Efficacy of gene delivery to the brain using AAV and ultrasound depends on serotypes and brain areas.

Focused ultrasound combined with intravenously injected microbubbles (FUS) is known to non-invasively, locally, and transiently increase the permeability of the blood-brain barrier (BBB). A promising approach for non-invasive gene delivery to the brain is to administer recombinant adeno-associated viruses (AAVs) intravenously and allow them to cross the BBB at precise FUS-targeted brain regions. FUS-AAV delivery has been achieved in animal models; however, the key elements influencing, guiding, and monitoring the success of FUS-AAV delivery to the brain remain largely unknown. We systematically compared the ability of AAV1, AAV2, AAV5, AAV8, AAV9, and AAVrg to enter four specific brain regions and transduce two main cell types: neurons and astrocytes. Our results demonstrate that the AAV serotype, the extent of FUS-induced BBB permeability, and the intrinsic properties of the targeted brain tissue influence the observed biodistribution, diffusion and transduction of AAV to cells of the cerebrovasculature and brain parenchyma. Non-invasive contrast-enhanced MR imaging was found to predict the efficacy of FUS-AAV delivery. Notably, we also found that AAVs with high biodistribution to peripheral organs result in low gene delivery to the brain when combined with FUS. Gene delivery by AAV1, AAV2, AAV5, AAV8 and AAV9 was highly and selectively localized to FUS-targeted brain areas. To obtain non-invasive gene delivery to multiple brain regions with one area of FUS-BBB modulation, we combined a modified AAV2 vector harboring enhanced retrograde transport properties (AAVrg) with FUS-mediated brain delivery. This allowed for gene delivery from the FUS-targeted site to multiple connected brain regions. This study demonstrates that MR imaging can be used as a non-invasive indication of AAV delivery to the brain, and that the properties of AAV serotypes influence the efficacy of gene delivery to the brain with FUS. AAVs that have minimal peripheral biodistribution are ideal candidates for enhanced, and perhaps exclusive with future serotypes, delivery to the brain with FUS. The characterization of parameters influencing FUS-AAV delivery to the brain are critical to the design of safe and efficient gene therapies, from preclinical studies to future clinical applications.

Transgene distribution and immune response after ultrasound delivery of rAAV9 and PHP.B to the brain in a mouse model of amyloidosis.

Efficient disease-modifying treatments for Alzheimer disease, the most common form of dementia, have yet to be established. Gene therapy has the potential to provide the long-term production of therapeutic in the brain following a single administration. However, the blood-brain barrier poses a challenge for gene delivery to the adult brain. We investigated the transduction efficiency and immunological response following non-invasive gene-delivery strategies to the brain of a mouse model of amyloidosis. Two emerging technologies enabling gene delivery across the blood-brain barrier were used to establish the minimal vector dosage required to reach the brain: (1) focused ultrasound combined with intravenous microbubbles, which increases the permeability of the blood-brain barrier at targeted sites and (2) the recombinant adeno-associated virus (rAAV)-based capsid named rAAV-PHP.B. We found that equal intravenous dosages of rAAV9 combined with focused ultrasound, or rAAV-PHP.B, were required for brain gene delivery. In contrast to rAAV9, focused ultrasound did not decrease the rAAV-PHP.B dosage required to transduce brain cells in a mouse model of amyloidosis. The non-invasive rAAV delivery to the brain using rAAV-PHP.B or rAAV9 with focused ultrasound triggered an immune reaction including major histocompatibility complex class II expression, complement system and microglial activation, and T cell infiltration.