Stably engineered nanobubbles and ultrasound - An effective platform for enhanced macromolecular delivery to representative cells of the retina

Recent Advances in Nanomedicine for Ocular Fundus Neovascularization Disease Management

As an indispensable part of the human sensory system, visual acuity may be impaired and even develop into irreversible blindness due to various ocular pathologies. Among ocular diseases, fundus neovascularization diseases (FNDs) are prominent etiologies of visual impairment worldwide. Intravitreal injection of anti-vascular endothelial growth factor drugs remains the primary therapy but is hurdled by common complications and incomplete potency. To renovate the current therapeutic modalities, nanomedicine emerged as the times required, which is endowed with advanced capabilities, able to fulfill the effective ocular fundus drug delivery and achieve precise drug release control, thus further improving the therapeutic effect. This review provides a comprehensive summary of advances in nanomedicine for FND management from state-of-the-art studies. First, the current therapeutic modalities for FNDs are thoroughly introduced, focusing on the key challenges of ocular fundus drug delivery. Second, nanocarriers are comprehensively reviewed for ocular posterior drug delivery based on the nanostructures: polymer-based nanocarriers, lipid-based nanocarriers, and inorganic nanoparticles. Thirdly, the characteristics of the fundus microenvironment, their pathological changes during FNDs, and corresponding strategies for constructing smart nanocarriers are elaborated. Furthermore, the challenges and prospects of nanomedicine for FND management are thoroughly discussed.

Luminal Delivery of Pectin-Modified Oxygen Microbubbles Mitigates Rodent Experimental Intestinal Ischemia

INTRODUCTION

Ischemic gut injury is common in the intensive care unit, impairs gut barrier function, and contributes to multiorgan dysfunction. One novel intervention to mitigate ischemic gut injury is the direct luminal delivery of oxygen microbubbles (OMB). Formulations of OMB can be modified to control the rate of oxygen delivery. This project examined whether luminal delivery of pectin-modified OMB (OMBp5) can reduce ischemic gut injury in a rodent model.

METHODS

The OMBp5 formulation was adapted to improve delivery of oxygen along the length of small intestine. Adult Sprague-Dawley rats (n = 24) were randomly allocated to three groups: sham-surgery (SS), intestinal ischemia (II), and intestinal ischemia plus luminal delivery of OMBp5 (II + O). Ischemia-reperfusion injury was induced by superior mesenteric artery occlusion for 45 min followed by reperfusion for 30 min. Outcome data included macroscopic score of mucosal injury, the histological score of gut injury, and plasma biomarkers of intestinal injury.

RESULTS

Macroscopic, microscopic data, and intestinal injury biomarker results demonstrated minimal intestinal damage in the SS group and constant damage in the II group. II + O group had a significantly improved macroscopic score throughout the gut mucosa (P = 0.04) than the II. The mean histological score of gut injury for the II + O group was significantly improved on the II group (P ≤ 0.01) in the proximal intestine only, within 30 cm of delivery. No differences were observed in plasma biomarkers of intestinal injury following OMBp5 treatment.

CONCLUSIONS

This proof-of-concept study has demonstrated that luminal OMBp5 decreases ischemic injury to the proximal small intestine. There is a need to improve oxygen delivery over the full length of the intestine. These findings support further studies with clinically relevant end points, such as systemic inflammation and vital organ dysfunction.

Nanomaterial-based ophthalmic drug delivery

The low bioavailability and side effects of conventional drugs for eye disease necessitate the development of efficient drug delivery systems. Accompanying the developments of nanofabrication techniques, nanomaterials have been recognized as promising tools to overcome these challenges due to their flexible and programmable properties. Given the advances achieved in material science, a broad spectrum of functional nanomaterials capable of overcoming various ocular anterior and posterior segment barriers have been explored to satisfy the demands for ocular drug delivery. In this review, we first highlight the unique functions of nanomaterials suitable for carrying and transporting ocular drugs. Then, various functionalization strategies are emphasized to endow nanomaterials with superior performance in enhanced ophthalmic drug delivery. The rational design of several affecting factors is essential for ideal nanomaterial candidates and is depicted as well. Lastly, we introduce the current applications of nanomaterial-based delivery systems in the therapy of different ocular anterior and posterior segment diseases. The limitations of these delivery systems as well as potential solutions are also discussed. This work will inspire innovative design thinking for the development of nanotechnology-mediated strategies for advanced drug delivery and treatment toward ocular diseases.

Characterization of Oxygen Nanobubbles and In Vitro Evaluation of Retinal Cells in Hypoxia

Purpose

Vein or artery occlusion causes a hypoxic environment by preventing oxygen delivery and diffusion to tissues. Diseases such as retinal vein occlusion, central retinal artery occlusion, or diabetic retinopathy create a stroke-type condition that leads to functional blindness in the effected eye. We aim to develop an oxygen delivery system consisting of oxygen nanobubbles (ONBs) that can mitigate retinal ischemia during a severe hypoxic event such as central retinal artery occlusion.

Methods

ONBs were synthesized to encapsulate oxygen saturated molecular medical grade water. Stability, oxygen release, biocompatibility, reactive oxygen species, superoxide, MTT, and terminal uridine nick-end labeling assays were performed. Cell viability was evaluated, and safety experiments were conducted in rabbits.

Results

The ONBs were approximately 220 nm in diameter, with a zeta potential of -58.8 mV. Oxygen release studies indicated that 74.06 µg of O2 is released from the ONBs after 12 hours at 37°C. Cell studies indicated that ONBs are safe and cells are viable. There was no significant increase in reactive oxygen species, superoxide, or double-stranded DNA damage after ONB treatment. ONBs preserve mitochondrial function and viability. Histological sections from rabbit eyes indicated that ONBs were not toxic.

Conclusions

The ONBs proposed have excellent oxygen holding and release properties to mitigate ischemic conditions in the retina. They are sterile, stable, and nontoxic.

Translation Relevance

ONB technology was evaluated for its physical properties, oxygen release, sterility, stability, and safety. Our results indicate that ONBs could be a viable treatment approach to mitigate hypoxia during ischemic conditions in the eye upon timely administration.

Investigating the influence of ultrasound parameters on ibuprofen drug release from hydrogels

Hydrogels are promising ultrasound-responsive drug delivery systems. In this study, we investigated how different ultrasound parameters affected drug release and structural integrity of self-healing hydrogels composed of alginate or poloxamers. The effects of amplitude and duty cycle at low frequency (24 kHz) ultrasound stimulation were first investigated using alginate hydrogels at 2% w/v and 2.5% w/v. Increasing ultrasound amplitude increased drug release from these gels, although high amplitudes caused large variations in release and damaged the gel structure. Increasing duty cycle also increased drug release, although a threshold was observed with the lower pulsed 50% duty cycle achieving similar levels of drug release to a continuous 100% duty cycle. Poloxamer-based hydrogels were also responsive to the optimised parameters at low frequency (24 kHz, 20% amplitude, 50% duty cycle for 30 s) and showed similar drug release results to a 2.5% w/v alginate hydrogel. Weight loss studies demonstrated that the 2% w/v alginate hydrogel underwent significant erosion following ultrasound application, whereas the 2.5% w/v alginate and the poloxamer gels were unaffected by application of the same parameters (24 kHz, 20% amplitude, 50% duty cycle for 30 s). The rheological properties of the hydrogels were also unaffected and the FTIR spectra remained unchanged after low frequency ultrasound stimulation (24 kHz, 20% amplitude, 50% duty cycle for 30 s). Finally, high-frequency ultrasound stimulation (1 MHz, 3 W.cm^-2, 50% duty cycle) was also trialled; the alginate gels were less responsive to this frequency, while no statistically significant impact on drug release was observed from the poloxamer gels. This study demonstrates the importance of ultrasound parameters and polymer selection in designing ultrasound-responsive hydrogels.

Current Advances in Nano-Based and Polymeric Stimuli-Responsive Drug Delivery Targeting the Ocular Microenvironment: A Review and Envisaged Future Perspectives

Optimal vision remains one of the most essential elements of the sensory system continuously threatened by many ocular pathologies. Various pharmacological agents possess the potential to effectively treat these ophthalmic conditions; however, the use and efficacy of conventional ophthalmic formulations is hindered by ocular anatomical barriers. Recent novel designs of ophthalmic drug delivery systems (DDS) using nanotechnology show promising prospects, and ophthalmic formulations based on nanotechnology are currently being investigated due to their potential to bypass these barriers to ensure successful ocular drug delivery. More recently, stimuli-responsive nano drug carriers have gained more attention based on their great potential to effectively treat and alleviate many ocular diseases. The attraction is based on their biocompatibility and biodegradability, unique secondary conformations, varying functionalities, and, especially, the stimuli-enhanced therapeutic efficacy and reduced side effects. This review introduces the design and fabrication of stimuli-responsive nano drug carriers, including those that are responsive to endogenous stimuli, viz., pH, reduction, reactive oxygen species, adenosine triphosphate, and enzymes or exogenous stimuli such as light, magnetic field or temperature, which are biologically related or applicable in clinical settings. Furthermore, the paper discusses the applications and prospects of these stimuli-responsive nano drug carriers that are capable of overcoming the biological barriers of ocular disease alleviation and/or treatment for in vivo administration. There remains a great need to accelerate the development of stimuli-responsive nano drug carriers for clinical transition and applications in the treatment of ocular diseases and possible extrapolation to other topical applications such as ungual or otic drug delivery.

Ultrasound-assisted ocular drug delivery: A review of current evidence

Efficient ocular drug delivery is a challenging clinical problem with various therapeutic options but no clearly preferred methodology. Given the ubiquity of ultrasound as a diagnostic technique, the safety profile of ultrasound in an ocular context, and the prospect of custom-made ultrasound-sensitive contrast agents, ultrasound presents an attractive ocular drug delivery modality. In this review, we evaluate our present understanding of ultrasound as it relates to ocular drug delivery and significant knowledge gaps in the field. In doing so, we hope to call attention to a potentially novel drug delivery pathway that could be manipulated to treat or cure ocular diseases.

A comprehensive update of micro- and nanobubbles as theranostics in oncology

Advances in diagnostic and imaging capabilities have allowed cancers to be detected earlier and characterized more robustly. These strategies have recently branched into theranostics whereby contrast agents traditionally used for imaging have been co-loaded with therapeutics to simultaneously diagnose and treat cancers in a patient-specific manner. Microbubbles (MB) and nanobubbles (NB) are contrast agents which can be modulated to meet the theranostic needs particularly in the realm of oncology. The current review focuses on the ultrasound-responsive MB/NB platforms used as a theranostic tool in oncology. We discuss in detail the key parameters that influence the utility of MB/NB formulations and implications of such treatment modalities. Recent advances in composition strategies, latest works in the pre-clinical stages and multiple paradigm-shifting innovations in the field of MB/NB are discussed in-depth in this review. The clinical application of MB/NB is currently limited to diagnostic imaging. Surface chemistry modification strategies will help tune the formulations toward therapeutic applications. It is also anticipated that MB/NB will see increased use to deliver gas therapeutics. Scalability and stability considerations will be at the forefront as these particles get introduced into the clinical theranostic toolbox.

Ultrasound and Microbubbles for the Treatment of Ocular Diseases: From Preclinical Research towards Clinical Application

The unique anatomy of the eye and the presence of various biological barriers make efficacious ocular drug delivery challenging, particularly in the treatment of posterior eye diseases. This review focuses on the combination of ultrasound and microbubbles (USMB) as a minimally invasive method to improve the efficacy and targeting of ocular drug delivery. An extensive overview is given of the in vitro and in vivo studies investigating the mechanical effects of ultrasound-driven microbubbles aiming to: (i) temporarily disrupt the blood–retina barrier in order to enhance the delivery of systemically administered drugs into the eye, (ii) induce intracellular uptake of anticancer drugs and macromolecules and (iii) achieve targeted delivery of genes, for the treatment of ocular malignancies and degenerative diseases. Finally, the safety and tolerability aspects of USMB, essential for the translation of USMB to the clinic, are discussed.

Intellective and stimuli-responsive drug delivery systems in eyes

Stimuli-responsive drug delivery systems have attracted widespread attention in recent years since they can control drug release in a spatiotemporal manner and can achieve tunable drug release according to patient's physiological or pathological condition. In this review, we briefly introduce the drug delivery barriers and drug delivery systems in the anterior and posterior segment of eyes, and collect the recent advances in stimuli-responsive drug delivery systems in eyes for controlled drug release in response to exogenous stimuli (ultrasound, magnetic stimulus, electrical stimulus, and light) or endogenous stimuli (enzyme, active oxygen species, temperature, ions, and pH). In addition, the design and mechanisms of the stimuli-responsive drug delivery systems have been summarized in this review, and the advantages and limitations are also briefly discussed.

The influence of hyperglycemia on the safety of ultrasound in retinal pigment epithelial cells

Ultrasound (US) assisted drug delivery is receiving interest in treating posterior eye diseases, such as diabetic retinopathy due to its ability to maximize drug penetration into difficult to reach tissues. Despite its promise, the technique has only been investigated using healthy cell and tissue models, with no evidence to date about its safety in active disease. As a result, the aim of this study was to evaluate the safety of US administration in vitro in retinal pigment epithelial cells under normal and high glucose conditions. US protocols within the presently accepted safety threshold were applied and their influence on cell membrane and tight junction integrity as well as intracellular inflammation was evaluated using lactate dehydrogenase (LDH), zona occludens-1 (ZO-1), fluorescein isothiocyanate (FITC)-dextran dye leak and nuclear factor-kappaB (NF-κB) assays, respectively. Under high glucose conditions, US application increased LDH release and resulted in loss of ZO-1 labeling at 2 h; however, normal levels were restored within 24 h. US within its safety parameters did not induce any FITC-dextran dye leak or NF-κB nuclear translocation in normal or high glucose conditions. In conclusion, our results suggest that while high glucose conditions increase cell susceptibility to US-mediated stress, basal conditions can be restored within 24 h without long-lasting cell damage.

Shell properties and concentration stability of acoustofluidic delivery agents

This paper investigates the shell elastic properties and the number-concentration stability of a new acoustofluidic delivery agent liposome in comparison to Definity™, a monolayer ultrasonic contrast agent microbubble. The frequency dependent attenuation of an acoustic beam passing through a microbubble suspension was measured to estimate the shell parameters. The excitation voltage was adjusted to ensure constant acoustic pressure at all frequencies. The pressure was kept at the lowest possible magnitude to ensure that effects from nonlinear bubble behaviour which are not considered in the analytical model were minimal. The acoustofluidic delivery agent shell stiffness S_p and friction S_f parameters were determined as (S_p = 0.11 N/m, S_f = 0.31 × 10^-6 Kg/s at 25 °C) in comparison to the Definity™ monolayer ultrasound contrast agent which were (S_p = 1.53 N/m, S_f = 1.51 × 10^-6 Kg/s at 25 °C). When the temperature was raised to physiological levels, the friction coefficient S_f decreased by 28% for the monolayer microbubbles and by only 9% for the liposomes. The stiffness parameter S_p of the monolayer microbubble decreased by 23% while the stiffness parameter of the liposome increased by a similar margin (27%) when the temperature was raised to 37 °C. The size distribution of the bubbles was measured using Tunable Resistive Pulse Sensing (TRPS) for freshly prepared microbubbles and for bubble solutions at 6 h and 24 h after activation to investigate their number-concentration stability profile. The liposome maintained >80% of their number-concentration for 24 h at physiological temperature, while the monolayer microbubbles maintained only 27% of their number-concentration over the same period. These results are important input parameters for the design of effective acoustofluidic delivery systems using the new liposomes.

Application of Nanomaterials in Biomedical Imaging and Cancer Therapy

Nanomaterials, such as nanoparticles, nanorods, nanosphere, nanoshells, and nanostars, are very commonly used in biomedical imaging and cancer therapy. They make excellent drug carriers, imaging contrast agents, photothermal agents, photoacoustic agents, and radiation dose enhancers, among other applications. Recent advances in nanotechnology have led to the use of nanomaterials in many areas of functional imaging, cancer therapy, and synergistic combinational platforms. This review will systematically explore various applications of nanomaterials in biomedical imaging and cancer therapy. The medical imaging modalities include magnetic resonance imaging, computed tomography, positron emission tomography, single photon emission computerized tomography, optical imaging, ultrasound, and photoacoustic imaging. Various cancer therapeutic methods will also be included, including photothermal therapy, photodynamic therapy, chemotherapy, and immunotherapy. This review also covers theranostics, which use the same agent in diagnosis and therapy. This includes recent advances in multimodality imaging, image-guided therapy, and combination therapy. We found that the continuous advances of synthesis and design of novel nanomaterials will enhance the future development of medical imaging and cancer therapy. However, more resources should be available to examine side effects and cell toxicity when using nanomaterials in humans.

Focused ultrasound as a novel strategy for noninvasive gene delivery to retinal Müller glia

Müller glia are specialized retinal cells with stem cell properties in fish and frogs but not in mammals. Current efforts to develop gene therapies to activate mammalian Müller glia for retinal repair will require safe and effective delivery strategies for recombinant adeno-associated viruses (AAVs), vectors of choice for clinical translation. Intravitreal and subretinal injections are currently used for AAV gene delivery in the eye, but less invasive methods efficiently targeting Müller glia have yet to be developed. Methods: As gene delivery strategies have been more extensively studied in the brain, to validate our vectors, we initially compared the glial tropism of AAV-PHP.eB, an AAV9 that crosses the blood-brain and blood-retinal barriers, for its ability to drive fluorescent protein expression in glial cells in both the brain and retina. We then tested the glial transduction of AAV2/8-GFAP-mCherry, a virus that does not cross blood-brain and blood-retinal barriers, for its effectiveness in transducing Müller glia in murine retinal explants ex vivo. For in vivo assays we used larger rat eyes, performing invasive intravitreal injections, and non-invasive intravenous delivery using focused ultrasound (FUS) (pressure amplitude: 0.360 - 0.84 MPa) and microbubbles (Definity, 0.2 ml/kg). Results: We showed that AAV-PHP.eB carrying a ubiquitous promoter (CAG) and green fluorescent protein (GFP) reporter, readily crossed the blood-brain and blood-retinal barriers after intravenous delivery in mice. However, murine Müller glia did not express GFP, suggesting that they were not transduced by AAV-PHP.eB. We thus tested an AAV2/8 variant, which was selected based on its safety record in multiple clinical trials, adding a glial fibrillary acidic protein (GFAP) promoter and mCherry (red fluorescent protein) reporter. We confirmed the glial specificity of AAV2/8-GFAP-mCherry, showing effective expression of mCherry in astrocytes after intracranial injection in the mouse brain, and of Müller glia in murine retinal explants. For in vivo experiments we switched to rats because of their larger size, injecting AAV2/8-GFAP-mCherry intravitreally, an invasive procedure, demonstrating passage across the inner limiting membrane, leading to Müller glia transduction. We then tested an alternative non-invasive delivery approach targeting a different barrier - the inner blood-retinal-barrier, applying focused ultrasound (FUS) to the retina after intravenous injection of AAV2/8 and microbubbles in rats, using magnetic resonance imaging (MRI) for FUS targeting. FUS permeabilized the rat blood-retinal-barrier and allowed the passage of macromolecules to the retina (Evans blue, IgG, IgM), with minimal extravasation of platelets and red blood cells. Intravenous injection of microbubbles and AAV2/8-GFAP-mCherry followed by FUS resulted in mCherry expression in rat Müller glia. However, systemic delivery of AAV2/8 also had off-target effects, transducing several murine peripheral organs, particularly the liver. Conclusions: Retinal permeabilisation via FUS in the presence of microbubbles is effective for delivering AAV2/8 across the inner blood-retinal-barrier, targeting Müller glia, which is less invasive than intravitreal injections that bypass the inner limiting membrane. However, implementing FUS in the clinic will require a comprehensive consideration of any off-target tropism of the AAV in peripheral organs, combined ideally, with the development of Müller glia-specific promoters.

Biomedical Applications for Gas-Stabilizing Solid Cavitation Agents

For over a decade, advancements in ultrasound-enhanced drug delivery strategies have demonstrated remarkable success in providing targeted drug delivery for a broad range of diseases. In order to achieve enhanced drug delivery, these strategies harness the mechanical effects from bubble oscillations (i.e., cavitation) of a variety of exogenous cavitation agents. Recently, solid cavitation agents have emerged due to their capacity for drug-loading and sustained cavitation duration. Unlike other cavitation agents, solid cavitation agents stabilize gaseous bubbles on hydrophobic surface cavities. Thus, the design of these particles is crucial. In this Review, we provide an overview of the different designs for solid cavitation agents such as nanocups, nanocones, and porous structures, as well as the current status of their development. Considering the numerous advantages of solid cavitation agents, we anticipate further innovations for this new type of cavitation agent across a broad range of biomedical applications.

Sink or float? Characterization of shell-stabilized bulk nanobubbles using a resonant mass measurement technique

Nano-sized shell-stabilized gas bubbles have applications in various fields ranging from environmental science to biomedical engineering. A resonant mass measurement (RMM) technique is demonstrated here as a new and only method capable of simultaneously measuring the size and concentration of buoyant and non-buoyant particles in a nanobubble sample used as a next-generation ultrasound contrast agent.

Cavitation-threshold Determination and Rheological-parameters Estimation of Albumin-stabilized Nanobubbles

Nanobubbles (NBs) are of high interest for ultrasound (US) imaging as contrast agents and therapy as cavitation nuclei. Because of their instability (Laplace pressure bubble catastrophe) and low sensitivity to US, reducing the size of commonly used microbubbles to submicron-size is not trivial. We introduce stabilized NBs in the 100-250-nm size range, manufactured by agitating human serum albumin and perfluoro-propane. These NBs were exposed to 3.34- and 5.39-MHz US, and their sensitivity to US was proven by detecting inertial cavitation. The cavitation-threshold information was used to run a numerical parametric study based on a modified Rayleigh-Plesset equation (with a Newtonian rheology model). The determined values of surface tension ranged from 0 N/m to 0.06 N/m. The corresponding values of dilatational viscosity ranged from 5.10-10 Ns/m to 1.10-9 Ns/m. These parameters were reported to be 0.6 N/m and 1.10-8 Ns/m for the reference microbubble contrast agent. This result suggests the possibility of using albumin as a stabilizer for the nanobubbles that could be maintained in circulation and presenting satisfying US sensitivity, even in the 3-5-MHz range.

Cryo-EM Visualization of Lipid and Polymer-Stabilized Perfluorocarbon Gas Nanobubbles - A Step Towards Nanobubble Mediated Drug Delivery

Gas microbubbles stabilized with lipids, surfactants, proteins and/or polymers are widely used clinically as ultrasound contrast agents. Because of their large 1-10 µm size, applications of microbubbles are confined to the blood vessels. Accordingly, there is much interest in generating nanoscale echogenic bubbles (nanobubbles), which can enable new uses of ultrasound contrast agents in molecular imaging and drug delivery, particularly for cancer applications. While the interactions of microbubbles with ultrasound have been widely investigated, little is known about the activity of nanobubbles under ultrasound exposure. In this work, we demonstrate that cryo-electron microscopy (cryo-EM) can be used to image nanoscale lipid and polymer-stabilized perfluorocarbon gas bubbles before and after their destruction with high intensity ultrasound. In addition, cryo-EM can be used to observe electron-beam induced dissipation of nanobubble encapsulated perfluorocarbon gas.