Polymorphism of Indomethacin in Semicrystalline Dispersions

Article Cite This: Mol. Pharmaceutics XXXX, XXX, XXX−XXX

Polymorphism of Indomethacin in Semicrystalline Dispersions: Formation, Transformation, and Segregation Tu Van Duong,†,‡ David Lüdeker,† Pieter-Jan Van Bockstal,§ Thomas De Beer,§ Jan Van Humbeeck,⊥ and Guy Van den Mooter*,† †

Drug Delivery and Disposition, Department of Pharmaceutical and Pharmacological Sciences, KU Leuven, Campus Gasthuisberg O&N2, Herestraat 49 b921, 3000 Leuven, Belgium ‡ Department of Pharmaceutics, Hanoi University of Pharmacy, 13-15 Le Thanh Tong, Hoan Kiem, Ha Noi, Vietnam § Laboratory of Pharmaceutical Process Analytical Technology, Department of Pharmaceutical Analysis, Faculty of Pharmaceutical Sciences, Ghent University, Ottergemsesteenweg 460, 9000 Ghent, Belgium ⊥ Department of Materials Engineering, KU Leuven, Campus Arenberg, Kasteelpark Arenberg 44 b2450, 3001 Heverlee, Belgium S Supporting Information *

ABSTRACT: The crystallization of metastable crystal polymorphs in polymer matrices has been extensively reported in literature as a possible approach to enhance the solubility of poorly water-soluble drug compounds, yet no clarification of the mechanism of the polymorph formation has been proposed. The current work aims to elucidate the polymorphism behavior of the model compound indomethacin as well as the mechanism of polymorph selection of drugs in semicrystalline systems. Indomethacin crystallized as either the α- or τ-form, a new metastable form, or a mixture of the two polymorphs in dispersions containing different drug loadings in polyethylene glycol, poloxamer, or Gelucire as the result of the variation in the mobility of drug molecules. As a general rule, low molecular mobility of the amorphous drug favors the crystallization into thermodynamically stable forms whereas metastable crystalline polymorphs are preferred when the molecular mobility of the drug is sufficiently high. This rule provides insight into the polymorph selection of numerous active pharmaceutical ingredients in semicrystalline dispersions and can be used as a guide for polymorphic screening from melt crystallization by tuning the mobility of drug molecules. In addition, the drug crystallized faster while the polymer crystallized slower as the drug-loading increased with the maxima of drug crystallization rate in 70% indomethacin dispersion. Increasing the drug content in solid dispersions reduced the τ to α polymorphic transition rate, except for when the more stable form was initially dominant. The segregation of τ and α polymorphs as well as the polymorphic transformation during storage led to the inherent inhomogeneity of the semicrystalline dispersions. This study highlights and expands our understanding about the complex crystallization behavior of semicrystalline systems and is crucial for preparation of solid dispersions with reproducible and consistent physicochemical properties and pharmaceutical performance. KEYWORDS: solid dispersions, amorphous, semicrystalline, nucleation, crystallization, polymorphism, metastable, molecular mobility, interfacial energy, Gibbs energy, activation energy, indomethacin, polyethylene glycol, poloxamer

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INTRODUCTION The appearance of a growing number of poorly water-soluble drugs emphasizes the importance of solid dispersions as a powerful formulation strategy to enhance the solubility and dissolution rate of this class of active pharmaceutical ingredients (APIs).1,2 However, the low reproducibility and consistency in the quality of solid dispersions, leading to variations in the bioavailability, remain underlying hurdles for their extensive application.3 The mechanistic insight and comprehensive understanding are thus vital for adequate control of the performance of drug-carrier dispersions. In binary systems of APIs and carriers, each component might influence the behavior of the other and physicochemical © XXXX American Chemical Society

properties of both components dictate the overall performance of solid dispersions.4 In a recent study, we found that indomethacin (IMC) inhibited the crystallization of polyethylene glycol (PEG)5 due to the formation of hydrogenbonding between the drug and the carrier in the liquid (molten) state.6 The presence of PEG has been shown to either accelerate, slow down, or have no influence on the crystallization process of Received: Revised: Accepted: Published: A

October 25, 2017 January 3, 2018 January 7, 2018 January 8, 2018 DOI: 10.1021/acs.molpharmaceut.7b00930 Mol. Pharmaceutics XXXX, XXX, XXX−XXX

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Molecular Pharmaceutics APIs.7 It is therefore interesting to elucidate how PEG affects the crystallization behavior of IMC in their dispersions. The crystallization of metastable crystal polymorphs in the polymer matrix has been proposed as a possible mechanism to increase the solubility of poorly water-soluble APIs.3,8 The polymorphism of APIs in semicrystalline dispersions was widely reported in literature. For instance, Ford and Rubinstein found a mixture of form I (γ-form) and form II (α-form) of IMC in high drug-loading dispersions with PEG, while only α-IMC was observed in samples containing less than 75% drug.9 Allen and Kwan10 also reported the generation of the α-polymorph in 10% IMC dispersion, whereas Fini et al.11 and Valizadeh et al.12 described the crystallization of the drug into the γ-form in the sample containing the same drug content. Beside IMC, numerous APIs have been found to form polymorphs in semicrystalline dispersions, such as carbamazepine,13−15 diflunisal,16 nimodipine,17 chlopropamide,18 topiramate,19 probucol,20 mefenamic acid,21 acetaminophen,22 and ursolic acid.23 Surprisingly, there has been no clarification of the mechanism of polymorph formation of the APIs in these systems. The objectives of the current work are therefore to elucidate the polymorphism behavior of IMC as well as the mechanism of polymorph formation of APIs in general in semicrystalline dispersions.

and analyzed by Universal Analysis software (version 4.4, TA Instruments). Powder X-ray Diffraction (PXRD). Bulk samples for phase identification were analyzed by PXRD on a PANalytical X’pert PRO powder diffractometer using an energy dispersive X’Cellerator detector (PANalytical, Almelo, The Netherlands). Experimental conditions: Cu Kα1 radiation (λ = 1.540598 Å); 45 kV; 40 mA; scanning interval 4° ≤ 2θ ≤ 40° at a step size of 0.0167° and 200 s counting time per step; T = 25 °C in transmission geometry using Kapton polyimide foil sample holders. The X’pert Data Collector was used for data acquisition, and data analysis was performed using the X’Pert Data Viewer and X’Pert HighScore Plus (PANalytical, Almelo, The Netherlands). In addition, all samples for structure solution were analyzed using the following experimental conditions: scanning interval 2° ≤ 2θ ≤ 70° at a step size of 0.0167° and 800 s counting time per step; T = 25 °C in the capillary mode (borosilicate glass; 0.5 mm diameter). Indexing of the powder patterns was performed using NTREOR25 and further confirmed with DICVOL06.26 For space group determination, Pawley refinement was used for whole pattern fitting thereby extracting the integrated peak intensities and their correlations using the EXPO2014 software package. Attempted structure solution was performed in direct space using the simulated annealing approach as implemented in EXPO2014, allowing flexible torsion angles as well as degrees of freedom for rotation and translation, respectively, for each molecule. Ultraviolet−Visible (UV−vis) Spectroscopy. The UV−vis absorption spectra of IMC were recorded in the range of 200− 500 nm using a GENESYS 10S UV−vis spectrophotometer (Thermo Scientific, Rochester, NY, USA). Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were acquired on a Vertex 70 spectrophotometer (Bruker Optics GmbH, Ettlingen, Germany) utilizing a single attenuated total reflectance accessory covering a wavenumber range from 4000 to 400 cm−1. The final spectrum was the average of 64 scans accumulated using Bruker’s Opus software 7.0, taken at 4 cm−1 resolution. Raman Microscopy. The distribution of API and PEG in their dispersions was evaluated using a Raman Rxn1 Microprobe (Kaiser Optical System, Michigan, USA) equipped with an aircooled CCD detector. The laser wavelength employed was 785 nm from an Invictus NIR diode laser having a laser power of 400 mW. The selected areas of samples were scanned by a 10× and 50× lens (spot size of 50 and 10 μm, respectively) using an exposure time of 5 s and 3 accumulations. Data collection were automated using HoloGRAMS data collection software (version 2.3.5, Kaiser Optical Systems). Polarized Light Microscopy (PLM). Sample morphology and birefringence were investigated using an Olympus BX60 polarizing optical microscope (Olympus, Hamburg, Germany) equipped with a THMS600 hot stage and a TMS93 temperature controller (Linkam Scientific, Tadworth, UK). The presence of birefringence under polarized light was visualized and captured using a digital camera. Viscosity Measurements. Steady shear viscosity of PEGIMC dispersions as a function of temperature was determined with oscillatory measurements on a ARES strain-controlled rheometer (TA Instruments, New Castle, DE, USA) using a 25 mm cone geometry with a cone angle of 0.04 radians. Strain sweeps were performed to determine the linear regime. The

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EXPERIMENTAL SECTION Materials. The γ-form of IMC was purchased from Fagron (Saint-Denis, France) while the α-polymorph of the drug was prepared by the method described by Kaneniwa et al.24 Amorphous IMC was produced by melting γ-IMC at 165 °C during 3 min, followed by quench cooling in liquid nitrogen. The material was then stored over phosphorus pentoxide at −25 °C. PEGs with the molecular weight of 6000 g/mol (PEG6000) and 400 g/mol (PEG400) (PEG is the abbreviation for PEG6000 unless otherwise stated) were obtained from Sigma-Aldrich (Geel, Belgium). The other semicrystalline polymers, namely poloxamer 407 (Lutrol F127) and Gelucire 50/13 were purchased from BASF (Ludwigshafen, Germany) and Gattefossé (Saint-Priest, France), respectively. Sample Preparation. Dispersions of IMC and polymers were prepared by heating the mixture of the two components to 165 °C under stirring. The samples were kept at this temperature for 3 min to ensure complete melting, followed by cooling the melt in ambient atmosphere to room temperature and storage in a desiccator containing phosphorus pentoxide. Modulated Differential Scanning Calorimetry (m-DSC). Thermal properties of dispersions were analyzed on a Q2000 mDSC (TA Instruments, Leatherhead, UK), equipped with a refrigerated cooling system RCS90 under inert dry nitrogen purge at a flow rate of 50 mL/min. Indium and n-octadecane were used for temperature calibration. Melting enthalpy and heat capacity were calibrated using indium and sapphire disks, respectively. The samples were crimped in aluminum pans and were then subjected to heating from 20 to 165 °C with a heating rate of 1 °C/min. For determination of the glass transition temperature (Tg), the samples were kept isothermal at 165 °C for 3 min and subsequently cooled to −75 °C at a cooling rate of 20 °C/min. Afterward, the samples were subjected to heating from −75 to 165 °C using an underlying heating rate of 5 °C/min with a modulation amplitude and period of ±0.636 °C and 40 s, respectively. All samples were analyzed in duplicate. DSC thermograms were acquired using Thermal Advantage software B

DOI: 10.1021/acs.molpharmaceut.7b00930 Mol. Pharmaceutics XXXX, XXX, XXX−XXX

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Molecular Pharmaceutics

Figure 1. PXRD diffractograms of PEG−IMC dispersions containing different drug loadings in the 2θ range of 4−40° (A) and in the zoomed region from 4 to 19° (B).

PEG−IMC cocrystals. As mentioned earlier, drug polymorphism in dispersions with PEG has been widely reported. In contrast, cocrystals of small molecular drugs and PEG are uncommon yet have been found in certain cases, including dispersions of the polymer and hydroxybenzenes,31,32 mavacoxib,33 and griseofulvin.34 In order to ascertain the nature of IMC in the 60% drug sample, a dispersion of the drug and liquid PEG400 containing the identical drug-loading was prepared. PXRD data of the dispersions of IMC in PEG6000 and PEG400 (Figure 2) illustrate indistinguishable diffraction patterns despite the vanishing of PEG6000 signals, most notably at 19.2 and 23.3°, in the dispersion of IMC with liquid PEG400.

shear rate was set at 0.1/s. The measurement was carried out in the temperature range between 165 and 25 °C at a cooling rate of 2 °C/min. High-Performance Liquid Chromatography (HPLC). Drug concentrations were determined by HPLC-analysis using a Merk-Hitachi LaCrom system consisting of a D-7000 interface, a L-7420 UV−vis detector, a L-7200 auto sampler, and a L-7100 pump. A Denali C18 120A 15 μm (250 mm × 4.6 mm) column from Grace Davison Discovery Sciences (Deerfield, Illinois, USA) was utilized. The mobile phase was 20 mM phosphate buffer pH 6.8:acetonitrile (55/45 v/v). The buffer was filtered over cellulose acetate filters (0.45 μm) purchased from Sartorius Stedim Biotech GmbH (Göttingen, Germany) and subsequently sonicated for 30 min before use. The injection volume and flow rate were 20 μL and 1 mL/min, respectively. The detection wavelength was set at 252 nm. Data acquisition was performed using Merck LaChrom D-7000 system Manager Software (version 4.1).

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RESULTS Formation of a New IMC Polymorph. Figure 1 shows the PXRD diffractograms of PEG−IMC dispersions containing various drug-loadings. In samples consisting of up to 50% IMC, the drug crystallized as the α-form. When the weight fraction of IMC increased to 60%, the diffractogram showed substantial new Bragg peaks, e.g., at 4.1, 5.4, 6.0, 7.4, 8.2, 10.6, 12.3, 16.5, 17.7, and 18.2°, indicated by vertical lines (Figure 1). No Bragg reflections indicating the presence of α-IMC can be observed in this dispersion. Samples containing 70−90% IMC displayed both new Bragg peaks found in the 60% drug sample as well as signals corresponding to α-IMC with intensities of the latter becoming more dominant while those of the former diminished as the drugloading increased. IMC has been reported to exist in various polymorphic crystal forms, namely α, β, γ, δ, ε, ζ, and η.27−30 The γ-form is the thermodynamically stable polymorph while α-IMC is the most commonly observed metastable form. None of the reported IMC polymorphs show Bragg peaks at the positions found in the diffractogram of the 60% drug sample, indicating the formation of a new IMC entity in this dispersion. The distinct X-ray diffractogram of 60% IMC dispersion most likely indicates the formation of either a new IMC polymorph or

Figure 2. PXRD diffractograms of dispersions containing 60% IMC in PEG6000 and PEG400.

Spectroscopic investigation revealed strong hydrogen-bonding between IMC and PEG in their molten dispersions.6 Nevertheless, the drug-carrier interactions were disrupted upon solidification and polymer crystallization. The absence of PEG− IMC interactions in the solid state combined with the appearance of identical X-ray diffraction peaks in dispersions of IMC with solid PEG6000 and liquid PEG400 reject the possibility of PEG− IMC cocrystals formation. The new crystalline IMC phase in the 60% drug dispersion with PEG must therefore be a new C


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sample where a mixture of mainly α-IMC and a small fraction of the τ-form was detected (Supporting Information, Figure S4A,B). Crystal Structural Aspects of τ-IMC. To date, the growth of single crystals of the τ-form suitable for single crystal X-ray analysis was unsuccessful and merely yielded polycrystalline samples. Additionally, attempted isolation of τ-IMC from PEG400 by washing the 60% IMC dispersion with water and acidic solution was unsuccessful due to the rapid transition of the τ-form to α-IMC. For these reasons, ab initio structure solution from PXRD data was attempted. Indexing of the diffraction data considering the most intense 20 reflections in the range of 2° ≤ 2θ ≤ 25° provided monoclinic cell parameters: (a = 21.9493 Å; b = 4.7693 Å; c = 16.8264 Å; α = 90.000°; β = 102.078°; γ = 90.000°; V = 1722.44 Å3). Subsequent space group determination yielded the space group P21 (the same space group as for α-IMC) as the most probable solution. The structure-free fit converged with an excellent agreement of experimental and theoretical data (weighted profile Rwp = 3.6%; Figure 3). The theoretical spatial volume containing a single molecule can be calculated from volume increments considering the sum formula of IMC (C19H16Cl1N1O4: 427.97 Å3). In addition, similar values can be deduced from the known crystal structures of α- and γ-IMC by dividing the cell volume by the number of molecules (α-IMC: 2491.95/6 = 415.33 Å3; γ-IMC: 847.87/2 = 424.93 Å3). Consequently, a theoretical cell volume of V = 1722.44 Å3 suggests the presence of four molecules in the unit cell (Z = 4). Taking into account the multiplicity of the determined space group (M = 2), the asymmetric unit of the crystal structures likely contains two molecules (Z′ = 2). At this point, it should be stated that attempted structure solution using direct-space methods like simulated annealing did not result in reliable crystal structure solutions. This may be caused by several reasons, such as indexing failures due to insufficient resolution and peak overlap, too many degrees of

polymorphic form of the drug that was named the “τ-form”. Since liquid PEG400 causes no Bragg scattering in X-ray diffractograms, all reflections of 60% IMC sample in PEG400 in Figure 2 can be assigned to τ-IMC (Table 1). Table 1. Unique X-ray Diffraction Peaks of Indomethacin Polymorphs polymorphic form

unique X-ray diffraction peaks [2θ (deg)]

τ α γ δa ξa ηa

4.1, 5.4, 6.0, 7.4, 8.2, 10.6, 12.3, 16.5, 17.7, 18.2 6.9, 8.5, 11.5, 11.9, 13.9, 14.2, 17.6, 18.0 10.2, 11.7, 16.7, 19.6, 20.5, 21.9 9.6, 10.5, 11.3, 13.0, 14.9 6.5, 11.0, 11.8, 12.8, 14.4, 16.4 9.1, 9.3, 12.2, 18.2, 20.5

Data obtained from Surwase et al.29 bThe ε form is not sufficiently stable for X-ray diffractogram to be recorded.29 Nobody is able to reproduce the β-form as reported by Yamamoto35 which was thereafter considered a benzene solvate.30 a

HPLC analysis provided comparable chromatograms of the τ-, α-, and γ-polymorphs with a sole peak at the retention time of ca. 3.8 min and peak areas within less than 2.0% variation (Supporting Information, Figure S1). In addition, all three polymorphs exhibited identical UV−vis spectra in the range of 200−500 nm (Supporting Information, Figure S2), eliminating the degradation of IMC during sample preparation and measurements as a possible cause for changes in the PXRD data. The τ-polymorph also appeared in dispersions of IMC and other semicrystalline polymers, such as poloxamer 407 and Gelucire 50/13. In dispersions with poloxamer, only τ-IMC was formed in the 60% drug sample, similarly to the behavior of the API in the dispersion with PEG containing the same drugloading. When the drug content increased or decreased, a mixture of τ- and α-IMC was observed (Supporting Information, Figure S3A,B). In Gelucire-based dispersions, IMC crystallized as the τ-polymorph at all drug-loadings, except in 90% drug

Figure 3. Structure-free fit of the monoclinic unit cell (V = 1722.44 Å3) in the space group P21 of τ-IMC. Red crosses: observed data points; solid blue line: calculated intensity; solid black line: background/difference (Yobs − Ycalc); blue tick marks: theoretical τ-IMC phase. D


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Figure 4. FTIR spectra of IMC polymorphs in the spectral region between 4000 and 400 cm−1 (A) and in the carbonyl stretching region (B).

freedom hindering the simulated annealing algorithm to find the global minimum, dynamical occupancy, or strong preferred orientation influencing the PXRD pattern. Especially, the latter one likely is an issue as τ-IMC was in situ crystallized from the melt of IMC and PEG400 in the capillary because of the problematic packing of tacky and viscous mixture of crystalline τIMC and PEG400. Referring to potential peak overlap and insufficient resolution, synchrotron radiation may improve the quality of the data. Since polymorphism of organic molecules has an impact on hydrogen-bonding motives and strengths, infrared spectroscopy may reveal additional structural insights. In the case of IMC, the functional groups involved in intermolecular hydrogen-bonding are suitable to probe the occurring polymorphism. PEG400 does not significantly influence the FTIR spectra of IMC, especially in the carbonyl stretching region, as it can be observed for α- and γ-IMC (Supporting Information, Figure S5). Thus, the carbonyl stretching region of 60% IMC dispersion in PEG400 can be assigned to the primitive τ polymorph. Figure 4 shows the FTIR spectra of various IMC polymorphs. IMC exhibits rather broad OH vibration bands in the region of 3400 to 2500 cm−1. In addition, this stretching region is superimposed by CH bands (Figure 4A). The differences in the shape of these bands likely suggest variations in hydrogenbonding pattern in different IMC forms. The molecular structure of IMC comprises a carboxylic acid and an amide carbonyl group that are capable of formation of hydrogen-bonding. The differences in the carbonyl stretching region are the results of various hydrogen-bonding arrangements in IMC polymorphs. The crystal structure of γ-IMC comprises one molecule in the asymmetric unit (Z′ = 1)36 forming intermolecular R22(8) carboxylic acid homo synthons (Figure 5), resulting in a single vibration mode for the amide carbonyl group at ν = 1690 cm−1 and another stretching band for the carboxylic acid carbonyl group at ν = 1713 cm−1 (Figure 4B).37 The crystal structure of α-IMC consists of three molecules in the asymmetric unit (Z′ = 3) involving a R22(8) carboxylic acid homo synthon.38 In addition, a third molecule connects to one of the amide oxygen atoms of the dimer involving discrete COOH···OCamide hydrogen bonds (Figure 6). For this polymorph, four different vibration modes in the carbonyl stretching region can be observed, which can be assigned to the

Figure 5. Hydrogen-bonding motive in the triclinic crystal structure of γ-IMC (Z′ = 1; Z = 2): two IMC molecules are connected via a R22(8) carboxylic acid homo synthon.

Figure 6. Hydrogen-bonding motives in the monoclinic crystal structure of α-IMC (Z′ =3; Z = 6): two IMC molecules are connected via a R22(8) carboxylic acid homo synthon. In addition, a third molecule connects to one of the amide oxygen atoms of the dimer involving discrete COOH··· OCamide hydrogen bonds. Short contacts to chlorine (green) and oxygen (red) atoms are indicated via red dashed lines.

non-hydrogen-bonded amide carbonyl group (1691 cm−1), the hydrogen-bonded carboxylic acid carbonyl group, or the hydrogen-bonded amide carbonyl group (1680 or 1649 cm−1) and the non-hydrogen-bonded carboxylic acid carbonyl group of the third molecule in the asymmetric unit (1734 cm−1).37 Amorphous IMC basically possesses R22(8) carboxylic acid dimers (1707 cm−1) and a minor fraction of discrete COOH···O = CN hydrogen bonds (CNO: ν = 1679 cm−1; HOCO: ν = 1735 cm−1).37,39 E


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Figure 7. Crystallinity of (a) PEG and (b) IMC in their dispersions as a function of drug-loading and storage time at room temperature.

The IR spectrum of τ-IMC appears to resemble that of amorphous IMC with a strong vibration band at 1676 cm−1 that can be assigned as amide carbonyl, a second peak at 1696 cm−1 corresponds to the asymmetric stretch of hydrogen-bonded acid carbonyl, and a rather weak signal of non-hydrogen-bonded acid carbonyl at 1733 cm−1. The presence of stretching vibrations of not only hydrogen-bonded but also non-hydrogen-bonded acid carbonyl indicates at least two hydrogen-bonding interactions in the τ-crystals, one between two carboxylic acid groups and the other between the carboxylic acid group and the amide carbonyl moiety. In other words, there should be no less than three IMC molecules in a unit cell of the crystals, which is consistent with the theoretical cell volume that suggests the presence of four molecules in the unit cell. Crystallization Kinetics of PEG and IMC as a Function of Drug-Loading. The crystallization process of PEG and IMC in their dispersions containing different drug loadings was monitored via measuring the crystallinity of the two components during storage. The crystallinity of α-IMC, τ-IMC, and PEG was calculated from the ratio of the background corrected intensities of the distinct X-ray diffraction peaks at 8.5, 10.6, and 19.2°, respectively, after a certain storage time to those of one month old samples when the crystallization was considered completed. In samples containing both α-IMC and τ-IMC (70−90% dispersions), the ratio of the peak intensities at 8.5° and 10.6° remained almost unchanged during the crystallization process, hence, only the intensity of the dominant Bragg peak was included in the calculation. Figure 7 shows the crystallinity of PEG and IMC in their dispersions as a function of drug-loading and time at room temperature. PEG generally crystallized slower as the drugloading increased which was consistent with the crystallization inhibition of IMC on PEG reported previously.5 Samples containing up to 50% IMC exhibited immediate PEG crystallization without induction time, whereas the first signal of the polymer at 19.2° in 60−70% drug dispersions only appeared after ca. 24 h. When the drug-loading increased to 80 and 90%, the induction times prolonged to ca. 48 and 96 h, respectively. For the drug, the crystallization rate increased with the IMCloading due to the higher driving force for crystallization. However, when %IMC rose to 80% and especially 90%, the drug crystallized more slowly. Amorphous IMC did not crystallize

within the time frame of experiments. Notably, the maxima in the crystallization rate of the drug appeared in 70% IMC dispersion. Polymorphic Transformation. The τ-form of IMC is metastable to the α-polymorph as the τ to α transition occurred during storage. At room temperature, the transition process was remarkably slow with negligible change after 6 months (data not shown). In order to accelerate the transformation, samples were kept at room temperature during one month to allow complete crystallization, followed by storage at 55 °C, which is slightly lower than the melting point of PEG (ca. 60 °C). PXRD diffractograms of the dispersion containing 60% IMC in Figure 8 show that upon storage at 55 °C, the Bragg peaks

Figure 8. PXRD diffractograms of 1-month-old 60% IMC dispersion during storage at 55 °C.

corresponding to τ-IMC (indicated by dash lines) diminished concurrently with the evolution of α-IMC signals (indicated by solid lines). No signals of the τ-form could be detected after 3 months, indicating the complete τ to α transition. Figure 9 shows τ to α transformation kinetics as a function of drug-loading and storage time. The degree of polymorphic transformation at time t, denoted as ϕt, is given by the following equation: F


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The trend was, however, opposite in samples containing 80− 90% drug: the τ to α conversion in these dispersions was much faster than in samples with lower drug contents. Polymorphic Segregation. In samples containing at least 70% IMC, there exists a mixture of τ- and α-polymorphs of the drug as shown by PXRD diffractograms in Figure 1. It would be interesting to find out how the two polymorphs were distributed in the dispersions. Under the polarized light microscope, the dispersion exhibited a complex structure with mainly large and round spherulites surrounded by a crystalline matrix (Figure 10). Additionally, smaller compact spherulites with different morphology (indicated by the arrows) were also observed. Raman spectra recorded at different spots in the sample to interpret their nature (Figure 11) demonstrate that spots inside the large and round spherulites (A to D) exhibited signals of the τ-form while α-IMC was detected in smaller compact ones (G to J). The matrix surrounded spherulites (spots E, F) consisted mainly of the τ-polymorph with minimal traces of the α-form. The nature of aforementioned spots was confirmed by examining the melting process of the dispersion (Figure 12). The DSC thermogram showed the first endothermic event at 57 °C corresponding to the melting of PEG, immediately followed by a shallow endothermic dip extending until the melting point of τ-IMC at 136 °C. This endothermic dip coincided with the slow dissolution of the drug in molten polymer as revealed by polarized light microscopy. The highest endotherm at 148 °C was assigned as the melting of α-IMC spherulites. Molten PEG exhibited no birefringence under polarized light but the contours of liquid droplets of the polymer were observable (Figure 13). PEG appeared to form a layer on top of the IMC ground, suggesting drug-polymer phase separation.

Figure 9. τ to α transformation kinetics as a function of drug-loading and storage time at 55 °C.

ϕt = 100 ×

Iα Iα + Iτ

(1)

where Iα and Iτ are the background corrected intensities of distinct Bragg peaks of α-IMC at 10.6° and τ-IMC at 8.5°, respectively. The intensity ratio was preferred as opposed to individual peak intensities for monitoring the transformation process in order to compensate for potential variations in sample irradiance. ϕt varies from 0%, when the samples contains solely the τ-form, to 100%, when the τ to α transition has finished. The dispersions containing 60−68% IMC exhibited no traces of α-IMC at room temperature. Upon storage at 55 °C, the percentage of the α-form increased gradually until 76 days. From 82 days, when ϕt approached 50%, meaning that α-IMC was becoming the dominant polymorph in the samples, the transformation was much more rapid. The rate of transition was in general inversely proportional to the weight fraction of the drug. This trend continued in 70−76% IMC dispersions that already contained certain fractions of α-IMC from the beginning. For instance, the ϕt of 70% IMC dispersion increased from 26.6% at day 0 to 56.0% after 132 days, whereas the sample containing 76% drug had higher initial ϕt of 38.5% but it rose to only 45.9% during the same period.

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DISCUSSION Polymorph Formation and Crystallization Kinetics. Based on the PXRD data, Figure 14 summarizes the polymorphism of IMC as a function of drug-loading. Briefly, in dispersions containing up to 54% IMC, the drug crystallized as the α-form whereas in 60−68% IMC sample, τ-polymorph was generated. The mixture of τ- and α-IMC was found in samples containing 56−58% drug. As the drug concentration further increased to above 68%, τ-IMC diminished coincidentally with the increase of the α-form. Pure amorphous drug crystallized as the γ-polymorph at room temperature.

Figure 10. Polarized light microscopic illustration of 70% IMC dispersions. G


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Figure 11. Raman spectra recorded at different locations in 70% IMC dispersion.

Figure 12. DSC thermogram (left) and polarized light microscopic illustration (right) of 70% IMC dispersions upon heating.

Figure 13. Microscopic illustration of molten droplets of PEG during the melting of 70% IMC dispersions.

the appearance of the least stable τ- and metastable α-IMC is governed by the kinetic driving force. The tendency of an amorphous drug to crystallize to a metastable form under conditions of kinetic control was expressed by Ostwald’s rule of stages, stating that “when leaving

Polymorphism of IMC in PEG-based dispersions must be explained in the context of the factors that determine crystallization kinetics. Thermodynamics only cannot elucidate this phenomenon because it always favors the formation of the most stable γ-polymorph with the lowest Gibbs energy. Instead, H


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⎛ s*ΔG* ⎞ ⎟ τ = τ∞exp⎜ c ⎝ kBTSc ⎠

Here, τ∞ is the relaxation time constant at infinite temperature, s*c is the critical entropy of the smallest group of molecules that can undergo a rearrangement, ΔG* is the activation energy, kB is the Boltzmann constant, and T is temperature. Given that the mobility of drug molecules in a crystal phase is constant at a certain temperature, the crystal−amorphous interfacial energy and Gibbs energy will also scale with the mobility of the drug in the amorphous state. In the 60% IMC dispersion, the amount of the drug was sufficient to inhibit the crystallization of the polymer to a significant extent;5 consequently, both IMC and PEG remained amorphous until few hours of storage. Owing to the low Tg (−17.9 °C) of the sample containing 60% drug (Supporting Information, Figure S6), the sample stored at room temperature, which is ca. 43 °C above the Tg of the dispersion, existed in the supercooled liquid state with considerably high mobility of drug molecules. As a result, once nucleated, the crystallization of IMC was rather fast (Figure 7B). More interestingly, the drug crystallized as the least stable τ-form (Figure 1). The formation of τ-IMC in the 60% dispersion is most likely due to the fact that the least stable τ-nucleus has the highest state of disorder compared to that of the metastable α- and stable γnuclei, and hence it has the lowest interfacial energy σ at the nucleus−amorphous matrix barrier. A greater decrease in Gibbs energy ΔGv occurs with crystallization to the α- or γ-form, but is not sufficient to overcome the more favorable σ term for the τIMC formation. Therefore, the nucleation barrier for the τpolymorph is lower than that for the α- and γ-forms. When the drug-loading was below 60%, the dispersions exhibited instant PEG crystallization (Figure 7A) because the amount of IMC was insufficient to inhibit the crystallization of the amorphous polymer in excessively low Tg systems (Supporting Information, Figure S6). The mobility of IMC molecules would therefore be low in the highly viscous crystalline polymer matrices as exhibited by the sharp escalation in the viscosity of the dispersions as a result of the crystallization of PEG during cooling (Figure 15). This leads to two consequences. First, the crystallization of the drug was retarded because of the lack of mobility required for nucleation; thus, it

Figure 14. Summary of IMC polymorph formation in dispersions with PEG as a function of drug-loading.

a given state and in transforming to another state, the state which is sought out is not the thermodynamically stable one, but the state nearest in stability to the original state”.40,41 This rule implies that during crystallization, the least stable form with the largest Gibbs energy whose structural organization most strongly resembles the short-range order of the amorphous state will appear first. In general, both nucleation and crystal growth kinetics might be responsible for polymorph formation during crystallization.42 Polymorph selection of IMC is, however, nucleation controlled as pointed out by Andronis et al.43 Therefore, Ostwald’s step rule is explained by the competition to nucleate among the polymorphs: the form with the lowest activation energy barrier for nucleation and highest nucleation rate will crystallize preferentially. According to the classical nucleation theory,44,45 the activation energy barrier for nucleation, ΔG*, can be expressed as follows: ΔG* =

16πσ 3 3ΔGv2

(3)

(2)

where σ is the interfacial energy at the crystal−amorphous interface and ΔGv is the Gibbs energy difference between the amorphous and crystalline phases. ΔGv reflects the negative bulk thermodynamic driving force for nucleation: the greater this parameter, the higher tendency for and rate of nucleation. ΔGv is always higher for the more stable form. The unfavorable positive interfacial energy, σ, represents the work required to create the crystal−amorphous interface and exhibits an opposite trend: the greater this value, the lower the rate of nucleation. In principle, σ should be lower for a metastable form whose structure is most readily derived from the arrangement of the amorphous state because the tension of the interface it forms with the amorphous phase, and hence the work necessary to construct the amorphous−crystal interface, is smaller. For IMC, it was reported that the crystal−amorphous interfacial energy is higher for the γ-polymorph than for the α-form.43 The nucleation barrier is the result of the interplay between the interfacial energy and Gibbs energy. The generation of metastable polymorphs is only possible under conditions where the nucleation process is kinetically favored over the thermodynamically preferred formation of the stable forms. In other words, the Ostwald’s rule of stages has mainly a kinetic origin modulated by the crystal−amorphous interface. In melt crystallization, the relative magnitudes of crystal− amorphous interfacial energy and Gibbs energy would reflect that they often scale as the energy46 or entropy47 difference between the two phases. The entropy difference between crystalline and amorphous phases is primarily configurational in nature.47 Within the context of the Adam−Gibbs theory,48 the molecular mobility, expressed as the reciprocal of the molecular relaxation time, τ, is related to the configurational entropy Sc by the equation:

Figure 15. Viscosity of PEG−IMC dispersions as a function of drugloading and temperature. I


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Molecular Pharmaceutics took 48−96 h, depending on the concentration of the drug, to detect the crystalline drug (Figure 7B). Second, nucleus− amorphous interfacial energy σ in lower drug-loaded dispersions decreased somewhat relative to the 60% dispersion. Therefore, the impact of σ on the nucleation of τ would be expected to be reduced in favor of the ΔGv term and the formation of α. As the thermodynamic driving force for crystallization of IMC increased with the drug-loading, the crystallization rate of the drug in 70% IMC dispersion became higher than that in the sample containing 60% IMC (Figure 7B). Generally, slow crystadllization favors thermodynamically stable crystalline forms whereas rapid transformation will produce metastable polymorphs.49 Thus, the crystalline form of IMC that is expected to nucleate in 70% dispersion would be the least stable τ-IMC. Despite of the fastest crystallization of IMC at this drug-loading, a fraction of metastable α-IMC developed along with the major portion of τ-form (Figure 1). The formation of the metastable αIMC fraction could be attributed to the decrease in the molecular mobility of the drug as reflected in the rapid increase in viscosity of ca. 1 order of magnitude (Figure 15). In this case, it is interesting to find out that thermodynamic driving force dominantly controls the nucleation and crystal growth, whereas the kinetic factor dictates the polymorph selection of the crystalline drug. The mobility of drug molecules in dispersions containing 80 and 90% IMC decreased drastically as revealed by a 2 and 4 orders of magnitude rise in viscosity, respectively, compared to that of the 70% IMC sample (Figure 15). Consequently, kinetics became the dominant factor controlling both the crystallization rate of IMC as well as the polymorph formation: the drug crystallized more slowly (Figure 7B) and preferably into the metastable α-form (Figure 1 and Figure 14). The fact that both τand α-IMC appear in dispersions containing 70−90% drug reflects the subtle balance in the interfacial energy and Gibbs energy. Pure amorphous IMC with strikingly high viscosity and Tg above room temperature was quite stable due to its extremely low molecular mobility. Consequently, the crystallization of glassy drug was observed only after months to years of storage, and the polymorph that formed was the stable γ-IMC. The hypothetical interplay between the interfacial energy and Gibbs energy as a function of molecular mobility is illustrated in Figure 16. When the mobility of drug molecules is relatively low, the difference in the crystal−amorphous interfacial energy (στ, L < σα, L < σγ, L) is less significant than the change in Gibbs energy for the transformation from the amorphous to the crystalline polymorphs (ΔGv, τ, L < ΔGv, α, L < ΔGv, γ, L). Therefore, the nucleation barrier will be lower for more stable forms (|ΔG*γ,L| < * | < |ΔGτ,L * |) and the slower the mobility of drug molecules |ΔGα,L is, the more stable polymorph is preferred. On the contrary, if the molecular mobility of the drug is sufficiently high, the discrepancy in the σ term (στ, H < σα, H < σγ, H) is much more pronounced than the change in the ΔGv term (ΔGv, τ, H < ΔGv, α, H < ΔGv, γ, H). Consequently, the activation energy barrier * |< for nucleation of less stable polymorphs will be lower (|ΔGτ,H |ΔG*α,H| < |ΔG*γ,H|) and the crystallization of the amorphous drug into metastable forms is favored. The model shown in Figure 16 provides an intuitive approach to predict the polymorph selection from melt crystallization based on the mobility of amorphous drug molecules. Briefly, stable polymorphs are thermodynamically preferred in the low mobility zone whereas metastable crystalline forms are kinetically favored when the molecular mobility of drug molecules is high.

Figure 16. Interplay between the crystal−amorphous interfacial energy and Gibbs energy as a function of molecular mobility. Key: L: low mobility; H: high mobility. Numerator and denominator refer to the eq 2.

The mixture of two polymorphs can occur if the molecular mobility lies between the low and the high mobility zones. This approach might help to conveniently interpret the crystallization behavior of IMC from the amorphous state below and above its Tg: the drug crystallized into the most stable γ-polymorph when stored below Tg due to the low mobility of drug molecules while the metastable α-form also appeared with γ-IMC at storage temperatures near and above Tg as the result of increased molecular mobility.43,50−52 In the same way, the stable γ-crystal form appeared below 43% RH, whereas the metastable α-crystal was generated at higher RH.47,53,54 The polymorph formation of IMC in dispersions with PEG 6000 was previously reported. Ford and Rubinstein9 found that in solid dispersions containing 80−90% IMC, a mixture of form I (γ-IMC) and form II (α-IMC) appeared. The proportion of the α-form increased at higher PEG concentrations and in samples containing more than 25% PEG, the drug crystallized into α-IMC only, which was consistent with the observation of Allen and Kwan that the α-polymorph occurred in 10% IMC sample.10 The formation of α-IMC in low drug-loading samples reported by these workers is compatible to our findings. Nevertheless, the appearance of γ and α mixture in 80−90% drug dispersions as well as the formation of α-IMC solely in the samples containing less than 75% drug are, to a certain extent, in contradiction to the data presented in this work. There might be some explanation for these discrepancies. First, the generation of α-IMC only in dispersions containing less than 75% drug, rather than α-IMC formed in samples consisting of less than 54% IMC and τpolymorph in 60−68% IMC dispersions, might originate from the transformation of τ-IMC (if any) to the α-form during the unrevealed storage period before the measurements. Second, Ford and Rubinstein ascribed the two endotherms between 140 and 160 °C in the DSC thermograms as the melting peaks of γ- and α-polymorphs without confirmation of the crystal forms by other techniques, such as PXRD. As shown in Figures 11 and 12, those endothermic events must be attributed to the melting of α- and τ-polymorphs. Another possibility is that in the work of Ford and Rubinstein, crystalline γ-IMC did not completely melt during sample preparation and the residual unmelted γ-nuclei induced the crystallization of a fraction of J


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Molecular Pharmaceutics amorphous drug into the γ-form along with the α-polymorph. The importance of residual crystalline drug in dictating the polymorphism formation from melt crystallization has been discussed by Mahieu et al.55 This research group reported that the polymorph of griseofulvin generated upon crystallization from the melt strongly depends on the time spent above its melting point. The drug (form I) melted during a short period crystallized to the most stable form I, triggering by crystalline traces of the initial material which persisted during the melting process. When the annealing time above the melting point of griseofulvin was sufficiently longer, all traces of the crystalline drug disappeared and crystallization of the melt into form I no longer occurred. Alternatively, the development of metastable form II and form III were observed.55 In another case study, amorphous IMC that was obtained by quench cooling molten γ-IMC or by grinding the γ-form, contained no nuclei after sample preparation and thus crystallized as γ-polymorph at temperatures below the Tg of the drug, conforming with Ostwald’s step rule. The amorphous drug produced by grinding α-IMC that was consisting of nuclei of the original compound transformed into the α-polymorph.54 These examples highlight the significance of residual crystalline nuclei in the initial amorphous state in the recrystallization behavior of drug compounds. Any remaining traces of the γ-form in molten PEG-IMC dispersions will thereby preferentially induce the crystallization of the drug into the most stable γ-polymorph as observed by Ford and Rubinstein.9 The appearance of IMC polymorphs in dispersions with PEG has been sometimes misinterpreted. Fini et al.11 reported the crystallization of the drug in 10% IMC dispersion with PEG (with the molecular weight of 4000, 5000, and 6000 g/mol) into the γ-form, which is incompatible with the formation of α-IMC in the identical dispersion in our current work. Although differences were obvious in PXRD diffractograms of 10% IMC dispersion and the corresponding physical mixture, the authors argued that these variations originated from the poor separation of Bragg peaks in the drug-carrier dispersion than in the physical mixture.11 In fact, the occurrence of the strong signal at ca. 12° and other distinct Bragg peaks, e.g., at ca. 11.5, 8.5, and 7.0°, were the evidence of the α-polymorph rather than γ-IMC in 10% dispersion (Table 1). Likewise, Valizadeh et al. reported that the crystalline nature of γ-IMC in 10% dispersion with PEG6000 prepared by the fusion method was still maintained although the crystal orientation changed and the quality of IMC crystals was reduced as revealed by the relative reduction of diffraction intensity.12 Nevertheless, the remarkable decrease in PXRD signal intensities of initial γ-IMC concomitantly with the development of new Bragg peaks at ca. 12° and 8.5° that was probably being overlooked apparently indicated the formation of the α-polymorph in 10% dispersion with PEG (Table 1). Polymorphism of APIs in semicrystalline dispersions has been widely reported without clear explanation of the mechanism of polymorph formation. A typical example is carbamazepine−PEG solid dispersions. El-Zein et al. found that in solid dispersions with PEG6000 prepared by fusion, carbamazepine exhibited higher dissolution rate which was attributed to the possible polymorphic change with the drug crystallizing in a metastable form.13 This was subsequently confirmed by Naima et al. that samples containing low amount of drug favors the crystallization of carbamazepine into the metastable form II while at drug loadings of more than 95%, both form II and stable form III were detected.14,56 The authors ambiguously argued that drug-carrier interactions appeared to be responsible for the crystallization of

the metastable carbamazepine form II from the molten mixtures with PEG. In the light of the polymorph selection model proposed in the current work (Figure 16), it is now obvious that the formation of metastable form II in dispersions containing low drug-loadings derives from relatively higher mobility of the drug molecules, whereas at drug concentrations above 95%, the viscosity of the systems increased and the mobility of drug molecules became lower, triggering crystallization of a fraction of amorphous carbamazepine into more stable form III along with metastable form II. For the same reason, diflunisal crystallized in metastable form III at low concentrations of the drug in dispersions with PEG4000 prepared by the melting method while stable polymorph I was mainly obtained as the drug content increased.16 The significance of mobility of drug molecules in polymorph selection during melt crystallization elucidates the polymorphism of IMC in semicrystalline dispersions with poloxamer 407 and Gelucire 50/13 as mentioned earlier. In the samples containing the same drug-loading of IMC in various polymers, the molecular mobility of the drug might be different due to the diverse microstructure and rigidity of the polymer matrices. The current model also explains the polymorph formation of nimodipine with PEG4000,17 chlopropamide in PEG3350,18 topiramate in PEG8000,19 probucol in poly(ethylene oxide) (molecular weight of 100 000 g/mol),20 mefenamic acid in a copolymer of polyoxyethylene−polyoxypropylene (Lutrol F68),21 acetaminophen in PEG3350, poloxaer 188 or poloxamer 407,22 and ursolic in PEG6000 or poloxamer 407.23 The controlling role of molecular mobility in polymorph selection, however, does not hold true during crystallization from solution. It was widely reported that IMC crystallized from aqueous environments into the metastable α-form rather than the most stable γ-polymorph despite its extremely high molecular mobility.57−59 In solid dispersions with poly (vinylpyrrolidone) (PVP), the polymorph selection during crystallization of amorphous IMC is strongly preparation method dependent. In IMC−PVP dispersions containing up to 5% PVP prepared by quench cooling, IMC crystallized as the γ-form, which is consistent with the proposed model since the presence of PVP increased the Tg of the systems and thus reduced the molecular mobility of drug molecules.60 On the contrary, the dispersions containing 5−20% PVP prepared by solvent evaporation showed only amorphous to α-crystals transformation upon crystallization.61 The appearance of the metastable α-form instead of the most thermodynamically stable γ-polymorph in aqueous media as well as in dispersions prepared by solvent evaporation suggested a different mechanism of nucleation of the drug in the solution in which molecular mobility does not play the controlling role. Polymorphic Transformation. Figure 4 shows that τ-IMC has the infrared spectra that most resembles the spectra of the amorphous drug, suggesting that the τ-polymorph should be the least stable form which is closest in Gibbs energy to the parent amorphous phase. Being the least stable polymorph, τ-IMC exhibits a high propensity to convert to a more stable polymorph with adjacent free energy level which is the α-form. In term of stability, the three polymorphs of IMC reported in this work follow the order: τ < α < γ. As can be seen in Figure 9, the rate of τ to α polymorphic transformation is generally inversely proportional to the drugloading. This is due to the fact that the mobility of drug molecules decreases as the content of the drug increases. However, this K


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Figure 17. Segregation of spherulites of τ- and α-IMC in the dispersion containing 70% drug. Spot X and Z indicate τ-spherulites while spot Y shows the α-spherulite.

observation only holds true in samples where α-IMC has not yet been the predominant polymorph because once ϕt approaches 50% as the α-form becomes the dominant form, it will accelerate the transition process. This explains why 80 and 90% IMC dispersions that contain rather high initial ϕt of 63.4 and 89.9%, respectively, showed much more rapid τ to α conversion than in lower drug-loading samples. Since polymorphic transformation is slower as the drugloading increases, it can be extrapolated for pure τ-IMC that the τ-polymorph would be relatively stable if it contains no trace of αform in the beginning. Polymorphic Segregation. As previously illustrated in Figure 11, in 70% IMC dispersions, large spherulites of the τform and smaller ones of the α-polymorph with different morphologies were segregated. The polymorphic segregation of IMC might be explained by observing the crystallization process of the systems. Figure 17 shows the evolution of spherulites of τIMC (spot X and Z) and α-IMC (spot Y) with obviously different morphologies and growth rates. The τ-spherulites evolved almost three time faster than the α-spherulite (2.84 vs. 1.03 μm/hour, respectively). Initially, the spherulite of the less stable τ-polymorph developed, followed by the evolution of a nearby α-spherulite. An interesting question is why both τ and α-IMC appeared in 70% drug dispersion while only the τ-form was identified in the sample containing 60% IMC. Figure 18 shows that in 70% IMC dispersion, τ-IMC and PEG started crystallizing at the same time (9 h after preparation) whereas the crystalline α-form was detected later at 15 h. In addition, the crystal growth rates of τ-IMC, α-IMC, and PEG follow the order: τ > PEG > α. In spot X (Figure 17) where τIMC and PEG crystallized, due to the fact that the crystal growth rate of τ-IMC was faster than that of PEG, the amorphous region surrounding the spherulite of τ-IMC became polymer-rich with the ratio of amorphous PEG to amorphous IMC increasing as the crystallization progressed. Consequently, the mobility of IMC molecules in this polymer-rich domain also increased and the drug continued to crystallize as the metastable τ-form. In spot Y, PEG crystallized first, generating an amorphous drug-rich domain around the spherulite with reduced mobility of the drug molecules so that IMC crystallized as the more stable αpolymorph. As PEG had faster crystal growth rate than α-IMC, the ratio of amorphous PEG to amorphous IMC in the drug-rich

Figure 18. Normalized intensities of distinct Bragg peaks of τ-, α-IMC, and PEG in dispersions containing 60 and 70% drug as a function of time.

domain decreased during crystallization, and hence the mobility of IMC molecules became slower, promoting the continuous crystallization of the drug into the α-form. In the sample containing 60% drug, the growth rates of τ-IMC and PEG were comparable (Figure 18), so that the ratio of drug to polymer in the amorphous region, and hence the mobility of IMC molecules, remained unchanged during the entire crystallization process. Therefore, only the τ-form developed in this sample. The identical growth rate of the drug and the polymer also explained why during the crystallization of 60% IMC sample, a single Tg was detected in DSC thermograms, whereas in the samples containing 45−50% IMC, two Tgs evolved as a result of amorphous−amorphous phase separation5 that in turn originated from much faster crystallization of PEG than IMC in these dispersions as shown in Figure 7. The segregation of τ- and α-IMC into spherulites with the size up to hundreds of micrometers combined with the formation of a layer of PEG on top of IMC ground suggests the inhomogeneity of PEG-IMC dispersions and the nonuniform distribution of the drug and the polymer in the systems. These seem to be the inherent characteristics of semicrystalline dispersions where more than one polymorphic form coexists as also reported for L


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Molecular Pharmaceutics PEG−nimodipine mixtures.17 Furthermore, polymorphic transformation during storage contributes to the variations in the physicochemical properties of the systems, implying that the pharmaceutical performance of semicrystalline dispersions might therefore be less consistent and less reproducible.

407-IMC dispersions containing different drug loadings in the 2θ range of 4−40°, PXRD diffractograms of Poloxamer 407-IMC dispersions containing different drug loadings in the zoomed region from 4 to 17°, PXRD diffractograms of Gelucire 50/13-IMC dispersions containing different drug loadings in the 2θ range of 4−40°, PXRD diffractograms of Gelucire 50/13-IMC dispersions containing different drug loadings in the zoomed region from 4 to 18°, FTIR spectra of γ- and α-IMC in PEG400 (60% IMC w/w) in the carbonyl stretching region, and glass transition temperature of PEG-IMC dispersions as a function of drugloading. (PDF)

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CONCLUSIONS Polymorphism of IMC in semicrystalline dispersions with PEG and other polymers, such as poloxamer and Gelucire, was characterized by numerous techniques. The formation of different polymorphic forms of IMC as a function of drugloading was reported and explained by the variation in the mobility of drug molecules, thereby changing the interplay between the interfacial energy at the crystal−amorphous interface and the Gibbs energy difference between the two phases, thus modifying the activation energy barrier for nucleation. Depending on the drug-loading and the distinct polymer, IMC may crystallize as the α-form, τ-form, or a mixture of the two polymorphs in semicrystalline systems. We proposed a model as an intuitive tool to predict the polymorph selection from melt crystallization based on the mobility of drug molecules. Briefly, stable polymorphs are thermodynamically preferred in the low mobility zone, whereas metastable crystalline forms are kinetically favored when the molecular mobility of drug molecules is sufficiently high. The mixture of two polymorphs can occur if the molecular mobility lies between the low and the high mobility zones. The significance of mobility of drug molecules in polymorph formation elucidates the polymorphism of numerous APIs in semicrystalline dispersions that has been extensively reported in literature. Nevertheless, quantitative data are needed to verify the mechanism of polymorph selection as a function of the molecular mobility of drugs during melt crystallization. The τ polymorph most resembles the “structure” of the amorphous drug, suggesting that this should be the least stable form. The τ- to α-transition occurs during storage with the rate of polymorphic transformation generally inversely proportional to the drug-loading, except for when the more stable form is dominant. The segregation of τ- and α-IMC into spherulites, with the size up to hundreds of micrometers combined with the formation of a layer of polymer on top of the drug ground as well as the polymorphic transformation during storage, implies the inherent inhomogeneity and the nonuniform distribution of the drug and the polymer in semicrystalline systems. Consequently, the pharmaceutical performance of semicrystalline dispersions will be less consistent and less reproducible. These findings expand our understanding about the complicated crystallization behavior of semicrystalline systems and are critical for preparation of solid dispersions with reproducible and consistent pharmaceutical performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +32 16 330 304; Fax: +32 16 330 305. ORCID

Tu Van Duong: 0000-0002-8918-0268 Pieter-Jan Van Bockstal: 0000-0002-0363-3618 Guy Van den Mooter: 0000-0001-9166-6075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from Research Foundation Flanders (FWO-Vlaanderen). Tu Van Duong gratefully acknowledges the scholarship (G.0764.13) and travel grant (K1H0416N) awarded by FWO-Vlaanderen. We thank Leuven Chem&Tech for providing access to the infrared spectrophotometer and rheometer.

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ABBREVIATIONS API, active pharmaceutical ingredient; FTIR, Fourier transform infrared; HPLC, high-performance liquid chromatography; IMC, indomethacin; m-DSC, modulated differential scanning calorimetry; PEG, polyethylene glycol; PLM, polarized light microscopy; PVP, poly (vinylpyrrolidone); PXRD, powder X-ray diffraction; Tg, glass transition temperatures; UV−vis, ultraviolet−visible

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.molpharmaceut.7b00930. HPLC chromatograms of IMC polymorphs (0.5 μg/mL) in 20 mM phosphate buffer pH 6.8:acetonitrile 55/45 v/v, UV spectra of IMC polymorphs (35 μg/mL in 20 mM phosphate buffer pH 6.8:acetonitrile 55/45 v/v) in the range of 200−500 nm, PXRD diffractograms of Poloxamer M


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Polymorphism of Indomethacin in Semicrystalline Dispersions

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