Inulin is a promising cryo- and lyoprotectant for PEGylated lipoplexes

The aim of this study was to investigate whether the oligosaccharides dextran and inulin are able to prevent aggregation of lipoplexes based on 1,2-dioleoyl-3-trimethylammonium-propane and dioleoylphosphatidyl-ethanolamine with and without distearoylphosphatidylethanolamine-polyethyleneglycol (PEGylated and nonPEGylated lipoplexes, respectively) during storage. The lipoplexes, dispersed in the oligosaccharide solution were frozen and subsequently stored at subzero temperature or freeze dried and subsequently stored at 37 °C. When lipoplexes in frozen dispersions were stored below the glass transition temperature of the maximally freeze concentrated fraction (Tg') of the oligosaccharide solutions severe aggregation of the nonPEGylated lipoplexes was prevented for 3 months by both inulin and dextran. However, while dextran failed to stabilize the frozen PEGylated lipoplexes (as in most cases full aggregation occurred in short time) inulin successfully protected them against aggregation. Compared to dextran, inulin was also a superior lyoprotectant of PEGylated lipoplexes: during freeze drying and subsequent storage at 37 °C of the dried powders for 3 months the PEGylated lipoplexes maintained their original size when dispersed in inulin matrices while in dextran matrices they fully aggregated in most cases. It is hypothesized that the aggregation of the PEGylated lipoplexes in dextran solutions is caused by the well known incompatibility between dextrans and PEG. This is further supported by the observation that inulins and PEG are compatible. It is concluded that oligosaccharides can prevent severe aggregation of nonPEGylated lipoplexes. The same holds for PEGylated lipoplexes provided that the oligosaccharide is compatible with PEG. Finally, this work also shows that the higher Tg' of oligosaccharides makes them more versatile cryoprotectants than disaccharides like sucrose or trehalose as the frozen dispersions can be stored at higher temperatures for prolonged periods of time. Furthermore, it is proposed that oligosaccharides are also more versatile lyoprotectants than the disaccharides because they can be exposed to higher relative humidities without passing the glass transition temperature.