Biophysical and Physiological Properties of Porcine Surfactant Enriched with Polymyxin B

Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure

The use of an exogenous pulmonary surfactant (EPS) to deliver other relevant drugs to the lungs is a promising strategy for combined therapy. We evaluated the interaction of polymyxin B (PxB) with a clinically used EPS, the poractant alfa Curosurf (PSUR). The effect of PxB on the protein-free model system (MS) composed of four phospholipids (diC16:0PC/16:0-18:1PC/16:0-18:2PC/16:0-18:1PG) was examined in parallel to distinguish the specificity of the composition of PSUR. We used several experimental techniques (differential scanning calorimetry, small- and wide-angle X-ray scattering, small-angle neutron scattering, fluorescence spectroscopy, and electrophoretic light scattering) to characterize the binding of PxB to both EPS. Electrostatic interactions PxB-EPS are dominant. The results obtained support the concept of cationic PxB molecules lying on the surface of the PSUR bilayer, strengthening the multilamellar structure of PSUR as derived from SAXS and SANS. A protein-free MS mimics a natural EPS well but was found to be less resistant to penetration of PxB into the lipid bilayer. PxB does not affect the gel-to-fluid phase transition temperature, Tm, of PSUR, while Tm increased by ∼+ 2 °C in MS. The decrease of the thickness of the lipid bilayer (dL) of PSUR upon PxB binding is negligible. The hydrophobic tail of the PxB molecule does not penetrate the bilayer as derived from SANS data analysis and changes in lateral pressure monitored by excimer fluorescence at two depths of the hydrophobic region of the bilayer. Changes in dL of protein-free MS show a biphasic dependence on the adsorbed amount of PxB with a minimum close to the point of electroneutrality of the mixture. Our results do not discourage the concept of a combined treatment with PxB-enriched Curosurf. However, the amount of PxB must be carefully assessed (less than 5 wt % relative to the mass of the surfactant) to avoid inversion of the surface charge of the membrane.

Structural hallmarks of lung surfactant: Lipid-protein interactions, membrane structure and future challenges

Lung surfactant (LS) is an outstanding example of how a highly regulated and dynamic membrane-based system has evolved to sustain a wealth of structural reorganizations in order to accomplish its biophysical function, as it coats and stabilizes the respiratory air-liquid interface in the mammalian lung. The present review dissects the complexity of the structure-function relationships in LS through an updated description of the lipid-protein interactions and the membrane structures that sustain its synthesis, secretion, interfacial performance and recycling. We also revise the current models and the biophysical techniques employed to study the membranous architecture of LS. It is important to consider that the structure and functional properties of LS are often studied in bulk or under static conditions, in spite that surfactant function is strongly connected with a highly dynamic behaviour, sustained by very polymorphic structures and lipid-lipid, lipid-protein and protein-protein interactions that reorganize in precise spatio-temporal coordinates. We have tried to underline the evidences available of the existence of such structural dynamism in LS. A last important aspect is that the synthesis and assembly of LS is a strongly regulated intracellular process to ensure the establishment of the proper interactions driving LS surface activity, while protecting the integrity of other cell membranes. The use of simplified lipid models or partial natural materials purified from animal tissues could be too simplistic to understand the true molecular mechanisms defining surfactant function in vivo. In this line, we will bring into the attention of the reader the methodological challenges and the questions still open to understand the structure-function relationships of LS at its full biological relevance.

Restoration of surfactant activity by polymyxin B in lipopolysaccharide-potentiated injury of immature rabbit lungs

During postnatal adaptation pulmonary surfactant may be inactivated by lipopolysaccharide (LPS). We evaluated the effect of surfactant therapy in combination with antibiotic polymyxin B (PxB) in double-hit model of neonatal lung injury. Surfactant (poractant alfa, Curosurf) was exposed to smooth (S) LPS without/with PxB and tested in captive bubble surfactometer. Preterm rabbits received intratracheally saline (control) or S-LPS and were ventilated with 100% oxygen. After 30 min, LPS-treated animals received no treatment, or surfactant (200 mg/kg) without/with 3% PxB; controls received the same dose of surfactant. Animals were ventilated for further 2 h. In vitro, addition of 5% S-LPS to surfactant increased minimum surface tension (γmin) and addition of 1-3% PxB to surfactant/S-LPS mixture restored γmin to low values. Animals only given S-LPS had lower lung compliance and lung gas volume (LGV) compared to surfactant groups. Treatment with surfactant/PxB, but not with surfactant only, restored LGV. Addition of PxB to the surfactant increased the alveolar expansion. S-LPS interferes with surface activity of the pulmonary surfactant and PxB improves the resistance of surfactant to LPS-induced inactivation. In our neonatal model of respiratory distress syndrome surfactant gives positive response even in simultaneous exposure to S-LPS, when enriched with PxB.

Non-Reflex Defense Mechanisms of Upper Airway Mucosa: Possible Clinical Application

The sinonasal mucosa has an essential role in defense mechanisms of the upper respiratory tract. The innate immune system presents the primary defense against noxious microorganisms followed by induction of the adaptive immune mechanisms as a consequence of the presence of pathogens. This well-known activation of adaptive immune system in response to presence of the antigen on mucosal surfaces is now broadly applicated in vaccinology research. Prevention of infectious diseases belongs to substantial challenges in maintaining the population health. Non-invasive, easily applicable mucosal vaccination purposes various research opportunities that could be usable in daily practice. However, the existence of multiple limitations such as rapid clearance of vaccine from nasal mucosa by means of mucociliary transport represents a great challenge in development of safe and efficient vaccines. Here we give an updated view on nasal functions with focus on nasal mucosal immunity and its potential application in vaccination in nearly future.

The Effect of Modified Porcine Surfactant Alone or in Combination with Polymyxin B on Lung Homeostasis in LPS-Challenged and Mechanically Ventilated Adult Rats

The study aimed to prove the hypothesis that exogenous surfactant and an antibiotic polymyxin B (PxB) can more effectively reduce lipopolysaccharide (LPS)-induced acute lung injury (ALI) than surfactant treatment alone, and to evaluate the effect of this treatment on the gene expression of surfactant proteins (SPs). Anesthetized rats were intratracheally instilled with different doses of LPS to induce ALI. Animals with LPS 500 μg/kg have been treated with exogenous surfactant (poractant alfa, Curosurf^®, 50 mg PL/kg b.w.) or surfactant with PxB 1% w.w. (PSUR + PxB) and mechanically ventilated for 5 hrs. LPS at 500 μg/kg increased lung edema, oxidative stress, and the levels of proinflammatory mediators in lung tissue and bronchoalveolar lavage fluid (BALF). PSUR reduced lung edema and oxidative stress in the lungs and IL-6 in BALF. This effect was further potentiated by PxB added to PSUR. Exogenous surfactant enhanced the gene expression of SP-A, SP-B, and SP-C, however, gene expression for all SPs was reduced after treatment with PSUR + PxB. In mechanically ventilated rats with LPS-induced ALI, the positive effect of exogenous surfactant on inflammation and oxidative stress was potentiated with PxB. Due to the tendency for reduced SPs gene expression after surfactant/PxB treatment topical use of PxB should be considered with caution.

Inhibition and counterinhibition of Surfacen, a clinical lung surfactant of natural origin

Inactivation of pulmonary surfactant by different components such as serum, cholesterol or meconium contributes to severe respiratory pathologies through destabilization and collapse of airspaces. Recent studies have analyzed in detail how the interfacial properties of natural surfactant purified from animal lungs are altered as a consequence of its exposure to serum proteins or meconium-mobilized cholesterol. It has been also demonstrated that pre-exposure of surfactant to polymers such as hyaluronic acid provides resistance to inactivation by multiple inhibitory agents. In the current work, we have extended these studies to the analysis of Surfacen, a clinical surfactant currently in use to rescue premature babies suffering or at risk of respiratory distress due to congenital lack of surfactant. This surfactant is also strongly inhibited by both meconium and serum when tested in the captive bubble surfactometer (CBS) under conditions mimicking respiratory dynamics. As it occurs with native surfactant, Surfacen is markedly protected from inhibition by pre-exposure to hyaluronic acid, confirming that clinical surfactants can be improved to treat pathologies associated with strongly deactivating contexts, such as those associated with lung injury and inflammation. Remarkably, we found that, under physiologically-mimicking conditions, a cholesterol-free clinical surfactant such as Surfacen is less susceptible to inhibition by cholesterol-mobilizing environments than cholesterol-containing natural surfactant, as a consequence of a markedly reduced susceptibility to incorporation of exogenous cholesterol.

The Perturbation of Pulmonary Surfactant by Bacterial Lipopolysaccharide and Its Reversal by Polymyxin B: Function and Structure

After inhalation, lipopolysaccharide (LPS) molecules interfere with a pulmonary surfactant, a unique mixture of phospholipids (PLs) and specific proteins that decreases surface tension at the air⁻liquid interphase. We evaluated the behaviour of a clinically used modified porcine pulmonary surfactant (PSUR) in the presence of LPS in a dynamic system mimicking the respiratory cycle. Polymyxin B (PxB), a cyclic amphipathic antibiotic, is able to bind to LPS and to PSUR membranes. We investigated the effect of PxB on the surface properties of the PSUR/LPS system. Particular attention was paid to mechanisms underlying the structural changes in surface-reducing features. The function and structure of the porcine surfactant mixed with LPS and PxB were tested with a pulsating bubble surfactometer, optical microscopy, and small- and wide-angle X-ray scattering (SAXS/WAXS). Only 1% LPS (w/w to surfactant PLs) prevented the PSUR from reaching the necessary low surface tension during area compression. LPS bound to the lipid bilayer of PSUR and disturbed its lamellar structure by swelling. The structural changes were attributed to the surface charge unbalance of the lipid bilayers due to LPS insertion. PxB acts as an inhibitor of structural disarrangement induced by LPS and restores original lamellar packing, as detected by polarised light microscopy and SAXS.

Physiology of nitric oxide in the respiratory system

Nitric oxide (NO) is an important endogenous neurotransmitter and mediator. It participates in regulation of physiological processes in different organ systems including airways. Therefore, it is important to clarify its role in the regulation of both airway and vascular smooth muscle, neurotransmission and neurotoxicity, mucus transport, lung development and in the. surfactant production. The bioactivity of NO is highly variable and depends on many factors: the presence and activity of NO-producing enzymes, activity of competitive enzymes (e.g. arginase), the amount of substrate for the NO production, the presence of reactive oxygen species and others. All of these can change NO primary physiological role into potentially harmful. The borderline between them is very fragile and in many cases not entirely clear. For this reason, the research focuses on a comprehensive understanding of NO synthesis and its metabolic pathways, genetic polymorphisms of NO synthesizing enzymes and related effects. Research is also motivated by frequent use of exhaled NO monitoring in the clinical manifestations of respiratory diseases. The review focuses on the latest knowledge about the production and function of this mediator and understanding the basic physiological processes in the airways.

Pulmonary Surfactant and Bacterial Lipopolysaccharide: The Interaction and its Functional Consequences

The respiratory system is constantly exposed to pathogens which enter the lungs by inhalation or via blood stream. Lipopolysaccharide (LPS), also named endotoxin, can reach the airspaces as the major component of the outer membrane of Gram-negative bacteria, and lead to local inflammation and systemic toxicity. LPS affects alveolar type II (ATII) cells and pulmonary surfactant and although surfactant molecule has the effective protective mechanisms, excessive amount of LPS interacts with surfactant film and leads to its inactivation. From immunological point of view, surfactant specific proteins (SPs) SP-A and SP-D are best characterized, however, there is increasing evidence on the involvement of SP-B and SP-C and certain phospholipids in immune reactions. In animal models, the instillation of LPS to the respiratory system induces acute lung injury (ALI). It is of clinical importance that endotoxin-induced lung injury can be favorably influenced by intratracheal instillation of exogenous surfactant. The beneficial effect of this treatment was confirmed for both natural porcine and synthetic surfactants. It is believed that the surfactant preparations have anti-inflammatory properties through regulating cytokine production by inflammatory cells. The mechanism by which LPS interferes with ATII cells and surfactant layer, and its consequences are discussed below.

Biophysical activity of animal-derived exogenous surfactants mixed with rifampicin

Exogenous pulmonary surfactant is a potential delivery system for topical medications via the conducting airways. Due to the sensitivity to inactivation of surfactant, mutual interaction with the shipped drug should be evaluated. Little is known about the interactions between surfactant and antimicrobial drugs. The aim of the present study was to evaluate whether biophysical properties of animal-derived surfactants are modified by the bactericidal antibiotic rifampicin. An intracellular activity and a broad antimicrobiotic spectrum toward Gram-negative and Gram-positive bacteria make rifampicin an interesting substance against pulmonary infections. Curosurf® (porcine surfactant from minced lungs) and Survanta® (bovine surfactant extract) were diluted to 2.5-5.0 mg/ml of phospholipids in 0.9 % NaCl and rifampicin (RIF) was added at 1, 5, and 10 % (w/w). Minimum (γ(min)) and maximum (γ(max)) surface tension of a cyclically compressed bubble in the mixture was assessed with a pulsating bubble surfactometer. After 5 min, γ(min) of Survanta at a concentration of 3 mg/ml was significantly increased after addition of 5 and 10 % RIF (both p < 0.001). At 1 % RIF, the γ(min) of Survanta was ≈10 mN/m and this value was not significantly different to that of Survanta alone. The γ(min) of Curosurf at 3 mg/ml was increased with 10 % RIF (p < 0.001), but not with 1 and 5 %. At 5 mg/ml Survanta was inhibited by 10 % RIF (p < 0.05), while γ(min) of Curosurf was low (<5 mN/m) in all mixtures. In conclusion, Curosurf and Survanta interfere with RIF in a concentration-dependent manner. At the appropriate phospholipid concentration, especially porcine-derived surfactant is able to retain good surface activity when mixed with antibiotics.

Pulmonary surfactant preserves viability of alveolar type II cells exposed to polymyxin B in vitro

Background

Exogenous surfactant derived from animal lungs is applied for treatment of surfactant deficiency. By means of its rapid spreading properties, it could transport pharmaceutical agents to the terminal air spaces. The antimicrobial peptide Polymyxin B (PxB) is used as a topical antibiotic for inhalation therapy. Whereas it has been shown that PxB mixed with surfactant is not inhibiting surface activity while antimicrobiotic activity is preserved, little is known concerning the effects on synthesis of endogenous surfactant in alveolar type II cells (ATIIC).

Objective

To investigate ATIIC viability and surfactant-exocytosis depending on PxB and/or surfactant exposure.

Methods

ATIIC were isolated from rat lungs as previously described and were cultivated for 48 h. After incubation for a period of 1–5 h with either PxB (0.05 or 0.1 mg/ml), modified porcine surfactant (5 or 10 mg/ml) or mixtures of both, viability and exocytosis (spontanously and after stimulation) were determined by fluorescence staining of intracellular surfactant.

Results

PxB 0.1 mg/ml, but not porcine surfactant or porcine surfactant plus PxB reduces ATIIC-viability. Only PxB alone, but not in combination with porcine surfactant, rapidly reduces fluorescence in ATIIC at maximum within 3 h, indicating stimulation of exocytosis. Subsequent ionomycin-stimulation does not further increase exocytosis of PxB incubated ATIIC. In presence of surfactant, stimulating effects of PxB and ionomycin on exocytosis are reduced.

Conclusion

PxB alone shows negative effects on ATIIC, which are counterbalanced in mixtures with surfactant. So far, our studies found no results discouraging the concept of a combined treatment with PxB and surfactant mixtures.

Fluorocarbon-hybrid pulmonary surfactants for replacement therapy--a Langmuir monolayer study

Effective additives to pulmonary surfactant (PS) preparations for therapy of respiratory distress syndrome (RDS) are being intensively sought. We report here the investigation of the effects of partially fluorinated amphiphiles (PFA) on the surface behavior of a model PS formulation. When small amounts of a partially fluorinated alcohol C(8)F(17)C(m)H(2m)OH (F8HmOH, m = 5 and 11) are added to the PS model preparation (a dipalmitoylphosphatidylcholine (DPPC)/Hel 13-5 peptide mixture) considered here, the effectiveness of the latter in in vitro pulmonary functions is enhanced. The mechanism for the improved efficacy depends on the hydrophobic chain length of the added PFA molecules. The shorter PFA, F8H5OH, when incorporated in the monolayer of the PS model preparation, promotes a disordered liquid-expanded (LE) phase upon lateral compression (fluidization). In contrast, the addition of the longer PFA, F8H11OH, reduces the disordered LE/ordered liquid-condensed (LC) phase transition pressure and promotes the growth of ordered domains (solidification). Furthermore, compression-expansion cycles suggest that F8H5OH, when incorporated in the PS model preparation, undergoes an irreversible elimination into the subphase, whereas F8H11OH enhances the squeeze-out phenomenon of the SP-B mimicking peptide, which is important in pulmonary functions and is related to the formation of a solid-like monolayer at the surface and of a surface reservoir just below the surface. F8H11OH particularly reinforces the effectiveness of DPPC in terms of minimum reachable surface tension, and of preservation of the integrated hysteresis area between compression and expansion isotherms, the two latter parameters being generally accepted indices for assessing PS efficacy. We suggest that PFA amphiphiles may be useful potential additives for synthetic PS preparations destined for treatment of RDS in premature infants and in adults.

Prophylactic intratracheal polymyxin B/surfactant prevents bacterial growth in neonatal Escherichia coli pneumonia of rabbits

In neonatal pneumonia, the surface activity of pulmonary surfactant is impaired and microorganisms may invade by passing the air-liquid interface. Previously, we have shown that addition of the antimicrobial peptide polymyxin B (PxB) to modified porcine surfactant (pSF) improves resistance to surfactant inactivation in vitro while antimicrobial activity of PxB is maintained. In this study, we investigated pSF/PxB in vivo. Neonatal near-term rabbits were treated with intratracheal pSF and/or PxB. Rabbits treated with only saline served as controls. Animals were ventilated with standardized tidal volumes and received ∼10(7) Escherichia coli intratracheally. Plethysmographic pressure-volume curves were recorded every 30 min. After 240 min, animals were killed, the right lung and left kidney were excised, and bacterial growth was determined. The left lung was used for histologic analysis. Intratracheal administration of PxB ± pSF significantly reduced the growth of E. coli compared with control animals or animals receiving only pSF. This was accompanied by reduction of severe inflammatory tissue destruction and significantly reduced bacterial translocation to the left kidney. Animals receiving pSF + PxB had no difference in lung compliance compared with the pSF- or PxB-treated group. Mixtures of PxB and pulmonary surfactant show antimicrobial effects in neonatal rabbits and prevent systemic spreading of E. coli.

Bronchoalveolar lavage with pulmonary surfactant/dextran mixture improves meconium clearance and lung functions in experimental meconium aspiration syndrome

Surfactant lung lavage is a promising approach in the treatment of meconium aspiration syndrome (MAS). We hypothesise that the enrichment of modified natural surfactant with dextran will enhance meconium clearance from the airspaces during lung lavage and improve lung function in experimental MAS. Human meconium (30 mg/ml; 4 ml/kg) was instilled into the tracheal cannula of anaesthetised and paralysed adult rabbits to induce respiratory failure. The animals were then lavaged with saline (Sal), surfactant without (Surf) and with dextran (Surf+dex). Lung lavage (10 ml/kg in three portions) was performed with diluted surfactant (Curosurf, 10 mg/ml, 100 mg/kg) without or with dextran (3 mg/mg of surfactant phospholipids) or saline and the animals were conventionally ventilated with 100% O(2) for an additional hour. Lung functions were measured prior to and after meconium instillation, and 10, 30 and 60 min after lavage. The recovery of meconium in bronchoalveolar lavage (BAL) fluid was quantified. More meconium solids was recovered in the surfactant-lavaged than in the saline-lavaged groups (Surf: 12.4 +/- 3.9% and Surf+dex: 17.5 +/- 3.5% vs. Sal: 4.8 +/- 1.0%; both P < 0.01). Moreover, more meconium solids was obtained by Curosurf/dextran than by Curosurf-only lavage (P < 0.05). In the Surf group, the values for PaO(2)/FiO(2) were significantly higher than in the controls (at 60 min: 24.5 +/- 4.2 kPa vs.9.1 +/- 2.2 kPa, P < 0.01). An additional increase in oxygenation was seen in the Surf+dex group (at 60 min: 34.2 +/- 8.1 kPa, P vs. Surf group <0.01). The lung-thorax compliance was higher in the Surf+dex group in comparison with the Sal and Surf groups (at 60 min: 9.6 +/- 0.9 vs.7.6 +/- 1.2, P < 0.01 and 8.0 +/- 0.7 ml/kPa/kg, P < 0.05). The enrichment of Curosurf with dextran improves meconium clearance and lung functions in surfactant-lavaged rabbits with meconium aspiration.

What's new in surfactant? A clinical view on recent developments in neonatology and paediatrics

Surfactant therapy has significantly changed clinical practice in neonatology over the last 25 years. Recent trials in infants with respiratory distress syndrome (RDS) have not shown superiority of any natural surfactant over another. Advancements in the development of synthetic surfactants are promising, yet to date none has been shown to be superior to natural preparations. Ideally, surfactant would be administered without requiring mechanical ventilation. An increasing number of studies investigate the roles of alternative modes of administration and the use of nasal continuous positive airway pressure to minimise the need for mechanical ventilation. Whether children with other lung diseases benefit from surfactant therapy is less clear. Evidence suggests that infants with meconium aspiration syndrome and children with acute lung injury/acute respiratory distress syndrome may benefit, while no positive effect of surfactant is seen in infants with congenital diaphragmatic hernia. However, more research is needed to establish potential beneficial effects of surfactant administration in children with lung diseases other than RDS. Furthermore, genetic disorders of surfactant metabolism have recently been linked to respiratory diseases of formerly unknown origin. It is important to consider these disorders in the differential diagnosis of unexplained respiratory distress although no established treatment is yet available besides lung transplantation for the most severe cases.

CONCLUSION

Research around surfactant is evolving and recent developments include further evolution of synthetic surfactants, evaluation of surfactant as a therapeutic option in lung diseases other than RDS and the discovery of genetic disorders of surfactant metabolism. Ongoing research is essential to continue to improve therapeutic prospects for children with serious respiratory disease involving disturbances in surfactant.

Polymyxin B/pulmonary surfactant mixtures have increased resistance to inactivation by meconium and reduce growth of gram-negative bacteria in vitro

Pulmonary surfactant is inactivated in meconium aspiration syndrome and neonatal pneumonia. Development of an exogenous surfactant less sensitive to inactivation might be useful for treating these diseases. We investigated in vitro whether addition of the cationic cyclic membrane cross-linking peptide polymyxin B (PxB) and/or calcium chloride (CaCl2) to modified porcine surfactant Curosurf increases resistance to meconium-induced inactivation of surface activity while antimicrobial activity of PxB is maintained. To study bacterial proliferation, Escherichia coli, group B streptococci (GBS), or Staphylococcus aureus were incubated 0-5 h in saline or in meconium in the presence or absence of Curosurf with or without PxB. PxB and CaCl2 improved spreading and adsorption of Curosurf. Curosurf plus CaCl2/PxB needed a 4-fold increase of meconium concentration to increase dynamic surface tension significantly compared with Curosurf plus CaCl2 alone, indicating that PxB further increases the resistance of Curosurf to meconium-induced inactivation. Meconium alone like meconium/Curosurf promoted growth of E. coli and GBS, but addition of Curosurf/PxB or PxB alone significantly reduced the growth of E. coli. Biophysical and antibacterial properties of Curosurf and PxB may be combined into a useful adjunct in the treatment of neonatal Gram-negative pneumonia and/or meconium aspiration syndrome.

New Synthetic Surfactants – Basic Science

The hydrophobic surfactant proteins, SP-B and SP-C, promote adsorption of surface-active lipids to the air-liquid interface of the alveoli and are essential for alveolar stability and gas exchange. Synthetic surfactant preparations must contain at least one of these hydrophobic proteins, or analogs thereof, to have optimal effects when administered into the airways of patients with lung diseases. However, development of clinically active artificial surfactants has turned out to be more complicated than initially anticipated since the native hydrophobic proteins are structurally complex or unstable in pure form. The proteins have been replaced by different analogs which have the right conformation without forming oligomers. Increased understanding of the surfactant proteins will hopefully lead to development of effective synthetic surfactants which can be produced in large quantities for treatment of a wide range of respiratory disorders. Furthermore, the lipid composition seems to be important, as well as a high lipid concentration in the suspension. For successful treatment of many respiratory diseases, it is also desirable that the synthetic surfactant resists inactivation by plasma components leaking into the alveoli.