pH-triggered release from nonphospholipid LUVs modulated by the pKa of the included fatty acid

Development of hyaluronic acid hydrogel containing prednisolone-encapsulated nonphospholipid liposomes for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) requires therapeutic approaches that alleviate symptoms and inhibit the progression of joint damage. Glucocorticoids (GCs) have been a cornerstone of RA treatment, yet their use is often limited by side effects. Recent advancements suggest that liposome-based delivery systems can improve GC biodistribution, minimizing toxicity. This study introduces an innovative tool for RA treatment using prednisone-encapsulated nonphospholipid liposomes (NPLs) in combination with a hyaluronic acid (HA) hydrogel. Our methodology involved incorporating prednisone (PR) with palmitic acid and cholesterol to formulate stable NPLs using a thin-film hydration technique. The synthesized PR-NPLs, characterized by a mean size of 150 nm, demonstrated uniform distribution and higher drug encapsulation in comparison with conventional phospholipid liposomes. In vitro assays revealed that PR-NPL markedly reduced inflammatory responses in macrophages. Additionally, we successfully incorporated PR-NPL into an HA hydrogel, employing a photoinitiated cross-linking process. This novel composite offered modulable PR release, governed by the degree of hydrogel cross-linking. The developed system presents a promising advancement in RA management, especially suited for intraarticular injections. It potentially enables targeted, controlled drug release with a reduced risk of side effects, signifying a significant improvement over existing RA therapies.

Mechanistic Insight into How PEGylation Reduces the Efficacy of pH-Sensitive Liposomes from Molecular Dynamics Simulations

Liposome-based drug delivery systems composed of DOPE stabilized with cholesteryl hemisuccinate (CHMS) have been proposed as a drug delivery mechanism with pH-triggered release as the anionic form (CHSa) is protonated (CHS) at reduced pH; PEGylation is known to decrease this pH sensitivity. In this manuscript, we set out to use molecular dynamics (MD) simulations with a model with all-atom resolution to provide insight into why incorporation of poly(ethyleneglycol) (PEG) into DOPE–CHMS liposomes reduces their pH sensitivity; we also address two additional questions: (1) How CHSa stabilizes DOPE bilayers into a lamellar conformation at a physiological pH of 7.4? and (2) how the change from CHSa to CHS at acidic pH triggers the destabilization of DOPE bilayers? We found that (A) CHSa stabilizes the DOPE lipid membrane by increasing the hydrophilicity of the bilayer surface, (B) when CHSa changes to CHS by pH reduction, DOPE bilayers are destabilized due to a reduction in bilayer hydrophilicity and a reduction in the area per lipid, and (C) PEG stabilizes DOPE bilayers into the lamellar phase, thus reducing the pH sensitivity of the liposomes by increasing the area per lipid through penetration into the bilayer, which is our main focus.

Lipid constituents of model protocell membranes

Primitive life must have possessed the essential features of modern cellular life, but without highly evolved proteins to perform dynamic functions such as nutrient transport and membrane remodeling. Here, we consider the membrane properties of protocells — minimal cells with hereditary material, capable of growth and division — and how these properties place restrictions on the components of the membrane. For example, the lipids of modern membranes are diacyl amphiphilic molecules containing well-over 20 carbons in total. Without proteins, these membranes are very stable and kinetically trapped. This inertness, combined with the need for enzymes to synthesize them, makes modern diacyl amphiphiles unsuitable candidates for the earliest membranes on Earth. We, therefore, discuss the progress made thus far with single-chained amphiphiles, including fatty acids and mixtures of fatty acids with related molecules, and the membrane-related research that must be undertaken to gain more insight into the origins of cellular life.

Mechanism of Long-Chain Free Fatty Acid Protonation at the Membrane-Water Interface

Long-chain free fatty acids (FFAs) play an important role in several physiological and pathological processes such as lipid fusion, adjustments of membrane permeability and fluidity, and the regulation of enzyme and protein activities. FFA-facilitated membrane proton transport (flip-flop) and FFA-dependent proton transport by membrane proteins (e.g., mitochondrial uncoupling proteins) are governed by the difference between FFA's intrinsic pK_a value and the pH in the immediate membrane vicinity. Thus far, a quantitative understanding of the process has been hampered, because the pK_a value shifts upon moving the FFA from the aqueous solution into the membrane. For the same FFA, pK_a values between 5 and 10.5 were reported. Here, we systematically evaluated the dependence of pK_a values on chain length and number of double bonds by measuring the ζ-potential of liposomes reconstituted with FFA at different pH values. The experimentally obtained intrinsic pK_a values (6.25, 6.93, and 7.28 for DOPC membranes) increased with FFA chain length (C16, C18, and C20), indicating that the hydrophobic energy of transfer into the bilayer is an important pK_a determinant. The observed pK_a decrease in DOPC with increasing number of FFA double bonds (7.28, 6.49, 6.16, and 6.13 for C20:0, C20:1, C20:2, and C20:4, respectively) is in line with a decrease in transfer energy. Molecular dynamic simulations revealed that the ionized carboxylic group of the FFAs occupied a fixed position in the bilayer independent of chain length, underlining the importance of Born energy. We conclude that pK_a is determined by the interplay between the energetic costs for 1) burying the charged moiety into the lipid bilayer and 2) transferring the hydrophobic protonated FFA into the bilayer.

Preparation of photothermal palmitic acid/cholesterol liposomes

Indocyanine green (ICG) is the only FDA-approved near-infrared dye and it is currently used clinically for diagnostic applications. However, there is significant interest in using ICG for triggered drug delivery applications and heat ablation therapy. Unfortunately, free ICG has a short half-life in vivo and is rapidly cleared from circulation. Liposomes have been frequently used to improve ICG's stability and overall time of effectiveness in vivo, but they have limited stability due to the susceptibility of phospholipids to hydrolysis and oxidation. In this study, nonphospholipid liposomes were used to encapsulate ICG, and the resulting liposomes were characterized for size, encapsulation efficiency, stability, and photothermal response. Using the thin-film hydration method, an ICG encapsulation efficiency of 54% was achieved, and the liposomes were stable for up to 12 weeks, with detectable levels of encapsulated ICG up to week 4. Additionally, ICG-loaded liposomes were capable of rapidly producing a significant photothermal response upon exposure to near-infrared light, and this photothermal response was able to induce changes in the mechanical properties of thermally responsive hydrogels. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1384-1392, 2019.

Effect of pH and Salt on Surface pKa of Phosphatidic Acid Monolayers

The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pK_a) of the protic functional group(s) present. Here, we use three independent methods-compression isotherms, surface tension pH titration, and infrared reflection-absorption spectroscopy (IRRAS)-to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pK_a to remove the second proton of DPPA (surface pK_a2) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pK_a2 values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pK_a2 of 11.5, a value higher than previously reported (∼7.9-8.5). This surface pK_a2 was further altered by the presence of Na⁺ cations in the aqueous subphase, which reduced the surface pK_a2 from 11.5 to 10.5. It was also found that the surface pK_a2 value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pK_a2 as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pK_a2 values are also found to be in agreement with those predicted by Gouy-Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pK_a2.

Biofluid-Triggered Burst Release from an Adaptive Covalently Assembled Dipeptide Nanocontainer for Emergency Treatment

The construction of quickly dissociating containers holding bioactive components that meet the extreme requirements of emergency treatment is highly desirable but remains a great challenge. Here the use of small-molecule-induced dynamic covalent assembly is reported for simple and tunable fabrication of a biocompatible diphenylalanine-based nanocontainer toward rapidly responsive cargo delivery. The assembled nanocontainer can adaptively encapsulate various charged or neutral molecules. Upon biofluid trigger, the encapsulated molecules and bioactive proteins are released in a burst (within 5 s) from the nanocontainer due to highly sensitive deprotonation-mediated disruption of hydrogen bonding. This highlighted feature allows the nanocontainer as an excellent "fast dissolving" delivery vehicle available in spray dosage form for medical emergencies, as demonstrated by in vivo application for massive hemorrhage.

Acid-responsive nanospheres from an asparagine-derived amphiphile

We describe the synthesis and self-assembly of an asparagine-derived amphiphile. The self-assembled systems formulated with the inclusion of cholesterol (0-50 mol%) show encapsulation for a hydrophobic model drug and rapidly disintegrate in response to mild acidic conditions.

Nonphospholipid fluid liposomes with switchable photocontrolled release

We created novel nonphospholipid photosensitive liposomes from a mixture of a monoacylated azobenzene amphiphile (AzoC10N(+)) and cholesterol sulfate (Schol). This system belongs to the family of sterol-enriched nonphospholipid liposomes that were shown to form stable large unilamellar vesicles (LUVs) with enhanced impermeability. Fluid bilayers were successfully prepared from AzoC10N(+)/Schol (25/75 molar ratio) mixtures, and LUVs could be derived at room temperature using standard extrusion methods. The isomerization process of the bilayer-inserted AzoC10N(+) was characterized. Leakage from these liposomes could be induced by the photoconversion of AzoC10N(+) from its trans form to its cis form. This photocontrolled release from fluid liposomes contrasts with the case of phospholipid-based azo-containing liposomes, which are generally required to be in the gel phase to be photosensitive. It is proposed that the very high degree of conformational order of the monoalkylated amphiphile and the tight packing of the hydrophobic core of the AzoC10N(+)/Schol liposomes make them responsive to the presence of the bulky cis azo isomer. Interestingly, the liposome impermeability could be fully restored by the photoisomerization of the cis form back to the trans form, providing a sharp on-and-off control of payload release. In addition, these nonphospholipid liposomes display a very limited passive release. Therefore, it is shown that AzoC10N(+)/Schol LUVs can be used as nanocontainers, whose content can be released by light in a controlled and switchable manner.

Formation, stability, and pH sensitivity of free-floating, giant unilamellar vesicles using palmitic acid-cholesterol mixtures

Despite the fact that palmitic acid (PA) and cholesterol (Chol) do not form fluid bilayers once hydrated individually, giant unilamellar vesicles (GUVs) were formed from a mixture of palmitic acid and cholesterol, 30/70 mol/mol. These free-floating GUVs were stable over weeks, did not aggregate and were shown to be highly stable in alkaline pH compared to conventional phospholipid-based GUVs. Acidic pH-triggered payload release from the GUVs was associated with the protonation state of palmitic acid that dictated the mixing lipid properties, thus affecting the stability of the fluid lamellar phase. The successful formation of PA-Chol GUVs reveals the possibility to create monoalkylated amphiphile-based GUVs with distinct pH stability/sensitivity.

Impact of interfacial cholesterol-anchored polyethylene glycol on sterol-rich non-phospholipid liposomes

HYPOTHESIS

Liposomes made of single-chain amphiphiles and a large amount of sterols display several advantages including a limited permeability. In the present paper, we examine the possibility to prepare such non-phospholipid liposomes with interfacial polyethylene glycol (PEG) in order to improve their circulation in the blood stream. Cholesterol (Chol) was chosen as the PEG anchor.

EXPERIMENTS

The phase behavior of mixtures of palmitic acid (PA) and cholesterol including various proportions of PEGylated cholesterol (PEG-Chol) was characterized. In conditions leading to the formation of fluid bilayers, properties of the resulting liposomes were assessed.

FINDINGS

Up to 20 mol% of PEGylated cholesterol could be introduced without significant perturbations in fluid bilayers made of PA and cholesterol. With 10 mol% PEG-Chol, PA/Chol/PEG-Chol liposomes showed a very limited permeability to calcein and doxorubicin. Doxorubicin could be actively loaded in PA/Chol/PEG-Chol liposomes with a high drug loading efficiency and a high drug to lipid ratio. Pharmaco-kinetic experiments in rats indicated that interfacial PEG reduced the clearance of PA/Chol liposomes compared to the naked ones. However the lifetime of these non-phospholipid liposomes in the blood circulation was considerably shorter than that observed for control PEGylated phospholipid liposomes, a phenomenon associated with the negative interfacial charge of the PA/Chol/PEG-Chol liposomes.

Self-assembly of fatty acids: from foams to protocell vesicles

Stable aqueous dispersions of fatty acids can now be obtained and yield multiple applications.

Lamellar self-assemblies of single-chain amphiphiles and sterols and their derived liposomes: distinct compositions and distinct properties

Typically, single-chain amphiphiles and sterols do not form fluid lamellar phases once hydrated individually. Most of the single-chain amphiphiles form actually micelles in aqueous environments, while sterols display a very limited solubility in water. However, under certain conditions, mixtures of single-chain amphiphiles and sterols lead to the formation of stable fluid bilayers. Over the past decade, several of these systems leading to fluid lamellar self-assemblies have been identified and this article reviews the current knowledge relative to these non-phospholipid bilayers made of single-chain amphiphiles and sterols. It presents an integrated view about the molecular features that are required for their stability, the properties they share, and the origin of these characteristics. It was also shown that these lamellar systems could lead to the formation of unilamellar vesicles, similar to phospholipid based liposomes. These vesicles display distinct properties that make them potentially appealing for technological applications; they display a limited permeability, they are stable, they are formed with molecules that are relatively chemically inert (and relatively cheap), and they can be readily functionalized. The features of these distinct liposomes and their technological applications are reviewed. Finally, the putative biological implications of these non-phospholipid fluid bilayers are also discussed.

Atomistic simulations of pH-dependent self-assembly of micelle and bilayer from fatty acids

Detailed knowledge of the self-assembly and phase behavior of pH-sensitive surfactants has implications in areas such as targeted drug delivery. Here we present a study of the formation of micelle and bilayer from lauric acids using a state-of-the-art simulation technique, continuous constant pH molecular dynamics (CpHMD) with conformational sampling in explicit solvent and the pH-based replica-exchange protocol. We find that at high pH conditions a spherical micelle is formed, while at low pH conditions a bilayer is formed with a considerable degree of interdigitation. The mid-point of the phase transition is in good agreement with experiment. Preliminary investigation also reveals that the effect of counterions and salt screening shifts the transition mid-point and does not change the structure of the surfactant assembly. Based on these data we suggest that CpHMD simulations may be applied to computational design of surfactant-based nano devices in the future.

Formation of pH-sensitive cationic liposomes from a binary mixture of monoalkylated primary amine and cholesterol

It has been shown that mixtures of monoalkylated amphiphiles and sterols can form liquid-ordered (lo) lamellar phases. These bilayers can be extruded using conventional methods to obtain large unilamellar vesicles (LUVs) that have very low permeability and a specific response to a given stimulus. For example, pH variations can trigger the release from LUVs formed with palmitic acid and sterols. In the present work, the possibility to form non phospholipid liposomes with mixtures of stearylamine (SA) and cholesterol (Chol) was investigated. The phase behavior of these mixtures was characterized by differential scanning calorimetry, infrared, and (2)H NMR spectroscopy. It is found that this particular mixture can form a lo lamellar phase that is pH-sensitive as the system undergoes a transition from a lo phase to a solid state when pH is increased from 5.5 to 12. LUVs have been successfully extruded from equimolar SA/Chol mixtures. Release experiments as a function of time revealed the relatively low permeability of these systems. The fact that the stability of these liposomes is pH dependent implies that these LUVs display an interesting potential as new cationic carriers for pH-triggered release. This is the first report of non phospholipid liposomes with high sterol content combining an overall positive charge and pH-sensitivity.

Wet-spinning of amyloid protein nanofibers into multifunctional high-performance biofibers

Amyloid nanofibers derived from hen egg white lysozyme were processed into macroscopic fibers in a wet-spinning process based on interfacial polyion complexation using a polyanionic polysaccharide as cross-linker. As a result of their amyloid nanostructure, the hierarchically self-assembled protein fibers have a stiffness of up to 14 GPa and a tensile strength of up to 326 MPa. Fine-tuning of the polyelectrolytic interactions via pH allows to trigger the release of small molecules, as demonstrated with riboflavin-5'-phophate. The amyloid fibrils, highly oriented within the gellan gum matrix, were mineralized with calcium phosphate, mimicking the fibrolamellar structure of bone. The formed mineral crystals are highly oriented along the nanofibers, thus resulting in a 9-fold increase in fiber stiffness.