The use of dynamic vapour sorption and near infra-red spectroscopy (DVS-NIR) to study the crystal transitions of theophylline and the report of a new solid-state transition

Routes of Theophylline Monohydrate Dehydration Process Proposed by Mid-Frequency Raman Difference Spectra

Two routes of the dehydration process of theophylline monohydrate have been proposed in this work from mid-frequency Raman difference spectra (MFRDS) results and experiments. MFRDS can establish short-range order correlations among various theophylline crystal forms. MFRDS results indicate that the short-range order of metastable Form III is most similar to that of monohydrate, which explains that Form III is the main dehydration products in the mild dehydration process. The phenomenon that unstable amorphous theophylline intermediate phase would appear during the dehydration process of theophylline monohydrate was confirmed indirectly by Powder X-ray Diffraction (PXRD) and optical microscope and reported in the previous reports, which could cause the nucleation of Form II, as MFRDS results indicate short-range order of amorphous solid dispersion of theophylline is most similar to that of Form II. MFRDS analysis shows the advantages in studying the phase transformation of small organic molecule crystals.

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.

Pre-nucleation aggregation based on solvent microheterogeneity

The microheterogeneous region of aqueous acetonitrile leads to preferred localisation and aggregation of caffeine and theophylline on the interface.

Crystal Structures and Vapor-Induced Crystalline Transformation of 7-Ethyl-10-Hydroxycamptothecin Pseudopolymorphs

Solvent-induced 3 frameworks (the monohydrate, methanol solvate, and dimethyl formamide [DMF] solvate) of an anticancer drug, 7-ethyl-10-hydroxycamptothecin (SN38), were constructed. The crystal structures of the 3 pseudopolymorphs were characterized by single-crystal X-ray diffraction technique for the first time. Crystal structure analysis of all these polymorphs revealed that the monohydrate showed 3D frameworks linked by hydrogen bonds. In the methanol solvate, the 1D molecular chains were linked by the methanol molecules through intermolecular hydrogen bonds to form a 2D sheet. The DMF solvate exhibits the 1D hydrogen bonding-linked chains along b-axis. Vapor-induced transformation has been investigated for the monohydrate, the methanol solvate, and the DMF solvate. The conversion relationships among the 3 solid frameworks are illustrated. Insights regarding transformation mechanisms have been established from consideration of the crystal structures.

On-line monitoring of vacuum drying of theophylline using NIR spectroscopy: solid-state transitions, water content and semi-empirical modeling

The aim of this work was to monitor in-line and at a real time, the solid-state forms during pharmaceuticals manufacturing. It concerns the dehydration behavior and the solid-state transitions of theophylline in an agitated vacuum contact dryer. First, a near infrared spectroscopy (NIRS) method was performed using a reflectance diffuse probe to measure the in-line and in-situ exact composition of the mixture of different forms of theophylline and water content during drying. A multivariate modeling has been investigated to build a robust model which can predict four components at the same time during drying process. The XRPD analysis was used as a reference method in the process of calibration of NIRS. The indicators of the accuracy in quantitative spectral analysis confirm the robustness of the model and the efficiency of the method of calibration. Second, the kinetics of solid state transformations were investigated. It was shown that the dehydration advanced first by the formation of the metastable anhydrate and after a lag time of the stable one. Once the stable form appeared, formation of the metastable form came to an end. The temperature was found out to be the main factor controlling the overall process rate but also the final contents of the stable and metastable anhydrates for the considered dryer and operating conditions range. Finally, a semi-empirical drying model was proposed and significant quantitative differences were found, particularly at the product temperature which was probably caused by the excessive simplicity of the model.

Concurrent oral and inhalation drug delivery using a dual formulation system: the use of oral theophylline carrier with combined inhalable budesonide and terbutaline

A novel approach to concurrently deliver oral and inhaled drugs as a single formulation is presented. A triple therapy containing theophylline (THEO; orally delivered) with budesonide (BUD) and terbutaline (TERB; as single and co-spray-dried inhaled powders) was prepared as an ordered mix, with THEO acting as a carrier. The aerosolisation performance of THEO formulations containing BUD and TERB alone, physical mix and co-spray-dried powder was evaluated using a next-generation impactor (NGI). Physicochemical properties were investigated using electron microscopy, laser diffraction, dynamic vapour sorption and thermal analysis. NGI analysis indicated that >99 % of the THEO powder was >4.46 μm, with >90 % dissolved within 5 min. Particle size analysis showed TEB and BUD samples were of a suitable size for inhalation. Thermal and moisture analysis suggested powders to be stable at room temperature up to 70 % RH. Aerosol studies indicated a different performance of BUD and TERB depending on the mixing procedure. The co-spray-dried formulation showed the highest performance, with a fine particle fraction (≤4.46 μm) of BUD and TERB of 34.39 ± 3.56 and 33.61 ± 5.67 %, respectively. Such observations suggest that this multicomponent drug delivery system could be developed to concomitantly deliver oral and inhaled drugs, an approach that, to date, does not exist. Ultimately, this technology potentially reduces the requirement for multiple therapies and increases patient compliance.

Cocrystal dissociation in the presence of water: a general approach for identifying stable cocrystal forms

In previous studies, cocrystals have been shown to be susceptible to dissociation at high humidity because of differences in the solubilities of the two coformer molecules, especially when these molecules can form hydrates. Contrastingly, however, the propensity of the pharmaceutically active compound caffeine to hydrate formation is reduced by cocrystallization with oxalic acid. Here, the stability of the oxalic acid cocrystal of caffeine is investigated from a thermodynamic perspective through the use of aqueous slurries of caffeine hydrate and oxalic acid dihydrate. Conversion to the anhydrous caffeine-oxalic acid cocrystal occurred under these conditions confirming that this form is thermodynamically stable in an aqueous environment. The slurry methodology was further developed as a general approach to screening for cocrystals that are not susceptible to dissociation at high humidity. In this manner, cocrystals of the hydrate-forming molecules theophylline, carbamazepine, and piroxicam that are stable at high humidity, indefinitely avoiding hydrate formation, were identified.

Analysis of solid-state transformations of pharmaceutical compounds using vibrational spectroscopy

Objectives

Solid-state transformations may occur during any stage of pharmaceutical processing and upon storage of a solid dosage form. Early detection and quantification of these transformations during the manufacture of solid dosage forms is important since the physical form of an active pharmaceutical ingredient can significantly influence its processing behaviour, including powder flow and compressibility, and biopharmaceutical properties such as solubility, dissolution rate and bioavailability.

Key findings

Vibrational spectroscopic techniques such as infrared, near-infrared, Raman and, most recently, terahertz pulsed spectroscopy have become popular for solidstate analysis since they are fast and non-destructive and allow solid-state changes to be probed at the molecular level. In particular, Raman and near-infrared spectroscopy, which require no sample preparation, are now commonly used coupled to fibreoptic probes and are able to characterise solid-state conversions in-line. Traditionally, uni- or bivariate approaches have been used to analyse spectroscopic data sets; however, recently the simultaneous detection of several solid-state forms has been increasingly performed using multivariate approaches where even overlapping spectral bands can be analysed.

Summary

This review discusses the applications of different vibrational spectroscopic techniques to detect and monitor solid-state transformations possible for crystalline polymorphs, hydrates and amorphous forms of pharmaceutical compounds. In this context, the theoretical basis of solid-state transformations and vibrational spectroscopy and common experimental approaches are described, including recent methods of data analysis.

A review of near infrared spectroscopy and chemometrics in pharmaceutical technologies

Near-infrared spectroscopy (NIRS) is a fast and non-destructive analytical method. Associated with chemometrics, it becomes a powerful tool for the pharmaceutical industry. Indeed, NIRS is suitable for analysis of solid, liquid and biotechnological pharmaceutical forms. Moreover, NIRS can be implemented during pharmaceutical development, in production for process monitoring or in quality control laboratories. This review focuses on chemometric techniques and pharmaceutical NIRS applications. The following topics are covered: qualitative analyses, quantitative methods and on-line applications. Theoretical and practical aspects are described with pharmaceutical examples of NIRS applications.

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%.

Hyphenation of Raman spectroscopy with gravimetric analysis to interrogate water–solid interactions in pharmaceutical systems

A moisture sorption gravimetric analyzer has been combined with a Raman spectrometer to better understand the various modes of water-solid interactions relevant to pharmaceutical systems. A commercial automated moisture sorption balance was modified to allow non-contact monitoring of the sample properties by interfacing a Raman probe with the sample holder. This hybrid instrument allows for gravimetric and spectroscopic changes to be monitored simultaneously. The utility of this instrument was demonstrated by investigating different types of water-solid interactions including stoichiometric and non-stoichiometric hydrate formation, deliquescence, amorphous-crystalline transformation, and capillary condensation. In each of the model systems, sulfaguanidine, cromolyn sodium, ranitidine HCl, amorphous sucrose and silica gel, spectroscopic changes were observed during the time course of the moisture sorption profile. Analysis of spectroscopic data provided information about the origin of the observed changes in moisture content as a function of relative humidity. Furthermore, multivariate data analysis techniques were employed as a means of processing the spectroscopic data. Principle components analysis was found to be useful to aid in data processing, handling and interpretation of the spectral changes that occurred during the time course of the moisture sorption profile.