Quantification of transient increase of the blood-brain barrier permeability to macromolecules by optimized focused ultrasound combined with microbubbles.
Authors: Shi L, Palacio-Mancheno P, Badami J, Shin DW, Zeng M, Cardoso L, Tu R, Fu BM
Radioimmunotherapy using a radiolabeled monoclonal antibody that targets tumor cells has been shown to be efficient for the treatment of many malignant cancers, with reduced side effects. However, the blood-brain barrier (BBB) inhibits the transport of intravenous antibodies to tumors in the brain. Recent studies have demonstrated that focused ultrasound (FUS) combined with microbubbles (MBs) is a promising method to transiently disrupt the BBB for the drug delivery to the central nervous system. To find the optimal FUS and MBs that can induce reversible increase in the BBB permeability, we employed minimally invasive multiphoton microscopy to quantify the BBB permeability to dextran-155 kDa with similar molecular weight to an antibody by applying different doses of FUS in the presence of MBs with an optimal size and concentration. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a micro-grinder. About 5 minutes after applying the FUS on the thinned skull in the presence of MBs for 1 minute, TRITC (tetramethylrhodamine isothiocyanate)-dextran-155 kDa in 1% bovine serum albumin in mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump. Simultaneously, the temporal images were collected from the brain parenchyma ~100-200 μm below the pia mater. Permeability was determined from the rate of tissue solute accumulation around individual microvessels. After several trials, we found the optimal dose of FUS. At the optimal dose, permeability increased by ~14-fold after 5 minutes post-FUS, and permeability returned to the control level after 25 minutes. FUS without MBs or MBs injected without FUS did not change the permeability. Our method provides an accurate in vivo assessment for the transient BBB permeability change under the treatment of FUS. The optimal FUS dose found for the reversible BBB permeability increase without BBB disruption is reliable and can be applied to future clinical trials.
Introduction
Purpose
Drug delivery with BBB opening
Study Objective
To accurately quantify and identify optimal focused ultrasound and microbubble conditions that produce a reversible increase in blood–brain barrier permeability to antibody-sized macromolecules using minimally invasive multiphoton microscopy.
Animal model / Human subject
Rat, Sprague-Dawley, 3-4 months, female
Disease model
Healthy
Targeted brain region(s)
Frontoparietal Cortex
Cargo name and characteristics
TRITC-dextran 155 kDa
Route of administration
intra-carotid (injection into the ipsilateral carotid artery via syringe pump)
Outcomes and Safety
Summary of Outcomes
Focused ultrasound (FUS) combined with microbubbles transiently increased BBB permeability to 155 kDa dextran ~14-fold at 5 min, with permeability peaking early and returning to baseline by ~25 min (FUS or MBs alone had no effect). The reversible, non‑disruptive parameters were MBs ~2 μm at ~1.7×10^6 bubbles/mL, 1 min sonication, 5% duty cycle and acoustic power ≈6.3 W/cm^2 (≈0.3 MPa) achieved with voltage = 1.25 V and BRR = 1000 kHz; by contrast ≈13.1 W/cm^2 (4 V, BRR 500 kHz) caused vessel disruption, 6.3 W/cm^2 at 2.5 V/BRR 500 kHz produced a non‑reversible increase, and ≈3.4 W/cm^2 produced no change.
Duration of biological effect
25 minutes
Safety-related matter
Optimaized FUS parameters produced reversible increase in BBB permeability without visible vascular, tissue damage. Higher acoustic dose does not result in reversible
Brain Region
Ultrasound Parameters
Ultrasound instrument
Spherically 1 MHz FUS transducer (Ws50-P50; Ultran, State College, PA, USA)
FUS Frequency
1 MHz
FUS Intensity
6.3 W/cm2
FUS Pressure
0.3 MPa
FUS Mode
pulsed
Duration of a single FUS session
1 minute
Focal Characteristics
Focal depth: none; Focal length: 50 mm; Aperture size: None
Treatment frequency
Single
Mechanical index
0.3
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