Freeze drying of nanosuspensions, 2: the role of the critical formulation temperature on stability of drug nanosuspensions and its practical implication on process design.

The present study investigates whether controlling the product temperature below the critical formulation temperature (CFT) during primary drying in a freeze drying cycle is a prerequisite for the stabilization of drug nanoparticles. For that purpose, the CFT of four drug nanosuspensions stabilized with different types (amorphous and crystalline) and concentrations of steric stabilizers and either of the disaccharides, trehalose and sucrose, was determined by differential scanning calorimetry and freeze-dry microscopy. Freeze-drying experiments were performed such that product temperatures during primary drying remained either below or well above the CFT of individual mixtures. It was found that glass formation did not influence the stability of the nanoparticles, suggesting that an adequate type of steric stabilizer and lyoprotectant concentration is present. Freeze drying could also be performed above the eutectic temperature without compromising on the final product quality profile, such as nanoparticle size and structural preservation of the lyophilized cake. The high concentration of solid drug nanoparticles provided additional cake stability. The results of this study confirm for the first time that primary drying for drug nanosuspensions can be greatly shortened because induced viscous flow or even meltback is not a limitation for nanoparticle stability and cake elegancy.