Crystallization kinetics and molecular mobility of an amorphous active pharmaceutical ingredient: A case study with Biclotymol

Stability of Rapidly Crystallizing Sulfonamides Glasses by Fast Scanning Calorimetry: Crystallization Kinetics and Glass-Forming Ability

Production and evaluation of the kinetic stability of the amorphous forms of active pharmaceutical ingredients are among the current challenges of modern pharmaceutical science. In the present work, amorphous forms of several sulfonamides were produced for the first time using Fast Scanning calorimetry. The parameters, characterizing the glass-forming ability of the compounds, i.e. the critical cooling rate of the melt and the kinetic fragility, were determined. The cold crystallization kinetics was studied using both isothermal and non-isothermal approaches. The results of the present study will contribute to the development of approaches for producing amorphous forms of rapidly crystallizing active pharmaceutical ingredients.

The Study of Amorphous Kaempferol Dispersions Involving FT-IR Spectroscopy

Attenuated total reflection-Mid-Fourier transform-infrared (ATR-Mid-FT-IR) spectroscopy combined with principal component analysis (PCA) has been applied for the discrimination of amorphous solid dispersion (ASD) of kaempferol with different types of Eudragit (L100, L100-55, EPO). The ASD samples were prepared by ball milling. Training and test sets for PCA consisted of a pure compound, physical mixture, and incomplete/complete amorphous solid dispersion. The obtained results confirmed that the range 400-1700 cm-1 was the major contributor to the variance described by PC1 and PC2, which are the fingerprint region. The obtained PCA model selected fully amorphous samples as follows: five for KMP-EL100, two for KMP-EL100-55, and six for KMP-EPO (which was confirmed by the XRPD analysis). DSC analysis confirmed full miscibility of all ASDs (one glass transition temperature). FT-IR analysis confirmed the formation of hydrogen bonds between the -OH and/or -CH groups of KMP and the C=O group of Eudragits. Amorphization improved the solubility of kaempferol in pH 6.8, pH 5.5, and HCl 0.1 N.

Mechanistic insights into the crystallization of coamorphous drug systems

In our previous study, the coamorphous formulation of lurasidone hydrochloride (LH) with saccharin (SAC) showed significantly enhanced dissolution and physical stability compared to crystalline/amorphous LH. However, the coamorphous system is still in amorphous state, and has the tendency to recrystallization, which will in turn result in the loss of above advantages. In this study, the crystallization kinetics under isothermal and non-isothermal conditions was investigated. Compared to amorphous LH, coamorphous LH-SAC showed 68.3-361.2 and 2.6-6.1 times lower crystallization rates in glassy state and supercooled liquid state, respectively. After co-amorphization, the addition of SAC changed the crystallization mechanism of amorphous LH from nucleation-controlled to diffusion-controlled manner. Amorphous LH followed the site-saturated nucleation, whereas the coamorphous system exhibited a fixed number of nuclei. The non-isothermal crystallization indicated amorphous LH and coamorphous LH-SAC showed two-dimensional (JMAEK 2) and three-dimensional (JMAEK 3) growth of nuclei, respectively. Furthermore, coamorphous LH-SAC exhibited higher molecular mobility and dynamic fragility (m_D) than amorphous LH, which is kinetically unfavorable for its physical stability. However, from thermodynamic perspective, coamorphous LH-SAC had a higher configurational entropy, i.e., a higher entropy barrier for crystallization, which is beneficial to hinder its crystallization. Therefore, it was concluded that the higher configurational entropy rather than the molecular mobility was proposed to be responsible for its improved stability. In addition, molecular dynamics simulations with miscibility, radial distribution function and binding energy calculations suggested coamorphous components exhibited good miscibility and strong intermolecular interactions, which was also conductive to the enhancement in its stability. This study offers an in-depth understanding about the effect of the coformer on the crystallization kinetics of coamorphous systems, and points out the important contribution of the configurational entropy in stabilizing the coamorphous systems.

The Effect of Various Poly (N-vinylpyrrolidone) (PVP) Polymers on the Crystallization of Flutamide

In this study, several experimental techniques were applied to probe thermal properties, molecular dynamics, crystallization kinetics and intermolecular interactions in binary mixtures (BMs) composed of flutamide (FL) and various poly(N-vinylpyrrolidone) (PVP) polymers, including a commercial product and, importantly, samples obtained from high-pressure syntheses, which differ in microstructure (defined by the tacticity of the macromolecule) from the commercial PVP. Differential Scanning Calorimetry (DSC) studies revealed a particularly large difference between the glass transition temperature (T_g) of FL+PVPsynth. mixtures with 10 and 30 wt% of the excipient. In the case of the FL+PVPcomm. system, this effect was significantly lower. Such unexpected findings for the former mixtures were strictly connected to the variation of the microstructure of the polymer. Moreover, combined DSC and dielectric measurements showed that the onset of FL crystallization is significantly suppressed in the BM composed of the synthesized polymers. Further non-isothermal DSC investigations carried out on various FL+10 wt% PVP mixtures revealed a slowing down of FL crystallization in all FL-based systems (the best inhibitor of this process was PVP M_n = 190 kg/mol). Our research indicated a significant contribution of the microstructure of the polymer on the physical stability of the pharmaceutical—an issue completely overlooked in the literature.

Combinations of Freeze-Dried Amorphous Vardenafil Hydrochloride with Saccharides as a Way to Enhance Dissolution Rate and Permeability

To improve physicochemical properties of vardenafil hydrochloride (VAR), its amorphous form and combinations with excipients-hydroxypropyl methylcellulose (HPMC) and β-cyclodextrin (β-CD)-were prepared. The impact of the modification on physicochemical properties was estimated by comparing amorphous mixtures of VAR to their crystalline form. The amorphous form of VAR was obtained as a result of the freeze-drying process. Confirmation of the identity of the amorphous dispersion of VAR was obtained through the use of comprehensive analysis techniques-X-ray powder diffraction (PXRD) and differential scanning calorimetry (DSC), supported by FT-IR (Fourier-transform infrared spectroscopy) coupled with density functional theory (DFT) calculations. The amorphous mixtures of VAR increased its apparent solubility compared to the crystalline form. Moreover, a nearly 1.3-fold increase of amorphous VAR permeability through membranes simulating gastrointestinal epithelium as a consequence of the changes of apparent solubility (P_{app crystalline VAR} = 6.83 × 10^-6 cm/s vs. P_{app amorphous VAR} = 8.75 × 10^-6 cm/s) was observed, especially for its combinations with β-CD in the ratio of 1:5-more than 1.5-fold increase (P_{app amorphous VAR} = 8.75 × 10^-6 cm/s vs. P_{app amorphous VAR:β-CD 1:5} = 13.43 × 10^-6 cm/s). The stability of the amorphous VAR was confirmed for 7 months. The HPMC and β-CD are effective modifiers of its apparent solubility and permeation through membranes simulating gastrointestinal epithelium, suggesting a possibility of a stronger pharmacological effect.

Crystallization kinetics and glass-forming ability of rapidly crystallizing drugs studied by Fast Scanning Calorimetry

The use of the amorphous forms of drugs is a modern approach for the enhancement of bioavailability. At the same time, the high cooling rate needed to obtain the metastable amorphous state often prevents its investigation using conventional laboratory methods such as differential scanning calorimetry, X-ray powder diffractometry. One of the ways to overcome this problem may be the application of Fast Scanning Calorimetry. This method allows direct determination of the critical cooling rate of the melt and kinetic parameters of the crystallization for bad glass formers. In the present work, the amorphous states of dopamine hydrochloride and atenolol were created using Fast Scanning Calorimetry for the first time. Critical cooling rates and glass transition temperatures of these drugs were determined. Based on the values of the kinetic fragility parameter, dopamine hydrochloride glass can be considered strong, while atenolol glass is moderately strong. Both model-based and model-free approaches were employed to determine the kinetic parameters of cold crystallization of dopamine and atenolol. The results were compared with the data from isothermal crystallization experiments. The Nakamura crystallization model provides the best description of the crystallization process and can be used to predict the long term stability of the amorphous forms of the drugs. The presented approaches may find applications in predicting the storage time and choosing the optimal storage conditions of the amorphous drugs prone to crystallization.

Molecular Mobility and Stability Studies of Amorphous Imatinib Mesylate

The proposed study examined the characterization and stability of solid-state amorphous imatinib mesylate (IM) after 15 months under controlled relative humidity (60 ± 5%) and temperature (25 ± 2 °C) conditions. After 2 weeks, and 1, 3, 6, and 15 months, the samples were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray powder diffractometry (XRPD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). Additionally, the amorphous form of imatinib mesylate was obtained via supercooling of the melt in a DSC apparatus, and aged at various temperatures (3, 15, 25 and 30 °C) and time periods (1–16 h). Glass transition and enthalpy relaxation were used to calculate molecular-relaxation-time parameters. The Kohlrausch–Williams–Watts (KWW) equation was applied to fit the experimental enthalpy-relaxation data. The mean molecular-relaxation-time constant (τ) increased with decreasing ageing temperature. The results showed a high stability of amorphous imatinib mesylate adequate to enable its use in solid dosage form.

Investigation of Drug-Excipient Interactions in Biclotymol Amorphous Solid Dispersions

The effect of low molecular weight excipients on drug-excipient interactions, molecular mobility, and propensity to recrystallization of an amorphous active pharmaceutical ingredient is investigated. Two structurally related excipients (α-pentaacetylglucose and β-pentaacetylglucose), five different drug:excipient ratios (1:5, 1:2, 1:1, 2:1, and 5:1, w/w), and three different solid state characterization tools (differential scanning calorimetry, X-ray powder diffraction, and dielectric relaxation spectroscopy) were selected for the present research. Our investigation has shown that the excipient concentration and its molecular structure reveal quasi-identical molecular dynamic behavior of solid dispersions above and below the glass transition temperature. Across to complementary quantum mechanical simulations, we point out a clear indication of a strong interaction between biclotymol and the acetylated saccharides. Moreover, the thermodynamic study on these amorphous solid dispersions highlighted a stabilizing effect of α-pentaacetylglucose regardless of its quantity while an excessive concentration of β-pentaacetylglucose revealed a poor crystallization inhibition. Finally, through long-term stability studies, we also showed the limiting excipient concentration needed to stabilize our amorphous API. Herewith, the developed procedure in this paper appears to be a promising tool for solid-state characterization of complex pharmaceutical formulations.

Molecular Relaxations in Supercooled Liquid and Glassy States of Amorphous Quinidine: Dielectric Spectroscopy and Density Functional Theory Approaches

In this article, we conduct a comprehensive molecular relaxation study of amorphous Quinidine above and below the glass-transition temperature (Tg) through broadband dielectric relaxation spectroscopy (BDS) experiments and theoretical density functional theory (DFT) calculations, as one major issue with the amorphous state of pharmaceuticals is life expectancy. These techniques enabled us to determine what kind of molecular motions are responsible, or not, for the devitrification of Quinidine. Parameters describing the complex molecular dynamics of amorphous Quinidine, such as Tg, the width of the α relaxation (βKWW), the temperature dependence of α-relaxation times (τα), the fragility index (m), and the apparent activation energy of secondary γ relaxation (Ea-γ), were characterized. Above Tg (> 60 °C), a medium degree of nonexponentiality (βKWW = 0.5) was evidenced. An intermediate value of the fragility index (m = 86) enabled us to consider Quinidine as a glass former of medium fragility. Below Tg (< 60 °C), one well-defined secondary γ relaxation, with an apparent activation energy of Ea-γ = 53.8 kJ/mol, was reported. From theoretical DFT calculations, we identified the most reactive part of Quinidine moieties through exploration of the potential energy surface. We evidenced that the clearly visible γ process has an intramolecular origin coming from the rotation of the CH(OH)C9H14N end group. An excess wing observed in amorphous Quinidine was found to be an unresolved Johari-Goldstein relaxation. These studies were supplemented by sub-Tg experimental evaluations of the life expectancy of amorphous Quinidine by X-ray powder diffraction and differential scanning calorimetry. We show that the difference between Tg and the onset temperature for crystallization, Tc, which is 30 K, is sufficiently large to avoid recrystallization of amorphous Quinidine during 16 months of storage under ambient conditions.

Amorphous powders for inhalation drug delivery

For inhalation drug delivery, amorphous powder formulations offer the benefits of increased bioavailability for poorly soluble drugs, improved biochemical stability for biologics, and expanded options of using various drugs and their combinations. However, amorphous formulations usually have poor physicochemical stability. This review focuses on inhalable amorphous powders, including the production methods, the active pharmaceutical ingredients and the excipients with a highlight on stabilization of the particles.

An investigation into the crystallization tendency/kinetics of amorphous active pharmaceutical ingredients: A case study with dipyridamole and cinnarizine

Amorphous drug formulations have great potential to enhance solubility and thus bioavailability of BCS class II drugs. However, the higher free energy and molecular mobility of the amorphous form drive them towards the crystalline state which makes them unstable. Accurate determination of the crystallization tendency/kinetics is the key to the successful design and development of such systems. In this study, dipyridamole (DPM) and cinnarizine (CNZ) have been selected as model compounds. Thermodynamic fragility (mT) was measured from the heat capacity change at the glass transition temperature (Tg) whereas dynamic fragility (mD) was evaluated using methods based on extrapolation of configurational entropy to zero [Formula: see text] , and heating rate dependence of Tg [Formula: see text] . The mean relaxation time of amorphous drugs was calculated from the Vogel-Tammann-Fulcher (VTF) equation. Furthermore, the correlation between fragility and glass forming ability (GFA) of the model drugs has been established and the relevance of these parameters to crystallization of amorphous drugs is also assessed. Moreover, the crystallization kinetics of model drugs under isothermal conditions has been studied using Johnson-Mehl-Avrami (JMA) approach to determine the Avrami constant 'n' which provides an insight into the mechanism of crystallization. To further probe into the crystallization mechanism, the non-isothermal crystallization kinetics of model systems were also analysed by statistically fitting the crystallization data to 15 different kinetic models and the relevance of model-free kinetic approach has been established. The crystallization mechanism for DPM and CNZ at each extent of transformation has been predicted. The calculated fragility, glass forming ability (GFA) and crystallization kinetics are found to be in good correlation with the stability prediction of amorphous solid dispersions. Thus, this research work involves a multidisciplinary approach to establish fragility, GFA and crystallization kinetics as stability predictors for amorphous drug formulations.

Transformation of an active pharmaceutical ingredient upon high-energy milling: A process-induced disorder in Biclotymol

This study investigates for the first time the thermodynamic changes of Biclotymol upon high-energy milling at various levels of temperature above and below its glass transition temperature (Tg). Investigations have been carried out by temperature modulated differential scanning calorimetry (TM-DSC) and X-ray powder diffraction (XRPD). Results indicate that Biclotymol undergoes a solid-state amorphization upon milling at Tg-45 °C. It is shown that recrystallization of amorphous milled Biclotymol occurs below the glass transition temperature of Biclotymol (Tg=20 °C). This displays molecular mobility differences between milled Biclotymol and quenched liquid. A systematic study at several milling temperatures is performed and the implication of Tg in the solid-state transformations generally observed upon milling is discussed. Influence of analysis temperature with respect to interpretation of results was investigated. Finally, it is shown that co-milling Biclotymol with only 20 wt% of amorphous PVP allows a stable amorphous dispersion during at least 5 months of storage.