Synthesis and Characterization of Multi-Modal Phase-Change Porphyrin Droplets.

Phase-change droplets are a class of ultrasound contrast agents that can convert into echogenic microbubbles in situ with the application of sufficient acoustic energy. Droplets are smaller and more stable than their microbubble counterparts. However, traditional ultrasound contrast agents are not trackable beyond acoustic feedback measurements, which makes quantifying contrast agent bio-distribution or accumulation ex vivo difficult. Researchers may have to rely on fluorescent or optically absorbent companion diagnostic particles to infer bio-distribution. The purpose of this protocol is to detail steps for creating multi-modal phase-change porphyrin droplets using a condensation method. Porphyrins are fluorescent molecules with distinct absorbance bands that can be conjugated onto lipids and incorporated into droplets to extend droplet versatility, enabling more robust bio-distribution while retaining acoustic properties. Seven formulations with varying porphyrin-lipid and base lipid contents were made to investigate microbubble and droplet size distributions. Characterizations suited to porphyrin-containing structures are also described in the protocol to demonstrate their analytic versatility in-solution. Sizing showed that the post-condensed mean diameters were 1.72 to 2.38 times smaller than precursor populations. Absorbance characterization showed intact assemblies had a Q-band peak of 700 nm while disrupted samples had an absorbance peak at 671 nm. Fluorescence characterization showed intact 30% porphyrin-lipid assemblies were fluorescently quenched (>97%), with fluorescence recovery achieved upon disruption. Acoustic vaporization showed that porphyrin droplets were non-echogenic at lower pressures and could be converted into echogenic microbubbles with sufficient pressures. These characterizations show the potential for porphyrin droplets to eliminate the need for absorbance or fluorescence-based companion diagnostic strategies to quantify ultrasound contrast agent bio-distribution for delivery or therapeutic applications in vivo or ex vivo.

Investigating the Accumulation of Submicron Phase-Change Droplets in Tumors.

Submicron phase-change droplets are an emerging class of ultrasound contrast agent. Compared with microbubbles, their relatively small size and increased stability offer the potential to passively extravasate and accumulate in solid tumors through the enhanced permeability and retention effect. Under exposure to sufficiently powerful ultrasound, these droplets can convert into in situ gas microbubbles and thus be used as an extravascular-specific contrast agent. However, in vivo imaging methods to detect extravasated droplets have yet to be established. Here, we develop an ultrasound imaging pulse sequence within diagnostic safety limits to selectively detect droplet extravasation in tumors. Tumor-bearing mice were injected with submicron perfluorobutane droplets and interrogated with our imaging-vaporization-imaging sequence. By use of a pulse subtraction method, median droplet extravasation signal relative to the total signal within the tumor was estimated to be Etumor=37±5% compared with the kidney Ekidney=-2±8% (p < 0.001). This work contributes toward the advancement of volatile phase-shift droplets as a next-generation ultrasound agent for imaging and therapy.

Impact of Encapsulation on in vitro and in vivo Performance of Volatile Nanoscale Phase-Shift Perfluorocarbon Droplets.

Phase-shift droplets can be converted by sound from low-echogenicity, liquid-core agents into highly echogenic microbubbles. Many proposed applications in imaging and therapy take advantage of the high spatiotemporal control over this dynamic transition. Although some studies have reported increased circulation time of the droplets compared with microbubbles, few have directly explored the impact of encapsulation on droplet performance. With the goal of developing nanoscale droplets with increased circulatory persistence, we first evaluate the half-life of several candidate phospholipid encapsulations in vitro at clinical frequencies. To evaluate in vivo circulatory persistence, we develop a technique to periodically measure droplet vaporization from high-frequency B-mode scans of a mouse kidney. Results show that longer acyl chain phospholipids can dramatically reduce droplet degradation, increasing median half-life in vitro to 25.6 min-a 50-fold increase over droplets formed from phospholipids commonly used for clinical microbubbles. In vivo, the best-performing droplet formulations showed a median half-life of 18.4 min, more than a 35-fold increase in circulatory half-life compared with microbubbles with the same encapsulation in vivo. These findings also point to possible refinements that may improve nanoscale phase-shift droplet performance beyond those measured here.

Characterization and evaluation of a peptide-based siRNA delivery system in vitro.

Since its inception more than a decade ago, gene silencing mediated by double-stranded small interfering RNA (siRNA) has been widely investigated as a potential therapeutic approach for a variety of diseases. However, the use of siRNA is hampered by its rapid degradation and poor cellular uptake in vitro and in vivo. Recently, peptide-based carriers have been applied to siRNA delivery, as an alternative to the traditional delivery systems. Here, a histidine-containing amphipathic amino acid pairing peptide, C6M3, which can form complexes with siRNA, was used as a new siRNA delivery system. This peptide exhibited a high affinity for siRNA and ability to efficiently deliver siRNA into the cells. The interaction of C6M3 with siRNA was investigated to determine the loading capacity of C6M3 at different peptide/siRNA molar ratios. At C6M3/siRNA molar ratio of 10/1, siRNA molecules were entirely associated with C6M3 as indicated by a gel electrophoretic assay and further confirmed by zeta potential analysis. The particle size distribution of the C6M3-siRNA complexes was studied using dynamic light scattering, which showed an intensity-based size distribution peaked approximately at 100 nm in RNase-free water and 220 nm in the Opti-MEM medium. C6M3 adopted a helical secondary structure in RNase-free water and became more so after forming complexes with siRNA. The interaction of siRNA with C6M3 is an entropy-driven spontaneous process, as determined by isothermal titration calorimetry (ITC) study. The efficiency of cellular uptake of the siRNA complexes at different C6M3/siRNA molar ratios was evaluated, and the results showed that C6M3 promoted efficient cellular uptake of siRNA into cells. Furthermore, a significant level of GAPDH gene silencing efficiency (69%) was achieved in CHO-K1 cells, with minimal cytotoxicity.

More Than Bubbles: Creating Phase-Shift Droplets from Commercially Available Ultrasound Contrast Agents.

Phase-shift perfluorocarbon droplets have been investigated for over 20 years as pre-clinical ultrasound contrast agents with distinctive advantages in imaging and therapy. A number of formulation strategies exist, each with inherent advantages and limitations. In this note, we demonstrate a unique opportunity: that phase-shift droplets can be generated directly from commercially available microbubbles. This may facilitate pre-clinical and translational development by reducing the in-house synthesis expertise and resources required to generate high concentration droplet emulsions. Proof-of-principle in vitro and in vivo is given using droplets created from Definity and MicroMarker. The results demonstrate the role of perfluorocarbon choice in the trade-off between thermal stability and vaporization threshold, and suggest that commercial microbubbles with decafluorobutane cores may be ideal for this approach.

Image-Guided Ultrasound Characterization of Volatile Sub-Micron Phase-Shift Droplets in the 20-40 MHz Frequency Range.

Phase-shift perfluorocarbon droplets are designed to convert from the liquid to the gas state by the external application of acoustic or optical energy. Although droplet vaporization has been investigated extensively at ultrasonic frequencies between 1 and 10 MHz, few studies have characterized performance at the higher frequencies commonly used in small animal imaging. In this study, we use standard B-mode imaging sequences on a pre-clinical ultrasound platform to both image and activate sub-micron decafluorobutane droplet populations in vitro and in vivo at center frequencies in the range of 20-40 MHz. Results show that droplets remain stable against vaporization at low imaging pressures but are vaporized at peak negative pressures near 3.5 MPa at the three frequencies tested. This study also found that a small number of size outliers present in the distribution can greatly influence droplet performance. Removal of these outliers results in a more accurate assessment of the vaporization threshold and produces free-flowing microbubbles upon vaporization in the mouse kidney.