Interactions of serum with lung surfactant extract in the bronchiolar and alveolar airway models

Pulmonary surfactant: a unique biomaterial with life-saving therapeutic applications

Pulmonary surfactant is a complex lipoprotein mixture secreted into the alveolar lumen by type 2 pneumocytes, which is composed by tens of different lipids (approximately 90% of its entire mass) and surfactant proteins (approximately 10% of the mass). It is crucially involved in maintaining lung homeostasis by reducing the values of alveolar liquid surface tension close to zero at end-expiration, thereby avoiding the alveolar collapse, and assembling a chemical and physical barrier against inhaled pathogens. A deficient amount of surfactant or its functional inactivation is directly linked to a wide range of lung pathologies, including the neonatal respiratory distress syndrome. This paper reviews the main biophysical concepts of surfactant activity and its inactivation mechanisms, and describes the past, present and future roles of surfactant replacement therapy, focusing on the exogenous surfactant preparations marketed worldwide and new formulations under development. The closing section describes the pulmonary surfactant in the context of drug delivery. Thanks to its peculiar composition, biocompatibility, and alveolar spreading capability, the surfactant may work not only as a shuttle to the branched anatomy of the lung for other drugs but also as a modulator for their release, opening to innovative therapeutic avenues for the treatment of several respiratory diseases.

Lipopolymers and lipids from lung surfactants in association with N-acetyl-l-cysteine: Characterization and cytotoxicity

In the present work, we obtained polymeric diacetylene liposomes that can associate N-Acetyl-l-Cysteine (NAC), a broad spectrum mucolytic. The reason for studying these formulations is that they could be applied in the future as NAC delivery systems, with a possible dose reduction but maintaining its effect. Liposomes used herein are obtained by a photopolymerization reaction, thus gaining stability and rigidity. Lipids belonging to lung surfactant were added in different ratios to the formulations in order to maximize its possible interaction with the lung tissue. Because of lipopolymer stability, the oral or nasal route could be appropriated. This formulation could efficiently transport NAC to exert its mucolytic activity and help in diseases such as cystic fibrosis, which has abnormal mucus production. Also, this type of treatment could be useful in other types of diseases, interacting with the mucus layer and making the lung tissue more permeable to other therapies. Formulations so obtained presented high levels of polymerization. Also, they present small hollow fibers structures with a high number of polymeric units. These types of arrangements could present advantages in the field of drug delivery, giving the possibility of a controlled release. Lipopolymers with lipids from lung surfactant associated with NAC are promising complexes in order to treat not only respiratory illnesses. The stability of the formulation would allow its inoculation through other routes such as the oral one, helping the reposition of NAC as an antioxidant drug. Finally, these formulations are non-toxic and easy to produce.

Mitigation of Hydrochloric Acid (HCl)-Induced Lung Injury in Mice by Aerosol Therapy of Surface-Active Nanovesicles Containing Antioxidant and Anti-inflammatory Drugs

Acute lung injury leading to alveolar inflammation and surfactant dysfunction remains a medical challenge. Surface-active lipid nanovesicles of 200-250 nm size with antioxidant D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) and anti-inflammatory drug dexamethasone disodium phosphate (DXP) dual combination (Dual-NV) were developed for delivery as aerosols by nebulization in acid lung injury models. Drug deposition studies showed Dual-NV deposited ∼2.5 times more DXP compared to equivalent DXP solution. Nanovesicles are actively internalized by A549 cells through ATP- and clathrin-dependent pathways. The nanovesicles could be phagocytosed by RAW 264.7 macrophages and were nonimmunogenic and did not elicit overproduction of TNF-α, IL-1β, and IL-6. Dual-NV aerosol therapy at 200 mg/kg body weight, in HCl acid-induced lung injury in mice, markedly reduced pulmonary hemorrhage and protein leakage and improved capillary (airway) patency to ∼96%. Dual-NV aerosol therapy also significantly lowered production of inflammatory cytokine IL-1β, IL-6, and TNF-α and reduced oxidative stress by ∼95% in the injured group. Surface-active Dual-NV aerosol therapy is promising for replenishing the dysfunctional surfactant pool and mitigating inflammation and oxidative stress in lung injuries.

Insights on animal models to investigate inhalation therapy: Relevance for biotherapeutics

Acute and chronic respiratory diseases account for major causes of illness and deaths worldwide. Recent developments of biotherapeutics opened a new era in the treatment and management of patients with respiratory diseases. When considering the delivery of therapeutics, the inhaled route offers great promises with a direct, non-invasive access to the diseased organ and has already proven efficient for several molecules. To assist in the future development of inhaled biotherapeutics, experimental models are crucial to assess lung deposition, pharmacokinetics, pharmacodynamics and safety. This review describes the animal models used in pulmonary research for aerosol drug delivery, highlighting their advantages and limitations for inhaled biologics. Overall, non-clinical species must be selected with relevant scientific arguments while taking into account their complexities and interspecies differences, to help in the development of inhaled medicines and ensure their successful transposition in the clinics.

Overcoming inactivation of the lung surfactant by serum proteins: a potential role for fluorocarbons?

In many pulmonary conditions serum proteins interfere with the normal adsorption of components of the lung surfactant to the surface of the alveoli, resulting in lung surfactant inactivation, with potentially serious untoward consequences. Here, we review the strategies that have recently been designed in order to counteract the biophysical mechanisms of inactivation of the surfactant. One approach includes protein analogues or peptides that mimic the native proteins responsible for innate resistance to inactivation. Another perspective uses water-soluble additives, such as electrolytes and hydrophilic polymers that are prone to enhance adsorption of phospholipids. An alternative, more recent approach consists of using fluorocarbons, that is, highly hydrophobic inert compounds that were investigated for partial liquid ventilation, that modify interfacial properties and can act as carriers of exogenous lung surfactant. The latter approach that allows fluidisation of phospholipid monolayers while maintaining capacity to reach near-zero surface tension definitely warrants further investigation.

Effect of serum, cholesterol and low density lipoprotein on the functionality and structure of lung surfactant films

Lung surfactant is a complex mixture of lipid and protein, responsible for alveolar stability, becomes dysfunctional due to alteration of its structure and function by leaked serum materials in disease. Serum proteins, cholesterol and low density lipoprotein (LDL) were studied with bovine lipid extract surfactant (BLES) using Langmuir films, and bilayer dispersions using Raman spectroscopy. While small amount of cholesterol (10 wt%) and LDL did not significantly affect the adsorption and surface tension lowering properties of BLES. However serum lipids, whole serum as well as higher amounts of cholesterol, and LDL dramatically altered the surface properties of BLES films, as well as gel-fluid structures formed in such films observed using atomic force microscopy (AFM). Raman-spectroscopic studies revealed that serum proteins, LDL and excess cholesterol had fluidizing effects on BLES bilayers dispersion, monitored from the changes in hydrocarbon vibrational modes during gel-fluid thermal phase transitions. This study clearly suggests that patho-physiological amounts of serum lipids (and not proteins) significantly alter the molecular arrangement of surfactant in films and bilayers, and can be used to model lung disease.

Effect of serum lipoproteins and cholesterol on an exogenous pulmonary surfactant. ESR analysis of structural changes and their relation with surfactant activity

The study of structural changes in the surfactant may help to understand the mechanisms by which the surfactant is inactivated by serum. Here, we compared the in vitro effects of serum, albumin, lipoproteins (VLDL, LDL, HDL) and cholesterol on the dynamic and structural properties of surfactant suspensions by electronic spin resonance and surface tension measurements. Our results showed that albumin seems to be responsible for macrostructure disaggregation and increased rigidity in the hydrophobic region, but it did not affect surfactant activity. Fluidity in the polar area seems to be critical for proper physiological activity, and the changes induced by serum observed in this area would be generated by HDL or cholesterol, but the amount of cholesterol transferred by serum is not significant. Statistical analysis showed that surfactant activity correlated with the fluidity in the polar area but not with that in the hydrophobic region. We obtained strong evidence that among all the serum components tested, HDL is the one that causes the structural changes that compromise surfactant performance.

Nanovesicle aerosols as surfactant therapy in lung injury

Acute lung injury causes inactivation of pulmonary surfactant due to leakage of albumin and other markers. Current surfactants are ineffective in this condition and are instilled intratracheally. Nanovesicles of 300 ± 50 nm composed of nonlamellar phospholipids were developed as pulmonary surfactant aerosols for therapy in acid-induced lung injury. A combination of dipalmitoyl phosphatidylcholine and dioleoyl phosphatidylethanolamine was used. The size and composition of the nanovesicles were optimized for an improved airway patency in the presence of albumin and serum. In an acid-induced lung injury model in mice, on treatment with nanovesicle aerosols at a dose of 200 mg/kg, the alveolar protein leakage decreased from 8.62 ± 0.97 μg/mL to 1.94 ± 0.74 μg/mL, whereas the airway patency of the bronchoalveolar lavage fluid increased from 0.6 ± 0.0% to 91.7 ± 1.05%. Nanovesicle aerosols of nonlamellar lipids improved the resistance of pulmonary surfactants to inhibition and were promising as a noninvasive aerosol therapy in acute lung injury.

FROM THE CLINICAL EDITOR

In acute lung injury, intrinsic surfactants are inactivated via albumin leakage and other mechanisms. Currently existing intratracheal surfactants are ineffective in this condition. The authors demonstrate that novel nanovesicle aerosols of nonlamellar lipids improved the resistance of pulmonary surfactants to inhibition and are promising as a noninvasive aerosol therapy in acute lung injury.

Effect of D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) on surfactant monolayers

In the present study, the effects of an amphiphilic polymer, d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) on model surfactant monolayers dipalmitoylphosphatidylcholine (DPPC), a binary mixture of DPPC with palmitoyloleoyl phosphatidylglycerol (DPPC-POPG) 9:1 (w/w) and binary mixture of DPPC and oleic acid (DPPC-OA) were evaluated. The ability of TPGS to act as an antioxidant adjuvant for pulmonary surfactants was also evaluated. Compression isotherms of surfactant monolayers at 37 °C in a Langmuir-Blodgett trough showed that DPPC and DPPC:TPGS mixed monolayers (1:0.25-1:1, w/w) exhibited low minimum surface tensions (MST) of 1-2 mN/m. Similarly [DPPC:POPG (9:1, w/w)]:TPGS mixed films of 1:0.25-1:1 weight ratios reached 1-2 mN/m MST. DPPC:POPG:TPGS liposomes adsorbed to surface tensions of 29-31 mN/m within 1s. While monolayers of DPPC:OA (1:1, w/w) reached high MST of ∼11 mN/m, DPPC:OA:TPGS (1:1:0.25, w/w) film reached near zero MST suggesting that low concentrations of TPGS reverses the effect of OA on DPPC monolayer. Capillary surfactometer studies showed DPPC:TPGS and [DPPC:POPG (9:1, w/w)]:TPGS liposomes maintained 84-95% airway patency. Fluorescence spectroscopy of Laurdan loaded DPPC:TPGS and DPPC:POPG:TPGS liposomes revealed no segregation of lipid domains in the lipid bilayer. Addition of TPGS to soybean liposome significantly reduced thiobarbituric acid reactive substance (TBARS) by 29-39% confirming its antioxidant nature. The results suggest a potential use of TPGS as an adjuvant to improve the surfactant activity as well as act as an antioxidant by scavenging free radicals.

Influence of serum protein and albumin addition on the structure and activity of an exogenous pulmonary surfactant

The comparative analysis of the deleterious action of albumin and total serum proteins (SP) might help to understand the nature of the interaction surfactant--SP. This study evaluated the effects of serum proteins and albumin on bulk shear viscosity, surface tension, surface area reduction, and the ratio between the light and heavy subtypes of surfactant suspensions. Our results showed a correlation between the bulk viscosity and aggregation degree of surfactant suspensions. The addition of albumin or SP induced the transformation from the heavy to the light subtype, reducing the viscosity. SP caused disaggregation and inactivation, whereas albumin caused only disaggregation without loss of surface activity. When SP were removed, the heavy fraction obtained recovered its surface activity. We conclude that the disaggregation may not be the primary cause for the loss of surface activity. Surfactant inactivation by a serum component, different from albumin, would be probably due to a physical interaction, a phenomenon that is reversed when SP are removed.

Physicochemical studies on the interaction of serum albumin with pulmonary surfactant extract in films and bulk bilayer phase

Functionality, structure and composition of the adsorbed films of bovine lipid extract surfactant (BLES), in the absence and presence of bovine serum albumin (BSA), at the air-buffer interface was characterized through surface tension, atomic force microscopy and time of flight secondary ion mass spectrometric methods. Gel and fluid domains of BLES films were found to be altered significantly in the presence of BSA. Differential scanning calorimetric studies on BLES dispersions in presence of BSA revealed that the perturbations of the lipid bilayer structures were significant only at higher amount of BSA. FTIR studies on the BLES dispersions in buffer solution revealed that BSA could affect the lipid head-group hydrations in bilayer as well as the methylene and methyl vibration modes of fatty acyl chains of the phospholipids present in BLES. Serum albumin could perturb the film structure at pathophysiological concentration while higher amount of BSA was required in perturbing the bilayer structures. The studies suggest a connected perturbed bilayer to monolayer transition model for surfactant inactivation at the alveolar-air interface in dysfunctional surfactants.

Restoring the charge and surface activity of bovine lung extract surfactants with cationic and anionic polysaccharides

Chitosan, a cationic polysaccharide, has been found to improve the surface activity of lung surfactant extracts in the presence of various inhibitors. It has been proposed that chitosan binds to anionic lipids (e.g. phosphatidyl glycerols) in lung surfactants, producing stable lipid films at the air-water interface. This binding also reverses the net charge of the surfactant aggregates, from negative to positive. Unfortunately, positively charged aggregates may adsorb or interact with the negatively charged epithelial tissue, leading to poor surfactant performance. To address this issue an anionic polysaccharide, dextran sulfate (dexS), was used as a secondary coating to reverse the charge of chitosan-lung surfactant extracts without affecting the surface activity of the preparation. The dynamic surface tension and zeta potential of bovine lipid extract surfactant (BLES) containing chitosan chloride (chiCl) and dexS were evaluated as a function of dexS concentration. These studies were conducted in the absence and presence of sodium bicarbonate buffer, and in the absence and presence of bovine serum used as model inhibitor. It was determined that using an appropriate concentration of dexS, especially at physiological pH, it is possible to restore the negative charge of the surfactant aggregates, and retain their surface activity, even in the presence of bovine serum. High concentrations of dexS affect the binding of chiCl to BLES, and the surface activity of the preparation.

Atomic force microscopy studies of functional and dysfunctional pulmonary surfactant films, II: albumin-inhibited pulmonary surfactant films and the effect of SP-A

Pulmonary surfactant (PS) dysfunction because of the leakage of serum proteins into the alveolar space could be an operative pathogenesis in acute respiratory distress syndrome. Albumin-inhibited PS is a commonly used in vitro model for studying surfactant abnormality in acute respiratory distress syndrome. However, the mechanism by which PS is inhibited by albumin remains controversial. This study investigated the film organization of albumin-inhibited bovine lipid extract surfactant (BLES) with and without surfactant protein A (SP-A), using atomic force microscopy. The BLES and albumin (1:4 w/w) were cospread at an air-water interface from aqueous media. Cospreading minimized the adsorption barrier for phospholipid vesicles imposed by preadsorbed albumin molecules, i.e., inhibition because of competitive adsorption. Atomic force microscopy revealed distinct variations in film organization, persisting up to 40 mN/m, compared with pure BLES monolayers. Fluorescence confocal microscopy confirmed that albumin remained within the liquid-expanded phase of the monolayer at surface pressures higher than the equilibrium surface pressure of albumin. The remaining albumin mixed with the BLES monolayer so as to increase film compressibility. Such an inhibitory effect could not be relieved by repeated compression-expansion cycles or by adding surfactant protein A. These experimental data indicate a new mechanism of surfactant inhibition by serum proteins, complementing the traditional competitive adsorption mechanism.

Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation

Pulmonary surfactant (PS) is a complicated mixture of approximately 90% lipids and 10% proteins. It plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension to near-zero values. Supplementing exogenous surfactant to newborns suffering from respiratory distress syndrome (RDS), a leading cause of perinatal mortality, has completely altered neonatal care in industrialized countries. Surfactant therapy has also been applied to the acute respiratory distress syndrome (ARDS) but with only limited success. Biophysical studies suggest that surfactant inhibition is partially responsible for this unsatisfactory performance. This paper reviews the biophysical properties of functional and dysfunctional PS. The biophysical properties of PS are further limited to surface activity, i.e., properties related to highly dynamic and very low surface tensions. Three main perspectives are reviewed. (1) How does PS permit both rapid adsorption and the ability to reach very low surface tensions? (2) How is PS inactivated by different inhibitory substances and how can this inhibition be counteracted? A recent research focus of using water-soluble polymers as additives to enhance the surface activity of clinical PS and to overcome inhibition is extensively discussed. (3) Which in vivo, in situ, and in vitro methods are available for evaluating the surface activity of PS and what are their relative merits? A better understanding of the biophysical properties of functional and dysfunctional PS is important for the further development of surfactant therapy, especially for its potential application in ARDS.