Structures and characterization of m-nisoldipine polymorphs

Article abstract of DOI:10.1039/c2ce06359j

m-Nisoldipine, a dihydropyridine calcium ion antagonist, first synthesized in the School of Pharmaceutical Sciences, Hebei Medical University, exhibits two polymorphs A and B with different colors and morphologies. The single-crystal X-ray structure analysis reveals that polymorph A crystallizes in a monoclinic P2₁/c space group with a = 9.3045(2) A, b = 16.5991(5) A, c = 13.0018(3) A, β = 91.539(2)° and Z = 4, while B crystallizes in a triclinic space group of P1 with a = 7.4965(2) A, b = 11.4692(4) A, c = 12.3648(5) A, α = 68.093(2)°, β = 88.655(2)°, γ = 81.853(2)° and Z = 2. The m-nisoldipine molecules are connected through N-H...O hydrogen bonds between the carbonyl group and the amine group to form either a wavy chain for A or a linear chain for B. The two polymorphs show distinct physicochemical properties as characterized by IR, Raman, DSC and terahertz pulse spectroscopy. DFT theoretical calculations are also used to simulate the terahertz spectroscopy of the two polymorphs. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ce06359j

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PAPER
Structures and characterization of m-nisoldipine polymorphs†
a
b
c
a
Caiqin Yang, Zhenwei Zhang, Yanli Zeng, Jing Wang, Yongli Wang and Baoqing Ma
a
d
*
*
Received 13th October 2011, Accepted 30th November 2011
DOI: 10.1039/c2ce06359j
m-Nisoldipine, a dihydropyridine calcium ion antagonist, first synthesized in the School of
Pharmaceutical Sciences, Hebei Medical University, exhibits two polymorphs A and B with different
colors and morphologies. The single-crystal X-ray structure analysis reveals that polymorph A
ꢀ
ꢀ
ꢀ
crystallizes in a monoclinic P21/c space group with a ¼ 9.3045(2) A, b ¼ 16.5991(5) A, c ¼ 13.0018(3) A,
ꢀ
ꢁ
ꢀ
b ¼ 91.539(2) and Z ¼ 4, while B crystallizes in a triclinic space group of P1 with a ¼ 7.4965(2) A,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
b ¼ 11.4692(4) A, c ¼ 12.3648(5) A, a ¼ 68.093(2) , b ¼ 88.655(2) , g ¼ 81.853(2) and Z ¼ 2. The
m-nisoldipine molecules are connected through N–H/O hydrogen bonds between the carbonyl group
and the amine group to form either a wavy chain for A or a linear chain for B. The two polymorphs
show distinct physicochemical properties as characterized by IR, Raman, DSC and terahertz pulse
spectroscopy. DFT theoretical calculations are also used to simulate the terahertz spectroscopy of the
two polymorphs.
with shelf life and solubility of pharmaceuticals.10 For poorly
soluble compounds such as m-nisoldipine, polymorphism can
lead to large variations in bioavailability among different
forms.11 The pharmacokinetics and relative bioavailability of
m-nisoldipine polymorphs in rats were investigated using a vali-
dated LC-MS-MS method.6 It indicated that polymorphs A and
B of m-nisoldipine were not bioequivalent. In order to optimize
the m-nisoldipine drug performance and execute the quality
control during manufacture, a series of physicochemical char-
acterizations on the two polymorphs are conducted to gain more
understanding of their polymorphic behaviors. Herein we report
their IR, Raman, DSC, single-crystal X-ray structures and
terahertz spectroscopy coupled with DFT calculations.
Introduction
Polymorphism is the existence of the same chemical compound in
more than one crystal structure in the solid-state. In the phar-
maceutical industry, the identification of polymorphic forms of
drug compounds is of crucial importance. Polymorphs, hydrates
and solvates can be produced by a variety of standard pharma-
ceutical processes.1–5 Polymorphs of the same drug compound
may have very different physical properties that affect bioavail-
ability or processability (e.g., tabletting),6 and drug regulatory
authorities demand detailed information about polymorphism
before granting licenses for product distribution.
m-Nisoldipine, 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-
3,5-pyridinedicarboxylic acid methyl 2-methylpropyl ester, is
a new dihydropyridine calcium ion antagonist, which was first
synthesized in the School of Pharmaceutical Sciences, Hebei
Medical University.7 Compared with its analogues, m-nisoldi-
pine significantly increases cardiac output and cardiac index,
dramatically reduces the negative inotropic effect on myocar-
dium and has relatively higher selectivity on the thoracic aorta.8,9
Two polymorphs of m-nisoldipine were obtained. It is well
known that polymorphism is especially important in connection
Results and discussion
Polymorphs A and B had initially been obtained from different
crops of ethanol solvent. Much effort had been made to screen
the possible polymorphs from a variety of single solvents or
combinations thereof, but only two polymorphs A and B have
been obtained so far. Good quality crystals of form A can be
obtained from a solution of acetone–EtOH (1 : 1) by a slow
evaporation process and polymorph B crystals were obtained
from a solution of EtOAc–hexane (1 : 1). The two polymorphs
have different colors and morphologies: form A is a yellowish
rod and form B is a colorless needle as shown in Fig. 1. As such,
aSchool of Pharmaceutical Sciences, Hebei Medical University,
Shijiazhuang, 050017, China. E-mail: jingwang@home.ipe.ac.cn; Fax:
+86311-86266419; Tel: +86311-86265627
bDepartment of Physics, Capital Normal University, Beijing, China
cSchool of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang, China
polymorph
A and B can be visibly identified by their
appearances.
dCurrently with Sanofi Animal Division, 631 Route 1, North Brunswick,
NJ, 08902, USA. E-mail: baoqing.ma@merial.com
Infrared and Raman spectra
† Electronic supplementary information (ESI) available. CCDC
reference number 802078. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2ce06359j
The IR and Raman spectra of the two polymorphs display
distinct features, which can be employed to distinguish the two
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CrystEngComm, 2012, 14, 2589–2594 | 2589

polymorph has the lower heat of fusion, then the two poly-
morphs are enantiotropic, forms A and B are enantiotropically
related. The conversion temperature in solution is about 47 ꢀC as
obtained from van’t Hoff plot.14 Form A is the thermodynami-
cally stable one below the conversion temperature. The phar-
macokinetic studies of oral dosage of polymorphs A and B
administrated to rats show that form B has relatively higher
bioavailability compared with form A.6 It is consistent with
thermodynamic stability of form A, which has low solubility and
dissolution rate. It also indicates the importance of polymorph
screening and form selection in drug formulation.
Single-crystal X-ray diffraction
The single-crystal X-ray diffraction analyses reveal that the two
polymorphs crystallize in different crystal systems. Form A
belongs to a monoclinic P21/c, while form B crystallizes in
ꢁ
a triclinic P1. Their molecular structures are illustrated in Fig. 2.
They exhibit a different molecular conformation as presented by
their superimposed structures in Fig. 3. The two carbonyl groups
from the isobutyl ester and methyl ester point in the same
direction in form B, while they are opposed in form A due to the
rotation of the single C–C bond. The nitrobenzene plane in both
forms is almost perpendicular to the diene plane with slightly
different intersecting angles of 84.23ꢀ and 89.55ꢀ for A and B,
respectively. In both polymorphs, the m-nisoldipine molecules
are linked through the N–H/O hydrogen bonds [N2/O3a ¼
ꢀ
ꢀ
ꢀ
ꢀ
2.927(2) A and 3.089 A, N2–H2A/O3a ¼ 139.1 and 171.0 for
A and B, respectively; a ¼ x, ꢁy + 1/2, z + 1/2 for A and x + 1,
Fig. 1 The morphologies of polymorph A (a) and B (b).
polymorphs for quantitative content analysis. Polymorph A
exhibits a single peak at 694 cmꢁ1 and a single peak at 1635 cmꢁ1
with a shoulder at 1650 cmꢁ1, as opposed to polymorph B which
has doublets at 689 cmꢁ1 and 704 cmꢁ1 and a singlet at 1655 cmꢁ1
.
Polymorph B has a single peak at 1380 cmꢁ1 with a shoulder at
1394 cmꢁ1, whereas polymorph A shows a single peak at
1399 cmꢁ1 with a shoulder at 1380 cmꢁ1. Additional strong peaks
for polymorph B occur at 2835, 1326, 1305, 1147, 1254 and
1699 cmꢁ1. Peaks at 586 cmꢁ1 and 636 cmꢁ1 for polymorph B are
stronger than those of polymorph A.
The Raman spectrum for polymorph A exhibits a significant
profile difference from that of polymorph B. The feature peaks at
586 cmꢁ1, 1669 cmꢁ1 and 1698 cmꢁ1 for form A contrast with
those at 1222 cmꢁ1 and 1677 cmꢁ1 for form B. The striking
Raman peak features for form A and B provide a practical
application in polymorph identification, drug content uniformity
and PAT as the Raman technique coupled with microscopic
images gains broad and intensive use in modern pharmaceutical
industries.
DSC analysis
Differential scanning calorimetry (DSC) showed a melting point
of 135.8 ꢀC and 129.2 ꢀC for polymorphs A and B with the
corresponding enthalpy of 82.3 J gꢁ1 and 88.3 J gꢁ1, respectively.
No thermal-induced polymorphic transformation is observed
during the course of heating. According to the polymorph rule of
heat of fusion,12,13 which states that if the higher melting
Fig. 2 The molecular structure of m-nisoldipine polymorph A (a) and B
(b) with 35% ellipsoid probability.
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Fig. 3 An overlapping of m-nisoldipine in polymorph A and B.
y, z for B], resulting in a 1D hydrogen-bonded structure (Fig. 4).
Although the connections between m-nisoldipine molecules are
same, the two 1D structures are quite different. In A, the
hydrogen bonding angle is bent and the adjacent m-nisoldipine
molecules are twisted, leading to a wavy chain. The dangling
isobutyl groups from adjacent wavy chains are intercalated
together by van der Waals interactions, giving rise to an undu-
lating layer. These layers are further stacked to produce a 3D
packing (Fig. 5a). However, in B, the hydrogen bond orientation
is close to linear and the adjacent m-nisoldipine molecules form
a linear ribbon motif. The nitrobenzene moieties from the
neighboring ribbon are held together by p–p stacking with the
ꢀ
centroid distance being 4.056 A (shown in Fig. 6) to form
a ladder-like motif. These ladders are packed together by van der
Waals interactions to generate a 3D network (Fig. 5b).
The calculated density of form B is 1.322 g cmꢁ3, which is
significantly larger than that of A (1.285 g cmꢁ3). This could be
related to the presence of strong aromatic p–p stacking in form
B, which gives the closest packing. Obviously, the density rule is
Fig. 5 The overall packing diagram of m-nisoldipine in the unit cell for
polymorph A (a) and B (b).
not applicable herein due to the presence of hydrogen bonding
interactions in the structure.12
TPS characterization and DFT theoretical calculations
Terahertz spectroscopy was carried out for polymorphs A and B
as this technique is very sensitive to the intermolecular interac-
tions. An overlay of the terahertz spectra for polymorphs A and
B is shown in Fig. 7. The spectrum of form A exhibits major
peaks at 0.23, 0.75, 1.81 THz, and smaller peaks at 0.6 and
1.1 THz, while the form B spectrum has prominent peaks at 0.50,
0.70, 0.88, 1.23 and 1.43 THz with low intensity peaks at 1.60 and
1.77 THz. In order to understand and interpret these terahertz
absorption spectra, DFT calculations were performed to simu-
late the structural interaction and the corresponding absorption,
which is used to assign the determined terahertz absorption to
vibrational spectra. In polymorph A, the low frequency features
at 0.23 and 0.4 THz are attributed a symmetric bending of the
N–H/O hydrogen bond at 0.25 and 0.34 THz. The latter results
from the vibrations of whole molecule frameworks in the dimer
(Fig. 8a). The strong peak at 0.75 THz is assigned to an asym-
metric bending mode of the N–H/O hydrogen bond at
0.73 THz (Fig. 8b). The absorption at 1.81 THz could come from
the vibration of the single molecule framework in the dimer
(Fig. 8c:A, 1.81 THz).
Fig. 4 The 1D chain constructed by N–H/O hydrogen bonding
between m-nisoldipine in polymorph A (a) and B (b).
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Fig.
8 Modes of vibration of the N–H/O hydrogen bond in
polymorph A.
Fig. 6 The 1D ladder-like structure connected by both hydrogen
bonding and the p–p stacking between the phenyl ring in polymorph B
(a) together with the side view (b).
The structural and spectral DFT simulation were able to
assign most of the experimental spectral features to one or more
of the calculated vibrational modes. It provides us with a good
insight into the molecular interactions in polymorphs A and B.
The markedly different terahertz features of form A and B
suggest that the terahertz spectrum is a useful tool to identify
polymorphs A and B.
Fig. 7 Absorbance spectra of m-nisoldipine polymorph A and B The
insert highlights the region from 0.7 to 1.8 THz where there are consid-
erable changes in the terahertz spectrum between the two polymorphs.
In form B, there are four peaks at 0.30, 0.51, 0.70 and 0.88 THz
from 0.2 to 0.9 THz. The corresponding calculation has four
modes at 0.25, 0.51, 0.69 and 0.93 THz. It was noted that the THz
spectrum in form B has an equal spaced absorption at an
approximate interval of 0.2 THz in this range. The calculation
indicates that these modes are predominantly involved with the
motions of p–p stacking between the phenyl rings in the dimers
related by an inversion center (Fig. 9). The peaks at 1.23 and
1.43 THz are assigned to the calculated asymmetric bending
mode of the N–H/O hydrogen bond at 1.09 and 1.45 THz
(Fig. 10a,b). The strong peak at 2.32 THz is associated to the
calculated stretch vibration of the N–H/O hydrogen bond
(Fig. 10c).
Fig. 9 Modes of vibration of p–p stacking between the phenyl rings in
polymorph B.
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gas at a flow rate of 20 mL minꢁ1. Infrared spectra were recorded
from 4000–400 cmꢁ1 for form A and B using a EQUINOX 55
(Bruker) FT-IR spectrometer. Each spectrum was the average of
200 scans using a spectral resolution of 4 cmꢁ1. Raman spectra
were recorded in the 500–2000 cmꢁ1 range by measuring in
backscattering geometry and using a Bruker spectrometer using
a 647.1 nm mixed argon–krypton laser (50 mW). All terahertz
measurements were made using a TPIꢀ spectra 1000 trans-
mission spectrometer (TeraView Limited, Cambridge, UK).
Samples were measured at an instrument resolution of 2–3 cmꢁ1
over the range of 0–3 THz. Data was acquired and processed
using OPUSꢀ (Bruker Optics, Germany) software.
DFT calculations
Calculations were performed on polymorphs A and B, taking
molecular structures from the X-ray determined coordinates as
the starting geometries. Then, equilibrium geometries were fully
optimized at the DFT B3LYP/6-31G** level using the
GAUSSIAN-03 program package.15 Harmonic frequencies were
calculated based on the equilibrium geometries.
Crystallography
Fig. 10 Modes of vibration of the N–H/O hydrogen bond in poly-
morph B.
Crystal data: Form A, C20H24N2O6, Mr 388.41, Monoclinic,
ꢀ
ꢀ
P21/c, T ¼ 298(2) K, a ¼ 9.3045(2) A, b ¼ 16.5991(5) A, c ¼
Conclusions
ꢀ
3
ꢀ
ꢀ
13.0018(3) A, b ¼ 91.539(2) , Z ¼ 4, V ¼ 2007.36(9) A , rcalc
¼
1.285 Mg mꢁ3, R1 [I > 2s(I)] ¼ 0.0688. Form B, C20H24N2O6, Mr
Two polymorphs of m-nisoldipine were discovered and physi-
cally characterized by a series of solid-state techniques. They can
be discriminated by their visible appearance as they have
different colors and morphologies. The single-crystal structure
shows that the two polymorphs are classified as conformational
polymorphs, with different packing motifs. Forms A and B have
an ꢀenantiotropic relationship with a conversion temperature of
47 C and form A is the thermodynamically stable form below
this temperature. The IR, Raman and terahertz spectra show
distinctive peaks, which can be used to distinguish polymorphs A
and B. The terahertz absorption was further interpreted based on
the DFT calculations and more insight into the intermolecular
interaction was gained.
ꢁ
ꢀ
388.41, Triclinic, P1, T ¼ 298(2) K, a ¼ 7.4965(2) A, b ¼ 11.4692
ꢀ
ꢀ
ꢀ
ꢀ
(4) A, c ¼ 12.3648(5) A, a ¼ 68.093(2) , b ¼ 88.655(2) , g ¼
81.853(2) , Z ¼ 2, V ¼ 975.89(6) A , rcalc ¼ 1.322 Mg mꢁ3, R1 [I >
2s(I)] ¼ 0.0676. X-ray reflections were collected on Bruker
SMART-APEX CCD diffractometer (Mo-Ka radiation, l ¼
ꢀ
3
ꢀ
ꢀ
0.71073 A) with a 10 s exposure time. Empirical absorption
corrections were applied using the multi-scan method. The
structures were solved by the direct method and refined by full-
matrix least-squares on F2 using the SHELX program with
anisotropic thermal parameters for all non-hydrogen atoms.
Hydrogen atoms were placed on calculation positions.
Acknowledgements
Experimental section
The financial support for this work by the Hebei Province
Natural Science Fund of China (2008001072), Science Fund
(2009148) of the Education Department and Science Fund
(20090059) of the Health Department of Hebei Province of
China are greatly acknowledged. This work was also funded by
the National Keystone Basic Research Program (973 Program)
under Grant No. 2007CB310408.
Preparation
The compound 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-
pyridinedicarboxylic acid methyl 2-methylpropyl ester was
synthesized according to the procedure given in the Chinese
patent.7 Efforts were made to crystallize the compound in
different solvent systems, such as EtOH, acetone, EtOAc–hexane
and acetone–EtOH to evaluate the influence of change in polarity
of the solvents. Single crystals of polymorph A were grown from
a solution of acetone–EtOH (1 : 1) by a slow evaporation process
and those of the polymorph B were obtained from a solution of
EtOAc–hexane (1 : 1).
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