Investigations on the Polymorphism and Pseudopolymorphism of the Glucocorticoid Triamcinolone: New Findings for a Well-Known Drug Viktor Suitchmezian, Inke Jess, and Christian Na¨ther*
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 69-74
Institut fu¨r Anorganische Chemie der Christian-Albrechts-UniVersita¨t zu Kiel, Olshausenstrasse 40, D-24098 Kiel, Germany ReceiVed July 8, 2006; ReVised Manuscript ReceiVed October 18, 2006
ABSTRACT: The glucocorticoid triamcinolone was investigated for polymorphism. If crystalline suspensions of the commercial available drug (form A) are stirred in different solvents, a new and hitherto unknown modification (form B) is always obtained. However, in water, a third form is isolated, which is a monohydrate (form C). Form B represents the thermodynamically most stable solvent-free modification at room temperature, whereas the commercially available form A is metastable. The stable form B crystallizes in the chiral, non-centrosymmetric monoclinic space group P21 with two crystallographically independent molecules in the asymmetric unit. In the crystal structure, the molecules are connected by O-H‚‚‚O hydrogen bonding into a three-dimensional network. Dehydration of the modification C by thermogravimetry leads to its transformation into the metastable form A. The differential scanning calorimetry (DSC) thermograms of forms B and C exhibit three endothermic events, while form A shows two endothermic events. The last endothermic signal is assigned to the melting of the compounds. The X-ray powder diffraction investigations of the residues formed in these thermal events indicate the formation of at least one additional solvent-free modification. Introduction Polymorphism and pseudopolymorphism are widespread phenomena1-16 and for several reasons of extremely importance in a number of areas such as pharmaceutical chemistry.17-22 Therefore, these phenomena have to be investigated in detail, which initially constitutes experiments to discover how many different forms can exist for a given special active agent and which of these forms represent the thermodynamically most stable form at room temperature. It is also essential to determine in which temperature range a given modification is stable and if one modification can be transformed into another. Finally, it has to be investigated how each of the different modifications can be prepared as a crystalline pure material. In this context, investigations on the polymorphism and pseudopolymorphism of drugs used for a very long time in therapy can be of interest, if their polymorphism and pseudopolymorphism were not investigated in detail and significant changes are made in their processing. This is the case for example for glucocorticoids, which belong to the most effective and versatile drugs and used in therapy for several years.23,24 Triamcinolone for example is a synthetic glucocorticoid agonist and is an inducer of gene expression and apoptosis. In a human airway epithelial cell line, triamcinolone inhibits prostaglandin synthesis via specific reduction in COX2 synthesis. It impairs tumor necrosis factor (TNF)-R-induced degradation of κB-R without affecting DNA binding of NF-κB. It potentates the differentiation-inducing effects of bone morphogenetic proteins (BMP-2, -4, -6).25-27 During processing, these drugs have to be sterilized for which different methods exist. One very elegant method, used in the past few years for glucocorticoids, is sterile filtration in which the drug is dissolved in a given solvent and filtered using a very small filter. Afterward, the solvent is vaporized and the solid drug is dried and micronized. In most cases, it must be guaranteed that in this process only that form is produced that * To whom correspondence should be addressed. Fax: +49-(0)4318801520. E-mail:
[email protected].
is used in therapy or that is preferred by the producer. The investigations on pseudopolymorphism are of importance in view of the above procedure because it has to be determined which solvent-free modification is formed after the removal of the solvent. Depending on the pseudopolymorph, this can lead to stable as well as metastable modifications. As a part of our ongoing investigations in this area, we have investigated different glucocorticoids for their polymorphism and pseudopolymorphism. For triamcinolone acetonide, we have found that the commercially available form is in fact a hydrate and not a solvent-free form.28 This form contains a small amount of water, which is responsible for the stability of this form. If the water is removed from this trigonal form, a new and hitherto unknown tetragonal modification is obtained. Both modifications were structurally characterized because only the structure of a methanol solvate was known.29,30 For the diacetate of triamcinolone, we have found only one solvent-free form but also a large number of pseudopolymophic solvate forms, which crystallize in three different structure types.31-33 If the solvent is removed from the solvates, in most cases the thermodynamically most stable form is obtained, but some solvates showed evidence for the formation of further solvent-free modifications. In the present contribution, we report on our results on triamcinolone (Scheme 1), used in therapy. Polymorphism and pseudopolymorphism have not been investigated in detail, and no crystal structures are available for this drug in the Cambridge Structural Database. For this compound, an orthorhombic symmetry was found, and in further investigations it was assumed that two forms might exist.34 Thermal measurements showed a wide melting range of 248-262 °C,35-37 and some investigations on the crystal habits have also been performed.34 We have found that triamcinolone definitely exists in two different solvent-free forms and also as a monohydrate. Here we report on our investigations. Experimental Section Crystal Structure Determination. The data were measured at 170 K using a STOE IPDS-1 Imaging Plate Diffraction system. Structure solutions were performed with direct methods using SHELXS-97.38
10.1021/cg060434r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006
70 Crystal Growth & Design, Vol. 7, No. 1, 2007
The structure refinements were performed against F2 using SHELXL97.39 All non-hydrogen atoms were refined using anisotropic displacement parameters. The C-H hydrogen atoms were positioned with idealized geometry and refined with isotropic displacement parameters (Uiso(C) ) 1.2 × Ueq(Cmethin/methylene) ) 1.5 × Ueq(Cmethyl) using a riding model with C-Hmethin ) 0.95 Å, C-Hmethylene ) 0.99 Å, and C-Hmethyl ) 0.98 Å. The O-H hydrogen atoms were located in difference map but positioned with idealized geometry and were allowed to rotate but not tip with an isotropic displacement parameter using a riding model with O-H ) 0.84 Å. Because no strong anomalous scattering atoms are present, the absolute configuration cannot be determined. Therefore, Friedel equivalents were merged in the refinement, and the absolute configuration was assigned based on the known absolute configuration of the starting compound. The technical details of data acquisition and some selected refinement results are summarized in Table 1. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 623585). Copies may be obtained free of charge on application to the director, CCDC, 12 Union Road, Cambridge CB2 1E2, UK (fax: Int. Code +(44)01223/3 36-033; e-mail:
[email protected]). X-ray Powder Diffraction Experiments. X-ray powder diffraction experiments were performed using a STOE STADI P transmission powder diffractometer with an 4° or 45° position sensitive detector (PSD) using Cu KR radiation (λ ) 1.540598 Å) and a graphite monochromator. The samples were rotated during measurement, and the measurement time was optimized to have at least 10 000 counts above background. The solvent-free samples were light grinded in a mortar, and about 5 mg was used in each measurement. All data were analyzed using WinXPOW form STOE&CIE. DSC Investigations. Differential scanning calorimetry (DSC) investigations were performed with the DSC 204/1/F device from Netzsch. The measurements were performed in Al pans with heating rates of 3 °C /min. The instrument was calibrated using standard reference materials. Differential Thermal Analysis, Thermogravimetry, and Mass Spectroscopy. Differential thermal analysis-thermogravimetry (DTATG) measurements were performed in Al2O3 crucibles using a STA409CD thermo balance from Netzsch. Several measurements under nitrogen atmosphere (purity 5.0) with different heating rates were performed. For MS measurements, this instrument is equipped with Skimmer coupling and a quadrupole mass spectrometer from Balzers. The MS measurements were performed in analog and trend scan mode in Al2O3 crucibles under a dynamic nitrogen (purity 5.0) atmosphere using heating rates of 4 °C/min. All measurements were performed with a flow rate of 75 mL/min and were corrected for buoyancy and current effects. Chemicals. Triamcinolone is commercially available and was procured from HPP pharmaceuticals, Magdeburg, Germany. All solvents used were of analytical grade.
Results and Discussion To determine if the commercially available form A represents the thermodynamically most stable form at room temperature or if additional modifications exist, we investigated crystalline suspensions of the commercial available form A by stirring it in a variety of solvents at room temperature. The solid residues thus obtained were examined by X-ray powder diffraction (Table 2).
Suitchmezian et al.
Figure 1. X-ray powder pattern of the commercially available form A (top) and of the residues formed by stirring crystalline suspensions in acetonitrile (mid) and water (bottom). Table 1. Crystal Data and Results of the Structure Refinement for Form B of Triamcinolone empirical formula MW/g mol-1 crystal color crystal size/mm3 crystal system space group A/Å B/Å C/Å β/° V/Å3 temp/K Z Dcalculated/mg cm-3 F(000) 2θ-range h/k/l ranges
C21H27FO6 394.43 colorless 0.2 × 0.15 × 0.2 monoclinic P21 6.0440(3) 24.0156(15) 12.7206(6) 101.937(6) 1806.47 200(2) 4 1.450 840 2.36 to 28.09° -7/7 -31/31 -14/16 0.11 11779 0.0490 4253 3513 514 0.0429 0.1024 1.042 0.322 and -0.272
µ(MoKR)/mm-1 reflections collected Rint Independent refl. reflns with I > 4σ(I) refined parameters R1 [I > 2σ(I)] wR2 [all data] GoF min/max res /e Å-3
Table 2. Results of the Crystallization Experiments solvent 1-butanol 1-propanol 2-butanol 2-propanol acetonitrile chloroform ether N,N-dimethyl-formamide ethyl acetate ethyl methyl ketone
form B B B B B B B B B B
solvent 3-methyl-1-butanol methylene chloride 1-pentanol carbon tetrachloride acetone ethyl alcohol methyl alcohol H2O HCl 0.1 M NaCl 150 mM
form B B B B B B B C C C
Surprisingly, the X-ray powder patterns of the residues formed in most solvents are identical but completely different from that of the commercially available form A (Figure 1) indicating that the residues constitute a new and hitherto unknown polymorphic modification (form B) of triamcinolone. TG measurements add further credence to this result as these residues do not contain any solvent. In addition, these experiments clearly show that this new form represents the thermodynamically most stable
Polymorphism of Glucocorticoid Triamcinolone
Crystal Growth & Design, Vol. 7, No. 1, 2007 71
Figure 2. Microscopic image of the crystals of form B obtained in methyl alcohol.
form at room temperature and that the commercial form is metastable. This is really surprising because the question arises as to how the metastable form A can be prepared. However, the residues formed in water, 2 M hydrochloric acid, or isotonic NaCl solution are quite different from both forms A and B, clearly indicating that triamcinolone can exist as a third modification (form C) (Figure 1 and Table 2). TG measurements coupled to MS clearly show that this form represents a monohydrate. In further investigations, crystallization experiments were performed using different conditions. In most solvents in which modification B is formed, we obtained only bundles of very small needles, too small for single-crystal analysis (not shown). Suitable crystals of form B were obtained from methyl alcohol, in which long needles up to 2 mm can be grown, which are suitable for single-crystal X-ray diffraction (Figure 2). As the commercially available form A always transforms to the stable form B, single crystals of this form could not be prepared. Crystal Structures. Crystal structure analysis of the needles shows that the thermodynamically most stable form B crystallizes in the primitive, orthorhombic and non-centrosymmetric space group P21 with Z ) 4 formula units in the unit cell (Table 1). There are two crystallographically independent molecules in the asymmetric unit, which differ predominantly in the conformation of the side chain and also in their crystal environment (Figure 3, top and bottom). In the crystal structure of form B, the molecules are connected via intermolecular O-H‚‚‚O hydrogen bonding, in which most of the hydroxyl groups and all carbonyl oxygen atoms are involved (Table 3). Some of these hydroxyl groups act as donors as well as acceptors (Table 3). Each of the two crystallographically independent molecules are connected via O1‚‚‚H4-O4 respectively O11‚‚‚H14-O14 interactions into chains that elongate in the direction of the crystallographic c-axis (Figure 4). According to the Graph-Set Notation the chains can be described as N1 ) C(12).40-42 These chains are connected via O‚‚‚H-O hydrogen bonding between the acetate group in one chain and each one hydroxyl group of two different molecules of the neighbored chain into double layers, which are parallel to the a-c plane. From this arrangement, two crystallographically independent rings are formed in which, three
Figure 3. Molecular structure of both crystallographically independent molecules in form B with labeling and displacement ellipsoids drawn at the 50% probability level (top) and drawing with both molecules superimposed onto each other (bottom). Table 3. Hydrogen-Bonding Interactions with Bond Lengths (Å) and Bond Angles (°)a O-H‚‚‚O
d(O-H)
d(H‚‚‚O)
