The study of phase separation in amorphous freeze-dried systems, part 2: investigation of Raman mapping as a tool for studying amorphous phase separation in freeze-dried protein formulations.

Phase separation in amorphous freeze-dried proteins systems is speculated to occur, but limitations in methods of detection have hindered progress in identifying and studying this phenomenon. We present here preliminary results obtained on model two-component protein-stabilizer systems that illustrate the potential of a novel confocal Raman mapping technique in detecting phase separation in protein-containing formulations. The Raman technique detected the presence of phase separation in Ficoll/bovine serum albumin (BSA) samples at ratios of 1:2 to 2:1 and in trehalose/lysozyme samples at weight ratios of 1:1 and 3:1. A model physical mixture of Ficoll and BSA was used as a positive control to illustrate extreme phase separation. The results suggest that Raman mapping of freeze-dried protein systems offers a means of identifying phase separation in susceptible formulations. Contrary to the use of differential scanning calorimetry, Raman does not rely upon a change in heat capacity. This is a strong advantage over calorimetry, which is not capable of detecting phase separation in systems that result in at least one protein-rich phase because of the weak change in heat capacity and the broad temperature range associated with the glass transition temperature in these systems.