Safety and Feasibility of Repeated and Transient Blood-Brain Barrier Disruption by Pulsed Ultrasound in Patients with Recurrent Glioblastoma

Pilot study of repeated blood-brain barrier disruption in patients with mild Alzheimer's disease with an implantable ultrasound device

Background

Temporary disruption of the blood-brain barrier (BBB) using pulsed ultrasound leads to the clearance of both amyloid and tau from the brain, increased neurogenesis, and mitigation of cognitive decline in pre-clinical models of Alzheimer’s disease (AD) while also increasing BBB penetration of therapeutic antibodies. The goal of this pilot clinical trial was to investigate the safety and efficacy of this approach in patients with mild AD using an implantable ultrasound device.

Methods

An implantable, 1-MHz ultrasound device (SonoCloud-1) was implanted under local anesthesia in the skull (extradural) of 10 mild AD patients to target the left supra-marginal gyrus. Over 3.5 months, seven ultrasound sessions in combination with intravenous infusion of microbubbles were performed twice per month to temporarily disrupt the BBB. ¹⁸F-florbetapir and ¹⁸F-fluorodeoxyglucose positron emission tomography (PET) imaging were performed on a combined PET/MRI scanner at inclusion and at 4 and 8 months after the initiation of sonications to monitor the brain metabolism and amyloid levels along with cognitive evaluations. The evolution of cognitive and neuroimaging features was compared to that of a matched sample of control participants taken from the Alzheimer’s Disease Neuroimaging Initiative (ADNI).

Results

A total of 63 BBB opening procedures were performed in nine subjects. The procedure was well-tolerated. A non-significant decrease in amyloid accumulation at 4 months of − 6.6% (SD = 7.2%) on ¹⁸F-florbetapir PET imaging in the sonicated gray matter targeted by the ultrasound transducer was observed compared to baseline in six subjects that completed treatments and who had evaluable imaging scans. No differences in the longitudinal change in the glucose metabolism were observed compared to the neighboring or contralateral regions or to the change observed in the same region in ADNI participants. No significant effect on cognition evolution was observed in comparison with the ADNI participants as expected due to the small sample size and duration of the trial.

Conclusions

These results demonstrate the safety of ultrasound-based BBB disruption and the potential of this technology to be used as a therapy for AD patients. Research of this technique in a larger clinical trial with a device designed to sonicate larger volumes of tissue and in combination with disease-modifying drugs may further enhance the effects observed.

Trial registration

ClinicalTrials.gov, NCT03119961

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00981-1.

“Focused Ultrasound-mediated Drug Delivery in Humans – a Path Towards Translation in Neurodegenerative Diseases”

The blood-brain barrier (BBB) has a major protective function in preventing the entry of harmful molecules into the brain, but is simultaneously limiting the delivery of drugs, restricting their potential clinical application in neurodegenerative diseases. Recent preclinical evidence demonstrates that following application of focused ultrasound with microbubbles (FUS+MB), the BBB becomes reversibly accessible to compounds that normally are brain-impermeable, suggesting FUS+MB as a promising new platform for delivery of therapeutic agents into the central nervous system. As a step towards translation, small cohort clinical studies were performed demonstrating safe BBB opening in Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis (ALS) patients following FUS+MB, however improved drug delivery has not yet been achieved in human. Simultaneously, rapid progress in the human induced pluripotent stem cell (hiPSC) modeling technology allowed for development of novel Alzheimer’s disease patient-derived BBB in vitro model that reacts to FUS+MB with BBB opening and can be used to answer fundamental questions of human BBB responses to FUS+MB in health and disease. This review summarizes key features of the BBB that contribute to limited drug delivery, recapitulates recent advances in the FUS+MB mediated human BBB opening in vivo and in vitro in the context of neurodegenerative disorders, and highlights potential strategies for fast-track translation of the FUS+MB to improve bioavailability of drugs to the human brain. With safe and effective application, this innovative FUS+MB technology may open new avenues for therapeutic interventions in neurodegenerative diseases leading to improved clinical outcomes for patients.

Endothelial Unc5B controls blood-brain barrier integrity

Blood-brain barrier (BBB) integrity is critical for proper function of the central nervous system (CNS). Here, we show that the endothelial Unc5B receptor controls BBB integrity by maintaining Wnt/β-catenin signaling. Inducible endothelial-specific deletion of Unc5B in adult mice leads to BBB leak from brain capillaries that convert to a barrier-incompetent state with reduced Claudin-5 and increased PLVAP expression. Loss of Unc5B decreases BBB Wnt/β-catenin signaling, and β-catenin overexpression rescues Unc5B mutant BBB defects. Mechanistically, the Unc5B ligand Netrin-1 enhances Unc5B interaction with the Wnt co-receptor LRP6, induces its phosphorylation and activates Wnt/β-catenin downstream signaling. Intravenous delivery of antibodies blocking Netrin-1 binding to Unc5B causes a transient BBB breakdown and disruption of Wnt signaling, followed by neurovascular barrier resealing. These data identify Netrin-1-Unc5B signaling as a ligand-receptor pathway that regulates BBB integrity, with implications for CNS diseases.

The authors show that Netrin-1-Unc5B signaling controls blood-brain barrier integrity by maintaining Wnt/b-catenin signaling and that delivery of antibodies blocking Netrin-1 binding to Unc5B causes transient and size-selective BBB breakdown.

The roles of thermal and mechanical stress in focused ultrasound-mediated immunomodulation and immunotherapy for central nervous system tumors

BACKGROUND

Focused ultrasound (FUS) is an emerging technology, offering the capability of tuning and prescribing thermal and mechanical treatments within the brain. While early works in utilizing this technology have mainly focused on maximizing the delivery of therapeutics across the blood-brain barrier (BBB), the potential therapeutic impact of FUS-induced controlled thermal and mechanical stress to modulate anti-tumor immunity is becoming increasingly recognized.

OBJECTIVE

To better understand the roles of FUS-mediated thermal and mechanical stress in promoting anti-tumor immunity in central nervous system tumors, we performed a comprehensive literature review on focused ultrasound-mediated immunomodulation and immunotherapy in brain tumors.

METHODS

First, we summarize the current clinical experience with immunotherapy. Then, we discuss the unique and distinct immunomodulatory effects of the FUS-mediated thermal and mechanical stress in the brain tumor-immune microenvironment. Finally, we highlight recent findings that indicate that its combination with immune adjuvants can promote robust responses in brain tumors.

RESULTS

Along with the rapid advancement of FUS technologies into recent clinical trials, this technology through mild-hyperthermia, thermal ablation, mechanical perturbation mediated by microbubbles, and histotripsy each inducing distinct vascular and immunological effects, is offering the unique opportunity to improve immunotherapeutic trafficking and convert immunologically "cold" tumors into immunologically "hot" ones that are prone to generate prolonged anti-tumor immune responses.

CONCLUSIONS

While FUS technology is clearly accelerating concepts for new immunotherapeutic combinations, additional parallel efforts to detail rational therapeutic strategies supported by rigorous preclinical studies are still in need to leverage potential synergies of this technology with immune adjuvants. This work will accelerate the discovery and clinical implementation of new effective FUS immunotherapeutic combinations for brain tumor patients.

Nanotherapeutics and the Brain

Brain disease remains a significant health, social, and economic burden with a high failure rate of translation of therapeutics to the clinic. Nanotherapeutics have represented a promising area of technology investment to improve drug bioavailability and delivery to the brain, with several successes for nanotherapeutic use for central nervous system disease that are currently in the clinic. However, renewed and continued research on the treatment of neurological disorders is critically needed. We explore the challenges of drug delivery to the brain and the ways in which nanotherapeutics can overcome these challenges. We provide a summary and overview of general design principles that can be applied to nanotherapeutics for uptake and penetration in the brain. We next highlight remaining questions that limit the translational potential of nanotherapeutics for application in the clinic. Lastly, we provide recommendations for ongoing preclinical research to improve the overall success of nanotherapeutics against neurological disease. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Innovating Strategies and Tailored Approaches in Neuro-Oncology

Diffuse gliomas, the most frequent and aggressive primary central nervous system neoplasms, currently lack effective curative treatments, particularly for cases lacking the favorable prognostic marker IDH mutation. Nonetheless, advances in molecular biology allowed to identify several druggable alterations in a subset of IDH wild-type gliomas, such as NTRK and FGFR-TACC fusions, and BRAF hotspot mutations. Multi-tyrosine kinase inhibitors, such as regorafenib, also showed efficacy in the setting of recurrent glioblastoma. IDH inhibitors are currently in the advanced phase of clinical evaluation for patients with IDH-mutant gliomas. Several immunotherapeutic approaches, such as tumor vaccines or checkpoint inhibitors, failed to improve patients' outcomes. Even so, they may be still beneficial in a subset of them. New methods, such as using pulsed ultrasound to disrupt the blood-brain barrier, gene therapy, and oncolytic virotherapy, are well tolerated and may be included in the therapeutic armamentarium soon.

Ultrasounds induce blood–brain barrier opening across a sonolucent polyolefin plate in an in vitro isolated brain preparation

The blood–brain barrier (BBB) represents a major obstacle to the delivery of drugs to the central nervous system. The combined use of low-intensity pulsed ultrasound waves and intravascular microbubbles (MB) represents a promising solution to this issue, allowing reversible disruption of the barrier. In this study, we evaluate the feasibility of BBB opening through a biocompatible, polyolefin-based plate in an in vitro whole brain model. Twelve in vitro guinea pig brains were employed; brains were insonated using a planar transducer with or without interposing the polyolefin plate during arterial infusion of MB. Circulating MBs were visualized with an ultrasonographic device with a linear probe. BBB permeabilization was assessed by quantifying at confocal microscopy the extravasation of FITC-albumin perfused after each treatment. US-treated brains displayed BBB permeabilization exclusively in the volume under the US beam; no significant differences were observed between brains insonated with or without the polyolefin plate. Control brains not perfused with MB did not show signs of FITC-albumin extravasation. Our preclinical study suggests that polyolefin cranial plate could be implanted as a skull replacement to maintain craniotomic windows and perform post-surgical repeated BBB opening with ultrasound guidance to deliver therapeutic agents to the central nervous system.

Ultrasonic Thermal Monitoring of the Brain Using Golay-Coded Excitations-Feasibility Study

Thermal monitoring during focused ultrasound (FUS) transcranial procedures is mandatory and commonly performed by MRI. Transcranial ultrasonic thermal monitoring is an attractive alternative. Furthermore, using the therapeutic FUS transducer itself for this task is highly desirable. Nonetheless, such application is challenged by massive skull-induced signal attenuation and aberrations. This study examined the feasibility of implementing the Golay-coded excitations (CoE) for temperature monitoring in bovine brain samples in the range of 35 °C-43 °C (hyperthermia). Feasibility was assessed using computer simulations, water-based phantoms, and ex vivo bovine brain white-matter samples. The samples were gradually heated to about 45 °C and sonicated during cool down with a 1-MHz therapeutic FUS implementing Golay CoE. Initially, a calibration curve correlating the normalized time-of-flight (TOF) changes and the temperature was generated. Next, a bovine bone was positioned between the FUS and the brain samples, and the scanning process was repeated for different fresh samples. The calibration curve was then used as a mean for estimating the temperature, which was compared to thermocouple measurements. The simulations demonstrated a substantial improvement in signal-to-noise ratio (SNR) and suggested that the implementation of 4-bit sequences is advantageous. The experimental measurements with bone demonstrated good temperature estimation with an average absolute error for the water phantoms and brains of 1.46 °C ± 1.22 °C and 1.23 °C ± 0.99 °C, respectively. In conclusion, a novel noninvasive method utilizing the Golay CoE for ultrasonic thermal monitoring using a therapeutic FUS transducer is introduced. This method can lead to the development of an acoustic tool for brain thermal monitoring.

Profiling of the immune landscape in murine glioblastoma following blood brain/tumor barrier disruption with MR image-guided focused ultrasound

PURPOSE

Glioblastoma (GB) poses formidable challenges to systemic immunotherapy approaches owing to the paucity of immune infiltration and presence of the blood brain/tumor barriers (BBB/BTB). We hypothesize that BBB/BTB disruption (BBB/BTB-D) with focused ultrasound (FUS) and microbubbles (MB) increases immune infiltration in GB. As a prelude to rational combination of FUS with ITx, we herein investigate the impact of localized BBB/BTB-D on innate and adaptive immune responses in an orthotopic murine GB model.

METHODS

Mice with GL261 gliomas received i.v. MB and underwent FUS BBB/BTB-D (1.1 MHz, 0.5 Hz pulse repetition frequency, 10 ms bursts, 0.4-0.6 MPa). Brains, meninges, and peripheral lymphoid organs were excised and examined by flow cytometry 1-2 weeks following FUS.

RESULTS

The number of dendritic cells (DC) was significantly elevated in GL261 tumors and draining cervical LN in response to sonication. CD86 + DC frequency was also upregulated with 0.6 MPa FUS, suggesting increased maturity. While FUS did not significantly alter CD8 + T cell frequency across evaluated organs, these cells upregulated checkpoint molecules at 1 week post-FUS, suggesting increased activation. By 2 weeks post-FUS, we noted emergence of adaptive resistance mechanisms, including upregulation of TIGIT on CD4 + T cells and CD155 on non-immune tumor and stromal cells.

CONCLUSIONS

FUS BBB/BTB-D exerts mild, transient inflammatory effects in gliomas-suggesting that its combination with adjunct therapeutic strategies targeting adaptive resistance may improve outcomes. The potential for FUS-mediated BBB/BTB-D to modify immunological signatures is a timely and important consideration for ongoing clinical trials investigating this regimen in GB.

Nanocarriers Call the Last Shot in the Treatment of Brain Cancers

Our brain is protected by physio-biological barriers. The blood–brain barrier (BBB) main mechanism of protection relates to the abundance of tight junctions (TJs) and efflux pumps. Although BBB is crucial for healthy brain protection against toxins, it also leads to failure in a devastating disease like brain cancer. Recently, nanocarriers have been shown to pass through the BBB and improve patients’ survival rates, thus becoming promising treatment strategies. Among nanocarriers, inorganic nanocarriers, solid lipid nanoparticles, liposomes, polymers, micelles, and dendrimers have reached clinical trials after delivering promising results in preclinical investigations. The size of these nanocarriers is between 10 and 1000 nm and is modified by surface attachment of proteins, peptides, antibodies, or surfactants. Multiple research groups have reported transcellular entrance as the main mechanism allowing for these nanocarriers to cross BBB. Transport proteins and transcellular lipophilic pathways exist in BBB for small and lipophilic molecules. Nanocarriers cannot enter via the paracellular route, which is limited to water-soluble agents due to the TJs and their small pore size. There are currently several nanocarriers in clinical trials for the treatment of brain cancer. This article reviews challenges as well as fitting attributes of nanocarriers for brain tumor treatment in preclinical and clinical studies.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.

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.

CAR-T cells for pediatric brain tumors: Present and future

Chimeric Antigen Receptor T (CAR-T) cells are currently approved for B cell malignancies only, in children and adults. Despite a lack of robust evidence to approve such cellular immunotherapy for pediatric solid tumors, there is a growing interest for this approach in the treatment of pediatric brain tumors. Following the identification of tumor antigens as targets, the first clinical trials demonstrated some degree of clinical and biological responses to CAR-T cells for such tumor types. Additionaly, several preclinical studies have recently identified new attractive targets and antigen combination strategies, along with a superior tumor trafficking following locoregional administration. We review here the preclinical and clinical knowledge at the basis of the current clinical development of CAR-T cells for pediatric brain tumors.

Blood-Brain Barrier in Brain Tumors: Biology and Clinical Relevance

The presence of barriers, such as the blood–brain barrier (BBB) and brain–tumor barrier (BTB), limits the penetration of antineoplastic drugs into the brain, resulting in poor response to treatments. Many techniques have been developed to overcome the presence of these barriers, including direct injections of substances by intranasal or intrathecal routes, chemical modification of drugs or constituents of BBB, inhibition of efflux pumps, physical disruption of BBB by radiofrequency electromagnetic radiation (EMP), laser-induced thermal therapy (LITT), focused ultrasounds (FUS) combined with microbubbles and convection enhanced delivery (CED). However, most of these strategies have been tested only in preclinical models or in phase 1–2 trials, and none of them have been approved for treatment of brain tumors yet. Concerning the treatment of brain metastases, many molecules have been developed in the last years with a better penetration across BBB (new generation tyrosine kinase inhibitors like osimertinib for non-small-cell lung carcinoma and neratinib/tucatinib for breast cancer), resulting in better progression-free survival and overall survival compared to older molecules. Promising studies concerning neural stem cells, CAR-T (chimeric antigen receptors) strategies and immunotherapy with checkpoint inhibitors are ongoing.

Modulating the Blood-Brain Barrier: A Comprehensive Review

The highly secure blood-brain barrier (BBB) restricts drug access to the brain, limiting the molecular toolkit for treating central nervous system (CNS) diseases to small, lipophilic drugs. Development of a safe and effective BBB modulator would revolutionise the treatment of CNS diseases and future drug development in the area. Naturally, the field has garnered a great deal of attention, leading to a vast and diverse range of BBB modulators. In this review, we summarise and compare the various classes of BBB modulators developed over the last five decades-their recent advancements, advantages and disadvantages, while providing some insight into their future as BBB modulators.

From Focused Ultrasound Tumor Ablation to Brain Blood Barrier Opening for High Grade Glioma: A Systematic Review

BACKGROUND

Focused Ultrasound (FUS) is gaining a therapeutic role in neuro-oncology considering its novelty and non-invasiveness. Multiple pre-clinical studies show the efficacy of FUS mediated ablation and Blood-Brain Barrier (BBB) opening in high-grade glioma (HGG), but there is still poor evidence in humans, mainly aimed towards assessing FUS safety.

METHODS

With this systematic review our aim is, firstly, to summarize how FUS is proposed for human HGG treatment. Secondly, we focus on future perspectives and new therapeutic options. Using PRISMA 2020 guidelines, we reviewed case series and trials with description of patient characteristics, pre- and post-operative treatments and FUS outcomes. We considered nine case series (five about tumor ablation and four about BBB opening) with FUS-treated HGG patients between 1991 and 2021.

RESULTS

Sixty-eight patients were considered in total, mostly males (67.6%), with a mean age of 50.5 ± 15.3 years old. Major complication rates were found in the tumor ablation group (26.1%). FUS has been rarely applied for direct tumoral ablation in human HGG patients with controversial results, but at the best of current studies, FUS-mediated BBB opening is showing good results with very low complication rates, paving the way for a new reliable technique to improve local chemotherapy delivery and antitumoral immune response.

CONCLUSIONS

FUS can become a complementary technique to surgical resection and standard radiochemotherapy in recurrent HGG. Ongoing trials could provide in the near future more data on FUS-mediated BBB opening impact on progression-free survival, overall survival and potential drug-delivery capacities.

Synergies between therapeutic ultrasound, gene therapy and immunotherapy in cancer treatment

Due to the ease of use and excellent safety profile, ultrasound is a promising technique for both diagnosis and site-specific therapy. Ultrasound-based techniques have been developed to enhance the pharmacokinetics and efficacy of therapeutic agents in cancer treatment. In particular, transfection with exogenous nucleic acids has the potential to stimulate an immune response in the tumor microenvironment. Ultrasound-mediated gene transfection is a growing field, and recent work has incorporated this technique into cancer immunotherapy. Compared with other gene transfection methods, ultrasound-mediated gene transfection has a unique opportunity to augment the intracellular uptake of nucleic acids while safely and stably modulating the expression of immunostimulatory cytokines. The development and commercialization of therapeutic ultrasound systems further enhance the potential translation. In this Review, we introduce the underlying mechanisms and ongoing preclinical studies of ultrasound-based techniques in gene transfection for cancer immunotherapy. Furthermore, we expand on aspects of therapeutic ultrasound that impact gene therapy and immunotherapy, including tumor debulking, enhancing cytokines and chemokines and altering nanoparticle pharmacokinetics as these effects of ultrasound cannot be fully dissected from targeted gene therapy. We finally explore the outlook for this rapidly developing field.

[18F]2-Fluoro-2-deoxy-sorbitol PET Imaging for Quantitative Monitoring of Enhanced Blood-Brain Barrier Permeability Induced by Focused Ultrasound

Focused ultrasound in combination with microbubbles (FUS) provides an effective means to locally enhance the delivery of therapeutics to the brain. Translational and quantitative imaging techniques are needed to noninvasively monitor and optimize the impact of FUS on blood-brain barrier (BBB) permeability in vivo. Positron-emission tomography (PET) imaging using [¹⁸F]2-fluoro-2-deoxy-sorbitol ([¹⁸F]FDS) was evaluated as a small-molecule (paracellular) marker of blood-brain barrier (BBB) integrity. [¹⁸F]FDS was straightforwardly produced from chemical reduction of commercial [¹⁸F]2-deoxy-2-fluoro-D-glucose. [¹⁸F]FDS and the invasive BBB integrity marker Evan’s blue (EB) were i.v. injected in mice after an optimized FUS protocol designed to generate controlled hemispheric BBB disruption. Quantitative determination of the impact of FUS on the BBB permeability was determined using kinetic modeling. A 2.2 ± 0.5-fold higher PET signal (n = 5; p < 0.01) was obtained in the sonicated hemisphere and colocalized with EB staining observed post mortem. FUS significantly increased the blood-to-brain distribution of [¹⁸F]FDS by 2.4 ± 0.8-fold (V_T; p < 0.01). Low variability (=10.1%) of V_T values in the sonicated hemisphere suggests reproducibility of the estimation of BBB permeability and FUS method. [¹⁸F]FDS PET provides a readily available, sensitive and reproducible marker of BBB permeability to noninvasively monitor the extent of BBB disruption induced by FUS in vivo.

Current state of therapeutic focused ultrasound applications in neuro-oncology

INTRODUCTION

Despite manifold advances in oncology, cancers of the central nervous system remain among the most lethal. Unique features of the brain, including distinct cellular composition, immunological privilege, and physical barriers to therapeutic delivery, likely contribute to the poor prognosis of patients with neuro-oncological disease. Focused ultrasound is an emerging technology that allows transcranial delivery of ultrasound energy to focal brain targets with great precision.

METHODS

A review of the clinical and preclinical focused ultrasound literature was performed to obtain data regarding the current state of the focused ultrasound in context of neuro-oncology. A narrative review was then constructed to provide an overview of current and future applications of this technology.

RESULTS

Focused ultrasound can facilitate direct control of tumors by thermal or mechanical ablation, as well as enhance delivery of diverse therapeutics by disruption of the blood-brain barrier without local tissue damage. Indeed, ultrasound-sensitive drug formulations or sonosensitizers may be combined with ultrasound blood-brain barrier disruption to achieve high local drug concentration while limiting systemic exposure to therapeutics. Furthermore, focused ultrasound can induce radiosensitization, immunomodulation, and neuromodulation. Here we review applications of focused ultrasound with a focus on approaches currently under clinical investigation for the treatment of neuro-oncological disease, such as blood-brain barrier disruption for drug delivery and thermal ablation. We also discuss design of clinical trials, selection of patient cohorts, and emerging approaches to improve the efficacy of transcranial ultrasound, such as histotripsy, as well as combinatorial strategies to exploit synergistic biological effects of existing cancer therapies and ultrasound.

CONCLUSIONS

Focused ultrasound is a promising and actively expanding therapeutic modality for diverse neuro-oncological diseases.

MR-guided focused ultrasound enhances delivery of trastuzumab to Her2-positive brain metastases

[Figure: see text].

Does interventional MRI-guided brain cryotherapy cause a blood-brain barrier disruption? Radiological analysis and perspectives

Glioblastoma is the most common primary malignant brain tumor with an incidence of 5/100,000 inhabitants/year and a 5-year survival rate of 6.8%. Despite recent advances in the molecular biology understanding of glioblastoma, CNS chemotherapy remains challenging because of the impermeable blood-brain barrier (BBB). Interventional MRI-guided brain cryotherapy (IMRgC) is technique that creates a tissue lesion by making a severe targeted hypothermia and possibly a BBB disruption. This study goal was to analyze the effect of IMRgC on human BBB glioblastoma through its gadolinium enhancing features. All patients harboring a local glioblastoma recurrence and meeting all the inclusion criteria were consecutively included into this retrospective study during a 2-year period. The primary endpoint was to analyze the modification of the gadolinium enhancement on MRI T1 sequences using MR perfusion weighted images during follow-up. The secondary endpoint was to assess any ischemic/hemorrhagic complication following cryotherapy procedure using diffusion weighted imaging (DWI), susceptibility weighted imaging (SWI), or fluid-attenuated inversion recovery (FLAIR). Among the 6 patients studied, all (100%) showed a BBB disruption on the cryotherapy site through the analysis of the perfusion weighted images with an average delay of 2.83 months following the procedure. The gadolinium enhancement located around the cavity then spontaneously decreased in 4/6 patients (67%). No ischemic or hemorrhagic complication was recorded. This study confirms the IMRgC capacity to disrupt BBB as already suggested by the literature. IMRgC might represent a new option in the management of GBM allowing the combined effect of direct cryoablation and enhanced chemotherapy.

Ultrasound and microbubbles to beat barriers in tumors: Improving delivery of nanomedicine

Successful delivery of drugs and nanomedicine to tumors requires a functional vascular network, extravasation across the capillary wall, penetration through the extracellular matrix, and cellular uptake. Nanomedicine has many merits, but penetration deep into the tumor interstitium remains a challenge. Failure of cancer treatment can be caused by insufficient delivery of the therapeutic agents. After intravenous administration, nanomedicines are often found in off-target organs and the tumor extracellular matrix close to the capillary wall. With circulating microbubbles, ultrasound exposure focused toward the tumor shows great promise in improving the delivery of therapeutic agents. In this review, we address the impact of focused ultrasound and microbubbles to overcome barriers for drug delivery such as perfusion, extravasation, and transport through the extracellular matrix. Furthermore, we discuss the induction of an immune response with ultrasound and delivery of immunotherapeutics. The review discusses mainly preclinical results and ends with a summary of ongoing clinical trials.

MR-guided focused ultrasound liquid biopsy enriches circulating biomarkers in patients with brain tumors

BACKGROUND

Liquid biopsy is promising for early detection, monitoring of response, and recurrence of cancer. The blood-brain barrier (BBB) limits the shedding of biomarker, such as cell-free DNA (cfDNA), into the blood from brain tumors, and their detection by conventional assays. Transcranial MR-guided focused ultrasound (MRgFUS) can safely and transiently open the BBB, providing an opportunity for less-invasive access to brain pathology. We hypothesized that MRgFUS can enrich the signal of circulating brain-derived biomarkers to aid in liquid biopsy.

METHODS

Nine patients were treated in a prospective single-arm, open-label trial to investigate serial MRgFUS and adjuvant temozolomide combination in patients with glioblastoma (NCT03616860). Blood samples were collected as an exploratory measure within the hours before and after sonication, with control samples from non-brain tumor patients undergoing BBB opening (BBBO) alone (NCT03739905).

RESULTS

Brain regions averaging 7.8 ± 6.0 cm3 (range 0.8-23.1 cm3) were successfully treated within 111 ± 39 minutes without any serious adverse events. We found MRgFUS acutely enhanced plasma cfDNA (2.6 ± 1.2-fold, P < .01, Wilcoxon signed-rank test), neuron-derived extracellular vesicles (3.2 ± 1.9-fold, P < .01), and brain-specific protein S100b (1.4 ± 0.2-fold, P < .01). Further comparison of the cfDNA methylation profiles suggests a signature that is disease- and post-BBBO-specific, in keeping with our hypothesis. We also found cfDNA-mutant copies of isocitrate dehydrogenase 1 (IDH1) increased, although this was in only one patient known to harbor the tumor mutation.

CONCLUSIONS

This first-in-human proof-of-concept study shows MRgFUS enriches the signal of circulating brain-derived biomarkers, demonstrating the potential of the technology to support liquid biopsy for the brain.

Localized blood-brain barrier opening in infiltrating gliomas with MRI-guided acoustic emissions-controlled focused ultrasound

Pharmacological treatment of gliomas and other brain-infiltrating tumors remains challenging due to limited delivery of most therapeutics across the blood-brain barrier (BBB). Transcranial MRI-guided focused ultrasound (FUS), an emerging technology for noninvasive brain treatments, enables transient opening of the BBB through acoustic activation of circulating microbubbles. Here, we evaluate the safety and utility of transcranial microbubble-enhanced FUS (MB-FUS) for spatially targeted BBB opening in patients with infiltrating gliomas. In this Phase 0 clinical trial (NCT03322813), we conducted comparative and quantitative analyses of FUS exposures (sonications) and their effects on gliomas using MRI, histopathology, microbubble acoustic emissions (harmonic dose [HD]), and fluorescence-guided surgery metrics. Contrast-enhanced MRI and histopathology indicated safe and reproducible BBB opening in all patients. These observations occurred using a power cycling closed feedback loop controller, with the power varying by nearly an order of magnitude on average. This range underscores the need for monitoring and titrating the exposure on a patient-by-patient basis. We found a positive correlation between microbubble acoustic emissions (HD) and MR-evident BBB opening (P = 0.07) and associated interstitial changes (P < 0.01), demonstrating the unique capability to titrate the MB-FUS effects in gliomas. Importantly, we identified a 2.2-fold increase of fluorescein accumulation in MB-FUS-treated compared to untreated nonenhancing tumor tissues (P < 0.01) while accounting for vascular density. Collectively, this study demonstrates the capabilities of MB-FUS for safe, localized, controlled BBB opening and highlights the potential of this technology to improve the surgical and pharmacologic treatment of brain tumors.

Cavitation-modulated inflammatory response following focused ultrasound blood-brain barrier opening

Focused ultrasound (FUS) in combination with systemically injected microbubbles can be used to non-invasively open the blood-brain barrier (BBB) in targeted regions for a variety of therapeutic applications. Over the past two decades, preclinical research into the safety and efficacy of FUS-induced BBB opening has proven this technique to be transient and efficacious, propelling FUS-induced BBB opening into several clinical trials in recent years. However, as clinical trials further progress, the neuroinflammatory response to FUS-induced BBB opening needs to be better understood. In this study, we provide further insight into the relationship of microbubble cavitation and the resulting innate immune response to FUS-induced BBB opening. By keeping ultrasound parameters fixed (i.e. frequency, pressure, pulse length, etc.), three groups of mice were sonicated using a real-time cavitation controller until a target cavitation dose was reached (1 x 107 V2•s, 5 x 107 V2•s, 1 x 108 V2•s). The change in relative gene expression of the mouse inflammatory cytokines and receptors were evaluated at three different time-points (6 h, 24 h, and 72 h) after FUS. At both 6 and 24 h time-points, significant changes in relative gene expression of inflammatory cytokines and receptors were observed across all cavitation groups. However, the degree of changes in relative expression levels and the number of genes with significant changes in expression varied across the cavitation groups. Groups with a higher cavitation dose exhibited both greater changes in relative expression levels and greater number of significant changes. By 72 h post-opening, the gene expression levels returned to baseline in all cavitation dose groups, signifying a transient inflammatory response to FUS-induced BBB opening at the targeted cavitation dose levels. Furthermore, the real-time cavitation controller was able to produce consistent and significantly different BBB permeability enhancement volumes across the three different cavitation dose groups. These results indicate that cavitation monitoring and controlling during FUS-induced BBB opening can be used to potentially modulate or limit the degree of neuroinflammation, further emphasizing the importance of implementing cavitation controllers as FUS-induced BBB opening is translated into the clinic.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

Converging technologies: targeting the hallmarks of cancer using ultrasound and microbubbles

Various complex biological effects occur when ultrasonic compression waves travel through biological material. The myriad of biological outcomes instigated by ultrasound are evident when viewed through the lens of the hallmarks of cancer. Herein, we summarise the therapeutic potential of ultrasound, enhanced by microbubbles, for the treatment of cancer.

Focused ultrasound: growth potential and future directions in neurosurgery

Over the past two decades, vast improvements in focused ultrasound (FUS) technology have made the therapy an exciting addition to the neurosurgical armamentarium. In this time period, FUS has gained US Food and Drug Administration (FDA) approval for the treatment of two neurological disorders, and ongoing efforts seek to expand the lesion profile that is amenable to ultrasonic intervention. In the following review, we highlight future applications for FUS therapy and compare its potential role against established technologies, including deep brain stimulation and stereotactic radiosurgery. Particular attention is paid to tissue ablation, blood-brain-barrier opening, and gene therapy. We also address technical and infrastructural challenges involved with FUS use and summarize the hurdles that must be overcome before FUS becomes widely accepted in the neurosurgical community.

Emerging Therapeutic Strategies for Brain Tumors

Nearly thirty thousand incidences of primary and 300 thousand incidences of metastatic brain cancer are diagnosed in the USA each year. It has a high mortality rate and is often unresponsive to the standard of care, which includes surgical resection, radiation, and chemotherapy. These treatment strategies are also hindered by their invasiveness and toxic effects on healthy cells and tissues. Furthermore, the blood-brain/tumor barrier severely limits delivery of anti-cancer therapeutics administered intravenously to brain tumors, resulting in poor tumor response to the treatment. There is a critical need to develop new approaches to brain cancer therapy that can overcome these limitations. Focused ultrasound has emerged as a modality that addresses many of these limitations and has the potential to alter the treatment paradigm for brain cancer. Ultrasound transmitted through the skull can be focused on tumors and used for targeted ablation or opening the vascular barriers for drug delivery. This review provides insight on the current status of these unique ultrasound techniques, different strategies of using this technique for brain cancer, experience in preclinical models, and potential for clinical translation. We also debate the safety perspective of these techniques and discuss potential avenues for future work in noninvasive planning, monitoring, and evaluation of the ultrasonic neurointervention.

Immune Microenvironment Landscape in CNS Tumors and Role in Responses to Immunotherapy

Despite the important evolution of immunotherapeutic agents, brain tumors remain, in general, refractory to immune therapeutics. Recent discoveries have revealed that the glioma microenvironment includes a wide variety of immune cells in various states that play an important role in the process of tumorigenesis. Anti-tumor immune activity may be occurring or induced in immunogenic hot spots or at the invasive edge of central nervous system (CNS) tumors. Understanding the complex heterogeneity of the immune microenvironment in gliomas will likely be the key to unlocking the full potential of immunotherapeutic strategies. An essential consideration will be the induction of immunological effector responses in the setting of the numerous aspects of immunosuppression and evasion. As such, immune therapeutic combinations are a fundamental objective for clinical studies in gliomas. Through immune profiling conducted on immune competent murine models of glioma and ex vivo human glioma tissue, we will discuss how the frequency, distribution of immune cells within the microenvironment, and immune modulatory processes, may be therapeutically modulated to lead to clinical benefits.

Establishing C. elegans as a Model for Studying the Bioeffects of Therapeutic Ultrasound

Ultrasound is widely used in diagnostic and therapeutic medical procedures and it is becoming an important tool in biomedical research. During exposure, as an ultrasound beam interacts with the tissues in its path, changes known as "bioeffects" can result. Animal studies have suggested that these changes can alter survival, movement, reproduction, development and learning in various species. Additional studies in animals could provide valuable information about the mechanisms of therapeutic ultrasound and may contribute to the development of additional exciting laboratory techniques. Therefore, we developed methods for exposing C. elegans nematode worms to ultrasound and observed that they exhibited exposure-dependent reductions in movement, fecundity and survival. These effects were prevented by polyvinyl alcohol, which suggested that cavitation was the main mechanism of damage. This work provides a foundation for capitalizing on the advantages of C. elegans as a model to thoroughly characterize ultrasound's bioeffects at the cellular and molecular levels.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Safety evaluation of a clinical focused ultrasound system for neuronavigation guided blood-brain barrier opening in non-human primates

An emerging approach with potential in improving the treatment of neurodegenerative diseases and brain tumors is the use of focused ultrasound (FUS) to bypass the blood-brain barrier (BBB) in a non-invasive and localized manner. A large body of pre-clinical work has paved the way for the gradual clinical implementation of FUS-induced BBB opening. Even though the safety profile of FUS treatments in rodents has been extensively studied, the histological and behavioral effects of clinically relevant BBB opening in large animals are relatively understudied. Here, we examine the histological and behavioral safety profile following localized BBB opening in non-human primates (NHPs), using a neuronavigation-guided clinical system prototype. We show that FUS treatment triggers a short-lived immune response within the targeted region without exacerbating the touch accuracy or reaction time in visual-motor cognitive tasks. Our experiments were designed using a multiple-case-study approach, in order to maximize the acquired data and support translation of the FUS system into human studies. Four NHPs underwent a single session of FUS-mediated BBB opening in the prefrontal cortex. Two NHPs were treated bilaterally at different pressures, sacrificed on day 2 and 18 post-FUS, respectively, and their brains were histologically processed. In separate experiments, two NHPs that were earlier trained in a behavioral task were exposed to FUS unilaterally, and their performance was tracked for at least 3 weeks after BBB opening. An increased microglia density around blood vessels was detected on day 2, but was resolved by day 18. We also detected signs of enhanced immature neuron presence within areas that underwent BBB opening, compared to regions with an intact BBB, confirming previous rodent studies. Logistic regression analysis showed that the NHP cognitive performance did not deteriorate following BBB opening. These preliminary results demonstrate that neuronavigation-guided FUS with a single-element transducer is a non-invasive method capable of reversibly opening the BBB, without substantial histological or behavioral impact in an animal model closely resembling humans. Future work should confirm the observations of this multiple-case-study work across animals, species and tasks.

Focused Ultrasound Combined with Microbubbles in Central Nervous System Applications

The blood–brain barrier (BBB) protects the central nervous system (CNS) from invasive pathogens and maintains the homeostasis of the brain. Penetrating the BBB has been a major challenge in the delivery of therapeutic agents for treating CNS diseases. Through a physical acoustic cavitation effect, focused ultrasound (FUS) combined with microbubbles achieves the local detachment of tight junctions of capillary endothelial cells without inducing neuronal damage. The bioavailability of therapeutic agents is increased only in the area targeted by FUS energy. FUS with circulating microbubbles is currently the only method for inducing precise, transient, reversible, and noninvasive BBB opening (BBBO). Over the past decade, FUS-induced BBBO (FUS-BBBO) has been preclinically confirmed to not only enhance the penetration of therapeutic agents in the CNS, but also modulate focal immunity and neuronal activity. Several recent clinical human trials have demonstrated both the feasibility and potential advantages of using FUS-BBBO in diseased patients. The promising results support adding FUS-BBBO as a multimodal therapeutic strategy in modern CNS disease management. This review article explores this technology by describing its physical mechanisms and the preclinical findings, including biological effects, therapeutic concepts, and translational design of human medical devices, and summarizes completed and ongoing clinical trials.

Mapping Knowledge Structure and Research Frontiers of Ultrasound-Induced Blood-Brain Barrier Opening: A Scientometric Study

Background: Among the effective approaches developed for blood-brain barrier (BBB) opening, ultrasound is recognized as a non-invasive technique that can induce localized BBB opening transiently and repeatedly. This technique has aroused broad attention from researchers worldwide, and numerous articles have been published recently. However, no existing study has systematically examined this field from a scientometric perspective. The aim of this study was to summarize the knowledge structure and identify emerging trends and potential hotspots in this field. Methods: Publications related to ultrasound-induced BBB opening published from 1998 to 2020 were retrieved from Web of Science Core Collection. The search strategies were as follows: topic: ("blood brain barrier" OR "BBB") AND topic: (ultrasound OR ultrason* OR acoustic* OR sonopora*). The document type was set to articles or reviews with language restriction to English. Three different analysis tools including one online platform, VOS viewer1.6.16, and CiteSpace V5.7.R2 software were used to conduct this scientometric study. Results: A total of 1,201 valid records were included in the final analysis. The majority of scientific publication was produced by authors from North America, Eastern Asia, and Western Europe. Ultrasound in Medicine and Biology was the most prominent journal. The USA, China, and Canada were the most productive countries. Hynynen K, and Mcdannold N were key researchers with considerable academic influence. According to analysis of keywords, four main research directions were identified: cluster 1 (microbubbles study), cluster 2 (management of intracranial tumors), cluster 3 (ultrasound parameters and mechanisms study), and cluster 4 (treatment of neurodegenerative diseases). The current research hotspot has shifted from the basic research of ultrasound and microbubbles to management of intracranial tumors and neurodegenerative diseases. Burst detection analysis showed that Parkinson's disease, doxorubicin, gold nanoparticle, glioblastoma, gene therapy, and Alzheimer's disease may continue to be the research frontiers. Conclusion: Ultrasound-induced BBB opening research is in a period of robust development. This study is a starting point, providing a comprehensive overview, development landscape, and future opportunities of this technology, which standout as a useful reference for researchers and decision makers interested in this area.

Glioblastoma Multiforme-A Look at the Past and a Glance at the Future

Gliomas are the most common type of brain tumor that occur in adults and children. Glioblastoma multiforme (GBM) is the most common, aggressive form of brain cancer in adults and is universally fatal. The current standard-of-care options for GBM include surgical resection, radiotherapy, and concomitant and/or adjuvant chemotherapy. One of the major challenges that impedes success of chemotherapy is the presence of the blood–brain barrier (BBB). Because of the tightly regulated BBB, immune surveillance in the central nervous system (CNS) is poor, contributing to unregulated glioma cell growth. This review gives a comprehensive overview of the latest advances in treatment of GBM with emphasis on the significant advances in immunotherapy and novel therapeutic delivery strategies to enhance treatment for GBM.

Glioblastoma Multiforme-A Look at the Past and a Glance at the Future

Gliomas are the most common type of brain tumor that occur in adults and children. Glioblastoma multiforme (GBM) is the most common, aggressive form of brain cancer in adults and is universally fatal. The current standard-of-care options for GBM include surgical resection, radiotherapy, and concomitant and/or adjuvant chemotherapy. One of the major challenges that impedes success of chemotherapy is the presence of the blood-brain barrier (BBB). Because of the tightly regulated BBB, immune surveillance in the central nervous system (CNS) is poor, contributing to unregulated glioma cell growth. This review gives a comprehensive overview of the latest advances in treatment of GBM with emphasis on the significant advances in immunotherapy and novel therapeutic delivery strategies to enhance treatment for GBM.

Focused Ultrasound Strategies for Brain Tumor Therapy

BACKGROUND

A key challenge in the medical treatment of brain tumors is the limited penetration of most chemotherapeutic agents across the blood-brain barrier (BBB) into the tumor and the infiltrative margin around the tumor. Magnetic resonance-guided focused ultrasound (MRgFUS) is a promising tool to enhance the delivery of chemotherapeutic agents into brain tumors.

OBJECTIVE

To review the mechanism of FUS, preclinical evidence, and clinical studies that used low-frequency FUS for a BBB opening in gliomas.

METHODS

Literature review.

RESULTS

The potential of externally delivered low-intensity ultrasound for a temporally and spatially precise and predictable disruption of the BBB has been investigated for over a decade, yielding extensive preclinical literature demonstrating that FUS can disrupt the BBB in a spatially targeted and temporally reversible manner. Studies in animal models documented that FUS enhanced the delivery of numerous chemotherapeutic and investigational agents across the BBB and into brain tumors, including temozolomide, bevacizumab, 1,3-bis (2-chloroethyl)-1-nitrosourea, doxorubicin, viral vectors, and cells. Chemotherapeutic interventions combined with FUS slowed tumor progression and improved animal survival. Recent advances of MRgFUS systems allow precise, temporally and spatially controllable, and safe transcranial delivery of ultrasound energy. Initial clinical evidence in glioma patients has shown the efficacy of MRgFUS in disrupting the BBB, as demonstrated by an enhanced gadolinium penetration.

CONCLUSION

Thus far, a temporary disruption of the BBB followed by the administration of chemotherapy has been both feasible and safe. Further studies are needed to determine the actual drug delivery, including the drug distribution at a tissue-level scale, as well as effects on tumor growth and patient prognosis.

An Advanced In Situ Magnetic Resonance Imaging and Ultrasonic Theranostics Nanocomposite Platform: Crossing the Blood-Brain Barrier and Improving the Suppression of Glioblastoma Using Iron-Platinum Nanoparticles in Nanobubbles

Glioblastoma (GBM) is one of the deadliest and most invasive brain cancers/gliomas, and there is currently no established way to treat this disease. The treatment of GBM typically involves intracranial surgery followed by chemotherapy. However, the blood-brain barrier (BBB) impedes the delivery of the chemotherapeutic drug, making the treatment challenging. In this study, we embedded a chemotherapeutic drug and other nanomaterials into a nanobubble (NB), utilized active tracking and other guidance mechanisms to guide the nanocomposite to the tumor site, and then used high-intensity focused ultrasound oscillation to burst the nanobubbles, generating a transient cavitation impact on the BBB and allowing the drug to bypass it and reach the brain. FePt enhances the resolution of T2-weighted magnetic resonance imaging images and has magnetic properties that help guide the nanocomposite to the tumor location. FePt nanoparticles were loaded into the hydrophobic core of the NBs along with doxorubicin to form a bubble-based drug delivery system (Dox-FePt@NB). The surface of the NBs is modified with a targeting ligand, transferrin (Dox-FePt@NB-Tf), giving the nanocomposite active tracking abilities. The Dox-FePt@NB-Tf developed in the present study represents a potential breakthrough in GBM treatment through improved drug delivery and biological imaging.

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.

Leptomeningeal Metastases from Solid Tumors: Recent Advances in Diagnosis and Molecular Approaches

Leptomeningeal metastases (LM) from solid tumors represent an unmet need of increasing importance due to an early use of MRI for diagnosis and improvement of outcome of some molecular subgroups following targeted agents and immunotherapy. In this review, we first discussed factors limiting the efficacy of targeted agents in LM, such as the molecular divergence between primary tumors and CNS lesions and CNS barriers at the level of the normal brain, brain tumors and CSF. Further, we reviewed pathogenesis and experimental models and modalities, such as MRI (with RANO and ESO/ESMO criteria), CSF cytology and liquid biopsy, to improve diagnosis and monitoring following therapy. Efficacy and limitations of targeted therapies for LM from EGFR-mutant and ALK-rearranged NSCLC, HER2-positive breast cancer and BRAF-mutated melanomas are reported, including the use of intrathecal administration or modification of traditional cytotoxic compounds. The efficacy of checkpoint inhibitors in LM from non-druggable tumors, in particular triple-negative breast cancer, is discussed. Last, we focused on some recent techniques to improve drug delivery.

Quantitative analysis of in-vivo microbubble distribution in the human brain

Microbubbles (MB) are widely used as contrast agents to perform contrast-enhanced ultrasound (CEUS) imaging and as acoustic amplifiers of mechanical bioeffects incited by therapeutic-level ultrasound. The distribution of MBs in the brain is not yet fully understood, thereby limiting intra-operative CEUS guidance or MB-based FUS treatments. In this paper we describe a robust platform for quantification of MB distribution in the human brain, allowing to quantitatively discriminate between tumoral and normal brain tissues and we provide new information regarding real-time cerebral MBs distribution. Intraoperative CEUS imaging was performed during surgical tumor resection using an ultrasound machine (MyLab Twice, Esaote, Italy) equipped with a multifrequency (3-11 MHz) linear array probe (LA332) and a specific low mechanical index (MI < 0.4) CEUS algorithm (CnTi, Esaote, Italy; section thickness, 0.245 cm) for non-destructive continuous MBs imaging. CEUS acquisition is started by enabling the CnTI PEN-M algorithm automatically setting the MI at 0.4 with a center frequency of 2.94 MHz-10 Hz frame rate at 80 mm-allowing for continuous non-destructive MBs imaging. 19 ultrasound image sets of adequate length were selected and retrospectively analyzed using a custom image processing software for quantitative analysis of echo power. Regions of interest (ROIs) were drawn on key structures (artery-tumor-white matter) by a blinded neurosurgeon, following which peak enhancement and time intensity curves (TICs) were quantified. CEUS images revealed clear qualitative differences in MB distribution: arteries showed the earliest and highest enhancement among all structures, followed by tumor and white matter regions, respectively. The custom software built for quantitative analysis effectively captured these differences. Quantified peak intensities showed regions containing artery, tumor or white matter structures having an average MB intensity of 0.584, 0.436 and 0.175 units, respectively. Moreover, the normalized area under TICs revealed the time of flight for MB to be significantly lower in brain tissue as compared with tumor tissue. Significant heterogeneities in TICs were also observed within different regions of the same brain lesion. In this study, we provide the most comprehensive strategy for accurate quantitative analysis of MBs distribution in the human brain by means of CEUS intraoperative imaging. Furthermore our results demonstrate that CEUS imaging quantitative analysis enables discernment between different types of brain tumors as well as regions and structures within the brain. Similar considerations will be important for the planning and implementation of MB-based imaging or treatments in the future.

MR-guided blood-brain barrier opening induced by rapid short-pulse ultrasound in non-human primates

Background

Opening the blood-brain barrier (BBB) with focused ultrasound and microbubbles (MBs) has potential use in non-invasive targeted therapy for central nervous system (CNS) diseases. Rapid short-pulse (RaSP) ultrasound with a microsecond sequence has been proposed as a minimally disruptive and efficient method for opening the BBB. This work aimed to test the feasibility and safety of BBB opening in a non-human primate model using combined RaSP ultrasound sequence and MBs.

Methods

The BBB of 2 rhesus macaques were opened with RaSP and the commonly used 10 millisecond long pulse (LP), combined with microbubble (SonoVueTM, 0.2 µL/g) injection in a bolus. The transducer's central frequency was 300 kHz, and the acoustic pressure was set to 0.56 MPa calibrated in water. The BBB opening procedure was guided and evaluated with contrast-enhanced magnetic resonance imaging. The relative signal enhancement was compared between RaSP and LP sonication. T2-weighted fast-spin echo (FSE) and T2*-weighted gradient echo (GRE) sequences were scanned to evaluate edema and micro-bleeding at the end of the procedure.

Results

The relative signal enhancement was significantly higher (P<0.01) in the focal area compared to a similar area of the opposite hemisphere at all time points after sonication in each monkey, indicating the successful opening of the BBB. The relative signal enhancement in RaSP reached more than 60% of that with LP in our experiment, while the energy deposition was only 6% of LP. No edema or hemorrhage was found on magnetic resonance images after RaSP.

Conclusions

Combined RaSP ultrasound and MBs for the BBB opening is a practical method in large animal models.

Opening of the Blood-Brain Barrier Using Low-Intensity Pulsed Ultrasound Enhances Responses to Immunotherapy in Preclinical Glioma Models

PURPOSE

The blood-brain barrier (BBB) inhibits adequate dosing/penetration of therapeutic agents to malignancies in the brain. Low-intensity pulsed ultrasound (LIPU) is a safe therapeutic method of temporary BBB disruption (BBBD) to enhance chemotherapeutic delivery to the tumor and surrounding brain parenchyma for treatment of glioblastoma.

EXPERIMENTAL DESIGN

We investigated if LIPU could enhance therapeutic efficacy of anti-PD-1 in C57BL/6 mice bearing intracranial GL261 gliomas, epidermal growth factor receptor variant III (EGFRvIII) chimeric antigen receptor (CAR) T cells in NSG mice with EGFRvIII-U87 gliomas, and a genetically engineered antigen-presenting cell (APC)-based therapy producing the T-cell attracting chemokine CXCL10 in the GL261-bearing mice.

RESULTS

Mice treated with anti-PD-1 and LIPU-induced BBBD had a median survival duration of 58 days compared with 39 days for mice treated with anti-PD-1, and long-term survivors all remained alive after contralateral hemisphere rechallenge. CAR T-cell administration with LIPU-induced BBBD resulted in significant increases in CAR T-cell delivery to the CNS after 24 (P < 0.005) and 72 (P < 0.001) hours and increased median survival by greater than 129%, in comparison with CAR T cells alone. Local deposition of CXCL10-secreting APCs in the glioma microenvironment with LIPU enhanced T-cell glioma infiltration during the therapeutic window (P = 0.004) and markedly enhanced survival (P < 0.05).

CONCLUSIONS

LIPU increases immune therapeutic delivery to the tumor microenvironment with an associated increase in survival and is an emerging technique for enhancing novel therapies in the brain.

Closed-loop trans-skull ultrasound hyperthermia leads to improved drug delivery from thermosensitive drugs and promotes changes in vascular transport dynamics in brain tumors

Effective drug delivery in brain tumors remains a major challenge in oncology. Although local hyperthermia and stimuli-responsive delivery systems, such as thermosensitive liposomes, represent promising strategies to locally enhance drug delivery in solid tumors and improve outcomes, their application in intracranial malignancies remains unexplored. We hypothesized that the combined abilities of closed-loop trans-skull Magnetic Resonance Imaging guided Focused Ultrasound (MRgFUS) hyperthermia with those of thermosensitive drugs can alleviate challenges in drug delivery and improve survival in gliomas.

Methods: To conduct our investigations, we first designed a closed loop MR-guided Focused Ultrasound (MRgFUS) system for localized trans-skull hyperthermia (ΔT < 0.5 °C) in rodents and established safety thresholds in healthy mice. To assess the abilities of the developed system and proposed therapeutic strategy for FUS-triggered chemotherapy release we employed thermosensitive liposomal Dox (TSL-Dox) and tested it in two different glioma tumor models (F98 in rats and GL261 in mice). To quantify Dox delivery and changes in the transvascular transport dynamics in the tumor microenvironment we combined fluorescent microscopy, dynamic contrast enhanced MRI (DCE-MRI), and physiologically based pharmacokinetic (PBPK) modeling. Lastly, to assess the therapeutic efficacy of the system and of the proposed therapeutic strategy we performed a survival study in the GL261 glioma bearing mice.

Results: The developed closed-loop trans-skull MRgFUS-hyperthermia system that operated at 1.7 MHz, a frequency that maximized the brain (FUS-focus) to skull temperature ratio in mice, was able to attain and maintain the desired focal temperature within a narrow range. Histological evidence (H&E and Nissl) suggests that focal temperature at 41.5 ± 0.5 °C for 10 min is below the threshold for tissue damage. Quantitative analysis of doxorubicin delivery from TSLs with MRgFUS-hyperthermia demonstrated 3.5-fold improvement in cellular uptake in GL261 glioma mouse tumors (p < 0.001) and 5-fold increase in delivery in F98 glioma rat tumors (p < 0.05), as compared to controls (TSL-Dox-only). Moreover, PBPK modeling of drug transport that was calibrated using the experimental data indicated that thermal stress could lead to significant improvement in the transvascular transport (2.3-fold increase in the vessel diffusion coefficient; P < 0.001), in addition to promoting targeted Dox release. Prospective experimental investigations with DCE-MRI during FUS-hyperthermia, supported these findings and provided evidence that moderate thermal stress (≈41 °C for up to 10 min) can promote acute changes in the vascular transport dynamics in the brain tumor microenvironment (K^trans value for control vs. FUS was 0.0097 and 0.0148 min^-1, respectively; p = 0.026). Crucially, survival analysis demonstrated significant improvement in the survival in the TSL-Dox-FUS group as compared to TSL-Dox-only group (p < 0.05), providing supporting evidence on the therapeutic potential of the proposed strategy.

Conclusions: Our investigations demonstrated that spatially controlled thermal stress can be attained and sustained in the mouse brain, using a trans-skull closed-loop MRgFUS system, and used to promote the effective delivery of chemotherapy in gliomas from thermosensitive drugs. This system also allowed us to conduct mechanistic investigations that resulted in the refinement of our understanding on the role of thermal stress in augmenting mass and drug transport in brain tumors. Overall, our study established a new paradigm for effective drug delivery in brain tumors based on closed-loop ultrasound-mediated thermal stress and thermosensitive drugs.

Breaching the Blood-Brain Tumor Barrier for Tumor Therapy

Tumors affecting the central nervous system (CNS), either primary or secondary, are highly prevalent and represent an unmet medical need. Prognosis of these tumors remains poor, mostly due to the low intrinsic chemo/radio-sensitivity of tumor cells, a meagerly known role of the microenvironment and the poor CNS bioavailability of most used anti-cancer agents. The BBTB is the main obstacle for anticancer drugs to achieve therapeutic concentrations in the tumor tissues. During the last decades, many efforts have been devoted to the identification of modalities allowing to increase drug delivery into brain tumors. Until recently, success has been modest, as few of these approaches reached clinical testing and even less gained regulatory approval. In recent years, the scenario has changed, as various conjugates and drug delivery technologies have advanced into clinical testing, with encouraging results and without being burdened by a heavy adverse event profile. In this article, we review the different approaches aimed at increasing drug delivery to brain tumors, with particular attention to new, promising approaches that increase the permeability of the BBTB or exploit physiological transport mechanisms.

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.