Artificial surfactants based on analogues of SP-B and SP-C

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.

Biomimetic N-terminal alkylation of peptoid analogues of surfactant protein C

Surfactant protein C (SP-C) is a hydrophobic lipopeptide that is critical for lung function, in part because it physically catalyzes the formation of surface-associated surfactant reservoirs. Many of SP-C's key biophysical properties derive from its highly stable and hydrophobic α-helix. However, SP-C's posttranslational modification with N-terminal palmitoyl chains also seems to be quite important. We created a new (to our knowledge) class of variants of a synthetic, biomimetic family of peptide mimics (peptoids) that allow us to study the functional effects of biomimetic N-terminal alkylation in vitro. Mimics were designed to emulate the amphipathic patterning, helicity, and hydrophobicity of SP-C, and to include no, one, or two vicinal amide-linked, N-terminal octadecyl chains (providing a reach equivalent to that of natural palmitoyl chains). Pulsating bubble surfactometry and Langmuir-Wilhelmy surface balance studies showed that alkylation improved biomimetic surface activities, yielding lower film compressibility and lower maximum dynamic surface tensions. Atomic force microscopy studies indicated that alkyl chains bind to and retain segregated interfacial surfactant phases at low surface tensions by inducing 3D structural transitions in the monolayer's fluid-like phase, forming surfactant-associated reservoirs. Peptoid-based SP-C mimics are easily produced and purified, and offer much higher chemical and secondary structure stability than polypeptide-based mimics. In surfactant replacements intended for medical use, synthetic SP mimics reduce the odds of pathogen contamination, which may facilitate the wider use of surfactant treatment of respiratory disorders and diseases.

Biomimicry of surfactant protein C

Since the widespread use of exogenous lung surfactant to treat neonatal respiratory distress syndrome, premature infant survival and respiratory morbidity have dramatically improved. Despite the effectiveness of the animal-derived surfactant preparations, there still remain some concerns and difficulties associated with their use. This has prompted investigation into the creation of synthetic surfactant preparations. However, to date, no clinically used synthetic formulation is as effective as the natural material. This is largely because the previous synthetic formulations lacked analogues of the hydrophobic proteins of the lung surfactant system, SP-B and SP-C, which are critical functional constituents. As a result, recent investigation has turned toward the development of a new generation of synthetic, biomimetic surfactants that contain synthetic phospholipids along with a mimic of the hydrophobic protein portion of lung surfactant. In this Account, we detail our efforts in creating accurate mimics of SP-C for use in a synthetic surfactant replacement therapy. Despite SP-C's seemingly simple structure, the predominantly helical protein is extraordinarily challenging to work with given its extreme hydrophobicity and structural instability, which greatly complicates the creation of an effective SP-C analogue. Drawing inspiration from Nature, two promising biomimetic approaches have led to the creation of rationally designed biopolymers that recapitulate many of SP-C's molecular features. The first approach utilizes detailed SP-C structure-activity relationships and amino acid folding propensities to create a peptide-based analogue, SP-C33. In SP-C33, the problematic and metastable polyvaline helix is replaced with a structurally stable polyleucine helix and includes a well-placed positive charge to prevent aggregation. SP-C33 is structurally stable and eliminates the association propensity of the native protein. The second approach follows the same design considerations but makes use of a non-natural, poly-N-substituted glycine or "peptoid" scaffold to circumvent the difficulties associated with SP-C. By incorporating unique biomimetic side chains in a non-natural backbone, the peptoid mimic captures both SP-C's hydrophobic patterning and its helical secondary structure. Despite the differences in structure, both SP-C33 and the SP-C peptoid mimic capture many requisite features of SP-C. In a surfactant environment, these analogues also replicate many of the key surface activities necessary for a functional biomimetic surfactant therapy while overcoming the difficulties associated with the natural protein. With improved stability, greater production potential, and elimination of possible pathogenic contamination, these biomimetic surfactant formulations offer not only the potential to improve the treatment of respiratory distress syndrome but also the opportunity to treat other respiratory-related disorders.

A multicenter, randomized, controlled trial of lucinactant versus poractant alfa among very premature infants at high risk for respiratory distress syndrome

BACKGROUND

Available therapeutic surfactants are either animal-derived or non-protein-containing synthetic products. Animal-derived surfactants contain variable amounts of surfactant apoproteins, whereas the older-generation synthetic products contain only phospholipids and lack surfactant proteins (SPs). Both decrease morbidity and mortality rates associated with respiratory distress syndrome (RDS) among preterm infants, compared with placebo. However, excess mortality rates have been observed with non-protein-containing synthetic surfactants, compared with the animal-derived products. Evidence suggests that synthetic surfactants consisting solely of phospholipids can be improved with the addition of peptides that are functional analogs of SPs. Lucinactant is a new synthetic peptide-containing surfactant that contains sinapultide, a novel, 21-amino acid peptide (leucine and lysine repeating units, KL4 peptide) designed to mimic human SP-B. It is completely devoid of animal-derived components.

OBJECTIVE

We hypothesized that the outcomes for premature infants treated with lucinactant and poractant alfa would be similar. Therefore, we compared lucinactant (Surfaxin; Discovery Laboratories, Doylestown, PA) with porcine-derived, poractant alfa (Curosurf; Chiesi Farmaceutici, Parma, Italy) in a trial to test for noninferiority.

METHODS

A total of 252 infants born between 24 and 28 weeks of completed gestation, with birth weights between 600 and 1250 g, were assigned randomly in a multicenter, multinational, noninferiority, randomized, controlled study to receive either lucinactant (n = 124) or poractant alfa (n = 128) within 30 minutes of life. The primary outcome was the incidence of being alive without bronchopulmonary dysplasia (BPD) through 28 days of age. Key secondary outcomes included death at day 28 and 36 weeks postmenstrual age (PMA), air leaks, neuroimaging abnormalities, and other complications related to either prematurity or RDS. An independent, international, data and safety monitoring committee monitored the trial.

RESULTS

The treatment difference between lucinactant and poractant alfa for survival without BPD through 28 days was 4.75% (95% confidence interval [CI]: -7.3% to 16.8%) in favor of lucinactant, with the lower boundary of the 95% CI for the difference, ie, -7.3%, being greater than the prespecified noninferiority margin of -14.5%. At 28 days, 45 of 119 infants given lucinactant were alive without BPD (37.8%; 95% CI: 29.1-46.5%), compared with 41 of 124 given poractant alfa (33.1%; 95% CI: 24.8-41.3%); at 36 weeks PMA, the rates were 64.7% and 66.9%, respectively. The corresponding mortality rate through day 28 for the lucinactant group was lower than that for the poractant alfa group (11.8% [95% CI: 6.0-17.6%] vs 16.1% [95% CI: 9.7-22.6%]), as was the rate at 36 weeks PMA (16% and 18.5%, respectively). There were no differences in major dosing complications. In addition, no significant differences were observed in the incidences of common complications of prematurity, including intraventricular hemorrhage (grades 3 and 4) and cystic periventricular leukomalacia (lucinactant: 14.3%; poractant alfa: 16.9%).

CONCLUSIONS

Lucinactant and poractant alfa were similar in terms of efficacy and safety when used for the prevention and treatment of RDS among preterm infants. The ability to enhance the performance of a synthetic surfactant with the addition of a peptide that mimics the action of SP-B, such as sinapultide, brings potential advantages to exogenous surfactant therapy.

Effect of pulmonary surfactant protein SP-B on the micro- and nanostructure of phospholipid films

Monolayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC/dipalmitoylphosphatidylglycerol (DPPG) (7:3, w/w) in the absence or in the presence of 2, 5, 10, or 20 weight percent of porcine surfactant protein SP-B were spread at the air-liquid interface of a surface balance, compressed up to surface pressures in the liquid-expanded/liquid-condensed (LE-LC) plateau of the isotherm, transferred onto mica supports, and analyzed by scanning force microscopy. In the absence of protein, the films showed micrometer-sized condensed domains with morphology and size that were analogous to those observed in situ at the air-liquid interface by epifluorescence microscopy. Scanning force microscopy permits examination of the coexisting phases at a higher resolution than previously achieved with fluorescent microscopy. Both LE and LC regions of DPPC films were heterogeneous in nature. LC microdomains contained numerous expanded-like islands whereas regions apparently liquid-expanded were covered by a condensed-like framework of interconnected nanodomains. Presence of increasing amounts of pulmonary surfactant protein SP-B affected the distribution of the LE and LC regions of DPPC and DPPC/DPPG films both at the microscopic and the nanoscopic level. The condensed microdomains became more numerous but their size decreased, resulting in an overall reduction of the amount of total LC phase in both DPPC and DPPC/DPPG films. At the nanoscopic level, SP-B also caused a marked reduction of the size of the condensed-like nanodomains in the LE phase and an increase in the length of the LE/LC interface. SP-B promotes a fine nanoscopic framework of lipid and lipid-protein nanodomains that is associated with a substantial mechanical resistance to film deformation and rupture as observed during film transference and manipulation. The effect of SP-B on the nanoscopic structure of the lipid films was greater in DPPC/DPPG than in pure DPPC films, indicating additional contributions of electrostatic lipid-protein interactions. The alterations of the nanoscopic structures of phospholipid films by SP-B provide the structural framework for the protein simultaneously sustaining structural stability as well as dynamical flexibility in surfactant films at the extreme conditions imposed by the respiratory mechanics. SP-B also formed segregated two-dimensional clusters that were associated with the boundaries between LC microdomains and the LE regions of DPPC and DPPC/DPPG films. The presence of these clusters at protein-to-lipid proportions above 2% by weight suggests that the concentration of SP-B in the surfactant lipid-protein complexes may be close to the solubility limit of the protein in the lipid films.

Exogenous surfactant therapy: newer developments

There are numerous pulmonary conditions in which qualitative or quantitative anomalies of the surfactant system have been demonstrated. In premature newborns with immature lungs, a functional deficit in surfactant is the main physiopathologic mechanism of the neonatal respiratory distress syndrome (RDS). Since the landmark pilot study of Fujiwara, published more than 20 years ago, the efficacy of exogenous surfactant for the treatment of neonatal RDS has been established by numerous controlled studies and meta-analyses. Promising results have also been reported in infants suffering from other lung disorders in which endogenous surfactant function is compromised. Enlightened by a growing insight into both the structure and function of the different surfactant components, a new generation of synthetic surfactants has been developed. Various complementary approaches have confirmed the fundamental role of the two hydrophobic proteins, SP-B and SP-C, in the surfactant system, thus opening the way to the design of analogues, either by chemical synthesis or expression in a prokaryotic system. These peptide-containing synthetic surfactant preparations are presently undergoing clinical trials, and may eventually replace the animal-derived surfactants currently used for the treatment of RDS.

Location of structural transitions in an isotopically labeled lung surfactant SP-B peptide by IRRAS

Pulmonary surfactant, a lipid/protein complex that lines the air/water interface in the mammalian lung, functions to reduce the work of breathing. Surfactant protein B (SP-B) is a small, hydrophobic protein that is an essential component of this mixture. Structure-function relationships of SP-B are currently under investigation as the protein and its peptide analogs are being incorporated into surfactant replacement therapies. Knowledge of the structure of SP-B and its related peptides in bulk and monolayer phases will facilitate the design of later generation therapeutic agents. Prior infrared reflection-absorption spectroscopic studies reported notable, reversible surface pressure-induced antiparallel beta-sheet formation in a synthetic peptide derived from human SP-B, residues 9-36 (SP-B(9-36)). In the current work, infrared reflection-absorption spectroscopy is applied in conjunction with isotopic labeling to detect the site and pressure dependence of the conformational change. SP-B(9-36), synthesized with (13)C=O-labeled Ala residues in positions 26, 28, 30, and 32, shifted the beta-sheet marker band to approximately 1600 cm(-1) and thus immediately identified this structural element within the labeled region. Surface pressure-induced alterations in the relative intensities of Amide I band constituents are interpreted using a semiempirical transition dipole coupling model. In addition, electron micrographs reveal the formation of tubular myelin structures from in vitro preparations using SP-B(9-36) in place of porcine SP-B indicating that the peptide has the potential to mimic this property of the native protein.

Molecular dynamics simulations of a pulmonary surfactant protein B peptide in a lipid monolayer

Pulmonary surfactant is a complex mixture of lipids and proteins that lines the air/liquid interface of the alveolar hypophase and confers mechanical stability to the alveoli during the breathing process. The desire to formulate synthetic mixtures for low-cost prophylactic and therapeutic applications has motivated the study of the specific roles and interactions of the different components. All-atom molecular dynamics simulations were carried out on a model system composed of a monolayer of palmitic acid (PA) and a surfactant protein B peptide, SP-B(1-25). A detailed structural characterization as a function of the lipid monolayer specific area revealed that the peptide remains inserted in the monolayer up to values of specific area corresponding to an untilted condensed phase of the the pure palmitic acid monolayer. The system remains stable by altering the conformational order of both the anionic lipid monolayer and the peptide secondary structure. Two elements appear to be key for the constitution of this phase: an electrostatic interaction between the cationic peptide residues with the anionic headgroups, and an exclusion of the aromatic residues on the hydrophobic end of the peptide from the hydrophilic and aqueous regions.