Binding and detachment dynamics of microbubbles targeted to P-selectin under controlled shear flow

This study was performed to assess the binding kinetics of a targeted microbubble contrast agent exposed to shear stress. An ultrasound contrast targeted to P-selectin was designed by conjugating monoclonal antibodies against murine P-selectin (RB40.34) to the lipid monolayer shell of the microbubble using poly(ethylene glycol)-biotin-streptavidin. The attachment and detachment of targeted microbubbles to P-selectin immobilized on a culture dish were assessed in a parallel-plate flow chamber. Targeted microbubbles (5×10⁶ particles/ml) drawn through the flow chamber coated with P-selectin (109 sites/μm²) at a shear stress of 0.3 dyn/cm ² accumulated at a rate of 565 mm^-2 min^-1. Attachment rates increased at higher plate surface densities of P-selectin, and microbubble detachment was reduced. Accumulation rate first increased with shear stress, reached a maximum at ∼0.6 dyn/cm² and then decreased. Control experiments on a plate that lacked P-selectin, or was blocked with mAb RB40.34, resulted in minimal bubble attachment. Microbubble detachment was tested by ramping up shear stress at 30-s intervals. Half-maximal detachment was reached at 34 dyn/cm². Overall, accumulation and retention of targeted ultrasound contrast agents is possible under physiologic flow conditions and is strongly influenced by shear stress and surface density of the target receptor.