Focused ultrasound and microbubbles for enhanced extravasation

Abstract

The permeability of blood vessels for albumin can be altered by using ultrasound and polymer or lipid-shelled microbubbles. The region in which the microbubbles were destroyed with focused ultrasound was quantified in gel phantoms as a function of pressure, number of cycles and type of microbubble. At 2MPa the destruction took place in a fairly wide area for a lipid-shelled agent, while for polymer-shelled agents at this setting, distinct destruction spots with a radius of only 1mm were obtained. When microbubbles with a thicker shell were used, the pressure above which the bubbles were destroyed shifts to higher values. In vivo both lipid and polymer microbubbles increased the extravasation of the albumin binding dye Evans Blue, especially in muscle leading to about 6-8% of the injected dose to extravasate per gram muscle tissue 30 min after start of the treatment, while no Evans Blue could be detected in muscle in the absence of microbubbles. Variation in the time between ultrasound treatment and Evans Blue injection, demonstrated that the time window for promoting extravasation is at least an hour at the settings used. In MC38 tumors, extravasation already occurred without ultrasound and only a trend towards enhancement with about a factor of 2 could be established with a maximum percentage injected dose per gram of 3%. Ultrasound mediated microbubble destruction especially enhances the extravasation in the highly vascularized outer part of the MC38 tumor and adjacent muscle and would, therefore, be most useful for release of, for instance, anti-angiogenic drugs.

Figure 1

Schematic overview of the therapy and imaging probe system with the microubble containing gel positioned under the transducer. The therapeutic transducer (in black) is generating focused ultrasound, driven by the control unit. In the opening of the therapy transducer (light grey) an imaging transducer can be placed. In the gel block the measurement direction is indicated, parallel (P-plane) and perpendicular (S-plane) to the focused ultrasound beam (TIPS).

Figure 2

seven point ultrasound scan pattern used in gels and on the mouse leg.

Figure 3

size distribution of microbubbles: lipid-shelled and polymer-shelled with a 1:8 shell to core ratio (A) and polymer microbubbles with 3 different shell to core ratio’s.

Figure 4

Destruction pattern in microbubble containing gels after 7 point exposures, 10000 cycles, 11 loops, 1.2 MHz at 0.5, 1, 2, 3, 4 MPa for lipid-shelled microbubbles and polymer-shelled microbubbles with three shell to core ratios.

Figure 5

diameter of destruction spots as a function of the applied pressure for lipid-shelled microbubbles and polymer-shelled microbubbles with three different shell to core ratios.

Figure 6

destruction patterns in gels of polymer-shelled microbubbles, 10000 cycle prf 10, 2 seconds exposure at 1, 2, 3 and 4 MPa, View in the direction of the transducer (P-plane), see figure 1.

Figure 7

% ID/g of Evans Blue in tumor, muscle around the tumor for lipid-shelled and polymer-shelled microbubbles with ultrasound, ultrasound without microbubbles and polymer microbubbles without ultrasound. MuscleR indicates the muscle on the right leg on which no tumor was grown, which was exposed to ultrasound and polymer microbubbles. Average of three experiments. The pictures give an impression of the blue coloration around the tumor and in the muscle for polymer-shelled microbubbles. Ultrasound settings: 1.2 MHz, 10000 cycle, 2 MPa, 11 loops.