Abstract
Ultrasound has gained popularity in recent years due to its ever-increasing therapeutic potential. Therapeutic ultrasound is capable of producing a range of bioeffects, such as localized heating or non-thermal disruption of the vasculature. Here, we propose an exploration of two distinct ultrasound-based cancer treatments: transcranial blood-brain barrier disruption (BBBD) for therapeutic nanoparticle delivery to primary brain tumors and immunotherapeutic tumor microenvironment modulation with enhanced drug delivery for melanoma.
Drug delivery to the brain:
The brain has long presented a unique challenge for drug delivery in the form of the blood-brain barrier (BBB). The BBB prevents the vast majority of chemotherapeutics from entering the brain, and most treatment modalities (direct injection, convection enhanced delivery, surgery) are invasive. In BBBD, low intensity ultrasound is focused through the intact skull. Microbubbles (MBs) injected into the blood stream expand and contract in the focal region within the brain, producing mechanical forces that interact with the vessel walls. As a result of these forces, the blood-brain barrier is disrupted in a localized, reversible and non-invasive manner, allowing delivery of systemically administered chemotherapeutics. We determined whether focused ultrasound (FUS) is capable of delivering high specialized “brain-penetrating nanoparticles” (BPNs), designed to diffuse within the brain parenchyma, across the BBB in a rodent model of glioblastoma. First, we defined a safe, repeatable protocol for MR-guided FUS-mediated BBBD in the rat brain based on T1 and T2*-weighted MR images, MR thermometry and histology. Next, detailed analysis of confocal microscopy images taken from treated brains demonstrated that FUS in combination with MBs is capable of delivering 60 nm fluorescently tagged BPNs across the BBB in normal brain and across the blood-tumor barrier in orthotopic glioblastoma. Finally, we determined that ultrasound-mediated delivery of drug-loaded biodegradable BPNs in an intracranial rat model of glioblastoma produces significant tumor growth control and survival benefit.
Immunomodulation in melanoma:
While there are currently several immune-based therapeutics available for the treatment of melanoma, a large number of patients do not respond to treatment. This is often attributed to a poor pre-treatment antitumor immune response, and it has been postulated that increasing this baseline immune activity may enhance the efficacy of these immunotherapeutics as well as increase the percentage of responders. It has been shown that high-intensity ultrasound generates an increased antitumor immune response, although the mechanisms by which this occurs are poorly understood. We investigated the effects of ultrasound-based immunomodulation in a mouse model of melanoma. First, we developed a low-intensity microbubble-enhanced ultrasound protocol that provided improved tumor growth control and animal survival compared to an FDA-approved immunotherapeutic (anti-PD-1). Next, we demonstrated that ultrasound alone significantly improved tumor growth control compared to a combination of ultrasound and anti-PD-1. To better understand the mechanisms controlling ultrasound-mediated tumor growth control, we performed flow cytometry on treated tumors and showed that ultrasound increases the infiltration of regulatory T cells, helper T cells, cytotoxic T cells, natural killer cells and macrophages. To determine which cell type is responsible for ultrasound-mediated tumor growth control, we have designed three experiments: 1) examine the growth of ultrasound vs. untreated tumors in Rag-1 knock out mice (lacking an adaptive immune system) 2) perform flow cytometry on ultrasound treated animals who have received tagged T cells and 3) perform flow cytometry on ultrasound treated animals who have received FTY720 (which prevented T cell trafficking). With these three experiments, we determined that US-mediated tumor growth control is dependent on trafficking of adaptive immune cells.
