Moisture sorption behavior of selected bulking agents used in lyophilized products

To develop a rational approach for the formulation of lyophilized products, six bulking agents commonly used in freeze-dried formulations were lyophilized under identical conditions, and their moisture sorption behavior, before and after lyophilization, were determined as a function of relative humidity at 25 degrees C. The bulking agents evaluated were mannitol, anhydrous lactose, sucrose, D(+)-trehalose, dextran 40 and povidone (PVP K24). The materials were also characterized for their crystal and thermal properties by powder X-ray diffraction, DSC and TG after exposure to various relative humidity conditions. Mannitol was crystalline and non-hygroscopic both before and after lyophilization with total moisture contents of 0.1 to 0.3% w/w between 10 and 60% RH. Anhydrous lactose, sucrose and trehalose were crystalline prior to lyophilization with moisture contents of 0.86, 0.15 and 9.2%, respectively, and the crystalline materials were relatively non-hygroscopic. Upon lyophilization, they converted to the amorphous form and had moisture contents of 1.6, 2.5 and 1.2%, respectively. The amorphous materials sorbed moisture rapidly upon exposure to increasing relative humidity conditions. The amorphous lactose converted to its crystalline hydrate form at 55% RH after sorption of an additional 10% moisture. This conversion to the crystalline hydrate form was accompanied by desorption of practically all the moisture sorbed by the amorphous form. Similarly, lyophilized sucrose converted to its crystalline form after the sorption of additional 4.5% moisture at 50% RH, and the lyophilized trehalose sorbed additional 10% moisture prior to its conversion to a crystalline hydrate form at 50% RH. Dextran and povidone were amorphous and hygroscopic both before and after lyophilization and they sorbed as much as 10-20% moisture at 50% RH. It is well established that different drugs, especially proteins, need different levels of moisture for optimal stability. The results of the present study show that moisture contents of lyophilized cakes may be varied and optimized by the selection of suitable excipients.