Acoustic radiation force enhances adhesion of microbubbles targeted to P-selectin

Ultrasound contrast microbubbles can be targeted to regions of intravascular pathology. However, the efficiency of microbubble attachment to molecular targets tends to be low, and adhesion in large, high-flow vessels has not been demonstrated. The efficacy of targeted contrast enhanced ultrasound imaging may be improved by increasing the fraction of free-flowing microbubbles that directly contact the vessel wall at the target site. Several studies have proven that low-intensity acoustic radiation can be used as a mechanism to force free-stream microbubbles toward a surface. In the current work, we present evidence that acoustic radiation increases the specific delivery of targeted microbubbles in vitro and in vivo. Lipid shell microbubbles bearing a monoclonal antibody as a targeting ligand were infused through a capillary flow chamber coated with the pro-inflammatory endothelial protein P-selectin. A 2.0 MHz ultrasonic pulse train was applied perpendicular to the flow direction, and microbubble accumulation was observed on the flow chamber surface opposite the transducer. We observed that acoustic pressures between 70 and 170 kPa improve microbubble adhesion up to 60-fold at microbubble concentrations between 0.25 and 75×10⁶ mL^-1. We found that adhesion was highly dependent upon microbubble concentration, and that acoustic pressure mediated the greatest enhancement in adhesion at concentrations within the clinical dosing range. Acoustic pressure enhanced adhesion to P-selectin nearly 80-fold at a wall shear rate of 1244 s^-1, suggesting that this mechanism is appropriate for achieving targeted microbubble delivery in very fast-flow vessels. We examined microbubble adhesion in a murine model of microvascular inflammation using intravital microscopy, and observed a six-fold increase in targeted microbubble adhesion in inflamed venules with acoustic radiation.