Identification of Pseudopolymorphism of Magnesium Stearate by Using Low-Frequency Raman Spectroscopy

Method for determining resin cure kinetics with low-frequency Raman spectroscopy

Characterizing resin extent of cure kinetics is critical to understanding the structure-property-processing relationships of polymers. The disorder band present in the low-frequency region of the Raman spectrum is directly related to conformational entropy and the modulus of amorphous materials, both of which change as the resin polymerizes. Normalizing the disorder band to its shoulder (∼85 cm-1) provides structural conversion kinetics, which we can directly correlate to chemical conversion kinetics for methacrylate and epoxy-amine based resin systems. In addition to fitting both the structural and chemical conversion data to a phenomenological kinetic rate equation, we also demonstrate a relationship between the chemical and structural kinetics which appears to relate to the softness of the material. Lastly, we use the method to investigate a methacrylate/epoxy interpenetrating polymer network resin system. We find that the structural and chemical conversions occur simultaneously during the formation of the primary (methacrylate) network, but there is a lag between the two during the formation of the secondary (epoxy-amine) network.

Monitoring of Cocrystal Dissociation during the Wet Granulation Process in the Presence of Disintegrants by Using Low-Frequency Raman Spectroscopy

The aim of this study was to evaluate the effect of three coformers and five disintegrants in the granulation formulation on the dissociation of cocrystal during the granulation process by monitoring wet granulation with probe-type low-frequency Raman (LF-Raman) spectroscopy. As model cocrystals, paracetamol (APAP)-oxalic acid (OXA), APAP-maleic acid (MLA), and APAP-trimethylglycine (TMG) were used. The monitoring of the granulation recipe containing cocrystals during wet granulation was performed over time with high-performance LF-Raman spectrometry and the dissociation rate was calculated from the results of multivariate analysis of LF-Raman spectra. The dissociation rate decreased in the order of APAP-TMG, APAP-OXA, and APAP-MLA, showing the same order as observed in Powder X-ray diffraction measurements. Furthermore, to compare the effect of disintegrants on the dissociation rate of APAP-OXA, LF-Raman monitoring was performed for the granulation recipes containing five typical disintegrants (two low-substitution hydroxypropyl cellulose (HPC), cornstarch (CSW), carmellose sodium (CMC), and crospovidone (CRP)). The dissociation rate of APAP-OXA decreased in the order of CSW, HPCs, CMC, and CRP. This difference in the dissociation rate of APAP-OXA was thought to be due to the disintegration mechanism of the disintegrants and the water absorption ratio, which was expected to affect the water behavior on the disintegrant surface during wet granulation. These results suggested that probe-type LF-Raman spectroscopy is useful to monitor the dissociation behavior of cocrystals during wet granulation and can compare the relative stability of cocrystal during wet granulation between different formulations.

Solid-State Analysis of Alpha-Cyclodextrin Inclusion Complexes Using Low-Frequency Raman Spectroscopy

A rapid and nondestructive analytical technique is critical for the analysis of cyclodextrin inclusion complexes in solid dosage forms. This study proposed a newly developed low-frequency Raman spectroscopy as a candidate technique for the analysis of cyclodextrin inclusion complexes. In this study, we selected a typical series of five crystalline cyclodextrin inclusion complexes and reported the usefulness of Raman spectroscopy for analyzing these inclusion complexes. Some inclusion complexes clearly differed from the raw materials in conventional Raman spectra. In another case, though specific differences were not observed between inclusion complexes and raw materials in conventional Raman spectra, clear differences were observed in low-frequency Raman spectra. Moreover, no characteristic differences between inclusion complexes consisting of different guest molecules were observed in conventional Raman spectra. The characteristic differences were observed only in low-frequency Raman spectra. Therefore, low-frequency Raman spectroscopy is a useful technique for solid-state analysis of crystalline inclusion complexes.

In situ monitoring of the crystalline state of active pharmaceutical ingredients during high-shear wet granulation using a low-frequency Raman probe

Optimization of manufacturing processes based on scientific evidence is important in the quality control of active pharmaceutical ingredients (APIs) and drug products, particularly when crystal forms change during production, which could affect subsequent drug performance. In this study, we verified crystalline states using various crystal faces and excipients during high-shear wet granulation based on non-contact low-frequency (LF) Raman probe monitoring. Four model drugs [indomethacin (IND), acetaminophen (APAP), theophylline (TP), and caffeine (CAF) polymorphs and cocrystals] were mixed with microcrystalline cellulose and hydroxypropyl cellulose with the addition of water over time. The LF Raman probe showed comparatively high sensitivity in monitoring 5-20% APAP and IND in a wet mass. Notably, as observed from the characteristic LF Raman peak shifts, form I TP and CAF and their cocrystals were more susceptible to transformation to the monohydrate form than form II. This method was also shown to be applicable in monitoring a commercial formulation of eight excipients and revealed crystalline transformations after 15 min of mixing. Therefore, probe-type LF Raman spectroscopy can be successfully employed to distinguish and monitor the crystalline state of APIs in real time during high-shear wet granulation, in which there is a risk of crystal transformation.

Comparative Evaluation of the Photostability of Carbamazepine Polymorphs and Cocrystals

Carbamazepine (CBZ), a widely used antiepileptic, is known to be sensitive to light. The aim of this study was to evaluate the photostabilities of three cocrystals of CBZ (CBZ–succinic acid (SUC), CBZ–saccharin (SAC) form I, and CBZ–SAC form II) illuminated with a D65 fluorescent lamp compared with those of the conventional solid forms: CBZ polymorphs (forms I, II, and III). The order of discoloration determined using a colorimetric measurement was almost consistent with that of the degradation rates estimated using Fourier-transform infrared reflection–absorption spectroscopy, and these parameters of CBZ polymorphs increased in the order of form III, form I, and form II. CBZ–SUC and CBZ–SAC form I significantly suppressed the discoloration and degradation of CBZ compared with the raw CBZ, while CBZ–SAC form II facilitated the discoloration and degradation of CBZ. These results were supported by the results from the low-frequency Raman spectroscopy. The molecular mobility estimated using solid-state nuclear magnetic resonance 1H spin–lattice relaxation time strongly correlated with the degradation rate constant, indicating that molecular mobility significantly decreased following the formation of CBZ–SUC and CBZ–SAC form I and resulted in higher photostability. Overall, CBZ–SUC and CBZ–SAC form I are photostable forms and cocrystallization was proven to be an effective approach to improving the photostability of a photolabile drug.

Two-Dimensional Correlation Spectroscopy (2D-COS) Studies of Solution Mixtures in the Low Frequency Raman Region

Raman spectra of a series of binary solution mixtures, including chloroform (CHCl₃), ethanol (EtOH), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), were analyzed using the two-dimensional correlation spectroscopic (2D-COS) technique in the low frequency region. Numerous asynchronous cross-peaks ubiquitously appeared in the concentration-dependent Raman spectra of these organic solvent mixtures. The result clearly demonstrated a deviation from ideal solution behavior, reflecting the presence of specific molecular interactions causing a subtle nonlinear spectral intensity response of Raman bands to the concentration changes. Furthermore, the combination of 2D-COS and low frequency Raman spectroscopy was extended to poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) copolymer solutions in CHCl₃-HFIP co-solvents. The results suggest the existence of hydrogen bonding interaction between the PHBHx and HFIP, which is consistent with the previous infrared spectroscopic study of PHBHx solutions.

Screening a trace amount of pharmaceutical cocrystals by using an enhanced nano-spot method

Cocrystallization is an attractive and promising technology that can improve the physical properties of formulations of active pharmaceutical ingredients (APIs). We have developed a "nano-spot method" that can evaluate the crystalline form on the nanogram scale. In this study, the following studies were performed to obtain versatile and comprehensive improvements to the nano-spot method: modification of the sample solution, application of solvent vapor exposure to attempt the precipitation of various states of crystals, and adoption of low-frequency Raman spectroscopy. Carbamazepine was used as a model API and cocrystallization screening was examined with 12 cocrystal formers (coformers). In the case of combinations that are already known to form cocrystals, spectra similar to those of previously reported cocrystals or new spectra were obtained. It was considered that the reported cocrystals or new polymorphs were obtained. In contrast, in the case of the combination which has been reported not to form a cocrystal, the spectra were consistent with that for the physical mixture of API and coformer, suggesting that a cocrystal also did not form in this screening. In addition, the newly adopted low-frequency Raman spectroscopy enabled the high-sensitive detection of the crystalline form.