A novel ultrasound microbubble carrying gene and Tat peptide: preparation and characterization

Localized Delivery of Caveolin-1 Peptide Assisted by Ultrasound-Mediated Microbubble Destruction Potentiates the Inhibition of Nitric Oxide-Dependent Vasodilation Response

In the endothelium, nitric oxide synthase (eNOS) is the enzyme that generates nitric oxide, a key molecule involved in a variety of biological functions and cancer-related events. Therefore, selective inhibition of eNOS represents an attractive therapeutic approach for NO-related diseases and anticancer therapy. Ultrasound-mediated microbubble destruction (UMMD) conjugated with cell-permeable peptides has been investigated as a drug delivery system for effective delivery of anticancer molecules. We investigated the feasibility of loading antennapedia-caveolin-1 peptide (AP-Cav), a specific eNOS inhibitor, onto microbubbles to be delivered by UMMD in rat aortic endothelium. AP-Cav-loaded microbubbles (AP-Cav-MBs) and US parameters were characterized. Aortas were treated with UMMD for 30 s with 1.3 × 10⁸ MBs/mL AP-Cav (8 μM)-MBs at 100-Hz pulse repetition frequency, 0.5-MPa acoustic pressure, 0.5 mechanical index and 10% duty cycle. NO-dependent vascular responses were assessed using an isolated organ system, 21 h post-treatment. Maximal relaxation response was inhibited 61.8% ± 1.6% in aortas treated with UMMD-AP-Cav-MBs, while in aortas treated with previously disrupted AP-Cav-MBs and then US, the inhibition was 31.6% ± 1.6%. The vascular contractile response was not affected. The impact of UMMD was evaluated in aortas treated with free AP-Cav; 30 μM of free AP-Cav was necessary to reach an inhibition response similar to that obtained with UMMD-AP-Cav-MBs. In conclusion, UMMD enhances the delivery and potentiates the effect of AP-Cav in the endothelial layer of rat aorta segments.

Cell-Penetrating Peptides Delivering siRNAs: An Overview

Cell-Penetrating Peptides (CPP) are valuable tools capable of crossing the plasma membrane to deliver therapeutic cargo inside cells. Small interfering RNAs (siRNA) are double-stranded RNA molecules capable of silencing the expression of a specific protein triggering the RNA interference (RNAi) pathway, but they are unable to cross the plasma membrane and have a short half-life in the bloodstream. In this overview, we assessed the many different approaches used and developed in the last two decades to deliver siRNA through the plasma membrane through different CPPs sorted according to three different loading strategies: covalent conjugation, complex formation, and CPP-decorated (functionalized) nanocomplexes. Each of these strategies has pros and cons, but it appears the latter two are the most commonly reported and emerging as the most promising strategies due to their simplicity of synthesis, use, and versatility. Recent progress with siRNA delivered by CPPs seems to focus on targeted delivery to reduce side effects and amount of drugs used, and it appears to be among the most promising use for CPPs in future clinical applications.

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.

Construction of CNA35 Collagen-Targeted Phase-Changeable Nanoagents for Low-Intensity Focused Ultrasound-Triggered Ultrasound Molecular Imaging of Myocardial Fibrosis in Rabbits

Myocardial fibrosis plays an important role in the development of heart failure and malignant arrhythmia, which potentially increases the incidence of sudden cardiac death. Therefore, early detection of myocardial fibrosis is of great significance for evaluating the prognosis of patients and formulating appropriate treatment strategies. Late gadolinium-enhanced magnetic resonance imaging is considered as the currently effective strategy for noninvasive detection of myocardial fibrosis, but it still suffers from some critical issues. In this work, multifunctional CNA35-labeled perfluoropentane nanoparticles (CNA35-PFP NPs) have been elaborately designed and constructed for molecular imaging of fibrotic myocardium based on ultrasound imaging. These as-constructed CNA35-PFP NPs are intravenously infused into rabbit circulation with an animal model of myocardial infarction. Especially, these targeted CNA35-PFP NPs with nanoscale size could efficiently pass through the endothelial cell gap and adhere to the surface of fibroblasts in the fibrotic myocardium. Importantly, followed by low-intensity focused ultrasound irradiation on the myocardium, these intriguing CNA35-PFP NPs could transform from liquid into gaseous microbubbles, which further significantly enhanced the ultrasound contrast in the fibrotic area, facilitating the detection by diagnostic ultrasound imaging. Therefore, this work provides a desirable noninvasive, economical, and real-time imaging technique for the assessment of cardiac fibrosis with diagnostic ultrasound based on the rational design of liquid-to-gas phase-changeable nanoplatforms.

Tumour targeted contrast enhanced ultrasound imaging dual-modal microbubbles for diagnosis and treatment of triple negative breast cancer

At present, the treatment of triple negative breast cancer (TNBC) is a worldwide problem, urgently requiring early precise diagnosis and effective treatment methods. In our study, we designed a type of tumour targeted dual-modal microbubbles, paclitaxel (PTX)-loaded RGD-lipid microbubbles (PTX@RGD-MBs), combined with ultrasonic targeted microbubble destruction (UTMD) to precisely diagnose TNBC and to improve the curative effect. As the first-line drug, PTX, lacking specific tumour targeting and the ability to be effectively internalized by TNBC cells, is still not effective in killing TNBC cells. For this reason, we used the tumour active targeting peptide RGD to precisely guide MBs around TNBC cells through the high affinity to the integrin α_vβ₃ receptor, and use UTMD to generate shear stress on cells to open transient pores in the cellular membrane, so as to improve MB penetration into the cellular membrane. Sulfur hexafluoride (SF₆) as the internal gas of MBs can greatly improve the resolution and sensitivity of conventional US images through nonlinear harmonics to enhance the comparison between lesions and surrounding normal tissues. The results in our study demonstrated that RGD-MBs with UTMD were internalized by TNBC cells more effectively, leading to significant increase in intercellular drug concentrations of TNBC cells, thus achieving the best inhibitory effect on TNBC cells in vitro. Ultrasonic experiment showed that PTX@RGD-MBs produced high quality contrast enhanced ultrasound (CEUS) images in vitro and in vivo, providing a better method for diagnosis and evaluation of the TNBC. Therefore, we conclude the advantages of excellent anti-tumour effect and CEUS imaging of PTX@RGD-MBs provided a better application for diagnosis and treatment of TNBC.

Tumour targeted contrast-enhanced ultrasound imaging dual-modal microbubbles (PTX@RGD-MBs) combined with UTMD for diagnosing and treating triple negative breast cancer.

Low-intensity focused ultrasound (LIFU)-activated nanodroplets as a theranostic agent for noninvasive cancer molecular imaging and drug delivery

Theranostics is a new trend in the tumor research field, which involves the integration of diagnostic and therapeutic functions using imageable nanoparticles coupled with therapeutic drugs. Imaging-guided targeted delivery of therapeutics and diagnostics using nanocarriers hold great promise to minimize the side effects of conventional chemotherapy. Ultrasound microbubbles have been employed as theranostic agents over the last decade, which provide both real-time dynamic imaging for diagnosis and precise control for targeted tumor therapy. However, the intrinsic defects of microbubbles such as poor tissue penetration, short circulation time and instability hinder microbubble-based theranostic applications. In recent years, liquid-to-gas transitional perfluorocarbon nanoparticles have been developed as promising diagnostic and therapeutic nanoagents to solve the abovementioned problems. In this study, phase-changeable, folate-targeted perfluoropentane nanodroplets loaded with 10-hydroxycamptothecin (HCPT) and superparamagnetic Fe3O4 (denoted as FA-HCPT-Fe3O4-PFP NDs) are prepared and investigated for multimodal tumor imaging and targeted therapy. After intravenous administration into nude mice bearing SKOV3 ovarian cancer, FA-HCPT-Fe3O4-PFP NDs exhibit the ability to enhance MR and PA imaging. Furthermore, after the phase transition activated by low-intensity focused ultrasound (LIFU) sonication, FA-HCPT-Fe3O4-PFP NDs remarkably enhance US imaging at the tumor location. Meanwhile, the HCPT released from FA-HCPT-Fe3O4-PFP NDs during the liquid-to-gas transition provides a therapeutic effect on tumor cells with relatively low side effects to normal tissue. Therefore, the combination of LIFU and FA-HCPT-Fe3O4-PFPNDs presents an ideal modality for tumor-targeted theranostics.

Cell-penetrating Peptide-modified Targeted Drug-loaded Phase-transformation Lipid Nanoparticles Combined with Low-intensity Focused Ultrasound for Precision Theranostics against Hepatocellular Carcinoma

Objective: Prepare a multifunctional ultrasound molecular probe, hyaluronic acid-mediated cell-penetrating peptide-modified 10-hydroxycamptothecin-loaded phase-transformation lipid nanoparticles (HA/CPPs-10-HCPT-NPs), and to combine HA/CPPs-10-HCPT-NPs with low-intensity focused ultrasound (LIFU) for precision theranostics against hepatocellular carcinoma (HCC).

Methods: HA/CPPs-10-HCPT-NPs were prepared using thin-film dispersion, ultrasound emulsification, and electrostatic effects. HA/CPPs-10-HCPT-NPs were characterized for particle size, zeta potential, encapsulation efficiency and drug-loading efficiency. In vitro, HA/CPPs-10-HCPT-NPs were tested for acoustic droplet vaporization (ADV) at different time points/acoustic intensities; the ability of HA/CPPs-10-HCPT-NPs to target SMMC-7721 cells was detected by confocal laser scanning microscopy (CLSM); the penetrating ability of CG-TAT-GC-modified NPs was verified by CLSM in a 3D multicellular tumor spheroid (MCTS) model; the effect of HA/CPPs-10-HCPT-NPs combined with LIFU on killing SMMC-7721 cells was measured by CCK-8 and flow cytometry. In vivo, the tumor-target efficiency of HA/CPPs-10-HCPT-NPs was evaluated by a small-animal fluorescence imaging system and CLSM; the enhanced ultrasound imaging efficiency of HA/CPPs-10-HCPT-NPs combined with LIFU was measured by an ultrasound imaging analyzer; the therapeutic effect of HA/CPPs-10-HCPT-NPs combined with LIFU was evaluated by tumor volume, tumor inhibition rate, and staining (hematoxylin and eosin (H & E), proliferating cell nuclear antigen (PCNA) and TUNEL).

Results: Mean particle size and mean zeta potential of HA/CPPs-10-HCPT-NPs were 284.2±13.3 nm and - 16.55±1.50 mV, respectively. HA/CPPs-10-HCPT-NPs could bind to SMMC-7721 cells more readily than CPPs-10-HCPT-NPs. Penetration depth into 3D MCTS of HA/CPPs-10-HCPT-NPs was 2.76-fold larger than that of NPs without CG-TAT-GC. HA/CPPs-10-HCPT-NPs could enhance ultrasound imaging by undergoing ADV triggered by LIFU. HA/CPPs-10-HCPT-NPs+LIFU group demonstrated significantly higher efficiency of anti-proliferation and apoptosis percentage than all other groups. In mouse liver tumor xenografts, HA/CPPs-10-HCPT-NPs could target tumor sites and enhance ultrasound imaging under LIFU. HA/CPPs-10-HCPT-NPs+LIFU group had a significantly smaller tumor volume, lower proliferative index (PI), and higher tumor inhibition and apoptotic index (AI) than all other groups.

Conclusions: Combined application of HA/CPPs-10-HCPT-NPs and LIFU should be a valuable and promising strategy for precise HCC theranostics.

Lipid Microbubbles as Ultrasound-Stimulated Oxygen Carriers for Controllable Oxygen Release for Tumor Reoxygenation

Microbubbles are proposed as a potentially novel method for oxygen delivery in vivo in initial studies. The lack of commercial microbubbles for oxygen delivery in preclinical research prompted us to fabricate an oxygen-loaded lipid microbubble. We aimed to extend the innovative strategy to modulate the tumor hypoxic microenvironment, using microbubbles intravenously as an oxygen carrier for the controllable tumor-specific delivery of oxygen by ultrasound (US). In our experiment, an oxygen-loaded lipid-coated microbubble (OLM) with mixed gas (O₂/C₃ F₈, 5:1 v/v) was fabricated and exhibited a higher rate of oxygen release to a desaturated solution through burst by US than that in the absence of US. Although in in vivo studies, OLMs could be imaged and triggered by US to elevate the pO₂ level in the breast VX2 tumor dramatically within a matter of minutes. The added presence of US-activated OLMs elicited a nearly six-fold increase in pO₂ levels within 1 min compared with that of the pre-injection. Owing to the high oxygen payload, great acoustic stability and acoustic properties, OLMs may be proposed as an ideal radio-sensitizer. We conclude that oxygen release mediated by ultrasound-targeted microbubble destruction is feasible and shows potential in image-guided, site-specific cancer radiotherapy.

Low-intensity focused ultrasound (LIFU)-induced acoustic droplet vaporization in phase-transition perfluoropentane nanodroplets modified by folate for ultrasound molecular imaging

The commonly used ultrasound (US) molecular probes, such as targeted microbubbles and perfluorocarbon emulsions, present a number of inherent problems including the conflict between US visualization and particle penetration. This study describes the successful fabrication of phase changeable folate-targeted perfluoropentane nanodroplets (termed FA-NDs), a novel US molecular probe for tumor molecular imaging with US. Notably, these FA-NDs can be triggered by low-intensity focused US (LIFU) sonication, providing excellent US enhancement in B-mode and contrast-enhanced US mode in vitro. After intravenous administration into nude mice bearing SKOV3 ovarian carcinomas, 1,1′-dioctadecyl-3,3,3′,3′ -tetramethylindotricarbocya-nine iodide-labeled FA-NDs were found to accumulate in the tumor region. FA-NDs injection followed by LIFU sonication exhibited remarkable US contrast enhancement in the tumor region. In conclusion, combining our elaborately developed FA-NDs with LIFU sonication provides a potential protocol for US molecular imaging in folate receptor-overexpressing tumors.

Efficient siRNA Delivery Using Novel Cell-Penetrating Peptide-siRNA Conjugate-Loaded Nanobubbles and Ultrasound

Because of the absence of tolerable and effective carriers for in vivo delivery, the applications of small interfering RNA (siRNA) in the clinic for therapeutic purposes have been limited. In this study, development of a novel siRNA delivery system based on ultrasound-sensitive nanobubbles (NBs, nano-sized echogenic liposomes) and cell-permeable peptides (CPPs) is described. A CPP-siRNA conjugate was entrapped in an NB, (CPP-siRNA)-NB, and the penetration of CPP-siRNA was temporally masked; local ultrasound stimulation triggered the release of CPP-siRNA from the NBs and activated its penetration. Subsequent research revealed that the (CPP-siRNA)-NBs had a mean particle size of 201 ± 2.05 nm and a siRNA entrapment efficiency >85%. In vitro release results indicated that >90% of the encapsulated CPP-siRNA was released from NBs in the presence of ultrasound, whereas <1.5% (30 min) was released in the absence of ultrasound. Cell experiments indicated higher cellular CPP-siRNA uptake of (CPP-siRNA)-NBs with ultrasound among the various formulations in human breast adenocarcinoma cells (HT-1080). Additionally, after systemic administration in mice, (CPP-siRNA)-NBs accumulated in the tumor, augmented c-myc silencing and delayed tumor progression. In conclusion, the application of (CPP-siRNA)-NBs with ultrasound may constitute an approach to selective targeted delivery of siRNA.

Integration of surface-active, periodically sequenced peptides into lipid-based microbubbles

The development of microbubbles toward functional, "theranostic" particles requires the incorporation of constituents with high binding specificity and therapeutic efficacy. Integrating peptides or proteins into the shell of lipid-based microbubbles can provide a means to access both receptor-ligand interactions and therapeutic properties. Simultaneously, peptides or proteins can define the characteristic monolayer mechanics of lipid bubbles and eliminate the need for post-bubble generation modification. The ability to engineer peptide sequences de novo that effectively partition into the bubble monolayer remains parametrically daunting. This work contributes to this effort using two simple amphipathic helical peptides that examine the role of local electrostatics and secondary structure. The two periodically sequenced peptides both have three positive charges, but peptide "K-2.5" spaces those charges 2.5 amino acids apart, while peptide "K-6.0" spaces the charges six amino acids apart. Size populations were determined for bubbles containing each peptide species using light scattering, and a quantitative method was developed to clearly define the fraction of peptides binding onto the microbubble monolayer. The impact of both the initial peptide concentration and the zwitterionic:anionic lipid ratio on peptide binding was also evaluated. Our results indicate that the lipid ratio affected only K-6.0 binding, which appears to be an outcome of the greater ensemble average α-helical population of the K-6.0. These findings provide further insights into the role of charge separation on peptide secondary structure, establishing a simple design metric for peptide binding onto microbubble systems.

A targeted ultrasound contrast agent carrying gene and cell-penetrating peptide: preparation and gene transfection in vitro

Targeted and high efficient gene delivery is a main issue in gene treatment. Taking advantage of ischemic memory target P-selectin and our previous study-synergistic effects of ultrasound-targeted microbubble destruction (UTMD) and TAT peptide on gene transfection, which were characterized by targeted aggregation and high efficient gene transfection, we set up a 'smart' gene delivery system-targeted ultrasound contrast agent (UCA) carrying gene and cell-permeable peptides (CPP). Such UCA had a strong binding force with DNA which was protected from being hydrolysed by nuclease. Moreover, synergistic effects of UTMD and TAT peptide increased gene transfection. Specifically, the UCA were reacted with an ischemic memory target P-selectin overexpressed by ischemic issues (including ischemic heart disease) and loaded with gene and CPP, which enabled targeted localization and gene delivery to ischemic cells overexpressing P-selectin. We demonstrated their targeting affinity for hypoxia human umbilical vein endothelial cell (HUVEC) and gene transfection in vitro. The results of confocal laser scanning microscopy (CLSM) showed that gene and CPP were distributed on the shell of UCA. Red fluorescence was observed on the surface of targeted UCA using immunofluorescent microscopy, which demonstrated that the antibody was successfully connected to the UCA. The targeted UCA was specifically and tightly binded to hypoxia HUVEC, while there were no or little non-targeted UCA binding around hypoxia HUVEC. 24h after transfection, gene transfection efficiency detected by FCM was higher in targeted group than non-targeted group. Overall, the targeted UCA carrying gene and CPP was prepared successfully. It had a strong target binding capacity to hypoxia HUVEC and high efficient gene transfection, which maybe provide a novel strategy for gene therapy.

Acoustic behavior of microbubbles and implications for drug delivery

Ultrasound contrast agents are valuable in diagnostic ultrasound imaging, and they increasingly show potential for drug delivery. This review focuses on the acoustic behavior of flexible-coated microbubbles and rigid-coated microcapsules and their contribution to enhanced drug delivery. Phenomena relevant to drug delivery, such as non-spherical oscillations, shear stress, microstreaming, and jetting will be reviewed from both a theoretical and experimental perspective. Further, the two systems for drug delivery, co-administration and the microbubble as drug carrier system, are reviewed in relation to the microbubble behavior. Finally, future prospects are discussed that need to be addressed for ultrasound contrast agents to move from a pre-clinical tool into a clinical setting.

Manufacture of concentrated, lipid-based oxygen microbubble emulsions by high shear homogenization and serial concentration

Gas-filled microbubbles have been developed as ultrasound contrast and drug delivery agents. Microbubbles can be produced by processing surfactants using sonication, mechanical agitation, microfluidic devices, or homogenization. Recently, lipid-based oxygen microbubbles (LOMs) have been designed to deliver oxygen intravenously during medical emergencies, reversing life-threatening hypoxemia, and preventing subsequent organ injury, cardiac arrest, and death. We present methods for scaled-up production of highly oxygenated microbubbles using a closed-loop high-shear homogenizer. The process can produce 2 L of concentrated LOMs (90% by volume) in 90 min. Resulting bubbles have a mean diameter of ~2 μm, and a rheologic profile consistent with that of blood when diluted to 60 volume %. This technique produces LOMs in high capacity and with high oxygen purity, suggesting that this technique may be useful for translational research labs.

Methotrexate-loaded PLGA nanobubbles for ultrasound imaging and Synergistic Targeted therapy of residual tumor during HIFU ablation

High intensity focused ultrasound (HIFU) has attracted the great attention in tumor ablation due to its non-invasive, efficient and economic features. However, HIFU ablation has its intrinsic limitations for removing the residual tumor cells, thus the tumor recurrence and metastasis cannot be avoided in this case. Herein, we developed a multifunctional targeted poly(lactic-co-glycolic acid) (PLGA) nanobubbles (NBs), which not only function as an efficient ultrasound contrast agent for tumor imaging, but also a targeted anticancer drug carrier and excellent synergistic agent for enhancing the therapeutic efficiency of HIFU ablation. Methotrexate (MTX)-loaded NBs were synthesized and filled with perfluorocarbon gas subsequently using a facile but general double emulsion evaporation method. The active tumor-targeting monoclonal anti-HLA-G antibodies (mAbHLA-G) were further conjugated onto the surface of nanobubbles. The mAbHLA-G/MTX/PLGA NBs could enhance the ultrasound imaging both in vitro and in vivo, and the targeting efficiency to HLA-G overexpressing JEG-3 cells has been demonstrated. The elaborately designed mAbHLA-G/MTX/PLGA NBs can specifically target to the tumor cells both in vitro and in vivo, and their blood circulation time in vivo was much longer than non-targeted MTX/PLGA NBs. Further therapeutic evaluations showed that the targeted NBs as a synergistic agent can significantly improve the efficiency of HIFU ablation by changing the acoustic environment, and the focused ultrasound can promote the on-demand MTX release both in vitro and in vivo. The in vivo histopathology test and immunohistochemical analysis showed that the mAbHLA-G/MTX/PLGA NBs plus HIFU group presented most serious coagulative necrosis, the lowest proliferation index and the highest apoptotic index. Therefore, the successful introduction of targeted mAbHLA-G/MTX/PLGA NBs provides an excellent platform for the highly efficient, imaging-guided and non-invasive HIFU synergistic therapy of cancer with the supplementary functions of killing residual tumor cells and preventing tumor recurrence/metastasis.

Micro- and nanobubbles: a versatile non-viral platform for gene delivery

Micro- and nanobubbles provide a promising non-viral strategy for ultrasound mediated gene delivery. Microbubbles are spherical gas-filled structures with a mean diameter of 1-8 μm, characterised by their core-shell composition and their ability to circulate in the bloodstream following intravenous injection. They undergo volumetric oscillations or acoustic cavitation when insonified by ultrasound and, most importantly, they are able to resonate at diagnostic frequencies. It is due to this behaviour that microbubbles are currently being used as ultrasound contrast agents, but their use in therapeutics is still under investigation. For example, microbubbles could play a role in enhancing gene delivery to cells: when combined with clinical ultrasound exposure, microbubbles are able to favour gene entry into cells by cavitation. Two different delivery strategies have been used to date: DNA can be co-administered with the microbubbles (i.e. the contrast agent) or 'loaded' in purposed-built bubble systems - indeed a number of different technological approaches have been proposed to associate genes within microbubble structures. Nanobubbles, bubbles with sizes in the nanometre order of magnitude, have also been developed with the aim of obtaining more efficient gene delivery systems. Their small sizes allow the possibility of extravasation from blood vessels into the surrounding tissues and ultrasound-targeted site-specific release with minimal invasiveness. In contrast, microbubbles, due to their larger sizes, are unable to extravasate, thus and their targeting capacity is limited to specific antigens present within the vascular lumen. This review provides an overview of the use of microbubbles as gene delivery systems, with a specific focus on recent research into the development of nanosystems. In particular, ultrasound delivery mechanisms, formulation parameters, gene-loading approaches and the advantages of nanometric systems will be described.

Synergistic effects of ultrasound-targeted microbubble destruction and TAT peptide on gene transfection: an experimental study in vitro and in vivo

Cell-permeable peptides (CPPs) and ultrasound-targeted microbubble destruction (UTMD) have tremendous potential for gene delivery. However, their applications are limited due to nonspecificity of CPPs and low transfection efficiency of UTMD. Here, we developed a 'smart' gene delivery system by encapsulating TAT peptide (TATp) and hepatocyte growth factor (HGF) gene within lipid microbubbles, in which TATp was protected from being enzymatically cleaved and HGF gene was protected from degradation. This new strategy had synergistic effects of UTMD and TATp on gene transfection. We investigated the efficacy and safety of HGF gene transfection mediated by the combination of UTMD and TATp in vitro and in vivo. The results from MTT assay and flow cytometry analyses indicated that the combination of UTMD and TATp could enhance HGF gene expression in HUVECs without any significant side effect on cell viability. In rat myocardial infarction models, we demonstrated that the protein and mRNA expressions of HGF in myocardium caused by the combination of UTMD and TATp were the highest. Histopathological findings demonstrated that the combination of UTMD and TATp enhanced myocardial microvasculature and ameliorated myocardial fibrosis. In conclusion, the combination of UTMD and TATp might be a safe and efficient technique for gene delivery.

Doxorubicin loaded superparamagnetic PLGA-iron oxide multifunctional microbubbles for dual-mode US/MR imaging and therapy of metastasis in lymph nodes

Current strategies for tumor-induced sentinel lymph node detection and metastasis therapy have limitations. In this work, we co-encapsulated iron oxide nanoparticles and chemotherapeutic drug into poly(lactic-co-glycolic acid) (PLGA) microbubbles to form multifunctional polymer microbubbles (MPMBs) for both tumor lymph node imaging and therapy. Fe(3)O(4) nanoparticles and doxorubicin (DOX) co-encapsulated PLGA microbubbles were prepared and filled with perfluorocarbon gas. Enhancement of ultrasound (US)/magnetic resonance (MR) imaging and US triggered drug delivery were evaluated both in vitro and in vivo. The MPMBs exhibited characters like narrow size distribution and smooth surface with a mean diameter of 868.0 ± 68.73 nm. In addition, varying the concentration of Fe(3)O(4) nanoparticles in the bubbles did not significantly influence the DOX encapsulation efficiency or drug loading efficiency. Our in vitro results demonstrated that these MPMBs could enhance both US and MR imaging which was further validated in vivo showing that these MPMBs enhanced tumor lymph nodes signals. The anti-tumor effect of MPMBs mediated chemotherapy was assessed in vivo using end markers like tumor proliferation index, micro blood vessel density and micro lymphatic vessel density, which were shown consistently the lowest after the MPMBs plus sonication treatment compared to controls. In line with these findings, the tumor cell apoptotic index was found the largest after the MPMBs plus sonication treatment. In conclusion, we have successfully developed a doxorubicin loaded superparamagnetic PLGA-Iron Oxide multifunctional theranostic agent for dual-mode US/MR Imaging of lymph node, and for low frequency US triggered therapy of metastasis in lymph nodes, which might provide a strategy for the imaging and chemotherapy of primary tumor and their metastases.

Effective gene transfer into central nervous system following ultrasound-microbubbles-induced opening of the blood-brain barrier

To investigate whether ultrasound-targeted microbubble destruction (UTMD) could transfer gene into central nervous system (CNS) following blood-brain barrier disruption (BBBD), DNA-loaded microbubbles were infused into the mice intravenously following ultrasonic exposure. Opening of the BBB, changes of mRNA and expression of enhanced green fluorescent protein (EGFP), and safety evaluation were measured. By UTMD, EGFP were substantially expressed in the cytoplasm of the neurons at the sonicated area with minor erythrocytes extravasation and the mRNA and expression of EGFP were markedly enhanced by about 15-fold and 10-fold, respectively, than that with US alone (p < 0.01). No EGFP was detected in the mice treated with DNA-loaded microbubbles or plasmid alone. The gene expression reached a climax at 48 h, gradually reduced to a much lower level thereafter. These results demonstrated UTMD could effectively enhance exogenous gene delivery and expression in CNS following BBBD, and this technique may provide a new method for CNS gene therapy.

Hematoporphyrin encapsulated PLGA microbubble for contrast enhanced ultrasound imaging and sonodynamic therapy

The purpose of this study was to develop a sonosensitizer-loaded multi-functional ultrasound (US) contrast agent for both tumour therapy and imaging. The hematoporphyrin (HP)-encapsulated poly(lactic-co-glycolic acid) microbubbles (HP-PLGA-MBs) were prepared and filled with perfluorocarbon gases. The enhancement of US imaging and its sonodynamically induced anti-tumour effect were evaluated by both in vitro and in vivo experiments. The HP-PLGA-MBs have a narrow size distribution and smooth surface with a mean diameter of 702.6 ± 56.8 nm and HP encapsulation efficiency of 63.50 ± 1.26% and drug-loading efficiency of 2.15 ± 0.13%. The HP-PLGA-MBs could well enhance the ultrasound imaging both in vitro and in vivo. A significant anti-tumour effect was obtained by HP-PLGA-MBs mediated sonodynamic therapy. The tumour growth rate and the tumour proliferation index were the lowest in the HP-PLGA-MBs plus sonication group. And the tumour cell apoptotic index was the biggest in the HP-PLGA-MBs plus sonication group. In conclusion, a sonosensitizer-loaded multi-functional contrast agent was constructed and the feasibility was demonstrated, which might provide a novel strategy for tumour imaging and therapy.

Ultrasound-triggered microbubble destruction in combination with cationic lipid microbubbles enhances gene delivery

This study aimed to examine the preparation of cationic lipid microbubble (CLM), and evaluate its physical and chemical properties and toxicity, measure the gene transfection efficiency by ultrasound triggered microbobble destruction (UTMD) in combination with CLM. The CLM was prepared by the method of the thin film hydration, and its morphology was observed under the electron microscopy at 1st, 3rd, 7th, 10th, and 14th day after preparation, respectively. The size, Zeta potential and stability of CLM were tested. The acute toxicity of CLM was assessed. The green fluorescent protein gene (EGFP) transfection efficiency was evaluated. The experiment grouping was as follows: naked plasmid group (P group), ultrasonic irradiation plus naked plasmid group (P-US group), naked plasmid plus CLM group (P-CLM group), naked plasmid plus ultrasound and CLM group (UTMD group). The expression of EGFP was detected by fluorescent microscopy and flow cytometry. The results showed that CLMs were spherical in shape, with the similar size and good distribution degree under the light and electron microscopies. The size of CLMs was varied from 250.4±88.3 to 399.0±99.8 nm and the Zeta potential of CLMs from 18.80±4.97 to 20.1±3.1 mV. The EGFP expression was the strongest in the UTMD group, followed by the P-CLM group, P-US group and P group. Flow cytometry results were consistent with those of fluorescent microscopy. The transfection efficiency was substantially increased in the P-US group, P-CLM group and UTMD group as compared with that in the P group, almost 7 times, 10 times and 30 times higher than that in the P group respectively. It is suggested that CLMs prepared by the method of thin film hydration are uniform in diameter, and proved non-toxic. UTMD combined with CLM can significantly increase the transfection efficiency of EGFP to targeted cells.

Delivery of water-soluble drugs using acoustically triggered perfluorocarbon double emulsions

PURPOSE

Ultrasound can be used to release a therapeutic payload encapsulated within a perfluorocarbon (PFC) emulsion via acoustic droplet vaporization (ADV), a process whereby the PFC phase is vaporized and the agent is released. ADV-generated microbubbles have been previously used to selectively occlude blood vessels in vivo. The coupling of ADV-generated drug delivery and occlusion has therapeutically synergistic potentials.

METHODS

Micron-sized, water-in-PFC-in-water (W(1)/PFC/W(2)) emulsions were prepared in a two-step process using perfluoropentane (PFP) or perfluorohexane (PFH) as the PFC phase. Fluorescein or thrombin was contained in the W(1) phase.

RESULTS

Double emulsions containing fluorescein in the W(1) phase displayed a 5.7±1.4-fold and 8.2±1.3-fold increase in fluorescein mass flux, as measured using a Franz diffusion cell, after ADV for the PFP and PFH emulsions, respectively. Thrombin was stably retained in four out of five double emulsions. For three out of five formulations tested, the clotting time of whole blood decreased, in a statistically significant manner (p < 0.01), when incubated with thrombin-loaded emulsions exposed to ultrasound compared to emulsions not exposed to ultrasound.

CONCLUSIONS

ADV can be used to spatially and temporally control the delivery of water-soluble compounds formulated in PFC double emulsions. Thrombin release could extend the duration of ADV-generated, microbubble occlusions.

Ultrasound microbubbles enhance recombinant adeno-associated virus vector delivery to retinal ganglion cells in vivo

RATIONALE AND OBJECTIVES

The aim of this study was to investigate whether ultrasound-mediated microbubble destruction enhances the transduction efficiency of recombinant adeno-associated virus (rAAV)-mediated enhanced green fluorescent protein (EGFP) gene into retinal ganglion cells (RGCs) of rats and whether it causes relevant adverse effects.

MATERIALS AND METHODS

Thirty-two adult Sprague-Dawley rats were divided into four groups with different ultrasound powers, and retinal flat mounts and hematoxylin and eosin staining sections were made for optimization of parameters. A further 70 adult Sprague-Dawley rats were divided into four groups randomly. The first group (group A) was used as a normal control with 10 rats, and the remaining rats were evenly divided into groups B, C, and D. Each group included 20 rats. Groups B and C received rAAV-encoding EGFP gene (rAAV(2)-EGFP) in phosphate-buffered saline without and with ultrasound to the retina, respectively. Group D received microbubbles and rAAV(2)-EGFP mixture and ultrasound to the retina. The injection approach was intravitreal injection for all eyes. After 21 days, RGCs were labeled retrogradely with Fluoro-Gold. After 28 days, retinal flat mounts, frozen sections, and pathologic sections were assessed in each group. Expression of EGFP reporter gene was observed on laser confocal microscopy and evaluated according to average optical density and transfected RGC rate. To evaluate adverse effects with retinal flat mounts, labeled RGCs were counted, and retinal pathologic sections were examined.

RESULTS

When ultrasound parameters (frequency, 0.3 MHz; power, 0.5 W/cm(2); total time, 60 seconds [irradiation time, 5 seconds; interval time, 10 seconds; four times]) were selected, EGFP expression was stronger, and retinas were not damaged. In the second part of the experiment, RGCs were labeled with Fluoro-Gold successfully. Green fluorescence can be observed in labeled RGCs in groups B to D. While average optical density and transfected RGC rate in group D were the highest compared to the other groups, no significant reduction in RGC number was detected with retrograde labeling. No obvious damage was observed with pathologic sections.

CONCLUSIONS

Ultrasound-mediated microbubble destruction can effectively and safely enhance rAAV delivery to RGCs in rats, and it may represent a novel gene delivery method in gene therapy for glaucomatous optic neuroprotection.