Phase transitions during freeze-drying: Influence of processing conditions and formulation variables

The effect of processing conditions and formulation variables on the phase transitions during the various stages of freeze-drying (viz. during cooling, annealing, primary and secondary drying) have been studied in aqueous binary and multicomponent systems. The model compounds were glycine, mannitol, trehalose and cefazolin sodium either alone or in some combinations. The crystallization of mannitol below the Tg′ (glass transition temperature of maximally freeze-concentrated amorphous phase), was investigated in detail. Though mannitol was retained amorphous following rapid cooling, annealing below Tg′ led to the crystallization of mannitol hydrate. X-ray powder diffractometry revealed crystallization of mannitol hydrate during primary drying below the Tg′. In case of trehalose, a non-crystallizing solute, the effect of annealing on enthalpic recovery was investigated. In mannitol-trehalose systems, at trehalose concentrations ≥5% w/w, annealing below Tg′ led to nucleation but not crystallization of mannitol. The crystallization of mannitol during primary drying was found to be dependent both on the trehalose concentration and the cooling rate. We postulated that annealing below Tg ′ facilitated ice crystallization which led to supersaturation and finally nucleation. At high trehalose concentration (>5% w/w), prolonged annealing was necessary for the crystallization of mannitol. In case of glycine, another popular bulking agent, thermal history strongly influenced its solid-state in frozen aqueous solutions and in the final lyophile. Polymorphic transitions were observed only in rapidly cooled solutions of glycine during annealing or during primary drying. In systems containing mannitol and glycine, the crystallization of both these solutes were inhibited by sodium phosphate buffers thereby providing improved lyoprotection for lactate dehydrogenase. Cefazolin sodium, a model cephalosporin antibiotic, forms poorly crystalline lyophiles. Different approaches (viz. annealing, addition of organic solvent or crystalline bulking agents) were taken to facilitate its crystallization.