Mechanistic Insight into Sonoporation with Ultrasound-Stimulated Polymer Microbubbles

High-Speed Optical Characterization of Protein-and-Nanoparticle-Stabilized Microbubbles for Ultrasound-Triggered Drug Release

OBJECTIVE

Ultrasound-triggered bubble-mediated local drug delivery has shown potential to increase therapeutic efficacy and reduce systemic side effects, by loading drugs into the microbubble shell and triggering delivery of the payload on demand using ultrasound. Understanding the behavior of the microbubbles in response to ultrasound is crucial for efficient and controlled release.

METHODS

In this work, the response of microbubbles with a coating consisting of poly(2-ethyl-butyl cyanoacrylate) (PEBCA) nanoparticles and denatured casein was characterized. High-speed recordings were taken of single microbubbles, in both bright field and fluorescence.

RESULTS

The nanoparticle-loaded microbubbles show resonance behavior, but with a large variation in response, revealing a substantial interbubble variation in mechanical shell properties. The probability of shell rupture and the probability of nanoparticle release were found to strongly depend on microbubble size, and the most effective size was inversely proportional to the driving frequency. The probabilities of both rupture and release increased with increasing driving pressure amplitude. Rupture of the microbubble shell occurred after fewer cycles of ultrasound as the driving pressure amplitude or driving frequency was increased.

CONCLUSION

The results highlight the importance of careful selection of the driving frequency, driving pressure amplitude and duration of ultrasound to achieve the most efficient ultrasound-triggered shell rupture and nanoparticle release of protein-and-nanoparticle-stabilized microbubbles.

In Vivo Sonoporation Effect Under the Presence of a Large Amount of Micro-Nano Bubbles in Swine Liver

OBJECTIVES

Sonoporation as a method of intracellular drug and gene delivery has not yet progressed to being used in vivo. The aim of this study was to prove the feasibility of sonoporation at a level practical for use in vivo by using a large amount of carbon dioxide micro-nano bubbles.

METHODS

The carbon dioxide micro-nano bubbles and 100 mg of cisplatin were intra-arterially injected to the swine livers, and ultrasound irradiation was performed from the surface of the liver under laparotomy during the intra-arterial injection. After the intra-arterial injection, ultrasound-irradiated and nonirradiated liver tissues were immediately excised. Tissue platinum concentration was measured using inductively coupled plasma mass spectrometry. Liver tissue platinum concentrations were compared between the irradiated tissue and nonirradiated tissue using the Wilcoxon signed rank test.

RESULTS

The mean (SD) liver tissue platinum concentration was 6.260*103 (2.070) ng/g in the irradiated liver tissue and 3.280*103 (0.430) ng/g in the nonirradiated liver tissue, showing significantly higher concentrations in the irradiated tissue ( P = 0.004).

CONCLUSIONS

In conclusion, increasing the tissue concentration of administered cisplatin in the livers of living swine through the effect of sonoporation was possible in the presence of a large amount of micro-nano bubbles.

mRNA Sonotransfection of Tumors with Polymeric Microbubbles: Co-Formulation versus Co-Administration

Despite its high potential, non-viral gene therapy of cancer remains challenging due to inefficient nucleic acid delivery. Ultrasound (US) with microbubbles (MB) can open biological barriers and thus improve DNA and mRNA passage. Polymeric MB are an interesting alternative to clinically used lipid-coated MB because of their high stability, narrow size distribution, and easy functionalization. However, besides choosing the ideal MB, it remains unclear whether nanocarrier-encapsulated mRNA should be administered separately (co-administration) or conjugated to MB (co-formulation). Therefore, the impact of poly(n-butyl cyanoacrylate) MB co-administration with mRNA-DOTAP/DOPE lipoplexes or their co-formulation on the transfection of cancer cells in vitro and in vivo is analyzed. Sonotransfection improved mRNA delivery into 4T1 breast cancer cells in vitro with co-administration being more efficient than co-formulation. In vivo, the co-administration sonotransfection approach also resulted in higher transfection efficiency and reached deeper into the tumor tissue. On the contrary, co-formulation mainly promoted transfection of endothelial and perivascular cells. Furthermore, the co-formulation approach is much more dependent on the US trigger, resulting in significantly lower off-site transfection. Thus, the findings indicate that the choice of co-administration or co-formulation in sonotransfection should depend on the targeted cell population, tolerable off-site transfection, and the therapeutic purpose.

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

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

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

A multicellular brain spheroid model for studying the mechanisms and bioeffects of ultrasound-enhanced drug penetration beyond the blood‒brain barrier

The blood‒brain barrier (BBB) acts as a hindrance to drug therapy reaching the brain. With an increasing incidence of neurovascular diseases and brain cancer metastases, there is a need for an ideal in vitro model to develop novel methodologies for enhancing drug delivery to the brain. Here, we established a multicellular human brain spheroid model that mimics the BBB both architecturally and functionally. Within the spheroids, endothelial cells and pericytes localized to the periphery, while neurons, astrocytes, and microglia were distributed throughout. Ultrasound-targeted microbubble cavitation (UTMC) is a novel noninvasive technology for enhancing endothelial drug permeability. We utilized our three-dimensional (3D) model to study the feasibility and mechanisms regulating UTMC-induced hyperpermeability. UTMC caused a significant increase in the penetration of 10 kDa Texas red dextran (TRD) into the spheroids, 100 µm beyond the BBB, without compromising cell viability. This hyperpermeability was dependent on UTMC-induced calcium (Ca2+) influx and endothelial nitric oxide synthase (eNOS) activation. Our 3D brain spheroid model, with its intact and functional BBB, offers a valuable platform for studying the bioeffects of UTMC, including effects occurring spatially distant from the endothelial barrier.

Polymeric materials for ultrasound imaging and therapy

Ultrasound (US) is routinely used for diagnostic imaging and increasingly employed for therapeutic applications. Materials that act as cavitation nuclei can improve the resolution of US imaging, and facilitate therapeutic US procedures by promoting local drug delivery or allowing temporary biological barrier opening at moderate acoustic powers. Polymeric materials offer a high degree of control over physicochemical features concerning responsiveness to US, e.g. via tuning chain composition, length and rigidity. This level of control cannot be achieved by materials made of lipids or proteins. In this perspective, we present key engineered polymeric materials that respond to US, including microbubbles, gas-stabilizing nanocups, microcapsules and gas-releasing nanoparticles, and discuss their formulation aspects as well as their principles of US responsiveness. Focusing on microbubbles as the most common US-responsive polymeric materials, we further evaluate the available chemical toolbox to engineer polymer shell properties and enhance their performance in US imaging and US-mediated drug delivery. Additionally, we summarize emerging applications of polymeric microbubbles in molecular imaging, sonopermeation, and gas and drug delivery, based on refinement of MB shell properties. Altogether, this manuscript provides new perspectives on US-responsive polymeric designs, envisaging their current and future applications in US imaging and therapy.

MRI-Guided Focused Ultrasound Blood-Brain Barrier Opening Increases Drug Delivery and Efficacy in a Diffuse Midline Glioma Mouse Model

Diffuse intrinsic pontine glioma (DIPG) is the most common and deadliest pediatric brainstem tumor and is difficult to treat with chemotherapy in part due to the blood-brain barrier (BBB). Focused ultrasound (FUS) and microbubbles (MBs) have been shown to cause BBB disruption (BBBD), allowing larger chemotherapeutics to enter the parenchyma. Panobinostat is an example of a promising in vitro agent in DIPG with poor clinical efficacy due to low BBB penetrance. In this study, we hypothesized that using FUS to disrupt the BBB allows higher concentrations of panobinostat to accumulate in the tumor, providing a therapeutic effect. Mice were orthotopically injected with a patient-derived DMG cell line, BT-245. MRI was used to guide FUS/MB (1.5 MHz, 0.615 MPa PNP, 1 Hz PRF, 10 ms PL, 3 min treatment time) / (25 µL/kg, IV) targeting to the tumor location. In animals receiving panobinostat (10 mg/kg, IP) in combination with FUS/MB, a 3-fold increase in tumor panobinostat concentration was observed, with only insignificant increase of the drug in the forebrain. In mice receiving three weekly treatments, the combination of panobinostat and FUS/MB led to a 71% reduction of tumor volumes by MRI ( p = 0.01). Furthermore, FUS/MB improved the mean survival from 21 to 31 days ( p < 0.0001). Our study demonstrates that FUS-mediated BBBD can increase the delivery of panobinostat to an orthotopic DMG tumor, providing a strong therapeutic effect and increased survival.

One Sentence Summary

FUS and microbubbles can increase the delivery of panobinostat to a patient-derived xenograft (PDX) orthotopic DMG tumor, providing a strong therapeutic effect and increased survival.

Buckling of lipidic ultrasound contrast agents under quasi-static load

Collapse of lipidic ultrasound contrast agents under high-frequency compressive load has been historically interpreted by the vanishing of surface tension. By contrast, buckling of elastic shells is known to occur when costly compressible stress is released through bending. Through quasi-static compression experiments on lipidic shells, we analyse the buckling events in the framework of classical elastic buckling theory and deduce the mechanical characteristics of these shells. They are then compared with that obtained through acoustic characterization. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'.

Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy

OBJECTIVE

Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. Because of the potentially longer circulation time and relative ease of storage and reconstitution of polymer-shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer microbubbles and their therapeutic potential for sonoreperfusion (SRP) therapy.

METHODS

The fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures and MB concentrations was recorded via high-speed microscopy and passive cavitation detection (PCD). SRP efficacy of the polymer MBs was investigated in an in vitro flow system and compared with that of lipid MBs.

DISCUSSION

Microscopy videos indicated that polymer MBs formed gas-filled clusters that continued to oscillate, fragment and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with pulse duration and acoustic pressure similarly to that with lipid MBs but no significant differences were found between polymer and lipid MBs.

CONCLUSION

These data suggest that persistent cavitation activity from polymer MBs during long tone-burst US excitation confers excellent reperfusion efficacy.

Enhanced Local Delivery of microRNA-145a-5P into Mouse Aorta via Ultrasound-Targeted Microbubble Destruction Inhibits Atherosclerotic Plaque Formation

Abnormal proliferation and migration of vascular smooth muscle cells (VSMCs) play a key role in the formation and rupture of atherosclerotic plaques. Previous studies have confirmed that microRNA-145 (miR-145) is involved in the phenotypic regulation of VSMCs and reduction of atherosclerosis. At present, seeking safe and effective gene delivery remains a key problem restricting the development of gene therapy. In recent years, ultrasound-targeted microbubble destruction (UTMD) has become a safe and effective transfection method that is widely used in the basic research of gene therapy for heart and tumor diseases. Here, we synthesized cationic microbubbles to encapsulate miR-145 and targeted their release into VSMCs in vitro and in vivo using ultrasound. The feasibility of this gene therapy was verified by fluorescence microscopy and an in vivo imaging system. The results showed that treatment with miR-145 delivered via UTMD considerably improved the gene transfection efficiency and promoted the contraction phenotype of VSMCs in vitro. In vivo, this treatment reduced the atherosclerotic plaque area by 48.04% compared with treatment with free miR-145. Therefore, UTMD-mediated miRNA therapy may provide a new targeted therapeutic approach for atherosclerotic plaques.

MRI-guided focused ultrasound blood-brain barrier opening increases drug delivery and efficacy in a diffuse midline glioma mouse model

Background

Diffuse intrinsic pontine glioma (DIPG) is the most common and deadliest pediatric brainstem tumor and is difficult to treat with chemotherapy in part due to the blood-brain barrier (BBB). Focused ultrasound (FUS) and microbubbles (MBs) have been shown to cause BBB opening, allowing larger chemotherapeutics to enter the parenchyma. Panobinostat is an example of a promising in vitro agent in DIPG with poor clinical efficacy due to low BBB penetrance. In this study, we hypothesized that using FUS to disrupt the BBB allows higher concentrations of panobinostat to accumulate in the tumor, providing a therapeutic effect.

Methods

Mice were orthotopically injected with a patient-derived diffuse midline glioma (DMG) cell line, BT245. MRI was used to guide FUS/MB (1.5 MHz, 0.615 MPa peak negative pressure, 1 Hz pulse repetition frequency, 10-ms pulse length, 3 min treatment time)/(25 µL/kg, i.v.) targeting to the tumor location.

Results

In animals receiving panobinostat (10 mg/kg, i.p.) in combination with FUS/MB, a 3-fold increase in tumor panobinostat concentration was observed, without significant increase of the drug in the forebrain. In mice receiving 3 weekly treatments, the combination of panobinostat and FUS/MB led to a 71% reduction of tumor volumes (P = .01). Furthermore, we showed the first survival benefit from FUS/MB improved delivery increasing the mean survival from 21 to 31 days (P < .0001).

Conclusions

Our study demonstrates that FUS-mediated BBB disruption can increase the delivery of panobinostat to an orthotopic DMG tumor, providing a strong therapeutic effect and increased survival.

Evaluation of the potential of ultrasound-mediated drug delivery for the treatment of ovarian cancer through preclinical studies

Ovarian cancer (OC) has the greatest mortality rate among gynecological cancers, with a five-year survival rate of &lt;50%. Contemporary adjuvant chemotherapy mostly fails in the case of OCs that are refractory, metastatic, recurrent, and drug-resistant. Emerging ultrasound (US)-mediated technologies show remarkable promise in overcoming these challenges. Absorption of US waves by the tissue results in the generation of heat due to its thermal effect causing increased diffusion of drugs from the carriers and triggering sonoporation by increasing the permeability of the cancer cells. Certain frequencies of US waves could also produce a cavitation effect on drug-filled microbubbles (MBs, phospholipid bilayers) thereby generating shear force and acoustic streaming that could assist drug release from the MBs, and promote the permeability of the cell membrane. A new class of nanoparticles that carry therapeutic agents and are guided by US contrast agents for precision delivery to the site of the ovarian tumor has been developed. Phase-shifting of nanoparticles by US sonication has also been engineered to enhance the drug delivery to the ovarian tumor site. These technologies have been used for targeting the ovarian cancer stem cells and protein moieties that are particularly elevated in OCs including luteinizing hormone-releasing hormone, folic acid receptor, and vascular endothelial growth factor. When compared to healthy ovarian tissue, the homeostatic parameters at the tissue microenvironment including pH, oxygen levels, and glucose metabolism differ significantly in ovarian tumors. US-based technologies have been developed to take advantage of these tumor-specific alterations for precision drug delivery. Preclinical efficacy of US-based targeting of currently used clinical chemotherapies presented in this review has the potential for rapid human translation, especially for formulations that use all substances that are deemed to be generally safe by the U.S. Food and Drug Administration.

An Overview of Cell Membrane Perforation and Resealing Mechanisms for Localized Drug Delivery

Localized and reversible plasma membrane disruption is a promising technique employed for the targeted deposition of exogenous therapeutic compounds for the treatment of disease. Indeed, the plasma membrane represents a significant barrier to successful delivery, and various physical methods using light, sound, and electrical energy have been developed to generate cell membrane perforations to circumvent this issue. To restore homeostasis and preserve viability, localized cellular repair mechanisms are subsequently triggered to initiate a rapid restoration of plasma membrane integrity. Here, we summarize the known emergency membrane repair responses, detailing the salient membrane sealing proteins as well as the underlying cytoskeletal remodeling that follows the physical induction of a localized plasma membrane pore, and we present an overview of potential modulation strategies that may improve targeted drug delivery approaches.

Sonoporation: Past, Present, and Future

A surge of research in intracellular delivery technologies is underway with the increased innovations in cell-based therapies and cell reprogramming. Particularly, physical cell membrane permeabilization techniques are highlighted as the leading technologies because of their unique features, including versatility, independence of cargo properties, and high-throughput delivery that is critical for providing the desired cell quantity for cell-based therapies. Amongst the physical permeabilization methods, sonoporation holds great promise and has been demonstrated for delivering a variety of functional cargos, such as biomolecular drugs, proteins, and plasmids, to various cells including cancer, immune, and stem cells. However, traditional bubble-based sonoporation methods usually require special contrast agents. Bubble-based sonoporation methods also have high chances of inducing irreversible damage to critical cell components, lowering the cell viability, and reducing the effectiveness of delivered cargos. To overcome these limitations, several novel non-bubble-based sonoporation mechanisms are under development. This review will cover both the bubble-based and non-bubble-based sonoporation mechanisms being employed for intracellular delivery, the technologies being investigated to overcome the limitations of traditional platforms, as well as perspectives on the future sonoporation mechanisms, technologies, and applications.

Applications of Ultrasound-Mediated Drug Delivery and Gene Therapy

Gene therapy has continuously evolved throughout the years since its first proposal to develop more specific and effective transfection, capable of treating a myriad of health conditions. Viral vectors are some of the most common and most efficient vehicles for gene transfer. However, the safe and effective delivery of gene therapy remains a major obstacle. Ultrasound contrast agents in the form of microbubbles have provided a unique solution to fulfill the need to shield the vectors from the host immune system and the need for site specific targeted therapy. Since the discovery of the biophysical and biological effects of microbubble sonification, multiple developments have been made to enhance its applicability in targeted drug delivery. The concurrent development of viral vectors and recent research on dual vector strategies have shown promising results. This review will explore the mechanisms and recent advancements in the knowledge of ultrasound-mediated microbubbles in targeting gene and drug therapy.

Ultrasound molecular imaging-guided tumor gene therapy through dual-targeted cationic microbubbles

The success of gene therapy depends largely on the development of gene vectors and effective gene delivery systems. It has been demonstrated that cationic microbubbles can be loaded with negatively charged plasmid DNA and thus improve gene transfection efficiency. In this study, we developed dual-targeting cationic microbubbles conjugated with iRGD peptides(Cyclo(Cys-Arg-Gly-Asp-Lys-Gly-Pro-Asp-Cys)) and CCR2 (chemokine (C-C motif) receptor 2) antibodies (MB_iRGD/CCR2) for ultrasound molecular imaging and targeted tumor gene therapy. The ultrasound molecular imaging experiments showed that there were significantly enhanced ultrasound molecular imaging signals in the tumor that received MB_iRGD/CCR2, compared with those that received MB_iRGD, MB_CCR2, or MB_control. As a therapy plasmid, pGPU6/GFP/Neo-shAKT2, carrying an expression cassette for the human AKT2 RNA interference sequence, was used. Our results demonstrated that MB_iRGD/CCR2 had a significantly higher gene transfection efficiency than MB_iRGD, MB_CCR2, or MB_control under ultrasound irradiation, resulting in much lower AKT2 protein expression and stronger tumor growth inhibition effects in vivo and in vitro. In conclusion, our study demonstrated a novel gene delivery system via MB_iRGD/CCR2 for ultrasound molecular-imaging-guided gene therapy of breast cancer.

Sonoporation: Underlying Mechanisms and Applications in Cellular Regulation

Ultrasound combined with microbubble-mediated sonoporation has been applied to enhance drug or gene intracellular delivery. Sonoporation leads to the formation of openings in the cell membrane, triggered by ultrasound-mediated oscillations and destruction of microbubbles. Multiple mechanisms are involved in the occurrence of sonoporation, including ultrasonic parameters, microbubbles size, and the distance of microbubbles to cells. Recent advances are beginning to extend applications through the assistance of contrast agents, which allow ultrasound to connect directly to cellular functions such as gene expression, cellular apoptosis, differentiation, and even epigenetic reprogramming. In this review, we summarize the current state of the art concerning microbubble‐cell interactions and sonoporation effects leading to cellular functions.

Combination CTLA-4 immunoglobulin treatment and ultrasound microbubble-mediated exposure improve renal function in a rat model of diabetic nephropathy

OBJECTIVE

This study explored the therapeutic impact of combined cytotoxic T lymphocyte-associated antigen 4 immunoglobulin (CTLA-4-Ig) treatment and microbubble-mediated exposure in a rat model of diabetic nephropathy (DN).

METHOD

We treated rats using CTLA-4-Ig and/or microbubble exposure. At 8 weeks post-intervention, key parameters were evaluated including blood biochemistry, damage to renal tissue, renal parenchymal elasticity, ultrastructural changes in podocytes, and renal parenchymal expression of CD31, CD34, IL-6, Fn, Collagen I, Talin, Paxillin, α3β1, podocin, nephrin, and B7-1.

RESULT

We found that renal function in the rat model of DN can be significantly improved by CTLA-4-Ig and CTLA-4-Ig + ultrasound microbubble treatment. Treatment efficacy was associated with reductions in renal parenchymal hardness, decreases in podocyte reduction, decreased IL-6, Fn and Collagen I expression, increased Talin, Paxillin and α3β1 expression, elevated podocin and nephrin expression, and decreased B7-1 expression. In contrast, these treatments did not impact CD31 or CD34 expression within the renal parenchyma.

CONCLUSION

These findings clearly emphasize that CTLA-4-Ig can effectively prevent podocyte damage, inhibiting inflammation and fibrosis, and thereby treating and preventing DN. In addition, ultrasound microbubble exposure can improve the ability of CTLA-4-Ig to pass through the glomerular basement membrane in order to access podocytes such that combination CTLA-4-Ig + microbubble exposure treatment is superior to treatment with CTLA-4-Ig only.

Ultrasound Contrast: Gas Microbubbles in the Vasculature

Gas-filled microbubbles are currently in clinical use as blood pool contrast agents for ultrasound imaging. The goal of this review is to discuss the trends and issues related to these relatively unusual intravascular materials, which are not small molecules per se, not polymers, not even nanoparticles, but larger micrometer size structures, compressible, flexible, elastic, and deformable. The intent is to connect current research and initial studies from 2 to 3 decades ago, tied to gas exchange between the bubbles and surrounding biological medium, in the following areas of focus: (1) parameters of microbubble movement in relation to vasculature specifics; (2) gas uptake and loss from the bubbles in the vasculature; (3) adhesion of microbubbles to target receptors in the vasculature; and (4) microbubble interaction with the surrounding vessels and tissues during insonation.Microbubbles are generally safe and require orders of magnitude lower material doses than x-ray and magnetic resonance imaging contrast agents. Application of microbubbles will soon extend beyond blood pool contrast and tissue perfusion imaging. Microbubbles can probe molecular and cellular biomarkers of disease by targeted contrast ultrasound imaging. This approach is now in clinical trials, for example, with the aim to detect and delineate tumor nodes in prostate, breast, and ovarian cancer. Imaging of inflammation, ischemia-reperfusion injury, and ischemic memory is also feasible. More importantly, intravascular microbubbles can be used for local deposition of focused ultrasound energy to enhance drug and gene delivery to cells and tissues, across endothelial barrier, especially blood-brain barrier.Overall, microbubble behavior, stability and in vivo lifetime, bioeffects upon the action of ultrasound and resulting enhancement of drug and gene delivery, as well as targeted imaging are critically dependent on the events of gas exchange between the bubbles and surrounding media, as outlined in this review.

STAT3 decoy oligonucleotide-carrying microbubbles with pulsed ultrasound for enhanced therapeutic effect in head and neck tumors

Signal transducer and activator of transcription-3 (STAT3) is an oncogenic transcription factor implicated in carcinogenesis, tumor progression, and drug resistance in head and neck squamous cell carcinoma (HNSCC). A decoy oligonucleotide targeting STAT3 offers a promising anti-tumor strategy, but achieving targeted tumor delivery of the decoy with systemic administration poses a significant challenge. We previously showed the potential for STAT3 decoy-loaded microbubbles, in conjunction with ultrasound targeted microbubble cavitation (UTMC), to decrease tumor growth in murine squamous cell carcinoma. As a next step towards clinical translation, we sought to determine the anti-tumor efficacy of our STAT3 decoy delivery platform against human HNSCC and the effect of higher STAT3 decoy microbubble loading on tumor cell inhibition. STAT3 decoy was loaded on cationic lipid microbubbles (STAT3-MB) or loaded on liposome-conjugated lipid microbubbles to form STAT3-loaded liposome-microbubble complexes (STAT3-LPX). UTMC treatment efficacy with these two formulations was evaluated in vitro using viability and apoptosis assays in CAL33 (human HNSCC) cells. Anti-cancer efficacy in vivo was performed in a CAL33 tumor murine xenograft model. UTMC with STAT3-MB caused significantly lower CAL33 cell viability compared to UTMC with STAT3-LPX (56.8±8.4% vs 84.5±8.8%, respectively, p<0.05). In vivo, UTMC with STAT3-MB had strong anti-tumor effects, with significantly less tumor burden and greater survival compared to that of UTMC with microbubbles loaded with a mutant control decoy and untreated control groups (p<0.05). UTMC with STAT3 decoy-loaded microbubbles significantly decreases human HNSSC tumor progression. These data set the stage for clinical translation of our microbubble platform as an imaged-guided, targeted delivery strategy for STAT3 decoy, or other nucleotide-based therapeutics, in human cancer treatment.

Antitumor effect of VEGFR2-targeted microbubble destruction with gemcitabine using an endoscopic ultrasound probe: In vivo mouse pancreatic ductal adenocarcinoma model

BACKGROUND

Ultrasound-targeted microbubble destruction (UTMD) induces cellular inflow of drugs at low intensity, while high intensity eradicates tumor vessels. Since vascular endothelial growth factor receptor 2 (VEGFR2) is highly expressed in pancreatic ductal adenocarcinoma (PDAC), VEGFR2-targeted microbubble (MB) might additionally increase the tissue specificity of drugs and thus improve antitumor effects. In addition, fixing the dual pulse intensity could maximize MB properties. This study evaluated the one-off (experiment 1) and cumulative (experiment 2) treatment effect of UTMD by regulating the dual pulse output applied to PDAC using VEGFR2-targeted MB.

METHODS

C57BL/6 mice inoculated with Pan-02 cells were allocated to five groups: VEGFR2-targeted MB+ gemcitabine (GEM), VEGFR2-targeted MB, non-targeted MB+GEM, GEM, and control groups. After injection of GEM or GEM and either VEGFR2-targeted or non-targeted MB, UTMD was applied for several minutes at low intensity followed by high intensity application. In experiment 1, mice were treated by the protocol described above and then euthanized immediately or at the tumor diameter doubling time (TDT). In experiment 2, the same protocol was repeated weekly and mice were euthanized at TDT regardless of protocol completion. Histological analysis by CD31 and VEGFR2 staining provided microvascular density (MVD) and VEGFR2 expression along vessels (VEGFR2v) or intra/peripheral cells (VEGFR2c).

RESULTS

In experiment 1, TDT was significantly longer in the VEGFR2-targeted MB+GEM group compared to the non-targeted MB+GEM, GEM, and control groups, while the VEGFR2-targeted MB group showed no statistical significance. MVD and VEGFR2v in the immediate euthanasia was significantly lower in the VEGFR2-targeted MB+GEM and VEGFR2-targeted MB groups than other conditions. In experiment 2, the VEGFR2-targeted MB+GEM group produced significantly longer TDT than the GEM or control groups, whereas the VEGFR2-targeted MB group showed no significant difference. Histology revealed significantly reduced VEGFR2v and VEGFR2c in the VEGFR2-targeted and non-targeted MB+GEM groups, while only VEGFR2v was significantly less in the VEGFR2-targeted MB group.

CONCLUSIONS

UTMD-mediated GEM therapy with the dual pulse application using VEGFR2-targeted MB substantially suppresses PDCA growth.

Increasing Doxorubicin Loading in Lipid-Shelled Perfluoropropane Nanobubbles via a Simple Deprotonation Strategy

Drug delivery to solid tumors using echogenic nanobubbles (NBs) and ultrasound (US) has recently gained significant interest. The approach combines attributes of nanomedicine and the enhanced permeation and retention (EPR) effect with the documented benefits of ultrasound to improve tumor drug distribution and treatment outcomes. However, optimized drug loading strategies, the drug-carrying capacity of NBs and their drug delivery efficiency have not been explored in depth and remain unclear. Here, we report for the first time on the development of a novel deprotonated hydrophobic doxorubicin-loaded C₃F₈ nanobubble (hDox-NB) for more effective US-mediated drug delivery. In this study, the size distribution and yield of hDox-NBs were measured via resonant mass measurement, while their drug-loading capacity was determined using a centrifugal filter technique. In vitro acoustic properties including contrast-imaging enhancement, initial echogenic signal, and decay were assessed and compared to doxorubicin hydrochloride loaded-NBs (Dox.HCl-NBs). In addition, in vitro therapeutic efficacy of hDox-NBs was evaluated by cytotoxicity assay in human ovarian cancer cells (OVCAR-3). The results showed that the hDox-NBs were small (300.7 ± 4.6 nm), and the drug loading content was significantly enhanced (2 fold higher) compared to Dox.HCl-NBs. Unexpectedly, the in vitro acoustic performance was also improved by inclusion of hDox into NBs. hDox-NB showed higher initial US signal and a reduced signal decay rate compared to Dox.HCl-NBs. Furthermore, hDox-NBs combined with higher intensity US exhibited an excellent therapeutic efficacy in human ovarian cancer cells as shown in a reduction in cell viability. These results suggest that hDox-NBs could be considered as a promising theranostic agent to achieve a more effective noninvasive US-mediated drug delivery for cancer treatment.

Opening of endothelial cell-cell contacts due to sonoporation

Ultrasound insonification of microbubbles can locally increase vascular permeability to enhance drug delivery. To control and optimize the therapeutic potential, we need to better understand the underlying biological mechanisms of the drug delivery pathways. The aim of this in vitro study was to elucidate the microbubble-endothelial cell interaction using the Brandaris 128 ultra-high-speed camera (up to 25 Mfps) coupled to a custom-built Nikon confocal microscope, to visualize both microbubble oscillation and the cellular response. Sonoporation and opening of cell-cell contacts by single α_Vβ₃-targeted microbubbles (n = 152) was monitored up to 4 min after ultrasound insonification (2 MHz, 100-400 kPa, 10 cycles). Sonoporation occurred when microbubble excursion amplitudes exceeded 0.7 μm. Quantification of the influx of the fluorescent model drug propidium iodide upon sonoporation showed that the size of the created pore increased for larger microbubble excursion amplitudes. Microbubble-mediated opening of cell-cell contacts occurred as a cellular response upon sonoporation and did not correlate with the microbubble excursion amplitude itself. The initial integrity of the cell-cell contacts affected the susceptibly to drug delivery, since cell-cell contacts opened more often when cells were only partially attached to their neighbors (48%) than when fully attached (14%). The drug delivery outcomes were independent of nonlinear microbubble behavior, microbubble location, and cell size. In conclusion, by studying the microbubble-cell interaction at nanosecond and nanometer resolution the relationship between drug delivery pathways and their underlying mechanisms was further unraveled. These novel insights will aid the development of safe and efficient microbubble-mediated drug delivery.

Physiologic Factors Affecting the Circulatory Persistence of Copolymer Microbubbles and Comparison of Contrast-Enhanced Effects between Copolymer Microbubbles and Sonovue

Ultrasound contrast agents have been widely used in clinical diagnosis. Knowledge of the physiologic factors affecting circulatory persistence is helpful in preparing long-lasting microbubbles (MBs) for blood perfusion and drug delivery research. In the study described here, we prepared copolymer MBs, compared their characteristics and contrast-enhanced effects with those of SonoVue and investigated the influence of external pressure, temperature, plasma components, renal microcirculation and cardiac motion on their circulatory persistence. The mean size of the copolymer MBs was 3.57 μm, larger than that of SonoVue. The copolymer MBs had longer circulatory persistence than SonoVue. At external pressures of 110 and 150 mm Hg, neither the quantity nor the morphology of the copolymer MBs changed. Further, their quantity and size were similar after incubation at 4°C and 39.4°C and when rabbit plasma and saline were compared. In vivo contrast-enhanced ultrasonography revealed a slightly larger area under the curve for the renal artery than for the renal vein. Thus, copolymer MBs exhibited good stability. However, the quantity of copolymer MBs decreased significantly after 180 s of circulation in an isolated toad heart perfusion model, indicating that cardiac motion was the main factor affecting their circulatory persistence.

PD-L1-targeted microbubbles loaded with docetaxel produce a synergistic effect for the treatment of lung cancer under ultrasound irradiation

Immunotherapy is gradually becoming as important as traditional therapy in the treatment of cancer, but adverse drug reactions limit patient benefits from PD1/PD-L1 checkpoint inhibitor drugs in the treatment of non-small cell lung cancer (NSCLC). As a chemotherapeutic drug for NSCLC, docetaxel (DTX) can synergize with PD1/PD-L1 checkpoint inhibitors but increase haematoxicity and neurotoxicity. Herein, anti-PD-L1 monoclonal antibody (mAb)-conjugated and docetaxel-loaded multifunctional lipid-shelled microbubbles (PDMs), which were designed with biologically safe phospholipids to produce synergistic antitumour effects, reduced the incidence of side effects and promoted therapeutic effects under ultrasound (US) irradiation. The PDMs were prepared by the acoustic-vibration method and then conjugated with an anti-PD-L1 mAb. The material features of the microbubbles and their cytotoxic effects, cellular apoptosis and cell cycle inhibition were studied. A subcutaneous tumour model was established to test the drug concentration-dependent and antitumour effects of the PDMs combined with US irradiation, and an orthotopic lung tumour model simultaneously confirmed the antitumour effect of this synergistic treatment. The PDMs achieved higher cellular uptake than free DTX, especially when combined with US irradiation. The PDMs combined with US irradiation also induced an increased rate of cellular apoptosis and an elevated G2-M arrest rate in cancer cells, which was positively correlated with PD-L1 expression. An in vivo study showed that synergistic treatment had relatively strong effects on tumour growth inhibition, increased survival time and decreased adverse effect rates. Our study possibly provides a well-controlled design for immunotherapy and chemotherapy and has promising potential for clinical application in NSCLC treatment.

Nanoparticles for mucosal vaccine delivery

Mucosal surfaces represent important routes of entry into the human body for the majority of pathogens, and they constitute unique sites for targeted vaccine delivery. Nanoparticle-based drug delivery systems are emerging technologies for delivering and improving the efficacy of mucosal vaccines. Recent studies have provided new insights into formulation and delivery aspects of importance for the design of safe and efficacious mucosal subunit vaccines based on nanoparticles. These include novel nanomaterials, their physicochemical properties and formulation approaches, nanoparticle interaction with immune cells in the mucosa, and mucosal immunization and delivery strategies. Here, we present recent progress in the application of nanoparticle-based approaches for mucosal vaccine delivery and discuss future research challenges and opportunities in the field.

Nano-Enhanced Drug Delivery and Therapeutic Ultrasound for Cancer Treatment and Beyond

While ultrasound is most widely known for its use in diagnostic imaging, the energy carried by ultrasound waves can be utilized to influence cell function and drug delivery. Consequently, our ability to use ultrasound energy at a given intensity unlocks the opportunity to use the ultrasound for therapeutic applications. Indeed, in the last decade ultrasound-based therapies have emerged with promising treatment modalities for several medical conditions. More recently, ultrasound in combination with nanomedicines, i.e., nanoparticles, has been shown to have substantial potential to enhance the efficacy of many treatments including cancer, Alzheimer disease or osteoarthritis. The concept of ultrasound combined with drug delivery is still in its infancy and more research is needed to unfold the mechanisms and interactions of ultrasound with different nanoparticles types and with various cell types. Here we present the state-of-art in ultrasound and ultrasound-assisted drug delivery with a particular focus on cancer treatments. Notably, this review discusses the application of high intensity focus ultrasound for non-invasive tumor ablation and immunomodulatory effects of ultrasound, as well as the efficacy of nanoparticle-enhanced ultrasound therapies for different medical conditions. Furthermore, this review presents safety considerations related to ultrasound technology and gives recommendations in the context of system design and operation.

Combined Confocal Microscope and Brandaris 128 Ultra-High-Speed Camera

Controlling microbubble-mediated drug delivery requires the underlying biological and physical mechanisms to be unraveled. To image both microbubble oscillation upon ultrasound insonification and the resulting cellular response, we developed an optical imaging system that can achieve the necessary nanosecond temporal and nanometer spatial resolutions. We coupled the Brandaris 128 ultra-high-speed camera (up to 25 million frames per second) to a custom-built Nikon A1R+ confocal microscope. The unique capabilities of this combined system are demonstrated with three experiments showing microbubble oscillation leading to either endothelial drug delivery, bacterial biofilm disruption, or structural changes in the microbubble coating. In conclusion, using this state-of-the-art optical imaging system, microbubble-mediated drug delivery can be studied with high temporal resolution to resolve microbubble oscillation and high spatial resolution and detector sensitivity to discern cellular response. Combining these two imaging technologies will substantially advance our knowledge on microbubble behavior and its role in drug delivery.

Engineered 3D Microvascular Networks for the Study of Ultrasound-Microbubble-Mediated Drug Delivery

Localized and targeted drug delivery can be achieved by the combined action of ultrasound and microbubbles on the tumor microenvironment, likely through sonoporation and other therapeutic mechanisms that are not well understood. Here, we present a perfusable in vitro model with a realistic 3D geometry to study the interactions between microbubbles and the vascular endothelium in the presence of ultrasound. Specifically, a three-dimensional, endothelial-cell-seeded in vitro microvascular model was perfused with cell culture medium and microbubbles while being sonicated by a single-element 1 MHz focused transducer. This setup mimics the in vivo scenario in which ultrasound induces a therapeutic effect in the tumor vasculature in the presence of flow. Fluorescence and bright-field microscopy were employed to assess the microbubble-vessel interactions and the extent of drug delivery and cell death both in real time during treatment as well as after treatment. Propidium iodide was used as the model drug while calcein AM was used to evaluate cell viability. There were two acoustic parameter sets chosen for this work: (1) acoustic pressure: 1.4 MPa, pulse length: 500 cycles, duty cycle: 5% and (2) acoustic pressure: 0.4 MPa, pulse length: 1000 cycles, duty cycle: 20%. Enhanced drug delivery and cell death were observed in both cases while the higher pressure setting had a more pronounced effect. By introducing physiological flow to the in vitro microvascular model and examining the PECAM-1 expression of the endothelial cells within it, we demonstrated that our model is a good mimic of the in vivo vasculature and is therefore a viable platform to provide mechanistic insights into ultrasound-mediated drug delivery.

Advances in mechanism studies on ultrasonic gene delivery at cellular level

Ultrasound provides a means for intracellular gene delivery, contributing to a noninvasive and spatiotemporally controllable strategy suitable for clinical applications. Many studies have been done to provide mechanisms of ultrasound-mediated gene delivery at the cellular level. This review summarizes the studies on the important aspects of the mechanisms, providing an overview of recent progress in cellular experiment of ultrasound-mediated gene delivery.