Samantha M. Fix

Therapeutic gas delivery via microbubbles and liposomes

Gaseous molecules including nitric oxide, hydrogen sulfide, carbon monoxide and oxygen mediate numerous cell signaling pathways and have important physiological roles. Several noble gasses have been shown to elicit biological responses. These bioactive gasses hold great therapeutic potential, however, their controlled delivery remains a significant challenge. Recently, researchers have begun using microbubbles and liposomes to encapsulate such gasses for parenteral delivery. The resultant particles are acoustically active, and ultrasound can be used to stimulate and/or image gas release in a targeted region. This review provides a summary of recent advances in therapeutic gas delivery using microbubbles and liposomes.

Accelerated Clearance of Ultrasound Contrast Agents Containing Polyethylene Glycol is Associated with the Generation of Anti-Polyethylene Glycol Antibodies

Emerging evidence suggests that the immune system can recognize polyethylene glycol (PEG), leading to the accelerated blood clearance (ABC) of PEGylated particles. Our aim here was to study the generation of anti-PEG immunity and changes in PEGylated microbubble pharmacokinetics during repeated contrast-enhanced ultrasound imaging in rats. We administered homemade PEGylated microbubbles multiple times over a 28-d period and observed dramatically accelerated clearance (4.2 × reduction in half-life), which was associated with robust anti-PEG IgM and anti-PEG IgG antibody production. Dosing animals with free PEG as a competition agent before homemade PEGylated microbubble administration significantly prolonged microbubble circulation, suggesting that ABC was largely driven by circulating anti-PEG antibodies. Experiments with U.S. Food and Drug Administration-approved Definity microbubbles similarly resulted in ABC and the generation of anti-PEG antibodies. Experiments repeated with non-PEGylated Optison microbubbles revealed a slight shift in clearance, indicating that immunologic factors beyond anti-PEG immunity may play a role in ABC, especially of non-PEGylated agents.

Focused ultrasound-facilitated brain drug delivery using optimized nanodroplets: Vaporization efficiency dictates large molecular delivery

Focused ultrasound with nanodroplets could facilitate localized drug delivery after vaporization with potentially improved in vivo stability, drug payload, and minimal interference outside of the focal zone compared with microbubbles. While the feasibility of blood-brain barrier (BBB) opening using nanodroplets has been previously reported, characterization of the associated delivery has not been achieved. It was hypothesized that the outcome of drug delivery was associated with the droplet sensitivity to acoustic energy, and can be modulated with the boiling point of the liquid core. Therefore, in this study, highly-efficient octafluoropropane (OFP) and less-efficient decafluorobutane (DFB) nanodroplets were used in vivo for delivering molecules with a size relevant to proteins (40-kDa dextran) to the murine brain. It was found that successful delivery was achieved with OFP droplets at 300 kPa or higher safely using 1/4 dosage compared to DFB droplets at 900 kPa where inertial cavitation caused damage. As a result, the OFP droplets due to the higher vaporization efficiency served as better acoustic agents to deliver large molecules safely and efficiently to the brain compared with the DFB droplets.

Oxygen microbubbles improve radiotherapy tumor control in a rat fibrosarcoma model – A preliminary study

Cancer affects 39.6% of Americans at some point during their lifetime. Solid tumor microenvironments are characterized by a disorganized, leaky vasculature that promotes regions of low oxygenation (hypoxia). Tumor hypoxia is a key predictor of poor treatment outcome for all radiotherapy (RT), chemotherapy and surgery procedures, and is a hallmark of metastatic potential. In particular, the radiation therapy dose needed to achieve the same tumor control probability in hypoxic tissue as in normoxic tissue can be up to 3 times higher. Even very small tumors (<2–3 mm3) comprise 10–30% of hypoxic regions in the form of chronic and/or transient hypoxia fluctuating over the course of seconds to days. We investigate the potential of recently developed lipid-stabilized oxygen microbubbles (OMBs) to improve the therapeutic ratio of RT. OMBs, but not nitrogen microbubbles (NMBs), are shown to significantly increase dissolved oxygen content when added to water in vitro and increase tumor oxygen levels in vivo in a rat fibrosarcoma model. Tumor control is significantly improved with OMB but not NMB intra-tumoral injections immediately prior to RT treatment and effect size is shown to depend on initial tumor volume on RT treatment day, as expected.

Tumor hypoxia modulation dynamics using intra-tumoral, intra-peritoneal and intra-venous oxygen microbubbles administrations — In vivo real-time measurements via spectroscopic absorbance on a rat subcutaneous fibrosarcoma model

Solid tumor hypoxia is a poor outcome predictor for radiotherapy, chemotherapy and also surgical treatment options. Recently, oxygen microbubbles (OMB) have shown promise as an adjuvant therapy to relieve such tumor hypoxia. However, a key limiting factor remains the lack of available data in vivo of the dose-response and time-dynamics of this OMB-induced reoxygenation. Here we study the kinetics of OMB-induced hypoxia-modulation by continuously measuring tumor hypoxia in vivo before, during and after OMB administration.

Daily intra-tumoral administration of oxygen microbubbles slows tumor growth in the absence of other therapy in a rat subcutaneous fibrosarcoma model

Oxygen microbubbles (OMBs) have recently been used as adjunct treatment for solid tumors. Indeed, their ability to relieve tumor hypoxia was shown to benefit sonodynamic and other treatment outcomes in vivo. However, OMBs are often administered via intratumoral injection, and the effect of repeated OMB injections in tumors has not been investigated. Here we report on the paradoxical finding that daily OMB injections significantly reduces tumor growth rate in the absence of other therapy in vivo.

Accelerated clearance of ultrasound contrast agents containing polyethylene glycol (PEG) is associated with a PEG-specific immune response

Lipid-shelled microbubbles (MBs) are routinely used as contrast agents for ultrasound imaging. Such MBs are often formulated with polyethylene glycol (PEG), with the intention of shielding them from recognition and clearance by the innate immune system, prolonging their intravenous circulation time. Paradoxically, the immune system is able to generate specific antibodies that bind PEG. This has been associated with accelerated blood clearance of PEGylated particles when dosed repeatedly over multiple days. Here we aim to (1) study how PEGylated MB pharmacokinetics changes during repeat contrast-enhanced ultrasound imaging schedules and (2) investigate whether administration of these MBs generates an anti-PEG antibody response in rats.

Oxygen microbubbles improve tumor control after radiotherapy in a rat fibrosarcoma model

It is well-established that tumor hypoxia negatively impacts treatment outcome for radiation therapy (RT). In particular, tumor cell resistance to radiotherapy is increased 3-fold under anoxic conditions. Hypoxia hinders direct “oxygen radiosensitization” (increased free radical oxidative damage and formation of difficult-to-repair organic peroxides during RT), but also promotes radioresistance through biological HIF-1 complex signaling. Oxygen microbubbles (OMB) have been successfully used as an oxygen-delivery vehicle, improving the efficacy of sonodynamic therapy. Furthermore, the robust oxygen-delivery potential of OMB is demonstrated by their ability to sustain asphyxiated animals for over two hours. Here we assess whether these microbubbles can be used to relieve tumor hypoxia and thereby improve RT outcome.

Focused ultrasound-facilitated brain drug delivery using optimized nanodroplets

Focused ultrasound with nanodroplets could facilitate localized drug delivery after vaporization with potentially improved in vivo stability, drug payload, and minimal interference outside of the focal zone compared with microbubbles. While the feasibility of blood-brain barrier (BBB) opening using nanodroplets has been previously reported, characterization of the associated delivery has not been achieved. It was hypothesized that the outcome of drug delivery was associated with the droplet sensitivity to acoustic energy, and can be modulated with the boiling point of the liquid core. Therefore, in this study, highly-efficient octafluoropropane (OFP) and less-efficient decafluorobutane (DFB) nanodroplets were used in vivo for delivering molecules with a size relevant to proteins (40-kDa dextran) to the murine brain. It was found that successful delivery was achieved with OFP droplets at 300 kPa or higher safely using 1/4 dosage compared to DFB droplets at 900 kPa where inertial cavitation caused damage. As a result, the OFP droplets due to the higher vaporization efficiency served as better acoustic agents to deliver large molecules safely and efficiently to the brain compared with the DFB droplets.

Focused ultrasound-facilitated molecular delivery to the brain using drug-loaded nanodroplets

Acoustically-activated nanodroplets facilitate localized drug delivery after vaporization with improved in vivo stability, drug payload, and minimal interference outside of the ultrasound focal zone compared with microbubbles. They are new acoustic mediators to induce blood-brain barrier (BBB) opening for drug delivery to the brain, with promising potential of extravasation to enhance targeted delivery in the extravascular space due to the nano sizes.)