Ultrasonic Microbubble Cavitation Enhanced Tissue Permeability and Drug Diffusion in Solid Tumor Therapy

External stimuli-responsive drug delivery to the posterior segment of the eye

Posterior segment eye diseases represent the leading causes of vision impairment and blindness globally. Current therapies still have notable drawbacks, including the need for frequent invasive injections and the associated risks of severe ocular complications. Recently, the utility of external stimuli, such as light, ultrasound, magnetic field, and electric field, has been noted as a promising strategy to enhance drug delivery to the posterior segment of the eye. In this review, we briefly summarize the main physiological barriers against ocular drug delivery, focusing primarily on the recent advancements that utilize external stimuli to improve treatment outcomes for posterior segment eye diseases. The advantages of these external stimuli-responsive drug delivery strategies are discussed, with illustrative examples highlighting improved tissue penetration, enhanced control over drug release, and targeted drug delivery to ocular lesions through minimally invasive routes. Finally, we discuss the challenges and future perspectives in the translational research of external stimuli-responsive drug delivery platforms, aiming to bridge existing gaps toward clinical use.

Ultrasound and Microbubble Mediated Delivery of Virus-Sensitizing Drugs Improves In Vitro Oncolytic Virotherapy Against Breast Cancer Cells

OBJECTIVE

Oncolytic virotherapy is an emerging form of cancer treatment that uses replication-competent viruses to kill cancer cells. However, as for other cancer therapies, oncolytic viruses are not effective against all cancers and there is a need to further improve treatment efficacy while maintaining low toxicity. Viral sensitizers are drugs that potentiate viral replication in tumor cells. While various studies have shown their synergy with oncolytic virotherapy, the risks associated with systemic toxicities that vary according to the drug used limit the clinical translation of the approach. In this study, we used an ultrasound and image-guided approach in which we loaded viral-sensitizing drugs onto microbubbles which are then cavitated by ultrasound to deliver the encapsulated drugs to tumor cells, which improves in vitro oncolytic virotherapy efficacy in the 4T1 breast cancer model.

METHODS

In this study, we loaded two viral sensitizers, paclitaxel and volasertib, onto lipid microbubbles and comprehensively characterized their effect on the infection of 4T1 murine mammary carcinoma cells by oncolytic Vesicular stomatitis virus in vitro.

RESULTS

We synthesized lipid microbubbles with high and moderate encapsulation efficiency for paclitaxel (83.7%) and volasertib (28.6%), respectively. Stability assessments demonstrated excellent retention in various conditions, highlighting their potential for in vivo use. In vitro studies confirmed their acoustic responsiveness essential for controlled drug release at targeted sites. Paclitaxel and volasertib release from viral sensitizer-loaded microbubbles following ultrasound-triggered cavitation significantly increased viral replication (57-fold, p < 0.0001 and 27-fold, p < 0.01, respectively), as well as tumor cell killing compared to virus-infected untreated cells.

CONCLUSION

Altogether, our data show that drug-loaded microbubble cavitation and free drugs both sensitize cancer cells to oncolytic viruses to equivalent levels. These findings provide a proof of concept for the use of ultrasound-guided microbubble drug delivery in combination with oncolytic virotherapy.

Advances in Ultrasound-Targeted Microbubble Destruction (UTMD) for Breast Cancer Therapy

Breast cancer is one of the most common types of cancer in women worldwide and is a leading cause of cancer deaths among women. As a result, various treatments have been developed to combat this disease. Breast cancer treatment varies based on its stage and type of pathology. Among the therapeutic options, ultrasound has been employed to assist in the treatment of breast cancer, including radiation therapy, chemotherapy, targeted immunotherapy, hormonal therapy, and, more recently, radiofrequency ablation for early-stage and inoperable patients. One notable advancement is ultrasound-targeted microbubble destruction (UTMD), which is gradually becoming a highly effective and non-invasive anti-tumor modality. This technique can enhance chemical, genetic, immune, and anti-vascular therapies through its physical and biological effects. Specifically, UTMD improves drug transfer efficiency and destroys tumor neovascularization while reducing toxic side effects on the body during tumor treatment. Given these developments, the application of ultrasound-assisted therapy to breast cancer has gained significant attention from research scholars. In this review, we will discuss the development of various therapeutic modalities for breast cancer and, importantly, highlight the application of ultrasound microbubble-targeted disruption techniques in breast cancer treatment.

Low-intensity focused ultrasound combined with microbubbles for non-invasive downregulation of rabbit carotid body activity in the treatment of hypertension

Targeting the carotid body (CB) is a new approach in treating hypertension. This study investigates the efficacy and safety of ultrasound combined with microbubbles in targeting CB to treat hypertension. Twenty-seven hypertensive rabbits were randomly assigned to three groups: microbubbles only (sham group, n = 11), ultrasound plus microbubbles (LIFU group, n = 11), and bilateral carotid sinus nerve denervation (CSND group, n = 5). Four weeks post-intervention, blood pressure, hypoxic ventilatory response (HVR), blood pressure variability (BPV), heart rate variability (HRV), biochemical indicators, neurohormones, and histopathology were assessed in all groups. The results indicated significant reductions in systolic and diastolic blood pressure in the LIFU and CSND groups post-intervention, along with decreases in BPV, HRV, and catecholamines. HVR results showed a 35.10% reduction in CB activity in the LIFU group compared to the sham group, which was significantly lower than the reduction in the CSND group compared to the sham group (73.85%). Histopathology and transmission electron microscopy confirmed CB damage and cell apoptosis, with immunofluorescence showing a reduction in type I and II cells. In conclusion, LIFU combined with microbubbles can reduce blood pressure by lowering CB and sympathetic nerve activity.

Enhancing the therapeutic efficacy of gefitinib on subcutaneously transplanted SKOV3 ovarian cancer tumors in nude mice via ultrasound‑stimulated microbubble cavitation

The present study aimed to explore the effect of ultrasound-stimulated microbubble cavitation (USMC) on drug concentration and therapeutic efficacy of oral gefitinib in treating subcutaneously transplanted SKOV3 ovarian cancer tumors in nude mice. The present study employed the VINNO70 ultrasonic diagnostic and treatment integrated machine for USMC therapy. Firstly, the mechanical index was set at 0.25, and the therapeutic efficacy of USMC treatment was assessed at intervals of 5, 10 and 20 min. Briefly, 72 nude mice were randomized into the following four groups (n=18/group): Control group, USMC5 min group, USMC10 min group and USMC20 min group, and the therapeutic response to USMC treatment was evaluated by comparing pre-and post-intervention effects. Additionally, the combined therapeutic efficacy of USMC and gefitinib was investigated by randomly dividing 96 tumor-bearing mice into the following four groups (n=24/group): Control group, USMC group, gefitinib group and USMC + gefitinib group. Contrast-enhanced ultrasound, hematoxylin and eosin staining, western blotting, immunofluorescence staining, TUNEL staining, ELISA and liquid chromatography-mass spectrometry were performed in the present study. The results showed that USMC combined with gefitinib had the best treatment effect; the tumor inhibition rate was higher than that of gefitinib alone and the overall survival time was prolonged. In addition, the drug concentration in the tumor tissue obtained from the USMC + gefitinib group was revealed to be ~1.4 times higher than that detected in the group treated with gefitinib alone. The experimental results also confirmed that the strongest tumor inhibition rate and longest overall survival time was observed in the USMC + gefitinib group, followed by the gefitinib group and USMC group. STAT3 is an important signaling transducer and transcription factor, which, when phosphorylated, can lead to abnormal cell proliferation and malignant transformation. In addition, the upregulation of phosphorylated (p)-STAT3 is consider a reason for the poor efficacy of gefitinib in treating ovarian cancer. The present study revealed that ultrasound microbubble therapy could overcome this side effect. In conclusion, USMC improved the effects of oral gefitinib on subcutaneously transplanted SKOV3 ovarian cancer tumors in nude mice and increased drug penetration. In addition, USMC overcame the gefitinib-induced side effect of upregulated STAT3 phosphorylation and reduced the expression levels of p-STAT3 in the tumor.

Engineered microbubbles decorated with red emitting carbon nanoparticles for efficient delivery and imaging

Altering the route of uptake by the cells is an attractive strategy to overcome drug-receptor adaptation problems. Carbon nanoparticles (CNPs) with emission beyond tissue autofluorescence for imaging biological tissues were used to study the phenomenon of uptake by the cells. In this regard, red-emitting carbon nanoparticles (CNPs) were synthesized and incorporated onto lipid microbubbles (MBs). The CNPs showed red emissions in the range of 640 nm upon excitation with 480 nm wavelength of light. Atomic force microscopic and confocal microscopic images showed the successful loading of CNPs onto the MB. Carbon nanoparticle loaded microbubbles (CNP-MBs) were treated with NIH 3 T3 cells at different concentrations. Confocal microscopic imaging studies confirm the presence of CNPs inside the treated cells. Cytotoxicity studies revealed that the CNPs showed minimal toxicity towards cells after loading onto MBs. The CNPs are usually taken up by the cells through the clathrin-mediated (CME) pathway, but when loaded onto MBs, the mechanism of uptake of CNPs is altered, and the uptake by the cells was observed even in the presence of inhibitors for the CME pathway. Loading CNPs onto MBs resulted in the uptake of CNPs by the cell through micropinocytosis and sonophoresis in the presence of ultrasound. The in vivo uptake CNP-MBs were performed in Danio rerio (Zebrafish larvae). This study provides insights into altering the uptake pathway through reformulation by loading nanoparticles onto MBs.

Ultrasound imaging guided targeted sonodynamic therapy enhanced by magnetophoretically controlled magnetic microbubbles

Sonodynamic therapy (SDT) utilizes ultrasonic excitation of a sensitizer to generate reactive oxygen species (ROS) to destroy tumor. Two dimensional (2D) black phosphorus (BP) is an emerging sonosensitizer that can promote ROS production to be used in SDT but it alone lacks active targeting effect and showed low therapy efficiency. In this study, a stable dispersion of integrated micro-nanoplatform consisting of BP nanosheets loaded and Fe3O4 nanoparticles (NPs) connected microbubbles was introduced for ultrasound imaging guided and magnetic field directed precision SDT of breast cancer. The targeted ultrasound imaging at 18 MHz and efficient SDT effects at 1 MHz were demonstrated both in-vitro and in-vivo on the breast cancer. The magnetic microbubbles targeted deliver BP nanosheets to the tumor site under magnetic navigation and increased the uptake of BP nanosheets by inducing cavitation effect for increased cell membrane permeability via ultrasound targeted microbubble destruction (UTMD). The mechanism of SDT by magnetic black phosphorus microbubbles was proposed to be originated from the ROS triggered mitochondria mediated apoptosis by up-regulating the pro-apoptotic proteins while down-regulating the anti-apoptotic proteins. In conclusion, the ultrasound theranostic was realized via the magnetic black phosphorus microbubbles, which could realize targeting and catalytic sonodynamic therapy.

Nonlinear effects of dual-frequency focused ultrasound on the on-demand regulation of acoustic droplet vaporization

Dual-frequency ultrasound has been widely employed to enhance and regulate acoustic droplet vaporization (ADV) but the role of ultrasonic nonlinear effects on it remains unclear. The main objective of this study is to investigate the influence of nonlinear effects on the control of ADV nucleation under different dual-frequency focused ultrasound conditions. ADV nucleation of PFC nanodroplets activated by nonlinear dual-frequency ultrasound was modeled and parametric studies were conducted to investigate the influence of dual-frequency ultrasound frequency and acoustic power on the degree of nonlinearity (DoN), nucleation rates and dimensions of the nucleation region in a wide parameter range. The results showed that the ultrasonic nonlinearity caused a significant decrease in peak negative pressure due to waveform distortion, which leads to a lower nucleation rate in the nonlinear model compared to that in the linear model. Furthermore, the distributions of nucleation regions were also affected by the interaction between waves of different frequencies and cloud-like spatial distributions were produced, which could be modulated by the dual-frequency ultrasound parameters and have great potentials in the spatial regulation of the ADV and customized treatment protocols in clinical applications. In addition, represented by 1.5 MHz + 3 MHz, such a dual-frequency combination of fundamental and second harmonic could effectively enhance ultrasonic nonlinear effects with relatively lower peak negative pressure and higher DoN. Therefore, nonlinear effect of the dual-frequency ultrasound plays an important role in the ADV regulation, which should be considered in the numerical model and practical applications.

Ultrasound combined with microbubble mediated immunotherapy for tumor microenvironment

The tumor microenvironment (TME) plays an important role in dynamically regulating the progress of cancer and influencing the therapeutic results. Targeting the tumor microenvironment is a promising cancer treatment method in recent years. The importance of tumor immune microenvironment regulation by ultrasound combined with microbubbles is now widely recognized. Ultrasound and microbubbles work together to induce antigen release of tumor cell through mechanical or thermal effects, promoting antigen presentation and T cells' recognition and killing of tumor cells, and improve tumor immunosuppression microenvironment, which will be a breakthrough in improving traditional treatment problems such as immune checkpoint blocking (ICB) and himeric antigen receptor (CAR)-T cell therapy. In order to improve the therapeutic effect and immune regulation of TME targeted tumor therapy, it is necessary to develop and optimize the application system of microbubble ultrasound for organs or diseases. Therefore, the combination of ultrasound and microbubbles in the field of TME will continue to focus on developing more effective strategies to regulate the immunosuppression mechanisms, so as to activate anti-tumor immunity and/or improve the efficacy of immune-targeted drugs, At present, the potential value of ultrasound combined with microbubbles in TME targeted therapy tumor microenvironment targeted therapy has great potential, which has been confirmed in the experimental research and application of breast cancer, colon cancer, pancreatic cancer and prostate cancer, which provides a new alternative idea for clinical tumor treatment. This article reviews the research progress of ultrasound combined with microbubbles in the treatment of tumors and their application in the tumor microenvironment.

Effectiveness of Ultrasonic and Manual Instrumentation in Nonsurgical Periodontal Therapy: Are Additional Therapies More Effective? A Systematic Review

Nonsurgical periodontal therapy aims to remove supragingival and subgingival biofilm to restore periodontal health. This systematic review aims to assess the clinical effectiveness of manual and/or ultrasonic instruments and to determine whether other therapies can improve periodontal clinical outcomes. Case-control, cross-sectional and cohort studies and clinical trials of patients undergoing nonsurgical periodontal therapy with ultrasonic and/or manual instruments (and any adjunctive therapies, such as glycine, erythritol, ozone, laser and glycine) from 2013 to 2023 were analyzed using Pub-med/MEDLINE, Scopus and Google Scholar. To assess the risk of bias in this review, blinding, randomization, allocation concealment, outcome data and outcome recording were assessed. No differences between treatments were found; all methods, including manual and ultrasound, were helpful in improving clinical parameters (primary outcome). Although the results were mixed, adjunctive therapies seemed to be helpful in the treatment of periodontal disease. The results of this systematic review are consistent with the previous scientific literature and have shown that both manual and ultrasonic instruments are effective in nonsurgical periodontal therapy. This review could not show how complementary therapies could further improve nonsurgical periodontal therapy. For future research, it would be good to standardize the sample with regard to the degree and stage of periodontal disease and to evaluate the risks and benefits of the instruments (manual and ultrasonic scalers).

Nanotherapeutic approaches for delivery of long non-coding RNAs: an updated review with emphasis on cancer

The long noncoding RNAs (lncRNAs) comprise a wide range of RNA species whose length exceeds 200 nucleotides, which regulate the expression of genes and cellular functions in a wide range of organisms. Several diseases, including malignancy, have been associated with lncRNA dysregulation. Due to their functions in cancer development and progression, lncRNAs have emerged as promising biomarkers and therapeutic targets in cancer diagnosis and treatment. Several studies have investigated the anti-cancer properties of lncRNAs; however, only a few lncRNAs have been found to exhibit tumor suppressor properties. Furthermore, their length and poor stability make them difficult to synthesize. Thus, to overcome the instability of lncRNAs, poor specificity, and their off-target effects, researchers have constructed nanocarriers that encapsulate lncRNAs. Recently, translational medicine research has focused on delivering lncRNAs into tumor cells, including cancer cells, through nano-drug delivery systems in vivo. The developed nanocarriers can protect, target, and release lncRNAs under controlled conditions without appreciable adverse effects. To deliver lncRNAs to cancer cells, various nanocarriers, such as exosomes, microbubbles, polymer nanoparticles, 1,2-dioleyl-3-trimethylammoniumpropane chloride nanocarriers, and virus-like particles, have been successfully developed. Despite this, every nanocarrier has its own advantages and disadvantages when it comes to delivering nucleic acids effectively and safely. This article examines the current status of nanocarriers for lncRNA delivery in cancer therapy, focusing on their potential to enhance cancer treatment.

Ultrasound-mediated multifunctional magnetic microbubbles for drug delivery of celastrol in VX2 liver transplant tumors

Celastrol (CST) has positive pharmacological effects on various cancers, but clinical application is limited because of poor water solubility and systemic toxicity. Ferric oxide (Fe3O4) has a large specific surface area and can be functionalized by inorganic modification to form complex magnetic drug delivery systems. Herein, Fe3O4 was surface-modified with citric acid and polyethylene glycol (PEG) (via) the Mitsunobu reaction and then covalently bound to CST. Finally, magnetic microbubbles (MMBs) containing perfluoropropane (C3F8) and Fe3O4-PEG2K-CST particles were constructed with poly(lactic-co-glycolic acid) (PLGA) as the shell membrane. In vitro studies showed that ultrasound-mediated MMBs exhibited improved inhibition of VX2 cell proliferation compared to inhibition achieved using MMBs without ultrasound mediation, blank MMBs, or free CST. In ultrasound mode, MMBs have favorable imaging properties. After the application of a high mechanical index, MMBs collapse through the cavitation effect, releasing their internal Fe3O4-PEG2K-CST. The CST is then delivered to the tumor microenvironment under acidic conditions. In magnetic resonance imaging T2 mode, a specific hypointense signal was observed in the tumor area compared with that before treatment, whereas no significant change occurred in the signal intensity of the surrounding organs. After treatment, pathological examination of tumor-bearing rabbit tissues showed that iron elements accumulated in several apoptosis cells in the tumor area, with no apparent abnormalities found in other areas. Thus, ultrasound-mediated MMBs could significantly improve the drug uptake of solid tumors and inhibit tumor growth with favorable biological safety.

Ultrasound robotics for precision therapy

In recent years, the application of microrobots in precision therapy has gained significant attention. The small size and maneuverability of these micromachines enable them to potentially access regions that are difficult to reach using traditional methods; thus, reducing off-target toxicities and maximizing treatment effectiveness. Specifically, acoustic actuation has emerged as a promising method to exert control. By harnessing the power of acoustic energy, these small machines potentially navigate the body, assemble at the desired sites, and deliver therapies with enhanced precision and effectiveness. Amidst the enthusiasm surrounding these miniature agents, their translation to clinical environments has proven difficult. The primary objectives of this review are threefold: firstly, to offer an overview of the fundamental acoustic principles employed in the field of microrobots; secondly, to assess their current applications in medical therapies, encompassing tissue targeting, drug delivery or even cell infiltration; and lastly, to delve into the continuous efforts aimed at integrating acoustic microrobots into in vivo applications.

Image-guided focused ultrasound-mediated molecular delivery to breast cancer in an animal model

Tumors become inoperable due to their size or location, making neoadjuvant chemotherapy the primary treatment. However, target tissue accumulation of anticancer agents is limited by the physical barriers of the tumor microenvironment. Low-intensity focused ultrasound (FUS) in combination with microbubble (MB) contrast agents can increase microvascular permeability and improve drug delivery to the target tissue after systemic administration. The goal of this research was to investigate image-guided FUS-mediated molecular delivery in volume space. Three-dimensional (3-D) FUS therapy functionality was implemented on a programmable ultrasound scanner (Vantage 256, Verasonics Inc.) equipped with a linear array for image guidance and a 128-element therapy transducer (HIFUPlex-06, Sonic Concepts). FUS treatment was performed on breast cancer-bearing female mice (N= 25). Animals were randomly divided into three groups, namely, 3-D FUS therapy, two-dimensional (2-D) FUS therapy, or sham (control) therapy. Immediately prior to the application of FUS therapy, animals received a slow bolus injection of MBs (Definity, Lantheus Medical Imaging Inc.) and near-infrared dye (IR-780, surrogate drug) for optical reporting and quantification of molecular delivery. Dye accumulation was monitored viain vivooptical imaging at 0, 1, 24, and 48 h (Pearl Trilogy, LI-COR). Following the 48 h time point, animals were humanely euthanized and tumors excised forex vivoanalyzes. Optical imaging results revealed that 3-D FUS therapy improved delivery of the IR-780 dye by 66.4% and 168.1% at 48 h compared to 2-D FUS (p= 0.18) and sham (p= 0.047) therapeutic strategies, respectively.Ex vivoanalysis revealed similar trends. Overall, 3-D FUS therapy can improve accumulation of a surrogate drug throughout the entire target tumor burden after systemic administration.

Targeted Microbubbles for Drug, Gene, and Cell Delivery in Therapy and Immunotherapy

Microbubbles are 1-10 μm diameter gas-filled acoustically-active particles, typically stabilized by a phospholipid monolayer shell. Microbubbles can be engineered through bioconjugation of a ligand, drug and/or cell. Since their inception a few decades ago, several targeted microbubble (tMB) formulations have been developed as ultrasound imaging probes and ultrasound-responsive carriers to promote the local delivery and uptake of a wide variety of drugs, genes, and cells in different therapeutic applications. The aim of this review is to summarize the state-of-the-art of current tMB formulations and their ultrasound-targeted delivery applications. We provide an overview of different carriers used to increase drug loading capacity and different targeting strategies that can be used to enhance local delivery, potentiate therapeutic efficacy, and minimize side effects. Additionally, future directions are proposed to improve the tMB performance in diagnostic and therapeutic applications.

On the application of hydrodynamic cavitation on a chip in cellular injury and drug delivery

Hydrodynamic cavitation (HC) is a phase change phenomenon, where energy release in a fluid occurs upon the collapse of bubbles, which form due to the low local pressures. During recent years, due to advances in lab-on-a-chip technologies, HC-on-a-chip (HCOC) and its potential applications have attracted considerable interest. Microfluidic devices enable the performance of controlled experiments by enabling spatial control over the cavitation process and by precisely monitoring its evolution. In this study, we propose the adjunctive use of HC to induce distinct zones of cellular injury and enhance the anticancer efficacy of Doxorubicin (DOX). HC caused different regions (lysis, necrosis, permeabilization, and unaffected regions) upon exposure of different cancer and normal cells to HC. Moreover, HC was also applied to the confluent cell monolayer following the DOX treatment. Here, it was shown that the combination of DOX and HC exhibited a more pronounced anticancer activity on cancer cells than DOX alone. The effect of HC on cell permeabilization was also proven by using carbon dots (CDs). Finally, the cell stiffness parameter, which was associated with cell proliferation, migration and metastasis, was investigated with the use of cancer cells and normal cells under HC exposure. The HCOC offers the advantage of creating well-defined zones of bio-responses upon HC exposure simultaneously within minutes, achieving cell lysis and molecular delivery through permeabilization by providing spatial control. In conclusion, micro scale hydrodynamic cavitation proposes a promising alternative to be used to increase the therapeutic efficacy of anticancer drugs.

The role of acoustofluidics and microbubble dynamics for therapeutic applications and drug delivery

Targeted drug delivery is proposed to reduce the toxic effects of conventional therapeutic methods. For that purpose, nanoparticles are loaded with drugs called nanocarriers and directed toward a specific site. However, biological barriers challenge the nanocarriers to convey the drug to the target site effectively. Different targeting strategies and nanoparticle designs are used to overcome these barriers. Ultrasound is a new, safe, and non-invasive drug targeting method, especially when combined with microbubbles. Microbubbles oscillate under the effect of the ultrasound, which increases the permeability of endothelium, hence, the drug uptake to the target site. Consequently, this new technique reduces the dose of the drug and avoids its side effects. This review aims to describe the biological barriers and the targeting types with the critical features of acoustically driven microbubbles focusing on biomedical applications. The theoretical part covers the historical developments in microbubble models for different conditions: microbubbles in an incompressible and compressible medium and bubbles encapsulated by a shell. The current state and the possible future directions are discussed.

Microbubble Delivery Platform for Ultrasound-Mediated Therapy in Brain Cancers

The blood-brain barrier (BBB) is one of the most selective endothelial barriers that protect the brain and maintains homeostasis in neural microenvironments. This barrier restricts the passage of molecules into the brain, except for gaseous or extremely small hydrophobic molecules. Thus, the BBB hinders the delivery of drugs with large molecular weights for the treatment of brain cancers. Various methods have been used to deliver drugs to the brain by circumventing the BBB; however, they have limitations such as drug diversity and low delivery efficiency. To overcome this challenge, microbubbles (MBs)-based drug delivery systems have garnered a lot of interest in recent years. MBs are widely used as contrast agents and are recently being researched as a vehicle for delivering drugs, proteins, and gene complexes. The MBs are 1-10 μm in size and consist of a gas core and an organic shell, which cause physical changes, such as bubble expansion, contraction, vibration, and collapse, in response to ultrasound. The physical changes in the MBs and the resulting energy lead to biological changes in the BBB and cause the drug to penetrate it, thus enhancing the therapeutic effect. Particularly, this review describes a state-of-the-art strategy for fabricating MB-based delivery platforms and their use with ultrasound in brain cancer therapy.

Microbubble–Nanoparticle Complexes for Ultrasound-Enhanced Cargo Delivery

Targeted delivery of therapeutics to specific tissues is critically important for reducing systemic toxicity and optimizing therapeutic efficacy, especially in the case of cytotoxic drugs. Many strategies currently exist for targeting systemically administered drugs, and ultrasound-controlled targeting is a rapidly advancing strategy for externally-stimulated drug delivery. In this non-invasive method, ultrasound waves penetrate through tissue and stimulate gas-filled microbubbles, resulting in bubble rupture and biophysical effects that power delivery of attached cargo to surrounding cells. Drug delivery capabilities from ultrasound-sensitive microbubbles are greatly expanded when nanocarrier particles are attached to the bubble surface, and cargo loading is determined by the physicochemical properties of the nanoparticles. This review serves to highlight and discuss current microbubble–nanoparticle complex component materials and designs for ultrasound-mediated drug delivery. Nanocarriers that have been complexed with microbubbles for drug delivery include lipid-based, polymeric, lipid–polymer hybrid, protein, and inorganic nanoparticles. Several schemes exist for linking nanoparticles to microbubbles for efficient nanoparticle delivery, including biotin–avidin bridging, electrostatic bonding, and covalent linkages. When compared to unstimulated delivery, ultrasound-mediated cargo delivery enables enhanced cell uptake and accumulation of cargo in target organs and can result in improved therapeutic outcomes. These ultrasound-responsive delivery complexes can also be designed to facilitate other methods of targeting, including bioactive targeting ligands and responsivity to light or magnetic fields, and multi-level targeting can enhance therapeutic efficacy. Microbubble–nanoparticle complexes present a versatile platform for controlled drug delivery via ultrasound, allowing for enhanced tissue penetration and minimally invasive therapy. Future perspectives for application of this platform are also discussed in this review.