Multiphoton imaging of ultrasound/Optison mediated cerebrovascular effects in vivo

The impact of pulse repetition frequency on microbubble activity and drug delivery during focused ultrasound-mediated blood-brain barrier opening

Objective.Pulsed focused ultrasound (FUS) can deliver therapeutics to the brain by using intravenous microbubbles (MBs) to open the blood-brain barrier (BBB). MB emissions indicate treatment outcomes, like BBB opening (harmonics) and damage (broadband). Typically, a pulse repetition frequency (PRF) of 1 Hz is used, but the effect of PRF on MBs is not fully understood. We investigated the effect of PRF on MB activity and tracer delivery.Approach.The effect of PRF (0.125, 0.25, 0.5, 1, and 2 Hz) on MB activity was monitored through harmonic and wideband emissions during FUS sonications of the rat brain at 274.3 kHz. BBB opening was quantified through fluorescence imaging to estimate the concentration of Trypan Blue (TB) dye following a 75-pulse FUS exposure for PRFs of 1 and 0.25 Hz.Main results.At a fixed acoustic pressure, the percentage change in maximum harmonic amplitude compared to the control (PRF = 1 Hz) decreased with increasing PRF, with a median change of 73.8% at 0.125 Hz and -38.3% at 2 Hz. There was no difference in the pressure threshold for broadband emissions between PRFs of 0.25 and 1 Hz. PRF = 0.25 Hz, led to a 68.2% increase in the mean concentration of TB measured after FUS, with a 53.9% increase in the mean harmonic sum, compared with PRF = 1 Hz. Harmonic emissions-based control at PRF = 0.25 Hz yielded similar TB delivery, with less damage at histology, compared with 1 Hz.Significance.For a fixed number of FUS pulses, reducing the PRF was shown to increase the magnitude of harmonic emissions and TB delivery, but not the threshold for broadband emissions. While further research is necessary to understand the mechanisms involved, these results may be useful to improve clinical safety margins and sensitivity to detecting small harmonic signals from cavitating MBs.

Label free, capillary-scale blood flow mapping in vivo reveals that low intensity focused ultrasound evokes persistent dilation in cortical microvasculature

Non-invasive, low intensity focused ultrasound (FUS) is an emerging neuromodulation technique that offers the potential for precision, personalized therapy. An increasing body of research has identified mechanosensitive ion channels that can be modulated by FUS and support acute electrical activity in neurons. However, neuromodulatory effects that persist from hours to days have also been reported. The brain's ability to provide targeted blood flow to electrically active regions involve a multitude of non-neuronal cell types and signaling pathways in the cerebral vasculature; an open question is whether persistent effects can be attributed, at least partly, to vascular mechanisms. Using a novel in vivo optical approach, we found that microvascular responses, unlike larger vessels which prior investigations have explored, exhibit persistent dilation. This finding and approach offers a heretofore unseen aspect of the effects of FUS in vivo and indicate that concurrent changes in neurovascular function may partially underly persistent neuromodulatory effects.

Alzheimer's disease brain endothelial-like cells reveal differential drug transporter expression and modulation by potentially therapeutic focused ultrasound

The blood-brain barrier (BBB) has a key function in maintaining homeostasis in the brain, partly modulated by transporters, which are highly expressed in brain endothelial cells (BECs). Transporters mediate the uptake or efflux of compounds to and from the brain and they can also challenge the delivery of drugs for the treatment of Alzheimer's disease (AD). Currently there is a limited understanding of changes in BBB transporters in AD. To investigate this, we generated brain endothelial-like cells (iBECs) from induced pluripotent stem cells (iPSCs) with familial AD (FAD) Presenilin 1 (PSEN1) mutation and identified AD-specific differences in transporter expression compared to control (ctrl) iBECs. We first characterized the expression levels of 12 BBB transporters in AD-, Ctrl-, and isogenic (PSEN1 corrected) iBECs to identify any AD specific differences. We then exposed the cells to focused ultrasound (FUS) in the absence (FUSonly) or presence of microbubbles (MB) (FUS+MB), which is a novel therapeutic method that can be used to transiently open the BBB to increase drug delivery into the brain, however its effects on BBB transporter expression are largely unknown. Following FUSonly and FUS+MB, we investigated whether the expression or activity of key transporters could be modulated. Our findings demonstrate that PSEN1 mutant FAD (PSEN1AD) possess phenotypical differences compared to control iBECs in BBB transporter expression and function. Additionally, we show that FUSonly and FUS+MB can modulate BBB transporter expression and functional activity in iBECs, having potential implications on drug penetration and amyloid clearance. These findings highlight the differential responses of patient cells to FUS treatment, with patient-derived models likely providing an important tool for modelling therapeutic effects of FUS.

Spatial and Temporal Comparisons of Calcium Channel and Intrinsic Signal Imaging During in Vivo Cortical Spreading Depolarizations in Healthy and Hypoxic Brains

BACKGROUND

Spreading depolarizations (SDs) can be viewed at a cellular level using calcium imaging (CI), but this approach is limited to laboratory applications and animal experiments. Optical intrinsic signal imaging (OISI), on the other hand, is amenable to clinical use and allows viewing of large cortical areas without contrast agents. A better understanding of the behavior of OISI-observed SDs under different brain conditions is needed.

METHODS

We performed simultaneous calcium and OISI of SDs in GCaMP6f mice. SDs propagate through the cortex as a pathological wave and trigger a neurovascular response that can be imaged with both techniques. We imaged both mechanically stimulated SDs (sSDs) in healthy brains and terminal SDs (tSDs) induced by system hypoxia and cardiopulmonary failure.

RESULTS

We observed a lag in the detection of SDs in the OISI channels compared with CI. sSDs had a faster velocity than tSDs, and tSDs had a greater initial velocity for the first 400 µm when observed with CI compared with OISI. However, both imaging methods revealed similar characteristics, including a decrease in the sSD (but not tSD) velocities as the wave moved away from the site of initial detection. CI and OISI also showed similar spatial propagation of the SD throughout the image field. Importantly, only OISI allowed regional ischemia to be detected before tSDs occurred.

CONCLUSIONS

Altogether, data indicate that monitoring either neural activity or intrinsic signals with high-resolution optical imaging can be useful to assess SDs, but OISI may be a clinically applicable way to predict, and therefore possibly mitigate, hypoxic-ischemic tSDs.

Effects of low-intensity ultrasound opening the blood-brain barrier on Alzheimer's disease-a mini review

Due to the complex pathological mechanisms of Alzheimer's disease (AD), its treatment remains a challenge. One of the major difficulties in treating AD is the difficulty for drugs to cross the blood-brain barrier (BBB). Low-intensity ultrasound (LIUS) is a novel type of ultrasound with neuromodulation function. It has been widely reported that LIUS combined with intravenous injection of microbubbles (MB) can effectively, safely, and reversibly open the BBB to achieve non-invasive targeted drug delivery. However, many studies have reported that LIUS combined with MB-mediated BBB opening (LIUS + MB-BBBO) can improve pathological deposition and cognitive impairment in AD patients and mice without delivering additional drugs. This article reviews the relevant research studies on LIUS + MB-BBBO in the treatment of AD, analyzes its potential mechanisms, and summarizes relevant ultrasound parameters.

Real-Time Intravital Imaging of Acoustic Cluster Therapy-Induced Vascular Effects in the Murine Brain

OBJECTIVE

The blood-brain barrier (BBB) is an obstacle for cerebral drug delivery. Controlled permeabilization of the barrier by external stimuli can facilitate the delivery of drugs to the brain. Acoustic Cluster Therapy (ACT®) is a promising strategy for transiently and locally increasing the permeability of the BBB to macromolecules and nanoparticles. However, the mechanism underlying the induced permeability change and subsequent enhanced accumulation of co-injected molecules requires further elucidation.

METHODS

In this study, the behavior of ACT® bubbles in microcapillaries in the murine brain was observed using real-time intravital multiphoton microscopy. For this purpose, cranial windows aligned with a ring transducer centered around an objective were mounted to the skull of mice. Dextrans labeled with 2 MDa fluorescein isothiocyanate (FITC) were injected to delineate the blood vessels and to visualize extravasation.

DISCUSSION

Activated ACT® bubbles were observed to alter the blood flow, inducing transient and local increases in the fluorescence intensity of 2 MDa FITC-dextran and subsequent extravasation in the form of vascular outpouchings. The observations indicate that ACT® induced a transient vascular leakage without causing substantial damage to the vessels in the brain.

CONCLUSION

The study gave novel insights into the mechanism underlying ACT®-induced enhanced BBB permeability which will be important considering treatment optimization for a safe and efficient clinical translation of ACT®.

An optical and acoustic investigation of microbubble cavitation in small channels under therapeutic ultrasound conditions

Therapeutic focused ultrasound in combination with encapsulated microbubbles is being widely investigated for its ability to elicit bioeffects in the microvasculature, such as transient permeabilization for drug delivery or at higher pressures to achieve 'antivascular' effects. While it is well established that the behaviors of microbubbles are altered when they are situated within sufficiently small vessels, there is a paucity of data examining how the bubble population dynamics and emissions change as a function of channel (vessel) diameter over a size range relevant to therapeutic ultrasound, particularly at pressures relevant to antivascular ultrasound. Here we use acoustic emissions detection and high-speed microscopy (10 kframes/s) to examine the behavior of a polydisperse clinically employed agent (Definity®) in wall-less channels as their diameters are scaled from 1200 to 15 µm. Pressures are varied from 0.1 to 3 MPa using either a 5 ms pulse or a sequence of 0.1 ms pulses spaced at 1 ms, both of which have been previously employed in an in vivo context. With increasing pressure, the 1200 µm channel - on the order of small arteries and veins - exhibited inertial cavitation, 1/2 subharmonics and 3/2 ultraharmonics, consistent with numerous previous reports. The 200 and 100 µm channels - in the size range of larger microvessels less affected by therapeutic focused ultrasound - exhibited a distinctly different behavior, having muted development of 1/2 subharmonics and 3/2 ultraharmonics and reduced persistence. These were associated with radiation forces displacing bubbles to the distal wall and inducing clusters that then rapidly dissipated along with emissions. As the diameter transitioned to 50 and then 15 µm - a size regime that is most relevant to therapeutic focused ultrasound - there was a higher threshold for the onset of inertial cavitation as well as subharmonics and ultraharmonics, which importantly had more complex orders that are not normally reported. Clusters also occurred in these channels (e.g. at 3 MPa, the mean lateral and axial sizes were 23 and 72 µm in the 15 µm channel; 50 and 90 µm in the 50 µm channel), however in this case they occupied the entire lumens and displaced the wall boundaries. Damage to the 15 µm channel was observed for both pulse types, but at a lower pressure for the long pulse. Experiments conducted with a 'nanobubble' (<0.45 µm) subpopulation of Definity followed broadly similar features to 'native' Definity, albeit at a higher pressure threshold for inertial cavitation. These results provide new insights into the behavior of microbubbles in small vessels at higher pressures and have implications for therapeutic focused ultrasound cavitation monitoring and control.

Evidence of cerebral hypoperfusion consecutive to ultrasound-mediated blood-brain barrier opening in rats

PURPOSE

This work aims to explore the effect of Blood Brain Barrier (BBB) opening using ultrasound combined with microbubbles injection on cerebral blood flow in rats.

METHODS

Two groups of n = 5 rats were included in this study. The first group was used to investigate the impact of BBB opening on the Arterial Spin Labeling (ASL) signal, in particular on the arterial transit time (ATT). The second group was used to analyze the spatiotemporal evolution of the change in cerebral blood flow (CBF) over time following BBB opening and validate these results using DSC-MRI.

RESULTS

Using pCASL, a decrease in CBF of up to 29 . 6 ± 15 . 1 % $$ 29.6\pm 15.1\% $$ was observed in the target hemisphere, associated with an increase in arterial transit time. The latter was estimated to be 533 ± 121ms $$ 533\pm 12\mathrm{1ms} $$ in the BBB opening impacted regions against 409 ± 93ms $$ 409\pm 93\mathrm{ms} $$ in the contralateral hemisphere. The spatio-temporal analysis of CBF maps indicated a nonlocal hypoperfusion. DSC-MRI measurements were consistent with the obtained results.

CONCLUSION

This study provided strong evidence that BBB opening using microbubble intravenous injection induces a transient hypoperfusion. A spatiotemporal analysis of the hypoperfusion changes allows to establish some points of similarity with the cortical spreading depression phenomenon.

Spatially targeted brain cancer immunotherapy with closed-loop controlled focused ultrasound and immune checkpoint blockade

Despite the challenges in treating glioblastomas (GBMs) with immune adjuvants, increasing evidence suggests that targeting the immune cells within the tumor microenvironment (TME) can lead to improved responses. Here, we present a closed-loop controlled, microbubble-enhanced focused ultrasound (MB-FUS) system and test its abilities to safely and effectively treat GBMs using immune checkpoint blockade. The proposed system can fine-tune the exposure settings to promote MB acoustic emission-dependent expression of the proinflammatory marker ICAM-1 and delivery of anti-PD1 in a mouse model of GBM. In addition to enhanced interaction of proinflammatory macrophages within the PD1-expressing TME and significant improvement in survival (P < 0.05), the combined treatment induced long-lived memory T cell formation within the brain that supported tumor rejection in rechallenge experiments. Collectively, our findings demonstrate the ability of MB-FUS to augment the therapeutic impact of immune checkpoint blockade in GBMs and reinforce the notion of spatially tumor-targeted (loco-regional) brain cancer immunotherapy.

First Metabolomic Signature of Blood-Brain Barrier Opening Induced by Microbubble-Assisted Ultrasound

Microbubble (MB)-assisted ultrasound (US) is a promising physical method to increase non-invasively, transiently, and precisely the permeability of the blood-brain barrier (BBB) to therapeutic molecules. Previous preclinical studies established the innocuity of this procedure using complementary analytical strategies including transcriptomics, histology, brain imaging, and behavioral tests. This cross-sectional study using rats aimed to investigate the metabolic processes following acoustically-mediated BBB opening in vivo using multimodal and multimatrices metabolomics approaches. After intravenous injection of MBs, the right striata were exposed to 1-MHz sinusoidal US waves at 0.6 MPa peak negative pressure with a burst length of 10 ms, for 30 s. Then, the striata, cerebrospinal fluid (CSF), blood serum, and urine were collected during sacrifice in three experimental groups at 3 h, 2 days, and 1 week after BBB opening (BBBO) and were compared to a control group where no US was applied. A well-established analytical workflow using nuclear magnetic resonance spectrometry and non-targeted and targeted high-performance liquid chromatography coupled to mass spectrometry were performed on biological tissues and fluids. In our experimental conditions, a reversible BBBO was observed in the striatum without physical damage or a change in rodent weight and behavior. Cerebral, peri-cerebral, and peripheral metabolomes displayed specific and sequential metabolic kinetics. The blood serum metabolome was more impacted in terms of the number of perturbated metabolisms than in the CSF, the striatum, and the urine. In addition, perturbations of arginine and arginine-related metabolisms were detected in all matrices after BBBO, suggesting activation of vasomotor processes and bioenergetic supply. The exploration of the tryptophan metabolism revealed a transient vascular inflammation and a perturbation of serotoninergic neurotransmission in the striatum. For the first time, we characterized the metabolic signature following the acoustically-mediated BBBO within the striatum and its surrounding biological compartments.

Low-Intensity Focused Ultrasound Technique in Glioblastoma Multiforme Treatment

Glioblastoma is one of the central nervous system most aggressive and lethal cancers with poor overall survival rate. Systemic treatment of glioblastoma remains the most challenging aspect due to the low permeability of the blood-brain barrier (BBB) and blood-tumor barrier (BTB), limiting therapeutics extravasation mainly in the core tumor as well as in its surrounding invading areas. It is now possible to overcome these barriers by using low-intensity focused ultrasound (LIFU) together with intravenously administered oscillating microbubbles (MBs). LIFU is a non-invasive technique using converging ultrasound waves which can alter the permeability of BBB/BTB to drug delivery in a specific brain/tumor region. This emerging technique has proven to be both safe and repeatable without causing injury to the brain parenchyma including neurons and other structures. Furthermore, LIFU is also approved by the FDA to treat essential tremors and Parkinson's disease. It is currently under clinical trial in patients suffering from glioblastoma as a drug delivery strategy and liquid biopsy for glioblastoma biomarkers. The use of LIFU+MBs is a step-up in the world of drug delivery, where onco-therapeutics of different molecular sizes and weights can be delivered directly into the brain/tumor parenchyma. Initially, several potent drugs targeting glioblastoma were limited to cross the BBB/BTB; however, using LIFU+MBs, diverse therapeutics showed significantly higher uptake, improved tumor control, and overall survival among different species. Here, we highlight the therapeutic approach of LIFU+MBs mediated drug-delivery in the treatment of glioblastoma.

Ultrasound-Mediated Blood-Brain Barrier Disruption for Drug Delivery: A Systematic Review of Protocols, Efficacy, and Safety Outcomes from Preclinical and Clinical Studies

Ultrasound-mediated blood-brain barrier (BBB) disruption has garnered focus as a method of delivering normally impenetrable drugs into the brain. Numerous studies have investigated this approach, and a diverse set of ultrasound parameters appear to influence the efficacy and safety of this approach. An understanding of these findings is essential for safe and reproducible BBB disruption, as well as in identifying the limitations and gaps for further advancement of this drug delivery approach. We aimed to collate and summarise protocols and parameters for achieving ultrasound-mediated BBB disruption in animal and clinical studies, as well as the efficacy and safety methods and outcomes associated with each. A systematic search of electronic databases helped in identifying relevant, included studies. Reference lists of included studies were further screened to identify supplemental studies for inclusion. In total, 107 articles were included in this review, and the following parameters were identified as influencing efficacy and safety outcomes: microbubbles, transducer frequency, peak-negative pressure, pulse characteristics, and the dosing of ultrasound applications. Current protocols and parameters achieving ultrasound-mediated BBB disruption, as well as their associated efficacy and safety outcomes, are identified and summarised. Greater standardisation of protocols and parameters in future preclinical and clinical studies is required to inform robust clinical translation.

A review of bioeffects induced by focused ultrasound combined with microbubbles on the neurovascular unit

Focused ultrasound combined with circulating microbubbles (FUS+MB) can transiently enhance blood-brain barrier (BBB) permeability at targeted brain locations. Its great promise in improving drug delivery to the brain is reflected by a rapidly growing number of clinical trials using FUS+MB to treat various brain diseases. As the clinical applications of FUS+MB continue to expand, it is critical to have a better understanding of the molecular and cellular effects induced by FUS+MB to enhance the efficacy of current treatment and enable the discovery of new therapeutic strategies. Existing studies primarily focus on FUS+MB-induced effects on brain endothelial cells, the major cellular component of BBB. However, bioeffects induced by FUS+MB expand beyond the BBB to cells surrounding blood vessels, including astrocytes, microglia, and neurons. Together these cell types comprise the neurovascular unit (NVU). In this review, we examine cell-type-specific bioeffects of FUS+MB on different NVU components, including enhanced permeability in endothelial cells, activation of astrocytes and microglia, as well as increased intraneuron protein metabolism and neuronal activity. Finally, we discuss knowledge gaps that must be addressed to further advance clinical applications of FUS+MB.

Translational Stroke Research Review: Using the Mouse to Model Human Futile Recanalization and Reperfusion Injury in Ischemic Brain Tissue

The approach to reperfusion therapies in stroke patients is rapidly evolving, but there is still no explanation why a substantial proportion of patients have a poor clinical prognosis despite successful flow restoration. This issue of futile recanalization is explained here by three clinical cases, which, despite complete recanalization, have very different outcomes. Preclinical research is particularly suited to characterize the highly dynamic changes in acute ischemic stroke and identify potential treatment targets useful for clinical translation. This review surveys the efforts taken so far to achieve mouse models capable of investigating the neurovascular underpinnings of futile recanalization. We highlight the translational potential of targeting tissue reperfusion in fully recanalized mouse models and of investigating the underlying pathophysiological mechanisms from subcellular to tissue scale. We suggest that stroke preclinical research should increasingly drive forward a continuous and circular dialogue with clinical research. When the preclinical and the clinical stroke research are consistent, translational success will follow.

Multiple regression analysis of a comprehensive transcriptomic data assembly elucidates mechanically- and biochemically-driven responses to focused ultrasound blood-brain barrier disruption

Background: Focused ultrasound (FUS) blood brain barrier disruption (BBBD) permits the noninvasive, targeted, and repeatable delivery of drugs to the brain. FUS BBBD also elicits secondary responses capable of augmenting immunotherapies, clearing amyloid-β and hyperphosphorylated tau, and driving neurogenesis. Leveraging these secondary effects will benefit from an understanding of how they correlate to the magnitude of FUS BBBD and are differentially affected by the mechanical and biochemical stimuli imparted during FUS BBBD.

Methods: We aggregated 75 murine transcriptomes in a multiple regression framework to identify genes expressed in proportion to biochemical (i.e. contrast MR image enhancement (CE)) or mechanical (i.e. harmonic acoustic emissions from MB-activation (MBA)) stimuli associated with FUS BBBD. Models were constructed to control for potential confounders, such as sex, anesthesia, and sequencing batch.

Results: MBA and CE differentially predicted expression of 1,124 genes 6 h or 24 h later. While there existed overlap in the transcripts correlated with MBA vs CE, MBA was principally predictive of expression of genes associated with endothelial reactivity while CE chiefly predicted sterile inflammation gene sets. Over-representation analysis identified transcripts not previously linked to BBBD, including actin filament organization, which is likely important for BBB recovery. Transcripts and pathways associated with neurogenesis, microglial activation, and amyloid-β clearance were significantly correlated to BBBD metrics.

Conclusions: The secondary effects of BBBD may have the potential to be tuned by modulating FUS parameters during BBBD, and MBA and CE may serve as independent predictors of transcriptional reactions in the brain.

Dynamic nanoassemblies for imaging and therapy of neurological disorders

The past decades have witnessed an increased incidence of neurological disorders (NDs) such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, ischemic stroke, and epilepsy, which significantly lower patients' life quality and increase the economic and social burden. Recently, nanomedicines composed of imaging and/or therapeutic agents have been explored to diagnose and/or treat NDs due to their enhanced bioavailability, blood-brain barrier (BBB) permeability, and targeting capacity. Intriguingly, dynamic nanoassemblies self-assembled from functional nanoparticles to simultaneously interfere with multiple pathogenic substances and pathological changes, have been regarded as one of the foremost candidates to improve the diagnostic and therapeutic efficacy of NDs. To help readers better understand this emerging field, in this review, the pathogenic mechanism of different types of NDs is briefly introduced, then the functional nanoparticles used as building blocks in the construction of dynamic nanoassemblies for NDs theranostics are summarized. Furthermore, dynamic nanoassemblies that can actively cross the BBB to target brain lesions, sensitively and efficiently diagnose or treat NDs, and effectively promote neuroregeneration are highlighted. Finally, we conclude with our perspectives on the future development in this field.

Ultrasound-mediated disruption of the blood tumor barrier for improved therapeutic delivery

The blood-brain barrier (BBB) is a major anatomical and physiological barrier limiting the passage of drugs into brain. Central nervous system tumors can impair the BBB by changing the tumor microenvironment leading to the formation of a leaky barrier, known as the blood-tumor barrier (BTB). Despite the change in integrity, the BTB remains effective in preventing delivery of chemotherapy into brain tumors. Focused ultrasound is a unique noninvasive technique that can transiently disrupt the BBB and increase accumulation of drugs within targeted areas of the brain. Herein, we summarize the current understanding of different types of targeted ultrasound mediated BBB/BTB disruption techniques. We also discuss influence of the tumor microenvironment on BBB opening, as well as the role of immunological response following disruption. Lastly, we highlight the gaps between evaluation of the parameters governing opening of the BBB/BTB. A deeper understanding of physical opening of the BBB/BTB and the biological effects following disruption can potentially enhance treatment strategies for patients with brain tumors.

Sonopermeation Enhances Uptake and Therapeutic Effect of Free and Encapsulated Cabazitaxel

Delivery of drugs and nanomedicines to tumors is often heterogeneous and insufficient and, thus, of limited efficacy. Microbubbles in combination with ultrasound have been found to improve delivery to tumors, enhancing accumulation and penetration. We used a subcutaneous prostate cancer xenograft model in mice to investigate the effect of free and nanoparticle-encapsulated cabazitaxel in combination with ultrasound and microbubbles with a lipid shell or a shell of nanoparticles. Sonopermeation reduced tumor growth and prolonged survival (26%-100%), whether the free drug was co-injected with lipid-shelled microbubbles or the nanoformulation was co-injected with lipid-shelled or nanoparticle-shelled microbubbles. Coherently with the improved therapeutic response, we found enhanced uptake of nanoparticles directly after ultrasound treatment that lasted several weeks (2.3 × -15.8 × increase). Neither cavitation dose nor total accumulation of nanoparticles could explain the variation within treatment groups, emphasizing the need for a better understanding of the tumor biology and mechanisms involved in ultrasound-mediated treatment.

Therapeutic Agent Delivery Across the Blood-Brain Barrier Using Focused Ultrasound

Specialized features of vasculature in the central nervous system greatly limit therapeutic treatment options for many neuropathologies. Focused ultrasound, in combination with circulating microbubbles, can be used to transiently and noninvasively increase cerebrovascular permeability with a high level of spatial precision. For minutes to hours following sonication, drugs can be administered systemically to extravasate in the targeted brain regions and exert a therapeutic effect, after which permeability returns to baseline levels. With the wide range of therapeutic agents that can be delivered using this approach and the growing clinical need, focused ultrasound and microbubble (FUS+MB) exposure in the brain has entered human testing to assess safety. This review outlines the use of FUS+MB-mediated cerebrovascular permeability enhancement as a drug delivery technique, details several technical and biological considerations of this approach, summarizes results from the clinical trials conducted to date, and discusses the future direction of the field.

Blood-brain barrier disruption with low-intensity pulsed ultrasound for the treatment of pediatric brain tumors: a review and perspectives

Pediatric brain tumors are the most common solid tumor and the first cause of cancer death in childhood, adolescence, and young adulthood. Current treatments are far from optimal in most of these tumors and the prognosis remains dismal for many of them. One of the main causes of the failure of current medical treatments is in part due to the existence of the blood-brain barrier (BBB), which limits drug delivery to tumors. Opening of the BBB with low-intensity pulsed ultrasound (LIPU) has emerged during the last 2 decades as a promising technique for enhancing drug delivery to the brain. In preclinical models, enhanced delivery of a wide range of therapeutic agents, from low-molecular-weight drugs, to antibodies and immune cells, has been observed as well as tumor control and increased survival. This technique has recently entered clinical trials with extracranial and intracranial devices. The safety and feasibility of this technique has furthermore been shown in patients treated monthly for recurrent glioblastoma receiving carboplatin chemotherapy. In this review, the characteristics of the BBB in the most common pediatric brain tumors are reviewed. Then, principles and mechanisms of BBB disruption with ultrasound (US) are summarized and described at the histological and biological levels. Lastly, preclinical studies that have used US-induced BBB opening in tumor models, recent clinical trials, and the potential use of this technology in pediatrics are provided.

Novel Treatment Approaches for Brain Tumour from a Blood-Brain Barrier Perspective

For a chemotherapeutic agent to be effective, it must conquer the presence of blood-brain barrier (BBB), which limits the penetration of drugs into the brain. Tumours in the brain compromise the integrity of BBB and result in a highly heterogeneous vasculature, known as blood-brain tumour barrier (BBTB). In this chapter, we firstly highlight the cellular and molecular characteristics of the BBB and BBTB as well as the challenges aroused by BBB/BBTB for drug delivery. Secondly, we discuss the current strategies overcoming the challenges in invasive and non-invasive manners. Finally, we highlight the emerging strategy using focused ultrasound (FUS) with systemic microbubbles to transiently and reversibly enhance the permeability of these barriers for drug delivery.

Microbubble formulation influences inflammatory response to focused ultrasound exposure in the brain

Focused ultrasound and microbubble (FUS + MB)-mediated blood-brain barrier (BBB) permeability enhancement can facilitate targeted brain-drug delivery. While controlling the magnitude of BBB permeability enhancement is necessary to limit tissue damage, little work has attempted to decouple these concepts. This work investigated the relationship between BBB permeability enhancement and the relative transcription of inflammatory mediators 4 h following sonication. Three microbubble formulations, Definity, BG8774, and MSB4, were compared, with the dose of each formulation normalized to gas volume. While changes in the transcription of key proinflammatory mediators, such as Il1b, Ccl2, and Tnf, were correlated to the magnitude of BBB permeability enhancement, these correlations were not independent of microbubble formulation; microbubble size distribution may play an important role, as linear regression analyses of BBB permeability magnitude versus differential gene expression for these proinflammatory mediators revealed significantly greater slopes for MSB4, a monodisperse microbubble with mean diameter of 4 μm, compared to Definity or BG8774, both polydisperse microbubbles with mean diameters below 2 μm. Additionally, the function of an acoustic feedback control algorithm, based on the detection threshold of ultraharmonic emissions, was assessed. While this control strategy was effective in limiting both wideband emissions and red blood cell extravasation, microbubble formulation was found to influence the magnitude of BBB leakage and correlations to acoustic emissions. This work demonstrates that while the initial magnitude of FUS + MB-mediated BBB permeability enhancement has a clear influence on the subsequent inflammatory responses, microbubble characteristics influence these relationships and must also be considered.

Concurrent visual and acoustic tracking of passive and active delivery of nanobubbles to tumors

Background: There has been growing interest in nanobubbles for their potential to extend bubble-mediated ultrasound approaches beyond that of their larger microbubble counterparts. In particular, the smaller scale of nanobubbles may enable them to access the tumor extravascular compartment for imaging and therapy in closer proximity to cancer cells. Compelling preliminary demonstrations of the imaging and therapeutic abilities of nanobubbles have thus emerged, with emphasis on their ability to extravasate. However, studies to date rely on indirect histologic evidence that cannot confirm whether the structures remain intact beyond the vasculature - leaving their extravascular potential largely untapped.

Methods: Nanobubble acoustic scattering was assessed using a recently reported ultra-stable formulation at low concentration (10⁶ mL^-1) and frequency (1 MHz), over a range of pressures (100-1500 kPa) in a channel phantom. The pressure-dependent response was utilized as a basis for in vivo experiments where ultrasound transmitters and receivers were integrated into a window chamber for simultaneous intravital multiphoton microscopy and acoustic monitoring in tumor-affected microcirculation. Microscopy and acoustic data were utilized to assess passive and active delivery of nanobubbles and determine whether they remained intact beyond the vasculature.

Results: Nanobubbles exhibit pressure-dependent nonlinear acoustic scattering. Nanobubbles are also found to have prolonged acoustic vascular pharmacokinetics, and passively extravasate intact into tumors. Ultrasound stimulation of nanobubbles is shown to actively enhance the delivery of both intact nanobubbles and shell material, increasing their spatial bioavailability deeper into the extravascular space. A range of acute vascular effects were also observed.

Conclusion: This study presents the first direct evidence that nanobubbles passively and actively extravasate intact in tumor tissue, and is the first to directly capture acute vascular events from ultrasound-stimulation of nanobubbles. The insights gained here demonstrate an important step towards unlocking the potential of nanobubbles and extending ultrasound-based applications.

Endothelial Cells, First Target of Drug Delivery Using Microbubble-Assisted Ultrasound

Microbubble-assisted ultrasound has emerged as a promising method for local drug delivery. Microbubbles are intravenously injected and locally activated by ultrasound, thus increasing the permeability of vascular endothelium for facilitating extravasation and drug uptake into the treated tissue. Thereby, endothelial cells are the first target of the effects of ultrasound-driven microbubbles. In this review, the in vitro and in vivo bioeffects of this method on endothelial cells are described and discussed, including aspects on the permeabilization of biologic barriers (endothelial cell plasma membranes and endothelial barriers), the restoration of their integrity, the molecular and cellular mechanisms involved in both these processes, and the resulting intracellular and intercellular consequences. Finally, the influence of the acoustic settings, microbubble parameters, treatment schedules and flow parameters on these bioeffects are also reviewed.

Simultaneous Intravital Optical and Acoustic Monitoring of Ultrasound-Triggered Nanobubble Generation and Extravasation

Ultrasound-activated nanobubbles are being widely investigated as contrast agents and therapeutic vehicles. Nanobubbles hold potential for accessing the tumor extravascular compartment, though this relies on clinically debated passive accumulation for which evidence to date is indirect. We recently reported ultrasound-triggered conversion of high payload porphyrin-encapsulated microbubbles to nanobubbles, with actively enhanced permeability for local delivery. This platform holds implications for optical/ultrasound-based imaging and therapeutics. While promising, it remains to be established how nanobubbles are generated and whether they extravasate intact. Here, insights into the conversion process are reported, complemented by novel simultaneous intravital and acoustic monitoring in tumor-affected functional circulation. The first direct acoustic evidence of extravascular intact nanobubbles are presented. These insights collectively advance this delivery platform and multimodal micro- and nanobubbles, extending their utility for imaging and therapeutics within and beyond the vasculature.

Secondary effects on brain physiology caused by focused ultrasound-mediated disruption of the blood-brain barrier

Focused ultrasound (FUS) combined with microbubbles is a non-invasive method for targeted, reversible disruption of the blood-brain barrier (FUS-BBB opening). This approach holds great promise for improving delivery of therapeutics to the brain. In order to achieve this clinically important goal, the approach necessarily breaks a protective barrier, temporarily, which plays a fundamental role in maintaining a homeostatic environment in the brain. Preclinical and clinical research has identified a set of treatment parameters under which this can be performed safely, whereby the BBB is disrupted to the point of being permeable to normally non-penetrant agents without causing significant acute damage to endothelial or neuronal cells. Much of the early work in this field focused on engineering questions around how to achieve optimal delivery of therapeutics via BBB disruption. However, there is increasing interest in addressing biological questions related to whether and how various aspects of neurophysiology might be affected when this fundamental protective barrier is compromised by the specific mechanisms of FUS-BBB opening. Improving our understanding of these secondary effects is becoming vital now that FUS-BBB opening treatments have entered clinical trials. Such information would help to safely expand FUS-BBB opening protocols into a wider range of drug delivery applications and may even lead to new types of treatments. In this paper, we will critically review our current knowledge of the secondary effects caused by FUS-BBB opening on brain physiology, identify areas that remain understudied, and discuss how a better understanding of these processes can be used to safely advance FUS-BBB opening into a wider range of clinical applications.

Ultrasound-induced blood-brain barrier disruption for the treatment of gliomas and other primary CNS tumors

The treatment of primary brain tumors, especially malignant gliomas, remains challenging. The failure of most treatments for this disease is partially explained by the blood-brain barrier (BBB), which prevents circulating molecules from entering the brain parenchyma. Ultrasound-induced BBB disruption (US-BBBD) has recently emerged as a promising strategy to improve the delivery of therapeutic agents to brain tumors. A large body of preclinical studies has demonstrated that the association of low-intensity pulsed ultrasound with intravenous microbubbles can transiently open the BBB in a localized manner. The safety of this technique has been assessed in numerous preclinical studies in both small and large animal models. A large panel of therapeutic agents have been delivered to the brain in preclinical models, demonstrating both tumor control and increased survival. This technique has recently entered clinical trials with encouraging preliminary data. In this review, we describe the mechanisms and histological effects of US-BBBD and summarize the preclinical studies published to date. We furthermore provide an overview of the current clinical development and future potential of this promising technology.

Effect of Ultrasound on the Vasculature and Extravasation of Nanoscale Particles Imaged in Real Time

Ultrasound and microbubbles have been found to improve the delivery of drugs and nanoparticles to tumor tissue. To obtain new knowledge on the influence of vascular parameters on extravasation and to elucidate the effect of acoustic pressure on extravasation and penetration of nanoscale particles into the extracellular matrix, real-time intravital multiphoton microscopy was performed during sonication of tumors growing in dorsal window chambers. The impact of vessel diameter, vessel structure and blood flow was characterized. Fluorescein isothiocyanate-dextran (2 MDa) was injected to visualize blood vessels. Mechanical indexes (MI) of 0.2-0.8 and in-house-made, nanoparticle-stabilized microbubbles or Sonovue were applied. The rate and extent of penetration into the extracellular matrix increased with increasing MI. However, to achieve extravasation, smaller vessels required MIs (0.8) higher than those of blood vessels with larger diameters. Ultrasound changed the blood flow rate and direction. Interestingly, the majority of extravasations occurred at vessel branching points.

Scanning ultrasound in the absence of blood-brain barrier opening is not sufficient to clear β-amyloid plaques in the APP23 mouse model of Alzheimer's disease

A major challenge in treating brain diseases is presented by the blood-brain barrier (BBB) that constitutes an efficient barrier not only for toxins but also a wide range of therapeutic agents. In overcoming this impediment, ultrasound in combination with intravenously injected microbubbles has emerged as a powerful technology that allows for the selective brain uptake of blood-borne factors and therapeutic agents by transient opening of the blood-brain barrier. We have previously shown that ultrasound in combination with microbubbles, but in the absence of a therapeutic agent, can effectively clear protein aggregates such as the hallmark lesions of Alzheimer's disease, amyloid-β (Aβ) plaques and Tau-containing neurofibrillary tangles. We have also demonstrated that the associated memory and motor impairments can be ameliorated or even restored. These studies included a negative sham control that received microbubbles in the absence of ultrasound. However, considering that ultrasound on its own is a pressure wave which has bioeffects, the possibility remained that ultrasound, without microbubbles, would also clear amyloid. We addressed this by performing repeated ultrasound only treatments of one brain hemisphere of Aβ-depositing APP23 mice, using the contralateral hemisphere as the unsonicated control. This was followed by an extensive histological analysis of fibrillar and non-fibrillar amyloid. We found that ultrasound on its own was not sufficient to clear amyloid. This implies that although ultrasound on its own has neuromodulatory effects, exogenously supplied microbubbles are required for the clearance of Aβ deposits.

Intracranial Non-thermal Ablation Mediated by Transcranial Focused Ultrasound and Phase-Shift Nanoemulsions

High intensity focused ultrasound (HIFU) mechanical ablation is an emerging technique for non-invasive transcranial surgery. Lesions are created by driving inertial cavitation in tissue, which requires significantly less peak pressure and time-averaged power compared with traditional thermal ablation. The utility of mechanical ablation could be extended to the brain provided the pressure threshold for inertial cavitation can be reduced. In this study, the utility of perfluorobutane (PFB)-based phase-shift nanoemulsions (PSNEs) for lowering the inertial cavitation threshold and enabling focal mechanical ablation in the brain was investigated. We successfully achieved vaporization of PFB-based PSNEs at 1.8 MPa with a 740 kHz focused transducer with a pulsed sonication protocol (duty cycle = 1.5%, 10 min sonication) within intact CD-1 mice brains. Evidence is provided showing that a single bolus injection of PSNEs could be used to initiate and sustain inertial cavitation in cerebrovasculature for at least 10 min. Histologic analysis of brain slices after HIFU exposure revealed ischemic and hemorrhagic lesions with dimensions that were comparable to the focal zone of the transducer. These results suggest that PFB-based PSNEs may be used to significantly reduce the inertial cavitation threshold in the cerebrovasculature and, when combined with transcranial focused ultrasound, enable focal intracranial mechanical ablation.

Safety and efficacy of focused ultrasound induced blood-brain barrier opening, an integrative review of animal and human studies

The blood-brain barrier, while fundamental in maintaining homeostasis in the central nervous system, is a bottleneck to achieving efficacy for numerous therapeutics. Improved brain penetration is also desirable for reduced dose, cost, and systemic side effects. Transient disruption of the blood-brain barrier with focused ultrasound (FUS) can facilitate drug delivery noninvasively with precise spatial and temporal specificity. FUS technology is transcranial and effective without further drug modifications, key advantages that will accelerate adoption and translation of existing therapeutic pipelines. In this review, we performed a comprehensive literature search to build a database and provide a synthesis of ultrasound parameters and drug characteristics that influence the safety and efficacy profile of FUS to enhance drug delivery.

The neurovascular response is attenuated by focused ultrasound-mediated disruption of the blood-brain barrier

Focused ultrasound (FUS)-induced disruption of the blood-brain barrier (BBB) is a non-invasive method to target drug delivery to specific brain areas that is now entering into the clinic. Recent studies have shown that the method has several secondary effects on local physiology and brain function beyond making the vasculature permeable to normally non-BBB penetrant molecules. This study uses functional MRI methods to investigate how FUS BBB opening alters the neurovascular response in the rat brain. Nine rats underwent actual and sham FUS induced BBB opening targeted to the right somatosensory cortex (SI) followed by four runs of bilateral electrical hind paw stimulus-evoked fMRI. The neurovascular response was quantified using measurements of the blood oxygen level dependent (BOLD) signal and cerebral blood flow (CBF). An additional three rats underwent the same FUS-BBB opening followed by stimulus-evoked fMRI with high resolution BOLD imaging and BOLD imaging of a carbogen-breathing gas challenge. BOLD and CBF measurements at two different stimulus durations demonstrate that the neurovascular response to the stimulus is attenuated in both amplitude and duration in the region targeted for FUS-BBB opening. The carbogen results show that the attenuation in response amplitude, but not duration, is still present when the signaling mechanism originates from changes in blood oxygenation instead of stimulus-induced neuronal activity. There is some evidence of non-local effects, including a possible global decrease in baseline CBF. All effects are resolved by 24 h after FUS-BBB opening. Taken together, these results suggest that FUS-BBB opening alters that state of local brain neurovascular physiology in such a way that hinders its ability to respond to demands for increased blood flow to the region. The mechanisms for this effect need to be elucidated.

Resting state functional connectivity changes after MR-guided focused ultrasound mediated blood-brain barrier opening in patients with Alzheimer's disease

MR-guided focused ultrasound (MRgFUS) can temporarily permeabilize the blood-brain barrier (BBB), noninvasively, to allow therapeutics access to the central nervous system. However, its secondary and potential neuromodulation effects are not well understood. We aimed to characterize the functional impact of MRgFUS BBB opening in human subjects, based on the phase I trial in patients with Alzheimer's disease. We analyzed for changes in bilateral frontoparietal networks in resting state functional MRI from five subjects after BBB opening in the right frontal lobe. We found a transient functional connectivity decrease within only the ipsilateral frontoparietal network that was recovered by the next day. Additionally, baseline to month three comparisons did not reveal any significant differences from matched-controls from the Alzheimer's Disease Neuroimaging Initiative. Overall, MRgFUS may transiently affect neurologic function, but the functional organization is restored at one day and remains unchanged at three months. This first in human data has implications for the development of MRgFUS as a drug delivery platform to pathologic brain tissue and potential use for non-invasive neuromodulation.

Applications of Ultrasound to Stimulate Therapeutic Revascularization

Many pathological conditions are characterized or caused by the presence of an insufficient or aberrant local vasculature. Thus, therapeutic approaches aimed at modulating the caliber and/or density of the vasculature by controlling angiogenesis and arteriogenesis have been under development for many years. As our understanding of the underlying cellular and molecular mechanisms of these vascular growth processes continues to grow, so too do the available targets for therapeutic intervention. Nonetheless, the tools needed to implement such therapies have often had inherent weaknesses (i.e., invasiveness, expense, poor targeting, and control) that preclude successful outcomes. Approximately 20 years ago, the potential for using ultrasound as a new tool for therapeutically manipulating angiogenesis and arteriogenesis began to emerge. Indeed, the ability of ultrasound, especially when used in combination with contrast agent microbubbles, to mechanically manipulate the microvasculature has opened several doors for exploration. In turn, multiple studies on the influence of ultrasound-mediated bioeffects on vascular growth and the use of ultrasound for the targeted stimulation of blood vessel growth via drug and gene delivery have been performed and published over the years. In this review article, we first discuss the basic principles of therapeutic ultrasound for stimulating angiogenesis and arteriogenesis. We then follow this with a comprehensive cataloging of studies that have used ultrasound for stimulating revascularization to date. Finally, we offer a brief perspective on the future of such approaches, in the context of both further research development and possible clinical translation.

Temporary blood-brain barrier disruption by low intensity pulsed ultrasound increases carboplatin delivery and efficacy in preclinical models of glioblastoma

INTRODUCTION

Glioblastoma (GBM) is the most common and aggressive primary brain cancer in adults. Few cytotoxic chemotherapies have been shown to be effective against GBM, due in part to the presence of the blood-brain barrier (BBB), which reduces the penetration of chemotherapies from the blood to the brain. Ultrasound-induced BBB opening (US-BBB) has been shown to increase the penetration of multiple chemotherapeutic agents in the brain in animal models. In the current study, the anti-tumor activity of carboplatin chemotherapy with and without US-BBB was investigated in several GBM mouse models.

METHODS

First, the IC50 of two commercial (U87 and U251) and six patient-derived GBM cell lines (PDCL) to carboplatin was measured. Next, U87 was subcutaneously grafted to a nude mouse model to test the in vivo response of the tumor to carboplatin in the absence of the BBB. Lastly, nude mice bearing orthotopically xenografted GBM cell lines (U87 or a PDCL) were randomized to four experimental groups: (i) untreated, (ii) US-BBB alone, (iii) carboplatin alone and, (iv) carboplatin + US-BBB. Mice were treated once weekly for 4 weeks and monitored for toxicity, tumor growth, and survival.

RESULTS

Carboplatin plus US-BBB enhanced survival (p = 0.03) and delayed tumor growth (p < 0.05) of GBM-bearing mice compared to carboplatin alone, with a 4.2-fold increase of carboplatin penetration in the brain, without evidence of significant neurological or systemic toxicity.

CONCLUSIONS

Carboplatin efficacy was enhanced in GBM mouse models with US-BBB and appears to be a promising chemotherapy for this approach.

Amyloid-β and Tau in Alzheimer's Disease: Novel Pathomechanisms and Non-Pharmacological Treatment Strategies

Accumulation of the peptide amyloid-β (Aβ) and the protein tau in Alzheimer's disease (AD) brains is a gradual process that involves the post-translational modification and assembly of monomeric forms into larger structures that eventually form fibrillar inclusions. This process is thought to both drive and initiate AD. However, why the axonally enriched tau in the course of AD accumulates in the somatodendritic domain is not fully understood. We discuss new data that provide a possible explanation that involves de novo protein synthesis, induced by Aβ and mediated through the kinase Fyn. We further discuss how in a pathological state, tau, being a scaffolding protein, impairs nuclear and mitochondrial functions and reduces action potential generation at the axon initial segment. Pathological tau can further be packaged into exosomes, released by one neuron and taken up by another, contributing to its pathogenicity. We also present our new work that suggests ultrasound as a new treatment modality to clear pathological Aβ and tau. We put this work into perspective, discussing current vaccination strategies and improved brain delivery methods involving antibody engineering and viral approaches. We propose that rather than reducing post-translational modifications of tau, its levels and de novo synthesis need to be reduced. We anticipate a surge in combinatorial strategies, simultaneously targeting multiple pathologies, and an improved drug delivery to the brain facilitated by emerging technologies such as ultrasound.

Monitoring of acoustic cavitation in microbubble-presented focused ultrasound exposure using gradient-echo MRI

BACKGROUND

Gadolinium-based contrast agents can be used to identify the blood-brain barrier (BBB) opening after inducing a focused ultrasound (FUS) cavitation effect in the presence of microbubbles. However, the use of gadolinium may be limited for frequent routine monitoring of the BBB opening in clinical applications.

PURPOSE

To use a gradient-echo sequence without contrast agent administration for monitoring of acoustic cavitation.

STUDY TYPE

Animal and phantom prospective.

PHANTOM/ANIMAL MODEL

Static and flowing gel phantoms; six normal adult male Sprague-Dawley rats.

FIELD STRENGTH/SEQUENCE

3T, 7T; fast low-angle shot sequence.

ASSESSMENT

Burst FUS with acoustic pressures = 1.5, 2.2, 2.8 MPa; pulse repetition frequencies = 1, 10,100 Hz; and duty cycles = 2%, 5%, 10% were transmitted to the chamber of a static phantom with microbubble concentrations = 10%, 1%, 0.1%. MR slice thicknesses = 3, 6, 8 mm were acquired. In flowing phantom experiments, 0.1%, 0.25%, 0.5%, 0.75%, and 1% microbubbles were infused and transmitted by burst FUS with an acoustic pressure = 0.4 and 1 MPa. In in vivo experiments, 0.25% microbubbles was infused and 0.8 MPa burst FUS was transmitted to targeted brain tissue beneath the superior sagittal sinus. The mean signal intensity (SI) was normalized using the mean SI from pre-FUS.

STATISTICAL TESTS

Two-tailed Student's t-test. P < 0.05 was considered statistically significant.

RESULTS

In the static phantom, the time courses of normalized SI decreases to minimum SI levels of 70-80%. In the flowing phantom, substantial normalized SI of 160-230% was present with variant acoustic pressures and microbubble concentrations. Compared with in vivo control rats, the brain tissue of experimental rats with transmission of FUS pulses exhibited considerable decreases of normalized SI (P < 0.001) because of the cavitation-induced perturbation of flow.

DATA CONCLUSION

Observing gradient-echo SI changes can help monitor the targeted location of microbubble-enhanced FUS, which in turn assists the monitoring of the BBB opening.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:311-318.

Imaging Hyperpolarized Pyruvate and Lactate after Blood-Brain Barrier Disruption with Focused Ultrasound

Imaging of hyperpolarized ¹³C-labeled substrates has emerged as an important magnetic resonance (MR) technique to study metabolic pathways in real time in vivo. Even though this technique has found its way to clinical trials, in vivo dynamic nuclear polarization is still mostly applied in preclinical models. Its tremendous increase in signal-to-noise ratio (SNR) overcomes the intrinsically low MR sensitivity of the ¹³C nucleus and allows real-time metabolic imaging in small structures like the mouse brain. However, applications in brain research are limited as delivery of hyperpolarized compounds is restrained by the blood–brain barrier (BBB). A local noninvasive disruption of the BBB could facilitate delivery of hyperpolarized substrates and create opportunities to study metabolic pathways in the brain that are generally not within reach. In this work, we designed a setup to apply BBB disruption in the mouse brain by MR-guided focused ultrasound (FUS) prior to MR imaging of ¹³C-enriched hyperpolarized [1-¹³C]-pyruvate and its conversion to [1-¹³C]-lactate. To overcome partial volume issues, we optimized a fast multigradient-echo imaging method (temporal resolution of 2.4 s) with an in-plane spatial resolution of 1.6 × 1.6 mm², without the need of processing large amounts of spectroscopic data. We demonstrated the feasibility to apply ¹³C imaging in less than 1 h after FUS treatment and showed a locally disrupted BBB during the time window of the whole experiment. From detected hyperpolarized pyruvate and lactate signals in both FUS-treated and untreated mice, we conclude that even at high spatial resolution, signals from the blood compartment dominate in the ¹³C images, leaving the interpretation of hyperpolarized signals in the mouse brain challenging.

Concurrent Osteosarcoma Theranostic Strategy Using Contrast-Enhanced Ultrasound and Drug-Loaded Bubbles

Osteosarcoma (OS) is the most common bone tumor in children and teenagers. The multidrug resistant property of OS produces a major obstacle to chemotherapy, since the effective drug dose cannot be achieved via conventional drug delivery routes without serious systemic cytotoxicity. Microbubbles in conjunction with ultrasound (US) has recently been shown to spatially and temporally permeabilize the cellular membrane, promoting drug penetration into tumors. Here, we investigated whether drug (doxorubicin, DOX)-loaded bubbles (DOX-bubbles) can serve as drug-loaded carriers in combination with US in order to facilitate tumor drug delivery. The proposed bubbles have a high payload capacity (efficiency of 69.4 ± 9.1%, payload of 1.4 mg/mL) for DOX. In vitro data revealed that when used in combination with US (1-MHz), these DOX-bubbles facilitate DOX entering into tumor cells. In tumor-bearing animals, DOX-bubbles + US could provide 3.7-fold suppression of tumor growth compared with the group without insonation (1.8 ± 0.9 cm³ vs. 8.5 ± 2.2 cm³) because of the acceleration of DOX-induced tumor necrosis. In the meantime, the tumor perfusion and volume can be monitored by DOX-bubbles with contrast-enhanced ultrasound imaging. Our data provide useful information in support of translating the use of theranostic US-responsive bubbles for regulated tumor drug delivery into clinical use.

Modulation of brain function by targeted delivery of GABA through the disrupted blood-brain barrier

The technology of transcranial focused ultrasound (FUS) enables a novel approach to neuromodulation, a tool for selective manipulation of brain function to be used in neurobiology research and with potential applications in clinical treatment. The method uses transcranial focused ultrasound to non-invasively open the blood-brain barrier (BBB) in a localized region such that a systemically injected neurotransmitter chemical can be delivered to the targeted brain site. The approach modulates the chemical signaling that occurs in and between neurons, making it complimentary to most other neuromodulation techniques that affect the electrical properties of neuronal activity. Here, we report delivering the inhibitory neurotransmitter GABA to the right somatosensory cortex of the rat brain during bilateral hind paw electrical stimulation and measure the inhibition of activation using functional MRI (fMRI). In a 2 × 2 factorial design, we evaluated conditions of BBB Closed vs BBB Open and No GABA vs GABA. Results from fMRI measurements of the blood oxygen level-dependent (BOLD) signal show: 1) intravenous GABA injection without FUS-mediated BBB opening does not have an effect on the BOLD response; 2) FUS-mediated BBB opening alone significantly alters the BOLD signal response to the stimulus, both in amplitude and shape of the time course; 3) the combination of FUS-mediated BBB opening and GABA injection further reduces the peak amplitude and spatial extent of the BOLD signal response to the stimulus. The data support the thesis that FUS-mediated opening of the BBB can be used to achieve non-invasive delivery of neuroactive substances for targeted manipulation of brain function.

Theranostic Strategy of Focused Ultrasound Induced Blood-Brain Barrier Opening for CNS Disease Treatment

Focused Ultrasound (FUS) in combination with gaseous microbubbles has emerged as a potential new means of effective drug delivery to the brain. Recent research has shown that, under burst-type energy exposure with the presence of microbubbles, this modality can transiently permeate the blood-brain barrier (BBB). The bioavailability of therapeutic agents is site-specifically augmented only in the zone where the FUS energy is targeted. The non-invasiveness of this approach makes FUS-induced BBB opening a novel and attractive means to perform localized CNS therapeutic agent delivery. Over the past decade, FUS-BBB opening has been preclinically confirmed to successfully enhance CNS penetration of therapeutic agents including chemotherapeutic agents, therapeutic peptides, monoclonal antibodies, and nanoparticles. Recently, a number of clinical human trials have begun to explore clinical utility. This review article, explores this technology through its physical mechanisms, summarizes the existing preclinical findings (including current medical device designs and technical approaches), and summarizes current ongoing clinical trials.

Experimental Models of Tauopathy - From Mechanisms to Therapies

Animal models have been instrumental in reproducing key aspects of human tauopathy. In pursuing these efforts, the mouse continues to have a prominent role. In this chapter, we focus on models that overexpress wild-type or mutant forms of tau, the latter being based on mutations found in familial cases of frontotemporal dementia. We review some of these models in more detail and discuss what they have revealed about the underlying pathomechanisms, as well as highlighting new developments that exploit gene editing tools such as TALEN and CRISPR. Interestingly, when investigating the role of tau in impairing cellular functions, common themes emerge. Because tau is a scaffolding protein that aggregates in the somatodendritic domain under pathological conditions, it traps proteins such as parkin and JIP1, preventing them from executing their normal function in mitophagy and axonal transport, respectively. Another aspect is the emerging role of tau in the translational machinery and the finding that the somatodendritic accumulation of tau in Alzheimer's disease may in part be due to the induction of the de novo synthesis of tau by amyloid-β via the Fyn/ERK/S6 pathway. We further discuss treatment strategies such as tau-based vaccinations and therapeutic ultrasound and conclude by discussing whether there is a future for animal models of tauopathies.

Sonopermeation to improve drug delivery to tumors: from fundamental understanding to clinical translation

INTRODUCTION

Ultrasound in combination with microbubbles can make cells and tissues more accessible for drugs, thereby achieving improved therapeutic outcomes. In this review, we introduce the term 'sonopermeation', covering mechanisms such as pore formation (traditional sonoporation), as well as the opening of intercellular junctions, stimulated endocytosis/transcytosis, improved blood vessel perfusion and changes in the (tumor) microenvironment. Sonopermeation has gained a lot of interest in recent years, especially for delivering drugs through the otherwise impermeable blood-brain barrier, but also to tumors.

AREAS COVERED

In this review, we summarize various in vitro assays and in vivo setups that have been employed to unravel the fundamental mechanisms involved in ultrasound-enhanced drug delivery, as well as clinical trials that are ongoing in patients with brain, pancreatic, liver and breast cancer. We summarize the basic principles of sonopermeation, describe recent findings obtained in (pre-) clinical trials, and discuss future directions.

EXPERT OPINION

We suggest that an improved mechanistic understanding, and microbubbles and ultrasound equipment specialized for drug delivery (and not for imaging) are key aspects to create more effective treatment regimens by sonopermeation. Real-time feedback and tools to predict therapeutic outcome and which tumors/patients will benefit from sonopermeation-based interventions will be important to promote clinical translation.

Targeting Early Dementia: Using Lipid Cubic Phase Nanocarriers to Cross the Blood⁻Brain Barrier

Over the past decades, a frequent co-morbidity of cerebrovascular pathology and Alzheimer's disease has been observed. Numerous published studies indicate that the preservation of a healthy cerebrovascular endothelium can be an important therapeutic target. By incorporating the appropriate drug(s) into biomimetic (lipid cubic phase) nanocarriers, one obtains a multitasking combination therapeutic, which targets certain cell surface scavenger receptors, mainly class B type I (i.e., SR-BI), and crosses the blood⁻brain barrier. This targeting allows for various cell types related to Alzheimer's to be simultaneously searched out for localized drug treatment in vivo.

Safety and Efficacy of Scanning Ultrasound Treatment of Aged APP23 Mice

Deposition of amyloid-β (Aβ) peptide leads to amyloid plaques that together with tau deposits characterize the brains of patients with Alzheimer's disease (AD). In modeling this pathology, transgenic animals such as the APP23 strain, that expresses a mutant form of the amyloid precursor protein found in familial cases of AD, have been instrumental. In previous studies, we have shown that repeated treatments with ultrasound in a scanning mode (termed scanning ultrasound or SUS) were effective in removing Aβ and restoring memory functions, without the need for a therapeutic agent such as an Aβ antibody. Considering that age is the most important risk factor for AD, we extended this study in which the mice were only 12 months old at the time of treatment by assessing a cohort of 2 year-old mice. Interestingly, at this age, APP23 mice are characterized by cerebral amyloid angiopathy (CAA) and the presence of occasional microbleeds. We found that SUS in aged mice that have been exposed to four SUS sessions that were spread out over 8 weeks and analyzed 4 weeks later did not show evidence of increased CAA or microbleeds. Furthermore, amyloid was reduced as assessed by methoxy-XO4 fluorescence. In addition, plaque-associated microglia were more numerous in SUS treated mice. Together this adds to the notion that SUS may be a treatment modality for human neurodegenerative diseases.

Nanotherapy for Alzheimer's disease and vascular dementia: Targeting senile endothelium

Due to the complexity of Alzheimer's disease, multiple cellular types need to be targeted simultaneously in order for a given therapy to demonstrate any major effectiveness. Ultrasound-sensitive coated microbubbles (in a targeted lipid nanoemulsion) are available. Versatile small molecule drug(s) targeting multiple pathways of Alzheimer's disease pathogenesis are known. By incorporating such drug(s) into the targeted "lipid-coated microbubble" [LCM]/"nanoparticle-derived" [ND] (or LCM/ND) nanoemulsion type, one obtains a multitasking combination therapeutic for translational medicine. This multitasking therapeutic targets cell-surface scavenger receptors (mainly class B type I), or SR-BI, making possible for various Alzheimer's-related cell types to be simultaneously searched out for localized drug treatment in vivo. Besides targeting cell-surface SR-BI, the proposed LCM/ND-nanoemulsion combination therapeutic(s) include a characteristic lipid-coated microbubble [LCM] subpopulation (i.e., a stable LCM suspension); such film-stabilized microbubbles are well known to substantially reduce the acoustic power levels needed for accomplishing temporary noninvasive (transcranial) ultrasound treatment, or sonoporation, if additionally desired for the Alzheimer's patient.

Hemodynamics Associated With Intracerebral Arteriovenous Malformations: The Effects of Treatment Modalities

The understanding of the physiology of cerebral arteriovenous malformations (AVMs) continues to expand. Knowledge of the hemodynamics of blood flow associated with AVMs is also progressing as imaging and treatment modalities advance. The authors present a comprehensive literature review that reveals the physical hemodynamics of AVMs, and the effect that various treatment modalities have on AVM hemodynamics and the surrounding cortex and vasculature. The authors discuss feeding arteries, flow through the nidus, venous outflow, and the relative effects of radiosurgical monotherapy, endovascular embolization alone, and combined microsurgical treatments. The hemodynamics associated with intracranial AVMs is complex and likely changes over time with changes in the physical morphology and angioarchitecture of the lesions. Hemodynamic change may be even more of a factor as it pertains to the vast array of single and multimodal treatment options available. An understanding of AVM hemodynamics associated with differing treatment modalities can affect treatment strategies and should be considered for optimal clinical outcomes.

Behavioral alterations following blood-brain barrier disruption stimulated by focused ultrasound

The purpose of this study was to investigate the behavioral alterations and histological changes of the brain after FUS-induced BBB disruption (BBBD). Rats were behaviorally tested using the open field, hole-board, and grip strength tests from day 1 through day 32 after undergoing BBBD induced by FUS with either a mild or heavy parameter. In the open field test, we found an increase in center entries on day 1 and day 9 following heavy FUS treatment and a decrease in center entries at day 18 following mild FUS treatment. With regard to memory-related alterations, rats subjected to heavy FUS treatment exhibited longer latency to start exploring and to find the first baited hole. However, rats subjected to mild FUS treatment exhibited no significant differences in terms of memory performance or grip force. The obtained data suggest that heavy FUS treatment might induce hyperactivity, spatial memory impairment, and forelimb gripping deficits. Furthermore, while mild FUS treatment may have an impact on anxiety-related behaviors, the data suggested it had no impact on locomotor activity, memory, or grip force. Thus, the behavioral alterations following FUS-induced BBBD require further investigation before clinical application.

Alzheimer’s Disease, Brain Injury, and C.N.S. Nanotherapy in Humans: Sonoporation Augmenting Drug Targeting

Owing to the complexity of neurodegenerative diseases, multiple cellular types need to be targeted simultaneously in order for a given therapy to demonstrate any major effectiveness. Ultrasound-sensitive coated microbubbles (in a targeted nanoemulsion) are available. Versatile small-molecule drug(s) targeting multiple pathways of Alzheimer’s disease pathogenesis are known. By incorporating such drug(s) into the targeted lipid-coated microbubble/nanoparticle-derived (LCM/ND) lipid nanoemulsion type, one obtains a multitasking combination therapeutic for translational medicine. This multitasking therapeutic targets cell-surface scavenger receptors (mainly scavenger receptor class B type I (SR-BI)), making it possible for various Alzheimer’s-related cell types to be simultaneously sought for localized drug treatment in vivo. Besides targeting cell-surface SR-BI, the proposed LCM/ND-nanoemulsion combination therapeutic(s) include a characteristic lipid-coated microbubble (LCM) subpopulation (i.e., a stable LCM suspension); such LCM substantially reduce the acoustic power levels needed for accomplishing temporary noninvasive (transcranial) ultrasound treatment, or sonoporation, if additionally desired for the Alzheimer’s patient.

Closed-loop control of targeted ultrasound drug delivery across the blood-brain/tumor barriers in a rat glioma model

Cavitation-facilitated microbubble-mediated focused ultrasound therapy is a promising method of drug delivery across the blood-brain barrier (BBB) for treating many neurological disorders. Unlike ultrasound thermal therapies, during which magnetic resonance thermometry can serve as a reliable treatment control modality, real-time control of modulated BBB disruption with undetectable vascular damage remains a challenge. Here a closed-loop cavitation controlling paradigm that sustains stable cavitation while suppressing inertial cavitation behavior was designed and validated using a dual-transducer system operating at the clinically relevant ultrasound frequency of 274.3 kHz. Tests in the normal brain and in the F98 glioma model in vivo demonstrated that this controller enables reliable and damage-free delivery of a predetermined amount of the chemotherapeutic drug (liposomal doxorubicin) into the brain. The maximum concentration level of delivered doxorubicin exceeded levels previously shown (using uncontrolled sonication) to induce tumor regression and improve survival in rat glioma. These results confirmed the ability of the controller to modulate the drug delivery dosage within a therapeutically effective range, while improving safety control. It can be readily implemented clinically and potentially applied to other cavitation-enhanced ultrasound therapies.