Modulation of Nonisduced and Phorbol Ester-Induced Generation of Superoxide Anion by Free Liposomes and Liposomes Containing Dexamethasone.

We tested the relative efficacy of free dexamethasone, dexamethasone containing liposomes and free liposomes in preventing superoxide anion, O^-₂ generation by neutrophils. O^-₂ production by 5 ± 10⁵ neutrophils, whether primed or not with lipopolysacchaiide, was stimulated by phorbol 12-myristate 13-acetate (PMA) to 13.4 ± 1.3 nmoles after 15 minutes compared to 1.2 ± 0.3 nmoles with nonstimulated cells. Free liposomes but not dexamethasone (dexa) decreased non-stimulated as well as PMA-induced O^-₂ generation. Dexa-containing phosphatidylcholine from egg yolk: phosphatidylserine from bovine brain (POPS 7:3) liposomes, unlike free dexa, diminished PMA-stimulated O^-₂ production in a dose-dependent manner with a maximal effect at 37.5 μg/ml phospholipid (6.6 ± 1.6 nmoles). The kinetics of cytochrome-c reduction revealed that decreased O^-₂ production resulted from an extended lag-time of release to almost 8 minutes with PMA induction and consequently led to the conclusion that liposomes modified the activity of NADPH oxidase as well as that of protein kinase C. Liposomes prepared with PC and PS of natural origin had a greater inhibitory effect on O^-₂ generation by neutrophils than dipal-mitoylphosphatidylcholine (DPPC) and phosphatidylethanolamine from egg yolk (PE):PC (3:1) liposomes. When 100 μM of Ca²⁺ was added to the medium, the inhibitory action of liposomes prepared with egg yolk PC and DPPC was increased by 30 and 60% respectively, while that of PS and PE:PC was prevented. We also verified that liposomes by themselves, even if phagocytized, did not induce O^-₂ generation or its concentration was too low to be detected by this technique. From the clinical point of view, some formulations delayed non-induced and PMA-induced O^-₂ generation, thus adding to the anti-inflammatory effect of the glucocorticoid they transported.