Self-assembled liposome-loaded microbubbles: The missing link for safe and efficient ultrasound triggered drug-delivery

Enhanced Antitumor Efficacy and Reduced Cardiotoxicity of Ultrasound-Mediated Doxorubicin Delivery by Microbubble-Liposome Complexes

OBJECTIVE

Doxorubicin (Dox) is standard of care for treatment of sarcomas, but cumulative dosing is often limited by cardiotoxicity. We hypothesized that ultrasound targeted microbubble (MB) cavitation (UTMC) of a liposomal doxorubicin (LDox) conjugated polymer microbubble complex (DoxLPX) would enhance tumor inhibition and limit Dox cardiotoxicity.

METHODS

DoxLPX was intravenously injected in MCA205 sarcoma-bearing mice and concurrent ultrasound was delivered to the tumor site (DoxLPX + UTMC). Other mice received equivalent dosages of free Dox, LDox, or LDox + MB co-administration with UTMC (LDox + MB + UTMC). Tumor size and cardiac function were serially imaged with ultrasound. Postmortem cardiac tissue was analyzed for apoptosis. Biodistribution of Dox was performed with bioluminescence imaging postmortem where Cy5.5 was used as a fluorescent Dox analog.

RESULTS

DoxLPX + UTMC showed increased drug concentration in the tumor, a significant slowdown in tumor growth and prolonged median survival time. LDox and DoxLPX formulations had reduced drug extravasation into the myocardium. LDox + MB + UTMC also demonstrated superior tumor growth inhibition compared to free Dox and LDox. Three weeks after treatment commenced, DoxLPX + UTMC group showed significantly better left ventricular function indices than the free Dox group, consistent with biodistribution findings. Concordantly, heart tissue showed normal architecture of cardiac myocytes and significantly less interstitial/perivascular fibrosis in the DoxLPX + UTMC group.

CONCLUSIONS

DoxLPX formulation in conjunction with ultrasound provides a targeted drug delivery platform with superior anti-tumor efficacy and reduced cardiac toxicity compared with systemic administration of free Dox.

Comprehensive Review on Bubbles: Synthesis, Modification, Characterization and Biomedical Applications

Accurate detection, treatment, and imaging of diseases are important for effective treatment outcomes in patients. In this regard, bubbles have gained much attention, due to their versatility. Bubbles usually 1 nm to 10 μm in size can be produced and loaded with a variety of lipids, polymers, proteins, and therapeutic and imaging agents. This review details the different production and loading methods for bubbles, for imaging and treatment of diseases/conditions such as cancer, tumor angiogenesis, thrombosis, and inflammation. Bubbles can also be used for perfusion measurements, important for diagnostic and therapeutic decision making in cardiac disease. The different factors important in the stability of bubbles and the different techniques for characterizing their physical and chemical properties are explained, for developing bubbles with advanced therapeutic and imaging features. Hence, the review provides important insights for researchers studying bubbles for biomedical applications.

Drug Release via Ultrasound-Activated Nanocarriers for Cancer Treatment: A Review

Conventional cancer chemotherapy often struggles with safely and effectively delivering anticancer therapeutics to target tissues, frequently leading to dose-limiting toxicity and suboptimal therapeutic outcomes. This has created a need for novel therapies that offer greater efficacy, enhanced safety, and improved toxicological profiles. Nanocarriers are nanosized particles specifically designed to enhance the selectivity and effectiveness of chemotherapy drugs while reducing their toxicity. A subset of drug delivery systems utilizes stimuli-responsive nanocarriers, which enable on-demand drug release, prevent premature release, and offer spatial and temporal control over drug delivery. These stimuli can be internal (such as pH and enzymes) or external (such as ultrasound, magnetic fields, and light). This review focuses on the mechanics of ultrasound-induced drug delivery and the various nanocarriers used in conjunction with ultrasound. It will also provide a comprehensive overview of key aspects related to ultrasound-induced drug delivery, including ultrasound parameters and the biological effects of ultrasound waves.

Microbubbles bound to drug-eluting beads enable ultrasound imaging and enhanced delivery of therapeutics

Transarterial chemoembolization (TACE) is an image-guided minimally invasive treatment for liver cancer which involves delivery of chemotherapy and embolic material into tumor-supplying arteries to block blood flow to a liver tumor and to deliver chemotherapy directly to the tumor. However, the released drug diffuses only less than a millimeter away from the beads. To enhance the efficacy of TACE, the development of microbubbles electrostatically bound to the surface of drug-eluting beads loaded with different amounts of doxorubicin (0-37.5 mg of Dox/mL of beads) is reported. Up to 400 microbubbles were bound to Dox-loaded beads (70-150 microns). This facilitated ultrasound imaging of the beads and increased the release rate of Dox upon exposure to high intensity focused ultrasound (HIFU). Furthermore, ultrasound exposure (1 MPa peak negative pressure) increased the distance at which Dox could be detected from beads embedded in a tissue-mimicking phantom, compared with a no ultrasound control.

Advances in nanobubbles for cancer theranostics: Delivery, imaging and therapy

Maximizing treatment efficacy and forecasting patient prognosis in cancer necessitates the strategic use of targeted therapy, coupled with the prompt precise detection of malignant tumors. Theutilizationof gaseous systems as an adaptable platform for creating nanobubbles (NBs) has garnered significant attention as theranostics, which involve combining contrast chemicals typically used for imaging with pharmaceuticals to diagnose and treattumorssynergistically in apersonalizedmanner for each patient. This review specifically examines the utilization of oxygen NBsplatforms as a theranostic weapon in the field of oncology. We thoroughly examine the key factors that impact the effectiveness of NBs preparations and the consequences of these treatment methods. This review extensively examines recent advancements in composition schemes, advanced developments in pre-clinical phases, and other groundbreaking inventions in the area of NBs. Moreover, this review offers a thorough examination of the optimistic future possibilities, addressing prospective methods for improvement and incorporation into widely accepted therapeutic practices. As we explore the ever-changing field of cancer theranostics, the incorporation of oxygen NBs appears as a promising development, providing new opportunities for precision medicine and marking a revolutionary age in cancer research and therapy.

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.

The Evolution and Recent Trends in Acoustic Targeting of Encapsulated Drugs to Solid Tumors: Strategies beyond Sonoporation

Despite recent advancements in ultrasound-mediated drug delivery and the remarkable success observed in pre-clinical studies, no delivery platform utilizing ultrasound contrast agents has yet received FDA approval. The sonoporation effect was a game-changing discovery with a promising future in clinical settings. Various clinical trials are underway to assess sonoporation's efficacy in treating solid tumors; however, there are disagreements on its applicability to the broader population due to long-term safety issues. In this review, we first discuss how acoustic targeting of drugs gained importance in cancer pharmaceutics. Then, we discuss ultrasound-targeting strategies that have been less explored yet hold a promising future. We aim to shed light on recent innovations in ultrasound-based drug delivery including newer designs of ultrasound-sensitive particles specifically tailored for pharmaceutical usage.

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.

Ultrasound-mediated nano drug delivery for treating cancer: Fundamental physics to future directions

The application of biocompatible nanocarriers in medicine has provided several benefits over conventional treatment methods. However, achieving high treatment efficacy and deep penetration of nanocarriers in tumor tissue is still challenging. To address this, stimuli-responsive nano-sized drug delivery systems (DDSs) are an active area of investigation in delivering anticancer drugs. While ultrasound is mainly used for diagnostic purposes, it can also be applied to affect cellular function and the delivery/release of anticancer drugs. Therapeutic ultrasound (TUS) has shown potential as both a stand-alone anticancer treatment and a method to induce targeted drug release from nanocarrier systems. TUS approaches have been used to overcome various physiological obstacles, including endothelial barriers, the tumor microenvironment (TME), and immunological hurdles. Combining nanomedicine and ultrasound as a smart DDS can increase in situ drug delivery and improve access to impermeable tissues. Furthermore, smart DDSs can perform targeted drug release in response to distinctive TMEs, external triggers, or dual/multi-stimulus. This results in enhanced treatment efficacy and reduced damage to surrounding healthy tissue or organs at risk. Integrating DDSs and ultrasound is still in its early stages. More research and clinical trials are required to fully understand ultrasound's underlying physical mechanisms and interactions with various types of nanocarriers and different types of cells and tissues. In the present review, ultrasound-mediated nano-sized DDS, specifically focused on cancer treatment, is presented and discussed. Ultrasound interaction with nanoparticles (NPs), drug release mechanisms, and various types of ultrasound-sensitive NPs are examined. Additionally, in vitro, in vivo, and clinical applications of TUS are reviewed in light of the critical challenges that need to be considered to advance TUS toward an efficient, secure, straightforward, and accessible cancer treatment. This study also presents effective TUS parameters and safety considerations for this treatment modality and gives recommendations about system design and operation. Finally, future perspectives are considered, and different TUS approaches are examined and discussed in detail. This review investigates drug release and delivery through ultrasound-mediated nano-sized cancer treatment, both pre-clinically and clinically.

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.

Microbubble-Assisted Ultrasound for Imaging and Therapy of Melanoma Skin Cancer: A Systematic Review

Recent technological developments in ultrasound (US) imaging and ultrasound contrast agents (UCAs) have improved diagnostic confidence in echography. In the clinical management of melanoma, contrast-enhanced ultrasound (CEUS) imaging complements conventional US imaging (i.e., high-resolution US and Doppler imaging) for clinical examination and therapeutic follow-up. These developments have set into motion the combined use of ultrasound and UCAs as a new modality for drug delivery. This modality, called sonoporation, has emerged as a non-invasive, targeted and safe method for the delivery of therapeutic drugs into melanoma. This review focuses on the results and prospects of using US and UCAs as dual modalities for CEUS imaging and melanoma treatment.

Nanomedicine Penetration to Tumor: Challenges, and Advanced Strategies to Tackle This Issue

Nanomedicine has been under investigation for several years to improve the efficiency of chemotherapeutics, having minimal pharmacological effects clinically. Ineffective tumor penetration is mediated by tumor environments, including limited vascular system, rising cancer cells, higher interstitial pressure, and extra-cellular matrix, among other things. Thus far, numerous methods to increase nanomedicine access to tumors have been described, including the manipulation of tumor micro-environments and the improvement of nanomedicine characteristics; however, such outdated approaches still have shortcomings. Multi-functional convertible nanocarriers have recently been developed as an innovative nanomedicine generation with excellent tumor infiltration abilities, such as tumor-penetrating peptide-mediated transcellular transport. The developments and limitations of nanomedicines, as well as expectations for better outcomes of tumor penetration, are discussed in this review.

Evaluation of Liposome-Loaded Microbubbles as a Theranostic Tool in a Murine Collagen-Induced Arthritis Model

Rheumatoid arthritis (RA) is an autoimmune disease characterized by severe inflammation of the synovial tissue. Here, we assess the feasibility of liposome-loaded microbubbles as theranostic agents in a murine arthritis model. First, contrast-enhanced ultrasound (CEUS) was used to quantify neovascularization in this model since CEUS is well-established for RA diagnosis in humans. Next, the potential of liposome-loaded microbubbles and ultrasound (US) to selectively enhance liposome delivery to the synovium was evaluated with in vivo fluorescence imaging. This procedure is made very challenging by the presence of hard joints and by the limited lifetime of the microbubbles. The inflamed knee joints were exposed to therapeutic US after intravenous injection of liposome-loaded microbubbles. Loaded microbubbles were found to be quickly captured by the liver. This resulted in fast clearance of attached liposomes while free and long-circulating liposomes were able to accumulate over time in the inflamed joints. Our observations show that murine arthritis models are not well-suited for evaluating the potential of microbubble-mediated drug delivery in joints given: (i) restricted microbubble passage in murine synovial vasculature and (ii) limited control over the exact ultrasound conditions in situ given the much shorter length scale of the murine joints as compared to the therapeutic wavelength.

Ultrasound-Mediated Cancer Therapeutics Delivery using Micelles and Liposomes: A Review

BACKGROUND

Existing cancer treatment methods have many undesirable side effects that greatly reduce the quality of life of cancer patients.

OBJECTIVE

This review will focus on the use of ultrasound-responsive liposomes and polymeric micelles in cancer therapy.

METHODS

This review presents a survey of the literature regarding ultrasound-triggered micelles and liposomes using articles recently published in various journals, as well as some new patents in this field.

RESULTS

Nanoparticles have proven promising as cancer theranostic tools. Nanoparticles are selective in nature, have reduced toxicity, and controllable drug release patterns making them ideal carriers for anticancer drugs. Numerous nanocarriers have been designed to combat malignancies, including liposomes, micelles, dendrimers, solid nanoparticles, quantum dots, gold nanoparticles, and, more recently, metal-organic frameworks. The temporal and spatial release of therapeutic agents from these nanostructures can be controlled using internal and external triggers, including pH, enzymes, redox, temperature, magnetic and electromagnetic waves, and ultrasound. Ultrasound is an attractive modality because it is non-invasive, can be focused on the diseased site, and has a synergistic effect with anticancer drugs.

CONCLUSION

The functionalization of micellar and liposomal surfaces with targeting moieties and the use of ultrasound as a triggering mechanism can help improve the selectivity and enable the spatiotemporal control of drug release from nanocarriers.

Ultrasound-Triggered Delivery of Iproplatin from Microbubble-Conjugated Liposomes

The PtIV prodrug iproplatin has been actively loaded into liposomes using a calcium acetate gradient, achieving a 3-fold enhancement in drug concentration compared to passive loading strategies. A strain-promoted cycloaddition reaction (azide- dibenzocyclooctyne) was used to attach iproplatin-loaded liposomes L(Pt) to gas-filled microbubbles (M), forming an ultrasound-responsive drug delivery vehicle [M-L(Pt)]. Ultrasound-triggered release of iproplatin from the microbubble-liposome construct was evaluated in cellulo. Breast cancer (MCF-7) cells treated with both free iproplatin and iproplatin-loaded liposome-microbubbles [M-L(Pt)] demonstrated an increase in platinum concentration when exposed to ultrasound. No appreciable platinum uptake was observed in MCF-7 cells following treatment with L(Pt) only or L(Pt)+ultrasound, suggesting that microbubble-mediated ultrasonic release of platinum-based drugs from liposomal carriers enables greater control over drug delivery.

Sub-millimetre precision of drug delivery in the brain from ultrasound-triggered nanodroplets

Drug-loaded nanoscale cavitation agents, called nanodroplets, are an attractive solution to enhance and localize drug delivery, offering increased stability and prolonged half-life in circulation compared to microbubbles. However, the spatial precision with which drug can be released and delivered into brain tissue from such agents has not been directly mapped. Decafluorobutane lipid-shell droplets (206 +/- 6 nm) were loaded with a fluorescent blood-brain barrier (BBB)-penetrating dye (Nile Blue) and vaporized with ultrasound (1.66 MHz, 10 ms pulse length, 1 Hz pulse repetition frequency), generating transient echogenic microbubbles and delivering the encapsulated dye. The distribution and intensity of released fluorophore was mapped in a tissue-mimicking phantom, and in the brain of rats (Sprague Dawley, N = 4, n = 16). The release and distribution of dye was found to be pressure-dependent (0.2-3.5 MPa) and to occur only above the vaporization threshold of the nanodroplets (1.5 +/- 0.25 MPa in vitro, 2.4 +/- 0.05 MPa in vivo). Dye delivery was achieved with sub-millimetre spatial precision, covering an area of 0.4 to 1.5 mm in diameter, determined by the sonication pressure. The distribution and intensity of dye released at depth in the brain followed the axial pressure profile of the ultrasound beam. Nile Blue (354 Da, LogP 2.7) was compared to Nile Red (318 Da, LogP 3.8) and Quantum Dots (CdSe/ZnS, 5 nm diameter) to visualize the role of molecule size and lipophilicity in crossing the intact BBB following triggered release. Acoustic emissions were shown to predict the successful delivery of the BBB-penetrating dye and the extent of the distribution, demonstrating the theranostic capabilities of nanoscale droplets to precisely localize drug delivery in the brain.

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.

Nanomedicine against Alzheimer's and Parkinson's Disease

Alzheimer's and Parkinson's are the two most rampant neurodegenerative disorders worldwide. Existing treatments have a limited effect on the pathophysiology but are unable to fully arrest the progression of the disease. This is due to the inability of these therapeutic molecules to efficiently cross the blood-brain barrier. We discuss how nanotechnology has enabled researchers to develop novel and efficient nano-therapeutics against these diseases. The development of nanotized drug delivery systems has permitted an efficient, site-targeted, and controlled release of drugs in the brain, thereby presenting a revolutionary therapeutic approach. Nanoparticles are also being thoroughly studied and exploited for their role in the efficient and precise diagnosis of neurodegenerative conditions. We summarize the role of different nano-carriers and RNAi-conjugated nanoparticle-based therapeutics for their efficacy in pre-clinical studies. We also discuss the challenges underlying the use of nanomedicine with a focus on their route of administration, concentration, metabolism, and any toxic effects for successful therapeutics in these diseases.

The promising shadow of microbubble over medical sciences: from fighting wide scope of prevalence disease to cancer eradication

Microbubbles are typically 0.5-10 μm in size. Their size tends to make it easier for medication delivery mechanisms to navigate the body by allowing them to be swallowed more easily. The gas included in the microbubble is surrounded by a membrane that may consist of biocompatible biopolymers, polymers, surfactants, proteins, lipids, or a combination thereof. One of the most effective implementation techniques for tiny bubbles is to apply them as a drug carrier that has the potential to activate ultrasound (US); this allows the drug to be released by US. Microbubbles are often designed to preserve and secure medicines or substances before they have reached a certain area of concern and, finally, US is used to disintegrate microbubbles, triggering site-specific leakage/release of biologically active drugs. They have excellent therapeutic potential in a wide range of common diseases. In this article, we discussed microbubbles and their advantageous medicinal uses in the treatment of certain prevalent disorders, including Parkinson's disease, Alzheimer's disease, cardiovascular disease, diabetic condition, renal defects, and finally, their use in the treatment of various forms of cancer as well as their incorporation with nanoparticles. Using microbubble technology as a novel carrier, the ability to prevent and eradicate prevalent diseases has strengthened the promise of effective care to improve patient well-being and life expectancy.

Evaluation of Loading Strategies to Improve Tumor Uptake of Gemcitabine in a Murine Orthotopic Bladder Cancer Model Using Ultrasound and Microbubbles

In this study we compared three different microbubble-based approaches to the delivery of a widely used chemotherapy drug, gemcitabine: (i) co-administration of gemcitabine and microbubbles (Gem+MB); (ii) conjugates of microbubbles and gemcitabine-loaded liposomes (GemlipoMB); and (iii) microbubbles with gemcitabine directly bound to their surfaces (GembioMB). Both in vitro and in vivo investigations were carried out, respectively, in the RT112 bladder cancer cell line and in a murine orthotopic muscle-invasive bladder cancer model. The in vitro (in vivo) ultrasound exposure conditions were a 1 (1.1) MHz centre frequency, 0.07 (1.0) MPa peak negative pressure, 3000 (20,000) cycles and 100 (0.5) Hz pulse repetition frequency. Ultrasound exposure produced no significant increase in drug uptake either in vitro or in vivo compared with the drug-only control for co-administered gemcitabine and microbubbles. In vivo, GemlipoMB prolonged the plasma circulation time of gemcitabine, but only GembioMB produced a statistically significant increase in cleaved caspase 3 expression in the tumor, indicative of gemcitabine-induced apoptosis.

Ultrasound-Responsive Nanocarriers in Cancer Treatment: A Review

The safe and effective delivery of anticancer agents to diseased tissues is one of the significant challenges in cancer therapy. Conventional anticancer agents are generally cytotoxins with poor pharmacokinetics and bioavailability. Nanocarriers are nanosized particles designed for the selectivity of anticancer drugs and gene transport to tumors. They are small enough to extravasate into solid tumors, where they slowly release their therapeutic load by passive leakage or biodegradation. Using smart nanocarriers, the rate of release of the entrapped therapeutic(s) can be increased, and greater exposure of the tumor cells to the therapeutics can be achieved when the nanocarriers are exposed to certain internally (enzymes, pH, and temperature) or externally (light, magnetic field, and ultrasound) applied stimuli that trigger the release of their load in a safe and controlled manner, spatially and temporally. This review gives a comprehensive overview of recent research findings on the different types of stimuli-responsive nanocarriers and their application in cancer treatment with a particular focus on ultrasound.

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.

Recent advances in ultrasound-triggered drug delivery through lipid-based nanomaterials

The high prescribed dose of anticancer drugs and their resulting adverse effects on healthy tissue are significant drawbacks to conventional chemotherapy (CTP). Ideally, drugs should have the lowest possible degree of interaction with healthy cells, which would diminish any adverse effects. Therefore, an ideal scenario to bring about improvements in CTP is the use of technological strategies to ensure the efficient, specific, and selective transport and/or release of drugs to the target site. One practical and feasible solution to promote the efficiency of conventional CTP is the use of ultrasound (US). In this review, we highlight the potential role of US in combination with lipid-based carriers to achieve a targeted CTP strategy in engineered smart drug delivery systems.

Non-invasive molecularly-specific millimeter-resolution manipulation of brain circuits by ultrasound-mediated aggregation and uncaging of drug carriers

Non-invasive, molecularly-specific, focal modulation of brain circuits with low off-target effects can lead to breakthroughs in treatments of brain disorders. We systemically inject engineered ultrasound-controllable drug carriers and subsequently apply a novel two-component Aggregation and Uncaging Focused Ultrasound Sequence (AU-FUS) at the desired targets inside the brain. The first sequence aggregates drug carriers with millimeter-precision by orders of magnitude. The second sequence uncages the carrier's cargo locally to achieve high target specificity without compromising the blood-brain barrier (BBB). Upon release from the carriers, drugs locally cross the intact BBB. We show circuit-specific manipulation of sensory signaling in motor cortex in rats by locally concentrating and releasing a GABA_A receptor agonist from ultrasound-controlled carriers. Our approach uses orders of magnitude (1300x) less drug than is otherwise required by systemic injection and requires very low ultrasound pressures (20-fold below FDA safety limits for diagnostic imaging). We show that the BBB remains intact using passive cavitation detection (PCD), MRI-contrast agents and, importantly, also by sensitive fluorescent dye extravasation and immunohistochemistry.

Ultrasound-triggered therapeutic microbubbles enhance the efficacy of cytotoxic drugs by increasing circulation and tumor drug accumulation and limiting bioavailability and toxicity in normal tissues

Most cancer patients receive chemotherapy at some stage of their treatment which makes improving the efficacy of cytotoxic drugs an ongoing and important goal. Despite large numbers of potent anti-cancer agents being developed, a major obstacle to clinical translation remains the inability to deliver therapeutic doses to a tumor without causing intolerable side effects. To address this problem, there has been intense interest in nanoformulations and targeted delivery to improve cancer outcomes. The aim of this work was to demonstrate how vascular endothelial growth factor receptor 2 (VEGFR2)-targeted, ultrasound-triggered delivery with therapeutic microbubbles (thMBs) could improve the therapeutic range of cytotoxic drugs. Methods: Using a microfluidic microbubble production platform, we generated thMBs comprising VEGFR2-targeted microbubbles with attached liposomal payloads for localised ultrasound-triggered delivery of irinotecan and SN38 in mouse models of colorectal cancer. Intravenous injection into tumor-bearing mice was used to examine targeting efficiency and tumor pharmacodynamics. High-frequency ultrasound and bioluminescent imaging were used to visualise microbubbles in real-time. Tandem mass spectrometry (LC-MS/MS) was used to quantitate intratumoral drug delivery and tissue biodistribution. Finally, 89Zr PET radiotracing was used to compare biodistribution and tumor accumulation of ultrasound-triggered SN38 thMBs with VEGFR2-targeted SN38 liposomes alone. Results: ThMBs specifically bound VEGFR2 in vitro and significantly improved tumor responses to low dose irinotecan and SN38 in human colorectal cancer xenografts. An ultrasound trigger was essential to achieve the selective effects of thMBs as without it, thMBs failed to extend intratumoral drug delivery or demonstrate enhanced tumor responses. Sensitive LC-MS/MS quantification of drugs and their metabolites demonstrated that thMBs extended drug exposure in tumors but limited exposure in healthy tissues, not exposed to ultrasound, by persistent encapsulation of drug prior to elimination. 89Zr PET radiotracing showed that the percentage injected dose in tumors achieved with thMBs was twice that of VEGFR2-targeted SN38 liposomes alone. Conclusions: thMBs provide a generic platform for the targeted, ultrasound-triggered delivery of cytotoxic drugs by enhancing tumor responses to low dose drug delivery via combined effects on circulation, tumor drug accumulation and exposure and altered metabolism in normal tissues.

Ultrasound-Responsive Carriers for Therapeutic Applications

Ultrasound (US)-responsive carriers have emerged as promising theranostic candidates because of their ability to enhance US-contrast, promote image-guided drug delivery, cause on-demand pulsatile release of drugs in response to ultrasound stimuli, as well as to enhance the permeability of physiological barriers such as the stratum corneum, the vascular endothelium, and the blood-brain barrier (BBB). US-responsive carriers include microbubbles MBs, liposomes, droplets, hydrogels, and nanobubble-nanoparticle complexes and have been explored for cavitation-mediated US-responsive drug delivery. Recently, a transient increase in the permeability of the BBB by microbubble (MB)-assisted low-frequency US has shown promise in enhancing the delivery of therapeutic agents in the case of neurological disorders. Further, the periodic mechanical stimulus generated by US-responsive MBs have also been explored in tissue engineering and has directly influenced the differentiation of mesenchymal stem cells into cartilage. This Review discusses the various types of US-responsive carriers and explores their emerging roles in therapeutics ranging from drug delivery to tissue engineering.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Magnetic Targeting and Ultrasound Activation of Liposome-Microbubble Conjugate for Enhanced Delivery of Anticancer Therapies

Effective delivery of chemotherapeutics with minimal toxicity and maximal outcome is clinically important but technically challenging. Here, we synthesize a complex of doxorubicin (DOX)-loaded magneto-liposome (DOX-ML) microbubbles (DOX-ML-MBs) for magnetically responsive and ultrasonically sensitive delivery of anticancer therapies with enhanced efficiency. Citrate-stabilized iron oxide nanoparticles (MNs) of 6.8 ± 1.36 nm were synthesized, loaded with DOX in the core of oligolamellar vesicles of 172 ± 9.2 nm, and covalently conjugated with perfluorocarbon (PFC)-gas-loaded microbubbles to form DOX-ML-MBs of ∼4 μm. DOX-ML-MBs exhibited significant magnetism and were able to release chemotherapeutics and DOX-MLs instantly upon exposure to ultrasound (US) pulses. In vitro studies showed that DOX-ML-MBs in the presence of US pulses promoted apoptosis and were highly effective in killing both BxPc-3 and Panc02 pancreatic cancer cells even at a low dose. Significant reduction in the tumor volume was observed after intravenous administration of DOX-ML-MBs in comparison to the control group in a pancreatic cancer xenograft model of nude mice. Deeply penetrated iron oxide nanoparticles throughout the magnetically targeted tumor tissues in the presence of US stimulation were clearly observed. Our study demonstrated the potential of using DOX-ML-MBs for site-specific targeting and controlled drug release. It opens a new avenue for the treatment of pancreatic cancer and other tissue malignancies where precise delivery of therapeutics is necessary.

Indium-111 labelling of liposomal HEGF for radionuclide delivery via ultrasound-induced cavitation

The purpose of this exploratory study was to investigate the combination of a radiopharmaceutical, nanoparticles and ultrasound (US) enhanced delivery to develop a clinically viable therapeutic strategy for tumours overexpressing the epidermal growth factor receptor (EGFR). Molecularly targeted radionuclides have great potential for cancer therapy but are sometimes associated with insufficient delivery resulting in sub-cytotoxic amounts of radioactivity being delivered to the tumour. Liposome formulations are currently used in the clinic to reduce the side effects and improve the pharmacokinetic profile of chemotherapeutic drugs. However, in contrast to non-radioactive agents, loading and release of radiotherapeutics from liposomes can be challenging in the clinical setting. US-activated cavitation agents such as microbubbles (MBs) have been used to release therapeutics from liposomes to enhance the distribution/delivery in a target area. In an effort to harness the benefits of these techniques, the development of a liposome loaded radiopharmaceutical construct for enhanced delivery via acoustic cavitation was studied. The liposomal formulation was loaded with peptide, human epidermal growth factor (HEGF), coupled to a chelator for subsequent radiolabelling with ¹¹¹Indium ([¹¹¹In]In³⁺), in a manner designed to be compatible with preparation in a radiopharmacy. Liposomes were efficiently radiolabelled (57%) within 1 h, with release of ~12% of the radiopeptide following a 20 s exposure to US-mediated cavitation in vitro. In clonogenic studies this level of release resulted in cytotoxicity specifically in cells over-expressing the epidermal growth factor receptor (EGFR), with over 99% reduction in colony survival compared to controls. The formulation extended the circulation time and changed the biodistribution compared to the non-liposomal radiopeptide in vivo, although interestingly the biodistribution did not resemble that of liposome constructs currently used in the clinic. Cavitation of MBs co-injected with liposomes into tumours expressing high levels of EGFR resulted in a 2-fold enhancement in tumour uptake within 20 min. However, owing to the poor vascularisation of the tumour model used the same level of uptake was achieved without US after 24 h. By combining acoustic-cavitation-sensitive liposomes with radiopharmaceuticals this research represents a new concept in achieving targeted delivery of radiopharmaceuticals.

Ultrasound-Activated Nanomaterials for Therapeutics

Ultrasound has attracted much attention in recent years as an external stimulus capable of activating different types of nanomaterials for therapeutic application. One of the characteristics that makes ultrasound an especially appealing triggering stimulus for nanomedicine is its capacity to be non-invasively applied in a focused manner at deep regions of the body. Combining ultrasound with nanoparticles, different biological effects can be achieved. In this work, an overview of the four main types of inducible responses will be provided: inducing drug release, producing ultrasound-derived biological effects, modifying nanoparticle biodistribution and developing theranostic agents. Several examples of each one of these applications are presented here to illustrate the key concepts underlying recent developments in the discipline.

Lipid-based microbubbles and ultrasound for therapeutic application

Microbubbles with diagnostic ultrasound have had a long history of use in the medical field. In recent years, the therapeutic application of the combination of microbubbles and ultrasound, called sonoporation, has received increased attention as microbubble oscillation or collapse close to various barriers in the body was recognized to potentially open those barriers, increasing drug transport across them. In this review, we aimed to describe the development of lipid-stabilized microbubbles equipped with functions, such as long circulation and drug loading, and the therapeutic application of sonoporation for tumor-targeted therapy, brain-targeted therapy, and immunotherapy. We also attempted to discuss the current status of the field and potential future developments.

Phospholipid prodrug conjugates of insoluble chemotherapeutic agents for ultrasound targeted drug delivery

The hydrophobicity and high potency of many therapeutic agents makes them difficult to use effectively in clinical practice. This work focuses on conjugating phospholipid tails (2T) onto podophyllotoxin (P) and its analogue (N) using a linker and characterizing the effects of their incorporation into lipid-based drug delivery vehicles for triggered ultrasound delivery. Differential Scanning Calorimetry results show that successfully synthesized lipophilic prodrugs, 2T-P (~28 % yield) and 2T-N(~26 % yield), incorporate within the lipid membranes of liposomes. As a result of this, increased stability and incorporation are observed in 2T-P and 2T-N in comparison to the parent compounds P and N. Molecular dynamic simulation results support that prodrugs remain within the lipid membrane over a relevant range of concentrations. 2T-N's (IC₅₀: 20 nM) biological activity was retained in HeLa cells (cervical cancer), whereas 2T-P's (IC₅₀: ~4 µM) suffered, presumably due to steric hindrance. Proof-of-concept studies using ultrasound in vitro microbubble and nanodroplet delivery vehicles establish that these prodrugs are capable of localized drug delivery. This study provides useful information about the synthesis of double tail analogues of insoluble chemotherapeutic agents to facilitate incorporation into drug delivery vehicles. The phospholipid attachment strategy presented here could be applied to other well suited drugs such as gemcitabine, commonly known for its treatment of pancreatic cancer.

Sonoprinting liposomes on tumor spheroids by microbubbles and ultrasound

Ultrasound-triggered drug-loaded microbubbles have great potential for drug delivery due to their ability to locally release drugs and simultaneously enhance their delivery into the target tissue. We have recently shown that upon applying ultrasound, nanoparticle-loaded microbubbles can deposit nanoparticles onto cells grown in 2D monolayers, through a process that we termed "sonoprinting". However, the rigid surfaces on which cell monolayers are typically growing might be a source of acoustic reflections and aspherical microbubble oscillations, which can influence microbubble-cell interactions. In the present study, we aim to reveal whether sonoprinting can also occur in more complex and physiologically relevant tissues, by using free-floating 3D tumor spheroids as a tissue model. We show that both monospheroids (consisting of tumor cells alone) and cospheroids (consisting of tumor cells and fibroblasts, which produce an extracellular matrix) can be sonoprinted. Using doxorubicin-liposome-loaded microbubbles, we show that sonoprinting allows to deposit large amounts of doxorubicin-containing liposomes to the outer cell layers of the spheroids, followed by doxorubicin release into the deeper layers of the spheroids, resulting in a significant reduction in cell viability. Sonoprinting may become an attractive approach to deposit drug patches at the surface of tissues, thereby promoting the delivery of drugs into target tissues.

A Responsive Mesoporous Silica Nanoparticle Platform for Magnetic Resonance Imaging-Guided High-Intensity Focused Ultrasound-Stimulated Cargo Delivery with Controllable Location, Time, and Dose

Magnetic resonance imaging (MRI) is an essential modality for clinical diagnosis, and MRI-guided high-intensity focused ultrasound (MRgHIFU) is a powerful technology for targeted therapy. Clinical applications of MRgHIFU primarily utilize hyperthermia and ablation to treat cancerous tissue, but for drug delivery applications thermal damage is undesirable. A biofriendly MRgHIFU-responsive mesoporous silica nanoparticle (MSN) platform that is stimulated within a physiological safe temperature range has been developed, reducing the possibility of thermal damage to the surrounding healthy tissues. Biocompatible polyethylene glycol (PEG) was employed to cap the pores of MSNs, and the release of cargo molecules by HIFU occurs without substantial temperature increase (∼4 °C). To visualize by MRI and measure the stimulated delivery in situ, a U.S. Food and Drug Administration (FDA)-approved gadolinium-based contrast agent, gadopentetate dimeglumine (Gd(DTPA)^2-), was used as the imageable cargo. Taking advantage of the three-dimensional (3-D) imaging and targeting capabilities of MRgHIFU, the release of Gd(DTPA)^2- stimulated by HIFU was pinpointed at the HIFU focal point in 3-D space in a tissue-mimicking gel phantom. The amount of Gd(DTPA)^2- released was controlled by HIFU stimulation times and power levels. A positive correlation between the amount of Gd(DTPA)^2- released and T₁ was found. The MRgHIFU-stimulated cargo release was further imaged in a sample of ex vivo animal tissue. With this technology, the biodistribution of the nanocarriers can be tracked and the MRgHIFU-stimulated cargo release can be pinpointed, opening up an opportunity for future image-guided theranostic applications.

Improving accessibility of EPR-insensitive tumor phenotypes using EPR-adaptive strategies: Designing a new perspective in nanomedicine delivery

The enhanced permeability and retention (EPR) effect has underlain the predominant nanomedicine design philosophy for the past three decades. However, growing evidence suggests that it is over-represented in preclinical models, and agents designed solely using its principle of passive accumulation can only be applied to a narrow subset of clinical tumors. For this reason, strategies that can improve upon the EPR effect to facilitate nanomedicine delivery to otherwise non-responsive tumors are required for broad clinical translation. EPR-adaptive nanomedicine delivery comprises a class of chemical and physical techniques that modify tumor accessibility in an effort to increase agent delivery and therapeutic effect. In the present review, we overview the primary benefits and limitations of radiation, ultrasound, hyperthermia, and photodynamic therapy as physical strategies for EPR-adaptive delivery to EPR-insensitive tumor phenotypes, and we reflect upon changes in the preclinical research pathway that should be implemented in order to optimally validate and develop these delivery strategies.

Drug-Loaded Microbubbles Combined with Ultrasound for Thrombolysis and Malignant Tumor Therapy

Cardiac-cerebral thrombosis and malignant tumor endanger the safety of human life seriously. Traditional chemotherapy drugs have side effects which restrict their applications. Drug-loaded microbubbles can be destroyed by ultrasound irradiation at the focus position and be used for thrombolysis and tumor therapy. Compared with traditional drug treatment, the drug-loaded microbubbles can be excited by ultrasound and release drugs to lesion sites, increasing the local drug concentration and the exposure dose to nonfocal regions, thus reducing the cytotoxicity and side effects of drugs. This article reviews the applications of drug-loaded microbubbles combined with ultrasound for thrombolysis and tumor therapy. We focus on highlighting the advantages of using this new technique for disease treatment and concluding with recommendations for future efforts on the applications of this technology.

Enhancing Tumor Drug Distribution With Ultrasound-Triggered Nanobubbles

Issues with limited intratumoral drug penetration and heterogeneous drug distribution continue to impede the therapeutic efficacy of nanomedicine-based delivery systems. Ultrasound (US)-enhanced drug delivery has emerged as one effective means of overcoming these challenges. Acoustic cavitation in the presence of nanoparticles has shown to increase the cellular uptake and distribution of chemotherapeutic agents in vivo. In this study, we investigated the potential of a drug-loaded echogenic nanoscale bubbles in combination with low frequency (3 MHz), high energy (2 W/cm2) US for antitumor therapy. The doxorubicin-loaded nanobubbles (Dox-NBs) stabilized with an interpenetrating polymer mesh were 171.5 ± 20.9 nm in diameter. When used in combination with therapeutic US, Dox-NBs combined with free drug showed significantly higher (*p < 0.05) intracellular uptake and therapeutic efficacy compared with free drug. When injected intravenously in vivo, Dox-NBs + therapeutic US showed significantly higher (*p < 0.05) accumulation and better distribution of Dox in tumors when compared with free drug. This strategy provides an effective and simple method to increase the local dose and distribution of otherwise systemically toxic chemotherapeutic agents for cancer therapies.

Ultrasound-Triggered Spatiotemporal Delivery of Topotecan and Curcumin as Combination Therapy for Cancer

Chemotherapy is limited by the low availability of drug at the tumor site and drug resistance by the tumor. In this report we describe a combination therapy for codelivery of two anticancer drugs with spatiotemporal control by ultrasound pulses. We developed curcumin and topotecan-coencapsulated nanoconjugates Cur_Tpt_NC.MB to have an ultrasound contrast property. MDA MB 231 and B16F10 cells were incubated with Cur_Tpt_NC.MB and exposed to ultrasound. Ultrasound exposure reduced the IC50 concentration of topotecan and curcumin significantly (P < 0.05) compared with free drug. Antitumor efficacy study of the Cur_Tpt_NC.MB in B16F10 melanoma tumor-bearing C57BL/6 mice showed that ultrasound exposure of right tumor reduced growth by 3.5 times compared with the unexposed left tumor of same mice and 14.8 times compared with a group treated with a physical mixture of drugs. These results suggest that the nanotherapeutic system we developed induces site-specific inhibition of tumor growth at a high rate and has the potential to be used as a therapeutic regimen for spatiotemporal delivery of dual drugs for treatment of cancer.

Formulation and Characterization of Chemically Cross-linked Microbubble Clusters

The purpose of this study is to introduce a new concept of chemically cross-linked microbubble clusters (CCMCs), which are individual microbubble ultrasound contrast agents (UCAs) physically tethered together. We demonstrate a facile means of their production, characterize their size and stability, and describe how they can potentially be used in biomedical applications. By tethering UCAs together into CCMCs, we propose that novel methods of ultrasound mediated imaging and therapy can be developed through unique interbubble interactions in an ultrasound field. One of the major challenges in generating CCMCs is controlling aggregate sizes and maintaining stability against Ostwald ripening and coalescence. In this study, we demonstrate that chemically cross-linked microbubble clusters can produce small (<10 μm) quasi-stable complexes that slowly fuse into bubbles with individual gas cores. Furthermore, we demonstrate that this process can be driven with low-intensity ultrasound pulses, enabling a rapid fusion of clusters which could potentially be used to develop novel ultrasound contrast imaging and drug delivery strategies in future studies. The development of novel microbubble clusters presents a simple yet robust process for generating novel UCAs with a design that could allow for more versatility in contrast-enhanced ultrasound (CEUS), molecular imaging, and drug delivery applications. Additionally, microbubble clustering is a unique way to control size, shell, and gas compositions that can be used to study bubble ripening and coalescence in a highly controlled environment or study the behavior of mixed-microbubble populations.

Minireview: Biophysical Mechanisms of Cell Membrane Sonopermeabilization. Knowns and Unknowns

Microbubble-assisted ultrasound has emerged as a promising method for the delivery of low-molecular-weight chemotherapeutic molecules, nucleic acids, therapeutic peptides, and antibodies in vitro and in vivo. Its clinical applications are under investigation for local delivery drug in oncology and neurology. However, the biophysical mechanisms supporting the acoustically mediated membrane permeabilization are not fully established. This review describes the present state of the investigations concerning the acoustically mediated stimuli (i.e., mechanical, chemical, and thermal stimuli) as well as the molecular and cellular actors (i.e., membrane pores and endocytosis) involved in the reversible membrane permeabilization process. The different hypotheses, which were proposed to give a biophysical description of the membrane permeabilization, are critically discussed.

Molecular Effects of Glycerol on Lipid Monolayers at the Gas-Liquid Interface: Impact on Microbubble Physical and Mechanical Properties

The production and stability of microbubbles (MBs) is enhanced by increasing the viscosity of both the formation and storage solution, respectively. Glycerol is a good candidate for biomedical applications of MBs, since it is biocompatible, although the exact molecular mechanisms of its action is not fully understood. Here, we investigate the influence glycerol has on lipid-shelled MB properties, using a range of techniques. Population lifetime and single bubble stability were studied using optical microscopy. Bubble stiffness measured by AFM compression is compared with lipid monolayer behavior in a Langmuir-Blodgett trough. We deduce that increasing glycerol concentrations enhances stability of MB populations through a 3-fold mechanism. First, binding of glycerol to lipid headgroups in the interfacial monolayer up to 10% glycerol increases MB stiffness but has limited impact on shell resistance to gas permeation and corresponding MB lifetime. Second, increased solution viscosity above 10% glycerol slows down the kinetics of gas transfer, markedly increasing MB stability. Third, above 10%, glycerol induces water structuring around the lipid monolayer, forming a glassy layer which also increases MB stiffness and resistance to gas loss. At 30% glycerol, the glassy layer is ablated, lowering the MB stiffness, but MB stability is further augmented. Although the molecular interactions of glycerol with the lipid monolayer modulate the MB lipid shell properties, MB lifetime continually increases from 0 to 30% glycerol, indicating that its viscosity is the dominant effect on MB solution stability. This three-fold action and biocompatibility makes glycerol ideal for therapeutic MB formation and storage and gives new insight into the action of glycerol on lipid monolayers at the gas-liquid interface.

Colloids, nanoparticles, and materials for imaging, delivery, ablation, and theranostics by focused ultrasound (FUS)

This review focuses on different materials and contrast agents that sensitize imaging and therapy with Focused Ultrasound (FUS). At high intensities, FUS is capable of selectively ablating tissue with focus on the millimeter scale, presenting an alternative to surgical intervention or management of malignant growth. At low intensities, FUS can be also used for other medical applications such as local delivery of drugs and blood brain barrier opening (BBBO). Contrast agents offer an opportunity to increase selective acoustic absorption or facilitate destructive cavitation processes by converting incident acoustic energy into thermal and mechanical energy. First, we review the history of FUS and its effects on living tissue. Next, we present different colloidal or nanoparticulate approaches to sensitizing FUS, for example using microbubbles, phase-shift emulsions, hollow-shelled nanoparticles, or hydrophobic silica surfaces. Exploring the science behind these interactions, we also discuss ways to make stimulus-responsive, or "turn-on" contrast agents for improved selectivity. Finally, we discuss acoustically-active hydrogels and membranes. This review will be of interest to those working in materials who wish to explore new applications in acoustics and those in acoustics who are seeking new agents to improve the efficacy of their approaches.

Ultrasound-activated microbubbles as a novel intracellular drug delivery system for urinary tract infection

The development of new modalities for high-efficiency intracellular drug delivery is a priority for a number of disease areas. One such area is urinary tract infection (UTI), which is one of the most common infectious diseases globally and which imposes an immense economic and healthcare burden. Common uropathogenic bacteria have been shown to invade the urothelial wall during acute UTI, forming latent intracellular reservoirs that can evade antimicrobials and the immune response. This behaviour likely facilitates the high recurrence rates after oral antibiotic treatments, which are not able to penetrate the bladder wall and accumulate to an effective concentration. Meanwhile, oral antibiotics may also exacerbate antimicrobial resistance and cause systemic side effects. Using a human urothelial organoid model, we tested the ability of novel ultrasound-activated lipid microbubbles to deliver drugs into the cytoplasm of apical cells. The gas-filled lipid microbubbles were decorated with liposomes containing the non-cell-permeant antibiotic gentamicin and a fluorescent marker. The microbubble suspension was added to buffer at the apical surface of the bladder model before being exposed to ultrasound (1.1 MHz, 2.5 Mpa, 5500 cycles at 20 ms pulse duration) for 20 s. Our results show that ultrasound-activated intracellular delivery using microbubbles was over 16 times greater than the control group and twice that achieved by liposomes that were not associated with microbubbles. Moreover, no cell damage was detected. Together, our data show that ultrasound-activated microbubbles can safely deliver high concentrations of drugs into urothelial cells, and have the potential to be a more efficacious alternative to traditional oral antibiotic regimes for UTI. This modality of intracellular drug delivery may prove useful in other clinical indications, such as cancer and gene therapy, where such penetration would aid in treatment.

Ultrasound responsive mesoporous silica nanoparticles for biomedical applications

Nanotechnology, which has already revolutionised many technological areas, is expected to transform life sciences. In this sense, nanomedicine could address some of the most important limitations of conventional medicine. In general, nanomedicine includes three major objectives: (1) trap and protect a great amount of therapeutic agents; (2) carry them to the specific site of disease avoiding any leakage; and (3) release on-demand high local concentrations of therapeutic agents. This feature article will make special emphasis on mesoporous silica nanoparticles that release their therapeutic cargo in response to ultrasound.

Pentagalloyl Glucose and Its Functional Role in Vascular Health: Biomechanics and Drug-Delivery Characteristics

Pentagalloyl glucose (PGG) is an elastin-stabilizing polyphenolic compound that has significant biomedical benefits, such as being a free radical sink, an anti-inflammatory agent, anti-diabetic agent, enzymatic resistant properties, etc. This review article focuses on the important benefits of PGG on vascular health, including its role in tissue mechanics, the different modes of pharmacological administration (e.g., oral, intravenous and endovascular route, intraperitoneal route, subcutaneous route, and nanoparticle based delivery and microbubble-based delivery), and its potential therapeutic role in vascular diseases such as abdominal aortic aneurysms (AAA). In particular, the use of PGG for AAA suppression and prevention has been demonstrated to be effective only in the calcium chloride rat AAA model. Therefore, in this critical review we address the challenges that lie ahead for the clinical translation of PGG as an AAA growth suppressor.

Microbubble-Mediated Enhanced Delivery of Curcumin to Cervical Cancer Cells

The major bottleneck in the current chemotherapy treatment of cancer is the low bioavailability and high cytotoxicity. Targeted delivery of drug to the cancer cells can reduce the cytotoxicity and increase the bioavailability. In this context, microbubbles are currently being explored as drug-delivery vehicles to effectively deliver drug to the tumors or cancerous cells. Microbubbles when used along with ultrasound can enhance drug uptake and inhibit the growth of tumor cells. Several potential anticancer molecules exhibit poor water solubility, which limits their use in therapeutic applications. Such poorly water soluble molecules can be coadministered with microbubbles or encapsulated within or loaded on the microbubbles surface, to enhance the effectiveness of these molecules against cancer cells. Curcumin is one of such potential anticancer molecules obtained from the rhizome of herbal spice, turmeric. In this work, curcumin-loaded protein microbubbles were synthesized and examined for effective in vitro delivery of curcumin to HeLa cells. Microbubbles in the size range of 1-10 μm were produced using perfluorobutane as core gas and bovine serum albumin (BSA) as shell material and were loaded with curcumin. The amount of curcumin loaded on the microbubble surface was estimated using UV-vis spectroscopy, and the average curcumin loading was found to be ∼54 μM/10⁸ microbubbles. Kinetics of in vitro curcumin release from microbubble surface was also estimated, where a 4-fold increase in the rate of curcumin release was obtained in the presence of ultrasound. Sonication and incubation of HeLa cells with curcumin-loaded BSA microbubbles enhanced the uptake of curcumin by ∼250 times. Further, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay confirmed ∼71% decrease in cell viability when HeLa cells were sonicated with curcumin-loaded microbubbles and incubated for 48 h.