Ultrasound-Stimulated microbubble cavitation improved tumor perfusion and promoted tumor vascular normalization in a rabbit VX2 tumor model.

Tumor hypoxia poses a major challenge in tumor therapy. Many strategies have been explored to alleviate tumor hypoxic microenvironment to improve the efficacy of tumor therapy. Ultrasound-stimulated microbubbles cavitation (USMC) was proved to improve tumor perfusion, and thus to alleviate tumor hypoxia. The synergistic role of USMC in tumor therapy has been identified by several preclinical and clinical studies. The effect of USMC on improving tumor perfusion is influenced by many factors, and the stability and reproducibility of this effect in long-term tumor treatment remain to be explored. In this study, we established rabbit VX2 tumor model. Fifteen tumor-bearing rabbits were enrolled to compare the effects of USMC with two different mechanical indexes (MIs). The results of contrast-enhanced ultrasound (CEUS) imaging showed that USMC with MI of 0.25 could improve the tumor perfusion better compared with MI 0.40. Then we conducted repeated USMC treatments on tumor-bearing rabbits once a week for six weeks. The results of CEUS showed that USMC with appropriate parameters could always enhance the tumor perfusion although the tumor had developed. Transmission electron microscopy revealed that tumors received multiple USMC treatments had more integral vascular structures compared with the control. Immunofluorescence staining showed that tumors received multiple USMC treatments had higher overlap coefficient of CD31 and intercellular cell adhesion molecule-1, indicating the normalization of tumor vasculature. In conclusion, USMC with appropriate parameters has stability and reproducibility in improving tumor perfusion. And multiple USMC treatments could potentially promote tumor vascular normalization, which is beneficial for tumor therapy.