Partitioning of surfactant into drug-rich nanodroplets and its impact on drug thermodynamic activity and droplet size

Excipient effects on supersaturation, particle size dynamics, and thermodynamic activity of multidrug amorphous formulations

Multidrug formulations enhance patient compliance and extend the life cycle of pharmaceutical products. To overcome solubility challenges for multidrug combinations, amorphous formulations are commonly used. However, the excipients for creating amorphous formulations are often selected without an understanding of their effects on the bioavailability of the drugs. In this context, we investigated the impact of three types of excipients (polymers, surfactants and amino acids) on the supersaturation and thermodynamic activity of multidrug amorphous formulations. Additionally, we studied the particle size dynamics of the colloidal phase formed as a result of liquid-liquid phase separation. The amorphous solubility of two drugs, atazanavir and ritonavir, was determined in solutions containing predissolved excipients and the particle size dynamics of the colloidal particles was measured by dynamic light scattering. Dissolution experiments of atazanavir and ritonavir were conducted in predissolved sodium dodecyl sulfate (SDS), an anionic surfactant, and alanine solutions under non-sink conditions. Membrane transport of the drugs was evaluated using a MicroFLUX setup. The polymers had only minor effects on the amorphous solubility, but SDS significantly increased the solubilities of both drugs. In contrast, the other non-ionic surfactants and amino acids reduced the solubility of atazanavir but had no negative effect on ritonavir. Polymers were effective in maintaining supersaturation and preventing the coarsening of the colloidal particles. Conversely, alanine was neither able to inhibit the solution crystallization nor increase the flux of either drug. Despite the increase in the amorphous solubility of both drugs in SDS, flux was reduced. These results highlight the importance of properly selecting excipients for supersaturating amorphous formulations. The choice of excipient impacts the thermodynamic activity, the phase behaviour of the drugs and hence, the resulting absorption after oral intake.

Amorphous Solubility Advantage: Theoretical Considerations, Experimental Methods, and Contemporary Relevance

Twenty-five years ago, Hancock and Parks asked a provocative question: "what is the true solubility advantage for amorphous pharmaceuticals?" Difficulties in determining the amorphous solubility have since been overcome due to significant advances in theoretical understanding and experimental methods. The amorphous solubility is now understood to be the concentration after the drug undergoes liquid-liquid or liquid-glass phase separation, forming a water-saturated drug-rich phase in metastable equilibrium with an aqueous phase containing molecularly dissolved drug. While crystalline solubility is an essential parameter impacting the absorption of crystalline drug formulations, amorphous solubility is a vital factor for considering absorption from supersaturating formulations. However, the amorphous solubility of drugs is complex, especially in the presence of formulation additives and gastrointestinal components, and concentration-based measurements may not indicate the maximum drug thermodynamic activity. This review discusses the concept of the amorphous solubility advantage, including a historical perspective, theoretical considerations, experimental methods for amorphous solubility measurement, and the contribution of supersaturation and amorphous solubility to drug absorption. Leveraging amorphous solubility and understanding the associated physicochemical principles can lead to more effective development strategies for poorly water-soluble drugs, ultimately benefiting therapeutic outcomes.

Enhanced Dissolution of Amphotericin B through Development of Amorphous Solid Dispersions Containing Polymer and Surfactants

Amphotericin B (AmB) is the gold standard for antifungal therapy; however, its poor solubility limits its administration via intravenous infusion. A promising formulation strategy to achieve an oral formulation is the development of amorphous solid dispersions (ASDs) via spray-drying. Inclusion of surfactants into ASDs is a newer concept, yet it offers increased dissolution opportunities when combined with a polymer (HPMCAS 912). We developed both binary ASDs (AmB:HPMCAS 912 or AmB:surfactant) and ternary ASDs (AmB:HPMCAS 912:surfactant) using a variety of surfactants to determine the optimal surfactant carbon chain length and functional group for achieving maximal AmB concentration during in vitro dissolution. The ternary ASDs containing surfactants with a carbon chain length of 14 ± 2 carbons and a sulfate functional group increased the dissolution of AmB by 90-fold compared to crystalline AmB. These same surfactants, when added to a binary ASD, however, were only able to achieve up to a 40-fold increase, alluding to a potential interaction occurring between excipients or excipient and drug. This potential interaction was supported by dynamic light scattering data, in which the ternary formulation produced a single peak at 895.2 dnm. The absence of more than one peak insinuates that all three components are interacting in some way to form a single structure, which may be preventing AmB self-aggregation, thus improving the dissolution concentration of AmB.

Impact of Colloidal Drug-Rich Droplet Size and Amorphous Solubility on Drug Membrane Permeability: A Comprehensive Analysis

This study aimed to investigate the impact of amorphous solubility and colloidal drug-rich droplets on drug absorption. The amorphous solubility of cilnidipine (CND) in AS-HF grade of hypromellose acetate succinate (HPMC-AS) solution was significantly reduced compared to that in non-polymer solution due to AS-HF partitioning into the CND-rich phase. In contrast, AS-LF grade of HPMC-AS has minimal effect on the amorphous solubility. The size of colloidal CND-rich droplets formed in the CND-supersaturated solution was less than 100 nm in the presence of AS-HF, while 200-450 nm in the presence of AS-LF. When the CND concentrations were near the amorphous solubility, CND membrane flux was reduced in the presence of AS-HF due to the decrease in the amorphous solubility of CND. However, the CND flux increased with the increase in CND-rich droplets, especially in the AS-HF solution. The size reduction of the CND-rich droplets led to their effective diffusion into the unstirred water layer, enhancing CND flux. In higher CND concentration regions, the CND flux became higher in the AS-HF solution than in the AS-LF solution. Thus, it is essential to elucidate the drug concentration-dependent impact of the colloidal drug-rich droplets on the drug absorption performance to optimize supersaturating formulations.

Mechanistic insights on the antibacterial action of the kyotorphin peptide derivatives revealed by in vitro studies and Galleria mellonella proteomic analysis

OBJECTIVES

The selected kyotorphin derivatives were tested to improve their antimicrobial and antibiofilm activity. The antimicrobial screening of the KTP derivatives were ascertained in the representative strains of bacteria, including Streptococcus pneumoniae, Streptococcus pyogenes, Escherichia coli and Pseudomonas aeruginosa.

METHODS

Kyotorphin derivatives, KTP-NH2, KTP-NH2-DL, IbKTP, IbKTP-NH2, MetKTP-DL, MetKTP-LD, were designed and synthesized to improve lipophilicity and resistance to enzymatic degradation. Peptides were synthesized by standard solution or solid-phase peptide synthesis and purified using RP-HPLC, which resulted in >95 % purity, and were fully characterized by mass spectrometry and 1H NMR. The minimum inhibitory concentrations (MIC) determined for bacterial strains were between 20 and 419 μM. The direct effect of IbKTP-NH2 on bacterial cells was imaged using scanning electron microscopy. The absence of toxicity, high survival after infection and an increase in the hemocytes count was evaluated by injections of derivatives in Galleria mellonella larvae. Proteomics analyses of G. mellonella hemolymph were performed to investigate the underlying mechanism of antibacterial activity of IbKTP-NH2 at MIC.

RESULTS

IbKTP-NH2 induces morphological changes in bacterial cell, many differentially expressed proteins involved in DNA replication, synthesis of cell wall, and virulence were up-regulated after the treatment of G. mellonella with IbKTP-NH2.

CONCLUSION

We suggest that this derivative, in addition to its physical activity on the bacterial membranes, can elicit a cellular and humoral immune response, therefore, it could be considered for biomedical applications.

Quantitative Investigation of Intestinal Drug Absorption Enhancement by Drug-Rich Nanodroplets Generated via Liquid-Liquid Phase Separation

Drug-rich droplets formed through liquid-liquid phase separation (LLPS) have the potential to enhance the oral absorption of drugs. This can be attributed to the diffusion of these droplets into the unstirred water layer (UWL) of the gastrointestinal tract and their reservoir effects on maintaining drug supersaturation. However, a quantitative understanding of the effect of drug-rich droplets on intestinal drug absorption is still lacking. In this study, the enhancement of intestinal drug absorption through the formation of drug-rich droplets was quantitatively evaluated on a mechanistic basis. To obtain fenofibrate (FFB)-rich droplets, an amorphous solid dispersion (ASD) of FFB/hypromellose (HPMC) was dispersed in an aqueous medium. Physicochemical characterization confirmed the presence of nanosized FFB-rich droplets in the supercooled liquid state within the FFB/HPMC ASD dispersion. An in situ single-pass intestinal perfusion (SPIP) assay in rats demonstrated that increased quantities of FFB-rich nanodroplets enhanced the intestinal absorption of FFB. The effective diffusion of FFB-rich nanodroplets through UWL would partially contribute to the improved FFB absorption. Additionally, confocal laser scanning microscopy (CLSM) of cross sections of the rat intestine after the administration of fluorescently labeled FFB-rich nanodroplets showed that these nanodroplets were directly taken up by small intestinal epithelial cells. Therefore, the direct uptake of drug-rich nanodroplets by the small intestine is a potential mechanism for improving FFB absorption in the intestine. To quantitatively evaluate the impact of FFB-rich droplets on the FFB absorption enhancement, we determined the apparent permeabilities of the FFB-rich nanodroplets and dissolved FFB based on the SPIP results. The apparent permeability of the FFB-rich nanodroplets was 110-130 times lower than that of dissolved FFB. However, when the FFB-rich nanodroplet concentration was several hundred times higher than that of dissolved FFB, the FFB-rich nanodroplets contributed significantly to FFB absorption improvement. The present study highlights that drug-rich nanodroplets play a direct role in enhancing drug absorption in the gastrointestinal tract, indicating their potential for further improvement of oral absorption from ASD formulations.

The biomolecular gastrointestinal corona in oral drug delivery

The formation of a biomolecular corona on exogenous particles in plasma is well studied and is known to dictate the biodistribution and cellular interactions of nanomedicine formulations. In contrast, while the oral route is the most favorable administration method for pharmaceuticals, little is known about the formation and composition of the corona formed by biomolecules on particles within the gastrointestinal tract. This work reviews the current literature understanding of (1) the formation of drug particles after oral administration, (2) the formation of a biomolecular corona within the gastrointestinal tract ("the gastrointestinal corona"), and (3) the possible implications of the formation of a gastrointestinal corona on the interactions of drug particles with their biological environment. In doing so, this work aims to establish the significance of the formation of a gastrointestinal corona in oral drug delivery to ultimately arrive at new avenues to control the behavior of orally administered pharmaceuticals.

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.

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.

Quantitative Analysis of Drug Supersaturation Region by Temperature-Variable Nuclear Magnetic Resonance Measurements, Part 2: Effects of Solubilizer

This study utilized temperature-variable nuclear magnetic resonance (NMR) spectroscopy to investigate the effects of a solubilizing agent on the ketoprofen (KTP) supersaturation region. Quantitative NMR analysis showed that the solubilizing agent cetyltrimethylammonium bromide (CTAB) increased both the crystalline and amorphous solubilities of KTP, shifting the KTP supersaturation region to a higher KTP concentration range. The amorphous solubility of KTP was found to be independent of the enantiomeric composition of KTP, even in the presence of CTAB. However, the supersaturation region of the S-enantiomer of KTP (s-KTP) in CTAB solutions was smaller than that of the racemic form of KTP (rac-KTP), likely because of the higher crystalline solubility of s-KTP. When KTP formed a KTP-rich phase via liquid-liquid phase separation from KTP-supersaturated solutions, CTAB was observed to be distributed into the KTP-rich phase, decreasing the chemical potential of KTP and the maximum thermodynamic activity of KTP in the aqueous phase. Additionally, the incorporation of CTAB into the KTP-rich phase diminished the solubilization effect of CTAB micelles in the aqueous phase, narrowing the KTP supersaturation region to a greater extent at higher KTP dose concentrations. Furthermore, the upper-temperature limit of the supersaturated dissolvable region of KTP was lowered in the presence of CTAB, which was rationalized by the melting point depression of the KTP crystal upon mixing with CTAB. The findings of this study highlight the importance of considering the molecular-level impact of solubilizing agents on the drug supersaturation region to fully exploit the potential benefits of supersaturated formulations.

Quantitative Analysis of Drug Supersaturation Region by Temperature-Variable Nuclear Magnetic Resonance Measurements, Part 1: Effects of Polymer and Drug Chiralities

We examined the effects of the polymer-additive and drug chiralities on the ketoprofen (KTP) supersaturation region using temperature-variable nuclear magnetic resonance (NMR). Quantitative NMR analysis revealed that the racemic KTP and corresponding S-enantiomer (rac- and s-KTP) exhibited similar amorphous solubilities in a buffer, while the crystalline solubility of s-KTP was higher than that of rac-KTP. Therefore, rac-KTP exhibited a larger supersaturation region than s-KTP. In contrast, polyvinylpyrrolidone (PVP) reduced the amorphous solubility of both rac- and s-KTP, whereas the crystalline solubility of KTP remained unchanged. Partitioning PVP into the KTP-rich phase reduced the chemical potential of KTP in the KTP-rich phase and the amorphous solubility of KTP. At higher temperatures, the distribution of PVP into the KTP-rich phase became more significant, which considerably reduced the amorphous solubility. Because the upper limit of the KTP supersaturation decreased, PVP narrowed the KTP supersaturation region. The maximum KTP supersaturation ratio decreased with increasing temperature, and the supersaturated dissolvable area of KTP finally disappeared. The maximum temperature at which KTP can form the supersaturation was lowered by replacing rac- with s-KTP and the addition of PVP. The maximum supersaturation temperature was dominated by the melting behavior of crystalline KTP in an aqueous solution. The present study highlighted that a quantitative understanding of the supersaturation region is essential to determine whether supersaturated formulations are beneficial for improving the oral absorption of poorly water-soluble drugs.

Supersaturation and phase behavior during dissolution of amorphous solid dispersions

Amorphous solid dispersion (ASD) is a promising strategy to enhance solubility and bioavailability of poorly water-soluble drugs. Due to higher free energy of ASD, supersaturated drug solution could be generated during dissolution. When amorphous solubility of a drug is exceeded, drug-rich nanodroplets could form and act as a reservoir to maintain the maximum free drug concentration in solution, facilitating the absorption of the drug in vivo. Dissolution behavior of ASD has received increasing interests. This review will focus on the recent advances in ASD dissolution, including the generation and maintenance of supersaturated drug solution in absence or presence of liquid-liquid phase separation. Mechanism of drug release from ASD including polymer-controlled dissolution and drug-controlled dissolution will be introduced. Formation of amorphous drug-rich nanodroplets during dissolution and the underlying mechanism will be discussed. Phase separation morphology of hydrated ASD plays a critical role in dissolution behavior of ASD, which will be highlighted. Supersaturated drug solution shows poor physical stability and tends to crystallize. The effect of polymer and surfactant on supersaturated drug solution will be demonstrated and some unexpected results will be shown. Physicochemical properties of drug and polymer could impact ASD dissolution and some of them even show opposite effect on dissolution and physical stability of ASD in solid state, respectively. This review will contribute to a better understanding of ASD dissolution and facilitate a rational design of ASD formulation.

Liquid-liquid phase separation drug aggregate: Merit for oral delivery of amorphous solid dispersions

As a promising strategy, amorphous solid dispersion has been extensively employed in improving the oral bioavailability of insoluble drugs. Despite the numerous advantages, the problems associated with supersaturation stability limit its further application. Recently, the formation and stability of the liquid-liquid phase separation drug aggregate (LLPS-DA) have been found to be vital for supersaturation maintenance. An in-depth review of LLPS-DA was required to further explore the supersaturation maintenance mechanism in vivo. Hence, this study aimed to present a short review to introduce the LLPS-DA, highlight the in vivo advantages for oral administration, and discuss the prospects to help understand the in vivo behavior of LLPS-DA.

Amorphous Solid Dispersions: Role of the Polymer and Its Importance in Physical Stability and In Vitro Performance

Amorphous solid dispersions stabilized by one or more polymer(s) have been widely used for delivering amorphous drugs with poor water solubilities, and they have gained great market success. Polymer selection is important for preparing robust amorphous solid dispersions, and considerations should be given as to how the critical attributes of a polymer can enhance the physical stability, and the in vitro and in vivo performances of a drug. This article provides a comprehensive overview for recent developments in the understanding the role of polymers in amorphous solid dispersions from the aspects of nucleation, crystal growth, overall crystallization, miscibility, phase separation, dissolution, and supersaturation. The critical properties of polymers affecting the physical stability and the in vitro performance of amorphous solid dispersions are also highlighted. Moreover, a perspective regarding the current research gaps and novel research directions for better understanding the role of the polymer is provided. This review will provide guidance for the rational design of polymer-based amorphous pharmaceutical solids with desired physicochemical properties from the perspective of physical stability and in vitro performance.

Role of surfactants in improving release from higher drug loading amorphous solid dispersions

Amorphous solid dispersion formulations (ASD) are increasingly being used as a formulation strategy to improve bioavailability of poorly soluble drugs. One of the limitations of ASDs, in particular for high glass transition temperature (T_g) compounds, is the drug loading threshold (termed the limit of congruency, LoC) below which rapid, complete and congruent release of drug and polymer is achieved. In this study, several ionic and non-ionic surfactants were added to atazanavir-copovidone ASDs with the main goal of increasing the limit of congruency. Atazanavir (ATZ) is a relatively high T_g compound with a LoC of 5 % drug loading (DL). Surface normalized dissolution studies revealed that addition of 5 % w/w of surfactant, sodium dodecyl sulfate (SDS) or cetrimonium bromide (CTAB), to the binary copovidone-based ASD doubled the LoC (from 5 to 10 % DL), resulting in a more than 30-fold increase in total release compared to the corresponding binary ASD. Moreover, addition of 5 % of Span®80 increased the LoC to 15 % DL. ASD T_g was found to decrease upon addition of surfactants and water sorption extent was found to increase. We speculate that surfactants act as plasticizers, which may facilitate polymer release from ASDs containing a high T_g drug, providing a possible explanation for the observed enhancement in drug release from ternary ASDs and the increase in LoC.

Unusual Correlation between the Apparent Amorphous Solubility of a Drug and Solubilizer Concentration Revealed by NMR Analysis

Herein, we investigated the effect of the solubilizers, cetyltrimethylammonium bromide (CTAB) and amino methacrylate copolymer (Eudragit E PO, EUD-E), on the apparent amorphous solubility of ketoprofen (KTP) and free KTP concentrations in an aqueous phase when a KTP-rich phase was generated by liquid-liquid phase separation. Quantitative analysis by solution nuclear magnetic resonance (NMR) revealed that the apparent amorphous solubility of KTP increased with increasing EUD-E concentrations by the solubilization of KTP into the EUD-E micelles; this was reminiscent of the improvement in the apparent crystalline solubility of KTP observed when EUD-E was added. In contrast, the apparent amorphous solubility of KTP decreased with increasing CTAB concentrations, although the solubilizing ability of CTAB was stronger than that of EUD-E when the KTP-rich phase was absent. NMR analysis revealed that CTAB was distributed into the KTP-rich phase to a relatively large extent. This resulted in a significant reduction of the chemical potential of KTP in the KTP-rich phase in the CTAB solution. Thus, the maximum free KTP concentration in the aqueous phase was reduced more significantly in the CTAB solution than in the EUD-E solution. Moreover, the solubilization effect of KTP by the CTAB micelles in the aqueous phase was drastically diminished due to the distribution of CTAB into the KTP-rich phase. As a result, the apparent amorphous solubility of KTP reached a minimum at a CTAB concentration of 200 μg/mL. A further increase in the CTAB concentration resulted in an improvement in the apparent amorphous solubility of KTP due to the solubilization effect of CTAB remaining in the aqueous phase. The present study highlights the impact of solubilizer selection on the apparent amorphous solubility and attainable supersaturation of the drug, which should be considered during the development of supersaturating formulations to obtain preferable oral absorption.

Water-Resistant Drug-Polymer Interaction Contributes to the Formation of Nano-Species during the Dissolution of Felodipine Amorphous Solid Dispersions

Drug-polymer interactions are of great importance in amorphous solid dispersion (ASD) formulation for both dissolution performance and physical stability considerations. In this work, three felodipine ASD systems with drug loading ranging from 5 to 20% were prepared using PVP, PVP-VA, or HPMC-AS as the polymer matrix. The amorphization and homogeneity were confirmed by differential scanning calorimetry and powder X-ray diffraction. The intrinsic dissolution behavior of these ASDs was studied in 0.05 M HCl and phosphate-buffered saline (PBS) (pH 6.5). In 0.05 M HCl, PVP-VA ASDs with low drug loading (<15%) showed rapid dissolution accompanied with nano-species generation, while in the PVP system, rapid dissolution and nano-species generation were observed only when drug loading was less than 10%, and HPMC-AS ASDs always released slowly with no nano-species formation. In PBS, PVP-VA ASDs with drug loading less than 10% showed rapid dissolution accompanied with nano-species generation, while for PVP ASDs, rapid dissolution and nano-species generation were observed only when drug loading was 5%. However, 20% drug loading HPMC-AS ASDs exhibited rapid dissolution of felodipine and nano-species generation. When the drug loading was above the transition point of PVP-VA ASDs and PVP ASDs, the release rate was significantly lowered, and no nano-species was generated. To understand this phenomenon, drug-polymer interactions were studied using the melting point depression method and the Flory-Huggins model fitting. The Flory-Huggins interaction parameters (χ) for felodipine/HPMC-AS, felodipine/PVP, and felodipine/PVP-VA were determined to be 0.62 ± 0.07, -0.55 ± 0.20, and -1.02 ± 0.21, respectively, indicating the existence of the strongest attractive molecular interaction between felodipine and PVP-VA, followed by felodipine/PVP, but not in felodipine/HPMC-AS. Furthermore, dynamic vapor sorption further revealed that the molecular interactions between felodipine and PVP or PVP-VA were resistant to water. We concluded that water-resistant drug-polymer interactions in felodipine/polymer systems were responsible for the formation of nano-species, which further facilitated the rapid initial drug dissolution.

Phase separation in surfactant-containing amorphous solid dispersions: Orthogonal analytical methods to probe the effects of surfactants on morphology and phase composition

Amorphous-amorphous phase separation (AAPS) is an important phase transition process for amorphous solid dispersion (ASD) performance both in terms of drug release as well as physical and chemical stability during storage. Addition of surfactants to ASD systems can impact both of these processes. One possible mechanism through which surfactants affect ASD performance is via their impact on AAPS. Unfortunately, despite their increasing usage in ASD formulations, the effect of surfactant on AAPS is still poorly understood, and there are limited analytical techniques that provide microstructural and composition information about phase separated ASDs. In this study, the impact of four surfactants (sodium dodecyl sulfate, Tween 80, Span 20 and Span 85) on water-induced phase separation in ASDs formulated with ritonavir and polyvinylpyrrolidone/vinyl acetate (PVPVA) was investigated using a variety of orthogonal analytical methods. Transparent films of ASDs with different compositions were prepared by spin coating. Fluorescence confocal microscopy in combination with an in situ humidity chamber was used to monitor the kinetics and morphology of phase separation following exposure to high relative humidity. Optical photothermal IR analysis of phase separated films enabled characterization of domain composition and surfactant distribution. Liquid-liquid phase separation concentration, zeta potential and solution nuclear magnetic resonance spectroscopy measurements enabled interpretation of interaction with and partition of surfactants into the drug-rich phase. It was found that phase separation kinetics and morphology were notably changed by the surfactants. Further, the surfactants showed different affinities for the drug-rich versus the aqueous/polymer-rich phases. The employed analytical techniques were found to be complementary in providing insight into surfactant location in phase separated systems. This study highlights the complexity of phase separation, especially in the presence of surfactants, and provides a foundation to understand the impact of AAPS on ASD performance.

Impact of Surfactants on the Performance of Clopidogrel-Copovidone Amorphous Solid Dispersions: Increased Drug Loading and Stabilization of Nanodroplets

PURPOSE

Surfactants are increasingly being added to amorphous solid dispersion (ASDs) formulations to enhance processability and release performance. The goal of the current work was to investigate the impact of cationic, anionic and non-ionic surfactants on the rate and extent of clopidogrel (CPD) release from copovidone-based ASDs.

METHODS

CPD release was evaluated for ASDs with different drug loadings using a surface normalized intrinsic dissolution apparatus. Studies were also carried out using dynamic light scattering, zeta potential measurements, and nuclear magnetic resonance spectroscopy to probe the impact of surfactants on drug-rich nanodroplet physical stability and clopidogrel-surfactant interactions.

RESULTS

CPD ASDs showed good release for drug loadings as high as 40%, before the release fell off a cliff at higher drug loadings. Only sodium dodecyl sulfate, added at a 5% level, was able to improve the release at 50% drug loading, with other surfactants proving to be ineffective. However, some of the surfactants evaluated did show some benefits in improving nanodroplet stability against size enlargement. Ionic and non-ionic surfactants were observed to interact differently with CPD-rich nanodroplets, and variations in the kinetics and morphology of water-induced phase separation were noted in the presence and absence of surfactants in ASD films.

CONCLUSIONS

In summary, addition of surfactants to ASD formulations may lead to some improvements in formulation performance, but predictive capabilities and mechanisms of surfactant effect still require further studies.

Transition from Amorphous Cyclosporin A Nanoparticles to Size-Reduced Stable Nanocrystals in a Poloxamer 407 Solution

Amorphous drug nanoparticles usually exhibit low storage stability. A comprehensive understanding of the molecular states and physicochemical properties of the product is indispensable for designing stable formulations. In the present study, an amorphous cyclosporin A (CyA) nanosuspension with a mean particle size of approximately 370 nm was prepared by wet bead milling with poloxamer 407 (P407). Interestingly, the prepared amorphous CyA nanoparticles were transformed into uniform CyA nanocrystals with a reduced mean particle size of approximately 200 nm during storage at 25 °C. The CyA nanocrystals were stably maintained for at least 1 month. The particle morphologies and molecular structures of the CyA nanosuspensions before and after storage were thoroughly characterized by cryogenic transmission electron microscopy and magic-angle spinning nuclear magnetic resonance spectroscopy, respectively. They revealed that the freshly prepared amorphous CyA nanoparticles (∼370 nm) were secondary particles composed of aggregated primary particles with an estimated size of 50 nm. A portion of P407 was found to be entrapped at the gaps between the primary particles due to aggregation, while most of P407 was dissolved in the solution either adsorbing at the solid/liquid interface or forming polymeric micelles. The entrapped P407 is considered to play an important role in the destabilization of the amorphous CyA nanoparticles. The resultant CyA nanocrystals (∼200 nm) were uniform single crystals of Form 2 hydrate and showed corner-truncated bipyramidal features. Owing to the narrow particle size distribution of the CyA nanocrystals, the rate of Ostwald ripening was slow, giving long-term stability to the CyA nanocrystals. This study provides new insights into the destabilization mechanism of amorphous drug nanoparticles.

Recent Advances in Enhancement of Dissolution and Supersaturation of Poorly Water-Soluble Drug in Amorphous Pharmaceutical Solids: A Review

Amorphization is one of the most effective pharmaceutical approaches to enhance the dissolution and oral bioavailability of poorly water-soluble drugs. In recent years, amorphous formulations have been experiencing rapid development both in theoretical and practical application. Based on using different types of stabilizing agents, amorphous formulations can be mainly classified as polymer-based amorphous solid dispersion, coamorphous formulation, mesoporous silica-based amorphous formulation, etc. This paper summarizes recent advances in the dissolution and supersaturation of these amorphous formulations. Moreover, we also highlight the roles of stabilizing agents such as polymers, low molecular weight co-formers, and mesoporous silica. Maintaining supersaturation in solution is a key factor for the enhancement of dissolution profile and oral bioavailability, and thus, the strategies and challenges for maintaining supersaturation are also discussed. With an in-depth understanding of the inherent mechanisms of dissolution behaviors, the design of amorphous pharmaceutical formulations will become more scientific and reasonable, leading to vigorous development of commercial amorphous drug products.

Mechanistic Investigation of Drug Supersaturation in the Presence of Polysorbates as Solubilizing Additives by Solution Nuclear Magnetic Resonance Spectroscopy

The introduction of solubilizing additives has historically been an attractive approach to address the ever-growing proportion of poorly water-soluble drug (PWSD) compounds within the modern drug discovery pipeline. Lipid-formulations, and more specifically micelle formulations, have garnered particular interest because of their simplicity, size, scalability, and avoidance of solid-state limitations. Although micelle formulations have been widely utilized, the molecular mechanism of drug solubilization in surfactant micelles is still poorly understood. In this study, a series of modern nuclear magnetic resonance (NMR) methods are utilized to gain a molecular-level understanding of intermolecular interactions and kinetics in a model system. This approach enabled the understanding of how a PWSD, 17β-Estradiol (E2), solubilizes within a nonionic micelle system composed of polysorbate 80 (PS80). Based on one-dimensional (1D) ¹H chemical shift differences of E2 in PS80 solutions, as well as intermolecular correlations established from 1D selective nuclear Overhauser effect (NOE) and two-dimensional NOE spectroscopy experiments, E2 was found to accumulate within the palisade layer of PS80 micelles. A potential hydrogen-bonding interaction between a hydroxyl group of E2 and a carbonyl group of PS80 alkane chains may allow for stabilizing E2-PS80 mixed micelles. Diffusion and relaxation NMR analysis and particle size measurements using dynamic light scattering indicate a slight increase in the micellar size with increasing degrees of supersaturation, resulting in slower mobility of the drug molecule. Based on these structural findings, a theoretical orientation model of E2 molecules with PS80 molecules was developed and validated by computational docking simulations.

Drug crystal growth in ternary amorphous solid dispersions: Effect of surfactants and polymeric matrix-carriers

The present study evaluates the crystal growth rate of amorphous drugs when dispersed in different ternary polymeric amorphous solid dispersions (ASDs) in the presence of surfactants. Specifically, ternary ASDs of aprepitant (APT, selected as a model drug) were prepared via melt-quench cooling by evaluating three commonly used ASDs matrix/carriers, namely hydroxypropyl cellulose (HPC), poly(vinylpyrrolidone) (PVP) and the copolymer Soluplus® (SOL), and two suitable surfactants, namely d-alpha tocopheryl polyethylene glycol 1000 succinate (TPGS) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P407). Results showed that all components were completely miscible (verified via hot stage polarized microscopy) and both surfactants were acting as plasticizers to the API. APT's crystal growth rate was increased in the presence of both P407 and TPGS, while PVP was identified as the matrix/carrier with the greatest impact API's crystal growth rate inhibition. Interestingly, TPGS presented a noticeable synergistic effect when combined with PVP resulting in a further reduction of APT's crystal growth rate. Furthermore, evaluation of APT's nucleation induction time in dissolution medium (PBS pH 6.8) revealed PVP as the most effective crystallization inhibitor, whereas the addition of TPGS showed to improve PVP's ability to inhibit APT's recrystallization. Finally, the formation of intermolecular interactions in the ternary APT-PVP-TPGS provided an explanation for the observed PVP-TPGS synergistic effects, with molecular dynamics simulations being able to unravel the type and extent of these interactions on a theoretical basis.

Impact of Simulated Intestinal Fluids on Dissolution, Solution Chemistry, and Membrane Transport of Amorphous Multidrug Formulations

The solution behavior and membrane transport of multidrug formulations were herein investigated in a biorelevant medium simulating fasted conditions. Amorphous multidrug formulations were prepared by the solvent evaporation method. Combinations of atazanavir (ATV) and ritonavir (RTV) and felodipine (FDN) and indapamide (IPM) were prepared and stabilized by a polymer for studying their dissolution (under non-sink conditions) and membrane transport in fasted state simulated intestinal fluid (FaSSIF). The micellar solubilization by FaSSIF enhanced the amorphous solubility of the drugs to different extents. Similar to buffer, the maximum achievable concentration of drugs in combination was reduced in FaSSIF, but the extent of reduction was affected by the degree of FaSSIF solubilization. Dissolution studies of ATV and IPM revealed that the amorphous solubility of these two drugs was not affected by FaSSIF solubilization. In contrast, RTV was significantly affected by FaSSIF solubilization with a 30% reduction in the maximum achievable concentration upon combination to ATV, compared to 50% reduction in buffer. This positive deviation by FaSSIF solubilization was not reflected in the mass transport–time profiles. Interestingly, FDN concentrations remain constant until the amount of IPM added was over 1000 μg/mL. No decrease in the membrane transport of FDN was observed for a 1:1 M ratio of FDN-IPM combination. This study demonstrates the importance of studying amorphous multidrug formulations under physiologically relevant conditions to obtain insights into the performance of these formulations after oral administration.

Variable-Temperature NMR Analysis of the Thermodynamics of Polymer Partitioning between Aqueous and Drug-Rich Phases and Its Significance for Amorphous Formulations

We previously reported that the polymers used in amorphous solid dispersion (ASD) formulations, such as polyvinylpyrrolidone (PVP), polyvinylpyrrolidone/vinyl acetate (PVP-VA), and hypromellose (HPMC), distribute into the drug-rich phase of ibuprofen (IBP) formed by liquid-liquid phase separation, resulting in a reduction in the maximum drug supersaturation in the aqueous phase. Herein, the mechanism underlying the partitioning of the polymer into the drug-rich phase was investigated from a thermodynamic perspective. The dissolved IBP concentration in the aqueous phase and the amount of polymer distributed into the IBP-rich phase were quantitatively analyzed in IBP-supersaturated solutions containing different polymers using variable-temperature solution-state nuclear magnetic resonance (NMR) spectroscopy. The polymer weight ratio in the IBP-rich phase increased at higher temperatures, leading to a more notable reduction of IBP amorphous solubility. Among the polymers, the amorphous solubility reduction was the greatest for the PVP-VA solution at lower temperatures, while HPMC reduced the amorphous solubility to the greatest extent at higher temperatures. The change in the order of polymer impact on the amorphous solubility resulted from the differences in the temperature dependency of polymer partitioning. The van't Hoff plot of the polymer partition coefficient revealed that both enthalpy and entropy changes for polymer transfer into the IBP-rich phase from the aqueous phase (ΔH_{aqueous→IBP-rich} and ΔS_{aqueous→IBP-rich}) gave positive values for most of the measured temperature range, indicating that polymer partitioning into the IBP-rich phase was an endothermic but entropically favorable process. The polymer transfer into the IBP-rich phase was more endothermic for HPMC than for PVP and PVP-VA. The solid-state NMR analysis of the IBP/polymer ASD implied that the newly formed IBP/polymer interactions in the IBP-rich phase upon polymer incorporation were weaker for HPMC, providing a rationale for the larger positive transfer enthalpy for HPMC. The change in Gibbs free energy for polymer transfer (ΔG_{aqueous→IBP-rich}) showed negative values across the experimental temperature range, decreasing with an increase in temperature, indicating that the distribution of the polymer into the IBP-rich phase is favored at higher temperatures. Moreover, ΔG_{aqueous→IBP-rich} for HPMC showed the greatest decrease with the temperature, likely reflecting the temperature-induced dehydration of HPMC in the aqueous phase. This study contributes fundamental insights into the phenomenon of polymer partitioning into drug-rich phases, furthering the understanding of achievable supersaturation levels and ultimately providing information on polymer selection for ASD formulations.

Role of Permeability on the Biopredictive Dissolution of Amorphous Solid Dispersions

An ideal dissolution test for amorphous solid dispersions (ASDs) should reflect physicochemical, physiological, and hydrodynamic conditions which accurately represent in vivo dissolution. However, this is confounded by the evolution of different molecular and colloidal species during dissolution, generating a supersaturated state of the drug. The supersaturated state of a drug is thermodynamically unstable which drives the process of precipitation resulting in a loss of solubility advantage. Maintaining a supersaturated state of the drug with the help of precipitation inhibiting excipients is a key component in the design of ASDs. Therefore, a biopredictive dissolution test is critical for proper risk assessment during the development of an optimal ASD formulation. One of the overlooked components of biopredictive dissolution is the role of drug permeability. The kinetic changes in the phase behavior of a drug during dissolution of ASDs are influenced by drug permeability across a membrane. Conventionally, drug dissolution and permeation are analyzed separately although they occur simultaneously in vivo. The kinetic phase changes occurring during dissolution of ASDs can influence the thermodynamic activity and membrane flux of a drug. The present review evaluates the feasibility, predictability, and practicability of permeability/dissolution for the optimal development and risk assessment of ASD formulations.

Impact of surfactant selection and incorporation on in situ nanoparticle formation from amorphous solid dispersions

Spray dried amorphous solid dispersions (ASDs) stand as one of the most effective formulation strategies to address issues of low aqueous solubility when developing new chemical entities.An emerging research topic focusing on the formation of amorphous nanoparticles or nanodroplets from ASD formulations has attracted attention recently. These ASD nanoparticlescan be highly beneficial and able to further increase oral bioavailability. The incorporation of surfactants in ASD formulations has been shown to facilitate the formation of these nanoparticles. Therefore, understanding the mechanism of surfactant-promoted nanoparticle formation becomes critical for the rational design of ASD formulations. This work demonstrated the importance of inclusion of the surfactant within the ASD composition for nanoparticle formation. In contrast, when a surfactant is added externally (e.g., by inclusion in the dosing vehicle), only a limited degree of nanoparticle formation was observed even at the optimized surfactant-to-drug ratios. A variety of different surfactants were also assessed for understanding their impact on ASD nanoparticle formation. The spray drying systems containing nonionic surfactants, Tween 80 and Vitamin E TPGS, produced higher amounts of in situ ASD nanoparticles when compared to an anionic surfactant, sodium lauryl sulfate (SLS). The ASD nanoparticles produced by the Genentech developmental compound, GDC-0334, were highly stable and retained their original particle size and amorphous feature for at least 18 h under biorelevant conditions. The high degree of nanoparticle formation from spray dried GDC-0334 containing Tween 80 combined with the superior physical stability of the nanoparticles also translated to enhanced in vivo performance in a rat pharmacokinetics study.

A Mechanistic Study of Drug Mass Transport from Supersaturated Solutions Across PAMPA Membranes

There is an increasing shift from dissolution testing to dissolution-permeation testing of formulations during formulation development and this has led increasing application of permeability measurements using parallel artificial membrane permeability assay (PAMPA) membranes. However, there is a lack of thorough analysis of the impact of variabilities in the PAMPA setup on the mass flow rate outcomes, particularly for complex solubility-enabling formulations. In this study, we investigated the impact of amorphous drug-rich nanodroplets, formed in supersaturated solutions by liquid-liquid phase separation, on membrane transport by measuring mass flow rate across PAMPA membranes. In addition, we explored the impact of PAMPA variants such as lipid composition, hydrophobicity and pore size of the filter support, as well as receiver sink properties on membrane mass flow rates of solutions containing amorphous nanodroplets. Filter properties and lipid composition did not show a notable influence on the mass flow rates for lipophilic molecules, while a marked impact was observed for hydrophilic molecules. High sink conditions in the receiver compartment, arising from addition of micellar surfactant, altered the membrane integrity for lipid-impregnated hydrophilic membranes. In contrast, no such effect was observed for a hydrophobic filter support. Membrane integrity tests also suggested that monitoring water transport may be an improved approach over using Lucifer yellow. Furthermore, high sink conditions in the receiver compartment resulted in an increase in the overall mass flow rate. This was due to the effect of asymmetric conditions, generated across the membrane, on mass transport kinetics. Linearity between mass flow rate and donor concentration was observed until the donor concentration reached the amorphous solubility. Above the amorphous solubility, a gradual increase in mass flow rate was observed i.e., with an increasing number of nanodroplets in the solution. This was attributed to decrease in the permeability barrier across unstirred water layer due to reduction of the concentration gradient as nanodroplets dissolved to replenish absorbed drug. Observations made in this study provide insights into the mechanisms associated with mass transport of supersaturated solutions across PAMPA membranes, which are critical for improved evaluation of enabling formulations.

Amorphous Drug Solubility and Maximum Free Drug Concentrations in Cyclodextrin Solutions: A Quantitative Study Using NMR Diffusometry

Cyclodextrin (CD) has been widely used as a solubilizing agent for poorly water-soluble drugs. In the present study, the effect of CD on the amorphous drug solubility and the maximum thermodynamic activity of the drug in the aqueous phase when the drug concentration exceeded the liquid-liquid phase separation (LLPS) concentration was investigated using three chemically diverse CDs, β-cyclodextrin (β-CD), dimethyl-β-CD (DM-β-CD), and hydroxypropyl-β-CD (HP-β-CD). The amorphous solubility of ibuprofen (IBP) increased substantially linearly with the increase in the CD concentration due to IBP/CD complex formation. Surprisingly, although the crystalline solubility of IBP in the β-CD solution reached a plateau at β-CD concentrations above 3 mM (B_S-type solubility diagram) because of the limited crystalline solubility of the IBP/β-CD complex, the amorphous solubility of IBP increased linearly even when the β-CD concentration was higher than 3 mM. The amorphous solubility of IBP in CD solutions was influenced primarily by the phase separation of the IBP-supersaturated solution to the aqueous phase and the other phase mainly composed of IBP, namely, the IBP-rich phase, via LLPS. NMR spectroscopy revealed that DM-β-CD was distributed into the IBP-rich phase when the IBP concentration exceeded its amorphous solubility, while β-CD and HP-β-CD showed minimal mixing with the IBP-rich phase. NMR diffusometry showed that the maximum free IBP concentration was reduced in the DM-β-CD solution compared to that in the buffer. The mixing of DM-β-CD with the IBP-rich phase reduced the chemical potential of IBP in the IBP-rich phase, which in turn reduced the maximum thermodynamic activity of IBP in the aqueous phase. In contrast, the maximum free IBP concentration was unchanged when β-CD or HP-β-CD was present. The hydrophobic nature of the DM-β-CD substituent may contribute to its partitioning into the IBP-rich phase. The present study highlights the impact of CD on the maximum thermodynamic activity of drugs as well as the apparent amorphous solubility of the drug. This aspect should be considered for improving the effective absorption of poorly water-soluble drugs.

Drug-Rich Phases Induced by Amorphous Solid Dispersion: Arbitrary or Intentional Goal in Oral Drug Delivery?

Among many methods to mitigate the solubility limitations of drug compounds, amorphous solid dispersion (ASD) is considered to be one of the most promising strategies to enhance the dissolution and bioavailability of poorly water-soluble drugs. The enhancement of ASD in the oral absorption of drugs has been mainly attributed to the high apparent drug solubility during the dissolution. In the last decade, with the implementations of new knowledge and advanced analytical techniques, a drug-rich transient metastable phase was frequently highlighted within the supersaturation stage of the ASD dissolution. The extended drug absorption and bioavailability enhancement may be attributed to the metastability of such drug-rich phases. In this paper, we have reviewed (i) the possible theory behind the formation and stabilization of such metastable drug-rich phases, with a focus on non-classical nucleation; (ii) the additional benefits of the ASD-induced drug-rich phases for bioavailability enhancements. It is envisaged that a greater understanding of the non-classical nucleation theory and its application on the ASD design might accelerate the drug product development process in the future.