Amorphous solid dispersions containing residual crystallinity: Influence of seed properties and polymer adsorption on dissolution performance

Roles of Supersaturation and Liquid-Liquid Phase Separation for Enhanced Oral Absorption of Poorly Soluble Drugs from Amorphous Solid Dispersions

Amorphous solid dispersion (ASD) is one of the most important enabling formulation technologies for the development of poorly soluble drugs. Because of its thermodynamically unstable nature in both solid and wet states, the evaluation and optimization of the formulation performance involves some difficulties. The dissolution process is sensitively influenced by various factors, including the applied dose, medium composition, and pH. Supersaturated solutions can cause liquid-liquid phase separation (LLPS) and/or crystallization, which complicates the comprehension of the dissolution process. However, LLPS should be evaluated carefully because it is closely related to oral absorption. As LLPS concentration is analogous to amorphous solubility, it can be a key factor in predicting oral absorption from ASDs, if absorption is limited by solubility. Moreover, LLPS droplets are expected to increase transmembrane flux by increasing the drug concentration near the epithelial cell membrane. In this review, recently updated knowledge on the dissolution, membrane permeation, and oral absorption behaviors of ASDs is discussed with an emphasis on LLPS behavior.

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.

Professor Lynne S. Taylor: Scientist, educator, and adventurer

This special edition of the Journal of Pharmaceutical Sciences is dedicated to Professor Lynne S. Taylor (Retter Distinguished Professor of Pharmacy, Department of Industrial and Molecular Pharmaceutics, Purdue University), to honor her distinguished career as a pharmaceutical scientist and educator. The goal of this commentary is to provide an overview of Professor Taylor's career path, summarize her key research contributions, and provide some insight into her personal and professional contributions as an educator, mentor, wife, mother, friend, and adventurer.

Crystallization inhibition in molecular liquids by polymers above the overlap concentration (c*): Delay of the first nucleation event

Low concentration polymer additives can significantly alter crystal growth kinetics of molecular liquids and glasses. However, the effect of polymer concentration on nucleation kinetics remains poorly understood. Based on an experimentally determined first nucleation time (time to form the first critical nucleus, t0), we show that the polymer overlap concentration, c*, where polymer coils in the molecular liquid start to overlap with each other, is a critical polymer concentration for efficient inhibition of crystallization of a molecular liquid. The value of t0 is approximately equal to that of the neat molecular liquid when the polymer concentration, c, is below c*, but increases significantly when c > c*. This finding is relevant for effective polymer screening and performance prediction of engineered multicomponent amorphous materials, particularly pharmaceutical amorphous solid dispersions.

Assessing processability of milled HME extrudates: Consolidating the effect of extrusion temperature, drug loading, and particle size via Non-dimensional cohesion

The downstream processability of Hot Melt Extrusion (HME) Amorphous Solid Dispersions (ASD), an underexplored topic of importance, was assessed through a multi-faceted particle engineering approach. Extrudates, comprised of griseofulvin (GF), a model poorly water-soluble drug, and hydroxypropyl cellulose (HPC), were prepared at four drug concentrations and three HME temperature profiles to yield cases with and without residual crystallinity and subsequently milled to five sieve cuts ranging from < 45 μm to 355 - 500 μm. Solid state characterization was performed with XRPD, FT-IR, and TGA. Particle scale properties of the milled extrudates were evaluated including particle size, density, surface energy, and morphologies imaged via SEM. It was observed that regardless of sieve cut size, drug concentration and HME conditions impacted the flowability trends, quantified via Flow Function Coefficient (FFC) and bulk density. As a novelty, the effects of various process parameters and drug loadings were consolidated into a dimensionless interparticle cohesion measure, granular Bond Number (Bog), to better correlate them with bulk powder properties. The significant contrast in particle morphologies, particle size, and densities among selected cases demonstrated that particle size alone should not be the sole consideration when correlating particle scale to bulk powder scale properties of milled extrudates. Instead, the HME temperature profile and ASD drug loading may be more suitable parameters affecting the bulk powder properties of the milled extrudates.

Melt-extruded formulations of fenofibrate with various grades of hydrogenated phospholipid exhibit promising in-vitro biopharmaceutical behavior

In the current study, it was demonstrated that three commercially available grades of hydrogenated phospholipids (HPL) differing in their content of phosphatidylcholine may be used as components for hot melt-extruded binary (HPL as sole excipient) or ternary (in combination with copovidone) solid dispersions of fenofibrate (FEN) at mass fractions between 0.5 and 20% (ternary) or 80% (binary). X-ray powder diffraction indicated complete conversion of crystalline fenofibrate into the amorphous state by hot melt extrusion for all ternary blends. In contrast, both the binary blends (HPL- and copovidone-based) contained minor remaining crystallites. Irrespectively, all solid dispersions induced during dissolution studies a supersaturated state of FEN, where the ternary ASDs showed enhanced and more complete release of FEN as compared to the binary blends and, even more pronounced, in comparison to the marketed micronized and nano-milled formulations. In terms of the cumulated amount permeated, there were marginal differences between the various formulations when combined dissolution/permeation was done using FeSSIF as donor medium; with FaSSIF as donor medium, the binary HPL-ASD containing the grade with the highest phosphatidylcholine fraction performed best in terms of permeation, even significantly better than the marketed nano-crystal formulation. Otherwise, no significant differences were seen between the various grades of HPL when FEN dissolution and permeation were analyzed for ternary solid dispersions. In conclusion, the in-vitro biopharmaceutical behaviour of hydrogenated phospholipid-containing blends manufactured by hot melt extrusion appears promising. They can be a viable formulation option for poorly water-soluble and lipophilic drug compounds like FEN.

Role of rheology in formulation and process design of hot melt extruded amorphous solid dispersions

Hot melt extrusion (HME) has been widely used as a continuous and highly flexible pharmaceutical manufacturing process for the production of a variety of dosage forms. In particular, HME enables preparation of amorphous solid dispersions (ASDs) which can improve bioavailability of poorly water-soluble drugs. The rheological properties of drug-polymer mixtures can significantly influence the processability of drug formulations via HME and eventually the end-use product properties such as physical stability and drug release. The objective of this review is to provide an overview of various rheological techniques and properties that can be used to evaluate the flow behavior and processability of the drug-polymer mixtures as well as formulation characteristics such as drug-polymer interactions, miscibility/solubility, and plasticization to improve the HME processability. An overview of the thermodynamics and kinetics of ASD processing by HME is also provided, as well as aspects of scale-up and process modeling, highlighting rheological properties on formulation design and process development. Overall, this review provides valuable insights into critical rheological properties which can be used as a predictive tool to optimize the HME processing conditions.

Drug Dissolution in Oral Drug Absorption: Workshop Report

The in-person workshop "Drug Dissolution in Oral Drug Absorption" was held on May 23-24, 2023, in Baltimore, MD, USA. The workshop was organized into lectures and breakout sessions. Three common topics that were re-visited by various lecturers were amorphous solid dispersions (ASDs), dissolution/permeation interplay, and in vitro methods to predict in vivo biopharmaceutics performance and risk. Topics that repeatedly surfaced across breakout sessions were the following: (1) meaning and assessment of "dissolved drug," particularly of poorly water soluble drug in colloidal environments (e.g., fed conditions, ASDs); (2) potential limitations of a test that employs sink conditions for a poorly water soluble drug; (3) non-compendial methods (e.g., two-stage or multi-stage method, dissolution/permeation methods); (4) non-compendial conditions (e.g., apex vessels, non-sink conditions); and (5) potential benefit of having both a quality control method for batch release and a biopredictive/biorelevant method for biowaiver or bridging scenarios. An identified obstacle to non-compendial methods is the uncertainty of global regulatory acceptance of such methods.

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.

Review of industrially recognized polymers and manufacturing processes for amorphous solid dispersion based formulations

Evolving therapeutic landscape through combinatorial chemistry and high throughput screening have resulted in an increased number of poorly soluble drugs. Drug delivery strategies quickly adapted to convert these drugs into successful therapies. Amorphous solid dispersion (ASD) technology is widely employed as a drug delivery strategy by pharmaceutical industries to overcome the challenges associated with these poorly soluble drugs. The development of ASD formulation requires an understanding of polymers and manufacturing techniques. A review of US FDA-approved ASD-based products revealed that only a limited number of polymers and manufacturing technologies are employed by pharmaceutical industries. This review provides a comprehensive guide for the selection and overview of polymers and manufacturing technologies adopted by pharmaceutical industries for ASD formulation. The various employed polymers with their underlying mechanisms for solution-state and solid-state stability are discussed. ASD manufacturing techniques, primarily implemented by pharmaceutical industries for commercialization, are presented in Quality by Design (QbD) format. An overview of novel excipients and progress in manufacturing technologies are also discussed. This review provides insights to the researchers on the industrially accepted polymers and manufacturing technology for ASD formulation that has translated these challenging drugs into successful therapies.

Amorphous Solid Dispersion as Drug Delivery Vehicles in Cancer

Cancer treatment has improved over the past decades, but a major challenge lies in drug formulation, specifically for oral administration. Most anticancer drugs have poor water solubility which can affect their bioavailability. This causes suboptimal pharmacokinetic performance, resulting in limited efficacy and safety when administered orally. As a result, it is essential to develop a strategy to modify the solubility of anticancer drugs in oral formulations to improve their efficacy and safety. A promising approach that can be implemented is amorphous solid dispersion (ASD) which can enhance the aqueous solubility and bioavailability of poorly water-soluble drugs. The addition of a polymer can cause stability in the formulations and maintain a high supersaturation in bulk medium. Therefore, this study aimed to summarize and elucidate the mechanisms and impact of an amorphous solid dispersion system on cancer therapy. To gather relevant information, a comprehensive search was conducted using keywords such as "anticancer drug" and "amorphous solid dispersion" in the PubMed, Scopus, and Google Scholar databases. The review provides an overview and discussion of the issues related to the ASD system used to improve the bioavailability of anticancer drugs based on molecular pharmaceutics. A thorough understanding of anticancer drugs in this system at a molecular level is imperative for the rational design of the products.

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.

Composites of N-butyl-N-methyl-1-phenylpyrrolo[1,2-a]pyrazine-3-carboxamide with Polymers: Effect of Crystallinity on Solubility and Stability

This work aimed to develop and characterize a water-soluble, high-release active pharmaceutical ingredient (API) composite based on the practically water-insoluble API N-butyl-N-methyl-1-phenylpyrrolo[1,2-a]pyrazine-3-carboxamide (GML-3), a substance with antidepressant and anxiolytic action. This allows to ensure the bioavailability of the medicinal product of combined action. Composites obtained by the method of creating amorphous solid dispersions, where polyvinylpyrrolidone (PVP) or Soluplus® was used as a polymer, were studied for crystallinity, stability and the release of API from the composite into purified water. The resulting differential scanning calorimetry (DSC), powder X-ray diffractometry (PXRD), and dissolution test data indicate that the resulting composites are amorphous at 1:15 API: polymer ratios for PVP and 1:5 for Soluplus®, which ensures the solubility of GML-3 in purified water and maintaining the supercritical state in solution.

Advances in the development of amorphous solid dispersions: The role of polymeric carriers

Amorphous solid dispersion (ASD) is one of the most effective approaches for delivering poorly soluble drugs. In ASDs, polymeric materials serve as the carriers in which the drugs are dispersed at the molecular level. To prepare the solid dispersions, there are many polymers with various physicochemical and thermochemical characteristics available for use in ASD formulations. Polymer selection is of great importance because it influences the stability, solubility and dissolution rates, manufacturing process, and bioavailability of the ASD. This review article provides a comprehensive overview of ASDs from the perspectives of physicochemical characteristics of polymers, formulation designs and preparation methods. Furthermore, considerations of safety and regulatory requirements along with the studies recommended for characterizing and evaluating polymeric carriers are briefly discussed.

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.

Impact of Gastric pH Variations on the Release of Amorphous Solid Dispersion Formulations Containing a Weakly Basic Drug and Enteric Polymers

Enteric polymers are widely used in amorphous solid dispersion (ASD) formulations. The aim of the current study was to explore ASD failure mechanisms across a wide range of pH conditions that mimic in vivo gastric compartment variations where enteric polymers such as hydroxypropyl methylcellulose phthalate (HPMCP) and hydroxypropyl methylcellulose acetate succinate (HPMCAS) are largely insoluble. Delamanid (DLM), a weakly basic drug used to treat tuberculosis, was selected as the model compound. Both DLM free base and the edisylate salt were formulated with HPMCP, while DLM edisylate ASDs were also prepared with different grades of HPMCAS. Two-stage release testing was conducted with the gastric stage pH varied between pH 1.6 and 5.0, prior to transfer to intestinal conditions of pH 6.5. ASD particles were collected following suspension in the gastric compartment and evaluated using X-ray powder diffraction and scanning electron microscopy. Additional samples were also evaluated with polarized light microscopy. In general, ASDs with HPMCP showed improved overall release for all testing conditions, relative to ASDs with HPMCAS. ASDs with the edisylate salt likewise outperformed those with DLM free base. Impaired release for certain formulations at intestinal pH conditions was attributed to surface drug crystallization that initiated during suspension in the gastric compartment where the polymer is insoluble; crystallization appeared more extensive for HPMCAS ASDs. These findings suggest that gastric pH variations should be evaluated for ASD formulations containing weakly basic drugs and enteric polymers.

Effect of Different Seed Crystals on the Supersaturation State of Ritonavir Tablets Prepared by Hot-Melt Extrusion

Hot-melt extrusion (HME) is a technology increasingly common for the commercial production of pharmaceutical amorphous solid dispersions (ASDs), especially for poorly water-soluble active pharmaceutical ingredients (APIs). However, recrystallization of the APIs during dissolution must be prevented to maintain the supersaturation state enabled by ASD. Unfortunately, the amorphous formulation may be contaminated by seed crystals during the HME manufacturing process, which could lead to undesirable crystal growth during the dissolution process. In this study, the dissolution behavior of ritonavir ASD tablets prepared using both Form I and Form II polymorphs was examined, and the effects of different seed crystals on crystal growth rates were investigated. The aim was to understand how the presence of seed crystals can impact the dissolution of ritonavir, and to determine the optimal polymorph and seeding conditions for the production of ASDs. The results showed that both Form I and Form II ritonavir tablets had similar dissolution profiles, which were also similar to the reference listed drug (RLD). However, it was observed that the presence of seed crystals, particularly the metastable Form I seed, led to more precipitation compared to the stable Form II seed in all formulations. The Form I crystals that precipitated from the supersaturated solution were easily dispersed in the solution and could serve as seeds to facilitate crystal growth. On the other hand, Form II crystals tended to grow more slowly and presented as aggregates. The addition of both Form I and Form II seeds could affect their precipitation behaviors, and the amount and form of the seeds had significant effects on the precipitation process of the RLD tablets, as are the tablets prepared with different polymorphs. In conclusion, the study highlights the importance of minimizing the contamination risk of seed crystals during the manufacturing process and selecting the appropriate polymorph for the production of ASDs.

Nanoseeded Desupersaturation and Dissolution Tests for Elucidating Supersaturation Maintenance in Amorphous Solid Dispersions

The impact of residual drug crystals that are formed during the production and storage of amorphous solid dispersions (ASDs) has been studied using micron-sized seed crystals in solvent-shift (desupersaturation) and dissolution tests. This study examines the impacts of the seed size loading on the solution-mediated precipitation from griseofulvin ASDs. Nanoparticle crystals (nanoseeds) were used as a more realistic surrogate for residual crystals compared with conventional micron-sized seeds. ASDs of griseofulvin with Soluplus (Sol), Kollidon VA64 (VA64), and hydroxypropyl methyl cellulose (HPMC) were prepared by spray-drying. Nanoseeds produced by wet media milling were used in the dissolution and desupersaturation experiments. DLS, SEM, XRPD, and DSC were used for characterization. The results from the solvent-shift tests suggest that the drug nanoseeds led to a faster and higher extent of desupersaturation than the as-received micron-sized crystals and that the higher seed loading facilitated desupersaturation. Sol was the only effective nucleation inhibitor; the overall precipitation inhibition capability was ranked: Sol > HPMC > VA64. In the dissolution tests, only the Sol-based ASDs generated significant supersaturation, which decreased upon an increase in the nanoseed loading. This study has demonstrated the importance of using drug nanocrystals in lieu of conventional coarse crystals in desupersaturation and dissolution tests in ASD development.

Dissolution Mechanisms of Amorphous Solid Dispersions: Role of Drug Load and Molecular Interactions

High drug load amorphous solid dispersions (ASDs) have been a challenge to formulate partially because drug release is inhibited at high drug loads. The maximum drug load prior to inhibition of release has been termed the limit of congruency (LoC) and has been most widely studied for copovidone (PVPVA)-based ASDs. The terminology was derived from the observation that below LoC, the polymer controlled the kinetics and the drug and the polymer released congruently, while above LoC, the release rates diverged and were impaired. Recent studies show a correlation between the LoC value and drug-polymer interaction strength, where a lower LoC was observed for systems with stronger interactions. The aim of this study was to investigate the causality between drug-PVPVA interaction strength and LoC. Four chemical analogues with diverse abilities to interact with PVPVA were used as model drugs. The distribution of the polymer between the dilute aqueous phase and the insoluble nanoparticles containing drug was studied with solution nuclear magnetic resonance spectroscopy and traditional separation techniques to understand the thermodynamics of the systems in a dilute environment. Polymer diffusion to and from ASD particles suspended in aqueous solution was monitored for drug loads above the LoC to investigate the thermodynamic driving force for polymer release. The surface composition of ASD compacts before and after exposure to buffer was studied with Fourier transform infrared spectroscopy to capture potential kinetic barriers to release. It was found that ASD compacts with drug loads above the LoC formed an insoluble barrier on the surface that was in pseudo-equilibrium with the aqueous phase and prevented further release of drugs and polymers during dissolution. The insoluble barrier contained a substantial amount of the polymer for the strongly interacting drug-polymer systems. In contrast, a negligible amount was found for the weakly interacting systems. This observation provides an explanation for the ability of strongly interacting systems to form an insoluble barrier at lower drug loads. The study highlights the importance of thermodynamic and kinetic factors on the dissolution behavior of ASDs and provides a potential framework for maximizing the drug load in ASDs.

Impact of Aluminum Oxide Nanocoating on Drug Release from Amorphous Solid Dispersion Particles

Atomic layer coating (ALC) is emerging as a particle engineering strategy to inhibit surface crystallization of amorphous solid dispersions (ASDs). In this study, we turn our attention to evaluating drug release behavior from ALC-coated ASDs, and begin to develop a mechanistic framework. Posaconazole/hydroxypropyl methylcellulose acetate succinate was used as a model system at both 25% and 50% drug loadings. ALC-coatings of aluminum oxide up to 40 nm were evaluated for water sorption kinetics and dissolution performance under a range of pH conditions. Scanning electron microscopy with energy dispersive X-ray analysis was used to investigate the microstructure of partially released ASD particles. Coating thickness and defect density (inferred from deposition rates) were found to impact water sorption kinetics. Despite reduced water sorption kinetics, the presence of a coating was not found to impact dissolution rates under conditions where rapid drug release was observed. Under slower releasing conditions, underlying matrix crystallization was reduced by the coating, enabling greater levels of drug release. These results demonstrate that water was able to penetrate through the ALC coating, hydrating the amorphous solid, which can initiate dissolution of drug and/or polymer (depending on pH conditions). Swelling of the ASD substrate subsequently occurs, disrupting and cracking the coating, which serves to facilitate rapid drug release. Water sorption kinetics are highlighted as a potential predictive tool to investigate the coating quality and its potential impact on dissolution performance. This study has implications for formulation design and evaluation of ALC-coated ASD particles.

Detecting Crystallinity in Amorphous Solid Dispersions Using Dissolution Testing: Considerations on Properties of Drug Substance, Drug Product, and Selection of Dissolution Media

Dissolution testing has long been used to monitor product quality. Its role in quality control of amorphous solid dispersion (ASD) formulations is relatively new. In the presence of the crystalline phase, the dissolution of ASDs is determined by the dynamics between the dissolution rate of the amorphous solids and the rate of crystal growth. The detection of crystalline phase by dissolution test has not been well understood in the context of drug properties, formulation characteristics and dissolution test variables. This study systematically evaluated the impact of key parameters such as intrinsic crystallization tendency of the API, drug loading, extent of dissolution sink conditions and level of crystallinity on the ASD dissolution behavior. The results indicated diverse dissolution behaviors due to the differences in the intrinsic crystallization propensity of the drug, the drug loading, the ASD polymers and the dissolution sink index. Each of the complex dissolution profiles were interpreted based on visual observations during dissolution, the appropriate sink index based on the amorphous solubility, and the competition between drug dissolution versus crystallization. The findings of this study provide insights towards the various considerations that should be taken into account towards rationally developing a discriminatory dissolution method.

Formulation and Processing Strategies which Underpin Susceptibility to Matrix Crystallization in Amorphous Solid Dispersions

Through matrix crystallization, an amorphous solid may transform directly into its more stable crystalline state, reducing the driving force for dissolution. Herein, the mechanism of matrix crystallization in an amorphous solid dispersion (ASD) was probed. ASDs of bicalutamide/copovidone were prepared by solvent evaporation and hot melt extrusion, and sized by mortar and pestle or cryomilling techniques, modulating the level of mechanical activation experienced by the sample. Drug loading (DL) of the binary ASD was varied from 5-50%, and ternary systems were formulated at 30% DL with two surfactants (sodium dodecyl sulfate, Vitamin E TPGS). Imaging of partially dissolved or crystallized compacts by scanning electron microscopy with energy-dispersive X-ray analysis and confocal fluorescence microscopy was performed to investigate pathways of hydration, phase separation, and crystallization. Monitoring drug and polymer release of ASD powder under non-sink conditions provided insight into supersaturation and desupersaturation profiles. Systems at the greatest risk of matrix crystallization had high DLs, underwent mechanical activation, and/or contained surfactant. Systems having greatest resistance to matrix crystallization had rapid and congruent drug and polymer release. This study has implications for formulation and process design of ASDs and risk assessment of matrix crystallization.

Challenges and Strategies for Solubility Measurements and Dissolution Method Development for Amorphous Solid Dispersion Formulations

This manuscript represents the view of the Dissolution Working Group of the IQ Consortium on the challenges of and recommendations on solubility measurements and development of dissolution methods for immediate release (IR) solid oral dosage forms formulated with amorphous solid dispersions. Nowadays, numerous compounds populate the industrial pipeline as promising drug candidates yet suffer from low aqueous solubility. In the oral drug product development process, solubility along with permeability is a key determinant to assure sufficient drug absorption along the intestinal tract. Formulating the drug candidate as an amorphous solid dispersion (ASD) is one potential option to address this issue. These formulations demonstrate the rapid onset of drug dissolution and can achieve supersaturated concentrations, which poses significant challenges to appropriately characterize solubility and develop quality control dissolution methods. This review strives to categorize the different dissolution and solubility challenges for ASD associated with 3 different topics: (i) definition of solubility and sink conditions for ASD dissolution, (ii) applications and development of non-sink dissolution (according to conventional definition) for ASD formulation screening and QC method development, and (iii) the advantages and disadvantages of using dissolution in detecting crystallinity in ASD formulations. Related to these challenges, successful examples of dissolution experiments in the context of control strategies are shared and may lead as an example for scientific consensus concerning dissolution testing of ASD.

Atomic Layer Coating to Inhibit Surface Crystallization of Amorphous Pharmaceutical Powders

Preventing crystallization is a primary concern when developing amorphous drug formulations. Recently, atomic layer coatings (ALCs) of aluminum oxide demonstrated crystallization inhibition of high drug loading amorphous solid dispersions (ASDs) for over 2 years. The goal of the current study was to probe the breadth and mechanisms of this exciting finding through multiple drug/polymer model systems, as well as particle and coating attributes. The model ASD systems selected provide for a range of hygroscopicity and chemical functional groups, which may contribute to the crystallization inhibition effect of the ALC coatings. Atomic layer coating was performed to apply a 5-25 nm layer of aluminum oxide or zinc oxide onto ASD particles, which imparted enhanced micromeritic properties, namely, reduced agglomeration and improved powder flowability. ASD particles were stored at 40 °C and a selected relative humidity level between 31 and 75%. Crystallization was monitored by X-ray powder diffraction and scanning electron microscopy (SEM) up to 48 weeks. Crystallization was observable by SEM within 1-2 weeks for all uncoated samples. After ALC, crystallization was effectively delayed or completely inhibited in some systems up to 48 weeks. The delay achieved was demonstrated regardless of polymer hygroscopicity, presence or absence of hydroxyl functional groups in drugs and/or polymers, particle size, or coating properties. The crystallization inhibition effect is attributed primarily to decreased surface molecular mobility. ALC has the potential to be a scalable strategy to enhance the physical stability of ASD systems to enable high drug loading and enhanced robustness to temperature or relative humidity excursions.

Benchmarking the Solubility Enhancement and Storage Stability of Amorphous Drug–Polyelectrolyte Nanoplex against Co-Amorphous Formulation of the Same Drug

Amorphization, typically in the form of amorphous solid dispersion (ASD), represents a well-established solubility enhancement strategy for poorly soluble drugs. Recently, two amorphous drug formulations, i.e., the amorphous drug–polyelectrolyte nanoparticle complex (nanoplex) and co-amorphous system, have emerged as promising alternatives to circumvent the issues faced by ASD (i.e., large dosage requirement, high hygroscopicity). In the present work, the nanoplex was benchmarked against the co-amorphous system in terms of the preparation efficiency, drug payload, thermal stability, dissolution rate, supersaturation generation, and accelerated storage stability. Weakly acidic curcumin (CUR) and weakly basic ciprofloxacin (CIP) were used as the model poorly soluble drugs. The CUR and CIP nanoplexes were prepared using chitosan and sodium dextran sulfate as the polyelectrolytes, respectively. The co-amorphous CUR and CIP were prepared using tannic acid and tryptophan as the co-formers, respectively. The benchmarking results showed that the amorphous drug nanoplex performed as well as, if not better than, the co-amorphous system depending on the drug in question and the aspects being compared. The present work successfully established the nanoplex as an equally viable amorphous drug formulation as the more widely studied co-amorphous system to potentially serve as an alternative to ASD.

Rational Fabrication of Folate-Conjugated Zein/Soy Lecithin/Carboxymethyl Chitosan Core-Shell Nanoparticles for Delivery of Docetaxel

The objective of this work is to design and fabricate a natural zein-based nanocomposite with core-shell structure for the delivery of anticancer drugs. As for the design, folate-conjugated zein (Fa-zein) was synthesized as the inner hydrophobic core; soy lecithin (SL) and carboxymethyl chitosan (CMC) were selected as coating components to form an outer shell. As for fabrication, a novel and appropriate atomizing/antisolvent precipitation process was established. The results indicated that Fa-zein/SL/CMC core-shell nanoparticles (FZLC NPs) were successfully produced at a suitable mass ratio of Fa-zein/SL/CMC (100:30:10) and the freeze-dried FZLC powder showed a perfect redispersibility and stability in water. After that, docetaxel (DTX) as a model drug was encapsulated into FZLC NPs at different mass ratios of DTX to FZLC (MR). When MR = 1:15, DTX/FZLC NPs were obtained with high encapsulation efficiency (79.22 ± 0.37%), small particle size (206.9 ± 48.73 nm), and high zeta potential (-41.8 ± 3.97 mV). DTX was dispersed in the inner core of the FZLC matrix in an amorphous state. The results proved that DTX/FZLC NPs could increase the DTX dissolution, sustain the DTX release, and enhance the DTX cytotoxicity significantly. The present study provides insight into the formation of zein-based complex nanocarriers for the delivery of anticancer drugs.

Predicting in vivo performance of fenofibrate amorphous solid dispersions using in vitro non-sink dissolution and dissolution permeation setup

Amorphous solid dispersion (ASD) is emerging as a useful formulation strategy to increase the bioavailability of active pharmaceutical ingredients with poor solubility. In vitro dissolution testing under non-sink conditions has often been used to evaluate the ability of ASDs to generate and maintain supersaturation to predict the in vivo performance. However, such a single compartment dissolution setup can fail to predict the oral bioavailability, due to an interdependence between precipitation and permeation. Hence, the use of two compartment dissolution-permeation setups is emerging. In this study, three ASDs containing fenofibrate as model drug substance were developed using Soluplus®, and Hypromellose Acetate Succinate in two different grades (high and low), respectively. The aim was to compare the use of a small-scale in vitro non-sink dissolution setup and a small-scale in vitro dissolution-permeation setup to predict the in vivo oral exposure of the ASDs in rats. The maximum concentration (C_max) and area under curve (AUC) obtained in the in vitro studies were used to predict the in vivo rank order of the formulations. The results showed that the two in vitro studies resulted in the same rank order based on both C_max and AUC. Interestingly, C_max resulted in a better in vitro/in vivo correlation than the in vitro AUC, and based on the in vitro C_max, the in vivo rank order was predicted.

pH-Dependent supersaturation from amorphous solid dispersions of weakly basic drugs

PURPOSE

In amorphous solid dispersions (ASDs), the chemical potential of a drug can be reduced due to mixing with the polymer in the solid matrix, and this can lead to reduced drug release when the polymer is insoluble in the dissolution media. If both the drug and the polymer composing an ASD are ionizable, drug release from the ASD becomes pH-dependent. The goal of this study was to gain insights into the pH-dependent solubility suppression from ASD formulations.

METHODS

The maximum release of clotrimazole, a weakly basic drug, from ASDs formulated with insoluble and pH-responsive polymers, was determined as a function of solution pH. Drug-polymer interactions in ASDs were probed using melting point depression, moisture sorption, and solid-state Nuclear Magnetic Resonance spectroscopy (SSNMR) measurements.

RESULTS

The extent of solubility suppression was dependent on polymer type and drug loading. The strength of drug-polymer interactions was found to correlate well with the degree of solubility suppression. For the same ASD, the degree of solubility suppression was nearly constant across the solution pH range studied, suggesting that polymer-drug interactions in residual ASD solids was independent of solution pH. The total drug release agrees with the Henderson-Hasselbalch relationship if the suppressed amorphous solubility of the free drug is independent of solution pH.

CONCLUSIONS

The mechanism of solubility suppression at different solution pHs appeared to be drug-polymer interactions in the solid-state, where the concentration of the free drug remains the same at variable pHs and the total drug concentration follows the Henderson-Hasselbalch relationship.

Formulating a heat- and shear-labile drug in an amorphous solid dispersion: Balancing drug degradation and crystallinity

We seek to further addresss the questions posed by Moseson et al. regarding whether any residual crystal level, size, or characteristic is acceptable in an amorphous solid dispersion (ASD) such that its stability, enhanced dissolution, and increased bioavailability are not compromised. To address this highly relevant question, we study an interesting heat- and shear-labile drug in development, LY3009120. To study the effects of residual crystallinity and degradation in ASDs, we prepared three compositionally identical formulations (57–1, 59–4, and 59–5) using the KinetiSol process under various processing conditions to obtain samples with various levels of crystallinity (2.3%, 0.9%, and 0.1%, respectively) and degradation products (0.74%, 1.97%, and 3.12%, respectively). Samples with less than 1% crystallinity were placed on stability, and we observed no measurable change in the drug's crystallinity, dissolution profile or purity in the 59–4 and 59–5 formulations over four months of storage under closed conditions at 25 °C and 60% humidity. For formulations 57–1, 59–4, and 59–5, bioavailability studies in rats reveal a 44-fold, 55-fold, and 62-fold increase in mean AUC, respectively, compared to the physical mixture. This suggests that the presence of some residual crystals after processing can be acceptable and will not change the properties of the ASD over time.

Enhanced Supersaturation via Fusion-Assisted Amorphization during FDM 3D Printing of Crystalline Poorly Soluble Drug Loaded Filaments

Filaments loaded with griseofulvin (GF), a model poorly water-soluble drug, were prepared and used for 3D printing via fused deposition modeling (FDM). GF was selected due to its high melting temperature, enabling lower temperature hot-melt extrusion (HME) keeping GF largely crystalline in the filaments, which could help mitigate the disadvantages of high HME processing temperatures such as filament quality, important for printability and the adverse effects of GF recrystallization on tablet properties. Novel aspects include single-step fusion-assisted ASDs generation during FDM 3D printing and examining the impact of tablet surface areas (SA) through printing multi-mini and square-pattern perforated tablets to further enhance drug supersaturation during dissolution. Kollicoat protect and hydroxypropyl cellulose were selected due to their low miscibility with GF, necessary to produce crystalline filaments. The drug solid-state was assessed via XRPD, DSC and FT-IR. At 165 °C HME processing temperature, the filaments containing ~80% crystalline GF were printable. Fusion-assisted 3D printing led to GF supersaturation of ~153% for cylindrical tablets and ~293% with the square-pattern perforated tablets, indicating strong monotonous impact of tablet SA. Dissolution kinetics of drug release profiles indicated Fickian transport for tablets with higher SA, demonstrating greater SA-induced drug supersaturation for well-designed 3D printed tablets.

Enhanced Dissolution of Sildenafil Citrate Using Solid Dispersion with Hydrophilic Polymers: Physicochemical Characterization and In Vivo Sexual Behavior Studies in Male Rats

Sildenafil citrate (SLC) is a frequently used medication (Viagra®) for the treatment of erectile dysfunction (ED). Due to its poor solubility, SLC suffers from a delayed onset of action and poor bioavailability. Hence, the aim of the proposed work was to prepare and evaluate solid dispersions (SDs) with hydrophilic polymers (Kolliphor® P188, Kollidon® 30, and Kollidon®-VA64), in order to enhance the dissolution and efficacy of SLC. The SLC-SDs were prepared using a solvent evaporation method (at the ratio drug/polymer, 1:1, w/w) and characterized by Differential Scanning Calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscope (SEM), drug content, yield, and in vitro release studies. Based on this evaluation, SDs (SLC-KVA64) were optimized, with a maximum release of drug (99.74%) after 2 h for all the developed formulas. The SDs (SLC-KVA64) were further tested for sexual behavior activity in male rats, and significant enhancements in copulatory efficiency (81.6%) and inter-copulatory efficiency (44.9%) were noted in comparison to the pure SLC drug, when exposed to the optimized SLC-KVA64 formulae. Therefore, SD using Kollidon®-VA64 could be regarded as a potential strategy for improving the solubility, in vitro dissolution, and therapeutic efficacy of SLC.

Effect of Counterions on Dissolution of Amorphous Solid Dispersions Studied by Surface Area Normalized Dissolution

Solubility enhancement has become a common requirement for formulation development to deliver poorly water soluble drugs. Amorphous solid dispersions (ASDs) and salt formation have been two successful strategies, yet there are opportunities for further development. For ASDs, drug-polymer phase separation may occur at high drug loadings during dissolution, limiting the increase of drug loadings in ASD formulations. For salt formation, a salt form with high crystallinity and sufficient solid-state stability is required for solid dosage form development. This work studied the effect of counterions on the dissolution performance of ASDs. Surface area normalized dissolution or intrinsic dissolution methodology was employed to eliminate the effect of particle size and provide a quantitative comparison of the counterion effect on the intrinsic dissolution rate. Using indomethacin (IMC)-poly(vinylpyrrolidone-co-vinyl acetate) ASD as a model system, the effect of different bases incorporated into the ASD during preparation, the molar ratios between the base and IMC, and the drug loadings in the ASD were systematically studied. Strong bases capable of ionizing IMC significantly enhanced drug dissolution, while a weak base did not. A physical mixture of a strong base and the ASD also enhanced the dissolution rate, but the effect was less pronounced. At different base to IMC molar ratios, dissolution enhancement increased with the base to IMC ratio. At different drug loadings, without a base, the IMC dissolution rate decreased with the increase of drug loading. After incorporating a strong base, it increased with the increase of drug loading. The observations from this study were thought to be related to both the ionization of IMC in ASDs and the increase of microenvironment pH by the incorporated bases. With the significant enhancement of the drug dissolution rate, our work provides a promising approach of overcoming the dissolution limitation of ASD formulations at high drug loadings.

Amorphous Solid Dispersions Containing Residual Crystallinity: Competition Between Dissolution and Matrix Crystallization

Crystallinity in an amorphous solid dispersion (ASD) may negatively impact dissolution performance by causing lost solubility advantage and/or seeding crystal growth leading to desupersaturation. The goal of the study was to evaluate underlying dissolution and crystallization mechanisms resulting from residual crystallinity contained within bicalutamide (BCL)/polyvinylpyrrolidone vinyl acetate copolymer (PVPVA) ASDs produced by hot melt extrusion (HME). In-line Raman spectroscopy, polarized light microscopy, and scanning electron microscopy were used to characterize crystallization kinetics and mechanisms. The fully amorphous ASD (0% crystallinity) did not dissolve completely, and underwent crystallization to the metastable polymorph (form 2), initiating in the amorphous matrix at the interface of the amorphous solid with water. Under non-sink conditions, higher extents of supersaturation were achieved because dissolution initially proceeded unhindered prior to nucleation. ASDs containing residual crystallinity had markedly reduced supersaturation. Solid-mediated crystallization (matrix crystallization) consumed the amorphous solid, growing the stable polymorph (form 1). Under sink conditions, both the fully amorphous ASD and crystalline physical mixture achieve faster release than the ASDs containing residual crystallinity. In the latter systems, matrix crystallization leads to highly agglomerated crystals with high relative surface area. Solution-mediated crystallization was not a significant driver of concentration loss, due to slow crystal growth from solution in the presence of PVPVA. The high risk stemming from residual crystallinity in BCL/PVPVA ASDs stems from (1) fast matrix crystallization propagating from crystal seeds, and (2) growth of the stable crystal form. This study has implications for dissolution performance outcomes of ASDs containing residual crystallinity.

Characterization of Amorphous Solid Dispersions and Identification of Low Levels of Crystallinity by Transmission Electron Microscopy

Amorphous solid dispersions (ASDs) are used to increase the solubility of oral medicines by kinetically stabilizing the more soluble amorphous phase of an active pharmaceutical ingredient with a suitable amorphous polymer. Low levels of a crystalline material in an ASD can negatively impact the desired dissolution properties of the drug. Characterization techniques such as powder X-ray diffraction (pXRD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) are often used to detect and measure any crystallinity within ASDs. These techniques are unable to detect or quantify very low levels because they have limits of detection typically in the order of 1-5%. Herein, an ASD of felodipine (FEL) and polyvinylpyrrolidone/vinyl acetate copolymer (PVP/VA) prepared via a hot melt extrusion (HME) in a mass ratio of 30:70 was characterized using a range of techniques. No signs of residual crystallinity were found by pXRD, DSC, or FTIR. However, transmission electron microscopy (TEM) did identify two areas containing crystals at the edges of milled particles from a total of 55 examined. Both crystalline areas contained Cl Kα X-ray peaks when measured by energy-dispersive X-ray spectroscopy, confirming the presence of FEL (due to the presence of Cl atoms in FEL and not in PVP/VA). Further analysis was carried out by TEM using conical dark field (DF) imaging of a HME ASD of 50:50 FEL-PVP/VA to provide insights into the recrystallization process that occurs at the edges of particles during accelerated ageing conditions in an atmosphere of 75% relative humidity. Multiple metastable polymorphs of recrystallized FEL could be identified by selected area electron diffraction (SAED), predominately form II and the more stable form I. Conical DF imaging was also successful in spatially resolving and sizing crystals. This work highlights the potential for TEM-based techniques to improve the limit of detection of crystallinity in ASDs, while also providing insights into transformation pathways by identifying the location, size, and form of any crystallization that might occur on storage. This opens up the possibility of providing an enhanced understanding of a drug product's stability and performance.

Applications of Supercritical Anti-Solvent Process in Preparation of Solid Multicomponent Systems

Solid multicomponent systems (SMS) are gaining an increasingly important role in the pharmaceutical industry, to improve the physicochemical properties of active pharmaceutical ingredients (APIs). In recent years, various processes have been employed for SMS manufacturing. Control of the particle solid-state properties, such as size, morphology, and crystal form is required to optimize the SMS formulation. By utilizing the unique and tunable properties of supercritical fluids, supercritical anti-solvent (SAS) process holds great promise for the manipulation of the solid-state properties of APIs. The SAS techniques have been developed from batch to continuous mode. Their applications in SMS preparation are summarized in this review. Many pharmaceutical co-crystals and solid dispersions have been successfully produced via the SAS process, where the solid-state properties of APIs can be well designed by controlling the operating parameters. The underlying mechanisms on the manipulation of solid-state properties are discussed, with the help of on-line monitoring and computational techniques. With continuous researching, SAS process will give a large contribution to the scalable and continuous manufacturing of desired SMS in the near future.

The interplay of dissolution, solution crystallization and solid-state transformation of amorphous indomethacin in aqueous solution

Supersaturation profiles of amorphous indomethacin in aqueous solution containing 0.4 wt% and 4 wt% of isopropanol were predicted by combining separately-determined kinetics for dissolution, solution crystallization, and solid-state transformation. The kinetics of solid-state transformation were measured and compared to various data from the literature. The proposed kinetic model accounts for dissolution, solution crystallization and amorphous-to-crystalline solid-state transformation. It was validated for different initial amounts of amorphous and crystalline material and systems with different isopropanol contents. Furthermore, the influence of polyethylene glycol on the supersaturation behavior was investigated. The results clearly show the robustness of the model and give insight into the interplay of dissolution, solution crystallization, and solid-state transformation of. In particular, the influence of solid-state transformation on the overall supersaturation profile was elucidated in a quantitative manner. An amorphicity function φ(t) is proposed to account for the kinetics of the solid-state transformation. Its general form could be derived consistently from different sets of experimental data and seems to be independent of the particle size of the amorphous material and hydrodynamic conditions. This work is among the first of its kind to successfully integrate dissolution, crystallization from solution and solid-state transformation in a model that shows good predictability.

Overview of Extensively Employed Polymeric Carriers in Solid Dispersion Technology

Solid dispersion is the preferred technology to prepare efficacious forms of BCS class-II/IV APIs. To prepare solid dispersions, there exist a wide variety of polymeric carriers with interesting physicochemical and thermochemical characteristics available at the disposal of a formulation scientist. Since the advent of the solid dispersion technology in the early 1960s, there have been more than 5000 scientific papers published in the subject area. This review discusses the polymeric carrier properties of most extensively used polymers PVP, Copovidone, PEG, HPMC, HPMCAS, and Soluplus® in the solid dispersion technology. The literature trends about preparation techniques, dissolution, and stability improvement are analyzed from the Scopus® database to enable a formulator to make an informed choice of polymeric carrier. The stability and extent of dissolution improvement are largely dependent upon the type of polymeric carrier employed to formulate solid dispersions. With the increasing acceptance of transfer dissolution setup in the research community, it is required to evaluate the crystallization/precipitation inhibition potential of polymers under dynamic pH shift conditions. Further, there is a need to develop a regulatory framework which provides definition and complete classification along with necessarily recommended studies to characterize and evaluate solid dispersions.