MRI Guided Focused Ultrasound-Mediated Delivery of Therapeutic Cells to the Brain: A Review of the State-of-the-Art Methodology and Future Applications

Abstract

Stem cell and immune cell therapies are being investigated as a potential therapeutic modality for CNS disorders, performing functions such as targeted drug or growth factor delivery, tumor cell destruction, or inflammatory regulation. Despite promising preclinical studies, delivery routes for maximizing cell engraftment, such as stereotactic or intrathecal injection, are invasive and carry risks of hemorrhage and infection. Recent developments in MRI-guided focused ultrasound (MRgFUS) technology have significant implications for treating focal CNS pathologies including neurodegenerative, vascular and malignant processes. MRgFUS is currently employed in the clinic for treating essential tremor and Parkinson's Disease by producing precise, incisionless, transcranial lesions. This non-invasive technology can also be modified for non-destructive applications to safely and transiently open the blood-brain barrier (BBB) to deliver a range of therapeutics, including cells. This review is meant to familiarize the neuro-interventionalist with this topic and discusses the use of MRgFUS for facilitating cellular delivery to the brain. A detailed and comprehensive description is provided on routes of cell administration, imaging strategies for targeting and tracking cellular delivery and engraftment, biophysical mechanisms of BBB enhanced permeability, supportive proof-of-concept studies, and potential for clinical translation.

Keywords: MRI-guided focused ultrasound; blood-brain barrier; cellular therapy; cellular tracking; central nervous system diseases.

Figure 1

Set-up of MRgFUS patient treatment. (A) A schematic representation of patient lying supine on MR table being fitted with FUS phased-array transducer array; (B), Close up of the 1,024 ultrasound element array for electronic steering of the ultrasound beam. (C) A schematic 2-dimensional representation of the multiple ultrasound beams focused non-invasively through the skull (bright green) to a single target. The image of the skull is obtained from a prior computed tomographic scan that is mechanically registered to the MR image. Information from the skull is used by the planning software to correct for aberrations to the beam paths and accurately position the focus at the desired target. Adapted from Fishman and Frenkel, Journal of Central Nervous System Disease 2017 (68). Reprinted with permission from SAGE Publishing.

Figure 2

Typical timeline for an MRgFUS preclinical study investigating cellular delivery. Adapted from Shen et al. Cell Transplantation 2017 (81). Reprinted with permission from SAGE Publishing.

Figure 3

MRgFUS mediated delivery of dual labeled (fluorescence & SPION) NPCs in the rodent brain. (A,B) Representative screen captures from an MRgFUS system graphic user interface. (A) T2 weighted axial MRI image of a rat brain showing treatment target (arrow) overlay. (B) T1 contrast MRI image showing hyperintense signal from gadolinium extravasation at location of treatment (arrow), indicating successful BBBO. Signal coincides with treatment target in “A”. (C) Whole brain coronal section indicating successful BBBO, evidenced by Evans blue dye (arrows) that extravasated in the region of the focal zone. (D) H&E stained brightfield histological section demonstrating unaffected tissue in the region of MRgFUS treatment. (E) higher magnification region from “D” (inset). (F–I) Fluorescence microscopy images of fluorescently labeled NPCs in the brain. (F) Fluorescently labeled NPCs in the dorsal cortex. (G) higher magnification of inset in “F”. (H) Fluorescent signals detected from labeled human cytoplasmic antigen (SC121). (I) Higher magnification of fluorescently labeled NPCs. Co-localization of fluorescent signals in “H” and “I” provide evidence that labeled cells are NPCs (human). (J–L) Brightfield microscopy images of Prussian blue stained histological sections (for SPION) indicating the presence of NPCs. (J) Low magnification image. (K) Inset in “J.” (L) Inset in “K.” Individual cells (blue) are seen (arrows). Scale bars: A, B = 10 mm; C = 2 mm; D = 2 mm; E = 200 μm; F = 400 μm; G = 200 μm; H, I = 20 μm; J = 1 mm; K = 50 μm; L = 5 μm. Adapted from Shen et al. Cell Transplantation 2017 (81). Reprinted with permission from SAGE Publishing.

Figure 4

FUS mediated delivery of MSCs in murine model of CLI in skeletal muscle (A) Stacked box plots comparing proteomic responses of CLI muscle between FUS treatment and untreated controls. The chemokines, cytokines, trophic factors, and cell adhesion molecules listed are those have significantly higher levels than those in normal muscle after CLI alone (n = 6). (B) SPION labeled MSCs in control and FUS treated skeletal muscle in CLI mice. Significantly greater numbers of MSCs were observed in FUS treated animals, based on Prussian blue staining of cells (n = 5). (C) Temporal changes in normalized perfusion comparing control and FUS treated CLI mice. Results are based on laser Doppler perfusion imaging (LDPI) indicating reperfusion that occurred in FUS treated animals only (n = 7). (D) Representative LDPI images at week 5 in the study for each experimental group. Adapted from Tebebi et al. Sci Rep 2017 (91). Reprinted with permission from Nature Research.

Figure 5

Representative examples of image-based cell tracking methods in the brain. (A) T2* MR images of rat brains with experimentally induced inflammation and infused with SPION-labeled human glial precursor cells (hGP). At 30 min. post-infusion, greater numbers of targeted (eng.) cells are observed in the inflamed tissue compared to naïve cells (arrows, hypointense signal). The results are supported by pixel-by-pixel analysis, comparing pre- and post-infusion MR images, which can be quantitatively compared. Modified, with permission, from “Gorelik et al. Use of MR cell tracking to evaluate targeting of glial precursor cells to inflammatory tissue by exploiting the very late antigen-4 docking receptor. Radiology 2012; 265: 175-185” (105). (B) (left) 19F MRI and (right) 19F MRI & T2 weighted MR images of fluorine-19 labeled glial progenitor cells injected into a mouse brain striatum. “Hot spot” in each image is clearly identified, indicating the presence of the labeled cells. Figure adapted from Richard et al. Stem Cells Translational Medicine 2019. Reprinted under creative commons license (106). (C) SPECT signals (arrows) from 111-In labeled NSCs administered into control mice (lower) and those with a glioma model (upper), to which the cells have homed. In both panels, SPECT images are overlayed on CT scans. Figure adapted from Cheng et al. 2016 (107). This research was originally published in the Journal of Nuclear Medicine. (D) MPI imaging of SPION labeled MSCs in the left hemisphere (1 × 10⁵ cells) and right hemisphere (5 × 10⁴ cells) transplanted in a mouse brain. Lower panel shows MPI signals (upper panel) superimposed on T2* MR image (middle panel). Figure adapted from Bulte et al. Tomography 2015 (108). Reprinted under creative commons license.

Figure 6

Schematic representation of the anticipated preclinical and clinical stages involved for clinical translation in using MRgFUS for enhancing cellular therapy.