Importance of hydrophobic apoproteins as constituents of clinical exogenous surfactants

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Optimizing Exogenous Surfactant as a Pulmonary Delivery Vehicle for Chicken Cathelicidin-2

The rising incidence of antibiotic-resistant lung infections has instigated a much-needed search for new therapeutic strategies. One proposed strategy is the use of exogenous surfactants to deliver antimicrobial peptides (AMPs), like CATH-2, to infected regions of the lung. CATH-2 can kill bacteria through a diverse range of antibacterial pathways and exogenous surfactant can improve pulmonary drug distribution. Unfortunately, mixing AMPs with commercially available exogenous surfactants has been shown to negatively impact their antimicrobial function. It was hypothesized that the phosphatidylglycerol component of surfactant was inhibiting AMP function and that an exogenous surfactant, with a reduced phosphatidylglycerol composition would increase peptide mediated killing at a distal site. To better understand how surfactant lipids interacted with CATH-2 and affected its function, isothermal titration calorimetry and solid-state nuclear magnetic resonance spectroscopy as well as bacterial killing curves against Pseudomonas aeruginosa were utilized. Additionally, the wet bridge transfer system was used to evaluate surfactant spreading and peptide transport. Phosphatidylglycerol was the only surfactant lipid to significantly inhibit CATH-2 function, showing a stronger electrostatic interaction with the peptide than other lipids. Although diluting the phosphatidylglycerol content in an existing surfactant, through the addition of other lipids, significantly improved peptide function and distal killing, it also reduced surfactant spreading. A synthetic phosphatidylglycerol-free surfactant however, was shown to further improve CATH-2 delivery and function at a remote site. Based on these in vitro experiments synthetic phosphatidylglycerol-free surfactants seem optimal for delivering AMPs to the lung.

Effects of surfactant on propagation and rupture of a liquid plug in a tube

Surfactant-laden liquid plug propagation and rupture occurring in lower lung airways are studied computationally using a front-tracking method. The plug is driven by an applied constant pressure in a rigid axisymmetric tube whose inner surface is coated by a thin liquid film. The evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations are solved in the front-tracking framework. The numerical method is first validated for a surfactant-free case and the results are found to be in good agreement with the earlier simulations of Fujioka et al. (2008) and Hassan et al. (2011). Then extensive simulations are performed to investigate the effects of surfactant on the mechanical stresses that could be injurious to epithelial cells such as pressure and shear stress. It is found that the liquid plug ruptures violently to induce large pressure and shear stress on airway walls and even a tiny amount of surfactant significantly reduces the pressure and shear stress and thus improves cell survivability. However, addition of surfactant also delays the plug rupture and thus airway reopening.

Helical side chain chemistry of a peptoid-based SP-C analogue: Balancing structural rigidity and biomimicry

Surfactant protein C (SP-C) is an important constituent of lung surfactant (LS) and, along with SP-B, is included in exogenous surfactant replacement therapies for treating respiratory distress syndrome (RDS). SP-C's biophysical activity depends upon the presence of a rigid C-terminal helix, of which the secondary structure is more crucial to functionality than precise side-chain chemistry. SP-C is highly sequence-conserved, suggesting that the β-branched, aliphatic side chains of the helix are also important. Nonnatural mimics of SP-C were created using a poly-N-substituted glycine, or "peptoid," backbone. The mimics included varying amounts of α-chiral, aliphatic side chains and α-chiral, aromatic side chains in the helical region, imparting either biomimicry or structural rigidity. Biophysical studies confirmed that the peptoids mimicked SP-C's secondary structure and replicated many of its surface-active characteristics. Surface activity was optimized by incorporating both structurally rigid and biomimetic side chain chemistries in the helical region indicating that both characteristics are important for activity. By balancing these features in one mimic, a novel analogue was created that emulates SP-C's in vitro surface activity while overcoming many of the challenges related to natural SP-C. Peptoid-based analogues hold great potential for use in a synthetic, biomimetic LS formulation for treating RDS.

Surfactant

Exogenous pulmonary surfactant, widely used in neonatal care, is one of the best-studied treatments in neonatology, and its introduction in the 1990s led to a significant improvement in neonatal outcomes in preterm infants, including a decrease in mortality. This chapter provides an overview of surfactant composition and function in health and disease and summarizes the evidence for its clinical use.

Molecular docking methodologies

Molecular docking represents an important technology for structure-based drug design. Docking is a computational technique aimed at the prediction of the most favorable ligand-target spatial configuration and an estimate of the corresponding complex free energy, although as stated at the beginning accurate scoring methods remain still elusive. In this chapter, the state of art of molecular docking methodologies and their applications in drug discovery is summarized.

Influence of film composition on the morphology, mechanical properties, and surfactant recovery of phase-separated phospholipid-perfluorinated fatty acid mixed monolayers

Monolayer surfactant films composed of a mixture of phospholipids and perfluorinated (or partially fluorinated) surfactants are of potential utility for applications in pulmonary lung surfactant-based therapies. As a simple, minimal model of such a lung surfactant system, binary mixed monolayer films composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F) prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM NaCl) have been characterized in terms of mixing thermodynamics and compressibility (measured through π–A compression isotherms), film morphology (via atomic force, fluorescence, and Brewster angle microscopy), as well as spreading rate and hysteresis response to repeated expansion–contraction cycles for a variety of compositions of mixed films. Under all mixing conditions, films and their components were found to be completely immiscible and phase-separated, though there were significant changes in the aforementioned film properties as a function of composition. Of particular note was the existence of a maximum in the extent of immiscibility (characterized by ΔG(ex)(π) values) and enhanced surfactant recovery during hysteresis experiments at χ(C18F) ≥ 0.30. The latter was attributed to the relatively rapid respreading rate of the perfluorinated amphiphile in comparison with DPPC alone at the air–water interface, which enhances the performance of this mixture as a potential pulmonary lung surfactant. Further, monolayer film structure could be tracked dynamically as a function of compression at the air–water interface via Brewster angle microscopy, with the C18F component being preferentially squeezed out of the film with compression, but returning rapidly upon re-expansion. In general, addition of C18F to DPPC monolayers resulted in improvements to mechanical, structural, and respreading properties of the film, indicating the potential value of these compounds as additives to pulmonary lung surfactant formulations.

Surfactant therapy for acute lung injury and acute respiratory distress syndrome

This article examines exogenous lung surfactant replacement therapy and its usefulness in mitigating clinical acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS). Surfactant therapy is beneficial in term infants with pneumonia and meconium aspiration lung injury, and in children up to age 21 years with direct pulmonary forms of ALI/ARDS. However, extension of exogenous surfactant therapy to adults with respiratory failure and clinical ALI/ARDS remains a challenge. This article reviews clinical studies of surfactant therapy in pediatric and adult patients with ALI/ARDS, focusing on its potential advantages in patients with direct pulmonary forms of these syndromes.

Synthesis and activity of a novel diether phosphonoglycerol in phospholipase-resistant synthetic lipid:peptide lung surfactants()

This paper reports the chemical synthesis and purification of a novel phospholipase-resistant C16:0, C16:1 diether phosphonoglycerol with structural analogy to ester-linked anionic phosphatidylglycerol (PG) in endogenous pulmonary surfactant. This diether phosphonoglycerol (PG 1) is studied for phospholipase A(2) (PLA(2)) resistance and for surface activity in synthetic exogenous surfactants combined with Super Mini-B (S-MB) peptide and DEPN-8, a previously-reported diether phosphonolipid analog of dipalmitoyl phosphatidylcholine (DPPC, the major zwitterionic phospholipid in native lung surfactant). Activity experiments measured both adsorption and dynamic surface tension lowering due to the known importance of these surface behaviors in lung surfactant function in vivo. Synthetic surfactants containing 9 : 1 DEPN-8:PG 1 + 3% S-MB were resistant to degradation by PLA(2) in chromatographic studies, while calf lung surfactant extract (CLSE, the substance of the bovine clinical surfactant Infasurf®) was significantly degraded by PLA(2). The 9 : 1 DEPN-8:PG 1 + 3% S-MB mixture also had small but consistent increases in both adsorption and dynamic surface tension lowering ability compared to DEPN-8 + 3% S-MB. Consistent with these surface activity increases, molecular dynamics simulations using Protein Modeller, GROMACS force-field, and PyMOL showed that bilayers containing DPPC and palmitoyl-oleoyl-PC (POPC) as surrogates of DEPN-8 and PG 1 were penetrated to a greater extent by S-MB peptide than bilayers of DPPC alone. These results suggest that PG 1 or related anionic phosphono-PG analogs may have functional utility in phospholipase-resistant synthetic surfactants targeting forms of acute pulmonary injury where endogenous surfactant becomes dysfunctional due to phospholipase activity in the innate inflammatory response.

Myth: all surfactants are alike

There are several surfactant preparations available to the clinician, none of which are alike. They differ in their phospholipid and surfactant protein (SP) composition as well as dosing, yet they all have been shown to be clinically effective as surfactants. Head-to-head randomized clinical trials comparing surfactants have shown some advantages of preparations that contain SP-B and SP-C, primarily in short-term clinical outcomes. A new synthetic surfactant that contains a phospholipid mixture and a peptide resembling SP-B has shown promise as a potential alternative to animal-derived surfactants.

Pulmonary surfactant proteins and polymer combinations reduce surfactant inhibition by serum

Acute respiratory distress syndrome (ARDS) is an inflammatory condition that can be associated with capillary leak of serum into alveoli causing inactivation of surfactant. Resistance to inactivation is affected by types and concentrations of surfactant proteins, lipids, and polymers. Our aim was to investigate the effects of different combinations of these three components. A simple lipid mixture (DPPC/POPG) or a more complex lipid mixture (DPPC/POPC/POPG/cholesterol) was used. Native surfactant proteins SP-B and SP-C obtained from pig lung lavage were added either singly or combined at two concentrations. Also, non-ionic polymers polyethylene glycol and dextran and the anionic polymer hyaluronan were added either singly or in pairs with hyaluronan included. Non-ionic polymers work by different mechanisms than anionic polymers, thus the purpose of placing them together in the same surfactant mixture was to evaluate if the combination would show enhanced beneficial effects. The resulting surfactant mixtures were studied in the presence or absence of serum. A modified bubble surfactometer was used to evaluate surface activities. Mixtures that included both SP-B and SP-C plus hyaluronan and either dextran or polyethylene glycol were found to be the most resistant to inhibition by serum. These mixtures, as well as some with either SP-B or SP-C with combined polymers were as or more resistant to inactivation than native surfactant. These results suggest that improved formulations of lung surfactants are possible and may be useful in reducing some types of surfactant inactivation in treating lung injuries.

Pharmacotherapy for prevention and treatment of acute respiratory distress syndrome: current and experimental approaches

The acute respiratory distress syndrome (ARDS) arises from direct and indirect injury to the lungs and results in a life-threatening form of respiratory failure in a heterogeneous, critically ill patient population. Critical care technologies used to support patients with ARDS, including strategies for mechanical ventilation, have resulted in improved outcomes in the last decade. However, there is still a need for effective pharmacotherapies to treat ARDS, as mortality rates remain high. To date, no single pharmacotherapy has proven effective in decreasing mortality in adult patients with ARDS, although exogenous surfactant replacement has been shown to reduce mortality in the paediatric population with ARDS from direct causes. Several promising therapies are currently being investigated in preclinical and clinical trials for treatment of ARDS in its acute and subacute, exudative phases. These include exogenous surfactant therapy, β₂-adrenergic receptor agonists, antioxidants, immunomodulating agents and HMG-CoA reductase inhibitors (statins). Recent research has also focused on prevention of acute lung injury and acute respiratory distress in patients at risk. Drugs such as captopril, rosiglitazone and incyclinide (COL-3), a tetracycline derivative, have shown promising results in animal models, but have not yet been tested clinically. Further research is needed to discover therapies to treat ARDS in its late, fibroproliferative phase. Given the vast number of negative clinical trials to date, it is unlikely that a single pharmacotherapy will effectively treat all patients with ARDS from differing causes. Future randomized controlled trials should target specific, more homogeneous subgroups of patients for single or combination therapy.

Close mimicry of lung surfactant protein B by "clicked" dimers of helical, cationic peptoids

A family of peptoid dimers developed to mimic SP-B is presented, where two amphipathic, cationic helices are linked by an achiral octameric chain. SP-B is a vital therapeutic protein in lung surfactant replacement therapy, but its large-scale isolation or chemical synthesis is impractical. Enhanced biomimicry of SP-B's disulfide-bonded structure has been previously attempted via disulfide-mediated dimerization of SP-B(1-25) and other peptide mimics, which improved surface activity relative to the monomers. Herein, the effects of disulfide- or "click"-mediated (1,3-dipolar cycloaddition) dimerization, as well as linker chemistry, on the lipid-associated surfactant activity of a peptoid monomer are described. Results revealed that the 'clicked' peptoid dimer enhanced in vitro surface activity in a DPPC:POPG:PA lipid film relative to its disulfide-bonded and monomeric counterparts in both surface balance and pulsating bubble surfactometry studies. On the pulsating bubble surfactometer, the film containing the "clicked" peptoid dimer outperformed all presented peptoid monomers and dimers, and two SP-B derived peptides, attaining an adsorbed surface tension of 22 mN m(-1), and maximum and minimum cycling values of 42 mN m(-1) and near-zero, respectively.

Randomized clinical trial comparing two natural surfactant preparations to treat respiratory distress syndrome

OBJECTIVES

Natural surfactant preparations have been shown to reduce the severity and mortality of respiratory distress syndrome (RDS) in preterm infants. The objective of this study was to compare the efficacy of two natural surfactants, namely SF-RI 1 (Alveofact) and barectant (Survanta), on the incidence of chronic lung disease (CLD) and other associated complications of RDS in preterm infants.

METHODS

Preterm infants with RDS requiring artificial ventilation were randomly selected to receive an initial dose of either Alveofact or Survanta. The two treatment groups were tested for variation in gas exchange, ventilatory settings and neonatal complications such as CLD and mortality.

RESULTS

After 5 days the Survanta-treated infants had a lower fraction of inspired oxygen (FiO2) compared with the Alveofact-treated infants. There were no differences in the ventilatory settings. More infants in the Survanta group were extubated at 3 days and fewer required the use of postnatal steroids. Less CLD and duration of oxygenation were experienced by the Survanta-treated group.

CONCLUSIONS

Improved oxygenation and reduced ventilatory requirements were greater with Survanta compared to Alveofact, which in turn was associated with a trend towards a lower incidence of serious pulmonary complications.

Post hoc analysis of calfactant use in immunocompromised children with acute lung injury: Impact and feasibility of further clinical trials

OBJECTIVE

To assess the impact of calfactant (a modified natural bovine lung surfactant) in immunocompromised children with acute lung injury and to determine the number of patients required for a definitive clinical trial of calfactant in this population.

DESIGN

Post hoc analysis of data from a previous randomized, control trial.

SETTING

Tertiary care pediatric intensive care units.

PATIENTS

All children, defined as immunocompromised, enrolled in a multicenter, masked, randomized, control trial of calfactant for acute lung injury conducted between July 2000 and July 2003.

INTERVENTIONS

Patients received either an intratracheal instillation of calfactant or an equal volume of air placebo in a protocolized manner.

MEASUREMENTS AND MAIN RESULTS

Eleven of 22 (50%) calfactant-treated patients died when compared with 18 of 30 (60%) placebo patients (absolute risk reduction 10.0%, 95% confidence interval [CI] -17.3, 37.3). Among the 23 patients with an initial oxygen index (OI) >/=13 and </=37, 44% (4 of 9) of calfactant-treated patients died in comparison with 71% (10 of 14) of placebo (absolute risk reduction 27.0%, 95% CI -13.2, 67.2). Only 33% (3 of 9) of calfactant patients died before intensive care discharge in comparison with 71% (10 of 14) of placebo (absolute risk reduction 38.1%, 95% CI -0.7, 76.9). Calfactant therapy was associated with improved oxygenation in these 23 patients. Using an OI entry criterion of (13 </= OI </= 37), stratifying on the presence of hematopoietic stem cell transplantation, and accepting the 27% difference in mortality observed in this analysis, 63 patients would be required in each arm of a randomized, control trial to demonstrate a significant effect of calfactant on mortality in this patient population assuming a two-sided alpha of 0.05 and a power of 0.85.

CONCLUSIONS

These preliminary data suggest a potential benefit of calfactant in this high-risk population. A clinical trial powered to appropriately assess these findings seems warranted and feasible.

The biophysical function of pulmonary surfactant

Pulmonary surfactant lowers surface tension in the lungs. Physiological studies indicate two key aspects of this function: that the surfactant film forms rapidly; and that when compressed by the shrinking alveolar area during exhalation, the film reduces surface tension to very low values. These observations suggest that surfactant vesicles adsorb quickly, and that during compression, the adsorbed film resists the tendency to collapse from the interface to form a 3D bulk phase. Available evidence suggests that adsorption occurs by way of a rate-limiting structure that bridges the gap between the vesicle and the interface, and that the adsorbed film avoids collapse by undergoing a process of solidification. Current models, although incomplete, suggest mechanisms that would partially explain both rapid adsorption and resistance to collapse as well as how different constituents of pulmonary surfactant might affect its behavior.

Surfactant for pediatric acute lung injury

This article reviews exogenous surfactant therapy and its use in mitigating acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) in infants, children, and adults. Biophysical and animal research documenting surfactant dysfunction in ALI/ARDS is described, and the scientific rationale for treatment with exogenous surfactant is discussed. Major emphasis is placed on reviewing clinical studies of surfactant therapy in pediatric and adult patients who have ALI/ARDS. Particular advantages from surfactant therapy in direct pulmonary forms of these syndromes are described. Also discussed are additional factors affecting the efficacy of exogenous surfactants in ALI/ARDS.

Biomechanics of liquid-epithelium interactions in pulmonary airways

The delicate structure of the lung epithelium makes it susceptible to surface tension induced injury. For example, the cyclic reopening of collapsed and/or fluid-filled airways during the ventilation of injured lungs generates hydrodynamic forces that further damage the epithelium and exacerbate lung injury. The interactions responsible for epithelial injury during airway reopening are fundamentally multiscale, since air-liquid interfacial dynamics affect global lung mechanics, while surface tension forces operate at the molecular and cellular scales. This article will review the current state-of-knowledge regarding the effect of surface tension forces on (a) the mechanics of airway reopening and (b) epithelial cell injury. Due to the complex nature of the liquid-epithelium system, a combination of computational and experimental techniques are being used to elucidate the mechanisms of surface-tension induced lung injury. Continued research is leading to an integrated understanding of the biomechanical and biological interactions responsible for cellular injury during airway reopening. This information may lead to novel therapies that minimize ventilation induced lung injury.

Influence of lipid saturation grade and headgroup charge: a refined lung surfactant adsorption model

Rapid adsorption of surfactant material to the air/liquid interface of the lung is essential for maintaining normal lung function. The detailed mechanism of this process, however, remains unclear. In this study, we elucidate the influence of lipid saturation grade and headgroup charge of surface layer lipids on surfactant protein (SP)-induced vesicle insertion into monolayers spread at the air/water interface of a film balance. We used dipalmitoylphosphatidlycholine (DPPC),1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) as monolayer lipids doped with either hydrophobic surfactant-specific protein SP-B or SP-C (0.2 and 0.4 mol %, respectively). Vesicles consisting of DPPC/DPPG (4:1, mol ratio) were injected into a stirred subphase to quantify adsorption kinetics. Based on kinetic film balance and fluorescence measurements, a refined model describing distinct steps of vesicle adsorption to surfactant monolayers is presented. First, in a protein-independent step, lipids from vesicles bridged to the interfacial film by Ca(2+) ions are inserted into defects of a disordered monolayer at low surface pressures. Second, in a SP-facilitated step, active material insertion involving an SP-B- or SP-C-induced flip-flop of lipids occurs at higher surface pressures. Negatively charged lipids obviously influence the threshold pressures at which this second protein-mediated adsorption mechanism takes place.

Dynamic surface activity of a fully synthetic phospholipase-resistant lipid/peptide lung surfactant

Background

This study examines the surface activity and resistance to phospholipase degradation of a fully-synthetic lung surfactant containing a novel diether phosphonolipid (DEPN-8) plus a 34 amino acid peptide (Mini-B) related to native surfactant protein (SP)-B. Activity studies used adsorption, pulsating bubble, and captive bubble methods to assess a range of surface behaviors, supplemented by molecular studies using Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD), and plasmon resonance. Calf lung surfactant extract (CLSE) was used as a positive control.

Results

DEPN-8+1.5% (by wt.) Mini-B was fully resistant to degradation by phospholipase A₂ (PLA₂) in vitro, while CLSE was severely degraded by this enzyme. Mini-B interacted with DEPN-8 at the molecular level based on FTIR spectroscopy, and had significant plasmon resonance binding affinity for DEPN-8. DEPN-8+1.5% Mini-B had greatly increased adsorption compared to DEPN-8 alone, but did not fully equal the very high adsorption of CLSE. In pulsating bubble studies at a low phospholipid concentration of 0.5 mg/ml, DEPN-8+1.5% Mini-B and CLSE both reached minimum surface tensions <1 mN/m after 10 min of cycling. DEPN-8 (2.5 mg/ml)+1.5% Mini-B and CLSE (2.5 mg/ml) also reached minimum surface tensions <1 mN/m at 10 min of pulsation in the presence of serum albumin (3 mg/ml) on the pulsating bubble. In captive bubble studies, DEPN-8+1.5% Mini-B and CLSE both generated minimum surface tensions <1 mN/m on 10 successive cycles of compression/expansion at quasi-static and dynamic rates.

Conclusions

These results show that DEPN-8 and 1.5% Mini-B form an interactive binary molecular mixture with very high surface activity and the ability to resist degradation by phospholipases in inflammatory lung injury. These characteristics are promising for the development of related fully-synthetic lipid/peptide exogenous surfactants for treating diseases of surfactant deficiency or dysfunction.

Activity and inhibition resistance of a phospholipase-resistant synthetic surfactant in rat lungs

This study investigates the activity and inhibition resistance in excised rat lungs of a novel synthetic surfactant containing the phospholipase-resistant diether phosphonolipid DEPN-8 plus 1.5% bovine surfactant protein (SP)-B/C compared to calf lung surfactant extract (CLSE). DEPN-8 + 1.5% SP-B/C surpassed CLSE in normalizing surfactant-deficient pressure-volume (P-V) deflation mechanics in lavaged excised lungs in the presence of phospholipase A(2) (PLA(2)) or C18:1 lyso-phosphatidylcholine (LPC). DEPN-8 + 1.5% SP-B/C had activity equal to CLSE in normalizing P-V mechanics in the absence of inhibitors or in the presence of serum albumin. These physiologic activity findings were directly consistent with surface activity measurements on the pulsating bubble surfactometer. In the absence of inhibitors, DEPN-8 + 1.5% SP-B/C and CLSE rapidly reached minimum surface tensions < 1 mN/m (0.5 and 2.5 mg surfactant phospholipid/ml). DEPN-8 + 1.5% SP-B/C maintained its high surface activity in the presence of PLA(2), while the surface activity of CLSE was significantly inhibited by exposure to this enzyme. DEPN-8 + 1.5% SP-B/C also had greater surface activity than CLSE in the presence of LPC, and the two surfactants had equivalent surface activity in the presence of albumin. DEPN-8 + 1.5% SP-B/C also had slightly greater surface activity than CLSE when exposed to peroxynitrite in pulsating bubble studies. These results support the potential of developing highly active and inhibition-resistant synthetic exogenous surfactants containing DEPN-8 + apoprotein/peptide constituents for use in treating direct pulmonary forms of clinical acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS).

Animal-derived surfactants versus past and current synthetic surfactants: current status

In this review, the authors assess major outcomes resulting from head-to-head comparison trials of animal-derived surfactants with previous and newer synthetic surfactants and among them. They also pay special attention to issues of study design and quality of the trials reviewed. Animal-derived surfactants that contain surfactant proteins (Survanta, Infasurf, and Curosurf) perform clinically better than Exosurf, a synthetic surfactant containing only phospholipids, primarily in outcomes related to acute management of respiratory distress syndrome (RDS; faster weaning and pneumothorax) but not in overall mortality or incidence of bronchopulmonary dysplasia (BPD). Trials comparing various animal-derived surfactants that provide different amounts of surface protein B (SP-B) or phospholipids have shown minor differences in outcomes related to the management of RDS or none at all. The exception is the suggestion of better survival using a high initial dose of Curosurf when compared with Survanta. This observation is based on analysis of trials of relatively lesser quality that have included a smaller number of infants than other surfactant comparisons, however. Data from recent trials comparing a new-generation synthetic surfactant that contains a peptide mimicking the action of SP-B, Surfaxin, have shown that it performs better than Exosurf (faster weaning and less BPD) and at least as well as the animal-derived surfactants Survanta and Curosurf. The ideal surfactant comparison trial to demonstrate which surfactant is better has yet to be conducted. Future surfactant comparison trials should pay particular attention to study design, be appropriately sized, and include long-term follow-up.

Repeat surfactant therapy for postsurfactant slump

OBJECTIVE

To evaluate repeat surfactant therapy for the treatment of respiratory failure associated with postsurfactant slump in extremely low birth weight infants (ELBW) by characterizing the population of premature infants who develop postsurfactant slump and measuring their response to a secondary course of surfactant therapy.

STUDY DESIGN

A retrospective analysis of a cohort of all patients admitted over a 3-year period with birth weights <1000 g (ELBW infants). Information was collected by chart review and the patients were categorized into three distinct groups for analysis. Initial surfactant only, patients who received surfactant replacement therapy only for respiratory distress syndrome (RDS); repeat surfactant, patients who received both initial surfactant replacement for RDS and repeat surfactant therapy for postsurfactant slump (defined as respiratory failure after 6 days of age), and no surfactant, patients in whom no surfactant was ever administered. A respiratory severity score (RSS) was used to measure the severity of lung disease and response to surfactant therapy.

RESULTS

Over 3 years, there were 165 ELBW infants who could develop postsurfactant slump and be eligible for repeat surfactant therapy. There were 39 infants who never received any surfactant therapy estimated gestational age (EGA) 27.7 +/- 1.7, birth weight 856 +/- 109 g) either at birth or after 6 days of life. There were 126 patients treated for RDS with initial surfactant replacement therapy (EGA 25.6 +/- 1.9 weeks, birth weight 713 +/- 179 g). Out of these RDS patients, 101 improved with an initial course of surfactant therapy (EGA 26 +/- 1.8, birth weight 751 +/- 143 g), but 25 (20% of the patients with RDS) developed postsurfactant slump and received a repeat course of surfactant therapy (EGA 24.7 +/- 1.2, birth weight 647 +/- 120 g). The repeat surfactant group (postsurfactant slump) was significantly more premature and had significantly lower birth weights compared to both the initial surfactant only group and the no surfactant ever group. Logistic regression analysis revealed that lack of antenatal steroids, earlier gestational age, and the receiving of 2 or more doses of surfactant to treat the initial RDS were significantly associated with receiving repeat surfactant therapy for postsurfactant slump. Of the 25 patients treated with a repeat course of surfactant therapy more than 70% of patients (n = 18) had an improvement in their lung disease with a 15% reduction in their RSS. This improvement was significant at all time points evaluated (12, 24, and 48 h).

CONCLUSION

We found that a repeat course of surfactant therapy, after day of life 6, led to a significant improvement in hypoxemic respiratory failure in premature infants with postsurfactant slump. Infants who received repeat surfactant therapy were born at a significantly earlier gestational age, had significantly smaller birth weight and had significantly worse lung disease. They were significantly less likely to have received antenatal steroids and were significantly more likely to have received multiple doses of surfactant to treat their initial RDS. A repeat course of surfactant therapy for patients with postsurfactant slump appeared beneficial in the short-term. These initial findings would support performing randomized control trials of repeat surfactant therapy for postsurfactant slump.

Surface properties of sulfur- and ether-linked phosphonolipids with and without purified hydrophobic lung surfactant proteins

Two novel C16:0 sulfur-linked phosphonolipids (S-lipid and SO(2)-lipid) and two ether-linked phosphonolipids (C16:0 DEPN-8 and C16:1 UnDEPN-8) were studied for surface behavior alone and in mixtures with purified bovine lung surfactant proteins (SP)-B and/or SP-C. Synthetic C16:0 phosphonolipids all had improved adsorption and film respreading compared to dipalmitoyl phosphatidylcholine, and SO(2)-lipid and DEPN-8 reached maximum surface pressures of 72mN/m (minimum surface tensions of <1mN/m) in compressed films on the Wilhelmy balance (23 degrees C). Dispersions of DEPN-8 (0.5mg/ml) and SO(2)-lipid (2.5mg/ml) also reached minimum surface tensions of <1mN/m on a pulsating bubble surfactometer (37 degrees C, 20cycles/min, 50% area compression). Synthetic lung surfactants containing DEPN-8 or SO(2)-lipid+0.75% SP-B+0.75% SP-C had dynamic surface activity on the bubble equal to that of calf lung surfactant extract (CLSE). Surfactants containing DEPN-8 or SO(2)-lipid plus 1.5% SP-B also had very high surface activity, but less than when both apoproteins were present together. Adding 10wt.% of UnDEPN-8 to synthetic lung surfactants did not improve dynamic surface activity. Surfactants containing DEPN-8 or SO(2)-lipid plus 0.75% SP-B/0.75% SP-C were chemically and biophysically resistant to phospholipase A(2) (PLA(2)), while CLSE was severely inhibited by PLA(2). The high activity and inhibition resistance of synthetic surfactants containing DEPN-8 or SO(2)-lipid plus SP-B/SP-C are promising for future applications in treating surfactant dysfunction in inflammatory lung injury.

Molecular dynamics simulations of the anchoring and tilting of the lung-surfactant peptide SP-B1-25 in palmitic acid monolayers

We have performed molecular dynamics simulations of multiple copies of the lung-surfactant peptide SP-B1-25 in a palmitic acid (PA) monolayer. SP-B1-25 is a shorter version of lung-surfactant protein B, an important component of lung surfactant. Up to 30 ns simulations of 20 wt % SP-B1-25 in the PA monolayers were performed with different surface areas of PA, extents of PA ionization, and various initial configurations of the peptides. Starting with initial peptide orientation perpendicular to the monolayer, the predicted final tilt angles average 54 degrees approximately 62 degrees with respect to the monolayer normal, similar to those measured experimentally by Lee et al. (Biophysical Journal. 2001. Synchrotron x-ray study of lung surfactant-specific protein SP-B in lipid monolayers. 81:572-585). In their final conformations, hydrogen-bond analysis and amino acid mutation studies show that the peptides are anchored by hydrogen bond interactions between the cationic residues Arg-12 and Arg-17 and the hydrogen bond acceptors of the ionized PA headgroup, and the tilt angle is affected by the interactions of Tyr-7 and Gln-19 with the PA headgroup. Our work indicates that the factors controlling orientation of small peptides in lipid layers can now be uncovered through molecular dynamics simulations.

Comparison of Infasurf (calfactant) and Survanta (beractant) in the prevention and treatment of respiratory distress syndrome

BACKGROUND

In biophysical and animal testing, Infasurf develops lower surface tension and restores total surfactant activity better than Survanta.

METHODS

We performed 2 prospective, randomized, masked clinical trials; 1 trial used a prophylactic strategy aimed at prevention of respiratory distress syndrome (prophylaxis trial) for infants who were born between 23 weeks, 0 days and 29 weeks, 6 days of gestation, and the second trial used a treatment strategy (treatment trial) for intubated infants with a birth weight of 401 to 2000 g who required fractional inspired oxygen of >0.4 to maintain an arterial oxygen saturation of >90% (or an arterial/alveolar oxygen ratio of <0.2) at any time before 36 hours of age. Our purpose was to determine if Infasurf (calfactant) was more effective than Survanta (beractant) at increasing the proportion of patients alive and not receiving supplemental oxygen at 36 weeks' postmenstrual age. Informed, written, parental consent was required, and protocols were approved by the institutional review boards of all participating institutions. The dose of surfactant was 4 mL/kg (100 mg/kg) for Survanta and 3 mL/kg (105 mg/kg) for Infasurf for both trials. The assigned drug was drawn into 2 masked syringes and administered by a health care professional who, in most cases, was not directly responsible for caring for the patient. A maximum of 3 repeat treatments, at least 6 hours apart, were permitted if the neonate required fractional inspired oxygen of >0.30 to maintain an arterial oxygen saturation of >90% (or an arterial/alveolar oxygen ratio of 0.33) and the infant remained intubated for respiratory distress syndrome.

RESULTS

Both trials were halted for not meeting enrollment targets after a 32-month recruitment period. The decision to end recruitment was made after the interim analysis of the treatment trial. We enrolled 749 infants in the prophylaxis trial and 1361 infants in the treatment trial. The primary outcome (alive and not receiving supplemental oxygen at 36 weeks' postmenstrual age) rate in the prophylaxis trial was 52.1% for group 1 and 52.4% for group 2. In the treatment trial, the primary outcome rate was 58.7% in group 1 and 56.8% in group 2. Based on sample-size requirements for a conclusion of similarity, and the lack of statistical significance to the differences noted in the primary outcome, we have chosen not to break the investigator blind but to report the results as groups 1 and 2.

CONCLUSION

Early trial closure prevents us from either accepting or rejecting our null hypothesis.

Molecular dynamics study of the lung surfactant peptide SP-B1-25 with DPPC monolayers: insights into interactions and peptide position and orientation

We have performed molecular dynamics simulations of the interactions of the peptide SP-B(1-25), which is a truncated version of the full pulmonary surfactant protein SP-B, with dipalmitoylphosphatidylcholine monolayers, which are the major lipid components of lung surfactant. Simulations of durations of 10-20 ns show that persistent hydrogen bonds form between the donor atoms of the protein and the acceptors of the lipid headgroup and that these bonds determine the position, orientation, and secondary structure of the peptide in the membrane environment. From an ensemble of initial conditions, the most probable equilibrium orientation of the alpha-helix of the peptide is predicted to be parallel to the interface, matching recent experimental results on model lipid mixtures. Simulations of a few mutated analogs of SP-B(1-25) also suggest that the charged amino acids are important in determining the position of the peptide in the interface. The first eight amino acids of the peptide, also known as the insertion sequence, are found to be essential in reducing the fluctuations and anchoring the peptide in the lipid/water interface.

A multicenter, randomized, masked, comparison trial of lucinactant, colfosceril palmitate, and beractant for the prevention of respiratory distress syndrome among very preterm infants

BACKGROUND AND OBJECTIVE

Evidence suggests that synthetic surfactants consisting solely of phospholipids can be improved through the addition of peptides, such as sinapultide, that mimic the action of human surfactant protein-B (SP-B). A synthetic surfactant containing a mimic of SP-B may also reduce the potential risks associated with the use of animal-derived products. Our objective was to compare the efficacy and safety of a novel synthetic surfactant containing a functional SP-B mimic (lucinactant; Discovery Laboratories, Doylestown, PA) with those of a non-protein-containing synthetic surfactant (colfosceril palmitate; GlaxoSmithKline, Brentford, United Kingdom) and a bovine-derived surfactant (beractant; Abbott Laboratories, Abbott Park, IL) in the prevention of neonatal respiratory distress syndrome (RDS) and RDS-related death.

METHODS

We assigned randomly (double-masked) 1294 very preterm infants, weighing 600 to 1250 g and of < or =32 weeks gestational age, to receive colfosceril palmitate (n = 509), lucinactant (n = 527), or beractant (n = 258) within 20 to 30 minutes after birth. Primary outcome measures were the rates of RDS at 24 hours and the rates of death related to RDS during the first 14 days after birth. All-cause mortality rates, bronchopulmonary dysplasia (BPD) rates, and rates of other complications of prematurity were prespecified secondary outcomes. Primary outcomes, air leaks, and causes of death were assigned by an independent, masked, adjudication committee with prespecified definitions. The study was monitored by an independent data safety monitoring board.

RESULTS

Lucinactant reduced significantly the incidence of RDS at 24 hours, compared with colfosceril (39.1% vs 47.2%; odds ratio [OR]: 0.68; 95% confidence interval [CI]: 0.52-0.89). There was no significant difference in comparison with beractant (33.3%). However, lucinactant reduced significantly RDS-related mortality rates by 14 days of life, compared with both colfosceril (4.7% vs 9.4%; OR: 0.43; 95% CI: 0.25-0.73) and beractant (10.5%; OR: 0.35; 95% CI: 0.18-0.66). In addition, BPD at 36 weeks postmenstrual age was significantly less common with lucinactant than with colfosceril (40.2% vs 45.0%; OR: 0.75; 95% CI: 0.56-0.99), and the all-cause mortality rate at 36 weeks postmenstrual age was lower with lucinactant than with beractant (21% vs 26%; OR: 0.67; 95% CI: 0.45-1.00).

CONCLUSIONS

Lucinactant is a more effective surfactant preparation than colfosceril palmitate for the prevention of RDS. In addition, lucinactant reduces the incidence of BPD, compared with colfosceril palmitate, and decreases RDS-related mortality rates, compared with beractant. Therefore, we conclude that lucinactant, the first of a new class of surfactants containing a functional protein analog of SP-B, is an effective therapeutic option for preterm infants at risk for RDS.

A common mutation in the surfactant protein C gene associated with lung disease

OBJECTIVE

To determine the contribution of the surfactant protein C (SP-C) I73T mutation to lung disease.

STUDY DESIGN

Genomic DNA was obtained from 116 children with interstitial lung disease (ILD) or chronic lung disease of unclear cause and from 166 control subjects and was screened for the I73T mutation using an allele-specific polymerase chain reaction assay.

RESULTS

The I73T mutation was found on 7 of 232 SP-C alleles from 7 unrelated children with ILD but was not found on 332 control SP-C alleles ( P < .01, Fisher exact test). The I73T mutation segregated with lung disease in one kindred with familial ILD. The I73T mutation was found in an asymptomatic parent from two different families with affected children consistent with variable penetrance, but it was not found in either asymptomatic parent of two other unrelated affected children consistent with a de novo mutation. Analysis of single nucleotide polymorphisms indicated diverse genetic backgrounds of the I73T alleles. Immunohistochemical analysis of lung tissue from an infant with the I73T mutation demonstrated normal staining patterns for proSP-B, SP-B, and proSP-C.

CONCLUSIONS

These findings support the hypothesis that the I73T mutation predisposes to or causes lung disease.

Effect of antitubercular drugs on dipalmitoylphosphatidylcholine monolayers: implications for drug loaded surfactants

The year long chemotherapy in pulmonary tuberculosis results in dose related side effects and may not reach atelectatic areas. On account of its spreading properties and the ability to re-expand atelectatic areas, exogenous surfactant may act as a pulmonary drug delivery agent. We investigated the interactions between antitubercular drugs and the main surfactant component, dipalmitoylphosphatidylcholine (DPPC) with the aim of developing more effective antitubercular drug loaded surfactants. The surface properties were evaluated using a Langmuir-Blodgett trough and Wilhelmy balance at 37 degrees C. Lung surfactant was modeled as DPPC monolayers. The isoniazid (INH)-DPPC combination in 1:1 ratio by weight significantly improved the adsorption of DPPC, reached a minimum surface tension of zero, formed a low compressibility film and required 32.7% area change to decrease surface tension from 30 to 10 mN/m. The triple drug (INH-rifampicin-ethambutol in 1:2:3 ratio by weight) DPPC combination when used in 1:1 or 1:2 ratios by weight also achieved surface properties superior to those of DPPC alone. A significant improvement in the adsorption was observed (surface tensions of 34.7 mN/m for 1:1 and 32.0 mN/m for 1:2 triple drug: DPPC combinations in the first second), and the films had low compressibility reaching a minimum surface tension of zero on compression. Thus, we observed statistically significant improvements in all the surface parameters and we feel encouraged to continue developing a tuberculosis therapy consisting of surfactant liposomes carrying antitubercular drugs.

Randomized trial comparing two natural surfactants (Survanta vs. bLES) for treatment of neonatal respiratory distress syndrome

Exogenous surfactants have been used as an effective treatment of neonatal respiratory distress syndrome (RDS). Different preparations of surfactant carry different biophysical and clinical properties. To study the response pattern and treatment outcome of two natural surfactants (bLES and Survanta) for the treatment of RDS, we conducted a randomized clinical trial at the neonatal unit of a university teaching hospital. Premature babies with birth weight between 500-1,800 g who developed RDS requiring mechanical ventilation with an oxygen requirement of more than 30% within 6 hr of life were randomized into two treatment groups. Oxygenation indices (OIs) within 12 hr of treatment were compared as primary outcomes, while neonatal complications were analyzed as secondary outcomes of the study. Sixty babies were recruited, with 29 in the bLES and 31 in the Survanta treatment group. Both groups had significant and sustained improvements in OI after surfactant replacement therapy (SRT), while the bLES group was associated with a significantly lower OI throughout the initial 12 hr after treatment compared with the Survanta group. There was no difference in secondary outcomes including mortality, ventilator days, and occurrence of chronic lung disease. We conclude that infants with RDS respond favorably to both types of surfactant replacement, and that bLES achieved a faster clinical response in terms of improvement in OI than Survanta.

Simple, Helical Peptoid Analogs of Lung Surfactant Protein B

The helical, amphipathic surfactant protein, SP-B, is a critical element of pulmonary surfactant and hence is an important therapeutic molecule. However, it is difficult to isolate from natural sources in high purity. We have created and studied three different, nonnatural analogs of a bioactive SP-B fragment (SP-B(1-25)), using oligo-N-substituted glycines (peptoids) with simple, repetitive sequences designed to favor the formation of amphiphilic helices. For comparison, a peptide with a similar repetitive sequence previously shown to be a good SP mimic was also studied, along with SP-B(1-25) itself. Surface pressure-area isotherms, surfactant film phase morphology, and dynamic adsorption behavior all indicate that the peptoids are promising mimics of SP-B(1-25). The extent of biomimicry appears to correlate with peptoid helicity and lipophilicity. These biostable oligomers could serve in a synthetic surfactant replacement to treat respiratory distress syndrome.

Surfactant therapy for respiratory distress syndrome in premature neonates: a comparative review

Exogenous surfactant therapy has been part of the routine care of preterm neonates with respiratory distress syndrome (RDS) since the beginning of the 1990s. Discoveries that led to its development as a therapeutic agent span the whole of the 20th century but it was not until 1980 that the first successful use of exogenous surfactant therapy in a human population was reported. Since then, randomized controlled studies demonstrated that surfactant therapy was not only well tolerated but that it significantly reduced both neonatal mortality and pulmonary air leaks; importantly, those surviving neonates were not at greater risk of subsequent neurological impairment. Surfactants may be of animal or synthetic origin. Both types of surfactants have been extensively studied in animal models and in clinical trials to determine the optimum timing, dose size and frequency, route and method of administration. The advantages of one type of surfactant over another are discussed in relation to biophysical properties, animal studies and results of randomized trials in neonatal populations. Animal-derived exogenous surfactants are the treatment of choice at the present time with relatively few adverse effects related largely to changes in oxygenation and heart rate during surfactant administration. The optimum dose of surfactant is usually 100 mg/kg. The use of surfactant with high frequency oscillation and continuous positive pressure modes of respiratory support presents different problems compared with its use with conventional ventilation. The different components of surfactant have important functions that influence its effectiveness both in the primary function of the reduction of surface tension and also in secondary, but nonetheless just as important, role of lung defense. With greater understanding of the individual surfactant components, particularly the surfactant-associated proteins, development of newer synthetic surfactants has been made possible. Despite being an effective therapy for RDS, surfactant has failed to have a significant impact on the incidence of chronic lung disease in survivors. Paradoxically the cost of care has increased as surviving neonates are more immature and consume a greater proportion of neonatal intensive care resources. Despite this, surfactant is considered a cost-effective therapy for RDS compared with other therapeutic interventions in premature infants.

Comparison of three treatment regimens of natural surfactant preparations in neonatal respiratory distress syndrome

The aim of the study was to compare the treatment regimen of three natural surfactants of different extraction and formulation (Alveofact [Surfactant A = SA], Poractant [Surfactant B = SB] and Beractant [Surfactant C = SC]) in neonatal respiratory distress syndrome (RDS). Premature infants of </=32 weeks' gestation with birth weight of </=2,000 g and with established RDS requiring artificial ventilation with a FiO2 >/=0.3 were randomly assigned to receive at least two doses of SA, SB or SC (100 mg/kg per dose). Infants who remained dependent on artificial ventilation with a FiO2 >/=0.3 received up to two additional doses. There were no differences among the groups regarding the necessity for more than two doses. The SA and the SB groups spent fewer days on a ventilator (p-value SA/SB 0.7, SA/SC 0.05, SB/SC 0.043) compared with the SC group, needed fewer days of oxygen administration (p-value SA/SB 0.14, SA/SC 0.05, SB/SC 0.04) and spent fewer days in hospital (p-value SA/SB 0.65, SA/SC 0.04, SB/SC 0.027). There were no statistically significant differences in the incidence of mortality, chronic lung disease, air leaks, necrotising enterocolitis, retinopathy of prematurity and intraventricular haemorrhage among the three groups.

CONCLUSION

The Alveofact and Poractant groups spent fewer days on the ventilator, needed fewer days of oxygen administration and spent fewer days in hospital compared with the Beractant group but no differences were observed among the three groups with regards to mortality and morbidity.

Lung surfactants for neonatal respiratory distress syndrome: animal-derived or synthetic agents?

Exogenous surfactant therapy is widely used in the management of neonatal respiratory distress syndrome. Two types of surfactants are available: synthetic surfactants, and those derived from animal sources ("natural" surfactants). Both of these surfactants have been shown to be effective. In this article, we review the evidence to compare the two types of surfactants in terms of their physical properties, physiologic effects, and clinical outcomes. Natural surfactants have been shown to have advantages over synthetic surfactants in their physical properties and physiologic effects in animals, as well as in humans. A systematic review of 11 randomized clinical trials comparing natural and synthetic surfactants demonstrated that the use of natural surfactant preparations results in greater clinical benefits compared with synthetic surfactants. These benefits include a more rapid improvement in oxygenation and lung compliance after surfactant therapy, a decrease in the risk of mortality (typical relative risk 0.87; typical risk difference -0.02), and a decrease in the risk of pneumothorax (typical relative risk 0.63; typical risk difference -0.04). Although the use of natural surfactants results in a slightly increased risk of intraventricular hemorrhage (typical relative risk 1.09; typical risk difference 0.03), there is no increase in the risk of grade 3 or 4 intraventricular hemorrhage. There are theoretical but unproven risks of natural surfactants, such as transmission of infectious agents, immunogenicity and impurities in composition. The use of natural surfactants is preferred in most situations. In addition, clinicians should determine the costs of different types of surfactants in their individual practice settings and use this information in decision-making.

Specific mode of interaction between components of model pulmonary surfactants using computer simulations

Atomistic molecular dynamics simulations and structural bioinformatics tools enable the identification of the exact mode of interaction between model pulmonary surfactant components. Two nanosecond long simulations of the N-terminal region of human surfactant protein-B (SP-B(1-25)) in dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) monolayers of different lipid surface densities reveal the preferential affinity of SP-B(1-25) for anionic phospholipids. In particular, arginine 12 and lysine 24 interact strongly and with high specificity with the phosphate group of the DPPG lipids, stabilizing the position, the orientation, and the secondary structure of the peptide in the monolayer. The peptide lies at an oblique angle to the interfacial plane, ranging between 47 degrees and 62 degrees, increasing with decreasing lipid surface density. In DPPC monolayers the interaction is largely determined by hydrophobic interactions. The non-specific nature of DPPC-SP-B(1-25) interactions allows for significant flexibility in the topology of the peptide in the lipid matrix. Bioinformatics tools are employed to generalize the simulation results to the sequences of SP-B(1-25) in other organisms. The importance of specific residues, and the role of the largely helical and amphiphilic nature of the peptide in the functionality of SP-B(1-25) are established. The synergy of classical mechanics tools with bioinformatics methods greatly enhances the molecular-level interpretation of pulmonary surfactant action and facilitates the development of design rules for synthetic surfactant analogues.

Palmitoylation and processing of the lipopeptide surfactant protein C

Pulmonary surfactant, a mixture of lipids and proteins, reduces the surface tension at the air-water interface of the lung alveoli by forming a surface active film. This way, it prevents alveoli from collapsing and facilitates the work of breathing. Surfactant protein C (SP-C) plays an important role in this surfactant function. SP-C is expressed as a proprotein (proSP-C), which becomes posttranslationally modified with palmitate and undergoes several rounds of proteolytical cleavage. This results in the formation of mature SP-C, which is stored in the lamellar bodies (LB) and finally secreted into the alveolar space. Recently, new insights into the sorting, processing and palmitoylation of proSP-C have been obtained by mutagenesis studies. Moreover, reports on the association of development of lung disease with SP-C deficiency have led to new insights into the importance of SP-C for proper surfactant homeostasis. In addition, new information has become available on the role of the palmitoyl chains of SP-C in surface activity. This review summarizes these recent developments in the processing and function of SP-C, with particular emphasis on the signals for and role of palmitoylation of SP-C.

Pulmonary surfactant: phase behavior and function

Pulmonary surfactant functions by first flowing rapidly into the alveolar air/water interface, but then resisting collapse from the surface when the adsorbed interfacial film is compressed during exhalation. Widely accepted models emphasize the importance of phase behavior in both processes. Recent studies show, however, that fluidity is a relatively minor determinant of adsorption and that solid films, which resist collapse, can form by kinetic processes unrelated to equilibrium phase behavior.

Bulk shear viscosities of endogenous and exogenous lung surfactants

Bulk shear viscosities were measured with a cone and plate microviscometer as a function of concentration, shear rate, and temperature for lavaged calf lung surfactant (LS), Exosurf, Infasurf, Survanta, and synthetic lipid mixtures dispersed in normal saline. Viscosity increased with phospholipid concentration for all surfactants, but its magnitude and shear dependence varied widely among the different preparations. Saline dispersions of Exosurf and synthetic phospholipids had low viscosities of only a few centipoise (cp) and exhibited minimal shear dependence. LS, Infasurf, Survanta, and lipid mixtures containing palmitic acid and tripalmitin had larger non-Newtonian viscosities that increased as shear rate decreased. At 35 mg of phospholipid/ml and 37 degrees C, viscosity values were 52.3 cp (Survanta), 31.1 cp (LS), and 25 cp (Infasurf) at a shear rate of 77 s(-1) and 16.9 cp (Survanta), 10.1 cp (LS), and 6.6 cp (Infasurf) at 770 s(-1). At 25 mg of phospholipid/ml and 37 degrees C, viscosity values at 77 s(-1) were 28.8 cp (Survanta), 4.7 cp (LS), and 12.5 cp (Infasurf). At fixed shear rate, viscosity was substantially decreased at 23 degrees C compared with 37 degrees C for LS and Infasurf but was increased for Survanta. Calcium (5 mM) greatly reduced the viscosity of both Survanta and Infasurf at 37 degrees C. Studies on synthetic mixtures indicated that phospholipid/apoprotein interactions were important in the rheology of lung-derived surfactants and that palmitic acid and tripalmitin contributed to the increased viscosity of Survanta. The viscous behavior of clinical exogenous surfactants potentially influences their delivery and distribution in lungs and varies significantly with composition, concentration, temperature, ionic environment, and physical formulation.

Component-specific surface and physiological activity in bovine-derived lung surfactants

Composition, surface activity and effects on pressure-volume (P-V) mechanics are examined for lavaged calf lung surfactant (LS) and the clinical exogenous surfactants Infasurf and Survanta. Lavaged LS and Infasurf had closely-matching compositions of phospholipids and neutral lipids. Survanta had higher levels of free fatty acids and triglycerides consistent with its content of added synthetic palmitic acid and tripalmitin. Infasurf and Survanta both contained less total protein than LS because of extraction with hydrophobic solvents, but the total protein content relative to phospholipid in Survanta was about 45% lower than in Infasurf. This difference was primarily due to surfactant protein (SP)-B, which was present by ELISA at a mean weight percent relative to phospholipid of 1.04% in LS, 0.90% in Infasurf, and 0.044% in Survanta. Studies on component fractions separated by gel permeation chromatography showed that SP-B was a major contributor to the adsorption, dynamic surface activity, and P-V mechanical effects of Infasurf, which approached whole LS in magnitude. Survanta had lower adsorption, higher minimum surface tension, and a smaller effect on surfactant-deficient P-V mechanics consistent with minimal contributions from SP-B. Addition of 0.05% by weight of purified bovine SP-B to Survanta did not improve surface or physiological activity, but added 0.7% SP-B improved adsorption, dynamic surface tension lowering, and P-V activity to levels similar to Infasurf. The SP-B content of lung surfactants appears to be a crucial factor in their surface activity and efficacy in improving surfactant-deficient pulmonary P-V mechanics.

Calfactant

Surfactant administration has proven remarkably effective in the prevention and treatment of infantile respiratory distress syndrome (IRDS) and may also be beneficial in other forms of acute lung injury. Several surfactant products are available commercially along with others in various phases of development and clinical trials. While all of these products share an ability to lower surface tension in vitro, there are substantial compositional differences that appear to affect their in vivo efficacy. At present, the 'modified natural' surfactants containing the hydrophobic surfactant proteins SP-B and SP-C appear most effective. Calfactant may have a particular advantage because of its relatively high content of SP-B and its lack of contamination with non-surfactant lipids and proteins.

Thermodynamic effects of the hydrophobic surfactant proteins on the early adsorption of pulmonary surfactant

We determined the influence of the two hydrophobic proteins, SP-B and SP-C, on the thermodynamic barriers that limit adsorption of pulmonary surfactant to the air-water interface. We compared the temperature and concentration dependence of adsorption, measured by monitoring surface tension, between calf lung surfactant extract (CLSE) and the complete set of neutral and phospholipids (N&PL) without the proteins. Three stages generally characterized the various adsorption isotherms: an initial delay during which surface tension remained constant, a fall in surface tension at decreasing rates, and, for experiments that reached approximately 40 mN/m, a late acceleration of the fall in surface tension to approximately 25 mN/m. For the initial change in surface tension, the surfactant proteins accelerated adsorption for CLSE relative to N&PL by more than ten-fold, reducing the Gibbs free energy of transition (DeltaG(O)) from 119 to 112 kJ/mole. For the lipids alone in N&PL, the enthalpy of transition (DeltaH(O), 54 kJ/mole) and entropy (-T. DeltaS, 65 kJ/mole at 37 degrees C) made roughly equal contributions to DeltaG(O). The proteins in CLSE had little effect on -T. DeltaS(O) (68 kJ/mole), but lowered DeltaG(O) for CLSE by reducing DeltaH(O) (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorption must meet these thermodynamic constraints.

Synchrotron X-ray study of lung surfactant-specific protein SP-B in lipid monolayers

This work reports the first x-ray scattering measurements to determine the effects of SP-B(1-25), the N-terminus peptide of lung surfactant-specific protein SP-B, on the structure of palmitic acid (PA) monolayers. In-plane diffraction shows that the peptide fluidizes a portion of the monolayer but does not affect the packing of the residual ordered phase. This implies that the peptide resides in the disordered phase, and that the ordered phase is essentially pure lipid, in agreement with fluorescence microscopy studies. X-ray reflectivity shows that the peptide is oriented in the lipid monolayer at an angle of approximately 56 degrees relative to the interface normal, with one end protruding past the hydrophilic region into the fluid subphase and the other end embedded in the hydrophobic region of the monolayer. The quantitative insights afforded by this study lead to a better understanding of the lipid/protein interactions found in lung surfactant systems.

Surfactant protein interactions with neutral and acidic phospholipid films

The captive bubble tensiometer was employed to study interactions of phospholipid (PL) mixtures of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) at 50 microg/ml with physiological levels of the surfactant protein (SP) A SP-B, and SP-C alone and in combination at 37 degrees C. All surfactant proteins enhanced lipid adsorption to equilibrium surface tension (gamma), with SP-C being most effective. Kinetics were consistent with the presence of two adsorption phases. Under the conditions employed, SP-A did not affect the rate of film formation in the presence of SP-B or SP-C. Little difference in gamma(min) was observed between the acidic POPG and the neutral POPC systems with SP-B or SP-C with and without SP-A. However, gamma(max) was lower with the acidic POPG system during dynamic, but not during quasi-static, cycling. Considerably lower compression ratios were required to generate low gamma(min) values with SP-B than SP-C. DPPC-POPG-SP-B was superior to the neutral POPC-SP-B system. Although SP-A had little effect on film formation with SP-B, surface activity during compression was enhanced with both PL systems. In the presence of SP-C, lower compression ratios were required with the acidic system, and with this mixture, SP-A addition adversely affected surface activity. The results suggest specific interactions between SP-B and phosphatidylglycerol, and between SP-B and SP-A. These observations are consistent with the presence of a surface-associated surfactant reservoir which is involved in generating low gamma during film compression and lipid respreading during film expansion.