Salts and ionic liquid of the antituberculosis drug S,S-ethambutol

Article abstract of DOI:10.1021/cg400071d

A salt screen of the antituberculosis chiral basic drug Ethambutol with protic acids resulted in the formation of several salts and an ionic liquid. The protic salt/ionic liquid product was characterized by spectroscopic (ATR-IR and ss-NMR), thermal (DSC and TGA), and X-ray diffraction. Similar to the marketed Ethambutol dihydrochloride salt of the drug, all the new salts were found to be hygroscopic. Moisture-free conditions of the desiccator and rotavapor gave nonhygroscopic materials in a few cases. X-ray crystal structures of two new salts were determined and that of the Ethambutol base and Ethambutol dihydrochloride salt were redetermined in this work.

Full text of DOI:10.1021/cg400071d

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Article
Salts and Ionic Liquid of The Anti-Tuberculosis Drug S,S-Ethambutol
Suryanarayan Cherukuvada, and Ashwini Nangia
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400071d • Publication Date (Web): 07 Mar 2013
Downloaded from http://pubs.acs.org on March 8, 2013
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Salts and Ionic Liquid of The Anti-Tuberculosis Drug S,S-Ethambutol
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Suryanarayan Cherukuvada and Ashwini Nangia*
School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road, Central University
PO, Gachibowli, Hyderabad 500 046, India
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Eꢀmail: ashwini.nangia@gmail.com
Abstract
A salt screen of the antiꢀtuberculosis chiral basic drug Ethambutol with protic acids
resulted in the formation of several salts and an ionic liquid. The protic salt/ionic liquid
product was characterized by spectroscopic (ATRꢀIR and ssꢀNMR), thermal (DSC and
TGA) and Xꢀray diffraction. Similar to the marketed Ethambutol dihydrochloride salt of
the drug, all the new salts were found to be hygroscopic. Moistureꢀfree conditions of
desiccator and rotavapor gave nonꢀhygroscopic materials in a few cases. Xꢀray crystal
structures of two new salts were determined and that of Ethambutol base and Ethambutol
dihydrochloride salt were redetermined in this work.
Introduction
Ethambutol (abbreviated as EMB; Figure 1) is the firstꢀline antiꢀtuberculosis drug
administered with Rifampicin, Isoniazid and Pyrazinamide in Fixed Dose Combinations
(FDC).1 It is a chiral basic drug (Figure 1) with the therapeutically active S,Sꢀenantiomer
whereas the R,Rꢀform is inactive.2 It is formulated as the dihydrochloride salt (S,SꢀEDH)
which is synonymous with the drug.1,3 AntiꢀTB FDC products are reported to be unstable
due to drug–drug interactions between the component drugs.3 The hygroscopicity of
ethambutol dihydrochloride catalyzes the degradation of rifampicin and isoniazid in the
FDC formulation, resulting in the loss of drug potency upon storage.1a,3 As a consequence,
individual drugs of the FDC formulation must be separately coated with polymers to
avoid mutual interactions and water uptake, and then blended to make up the final
product formulation.4 Several salts of ethambutol base (dinitrate,5 dihydrobromide,6
oxalate pentahydrate,7 and metal salts8) including a drug–drug methanesulfonic acid salt
with isoniazid9 are reported but without any discussion of their physicoꢀchemical
properties. The dihydrochloride salt of ethambutol is tetramorphic,10 but neither the Xꢀray
crystal structure nor any polymorphs of S,Sꢀethambutol base are reported.
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SO3H
H2SO4
SO3H
OH
H
N
Sulfuric acid
N
H
OH
H3C SO3H
p-Toluenesulfonic acid
Benzenesulfonic acid
HOOC
Ethambutol
Methanesulfonic acid
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HOOC
COOH
COOH
Fumaric acid
Adipic acid
Figure 1 Molecular structure of ethambutol and protic acids used in this study.
In an attempt to solve the hygroscopicity problem of S,SꢀEDH, we performed a
salt screen of EMB base with several saltꢀforming protic acids (Figure 1) to find new
salts having lower hygroscopicity. We obtained five salts (sulfate, dimesylate, ditosylate,
dibesylate, and fumarate) and two ionic liquids (dibenzoate11 and adipate), but these new
products were also found to be hygroscopic. Trimorphs of ethambutol dibenzoate ionic
liquid were reported in a preliminary communication.11 Ionic liquids (ILs) are salts with
melting point below 100 °C12 and if they are liquids at room temperature (m.p. ≤ 25 °C)
then they are termed as ‘room temperature ionic liquids’ (RTILs).13 The hygroscopic
nature of the salts and ionic liquids was assessed by visual inspection and Karl Fischer
(KF) titration. Out of the six salts, including the ionic liquid, of this report, two salts
crystallized as diffraction quality single crystals (dimesylate and fumarate hydrate), and
their Xꢀray crystal structures are reported.
Results and Discussion
The salts were prepared by different methods such as (i) direct reaction of the
base (EMB) with an acid (the salt former) in a suitable solvent, (ii) coꢀgrinding,14 (iii) coꢀ
melting, (iv) solution crystallization, (v) crystallization in desiccator or rotavapor, etc. A
summary of the experimental conditions is given in Table 1 and the methodology is
detailed in the Experimental Section. All the product salts were obtained both in solid and
liquid state (due to hygroscopic nature) in these experiments. The dimesylate, ditosylate,
dibesylate and fumarate EMB salts crystallized as solids in a majority of experiments,
whereas sulfate and adipate crystallized in fewer conditions (Table 1). Out of the six salts,
five are high melting solids (sulfate, dimesylate, ditosylate, dibesylate and fumarate; m.p.
> 100 °C) and one is an ionic liquid (adipate; m.p. 87 °C). The physical characterization
of the protic salt/ionic liquid15 was done by spectroscopic (ATRꢀIR and ssꢀNMR),
thermal (DSC and TGA) and Xꢀray diffraction.
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Table 1 Summary of salt screening experiments.
Synthesis by
direct reaction
of base and acid
solid producta
Solution
Storage of
physical
Crystallization Crystallization
S. No.
Salt
crystallization in
MeOH/ EtOH
viscous liquid
solidd; single
Co-grinding
Co-melting
in desiccator
viscous liquid
solidd
in rotavapor
semisolid
solidd
mixture
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c
c
1
2
Ethambutol sulfate
Ethambutol
viscous liquidb
slight sticky solidb
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solid productd
ꢀdoꢀ
ꢀdoꢀ
semisolid
ꢀdoꢀ
dimesylate
crystals at –20 °C
Ethambutol
dibesylate
Ethambutol
ditosylate
3
4
ꢀdoꢀ
solidd
ꢀdoꢀ
ꢀdoꢀ
liquid extract
solidd
viscous liquide
ꢀdoꢀ
semisolid
ꢀdoꢀ
ꢀdoꢀ
ꢀdoꢀ
Ethambutol
fumarate (hydrate)
Ethambutol adipate
f
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ꢀdoꢀ
solid producta
solidd
single crystals
semisolid
semisolid
ꢀdoꢀ
ꢀdoꢀ
ꢀdoꢀ
ꢀdoꢀ
ꢀdoꢀ
viscous liquid
semisolid
a
b
Tends to liquefy during extraction upon concomitant exposure to atmospheric moisture, but crystalline solid by PXRD; Acetoneꢀassisted
grinding; c Not feasible as the acid component is liquid; d Crystalline solid by PXRD; e Neat grinding and concomitant exposure to atmospheric
moisture; f Not done since the acid component decomposes upon melting.
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X-ray crystal structures
(+)ꢀEthambutol is a chiral molecule with the S,Sꢀconfiguration (Figure 1). The
reported crystal structures of EMB and its salts (dihydrochloride, dinitrate, dibromide,
oxalate pentahydrate) in the Cambridge Structural Database16 have no mention of the
Flack parameter.17 In this study, we inferred the absolute configuration of the drug by
specific rotation measurement and the Flack parameter for the Xꢀray crystal structure of
ethambutol dihydrochloride (EDH), salt which is the starting material in our experiments.
Commercial EDH was neutralized to ethambutol base for making new salts (detailed in
the Experimental Section). There is no information on the stereochemistry of the salt on
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the commercial package label. The chirality of the base and the dihydrochloride salt were
25
D
found to be S,S by matching their specific rotation with the reported values ([
α]
:
ethambutol expt. +13.4° (+13.7°), ethambutol dihydrochloride expt. +7.8° (+7.6°).18 A
reliable Flack parameter to infer the absolute structure17 was obtained for the
dihydrochloride and dimesylate salts because they contain a heavy atom Cl and S (atomic
number > Si). The Flack parameter from the Xꢀray crystal structure is 0.09(17) and –
0.04(16), respectively, which confirms the assigned S,Sꢀchirality for the salts. Xꢀray
crystallographic parameters are listed in Table 2 and hydrogen bonds in Table 3. The
space group and crystallographic features of the dihydrochloride salt (EDH) are the same
as those for the reported structure10,19 (CCDC Refcode CURJEE02, CURJEE)16 (Figure
2). The Flack parameter of EMB base structure [–0.4(3)] is not reliable due to the
absence of heavy atoms, but the absolute chirality is assumed to be the same as that in the
salt. The crystal structure of EMB matches with that reported20 for CCDC Refcode
GEJHOT16 (Figure 3).
(a)
(b)
Figure 2 (a) N–H+ꢁꢁꢁCl– and O–HꢁꢁꢁCl– interactions connect translation related molecules
along the cꢀaxis in S,SꢀEDH. (b) Screw related molecules form zigzag tapes through C–
HꢁꢁꢁO interactions along the bꢀaxis which further extend into 2D sheets.
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(a)
(b)
Figure 3 (a) Translation related molecules form a tape through N–HꢁꢁꢁO bonds in S,Sꢀ
EMB. Screw related molecules through O–HꢁꢁꢁN bonds make parallel tapes which extend
into (b) 2D sheets (shown in different color) along the cꢀaxis through close pack.
[S,S]-Ethambutol dimesylate: A 1:2 salt of ethambutol and methanesulfonic acid was
crystallized from methanol at –20 °C (Table 1). The Xꢀray crystal structure showed
protonation of the secondary amine NHs of ethambutol by methanesulfonic acid. Linear
tapes of ethambutol cations and mesylate anions are bonded via N+–HꢁꢁꢁO– and O–HꢁꢁꢁO
hydrogen bonds (Figure 4a). These tapes make alternating sheets of drug and mesylate
molecules (Figure 4b) such that there are strong heteromolecular interactions between the
drug and mesylate ions.
(a)
(b)
Figure 4 (a) Linear tapes of translationꢀrelated ethambutol molecules and screwꢀrelated
mesylate ions are connected by N+–HꢁꢁꢁO– and O–HꢁꢁꢁO interactions. (b) Alternate sheets
of ethambutol and mesylate molecules along the aꢀaxis.
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[S,S]-Ethambutol fumarate hydrate (2:2:1.145): A salt hydrate was obtained when
ethambutol and fumaric acid were crystallized in 1:1 ratio from methanol in a desiccator.
The same salt hydrate also crystallized at –20 °C (Table 1). The Xꢀray crystal structure
shows two molecules of ethambutol and fumarate, one water molecule and a second
water with 0.145 site occupancy of O atom (s.o.f. was derived from the electron density
refinement). Each fumaric acid donates proton to ethambutol molecules lying above and
below (Figure 5). The ethambutol molecules form parallel tapes along the bꢀaxis through
N+–HꢁꢁꢁO– and O–HꢁꢁꢁO bonds. Water molecules are connected through O–HꢁꢁꢁO– and C–
HꢁꢁꢁO bonds to fumarate and ethambutol to make channels along the cꢀaxis.
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Figure 5 Linear tapes of ethambutol and fumarate crystallographic unique molecules
(shown in different color) along the bꢀaxis propagate through N+–HꢁꢁꢁO– and O–HꢁꢁꢁO
interactions. Water molecules (magenta and red of 1.0 and 0.145 s.o.f.) form channels
parallel to the cꢀaxis.
Table 2 Crystallographic parameters.
Ethambutol
Ethambutol
Ethambutol
dimesylate
Compound
Ethambutol
fumarate
hydrate
C28H58N4O13
658.78
triclinic
P1
dihydrochloride
empirical formula C10H26Cl2N2O2
C10H24N2O2
204.31
C12H32N2O8S2
396.52
formula weight
277.23
crystal system
orthorhombic
P21212
monoclinic
P21
orthorhombic
P21212
space group
Z*
6
2
6
5
T/K
a/Å
b/Å
100(2)
298(2)
100(2)
100(2)
6.432(2)
22.966(9)
7.1428(3)
8.4149(3)
12.239(3)
15.151(3)
8.5813(6)
10.5252(8)
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c/Å
/°
β/°
/°
V/Å3
Dcalc/g cm–3
ꢀ/mm–1
5.0934(19)
90
10.1973(4)
90
5.2980(11)
10.9210(8)
70.6260(10)
68.8720(10)
73.8670(10)
853.82(11)
1.281
90
α
90
95.633(4)
90
90
90
90
γ
752.4(5)
1.224
0.423
2891
609.96(4)
1.112
982.4(3)
1.340
0.310
10263
1951
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0.613
0.101
reflns. collected
unique reflns.
observed reflns.
R1[I > 2σ(I)]
wR2 [all]
3541
8775
1445
2201
6434
1282
2157
1763
6382
0.0588
0.1378
1.032
0.09(17)
Bruker
Apex
0.0506
0.1438
1.075
0.0646
0.1242
1.156
–0.04(16)
0.0326
0.0859
goodnessꢀofꢀfit
flack parameter
1.041
–0.4(3)
0.1(5)
Smartꢀ Oxford Xcalibur Bruker Smartꢀ Bruker Smartꢀ
Gemini Apex Apex
diffractometer
* Z = Z'' (Number of crystallographically nonꢀequivalent molecules of any type in the
asymmetric unit)21 × Number of independent general positions of the space group.
Table 3 Hydrogen bonds in the crystal structures.
Interaction
H···A/Å
D···A/Å
Symmetry code
∠D–H···A/°
Ethambutol dihydrochloride
N1–H1AꢁꢁꢁCl1
N1–H1BꢁꢁꢁCl1
O1–H1ꢁꢁꢁCl1
2.20(5)
2.35(5)
2.32(7)
2.27(5)
2.69
3.046(4)
3.134(4)
3.055(3)
3.234(5)
3.652(5)
161(4)
159(4)
169(7)
160(4)
165
*
x, y, –1+z
–1+x, y, –1+z
x, y, 1+z
C3–H3ꢁꢁꢁO1
C1–H1DꢁꢁꢁO1
Ethambutol
–1/2+x, 1/2+y, –z
N2–H2ꢁꢁꢁO1
2.22(2)
1.79(4)
1.98(3)
3.00(2)
156(2)
168(3)
176(3)
1+x, y, z
O1–H1DꢁꢁꢁN2
O2–H2CꢁꢁꢁN1
Ethambutol dimesylate
N1–H1AꢁꢁꢁO2
N1–H1BꢁꢁꢁO2
N1–H1BꢁꢁꢁO3
O1–H1ꢁꢁꢁO4
2.763(2)
2.874(2)
1–x, 1/2+y, 1–z
2–x, 1/2+y, 1–z
1.92(4)
2.48(3)
2.06(4)
1.98(5)
2.53
2.744(4)
3.060(4)
2.864(4)
2.733(4)
3.457(5)
3.621(6)
3.450(5)
159(3)
128(3)
168(3)
176(4)
155
*
x, y, –1+z
x, y, –1+z
1/2+x, 1/2–y, 2–z
1/2+x, 1/2–y, 2–z
1/2–x, –1/2+y, 2–z
1/2+x, 1/2–y, 1–z
C5–H5AꢁꢁꢁO3
C1–H1CꢁꢁꢁO3
C6–H6BꢁꢁꢁO3
2.64
177
2.55
152
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C6–H6CꢁꢁꢁO1
2.69
3.543(5)
145
1–x, –y, z
Ethambutol fumarate hydrate
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8
9
N1–H1AꢁꢁꢁO3
N1–H1BꢁꢁꢁO9
N2–H2AꢁꢁꢁO11
N2–H2BꢁꢁꢁO5
N3–H3AꢁꢁꢁO9
N3–H3BꢁꢁꢁO12
N4–H4CꢁꢁꢁO4
N4–H4DꢁꢁꢁO5
O1–H1ꢁꢁꢁO6
1.87(3)
2.07(2)
1.80(3)
1.90(3)
1.82(3)
1.82(2)
1.84(2)
1.77(3)
1.78(4)
1.82(2)
1.88(3)
1.90(3)
1.82(3)
1.59(4)
2.63
2.688(2)
2.881(2)
2.693(2)
2.845(3)
2.742(2)
2.701(2)
2.689(2)
2.686(2)
2.692(2)
2.648(2)
2.684(2)
2.782(2)
2.897(2)
2.744(2)
3.136(3)
3.650(1)
3.348(2)
3.225(2)
3.153(9)
3.347(2)
3.362(2)
3.471(3)
3.251(2)
158(2)
155(2)
167(2)
166(3)
166(2)
172(2)
168(2)
164(2)
169(3)
175(2)
173(3)
177(2)
169(2)
173(4)
112
*
*
x, 1+y, z
x, 1+y, z
*
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1+x, y, z
x, 1+y, z
1+x, 1+y, z
x, 1+y, z
1+x, –1+y, z
*
O2–H2ꢁꢁꢁO10
O7–H7AꢁꢁꢁO10
O8–H8ꢁꢁꢁO7
x, y, 1+z
x, y, 1+z
x, y, 1+z
*
O13–H13AꢁꢁꢁO11
O13–H13BꢁꢁꢁO4
C1–H1DꢁꢁꢁO13
C5–H5AꢁꢁꢁO14
C6–H6AꢁꢁꢁO4
C6–H6BꢁꢁꢁO9
C9–H9CꢁꢁꢁO14
C18–H18BꢁꢁꢁO2
C19–H19AꢁꢁꢁO13
C23–H23AꢁꢁꢁO7
C26–H26ꢁꢁꢁO6
2.70
159
x, y, 1+z
*
2.55
138
2.42
138
*
2.54
121
x, y, –1+z
x, 1+y, 2+z
x, 1+y, 2+z
x, y, 1+z
x, 1+y, z
2.42
156
2.46
151
2.67
139
2.59
127
* Molecules/ions in the same asymmetric unit.
Spectroscopic and Thermal analysis
A salt screen of EMB gave both solid and liquid products (Table 1) which were
characterized as salts by ATRꢀIR spectroscopy. A shift in the vibrational frequency for
the product compared to the starting materials indicates the formation of a multiꢀ
component adduct.22 For example, the characteristic C=O absorption peak of COOH
group at 1700 cm–1 disappeared and gave two peaks in the IR spectrum of the salt at 1550
and 1400 cm–1 for the asymmetric and symmetric stretch of COO– group22 for ethambutol
adipate ionic liquid (Figure 6). Similarly, shifts in S=O vibrations of SO3H group are
indicative of sulfonate salts (e.g. ditosylate salt, Figure 7).
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Figure 6 Ethambutol adipate liquid product (red) exhibits distinct ATR FTꢀIR spectrum
compared to its parent compounds ethambutol (green) and adipic acid (blue) with respect
to C=O, N–H, O–H and C–H bands.
Figure 7 Ethambutol ditosylate salt (red) exhibits distinct ATR FTꢀIR spectrum
compared to its parent compounds ethambutol (blue) and pꢀtoluenesulfonic acid (green)
with respect to S=O, O–H and N–H bands.
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Solidꢀstate NMR spectroscopy,23 particularly 15N nucleus chemical shifts, are
useful to establish salt formation (protonation at basic N site). The conversion of the
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secondary amine of EMB base to NH2 moves the nitrogen peak upfield.11,23a As such, an
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secondary NHs of S,SꢀEMB base are nonꢀequivalent in the crystal structure since one NH
acts as a hydrogen bond donor and acceptor and the other is an acceptor only, and
accordingly two 15N peaks were observed. In case of S,Sꢀethambutol dihydrochloride and
dimesylate salts, the protonated amines are equivalent in the crystal structure and
therefore a single upfield signal was noted (Figure 8a). Similarly, the occurrence of an
upfield shift in other cases confirms their salt nature (Figure 8b). PXRD of these salts
established their crystalline nature (Figure 9). DSC showed distinct melting point
endotherm and in two cases, sulfate and ditosylate salts, endothermic phase transitions
were observed before melting (Figure 10). These transitions are attributed to polymorphic
transformation of the salts at high temperature24 since there is no weight loss in TGA in
that temperature range (Figure 11).
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(b)
Figure 8 15N ssꢀNMR spectra of ethambutol (blue) and its salts (a) ethambutol
dihydrochloride (red), dimesylate (green) and fumarate hydrate (purple), and (b) sulfate
(red), dibesylate (green), ditosylate (purple) and adipate (yellow) show characteristic
upfield shift of secondary amine NH in the salts.
Figure 9 Experimental PXRD patterns of ethambutol salts.
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Figure 10 DSC of ethambutol sulfate (black), fumarate hydrate (magenta), dibesylate
(green), ditosylate (blue) and dimesylate (red) salts.
(a)
(b)
Figure 11 DSC (red) and TGA (blue) of (a) ethambutol sulfate and (b) ethambutol
tosylate indicate polymorphic transformation of the salts at high temperature prior to
melting as evident by no weight loss in the phase transition temperature range.
A hygroscopic salt can exist in the liquid state at room temperature due to
deliquescence and the same is true for an ionic liquid, even though it is not an RTIL. If
the melting point of a hygroscopic salt is less than 100 °C, then the solid residue upon
water loss may not be visually observed if water loss and melting are concomitant (occur
together) upon heating. Thus, a hygroscopic salt of m.p. <100 °C may be confused for an
RTIL. A heat−cool−reheat DSC run will evolve any residual water and give Tg (glass
transition temperature) or Tm (melting temperature) of the sample,11,12b thereby
eliminating interference/ confusion from water escape for thermal phase transitions in
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DSC. Thermal transitions of ethambutol adipate were observed in the DSC (Figure 12)
for a sample containing 38.6% water content by KF titration. The thermal transitions
suggested that (i) water is lost at the broad endotherm between 60ꢀ100 °C in the heat run,
(ii) the anhydrous liquid solidified during the broad exotherm around –20 °C in the
cooling cycle, and (iii) the solidified mass, which is an amorphous/glassy phase,
transformed into a rubbery material after glass transition (Tg = –20 °C) and finally melted
at 85 °C (Figure 12). The transitions of the hygroscopic sample were validated by DSC of
the crystalline material which showed clean melting at 87 °C (Figure 12). Therefore,
ethambutol adipate with Tm = 87 °C can be classified as an ionic liquid, more precisely as
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a protic ionic liquid,15 evident from the proton transfer analyzed by ATRꢀIR and N ssꢀ
NMR spectroscopy.
Figure 12 DSC of ethambutol adipate. (a) Heating and cooling of the hygroscopic liquid
(blue trace) shows a broad endotherm at 100 °C indicative of water loss and a broad
exotherm around –20 °C for solidification. Reheating (red trace) shows glass transition at
–20 °C and melting at 85 °C. Crystalline sample (black trace) shows a single melting
endotherm at 87 °C.
Conclusions
A salt screen of ethambutol base with several protic acids was carried out with the
intent of solving the hygroscopicity problem of EDH. The tendency of ethambutol to
form protic salts is so high that even a simple physical mixture of EMB and protic acid
shows evidence of salt formation. Rotavaporization, a recent technique for new
polymorphs11,25 and cocrystals,26 is successfully utilized to prepare salts in this study.
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Polymorphism of ethambutol salts (EDH,10 dibenzoate ionic liquid11 and sulfate and
tosylate salts of this study) warrants more studies in that ethambutol itself is not
polymorphic to date. The salts of this study were found to be hygroscopic when exposed
to ambient humidity, and therefore do not solve the original problem stated in the
introduction of finding new nonꢀhygroscopic salts of EMB. The common problem of
hygroscopicity with salts/ILs12,27 suggests that explorations of other solid forms such as
cocrystals,28 eutectics,29 and solid dispersions29c,30 are equally important in drug
formulation. We are searching for improved stability and minimal hygroscopicity form of
ethambutol dihydrochloride in solid screens.
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Experimental Section
Materials and Methods
Ethambutol dihydrochloride (Lot#090M0189V) was purchased from SigmaꢀAldrich
(Hyderabad, India) and used without further purification. All other chemicals were of
analytical or chromatographic grade. Water purified from a deionizer cum mixed bed
purification system (AquaDM, Bhanu, Hyderabad, India) was used for experiments.
Preparation and Crystallization of Ethambutol Base
The free base of ethambutol was prepared as per the method reported by Bhutani et al.1a
Ethambutol dihydrochloride (2 g) was added to 5N aqueous NaOH (20 mL). The mixture
was stirred for 10ꢀ15 min and extracted with CH2Cl2 (25 mL). The organic extract was
dried (Na2SO4) and left for evaporation at ambient temperature to afford blockꢀshape
crystals of ethambutol after one day. The product crystals were characterized by DSC
(m.p. 88 °C), NMR and XRD. Recrystallization from acetone resulted in diffraction
quality plate morphology crystals but they were very often twinned. Several crystals were
checked for identical unit cell parameters during Xꢀray cell check and only that crystal
which did not show a doubling of the unit cell was continued for Xꢀray data collection.
Ethambutol Dihydrochloride (C10H26Cl2N2O2)
Colorless plate crystals of the salt, suitable for single crystal Xꢀray diffraction, were
obtained from a methanolic solution, kept at –20 °C, after complete evaporation of the
solvent.
1H NMR (DMSOꢀd6):
δ
0.92 (3H, t, J 8), 1.68 (2H, m), 3.06 (1H, m), 3.71 (2H, m), 5.40
+
(1H, s), 9.29 (2H, d, J 80 (N–H coupling)). Protons of CH2 group (attached to NH2
group) merged with water peak of DMSOꢀd6.
13C NMR (DMSOꢀd6):
Ethambutol (C10H24N2O2)
1H NMR (DMSOꢀd6):
0.82 (3H, t, J 8), 1.31 (2H, m), 2.32 (1H, m), 2.55 (2H, s), 3.27
δ 10.24, 20.78, 41.27, 57.99, 60.74.
δ
(2H, m). Exchange of NH and OH protons in the solvent.
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13C NMR (DMSOꢀd6):
δ
10.51, 24.22, 47.31, 60.72, 63.07.
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Salt Synthesis
a. Synthesis by direct reaction of base and acid
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Ethambutol and the saltꢀforming acid (in 1:1 molar ratio for diacids and 1:2 molar ratio
for monoacids) were separately dissolved in acetone and kept at –20 °C. After incubation
for 10 min, the two solutions were mixed and stirred for 5ꢀ10 min in ice cold bath. The
product salt was obtained as a white precipitate and filtered on 2.5 ꢂm Whatman filter
paper and washed with acetone and dried.
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b. Solution crystallization
Ethambutol and acid (in 1:1 molar ratio for diacids and 1:2 molar ratio for monoacids)
were directly dissolved in methanol/ethanol. The solutions were kept for slow
crystallization at (i) room temperature, in (ii) a –20 °C refrigerator and (iii) a desiccator
separately. For fast crystallization of salts, the alcoholic solutions were subjected to
rotavaporization.
c. Co-grinding
Ethambutol and acid (in 1:1 molar ratio for diacids and 1:2 molar ratio for monoacids)
were subjected to grinding using a mortarꢀpestle for 1ꢀ2 min.
d. Co-melting
Ethambutol and acid (in 1:1 molar ratio for diacids and 1:2 molar ratio for monoacids)
were taken together in a sublimation tube and heated in an oil/salt bath beyond the
melting point of the higher melting compound. The tube was held until a uniform liquid
with no trace of solid material is formed and then kept aside for ambient cooling.
e. Physical mixture
Ethambutol and acid (in 1:1 molar ratio for diacids and 1:2 molar ratio for monoacids)
were gently mixed together with a spatula on a glass plate and the material was
transferred to a vial for storage at ambient temperature and humidity.
Ethambutol Sulfate (C10H26N2O6S)
1H NMR (D2O):
m). Exchange of NH and OH protons in the solvent.
13C NMR (D2O):
9.00, 20.37, 40.82, 57.86, 61.40.
Ethambutol Dimesylate (C12H32N2O8S2)
δ 0.94 (3H, t, J 8), 1.70 (2H, m), 3.25 (1H, m), 3.48 (2H, s), 3.73 (2H,
δ
1H NMR (DMSOꢀd6):
δ
0.90 (3H, t, J 8), 1.60 (2H, m), 2.39 (2H, s), 3.09 (1H, m), 3.42
–
(2H, m). Protons of CH3 group (attached to SO3 group) merged with dissolved water
peak of DMSOꢀd6. Exchange of NH and OH protons in the solvent.
13C NMR (DMSOꢀd6):
Ethambutol Dibesylate (C22H36N2O8S2)
δ 10.14, 20.64, 39.10, 40.99, 57.99, 60.73.
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1H NMR (DMSOꢀd6):
δ
0.86 (3H, t, J 8), 1.57 (2H, m), 3.07 (1H, m), 3.30 (2H, s), 7.35
+
(2H, d, J 4), 7.64 (2H, d, J 4), 8.55 (1H, s). Protons of CH2 group (attached to NH2
group) merged with dissolved water peak of DMSOꢀd6. Exchange of NH and OH protons
in the solvent.
13C NMR (DMSOꢀd6):
δ 10.07 (2C), 20.60 (2C), 41.02 (2C), 57.95 (2C), 60.79 (2C),
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125.89 (4C), 128.43 (4C), 129.65 (2C), 147.32 (2C).
Ethambutol Ditosylate (C24H40N2O8S2)
1H NMR (DMSOꢀd6):
δ
0.86 (3H, t, J 8), 1.57 (2H, m), 2.27 (3H, s), 3.05 (1H, m), 3.29
+
(2H, s), 7.14 (2H, d, J 8), 7.51 (2H, d, J 8). Protons of CH2 group (attached to NH2
group) merged with dissolved water peak of DMSOꢀd6. Exchange of NH and OH protons
in the solvent.
13C NMR (DMSOꢀd6):
δ 10.09 (2C), 20.63 (2C), 21.23 (2C), 41.06 (2C), 57.99 (2C),
60.77 (2C), 125.92 (4C), 128.82 (4C), 139.05 (2C), 144.73 (2C).
Ethambutol Fumarate (C14H28N2O6)
1H NMR (DMSOꢀd6):
δ
0.86 (3H, t, J 8), 1.47 (2H, m), 2.69 (1H, m), 2.92 (2H, s), 3.39
(2H, m), 6.42 (1H, s). Exchange of NH and OH protons in the solvent.
13C NMR (DMSOꢀd6):
10.47, 22.30, 43.58, 60.39, 60.45, 135.88, 169.28.
Ethambutol Adipate (C16H34N2O6)
1H NMR (DMSOꢀd6):
0.83 (3H, t, J 8), 1.39 (2H, m), 1.43 (2H, d, J 4), 2.08 (2H, d, J
δ
δ
4), 2.73 (1H, m), 3.15 (2H, s), 3.36 (2H, m). Exchange of NH and OH protons in the
solvent.
13C NMR (DMSOꢀd6):
X-ray Crystallography
δ 10.51, 24.11, 35.29, 45.05, 49.05, 60.45, 61.55, 176.15.
Xꢀray reflections for ethambutol dihydrochloride, ethambutol dimesylate and ethambutol
fumarate hydrate were collected at 100 K on a Bruker SMARTꢀAPEX CCD
diffractometer equipped with a graphite monochromator and MoꢀKα fineꢀfocus sealed
tube (λ = 0.71073 Å). Data reduction was performed using Bruker SAINT software.31
Intensities were corrected for absorption using SADABS.32 Structures were solved and
refined using SHELXꢀ97.33 Xꢀray reflections for ethambutol were collected at 298 K on
an Oxford Xcalibur Gemini Eos CCD diffractometer using CuꢀKα radiation (λ = 1.5418
Å). Data reduction was performed using CrysAlisPro (version 1.171.33.55)34 and
OLEX2ꢀ1.035 was used to solve and refine the structure. All nonꢀhydrogen atoms were
refined anisotropically. In the crystal structure of ethambutol fumarate hydrate, site
occupancy of 0.145 for O atom of the second water molecule was obtained by least
squares refinement of electron density using FVAR command. Hydrogen atoms on
heteroatoms were located from difference electron density maps and all C–H hydrogens
were fixed geometrically. Flack parameter17 was obtained upon structure solution and
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refinement of Xꢀray data. The final CIF files and hydrogen bond geometries were
validated in PLATON.36 XꢀSeed37 was used to prepare packing diagrams.
Powder X-ray Diffraction
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PXRD were recorded on Bruker D8 Advance diffractometer using CuꢀKα Xꢀradiation (λ
= 1.5406 Å) at 40 kV and 30 mA. Diffraction patterns were collected over 2θ range of 5ꢀ
50° at scan rate of 1° min–1. Powder cell 2.438 was used to plot the diffraction patterns.
Spectroscopy
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Nicolet 6700 FTꢀIR spectrometer equipped with a single bounce ATR accessory was
used to record IR spectra. IR spectra were recorded on samples placed on ZnꢀSe crystal.
Solution and solid state NMR spectra were recorded on a Bruker Avance spectrometer at
400 MHz. SSꢀNMR spectra were recorded on a Bruker 4 mm double resonance CPꢀMAS
probe in zirconia rotors at 5.0 kHz with a crossꢀpolarization contact time of 2.5 ms and a
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recycle delay of 8 s. N CPꢀMAS spectra were recorded at 40 MHz and referenced to
glycine N atom, and then the chemical shifts were recalibrated to nitromethane N (δglycine
= –347.6 ppm). Additionally the identity and stoichiometry of the components of the salts
were established through solution 1H NMR integration and 13C NMR spectra.
Thermal Analysis
DSC was performed on a Mettler Toledo DSC 1 module calibrated with indium (Tm =
156.60 °C; ꢃHf = 28.45 J g–1) and zinc (Tm = 419.50 °C; ꢃHf = 107.50 J g–1) as per the
manufacturer’s specifications. TGA was performed on a Mettler Toledo TGA/SDTA
851e module calibrated with indium (Tm = 156.60 °C) and aluminium (Tm = 660.30 °C).
The temperature range used for solid samples in both DSC and TGA is 30–400 °C at 5 °C
min–1. For liquid samples, a heatꢀcoolꢀreheat DSC cycle through liquid nitrogen cooling
in the temperature range –100 to +150 °C at 5 °C min–1 was employed. The typical
sample size is 5–10 mg for DSC and 10–15 mg for TGA. Samples were placed in
crimped but vented aluminium pans for DSC and open alumina pans for TGA and were
purged by a stream of dry nitrogen flowing at 50 mL min–1.
Karl Fischer (KF) Titration
Water content of the samples was determined using a Spectralab volumetric MA 101 C
Karl Fischer titrator with KF reagent (single solution) as the titrant and anhydrous
methanol as the solvent.
Supporting Information Available
Crystallographic information (.cif files) are available free of charge via the Internet at
http://pubs.acs.org.
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Acknowledgments
SC thanks the ICMR for fellowship. We thank the DST (JC Bose SR/S2/JCBꢀ06/2009)
and CSIR (Pharmaceutical cocrystals 01(2410)/10/EMRꢀII) for research funding. DST
(IRPHA) and UGC (PURSE grant) are thanked for providing instrumentation and
infrastructure facilities.
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References
1. (a) Bhutani, H.; Singh, S.; Jindal, K. C.; Charkraborti, A. K. J. Pharm. Biomed. Anal.
2005, 39, 892. (b) Blomberg, B.; Spinachi, S.; Fourie, B.; Laing, R. Bull. World Health
Organ. 2001, 79, 61.
2. Wilkinson, R. G.; Shepherd, R. G.; Thomas J. P.; Baughn, C. J. Amer. Chem. Soc. 1961,
83, 2212.
3. Singh S.; Mohan, B. Int. J. Tuberc. Lung Dis. 2003, 7, 298.
4. http://apps.who.int/prequal/whopar/whoparproducts/TB168Part6v1.pdf.
5. Bai, G. ꢀY.; Ning, H. ꢀS.; Zhang, Y. ꢀC.; Zeng T.; Li, J. ꢀS. Acta Crystallogr. 2006, E62,
o3364.
6. Godfrey, R.; Hargreaves, R.; Hitchcock, P. B. Acta Crystallogr. 1992, C48, 79.
7. Bai, G. ꢀY.; Ning, H. ꢀS.; Simpson, J.; Kin, X. ꢀY.; Li, N. Acta Crystallogr. 2006, E62,
o4567.
8. Mitsui Toatsu Inc. and Mitsui Pharmaceuticals Inc., GB Patent 1365473A, 1973.
9. Park, S. ꢀW.; Korean Advanced Institute of Science and Technology, US Patent 4450274,
1984.
10. RubinꢀPreminger, J. M.; Bernstein, J.; Harris, R. K.; Evans, I. R.; Ghi, P. Y. Cryst.
Growth Des. 2004, 4, 431. (CCDC Refcode CURJEE02)
11. Cherukuvada, S.; Nangia, A. CrystEngComm 2012, 14, 7840.
12. (a) Stoimenovski, J.; MacFarlane, D. R.; Bica, K.; Rogers, R. D. Pharm. Res. 2010, 27,
521. (b) Bica, K.; Rogers, R. D. Chem. Commun. 2010, 46, 1215; (c) Rogers, R. D.;
MacFarlane, D.; Zhang, S. Guest Eds., Themed Issue on Ionic Liquids, Chem. Commun.
2012, 48.
13. Silvester, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247.
14. Trask, A. V.; Haynes, D. A.; Motherwell, W. D. S.; Jones, W. Chem. Commun. 2006, 51.
15. (a) Stoimenovski, J.; Dean, P. M.; Izgorodina, E. I.; MacFarlane, D. R. Faraday Discuss.
2012, 154, 335. (b) Yoshizawa, M.; Xu, W.; Angell, C. A. J. Amer. Chem. Soc. 2003,
125, 15411.
16. Cambridge Structural Database, ver. 5.33, ConQuest 1.14, November 2011 release,
August 2012 update; www.ccdc.cam.ac.uk.
17. (a) Flack, H. D.; Bernardinelli, G. Acta Crystallogr. 1999, A55, 908. (b) Flack, H. D.;
Bernardinelli, G. J. Appl. Cryst. 2000, 33, 1143.
18. The Merck Index, An Encyclopedia of Chemicals, Drugs, and Biologicals, 14th ed.; Merck
Research Laboratories, 2006.
18
ACS Paragon Plus Environment

Page 19 of 20
Crystal Growth & Design
1
2
3
4
5
19. Hamalainen, R.; Lehtinen, M.; Ahlgren, M. Arch. Pharm. 1985, 318, 26. (CCDC Refcode
CURJEE)
6
7
8
9
20. (a) Bai, G. ꢀY.; Zhang, C. ꢀF.; Zhang, Y. ꢀC.; Zeng, T.; Li, J. ꢀS. Acta Crystallogr. 2006,
E62, o2173. (b) Bai, G. ꢀY.; Zhang, C. ꢀF.; Zhang, Y. ꢀC.; Zeng, T.; Li, J. ꢀS. Acta
Crystallogr. 2007, E63, e2.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
21. van Eijck, B. P.; Kroon, J. Acta Crystallogr. 2000, B56, 535.
22. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic
Compounds, John Wiley & Sons, NJ, 2005.
23. (a) Li, Z. J.; Abramov, Y.; Bordner, J.; Leonard, J.; Medek, A.; Trask, A. V. J. Am.
Chem. Soc. 2006, 128, 8199. (b) Vogt, F. G.; Clawson, J. S.; Strohmeier, M.; Edwards,
A. J.; Pham, T. N.; Watson, S. A. Cryst. Growth Des. 2009, 9, 921.
24. Maddileti, D.; Thakuria, R.; Cherukuvada, S.; Nangia, A. CrystEngComm 2012, 14,
2367.
25. (a) Nath, N. K.; Nilapwar, S.; Nangia, A. Cryst. Growth Des. 2012, 12, 1613. (b) Bag, P.
P.; Reddy, C. M. Cryst. Growth Des. 2012, 12, 2740.
26. Bag, P. P.; Patni, M.; Reddy, C. M. CrystEngComm 2011, 13, 5650.
27. (a) Bastin, R. J.; Bowker, M. J.; Slater, B. J. Org. Pro. Res. Dev. 2000, 4, 427. (b) Gould,
P. L. Int. J. Pharm. 1986, 33, 201. (c) Maiti, A.; Kumar, A.; Rogers, R. D. Phys. Chem.
Chem. Phys. 2012, 14, 5139. (d) Huddleston, J. G.; Visser, A. E.; Riechert, W. M.;
Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (e) Qian, Q.;
Yu, P.; Cheng, H.; Wang, X.; Yang, L.; Mao, L. Electroanal. 2011, 23, 2870.
28. (a) Schultheiss, N.; Newman, A. Cryst. Growth Des. 2009, 9, 2950. (b) Babu, N. J.;
Nangia, A. Cryst. Growth Des. 2011, 11, 2662.
29. (a) Górniak, A.; Wojakowska, A.; Karolewicz, B.; Pluta, J. J. Ther. Anal. Calorim. 2011,
104, 1195. (b) Cherukuvada, S.; Nangia, A. CrystEngComm 2012, 14, 2579. (c) Moore,
M. D.; Wildfong, P. L. D. J. Pharm. Innov. 2009, 4, 36.
30. (a) Vasconcelos, T.; Sarmento, B.; Costa, P. Drug Disc. Today 2007, 12, 1068. (b)
Janssens, S.; Roberts, C.; Smith, E. F.; Mooter, G. V. Int. J. Pharm. 2008, 355, 100.
31. SAINTꢁPlus, version 6.45; Bruker AXS Inc.: Madison, Wisconsin, USA, 2003.
32. SADABS, Program for Empirical Absorption Correction of Area Detector Data,
Sheldrick, G. M. University of Göttingen, Göttingen, Germany, 1997.
33. (a) SMART, version 5.625 and SHELXꢁTL, version 6.12, 2000, Bruker AXS Inc.:
Madison, Wisconsin, USA, 2000. (b) Sheldrick, G. M. SHELXSꢁ97 and SHELXLꢁ97,
University of Göttingen, Göttingen, Germany, 1997.
34. CrysAlis CCD and CrysAlis RED, versions 1.171.33.55, Oxford Diffraction Ltd: Yarnton,
Oxfordshire, UK, 2008.
35. Dolomanov, O. V.; Blake, A. J.; Champness, N. R.; Schröder, M. J. Appl. Crystallogr.
2003, 36, 1283.
36. Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University:
Utrecht, Netherlands, 2002.
19
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Crystal Growth & Design
Page 20 of 20
1
2
3
4
5
6
7
8
9
37. Barbour, L. J. XꢁSeed, Graphical Interface to SHELXꢁ97 and POVꢁRay, Program for
Better Quality of Crystallographic Figures; University of MissouriꢀColumbia: Missouri,
USA, 1999.
38. PowderCell, Program for Structure Visualization, Powder Pattern Calculation and
Profile Fitting, http://www.ccp14.ac.uk/index.html.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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32
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TOC and Synopsis
Salt screen of antiꢀtuberculosis chiral basic drug Ethambutol resulted in high melting salts
15
(m.p. > 100 °C) and an ionic liquid (m.p. 87 °C). The upfield shift in the N ssꢀNMR
spectra established the products as protic salt/ionic liquid.
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