The human lung is composed of a large number of small sacs or alveoli in which gases are exchanged between the blood and the air spaces of the lung. In healthy individuals, this exchange is mediated by the presence of a protein containing surfactant complex which is synthesized in the microsomal membranes of type II alveolar cells. In the absence of adequate levels of this complex, a lung cannot properly function--i.e., the alveoli collapse during exhalation, and cannot be subsequently re-inflated by inhaling. Thus, the untreated inability to synthesize this complex may result in death or in severe physical damage.
The best documented instance of inadequate surfactant complex levels occurs in premature infants and infants born after complicated pregnancies, and is widely known as respiratory distress syndrome (RDS). A widely publicized form of this syndrome has been designated hyaline membrane disease, or idiopathic RDS. RDS is currently the leading cause of infant mortality and morbidity in the United States and in other developed countries, and substantial efforts have been directed to diagnosis and treatment. Current treatment has focused on mechanical (pressure) ventilation which, at best, is an invasive stop-gap measure that often results in damage to the lung and other deleterious side effects, including complications such as bronchopulmonary dysplasia, interstitial emphysema and pneumothorax. Mental retardation has also resulted on occasion when this treatment was used (Hallman, M., et al, Pediatric Clinics of North America (1982) 29: 1057-1075).
Limited attempts have been made to treat the syndrome by surfactant substitution. This would be a method of choice, as, in general, only one administration is required, and the potential for damage is reduced. For example, Fujwara, et al, Lancet (1980) 1: 55 -used a protein-depleted surfactant preparation derived from bovine lungs; the preparation is effective but immunogenic. Hallman, M., et al, Pediatrics (1983) 71: 473-482 used a surfactant isolate from human amniotic fluid to treat a limited number of infants with some success. U.S. Pat. No. 4,312,860 to Clements discloses an artificial surfactant which contains no protein and is said to be useful in this approach although no data are shown. In short, surfactant substitution has not been widely used clinically.
The preferred surfactant substitute would be the lung surfactant complex itself. This complex is composed of apoprotein, two phospholipids (dipalmitoyl phosphocholine (DPPC) and phosphatidyl-glycerol (PG)) which are present in major amount, several lipid components present in only very minor amount, and calcium ions. The apoprotein contains proteins having molecular weights of the order of 32,000 daltons and very hydrophobic proteins of the order of about 10,000 daltons (King, R. J. et al, Am J Physiol (1973) 224: 788-795). The 32,000 dalton protein is glycosylated and contains hydroxyproline.
A major reason for the limited progress in surfactant replacement therapy has been the lack of availability of the protein portion of the complex. Replacement therapies have focused on attempts to use the lipid components alone, and it appears that the performance of such treatment can be markedly improved by addition of the apoprotein (Hallman, M., et al, Pediatric Clinics of North America (1982) (supra)). At present, however, these proteins are available only from normal adult human lung, and from amniotic fluid. Even efficient isolation procedures would not provide an adequate supply. Thus, it would be desirable to have available a method for producing practical quantities of apoprotein for use alone or in conjunction with the saturated phospholipid portion of the complex.