Optimization of selected liposome-encapsulated diketopiperazines

Abstract

Response surface methodology (RSM) is an effective way to optimize various processes and formulations were investigators are able to determine how variables influence the formulation by assessing interaction effects in order to obtain an optimum formulation. Cyclic dipeptides have been shown to inhibit tumor cells but problems with their physicochemical characteristics have limited further development of these compounds clinically. The aim was to optimize a liposome-based drug delivery system incorporating cyclo(His-Gly) and cyclo(His-Ala) using response surface methodology. Liposomes, containing varying compositions of phosphatidylcholine, cholesterol, stearylamine, vitamin E and PEG2000-phosphatidylethanolamine were prepared according to the thin film hydration and extrusion method. Drugs were passively encapsulated and separated from free drug using Sephadex G50 columns. RSM, utilizing a central composite rotatable design (CCRD) was used to assess the influence of selected variables on encapsulation, uptake, membrane leakage, polydispersity and zeta potential. A quadratic model was used to fit data for all responses. The most significant interaction was between cholesterol and stearylamine content with cellular uptake, indicating that these parameters cannot be altered independently of each other. Optimization of the formulation yielded stable liposomes with an average encapsulation of 0.123 mg drug/mg lipid.