Effects of sterilizing-grade filters on the physico-chemical properties of onion-like vesicles

Spherulites™ are new promising multilamellar vesicles that we study in a drug delivery context. The sterilization of spherulites™ suspensions is a necessary step before biological tests and later, before pharmaceutical applications (for example, parenteral or local injections). Among all sterilizing operations, the filtration through 0.22 μm sterilizing-grade filters (of the type Millex® (Ø 4 mm) by Millipore) is easy and rapid, and we decided to study it as a mean to obtain sterile suspensions. The spherulites™ diameter is usually comprised between 0.2 and 0.5 μm but bigger vesicles occur and reach Ø 1 μm. The effects of such filters on vesicles’ size and lipids’ concentration were then compromised. After examination of this challenging operation, results proved that the sterilizing filtration had no effect on these two parameters whatever the formulation chosen. Then, the possible release of amaranth, an encapsulated hydrophilic dye was followed. With the formulations and in spite of a filter diameter inferior to that of the vesicles, the encapsulation yields were not significantly different before and after the filtration and no leakage could be detected. Finally, the spherulites™’ functionality after sterilizing filtration was studied under the chemical angle: vesicles containing an amphiphilic reactive anchor (CholE3ONH2) were still able to bind covalently a peptidic molecular recognition pattern. The ligation was quantified by fluorimetry as high as for non-filtrated suspensions. Thus, though spherulites™ can present a diameter superior to that of the sterilizing filters, their passage through them do not alter the physico-chemical properties of these vesicles.