Imaging the microvascular response to ultrasound-stimulated therapy in a preclinical animal model of breast cancer

Ultrasound (US)-stimulated therapy has been shown beneficial for improving drug delivery in solid tumors; however there are potential bioeffects that are not fully understood. This study evaluates the presence of bioeffects associated with US-stimulated therapy through the use of multimodal methods. Tumor-bearing mice underwent US-stimulated therapy by exposing systemically circulating microbubbles (MBs) to US treatment at various acoustic pressures of 0.4 MPa (low) or 3.4 MPa (high). Mice were imaged at baseline, day 1, or day 2 with multimodal methods to evaluate tumor response to US-stimulated therapy. Contrast-enhanced T1-weighted magnetic resonance (MR) imaging evaluated vascular permeabilization. Contrast-enhanced US imaging calculated temporal changes in tumor perfusion. Immunohistochemistry was utilized to evaluate microvessel density, red blood cell extravasation, inflammation, apoptosis, and stress. At low acoustic pressures, US-stimulated therapy resulted in a 137.6% increase in tumor perfusion as measured with contrast-enhanced US imaging on day 1, which subsided to 12.4% by day 2. Conversely, high pressure US-stimulated therapy progressively decreased perfusion by 13.1% and 60.5% on days 1 and 2, respectively, which matched patterns observed using MR imaging. Low pressure US-stimulated therapy produced intensity increases of 132.0% (day 1) and 43.6% (day 2) from MR contrast agent accumulation at the tumor site. Histological analysis of CD31 confirmed decreased microvessels in the high pressure US-stimulated therapy group. Overall, US-stimulated therapy employing high pressure US pulse sequences produced a decrease in tumor perfusion that inhibited molecular delivery; however low pressure US-stimulated therapy temporarily increases both tumor perfusion and vascular permeability.