Photoacoustic-derived biomarkers of nanobubble-mediated cancer treatment response

Gas-filled nanobubbles (NBs) have been used for contrast enhanced ultrasound (US) molecular imaging similar to their microbubble (MB) counterparts. In this work we explore the use of NBs as cancer therapeutic vehicles to enhance radiation therapy and examine the role of photoacoustic (PA) imaging for quantifying the tissue response. Experiments were conducted in 58 mice bearing prostate cancer tumors. The treatment groups consisted of untreated controls (n=14), 8Gy radiation (n=14), US+MB (n=8), US+NB (n=5), US+MB+8Gy (n=12) and US+NB+8Gy (n=5). PA imaging was conducted using the VevoLAZR 21 MHz probe at pre-treatment, 2h and 24h post-treatment. To examine the changes in the tumor microenvironment post-treatment, we measured the tumor oxygen saturation (sO2), total hemoglobin (HbT) and the spectral slope (SS) as a surrogate measurement of vessel size. Histology measures of tumor vascularity/vessel size (CD31) and cellular death (TUNEL) allowed us to interpret the PA biomarker changes. The PA biomarkers revealed the following changes for the US+NB+8Gy group: (a) a 20% decrease in tumor sO2, (b) a 30% increase in tHb and (c) 20% decrease SS. These results suggest that NBs induce vessel damage from US-induced cavitation. Histology elucidates these findings: CD31 staining measuring vascularity decreases by 80% at 24h–this causes the drop in sO2 and increase in HbT; the size of vessels decreases by 10%–this is captured by the SS change. This work shows that PA imaging biomarkers can quantify these vascular changes which in turn enhance the radiation-induced cell death (up to 40%, as measured by TUNEL staining).