Consistent opening of the blood brain barrier using focused ultrasound with constant intravenous infusion of microbubble agent.

The blood brain barrier (BBB) is a major obstacle to the delivery of therapeutics to the brain. Focused ultrasound (FUS) in combination with microbubbles can non-invasively open the BBB in a targeted manner. Bolus intravenous injections of microbubbles are standard practice, but dynamic influx and clearance mechanisms prevent delivery of a uniform dose with time. When multiple targets are selected for sonication in a single treatment, uniform serum concentrations of microbubbles are important for consistent BBB opening. Herein, we show that bubble infusions were able to achieve consistent BBB opening at multiple target sites. FUS exposures were conducted with different Definity microbubble concentrations at various acoustic pressures. To quantify the effects of infusion on BBB opening, we calculated the MRI contrast enhancement rate. When infusions were performed at rates of 7.2 µl microbubbles/kg/min or below, we were able to obtain consistent BBB opening without injury at all pressures. However, when infusion rates exceeded 20 µl/kg/min, signs of injury occurred at pressures from 0.39 to 0.56 MPa. When compared to bolus injections, a bubble infusion offers a more controlled and consistent approach to multi-target BBB disruption.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To evaluate whether continuous microbubble infusions enable consistent, controlled multi-target focused ultrasound blood–brain barrier opening and to compare their efficacy and safety with bolus injections.

Animal model / Human subject Mouse (Mus musculus), Swiss Webster strain, 3–4 months old, both sexes

Disease model Healthy

MRI or image guidance method Stereotactic-guided system (RK-50) using skull landmark registration (lambda and bregma) to a mouse brain atlas

Targeted brain region(s) Hippocampus And Frontal Cortex

Cargo name and characteristics Gadobutrol (Gadovist) — a gadolinium-based small-molecule MRI contrast agent administered intravenously (1.0 mmol/kg) to assess blood–brain barrier opening

Route of administration Intravenous (tail vein catheter)

Outcomes and Safety

Summary of Outcomes Microbubble infusions during focused ultrasound produced consistent, reproducible BBB opening across multiple brain targets with less variability than bolus injections; infusions at rates ≤7.2 µl/kg/min achieved consistent opening without injury, while infusion rates >20 µl/kg/min caused haemorrhage/edema at 0.39–0.56 MPa. Enhancement increased with microbubble influx up to apparent optima (≈32.7 µl/kg/min at 0.39 MPa; ≈15.9 µl/kg/min at 0.48 MPa; ≈22.7 µl/kg/min at 0.56 MPa) and declined at higher doses.

Safety-related matter Infusions at ≤7.2 µl/kg/min produced consistent BBB opening without injury, whereas infusion rates above ~20 µl/kg/min were associated with signs of injury including oedema and haemorrhage. T2-weighted MRI showed haemorrhage was significantly more common at high infusion rates (83% n=6 vs 0% n=3, p=0.0183), and circulating microbubbles persisted >15 min after stopping infusion, risking inadvertent accumulation.

Brain Region

Ultrasound Parameters

Ultrasound instrument RK-50 stereotactic-guided FUS system (FUS Instruments, Toronto, ON, Canada); transducer: 1.43 MHz, 35 mm diameter, 24.5 mm radius of curvature

FUS Frequency 1.43 MHz

FUS Pressure 0.39 to 0.56 MPa

FUS Mode pulsed

Pulse duration 10 ms

Duration of a single FUS session 6-10 minutes

Focal Characteristics Focal depth: None; Focal length: 24.5 mm (radius of curvature); Aperture size: 35 mm (diameter)

Treatment frequency single

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