Effect of Additives on Crystal Growth and Nucleation of Amorphous

Article pubs.acs.org/crystal

Effect of Additives on Crystal Growth and Nucleation of Amorphous Flutamide Niraj S. Trasi and Lynne S. Taylor* Department of Industrial and Physical Pharmacy, Purdue University, 575 Stadium Mall Drive, West Lafayette, Indiana 47907, United States ABSTRACT: Understanding the effect of polymers on the crystallization behavior of low molecular weight organic compounds is important to a number of areas. Since crystallization is a combination of nucleation and growth, it is of interest to evaluate how these additives influence both these processes. In this study, the crystallization of supercooled liquid flutamide in the presence and absence of a number of synthetic vinyl polymers with different functional groups was investigated, at a constant molar ratio of polymer functional group. The impact of a structurally similar small molecule compound, nilutamide, was also evaluated. Isothermal crystal growth rates were measured using hot stage microscopy, while nucleation behavior was more qualitatively investigated using differential scanning calorimetry. Polymers that formed better hydrogen bonds via their acceptor groups, as determined by infrared spectroscopy, were found to be more effective at reducing crystal growth rates, while nilutamide influenced growth rates mostly by increasing the blend glass transition temperature. Only two polymers had a noticeable impact on nucleation behavior, while nilutamide was by far the most effective at reducing nucleation rates, most likely due to its low molecular size. This study has further highlighted the correlation between drug−polymer hydrogen bonding interactions and crystal growth, and the corresponding lack of correlation between this factor and nucleation behavior.

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shelf life.4 This is a not a practical solution since, for many compounds, the Tg is low. Furthermore, there are several examples reporting crystallization at temperatures substantially below Tg.5 Therefore, various other strategies have been developed to inhibit crystallization of amorphous compounds, the most effective method being the addition of polymers.6−8 Different mechanisms have been proposed as the basis for the crystallization inhibition of the amorphous solid by polymers including increasing the dispersion Tg and thus reducing mobility at the storage temperature,9 by acting as a diluent thus decreasing the chemical potential of the drug and by forming specific drug−polymer interaction and thus restricting the mobility of the drug at a local scale.10 Recently, it has been observed that among these various factors, correlations appear to exist between crystal growth rates and drug−polymer hydrogen bonding interactions.11 However, in order to confirm this relationship, it is necessary to study additional compounds with more diverse chemistry. Moreover, most studies to date have evaluated polymer performance by comparing polymer effectiveness at a given weight percent. This means that polymers that have high molecular weight monomers, will potentially have fewer functional groups per unit weight compared to those polymers that have smaller repeating units. Unfortunately,

INTRODUCTION Glasses and supercooled liquids, collectively known as amorphous systems, are attracting increasing attention in the area of drug delivery. While most active pharmaceutical ingredients (APIs) are purified as crystalline forms, compounds with low solubility and dissolution rate may subsequently be formulated into an amorphous solid dosage form.1 The overall free energy of the amorphous form is significantly higher than the crystalline counterpart resulting in a higher transient solubility and faster dissolution rates.2 The greatest challenge in making amorphous dosage forms is ensuring the physical stability, that is, slowing or preventing the inevitable transformation (from a thermodynamic perspective) to the lower energy crystalline state. The two main factors influencing the kinetics of the transformation to the crystalline phase are the thermodynamic driving force/barriers and the molecular mobility of the supercooled liquid/glass. When a sample is cooled below the melting point, the thermodynamic driving force for crystallization increases as the temperature is lowered and becomes increasingly far from equilibrium. However, the molecular mobility also decreases strongly with decreasing temperature and the system falls out of equilibrium at the glass transition temperature, Tg, forming a glass with limited molecular mobility relative to the supercooled liquid.3 Thus one way of stabilizing an amorphous solid is by storing the sample at very low temperatures where the molecular mobility is low enough to prevent crystallization over the intended © 2012 American Chemical Society

Received: March 20, 2012 Revised: April 26, 2012 Published: May 1, 2012 3221

dx.doi.org/10.1021/cg300370q | Cryst. Growth Des. 2012, 12, 3221−3230

Crystal Growth & Design

Article

many pharmaceutically relevant polymers have complex structures with multiple functional groups capable of participating in hydrogen bond interactions. Therefore, in order to better assess the impact of hydrogen bonding on crystal growth, more systematic studies should be performed with chemically simple polymers that consider the number as well as type of functional groups available for interaction. The objective of the current study was thus to evaluate the ability of different polymers to inhibit crystal growth of a model compound, comparing the polymers at the same molar ratio (based on polymer monomer unit). The hypothesis to be tested is that the polymer that can form the strongest/most numerous hydrogen bonds with the drug will be the best inhibitor of crystal growth in the supercooled liquid. Flutamide, 2-methyl-N-[4-nitro-3-(trifluoromethyl)phenyl]-propanamide, a low molecular weight organic compound (276.2 Da) with both hydrogen bond donor and acceptor groups, was selected as the model compound, and the ability of various chemically simple polymers, added at the same molar ratio, to reduce crystal growth rates from the supercooled liquid was investigated. More complex polymers of pharmaceutical interest were also studied on a weight basis. The effect of these polymers on the nucleation behavior was also qualitatively evaluated.

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Figure 1. Molecular structures of (a) flutamide, (b) nilutamide, (c) PDMA, (d) PVAc, (e) PAA, (f) PVPH, (g) PIPA, and (h) PVP.

MATERIALS AND METHODS

Flutamide (2-methyl-N-[4-nitro-3-(trifluoromethyl)phenyl]-propanamide) and nilutamide (5,5-dimethyl-3-[4-nitro-3-(trifluoromethyl)phenyl] imidazolidine-2,4-dione) were purchased from Sigma Aldrich (St. Louis, MO). The various chemically simple polymers used were poly (N,N-dimethyl acrylamide) (PDMA) (Mw 100 000), poly(vinyl acetate) (PVAc, Mw 350 000), poly(acrylic acid) (PAA) (Mw 450 000), poly(4-vinylphenol) (PVPH), poly(N-iso-propylacrylamide) (PIPA) and poly(vinylpyrrolidone) K12 (PVP, Mw 2000−3000). PDMA and PVAc were purchased from Scientific Polymer Products Inc. (Ontario, NY), PAA was purchased from Sigma-Aldrich (St. Louis, MO), PVP was obtained from BASF Chemicals (New Jersey, USA), and PVPH and PIPA were procured from Polysciences Inc. (Warrington, PA). The chemical structures of the drugs and the polymers are shown in Figure 1. The pharmaceutically relevant more complex polymers tested were hydroxypropyl methylcellulose (HPMC, Hypromellose USP, substitution type 2910) and hydroxypropyl methylcellulose-acetate succinate (HPMCAS, type AS-MF, Mw 18 000), which were generous gifts from Shin-Etsu Chemicals (Niigata, Japan), and PVP-vinyl acetate (PVPVA) (Kollidon VA64, Mw 45 000−47 000), which was supplied by BASF Chemicals (New Jersey, USA). Mixtures of flutamide with the simple polymers and nilutamide were prepared at 9:1 (drug/polymer) molar ratio by first gently mixing the components in a mortar and pestle and then ensuring complete mixing by cryomilling for 10 min in a 6750 freezer mill (Spex Sampleprep, Metuchen, New Jersey). On a weight basis, this resulted in the following concentrations of polymer being added: PAA (2.8%), PVP (4.3%), PDMA (3.8%), PVPH (4.6%), PVAc (3.3%), PIPA (4.6%), and nilutamide (11.3%). Mixtures of drugs with pharmaceutical polymers were prepared at 5%w/w polymer in drug by the same method. Growth Rate Measurements. Crystal growth rate measurements were performed by melting approximately 3 mg of sample between two clean coverslips at around 120−125 °C and immediately quenching the melt to room temperature to give a clear film. The samples were confirmed to be noncrystalline by visual observation under crosspolarized light using a microscope (Nikon Eclipse E600 POL microscope, Nikon Corp, Tokyo, Japan). Because the samples showed formation of nuclei and slow growth at room temperature, the growth rate experiments were carried out immediately after preparation. The samples were heated to and maintained at different temperatures ranging from 40 to 70 °C using a hot stage (Linkam THMS 600, Surrey, UK). The growth of the spherulite was recorded by time lapse

photography and the growth rate was determined from the slope of the line obtained by plotting the diameter against time for the different temperatures. The change in diameter with time was found to be linear. All growth rates experiments were performed in triplicate. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectroscopy was performed in transmission mode using a Bio-Rad FTS 6000 (Bio-Rad, Cambridge, MA). 1:1 molar mixtures of flutamide and the simple polymers were dissolved in ethanol. The solutions were then spin coated on Zn−Se substrates using a KW-4A spin coater (Chemat Technology Inc., Northridge, CA) at a spin rate of about 2000 rpm. The fast spinning action resulted in a thin film and quick evaporation of the solvent. The spectra were then obtained by coadding 128 scans over the wavenumber region of 4000−400 cm−1 while maintaining 0% RH with a dry air purge. Thermal Analysis. Thermal analysis of the drug and its mixtures was performed using a differential scanning calorimeter (DSC) (TA Q2000, TA Instruments, New Castle, DE, USA) attached to a refrigerated cooling system. Dry nitrogen was used as the purge gas and was maintained at a flow rate of 50 mL/min. The instrument was calibrated for temperature using indium and tin, and enthalpy was calibrated using indium at the heating rate used. The samples were placed in sealed aluminum pans with a pinhole in the lid. Determination of the glass transition temperature (Tg) was performed by cooling the melt at 15 °C/min and then reheating at 10 °C/min. Nucleation temperature zone measurements were performed by cooling the melt to different temperatures at 5 °C/min followed by reheating at 10 °C/min to determine if crystallization occurred. Computational Analysis. The lowest-energy single-molecule conformation of flutamide in the ground state and the partial atomic charges on the different atoms on flutamide and the polymer monomers were calculated from density functional theory and NBO population analysis using SPARTAN’08 software (Wave function Inc., Irvine, CA, USA). The geometry was optimized (in vacuum) at the B3LYP/6-311++G (3d2f,2p) level, and the partial atomic charges for the atoms were determined. The energy profile for different conformers of flutamide was determined by varying the dihedral angle between the phenyl ring and the amide group in the molecule in 20° steps from 0 to 360° and calculating the ground state energy by minimizing using B3LYP calculations with a 6-311++G** level basis set. 3222



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RESULTS

Growth Rate Kinetics. The growth rate kinetics of flutamide crystals alone and in the presence of the simple polymers is shown in Figure 2. Raman spectroscopic analysis of

Figure 3. Diameter growth rates of pure flutamide (blue diamond) and in the presence of 5% w/w PAA (red solid square), HPMC (purple ×), HPMCAS (blue ×), PVPVA (orange solid circle), and PVP K12 (green triangle). It should be noted that another polymorph appears to grow in the presence of HPMC.

Thermal Analysis. Thermal analysis of flutamide and the dispersions was carried out to determine the effect of polymers on the Tg. The values are shown in Table 1. The Tg of pure

Figure 2. Diameter growth rates of pure flutamide (blue diamond) and in the presence of 10% (molar) PAA (orange solid circle), PVAc (purple ×), PVPH (orange solid square), nilutamide (red solid square), PIPA (blue ×), PDMA (+), and PVP K12 (green triangle).

Table 1. Onset Glass Transition and Melting Temperatures Measured at 10°C/min

the material crystallized between quartz coverslips indicated that the same form (Cambridge Structural Database ref. code WEZCOT) had crystallized for all the samples except for HPMC which resulted in a different form which was identified by small shifts in certain Raman peaks. It can be seen that pure flutamide has a very high growth rate which decreases by nearly 2 orders of magnitude when cooled from 70 to 40 °C. The effect of the polymeric additives was diverse. The least effective polymer was PAA which resulted in a slightly higher average growth rate than for the pure drug, although the difference is not significant. Interestingly, the sample with PAA also partially crystallized during cooling of the melt, a phenomenon not seen for other samples. The most effective polymer at reducing crystal growth rates at 10% (molar) concentration was PVP, which reduced the growth rate by approximately 1 order of magnitude. The next two most effective polymers were PDMA and PIPA, both of which had a very similar effect on reducing growth. PVAc had very little effect on the crystal growth rates, while PVPH had a slightly better growth retarding effect than PVAc. Interestingly, the small molecule additive, nilutamide was more effective than PVAc and PVPH in reducing growth rates and around half as effective as PVP. The order of effectiveness in reducing growth rate for the additives was therefore: PVP > PDMA = PIPA > nilutamide > PVPH > PVAc > PAA. It thus appears that polymers with strong acceptor groups are more effective than those with donor groups, even though flutamide has both acceptor and donor moieties. Growth rates of flutamide in the presence of pharmaceutically relevant polymers at 5% w/w (Figure 3) showed that all the polymers were effective at reducing the growth rates. Again, PVP was the most effective polymer at reducing growth rates, this time by more than 1 order of magnitude. PVPVA which is a copolymer of containing monomers present in PVP and PVAc had a lower effect on growth inhibition than PVP, as expected from the results with the simple polymers where PVAc was found to be not very effective. HPMCAS reduced the growth rates by around 4 times, while HPMC was the least effective polymer reducing the growth rate by around 2.5 times.

a

sample

Tg (°C)

Tm (°C)

flutamide 10% PDMA 10% PVAc 10% PAA 10% PVPH 10% PIPA 10% PVP 10% Nilutamide

−3.1 −0.2 −1.07 crystallized on cooling −0.18 −0.47 0.6 2.5

110 109.3 109.3 110.1 109.8 108 108 Eutectic

Standard deviations were within 1°C for Tg and Tmax.

flutamide was found to be around −3 °C. The presence of polymers at a 10% molar level resulted in a slight increase of a few degrees, with little differentiation between the polymers. The greatest increase of around 6 °C was seen with nilutamide, the amorphous form of which has a Tg at around 32 °C. Under the analysis conditions employed, it can be seen that the melting temperature of flutamide changed little in the presence of 10% (molar) polymer. It was also noted that PAA resulted in crystallization of flutamide during the cooling phase. While pure amorphous flutamide appeared to show one exothermermic event followed by melting of the starting form when heated at 10 °C/min, it has been suggested previously that the exotherm actually consists of two overlapping events, crystallization of the amorphous material to a metastable form followed by a rapid transition to the stable starting form.12 The transition is more observable when samples are heated at slower rates, and the two forms have been suggested to be polymorphs composed of either the higher or low energy conformer of flutamide. Interestingly, in the presence of PDMA, the transition between the polymorphs was somewhat delayed, resulting in the appearance of a small exotherm at a higher temperature, while for the other polymers this exotherm occurred at the trailing end of the crystallization exotherm and was not very visible (Figure 4). DSC analysis performed on the 5%w/w flutamide−pharmaceutical polymer mixtures (data not shown) showed that 3223



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Figure 4. DSC thermograms of (a) pure flutamide and mixtures of flutamide with 10% molar (b) PVAc, (c) PVPH, (d) PDMA, (e) PIPA, and (f) PVP.

Figure 5. Schematic of nucleation (J) and growth zone (U) of flutamide (a) and flutamide in the presence of 10 molar % (b) PDMA, (c) PVAc, (d) PVPH, (e) PIPA, (f) PVP, and (g) nilutamide.

flutamide partially crystallized during the cooling in the presence of HPMCAS. In the presence of HPMC, the metastable polymorph appeared to be highly stabilized, and thus its melting point could be observed with an onset temperature of 90 °C.

Nucleation zone experiments showed that the flutamide nucleation zone occurred between 10 and 40 °C. A schematic of the effect of polymers on nucleation is shown in Figure 5, where we see that the only major effect that the addition of polymers had on the nucleation zone was to narrow the 3224



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interact with the drug. For pure flutamide, significant differences in the spectra were observed between the crystal and supercooled liquid phases. In the crystal, the NH stretch peak is very sharp and is observed at 3360 cm−1 and the CO stretch is present as a sharp peak at 1716 cm−1. In the supercooled liquid, the NH stretching peak is broadened and significantly shifted to a lower wavelength (3315 cm−1) and the CO stretch is split into two peaks with the more intense peak occurring at a lower wavenumber of 1681 cm−1 and the second occurring as shoulder to this peak with a maximum at 1710 cm−1 (Figure 7). This

nucleation zone slightly. PAA at 10% (molar) resulted in crystallization during cooling from the melt and thus seemed to promote nucleation. Nilutamide, on the other hand, appeared to be an effective nucleation inhibitor, resulting in very little crystallization during the DSC experiment. This is most likely due to a reduction in the nucleation rate to one where very little nucleation occurs at the heating and cooling rates used in the experiment. It is known that the number of nuclei will influence the temperature at which crystallization occurs during a DSC experiment.13 Overlaying the DSC results with the crystal growth rate experiments allows us to determine the crystal growth rates at the Tmax of the crystallization exotherm. A change in the number of nuclei will result in a change in the crystallization maximum and thus the corresponding crystal growth rate at which it occurs.14 Thus if the effect of the polymers on growth rate as a function of temperature is known, inferences about the relative effects of the polymers on the nucleation rate of flutamide can be made. As shown in Table 2, there was very little difference in the crystal Table 2. Average Growth Rate at Tmax in the Presence of Polymers sample

avg Tmax (°C)a

10% PDMA 10% PVAc 10% PVPH 10% PIPA 10% PVP flutamide

60.6 53.2 53.3 65.5 68.4 46.8

a

growth rate(diameter) at Tmax (μm/s) 4.1 7.7 5.9 7.0 5.4 4.4

(0.2) (0.1) (0.1) (0.3) (0.3) (0.6)

effect on nucleation rate (P = 0.05) no effect decrease no effect decrease no effect

Standard deviations of the Tmax were within 1 °C, n = 2.

growth rate at the Tmax for PDMA, PVPH, and PVP when compared at a P value of 0.05, indicating that the polymers did not appear to significantly impact the nucleation rate of flutamide. PVAc and PIPA, however, reduced the nucleation rate which in turn reduced the number of nuclei in the sample resulting a higher growth rate at Tmax. The drastic effect of nilutamide on nucleation inhibition is seen in Figure 6 which shows a snapshot of the samples melted between coverslips, cooled at 15 °C/min to −30 °C and reheated at 10 °C/min to around 40−60 °C. FTIR Spectroscopy. Infrared spectroscopy has been commonly used to study intermolecular interaction in organic amorphous dispersions especially with respect to hydrogen bond interactions.10,15 Flutamide has one hydrogen bond donor, the NH group, and two acceptors which are the carbonyl and the nitro group. Therefore, based on molecular structure alone, it is expected that all the polymers, irrespective of whether they have acceptors or donors, should be able to

Figure 7. A comparison of the FTIR spectra in the (a) NH stretching region and (b) CO stretching region of crystalline and amorphous flutamide.

result suggests that hydrogen bonding is, on average, stronger in the amorphous material than in the crystal, a somewhat unexpected, but certainly not unprecedented observation. The spectroscopic results can be more readily interpreted by consideration of the crystal structure. In the crystalline state (ref code WEZCOT), the NH moiety of the amide functional group interacts intermolecularly with the NO2 group which is a relatively weak acceptor, while the CO group forms an intramolecular interaction with an activated aromatic CH, which is rendered more acidic than normal due to the electron withdrawing groups present (nitro and trifluoro groups). These interactions account for the relatively high wavenumber

Figure 6. Nuclei formed in a small region of (a) pure flutamide, (b) 10% PIPA, and (c) 10% nilutamide sample upon melting and cooling to −30 °C and reheating to 40−60 °C at constant rates. Only one nucleus is observed in the presence of nilutamide. 3225



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position of the NH group which is involved in a hydrogen bond with a relatively poor acceptor. In the amorphous material, the crystal packing constraints are removed, and it appears that the NH group now interacts with the amide CO group, resulting in an improved hydrogen bond. It is clear that there is a population of carbonyl groups that have a similar interaction as found in the crystal, that is, do not hydrogen bond with the amide NH group. The more extensive intermolecular hydrogen bonding in amorphous flutamide might contribute to flutamide forming an amorphous solid on cooling; it has a very simple structure and would be expected to crystallize during cooling based on the behavior of other small molecules.16 In the presence of the polymers, changes in the spectrum of the dispersion relative to the spectra of amorphous flutamide and the polymer will indicate the formation of specific interactions between the two components. The NH peak position of flutamide in the presence of all the polymers is listed in Table 3. Table 3. Peak Position of the NH Stretch in Pure Flutamide and in 1:1 Molar Ratio with Polymers sample

NH peak position

flutamide flutamide−PVP flutamide−PDMA flutamide−PVAc flutamide−PVPH flutamide−nilutamide flutamide−PIPA flutamide−PAA

3315 3287 3289 3322 3318 3321 3293 3315

Figure 9. Effect of PDMA at a 1:1 molar ratio on the (a) NH stretch and (b) carbonyl stretch region in the FTIR spectra of flutamide.

to 1620 cm−1 which is also accompanied by a corresponding increase in the relative intensity of the higher wavenumber shoulder of the flutamide CO stretch. This is likely due to a decrease in the extent of bonding between drug molecules and an increase in drug-polymer interactions in the dispersions. Clearly, it appears that the CO groups in the polymers act as competitive acceptors for the NH group in flutamide, thus forming hydrogen bonds resulting in a decrease in the NH stretching wavenumber. The FTIR spectra in the presence of PVAc (Figure 10), which has the least effect on reducing crystal

The maximum shift in this peak (28 cm−1) is observed in the presence of PVP, and the corresponding spectrum is shown in Figure 8. Here it can be observed that the carbonyl stretching

Figure 8. Effect of PVP at a 1:1 molar ratio on the (a) NH stretch and (b) carbonyl stretch region in the FTIR spectra of flutamide. Figure 10. Effect of PVAc at a 1:1 molar ratio on the (a) NH stretch and (b) CO stretch region in the FTIR spectra of flutamide.

vibration of the polymer is correspondingly shifted from 1679 cm−1 to 1658 cm−1. This indicates a more favorable interaction between the NH group of flutamide and the CO group of PVP than is present in either of the pure components. Figure 9 shows the NH shift in the presence of PDMA as well as the shift in CO stretch of the polymer from 1641 cm−1

growth rates, however shows a slight increase in the NH vibration frequency even though this polymer also has a carbonyl group. This is essentially because this functional group 3226



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is directly attached to an oxygen group which reduces the negative charge on the carbonyl and results in weak hydrogen bond with flutamide. The carbonyl region is more difficult to interpret since it is impossible to determine if the increase in intensity of the peak at 1710 cm−1 is due to the lowering of the polymer carbonyl peak stretching due to hydrogen bonding with the drug, or due to the disruption of drug−drug interactions, or some combination of both. In the presence of PAA there was no change in the peak positions of any of the functional groups involved in hydrogen bonding, as seen in Figure 11. The NH peak was not expected

Article

DISCUSSION

A mechanistic understanding of the role of polymers in inhibiting phase transformations in amorphous drug formulations is critical for the development of robust formulations. Since crystallization requires both crystal nucleation and crystal growth, improved understanding of the effect of polymers on both of these processes is important. In this study, we attempt to gain a better molecular level understanding of the impact of polymers on crystallization by investigating blends of drug with simple polymers containing only one functional group, and with well-defined drug: polymer (relative to monomer unit) molar ratios. It is difficult to prepare well-defined molar mixtures using pharmaceutical polymers since these polymers are often complex with multiple functional groups and degrees of substitution. Clearly, polymers with more than one functional group can also complicate interpretation of drug−polymer interactions and the role of such interactions in impacting crystallization. When evaluated at the same molar ratio, it is clear that the various polymers influence growth rates to different extents. Before discussing if there are correlations between drug−polymer specific interactions and crystal growth rates, as hypothesized, other factors will briefly be considered. First we note that the molar ratio of the drug and the polymer monomer unit is constant for all the polymers; hence any dilution effect is the same based on this metric. Second, as seen in Table 1, the glass transition temperature of the binary systems are increased by only a few degrees relative to the Tg of pure flutamide and while this will decrease the molecular mobility and thus reduce growth rates, the difference in Tg between the dispersions is not large enough to explain the difference in growth rates. Also, it should be noted that the additive that had the greatest effect on the dispersion Tg, nilutamide, was not the most effective additive at inhibiting crystal growth, suggesting that effects on mobility as manifested by changes in Tg cannot completely explain effects on crystal growth rate. The minor change in flutamide melting point in the presence of the polymers also indicated that the polymers do not substantially change the degree of supercooling of the system. Therefore we return to further examine the original hypothesis that drug−polymer specific interactions correlate with the extent of growth rate inhibition. With respect to specific interactions, the various polymers used have a variety of functional groups, while flutamide, as the name suggests, has an amide group that can interact with the polymer as both a donor and an acceptor, as well as a nitro acceptor group (Figure 1). At first glance, it appears that since flutamide has both donor and acceptor groups, it should be able to interact well with all of the polymers which in turn should reduce crystal growth according to our hypothesis. However, it is apparent that the two polymers with the best hydrogen bond donor groups, namely, PAA and PVPH, are ineffective and poorly effective at inhibiting growth respectively, suggesting either that our hypothesis is incorrect, or that these polymers do not form good hydrogen bonds with the amide carbonyl of flutamide which is the best acceptor group present in drug, and a better acceptor group than found in either of the polymers. FTIR spectra show little change in the carbonyl peak of the drug in the presence of the polymers, consistent with a lack of interaction with this functional group. Interestingly, both PAA and PVPH were found to be very effective growth inhibitors for acetaminophen, which also contains an amide function, and in this case it did appear that a drug amide carbonyl-polymer OH

Figure 11. Effect of PAA at a 1:1 molar ratio on the (a) NH stretch and (b) CO stretch region in the FTIR spectra of flutamide.

to change much since PAA does not have a good acceptor group, although it does have a very strong donor group which was expected to interact with the amide carbonyl group in flutamide. Acetaminophen, which also has a similar amide structure as flutamide, in fact interacts equally well with both PVP and PAA.17 However, this clearly was not the case with flutamide and the spectrum of the dispersion represents the simple summation of the spectra of the two pure components, typically the case for immiscible systems.18 The other acidic polymer investigated, PVPH, also did not result in any change in the carbonyl stretching wavenumber of flutamide, but did show some small changes in the NH/OH region (Table 3), where the dispersion had a slightly different spectra from the sum of the individual component spectra indicating some level of interaction. In the presence of nilutamide, the carbonyl stretching region becomes more complicated since there are now three CO groups. However, it was observed that the vibration of the carbonyl stretch for nilutamide moved from 1717 to 1726 cm−1 while that for flutamide moved from 1680 to 1686 cm−1. Clearly there is some interaction between the two components, but it does not appear as if the hydrogen bond interaction between the two drugs is stronger or even equal to that between the drugs themselves. It was also observed that the NH peaks were shifted to higher wavenumbers. 3227



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interaction, we turn our attention to the ability of the NH group to interact with the polymers. FTIR spectroscopy clearly showed interactions between flutamide and the polymer hydrogen bond acceptor groups, with the downward shifts in the flutamide NH peak and polymer CO groups being characteristic of stronger hydrogen bonding interactions. There is a correlation between the change in NH peak position and the order of growth inhibition for the carbonyl bearing polymers, with larger decreases in NH peak position being associated with greater growth rate inhibition. The calculated charges on the oxygen in the carbonyl functional group of the polymers, shown in Table 4,

group hydrogen bonding interaction occurred at least for PAA.19 To make sense of these observations, the hydrogen bonding pattern of flutamide in the crystal was evaluated. In the crystalline state, the CO group is involved in a bifurcated Hbond with one intermolecular CH···O bond and one intramolecular CH···O bond.20 This intramolecular bond is responsible for the planar configuration of the molecule. The lowest energy molecular conformation of flutamide calculated in a vacuum indicated that the planar structure and the intramolecular bond is maintained, suggesting that it is highly likely to also occur in the amorphous state as well. The energy profile of the flutamide molecule upon rotation of the bond between the phenyl ring and the amide group showed that the overall energy is the least when the carbonyl group is present adjacent to the hydrogen on C(5), opposite the CF3 functional group (Figure 12). The second minimum

Table 4. Partial Atomic Charges on the Oxygen of the Carbonyl Functional Group of the Polymers compound/polymer

O charge in CO

PVAc PAA PDMA PIPA PVP

−0.598 −0.606 −0.64 −0.647 −0.65

also match the extent to which the polymer reduced the growth rates. Both of these pieces of evidence strongly support our hypothesis that specific interactions between the drug and the polymer are extremely important in determining the extent of crystal growth inhibition. Growth rate measurements obtained with pharmaceutically acceptable polymers at a 5%w/w ratio are in line with the observations with the simple polymers. PVPVA, having a fewer number of PVP monomers in the polymer chain, was less effective than the same wt% of PVP, presumably since the comonomer, vinyl acetate, forms much weaker interactions than the vinyl pyrrolidone monomer (see Table 4). The cellulose polymers were less effective, presumably due to the lower hydrogen bond acceptor basicity of the acceptor groups present on these polymers.22 In contrast the impact of nilutamide on the crystal growth of flutamide can be explained by the combination of a dilution effect and an increase in Tg. If the growth rate curve of flutamide in the presence of nilutamide is corrected for dilution and increase in Tg, it overlaps with the experimental results obtained for pure flutamide. This supposition is consistent with IR results which indicate that there are no hydrogen bonds formed between the two molecules that are more favorable than in the individual components. While the polymers had only a minor effect on the temperature and rates of nucleation, nilutamide had a very significant impact on inhibiting the nucleation of flutamide (Figure 6). The factors contributing to the nucleation behavior in supercooled liquids are not well understood and the presence of additives contributes even more complexity. Most studies on the effects of impurities on crystallization behavior have been performed with inorganics or with organics from the solution state. While hydrogen bonding between the drug and additive has a very clear effect on crystal growth behavior, a similar correlation is not observed in this study with respect to the nucleation behavior of the drug. Similar observations have been made previously.23 Nucleation begins with the formation of a prenuclei embryo leading to the formation of a short-range order matching the crystalline motif. Many prenuclei return back to the supercooled liquid since the

Figure 12. Energy profile of flutamide upon rotation along the bond between the phenyl and amide group. The highlighted point represents the starting conformation as shown in the inset.

occurs when the amide group is rotated by 180° and presumably interacts with the hydrogen attached to C(3) which shows that the interaction with the carbonyl group is stronger for the hydrogen attached to the C(5) atom, as noted previously.12 This is because of the higher positive charge for that atom due to the positions of the two electron withdrawing substituents on the phenyl ring. Therefore, it appears that part of the negative charge on the oxygen in the CO group of flutamide is utilized in interacting intramolecularly with the aromatic CH, leaving only a partial negative charge on the CO group available for intermolecular interactions. Clearly, the intramolecular hydrogen bond renders the amide carbonyl a much less favorable acceptor group and hence it is less able to compete for polymer hydrogen bond donors. In particular, PAA is known to selfassociate by forming dimers,21 and it is very likely that the available charge on the CO group in flutamide is not greater than that for the CO of the polymer leading to a lack of interaction between the two components. In fact, at higher drug−polymer ratios, it appears that flutamide-PAA blends are immiscible based on the IR spectra where the spectrum of the blend appears to be the summation of the individual component spectra, and DSC measurements where a 50 wt % blend had a Tg identical to that of flutamide. Having established that the carbonyl acceptor group in flutamide is a poor acceptor group due to intramolecular 3228



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(6) Thybo, P.; Pedersen, B. L.; Hovgaard, L.; Holm, R.; Mullertz, A. Characterization and physical stability of spray dried solid dispersions of probucol and PVP-K30. Pharm. Dev. Technol. 2008, 13, 375−386. (7) Matsumoto, T.; Zografi, G. Physical properties of solid molecular dispersions of indomethacin with poly(vinylpyrrolidone) and poly(vinylpyrrolidone-co-vinylacetate) in relation to indomethacin crystallization. Pharm. Res. 1999, 16, 1722−1728. (8) Crowley, K. J.; Zografi, G. The effect of low concentrations of molecularly dispersed poly(vinylpyrrolidone) on indomethacin crystallization from the amorphous state. Pharm. Res. 2003, 20, 1417− 1422. (9) Van den Mooter, G.; Wuyts, M.; Blaton, N.; Busson, R.; Grobet, P.; Augustijns, P.; Kinget, R. Physical stabilisation of amorphous ketoconazole in solid dispersions with polyvinylpyrrolidone K25. Eur. J. Pharm. Sci. 2001, 12, 261−269. (10) Taylor, L. S.; Zografi, G. Spectroscopic characterization of interactions between PVP and indomethacin in amorphous molecular dispersions. Pharm. Res. 1997, 14, 1691−1698. (11) Kestur, U. S.; Taylor, L. S. Role of polymer chemistry in influencing crystal growth rates from amorphous felodipine. CrystEngComm 2010, 12, 2390−2397. (12) Ceolin, R.; Agafonov, V.; GonthierVassal, A.; Szwarc, H.; Cense, J. M.; Ladure, P. Solid-state studies on crystalline and glassy flutamide Thermodynamic evidence for dimorphism. J. Therm. Anal. 1995, 45, 1277−1284. (13) Hernández Sánchez, F.; Molina Mateo, J.; Romero Colomer, F. J.; Salmerón Sánchez, M.; Gómez Ribelles, J. L.; Mano, J. F. Influence of low-temperature nucleation on the crystallization process of poly(l-lactide). Biomacromolecules 2005, 6, 3283−3290. (14) Masirek, R.; Piorkowska, E.; Galeski, A.; Mucha, M. Influence of thermal history on the nonisothermal crystallization of poly(L-lactide). J. Appl. Polym. Sci. 2007, 105, 282−290. (15) Andrews, G. P.; Abudiak, O. A.; Ones, D. S. Physicochemical characterization of hot melt extruded bicalutamide-polyvinylpyrrolidone solid dispersions. J. Pharm. Sci. 2010, 99, 1322−1335. (16) Baird, J. A.; Van Eerdenbrugh, B.; Taylor, L. S. A classification system to assess the crystallization tendency of organic molecules from undercooled melts. J. Pharm. Sci. 2010, 99, 3787−3806. (17) Miyazaki, T.; Yoshioka, S.; Aso, Y.; Kojima, S. Ability of polyvinylpyrrolidone and polyacrylic acid to inhibit the crystallization of amorphous acetaminophen. J. Pharm. Sci. 2004, 93, 2710−2717. (18) Rumondor, A. C. F.; Ivanisevic, I.; Bates, S.; Alonzo, D. E.; Taylor, L. S. Evaluation of drug-polymer miscibility in amorphous solid dispersion systems. Pharm. Res. 2009, 26, 2523−2534. (19) Trasi, N. S.; Taylor, L. S. Effect of polymers on nucleation and crystal growth of amorphous acetaminophen. CrystEngComm 2012, Submitted. (20) Cense, J. M.; Agafonov, V.; Ceolin, R.; Ladure, P.; Rodier , N. Crystal and molecular structure analysis of flutamide. Bifurcated helicoidal C-H···O hydrogen bonds. Struct. Chem. 1994, 5, 79−84. (21) Dong, J.; Ozaki, Y.; Nakashima, K. Infrared, Raman, and nearinfrared spectroscopic evidence for the coexistence of various hydrogen-bond forms in poly(acrylic acid). Macromolecules 1997, 30, 1111−1117. (22) Van Eerdenbrugh, B.; Taylor, L. S. An ab initio polymer selection methodology to prevent crystallization in amorphous solid dispersions by application of crystal engineering principles. CrystEngComm 2011, 13, 6171−6178. (23) Konno, H.; Taylor, L. S. Influence of different polymers on the crystallization tendency of molecularly dispersed amorphous felodipine. J. Pharm. Sci. 2006, 95, 2692−2705. (24) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann: Oxford, 2001. (25) Weissbuch, I.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L.; Rehovot. Understanding and control of nucleation, growth, habit,

negative volume excess free energy between the particle and the melt is lower than the positive interface excess free energy between the newly forming crystal nuclei and the supercooled liquid surrounding it, resulting in an overall positive change in free energy.24 For the prenucleus to form a stable nucleus, it has to increase beyond a critical radius. Since nilutamide has a molecular structure which is very similar to flutamide, favorable molecular recognition, which is a very important factor for nucleation, may allow nilutamide to interact and result in incorporation in the prenucleus. Since the molecular structure of nilutamide is larger than that of flutamide it is quite possible that nilutamide disrupts the nucleus by not allowing additional growth. Such a mechanism has been proposed for inhibition of nucleation by impurities from the solution state.25,26

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CONCLUSIONS The effect of chemically diverse polymers on the crystal growth rate of flutamide has been evaluated. Even though flutamide has both hydrogen bond donor and acceptor groups, effective reduction in growth rates was observed only for polymers having good hydrogen bond acceptor groups. Hydrogen bond formation between these polymers and flutamide was verified by infrared spectroscopy. Polymers with good hydrogen bond donor groups were not effective growth rate inhibitors, most likely due to the involvement of the acceptor group of flutamide in an intramolecular hydrogen bond interaction which reduces the ability of this group to participate in intermolecular interactions. Polymers effective at reducing crystal growth rates were not found to have a similar impact on nucleation. This study thus points to the importance of drug−polymer interactions for reducing crystallization rates in amorphous formulations. It also highlights the importance of considering not only the molecular structure of the crystallizing compound but also the availability of functional groups to participate in intermolecular interactions.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +1 (765) 494-6545; tel: +1 (765) 496-6614; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation Engineering Research Center for Structured Organic Particulate Systems (NSF ERC-SOPS) (EEC-0540855) for financial support.

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REFERENCES

(1) Yu, L. Amorphous pharmaceutical solids: preparation, characterization and stabilization. Adv. Drug Delivery Rev. 2001, 48, 27−42. (2) Hancock, B. C.; Zograf, G. Characteristics and significance of the amorphous state in pharmaceutical systems. J. Pharm. Sci. 1997, 86, 1−12. (3) Ediger, M. D.; Angell, C. A.; Nagel, S. R. Supercooled liquids and glasses. J. Phys. Chem. 1996, 100, 13200−13212. (4) Yoshioka, M.; Hancock, B. C.; Zografi, G. Inhibition of indomethacin crystallization in poly(vinylpyrrolidone) coprecipitates. J. Pharm. Sci. 1995, 84, 983−986. (5) Vyazovkin, S.; Dranca, I. Effect of physical aging on nucleation of amorphous indomethacin. J. Phys. Chem. B 2007, 111, 7283− 7287. 3229



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dissolution and structure of two- and three-dimensional crystals using 'tailor-made' auxiliaries. Acta Crystallogr. Sect. B 1995, 51, 115−148. (26) Anwar, J.; Boateng, P. K.; Tamaki, R.; Odedra, S. Mode of action and design rules for additives that modulate crystal nucleation. Angew. Chem.-Int. Ed. 2009, 48, 1596−1600.

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