Focused Ultrasound Enables Nonsurgical Brain Delivery of scFvMC1 Gene Therapy That Reduces Cortical Tau Pathology in Tauopathy Mice

A preclinical study presented at the 2026 AD/PD International Conference on Alzheimer’s and Parkinson’s Diseases, held March 17-21 in Copenhagen, Denmark, demonstrated that MRI-guided focused ultrasound (FUS) can be used to direct an intravenously administered adeno-associated virus (AAV) carrying an antitau gene therapy to discrete brain regions in a mouse model of tauopathy — and that doing so produces regionally specific reductions in both soluble and insoluble tau species.¹ The findings offer early proof-of-principle for a nonsurgical approach to precision gene delivery in the context of tauopathy.

Study Design and Animal Model

According to the AD/PD abstract, the study utilized P301S transgenic mice. Four groups of animals were studied: P301S mice treated with AAV9.CAG.scFvMC1 plus FUS (n=17); P301S mice treated with saline plus FUS (n=14); untreated P301S mice (n=14); and age-matched wild-type C57Bl/6J mice (n=15). MRI-guided FUS was used to direct intravenously administered AAV9.CAG.scFvMC1 — at one of 2 doses (5×10¹² or 5×10¹³ genome copies per kilogram) — to 2 anatomically distinct target regions: the frontal cortex and the hippocampus. scFvMC1 expression was characterized using quantitative PCR and RNAscope. Total tau levels and phosphorylated tau species across the soluble and insoluble fractions in each brain region were quantified by ELISA, with phosphorylation assessed at pThr231, pSer396/404, and pSer202.

Key Findings: Cortical Efficacy, Hippocampal Silence

In the frontal cortex, treatment with AAV9.CAG.scFvMC1 delivered via FUS produced statistically significant reductions across both soluble and insoluble tau fractions compared with controls. Against untreated P301S mice, the treated group demonstrated 30% to 65% reductions in total soluble tau (P = .028), pThr231 (P = .016), pSer396/404 (P = .0012), and pSer202 (P = .0022). When compared with the saline-plus-FUS group, the treated group exhibited 43% to 65% reductions in total insoluble tau (P = .0022), pThr231 (P = .0053), pSer396/404 (P = .0017), and pSer202 (P = .0007). The FUS-only (saline) group showed no significant effect on tau levels in either brain region, suggesting that the observed reductions were attributable to scFvMC1 expression rather than to ultrasound exposure.

In the hippocampus — the other FUS-targeted region — no statistically significant reductions in soluble or insoluble tau were detected. The basis for this regional discrepancy is not yet understood and constitutes one of the primary open questions raised by the study. The authors describe this as an area requiring further investigation.

“This study establishes proof-of-principle that scFvMC1 gene therapy delivered to the brain using FUS is effective at attenuating tau pathology,” first author Laura Vecchio, PhD, a research associate at Sunnybrook Research Institute, and colleagues concluded.¹ “Further study is needed to understand the basis of regional variation in efficacy.”

Disease Burden and the Ongoing Unmet Need in Tauopathy

Tau pathology is a defining neuropathological feature of Alzheimer disease (AD) — the leading cause of dementia globally — as well as of numerous primary tauopathies including progressive supranuclear palsy, corticobasal degeneration, frontotemporal dementia with tau inclusions, and chronic traumatic encephalopathy. The tau protein undergoes pathological changes in AD and other tauopathies that eventually lead to functional impairments, and over the years several therapeutic approaches have been examined to slow or halt progression of tau pathology but have yet to lead to an approved disease-modifying treatment.² A major challenge for passive tau immunotherapy is achieving adequate central nervous system exposure from systemically administered antibodies: the intact blood brain barrier (BBB) limits the fraction of circulating antibody that can enter the brain, typically estimated at less than 0.1% to 0.5% of the plasma concentration for large IgG molecules. Vectorized delivery of scFv constructs directly to the brain, circumventing the BBB penetration problem entirely, is one proposed solution to this limitation.

The broader tau immunotherapy landscape reflects this challenge. While many phase 1 trials have been successfully completed, a few trials targeting primary tauopathies and 1 AD trial have been halted due to lack of efficacy.³

REFERENCES

1. Vecchio L, Habib N, Apa A, et al. Focused ultrasound-mediated delivery of scFvMC1 gene therapy results in region-specific reductions of soluble and insoluble tau. Presented at:AD/PD International Conference on Alzheimer’s and Parkinson’s Diseases, March 17-21, Copenhagen, DE. Abstract #2976

2. Sigurdsson EM. Tau immunotherapies for Alzheimer's disease and related tauopathies: status of trials and insights from preclinical studies. J Alzheimers Dis. 2024;101(S1):S129–S140. https://doi.org/10.3233/JAD-231238 [PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC11587787/]

3. Ji C, Sigurdsson EM. Current status of clinical trials on tau immunotherapies. Drugs. 2021;81(10):1135–1152. https://doi.org/10.1007/s40265-021-01546-6 [PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC8752054/]