High-Resolution Imaging of Intracellular Calcium Fluctuations Caused by Oscillating Microbubbles

Focused ultrasound and microbubble-mediated delivery of CRISPR-Cas9 ribonucleoprotein to human induced pluripotent stem cells

CRISPR-Cas9 ribonucleoproteins (RNPs) have been heavily considered for gene therapy due to their high on-target efficiency, rapid activity, and lack of insertional mutagenesis relative to other CRISPR-Cas9 delivery formats. Genetic diseases such as hypertrophic cardiomyopathy currently lack effective treatment strategies and are prime targets for CRISPR-Cas9 gene editing technology. However, current in vivo delivery strategies for Cas9 pose risks of unwanted immunogenic responses. This proof-of-concept study aimed to demonstrate that focused ultrasound (FUS) in combination with microbubbles can be used to deliver Cas9-sgRNA (single-guide RNA) RNPs and functionally edit human induced pluripotent stem cells (hiPSCs) in vitro, a model system that can be expanded to cardiovascular research via hiPSC-derived cardiomyocytes. Here, we first determine acoustic conditions suitable for the viable delivery of large proteins to hiPSCs with clinical Definity microbubble agents using our customized experimental platform. From here, we delivered Cas9-sgRNA RNP complexes targeting the EGFP (enhanced green fluorescent protein) gene to EGFP-expressing hiPSCs for EGFP knockout. Simultaneous acoustic cavitation detection during treatment confirmed a strong correlation between microbubble disruption and viable FUS-mediated protein delivery in hiPSCs. This study shows for the first time the potential for an FUS-mediated technique for targeted and precise CRISPR-Cas9 gene editing in human stem cells.

Reversible Ca2+ signaling and enhanced paracellular transport in endothelial monolayer induced by acoustic bubbles and targeted microbeads

Ultrasound and microbubble mediated blood brain barrier opening is a non-invasive and effective technique for drug delivery to targeted brain region. However, the exact mechanisms are not fully resolved. The influences of Ca2+ signaling on sonoporation and endothelial tight junctional regulation affect the efficiency and biosafety of the technique. Therefore, an improved understanding of how ultrasound evokes Ca2+ signaling in the brain endothelial monolayer, and its correlation to endothelial permeability change is necessary. Here, we examined the effects of SonoVue microbubbles or integrin-targeted microbeads on ultrasound induced bioeffects in brain microvascular endothelial monolayer using an acoustically-coupled microscopy system, where focused ultrasound exposure and real-time recording of Ca2+ signaling and membrane perforation were performed. Microbubbles induced robust Ca2+ responses, often accompanied by cell poration, while ultrasound with microbeads elicited reversible Ca2+ response without membrane poration. At the conditions evoking reversible Ca2+ signaling, intracellular Ca2+ release and reactive oxygen species played key roles for microbubbles induced Ca2+ signaling while activation of mechanosensitive ion channels was essential for the case of microbeads. Trans-well diffusion analysis revealed significantly higher trans-endothelial transport of 70 kDa FITC-dextran for both integrin-targeted microbeads and microbubbles compared to the control group. Further immunofluorescence staining showed disruption of cell junctions with microbubble stimulation and reversible remodeling of many cell junctions by ultrasound with integrin-targeted microbeads. This investigation provides new insights for ultrasound induced Ca2+ signaling and its influence on endothelial permeability, which may help develop new strategies for safe and efficient drug/gene delivery in the vascular system.

Characterizing Microbubble-Mediated Permeabilization in a Vessel-on-a-Chip Model

Drug transport from blood to extravascular tissue can locally be achieved by increasing the vascular permeability through ultrasound-activated microbubbles. However, the mechanism remains unknown, including whether short and long cycles of ultrasound induce the same onset rate, spatial distribution, and amount of vascular permeability increase. Accurate models are necessary for insights into the mechanism so a microvessel-on-a-chip is developed with a membrane-free extravascular space. Using these microvessels-on-a-chip, distinct differences between 2 MHz ultrasound treatments are shown with 10 or 1000 cycles. The onset rate is slower for 10 than 1000 cycles, while both cycle lengths increase the permeability in spot-wise patterns without affecting microvessel viability. Significantly less vascular permeability increase and sonoporation are induced for 10 versus 1000 cycles at 750 kPa (i.e., the highest studied peak negative acoustic pressure (PNP)). The PNP threshold for vascular permeability increases is 750 versus 550 kPa for 10 versus 1000 cycles, while this is 750 versus 220 kPa for sonoporation. Vascular permeability increases do not correlate with αvβ3-targeted microbubble behavior, while sonoporation correlates with αvβ3-targeted microbubble clustering. In conclusion, the further mechanistic unraveling of vascular permeability increase by ultrasound-activated microbubbles in a developed microvessel-on-a-chip model aids the safe and efficient development of microbubble-mediated drug transport.

Manipulating long-term fates of sonoporated cells by regulating intracellular calcium for improving sonoporation-based delivery

Sonoporation-based delivery has great promise for noninvasive drug and gene therapy. After short-term membrane resealing, the long-term function recovery of sonoporated cells affects the efficiency and biosafety of sonoporation-based delivery. It is necessary to identify the key early biological signals that influence cell fate and to develop strategies for manipulating the long-term fates of sonoporated cells. Here, we used a customized experimental platform with a single cavitating microbubble induced by a single ultrasound pulse (frequency: 1.5 MHz, pulse length:13.33 μs, peak negative pressure: ∼0.40 MPa) to elicit single-site reversible sonoporation on a single HeLa cell model. We used a living-cell microscopic imaging system to trace the long-term fates of sonoporated HeLa cells in real-time for 48 h. Fluorescence from intracellular propidium iodide and Fluo-4 was used to evaluate the degree of sonoporation and intracellular calcium fluctuation (ICF), respectively. Changes in cell morphology were used to assess the long-term cell fates (i.e., proliferation, arrest, or death). We found that heterogeneously sonoporated cells had different long-term fates. With increasing degree of sonoporation, the probability of normal (proliferation) and abnormal fates (arrest and death) in sonoporated cells decreased and increased, respectively. We identified ICF as an important early event for triggering different long-term fates. Reversibly sonoporated cells exhibited stronger proliferation and restoration at lower extents of ICF. We then regulated ICF dynamics in sonoporated cells using 2-APB or BAPTA treatment to reduce calcium release from intracellular organelles and enhance intracellular calcium clearance, respectively. This significantly enhanced the proliferation and restoration of sonoporated cells and reduced the occurrence of cell-cycle arrest and death. Finally, we found that the long-term fates of sonoporated cells at multiple sites and neighboring cells were also dependent on the extent of ICF, and that 2-APB significantly enhanced their viability and reduced death. Thus, using a single HeLa cell model, we demonstrated that regulating intracellular calcium can effectively enhance the proliferation and restoration capabilities of sonoporated cells, therefore rescuing the long-term viability of sonoporated cells. These findings add to our understanding of the biophysical process of sonoporation and help design new strategies for improving the efficiency and biosafety of sonoporation-based delivery.

Immunomodulation of human T cells by microbubble-mediated focused ultrasound

While met with initial and ground-breaking success targeting blood borne cancers, cellular immunotherapy remains significantly hindered in the context of solid tumors by the tumor microenvironment. Focused ultrasound, in conjunction with microbubbles, has found tremendous potential as a targeted and local drug/gene delivery technique for cancer therapy. The specific immunomodulating effects of this technique on immune cells, including T-cells, remain unexplored. Here, with freshly isolated human immune cells, we examine how focused ultrasound can viably modulate immune cell membrane permeability and influence the secretion of over 90 cytokines, chemokines and other analytes relevant to a potent immune response against cancer. We determine that microbubble-mediated focused ultrasound modulates the immune cell secretome in a time-dependent manner - ranging in ~0.1-3.6-fold changes in the concentration of a given cytokine compared to sham controls over 48 hours post-treatment (e.g. IL-1β, TNF-α, CX3CL1, CCL21). Further, we determine the general trend of a negative correlation between secreted cytokine concentration and viable ultrasound-assisted membrane permeability with negligible loss of cell viability. Taken together, the data presented here highlights the potential of microbubble-mediated focused ultrasound to viably enhance T-cell permeability and modulate key pro-immune pathways, offering a novel approach to augment targeted cellular therapies for solid tumors.

RGD-coated polymeric microbubbles promote ultrasound-mediated drug delivery in an inflamed endothelium-pericyte co-culture model of the blood-brain barrier

Drug delivery to central nervous pathologies is compromised by the blood-brain barrier (BBB). A clinically explored strategy to promote drug delivery across the BBB is sonopermeation, which relies on the combined use of ultrasound (US) and microbubbles (MB) to induce temporally and spatially controlled opening of the BBB. We developed an advanced in vitro BBB model to study the impact of sonopermeation on the delivery of the prototypic polymeric drug carrier pHPMA as a larger molecule and the small molecule antiviral drug ribavirin. This was done under standard and under inflammatory conditions, employing both untargeted and RGD peptide-coated MB. The BBB model is based on human cerebral capillary endothelial cells and human placental pericytes, which are co-cultivated in transwell inserts and which present with proper transendothelial electrical resistance (TEER). Sonopermeation induced a significant decrease in TEER values and facilitated the trans-BBB delivery of fluorescently labeled pHPMA (Atto488-pHPMA). To study drug delivery under inflamed endothelial conditions, which are typical for e.g. tumors, neurodegenerative diseases and CNS infections, tumor necrosis factor (TNF) was employed to induce inflammation in the BBB model. RGD-coated MB bound to and permeabilized the inflamed endothelium-pericyte co-culture model, and potently improved Atto488-pHPMA and ribavirin delivery. Taken together, our work combines in vitro BBB bioengineering with MB-mediated drug delivery enhancement, thereby providing a framework for future studies on optimization of US-mediated drug delivery to the brain.

Cardiac gene delivery using ultrasound: State of the field

Over the past two decades, there has been tremendous and exciting progress toward extending the use of medical ultrasound beyond a traditional imaging tool. Ultrasound contrast agents, typically used for improved visualization of blood flow, have been explored as novel non-viral gene delivery vectors for cardiovascular therapy. Given this adaptation to ultrasound contrast-enhancing agents, this presents as an image-guided and site-specific gene delivery technique with potential for multi-gene and repeatable delivery protocols-overcoming some of the limitations of alternative gene therapy approaches. In this review, we provide an overview of the studies to date that employ this technique toward cardiac gene therapy using cardiovascular disease animal models and summarize their key findings.

Shear stress preconditioning and microbubble flow pattern modulate ultrasound-assisted plasma membrane permeabilization

The recent and exciting success of anti-inflammatory therapies for ischemic heart disease (e.g. atherosclerosis) is hindered by the lack of site-specific and targeted therapeutic deposition. Microbubble-mediated focused ultrasound, which uses circulating, lipid-encapsulated intravascular microbubbles to locally enhance endothelial permeability, offers an exciting approach. Atherosclerotic plaques preferentially develop in regions with disturbed blood flow, and microbubble-endothelial cell membrane interactions under such flow conditions are not well understood. Here, using an acoustically-coupled microscopy system, endothelial cells were sonicated (1 MHz, 20 cycle bursts, 1 ms PRI, 4 s duration, 300 kPa peak-negative pressure) under perfusion with Definity™ bubbles to examine microbubble-mediated endothelial permeabilization under a range of physiological conditions. Endothelial preconditioning under prolonged shear influenced physiology and the secretome, inducing increased expression of pro-angiogenesis analytes, decreasing levels of pro-inflammatory ones, and increasing the susceptibility of ultrasound therapy. Ultrasound treatment efficiency was positively correlated with concentrations of pro-angiogenic cytokines (e.g. VEGF-A, EGF, FGF-2), and negatively correlated with pro-inflammatory chemokines (e.g. MCP-1, GCP-2, SDF-1). Furthermore, ultrasound therapy under non-reversing pulsatile flow (∼4-8 dyne/cm2, 0.5-1 Hz) increased permeabilization up to 2.4-fold compared to shear-matched laminar flow, yet treatment under reversing oscillatory flow resulted in more heterogeneous modulation. This study provides insight into the role of vascular physiology, including endothelial biology, into the design of a localized ultrasound drug delivery system for ischemic heart disease.

Current clinical investigations of focused ultrasound blood-brain barrier disruption: A review

The blood-brain barrier (BBB) presents a formidable challenge in delivering therapeutic agents to the central nervous system. Ultrasound-mediated BBB disruption has emerged as a promising non-invasive technique to enhance drug delivery to the brain. This manuscript reviews fundamental principles of ultrasound-based techniques and their mechanisms of action in temporarily permeabilizing the BBB. Clinical trials employing ultrasound for BBB disruption are discussed, summarizing diverse applications ranging from the treatment of neurodegenerative diseases to targeted drug delivery for brain tumors. The review also addresses safety considerations, outlining the current understanding of potential risks and mitigation strategies associated with ultrasound exposure, including real-time monitoring and assessment of treatment efficacy. Among the large number of studies, significant successes are highlighted thus providing perspective on the future direction of the field.

Focused ultrasound-assisted delivery of immunomodulating agents in brain cancer

Focused ultrasound (FUS) combined with intravascularly circulating microbubbles can transiently increase the permeability of the blood-brain barrier (BBB) to enable targeted therapeutic delivery to the brain, the clinical testing of which is currently underway in both adult and pediatric patients. Aside from traditional cancer drugs, this technique is being extended to promote the delivery of immunomodulating therapeutics to the brain, including antibodies, immune cells, and cytokines. In this manner, FUS approaches are being explored as a tool to improve and amplify the effectiveness of immunotherapy for both primary and metastatic brain cancer, a particularly challenging solid tumor to treat. Here, we present an overview of the latest groundbreaking research in FUS-assisted delivery of immunomodulating agents to the brain in pre-clinical models of brain cancer, and place it within the context of the current immunotherapy approaches. We follow this up with a discussion on new developments and emerging strategies for this rapidly evolving approach.

Ultrasound-responsive microbubbles and nanodroplets: A pathway to targeted drug delivery

Ultrasound-responsive agents have shown great potential as targeted drug delivery agents, effectively augmenting cell permeability and facilitating drug absorption. This review focuses on two specific agents, microbubbles and nanodroplets, and provides a sequential overview of their drug delivery process. Particular emphasis is given to the mechanical response of the agents under ultrasound, and the subsequent physical and biological effects on the cells. Finally, the state-of-the-art in their pre-clinical and clinical implementation are discussed. Throughout the review, major challenges that need to be overcome in order to accelerate their clinical translation are highlighted.

Development of an In Vitro Model to Study Mechanisms of Ultrasound-Targeted Microbubble Cavitation-Mediated Blood-Brain Barrier Opening

OBJECTIVE

Ultrasound-targeted microbubble cavitation (UTMC)-mediated blood-brain barrier (BBB) opening is being explored as a method to increase drug delivery to the brain. This strategy has progressed to clinical trials for various neurological disorders, but the underlying cellular mechanisms are incompletely understood. In the study described here, a contact co-culture transwell model of the BBB was developed that can be used to determine the signaling cascade leading to increased BBB permeability.

METHODS

This BBB model consists of bEnd.3 cells and C8-D1A astrocytes seeded on opposite sides of a transwell membrane. Pulsed ultrasound (US) is applied to lipid microbubbles (MBs), and the change in barrier permeability is measured via transendothelial electrical resistance and dextran flux. Live cell calcium imaging (Fluo-4 AM) is performed during UTMC treatment.

RESULTS

This model exhibits important features of the BBB, including endothelial tight junctions, and is more restrictive than the endothelial cell (EC) monolayer alone. When US is applied to MBs in contact with the ECs, BBB permeability increases in this model by two mechanisms: UTMC induces pore formation in the EC membrane (sonoporation), leading to increased transcellular permeability, and UTMC causes formation of reversible inter-endothelial gaps, which increases paracellular permeability. Additionally, this study determines that calcium influx into ECs mediates the increase in BBB permeability after UTMC in this model.

CONCLUSION

Both transcellular and paracellular permeability can be used to increase drug delivery to the brain. Future studies can use this model to determine how UTMC-induced calcium-mediated signaling increases BBB permeability.

Initiating and imaging cavitation from infused echo contrast agents through the EkoSonic catheter

Ultrasound-enhanced delivery of therapeutic-loaded echogenic liposomes is under development for vascular applications using the EkoSonic Endovascular System. In this study, fibrin-targeted echogenic liposomes loaded with an anti-inflammatory agent were characterized before and after infusion through an EkoSonic catheter. Cavitation activity was nucleated by Definity or fibrin-targeted, drug-loaded echogenic liposomes infused and insonified with EkoSonic catheters. Passive cavitation imaging was used to quantify and map bubble activity in a flow phantom mimicking porcine arterial flow. Cavitation was sustained during 3-min infusions of Definity or echogenic liposomes along the distal 6 cm treatment zone of the catheter. Though the EkoSonic catheter was not designed specifically for cavitation nucleation, infusion of drug-loaded echogenic liposomes can be employed to trigger and sustain bubble activity for enhanced intravascular drug delivery.

Tumour associated vasculature-on-a-chip for the evaluation of microbubble-mediated delivery of targeted liposomes

The vascular system is the primary route for the delivery of therapeutic drugs throughout the body and is an important barrier at the region of disease interest, such as a solid tumour. The development of complex 3D tumour cultures has progressed significantly in recent years however, the generation of perfusable vascularised tumour models still presents many challenges. This study presents a microfluidic-based vasculature system that can be induced to display properties of tumour-associated blood vessels without direct incorporation of tumour cells. Conditioning healthy endothelial-fibroblast cell vasculature co-cultures with media taken from tumour cell cultures was found to result in the formation of disorganised, tortuous networks which display characteristics consistent with those of tumour-associated vasculature. Integrin αvβ3, a cell adhesion receptor associated with angiogenesis, was found to be upregulated in vasculature co-cultures conditioned with tumour cell media (TCM) - consistent with the reported αvβ3 expression pattern in angiogenic tumour vasculature in vivo. Increased accumulation of liposomes (LSs) conjugated to antibodies against αvβ3 was observed in TCM networks compared to non-conditioned networks, indicating αvβ3 may be a potential target for the delivery of drugs specifically to tumour vasculature. Furthermore, the use of microbubbles (MBs) and ultrasound (US) to further enhance the delivery of LSs to TCM-conditioned vasculature was investigated. Quantification of fluorescent LS accumulation post-perfusion of the vascular network showed 3-fold increased accumulation with the use of MBs and US, suggesting that targeted LS delivery could be further improved with the use of locally administered MBs and US.

Barrier-breaking effects of ultrasonic cavitation for drug delivery and biomarker release

Recently, emerging evidence has demonstrated that cavitation actually creates important bidirectional channels on biological barriers for both intratumoral drug delivery and extratumoral biomarker release. To promote the barrier-breaking effects of cavitation for both therapy and diagnosis, we first reviewed recent technical advances of ultrasound and its contrast agents (microbubbles, nanodroplets, and gas-stabilizing nanoparticles) and then reported the newly-revealed cavitation physical details. In particular, we summarized five types of cellular responses of cavitation in breaking the plasma membrane (membrane retraction, sonoporation, endocytosis/exocytosis, blebbing and apoptosis) and compared the vascular cavitation effects of three different types of ultrasound contrast agents in breaking the blood-tumor barrier and tumor microenvironment. Moreover, we highlighted the current achievements of the barrier-breaking effects of cavitation in mediating drug delivery and biomarker release. We emphasized that the precise induction of a specific cavitation effect for barrier-breaking was still challenged by the complex combination of multiple acoustic and non-acoustic cavitation parameters. Therefore, we provided the cutting-edge in-situ cavitation imaging and feedback control methods and suggested the development of an international cavitation quantification standard for the clinical guidance of cavitation-mediated barrier-breaking effects.

Faster calcium recovery and membrane resealing in repeated sonoporation for delivery improvement

In sonoporation-based macromolecular delivery, repetitive microbubble cavitation in the bloodstream results in repeated sonoporation of cells or sonoporation of non-sonoporated neighboring cells (i.e., adjacent to the sonoporated host cells). The resealing and recovery capabilities of these two types of sonoporated cells affect the efficiency and biosafety of sonoporation-based delivery. Therefore, an improved understanding of the preservation of viability in these sonoporated cells is necessary. Using a customized platform for single-pulse ultrasound exposure (pulse length 13.33 μs, peak negative pressure 0.40 MPa, frequency 1.5 MHz) and real-time recording of membrane perforation and intracellular calcium fluctuations (using propidium iodide and Fluo-4 fluorescent probes, respectively), spatiotemporally controlled sonoporation was performed to administer first and second single-site sonoporations of a single cell or single-site sonoporation of a neighboring cell. Two distinct intracellular calcium changes, reversible and irreversible calcium fluctuations, were identified in cells undergoing repeat reversible sonoporation and in neighboring cells undergoing reversible sonoporation. In addition to an increased proportion of reversible calcium fluctuations that occurred with repeated sonoporation compared with that in the initial sonoporation, repeated sonoporation resulted in significantly shorter calcium fluctuation durations and faster membrane resealing than that produced by initial sonoporation. Similarly, compared with those in sonoporated host cells, the intracellular calcium fluctuation recovery and membrane perforation resealing times were significantly shorter in sonoporated neighboring cells. These results demonstrated that the function recovery and membrane resealing capabilities after a second sonoporation or sonoporation of neighboring cells were potentiated in the short term. This could aid in sustaining the long-term viability of sonoporated cells, therefore improving delivery efficiency and biosafety. This investigation provides new insight into the resealing and recovery capabilities in re-sonoporation of sonoporated cells and sonoporation of neighboring cells and can help develop safe and efficient strategies for sonoporation-based drug delivery.

Photonic and magnetic materials for on-demand local drug delivery

Nanomedicine has been considered a promising tool for biomedical research and clinical practice in the 21st century because of the great impact nanomaterials could have on human health. The generation of new smart nanomaterials, which enable time- and space-controlled drug delivery, improve the limitations of conventional treatments, such as non-specific targeting, poor biodistribution and permeability. These smart nanomaterials can respond to internal biological stimuli (pH, enzyme expression and redox potential) and/or external stimuli (such as temperature, ultrasound, magnetic field and light) to further the precision of therapies. To this end, photonic and magnetic nanoparticles, such as gold, silver and iron oxide, have been used to increase sensitivity and responsiveness to external stimuli. In this review, we aim to report the main and most recent systems that involve photonic or magnetic nanomaterials for external stimulus-responsive drug release. The uniqueness of this review lies in highlighting the versatility of integrating these materials within different carriers. This leads to enhanced performance in terms of in vitro and in vivo efficacy, stability and toxicity. We also point out the current regulatory challenges for the translation of these systems from the bench to the bedside, as well as the yet unresolved matter regarding the standardization of these materials.

Opportunities and challenges in delivering biologics for Alzheimer's disease by low-intensity ultrasound

Low-intensity ultrasound combined with intravenously injected microbubbles (US^+MB) is a novel treatment modality for brain disorders, including Alzheimer's disease (AD), safely and transiently allowing therapeutic agents to overcome the blood-brain barrier (BBB) that constitutes a major barrier for therapeutic agents. Here, we first provide an update on immunotherapies in AD and how US^+MB has been applied to AD mouse models and in clinical trials, considering the ultrasound and microbubble parameter space. In the second half of the review, we compare different in vitro BBB models and discuss strategies for combining US^+MB with BBB modulators (targeting molecules such as claudin-5), and highlight the insight provided by super-resolution microscopy. Finally, we conclude with a short discussion on how in vitro findings can inform the design of animal studies, and how the insight gained may aid treatment optimization in the clinical ultrasound space.

Ultrasound-Targeted Microbubble Destruction: Modulation in the Tumor Microenvironment and Application in Tumor Immunotherapy

Tumor immunotherapy has shown strong therapeutic potential for stimulating or reconstructing the immune system to control and kill tumor cells. It is a promising and effective anti-cancer treatment besides surgery, radiotherapy and chemotherapy. Presently, some immunotherapy methods have been approved for clinical application, and numerous others have demonstrated promising in vitro results and have entered clinical trial stages. Although immunotherapy has exhibited encouraging results in various cancer types, however, a large proportion of patients are limited from these benefits due to specific characteristics of the tumor microenvironment such as hypoxia, tumor vascular malformation and immune escape, and current limitations of immunotherapy such as off-target toxicity, insufficient drug penetration and accumulation and immune cell dysfunction. Ultrasound-target microbubble destruction (UTMD) treatment can help reduce immunotherapy-related adverse events. Using the ultrasonic cavitation effect of microstreaming, microjets and free radicals, UTMD can cause a series of changes in vascular endothelial cells, such as enhancing endothelial cells' permeability, increasing intracellular calcium levels, regulating gene expression, and stimulating nitric oxide synthase activities. These effects have been shown to promote drug penetration, enhance blood perfusion, increase drug delivery and induce tumor cell death. UTMD, in combination with immunotherapy, has been used to treat melanoma, non-small cell lung cancer, bladder cancer, and ovarian cancer. In this review, we summarized the effects of UTMD on tumor angiogenesis and immune microenvironment, and discussed the application and progress of UTMD in tumor immunotherapy.

Internalization of targeted microbubbles by endothelial cells and drug delivery by pores and tunnels

Ultrasound insonification of microbubbles can locally enhance drug delivery by increasing the cell membrane permeability. To aid development of a safe and effective therapeutic microbubble, more insight into the microbubble-cell interaction is needed. In this in vitro study we aimed to investigate the initial 3D morphology of the endothelial cell membrane adjacent to individual microbubbles (n = 301), determine whether this morphology was affected upon binding and by the type of ligand on the microbubble, and study its influence on microbubble oscillation and the drug delivery outcome. High-resolution 3D confocal microscopy revealed that targeted microbubbles were internalized by endothelial cells, while this was not the case for non-targeted or IgG1-κ control microbubbles. The extent of internalization was ligand-dependent, since α_vβ₃-targeted microbubbles were significantly more internalized than CD31-targeted microbubbles. Ultra-high-speed imaging (~17 Mfps) in combination with high-resolution confocal microscopy (n = 246) showed that microbubble internalization resulted in a damped microbubble oscillation upon ultrasound insonification (2 MHz, 200 kPa peak negative pressure, 10 cycles). Despite damped oscillation, the cell's susceptibility to sonoporation (as indicated by PI uptake) was increased for internalized microbubbles. Monitoring cell membrane integrity (n = 230) showed the formation of either a pore, for intracellular delivery, or a tunnel (i.e. transcellular perforation), for transcellular delivery. Internalized microbubbles caused fewer transcellular perforations and smaller pore areas than non-internalized microbubbles. In conclusion, studying microbubble-mediated drug delivery using a state-of-the-art imaging system revealed receptor-mediated microbubble internalization and its effect on microbubble oscillation and resulting membrane perforation by pores and tunnels.

Spatiotemporal Dynamics and Mechanisms of Actin Cytoskeletal Re-modeling in Cells Perforated by Ultrasound-Driven Microbubbles

To develop new strategies for improving the efficacy and biosafety of sonoporation-based macromolecule delivery, it is essential to understand the mechanisms underlying plasma membrane re-sealing and function recovery of the cells perforated by ultrasound-driven microbubbles. However, we lack a clear understanding of the spatiotemporal dynamics of the disrupted actin cytoskeleton and its role in the re-sealing of sonoporated cells. Here we used a customized experimental setup for single-pulse ultrasound (133.33-µs duration and 0.70-MPa peak negative pressure) exposure to microbubbles and for real-time recording of single-cell (human umbilical vein endothelial cell) responses by laser confocal microscopic imaging. We found that in reversibly sonoporated cells, the locally disrupted actin cytoskeleton, which was spatially correlated with the perforated plasma membrane, underwent three successive phases (expansion; contraction and re-sealing; and recovery) to re-model and that each phase of the disrupted actin cytoskeleton was approximately synchronized with that of the perforated plasma membrane. Moreover, compared with the closing time of the perforated plasma membrane, the same time was used for the re-sealing of the actin cytoskeleton in mildly sonoporated cells and a longer time was required in moderately sonoporated cells. Further, the generation, directional migration, accumulation and re-polymerization of globular actin polymers during the three phases drove the re-modeling of the actin cytoskeleton. However, in irreversibly sonoporated cells, the actin cytoskeleton, which underwent expansion and no contraction, was progressively de-polymerized and could not be re-sealed. Finally, we found that intracellular calcium transients were essential for the recruitment of globular actin and the re-modeling of the actin cytoskeleton. These results provide new insight into the role of actin cytoskeleton dynamics in the re-sealing of sonoporated cells and serve to guide the design of new strategies for sonoporation-based delivery.

Spatiotemporal control of myofibroblast activation in acoustically-responsive scaffolds via ultrasound-induced matrix stiffening

Hydrogels are often used to study the impact of biomechanical and topographical cues on cell behavior. Conventional hydrogels are designed a priori, with characteristics that cannot be dynamically changed in an externally controlled, user-defined manner. We developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), that enables non-invasive, spatiotemporally controlled modulation of mechanical and morphological properties using focused ultrasound. An ARS consists of a phase-shift emulsion distributed in a fibrin matrix. Ultrasound non-thermally vaporizes the emulsion into bubbles, which induces localized, radial compaction and stiffening of the fibrin matrix. In this in vitro study, we investigate how this mechanism can control the differentiation of fibroblasts into myofibroblasts, a transition correlated with substrate stiffness on 2D substrates. Matrix compaction and stiffening was shown to be highly localized using confocal and atomic force microscopies, respectively. Myofibroblast phenotype, evaluated by α-smooth muscle actin (α-SMA) immunocytochemistry, significantly increased in matrix regions proximal to bubbles compared to distal regions, irrespective of the addition of exogenous transforming growth factor-β1 (TGF-β1). Introduction of the TGF-β1 receptor inhibitor SB431542 abrogated the proximal enhancement. This approach providing spatiotemporal control over biophysical signals and resulting cell behavior could aid in better understanding fibrotic disease progression and the development of therapeutic interventions for chronic wounds. STATEMENT OF SIGNIFICANCE: Hydrogels are used in cell culture to recapitulate both biochemical and biophysical aspects of the native extracellular matrix. Biophysical cues like stiffness can impact cell behavior. However, with conventional hydrogels, there is a limited ability to actively modulate stiffness after polymerization. We have developed an ultrasound-based method of spatiotemporally-controlling mechanical and morphological properties within a composite hydrogel, termed an acoustically-responsive scaffold (ARS). Upon exposure to ultrasound, bubbles are non-thermally generated within the fibrin matrix of an ARS, thereby locally compacting and stiffening the matrix. We demonstrate how ARSs control the differentiation of fibroblasts into myofibroblasts in 2D. This approach could assist with the study of fibrosis and the development of therapies for chronic wounds.

Antibacterial Sonodynamic Therapy: Current Status and Future Perspectives

Multidrug-resistant bacteria have emerged in both community and hospital settings, partly due to the misuse of antibiotics. The inventory of viable antibiotics is rapidly declining, and efforts toward discovering newer antibiotics are not yielding the desired outcomes. Therefore, alternate antibacterial therapies based on physical mechanisms such as light and ultrasound are being explored. Sonodynamic therapy (SDT) is an emerging therapeutic approach that involves exposing target tissues to a nontoxic sensitizing chemical and low-intensity ultrasound. SDT can enable site-specific cytotoxicity by producing reactive oxygen species (ROS) in response to ultrasound, which can be harnessed for treating bacterial infections. This approach can potentially be used for both superficial and deep-seated microbial infections. The majority of the sonosensitizers reported are nonpolar, exhibiting limited bioavailability and a high clearance rate in the body. Therefore, targeted delivery agents such as nanoparticle composites, liposomes, and microbubbles are being investigated. This article reviews recent developments in antibacterial sonodynamic therapy, emphasizing biophysical and chemical mechanisms, novel delivery agents, ultrasound exposure and image guidance strategies, and the challenges in the pathway to clinical translation.

DNA Double-Strand Breaks in Murine Mammary Tumor Cells Induced by Combined Treatment with Doxorubicin and Controlled Stable Cavitation

Chemotherapeutic agents such as doxorubicin induce cell cytotoxicity through induction of DNA double-strand breaks. Recent studies have reported the occurrence of DNA double-strand breaks in different cell lines exposed to cavitational ultrasound. As ultrasound stable cavitation can potentiate the therapeutic effects of cytotoxic drugs, we hypothesized that combined treatment with unseeded stable cavitation and doxorubicin would lead to increased DNA damage and would reduce cell viability and proliferation in vitro. In this study, we describe how we determined, using 4T1 murine mammary carcinoma as a model cell line, that unseeded stable cavitation combined with doxorubicin leads to additive DNA double-strand break induction. Combined treatment with doxorubicin and unseeded stable cavitation significantly reduced cell viability and proliferation at 72 h. A mechanistic study of the potential mechanisms of action of the combined treatment identified the presence of cavitation necessary to increase early DNA double-strand break induction, likely mediated by a bystander effect with release of extracellular calcium.

Ultrasound-Enabled Therapeutic Delivery and Regenerative Medicine: Physical and Biological Perspectives

The role of ultrasound in medicine and biological sciences is expanding rapidly beyond its use in conventional diagnostic imaging. Numerous studies have reported the effects of ultrasound on cellular and tissue physiology. Advances in instrumentation and electronics have enabled successful in vivo applications of therapeutic ultrasound. Despite path breaking advances in understanding the biophysical and biological mechanisms at both microscopic and macroscopic scales, there remain substantial gaps. With the progression of research in this area, it is important to take stock of the current understanding of the field and to highlight important areas for future work. We present herein key developments in the biological applications of ultrasound especially in the context of nanoparticle delivery, drug delivery, and regenerative medicine. We conclude with a brief perspective on the current promise, limitations, and future directions for interfacing ultrasound technology with biological systems, which could provide guidance for future investigations in this interdisciplinary area.

Phospholipid-coated targeted microbubbles for ultrasound molecular imaging and therapy

Phospholipid-coated microbubbles are ultrasound contrast agents that, when functionalized, adhere to specific biomarkers on cells. In this concise review, we highlight recent developments in strategies for targeting the microbubbles and their use for ultrasound molecular imaging (UMI) and therapy. Recently developed novel targeting strategies include magnetic functionalization, triple targeting, and the use of several new ligands. UMI is a powerful technique for studying disease progression, diagnostic imaging, and monitoring of therapeutic responses. Targeted microbubbles (tMBs) have been used for the treatment of cardiovascular diseases and cancer, with therapeutics either coadministered or loaded onto the tMBs. Regardless of which disease was treated, the use of tMBs always resulted in a better therapeutic outcome than non-tMBs when compared in vitro or in vivo.

Opening doors with ultrasound and microbubbles: Beating biological barriers to promote drug delivery

Apart from its clinical use in imaging, ultrasound has been thoroughly investigated as a tool to enhance drug delivery in a wide variety of applications. Therapeutic ultrasound, as such or combined with cavitating nuclei or microbubbles, has been explored to cross or permeabilize different biological barriers. This ability to access otherwise impermeable tissues in the body makes the combination of ultrasound and therapeutics very appealing to enhance drug delivery in situ. This review gives an overview of the most important biological barriers that can be tackled using ultrasound and aims to provide insight on how ultrasound has shown to improve accessibility as well as the biggest hurdles. In addition, we discuss the clinical applicability of therapeutic ultrasound with respect to the main challenges that must be addressed to enable the further progression of therapeutic ultrasound towards an effective, safe and easy-to-use treatment tailored for drug delivery in patients.

Sonoporation generates downstream cellular impact after membrane resealing

Sonoporation via microbubble-mediated ultrasound exposure has shown potential in drug and gene delivery. However, there is a general lack of mechanistic knowledge on sonoporation-induced cellular impact after membrane resealing, and this issue has made it challenging to apply sonoporation efficiently in practice. Here, we present new evidence on how sonoporation, without endangering immediate cell viability, may disrupt downstream cellular hemostasis in ways that are distinguished from the bioeffects observed in other sonicated and unsonoporated cells. Sonoporation was realized on HL-60 leukemia cells by delivering pulsed ultrasound (1 MHz frequency, 0.50 MPa peak negative pressure; 10% duty cycle; 30 s exposure period; 29.1 J/cm² acoustic energy density) in the presence of lipid-shelled microbubbles (1:1 cell-to-bubble ratio). Results showed that 54.6% of sonoporated cells, despite remaining initially viable, underwent apoptosis or necrosis at 24 h after sonoporation. Anti-proliferation behavior was also observed in sonoporated cells as their subpopulation size was reduced by 43.8% over 24 h. Preceding these cytotoxic events, the percentages of sonoporated cells in different cell cycle phases were found to be altered by 12 h after exposure. As well, for sonoporated cells, their expressions of cytoprotective genes in the heat shock protein-70 (HSP-70) family were upregulated by at least 4.1 fold at 3 h after exposure. Taken altogether, these findings indicate that sonoporated cells attempted to restore homeostasis after membrane resealing, but many of them ultimately failed to recover. Such mechanistic knowledge should be taken into account to devise more efficient sonoporation-mediated therapeutic protocols.

Plasma Membrane Blebbing Dynamics Involved in the Reversibly Perforated Cell by Ultrasound-Driven Microbubbles

The perforation of plasma membrane by ultrasound-driven microbubbles (i.e., sonoporation) provides a temporary window for transporting macromolecules into the cytoplasm that is promising with respect to drug delivery and gene therapy. To improve the efficacy of delivery while ensuring biosafety, membrane resealing and cell recovery are required to help sonoporated cells defy membrane injury and regain their normal function. Blebs are found to accompany the recovery of sonoporated cells. However, the spatiotemporal characteristics of blebs and the underlying mechanisms remain unclear. With a customized platform for ultrasound exposure and 2-D/3-D live single-cell imaging, localized membrane perforation was induced with ultrasound-driven microbubbles, and the cellular responses were monitored using multiple fluorescent probes. The results indicated that localized blebs undergoing four phases (nucleation, expansion, pausing and retraction) on a time scale of tens of seconds to minutes were specifically involved in the reversibly sonoporated cells. The blebs spatially correlated with the membrane perforation site and temporally lagged (about tens of seconds to minutes) the resealing of perforated membrane. Their diameter (about several microns) and lifetime (about tens of seconds to minutes) positively correlated with the degree of sonoporation. Further studies revealed that intracellular calcium transients might be an upstream signal for triggering blebbing nucleation; exocytotic lysosomes not only contributed to resealing of the perforated membrane, but also to the increasing bleb volume during expansion; and actin components accumulation facilitated bleb retraction. These results provide new insight into the short-term strategies that the sonoporated cell employs to recover on membrane perforation and to remodel membrane structure and a biophysical foundation for sonoporation-based therapy.

Cytosolic Ca2+ transients during pulsed focused ultrasound generate reactive oxygen species and cause DNA damage in tumor cells

Mechanical forces from non-ablative pulsed focused ultrasound (pFUS) generate pro-inflammatory tumor microenvironments (TME), marked by increased cytokines, chemokines, and trophic factors, as well as immune cell infiltration and reduced tumor growth. pFUS also causes DNA damage within tumors, which is a potent activator of immunity and could contribute to changes in the TME. This study investigated mechanisms behind the mechanotransductive effects of pFUS causing DNA damage in several tumor cell types. Methods: 4T1 (murine breast tumor), B16 (murine melanoma), C6 (rat glioma), or MDA-MB-231 (human breast tumor) cells were sonicated in vitro (1.1MHz; 6MPa PNP; 10ms pulses; 10% duty cycle; 300 pulses). DNA damage was detected by TUNEL, apoptosis was measured by immunocytochemistry for cleaved caspase-3. Calcium, superoxide, and H₂O₂ were detected by fluorescent indicators and modulated by BAPTA-AM, mtTEMPOL, or Trolox, respectively. Results: pFUS increased TUNEL reactivity (range = 1.6-2.7-fold) in all cell types except C6 and did not induce apoptosis in any cell line. All lines displayed cytosolic Ca²⁺ transients during sonication. pFUS increased superoxide (range = 1.6-2.0-fold) and H₂O₂ (range = 2.3-2.8-fold) in all cell types except C6. BAPTA-AM blocked increased TUNEL reactivity, superoxide and H₂O₂ formation, while Trolox also blocked increased TUNEL reactivity increased after pFUS. mtTEMPOL allowed H₂O₂ formation and did not block increased TUNEL reactivity after pFUS. Unsonicated C6 cells had higher baseline concentrations of cytosolic Ca²⁺, superoxide, and H₂O₂, which were not associated with greater baseline TUNEL reactivity than the other cell lines. Conclusions: Mechanotransduction of pFUS directly induces DNA damage in tumor cells by cytosolic Ca²⁺ transients causing formation of superoxide and subsequently, H₂O₂. These results further suggest potential clinical utility for pFUS. However, the lack of pFUS-induced DNA damage in C6 cells demonstrates a range of potential tumor responses that may arise from physiological differences such as Ca²⁺ or redox homeostasis.

FRET-Based Ca2+ Biosensor Single Cell Imaging Interrogated by High-Frequency Ultrasound

Fluorescence resonance energy transfer (FRET)-based biosensors have advanced live cell imaging by dynamically visualizing molecular events with high temporal resolution. FRET-based biosensors with spectrally distinct fluorophore pairs provide clear contrast between cells during dual FRET live cell imaging. Here, we have developed a new FRET-based Ca²⁺ biosensor using EGFP and FusionRed fluorophores (FRET-GFPRed). Using different filter settings, the developed biosensor can be differentiated from a typical FRET-based Ca²⁺ biosensor with ECFP and YPet (YC3.6 FRET Ca²⁺ biosensor, FRET-CFPYPet). A high-frequency ultrasound (HFU) with a carrier frequency of 150 MHz can target a subcellular region due to its tight focus smaller than 10 µm. Therefore, HFU offers a new single cell stimulations approach for FRET live cell imaging with precise spatial resolution and repeated stimulation for longitudinal studies. Furthermore, the single cell level intracellular delivery of a desired FRET-based biosensor into target cells using HFU enables us to perform dual FRET imaging of a cell pair. We show that a cell pair is defined by sequential intracellular delivery of the developed FRET-GFPRed and FRET-CFPYPet into two target cells using HFU. We demonstrate that a FRET-GFPRed exhibits consistent 10–15% FRET response under typical ionomycin stimulation as well as under a new stimulation strategy with HFU.