Formulation development of lyophilized, long-term stable siRNA/oligoaminoamide polyplexes

Polyplexes based on precise oligoaminoamides exhibited promising results in non-viral siRNA delivery. However, one serious limitation is insufficient stability of polyplexes in liquid, which raises the demand for lyophilized, long-term stable formulations. Two different siRNA/oligoaminoamide polyplexes were prepared. Freeze-thaw experiments were performed, in order to test various formulations containing sucrose, trehalose, lactosucrose, and hydroxypropyl-β-cyclodextrin for their cryoprotective potential and to investigate the influence of the oligoaminoamide structure on particle stability. Selected formulations were lyophilized and tested for storage stability up to 6 months. Moreover, reconstitution of the lyophilisates in reduced volume as a technique to prepare higher concentration formulations was studied. Samples were analyzed for particle size, gene silencing, cytotoxicity, turbidity, subvisible particles, osmolarity, residual moisture content, glass transition temperature, and morphology. Depending on the oligoaminoamide, siRNA polyplexes maintained particle size and gene silencing efficiency in the absence or presence of low amounts (7%) of stabilizers after freeze-thawing, lyophilization, and reconstitution. Particle stability was highly dependent on the oligoaminoamide used, but independent of the presence of cysteines that form intra-particular disulfide bridges. In contrast to all other excipients, hydroxypropyl-β-cyclodextrin did not provide sufficient stability. For lyophilized 5%/10% sucrose and 7% lactosucrose formulations, long-term stability was demonstrated at 40 °C with retained particle size, retained gene silencing activity, unchanged turbidity values, low numbers of subvisible particles, low residual moisture level, and sufficiently high glass transition temperature. Hence, this work is a promising approach in order to provide long-term stable siRNA polyplex formulations that are ready to use after a simple reconstitution step.