Toward Novel Pseudo-Polymorphs of Nifedipine: Elucidation of a Slow

May 13, 2010 - ... for Materials Research and Testing, Richard-Willstaetter-Strasse 11, ... Shweta A. Raina , David E. Alonzo , Geoff G. Z. Zhang , Yi...
0 downloads 0 Views 3MB Size
Download PDF
DOI: 10.1021/cg100186v

Published as part of a virtual special issue of selected papers presented in celebration of the 40th Anniversary Conference of the British Association for Crystal Growth (BACG), which was held at Wills Hall, Bristol, UK, September 6-8, 2009.

2010, Vol. 10 2693–2698

Toward Novel Pseudo-Polymorphs of Nifedipine: Elucidation of a Slow Crystallization Process Maria Klimakow,† Klaus Rademann,‡ and Franziska Emmerling*,† †

BAM Federal Institute for Materials Research and Testing, Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany, and ‡Department of Chemistry, Humboldt-Universitaet, Brook-Taylor-Strasse 2, 12489 Berlin, Germany Received February 5, 2010; Revised Manuscript Received April 20, 2010

ABSTRACT: The crystallization of nifedipine was studied by means of synchrotron-X-ray diffraction, single-crystal X-ray structural analysis, and Raman spectroscopy. The results of slow evaporation (24 h in minimum) using dimethyl sulfoxide (DMSO) are presented. Since fast crystallization processes (typically minutes) in different solvents always led to the final formation of the thermodynamically most stable R-polymorph of nifedipine, we observed a novel pseudo-polymorph due to slow crystallization from DMSO. The single-crystal X-ray structure of the solvated species nifedipine 3 DMSO (1:1) is reported for the first time. In addition, the crystallization process on surfaces was followed by means of light microscopy and environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray spectroscopy (EDS) analysis. Different diffractions pattern and Raman spectra were observed for crystals grown from stock solution and those obtained by drying the solution on soda lime silicate surfaces.

Introduction Polymorphism is defined as the ability of a chemical substance to crystallize in two or more different crystal structures with deviating arrangements of the molecules in the unit cell.1 As soon as molecules of solvent are contained in the crystal lattice, the term pseudo-polymorph, solvate, or solvatomorph is appropriate.2 Polymorphs and pseudo-polymorph exhibit different physical properties in the solid-state, such as color, solubility, stability, or reactivity. Consequently, polymorphism of organic compounds is of great interest both for basic research and industrial applications, especially for the pharmaceutical industry.3,4 Since nucleation plays a key role in the selective crystallization of polymorphs and in the stabilization of intermediates, detailed studies of the crystallization process are essential when characterizing intermolecular interactions and identifying solvents that may promote the crystallization of a specific polymorph or solvate. Furthermore, polymorphism strongly affects important properties of drug substances such as toxicity and bioavailability. There is a need to find new structures for pharmaceutical applications. However, a basic understanding for crystallization processes and structure determination is far from being complete.5 An effort to determine if a given compound exists in polymorphic forms is the so-called polymorph screening. This procedure is extensively applied, for example, by Stahly et al.6 About 90% of the organic compounds exhibited multiple

Figure 1. Structure of nifedipine (4-(2-nitrophenyl)-2,6-dimethyl3,5-dicarbomethoxy-1,4-dihydro-pyridine).

*To whom correspondence should be addressed. E-mail: franziska. [email protected].

crystalline and noncrystalline forms, with about 50% showing polymorphism. Owing to the fact that polymorphs and pseudo-polymorph can be distinguished by their different crystal structures, X-ray diffraction (XRD) is in most cases the primary method utilized to study polymorphic transitions or solvation. To elucidate the structure of transient crystalline or amorphous phases, additional techniques are required. Hence, vibrational spectroscopy has found broad application in the characterization of pharmaceuticals and their matrices.7 Being well-known for its (pseudo-) polymorphism, the compound nifedipine (4-(2-nitrophenyl)-2,6-dimethyl-3,5-dicarbomethoxy-1,4-dihydropyridine, C17H18N2O6, see Figure 1) was chosen as a model system for the current study. In medical treatments, nifedipine acts as a dihydropyridine calcium antagonist.8 Despite the wide use of the substance and the knowledge of its polymorphism, the crystal structures of

r 2010 American Chemical Society

Published on Web 05/13/2010

pubs.acs.org/crystal

2694

Crystal Growth & Design, Vol. 10, No. 6, 2010

the nifedipine polymorphs are still unrevealed. Only the crystal structure of R-nifedipine (form I) and a dioxane solvatomorph is reported.9,10 The structural characterization, physicochemical properties, and thermal stability of the solvated modification of nifedipine was investigated by Caira et al.10 The polymorphism and devitrification of nifedipine under controlled humidity was characterized by a combined FTRaman, IR, and Raman microscopic investigation, reporting the existence of two other crystalline polymorphs ( β, γ) and glassy nifedipine (g).11 These intermediated phases are kinetically favored and therefore formed preferentially under fast evaporation conditions. In a previous work, we investigated the crystallization process of nifedipine in solvents with high evaporation rates to corroborate this assumption.12 The measurements were carried out in situ with a combined synchrotron-XRD/Raman setup using an acoustic levitator as a sample holder. The results indicate solvent-dependent transitions between different polymorphs. The obtained modifications in this experiment showed no inclusion of solvent molecules. Hence, the way toward novel solvatomorphs of nifedipine has to be found in slow evaporating crystallization processes. In this study, we therefore report on the crystallization process of nifedipine using the slow evaporating, polar and aprotic solvent dimethyl sulfoxide. Experimental Section Materials. Nifedipine (4-(2-nitrophenyl)-2,6-dimethyl-3,5-dicarbomethoxy-1,4-dihydropyridine) was purchased from Sigma Aldrich (CAS 21829-25-4) and used without further purification. The X-ray diffraction pattern (Bruker AXS, D5000, Cu-KR radiation) revealed that the sample consisted of R-polymorph (P21/c, a = 10.923(5) A˚, b = 10.326(6) A˚, c = 14.814(7) A˚, β = 92.70(6), V = 1669.03(494) A˚3).9 Sample Preparation. Drying experiments using a solution of nifedipine in dimethyl sulfoxide (50 mM) on soda lime silicate glass surfaces were performed. The samples were heated to 70 C for 1 h and the crystallization process was investigated during the following 24 h at room temperature. The stock solution was stored in darkness and yellow, block-shaped crystals started to grow after several month. They were isolated from the solution and prepared for XRD analysis. Methods. Raman measurements of the crystallized final products were performed with a LabRam single stage spectrograph (Horiba Jobin-Yvon, Bensheim, Germany) at the excitation wavelengths 785 and 633 nm using a notch filter and a liquid nitrogen cooled CCD detector (256  1024 pixels). At a laser power of 1.8 mW, respectively 0.5 mW, an acquisition time of 10  10 s was sufficient for most of the samples. The single crystal X-ray data collection was carried out on a Bruker AXS SMART diffractometer at room temperature using Mo KR radiation (λ = 0.71073 A˚), monochromatized by a graphite crystal. A total of 27 980 reflection intensities were measured with an exposure time of 30 s per frame. The data reduction was performed by using the Bruker AXS SAINT and SADABS packages. The structure was solved by direct methods and refined by full-matrix least-squares calculation using SHELX. Anisotropic thermal parameters were employed for non-hydrogen atoms. The hydrogen atoms were treated isotropically with Uiso=1.2 times the Ueq value of the parent atom. In case of methylen groups Uiso=1.5 times the Ueq value was chosen. Crystal data and refinement details are summarized in Table 1. Selected bond lengths and angles are given in Table S2 (see Supporting Information). CCDC-764292 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.cdc.cam.ac.uk/data_request/cif. Additional X-ray diffraction experiments were performed at the synchrotron micro focus beamline μSpot (BESSY II of the Helmholtz Centre Berlin for Materials and Energy). Providing a divergence of less than 1 mrad (horizontally and vertically), the focusing

Klimakow et al. Table 1. Crystal Data and Structure Refinement of the Nifedipine Dimethyl Sulfoxide Solvate formula formula weight temperature [K] wavelength [A˚] crystal system space group lattice constants [A˚, ] Z volume [A˚3] calculated density [g/cm3] absorption coefficient [mm-1] θ range for data collection [] limiting indices reflections collected reflections unique absorption correction refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole [e/A˚3]

C19H24N2O7S 424.47 296(2) 0.71073 triclinic P1 (No. 2) a = 8.0260(6), R = 68.282(4) b = 11.9102(9), β = 76.974(4) c = 12.2806(9), γ = 81.013(4) 2 1059.08(14) 1.331 0.195 1.82-28.74 -10 e h e 10, -16 e k e 16, -16 e l e 16 27980 5448 [R(int) = 0.1474] empirical, ψ-scan full-matrix least-squares on F2 5448/0/268 0.948 R1 = 0.0505, wR2 = 0.1292 R1 = 0.0980, wR2 = 0.1475 0.229 and -0.333

scheme of the beamline is designed to provide a beam diameter of 20-100 μm at a photon flux of 1  109 s-1, at a ring current of 100 mA. The experiments were carried out by employing a wavelength of 1.03358 A˚, using a double crystal monochromator (Si 111). Scattered intensities were collected 200 mm behind the sample position with a two-dimensional X-ray detector (MarMosaic, CCD 3072  3072 pixel and a point spread function width of about 100 μm). The typical data acquisition time for one measurement amounts to 60 s, which allows one to measure specimens on different surfaces on a routine basis. A more detailed description of the beamline can be found in ref 13. The obtained scattering images were processed and converted into diagrams of scattered intensities versus scattering vector q (q is defined in terms of the scattering angle θ and the wavelength λ of the radiation, thus q = 4π/λ sin θ) employing an algorithm of the computer program FIT2D.14 Environmental scanning electron microscopy (ESEM) experiments were carried out on a tabletop microscope TM-1000 (Hitachi High-Technologies Europe GmbH) using a precentered cartridge filament electron gun and a high-sensitive semiconductor BSE (solid-state backscattered electron) detector. The accelerating voltage was 15 kV and the obtained magnification was between 20 and 10000. The evacuation system consisted of a turbo molecular pump (30 L/s) and a diaphragm pump (1 m3/h). The samples were prepared by drying the solutions mentioned above on glass surfaces. Coupled to the ESEM system is a Bruker Quantax 50 for energydispersive X-ray spectroscopy (EDS) to perform qualitative and quantitative analyses. It consists of a customized silicon drift detector (XFlash) with an active detector area of 30 mm2. The element range for the analysis is from boron to americium.

Results and Discussion First investigations on the crystallization process of nifedipine from DMSO solution were carried out by drying experiments on glass surfaces (soda lime silicate glass). This procedure allows for the characterization of the final crystallization product by XRD of different areas. The experiments led to honey yellow deposits of solid consistency. As an example, Figure 2 shows different areas of a typical deposit of nifedipine solved in dimethyl sulfoxide and dried on glass. Various samples were analyzed with synchrotron radiation to reveal whether the sample is crystalline or amorphous. In Figure 2, the micro focus of the X-ray beam is symbolized by white circles (100 μm in diameter).

Article

Crystal Growth & Design, Vol. 10, No. 6, 2010

2695

Figure 2. Microscope images of dried nifedipine on a glass surface (left) with indicated measuring points (circles with diameter of 100 μm) marking different morphologies (a-c, middle) and observed correlating diffraction patterns (right, inset: corresponding 2D scattering images).

There were no significant differences between samples of the border and the inner region of the deposit. The 2D scattering images consist of a small number of reflexes with relatively high intensity and weak Laue rings. Comparison of the diffractograms with the theoretical diffractogram of the R-polymorph of nifedipine, calculated from the crystal structure (CSD entry: BICCIZ), showed consistency for most of the reflections, but was not unquestionably established. The polymorphism of nifedipine is intensively studied via Raman spectroscopy, since the prominent CdO and CdC stretching modes of the ester carbonyls and the dihydropyridine rings can be used as sensitive indicators of intermolecular H-bond formation, which differs in the different nifedipine polymorphs, as was shown by Chan et al.11 In R-nifedipine, the frequency of the CdO stretching doublet at ∼1680 cm-1 results from strong intermolecular hydrogen bonds between ester carbonyls of one molecule with the NH-group of another molecule. While the crystal structures of other polymorphs are missing, the Raman spectra of many polymorphs are known. And even though some powder patterns have been published and are denoted as form A, B, C, and an amorphous form,15 there is still no proof for the suggestions that they match the Raman spectra for β-, γ-, and glassy-nifedipine described by Chan et al.11 Raman experiments were carried out for the samples dried on glass. In Figure 3, a Raman spectrum of those aggregates is shown for which the XRD results are presented in Figure 2. For comparison, the intensity of the measured spectrum is adjusted to the spectra obtained from the literature.11 In Table S1 (see Supporting Information) selected wave numbers of the Raman shifts and their assignments are shown for the known polymorphs from the literature (R-, β-, γ-, and g-nifedipine) in comparison to crystals obtained from dimethyl sulfoxide and the sample dried on glass. It shows that the spectrum of the sample from DMSO on glass looks different to all the known polymorphs (see Figure 3). It exhibits a pronounced background scattering signal, and while many bands are shifted, others are missing in comparison to the spectra from the literature. Hence, having indications for crystallinity via XRD and showing a Raman spectrum different from known spectra, the

Figure 3. Micro-Raman spectra of nifedipine obtained from dimethyl sulfoxide, dried on glass (same sample as shown in Figure 2), compared to known Raman spectra of nifedipine polymorphs from the literature.

material appears to be an interesting candidate for a new solvatomorphic structure of nifedipine. In-Situ Observation of the Crystallization Process. The crystallization process of the solution of nifedipine in DMSO was investigated via ESEM and a light microscope over a time span of 24 h (see Figures 4 and 5). In a typical experiment, 50 μL of a solution of nifedipine in DMSO (50 mM) were placed on soda lime glass surfaces and heated for 1 h at 70 C. Subsequently, the crystallization process upon evaporation of the solvent at room temperature was observed. Directly after the heating, only liquid areas are visible. After 2 h at room temperature, first small seed crystals begin to grow at the boundary of the droplet, only detectable with the strong magnification of the ESEM (see Figure 5a). Within the next 20 h, the solvent evaporates constantly and the concentrated solution transforms into an amorphous film. In the following, more small crystallites start to grow, become larger, and agglomerate. The evaporation process finishes after 24 h at room temperature and yellow, block-shaped single crystals can be observed.

2696

Crystal Growth & Design, Vol. 10, No. 6, 2010

Figure 4. Light-microscope images of the crystallization process of nifedipine in DMSO heated for 1 h at 70 C. (a) After 16 h at room temperature, (b) after 20 h at room temperature, (c, d) after 24 h at room temperature. Total image length: 600 μm (a and c), 200 μm (b and d).

Figure 5. ESEM pictures of the crystallization process of nifedipine in DMSO. (a) After 2 h at room temperature. (b) After 20 h at room temperature. (c, d) After 24 h at room temperature. White circles indicate the measuring points for EDS analysis.

EDS analyses, carried out simultaneously with the ESEM investigations, showed that the amorphous film consists of a high percentage of the solvent DMSO, as a large amount of sulfur can be detected. When analyzing a crystalline region, the amount of sulfur is lower and the signals for carbon and nitrogen become stronger. The EDS results indicate that a small amount of the solvent remains inside the crystal structure, revealing again the assumption of a solvatomorphic structure. Structure Determination. Single crystal XRD of yellow crystals obtained from the DMSO stock solution resting in darkness after several months revealed a cocrystal structure of nifedipine and the solvent DMSO. The compound crystallizes in the triclinic space group P1 (No. 2). Details of the data collection and structure refinement are given in Table 1. The conformation of the nifedipine molecule is shown in Figure 6. The observed conformation is very similar to that found in the crystal structure of R-nifedipine and in the

Klimakow et al.

Figure 6. Conformation and atomic numbering of the nifedipine and the dimethyl sulfoxide molecule in the crystal structure of the dimethyl sulfoxide solvate. Thermal ellipsoid plots are drawn at the 30% probability level.

1,4-dioxane solvate. Obviously, the interaction between the nifedipine molecule and the DMSO molecule does not lead to any conformational changes. It can be concluded that the conformation found in all three compounds is the most stable one. The bond distances and angles are comparable to the values for R-nifedipine and the 1,4-dioxane solvate. Like in these crystal structures, the dihydropyridine ring shows a pronounced distortion from planarity, which can be estimated from the torsion angles calculated about the ring bonds (see Table S2 in the Supporting Information). The angle between the ring planes of the phenyl ring and the dihydropyridine ring amounts to 89.56 in the DMSO solvate, 87.53 in R-nifedipine, and 88.21 in the dioxane solvate. The atoms most affected from the distortion in the dihydropyridine ring are the pyridyl-nitrogen and the ternary carbon atom. The distances for the nitrogen atoms from the ring plane are 0.120 A˚ in the DMSO solvate, 0.091 A˚ in the R-polymorph, and 0.060 A˚ in the dioxane solvate. For the ternary carbon atom, the distances are 0.163 A˚ in the DMSO solvate, 0.131 A˚ in the R-polymorph, and 0.082 A˚ in the 1,4-dioxane solvate. The overlay structures given in Figure 7 clarify the differences between the new solvate structure on the one hand and the crystal structure of R-nifedipine and the 1,4-dioxane solvate on the other hand. The geometries of the drug molecules in the first case are nearly congruent. The structural differences between the two different solvate molecules are more pronounced. The acetate group is rotated by 180 with respect to the phenyl ring. Therefore, the distances between the center of the phenyl rings and the methyl-carbon atoms of the acetate groups deviate. In case of the DMSO solvate, the distances amount to 5.229 and 5.992 A˚, whereas in the dioxane solvate the values amount to 4.690 and 5.760 A˚. The unit cell contains one nifedipine and one DMSO molecule in the general position. As indicated in Figure 8, hydrogen bonds are generated between the pyridine hydrogen atom of nifedipine and the dimethyl sulfoxide molecules (N1-H1 3 3 3 O1; with d(D 3 3 3 A) = 2.877(2) A˚, d(H 3 3 3 A) = 1.94 A˚, and — (D-H 3 3 3 A) = 178). These hydrogen bonds are slightly shorter than those found in the crystal structure of the dioxane solvate. Since the simulated powder pattern of the novel pseudopolymorph is strikingly different from those obtained from

Article

Crystal Growth & Design, Vol. 10, No. 6, 2010

2697

Figure 9. Simulated powder pattern of the obtained single crystal from DMSO in comparison to the theoretical pattern from the crystal structure of R-nifedipine.

Figure 7. (a) Structural overlay of the nifedipine molecule in the crystal structure of the DMSO solvate (blue) and the R-polymorph (gray). (b) Structural overlay of the nifedipine molecule in the crystal structure of the DMSO solvate (blue) and the dioxane solvate (gray). Upper row: overlay of the phenyl rings, lower row: overlay of the dihydropyridine rings. Figure 10. Raman spectrum of nifedipine 3 DMSO single crystals, compared to known Raman spectra of nifedipine polymorphs from the literature.11

normalized to spectra obtained from the literature.11 In Table S1 selected wave numbers of the Raman shifts are shown for the known polymorphs from the literature (R-, β-, γ-, and g-nifedipine), and the sample in DMSO dried on glass, in comparison to the single crystals obtained from the dimethyl sulfoxide stock solution. As documented the wave numbers are assigned to the molecule vibrations. The spectrum of the novel pseudo-polymorph fits well with the known spectra and shows some small deviations to some specific bands. Conclusions

Figure 8. Crystal structure of the 1:1 nifedipine-dimethyl sulfoxide solvate shown along the a-axis. Hydrogen bonds between the nifedipine and dimethyl sulfoxide pairs are shown as green dotted lines.

the drying experiments at a glass surface and the theoretical powder pattern calculated from the crystal structure of the R-polymorph of nifedipine (see Figure 9), additional Raman spectroscopic investigations of the single crystals were performed. Figure 10 shows a Raman spectrum of nifedipine single crystals obtained from DMSO. The measured spectrum is

While nifedipine exhibits a great variety of morphologies and forms several polymorphs, only the crystal structure of one pseudo-polymorph (nifedipine 1,4-dioxane solvate (2:1)) has been reported. In this study, we present the first example of an 1:1 solvate, namely, Nifedipine DMSO and the characterization of its crystal structure via X-ray structural analysis. The triclinic crystal structure consists of nifedipine DMSO dimers, connected via hydrogen bonds. The conformation of the nifedipine solvate is comparable to the conformation in the R-modification and in the other known 2:1 solvate with 1,4-dioxane. Single-crystals of this novel pseudo-polymorph of nifedipine have been grown by a method which can be used in general for the formation of further solvatomorphs and cocrystals. It is suitable for the reliable formation of large single-crystals. The method is based on the controlled slow

2698

Crystal Growth & Design, Vol. 10, No. 6, 2010

evaporation of solvents from supersaturated solutions. For spectroscopic studies (Raman, for instance), thin films containing smaller crystals of solvatomorphs, can be easily produced via solvent-assisted drop casting. While Raman spectroscopy is well suited for the identification of interacting molecules, ESEM and light microscopy have been applied successfully for distinguishing amorphous and crystalline matter in thin films. Hence, the combination of these methods is very powerful for the characterization of novel solvatomorphs and polymorphs, as well as the identification of solvents that may lead to structures stabilized by molecular interactions. Finally, it is remarkable that this type of solvate cannot be obtained by use of fast evaporating solvents, such as dichloromethane, acetone, or acetonitrile. This novel pseudo-polymorph deserves further investigations with regard to the molecular recognition and supramolecular chemistry of nifedipine. Acknowledgment. We gratefully acknowledge financial support of this work by the Deutsche Forschungsgemeinschaft DFG through the program SPP1415 (EM-198/41 and RA 494-15/1). We thank Prof. J. Kneipp for providing the Raman data. The authors are grateful to Dr. J. Leiterer for technical support with the synchrotron experiments. Supporting Information Available: A table containing the respective wave numbers and suggested assignments derived from the Raman spectra and a table containing selected bond lengths and

Klimakow et al. angles of the pseudo-polymorph. This information is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University Press: New York, 2002. (2) Nangia, A.; Desiraju, G. R. Chem. Commun. 1999, 605. (3) Haleblian, J.; McCrone, W. J. Pharm. Sci. 1969, 58, 911. (4) Brittain, H. G. J. Pharm. Sci. 2009, 98, 1617. (5) Brittain, H. G. Polymorphism in Pharmaceutical Solids, 2nd ed.; Informa Healthcare: New York, 2009; Vol. 192. (6) Stahly, G. P. Cryst. Growth Des. 2007, 7, 1007. (7) Rodriguez-Spong, B.; Price, C. P.; Jayasankar, A.; Matzger, A. J.; Rodriguez-Hornedo, N. Adv. Drug Delivery Rev. 2004, 56, 241. (8) Vater, W.; Schlossm, K; Stoepel, K.; Hoffmeis, F; Kroneber, G; Puls, W.; Kaller, H.; Oberdorf, A.; Meng, K. Arznei-Forschung 1972, 22, 1. (9) Triggle, A. M.; Shefter, E.; Triggle, D. J. J. Med. Chem. 1980, 23, 1442. (10) Caira, M. R.; Robbertse, Y.; Bergh, J. J.; Song, M. N.; De Villiers, M. M. J. Pharm. Sci. 2003, 92, 2519. (11) Chan, K. L. A.; Fleming, O. S.; Kazarian, S. G.; Vassou, D.; Chryssikos, G. D.; Gionis, V. J. Raman Spectrosc. 2004, 35, 353. (12) Klimakow, M.; Leiterer, J.; Kneipp, J.; R€ ossler, E. A.; Panne, U.; Rademann, K. Emmerling, F. Langmuir 2010, DOI: 10.1021/ la100540q. (13) Paris, O.; Li, C.; Siegel, S.; Weseloh, G.; Emmerling, F.; Riesemeier, H.; Erko, A.; Fratzl, P. J. Appl. Crystallogr. 2007, 40, S466. (14) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. High Pressure Res. 1996, 14, 235. (15) Grooff, D.; De Villiers, M. M.; Liebenberg, W. Thermochim. Acta 2007, 454, 33.