The Preparation of Tissue-Type Plasminogen Activator (t-PA) Containing Liposomes: Entrapment Efficiency and Ultracentrifugation Damage

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Abstract
In this study, a method was developed for the efficient entrapment of active tissue-type Plasminogen Activator (t-PA) into liposomes. Experimental conditions were varied to optimize t-PA entrapment: different buffer solutions were used (pH 4 and 7.5), the effect of the incubation concentrations of phospholipid (PL) and t-PA was monitored and the influence of liposome-size was examined. Furthermore, the effect of ultracentrifugation on t-PA containing liposomes was determined in the presence and absence of Tween 80. t-PA entrapment strongly depended on experimental conditions and ranged from 30 up to 90%. Almost quantitative (90%) entrapment (entrapment percentage defined as absolute entrapment (IU t-PA/μmol PL) divided by total incubation ratio (IU t-PA/μmol PL), times 100%) was obtained in Hepes buffer pH 7.5, devoid of arginine, with low ionic strength. Ultracentrifugation, used for removal of non-entrapped t-PA, was shown to have a damaging effect on the liposomes (especially in the presence of 0.05% Tween 80), leading to t-PA loss. However, because acceptable alternatives were not available, ultracentrifugation was used during this study. Therefore, the encapsulation-percentage values shown in this study are in fact underestimates for the true entrapment of t-PA. In conclusion: almost quantitative t-PA entrapment in liposomes can be achieved by selecting the proper milieu and inducing a strong interaction between t-PA and bilayer.