Characterization of polymorphic solid-state changes using variable temperature X-ray powder diffraction

Understanding the Metastability of Theophylline FIII by Means of Low-Frequency Vibrational Spectroscopy

While theophylline has been extensively studied with multiple polymorphs discovered, there is still currently no conclusive structure for the metastable theophylline form III. In this present work, by combining more widely used techniques such as X-ray diffraction and thermogravimetric analysis with more emerging techniques like low-frequency Raman and terahertz time-domain spectroscopy, to analyze the structure and dynamics of a crystalline system, it was possible to provide further evidence that the form III structure has a theophylline monohydrate structure with the water molecules removed. Solid-state density functional theory simulations were paramount in proving that this proposed structure is correct and explain how vibrational modes within the crystal structures feature and govern polymorphic transitions and the metastable form III. Through the insight provided by both simulated and experimental results, it was possible to decisively conclude the elusive crystal structure of theophylline form III. It was also shown that the correct space group for theophylline monohydrate is not P21/n but, in fact, Pc.

Thermodynamic and kinetic studies on the polymorphic transformations of puerarin hydrates

Puerarin (PUE), a bioactive flavonoid from the plant Pueraria lobata, exists in two hydrated forms: monohydrate (PUEMH) and dihydrate (PUEDH). The aim of the present work was to explore the thermodynamic and kinetic mechanism of the polymorphic transformation of PUE, including the solvent-mediated polymorphic transformation (SMPT) of PUEMH to PUEDH and the solid-state polymorphic transformations (SSPTs) of PUEMH and PUEDH. PUEMH and PUEDH were identified as isolated and channel hydrate, respectively. The thermodynamic parameters (ΔG < 0, ΔH < 0, and ΔS < 0) indicated that the SMPT was a spontaneous, exothermic and entropy-decreased reaction. The facilitating roles of stirring rate and temperature on the SMPT were favored by the primary and secondary nucleation process of PUEDH. In addition, the results of SSPTs suggested that PUEMH and PUEDH would transform to two different anhydrates (PUEAH-I and PUEAH-II) upon heating, respectively. The dehydration rate of PUEMH was slower than that of PUEDH due to the stronger hydrogen bond interactions. The rate-limiting step for the dehydration of PUEMH was the diffusion of water molecules, resulting in the increased dehydration activation during the dehydration process, while the dehydration activation energy of PUEDH showed opposite trend due to the complicated crystallization process of PUEAH-II.

Monitoring the Isothermal Dehydration of Crystalline Hydrates Using Low-Frequency Raman Spectroscopy

Detection of the solid-state forms of pharmaceutical compounds is important from the drug performance point of view. Low-frequency Raman (LFR) spectroscopy has been demonstrated to be very sensitive in detecting the different solid-state forms of pharmaceutically relevant compounds. The potential of LFR spectroscopy to probe the in situ isothermal dehydration was studied using piroxicam monohydrate (PXM) and theophylline monohydrate (TPMH) as the model drugs. The dehydration of PXM and TPMH at four different temperatures (95, 100, 105, and 110 °C and 50, 60, 70, and 80 °C, respectively) was monitored in both the low- (20-300 cm^-1) and mid-frequency (335-1800 cm^-1) regions of the Raman spectra. Principal component analysis and multivariate curve resolution were applied for the analysis of the Raman data. Spectral differences observed in both regions highlighted the formation of specific anhydrous forms of piroxicam and theophylline from their respective monohydrates. The formation of the anhydrous forms was detected on different timescales (approx. 2 min) between the low and mid-frequency Raman regions. This finding highlights the differing nature of the vibrations being detected between these two spectral regions. Computational simulations performed were also in agreement with the experimental results, and allowed elucidating the origin of different spectral features.

Avoiding irreversible 5-fluorocytosine hydration via supramolecular synthesis of pharmaceutical cocrystals

Supramolecular reaction of 5-FC with caffeine, p-aminobenzoic and caprylic acid gave rise to solid forms physically stable in humid environments.

Modelling temperature-dependent properties of polymorphic organic molecular crystals

We present a large-scale study of the temperature-dependence of structures, free energy differences and properties of polymorphic molecular organic crystals. Lattice-vibrational Gibbs free energy differences between 475 pairs of polymorphs of organic molecular crystals have been calculated at 0 K and at their respective melting points, using a highly accurate anisotropic multipole-based force field and including thermal expansion through the use of a (negative) thermal pressure. Re-ranking of the relative thermodynamic stability of the polymorphs in each pair indicates the possibility of an enantiotropic phase transition between the crystal structures, which occurs in 21% of the studied systems. While vibrational contributions to free energies can have a significant effect on thermodynamic stability, the impact of thermal expansion on polymorph free energy differences is generally very small. We also calculate thermal expansion coefficients for the 864 crystal structures and investigate the temperature-dependence of mechanical properties, and pairwise differences in these properties between polymorphs.

Monitoring model drug microencapsulation in PLGA scaffolds using X-ray powder diffraction

The microencapsulation of three model drugs; metronidazole, paracetamol and sulphapyridine into Poly (dl-Lactide-Co-Glycolide) (PLGA) scaffolds were probed using X-ray Powder Diffraction (XRPD). Changes in the diffraction patterns of the PLGA scaffolds after encapsulation was suggestive of a chemical interaction between the pure drugs and the scaffolds and not a physical intermixture.

Understanding pharmaceutical polymorphic transformations I: influence of process variables and storage conditions

The active pharmaceutical ingredient (API) of a dosage form is affected by number of mechanical and environmental factors which have a tendency to alter its crystalline state. Polymorphic transitions have been observed to occur during various unit operations like granulation, milling and compression. Forces of pressure, shear and temperature have an ability to induce alterations in crystal habit. A conversion in polymorphic form during a unit operation is very likely to affect the handling of API in the subsequent unit operation. Transitions have also been observed during storage of formulations where the relative humidity and temperature play a major role. An increase in temperature during storage can dehydrate or desolvate the crystal and hence produce crystal defects, whilst, high humidity conditions produce higher molecular mobility leading to either crystallization of API or alteration of its crystalline form.

Integrated approach to study the dehydration kinetics of nitrofurantoin monohydrate

There is a need for thorough knowledge of solid-state transformations in order to implement quality by design (QbD) methodology in drug development. The present study was aimed at gaining a mechanistic understanding of the dehydration of nitrofurantoin monohydrate II (NF-MH). The dehydration was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hot-stage microscopy (HSM), and variable temperature X-ray powder diffraction (VT-XRPD). Isothermal TGA data were used to study dehydration kinetics using model-fitting and model-free approaches. Model-fitting analysis indicated a good fit for several models derived from nucleation-growth and/or geometric contraction mechanisms. However, based on visual observations during HSM, Avrami-Erofeyev equations A3 and A4, indicating nucleation-growth phenomenon, were found to be the most suitable kinetic models. HSM showed initiation of dehydration with random nucleation, and nuclei coalesced with the progress of dehydration reaction. VT-XRPD revealed formation of anhydrate beta form on dehydration of NF-MH. The phenomenon of random nucleation is justified based on the crystal structure of NF-MH, which showed presence of water molecules in an isolated manner, prohibiting directional dehydration. It was found that supplementary information from HSM and VT-XRPD can be valuable to gain a better understanding of dehydration from formal solid-state kinetics analysis.

Formation of bicalutamide nanodispersion for dissolution rate enhancement

Bicalutamide was loaded on hydrophilic excipients to form nanodispersions via a combination of anti-solvent precipitation and spray drying method. The particle size, BET surface area, contact angles and dissolution rate of the nanodispersions were analyzed. The results indicated that lactose was a suitable matrix to prevent the bicalutamide particles growth and aggregation. The lactose loaded particles had a mean size of 330 nm within a narrow distribution. X-ray diffraction (XRD), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) characterization indicated the nanodispersion exhibited unchanged crystalline and chemical structure. Dissolution rate of bicalutamide nanodispersion was significantly faster than that of commercial products. It increased to 94% in 10 min while both commercial formulas Casodex and bicalutamide tablets dissolved 60% and 38% respectively at the same period. It was proposed that the enhanced dissolution rate of bicalutamide nanodispersion contribute to high surface area and well-wetted state of drug particles.

Preparation and characterization of salt forms of enalapril

Selection of an optimal salt form of a drug candidate is a vital component of preformulation stage of drug development. In this study, six salts of enalapril--citrate, mesylate, tartrate, malate, besylate and tosylate--were prepared and characterized by Mass Spectroscopy, Differential Scanning Calorimetry, Thermogravimetric Analysis, Microscopy, Powder X-ray Diffraction, Karl Fischer Titration, High Performance Liquid Chromatography, Fourier-Transform Infra-red Spectroscopy and Head Space Gas Chromatography. All the six salts were subjected to a tiered screening involving five stages in the following order: crystallinity, hygroscopicity, solubility, stability and flow/compactability. Enalapril malate showed encouraging profile because of its lower hygroscopicity, higher solubility, good solid state stability, and better flow and compactability, in comparison to the marketed maleate salt.

Fluorescence studies of the dehydration of cefadroxil monohydrate

Although cefadroxil does not exhibit the phenomenon of photoluminescence when dissolved in a fluid medium, the compound has been found to exhibit fluorescence in its solid-state monohydrate crystal form. The monohydrate was found to exhibit complicated photoluminescence, where two different sets of emission spectra could be obtained upon irradiation with an appropriate excitation wavelength. One of these photophysical systems became strongly suppressed when the monohydrate was half-dehydrated, and only one of the photophysical systems could be observed in this hemihydrate. In the fully dehydrated state, both photophysical pathways became almost totally suppressed, so that the nonsolvated cefadroxil became effectively nonfluorescent.

Probing pseudopolymorphic transitions in pharmaceutical solids using Raman spectroscopy: hydration and dehydration of theophylline

Theophylline is known to undergo vapor phase induced hydrate-anhydrate pseudopolymorphic transformations, which can affect its bioavailability. In this work, the kinetics of the pseudopolymorphic transitions of theophylline crystals in different storage conditions is studied using a vibrational spectroscopic technique. While the hydration is a single-step process with a half-life time of ca. 5 h, the dehydration occurs through a two-step mechanism. In addition, the phase stability of hydrate-anhydrate systems in different relative humidity (RH) conditions was probed. The critical RH for anhydrous teophylline was found to be at ca. 79%, while the critical RH for dehydration is ca. 30%.

Polymorphism and solvatomorphism 2005

Papers and patents that deal with polymorphism (crystal systems for which a substance can exist in structures characterized by different unit cells, but where each of the forms consists of exactly the same elemental composition) and solvatomorphism (systems where the crystal structures of the substance are defined by different unit cells, but where these unit cells differ in their elemental composition through the inclusion of one or molecules of solvent) have been summarized in an annual review. The works cited in this review were published during 2005, and were drawn primarily from the major physical, crystallographic, and pharmaceutical journals. The review is divided into sections that cover articles of general interest, computational and theoretical studies, preparative and isolation methods, structural characterization and properties of polymorphic and solvatomorphic systems, studies of phase transformations, effects associated with secondary processing, and United States patents issued during 2005.

Processing-Induced Phase Transitions of Theophylline—Implications on the Dissolution of Theophylline Tablets

Aqueous wet massing of stable anhydrous theophylline (A) with polyvinylpyrrolidone (PVP) resulted in its complete transformation to theophylline monohydrate (M). Drying at 45 degrees C, resulted in the formation of metastable anhydrous theophylline (A*) which then transformed to A. PVP, a known crystallization inhibitor, was effective in inhibiting the A* --> A transition. The higher molecular weight polymer, PVP K90, was more effective in inhibiting the A* --> A transition as compared to PVP K17. The disappearance of M, and the formation of A* and A was simultaneously monitored by XRD. An increase in the drying temperature from 45 to 55 degrees C accelerated the A* --> A transition. In granules prepared by the high-shear process, approximately 50% of theophylline existed as A and the rest as A*. In contrast, the fluid-bed granulation process yielded granules containing only A. Thus, the physical form of theophylline in tablets was influenced by the molecular weight of the binding agent, the granulation method, and the drying temperature. Using A as the starting material, tablets were manufactured by high-shear aqueous wet granulation process and the A* content was quantified. These tablets were stored under various relative humidity (RH) conditions at 25 degrees C for 2 weeks. Storage at RH >or= 33% caused complete A* --> A conversion accompanied by a pronounced decrease in the initial dissolution rate indicating that phase transitions during processing and storage can have a significant influence on product performance.

Investigation of the multi-step dehydration reaction of theophylline monohydrate using 2-dimensional powder X-ray diffractometry

PURPOSE

(i) To study the dehydration kinetics of theophylline monohydrate using 2-dimensional (2D) powder X-ray diffractometry (XRD), and (ii) to investigate the effect of polyvinylpyrrolidone (PVP) on the dehydration pathway and kinetics.

METHODS

Theophylline monohydrate (C(7)H(8)N(4)O(2).H(2)O; M) was recrystallized from aqueous PVP solutions of different concentrations. Dehydration kinetics was studied isothermally, at several temperatures, from 35 to 130 degrees C. The experimental set-up comprised of a high intensity X-ray source (synchrotron radiation or 8 kW rotating anode), a heating chamber, and a 2D area detector. Diffraction patterns were collected continuously, with a time resolution ranging from 40 ms to 30 s, over the angular range of 3 to 27 degrees 2theta.

RESULTS

Dehydration of M resulted in either the stable (C(7)H(8)N(4)O(2); A), or the metastable anhydrate (A*), with the latter having a tendency to transform to A. The XRD technique allowed simultaneous quantification of M, A* and A during the dehydration reaction. The rate constants for individual reaction steps (M-->A*; M-->A and A*-->A) were determined by fitting the data to solid-state reaction models. In presence of PVP, there was a decrease in the magnitude of the rate constant associated with the M-->A transition, resulting in an increased build-up of A* in the product. The inhibitory effect of PVP on M-->A transition was more pronounced at lower dehydration temperatures, and was proportional to the concentration of PVP.

CONCLUSIONS

Two dimensional powder X-ray diffractometry, using a high intensity source, is a powerful technique to study kinetics of rapid solid-state reactions. The inhibitory effect of excipients can have profound effect on phases formed during pharmaceutical processing.

Multivariate data analysis as a fast tool in evaluation of solid state phenomena

A thorough understanding of solid state properties is of growing importance. It is often necessary to apply multiple techniques offering complementary information to fully understand the solid state behavior of a given compound and the relations between various polymorphic forms. The vast amount of information generated can be overwhelming and the need for more effective data analysis tools is well recognized. The aim of this study was to investigate the use of multivariate data analysis, in particular principal component analysis (PCA), for fast analysis of solid state information. The data sets analyzed covered dehydration phenomena of a set of hydrates followed by variable temperature X-ray powder diffractometry and Raman spectroscopy and the crystallization of amorphous lactose monitored by Raman spectroscopy. Identification of different transitional states upon the dehydration enabled the molecular level interpretation of the structural changes related to the loss of water, as well as interpretation of the phenomena related to the crystallization. The critical temperatures or critical time points were identified easily using the principal component analysis. The variables (diffraction angles or wavenumbers) that changed could be identified by the careful interpretation of the loadings plots. The PCA approach provides an effective tool for fast screening of solid state information.

Role of water in the physical stability of solid dosage formulations

The interaction of moisture with pharmaceutical solids is highly crucial to an understanding of water-based processes, for example, manufacturing processes or prediction of solid dosage form stability and shelf life. Both the active pharmaceutical ingredient (API) and excipients in the formulation have different moisture sorption properties that can result in unexpected processing-induced phase transitions and they can affect solid-state phase transitions in the final dosage forms. The character of excipient effects on the stability of formulation. Phase transformations in formulations can lead to instability in physicochemical, biopharmaceutical, and processing properties of products. The aim of the present study was to investigate the water sorption properties of different excipients, model the sorption isotherms, examine the phase transitions, and identify differences of excipients in solid dosage form stability using dynamic vapor sorption analysis, near-infrared spectroscopy, and X-ray diffraction methods. The thermal processing was carried out with a variable temperature X-ray powder diffractometer to compare the dehydration behavior of wet excipients and evaluate solid-state properties during heating. These results showed that despite some limitations, moisture sorption isotherms of excipients are useful in predicting solid-state stability, interactions at early stages of formulation development, and effects of moisture on physicochemical properties of the final dosage forms.