Design of Tunable Multicomponent Polymers as Modular Vehicles To

Article pubs.acs.org/Macromolecules

Design of Tunable Multicomponent Polymers as Modular Vehicles To Solubilize Highly Lipophilic Drugs Jeffrey M. Ting,† Tushar S. Navale,‡ Frank S. Bates,*,† and Theresa M. Reineke*,‡ †

Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: Synthetic and natural polymers hold tremendous potential to improve therapeutic potency, bioavailability, stability, and safety through aiding the solubility of lipophilic drug candidates that may otherwise be clinically inaccessible. For the leading pharmaceutical delivery method (oral administration), one such approach involves maintaining drugs in an amorphous, nonequilibrium state using spraydried dispersions (SDDs). However, few well-understood vehicles exist, and available formulations employ Edisonian approaches without regard to examining chemical, thermodynamic, and kinetic phenomena. Herein, we present a rational approach to study polymer−drug interactions with a multicomponent polymer platform, inspired by hydroxypropyl methylcellulose acetate succinate (an excipient increasingly utilized as a delivery vehicle). The controlled syntheses of these modular analogs were strategically defined with (i) hydroxypropyl, (ii) methoxy, (iii) acetyl, (iv) succinoyl, and (v) glucose groups to tune the amphiphilicity balance (i−v), ionization near gastrointestinal pH levels (iv), hydrogen bonding (i, iii, iv, v), and glass transition temperature (v). We examined how polymer architecture produces amorphous SDDs with a highly hydrophobic drug model (probucol, log P = 8.9). Dissolution experiments revealed dramatic differences in bioavailability as a function of polymeric chemical specificity. We identify chemically driven interactions as crucial ingredients for facilitating amorphous phase behavior and supersaturation maintenance. In particular, increasing the fraction of ionizable carboxylic acid moieties and selective deprotection of glucose acetates into hydroxyls established stabilizing ionic character and polar interactions. Our results show the utility of rationally designed polymer platforms, which we can precisely tune via monomer selection and functionality, as direct handles for elucidating important structure−property relationships in oral delivery.

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INTRODUCTION Over the past decades, technological advances in combinatorial chemistry and high-throughput screening of new molecular entities (NMEs) have revolutionized the drug discovery landscape. These methods have enabled an unprecedented growth of newly discovered compound libraries and, consequently, increased the number of potential drug candidates reaching the formulation stage as active pharmaceutical ingredients (APIs).1,2 However, this success in the field of drug discovery has not translated into higher approval rates for new drug applications, which remarkably remain equivalent to that of 50 years ago.3 Enormous costs associated with investment and bringing a new drug to market further solidify the stagnancy in therapeutic innovation.4,5 Together, the high attrition rates of NMEs coupled with unfavorable comparative returns in R&D productivity represent key therapeutic and economic bottlenecks in the pharmaceutical pipeline. Oral drug delivery strategies offer the simplest way to administer drugs in solid dose form with high patient compliance. It has been shown that 40−60% of potential drug candidates have poor aqueous solubility in the gastrointestinal (GI) tract, limiting their systemic bioavailability.6,7 © XXXX American Chemical Society

The majority of these materials are classified as Biopharmaceutical Classification System (BCS) Class II compounds8 or materials with high GI permeability but low solubility. Solid dispersions (solid/solid mixtures of a drug and encapsulating excipient) have the ability to increase the shelf life and bioavailability of BCS Class II drugs. These materials are simple to produce, amenable to scale-up production, and broadly applicable in solubilizing all types of drug molecules.9 Spray drying is a common method of formulating polymer-based dispersions (known as spray-dried dispersions, or SDDs), where drug molecules are distributed throughout a carrier polymer matrix in a thermodynamically more soluble, albeit energetically unstable, amorphous state. Polymer−drug interactions stabilize the drug molecules by inhibiting nucleation and crystal growth, which returns the drug to its less soluble and bioavailable form that can affect toxicity and undesirable side effects. Thus, selection of an appropriate polymer to Received: September 4, 2014 Revised: September 12, 2014

A

dx.doi.org/10.1021/ma501839s | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Multicomponent Polymers (A) Prepared Using RAFT Polymerization, Based on the (B) Pendant Functional Groups of HPMCAS,a and Subsequent (C) Selective Deprotection Reaction

a

This depiction of the chemical structure of HPMCAS does not represent the actual regiochemistry. The pendant methoxy, hydroxypropyl, acetyl, succinoyl, and glucose groups along the cellulosic backbone are statistical in positioning.

in HPMCAS due to its overall heterogeneity, i.e., ill-defined length, cross-linking, and placement of functional groups along the polymer molecule. In this study, we aimed to create a vehicle platform that can be customized in a well-defined manner with similar chemical attributes to HPMCAS. We hypothesized that the judicious incorporation of hydroxypropyl, methoxy, acetyl, succinoyl, and glucose monomers into polymers with controlled length (Scheme 1) can provide insight into interactions that (i) prevent drug crystallization and precipitation, (ii) achieve high drug release, and (iii) maintain drug supersaturation. SDDs were prepared and characterized with a BCS Class II model drug, probucol. This anti-hyperlipidemic drug was developed in the 1970s to treat coronary artery disease after exhibiting potent antioxidant and anti-inflammatory properties; despite its discontinued clinical use in 1995, probucol contains unusual pharmacological properties that have sustained therapeutic interest.16 As illustrated graphically in Figure 1, we demonstrate how tuning simple structural variables in polymer vehicles can lead to extraordinary enhancements in drug dissolution and bioavailability, thereby challenging the traditional, indiscriminate mass-screening nature of the pharmaceutical sector with a paradigm shift toward formulating rationally constructed excipients.

combat drug crystallization is critical to the practical utility of SDDs. While polymer excipients have an immense and impactful role in the pharmaceutical development pipeline, few vehicles are available for screening at the formulation stagearound a dozen solid dispersions are commercially available on the market.10 Indeed, of those available, little is known about general structure−activity relationships for each API structure. Hydroxypropyl methyl cellulose acetate succinate (HPMCAS) has been shown to be leading excipient platform for API storage and delivery. For example, Curatolo and co-workers screened a library of 41 excipients for promoting lipophilic drug solubility and concluded that HPMCAS SDDs were generally better at preventing drug crystallization and establishing in vitro supersaturation as compared to common precipitation inhibitors (e.g., cellulosics, Pluronics, and surfactants).11 Additionally, HPCMAS has a demonstrated nonclinical safety record with the U.S. Food and Drug Administration, which supports its use under in vivo and clinical settings. Qian et al. compared the pharmacokinetic performance of HPMCAS to another common drug delivery excipient, poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP−VA), in dogs and found that HPMCAS SDDs better maintained drug supersaturation levels and minimized drug crystallization.12 Although exemplary in performance for many drugs, several key factors limit HPMCAS as a universal excipient. As it is industrially produced, HPMCAS exhibits high material dispersity, rendering its physiochemical properties challenging to characterize13,14 and even more difficult to optimize compared to synthetic polymer counterparts. Because of its ionizable succinate groups at intestinal pH levels, the underlying mechanism responsible for its effectiveness against desupersaturation is speculated to be the formation of nanoaggregates and colloidal assemblies in solution,15 but this phenomenon remains difficult to quantify and understand

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RESULTS Polymer Synthesis and Characterization of HPMCAS Analogs. Previously, we have reported a systematic approach to prepare well-defined, multimonomeric statistical systems with precise molecular weights and chemical compositions using reversible addition−fragmentation chain transfer (RAFT) polymerization.17 RAFT chemistry is a type of reversibledeactivation radical polymerization that enables exquisite control of the polymer lengths (molecular weights) and B


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polymers containing acrylic acid have been studied in mucoadhesive drug delivery applications because the carboxylic acid group exhibits strong adhesiveness to GI tract tissues.19,20 We hypothesize that similar complexation−decomplexation behavior may occur with CEA-containing systems. In highly acidic settings, e.g., fasted or fed stomach conditions, intramolecular hydrogen bonding restricts polymer conformations to collapsed conformations; by comparison, in the higher pH levels of the upper GI tract, pendant carboxylic acid groups ionize and the complexes dissociate. GATA was synthesized from D-(+)-glucose in a series of three steps according a similar procedure to Mahkam et al. (Scheme S-1).21 The glucose provides another accepting source of hydrogen bonding, steric rigidity, and a high-Tg component to facilitate spray drying and prolong SDD shelf life. First, we conducted binary free-radical polymerizations for each monomer pair with initiator 2,2′-azobis(2-methylpropionitrile) (AIBN) in dimethylformamide (DMF) at 70 °C to calculate the reactivity ratios. We analyzed our five-component polymers with the predictive Walling−Briggs22 and Skeist23 models and concluded that our systems are statistical in nature, capturing the random spatial placement of functional groups as in HPMCAS (Supporting Information). Two RAFT polymerizations with a degree of polymerization (DP = total monomer concentration/chain transfer agent concentration) of 220 were c o n d u c t e d w i t h A I B N i n i t ia t o r a n d 4 - c y a n o - 4 (propylsulfanylthiocarbonyl)sulfanylpentanoic acid (CPP) chain transfer agent in DMF at 70 °C (see Figure S-4 for a representative 1H NMR spectrum). For the HPMCAS analogs, we included all five monomers, distinguishing the mole ratio of PAA to CEA as low (A-L) or high (A-H). We hypothesized that these monomers, which are analogous to the respective acetate and succinate groups in HPMCAS, impart its unique excipient properties in facilitating drug release and supersaturation maintenance by providing hydrophobic groups and anionic stability. A control homopolymer P(GATA) with a DP of 120 was also synthesized under the same reaction conditions. Furthermore, exact control of the polymer chemical composition enables us to explore the amphiphilic nature of HPMCAS. To investigate this in greater detail, we selectively deprotected the acetoxy groups on the sugar GATA groups using basecatalyzed hydrolysis in a MeOH/CHCl3 mixture (1:1, v/v) at 22 °C for ∼30 min, shown in Scheme 1. This reaction was

Figure 1. Schematic representation of the utilization of solid dispersion vehicles for oral drug delivery. A polymeric excipient platform with structurally tailorable architectures enables precise control over drug delivery and release upon patient administration.

dispersity.18 More importantly, control of monomeric incorporation for statistical polymers involves pairwise reactivity ratio measurements and predictive modeling. The reactivity ratio of a monomer is an intrinsic measure of its relative probabilities of self-propagation to cross-propagation under specific solvent and temperature conditions. These values govern the resultant chemical architecture for any number of monomers. We applied these principles for five acrylic monomers: methyl acrylate (MA), 2-carboxyethyl acrylate (CEA), 2-hydroxypropyl acrylate (HPA), 2-propylacetyl acetate (PAA), and glucose-6-acrylate-1,2,3,4-tetraacetate (GATA), as shown in Scheme 1. Monomers MA, CEA, HPA, PPA, and GATA reflect the functional moieties and statistical configuration along HPMCAS, with possible interactions as follows: MA provides hydrophobicity, CEA is ionizable near GI pH levels and acts as a hydrogen bond donor and acceptor, HPA increases hydrophilicity and can hydrogen bond, PAA contains a hydrophobic hydrogen bond acceptor group, and GATA tunes the glass transition temperature (Tg). Additionally,

Table 1. Molecular Characterization of the Acrylic HPMCAS Analogues % mol pol compg

Mn (kg/mol) system

total monomer conv

A-L DA-L A-H DA-H P(GATA) D-P(GATA)

0.92 0.92 0.81 0.81 0.80 0.80

a

calcd

b

34.6 27.5 28.9 23.2 38.6 23.6

NMR

SEC

Đ

32.0 26.4 39.4 23.4 21.9 18.5

30.6 −h 43.9 −h 25.8 −h

1.32 −h 1.09 −h 1.04 −h

c

d

e

f

solubility (mg/mL)

(MA/CEA/HPA,PAA/GATA)

0.08 10 0.07 0.1 insoluble 40

53/11/15/21 53/11/15/21 61/5/14/20 61/5/14/20 0/0/0/100 0/0/0/100

a Total monomer conversion, determined from 1H NMR of the reaction mixture. bCalculated Mn, determined by Mn = (sum of monomer molecular weight times measured polymer composition) × (conversion) × (degree of polymerization); the average of the HPA and PAA molar mass was used since their individual compositions could not be resolved. cDetermined by 1H NMR end-group analysis assuming one CTA end group per chain; the average of the HPA and PAA molar mass was assumed. dDetermined by SEC-MALS using tetrahydrofuran (THF) as the elutant at 25 °C with measured dn/dc values of 0.0726, 0.0495, and 0.0844 for A-L, A-H, and P(GATA), respectively. eDispersity, from SEC-MALS. fAssessed visually in phosphate buffer saline solution only (without FaSSIF). gPolymer composition by mole percent, calculated from 1H NMR of the isolated polymer; the molar composition of HPA and PAA were unable to be distinguished, so the total composition of HPA and PAA is reported. hUnavailable due to insolubility in THF.

C


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monitored with 1H NMR by tracking the disappearance of the GATA acetoxy peak at 2.10 ppm (Figure S-5). The replacement hydroxyl groups increases the polymer hydrophilicity in the deprotected form (DA-L, DA-H, and DP(GATA)). These systems serve as direct comparisons to the sugar-protected HPMCAS analogs. The molecular characterization of all six investigated polymers in Table 1 details the ability of RAFT polymerization to strategically tailor the length and spatial arrangement of statistical multimonomeric excipients. The number-average molecular weight (Mn) and dispersity (Đ) of the polymers were calculated by 1H NMR end-group analysis and size exclusion chromatography (SEC) using tetrahydrofuran (THF) as the mobile phase. The experimentally measured Mn values were in good agreement with low Đ, characteristic of RAFT polymerization. This demonstrates the comprehensive control in preparing uniform polymeric excipients with tunable molecular weights and chemical compositions. Additionally, the deprotected analogs all exhibited marked increase in aqueous solubility, supporting 1H NMR analysis of GATA acetate removal. Spray Drying and Solid-State Characterization. Spray drying is a traditional unit operation in process engineering, used to produce powders from solutions, slurries, emulsions, etc., by rapid removal of volatile solvents. Specifically, this atomization process involves the conversion of fluid into fine droplets through a nozzle with simultaneous exposure to a hot gas stream. Because of its relative simplicity, high efficiency, and scalability to kilogram quantities, spray drying has been historically used for applications such as coffee,24 paint pigments,25 and vitamins.26 Recently, spray drying has entered the pharmaceutical realm as a viable route to prepare bulk active ingredients by converting crystalline drugs into a kinetically trapped metastable amorphous form, increasing the apparent solubility with no decrease in the apparent permeability in the intestinal membrane.27 Hancock and Parks have shown that, thermodynamically, the amorphous state of drug compounds offers substantial solubility enhancement, though theoretical predictions may not reflect what is experimentally accessible depending on the drug molecule.28 Nevertheless, SDDs offer a distinct solubility advantage, so long as drugs are kinetically stabilized from transitioning back into their more-stable crystalline form. The framework of SDD preparation is established in particle engineering29 and corroborated with computational fluid dynamic simulations30 and design of experiments.31 Furthermore, the in vitro−in vivo correlation based on this supersaturation maintenance approach shows excellent agreement to a first approximation for most drugs.32 We used the six protected and deprotected polymers with BCS Class II drug probucol at 10, 25, and 50 wt % loading. Probucol has a calculated octanol/water partition coefficient (log P) and pKa value of 8.9 and 10.29, respectively.33 It has two melting points (125 and 116 °C), which Gerber et al. correlated to its polymorphs, Form I and Form II, respectively.34 In other words, this is a highly insoluble drug in aqueous settings with moderate crystal lattice strength. The chemical structure of probucol is shown in Figure 2. To prepare the sugar-protected SDDs, measured probucol was dissolved into a solution of acetone and A-L, A-H, or P(GATA) polymer. The resultant mixture was stirred for 24 h until a clear solution was obtained. Meanwhile, for sugardeprotected SDDs, probucol was dissolved into methanol for

Figure 2. Chemical structure of BCS Class II drug probucol.

D-AL and D-P(GATA), while a tetrahydrofuran/methanol (1:1 v/v) mixture was used for D-AH. For all samples, laboratory scale spray drying achieved 75−90% yield with the inlet temperature set at 80 °C, nitrogen gas flow rate at 12.8 SLPM, and solution feed rate at 0.65 mL/min. Minor loss of SDDs was attributed to the filter paper pore size. By thermogravimetric analysis, particles consistently contained ∼2 wt % residual solvent. Upon scale-up, we expect this to be minimized with optimized process control and more precise control of the spray-drying outlet temperature.35 We examined the particle size and morphologies of our prepared SDDs with scanning electron microscopy (SEM). Particle size directly affects dissolution performance: smaller particles have larger surface area, which consequently increases wettability and facilitates a rapid drug release.36 As seen in Figure 3A−C, SDDs at 10 wt % loading exhibited similar

Figure 3. SEM images of protected and deprotected SDDs for (A) AL and DA-L, (B) A-H and DA-H, and (C) P(GATA) and DP(GATA), respectively, with probucol at 10 wt % loading. All scale bars in (A)−(C) denote 3 μm. (D) SEM images of spray-dried HPMCAS at 10 wt % probucol and unprocessed probucol are shown with respective scale bars of 3 and 10 μm. D


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morphologies with polydisperse diameters. For comparison, Figure 3D shows HPMCAS at 10 wt % probucol loading and unprocessed probucol. Qualitatively, the particles showed spherical, discrete particles, while sugar homopolymers P(GATA) and D-P(GATA) gave rise to a wrinkled, folded morphology like HPMCAS. This is a result of a core/shell formation of high-Tg cellulosic excipients during the evaporation-induced atomization process.29 SEM images and tabulated size distributions of SDDs at higher drug loadings are provided in the Supporting Information. At 50 wt % loading, we observed the onset of coalescence visually and an increase in the particle size across all systems (Figure S-10 and Table S-3). We attribute this to the decreased quantity of stabilizing polymeric excipient in the microenvironment during the atomization process. Table 2 contains the particle size Table 2. Particle Size Distributiona for Prepared SDDs at 10 wt % Loading system

mean diameter (nm)

median diameter (nm)

interquartile range (nm)

A-L DA-L A-H DA-H P(GATA) D-P(GATA) HPMCAS probucol only

630 660 800 630 670 770 990 10400

470 370 730 520 390 440 550 8210

270−790 240−940 490−940 310−820 270−850 290−1190 300−1120 5760−12800

Figure 4. PXRD patterns of (A) A-L and (B) DA-L SDDs at 10, 25, and 50 wt % probucol loading, compared to spray-dried polymer (0 wt %) and neat probucol (100 wt %).

values of 11.10°, 13.68°, 16.68°, 17.26°, and 19.54°. The formation of the unstable secondary polymorph of probucol implies that the spray-drying process successfully deconstructed probucol’s crystal lattice packing. Similar PXRD results for probucol-based solid dispersions have been reported.37 Furthermore, the presence of polymers appears to stabilize this metastable polymorph of probucol from transitioning back into Form I, demonstrating polymer−drug miscibility throughout the particle matrix. To further probe the solid-state properties of the SDDs, various analytical techniques have been conventionally utilized, such as differential scanning calorimetry (DSC), dielectric spectroscopy, and other emerging technologies summarized concisely by Baird and Taylor.38 We employed modulated differential scanning calorimetry (MDSC) to analyze the homogeneity of drug molecules throughout the polymeric matrix and to quantify the amorphicity of probucol in the SDD. MDSC grants access to nonergodic DSC thermal traces and enables the complete separation of the apparent glass transition temperature (Tg) from other nonreversing processes, e.g., trace solvent evaporation or enthalpic relaxation endotherms. For SDDs, Tg is a common metric for determining the physical and chemical stability of the system in terms of the longevity of a drug’s shelf life against humidity or other plasticizing events.39 Babcock et al. suggest a general Tg guideline of 30 °C or greater to ensure low mobility upon storage.40 Conventionally, for DSC Tg studies thermal cycling is utilized; that is, samples are first annealed to remove their past thermal history and stresses for reproducibility.41 However, first heating curves for SDDs are reported herein because this captures the Tg of the material as prepared by spray drying and prevents alteration effects, such as plasticization from polymer degradation at elevated DSC temperatures. Royall et al. have shown how MDSC advantageously provided a noninvasive evaluation of Tg for amorphous drug saquinavir in the first heating scan, given a carefully chosen heating rate, amplitude, and period.42 Figure 5 shows a representative MDSC curve for A-L at all investigated probucol-loading levels. In the 50 wt % probucol thermal trace, the polymorphic transition of Form II probucol into the more thermodynamically stable Form I probucol is observed as a secondary exothermic peak. For our polymers, by second heating DSC scans, the Tg of AL, DA-L, A-H, and DA-H ranged from 47 to 70 °C, while the

a

Particles were randomly measured (n = 70). The distribution was not normal and skewed to the right, so the center of the data is estimated using the calculated interquartile range, or the spread between the smallest and largest value in the middle 50% of data.

distributions for these SDDs. The selective deprotection reaction did not appear to affect the size distribution, as the mean and median particle diameters were comparatively similar. For instance, A-L and DA-L SDDs had a mean particle diameter of 630 and 660 nm, respectively. To best capture the spread of the distribution and minimize outlier effects, the interquartile range (IQR) of each system was calculated. In the same example, A-L and DA-L exhibited a respective IQR of 270−790 and 240−940 nm. No bulk unprocessed probucol crystals were visually observed. Powder X-ray diffraction (PXRD) was employed to determine the presence of crystalline probucol in our prepared SDDs. If crystalline lattice arrangements are present in SDDs, distinct sharp peaks corresponding to the one-dimensional crystal diffraction pattern should appear in an otherwise broad scattering profile. Furthermore, comparison of peak positioning to calculated crystal patterns of probucol from single-crystal Xray studies34 enables polymorphic determination. We compared the 0, 10, 25, and 50 wt % probucol loaded particles PXRD patterns against neat crystalline probucol. This unprocessed probucol exhibits sharp peaks (2θ = 13.04°, 15.88°, 17.94°, 18.98°, and 22.38°) corresponding to the more-stable primary polymorph (Form I, Tm = 125 °C). In Figure 4, for the A-L and DA-L at 0, 10, and 25 wt % loaded samples, broad featureless diffraction patterns indicate that these SDDs are amorphous within resolution of PXRD detection. In comparison, at 50 wt % loading sharp crystalline diffraction peaks are observed, with shifted diffraction peaks akin to the secondary polymorph (Form II, Tm = 116 °C). For instance, A-L exhibits peaks at 2θ E


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workers reported similar MDSC findings with their novel amphiphilic copolymers with probucol and antiepileptic BSC Class II drug phenytoin.43 This is an unusual finding and the subject of ongoing investigation. Furthermore, with thermal analysis we can precisely calculate the amorphicity of our SDDs. Thus, we can calculate the fraction of crystalline drug initially present in particles using MDSC. The specific heat of fusion (ΔĤ m) for 100% crystalline probucol was calculated to be 68.4 J/g by integrating the endothermic peak at Tm = 126 °C using first heating DSC. Thus, the fraction of crystalline drug in each SDD was calculated by eq 1: % crystallinity = Figure 5. MDSC first heating curves for A-L SDDs showing the (A) reversing heat flow and (B) total heat flow at 0, 10, 25, and 50 wt % probucol loading. Arrows indicate the Tg of the SDD, determined to be 70, 67, 66, and 67 °C for A-L at 0, 10, 25, and 50 wt % probucol, respectively.

(ΔĤ m − ΔĤc)SDD (ΔĤ m)drug (fraction drug loading)

× 100 (1)

Here, the ΔĤ m and ΔĤ c of a completely amorphous SDD would be equivalent in magnitude. Physically, this means that for an amorphous SDD the energy shown in the endothermic peak corresponding to the heat of fusion was transferred from the exothermic cold crystallization. All MDSC results are tabulated in the Supporting Information. Figure 6 shows the

Tg values of the controls P(GATA) and D-P(GATA) were over 100 °C. The Tg of amorphous probucol was 25 °C upon second heating (Figure S-13); no specific exothermic crystallization peak, or enthalpy of cold crystallization (ΔĤ c), was observed during the first cooling cycle at 5 °C/min, verifying that the neat probucol as received is completely crystalline. Upon spray drying, all systems exhibited a single Tg by first heating MDSC, suggesting that probucol was homogeneously dispersed throughout the polymeric excipient matrix to a first approximation. All measured Tg values are reported in Table 3. In general, small drug molecules act as Table 3. Glass Transitions of All Systems Tga (°C) system

0 wt %

10 wt %

25 wt %

50 wt %

A-L DA-L A-H DA-H P(GATA) D-P(GATA)

70 53 62 47 111 124

67 55 58 53 106 122

66 57 59 55 87 119

67 59 59 57 75 108

Figure 6. Crystallinity content of SDDs across all prepared systems at 10, 25, and 50 wt % probucol loading. The fraction of crystalline drug was determined using the first heating MDSC total heat curve with eq 1. *These systems exhibited no crystallinity by MDSC.

a

For 0 wt % (or spray-dried polymer only), second heating DSC traces were used to calculate Tg. For all other loadings, first heating reversing MDSC traces were used to calculate Tg.

plasticizing agents that should effectively lower the overall Tg by increasing the free volume of the polymer system. For the sugar-protected A-L, A-H, and P(GATA) systems, this was consistent: the overall Tg decreased with increased drug loading. However, this trend was inverted (Tg increased with drug loading) for sugar-deprotected HPMCAS analogs DA-L and DA-H in SDDs. We attributed this relationship to the increased intermolecular hydrogen bonding between polymer and drug. Since hydroxyl groups constitute a significant portion of the sugar-deprotected polymers, both hydrogen bond donating and accepting capabilities are greatly enhanced upon deprotection. Interestingly, this was not the case for the D-P(GATA) SDDs, where additional drug decreased the Tg. We believed that probucol molecules disrupted intramolecular hydrogen bonding between the D-P(GATA) hydroxyl groups. Dalsin and co-

crystallinity content of our SDDs. At 10 wt % probucol, P(GATA) SDDs were the most effective among the protected series at keeping probucol amorphous. A-L and A-H SDDs maintained 96% and 86% of probucol in the amorphous state, respectively. Upon deprotection, the amorphicity was increased to a respective 100% and 93%. At increasingly higher drug loading, there is less available polymer to molecularly stabilize drug molecules. This is consistent across all systems at 25 and 50 wt % loading, with the exception of D-P(GATA). At 25 wt % loading this polymer was able to suppress crystallization, and at 50 wt % loading, only 15% of probucol had crystallized. We attributed this to the strong solid-state hydrogen bonding interactions between hydroxyl groups of the homopolymer and probucol. Finally, Fourier transform infrared (FTIR) spectroscopy revealed hydrogen bonding between probucol and the acrylic F


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O−H stretching vibrations. In comparison, all spray-dried polymer except for P(GATA) showed weak O−H stretching at 3600 cm−1. With the drug-loaded SDDs, this peak was shifted toward lower wavenumbers and broadened as the polymer content increases, indicative of hydrogen bonding.44 For instance, at 10 wt % probucol loading in Figure 7, the O−H peak was lowered to 3510 cm−1 with significant band broadening. We can qualitatively classify the interspecies hydrogen bonding using rules proposed by Eerdenbrugh and Taylor for polymers and drugs.45 The hydroxyl groups on probucol act as medium acceptors and strong donors of hydrogen-bonding to (1) pendant acetate carbonyl groups as medium acceptors on PAA and GATA, (2) carboxylic groups as very strong donors and medium acceptors on CEA, and (3) hydroxy groups as strong donors and medium acceptors on HPA and deprotected GATA. Although it was difficult to pinpoint which monomer plays the most significant role relative to its neighbors, the presence of hydrogen bonding in all SDD systems translated to favorable polymer−drug interactions that inhibited molecular mobility and stabilized systems from bulk phase separation. Drug Dissolution Performance and Analysis. Microcentrifuge dissolution testing was conducted on the prepared SDDs or crystalline drug to assess the apparent concentration of released probucol and the ability of our polymers to release and maintain in vitro supersaturation levels. All dissolution tests were carried out under a total drug concentration (Ctot) of 1000 μg/mL in phosphate buffer saline (PBS, pH = 6.5) with 0.5 wt

Figure 7. FTIR spectra of A-L at 0, 10, 25, and 50 wt % drug loading compared to neat probucol (100 wt %). Arrows indicate the O−H stretching vibrations.

polymers. Figure 7 shows representative FTIR spectra of the 0, 10, 25, and 50 wt % A-L SDD with unprocessed probucol as a comparison. Based on its chemical structure, neat probucol exhibited characteristic aromatic C−H stretching at 2960 cm−1 and a sharp (non-hydrogen-bonded) singlet at 3650 cm−1 for

Figure 8. Dissolution concentration−time curves for (A−C) all investigated acrylic systems and (D) HPMCAS at 10, 25, and 50 wt % probucol loading, with the (E) AUC enhancement at 360 min for all investigated systems. Circles in the dissolution plots signify the average concentration taken experimentally. Error bars in the dissolution plots and bar graph denote the range of measured data. G


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% fasted simulated intestinal fluid powder (FaSSIF) at 37 °C. We assumed the probucol present in the supernatant to be solublized and available for absorption. This apparent drug concentration was measured over time by reverse-phase highperformance liquid chromatography (HPLC). Furthermore, we calculated the pharmacokinetic area under the curve (AUC) and AUC enhancement, defined as a ratio of the SDD AUC to the crystalline drug AUC. Thus, at unity the AUC enhancement indicates no solubility enhancement over crystalline drug, while an AUC enhancement of 100 shows a 100-fold increase in the drug solubility. The experimentally measured solubility of probucol in PBS/FaSSIF solution remained at 5 μg/mL over 360 min. As seen in Figure 8, the dissolution profiles for the sugarprotected and sugar-deprotected systems exhibit distinct behaviors, dependent on the chemical specificity. In principle, the presence of a polymeric excipient is responsible for the solubility advantage of SDDs. All investigated SDDs in PBS showed marked improvement over the solubility of neat crystalline probucol. As the drug loading increases, the amount of crystallized drug increases, and the attainable Ctot decreases accordingly. For example, if a 25 wt % loaded SDD contained 20% crystalline drug after spray drying, the maximum achievable Ctot would be reduced to 800 μg/mL. Furthermore, crystalline or crystal-rich regions in the solid state can catalyze crystal growth in solution by providing locally accessible nucleating sites, potentially lowering the Ctot even further. MDSC analysis of the fraction of crystalline drug in each SDD from Figure 6 is in good agreement with this loading− performance relationship. As a common the basis of comparison for the following analysis, we will focus on the 10 wt % loaded SDDs across all systems. See the Supporting Information for tabulated data of the maximum concentration achieved (Cmax), final concentration at 360 min (C360 min), AUC, and AUC enhancement. For HPMCAS analog systems A-L and DA-L (Figure 8A), the polymer backbone contains 11 mol % CEA, the monomer containing the ionizable carboxylic acid group. In the sugarprotected form, A-L SDDs exhibited first-order release to Cmax = C360 min = Ctot at the end of the dissolution test. Here, we believe that the classic diffusion-limited release kinetics originates from the gradual, steady-state hydration of the amphiphilic polymeric particles. Because of the monotonic increase in concentration over time, A-L polymer was successful at actively inhibiting drug precipitation. Upon GATA acetate deprotection into hydroxyl groups, the polymer became predominately hydrophilic and more capable of hydrogen bonding. As a result, DA-L SDDs underwent a burst release to Cmax = Ctot, and supersaturation levels were entirely maintained at this concentration over 360 min. The AUC enhancement of DA-L over crystalline probucol was calculated to be 200 ± 4. This trend is more pronounced in our A-H and DA-H systems (Figure 8B). We reduced the number of CEA monomers incorporated by approximately half, relative to the other hydrophobic components, so that the polymer became less soluble in aqueous settings. As expected, we found that AH SDDs offered limited improvement over crystalline probucol in terms of solubility enhancement. Although concentration monotonically increased to Cmax = C360 min = 76 ± 34 μg/mL, the AUC enhancement was only 12 ± 3. Because particles remained insoluble in the PBS solution, drug release into the supernatant was limited. However, upon GATA deprotection initial release to Cmax = Ctot was achieved with supersaturation

maintenance over 360 min. The AUC enhancement increased to 180 ± 3. Again, more hydrophilic polymers promoted nearinstantaneous swelling so that colloidal polymer−drug interactions could maintain supersaturation. However, polymer hydrophilicity is not solely responsible for achieving high drug release and supersaturation maintenance. When hydrophobic P(GATA) and hydrophilic D-P(GATA) control SDDs were subject to dissolution testing, less than 10% of Ctot for probucol was released in both cases (Figure 8C). As seen in Figure 6, despite being able to keep probucol completely amorphous at 10 wt % loading upon spray drying, there is limited apparent solubility enhancement. The AUC enhancement for P(GATA) and D-P(GATA) was 9 ± 0 and 13 ± 0, respectively. Thus, the combination of the methoxy, succinoyl, hydroxypropyl, and acetyl functional groups in HPMCAS is largely responsible for its unique ability to thwart precipitation for poorly water-soluble APIs. Finally, we compared our results to an industrial-grade HPMCAS sample (AFFINISOL HPMCAS 912 G). Its dissolution behavior exhibited a high initial burst release to about 90% of Ctot followed by maintenance of supersaturation levels over 360 min (Figure 8D). The calculated AUC enhancement of HPMCAS was 180 ± 5. By comparison, DA-H was similar to this grade of HPMCAS in terms of improving the apparent solubility of probucol, and DA-L remarkably outperformed it at solubility enhancement capability.

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DISCUSSION Recently, HPMCAS has entered the excipient market as a platform to promote actives solubility.46,47 Many studies comparing this excipient to other common drug delivery agents strongly support the utility of this excipient to aid drug formulation and development.11,15,48,49 While the acetyl and succinoyl substitution levels of HPMCAS can be screened and potentially optimized on a case-by-case basis for a drug candidate,50 the dramatic differences in the results of our dissolution kineticsdriven by subtle changes in well-defined polymer chemistryshow the immense potential of customizing excipients to incorporate the advantageous properties of HPMCAS with conceivably limitless monomers and functionalities. But how are individual, noncovalent interactions in HPMCAS SDDs contributing toward drug release and supersaturation maintenance? The complexity of analyzing metastable SDD behavior in solution alone remains challenging to fully graspthe inclusion of a heterogeneous excipient further complicates this fundamental problem. From our studies using this HPMCAS analog platform, we establish several structure−property relationships that elucidate some of the underlying molecular interactions and, moreover, generalize these features into vehicle design principles to solubilize highly lipophilic drugs for oral drug delivery. These are graphically illustrated in Figure 9 and discussed as follows. First, the constituent monomers of an ideal excipient should be capable of kinetically stabilizing amorphous drug molecules in the solid state, thereby prolonging the shelf life of drug formulations. One avenue to fulfill this need is to introduce stabilizing hydrogen-bonding capability in excipient formulation. Because probucol has two hydroxyl groups in its structure, the deprotected analogs (DA-L and DA-H, of which features deprotected hydroxyls on the sugar moiety) were able to hydrogen bond and effectively decrease the probucol crystallinity at low drug loadings by PXRD and MDSC. H


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amorphous probucol in the supernatant. However, for a vehicle that is entirely a synthetic polysaccharide model such as hydrophilic D-P(GATA), upon introduction into aqueous conditions, this system abandoned the probucol molecules and gave poor AUC enhancement. The ionic groups within the polymer models A-H and A-L appear to promote solubility: by nearly doubling the fraction of ionizable CEA between A-H and A-L, the first-order release of probucol toward supersaturation became more instantaneous. Recent works have also reiterated the importance of establishing a rapid rate of supersaturation. For instance, Sun and Lee modeled and performed dissolution experiments on multiple lipophilic BSC Class II drugs with hydrophilic polyvinylpyrrolidone as the excipient; they discovered optimal supersaturation rates to maximize the concentration−time AUC assuming linear drug dissolution.53 In short, the rate of supersaturation for a drug is central to the bioavailability enhancement, which we can increase by adjusting the polymeric carrier’s hydrophobic−hydrophilic balance and ionization. Finally, the selected monomer components must provide cohesive interactions to prevent drug molecules in solution from precipitation. This means that intermolecular interactions provided by the polymer “parachute” need to overcome drug nucleation and crystal growth. From our dissolution results, we believe that the formation of nanoaggregates, supported by polymer−drug hydrogen bonds and stabilized with ionizable carboxylic acid groups, successfully inhibited crystallization. For instance, despite the difference in AUC enhancement for probucol between A-H and DA-H, both release profiles maintained supersaturation levels over 6 h without returning to the thermodynamic solubility concentration. This means that the intermolecular interactions provided by the amphiphilic HPMCAS analogs were successful in maintaining the solubility enhancement of probucol. In general, additives or polymers have been shown to deposit onto growing drug nuclei by hydrogen bonding, van der Waal interactions, hydrophobic effects, and ionic interactions.54−56 Several groups examining the behavior of solid dispersions in solution have observed similar findings to our results. For example, Yin and Hillmyer studied hydroxypropyl methylcellulose esters of chemically substituted succinates and found that the highest degree of succinic substitution was most effective at solubilizing BCS Class II drug phenytoin.57 Warren et al. also emphasized the importance of incorporating ionic interactions and hydrogen bonding in solid dispersions after evaluating an extensive polymer library against model lipophilic drugs with a principal component analysis model.48 In this discussion, we have outlined key relationships from our solid- and solution-state studies using a tunable multicomponent polymer equipped with specific chemical moieties. The future viability of solid dispersions depends on the advancement of more in-depth techniques to better understand molecular-level interactions during drug dissolution. Recent solution-state NMR studies using the 1H difference nuclear Overhauser effect have been capable of showing specific polymer−drug associations between molecules.58,59 Computational screening tools can also provide new mechanistic insight into aqueous polymer−drug interactions. Jha and Larson conducted atomistic molecular dynamic simulations of HPMCAS oligomers and phenytoin complexation.60 The authors reported polymer−drug associations in the simulation length and time scales precluding nucleation and crystal growth events, where drug aggregation and diffusivity were inhibited as

Figure 9. Schematic illustration of the fundamental structure− property relationships for modular oral excipients during dissolution into supersaturated solutions. Depending on the carefully constructed chemical architecture of the polymer−drug solid dispersion, the following dissolution profiles can be expected: (i) complete initial release and supersaturation maintenance by cohesive polymer−drug interactions, (ii) gradual, diffusion-limited release facilitated by intermolecular interactions, or (iii) limited or poor release near the equilibrium drug solubility in the absence of effective polymer functionalities. (iv) Without an excipient vehicle, the drug concentration is limited to the equilibrium drug solubility level.

Furthermore, the stark difference in probucol crystallinity when encapsulated in either the protected or deprotected sugar homopolymers (P(GATA) and D-P(GATA), respectively) supports the role of hydrogen bonding in producing amorphous SDDs. At 50 wt % drug loading, over half of the spray-dried probucol crystallized in all systems except for DP(GATA), which performed the best at transitioning probucol into the amorphous state. In all other cases, as a result of insufficient polymer, solid-state phase separation promoted rapid drug nucleation and crystal growth in dissolution studies, which compromised overall SDD efficacy. As an alternative to hydrogen bonding, Ilevbare et al. compared semirigid cellulosic polymers to flexible synthetic structures and identified polymer rigidity as another factor in facilitating amorphous drug molecules.51 By comparing our synthetic analogs to HPMCAS in terms of maintaining probucol amorphicity in SDDs, we show that the inclusion of sufficient hydrogen acceptors and donors in the polymeric excipient provides comparable stability for SDD formulations. Next, designed vehicles need to achieve high drug release and generate supersaturation. Conventionally, a “spring-parachute” analogy has been used to describe the concentration−time dissolution behavior: the solublized drug in the high-energy “spring” state thermodynamically transitions toward precipitation out of the supersaturated solution, while the polymeric precipitation inhibitor (“parachute”) kinetically impedes decline in the supersaturation levels over pharmaceutically relevant time scales.52 Mechanistically, the rate of polymer swelling correlates to the “spring” efficacy in supersaturation generation. Our HPMCAS analogs enable the ability to tune this parameter with precise control of polymer hydrophilicity and ionization. The more hydrophilic DA-L and DA-H systems demonstrated a burst release of probucol and increased the accessibility of I


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excipient technologies for solubilizing otherwise intractable drugs.

a strong function of variations in methoxy, hydroxypropyl, acetate, and succinate substitution levels. They concluded that HPMCAS oligomers with the highest acetate content were ideal due to hydrophobic−hydrophobic affinity. The challenge to unify molecular-level interactions with macroscopic drug delivery and controlled release can be met with tunable, wellunderstood polymer platforms. Indeed, understanding the fundamental interactions of such platforms that decrease drug crystallization and promote solubility will lead to formulation innovation that improves bioavailability and clinical approval of pharmaceutical candidates previously thought to be unattainable for oral administration.

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EXPERIMENTAL SECTION

Materials. The following chemicals were reagent grade and used as received unless otherwise specified: methyl acrylate (MA, Aldrich, 99%), 2-carboxyethyl acrylate (CEA, Aldrich), 2-hydroxypropyl acrylate (HPA, Polysciences Inc.), D-(+)-glucose (Aldrich, ≥99.5% GC), trityl chloride (Aldrich, 97%), HBr (Aldrich, 33 wt % in acetic acid), triethylamine (Aldrich, ≥99.5%), acryloyl chloride (Aldrich, 97%), acetyl chloride (Aldrich, ≥99%), 4-(dimethylamino)pyridine (DMAP, Aldrich, ≥99%), 1-propanethiol (Aldrich, 99%), potassium hydroxide (Aldrich, 90%), p-toluenesulfonyl chloride (Aldrich, ≥99%), 4,4′-azobis(4-cyanopentanoic acid) (Aldrich, ≥75%), magnesium sulfate (Aldrich, ≥99.5%), carbon disulfide (Aldrich, ≥99.9%), 2,2′azobis(2-methylpropionitrile) (AIBN, Aldrich, 98%), magnesium sulfate (MgSO4, Aldrich, ≥99.5%), acetic acid (Aldrich, ≥99.7%), sodium methoxide (NaOMe, Aldrich, 25 wt % in methanol), DOWEX-H+ resin (Aldrich), probucol (Aldrich), chloroform-d (Aldrich, 99.8 atom % D), methanol (MeOH, Aldrich, 99.8%), chloroform (CHCl3, Aldrich, ≥99%), tetrahydrofuran (THF, Aldrich, ≥99.9%), and dimethylformamide (DMF, Aldrich, 99.8%). R AFT agent 4-c yano- 4-(p ro pylsulfanylth io ca rbony l) sulfanylpentanoic acid (CPP) was synthesized and purified as described.61 HPMCAS (AFFINISOL 912 G) was provided by The Dow Chemical Company. Phosphate buffered saline (PBS, pH = 6.5) was prepared from 82 mM sodium chloride (Fisher, ≥99.0%), 20 mM sodium phosphate dibasic heptahydrate (Fisher, 98%), and 47 mM potassium phosphate monobasic (J.T. Baker, ≥99.0%). Fasted simulated intestinal fluid powder (FaSSIF, consisting of 3 mM sodium taurocholate, 0.2 mM lecithin, 34.8 mM sodium hydroxide, 68.62 mM sodium chloride, and 19.12 mM maleic acid) was purchased from Biorelevant (Surrey, UK). Monomer and Polymer Synthesis. All experiments were performed in anhydrous solvents under a nitrogen atmosphere in oven-dried glassware. Methyl acrylate (MA), 2-carboxyethyl acrylate (CEA), 2-hydroxypropyl acrylate (HPA), and 2-propylacetyl acrylate (PAA) were passed through an activated alumina column to remove inhibitor and stored at −20 °C. HPA was used to prepare PAA by acetylation with DMAP at 0 °C with monomethylhydroquinone inhibitor (∼200 ppm). Reactivity ratio measurements and polymer composition/sequencing assessments with the combined Walling− Briggs−Skeist model for reversible addition−fragmentation chain transfer (RAFT) chemistry were performed according to our previous work.17 Details are provided in the Supporting Information. For the sugar acetoxy group deprotection chemistry, a freshly prepared solution of NaOMe was added dropwise to a predissolved polymer solution, e.g., A-L (2.7 g) in a dry MeOH/CHCl3 mixture (50 mL, 2:1, v/v), until pH = ∼9 was established. The reaction progress was monitored by taking ∼2 mL of the reaction mixture and quenching it with DOWEX-H+ resin. The resin was removed by filtration, the filtrate was evaporated, and the product was analyzed by 1H NMR; the complete disappearance of only sugar acetate peak at 2.10 ppm suggested the completion of reaction (∼30 min). The reaction was quenched by adding 4 g of DOWEX H+ resin and 5 mL of water in excess. The solid resin was removed by filtration, and the filtrate was evaporated to afford sugar-deprotected polymer (e.g., ∼1.9 g of DA-L from the example above), which was further dried in vacuo (10 mTorr) overnight for at least 12 h. Polymer Molecular Characterization. The molecular weights and chemical compositions were analyzed by size exclusion chromatography (SEC) and proton nuclear magnetic resonance (1H NMR) spectroscopy, respectively. SEC measurements were conducted on an Agilent 1260 Infinity liquid chromatogram equipped with one Waters Styragel guard column and three Waters Stryragel columns (HR6, HR4, and HR1) with pore sizes suitable for materials with effective molecular weights from 100 to 10 000 000 g/mol. The SEC is equipped with an Agilent 1260 Infinity Variable Wavelength Detector monitoring at 254 nm (80 Hz data collection frequency), a Wyatt

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CONCLUSIONS Altogether, we have designed a chemically tunable polymeric excipient platform that has allowed a fundamental examination of polymer−drug interactions for enhancing the bioavailability of oral drug formulations. The use of controlled polymer chemistry to exploit polymer properties such as amphiphilicity represents a powerful tool for tailoring desirable drug release kinetics and revealing underlying release mechanisms. RAFT polymerization of multimonomeric systems to develop HPMCAS analogs enabled well-defined, multicomponent systems to be prepared, and successful selective deprotection chemistry converted the sugar acetoxy groups to more hydrophilic hydroxyl functionalities. In the solid- and solution-state studies, carboxylic acid and hydroxyl functional groups appear essential for the kinetic stabilization of amorphous drugs, which contribute toward the general effectiveness of HPMCAS as an excipient. For in vitro dissolution studies, systems that contained more of these chemical moieties exhibited burst release to Ctot and supersaturation maintenance, resulting in high bioavailability that was comparable to HPMCAS. We attributed this to the ability of the polymer matrix to swell immediately and hinder nucleation and crystal growth with ionic, repulsive interactions and hydrogen bonding to the hydroxyl groups of probucol molecules. Furthermore, we showed the importance of balancing the polymer-to-drug ratio, as SDDs with 50 wt % loaded drug yielded high crystallinity upon spray drying by PXRD and MDSC. This resulted in hampered dissolution performance due to insufficient surrounding excipient to prevent bulk precipitation. Industrial-scale optimization of such spray drying parameters can be accomplished to afford SDDs with high efficacy over pharmaceutically relevant shelf lives. The mutual simplicity and versatility of precisely modulating polymer structure promise to uncover polymer−drug interactions and improve predictability in excipient design. Moreover, a myriad of monomeric components can be judiciously incorporated to form robust, hybrid polymers that contain specific molecular functionalities to complement drug chemical structures and precipitation mechanisms. Hydrogen bonding, hydrophobicity−hydrophilicity balance, and ionic stabilization: these noncovalent interactions that we have identified and discussed herein are crucial to bioavailability. They also parallel prevalent motifs in Nature as the driving forces behind complex biological processes (e.g., polypeptide folding, self-assembly of macromolecular machinery, and ligand binding/activation). We believe that employing such tools in tailorable polymeric systems to better understand polymer-mediated drug release will ultimately lead to concrete pharmaceutical guidelines with structure−property design rules and revolutionary oral J


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Dawn Heleos II multiangle laser light scattering (MALS) detector at a laser wavelength of 663.6 nm (18 angles from 10° to 160°), and a Wyatt Optilab T-rEX refractive index detector operating at 658 nm. Tetrahydrofuran (THF) was run as the mobile phase at 1.0 mL/min at 25 °C. The dn/dc values were measured using an Abbe refractometer with a red light-emitting diode as a light source in concentrations using SEC-grade THF at 25 °C. 1H NMR measurements were carried out in a Varian Inova 500 spectrometer at 22 °C. Polymer solubility in phosphate buffer saline at 37 °C was estimated by visual clarity. From a basis of 10 mg/mL polymer, a series of dilutions in the concentration were conducted to 8.0, 6.0, 4.0, 3.0, 2.0, 1.0, and 0.5 mg/mL with 5 min of sonication and 15 min of magnetic stirring in between dilutions until the solution became visually clear. Spray Drying. Spray drying was performed on a laboratory scale using a Bend Research Mini Spray Dryer (Bend, OR). A 2 wt % of polymer and drug solution at 10, 25, and 50 wt % drug loading in either acetone or acetone/methanol mixture (1:1, v/v) was prepared, with the following as an example: 20 mg of probucol and 180 mg of AL polymer were combined with 9.8 g of acetone under rigorous magnetic stirring to prepare a 10 wt % spray-dried dispersion (SDD) from 2 wt % total solute. The solution was transferred to a 20.0 mL syringe and injected at the following processing parameters: solution feed rate = 0.65 mL min−1, inlet temperature = 90 °C, and nitrogen feed rate = 12.8 standard liters per minute (SLPM). The outlet temperature was not controlled and ranged from 24 to 29 °C. SDDs were removed from the 1.5 in. Whatman filter paper with assistance from an antistatic bar, dried in vacuo (10 mTorr) overnight for at least 12 h, and stored in a vacuum desiccator at 22 °C. Residual solvent in the SDDs was analyzed by thermogravimetric analysis with a Pyris Diamond (PerkinElmer) thermogravimetric analyzer model TGA7. Nitrogen was used as a purge gas at a flow rate of 20 mL/min, and a heating rate of 15 °C/min was used for all samples. Scanning Electron Microscopy (SEM). All samples were placed on carbon tape and sputter-coated with a conductive ∼100 Å gold/ palladium (60:40, %, w/w) coating using a Denton DV-502A high vacuum deposition system. SEM images were obtained on a Hitachi S900 field emission gun SEM equipped with a backscattering detector with Autrata modified YAG crystal. Images were taken at an accelerating voltage of 3.0 kV. Particle size was measured using ImageJ 1.47v.62 The longest length scale for 70 randomly chosen particles was recorded as a representative sample population. The gold/palladium coating thickness (∼100 Å) was subtracted from the diameter of the particles. Histograms and statistics were prepared using JMP 9.0.1v.63 Powder X-ray Diffraction (PXRD). PXRD experiments were carried out on a Bruker-AXS (Siemens) D5005 diffractometer with 2.2 kW sealed Cu (λ = 1.54 Å) source equipped with a scintillation counter detector. SDD samples (∼50 mg) were packed evenly into standard glass holders. Measurements were taken at a voltage of 40 kW and current of 45 mA. Samples were analyzed in the 2θ angle range of 5°−40° with a step size of 0.02 and scan step time of 0.5 s. Differential Scanning Calorimetry (DSC) and Modulated DSC (MDSC). DSC and MDSC measurements were analyzed using a TA Instruments Discovery DSC. All samples (∼5−10 mg) were hermetically crimped in T-zero aluminum pans. For polymers-only SDDs with DSC, second heating experiments were conducted from −80 to 180 °C; temperatures were ramped at a rate of 5 °C/min. For all other SDDs with MDSC, the total and reversing heat capacity signals under first heating were collected in modulated heating mode, modulated with ±1 °C amplitude every 40 s from 0 to 150 °C; temperatures were ramped at a rate of 10 °C/min. TA TRIOS software (Version 2.2) was used to analyze the thermal transitions. The glass transition temperature of SDDs was determined using the reversing thermogram in MDSC during the first heating step. For polymer only samples, the second heating scan in DSC was reported to calculate the glass transition. Fourier Transform Infrared (FTIR) Spectroscopy. Infrared spectra obtained on a Nicolet Series II Magna-IR System 750 FTIR

spectrometer equipped with a KBr beam splitter. Measurements were taken with 2 cm−1 spectral resolution. Dissolution of Solid Dispersions. Dissolution tests were carried out using a microcentrifuge dissolution test method under nonsink conditions.11 Measured samples were weighed in duplicate into 2.0 mL plastic conical microcentrifuge tubes. Phosphate buffer saline (PBS, pH 6.5) with 0.5 wt % fasted simulated intestinal fluid powder (FaSSIF) at 37 °C was added (time = 0 min) to achieve a final drug concentration of 1000 μg/mL if all material were fully dissolved (e.g., 7.2 mg of a A-L SDD loaded with 25 wt % probucol, consisting of 1.8 mg of drug and 5.4 mg of polymer, was carefully measured into a conical tube and diluted with 1.8 mL of PBS and FaSSIF solution). Samples were vortexed for 30 s and incubated in an isothermal aluminum heating block at 37 °C. At each time point (4, 10, 20, 40, 90, 180, and 360 min), samples were centrifuged for 1 min at 13 000 rpm, and a 50 μL aliquot was taken from the supernatant and diluted with 250 μL of methanol. The samples were then vortexed again for an additional 30 s and held at 37 °C until the next time point. Solublized drug concentration in each aliquot sample was determined by reverse phase high-performance liquid chromatography (HPLC) analysis. The HPLC consisted of a reversed-phase EC-C18 column (Poroshell 120, 4.6 × 50 mm, 2.7 μm, Agilent) and a mobile phase of acetonitrile/water (96:4, %, v/v) pumped at a flow rate of 1.0 mL/min at 30 °C. A 10 μL aliquot of sample was injected, and the column effluent was detected at 241 nm with a UV detector (1260 Infinity Multiple Wavelength Detector, Agilent). The sample drug concentrations were determined using a calibration curve for probucol from 0.1 to 500 μg/mL (see Supporting Information). The concentration−time area under the curve (AUC) from 0 to 360 min was determined using the trapezoidal rule.

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ASSOCIATED CONTENT

S Supporting Information *

Supplemental synthetic schemes, reactivity ratio studies of pairwise monomers, select 1H NMR spectra and SEC chromatograms of synthesized polymers, SEM images of neat probucol and SDDs at 25/50 wt % loading, additional PXRD/ MDSC/FTIR of SDDs, probucol calibration curve, detailed tabulation of Cmax/C360 min/AUC360 min for dissolution studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel 612-624-0839 (F.S.B.). *E-mail [email protected]; Tel 612-624-8042 (T.M.R.). Author Contributions

J.M.T. and T.S.N. contributed equally to the work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge The Dow Chemical Company for funding this project. We thank Prof. Marc A. Hillmyer and Prof. Timothy P. Lodge at the University of Minnesota, as well as Drs. Steven Guillaudeu, Robert Schmitt, and William Porter III at The Dow Chemical Company, for helpful discussions and feedback. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 00006595. Parts of this work were carried out in the Characterization Facility at the University of Minnesota, a member of the NSF-funded Materials Research Facilities Network via the MRSEC program. K


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