Influence of process conditions on the crystallization and transition of metastable mannitol forms in protein formulations during lyophilization.

PURPOSE: To study the impact of different process conditions and formulation compositions on metastable mannitol forms in protein formulations during lyophilization.
METHODS: Mannitol was studied with and without other formulation components. A cryostage was used to mimic the different processing steps during lyophilization. The different mannitol forms were monitored and quantified with an in situ Raman spectroscopic method. In addition, a Raman imaging method was developed to characterize the spatial distribution of mannitol forms in final lyophilization samples from the freeze-drying stage.
RESULTS: Amorphous mannitol was observed during fast cooling (10 °C/min) and with the addition of other formulation component. Amorphous mannitol crystallized into mainly δ and hemihydrate forms during annealing at -20 °C. Under vacuum without moisture, dried amorphous mannitol could transform to mainly α form at 45 °C and greater. The transformation mechanism of the hemihydrate mannitol was similar to that of amorphous form.
CONCLUSION: Mannitol tends to crystallize into stable crystalline forms by itself, but the addition of lyoprotectant (e.g. sucrose) and protein helps stabilize the metastable forms (hemihydrate and amorphous). The metastable forms are capable of transforming into mixtures of different forms, with heat and moisture being the critical processing factors.