Acoustic Droplet Vaporization of Perfluorohexane Emulsions Induced by Heterogeneous Nucleation at an Ultrasonic Frequency of 1.1 MHz

Droplets made of liquid perfluorocarbon undergo a phase transition and transform into microbubbles when triggered by ultrasound of intensity beyond a critical threshold; this mechanism is called acoustic droplet vaporization (ADV). It has been shown that if the intensity of the signal coming from high ultrasonic harmonics are sufficiently high, superharmonic focusing is the mechanism leading to ADV for large droplets (>3 μm) and high frequencies (>1.5 MHz). In such a scenario, ADV is initiated due to a nucleus occurring at a specific location inside the droplet volume. But the question on what induces ADV in the case of nanometer-sized droplets and/or at low ultrasonic frequencies (<1.5 MHz) still remains. We investigated ADV of perfluorohexane (PFH) nano- and microdroplets at a frequency of 1.1 MHz and at conditions where there is no superharmonic focusing. Three types of droplets produced by microfluidics were studied: plain PFH droplets, PFH droplets containing many nanometer-sized water droplets, and droplets made of a PFH corona encapsulating a single micron-sized water droplet. The probability to observe a vaporization event was measured as a function of acoustic pressure. As our experiments were performed on droplet suspensions containing a population of monodisperse droplets, we developed a statistical model to extrapolate, from our experimental curves, the ADV pressure thresholds in the case where only one droplet would be insonified. We observed that the value of ADV pressure threshold decreases as the radius of a plain PFH droplet increases. This value was further reduced when a PFH droplet encapsulates a micron-sized water droplet, while the encapsulation of many nanometer-sized water droplets did not modify the threshold. These results cannot be explained by a model of homogeneous nucleation. However, we developed a heterogeneous nucleation model, where the nucleus appears at the surface in contact with PFH, that successfully predicts our experimental ADV results.

Assessing Enhanced Acoustic Absorption From Sonosensitive Perfluorocarbon Emulsion With Magnetic Resonance-Guided High-Intensity Focused Ultrasound and a Percolated Tissue-Mimicking Flow Phantom

OBJECTIVE

Sonosensitive high-boiling point perfluorocarbon F₈TAC₁₈-PFOB emulsions previously exhibited thermal enhancement during focused ultrasound heating in ex vivo pig livers, kidneys and a laminar flow phantom. The main objectives of this study were to evaluate heating under turbulent conditions, observe perfusion effects, quantify heating in terms of acoustic absorption and model the experimental data.

METHODS

In this study, similar perfluorocarbon emulsions were circulated at incremental concentrations of 0.07, 0.13, 0.19 and 0.25% v:v through a percolated turbulent flow phantom, more representative of the biological tissue than a laminar flow phantom. The concentrations represent the droplet content in only the perfused fluid, rather than the droplet concentration throughout the entire cross-section. The temperature was measured with magnetic resonance thermometry, during focused ultrasound sonications of 67 W, 95% duty cycle and 33 s duration. These were used in Bioheat equation simulations to investigate in silico the thermal phenomena. The temperature change was compared with the control condition by circulating de-gassed and de-ionized water through the flow phantom without droplets.

RESULTS

With these 1.24 µm diameter droplets at 0.25% v:v, the acoustic absorption coefficient increased from 0.93 ± 0.05 at 0.0% v:v to 1.82 ± 0.22 m^-1 at 0.25% v:v using a 0.1 mL s^-1 flow rate. Without perfusion at 0.25% v:v, an increase was observed from 1.23 ± 0.07 m^-1 at 0.0% v:v to 1.65 ± 0.17 m^-1.

CONCLUSION

The results further support previously reported thermal enhancement with F₈TAC₁₈-PFOB emulsion, quantified the increased absorption at small concentration intervals, illustrated that the effects can be observed in a variety of visceral tissue models and provided a method to simulate untested scenarios.

PFOB sonosensitive microdroplets: determining their interaction radii with focused ultrasound using MR thermometry and a Gaussian convolution kernel computation

Purpose: Micron-sized perfluorocarbon droplet adjuvants to focused ultrasound therapies allow lower applied power, circumvent unwanted prefocal heating, and enhance thermal dose in highly perfused tissues. The heat enhancement has been shown to saturate at increasing concentrations. Experiments were performed to empirically model the saturating heating effects during focused ultrasound.Materials and methods: The measurements were made at varying concentrations using magnetic resonance thermometry and focused ultrasound by circulating droplets of mean diameter 1.9 to 2.3 µm through a perfused phantom. A simulation was performed to estimate the interaction radius size, empirically.Results: The interaction radius, representing the radius of a sphere encompassing 90% of the probability for the transformation of acoustic energy into heat deposition around a single droplet, was determined experimentally from ultrasonic absorption coefficient measurements The simulations suggest the interaction radius was approximately 12.5-fold larger than the geometrical radius of droplets, corresponding to an interaction volume on the order of 2000 larger than the geometrical volume.Conclusions: The results provide information regarding the dose-response relationship from the droplets, a measure with 15% precision of their interaction radii with focused ultrasound, and subsequent insights into the underlying physical heating mechanism.

Perfluorocarbon Emulsion Contrast Agents: A Mini Review

Perfluorocarbon emulsions offer a variety of applications in medical imaging. The substances can be useful for most radiological imaging modalities; including, magnetic resonance imaging, ultrasonography, computed tomography, and positron emission tomography. Recently, the substance has gained much interest for theranostics, with both imaging and therapeutic potential. As MRI sequences improve and more widespread access to ¹⁹F-MRI coils become available, perfluorocarbon emulsions have great potential for new commercial imaging agents, due to high fluorine content and previous regulatory approval as antihypoxants and blood substitutes. This mini review aims to discuss the chemistry and physics of these contrast agents, in addition to highlighting some of the past, recent, and potential applications.

Perfluorocarbon emulsion enhances MR-ARFI displacement and temperature in vitro: Evaluating the response with MRI, NMR, and hydrophone

Sonosensitive perfluorocarbon F₈TAC₁₈-PFOB emulsion is under development to enhance heating, increase thermal contrast, and reduce treatment times during focused ultrasound tumor ablation of highly perfused tissue. The emulsion previously showed enhanced heating during ex vivo and in vitro studies. Experiments were designed to observe the response in additional scenarios by varying focused ultrasound conditions, emulsion concentrations, and surfactants. Most notably, changes in acoustic absorption were assessed with MR-ARFI. Phantoms were developed to have thermal, elastic, and relaxometry properties similar to those of ex vivo pig tissue. The phantoms were embedded with varying amounts of F₈TAC₁₈-PFOB emulsion or lecithin-PFOB emulsion, between about 0.0-0.3% v:w, in 0.05% v:w increments. MR-ARFI measurements were performed using a FLASH-ARFI-MRT sequence to obtain simultaneous displacement and temperature measurements. A Fabry-Perot hydrophone was utilized to observe the acoustic emissions. Susceptibility-weighted imaging and relaxometry mapping were performed to observe concentration-dependent effects. ¹⁹F diffusion-ordered spectroscopy NMR was used to measure the diffusion coefficient of perfluorocarbon droplets in a water emulsion. Increased displacement and temperature were observed with higher emulsion concentration. In semi-rigid MR-ARFI phantoms, a linear response was observed with low-duty cycle MR-ARFI sonications and a mono-exponential saturating response was observed with sustained sonications. The emulsifiers did not have a significant effect on acoustic absorption in semi-rigid gels. Stable cavitation might also contribute to enhanced heating.

Hybrid Fluorocarbon-Hydrocarbon Surfactants: Synthesis and Colloidal Characterization

Four double-tailed hybrid fluorocarbon-hydrocarbon (F-H) surfactants with a poly(ethylene glycol) (PEG) polar headgroup were synthesized. The hydrophobic scaffold consists of an amino acid core, onto which were grafted both fluorocarbon and hydrocarbon chains of different lengths. The PEG polar head was connected to the hydrophobic scaffold through a copper(I)-mediated click reaction. The four derivatives exhibit aqueous solubility >100 g/L and self-assemble into micellar aggregates with micromolar critical micellar concentration (CMC) values, as demonstrated by isothermal titration calorimetry (ITC), surface tension (ST) measurements, and steady-state fluorescence spectroscopy. The CMC value decreased by a factor of ∼6 for each additional pair of CH₂ groups, whereas a decrease by a factor of ∼2.5 was observed when the size of the PEG polar head was reduced from 2000 to 750 g/mol. Dynamic light scattering (DLS) showed unimodal micelle populations with hydrodynamic diameters of 10-15 nm, in agreement with results obtained from size-exclusion chromatography (SEC). The aggregation number increased with the hydrocarbon chain length but decreased with increasing PEG chain lengths. The combination in one molecular design of both low CMC and high water solubility makes these new surfactants promising systems for novel drug-delivery systems.

Ultrasound-triggered delivery of paclitaxel encapsulated in an emulsion at low acoustic pressures

We investigated the in vitro ultrasound-triggered delivery of paclitaxel, a well known anti-cancerous drug, encapsulated in an emulsion and in the presence of CT26 tumor cells. The emulsion was made of nanodroplets, whose volume comprised 95% perfluoro-octyl bromide and 5% tributyl O-acetylcitrate, in which paclitaxel was solubilized. These nanodroplets, prepared using a high-pressure microfluidizer, were stabilized by a tailor-made and recently patented biocompatible fluorinated surfactant. The delivery investigations were performed at 37 °C using a high intensity focused ultrasound transducer at a frequency of 1.1 MHz. The ultrasonic pulse was made of 275 sinusoidal periods and the pulse repetition frequency was 200 Hz with a duty cycle of 5%. The measured viabilities of CT26 cells showed that paclitaxel delivery was achievable for peak-to-peak pressures of 0.4 and 3.5 MPa, without having to vaporize the perfluorocarbon part of the droplet or to induce inertial cavitation.

From Electronic Waste to Suzuki-Miyaura Cross-Coupling Reaction in Water: Direct Valuation of Recycled Palladium in Catalysis

From electronic waste to Pd-catalyzed reaction! The straightforward valuation of palladium recovered from electronic waste is reported here. Following a classical leaching stage, palladium is selectively extracted from a complex aqueous mixture of metallic cations into an organic phase. Afterwards, the judicious choice of a surfactant enables stabilization of palladium during back extraction cycles, and the direct preparation of an aqueous micellar solution, which can be employed in a model Suzuki-Miyaura cross-coupling reaction. Clean phase separation is observed, and distribution of all components between organic and aqueous phases is mastered. The proposed process avoids several waste generating steps dedicated to palladium isolation and ultimate purification, as well as the preparation of palladium pre-catalyst. This novel approach enables a better use of both natural resources and industrial wastes, through new cycles in circular economy.

From Electronic Waste to Suzuki-Miyaura Cross-Coupling Reaction in Water: Direct Valuation of Recycled Palladium in Catalysis

Invited for this month's cover is the group of Damien Bourgeois at the Marcoule Institute for Separation Chemistry (ICSM). The image shows how a short process chain can efficiently transform our waste into an active catalyst. The Full Paper itself is available at 10.1002/cssc.202001155.

Enhancement of HIFU thermal therapy in perfused tissue models using micron-sized FTAC-stabilized PFOB-core endovascular sonosensitizers

BACKGROUND

High intensity focused ultrasound (HIFU) is clinically accepted for the treatment of solid tumors but remains challenging in highly perfused tissue due to the heat sink effect. Endovascular liquid-core sonosensitizers have been previously suggested to enhance the thermal energy deposition at the focal area and to lower the near-/far-field heating. We are investigating the therapeutic potential of PFOB-FTAC micro-droplets in a perfused tissue-mimicking model and postmortem excised organs.

METHOD

A custom-made in vitro perfused tissue-mimicking model, freshly excised pig kidneys (n = 3) and liver (n = 1) were perfused and subjected to focused ultrasound generated by an MR-compatible HIFU transducer. PFOB-FTAC sonosensitizers were injected in the perfusion fluid up to 0.235% v/v ratio. Targeting and on-line PRFS thermometry were performed on a 3 T MR scanner. Assessment of the fluid perfusion was performed with pulsed color Doppler in vitro and with dynamic contrast-enhanced (DCE)-MRI in excised organs.

RESULTS

Our in vitro model of perfused tissue demonstrated re-usability. Sonosensitizer concentration and perfusion rate were tunable in situ. Differential heating under equivalent HIFU sonications demonstrated a dramatic improvement in the thermal deposition due to the sonosensitizers activity. Typically, the energy deposition was multiplied by a factor between 2.5 and 3 in perfused organs after the administration of micro-droplets, while DCE-MRI indicated an effective perfusion.

CONCLUSION

The current PFOB-FTAC micro-droplet sonosensitizers provided a large and sustained enhancement of the HIFU thermal deposition at the focal area, suggesting solutions for less technological constraints, lower risk for the near-/far- field heating. We also report a suitable experimental model for other MRgHIFU studies.

Micron-sized PFOB liquid core droplets stabilized with tailored-made perfluorinated surfactants as a new class of endovascular sono-sensitizers for focused ultrasound thermotherapy

The effect of micro-droplet concentration on HIFU beam absorption.

Molecular oxygen loading in candidate theranostic droplets stabilized with biocompatible fluorinated surfactants: Particle size effect and application to in situ19F MRI mapping of oxygen partial pressure

OBJECTIVE

Perfluorocarbon nano- and micron-sized emulsions are a new field of investigation in cancer treatment due to their ability to be used as imaging contrast agents, or as delivery vectors for pharmaceuticals. They also demonstrated capability to enhance the efficiency of high intensity focused ultrasound thermo-therapy. In the context of new biomedical applications we investigated perfluorooctyl bromide (PFOB) theranostic droplets using ¹⁹F NMR. Each droplet contains biocompatible fluorinated surfactants composed of a polar Tris(hydroxymethyl)aminomethane head unit and hydrophobic perfluorinated tail (abbreviated as F-TAC). The influence of the droplet size on the oxygen loading capacity was determined from longitudinal relaxation (T₁) data of ¹⁹F NMR signal.

MATERIAL AND METHODS

Liquid PFOB and five samples of PFOB droplets of average diameter 0.177, 0.259, 1.43, 3.12 and 4.53 µm were tested with different oxygen levels. A dedicated gas exchange system was validated to maintain steady state oxygen concentrations, including a spatial gradient of oxygen concentration. A prototyped transmit-receive switchable ¹⁹F/¹H quadrature coil was integrated on a 3 T clinical scanner. The coil is compatible with focused ultrasound sonication for future application. A spectroscopy FID inversion-recovery (IR) sequence was used to measure the T₁ value per sample and per value of equilibrium oxygen pressure. Pixel wise, spatial T₁ mapping was performed with magnetization prepared 2D gradient echo sequences in tissue mimicking gels doped with theranostic droplets.

RESULTS

Experimental data indicated that the longitudinal relaxation rate of ¹⁹F signal of the investigated theranostic droplets depended approximately linearly on the oxygen level and its slope decreased with the particle size according to a second order polynomial over the investigated range. This semi-empirical model was derived from general thermodynamics and weak electrostatic forces theory and fitted the experimental data within 0.75% precision. The capacity of oxygen transportation for the described theranostic droplets tended to that of pure PFOB, while micron-sized droplets lost up to 50% of this capacity. In a specific setup producing a steady state gradient of oxygen concentration, we demonstrated spatial mapping of oxygen pressure gradient of 6 kPa/mm with 1 mm in-plane resolution.

CONCLUSION

The size-tunable PFOB theranostic droplets stabilized with F-TAC surfactants could be characterized by ¹⁹F MRI in a clinical setup readily compatible with interventional in vivo studies under MR guidance. Current precision and spatial resolution of T₁ mapping are promising. A potential challenge for further in vivo studies is the reduction of the imaging time.

Effect of Dimethyl Sulfoxide on the Binding of 1-Adamantane Carboxylic Acid to β- and γ-Cyclodextrins

Most therapeutic targets are proteins whose binding sites are hydrophobic cavities. For this reason, the majority of drugs under development are hydrophobic molecules exhibiting low solubility in water. To tackle this issue, a few percent of cosolvent, such as dimethyl sulfoxide (DMSO), is usually employed to increase drug solubility during the drug screening process. However, the few published studies dealing with the effect of adding DMSO showed that the affinity of hydrophobic ligands is systematically underestimated. To better understand the effect of DMSO, there is a need of studying its effect on a large range of systems. In this work, we used β- and γ-cyclodextrins (made of 6 and 7 α-d-glucopyranoside units, respectively) as models of hydrophobic cavities to investigate the effect of the addition 5% DMSO on the affinity of 1-adamantane carboxylic acid (ADA) to these cyclodextrins. The two systems differ by the size of the cyclodextrin cavity. The evaluation of binding constants was performed using ultrasound velocimetry, nuclear magnetic resonance spectroscopy, and molecular simulations. All techniques show that the presence of 5% DMSO does not significantly modify the affinity of ADA for γ-cyclodextrin, while the affinity is dramatically reduced for β-cyclodextrin. The bias induced by the presence of DMSO is thus more important when the ligand volume better fits the cyclodextrin cavity. Our work also suggests that free energy calculations provide a sound alternative to experimental techniques when dealing with poorly water-soluble drugs.

Perfluorocarbon nanodroplets stabilized by fluorinated surfactants: characterization and potentiality as theranostic agents

We aim to produce emulsions that can act as contrast agents and drug carriers for cancer imaging and therapy.

Preliminary biological evaluations of new thalidomide analogues for multiple sclerosis application

The present work deals with the synthesis of a new series of thalidomide derivatives for therapeutic applications. These compounds were evaluated in vitro on a human endothelial cell line EA.hy926 for their antiproliferative potential and in vivo on an experimental animal multiple sclerosis model called EAE as angiogenesis inhibitors. The preliminary results obtained on EAE assays seem to validate that anti-angiogenesis compounds could be promising tools for the treatment of MS.

Design, synthesis and preliminary biological evaluations of novel amphiphilic drug carriers

The synthesis of a new fluorocarbon amphiphilic drug carrier is described. A polyfunctional amino acid endowed with a fluorocarbon chain and a sugar moiety providing the amphiphilic character constitutes the central element of this structure. A (14)C-radiolabelled acetyl group was grafted onto the third function and the bioavailability of this molecule was specified in mice after IV administration. This amphiphilic drug carrier exhibits a rapid and homogeneous distribution to the whole tissues and slow elimination half-lives (higher than one day) through a biliary excretion without any toxicity (no measured DL 50 for concentrations up to 500 mg/kg).

Inhibition of angiogenesis by THAM-derived cotelomers endowed with thalidomide moieties

The synthesis of a tris(hydroxymethyl)acrylamidomethane (THAM)-derived cotelomer endowed with thalidomide units and a preliminary assessment of its biological activity are described. 4-Carboxy thalidomide and 4-(N-acryloyl) lysine thalidomide derivatives were prepared. The polymerization of these compounds with THAM in the presence of octanethiol as transfer reagent provided a water-soluble telomer bearing several thalidomide units. The ability of this telomer to inhibit angiogenesis in a mouse model of corneal neovascularization was compared to 4-carboxy thalidomide and thalidomide. A significant inhibition in area of neovascularization stimulated by a bFGF pellet was observed only in the mice treated with the telomer.

In vitro and in vivo evaluations of THAM derived telomers bearing RGD and Ara-C for tumour neovasculature targeting

As an approach to the development of specific drug delivery systems, a new class of low macromolecular carriers called 'telomers' endowed with an antitumour agent, such as arabinofuranosylcytosine (Ara-C), RGDSK peptidic sequences, as tumour targeting moieties, and tyrosine groups labelled with 125I atoms allowing the in vivo scintigraphic follow up, were synthesized. Their tumour targeting ability was assessed in vivo in mice bearing a murine B16 melanoma. The biological results showed that the presence of RGDSK sequences onto the macromolecules leads to the selective targeting and the accumulation of telomers within the vascularized zone of the tumour. Moreover, such compounds exhibited in vitro a better IC(50) (0.015 muM) than pure Ara-C and in vivo an oncostatic index higher than 160%.

Amphiphilic oligomers: a new kind of macromolecular carrier of antimitotic drugs

The last three decades have seen the development of new concepts in the biomedical and medical field based on the principle of drug carrying and in vivo pharmaco-targeting using synthetic carriers such as nanoparticles, liposomes or polymers. This review deals with the synthesis and biomedical applications of a new kind of macromolecular compounds: end-capped oligomers called telomers. The telomers, derived from Tris(hydroxymethyl)aminomethane bearing either a hydro- or a fluorocarbon tail, are described. Their functionality and biocompatibility enhance their potential in the biomedical and medical fields. Such compounds exhibit no cytotoxicity and are able to cross cell membranes by endocytosis. After i.v or per os administration in the animal, they exhibit a very good bio-availability and a homogeneous distribution in all tissues without any accumulation. Their ability to carry in vivo antimitotic drugs has been shown. Moreover, the grafting of different glycoside moieties onto the hydroxyl groups of telomers allows the selective targeting of specific receptors called integrins. Finally, we have shown the potential use of telomers bearing specific peptide ligands, such as RGD sequences, in the selective carrying of antimitotic drugs to the angiogenic zone surrounding tumors.

Synthesis and preliminary biological assessments of RGD bearing biocompatible telomers

Work reported herein deals with the synthesis and preliminary biological assessments of a new class of biocompatible telomeric carriers bearing peptidic RGDSK sequences as tumor cell targeting and tyrosine moieties labelled with 125I as in vivo probe. The radioactivity levels obtained in several tissues, after the intravenous injections of these telomers in mice bearing grafted B16 syngenic melanoma showed that the addition of a RGD residue to a telomeric structure confers it an increased affinity for the highly vascularized zone surrounding the tumor.