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A systematic study of molecular interactions of anionic drugs with a dimethylaminoethyl methacrylate copolymer regarding solubility enhancement Wiebke Saal, Alfred Ross, Nicole Wyttenbach, Jochem Alsenz, and Martin Kuentz Mol. Pharmaceutics, Just Accepted Manuscript • DOI: 10.1021/acs.molpharmaceut.6b01116 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Molecular Pharmaceutics

1

A systematic study of molecular interactions of anionic drugs with

2

a

3

solubility enhancement

dimethylaminoethyl

methacrylate

copolymer

regarding

4 5

Wiebke Saal1,2, Alfred Ross3, Nicole Wyttenbach3, Jochem Alsenz3, Martin Kuentz1*

6

7

1

8

Technology, Gründenstrasse 40, 4132 Muttenz, Switzerland

9

2

University of Applied Sciences and Arts Northwestern Switzerland, Institute of Pharma

University of Basel, Institute of Pharmaceutical Technology, Klingelbergstrasse 50, 4056

10

Basel, Switzerland

11

3

12

Center Basel, F. Hoffmann-La Roche Ltd, Grenzacherstrasse 124, 4070 Basel, Switzerland

Roche Pharmaceutical Research & Early Development, Pre-Clinical CMC, Roche Innovation

13 14

Prof. Dr. Martin Kuentz

15

University of Applied Sciences and Arts Northwestern Switzerland

16

Institute of Pharma Technology

17

Gründenstrasse 40

18

4132 Muttenz / Switzerland

19

T +41 (61) - 467 46 88 / F +41 (61) - 467 47 01

20

[email protected]

21 22 23

1 ACS Paragon Plus Environment


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For Table of Contents Use Only

25 26

A systematic study of molecular interactions of anionic drugs with a

27

dimethylaminoethyl

28

enhancement

methacrylate

copolymer

regarding

solubility

29 30

Wiebke Saal1,2, Alfred Ross3, Nicole Wyttenbach3, Jochem Alsenz3, Martin Kuentz1*

31

1

32

Gründenstrasse 40, 4132 Muttenz, Switzerland

33

2

34

Switzerland

35

3

36

Basel, F. Hoffmann-La Roche Ltd, Grenzacherstrasse 124, 4070 Basel, Switzerland

University of Applied Sciences and Arts Northwestern Switzerland, Institute of Pharma Technology,

University of Basel, Institute of Pharmaceutical Technology, Klingelbergstrasse 50, 4056 Basel,

Roche Pharmaceutical Research & Early Development, Pre-Clinical CMC, Roche Innovation Center

37

38


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39

Abstract

40

The methacrylate-copolymer Eudragit® EPO (EPO) has raised interest in solubility

41

enhancement of anionic drugs. Effects on aqueous drug solubility at rather low polymer

42

concentrations are barely known despite of their importance upon dissolution and dilution of

43

oral dosage forms. We provide evidence for substantial enhancement (factor 4-230) of

44

aqueous solubility of poorly water-soluble anionic drugs induced by low (0.1-5% (w/w))

45

concentration of EPO for a panel of seven acidic crystalline drugs. Diffusion data (determined

46

by 1H nuclear magnetic resonance spectroscopy) indicate that the solubility increasing effect

47

monitored by quantitative ultra-pressure liquid chromatography was caused primarily by

48

molecular API polymer interactions in the bulk liquid phase. Residual solid API remained

49

unaltered as tested by X-ray powder diffraction. The solubility enhancement (SE) revealed a

50

significant rank correlation (rSpearman= -0.83) with rDiffAPI, where SE and rDiffAPI are defined

51

ratios of solubility and diffusion coefficient in the presence and absence of EPO. SE

52

decreased in the order of indomethacin, mefenamic acid, warfarin, piroxicam, furosemide,

53

bezafibrate, and tolbutamide. The solubilizing effect was attributed to both ionic and

54

hydrophobic interactions between drugs and EPO. The excellent solubilizing properties of

55

EPO are highly promising for pharmaceutical development and the dataset provides first steps

56

towards an understanding of drug-excipient interaction mechanisms.

57

Keywords:

58

acidic drugs, solubility enhancement.

Nuclear magnetic resonance (NMR) spectroscopy, polymer-drug interaction,

59



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60

Introduction

61

An increasing number of new active pharmaceutical compounds (APIs) exhibit low water

62

solubility which may lead to poor oral bioavailability.1 Various approaches have been

63

reported to increase drug solubility for these candidates. Formulations were designed to

64

promote intestinal solubility like micro- and nanosuspensions,2 cyclodextrin complexes,3-5 and

65

lipid-based formulations.6, 7 Formulations can also target primarily a transient increase of bulk

66

concentrations by drug supersaturation, which thereby enhances absorptive flux.

67

Supersaturated states can be obtained in vivo by pH change when the dissolved compound is

68

transported from the acid stomach into the small intestine or by a loss of solubilization

69

capacity of a formulation as upon dilution, dispersion or digestion in the gastrointestinal

70

tract.8 Supersaturation is only beneficial if drug crystallization is slower than drug

71

absorption.9 The most common supersaturating formulations are amorphous solid dispersions

72

that are used to enhance the bioavailability of compounds belonging to biopharmaceutical

73

classification system (BCS) class II or IV.8, 10

74

This work focusses on the copolymer EPO that belongs to the family of methacrylic acid

75

copolymers. EPO is composed of dimethylaminoethyl methacrylate, butyl methacrylate, and

76

methyl methacrylate at molar ratios of 2:1:1 and is positively charged at physiological pH in

77

aqueous media. Figure 1 displays the molecular structure of this copolymer. EPO has been

78

widely used as pharmaceutical excipient for taste masking, moisture protection, enteric film-

79

coating, and sustained release drug delivery.11


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80 81

Figure 1. Molecular structure of Eudragit® EPO.

82

EPO was more recently used to prepare solid dispersions, especially with anionic drugs12-14,

83

and outstanding results were obtained in terms of solubility and bioavailability

84

enhancement.12 Priemel et al. also reported in situ amorphization with EPO and

85

indomethacin15 and recently in situ amorphization with naproxen and ibuprofen was

86

evidenced.16

87

Different studies attempted to gain insights into the molecular interactions of the copolymer

88

EPO with anionic drugs. Kojima et al. and Higashi et al. used for example nuclear magnetic

89

resonance (NMR) spectroscopy to better understand the molecular basis of solubility

90

enhancement (SE) of solid dispersions and of supersaturated solutions using EPO and

91

mefenamic acid.12,

92

carrier in solid dispersions but also affects drug solubilization and supersaturation of acidic

93

drugs. Formulations with EPO therefore seem to become a research field in its own right.

94

However so far, only few compounds, exclusively in amorphous state, were studied.

95

Moreover, data for rather low polymer concentrations are rare, for example concentrations

96

that would result from dilution of oral dosage forms in the gastrointestinal tract. Another

97

aspect is that combinations of EPO and acidic drugs were primarily studied in the context of

13

These important pioneer studies showed that EPO acts not only as a



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12

98

solid dispersions.

The molecular interactions of the polymer and acidic drugs seem to be

99

also promising for other formulation types but such data are currently lacking.

100

We therefore studied the influence of EPO in solution on the solubility and solid state of

101

seven poorly water-soluble, acidic, chemically diverse drugs. Our final aim was to better

102

mechanistically understand how the molecular interactions of acidic drugs and EPO would

103

result in specific SE.

104

105

Experimental Section

106

Materials

107

Indomethacin (IMC), mefenamic acid (MFA), tolbutamide (TLB) and warfarin (WFN) were

108

obtained from Sigma Aldrich (St. Louis, USA), while bezafibrate (BZF) and piroxicam (PRX)

109

were from TCI Europe N.V. (Zwijndrecht, Belgium). Furosemide (FRS) was purchased from

110

Molekula GmbH (München, Germany) and amino alkyl metacrylate copolymer E, Eudragit®

111

EPO, (EPO) was obtained from Evonik (Darmstadt, Germany). Chemical structures of all

112

model compounds are shown in Figure 2. Hydrochloric acid (0.1 M) and sodium hydroxide

113

solution (0.1 M) were supplied by Merck KGaA (Darmstadt, Germany).

114 6 ACS Paragon Plus Environment

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115

Figure 2. Chemical structure of model drugs.

116

Table 1. List of model drugs and selected physicochemical properties.

Compound Bezafibrate (BZF) Furosemide (FRS) Indomethacin (IMC) Mefenamic acid (MFA) Piroxicam (PRX) Tolbutamide (TLB) Warfarin (WFN) 117 118

Mw [g/mol] 361.8 330.7 357.8 241.3 331.4 270.4 308.3

pKaa 3.2 3.5 4.5 4.2 2.3 5.1 5.0

LogDb (pH 6.0) 1.8 0.0 1.4 3.3 -0.4 1.4 2.2

a

Measured pKa-values via photometric titration. Values calculated by the Marvin program suite (V.16.5.30) (ChemAxon Ltd., USA).

b

119

120

Methods

121

Sample preparation

122

Polymer solutions were prepared by dissolving EPO (0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%

123

(w/w)) in deionized water and adjusting the pH of the solutions to pH 6.0 with hydrochloric

124

acid or sodium hydroxide at 25°C. All solutions were visually inspected for absence of

125

residual particles.

126

127

Viscosity measurements

128

Apparent viscosity was measured with a cone-plate-rheometer (Physica MCR 301, Anton

129

Paar GmbH, Graz, Austria) by applying a shear rate of 1000 min-1 for 2 min.

130 131

Solubility and residual solid analysis

132

Solubility of compounds in EPO-solutions was determined using a slightly modified 96-well

133

SORESOS17 assay, which measures both equilibrium solubility and solid form of the residual 7 ACS Paragon Plus Environment


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solid. In brief, APIs were dispensed using the powder-picking-method18 in 96-well flat bottom

135

plates (Corning Inc., Durham, USA) and stirring bars and excipient vehicles (150 µL) were

136

added. The plate was then sealed with pre-slit silicon caps and the mixtures were agitated by

137

head-over-head rotation for 24 h, 48 h and one week at room temperature. After equilibration,

138

the suspensions were carefully transferred into 96-well filter plates and liquid was separated

139

from residual solid by centrifugation. Collected filtrates were diluted with N-methyl-2-

140

pyrrolidone and drug content was determined using a Waters Acquity Ultra Performance

141

Liquid Chromatographic (UPLC) system equipped with a 2996 Photodiode Array Detector

142

and an Acquity UPLC BEH C18 column (2.1x50 mm, 1.7 µm particle size) from Waters

143

(Milford, USA). Table 2 summarizes the experimental conditions (solvents, composition of

144

mobile phase, detection wave length) used for the drugs. An isocratic flow of a mixture of

145

solvent A and solvent B was applied for 0.3 min at a flow rate of 0.75 mL/min. Subsequently,

146

the concentration of solvent B was linearly increased to 100% within 0.5 min. Solid state

147

analysis of residual solid was performed by X-ray powder diffraction (XRPD) using a STOE

148

Stadi P Combi diffractometer with a primary Ge-monochromator (Cu Kα radiation), imaging

149

plate position sensitive detector (IP-PSD), and a 96-well sample stage. The IP-PSD allowed

150

simultaneous recording of the diffraction pattern on both sides of the primary beam which

151

were summed up by the software STOE WinXPOW to reduce effects related to poor crystal

152

orientation statistics. Samples were analyzed directly in the 96-well filter plate with an

153

exposure time of 5 min per well.

154 155

Table 2. Experimental conditions used for UPLC analysis. Compound

Composition a

Detection

(A:B) [%]

wavelength [nm]

BZF

50:50

254

FRS

75:25

274

IMC

50:50

318


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156 157


MFA

40:60

352

PRX

65:35

342

TLB

60:40

265

WFN

50:50

283

a

Mobile phase A: deionized water with 0.1% (v/v) triethylamine adjusted to pH 2.2 with methanesulfonic acid, mobile phase B: acetonitrile.

158 159

1

160

Solutions for NMR analyses were prepared by incubating APIs for 24 h in a 0.5% (w/w)

161

EPO-solution in deuterium oxide (D2O) at pH 6.0. Samples were then centrifuged and

162

supernatants (550 µl) were transferred to short disposable 5 mm NMR tubes.

163

All NMR measurements were performed with a Bruker 600 MHz Avance II spectrometer

164

equipped with a cryogenic QCI probe head at a temperature of 300 K. Spectrometer operation

165

and data processing was done by Topsin 2.1 software (Bruker, Fällanden, Switzerland). For

166

all samples matching/tuning of the probe head and the 90° pulse were determined fully

167

automated. Pseudo 2D 1H diffusion ordered spectroscopy with bipolar gradient pulse pairs

168

and 2 spoil gradients19 was measured for all samples with presaturation of residual water.

169

Data points (32k) were acquired over 18 ppm sweep-width. The interscan delay was set to 1.5

170

s and the SMSQ10.100 shaped bipolar gradient was ramped from 2.65 to 50.35 gauss/cm in

171

16 equidistant steps. A diffusion time of 300 ms was used. Spectra were processed with a lb =

172

1 exponential filtering.

173

Diffusion coefficient D was fitted by use of the T1/T2 relaxation module implemented within

174

the Topsin 2.1 (Bruker, Switzerland). For most molecules at least one API and excipient

175

related NMR signal was identified by visual inspection.

H NMR spectroscopy

176

177

Statistical analysis and molecular modeling 9 ACS Paragon Plus Environment


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178

The program STATGRAPHICS Centurion XVI ed. Professional (V. 16.1.15) from Statpoint

179

Technologies Inc. (Warrenton, USA) was used for statistical rank correlation (Spearman)

180

testing.

181

For graphical representation of the mefenamic acid interaction with EPO, both ionized

182

molecules were assigned to an AMBER force field using the software Chemsite (V.10.4.,

183

Norgwyn Montgomery Software Inc., Northwales, USA). The starting configuration was

184

based on docking the anionic model drug to positively charged nitrogen of the polymer that

185

was modeled as residue of 10 monomers. Following energy minimization, the molecular

186

dynamics simulation was running for 10 ns while using an implicit Born solvation model

187

(with a relative permittivity of 78.4) at a temperature of 300 K.

188

189

Results

190

Viscosity

191

The apparent viscosity was measured under shear (Table 3) for all solutions of EPO. A slight

192

increase in viscosity with polymer concentration was observed, however all solutions had a

193

rather low viscosity. These EPO-solutions were used to determine drug solubility after 24 and

194

48 h. Most drugs reached equilibrium solubility within 24 h, some compounds (MFA, FRS,

195

TLB) showed differences between the selected time points especially at higher polymer

196

concentration. For those compounds solubility was additionally determined after one week to

197

prove that equilibrium was reached. Finally, 48 h was selected as incubation time for all

198

solubility experiments (shown in Table 4 and Figures 3 and 4) because all systems have

199

reached equilibrium after this time.

200 201

Table 3. Apparent viscosity of EPO-solutions (25°C, 1000s-1).


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Concentration of

Viscosity [mPas]

EPO [%, (w/w)]

(Standard deviation, n=3)

0.1

1.1 (0.01)

0.5

1.1 (0.01)

1.0

1.2 (0.01)

2.0

1.5 (0.02)

3.0

1.9 (0.05)

4.0

2.1 (0.01)

5.0

2.3 (0.00)

202 203

Solubility and residual solid analysis

204

All test solutions were adjusted to pH 6 and following equilibration, the residual solid was

205

analyzed by means of XRPD. Compared to water, all model compounds displayed an

206

enormous SE in the different EPO-solutions (Fig. 3-4 and Table 4) with SE factors of

207

approximately 4 - 230 fold.

208 209

Table 4. Drug solubility and pH in water after incubation for 48h.

Compound

Solubility in water

pH in water (standard

(standard deviation, n=3)

deviation, n=3)

[mg/ml] BZF

0.016 (0.003)

4.6 (0.1)

FRS

0.024 (0.007)

4.4 (0.0)

IMC

0.007 (0.003)

5.0 (0.2)

< 0.001

6.1 (0.1)

PRX

0.008 (0.001)

5.4 (0.1)

TLB

0.083 (0.007)

5.0 (0.1)

WFN

0.005 (0.001)

5.4 (0.2)

MFA*

210

*Aqueous solubility was below the limit of detection.

211 212



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213 214

Figure 3. Solubility of BZF, IMC, PRX, and WFN in EPO-solutions after 48 h equilibration

215

at room temperature.

216 217

BZF, IMC, PRX and WFN (Figure 3) reached maximum solubility at 2% EPO (w/w) and

218

their solubility increased with the polymer concentration. Above 2% EPO, solubility

219

decreased again and finally reached a kind of plateau. In contrast, FRS and MFA (Figure 4)

220

showed a constant increase of solubility with polymer concentration up to 5% EPO. TLB

221

(Figure 4) was the only tested compound that reached a kind of plateau at 2% EPO where the

222

solubility did not decrease at higher EPO concentrations.

223 224

The residual solid analysis confirmed that none of the tested compounds changed its

225

polymorphic form. Therefore, it is likely that the SE resulted from drug-EPO interactions in

226

the bulk phase and not from stabilization of a metastable polymorphic form. 12 ACS Paragon Plus Environment

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227

228 229

Figure 4. Solubility of FRS, MFA, and TLB in EPO-solutions after 48 h equilibration at room

230

temperature.

231 232

To address potential pH effects on drug solubility, the pH was measured after 48 h for all

233

substances in EPO (0.5%, 2%, and 5%) (Table 5). For most compounds the pH dropped about

234

0.5 units. Maximum changes were observed in FRS suspensions where the pH dropped to pH

235

3.6 in 2% EPO and to pH 4.1 in 0.5% and 5% EPO. 13 ACS Paragon Plus Environment


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236 237

Table 5. pH of drug suspensions in EPO after 48 h at room temperature. Compound

pH in EPO 0.5% after 48 h

pH in EPO 2% after 48 h

pH in EPO 5% after 48 h




BZF

4.5 (0.0)

4.6 (0.3)

4.7 (0.0)

FRS

4.1 (0.0)

3.6 (0.0)

4.1 (0.1)

IMC

4.8 (0.0)

4.4 (0.1)

4.9 (0.1)

MFA

5.6 (0.1)

5.2 (0.0)

5.4 (0.0)

PRX

5.4 (0.0)

5.5 (0.1)

5.6 (0.1)

TLB

5.2 (0.1)

5.4 (0.1)

5.6 (0.0)

WFN

5.4 (0.1)

5.4 (0.1)

5.7 (0.1)

238 239

As a trend, the pH was slightly higher in 5% EPO compared to the pH in 0.5% and 2% EPO.

240

The dimethylaminoethyl groups in EPO have a rather high pKa value (8.4)20 and thus

241

concentration-dependent interactions with the dissolving acidic drugs are likely to affect

242

equilibrium pH.

243 244

1

H NMR spectroscopy

245

1

H-NMR measurements were performed to evaluate the interactions between EPO and the

246

compounds in solution. Peaks originating from protons of aromatic ring systems (present in

247

all API molecules investigated) were observed between 7.00 and 8.25 ppm in D2O for PRX

248

and 6.75 to 8.00 ppm for WFN.


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249 250

Figure 5. Solution-state 1H-NMR spectra of WFN and PRX in presence of EPO and in D2O

251

alone.

252

The NMR signals of WFN and PRX in D2O were very sharp (see Fig. 5), indicating that API-

253

molecules were dispersed in D2O without substantial aggregation. This was also observed for

254

the other compounds (spectra not shown). All drugs had in common that API-related signals

255

displayed changes in line-width and/or chemical shift in presence of EPO as shown for two

256

examples, WFN and PRX, in Figure 5. Peaks derived from compounds could be still clearly

257

observed although the peaks’ shapes were comparatively much broader.

258

1

259

without 0.5% EPO and results are displayed in Table 6.

H-NMR was further used to measure the diffusion coefficient of the APIs in D2O with and

260

261

Table 6. Diffusion coefficient of APIs in D2O with and without EPO. Compound BZF

Diffusion coefficient in D2O × 1010 [m2/s] 4.65

Diffusion coefficient in EPO 0.5% × 1010 [m2/s] 0.56

FRS

4.73

0.36

IMC

5.25

0.34

MFA

5.50

0.34

PRX

5.43

0.63



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TLB

5.40

1.32

WFN

7.00

0.48

EPO

0.38

-

262

263

As expected, the much larger polymer EPO showed a lower diffusion coefficient

264

(10 to 15-fold) than the APIs alone and therefore moves slower. The EPO diffusion

265

coefficient was practically unaffected by the different drugs (in the range 5.3·10-11 to 3.7·10-11

266

m2/s). By contrast, the diffusion coefficient of the APIs decreased substantially in the

267

presence of EPO down to or even slightly below the diffusion coefficient of EPO.

268 269

Discussion

270

It is a reasonable assumption that the ionic interaction between the deprotonated drug and

271

protonated amino alkyl group of EPO provides important contribution to the whole

272

interaction. As an example of such a likely molecular association of drug and excipient, the

273

potential EPO-MFA interaction is depicted in Figure 6.

274 275

Figure 6. Graphical representation of the interaction of ionized molecules and ionized EPO

276

using mefenamic acid as model (light gray).


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277

MFA was selected as an example since its interaction with EPO has been investigated before

278

in studies addressing supersaturation.13 Figure 6 shows that apart from association of the ionic

279

groups, additional interactions e.g. between side chain methyl groups and the aromatic ring,

280

are likely in case of MFA. Especially, the amino group may increase the polarization of the

281

methyl C-H group and thereby favor interaction with an aromatic ring. Similarly hydrophobic

282

interactions between the dangling EPO side chains and aromatic drug moieties may further

283

strengthen overall association. Higashi et al. studied the interaction mechanism of MFA with

284

EPO by high resolution magic-angle spinning NMR and also divided the molecular

285

interactions into ionic and hydrophobic interactions.13 They also identified peaks that indicate

286

interactions between the amino alkyl group of EPO and the aromatic systems of MFA.13 In

287

line with these results, Figure 6 shows that interactions with at least three spatial contacts

288

could be viewed as a kind of molecular recognition in the case of MFA and EPO. Thus, EPO

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might form a kind of a polymeric pocket for MFA and additional interactions are conceivable

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that further shield MFA from the aqueous environment. Based on the example of MFA, we

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assume that such specific interaction motifs may also occur with other acidic drugs. In

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summary, these motifs are the interaction of acidic and positively charged amino alkyl group,

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the interaction of methyl amino group and aromatic rings, and additional hydrophobic

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interactions. Even though all tested compounds can in principle show such interaction motifs,

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the molecular structures are quite diverse and these structural differences are very likely to

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influence the specific strength of molecular association between polymer and compound.

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MFA has a carboxylic acid group whereas other compounds, like PRX, TLB, and WFN, have

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alternative acidic groups (enolate or sulfonamide). When looking at Table 6, it is quite

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remarkable that the diffusion coefficient of TLB decreases the least in presence of polymer.

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Comparing all the structures, TLB is the only compound that has only one aromatic ring. All

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other compounds have at least two aromatic rings. Thus the number of aromatic rings could

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be important for the polymer-drug interaction, for example in case of MFA, the interaction of 17 ACS Paragon Plus Environment


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the methyl amino group and of the aromatic ring. Such interaction motifs also require that an

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aromatic ring is spatially arranged close to the amino alkyl side chain of EPO. Therefore

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molecular flexibility and hence rotatable bonds adjacent to aromatic systems are also very

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likely associated with a strong polymer-drug interaction and in turn also with a good SE. FRS

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and WFN showed excellent SE and these drugs are also flexible regarding their spatial

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arrangement of their aromatic groups.

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Theoretically, a certain initial conformation of the polymer might be optimal for drug

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solubilization. However, at the same time, the presence of API molecules may also influence

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conformation of the polymer. It is therefore expected that due to these complex molecular

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interactions, no simple relationship can be established between the polymer concentration and

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drug solubility. Concentration dependent solubility data were indeed in agreement with this

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view since there was no general solubilization pattern evidenced for the different compounds

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(Figures 3-4). Nonetheless, it is remarkable that most acidic drugs show a solubility maximum

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at 2% EPO. Drug solubility was measured in a broad concentration range of EPO and

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solubility could be affected by both the initial conformational structure of the polymer in

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solution and by the conformation induced by its interaction with the drug.

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For most compounds we observed a maximum of SE within the range of polymer

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concentration investigated. For those which do not show this maximum the curvature of the

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solubility increase proposed a maximum (or plateau) outside the range of polymer

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concentration investigated. Eventually, these compounds will show beyond the tested range

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also a maximum for SE at a certain polymer concentration. These observed maxima can be

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explained by steric hindrance of the interactions between EPO and the APIs occurring at high

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polymer loads. In addition conformational changes of the polymer at high concentration may

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lead to limited accessibility of the polymeric cavities that enable strong drug association.

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The molecular interactions discussed and exemplarily shown in Figure 6 were in agreement

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with our 1H-NMR results. Notable was for example that all drugs revealed changes in the

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aromatic regions, where a change in line-width and/or chemical shift for peaks originated

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from protons of aromatic rings in presence of EPO. These results are in line with former

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results, where similar line broadening was observed for MFA due to strong interaction with

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EPO.13 Besides spectral information, drug diffusion coefficients of the compounds in presence

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and absence of EPO were used to monitor the strength of the API polymer interactions in

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solution.

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Figure 7. SE in presence of polymer (0.5% (w/w)) plotted against the rDiffAPI defined as the

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diffusion coefficient of drug with polymer divided by the value of pure drug in D2O.

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A relative diffusion coefficient (rDiffAPI) was considered that holds for the observed value of

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drug in the presence of polymer normalized by the diffusion coefficient of drug alone (in

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D2O). When rDiffAPI is plotted against the relative SE of the drug (drug solubility in polymer

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solution normalized by the pure aqueous solubility) there appears to be a non-linear

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relationship. A Spearman rank correlation provided a coefficient of rSpearman = -0.83 that 19 ACS Paragon Plus Environment


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supported the view of a significant relationship (p= 0.04). Thus, a decrease of diffusion

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coefficient ratio correlates well with an observed excipient SE (Figure 7). It is expected that

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APIs diffusion will be increasingly reduced by stronger interaction with EPO and therefore, a

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relative diffusion coefficient might be used as marker for the strength of drug-excipient

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interaction in the bulk phase. The approach seems promising because no additional effects of

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the drug solid state were evidenced in presence of polymer. Based on the identified

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correlation, it might be in the future possible to formulate an empirical power law that

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describes SE as a function of rDiffAPI but this should be based on an even larger dataset than

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presented in the current study.

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The non-linear nature of the relationship is likely due to a complex solid-liquid equilibrium

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where drug in the bulk phase can be either interacting or non-interacting with polymer. It is

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possible that presence of drug may induce conformational change of the polymer, thereby

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influencing its binding. On top of such multiphase consideration, one may also have to

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account for fractal physics of drug dissolution and diffusion.21-23 Such further theoretical

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research is beyond the scope of the present article. The observed empirical correlation has in

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any case a great practical relevance because it suggests how the strength of the molecular

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interaction, expressed by rDiffAPI, may translate into SE. It is here interesting that a moderate

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change in the strength of molecular interaction can exert a huge effect on solubilization.

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Further possible correlations were studied between characteristics of the compounds (used for

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general solubility prediction24) like logP, melting point, or pKa of the APIs and the solubility

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enhancement by EPO (data shown in supplementary data). As a result, the present dataset did

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not point to a comparatively simple relationship of such a molecular parameter with SE, 20 ACS Paragon Plus Environment

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which was probably due to the complex nature of ionic and relevant hydrophobic interactions

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of drug and polymer.

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Conclusion

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In former studies it was shown that EPO is a very potent polymer for stabilizing amorphous

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acidic compounds, which can increase oral bioavailability.12, 13 We studied the influence of

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EPO in solution on seven crystalline acidic drugs and could show that the observed huge SE

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was enabled by specific molecular interactions of drug and polymer. Thus, EPO seems to be a

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very well suitable polymer to formulate anionic drugs in both solid dispersions and in less

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complex formulations such as solutions or suspensions. The exceptional drug solubilization

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by EPO may even replace solid dispersions in many cases. This is attractive because simple

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formulations that are easy to produce and stable upon storage are clearly preferred particularly

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for early stage development. An important finding was also that a ratio of diffusion

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coefficients of an API measured by NMR in presence and absence of EPO correlated well

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with the obtained excipient solubility enhancement. The findings suggested that small

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changes in the strength of molecular interaction between APIs and EPO may substantially

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affect drug solubility in the presence of EPO. Based on the comparison of the acidic drugs, a

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better molecular understanding was achieved for the association of drug and polymer to target

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maximum solubility. Such findings of molecular pharmaceutics are of practical relevance and

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it is particularly the early development phase that benefits from such knowledge to formulate

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poorly soluble drugs, while coping with challenges of limited compound availability and

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stretched timelines. The results are also interesting for oral solid dosage forms in the later

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stage of pharmaceutical development where EPO could be used as well for SE.

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Figure 1. Molecular structure of Eudragit® EPO. EPO was more recently used to 37x29mm (600 x 600 DPI)

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Figure 2. Chemical structure of model drugs. Chemical structures of all mod 58x22mm (600 x 600 DPI)


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Figure 3. Solubility of BZF, IMC, PRX, and WFN in EPO-solutions after 48 h equilibration at room temperature. BZF, IMC, PRX and WFN (Figure 118x87mm (600 x 600 DPI)



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Figure 4. Solubility of FRS, MFA, and TLB in EPO-solutions after 48 h equilibration at room temperature To address potential pH effect 163x333mm (600 x 600 DPI)


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Figure 5. Solution-state 1H-NMR spectra of WFN and PRX in presence of EPO and in D2O alone. The NMR signals of WFN and PRX 65x52mm (600 x 600 DPI)



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Figure 6. Graphical representation of the interaction of ionized molecules and ionized EPO using mefenamic acid as model (light gray). MFA was selected as an example 73x66mm (600 x 600 DPI)


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Figure 7. SE in presence of polymer (0.5% (w/w)) plotted against the rDiffAPI defined as the diffusion coefficient of drug with polymer divided by the value of pure drug in D2O. A relative diffusion coefficie 92x57mm (600 x 600 DPI)



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Graphical abstract 32x11mm (600 x 600 DPI)


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