Insonation of Targeted Microbubbles Produces Regions of Reduced Blood Flow Within Tumor Vasculature

Objectives: In ultrasound molecular imaging, a sequence of high-pressure ultrasound pulses is frequently applied to destroy bound targeted microbubbles, to quantify accumulated microbubbles or to prepare for successive microbubble injections; however, the potential for biological effects from such a strategy has not been fully investigated. Here, we investigate the effect of high-pressure insonation of bound microbubbles and the potential for thrombogenic effects.

Materials And Methods: A total of 114 mice carrying either Met-1 or neu deletion mutant (NDL) tumors was insonified (Siemens Sequoia system, 15L8 transducer, 5-MHz color-Doppler pulses, 4 or 2 MPa peak-negative pressure, 8.1-millisecond pulse repetition period, 6-cycle pulse length, and 900-millisecond insonation). Microbubbles conjugated with cyclic-arginine-glycine-aspartic acid (cRGD) or cyclic-aspartic-acid-glycine-tyrosine (3-NO)-glycine-hydroxyproline-asparagine (LXY-3) peptides or control (no peptide) microbubbles were injected, and contrast pulse sequencing was used to visualize the flowing and bound microbubbles. An anti-CD41 antibody was injected in a subset of animals to block potential thrombogenic effects.

Results: After the accumulation of targeted microbubbles and high-pressure (4 MPa) insonation, reduced blood flow, as demonstrated by a reduction in echoes from flowing microbubbles, was observed in 20 Met-1 mice (71%) and 4 NDL mice (40%). The area of low image intensity increased from 22 ± 13% to 63 ± 17% of the observed plane in the Met-1 model (P < 0.01) and from 16 ± 3% to 45 ± 24% in the NDL model (P < 0.05). Repeated microbubble destruction at 4 MPa increased the area of low image intensity to 76.7 ± 13.4% (P < 0.05). The fragmentation of bound microbubbles with a lower peak-negative pressure (2 MPa) reduced the occurrence of the blood flow alteration to 28% (5/18 Met-1 tumor mice). The persistence of the observed blood flow change was approximately 30 minutes after the microbubble destruction event. Dilated vessels and enhanced extravasation of 150 kDa fluorescein-isothiocyanate (FITC)-dextran were observed by histology and confocal microscopy. Preinjection of an anti-CD41 antibody blocked the reduction of tumor blood flow, where a reduction in blood flow was observed in only 1 of 26 animals.

Conclusion: High-pressure fragmentation of microbubbles bound to tumor endothelial receptors reduced blood flow within 2 syngeneic mouse tumor models for ∼30 minutes. Platelet activation, likely resulting from the injury of small numbers of endothelial cells, was the apparent mechanism for the flow reduction.