Activity and Inhibition Resistance of a Phospholipase-Resistant Synthetic Surfactant in Rat Lungs

Abstract

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 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 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 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 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 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 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 1 apoprotein/peptide constituents for use in treating direct pulmonary forms of clinical acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS).

The Future of Exogenous Surfactant Therapy

“…In addition, several synthetic exogenous surfactants are under clinical study and laboratory research. [16][17][18][19][20][21][22][23][24] …”

“…One particularly active synthetic lipid analog of this kind is DEPN-8, a phospholipase-resistant diether lipid developed by Notter and co-workers. 19,21,23,46,47 Synthetic surfactants containing DEPN-8 or other phospholipase-resistant lipids plus active SP-B peptides have the potential for particular utility in ALI/ ARDS, 19,21,23,[48][49][50] where these lytic enzymes can be elaborated in high concentrations during the inflammatory response in injured lungs. [51][52][53][54][55][56][57] Regardless of category (animal-derived or synthetic), the requirements for an effective therapeutic surfactant in ALI/ARDS are more stringent than is the case for RDS.…”

“…• Alterations in alveolar surfactant aggregates, whereby the most active large aggregate forms of surfactant are reduced in activity and/or percent content, while less active small aggregate forms of surfactant become more prevalent 72,80,[85][86][87][88][89][90][91][92][93] • Biophysical inhibition and/or chemical alteration of components in the alveolar surfactant film induced by cell membrane lipids, 75,79,83,[94][95][96] meconium, 97 or other substances present during the innate pulmonary inflammatory response, such as proteases, 98 phospholipases, 23,99,100 or reactive oxygen/nitrogen species 84,[101][102][103] • Altered synthesis, secretion, or composition of active surfactant due to injury-induced changes in alveolar type II pneumocytes, which are the primary cells of lung surfactant metabolism [104][105][106][107] All of the above mechanistic pathways of surfactant dysfunction are potentially present in the complex pathology of ALI/ARDS lung injury, and abnormalities in surfactant activity, large aggregate content, or composition have been well documented in bronchoalveolar lavage from patients with ALI/ARDS. 92,93,[108][109][110][111][112][113] Regardless of mechanism, the practical consequences of surfactant dysfunction in ALI/ARDS are not dissimilar to those in RDS.…”

Surfactant for Pediatric Acute Lung Injury

“…Ester-linked glycerophospholipids of the kind found in native surfactant are the primary lipids used in current synthetic exogenous surfactants. However, synthetic ether-linked lipids are now available that have direct structural analogy to lung surfactant lipids plus designed molecular behavior that enhances adsorption and spreading while maintaining very high dynamic surface tension lowering in surface films (e.g., [71,[205][206][207][208][209][210][211]). Moreover, such lipids have structural features making them resistant to phospholipases such as phospholipase A 2 , which has been implicated in the pathology of ALI/ARDS [65,[212][213][214][215][216][217][218].…”

“…Phospholipase-induced degradation of lung surfactant glycerophospholipids not only reduces the concentration of active components, but also generates reaction products such as lysophosphatidylcholine and fluid free fatty acids that can further decrease surface activity by interacting biophysically with remaining surfactant at the alveolar interface [55,59,70]. Synthetic exogenous surfactants containing such phosphonolipids combined with purified surfactant proteins or synthetic peptides have very high overall surface activity plus an ability to resist phospholipases in ALI/ARDS [71,[208][209][210][211]219]. On-going basic research is continuing to design and synthesize peptides related to SP-B and other surfactant proteins for use in highly-active fully-synthetic exogenous surfactants in combination with either normal glycerophospholipids or phospholipase-resistant analogs.…”