Bilayer Composition, Temperature, Speciation Effects and the Role of Bilayer Chain Ordering on Partitioning of Dexamethasone and its 21-Phosphate

Hyaluronic acid-conjugated liposomes loaded with dexamethasone: a promising approach for the treatment of inflammatory diseases

Dexamethasone is a well-known anti-inflammatory drug readily used to treat many lung diseases. However, its side effects and poor lower airway deposition and retention are significant limitations to its usage. In this work, we developed lipid nanoparticulate platforms loaded with dexamethasone and evaluated their behavior in inflammatory lung models in vitro and in vivo. Dexamethasone-loaded liposomes with an average diameter below 150 nm were obtained using a solvent injection method. Three different formulations were produced with a distinct surface coating (polyethylene glycol, hyaluronic acid, or a mixture of both) as innovative strategies to cross the pulmonary mucus layer and/or target CD44 expressed on alveolar proinflammatory macrophages. Interestingly, while electron paramagnetic spectroscopy showed that surface modifications did not induce any molecular changes in the liposomal membrane, drug loading analysis revealed that adding the hyaluronic acid in the bilayer led to a decrease of dexamethasone loading (from 3.0 to 1.7w/w%). In vitro experiments on LPS-activated macrophages demonstrated that the encapsulation of dexamethasone in liposomes, particularly in HA-bearing ones, improved its anti-inflammatory efficacy compared to the free drug. Subsequently, in vivo data revealed that while intratracheal administration of free dexamethasone led to an important inter-animals variation of efficacy, dexamethasone-loaded liposomes showed an improved consistency within the results. Our data indicate that encapsulating dexamethasone into lipid nanoparticles is a potent strategy to improve its efficacy after lung delivery.

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

In vitro methods for predicting the bioconcentration of xenobiotics in aquatic organisms

The accumulation of anthropogenic chemical substances in aquatic organisms is an immensely important issue from the point of view of environmental protection. In the context of the increasing number and variety of compounds that may potentially enter the environment, there is a need for efficient and reliable solutions to assess the risks. However, the classic approach of testing with fish or other animals is not sufficient. Due to very high costs, significant time and labour intensity, as well as ethical concerns, in vivo methods need to be replaced by new laboratory-based tools. So far, many models have been developed to estimate the bioconcentration potential of chemicals. However, most of them are not sufficiently reliable and their predictions are based on limited input data, often obtained with doubtful quality. The octanol-water partition coefficient is still often used as the main laboratory tool for estimating bioconcentration. However, according to current knowledge, this method can lead to very unreliable results, both for neutral species and, above all, for ionic compounds. It is therefore essential to start using new, more advanced and credible solutions on a large scale. Over the last years, many in vitro methods have been newly developed or improved, allowing for a much more adequate estimation of the bioconcentration potential. Therefore, the aim of this work was to review the most recent laboratory methods for assessing the bioconcentration potential and to evaluate their applicability in further research.

Affinity of Lipophilic Drugs to Mixed Lipid Aggregates in Simulated Gastrointestinal Fluids

Mixed lipid aggregates, comprising of bile salts and phospholipids, present in the small intestine assist in drug solubilization and subsequent drug dissolution and absorption through the intestinal epithelium. The increased variability in their levels, observed physiologically, may create challenges not only for in vivo bioavailability and bioequivalence studies, but also for in vitro bio-predictive studies as correlations between in vitro and in vivo data are not always successful. The current study investigated the impact of biorelevant dissolution media, with physiologically relevant sodium taurocholate and lecithin levels, on the apparent solubility and affinity of lipophilic compounds with a wide range of physicochemical properties (drug ionization, drug lipophilicity, molecular weight) to mixed lipid aggregates. Apparent solubility data in biorelevant dissolution media for the studied neutral drugs, weak bases and weak acids were compared against a phosphate buffer pH 6.5 in the absence of these lipidic components. Presence of mixed lipid aggregates enhanced the apparent solubility of the majority of compounds and the use of multivariate data analysis identified the significant parameters affecting drug affinity to mixed lipid aggregates based on the chemical class of the drug. For neutral drugs, increasing bile salt concentrations and/or drug lipophilicity resulted in greater enhancement in apparent solubility at 24-hr. For weak bases and weak acids, the effect of increasing bile salt levels on apparent solubility depended mostly on an interplay between drug lipophilicity and drug ionization.

Rationalizing Steroid Interactions with Lipid Membranes: Conformations, Partitioning, and Kinetics

Steroids have numerous physiological functions associated with cellular signaling or modulation of the lipid membrane structure and dynamics, and as such, they have found broad pharmacological applications. Steroid-membrane interactions are relevant to multiple steps of steroid biosynthesis and action, as steroids are known to interact with neurotransmitter or membrane steroid receptors, and steroids must cross lipid membranes to exert their physiological functions. Therefore, rationalizing steroid function requires understanding of steroid-membrane interactions. We combined molecular dynamics simulations and isothermal titration calorimetry to characterize the conformations and the energetics of partitioning, in addition to the kinetics of flip-flop transitions and membrane exit, of 26 representative steroid compounds in a model lipid membrane. The steroid classes covered in this study include birth control and anabolic drugs, sex and corticosteroid hormones, neuroactive steroids, as well as steroids modulating the lipid membrane structure. We found that the conformational ensembles adopted by different steroids vary greatly, as quantified by their distributions of tilt angles and insertion depths into the membrane, ranging from well-defined steroid conformations with orientations either parallel or normal to the membrane, to wide conformational distributions. Surprisingly, despite their chemical diversity, the membrane/water partition coefficient is similar among most steroids, except for structural steroids such as cholesterol, leading to similar rates for exiting the membrane. By contrast, the rates of steroid flip-flop vary by at least 9 orders of magnitude, revealing that flip-flop is the rate-limiting step during cellular uptake of polar steroids. This study lays the ground for a quantitative understanding of steroid-membrane interactions, and it will hence be of use for studies of steroid biosynthesis and function as well as for the development and usage of steroids in a pharmacological context.

Surviving nebulization-induced stress: dexamethasone in pH-sensitive archaeosomes

Aim: To increase the subcellular delivery of dexamethasone phosphate (DP) and stability to nebulization stress, pH-sensitive nanoliposomes (LpH) exhibiting archaeolipids, acting as ligands for scavenger receptors (pH-sensitive archaeosomes [ApH]), were prepared. Materials & methods: The anti-inflammatory effect of 0.18 mg DP/mg total lipid, 100–150 nm DP-containing ApH (dioleylphosphatidylethanolamine: Halorubrum tebenquichense total polar archaeolipids:cholesteryl hemisuccinate 4.2:2.8:3 w:w) was tested on different cell lines. Size and HPTS retention of ApH and conventional LpH (dioleylphosphatidylethanolamine:cholesteryl hemisuccinate 7:3 w:w) before and after nebulization were determined. Results & conclusion: DP-ApH suppressed IL-6 and TNF-α on phagocytic cells. Nebulized after 6-month storage, LpH increased size and completely lost its HPTS while ApH3 conserved size and polydispersity, fully retaining its original HPTS content.

Mechanistic model and analysis of doxorubicin release from liposomal formulations

Reliable and predictive models of drug release kinetics in vitro and in vivo are still lacking for liposomal formulations. Developing robust, predictive release models requires systematic, quantitative characterization of these complex drug delivery systems with respect to the physicochemical properties governing the driving force for release. These models must also incorporate changes in release due to the dissolution media and methods employed to monitor release. This paper demonstrates the successful development and application of a mathematical mechanistic model capable of predicting doxorubicin (DXR) release kinetics from liposomal formulations resembling the FDA-approved nanoformulation DOXIL® using dynamic dialysis. The model accounts for DXR equilibria (e.g. self-association, precipitation, ionization), the change in intravesicular pH due to ammonia release, and dialysis membrane transport of DXR. The model was tested using a Box-Behnken experimental design in which release conditions including extravesicular pH, ammonia concentration in the release medium, and the dilution of the formulation (i.e. suspension concentration) were varied. Mechanistic model predictions agreed with observed DXR release up to 19h. The predictions were similar to a computer fit of the release data using an empirical model often employed for analyzing data generated from this type of experimental design. Unlike the empirical model, the mechanistic model was also able to provide reasonable predictions of release outside the tested design space. These results illustrate the usefulness of mechanistic modeling to predict drug release from liposomal formulations in vitro and its potential for future development of in vitro - in vivo correlations for complex nanoformulations.

Dynamic, nonsink method for the simultaneous determination of drug permeability and binding coefficients in liposomes

Drug release from liposomal formulations is governed by a complex interplay of kinetic (i.e., drug permeability) and thermodynamic factors (i.e., drug partitioning to the bilayer surface). Release studies under sink conditions that attempt to mimic physiological conditions are insufficient to decipher these separate contributions. The present study explores release studies performed under nonsink conditions coupled with appropriate mathematical models to describe both the release kinetics and the conditions in which equilibrium is established. Liposomal release profiles for a model anticancer agent, topotecan, under nonsink conditions provided values for both the first-order rate constant for drug release and the bilayer/water partition coefficient. These findings were validated by conducting release studies under sink conditions via dynamic dialysis at the same temperature and buffer pH. A nearly identical rate constant for drug release could be obtained from dynamic dialysis data when appropriate volume corrections were applied and a mechanism-based mathematical model was employed to account for lipid bilayer binding and dialysis membrane transport. The usefulness of the nonsink method combined with mathematical modeling was further explored by demonstrating the effects of topotecan dimerization and bilayer surface charge potential on the bilayer/water partition coefficient at varying suspension concentrations of lipid and drug.

Determination of drug release kinetics from nanoparticles: overcoming pitfalls of the dynamic dialysis method

Dynamic dialysis is one of the most common methods for the determination of release kinetics from nanoparticle drug delivery systems. Drug appearance in the "sink" receiver compartment is a consequence of release from the nanoparticles into the dialysis chamber followed by diffusion across the dialysis membrane. This dual barrier nature inherent in the method complicates data interpretation and may lead to incorrect conclusions regarding nanoparticle release half-lives. Although the need to consider the barrier properties of the dialysis membrane has long been recognized, there is insufficient quantitative appreciation for the role of the driving force for drug transport across that membrane. Reversible nanocarrier binding of the released drug reduces the driving force for drug transport across the dialysis membrane leading to a slower overall apparent release rate. This may lead to the conclusion that a given nanoparticle system will provide a sustained release in vivo when it will not. This study demonstrates these phenomena using model lipophilic drug-loaded liposomes varying in lipid composition to provide variations in bilayer permeability and membrane binding affinities. Model simulations of liposomal transport as measured by dynamic dialysis were conducted to illustrate the interplay between the liposome concentration, membrane/water partition coefficient, and the apparent release rate. Reliable determination of intrinsic liposomal bilayer permeability coefficients for lipophilic drugs by dynamic dialysis requires validation of drug release kinetics at varying nanoparticle concentration and the determination of membrane binding coefficients along with appropriate mechanism-based mathematical modeling to ensure the reliability and proper interpretation of the data.