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.

Improved Tumor Control Following Radiosensitization with Ultrasound-Sensitive Oxygen Microbubbles and Tumor Mitochondrial Respiration Inhibitors in a Preclinical Model of Head and Neck Cancer

Tumor hypoxia (oxygen deficiency) is a major contributor to radiotherapy resistance. Ultrasound-sensitive microbubbles containing oxygen have been explored as a mechanism for overcoming tumor hypoxia locally prior to radiotherapy. Previously, our group demonstrated the ability to encapsulate and deliver a pharmacological inhibitor of tumor mitochondrial respiration (lonidamine (LND)), which resulted in ultrasound-sensitive microbubbles loaded with O₂ and LND providing prolonged oxygenation relative to oxygenated microbubbles alone. This follow-up study aimed to evaluate the therapeutic response to radiation following the administration of oxygen microbubbles combined with tumor mitochondrial respiration inhibitors in a head and neck squamous cell carcinoma (HNSCC) tumor model. The influences of different radiation dose rates and treatment combinations were also explored. The results demonstrated that the co-delivery of O₂ and LND successfully sensitized HNSCC tumors to radiation, and this was also enhanced with oral metformin, significantly slowing tumor growth relative to unsensitized controls (p < 0.01). Microbubble sensitization was also shown to improve overall animal survival. Importantly, effects were found to be radiation dose-rate-dependent, reflecting the transient nature of tumor oxygenation.

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.

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.

Emerging Applications of Ultrasound-Contrast Agents in Radiation Therapy

Radiation therapy (RT) causes DNA damage through ionization, leading to double-strand breaks. In addition, it generates reactive oxygen species (ROS), which are toxic to tumor cells and the vasculature. However, hypoxic regions in the tumor have been shown to not only decrease treatment response but also increase the likelihood of recurrence and metastasis. Ultrasound-sensitive micro-bubbles are emerging as a useful diagnostic and therapeutic tool within RT. Contrast-enhanced ultrasound (CEUS) has shown great promise in early prediction of tumor response to RT. Ultrasound-triggered micro-bubble cavitation has also been shown to induce bio-effects that can sensitize angiogenic tumor vessels to RT. Additionally, ultrasound can trigger the release of drugs from micro-bubble carriers via localized micro-bubble destruction. This approach has numerous applications in RT, including targeted oxygen delivery before radiotherapy. Furthermore, micro-bubbles can be used to locally create ROS without radiation. Sonodynamic therapy uses focused ultrasound and a sonosensitizer to selectively produce ROS in the tumor region and has been explored as a treatment option for cancer. This review summarizes emerging applications of ultrasound contrast agents in RT and ROS augmentation.

Breast Cancer Brain Metastasis Response to Radiation After Microbubble Oxygen Delivery in a Murine Model

OBJECTIVES

Hypoxic cancer cells have been shown to be more resistant to radiation therapy than normoxic cells. Hence, this study investigated whether ultrasound (US)-induced rupture of oxygen-carrying microbubbles (MBs) would enhance the response of breast cancer metastases to radiation.

METHODS

Nude mice (n = 15) received stereotactic injections of brain-seeking MDA-MB-231 breast cancer cells into the right hemisphere. Animals were randomly assigned into 1 of 5 treatment groups: no intervention, 10 Gy radiation using a small-animal radiation research platform, nitrogen-carrying MBs combined with US-mediated MB rupture immediately before 10 Gy radiation, oxygen-carrying MBs immediately before 10 Gy radiation, and oxygen-carrying MBs with US-mediated MB rupture immediately before 10 Gy radiation. Tumor progression was monitored with 3-dimensional US, and overall survival was noted.

RESULTS

All groups except those treated with oxygen-carrying MB rupture and radiation had continued rapid tumor growth after treatment. Tumors treated with radiation alone showed a mean increase in volume ± SD of 337% ± 214% during the week after treatment. Tumors treated with oxygen-carrying MBs and radiation without MB rupture showed an increase in volume of 383% ± 226%. Tumors treated with radiation immediately after rupture of oxygen-carrying MBs showed an increase in volume of only 41% ± 1% (P = 0.045), and this group also showed a 1 week increase in survival time.

CONCLUSIONS

Adding US-ruptured oxygen-carrying MBs to radiation therapy appears to delay tumor progression and improve survival in a murine model of metastatic breast cancer.

Preserving the Integrity of Surfactant-Stabilized Microbubble Membranes for Localized Oxygen Delivery

Ultrasound contrast agents consist of stabilized microbubbles. We are developing a surfactant-stabilized microbubble platform with a shell composed of Span 60 (Sorbitan monostearate) and an emulsifying agent, water-soluble vitamin E (α-tocopheryl poly(ethylene glycol) succinate, abbreviated as TPGS), named SE61. The microbubbles act both as an imaging agent and a vehicle for delivering oxygen to hypoxic areas in tumors. For clinical use, it is important that a platform be stable under storage at room temperature. To accomplish this, a majority of biologicals are prepared as freeze-dried powders, which also eliminates the necessity of a cold chain. The interfaces among the surfactants, gas, and liquids are subject to disruption in both the freezing and drying phases. Using thermocouples to monitor temperature profiles, differential scanning calorimetry to determine the phase transitions, and acoustic properties to gauge the degree of microbubble disruption, the effects of the freezing rate and the addition of different concentrations of lyoprotectants were determined. Slower cooling rates achieved by freezing the samples in a -20 °C bath were found to be reproducible and produce contrast agents with acceptable acoustical properties. The ionic strength of the solutions and the concentration of the lyoprotectant determined the glass-transition temperature (Tg') of the frozen sample, which determines at what temperature samples can be dried without collapse. Crucially, we found that the shelf stability of surfactant-shelled oxygen microbubbles can be enhanced by increasing the lyoprotectant (glucose) concentration from 1.8 to 5.0% (w/v), which prevents the melt temperature (Tm) of the TPGS phase from rising above room temperature. The increase in glucose concentration results in a lowering of Tm of the emulsifying agent, preventing a phase change in the liquid-crystalline phase and allowing for more stable bubbles. We believe that preventing this phase change is necessary to producing stabilized freeze-dried microbubbles.

Sensitization of Hypoxic Tumors to Radiation Therapy Using Ultrasound-Sensitive Oxygen Microbubbles

PURPOSE

Much of the volume of solid tumors typically exists in a chronically hypoxic microenvironment that has been shown to result in both chemotherapy and radiation therapy resistance. The purpose of this study was to use localized microbubble delivery to overcome hypoxia prior to therapy.

MATERIALS AND METHODS

In this study, surfactant-shelled oxygen microbubbles were fabricated and injected intravenously to locally elevate tumor oxygen levels when triggered by noninvasive ultrasound in mice with human breast cancer tumors. Changes in oxygen and sensitivity to radiation therapy were then measured.

RESULTS

In this work, we show that oxygen-filled microbubbles successfully and consistently increase breast tumor oxygenation levels in a murine model by 20 mmHg, significantly more than control injections of saline solution or untriggered oxygen microbubbles (P < .001). Using photoacoustic imaging, we also show that oxygen delivery is independent of hemoglobin transport, enabling oxygen delivery to avascular regions of the tumor. Finally, we show that overcoming hypoxia by this method immediately prior to radiation therapy nearly triples radiosensitivity. This improvement in radiosensitivity results in roughly 30 days of improved tumor control, providing statistically significant improvements in tumor growth and animal survival (P < .03).

CONCLUSIONS

Our findings demonstrate the potential advantages of ultrasound-triggered oxygen delivery to solid tumors and warrant future efforts into clinical translation of the microbubble platform.

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.

Nanoparticle Loaded Polymeric Microbubbles as Contrast Agents for Multimodal Imaging

Ultrasound contrast agents are typically microbubbles (MB) with a gas core that is stabilized by a shell made of lipids, proteins, or polymers. The high impedance mismatch between the gas core and an aqueous environment produces strong contrast in ultrasound (US). Poly(lactic acid) (PLA) MB, previously developed in our laboratory, have been shown to be highly echogenic both in vitro and in vivo. Combining US with other imaging modalities such as fluorescence, magnetic resonance imaging (MRI), or computerized tomography (CT) could improve the accuracy of many US applications and provide more comprehensive diagnostic information. Furthermore, our MB have the capacity to house a drug in the PLA shell and create drug-loaded nanoparticles in situ when passing through an ultrasound beam. To create multimodal contrast agents, we hypothesized that the polymer shell of our PLA MB platform could accommodate additional payloads. In this study, we therefore modified our current MB by encapsulating nanoparticles including aqueous or organic quantum dots (QD), magnetic iron oxide nanoparticles (MNP), or gold nanoparticles (AuNP) to create bimodality platforms in a manner that minimally compromised the performance of each individual imaging technique.

Preservation of imaging capability in sensitive ultrasound contrast agents after indirect plasma sterilization

Many injectables are not amenable to standard sterilization methods, which destroy sensitive materials. This is particularly true for ultrasound contrast agents (UCA) consisting of gas bubbles stabilized by a surfactant or polymer shell. We investigated a new method to achieve safe and effective sterilization in production by introducing dielectric-barrier discharge non-thermal plasma. A dielectric-barrier discharge was generated to first produce plasma-treated phosphate-buffered saline (PTPBS), which was used as a sterilant solution for our UCA SE61, avoiding direct heat, pressure, chemicals, or radiation. Treated samples were tested for acoustic properties in vitro and in a flow phantom, and for sterility by standard methods. Three minutes plasma treatment of phosphate-buffered saline (PBS) proved effective. The samples showed significant inactivation of inoculated bacteria upon PTPBS treatment as compared to un-treated-PBS (p=0.0022). The treated and untreated samples showed no statistical significance (p>0.05) in acoustic response or bubble diameter (mean±SEM: 2.52±0.31 μm). Nile Red was used to model intercalation of drug in the hydrophobic shell, intercalated successfully into SE61, and was unaffected by plasma treatment. The PTPBS completely sterilized suspensions of UCA, and it did not compromise the acoustic properties of the agent or its ability to retain a hydrophobic compound.

Strategies for neurotrophin-3 and chondroitinase ABC release from freeze-cast chitosan-alginate nerve-guidance scaffolds

Freeze casting, or controlled unidirectional solidification, can be used to fabricate chitosan-alginate (C-A) scaffolds with highly aligned porosity that are suitable for use as nerve-guidance channels. To augment the guidance of growth across a spinal cord injury lesion, these scaffolds are now evaluated in vitro to assess their ability to release neurotrophin-3 (NT-3) and chondroitinase ABC (chABC) in a controlled manner. Protein-loaded microcapsules were incorporated into C-A scaffolds prior to freeze casting without affecting the original scaffold architecture. In vitro protein release was not significantly different when comparing protein loaded directly into the scaffolds with release from scaffolds containing incorporated microcapsules. NT-3 was released from the C-A scaffolds for 8 weeks in vitro, while chABC was released for up to 7 weeks. Low total percentages of protein released from the scaffolds over this time period were attributed to limitation of diffusion by the interpenetrating polymer network matrix of the scaffold walls. NT-3 and chABC released from the scaffolds retained bioactivity, as determined by a neurite outgrowth assay, and the promotion of neurite growth across an inhibitory barrier of chondroitin sulphate proteoglycans. This demonstrates the potential of these multifunctional scaffolds for enhancing axonal regeneration through growth-inhibiting glial scars via the sustained release of chABC and NT-3. Copyright © 2014 John Wiley & Sons, Ltd.

In vitro gene delivery with ultrasound-triggered polymer microbubbles

In the work described here, gene delivery using polymer microbubbles triggered by ultrasound in vitro was investigated. The effects of pressure amplitude (0-2 MPa), center frequency (1-5 MHz), pulse length (3-12,000 μs), pulse repetition frequency (5-20,000 Hz) and exposure time (0-30 s) on transfection efficiency and cell viability were examined. The effects of radiation force, calcium ion concentration and timing of treatments were also examined. Cells were successfully transfected with pressure amplitudes as low as 250 kPa. Transfection was most efficient at lower frequencies and longer pulse lengths, with a transfection efficiency of 24.2 ± 2.0% achieved using a center frequency of 1 MHz, pressure amplitude of 1 MPa, pulse length of 12,000 μs and pulse repetition frequency of 5 Hz. Gene delivery was also affected by the extracellular calcium ion concentration and the timing of treatments.

An ice-templated, linearly aligned chitosan-alginate scaffold for neural tissue engineering

Several strategies have been investigated to enhance axonal regeneration after spinal cord injury, however, the resulting growth can be random and disorganized. Bioengineered scaffolds provide a physical substrate for guidance of regenerating axons towards their targets, and can be produced by freeze casting. This technique involves the controlled directional solidification of an aqueous solution or suspension, resulting in a linearly aligned porous structure caused by ice templating. In this study, freeze casting was used to fabricate porous chitosan-alginate (C/A) scaffolds with longitudinally oriented channels. Chick dorsal root ganglia explants adhered to and extended neurites through the scaffold in parallel alignment with the channel direction. Surface adsorption of a polycation and laminin promoted significantly longer neurite growth than the uncoated scaffold (poly-L-ornithine + Laminin = 793.2 ± 187.2 μm; poly-L-lysine + Laminin = 768.7 ± 241.2 μm; uncoated scaffold = 22.52 ± 50.14 μm) (P < 0.001). The elastic modulus of the hydrated scaffold was determined to be 5.08 ± 0.61 kPa, comparable to reported spinal cord values. The present data suggested that this C/A scaffold is a promising candidate for use as a nerve guidance scaffold, because of its ability to support neuronal attachment and the linearly aligned growth of DRG neurites.

Cellular signal transduction can be induced by TRAIL conjugated to microcapsules

The extracellular agent tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis in tumor cells but spare normal cells. Ligation of TRAIL to a nanoparticle would serve to facilitate targeting to an extravascular site. Polymeric ultrasound contrast agents (UCA) (microencapsulated gas bubbles) can be tracked by ultrasound imaging, and fragmented into nanoparticles by focused ultrasound. This tumor-targeted delivery system has been shown to deliver more efficiently than solid nanoparticles. Additionally, small molecule inhibitors such as bortezomib, shown to sensitize TRAIL-resistant cells, could be co-administered within these UCA. In this pilot study, TRAIL was conjugated to UCA while preserving the agent's sensitivity to ultrasound. Human cancer cell lines, OVCAR-3 and A2058, were bathed with the TRAIL-UCA with and without the addition of bortezomib. Apoptosis was quantified using flow cytometry. OVCAR-3 treated with TRAIL-UCA exhibit significant (p < 0.05) apoptotosis compared to unmodified UCA, equal to positive controls, but no synergistic effect when combined with bortezomib. A2058 cells treated with TRAIL-UCA also exhibited significant apoptosis (p < 0.01) compared to unmodified UCA, similar to positive controls and bortezomib significantly increased apoptosis in combination with TRAIL-UCA. We conclude that TRAIL-ligated UCA show exciting potential as a new therapy.

Disposition of ultrasound sensitive polymeric drug carrier in a rat hepatocellular carcinoma model

RATIONALE AND OBJECTIVES

A doxorubicin-loaded microbubble has been developed that can be destroyed with focused ultrasound resulting in fragments, or "nanoshards" capable of escaping through the leaky tumor vasculature, promoting accumulation within the interstitium. This study uses a rat liver cancer model to examine the biodistribution and tumoral delivery of this microbubble platform compared with de novo drug-loaded polymer nanoparticles and free doxorubicin.

MATERIALS AND METHODS

Microbubbles (1.8 μm) and 217-nm nanoparticles were prepared containing 14-C labeled doxorubicin. Microbubbles, nanoparticles, a combination of the two, or free doxorubicin were administered intravenously in rats bearing hepatomas, concomitant with tumor insonation. Doxorubicin levels in plasma, organs, and tumors were quantified after 4 hours and 7 and 14 days. Tumors were measured on sacrifice and evaluated with autoradiography and histology.

RESULTS

Animals treated with microbubbles had significantly lower plasma doxorubicin concentrations (0.466 ± 0.068%/mL) compared with free doxorubicin (3.033 ± 0.612%/mL, P = .0019). Drug levels in the myocardium were significantly lower in animals treated with microbubbles compared to free doxorubicin (0.168%/g tissue vs. 0.320%/g, P = .0088). Tumors treated with microbubbles showed significantly higher drug levels than tumors treated with free doxorubicin (2.491 ± 0.501 %/g vs. 0.373 ± 0.087 %/g, P = .0472). These tumors showed significantly less growth than tumors treated with free doxorubicin (P = .0390).

CONCLUSIONS

Doxorubicin loaded microbubbles triggered with ultrasound provided enhanced, sustained drug delivery to tumors, reduced plasma and myocardium doxorubicin levels, and arresting tumor growth. The results suggest that in situ generation of nano particles provides a superior treatment over injection of free drug and also de novo synthesized nanoparticles.

Doxorubicin and paclitaxel loaded microbubbles for ultrasound triggered drug delivery

A polymer ultrasound contrast agent (UCA) developed in our lab has been shown to greatly reduce in size when exposed to ultrasound, resulting in nanoparticles less than 400 nm in diameter capable of escaping the leaky vasculature of a tumor to provide a sustained release of drug. Previous studies with the hydrophilic drug doxorubicin (DOX) demonstrated enhanced drug delivery to tumors when triggered with ultrasound. However the therapeutic potential has been limited due to the relatively low payload of DOX. This study compares the effects of loading the hydrophobic drug paclitaxel (PTX) on the agent's acoustic properties, drug payload, tumoricidal activity, and the ability to deliver drugs through 400 nm pores. A maximum payload of 129.46 ± 1.80 μg PTX/mg UCA (encapsulation efficiency 71.92 ± 0.99%) was achieved, 20 times greater than the maximum payload of DOX (6.2 μg/mg), while maintaining the acoustic properties. In vitro, the tumoricidal activity of paclitaxel loaded UCA exposed to ultrasound was significantly greater than controls not exposed to ultrasound (p<0.0016). This study has shown that PTX loaded UCA triggered with focused ultrasound have the potential to provide a targeted and sustained delivery of drug to tumors.

Neural progenitor cells grown on hydrogel surfaces respond to the product of the transgene of encapsulated genetically engineered fibroblasts

Engineered tissue strategies for central nervous system (CNS) repair have the potential for localizing treatment using a wide variety of cells or growth factors. However, these strategies are often limited by their ability to address only one aspect of the injury. Here we report the development of a novel alginate construct that acts as a multifunctional tissue scaffold for CNS repair, and as a localized growth factor delivery vehicle. We show that the surface of this alginate construct acts as an optimal growth environment for neural progenitor cell (NPC) attachment, survival, migration, and differentiation. Importantly, we show that tailor-made alginate constructs containing brain-derived neurotrophic factor or neurotrophin-3 differentially direct lineage fates of NPCs and may therefore be useful in treating a wide variety of injuries. It is this potential for directed differentiation of a scaffold prior to implantation at the injury site that we explore here.

Targeted binding of PEG-lipid modified polymer ultrasound contrast agents with tiered surface architecture

In order for site-directed polymer ultrasound contrast agents (UCAs) to provide acoustic enhancement at disease sites to distinguish normal tissue from diseased tissue, the surface of these agents must be functionalized with mixtures of grafted polymers. Here a combination of longer liganded polyethylene glycol (PEG)-lipids and shorter unliganded PEG-lipids were introduced into the oil phase of a modified solvent evaporation double emulsion method for preparing UCAs. UCAs with different lengths of both liganded and unliganded lipids were imaged under 7.5 MHz ultrasound. The B-mode image brightness of the mixed PEG-lipid UCAs was within 1 dB the brightness of the unliganded surface. After 15 min of continuous insonation, 70% of the contrast signal remained. The peptide arginine-glycine-aspartic acid (RGD) was added to the surface of these UCAs through a biotin-avidin linkage and binding was assessed under static and shear conditions. Binding was significant after 30 min of static incubation and the adherence of the UCA increased under shear flow from 3 UCA/cell (static) to 5 UCA/cell (shear).

Preserving enhancement in freeze-dried contrast agent ST68: Examination of excipients

The perfluorcarbon (perfluorobutane) ultrasound contrast agent ST68, composed of sonicated mixtures of non-ionic surfactants, is stable in solution for only a few weeks at 4 degrees C. Freeze-drying critically diminished ST68's ability to reflect ultrasound (its echogenicity). A method of incorporating specific lyoprotectants before lyophilization was investigated. Reintroduction of perfluorobutane to the protected freeze-dried sample, followed by reconstituting with water preserved echogenicity. Glucose, trehalose, sucrose, and mannitol were tested at 100mM and in vitro echogenicity data was collected from samples with dose concentrations of 50-300microl/l. Glucose was found to be the best lyoprotectant providing an average (n=3) maximum peak enhancement of 23.2+/-1.2dB in vitro, measured at 5MHz, 684kPa, and a pulse repetition frequency (PRF) of 100Hz (p<0.05 over freeze-dried ST68 control) and 20.8+/-0.8dB in vivo in New Zealand white rabbits at 5MHz and a PRF of 6.7kHz. Pulse inversion harmonic US images of a rabbit kidney, pre- and post-contrast injection (0.1ml/kg), showed excellent enhancement and clear vascular delineation, similar to that of the original agent. For the first time this contrast agent can be successfully freeze-dried yielding a longer self-life without the need for refrigeration.

Nano-sized ultrasound contrast agent: salting-out method

Tumor imaging by ultrasound is greatly enhanced by the use of ultrasound contrast agents (UCAs), stabilized, gas-filled bodies. They are generally less than 7 microm to pass freely through the capillary bed. Development of a nano-sized agent would enable them to extravasate through the leaky pores of angiogenic vessels. We describe the development of an echogenic, nano-sized polylactic acid UCA by adaptation of a salting-out method. The viscosity of the initial colloidal suspension (concentration and molecular weight of protective colloid [polyvinyl alcohol (PVA)] and concentration of polymer) was key in determining particle size and polydispersity (increasing viscosity increased both). Addition of the porogens ammonium carbonate and camphor, required to produce hollow echogenic capsules, also increased the size (eg, 5 wt% PVA, mean solid nanocapsule size 386 +/- 25 nm, polydispersity index 0.367 +/- 0.14, and mean UCA size 640 +/- 18 nm, polydispersity index 0.308 +/- 0.027). Viscosity had the opposite effect on echogenicity of the resultant nano-UCA, which ranged from 21.69 +/- 0.78 dB for 2% PVA to 12.1 +/- 0.8 dB for 10% PVA. The UCA prepared with 10% PVA, however, had a longer half-life in the ultrasound beam (t(1/2) > 15 minutes vs t(1/2) < 10 minutes), suggesting a thicker shell. Optimization will require compromise among size, echogenicity, and stability.

Development and optimization of a doxorubicin loaded poly(lactic acid) contrast agent for ultrasound directed drug delivery

An echogenic, intravenous drug delivery platform is proposed in which an encapsulated chemotherapeutic can travel to a desired location and drug delivery can be triggered using external, focused ultrasound at the area of interest. Three methods of loading poly(lactic acid) (PLA) shelled ultrasound contrast agents (UCA) with doxorubicin are presented. Effects on encapsulation efficiency, in vitro enhancement, stability, particle size, morphology and release during UCA rupture are compared by loading method and drug concentration. An agent containing doxorubicin within the shell was selected as an ideal candidate for future hepatocellular carcinoma studies. The agent achieved a maximal drug load of 6.2 mg Dox/g PLA with an encapsulation efficiency of 20.5%, showed a smooth surface morphology and tight size distribution (poly dispersity index=0.309) with a peak size of 1865 nm. Acoustically, the agent provided 19 dB of enhancement in vitro at a dosage of 10 microg/ml, with a half life of over 15 min. In vivo, the agent provided ultrasound enhancement of 13.4+/-1.6 dB within the ascending aorta of New Zealand rabbits at a dose of 0.15 ml/kg. While the drug-incorporated agent is thought to be well suited for future drug delivery experiments, this study has shown that agent properties can be tailored for specific applications based on choice of drug loading method.

Preclinical acute toxicology study of surfactant-stabilized ultrasound contrast agents in adult rats

Gas-filled microbubbles are used as contrast agents in diagnostic ultrasound imaging. A preclinical, acute toxicity study of 2 surfactant-stabilized ultrasound contrast agents (ST68 and ST44) was conducted. Subjects were 104 Sprague-Dawley rats (experimental doses, 0.1, 0.2, 0.8, and 1.0 mL/kg; control, 1.0 mL/kg saline) that were studied for 14 days after contrast; clinical signs, weight, blood, and urine were evaluated. Histopathology was performed following euthanasia. Of the 40 animals receiving ST44, 4 died prematurely and a dose dependency was demonstrated (P = .011), whereas in the ST68 groups only 1 death occurred (no dose dependency; P = .48). Only the weight of rats injected with ST44 varied significantly (P = .0003). This dependency was also found for 3 of 5 urine parameters and 4 of 36 blood parameters (P < .05). For ST68, only 1 urine parameter showed significance (P < .0001). Giant cell infiltration in the lungs was significantly higher than controls in the ST44 0.1 mL/kg and the ST68 0.8-1.0 mL/kg groups (P < .01). It is concluded that the prudent choice for future nonrodent, toxicology studies and potentially for human clinical trials is ST68 (given the deaths in the ST44 groups).

Ultrasound triggered cell death in vitro with doxorubicin loaded poly lactic-acid contrast agents

Traditional chemotherapy generally results in systemic toxicity, which also limits drug levels at the area of need. Two ultrasound contrast agents (UCA), with diameters between 1-2 microm in diameter and shell thicknesses of 100-200 nm, composed of poly lactic-acid (PLA), one loaded by surface adsorption and the other loaded by drug incorporation in the shell, were compared in vitro for potential use in cancer therapy. These poly lactic-acid (PLA) UCA platforms contain a gas core that in an ultrasound (US) field can cause the UCA to oscillate or rupture. Following a systemic injection of drug loaded UCA with external application of US focused at the area of interest, this platform could potentially increase drug toxicity at the area of need, while protecting healthy tissue through microencapsulation of the drug. In vitro toxicity in MDA-MB-231 breast cancer cells of the surface-adsorbed and shell-incorporated doxorubicin (Dox) loaded UCA were examined at 5 MHz insonation using a pulse repetition frequency of 100 Hz at varying pressure amplitudes. Both platforms resulted in equivalent cell death compared to free Dox and US when insonated at peak positive pressure amplitudes of 1.26 MPa and above. While no significant changes in cell death were seen for surface adsorbed Dox-UCA with or without insonation, cell death using the platform with Dox incorporated within the shell increased from 16.12% to 25.78% (p=0.0272), approaching double the potency of the platform when insonated at peak positive pressure amplitudes of 1.26 MPa and above. This mechanism is believed to be the result of UCA rupture at higher insonation pressure amplitudes, resulting in more exposed drug and shell surface area as well as increased cellular uptake of Dox containing polymer shell fragments. This study has shown that a polymer UCA with drug housed within the shell may be used for US-triggered cell death. US activation can be used to make a carrier significantly more potent once in the area of need.

Plasma sterilization of poly lactic acid ultrasound contrast agents: surface modification and implications for drug delivery

Poly lactic acid (PLA) ultrasound contrast agents (CA) have been developed previously in our laboratory for ultrasound (US) imaging, as well as surface coated with doxorubicin to create a potential targeted platform of chemotherapeutic delivery using focused US. However, we have previously found it impossible to sterilize these agents while at the same time maintaining their acoustic properties, a task that would probably require fabrication within a clean facility. The purpose of this paper is to investigate the feasibility of using plasma to sterilize these CA while maintaining maximum echogenicity, a step that would greatly facilitate in vivo investigations. Effects of plasma exposure time (1, 3 and 6 min) and intensity (low-10 mA, 6.8 W; medium-15 mA, 10.5 W; and high-25 mA, 18 W) on the CAs' acoustic properties, surface morphology, zeta potential, capacity to carry chemotherapeutics and overall sterility are described. Both increases in plasma intensity and exposure time increased CA zeta potential and also significantly increased drug payload. High-intensity plasma exposure for 3 min was found to be an optimal sterilization protocol for maximal (100%) preservation of CA echogenicity. Plasma exposure resulted in sterile samples and maintained original CA enhancement of 20 dB and acoustic half-life over 75 min, while increasing CA zeta potential by 11 mV and doxorubicin loading efficiency by 10%. This study not only shows how a highly temperature- and pressure-sensitive agent can be sterilized using plasma, but also that surface modification can be used to increase surface binding of the drug.

Delivery of encapsulated Doxorubicin by ultrasound-mediated size reduction of drug-loaded polymer contrast agents

Low delivery efficiency combined with systemic toxicity of traditional chemotherapy provides a need for improved chemotherapeutic delivery. Within our laboratory, we have developed polymer ultrasound contrast agents (1.2-1.8 mum in diameter) containing doxorubicin (Dox) within the shell (100-150 nm). In vivo this platform is expected to circulate through the vasculature until activated at the tumor site with external focused ultrasound (US). In vitro, the agent is responsive to US and when insonated at peak positive pressure amplitudes of 0.69 MPa and above, shows dramatic size reduction, eventually reaching a mean particle size of 350 nm, presumably due to fragmentation of, or gas release from the agent. The resulting Dox-polymer particles retain the drug and are small enough to pass through the leaky pores (350-400 nm) within the tumor vasculature, providing a sustained intratumoral release of chemotherapeutic as the polymer degrades. In vivo studies using a VX2 liver tumor model have shown that the combination of the agent and US results in nearly 50% less drug delivered to the nontargeted, healthy liver ( p = 0.009) and a 110% increase ( p = 0.004) in Dox delivery to the viable peripheral tissue of the tumor, relative to the uninsonated controls. This study shows how US-mediated destruction of drug-loaded polymer contrast agent can be used to deliver encapsulated drug for potential sustained release. Penetration mechanisms of these resulting particles and their ability to provide a sustained release from the tumor interstia will be explored in the future.

Formulation of polylactide-co-glycolic acid nanospheres for encapsulation and sustained release of poly(ethylene imine)-poly(ethylene glycol) copolymers complexed to oligonucleotides

Antisense oligonucleotides (AOs) have been shown to induce dystrophin expression in muscles cells of patients with Duchenne Muscular Dystrophy (DMD) and in the mdx mouse, the murine model of DMD. However, ineffective delivery of AOs limits their therapeutic potential. Copolymers of cationic poly(ethylene imine) (PEI) and non-ionic poly(ethylene glycol) (PEG) form stable nanoparticles when complexed with AOs, but the positive surface charge on the resultant PEG-PEI-AO nanoparticles limits their biodistribution. We adapted a modified double emulsion procedure for encapsulating PEG-PEI-AO polyplexes into degradable polylactide-co-glycolic acid (PLGA) nanospheres. Formulation parameters were varied including PLGA molecular weight, ester end-capping, and sonication energy/volume. Our results showed successful encapsulation of PEG-PEI-AO within PLGA nanospheres with average diameters ranging from 215 to 240 nm. Encapsulation efficiency ranged from 60 to 100%, and zeta potential measurements confirmed shielding of the PEG-PEI-AO cationic charge. Kinetic measurements of 17 kDa PLGA showed a rapid burst release of about 20% of the PEG-PEI-AO, followed by sustained release of up to 65% over three weeks. To evaluate functionality, PEG-PEI-AO polyplexes were loaded into PLGA nanospheres using an AO that is known to induce dystrophin expression in dystrophic mdx mice. Intramuscular injections of this compound into mdx mice resulted in over 300 dystrophin-positive muscle fibers distributed throughout the muscle cross-sections, approximately 3.4 times greater than for injections of AO alone. We conclude that PLGA nanospheres are effective compounds for the sustained release of PEG-PEI-AO polyplexes in skeletal muscle and concomitant expression of dystrophin, and may have translational potential in treating DMD.

Polymeric ultrasound contrast agents targeted to integrins: importance of process methods and surface density of ligands

The use of injectable gas-filled microbubbles during ultrasound imaging is accepted as a good method to increase image contrast. Site-targeted microbubbles are expected to provide higher sensitivity and specificity than blood pool contrast agents (CAs). We have shown that covalent attachment of GRGDS peptide fragments to the surface of poly(lactic acid) CAs facilitates attachment to MDA-MB-231 human breast cancer cells in vitro. This paper examines the effect of process conditions during microbubble fabrication and ligand attachment and also changes in ligand surface density and shows that they have important effects on in vitro acoustic response and CA adhesion to breast cancer and cell lines. Use of intermittent sonication in the emulsion step, shortening of reaction times, and increase in freeze-drying times allows for a reduction of 50% in the dose of GRGDS-modified capsules (from 0.16 to 0.012 mg/mL) required to achieve a maximum enhancement of 20 dB; signal loss after 15 min insonation of GRGDS-modified capsules is reduced from a loss of 60% to a loss of 40%, and cell attachment after 10 min contact time is increased from an average of 1.4 +/- 0.86 to 1.8 +/- 0.17 capsules/cell. Optimal attachment is achieved with a molar ratio of total -COOH groups to GRGDS of 1:0.5. The effect of process conditions during microcapsule fabrication, ligand attachment, and ligand surface density on in vitro acoustic response and CA adhesion to breast cancer cell lines in tissue culture are shown to be important parameters that can aid in the future design of an ultrasound CA that allows both cancer detection and treatment, potentially by targeted drug delivery.

Comparison of in vitro and in vivo acoustic response of a novel 50:50 PLGA contrast agent

A comparison between in vitro and in vivo experiments conducted to investigate the acoustic properties of a novel, 1.2 microm diameter poly(lactic-co-glycolic acid) (50:50) (PLGA) ultrasound contrast agent, the development of which was described previously by us, is presented. A pulse-echo setup was used to determine enhancement in vitro. Additional in vitro studies further characterized the hollow microcapsules, including resonance frequency from attenuation measurements (from 2.25 to 15 MHz) and temperature effects (25 degrees C vs. 37 degrees C). In vivo, four rabbits received intravenous injections of the agent (dose range: 0.005-0.13 ml/kg). Quantitative in vivo dose-responses were calculated off-line using spectral power analysis of audio Doppler signals acquired from a custom-made 10 MHz cuff transducer placed around the surgically exposed distal aorta. This frequency was chosen since the very shallow scanning depths encountered in rabbits, in particular for the cuff transducer placed directly around the vessel, necessitates the use of high frequency imaging devices with sufficient spatial resolution to enable meaningful measurements. For qualitative assessments, two rabbits were imaged pre- and post-contrast administration (dose: 0.1 ml/kg) in power Doppler mode. Significant acoustic enhancements (up to 24 dB) were reported both in vitro and in vivo. Moreover, the rabbits did not show any adverse side effects from multiple injections (>20) of the agent. Measured in vitro resonance frequency between 3.09 and 3.49 MHz was lower than predicted for a similar sized free bubble, potentially due to capsule wall structure. Minimal loss of signal (approximately 4 dB) was observed at 25 degrees C over 20 min of insonation at 5 MHz but at 37 degrees C the signal dropped close to base line within the first 5 min. This temperature sensitivity could be due to loss of capsule integrity (and hence loss of gas). Potential causes include increased hydrolysis or polymer softening and increased water uptake by the shell at temperatures closer to the glass transition temperature (T(g)).

Surfactant-stabilized contrast agent on the nanoscale for diagnostic ultrasound imaging

Ultrasound contrast agents (CA) are generally micron-sized stabilized gas bubbles, injected IV. However, to penetrate beyond the vasculature and accumulate in targets such as tumors, CA must be an order of magnitude smaller. We describe a method of achieving nanometer-sized, surfactant-stabilized CA by differential centrifugation. High g force was shown to destroy bubble integrity. Optimal conditions (300 rpm for 3 min) produced an agent with a mean diameter of 450 nm, which gave 25.5 dB enhancement in vitro at a dose of 10 microL/mL, with a 13 min half-life. In vivo, the CA produced excellent power Doppler and grey-scale pulse inversion harmonic images at low acoustic power when administered. In vivo dose-response curves obtained in three rabbits showed enhancement between 20 and 25 dB for dosages above 0.025 mL/kg. These results encourage further investigation of the possible diagnostic and therapeutic benefits of using nanoparticles as CA, including passive targeting and accumulation in tumors.

Alginate Encapsulated BDNF-Producing Fibroblast Grafts Permit Recovery of Function after Spinal Cord Injury in the Absence of Immune Suppression

Encapsulation of cells has the potential to provide a protective barrier against host immune cell interactions after grafting. Previously we have shown that alginate encapsulated BDNF-producing fibroblasts (Fb/BDNF) survived for one month in culture, made bioactive neurotrophins, survived transplantation into the injured spinal cord in the absence of immune suppression, and provided a permissive environment for host axon growth. We extend these studies by examining the effects of grafting encapsulated Fb/BDNF into a subtotal cervical hemisection on recovery of forelimb and hindlimb function and axonal growth in the absence of immune suppression. Grafting of encapsulated Fb/BDNF resulted in partial recovery of forelimb usage in a test of vertical exploration and of hindlimb function while crossing a horizontal rope. Recovery was significantly greater compared to animals that received unencapsulated Fb/BDNF without immune suppression, but similar to that of immune suppressed animals receiving unencapsulated Fb/BDNF. Immunocytochemical examination revealed neurofilament (RT-97), 5-HT, CGRP and GAP-43 containing axons surrounding encapsulated Fb/BDNF within the injury site, indicating axonal growth. BDA labeling however showed no evidence of regeneration of rubrospinal axons in recipients of encapsulated Fb/BDNF, presumably because the amounts of BDNF available from the encapsulated grafts are substantially less than those provided by the much larger numbers of Fb/BDNF grafted in a gelfoam matrix in the presence of immune suppression. These results suggest that plasticity elicited by the BDNF released from the encapsulated cells contributed to reorganization that led to behavioral recovery in these animals and that the behavioral recovery could proceed in the absence of rubrospinal tract regeneration. Alginate encapsulation is therefore a feasible strategy for delivery of therapeutic products produced by non-autologous engineered fibroblasts and provides an environment suitable for recovery of lost function in the injured spinal cord.

Effect of shell type on the in vivo backscatter from polymer-encapsulated microbubbles

This study compared in vivo enhancement from four different polymer-encapsulated ultrasound (US) contrast agents. The agents were produced with a rigid shell composed of the biodegradable block copolymer poly[D,L-lactide-co-glycolide] (PLGA) with the lactic and glycolic acid ratios 50:50, 75:25, 85:15 and 100:0 (i.e., increasingly hydrophobic shell compositions). Approximately the same bubble diameter (1.2 microm) and concentration (0.4 g/mL) were obtained for each agent. In four rabbits, audio Doppler signals were acquired from a 10 MHz cuff transducer placed around a surgically exposed vessel (contrast dose: 0.0125 to 0.15 mL/kg). In vivo dose responses were calculated off-line (in dB). Nine rabbit kidneys were imaged during contrast administration (0.1 mL/kg) in power Doppler and grey-scale pulse inversion harmonic (PIHI) modes using an HDI 5000 scanner (Philips Medical Systems, Bothell, WA). Time-intensity curves were produced and the time-to-peak, peak intensity, slope, area under the curve (AUC) and total duration of enhancement for each agent were compared. All agents produced marked Doppler enhancement with increasing duration from the 50:50 agent (48 +/- 10 s) to the 75:25 agent (166 +/- 46 s), the 85:15 agent (403 +/- 83 s) and with the 100:0 agent (603 +/- 93 s) lasting longest (p < 0.02). No other parameters changed significantly, except the AUC of the 85:15 agent, which was greater than that of the 50:50 agent (190.75 vs. 61.58; p = 0.02). The in vivo dose-response curves were similar for all agents, with mean enhancement up to 20.6 +/- 1.11 dB (p = 0.17). In conclusion, contrast duration increases by an order of magnitude as the lactic acid component in the polymer-encapsulated bubbles increases and the shell, thus, becomes increasingly hydrophobic.

Ultrasound degradation of novel polymer contrast agents

This report describes an investigation into factors affecting the degradation of novel poly(lactic-co-glycolic acid) (PLGA) contrast agents. Contrast agents fabricated by two different methods and varying in acoustic properties were compared. The effect of ultrasound frequency (5 and 10 MHz) on degradation of the microcapsules was also studied. High-performance liquid chromatography was used to quantify the production of lactic and glycolic acid to monitor agent degradation. The degradation pattern from the microcapsules was found to be closely related to capsule morphology; the more acoustically efficient capsules (maximum enhancement of 25 dB at 5 MHz with 0.004 mg/mL) degraded at a faster rate than those with lower acoustical efficiency (maximum enhancement of 25 dB at 5 MHz only achieved with 0.6 mg/mL). The capsules also degraded fastest when insonated at the frequency at which they gave highest backscatter. In addition, despite the use of a 50:50 PLGA copolymer, more glycolic than lactic acid was released at early time points, which reflects the greater hydrophilicity of the glycolic acid residues, and greater degradation rate of glycolic acid repeat units. The results from this study provided unique insight into the degradation behavior of hollow PLGA microcapsules, and their potential in ultrasound diagnosis and therapy.

Development and characterization of a nano-scale contrast agent

Agents injected parenterally must be less than approximately 8 microm diameter in order to traverse the capillaries in the pulmonary bed, but these agents remain in the vasculature until they are eliminated from the body by a variety of mechanisms. Targeting of cells outside the capillaries requires agent diameters of less than approximately 700 nm to enable escape through the larger-than-usual pores that have been noted in the leaky vasculature of a tumor. The objective of this study was to test the feasibility of creating a surfactant-stabilized nano-bubble with favorable acoustic properties, and identify the key parameters that influence size, yield and stability. Size distribution was characterized using laser light scattering. In vitro acoustic enhancement was assessed by generation of dose and time response curves. We previously developed a successful protocol to generate gas-filled microbubbles (containing perfluorocarbon, sulfur hexafluoride or air) with mean diameter of 1.5 microm, using sonication of carefully selected surfactant mixtures. This presentation describes generation of nano-bubbles with mean diameters ranging from 700 to 450 nm, depending on process variables. In all cases a centrifugation step was employed to separate the nano-sized particles. The in vitro dose response curves gave a maximum of 23-27 dB enhancement compared to buffer in the absence of agent, with the maximum enhancement and presence of shadowing at higher doses being dependent on the fabrication protocol. The effect of sonication time for solutions containing a mixture of the surfactants (Span 60 and Tween 80) was also tested, but was determined not to be an influencing factor. Future studies will involve development of a mathematical model characterizing the mean size as a function of centrifugal force, spin time and initial size distribution. Future work will also include imaging of tumor-bearing mice and measuring imaging potential in vivo in New Zealand white rabbits using power Doppler.

Polymeric contrast agent with targeting potential

The Arg-Gly-Asp (RGD) peptide sequence was conjugated to poly (lactid acid), (PLA), microcapsules. These hollow, biodegradable PLGA microcapsules were developed in our laboratory for use as ultrasound contrast agents. By modifying the surface of the agent with a targeting ligand, it can be targeted to a specific address within the body. This application is ideal for both targeted imaging and/or targeted drug delivery. Integrins are membrane-spanning proteins in cells that play a vital role in cell attachment and many other processes. The RGD peptide sequence targets integrins expressed during angiogenesis, alphavbeta3 and alphavbeta5. The integrins specific to angiogenesis are more active during cancer and can be used as receptors for the RGD-conjugated contrast agents. Although the generic RGD sequence is not specific to only alphavbeta3 and alphavbeta5 integrins, it is an excellent candidate for proof of concepts studies such as described here. Preliminary in vitro results indicate that the modified capsules remain highly echogenic (maximum enhancement of 20 dB in vitro) and adhere specifically to a breast cancer cell line MDA-MB-231 in static experiments. However, no adherence is seen with either unmodified capsules (negative control), or when cells that have been pre-saturated with RGD ligand are contacted with modified capsules (positive control). Specific targeting of ultrasound contrast agents could lead the way to imaging as a method for discrimination of malignant from benign.

Peptide-modified alginate surfaces as a growth permissive substrate for neurite outgrowth

Different strategies are being investigated for treatment of spinal cord injuries, one of the most promising being application of neurotrophic factors, which have been shown to prevent neuronal death and stimulate regeneration of injured axons. Ex vivo gene therapy has emerged as the leading delivery method at the site of the injury, and we have shown previously that encapsulating genetically engineered fibroblasts in an immunoprotective alginate capsule can permit implantation of the factor-secreting cells without need for immunosuppression. This strategy could be greatly enhanced by providing the sprouting neurons with a permissive substrate upon which to attach and grow. We report here studies on the modification of an alginate gel surface by either coating it with laminin or by covalent attachment of YIGSR peptide. Using NB2a neuroblastoma cells, we found that native alginate elicited minimal cell attachment ( approximately 1.5%); however, YIGSR-alginate conjugate elicited a fivefold increase in numbers of cells attached using peptide ratios of 0.5 and 1 mg/g alginate, ranging from 9.5% of the cells at the lower ratio, to about 44% at the higher. Only a further 19% increase was obtained at an increased peptide density of 2 mg/g alginate ( approximately 63% over control). Laminin-coated gels showed approximately 60% cell attachment. However, laminin coating did not stimulate differentiation and neurite growth, whereas both numbers and lengths of outgrowths increased with increasing peptide density on peptide-modified alginate. We demonstrate here the ability of the peptide-modified alginate gels to allow adhesion of NB2a neuroblastoma cells and to promote neurite outgrowth from these cells when attached to the peptide-modified alginate surface. Also, we show that the adhesion of NB2a neuroblastoma cells and neurite outgrowth from the attached cells is a function of the peptide density on the gel surface.

Development of a novel method for synthesis of a polymeric ultrasound contrast agent

Medical ultrasound is a highly valuable diagnostic tool, especially when compared with other imaging modalities. It is a noninvasive, real-time, portable, extremely safe method compared with X-ray and inexpensive relative to magnetic resonance imaging. However, ultrasound is limited in its ability to distinguish between diseased and normal tissue. This limitation has led to the development of contrast agents. We have produced novel poly (lactic-co-glycolic) acid air-filled microcapsules that work well as ultrasound contrast agents, giving up to 24 and 25 dB enhancement when insonated in the medical imaging range at 5 and 7.5 MHz, respectively. The capsules were fabricated by modifying a double emulsion method to encapsulate camphor in the oil phase and ammonium carbonate in the aqueous phase, and later sublime the encapsulated material, leaving voids capable of being filled with a gas in their place. The role of the surfactant, poly vinyl alcohol, solution temperature, was studied and found to play an important role in the morphology of the capsules, altering their acoustic response.

Microencapsulated liposomes in controlled drug delivery: strategies to modulate drug release and eliminate the burst effect

The release of fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) from alginate-microencapsulated liposomes was studied to evaluate the properties of this system for controlled drug delivery. Liposomes composed of phosphatidylcholine (PC) and cholesterol (Chol) (molar ratio 7:3) and of PC, phosphatidylglycerol (PG), and cholesterol (6:1:3) were encapsulated in alginate (Alg) crosslinked with Ca(2+) (Ca-Alg), Al(3+) (Al-Alg), and Ba(2+) (Ba-Alg). Capsules were coated with poly(l-ornithine) followed by a final alginate coat. A rapid initial burst of protein release was observed from liposomes encapsulated in Ca-Alg and Al-Alg. No burst was observed when liposomes were encapsulated in Ba-Alg, indicating that the crosslinking ions could significantly affect the release of entrapped protein. Also, the release from encapsulated liposomes varied significantly with liposome composition, especially with Ca-Alg as observed with encapsulation of PC, dioleoylphosphatidylcholine (DOPC), and DOPC/Chol liposomes. Cholesterol increased the leakiness of the liposomes after encapsulation. In all cases, the release from microencapsulated liposomes was much faster than that from free liposomes suggesting an interaction between the liposomes and the alginate. Differential scanning calorimetry supports the hypothesis that alginate was inserted into the lipid bilayer resulting in a rapid release of protein from microencapsulated liposomes. Moreover, it was observed that the degree of interaction between liposomes and alginate varied with liposome composition.

Generation of ultraharmonics in surfactant based ultrasound contrast agents: use and advantages

A unique distinction between surfactant stabilized ultrasound contrast agent ST68 and water (or tissue), is the enhanced ability of the agent to generate non-linear frequencies such as sub-harmonics (f0/2), higher harmonics (2fo, 3fo, 4fo,...), and ultraharmonics (3f0/2, Sf0/2, 7f0/2,...), when insonated with fundamental frequency f0. Currently, second harmonics (2f0) have been predominantly researched, to exploit the diagnostic benefits of the contrast-specific non-linear imaging. However, we found that at normal imaging pressures (100 kPa-1 MPa), ST68 agent-generated second harmonic enhancements dropped to approximately 8 dB at 100 kPa and approximately 2 dB at 1 MPa. Moreover, at these pressures water (or tissue) produced strong second harmonics due to non-linear propagation. Ultraharmonics and sub-harmonics on the other hand, were generated only by the agent, and were not produced due to the non-linear propagation of ultrasound in either water or tissue. Additionally, ultraharmonic (3f0/2) enhancements of approximately 23 dB at 100 kPa, approximately 35 dB at 0.5 MPa and approximately 41dB at 1.1 MPa for ST68-PFC, offer much greater signal to noise ratio than higher harmonics.

Grafting of encapsulated BDNF-producing fibroblasts into the injured spinal cord without immune suppression in adult rats

Grafting of genetically modified cells that express therapeutic products is a promising strategy in spinal cord repair. We have previously grafted BDNF-producing fibroblasts (FB/BDNF) into injured spinal cord of adult rats, but survival of these cells requires a strict protocol of immune suppression with cyclosporin A (CsA). To develop a transplantation strategy without the detrimental effects of CsA, we studied the properties of FB/BDNF that were encapsulated in alginate-poly-L-ornithine, which possesses a semipermeable membrane that allows production and diffusion of a therapeutic product while protecting the cells from the host immune system. Our results show that encapsulated FB/BDNF, placed in culture, can survive, secrete bioactive BDNF and continue to grow for at least one month. Furthermore, encapsulated cells that have been stored in liquid nitrogen retain the ability to grow and express the transgene. Encapsulated FB/BDNF survive for at least one month after grafting into an adult rat cervical spinal cord injury site in the absence of immune suppression. Transgene expression decreased within two weeks after grafting but resumed when the cells were harvested and re-cultured, suggesting that soluble factors originating from the host immune response may contribute to the downregulation. In the presence of capsules that contained FB/BDNF, but not cell-free control capsules, there were many axons and dendrites at the grafting site. We conclude that alginate encapsulation of genetically modified cells may be an effective strategy for delivery of therapeutic products to the injured spinal cord and may provide a permissive environment for host axon growth in the absence of immune suppression.

Optimization of spray drying by factorial design for production of hollow microspheres for ultrasound imaging

A process for producing hollow microcapsules as ultrasound contrast agents was optimized using a 2(3) factorial experimental design method with two replicates. Spray drying, a conveniently scalable encapsulation technique, was used to encapsulate a volatile core material, such as ammonium carbonate, using biodegradable 50-50 poly(D,L-lactide-co-glycolide). Various effects due to changes in processing variables and their interactions were studied using the factorial grid. The high- and low-incremented variables examined included the temperature difference between the inlet and outlet of the spray dryer (5 degrees and 15 degrees C), air atomization pressure (80 and 100 psi), and polymer concentration in solvent (0.005 and 0.025 g/mL). Responses analyzed for computing the main effects and interactions were microcapsule morphology, yield, mean size, and zeta potential. Experimental results showed that polymer concentration was most important for determining microcapsule morphology. The temperature difference for drying prominently affected mean size, and atomization pressure was the main effect for microcapsule yield. Interactions among variables were not present in this case. The best conditions for producing PLGA microcapsules was a temperature difference of 5 degrees C, an initial polymer concentration of 0.005 g/mL, and an atomization pressure of 80 psi. The microcapsule zeta potentials were unaffected by spray-drying conditions.

Methylmercury and the health of indigenous peoples: a risk management challenge for physical and social sciences and for public health policy

Methylmercury in aquatic ecosystems and bio-accumulated in aquatic biota, especially fish, is a major public health concern internationally. Precautionary efforts are currently underway internationally to reduce the anthropogenic release of mercury, which in turn, over time, will reduce human exposure. However, at the present time, it is important to address the issue of management of the risks of exposure as they exist now. Of particular concern are the impacts of methylmercury on indigenous populations which depend on fish as a subsistence food source, both in remote areas of developed countries, such as Canada, and in developing countries such as Brazil. Research into these impacts over the past two or three decades has shown that, other than in very severe pollution situations such as occurred in Minamata, Japan, the direct impacts on human health are difficult to prove. On the other hand, the indirect negative effects of methylmercury on health, mediated through the disruption of lifestyle and eating patterns and the associated socio-cultural and socio-economic consequences among the affected native populations, have, in many cases, been significant. These social factors have raised serious challenges in determining practical public health policies on the issue. Policy development relating to environmental contaminants has been presented, with the problem of assessing the role of the various factors which contribute to the impact on health as a result of socio-cultural disruption. These factors include changes in diet and lifestyle due to methylmercury in the environment and its real or perceived risk. The standard physical sciences risk assessment process, based on the lowest observed adverse effects level (LOAEL) or no observed adverse effects level (NOAEL) used in defining health policies may be seen as over-simplistic theoretical extrapolations when viewed in the context of the concerns of the social sciences. Both approaches, however, have relevance to health policies that address the risks posed by environmental methylmercury. Therefore, the standard physical sciences approach of the past three decades now needs to be linked with the social sciences approach, with its focus on the indirect impacts of exposure to methylmercury, to provide a comprehensive approach to public health policy development. With this objective in mind, this paper reviews methylmercury-related data from both physical and social sciences. It attempts to draw on the findings in both disciplines to provide suggestions for an integrated approach in policy development relating to human health and human exposure to methylmercury, especially among indigenous peoples in remote areas and in developing countries. An integrated approach such as this may help to limit adverse health effects in the indigenous communities affected.