Effect of superoxide dismutase encapsulated in liposomes or conjugated with polyethylene glycol on neutrophil bactericidal activity in vitro and bacterial clearance in vivo.

Encapsulation in liposomes (LIP) or conjugation with polyethylene glycol (PEG) are methods being used to increase circulating half-life periods and/or improve delivery of antioxidant enzymes that could decrease lung injury induced by O2 metabolites. We found that pretreatment with LIP, with or without the antioxidant enzyme, superoxide dismutase (SOD), decreased killing of Staphylococcus aureus 502A by neutrophils in vitro and clearance of intravenously injected S. aureus in rabbits in vivo. In contrast, pretreatment with PEG (with or without SOD) had no effect on neutrophil bactericidal activity in vitro or bacterial clearance in vivo. Our results suggest that conjugation with PEG may be a better way than encapsulation in liposomes for delivering antioxidant enzymes, especially if one is concerned about preserving host-defense mechanisms.