Selectin ligands promote ultrasound contrast agent adhesion under shear flow

Contrast-enhanced ultrasound imaging has shown promise in the field of molecular imaging. This technique relies upon the adhesion of ultrasound contrast agent (UCA) to targeted molecular markers of disease. This is accomplished by coating the surface of the contrast agent with a ligand that specifically binds to the intended molecular marker. Most UCA particles remain in the blood space, and their retention is influenced by the forces imposed by blood flow. For a UCA bound to a molecular target on the vascular endothelium, blood flow imposes a dislodging force that counteracts retention. Additionally, contrast agent adhesion to the molecular marker requires rapid binding kinetics, especially in rapid blood flow. The ability of a ligand:target bond complex to mediate fast adhesion and withstand dislodging force is necessary for efficient ultrasound-based molecular imaging. In the current study, we describe a flow-based adhesion assay which, combined with a novel automated tracking algorithm, enables quick determination of the ability of a targeting ligand to mediate effective contrast agent adhesion. This system was used to explore the adhesion of UCA targeted to the proinflammatory endothelial protein P-selectin via four targeting ligands, which revealed several interesting adhesive behaviors. Contrast agents targeted with glycoconjugate ligands modeled on P-selectin glycoprotein ligand 1 exhibited primarily unstable or transient adhesion, while UCA targeted with an anti-P-selectin monoclonal antibody exhibited primarily firm adhesion, although the efficiency with which these agents were recruited to the target surface was relatively low.