Characterization of phase transitions during freeze-drying by in situ X-ray powder diffractometry.

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 degrees C, resulted in crystallization of the solute as 'sodium nafcillin hydrate' (unknown stoichiometry). Primary drying at -10 degrees 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 degrees 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 degrees C, mannitol crystallized as an anhydrous mixture of the delta- and beta-polymorphs. In a second set of experiments, the frozen solutions were warmed to -25 degrees 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 degrees 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.

Characterization of frozen aqueous solutions by low temperature X-ray powder diffractometry.

PURPOSE: A low temperature X-ray powder diffractometric (XRD) technique has been developed which permits in situ characterization of the solid-state of solutes in frozen aqueous solutions. METHODS: A variable temperature stage, with a working temperature range of -190 to 300 degrees C, was attached to a wide-angle XRD. The stage was calibrated with a sodium chloride-water binary system. RESULTS: When aqueous nafcillin sodium solution (22% w/w) was frozen, eutectic crystallization of the solute was not observed. However, annealing at -4 degrees C, caused crystallization of the solute. With increasing annealing time, there was a progressive increase in the crystallinity of the solute. Studies were carried out with sodium nafcillin solutions ranging in concentration from 20 to 50% w/w. The solid-state of the phase crystallizing from solution was independent of the solute concentration. Next, solutions of mono- and disodium hydrogen phosphate were individually frozen. Only the latter crystallized as the dodecahydrate (Na2HPO4.12H2O). However when an aqueous buffer mixture of mono- and disodium hydrogen phosphate was frozen, the former inhibited the crystallization of the latter. CONCLUSIONS: Since freezing of solutions is the first step in lyophilization, the XRD technique can provide a mechanistic understanding of the alterations in solid-state that occur during freeze-drying. DSC has so far been the technique of choice to study frozen systems. The advantage of XRD is that it not only permits unambiguous identification of the crystalline solid phase(s), but it also provides information about the degree of crystallinity. While overlapping thermal events are difficult to interpret in DSC, XRD does not suffer from such a limitation.

Identification of drugs in pharmaceutical dosage forms by X-ray powder diffractometry.

A simple X-ray powder diffractometric (XRD) method was developed for the identification of the active ingredient in a variety of dosage forms. The method was successfully used to unambiguously identify the active ingredient(s) in tablet, capsule, suppository and ointment formulations. The unique feature of the method is that it provides information about the solid-state of the drug. Thus, a capsule formulation containing anhydrous ampicillin was readily distinguished from that containing ampicillin trihydrate. The USP stipulates the use of the beta-polymorphic form of anhydrous carbamazepine in carbamazepine tablets. Contamination by the alpha-polymorph (down to a level of 1.4% w/w of the formulation) could be detected. In some of the multicomponent formulations, there was a pronounced overlap of the powder patterns of ingredients which made identification difficult. This problem was solved by using a pattern subtraction technique, which permitted selective subtraction of the XRD pattern of the constituents of the formulation from the overall XRD pattern. Such an approach enabled identification of the drug even when it constituted only 5% w/w of the formulation. The method also permitted simultaneous identification of the multiple active ingredients in trimethoprim-sulfamethoxazole and acetaminophen-aspirin-caffeine formulations.