Cocktail-Solvent Screening to Enhance Solubility, Increase Crystal Yield, and Induce Polymorphs - Pharmaceutical Technology

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Cocktail-Solvent Screening to Enhance Solubility, Increase Crystal Yield, and Induce Polymorphs
The authors propose extending initial solvent screening for a single-solvent system to the cocktail solvent screening of binary and ternary solvent mixtures.


Pharmaceutical Technology
Volume 32, Issue 1

Crystallization from solutions is not only an important step in the fabrication of various functional materials in biological systems, but also a key separation and purification process in the manufacture of many fine chemicals and specialty chemicals, especially pharmaceuticals (1, 2). Pharmaceutical crystallizations are often carried out in batches of organic solvents or mixtures of solvents through temperature cooling (3). Because of the excess properties (i.e., the difference between the real properties and the ideal properties) for a real solution, the solubility of an active pharmaceutical ingredient (API) in a solvent mixture is sometimes higher than its solubility in a single solvent as the activity coefficient decreases (4, 5). The solubility enhancement that the solvent mix offers can bring three main advantages to pharmaceutical batch crystallization:

  • It replaces excellent but environmentally harmful solvents such as tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and N, N-dimethylacetamide (DMA) with greener solvents such as alcohols, acetonitrile, and water
  • It gives a higher crystal yield
  • It increases the degree of supersaturation, thus allowing nucleation to be controlled by the lowest nucleation barrier so that the metastable, instead of the thermodynamically stable crystalline phase, is induced (6–8).

Binary-solvent mixtures result when a second solvent (i.e., an antisolvent or cosolvent) is added to a subsaturated solution until the degree of supersaturation is high enough for crystallization to take place under isothermal conditions. Performing further crystallization in ternary solvent mixtures may be a new method of influencing the nucleation rate, the shape of the product crystals, the size distribution of the entire crystallized mass, aggregate and agglomeration properties, the purity of the crystals, and polymorphism (9–14).

Because solubility, crystal yield, and polymorphism are solvent dependent, solution recrystallization by solvent screening is of fundamental and of foremost importance to many chemical process industries. The process is especially important for manufacturing APIs (15–22). Pharmaceutical companies have a limited amount of time and materials to advance new chemical entities from candidacy to Phase I clinical trials (23).

The aim of this article is to extend initial solvent screening for a single-solvent system to the cocktail solvent screening of binary and ternary solvent mixtures on a small scale through temperature cooling from 60 to 25 C, which is often used in drug development and manufacturing (24).


Figure 1 (IMAGES AND FIGURES ARE COURTESY OF THE AUTHORS)
Sulfathiazole, a valuable antimicrobical sulfa drug used in veterinary medicine (see Figure 1), was chosen for this study mainly because of its commercial value and its five polymorphs, which are well-characterized by Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC) (10, 15, 17).

Under the initial solvent-screening strategy of single solvent systems, 24 solvents, mostly useful for scale-up, were selected (25). If all 24 solvents had been taken into account, the possible combinations for binary and ternary solvent mixtures would have been 24! (22!2!) = 276 and 24! (21!3!) = 2024, respectively. However, for the purpose of showing the feasibility of cocktail-solvent screening, the authors limited the screening to three miscible green solvents (acetonitrile, n-propanol, and water) and their combinations. N-propanol and acetonitrile produce Form I and Form IV sulfathiazole crystals, respectively. Water is a common solvent in the manufacture of sulfathiazole (15, 26).


Figure 2
At first, the solubility of sulfathiazole in each solvent or combination of solvent mixtures at 15, 25, 40, and 60 C was measured by gravimetric titration (see Figure 2). This method did not require any calibration and was more robust than weighing the dry-residue mass, which might prompt the formation of sulfathiazole solvates during the drying process (11, 19). The crystal yield was estimated by finding the solubility difference between 60 and 25 C for each solvent or solvent mixture. Solid generation of sulfathiazole in each solvent or solvent mixture was then achieved in a 20-mL scintillation vial by gentle shaking and cooling from 60 C in a water bath to 25 C in another water bath. The cooling rate of a solution with a volume of less than 20 mL was almost independent of the volume and the nature of a solvent. The cooling profile could be approximated by an exponential decay determined experimentally as:

T = 26 + 31 exp (-t 0.9)

where T = temperature (C) and t = time (min). The relatively rapid decrease in temperature served as an ideal way to preserve kinetically induced polymorphs by a sudden surge of supersaturation (8). Solids produced were vacuum filtered at once and oven dried at 40 C for 4 h. DSC and thermogravimetric analysis (TGA) were used mainly to determine the polymorphism of sulfathiazole solid samples. Optical microscopy (OM) was used for crystal-habit imaging.

Materials and methods


Table I: Sources of solvents used in this experiment.
Solvents. Table I lists the solvents used in this study and the companies that provided them. Reversible osmosis (RO) water was clarified with a water purification system (Milli-RO Plus, Millipore, Billerica, MA).

Active pharmaceutical ingredient. Sulfathiazole (Form III) white crystalline powders (C9H9N3O2S2, MW: 255.31, m.p. = 201–204 C, 98%, Lot: 410504/1 51804006) were purchased from Fluka (Buchs, Switzerland).

Solubility and crystal-yield studies. About 10 mg of sulfathiazole Form III crystals were weighed in a 20-mL scintillation vial. Drops of solvent or solvent mixture were titrated into the vial carefully by micropipette and shaken intermittently until all sulfathiazole Form III solids were dissolved. The solubility of sulfathiazole Form III solids at a given temperature was calculated as the weight of sulfathiazole Form III solids in a vial divided by the total volume of solvent or solvent mixture added to a vial. The solubility of sulfathiazole Form III solids in the same solvent or solvent mixture at 15, 25, 40, and 60 C was determined. The crystal yield was calculated as the difference between the solubility at 60 C and at 25 C. All temperatures were maintained and controlled by a water bath. Despite the inherent inaccuracy (20%) of measuring volume by sight, this method provided a rapid and robust technique for process scale-up (24).

Solvent-miscibility studies. Of the 24 solvents, about 1-mL portions of each solvent in a pair were shaken together for approximately 1 min in a 20-mL scintillation vial at 25 C at 1 atm. Assuming that solvent miscibility was independent of the weight fraction of solvent, the solvent pair was considered to be miscible if no interfacial meniscus was observed after the contents of the vial were allowed to settle. If a meniscus was observed without apparent change in the volume of either solvent, the pair was regarded as immiscible (24).

Polymorph studies. A saturated sulfathiazole solution of a particular solvent or solvent mixture was prepared in a 20-mL scintillation vial in accordance with the solubility values determined at 60 C from the experiments in "Solubility and crystal-yield studies" above. Solids were generated by cooling from 60 C in a water bath to 25 C in another water bath and intermittent shaking. Solids were immediately vacuum filtered and oven dried at 40 C for 4 h. Polymorphism and crystal habits were determined by DSC, TGA, and OM.

DSC. A differential scanning calorimeter (DSC 7, Perkin Elmer, Norwalk, CT) was used to monitor thermal events during heating. The instrument was calibrated with indium (8–10 mg, 99.999% pure, extrapolated melting onset at 156.6 C). All samples were run in crimped aluminum pans under a constant nitrogen purge. Each sample was heated at 10 C/min.

TGA. A thermogravimetric analyzer (TGA 7, Perkin Elmer) was used to measure changes in the weight of a specimen while varying temperature in a controlled nitrogen atmosphere. About 2 to 3 mg of sample were placed on an open platinum pan and heated at 10 C/min.

OM. An optical microscope (SZII, Olympus, Tokyo, Japan) equipped with a charge-coupled-device camera (SSC-DC50A, SONY, Tokyo, Japan) was used to take images of crystal habit.

Results and discussion

The 24 solvents, arranged in ascending order by their total Hildebrand values, were n-heptane, xylene, p-xylene, ethyl acetate, toluene, methyl tert-butyl ether (MTBE), benzene, methyl ethyl ketone (MEK), chloroform, THF, DMA, acetone, 1, 4-dioxane, nitrobenzene, n-butyl alcohol, isopropyl alcohol (IPA), benzyl alcohol, acetonitrile, n-propanol, DMF, ethanol, dimethyl sulfoxide (DMSO), methanol, and water (24).

Sulfathiazole Form III crystals dissolved well in 11 solvents (labeled "good solvents"): MEK, THF, acetone, benzyl alcohol, acetonitrile, n-propanol, DMF, ethanol, DMSO, methanol, and water. The crystals dissolved only slightly in 13 solvents (labeled "bad solvents"): n-heptane, xylene, p-xylene, ethyl acetate, toluene, MTBE, benzene, chloroform, DMA, 1, 4-dioxane, nitrobenzene, n-butyl alcohol, and isopropyl alcohol. The crystals' solubility in these solvents was < 0.5 mg/mL at 25 C.

Based on the solvent-miscibility studies of the solvent pairs of the 24 solvents, there were 38 gray boxes 2 = 19 immiscible pairs (because of the symmetry in Table II). The pure-solvent systems are represented by the 24 diagonal boxes in Table II. The form space (i.e., a probable condition of discovering a new polymorph) of the pure-solvent systems for our initial solvent screening was limited to the number of good single solvents, represented by the yellow boxes in Table II (24). Therefore, the probable condition of discovering a new polymorph for sulfathiazole was 11. However, if the good cosolvent systems (i.e., binary miscible mixtures of good solvents) are taken into account, the form space would be extended to the number of blue boxes in Table II 2 = 54. In addition, if the antisolvent systems (i.e., binary miscible mixtures of a good and a bad solvent) are also considered, the form space of the antisolvent systems would be calculated as the number of green boxes in Table II 2 = 126.


Table II: The 24 3 24 form space of sulfathiazole Form III crystals at 25 8C.
Currently, the total form space should then be at least equal to 11 + 54 + 126 = 191. The total form space is expected to expand dramatically if various solvent compositions of binary mixtures, temperatures, and ternary solvent systems are also considered (14). Solid generation by cooling can be applied in the yellow and blue regions in the form space (see Table II) if solubility curves are available. However, isothermal condition is usually employed if solid generation is achieved by adding antisolvent to the green domain (see Table II). Generally, no attempts are made to generate solids in the regions of immiscible solvent pairs (gray boxes), bad solvents (red boxes), and cosolvents of bad solvents (white boxes) (i.e., binary mixture of miscible bad solvents) in the form space (see Table II).


Figure 3
Therefore, only 11 solubility curves of sulfathiazole Form III crystals in 11 kinds of good solvent, based on their solubility at 15, 25, 40, and 60 C, were constructed and grouped by their solubility ranges for ease of comparison (see Figure 3). The crystal yield was calculated as the solubility difference between 60 and 25 C, based on Figure 3 for each solvent (see Figure 4). Although solvents such as DMF and DMSO gave high crystal yields, they are environmentally harmful (6).


Figure 4
Since the 24 24 form space in Table II did not take the solvent compositions and the ternary solvent mixtures into account, the authors developed a "triangular form space" for given ternary solvent mixtures. Three of the 11 good solvents (acetonitrile, n-propanol, and water) were used to concoct three different binary mixtures and 10 different ternary mixtures (see Figure 2). Consequently, there were 3 + 10 = 13 combinatorial mixtures in total. The compositions of each mixture were expressed by mole fraction ratios of acetonitrile:n-propanol:water. The 13 points and three single solvents were coordinated on a triangular graph (see Figure 2). The corresponding 16 solubility curves of sulfathiazole Form III crystals were constructed and grouped by their solubility ranges at 15, 25, 40, and 60 C for ease of comparison (see Figure 5). Solubility curves at 15, 25, 40, and 60 C for the binary and ternary mixtures as a function of mole fractions are illustrated in Figure 6. The crystal yield, based on the solubility difference from 60 to 25C in Figure 5, is shown in Figure 7. The crystal yields of the solvent mixtures such as (80:10:10), (60:20:20), (45:10:45), and (50:0:50) were much higher than that of acetonitrile, n-propanol, and water individually, and almost as high as that of THF.


Figure 5
Comparing the solubility of sulfathiazole Form III crystals in each solvent at 25 C (see Figure 3) with the solvent properties, the authors found that the dispersion-force contribution (Hansen dispersion parameter d) was small in all cases, but the polar-force contribution (Hansen polar parameter p) and the hydrogen-bonding contribution (Hansen hydrogen-bonding parameter h) were critical (24). The solvent needed relatively strong polar force, hydrogen bonding, or both to achieve acceptable solubility by disrupting the hydrogen-bonded network in crystalline sulfathiazole (9). This criterion is shown by the fact that the highest solubility was about 0.03 g/mL sulfathiazole Form III crystals in ternary solvent mixtures of acetonitrile, n-propanol, and water with mole fraction ratios of (45:10:45) (see Figure 6e) and (50:0:50) (see Figure 6a) at 25 C. Acetonitrile and water individually had high values of 18.0 MPa and 42.3 MPa for δp and δh at 25 C, respectively (24). Only when they were both blended at equally high mole fractions could the resultant mixed-solvent system exhibit relatively high solubility (see Figure 5). In this case, the solubility of the solvent mixtures (see Figure 5) was generally better than that of the single-solvent systems (see Figure 3).


Figure 6
Solids generated by the cooling method from the 11 good solvents (i.e., the yellow boxes in Table II) and 13 solvent mixtures (i.e., points 4–16 in Figure 2) were isolated and then analyzed by DSC and TGA. Although sulfathiazole forms solvates, TGA showed that none of the solids were solvates or hydrates because no weight loss occured from 50 to 205 C for all solids (19). Based on the five general melting points of Forms I–V sulfathiazole crystals at around 203, 198, 175, 166 (broad), and 148 C (broad), respectively, DSC responses in Figure 8 illustrated the various sulfathiazole crystal forms grown in the 11 single solvents: MEK (Forms I, III, and V), THF (Form III), acetone (Forms I and V), benzyl alcohol (Form I), acetonitrile (Forms I, III, and IV), n-propanol (Forms I and V), DMF (Form III), ethanol (Forms I, III, IV, and V), DMSO (Forms I, III, and V), methanol (Forms I, III, and V), and water (Forms I and III) (15).


Figure 7
Form II was not observed at all in any solvent. The endotherm at 203 C was the melting point of Form I, which might have been produced by the enantiotropic transformation of Forms II, III, IV (and possibly V) upon heating in the DSC (27). For example, a specimen of pure Form III generated in water that was free from seeds of Form I surpassed the transition point, melted at its melting point, and immediately underwent exothermic recrystallization, as indicated by the exotherm at about 182 C in Figure 8k. But sometimes crystals did not resolidify as polymorphs with higher melting points when the system was kinetically trapped, as in the cases of sulfathiazole solids grown in THF and DMF (see Figures 8b and 8g) (28, 29).

Using the reported relative solubilities in the order of I > II > IV > III, the thermodynamic stability of the structures should increase in the order of I < II < IV < III (16). For sulfathiazole, crystallization from any solvent at a given temperature should initially give rise to the least stable form, followed by the stepwise conversion through the other metastable forms to the thermodynamically most stable Form III. But specific solvent–solute interactions might be needed to stabilize a particular form by inhibiting the nucleation and growth of the next most stable structure. Anwar reported that n-propanol could interfere with the completion of the β-sulfathiazole dimer in structures of Forms II, III, and IV because of its donor–acceptor duality (17). This could be why the efficiency of blocking the formation of a β ring in structures of Forms II, III, and IV increased in the order of water < methanol < ethanol < n-propanol < benzyl alcohol, as reflected by the decreasing amount of Forms III and IV in the DSC profiles (see Figures 8d, 8f, 8h, 8j, and 8k). On the other hand, solvents such as THF and DMF that lack a donor hydrogen and have lower Hansen hydrogen-bonding parameter (δh) values were unable to stabilize the transition state and gave a pure Form III structure without any trace of Form I seeds (see Figures 8b and 8g).

The kinetics effects of solvents on the crystallization of sulfathiazole polymorphs were also noticed in binary and ternary systems (16). Although sulfathiazole forms solvates, TGA scans verified that all sulfathiazole crystals produced were neither solvates nor hydrates because no weight loss upon heating was observed before the melting point of 203 C (19). DSC responses of the sulfathiazole crystals grown in the 13 solvent mixtures of (acetonitrile:n-propanol:water) in Figure 9 showed that the crystals contained (polymorphs): (100:0:0) (Forms I, III, and IV), (0:100:0) (Forms I and V), (0:0:100) (Forms I and III), (50:50:0) (Forms I, III, and IV), (80:10:10) (Forms II, III, IV, and V), (45:45:10) (Forms I and V), (10:80:10) (Forms I, III, and IV), (60:20:20) (Forms I and V), (20:60:20) (Forms I, III, and IV), (33:33:33) (Forms II and III), (10:45:45) (Forms I and VI), (0:50:50) (Forms I, III, and IV), (45:10:45) (Forms I, III, and IV), (20:20:60) (Forms I, III, and IV), (50:0:50) (Forms I, III, IV, and V) and (10:10:80) (Forms I, III, and IV).

The endotherm at 203 C was the melting point of Form I, which also might have been produced by the enantiotropic transformation of Forms III, IV, V, and VI upon heating in the DSC (29). But this time, Form II was observed for solids generated by the ternary solvent mixtures of (80:10:10) and (33:33:33) (see Figures 9e and 9j). Moreover, an endotherm that appeared at a relatively low 135 C for the ternary solvent mixture of (10:45:45) (see Figure 9k) was considered a newly discovered polymorph for sulfathiazole, conventionally Form VI (29).

The crystal habits and length–breadth aspect ratios of sulfathiazole crystals produced by the 11 pure single solvents and 13 solvent mixtures of acetonitrile, n-propanol, and water by cooling are shown in Figures 10 and 11, respectively. In general, Form I possessed a needle-like morphology. Form II had a cuboid morphology. The morphology of Form III was a truncated hexagon and Form IV had a plate-like hexagonal morphology (9). If crystal habits alone are used as a visual guide, the major phases grown from the 11 pure single solvents were: Form I in MEK, acetone, benzyl alcohol, acetonitrile, and n-propanol; Form III in THF, DMF, ethanol, DMSO, and water; and Form IV in methanol. Clearly, the observation that Form I appeared with the same crystal habit from more than one solvent suggested that none of the available solvent–surface interactions could inhibit the addition of sulfathiazole molecules to the {010} face (9). A similar conclusion could be drawn for Form III and its fast-growing {110} face (9).


Figure 8
In the solvent mixture systems of acetonitrile:n-propanol:water, Form I was the major phase in the needle-like crystals grown in the solvent mixtures of (50:50:0), (45:45:10), (60:20:20), (0:50:50), and (50:0:50), judging by the crystal habits. Form III was the major phase in the truncated hexagonal crystals produced by mixtures of (80:10:10), (10:80:10), (33:33:33), (20:20:60), and (10:10:80). Form IV was the major phase in the plate-like hexagonal crystals generated in the solvent mixtures of (10:80:10), (10:45:45), (45:10:45), and (10:10:80). Unlike the crystal habits in the pure solvent systems, not all of the crystal habits in the solvent mixtures were well defined.


Figure 9
The authors speculated that the lack of definition was caused by the presence of various polymorphs in a crystal, as indicated by the DSC scans (see Figures 8 and 9), and not by the solvent–surface interactions. The average size of crystals grown in the solvent mixtures was larger than those grown in pure solvent systems. Because the DSC traces in Figures 8 and 9 were derived from the sulfathiazole crystals produced by spontaneous nucleation, if the high crystal yield given by a solvent mixture is desired, the polymorphic purity of sulfathiazole can be ensured by seeding (3).

Conclusion


Figure 10
Successful large-scale preparation of fine chemicals, and the manufacture of pharmaceuticals in particular, depends heavily on the solubility, crystal yield, and polymorphism of solid compounds and active pharmaceutical ingredients. The cocktail solvent screening method is a simple and inexpensive technique on a laboratory scale. It should be implemented together with the initial solvent screening approach to enhance the solubility of green solvents, increase crystal yield, and optimize the chances of finding the number of polymorphs by coupling with DSC.

Acknowledgements


Figure 11
This work was supported by a grant from the National Science Council of Taiwan (NSC 93-2113-M-008-012-MY2). Suggestions from Ms. Jui-Mei Huang of the National Central University Precision Instrument Center regarding DSC and TGA are gratefully acknowledged.

Tu Lee is an assistant professor and Shi Ting Hung was a graduate student in the Department of Chemical and Materials Engineering and the Institute of Materials Science and Engineering, National Central University, 300 Jhong-Da Rd., Jhong-Li City 320, Taiwan, China, tel. 1886 3 422 7151 ext. 34204, fax 1886 3 425 2296,

*To whom all correspondence should be addressed.

Submitted: Mar. 26, 2007. Accepted: Apr. 27, 2007.

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