Reduced blood flow in murine tumors after the destruction of bound, targeted microbubbles

The insonation of circulating microbubbles (MBs) by low frequency (1 MHz) ultrasound (US) pulses has previously been associated with changes in vascular permeability and local changes in blood flow. Here, using a clinical scanner, 5 MHz insonation of bound, targeted MBs is demonstrated to locally alter blood flow in murine tumors. Peptide-targeted MBs were administrated into murine Met-1 and NDL tumor models via tail vein injection (5×10⁷ MBs). Thirty frames of CPS contrast images (Siemens Sequoia 512, 0.09 MI, 10 Hz frame rate) were recorded to assess tumor blood flow. Seven minutes after injection, freely-circulating MBs had cleared from the blood stream leaving bound MBs that had accumulated in the tumor vasculature. At this time, 5 MHz pulses with a peak negative pressure (PNP) of 2 or 4 MPa, a pulse length of 5 cycles and a pulse repetition period of 8.1 ms were transmitted for 0.9 second. Five minutes after the high-pressure pulse sequence, a second dose of MBs was injected and 30 frames of CPS images were acquired. Optical images of systemically-injected FITC-dextran (MW=150,000), pre-administration of an anti-CD41 antibody, and histology were used to investigate the possible mechanism for the vascular changes. After the insonation of bound MBs with a 4 MPa PNP, additional regions of reduced blood flow were observed in 71% of Met-1 tumors (n=28) and 40% of NDL tumors (n = 10). In Met-1 tumors insonified with 4 MPa pulses, the area over which reduced blood flow was observed increased from 22±13% to 63±17% (pμ0.01) of the tumor region of interest. Decreasing the PNP to 2 MPa decreased the percentage of Met-1 tumors with additional regions of reduced blood flow from 71% to 28%. Histological analysis of Met-1 tumors after 4 MPa insonation demonstrated that the mean microvascular diameter in insonified tumors was approximately 17±8 μm, compared to 7±4 μm in control tumors (pμ0.01).- Extravasation of FITC-dextran was observed in 4 MPa insonified, but not control, Met-1 tumors. The results suggest that high-pressure insonation of targeted MBs which had accumulated at high concentration, may result in changes in blood flow.