Communication pubs.acs.org/crystal
Spherical Crystallization of Carbamazepine/Saccharin Co-Crystals: Selective Agglomeration and Purification through Surface Interactions Sudhir K. Pagire,† Sachin A. Korde,† Benjamin R. Whiteside,‡ John Kendrick,§ and Anant Paradkar*,† †
Centre for Pharmaceutical Engineering Science, University of Bradford, Bradford BD7 1DP, U.K. Polymer Micro and Nano Technology, University of Bradford, Bradford BD7 1DP, U.K. § Department of Chemistry and Forensic Science, University of Bradford, Bradford BD7 1DP, U.K. ‡
S Supporting Information *
ABSTRACT: Spherical crystallization involves crystallization and simultaneous agglomeration of a crystalline particle using an immiscible phase, which has preferential affinity for the crystal surface. Here, we report application of a spherical crystallization technique to the field of cocrystallization. Carbamazepine/saccharin (CBZ/SAC) co-crystals were generated using reverse antisolvent addition and agglomerated using different bridging liquids. Two crystal forms of CBZ/SAC co-crystals were formed, depending on the levels of supersaturation achieved during processing. The selective agglomeration of co-crystal occurred during the agglomeration stage, depending on the relative interaction between bridging liquid and the crystal surfaces. The computational investigation of isosteric heats of adsorption of the bridging liquids at the prominent crystal surfaces proved to be a useful tool in understanding the surface interactions. The spherical crystallization technique shows opportunity to generate co-crystals and its purification through selective agglomeration.
Spherical crystallization (SC), a technique which involves crystallization of a chemical substance and simultaneous agglomeration of crystals, was developed by Kawashima et al.1,2 This technique involves the addition of a solution of the substance in a good solvent to a poor solvent, resulting in crystallization and simultaneous agglomeration using an immiscible bridging liquid phase in the form of droplets. The agglomeration process involves wetting of crystals by an immiscible bridging liquid phase, coalescence of the wetted crystals, squeezing of bridging liquid and compression due to shear provided by an agitator, resulting in spherical agglomerates3 (Figure 1). This process enables manipulation of both primary and secondary properties of the particles and has been well-explored to achieve the desired physicochemical, micromeritic, and mechanical properties of pharmaceutically active compounds.4−6 There are few reports about the generation of agglomerates containing two different crystalline entities.7−9 Desiraju introduced the term “co-crystal” to describe multicomponent, molecular, crystalline systems.10 Co-crystals consist of more than one component in a definite stoichiometric ratio held together by noncovalent interactions within a single crystal lattice.11 For pharmaceutical applications, co-crystals contain a pharmaceutical agent (PA) and a co-former (CF), which is a substance generally recognized as safe (GRAS) for human © XXXX American Chemical Society
Figure 1. Representation of spherical crystallization process.
consumption. These are designed to tailor solubility,12,13 stability,14,15 and mechanical properties of the materials.16 Here, we report for the first time the formation of co-crystals by a reverse antisolvent technique, along with selective agglomeration using a bridging liquid. Previous reports on SC concern crystallization of a single component and its Received: May 24, 2013 Revised: July 17, 2013
A
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relatively less soluble at 20 °C in methanol and ethanol than DMSO but they both show a sharp rise in solubility with temperature, therefore solutions were warmed to 60 °C, which dissolved the solids completely. The crystalline product was characterized using powder Xray diffractometry (PXRD) (refer to Figure SI.3 of the Supporting Information). The phase diagram was generated based on the PXRD results. The phase diagram (Figure 2) shows crystalline phases obtained under different solvent composition and stoichiometric ratios (for more details see Figure SI.3 of the Supporting Information). It is observed that a higher proportion of good solvent favors formation of FI and in some cases carbamazepine dihydrate (CBZ-D). It is clear from the phase diagram that 2 parts of the good solvent with a high proportion of SAC-favored formation of FII in all solvent systems. The formation of crystal forms depends on the level of supersaturation achieved with respect to both the reacting components and the co-crystal. The formation of FII is favored at high supersaturation with respect to individual components and co-crystal compared to FI. The formation of CBZ-D was observed when a noncongruent state was achieved. In the cases, where SAC is not available for CBZ in the molar proportion, it interacts with water, leading to formation of CBZ-D.20 This was observed particularly in the case of DMSO where the solubility difference between CBZ and SAC is higher compared to methanol and ethanol. The incongruent solubility behavior required the use of higher stoichiometric ratios to achieve phase pure co-crystal systems.21 The higher stoichiometric ratios of SAC were required to achieve higher supersaturation, given the higher solubility of SAC as compared to CBZ in good as well as poor solvents. All solvents show a variation in crystal form with a stoichiometric ratio, Figure 3 (refer to Figure SI.2 of the Supporting Information for the remaining PXRD). Methanol and ethanol show FII at lower stoichiometric ratios than DMSO (Figures SI.2 and SI.3 of the Supporting Information), which can be attributed to the lower level of supersaturation achieved in DMSO because of the higher solubility of both the components and the low vapor pressure of the DMSO. A low vapor pressure means that less solvent is lost by evaporation during the experiment. In the case of a high vapor pressure solvent, loss of solvent will lead to an increase in supersaturation. We observed the formation of FII at a stoichiometric ratio of 1:1.5 when the volume of DMSO was reduced by half to 10 mL, which clearly indicates a relationship between levels of supersaturation and crystal form. Differences in the solubilities of components are always a major challenge in antisolvent addition crystallization. Sander et al.22 prepared nano co-crystals of caffeine and 2,4-dihydroxyaminobenzoic acid by antisolvent addition. They used ultrasound and surfactants to produce nano-sized co-crystals and to overcome the issues associated with the inherent solubility differences between PA and CF. The good solvent to bad solvent ratio was fixed at 2:9 for the further experiments as it has minimum amount of good solvent required to dissolve both the components, chances of generating FII, and providing maximum yield of co-crystals. The solubility data of the individual components and cocrystals in pure solvents and solvent mixtures in the 2:9 ratio in the presence of SAC was generated to provide mechanistic understanding of the crystallization process. Rodriguez group has shown that the solubility of the co-crystal phase decreases
agglomeration using bridging liquids. Application of the SC concept for co-crystallization is challenging because of the presence of many components, including the co-crystal, PA, and CF. With the use of the antisolvent, co-crystallization achieving equimolar supersaturation of both the components is difficult because of the difference in solubilities of PAs and CFs, which generally results in contamination of the co-crystal phase with one of the components. The bridging liquid will preferentially agglomerate one component over the others; this selective agglomeration can be used to separate a desired component from the multicomponent system. We selected the carbamazepine/saccharin (CBZ/SAC), a well-studied pair,17 because of its suitability for antisolvent crystallization with water. CBZ/SAC co-crystals exist in two forms, forms I (FI) and II (FII). 18 FI is generally obtained by solvent crystallization. There is only one report about FII obtained using polymeric heteronuclei in a solvent evaporation technique.18 FI is stable relative to FII, and FII may therefore exhibit improved bioavailability.18 The generation of a desired polymorph and its selective agglomeration are important challenges in SC of CBZ/SAC co-crystals and requires control of the kinetic and thermodynamic processes occurring in solution and the immiscible bridging liquid phase. We report the successful production of spherical agglomerates of FI and FII separately using a simple antisolvent crystallization technique. Recently, Lee et al. have reported generation of CBZ/SAC co-crystal FI by antisolvent crystallization.19 As antisolvent crystallization enables achievement of high levels of supersaturation, we explored the generation of FII by this technique during our preliminary crystallization experiments before developing SC. The selection of three solvents for this biphasic process was carried out on the basis of the solubility of the components, miscibility, partitioning, and vapor pressure (refer to Figure SI.8 of the Supporting Information). After initial screening, we considered dimethylsulfoxide (DMSO), ethanol, and methanol to be good solvents, water as the poor solvent, and benzene (BEN), dichloromethane (DCM), and ethyl acetate (EA) as the bridging liquids (Table 1). Table 1. Solubility of CBZ, SAC, FI, and FII in the Selected Solvent Systems solubility (M/L) solvent
CBZ
SAC
FI
F II
water DMSO methanol ethanol DCM EA BEN
0.0004 0.345 0.264 0.0804 0.581 0.036 0.017
0.015 5.397 0.389 0.268 0.013 0.278 0.008
0.006 2.727 0.895 0.454 0.170 0.076 0.011
0.007 4.025 0.373 0.202 0.101 0.079 0.019
The preliminary crystallization experiments were conducted to understand the phase behavior of CBZ/SAC in different stoichiometry and in different solvent compositions. The experiments involved addition of the CBZ/SAC solution in a good solvent to 90 mL of a poor solvent (water). The amounts of good solvents were 10, 20, 30, and 40 mL, depending on the solvent composition. The experiments were conducted at 20 °C using three different stoichiometric ratios of CBZ/SAC (1:1, 1:1.5, and 1:2) in a jacketed vessel with an overhead propeller agitator. The solution was stirred at 500 rpm for 5 min. CBZ is B
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Figure 2. Phase diagram showing the crystal forms obtained at different solvent and CBZ/SAC compositions.
Figure 3. PXRD pattern of precipitates obtained from DMSO solution containing CBZ and SAC with stoichiometries of (a) 1:1, (b) 1:1.5, and (c) 1:2 molar ratios.
in the presence of the coformer with an in-depth investigation of carbamazepine co-crystals.23,24 Lee et al. has also reported the formation of indomethacin/saccharin and carbamazepine/ saccharin co-crystals by antisolvent addition method, where it shows significant effect of antisolvent on the solubility of the co-crystal phase and following the same pattern in the presence of the coformer.19,21 The solubility of CBZ and CBZ co-crystals in the presence of different levels of SAC in good solvents and their mixture with bad solvent was determined. The solubility of CBZ and cocrystals decreases with an increase in the SAC concentration, as suggested by previous publications.19 The solubility of CBZ and the co-crystal is highest in DMSO, followed by methanol and ethanol. The drop in solubility of the co-crystal in the presence of a bad solvent acts as a driving force for co-crystallization in antisolvent crystallization experiments. Figure 4 shows the shift in the phase solubility of FI and FII in the methanol/water system during antisolvent crystallization experiments. The
Figure 4. Phase solubility shift of FI and FII during antisolvent crystallization in the methanol/water (2/9) system.
similar studies of other solvent systems are detailed in Figure SI.4 of the Supporting Information. We have used SAC in excess to maintain its supersaturation during the crystallization so as to get FII, and thus we need to determine the amount of free SAC in the final product. The PXRD patterns in most cases showed the absence of CBZ or CBZ dihydrate but failed to provide any estimate of the level of SAC present because of the presence of overlapping characteristic peaks. The absence of a CBZ peak provided assurance that CBZ has completely converted into a co-crystal form. We estimated the excess of SAC in the product using highC
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performance liquid chromatography (HPLC) (for details of the HPLC method, see Figure SI.4 of the Supporting Information) and expressed it in terms of the molar ratio of SAC to CBZ. The values of this ratio were observed to be well above 1, confirming the presence of unreacted SAC in the product (Table 2). SAC has a significantly high solubility in DMSO as compared to the alcohols, which resulted in relatively less unreacted SAC. Table 2. Molar Ratio of SAC to CBZ by HPLC Analysis Molar ratio of SAC to CBZ bridging liquid
methanol
ethanol
DMSO
no addition BEN EA DCM
1.56 1.004 1.343 0.957
1.382 0.997 1.079 1.033
1.15 1.039 1.160 1.145
To understand the role of bridging liquid in SC, 1:2 stoichiometry of CBZ/SAC was chosen as these conditions had made FII in all three good solvents. During these experiments, the selected bridging liquids were added dropwise under continuous stirring in a crystallization vessel with baffles. The bridging liquid was added immediately after addition of solution of CBZ/SAC in good solvent to poor solvent. The amount of bridging liquid used was sufficient to generate agglomerates. The amounts of bridging liquid required to cause agglomeration differed depending on its miscibility with water, the higher the miscibility, the greater the amount required. The amounts required were EA 15 mL, BEN 5 mL, and DCM 6 mL. Agglomerates were then separated by filtration and dried. The agglomerates were spherical in the size range of 430 μm to 1.6 mm (Figures 5 and 6 and Figure SI.7 of the Supporting
Figure 6. SEM images of spherical agglomerates of CBZ/SAC II prepared using ethanol as a good solvent and agglomeration with (a) EA, (b) BEN, (c) DCM, and (d) BEN with methanol as a good solvent.
Table 3. Co-Crystal Forms Obtained by SC bridging liquid good solvents
BEN
EA
DCM
DMSO methanol ethanol
FI FII FII
FI + FII FII FII
FI FI +FII FI + FII
Figure 5. Spherical agglomerates of CBZ/SAC II prepared using ethanol as a good solvent and agglomeration with BEN.
Information). The crystal forms obtained were confirmed by PXRD (Table 3 and Figure 7). For the PXRD patterns of remaining batches refer to Figure SI.5 of the Supporting Information. Comparison of the PXRD patterns of the crystallization experiment batches with the SC batches of the same compositions reveals that the presence of bridging liquid has affected the crystal form obtained (Table 3 and Figure SI.3 of the Supporting Information). In absence of a bridging liquid, the supersaturation levels generated upon addition to water are high enough for the formation of FII. However, CBZ, SAC, and co-crystal phase have some solubility in the bridging liquids. This leads to an overall change in the supersaturated state of
Figure 7. PXRD patterns of spherical agglomerated co-crystals obtained by addition of methanol solution of CBZ/SAC in 1:2 molar mixtures in water and agglomeration with (a) BEN, (b) DCM, (c) EA, and from DMSO solution and agglomeration with (d) BEN immediate addition and (e) BEN after 10 min.
the solution, favoring the formation of FI. To investigate further the effect of bridging liquid on supersaturation, we delayed addition of the bridging liquid. Addition of the bridging liquid 10 min after adding the solution of CBZ/SAC in DMSO to water resulted in the formation of FII for all bridging liquids D
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computational calculations, refer to Figure 6 of the Supporting Information.
studied (Figure 7 and Figure SI.5 of the Supporting Information). This confirms that the addition of the bridging liquid disturbs the supersaturation level and in turn the crystal form generated. The molar ratio of SAC:CBZ in agglomerates was estimated using HPLC and the results reported in Table 3. It is clear that SAC excess in the product was significantly reduced when crystallization was carried out in the presence of a bridging liquid, as indicated by molar ratio values close to 1 (Table 2). Agglomerates obtained using EA as a bridging liquid showed relatively higher unreacted SAC, which may be attributed to the higher solubility of SAC (compared to CBZ), favoring its partitioning in the EA phase. In the case of BEN, where the difference in the solubilities of the two components is small, the agglomerates show molar ratios close to 1, indicating an almost phase pure system. The use of DCM as a bridging liquid resulted in a mixture of FI and FII because of the high difference in solubility of the two components, as well as higher solubility for FI and FII compared to BEN and EA. This indicates that imbalance in the supersaturation level of any component will disturb the formation of phase pure co-crystal. In summary, the bridging liquid, depending on the difference in the solubility of the components and the time of addition, may disturb the level of supersaturation in aqueous phase, thereby affecting the crystal form formed. The reduction in amount of unreacted SAC in the agglomerates (Table 2) suggests the selective agglomeration of FII by the bridging liquid during SC. The process of selective agglomeration will depend to a great extent on the interaction between the bridging liquid and the crystalline components. We therefore undertook a computational investigation into the possible nature of these interactions, by calculating the isosteric heats of adsorption of the bridging liquids with the important surfaces of FI, FII, CBZ, and SAC crystals. Full details of the calculations are reported in the Figure 6 of the Supporting Information. The Forcite module of Materials Studio 4.125 was used to determine the optimized unit cells of FI, FII, CBZ, and SAC, using a variety of force fields and atomic charge models. The force field and charge model which gave the least deviation between the experimental starting crystal structure and the final optimized structure was used to perform the other calculations reported here. Using the Morphology module, calculations of the growth morphology for all four crystals were performed by calculating the attachment energies of all the low index surfaces of each crystal. The surfaces with the lowest energies were used to explore their interaction with a variety of sorbent molecules, including water, BEN, DCM, and EA. Based on the calculated growth morphologies, the dominant surface was determined for each crystal. For CBZ, SAC, and the FI and FII co-crystals, the {011}, {100}, {001}, and {200} families of surfaces were calculated to provide 49%, 41%, 56%, and 53% of the surface area of the crystal, respectively, and were therefore subjected to further study. This approach is a gross assumption as the actual important crystal surfaces are unknown and subject to the experimental conditions. However, such an assumption allows the exploration of the possible stabilization of the crystal by the bridging liquid and allows us to test simple models for the interaction between surfaces and solvents. Constant load sorption calculations were carried out by introducing 10 gas phase sorbent molecules into a vacuum slit between a slab whose surfaces represented those of interest, and the resulting isosteric heats of adsorption are shown in Table 4. For full details of the methodology adopted for
Table 4. Isosteric Heats of Adsorption (kcal/mol) Constant Loading sorbent
CBZ (011)
SAC (100)
FI (001)
FII (200)
water DCM EA BEN
1.6 7.1 11.9 12.7
1.5 5.1 7.3 7.4
1.1 4.6 6.6 6.8
1.3 7.5 9.9 11.4
BEN showed the strongest adsorption for CBZ {011} and CBZ/SAC II {200}; this correlates well with the HPLC analysis, which suggests selective agglomeration of CBZ/SAC II when BEN was used as a bridging liquid. As can be seen in Figure 8, the surfaces of CBZ and FII show a similar aromatic rich nature, which is responsible for the high calculated sorption energies of BEN.
Figure 8. Side view of (a) CBZ (011) surface and (b) FII (200) surface.
Compared to water, the bridging liquids show significantly higher affinity for the crystal surfaces, which is essential for agglomeration to occur. During processing, we observed that BEN quickly picked up the crystals resulting in a clear aqueous phase, whereas EA formed finer droplets and required more time to reach the clear aqueous state. Though both bridging liquids have similar high affinity for FII, the high miscibility of EA with water is probably responsible for this difference. The size and density of agglomerates also varied with bridging liquid (Figure 6). BEN provided dense well-compressed agglomerates, whereas they were lightly compressed in the case of DCM. The fast pickup of FII by BEN is in accord with the calculated heat of adsorption. As the crystallites agglomerate, the bridging liquid is squeezed to the surface making more bridging liquid available to pick up more crystallites from the solution. The E
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(13) Smith, A. J.; Kavuru, P.; Wojtas, L.; Zaworotko, M. J.; Shytle, R. D. Mol. Pharmaceutics 2011, 8, 1867−1876. (14) Trask, A. V.; Motherwell, D. W.; Jones, W. Int. J. Pharm. 2006, 320, 114−123. (15) Babu, N. J.; Sanphui, P.; Nangia, A. Chem. Asian J. 2012, 7, 2274−2285. (16) Karki, S.; Friscic, T.; Fabian, L.; Laity, P. R.; Day, G. M.; Jones, W. Adv. Mater. 2009, 21, 3905−3909. (17) Alhalaweh, A.; Roy, L.; Rodriguez-Hornedo, N.; Velaga, S. P. Mol. Pharmaceutics 2012, 9, 2605−2612. (18) Porter, W. W., III; Elie, S. C.; Matzger, A. J. Cryst Growth Des. 2008, 8, 14−16. (19) Wang, I.; Lee, M.; Sim, S.; Kim, W.; Chun, N.; Choi, G. Int. J. Pharm. 2013, 450, 311−322. (20) Rager, T.; Hilfiker, R. Cryst Growth Des. 2010, 10, 3237−3241. (21) Lee, M.; Chun, N.; Wang, I.; Liu, J.; Jeong, M.; Choi, G. Cryst Growth Des. 2013, 13, 2067−2074. (22) Sander, J. R. G.; Bucar, D.; Henry, R. F.; Zhang, G. G. Z.; MacGillivray, L. R. Angew. Chem. Int. Ed. 2010, 49, 7284−7288. (23) Nehm, S.; Rodriguez-Spong, B.; Rodriguez-Hornedo, N. Cryst Growth Des. 2006, 6, 592−600. (24) Alhalaweh, A.; Sokolowski, A.; Rodriguez-Hornedo, N.; Velaga, S. Cryst Growth Des. 2011, 11, 3923−3929. (25) Material Studio 4.1; Accelrys Inc, San Diego, 2007.
vaporization of BEN is slower than DCM, resulting in a longer time for compression. In the case of EA, although there is high affinity, due to higher water miscibility the final droplet size was small and the agglomerates were not highly compressed. In general, the process of spherical crystallization for cocrystals requires careful control at different stages of the process. The stoichiometry of the coformers needs to be controlled in the light of their solubility in the solvent, antisolvent, and bridging liquid. The wetting of the crystal surfaces by the bridging liquid determines the rate at which crystals are removed from the solvent/antisolvent system and this modifies both the supersaturation levels and the nature of the final agglomerate. The squeezing and compression depends on shear provided by the agitator. The rate of evaporation of the bridging liquid, which depends on its vapor pressure, is also important because too fast of evaporation may remove the bridging liquid completely before completion of compression will result in deagglomeration or generation of loose agglomerates, as observed in the case of DCM. To conclude, we have successfully generated an almost phase-pure form II of a carbamazepine/saccharin co-crystal during spherical crystallization by a reverse antisolvent method.
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ASSOCIATED CONTENT
S Supporting Information *
Information concerning the details of HPLC analysis and solubility experiments (SI 4), PXRD results phase diagram (SI 3), and computational study (SI 6). The calculated PXRD patterns of pure components and two forms of the CBZ/SAC co-crystal (SI 1), along with the remaining PXRD patterns of crystal forms obtained during the crystallization experiment performed without and with addition of bridging liquids (SI 2 and SI 5, respectively). SEM images (SI 7). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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