Towards a nanoscale mammographic contrast agent: development of a modular pre-clinical dual optical/x-ray agent

A new breast phantom suitable for digital mammography, contrast-enhanced digital mammography and digital breast tomosynthesis

Our objective is to report a new breast phantom that provides the objective assessment for three types of clinical mammography, i.e. digital mammography (DM), contrast-enhanced digital mammography (CEDM), and digital breast tomosynthesis (DBT). The tissue-equivalent materials are used to represent the corresponding tissue, and the layer-by-layer structure with separate regions is designed for image quality assessment of different mammography modes. For DM imaging, substitutes for microcalcifications and fibroglandular tissue of different sizes are used to simulate the conventional breast. For CEDM imaging, the tumor module that can be injected with imaging contrast agents is adopted to distinguish normal tissue and diseased tissue in the dense breast. For DBT imaging, the overlapping breast mass module with multiple layers is designed to perform the layer-by-layer imaging of overlapping tissue. In addition, the quantitative assessment module of image quality is designed based on contrast-to-noise ratio, modulation transfer function and artifact spread function. This phantom allows image quality to be evaluated objectively for three different types of the clinical mammography, while it provides an effective tool for optimizing the dose-image quality relationship of patients.

Phase-shift nanodroplets as an emerging sonoresponsive nanomaterial for imaging and drug delivery applications

Nanodroplets - emerging phase-changing sonoresponsive materials - have attracted substantial attention in biomedical applications for both tumour imaging and therapeutic purposes due to their unique response to ultrasound. As ultrasound is applied at different frequencies and powers, nanodroplets have been shown to cavitate by the process of acoustic droplet vapourisation (ADV), causing the development of mechanical forces which promote sonoporation through cellular membranes. This allows drugs to be delivered efficiently into deeper tissues where tumours are located. Recent reviews on nanodroplets are mostly focused on the mechanism of cavitation and their applications in biomedical fields. However, the chemistry of the nanodroplet components has not been discussed or reviewed yet. In this review, the commonly used materials and preparation methods of nanodroplets are summarised. More importantly, this review provides examples of variable chemistry components in nanodroplets which link them to their efficiency as ultrasound-multimodal imaging agents to image and monitor drug delivery. Finally, the drawbacks of current research, future development, and future direction of nanodroplets are discussed.

Polyelectrolyte Coatings Can Control Charged Fluorocarbon Nanodroplet Stability and Their Interaction with Macrophage Cells

Fluorocarbon nanodroplets, ∼100 to ∼400 nm in diameter, are of immense interest in a variety of medical applications including the imaging and therapy of cancer and inflammatory diseases. However, fluorocarbon molecules are both hydrophobic and lipophobic; therefore, it is challenging to synthesize fluorocarbon nanodroplets with the optimal stability and surface properties without the use of highly specialized surfactants. Here, we hypothesize that we can decouple the control of fluorocarbon nanodroplet size and stability from its surface properties. We use a simple, two-step procedure where standard, easily available anionic fluorosurfactants are used to first stabilize the fluorocarbon nanodroplets, followed by electrostatically attaching functionalized polyelectrolytes to the nanodroplet surfaces to independently control their surface properties. Herein, we demonstrate that PEGylated polyelectrolyte coatings can effectively alter the fluorocarbon nanodroplet surface properties to reduce coalescence and its uptake into phagocytic cells in comparison with non-PEGylated polyelectrolyte coatings and uncoated nanodroplets, as measured by flow cytometry and fluorescence microscopy. In this study, perfluorooctyl bromide (PFOB) was used as a representative fluorocarbon material, and PEGylated PFOB nanodroplets with diameters between 250 and 290 nm, depending on the poly(ethylene glycol) block length, were prepared. The PEGylated PFOB nanodroplets had superior size stability in comparison with uncoated and non-PEGylated polyelectrolyte nanodroplets in saline and within macrophage cells. Of significance, non-PEGylated nanodroplets were rapidly internalized by macrophage cells, whereas PEGylated nanodroplets were predominantly colocalized on the cell membrane. This suggests that the PEGylated-polyelectrolyte coating on the charged PFOB nanodroplets may afford adjustable shielding from cells of the reticuloendothelial system. This report shows that using the same fluorosurfactant as a base layer, modularly assembled PFOB nanodroplets tailored for a variety of end applications can be created by selecting different polyelectrolyte coatings depending on their unique requirements for stability and interaction with phagocytic cells.

Perfluorocarbon nanodroplets can reoxygenate hypoxic tumors in vivo without carbogen breathing

Nanoscale perfluorocarbon (PFC) droplets have enormous potential as clinical theranostic agents. They are biocompatible and are currently used in vivo as contrast agents for a variety of medical imaging modalities, including ultrasound, computed tomography, photoacoustic and 19F-magnetic resonance imaging. PFC nanodroplets can also carry molecular and nanoparticulate drugs and be activated in situ by ultrasound or light for targeted therapy. Recently, there has been renewed interest in using PFC nanodroplets for hypoxic tumor reoxygenation towards radiosensitization based on the high oxygen solubility of PFCs. Previous studies showed that tumor oxygenation using PFC agents only occurs in combination with enhanced oxygen breathing. However, recent studies suggest that PFC agents that accumulate in solid tumors can contribute to radiosensitization, presumably due to tumor reoxygenation without enhanced oxygen breathing. In this study, we quantify the impact of oxygenation due to PFC nanodroplet accumulation in tumors alone in comparison with other reoxygenation methodologies, in particular, carbogen breathing. Methods: Lipid-stabilized, PFC (i.e., perfluorooctyl bromide, CF3(CF2)7Br, PFOB) nanoscale droplets were synthesized and evaluated in xenograft prostate (DU145) tumors in male mice. Biodistribution assessment of the nanodroplets was achieved using a fluorescent lipophilic indocarbocyanine dye label (i.e., DiI dye) on the lipid shell in combination with fluorescence imaging in mice (n≥3 per group). Hypoxia reduction in tumors was measured using PET imaging and a known hypoxia radiotracer, [18F]FAZA (n≥ 3 per group). Results: Lipid-stabilized nanoscale PFOB emulsions (mean diameter of ~250 nm), accumulated in the xenograft prostate tumors in mice 24 hours post-injection. In vivo PET imaging with [18F]FAZA showed that the accumulation of the PFOB nanodroplets in the tumor tissues alone significantly reduced tumor hypoxia, without enhanced oxygen (i.e., carbogen) breathing. This reoxygenation effect was found to be comparable with carbogen breathing alone. Conclusion: Accumulation of nanoscale PFOB agents in solid tumors alone successfully reoxygenated hypoxic tumors to levels comparable with carbogen breathing alone, an established tumor oxygenation method. This study confirms that PFC agents can be used to reoxygenate hypoxic tumors in addition to their current applications as multifunctional theranostic agents.

Perfluorooctylbromide nanoparticles for ultrasound imaging and drug delivery

Perfluorooctylbromide nanoparticles (PFOB NPs) are a type of multifunctional nanotechnology that has been studied for various medical applications. Commercial ultrasound contrast agents (UCAs) suffer from the following limitations: short half-lives in vivo, high background signal and restricted distribution in the vascular circulation due to their micrometer dimensions. PFOB NPs are new potential UCAs that persist for long periods in the circulatory system, possess a relatively stable echogenic response without increasing the background signal and exhibit lower acoustic attenuation than commercial UCAs. Furthermore, PFOB NPs may also serve as drug delivery vehicles in which drugs are dissolved in the outer lipid or polymer layer for subsequent delivery to target sites in site-targeted therapy. The use of PFOB NPs as carriers has the potential advantage of selectively delivering payloads to the target site while improving visualization of the site using ultrasound (US) imaging. Unfortunately, the application of PFOB NPs to the field of ultrasonography has been limited because of the low intensity of US reflection. Numerous researchers have realized the potential use of PFOB NPs as UCAs and thus have developed alternative approaches to apply PFOB NPs in ultrasonography. In this article, we review the latest approaches for using PFOB NPs to enhance US imaging in vivo. In addition, this article emphasizes the application of PFOB NPs as promising drug delivery carriers for cancer and atherosclerosis treatments, as PFOB NPs can transport different drug payloads for various applications with good efficacy. We also note the challenges and future study directions for the application of PFOB NPs as both a delivery system for therapeutic agents and a diagnostic agent for ultrasonography.

18F-Labeled perfluorocarbon droplets for positron emission tomography imaging

INTRODUCTION

Nanoscale perfluorocarbon (PFC) droplets have been used to create imaging agents and drug delivery vehicles. However, development and characterization of new formulations of PFC droplets are hindered because of the lack of simple methods for quantitative and sensitive assessment of whole body tissue distribution and pharmacokinetics of the droplets. To address this issue, a general-purpose method for radiolabeling the inner core of nanoscale perfluorocarbon droplets with a hydrophobic and lipophobic fluorine-18 compound was developed, so that positron emission tomography (PET) and quantitative biodistribution studies can be employed to evaluate PFC nanodroplets in vivo.

METHODS

A robust method to produce [¹⁸F]CF₃(CF₂)₇(CH₂)₃F from a tosylate precursor using [¹⁸F]F^- was developed. The product's effectiveness as a general label for different PFCs and its ability to distinguish the in vivo behavior of different PFC droplet formulations was evaluated using two types of PFC nanodroplets: fluorosurfactant-stabilized perfluorohexane (PFH) nanodroplets and lipid-stabilized perfluorooctylbromide (PFOB) nanodroplets. In vivo assessment of the ¹⁸F-labeled PFH and PFOB nanodroplets were conducted in normal mice following intravenous injection using small animal PET imaging and gamma counting of tissues and fluids.

RESULTS

[¹⁸F]CF₃(CF₂)₇(CH₂)₃F was produced in modest yield and was stable with respect to loss of fluoride in vitro. The labeled fluorocarbon was successfully integrated into PFH nanodroplets (~175 nm) and PFOB nanodroplets (~260 nm) without altering their mean sizes, size distributions, or surface charges compared to their non-radioactive analogues. No leakage of the radiolabel from the nanodroplets was detected after droplet formation in vitro. PET imaging and biodistribution data for the two droplet types tested showed significantly different tissue uptake and clearance patterns.

CONCLUSION

A convenient method for producing ¹⁸F-labeled PFC droplets was developed. The results highlight the potential utility of the strategy for pre-clinical evaluation of different PFC droplet formulations through direct PFC core labeling using a fluorinated radiolabel.

Methods of Generating Submicrometer Phase-Shift Perfluorocarbon Droplets for Applications in Medical Ultrasonography

Continued advances in the field of ultrasound and ultrasound contrast agents have created new approaches to imaging and medical intervention. Phase-shift perfluorocarbon droplets, which can be vaporized by ultrasound energy to transition from the liquid to the vapor state, are one of the most highly researched alternatives to clinical ultrasound contrast agents (i.e., microbubbles). In this paper, part of a special issue on methods in biomedical ultrasonics, we survey current techniques to prepare ultrasound-activated nanoscale phase-shift perfluorocarbon droplets, including sonication, extrusion, homogenization, microfluidics, and microbubble condensation. We provide example protocols and discuss advantages and limitations of each approach. Finally, we discuss best practice in characterization of this class of contrast agents with respect to size distribution and ultrasound activation.

Synthesis of Stable Multifunctional Perfluorocarbon Nanoemulsions for Cancer Therapy and Imaging

Nanotechnology provides a promising platform for drug-delivery in medicine. Nanostructured materials can be designed with desired superparamagnetic or fluorescent properties in conjunction with biochemically functionalized moieties (i.e., antibodies, peptides, and small molecules) to actively bind to target sites. These multifunctional properties make them suitable agents for multimodal imaging, diagnosis, and therapy. Perfluorohexane nanoemulsions (PFH-NEs) are novel drug-delivery vehicles and contrast agents for ultrasound and photoacoustic imaging of cancer in vivo, offering higher spatial resolution and deeper penetration of tissue when compared to conventional optical techniques. Compared to other theranostic agents, our PFH-NEs are one of the smallest of their kind (<100 nm), exhibit minimal aggregation, long-term stability at physiological conditions, and provide a noninvasive cancer imaging and therapy alternative for patients. Here, we show, using high-resolution imaging and correlative techniques, that our PFH-NEs, when in tandem with silica-coated gold nanoparticles (scAuNPs), can be used as a drug-loaded therapeutic via endocytosis and as a multimodal imaging agent for photoacoustic, ultrasound, and fluorescence imaging of tumor growth.

Gold silver alloy nanoparticles (GSAN): an imaging probe for breast cancer screening with dual-energy mammography or computed tomography

Earlier detection of breast cancer reduces mortality from this disease. As a result, the development of better screening techniques is a topic of intense interest. Contrast-enhanced dual-energy mammography (DEM) is a novel technique that has improved sensitivity for cancer detection. However, the development of contrast agents for this technique is in its infancy. We herein report gold-silver alloy nanoparticles (GSAN) that have potent DEM contrast properties and improved biocompatibility. GSAN formulations containing a range of gold : silver ratios and capped with m-PEG were synthesized and characterized using various analytical methods. DEM and computed tomography (CT) phantom imaging showed that GSAN produced robust contrast that was comparable to silver alone. Cell viability, reactive oxygen species generation and DNA damage results revealed that the formulations with 30% or higher gold content are cytocompatible to Hep G2 and J774A.1 cells. In vivo imaging was performed in mice with and without breast tumors. The results showed that GSAN produce strong DEM and CT contrast and accumulated in tumors. Furthermore, both in vivo imaging and ex vivo analysis indicated the excretion of GSAN via both urine and feces. In summary, GSAN produce strong DEM and CT contrast, and has potential for both blood pool imaging and for breast cancer screening.

Direct incorporation of lipophilic nanoparticles into monodisperse perfluorocarbon nanodroplets via solvent dissolution from microfluidic-generated precursor microdroplets

Multifunctional medical agents based on imaging or therapy nanoparticles (NPs) incorporated into perfluorocarbon (PFC) droplets are promising new agents for cancer detection and treatment. For the first time, monodisperse PFC nanodroplets labeled with NPs have been produced. Lipophilic, as-synthesized, hydrocarbon-stabilized NPs are directly miscibilized into lipophobic PFCs using a removable cosolvent, diethyl ether (DEE), which eliminates the need of the typical time-consuming and expertise-specific NP surface modification steps previously required for NP incorporation into PFCs. This NP-DEE/PFC solution is then used to synthesize monodisperse, micrometer-scale, DEE-infused NP-PFC precursor droplets in water using microfluidics. After precursor microdroplet generation, the DEE cosolvent is removed by dissolution and evaporation, resulting in dramatically smaller, monodisperse, NP-labeled nanodroplets, with final droplet sizes far smaller than the minimum droplet size limit of the microfluidic system, and easily controlled by the amount of DEE mixed in the PFC phase prior to precursor droplet synthesis. Using this technique, unmodified lipophilic quantum dot (QD) NPs were integrated into monodisperse and PFC nanodroplets 165 times smaller in volume than the precursor microdroplets, with dimensions down to 470 nm. The final droplet sizes scaled with the PFC concentrations in the precursor microdroplets, and the QDs remain localized within the droplets after DEE is removed from the system. This method is robust and versatile, and it comprises a platform technology for other unmodified lipophilic NPs and molecules to be incorporated into different types of PFC droplets for the production of new NP-PFC hybrid agents for medical imaging and therapy applications.