Cryoprotectants are often required in lyophilization to reduce or eliminate agglomeration of solute or suspended materials. The aim of this study was to select a cryoprotecting agent and optimize its concentration in a solid lipid nanoparticle formulation. Progesterone-loaded stearic acid solid lipid nanoparticles (SA-P SLNs) were prepared by hot homogenization with high speed mixing and sonication. The stearic acid content was 4.6% w/w and progesterone was 0.46% w/w of the initial formulation. Multiple surfactants were evaluated, and a lecithin and sodium taurocholate system was chosen. Three concentrations of surfactant were then evaluated, and a concentration of 2% w/w was chosen based on particle size, polydispersity, and zeta potential. Agglomeration of SA-P SLNs after lyophilization was observed as measured by increased particle size. Dextran, glycine, mannitol, polyvinylpyrrolidone (PVP), sorbitol, and trehalose were evaluated as cryoprotectants by both an initial freeze-thaw analysis and after lyophilization. Once selected as the cryoprotectant, trehalose was evaluated at 5%, 10%, 15%, and 20% for optimal concentration, with 20% trehalose being finally selected as the level of choice. Evaluation by DSC confirmed intimate interaction between stearic acid and progesterone in the SA-P SLNs, and polarized light microscopy shows successful lyophilization of the trehalose/SA-P SLN. A short term 28-day stability study suggests the need for refrigeration of the final lyophilized SA-P SLNs in moisture vapor impermeable packaging.
Keywords: cryoprotectant; freezing rate; lyophilization; solid lipid nanoparticles; surfactants; trehalose.