Comparison of surface properties and physiological effects of a synthetic and a natural surfactant in preterm rabbits

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.

The role of exogenous surfactant in the treatment of acute lung injury

A number of conditions, such as pneumonia, trauma, or systemic sepsis arising from the gut, may result in the acute respiratory distress syndrome (ARDS). Because of its significant morbidity and mortality, ARDS has been the focus of extensive research. One specific area of interest has been the investigation of the role of the surfactant system in the pathophysiology of this disease. Several studies have demonstrated that alterations of surfactant contribute to the lung dysfunction associated with ARDS, which has led to investigations into the use of exogenous surfactant as a therapy for this syndrome. Clinical experience with surfactant therapy has been variable owing to a number of factors including the nature of the injury at the time of treatment, the specific surfactant preparation utilized, the dose and delivery method chosen, the timing of surfactant administration over the course of the disease, and the mode of ventilation used during and after surfactant administration.

Bronchoalveolar lavage surfactant protein a, B, and d concentrations in preterm infants ventilated for respiratory distress syndrome receiving natural and synthetic surfactants

Surfactant proteins (SPs) play an important role in surfactant metabolism and function. Understanding their relative contribution to clinical outcome remains incomplete. Exogenous surfactants differ in their SP content and physiologic effects. The aims of this study were to measure bronchoalveolar lavage (BAL) SP concentrations from preterm infants ventilated for respiratory distress syndrome and to assess their association with clinical outcome. Fifty preterm infants randomized to receive a natural or synthetic surfactant were lavaged each day for the first week and twice weekly thereafter using a standardized nonbronchoscopic technique. BAL SP-A, SP-B, and SP-D concentrations were measured using ELISA. Median BAL SP-A, SP-B, and SP-D concentrations for the whole cohort rose significantly during the first postnatal week (p < 0.05). SP-A concentration did not differ between outcome groups. BAL SP-B concentration rose significantly in lungs that were not supplemented with SP-B. Infants dying had significantly lower BAL SP-B concentrations on d 2 and 6 compared with survivors. BAL SP-D concentrations were significantly lower on d 2 and 3 among infants in supplemental oxygen on d 28 compared with those in air. BAL SP-A and SP-D concentrations did not differ significantly between infants randomized to receive a natural or synthetic surfactant. Lower BAL SP-B and SP-D but not SP-A concentrations were associated with worse clinical prognosis.

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.

Detectable IL-8 and IL-10 in bronchoalveolar lavage fluid from preterm infants ventilated for respiratory distress syndrome

Pro-inflammatory cytokines such as IL-8 play an important role in the inflammatory response to neonatal airway injury. Difficulty in detecting counter-regulatory cytokines such as IL-10 in lavage fluid from preterm infants led to the suggestion that its deficit may be a factor in the etiology of chronic lung disease of prematurity (CLD). The aim of the study was to determine IL-8 and IL-10 concentrations in lavage fluid from preterm infants ventilated for respiratory distress syndrome. Fifty infants <30 wk gestation were studied who had been randomized to receive a natural or synthetic surfactant. Lavage samples were collected daily for the first week and twice weekly thereafter. Samples were immediately centrifuged and stored at -70 degrees C. Cytokine concentrations were quantified in duplicate using commercially available sandwich ELISA kits. Lavage IL-10 concentration, at a minimum initially, rose significant over the first five postnatal days (p = 0.009). In the same samples, lavage IL-8 concentrations rose significantly over the first postnatal week (p < 0.001), the rise preceding that of IL-10. Infants dying or developing CLD had a significant early rise in both cytokine concentrations. Compared with infants developing CLD, lavage IL-10 concentrations were significantly higher on d 1 among those not developing CLD but significantly lower on d 4 (p < 0.05). To conclude, IL-10 is detectable in lavage fluid from ventilated preterm infants and its concentrations rise significantly over the first five postnatal days. In the same samples, IL-8 concentration also rises and this increase precedes the rise in IL-10.

SP-A-enriched surfactant for treatment of rat lung transplants with SP-A deficiency after storage and reperfusion

BACKGROUND

The function of pulmonary surfactant is affected by lung transplantation, contributing to impaired lung transplant function. A decreased amount of surfactant protein-A (SP-A) after reperfusion is believed to contribute to the impaired surfactant function. Surfactant treatment has been shown to improve lung transplant function, but the effect is variable. We investigated whether SP-A enrichment of surfactant improved the efficacy of surfactant treatment in lung transplantation.

METHODS

Left and right lungs of Lewis rats, inflated with 50% O2, were stored for 20 hr at 8 degrees C. Surfactant in bronchoalveolar lavage fluid from right lungs was investigated after storage (n=6). Left lungs were transplanted into syngeneic recipients and treated with SP-A-deficient surfactant (n=6) or SP-A-enriched surfactant (n=6) just before reperfusion. Air was instilled into untreated lung transplants (n=6). Sham operated (n=4) and normal (n=8) animals served as controls. Lung function was measured during 1 hr of reperfusion; surfactant components in bronchoalveolar lavage fluid were measured after reperfusion.

RESULTS

After storage the amount of SP-A decreased by 27%, whereas surfactant phospholipids changed minimally. After reperfusion a further decrease of SP-A was paralleled by profound changes in surfactant phospholipids. Lung transplant function, however, remained relatively good. After instillation of SP-A-enriched surfactant, PO2 values were reached that approximated sham control PO2 values, whereas after SP-A-deficient surfactant treatment, the PO2 values did not improve.

CONCLUSION

Enrichment of surfactant with SP-A for treatment of lung transplants improves the efficacy of surfactant treatment.

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.

Animal-derived or synthetic surfactant for the treatment of neonatal respiratory distress syndrome: a review

Animal-derived surfactants containing SP-B and SP-C are more effective in vitro and in animal models than their synthetic counterparts, but are not as effective as unmodified, naturally occurring surfactant. In clinical trials involving newborn babies with respiratory distress syndrome (RDS) these short-term differences are reflected as improvements in gas exchange and lung function. Treatment with animal-derived surfactants results in fewer air leaks and lower neonatal mortality. The evidence is now strong enough to recommend routine use of animal-derived surfactants in very preterm infants with RDS. The newer generation of synthetic surfactants may be important in the future as they have the advantages of currently available animal products with the addition of better resistance to inactivation.

Ultrastructure of exogenous surfactants using cryogenic scanning electron microscopy

Therapy with specialised biomaterials, exogenous surfactants, is known to significantly decrease the mortality rates in Respiratory Distress Syndrome (RDS). Surfactants available commercially vary widely in composition and biophysical properties. The present paper studies the ultrastructure of three exogenous surfactants used for the treatment of Respiratory Distress Syndrome, namely, Survanta, ALEC and Exosurf Neonatal with respect to their ability to form liposomes using cryogenic scanning electron microscopy. Liposomal organisation is more obvious in Exosurf than in Survanta and is most pronounced in ALEC. ALEC forms closed regular liposomes with an onion-ring-like internal bilayer arrangement. Survanta forms open membranous structures with wavy ribbon-like membranes. The complex membrane-like structures seen with Survanta may be due to the interaction of lipids with surfactant-specific proteins present in this surfactant which is derived from natural lung extracts and might indicate superior spreading at the lipid-water interface. Artificial protein-free surfactants (ALEC and Exosurf) did not appear to form these open membranous structures. Further study of the ultrastructure of possible biomaterials as surfactants could help in the development of new, improved artificial protein-free surfactants with open membranous structures that might facilitate spreading at the air-liquid interface of lungs.

Analysis of dynamic surface properties of therapeutic surfactants and lung phospholipids

Exogenous surfactant is a specialized biomaterial used for substitution of the lipoprotein mixture normally present in lungs--pulmonary surfactant. Respiratory Distress Syndrome is a disease of preterm infants mainly caused by a deficiency of mature lung surfactant. Pulmonary surfactant is known to stabilize small alveoli and prevent them from collapsing during expiration due to its unique surface properties. A pulsating bubble surfactometer was used for in vitro analysis of surface parameters of therapeutic surfactants and of test formulations to be used for exogenous therapy in Respiratory Distress Syndrome. Surface parameters that were considered for comparison were minimum surface tension (gamma(min)) at three different frequencies (20, 40 and 60 cpm), adsorption at two extreme bubble radii (Rmin and Rmax), stability index at the three frequencies, recruitment index and the surface viscoelastic parameters. Survanta, ALEC and Exosurf were compared with formulations consisting of the main phospholipids of pulmonary surfactant, namely dipalmitoyl phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) as well as binary mixtures of these phospholipids in the ratio 2:3. Survanta performed much better than the non-protein therapeutic surfactants in all parameters and at all three frequencies. Exosurf had a very low stability index and a very low modulus of surface dilatational elasticity at all three frequencies. The test compounds showed a frequency dependence in their performance. At 20 cpm, PC:PG (2:3) was the best test combination. It achieved a gamma(min) and stability index equivalent to Survanta at this frequency. None of the test compounds were comparable to Survanta at 40 and 60 cpm. These findings may have important therapeutic implications for exogenous surfactants.

Commercial versus native surfactants. Surface activity, molecular components, and the effect of calcium

Despite their broad clinical use, there is no standardized comparative study on the functional, biochemical, and morphologic differences of the various commercial surfactants in relation to native surfactant. We investigated these parameters in Alveofact, Curosurf, Exosurf, and Survanta, and compared them with native bovine (NBS) and porcine (NPS) surfactant. For Curosurf and Alveofact the concentrations necessary for minimal surface tensions < 5 mN/m were six to 12 times higher (1.5 and 3 mg/ml, respectively) than with NPS and NBS. Exosurf and Survanta only reached 22 and 8 mN/m, respectively. Increasing calcium to nonphysiologic concentrations artificially improved the function of Alveofact and Curosurf, but it had little effect on Exosurf and Survanta. Impaired surface activity of commercial versus native surfactants corresponded with their lack in surfactant protein SP-A and decreased SP-B/C. The higher surface activity of Curosurf compared with Alveofact corresponded with its higher concentration of dipalmitoylphosphatidylcholine (DPPC). Despite their enrichment in DPPC Survanta and Exosurf exhibited poor surface activity because of low or absent SP-B/C. Ultrastructurally, Curosurf and Alveofact consisted mainly of lamellar and vesicular structures, which were also present in NPS and NBS. Exosurf contained crystalline structures only, whereas the DPPC-enriched Survanta contained separate lamellar/vesicular and crystalline structures. We conclude that in vitro surface activity of commercial surfactants is impaired compared with native surfactants at physiologic calcium concentrations. In the presence of SP-B/C, surface activity corresponds to the concentration of DPPC. Our data underscore the importance of a standardized protocol at physiologic calcium concentrations for the in vitro assessment of commercial surfactants.

Aerosolized surfactant in lung-lavaged adult rats: factors influencing the therapeutic response

OBJECTIVE

To evaluate the effect of aerosolized modified natural surfactant in adult rats with respiratory failure.

METHODS

Lung-lavaged adult rats were treated with aerosolized surfactant, aerosolized saline or a bolus of surfactant. Surfactant was labelled with dimyristoylphosphatidylcholine (DMPC) and human serum albumin was given intravenously for evaluation of lung protein leakage. Blood gases and dynamic compliance were measured intermittently. At the end of ventilation, the lungs were either fixed by vascular perfusion for histological examination or washed for determination of total phospholipids, DMPC and human albumin in the lavage fluid.

RESULTS

Treatment with bolus surfactant led to a quick and sustained restoration of pre-lavage blood gas values in most animals. The response to aerosolized surfactant varied considerably, with an overall moderate improvement of gas exchange. The saline-treated group failed to show any significant recovery of lung function. No histopathological differences were found between any of the groups. On average 0.46% of total administered aerosolized surfactant could be recovered. Vascular-to-alveolar leakage of human albumin averaged 11%, with no significant differences between the groups. Final values for PaO2 were significantly correlated with total phospholipids in the lavage fluid, and inversely related to the vascular-to-alveolar leakage of albumin.

CONCLUSION

Neither bolus nor aerosolized surfactant influenced lung morphology. Nebulized surfactant improved lung function but the effect was inferior to that obtained with bolus surfactant, and the outcome depended on the balance between the combined pool size of exogenous and endogenous surfactant and the vascular-to-alveolar leakage of serum protein.

Pumactant and poractant alfa for treatment of respiratory distress syndrome in neonates born at 25-29 weeks' gestation: a randomised trial

BACKGROUND

Exogenous surfactant preparations vary in their constitution and biophysical properties. Synthetic and animal-derived preparations lower the rate of death compared with controls. No significant differences in mortality or important long-term clinical outcomes have been shown between them in randomised trials. We did a randomised controlled trial to compare pumactant, a synthetic surfactant, with poractant alfa, an animal-derived surfactant, both of which are widely used in the UK.

METHODS

We enrolled 212 neonates born between 25 weeks' and 29 weeks and 6 days' gestation who were intubated for presumed surfactant deficiency and were free from life-threatening malformations. We randomly assigned 105 neonates poractant alfa, and 107 pumactant. The primary outcome was duration of high-dependency care and mortality was a secondary outcome. Analysis was by intention to treat.

FINDINGS

Outcome data were analysed for 199 babies. The trial was stopped on the recommendation of the data and safety monitoring committee because mortality assumed a greater importance than the primary outcome. Predischarge mortality differed significantly between groups, in favour of poractant alfa (14.1 vs 31.0%, p=0.006; odds ratio 0.37 [95% CI 0.18-0.76). This difference was sustained after adjustment for centre, gestation, birthweight, sex, plurality, and use of antenatal steroids.

INTERPRETATION

Mortality was unexpectedly lower among neonates who received poractant alfa than among those who received pumactant, and was independent of all the variables we investigated. Stopping the trial early may have widened the difference between the treatment groups.

Natural vs synthetic surfactants in neonatal respiratory distress syndrome

This review examines the 11 randomised clinical trials that have compared different surfactant preparations. Seven trials, enrolling 2488 infants with respiratory distress syndrome (RDS), compared the natural surfactant beractant (Survanta) with the synthetic surfactant colfosceril palmitate (Exosurf Neonatal). Infants treated with beractant had lower oxygen requirements for at least 3 days than those treated with colfosceril palmitate. The infants treated with beractant also had lower risks of neonatal mortality [odds ratio (OR) 0.81; 95% confidence interval (CI) 0.65 to 1.01], retinopathy of prematurity (OR 0.81; 95% CI 0.66 to 0.99), and the combined endpoint of death or bronchopulmonary dysplasia (OR 0.86; 95% CI 0.75 to 0.99), compared with those treated with colfosceril palmitate. Calf lung surfactant extract (CLSE; Infasurf), another natural surfactant, has been compared with colfosceril palmitate in 2 studies: in one as prophylaxis and in the other as rescue therapy. Similar, although nonsignificant, advantages were found for the natural surfactant compared with the synthetic surfactant. In 6 of these 9 trials there was a significant reduction in the odds of pulmonary air leaks (OR 0.53; 95% CI 0.41 to 0.64) for infants treated with natural compared with synthetic surfactants. In 7 trials (3554 infants) comparing natural and synthetic surfactants to treat RDS (6 comparing beractant and colfosceril palmitate, and one CLSE and colfosceril palmitate), there was a significantly reduced risk of neonatal mortality (OR 0.80; 95% CI 0.66 to 0.97) with natural compared with synthetic surfactant treatment. In 2 further trials, different natural surfactant preparations have been compared. Reduced oxygen needs for 24 hours after treatment were found for CLSE and Curosurf (porcine-derived lung surfactant, PLS) when each was compared with beractant. Apparent longer term benefits from these surfactants were not statistically proven. Further trials are needed to be certain of the differences between the various surfactant preparations.

[Natural or artificial surfactants? Arguments in favour of natural surfactants]

The use of exogenous surfactant (ES) is an essential component for prevention and treatment of hyaline membrane disease (HMD). The ES available for clinical use are of two therapeutic classes: natural surfactants prepared from mammalian lung and artificial surfactants. The choice between these two classes of ES is controversial. In this overview, we present the arguments in favour of the preferential use of natural ES. The presence of hydrophobic specific proteins (SP-B and SP-C) provides to natural ES better surface tension properties than artificial ES. The in vitro greater efficacy of natural ES has been confirmed in vivo in experimental models of surfactant deficiency, human pharmacodynamic studies, and comparative clinical trials. Furthermore, the excellent clinical tolerance and harmlessness of natural ES has been firmly established. A meta-analysis of the comparative clinical trials between natural ES and one artificial ES (enrolling as many as 4400 babies treated for HMD) suggests that the use of natural ES compared to this artificial ES significantly reduces the neonatal mortality by 20%. In conclusion, all these arguments are in favor of the preferential use of natural ES for prevention and treatment of HMD.