Optimization of the ultrasound-induced blood-brain barrier opening.

Current treatments of neurological and neurodegenerative diseases are limited due to the lack of a truly non-invasive, transient, and regionally selective brain drug delivery method. The brain is particularly difficult to deliver drugs to because of the blood-brain barrier (BBB). The impermeability of the BBB is due to the tight junctions connecting adjacent endothelial cells and highly regulatory transport systems of the endothelial cell membranes. The main function of the BBB is ion and volume regulation to ensure conditions necessary for proper synaptic and axonal signaling. However, the same permeability properties that keep the brain healthy also constitute the cause of the tremendous obstacles posed in its pharmacological treatment. The BBB prevents most neurologically active drugs from entering the brain and, as a result, has been isolated as the rate-limiting factor in brain drug delivery. Until a solution to the trans-BBB delivery problem is found, treatments of neurological diseases will remain impeded. Over the past decade, methods that combine Focused Ultrasound (FUS) and microbubbles have been shown to offer the unique capability of noninvasively, locally and transiently open the BBB so as to treat central nervous system (CNS) diseases. Four of the main challenges that have been taken on by our group and discussed in this paper are: 1) assess its safety profile, 2) unveil the mechanism by which the BBB opens and closes, 3) control and predict the opened BBB properties and duration of the opening and 4) assess its premise in brain drug delivery. All these challenges will be discussed, findings in both small (mice) and large (non-human primates) animals are shown and finally the clinical potential for this technique is shown.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To evaluate and optimize focused ultrasound combined with microbubbles as a noninvasive, localized, and transient method for opening the blood-brain barrier to enable safe, controllable brain drug delivery.

Animal model / Human subject Mouse (Mus musculus); strain: None; age: None; sex: None

Disease model Healthy

MRI or image guidance method Transducer positioning using a computer-controlled positioner to sonicate the hippocampus (horizontal orientation) with acoustic emissions monitored confocally by a passive cavitation detector; MRI (9.4T T1/T2*/DCE) was used to confirm and assess BBB opening and permeability; for large-animal/human planning CT-based simulations with ex vivo skull CT and 3D brain-atlas registration were used to guide trans-skull targeting.

Targeted brain region(s) Hippocampus

Cargo name and characteristics Gadodiamide (Omniscan) — small-molecule Gd-based MRI contrast agent (~590 Da); Fluorescently labeled dextrans — polysaccharide tracers used as molecular cargo (examples: 3 kDa dextran and up to 2000 kDa dextrans, ~20 nm for the largest); Albumin — protein (≈67 kDa); Brain-derived neurotrophic factor (BDNF) — protein neurotrophin (mature BDNF ≈13–14 kDa)

Route of administration Intravenous (tail vein) and intraperitoneal (IP)

Outcomes and Safety

Summary of Outcomes Focused ultrasound (FUS) with microbubbles transiently and reproducibly opened the blood–brain barrier in targeted hippocampus (mice and non-human primates/transcranial), increasing permeability ≥100-fold and enabling intracellular delivery of 0.5–70 kDa molecules (e.g., Dextran, BDNF) that activated neuronal signaling with recovery within 24 h and no structural damage within optimized conditions. Successful FUS parameters included optimized pulse length and peak‑rarefactional pressure ranges (identified to avoid damage), use of species‑specific single‑element transducers/frequencies for trans‑skull focusing, very short pulse sequences, and appropriate microbubble sizes (with permeability and number of opening sites dependent on pressure and microbubble size).

Safety-related matter The authors note that microbubble inertial cavitation can induce irreversible vascular and cellular damage at high acoustic pressures. However, optimization experiments and H&E histology showed BBB opening without structural damage within a defined pulse-length/pressure range, with barrier recovery occurring within 24 hours.

Brain Region

Ultrasound Parameters

Ultrasound instrument FUS transducer, model cdc7411-3 (Imasonic, Besançon, France); center frequency 1.5 MHz; focal depth 60 mm; outer radius 30 mm; inner radius 11.2 mm

FUS Frequency 1.5 MHz; 500 kHz; 800 kHz; (also reported: high-pass filter cutoff 4 MHz; digitizer sampling 25 MHz

FUS Pressure 0.15 MPa, 0.30 MPa, 0.45 MPa, 0.60 MPa, 0.90 MPa; reported ranges: 0.30–0.45 MPa (threshold), 0.30–0.60 MPa (feasibility/safety window)

FUS Mode pulsed

Pulse duration 20 ms

Duration of a single FUS session 90 s (two 30-s sonication intervals with a 30-s intermittent delay)

Focal Characteristics focal depth: 60 mm; focal length: None; aperture size: outer radius 30 mm; inner radius 11.2 mm

Treatment frequency Single session

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