LIPOSOME-INCORPORATED DEXAMETHASONE PALMITATE - CHEMICAL AND PHYSICAL-PROPERTIES

Dexamethasone palmitate (DMP) has been prepared by esterification. This molecule and its isotopic marker have been encapsulated into 20 mg multilamellar liposomes (MLV) and small unilamellar vesicles (SUV) made of egg phosphatidylcholine (egg-PC) alone or of egg-PC and cholesterol (CH) in molar ratios of 4:3 and 4:6. Amounts of DMP ranging from 0.1 to 3000 mug/ml were added to the lipid film. Liposomes were formed in 1 ml phosphate-buffered saline, pH 7.4, 300 mOsm/kg H2O. MLV were first isolated by centrifugation. This method proved to be inappropriate, since DMP not only was encapsulated into MLV but also formed micellar structures and aggregates which could not be separated from MLV by centrifugation nor by filtration through a glass microfiber filter. An alternative rapid method was developed, attempting to isolate unencapsulated DMP complexes by minicolumn centrifugation from liposomal suspensions. This method was successfully applied to DMP associated SUV. The encapsulation efficiency was 65% for 4:3 SUV. This fell to 50% for a DMP concentration of 3000 mug/ml. This should be related to a maximal DMP encapsulation of 13 mol% (Fildes and Oliver, J. Pharm. Pharmacol., 30 (1978) 337-342) which corresponds to 2600 mug/ml under our experimental conditions. The 75% optimum efficiency observed for liposomes made of egg-PC alone suggested that DMP could replace CH in these vesicles. Conformational analysis indicated that DMP inserted into monolayers with its carbonyl group oriented towards the aqueous surface while the aliphatic chain became aligned parallel with the acyl chains of phospholipids. The consideration of the incorporation of DMP into an assembly of phospholipids as taking place in a manner similar to that of CH would appear to be reasonable. This reinforces the validity of the hypothesis of CH being replaced by DMP in liposomes composed of egg PC alone.