MR-guided transcranial focused ultrasound safely enhances interstitial dispersion of large polymeric nanoparticles in the living brain

Abstract

Generating spatially controlled, non-destructive changes in the interstitial spaces of the brain has a host of potential clinical applications, including enhancing the delivery of therapeutics, modulating biological features within the tissue microenvironment, altering fluid and pressure dynamics, and increasing the clearance of toxins, such as plaques found in Alzheimer's disease. Recently we demonstrated that ultrasound can non-destructively enlarge the interstitial spaces of the brain ex vivo. The goal of the current study was to determine whether these effects could be reproduced in the living brain using non-invasive, transcranial MRI-guided focused ultrasound (MRgFUS). The left striatum of healthy rats was treated using MRgFUS. Computer simulations facilitated treatment planning, and targeting was validated using MRI acoustic radiation force impulse imaging. Following MRgFUS treatments, Evans blue dye or nanoparticle probes were infused to assess changes in the interstitial space. In MRgFUS-treated animals, enhanced dispersion was observed compared to controls for 70 nm (12.8 ± 0.9 mm3 vs. 10.6 ± 1.0 mm3, p = 0.01), 200 nm (10.9 ± 1.4 mm3 vs. 7.4 ± 0.7 mm3, p = 0.01) and 700 nm (7.5 ± 0.4 mm3 vs. 5.4 ± 1.2 mm3, p = 0.02) nanoparticles, indicating enlargement of the interstitial spaces. No evidence of significant histological or electrophysiological injury was identified. These findings suggest that transcranial ultrasound can safely and effectively modulate the brain interstitium and increase the dispersion of large therapeutic entities such as particulate drug carriers or modified viruses. This has the potential to expand the therapeutic uses of MRgFUS.

Fig 1. Schematic representation of the MRgFUS system.

(A) The animal’s head was positioned between the MRI coil and the FUS water bolus. (B) The RF coil is built into the animal holder of the system. An 8-element, 1.5 MHz FUS transducer was positioned over the head of the animal. The transducer was coupled to the head via an inflated membrane containing degassed water.

Fig 2. Acoustic pressure field simulations.

Acoustic pressure fields generated by a 1.5 MHz FUS 8-element annular array transducer in (A) free field and (B) in the setting of the in vivo transcranial treatments. The dashed line outlines the contour of the skull.

Fig 3. MRgFUS treatment planning.

T2-weighted (A) coronal and (B) axial MR images acquired following positioning of the FUS transducer. (C) T2-weighted axial MR image depicting a schematic of the rastering protocol that was used for sonication of the striatum. Each raster point underwent sonication for a total duration of 120 s. Inset depicts a schematic of the order in which each point was treated.

Fig 4. Confirmation of targeting in vivo by MR-ARFI imaging.

(A) MR-ARFI color map generated by post-processing the MR planes following sonication. Inset depicts sonicated region under higher magnification. (B) Brain tissue displacements generated by MRgFUS pulses and measured by MR-ARFI imaging at acoustic pressures of (I) 1.95 MPa, (II) 2.25 MPa, and (III) 2.4 MPa (n = 5 per acoustic pressure). Error bars represent standard deviation.

Fig 5. Effect of MRgFUS pre-treatment on the volume of distribution of locally delivered EBD and nanoparticle probes.

(Upper) MRgFUS pre-treatment did not have a significant effect on the volume of distribution of EBD, but resulted in statistically significant increases in the volume of distribution of locally delivered 70 nm, 200 nm, and 700 nm blue-dyed, non-adhesive nanoparticles (n = 4–6 treated animals and 4–6 control animals per probe). * p < 0.05, ** p < 0.01. Error bars represent standard deviation. (Lower) Representative images of brain slices obtained 2 hours following the local delivery of EBD or nanoparticles, with or without MRgFUS pre-treatment.

Fig 6. Histological analysis of MRgFUS-treated brains.

A representative brain slice obtained 2 hours following MRgFUS treatment of the left striatum indicating a lack of visible differences between the treated (blue) and untreated (red) regions. Left scale bar = 250 μm, center and right scale bars = 25 μm.

Fig 7. Electrophysiological effects of MRgFUS.

(A) MRgFUS did not result in significant differences in the mean frequency of sPSCs recorded from treated animals (n = 13 neurons), compared to those from control animals (n = 10). (B) There was a significant reduction in the mean amplitudes of sPSCs recorded from these two groups. (C) Sample traces of sPSCs recorded from MRgFUS-treated and control animals. (D) Averaged I/V curves recorded from both groups (n = 15 each) indicate that there were no significant differences in whole-cell currents. Error bars represent standard deviation.