Effect of Polymer Species on Maximum Aqueous Phase Supersaturation Revealed by Quantitative Nuclear Magnetic Resonance Spectroscopy

A review on design rules for formulating amorphous solid dispersions based on drug-polymer interactions in aqueous environment

Amorphous solid dispersions (ASDs) are multi-component formulations in which a drug is molecularly dispersed in a carrier. ASDs undergo complex dissolution mechanisms to generate and sustain a supersaturated state of poorly soluble drugs. The link between enhanced solubility, supersaturation stability and drug-polymer interaction (DPI) is critical for the rational design of ASDs. The key mechanism responsible for a high bioavailability is the evolution of supersaturation during the dissolution of ASDs which is also the driving force for drug precipitation. A critical determinant of robust supersaturation generation and stability during dissolution is the molecular interaction between the drug and polymer. Characterization of DPI in a solution state is, however, challenging because of the poor hydrodynamic resolution of the techniques, traditionally used in solid-state analysis. Further, the dissolution conditions, such as the choice of buffer, pH and ionic strength may complicate the analyses and predictions. The role of DPI is a poorly understood aspect of ASD dissolution and therefore is an active area of research. DPI is critical for understanding the design rules for formulating an optimal ASD formulation. The review focuses on different aspects of DPI to stabilize the supersaturated state of a drug during the dissolution of ASDs.

A comprehensive overview of the role of intermolecular interactions in amorphous solid dispersions

Many recent studies have indicated that drug-polymer intermolecular interactions are an important aspect of amorphous solid dispersions (ASDs) and determine many of the properties of this type of formulations. In this review, a comprehensive overview is given of the latest insights with respect to intermolecular interactions in ASDs. The thermodynamic properties and theoretical considerations of the interactions are discussed, followed by a detailed and critical overview of the various solid-state analysis techniques used to probe interactions at the disposal of the formulation scientist. As the physical stability and the pharmaceutical performance of the ASD are its most crucial properties, the most recent understanding of the influence of drug-polymer interactions on these aspects is addressed as well. It is clear that intermolecular interactions may provide many advantages for ASDs but need to be weighed against the possible disadvantages. Further investigation into the interplay and trade-off between physical stability and dissolution properties is necessary in order to be able to take full advantage of the possible benefits of the interactions.

Impact of Microemulsion Oil Components on Liquid-Liquid Phase Separation of a Supersaturated Drug Revealed by Cryo-TEM and 1H NMR Analysis

Supersaturatable self-microemulsifying drug delivery system (S-SMEDDS) has recently been utilized to enhance the oral absorption of poorly water-soluble drugs. S-SMEDDS forms drug-incorporated microemulsions (MEs) during aqueous dispersion with the formation of drug supersaturation in the bulk water phase. However, the liquid-liquid phase separation (LLPS) behavior of the supersaturated drugs within MEs has not been well studied. This study investigated the impact of S-SMEDDS components on the LLPS of the supersaturated drug and the achievable supersaturation level of the drug in MEs. Fenofibrate (FFB)-loaded S-SMEDDS formulations composed of different oils, Labrafil M 1944 CS (M1944) and Labrafac PG (PG), were prepared and dispersed into water to form MEs (M1944 ME and PG ME). Cryo-TEM measurements revealed the coexistence of swelling micelles and nanosized FFB-rich droplets in highly FFB-loaded MEs, indicating that FFB underwent LLPS even in the MEs. The FFB-rich droplet size was significantly reduced in PG ME. NMR-based quantification of the solubilized FFB in swelling micelles and phase-separated FFB revealed that apparent amorphous solubility of FFB increased with increasing M1944 components in MEs, while that was almost constant regardless of PG contents. On the other hand, PG was largely partitioned into the FFB-rich phase, resulting in the reduction of the chemical potential of FFB in the FFB-rich phase and the maximum free FFB concentration in the bulk water phase. The mixing of PG with the FFB-rich phase would work to maintain the FFB-rich droplet as a smaller size. Meanwhile, M1944 was minimally distributed to the FFB-rich phase, keeping the maximum supersaturation level of FFB. This study highlights that the impact of S-SMEDDS oil components on the physicochemical properties of the drug-rich phase formed via LLPS and achievable drug supersaturation should be considered when designing S-SMEDDS formulations to enhance drug absorption.

Dissolution mechanisms of amorphous solid dispersions: Role of polymer molecular weight and identification of a new failure mode

The mechanisms of drug release from amorphous solid dispersions (ASDs) are complex and not fully explored, making it difficult to optimize for in vivo performance. A recurring behavior has been the limit of congruency (LoC), a drug loading above which the ASD surface forms an amorphous drug-rich barrier in the presence of water, which hinders release, especially in non-sink conditions. Drug-polymer interactions and drug glass transition temperature were reported to affect the LoC. However, the effect of polymer molecular weight has not been explored. ASDs of clotrimazole and different molecular weight grades of poly (vinylpyrrolidone) (PVP) were studied for their release to obtain their LoC drug loadings. Failure modes underpinning the LoC were investigated using fluorescence confocal microscopy to analyze the ASD/solution interface and phase behavior of ASD films at high relative humidity. ASDs with good release formed stable drug-rich nanodroplets at the ASD/solution interface, while ASDs with poor release were limited by one of two failure modes, depending on PVP molecular weight. In Failure Mode I the nanodroplets quickly agglomerated, while in Failure Mode II the system underwent phase inversion. This work highlights the importance of identifying the mechanisms underlying the LoC to improve the release of higher drug loading ASDs.

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.

The importance of surface composition and wettability on the dissolution performance of high drug loading amorphous dispersion formulations

One of the limitations with an amorphous solid dispersion (ASD) formulation strategy is low drug loading. Hydrophobic drugs have poor wettability and require a substantial amount of polymer to stabilize the amorphous drug and facilitate release. Using grazoprevir and hypromellose acetate succinate as model drug and polymer, respectively, the interplay between particle surface composition, particle wettability, and release performance was investigated. A hierarchical particle approach was used where the surfaces of high drug loading ASDs generated by either solvent evaporation or co-precipitation were further modified with a secondary excipient (i.e., polymer or wetting agent). The surface-modified particles were characterized for drug release, wettability, morphology, and surface composition using two-stage dissolution studies, contact angle measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. Despite surface modification with hydrophilic polymers, the hierarchical particles did not consistently exhibit good release performance. Contact angle measurements showed that the secondary excipient had a profound impact on particle wettability. Particles with good wettability showed improved drug release relative to particles that did not wet well, even with similar drug loadings. These observations underscore the intricate interplay between particle wettability and performance in amorphous dispersion formulations and illustrate a promising hierarchical particle approach to formulate high drug loading amorphous dispersions with improved dissolution performance.

Prediction of Successful Amorphous Solid Dispersion Pairs through Liquid State Nuclear Magnetic Resonance

Amorphous solid dispersions (ASDs) function in part via a "parachute effect", i.e., polymer-enabled prolonged drug supersaturation, presumably through drug-polymer interactions in the liquid state. We aim to expand the utility of liquid state nuclear magnetic resonance (1HNMR) to streamline polymer selection for ASDs. Our hypothesis is that strong molecular interactions between polymer and drug in 1HNMR anticipate reduced precipitation kinetics in supersaturation studies. For three drug-polymer pairs (i.e., etravirine with each HPMC, HPMCAS-M, and PVP-VA), 1HNMR findings were compared to more common supersaturation studies. Drug-polymer interactions were assessed by saturation transfer difference NMR (STD-NMR) and T1 relaxation time. 2D-1H NOESY experiments were also performed. Supersaturation studies involved precipitation inhibition using the solvent-shift methodology. The results from STD-NMR and T1 relaxation time indicate etravirine bound preferably to HPMCAS-M > HPMC ≫ PVP-VA. STD-NMR and T1 relaxation time yielded insight into which fragments of etravirine structure bind with HPMCAS-M and HPMC. The strong interactions from STD-NMR and T1 relaxation time changes indicated that HPMCAS-M and HPMC, but not PVP-VA, are suitable polymers to maintain etravirine supersaturation and inhibit drug precipitation. 2D-1H NOESY results corroborate the findings of STD-NMR and T1 relaxation time, showing that etravirine interacts preferably to HPMCAS-M than to PVP-VA. Supersaturation studies using solvent-shift technique corroborated our hypothesis as predissolved HPMCAS-M and HPMC, but to a less extent PVP-VA, markedly promoted etravirine supersaturation and inhibited drug precipitation. Supersaturation studies agreed with STD-NMR and T1 relaxation time predictions, as HPMC and HPMCAS-M maintained etravirine in solution for longer time than PVP-VA. The results show promise of 1HNMR to streamline polymer selection in a nondestructive and resource sparing fashion for subsequent ASD development.

Computational Support to Explore Ternary Solid Dispersions of Challenging Drugs Using Coformer and Hydroxypropyl Cellulose

A majority of drugs marketed in amorphous formulations have a good glass-forming ability, while compounds less stable in the amorphous state still pose a formulation challenge. This work explores ternary solid dispersions of two model drugs with a polymer (i.e., hydroxypropyl cellulose) and a coformer as stabilizing excipients. The aim was to introduce a computational approach by preselecting additives using solubility parameter intervals (i.e., overlap range of solubility parameter, ORSP) followed by more advanced COSMO-RS theory modeling. Thus, a mapping of calculated mixing enthalpy and melting points is proposed for in silico evaluation prior to hot melt extrusion. Following experimental testing of process feasibility, the selected formulations were tested for their physical stability using conventional bulk analytics and by confocal laser scanning and atomic force microscopy imaging. In line with the in silico screening, dl-malic and l-tartaric acid (20%, w/w) in HPC formulations showed no signs of early drug crystallization after 3 months. However, l-tartaric acid formulations displayed few crystals on the surface, which was likely a humidity-induced surface phenomenon. Although more research is needed, the conclusion is that the proposed computational small-scale extrusion approach of ternary solid dispersion has great potential in the formulation development of challenging drugs.

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.

Dissolution Mechanisms of Amorphous Solid Dispersions: Application of Ternary Phase Diagrams To Explain Release Behavior

The use of amorphous solid dispersions (ASDs) in commercial drug products has increased in recent years due to the large number of poorly soluble drugs in the pharmaceutical pipeline. However, the release behavior of ASDs is complex and remains not well understood. Often, the drug release from ASDs is rapid and complete at lower drug loadings (DLs) but becomes slow and incomplete at higher DLs. The DL where release becomes hindered is termed the limit of congruency (LoC). Currently, there are no approaches to predict the LoC. However, recent findings show that one potential cause leading to the LoC is a change in phase morphology after water-induced phase separation at the ASD/solution interface. In this study, the phase behavior of ASDs in contact with aqueous solutions was described thermodynamically by constructing experimental and computational ternary phase diagrams, and these were used to predict morphology changes and ultimately the LoC. Experimental ternary phase diagrams were obtained by equilibrating ASD/water mixtures over time. Computational ternary phase diagrams were obtained by Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT). The morphology of the hydrophobic phase was studied with fluorescence confocal microscopy. It was demonstrated that critical point (plait point) composition approximately corresponded to the ASD DL, where the hydrophobic phase, formed during phase separation, became interconnected and hindered ASD release. This work provides mechanistic insights into the ASD release behavior and highlights the potential of in silico ASD design using phase diagrams.

Crystallinity: A Complex Critical Quality Attribute of Amorphous Solid Dispersions

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

Crystallization inhibitory effects of konjac glucomannan, sodium alginate and xanthan gum on curcumin in supersaturated solution

Supersaturating drug delivery system (SDDS) is a promising approach to enhance the solubility of hydrophobic functional components. However, SDDS is thermodynamically unstable and crystallization tends to occur. In this work, curcumin was used as a model compound, and the crystallization inhibitory effect of konjac glucomannan (KGM), sodium alginate (SA) and xanthan gum (XTG) on curcumin in supersaturated solution was investigated. Amorphous solubility of curcumin was determined using ultraviolet extinction, fluorescence spectroscopy and dynamic light scattering methods. Nucleation induction time (NIT) and crystal growth rate of curcumin were evaluated using ultraviolet probe in the absence and presence of various natural polysaccharides (NPs). Results showed that amorphous solubility of curcumin was approximately 30 μg/mL in pH 6.8 phosphate buffer. NPs used in this work restrained nucleation or crystal growth of curcumin effectively. The NITs of curcumin in the absence of NPs and in the presence of XTG, KGM and SA (1 μg/mL) were 3.7, 60.7, 20.0 and 8.0 min, respectively. The crystal growth rate of curcumin in the absence of NPs and in the presence of XTG, SA and KGM (1 μg/mL) were 0.0103, 0.00752, 0.00286 and 0.000306 min^-1, respectively. The nucleation inhibitory effect of NPs on curcumin was ranked as XTG > KGM > SA. The order of crystal growth inhibition capacity of NPs was KGM > SA > XTG. In conclusion, NPs could be incorporated into SDDS to maintain supersaturation of hydrophobic components for enhanced bioavailability.

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.

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.

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.

Thermodynamic Correlation between Liquid-Liquid Phase Separation and Crystalline Solubility of Drug-Like Molecules

The purpose of the present study was to experimentally confirm the thermodynamic correlation between the intrinsic liquid−liquid phase separation (LLPS) concentration (S0LLPS) and crystalline solubility (S0c) of drug-like molecules. Based on the thermodynamic principles, the crystalline solubility LLPS concentration melting point (Tm) equation (CLME) was derived (log10S0C=log10S0LLPS−0.0095Tm−310 for 310 K). The S0LLPS values of 31 drugs were newly measured by simple bulk phase pH-shift or solvent-shift precipitation tests coupled with laser-assisted visual turbidity detection. To ensure the precipitant was not made crystalline at <10 s, the precipitation tests were also performed under the polarized light microscope. The calculated and observed log10S0C values showed a good correlation (root mean squared error: 0.40 log unit, absolute average error: 0.32 log unit).

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.

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.

In Vitro-In Silico Tools for Streamlined Development of Acalabrutinib Amorphous Solid Dispersion Tablets

Amorphous solid dispersion (ASD) dosage forms can improve the oral bioavailability of poorly water-soluble drugs, enabling the commercialization of new chemical entities and improving the efficacy and patient compliance of existing drugs. However, the development of robust, high-performing ASD dosage forms can be challenging, often requiring multiple formulation iterations, long timelines, and high cost. In a previous study, acalabrutinib/hydroxypropyl methylcellulose acetate succinate (HPMCAS)-H grade ASD tablets were shown to overcome the pH effect of commercially marketed Calquence in beagle dogs. This study describes the streamlined in vitro and in silico approach used to develop those ASD tablets. HPMCAS-H and -M grade polymers provided the longest acalabrutinib supersaturation sustainment in an initial screening study, and HPMCAS-H grade ASDs provided the highest in vitro area under the curve (AUC) in gastric to intestinal transfer dissolution tests at elevated gastric pH. In silico simulations of the HPMCAS-H ASD tablet and Calquence capsule provided good in vivo study prediction accuracy using a bottom–up approach (absolute average fold error of AUC_0-inf < 2). This streamlined approach combined an understanding of key drug, polymer, and gastrointestinal properties with in vitro and in silico tools to overcome the acalabrutinib pH effect without the need for reformulation or multiple studies, showing promise for reducing time and costs to develop ASD drug products.

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

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