Use of photoacoustic imaging for monitoring vascular disrupting cancer treatments

Vascular disrupting agents disrupt tumor vessels, blocking the nutritional and oxygen supply tumors need to thrive. This is achieved by damaging the endothelium lining of blood vessels, resulting in red blood cells (RBCs) entering the tumor parenchyma. RBCs present in the extracellular matrix are exposed to external stressors resulting in biochemical and physiological changes. The detection of these changes can be used to monitor the efficacy of cancer treatments. Spectroscopic photoacoustic (PA) imaging is an ideal candidate for probing RBCs due to their high optical absorption relative to surrounding tissue. The goal of this work is to use PA imaging to monitor the efficacy of the vascular disrupting agent 5,6-Dimethylxanthenone-4-acetic acid (DMXAA) through quantitative analysis. Then, 4T1 breast cancer cells were injected subcutaneously into the left hind leg of eight BALB/c mice. After 10 days, half of the mice were treated with 15 mg/kg of DMXAA and the other half were injected with saline. All mice were imaged using the VevoLAZR X PA system before treatment, 24 and 72 hours after treatment. The imaging was done at six wavelengths and linear spectral unmixing was applied to the PA images to quantify three forms of hemoglobin (oxy, deoxy and met-hemoglobin). After imaging, tumors were histologically processed and H&E and TUNEL staining were used to detect the tissue damage induced by the DMXAA treatment. The total hemoglobin concentration remained unchanged after treatment for the saline treated mice. For DMXAA treated mice, a 10% increase of deoxyhemoglobin concentration was detected 24 hours after treatment and a 22.6% decrease in total hemoglobin concentration was observed by 72 hours. A decrease in the PA spectral slope parameters was measured 24 hours after treatment. This suggests that DMXAA induces vascular damage, causing red blood cells to extravasate. Furthermore, H&E staining of the tumor showed areas of bleeding with erythrocyte deposition. These observations are further supported by the increase in TUNEL staining in DMXAA treated tumors, revealing increased cell death due to vascular disruption. This study demonstrates the capability of PA imaging to monitor tumor vessel disruption by the vascular disrupting agent DMXAA.