Studies on the Mechanism of Uptake of Liposomes Using An Isolated Perfused Liver System

Species difference in correlation between in vivo/in vitro liposome-complement interactions

The objective of this study was to investigate the correlation between in vitro and in vivo liposome-complement interactions. Third component of the complement (C3) fragments associated with hydrogenated egg phosphatidylcholine (HEPC)-based liposomes in vivo and complement-dependent destabilization in vitro were determined as an indication of liposome-complement interaction in vivo and in vitro, respectively. C3 fragments on the liposomes were detected in both rats and guinea pigs. Pretreatment with K76COOH (K76), a complement inactivating agent, reduced the binding of C3 fragments. These findings indicated that the liposomes remarkably activated the complement system in both animals in vivo. Interestingly, significant complement-dependent liposome destabilization was observed in rat serum, but not in guinea pig serum, indicating that the liposomes activated the complement system in rats, but not in guinea pigs in vitro. Taken together, it is apparent that in vitro complement activation by the liposomes is not in agreement with in vivo complement activation in ginea pigs. This discrepancy in the liposome-complement interaction would suggest the need for further investigation to utilize the information obtained from the liposome-complement interaction to predict in vivo behavior of the liposomes.

The complement system enhances the clearance of phosphatidylserine (PS)-liposomes in rat and guinea pig

In this study, we investigated the contribution of the complement system to the biodistribution of phosphatidylserine (PS)-containing liposomes in rat and guinea pig. It appeared that the inclusion of PS in the liposome formulation accelerates the rate of liposome uptake by liver, resulting in rapid elimination of the liposomes from blood circulation. Pretreatment with K76COOH (K76), an anti-complement agent, decreased the rapid uptake of PS-containing liposomes by guinea pig liver, resulting in increasing blood concentration of the liposomes. Significant complement-dependent liposome destabilization was observed in vitro in both animals, whereas the complement-dependent destabilization in vivo was likely only a part of the process of the clearance of the PS-containing liposomes. This discrepancy suggests that the rate of complement-dependent liposome uptake by liver is much faster than the rate of complement-dependent liposome destabilization in vivo. Pretreatment of K76 dramatically inhibited the binding of C3 fragments, one of dominant opsonins, to PS-containing liposomes in guinea pig under both in vivo and in vitro conditions. This finding suggests that the C3 fragments in the system are responsible for the clearance of the PS-containing liposomes in guinea pig. In rat, in contrast to guinea pig, in vivo binding of C3 fragments was not inhibited by K76-pretreatment, while in vitro binding was inhibited. This discrepancy may be due to different experimental conditions between in vitro and in vivo assay. Nevertheless, based on the observations in this study, the complement components are most likely involved in the clearance of the PS-containing liposomes in rat. Taken together, the activity of PS in enhancing the liposome clearance appears to be mediated by the complement components, presumably C3 fragments, in both guinea pig and rat. This is a first report showing the mechanism on the hepatic uptake of the PS-containing liposomes in guinea pig.

Biodistribution of liposomes and C3 fragments associated with liposomes: evaluation of their relationship

The biodistribution of liposomes with two different kind phospholipids (hydrogenated egg phosphatidylcholine and egg phosphatidylcholine) plus cholesterol (CHOL) were investigated after intravenous administration to rats. Elimination of liposomes from blood circulation was affected by the lipid composition. It appeared that the inclusion of CHOL in liposomes accelerates the rate of liposome uptake by liver, resulting in rapid elimination of liposomes. The amount of C3 fragments bound to liposomes was quantitatively determined to assess the contribution of the complement system to liposome accumulation into organs and liposome destabilization in vivo and in vitro. The amount of bound C3 fragments was directly proportional to CHOL content, and the amount was also proportional to the CLh, CLs as well as CLrel. This relationship suggests that the complement system is responsible for the elimination of liposomes from blood circulation, presumably as a consequence of opsonization by C3 fragments and assembly of membrane attack complex (MAC) onto liposomes. In addition, substitution of cholesteryl methyl ether into the liposome formulation for CHOL significantly diminished not only the binding of C3 fragments but also the CLh, CLs and CLrel, resulting in increased mean resident time (MRT) of the liposomes. This result suggests that the hydroxyl-group on CHOL is a binding site for C3 fragments on the liposomes and that CHOL in a liposome formulation promotes the accumulation of liposomes into the liver and spleen, probably due to their uptake by phagocytic cells, and impairs the stability of the liposomes in blood circulation, via a mechanism involving the complement system.

Identification of proteins mediating clearance of liposomes using a liver perfusion system

The objective of this paper is to identify the principal blood components governing the fate of liposomes in blood circulation. Information based on an isolated perfused liver system in rats has revealed the central role of the complement system in enhancing the uptake of liposomes by the liver. A species difference was an important factor in determining the uptake mechanisms of liposomes by the liver. Limited evidence revealed the tendency that opsonin-dependent hepatic uptake is principal in rats, while opsonin-independent or dysopsonin-dependent uptake governs in mice, although there are some exceptions. These studies provide us with important information for understanding the uptake mechanisms of liposomes by the liver, and useful insights in predicting the in vivo disposition of liposomes in humans.