tiotropium fumarate: an interesting pharmaceutical co-crystal

PHARMACEUTICAL TECHNOLOGY

Tiotropium Fumarate: An Interesting PharmaceuticalCo-Crystal

MIHAELA POP,1 PETER SIEGER,2 PETER W. CAINS1

1Avantium Technologies BV, Zekeringstraat 29, 1014 BV Amsterdam, The Netherlands

2Boehringer Ingelheim Pharma GmbH & Co KG, Birkendorfer Strasse 65, 88397 Biberach an der Riss, Germany

Received 28 April 2008; accepted 9 July 2008

Published online 9 September 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21531

Additional Sonline version o

Corresponden8039; Fax: þ31-E-mail: mihaela

Journal of Pharm

� 2008 Wiley-Liss

1820 JOURN

ABSTRACT: A new salt–co-crystal of tiotropium fumarate with fumaric acid has beendiscovered, and found to be the most stable solid form of tiotropium fumarate. This typeof structure consists of matched cations and anions (a salt) together with a nonionizedfree acid moiety as the co-former (co-crystal), and is unique amongst the large number oftiotropium salts that have been prepared and characterized. The stoichiometry cation/anion/co-former of 2:1:1 corresponds to a simple polymorph of the 1:1 salt, and itsidentity as a co-crystal has been established by single-crystal X-ray diffraction with somecorroborating evidence from the Raman and infrared spectra. A detailed investigation ofthe bonding and geometry of the three crystalline forms of the fumarate indicates thatthe hydrogen bonding motifs are very similar, and that conformational differencesarising from the packing of the two thiophene rings into the crystal structure is probablyimportant in determining their relative stabilities. A comparison with the structures ofother tiotropium salts indicates that there is a correlation of the dihedral angle betweenthe two tiotropium thiophene rings with the stability of the crystal forms. � 2008 Wiley-

Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:1820–1834, 2009

Keywords: co-crystals; crystal structur
e; crystallization; crystals; high throughputtechnologies; materials science; polymorphism; spectroscopy; crystallography
INTRODUCTION

The use of co-crystals as a new class of solid formsfor the consolidation of pharmaceutical actives(APIs) is an idea that has excited the industry inrecent years.1–3 The combination of an API with apharmacologically benign co-former offers anadditional dimension by which the physicalproperties of the solid, such as stability, solubility

upporting Information may be found in thef this article.ce to: Mihaela Pop (Telephone: þ31-20-586-20-586-8085;[email protected])

aceutical Sciences, Vol. 98, 1820–1834 (2009)

, Inc. and the American Pharmacists Association

AL OF PHARMACEUTICAL SCIENCES, VOL. 98, NO. 5, MAY

and dissolution rate, can be tailored to a givenformulation or end-use.4,5 Specifically, co-crystalshave been developed to modify and increase APIsolubility,6,7 to improve physical stability,8 and toincrease API plasma levels in animal tests.9,10 Thedevelopment and use of new co-crystal forms canoverlap and be combined with the conventionalrange of salts, with their common advantages ofhigher solubility and dissolution rate, to optimizea solid form.11 This trend is also extending theintellectual property landscape associated withsolid forms.12

We report here an interesting case of thetiotropium fumarate–fumaric acid salt–co-crystal.This appears to be made up of two monovalenttiotropium cations (Fig. 1) combined with a

2009

Figure 1. Tiotropium cation. Planar regions of themolecule used in analyzing the conformations areindicated.

TIOTROPIUM FUMARATE 1821

divalent fumarate anion to make the salt, plus anonionized free fumaric acid moiety to make theco-crystal. This structure seems to be stabilizedby virtue of the optimum conformation of thetiotropium cations within the crystal packing; theco-crystal structure creates the conditions for theoptimum conformation of the flexible parts ofthe molecule to be attained. The identity of thissalt–co-crystal could only be identified from astructure solution by single-crystal X-ray diffrac-tion, with some supporting evidence from Ramanand infrared spectroscopy. We have also foundand characterized a less stable 1:1 tiotropiumfumarate salt, and we discuss below the relation-ship between the two structures.

Tiotropium is a highly effective anticholinergicagent which may be used to treat respiratorycomplaints such as chronic obstructive pulmonarydisease (COPD) and asthma.13 It is prepared as thehydrobromide which can be crystallized as three

Table 1. Solid Salt Forms of Tiotropium for Which SingleObtained

Solid Form: Salt, Solvate, Co-Crystal Stoichiometrya S

Benzenesulfonate 1:1 BrTriflate 1:1 BrSalicylate monohydrate (I) 1:1:1 BrSalicylate (II) 1:1 XiHydrogen sulfate monohydrate (I)b 1:1:1 XiHydrogen sulfate (II)b XiDihydrogen phosphate monohydrate 1:1:1 FuEdisylate hydrateb 2:1:10 FuBromide—C 1:1 FuBromide—B 1:1 M

aRatio [tiotropiumþ]/[anion]/[solvent or co-former].bMore complex structures not included in Supplementary Inform

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nonsolvated forms, a monohydrate, and as solvateswith methanol, ethanol, n-butanol, isopropanol,tert-butanol, 1,2-propanediol, anisole, pyridine,THF, THP, dioxane, DMA, and DMF.13 A salt andpolymorph screen has been carried out to identifyalternatives to the bromide which may confer boththerapeutic advantages and improved physicalcharacteristics for formulation and delivery.During this screen, the fumarate salt–co-crystalhas been identified as more stable than thesimpler salt and salt–solvate fumarate forms.

RESULTS

Salt and Polymorph Screening

A salt screen has been carried out by replacingthe Br� anion with HCO�

3 using ion exchange inan aqueous solution, and converting to therequired anion by reaction of the bicarbonatewith an appropriate acid. Details are given inthe experimental section (below). A wide range ofcrystalline salt forms have been obtained. Table 1gives all solid forms for which single-crystalstructure solutions have been obtained. Summarydata for the structures not described in detail hereare provided in the supplementary information,except for the sulfates and edisylate which havemore complex structures with larger unit cellsthat we are continuing to investigate in moredetail.

Tiotropium fumarate has been prepared in fourcrystalline solid forms, as identified by theirpowder X-ray diffraction patterns. Crystal struc-ture determinations have been carried out onthree of them as summarized in Table 2. Thecompound was initially obtained as an amorphous

-Crystal X-Ray Structure Determinations Have Been

olid Form: Salt, Solvate, Co-Crystal Stoichiometrya

omide—E 1:1omide monohydrate—A 1:1:1omide dioxane solvate—D 1:1:1nafoate monohydrate (I) 1:1:1nafoate (II) 1:1nafoate monohydrate (III) 1:1:1marate ethanolate (I) 1:1:1marate (II) 1:1marate–fumaric acid co-crystala (IV) 2:1:1

esylate 1:1

ation.

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Table 2. Summary Crystal Data: Tiotropium Fumarate Forms I, II, and IV

Form I Ethanolate Form II (1:1) Salt Form IV Co-Crystal

Empirical formula C19H22NO4Sþ2 �

C4H3O�4 �C2H6O

C19H22NO4Sþ2 �

C4H3O�4

2 C19H22NO4S2þ�C4H2O2�

4 �C2H4O4

Fw 553.63 507.56 1015.12T [K] 293 (2) 120 (2)� 120 (2)�

l [A] 0.71073 0.71073 0.71073Crystal system Orthorhombic Triclinic MonoclinicSpace group Pbca P-1 P21/cUnit cell dimensionsa [A] 15.383 (7) 7.451 (2) 9.569 (2)b [A] 16.849 (7) 9.404 (3) 14.497 (3)c [A] 20.090 (12) 16.831 (6) 17.969 (3)a [8] — 102.53 (3) —b [8] — 96.40 (3) 115.939 (8)g [8] — 98.98 (5) —V [A3] 5207.1 (4) 1124.2 (6) 2241.6 (8)Z 8 2 4Dm [g/cm3] 1.412 1.499 1.504F(000) 2336 532 1064Crystal size [mm3] 0.2� 0.2� 0.08 0.4� 0.25� 0.15 0.45� 0.4� 0.3u range [8] 1!22.5 2.5! 32.6 1!35Reflections collected 6183 8173 25879Independent reflections 3285 [Rint¼ 0.0458] 6653 [Rint¼ 0.0307] 9159 [Rint¼ 0.0352]Data/restraints/parameters 3285/0/336 6653/0/416 9159/0/452S 1.040 1.090 1.030R [I> 2s(I)] R1¼ 0.0923 R1¼ 0.0579 R1¼ 0.0429

vR2¼ 0.2229 vR2¼ 0.1125 vR2¼ 0.1034R indices (all data) R1¼ 0.1243 R1¼ 0.0797 R1¼ 0.0570

vR2¼ 0.2489 vR2¼ 0.1231 vR2¼ 0.1119

�Structural data at 293 (2) K submitted as Supplementary Information.

1822 POP, SIEGER, AND CAINS

oil on evaporation of the aqueous solution in whichthe salt was prepared by the reaction of 1.1equivalents of fumaric acid with the bicarbonate.Recrystallization from absolute ethanol by coolingfrom 508C to room temperature and allowing tostand for several days gave the ethanolate solvateof the salt, Form (I). A reliable powder diffractionpattern of this material was not obtainable,because of the rapid desolvation of the powderduring measurement, and the quality of thesingle-crystal structure data obtainable has alsobeen limited by its instability. On desolvation,Form (I) transformed into the nonsolvated saltForm (II).

The nonsolvated 1:1 salt Form II was obtainedconsistently on recrystallizing the amorphous oilfrom ethanol over a period of several weeks. Itsproduction was also scaled up to obtain a 5 g batchas a starting point for the screen in whichForms III and IV were obtained. Tiotropiumfumarate Form (III) exhibited a similar butdistinct powder diffraction pattern to Form II,and was prepared by recrystallization of Form II

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 98, NO. 5, MAY 2009

from N-methylpyrrolidone (NMP) as part of theextended polymorph screen. It is believed to be anNMP solvate.

Tiotropium fumarate Form (IV) has beenobtained in 90% of the cooling and evaporativecrystallizations in the extended screen using awide range of solvents and crystallization condi-tions. Material used for the structural determina-tions and other analysis reported here wasobtained from methanol. At first sight thestoichiometric ratio of tiotropium to the anion,determined by solution 1H-NMR, indicated it to bea 1:1 salt, a polymorph of Form (II). However,structural measurements detailed below revealedthat it is really a 2:1 salt that contains a co-crystallized moiety of free fumaric acid.

Stabilities and Structures of the Fumarate Salt,Solvate and Co-Crystal Forms

DSC measurements carried out on Forms II andIV, and the thermochemical data derived from

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Figure 2. DSC traces for the fusion of Forms II andIV. (a) Transition Form II!Form IV, followed by melt-ing of Form IV; (b) melting endotherm, Form II. Dis-placed by �4 on the Y-axis for clarity; (c) meltingendotherm, Form IV. Displaced by �8 on the Y-axisfor clarity.

Figure 3. Tiotropium fumarate ethanolate (1,1,1),Form I. (a) ORTEP diagram. Ellipsoids represent 50%probabilities. (b) Packing diagram viewed along theb-axis. Fumarate and ethanol moieties are shown in bold.


them, are summarized respectively in Figure 2and Table 3. Figure 2c shows the fusion event ofthe co-crystal Form IV. While a single meltingpeak was obtained from some replicates of FormII, as in Figure 2b, it was more common to observea phase transition consisting of an endothermimmediately followed by an exotherm as shown inFigure 2a. This indicates transient fusion followedimmediately by recrystallization to a form morestable at the elevated temperature, in this caseForm IV. The enthalpy of fusion for Form II,Table 3, has been derived from a trace that doesnot show the recrystallization characteristic. TheDSC and associated TGA data indicate nosignificant changes or weight losses occurring atlower temperatures. The crystal density of FormIV (Tab. 2) is higher than that of Form II.

Summary crystallographic data on Forms I, II,and IV of the fumarate are given in Table 2.Figures 3a, 4a, and 5a respectively show theORTEP diagrams of tiotropium fumarates FormsI, II, and IV, and Figures 3b, 4b, and 5b therespective crystal packings. The atom numberings

Table 3. Thermochemical Data for TiotropiumFumarate Forms II and IV

FormMelting Point

(8C)Enthalpy of Fusion

DHf (kJ mol�1)

II 186 56.3IV 216 69.0

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of the tiotropium cation are consistent through allthese structures, and throughout the other saltstructures discussed in this article. The co-crystalForm IV and the 1:1 salt Form II have beenmeasured at both normal (293 K) and lowtemperature (120 K). The better-quality, low-temperature data sets are given in Table 2 andFigures 4 and 5. These low temperature structuredeterminations were carried out to increase thenumber of reflections per parameter, allowingmore reliable bond lengths and a more detaileddescription of the disorder present to be obtained.Disorder is present in the co-crystal Form IV atboth the inversion center at C44 and the freefumaric acid moiety at O41–C42–O43. Refittingthe data in the lower symmetry space groups P21

and P-1 did not result in any change of the model,showing that the proposed crystal structure andsymmetry are correct.

The structure of Form IV exhibits alternatingfumarate anions and free fumaric acid molecules


Figure 4. Tiotropium fumarate (1:1), Form II.(a) ORTEP diagram. Ellipsoids represent 50% probabil-ities. (b) Packing diagram viewed along the b-axis.Fumarate moieties are shown in bold.

Figure 5. Tiotropium fumarate–fumaric acid co-crystal, (2:1:1), Form IV. (a) ORTEP diagram. Ellipsoidsrepresent 50% probabilities. (b) Packing diagramviewed along the a-axis. Fumarate (blue) and fumaricacid (green) moieties are shown in bold.


(co-former) catenated along the [010] direction(Fig. 5b), forming a chain with the motif:

� � �A2�2 � � �A2H2 � � �A2�

2 � � �A2H2 � � �A2�2 � � �A2H2 � � �

(a)

On the other hand, the corresponding stoichio-metrically equivalent structure of the simple 1:1hydrogen fumarate salt Form II exhibits chainsalong [100] with the following motif:

� � �A2H� � � �A2H� � � �A2H� � � �A2H� � � �A2H� (b)

where A2 represents the fumarate anion. Whilewe accept that a definitive assignment of these


structures is difficult, and depends ultimately onthe position of a single proton, we offer thefollowing reasons and evidence for assigningstructure (a) to the chains of the salt–co-crystalForm IV:

(1) T
he inversion centers present in A2�2
and A2H2 do not allow pattern (b). Decreas-ing the symmetry to allow structure (b) doesnot alter the crystal model or improve thelevel of disorder.

(2) T
he C–O bond lengths measured in thecarboxylate functions O31–C32–O33, andO41–C42–O43 are listed in Table 4 and
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Table 4. C–O Bond Lengths in the Fumarate and Fumaric Acid CarboxylateFunctions of Tiotropium Fumarate Co-Crystal Form IV

C–O Bond Length Measured C–O Bond Length Reference 14

Bond Length (A) Bond Length (A)

C32–O31 1.2452 (13) C––C–CO�2 1.250 (17)

C32–O33 1.2691 (16) C––C–C––O (acid) 1.229 (17)C42–O41 1.307 (5), 1.310 (8)a C––C–C–OH (acid) 1.293 (19)C42–O43 1.215 (5), 1.218 (10)a

aStructures showing disorder, both values given.

DOI 10.


compared with reference values for anionand free acid functionalities.14 The O31–C32–O33 values resemble those of thecarboxylate anion, while those of O41–C42–O43 correspond to those of the freeacid.

We have also compared the infrared and Ramanspectra of Forms II and IV. The ATR-IR spectra ofthe carbonyl absorption region 1760–1450 cm�1 ofboth Forms is shown in Figure 6. Differences inthe carboxylic acid and carboxylate functionalitiesand their environments may be expected to showup particularly in the region of the C––O asym-

Figure 6. Carbonyl absorption region of theForms II and IV, 1760–1450 cm�1.

1002/jps J

metric stretch15 at 1750–1600 cm�1. The absorp-tion will move towards lower wave numbers if theoxygen atoms attached to the carbonyl becomeperturbed or ionized,16 as has been demonstratedfor carboxylate salts of terfenadine.17 The sharpfeatures at 1756 cm�1 (Form II) and 1748 cm�1

(Form IV) are probably the ester group in thetiotropium moiety (Fig. 1). The correspondingstretch modes for the fumaric acid and fumaratecarbonyls should occur at lower wave numbers,due to coupling with the C––C bond in theb-position,15 and they will be broadened byhydrogen bonding in crystalline structures. Wesuggest that the broad absorptions centered at

infrared spectrum of tiotropium fumarate


Figure 7. Raman spectra of tiotropium fumarate Forms II and IV in the range 1800–950 cm�1. Baseline of Form IV spectrum (lower) offset for clarity.


1695 cm�1 (Form II) and 1670 cm�1 (Form IV)probably originate from the nonionized COOHfunction, although the extent of broadening doesnot allow us to make more definitive assignments,apart from noting that they are different for thetwo forms. The broad absorptions centered at1630 cm�1 (Form II) and 1608 cm�1 (Form IV) maypossibly arise from the COO� anions, since boththe frequencies and the differences compared tothe COOH absorptions above are consistent withother data for these modes.15–17 However, thereare other possible absorptions in this region suchas C––C, and we cannot make the assignment withany confidence. There are also substantial differ-ences elsewhere in the spectra, particularly in thebroad O–H stretching band around 2780 cm�1

(Form II) and 2800 cm�1 (Form IV), which is alsoconsiderably broader in the latter case.

The Raman spectra of Forms II and IV in therange 1800–950 cm�1 are shown in Figure 7. Forthe salt Form II, the broad band centered at1671 cm�1 is probably the n(C––O) stretch of thecarboxylic acid, and is also exhibited by succinicacid, the unsaturated oleic and cis-vaccenic acids,and a number of unsaturated fats (esters).18 FormIV shows a similar feature at 1673 cm�1, and solidfumaric acid a very intense broad feature centeredat 1685 cm�1. The spectrum of disodium fuma-rate18 does not show this feature, but has anintense, sharp absorption at 1655–1657 cm�1


assigned to the symmetric C––C stretch. If thebroad C––O feature of fumaric acid occurs inconjunction with solid matrix effects such ashydrogen bonding, then it would be expected toeither differ or be absent from the sodiumfumarate spectrum (with no hydrogen bonding),as appears to be the case. Form IV also shows astrong feature at 1653 cm�1 that appears to bedistinct from the C––O absorption, and probablyarises from the symmetric C––C stretch mode.Such a vibration would occur in the symmetricsalt–co-crystal chain motif designated (a) above,but would not occur in the nonsymmetric 1:1 saltmotif (b). We infer that the presence of this featurein the Form IV spectrum and its absence from theForm II spectrum is consistent with our assign-ment of its structure.

Other assignable features in the Raman spec-trum include the intense absorptions at 1430 cm�1

(Form II) and 1422 cm�1 (Form IV), whichcorrespond to the symmetric n(COO�) stretch ofthe carboxylate anion,18 and appear in the sodiumfumarate spectrum but only weakly in fumaricacid. This feature is also present in the IR spectra.The features around 1073–1084 cm�1 do not occurin either the fumaric acid or fumarate spectra.Both of these reference spectra include a moderatesharp absorption at 953 cm�1 that is not present ineither Form II or Form IV, and that is difficult toassign in this complex, ‘‘fingerprinting’’ part of the

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Table 5. Calculated Bond Geometries for the Principal Intermolecular Hydrogen Bonding Modes in TiotropiumFumarate Forms I, II, and IV

Bonded Moieties Bond

Bond Lengths (A) Angle (8)

D–H H � � �A D � � �A

Form I: tiotropium fumarate ethanolate (1:1:1)Tiotrþ–Fum� O7–H7 � � �O36 0.82 1.86 2.6782 171Fum�–Fum� O30–H30A � � �O37 0.82 1.85 2.643 (1) 164EtOH–Fum� O38–H38A � � �O36 0.89 2.07 2.953 (1) 180

Form II: tiotropium fumarate (1:1)Tiotrþ–Fum� O7–H7 � � �O32 0.80 (3) 1.89 (3) 2.672 (2) 170 (3)Fum�–Fum� O36–H36 � � �O30 0.97 (4) 1.57 (4) 2.543 (2) 176 (4)

Form IV: tiotropium fumarate–fumaric acid (2:1:1) co-crystalTiotrþ–Fum� O7–H7 � � �O31 0.86 (3) 1.83 (3) 2.677 (3) 172 (3)Fum�–FumH O41–H41 � � �O33 0.94 (4) 1.49 (4) 2.423 (3) 169 (3)


spectrum. The features at 1755 cm�1 (Form II)and 1751 cm�1 (Form IV) correspond with IRabsorptions that are believed to arise from thetiotropium ester group.

Table 5 summarizes the calculated bond geo-metries for the principal intermolecular hydro-gen-bonding modes of the three fumaratestructures I, II, and IV. Very similar bondingmotifs are exhibited in all three cases, and thebond geometries are also very similar. In all cases,fumarate anions are hydrogen bonded to thetiotropium cation at –O7–H7, to each other and to

Table 6. Dihedral Angles between the PrinCation

Designation

P1P2R3R4P5

Pair of Planes

D

Form I Ethanolate F

P1–P2 73.7 (4)P1–R3 45.4 (7)P1–R4 30.0 (8)P1–P5 88.9 (3)R3–R4 67.1 (10)R3–P5 46.6 (7)R4–P5 81.5 (8)

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other molecular species (free fumaric acid orethanol) (Figs. 3a–5a).

The similarity in the hydrogen bonding patternsof the three forms, while the crystal symmetriesand packings are completely different, led us toconsider whether differences in stability and latticeenergy could arise from different conformations ofthe tiotropum cation, as would occur in cases ofconformational polymorphism.19 We have there-fore carried out a detailed analysis of the con-formations of the tiotropium cation in the threecrystal Forms I, II, and IV.

cipal Planar Motifs in the Tiotropium

Plane

C17–C18–C22–C23C18–C19–C21–C22C2–C3–C4–C5–S1

C8–C12–C11–C10–S9O14–C16–O15–C6–C13

ihedral Angle (8)

orm II 1:1 Salt Form IV Co-Crystal

74.18 (16) 73.64 (15)47.12 (15) 37.8 (4)44.93 (14) 61.43 (14)88.11 (15) 84.86 (13)76.83 (14) 89.2 (4)48.25 (14) 69.7 (4)56.01 (14) 69.55 (12)


Figure 8. Comparison of the conformations of the twothiophenes and –O7–H7 around the C6 center for FormsI, II, and IV. The ‘‘fixed’’ part of the molecule, the [3.2.1]chair structure, is shown in bold. The positions for FormI are shown as single-dashed lines, Form II as thin solidlines, and Form IV as double-dashed lines.


Table 6 gives the dihedral angles between theprincipal planar motifs in the tiotropium cation,calculated using PLATON.20 The motifs are listedin Table 6 and are shown in Figure 1. Planes P1and P2 belong to the bicyclic [3.2.1] chairstructure shown in the upper part of the moleculein Figure 1. The angle between P1 and P2 remainsessentially constant across the three formscompared, and the angles between P2 and theother planes essentially replicate the pattern ofbehavior indicated by the angles with the P1plane. P5 represents a coplanar arrangement ofatoms around the central ester group. R3 and R4represent the two thiophene rings attached to theC6 carbon atom which is also joined to O7participating in hydrogen bonding in all struc-tures. Free rotation of these three functionalitiesaround the relatively unhindered C6 centerappears to confer conformational flexibility tothese structures. Table 6 shows that the mostsignificant differences between the three formsare mediated by the position of the R4 ring (C8–C12–C11–C10–S9), with a difference of up to 308in the dihedral angles relative to P1 and R3.

Figure 8 shows an overlay of the conformation ofthe R3 and R4 thiophene rings and –O7–H7groups in the three Forms I, II, and IV viewedalong the C6–C13 bond. The bicyclic [3.2.1] chairstructure is held constant and represented by boldlines. The R3 ring (C2–C3–C4–C5–S1) shown onthe left of Figure 8 retains a similar orientationwith respect to the ‘‘fixed’’ part of the molecule, butthe differences in the R4 positions are clearlyvisible on the right of the illustration. The torsionangles of the thiophene rings around the C5–C6and C8–C6 bonds are given in Table 7 for the threeForms. Torsion angles measured for the C6–C13bond relative to the ester group showed that thelatter rotated freely relative to the C5–C6–C8motif. The torsion angles around C5–C6 (R3) aresimilar for the three crystal forms, with particu-larly good agreement between Forms I and II. Onthe other hand, the corresponding angles aroundC8–C6 (R4) varied considerably. These torsionalfeatures may also be seen in Figure 8 as the anglesthat the thiophene rings adopt relative to thebicyclic moiety.

The observed similarities in bonding motifs andvariations in orientations of the thiophene ringsaround the C6 center led us to examine to whatextent the relative stability of the Forms could becorrelated with the conformation of the cation. Wehave carried out a molecular mechanics calcula-tion on the (gas-phase) cation, to compare the


contributions of the torsion angles to the con-formational energy with other modes includingbond lengths and angles, using the AMBER forcefield. The calculation showed that the contribu-tions of the torsion angles to the conformationenergies are relatively small (7 kJ mol�1), andthat the latter are dominated by the bond lengthcontributions. As the bond lengths are prescribedby the force field, and are probably not represen-tative of the crystal structure, we do not believethat this calculation is representative enough todraw conclusions about the stability ordering orthe contributions that determine it. We havetherefore examined other crystalline tiotropiumsalts to compare the conformations more widely.

Comparison With Other Crystalline Tiotropium Salts

We have examined the structures of all the saltslisted in Table 1, except for the sulfates and

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Table 7. Torsion Angles (8) of the Thiophene Rings and Hydrogen-Bonded OH Group for Forms I, II, and IV

Bond Angle Form I Ethanolate Form II 1:1 Salt Form IV Co-Crystal

C5–C6 (R3) S1–C5–C6–C13 �132.52 �131.5 �133.03S1–C5–C6–C8 104.96 106.49 103.09S1–C5–C6–O7 �11.64 �11.38 �17.11

C8–C6 (R4) S9–C8–C6–C13 81.49 57.65 �150.15S9–C8–C6–C5 �151.94 �179.41 �29.29S9–C8–C6–O7 35.08 �59.17 88.73

Figure 9. R3–R4 dihedral angle plotted against den-sity for tiotropium salts. Trend lines are shown forindividual anions (Xin, Fum, Br) for which multiplesolid structures are available.


edisylate which are more complex structurescontaining multiple cations in the asymmetricunit. The nonhydrated structures all contain ahydrogen bond from the tiotropium O7–H7 to oneof the oxygens of the sulfonate, phosphate orcarboxylate anion, while in the monohydrates theO7–H7 function is hydrogen-bonded to the wateroxygen. All of these hydrogen-bonded motifs havevery similar geometries to those recorded for thefumarates in Table 5. In the monohydrates, one orboth of the water hydrogens are also furtherbonded to anion oxygen atoms.

Measurements of the torsion angles of the twothiophene rings around the C6–C13 bond gavevery different values for the various structures. Inall cases, the positioning of the two rings appearedto be determined by a combination of thepositioning of the above hydrogen bond withrespect to the anion and the crystal packing. Inthe benzenesulfonate, the three torsion anglesassociated with the R3 ring are very similar tothose for the fumarate Forms I and II in Table 7,while those of the R4 ring deviated considerably.It thus appears that the factors driving structurestability in this case are very similar to those forthe fumarates, with a ‘‘locked’’ R3 ring and a muchmore flexible R4 ring.

In the other structures examined, the R3 ringalso exhibited rotational flexibility, as indicatedby the variability of the torsion angles around S1–C5–C6. To accommodate this additional flexibil-ity, we focused on the dihedral angles betweenthe two thiophene rings R3 and R4, and theirinfluence on the crystal packing. Figure 9 showsthe R3–R4 dihedral angle plotted against densityfor all the salts in Table 1 except the sulfate andedisylate. Here, monohydrates and anhydratesare not distinguished, on the grounds that eitherwater or an oxyanion connect to the O7–H7hydrogen bond, and the positioning of the R3 andR4 thiophene rings are not otherwise affected bythe presence or absence of water. Trend lines aredrawn in Figure 9 to show the increase in the R3–R4 dihedral angle with increasing density for the

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salts that exhibit multiple forms—xinafoates,fumarates, and bromides. Figure 9 also showssome evidence of banding of these structures, inthe dihedral angle ranges 67–698, 75–788, and 85–898. Except for the salicylates, the other saltsrepresented—benzenesulfonate, triflate, metha-nesulfonate, and dihydrogen phosphate—showedno clear evidence of alternative solid forms in thepolymorph screens.

DISCUSSION

Tiotropium fumarate appears to be unique amongthe tiotropium salts identified in this investigationin forming a co-crystal of the 2:1 salt with a freeacid moiety. An interesting question arises as towhether this co-crystal can be considered poly-morphic with the less stable 1:1 tiotropiumfumarate salt, Form II. McCrone19 includes con-formational polymorphs under his general defini-tion of polymorphism, but Bernstein21 judges thatisomeric effects such as tautomerism and dynamicisomerism lie outside of the scope of polymorphism.However, a more recent investigation22 includestautomers within its definition of polymorphism.



Although the formal difference between Forms IIand IV in terms of molecular species consists of theposition of a single proton, the two forms invokedifferent formal form types and nomenclature andexhibit different symmetries. We have thereforenot used the term polymorphism to relate tiotro-pium fumarate Forms II and IV, and have not citedthe Burger–Ramberger rules23 to relate theirstabilities, but we judge nevertheless that theexperimental evidence shows that Form IV is themore stable under the temperature conditions ofthis investigation.

A recent detailed investigation of molecularsalts of (1R,2S)-ephedrine24 has shown that theusual guidelines for selecting candidate salt formsderived from the pKa values of the correspondingacids does not work in organic solvents such asmethanol, where the effective pKa values of acidscan be up to 5 pH units higher than in aqueoussolutions. It is also legitimate to question whethersalts formed under these conditions can always berepresented as simple ion pairs, or whether theycan also adopt bonding patterns characteristicof a co-crystal. The structural ambiguity betweensalts and co-crystals in general, for exampleinvolving aromatic nitrogen centers, has alsobeen noted,25 although the changes in propertiesbrought about by complete proton transfer and ionpair formation are likely to be very significant.

Conformational flexibility is conferred by freerotation about the C6 center, via which twothiophene rings are positioned to accommodatethe participation of the OH group in intermole-cular hydrogen bonding. It is the positioning ofthese rings determined by the geometric require-ments of the hydrogen bond and the crystalpacking that correlates with the relative stabi-lities of the solid forms.

It further appears that the position of one of thethiophene rings designated R3 (Fig. 8) varies littlebetween the three structures, while the other ring(R4) is clearly labile within the solid forms. Wesuggest that the positioning of this ring within thecrystal structure to minimize the conformationalenergy may determine the relative stability of thesolid form. We were unable to verify this withconformational energy calculations because thetorsional contributions within the molecularmechanics model were relatively small comparedwith other factors, and the force-field was notsufficiently reliable for us to make such distinc-tions with confidence.

We have been able to prepare and characterize arelatively large number of single crystals of


tiotropium salts. However, we have only a singleexample of a salt–co-crystal, which was preparedfrom a simple salt precursor via rearrangement ofa less stable simple salt form to a more stable salt–co-crystal. Comparisons of the fumarate struc-tures with other tiotropium salts indicated thatthere were no fixed or constant absolute molecularorientations of the thiophene rings, but that theirpositions with respect to each other probably exerta significant effect on relative stability. Compar-isons of densities with the dihedral anglesbetween the rings (Fig. 9) support this. For thebromides, additional stability tests13 show thatthe denser Form E, with the larger dihedral angle,is definitely the most stable.

The bonding of the anions to solvent (water,ethanol, dioxane, NMP) and co-crystal formermolecules (fumaric acid) is a general character-istic of the tiotropium solid structures, and occursin other structures reported in the CCDC data-base. A search of the database revealed 41crystalline structures that contained a combina-tion of an anion and the corresponding free acid.Of these, 16 contain hydrogen bis(anion) moieties,whereby two anions are linked by a hydrogen bondformed by ‘‘sharing’’ the proton from a single freeacid group, structure motifs very similar to thosein our co-crystal. In the L-arginine succinate—succinic acid monohydrate26 (2:1:1:1) salt–co-crystal–solvate (P-1, R¼ 3.60%), the L-argininemoieties are oriented in pairs that are arrangedhead-to-tail roughly parallel to the (001) plane,with the anions, acid and water molecules layeredbetween the dimers. Each L-arginine moleculeappears to be hydrogen bonded to a water oxygenvia the protonated amine function, while a shortcontact between a succinate carbonyl oxygenand a proton of the free acid (H � � �A¼ 1.258 A)represents a mode of linkage similar to that oftiotropium fumarate co-crystal Form IV. Anothersimilarity is that these structure characteristicsinvolve a diacid and its anion with terminalcarboxylate functions, which probably exhibitsimilar molecular recognition characteristics. Itis therefore possible that this is a further exampleof the molecular conformation in the solid statedetermining the most stable co-crystal structure.

The analytical methods commonly used todetermine the composition of salts, notably ionchromatography and solution 1H-NMR, wouldindicate a stoichiometric ratio base/counterion of1:1, and Form IV could be easily mistaken as amore stable polymorph of the anhydrous 1:1 salt.The true identity of the co-crystal was only

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established via a structure solution by single-crystal X-ray diffraction, with some corroboratingevidence by Raman and infrared spectroscopy.

In the absence of satisfactory single crystals,there is a need to investigate and develop otheranalytical techniques by which co-crystals can beidentified. Conventional ‘‘fingerprinting’’ by pow-der X-ray diffraction enables the appearance ofnew crystalline forms in crystallized mixtures ofAPI and co-formers to be identified, but gives noinformation on the new form, which could be a co-crystal or a polymorph of one of the constituents.However, the crystal structures of some cyclodex-trin complexes involving multiple molecularcomponents have been determined recently usingpowder diffraction with synchrotron radiation,27

and it may be possible to attempt such solutionsfrom powder data obtained with conventionallaboratory X-ray sources. We are also currentlyapplying a combination of Raman, ATR-IR andsolid-state NMR spectroscopies more generally asmethods for confirming the identity of co-crystalswhere single crystals suitable for X-ray structuredetermination cannot be prepared.

CONCLUSIONS

We present here an example of a salt–co-crystalthat is the most stable solid form of tiotropiumfumarate under ambient conditions. The struc-ture comprises a 2:1 salt of the diacid co-crystal-lized with a free fumaric acid moiety, and appearsto be unique among a large number of tiotropiumsalts prepared. The co-crystal is more stable thanthe corresponding simple 1:1 salt and an ethanolsolvate, and forms via rearrangement on recrys-tallizing the salt. The stability correlates with andappears to be conferred by the packing arrange-ment of the cations.

The determination of an overall 1:1 cation/anionstoichiometry, for example by ion chromatogra-phy or solution 1H-NMR, could lead to the falseconclusion that this form is a simple polymorph ofthe 1:1 salt. Structure solution by single-crystalX-ray diffraction shows that it is a 2:1 salt with anadditional free fumaric acid moiety, and that theanions and free acid co-formers are catenatedalong [010] between pairs of tiotropium cations.Raman and infrared spectra are also consistentwith a structure of this type. Detailed examina-tion of the hydrogen bonding motifs, the dihedraland torsion angles, indicates that the stability ofthis structure appears to be conferred by the free

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rotation of the two thiophene rings of the cationabout the C6 center. The positions of these ringsare determined by the hydrogen bonding of the–OH group also on C6 and by the crystal packing.The position of one of the thiophene rings isinvariant, while the other accommodates to thepacking.

Other salts of tiotropium exhibit similar char-acteristics around the C6 center, while the crystalsymmetries are completely different. The –OHgroup is hydrogen bonded, either to the anion or tothe water molecule of monohydrates. We find acorrelation of the dihedral angles between the twothiophene rings with the densities of the crystals,as indicators of relative stability. We show, fromour extensive data set, that there are nosignificant differences between the hydrogenbonding in a co-crystal and in other solid forms,and that stabilization probably arises frommolecular conformation and crystal packing. Wealso find some evidence of conformational bandingof the dihedral angle between the two thiophenefunctions over the data set of salts which exhibitconformational polymorphism.

EXPERIMENTAL

Preparation of Tiotropium Salts

The starting material for experiments was tio-tropium bromide.13,28 This was initially convertedto the bicarbonate by ion exchange. The ionexchange resin (IER), Dowex 1x8-100 Cl (Aldrich21,742-5, 1.5 eq./mL), was preconditioned by aflow of saturated NaHCO3 solution over a period of4–5 h, until an exit sample showed no Cl� ontesting with AgNO3, followed by rinsing withdemineralized water until a pH of 6–7 wasattained. The conditioned IER was kept anddispensed as a slurry.

One gram aliquots of tiotropium bromide weredissolved in 50 mL of demineralized water in aparallel reactor block (24� 250 mL) equippedwith magnetic stirring. Ten milliliters of IERslurry was added to each sample, followed by asecond 5 mL addition after 3–5 min. A silvernitrate test carried out on a selection of thereactors indicated the completion of the exchange.After stirring for an additional 5 min the slurrieswere transferred to a parallel vacuum filtrationunit equipped with PTFE filters with a pore size of10 mm.



The collector vials were previously charged with1.1 equivalents of the respective acids in 10 mL ofwater. The samples of filtrateþ acid were storedat 48C in a refrigerator overnight. The followingday, crystals had formed from the salts ofphosphoric, sulfuric, benzensulfonic, and trifluor-omethanesulfonic acids, comprising respectivelythe dihydrogen phosphate monohydrate, thehydrogen sulfate monohydrate (I), the benzene-sulfonate, and the triflate (trifluoromethanesul-fonate) (Tab. 1). These were filtered, washed withcold demineralized water and dried undervacuum. The remaining salts were isolated byremoval of water below 358C. The resulting oilswere further dried under vacuum to yieldamorphous hygroscopic solids except for thesalicylate and the edisylate (ethanedisulfonate)salts, which respectively formed crystals of thesalicylate monohydrate (I) and the edisylatedecahydrate, and the xinafoate (1-hydroxy-2-naphthoate) obtained as a powder. The integrityof the salts was checked by HPLC.

The six crystalline tiotropium salts obtained asabove could be immediately characterized bysingle crystal X-ray diffraction. For the remainingsalts, recrystallizations were carried out to obtainsuitable single crystals. Tiotropium fumarateswere successfully recrystallized, from ethanol asdescribed in the main text. One hundred milli-grams of the oil obtained as above was suspendedin absolute ethanol, and heated to 508C for 1 h tobring about complete dissolution. The solutionswere then cooled slowly to room temperature, andaged for several weeks to allow growth of thecrystallized products. Details of the preparation ofeach solid form of tiotropium fumarate are givenin the main text.

The xinafoate monohydrate (I) was prepared in asimilar way to the fumarate, except that aredissolution temperature (in ethanol) of 708Cwas employed. The remaining salt forms inTable 1 were obtained from (evaporative) crystal-lization screens starting with the salts prepared asabove. For each salt, a stock solution was preparedin acetone/water (80:20) or water. Ninety-six-wellplates were charged by dosing each tiotropium saltin two stages of 40 and 35mL, equivalent to a total of4.5 mg solid, in each well. The plates were placedin a vacuum chamber (1 kPa) at room temperaturefor 1 day between each dosing stage. After the stocksolvent was evaporated, 24 different crystallizationsolvents were added and each well was individuallysealed. The plates were then subjected to a series oftemperature profiles.


A total of 2304 experiments were carried out asabove in the salt screens, and a further 3840 in thesubsequent polymorph screens of 10 salts. Thesolids obtained were subsequently isolated byevaporation of the solvent at room temperaturein a vacuum chamber (13 kPa), and initiallyexamined by high-throughput X-ray powderdiffraction29 to identify new solid forms. Theadditional crystal forms for which the structureswere determined by single-crystal X-ray diffrac-tion, cf. Table 1, were recovered in this way fromthe wells crystallized from the following solvents:

Crystal (Tab. 1)
Solvent(s)
Salicylate (II)
3-ethoxy-1-propanol;1,2-propanediol
Hydrogen sulfate (II)
DMSO/water, 80:20 Xinafoate (II) Nitromethane/
water, 90:10
Xinafoate
monohydrate(III)
1-hexanol
Mesylate(methanesulfonate)

Nitromethane

In a scale-up experiment to produce 5 g oftiotropium fumarate Form II, 6.0 g of tiotropiumbromide monohydrate dissolved in 350 mL waterwas added to a slurry of 150 mL of bicarbonateloaded IER (preparation see above) in water(containing 100 g of resin). The mixture wasvigorously stirred for 5 min, the IER was removedby filtration and washed with another 150 mLwater. In the filtrate no bromide ions could bedetected on testing with AgNO3. Fumaric acid(1.5 g) dissolved in 50 mL water at 808C was addedto the filtrate containing the ion exchangedtiotropium bicarbonate. The pH of the resultingaqueous solution was measured to be 3.6. Theaqueous solution was evaporated at 40–508C untilan oily material was obtained (oil yield: 7.2 g). Theresidual oil was dissolved in 50 mL ethanol at608C. After a few minutes formation of crystalswas observed. A further 50 mL of ethanol wasadded and the resulting suspension stirred overnight at room temperature. The crystals formedwere filtered, washed with a small amount of coldethanol, and dried at 408C. The yield of tiotropiumfumarate Form II obtained was 5.7 g.

X-Ray Diffraction

Solids recovered were initially examined by X-raypowder diffraction (XRPD). The plates were

DOI 10.1002/jps


mounted on a Bruker GADDS diffractometerequipped with a Hi-Star area detector. Theplatform was calibrated using silver behenatefor the long d-spacings and corundum for the shortd-spacings. Data collection was carried out atroom temperature using monochromatic CuKa

radiation in the 2u region between 1.58 and 41.58.Suitable single crystals for structure determi-

nations were glued to a glass fiber, which was thenmounted on an X-ray diffraction goniometer.X-ray diffraction data were collected at a tem-peratures of 120 and 293 K, using a KappaCCDsystem and MoKa radiation, generated by aFR590 generator (Bruker Nonius, Delft, TheNetherlands). Unit-cell parameters and crystalstructures were determined and refined usingthe software package maXus.30

For the tiotropium fumarate 1:1 salt Form II(Fig. 4), all hydrogens were found on thedifference map and refined isotropically. For thesalt–co-crystal Form IV (Fig. 5), the aboveprocedure was applied to all hydrogen atoms forwhich no disorder was present. Where disorderwas present, the displacement parameters werefixed as follows: (i) H(C) as 120% of the displace-ment factor of the neighboring C atom; (ii) H(O) as150% of the displacement factor of the neighbor-ing O atom.

From the crystal structures the theoreticalX-ray powder diffraction patterns were calculatedusing Mercury for Windows version 1.531 tocompare with the measured powder patterns.

DSC

DSC thermograms were recorded with a heat fluxDSC822e instrument (Mettler-Toledo GmbH,Switzerland) calibrated for temperature andenthalpy with indium (m.p. 156.68C; DHf 28.45J g�1). Samples were sealed in standard 40 mLaluminum pans and heated from 25 to 3008C, at arate of 208C min�1. Dry N2 gas, at a flow rate of50 mL min�1, was used to purge the equipmentduring measurement.

Infrared and Raman Spectra

Infrared spectra were recorded on a Thermo-scientific Nicolet 6700 FTIR spectrophotometer inATR (attenuated total reflectance) mode usingsamples of crystalline material crushed to apowder in a mortar and pestle. The spectralresolution was set to 0.5 cm�1. Raman spectra

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were recorded on a Nicolet Almega XR DispersiveRaman spectrometer supplied by the Thermo-Electron corporation using an incident laser beamat 780 nm. These were recorded for crushedcrystal samples of tiotropium fumarate Forms IIand IV, powdered solid fumaric acid, and also for asingle large crystal of Form IV. The spectra ofcrushed and single-crystal samples of Form IVwere indistinguishable, showing no discernableface-specific effect.

SUPPLEMENTARY INFORMATION

Summary crystallographic data for the followingtiotropium salts: benzenesulfonate, triflate, mesy-late, salicylate monohydrate (I), salicylate (II),dihydrogen phosphate monohydrate, xinafoatemonohydrate (I), xinafoate (II), xinafoate mono-hydrate (III), bromide (E), bromide (B), bromide(E), bromide monohydrate (A), bromide dioxanesolvate (D). Summary data for RT (293 K)structure determinations of tiotropium fumarateForms II and IV. CIF files for fumarate Forms I, IIand IV in Table 2.

ACKNOWLEDGMENTS

Conformational energy calculations were carriedout by Menno Deij.

REFERENCES

1. Vishweshwar P, McMahon J, Bis JA, ZoworotkoMJ. 2006. Pharmaceutical co-crystals. J PharmSci 95:499–516.

2. Bucar D-K, Henry RF, Lou X, Borchardt TB, ZhangGGZ. 2007. A ‘‘hidden’’ co-crystal of caffeine andadipic acid. Chem Commun 5:525–527.

3. Almarrson O, Zaworotko MJ. 2004. Crystal engi-neering of the composition of pharmaceuticalphases. Do pharmaceutical cocrystals represent anew path to improved medicines? Chem Commun17:1889–1896.

4. Hickey MB, Peterson ML, Scoppettuolo LA, Morri-sette SL, Vetter A, Guzman HR, Remenar JF,Zhang Z, Tawa MD, Haley S, Zaworotko MJ,Almarrson O. 2007. Performance comparison of acocrystal of carbamazepine with the marketed pro-duct. Eur J Pharm Biopharm 67:112–119.

5. Remenar JF, Morrisette SL, Peterson ML, MoultonB, McPhee JM, Guzman HR, Almarrson O. 2003.Crystal engineering of novel cocrystals of a triazole



drug with 1,4-dicarboxylic acids. J Am Chem Soc125:8456–8457.

6. Almarsson O, Bourghol Hickey M, Peterson M,Zaworotko MJ, Moulton B, Rodriguez-Hornedo N.PCT Int. Pat. Appns. 2004. WO 2004078161,WO 2004078163.

7. Childs SL, Chyall LJ, Dunlap JT, Smolenskaya VN,Stahly BC, Stahly GP. 2004. Crystal engineeringapproach to forming cocrystals of amine hydrochlor-ides with organic acids. Molecular complexes offluoxetine hydrochloride with benzoic, succinic andfumaric acids. J Am Chem Soc 126:13335–13342.

8. Trask AV, Motherwell WDS, Jones W. 2005.Pharmaceutical co-crystallization: engineering aremedy for caffeine hydration. Cryst Growth Des5:1013–1021.

9. McNamara DP, Childs SL, Giordano J, Iarriccio A,Cassidy J, Shet MS, Mannion R, O’Donnel E, ParkA. 2006. Use of a glutaric acid cocrystal to improveoral bioavailability of a low solubility API. PharmRes 23:1888–1897.

10. Variankaval N, Wenslow R, Murry J, Hartman R,Helmy R, Kwong E, Clas S-D, Dalton C, Santos I.2006. Preparation and solid-state characterizationof nonstoichiometric cocrystals of a phosphodiester-ase-IV inhibitor and L-tartaric acid. Cryst GrowthDes 6:690–700.

11. Childs SL, Stahly GP, Park A. 2007. The salt—cocrystal continuum: the influence of crystal struc-ture on ionisation state. Mol Pharm 4:323–338.

12. Trask AV. 2007. An overview of pharmaceuticalcocrystals as intellectual property. Mol Pharm4:301–309.

13. Pop M, Mulder Houdayer S, Lamkadmi M. PCT Int.Pat. Appn. 2006. WO 2006117299.

14. Chemical Rubber Company (CRC) Handbook ofChemistry and Physics. 81st edition, CRC Press.2000–2001. pp 9-8–9-9.

15. Max J-J, Chapados C. 2004. Infrared spectroscopyof aqueous carboxylic acids: comparison betweendifferent acids and their salts. J Phys Chem A108:3324–3337.

16. Lecomte J. 1950. Infrared absorption spectra ofmetallic acetyl-acetonates. Discuss Faraday Soc9:125–131.

17. Ertel KD, Long DF. US Pat. Appn. No. 624362, 3rdJuly 1997.

18. De Gelder J, De Gussem K, Vandenabeele P, MoensL. 2007. Reference database of Raman spectra of


biological molecules. J Raman Spectrosc 38:1133–1147.

19. McCrone WC. 1965. Polymorphism. In: Fox D,Labes MM, Weissberger A, editors. Physics andchemistry of the organic solid state. Vol. 2. NewYork: Wiley Interscience. pp 725–767.

20. Spek AL. 2003. Single-crystal structure validationwith the program PLATON. J Appl Cryst 36:7–13.

21. Bernstein J. 2002. Polymorphism in molecular crys-tals. IUCr monograph on crystallography. UK:Oxford University Press.

22. Zhang ZQ, Njus JM, Sandman DJ, Guo C, FoxmanBM, Erk P, van Gelder R. 2004. Diiminoisoindoline:tautomerism, conformation and polymorphism.Chem Commun 7:886–887.

23. Burger A, Ramberger R. 1979. On the polymorph-ism of pharmaceuticals and other molecular crys-tals. Theory and applicability of thermodynamicrules. Mikrochim Acta II:259–316.

24. Black SN, Collier EA, Davey RJ, Roberts RJ. 2007.Structure, solubility, screening and synthesis ofmolecular salts. J Pharm Sci 96:1053–1068.

25. Aakeroy CB, Fasulo ME, Desper J. 2007. MolPharm 4:317–322.

26. Prasad GS, Vijayan M. 1990. X-ray studies oncrystalline complexes involving amino acids andpeptides. XIX. Effects of change in chirality inthe complexes of succinic acid with DL- and L-arginine. Int J Pept Protein Res 35:357–364.

27. Pop M, Goubitz K, Borodi G, Bogdan M, De RidderDJA, Peschar R, Schenk H. 2002. Crystal structureof the inclusion complex of b-cyclodextrin withmefenamic acid from high-resolution synchrotronpowder-diffraction data in combination with molec-ular mechanics calculations. Acta Cryst B 58: 1036–1043.

28. Pop M, Mulder Houdayer S, Sieger P, Pieper MP.US Pat. 2006. 0287530.

29. Blomsma E, van Langenvelde AJ. PCT Int.Pat. Appns. 2003. WO 2003/031959A2, WO 2003/060497A1.

30. Mackay S, Gilmore CJ, Edwards C, Stewart N,Shankland K. 1999. maXus Computer Programfor the Solution and Refinement of Crystal Struc-tures. Bruker Nonius, The Netherlands; Mac-Science, Japan & The University of Glasgow.

31. CCDC reference: http://www.ccdc.cam.ac.uk/pro-ducts/csd_system/mercury.

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tiotropium fumarate: an interesting pharmaceutical co-crystal

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