A simple in vitro model to study the release kinetics of liposome encapsulated material

A simple in vitro model was developed to study the release kinetics of liposome encapsulated material in the presence of biologic components. Liposomes were embedded in an agarose gel (bottom layer) formed in a glass vial and separated from the receptor compartment buffer by a second layer of agarose gel (top layer). To follow the release of liposomal contents, aqueous space markers differing in molecular weight (from 205 Dalton to 17500 Dalton) were encapsulated. The isotonic buffer in the receptor was completely changed at various time points and the amount of marker released from the agarose matrix containing the liposomes into the receptor medium determined. The release of non-encapsulated markers from the gel followed a time0.5 relationship with about 75% of a 17500 Dalton protein being released from the matrix in 48 h. In the same period, about 7% of the intact liposomes added to the agarose gel appeared in the receptor phase. The release of calcein from various liposome compositions including: (A) egg phosphatidylcholine (EPC)/egg phosphatidylglycerol (EPG) 9:1, (B) dioleoylphosphatidylethanolamine (DOPE)/cholesterylhemisuccinate (CHEMS) 2:1, and (C) dioleoylphosphatidylglycerol (DOPC)/dioleoylphosphatidylglycerol (DOPG) 2:1 was measured. Components of the biological milieu such as serum proteins and calcium influenced release of encapsulated material. This in vitro model is a convenient and reproducible system that permits the study of the release of high molecular weight molecules such as proteins from liposomal formulations in the presence of serum. It may find applications with respect to release of proteins from a variety of colloidal drug delivery systems.