Molecular Mobility Suppression of Ibuprofen-Rich Amorphous Nanodroplets by HPMC Revealed by NMR Relaxometry and Its Significance with Respect to Crystallization Inhibition

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.

An Artificial Gut/Absorption Simulator: Understanding the Impact of Absorption on In Vitro Dissolution, Speciation, and Precipitation of Amorphous Solid Dispersions

Upon dissolution, amorphous solid dispersions (ASDs) of poorly water-soluble compounds can generate supersaturated solutions consisting of bound and free drug species that are in dynamic equilibrium with each other. Only free drug is available for absorption. Drug species bound to bile micelles, polymer excipients, and amorphous and crystalline precipitate can reduce the drug solute's activity to permeate, but they can also serve as reservoirs to replenish free drug in solution lost to absorption. However, with multiple processes of dissolution, absorption, and speciation occurring simultaneously, it may become challenging to understand which processes lead to an increase or decrease in drug solution concentration. Closed, nonsink dissolution testing methods used routinely, in the absence of drug removal, allow only for static equilibrium to exist and obscure the impact of each drug species on absorption. An artificial gut simulator (AGS) introduced recently consists of a hollow fiber-based absorption module and allows mass transfer of the drug from the dissolution media at a physiological rate after tuning the operating parameters. In the present work, ASDs of varying drug loadings were prepared with a BCS-II model compound, ketoconazole (KTZ), and hypromellose acetate succinate (HPMCAS) polymer. Simultaneous dissolution and absorption testing of the ASDs was conducted with the AGS, and simple analytical techniques were utilized to elucidate the impact of bound drug species on absorption. In all cases, a lower amount of crystalline precipitate was formed in the presence of absorption relative to the nonsink dissolution "control". However, formation of HPMCAS-bound drug species and crystalline precipitate significantly reduced KTZ absorption. Moreover, at high drug loading, inclusion of an absorption module was shown to enhance ASD dissolution. The rank ordering of the ASDs with respect to dissolution was significantly different when nonsink dissolution versus AGS was used, and this discrepancy could be mechanistically elucidated by understanding drug dissolution and speciation in the presence of absorption.

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.

Use of Time-Domain NMR for 1H T1 Relaxation Measurement and Fitting Analysis in Homogeneity Evaluation of Amorphous Solid Dispersion

This study examined the usefulness of 1H T1 relaxation measurements for evaluating the homogeneity of amorphous solid dispersion (ASD). Indomethacin and polyvinylpyrrolidone were used to prepare two kinds of ASDs. One was inhomogeneous ASD (ASDmelt) prepared by a melt-quenching method, and the other was homogeneous ASD (ASDsolvent) prepared by a solvent evaporation method. The T1 relaxation was measured by the time-domain NMR (TD-NMR) technique using a low-field NMR system. Curve-fitting analysis of T1 relaxation plots was conducted using the Akaike information criterion. This fitting analysis revealed that the T1 relaxation of ASDmelt and ASDsolvent was biphasic and monophasic, respectively. ASDmelt and ASDsolvent were inhomogeneous and homogeneous on a nanometer scale, respectively, considering the spin diffusion of 1H nuclei. These T1 results were consistent with the Raman mapping of ASDs. From the fitting analysis of 1H T1 relaxation, we conclude that TD-NMR is a promising technique for evaluating ASD homogeneity.

Molecular-Level Structural Analysis of siRNA-Loaded Lipid Nanoparticles by 1H NMR Relaxometry: Impact of Lipid Composition on Their Structural Properties

1H NMR relaxometry was applied for molecular-level structural analysis of siRNA-loaded lipid nanoparticles (LNPs) to clarify the impact of the neutral lipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol, on the physicochemical properties of LNP. Incorporating DSPC and cholesterol in ionizable lipid-based LNP decreased the molecular mobility of ionizable lipids. DSPC reduced the overall molecular mobility of ionizable lipids, while cholesterol specifically decreased the mobility of the hydrophobic tails of ionizable lipids, suggesting that cholesterol filled the gap between the hydrophobic tails of ionizable lipids. The decrease in molecular mobility and change in orientation of lipid mixtures contributed to the maintenance of the stacked bilayer structure of siRNA and ionizable lipids, thereby increasing the siRNA encapsulation efficiency. Furthermore, NMR relaxometry revealed that incorporating those neutral lipids enhanced PEG chain flexibility at the LNP interface. Notably, a small amount of DSPC effectively increased PEG chain flexibility, possibly contributing to the improved dispersion stability and narrower size distribution of LNPs. However, cryogenic transmission electron microscopy represented that adding excess amounts of DSPC and cholesterol into LNP resulted in the formation of deformed particles and demixing cholesterol within the LNP, respectively. The optimal lipid composition of ionizable lipid-based LNPs in terms of siRNA encapsulation efficiency and PEG chain flexibility was rationalized based on the molecular-level characterization of LNPs. Moreover, the NMR relaxation rate of tertiary amine protons of ionizable lipids, which are the interaction site with siRNA, can be a valuable indicator of the encapsulated amount of siRNA within LNPs. Thus, NMR-based analysis can be a powerful tool for efficiently designing LNP formulations and their quality control based on the molecular-level elucidation of the physicochemical properties of LNPs.

Designing a Novel Coamorphous Salt Formulation of Telmisartan with Amlodipine to Enhance Permeability and Oral Absorption

Coamorphous formulation is a useful approach for enhancing the solubility of poorly water-soluble drugs via intermolecular interactions. In this study, a hydrogen-bonding-based coamorphous system was developed to improve drug solubility, but it barely changed the apparent permeability (Papp) of the drug. This study aimed to design a novel coamorphous salt using ionic interactions to improve drug permeability and absorption. Telmisartan (TMS), with an acidic group, was used to form a coamorphous salt with basic amlodipine (AML). Evaluation of the physicochemical properties confirmed the formation of a coamorphous salt via ionic interactions between the amine group of AML and the carboxyl group of TMS at a molar ratio of 1:1. The coamorphous salt of TMS/AML enhanced the partitioning of both drugs into octanol, indicating increased lipophilicity owing to the interaction between TMS and AML. The coamorphous salt dramatically enhanced TMS solubility (99.8 times that of untreated TMS) and decreased AML solubility owing to the interaction between TMS and AML. Although the coamorphous salt showed a decreased Papp in the permeation study in the presence of a thicker unstirred water layer (UWL) without stirring, Papp increased in the presence of a thinner UWL with stirring. The oral absorption of TMS from the coamorphous salt increased by up to 4.1 times compared to that of untreated TMS, whereas that of AML remained unchanged. Although the coamorphous salt with increased lipophilicity has a disadvantage in terms of diffusion through the UWL, the UWL is thin in human/animal bodies owing to the peristaltic action of the digestive tract. Dissociation of the coamorphous salt on the membrane surface could contribute to the partitioning of the neutral form of drugs to the membrane cells compared with untreated drugs. As a result, coamorphous salt formation has the advantage of improving the membrane permeation and oral absorption of TMS, owing to the enhanced solubility and supply of membrane-permeable free TMS on the surface of the membrane.

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

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

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.

Co-amorphous formulation of dipyridamole with p-hydroxybenzoic acid: Underlying molecular mechanisms, physical stability, dissolution behavior and pharmacokinetic study

Coamorphization has been proven to be an effective approach to improve bioavailability of poorly soluble active pharmaceutical ingredients (APIs) by virtue of solubilization, and also contributes to overcome limitation of physical stability associated with amorphous drug alone. In current work, a co-amorphous formulation of dipyridamole (DPM), a poor solubility drug, with p-hydroxybenzoic acid (HBA) was prepared and investigated. At a molar ratio of 1:2, DPM and HBA were melted result in the formation of a binary co-amorphous system. The DPM-HBA co-amorphous was structurally characterized by powder X-ray diffraction (PXRD), temperature modulated differential scanning calorimetry (mDSC), high performance liquid chromatography (HPLC) and solution state ¹H nuclear magnetic resonance (¹H NMR). The molecular mechanisms in the co-amorphous were further analysed via Fourier-transform infrared (FTIR) and Raman spectroscopies, as well as density functional theory (DFT) calculation. All the results consistently revealed the presence of hydrogen bonding interactions between -OH of DPM and -COOH on HBA. Accelerated test and glass transition kinetics showed excellent physical stability of DPM-HBA co-amorphous compared with amorphous DPM along with glass transition temperatures (T_g). The phase-solubility study indicated that complexation occurred between DPM and HBA in solution, which contributed to the solubility and dissolution enhancement of DPM in co-amorphous system. Pharmacokinetic study of co-amorphous DPM-HBA in mouse plasma revealed that the DPM exhibited 1.78-fold and 2.64-fold improvement in AUC_0‑∞ value compared with crystalline and amorphous DPM, respectively. This current study revealed coamorphization is an effective approach for DPM to improve the solubility and biopharmaceutical performance.

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.

The Impact of Water-Soluble Chitosan on the Inhibition of Crystal Nucleation of Alpha-Mangostin from Supersaturated Solutions

The use of an amorphous drugs system to generate supersaturated solutions is generally developed to improve the solubility and dissolution of poorly soluble drugs. This is because the drug in the supersaturation system has a high energy state with a tendency to precipitate. In the amorphous solid dispersion (ASD) formulation, it was discovered that polymer plays a critical role in inhibiting nucleation or crystal growth of the drugs. Therefore, this study aimed to evaluate the crystallization inhibition of water-soluble chitosan (WSC) on nucleation as well as crystal growth from alpha-mangostin (AM) and elucidate its inhibition mechanism in the supersaturated solutions. During the experiment, WSC was used as a polymer to evaluate its ability to inhibit AM nucleation. The interaction between WSC and AM was also estimated using FT-IR, NMR, and in silico study. The result showed that in the absence of polymer, the concentration of AM rapidly decreased due to the precipitation in one minute. Meanwhile, the addition of WSC effectively inhibited AM crystallization and maintained a supersaturated state for the long term. FT-IR measurement also revealed that the shift in the amine primer of WSC occurred because of the interaction between WSC and AM. In the 1H NMR spectra, the proton peaks of WSC showed an upfield shift with the presence of AM, indicating the intermolecular interactions between AM and WSC. Moreover, in silico study revealed the hydrogen bond interaction between the carbonyl group of AM with hydrocarbon groups of WSC. This indicated that WSC interacted with AM in the supersaturated solution and suppressed their molecular mobility, thereby inhibiting the formation of the crystal nucleus. Based on these results, it can be concluded that the interaction between drug polymers contributed to the maintenance of the drug supersaturation by inhibiting both nucleation and growth.

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.

Effect of Vehicle Composition on the Preparation of Different Types of Dapsone Crystals for Topical Drug Delivery

Topical formulations composed of API-pure crystals have been increasingly studied, especially in regards to the impact of particle size in penetration efficiency. Less attention, however, has been devoted to the solid-state properties of drugs delivered to the skin. In this study, we address the effect of formulation composition on the crystal form existing in topical products. Dapsone (DAP) gel formulations were prepared by mixing an organic solution containing DAP with an aqueous solution containing polymers and preservatives. The organic solvent was chosen as ethoxydiglycol (DEGEE), polyethylene glycol (PEG), or 1-methyl-2-pirrolidone (MPR) to assess the impact of composition on DAP crystal form. Such solvent variations resulted in different particulate matter. In terms of crystalline nature, the presence of DEGEE in formulations induced the crystallization of DAP hydrate, while PEG cocrystal and a mixture of hydrate and MPR solvate crystallized from the same amounts of PEG and MPR, respectively. Microscopic analysis of the gels showed heterogeneous particles with different characteristics. The behavior of gels after application to the skin was also tested. Interestingly, the different formulations seemed to accumulate in different regions of the skin. This could be the result of the effect of vehicle composition/excipients on the characteristics of the skin, such as hydration. The site-specific accumulation, however, was more pronounced in crystal-loaded gels as opposed to blank formulations. These results indicate that future studies should consider the effect of formulation composition on the API crystal form landscape as part of the strategies used to successfully target drug delivery to the skin.

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.

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.

New potential application of hydroxypropyl-β-cyclodextrin in solid self-nanoemulsifying drug delivery system and solid dispersion

The purpose of this study was to use hydroxypropyl-β-cyclodextrin (HP-β-CD) as a novel carrier in solid SNEDDS and solid dispersions to enhance the solubility and oral bioavailability of poorly water-soluble dexibuprofen. The novel dexibuprofen-loaded solid SNEDDS was composed of dexibuprofen, corn oil, polysorbate 80, Cremophor® EL, and HP-β-CD at a weight ratio of 45/35/50/15/100. This solid SNEDDS spontaneously formed a nano-emulsion with a size of approximately 120 nm. Unlike the conventional solid SNEDDS prepared with colloidal silica as a carrier, this dexibuprofen-loaded solid SNEDDS exhibited a spherical structure. Similar to the dexibuprofen-loaded solid dispersion prepared with HP-β-CD, the transformation of the crystalline drug to an amorphous state with no molecular interactions were observed in the solid SNEDDS. Compared to the solid dispersion and dexibuprofen powder, solid SNEDDS significantly enhanced drug solubility and AUC. Therefore, HP-β-CD is a novel potential carrier in SNEDDS for improving the oral bioavailability of dexibuprofen.

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.

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.

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

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

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

The polymer used in an amorphous solid dispersion (ASD) formulation impacts the maximum achievable drug supersaturation. Herein, the effect of dissolved polymer on drug concentration in the aqueous phase when a drug-rich phase was generated by liquid-liquid phase separation (LLPS) was investigated for different polymers at various concentrations of drug and polymer. Solution nuclear magnetic resonance (NMR) spectroscopy revealed that polyvinylpyrrolidone (PVP), polyvinylpyrrolidone/vinyl acetate (PVP-VA), and hypromellose (HPMC) distributed into the ibuprofen (IBP)-rich phase formed by LLPS when the amorphous solubility of IBP was exceeded. The amount of polymer in the drug-rich phase increased for higher-molecular-weight grades of PVP and HPMC. Moreover, PVP-VA showed a greater extent of distribution into the IBP-rich phase compared to PVP, and this is attributed to its reduced hydrophilicity resulting from the incorporation of vinyl acetate monomers. Direct quantification by NMR measurements indicated that the IBP concentration in the aqueous phase decreased as the amount of polymer in the IBP-rich phase increased. This can be attributed to a reduction of the chemical potential of IBP in the IBP-rich phase. The reduction in dissolved IBP concentration was greater for the IBP/PVP-VA system compared to the IBP/HPMC system, as a result of more extensive drug-polymer interactions in the former system. The present study highlights the impact of polymer selection on the attainable supersaturation of the drug and the factors that need to be considered in the formulation of ASDs to obtain optimized in vivo performance.

Interaction of Polymers with Enzalutamide Nanodroplets-Impact on Droplet Properties and Induction Times

Amorphous solid dispersions (ASDs), which consist of a drug dispersed in a polymeric matrix, are increasingly being applied to improve the in vivo performance of poorly water-soluble drugs delivered orally. The polymer is a critical component, playing several roles including facilitating drug release from the ASD, as well as delaying crystallization from the supersaturated solution generated upon dissolution. Certain ASD formulations dissolve to produce amorphous drug-rich nanodroplets. The interaction of the polymer with these nanodroplets is poorly understood but is thought to be important for inhibiting crystallization in these systems. In this study, the impact of ionic polymers on the crystallization kinetics of enzalutamide from supersaturated solutions containing different amounts of amorphous nanodroplets was evaluated by determination of nucleation induction times. The amount of the polymer associated with the drug nanodroplets was also determined. When comparing two polymers, hydroxypropylmethyl cellulose acetate succinate (HPMCAS) and Eudragit E PO, it was found that the crystallization tendency and physical properties of the drug nanodroplets varied in the presence of these two polymers. Both polymers distributed between the aqueous phase and the drug-rich nanodroplets. A greater amount of Eudragit E PO was associated with the drug-rich nanodroplets. Despite this, Eudragit E PO was a less-effective crystallization inhibitor than HPMCAS in systems containing nanodroplets. In conclusion, in supersaturated solutions containing amorphous nanodroplets, the extent of association of a polymer with the drug nanodroplet does not solely predict crystallization inhibition.

Exposing Hidden Conformations of Carbamazepine Appearing Due to Interaction With the Solid Phase by 2D 1H-15N HMBC NMR Spectroscopy

Two-dimensional ¹H-¹⁵N HMBC NMR spectra of the well-known anticonvulsant carbamazepine dissolved in different organic solvents, recorded on an NMR spectrometer prove the existence of hidden conformers in saturated solutions. Obtained conformer distribution arises due to the presence of the solid phase in saturated solution. A weak influence of ring currents was revealed for different molecular conformations of carbamazepine dissolved in a saturated solution, which provides a simple approach to discovering hidden conformations. Hidden conformers were found in three different solvents: dimethyl sulfoxide, chloroform, and dichloromethane.

Toward Developing Discriminating Dissolution Methods for Formulations Containing Nanoparticulates in Solution: The Impact of Particle Drift and Drug Activity in Solution

Enabling formulations are an attractive approach to increase the dissolution rate, solubility, and oral bioavailability of poorly soluble compounds. With the growing prevalence of poorly soluble drug compounds in the pharmaceutical pipeline, supersaturating drug delivery systems (SDDS), a subset of enabling formulations, have grown in popularity due to their properties allowing for drug concentrations greater than the corresponding crystalline solubility. However, the extent of supersaturation generated as the enabling formulation traverses the gastrointestinal (GI) tract is dynamic and poorly understood. The dynamic nature of supersaturation is a result of several competing kinetic processes such as dissolution, solubilization by formulation and endogenous surfactants, crystallization, and absorption. Ultimately, the free drug concentration, which is equivalent to the drug's inherent thermodynamic activity amid these kinetic processes, defines the true driving force for drug absorption. However, in cases where solubilizing agents are present (i.e., surfactants and bile salts), drug molecules may associate with colloidal nanoscale species, complicating drug activity determination. These nanoscale species can drift into the aqueous boundary layer (ABL), increasing the local API activity at the membrane surface, resulting in increased bioavailability. Herein, a novel approach was developed to accurately measure thermodynamic drug activity in complex media containing drug distributed in nanoparticulate species. This approach captures the influence of the ABL on the observed flux and, ultimately, the predicted unbound drug concentration. The results demonstrate that this approach can help to (1) measure the true extent of local supersaturation in complex systems containing solubilizing excipients and (2) elucidate the mechanisms by which colloidal aggregates can modulate the drug activity in solution and potentially enhance the flux observed across a membrane. The utilization of these techniques may provide development scientists with a strategy to evaluate formulation sensitivity to nanospeciation and allow formulators to maximize the driving force for absorption in a complex environment, perhaps enabling the development of dissolution methods with greater discrimination and correlation to pre-clinical and clinical data sets.

Polymer Type Impacts Amorphous Solubility and Drug-Rich Phase Colloidal Stability: A Mechanistic Study Using Nuclear Magnetic Resonance Spectroscopy

The polymer used in an amorphous solid dispersion (ASD) formulation plays a critical role in dosage form performance. Herein, drug-polymer interactions in aqueous solution were evaluated in order to better understand the dispersion stability of the colloidal drug-rich phase generated when the amorphous solubility is exceeded. The amorphous solubility (S_a,IBP) of ibuprofen (IBP) decreased when hypromellose (HPMC) or polyvinylpyrrolidone/vinyl acetate (PVP-VA) were present in solution. Solution nuclear magnetic resonance (NMR) spectroscopy revealed that a large amount of HPMC and PVP-VA distributed into the IBP-rich phase. The mixing of HPMC and PVP-VA with the IBP-rich phase led to the decreased S_a,IBP. In contrast, the charged amino methacrylate copolymer (Eudragit E PO, EUD-E) showed minimal mixing with the IBP-rich phase; however, this polymer did lead to solubilization of IBP in the bulk aqueous phase. Although the IBP-rich phase generated by dissolving IBP at concentrations above S_a,IBP rapidly coarsened followed by creaming in the absence of polymer, all of the polymers stabilized the IBP dispersion to some extent. The droplet size of the IBP-rich phase immediately after formation was around 300 nm in HPMC and PVP-VA solutions, and around 800 nm in the EUD-E solution. The mixing of the former two polymers with the drug-rich phase is thought to account for the smaller droplet size. Despite a smaller initial size, the dispersion stability of the IBP-rich droplets was relatively poor in the presence of PVP-VA. In contrast, the coalescence of the IBP-rich droplets was effectively suppressed by the steric repulsion and electrostatic repulsion derived from adsorbed HPMC and EUD-E, respectively. The present study highlights the complex effects of a polymer on the drug amorphous solubility and colloidal stability, which should be considered when optimizing ASD formulations with the goal of maximizing drug absorption.