Effect of Different Pretreatment Methods on the Measurement of the Entrapment Efficiency of Liposomes and Countermeasures in Free Drug Analysis

Further investigation into the methods for the analysis of the entrapment efficiency of liposomes has been prompted by the urgent need to improve traditional methods which offer more risk to liposome leakage. The aim of this study was to investigate the suitability of hollow fiber centrifugal ultrafiltration (HF-CF-UF) coupled with HPLC as an alternative method for the routine determination of the entrapment efficiency of liposomes and to compare it with previously developed nonequilibrium procedures based on size-exclusion chromatography (SEC). By comparison, evaluation of the entrapped fraction with SEC resulted in 3 or 4 % lower entrapment values. More importantly, the free drug concentrations were three- to sixfold higher than with HF-CF-UF. In addition, we investigated the reasons for the different values obtained with SEC and a dynamic equilibrium theory was put forward based on this. In addition, we have developed and characterized an equilibrium method of separating free and liposomal drugs for liposomal formulations based on HF-CF-UF. The method was validated over the concentration range of 7.9–235 μg mL−1 for indomethacin and 0.258–8.24 μg mL−1 for vitamin A. Inter- and intra-day precision (RSD %) were ≤1.2 % for indomethacin and ≤1.8 % for vitamin A, respectively. The recoveries of both free drug and total drug were higher than 96.0 % with RSD ≤1.3 % (n = 5). Taken together, our improved method differs from those previously reported with regard to higher recovery, small sample volume, less laborious, and most importantly without damaging or disturbing liposomes. Validation results suggested that our method was sufficiently accurate and sensitive to be used to evaluate the entrapment efficiency of a liposome formulation without complicated pretreatment.