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Structural characterization, physicochemical properties, and thermal stability of three crystal forms of nifedipine
Structural Characterization, Physicochemical Properties, and Thermal Stability of Three Crystal Forms of Nifedipine MINO R. CAIRA,1 YOLANDE ROBBERTSE,2 JACOBUS J. BERGH,2 MINGNA SONG,3 MELGARDT M. DE VILLIERS3 1
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
2
Department of Pharmaceutical Chemistry, School of Pharmacy, Potchefstroom University for Christian Higher Education, Potchefstroom 2520, South Africa
3
Department of Basic Pharmaceutical Sciences, School of Pharmacy, University of Louisiana at Monroe, Monroe, Louisiana 71209
Received 14 February 2003; revised 30 April 2003; accepted 22 May 2003
ABSTRACT: In this study the single-crystal X-ray structure of the solvated species (nifedipine)2 1,4-dioxane is reported for the first time. Included solvent molecules are located in isolated cavities in the crystal, yielding a very stable solvate. Desolvation of this species involves complete disruption of the crystal structure at the relatively high temperature of 150–1538C, i.e., 508 above the boiling point of 1,4-dioxane, and yields a monoclinic polymorph (Modification I) with a melting point of 1748C. When exposed to an aqueous medium for 48 h, the solvate transformed into a dihydrate. The aqueous solubilities of the above species were in the order: 1,4-dioxane solvate Modification I > dihydrate. The solubility of nifedipine was increased sixfold when transformed into an amorphous form by quenched fusion. This amorphous form was relatively stable at room temperature but converted to Modification I when suspended in water at pH 1. The fused materials also converted to Modification I through an intermediate, Modification III, within 6 days when kept at 408C for 6 days. XRPD analysis showed that grinding increased the crystallinity of the amorphous form due to partial transformation to Modification I. The pulverized amorphous powder was more stable at 408C and was approximately three times as soluble as Modification I. ß 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2519–2533, 2003
Keywords: solubility
nifedipine; polymorph; solvate; crystal structure; amorphous; stability;
INTRODUCTION Nifedipine is a dihydropyridine L-type calciumchannel blocker, widely used clinically as a coronary vasodilator.1 Pharmaceutical preparations are available as capsules, tablets, and solutions, and the drug is usually administered orally or intravenously. The therapeutic range in plasma is 25–100 mg/L. The drug is practically insoluble in
Correspondence to: Melgardt M. De Villiers (Telephone: 318-342-1727; Fax: 381-342-3255; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 92, 2519–2533 (2003) ß 2003 Wiley-Liss, Inc. and the American Pharmacists Association
water, and is reported to be highly light sensitive, but there is controversy regarding its degradation products and extent of degradation under different light conditions.2 Burger and Koller3 reported three monotropically related modifications of nifedipine: Modification I was reported to be thermodynamically stable at room temperature. Modification II and Modification III were unstable and were obtained only from the melt on an object slide. They also succeeded in preparing four different solvates (A, B, C, and D) through crystallization from 1,4-dioxane. Only solvate B was stable at room temperature. Solvate B was also monotropically related to solvate A. The desolvation of C at about 808C, which
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occurred in combination with an unusually high exothermic reaction, led to Modification I. Both Aso et al.4–6 and Hirayama et al.7 succeeded in preparing amorphous nifedipine by fusion and subsequent cooling. In these studies, they evaluated the use of isothermal microcalorimetry and 13C NMR to determine the physical stability of amorphous nifedipine. Both temperature and moisture decreased the stability of the amorphous form and the decrease in stability was interpreted on the basis of changes in the matrix viscosity, which is related to the molecular motion of nifedipine.4,7 To our knowledge, although the preparation of nifedipine polymorphs and the effect of temperature on the stability of amorphous nifedipine have been reported in the literature, as described above, relatively few studies have focused on structural characterization of nifedipine crystal forms and the effect of pharmaceutical processing on the solid-state stability. The objective of this study was therefore to investigate the solid-state stability and crystal transformations of a nifedipine polymorph, a 1,4-dioxane solvate, and an amorphous form upon suspension in an aqueous medium and grinding by pulverization in a mortar with a pestle. In this study, changes in the crystal properties of the three nifedipine species during these processes are related to differences in their structural and physicochemical properties.
The amorphous form was prepared according to the method described by Aso et al.4 Approximately 2 g nifedipine raw material was placed in a vessel made of aluminum foil and kept at 1808C for 20 min, followed by rapid cooling in a mixture of acetone and dry ice until the amorphous material precipitated. The amorphous material was used as prepared or pulverized using a mortar and pestle.
Scanning Electron Microscopy A Philips XL 30 scanning electron microscope (Philips, The Netherlands) was used to obtain photomicrographs of the various nifedipine crystal forms. Samples were adhered to a small piece of carbon tape mounted on a metal stub and coated under vacuum with carbon (Emscope TB500 sputter-coater) before being coated with a thin gold-palladium film (Eiko Engineering ion Coater IB-2). X-ray Powder Diffraction X-ray powder diffraction (XRPD) patterns (Figure 1) were obtained at room temperature on either a Philips PM 9901/00 (Philips, The
MATERIALS AND METHODS Preparation of Crystal Forms Rolab (Kempton Park, South Africa) generously donated Nifedipine powder, batch number 950361. Solvents used for recrystallization of nifedipine were of analytical reagent grade. From this powder three different crystal forms of nifedipine were prepared, namely a polymorph, a 1,4dioxane solvate, and an amorphous form. Modification I is the commercially available product, and was used as such. It was also crystallized from acetone, chloroform, ethyl acetate, and methyl chloride. To prepare the 1,4-dioxane solvate, nifedipine powder was added to 1,4-dioxane under constant stirring until a saturated solution was obtained. The solution was left at room temperature (208C) to slowly evaporate for approximately 2 weeks. Recrystallized material was removed from the solvent using a spatula and dried on filter paper to ensure evaporation of surface solvent molecules.
Figure 1. X-ray powder patterns for: (a) the dihydrate of nifedipine, (b) the 1,4-dioxane solvate, (c) Modification I, and (d) pulverized amorphous powder.
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Netherlands) or Bruker D8 Advance diffractometer (Bruker, Germany). The isothermal measurement conditions were: target, Cu; voltage, 40 kV; current, 30 mA; divergence slit, 2 mm; antiscatter slit, 0.6 mm; receiving slit, 0.2 mm; monochromator; detector slit, 0.1 mm; scanning speed, 28/min (step size 0.0258, step time, 1.0 s). Approximately 300 mg samples were weighed into aluminium sample holders, taking care to avoid introducing preferred orientation of the crystallites. The XRD traces of the samples (powders or crystals) were compared with regard to peak position and relative intensity, peak shifting, and the presence or lack of peaks in certain angular regions. Variable temperature XRPD patterns were recorded with an Anton Paar TTK 450 low-temperature camera (Anton Paar, Austria) attached to a Bruker D8 Advance diffractometer (Bruker, Germany). A heating rate of 108C/min was used during all determinations.
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oil (Fluka, Switzerland). Photographs were taken using a Pentax K 1000 SLR camera. Infrared Spectroscopy IR spectra of powdered samples were recorded on a NexusTM 470 spectrophotometer (Nicolet Instrument Corporation, Madison, WI) over a range of 4000–400 cm1 with the Avatar Diffuse Reflectance smart accessory or the KBr disc technique. For diffuse reflectance analysis, samples weighing approximately 2 mg were mixed with 200 mg of KBr (Merck, Darmstadt, Germany) by means of an agate mortar and pestle, and placed in sample cups for convenient, fast sampling. For the compressed disk technique samples weighing approximately 2 mg were mixed with 200 mg KBr (Merck, Darmstadt, Germany) by means of an agate mortar and pestle. Discs were pressed using a Beckman 00-25 press (Beckman, Scotland) at a pressure of 15 103 kg/cm2. Single Crystal X-ray Structural Analysis
Thermal Analysis Thermal analysis methods used in this study included differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and hot stage microscopy (HSM). DSC traces were recorded with a Shimadzu DSC-50 instrument (Shimadzu, Kyoto, Japan) or a DSC 2920 modulated DSC (TA Instruments, New Castle, DE). Samples weighing 3–5 mg were heated in crimped aluminum cells at a rate of 108C/ min under nitrogen gas flow of 35 mL/min. TGA analysis was performed on all samples indicated by DSC as being possible solvates or hydrates. TGA traces were recorded with a Shimadzu TGA-50 instrument (Shimadzu, Kyoto, Japan) or Hi-Res Modulated TGA 2950 (TA Instruments, New Castle, DE). The sample weight was approximately 5–8 mg and heating rates of 1–128C/min under nitrogen gas flow of 35 mL/min were used. HSM analysis was carried out on small amounts of samples with a Leitz Wetzlar Laborlux K thermomicroscope (Leitz Wetzlar, Germany), equipped with a Metratherm 1200 heating unit. The effects of temperature increase on the crystal behavior of the samples were studied by placing approximately 3 mg of each on an object glass, covering it with a cover slip, and gradually increasing the temperature to about 2008C at a heating rate of 108C/min. Dehydration and desolvation were observed with samples immersed in silicone
Bright yellow, prismatic crystals of the 1,4-dioxane solvate were examined microscopically and were found to have excellent quality for X-ray diffraction analysis. Slow spontaneous loss of included solvent was observed following removal of the crystals from their mother liquor, and therefore, all further experiments were carried out either with the crystals immersed in mother liquor or otherwise protected. X-ray photography (oscillation and precession ˚) methods) using CuKa radiation (l ¼ 1.5418 A revealed only a center of symmetry in the diffraction pattern, indicating the triclinic crystal system. Preliminary estimates of the unit cell volume were obtained from the photographic data. This volume was consistent with a solvated species after comparison with unit cell data for an unsolvated form of the drug.8 For intensity data collection, a single fragment of suitable size was cut from a large crystal specimen, which showed easy cleavage. Intensity data were collected on a Nonius Kappa CCD diffract˚ ) with ometer using MoKa radiation (l ¼ 0.71069 A the crystal mounted on a glass fibre, coated with Paratone oil, and cooled to 177(1)K in a continuous nitrogen stream by means of a Cryostream cooler (Oxford Cryosystems). These precautions were taken to optimize the quality of the X-ray data by minimizing crystal decomposition and reducing atomic thermal motion. Comparison of the unit cell
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data at ambient and low temperatures indicated that no phase change had occurred on cooling the crystal. A total of 14,828 reflection intensities were measured on 316 frames (20-s exposure per frame) by a series of j- and o-scans of 28 each, with the crystal at 40.0 mm from the detector. Datacollection strategy and data reduction followed standard procedures described by Otwinowski and Minor,9 using ‘‘Collect’’ data collection software, Nonius B.V. (1998). Accurate unit cell data were calculated by least-squares (LS) using all measured data. The structure was solved using program SHELXS-9710 and refined using the full-matrix LS procedure with SHELXL-97.11 Anisotropic thermal parameters were employed for nonhydrogen atoms and H atoms were treated isotropically with Uiso ¼ 1.2 times the Ueq value of the parent atom. In the final refinement, LS weights of the form w ¼ 1 / [s2)(Fo)2 þ (aP)2 þ bP], P ¼ [max(Fo2, 0) þ 2Fc2]/3 were employed. Solubility Measurements An amount of powder, enough to ensure that supersaturation could be obtained, namely 10 1 mg, were measured into 10-mL test tubes with screw caps. To each test tube, 5 mL Milli-Q water, whose pH was set at 1 using 0.1 N aqueous HCl, was added and the caps screwed on tightly. The test tubes were rotated at 60 rpm (Heidolph RZR-2000 rotator, Germany) in a thermostatically controlled (Julabo EM/4 thermostat, Germany) water bath at 30 18C. Samples were withdrawn and filtered through a 0.45-mm filter after 24 h. The concentrations of the filtered samples were determined by HPLC. The HPLC system consisted of a Hewlett Packard HP 1100 system with an HP 3395 integrator (Agilent, USA) equipped with a reverse phase Nucleosil1 C18 steel cartridge column (125 4 mm i.d., 5 mm particle size, Macherey-Nagel, Du¨ren, Germany). UV detection was set at 254 nm, the solvent flow rate at 0.7 mL/ min, column temperature at 208C and 20-mL samples were injected into the HPLC apparatus. DSC and HPLC analysis showed that the nifedipine was not decomposed during the time it took to measure the solubilities of the crystal forms (Figure 4, Tables 2 and 5). Results obtained from aqueous solubility studies were compared to identify possible differences between the various polymorphic forms. These results were analyzed statistically using the Newman-Keuls test (Statistica for Windows
5.1B) to determine the extreme of statistically significant differences. DSC traces of all samples were recorded before and after solubility determination to identify possible polymorphic transformations.
Karl Fischer Water Determination A Metrohm 831 Karl Fischer Automatic Coulometer (Metrohm, Switzerland) was used to determine trace amounts of water in the powdered samples. Results are the mean moisture content of 5 to 10 different amounts of the samples weighed to the nearest 0.1 mg with a validated balance.
RESULTS AND DISCUSSION Morphology of the Nifedipine Crystal Forms After preparation and recovery of the crystal forms, visual and microscopic evaluation (Figure 2) clearly showed differences in the morphology of the prepared powders. The crystals of Modification I (a polymorph) were not of a definite shape. The sample consisted of large particles with small particles in between, with a mean diameter of 2.79 2.26 mm. The solvate consisted of flatsurfaced crystals with well-defined and sharp edges. The mean particle size was 81.33 7.71 mm. The particles of the amorphous form were large glassy plates with a mean particle size of 52.18 6.53 mm that yielded irregular sharp-edged fragments when crushed.
X-ray Diffraction Analysis Each of the prepared crystal forms had a distinct and characteristic XRPD pattern as shown in Figure 1. Main X-ray diffraction peak angles and relative intensities for Modification I, the solvate, and hydrate are listed in Table 1. No discernible peaks were detected in the powder pattern of the fused nifedipine, confirming its amorphous nature. The formation of amorphous nifedipine with a diffuse XRPD pattern was attributed to the loss in crystallinity caused by the fusion and subsequent rapid cooling. A few peaks with measurable intensities were detected in the XRPD pattern of the amorphous form after grinding, as shown in Figure 3. Upon heating, the pulverized material converted to Modification I.
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Figure 2. SEM photomicrographs of: (a) Modification I, (b) the 1,4-dioxane solvate and (c) amorph.
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ed between the crystals and the raw material, the latter was used in further experiments. The endotherm at 1538C in the DSC trace of the solvate (Figure 4) is due to the loss of the solvent, 1,4-dioxane. Desolvation is followed by a melting endotherm at 1748C. Hot stage microscopy (HSM) confirmed desolvation as seen in Figure 5. A theoretical weight loss of 11.2%, calculated for a solvate consisting of two molecules of nifedipine sharing one molecule of 1,4-dioxane, corresponds to an experimental weight loss of 11.1%, as indicated by TGA (Figure 6) in the temperature range of 158– 1698C. Karl Fischer analysis confirmed a solvate, rather than a hydrate, because the mean moisture content of the crystals was only 0.097%. The DSC trace of the amorphous form before grinding (Figure 4) (Amorph A), exhibited an endothermic peak at 418C, which is the glass transition temperature. The exothermic peak at 818C is attributed to the transformation of the amorphous form to Modification I. The endothermic peak at 1698C is the melting point of this form. The ratio of the glass transition temperature and melting point of nifedipine amorph was 0.71 (¼ 314/442 K), and within the range (0.69–0.85) so far reported for other glassy compounds.12 No weight loss was observed during heating of the amorphous form. After grinding, the melting properties of the amorphous form changed; a small, sharp exothermic peak at 918C appeared while the glass transition at around 408C was much less marked. XRPD analysis (Figure 3) showed that upon grinding, some of the amorphous material was converted into crystalline material because small peaks began to appear in the XRPD pattern. This change evidently indicated a mixture of an intermediate form and Modification I. All attempts to isolate this intermediate form failed.
Thermal Analysis
Infrared Spectroscopy
A summary of the DSC data is given in Table 2. These data include all thermal events (i.e., desolvation and melting) and their corresponding heat requirements. The DSC trace of nifedipine raw material (Figure 4) showed a melting point of 1718C with no desolvation endotherms, indicating that Modification I is not a solvate but a polymorph. Modification I was also prepared by recrystallization from acetone, chloroform, ethyl acetate, and methyl chloride, and the melting point endotherm and enthalpy of fusion corresponded to that of the commercial sample. Because no significant differences could be detect-
The IR-spectra of all the nifedipine crystal forms were obtained with the KBr disc method and compared to identify significant differences. The main IR absorption peaks of the nifedipine crystal forms and their corresponding wavenumbers (cm1), are listed in Table 3. Absorption peaks for pure 1,4dioxane, described by Burger and Koller3 and Sadtler,13 and those found for the solvent in crystals of the 1,4-dioxane solvate are also listed. There are clear and definite differences between the IR-spectra of the three nifedipine crystal forms, especially in the region 2000–4000 cm1. Differences are also present in the carbonyl
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Table 1. Peak Intensity Values (I/I0) at Main PXRD Peak Angles (82y) of Nifedipine Crystal Forms Modification I Main Peaks 1 2 3 4 5 6 7 8 9 10
1,4-Dioxane Solvate
Dihydrate
82y
I/I0
82y
I/I0
82y
I/I0
11.77 19.59 24.65 11.93 16.20 19.77 8.08 10.41 22.72 25.90
100.0 88.4 77.4 72.0 58.6 56.5 55.2 51.0 47.4 43.9
23.95 7.91 23.75 19.56 23.62 24.15 24.24 15.96 20.17 12.79
100.0 89.9 65.1 61.7 45.7 22.9 22.2 21.9 16.2 13.9
24.31 19.49 8.00 25.73 24.50 22.64 19.81 11.62 10.29 14.62
100.0 98.0 97.6 89.6 85.3 78.1 78.1 74.2 67.5 67.0
stretching region (1500–1800 cm1), as well as in the fingerprint region below 1500 cm1. Some of the main absorption peaks of pure 1,4-dioxane are comparable to those in the spectrum of the solvate and these absorption peaks do not exist, or differ
drastically from those of Modification I and the fused amorphous form. The differences between the IR-spectra of the three nifedipine crystal forms correlate with the XRPD data, indicating that three distinctly different crystal forms of nifedipine were prepared. Single Crystal Structure of the 1,4-Dioxane Solvate
Figure 3. Variable temperature XRPD patterns showing the conversion of pulverized amorphous nifedipine to Modification I.
The crystal structure of Modification I was determined by Triggle et al.8 but this is the first report of the single crystal structure of a 1,4dioxane solvate of nifedipine. Crystal data and refinement details appear in Table 4. The conformation of the nifedipine molecule is shown in Figure 7, which includes the atomic numbering and the atoms represented by 50% thermal ellipsoids.14 The average planes through the dihydropyridine and phenyl rings are nearly orthogonal [interplanar angle: 88.21(4)8]. This conformation is remarkably similar to that found in the crystal structure of the polymorph of nifedipine described by Triggle et al.8 crystallizing in the space group P21/c with Z ¼ 4. Thus, because the interaction between the drug molecule and the included 1,4-dioxane molecule (see below) does not induce further conformational changes, it appears that the conformation shown in Figure 7 is a stable one. The unit cell contains two nifedipine molecules located in the general positions, and one 1,4dioxane molecule, which is necessarily located on a center of inversion on account of the centrosymmetric space group. The guest molecule is ordered, and adopts a chair conformation. Figure 8 shows the 2:1 complex unit in which each oxygen atom
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Table 2. DSC and TGA Data of the Three Nifedipine Crystal Forms with Respect to Thermal Events at a Certain Temperature with the Corresponding Heat Required
Crystal Form
Desolvation Endotherm (8C)
Heat (J/g)
Endo/ Exotherms (8C)
Melting Point Endotherm (8C)
Heat (J/g)
Modification I Solvate Amorph A Amorph B Dihydrate
— 153 — — 100–110
— 68 — — 28–32
— — 45; 80 40; 91–98 —
171 169 168 170 165
98 80 66 65 72
of the guest molecule is linked to a nifedipine molecule by an O HN hydrogen bond. This ˚ and the angle bond has O N 2.924(1)A NH O is 172(1)8. Figure 9 shows the crystal packing arrangement. Separate domains of host drug and guest 1,4-dioxane molecules are clearly evident. Using program Lazy Pulverix,15 we have used the output of the single crystal X-ray analysis to generate an idealized powder pattern for the 2:1 solvate. This pattern is shown in Figure 10a and serves as a reference pattern for future identification of this phase. The most intense peak (2y ¼ 24.688) corresponds to the (210) reflection. Inspection of packing diagrams shows that these are the planes populated by the dihydropyridine rings and their methyl and carbomethoxy groups as well as the oxygen atoms of the guest molecule.
Solid-State Stability of the 1,4-Dioxane Solvate Burger and Koller3 reported the melting points of four solvates prepared from 1,4-dioxane to be 169–1738C, 161–1638C, and 1358C, respectively. With the help of TGA curves they determined the thermal stability of the various solvates and found that the crystal lattice of their Form A was continuously diffused by dioxane between 70 and 1508C. The crystal lattice of Form B was also stable up to approximately 858C, the maximum dioxane release following at nearly 1508C. For the third form, C, the dioxane was released at about 408C, and at approximately 1008C the bulk of the 1,4-dioxane had already evaporated, and for the last form, D, dioxane was released between 50 and 1508C. Comparison of these results with the TGA and DSC results obtained in this study
Figure 4. DSC thermograms of Modification I, the 1,4-dioxane solvate, dihydrate and amorphous forms. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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Figure 5. Hot stage photographs showing the 1, 4-dioxane solvate at: (a) ambient temperature and (b) during desolvation at around 1538C.
(Table 2) showed that the onset of desolvation, between 135–1508C depending on the heating rate, and the melting point of the desolvated material, 1748C, of the 1,4-dioxane solvate pre-
pared in this study were higher. In addition, because there is no published X-ray data of the 1,4-dioxane solvates available for comparison, it was impossible to definitively characterize this solvate as either A, B, C, or D prepared by Burger and Koller.3 This shows that crystal solvates such as the 1,4dioxane solvate prepared in this study can evidently exhibit a wide range of behavior with regard to stability and desolvation. However, that described in this study has a well-defined composition and reproducible properties. A detailed study of the packing (Figure 9) shows that the guest 1,4dioxane molecules in the nifedipine solvate described here are located in isolated cavities in the crystal, successive cavities having their cen˚ ). ters separated by the length of the a-axis (&7.6 A The van der Waals envelopes of the guest mole˚ along this direction. cules are separated by 4.5 A Desolvation of the crystal must therefore involve complete disruption of the crystal structure shown in Figure 9 and should occur at relatively high temperature owing to strong host–guest hydrogen bonding, and location of guest molecules in isolated cavities. This is borne out in the DSC behavior, which shows the desolvation onset at around 1508C. This temperature exceeds the boiling point of the pure solvent 1,4-dioxane by approximately 508C, and indicates a very stable inclusion compound.16 The DSC exotherm that follows desolvation implies recrystallization of the desolvated species into a polymorph; this species eventually melts at around 1748C.
Figure 6. TGA thermograms of the 1,4-dioxane solvate at different heating rates. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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Table 3. Main Absorption Peaks in the IR-Spectra of the Different Nifedipine Crystal Forms, as Well as Main Absorption Peaks of 1,4-Dioxane, Found in the Literature and Obtained Experimentally Modification I
Amorph
Solvate
Dioxane
Lit.a
Exp.b
Lit.a
Exp.b
Lit.a
Exp.b
Lit.c
3333
3329
3346
3346
3102
3101
3106
3099
3296 3235 3101
2868
1701
1703 1680
1703 1680 1630
3292 3236 3099 2868 1701 1680 1653
1624 1383 1280
1378
1377 1254 1219
1250
1235
1150 1113 893 860S 831M 750
1130
1689 1649 1619 1380 1289
1684 1647 1622 1381
1236 1165
1226 1155
1239 1162
1217
1104
1118
1100
1114
746
858W 829M 744
1651 1620 1383 1282
858W 829M 748
750
891 863 833
a
Lit. ¼ Literature (Burger & Koller, 1996). Exp. ¼ IR-values obtained with the KBr disc method. Lit. ¼ Literature (Sadtler, 1967). W Weak peak, MMedium peak, SStrong peak. b c
The identity of the resulting polymorph was established in this study as follows: a sample of the 1,4-dioxane solvate was desolvated under controlled conditions (heated at 1308C under vacuum for 24 h), the residual material was then checked by thermogravimetric analysis and was found to be completely solvent-free. The X-ray powder pattern, Figure 10b, was then recorded. The XRD pattern for the known monoclinic polymorph was calculated from the crystal data for this form reported by Triggle et al.8 This is shown in Figure 10c, and matches closely the pattern of Figure10b. Differences in peak intensities are due to sample effects but the excellent match of the peak angular positions leaves no doubt that desolvation of the 1,4-dioxane solvate yields the known monoclinic polymorph. The desolvation kinetics of the nifedipine solvate was studied by subjecting the crystals to TGA heating rates of 5, 7, 10, 12, and 158C per minute. TGA traces for the solvate at different heating rates are shown in Figure 6. The activation energy (Ea) for the dehydration process was calculated from these TGA data according to the method described by Flynn and Wall.17 This
method involves the analysis of weight loss versus temperature at different heating rates (b) to determine the corresponding absolute temperatures at a constant weight loss (C). Graphs of negative logarithm of heating rates (expressed in8C/s) (log b) versus 1/T were plotted and the activation energy (Ea) calculated from the slope of the curves. The calculated activation energy (Ea) for the desolvation of the nifedipine solvate according to the TGA data was &102 kJ/mol. This value is similar to that of other 1,4-dioxane solvates determined by the Flynn and Wall17 method. For example, for the 1,4-dioxane solvate of 5-methoxysulfadiazine, which similarly has the solvent in isolated cavities, a value of approximately 94 kJ mol1 was obtained for the activation energy.18 Solid-State Stability of the Amorphous Form In contrast to crystalline materials, glassy or amorphous substances have a certain form and volume, but no specific internal structure, the constituent molecules occurring in an irregular array. Aso et al.4–6 studied the physical stability
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Table 4.
Crystal Data and Refinement Details C17H18N2O6 (C4H8O2)0.5
Complex Formula Formula weight Temperature Crystal system Space group Unit cell dimensions
Volume Z Density (calculated) Radiation, wavelength Absorption coefficient F (000) Crystal size Theta range for data collection Index ranges Reflections collected Observed reflections [I > 2s(I)] Independent reflections Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data) Largest diff. peak and hole
of amorphous nifedipine using isothermal microcalorimetry and 13C nuclear magnetic resonance relaxation time measurements as a function of temperature and humidity. The heat produc-
Figure 7. Atomic numbering and conformation of the nifedipine molecule in the crystal of the 1,4-dioxane solvate. Thermal ellipsoids are drawn at the 50% probability level.
390.39 177(1) K Triclinic P(1) ˚ a ¼ 7.6238(2) A ˚ b ¼ 11.1758(3) A ˚ c ¼ 11.8898(3) A a ¼ 73.524(2)8 b ¼ 73.417(2)8 g ¼ 75.603(2)8 ˚3 915.40(4) A 2 1.416 g cm3 ˚ MoKa, 0.71069 A 0.109 mm1 412.0 0.31 0.50 0.44 mm 2.34 to 27.518 9 h 9, 14 k 14, 14 l 15 14,828 6997 3951 full-matrix on F2 3951/0/258 1.039 R1 ¼ 0.0386, wR2 ¼ 0.0986 R1 ¼ 0.0523, wR2 ¼ 0.1070 ˚ 3 0.218, 0.624 eA
tion due to the crystallization of amorphous nifedipine and its transformation into a stable crystal form was detected at temperatures above its glass transition temperature. The rate of transformation increased as the temperature increased. As stated earlier variable temperature XRPD (Figure 3) at 108C/min of the pulverized material showed that some of the amorphous form converted to Modification 1 upon grinding and that it then completely converts to this form upon heating. DSC investigation also indicated that the
Figure 8. The 2:1 nifedipine–1,4-dioxane unit in the crystal of the solvate.
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Figure 9. Projection of the crystal structure of the 2:1 nifedipine–1,4-dioxane solvate. Four unit cells are drawn and hydrogen bonds are indicated as dotted lines.
thermal properties of the amorphous material differed before and after pulverization. As shown in Figure 4, the thermal features observed in the DSC trace of the fused material (amorph A) were
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very broad peaks with a prominent glass transition around 418C. A DSC curve for the pulverized material (amorph B) showed a much smaller glass transition with a sharper recrystallization around 91–988C of an intermediate form to Modification I. Samples of these two materials were kept at 25 and 408C, 60% relative humidity and protected from light. At 258C no change in the powders was observed but, as shown in Figure 11a and b, their transformation to Modification I differed significantly when stored close to the glass transition temperature at 408C. The pulverized material transformed into Modification I much more slowly because after 9 days a significant amount of the amorphous material was still detected by DSC. In contrast, no amorphous material was present in the fused material after 6 days. The transformation of the amorphous forms to Modification I, XRPD pattern similar to that shown in Figure 2c, was confirmed by variable temperature XRPD analysis (Figure 3). This was not a simple conversion because the appearance of other endo- and exotherms in the DSC traces (Figure 11) showed that for both the fused and pulverized material the transition to Modification I occurred through intermediates. All attempts to isolate these intermediates, including isolation on a hot stage microscope, recrystallization, and storage at elevated temperatures, failed but variable temperature XRPD patterns between 65–958C (Figure 3) showed the occurrence of peaks around 7 and 2482y similar to that reported by Hirayama et al.7 for a polymorph (Form B) they prepared by heating glassy nifedipine to 1108C. Based on DSC results this form is most probably also the same as Modification III reported by Burger and Koller.3 Solubility of Nifedipine Crystal Forms
Figure 10. X-ray powder patterns for: (a) the dioxane solvate of nifedipine, calculated from single crystal X-ray data, (b) the material obtained by desolvating the dioxane solvate, and (c) Modification I of nifedipine, calculated from single crystal X-ray data.
Two major problems hindering the effective use of nifedipine in dosage forms are its extremely low aqueous solubility and its chemical instability. Boje et al.19 reported the aqueous solubility of nifedipine (Modification I) as 5–10 mg/mL over the pH range 2.2–13. However, over this pH range the stability of nifedipine varies and is most stable at very low pH with a reported degradation halflife at a pH 1 of more than 20 h.20 For this reason the solubility of the three crystal forms was determined at pH 1. Solubility values are listed in Table 5. A statistical comparison of the solubilities of the different nifedipine crystal forms after 24 h, using the Newman-Keuls test, showed
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Figure 11. DSC thermograms showing the transformation of (a) Amorph A and (b) Amorph B at 408C.
Table 5. Saturation Solubility of the Nifedipine Crystal Forms in Water at 308C 18C and pH 1 Crystal Form Modification I 1,4-Dioxane solvate Amorphous form A Amorphous form B Dihydrate a
Solubility (mg/mL)
Assay (%)a
5.42 0.33 5.99 0.11 35.04 2.51 16.13 0.87 3.01 0.56
98.2 1.1 96.4 1.5b 97.2 1.6 97.6 2.1 97.8 0.8
HPLC analysis of undissolved material. 1,4-Dioxane solvate converted to the dihydrate.
b
that the solubilities of Modification I and the solvate are not significantly different ( p < 0.49166), but the fused amorphous form (A) is significantly more soluble (&6 times) than Modification I ( p < 0.00022) and the solvate ( p < 0.00024). After grinding, the solubility of the amorphous form decreased significantly to &16 mg/mL. However, this powder was still significantly more soluble than Modification I and the solvate. These results correlated with those reported for other metastable crystal forms, amorphous forms generally being more soluble.21,22 In addition HPLC
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Figure 12. HPLC chromatograms of undissolved material removed after solubility testing: (1) ground amorph B, (2) Modification I, (3) 1,4-dioxane solvate, (4) dihydrate, and (5–6) decomposed nifedipine obtained by leaving an aqueous suspension of Modification I exposed to light for 72 h.
analysis (Table 5 and Figure 12), confirmed that during solubility analysis nifedipine did not decompose. However, it is expected that metastable forms will return to the stable state, if the transformation is facilitated by an appropriate kinetic mechanism, such as recrystallization or conversion through contact with a solvent.22 To ascertain this, after solubility testing the powders that did not dissolve were recovered and DSC analysis performed. The glass transition (408C) seen for the fused and pulverized powders before the solubility measurement was not present after solubility testing. Only one endotherm at 1738C was present,
suggesting that during solubility testing the powders transformed to Modification I. In the DSC trace of the solvate after solubility testing, an endotherm at 1458C (about 108C lower than the desolvation temperature for 1,4-dioxane) appeared. This suggested transformation of the solvate to a hydrated form with a lower desolvation temperature. Because this conversion had not reached completion during the time course of the solubility test, it was not possible to quantify this transition during the solubility study. To investigate the transformation several samples of the solvate were kept in the solubility medium and analyzed by DSC and TGA after 24 and 48 h. TGA
Figure 13. TGA curves of the 1.4-dioxane solvate left in water with pH 1 showing the transformation of the solvate to a dihydrated form of nifedipine. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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analyses (Figure 13) showed that the solvate had changed to another crystal form that loses about 10% weight at 105–1108C after 48 h in the aqueous solution, protected from light. Theoretically, if the solvate transformed into a dihydrated form the weight loss should be 9.4%, which is very close to the observed value. Karl Fischer analysis confirmed this, because the water content of the recovered crystals increased from 0.09 to 7.9%. HPLC analysis (Figure 12) excluded the formation of a dihydrate of a degradation product. The solubility of the dihydrated form (Table 5) was significantly lower than that of Modification I and the 1,4-dioxane solvate. Because this is the first report of a hydrated crystal form of nifedipine the transformation of the 2:1 1,4-dioxane solvate to the dihydrate was measured three times. The transformation occurred in water or water at pH 1, but not in dioxane: water, ethanol:water, methanol:water, or acetone:water mixtures. This transformation was also specific for the solvate because Modification I and the amorphous form were not transformed to the dihydrate in the same solvents. The hydrated form was unstable and converted quickly to Modification I on removal from the aqueous medium. Although the XRPD pattern of the hydrated form (Figure 2a) closely resembles that of Modification I (Figure 2c) there are small differences in peak angular positions and large differences in peak intensities. There are also three large peaks, at 24.308, 19.818, and 14.628 with relative intensities above 60%, which are not present in the XRPD patterns of the solvate or the monoclinic polymorph. The dihydrate appears to be a ‘‘pseudopolymorph’’ because the overall PXRD profile closely resembles that of Modification I with the peaks shifted to lower 2theta angle, which implies a larger unit cell, as expected when water is incorporated into the anhydrous crystal.
In summary, the relationships between the various forms of nifedipine, under the conditions reported in this study, are shown in Figure 14.
CONCLUSIONS A well-defined nifedipine–1,4-dioxane solvate with drug–solvent stoichiometric ratio 2:1 was isolated, and its crystal structure and thermal properties were characterized. DSC analysis showed that desolvation of this species involves complete disruption of the crystal structure at relatively high temperature. This is consistent with the crystallographic data showing that the guest 1,4-dioxane molecules are located in isolated cavities in the crystal. XRPD data simulated from single crystal structure analysis supported DSC results and also showed that upon desolvation, the solvate transformed into a monoclinic polymorph known as Modification I. However, TGA and Karl Fischer water determination showed that upon exposure to an aqueous medium the solvate transformed into a dihydrate. The solubilities of the 1,4-dioxane solvate, Modification I, and the dihydrate in water at pH 1 were in the order: 1,4-dioxane solvate Modification I > dihydrate. The solubility of nifedipine was increased approximately sixfold when transformed into an amorphous form by quenched fusion from the melt at 1508C in an acetone dry ice bath. DSC and XRPD analysis showed that the amorphous form was relatively stable at room temperature but converted to Modification I upon exposure to an aqueous medium and when stored at 408C for 6 days. When pulverized, the crystallinity of the amorphous form increased slightly, indicating partial transformation to Modification I and an intermediate form, Modification III. The pulverized material was more stable at 408C, and was still approximately three times as soluble as Modification I.
ACKNOWLEDGMENTS
Figure 14. Stability relationships between the various forms of nifedipine reported in this study.
This work was supported by grants from the National Research Foundation (Pretoria, South Africa) and the Louisiana Board of Regents Enhancement Program (LEQSF(2001-02)-ENH-TR82). Rolab, Pty.Ltd., Kempton Park, South Africa, is thanked for the generous supply of nifedipine. Research support from the University of Cape Town is also acknowledged.
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REFERENCES 1. Katzung BG. 1998. Vasodilators and the treatment of angina pectoris. In: Katzung BG, editor. Basic and clinical pharmacology, 7th ed. Stamford: Appleton & Lange, pp 179–196. 2. Sadana GS, Ghogare AB. 1991. Mechanistic studies on photolytic degradation of nifedipine by use of 1 H-NMR and 13C-NMR spectroscopy. J Pharm Sci 70:195–199. 3. Burger A, Koller KT. 1996. Polymorphism and pseudopolymorphism on nifedipine. Sci Pharm 64: 293–301. 4. Aso Y, Yoshioka S, Otsuka T, Kojima S. 1995. The physical stability of amorphous nifedipine determined by isothermal microcalorimetry. Chem Pharm Bull 43:300–303. 5. Aso Y, Yoshioka S, Otsuka T, Kojima S. 2001. Explanation of the crystallization rate of amorphous nifedipine and Phenobarbital from their molecular mobility as measured by 13C nuclear magnetic resonance relaxation time and the relaxation time obtained from the heating rate dependence of the glass transition temperature. J Pharm Sci 90:798– 806. 6. Aso Y, Yoshioka S, Otsuka T, Kojima S. 2001. Feasibility of using isothermal microcalorimetry to evaluate the physical stability of amorphous nifedipine and phenobarbital. Them Acta 380:199– 204. 7. Hirayama F, Wang Z, Uekama K. 1994. Effect of 2-hydroxyl-b-cyclodextrin on crystallization and polymorphic transition of nifedipine in the solid state. Pharm Res 11:1766–1770. 8. Triggle AM, Schefter E, Triggle DY. 1980. Crystal structures of calcium channel antagonists: 2,6dimethyl-3,5-dicarbomethoxy-4-[2-nitro-, 3-cyano-, 4-(dimethylamino)- and 2,3,4,5,6-pentalfluorophenyl]-1,4-dihydropyridine. J Med Chem 23: 1442–1445. 9. Otwinowski Z, Minor W. 1997. Processing of x-ray diffraction data in oscillation mode. In: Carter CW, Sweet RM, editors. Methods in enzymology, vol. 276. New York: Academic Press, pp 307–326. 10. Sheldrick GM. 1985. SHELXS86. In: Kruger GM, Goddard R, Sheldrick GM, editors. Crystallographic computing 3. Oxford, UK: Oxford University Press, pp 175–178.
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11. Sheldrick GM. 1997. SHELXL97: Program for the refinement of crystal structures. Go¨ttingen, Germany: University of Go¨ttingen. 12. Fukuoka E, Makita M, Nakamura Y. 1991. Glassy states of pharmaceuticals: relaxation during cooling and heating of glass by differential scanning calorimetry. Chem Pharm Bull 39:2087–2090. 13. The Sadtler Standard Spectra, vol. 1. 1967. Philadelphia: Sadtler Research Laboratories. 14. Zsolnai L, Pritzkow H. 1994. ZORTEP: An interactive version of ORTEP. Heidelberg, Germany: University of Heidelberg. 15. Yvon K, Jeitschko W, Parthe E. 1977. LAZY PULVERIX: A computer program for calculating x-ray and neutron diffraction patterns. J Appl Crystallogr 10:73–86. 16. Caira MR, Nassimbeni LR. 1996. Phase transformations in inclusion compounds, kinetics and thermodynamics of enclathration. In: Attwood JL, Davies JED, Macnicol DD, Vo¨gtle F, editors. Comprehensive supramolecular chemistry, vol. 6. New York: Pergamon, pp 132–145. 17. Flynn JH, Wall AW. 1966. A quick, direct method for the determination of activation energy from thermogravimetric data. Polym Lett 4:323– 328. 18. Caira MR, Mohamed R. 1993. Inclusion compounds with the drug 5-methoxysulfadiazine: Structures, thermodynamics of decomposition and relationship to polymorphism of the sulfonamide. Supramol Chem 2:210–207. 19. Boje KM, Sak M, Fung H-L. 1988. Complexation of nifedipine with substituted phenolic ligands. Pharm Res 5:655–659. 20. Soons PA, Schellens JHM, Roosemalen MCM, Breimer DD. 1991. Analysis of nifedipine and its pyridine metabolite dehydronifedipine in blood and plasma: Review and improved high performance liquid chromatographic methodology. J Pharm Biomed Anal 9:475–484. 21. Chiou WL, Kyle LE. 1979. Differential thermal, solubility, and aging studies on various sources of digoxin and digitoxin powder: Biopharmaceutical implications. J. Pharm Sci 68:1224–1229. 22. Elamin AA, Ahlneck C, Alderborn G, Nystro¨m C. 1994. Increased metastable solubility of milled griseofulvin, depending on the formation of a disordered surface structure. Int J Pharm 111:159–170.
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Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
2
Department of Pharmaceutical Chemistry, School of Pharmacy, Potchefstroom University for Christian Higher Education, Potchefstroom 2520, South Africa
3
Department of Basic Pharmaceutical Sciences, School of Pharmacy, University of Louisiana at Monroe, Monroe, Louisiana 71209
Received 14 February 2003; revised 30 April 2003; accepted 22 May 2003
ABSTRACT: In this study the single-crystal X-ray structure of the solvated species (nifedipine)2 1,4-dioxane is reported for the first time. Included solvent molecules are located in isolated cavities in the crystal, yielding a very stable solvate. Desolvation of this species involves complete disruption of the crystal structure at the relatively high temperature of 150–1538C, i.e., 508 above the boiling point of 1,4-dioxane, and yields a monoclinic polymorph (Modification I) with a melting point of 1748C. When exposed to an aqueous medium for 48 h, the solvate transformed into a dihydrate. The aqueous solubilities of the above species were in the order: 1,4-dioxane solvate Modification I > dihydrate. The solubility of nifedipine was increased sixfold when transformed into an amorphous form by quenched fusion. This amorphous form was relatively stable at room temperature but converted to Modification I when suspended in water at pH 1. The fused materials also converted to Modification I through an intermediate, Modification III, within 6 days when kept at 408C for 6 days. XRPD analysis showed that grinding increased the crystallinity of the amorphous form due to partial transformation to Modification I. The pulverized amorphous powder was more stable at 408C and was approximately three times as soluble as Modification I. ß 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2519–2533, 2003
Keywords: solubility
nifedipine; polymorph; solvate; crystal structure; amorphous; stability;
INTRODUCTION Nifedipine is a dihydropyridine L-type calciumchannel blocker, widely used clinically as a coronary vasodilator.1 Pharmaceutical preparations are available as capsules, tablets, and solutions, and the drug is usually administered orally or intravenously. The therapeutic range in plasma is 25–100 mg/L. The drug is practically insoluble in
Correspondence to: Melgardt M. De Villiers (Telephone: 318-342-1727; Fax: 381-342-3255; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol. 92, 2519–2533 (2003) ß 2003 Wiley-Liss, Inc. and the American Pharmacists Association
water, and is reported to be highly light sensitive, but there is controversy regarding its degradation products and extent of degradation under different light conditions.2 Burger and Koller3 reported three monotropically related modifications of nifedipine: Modification I was reported to be thermodynamically stable at room temperature. Modification II and Modification III were unstable and were obtained only from the melt on an object slide. They also succeeded in preparing four different solvates (A, B, C, and D) through crystallization from 1,4-dioxane. Only solvate B was stable at room temperature. Solvate B was also monotropically related to solvate A. The desolvation of C at about 808C, which
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occurred in combination with an unusually high exothermic reaction, led to Modification I. Both Aso et al.4–6 and Hirayama et al.7 succeeded in preparing amorphous nifedipine by fusion and subsequent cooling. In these studies, they evaluated the use of isothermal microcalorimetry and 13C NMR to determine the physical stability of amorphous nifedipine. Both temperature and moisture decreased the stability of the amorphous form and the decrease in stability was interpreted on the basis of changes in the matrix viscosity, which is related to the molecular motion of nifedipine.4,7 To our knowledge, although the preparation of nifedipine polymorphs and the effect of temperature on the stability of amorphous nifedipine have been reported in the literature, as described above, relatively few studies have focused on structural characterization of nifedipine crystal forms and the effect of pharmaceutical processing on the solid-state stability. The objective of this study was therefore to investigate the solid-state stability and crystal transformations of a nifedipine polymorph, a 1,4-dioxane solvate, and an amorphous form upon suspension in an aqueous medium and grinding by pulverization in a mortar with a pestle. In this study, changes in the crystal properties of the three nifedipine species during these processes are related to differences in their structural and physicochemical properties.
The amorphous form was prepared according to the method described by Aso et al.4 Approximately 2 g nifedipine raw material was placed in a vessel made of aluminum foil and kept at 1808C for 20 min, followed by rapid cooling in a mixture of acetone and dry ice until the amorphous material precipitated. The amorphous material was used as prepared or pulverized using a mortar and pestle.
Scanning Electron Microscopy A Philips XL 30 scanning electron microscope (Philips, The Netherlands) was used to obtain photomicrographs of the various nifedipine crystal forms. Samples were adhered to a small piece of carbon tape mounted on a metal stub and coated under vacuum with carbon (Emscope TB500 sputter-coater) before being coated with a thin gold-palladium film (Eiko Engineering ion Coater IB-2). X-ray Powder Diffraction X-ray powder diffraction (XRPD) patterns (Figure 1) were obtained at room temperature on either a Philips PM 9901/00 (Philips, The
MATERIALS AND METHODS Preparation of Crystal Forms Rolab (Kempton Park, South Africa) generously donated Nifedipine powder, batch number 950361. Solvents used for recrystallization of nifedipine were of analytical reagent grade. From this powder three different crystal forms of nifedipine were prepared, namely a polymorph, a 1,4dioxane solvate, and an amorphous form. Modification I is the commercially available product, and was used as such. It was also crystallized from acetone, chloroform, ethyl acetate, and methyl chloride. To prepare the 1,4-dioxane solvate, nifedipine powder was added to 1,4-dioxane under constant stirring until a saturated solution was obtained. The solution was left at room temperature (208C) to slowly evaporate for approximately 2 weeks. Recrystallized material was removed from the solvent using a spatula and dried on filter paper to ensure evaporation of surface solvent molecules.
Figure 1. X-ray powder patterns for: (a) the dihydrate of nifedipine, (b) the 1,4-dioxane solvate, (c) Modification I, and (d) pulverized amorphous powder.
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Netherlands) or Bruker D8 Advance diffractometer (Bruker, Germany). The isothermal measurement conditions were: target, Cu; voltage, 40 kV; current, 30 mA; divergence slit, 2 mm; antiscatter slit, 0.6 mm; receiving slit, 0.2 mm; monochromator; detector slit, 0.1 mm; scanning speed, 28/min (step size 0.0258, step time, 1.0 s). Approximately 300 mg samples were weighed into aluminium sample holders, taking care to avoid introducing preferred orientation of the crystallites. The XRD traces of the samples (powders or crystals) were compared with regard to peak position and relative intensity, peak shifting, and the presence or lack of peaks in certain angular regions. Variable temperature XRPD patterns were recorded with an Anton Paar TTK 450 low-temperature camera (Anton Paar, Austria) attached to a Bruker D8 Advance diffractometer (Bruker, Germany). A heating rate of 108C/min was used during all determinations.
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oil (Fluka, Switzerland). Photographs were taken using a Pentax K 1000 SLR camera. Infrared Spectroscopy IR spectra of powdered samples were recorded on a NexusTM 470 spectrophotometer (Nicolet Instrument Corporation, Madison, WI) over a range of 4000–400 cm1 with the Avatar Diffuse Reflectance smart accessory or the KBr disc technique. For diffuse reflectance analysis, samples weighing approximately 2 mg were mixed with 200 mg of KBr (Merck, Darmstadt, Germany) by means of an agate mortar and pestle, and placed in sample cups for convenient, fast sampling. For the compressed disk technique samples weighing approximately 2 mg were mixed with 200 mg KBr (Merck, Darmstadt, Germany) by means of an agate mortar and pestle. Discs were pressed using a Beckman 00-25 press (Beckman, Scotland) at a pressure of 15 103 kg/cm2. Single Crystal X-ray Structural Analysis
Thermal Analysis Thermal analysis methods used in this study included differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and hot stage microscopy (HSM). DSC traces were recorded with a Shimadzu DSC-50 instrument (Shimadzu, Kyoto, Japan) or a DSC 2920 modulated DSC (TA Instruments, New Castle, DE). Samples weighing 3–5 mg were heated in crimped aluminum cells at a rate of 108C/ min under nitrogen gas flow of 35 mL/min. TGA analysis was performed on all samples indicated by DSC as being possible solvates or hydrates. TGA traces were recorded with a Shimadzu TGA-50 instrument (Shimadzu, Kyoto, Japan) or Hi-Res Modulated TGA 2950 (TA Instruments, New Castle, DE). The sample weight was approximately 5–8 mg and heating rates of 1–128C/min under nitrogen gas flow of 35 mL/min were used. HSM analysis was carried out on small amounts of samples with a Leitz Wetzlar Laborlux K thermomicroscope (Leitz Wetzlar, Germany), equipped with a Metratherm 1200 heating unit. The effects of temperature increase on the crystal behavior of the samples were studied by placing approximately 3 mg of each on an object glass, covering it with a cover slip, and gradually increasing the temperature to about 2008C at a heating rate of 108C/min. Dehydration and desolvation were observed with samples immersed in silicone
Bright yellow, prismatic crystals of the 1,4-dioxane solvate were examined microscopically and were found to have excellent quality for X-ray diffraction analysis. Slow spontaneous loss of included solvent was observed following removal of the crystals from their mother liquor, and therefore, all further experiments were carried out either with the crystals immersed in mother liquor or otherwise protected. X-ray photography (oscillation and precession ˚) methods) using CuKa radiation (l ¼ 1.5418 A revealed only a center of symmetry in the diffraction pattern, indicating the triclinic crystal system. Preliminary estimates of the unit cell volume were obtained from the photographic data. This volume was consistent with a solvated species after comparison with unit cell data for an unsolvated form of the drug.8 For intensity data collection, a single fragment of suitable size was cut from a large crystal specimen, which showed easy cleavage. Intensity data were collected on a Nonius Kappa CCD diffract˚ ) with ometer using MoKa radiation (l ¼ 0.71069 A the crystal mounted on a glass fibre, coated with Paratone oil, and cooled to 177(1)K in a continuous nitrogen stream by means of a Cryostream cooler (Oxford Cryosystems). These precautions were taken to optimize the quality of the X-ray data by minimizing crystal decomposition and reducing atomic thermal motion. Comparison of the unit cell
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data at ambient and low temperatures indicated that no phase change had occurred on cooling the crystal. A total of 14,828 reflection intensities were measured on 316 frames (20-s exposure per frame) by a series of j- and o-scans of 28 each, with the crystal at 40.0 mm from the detector. Datacollection strategy and data reduction followed standard procedures described by Otwinowski and Minor,9 using ‘‘Collect’’ data collection software, Nonius B.V. (1998). Accurate unit cell data were calculated by least-squares (LS) using all measured data. The structure was solved using program SHELXS-9710 and refined using the full-matrix LS procedure with SHELXL-97.11 Anisotropic thermal parameters were employed for nonhydrogen atoms and H atoms were treated isotropically with Uiso ¼ 1.2 times the Ueq value of the parent atom. In the final refinement, LS weights of the form w ¼ 1 / [s2)(Fo)2 þ (aP)2 þ bP], P ¼ [max(Fo2, 0) þ 2Fc2]/3 were employed. Solubility Measurements An amount of powder, enough to ensure that supersaturation could be obtained, namely 10 1 mg, were measured into 10-mL test tubes with screw caps. To each test tube, 5 mL Milli-Q water, whose pH was set at 1 using 0.1 N aqueous HCl, was added and the caps screwed on tightly. The test tubes were rotated at 60 rpm (Heidolph RZR-2000 rotator, Germany) in a thermostatically controlled (Julabo EM/4 thermostat, Germany) water bath at 30 18C. Samples were withdrawn and filtered through a 0.45-mm filter after 24 h. The concentrations of the filtered samples were determined by HPLC. The HPLC system consisted of a Hewlett Packard HP 1100 system with an HP 3395 integrator (Agilent, USA) equipped with a reverse phase Nucleosil1 C18 steel cartridge column (125 4 mm i.d., 5 mm particle size, Macherey-Nagel, Du¨ren, Germany). UV detection was set at 254 nm, the solvent flow rate at 0.7 mL/ min, column temperature at 208C and 20-mL samples were injected into the HPLC apparatus. DSC and HPLC analysis showed that the nifedipine was not decomposed during the time it took to measure the solubilities of the crystal forms (Figure 4, Tables 2 and 5). Results obtained from aqueous solubility studies were compared to identify possible differences between the various polymorphic forms. These results were analyzed statistically using the Newman-Keuls test (Statistica for Windows
5.1B) to determine the extreme of statistically significant differences. DSC traces of all samples were recorded before and after solubility determination to identify possible polymorphic transformations.
Karl Fischer Water Determination A Metrohm 831 Karl Fischer Automatic Coulometer (Metrohm, Switzerland) was used to determine trace amounts of water in the powdered samples. Results are the mean moisture content of 5 to 10 different amounts of the samples weighed to the nearest 0.1 mg with a validated balance.
RESULTS AND DISCUSSION Morphology of the Nifedipine Crystal Forms After preparation and recovery of the crystal forms, visual and microscopic evaluation (Figure 2) clearly showed differences in the morphology of the prepared powders. The crystals of Modification I (a polymorph) were not of a definite shape. The sample consisted of large particles with small particles in between, with a mean diameter of 2.79 2.26 mm. The solvate consisted of flatsurfaced crystals with well-defined and sharp edges. The mean particle size was 81.33 7.71 mm. The particles of the amorphous form were large glassy plates with a mean particle size of 52.18 6.53 mm that yielded irregular sharp-edged fragments when crushed.
X-ray Diffraction Analysis Each of the prepared crystal forms had a distinct and characteristic XRPD pattern as shown in Figure 1. Main X-ray diffraction peak angles and relative intensities for Modification I, the solvate, and hydrate are listed in Table 1. No discernible peaks were detected in the powder pattern of the fused nifedipine, confirming its amorphous nature. The formation of amorphous nifedipine with a diffuse XRPD pattern was attributed to the loss in crystallinity caused by the fusion and subsequent rapid cooling. A few peaks with measurable intensities were detected in the XRPD pattern of the amorphous form after grinding, as shown in Figure 3. Upon heating, the pulverized material converted to Modification I.
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Figure 2. SEM photomicrographs of: (a) Modification I, (b) the 1,4-dioxane solvate and (c) amorph.
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ed between the crystals and the raw material, the latter was used in further experiments. The endotherm at 1538C in the DSC trace of the solvate (Figure 4) is due to the loss of the solvent, 1,4-dioxane. Desolvation is followed by a melting endotherm at 1748C. Hot stage microscopy (HSM) confirmed desolvation as seen in Figure 5. A theoretical weight loss of 11.2%, calculated for a solvate consisting of two molecules of nifedipine sharing one molecule of 1,4-dioxane, corresponds to an experimental weight loss of 11.1%, as indicated by TGA (Figure 6) in the temperature range of 158– 1698C. Karl Fischer analysis confirmed a solvate, rather than a hydrate, because the mean moisture content of the crystals was only 0.097%. The DSC trace of the amorphous form before grinding (Figure 4) (Amorph A), exhibited an endothermic peak at 418C, which is the glass transition temperature. The exothermic peak at 818C is attributed to the transformation of the amorphous form to Modification I. The endothermic peak at 1698C is the melting point of this form. The ratio of the glass transition temperature and melting point of nifedipine amorph was 0.71 (¼ 314/442 K), and within the range (0.69–0.85) so far reported for other glassy compounds.12 No weight loss was observed during heating of the amorphous form. After grinding, the melting properties of the amorphous form changed; a small, sharp exothermic peak at 918C appeared while the glass transition at around 408C was much less marked. XRPD analysis (Figure 3) showed that upon grinding, some of the amorphous material was converted into crystalline material because small peaks began to appear in the XRPD pattern. This change evidently indicated a mixture of an intermediate form and Modification I. All attempts to isolate this intermediate form failed.
Thermal Analysis
Infrared Spectroscopy
A summary of the DSC data is given in Table 2. These data include all thermal events (i.e., desolvation and melting) and their corresponding heat requirements. The DSC trace of nifedipine raw material (Figure 4) showed a melting point of 1718C with no desolvation endotherms, indicating that Modification I is not a solvate but a polymorph. Modification I was also prepared by recrystallization from acetone, chloroform, ethyl acetate, and methyl chloride, and the melting point endotherm and enthalpy of fusion corresponded to that of the commercial sample. Because no significant differences could be detect-
The IR-spectra of all the nifedipine crystal forms were obtained with the KBr disc method and compared to identify significant differences. The main IR absorption peaks of the nifedipine crystal forms and their corresponding wavenumbers (cm1), are listed in Table 3. Absorption peaks for pure 1,4dioxane, described by Burger and Koller3 and Sadtler,13 and those found for the solvent in crystals of the 1,4-dioxane solvate are also listed. There are clear and definite differences between the IR-spectra of the three nifedipine crystal forms, especially in the region 2000–4000 cm1. Differences are also present in the carbonyl
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Table 1. Peak Intensity Values (I/I0) at Main PXRD Peak Angles (82y) of Nifedipine Crystal Forms Modification I Main Peaks 1 2 3 4 5 6 7 8 9 10
1,4-Dioxane Solvate
Dihydrate
82y
I/I0
82y
I/I0
82y
I/I0
11.77 19.59 24.65 11.93 16.20 19.77 8.08 10.41 22.72 25.90
100.0 88.4 77.4 72.0 58.6 56.5 55.2 51.0 47.4 43.9
23.95 7.91 23.75 19.56 23.62 24.15 24.24 15.96 20.17 12.79
100.0 89.9 65.1 61.7 45.7 22.9 22.2 21.9 16.2 13.9
24.31 19.49 8.00 25.73 24.50 22.64 19.81 11.62 10.29 14.62
100.0 98.0 97.6 89.6 85.3 78.1 78.1 74.2 67.5 67.0
stretching region (1500–1800 cm1), as well as in the fingerprint region below 1500 cm1. Some of the main absorption peaks of pure 1,4-dioxane are comparable to those in the spectrum of the solvate and these absorption peaks do not exist, or differ
drastically from those of Modification I and the fused amorphous form. The differences between the IR-spectra of the three nifedipine crystal forms correlate with the XRPD data, indicating that three distinctly different crystal forms of nifedipine were prepared. Single Crystal Structure of the 1,4-Dioxane Solvate
Figure 3. Variable temperature XRPD patterns showing the conversion of pulverized amorphous nifedipine to Modification I.
The crystal structure of Modification I was determined by Triggle et al.8 but this is the first report of the single crystal structure of a 1,4dioxane solvate of nifedipine. Crystal data and refinement details appear in Table 4. The conformation of the nifedipine molecule is shown in Figure 7, which includes the atomic numbering and the atoms represented by 50% thermal ellipsoids.14 The average planes through the dihydropyridine and phenyl rings are nearly orthogonal [interplanar angle: 88.21(4)8]. This conformation is remarkably similar to that found in the crystal structure of the polymorph of nifedipine described by Triggle et al.8 crystallizing in the space group P21/c with Z ¼ 4. Thus, because the interaction between the drug molecule and the included 1,4-dioxane molecule (see below) does not induce further conformational changes, it appears that the conformation shown in Figure 7 is a stable one. The unit cell contains two nifedipine molecules located in the general positions, and one 1,4dioxane molecule, which is necessarily located on a center of inversion on account of the centrosymmetric space group. The guest molecule is ordered, and adopts a chair conformation. Figure 8 shows the 2:1 complex unit in which each oxygen atom
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Table 2. DSC and TGA Data of the Three Nifedipine Crystal Forms with Respect to Thermal Events at a Certain Temperature with the Corresponding Heat Required
Crystal Form
Desolvation Endotherm (8C)
Heat (J/g)
Endo/ Exotherms (8C)
Melting Point Endotherm (8C)
Heat (J/g)
Modification I Solvate Amorph A Amorph B Dihydrate
— 153 — — 100–110
— 68 — — 28–32
— — 45; 80 40; 91–98 —
171 169 168 170 165
98 80 66 65 72
of the guest molecule is linked to a nifedipine molecule by an O HN hydrogen bond. This ˚ and the angle bond has O N 2.924(1)A NH O is 172(1)8. Figure 9 shows the crystal packing arrangement. Separate domains of host drug and guest 1,4-dioxane molecules are clearly evident. Using program Lazy Pulverix,15 we have used the output of the single crystal X-ray analysis to generate an idealized powder pattern for the 2:1 solvate. This pattern is shown in Figure 10a and serves as a reference pattern for future identification of this phase. The most intense peak (2y ¼ 24.688) corresponds to the (210) reflection. Inspection of packing diagrams shows that these are the planes populated by the dihydropyridine rings and their methyl and carbomethoxy groups as well as the oxygen atoms of the guest molecule.
Solid-State Stability of the 1,4-Dioxane Solvate Burger and Koller3 reported the melting points of four solvates prepared from 1,4-dioxane to be 169–1738C, 161–1638C, and 1358C, respectively. With the help of TGA curves they determined the thermal stability of the various solvates and found that the crystal lattice of their Form A was continuously diffused by dioxane between 70 and 1508C. The crystal lattice of Form B was also stable up to approximately 858C, the maximum dioxane release following at nearly 1508C. For the third form, C, the dioxane was released at about 408C, and at approximately 1008C the bulk of the 1,4-dioxane had already evaporated, and for the last form, D, dioxane was released between 50 and 1508C. Comparison of these results with the TGA and DSC results obtained in this study
Figure 4. DSC thermograms of Modification I, the 1,4-dioxane solvate, dihydrate and amorphous forms. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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Figure 5. Hot stage photographs showing the 1, 4-dioxane solvate at: (a) ambient temperature and (b) during desolvation at around 1538C.
(Table 2) showed that the onset of desolvation, between 135–1508C depending on the heating rate, and the melting point of the desolvated material, 1748C, of the 1,4-dioxane solvate pre-
pared in this study were higher. In addition, because there is no published X-ray data of the 1,4-dioxane solvates available for comparison, it was impossible to definitively characterize this solvate as either A, B, C, or D prepared by Burger and Koller.3 This shows that crystal solvates such as the 1,4dioxane solvate prepared in this study can evidently exhibit a wide range of behavior with regard to stability and desolvation. However, that described in this study has a well-defined composition and reproducible properties. A detailed study of the packing (Figure 9) shows that the guest 1,4dioxane molecules in the nifedipine solvate described here are located in isolated cavities in the crystal, successive cavities having their cen˚ ). ters separated by the length of the a-axis (&7.6 A The van der Waals envelopes of the guest mole˚ along this direction. cules are separated by 4.5 A Desolvation of the crystal must therefore involve complete disruption of the crystal structure shown in Figure 9 and should occur at relatively high temperature owing to strong host–guest hydrogen bonding, and location of guest molecules in isolated cavities. This is borne out in the DSC behavior, which shows the desolvation onset at around 1508C. This temperature exceeds the boiling point of the pure solvent 1,4-dioxane by approximately 508C, and indicates a very stable inclusion compound.16 The DSC exotherm that follows desolvation implies recrystallization of the desolvated species into a polymorph; this species eventually melts at around 1748C.
Figure 6. TGA thermograms of the 1,4-dioxane solvate at different heating rates. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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Table 3. Main Absorption Peaks in the IR-Spectra of the Different Nifedipine Crystal Forms, as Well as Main Absorption Peaks of 1,4-Dioxane, Found in the Literature and Obtained Experimentally Modification I
Amorph
Solvate
Dioxane
Lit.a
Exp.b
Lit.a
Exp.b
Lit.a
Exp.b
Lit.c
3333
3329
3346
3346
3102
3101
3106
3099
3296 3235 3101
2868
1701
1703 1680
1703 1680 1630
3292 3236 3099 2868 1701 1680 1653
1624 1383 1280
1378
1377 1254 1219
1250
1235
1150 1113 893 860S 831M 750
1130
1689 1649 1619 1380 1289
1684 1647 1622 1381
1236 1165
1226 1155
1239 1162
1217
1104
1118
1100
1114
746
858W 829M 744
1651 1620 1383 1282
858W 829M 748
750
891 863 833
a
Lit. ¼ Literature (Burger & Koller, 1996). Exp. ¼ IR-values obtained with the KBr disc method. Lit. ¼ Literature (Sadtler, 1967). W Weak peak, MMedium peak, SStrong peak. b c
The identity of the resulting polymorph was established in this study as follows: a sample of the 1,4-dioxane solvate was desolvated under controlled conditions (heated at 1308C under vacuum for 24 h), the residual material was then checked by thermogravimetric analysis and was found to be completely solvent-free. The X-ray powder pattern, Figure 10b, was then recorded. The XRD pattern for the known monoclinic polymorph was calculated from the crystal data for this form reported by Triggle et al.8 This is shown in Figure 10c, and matches closely the pattern of Figure10b. Differences in peak intensities are due to sample effects but the excellent match of the peak angular positions leaves no doubt that desolvation of the 1,4-dioxane solvate yields the known monoclinic polymorph. The desolvation kinetics of the nifedipine solvate was studied by subjecting the crystals to TGA heating rates of 5, 7, 10, 12, and 158C per minute. TGA traces for the solvate at different heating rates are shown in Figure 6. The activation energy (Ea) for the dehydration process was calculated from these TGA data according to the method described by Flynn and Wall.17 This
method involves the analysis of weight loss versus temperature at different heating rates (b) to determine the corresponding absolute temperatures at a constant weight loss (C). Graphs of negative logarithm of heating rates (expressed in8C/s) (log b) versus 1/T were plotted and the activation energy (Ea) calculated from the slope of the curves. The calculated activation energy (Ea) for the desolvation of the nifedipine solvate according to the TGA data was &102 kJ/mol. This value is similar to that of other 1,4-dioxane solvates determined by the Flynn and Wall17 method. For example, for the 1,4-dioxane solvate of 5-methoxysulfadiazine, which similarly has the solvent in isolated cavities, a value of approximately 94 kJ mol1 was obtained for the activation energy.18 Solid-State Stability of the Amorphous Form In contrast to crystalline materials, glassy or amorphous substances have a certain form and volume, but no specific internal structure, the constituent molecules occurring in an irregular array. Aso et al.4–6 studied the physical stability
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Table 4.
Crystal Data and Refinement Details C17H18N2O6 (C4H8O2)0.5
Complex Formula Formula weight Temperature Crystal system Space group Unit cell dimensions
Volume Z Density (calculated) Radiation, wavelength Absorption coefficient F (000) Crystal size Theta range for data collection Index ranges Reflections collected Observed reflections [I > 2s(I)] Independent reflections Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data) Largest diff. peak and hole
of amorphous nifedipine using isothermal microcalorimetry and 13C nuclear magnetic resonance relaxation time measurements as a function of temperature and humidity. The heat produc-
Figure 7. Atomic numbering and conformation of the nifedipine molecule in the crystal of the 1,4-dioxane solvate. Thermal ellipsoids are drawn at the 50% probability level.
390.39 177(1) K Triclinic P(1) ˚ a ¼ 7.6238(2) A ˚ b ¼ 11.1758(3) A ˚ c ¼ 11.8898(3) A a ¼ 73.524(2)8 b ¼ 73.417(2)8 g ¼ 75.603(2)8 ˚3 915.40(4) A 2 1.416 g cm3 ˚ MoKa, 0.71069 A 0.109 mm1 412.0 0.31 0.50 0.44 mm 2.34 to 27.518 9 h 9, 14 k 14, 14 l 15 14,828 6997 3951 full-matrix on F2 3951/0/258 1.039 R1 ¼ 0.0386, wR2 ¼ 0.0986 R1 ¼ 0.0523, wR2 ¼ 0.1070 ˚ 3 0.218, 0.624 eA
tion due to the crystallization of amorphous nifedipine and its transformation into a stable crystal form was detected at temperatures above its glass transition temperature. The rate of transformation increased as the temperature increased. As stated earlier variable temperature XRPD (Figure 3) at 108C/min of the pulverized material showed that some of the amorphous form converted to Modification 1 upon grinding and that it then completely converts to this form upon heating. DSC investigation also indicated that the
Figure 8. The 2:1 nifedipine–1,4-dioxane unit in the crystal of the solvate.
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Figure 9. Projection of the crystal structure of the 2:1 nifedipine–1,4-dioxane solvate. Four unit cells are drawn and hydrogen bonds are indicated as dotted lines.
thermal properties of the amorphous material differed before and after pulverization. As shown in Figure 4, the thermal features observed in the DSC trace of the fused material (amorph A) were
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very broad peaks with a prominent glass transition around 418C. A DSC curve for the pulverized material (amorph B) showed a much smaller glass transition with a sharper recrystallization around 91–988C of an intermediate form to Modification I. Samples of these two materials were kept at 25 and 408C, 60% relative humidity and protected from light. At 258C no change in the powders was observed but, as shown in Figure 11a and b, their transformation to Modification I differed significantly when stored close to the glass transition temperature at 408C. The pulverized material transformed into Modification I much more slowly because after 9 days a significant amount of the amorphous material was still detected by DSC. In contrast, no amorphous material was present in the fused material after 6 days. The transformation of the amorphous forms to Modification I, XRPD pattern similar to that shown in Figure 2c, was confirmed by variable temperature XRPD analysis (Figure 3). This was not a simple conversion because the appearance of other endo- and exotherms in the DSC traces (Figure 11) showed that for both the fused and pulverized material the transition to Modification I occurred through intermediates. All attempts to isolate these intermediates, including isolation on a hot stage microscope, recrystallization, and storage at elevated temperatures, failed but variable temperature XRPD patterns between 65–958C (Figure 3) showed the occurrence of peaks around 7 and 2482y similar to that reported by Hirayama et al.7 for a polymorph (Form B) they prepared by heating glassy nifedipine to 1108C. Based on DSC results this form is most probably also the same as Modification III reported by Burger and Koller.3 Solubility of Nifedipine Crystal Forms
Figure 10. X-ray powder patterns for: (a) the dioxane solvate of nifedipine, calculated from single crystal X-ray data, (b) the material obtained by desolvating the dioxane solvate, and (c) Modification I of nifedipine, calculated from single crystal X-ray data.
Two major problems hindering the effective use of nifedipine in dosage forms are its extremely low aqueous solubility and its chemical instability. Boje et al.19 reported the aqueous solubility of nifedipine (Modification I) as 5–10 mg/mL over the pH range 2.2–13. However, over this pH range the stability of nifedipine varies and is most stable at very low pH with a reported degradation halflife at a pH 1 of more than 20 h.20 For this reason the solubility of the three crystal forms was determined at pH 1. Solubility values are listed in Table 5. A statistical comparison of the solubilities of the different nifedipine crystal forms after 24 h, using the Newman-Keuls test, showed
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Figure 11. DSC thermograms showing the transformation of (a) Amorph A and (b) Amorph B at 408C.
Table 5. Saturation Solubility of the Nifedipine Crystal Forms in Water at 308C 18C and pH 1 Crystal Form Modification I 1,4-Dioxane solvate Amorphous form A Amorphous form B Dihydrate a
Solubility (mg/mL)
Assay (%)a
5.42 0.33 5.99 0.11 35.04 2.51 16.13 0.87 3.01 0.56
98.2 1.1 96.4 1.5b 97.2 1.6 97.6 2.1 97.8 0.8
HPLC analysis of undissolved material. 1,4-Dioxane solvate converted to the dihydrate.
b
that the solubilities of Modification I and the solvate are not significantly different ( p < 0.49166), but the fused amorphous form (A) is significantly more soluble (&6 times) than Modification I ( p < 0.00022) and the solvate ( p < 0.00024). After grinding, the solubility of the amorphous form decreased significantly to &16 mg/mL. However, this powder was still significantly more soluble than Modification I and the solvate. These results correlated with those reported for other metastable crystal forms, amorphous forms generally being more soluble.21,22 In addition HPLC
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Figure 12. HPLC chromatograms of undissolved material removed after solubility testing: (1) ground amorph B, (2) Modification I, (3) 1,4-dioxane solvate, (4) dihydrate, and (5–6) decomposed nifedipine obtained by leaving an aqueous suspension of Modification I exposed to light for 72 h.
analysis (Table 5 and Figure 12), confirmed that during solubility analysis nifedipine did not decompose. However, it is expected that metastable forms will return to the stable state, if the transformation is facilitated by an appropriate kinetic mechanism, such as recrystallization or conversion through contact with a solvent.22 To ascertain this, after solubility testing the powders that did not dissolve were recovered and DSC analysis performed. The glass transition (408C) seen for the fused and pulverized powders before the solubility measurement was not present after solubility testing. Only one endotherm at 1738C was present,
suggesting that during solubility testing the powders transformed to Modification I. In the DSC trace of the solvate after solubility testing, an endotherm at 1458C (about 108C lower than the desolvation temperature for 1,4-dioxane) appeared. This suggested transformation of the solvate to a hydrated form with a lower desolvation temperature. Because this conversion had not reached completion during the time course of the solubility test, it was not possible to quantify this transition during the solubility study. To investigate the transformation several samples of the solvate were kept in the solubility medium and analyzed by DSC and TGA after 24 and 48 h. TGA
Figure 13. TGA curves of the 1.4-dioxane solvate left in water with pH 1 showing the transformation of the solvate to a dihydrated form of nifedipine. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 92, NO. 12, DECEMBER 2003
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analyses (Figure 13) showed that the solvate had changed to another crystal form that loses about 10% weight at 105–1108C after 48 h in the aqueous solution, protected from light. Theoretically, if the solvate transformed into a dihydrated form the weight loss should be 9.4%, which is very close to the observed value. Karl Fischer analysis confirmed this, because the water content of the recovered crystals increased from 0.09 to 7.9%. HPLC analysis (Figure 12) excluded the formation of a dihydrate of a degradation product. The solubility of the dihydrated form (Table 5) was significantly lower than that of Modification I and the 1,4-dioxane solvate. Because this is the first report of a hydrated crystal form of nifedipine the transformation of the 2:1 1,4-dioxane solvate to the dihydrate was measured three times. The transformation occurred in water or water at pH 1, but not in dioxane: water, ethanol:water, methanol:water, or acetone:water mixtures. This transformation was also specific for the solvate because Modification I and the amorphous form were not transformed to the dihydrate in the same solvents. The hydrated form was unstable and converted quickly to Modification I on removal from the aqueous medium. Although the XRPD pattern of the hydrated form (Figure 2a) closely resembles that of Modification I (Figure 2c) there are small differences in peak angular positions and large differences in peak intensities. There are also three large peaks, at 24.308, 19.818, and 14.628 with relative intensities above 60%, which are not present in the XRPD patterns of the solvate or the monoclinic polymorph. The dihydrate appears to be a ‘‘pseudopolymorph’’ because the overall PXRD profile closely resembles that of Modification I with the peaks shifted to lower 2theta angle, which implies a larger unit cell, as expected when water is incorporated into the anhydrous crystal.
In summary, the relationships between the various forms of nifedipine, under the conditions reported in this study, are shown in Figure 14.
CONCLUSIONS A well-defined nifedipine–1,4-dioxane solvate with drug–solvent stoichiometric ratio 2:1 was isolated, and its crystal structure and thermal properties were characterized. DSC analysis showed that desolvation of this species involves complete disruption of the crystal structure at relatively high temperature. This is consistent with the crystallographic data showing that the guest 1,4-dioxane molecules are located in isolated cavities in the crystal. XRPD data simulated from single crystal structure analysis supported DSC results and also showed that upon desolvation, the solvate transformed into a monoclinic polymorph known as Modification I. However, TGA and Karl Fischer water determination showed that upon exposure to an aqueous medium the solvate transformed into a dihydrate. The solubilities of the 1,4-dioxane solvate, Modification I, and the dihydrate in water at pH 1 were in the order: 1,4-dioxane solvate Modification I > dihydrate. The solubility of nifedipine was increased approximately sixfold when transformed into an amorphous form by quenched fusion from the melt at 1508C in an acetone dry ice bath. DSC and XRPD analysis showed that the amorphous form was relatively stable at room temperature but converted to Modification I upon exposure to an aqueous medium and when stored at 408C for 6 days. When pulverized, the crystallinity of the amorphous form increased slightly, indicating partial transformation to Modification I and an intermediate form, Modification III. The pulverized material was more stable at 408C, and was still approximately three times as soluble as Modification I.
ACKNOWLEDGMENTS
Figure 14. Stability relationships between the various forms of nifedipine reported in this study.
This work was supported by grants from the National Research Foundation (Pretoria, South Africa) and the Louisiana Board of Regents Enhancement Program (LEQSF(2001-02)-ENH-TR82). Rolab, Pty.Ltd., Kempton Park, South Africa, is thanked for the generous supply of nifedipine. Research support from the University of Cape Town is also acknowledged.
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