Crystallization of probucol from solution and the glassy state

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.

Improved Dissolution Properties of Co-amorphous Probucol with Atorvastatin Calcium Trihydrate Prepared by Spray-Drying

A co-amorphous model drug was prepared by the spray-drying (SD) of probucol (PC) and atorvastatin calcium trihydrate salt (ATO) as low water solubility and co-former components, respectively. The physicochemical properties of the prepared samples were characterized by powder X-ray diffraction (PXRD) analysis, thermal analysis, Fourier transform infrared spectroscopy (FTIR), and dissolution tests. Stability tests were also conducted under a stress environment of 40 °C and 75% relative humidity. The results of PXRD measurements and thermal analysis suggested that PC and ATO form a co-amorphous system by SD. Thermal analysis also indicated an endothermic peak that followed an exotherm in amorphous PC and a physical mixture (PM) of amorphous PC and ATO; however, no endothermic peak was detected in the co-amorphous system. The dissolution profiles for PC in the co-amorphous sample composed of PC and ATO were improved compared to those for raw PC crystals or the PM. Stability tests indicated that the co-amorphous material formed by PC and ATO can be stored for 35 d without crystallization, whereas amorphous PC became crystallized within a day. Therefore, co-amorphization of PC and ATO prepared by SD is considered to be a useful method to improve the solubility of PC in water.

Preparation of a ternary amorphous solid dispersion using hot-melt extrusion for obtaining a stable colloidal dispersion of amorphous probucol nanoparticles

In our previous reports, ternary amorphous solid dispersions (ASDs) of probucol (PBC)/polymer/surfactant were prepared by spray-drying and cryo-grinding, and colloidal dispersions of amorphous PBC nanoparticles were obtained by dispersing the ternary ASD into water. In this study, hot-melt extrusion, which is a practical method for preparing ASD formulations, to obtain ternary ASDs and colloidal dispersions of amorphous PBC nanoparticles. Polyvinylpyrrolidone (PVP) K12, with a relatively low T_g, below 100°C, was used as a polymer, while poloxamer P407 (P407), which remains chemically stable during the hot-melt extrusion process, was utilized as a surfactant. Ternary ASDs were successfully produced with high-weight ratios of PVP and P407. A hydrogen bond between the PBC hydroxyl proton and PVP carbonyl oxygen in the ternary ASD was detected using solid-state NMR spectroscopy, which suggested that amorphous PBC was mainly stabilized by PVP. Stable colloidal dispersions of amorphous PBC nanoparticles were obtained from the PBC/PVP/P407 ASD, at a weight ratio of 1:4:2. The mean particle size was below 200 nm and the amorphous state of PBC remained stable upon storage at 25°C for 14 d. Solution-state ¹H NMR and zeta-potential measurements suggested that P407 mainly stabilized the colloidal dispersion of amorphous PBC nanoparticles, by steric hindrance at the solid/liquid interface. The findings of this study demonstrate that, similar to spray-drying, hot-melt extrusion can form practical ternary ASDs that provide colloidal dispersion of amorphous drug nanoparticles. Thus, this study advocates for the use of hot-melt extrusion in the design of an amorphous formulation for a variety of poorly water-soluble drugs.

Revealing the mechanism of morphological variation of amorphous drug nanoparticles formed by aqueous dispersion of ternary solid dispersion

Probucol (PBC)/hypromellose (HPMC)/sodium dodecyl sulfate (SDS) ternary solid dispersions (SDs) of various weight ratios were prepared and evaluated to unveil the effect of HPMC and SDS on the formation of amorphous PBC nanoparticles. The morphological variation of the PBC nanoparticles prepared using SDs of different compositions was determined using dynamic light scattering and cryogenic transmission electron microscopy (cryo-TEM). Statistical analysis of particle size versus roundness of PBC nanoparticles was carried out based on cryo-TEM images. A clear correlation was observed between the morphologies of the PBC nanoparticles and the amounts of HPMC and SDS, either admixed in SDs or pre-dissolved in an aqueous solution. The admixed HPMC in SDs was demonstrated to play the major role in determining the primary particle sizes of discrete amorphous PBC nanoparticles. Based on ¹³C solid-state NMR spectroscopy, this phenomenon should be due to the enlarged size of the PBC-rich domains in SDs, which depended on the decreasing amounts of admixed HPMC. Although the pre-dissolved part of HPMC had less impact on the primary particle sizes, it was found to inhibit the particle agglomeration and recrystallization of amorphous PBC nanoparticles. On the other hand, sufficient SDS admixed in SDs could suppress the size enhancement of the PBC-rich domains during water immersion and nanoparticle evolution (agglomeration and crystallization) after aqueous dispersion. The pre-dissolved SDS could restrain the agglomeration of amorphous PBC nanoparticles, ultimately forming hundreds of irregular nanometer-order structures. Since the increase in size during water immersion, their sizes were still slightly larger than those obtained with a high portion of admixed SDS. The findings of this study clarified the usefulness and necessity of adding polymers and surfactants to SDs to fabricate drug nanoparticle formulations.

High-Pressure Dielectric Studies-a Way to Experimentally Determine the Solubility of a Drug in the Polymer Matrix at Low Temperatures

In this work, we employed broad-band dielectric spectroscopy to determine the solubility limits of nimesulide in the Kollidon VA64 matrix at ambient and elevated pressure conditions. Our studies confirmed that the solubility of the drug in the polymer matrix decreases with increasing pressure, and molecular dynamics controls the process of recrystallization of the excess of amorphous nimesulide from the supersaturated drug–polymer solution. More precisely, recrystallization initiated at a certain structural relaxation time of the sample stops when a molecular mobility different from the initial one is reached, regardless of the temperature and pressure conditions. Finally, based on the presented results, one can conclude that by transposing vertically the results obtained at elevated pressures, one can obtain the solubility limit values corresponding to low temperatures. This approach was validated by the comparison of the experimentally determined points with the theoretically obtained values based on the Flory–Huggins theory.

Automated PET-RAFT Polymerization Towards Pharmaceutical Amorphous Solid Dispersion Development

In pharmaceutical oral drug delivery development, about 90% of drugs in the pipeline have poor aqueous solubility leading to severe challenges with oral bioavailability and translation to effective and safe drug products. Amorphous solid dispersions (ASDs) have been utilized to enhance the oral bioavailability of poorly soluble active pharmaceutical ingredients (APIs). However, a limited selection of regulatory-approved polymer excipients exists for the development and further understanding of tailor-made ASDs. Thus, a significant need exists to better understand how polymers can be designed to interact with specific API moieties. Here, we demonstrate how an automated combinatorial library approach can be applied to the synthesis and screening of polymer excipients for the model drug probucol. We synthesized a library of 25 random heteropolymers containing one hydrophilic monomer (2-hydroxypropyl acrylate (HPA)) and four hydrophobic monomers at varied incorporation. The performance of ASDs made by a rapid film casting method was evaluated by dissolution using ultra-performance liquid chromatography (UPLC) sampling at various time points. This combinatorial library and rapid screening strategy enabled us to identify a relationship between polymer hydrophobicity, monomer hydrophobic side group geometry, and API dissolution performance. Remarkably, the most effective synthesized polymers displayed slower drug release kinetics compared to industry standard polymer excipients, showing the ability to modulate the drug release profile. Future coupling of high throughput polymer synthesis, high throughput screening (HTS), and quantitative modeling would enable specification of designer polymer excipients for specific API functionalities.

Glass Transition Dynamics and Physical Stability of Amorphous Griseofulvin in Binary Mixtures with Low-Tg Excipients

Amorphization of drug formulations containing active pharmaceutical ingredients (APIs) and excipients has been proven to be an effective strategy to improve their poor aqueous solubility. The excipients can also impact the physical stability of the prepared amorphous forms. Generally, researchers are more apt to select excipients that have high values of glass transition temperature (T_g) because of the antiplasticization effect of the additives on APIs. In this article, we studied the glass transition dynamics as well as crystallization behavior in binary blends composed of griseofulvin (GSF) and two low-T_g additives, octaacetylmaltose (acMAL) and polyvinyl acetate (PVAc), with a particular focus on the plasticization effect. Effectively suppressed crystallization of GSF is observed in both systems when higher excipient contents are used. Our finding aims to encourage the use of specifically developed protocols in which suitable plasticizers are used as excipients for stabilizing the amorphous state of a drug.

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.

Effects of Solvent Stabilization on Pharmaceutical Crystallization: Investigating Conformational Polymorphism of Probucol Using Combined Solid-State Density Functional Theory, Molecular Dynamics, and Terahertz Spectroscopy

Solid-state density functional theory (DFT), molecular dynamics (MD), and terahertz (THz) spectroscopy were used to study the formation of enantiotropically related conformational Form I and Form II polymorphs of the pharmaceutical compound, probucol. DFT calculations were performed on the crystal systems to compare relative lattice energies and the solvent stabilization of the metastable Form II structure. The thermodynamics of solvent inclusion in the Form II·MeOH crystal system were determined from MD simulations, as was the favored conformation of molecular probucol in methanol and ethanol solutions. The findings from both solid-state DFT and MD calculations suggest that the preferred molecular orientations of the probucol molecule in solution and the probable inclusion of methanol in the crystal lattice during the crystallization process lead to the solvent selectivity of the probucol polymorph formation. The additional stabilization energy provided by the crystallization solvent facilitates the nucleation and growth of the Form II polymorph under conditions that favor this metastable crystal form over the thermodynamically stable Form I, despite the higher energy molecular and crystalline configurations of probucol Form II. We demonstrate the influence of solvent on the formation of pharmaceutical polymorphs and provide a molecular-level view of complex interactions leading to polymorphism using a combination of computational methods and THz spectral data.

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.