Abstract
The blood brain barrier (BBB) is a specialized protective structure in central nervous system, which is critical for maintaining brain homeostasis and low permeability that controls the passage of molecules from the circulation into the brain parenchyma and the efflux from the brain. However, the BBB also hinders the transportation of therapeutic agents and contrast agents from blood into brain tissue, lowering the treatment efficiency. Recently, focused ultrasound (FUS) sonication in the presence of microbubbles (MBs) has been proved to transiently open the BBB, allowing the penetration of administered agents into the brain. This thesis tried to induce blood-brain barrier disruption by selfmade multifunctional microbubbles and focused ultrasound (MB-FUS-BBBD) in small animal model, and focused on its safety issue and drug delivery application. The safety issue can be divided to two certain parts. The first part described in chapter 2 investigated the feasibility of using real-time ultrasound imaging to monitor the histological alterations of brain after receiving MB-FUS-BBBD process. We found that when FUS was over-excited, the MB-FUS-BBBD process accompanied extensive intracerebral hemorrhage (ICH) and blood flow shortage. In addition, the hemorrhage pattern and the location of blood flow shortage were represented by high-frequency ultrasound B-mode images and contrast-enhanced ultrasound images, respectively. The second part presented in chapter 3 attempted to use submicron bubbles and on-resonant frequency FUS synergistically to inhibit the occurrence of inertial cavitation during MB-FUS-BBBD. The results demonstrated that when submicron bubbles were exposed to resonant-frequency matched FUS, inertial cavitation could be reduced, avoiding the risk of ICH and brain damage. Feasibility of combined-use of multifunctional MBs with FUS for brain drug delivery was described in chapter 4. A 1,3-bis(2-chloroethyl)-1-nitrosoure (BCNU)-loaded and VEGF-A ligand conjugated MB (VEGF-BCNU-MB) was developed as a novel targeting drug carrier. These VEGF-BCNU-MBs accumulated actively in the brain tumor vasculatures where overactive angiogenesis was marked by overexpression of VEGF-R2 receptor. Sonicated by FUS, VEGF-BCNU-MBs not only opened the BBB but also released chemotherapeutic agent into tumor sites, thus achieving the goal of targeting therapy. Furthermore, these abundantly accumulated targeting bubbles had the potential to serve as tools for molecular ultrasound imaging for tumor detection. Further applied the combination of FUS exposure and VEGF-BCNU-MBs in a rat glioblastoma multiforme (GBM) model, obvious tumor growth suppression was found. Our findings gave the information for future MB design aimed at targeted brain tumor therapy or extending the application of MBs to theragnostic applications.