Gene delivery systems by the combination of lipid bubbles and ultrasound

A Review of Ultrasound-Mediated Checkpoint Inhibitor Immunotherapy

Over the past decade, immunotherapy has emerged as a major modality in cancer medicine. However, despite its unprecedented success, immunotherapy currently benefits only a subgroup of patients, may induce responses of limited duration and is associated with potentially treatment-limiting side effects. In addition, responses to immunotherapeutics are sometimes diminished by the emergence of a complex array of resistance mechanisms. The efficacy of immunotherapy depends on dynamic interactions between tumour cells and the immune landscape in the tumour microenvironment. Ultrasound, especially in conjunction with cavitation-promoting agents such as microbubbles, can assist in the uptake and/or local release of immunotherapeutic agents at specific target sites, thereby increasing treatment efficacy and reducing systemic toxicity. There is also increasing evidence that ultrasound and/or cavitation may themselves directly stimulate a beneficial immune response. In this review, we summarize the latest developments in the use of ultrasound and cavitation agents to promote checkpoint inhibitor immunotherapy.

Nanobubble-mediated cancer cell sonoporation using low-frequency ultrasound

Ultrasound insonation of microbubbles can form transient pores in cell membranes that enable the delivery of non-permeable extracellular molecules to the cells. Reducing the size of microbubble contrast agents to the nanometer range could facilitate cancer sonoporation. This size reduction can enhance the extravasation of nanobubbles into tumors after an intravenous injection, thus providing a noninvasive sonoporation platform. However, drug delivery efficacy depends on the oscillations of the bubbles, the ultrasound parameters and the size of the target compared to the membrane pores. The formation of large pores is advantageous for the delivery of large molecules, however the small size of the nanobubbles limit the bioeffects when operating near the nanobubble resonance frequency at the MHz range. Here, we show that by coupling nanobubbles with 250 kHz low frequency ultrasound, high amplitude oscillations can be achieved, which facilitate low energy sonoporation of cancer cells. This is beneficial both for increasing the uptake of a specific molecule and to improve large molecule delivery. The method was optimized for the delivery of four fluorescent molecules ranging in size from 1.2 to 70 kDa to breast cancer cells, while comparing the results to targeted microbubbles. Depending on the fluorescent molecule size, the optimal ultrasound peak negative pressure was found to range between 300 and 500 kPa. Increasing the pressure to 800 kPa reduced the fraction of fluorescent cells for all molecules sizes. The optimal uptake for the smaller molecule size of 4 kDa resulted in a fraction of 19.9 ± 1.8% of fluorescent cells, whereas delivery of 20 kDa and 70 kDa molecules yielded 14 ± 0.8% and 4.1 ± 1.1%, respectively. These values were similar to targeted microbubble-mediated sonoporation, suggesting that nanobubbles can serve as noninvasive sonoporation agents with a similar potency, and at a reduced bubble size. The nanobubbles effectively reduced cell viability and may thus potentially reduce the tumor burden, which is crucial for the success of cancer treatment. This method provides a non-invasive and low-energy tumor sonoporation theranostic platform, which can be combined with other therapies to maximize the therapeutic benefits of cancer treatment or be harnessed in gene therapy applications.

Functional evaluation of constructed pseudo-endogenous microRNA-targeted myocardial ultrasound nanobubble

Background

Stem cell transplantation is one of the treatment methods for acute myocardial infarction (AMI). MicroRNA-1 contributes to the study of the essential mechanisms of stem cell transplantation for treating AMI by targeted regulating the myocardial microenvironment after stem cell transplantation at the post-transcriptional level. Thus, microRNA-1 participates in regulating the myocardial microenvironment after stem cell transplantation, a promising strategy for the Stem cell transplantation treatment of AMI. However, the naked microRNA-1 synthesized is extremely unstable and non-targeting, which can be rapidly degraded by circulating RNase. Herein, to safely and effectively targeted transport the naked microRNA-1 synthesized into myocardial tissue, we will construct pseudo-endogenous microRNA-targeted myocardial ultrasound nanobubble pAd-AAV-9/miRNA-1 NB and evaluate its characteristics, targeting, and function.

Methods

The pAd-AAV-9/miRNA-1 gene complex was linked to nanobubble NBs by the "avidin-biotin bridging" method to prepare cardiomyocyte-targeted nanobubble pAd-AAV-9/miRNA-1 NB. The shape, particle size, dispersion, and stability of nanobubbles and the connection of pAd-AAV-9/miRNA-1 gene complex to nanobubble NB were observed. The virus loading efficiency was determined, and the myocardium-targeting imaging ability was evaluated using contrast-enhanced ultrasound imaging in vivo. The miRNA-1 expression level in myocardial tissue and other vital organs ex vivo of SD rats was considered by Q-PCR. Also, the cytotoxic effects were assessed.

Results

The particle size of NBs was 504.02 ± 36.94 nm, and that of pAd-AAV-9/miRNA-1 NB was 568.00 ± 37.39 nm. The particle size and concentration of pAd-AAV-9/miRNA-1 NBs did not change significantly within 1 h at room temperature (p > 0.05). pAd-AAV-9/miRNA-1 NB had the highest viral load rate of 86.3 ± 2.2% (p < 0.05), and the optimum viral load was 5 μL (p < 0.05). pAd-AAV-9/miRNA-1 NB had good contrast-enhanced ultrasound imaging in vivo. Quantitative analysis of miRNA-1 expression levels in vital organs ex vivo of SD rats by Q-PCR showed that pAd-AAV-9/miRNA-1 NB targeted the myocardial tissue. Q-PCR indicated that the expression level of miRNA-1 in the myocardium of the pAd-AAV-9/miRNA-1 NB + UTMD group was significantly higher than that of the pAd-AAV-9/miRNA-1 NB group (p < 0.05). pAd-AAV-9/miRNA-1 NB had no cytotoxic effect on cardiomyocytes (p > 0.05).

Conclusion

The pAd-AAV-9/miRNA-1 NB constructed in this study could carry naked miRNA-1 synthesized in vitro for targeted transport into myocardial tissue successfully and had sound contrast-enhanced imaging effects in vivo.

Targeted chitosan nanobubbles as a strategy to down-regulate microRNA-17 into B-cell lymphoma models

Introduction

MicroRNAs represent interesting targets for new therapies because their altered expression influences tumor development and progression. miR-17 is a prototype of onco-miRNA, known to be overexpressed in B-cell non-Hodgkin lymphoma (B-NHL) with peculiar clinic-biological features. AntagomiR molecules have been largely studied to repress the regulatory functions of up-regulated onco-miRNAs, but their clinical use is mainly limited by their rapid degradation, kidney elimination and poor cellular uptake when injected as naked oligonucleotides.

Methods

To overcome these problems, we exploited CD20 targeted chitosan nanobubbles (NBs) for a preferential and safe delivery of antagomiR17 to B-NHL cells.

Results

Positively charged 400 nm-sized nanobubbles (NBs) represent a stable and effective nanoplatform for antagomiR encapsulation and specific release into B-NHL cells. NBs rapidly accumulated in tumor microenvironment, but only those conjugated with a targeting system (antiCD20 antibodies) were internalized into B-NHL cells, releasing antagomiR17 in the cytoplasm, both in vitro and in vivo. The result is the down-regulation of miR-17 level and the reduction in tumor burden in a human-mouse B-NHL model, without any documented side effects.

Discussion

Anti-CD20 targeted NBs investigated in this study showed physico-chemical and stability properties suitable for antagomiR17 delivery in vivo and represent a useful nanoplatform to address B-cell malignancies or other cancers through the modification of their surface with specific targeting antibodies.

Ultrasound-Guided Microbubble-Mediated Locoregional Delivery of Multiple MicroRNAs Improves Chemotherapy in Hepatocellular Carcinoma

Rationale: To assess treatment effects of 4 complementary miRNAs (miRNA-100/miRNA-122/antimiRNA-10b/antimiRNA-21) encapsulated in a biodegradable PLGA-PEG nanoparticle, administered by an ultrasound-guided microbubble-mediated targeted delivery (UGMMTD) approach in mouse models of hepatocellular carcinoma (HCC). Methods:In vitro apoptotic index was measured in HepG2 and Hepa1-6 HCC cells treated with various combinations of the 4 miRNAs with doxorubicin. Three promising combinations were further tested in vivo by using UGMMTD. 63 HepG2 xenografts in mice were randomized into: group 1, miRNA-122/antimiRNA-10b/antimiRNA-21/US/doxorubicin; group 2, miRNA-100/miRNA-122/antimiRNA-10b/antimiRNA-21/US/doxorubicin; group 3, miRNA-100/miRNA-122/antimiRNA-10b/US/doxorubicin; group 4, miRNA-122/anitmiRNA-10b/antimiRNA-21/doxorubicin; group 5, miRNA-100/miRNA-122/antimiRNA-10b/antimiRNA-21/doxorubicin; group 6, miRNA-100/miRNA-122/antimiRNA-10b/doxorubicin; group 7, doxorubicin only treatment; and group 8, without any treatment. Tumor volumes were measured through 18 days. H&E staining, TUNEL assay, and qRT-PCR quantification for delivered miRNAs were performed. Results:In vivo results showed that UGMMTD of miRNAs with doxorubicin in groups 1-3 significantly (P<0.05) delayed tumor growth compared to control without any treatment, and doxorubicin only from day 7 to 18. On qRT-PCR, levels of delivered miRNAs were significantly (P<0.05) higher in groups 1-3 upon UGMMTD treatment compared to controls. TUNEL assay showed that upon UGMMTD, significantly higher levels of apoptotic cell populations were observed in groups 1-3 compared to controls. Toxicity was not observed in various organs of different groups. Conclusions: UGMMTD of miRNA-100/miRNA-122/antimiRNA-10b/antimiRNA-21 combination improved therapeutic outcome of doxorubicin chemotherapy in mouse models of HCC by substantial inhibition of tumor growth and significant increase in apoptotic index.

Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part II: Nanoacoustics for biomedical imaging and therapy

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part II of this two-part review, this paper concentrates on nanoacoustics in biomedical imaging and therapy applications, including molecular ultrasound imaging, photoacoustic imaging, ultrasound-mediated drug delivery and therapy, and photoacoustic drug delivery and therapy. Firstly, the recent developments of nanosized ultrasound and photoacoustic contrast agents as well as their various imaging applications are examined. Secondly, different types of nanomaterials/nanostructures as nanocarriers for ultrasound and photoacoustic therapies are discussed. Finally, a discussion of challenges and future research directions are provided for nanoacoustics in medical imaging and therapy.

Enhanced Vascular Permeability by Microbubbles and Ultrasound in Drug Delivery

Ultrasound and microbubbles, an ultrasound contrast agent, have recently increased attention to developing novel drug delivery systems. Ultrasound exposure can induce mechanical effects derived from microbubbles behaviors such as an expansion, contraction, and collapse depending on ultrasound conditions. These mechanical effects induce several biological effects, including enhancement of vascular permeability. For drug delivery, one promising approach is enhancing vascular permeability using ultrasound and microbubbles, resulting in improved drug transport to targeted tissues. This approach is applied to several tissues and drugs to cure diseases. This review describes the enhancement of vascular permeability by ultrasound and microbubbles and its therapeutic application, including our recent study. We also discuss the current situation of the field and its potential future perspectives.

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.

Ultrasound-microbubble cavitation facilitates adeno-associated virus mediated cochlear gene transfection across the round-window membrane

The round window of the cochlea provides an ideal route for delivering medicines and gene therapy reagents that can cross the round window membrane (RWM) into the inner ear. Recombinant adeno-associated viruses (rAAVs) have several advantages and are recommended as viral vectors for gene transfection. However, rAAVs cannot cross an intact RWM. Consequently, ultrasound-mediated microbubble (USMB) cavitation is potentially useful, because it can sonoporate the cell membranes, and increase their permeability to large molecules. The use of USMB cavitation for drug delivery across the RWM has been tested in a few animal studies but has not been used in the context of AAV-mediated gene transfection. The currently available large size of the ultrasound probe appears to be a limiting factor in the application of this method to the RWM. In this study, we used home-made ultrasound probe with a decreased diameter to 1.5 mm, which enabled the easy positioning of the probe close to the RWM. In guinea pigs, we used this probe to determine that (1) USMB cavitation caused limited damage to the outer surface layer or the RWM, (2) an eGFP-gene carrying rAAV could effectively pass the USMB-treated RWM and reliably transfect cochlear cells, and (3) the hearing function of the cochlea remained unchanged. Our results suggest that USMB cavitation of the RWM is a good method for rAAV-mediated cochlear gene transfection with clear potential for clinical translation. We additionally discuss several advantages of the small probe size.

General Characteristics of Microbubble-Adenovirus Vectors Carrying Genes

Introduction

Transferring genes safely, targeting cells and achieving efficient transfection are urgent problems in gene therapy that need to be solved. Combining microbubbles (MBs) and viruses to construct double vectors has become a promising approach for gene delivery. Understanding the characteristic performance of MBs that carry genes is key to promoting effective gene transfer. Therefore, in this study, we constructed MB-adenovirus vectors and discussed their general characteristics.

Methods

We constructed MB-adenovirus vectors carrying the chemokine (C-X-C motif) ligand 12 (Cxcl12) and bone morphogenetic protein-2 (Bmp2) genes (pAd-Cxcl12 and pAd-Bmp2, respectively) to explore the general characteristics of double vectors carrying genes.

Results

The MB-adenovirus vectors had stable physical properties, and no significant differences in diameter, concentration, or pH were noted compared with naked MBs (p > 0.05). Flow cytometry and RT-PCR were used to detect the gene-loading capacity of MBs. The gene-loading efficiency of MBs increased with increasing virus amounts and was highest (91%) when 10.0 µL of virus was added. Beyond 10.0 µL of added virus, the gene-loading efficiency of MBs decreased with the continuous addition of virus. The maximum amounts of pAd-Cxcl12 and pAd-Bmp2 in 100 µL of MBs were approximately 14 and 10 µL, respectively.

Conclusions

This study indicates that addition of an inappropriate viral load will result in low MB loading efficiency, and the maximum amount of genes loaded by MBs may differ based on the genes carried by the virus.

Effect of lipid shell composition in DSPG-based microbubbles on blood flow imaging with ultrasonography

Diagnostic ultrasound is non-invasive and provides real-time imaging. Microbubbles (MBs) are ultrasound contrast agents used to observe small blood flow, such as tumor tissue. However, MBs have short blood flow imaging time. This study developed lipid-based microbubbles (LMBs) with longer blood flow imaging time by focusing on their shell composition. Liposome research reported that addition 1,2-distearoyl-sn-glycero-3-phosphoglycerol (DSPG) to the lipid composition enhances liposome membrane stability. Therefore, we introduced DSPG at different ratios into the LMBs lipid shell. Results showed that the lipid shell composition of MBs affects stability in vivo. 60% DSPG-containing LMBs (DSPG60-LMBs) have sustained blood flow imaging time compared with LMBs, which have other DSPG ratios, Sonazoid® and SonoVue®. DSPG60-LMBs also showed less uptake into the liver compared with Sonazoid®. Therefore, DSPG60-LMBs can have long blood flow imaging time and can be effective diagnostic agents in ultrasound imaging.

Ultrasound-Aided Targeting Nanoparticles Loaded with miR-181b for Anti-Inflammatory Treatment of TNF-α-Stimulated Endothelial Cells

Gene therapy is an emerging therapeutic strategy used in clinics. Ultrasound-mediated gene transfection possesses great potential as a secure and available approach for gene delivery. However, transfection efficiency and targeting ability remain challenging. In this study, we developed a kind of ultrasound-aided and targeting nanoparticles for microRNA delivery. These nanoparticles carrying nucleic acids were prepared with cationic poly-(amino acid) encapsulated with perfluoropentane. The formulated nanoparticles were stabilized with negatively charged PGA-PEG-RGD peptide coating. Ultrasound imaging and specific gene transfection using this nanocarrier could be implemented simultaneously. Upon treatment with ultrasound irradiation, phase transition was induced in the nanoparticles and they generated acoustic cavitation, resulting in enhanced gene transfection against the endothelial cells. With the overexpression of miR-181b loaded by the nanoparticles, the TNF-α-stimulated endothelial cells were effectively rescued from the inflammatory state through the protection of cell viability and suppression of cell adhesion.

Low-frequency ultrasound-mediated cytokine transfection enhances T cell recruitment at local and distant tumor sites

Robust cytotoxic T cell infiltration has proven to be difficult to achieve in solid tumors. We set out to develop a flexible protocol to efficiently transfect tumor and stromal cells to produce immune-activating cytokines, and thus enhance T cell infiltration while debulking tumor mass. By combining ultrasound with tumor-targeted microbubbles, membrane pores are created and facilitate a controllable and local transfection. Here, we applied a substantially lower transmission frequency (250 kHz) than applied previously. The resulting microbubble oscillation was significantly enhanced, reaching an effective expansion ratio of 35 for a peak negative pressure of 500 kPa in vitro. Combining low-frequency ultrasound with tumor-targeted microbubbles and a DNA plasmid construct, 20% of tumor cells remained viable, and ∼20% of these remaining cells were transfected with a reporter gene both in vitro and in vivo. The majority of cells transfected in vivo were mucin 1+/CD45- tumor cells. Tumor and stromal cells were then transfected with plasmid DNA encoding IFN-β, producing 150 pg/106 cells in vitro, a 150-fold increase compared to no-ultrasound or no-plasmid controls and a 50-fold increase compared to treatment with targeted microbubbles and ultrasound (without IFN-β). This enhancement in secretion exceeds previously reported fourfold to fivefold increases with other in vitro treatments. Combined with intraperitoneal administration of checkpoint inhibition, a single application of IFN-β plasmid transfection reduced tumor growth in vivo and recruited efficacious immune cells at both the local and distant tumor sites.

pH- and Ultrasound-Responsive Paclitaxel-Loaded Carboxymethyl Chitosan Nanodroplets for Combined Imaging and Synergistic Chemoradiotherapy

Background

Synergistic chemoradiotherapy (CRT) has become a primary effective curative approach for many solid cancers. However, CRT is still associated with several obstacles, including the increases in side effects and systemic toxicity. Incorporating nanocarriers into CRT is a new and exciting approach to solve these obstacles. The purpose of the present study was to design a unique pH- and ultrasound-responsive perfluoropentane-encapsulated, paclitaxel (PTX)-loaded carboxymethyl chitosan nanodroplets (NDs) for combined imaging and synergistic CRT.

Materials and Methods

The NDs were prepared by a homogenization/emulsion method. Their physicochemical properties, echogenicity and biocompatibility were evaluated. PTX-loaded NDs with a high loading efficiency and encapsulation efficiency were prepared and their pH-responsive drug release profile was determined by dialysis sack method. Then, PC3 cells were exposed to (1) PTX (4 μg/mL), (2) NDs (30 μg/mL), (3) PTX-loaded NDs (34 μg/mL), (4) RT (6 Gy), (5) RT (10 Gy), (6) combination of PTX (4 μg/mL), ultrasound (0.5 W/cm², 30 s) and RT (6 Gy), (7) combination of NDs (30 μg/mL), ultrasound (0.5 W/cm², 30 s) and RT (6Gy), (8) combination of PTX-loaded NDs (30 μg/mL), ultrasound (0.5 W/cm², 30 s) and RT (6 Gy). 24 hrs later, CCK-8 assay, flow cytometry and migration assay were carried out to evaluate their therapeutic effects in CRT.

Results

The desired NDs were successfully prepared, which were with round, spherical shapes, relatively smooth surfaces, core-shell structures and uniform in sizes (<300 nm with PDI<0.3 when at pH≧6.0). The NDs exhibited good abilities in pH-dependent charge conversion, biocompatibility and ultrasound contrast echogenicity. The in vitro drug release from PTX-loaded NDs (the highest loading efficiency and encapsulation efficiency were 20.35% and 91.58%) was pH dependent and exhibited an initial burst followed by a sustained drug release. The results of the CCK-8 assay, flow cytometry and migration assay all showed PTX-loaded NDs combined ultrasound and RT significantly enhanced cell responses in CRT.

Conclusion

The pH- and ultrasound-responsive PTX-loaded NDs, which exhibited a high echogenicity, drug delivery ability and radiosensitization ability, could be a feasible option for combined imaging and novel enhancing approach in synergistic CRT.

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.

Effect of docetaxel-loaded lipid microbubble in combination with ultrasound-triggered microbubble destruction on the growth of a gastric cancer cell line

Although gastric cancer therapy has been improved, more efficient treatment strategies still need to be developed. In the present study, a docetaxel (DOC)-loaded lipid microbubble (DLLD) was prepared and the effect of DLLD combined with ultrasound-triggered microbubble destruction (UTMD) on the growth of a gastric cancer cell line was investigated. The following four groups were included in the present study: Control, DOC, DLLD and DLLD plus UTMD. The determined entrapment efficiency of DLLD is 76±3.5%. The present study demonstrated that treatment with DLLD plus UTMD could significantly inhibit the growth of the cultured gastric cancer cell line BGC-823 via arresting the cell cycle in G₂/M phase, inhibiting cell DNA synthesis, promoting cell apoptosis and disrupting mitochondrial membrane potential, as compared with treatment with DOC or DLLD alone. Furthermore, the expression of p53, p21 and Bax were identified to be significantly upregulated, while that of Bcl-2 was significantly downregulated in the DLLD plus UTMD group. Therefore, treatment with DLLD plus UTMD was more efficient in inhibiting cell proliferation and inducing cell apoptosis in the gastric cancer cell line, when compared with treatment with DOC or DLLD alone, suggesting that DLLD plus UTMD could serve as a promising strategy for the treatment of gastric cancer.

Ultrasound-Mediated Gene Therapy in Swine Livers Using Single-Element, Multi-lensed, High-Intensity Ultrasound Transducers

We have achieved significant enhancement of gene delivery into livers of large animals using ultrasound (US)-targeted microbubble (MB) destruction methods. An infusion of pGL4 (encoding a luciferase reporter gene) plasmid DNA (pDNA) and MBs into a portal-vein segmental branch of a porcine liver was exposed to US for 4 min. Therapeutic US induced cavitation of MBs to temporarily permeabilize the vascular endothelium and cell membranes, allowing entry of pDNA. We obtained a 64-fold enhancement in luciferase expression in pig livers compared to control without US using an unfocused, dual-element transducer (H105, center frequency [f_c] = 1.10 MHz) at 2.7 MPa peak negative pressure (PNP). However, input electrical energy was limited, and modified transducers were designed to have spherical (H185A, f_c = 1.10 MHz) or cylindrical foci (H185B, f_c = 1.10 MHz; H185D, f_c = 1.05 MHz) to enhance PNP output. The revised transducers required less electrical input to achieve 2.7 MPa PNP compared to H105, thereby allowing PNP outputs of up to 6.2 MPa without surpassing the piezo-material limitations. Subsequently, luciferase expression significantly improved up to 9,000-fold compared to controls with minor liver damage. These advancements will allow us to modify our current protocols toward minimally invasive US gene therapy.

Perspectives on cavitation enhanced endothelial layer permeability

Traditional drug delivery systems, where pharmaceutical agents are conveyed to the target tissue through the blood circulation, suffer of poor therapeutic efficiency and limited selectivity largely due to the low permeability of the highly specialised biological interface represented by the endothelial layer. Examples concern cancer therapeutics or degenerative disorders where drug delivery is inhibited by the blood-brain barrier (BBB). Microbubbles injected into the bloodstream undergo volume oscillations under localised ultrasound irradiation and possibly collapse near the site of interest, with no effect on the rest of the endothelium. The resulting mechanical action induces a transient increase of the inter-cellular spaces and facilitates drug extravasation. This approach, already pursed in in vivo animal models, is extremely expensive and time-consuming. On the other hand in vitro studies using different kinds of microfluidic networks are firmly established in the pharmaceutical industry for drug delivery testing. The combination of the in vitro approach with ultrasound used to control microbubbles oscillations is expected to provide crucial information for developing cavitation enhanced drug delivery protocols and for screening the properties of the biological interface in presence of healthy or diseased tissues. Purpose of the present review is providing the state of the art in this rapidly growing field where cavitation is exploited as a viable technology to transiently modify the permeability of the biological interface. After describing current in vivo studies, particular emphasis will be placed on illustrating characteristics of micro-devices, biological functionalisation, properties of the artificial endothelium and ultrasound irradiation techniques.

Cationic gas-filled microbubbles for ultrasound-based nucleic acids delivery

The use of ultrasound has gained great interest for nucleic acids delivery. Ultrasound can reach deep tissues in non-invasive manner. The process of sonoporation is based on the use of low-frequency ultrasound combined with gas-filled microbubbles (MBs) allowing an improved delivery of molecules including nucleic acids in the insonified tissue. For in vivo gene transfer, the engineering of cationic MBs is essential for creating strong electrostatic interactions between MBs and nucleic acids leading to their protection against nucleases degradation and high concentration within the target tissue. Cationic MBs must be stable enough to withstand nucleic acids interaction, have a good size distribution for in vivo administration, and enough acoustic activity to be detected by echography. This review aims to summarize the basic principles of ultrasound-based delivery and new knowledge acquired in these recent years about this method. A focus is made on gene delivery by discussing reported studies made with cationic MBs including ours. They have the ability for efficient delivery of plasmid DNA (pDNA), mRNA or siRNA. Last, we discuss about the key challenges that have to be faced for a fine use of this delivery system.

DNA-loaded microbubbles with crosslinked bovine serum albumin shells for ultrasound-promoted gene delivery and transfection

The microbubble is a kind of clinically applied ultrasound contrast agent in disease diagnosis that can also rupture under sonication to increase membrane permeability and promote gene entry into targeted cells. However, the development of ultrasound-mediated gene delivery might be restricted because genes and microbubbles were separated and would not reach the targeted cells simultaneously. Herein, a kind of crosslinked positive microbubbles (CPMBs) were prepared to load DNA as gene vectors to promote gene delivery efficiency. The BSA shell of the CPMBs was crosslinked with disulfide bonds, which obviously enhanced the stability of the CPMBs. Furthermore, the CPMBs revealed sonoporation effects comparable to those of clinically applied SonoVue microbubbles. As DNA and CPMBs were electrostatically linked as an entirety, they would reach cells simultaneously. Thus, with the aid of ultrasound, these DNA-loaded microbubbles promoted DNA entry into cytoplasm more effectively and obtained higher cellular uptake efficiency and better transfection efficiency than DNA-mixed microbubbles. Confocal microscopy results showed that rupturing of the CPMBs/DNA entire microbubbles under sonication could carry DNA directly into the cytoplasm or nucleus. All results indicated that the cytocompatible DNA-loaded microbubbles have promising prospects in ultrasound-mediated gene delivery.