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Page 10 of 20 Choi et al. Cancer Drug Resist. 2026;9:12

demonstrated increased tumor uptake of intravenously administered nanoparticles after focused US
treatment [Figure 4A(i)]. To examine whether enhanced accumulation was accompanied by improved
spatial distribution within tumors, confocal fluorescence imaging was performed, revealing deeper
intratumoral penetration of nanoparticles relative to tumor microvessels after US treatment [Figure 4A(ii)].
To evaluate whether enhanced interstitial flow could also improve gene-vector delivery, the authors
intravenously injected luciferase-bearing nanoparticles immediately before US treatment and measured
tumor transgene expression using ex vivo bioluminescence imaging. Focused-US with microbubble resulted
in ~4-fold increases in both total photon flux and average radiance compared with nanoparticles alone
[Figure 4A(iii)], demonstrating that US enhances not only tracer dispersion but also functional
gene-expression efficiency. They also demonstrated how focused US modulates interstitial transport by
analyzing time-series contrast-enhanced MRI data [Figure 4B(i)]. Using an MRI-guided focused US system
(3T MRI integrated with a 1.1 MHz spherical transducer and passive cavitation hydrophone), they selected
eight treatment points per tumor and applied 0.45-0.55 MPa peak negative pressure for 2 min in 0.5% duty
cycle. After BBB/BTB opening, serial T1-weighted MRI acquisitions with intravenously injected Gd-chelate
(~0.75 nm) enabled voxel-wise diffusion modeling. In both U87 glioma and intracranial B16F1ova
melanoma models, interstitial flow velocity significantly increased after US treatment, indicating US-driven
augmentation of solute transport within tumor tissue [Figure 4B(ii)]. Finally, to assess transport of larger
therapeutic carriers, the authors examined nanoparticles originally termed “brain-penetrating nanoparticles
(BPNs)” - ~50-nm particles engineered to disperse efficiently through tumor interstitial spaces after local
infusion. U87mCherry-bearing mice received systemic administration of BPNs carrying a ZsGreen reporter
gene, followed by focused US insonification. Histological analysis showed a two-fold increase in transfection
volume, demonstrating the feasibility that US-based interstitial-flow upregulation can improve the delivery
of large-molecule drugs into brain tumors.

Meng et al. extended US-modulated glymphatic research into humans by examining contrast transport
patterns following MR-guided focused US BBB opening in subjects with Alzheimer’s disease and
amyotrophic lateral sclerosis . In all 12 participants, reversible BBB opening was successfully achieved, and
[66]
a subset of subjects exhibited characteristic perivenous and subarachnoid contrast enhancement on
fluid-attenuated inversion recovery (FLAIR) MRI immediately after sonication [Figure 4C]. These
hyperintensities appeared as sheath-like signal encasement along large cortical draining veins and branching
enhancement within adjacent subarachnoid spaces, closely resembling glymphatic efflux routes previously
identified in rodents. Notably, these patterns consistently reappeared across repeated treatments yet resolved
within 24 h without clinical symptoms, suggesting a transient and physiologically driven response rather
than tissue injury. Collectively, these observations provide the first radiologic evidence in humans that
focused US may modulate glymphatic drainage along perivenous routes, supporting its translational
potential for enhancing solute clearance in neurodegenerative disease.

A summary of the in vivo and ex vivo studies, including protocols, US parameters, and key findings, is
provided in Table 2.

DISCUSSION AND FUTURE DIRECTIONS
Despite the substantial interest in the glymphatic system in the context of neurodegenerative diseases, only a
limited number of studies have examined its role in brain cancers. Conventional glymphatic research has
focused on the drainage and removal of metabolic waste in non-cancerous brain diseases, because these
processes are directly related to alleviating symptoms and slowing disease progression. In contrast, research
in brain cancer has traditionally emphasized angiogenic remodeling, immune interactions, and
tumor-specific molecular signaling, leaving the contribution of glymphatic transport poorly defined. With
respect to drug delivery, BBB opening has been the dominant strategy, and the idea of leveraging the

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