Development of an in Vitro Drug Release Assay That Accurately Predicts in Vivo Drug Retention for Liposome-based Delivery Systems

The therapeutic activity of numerous drugs can be dramatically improved by liposomal encapsulation. However, this requires that liposomes retain their encapsulated drugs following systemic administration. Often, in vitro drug release assays do not accurately predict the liposomal drug retention properties observed in vivo. We postulate that this discrepancy is due to the large membrane pool present in blood cells and tissues, into which drugs can distribute after in vivo administration. Herein we describe an in vitro drug release assay that more accurately predicts in vivo drug release from liposomes following systemic administration. Drug-encapsulated large unilamellar vesicles (LUVs) approximately 100 nm in diameter were incubated with a 100-fold excess of multilamellar vesicles (MLVs) containing 300 mM sucrose, which served as 'acceptors' for drug release and transfer from 'donor' LUVs. Following incubation at 37 degrees C, the donor and acceptor populations were separated with greater than 90% efficiency by centrifugation at 1600xg for 10 min. The amount of drug in the MLV pellet reflects the degree of drug leakage from the donor liposomes. Drug release profiles using this in vitro assay were compared to those obtained using dialysis-based assays and in vivo results following systemic administration to mice. Our results indicate that this release assay is a better predictor of in vivo drug transfer than dialysis-based systems. We also demonstrate its utility in measuring exchange of lipophilic components.