Abstract
Glioblastoma (GB) is the most common malignant primary brain tumor. Even with aggressive treatment, the median overall survival for GB patients is only 15 months. Furthermore, brain metastases develop in roughly 10-20% of all cancer patients. Development of brain metastases worsens overall prognosis and limits treatment options. Although promising new treatments for both primary and metastatic brain tumors, including gene therapy approaches, are constantly under development, brain neoplasms present tremendous challenges to effective therapeutic delivery.
There are three main physical barriers impede effective delivery of systemically administered agents into brain tumor tissue: 1.) the blood-tumor barrier (BTB) 2.) the blood-brain barrier (BBB) and 3.) the brain tissue barrier. The BTB is formed by leaky tumor vessels that contribute to high interstitial fluid pressures, limiting convective transport of circulating agents into the tissue. Despite areas of high vascular permeability, vessel leakiness varies throughout tumors and blood-brain barrier-like properties are retained within certain regions. Lastly, the transport of agents that have crossed into the brain tumor tissue compartment is limited by steric and adhesive interactions with the extracellular matrix (ECM).
Focused ultrasound (FUS) is a versatile tool that can be used to overcome the major obstacles to effective agent delivery to brain tumors. In this dissertation, we explore two FUS-based techniques for gene delivery applications in brain tumors and investigate the molecular responses to FUS application in brain tumor tissue. First, we develop a multifaceted approach for non-invasive, non-viral transfection of brain tumors. For this method, we apply FUS in the presence of intravascular microbubbles (MBs) to disrupt the blood-tumor/blood-brain barrier (BTB/BBB) and facilitate delivery of non-viral brain-penetrating nanoparticle (BPN) gene vectors, designed to rapidly penetrate the extracellular space and transfect large tissue volumes. We show that FUS + MB BTB/BBB disruption permits non-invasive delivery of BPNs into brain tumor tissue and results in a roughly 4-fold enhancement in transfection in both a primary and secondary brain tumor model. Furthermore, we identify enhanced convective transport in the tumor interstitium as a potential key mediator of FUS + MB tumor transfection with BPNs. Second, we aim to enhance transfection by utilizing FUS to modulate the tumor tissue interstitial space prior to BPN administration, a technique referred to as FUS preconditioning. Lastly, given the promise of FUS as a therapeutic delivery strategy for brain tumors and the progression of this technique into human clinical trials for patients with brain metastases, we investigate the effects of FUS + MB BTB/BBB opening on the brain tumor microenvironment in an intracranial melanoma tumor model.
