Salt and cocrystals of sildenafil with dicarboxylic acids: Solubility and pharmacokinetic advantage of the glutarate salt

Article abstract of DOI:10.1021/mp400516b

Sildenafil is a drug used to treat erectile dysfunction and pulmonary arterial hypertension. Because of poor aqueous solubility of the drug, the citrate salt, with improved solubility and pharmacokinetics, has been marketed. However, the citrate salt requires an hour to reach its peak plasma concentration. Thus, to improve solubility and bioavailability characteristics, cocrystals and salts of the drug have been prepared by treating aliphatic dicarboxylic acids with sildenafil; the N-methylated piperazine of the drug molecule interacts with the carboxyl group of the acid to form a heterosynthon. Salts are formed with oxalic and fumaric acid; salt monoanions are formed with succinic and glutaric acid. Sildenafil forms cocrystals with longer chain dicarboxylic acids such as adipic, pimelic, suberic, and sebacic acids. Auxiliary stabilization via C-H...O interactions is also present in these cocrystals and salts. Solubility experiments of sildenafil cocrystal/salts were carried out in 0.1N HCl aqueous medium and compared with the solubility of the citrate salt. The glutarate salt and pimelic acid cocrystal dissolve faster than the citrate salt in a two hour dissolution experiment. The glutarate salt exhibits improved solubility (3.2-fold) compared to the citrate salt in water. Solubilities of the binary salts follow an inverse correlation with their melting points, while the solubilities of the cocrystals follow solubilities of the coformer. Pharmacokinetic studies on rats showed that the glutarate salt exhibits doubled plasma AUC values in a single dose within an hour compared to the citrate salt. The high solubility of glutaric acid, in part originating from the strained conformation of the molecule and its high permeability, may be the reason for higher plasma levels of the drug.

Full text of DOI:10.1021/mp400516b

Article
pubs.acs.org/molecularpharmaceutics
Salt and Cocrystals of Sildenafil with Dicarboxylic Acids: Solubility
and Pharmacokinetic Advantage of the Glutarate Salt
Palash Sanphui, Srinu Tothadi, Somnath Ganguly, and Gautam R. Desiraju*
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
S
* Supporting Information
ABSTRACT: Sildenafil is a drug used to treat erectile
dysfunction and pulmonary arterial hypertension. Because of
poor aqueous solubility of the drug, the citrate salt, with
improved solubility and pharmacokinetics, has been marketed.
However, the citrate salt requires an hour to reach its peak
plasma concentration. Thus, to improve solubility and
bioavailability characteristics, cocrystals and salts of the drug
have been prepared by treating aliphatic dicarboxylic acids with
sildenafil; the N-methylated piperazine of the drug molecule
interacts with the carboxyl group of the acid to form a
heterosynthon. Salts are formed with oxalic and fumaric acid;
salt monoanions are formed with succinic and glutaric acid. Sildenafil forms cocrystals with longer chain dicarboxylic acids such as
adipic, pimelic, suberic, and sebacic acids. Auxiliary stabilization via C−H···O interactions is also present in these cocrystals and
salts. Solubility experiments of sildenafil cocrystal/salts were carried out in 0.1N HCl aqueous medium and compared with the
solubility of the citrate salt. The glutarate salt and pimelic acid cocrystal dissolve faster than the citrate salt in a two hour
dissolution experiment. The glutarate salt exhibits improved solubility (3.2-fold) compared to the citrate salt in water. Solubilities
of the binary salts follow an inverse correlation with their melting points, while the solubilities of the cocrystals follow solubilities
of the coformer. Pharmacokinetic studies on rats showed that the glutarate salt exhibits doubled plasma AUC values in a single
dose within an hour compared to the citrate salt. The high solubility of glutaric acid, in part originating from the strained
conformation of the molecule and its high permeability, may be the reason for higher plasma levels of the drug.
KEYWORDS: bioavailability, dissolution, crystal engineering, X-ray crystallography, hydrogen bond
FDA indicated that the cocrystals have to dissociate completely
from the dosage form before reaching the target site of action.18
Sildenafil (SLD) (Scheme 1) is an inhibitor of cyclic GMP
phosphodiesterase and was initially marketed as an anti-
hypertensive agent (Brand name Revatio) by Pfizer (UK).
The drug (Brand name Viagra) is now used mainly for the
treatment of erectile dysfunction in elderly patients.19−21 The
parent API is a potent selective inhibitor of the enzyme
phosphodiesterase (PDE-5). It destroys cyclic guanosine
monophosphate (cGMP) and dilates blood vessels in the
body. Due to its poor aqueous solubility (∼5−10 mg/L),
sildenafil exhibits incomplete absorption, has low bioavailability,
and leads to slow onset of action. The API has been marketed
as the citrate salt hydrate in 20−100 mg tablets for oral
administration. In 2011 sales of Viagra exceeded $2 billion
worldwide. However, the citrate salt exhibits moderate
solubility (3.5 g/L) and bioavailability (40%) and has a half-
life of 3−4 h. It requires approximate 1 h to reach its peak
plasma concentration because of its moderate solubility, and
INTRODUCTION
■
Drug molecules, active pharmaceutical ingredients (API), can
exist in different solid forms such as polymorphs, amorphous
phases, cocrystals, salts, and hydrate/solvates without changing
their intrinsic chemical structures.1−4 Each solid form is known
to exhibit different physicochemical properties, and this is often
determined by structural parameters.5−9 A clear understanding
of intermolecular interactions between the molecules in the
solid state, in terms of their geometry and energy is crucial in
the design of new pharmaceutical solid forms. These
fundamental principles of crystal engineering find a ready
application in the development of new solid state forms of
APIs. To improve the solubility of a drug, a viable approach is
the development of the salt form, provided the API has
ionizable acidic/basic functional groups. Salt forms are well-
known in the pharmaceutical industry and can improve
solubility by almost 100−1000 fold when compared to the
parent API.10−14 Salts can, however, be hygroscopic. To
counter the hygroscopicity problems of salts, cocrystals are an
alternate option as they are stable and can also improve
solubility to levels that are comparable to the amorphous
phase.5,15−17 Recently the US FDA identified pharmaceutical
cocrystals as a promising solid dosage form because of their
significance in pharmaceutical properties improvement. The US
Received: August 28, 2013
Revised: October 25, 2013
Accepted: October 29, 2013
Published: October 29, 2013
© 2013 American Chemical Society
4687
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
(generally regarded as safe molecules) list.30 Glutaric acid is a
natural component of dietary foodstuffs and is a well-known
metabolic intermediate of fatty acids metabolism. Pimelic acid
derivatives are used in lysine biosynthesis.31 Recently there have
been a few reports on pharmaceutical cocrystal/salts of
fluconazole,32 pyrimethamine,33 praziquantel,34 and temozolo-
mide35 with dicarboxylic acids with improved solubility and
stability. Chemical structures of the sildenafil and coformers are
displayed in Scheme 1. The cocrystals and salts are
characterized by Fourier transform infrared (FT-IR), differ-
ential scanning calorimetry (DSC), and powder and last single
crystal X-ray diffraction. Here, we present structural aspects,
hydrogen bonding, solubility, and pharmacokinetic behavior of
sildenafil salts and cocrystals.
Scheme 1. Chemical Structures of Sildenafil and Its
Coformers
RESULTS AND DISCUSSION
■
Stepanovs et al.36 have recently reported the crystal structure of
sildenafil. The API forms salts with both citric acid and
saccharin.25,26 Our attempt to cocrystallize saccharin with
sildenafil citrate (to decrease bitterness of the API) proved
unsuccessful. On the other hand, the product obtained was
sildenafil citrate monohydrate (mostly) and sildenafil saccha-
rinate acetonitrile solvate. In this work, aliphatic dicarboxylic
acids were ground with the citrate salt using solvent drop
assisted grinding. Stronger acids such as oxalic and fumaric
acids form sildenafil oxalate and fumarate salts by replacing
citric acid. Other long chain dicarboxylic acids did not produce
the same results, and this may be because of weaker acidity.
However, all of the coformers easily form cocrystals or salts
with the free base sildenafil. The experimental conditions of the
competitive supramolecular reactions are summarized in Table
1.
this may be a problem for patients with cardiopulmonary
diseases. However, the hydrate is not preferred as a solid dose
formulation because of the stability issue and low solubility as
compared to the anhydrous form.22−24 Further, sildenafil citrate
has a bitter taste, and hence tablets are commercially available
as films coated with artificial sweeteners. Although this
blockbuster drug has been in the news for over a decade,
relatively little work has been done on the structural aspects
and on its solubility and pharmacokinetic aspects.
The crystal structures of sildenafil citrate monohydrate25 and
solvates of the 1:1 saccharinate salt26 are reported in the
literature. A recent patent27 has disclosed a drug−drug cocrystal
of sildenafil with aspirin with improved solubility of the
cocrystal compared to the API in 0.1N HCl medium. Another
patent28 has disclosed a few salt polymorphs of sildenafil with
hydrochloric acid, sulfuric acid, ethanesulfonic acid, tartaric
acid, and fumaric acid along with their solubilities, and these are
compared to the sildenafil free base. Sawatdee et al.29 reported
a cyclodextrin nanosupension of the citrate salt with improved
solubility.
In a search for new solid forms with improved solubility and
bioavailability compared to the citrate salt form, both the
cocrystals and the salts have been screened in this study using
pharmaceutically acceptable dicarboxylic acids. Succinic and
adipic acids are included in the US FDA approved GRAS
Sildenafil36 contains a N-methylpiperazine fragment, and the
N atom (attached to Me group) is the most basic site (pKa =
8.7) and least hindered to form a hydrogen bond (in cocrystal)
or ionic bond (in salt). Based on the hydrogen bonding
possibilities of the API, dicarboxylic acids were selected to
obtain robust O−H···N heterosynthons37−39 in the cocrystals.
Oxalic acid (pKa = 1.2) and fumaric acid (pKa = 3.0) form salts,
but weaker acids such as succinic acid (pKa = 4.2) and glutaric
acid (pKa = 4.3) produce salt mono anions. Comparatively,
long chain acids such as adipic acid (pKa = 4.4), pimelic acid
Table 1. Experiments Attempted to Make Cocrystals of Sildenafil Citrate or Sildenafil
coformers attempted/crystallization
crystals obtained
sildenafil citrate
saccharin/MeOH
sildenafil citrate monohydrate
sildenafil saccharinate CH3CN solvate (1:1:1)
sildenafil oxalate (1:0.5)
saccharin/CH3CN
oxalic acid
succinic acid/CH3CN
fumaric acid/CH3CN, EtOH, THF
fumaric acid/CH3OH
glutaric acid/CH3CN
pimelic acid/MeOH
oxalic acid/MeOH
fumaric acid/MeOH
succinic acid/MeOH
glutaric acid/MeOH
adipic acid/MeOH
pimelic acid/MeOH
suberic acid/MeOH
sebacic acid/MeOH
sildenafil citrate monohydrate
sildenafil citrate monohydrate
sildenafil fumarate trihydrate (1:1:3)
sildenafil citrate monohydrate
sildenafil citrate monohydrate
sildenafil oxalate (1:0.5) salt
sildenafil fumarate trihydrate (1:1:3)
sildenafil succinate (1:1)
sildenafil
sildenafil glutarate (1:0.5)
sildenafil adipic acid (1:0.5)
sildenafil pimelic acid (1:0.5)
sildenafil suberic acid (1:0.5)
sildenafil sebacic acid (1:0.5)
4688
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
Table 2. Crystallographic Parameters of Sildenafil and Its Cocrystal/Salts
SLD−OXA
C22H31N6O4S,
0.5(C2O4)
SLD−FUM
SLD−SUC
SLD−GLU
C22 H32N6O4S,
0.5(C5H6O4)
SLD−ADP
C22 H31N6O4S,
0.5(C6H10O4)
emp. formula
C22 H31N6O4S, C4H3O4,
3(H2O)
C22 H31N6O4S,
C4H5O4
formula wt
crystal system
space group
T (K)
519.60
644.70
550.61
541.14
547.65
triclinic
P1
triclinic
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
P1
150(2)
7.965(6)
12.445(9)
13.466(9)
80.89(3)
75.26(2)
77.71(2)
1253.5(15)
1.377
150(2)
150(2)
19.17(3)
15.05(3)
9.862(18)
90
150(2)
150(2)
a (Å)
11.714(3)
11.894(3)
13.531(3)
101.666(9)
102.8040(10)
116.260(5)
1548.3(7)
1.383
19.850(6)
14.604(3)
9.684(4)
90
19.156(3)
14.846(2)
9.7216(15)
90
b (Å)
c (Å)
α (deg)
β (deg)
104.26(2)
90
100.608(13)
90
100.186(9)
90
γ (deg)
volume (Å3)
Dcalcd (g cm−3
Z
2757(8)
1.326
2759.4(15)
1.318
2721.1(7)
1.337
)
2
2
4
4
4
2θ range
2.8−27.5
2.0−27.5
0.0516, 0.1718
1.139
2.5−27.5
0.0903, 0.2796
1.099
2.5−27.5
0.0955, 0.2807
1.100
2.2−27.5
0.0545, 0.1584
1.079
R1 (I > 2σ(I)), wR2 0.0555, 0.1850
GOF
1.121
diffractometer
Rigaku-CCD
Rigaku-CCD
Rigaku-CCD
Rigaku-CCD
SLD−SEB
Rigaku-CCD
SLD−PIM
SLD−SUB
NBA−GLU
emp. formula
formula wt
crystal system
space group
T (K)
C22 H31N6O4S, 0.5(C7H12O4)
C22 H31N6O4S, 0.5(C8H14O4)
C22 H31N6O4S, 0.5(C10H18O4)
2(C7H6N2O3), C5H8O4
464.23
548.61
561.68
575.70
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
triclinic
P1
130(2)
150(2)
150(2)
150(2)
a (Å)
19.062(5)
14.864(4)
9.925(3)
90
19.031(10)
15.097(7)
9.915(5)
90
18.969(8)
14.820(6)
10.525(4)
90
7.0274(14)
11.049(2)
13.875(3)
72.431(5)
78.479(5)
87.944(6)
1006.0(3)
1.532
b (Å)
c (Å)
α (deg)
β (deg)
100.889(15)
90
104.746(4)
90
99.559(17)
90
γ (deg)
volume (Å3)
Dcalcd (g cm−3
Z
2761.6(13)
1.320
2755(2)
1.354
2918(2)
1.311
)
4
4
4
2
2θ range
1.7−27.5
0.0570, 0.1853
1.148
1.7−27.5
0.0480, 0.1563
1.131
1.8−27.5
0.0977, 0.2804
1.188
3.1−27.6
0.0682, 0.1745
1.018
R1 (I > 2σ(I)), wR2
GOF
diffractometer
Rigaku-CCD
Rigaku-CCD
Rigaku-CCD
Rigaku-CCD
(pKa = 4.5), suberic acid (pKa = 4.6), and sebacic acid (pKa =
4.7) form cocrystals with sildenafil. Except the fumarate salt
trihydrate (1:1:3) and the succinate salt (1:1), other multi-
component systems maintain a 1:0.5 stoichiometry. Crystallo-
graphic parameters and normalized hydrogen bonds for the
cocrystal/salts of sildenafil are summarized in Tables 2 and 3.
Sildenafil Oxalate Salt (SLD−OXA, 1:0.5). The crystal
structure (P1, Z = 2) contains a half equivalent of oxalate anion
which is located on a crystallographic inversion center. Proton
transfer from oxalic acid to the N-methylated piperazine ring of
sildenafil confirms salt formation. The NH proton forms
bifurcated hydrogen bonds with two oxalates through N+−H···
O− ionic interactions (N1−H1···O5: 1.60 Å, 161°; N1−H1···
O6: 2.46 Å, 113°); see Figure 1a. Sildenafil molecules form
dimers through auxiliary C−H···O hydrogen bonds and
connect to the next dimer via the oxalate anion (Figure 1b).
Sildenafil Fumarate Salt Trihydrate (SLD−FUM, 1:1:3).
The salt (P1, Z = 2) consists of one sildenafil cation, two half
equivalent fumarate anions binding through one common
hydrogen atom, and three water molecules. Proton transfer
from fumaric acid to N-methylpiperazine fragment (N2+−
H2N···O7−: 1.75 Å, 158°) confirms the ionic nature of the
fumarate salt (Figure 2a). Fumarate monoanions form 1D
chains along the crystallographic b-axis. Two asymmetric water
molecules form a hydrogen bonded tetramer40,41 via O−H···O
hydrogen bonds (O9−H9A···O10: 1.82 Å, 173°; O10−H10A···
O9: 1.85 Å, 173°), whereas the third water molecule forms a
hydrogen bond (O11−H11B···O4: 1.78 Å; 170°) with the API
carbonyl. Sildenafil molecules form a cavity in which fumarate
anions (chain) sit in a channel along the crystallographic b-axis
(Figure 2b).
Sildenafil−Succinate (SLD−SUC, 1:1) and Sildenafil−
Glutarate Salts (SLD−GLU, 1:0.5). In the crystal structures
of the salts of succinic and glutaric acid (P21/c, Z = 4), the
dicarboxylic acids show positional static disorder in the internal
carbon atoms at 150 K. An inversion center is present in the
special position intersecting the carboxylate ions in succinic
acid; see Figure 3a. The site occupancy factors (SOF) for
internal carbons are therefore 0.5 each. We were unable to
model the uncommon disorder in succinic acid precisely. It is
difficult to conclude whether SLD−SUC is a cocrystal or salt as
the CO bond distances are 1.21 and 1.23 Å. The FT-IR
4689
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
Table 3. Hydrogen Bond Geometrical Parameters of Crystal Structures (O−H, N−H, and C−H Distances Are Neutron
a
Normalized)
compound
interaction
H···A (Å)
D···A (Å)
∠D−H···A (deg)
symmetry
−1 + x, y, z
SLD−OXA
N1−H1···O5
1.60
2.46
1.86
2.51
2.46
2.31
2.57
2.36
2.45
2.26
2.50
1.75
1.98
1.82
1.86
1.85
1.86
1.78
1.65
1.85
2.51
2.39
2.44
2.56
2.25
1.62
1.86
2.54
2.55
2.50
1.83
1.67
2.42
2.54
2.34
2.53
2.41
1.84
1.78
2.41
2.51
2.40
2.44
2.32
2.576(3)
3.000(3)
2.666(3)
3.516(3)
2.924(3)
3.366(3)
3.561(3)
3.012(3)
2.923(3)
3.176(3)
2.915(3)
2.683(3)
2.650(3)
2.742(3)
2.856(3)
2.787(3)
2.888(3)
2.805(3)
2.635(2)
2.629(2)
2.959(3)
2.895(3)
3.339(3)
3.349(3)
3.330(3)
2.629(3)
2.632(3)
3.537(4)
3.203(4)
3.361(3)
2.629(2)
2.653(2)
2.867(3)
2.982(3)
3.420(3)
2.923(3)
3.402(3)
2.606(4)
2.642(5)
2.868(6)
2.910(6)
3.468(7)
3.385(6)
3.285(7)
161
113
134
153
105
164
155
117
105
141
101
158
123
173
174
173
175
170
174
131
104
106
140
131
172
174
136
172
117
144
133
173
103
103
178
100
152
141
166
104
101
169
145
148
N1−H1···O6
N4−H4···O3
−x, 1 − y, 1 − z
intramolecular
C2−H2B···O1
C3−H3A···O2
C3−H3B···O4
C4−H4A···O2
C4−H4B···O6
C5−H5B···O1
C7−H7···O5
C11−H11···O2
N2−H2N···O7
N4−H4···O3
O9−H9A···O10
O9−H9B···O2
O10−H10A···O9
O11−H11B···O9
O11−H11B···O4
O6−H6···N1
−x, −y, 1 − z
intramolecular
−x, 1 − y, −z
−x, −y, 1 − z
−x, 1 − y, 1 − z
intramolecular
1 − x, 1 − y, 1 − z
intramolecular
SLD−FUM
SLD−ADP
2 − x, 1 − y, 1 − z
intramolecular
−1 + x, −1 + y, z
1 − x, −y, −z
1 − x, 1 −y, −z
1 − x, −y, 1 − z
x, y, z
x, 1/2 − y, 1/2 + z
intramolecular
N4−H4···O3
C3−H3B···O1
C4−H4A···O2
C13−H13C···O2
C2−H2B···O5
C8−H8···O4
O5−H5···N1
N4−H4···O3
C8−H8···O4
intramolecular
intramolecular
−x, −y, 1 − z
1 − x, 1 − y, 1 − z
x, 1/2 − y, −1/2 + z
1 − x, 1 − y, 2 − z
intramolecular
SLD−PIM
SLD−SUB
2 − x, −1/2 + y, 3/2 − z
2 − x, −1/2 + y, 3/2 − z
2 − x, −y, 1 − z
intramolecular
C12−H12A···O4
C13−H13B···O1
N3−H3···O3
O5−H5···N1
x, 1/2 − y, −1/2 + z
intramolecular
C3−H3A···O1
C4−H4B···O2
C8−H8···O4
C11−H11···O2
C13−H13B···O1
N4−H4···O3
O6−H6A···N1
C3−H3A···O2
C7−H7···O1
C10−H10···O4
C3−H13···O2
C24−H24B···O1
intramolecular
−x, −1/2 + y, 1/2 − z
intramolecular
−x, 1 − y, −z
intramolecular
SLD−SEB
1 − x, 1 − y, 2 − z
intramolecular
intramolecular
−x, −1/2 + y, 1/2 − z
−x, 1 − y, −z
x, y, 1 + z
a
Note that H-bond parameters for succinate and glutarate salt are not added because of a disorder issue.
spectrum of SLD−SUC (discussed next) indicates the
possibility of one neutral carboxylic acid and a carboxylate
anion in the coformer. A hydrogen bond is expected between
N-methylpiperazine and succinic acid. In the sildenafil−
glutarate salt, two carboxylic acids are positionally disordered
between two sildenafil molecules. Unlike in the succinate salt,
proton specification is easier here (obtained by modeling
glutaric acid). The four C−O bond distances in glutaric acid are
1.26, 1.28, 1.21, and 1.32 Å, and this suggests that the sildenafil
glutarate salt exists as a salt monoanion (Figure 3b). The
glutarate is present in a folded conformation compared to the
usual extended zigzag chain.
Figure 1. (a) Hydrogen bonding of SLD−OXA salt (1:0.5). (b)
Oxalate anions bridge between two C−H···O hydrogen bonded
sildenafil dimers.
4690
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
Figure 2. (a) Hydrogen bonding of SLD−FUM salt. (b) Fumarate anions act as guest inside (shown in circle) the cavity formed by the SLD cations.
and N-methylpiperazine of the API, see Figure 5a, b). Sildenafil
molecules interact with adipic acid through C−H···O
Figure 3. (a) Hydrogen bonded tricomponent assembly between (a)
two sildenafil and one succinate salt monoanion (disorder) and (b)
glutarate salt monoanion.
Succinate and glutarate salt monoanions were further
confirmed by FT-IR spectroscopy. The sildenafil molecule
contains sulfone, amide carbonyl, and ring NH groups. The API
exhibits vibrational bands at 1166 cm−1 (SO2 symmetric), 1356
cm−1 (SO2 asymmetric), 1690 cm−1 (CO asym), and 3309
cm−1 (NH stretch) in the vibrational spectra. For SLD−SUC
and SLD−GLU, there are CO bands at 1724 and 1729 cm−1
(blue shift from coformers), which indicates salts, not cocrystals
(Figure 4). However, in the salts, there are no IR bands at 1920
Figure 5. Common O−H···N hydrogen bonding motif in (a) SLD−
ADP and (b) SLD−PIM cocrystals. (c) C−H···O hydrogen bonds
between SLD and carboxylic acid in SLD−ADP cocrystal.
interactions (d, 2.56 Å; θ, 131°) between the aliphatic CH2
of piperazine ring of SLD with the carbonyl of the acid. In the
SLD−ADP cocrystal, four sildenafil molecules form tetramers
through auxiliary C−H···N and C−H···O interactions (Figure
5c). A similar packing motif is also present in the SLD−PIM
cocrystal. There is a positional disorder of C25 carbon atom
(SOF 0.6 and 0.4) in pimelic acid. An inversion center
intersects pimelic acid from the middle carbon C26 with equal
SOF’s each of 0.5.
Sildenafil−Suberic and Sildenafil−Sebacic Acid Coc-
rystals (SLD−SUB, SLD−SEB, 1:0.5). The cocrystals (P21/c,
Figure 4. FT-IR vibrational frequency comparison between SLD−
Z = 4) consist of the common hydrogen bonding motif of O−
H···N interaction (O5−H5···N1: 1.67 Å, 173°, O6−H6A···N1:
1.78 Å, 166°) between carboxylic acid and N-methyl piperazine
of the API which results in a tricomponent assembly (Figure 6a,
b). In the suberic acid cocrystal, four sildenafil molecules form a
tetramer through C−H···O and C−H···π interactions and
interact with carbonyl acceptor of the dicarboxylic acid (Figure
6c). A similar hydrogen bonding motif is also present in the
sebacic acid cocrystal.
SUC and SLD−GLU salts.
and 2450 cm−1 which corresponds to the acid···piperazine
heterosynthon present in SLD cocrystals (Figure S4, ESI). But
as SLD−GLU is a salt monoanion (confirmed from the crystal
structure), we can expect the same for SLD−SUC salt because
of the similarity of CO bands.
Sildenafil−Adipic and Sildenafil−Pimelic Acid Cocrys-
tals (SLD−ADP, SLD−PIM, 1:0.5). The cocrystals (P21/c, Z =
4) consist of O−H···N hydrogen bonds (O6−H6···N1: 1.65 Å,
174°, O5−H5···N1: 1.62 Å, 174°) involving dicarboxylic acid
It is interesting to note that SLD cocrystals with adipic to
sebacic acid have similar cell parameters (Table 2), even though
there is a variation in alkyl chain length in the dicarboxylic
4691
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
was 34.3 g/L, which is the solubility of sildenafil hydrochloride
hydrate (SLD−HCL). SLD−HCL exhibits a characteristic peak
at 6.7 0.2° 2θ (see Figure S5a, SI), which is prominent in all
of the solid forms after 4 h of the solubility experiment. At 4 h,
the solubility of the SLD−CIT salt was 93.3 g/L. However,
after 24 h, the solubility of the citrate salt decreased to 36.7 g/L.
This happens because of a phase transformation of SLD−CIT
to SLD−HCL. Among all of the binary systems, SLD−OXA
showed the lowest solubility of 8.5 g/L and transformed to
SLD−HCL within 4 h. In comparison, the SLD−FUM salt is
stable up to 4 h in a slurry experiment, and the solubility
increased from 32.3 g/L (4 h) to 47.4 g/L (24 h). As expected,
sildenafil citrate exhibits a higher solubility than the oxalate and
fumarate salts because of higher solubility of the citric acid.
Comparatively, SLD−SUC salt exhibits moderate solubility (74
g/L). The SLD−GLU salt showed the highest solubility of 119
g/L at 4 h. Further, the SLD−GLU salt showed the highest
solubility (164 g/L) at 2 h, which is more than the SLD−CIT
salt hydrate (106 g/L, 2 h) (Figure 7a).
Figure 6. Common O−H···N hydrogen bonding motif in (a) SLD−
SUB and (b) SLD−SEB cocrystals. (c) C−H···O hydrogen bonds
between sildenafil and carboxylic acid in SLD−SUB cocrystals.
The solubility of a binary system may be affected by the
factors such as interactions in the crystal lattice, melting point
(mp), solubility of the coformer, and finally particle
morphology. Let us consider these factors in turn: considering
the bond distances between the API and coformer, the
interaction between sildenafil cation and oxalate anion is the
strongest (N+−H···O−, D, 2.57 Å), and hence the solubility of
the oxalate salt is expected to be the least among the salts
(Table 4). In the fumarate, glutarate, and citrate salts, longer
hydrogen bond distances (D, 2.67, 2.69, and 2.68 Å) between
the API and coformers are observed, which correlates with their
higher solubility compared to the oxalate salt. Considering the
solubillities of the coformers, the citrate salt showed a higher
solubility than the oxalate and fumarate salt due to the higher
solubility of citric acid. The oxalate salt should correspondingly
have a higher solubility than the fumarate salt. However, this
was not observed, and this also agrees with the higher melting
point (m.p.) of the oxalate salt (Table S4). Also, there is an
inverse correlation between mp and solubility of the SLD salts
(Figure 8a). SLD−GLU salt has the lowest mp and the highest
solubility among the SLD salts.
acids. It may be possible that the zigzag alkyl chain of
dicarboxylic acids occupy the space between the two APIs.
Further, positional disorder is present in the corresponding
salts/cocrystals with the coformer dicarboxylic acids, for
example, succinic (even, n = 2), glutaric (odd, n = 3), and
pimelic acid (odd, n = 5), because the coformers may not fit
properly in the available space between two SLD molecules.
Moreover, odd dicarboxylic acids are present in a disordered
structure possibly because of their high energy conformations
and strained trapezoid geometry. There may be certain
optimum distance required between the two API’s to form a
tricomponent assembly. Here adipic acid (n = 4) has the
optimum chain length required to avoid steric repulsion
between the two API molecules. All of the new solid forms
were further characterized by FT-IR, PXRD, and DSC; see the
Supporting Information.
Solubility and Stability Study. The solubility of sildenafil
is pH-dependent and decreases with increase of pH of the
medium.42 All of the solubility experiments were carried out in
0.1 N HCl medium at 4 and 24 h to examine the effect of acidic
pH (see Table 4). The apparent solubility of sildenafil after 4 h
Table 4. Solubility Profiles of Sildenafil and Its New Solid Phases in 0.1N HCl Medium
absorption
coefficient
solubility
(g/L)
at 24 h
crystalline
forms
apparent solubility
(g/L) at 4 h (2 h)
aqueous solubility distance, D (Å), N+−H···O−/O−H···N residue obtained after
a
(mM−1cm−1
)
(g/L) of coformers
(Å)
solubility expt. (4 h)
SLD
12.34
14.09
34.3
34.5
36.7
SLD + SLD−HCL
SLD−CIT
93.3 (106.1)
730
143
2.674, 2.911
2.574, 3.005
SLD−CIT + SLD−
HCL
SLD−OXA
11.40
8.5
7.6
SLD−HCL + SLD−
OXA
SLD−FUM
SLD−SUC
10.81
12.82
32.3
74.5
47.4
48.7
6.3
80
2.683
2.675
SLD−FUM
SLD−SUC + SLD−
HCL
SLD−GLU
SLD−ADP
SLD−PIM
SLD−SUB
SLD−SEB
12.57
14.33
13.33
12.40
13.31
118.8 (164.3)
59.6
73.7
38.6
57.2
65.0
42.9
430
14
2.691
2.635
2.629
2.654
2.642
SLD−GLU + SLD−
HCL
SLD−ADP + SLD−
HCL
95.2 (127.9)
76.4
71
SLD−PIM + SLD−
HCL
2.5
0.3
SLD−SUB + SLD−
HCL
58.0
SLD−SEB + SLD−
HCL
a
Note: Solubility values of coformers were obtained from literature.
4692
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
Figure 7. (a) Dissolution studies of SLD, its salts, and cocrystals in 0.1 N HCl aqueous medium. (b) Solubility comparisons in water at 2 h of
dissolution.
Figure 8. Solubilities (at 4h ) of the SLD salt/cocrystals correlations with their (a) melting points (mp) and (b) the solubility of the coformers. Note
that the inverse correlation between mp and solubility is valid for salts only. Except for oxalate salt, the solubility of other binary systems increases
with the coformer solubility.
Finally, morphology may play a role in improving the
dissolution rate of a solid form. Generally, block or plate
morphologies provide higher surface area than an acicular
morphology. This increases the exposure to the medium and
hence the dissolution. In the present context, SLD−GLU has a
thick plate morphology (single crystals) compared to the
acicular citrate salt. This may be another reason for improved
dissolution rate of the glutarate salt.
Considering the cocrystals, the SLD−PIM cocrystal showed
the highest solubility of 128 g/L (after the glutarate salt) at 2 h
of dissolution. The solubility of the cocrystals follows the same
order as the solubility of the coformers15 (Figure 8b).
Generally, cocrystals having longer alkyl chains and formed
from dicarboxylic acids (pimelic, suberic, sebacic acid) are more
hydrophobic and hence exhibit poor aqueous solubility. SLD−
SEB showed the least solubility (58.0 g/L) at 4 h of dissolution.
The reported aspirin cocrystal27 (SLD−ASP) showed a higher
solubility of 82 g/L compared to the citrate salt at 1 h but
transformed faster to the hydrochloride salt compared to the
highly soluble SLD−GLU salt and SLD−PIM cocrystal (Figure
S5, SI).
structures. Glutarate salts showed higher solubility than the
pimelic acid cocrystals in keeping with the solubilities of the
carboxylic acidsGLU (430 g/L) is more soluble than PIM
(71 g/L). The solubility experiment suggests that all of the
solid forms (except SLD−FUM) transformed to SLD−HCL
after 4 h in acidic medium. The stability of SLD−FUM salt (4
h) is due to the presence of three water molecules in the crystal
lattice, and hence there is a natural tendency to exclude water
molecules (external) during dissolution. However, after 24 h
slurry experiments, all of the solid forms transformed to SLD−
HCL as confirmed by PXRD comparisons (Figure S5, ESI).
The apparent solubility of SLD−CIT, SLD−GLU salts, and
SLD−PIM, SLD−ASP cocrystals were carried out in distilled
water (neutral pH) at 2 h. The solubility of the citrate salt is 4.7
g/L at 37 1 °C. SLD−GLU (solubility 15.1 g/L) and SLD−
PIM (solubility 6.4 g/L) are 3.2 and 1.3 times more soluble
than the citrate salt; see Figure 7b. However, the prior art
SLD−ASP cocrystals27 showed less solubility compared to the
citrate salt. All of the solid phases are stable in water as seen
after 2 h of solubility experiments, confirmed by the PXRD
comparisons. The rates of phase transformation of all SLD
cocrystals and salts to SLD−HCL are much faster in acidic
medium than in neutral medium.
Another important aspect is that multicomponent systems
with odd acids (glutaric, pimelic acid) showed higher solubility
than their even counterparts (succinic, adipic, suberic). This
could be because of the strained geometry and high energy and
hence high solubility of the coformers.43,44 Strained geometry is
also reflected in the glutarate salt and pimelic acid cocrystal
Conformations of Glutaric Acid and the High
Solubility. The high solubility of glutaric acid containing
salts can be explained by the possible occurrence of different
conformations of the glutaric acid in solution. Drug solubility is
4693
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
a function of molecular flexibility and/or polar surface area.
Hence increased rotational degrees of freedom may often
decrease crystallinity of the drug (solute), resulting in improved
aqueous solubility and absorption.2,45 In the case of cocrystals
and salts of the highly soluble glutaric acid, both enthalpy and
entropy play a role in their solubility. The coformer is more
soluble than the API. During dissolution, it is reasonable that
the coformer (here GLU) dissolves first, followed by the
transformation of the API into a higher energy amorphous
phase through rearrangement of molecules caused by the voids
created by dissolution of the coformers.46,47 The greater the
solubility of the coformer, the faster the API could transform to
the amorphous phase followed by supersaturation. An unusual
bent (semicircle) conformation of glutaric acid in 4-nitro-
benzamide−glutaric acid cocrystal was seen in our studies
(NBA−GLU, Figure 9). This is the only conformation of
could well be the reason for higher solubility of the acid itself
and of cocrystals and salts comprising the acid. However,
compared to the citrate salt and pimelic acid cocrystal, glutarate
salt follows the parachute model46,47 by increasing the lifetime
of the metastable states of the API, which is necessary for a
stable drug formulation.
Pharmacokinetic Study. Oral bioavailability, the fraction
of the oral dose that reaches the arterial blood in its active form,
is an important physical property for drug formulation.
Improving the solubility of a solid form by cocrystal/salts can
have a dramatic impact on absorption and bioavailability16,51−53
Thus, pharmacokinetic studies of the citrate and glutarate salts
were carried out on male Sprague−Dawley rats. Oral
bioavailability can be assessed by measuring AUC (area under
the curve) or Cmax (peak plasma concentration). At a single
administered dose of 5 mg/kg, glutarate salt showed a higher
peak plasma concentration (334 ng/mL) compared to the
citrate salt (214 ng/mL, Figure 10) within 1 h, while AUC0−24h
Figure 9. 4-Nitrobenzamide−glutaric acid (NBA−GLU, 2:1)
cocrystal. Notice the bent (semicircle) conformation of glutaric acid
connected to 4-nitrobenzamide via acid−amide synthons followed by
auxiliary C−H···O interactions.
glutaric acid (enthalpy effect), where both the carboxylic acid
groups are in the syn location because of acid−amide
heterosynthon formation between a glutaric acid and two 4-
nitrobenzamide molecules. The auxiliary C−H···O interactions
between the CH2 of GLU with the NO2 group of the
benzamide drives the bent conformation of the carboxylic acid.
The energy difference between the flat (chain) and bent
(semicircle) conformations is quite high (3.65 kcal/mol); see
Table S1 and Figure S1, SI. The conformational energy of the
observed GLU conformer in the SLD−GLU salt is 1 kcal/mol
higher than the flat (chain) geometry. Among all of the
aliphatic dicarboxylic acids (oxalic acid to sebacic acids), only
GLU (n = 5) has the optimum chain length, suitable for a bent
conformation. It can be hypothesized that when the higher
energy GLU (partially bent) in salt dissolves in aqueous
medium, the conformation of the GLU changes from the bent
to flat geometry. During dissolution, the trapped water (more
structured) in the bent conformer of GLU is expected to be
pushed to the bulk water with entropy gain. This gain results in
a lowering in free energy of the system and facilitates the
dissolution process of SLD−GLU.
The solubility (2 h) ratio of NBA−GLU cocrystal and NBA
is only 18, whereas the solubility ratio of SLD−GLU salt
compared to SLD is very high (3020-fold). Of course salts can
increase solubilities much more compared to cocrystals. Again,
the aqueous solubility of NBA−GLU cocrystal (1.69 g/L),
which is 1.3-fold more than the reported NBA−SUC (1.28 g/
L) and NBA−ADP (1.25 g/L) cocrystals48 after 30 min of
dissolution, correlates with the solubility of the coformers
themselves; the anomalous higher solubility of the glutaric acid
salts and cocrystals may be the effect of high energy bent
conformation of glutaric acid. Hence it is expected that, along
with the higher energy conformation of glutaric acid, salt
formation also contributes toward improving solubility. Again,
the melting point and density of glutaric acid are the lowest and
molar entropy is the highest in the series of malonic to pimelic
acids.49,50 The above unusual physical properties of glutaric acid
Figure 10. Mean plasma concentration−time profile of the citrate and
glutarate salt in rats.
almost doubled for the glutarate salt. Further, glutarate salt
showed improved shelf life of the drug more than twice
compared to the citrate salt; see Table 5. Amidon and
Lobenberg54 in their models have shown that oral bioavail-
̈
ability is a product of permeability and solubility. To obtain
high bioavailability, an increased level of solubility and
permeability are required. Glutaric acid is a highly soluble
and permeable dicarboxylic acid, which also correlates with the
high bioavailability of the glutarate salt. By improving AUC
values of the glutarate salt by a factor of 2, it is expected that the
bioavailability of sildenafil will go up.
CONCLUSIONS
■
Sildenafil is a well-known drug used to treat erectile dysfunction
and pulmonary arterial hypertension but faces problems of poor
aqueous solubility and bioavailability. To improve the solubility
of the API, multicomponent molecular crystals were prepared
through solvent assisted and slurry grinding methods. Among
the new synthesized solid forms, oxalic and fumaric acids form
salts with the API. However, the formation of cocrystals is more
predominant with increasing carbon chain length (succinic acid
to sebacic acid). Succinic and glutaric acids showed positional
4694
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
Table 5. Mean Pharmacokinetic Parameters of Glutarate and Citrate Salts Following Oral Administration to Male Sprague−
Dawley Rats
compound
route
dose (mg/kg)
Cmax (ng/mL)
Tmax (h)
T1/2 (h)
AUC0−24 (ng·h/mL)
SLD−GLU
SLD−CIT
oral
oral
5
5
334.10
214.01
1.0
1.0
15.06
6.70
4220
2200
static disorder in the corresponding salt monoanions. There is a
decrease in melting points with increasing chain length for the
cocrystals/salts with even dicarboxylic acids, but the same trend
is not observed in cocrystals/salts with the odd acids.
Sildenafil−glutarate salt and sildenafil−pimelic acid cocrystal
showed improved solubility and also inhibit phase trans-
formation when compared to the citrate salt in 0.1 N HCl
medium. The glutarate salt is more soluble than the citrate salt
in water. The solubility of the API salts follows an inverse
correlation with their melting points, while the cocrystals follow
the solubility of the coformers. Cocrystals/salts with odd
dicarboxylic acids showed better solubility than their even
counterparts. An unusual bent conformation of glutaric acid
was observed, which is relevant in demonstrating the
crystallization pathway of glutaric acid from a bent to straight
chain geometry and gives a possible reason for high solubility of
the glutarate salt. Further, the glutarate salt exhibits increased
plasma AUC and shelf life which is more than twice compared
to the citrate salt. Hence sildenafil glutarate salt is a promising
candidate for next stage of formulation development and can be
an alternate solid form of the citrate salt.
maps. PLATON58,59 software was used to check correct
symmetry. Disorder in glutarate and pimelic acid was modeled
successfully, but there were problems with the succinate salt. X-
Seed60 was used to prepare the packing diagrams of the
cocrystals and salts. Crystallographic CIF files (CCDC Nos.
956673−956681) are available at http://www.ccdc.cam.ac.uk/
data_request/cif or as part of the Supporting Information.
Thermal Analysis. DSC was performed on a Mettler
Toledo DSC 822e module. About 4−6 mg of the sample was
placed in crimped but vented aluminum pans of 40 μL, and the
temperature range scanned was 30−250 °C at a heating rate of
5 °C/min. The sample was purged by a stream of dry nitrogen
flowing at 50 mL/min. DSC endotherms of the API and its
cocrystals and salts are shown in Figure S2, SI. Melting points
(°C) of the API, salts, and cocrystals are summarized in Table
S4 (obtained from DSC data) and compared with the
coformers. Similar to the reported sildenafil citrate and
saccharinate salt,25,26 oxalate and fumarate salts also melt at a
temperature higher than the API, but all of the cocrystals melt
lower than the API. There is a downward trend of melting
points of cocrystal/salts of sildenafil (as coformers) with
increasing alkyl chain length of even dicarboxylic acids.
However, the same trend is not observed for odd carboxylic
acid cocrystals and salts. A 1:1 ground mixture of sildenafil and
succinic acid results a single endotherm at 179 °C, but same
stoichiometry of the API and glutaric acid melts at 159 °C
preceded by melting of glutaric acid (92−97 °C). The single
endotherm of the API and glutaric acid (1:0.5) ground mixture
in DSC further confirms the stoichiometry.
EXPERIMENTAL SECTION
■
Sildenafil citrate salt was supplied by Ranbaxy Laboratories
(Gurgaon, India). The free base was prepared by slurrying the
citrate salt in 0.05 (M) NaOH solutions for 3−4 days. The free
base sildenafil was crystallized from MeOH as parallelogram
plate crystals, which is confirmed with the reported crystal
structure.36
Powder X-ray Diffraction. Powder X-ray diffraction data
were collected on a Philips X’pert Pro X-ray powder
diffractometer equipped with X’cellerator detector. The scan
range, step size, and time per step were 2θ = 5.0 to 40°, 0.017,
and 30 s, respectively. The formation of cocrystal/salt was
monitored by the appearance of new diffraction lines using
X’pert HighScore software. The bulk phase purity of the
cocrystal/salts was confirmed from the comparison with the
calculated lines obtained from single crystal X-ray data (Figure
S3, SI). However, there are powder X-ray pattern differences
(2θ < 15°) between sildenafil succinate salt crystal structure
and its bulk material obtained through MeOH solvent assisted
grinding. This indicates a possibility of sildenafil succinate salt
polymorphs24,61 (see Figure S3d, SI).
Vibrational Spectroscopy. FT-IR spectra were recorded
on samples dispersed in diffused reflectance mode in the range
of 4000−400 cm−1 in a Perkin-Elmer FTIR spectrometer. Data
were analyzed using Spectrum software. IR bands at 1920 and
2450 cm−1 represent acid−piperazine heterosynthon present in
all of the cocrystals of adipic acid to sebacic acid. As expected
those bands are absent in the oxalate to glutarate salts (Figure
S4, SI). FT-IR vibrational frequencies (cm−1) of sildenafil and
its multicomponent systems are summarized in Table S3.
Solubility Study. The absorption coefficient of each solid
phase was measured from the slope of the absorbance vs
concentration of the five known concentrated solutions in 0.1
N HCl aqueous medium and distilled water and measured at
Sildenafil and aliphatic dicarboxylic acid coformers were
taken in a definite stoichiometric ratio (preferably 1:1 or 1:0.5)
and ground in a mortar-pestle for 15−30 min with a dropwise
addition of MeCN, MeOH, or EtOH. The final mixture was
characterized by FT-IR, PXRD, and DSC. Crystallization
conditions of the cocrystal/salts preparation are summarized
in Table S2.
4-Nitrobenzamide−glutaric acid (2:1) cocrystal. 4-Nitro-
benzamide and glutaric acid were taken in a 2:1 molar ratio
and ground after adding 2−3 drops of EtOH (solvent drop
grinding). The ground sample was dissolved in a minimum
amount of 1:1 mixture of CHCl3 and MeOH in vial. Later,
based on vapor diffusion method, the vial was kept in a CHCl3
containing beaker. Good quality crystals were obtained after six
days.
Single Crystal X-ray Diffraction. Single crystal X-ray data
for all of the binary systems were collected on a Rigaku
Mercury375/M CCD (XtaLAB mini) diffractometer using
graphite monochromated Mo−Kα radiation at 150 K. The data
were processed with the Rigaku Crystal clear software.55
Structure solution and refinements were executed using
SHELX-9756 using the WinGX57 suite of programs. Refinement
of coordinates and anisotropic thermal parameters of non-
hydrogen atoms were performed with the full-matrix least-
squares method. The different treatment of H in D−H in any
structure depends on the data quality. All of the hydrogen
atoms in NH, OH, and CH are located from difference Fourier
4695
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
(7) Schultheis, N.; Newman, A. Pharmaceutical Cocrystals and Their
Physicochemical Properties. Cryst. Growth Des. 2009, 9, 2950−2967.
̌ ̌ ́ ́ ́
(8) Karki, S.; Friscic, T.; Fabian, L.; Laity, P. R.; Day, G. M.; Jones,
W. Improving Mechanical Properties of Crystalline Solids by Cocrystal
Formation: New Compressible Forms of Paracetamol. Adv. Mater.
2009, 21, 3905−3909.
(9) Chen, J.; Wang, Z.; Chuan-Bin, W.; Li, S.; Lu, T. Crystal
engineering approach to improve the solubility of mebendazole.
CrystEngComm 2012, 14, 6221−6229.
(10) Banerjee, R.; Bhatt, P. M.; Ravindra, N. V.; Desiraju, G. R.
Saccharin Salts of Active Pharmaceutical Ingredients, Their Crystal
Structures, and Increased Water Solubilities. Cryst. Growth Des. 2005,
5, 2299−2309.
(11) Portell, A.; Barbas, R.; Font-Bardia, M.; Dalmases, P.; Prohens,
R.; Puigjaner, C. Ziprasidone malate, a new trimorphic salt with
improved aqueous solubility. CrystEngComm 2009, 11, 791−795.
(12) Thakuria, R.; Nangia, A. Highly soluble olanzapinium maleate
crystalline salts. CrystEngComm 2011, 13, 1759−1764.
(13) Prohotsky, D. L.; Zhao, F. A survey of top 200 drugs
inconsistent practice of drug strength expression for drugs containing
salt forms. J. Pharm. Sci. 2012, 101, 1−6.
293 nm in a Perkin-Elmer UV−vis spectrometer. The solubility
of each solid was measured at 4 h and also 24 h using the shake-
flask method.62 For SLD, SLD−CIT, SLD−GLU, and SLD−
PIM, apparent solubilities were measured at 1 and 2 h.
Similarly, solubility experiments were carried out in water
(neutral pH) at 2 h to observe the effect of pH on the SLD
cocrystals and salts.
Pharmacokinetic Study. Pharmacokinetic data of the
glutarate and citrate salts were evaluated in four healthy male
Sprague−Dawley rats (n = 2 per group, weighing 150−200 g)
following oral administration of 5 mg/kg dose. Oral
formulation prepared as a suspension using 0.25% w/v sodium
carboxy methyl cellulose with 0.1% (w/v) Tween 80 in water
was administered. A portion of 0.2 mL of blood samples was
collected from the saphanous vein at 0, 0.25, 0.5, 1, 2, 4, 6, 8,
and 24 h from each rat, immediately centrifuged for 10 min at
4000 rpm at 4 2 °C and stored below −70 °C until the assay.
Concentrations of the respective compounds in rat plasma
samples were determined by a validated HPLC method. All
pharmacokinetic parameters are reported as mean and standard
deviations.
(14) Elder, D. P.; Holm, R.; Diego, H. L. Use of pharmaceutical salts
and cocrystals to address the issue of poor solubility. Int. J.
Pharmaceutics 2013, 453, 88−100.
Molecule Conformation Energy Calculation. For
molecules geometrical optimization, except bent glutaric acid,
all of the glutaric acid molecules were directly taken from the
Crystal Structure Database (CSD). The molecules geometrical
optimization was carried out by B3LYP/6-311g (d, p) basis set
using the Gaussian 03 package. See the Supporting Information
for details.
(15) Good, D. J.; Rodríguez-Hornedo, N. Solubility Advantage of
Pharmaceutical Cocrystals. Cryst. Growth Des. 2009, 9, 2252−2264.
(16) McNamara, D. P.; Childs, S. L.; Giordano, J.; Iarriccio, A.;
Cassidy, J.; Shet, M. S.; Mannion, R.; O’Donnell, E.; Park, A. Use of a
glutaric acid cocrystal to improve oral bioavailability of a low solubility
API. Pharm. Res. 2006, 23, 1888−1897.
(17) Sanphui, P.; Goud, N. R.; Khandavilli, U. B. R.; Nangia, A. Fast
dissolving curcumin cocrystals. Cryst. Growth Des. 2011, 11, 4135−
4145.
(18) http://www.fda.gov/downloads/Drugs/Guidances/
UCM281764.pdf (accessed on July 26, 2013).
(19) Terrett, N. K.; Bell, A. S.; Brown, D.; Ellis, P. Sildenafil (Viagra),
a potent and selective inhibitor of Type 5 cGMP phosphodiesterase
with utility for the treatment of male erectile dysfunction. Bioorg. Med.
Chem. Lett. 1996, 6, 1819−1824.
ASSOCIATED CONTENT
* Supporting Information
Conformational energy calculation, DSC, FT-IR, and PXRD of
the cocrystals and salts. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Fax: +91 80 23602306. Tel.: +91 80 22933311. E-mail:
desiraju@sscu.iisc.ernet.in.
■
(20) Boolell, M.; Allen, M. J.; Ballard, S. A.; Gepi-Attee, S.; Muirhead,
G. J.; Naylor, A. M.; Osterloh, I. H.; Gingell, C. Sildenafil: an orally
active type 5 cyclic GMP-specific phosphodiesterase inhibitor for the
treatment of penile erectile dysfunction. Int. J. Impot. Res. 1996, 8, 47−
52.
Notes
(21) Barnett, C. F.; Machado, R. F. Sildenafil in the treatment of
pulmonary hypertension. Vasc. Health Risk Manage. 2006, 2, 411−422.
(22) Khankari, R. K.; Grant, D. J. W. Pharmaceutical Hydrates.
Thermochim. Acta 1995, 248, 61−79.
(23) Rodríguez-Hornedo, N.; Lechuga-Ballesteros, D.; Hsiu-Jean, W.
Phase transition and heterogeneous/epitaxial nucleation of hydrated
and anhydrous theophylline crystals. Int. J. Pharmaceutics 1992, 85,
149−162.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are thankful to Ranbaxy Pvt. Ltd. for providing the API
sildenafil citrate. P.S. thanks Ranbaxy for a fellowship. S.T.
thanks UGC for a SRF. S.G. thanks IISc for a fellowship.
G.R.D. thanks DST for a J. C. Bose fellowship.
(24) Sanphui, P.; Bolla, G.; Nangia, A. High Solubility Piperazine
Salts of the Nonsteroidal Anti-Inflammatory Drug (NSAID)
Meclofenamic Acid. Cryst. Growth Des. 2012, 12, 2023−2036.
(25) Yathirajan, H. S.; Nagaraj, B.; Nagaraja, P.; Bolte, M. Sildenafil
citrate monohydrate. Acta Crystallogr. 2005, E61, o489−o491.
(26) Banerjee, R.; Bhatt, P. M.; Desiraju, G. R. Solvates of Sildenafil
Saccharinate. A New Host Material. Cryst. Growth Des. 2006, 6, 1468−
1478.
(27) Zegarac, M.; Mestrovic, E.; Dumbovic, A.; Tudja, P.
Pharmaceutically acceptable co-crystalline form of sildenafil. WO
patent July 19, 2007, 080362 A1.
(28) Zegarac, M.; Mestrovic, E.; Devcic, M.; Tudja, P. Pharmaceuti-
cally acceptable salts and polymorphic form of sildenafil. WO patent
October 4, 2007, 110559 A1.
REFERENCES
■
(1) Byrn, S. R.; Pfeiffer, R. R.; Stowell, J. G. Solid-State Chemistry of
Drugs, 2nd ed.; SSCI, Inc.: West Lafayette, IN, 1999.
(2) Vippagunta, S. R.; Brittain, H. G.; Grant, D. J. W. Crystalline
solids. Adv. Drug Delivery Rev. 2001, 48, 3−26.
(3) Lipinski, C. Poor aqueous solubility-an industry wide problem in
drug discovery. Am. Pharm. Rev. 2002, 5, 82−85.
(4) Stahl, P. H.; Wermuth, C. G. Handbook of Pharmaceutical Salts:
Properties, Selection and Use; Wiley-VCH: New York, 2002.
(5) Remenar, J. F.; Morissette, S. L.; Peterson, M. L.; Moulton, B.;
Mac-Phee, J. M.; Guzman
̈
́
, H.; Almarsson, O. Crystal engineering of
novel cocrystals of a triazole drug with 1,4-dicarboxylic acids. J. Am.
Chem. Soc. 2003, 125, 8456−8457.
(6) Trask, A. V.; Sam, M. W. D.; Jones, W. Physical stability
enhancement of theophylline via cocrystallization. Int. J. Pharmaceutics
2006, 320, 114−123.
(29) Sawatdee, S.; Phetmung, H.; Srichana, T. Sildenafil citrate
monohydrate−cyclodextrin nanosuspension complexes for use in
metered-dose inhalers. Int. J. Pharmaceutics 2013, 455, 248−258.
4696
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697

Molecular Pharmaceutics
Article
(30) Generally Regarded as Safe chemicals by the US FDA: http://
w w w . f d a . g o v / F o o d / I n g r e d i e n t s P a c k a g i n g L a b e l i n g /
FoodAdditivesIngredients/ucm091048.htm (accessed on July 26,
2013).
(50) Sijpkes, A. H.; Rossum, P. V.; Raad, J. S.; Somsen, G. Heat
capacities and volumes of some polybasic carboxylic acids in water at
29.15 K. J. Chem. Thermodyn. 1989, 21, 1061−1067.
(51) Smith, A. J.; Kavuru, P.; Wojtas, L.; Zaworotko, M. J.; Shytle, R.
D. Cocrystals of quercetin with improved solubility and oral
bioavailability. Mol. Pharmaceutics 2011, 8, 1867−1876.
(52) Bak, A.; Gore, A.; Yanez, E.; Stanton, M.; Tufekcic, S.; Syed, R.;
Akrami, A.; Rose, M.; Surapaneni, S.; Bostick, T.; King, A.; Neervanan,
S.; Ostovic, D.; Koparkar, A. The co-crystal approach to improve the
exposure of a water-insoluble compound: AMG 517 sorbic acid co-
crystal characterization and pharmacokinetics. J. Pharm. Sci. 2008, 97,
3942−3956.
(31) http://en.wikipedia.org/wiki/Pimelic_acid (accessed on July 26,
2013).
̌
̌
(32) Kastelic, J.; Hodnik, Z.; Sket, P.; Plavec, J.; Lah, N.; Leban, I.;
Pajk, M.; Planinsek, O.; Kikelj, D. Fluconazole Cocrystals with
̌
Dicarboxylic Acids. Cryst. Growth Des. 2010, 10, 4943−4953.
(33) Delori, A.; Galek, P. T. A.; Pidcock, E.; Jones, W. Quantifying
Homo- and Heteromolecular Hydrogen Bonds as a Guide for Adduct
Formation. Chem.Eur. J. 2012, 18, 6835−6846.
(53) Venczel, M.; Szvoboda, I.; Podan
́
yi, B.; Valente, D.; Menegotto,
(34) Espinosa-Lara, J. C.; Guzman-Villanueva, D.; Arenas-García, J.
J.; Pintye-Hodi, K.; Ujhelyi, G. Formulation Possibilities of a Weak
́
I.; Herrera-Ruiz, D.; Rivera-Islas, J.; Roman
́
-Bravo, P.; Morales-Rojas,
Base with a Narrow Solubility Range. Cryst. Growth Des. 2012, 12,
1101−1110.
H.; Hofl, H. Cocrystals of Active Pharmaceutical Ingredients
̈
Praziquantel in Combination with Oxalic, Malonic, Succinic, Maleic,
Fumaric, Glutaric, Adipic, And Pimelic Acids. Cryst. Growth Des. 2013,
13, 169−185.
(54) Lobenberg, R.; Amidon, G. L. Modern bioavailability,
̈
bioequivalence and biopharmaceutics classification system. New
scientific approaches to international regulatory standards. Eur. J.
Pharm. Biopharm. 2000, 50, 3−12.
(35) Babu, N. J.; Sanphui, P.; Nangia, A. Crystal engineering of stable
Temozolomide cocrystals. Chem. Asian J. 2012, 7, 2274−2285.
(36) Stepanovs, D.; Mishnev, A. Molecular and Crystal Structure of
Sildenafil Base. Z. Naturforsch. 2012, 67b, 491−494.
(37) Nangia, A.; Desiraju, G. R. Supramolecular synthons and pattern
recognition. Top. Curr. Chem. 1998, 198, 57−95.
(55) RigakuMercury375R/M CCD, CrystalClear-SM Expert 2.0 rc14;
Rigaku Corporation: Tokyo, Japan, 2009.
(56) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr.
2008, A64, 112−122.
(57) Farrugia, L. J. WinGX suite for small-molecule single-crystal
crystallography. J. Appl. Crystallogr. 1999, 32, 837−838.
(58) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2002.
(38) McMahon, J. A.; Bis, J. A.; Vishweshwar, P.; Shattock, T. R.;
McLaughlin, O. L.; Zaworotko, M. J. Crystal engineering of the
composition of pharmaceutical phases. 3′. Primary amide supra-
molecular heterosynthons and their role in the design of
pharmaceutical co-crystals. Z. Kristallogr. 2005, 220, 340−350.
(59) Spek, A. L. Single crystal structure validation with the program
PLATON. J. Appl. Crystallogr. 2003, 36, 7−13.
(60) Barbour, L. J. X-Seed, Graphical Interface to SHELX-97 and
POV-Ray, a Program for Better Quality of Crystallographic Figures;
University of Missouri: Columbia, MO, 1999.
̌
(39) Bucar, D. K.; Henry, R. F.; Lou, X.; Duerst, R. W.; MacGillivray,
L. R.; Zhang, G. G. Z. Cocrystals of Caffeine and Hydroxybenzoic
Acids Composed of Multiple Supramolecular Heterosynthons:
Screening via Solution-Mediated Phase Transformation and Structural
Characterization. Cryst. Growth Des. 2009, 9, 1932−1943.
(61) Portell, A.; Barbas, R.; Font-Bardia, M.; Dalmases, P.; Prohens,
R.; Puigjaner, C. Ziprasidone malate, a new trimorphic salt with
improved aqueous solubility. CrystEngComm 2009, 11, 791−795.
(62) Glomme, A.; Marz, J.; Dressman, J. B. Comparison of a
miniaturized shake-flask solubility method with automated potentio-
metric acid/base titrations and calculated solubilities. J. Pharm. Sci.
2005, 94, 1−16.
(40) Varughese, S.; Desiraju, G. R. Using Water as a Design Element
in Crystal Engineering. Host-Guest Compounds of Hydrated 3,5-
Dihydroxybenzoic Acid. Cryst. Growth Des. 2010, 10, 4184−4196.
́
(41) Bathori, N. B.; Lemmerer, A.; Venter, G. A.; Bourne, S. A.;
Caira, M. R. Pharmaceutical Co-crystals with Isonicotinamide; Vitamin
B3, Clofibric Acid, and Diclofenac; and Two Isonicotinamide
Hydrates. Cryst. Growth Des. 2010, 11, 75−87.
(42) Assessment report for sildenafil ratiopharm; European Medicines
Agency Evaluation of Medicines for Human Use; European Medicines
Agency: London, 2009; Doc. Ref.: EMA/793633/2009.
(43) Thalladi, V. R.; Boese, R.; Weiss, H.-C. The Melting Point
Alternation in α,ω-Alkanedithiols. J. Am. Chem. Soc. 2000, 122, 1186−
1190.
(44) Mishra, M. K.; Varughese, S.; Ramamurty, U.; Desiraju, G. R.
Odd−Even Effect in the Elastic Modulii of α,ω-Alkanedicarboxylic
Acids. J. Am. Chem. Soc. 2013, 135, 8121−8124.
(45) Bauer, J.; Spanton, R.; Henry, J.; Quick, J.; Dzidi, W.; Porter, W.;
Morris, J. Ritonavir: an extraordinary example of conformational
polymorphism. Pharm. Res. 2001, 18, 859−866.
́
(46) Guzman, H. R.; Tawa, M.; Zhang, Z.; Ratanabanangkoon, P.;
̈
Shaw, P.; Gardner, C. R.; Chen, H.; Moreau, J. P.; Almarsson, O.;
Remenar, J. F. Combined use of crystalline salt forms and precipitation
inhibitors to improve oral absorption of celecoxib from solid oral
formulations. J. Pharm. Sci. 2007, 96, 2686−2702.
(47) Babu, N. J.; Nangia, A. Solubility Advantage of Amorphous
Drugs and Pharmaceutical Cocrystals. Cryst. Growth Des. 2011, 11,
2662−2679.
(48) Tothadi, S.; Desiraju, G. R. Designing ternary cocrystals with
hydrogen bonds and halogen bonds. Chem. Commun. 2013, 49, 7791−
7793.
(49) Schrodera, B.; Santos, L. M. N. B. F.; Marruchoa, I. M.;
Coutinho, J. A. P. Prediction of aqueous solubilities of solid carboxylic
acids with COSMO-RS. Fluid Phase Equilib. 2010, 289, 140−147.
4697
dx.doi.org/10.1021/mp400516b | Mol. Pharmaceutics 2013, 10, 4687−4697