Size distribution of microbubbles as a function of shell composition

Effect of Poly(ethylene glycol) Configuration on Microbubble Pharmacokinetics

Microbubbles (MBs) hold substantial promise for medical imaging and therapy; nonetheless, knowledge gaps persist between composition, structure, and in vivo performance, especially with respect to pharmacokinetics. Of particular interest is the role of the poly(ethylene glycol) (PEG) layer, which is thought to shield the MB against opsonization and rapid clearance but is also known to cause an antibody response upon multiple injections. The goal of this study was, therefore, to elucidate the role of the PEG layer in circulation persistence of MBs in the naïve animal (prior to an adaptive immune response). Here, we directly observe the number and size of individual MBs obtained from blood samples, unifying size and concentration into the microbubble volume dose (MVD) parameter. This approach enables direct evaluation of the pharmacokinetics of intact MBs, comprising both the lipid shell and gaseous core, rather than separately assessing the lipid or gas components. We examined the in vivo circulation persistence of 3 μm diameter phospholipid-coated MBs with three different mPEG2000 content: 2 mol % (mushroom), 5 mol % (intermediate), and 10 mol % (brush). MB size and concentration in the blood were evaluated by a hemocytometer analysis over 30 min following intravenous injections of 20 and 40 μL/kg MVD in Sprague-Dawley rats. Interestingly, pharmacokinetic analysis demonstrated that increasing PEG concentration on the MB surface resulted in faster clearance. This was evidenced by a 1.6-fold reduction in half-life and area under the curve (AUC) (p < 0.05) in the central compartment. Conversely, the AUC in the peripheral compartment increased with PEG density, suggesting enhanced MB trapping by the mononuclear phagocyte system. This was supported by an in vitro assay, which showed a significant rise in complement C3a activation with a higher PEG content. In conclusion, a minimal PEG concentration on the MB shell (mushroom configuration) was found to prolong circulation and mitigate immunogenicity.

Material Properties, Dissolution and Time Evolution of PEGylated Lipid-Shelled Microbubbles: Effects of the Polyethylene Glycol Hydrophilic Chain Configurations

Polyethylene glycol (PEG) is often added to the lipid coating of a contrast microbubble to prevent coalescence and improve circulation. At high surface density, PEG chains are known to undergo a transition from a mushroom configuration to an extended brush configuration. We investigated the effects of PEG chain configuration on attenuation and dissolution of microbubbles by varying the molar ratio of the PEGylated lipid in the shell with three (0%, 2% and 5%) in the mushroom configuration and two (10% and 20%) in the brush configuration. We measured attenuation through the bubble suspensions and used it to obtain the characteristic rheological properties of their shells according to two interfacial rheological models. The interfacial elasticity was found to be significantly lower in the brush regime (∼0.6 N/m) than in the mushroom regime (∼1.3 N/m), but similar in value within each regime. The dissolution behavior of microbubbles under acoustic excitation inside an air-saturated medium was studied by measuring the time-dependent attenuation. Total attenuation recorded a transient increase because of growth resulting from air influx and an eventual decrease caused by dissolution. Microbubble shell composition with varying PEG concentrations had significant effects on dissolution dynamics.

pH-controlled microbubble shell formation and stabilization

We report on microbubbles with a shell self-assembled from an anionic perfluoroalkylated surfactant, perfluorooctyl(ethyl)phosphate (F8H2Phos). Microbubbles were formed and effectively stabilized from aqueous solutions of F8H2Phos at pH 5.6-8.5. This range overlaps the domains of existence of the monosodic and disodic salts. The shell morphology of microbubbles formed spontaneously by heating aqueous solutions of F8H2Phos was monitored during cooling, directly on the microscope's stage. At pH 5.6, the shell collapses through nucleation of folds, as typical for insoluble surfactants. At pH 8.5, no folds were seen during shrinking. At higher pH, the microbubbles rapidly adsorb on the glass. The effect of pH (from 5.6 to 9.7) on adsorption kinetics of F8H2Phos at the air/water interface, and on the elasticity of its Gibbs films, was determined. At low pH, F8H2Phos is highly surface active. The interfacial film undergoes a dilute-to-condensed phase transition and a dramatic increase of elastic module, leading to extremely high values (up to 500 mN m(-1)). At high pH, the surfactant's adsorption is quasi-instantaneous, but interfacial tension lowering is limited, leading to very low elastic module (∼5 mN m(-1)). At pH 5.6 and 8.5, the interfacial tension of F8H2Phos adsorbed on millimetric bubbles and compressed at a rate similar to that exerted on micrometric bubbles during deflation is lower than the equilibrium interfacial tension. Langmuir monolayers of F8H2Phos are highly stable at low pH and feature a liquid expanded/liquid condensed transition; at high pH, they do not withstand compression. Both mono- and disodic F8H2Phos salts are needed to effectively stabilize microbubbles: the rapidly adsorbed disodic salt stabilizes a newly created air/water interface; the more surface active monosodic salt then replaces the more water-soluble disodic salt at the interface. During deflation, the surfactant shell undergoes a transition toward a highly elastic phase, which further contributes to bubble stabilization.