Relationship between Crystallization Tendencies during Cooling from Melt and Isothermal Storage: Toward a General Understanding of Physical Stability of Pharmaceutical Glasses

Specific intermolecular interaction with sodium glycocholate generates the co-amorphous system showing higher physical stability and aqueous solubility of Y5 receptor antagonist of neuropeptide Y, a brick dust molecule

Drugs with poor water and lipid solubility are termed "brick dust." We previously successfully developed a co-amorphous system of a novel neuropeptide Y5 receptor antagonist (AntiY5R), a brick dust molecule, using sodium taurocholate (NaTC) as a co-former. However, the maximum improvement in AntiY5R dissolution by the co-amorphous system was only approximately 10 times greater than that of the crystals. Therefore, in the current study, other bile salts, including sodium cholate (NaC), sodium chenodeoxycholate (NaCC), and sodium glycocholate (NaGC), were examined as co-formers to further improve AntiY5R dissolution. NaC, NaCC, and NaGC have glass transition temperatures above 150°C. All three co-amorphous systems prepared successfully retained the amorphous form of AntiY5R for 3 months at 40°C, but the co-amorphous system with NaGC (AntiY5R-NaGC; 1:9 molar ratio) provided the highest improvement in AntiY5R dissolution, which was approximately 50 times greater than that of the crystals. Possible intermolecular interactions via the glycine moiety of NaGC more than the other bile salts would contribute to the highest dissolution enhancement with AntiY5R-NaGC. Thus, NaGC would be a promising co-former for formulating stable co-amorphous systems to enhance the dissolution behavior of brick dust molecules.

Influence of Solvent Selection on the Crystallizability and Polymorphic Selectivity Associated with the Formation of the "Disappeared" Form I Polymorph of Ritonavir

The comparative crystallizability and polymorphic selectivity of ritonavir, a novel protease inhibitor for the treatment of acquired immune-deficiency syndrome, as a function of solvent selection are examined through an integrated and self-consistent experimental and computational molecular modeling study. Recrystallization at high supersaturation by rapid cooling at 283.15 K is found to produce the metastable "disappeared" polymorphic form I from acetone, ethyl acetate, acetonitrile, and toluene solutions in contrast to ethanol which produces the stable form II. Concomitant crystallization of the other known solid forms is not found under these conditions. Isothermal crystallization studies using turbidometric detection based upon classical nucleation theory reveal that, for an equal induction time, the required driving force needed to initiate solution nucleation decreases with solubility in the order of ethanol, acetone, acetonitrile, ethyl acetate, and toluene consistent with the expected desolvation behavior predicted from the calculated solute solvation free energies. Molecular dynamics simulations of the molecular and intermolecular chemistry reveal the presence of conformational interplay between intramolecular and intermolecular interactions within the solution phase. These encompass the solvent-dependent formation of intramolecular O-H...O hydrogen bonding between the hydroxyl and carbamate groups coupled with differing conformations of the hydroxyl's shielding phenyl groups. These conformational preferences and their relative interaction propensities, as a function of solvent selection, may play a rate-limiting role in the crystallization behavior by not only inhibiting to different degrees the nucleation process but also restricting the assembly of the optimal intermolecular hydrogen bonding network needed for the formation of the stable form II polymorph.

Long-Term Physical Stability of Amorphous Solid Dispersions: Comparison of Detection Powers of Common Evaluation Methods for Spray-Dried and Hot-Melt Extruded Formulations

Although physical stability can be a critical issue during the development of amorphous solid dispersions (ASDs), there are no established protocols to predict/detect their physical stability. In this study, we have prepared fenofibrate ASDs using two representative manufacturing methods, hot-melt extrusion and spray-drying, to investigate their physical stability for one year. Intentionally unstable ASDs were designed to compare the detection power of each evaluation method, including X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and dissolution study. Each method did not provide the same judgment results on physical stability in some cases because of their different evaluation principles and sensitivity, which has been well-comprehended only for one-component glass. This study revealed that the detection powers of each evaluation method significantly depended on the manufacturing methods. DSC was an effective method to detect a small amount of crystals for both types of ASDs in a quantitative manner. Although the sensitivity of XRPD was always lower compared to that of DSC, interpretation of the data was the easiest. SEM was very effective for observing the crystallization of the small amount of drug for hot-melt extruded products, as the drug crystal vividly appeared on the large grains. The dissolution performance of spray-dried products could change even without any indication of physical change including crystallization. The advantage/disadvantage and complemental roles of each evaluation method are discussed for deeper understanding on the physical stability data of ASDs.

Influence of Nucleation on Relaxation, Molecular Cooperativity, and Physical Stability of Celecoxib Glass

Although nucleation is considered the first step in the crystallization of glass materials, the structure and properties of the nuclei are not understood well. Influence of nucleation on the structure and dynamics of celecoxib glass was evaluated in this study. The nuclei for Form III were induced by annealing the glass at freezing temperature, and their impact on the relaxation behavior was investigated using thermal analysis and broadband dielectric spectroscopy to find accelerated α relaxation and suppressed β relaxation. In addition, observed after nucleation was a decrease in cooperativity of the molecular motion, presumably because of the appearance of void spaces in the glass structure. During long-term isothermal crystallization studies, crystal growth to Form III was accelerated in the presence of the nuclei, whereas this effect was less remarkable when a different crystal form dominated the crystallization behavior. These observations should provide more detailed insights into the nucleation mechanism and impact of nucleation on molecular dynamics including physical stability of pharmaceutical glasses. In addition, discussed is the remarkable acceleration of the crystallization rate of the celecoxib glass just below its Tg, which could be understood by diffusionless crystal growth.

Crystallinity: A Complex Critical Quality Attribute of Amorphous Solid Dispersions

Does the performance of an amorphous solid dispersion rely on having 100% amorphous content? What specifications are appropriate for crystalline content within an amorphous solid dispersion (ASD) drug product? In this Perspective, the origin and significance of crystallinity within amorphous solid dispersions will be considered. Crystallinity can be found within an ASD from one of two pathways: (1) incomplete amorphization, or (2) crystal creation (nucleation and crystal growth). While nucleation and crystal growth is the more commonly considered pathway, where crystals originate as a physical stability failure upon accelerated or prolonged storage, manufacturing-based origins of crystallinity are possible as well. Detecting trace levels of crystallinity is a significant analytical challenge, and orthogonal methods should be employed to develop a holistic assessment of sample properties. Probing the impact of crystallinity on release performance which may translate to meaningful clinical significance is inherently challenging, requiring optimization of dissolution test variables to address the complexity of ASD formulations, in terms of drug physicochemical properties (e.g., crystallization tendency), level of crystallinity, crystal reference material selection, and formulation characteristics. The complexity of risk presented by crystallinity to product performance will be illuminated through several case studies, highlighting that a one-size-fits-all approach cannot be used to set specification limits, as the risk of crystallinity can vary widely based on a multitude of factors. Risk assessment considerations surrounding drug physicochemical properties, formulation fundamentals, physical stability, dissolution, and crystal micromeritic properties will be discussed.

Crystallization of Amorphous Nifedipine Under Isothermal Conditions: Inter-laboratory Reproducibility and Investigation of the Factors Affecting Reproducibility

High inter-laboratory reproducibility is required for conducting collaborative experiments among several laboratories. The primary aim of our evaluation of the physical stability of amorphous drugs, conducted in co-operation with eight laboratories, was to establish a protocol for isothermal storage tests to obtain data of the same quality from all the participating laboratories. Sharing a protocol that contained the same level of detail as the experimental section of general papers was insufficient for high inter-laboratory reproducibility. We investigated the causes of variations in the data from the various laboratories and restricted the protocol step-by-step to achieve high inter-laboratory reproducibility. The various experimentalists had very different levels of awareness regarding how to control the temperature of a sample as the samples were transferred into and out of thermostatic chambers. Specific instructions on how to conduct this operation, such as regarding the time required for the transfer and thermal protection of the container during the transfer, helped to reduce variation. Improved inter-laboratory reproducibility revealed that the physical stabilities of amorphous drugs differed when samples were prepared in differently shaped aluminum pans designed for various differential scanning calorimeters.

Ritonavir Revisited: Melt Crystallization Can Easily Find the Late-Appearing Polymorph II and Unexpectedly Discover a New Polymorph III

Identification of a thermodynamically stable polymorph is an important step in the early stage of drug development. Ritonavir (RIT) is a well-known case where the most stable polymorph II emerged after being marketed, leading to a loss of $250 million. Herein, we report the findings that routine melt crystallization can reveal the late-appearing polymorph II of RIT at small supercooling, but the probability of nucleation is very low. The addition of 30-50% polyethylene glycol (PEG) promotes the crystallization of Form II as the only phase at low supercooling, making it easier to detect in polymorphism screening. During the course of our research, a new polymorph, denoted Form III, was unexpectedly discovered, crystallizing as the major phase from neat RIT melts. Single crystals of Form III were grown from melt microdroplets. Benefiting from the ability of synchrotron radiation to detect weak diffraction signals that cannot be accessible by a laboratory diffractometer, a reasonable structure of Form III was solved with slight disorder relative to thiazole groups (P1 space group and Z' = 4). The thermodynamic stability ranking of the three true polymorphs is Form II > Form I > Form III, as opposed to the order of solubility. The capacity to efficiently reveal rich polymorphs, especially the kinetically hindered polymorph, and rapidly grow single crystals of a new phase for structure determination together highlights the necessity of incorporating melt crystallization into routine methods for pharmaceutical polymorphism screening.

Nucleation During Storage Impeded Supersaturation in the Dissolution Process of Amorphous Celecoxib

The aqueous solubility of active pharmaceutical ingredients (APIs) is one of the most critical factors in determining the absorption of orally administered drugs. Amorphization of API may offer better drug absorption than the crystalline state owing to enhanced solubility. However, if crystal nuclei are formed during storage, they may develop into crystals upon contact with water, thus limiting the dissolution advantage. In an earlier study, we found that the nuclei of amorphous celecoxib (CEL) could be formed at freezing temperatures (FT) without further crystal growth. Following this finding, we compared the dissolution performances of amorphous CEL annealed at room temperature (RT, 25 °C) or FT (-20 °C). We found that only the RT-annealed CEL could achieve a supersaturated state effectively during the dissolution process, which could be explained by the fast conversion of the FT-annealed amorphous CEL to a crystalline state owing to the presence of nuclei. Investigation of the residual solids revealed that supersaturation could be maintained for a while after the appearance of the crystals, which could be explained by heterogeneous nucleation and competition between the dissolution of amorphous parts and crystallization. In addition, a new crystalline form of CEL was observed during dissolution.

Rigid Nuclei and Flexible Nuclei: Appearance and Disappearance of Nuclei in Indomethacin Glass Revealed by a Long-Term Annealing Study

In this study, nucleation/crystallization behaviors of indomethacin glass are discussed with emphasis on the fate of nuclei, which is explained using a notion of "rigid" and "flexible" nuclei. The observation was made mainly by thermal analysis of indomethacin glass after long-term annealing at various temperatures. Formation of nuclei was evaluated by observing cold crystallization behaviors of the annealed glasses, as they should be dominated by the "nuclei form" produced in the glass. Nuclei of forms α and γ, which have opposite stability trends, were found to appear over a wide temperature range. The nuclei of form α were stable even in the presence of other crystal forms, whereas those of form γ were likely to be easily integrated into other crystals during their growth, which was explained by a notion of rigid and flexible nuclei. Moreover, unconventionally fast crystallization in the glass-transition region and the finding of a new crystal form are also reported.

Formation of a Stable Co-Amorphous System for a Brick Dust Molecule by Utilizing Sodium Taurocholate with High Glass Transition Temperature

Brick dust molecules are usually poorly soluble in water and lipoidal components, making it difficult to formulate them in dosage forms that provide efficient pharmacological effects. A co-amorphous system is an effective strategy to resolve these issues. However, their glass transition temperatures (Tg) are relatively lower than those of polymeric amorphous solid dispersions, suggesting the instability of the co-amorphous system. This study aimed to formulate a stable co-amorphous system for brick dust molecules by utilizing sodium taurocholate (NaTC) with a higher Tg. A novel neuropeptide Y5 receptor antagonist (AntiY5R) and NaTC with Tg of 155 °C were used as the brick dust model and coformer, respectively. Ball milling formed a co-amorphous system for AntiY5R and NaTC (AntiY5R-NaTC) at various molar ratios. Deviation from the theoretical Tg value and peak shifts in Fourier-transform infrared spectroscopy indicated intermolecular interactions between AntiY5R and NaTC. AntiY5R-NaTC at equal molar ratios resulting in an 8.5-fold increase in AntiY5R solubility over its crystalline form. The co-amorphous system remained amorphous for 1 month at 25 °C and 40 °C. These results suggest that the co-amorphous system formed by utilizing NaTC as a coformer could stably maintain the amorphous state and enhance the solubility of brick dust molecules.

Influence of the crystallization tendencies of pharmaceutical glasses on the applicability of the Adam-Gibbs-Vogel and Vogel-Tammann-Fulcher equations in the prediction of their long-term physical stability

Amorphization is a powerful approach for improving the aqueous solubility and bioavailability of poorly water-soluble compounds. However, it can cause chemical and physical instability, the latter of which can lead to crystallization during storage, diminishing the solubility advantage of the amorphous state. As there is no standard method for predicting the physical stability of amorphous materials, a long-term stability study is needed in drug development. This study investigated the correlation between the physical stability of amorphous compounds and molecular mobility based on the assumption that physical stability is governed by the diffusional motion of a molecule. Model compounds were evaluated for crystallization onset time, structural relaxation time, fragility, and fictive temperature. The crystallization onset time of acetaminophen glass correlated with its relaxation time calculated from the Adam-Gibbs-Vogel equation; however, that of felodipine glass correlated with the relaxation time calculated from the Vogel-Tammann-Fulcher equation. The different crystallization tendencies of these compounds can be explained by the differences in the rate limiting steps in their crystallization processes, indicating the importance of distinguishing the critical process associated with crystallization. These findings will be useful for more accurate prediction of long-term physical stability of amorphous materials.

Design of a Stable Coamorphous System Using Lactose as an Antiplasticizing Agent for Diphenhydramine Hydrochloride with a Low Glass Transition Temperature

Coamorphous systems comprising small molecules are emerging as counterparts to polymeric solid dispersions. However, the glass transition temperatures (T_gs) of coamorphous materials are relatively low because of the lack of polymeric carriers with higher T_gs. This study aimed to investigate the applicability of lactose (LAC) as an antiplasticizing coformer to a coamorphous system. Diphenhydramine hydrochloride (DPH) was selected as a model drug (T_g = 16 °C). Differential scanning calorimetry showed a comelting point in addition to a decrease in the neat melting points depending on the composition of the physical mixtures, suggesting that the mixture of DPH-LAC was eutectic. The melting point of the eutectic mixture was calculated according to the Schröder-van Laar equation. The heat of fusion of the eutectic mixture was maximized at a 70:30 molar ratio of DPH to LAC; at this point, the melting peaks of the pure components disappeared. The heat flow profiles following the melting and cooling of DPH-LAC physical mixtures at the ratios from 10:90 to 90:10 showed a single T_g, suggesting the formation of a coamorphous system. Lactose showed a T_g of over 100 °C, and the T_g of DPH increased with the molar ratio of LAC; it was 84 °C at a 10:90 molar ratio of DPH to LAC. The Raman image indicated the formation of a homogeneous dispersion of DPH and LAC in the coamorphous system. Peak shifts in the infrared spectra indicated the presence of intermolecular interactions between the amino group of DPH and the hydroxyl group of LAC. Principal component analysis of the infrared spectra revealed a significant change at the 70:30 molar ratio of DPH to LAC, which was in agreement with the results of the thermal analysis. A stability test at 40 °C revealed rapid crystallization of the supercooled liquid DPH. The coamorphous samples containing 10-50% of LAC remained in an amorphous state for 21 days, and no crystallization was observed for the samples containing >60% of LAC for 28 days. The relatively lower T_g (less than 40 °C) of the coamorphous system containing 10-50% of LAC might have caused crystallization during storage. These findings indicate that LAC, which is a safe and widely used pharmaceutical excipient, can be applied to coamorphous systems as an antiplasticizing coformer.

Ternary Eutectic Ezetimibe-Simvastatin-Fenofibrate System and the Physical Stability of Its Amorphous Form

In this study, the phase diagram of the ternary system of ezetimibe–simvastatin–fenofibrate was established. It has been proven that the ternary composition recommended for the treatment of mixed hyperlipidemia forms a eutectic system. Since eutectic mixtures are characterized by greater solubility and dissolution rate, the obtained result can explain the marvelous medical effectiveness of combined therapy. Considering that another well-known method for improving the aqueous solubility is amorphization, the ternary system with eutectic concentration was converted into an amorphous form. Thermal properties, molecular dynamics, and physical stability of the obtained amorphous system were thoroughly investigated through various experimental techniques compared to both: neat amorphous active pharmaceutical ingredients (considered separately) and other representative concentrations of ternary mixture. The obtained results open up a new way of selecting the therapeutic concentrations for combined therapies, a path that considers one additional variable: eutecticity.

X-ray Absorption Near-Edge Spectroscopy Analysis of Indomethacin in Crystalline Forms and in Amorphous Solid Dispersions

Chlorine K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of identical active pharmaceutical ingredients (APIs) containing chloride atoms and their amorphous solid dispersions (ASDs). Indomethacin (IMC), of which three crystal forms (α, β, and γ) have been reported, was used as a model API. The shape of XANES spectra was unique to each IMC crystal. The analysis of the crystal structures of IMC revealed that chlorine atoms of the IMCα form had unique intermolecular interactions and halogen bonds with oxygen atoms, while those of the IMCγ form do not have any notable interactions. This result showed that XANES measurements can detect weak interatomic interactions. The shapes of the ASD spectra were clearly different from those of the crystals, suggesting that the environment around the Cl atom of IMC was different from that of the crystals. A thermal stress test was then performed to study the transformation from the amorphous form to the crystalline form of IMC in the ASD. The powder X-ray diffraction (PXRD) patterns indicated that amorphous IMC transformed into crystals during the thermal stress test. In accordance with the PXRD results, the XANES spectra also transformed from ASD to crystalline form. These results indicate that the IMC transformation could be monitored by XANES measurement. Our findings led us to conclude that XANES measurement is a novel approach for the evaluation of crystal polymorphs of APIs and the crystalline state of APIs in ASDs.

Influence of Adsorption State and Molecular Interaction on Physical Stability of Confined Amorphous Vortioxetine

The composites of amorphous vortioxetine (VXT) and ordered mesoporous silica were prepared. Three silica matrixes with different pore sizes were used here: Mobil Composition of Matter No.41 (MCM), Santa Barbara Amorphous No.15 (SBA), and mesostructured cellular foam (MCF). The amorphous composites behaved enhanced physical stability (303.15 K, 56.0 ± 0.4% RH) compared to bulk VXT amorphism. Interestingly, the physical stability of these amorphous composites showed a great difference. Amorphous VXT loaded in MCF crystallized within 1 week, while VXT-SBA composites could be stable over 3 months. The stability of VXT-MCM composites were somewhere in between. In addition, with VXT loading decreasing, the physical stability of confined amorphous VXT became better. Nitrogen adsorption measurements indicated that VXT molecules were adsorbed in SBA in a dispersive state while aggregated in MCM and MCF. VXT-VXT interactions in MCM could be stronger than that in SBA. ¹H-¹³C solid-state nuclear magnetic resonance experiments demonstrated the weaker VXT-VXT interactions in SBA. The dispersive adsorption state and weak VXT-VXT interactions were benefit to the physical stability of amorphous VXT in SBA channels. In addition, dissolution profiles of confined amorphous VXT and bulk crystalline VXT were determined and the dissolution rate of VXT loaded in nanopores was faster than the latter.

Isothermal Crystallization and Time-Temperature Transformation of Amorphous Nifedipine: A Case of Polymorphism Formation and Conversion

Crystallization of active pharmaceutical ingredients (APIs) from the supercooled liquid state is an important issue in determining the stability of amorphous pharmaceutical dispersions. In the present study, the isothermal crystallization from the supercooled liquid state of the pharmaceutical compound nifedipine was investigated by both rheological and differential scanning calorimetry (DSC) measurements, and the crystallization kinetics was fitted to the Johnson-Mehl-Avrami (JMA) equation. Both the crystallization induction time and completion time from the two methods were used to construct the time-temperature-transformation (TTT) diagram for nifedipine. A model based on a modification of classical homogeneous nucleation and crystal growth theory was employed to fit the induction and completion time curves. Both DSC and rheological methods give similar results for the crystallization kinetics of the nifedipine. From the crystallization kinetics modeling, the solid-liquid interfacial surface tension σ_SL of nifedipine was estimated and the value was found to be consistent with prior results obtained from melting point depression measurements as a function of crystal size. Evidence is shown that for temperatures below 110 °C, at the early stage of nucleation, NIF first nucleates into the metastable β'-form and later converts into the stable α-form during the isothermal crystallization. We are also able to report the heat of fusion of the γ'-NIF based on the calorimetric experiments.

Importance of Mesoporous Silica Particle Size in the Stabilization of Amorphous Pharmaceuticals-The Case of Simvastatin

In this paper, the role of mesoporous silica (MS) particle size in the stabilization of amorphous simvastatin (SVT) is revealed. For inhibiting recrystallization of the supercooled drug, the two MS materials (Syloid® XDP 3050 and Syloid® 244 FP) were employed. The crystallization tendency of SVT alone and in mixture with the MS materials was investigated by Differential Scanning Calorimetry (DSC) and Broadband Dielectric Spectroscopy (BDS). Neither confinement of the SVT molecules inside the MS pores nor molecular interactions between functional groups of the SVT molecules and the surface of the stabilizing excipient could explain the observed stabilization effect. The stabilization effect might be correlated with diffusion length of the SVT molecules in the MS materials that depended on the particle size. Moreover, MS materials possessing different particle sizes could offer free spaces with different sizes, which might influence crystal growth of SVT. All of these factors must be considered when mesoporous materials are used for stabilizing pharmaceutical glasses.

Glass engineering of aminotriazine-based materials with sub-ambient Tg and high kinetic stability

Designing molecular glasses with phenyl rings favors a lower T_g and higher glass stability compared to cyclohexyl analogues.

Ultraslow Cooling for the Stabilization of Pharmaceutical Glasses

Stabilization technology of glass structures is of great interest in the field of pharmaceutical science, as it may prevent poorly soluble candidate compounds from dropping out of the pipeline. Cooling rate from the melt has been recognized as one parameter to alter the energy state of the glass; however, the relationship between the physicochemical properties of glass and stabilization efficiency of the cooling rate has not been clarified yet. We have investigated the effect of cooling rate on the thermodynamic parameters of 13 pharmaceutical glasses, to find features of the compounds that are closely related to the stabilization efficiency. We have also analyzed the structural differences between slowly cooled and annealed glasses based on Fourier-transform infrared spectra and relaxation enthalpy. Although the degree of stabilization was lower for slowly cooled glasses compared to that for vapor-deposited ones, slow cooling was found to be a prominent method for producing stable glass and is applicable to bulk materials. In this observation, a strong correlation between fragility and the number of rotatable bonds was also found.

Crystallization Tendency of Pharmaceutical Glasses: Relevance to Compound Properties, Impact of Formulation Process, and Implications for Design of Amorphous Solid Dispersions

Amorphous solid dispersions (ASDs) are important formulation strategies for improving the dissolution process and oral bioavailability of poorly soluble drugs. Physical stability of a candidate drug must be clearly understood to design ASDs with superior properties. The crystallization tendency of small organics is frequently estimated by applying rapid cooling or a cooling/reheating cycle to their melt using differential scanning calorimetry. The crystallization tendency determined in this way does not directly correlate with the physical stability during isothermal storage, which is of great interest to pharmaceutical researchers. Nevertheless, it provides important insights into strategy for the formulation design and the crystallization mechanism of the drug molecules. The initiation time for isothermal crystallization can be explained using the ratio of the glass transition and storage temperatures (T_g/T). Although some formulation processes such as milling and compaction can enhance nucleation, the T_g/T ratio still works for roughly predicting the crystallization behavior. Thus, design of accelerated physical stability test may be possible for ASDs. The crystallization tendency during the formulation process and the supersaturation ability of ASDs may also be related to the crystallization tendency determined by thermal analysis. In this review, the assessment of the crystallization tendency of pharmaceutical glasses and its relevance to developmental studies of ASDs are discussed.

New Insight Into Thermodynamical Stability of Carbamazepine

Carbamazepine (CBZ)-an antiepileptic drug-belongs to Biopharmaceutics Classification System II Class. It has low solubility and consequently limited bioavailability. One of the ways to improve drugs solubility is amorphization of their structure. Herein, cooling CBZ-at different cooling rates-was investigated as a way to obtain glassy, better soluble form. During preliminary differential scanning calorimetry experiments, some peculiar behavior of the examined material, different from those stated in the literature, was observed. Further investigations using differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscope revealed that decomposition temperature of CBZ is about 30°C lower than previously assumed. Moreover, high-resolution thermogravimetric measurements indicate that some decomposition processes could start even below the temperature reported as the melting point of the form I of CBZ.

Impact of Supramolecular Aggregation on the Crystallization Kinetics of Organic Compounds from the Supercooled Liquid State

Despite numerous challenges in their theoretical description and practical implementation, amorphous drugs are of growing importance to the pharmaceutical industry. One such challenge is to gain molecular level understanding of the propensity of a molecule to form and remain as a glassy solid. In this study, a series of structurally similar diarylamine compounds was examined to elucidate the role of supramolecular aggregation on crystallization kinetics from supercooled liquid state. The structural similarity of the compounds makes it easier to isolate the molecular features that affect crystallization kinetics and glass forming ability of these compounds. To examine the role of hydrogen-bonded aggregation and motifs on crystallization kinetics, a combination of thermal and spectroscopic techniques was employed. Using variable temperature FTIR, Raman, and solid-state NMR spectroscopies, the presence of hydrogen bonding in the melt and glassy state was examined and correlated with observed phase transition behaviors. Spectroscopic results revealed that the formation of hydrogen-bonded aggregates involving carboxylic acid and pyridine nitrogen (acid-pyridine aggregates) between neighboring molecules in the melt state impedes crystallization, while the presence of carboxylic acid dimers (acid-acid dimers) in the melt favors crystallization. This study suggests that glass formation of small molecules is influenced by the type of intermolecular interactions present in the melt state and the kinetics associated with the molecules to assemble into a crystalline lattice. For the compounds that form acid-pyridine aggregates, the formation of energy degenerate chains, produced due to conformational flexibility of the molecules, presents a kinetic barrier to crystallization. The poor crystallization tendency of these aggregates stems from the highly directional hydrogen-bonding interactions needed to form the acid-pyridine chains. Conversely, for the compounds that form acid-acid dimers, the nondirectional van der Waals forces needed to construct a nucleus promote rapid assembly and crystallization.

Crystallization of probucol from solution and the glassy state

Crystallization of probucol (PBL) from both solution and glassy solid state was investigated. In the crystallization study from solution, six solvents and three methods, i.e., evaporation, addition of a poor solvent, and cooling on ice, were used to obtain various crystal forms. In addition to common two crystal forms (forms I and II), two further forms (forms III and cyclohexane-solvate) were found in this study, and their thermodynamic relationships were determined. Forms I and II are likely to be enantiotropically related with thermodynamic transition temperature below 5°C. Isothermal crystallization studies revealed that PBL glass initially crystallized into form III between 25 and 50°C, and then transformed to form I. The isothermal crystallization appears to be a powerful option to find uncommon crystal forms. The crystallization of PBL was identified to be pressure controlled, thus the physical stability of PBL glass is higher than that of typical compounds.

Physicochemical Properties of Solid Phospholipid Particles as a Drug Delivery Platform for Improving Oral Absorption of Poorly Soluble Drugs

PURPOSE

A novel drug delivery platform, mesoporous phospholipid particle (MPP), is introduced. Its physicochemical properties and ability as a carrier for enhancing oral absorption of poorly soluble drugs are discussed.

METHODS

MPP was prepared through freeze-drying a cyclohexane/t-butyl alcohol solution of phosphatidylcholine. Its basic properties were revealed using scanning electron microscopy, x-ray diffraction, thermal analysis, hygroscopicity measurement, and so on. Fenofibrate was loaded to MPP as a poorly soluble model drug, and effect of MPP on the oral absorption behavior was observed.

RESULTS

MPP is spherical in shape with a diameter typically in the range of 10-15 μm and a wide surface area that exceeds 10 m²/g. It has a bilayer structure that may accommodate hydrophobic drugs in the acyl chain region. When fenofibrate was loaded in MPP as a model drug, it existed partially in a crystalline state and improvement in the dissolution behavior was achieved in the presence of a surfactant, because of the formation of mixed micelles composed of phospholipids and surfactants in the dissolution media. MPP greatly improved the oral absorption of fenofibrate compared to that of the crystalline drug and its efficacy was almost equivalent to that of an amorphous drug dispersion.

CONCLUSION

MPP is a promising option for improving the oral absorption of poorly soluble drugs based on the novel mechanism of dissolution improvement.

Glass Forming Ability of Amorphous Drugs Investigated by Continuous Cooling and Isothermal Transformation

The aim of this study was to investigate the glass forming ability of 12 different drugs by the determination of continuous cooling and isothermal transformation diagrams in order to elucidate if an inherent differentiation between the drugs with respect to their the glass forming ability can be made. Continuous-cooling-transformation (CCT) and time-temperature-transformation (TTT) diagrams of the drugs were developed in order to predict the critical cooling rate necessary to convert the drug from the melt into an amorphous form. While TTT diagrams overestimated the actual critical cooling rate, they allowed an inherent differentiation of glass forming ability for the investigated drugs into drugs that are extremely difficult to amorphize (>750 °C/min), drugs that require modest cooling rates (>10 °C/min), and drugs that can be made amorphous even at very slow cooling rates (>2 °C/min). Thus, the glass forming ability can be predicted by the use of TTT diagrams. In contrast to TTT diagrams, CCT diagrams may not be suitable for small organic molecules due to poor separation of exothermic events, which makes it difficult to determine the zone of recrystallization. In conclusion, this study shows that glass forming ability of drugs can be predicted by TTT diagrams.

Isoenergetic Polymorphism: The Puzzle of Tolazamide as a Case Study

In the present case study of tolazamide we illustrate how many seemingly contradictory results that have been obtained from experimental observations and theoretical calculations can finally start forming a consistent picture: a "puzzle put together". For many years, tolazamide was considered to have no polymorphs. This made this drug substance unique among the large family of sulfonylureas, which was known to be significantly more prone to polymorphism than many other organic compounds. The present work employs a broad and in-depth analysis that includes the use of optical microscopy, single-crystal and powder X-ray diffraction, IR and Raman spectroscopies, DSC, semiempirical PIXEL calculations and DFT of three polymorphs of tolazamide. This case study shows how the polymorphs of a molecular crystal can be overlooked even if discovered serendipitously on one of numerous crystallizations, and how very different molecular packings can be practically isoenergetic but still crystallize quite selectively and transform one into another irreversibly upon heating.

Physical stability of drugs after storage above and below the glass transition temperature: Relationship to glass-forming ability

Amorphous materials are inherently unstable and tend to crystallize upon storage. In this study, we investigated the extent to which the physical stability and inherent crystallization tendency of drugs are related to their glass-forming ability (GFA), the glass transition temperature (T_g) and thermodynamic factors. Differential scanning calorimetry was used to produce the amorphous state of 52 drugs [18 compounds crystallized upon heating (Class II) and 34 remained in the amorphous state (Class III)] and to perform in situ storage for the amorphous material for 12 h at temperatures 20 °C above or below the T_g. A computational model based on the support vector machine (SVM) algorithm was developed to predict the structure-property relationships. All drugs maintained their Class when stored at 20 °C below the T_g. Fourteen of the Class II compounds crystallized when stored above the T_g whereas all except one of the Class III compounds remained amorphous. These results were only related to the glass-forming ability and no relationship to e.g. thermodynamic factors was found. The experimental data were used for computational modeling and a classification model was developed that correctly predicted the physical stability above the T_g. The use of a large dataset revealed that molecular features related to aromaticity and π–π interactions reduce the inherent physical stability of amorphous drugs.

Theory and practice of supersaturatable formulations for poorly soluble drugs

Candidate compounds with high activity do not always possess adequate physicochemical properties to be developed as commercial products. Notably, the development of candidates with poor aqueous solubility has been a great challenge in the past two decades. Formulations that offer supersaturated state during the dissolution process are considered effective for increasing the oral bioavailability of such candidates. Representative supersaturatable dosage forms include amorphous solid dispersions, nanocrystal formulations and self-(micro)emulsifying drug delivery systems. This review describes the characteristics of these formulations, with emphasis on the suitability of the candidates for each type of formulation, from a physicochemical viewpoint. Influence of developmental strategy on the formulation selection is also discussed. This review aims to provide guidance for selecting formulations for poorly soluble drugs based on both academic and practical backgrounds.

Correlation between glass-forming ability and fragility of pharmaceutical compounds

Fragility is a measure of the departure from non-Arrhenius behavior for supercooled liquids and glasses, and various simple methods are available for its quantification. However, the obtained values usually do not agree with each other. One of the purposes of this study was to compare the fragility values obtained by different methodologies. Thermodynamic fragility (FT) is a simple concept that is evaluated from the heat capacity change at the glass transition temperature (Tg). Dynamic fragility is evaluated using three methodologies in this study: extrapolation of the configurational entropy (Sc) to the Kauzmann temperature (Tk) (FDC), ramp-rate dependence of Tg (FDTg), and that of the fictive temperature (Tf) (FDTf). FT and FDC of 19 pharmaceutical compounds were correlated, whereas FDTg and FDTf did not correlate with either of them. This result seems reasonable because both FT and FDC are calculated from thermodynamic parameters in the quasi-equilibrium state, but FDTg and FDTf are likely affected by kinetics as well. Another goal of this study was to find the correlation between the glass-forming ability (GFA) and fragility. FDTg was shown to correlate with GFA, presumably because both were determined on the balance of thermodynamic and kinetic factors. This correlation suggests that fragile glass has low GFA. Furthermore, the relevance of fragility to isothermal crystallization is discussed. Compounds with small FDTg and FDTf tended to exhibit pressure-controlled crystallization, for which better storage stability can be expected relative to temperature-controlled compounds. Fragility was shown to be a useful parameter practically as well as scientifically.