Novel furosemide cocrystals and selection of high solubility drug forms

Article abstract of DOI:10.1002/jps.22805

Furosemide was screened in cocrystallization experiments with pharmaceutically acceptable coformer molecules to discover cocrystals of improved physicochemical properties, that is high solubility and good stability. Eight novel equimolar cocrystals of furosemide were obtained by liquid-assisted grinding with (i) caffeine, (ii) urea, (iii) p-aminobenzoic acid, (iv) acetamide, (v) nicotinamide, (vi) isonicotinamide, (vii) adenine, and (viii) cytosine. The product crystalline phases were characterized by powder x-ray diffraction, differential scanning calorimetry, infrared, Raman, near IR, and ¹³C solid-state NMR spectroscopy. Furosemide-caffeine was characterized as a neutral cocrystal and furosemide-cytosine an ionic salt by single crystal x-ray diffraction. The stability of furosemide-caffeine, furosemide-adenine, and furosemide-cytosine was comparable to the reference drug in 10% ethanol-water slurry; there was no evidence of dissociation of the cocrystal to furosemide for up to 48 h. The other five cocrystals transformed to furosemide within 24 h. The solubility order for the stable forms is furosemide-cytosine > furosemide-adenine > furosemide-caffeine, and their solubilities are approximately 11-, 7-, and 6-fold higher than furosemide. The dissolution rates of furosemide cocrystals were about two times faster than the pure drug. Three novel furosemide compounds of higher solubility and good phase stability were identified in a solid form screen.

Full text of DOI:10.1002/jps.22805

Novel Furosemide Cocrystals and Selection of High Solubility
Drug Forms
N. RAJESH GOUD,1 SWARUPA GANGAVARAM,2 KUTHURU SURESH,1 SHARMISTHA PAL,3 SULUR G. MANJUNATHA,3
SUDHIR NAMBIAR,3 ASHWINI NANGIA1,2
1School of Chemistry, University of Hyderabad, Prof. C.R. Rao Road, Central University PO, Gachibowli,
Hyderabad 500 046, India
2Technology Business Incubator, University of Hyderabad, Prof. C. R. Rao Road, Central University PO, Gachibowli,
Hyderabad 500 046, India
3Pharmaceutical Development, AstraZeneca India Pvt. Ltd., Bellary Road, Hebbal, Bangalore 560 024, India
Received 21 July 2011; revised 24 September 2011; accepted 13 October 2011
Published online 11 November 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.22805
ABSTRACT: Furosemide was screened in cocrystallization experiments with pharmaceutically
acceptable coformer molecules to discover cocrystals of improved physicochemical properties,
that is high solubility and good stability. Eight novel equimolar cocrystals of furosemide were
obtained by liquid-assisted grinding with (i) caffeine, (ii) urea, (iii) p-aminobenzoic acid, (iv)
acetamide, (v) nicotinamide, (vi) isonicotinamide, (vii) adenine, and (viii) cytosine. The prod-
uct crystalline phases were characterized by powder x-ray diffraction, differential scanning
calorimetry, infrared, Raman, near IR, and 13C solid-state NMR spectroscopy. Furosemide–
caffeine was characterized as a neutral cocrystal and furosemide–cytosine an ionic salt by
single crystal x-ray diffraction. The stability of furosemide–caffeine, furosemide–adenine, and
furosemide–cytosine was comparable to the reference drug in 10% ethanol–water slurry; there
was no evidence of dissociation of the cocrystal to furosemide for up to 48 h. The other five
cocrystals transformed to furosemide within 24 h. The solubility order for the stable forms is
furosemide–cytosine > furosemide–adenine > furosemide–caffeine, and their solubilities are
approximately 11-, 7-, and 6-fold higher than furosemide. The dissolution rates of furosemide
cocrystals were about two times faster than the pure drug. Three novel furosemide compounds
of higher solubility and good phase stability were identified in a solid form screen. © 2011 Wiley
Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:664–680, 2012
Keywords: bioavailability; co-crystal; crystallography; dissolution; furosemide; solid dosage
form; solubility; stability; thermal analysis; x-ray diffractometry
are classified into four categories in the Biopharma-
ceutics Classification System3,4 (BCS): class I (high
solubility, high permeability), class II (low solubility,
high permeability), class III (high solubility, low per-
meability), and class IV (low solubility, low permeabil-
ity). Low solubility drugs are those with a concentra-
tion of <20 mg/L in water. A well-accepted parameter
for solubility is the dimensionless quantity D0, or
dose number. D0 is the ratio of the highest drug
dose strength in the administered volume (taken as
250 mL = a glass of water) to the saturation solubil-
ity of that drug in water (measured in mg/L). D0 < 1
for high solubility drugs and D0 is 25 to 100 for low
solubility drugs. This value can go as high as 1000
or even 10,000. In addition to good oral absorption,
permeability in the gastrointestinal tract is equally
INTRODUCTION
Poor aqueous solubility is a major bottleneck in the
development of new drug molecules as pharmaceuti-
cal formulations.1 More than 80% of marketed drugs
are sold as tablets; 40% drugs in the marketplace have
poor solubility and, more alarmingly, almost 80–90%
of drug molecules that are at advanced stages of drug
development will pose a solubility problem.2 Drugs
Additional Supporting Information may be found in the online
version of this article. Supporting Information
Correspondence to: A. Nangia (Telephone: +91-40-23134854;
Fax: +91-40-23012460; E-mail: ashwini.nangia@gmail.com);
Sharmistha Pal (Telephone: +91-80-66213000; Fax: +91-80-
23621214; E-mail: sharmistha.pal@astrazeneca.com)
Journal of Pharmaceutical Sciences, Vol. 101, 664–680 (2012)
© 2011 Wiley Periodicals, Inc. and the American Pharmacists Association
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NOVEL FUROSEMIDE COCRYSTALS AND SELECTION OF HIGH SOLUBILITY DRUG FORMS
665
important so that the drug can bind to the recep-
tor target. The reference standard for defining high
or low permeability boundary is the n-octanol/water
partition coefficient for metoprolol, which has log
Pow = 1.72.
conditions.36 Even though there was good success in
achieving faster dissolution rates with furosemide
polymorphs, observations such as the transformation
of these metastable forms to the stable modification
during dissolution experiments or on storage made
these results less attractive for further drug devel-
opment. The fact remains that furosemide is still
marketed in its stable crystalline modification (Form
1)13 despite its poor aqueous solubility. In this back-
ground, we carried out a study on pharmaceutical
cocrystals37,38 of furosemide with the idea that being
crystalline in nature they are relatively more stable
and less prone to accidental phase transformations.
A pharmaceutical cocrystal is defined as a stoichio-
metric hydrogen-bonded complex in the solid state
between the active drug species and a suitable co-
former molecule that is safe for human consumption
(selected from the generally recognized as safe (GRAS)
list39 of U.S. Food and Drug Administration).
Furosemide (Lasix) is a loop diuretic drug com-
monly used in the treatment of hypertension and
edema. It is a BCS class IV drug,5,6 that is of low
solubility (6 mg/L in water) and low permeability (log
Pow 1.4). The highest dose strength of furosemide
is 80 mg, which means that it has a D0 number
of 53. The practiced approaches to improve the
solubility of furosemide may be classified into two
broad categories: (1) altering the physicochemical
property of the drug substance and (2) improving
the way in which the drug is processed or formu-
lated. The cocrystals strategy described in this
work is currently a popular approach to modify the
physicochemical properties of drugs.7,8 A closely
related method for solid form modification is that
of fast dissolving furosemide polymorphs,9–13 which
were first characterized more than 20 years ago.
The stabilization of nanoparticles14 and amor-
phous phases15 to achieve high dissolution rates
by particle size reduction, cogrinding, and copre-
cipitation of the drug with crospovidone,16 solid
dispersions with polymers,17 complexation with $
cyclodextrins,18–20 calix[n]arenes,21 emulsification,22
micelle formation,23 SEDDS (self-emulsifying
drug delivery systems),24 micronization and spray
drying,25 etc. have also been reported for furosemide.
The use of pharmaceutical cocrystals has gained
increasing popularity in the past decade26–28 as a
supramolecular approach to improve the physico-
chemical and pharmacokinetic behavior of drug sub-
stances. Furosemide has three functional groups
for hydrogen bonding to make cocrystals: COOH,
SO2NH2, and NH. Of these, the carboxylic acid and
sulfonamide functional groups are well studied and
known to give robust supramolecular synthons29,30
via O H···O and N H···O hydrogen bonds in cocrys-
tals. Our objective was to use COOH and SO2NH2
functional groups in heterosynthons with comple-
mentary coformer molecules, such as pyridine,31
CONH2,32 NH2,33 and pyridine-N-oxide34, etc. with
the idea that a multicomponent crystalline adduct
will be produced by self-assembly in the solid state.
Within the domain of solid form modification strate-
gies, we noted that apart from the metastable
polymorphs of furosemide with higher dissolution
rates,9,10 a crystal engineering approach to tune the
physicochemical characteristics of furosemide has not
been reported in the literature. Furosemide salts
with amino acids and their solubility characteristics
were reported in a U.S. patent 5182300.35 Furosemide
is relatively stable to photodegradation in alkaline
medium, but the molecule rapidly degrades in acidic
RESULTS AND DISCUSSION
Furosemide was cocrystallized with several coform-
ers containing CONH and COOH functional groups
with the intent of making cocrystals. The coformer
selection was kept as broad as possible to widen the
possibility for functional group pairing through hy-
drogen bonds. Non-GRAS molecules were excluded
because our end goal was to make cocrystals for
pharmaceutical form development. Thus even though
pyridine-N-oxides are known to form strong and re-
liable heterosynthons with the sulfonamide group,34
they were excluded because there is no pyridine-N-
oxide compound of the GRAS status.39 We obtained
the following new crystalline phases (Scheme 1) in
solution crystallization, manual grinding, and slurry
crystallization screen for novel furosemide cocrys-
tals: (i) furosemide –caffeine (FUROS–CAF), (ii)
furosemide –urea (FUROS–UREA), (iii) furosemide-
p-aminobenzoic acid (FUROS–PABA), (iv) furosemide
–acetamide (FUROS–ACT), (v) furosemide –nicoti-
namide (FUROS–NIC), (vi) furosemide –isonicoti-
namide (FUROS–INIC), (vii) furosemide –adenine
(FUROS–ADEN), and (viii) furosemide –cytosine
(FUROS–CYT). The solvents used for cocrystalliza-
tion are given in the Experimental section. A com-
plete list of all coformers attempted with furosemide
under different cocrystallization conditions is given
in the Supporting Information (Table S1).
Characterization of Cocrystals
All the above-mentioned crystalline phases were char-
acterized by powder x-ray diffraction (PXRD), in-
frared, near infrared and Raman spectroscopy (IR,
NIR, Raman), solid-state NMR spectroscopy (ss-
NMR), and differential scanning calorimetry (DSC)
techniques. Microcrystalline powders were obtained
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Scheme 1. Furosemide and coformers discussed in this study. The stoichiometry of
furosemide–coformer cocrystal is indicated in each case along with the analytical method used.
Compound abbreviations are used throughout the paper.
1
in all cases. Diffraction quality single crystals could
be grown for FUROS–CAF and FUROS–CYT. The
molecular composition and stoichiometry of these two
cocrystals was unambiguously confirmed from their
x-ray crystal structures. The nature of the cocrys-
tal–salt continuum,40 or the exact position of the H
atom in an acid–base crystal structure, is sometimes
difficult to ascertain except by accurate x-ray diffrac-
tion. Solid-state NMR41 and XPS (x-ray photoelec-
tron spectroscopy)42 are complementary techniques
to study the cocrystal/ salt state. FUROS–CAF was
confirmed to be a cocrystal (neutral complex) and
FUROS–CYT a salt (ionic structure) by x-ray crys-
tallography. The exact location of the acidic hydrogen
atom in other furosemide cocrystals could not be con-
clusively established due to nonavailability of single
crystals for x-ray diffraction.
The structures of new multicomponent solid phases
(i–viii) were confirmed by IR, NIR and Raman spec-
tral analysis of the product cocrystals. This was fol-
lowed by comparison of fingerprint lines in the PXRD
patterns, which showed clear differences in 2θ values.
A homogeneous cocrystal phase chemically different
from the drug and the coformer was indicated by a
sharp melting endotherm in DSC. The cocrystal com-
position was established, and stoichiometry was esti-
mated by using H NMR spectroscopy. Finally, solid-
state 13C NMR spectra were recorded to character-
ize all novel cocrystals. PXRD patterns, DSC ther-
mograms, and ss-NMR spectra are presented in the
paper, whereas IR, NIR, and Raman spectra are dis-
played in the Supporting Information. The structures
of FUROS–CAF and FUROS–CYT determined to be
cocrystal and salt, respectively, by x-ray crystallog-
raphy are discussed first, followed by the remaining
six cocrystals for which single crystal x-ray data are
unavailable at the present time.
FUROS–CAF
Crystallization of a ground mixture of furosemide
and caffeine from a MeOH–MeCN solvent mixture af-
forded diffraction quality single crystals, which solved
and refined in the triclinic space group P-1 (Table 1).
The crystal structure contains one molecule each
of furosemide and caffeine in the asymmetric unit
(Fig. S1), which confirms the cocrystal composition
and stoichiometry. The most acidic COOH donor of
furosemide is hydrogen bonded to the most basic N3
acceptor of caffeine via an O H···N hydrogen bond
(Fig. 1a). The primary sulfonamide NH donor hydro-
gen bonds to different caffeine C O acceptor groups
(Fig. 1b), and the secondary NH is bonded to one of the
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Table 1. X-Ray Crystal Structure Data of Furosemide–Caffeine
Cocrystal and Furosemide–Cytosine Salt
electron density maps. The purity of the bulk cocrys-
tal phase was confirmed by an excellent overlay of
the experimental PXRD pattern with the calculated
lines from the x-ray crystal structure (Fig. 2). Thus a
novel cocrystal of furosemide was obtained by liquid-
assisted grinding,43 and solution crystallization gave
diffraction quality single crystals of the same com-
pound. The furan moiety of furosemide was disor-
dered in the crystal structure, determined at 100 K
(Low Temperature (LT) structure). Since the phar-
maceutical cocrystal composition is relevant at room
temperature, we redetermined the structure at 298
K (Room Temperature (RT) structure). There was no
change in the proton state (neutral O H···N hydro-
gen bond) except that the disorder in the furan ring
portion was too severe to assign partial occupancies
in structure solution. The more accurate LT crystal
structure is reported in this paper.
The carbonyl peak of furosemide occurs at
1673 ncm−1 in the IR spectrum and that of caffeine
at 1658 and 1700 cm−1; the peaks are shifted to 1699
and 1650 cm−1 in the cocrystal adduct. There are dif-
ferences in the N H stretching region between 3200
and 2500 cm−1. The C N stretch is shifted from 1658
cm−1 in caffeine to 1650 cm−1 in the cocrystal and the
N H bending vibration from 1592 cm−1 in furosemide
to 1595 cm−1 in the product. IR, NIR, and Raman
spectra details are presented in the Supporting In-
formation (Tables S2, S3, and S4 and Figs. S2, S3,
and S4).
FUROS–CAF
FUROS–CYT
Empirical formula
Formula weight
Crystal system
Space group
C20 H18 Cl N6 O7
521.91
Triclinic
P-1
S
C16 H16 Cl N5 O6
441.85
Triclinic
P-1
S
T (K)
100(2)
298(2)
a (Å)
b (Å)
7.512(2)
9.462(3)
17.198(5)
95.387(5)
102.271(5)
110.101(5)
2
1103.0(6)
8290
4045
3388
363
0.0735
0.1874
1.065
7.909(4)
9.467(5)
12.484(7)
100.151(8)
95.950(8)
97.644(9)
2
904.2(8)
9245
3489
3153
290
0.0390
0.1070
1.060
c (Å)
α (◦)
β (◦)
γ (◦)
Z
V (Å3)
Reflections collected
Unique reflections
Observed reflections
Parameters
R1
wR2
GOF
CCDC No.
833554
833555
sulfonyl O acceptors (Fig. 1c). Hydrogen bond metrics
are listed in Table 2. The COOH group is in a neutral
state (C O 1.221(5) Å, C O 1.315(5) Å), and the H
atom is covalently bonded to the OH group and hy-
drogen bonded to the N acceptor (1.66 Å, 177◦). The
position of the acidic H atom was located in difference
Table 2. Hydrogen Bonds in Furosemide–Caffeine Cocrystal and Furosemide–Cytosine Salt Crystal Structures
Interaction
H···A (Å)
D···A (Å)
∠D–H···A (◦)
Symmetry Code
FUROS–CAF
O3 H3A···N3
N1 H4A···O1
N1 H4A···O4
N2 H5A···O6
N2 H5B···O7
C1 H1B···N3
C1 H1B···O3
C2 H2B···O6
C7 H7···O2
1.66
2.33
1.82
1.87
1.91
2.48
2.43
2.31
2.29
2.30
2.44
2.32
2.642(3)
3.001(4)
2.669(7)
2.849(3)
2.899(6)
2.937(6)
3.484(6)
2.726(5)
3.270(7)
3.190(7)
3.343(6)
2.790(7)
177
139
123
164
166
104
165
100
149
138
140
104
2–x, 1–y, 1–z
x,–1+y, z
Intramolecular
1 + x, y, z
1 + x, 1+ y, z
Intramolecular
2–x, 1–y, 1–z
Intramolecular
2–x, 2–y, 1–z
1+x, y, z
C8 H8C···O1
C11 H11···O5A
C14 H14···O2
FUROS–CYT
N1 H1···O2
N2 H2A···Cl1
N2 H2A···O6
N2 H2A···O5
N5 H4···O3
N5 H5···O3
N4 H6···O2
N3 H7···O6
C2 H2···O4
Intramolecular
Intramolecular
1.80
2.66
2.37
2.44
1.83
1.89
1.63
1.85
2.38
2.46
2.45
2.605(3)
3.257(2)
3.016(2)
3.349(9)
2.828(8)
2.849(2)
2.641(1)
2.857(8)
2.842(7)
3.413(2)
3.244(6)
134
118
121
149
173
157
178
172
104
146
129
Intramolecular
Intramolecular
1+x, y, z
2–x,–y,–z
1–x,–y,–z
x, 1+y, z
1–x,–y,1–z
1–x,–y,–z
Intramolecular
x, 1+y, 1+z
x, 1+y, z
C12 H12···O4
C15 H15···O4
Neutron-normalized distances are used in the paper.
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caffeine. Melting data from DSC thermograms are
summarized in Table 3.
The solid-state 13C NMR spectrum of the cocrystal
showed peaks for furosemide plus caffeine, but the
chemical shifts were moved upfield/downfield relative
to the pure components. Since short-range aggrega-
tion and shielding/deshielding will be different in the
starting components and product species, there are
small but discernible chemical shift differences in the
ss-NMR spectra (Fig. 4; see Table S5 for chemical shift
values). The equimolar stoichiometry of FUROS—
CAF cocrystal was confirmed by 1H NMR integration
(Fig. S5).
FUROS–CYT
Single crystals of furosemide–cytosine complex were
obtained from MeOH solution. The x-ray crystal
structure of FUROS–CYT was solved and refined in
the triclinic space group P-1 (Table 1). The ORTEP
diagram shows both the molecular components in
the crystal lattice (Fig. S6). The main difference be-
tween this crystal structure and that of FUROS—
CAF is that the proton is transferred from the COOH
group of the drug to the basic N4 nitrogen of cy-
tosine to make an ionic N H+···O− hydrogen bond
(1.63 Å, 178◦). FUROS–CYT is a salt. The protonation
state of the carboxylic acid and the basic nitrogen in
acid–base structures can be difficult to predict a pri-
ori in cocrystal structures. There are several cases
of borderline proton location and even a continuum
of O···H···N hydrogen bond states was noted.40,42,45
The electron density maps of x-ray diffraction showed
that the acidic H atom is transferred and covalently
bonded to the cytosine N and makes a hydrogen
bond with the COO− acceptor. The two C–O dis-
tances are nearly equal (1.247(2) Å, 1.262(2) Å) in
the carboxylate group. Hydrogen bonding is mediated
by the two-point carboxylate···aminopyridine synthon
of R22(8) geometry.46 Two furosemide molecules are
hydrogen-bonded to two cytosine coformers via the
carboxylate···amine R24(8) motif (Fig. 5). The cytosine
molecules pair up via the carboxamide N H···O dimer
synthon. The bulk crystalline material prepared by
liquid-assisted grinding matched with the single crys-
tal phase purity (Fig. 6).
Figure 1. Hydrogen bonding in FUROS–CAF crys-
tal structure. (a) COOH···N, (b) SO2NH···O C, and (c)
NH···O2S. The terminal furan ring is disordered over two
sites with 0.65: 0.35 site occupancy factor of C18, C20, and
O5 atoms in the low-temperature crystal structure data col-
lected at 100 K.
The DSC thermogram (Fig. 3) of the cocrystal
showed a single endotherm at a temperature different
from that of the components (Tpeak 225◦C). The post-
melting exotherm above 230◦C is due to decomposi-
tion of furosemide to 4-chloro-5-sulfamoylanthranilic
acid (saluamine) and other by-products upon heating
(Scheme 2).44 The single endotherm of the cocrystal is
at a higher temperature than that of furosemide but
lower than caffeine melting point. Furthermore, the
DSC of the cocrystal did not exhibit small thermal
events (e.g., polymorphic phase transitions), which
were observed in DSC of commercial furosemide and
IR, Raman and NIR spectra (Figures S7, S8, and
S9), DSC (Fig. 7), and NMR spectra (solid state in
Fig. 8 and solution spectrum in Fig. S10) were satis-
factory and consistent with the x-ray crystal structure
analysis. DSC endotherms for the salt appeared at a
different temperature from that for furosemide and
the coformer (Table 3).
FUROS–UREA, FUROS–PABA, FUROS–ACT,
FUROS–NIC, FUROS–INIC, and FUROS–ADEN
Scheme 2. Thermal degradation of furosemide to salu-
amine, furfuryl alcohol, and levulinic acid. This transfor-
mation occurs after melting of furosemide.
The next six cocrystal structures are described
briefly because the same techniques were used
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Figure 2. The experimental powder x-ray diffraction pattern of FUROS–CAF (black) and
calculated lines from the crystal structure (red) show an excellent overlay in 2θ and reasonably
good match of peak intensity. The small difference in peak position could be due to x-ray
reflections being collected at 100 K and the PXRD was recorded at 300 K. Reitveld refinement
gave Rp = 0.1953, Rwp = 0.2500, and Rexp = 0.0257.
as described above. That the structure and com-
position of furosemide–caffeine and furosemide–
cytosine determined by several analytical methods47
matched with the x-ray crystal structure in two cases
gave us the confidence to characterize novel cocrystal
phases, even in the absence of suitable single crys-
tal data. Diffraction quality single crystals can be
difficult to obtain for cocrystals, often due to mis-
matched solubility of the components, whereas mi-
crocrystalline powders are relatively easy to prepare
by neat or liquid-assisted grinding.48 The IR spectra
of these six cocrystals (Fig. S11) showed significant
differences compared to those of the drug and the co-
former, providing evidence for new crystalline phases.
Shifts in the CONH fragment of cocrystal spectra
compared with components were observed in NH
Table 3. Melting Points of Furosemide Cocrystals Compared with Those of the Drug and Coformers Used
Drug/Coformer
Melting Point of Coformera (◦C)
Cocrystal/Salt
Melting Point of New Phase (◦C)
FUROS
203
227
133
—
—
225
156
200
123
150, 166
154, 196
218
CAFb
FUROS–CAF
FUROS–UREA
FUROS–PABA
FUROS–ACT
FUROS–NIC
FUROS–INIC
FUROS–ADEN
FUROS–CYT
UREA
PABA
ACT (form 1, 2)
NICc (form 1, 2)
INIC (form 1, 2, 3)
ADEN
187
71, 81
106, 125
161
360
320
CYT
232
aMultiple melting points are given for polymorphic compounds.
bMelting point of caffeine monohydrate is 234◦C.
cAt least six polymorphic forms of nicotinamide are reported by hot stage microscopy in Kofler L, Kolfer A. 1943., Chem Ber 76:246—248,
having melting points (I) 129, (II) 110, (III) 113, (IV) 111, (V) 110, and (VI) 105◦C. Melting points quoted in the table are taken from Li et al.50
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Figure 3. DSC thermogram of furosemide–caffeine cocrystal. The compound melts at 225◦C
(Tpeak) followed by decomposition of furosemide between 227 and 240◦C.Melting point of
furosemide is 203–205◦C and of caffeine is 227–228◦C.
1H NMR integration (Fig. S15). The analysis of nicoti-
namide and isonicotinamide cocrystals is complicated
by the possibility of polymorphs and multiple stoi-
chiometry. The 1:1 predominant cocrystal composition
proposed is consistent with the available data.
Powder x-ray diffraction is a fingerprint technique
to characterize the solid state. This is the most quanti-
tative method for identifying novel crystalline phases
when the x-ray crystal structure is not feasible due to
microcrystalline nature of the sample. We observed
new diffraction peaks in the powder XRD patterns of
cocrystals compared with pure furosemide and the co-
former (Fig. 9a). The calculated diffraction lines from
x-ray crystal structures (Fig. 9b) are shown to detect
the starting materials or polymorphic phases in the
product cocrystal.
stretching and bending regions as well as for the car-
bonyl stretch peak, for example, in FUROS–UREA,
FUROS–ACT, and FUROS–INIC. Their Raman and
NIR spectra are compared in Figures S12 and S13
(in the Supporting Information). The melting point
of the cocrystal was determined, and the phase pu-
rity was ascertained in DSC thermograms (Table 3;
Fig. S14). All cocrystals exhibited sharp melting en-
dotherms at temperatures significantly different from
those for the drug and the coformer. The cocrystal
melting point was lower than that of furosemide for
urea, PABA (p-aminobenzoic acid), acetamide, nicoti-
namide, and isonicotinamide cocrystals but higher
than the drug melting point for caffeine, adenine,
and cytosine compounds. The intermediate melting
point of furosemide cocrystals, that is in-between the
drug and the coformer, is consistent with the general
trend in cocrystals.49 Multiple endotherms were ob-
served for nicotinamide and isonicotinamide cocrys-
tals, and a reason could be that polymorphs of the
coformer are produced in the cocrystallization ex-
periment by grinding. Polymorphs of nicotinamide
and isonicotinamide50 and multiple stoichiometries of
their cocrystals were reported in recent studies.51,52
The molecular composition of all the multicomponent
phases and their stoichiometry were ascertained by
To confirm the cocrystal structure and composi-
tion, 13C ss-NMR spectra41 were recorded to discern
short-range order differences and molecular mobil-
ity changes. The chemical shift values (δ scale) of ss-
NMR spectra (Fig. 10) recorded at cross-polarization
magic-angle spinning setting (CP-MAS) are convinc-
ing to confirm cocrystal formation. The δ values are
presented in Table S5 for a detailed peak-by-peak
comparison. The homogeneity of cocrystals and the
absence of starting materials were established by 13
C
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Figure 4. 13C ss-NMR spectrum of FUROS–CAF cocrystal compared with the individual
components.
nent systems are also multifunctional (COOH/COO−,
CONH) makes it difficult to assign the carbonyl peaks
individually. Whereas normally it is relatively easy to
differentiate between COOH and COO− by IR spec-
troscopy, it would have been difficult to make the
same assignment for the above-discussed cocrystal/
salt without knowledge of the x-ray crystal structures.
The carbonyl resonance occurred between δ 160 and
170 ppm in the NMR spectra for furosemide whether
the molecule is in the neutral (COOH) or ionized
(COO−) state. 15N ss-NMR41 and XPS42 should un-
equivocally clarify the cocrystal/salt nature of all the
furosemide adducts, and these measurements will be
carried out to clarify the ionization state. The main
objective of this preliminary study was to search for
novel solid-state forms of furosemide with GRAS co-
formers to discover crystalline materials of improved
solubility and stability.
ss-NMR spectra. A difference in the chemical shift
peak positions of the cocrystal with respect to the
individual components was considered as evidence
for the formation of a new phase. For example, the
carbonyl group of furosemide moved downfield in
FUROS–UREA (δ 176.9–178.6) whereas the carbonyl
group of urea moved upfield (δ 168.2–166.6), indicat-
ing a hydrogen-bonding interaction between the com-
ponents. Similarly, carbonyl groups of furosemide and
PABA were shifted in the cocrystal relative to the pure
components. The carbonyl peaks in FUROS–ACT at
(δ 174.7, 180.4) are shifted compared with individual
resonances (δ 176.9, 181.5). Cocrystals of furosemide
with nicotinamide, isonicotinamide, and adenine in-
dicated new cocrystal phases by NMR.
Of eight molecular complexes prepared in this
work, single crystals were obtained for two com-
pounds. Furosemide–caffeine is a cocrystal and
furosemide–cytosine is a salt. The structural details
of the neutral or ionic state were known after sin-
gle crystal x-ray diffraction. Surprisingly, the IR spec-
trum of furosemide–cytosine did not exhibit a broad
carboxylate band at 1600–1650 cm−1 and a sharper
peak for the carboxylic acid group at 1700 cm−1 for
furosemide–caffeine. The fact that the multicompo-
The melting endotherms in DSC for cocrystals
are sharp and occur in-between the furosemide
and the coformer (Table 3). This indicated cocrys-
tal formation.49 We believe that the examples dis-
cussed in this paper are cocrystals, not eutectic com-
positions. Generally, the melting point of eutectic is
lower than either of the components and, moreover,
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that are metastable and undergo phase transforma-
tion during the slurry conditions of dissolution. The
rate at which the equilibrium solubility is reached is
the dissolution rate, which is a kinetic parameter. The
IDR method overcomes the problems of drug particle
size and morphology effects. Dissolution rate stud-
ies rely on the supersaturation phenomenon, that is
the peak concentration of the drug delivered through
a suitable carrier (e.g., amorphous, salt, cocrystal,
nanoparticle)55 in a short period of time (say 2–4–6
h) depending on the stability of the drug form being
tested. The survival of the starting drug form during
the course of the test conditions and until the end of
the experiment must be verified independently, usu-
ally by PXRD or video microscopy. IDR measures the
rate of drug dissolution from a constant surface area
of the disk in a unit time. Because of the very low
solubility of furosemide in aqueous medium, all dis-
solution and solubility experiments were carried out
in 10% ethanol–water (Fig. 11).
The drug concentration was determined by UV–vis
spectroscopy from a calibrated concentration–inten-
sity curve. Furosemide shows maxima of decreasing
intensities at 230 nm, 275 nm, and a third shallow,
broad maximum at 335 nm. Among the coformers
in this study, caffeine, cytosine, and p-aminobenzoic
acid absorb between 240 and 300 nm but they exhibit
no maxima beyond this region even at the highest
concentration considered. There is a slight up-curve
above the base line at 230–240 nm due to the coformer
peaks, but this small contribution is no more than
10% (Fig. S16). This artifact can be corrected using
the drug:coformer concentration calibration curves.
To minimize possible errors due to overlapping peaks,
the maximum in furosemide UV–vis spectrum at 335
nm was used to calculate drug concentrations without
any interference from coformer peaks.
The solubility of the drug and cocrystal pow-
ders was determined at 24 h. Apart from
furosemide, which was stable to the slurry condi-
tions, furosemide–caffeine, furosemide–adenine, and
furosemide–cytosine were stable for the same pe-
riod (Fig. S17); there was no perceptible change in
the PXRD pattern even up to 48 h. The solubil-
ity of all cocrystals is superior to that of the pure
drug. The other five cocrystals with urea, PABA,
acetamide, nicotinamide, and isonicotinamide con-
verted to furosemide within 24 h. The solubility order
for the stable cocrystals (no phase transformation ob-
served) is furosemide–cytosine > furosemide–adenine
> furosemide–caffeine > furosemide, which have 11,
7, and 6 times higher solubility than furosemide. The
dissolution rate advantage is modest (about twofold
faster). Since there is no evidence of the solid-form
conversion for extended periods of time, the solubil-
ity numbers may be used as a safe guide to estimate
the higher aqueous exposure and bioavailability of
Figure 5. X-ray crystal structure of FUROS–CYT salt.
(a) R22(8) and R24(8) N H···O hydrogen bond ring mo-
tifs. (b) The SO2NH2 group donors engage in N H···O and
N H···B(furan) hydrogen bonds.
it does not depend on the stoichiometry in which the
components were mixed or reacted. This was not the
case for furosemide cocrystals. Another indication for
cocrystal instead of the eutectic phase is the stabil-
ity experiments (discussed next). The PXRD of the
stable cocrystals in this study, that is furosemide–
caffeine, furosemide–adenine, and furosemide–cyto-
sine, matched with those of the product solid forms,
but not with the individual components. This implies
that a novel crystalline adduct is present, not a eu-
tectic phase. A physical mixture would have shown
PXRD lines matching with those of the starting ma-
terials after the slurry experiment.
Solubility and Dissolution
Solubility and intrinsic dissolution rate (IDR) were
determined on a U.S. Pharmacopeia (USP) approved
dissolution tester.53,54 A typical measurement is de-
scribed in the Experimental section. Solubility is the
concentration of a substance in the solvent when the
dissolved and undissolved particles are in a state of
dynamic equilibrium. Solubility is a thermodynamic
quantity and usually taken as the concentration of the
solute at 24 or 48 h after mixing in a solvent. The sol-
ubility measurement is unsuited for those drug forms
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Figure 6. Experimental PXRD of FUROS–CYT (black trace) overlaid on the calculated diffrac-
tion lines from the x-ray crystal structure (red). The matching of the bulk material with the
single crystal phase is excellent. Reitveld refinement gave Rp = 0.1833, Rwp = 0.2514, and Rexp
= 0.0270.
cocrystals compared with the pure drug. The equilib-
rium solubility at 24 h and the dissolution rate from
the linear region of the IDR curve are presented in
Table 4.
the highest solubility of FUROS–CYT does not corre-
late with the low solubility of cytosine. On the other
hand, FUROS–CAF has lower solubility even though
caffeine is more soluble. A reason for these observa-
tions is that there are drastic changes in the crys-
tal structure and hydrogen bonding from neutral to
ionic in FUROS–CAF and FUROS–CYT. We believe
that the salt nature of the latter compound guides
its highest aqueous solubility. The solubility data of
Table 4 indicate that FUROS–CAF, FUROS–ADEN,
and FUROS–CYT are congruently dissolving systems
because they are pairs of similar solubility. The other
systems are incongruently dissolving due to the huge
difference in their components solubilities.59,60 Their
stabilities are consequently related. Congruent sys-
tems tend to be more stable in the slurry medium.
In incongruent systems, however, the highly solu-
ble coformer causes rapid dissolution and dissoci-
ation of the cocrystal to its components. FUROS–
PABA is an exception to the above-mentioned
analysis.
The reasons for solubility enhancement may be
described on the basis of the solubility of the co-
former in water (Table 4) and from crystal struc-
ture analysis (Figs. 1 and 5). The high solubility of
FUROS–NIC cocrystal is not surprising56 because
nicotinamide is one of the highest soluble coform-
ers that is used in pharmaceutical cocrystallization.
Hence the highly soluble coformer guides the high
concentration of the drug cocrystal.57,58 However, this
explanation, which is based on the linear relation-
ship between the solubility ratio of the components
plotted against the solubility of the former cocrystal
divided by the solubility of the API (Active Pharma-
ceutical Ingredient),57 has its limitations. The above
model is faithfully realized only when the cocrystals
whose solubility is being compared have similar hy-
drogen bonding and molecular packing. For example,
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Figure 7. DSC of FUROS–CYT. The salt exhibits a melting endotherm at 232◦C (Tpeak) fol-
lowed by decomposition of furosemide between 240 and 260◦C. Melting point of furosemide
is203–205◦C and of cytosine is 320–322◦C.
ture and the neutral/ionic state of the product are
difficult to know without a crystal structure. Struc-
ture solution from powder diffraction data is still not
a routine method for large and flexible molecules.
Furosemide–caffeine is a cocrystal, and furosemide–
cytosine is a salt as revealed by x-ray crystal struc-
ture analysis. A definitive answer about the neu-
tral/ionic state of other six furosemide compounds
is pending upto the availability of XPS/NMR mea-
surements or good quality single crystals. A practi-
cal advantage that cocrystals offer over metastable
CONCLUSIONS
Solid-state crystalline forms of an insoluble anti-
hypertensive drug furosemide were prepared using
liquid-assisted grinding and slurry methods. Single
crystal x-ray structure validation was performed on
two of eight materials. All the products were charac-
terized by DSC, ss-NMR, PXRD, IR, NIR, and Raman
measurements. Even as crystal engineering princi-
ples offer guidelines for the rational selection of co-
formers for a particular API, the exact cocrystal struc-
Table 4. Solubility and IDR in 10% Ethanol–Water
Solubility at 24 h in
10% EtOH–Water (mg/L)
Solubility in Water
(mg/mL)
Intrinsic Dissolution Rate in
Stability in 10% EtOH–Water
Slurry Medium
Compound
10% EtOH–Water (mg/cm2)/min
FUROS
118
0.006
22
1000
5
2000
1000
192
9
44 × 10−3
Stable after 24, 48 h
Stable after 24, 48 h
FUROS–CAF
FUROS–UREA
FUROS–PABA
FUROS–ACT
FUROS–NIC
FUROS–INIC
FUROS–ADEN
FUROS–CYT
720 (×6.1)
632 (×5.3)
370 (×3.1)
812 (×6.9)
1040 (×8.8)
856 (×7.2)
787 (×6.7)
1260 (×10.7)
87 × 10−3 (×1.9)
83 × 10−3 (×1.9)
53 × 10−3 (×1.2)
94 × 10−3 (×2.1)
111 × 10−3 (×2.5)
102 × 10−3 (×2.3)
91 × 10−3 (×2.1)
116 × 10−3 (×2.6)
Converted to furosemide within 24 h
Converted to furosemide within 24 h
Converted to furosemide within 24 h
Converted to furosemide within 24 h
Converted to furosemide within 24 h
Stable after 24, 48 h
8
Stable after 24, 48 h
Numbers in parentheses give the number of times solubility is higher compared to the pure drug.
Solubility of the coformer is given in water.
The stable cocrystal entries are highlighted in bold.
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Figure 8. 13C ss-NMR of FUROS–CYT and the individual components show that a novel
solid-state complex was formed upon grinding.
polymorphs or amorphous drug forms is that they are
relatively more stable owing to their crystalline na-
ture. Thus cocrystals can combine the twin aspects
of improved solubility and good stability for opti-
mal drug delivery. Furosemide–caffeine, furosemide–
adenine, and furosemide–cytosine exhibited com-
parable stability to the commercial drug in 10%
ethanol–water slurry medium up to 48 h. More-
over, their solubility is 6–11-fold higher relative to
furosemide. Thus cocrystals could offer a superior
strategy to improve the solubility of BCS class II/IV
drugs compared to metastable polymorphs and amor-
phous dispersions.
Preparation of Furosemide Cocrystals
Furosemide (34 mg) and caffeine (20 mg) (1:1 molar
ratio) were ground and mixed in a mortar–pestle for
20 min after adding 5–6 drops of acetonitrile, a liquid-
assisted grinding procedure. Cocrystal formation was
confirmed by changes in the signature peaks of FT-IR
and PXRD. Forty milligram of the ground material
was dissolved in 5 mL of MeOH–CH3CN solvent mix-
ture and left aside for evaporation at ambient con-
ditions. Single crystals suitable for x-ray diffraction
appeared after 4–5 days.
Furosemide (68 mg) and urea (12 mg) (1:1 molar
ratio) were ground and mixed together in a mortar–
pestle for 20 min after adding 5–6 drops of acetone, a
liquid-assisted grinding procedure. Cocrystal forma-
tion was confirmed by changes in the signature peaks
of FT-IR and PXRD.
Furosemide (34 mg) and p-aminobenzoic acid
(14 mg) (1:1 molar ratio) were ground and mixed
in a mortar–pestle for 20 min after adding 5–
6 drops of acetone, a liquid-assisted grinding proce-
dure. Cocrystal formation was confirmed by changes
in the signature peaks of FT-IR and PXRD. The
same cocrystal was also obtained by using the slurry
EXPERIMENTAL
Furosemide (purity >99.8%) was supplied by As-
traZeneca India Pvt. Ltd (Bangalore, India). Coform-
ers (purity >99.8%) were purchased from Sigma-
Aldrich (Hyderabad, India). Solvents (purity >99%)
were purchased from Hychem Laboratories (Hyder-
abad, India). Water filtered through a double deion-
ized purification system (AquaDM, Bhanu, Hyder-
abad, India) was used in all experiments.
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Figure 9. (a) Experimental PXRD plots of furosemide, cocrystal, and coformer to compare 2θ
values in the new crystalline phases. (b) Calculated PXRD lines form the x-ray crystal struc-
ture for furosemide, coformer polymorphs, and cocrystals for fingerprint matching of starting
materials in the product phases of (a).
crystallization method. PABA (14 mg) was dissolved
in 1.5-mL MeOH and 3.5 mL of H2O, and furosemide
(34 mg) was added with stirring. The formation of
cocrystal was completed after 30 min.
Furosemide (68 mg) and acetamide (12 mg) (1:1 mo-
lar ratio) were ground and mixed in a mortar–pestle
for 20 min after adding 5–6 drops of acetonitrile, a
liquid-assisted grinding procedure. Cocrystal forma-
tion was confirmed by changes in the signature peaks
of FT-IR and PXRD.
Furosemide (34 mg) and nicotinamide (13 mg) (1:1
molar ratio) were ground and mixed in a mortar–pes-
tle for 20 min after adding 5–6 drops of acetone, a
liquid-assisted grinding procedure. Cocrystal forma-
tion was confirmed by changes in the signature peaks
of FT-IR and PXRD.
Furosemide (34 mg) and isonicotinamide (13 mg)
(1:1 molar ratio) were ground and mixed in a mor-
tar–pestle for 20 min after adding 5–6 drops of ace-
tone, a liquid-assisted grinding procedure. Cocrystal
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Figure 11. Dissolution curve of furosemide and its cocrys-
tals for up to 6 h.
formation was confirmed by changes in the signature
peaks of FT-IR and PXRD.
Furosemide (34 mg) and adenine (14 mg) (1:1 molar
ratio) were ground and mixed in a mortar–pestle for
20 min after adding 5–6 drops of acetone, a liquid-
assisted grinding procedure. Cocrystal formation was
confirmed by changes in the signature peaks of FT-IR
and PXRD. The same cocrystal was also obtained by
the slurry crystallization method. Furosemide (110
mg) and adenine (40 mg) was added to 8-mL MeOH
with stirring. The formation of cocrystal was complete
after 7–8 h.
Furosemide (34 mg) and cytosine (12 mg) (1:1 mo-
lar ratio) were ground and mixed in a mortar–pestle
for 20 min after adding 5–6 drops of acetonitrile, a
liquid-assisted grinding procedure. Cocrystal forma-
tion was confirmed by changes in the signature peaks
of FT-IR and PXRD. Fifty milligram of the ground ma-
terial was dissolved in 6 mL of MeOH and left aside
for evaporation at ambient conditions. Single crystals
suitable for x-ray diffraction appeared after 5–6 days.
Vibrational Spectroscopy
A Thermo-Nicolet 6700 Fourier transform infrared
spectrophotometer with an NXR-Fourier transform
Raman module (Thermo Scientific, Waltham, MA)
was used to record IR, Raman, and NIR spectra. IR
and NIR spectra were recorded on samples dispersed
in KBr pellets. Raman spectra were recorded on sam-
ples contained in standard NMR diameter tubes or on
compressed samples contained in a gold-coated sam-
ple holder. Data were analyzed using the Omnic soft-
ware (Thermo Scientific, Waltham, MA).
Figure 10. 13C ss-NMR spectra of cocrystals, drug, and
coformers.
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GOUD ET AL.
tally located in difference electron density maps. All
C H atoms were fixed geometrically using HFIX com-
mand in SHELX-TL (Bruker-AXS). A check of the fi-
nal CIF file using PLATON66 did not show any missed
symmetry. Hydrogen bond distances shown in Table 2
are neutron normalized to fix the D–H distance to its
accurate neutron value in the x-ray crystal structures
(O–H 0.983 Å, N–H 1.009 Å, and C–H 1.083 Å). X-
Seed67 was used to prepare packing diagrams. Crys-
tallographic.cif files (CCDC Nos. 833554–833555) are
available at www.ccdc.cam.ac.uk/data or as part of
Supporting Information.
Solid-State NMR Spectroscopy
Solid-state 13C NMR spectra were obtained on a
Bruker Avance 400 MHz spectrometer (Bruker-
Biospin, Karlsruhe, Germany). Ss-NMR measure-
ments were carried out on Bruker 4-mm double reso-
nance CP-MAS probe in zirconia rotors with a Kel-F
cap at 5.0-kHz spinning rate with a cross-polarization
contact time of 2.5 ms and a delay of 8 s. 13C NMR
spectra were recorded at 100 MHz and referenced to
the methylene carbon of glycine (δglycine = 43.3 ppm).
Thermal Analysis
Dissolution and Solubility Measurements
Differential scanning calorimetry was performed on a
Mettler-Toledo DSC 822e module, and TGA was per-
formed on a Mettler Toledo TGA/SDTA 851e mod-
ule (Mettler-Toledo, Columbus, OH). Samples were
placed in sealed pin-pricked alumina pans for TG ex-
periments and in crimped but vented aluminum pans
for DSC experiments. The typical sample size is 3–5
mg for DSC and 8–12 mg for TGA. The temperature
range for the thermogram was 30–300◦C, and the
sample was heated at a rate of 5◦C/ min. Samples
were purged in a stream of dry nitrogen flowing at 80
mL/min for DSC and 50 mL/min for TGA. The TGA of
FUROS–UREA and FUROS–ACT is shown as repre-
sentative cases of cocrystals (Fig. S18) to monitor the
decomposition of furosemide in the postmelting stage.
The solubility of furosemide and its cocrystals were
determined according to the Higuchi and Connor
method68 in 10% ethanol–water medium at 30◦C.
First, the absorbance of a known concentration of
the cocrystal/drug was measured at the respective
λmax (furosemide 334 nm, furosemide–caffeine 330
nm, furosemide–urea 331 nm, furosemide–PABA 331
nm, furosemide–acetamide 330 nm, furosemide–ni-
cotinamide 330 nm, furosemide–isonicotinamide 329
nm, furosemide–adenine 330 nm) in 10% ethanol–wa-
ter medium on a Thermo Scientific Evolution 300
UV–vis spectrometer (Thermo Scientific, Waltham,
MA). These absorbance values were plotted against
several known concentrations to prepare the concen-
tration versus intensity calibration curve. From the
slope of the calibration curves, molar extinction co-
efficients (Table S6) for the cocrystal/drug were cal-
culated. An excess amount of the sample was added
to 6 mL of 10% ethanol–water medium. The super-
saturated solutions were stirred at 300 rpm using a
magnetic stirrer at 30◦C. After 24 h, the suspension
was filtered through Whatman’s filter paper No. 1.
The filtered aliquots were diluted sufficiently, and the
absorbance was measured at the respective λmax. Fi-
nally, the concentrations of furosemide and its cocrys-
tals were calculated at regular time intervals of 24
and 48 h using the relevant calibration curve. IDR
experiments were carried out on a USP-certified Elec-
trolab TDT-08L dissolution tester (Mumbai, India).
Dissolution experiments were performed for 6 h in
10% ethanol–water medium at 37◦C. Prior to IDR
estimation, standard curves for all the compounds
were obtained spectrophotometrically at their respec-
tive λmax. The respective molar extinction coefficients
were used to determine the IDR values. For IDR mea-
surements, 100 mg of the compound was taken in
the intrinsic attachment and compressed to a 0.5-cm2
disk using a hydraulic press at pressure of 4.0 ton/
in.2 for 5 min. The intrinsic attachment was placed
in a jar of 500-mL medium preheated to 37◦C and
rotated at 150 rpm. Five milliliter of aliquot was col-
lected at specific time intervals, and the concentra-
tion of the aliquots was determined with appropriate
Powder X-Ray Diffraction
Powder x-ray diffraction of all the samples was
recorded on a Bruker D8 advance diffractometer
(Bruker-AXS, Karlsruhe, Germany) using Cu K" x-
radiation (8 = 1.5406 Å) at 40 kV and 30 mA power.
X-ray diffraction patterns were collected over the 22
range 5–50◦ at a scan rate of 1◦/min. Powder Cell 2.461
(Federal Institute of Materials Research and Testing,
Berlin, Germany) was used for Rietveld refinement
of experimental PXRD and calculated lines from the
x-ray crystal structure. The calculated crystal struc-
tures of coformer polymorphs not determined in this
study were taken from the literature50 or extracted
from the CCDC (Cambridge Crystallographic Data
Centre).62
X-Ray Crystallography
X-ray reflections were collected on a Bruker Smart-
Apex CCD diffractometer (Bruker-AXS, Karlsruhe,
Germany). Mo K" x-radiation (λ = 0.71073 Å) was
used to collect x-ray reflections on the single crys-
tal. Data reduction was performed using the Bruker
SAINT software.63 Intensities for absorption were cor-
rected using SADABS,64 the Siemens area detector
absorption correction program (Bruker-AXS). Crystal
structures were solved and refined using SHELX-9765
with anisotropic displacement parameters for non-H
atoms. Hydrogen atoms on O and N were experimen-
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NOVEL FUROSEMIDE COCRYSTALS AND SELECTION OF HIGH SOLUBILITY DRUG FORMS
679
dilutions from the predetermined standard curves of
the respective compounds. The IDR of the compound
was calculated in the linear region of the dissolu-
tion curve (which is the slope of the curve or amount
of drug dissolved/surface area of the disk) per unit
time. The identity of the undissolved material after
the dissolution experiment was ascertained by PXRD.
The stability of the solid samples after disk compres-
sion and solubility measurements was confirmed by
PXRD.
12. Karami S, Li Y, Hughes DS, Hursthouse MB, Russell AE,
Threlfall TL, Claybourn M, Roberts R. 2006. Further errors
in polymorph identification: Furosemide and finasteride. Acta
Crystallogr B 62:689–691.
13. Babu NJ, Cherukuvada S, Thakuria R, Nangia A. 2010. Con-
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ACKNOWLEDGMENTS
17. Shin SC, Kim J. 2003. Physicochemical characterization of
solid dispersion of furosemide with TPGS. Int J Pharm
251:79–84.
AN thanks the Department of Science and Technol-
ogy, Government of India (DST) for research fund-
ing (SR/S1/OC-67/2006) and J.C. Bose fellowship (SR/
S2/JCB-06/2009) and Council of Scientific and In-
dustrial Research (CSIR) project (01/2410)/10/EMR-
II). DST (IRPHA) and University Grants Commis-
sion (UGC) (PURSE grant) are thanked for providing
instrumentation and infrastructure facilities at Uni-
versity of Hyderabad (UOH). NRG thanks CSIR, and
KS thanks UGC for a fellowship. Crystalin Research
Pvt. Ltd. (Hyderabad, India) thanks AstraZeneca
Pvt. Ltd. (Bangalore, India) for funding a research
project. We thank Dr. Mike Quayle and Dr. David
Berry (AstraZeneca PLC) for their critical comments
and helpful suggestions. This publication is assigned
AstraZeneca ATP No. 11/1199.
¨
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