Understanding the Impact of Water on the Miscibility and

Article pubs.acs.org/molecularpharmaceutics

Understanding the Impact of Water on the Miscibility and Microstructure of Amorphous Solid Dispersions: An AFM−LCR and TEM−EDX Study Na Li,† Christopher J. Gilpin,‡ and Lynne S. Taylor*,† †

Department of Industrial and Physical Pharmacy, Purdue University, 575 Stadium Mall Drive, West Lafayette, Indiana 47907, United States ‡ Life Science Microscopy Facility, Purdue University, 625 Agriculture Mall Drive, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Miscibility is critical for amorphous solid dispersions (ASDs). Phase-separated ASDs are more prone to crystallization, and thus can lose their solubility advantage leading to product failure. Additionally, dissolution performance can be diminished as a result of phase separation in the ASD matrix. Water is known to induce phase separation during storage for some ASDs. However, the impact of water introduced during preparation has not been as thoroughly investigated to date. The purpose of this study was to develop a mechanistic understanding of the effect of water on the phase behavior and microstructure of ASDs. Evacetrapib and two polymers were selected as the model system. Atomic force microscopy coupled with Lorentz contact resonance, and transmission electron microscopy with energy dispersive X-ray spectroscopy were employed to evaluate the microstructure and composition of phase-separated ASDs. It was found that phase separation could be induced via two routes: solution-state phase separation during ASD formation caused by water absorption during film formation by a hydrophilic solvent, or solid-phase separation following exposure to high RH during storage. Water contents of as low as 2% in the organic solvent system used to dissolve the drug and polymer were found to result in phase separation in the resultant ASD film. These findings have profound implications on lab-scale ASD preparation and potentially also for industrial production. Additionally, these highresolution imaging techniques combined with orthogonal analyses are powerful tools to visualize structural changes in ASDs, which in turn will enable better links to be made between ASD structure and performance. KEYWORDS: amorphous solid dispersions, miscibility, water, microstructure, AFM, TEM

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INTRODUCTION

however, can provide a significant solubility advantage without concomitantly decreasing the apparent intestinal membrane permeability14 and therefore has become a popular formulation strategy for suitable poorly water-soluble compounds. Drug−polymer miscibility is considered an essential property for ASDs, both in terms of maintaining the solid-state stability of the drug during storage, as well as for promoting the formation of a supersaturated solution upon dissolution.16,17 A

Advances in chemical synthesis and the use of target-based high-throughput screening assays have led to more and more poorly water-soluble compounds in the drug discovery pipeline.1−3 About 90% of new chemical entities and 33% of drugs listed in the US Pharmacopeia are poorly water-soluble compounds.4−7 As a result, various formulation strategies, including amorphous solid dispersions,8 nano suspensions,9 addition of surfactants,10 cyclodextrins,11 cocrystals,12 and salts13 are commonly used for solubility enhancement. Nevertheless, enhanced apparent solubility does not always correlate with increased passive transport rate across membranes.14,15 The use of amorphous solid dispersions (ASDs), © 2017 American Chemical Society

Received: Revised: Accepted: Published: 1691

December 21, 2016 March 9, 2017 April 10, 2017 April 10, 2017 DOI: 10.1021/acs.molpharmaceut.6b01151 Mol. Pharmaceutics 2017, 14, 1691−1705

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Molecular Pharmaceutics

Figure 1. Molecular structures of evacetrapib and polymers.

phase-separated ASD system is more prone to crystallization18 and would be expected to lead to a lower maximum achievable concentration during dissolution, both leading to compromised bioavailability.19 Therefore, being able to detect phase separation at an early stage in the formulation development process is critical in order to develop robust formulations. High-resolution imaging techniques (sub 100 nm) such as atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are powerful tools that can be used in the investigation of heterogeneous structures,20−25 such as those anticipated to be present in phase separated ASDs. TEM imaging was previously used to demonstrate heterogeneous features within a felodipine-polyvinylpyrrolidone amorphous film.24 A recent study reported the use of electron energy-loss spectroscopy (TEM-EELS) in determining concentration differences between submicron-size regions.25 Compared to conventional imaging, hybrid, orthogonal characterization techniques, which combine imaging with additional information, such as infrared and Raman spectroscopy, mechanical analysis, thermal analysis, and elemental analysis, are highly advantageous for the investigation of these microstructures, providing additional details about both the microstructure and composition of the different phases.22,26−32 Solvent evaporation methods, such as spin coating, have been widely used in miscibility investigations of ASDs. However, day-to-day variations in drug−polymer miscibility have been reported in such systems.26 To date, there is limited knowledge about the origin of these discrepancies and how to eliminate them. Solvent composition may also affect ASD quality following spray drying.33 Variations in miscibility have also been reported depending on ASD preparation technique.34 In addition, although ASDs are known to be prone to moistureinduced phase separation upon storage at high humidity conditions,20,24,35 detailed information on the microstructure and composition of these phase-separated ASDs is still lacking. Hence, the goal of this study was to provide a mechanistic understanding of the effect of water on the phase behavior and microstructure of ASDs prepared using solvent evaporation, using evacetrapib (ECP) as the model compound. The molecular structures of evacetrapib and the two polymers used are shown in Figure 1. Evacetrapib inhibits the cholesterylester transfer protein and has extremely low aqueous solubility.36 Hydroxypropyl methylcellulose (HPMC) and poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA) are widely used in commercial ASD formulations. In this study, the phase behavior of evacetrapib and polymers was investigated as a function of drug loading and environmental conditions. Microstructure was investigated using AFM topgraphical

imaging, AFM−Lorentz contact resonance (LCR) measurements, and TEM coupled with energy dispersive X-ray spectroscopy (EDX). LCR, which enables the measurement of differences in the viscoelastic properties of the sample surface, is a technique with a high spatial resolution (95%) was purchased from ChemShuttle (Wuxi, China). HPMC (E3 grade) was obtained from the Dow Chemical Company (Midland, MI). Kollidon VA 64 (PVPVA) was obtained from the BASF Corporation (Ludwigshafen, Germany). Methanol and dichloromethane were purchased from Mallinckrodt Baker (Phillipsburg, NJ). The water content in methanol was determined to be 0.03 ± 0.00% by Karl Fischer titration. Methods. Solubility Determination. The crystalline solubility of evacetrapib was measured by adding an excess amount of evacetrapib powder to 10 mL of 50 mM pH 6.8 sodium phosphate buffer solution. Samples were prepared in triplicate. The mixtures were then kept at 25 °C in a water bath shaker for 48 h. An Optima L-100XP ultracentrifuge (Beckman Coulter Inc., Brea, CA) was used to separate the undissolved solids and the supernatant. Samples were spun at 35000 rpm for 30 min at 25 °C. Upon ultracentrifugation, 1 mL of the supernatant was immediately taken for high performance liquid chromatography (HPLC) analysis. The onset concentration for liquid−liquid phase separation (LLPS) was determined by adding 10 μL of 10 mg/mL evacetrapib methanolic stock solution into 10 mL of pH 6.8 buffer with or without 20 μg/mL HPMC or PVPVA (polymer added to retard crystallization) at ambient temperature (22 ± 2 °C). The presence of 20 μg/mL HPMC or PVPVA was confirmed to have no impact on the LLPS onset concentration of evacetrapib (see Supporting Information, Table S1). Samples were vortexed and then centrifuged for 10 min at 35000 rpm and 25 °C using the parameters described above to minimize evacetrapib ionization. One milliliter of the supernatant was then immediately sampled for HPLC analysis. An Agilent 1260 Infinity series HPLC (Agilent Technologies, Santa Clara, CA) with a Waters XTerra RP C18 column (150 mm × 4.6 mm, i.d. 3.5 μm) (Waters Corp., Milford, MA) was 1692

DOI: 10.1021/acs.molpharmaceut.6b01151 Mol. Pharmaceutics 2017, 14, 1691−1705

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coated from 50 mg/mL binary solvents was in the range of 50 nm to 1 μm from 10% to 90% drug loading due to the high viscosity of HPMC. For TEM imaging purposes, thinner ECP− HPMC films were made for storage experiments. Atomic Force Microscopy (AFM) and Lorentz Contact Resonance (LCR) Characterization. A nanoIR2 AFM-IR instrument (Anasys Instruments, Inc., Santa Barbara, CA) was used to perform topographical imaging. Contact mode NIR2 probes (Model: PR-EX-NIR2, Anasys Instruments, Santa Barbara, CA) were used. A scan rate of 0.5 kHz was used for contact mode topographical imaging, with an x and y resolution of 256 points. The Analysis Studio software (version 3.10.5539, Anasys Instruments., Santa Barbara, CA) was used to collect and analyze these images. Surface roughness values were calculated using Gwyddion 2.47 (http://gwyddion.net/). To conduct LCR sweeps and imaging, the nanoIR2 AFM-IR instrument was used in LCR mode. Thermalever probes (Model: EXP-AN2-300, Anasys Instruments, Santa Barbara, CA) were used. The contact resonance frequency varies with the cantilever, and therefore, the same cantilever was used for the same set of experiments. Briefly, by driving the probe at a characteristic frequency that depends on the sample being investigated, LCR enables differentiation of the sample surface based on differences of mechanical properties of each domain.26,40 An LCR drive magnet was used to generate the magnetic field. To collect nanomechanical spectra, the probe was driven from 1 to 1000 kHz with a scan rate of 100 kHz/s. At least five mechanical spectra for each phase from multiple domains throughout the sample were collected. Data collection rate was 200 pt/s. To collect an LCR image, LCR drive strength was set at 50%. The probe was driven at a center frequency of interest, with a scan rate of 0.3 kHz. Transmission Electron Microscopy (TEM) and Electron Dispersive X-ray (EDX) Spectroscopy. Samples were spincoated onto 300 mesh carbon coated copper grids (SPI supplies, West Chester, PA). Solid dispersion samples were prepared at 18% RH and 50% RH, respectively, to make initially homogeneous and phase-separated films. Selected initially homogeneous ECP−PVPVA samples were then stored at 97% RH for 7 days, followed by vacuum drying at room temperature for 2 days prior to TEM imaging. At least six images were taken at different locations for each sample, and the most representative images were shown to present the general trend observed. For EDX, 10 spectra were collected for each phase-separated phase of interest in the initially phase separated systems as well as reference ECP and PVPVA films, and 20 spectra were collected for each phase in the sample stored at 97% RH. An FEI Tecnai G2 20 TEM instrument (FEI Company, Hillsboro, OR) operating at 200 kV equipped with a Gatan US100 2k × 2k charge-coupled device camera (Gatan Inc., Warrendale, PA) and an X-max 80 mm2 silicon drift detector (Oxford Instruments, Abingdon, Oxfordshire, UK) was used to conduct TEM imaging and EDX experiments. A copper singletilt holder was used for imaging purposes. A beryllium doubletilt holder was used throughout EDX experiments to eliminate interference of iron L peaks from the pole piece with fluorine K peaks from the sample. The Cu content detected in these samples (from Cu grid bars) was below 0.23% (atomic %). For EDX spectra collection, the instrument specific settings were a spot size of 4 and a process time of 4. These setting were used for all samples analyzed in order to achieve a high spatial resolution with a short deadtime and a relatively high count

used for solubility measurements. An isocratic elution method of 10% of 0.1% (v/v) trifluoroacetic acid aqueous solution and 90% acetonitrile was used. The injection volume was 50 μL, and the flow rate was 1 mL/min, with a run length of 6 min. The eluent was detected at 210 nm at 2.914 min. Two calibration curves from 20 to 100 ng/nL and 200−800 ng/mL were created to cover the concentration range for the crystalline and amorphous solubility of evacetrapib. Preparation of Drug−Polymer Solutions. Stock solutions with an evacetrapib−polymer ratio covering the range 10%− 90% (w/w) were prepared by dissolving evacetrapib and polymers (HPMC or PVPVA) in a 50:50 (v/v) methanol− dichloromethane solution to a final solid content of 50 mg/mL, unless specified elsewhere. To eliminate heterogeneity caused by different evaporation rate of solvents in a binary solvent mixture, ECP and ECP− PVPVA stock solutions were also prepared using methanol as a single solvent. The final concentration of these solutions was 5 mg/mL to fully dissolve evacetrapib. For solutions spiked with water, various amounts of water were added into 1 mL of the stock solution. The solution was mixed and immediately spin-coated onto substrates to minimize droplet coalescence and Oswald ripening. Spin Coating under a Controlled Environment. A KW-4A spin coater (Chemat Technology Inc., Northridge, CA) was placed in a glovebox with humidity control. A nitrogen purge was used prior to spin coating until the environmental humidity was equilibrated at the desired level. An RH indicator (RH range, 1 to 99%; accuracy, ±4% between 20 to 80% RH; resolution, 0.1% RH) was used to monitor the RH inside the glovebox. In the summer months, the ambient humidity in the lab was around 50−60% RH. With a nitrogen purge, the lowest achievable RH was around 18% RH. Therefore, 18% RH was chosen as a representative low RH level for all experiments, whereas 50% RH was selected as a representative high RH condition in this study, and 35% RH was chosen as an intermediate humidity condition. All experiments were carried out at ambient temperature (22 ± 2 °C). Silicon wafers with a mirror finish were used for spin coating. These substrates were cut into small pieces of approximately 1 cm2 in size, and were taped on steel stubs for AFM scans. Fifteen microliters of the stock solution of drug and polymer mixture or stock solution spiked with water was placed on the substrate. It was then spun for 6 s at 50 rpm, followed by 30 s at 2000 rpm. All spin coating experiments were carried out with humidity control at the desired RH condition. Variations exist at different locations for the same spin-coated film;38 therefore, the images shown are representative of the general trends observed throughout the sample. Freshly prepared ASD films were vacuum-dried at room temperature overnight prior to AFM analysis. For storage treated samples, freshly prepared ASD films were stored in a desiccator using a saturated solution of K2SO4 (97.30 ± 0.45% RH at 25 °C39) for 7 days, and then vacuum-dried for at least 2 days at room temperature prior to analysis. The lack of crystallinity of these ASDs were verified by the absence of birefringence using a polarized light microscope. To determine the thickness of spin-coated films used for AFM analysis, a razor blade was used to create a scratch across the surface of the film. An AFM scan was then performed to determine the thickness of the film. The thickness of the ECP− PVPVA films prepared from binary solvents at 50 mg/mL solid content were measured to be between 50 to 200 nm from 10% to 90% drug loading. The thickness of ECP−HPMC films spin 1693


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Figure 2. Surface topography images of evacetrapib−HPMC spin coated films prepared at (A) 18% RH during preparation, (B) 35% RH during preparation, and (C) 50% RH during preparation.

rate. With these instrument settings, the beam size was kept much smaller than the smallest surface features on the sample analyzed. A live time of 30 s was used. Quantitative estimate of the elemental analysis was performed using Oxford Instruments INCA software package (INCA suite version 4.15). The calculation is an enhanced Cliff−Lorimer method that includes an absorption correction term based on the density and thickness estimate of a thin specimen. Nanoparticle Tracking Analysis (NTA). The liquid−liquid phase separation induced by water addition in the organic stock solution was measured using a nanoparticle tracking analysis (NTA) system. Evacetrapib−HPMC at a drug to polymer ratio of 50:50 (w/w) was used as a model system. The stock solution was prepared in a 50:50 methanol−dichloromethane (v/v) solution at a solids content of 5% (w/v). A different amount of water was added to each sample and vigorously mixed with the stock solution for 2−5 s. One milliliter of each sample was injected into the flow-through cell stage immediately after sample preparation to minimize droplet aggregation. NTA measurements were performed using a NanoSight LM10 (Malvern Instruments, Westborough, MA) equipped with nanoparticle tracking analysis 3.2 software. A 75 mW green laser (532 nm) was used together with a flow-through cell

stage. Samples were analyzed at ambient temperature for 30 s. The camera settings were kept at a screen gain of 2.0 and camera level of 9.0. For analysis, the detector threshold was held at 6.9 and camera gain was kept at 9.0. Differential Scanning Calorimetry. The glass transition temperatures (Tgs) of evacetrapib, PVPVA, and ECP−PVPVA solid dispersions were measured using a TA Q2000 DSC equipped with a RCS90 refrigeration unit (TA Instruments, New Castle, DE). For pure evacetrapib and PVPVA, powder samples were weighed in Tzero aluminum pans. These samples were heated in the DSC at 20 °C/min to 170 °C, then cooled down to 40 °C and ramped up to 170 °C at 5 °C/min with a temperature modulation of ±1 °C/60 s. Bulk ASD samples were prepared for Tg measurements. Briefly, a total of 200 mg solids were weighed in a 20 mL scintillation vial. Approximately 2 mL of 50:50 dichloromethane−methanol (v/v) solvent mix was added to completely dissolve the solids. ASDs were prepared by rotary evaporation directly from the scintillation vial. All samples were freshly prepared and stored in a vacuum oven overnight prior to thermal analysis. 1694


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Figure 3. Surface roughness (Ra) and TEM images of ECP−HPMC systems. (A) Surface roughness as a function of drug loading for ECP−HPMC systems. TEM images of (B) homogeneous and a (C) phase-separated 80% DL ECP−HPMC film.

To determine the Tgs of freshly prepared ASDs, powder samples were weighed in Tzero aluminum pans with Tzero lids. The samples were then heated in the DSC at 5 °C/min to 170 °C with a temperature modulation of 1 °C/60 s. Preheating (to eliminate sample history) was not performed to avoid heatinduced miscibility. To determine the effect of moisture during storage, powder ASD samples were weighed in Tzero aluminum pans and placed in desiccators maintained at 97% RH by using a saturated solution of K2SO4 for 24 h. These samples were then dried in a vacuum oven for 2 days prior to DSC analysis. A temperature ramp of 5 °C/min to a final temperature of 20 °C above the second glass transition temperature was used, with a temperature modulation of 1 °C/60 s.

estimate of particle size distribution for a 90% drug loading ECP−HPMC film prepared at 35% RH is shown in Supporting Information (Figure S1). Similar round discrete domains were also seen in ECP−PVPVA systems (see Supporting Information, Figure S2). In general, the surface roughness increased as a function of RH and drug loading (Figure 3A), with a few exceptions at 70% drug loading and above. This may be because these discrete domains grow bigger and more crowded at high drug loadings, resulting in less continuous phase and shallower “valleys”. Similar RH effects on surface roughness were also seen in ECP−PVPVA systems (Supporting Information, Figure S3). The internal structure of 80% ECP−HPMC films prepared at 18% and 50% RH were examined using TEM as shown in Figure 3C,D. The sample prepared at low RH showed no distinct features, consistent with a single phase, miscible film. In contrast, dark spherical domains were seen in the film prepared at 50% RH, similar to the height features seen in AFM surface topographic images, indicating a phase separated system. These discrete domains were located not only on the surface but also in the interior of the film, as shown by the overlapping spherical features in the TEM micrograph. Round discrete domains were formed, following the general trend of increasing size with increasing drug loading (see Supporting Information, Figures S4 and S5). In this specific sample (Figure 3C), some droplet coalescence was observed as multiple droplets were bridged together. Some of the overlapping features remain discrete without being connected to other features (an example stereopair is shown in the Supporting Information to give a 3D display of these structures, Figure S6). The phase-separated domains disappeared when the film was heated at 170 °C for 1 min, and featureless surfaces were observed using the AFM (data not shown), suggesting that heat leads to mixing of ECP with the polymers. These experiments suggested that environmental humidity played a critical role in influencing if phase separation occurred in the spin-coated drug−polymer films. ASDs with higher drug loadings were found to be more prone to phase separation when formed in the presence of higher atmospheric moisture (50% RH). It is likely that the phase separation is caused by absorption of water by methanol, which is a hygroscopic solvent; when pure dichloromethane, which has very limited miscibility with water, was used as the solvent for ECP−HPMC systems, no phase separation was observed for samples spin-coated at 50% RH (data not shown). Water is completely miscible with methanol, whereas its solubility in dichloromethane is 1.76 g/L at 25 °C.41

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RESULTS Physicochemical Properties of Evacetrapib. Evacetrapib is a very hydrophobic molecule. At pH 6.8 and 25 °C, the experimental crystalline solubility of evacetrapib in water was determined to be 33 ± 7 ng/mL. The onset LLPS concentration (“amorphous solubility”) was measured to be 481 ± 98 ng/mL. Therefore, by formulating evacetrapib with a polymer as an amorphous solid dispersion, a substantial solubility advantage is anticipated. The glass transition temperature (Tg) and melting temperature of pure evacetrapib were measured to be 72 ± 1 and 104 ± 3 °C, respectively. It appears to be a very slow crystallizer; over the time frame of all the experiments conducted in this study (7 days at 97% RH), the drug remained amorphous as confirmed by polarized light microscopy. Effect of Environmental Moisture. The topographical images of evacetrapib−HPMC films prepared at various RHs are shown in Figure 2. At 18% RH, no surface features were observed, consistent with a miscible system. Thus, evacetrapib and HPMC dispersions appear to be miscible when prepared at low RH conditions for drug loadings between 10 and 90%, at least at the 40 nm spatial resolution of the AFM with the current experimental settings. When the environmental humidity increased to 35% RH during film formation, small round discrete domains were observed at drug loadings of 30% and above. As the drug loading increased, these domains became larger and taller, with domain height increasing from about 50 to 120 nm for 50% to 90% drug loadings. As the environmental humidity increased to 50%, these discrete round domains were observed at all drug loadings. Likewise, the diameter and height of these domains increased with drug loading over the range 10% to 90% drug (Figure 2C). An 1695


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Figure 4. Surface topography of evacetrapib−HPMC films prepared from a 50:50 v/v methanol−dicloromethane solvent solution spiked with water, prepared at 18% RH (first row, 10% ECP; second row, 50% ECP); the labeled amount of water was added into 1 mL of the stock solution.

additions up to 170 μL, indicating a homogeneous solution where both drug and polymer are molecularly dissolved. For higher amounts of water, scattering centers appeared, indicating that phase separation had occurred, and the particle count increased steeply as a function of added water. To compare the amount of water that induced visible phase separation in the solution phase prior to evaporation, to that which resulted in phase separation during organic solvent evaporation and hence visible phase separation in the resultant film, surface roughness values calculated from AFM images (an example is shown in Figure 4) were also plotted as a function of the amount of added water in Figure 5. The surface roughness values (Ra) of the films remained low at water additions of up to 10 μL, and then increased dramatically with increasing amounts of water added to the stock solution. Thus, the water content that eventually resulted in phase separation in the spin coated films was much lower than the water addition levels found to induce phase separation in the stock solutions. These results confirm the visual observations that the stock solutions used for spin coating were initially a single phase, even after spiking with up to 15% water, with phase separation occurring subsequently during the film formation process. The subsequent phase separation is presumably because water evaporated at a much slower rate as compared to the organic solvents used, methanol and dichloromethane. Thus, fast removal of the more volatile solvents led to concentration of drug, polymer, and water, resulting in phase separation during the film formation process. To confirm that absorption of atmospheric moisture by the solvent induced phase separation, spin-coated films were also made from single solvent methanolic solutions containing just evacetrapib, i.e., omitting the polymer, with results shown in Figure 6A,B. For evacetrapib alone, films prepared at 18% RH showed a flat featureless surface. In contrast, for films prepared at 50% RH, small regular round discrete domains were observed. The formation of these drug droplets most likely occurred in the stock solution during the initial stages of methanol evaporation following water absorption by the solvent. When methanol was completely removed, amorphous drug, in the form of small spherical particles, was deposited on the surface. When a trace amount of water was added to the binary solvent stock solution and films were spin-coated at 18% RH, phase separation was also seen (Figure 6C). Since the solid content in the binary solvent stock solution was higher (25 mg/ mL, Figure 6C) compared to that in the single solvent stock solution (5 mg/mL, Figure 6B), surface features were more

Impact of Water Added to the Stock Solution on Miscibility. To verify the hypothesis that water in the organic solvent solution facilitated phase separation during film formation, different amounts of water were added to the evacetrapib−HPMC stock solutions, which were then used to prepare films at 18% RH, with results shown in Figure 4. All of the stock solutions remained transparent after water addition with no evidence of precipitate formation prior to spin coating. It can be seen that phase separation occurred in the films when only trace amounts of water (20 μL) were added to 1 mL of the stock solution (50% drug loading). The domain size increased with an increase in the amount of water added. For a constant amount of water, the size of discrete domains increased with increasing drug loading. For the 50% ECP−HPMC system, and based on a comparison of the domain sizes observed for the systems following the water spiking experiments (Figure 4) with those obtained using solvent with no added water (Figure 2C) as well as surface roughness values, it is estimated that the amount of water brought in by the binary solvent during spincoating at 50% RH is between 10 and 30 μL per mL solvent. To determine the concentration of water that induces phase separation of the drug from the stock solution prior to any solvent evaporation, NTA measurements were performed as a function of the amount of added water. Stock solutions containing ECP and HPMC at a 50:50 weight ratio were used, and results are shown in Figure 5. Here it can be clearly seen that in the stock solution, the particle count was low at water

Figure 5. Phase separation in the solution and solid state for a 50% ECP−HPMC system. In the solution state, the number of particles tracked per mL of solution as a function of the amount of water added to 1 mL of stock solution was measured using NTA. For the solid state film samples, the surface roughness of films prepared at 18% RH as a function of the amount of water added to 1 mL of stock solution was calculated from AFM images. 1696


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Figure 6. Surface topography of pure evacetrapib films (A) prepared at 18% RH, (B) prepared at 50% RH with methanol as a single solvent at 5 mg/ mL concentration, and (C) spiked with 10 μL of H2O in 1 mL of 25 mg/mL ECP binary solvent solution, prepared at 18% RH.

Figure 7. Reference mechanical spectra and LCR images of (A) a phase-separated ECP film and (B) a phase-separated ECP−HPMC film. The brighter regions in the LCR image are for the component that gives higher response to the resonance frequency.

crowded in the film prepared from the stock solution containing higher solid content. To identify the drug-rich and polymer-rich domains, AFM− LCR scans were performed on a phase-separated film containing just ECP (Figure 7A) and an ECP−HPMC phaseseparated sample (Figure 7B). Two first flexural resonance mode peaks were chosen for ECP and either the silicon substrate or the polymer, and LCR images were obtained at these selected contact resonant frequencies with minor frequency shifts to obtain maximum contrast.26 Good contrast was obtained in all LCR images. The discrete domains yielded

stronger signals at 117 and 123.5 kHz for ECP in the two experiments shown in Figure 7, which were the resonance frequency peaks for ECP, indicating the dominance of the drug in these regions in both samples. It is worth noting that the peak frequencies used for LCR imaging are slightly shifted from the peak frequencies observed in the mechanical spectra. This is because during LCR imaging, a torque was induced by the friction between the sample and the cantilever when the probe is translating across the sample. This torque shifted the resonant frequencies of the cantilever alone as compared to when it is still during mechanical spectra collection.26 Also, as 1697


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Figure 8. TEM images of evacetrapib−PVPVA films (A) prepared at 50% RH using methanol as the solvent and a solids loading of 5 mg/mL and (B) prepared at 18% RH and stored at 97% RH for 7 days.

different cantilevers were used for the two samples and thus yielded different contact resonance peaks for ECP. The continuous phase yielded a stronger signal at 120 kHz for silicon and 121 kHz for HPMC, indicating either the substrate or the polymer is predominant in this phase. The silicon

evacetrapib and HPMC may be partially miscible, the presence of a second component in each phase in a phase-separated system may alter the peak resonant frequencies relative to those obtained from pure drug and polymer samples.26 The variation in the frequencies for the same material occurred because two 1698


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Figure 9. TEM images and average EDX spectra of (A) a homogeneous ECP film, (B) a homogeneous PVPVA film, (C) a 90% ECP−PVPVA film prepared at 50% RH with methanol as a solvent at a solids loading of 5 mg/mL, and (D) a 50% ECP−PVPVA film prepared at 18% RH and stored at 97% RH for 7 days.

substrate peak appeared at a higher frequency compared to that of evacetrapib, suggesting silicon is stiffer than ECP. Likewise, the first flexural resonance mode peak for HPMC had a lower frequency than that of ECP, indicating that ECP is stiffer than HPMC. Effect of Exposure to Moisture during Film Storage. It has been shown previously that environmental moisture can

induce phase separation in drug−polymer systems during storage.35,42−44 To compare the effect of water introduced during preparation versus during storage on ASD microstructure, two sets of samples were prepared: immiscible ECP− PVPVA films prepared at 50% RH using only methanol as the solvent, and initially miscible ECP−PVPVA films (miscibility was confirmed using AFM, data not shown) and then stored at 1699


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Molecular Pharmaceutics 97% RH. TEM images are shown in Figure 8. It is apparent that the surface morphology and internal structures of these two sets of samples are different, suggesting two distinct routes for phase separation. Distinct morphological features were also observed in phased separated ECP−HPMC systems formed during preparation and storage (Supporting Information, Figure S5). As shown in Figure 8A, spherical discrete domains were seen in phase-separated samples induced by water during preparation, with increasing domain size as drug loading increased. Interestingly, for systems containing 80% and 90% ECP, TEM images hinted at the possibility of three phase-separated domains: dark spherical particles, dark shade (net-like) in the background, and light holes in the background. An enlarged example is shown in Figure 9C. In contrast, for initially miscible samples exposed to 97% RH, samples containing 10 to 80% ECP showed net-like structures, whereas the 90% drug loading sample remained homogeneous. In systems containing 10% to 30% ECP, phase-separated domains were irregular. Regular round discrete features were found to be homogeneously distributed in samples containing 40% to 80% ECP, with the size of the holes decreasing with increasing drug loading. Similar net-like features were also seen on sample surfaces by AFM topographical imaging following storage at 75% RH for 10 days (see Supporting Information, Figure S7). TEM possesses very high spatial resolution, and compositional analysis is possible by obtaining EDX spectra, enabling qualitative chemical insight into the various domains observed in the samples. Some representative raw EDX spectra are shown in Figure 9. In a pure ECP film (Figure 9A), EDX detected C, N, O, and F in the energy range of 0.2 to 0.8 keV, for which the experimental F/N and F/O atomic ratios are 1.0 ± 0.2 and 2.8 ± 0.3, respectively; whereas in a PVPVP film (Figure 9B), F was absent in the EDX spectrum. In a 90% ECP−PVPVA film prepared at 50% RH, three phases were formed. EDX detected C, N, O, and F in all phases, with the F/ N atomic ratios being 0.15 ± 0.02 for the dark spherical particles, 0.060 ± 0.006 for the continuous phase in the background, and 0.053 ± 0.009 for the white holes appeared in the background. Therefore, the dark particles appear to be ECP-rich, with the background being ECP-lean. No significant difference was detected by EDX between the shade and light areas in the background. It could be that the compositional difference between these two phases is beyond the detection limit of TEM−EDX or that the image contrast represents thickness differences in the sample. TEM−EDX was also performed for some the samples stored at high RH (ECP−HPMC data is shown in Figure S5). For the 50% ECP−PVPVA sample stored at 97% RH with the net-like structure (Figure 8B), EDX detected C, N, O, and F in both continuous and dispersed phases. The F/N atomic ratios are 0.06 ± 0.01 for the dispersed phase, and 0.10 ± 0.02 for the continuous phase. The F/N atomic ratios obtained on a 40% ECP−PVPVA phase-separated film are 0.01 ± 0.01 and 0.09 ± 0.01 for the dispersed and continuous phases, respectively. F/O atomic ratios revealed similar trends with higher values being seen for the continuous phase. Hence, the dark continuous phase appears to be ECP-rich, while the holes appear to be polymer-rich. This supposition is consistent with the observation that the size of these holes decreased with increasing drug loading. DSC measurements were also performed before and after storage at high RH to confirm phase separation (Figure 10). Representative DSC thermo-

Figure 10. (A) Glass transition temperatures of ECP−PVPVA ASDs (red and blue, ASDs stored at 97% RH for 24 h; green, freshly prepared ASDs). (B) Amount of moisture absorbed for ECP−PVPVA ASDs in DSC pans after storage at 98% RH for 24 h.

grams are provided in the Supporting Information (Figure S8). It can be seen that all freshly prepared ASDs exhibited a single Tg. This result is consistent with evacetrapib being miscible with PVPVA over the drug loading range of 10% to 90% when prepared under low RH conditions. The Tg of the ASD decreased with increasing drug loading, as evacetrapib has a lower Tg as compared to PVPVA. After storage at 97% RH, two Tgs were observed in systems containing 10 to 50% evacetrapib, indicating phase separation. ASDs at 60 to 90% drug loadings showed one Tg. This is probably because the size of phaseseparated domains is beyond the detection limit of DSC, and/ or because rapid mixing occurred during heating.

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DISCUSSION Phase Behavior. To maintain the physical stability of ASDs, it is generally considered essential to prevent phase separation. For example, the drug-rich phase will be more prone to crystallization due to a lower local concentration of the polymer. The dissolution properties may also be different between miscible and phase separated systems. These factors can impair the solubility/dissolution rate advantage and resultant bioavailability of these products. The results of this study clearly indicate that the phase behavior of evacetrapib− HPMC and evacetrapib−PVPVA systems are very susceptible to the presence of water. Water, which is an antisolvent for evacetrapib, can lead to phase separation in the ASD through two routes: solution-state phase separation during preparation and solid-state phase separation during storage at high RH. When prepared under dry conditions or upon heating, the ECP−polymer ASDs exhibit full miscibility over the drug loading range from 10 to 90% (Figure 2A); in contrast, they 1700


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components will change as volatile solvents evaporate, and additional water can be simultaneously absorbed from the vapor phase if the atmosphere is not controlled, as shown in Figure 11. The simplest system to consider is a spin coated evacetrapib film prepared using a single solvent stock solution and no polymer (Figure 6B). This system consists of drug, methanol, and water, and the phase behavior can be represented by a ternary phase diagram, as shown in Figure 12A. The phase

become phase-separated in the presence of a trace amounts of water introduced during preparation (Figures 2B,C and 4). Following storage at 98% RH for 7 days, the originally miscible ASDs also became phase-separated over the drug loading range 10 to 80% (based on TEM results shown in Figure 8). The shape, size, and chemical composition of phase-separated domains were noted to vary depending on the route of phase separation. This is to be expected. In the solution, highly mobile solvated drug and polymer molecules of different relative solubilities were present in a solvent and water mixture, the composition of which is changing during drying. The high mobility and antisolvent effect appear to lead to the formation of spherical drug-rich particles, which become embedded in a polymer-rich matrix. In contrast, in the film exposed to high RH, the system is less mobile with only water present and no organic solvent. The lower mobility in this system perhaps explains the resultant distinct microstructure seen in the TEM images shown in Figure 8. Phase separation during solvent evaporation is a complex interplay between equilibrium thermodynamics and mass transfer kinetics; however, such phenomena are well documented and understood in the field of polymer science where phase separation is exploited to prepare porous membranes.45−48 To induce phase separation, the polymer is dissolved in a mixture of a volatile and a nonvolatile solvent, whereby the nonvolatile solvent is a poor solvent for the polymer. Following evaporation of the volatile solvent, phase separation of the polymer occurs. An alternative approach to induce phase separation is termed vapor induced phase separation (VIPS).45 A schematic of VIPS is shown in Figure 11. Here a water-insoluble polymer is dissolved in a water-

Figure 11. Schematic for vapor induced phase separation (VIPS). Figure adapted from ref 45. Copyright 2013 Royal Society of Chemistry.

Figure 12. Phase diagrams showing phase separation routes during (A) preparation and (B) storage.

miscible solvent, which evaporates slowly. A film is then cast and placed in a humid atmosphere. If evaporation of the solvent is slower than absorption of water from the atmosphere, phase separation of the polymer is induced. However, in the context of amorphous solid dispersions, the effect of water during ASD preparation has not been as thoroughly considered to date. Furthermore, due to the greater number of components in an ASD−solvent system, the impact of water on the phase behavior may be much more complex than that observed for ternary polymer−solvent−nonsolvent systems described above. During spin coating or other solvent evaporation processes, the system contains several components, which typically at a minimum comprise drug, polymer, organic solvent system, and water. Water can be initially present in the starting solution, introduced by hygroscopic excipients/solvents that have been exposed to the atmosphere or added deliberately to manipulate drug or excipient solubility. The concentration of these

diagram shows the phase boundaries, and kinetic factors are not taken into account. Taking point A as an example, in the absence of water, the drug is completely miscible with the solvent. When a trace amount of water is introduced into the system (either added to the solvent system, or absorbed into the solvent during spin coating at higher RHs), the system moves from point A to point B, where it still shows miscibility. During spin coating, the relative amount of water increases as the organic solvent evaporates more rapidly than water and the system moves along the line from B to D with phase separation occurring at point C. For the experiments where more and more water was added to the stock solution (Figure 5), the system moves along the ABE line, and phase separation occurs at point E. It can be seen that although the spin-coated system moved to point D eventually, which is in the one phase region where all the water has been removed, the drug droplets are 1701


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research. Similar moisture-induced phase separation during spin coating has been reported previously,26 and is something that we have observed widely in our laboratory for several drug− polymer systems. Many of the solvents employed to dissolve solids for film preparation, such as methanol, are very hygroscopic and hence can contain significant quantities of water. Therefore, when assessing drug−polymer miscibility, caution should be used during solvent transfer and handling, to eliminate the effect of residual water in the solvent. Spray drying, a solvent based method, is one of the two most widely used methods for commercial ASD production. Modest heating and short exposure times are normally used due to the low boiling point of the organic solvents employed.51 A nitrogen purge is usually used during atomization, hence the risk for moisture acquisition by the solvent may be much lower during the actual spray drying process. However, water may be added to the organic solvent system to increase the solubility of certain formulation components such as a specific polymer or surfactant. The added water could potentially cause phase separation during spray drying leading to an ASD that is not a true molecular dispersion; therefore, it is important to optimize the solvent system considering this aspect. Also, if the solvent used in the spray drying operation is exposed to the atmosphere during sample preparation, again there is a risk for water acquisition. Small amounts of water may not cause phase separation initially, but can induce phase separation due to concentration of water during spray drying due to the differential evaporation rates of water versus the organic solvent. In addition, we need to consider that spray drying typically involves primary drying to remove the bulk of the organic solvent, and then a secondary drying stage to reduce residual solvent to levels below those set by regulatory requirements. During the period between primary and secondary drying, residual solvent may lead to water being absorbed into the system with unknown consequences on miscibility. Clearly these are areas that need further investigation. Phase separation can also occur during the hydration step that occurs during the dissolution process.42 However, due to analytical challenges associated with samples containing water, the evolution of microstructure due to phase separation during dissolution has not been extensively investigated to date. It can be anticipated that if phase separation occurs upon storage at high RH, albeit with slower kinetics and no material transfer, that this may be predictive of the subsequent phase behavior during dissolution. It has been reported that ASD phase separation occurred immediately upon contact with water for both probucol and ritonavir ASDs with PVPVA at 50% drug loading.42 The rate of phase separation was faster than the dissolution rate.42 When phase separation occurs upon hydration, the polymer-rich regions are likely to exit the ASD matrix and dissolve, leaving the drug-rich phases behind. If the drug-rich phase formed small discrete domains during phase separation, then small undissolved nanosize drug-rich particles can be subsequently released into the dissolution medium, and these may dissolve faster compared to macrosize drug-rich particles. This mechanism has been suggested to be responsible for the presence of nanosized species of anacetrapib following dissolution of PVPVA dispersions.52 In contrast, if the continuous phase is drug-rich, e.g., similar to the net-like structure observed in Figure 8, it is likely to become a porous scaffold upon polymer exit, as hypothesized by Higuchi and coworkers, and the subsequent dissolution rate will be controlled

kinetically trapped due to lack of mobility in the absence of solvents, leading to the topography seen in Figure 6. Also, it is worth noting that less water is needed to induce phase separation in the BCD line (spin coating) as compared to the BE line (solution-state liquid−liquid phase separation). The amount of water brought in is highly dependent on the hydrophilicity of the solvent. For methanol−water mixtures, the water activity and mole fraction of water in these mixtures have been found to exhibit near ideal Raoult’s law behavior.49,50 Thus, 1 mol (32 g) of methanol is capable of bringing in almost 1 mol (18 g) of water when equilibrated at 50% RH. Therefore, with methanol as a solvent, a significant amount of water can be absorbed from the atmosphere, depending on the exposure time, leading to phase separation. In the presence of a polymer, water affected the miscibility of ASDs in a similar way through liquid−liquid phase separation; however, the quaternary phase diagram becomes much more complex. Moreover, it is apparent that the polymer can impact the miscibility of the system. For example, when a small amount of water was added to a stock solution of evacetrapib, phase separation occurred during spin coating (Figure 6C); in contrast, a film prepared under identical conditions showed no evidence of phase separation when HPMC was present (Figure 4). The presence of the polymer in the stock solution thus appears to decrease the tendency of the drug to phase separate from the solvent in the presence of water. In addition, the morphological properties of the film, such as domain size, shape, and porosity, are also affected by going through different composition paths during phase separation.46,48 During storage, the phase behavior of spin-coated films can be described by Figure 12B. The drug and the polymer were miscible initially, shown by line AC. For a high drug loading system, little water was absorbed in the film due to its hydrophobicity, as shown by line AB, and therefore, the system remained miscible after storage at 97% RH. However, for a low drug loading system, more moisture was absorbed in the film due to a larger amount of polymer present. The system underwent phase separation as shown by line CD. Low drug loading systems are prone to absorb more moisture due to the higher polymer content and therefore are more susceptible to phase separation than high drug loading systems. An alternative explanation is that the “miscibility” observed herein, may be a kinetically trapped thermodynamically metastable or unstable state that occurred during rapid solvent evaporation or cooling, instead of a representing a thermodynamically miscible region. In this scenario, the introduction of water during storage increased the molecular mobility of the system, and thus enabled phase separation, which is thermodynamically favored, to occur. However, we think that this scenario is less likely based on discussions presented previously for similar hydrophobic drug−polymer systems.35 Evidence against a kinetic route was found in indomethacin−PVP and ketoconazole−PVP systems exposed to environmental moisture without phase separation.35 Moisture-induced phase separation was found in initially miscible pimozole−PVP samples exposed to high RH at 4 °C, albeit with a lower rate as compared to that at 22 °C, suggesting thermodynamic factors are critical in determining whether an ASD would undergo moisture induced phase separation upon storage, while molecular mobility is a ratelimiting factor.35 Implications on ASD Preparation and Performance. Humidity control is critical in lab scale investigations since spin coating and solvent casting are widely used methods in ASD 1702


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Molecular Pharmaceutics by the drug.53 Therefore, understanding the changes in ASD microstructure in the presence of water is likely essential to help us to gain an improved mechanistic understanding of the complex ASD dissolution process.

Lynne S. Taylor: 0000-0002-4568-6021

CONCLUSIONS The miscibility and microstructure of evacetrapib−polymer ASDs are extremely sensitive to the presence of trace amounts of water present in the solvent during film formation. Nanosized drug-rich spherical particles readily form during solvent evaporation and become embedded in a polymer-rich continuous phase. By rigorously excluding water during film formation, miscible films can be formed, confirming the role of water in inducing phase separation. The role of water in promoting phase separation was further explored by storing initially miscible films at high RH, where phase separation was observed. In this instance, as shown by TEM−EDX measurements, the continuous phase is drug-rich, while the discrete regions are polymer-rich; hence, the microstructure is vastly different for phase separation via this route. Higher drug loading systems are more prone to phase separate when water is present in the solvent used for film formation but are more resistant to phase separation following exposure to high RH, presumably because moisture sorption is suppressed. By combining high-resolution imaging technique with orthogonal analyses, the microstructure of phase-separated ASDs can be revealed at a high spatial resolution. These findings about the phase behavior and microstructure of ASDs are anticipated to aid in the design and production of optimally performing ASDs.

ACKNOWLEDGMENTS The authors would like to thank the National Institutes of Health through grant numbers R41 GM100657-01A1 and R42 GM100657-03 for financial support. We gratefully thank Laurie Mueller for technical training and helpful discussions on TEM, and Dr. Alpana Thorat for assistance with Karl Fischer titration.

Notes

The authors declare no competing financial interest.

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ABBREVIATIONS AFM, atomic force microscopy; API, active pharmaceutical ingredients; ASD, amorphous solid dispersion; ECP, evacetrapib; EDX, energy dispersive X-ray spectroscopy; HPMC, hydroxypropyl methylcellulose; PVPVA, polyvinylpyrrolidone/ vinyl acetate; RH, relative humidity; TEM, transmission electron microscopy; Tg, glass transition temperature; VIPS, vapor induced phase separation

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.molpharmaceut.6b01151. LLPS concentration (“amorphous solubility”) of evacetrapib at pH 6.8 and 25 °C in water, particle size distribution of a 90% drug loading ECP−HPMC film prepared at 35%RH, AFM images of ECP−PVPVA films prepared at 50% RH using methanol as the solvent and a solids loading of 5 mg/mL and prepared at 18% RH and stored at 97% RH for 7 days, surface roughness (Ra) as a function of drug loading of ECP−PVPVA systems, a stereopair of TEM images showing the structure of a 80% ECP−HPMC film, surface topography of ECP− PVPVA films stored at 75% RH, AFM images of evacetrapib-PVPVA films prepared at 50% RH using methanol as the solvent and a solids loading of 5 mg/mL and prepared at 18% RH and stored at 97% RH for 7 days, TEM images of ECP−HPMC films prepared at 50% RH using binary solvent and a solids loading of 50 mg/mL and prepared at 18% RH and stored at 97% RH for 7 days with reduced thickness, EDX spectrum of the carbon coated copper grid used for TEM/EDX studies, and representative DSC thermograms for ECP−PVPVA ASDs after storage at 98% RH (PDF)

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REFERENCES

(1) Kawakami, K. Current status of amorphous formulation and other special dosage forms as formulations for early clinical phases. J. Pharm. Sci. 2009, 98 (9), 2875−2885. (2) Lipinski, C. A. Drug-like properties and the causes of poor solubility and poor permeability. J. Pharmacol. Toxicol. Methods 2000, 44 (1), 235−249. (3) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv. Drug Delivery Rev. 1997, 23 (1−3), 3−25. (4) Di, L.; Fish, P. V.; Mano, T. Bridging solubility between drug discovery and development. Drug Discovery Today 2012, 17 (9−10), 486−495. (5) Di, L.; Kerns, E. H.; Carter, G. T. Drug-Like Property Concepts in Pharmaceutical Design. Curr. Pharm. Des. 2009, 15 (19), 2184− 2194. (6) Loftsson, T.; Brewster, M. E. Pharmaceutical applications of cyclodextrins: effects on drug permeation through biological membranes. J. Pharm. Pharmacol. 2011, 63 (9), 1119−1135. (7) Rodriguez-Aller, M.; Guillarme, D.; Veuthey, J.-L.; Gurny, R. Strategies for formulating and delivering poorly water-soluble drugs. J. Drug Delivery Sci. Technol. 2015, 30 (Part B), 342−351. (8) Leuner, C.; Dressman, J. Improving drug solubility for oral delivery using solid dispersions. Eur. J. Pharm. Biopharm. 2000, 50 (1), 47−60. (9) Müller, R. H.; Jacobs, C.; Kayser, O. Nanosuspensions as particulate drug formulations in therapy: Rationale for development and what we can expect for the future. Adv. Drug Delivery Rev. 2001, 47 (1), 3−19. (10) Elworthy, P. H.; Patel, M. S. Demonstration of maximum solubilization in a polyoxyethylene alkyl ether series of non-ionic surfactants. J. Pharm. Pharmacol. 1982, 34 (9), 543−546. (11) Loftsson, T.; Brewster, M. E. Pharmaceutical Applications of Cyclodextrins. 1. Drug Solubilization and Stabilization. J. Pharm. Sci. 1996, 85 (10), 1017−1025. (12) Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J. Pharmaceutical Co-Crystals. J. Pharm. Sci. 2006, 95 (3), 499−516. (13) Berge, S. M.; Bighley, L. D.; Monkhouse, D. C. Pharmaceutical Salts. J. Pharm. Sci. 1977, 66 (1), 1−19. (14) Miller, J. M.; Beig, A.; Carr, R. A.; Spence, J. K.; Dahan, A. A Win−Win Solution in Oral Delivery of Lipophilic Drugs: Supersaturation via Amorphous Solid Dispersions Increases Apparent Solubility without Sacrifice of Intestinal Membrane Permeability. Mol. Pharmaceutics 2012, 9 (7), 2009−2016.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Na Li: 0000-0001-8941-4784 1703


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Molecular Pharmaceutics (15) Raina, S.; Zhang, G. Z.; Alonzo, D.; Wu, J.; Zhu, D.; Catron, N.; Gao, Y.; Taylor, L. S. Impact of solubilizing additives on supersaturation and membrane transport of drugs. Pharm. Res. 2015, 32, 1− 15. (16) Newman, A.; Knipp, G.; Zografi, G. Assessing the performance of amorphous solid dispersions. J. Pharm. Sci. 2012, 101 (4), 1355− 1377. (17) Qian, F.; Huang, J.; Hussain, M. A. Drug−polymer solubility and miscibility: Stability consideration and practical challenges in amorphous solid dispersion development. J. Pharm. Sci. 2010, 99 (7), 2941−2947. (18) Rumondor, A. F.; Stanford, L.; Taylor, L. S. Effects of Polymer Type and Storage Relative Humidity on the Kinetics of Felodipine Crystallization from Amorphous Solid Dispersions. Pharm. Res. 2009, 26 (12), 2599−2606. (19) Hancock, B.; Parks, M. What is the true solubility advantage for amorphous pharmaceuticals? Pharm. Res. 2000, 17 (4), 397−404. (20) Qi, S.; Moffat, J. G.; Yang, Z. Early Stage Phase Separation in Pharmaceutical Solid Dispersion Thin Films under High Humidity: Improved Spatial Understanding Using Probe-Based Thermal and Spectroscopic Nanocharacterization Methods. Mol. Pharmaceutics 2013, 10 (3), 918−930. (21) Qi, S.; Belton, P.; Nollenberger, K.; Clayden, N.; Reading, M.; Craig, D. Q. M. Characterisation and Prediction of Phase Separation in Hot-Melt Extruded Solid Dispersions: A Thermal, Microscopic and NMR Relaxometry Study. Pharm. Res. 2010, 27 (9), 1869−1883. (22) Lauer, M.; Grassmann, O.; Siam, M.; Tardio, J.; Jacob, L.; Page, S.; Kindt, J.; Engel, A.; Alsenz, J. Atomic Force Microscopy-Based Screening of Drug-Excipient Miscibility and Stability of Solid Dispersions. Pharm. Res. 2011, 28 (3), 572−584. (23) Karavas, E.; Georgarakis, M.; Docoslis, A.; Bikiaris, D. Combining SEM, TEM, and micro-Raman techniques to differentiate between the amorphous molecular level dispersions and nanodispersions of a poorly water-soluble drug within a polymer matrix. Int. J. Pharm. 2007, 340 (1−2), 76−83. (24) Marsac, P. J.; Rumondor, A. C. F.; Nivens, D. E.; Kestur, U. S.; Stanciu, L.; Taylor, L. S. Effect of temperature and moisture on the miscibility of amorphous dispersions of felodipine and poly(vinyl pyrrolidone). J. Pharm. Sci. 2010, 99 (1), 169−185. (25) Ricarte, R. G.; Lodge, T. P.; Hillmyer, M. A. Nanoscale Concentration Quantification of Pharmaceutical Actives in Amorphous Polymer Matrices by Electron Energy-Loss Spectroscopy. Langmuir 2016, 32 (29), 7411−7419. (26) Li, N.; Taylor, L. S. Nanoscale Infrared, Thermal, and Mechanical Characterization of Telaprevir−Polymer Miscibility in Amorphous Solid Dispersions Prepared by Solvent Evaporation. Mol. Pharmaceutics 2016, 13 (3), 1123−1136. (27) Purohit, H. S.; Taylor, L. S. Miscibility of ItraconazoleHydroxypropyl Methylcellulose Blends- Insights with High Resolution Analytical Methodologies. Mol. Pharmaceutics 2015, 12 (12), 4542− 4553. (28) Harrison, A. J.; Bilgili, E. A.; Beaudoin, S. P.; Taylor, L. S. Atomic Force Microscope Infrared Spectroscopy of Griseofulvin Nanocrystals. Anal. Chem. 2013, 85 (23), 11449−11455. (29) Fussell, A. L.; Grasmeijer, F.; Frijlink, H. W.; de Boer, A. H.; Offerhaus, H. L. CARS microscopy as a tool for studying the distribution of micronised drugs in adhesive mixtures for inhalation. J. Raman Spectrosc. 2014, 45 (7), 495−500. (30) Stöckle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Nanoscale chemical analysis by tip-enhanced Raman spectroscopy. Chem. Phys. Lett. 2000, 318 (1−3), 131−136. (31) Yeo, B.-S.; Stadler, J.; Schmid, T.; Zenobi, R.; Zhang, W. Tipenhanced Raman Spectroscopy − Its status, challenges and future directions. Chem. Phys. Lett. 2009, 472 (1−3), 1−13. (32) Takadama, H.; Kim, H.-M.; Kokubo, T.; Nakamura, T. TEMEDX study of mechanism of bonelike apatite formation on bioactive titanium metal in simulated body fluid. J. Biomed. Mater. Res. 2001, 57 (3), 441−448.

(33) Paudel, A.; Van den Mooter, G. Influence of Solvent Composition on the Miscibility and Physical Stability of Naproxen/ PVP K 25 Solid Dispersions Prepared by Cosolvent Spray-Drying. Pharm. Res. 2012, 29 (1), 251−270. (34) Janssens, S.; De Zeure, A.; Paudel, A.; Van Humbeeck, J.; Rombaut, P.; Van den Mooter, G. Influence of Preparation Methods on Solid State Supersaturation of Amorphous Solid Dispersions: A Case Study with Itraconazole and Eudragit E100. Pharm. Res. 2010, 27 (5), 775−785. (35) Rumondor, A. C. F.; Marsac, P. J.; Stanford, L. A.; Taylor, L. S. Phase Behavior of Poly(vinylpyrrolidone) Containing Amorphous Solid Dispersions in the Presence of Moisture. Mol. Pharmaceutics 2009, 6 (5), 1492−1505. (36) Cao, G.; Beyer, T. P.; Zhang, Y.; Schmidt, R. J.; Chen, Y. Q.; Cockerham, S. L.; Zimmerman, K. M.; Karathanasis, S. K.; Cannady, E. A.; Fields, T.; Mantlo, N. B. Evacetrapib is a novel, potent, and selective inhibitor of cholesteryl ester transfer protein that elevates HDL cholesterol without inducing aldosterone or increasing blood pressure. J. Lipid Res. 2011, 52 (12), 2169−2176. (37) Bell, D.; Garratt-Reed, A. Energy Dispersive X-ray Analysis in the Electron Microscope; Garland Science: 2003; Vol. 49. (38) Liu, T.; Ozisik, R.; Siegel, R. W. Phase separation and surface morphology of spin-coated films of polyetherimide/polycaprolactone immiscible polymer blends. Thin Solid Films 2007, 515 (5), 2965− 2973. (39) Greenspan, L. Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand., Sect. A 1977, 81 (1), 89−96. (40) Khanal, D.; Dillon, E.; Hau, H.; Fu, D.; Ramzan, I.; Chrzanowski, W. Lorentz contact resonance spectroscopy for nanoscale characterisation of structural and mechanical properties of biological, dental and pharmaceutical materials. J. Mater. Sci.: Mater. Med. 2015, 26 (12), 1−10. (41) Horvath, A. L. IUPAC-NIST Solubility Database-NIST Standard Reference Database 106. Solubility System: Dichloromethane with Water. https://srdata.nist.gov/solubility/sol_detail. aspx?goBack=Y&sysID=60_242. (42) Purohit, H. S.; Taylor, L. S. Phase Separation Kinetics in Amorphous Solid Dispersions Upon Exposure to Water. Mol. Pharmaceutics 2015, 12 (5), 1623−1635. (43) Rumondor, A. C. F.; Taylor, L. S. Effect of Polymer Hygroscopicity on the Phase Behavior of Amorphous Solid Dispersions in the Presence of Moisture. Mol. Pharmaceutics 2010, 7 (2), 477−490. (44) Rumondor, A. F.; Wikström, H.; Van Eerdenbrugh, B.; Taylor, L. S. Understanding the Tendency of Amorphous Solid Dispersions to Undergo Amorphous−Amorphous Phase Separation in the Presence of Absorbed Moisture. AAPS PharmSciTech 2011, 12 (4), 1209−1219. (45) Li, M.; Katsouras, I.; Piliego, C.; Glasser, G.; Lieberwirth, I.; Blom, P. W. M.; de Leeuw, D. M. Controlling the microstructure of poly(vinylidene-fluoride) (PVDF) thin films for microelectronics. J. Mater. Chem. C 2013, 1 (46), 7695−7702. (46) Young, T.-H.; Cheng, L.-P.; Lin, D.-J.; Fane, L.; Chuang, W.-Y. Mechanisms of PVDF membrane formation by immersion-precipitation in soft (1-octanol) and harsh (water) nonsolvents. Polymer 1999, 40 (19), 5315−5323. (47) Matsuyama, H.; Teramoto, M.; Nakatani, R.; Maki, T. Membrane formation via phase separation induced by penetration of nonsolvent from vapor phase. I. Phase diagram and mass transfer process. J. Appl. Polym. Sci. 1999, 74 (1), 159−170. (48) Matsuyama, H.; Teramoto, M.; Nakatani, R.; Maki, T. Membrane formation via phase separation induced by penetration of nonsolvent from vapor phase. II. Membrane morphology. J. Appl. Polym. Sci. 1999, 74 (1), 171−178. (49) Zhu, H.; Yuen, C.; Grant, D. J. W. Influence of water activity in organic solvent + water mixtures on the nature of the crystallizing drug phase. 1. Theophylline. Int. J. Pharm. 1996, 135 (1), 151−160. (50) Gölles, F. Examination and calculation of thermodynamic data from experimental measurements. I. The numerical integration of the 1704


Article

Molecular Pharmaceutics vapor-pressure curves of the system methanol-water. Monatsh. Chem. 1961, 92, 981−991. (51) Patel, B. B.; Patel, J. K.; Chakraborty, S.; Shukla, D. Revealing facts behind spray dried solid dispersion technology used for solubility enhancement. Saudi Pharm. J. 2015, 23 (4), 352−365. (52) Harmon, P.; Galipeau, K.; Xu, W.; Brown, C.; Wuelfing, W. P. Mechanism of Dissolution-Induced Nanoparticle Formation from a Copovidone-Based Amorphous Solid Dispersion. Mol. Pharmaceutics 2016, 13 (5), 1467−1481. (53) Simonelli, A. P.; Mehta, S. C.; Higuchi, W. I. Dissolution rates of high energy sulfathiazole-povidone coprecipitates II: Characterization of form of drug controlling its dissolution rate via solubility studies. J. Pharm. Sci. 1976, 65 (3), 355−361.

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