mol% Gd). Higher-resolution scans and scanning electron microscopy revealed that the nanoparticles were present as a thin layer over the clot surface. In vivo contrast enhancement under open-circulation conditions was assessed in dogs. The contrast-to-noise ratio between the targeted clot (20 mol% Gd-DTPA nanoparticles) and blood was ≈118±21, and that between the targeted clot and the control clot was 131±37. Conclusions These results suggest that molecular imaging of fibrin-targeted paramagnetic nanoparticles can provide sensitive detection and localization of fibrin and may allow early, direct identification of vulnerable plaques, leading to early therapeutic decisions.

In this study, the sensitivity of a novel fibrin-targeted contrast agent for fibrin detection was defined in vitro on human thrombus. The contrast agent was a lipid-encapsulated perfluorocarbon nanoparticle with numerous Gd-DTPA complexes incorporated into the outer surface. After binding to fibrin clots, scanning electron microscopy of treated clots revealed dense accumulation of nanoparticles on the clot surfaces. Fibrin clots with sizes ranging from 0.5‐7.0 mm were imaged at 4.7 T with or without treatment with the targeted contrast agent. Regardless of sizes, untreated clots were not detectable by T1-weighted MRI, while targeted contrast agent dramatically improved the detectability of all clots. Decreases in T1 and T2 relaxation times (20 ‐ 40%) were measured relative to the surrounding media and the control clots. These results suggest the potential for sensitive and specific detection of microthrombi that form on the intimal surfaces of unstable atherosclerotic plaque. Magn Reson Med 44:867‐ 872, 2000. © 2000 Wiley-Liss, Inc.

For 30 years, medical professionals have sought the ‘‘magic bullet’’ which would allow sensitive detection of disease and facilitate localized administration of potent chemotherapeutic agents without systemic toxicities. Ligand targeted ultrasonic contrast agents have the potential to improve diagnostic sensitivity, port drugs to specific pathologic tissues, and provide dosimetry of the delivered therapeutics. The authors have recently developed a ligand directed, lipid encapsulated perfluorocarbon emulsion particle (250‐nm diameter). These particles, when bound to biological surfaces, strikingly increase the acoustic reflectivity, although the particles have low inherent echogenicity. Contrast enhancement of the reflectivity of blood thrombi, fibrin clots, and nitrocellulose membranes has been tentatively explained by a transmission line model. Intravenous injection of the perfluorocarbon emulsion particles has localized and acoustically enhanced remote thrombi which were otherwise difficult to detect. The contrast agent has a 1‐h circulatory half‐life which greatly facilitates successful systemic targeting. The small particle size of these emulsions affords access to tiny capillary beds and vascular endothelial disruptions such as those following angioplasty. Collectively, these studies suggest perfluorocarbon contrast agents can target molecular epitopes within and proximate to the systemic circulation. These agents are creating new and exciting opportunities for ultrasound in medical diagnosis and therapy.