Investigation of the stabilisation of freeze-dried lysozyme and the physical properties of the formulations.

The long-term stability of a protein formulation requires that the glass transition temperature (Tg) of the formulation should be maximised and the perturbation of the protein native structure in the dried form after processing minimised. In the present study, the stabilisation of lysozyme structure conferred by excipients was monitored using second derivative Fourier transform infrared spectroscopy and the physical properties of protein formulations were investigated using differential scanning calorimetry. The results showed that the preservation of protein native structure during freeze-drying and the Tg of freeze-dried formulations were excipient- and excipient to enzyme mass ratio-dependent. The freeze-dried lysozyme appeared to be less effectively stabilised compared with the spray-dried enzyme when the excipients and the excipient to enzyme mass ratios were the same. In terms of the preservation of the secondary structure of lysozyme, glycerol and sucrose seemed to be more efficient than trehalose, although the Tg of trehalose-containing formulations were found to be higher than the Tg of the equivalent sucrose-based ones. With adding either trehalose or dextran to sucrose-containing formulations, the stabilisation of lysozyme native structure could be as effective as with sucrose alone, whilst the Tg could be enhanced. The results in this study suggested that lysozyme, processed by freeze-drying, is stabilised primarily by the water substitution mechanism. Copyright 2004 Elsevier B.V.