Interactions of water with the surfaces of crystal polymorphs

Solid-State Forms of Koumine Hydrochloride: Phase Transformations and the Crystal Structure and Properties of the Stable Form

To investigate the solid-state forms of koumine hydrochloride (KMY), solid form screening was performed, and one amorphous form and five crystalline forms (forms A, B, C, D, and E) were identified by powder X-ray diffraction. Form A was the dominant crystal product, and its crystal structure and packing pattern were determined by single-crystal X-ray diffraction. The crystals displayed an orthorhombic crystal system and symmetry of space group P2₁2₁2₁ with Z' = 1. The amorphous form transformed to form A at 105-120 °C or 75% RH, while forms B, C, D, and E could only be intermediate phases and readily transformed to form A at room temperature. Therefore, the phase transformations of KMY solid-state forms were established. The properties of the amorphous form and form A were further elucidated by applying vibrational spectroscopy, moisture sorption analysis, and thermal analysis. Accordingly, form A, the KMY anhydrate, was found to be the thermodynamically stable form with low hygroscopicity under ambient conditions. These characteristics are crucial in the manufacture and storage of active pharmaceutical ingredients.

Merits and Limitations of Dynamic Vapor Sorption Studies on the Morphology and Physicochemical State of Freeze-Dried Products

The goal of the present study was to assess the applicability of dynamic vapor sorption analysis of freeze-dried products. Water vapor sorption profiles of intact and ground cakes were recorded to determine the relevance of powder handling. Grinding prior to measurements appeared to be related with a more rapid uptake of water vapor and crystallization. Crystallization may be prevented when analyzing intact cakes. More hygroscopic materials appeared to require a longer time to achieve a constant mass. The specific surface area of different freeze-dried products was calculated from the sorption isotherms using water, organic solvents, and krypton. The specific surface areas calculated for mannitol with water and ethanol was in good agreement with krypton data. False high values were obtained from water vapor sorption of the investigated amorphous materials. The results were slightly improved by the application of vacuum. For trehalose and sucrose, no sorption and thus faulty results were detected with the studied organic solvents. The degree of crystallinity of mannitol within a binary formulation could not be determined by dynamic vapor sorption. Differences in sorption and crystallization tendencies of mannitol and sucrose that were freeze-dried separately and in a binary mixture were considered as the root cause.

Role of moisture on the physical stability of polymorphic olanzapine

The focus of this study was the understanding of the hydrate transformations of anhydrous olanzapine Forms I and II (the most common polymorphs) upon exposure to different moisture conditions (11, 53, 75, 93% RH) and direct contact with water (e.g. aqueous slurry) and the impact of hydration on the aqueous dissolution rates of the polymorphs. The kinetics of reversible transformations (anhydrate-hydrate phases) and the identification of polymorphs were evaluated by differential scanning calorimetry, thermogravimetry, infrared (DRIFT) and X-ray powder diffraction. The results showed that anhydrous Forms I and II have undergone water vapor phase induced transformations at 93% and 75% RH, respectively. At 93% RH Forms I and II showed to hydrate into dihydrates D and B, respectively, the latter with a higher hydration rate. The conversion of Form I into the dihydrate D showed to affect the dissolution rate of olanzapine (f2<50). As slurries both forms showed to hydrate into a mixture of two different Forms - dihydrate B and higher hydrate. The study provided an understanding of the conversion pathways of the different forms when they were exposed to humid air or aqueous environments, resembling the transformations that might occur during processing, storage or during the persecution of dissolution tests to assess the quality of dosage forms delivering olanzapine.

Solid-State Characterization and Interconversion of Recrystallized Amodiaquine Dihydrochloride in Aliphatic Monohydric Alcohols

Amodiaquine dihydrochloride monohydrate (AQ-DM) was obtained by recrystallizing amodiaquine dihydrochloride dihydrate (AQ-DD) in methanol, ethanol, and n-propanol. Solid-state characterization of AQ-DD and AQ-DM was performed using X-ray powder diffractometry, Fourier transform infrared spectroscopy, thermogravimetry, and differential scanning calorimetry. All recrystallized samples were identified as AQ-DM. Crystal habits of AQ-DD and AQ-DM were shown to be needle-like and rhombohedral crystals, respectively. When AQ-DD and AQ-DM were exposed to various relative humidity in dynamic vapor sorption apparatus, no solid-state interconversion was observed. However, AQ-DM showed higher solubility than AQ-DD when exposed to bulk water during solubility study, while excess AQ-DM was directly transformed back to a more stable AQ-DD structure. Heating AQ-DM sample to temperatures ≥190°C induced initial change to metastable amorphous form (AQ-DA) which was rapidly recrystallized to AQ-DD upon ≥80%RH moisture exposure. AQ-DD was able to be recrystallized in alcohols (C1-C3) as AQ-DM solid-state structure. In summary, AQ-DM was shown to have different solubility, moisture and temperature stability, and interconversion pathways when compared to AQ-DD. Thus, when AQ-DM was selected for any pharmaceutical applications, these critical transformation and property differences should be observed and closely monitored.

Investigation of Metformin HCl lot-to-lot variation on flowability differences exhibited during drug product processing

The purpose of this study was to determine the cause for flowability difference observed during drug product processing when different Metformin HCl drug substance batches of varying age were used. It was found that the lead time (age) between the final step (milling) in the manufacturing process of the Metformin HCl drug substance could be a factor. The lead time had an impact on flowability of Metformin/excipient blends during drug product processing even though these batches had no apparent differences in their release specifications. To study and understand the aging effect, two batches of Metformin HCl manufactured at different periods of time were selected. The surface energy values obtained by the density functional theory (DFT) method together with X-ray diffraction patterns, thermally stimulated current measurements, and dynamic vapor sorption isotherms indicated that the freshly manufactured Metformin HCl material contains detectable amounts of surface crystal defects, but are absent in aged sample, which could be the cause of flowability differences of Metformin/excipient blends observed during the drug product processing. Having identified the cause for different flow behavior, a method to destroy these defects was designed and the issue was resolved by rapid aging of Metformin HCl under humidity at room temperature.

Anisotropic surface chemistry of crystalline pharmaceutical solids

The purpose of this study was to establish the link between the wetting behavior of crystalline pharmaceutical solids and the localized surface chemistry. A range of conventional wetting techniques were evaluated and compared with a novel experimental approach: sessile drop contact angle measurements on the individual facets of macroscopic (>1 cm) single crystals. Conventional measurement techniques for determining surface energetics such as capillary rise and sessile drops on powder compacts were found not to provide reliable results. When the macroscopic crystal approach was used, major differences for advancing contact angles, theta(a), of water were observed-as low as 16 degrees on facet (001) and as high as 68 degrees on facet (010) of form I paracetamol. theta(a) trends were in excellent agreement with X-ray photoelectron spectroscopy surface composition and known crystallographic structures, suggesting a direct relationship to the local surface chemistry. Inverse gas chromatography (IGC) was further used to probe the surface properties of milled and unmilled samples, as a function of particle size. IGC experiments confirmed that milling exposes the weakest attachment energy facet, with increasing dominance as particle size is reduced. The weakest attachment energy facet was also found to exhibit the highest theta(a) for water and to be the most hydrophobic facet. This anisotropic wetting behavior was established for a range of crystalline systems: paracetamol polymorphs, aspirin, and ibuprofen racemates. theta(a) was found to be very sensitive to the local surface chemistry. It is proposed that the hydrophilicity/hydrophobicity of facets reflects the presence of functional groups at surfaces to form hydrogen bonds with external molecules.