Promote potential applications of nanoparticles as respiratory drug carrier: insights from molecular dynamics simulations

A Review on Investigating the Interactions between Nanoparticles and the Pulmonary Surfactant Monolayer with Coarse-Grained Molecular Dynamics Method

Pulmonary drug delivery has garnered significant attention due to its targeted local lung action, minimal toxic side effects, and high drug utilization. However, the physicochemical properties of inhaled nanoparticles (NPs) used as drug carriers can influence their interactions with the pulmonary surfactant (PS) monolayer, potentially altering the fate of the NPs and impairing the biophysical function of the PS monolayer. Thus, the objective of this review is to summarize how the physicochemical properties of NPs affect their interactions with the PS monolayer. Initially, the definition and properties of NPs, as well as the composition and characteristics of the PS monolayer, are introduced. Subsequently, the coarse-grained molecular dynamics (CGMD) simulation method for studying the interactions between NPs and the PS monolayer is presented. Finally, the implications of the hydrophobicity, size, shape, surface charge, surface modification, and aggregation of NPs on their interactions with the PS monolayer and on the composition of biomolecular corona are discussed. In conclusion, gaining a deeper understanding of the effects of the physicochemical properties of NPs on their interactions with the PS monolayer will contribute to the development of safer and more effective nanomedicines for pulmonary drug delivery.

Heterogeneous aggregation of carbon and silicon nanoparticles with benzo[a]pyrene modulates their impacts on the pulmonary surfactant film

Inhaled nanoparticles (NPs) can deposit in alveoli where they interact with the pulmonary surfactant (PS) and potentially induce toxicity. Although nano-bio interactions are influenced by the physicochemical properties of NPs, isolated NPs used in previous studies cannot accurately represent those found in atmosphere. Here we used molecular dynamics simulations to investigate the interplay between two types of NPs associated with benzo[a]pyrene (BaP) at the PS film. Silicon NPs (SiNPs), regardless of aggregation and adsorption, directly penetrated through the PS film with minimal disturbance. Meanwhile, BaPs adsorbed on SiNPs were rapidly solubilized by PS, increasing the BaP's bioaccessibility in alveoli. Carbon NPs (CNPs) showed aggregation and adsorption-dependent effects on the PS film. Compared to isolated CNPs, which extracted PS to form biomolecular coronas, aggregated CNPs caused more pronounced PS disruption, especially around irregularly shaped edges. SiNPs in mixture exacerbated the PS perturbation by piercing PS film around the site of CNP interactions. BaPs adsorbed on CNPs were less solubilized and suppressed PS extraction, but aggravated biophysical inhibition by prompting film collapse under compression. These results suggest that for proper assessment of inhalation toxicity of airborne NPs, it is imperative to consider their heterogeneous aggregation and adsorption of pollutants under atmospheric conditions.

Lessons from the history of inorganic nanoparticles for inhalable diagnostics and therapeutics

The respiratory tract is one of the most accessible ones to exogenous nanoparticles, yet drug delivery by their means to it is made extraordinarily challenging because of the plexus of aerodynamic, hemodynamic and biomolecular factors at cellular and extracellular levels that synergistically define the safety and efficacy of this process. Here, the use of inorganic nanoparticles (INPs) for inhalable diagnostics and therapies of the lung is viewed through the prism of the history of studies on the interaction of INPs with the lower respiratory tract. The most conceptually and methodologically innovative and illuminative studies are referred to in the chronological order, as they were reported in the literature, and the trends in the progress of understanding this interaction of immense therapeutic and toxicological significance are being deduced from it. The most outstanding actual trends delineated include the diminishment of toxicity via surface functionalization, cell targeting, tagging and tracking via controlled binding and uptake, hybrid INP treatments, magnetic guidance, combined drug and gene delivery, use as adjuvants in inhalable vaccines, and other. Many of the understudied research directions, which have been accomplished by the nanostructured organic polymers in the pulmonary niche, are discussed. The progress in the use of INPs as inhalable diagnostics or therapeutics has been hampered by their well-recognized inflammatory potential and toxicity in the respiratory tract. However, the annual numbers of methodologically innovative studies have been on the rise throughout the past two decades, suggesting that this is a prolific direction of research, its comparatively poor commercial takings notwithstanding. Still, the lack of consensus on the effects of many INP compositions at low but therapeutically effective doses, the plethora of contradictory reports on ostensibly identical chemical compositions and NP properties, and the many cases of antagonism in combinatorial NP treatments imply that the rational design of inhalable medical devices based on INPs must rely on qualitative principles for the most part and embrace a partially stochastic approach as well. At the same time, the fact that the most studied INPs for pulmonary applications have been those with some of the thickest records of pulmonary toxicity, e.g., carbon, silver, gold, silica and iron oxide, is a silent call for the expansion of the search for new inorganic compositions for use in inhalable therapies to new territories.

Molecular dynamics simulation insights into the cellular uptake of elastic nanoparticles through human pulmonary surfactant

The interaction between inhaled nanoparticles (NPs) and the pulmonary surfactant (PS) monolayer has drawn significant attention due to its potential in drug delivery design and application for respiratory therapeutics in active and passive cellular uptake pathways. Even though much attention has been given to explore the interaction between NPs and the PS monolayer, the effects of the NP elasticity on the translocation across the PS monolayer have not been thoroughly studied. Here, we performed a series of coarse-grained (CG) molecular dynamics simulations to study active or passive cellular uptake pathways of three NPs with different elasticities through a PS monolayer. The differences between active and passive pathways underly the enhanced targeting ability by ligand-receptor interaction (L-R interaction). In the active or passive cellular uptake pathways, it is found that the increase in stiffness level leads to a higher penetrability of NPs at the same time range. The soft NP has always been withheld inside the PS monolayer due to the lowest level of elasticity, while the other two types of NPs penetrate through the PS monolayer as the simulation progresses toward the end. The NPs in the active cellular uptake pathways take a longer time to penetrate the PS monolayer, resulting in a longer average penetration distance of approximately 40.55% and a higher average number of contacts, approximately 36.11%, than passive cellular uptake pathways, due to the L-R interaction. Moreover, it demonstrates that NPs in active cellular uptake pathways have a significantly higher targeting ability with the PS monolayer. We conclude that the level of NP elasticities has a substantial link to the penetrability in active or passive cellular uptake pathways. These results provide valuable insights into drug delivery and nanoprobe design for inhaled NPs within the lungs.

The lung surfactant activity probed with molecular dynamics simulations

The surface of pulmonary alveolar subphase is covered with a mixture of lipids and proteins. This lung surfactant plays a crucial role in lung functioning. It shows a complex phase behavior which can be altered by the interaction with third molecules such as drugs or pollutants. For studying multicomponent biological systems, it is of interest to couple experimental approach with computational modelling yielding atomic-scale information. Simple two, three, or four-component model systems showed to be useful for getting more insight in the interaction between lipids, lipids and proteins or lipids and proteins with drugs and impurities. These systems were studied theoretically using molecular dynamic simulations and experimentally by means of the Langmuir technique. A better understanding of the structure and behavior of lung surfactants obtained from this research is relevant for developing new synthetic surfactants for efficient therapies, and may contribute to public health protection.

Molecular basis of transport of surface functionalised gold nanoparticles to pulmonary surfactant

Ligands like alkanethiol (e.g. dodecanethiol, hexadecanethiol, etc.) and polymers (e.g. poly(vinyl pyrrolidone), polyethylene glycol-thiol) capped to the gold nanoparticles (AuNPs) are widely used in biomedical field as drug carriers and as promising materials for probing and manipulating cellular processes. Ligand functionalised AuNPs are known to interact with the pulmonary surfactant (PS) monolayer once reaching the alveolar region. Therefore, it is crucial to understand the interaction between AuNPs and PS monolayers. Using coarse-grained molecular dynamics simulations, the effect of ligand density, and ligand length have been studied for two classes of ligands on a PS model monolayer consisting of DPPC, POPG, cholesterol and SP-B (mini-peptide). The ligands considered in this study are alkanethiol and polyethylene glycol (PEG) thiol as examples of hydrophobic and hydrophilic ligands, respectively. It was observed that the interaction between AuNPs and PS changes the biophysical properties of PS monolayer in compressed and expanded states. The AuNPs with hydrophilic ligand, can penetrate through the monolayer more easily, while the AuNPs with hydrophobic ligand are embedded in the monolayer and participated in deforming the monolayer structure particularly the monolayer in the compressed state. The bare AuNPs hinder to lower the monolayer surface tension value at the interface, however introducing ligand to the bare AuNPs or increasing the ligand length and density have an impact of lowering of monolayer surface tension to a minor extent. The simulation results guide the design of ligand protected NPs as drug carriers and can identify the nanoparticles' potential side effects on lung surfactant.

Molecular-level observations of the behavior of ligand functionalised gold nanoparticles with a lipid monolayers.

The steroid mometasone alters protein containing lung surfactant monolayers in a concentration-dependent manner

Mometasone is an investigational anti-inflammatory steroidal drug to treat inflammation via pulmonary administration. For steroid drugs to be effective they need to be adsorbed by lung surfactants, a thin monolayer at the air-water interface in alveoli that reduces surface tension. Information on the molecular-level interactions of the drug with lung surfactants is useful to understand the mechanism of adsorption. In this study, we use coarse-grained molecular dynamics simulation to understand the concentration-dependent effect of mometasone on a lung surfactant monolayer (LSM) composed of lipids and surfactant proteins, under two different breathing conditions (exhalation, at surface tension 0 mNm^-1 and inhalation, surface tension 20-25 mNm^-1). A series of fixed-APL and fixed-surface tension simulations were used to demonstrate that in the absence of drugs, the model LSM reproduces the surface tensions for the compressed and expanded states, as well as compressibility at different surface tensions. In-depth analysis of simulations of a LSM in the presence of five different drug concentrations shows that mometasone alters the structure and dynamics of the LSM in a concentration-dependent manner. Mometasone induces a collapse in the monolayer that is affected by the surfactant protein and surface tension. Overall, these findings suggest that the surfactant proteins, surface tension and drug concentration are all critical components affecting monolayer stability and drug adsorption. The outcomes of this study may be beneficial for a more in-depth understanding of how mometasone is adsorbed by lung surfactants.

Computational Studies of Lipid-Wrapped Gold Nanoparticle Transport Through Model Lung Surfactant Monolayers

Colloidal nanoparticles, such as gold nanoparticles (AuNPs), are promising materials for the delivery of hydrophilic drugs via the pulmonary route. The inhaled nanoparticle drug carriers primarily deposit in lung alveoli and interact with the alveolar surface known as lung surfactants. Therefore, it is vital to understand the interactions of nanocarriers with the surfactant layer. To understand the interactions at the molecular level, here we simulated model lung surfactant monolayers with phospholipid (PL)-wrapped AuNPs at the vacuum-water interface using coarse-grained molecular dynamics simulations. The PL-wrapped AuNPs quickly adsorbed into the surfactant layer, altered the structural properties of the monolayer, and at high concentrations initiated the compressed monolayer to collapse/buckle. Among the surfactant monolayer lipid components, cholesterol adsorbed to the AuNPs preferentially over PL species. The position of the adsorbed PL-AuNPs within the monolayer, and subsequent monolayer perturbation, vary depending on the monolayer phase, monolayer composition, and species of PL used as a ligand. Information provided by these molecular dynamic simulations helps to rationalize why some colloidal nanoparticles work better as nanocarriers than others and aid the design of new ones, to avoid biological toxicity and improve efficacy for pulmonary drug delivery.

Protein Corona on Gold Nanoparticles Studied with Coarse-Grained Simulations

Understanding protein corona formation in an aqueous environment at the molecular and atomistic levels is critical to applications such as biomolecule-detection and drug delivery. In this work, we employed mesoscopic coarse-grained simulations to study ovispirin-1 and lysozyme protein coronas on bare gold nanoparticles. Our study showed that protein corona formation is governed by protein-surface and protein-protein interactions, as well as the surface hydrophobic effect. The corona structure was found to be dependent on protein types and the size of nanoparticles. Ovispirin proteins form homogeneous single-layered adsorption in comparison with the lysozyme's inhomogeneous multilayered aggregates on gold NP surfaces. The decrease in nanoparticle size leads to more angular degrees of freedom for protein adsorption orientation. Subsequent atomistic molecular dynamics simulations further demonstrate the loss of secondary structure of ovispirin upon adsorption and the heterogeneity of its local structure.

Finite Element Analysis for Predicting Skin Pharmacokinetics of Nano Transdermal Drug Delivery System Based on the Multilayer Geometry Model

Background

Skin pharmacokinetics is an indispensable indication for studying the drug fate after administration of transdermal drug delivery systems (TDDS). However, the heterogeneity and complex skin structured with stratum corneum, viable epidermis, dermis, and subcutaneous tissue inevitably leads the drug diffusion coefficient (Kp) to vary depending on the skin depth, which seriously limits the development of TDDS pharmacokinetics in full thickness skin.

Methods

A multilayer geometry skin model was established and the Kp of drug in SC, viable epidermis, and dermis was obtained using the technologies of molecular dynamics simulation, in vitro permeation experiments, and in vivo microdialysis, respectively. Besides, finite element analysis (FEA) based on drug Kps in different skin layers was applied to simulate the paeonol nanoemulsion (PAE-NEs) percutaneous dynamic penetration process in two and three dimensions. In addition, PAE-NEs skin pharmacokinetics profile obtained by the simulation was verified by in vivo experiment.

Results

Coarse-grained modeling of molecular dynamic simulation was successfully established and the Kp of PAE in SC was 2.00×10⁻⁶ cm²/h. The Kp of PAE-NE in viable epidermis and in dermis detected using penetration test and microdialysis probe technology, was 1.58×10⁻⁵ cm²/h and 3.20×10⁻⁵ cm²/h, respectively. In addition, the results of verification indicated that PAE-NEs skin pharmacokinetics profile obtained by the simulation was consistent with that by in vivo experiment.

Discussion

This study demonstrated that the FEA combined with the established multilayer geometry skin model could accurately predict the skin pharmacokinetics of TDDS.

Molecular Simulations of PEGylated Biomolecules, Liposomes, and Nanoparticles for Drug Delivery Applications

Since the first polyethylene glycol (PEG)ylated protein was approved by the FDA in 1990, PEGylation has been successfully applied to develop drug delivery systems through experiments, but these experimental results are not always easy to interpret at the atomic level because of the limited resolution of experimental techniques. To determine the optimal size, structure, and density of PEG for drug delivery, the structure and dynamics of PEGylated drug carriers need to be understood close to the atomic scale, as can be done using molecular dynamics simulations, assuming that these simulations can be validated by successful comparisons to experiments. Starting with the development of all-atom and coarse-grained PEG models in 1990s, PEGylated drug carriers have been widely simulated. In particular, recent advances in computer performance and simulation methodologies have allowed for molecular simulations of large complexes of PEGylated drug carriers interacting with other molecules such as anticancer drugs, plasma proteins, membranes, and receptors, which makes it possible to interpret experimental observations at a nearly atomistic resolution, as well as help in the rational design of drug delivery systems for applications in nanomedicine. Here, simulation studies on the following PEGylated drug topics will be reviewed: proteins and peptides, liposomes, and nanoparticles such as dendrimers and carbon nanotubes.

Nanoparticle translocation across the lung surfactant film regulated by grafting polymers

Nanoparticle-based pulmonary drug delivery has gained significant attention due to its ease of administration, increased bioavailability, and reduced side effects caused by a high systemic dosage. After being delivered into the deep lung, the inhaled nanoparticles first interact with the lung surfactant lining layer composed of phospholipids and surfactant proteins and then potentially cause the dysfunction of the lung surfactant. Conditioning the surface properties of nanoparticles with grafting polymers to avoid these side effects is of crucial importance to the efficiency and safety of pulmonary drug delivery. Herein, we perform coarse-grained molecular simulations to decipher the involved mechanism responsible for the translocation of the polymer-grafted Au nanoparticles across the lung surfactant film. The simulations illustrate that conditioning of the grafting polymers, including their length, terminal charge, and grafting density, can result in different translocation processes. Based on the energy analysis, we find that these discrepancies in translocation stem from the affinity of the nanoparticles with the lipid tails and heads and their contact with the proteins, which can be tuned by the surface polarity and surface charge of the nanoparticles. We further demonstrate that the interaction between the nanoparticles and the lung surfactant is related to the depletion of the lipids and proteins during translocation, which affects the surface tension of the surfactant film. The change in the surface tension in turn affects the nanoparticle translocation and the collapse of the surfactant film. These results can help understand the adverse effects of the nanoparticles on the lung surfactant film and provide guidance to the design of inhaled nanomedicines for improved permeability and targeting.

Molecular mechanism of HIV-1 TAT peptide and its conjugated gold nanoparticles translocating across lipid membranes

The trans-acting activator of transcription (TAT) peptide, which is derived from human immunodeficiency virus-1 (HIV-1), has been widely used as an effective nanocarrier to transport extracellular substances into cells. However, the underlying translocation mechanism of TAT peptide across cell membranes still remains controversial. Besides, the molecular process of TAT peptide facilitating the transport of extracellular substances into cells is largely unknown. In this study, we explore the interactions of TAT peptides and their conjugated gold nanoparticles with lipid membranes by coarse-grained molecular dynamics simulations. It is found that the TAT peptides can hardly penetrate through the membrane at low peptide concentrations; after the concentration increases to a threshold value, they can cross the membrane through an induced nanopore due to the transmembrane electrostatic potential difference. The translocation of TAT peptides is mainly caused by the overall structural changes of membranes. Furthermore, we demonstrate that the translocation of gold nanoparticles (AuNPs) across the membrane is significantly affected by the number of grafted TAT peptides on the particle surface. The transmembrane efficiency of AuNPs may even be reduced when a small number of peptides modify them; whereas, when the number of grafted peptides increases to a certain value, the TAT-AuNP complex can translocate across the membrane in a pore-mediated way. Based on our findings, an effective strategy has been proposed to enhance the delivery efficiency of AuNPs. The present study can improve our understanding of the interactions between TAT peptides and cell membranes; it may also give some insightful suggestions on the design and development of nanocarriers with high efficiency for the delivery of nanoparticles and drugs.

Poly(amidoamine) Dendrimer as a Respiratory Nanocarrier: Insights from Experiments and Molecular Dynamics Simulations

Pulmonary drug delivery is superior to the systemic administration in treating lung diseases. An optimal respiratory nanocarrier should be able to efficiently and safely cross the pulmonary surfactant film, which serves as the first biological barrier for respiratory delivery and plays paramount roles in maintaining the proper mechanics of breathing. In this work, we focused on the interactions between poly(amidoamine) (PAMAM) dendrimers and a model pulmonary surfactant. With combined Langmuir monolayer experiments and coarse-grained molecular dynamics simulations, we studied the effect of environmental temperature, size, and surface property of PAMAM dendrimers (G3-OH, G3-NH₂, G5-OH, and G5-NH₂) on the dipalmitoylphosphatidylcholine (DPPC) monolayer. Our simulations indicated that the environmental temperature could significantly affect the influence of PAMAM dendrimers on the DPPC monolayer. Therefore, results obtained at room temperature cannot be directly applied to elucidate interactions at body temperature. Simulations at body temperature found that all tested PAMAM dendrimers can easily penetrate the lipid monolayer during the monolayer expansion process (mimicking "inhalation"), and the cationic PAMAM dendrimers (-NH₂) show promising penetration ability during the monolayer compression process (mimicking "expiration"). Larger PAMAM dendrimers (G5) adsorbed onto the lipid monolayer tend to induce structural collapse and inhibit normal phase transitions of the lipid monolayer. These adverse effects could be mitigated in the subsequent expansion-compression cycle. These findings suggest that the PAMAM dendrimer may be used as a potential respiratory drug nanocarrier.

Role of Lipid Coating in the Transport of Nanodroplets across the Pulmonary Surfactant Layer Revealed by Molecular Dynamics Simulations

Hydrophilic drugs can be delivered into lungs via nebulization for both local and systemic therapies. Once inhaled, ultrafine nanodroplets preferentially deposit in the alveolar region, where they first interact with the pulmonary surfactant (PS) layer, with nature of the interaction determining both efficiency of the pulmonary drug delivery and extent of the PS perturbation. Here, we demonstrate by molecular dynamics simulations the transport of nanodroplets across the PS layer being improved by lipid coating. In the absence of lipids, bare nanodroplets deposit at the PS layer to release drugs that can be directly translocated across the PS layer. The translocation is quicker under higher surface tensions but at the cost of opening pores that disrupt the ultrastructure of the PS layer. When the PS layer is compressed to lower surface tensions, the nanodroplet prompts collapse of the PS layer to induce severe PS perturbation. By coating the nanodroplet with lipids, the disturbance of the nanodroplet on the PS layer can be reduced. Moreover, the lipid-coated nanodroplet can be readily wrapped by the PS layer to form vesicular structures, which are expected to fuse with the cell membrane to release drugs into secondary organs. Properties of drug bioavailability, controlled drug release, and enzymatic tolerance in real systems could be improved by lipid coating on nanodroplets. Our results provide useful guidelines for the molecular design of nanodroplets as carriers for the pulmonary drug delivery.

The role of surface charge in the interaction of nanoparticles with model pulmonary surfactants

Inhaled nanoparticles traveling through the airways are able to reach the respiratory zone of the lungs. In such an event, the incoming particles first come into contact with the liquid lining the alveolar epithelium, the pulmonary surfactant. The pulmonary surfactant is composed of lipids and proteins that are assembled into large vesicular structures. The question of the nature of the biophysicochemical interaction with the pulmonary surfactant is central to understand how the nanoparticles can cross the air-blood barrier. Here we explore the phase behavior of sub-100 nm particles and surfactant substitutes under controlled conditions. Three types of surfactant mimetics, including the exogenous substitute Curosurf®, a drug administered to infants with respiratory distress syndrome, are tested together with aluminum oxide (Al2O3), silicon dioxide (SiO2) and polymer (latex) nanoparticles. The main result here is the observation of spontaneous nanoparticle-vesicle aggregation induced by coulombic attraction. The role of the surface charges is clearly established. We also evaluate the supported lipid bilayer formation recently predicted and find that in the cases studied these structures do not occur. Pertaining to the aggregate internal structure, fluorescence microscopy shows that the vesicles and particles are intermixed at the nano- to microscale. With particles acting as stickers between vesicles, it is anticipated that the presence of inhaled nanomaterials in the alveolar spaces could significantly modify the interfacial and bulk properties of the pulmonary surfactant and interfere with lung physiology.

Penetration of antimicrobial peptides in a lung surfactant model

Molecular dynamics simulations were successfully performed to understand the absorption mechanism of antimicrobial peptides LL-37, CATH-2, and SMAP-29 in a lung surfactant model. The antimicrobial peptides quickly penetrate in the lung surfactant model in dozens or hundreds nanoseconds, but they electrostatically interact with the lipid polar heads during the simulation time of 2 μs. This electrostatic interaction should be the explanation for the inactivation of the antimicrobial peptides when co-administrated with lung surfactant. As they strongly interact with the lipid polar heads of the lung surfactant, there is no positive charge available on the antimicrobial peptide to attack the negatively charged bacteria membrane. In order to avoid the interaction of peptides with the lipid polar heads, sodium cholate was used to form nanoparticles which act as an absorption enhancer of all antimicrobial peptides used in this investigation. The nanoparticles of 150 molecules of sodium cholate with one peptide were inserted on the top of the lung surfactant model. The nanoparticles penetrated into the lung surfactant model, spreading the sodium cholate molecules around the lipid polar heads. The sodium cholate molecules seem to protect the peptides from the interaction with the lipid polar heads, leaving them free to be delivered to the water phase. The penetration of peptides alone or even the peptide nanoparticles with sodium cholate do not collapse the lung surfactant model, indicating to be a promisor drug delivery system to the lung. The implications of this finding are that antimicrobial peptides may only be co-administered with an absorption enhancer such as sodium cholate into lung surfactant in order to avoid inactivation of their antimicrobial activity.

Use of cilomilast-loaded phosphatiosomes to suppress neutrophilic inflammation for attenuating acute lung injury: the effect of nanovesicular surface charge

Background

Cilomilast is a phosphodiesterase 4 (PDE4) inhibitor for treating inflammatory lung diseases. This agent has a narrow therapeutic index with significant adverse effects on the nervous system. This study was conducted to entrap cilomilast into PEGylated phosphatidylcholine-rich niosomes (phosphatiosomes) to improve pulmonary delivery via the strong affinity to pulmonary surfactant film. Neutrophils were used as a cell model to test the anti-inflammatory activity of phosphatiosomes. In an in vivo approach, mice were given lipopolysaccharide to produce acute lung injury. The surface charge in phosphatiosomes that influenced the anti-inflammatory potency is discussed in this study.

Results

The average diameter of the phosphatiosomes was about 100 nm. The zeta potential of anionic and cationic nanovesicles was − 35 and 32 mV, respectively. Cilomilast in both its free and nanocapsulated forms inhibited superoxide anion production but not elastase release in activated neutrophils. Cationic phosphatiosomes mitigated calcium mobilization far more effectively than the free drug. In vivo biodistribution evaluated by organ imaging demonstrated a 2-fold ameliorated lung uptake after dye encapsulation into the phosphatiosomes. The lung/brain distribution ratio increased from 3 to 11 after nanocarrier loading. The intravenous nanocarriers deactivated the neutrophils in ALI, resulting in the elimination of hemorrhage and alveolar wall damage. Only cationic phosphatiosomes could significantly suppress IL-1β and TNF-α in the inflamed lung tissue.

Conclusions

These results suggest that phosphatiosomes should further be investigated as a potential nanocarrier for the treatment of pulmonary inflammation.

Supported pulmonary surfactant bilayers on silica nanoparticles: formulation, stability and impact on lung epithelial cells

Studies have shown that following exposure to particulate matter, ultrafine fractions (<100 nm) may deposit along the respiratory tract down to the alveolar region. To assess the effects of nanoparticles on the lungs, it is essential to address the question of their biophysicochemical interaction with the different pulmonary environments, including the lung lining fluids and the epithelia. Here we examine one of these interactive scenarios and study the role of supported lipid bilayers (SLB) in the effect of 40 nm fluorescent silica particles on living cells. We first study the particle phase behavior in the presence of Curosurf®, a pulmonary surfactant substitute used in replacement therapies. It is found that Curosurf® vesicles interact strongly with the nanoparticles, but do not spontaneously form SLBs. To achieve this goal, we use sonication to reshape the vesicular membranes and induce lipid fusion around the particles. Centrifugal sedimentation and electron microscopy are carried out to determine the optimum coating conditions and layer thickness. We then explore the impact of surfactant SLBs on the cytotoxic potential and interactions towards a malignant epithelial cell line. All in vitro assays indicate that SLBs mitigate the particle toxicity and internalization rates. In the cytoplasm, the particle localization is also strongly coating dependent. It is concluded that SLBs profoundly affect cellular interactions and functions in vitro and could represent an alternative strategy for particle coating. The current data also shed some light on the potential mechanisms pertaining to the particle or pathogen transport through the air-blood barrier.

Perturbation of the pulmonary surfactant monolayer by single-walled carbon nanotubes: a molecular dynamics study

Single-walled carbon nanotubes (SWCNTs) are at present synthesized on a large scale with a variety of applications. The increasing likelihood of exposure to SWCNTs, however, puts human health at a high risk. As the front line of the innate host defense system, the pulmonary surfactant monolayer (PSM) at the air-water interface of the lungs interacts with the inhaled SWCNTs, which in turn inevitably perturb the ultrastructure of the PSM and affect its biophysical functions. Here, using molecular dynamics simulations, we demonstrate how the diameter and length of SWCNTs critically regulate their interactions with the PSM. Compared to their diameters, the inhalation toxicity of SWCNTs was found to be largely affected by their lengths. Short SWCNTs with lengths comparable to the monolayer thickness are found to vertically insert into the PSM with no indication of translocation, possibly leading to accumulation of SWCNTs in the PSM with prolonged retention and increased inflammation potentials. The perturbation also comes from the forming water pores across the PSM. Longer SWCNTs are found to horizontally insert into the PSM during inspiration, and they can be wrapped by the PSM during deep expiration via a tube diameter-dependent self-rotation. The potential toxicity of longer SWCNTs comes from severe lipid depletion and the PSM-rigidifying effect. Our findings could help reveal the inhalation toxicity of SWCNTs, and pave the way for the safe use of SWCNTs as vehicles for pulmonary drug delivery.

Molecular Understanding of the Penetration of Functionalized Gold Nanoparticles into Asymmetric Membranes

In this work, the interactions between surface-functionalized gold nanoparticles (AuNPs) and asymmetric membranes and the associated cytotoxicity were explored by coarse-grained molecular dynamics simulations. Simulation results show that the surface chemistry of AuNPs and the asymmetry of lipid membranes play significant roles. AuNPs with different signs of charges spontaneously adhere to the membrane surface or penetrate the membrane core. Also, the asymmetric distribution of charged lipids in membranes can facilitate the penetration of cationic AuNPs. Increasing the surface charge density (SCD) of AuNPs can not only improve the penetration efficiency but also lead to more disruption of the membrane structure. Moreover, the flip-flop of charged lipids in the inner leaflet can be observed during the translocation of AuNPs with a high SCD. The breakdown of membrane asymmetry may hinder the cellular internalization of AuNPs in a direct penetration mechanism. More importantly, we demonstrate that the hydrophobic contact between protruding solvent-exposed lipid tails and the hydrophobic moieties of ligands can mediate the insertion of AuNPs with a low SCD into cell membranes, which will exhibit less cytotoxicity in most in vivo applications. This may open a new exciting avenue to developing nanocarriers with a higher translocation efficiency and a lower toxicity simultaneously for biomedical applications.

Transport of nanoparticles across pulmonary surfactant monolayer: a molecular dynamics study

Three types of nanoparticles, including hydrophobic nanoparticles, hydrophilic nanoparticles, and hydrophilic nanoparticles coated with lipids, were found by our molecular dynamics simulations to be transported across the pulmonary surfactant monolayer, but via different pathways, which affect their subsequent interactions with target cell membranes.

Surface patterning of single-walled carbon nanotubes enhances their perturbation on a pulmonary surfactant monolayer: frustrated translocation and bilayer vesiculation

In contrast to SWCNTs with unique surface properties, the surface patterning of SWCNTs is found to enhance their perturbation on the pulmonary surfactantsviafrustrated translocation and destructive bilayer vesiculation.

Prednisolone adsorption on lung surfactant models: insights on the formation of nanoaggregates, monolayer collapse and prednisolone spreading

The adsorption of prednisolone on a lung surfactant model was successfully performed using coarse grained molecular dynamics.

Modeling the Hydrophobicity of Nanoparticles and Their Interaction with Lipids and Proteins

We present a method of modeling nanoparticle (NP) hydrophobicity using coarse-grained molecular dynamics (CG MD) simulations, and apply this to the interaction of lipids with nanoparticles. To model at a coarse-grained level the wettability or hydrophobicity of a given material, we choose the MARTINI coarse-grained force field, and determine through simulation the contact angles of MARTINI water droplets residing on flat regular surfaces composed of various MARTINI bead types (C1, C2, etc.). Each surface is composed of a single bead type in each of three crystallographic symmetries (FCC, BCC, and HCP). While this method lumps together several atoms (for example, one cerium and two oxygens of CeO₂) into a single CG bead, we can still capture the overall hydrophobicity of the actual material by choosing the MARTINI bead type that gives the best fit of the contact angle to that of the actual material, as determined by either experimental or all-atom simulations. For different MARTINI bead types, the macroscopic contact angle is obtained by extrapolating the microscopic contact angles of droplets of eight different sizes (containing N_w = 3224-22978 water molecules) to infinite droplet size. For each droplet, the contact angle was computed from a best fit of a circular curve to the droplet interface extrapolated to the first layer of the surface. We then examine how small nanoparticles of differing wettability interact with MARTINI dipalmitoylphosphotidylcholine (DPPC) lipids and SP-C peptides (a component of lung surfactant). The DPPC shows a transition from tails coating the nanoparticle to a hemimicelle coating the water-wet NP, as the contact angle of a water droplet on the surface is lowered below ∼60°. The results are relevant to developing a taxonomy describing the potential nanotoxicity of nanoparticle interactions with components in the lung.

Domain Stability in Biomimetic Membranes Driven by Lipid Polyunsaturation

Biological membranes contain a broad variety of lipid species whose individual physicochemical properties and collective interactions ultimately determine membrane organization. A key aspect of the organization of cellular membranes is their lateral subdivision into domains of distinct structure and composition. The most widely studied membrane domains are lipid rafts, which are the biological manifestations of liquid-ordered phases that form in sterol-containing membranes. Detailed studies of biomimetic membrane mixtures have yielded wide-ranging insights into the physical principles behind lipid rafts; however, these simplified models do not fully capture the diversity and complexity of the mammalian lipidome, most notably in their exclusion of polyunsaturated lipids. Here, we assess the role of lipid acyl chain unsaturation as a driving force for phase separation using coarse-grained molecular dynamics (CGMD) simulations validated by model membrane experiments. The clear trends in our observations and good qualitative agreements between simulations and experiments support the conclusions that highly unsaturated lipids promote liquid-liquid domain stability by enhancing the differences in cholesterol content and lipid chain order between the coexisting domains. These observations reveal the important role of noncanonical biological lipids in the physical properties of membranes, showing that lipid polyunsaturation is a driving force for liquid-liquid phase separation.

Nanoparticles in the lung and their protein corona: the few proteins that count

The formation of protein coronae on nanoparticles (NPs) has been investigated almost exclusively in serum, despite the prevailing route of exposure being inhalation of airborne particles. In addition, an increasing number of nanomedicines, that exploit the airways as the site of delivery, are undergoing medical trials. An understanding of the effects of NPs on the airways is therefore required. To further this field, we have described the corona formed on polystyrene (PS) particles with different surface modifications and on titanium dioxide particles when incubated in human bronchoalveolar lavage fluid (BALF) from patients with pulmonary alveolar proteinosis (PAP). We show, using high-resolution quantitative mass spectrometry (MS(E)), that a large number of proteins bind with low copy numbers but that a few "core" proteins bind to all particles tested with high fidelity, averaging the surface properties of the different particles independent of the surface properties of the specific particle. The averaging effect at the particle surface means that differing cellular effects may not be due to the protein corona but due to the surface properties of the nanoparticle once inside the cell. Finally, the adherence of surfactant associated proteins (SP-A, B and D) suggests that there may be interactions with lipids and pulmonary surfactant (PSf), which could have potential in vivo health effects for people with chronic airway diseases such as asthma and chronic obstructive pulmonary disease (COPD), or those who have increased susceptibility toward other respiratory diseases.

Computer simulations of lung surfactant

Lung surfactant lines the gas-exchange interface in the lungs and reduces the surface tension, which is necessary for breathing. Lung surfactant consists mainly of lipids with a small amount of proteins and forms a monolayer at the air-water interface connected to bilayer reservoirs. Lung surfactant function involves transfer of material between the monolayer and bilayers during the breathing cycle. Lipids and proteins are organized laterally in the monolayer; selected species are possibly preferentially transferred to bilayers. The complex 3D structure of lung surfactant and the exact roles of lipid organization and proteins remain important goals for research. We review recent simulation studies on the properties of lipid monolayers, monolayers with phase coexistence, monolayer-bilayer transformations, lipid-protein interactions, and effects of nanoparticles on lung surfactant. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Lipid extraction mediates aggregation of carbon nanospheres in pulmonary surfactant monolayers

Our MD simulations demonstrate that the aggregation of carbon nanospheres in PSM is in fact size-dependent and mediated by lipid extractions.

Interaction between charged nanoparticles and vesicles: coarse-grained molecular dynamics simulations

Interactions between charged nanoparticles and curved zwitterionic lipid vesicles with different surface charge densities (ρ): insertion, repulsion, adsorption, and penetration.

Effects of graphene oxide nanosheets on the ultrastructure and biophysical properties of the pulmonary surfactant film

Graphene oxide (GO) is the most common derivative of graphene and has been used in a large range of biomedical applications. Despite considerable progress in understanding its cytotoxicity, its potential inhalation toxicity is still largely unknown. As the pulmonary surfactant (PS) film is the first line of host defense, interaction with the PS film determines the fate of the inhaled nanomaterials and their potential toxicity. Using a coarse-grained molecular dynamics model, we reported, for the first time, a novel mechanism of toxicity caused by the inhaled GO nanosheets. Upon deposition, the GO nanosheets induce pores in the PS film and thus have adverse effects on the ultrastructure and biophysical properties of the PS film. Notably, the pores induced by GO nanosheets result in increasing the compressibility of the PS film, which is an important indication of surfactant inhibition. In vitro experiments have also been conducted to study the interactions between GO and animal-derived natural PS films, qualitatively confirming the simulation results.

Barrier or carrier? Pulmonary surfactant and drug delivery

To consider the lung as a target for drug delivery and to optimise strategies directed at the pulmonary route, it is essential to consider the role of pulmonary surfactant, a thin lipid-protein film lining the respiratory surface of mammalian lungs. Membrane-based surfactant multilayers are essential for reducing the surface tension at the respiratory air-liquid interface to minimise the work of breathing. Different components of surfactant are also responsible for facilitating the removal of potentially pathological entities such as microorganisms, allergens or environmental pollutants and particles. Upon inhalation, drugs or nanoparticles first contact the surfactant layer, and these interactions critically affect their lifetime and fate in the airways. This review summarises the current knowledge on the possible role and effects of the pulmonary surfactant system in drug delivery strategies. It also summarises the evidence that suggests that pulmonary surfactant is far from being an insuperable barrier and could be used as an efficient shuttle for delivering hydrophobic and hydrophilic compounds deep into the lung and the organism.

Molecular modeling of interaction between lipid monolayer and graphene nanosheets: implications for pulmonary nanotoxicity and pulmonary drug delivery

Understanding how nanoparticles interact with the pulmonary surfactant monolayer (PSM) is of great importance for safe applications in biomedicine and for evaluation of both health and environment impacts.

Active targeted drug delivery for microbes using nano-carriers

Although vaccines and antibiotics could kill or inhibit microbes, many infectious diseases remain difficult to treat because of acquired resistance and adverse side effects. Nano-carriers-based technology has made significant progress for a long time and is introducing a new paradigm in drug delivery. However, it still has some challenges like lack of specificity toward targeting the infectious site. Nanocarriers utilized targeting ligands on their surface called 'active target' provide the promising way to solve the problems like accelerating drug delivery to infectious areas and preventing toxicity or side-effects. In this mini review, we demonstrate the recent studies using the active targeted strategy to kill or inhibit microbes. The four common nano-carriers (e.g. liposomes, nanoparticles, dendrimers and carbon nanotubes) delivering encapsulated drugs are introduced.

Lipid monolayer disruption caused by aggregated carbon nanoparticles

Carbon nanoparticles (CNP) have significant impact on the Pulmonary Surfactant (PS), the first biological barrier in the respiratory system.

Molecular dynamics of dibenz[a,h]anthracene and its metabolite interacting with lung surfactant phospholipid bilayers

Dibenz[a,h]anthracene and its metabolite may form aggregates, which have implications in the clearance process of the lung surfactant phospholipid bilayers.

Shape affects the interactions of nanoparticles with pulmonary surfactant

The interactions with the pulmonary surfactant, the initial biological barrier of respiratory pathway, determine the potential therapeutic applications and toxicological effects of inhaled nanoparticles (NPs). Although much attention has been paid to optimize the physicochemical properties of NPs for improved delivery and targeting, shape effects of the inhaled NPs on their interactions with the pulmonary surfactant are still far from clear. Here, we studied the shape effects of NPs on their penetration abilities and structural disruptions to the dipalmitoyl-phosphatidylcholine (DPPC) monolayer (being model pulmonary surfactant film) using coarse-grained molecular dynamics simulations. It is found that during the inspiration process (i.e., surfactant film expansion), shape effects are negligible. However, during the expiration process (i.e., surfactant film compression), NPs of different shapes show various penetration abilities and degrees of structural disruptions to the DPPC monolayer. We found that rod-like NPs showed the highest degree of penetration and the smallest side-effects to the DPPC monolayer. Our results may provide a useful insight into the design of NPs for respiratory therapeutics.

Biophysical inhibition of pulmonary surfactant function by polymeric nanoparticles: role of surfactant protein B and C

The current study investigated the mechanisms involved in the process of biophysical inhibition of pulmonary surfactant by polymeric nanoparticles (NP). The minimal surface tension of diverse synthetic surfactants was monitored in the presence of bare and surface-decorated (i.e. poloxamer 407) sub-100 nm poly(lactide) NP. Moreover, the influence of NP on surfactant composition (i.e. surfactant protein (SP) content) was studied. Dose-elevations of SP advanced the biophysical activity of the tested surfactant preparation. Surfactant-associated protein C supplemented phospholipid mixtures (PLM-C) were shown to be more susceptible to biophysical inactivation by bare NP than phospholipid mixture supplemented with surfactant protein B (PLM-B) and PLM-B/C. Surfactant function was hindered owing to a drastic depletion of the SP content upon contact with bare NP. By contrast, surface-modified NP were capable of circumventing unwanted surfactant inhibition. Surfactant constitution influences the extent of biophysical inhibition by polymeric NP. Steric shielding of the NP surface minimizes unwanted NP-surfactant interactions, which represents an option for the development of surfactant-compatible nanomedicines.