Dehydration Kinetics and Crystal Water Dynamics of Carbamazepine Dihydrate

Exploring the Solid-State Landscape of Carbamazepine during Dehydration: A Low Frequency Raman Spectroscopy Perspective

The solid-state landscape of carbamazepine during its dehydration was explored using Raman spectroscopy in the low- (-300 to -15, 15 to 300) and mid- (300 to 1800 cm^-1) frequency spectral regions. Carbamazepine dihydrate and forms I, III, and IV were also characterized using density functional theory with periodic boundary conditions and showed good agreement with experimental Raman spectra with mean average deviations less than 10 cm^-1. The dehydration of carbamazepine dihydrate was examined under different temperatures (40, 45, 50, 55, and 60 °C). Principal component analysis and multivariate curve resolution were used to explore the transformation pathways of different solid-state forms during the dehydration of carbamazepine dihydrate. The low-frequency Raman domain was able to detect the rapid growth and subsequent decline of carbamazepine form IV, which was not as effectively observed by mid-frequency Raman spectroscopy. These results showcased the potential benefits of low-frequency Raman spectroscopy for pharmaceutical process monitoring and control.

Dehydration kinetics and mechanism of the stable isonicotinamide hydrate revealed by terahertz spectroscopy and DFT calculation

The dehydration behavior of pharmaceutical hydrates has a great influence on its physiochemical properties such as stability, dissolution rate and bioavailability. However, how the intermolecular interactions vary during dehydration process remains elusive. In this work, we employed terahertz time-domain spectroscopy (THz-TDS) to probe the low-frequency vibrations and the dehydration process of isonicotinamide hydrate I (INA-H I). Theoretical solid-state DFT calculation was conducted to reveal its mechanism. Vibrational modes which are responsible for the THz absorption peaks were decomposed for better understanding the characters of these low-frequency modes. The result suggests translational motion is the dominant component for water molecules in THz region. Evolution of the THz spectrum of INA-H I during dehydration provides direct evidence of the variations of crystal structure. Based on the THz measurements, a two-step kinetics mode including first-rate reaction and three-dimensional nuclei growth is proposed. And we nure that the low-frequency vibrations of water molecules are the origin of dehydration process of hydrate.

Understanding stress-induced disorder and breakage in organic crystals: beyond crystal structure anisotropy

Crystal engineering has advanced the strategies for design and synthesis of organic solids with the main focus being on customising the properties of the materials. Research in this area has a significant impact on large-scale manufacturing, as industrial processes may lead to the deterioration of such properties due to stress-induced transformations and breakage. In this work, we investigate the mechanical properties of structurally related labile multicomponent solids of carbamazepine (CBZ), namely the dihydrate (CBZ·2H2O), a cocrystal of CBZ with 1,4-benzoquinone (2CBZ·BZQ) and the solvates with formamide and 1,4-dioxane (CBZ·FORM and 2CBZ·DIOX, respectively). The effect of factors that are external (e.g. impact stressing) and/or internal (e.g. phase transformations and thermal motion) to the crystals are evaluated. In comparison to the other CBZ multicomponent crystal forms, CBZ·2H2O crystals tolerate less stress and are more susceptible to breakage. It is shown that this poor resistance to fracture may be a consequence of the packing of CBZ molecules and the orientation of the principal molecular axes in the structure relative to the cleavage plane. It is concluded, however, that the CBZ lattice alone is not accountable for the formation of cracks in the crystals of CBZ·2H2O. The strength and the temperature-dependence of electrostatic interactions, such as hydrogen bonds between CBZ and coformer, appear to influence the levels of stress to which the crystals are subjected that lead to fracture. Our findings show that the appropriate selection of coformer in multicomponent crystal forms, targetting superior mechanical properties, needs to account for the intrinsic stress generated by molecular vibrations and not solely by crystal anisotropy. Structural defects within the crystal lattice, although highly influenced by the crystallisation conditions and which are especially difficult to control in organic solids, may also affect breakage.

Revealing the early stages of carbamazepine crystallization by cryoTEM and 3D electron diffraction

Time-resolved carbamazepine crystallization from wet ethanol has been monitored using a combination of cryoTEM and 3D electron diffraction. Carbamazepine is shown to crystallize exclusively as a dihydrate after 180 s. When the timescale was reduced to 30 s, three further polymorphs could be identified. At 20 s, the development of early stage carbamazepine dihydrate was observed through phase separation. This work reveals two possible crystallization pathways present in this active pharmaceutical ingredient.

Periodic DFT Calculations-Review of Applications in the Pharmaceutical Sciences

In the introduction to this review the complex chemistry of solid-state pharmaceutical compounds is summarized. It is also explained why the density functional theory (DFT) periodic calculations became recently so popular in studying the solid APIs (active pharmaceutical ingredients). Further, the most popular programs enabling DFT periodic calculations are presented and compared. Subsequently, on the large number of examples, the applications of such calculations in pharmaceutical sciences are discussed. The mentioned topics include, among others, validation of the experimentally obtained crystal structures and crystal structure prediction, insight into crystallization and solvation processes, development of new polymorph synthesis ways, and formulation techniques as well as application of the periodic DFT calculations in the drug analysis.

Influence of Carbamazepine Dihydrate on the Preparation of Amorphous Solid Dispersions by Hot Melt Extrusion

Amorphous solid dispersions (ASDs) are commonly used in the pharmaceutical industry to improve the dissolution and bioavailability of poorly water-soluble drugs. Hot melt extrusion (HME) has been employed to prepare ASD based products. However, due to the narrow processing window of HME, ASDs are normally obtained with high processing temperatures and mechanical stress. Interestingly, one-third of pharmaceutical compounds reportedly exist in hydrate forms. In this study, we selected carbamazepine (CBZ) dihydrate to investigate its solid-state changes during the dehydration process and the impact of the dehydration on the preparation of CBZ ASDs using a Leistritz micro-18 extruder. Various characterization techniques were used to study the dehydration kinetics of CBZ dihydrate under different conditions. We designed the extrusion runs and demonstrated that: 1) the dehydration of CBZ dihydrate resulted in a disordered state of the drug molecule; 2) the resulted higher energy state CBZ facilitated the drug solubilization and mixing with the polymer matrix during the HME process, which significantly decreased the required extrusion temperature from 140 to 60 °C for CBZ ASDs manufacturing compared to directly processing anhydrous crystalline CBZ. This work illustrated that the proper utilization of drug hydrates can significantly improve the processability of HME for preparing ASDs.

Low-frequency Raman spectrophotometer with wide laser illumination on the sample: A tool for pharmaceutical analytical analysis

This work describes an optical configuration for a Raman spectrophotometer, which permits variation of the laser spot size from 3 to 3000 μm, maintaining a high Raman photons throughput and allowing acquisitions with a short integration time. In addition, the instrument can acquire spectra from the low to middle frequency vibrational range (10 to 2000 cm^-1), on the Stokes and anti-Stokes sides. One of the features of this new optical configuration is the non-use of beam splitters to redirect the scattered light to the detector, which would sacrifice the laser power. The quantitative and qualitative analytical performances of the Raman spectrophotometer were evaluated using chemometric models to predict the concentrations of different active pharmaceutical ingredients (APIs) in mixtures with polymorphs and excipients, as well as by analysis of an API mixture employing hyperspectral imaging. This new optical configuration was shown to be versatile for pharmaceutical purposes and could be used in applications such as the characterization of new drugs or the quality control of raw materials and processes, using normal Raman measurements or SERS (surface-enhanced Raman scattering).

Dissolution enhancement of carbamazepine using twin-screw melt granulation

The current study explored the twin-screw melt granulation (TSMG) as a potential technology for the water solubility enhancement of biopharmaceutical classification system (BCS) class II drugs. As a model drug, carbamazepine (CBZ) was formulated with three different polymers as melt granules produced in a co-rotating twin-screw granulator. Polyethylene glycol 6000 (PEG 6000) and Kolliphor® (poloxamer) P407 were used as binding materials at two different granulation temperatures (T_max: 70 °C; 100 °C). Additionally, Soluplus® (polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer) was chosen as binder of higher melting/ granulation temperature (T_max: 140 °C). Temperature dependent polymorphic transition of CBZ during melt granulation was observed and identified using XRPD- (X-ray powder diffraction) and FTIR- (Fourier transform infrared spectroscopy) analysis. The effects of polymer type, polymer content (10, 15, 20% (w/w)) and granulation temperature on polymorphic transition, their impact on wettability (contact angle via drop shape-analysis), and the resulting dissolution performance at non-sink conditions in phosphate buffer (pH 6.8), were studied. This study showed that TSMG led to a crystalline system facilitating supersaturation when brought in solution, even when high drug loads (up to 90% (w/w)) were used. In general, for all granules produced, the supersaturation level and its duration varied with the extent of polymorphic transition and binder concentration. The results of this study indicated the importance of temperature control and polymer selection for tailoring desired dissolution profiles.

New Insights on Solid-State Changes in the Levothyroxine Sodium Pentahydrate during Dehydration and its Relationship to Chemical Instability

Levothyroxine sodium pentahydrate (LEVO) tablets have been on the US market since the mid-twentieth century and remain the most highly prescribed product. Unfortunately, levothyroxine sodium tablets have also been one of the most highly recalled products due to potency and dissolution failures on stability. In 2008, the assay limits were tightened, yet the recalls did not decline, which highlights the serious quality concerns remaining to be elucidated. The aim of the present investigation was to test the hypothesis that the solid-state physical instability of LEVO precedes the chemical instability leading to product failure. The failure mode was hypothesized to be the dehydration of the crystal hydrate, when exposed to certain humidity and temperature conditions, followed by the oxidation of the API through vacated channels. It was previously reported by the authors that LEVO degradation occurred in the presence of oxygen at a low relative humidity (RH). Furthermore, powder X-ray diffractometry shows changes in the crystal lattice of LEVO initially and through the dehydration stages. Storage of LEVO at RT and 40 °C at 4-6% RH for 12 days shows a decrease in d-spacing of the (00 l) planes. Based on a structure solution from the powder data of the dehydrated material, the basic packing motif persists to varying degrees even when fully dehydrated along with disordering. Therefore, the crystal structure changes of LEVO depend on RH and temperature and are now explicable at the structural level for the first time. This exemplifies the dire need for "new prior knowledge" in generic product development.

Computational Dehydration of Crystalline Hydrates Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have evolved to an increasingly reliable and accessible technique and are today implemented in many areas of biomedical sciences. We present a generally applicable method to study dehydration of hydrates based on MD simulations and apply this approach to the dehydration of ampicillin trihydrate. The crystallographic unit cell of the trihydrate is used to construct the simulation cell containing 216 ampicillin and 648 water molecules. This system is dehydrated by removing water molecules during a 2200 ps simulation, and depending on the computational dehydration rate, different dehydrated structures were observed. Removing all water molecules immediately and removing water relatively fast (10 water molecules/10 ps) resulted in an amorphous system, whereas relatively slow computational dehydration (3 water molecules/10 ps) resulted in a crystalline anhydrate. The structural changes could be followed in real time, and in addition, an intermediate amorphous phase was identified. The computationally identified dehydrated structure (anhydrate) was slightly different from the experimentally known anhydrate structure suggesting that the simulated computational structure could represent a kinetically trapped dehydration intermediate.

Structures of cefradine dihydrate and cefaclor dihydrate from DFT-D calculations

The crystal structure of cefradine dihydrate, C16H19N3O4S·2H2O, is considered in the pharmaceutical sciences to be the epitome of an isolated-site hydrate. The structure from single-crystal X-ray data was described in 1976, but atomic coordinates were not published. The atomic coordinates are determined here by combining the information available from the published single-crystal data with a dispersion-corrected density functional theory (DFT-D) method that has been validated to reproduce molecular crystal structures very accurately. Additional proof for the correctness of the structure comes from comparison with cefaclor dihydrate, C15H14ClN3O4S·2H2O, which is isomorphous and for which more complete single-crystal data are available. H-atom positions have not previously been published for either compound. The DFT-D calculations confirm that both cefradine and cefaclor are present in the zwitterionic form in the two dihydrate structures. A potential ambiguity concerning the orientation of the cyclohexadienyl ring in cefradine dihydrate is also clarified, and on the basis of the calculated energies it is shown that disorder should not be expected at room temperature. The DFT-D methods can be applied to recover full structural data in cases where only partial information is available, and where it may not be possible or desirable to obtain new experimental data.