Article pubs.acs.org/molecularpharmaceutics

Preparation and Performance of Hydroxypropyl Methylcellulose Esters of Substituted Succinates for in Vitro Supersaturation of a Crystalline Hydrophobic Drug Ligeng Yin and Marc A. Hillmyer* Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *

ABSTRACT: We prepared hydroxypropyl methylcellulose (HPMC) esters of substituted succinates and examined their performance for improving the aqueous solubility of crystalline hydrophobic drugs in spray-dried dispersions (SDDs). From one HPMC, we synthesized five HPMC esters using various monosubstituted succinic anhydrides. These HPMC esters along with a commercial HPMC acetate succinate (HPMCAS) were spray-dried from solutions with phenytoin. The SDDs with different matrices at 10 wt % loading had very similar bulk properties with a minimal amount of detectable crystalline phenytoin as revealed by scanning electron microscopy (SEM), powder X-ray diffraction (powder XRD), and differential scanning calorimetry (DSC). In solution, while the SDD with HPMCAS was very effective at achieving high levels of phenytoin supersaturation initially, it was not competent at maintaining such supersaturation due to the rapid crystallization of the dissolved phenytoin. Alternatively, SDDs with several synthesized HPMC esters of substituted succinates not only achieved rather high initial supersaturation but also maintained high concentrations for extended time (i.e., 1.5 h and longer). Such maintenance was largely ascribed to the inhibition of phenytoin nucleation. Structure−property relationships were established, and the most successful systems contained a high degree of substitution and a combination of a thioether with neighboring weak electronwithdrawing groups in the substituted succinic anhydrides. The effective maintenance of supersaturated solutions was only found in SDDs with rather low drug loadings, which indicates the significance of sufficiently high concentrations of polymer additives in the dissolution media. KEYWORDS: spray-dried dispersions, HPMCAS, HPMC esters of substituted succinates, phenytoin, supersaturated solutions, crystallization inhibition

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INTRODUCTION

and thus spray drying is suitable for a wide variety of compounds.8e The matrix plays a critical role in dictating the bulk and solution performance of a spray-dried dispersion (SDD). To date, numerous reports have shown that a cellulose ether ester, hydroxypropyl methylcellulose acetate succinate (HPMCAS), is more effective than many other candidates including synthetic homopolymers such as poly(ethylene glycol) (PEG) and poly(vinylpyrrolidone) (PVP) as well as other cellulose derivatives such as hydroxypropyl methylcellulose (HPMC) and hydroxylpropyl methylcellulose phthalate (HPMCP).8e,10 SDDs with HPMCAS as matrices generally exhibit strong resistance against moisture-induced crystallization of the dispersed amorphous drugs during storage.8e,10d Moreover, HPMCAS is very effective at both achieving high levels of supersaturation and inhibiting the recrystallization of dissolved

One of the biggest challenges in the pharmaceutical field is improving the aqueous solubility of active pharmaceutical ingredients (APIs). Currently, more than 40% of the drugs under development are estimated to suffer from insufficient solubility in water,1 and the percentage has been continuously increasing.2 Such low water solubility results in poor bioavailability of the drugs by oral administration. Efforts to address this problem include salt formation,3 prodrugs,4 selfemulsifying drug delivery systems,5 complexation with cyclodextrin,6 nanosuspensions,7 and amorphous solid dispersions.8 Among them, spray-drying from an organic solvent is an effective and versatile method for the preparation of amorphous solid dispersions.9 Due to rapid drying during processing, the hydrophobic drugs are rendered amorphous and are further stabilized within the matrix of choice (often a polymer). Compared to the crystalline form, an amorphous drug has higher free energy and thus is more apt to dissolve in water. The drug is molecularly dispersed within the matrix and can experience rapid dissolution.8b In addition, many poorly watersoluble drugs are readily soluble in common organic solvents, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 175

June 24, 2013 November 21, 2013 November 27, 2013 December 9, 2013 dx.doi.org/10.1021/mp4003656 | Mol. Pharmaceutics 2014, 11, 175−185

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(Aldrich, 99%), cyclohexylsuccinic acid (Aldrich, 96%), maleic anhydride (Fluka, 99+%), cyclohexyl thiol (Aldrich, 97%), phenyl thiol (Aldrich, 99+%), benzyl thiol (Aldrich, 99%), triethylamine (Sigma-Aldrich, 99.0+%), sulfuric acid (BDH, 95.0−98.0% min), succinic anhydride (Aldrich, 99+%), acetyl chloride (Fluka, 99+%), benzene (Sigma-Aldrich, anhydrous, 99.8%), acetic acid (Sigma-Aldrich, 99.7+%), sodium acetate (Sigma-Aldrich, 99.0+%), isopropyl alcohol (Sigma-Aldrich, anhydrous, 99.5%), hydroxypropyl methylcellulose (METHOCEL E3 Premium LV,16 The Dow Chemical Company), hydroxypropyl methylcellulose acetate succinate (AFFINISOL HPMCAS 912 G,16 The Dow Chemical Company), phenytoin (Sigma, 99+%), simulated intestinal fluid powder (Biorelevant), chloroform-d (Alrich, 99.96% atom D), acetic acid-d4 (Cambridge Isotope Laborotaries, 99.5% D), and dimethyl sulfoxided6 (Cambridge Isotope Laborotaries, 99.9% D) were used as received. Molecular Characterization. The degree of substitution (DS) of succinate and acetate in HPMCAS was determined following the monograph by United States Pharmacopeia and The National Formulary (USP−NF).17 HPMCAS was hydrolyzed using sodium hydroxide followed by the acidification of the solution, and the content of succinic acid and acetic acid was quantified by high-performance liquid chromatography (HPLC). The DS of methoxy and hydroxypropoxy in HPMC and HPMCAS were determined following the monograph by USP−NF. HPMC or HPMCAS was hydrolyzed using hydriodic acid, and the contents of methyl iodide and isopropyl iodide were quantified by HPLC. 1H and 13C NMR spectra were recorded on a Bruker AV-500, Varian Inova 500, or Varian Inova 300 spectrometer at 23 °C with DMSO-d6 or CDCl3 as the solvent unless otherwise specified. The SEC measurement of HPMC was carried out on an Agilent 1260 liquid chromatograph with 0.1 M Na2 SO 4 aqueous solution (supplemented with 1% acetic acid) as the mobile phase. The SEC housed a guard column and three separating columns with pore sizes of 1000, 300, and 100 Å, all of which were CATSEC series from Eprogen (Downers Grove, IL). The detectors included an Agilent 1260 MWD UV−vis detector, a Wyatt Dawn Heleos II light-scattering detector, and a Wyatt Optilab T-rEX refractive-index detector. The SEC measurement of HPMC esters of substituted succinates was carried out on an Agilent 1260 liquid chromatograph with tetrahydrofuran (THF) as the mobile phase. The SEC housed a Waters Styragel guard column and three separation columns that covered an effective molecular weight range of 100−10 000 000 g mol−1. The detectors included an Agilent 1260 VWD UV−vis detector, a Wyatt Dawn Heleos II light-scattering detector, and a Wyatt Optilab T-rEX refractive-index detector. Synthesis of Substituted Succinic Anhydrides. The synthesis of cyclohexylsuccinic anhydride (Cy-SA) was carried out following the published procedure toward cyclopentylsuccinic anhydride,18 and the product was purified via vacuum distillation.19 The synthesis of phenylthiosuccinic anhydride (Ph-S-SA)20 and benzylthiosuccinic anhydride (Bn-S-SA)20b was carried out following published procedures except that the product was purified via column chromatography followed by vacuum distillation. Ph-S-SA: Rf = 0.54 in hexane/ethyl acetate = 8:2 (v/v), bp = 120 °C at 65 mTorr. Bn-S-SA: Rf = 0.75 in hexane/ ethyl acetate = 8:2 (v/v), bp = 140 °C at 55 mTorr. The detailed synthetic procedure of cyclohexylthiosuccinic anhydride (Cy-S-SA) is described as below. A sample of 11.3

drugs in solution, which can significantly enhance the delivery efficacy in the gastrointestinal (GI) tract.8e,10 The outstanding performance of HPMCAS as the matrix in SDDs is often attributed to its multifunctional structure.8e,10c,i HPMCAS is a cellulose derivative of two ethers, methoxy (MeO) and hydroxypropoxy (HPO), and two esters, acetate (Ac) and succinate (Su). The hydrophobic acetate groups are thought to facilitate the molecular dispersions of hydrophobic drugs within the polymeric matrix, and the small amount of unreacted hydrophilic hydroxyls allows sufficient hydration of the resulting SDDs in solution. In addition, the carboxylic acids from succinates are ionized at intestinal pH (∼7). These negative charges can provide excellent colloidal stability of any polymer−drug nanostructures in solution, which can be important in preventing the “desupersaturation” of the dissolved drugs.8e However, efforts to understand and optimize HPMCAS are difficult due to the low precision in the composition control and unclear definition of molecular structure in such modified cellulosics. Modifying HPMC with a substituted succinic anhydride is a simple and effective method of synthesizing cellulose derivatives that simultaneously contain hydrophobic, hydrophilic, and pH-responsive functionalities. Such anhydrides allow for a reduction in the number of derivatizing reagents without sacrificing any functionality. The composition of the hydrophobic and pH-responsive moieties in the resulting materials is fixed at 1:1. Besides, these two groups are covalently neighboring to each other, rather than being roughly statistically distributed along the cellulose backbone, which result in better-defined structures for HPMC esters of substituted succinates (HPMC-(R-SA)). A few examples of starch11 or cellulose12 derivatives of substituted succinates have been reported. In early patent literature, Caldwell and Wurzburg first claimed the synthesis of polysaccharide derivatives of substituted succinates, in which the substituent was an alkyl or aromatic group.11a Later, Kalbe et al. used nonaqueous systems to modify cellulose, which allowed more effective incorporation of the substituted succinates.12c Nevertheless, mostly long-chain alkenylsuccinic anhydrides, such as 2-octen-1-yl succinic anhydride (C8) and 2dodecen-1-yl succinic anhydride (C12), were employed.11,12 Moreover, such modified polysaccharides were mainly targeting emulsifiers and viscosity modifiers,11a,b,d,f,12a,f paper coating,11b,12c,d dialysis membranes,12b biodegradable molded parts and films,11e,12c−e and composite applications.12g We demonstrate the synthesis of HPMC esters using a variety of monosubstituted succinic anhydrides and examined their efficacy as the matrices of SDDs. We chose the antiepileptic phenytoin as the model drug, which has a low aqueous solubility of 27.1 μg/mL.13 The c log P,14 which measures the lipophilicity of a compound, is moderately high at 1.9.13 Phenytoin has a high melting temperature (Tm) of 296 °C, which indicates strong intermolecular interactions within crystals that can prevent effective hydration of phenytoin in water.15 We have comprehensively characterized the SDDs with different matrices in bulk and in aqueous media and established structure−property relationships. Several of these new materials showed more effective supersaturation enhancement for phenytoin than HPMCAS.

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EXPERIMENTAL SECTION Reagents. All chemicals were reagent grade and used as received unless otherwise noted. Phenylsuccinic anhydride 176

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mL of cyclohexyl thiol (9.2 × 10−2 mol), 9.0 g of maleic anhydride (9.2 × 10−2 mol), and 256 μL of triethylamine (1.8 × 10−3 mol) were dissolved in 75 mL of benzene, and the reaction was allowed to proceed for 3.0 h at 70 °C. Then 200 μL of concentrated sulfuric acid was added into the mixture to quench the reaction. The mixture was filtered and concentrated to afford dark red crude product. Further purifications with flash chromatography (Rf = 0.49 in hexane/ethyl acetate = 8:2, v/v) followed by fractional distillation (bp = 110 °C at 55 mTorr) gave a nearly colorless, slightly yellow oil (conversion: 98% by 1H NMR, yield: 11.8 g, 60%). 1H NMR (in CDCl3, 300 MHz): δ (ppm): 4.01 (dd, 1H, J = 4.07, 9.48 Hz), 3.40 (dd, 1H, J = 9.48, 19.19 Hz), 3.15−3.05 (m, 1H), 2.73 (dd, 1H, J = 4.07, 19.19 Hz), 2.15−2.00 (m, 1H), 2.00−1.80 (m, 1H), 1.80−1.65 (m, 2H), 1.65−1.50 (m, 1H), 1.50−1.20 (m, 5H). 13 C NMR (75 MHz, CDCl3): 170.51, 168.72, 44.07, 37.81, 36.03, 33.00, 32.32, 25.60, 25.35, and 25.32 ppm. IR (melt, NaCl): νCO = 1863, 1785 cm−1. Elemental analysis calculated for C10H14O3S (%): C, 56.05, H, 6.59, S, 14.96; found: C, 56.26, H, 6.66, S, 15.14. The high purity of the anhydrides was affirmed by chromatography, 1H NMR spectroscopy, and elemental analysis. Synthesis of HPMC Esters of Substituted Succinates. Generally speaking, the composition of HPMC, sodium acetate, and acetic acid in the feed was kept the same, and a homogeneous mixture was first allowed to form at 85 °C ([−OH]0/[NaOAc]0 = 1:2, and [−OH]0 ≈ 0.4 M). Next, the specific amount of an anhydride was added to target the specific substituent and degree of substitution (Table S1 of the Supporting Information). The detailed procedure for synthesizing HPMC-CyS-0.72 is listed below as an example. Portions of 0.87 g of HPMC (pendent hydroxyl: 4.7 × 10−3 mol), 0.77 g of sodium acetate (9.4 × 10−3 mol), and 11.0 mL of glacial acetic acid were charged into a 50 mL round-bottom flask and allowed to form a homogeneous mixture at 85 °C with magnetic stirring. After that, 2.0 g of Cy-S-SA (9.4 × 10−3 mol) was added into the flask. The reaction was allowed to proceed at 85 °C for 3.0 h before it was quenched by adding 5 mL of DI water into the mixture. The mixture was first precipitated into 1.0 L of water, and then the filtered product was redissolved in 20 mL of THF, precipitated into 1.0 L of water once more, and further dried under vacuum at 22 °C until constant mass was achieved. The product was a white, slightly yellow powder. The conversion of the anhydride was 33% by 1H NMR. Yield: 1.45 g, 95%. The DS of substituted succinates on the resulting HPMC esters was determined by comparing the integration of signals from protons on the characteristic substituents (i.e., cyclohexyl, phenyl, and benzyl) to that of HPMC backbone in the δ = 3.8−2.6 region in 1H NMR spectroscopy. Preparation of Spray-Dried Dispersions (SDDs). SDDs were made at laboratory scale, and the following procedure for HPMC-CyS-0.72 at 10 wt % phenytoin loading serves as an example. Samples of 225 mg of HPMC-CyS-0.72 and 25.0 mg of phenytoin were first mixed in 12.25 g of acetone (corresponding to the concentration of total solids as 2.0 wt %) to form a homogeneous solution with magnetic stirring. The solution was loaded in a 20 mL syringe on a syringe pump, and SDDs were prepared with a mini-sprayer (Bend Research, Bend, OR) at the following conditions: inlet temperature, 68 °C; N2 flow rate, 12.8 L min−1; solution flow rate, 0.65 mL min−1. The outlet temperature was not controlled and recorded as between 22 and 28 °C. The spray-dried powders were collected with the aid of an antistatic bar and further dried

under vacuum (10 mTorr) for at least 12 h. SDDs with phenytoin were stored in a vacuum desiccator at 22 °C. The mass content of residual volatile solvents in the obtained SDDs were typically less than 0.6 wt % by monitoring the mass loss after holding samples at 160 °C for 5 min. Scanning Electron Microscopy (SEM). Powder samples were spread with a spatula onto a carbon conductive tape (Ted Pella Inc.) and then coated with 10 nm Au/Pd (60/40 by mass) in a 15 mTorr argon atmosphere using a Denton DV-502A high vacuum deposition system. Samples were imaged using a secondary electron detector on a Hitachi S-900 field emission gun SEM. The accelerating voltage was 1.2 or 3.0 kV. The magnification was between 3000 and 30 000. The particle size was determined by measuring the largest dimension of more than 100 individual particles in SEM images using ImageJ 1.47v. The thickness of the Au/Pd coating (∼20 nm) was subtracted from the diameter of each particle. Powder X-ray Diffraction (Powder XRD). Powder samples (∼50 mg) were packed in a 0.5 mm deep zerobackground holder and analyzed on a Bruker-AXS D5005 diffractometer at 22 °C. The X-ray source (Cu, λ = 1.54 Å) was operated at a voltage of 45 kV and a current of 40 mA. Data were collected from 5 to 40° (2θ) with a step size of 0.02° and a scan rate of 1 s/step. Differential Scanning Calorimetry (DSC). Samples of known mass (2−10 mg) were encapsulated in Tzero aluminum pans and analyzed on a Discovery DSC (TA Instruments). To determine the Tg of a neat polymer, normal pans were used, and the temperature was ramped between 20 and 160 °C at a rate of 10 °C min−1. Tg was determined using the second heating scan. Spray-dried dispersions were put inside hermetically sealed pans, and the temperature was ramped from 22 to 180 °C at a rate of 2.5 °C min−1. All analyses were carried out using TA TRIOS software version 2.2. Dissolution Tests. Samples (either a SDD or a crystalline drug) were weighed into 1.5 mL conical microcentrifuge tubes in triplicate. An appropriate amount of phosphate buffer solution (PBS, 82 mM sodium chloride, 20 mM sodium phosphate dibasic, 47 mM potassium phosphate monobasic, 0.5 wt % simulated intestinal fluid powder, adjusted to pH 6.5 with NaOH) at 37 °C was added to produce a final concentration of drug of 1000 μg/mL if all material was fully dissolved (e.g., 18.0 mg of spray-dried dispersion consisting of 1.8 mg of drug and 16.2 mg of polymer was diluted with 1.8 mL of buffer solution). Samples were vortexed for 1 min and then set in an isothermal aluminum sample holder at 37 °C. At each time point (4, 10, 20, 40, 90, 180, and 360 min), samples were centrifuged at 13 000 g for 1 min, and a 50 μL aliquot was removed and diluted with 250 μL of ethanol. The samples were again vortexed for 30 s and held at 37 °C until the next time point. The drug concentration in each aliquot was determined by reverse-phase HPLC. Samples were analyzed on an Agilent 1260 liquid chromatograph system with a multiwavelength UV−vis detector, 1260 MWD. The HPLC housed an Agilent Poroshell 120 EC-C18 column, whose dimension was 4.6 × 50 mm. The pore size was 120 Å, and the particle size was 2.7 μm. The chromatogram was monitored at 254 nm. For phenytoin, the mobile phase was 45:55 (v/v) of MeCN/Water, the elution volume of phenytoin was 1.08 mL, and a calibration was made in the range of 10−1000 μg/mL. The solubility of polymers in PBS (w/o SIF powder) was tested by visually examining the clarity of the solution at 37 °C. The concentration of the solution was sequentially diluted from 177

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Scheme 1. Synthesis of HPMC Esters of Substituted Succinates (HPMC-(R-SA))a

a

The structures of HPMC and HPMC-(R-SA) do not reflect the actual regiochemistrythe methyl, 2-hydroxypropyl, and substituted succinoyl groups are statistically distributed along the backbone.

Table 1. Molecular and Thermal Characteristics of HPMC and HPMC Esters sample HPMC HPMC-Cy-0.57 HPMC-Ph-0.63 HPMC-CyS-0.42 HPMC-CyS-0.57 HPMC-CyS-0.72 HPMC-CyS-0.99 HPMC-PhS-0.62 HPMC-BnS-0.57 HPMCAS

substituentsa

DSb

Cy Ph CyS CyS CyS CyS PhS BnS succinate acetate

0 0.57 0.63 0.42 0.57 0.72 0.99 0.62 0.57 S: 0.28 A: 0.57

Mn NMRc (kg/mol)

Mn SECd (kg/mol)

Mw SECd (kg/mol)

Đd

dn/dce (mL/mg)

Tg DSCf (°C)

17.8 18.2 17.0 18.9 20.8 24.1 19.3 19.2

11.8 16.9 17.2 17.9 19.8 21.3 23.5 17.9 19.5

15.7 26.2 26.2 26.0 28.3 30.8 33.3 25.3 28.5

1.33 1.56 1.52 1.45 1.43 1.44 1.42 1.41 1.46

0.146 0.085 0.098 0.092 0.089 0.094 0.099 0.129 0.106

132 120 124 111 100 94 94 100 96 121

a

The kind of ester substituents in each HPMC ester. All samples also contain two kinds of ether substituents, HPO and MeO. bDS of the ester substituents. The DS of substituted succinates in HPMC-(R-SA) were determined by 1H NMR spectroscopy (exemplified in Figure S2). cNumberaverage molar masses calculated based on the Mn of HPMC by SEC and the DS of the HPMC-(R-SA) by 1H NMR spectroscopy. The cellulose backbone was assumed to undergo no degradation or cross-linking during esterification or fractionation during purification. dNumber-average molar masses, weight-average molar masses, and dispersity as determined by SEC, which was equipped with a LS and a dRI detector. eDetermined by the dRI signal assuming 100% mass recovery. fApparent glass transition temperatures26 in the second heating scans at a rate of 10 °C min−1 by DSC.

Ph-S-SA, and Bn-S-SA) were synthesized using a one-step conjugate addition of a thiol to maleic anhydride (Scheme S2).20a From one batch of HPMC,22 we synthesized HPMC-(R-SA) samples with five different substituents but with similar DS of ∼0.6.23 We also made HPMC cyclohexylthiosuccinates with DS values between 0.4 and 1.0.24 The molecular and thermal data for HPMC and the HPMC-(R-SA) samples are summarized in Table 1. Using an aqueous SEC equipped with light scattering (LS) and differential refractive index (dRI) detectors, we determined Mw of HPMC to be 15.7 kg mol−1 and Đ to be 1.33 (Figure S3). HPMC samples with a rather low molar mass were used to target HPMC esters with low viscosity in organic solvents, a feature beneficial in controlling the particle size during spray drying.9b The DS of MeO and HPO per anhydro glucopyranose unit (AGU) was 1.91 and 0.25, respectively. The HPMC esters of substituted succinates are named as HPMC followed by the hydrophobic substituent of the anhydride and DS. For example, HPMC-CyS-0.57 stands for HPMC cyclohexylthiosuccinate with a DS of 0.57. All of these HPMC-(R-SA) showed monomodal elution curves by SEC in THF (Figure S4), from which Mw were determined to be 25− 33 kg mol−1 and Đ values were 1.4−1.6. In addition, we used AFFINISOL HPMCAS 912 G16 from The Dow Chemical Company as a reference material.25 All of the HPMC esters

9.0, 7.0, 5.0, 3.0, 2.0, 1.5, to 1.0 mg/mL until a clear solution was formed. The dissolution was aided by 15 min of sonication followed by magnetic stirring for at least 2 h. The solubility of phenytoin in the presence of polymers was tested by adding crystalline phenytoin into PBS (w/i SIF powder) with predissolved polymers. The concentration of polymers was 2.0 mg/mL, and the target concentration of phenytoin was 1.0 mg/mL. The mixture was allowed to equilibrate with magnetic stirring at 37 °C for 5 h. Undissolved phenytoin was removed by centrifuge at 13 000 g for 1 min, and aliquots were taken to determine the concentration of dissolved phenytoin by HPLC, similar to those in dissolution tests as described above.

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RESULTS Synthesis of HPMC Esters of Substituted Succinates. Similar to the synthesis of HPMCAS,21 we started with HPMC, used sodium acetate as the catalyst, and employed glacial acetic acid as the solvent to synthesize HPMC esters of substituted succinates, or HPMC-(R-SA). Five monosubstituted succinic anhydrides were used to prepare HPMC esters as shown in Scheme 1. Phenylsuccinic anhydride (Ph-SA) is commercially available, cyclohexylsuccinic anhydride (Cy-SA) was synthesized by dehydrating the dicarboxylic acid precursor (Scheme S1), and the three α-sulfur-substituted anhydrides (Cy-S-SA, 178

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Figure 1. SEM pictures of SDDs of phenytoin with (a) HPMCAS, (b) HPMC-Ph-0.63, and (c) HPMC-CyS-0.57 as the matrices at 10 wt % loading. The scale bars indicate 800 nm.

(HPMC-(R-SA) and HPMCAS) had Tg values between 94 and 124 °C and were soluble in acetone at a concentration of 2.0 wt %, and thus are well-suited for normal spray-drying conditions. SDDs with Phenytoin. All samples were spray-dried on a lab scale spray dryer. Initially we studied the effect of substituents and focused on HPMCAS and five HPMC-(RSA) samples with a DS of ∼0.6. At 10 wt % phenytoin loading,27 the SDDs with different polymeric matrices exhibited very similar bulk properties as revealed by scanning electron microscopy (SEM), powder X-ray diffraction (powder XRD), and differential scanning calorimetry (DSC). The SDDs resembled collapsed, grape-shaped particles by SEM (Figures 1 and S6). The size distribution of the particles was relatively broadthe larger particles were ca. 1−10 μm, and the smaller ones were ca. 20−100 nm. The average particle size ± one standard deviation was determined to be 660 ± 750, 670 ± 620, and 640 ± 750 nm for SDDs of HPMCAS, HPMC-Ph0.63, and HPMC-CyS-0.57, respectively (Figure S19). There were no apparent residual phenytoin crystals in the samples, which would exist as columnar particles with fine layered structures (Figure S5). The spray-dried dispersions presented broad, featureless powder XRD patterns (Figures 2 and S7). The combined SEM

Figure 3. DSC curves of SDDs with (a) HPMCAS, (b) HPMC-Ph0.63, and (c) HPMC-CyS-0.57 as the matrices at 10 wt % phenytoin loading. Shown here are the second heating scans. The arrows highlight glass transitions, and the Tg values were 78, 80, and 62 °C for the SDDs with HPMCAS, HPMC-Ph-0.63, and HPMC-CyS-0.57 as the matrices, respectively.

that the spray-dried particles with 10 wt % of phenytoin loading have excellent thermal stability up to 180 °C without any apparent macrophase separation. Therefore, similar to HPMCAS, the SDDs with HPMC-(R-SA) samples provided desired bulk properties in terms of sufficient drug loading and high Tg (i.e., 5−30 °C above common storage temperature, 40 °C8e). The performance of SDDs in solution, that is, how well SDDs achieve and maintain the in vitro supersaturation of phenytoin, reveals their potential to improve the in vivo bioavailability. In dissolution tests, all undissolved phenytoin was separated from the solution using a microcentrifuge method, and the concentration of dissolved phenytoin was monitored from 4 min to 6 h. From the dissolution profiles as shown in Figure 4 and summarized in Table 2, we observed three types of behavior from the six SDDs: (I) As exemplified by HPMCAS, the concentration of dissolved phenytoin achieved an initial maximum (cmax29), quickly decreased afterward, and plateaued at a value (c360min) that was much smaller than cmax. In the case of HPMCAS, the cmax was 970 μg/mL at 4 min, which corresponded to the dissolution of almost 100% of the loaded phenytoin and a 20fold improvement compared to the crystalline drug solubility alone in the aqueous buffer (48 μg/mL).30 AUC360min31 was 6.5 times as large as that of crystalline phenytoin. The SDDs with HPMC-Ph-0.63 and HPMC-PhS-0.62 as matrices exhibited similar behavior but provided smaller cmax, c360min, and AUC360min values than that with HPMCAS.

Figure 2. (From bottom to top) Powder XRD patterns of crystalline phenytoin and SDDs with HPMCAS, HPMC-Ph-0.63, and HPMCCyS-0.57 as the matrices at 10 wt % phenytoin loading.

and powder XRD evidence was consistent with amorphous solid dispersions containing insignificant content of crystalline phenytoin. The samples did not show any apparent demixing (e.g., cold crystallization) during the first heating scans (Figure S8), and the Tg values were in the range of 60−80 °C as determined by the second heating scans in DSC (Figure 3).28 The samples displayed homogeneous structures by SEM after thermal treatment on DSC (Figure S12). These data suggest 179

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was very effective at not only achieving rather high levels of supersaturation initially but also maintaining those concentrations and thus was very promising at improving the overall AUC360min. Type III was not very promising as the cmax was rather low and thus exhibited slight improvement of AUC360min. For the five HPMC-(R-SA) with similar DS but different substituents, the three sulfur-containing samples outperformed HPMCAS in terms of AUC360min. Effect of DS. We also studied the SDDs with matrices of HPMC-CyS with a DS of 0.42, 0.57, 0.72, and 0.99. At 10 wt % phenytoin loading, the samples showed bulk properties similar to those previously described samples as revealed by SEM (Figure S9), powder XRD (Figure S10), and DSC (Figure S11). However, the dissolution performance exhibited significant dependence on the DS (Figure 5 and Table 2). The

Figure 4. Dissolution profiles of SDDs at 10 wt % phenytoin loading. The polymeric matrices are HPMCAS and five HPMC-(R-SA) with a DS of ∼0.6 but different substituted succinates. The dissolution profile of crystalline phenytoin was also included as a reference. The target concentration of phenytoin was 1000 μg/mL. The samples were run in triplicate, and the data shown are the mean ± standard deviations.

(II) As exemplified by HPMC-CyS-0.57, the concentration of phenytoin also achieved rather high levels of supersaturation at early time points, but it stayed constant for a significant amount of time (1.5 h) before slowly decreasing; a plateau value was not achieved during the 6 h test. In the case of HPMC-CyS-0.57, the cmax was 740 μg/mL, somewhat smaller than that of HPMCAS, but the AUC360min was 2.1 × 105 min μg/mL, which was two times larger than that with HPMCAS. Similar behavior was observed in the SDD with HPMC-BnS0.57 as the matrix. (III) As in HPMC-Cy-0.57, the concentration of phenytoin increased over the course of 6 h, and cmax was 250 μg/mL, four times as much as that of crystalline phenytoin in the aqueous buffer. Correspondingly, the AUC360min was 8.9 × 104 min μg/ mL, which was smaller than the SDD with HPMCAS.32 Among the three types of dissolution performance, Type I allowed attainment of high levels of initial phenytoin supersaturation but was not very effective at maintenance and thus generated moderate enhancement of AUC360 min. Type II

Figure 5. Dissolution profiles of the SDDs at 10 wt % phenytoin loading. The polymeric matrices are HPMC cyclohexylthiosuccinates with a DS of 0.42, 0.57, 0.72, and 0.99. The target concentration of phenytoin was 1000 μg/mL, and the data shown are the mean ± standard deviations.

SDD with HPMC-CyS-0.42 as the matrix showed Type I behavior and was not effective at maintaining high levels of phenytoin supersaturation. As the DS increased to 0.72 and further to 0.99, the resulting SDDs presented Type II behavior, and the concentrations of dissolved phenytoin remained constant over 6 h (even longer than those from HPMC-CyS0.57). On the other hand, the cmax of these two samples were smaller than that of HPMC-CyS-0.57, which provided the highest AUC360 min among the four samples with varying DS. Effect of Drug Loading. SDDs at higher drug loading are desirable since less matrix material is incorporated for the same target drug concentration.33 We prepared SDDs with

Table 2. Dissolution Results of Crystalline Phenytoin and SDDs with Polymeric Matrices at 10 and 25 wt % Phenytoin Loadings 10 wt % polymer HPMCAS HPMC-Cy-0.57 HPMC-Ph-0.63 HPMC-PhS-0.62 HPMC-BnS-0.57 HPMC-CyS-0.57 HPMC-CyS-0.42 HPMC-CyS-0.72 HPMC-CyS-0.99

cmaxa (μg/mL) 970 250 620 910 550 740 890 550 410

25 wt %

c360minb (μg/mL)

AUC360minc (min μg/mL)

230 250 190 240 230 430 210 550 410

× × × × × × × × ×

1.1 8.9 8.0 1.2 1.5 2.1 1.2 1.9 1.4

5

10 104 104 105 105 105 105 105 104

EFd 6.5 5.3 4.8 7.0 8.9 13 7.0 12 8.5

cmaxa (μg/mL) 460 380 370 330 450 490 620 800 700

c360minb (μg/mL)

AUC360minc (min μg/mL)

EFd

190 180 130 130 190 200 190 240 290

× × × × × × × × ×

4.7 4.7 3.5 3.4 5.0 5.1 5.1 6.5 7.7

7.9 7.9 5.8 5.7 8.3 8.6 8.6 1.1 1.3

4

10 104 104 104 104 104 104 105 105

a

The maximum observed concentration of phenytoin. bConcentration of phenytoin at 360 min. cThe area under the drug concentration−time profile over the course of a 6 h dissolution test. dEnhancement factor defined as the ratio of AUC360min of a SDD to that of crystalline phenytoin. The cmax, c360min, and AUC360min of crystalline phenytoin was 49 μg/mL, 48 μg/mL, and 1.7 × 104 min μg/mL, respectively. 180

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HPMCAS and the nine HPMC-(R-SA) as listed in Table 1 as the matrices at 25 wt % phenytoin loading. These SDDs are also likely to be amorphous solid dispersions as revealed by SEM (Figure S13) and powder XRD (Figure S14). However, besides the glass transitions at ca. 70 °C, the SDDs also showed exothermic peaks at 140−170 °C during the first heating scans in the DSC analysis (Figure 6). Moreover, we observed

Gamorphous − Gcrystalline ≡ ΔGa → c ≈ RT ln

Samorphous Scrystalline

(1)

in which R is the gas constant, T is the absolute temperature, and S is the solubility. Further, ΔGa→c can be estimated from Tm and the heat of fusion of the crystalline drug, ΔHf, using the Hoffman equation35 as shown in eq 2. ΔGa → c = ΔHf

(Tm − T )T Tm 2

(2)

Using literature values,36 we calculated the Samorphous/Scrystalline of phenytoin to be 47.4 at 37 °C, which corresponds to an Samorphous of phenytoin of 1285 μg/mL in DI water (i.e., 27.1 μg/mL × 47.4).37 In other words, if the maximum solubility of amorphous phenytoin is achieved, the concentration of dissolved phenytoin would be 1280 μg/mL in DI water. However, the dissolved phenytoin above the crystalline solubility limit is thermodynamically unstable and has been well-documented to “desupersaturate” through crystallization.34 Among the nine samples investigated, three polymers (HPMCAS, HPMC-PhS-0.62, and HPMC-CyS-0.42) helped the resulting SDDs achieve a concentration of dissolved phenytoin close to 1000 μg/mL initially during the dissolution tests. More remarkably, the SDDs with four other polymers (HPMC-CyS-0.57, HPMC-CyS-0.72, HPMC-CyS-0.99, and HPMC-BnS-0.57) as the matrices effectively inhibited the desupersaturation at concentrations 8−15 times of the solubility of crystalline phenytoin (although the cmax was 30− 60% of the maximum solubility limit). Such supersaturation maintenance is critical in enhancing the AUC360min, which is related to the apparent in vitro bioavailability of phenytoin. The ability of the SDDs at 10 wt % loading to achieve high levels of supersaturation depends on the solubility of the polymeric matrix in phosphate-buffered solution (PBS). The diameter of the SDDs was in the range of 10 μm or less, much smaller than those typically produced industrially (50−100 μm). The specific surface areas of the SDDs are so large that the drug release is likely to be a dissolution-limited process, rather than a diffusion-limited process as in pressed tablets.10f Indeed, all of the SDDs at 10 wt % loading showed cmax within 20 min, and most occurred at the earliest time point of observation, 4 min. Besides, we found that the cmax of a SDD was quantitatively correlated with the solubility of the matrix polymer in PBS at 37 °C (Table S2). For example, the three SDDs that showed cmax of almost 100% of the target concentration at 4 min were from polymers (HPMCAS, HPMC-PhS-0.62, and HPMC-CyS-0.42) that had a solubility of more than 9 mg/mL in PBS, the administered polymer concentration at 10 wt % phenytoin (i.e., full dissolution of the polymer and the phenytoin in the SDD would give a polymer concentration of 9 mg/mL and a phenytoin concentration of 1 mg/mL or 1000 μg/mL). Correspondingly, such SDDs were completely dissolved, and clear solutions were formed at 4 min in the dissolution tests. In contrast, the solubility of HPMCCyS-0.57 in PBS was ∼7 mg/mL, lower than the administered polymer concentration. As such, we observed some undissolved SDD powder in solution, and cmax was thus smaller. As the DS increased from 0.42 to 0.99, the solubility of the HPMC-CyS polymers and the cmax values of corresponding SDDs decreased monotonously. The decreased solubility of HPMC-CyS was consistent with smaller hydrophilic hydroxyl content and larger

Figure 6. (From bottom to top) DSC curves of SDDs with HPMCAS, HPMC-Ph-0.63, and HPMC-CyS-0.57 (as representative materials) at 25 wt % phenytoin loading. The temperature was increased from 22 to 180 °C at a rate of 2.5 °C min−1. The black arrows and yellow circles highlight the glass transitions of the SDD and the cold crystallization of phenytoin, respectively.

columnar particles protruding from the surface of such thermally treated samples by SEM (Figure S15). These results led us to conclude that phenytoin in SDDs at 25 wt % loading underwent cold crystallization and thus had reduced thermal stability compared to those at 10 wt % loading. The SDDs with 25 wt % phenytoin loading also showed reduced dissolution performance compared to those at 10 wt % loading. As shown in Figure S16 and Table 2, all SDDs with 25 wt % phenytoin loading exhibited Type I behavior, and they were less effective at maintaining the high supersaturation levels of phenytoin achieved at 4 min than those with 10 wt % loading. For each polymeric matrix, the AUC360min value of the resulting SDD at 25 wt % loading was smaller than that at 10 wt % loading. Nevertheless, similar to the results obtained at 10 wt % loading, at DS of ∼0.6 the CyS samples provided the highest AUC360min value among the five HPMC-(R-SA) at 25 wt % phenytoin loading. Moreover, the c360min value monotonously increased as the DS increased from 0.57 to 0.99 in HPMC-CyS samples. As a result, the AUC360min value gradually increased as the DS increased from 0.57 to 0.99 in HPMC-CyS samples. The best performer, HPMC-CyS-0.99, provided a 65% increase of the AUC360min value compared to HPMCAS at 25 wt % loading, which was even 20% higher than HPMCAS at 10 wt % phenytoin.

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DISCUSSION Amorphous drugs have been well-recognized to exhibit markedly higher solubility than their crystalline counterparts.34 It is thermodynamically more favorable to solubilize the amorphous form of a drug than the crystalline form. The theoretical solubility ratio of an amorphous drug to the crystalline counterpart is related to the difference in the free energy of the two forms, ΔGa→c, as shown in eq 1,34 181

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BnS-0.57 maintained the cmax for prolonged time (1.5 h) and thus provided remarkable increase of AUC360 min compared to HPMCAS. On the other hand, those samples that do not contain sulfur (HPMC-Ph-0.63) or have an electron-withdrawing group connected with sulfur (HPMC-PhS-0.62) showed an induction time of less than 4 min. These results suggest that a combination of thioethers and weak electronwithdrawing groups is important for effective inhibition of nucleation. As discussed by Anwar et al., the nucleation event contains the formation of amorphous clusters of solute molecules followed by the reorganization into crystalline structures.43 Simulation results suggest that successful nucleation inhibitors have stronger affinity for the solute molecules than the solvent. Important parameters include the solubility parameter,44 a measure of the intermolecular cohesive interactions. For example, Ilevbare et al. found that successful polymeric nucleation inhibitors have similar solubility parameters to that of the drug (e.g., celecoxib, efavirenz, or ritonavir) in question.42a On the other hand, a rather significant amount of polymers were needed for effective nucleation inhibition as all of the SDDs at 25 wt % phenytoin loading (i.e., 75 wt % polymer) showed an induction time of less than 4 min. Similar concentration effects have been reported and were ascribed to the less-effective dispersion of phenytoin with the polymeric matrix in bulk as well as the decreased interaction between polymers and the solute clusters in solution.10b,38,45 Nevertheless, c360min and AUC360min monotonously increased as the DS increased from 0.57 to 0.99 in HPMC-CyS for the SDDs at 25 wt % loading. A higher DS of substituted succinates may allow more effective reduction of the growth of phenytoin crystals. Polymer additives are known to adsorb onto crystal surfaces via van der Waals forces and electrostatic forces, and this adsorption sterically prevents the diffusion of solute molecules onto the growing front.10i,45,46 At higher DS, the higher cyclohexylthio content may allow better coverage of the phenytoin crystals, and the ionized carboxylic acids may enhance colloidal stability of these nanoaggregates. Similarly, Miller et al. found that HPMCAS prepared from HPMC K-grade,47 compared to those from HPMC E-grade, had higher DS of acetate and succinate, and was more effective at inhibiting the desupersaturation of phenytoin.48 Ilevbare et al. also found that cellulose esters with higher DS of adipates gave a slower growth rate of ritonavir.10f Furthermore, the ability of cellulose adipates to inhibit drug nucleation42a and growth49 depended significantly on the pH of the dissolution media, which indicates the importance of ionizable carboxylic acid groups.

hydrophobic cyclohexylthio and pH-responsive carboxylic acid at a higher DS. Increased Scrystalline commonly occurs in systems using surfactants5 or cyclodextrin6 as additives, in which some extra drug is thermodynamically stabilized in solution through either the hydrophobic interaction with the micellar cores or by complexation. As such, the degree of supersaturation is decreased, and thus the thermodynamic force for crystallization is lowered. However, the measured solubility of phenytoin in PBS in the presence of selected cellulosic polymers was roughly the same as or in several cases lower than that in the absence of any polymeric additives (Table S3), and these data argued against this mechanism. Instead, we posit that the crystallization (i.e., nucleation and growth) of phenytoin was effectively retarded through the formation of colloidally stable polymer− drug nanoaggregates in solution. Polymer−drug colloids of 20− 300 nm,8e whose detailed structure is not yet fully understood, have been documented in supersaturated solutions from amorphous solid dispersions. Examples include HPMC with felodipine38 and HPMCAS with a variety of drugs including griseofulvin,10c,g danazol,10g and progesterone.39 Indeed, we observed a persistent bluish tinge in the solutions that showed prolonged maintenance of the supersaturation of phenytoin (e.g., the SDDs with HPMC-CyS-0.72 at 10 wt % loading), consistent with the existence of colloidally stable nanostructures. On the other hand, those solutions that showed rapid desupersaturation of phenytoin were essentially clear without apparent bluish tinge (e.g., the SDDS with HPMCAS at 10 wt % loading). It is likely that inhibiting the nucleation of phenytoin is critical in preventing the desupersaturation of these SDDs in the dissolution media. At a target concentration of 1000 μg/ mL, the SDDs with HPMC-CyS-0.72 and HPMC-CyS-0.99 as matrices at 10 wt % phenytoin loading showed no concentration decrease for at least 6 h, corresponding to significantly prolonged inhibited nucleation of crystalline phenytoin. On the other hand, the induction time in the presence of HPMCAS was less than 4 min, after which the fast growth of phenytoin crystals lead to rapidly decreased concentration. As the cmax values were significantly different in these three SDDs, it may be difficult to directly compare the inhibition ability of the matrix polymers. Dissolution tests at a lower target concentration, 500 μg/mL, gave similar results (Figure S17 and Table S4). The cmax of the three SDDs were similar, corresponding to similar initial degrees of supersaturation. The induction time in the presence of HPMCAS increased to ∼20 min but was still much shorter than those with HPMC-CyS-0.72 or HPMC-CyS-0.99, which were at least 6 h. Previous studies show that cellulosics of moderate hydrophobicity are often the most effective at both inhibiting nucleation and slowing crystal growth. For example, Alonzo et al. determined that the nucleation and growth rates of felodipine were slowed by a factor of 1000 and 2, respectively, in the presence of HPMC compared to those without any additives.40 Alternatively, the hydrophilic and synthetic polymer PVP typically influences growth to a greater extent than, and independently from, nucleation, as seen in the cases of bicalutamide,41 ritonavir,42 and efavirenz.42 These results clearly suggest the importance of suppressing nucleation in increasing the bioavailability of crystalline hydrophobic drugs using cellulosic polymers. Among the five HPMC esters of monosubstituted succinates with a similar DS of ∼0.6, only HPMC-CyS-0.57 and HPMC-

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CONCLUSION HPMC esters of substituted succinates are structurally analogous to HPMCAS and simultaneously contain hydrophobic, hydrophilic, and pH-responsive substituents. These multifunctional characteristics are important in enhancing the aqueous solubility of crystalline hydrophobic drugs. Starting from one HPMC, we synthesized five HPMC esters of monosubstituted succinates with a similar DS of ∼0.6 and HPMC cyclohexylthiosuccinates with varying DS of 0.4−1.0. In the formulated spray-dried dispersions with phenytoin, we found no significant difference in the bulk properties of samples with different matrices using SEM, powder XRD, and DSC. However, individual polymers exhibited vastly different performance in enhancing the supersaturation of phenytoin in solution at 10 wt % loading. The release of loaded phenytoin 182

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(3) Serajuddin, A. T. M. Salt formation to improve drug solubility. Adv. Drug Delivery Rev. 2007, 59, 603−616. (4) (a) Stella, V. J.; Nti-Addae, K. W. Prodrug strategies to overcome poor water solubility. Adv. Drug Delivery Rev. 2007, 59, 677−694. (b) Rautio, J.; Kumpulainen, H.; Heimbach, T.; Oliyai, R.; Oh, D.; Jarvinen, T.; Savolainen, J. Prodrugs: design and clinical applications. Nat. Rev. Drug Discovery 2008, 7, 255−270. (5) (a) Gursoy, R. N.; Benita, S. Self-emulsifying drug delivery systems (SEDDS) for improved oral delivery of lipophilic drugs. Biomed. Pharmacother. 2004, 58, 173−182. (b) Gao, P.; Morozowich, W. Development of supersaturatable self-emulsifying drug delivery system formulations for improving the oral absorption of poorly soluble drugs. Expert Opin. Drug Delivery 2006, 3, 97−110. (c) Kohli, K.; Chopra, S.; Dhar, D.; Arora, S.; Khar, R. K. Self-emulsifying drug delivery systems: an approach to enhance oral bioavailability. Drug Discovery Today 2010, 15, 958−965. (6) (a) Loftsson, T.; Brewster, M. E.; Masson, M. Role of cyclodextrins in improving oral drug delivery. Am. J. Drug Delivery 2004, 2, 261−275. (b) Carrier, R. L.; Miller, L. A.; Ahmed, M. The utility of cyclodextrins for enhancing oral bioavailability. J. Controlled Release 2007, 123, 78−99. (c) Loftsson, T.; Brewster, M. E. Cyclodextrins as functional excipients: Methods to enhance complexation efficiency. J. Pharm. Sci. 2012, 101, 3019−3032. (7) (a) Rabinow, B. E. Nanosuspensions in drug delivery. Nat. Rev. Drug Discovery 2004, 3, 785−796. (b) Kesisoglou, F.; Panmai, S.; Wu, Y. H. Nanosizing - Oral formulation development and biopharmaceutical evaluation. Adv. Drug Delivery Rev. 2007, 59, 631−644. (c) Merisko-Liversidge, E. M.; Liversidge, G. G. Drug Nanoparticles: Formulating Poorly Water-Soluble Compounds. Toxicol. Pathol. 2008, 36, 43−48. (d) Merisko-Liversidge, E.; Liversidge, G. G. Nanosizing for oral and parenteral drug delivery: A perspective on formulating poorly-water soluble compounds using wet media milling technology. Adv. Drug Delivery Rev. 2011, 63, 427−440. (8) (a) Leuner, C.; Dressman, J. Improving drug solubility for oral delivery using solid dispersions. Eur. J. Pharm. Biopharm. 2000, 50, 47−60. (b) Vasconcelos, T.; Sarmento, B.; Costa, P. Solid dispersions as strategy to improve oral bioavailability of poor water soluble drugs. Drug Discovery Today 2007, 12, 1068−1075. (c) Crowley, M. M.; Zhang, F.; Repka, M. A.; Thumma, S.; Upadhye, S. B.; Battu, S. K.; McGinity, J. W.; Martin, C. Pharmaceutical applications of hot-melt extrusion: Part I. Drug Dev. Ind. Pharm. 2007, 33, 909−926. (d) Repka, M. A.; Battu, S. K.; Upadhye, S. B.; Thumma, S.; Crowley, M. M.; Zhang, F.; Martin, C.; McGinity, J. W. Pharmaceutical applications of hot-melt extrusion: Part II. Drug Dev. Ind. Pharm. 2007, 33, 1043− 1057. (e) Friesen, D. T.; Shanker, R.; Crew, M.; Smithey, D. T.; Curatolo, W. J.; Nightingale, J. A. S. Hydroxypropyl Methylcellulose Acetate Succinate-Based Spray-Dried Dispersions: An Overview. Mol. Pharmaceutics 2008, 5, 1003−1019. (9) (a) Re, M. I. Formulating drug delivery systems by spray drying. Drying Technol. 2006, 24, 433−446. (b) Vehring, R. Pharmaceutical particle engineering via spray drying. Pharm. Res. 2008, 25, 999−1022. (c) Ormes, J. D.; Zhang, D.; Chen, A. M.; Hou, S.; Krueger, D.; Nelson, T.; Templeton, A. Design of experiments utilization to map the processing capabilities of a micro-spray dryer: particle design and throughput optimization in support of drug discovery. Pharm. Dev. Technol. 2013, 18, 121−129. (10) (a) Tanno, F.; Nishiyama, Y.; Kokubo, H.; Obara, S. Evaluation of hypromellose acetate succinate (HPMCAS) as a carrier in solid dispersions. Drug Dev. Ind. Pharm. 2004, 30, 9−17. (b) Konno, H.; Handa, T.; Alonzo, D. E.; Taylor, L. S. Effect of polymer type on the dissolution profile of amorphous solid dispersions containing felodipine. Eur. J. Pharm. Biopharm. 2008, 70, 493−499. (c) Curatolo, W.; Nightingale, J. A.; Herbig, S. M. Utility of Hydroxypropylmethylcellulose Acetate Succinate (HPMCAS) for Initiation and Maintenance of Drug Supersaturation in the GI Milieu. Pharm. Res. 2009, 26, 1419−1431. (d) Rumondor, A. C. F.; Stanford, L. A.; 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, 2599−2606. (e) Alonzo, D. E.;

from the SDDs depended on the solubility of the polymeric matrices in the dissolution media. Phenytoin has a strong crystallization tendency, and the ability to maintain supersaturation is crucial in improving the total area under the concentration−time curve in the dissolution tests, an indicator of bioavailability. HPMCAS was found to be effective at achieving initially but not at maintaining such high phenytoin supersaturation. Conversely, several HPMC esters of monosubstituted succinates not only achieved rather high initial supersaturation but also effectively maintained it for prolonged time (varying from 1.5 h to more than 6 h). The most successful systems contained a sufficiently high DS and a combination of thioether and weak electron-withdrawing groups as the substituents. The maintenance was largely ascribed to the effective nucleation inhibition of phenytoin from supersaturated solutions. Further, such performance was not observed at 25 wt % loading, and all SDDs showed an induction time of less than 4 min as the administered concentration of polymer was significantly reduced. Nonetheless, HPMC cyclohexylthiosuccinates with higher DS gave larger total areas under the curve, which are attributed to the more effective retardation of crystal growth and nucleation.

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ASSOCIATED CONTENT

S Supporting Information *

Reactivity of monosubstituted succinic anhydrides toward hydrolysis, additional synthetic schemes, 1H NMR spectra and SEC curves of HPMC and HPMC esters, SEM of crystalline phenytoin, additional SEM, DSC, powder XRD, and dissolution test data of SDDs, solubility of HPMC esters in PBS, and solubility of phenytoin in PBS in the absence and presence of several HPMC esters. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by The Dow Chemical Company (Dow) through Agreement 224249AT with the University of Minnesota. We gratefully acknowledge helpful discussions with Dr. Steven J. Guillaudeu, Dr. Robert L. Schmitt, and Dr. William Porter III at Dow. We also thank Lindsay M. Johnson for helpful discussions, carrying out the elemental analysis of Cy-S-SA, and determining the residual solvent content of SDDs. Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (http://www.mrfn.org).

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REFERENCES

(1) Hauss, D. J. Oral lipid-based formulations. Adv. Drug Delivery Rev. 2007, 59, 667−676. (2) (a) Lipinski, C. A. Drug-like properties and the causes of poor solubility and poor permeability. J. Pharmacol. Toxicol. Methods 2000, 44, 235−249. (b) 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. 2001, 46, 3−26. 183

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(17) USP32-NF27 Monograph: Hypromellose Acetate Succinate; United States Pharmacopeia: Rockville, MD, 2009. (18) Bergmeier, S. C.; Ismail, K. A. Synthesis of Monosubstituted Succinic Acids from tert-Butylsuccinate. Synthesis-Stuttgart 2000, 10, 1369−1371. (19) Rowley, J. M.; Lobkovsky, E. B.; Coates, G. W. Catalytic Double Carbonylation of Epoxides to Succinic Anhydrides: Catalyst Discovery, Reaction Scope, and Mechanism. J. Am. Chem. Soc. 2007, 129, 4948−4960. (20) (a) Zienty, F. B.; Vineyard, B. D.; Schleppnik, A. A. BaseCatalyzed Addition of Thiols to α,β-Unsaturated Anhydrides. J. Org. Chem. 1962, 27, 3140−3146. (b) Kaydos, J. A.; Smith, D. L. Preparation of dimethyl 2-phenylthiomaleate dimethyl 2-phenylthiofumarate, and their sulfoxides. J. Org. Chem. 1983, 48, 1096−1099. (21) Onda, Y.; Muto, H.; Maruyama, K. Ether-ester Derivatives of Cellulose and Their Applications. U.S. Patent US4226981, 1980. (22) METHOCEL E3 Premium LV from the The Dow Chemical Company (Midland, MI). Trademark of The Dow Chemical Company. (23) The anhydrides exhibited different reactivity towards alcoholysis−thiolated succinic anhydrides (e.g., Ph-S-SA and Bn-S-SA) are more reactive towards alcoholysis than the nonthiolated ones (e.g., PhSA) due to the electron-withdrawing thioethers at the α-position, as demonstrated in the model reactions with isopropyl alcohol (Supporting Information). Therefore, we adjusted the ratio of the anhydride to hydroxyls in the feed and kept the rest of the parameters (e.g., the concentration of hydroxyls, the amount of NaOAc) the same for different HPMC esters with the same target DS. The synthetic details are listed in Table S1. (24) The esterification is an equilibrium reaction, and we simply adjusted the ratio of the anhydride to the alcoholic hydroxyls on HPMC in the feed to target a specific degree of esterification. The synthetic details are listed in Table S1. (25) The DS of MeO and HPO of the HPMCAS was 1.94 and 0.25, respectively. The average number of unreacted hydroxyls per AGU unit was 0.21. (26) This method was likely to underestimate the Tgs as some degradation of HPMC and HPMC esters may occur at elevated temperatures and the resulting degraded product would act as plasticizers. The chemical identity and amount of the degraded product and quantitative analysis of their effect on the measured Tgs are currently under investigation. (27) The composition of a SDD is expressed as the weight fraction of the drug in the total solids. For example, at 10 wt % drug loading, the composition of an SDD is 1:9 of drug to polymer by mass. (28) The Tg values as predicted by Fox equation were all overestimates compared to those experimentally obtained values. The employed Tg value of phenytoin was 71 °C (ref 8e). The discrepancy was likely due to the small amount of residual acetone or water that acted as a plasticizer in the SDDs. (29) Defined as the maximum observed drug concentration. (30) The measured concentration of crystalline phenytoin in the aqueous buffer at 37 °C is larger than the literature value of its equilibrium solubility in water, 27.1 μg/mL as in ref 13. The increase is largely caused by the stabilization of extra phenytoin in the lipids as 0.5 wt % of simulated intestinal fluid (SIF) powder was added in the solution. (31) Defined as the area under the drug concentration−time profile over the course of a 6-h dissolution test. (32) cmax is related with the degree of supersaturation of phenytoin in solution, which is further related with the nucleation rate (ref 42a). Dissolution tests of SDDs at varying target concentrations showed that, if cmax was sufficiently low (e.g., 250 μg/mL), no apparent desupersaturation was observed within 6 h in those with HPMCAS matrix at 10 wt % phenytoin loading (Figure S18 and Table S5). Here we arbitrarily chose 300 μg/mL as the critical cmax between Type II and Type III behavior, which corresponds to an AUC360 min of 1.1 × 105 min μg/mL if no apparent decrease happened in the following 6 h, a

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value that is equal to that of the SDD with HPMCAS matrix at 10 wt % loading at a target concentration of 1000 μg/mL. (33) For the target concentration of 1000 μg/mL in the dissolution tests, the concentrations of added polymers are 9000 and 3000 μg/mL for SDDs with 10 and 25 wt % drug loadings, respectively. (34) Hancock, B. C.; Parks, M. What is the true solubility advantage for amorphous pharmaceuticals? Pharm. Res. 2000, 17, 397−404. (35) Hoffman, J. D. Thermodynamic driving force in nucleation and growth processes. J. Chem. Phys. 1958, 29, 1192−3. (36) ΔHf = 40.1 kJ mol−1, from ref 15. (37) It is practically difficult to prepare pure amorphous phenytoin by either quenching molten phenytoin (due to thermal decomposition) or spray-drying (incomplete compression of the crystalline form), and thus the solubility of purely amorphous phenytoin was not experimentally determined. (38) Alonzo, D. E.; Gao, Y.; Zhou, D. L.; Mo, H. P.; Zhang, G. G. Z.; Taylor, L. S. Dissolution and Precipitation Behavior of Amorphous Solid Dispersions. J. Pharm. Sci. 2011, 100, 3316−3331. (39) 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, 2009−2016. (40) Alonzo, D. E.; Raina, S.; Zhou, D.; Gao, Y.; Zhang, G. G. Z.; Taylor, L. S. Characterizing the Impact of Hydroxypropylmethyl Cellulose on the Growth and Nucleation Kinetics of Felodipine from Supersaturated Solutions. Cryst. Growth Des. 2012, 12, 1538−1547. (41) Lindfors, L.; Forssen, S.; Westergren, J.; Olsson, U. Nucleation and crystal growth in supersaturated solutions of a model drug. J. Colloid Interface Sci. 2008, 325, 404−413. (42) (a) Ilevbare, G. A.; Liu, H.; Edgar, K. J.; Taylor, L. S. Maintaining Supersaturation in Aqueous Drug Solutions: Impact of Different Polymers on Induction Times. Cryst. Growth Des. 2013, 13, 740−751. (b) Ilevbare, G. A.; Liu, H. Y.; Edgar, K. J.; Taylor, L. S. Impact of Polymers on Crystal Growth Rate of Structurally Diverse Compounds from Aqueous Solution. Mol. Pharmaceutics 2013, 10 (6), 2381−2393. (43) Anwar, J.; Boateng, P. K.; Tamaki, R.; Odedra, S. Mode of action and design rules for additives that modulate crystal nucleation. Angew. Chem., Int. Ed. 2009, 48, 1596−1600. (44) Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, 2007; p 276. (45) Raghavan, S. L.; Trividic, A.; Davis, A. F.; Hadgraft, J. Crystallization of hydrocortisone acetate: influence of polymers. Int. J. Pharmaceutics 2001, 212, 213−221. (46) Hasegawa, A.; Taguchi, M.; Suzuki, R.; Miyata, T.; Nakagawa, H.; Sugimoto, I. Supersaturation mechanism of drugs from solid dispersions with enteric coating agents. Chem. Pharm. Bull. 1988, 36, 4941−50. (47) Commercially available under the trademark of METHOCEL from The Dow Chemical Company. The DS of methoxy in HPMC of K grade is 1.15−1.50 and thus allows a total DS of acetate and succinate of 1.50−1.85 in the resulting HPMCAS. In comparison, the DS of methoxy in HPMC of E grade is 1.80−1.95, which allows a maximum total DS of acetate and succinate of 1.05−1.20. (48) Miller, W. K.; Lyon, D. K.; Friesen, D. T.; Caldwell, W. B.; Vodak, D. T.; Dobry, D. E. Hydroxypropyl methyl cellulose acetate succinate with enhanced acetate and succinate substitution. World Patent WO2011159626A1, 2011. (49) Ilevbare, G. A.; Liu, H.; Edgar, K. J.; Taylor, L. S. Inhibition of solution crystal growth of ritonavir by cellulose polymers - factors influencing polymer effectiveness. CrystEngComm 2012, 14, 6503− 6514.

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