DELIVERY OF SUPEROXIDE-DISMUTASE TO PULMONARY EPITHELIUM VIA PH-SENSITIVE LIPOSOMES

Respiratory insufficiency, when treated with oxygen supplementation, o r exposure to diverse pulmonary toxins can cause lung damage as a resu lt of increased oxygen radical production, Enzymes such as superoxide dismutase (SOD) may attenuate this pathological process, but the intra cellular delivery and antioxidant action of SOD is impeded by its inab ility to cross cellular membranes. One approach for facilitating intra cellular delivery of macromolecules is to entrap SOD into liposomes. T he delivery of SOD to lung cells was accomplished using pH-sensitive l iposomes, made with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOP E) and 1-oleoyl-2-oleoyl-sn-glycero-3-succinate (DOSG), added to cultu red fetal rat lung distal epithelial (FRLE) cells. FRLE cells, obtaine d from fetuses at day 19 gestation, expressed a high-affinity receptor for surfactant protein A (SP-A) with an apparent dissociation constan t (K-d) = 3.6 +/- 0.2 mu g/ml (5.5 x 10(-9) M) and a capacity of 130 /- 3 ng/10(6) cells (125,000 +/- 3,000 binding sites/cell). This recep tor was utilized for targeting liposomes to cells, after incorporating SP-A during liposome membrane formation. Liposomes were uniformly sma ll (180 +/- 77 nm; mean +/- SD) and stable at 4 degrees C for 1 wk, en trapping 10 +/- 4% of initially added SOD. After incubation of PH-sens itive liposomes containing entrapped SOD with cultured FRLE cells, cel l-associated SOD activity was increased 5.1-fold from 7.8 +/- 2.5 to 4 0.1 +/- 3.3 U SOD/mg cell protein. Incorporation of SP-A into liposome s increased by 6.2-fold the delivery of liposomal SOD to cells. The ce ll uptake of SP-A-containing liposomes was significantly reduced by ad dition of type IV collagen during cell incubations, decreasing to leve ls similar to control incubations. We conclude that SP-A-containing pH -sensitive liposomes, composed of DOPE and DOSG with entrapped SOD, ef ficiently deliver SOD to pulmonary alveolar epithelial cells in part b y a receptor-mediated process.