Balancing stealth and echogenic properties in an ultrasound contrast agent with drug delivery potential

Determining Ultrasound Parameters for Bursting Polymer Microbubbles for Future Use in Spinal Cord Injury

OBJECTIVE

We believe our poly(lactic acid) (PLA) microbubbles are well suited for therapeutic delivery to spinal cord injury (SCI) using ultrasound-triggered bursting. We investigated the feasibility of clinical ultrasound bursting in situ, the optimal bursting parameters in vitro and the loading and release of a model bio-active DNA.

METHODS

Microbubbles were tested using clinical ultrasound in a rat cadaver SCI model. Burst pressure thresholds were determined using the change in enhancement after ultrasound exposure. Resonance frequency, acoustic enhancement, sizing and morphology were evaluated by comparing two microbubble porogens, ammonium carbonate and ammonium carbamate. Oligonucleotides were loaded into the shell and released using the found optimized ultrasound bursting parameters.

RESULTS

In situ imaging and bursting were successful. In vitro bursting thresholds using frequencies 1, 2.25 and 5 MHz were identified between peak negative pressures 0.2 and 0.5 MPa, believed to be safe for spinal cord. The pressure threshold decreased with decreasing frequencies. PLA bursting was optimized near the resonance frequency of 2.5 to 3.0 MHz using 2.25 MHz and not at lower frequencies. PLA microbubbles, initially with a mean size of approximately 2 µm, remained in one piece, collapsed to between 0.5 and 1 µm and did not fragment. Significantly more oligonucleotide was released after ultrasound bursting of loaded microbubbles. Microbubble-sized debris was detected when using ammonium carbamate, leading to inaccurate microbubble concentration measurements.

CONCLUSION

PLA microbubbles made with ammonium carbonate and burst at appropriate parameters have the potential to safely improve intrathecal therapeutic delivery to SCI using targeted ultrasound.

Preparation of ultrasound contrast agents: The exploration of the structure-echogenicity relationship of contrast agents based on neural network model

There is a need to standardize the process of micro/nanobubble preparation to bring it closer to clinical translation. We explored a neural network-based model to predict the structure-echogenicity relationship for the preparation and fabrication of ultrasound-enhanced contrast agents. Seven formulations were screened, and 109 measurements were obtained. An artificial neural network-multilayer perceptron (ANN-MLP) model was used. The original data were divided into the training and testing groups, which included 73 and 36 groups of data, respectively. The hidden layer was selected from three hidden layers and included bias. The classification graph showed that the predicted values of the training and testing groups were 76.7% and 66.7%, respectively. According to the receiver operating characteristic curve, the accuracy of different imaging effects could achieve a prediction rate of 88.1–96.5%. The percentage graph showed that the data were gradually converging. The predictive analysis curves of different ultrasound effects gradually approached stable value of Gain. Normalized importance predicted contributions for the Pk1, poly-dispersity index (PDI), and intensity account were 100%, 98.5%, and 89.7%, respectively. The application of the ANN-MLP model is feasible and effective for the exploration of the synthesis process of ultrasound contrast agents. 1,2-Distearoyl-sn-glycero-3 phosphoethanolamine-N (methoxy[polyethylene glycol]-2000) (DSPE PEG-2000) correlated highly with the success rate of contrast agent synthesis.

Theranostic Trigger-Responsive Carbon Monoxide-Generating Microbubbles

Carbon monoxide (CO) is a gaseous signaling molecule that modulates inflammation, cell survival, and recovery after myocardial infarction. However, handling and dosing of CO as a compressed gas are difficult. Here, light-triggerable and magnetic resonance imaging (MRI)-detectable CO release from dimanganese decacarbonyl (CORM-1) are demonstrated, and the development of CORM-1-loaded polymeric microbubbles (COMB) is described as an ultrasound (US)- and MRI-imageable drug delivery platform for triggerable and targeted CO therapy. COMB are synthesized via a straightforward one-step loading protocol, present a narrow size distribution peaking at 2 µm, and show excellent performance as a CORM-1 carrier and US contrast agent. Light irradiation of COMB induces local production and release of CO, as well as enhanced longitudinal and transversal relaxation rates, enabling MRI monitoring of CO delivery. Proof-of-concept studies for COMB-enabled light-triggered CO release show saturation of hemoglobin with CO in human blood, anti-inflammatory differentiation of macrophages, reduction of hypoxia-induced reactive oxygen species (ROS) production, and inhibition of ischemia-induced apoptosis in endothelial cells and cardiomyocytes. These findings indicate that CO-generating MB are interesting theranostic tools for attenuating hypoxia-associated and ROS-mediated cell and tissue damage in cardiovascular disease.

Development of a Dual Drug-Loaded, Surfactant-Stabilized Contrast Agent Containing Oxygen

Co-delivery of cancer therapeutics improves efficacy and encourages synergy, but delivery faces challenges, including multidrug resistance and spatiotemporal distribution of therapeutics. To address these, we added paclitaxel to previously developed acoustically labile, oxygen-core, surfactant-stabilized microbubbles encapsulating lonidamine, with the aim of developing an agent containing both a therapeutic gas and two drugs acting in combination. Upon comparison of unloaded, single-loaded, and dual-loaded microbubbles, size (~1.7 µm) and yield (~2 × 10⁹ microbubbles/mL) (~1.7) were not statistically different, nor were acoustic properties (maximum in vitro enhancements roughly 18 dB, in vitro enhancements roughly 18 dB). Both drugs encapsulated above required doses calculated for head and neck squamous cell carcinoma, the cancer of choice. Interestingly, paclitaxel encapsulation efficiency increased from 1.66% to 3.48% when lonidamine was included. During preparation, the combination of single drug-loaded micelles gave higher encapsulation (µg drug/g microbubbles) than micelles loaded with either drug alone (lonidamine, 104.85 ± 22.87 vs. 87.54 ± 16.41), paclitaxel (187.35 ± 8.38 vs. 136.51 ± 30.66). In vivo intravenous microbubbles produced prompt ultrasound enhancement within tumors lasting 3–5 min, indicating penetration into tumor vasculature. The ability to locally destroy the microbubble within the tumor vasculature was confirmed using a series of higher intensity ultrasound pulses. This ability to locally destroy microbubbles shows therapeutic promise that warrants further investigation.

Making waves: how ultrasound-targeted drug delivery is changing pharmaceutical approaches

Administration of drugs through oral and intravenous routes is a mainstay of modern medicine, but this approach suffers from limitations associated with off-target side effects and narrow therapeutic windows. It is often apparent that a controlled delivery of drugs, either localized to a specific site or during a specific time, can increase efficacy and bypass problems with systemic toxicity and insufficient local availability. To overcome some of these issues, local delivery systems have been devised, but most are still restricted in terms of elution kinetics, duration, and temporal control. Ultrasound-targeted drug delivery offers a powerful approach to increase delivery, therapeutic efficacy, and temporal release of drugs ranging from chemotherapeutics to antibiotics. The use of ultrasound can focus on increasing tissue sensitivity to the drug or actually be a critical component of the drug delivery. The high spatial and temporal resolution of ultrasound enables precise location, targeting, and timing of drug delivery and tissue sensitization. Thus, this noninvasive, non-ionizing, and relatively inexpensive modality makes the implementation of ultrasound-mediated drug delivery a powerful method that can be readily translated into the clinical arena. This review covers key concepts and areas applied in the design of different ultrasound-mediated drug delivery systems across a variety of clinical applications.

Shaping the synthesis of surfactant-stabilized oxygen microbubbles to accommodate encapsulated drug

We have developed oxygen filled microbubbles, SE61_O2, for localized, ultrasound-triggered oxygen delivery to hypoxic tumors prior to radiation therapy. Microbubbles, created by sonication, have a shell composed of D-α-Tocopherol polyethylene glycol 1000 succinate (TPGS) and sorbitan monostearate. Preliminary studies in mice with breast tumor xenographs showed that increases in oxygen partial pressure levels lasted less than 3 min, which is insufficient for most clinical applications. Hence, we investigated the potential of incorporating a hydrophobic antiglycolytic drug, modeled with Nile red. A new fabrication method was developed by first creating drug-loaded TPGS micelles. The resulting microbubbles had similar shell compositions, physical size, morphology, and acoustic properties as the original method. However, microbubble yield was more than doubled, resulting in twice the encapsulation efficiency. For the TPGS micelle method these include similar shell compositions (94.4 ± 0.6 % Montane 60), physical size post freeze-drying and reconstitution (1.57 ± 0.42 μm), morphology (spherical), and acoustic properties (maximum enhancement 19.92 ± 0.55 dB). However, microbubble yield was more than doubled, resulting in twice the encapsulation efficiency (up to 10.49 %). We propose that a nonideal mixture is formed when the surfactants are combined by the standard method, resulting in the formation of mixed micelles that are more stable, making microbubble creation more difficult during the sonication step.

Gemcitabine-loaded microbubble system for ultrasound imaging and therapy

Ultrasound imaging presents many positive attributes, including safety, real-time imaging, universal accessibility, and cost. However, inherent difficulties in discrimination between soft tissues and tumors prompted development of stabilized microbubble contrast agents. This presents the opportunity to develop agents in which drug is entrapped in the microbubble shell. We describe preparation and characterization of theranostic poly(lactide) (PLA) and pegylated PLA (PEG-PLA) shelled microbubbles that entrap gemcitabine, a commonly used drug for pancreatic cancer (PDAC). Entrapping 6 wt% gemcitabine did not significantly affect drug activity, microbubble morphology, or ultrasound contrast activity compared with unmodified microbubbles. In vitro microbubble concentrations yielding ≥ 500nM entrapped gemcitabine were needed for complete cell death in MIA PaCa-2 PDAC drug sensitivity assays, compared with 62.5 nM free gemcitabine. In vivo administration of gemcitabine-loaded microbubbles to xenograft MIA PaCa-2 PDAC tumors in athymic mice was well tolerated and provided substantial tumoral image enhancement before and after destructive ultrasound pulses. However, no significant differences in tumor growth were observed among treatment groups, in keeping with the in vitro observation that much higher doses of gemcitabine are required to mirror free gemcitabine activity. STATEMENT OF SIGNIFICANCE: The preliminary results shown here are encouraging and support further investigation into increased gemcitabine loading. Encapsulation of gemcitabine within polylactic acid (PLA) microbubbles does not damage its activity towards pancreatic cancer (pancreatic ductal adenocarcinoma, PDAC) cells. Excellent imaging and evidence of penetration into the highly desmoplastic PDAC tumors is demonstrated. Microbubble destruction was confirmed in vivo, showing that elevated mechanical index shatters the microbubbles for enhanced delivery. The potential to slow PDAC growth in vivo is shown, but higher gemcitabine concentrations are required. Current efforts are directed at increasing drug loading by inclusion of drug-carrying nanoparticles for effective in vivo treatment.

Ultrasound-responsive lipid microbubbles for drug delivery: A review of preparation techniques to optimise formulation size, stability and drug loading

Lipid-shelled microbubbles have received extensive interest to enhance ultrasound-responsive drug delivery outcomes due to their high biocompatibility. While therapeutic effectiveness of microbubbles is well established, there remain limitations in sample homogeneity, stability profile and drug loading properties which restrict these formulations from seeing widespread use in the clinical setting. In this review, we evaluate and discuss the most encouraging leads in lipid microbubble design and optimisation. We examine current applications in drug delivery for the systems and subsequently detail shell compositions and preparation strategies that improve monodispersity while retaining ultrasound responsiveness. We review how excipients and storage techniques help maximise stability and introduce different characterisation and drug loading techniques and evaluate their impact on formulation performance. The review concludes with current quality control measures in place to ensure lipid microbubbles can be reproducibly used in drug delivery.

Lipid/PLGA Hybrid Microbubbles as a Versatile Platform for Noninvasive Image-Guided Targeted Drug Delivery

Microbubbles (MBs) have recently emerged as promising theranostic carriers for ultrasound contrast imaging and drug delivery. However, conventional lipid-based MBs have a poor drug encapsulation efficiency, and polymer-based MBs show a weak capability in contrast imaging and ultrasound-triggered drug release. Here, we developed a novel type of multiporous lipid/PLGA hybrid MBs (lipid/PLGA MBs) that solved the dilemma of MBs as imaging agents and drug carriers. The lipid/PLGA MBs were designed through regulating the elasticity of the bubble shells using lipids to incorporate into the PLGA shells and ammonium bicarbonate as a gas-generating agent. The softened shells and the porous bubble structure make them be able to generate stronger harmonic signals and be more vulnerable to ultrasound irradiation, leading to their excellent performance in ultrasound contrast imaging and ultrasound-triggered MB destruction in vitro and in vivo. By using doxorubicin (Dox) as a model drug, the Dox-loaded lipid/PLGA MBs (Dox-lipid/PLGA MBs) were prepared and achieved a high drug encapsulation efficiency. The real-time tracking of drug delivery and on-command controlled drug release by ultrasound were successfully realized in the tumor-bearing mice. A significantly enhanced tumor growth inhibition effect could be observed when using Dox-lipid/PLGA MBs combined with ultrasound irradiation, compared with free Dox and Dox-lipid/PLGA MBs without ultrasound. Our study provides an innovative multifunctional platform of MBs for ultrasound contrast imaging and drug delivery applications.

The Horizon of the Emulsion Particulate Strategy: Engineering Hollow Particles for Biomedical Applications

With their hierarchical structures and the substantial surface areas, hollow particles have gained immense research interest in biomedical applications. For scalable fabrications, emulsion-based approaches have emerged as facile and versatile strategies. Here, the recent achievements in this field are unfolded via an "emulsion particulate strategy," which addresses the inherent relationship between the process control and the bioactive structures. As such, the interior architectures are manipulated by harnessing the intermediate state during the emulsion revolution (intrinsic strategy), whereas the external structures are dictated by tailoring the building blocks and solidification procedures of the Pickering emulsion (extrinsic strategy). Through integration of the intrinsic and extrinsic emulsion particulate strategy, multifunctional hollow particles demonstrate marked momentum for label-free multiplex detections, stimuli-responsive therapies, and stem cell therapies.

Ultrasound-triggered antibiotic release from PEEK clips to prevent spinal fusion infection: Initial evaluations

Despite aggressive peri-operative antibiotic treatments, up to 10% of patients undergoing instrumented spinal surgery develop an infection. Like most implant-associated infections, spinal infections persist through colonization and biofilm formation on spinal instrumentation, which can include metal screws and rods for fixation and an intervertebral cage commonly comprised of polyether ether ketone (PEEK). We have designed a PEEK antibiotic reservoir that would clip to the metal fixation rod and that would achieve slow antibiotic release over several days, followed by a bolus release of antibiotics triggered by ultrasound (US) rupture of a reservoir membrane. We have found using human physiological fluid (synovial fluid), that higher levels (100–500 μg) of vancomycin are required to achieve a marked reduction in adherent bacteria vs. that seen in the common bacterial medium, trypticase soy broth. To achieve these levels of release, we applied a polylactic acid coating to a porous PEEK puck, which exhibited both slow and US-triggered release. This design was further refined to a one-hole or two-hole cylindrical PEEK reservoir that can clip onto a spinal rod for clinical use. Short-term release of high levels of antibiotic (340 ± 168 μg), followed by US-triggered release was measured (7420 ± 2992 μg at 48 h). These levels are sufficient to prevent adhesion of Staphylococcus aureus to implant materials. This study demonstrates the feasibility of an US-mediated antibiotic delivery device, which could be a potent weapon against spinal surgical site infection.

Manipulating multifaceted microbubble shell composition to target both TRAIL-sensitive and resistant cells

This study represents the first attempt to combine surface TRAIL expression and doxorubicin co-encapsulation in a single drug delivery agent in the form of ultrasound-responsive microbubbles that shatter into fragments, or nanoshards, in an ultrasound beam. We compare customized microbubbles of different polymeric shell compositions, and investigate the effect of both shell composition and incorporation of doxorubicin on action against TRAIL-sensitive MDA-MB-231 and TRAIL-resistant MCF7 human breast adenocarcinoma cells. Ligation of TRAIL only significantly impacted MDA-MB-231 cells predominantly by apoptosis, and had minimal effect on MCF12A (normal control) cells. For all shell types, nanoshards had a greater effect (apoptotic death ranging from approximately 25% for 1 wt % LipidPEG to 50% for 100% PLA), reflecting the greater surface area and larger number of particles that ultrasound generates. Encapsulation of doxorubicin generated necrosis in all cell lines, but PEGylation produced less effective necrosis in all cell lines. Co-encapsulation of doxorubicin within the contrast agent shell increased MDA-MB-231 cell death to approximately 40-80%, representing a marked increase over TRAIL alone, reflecting the dramatic effect of shell composition. Additionally, shells that co-encapsulated TRAIL and doxorubicin resulted in approximately 30-60% death in TRAIL-resistant MCF7 human breast adenocarcinoma cells, compared with little apoptotic response in these cells from shells encapsulating TRAIL alone, demonstrating the sensitization effect of the drug. This work has resulted in production of a library of effective ultrasound-triggered, minimally immunogenic, targeted drug delivery agents for potential use in cancer therapy, and represents a promising multifaceted treatment to better serve the population with solid tumors. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1903-1915, 2018.

Drug Release from Phase-Changeable Nanodroplets Triggered by Low-Intensity Focused Ultrasound

Background: As one of the most effective triggers with high tissue-penetrating capability and non-invasive feature, ultrasound shows great potential for controlling the drug release and enhancing the chemotherapeutic efficacy. In this study, we report, for the first time, construction of a phase-changeable drug-delivery nanosystem with programmable low-intensity focused ultrasound (LIFU) that could trigger drug-release and significantly enhance anticancer drug delivery. Methods: Liquid-gas phase-changeable perfluorocarbon (perfluoropentane) and an anticancer drug (doxorubicin) were simultaneously encapsulated in two kinds of nanodroplets. By triggering LIFU, the nanodroplets could be converted into microbubbles locally in tumor tissues for acoustic imaging and the loaded anticancer drug (doxorubicin) was released after the microbubble collapse. Based on the acoustic property of shell materials, such as shell stiffness, two types of nanodroplets (lipid-based nanodroplets and PLGA-based nanodroplets) were activated by different acoustic pressure levels. Ultrasound irradiation duration and power of LIFU were tested and selected to monitor and control the drug release from nanodroplets. Various ultrasound energies were introduced to induce the phase transition and microbubble collapse of nanodroplets in vitro (3 W/3 min for lipid nanodroplets; 8 W/3 min for PLGA nanodroplets). Results: We detected three steps in the drug-releasing profiles exhibiting the programmable patterns. Importantly, the intratumoral accumulation and distribution of the drug with LIFU exposure were significantly enhanced, and tumor proliferation was substantially inhibited. Co-delivery of two drug-loaded nanodroplets could overcome the physical barriers of tumor tissues during chemotherapy. Conclusion: Our study provides a new strategy for the efficient ultrasound-triggered chemotherapy by nanocarriers with programmable LIFU capable of achieving the on-demand drug release.

End-chain fluorination of polyesters favors perfluorooctyl bromide encapsulation into echogenic PEGylated nanocapsules

Fluorination of polyesters favors the encapsulation efficiency of perfluorooctyl bromide into nanocapsules.