Achieving Spatial and Molecular Specificity with Ultrasound-Targeted Biomolecular Nanotherapeutics

The precise targeting of cells in deep tissues is one of the primary goals of nanomedicine. However, targeting a specific cellular population within an entire organism is challenging due to off-target effects and the need for deep tissue delivery. Focused ultrasound can reduce off-targeted effects by spatially restricting the delivery or action of molecular constructs to specific anatomical sites. Ultrasound can also increase the efficiency of nanotherapeutic delivery into deep tissues by enhancing the permeability of tissue boundaries, promoting convection, or depositing energy to actuate cellular activity. In this review we focus on the interface between biomolecular engineering and focused ultrasound and describe the applications of this intersection in neuroscience, oncology, and synthetic biology. Ultrasound can be used to trigger the transport of therapeutic payloads into a range of tissues, including specific regions of the brain, where it can be targeted with millimeter precision through intact skull. Locally delivered molecular constructs can then control specific cells and molecular pathways within the targeted region. When combined with viral vectors and engineered neural receptors, this technique enables noninvasive control of specific circuits and behaviors. The penetrant energy of ultrasound can also be used to more directly actuate micro- and nanotherapeutic constructs, including microbubbles, vaporizable nanodroplets, and polymeric nanocups, which nucleate cavitation upon ultrasound exposure, leading to local mechanical effects. In addition, it was recently discovered that a unique class of acoustic biomolecules-genetically encodable nanoscale protein structures called gas vesicles-can be acoustically "detonated" as sources of inertial cavitation. This enables the targeted disruption of selected cells within the area of insonation by gas vesicles that are engineered to bind cell surface receptors. It also facilitates ultrasound-triggered release of molecular payloads from engineered therapeutic cells heterologously expressing intracellular gas vesicles. Finally, focused ultrasound energy can be used to locally elevate tissue temperature and activate temperature-sensitive proteins and pathways. The elevation of temperature allows noninvasive control of gene expression in vivo in cells engineered to express thermal bioswitches. Overall, the intersection of biomolecular engineering, nanomaterials and focused ultrasound can provide unparalleled specificity in controlling, modulating, and treating physiological processes in deep tissues.