Characterization of Phase Transitions During Freeze-Drying by In Situ X-ray Powder Diffractometry

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Abstract
The purpose of this research was to develop the technique of in situ freeze-drying in the sample chamber of an X-ray powder diffractometer (XRD) and to monitor the phase transitions during the freeze-drying of aqueous solutions of sodium nafcillin (I) and mannitol (II). Aqueous solutions of I and II were frozen under controlled conditions in the sample chamber of an XRD. This variable temperature XRD was modified so that the sample chamber could be evacuated and the samples dried under reduced pressures. Thus the entire freeze-drying cycle was carried out in the XRD holder and the solid-state was monitored during the various stages of the process. Frozen solutions of I when annealed at −4°C resulted in crystallization of the solute as 'sodium nafcillin hydrate' (unknown stoichiometry). Primary drying at −10°C, resulted in partial dehydration to a poorly crystalline sodium nafcillin hemihydrate. There was no crystallization of mannitol when solutions of II were cooled and subjected to primary drying at −50°C. During the drying, the intensities of the characteristic X-ray lines of ice (d-spacings of 3.94, 3.70 and 3.48 A) were quantified. This enabled real time monitoring of the complete sublimation of crystalline ice. When the secondary drying was carried out at −25°C, mannitol crystallized as an anhydrous mixture of the &dL-and β-polymorphs. In a second set of experiments, the frozen solutions were warmed to −25°C and subjected to primary drying. Mannitol crystallized and its XRD pattern matched that of mannitol hydrate reported recently (Yu et al., Pharm. Res., 14s (1997) S-445). When the secondary drying was carried out at −10°C, there was no change in the XRD pattern suggesting the formation of a dehydrated hydrate. This in situ XRD technique enabled us to characterize the phase transitions during freeze-drying. It would be useful in developing a mechanistic understanding of the alterations in the solid-state during freeze-drying of complex, multi-component, pharmaceutical systems.