Low-Frequency Raman Spectroscopic Study on Compression-Induced Destabilization in Melt-Quenched Amorphous Celecoxib

Calibration free approaches for rapid polymorph discrimination via low frequency (THz) Raman spectroscopy

Application of multivariate curve resolution to non-invasive Raman spectra has been investigated for rapid on-line analysis of crystallisation processes and high-throughput screening. Exploring quantification of mefenamic acid solid forms (form I, form II, and dimethylformamide solvate) from the Raman spectra indicated excellent agreement with off-line X-ray analysis.

Different or the same? exploring the physicochemical properties and molecular mobility of celecoxib amorphous forms

The influence of different preparation methods on the physicochemical properties of amorphous solid forms have gained considerable attention, especially with recent publications on pharmaceutical polyamorphism. In the present study, we have investigated the possible occurrence of polyamorphism in the drug celecoxib (CEL) by investigating the thermal behavior, morphology, structure, molecular mobility and physical stability of amorphous CEL obtained by quench-cooling (QC), ball milling (BM) and spray drying (SD). Similar glass transition temperatures but different recrystallization behaviors were observed for CEL-QC, CEL-BM and CEL-SD using modulated differential scanning calorimetry analysis. A correlation between the different recrystallization behaviors of the three CEL amorphous forms and the respective distinct powder morphologies, was also found. Molecular dynamics simulations however, reveal that CEL presents similar molecular conformational distributions when subjected to QC and SD. Moreover, the obtained molecular conformational distributions of CEL are different from the ones found in its crystal structure and also from the ones found in the lowest-energy structure obtained by quantum mechanical calculations. The type and strength of CEL hydrogen bond interactions found in CEL-QC and CEL-SD systems are almost identical, though different from the ones presented in the crystal structure. Pair distribution function analyses and isothermal microcalorimetry show similar local structures and structural relaxation times, respectively, for CEL-QC, CEL-BM and CEL-SD. The present work shows that not only similar physicochemical properties (glass transition temperature, and structural relaxation time), but also similar molecular conformational distributions were observed for all prepared CEL amorphous systems. Hence, despite their different recrystallization behaviors, the three amorphous forms of CEL did not show any signs of polyamorphism.

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.

Old Polymorph, New Technique: Assessing Ritonavir Crystallinity Using Low-Frequency Raman Spectroscopy

Two decades ago, postmarket discovery of a second crystal form of ritonavir with lower solubility had major implications for drug manufacturers and patients. Since then, ritonavir has been reformulated via the hot-melt-extrusion process in an amorphous form. Here, quantitative low- and mid-frequency Raman spectroscopy methods were developed to characterize polymorphs, form I and form II, in commercial ritonavir 100 mg oral tablets as an alternate analysis approach compared to X-ray powder diffraction (XRPD). Crystallization in three lots of ritonavir products obtained from four separate manufacturers was assessed after storage under accelerated conditions at 40 °C and 75% relative humidity (RH). Results were compared with quantitative XRPD methods developed and validated according to ICH Q2 (R1) guidelines. In a four-week open-dish study, form I crystallization occurred in two of the four products and form II crystallization was detected in another ritonavir product. The limits of detection for XRPD, low-frequency Raman (LFR), and mid-frequency Raman (MFR) were determined to be 0.7, 0.8, and 0.5% for form I and 0.6, 0.6, and 1% for form II, respectively. Root-mean-squared-error of predictions were 0.6-1.0 and 0.6-2.5% for LFR- and MFR-based partial least-squares models. Further, ritonavir polymorphs could also be identified and detected directly from ritonavir tablets using transmission LFR. In summary, LFR was applied for the assessment of polymorphism in real-world samples. While providing analytical performance similar to conventional techniques, LFR reduced the single measurement time from 66 min (XRPD) to 10 s (LFR) without the need for tedious sample preparation procedures.

Understanding the Effect of Nucleation in Amorphous Solid Dispersions through Time-Temperature Transformation

In an earlier investigation, the critical cooling rate to prevent drug crystallization (CRcrit) during the preparation of nifedipine (NIF) amorphous solid dispersions (ASDs) was determined through a time-temperature transformation (TTT) diagram (Lalge et al. Mol. Pharmaceutics 2023, 20 (3), 1806-1817). The current study aims to use the TTT diagram to determine the critical cooling rate to prevent drug nucleation (CRcrit N) during the preparation of ASDs. ASDs were prepared with each polyvinylpyrrolidone (PVP) and hydroxypropyl methylcellulose acetate succinate (HPMCAS). The dispersions were first stored under conditions promoting nucleation and then heated to the temperature that favors crystallization. The crystallization onset time (tC) was determined by differential scanning calorimetry and synchrotron X-ray diffractometry. TTT diagrams for nucleation were generated, which provided the critical nucleation temperature (50 °C) and the critical cooling rate to avoid nucleation (CRcrit N). The strength of the drug-polymer interactions as well as the polymer concentration affected the CRcrit N, with PVP having a stronger interaction than HPMCAS. The CRcrit of amorphous NIF was ∼17.5 °C/min. The addition of a 20% w/w polymer resulted in CRcrit of ∼0.05 and 0.2 °C/min and CRcrit N of ∼4.1 and 8.1 °C/min for the dispersions prepared with PVP and HPMCAS, respectively.

In Situ Monitoring of Drug Precipitation from Digesting Lipid Formulations Using Low-Frequency Raman Scattering Spectroscopy

Low-frequency Raman spectroscopy (LFRS) is a valuable tool to detect the solid state of amorphous and crystalline drugs in solid dosage forms and the transformation of drugs between different polymorphic forms. It has also been applied to track the solubilisation of solid drugs as suspensions in milk and infant formula during in vitro digestion. This study reports the use of LFRS as an approach to probe drug precipitation from a lipid-based drug delivery system (medium-chain self-nanoemulsifying drug delivery system, MC-SNEDDS) during in vitro digestion. Upon lipolysis of the digestible components in MC-SNEDDS containing fenofibrate as a model drug, sharp phonon peaks appeared at the low-frequency Raman spectral region (<200 cm^-1), indicating the precipitation of fenofibrate in a crystalline form from the formulation. Two multivariate data analysis approaches (principal component analysis and partial least squares discriminant analysis) and one univariate analysis approach (band ratios) were explored to track these spectral changes over time. The low-frequency Raman data produces results in good agreement with in situ small angle X-ray scattering (SAXS) measurements with all data analysis approaches used, whereas the mid-frequency Raman requires the use of PLS-DA to gain similar results. This suggests that LFRS can be used as a complementary, and potentially more accessible, technique to SAXS to determine the kinetics of drug precipitation from lipid-based formulations.

Impact of Crystal Nuclei on Dissolution of Amorphous Drugs

Amorphous drugs are used to improve bioavailability of poorly water-soluble drugs. Crystallization must be managed to take full advantage of this formulation strategy. Crystallization of amorphous drugs proceeds in a sequence of crystal nucleation and growth, with different kinetics. At low temperatures, crystal nucleation is fast, but crystal growth is slow. Therefore, amorphous drugs may generate dense but nanoscale crystal nuclei. Such tiny nuclei cannot be detected using routine powder X-ray diffraction (PXRD) and polarized light microscopy (PLM). However, they may negate the dissolution advantage of amorphous drugs. In this work, for the first time, the impact of crystal nuclei on dissolution of amorphous drugs was studied by monitoring the real-time dissolution from amorphous drug films, with and without crystal nuclei, and the evolving crystallinity in the films. Three model drugs (ritonavir/RTV, posaconazole/POS, and nifedipine/NIF) were chosen to represent different crystallization tendencies in the supercooled liquid state, namely, slow-nucleation-and-slow-growth (SN-SG), fast-nucleation-and-slow-growth (FN-SG), and fast-nucleation-and-fast-growth (FN-FG), respectively. We find that although the amorphous films containing nuclei do not show obvious differences from the nuclei-free films under PLM and PXRD before dissolution, they have inferior dissolution performance relative to the nuclei-free amorphous films. For SN-SG drug RTV, crystal nuclei have negligible impact on the crystallization of amorphous films, dissolution rate, and supersaturation achieved. However, they cause earlier de-supersaturation by inducing crystallization in solution as heterogeneous seeds. For FN-SG drug POS and FN-FG drug NIF, crystal nuclei accelerate crystallization in the amorphous films leading to lower supersaturation achieved with POS, and elimination of any supersaturation with NIF. Dissolution profiles of amorphous films can be further analyzed using a derivative function of the apparent dissolution rate, which yields amorphous solubility, initial intrinsic dissolution rate, and onset of crystallization in the amorphous films. This study highlights that although crystal nuclei are undetectable with routine analytical methods, they can significantly negate, or even eliminate, the dissolution advantage of amorphous drugs. Hence, understanding crystal nucleation process and developing approaches to prevent it are necessary to fully realize the benefits of amorphous solids.

Development of a multiparticulate drug delivery system for in situ amorphisation

In the current study, the concept of multiparticulate drug delivery systems (MDDS) was applied to tablets intended for the amorphisation of supersaturated granular ASDs in situ, i.e. amorphisation by microwave irradiation within the final dosage form. The MDDS concept was hypothesised to ensure geometric and structural stability of the dosage form and to improve the in vitro disintegration and dissolution characteristics. Granules were prepared in two sizes (small and large) containing the crystalline drug celecoxib (CCX) and polyvinylpyrrolidone/vinyl acetate copolymer (PVP/VA) at a 50 % w/w drug load as well as sodium dihydrogen phosphate monohydrate as the microwave absorbing excipient. The granules were subsequently embedded in an extra-granular tablet phase composed of either the filler microcrystalline cellulose (MCC) or mannitol (MAN), as well as the disintegrant crospovidone and the lubricant magnesium stearate. The tensile strength and disintegration time were investigated prior to and after 10 min of microwave irradiation (800 and 1000 W) and the formed ASDs were characterised by X-ray powder diffraction and modulated differential scanning calorimetry. Additionally, the internal structure was elucidated by X-ray micro-Computed Tomography (XµCT) and, finally, the dissolution performance of selected tablets was investigated. The MDDS tablets displayed no geometrical changes after microwave irradiation, however, the tensile strength and disintegration time increased. Complete amorphisation of CCX was achieved only for the MCC-based tablets at a power input of 1000 W, while MAN-based tablets displayed partial amorphisation independent of power input. The complete amorphisation of CCX was associated with the fusion of individual ASD granules within the tablets, which impacted the subsequent disintegration and dissolution performance. For these tablets, supersaturation was only observed after 60 min. On the other hand, the partially amorphised MDDS tablets displayed complete disintegration during the dissolution experiments, resulting in a fast onset of supersaturation within 5 min and an approx. 3.5-fold degree of supersaturation within the experimental timeframe (3 h). Overall, the MDDS concept was shown to potentially be a feasible dosage form for in situ amorphisation, however, there is still room for improvement to obtain a fully amorphous and disintegrating system.

Low- versus Mid-frequency Raman Spectroscopy for in Situ Analysis of Crystallization in Slurries

Slurry studies are useful for exhaustive polymorph and solid-state stability screening of drug compounds. Raman spectroscopy is convenient for monitoring crystallization in such slurries, as the measurements can be performed in situ even in aqueous environments. While the mid-frequency region (400–4000 cm^–1) is dominated by intramolecular vibrations and has traditionally been used for such studies, the low-frequency spectral region (<200 cm^–1) probes solid-state related lattice vibrations and is potentially more valuable for understanding subtle and/or complex crystallization behavior. The aim of the study was to investigate low-frequency Raman spectroscopy for in situ monitoring of crystallization of an amorphous pharmaceutical in slurries for the first time and directly compare the results with those simultaneously obtained with mid-frequency Raman spectroscopy. Amorphous indomethacin (IND) slurries were prepared at pH 1.2 and continuously monitored in situ at 5 and 25 °C with both low- and mid-frequency Raman spectroscopy. At 25 °C, both spectral regions profiled amorphous IND in slurries as converting directly from the amorphous form toward the α crystalline form. In contrast, at 5 °C, principal component analysis revealed a divergence in the detected conversion profiles: the mid-frequency Raman suggested a direct conversion to the α crystalline form, but the low-frequency region showed additional transition points. These were attributed to the appearance of minor amounts of the ε-form. The additional solid-state sensitivity of the low-frequency region was attributed to the better signal-to-noise ratio and more consistent spectra in this region. Finally, the low-frequency Raman spectrum of the ε-form of IND is reported for the first time.

Optimization of Methionine in Inhalable High-dose Spray-dried Amorphous Composite Particles using Response Surface Method, Infrared and Low frequency Raman Spectroscopy

The influence of amino acids, other than leucine, in improving aerosolization of inhalable powders has not been widely explored. This detailed study focused on the use of methionine, another promising endogenous amino acid, in high dose spray-dried co-amorphous powders by investigating the influence of methionine proportion (0 - 20% ^w/_w), and feed concentration (0.2 - 0.8% ^w/_v) on aerosolization of kanamycin, a model drug, using a design of experiment approach. Low frequency Raman spectroscopy was used to assess the stability of the powders stored at 25 °C/53% relative humidity over 28 days. An increase in concentration of methionine was associated with an increase in fine particle fraction (FPF), with the highest FPF of 84% being achieved at 20% ^w/_w and 0.2% ^w/_v feed concentration. With an increase in feed concentration, both yield and particle size increased for all formulations; the FPF did not change except for kanamycin only formulation in which it decreased. During storage at high humidity, similar aerosolization stabilities were offered by different proportions of methionine although methionine crystallized out in all formulations. Furthermore, the crystallization was accompanied by surface enrichment of methionine on the particles. This study suggests that there is a direct relationship between methionine content and aerosolization for kanamycin-methionine amorphous matrices but feed concentration has little effect. In addition, methionine proportion has no effect on physical stability of such matrices at high humidity.

Explanation for the selective crystallization from inosine solutions using mid-frequency Raman difference spectra analysis

Mid-frequency Raman difference spectra (MFRDS) analysis can be used to reveal the selective crystallization from solutions through determining the degree of similarity of the short-range orders between the assemblies of small organic molecules in solutions and their solid phases. Four solid phases of inosine (IR) (α-anhydrous IR (α-IR), β-anhydrous IR (β-IR), IR dihydrate (IRD), and amorphous IR (AmIR)) and two IR solutions (aqueous and 70 vol% DMSO aqueous solution) were prepared and characterized using MFRDS here. The MFRDS analysis results indicate that the selective formation of IRD and AmIR from IR aqueous solution and β-IR from IR 70 vol% DMSO solution are originated from the high similarity of their short-range structures. Moreover, we propose that the formation of α-IR from IR aqueous solution benefits from the appearance of AmIR as an intermediate phase. MFRDS is a robust tool to explain and predict the possible precipitation products from various solutions of small organic molecules.

The experimental phenomena that amorphous inosine (IR), α-IR, and IR dihydrate can form from IR aqueous solution and β-IR can crystallize from IR 70 vol% DMSO aqueous solution were explained using mid-frequency Raman difference spectra analysis.

Investigation on Formulation Strategies to Mitigate Compression-Induced Destabilization in Supersaturated Celecoxib Amorphous Solid Dispersions

Compression-induced destabilization was investigated in various celecoxib amorphous solid dispersions containing hydroxypropyl methylcellulose (HPMC), poly(vinylpyrrolidone)/vinyl acetate copolymer (PVP/VA), or poly(vinylpyrrolidone) (PVP) at a concentration range of 1-10% w/w. Pharmaceutically relevant (125 MPa pressure with a minimal dwell time) and extreme (500 MPa pressure with a 60 s dwell time) compression conditions were applied to these systems, and the changes in their physical stability were monitored retrospectively (i.e., in the supercooled state) using dynamic differential scanning calorimetry (DSC) and low-frequency Raman (LFR) measurements over a broad temperature range (-90 to 200 and -150 to 140 °C, respectively). Both techniques revealed similar changes in the crystallization behavior between samples, where the application of a higher compression force of 500 MPa resulted in a more pronounced destabilization effect that was progressively mitigated with increasing polymer content. However, other aspects such as more favorable intermolecular interactions did not appear to have any effect on reducing this undesirable effect. Additionally, for the first time, LFR spectroscopy was used as a viable technique to determine the secondary or local glass-transition temperature, T_g,β, a major indicator of the physical stability of neat amorphous pharmaceutical systems.

Microwave induced in situ amorphisation facilitated by crystalline hydrates

Amorphisation within the final dosage form, i.e. in situ amorphisation, seeks to circumvent the potential stability issues associated with poorly soluble drugs in amorphous solid dispersions (ASDs). Microwave irradiation has previously been shown to enable in situ preparation of ASDs, when a high amount of microwave absorbing water was introduced into the final dosage form by conditioning at high relative humidity. In this study, an alternative to this conditioning step was investigated by introducing crystal water in form of sodium dihydrogen phosphate (NaH₂PO₄) di-, and monohydrate, in compacts prepared with 30 % w/w celecoxib (CCX) in polyvinylpyrrolidone K12 (PVP). As controls, compacts prepared with NaH₂PO₄ anhydrate and without NaH₂PO₄ were included in the study. The quantification of amorphous CCX after microwave irradiation showed an increase in CCX amorphicity for compacts containing NaH₂PO₄ di-, and monohydrate with increasing irradiation time. Complete amorphisation of CCX in compacts containing NaH₂PO₄ di-, and monohydrate was observed after 6 min, while no appreciable amorphisation was observed for the control compacts containing NaH₂PO₄ anhydrate and without NaH₂PO₄. Modulated differential scanning calorimetric analysis revealed that a homogenous ASD was formed after 12 min and 6 min for compacts containing NaH₂PO₄ di-, and monohydrate, respectively. Thermal gravimetric analysis indicated that NaH₂PO₄ monohydrate showed higher dehydration rates compared to the dihydrate, which in turn resulted in higher compact temperatures, and overall increased the rate of amorphisation and reduced the microwave irradiation time necessary to achieve a homogenous ASD. The present results confirmed the suitability of NaH₂PO₄ di- and monohydrate as alternative sources of water, the primary microwave absorbing material, for in situ microwave amorphisation. The use of crystalline hydrates as water reservoirs for in situ amorphisation circumvents the time-consuming and highly impractical conditioning step previously reported in order to achieve complete amorphisation. Additionally, it allows for easier and more accurate adjustment of the compacts water content, which directly affects the temperature reached during microwave irradiation, and thus, the rate of amorphisation.

A New Frontier for Nondestructive Spatial Analysis of Pharmaceutical Solid Dosage Forms: Spatially Offset Low-Frequency Raman Spectroscopy

A new Raman subtechnique, spatially offset low-frequency Raman spectroscopy (SOLFRS), is demonstrated via an analysis of pharmaceutical solid dosage forms. Several different model systems comprised of celecoxib (a popular anti-inflammatory drug), α-lactose anhydrous stable form, α-lactose monohydrate, and polyvinylpyrrolidone (PVP) were used to represent tangible scenarios for the application of SOLFRS. Additionally, the challenges and limitations were highlighted in relation to its real-time use, and potential solutions to address them were also provided. Lastly, the future directions for this new variation of Raman spectroscopic technique were briefly discussed, including its potential for broader application in pharmaceutical analysis and other research fields.

Combined Effect of the Preparation Method and Compression on the Physical Stability and Dissolution Behavior of Melt-Quenched Amorphous Celecoxib

In an earlier investigation, amorphous celecoxib was shown to be sensitive to compression-induced destabilization. This was established by evaluating the physical stability of uncompressed/compressed phases in the supercooled state (Be̅rziņš . Mol. Pharmaceutics, 2019, 16(8), 3678-3686). In this study, we investigated the ramifications of compression-induced destabilization in the glassy state as well as the impact of compression on the dissolution behavior. Slow and fast melt-quenched celecoxib disks were compressed with a range of compression pressures (125-500 MPa) and dwell times (0-60 s). These were then monitored for crystallization using low-frequency Raman spectroscopy when kept under dry (∼20 °C; <5% RH) and humid (∼20 °C; 97% RH) storage conditions. Faster crystallization was observed from the samples, which were compressed using more severe compression parameters. Furthermore, crystallization was also affected by the cooling rate used to form the amorphous phases; slow melt-quenched samples exhibited higher sensitivity to compression-induced destabilization. The behavior of the melt-quench disks, subjected to different compression conditions, was continuously monitored during dissolution using low-frequency Raman and UV/vis for the solid-state form and dissolution properties, respectively. Surprisingly the compressed samples exhibited higher apparent dissolution (i.e., higher area under the dissolution curve and initial celecoxib concentration in solution) than the uncompressed samples; however, this is attributed to biaxial fracturing throughout the compressed compacts yielding a greater effective surface area. Differences between the slow and fast melt quenched samples showed some trends similar to those observed for their storage stability.

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.

Co-Amorphization of Kanamycin with Amino Acids Improves Aerosolization

Different formulation techniques have been investigated to prepare highly aerosolizable dry powders to deliver a high dose of antibiotics to the lung for treating local infections. In this study, we investigated the influence of the co-amorphization of a model drug, kanamycin, with selected amino acids (valine, methionine, phenylalanine, and tryptophan) by co-spray drying on its aerosolization. The co-amorphicity was confirmed by thermal technique. The physical stability was monitored using low-frequency Raman spectroscopy coupled with principal component analysis. Except for the kanamycin-valine formulation, all the formulations offered improved fine particle fraction (FPF) with the highest FPF of 84% achieved for the kanamycin-methionine formulation. All the co-amorphous formulations were physically stable for 28 days at low relative humidity (25 °C/<15% RH) and exhibited stable aerosolization. At higher RH (53%), even though methionine transformed into its crystalline counterpart, the kanamycin-methionine formulation offered the best aerosolization stability without any decrease in FPF. While further studies are warranted to reveal the underlying mechanism, this study reports that the co-amorphization of kanamycin with amino acids, especially with methionine, has the potential to be developed as a high dose kanamycin dry powder formulation.