Development and Scale-Up of Cocrystals Using Resonant Acoustic

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Development and Scale-Up of Cocrystals using Resonant Acoustic Mixing David J am Ende, Stephen R Anderson, and Jerry S Salan Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/op4003399 • Publication Date (Web): 14 Jan 2014 Downloaded from http://pubs.acs.org on January 15, 2014

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Organic Process Research & Development

Development and Scale-Up of Cocrystals using Resonant Acoustic Mixing David J. am Ende1, Stephen R. Anderson, and Jerry S. Salan Nalas Engineering Services, Inc., 85 Westbrook Rd, Centerbrook, CT, 06409

KEYWORDS: cocrystal, cocrystallization, coformer, resonant acoustic mixing, LabRAM, liquid assisted grinding, mechanochemistry, solvent drop grinding, scale-up Table of contents graphic

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author to whom correspondence should be addressed. [email protected],

telephone: 860-388-7275.

ACS Paragon Plus Environment

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ABSTRACT: In the present work resonance acoustic mixing was applied to afford a practical and environmentally-friendly approach to produce and scale-up cocrystals. Scale-up options for producing cocrystals are limited. Solution phase cocrystallizations, although amenable to scaleup in stirred tanks, may be limited due to multiple solubility constraints on both coformers and the product cocrystals resulting in challenges to find feasible processing conditions. While mechanochemical methods such as liquid assisted grinding (LAG), solid drop grinding (SDG), and ball-milling have shown to be more general than solution-phase methods, they are also more difficult and impractical to scale-up. In the present work a resonant acoustic mixer was used to intimately mix API compound and coformer at high frequency, in the presence of a small amount of solvent, to induce conversion to cocrystals with no grinding media required. Carbamazepine (CBZ) and nicotinamide (NCT) were used as a model system for successfully producing CBZ:NCT cocrystals. Thus it was shown that resonant acoustic mixing provides the mixing intensity required of lab-scale mechanochemical methods, such as liquid assisted grinding, but now on a platform more amenable to larger scale manufacture. Resonant acoustic mixing in general has been demonstrated to be scalable to volumes greater than 200 L and thus affords a potential new platform for cocrystallization processes.

Introduction: Pharmaceutical companies are seeking new ways to improve solubility of poorly solubility active pharmaceutical ingredients (APIs). For this reason, cocrystals are becoming increasingly important to the pharmaceutical industry because they provide an opportunity for tuning the physicochemical properties of APIs without altering their intended biological activity. At least one co-crystal drug is currently in development and in clinical trials. The synthetic C-aryl glycoside ertugliflozin is a sodium glucose cotransporter 2 (SGLT2) inhibitor currently in clinical development for the potential treatment of type 2 diabetes mellitus. A cocrystalline complex of the amorphous solid with L-pyroglutamic acid was developed to improve the physical properties of API for manufacturing and to “ensure robust API quality”.1 Cocrystals may be used to enhance solubility, dissolution, and bioavailability of a poorly soluble drug, to avoid an oil or amorphous phase, to simply improve chemical stability characteristics, or to attenuate a physical stability issue. Cocrystals have been shown to change the aforementioned


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attributes as well as change thermal behavior, hygroscopicity, particle size and shape, flow, and mechanical properties.2 Similarly for energetic materials, cocrystals provide an opportunity to tailor performance characteristics of explosives especially important for developing new insensitive munition (IM) applications. The energetic material must be formulated or prepared in such a way to reduce sensitivity to unintended initiation. In this way, cocrystals afford an opportunity to tune the sensitivity and overall performance characteristics of energetic materials. One of the main challenges that exist with cocrystallization methods is their ability to scale-up to multi-kilogram scale. Current methods of forming cocrystals in lab scale include direct solution phase crystallization, solid-state grinding [also termed solvent drop grinding (SDG)], liquid assisted grinding (LAG), and ultrasonic assisted methods3 . Other than the solution phase crystallization method, these other methods are not necessarily amenable to large-scale manufacturing. Solution phase crystallizations for cocrystals are common and similar to traditional crystallizations but have the added challenge of producing the desired cocrystal without concomitantly precipitating API or coformer. The development of the cocrystallization process can sometimes be accomplished through understanding of the ternary phase diagram for design while typically requiring precise seeding strategies.4,5,6

Early development and screening

cocrystals typically rely on slow evaporation and mechanochemical methods as among the most commonly used. 7 The underlying mechanism for the formation of cocrystal products through the application of mechanical stress has been described, and more accurately postulated, in numerous references and compiled in a recently published text.7 Not surprisingly the ability to produce cocrystal products through mechanochemical means requires that the energy input into the system exceeds the relative strength of the supramolecular interactions responsible for “holding” crystalline substrates together. The use of solvents to facilitate or push equilibrium to the cocrystal products improves the reaction kinetics and permits the formation of cocrystal systems not accessible through other methods. Supramolecular synthons include hydrogen bonding, halogen bonding, π - π stacking, and weaker dispersion force interactions are readily disrupted by moderate mechanical force.8


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It is postulated that cocrystal formation through neat grinding transverses one of three paths and may in fact simultaneously follow more than a single path. The coformers’ crystalline phases undergo molecular diffusion via a (i) gas, (ii) liquid eutectic or (iii) amorphous phase followed by cocrystal formation. The preparation of a cocrystal via neat grinding relies on the three higher energy transition states (relative to the crystalline state of the coformers) with increased molecular mobility.8 Vapor phase diffusion along with surface migration has been cited as the probable mechanism driving the formation of cocrystals of picric acid and aromatic hydrocarbons.9

Cocrystal

formation between p-benzoquinone and bis-β-naphthol requires mechanical activation presumably to overcome strong intermolecular forces binding the bis-β-naphthol molecules together in the crystalline state. Thus mechanical energy is used to form disordered layers on the crystal surfaces and subsequently removing them through shearing leaving a clean surface to repeat the process. 10 Charge transfer interactions are the primary forces responsible for the formation and color of pbenzoquinone cocrystals formed through the molecular diffusion of coformers such as 2,2’biphenol after mixing the components.10 Cocrystal formation through a eutectic phase was observed for the diphenylamine – benzophenone system.

A eutectic phase forms spontaneously at the interface between

diphenylamine – benzophenone crystals which is enhanced by mechanical stimulation. Additional mechanical energy promotes cocrystal precipitation from the eutectic liquid.11 Cocrystal formation through an intermediate amorphous phase has been employed by most pharmaceutical companies during the screening process and has been demonstrated in academic labs. The carbamazepine – saccharin system has been shown to go through an amorphous phase when mechanical activated at lower temperatures. Grinding at temperatures near the glass transition accelerates the cocrystal product formation.12 Numerous cocrystals formed through grinding have been shown to first form a metastable cocrystal of the same stoichiometry or a cocrystal of a different stoichiometry as an intermediate phase.13


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The addition of a small volume of a solvent (liquid assisted grinding, solvent drop grinding) facilitates the cocrystal product formation through accelerating kinetics by wetting coformer surfaces. The addition of small volumes of liquid provides a distinct advantage in that the cocrystal products have a tendency to be more crystalline than the neat grinding counterparts. In this way one can exercise control over the product stoichiometry, thus screening can be faster than evaporative slurry or solution methods, as well as the cocrystals can be prepared independent of the relative solubility of the coformers.8 Solid-state grinding to induce a chemical change (aka mechanochemistry) offers advantages because it can be used to produce cocrystals that are difficult or inefficient (energy) by other methods.8 Solid state grinding, either dry or with a small amount of solvent added (liquidassisted grinding) is typically achieved through mortar and pestle grinding or ball-mill grinding wherein stainless steel balls or ceramic grinding media are employed.

Solid-state grinding

approaches are difficult to scale-up to manufacturing multi-kilogram or larger quantities. For energetic materials especially, solid-state grinding with grinding media presents a potential hazard due to friction generated during grinding, potentially initiating an energetic material resulting in detonation or deflagration. Ultrasonics of powders have issues creating homogeneity across the sample or mixture and are also not easily scaled for powder systems. As Schultheiss and Newman6 rightly noted, “slow evaporation and grinding are the two most common techniques for cocrystal growth; however, these approaches possess obvious limitations upon scale-up, and thus additional routes must be formulated.” In the present work a resonant acoustic mixer was used to intimately mix API compound and coformer at high frequency, in the presence of a small amount of solvent, to induce conversion to cocrystals with no grinding media required. Carbamazepine (CBZ) and nicotinamide (NCT) were used as a model system for producing CBZ:NCT (I) cocrystal. The polymorph CBZ:NCT (I) system is characterized by a melting point of 160 ºC and a heat of fusion of 150.4 J/g. 14 Thus we set out to test the idea of employing resonant acoustic mixing (RAM) as a new technology replacement for liquid assisted grinding and to exemplify the idea via producing CBZ:NCT cocrystals. We followed a similar procedure as described by Weyna et. al. (2009) for their SDG/LAG experiments employing carbamazepine. Specifically, solids were weighed out in the desired stoichiometric proportions followed by addition of 20 microliters of solvent per 100 mg


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of total solids.13 The results of carbamazepine cocrystals prepared using resonant acoustic mixing are the subject of this paper. This approach was recently extended toward preparing energetic-energetic cocrystals (e.g. CL-20:HMX) in Nalas laboratories.

Specifically we

synthesized and confirmed the identification of energetic:energetic CL-20:HMX cocrystal prepared via resonant acoustic mixing. The details will be described in a separate publication. Experimental Details: Resonant Acoustic Mixer: Cocrystallization studies were performed in a LabRAM® resonant acoustic mixer (Resodyn™ Acoustic Mixers, Inc., Butte, MT) shown in Figure 1. The screening experiments were carried out as follows: Carbamazepine 100 mg was weighed into a vial followed by one molar equivalent of coformer. The coformers studied were nicotinamide, saccharin, and 4-aminobenzoic acid. Solvent was added to the solid mixture using a calibrated laboratory pipette. For each experiment, 20 microliters of solvent per 100 mg of total solids were added to the top of the solid mixture. Solvents studied were methanol, DMF, water, chloroform, DMSO, cyclohexane, toluene, and ethyl acetate. The vial was then sealed with a screw cap. The vial was placed in secondary containment and then placed in the sample holder and secured. The LabRAM® was turned on for desired intensity, typically near 80 to 100 G (where G = acceleration of gravity), in auto-resonance mode (typically near 60 Hz). Larger scale runs were performed with slight modifications to the small-scale runs. CBZ was weighed into a plastic vial followed by nicotinamide. The powders were pre-mixed in the LabRAM® at 30% intensity for five minutes. In this way when the solvent is added, it can be distributed across both materials more evenly. After the dry powder mix, the solvent was added on top of the powder. The cap was replaced, vial secured in LabRAM® and resonance started for one hour at 80% (or in initial tests two hours at 90%). During the time held at 80% intensity the G forces increased from 80 to 90 G. During the initial phases of resonance the wet solids appear to be “fluidized’ in the mixing chamber. By the end of the resonance period the CBZ:NCT cocrystal was typically “caked” at the bottom of the vial. For the scale-up runs a suitable reslurry wash solvent (acetonitrile) was added to fluidize the cocrystal wet-cake to dissolve any unreacted coformers or impurities. This reslurry was an added step after the cocrystallization. Specifically, acetonitrile was added (6


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volumes) to the wet-cake of cocrystals and agitated in LabRAM® for five minutes; the slurry was easily filtered, and the solids were typically dried under vacuum at 50 °C.

Figure 1: A: LabRAM® resonant acoustic mixer (Resodyn™ Acoustic Mixers, Inc., Butte, MT), B: several sample vials employed on LabRAM allows a wide range of lab scale-up. C: Secondary containers for the sample vials are shown with a 6-place sample holder.

Solid-State Characterization: Materials were characterized utilizing the following instruments:

A Rigaku MiniFlex600

powder x-ray diffractometer (PXRD) equipped with a copper source at 30 kV, and 15 mA typically scanned over a range of 2-40 degrees (2-Theta). A Mettler Toledo 822 Differential Scanning Calorimetry (DSC) was used for thermal analysis using 40 microliter aluminum sample and reference pans scanned at a rate of 10°C/min. Materials:


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Carbamazepine 98% was supplied from Alfa Aesar and used without further purification (Batch J62590 LOT 27Y004). Nicotinamide 99% was supplied from Alfa Aesar and used without further purification (Batch A15970 LOT KO3W063). 4-Aminobenzoic acid was purchased from Sigma 99% Lot SLBB3759V. Solvents were used without further purification.

Results: CBZ:NCT Cocrystal (MeOH) A series of experiments were performed utilizing the LabRAM® resonance acoustic mixer to produce cocrystals of carbamazepine. In the first example 100 mg of carbamazepine and 51.7 mg of nicotinamide were added to a 4-ml vial in a 1:1 molar stoichiometric ratio. 30 microliters of methanol were added to the vial via pipette. The vial was capped and placed securely into the resonant acoustic mixer. The LabRAM® was used to mix the powders for two hours at 90% intensity in auto-resonance mode. The powder was analyzed by powder x-ray diffraction. The PXRD patterns shown in Figure 2 show unique reflections for CBZ:NCT cocrystal at 5.2, 6.7, 8.9, 10.4, 12.3, 18.0, 20.7 (2-Theta). Carbamazepine Nicotinamide in LabRAM 30000

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Figure 2: carbamazepine:nicotinamide (MeOH) cocrystal formed in the presence of 30 µl of methanol as solvent, assisted using resonant acoustic mixing (LabRAM®) at 90% intensity (61 Hz and 100 G) after two hours of mixing.


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Following a similar procedure used in the previous example, a series of experiments were performed with carbamazepine and nicotinamide coformers while varying the solvent that is used during the cocrystallization in the LabRAM®. 100 mg of carbamazepine and 51.7 mg nicotinamide were mixed with 30 microliters of solvent. The following solvents were used neat and charged onto the powder at room temperature each in separate vials: chloroform (CHCl3), water, dimethylformamide (DMF), dimethylsulfoxide (DMSO), methanol (MeOH), cyclohexane, toluene, and ethyl acetate. The set of vials were simultaneously placed in a sample holder and subjected to resonance acoustic mixing for two hours at 90% intensity.

The powder x-ray

diffraction patterns for these eight experiments are shown in Figure 3. In all cases CBZ:NCT cocrystal was formed, with the exception of cyclohexane where little or no conversion was obtained. Increased solubility of the coformers in the solvent facilitates rate of conversion. Cyclohexane had the lowest predicted solubility for CBZ:NCT cocrystal based on COSMOtherm15 solubility calculations included in Supplementary Information. DSC thermograms of the cocrystals obtained after 2 hours of resonant acoustic mixing with each solvent are included in the supplementary information.

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Figure 3: Carbamazepine:nicotinamide cocrystallization (various solvents) assisted using resonant acoustic mixing (LabRAM®) at 90% intensity.


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CBZ:NCT Cocrystal (Water) Water as the cocrystallization solvent is an obvious choice from a Green Chemistry perspective and was one of the solvents tested as described above. In the small scale screening tests the cocrystal isolated using 20 microliters of deionized (DI) water per 100 mg of solids in the LabRAM®. The DSC of CBZ:NCT cocrystal is shown in Figure 4 with melting point (onset) of 157.2oC and heat of fusion of -139.5 J/g uncorrected residual for solvent content. êxo mW

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Figure 4: DSC of CBZ:NCT cocrystal prepared with DI water and resonance acoustic mixing

CBZ:NCT cocrystallization kinetics (Methanol) To test the kinetics of cocrystallization, a series of experiments were performed with varying times of mixing. In these experiments, 100 mg of carbamazepine and 51.7 mg nicotinamide were added to a 4-ml glass vial in a 1:1 stoichiometric ratio. 30 microliters of methanol was then added to the vial via pipette. The LabRam® was used to mix the powders at 90% intensity in auto-resonance mode.

The powder was analyzed by powder x-ray diffraction and DSC.

Individual vials were subjected to 5, 15, 30, 60, and 90 minutes of mixing. The data indicated that conversion was rapid with partial conversion occurring as early as five minutes as shown in Figure 5.


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Carbamazepine - Nicotinamide MeOH Kinetics 30000

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Figure 5: CBZ:NCT cocrystallization kinetics (MeOH) top: PXRD indicates a change in crystal structure starting after five minutes of resonant acoustic mixing. DSC profiles at different mixing times for CBZ:NCT cocrystal formed in the presence of 30 µl of methanol as solvent, assisted using resonant acoustic mixing (LabRAM®) at 90% intensity. Samples were run for 5, 15, 30, 60, 90, and 120 minutes of mixing. DSC shows increased purity obtained after 30 minutes (Pure carbamazepine melts at 190°C while nicotinamide melts at 128°C). DSC of the physical mixture and integrated peaks from Figure 5 can be found in the supplementary information.

From Figure 5 it can be seen that a small amount of nicotinamide is present as evident by the small melt/endotherm in the range of (Tonset) 118 – 126 °C. Note that these samples were not reslurried with acetonitrile, rather they were tested as-is after resonance mixing. In practice any unreacted coformer material would ideally be solubilized and washed away during the reslurry in an appropriate solvent and a filtration step for increased purification.

The amount of


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nicotinamide present that did not form CBZ:NCT Form I cocrystal as a result of RAM varies across the methanol kinetics experiments due to the non-homogenous distribution of methanol throughout the powder mass. Optimizing processing parameters such as the CBZ and NCT particle size and mass along with solvent volume and reaction time will facilitate complete conversion of the starting materials to the cocrystal product. A Separate overlay with the endotherm in the (Tonset) 118 – 126 °C regions integrated and individual DSC curves are included in the Supplemental Information.

It is interesting to note the presence of the low temperature shoulder in the 150 °C to 160 °C range in Figure 5. The CBZ:NCT Form I cocrystal has been reported to melt at 157.2°C and the CBZ:NCT Form II cocrystal has been reported to undergo melting at onset of approximately 131 °C followed by recrystallization to Form I.16,17 It is possible that the lower temperature shoulder is the result of another CBZ:NCT cocrystal form melting. The CBZ:NCT physical mixture described by Rahman et. al.2011 shows similar behavior.14

CBZ:NCT Cocrystal (DMF): As shown in Figure 6 using DMF resulted in the successful preparation of the CBZ:NCT cocrystal. This screening preparation resulted in high purity CBZ:NCT cocrystal based on DSC heat of fusion. Figure 6 shows DSC data from the CBZ:NCT screen with DMF as solvent (20 microliters per 100 mg) indicating a melting point of 158.1 °C and heat of fusion of 146.7 J/g.


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$]2[NAL-009-0981-G3 CBZ:NCT DMF dried50C NAL-009-0981-G3 CBZ:NCT DMF dried50C, 1.6290 mg

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Figure 6: The CBZ:NCT produced with DMF as solvent resulted in a heat of fusion of 146.7 J/g.

CBZ:NCT Cocrystal (DMF) + Reslurry The CBZ:NCT from DMF was repeated on 100-mg scale with the aim to develop a purification and isolation procedure to obtain clean cocrystal free from residual DMF, unreacted starting materials, and any impurities from the starting materials. A reslurry-wash of the solids was performed and then filtered and dried.

In this case the 100 mg carbamazepine plus 51.7 mg

nicotinamide were cocrystallized in the presence of 30 microliters of DMF. The experiment was stopped after one hour at 90% intensity in the LabRAM®. The cocrystals were hard-packed in the bottom of the vial; DSC indicated excellent conversion based on the melting point of 157.3°C. The solids were dried overnight at 70°C and 28 in Hg of vacuum. A DSC of the dried solids indicated a heat of fusion of 137.8 J/g and melting point onset of 157.3°C. The dry solids were then reslurried in the LabRAM® with 6 volumes of acetonitrile at 30% intensity for 5 minutes resulting in a fine slurry. The solids were filtered and dried at 65°C and 28 in Hg of vacuum overnight. The DSC curves are shown in Figure 7. The heat of fusion for the initial wet cake after 1 hr of resonant acoustic mixing was 123 J/g (DMF wet) and after drying was 138 J/g. The heat of fusion of the reslurried (acetonitrile), filtered, and dried material was 154 J/g.


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Figure 7: DSC: 3 curves shown a) CBZ:NCT (I) in DMF, b) after drying of residual DMF, c) after reslurry in acetonitrile, filtration, and drying of residual acetonitrile. Melting point and heat of fusion increases with purity. Measured heats of fusion were 1) 123 J/g, 2) 138 J/g, 3) 154 J/g, respectively.

CBZ:4ABA cocrystal (MeOH) CBZ:4ABA (1:1 cocrystal of carbamazepine and 4-aminobenzoic acid from methanol): Similar to the procedure used with carbamazepine and nicotinamide, 4-aminobenzoic acid (4ABA) was used as a coformer with carbamazepine. 1:1 cocrystals were prepared in the same way as CBZ:NCT experiments.

In the presence of 32 microliters of methanol a 1:1 CBZ:4ABA

cocrystal was produced during resonance acoustic mixing. The product formed as a blend of agglomerates/granules and powder. For the powder the melting onset was 142 °C and a heat of fusion of 113.8 J/g based on DSC and is shown in Figure 8.


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FIG 8: CBZ:4ABA cocrystal (MeOH) 1:1 cocrystal produced from MeOH and resonance acoustic mixing. (mp onset 142 °C and peak of 147 °C) Although the DSC of one of the granules was not as clean as the powder (see Figure 9), it is interesting to note how well-formed and spherical the granules appear. Under specific conditions resonance acoustic mixing can be utilized to produce spherical agglomerates or granules. This is an area of interest for formulations and an active area of research at Nalas. The agglomerates formed with water likely follow a traditional wet granulation mechanism wherein the binder (methanol) saturated with the solid changes the rheological properties of the mixture leading to increased “stickiness” of the wet mass; this stickiness leads to granule growth and agglomerate formation. Agglomerate formation is highly dependent on the solvent or binder properties and the combined surface properties with the solid, and thus agglomerates were only observed in certain cases.


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Figure 9: Carbamazepine:4-aminobenzoic acid (1:1) cocrystal; sample from spherical granules produced during cocrystallization of CBZ and 4ABA in the presence of methanol and resonant acoustic mixing at 90% Intensity for two hrs. The DSC trace is of one of the spherical granules. (mp onset 148.6 oC).

CBZ:SAC Cocrystal CBZ:SAC (1:1 cocrystal of carbamazepine (CBZ) and saccharin (SAC) from methanol): 1:1 cocrystals were prepared in the same way as CBZ:NCT experiments. In the presence of 37 microliters of methanol a 1:1 CBZ:SAC cocrystal was afforded during resonance acoustic mixing.

Based on DSC of the resulting wet cake, the melting point was 171°C as shown in

Figure 10.


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Figure 10: CBZ:SAC (1:1) in methanol produced during cocrystallization of CBZ and saccharin during resonant acoustic mixing at 90% Intensity for two hours. Melting point of 171 oC and heat of fusion of 121.8 J/g with sigmoidal baseline.

DSCs and powder x-ray diffraction (PXRD) patterns of the other cocrystal combinations tested at 100 mg (carbamazepine) scale are included in the supplementary information. Included are PXRD patterns and selected DSC traces from the matrix of experiments. Coformers in the matrix included the following: 4,4’Bipyridine, 4-aminobenzoic acid, 2,6, pyridine dicarboxylic acid, pbenzoquinone, terephalaldehyde, saccharin, nicotinamide, aspirin. The list of solvents included: chloroform, water, DMF, DMSO, methanol, cyclohexane, ethyl acetate, toluene. All combinations had PXRD pattern reported; thermal analysis was performed only on selected experiments (see Supplementary Information).

Cocrystallization Scale-up Results: In this paper “scale-up” refers to any experiment above the 100-mg scale used for proof of concept. Scale-up runs using resonant acoustic mixing for preparing cocrystals performed to


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date include 1.5 g of CBZ:NCT (water), 1.5 g of CBZ:NCT (DMF), and 22.5 g of CBZ:NCT (DMF). 1.5 g of CBZ:NCT (1:1) cocrystal in water:1.0075 g of carbamazepine and 0.516 g of nicotinamide were combined in a vial. 304 microliters (20 microliter/100 mg of solids) of DI water was added. The wet cake was mixed for 1 hour at 59 G’s (62 Hz). A blend of powder and granules was formed.

DSC indicated a small amount of nicotinamide still present so an

additional hour of mixing was provided. The granules were separated from the powder and tested separately. The heat of the fusion for the agglomerated granules was 130 J/g with a melting onset of 156.6 °C. The powder material had a heat of fusion of 148.5 J/g with a melting point of 156.7 °C. (DSC thermal profiles are included in supplemental information). Since a more uniform solid-phase cocrystal process is desired, and uncontrolled granulation is undesired, at least for the purposes of screening, our focus shifted from using water to DMF in an effort to identify a solvent (with low agglomerating potential) for performing proof of concept scale-up studies.

Specifically, because water as the solvent was resulting in significant

agglomeration and balling for CBZ:NCT cocrystals, we shifted our attention to another solvent where agglomeration was not an issue and selected DMF (dimethylformamide) for this purpose. A 1.5g scale of CBZ:NCT (1:1) in DMF: 1.00 g of carbamazepine and 0.517 g of nicotinamide were combined in a vial; the solids were premixed at 30% intensity for 5 minutes followed by the subsequent addition of 300 microliters of DMF to the solid mixture. The cocrystal wet cake was caked to the bottom of the vessel after running the LabRAM® for one hour at 80% intensity in auto resonance mode. Six volumes of acetonitrile was added to the wet cake. The slurry was agitated for approximately one minute at 20% intensity to reslurry the solids. Only a mild intensity was required to reslurry and suspend the solids. The slurry was then filtered and the solids dried on a watch-glass overnight at 50 °C and 28 in Hg of vacuum. The PXRD pattern of the isolated solids is shown in Figure 11. DSC of the dried cocrystal showed ∆Ηfusion of 140 J/g at 158.4 oC (melting onset). CBZ:NCT (1:1) in DMF process was further scaled to 22.75 g in the resonance acoustic mixer. In this experiment a 300-ml glass jar was used with a secondary plastic container. 15.0 grams of carbamazepine were weighed into sample jar. Next, nicotinamide was sieved through a 500


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micron sieve to break-up any hard lumps and 7.75 g of the sieved material was added to the sample jar providing a 1:1 molar ratio. The dry powder blend was mixed in the LabRAM® for 5 minutes at 30% intensity to mix the powders before solvent addition. The jar was opened, and to it was added 4.55 ml (4.82 g) of DMF solvent. The solvent was simply poured on top of the solids from a beaker. The resonance mixing was set for 80% intensity for one hour. After one hour the reaction was sampled for analysis by PXRD (Figure 11), confirming that the cocrystal product was formed.

1.0e+004

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8.0e+003

6.0e+003

4.0e+003

22 gram scale 2.0e+003

Reference CPP 1.5 gram scale

0.0e+000 10

20

30

2-theta (deg)

Figure 11. X-ray powder diffraction overlay of CBZ:NCT cocrystal prepared from DMF with acetonitrile reslurry on a 1.5 gram scale (red) and 22 gram scale (blue) by resonance acoustic mixing compared with a calculated powder pattern (CPP) from single crystal structure for CBZ:NCT (I) obtained from Fleischman et. al18.

During resonance the wet-cake appears to be “fluidized” as the solids and solvent are intimately mixed until eventually the wet solids become caked to the bottom of the vessel. Five volumes (115 ml) of acetonitrile were then added for the reslurry step. The sample container was placed back into the LabRAM® to reslurry the solids at 20% intensity for 5 minutes. The slurry was then transferred to a glass Buchner funnel and 1 volume (24 ml) of acetonitrile was used to rinse solids from the original sample container. The solids were transferred to the filter; the solids


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filtered fast, the filtrate was clear, and the solids were dried at 50 °C and 27 in Hg of vacuum overnight; The isolated yield of 86% includes physical losses and losses in filtrate. Some cake hardening was observed upon drying requiring that it be delumped through 500 micron sieve. A DSC of the cocrystal product showed a ∆Ηfusion of 141 J/g at 158.97 oC (melting onset). Photomicrographs of the physical mixture and cocrystal are shown in Figure 12.

Left (300X)

Right (300X)

Figure 12: Photomicrographs Left: Physical mixture of CBZ and NCT Right: CBZ:NCT (1:1) cocrystal after isolation and dried (from 22-g scale) HPLC analysis of the isolated cocrystal showed that the reslurry reduced the impurities brought in with the carbamazepine but did not completely eliminate them. (see supplementary information for chromatograms)

Discussion of the Results: CBZ:NCT is a convenient case study for testing new methods of cocrystallization; it forms cocrystals readily and is conducive to solution phase crystallization as well. Initial small-scale experiments with carbamazepine and nicotinamide and various solvents, assisted by resonant acoustic mixing resulted in CBZ:NCT cocrystal. The kinetic study of CBZ and NCT using methanol showed partial conversion as early as five minutes. In this work we consistently observed CBZ:NCT (I) based on x-ray powder diffraction and melting point data. Buanz et al17 report the melting point of CBZ:NCT (I) as 157.2 ± 0.3 ºC with a heat of fusion of 157.6 ± 4.5 ºC J/g and the melting point of CBZ:NCT (II) as 131.8 ± 0.3


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ºC with a heat of fusion of 125.1 ± 2.4 ºC J/g. Rahman7 et. al. reported 160 oC mp with a ∆Ηfusion of 150.4 J/g.

In the case of the small-scale experiments of CBZ:NCT from DMF, the resulting cocrystals had a measured heat of fusion of 154.0 J/g and melting point (onset) of 158.5 °C in reasonable agreement with measurements reported by Buanz et. al. The additional purification step raised the melting point and increased the measured heat of fusion. The rationale is that grinding often creates amorphous or disordered solids and the application of liquid assisted grinding not only increases the rate of conversion but also the crystallinity of the product as compared to dry-grinding. The re-slurry step allows any disordered or amorphous solids to dissolve and then crystallize on the very well “seeded” slurry. Thus an increase in crystallinity is to be expected. The solvent choice influences the kinetics of formation – in the methanol study it was shown by DSC and PXRD that the kinetics of cocrystallization during resonance acoustic mixing is reasonably rapid (15-90 min). Weyna et. al. suggests that the more soluble the coformers are in the solvent used for solvent-drop grinding (SDG) the more likely a cocrystal will be produced; it is expected that this concept extends to resonance acoustic mixing as well16. In the present work COSMOtherm (COSMOlogic GmbH & Co.) was utilized as a guide for solvent selection for cocrystal solubility19. A table of predicted solubilities for CBZ:NCT cocrystal and the individual coformers are included in the Supplementary Information. Certain solvents can promote agglomeration – i.e. water and CBZ:NCT as well as methanol with CBZ:4ABA resulted in some granule formation. Water is preferred from an environmentallyfriendly perspective for CBZ:NCT cocrystals but resulted in a higher prevalence of agglomeration for CBZ:NCT. Recently Lombardo showed LabRAM® could be used to simulate agglomeration behavior in agitated filter driers20. For the scale-up, the final product cocrystal was analyzed by high performance liquid chromatography (HPLC) using reverse phase chromatography with an acetonitrile:water gradient method.

By comparing the HPLC data of the starting materials and cocrystal product, there

were no apparent degradation products formed during cocrystallization. A small peak was found


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to correspond to residual DMF on the cocrystals after reslurry and drying. HPLC data indicates that 0.4 area% and 0.2 area% of DMF remained on the 22-g scale-up product and 1.5-g scale-up product, respectively. A small amount of impurities came in with the carbamazepine and these were reduced by the reslurry procedure but not completely eliminated. The HPLC chromatograms can be found in the supplementary materials. The acetonitrile worked well for suspending/reslurring the cocrystals but was not optimal for purging the impurities from the incoming carabamazepine. In future work, the reslurry wash solvent composition will need to be optimized for maximum purge of impurities. In some cases, two reslurries may be needed to selectively remove residual impurities. Alternatively cleaner input material may be needed. As with any API crystallization step, the design of the washes and isolation conditions would be an important development task. Although we did not perform a detailed kinetic study across all scales (100 mg, 1 g, 22 g scales), we also did not notice any attenuation in rate across those experiments but this still needs to be confirmed on multi-kg scale to understand whether significantly longer mixing times will be needed. Given the consistent intensity and uniformity of mixing, the solvent mediated kinetics may not be appreciably impacted, and depend more on how the solvent is distributed to the solid. The impacts of temperature rise and mixing on the product yield, particle morphology and impurity formation need to be understood at a larger scale as well.

Because resonant acoustic mixing is a convenient and scalable platform, the process to manufacture cocrystals is thus considered scalable.

In the present work, the process for

preparing CBZ:NCT cocrystals using resonant acoustic mixing was successfully scaled from 150-mg, 1.5-g, and 22-g scale. 100-g and 10-kg tests are also planned. Production scale resonant acoustic mixers can accommodate a 55-gallon drum container which could produce up to 35 kg of API cocrystal or more depending on the required wash solvent volume and head space required. Implementation of the process flow diagram depicted in Figure 13 is proposed for the successful development of a cocrystallization and isolation process.


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Load Solids (API + Coformer) Premix Solids

Add Solvent Resonant Acoustically Mix IPC Reslurry with Wash Solvent (+ Mix) Filter+Dry Sieve or Mill to achieve PSD

Figure 13: A process flow for developing a cocrystallization process utilizing resonance acoustic mixing. The process steps depicted in Figure 13 include: Loading solids: In general, it is recommended to have similar particle sizes between the two coformers (API and coformer or energetic materials). If one of the components is much larger than the other, it would be recommended to mill. If one of the components is agglomerated or has chunks, it should be milled or screened prior to use. Preblend: Adequately blend powders to ensure good homogeneity before solvent is added. Resonance acoustic mixing is convenient and efficient for blending powders. Here it assumed that only a low intensity resonance would be used simply to blend the materials (30% intensity was used for this step). The preblend is important especially if one component has appreciably higher solubility than the other with the solvent to be used. Solvent selection: solvent selection is critical for finding the desired solubility. Solvates of the API or coformer can result, no reaction, or excessive solubilization may result.

Weyna

suggested that increased solubility of the cocrystal in the solvent was most influential. Although not studied in the present work, there is assumed to be a balance between particle size, solubility, solvent volume, and rate of conversion for resonant acoustic mixing-assisted (RAM-assisted) cocrystallization.


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Solvent mode of addition: In the experiments included in the present work, solvent was simply added all-at-once via pipette or beaker. During resonance the wet cake would be suspended like a fluidized bed but would eventually form a packed material. Some cocrystals will tend to remain granular while in some cases agglomeration was observed. Although not tested here it is felt that for scale-up (especially on multi-kg scale) it would be advantageous to evenly distribute the liquid during the resonant mixing phase to equitably distribute liquid over the entire powder mass especially if there a significant difference in solubilities of the components. Reslurry Solvent: Reslurry is needed for purification as well as a providing a media for suspending and transferring the product.

The reslurry solvent should have relatively low

solubility of the cocrystal but have some solubility for individual compounds to remove excess unreacted starting materials. Multiple reslurries may be required for highest purity. Particle Size Distribution (PSD): Ideally the material can be dried in an agitated filter dryer (AFD), or cone dryer and delumped as necessary to achieve a desired PSD. Recent publications provide methodologies for minimizing agglomeration and attrition in AFD’s and how the final wash solvent can influence agglomeration potential.21 Future work includes scaling to multi kilogram quantities to better understanding important scale-up parameters. This includes developing a better understanding of the required mixing times as scale increases, sensitivity to the mode of liquid addition / distribution, and the effect of temperature on the rate of RAM-assisted cocrystallization.

Conclusions: A new method for preparing cocrystals utilizing resonant acoustic mixing (RAM) has been demonstrated22. The method was demonstrated via preparation for carbamazepine cocrystals on 150- mg, 1.5-g and 22-g scale. For screening trials, multiple vials can be prepared and processed simultaneously to evaluate coformers, stoichiometry, solvent composition, or loading, thus providing an ideal screening platform. The scale-up process includes dispensing solids in the desired ratio, premixing the powders, introducing solvent, RAM at high intensity, confirming cocrystal formation, adding a reslurry-wash solvent, mixing to homogenize the slurry, filtering, drying, sieving, delumping, or milling as needed to achieve target particle size. Thus, it was


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shown that resonance acoustic mixing can be used for cocrystallization with the apparent advantage of being amenable to larger scale manufacture, although this has yet to be demonstrated and confirmed on multi-kg scale. RAM was shown to be a convenient platform due to amenability to preblend, cocrystallize, and reslurry within the same equipment across a wide range of scales with the potential for cocrystallization applications in the pharmaceutical, fine chemical, and energetic-materials industries.

Supporting Information.

Powder x-ray diffraction patterns are shown for CBZ:4ABA,

CBZ:SAC, CBZ: 4-4’bipyridine, CBZ:p-benzoquinine, CBZ:terephthalaldehyde, CBZ:aspirin, CBZ:2,6 pyridine dicarboxylic acid prepared from various solvents subjected to two hours of mixing at 90% . DSCs of selected experiments are included as well. HPLC chromatograms of the CBZ:NCT (DMF) scale-ups.

Predicted Solubilities calculated from COSMOtherm

(CosmoLogic) of CBZ:NCT cocrystal and conformers in a variety of solvents. This material is available free of charge via Internet at http://pubs.acs.org. ACKNOWLEDGMENT: We gratefully acknowledge the contributions of our co-op student Mr. Andrew Wolek (Northeastern University) for performing many of the initial screening experiments and associated powder x-ray diffraction analyses. We also acknowledge Mr. Mark Delude for his technical input and analytical support.

1

Bowles, P. et al, Commercial Route Research and Development for SGLT2 Inhibitor Candidate Ertugliflozin, Organic Process R&D, Org. Process Res. Dev., Article ASAP, DOI: 10.1021/op4002802, Publication Date (Web): December 20, 2013, Copyright © 2013 American Chemical Society 2 Friscic, T., Jones, W., J. Pharm Pharmacology, 2010, 62, 1547-1559. 3 Goyal, S., Thorson, M. R., Zhang, G. Z.G., Gong, Y., Kenis, P. J. A., Crystal Growth & Design. 2012, 12, 12, 6023-6034. 4 Nehm, S.J., Rodríguez-Spong, B., Rodríguez-Hornedo, N., Crystal Growth and Design, 2006, 6, 592-600.


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Rager, T, Hilfiker, R., Z. Phys. Chem., 2009, 2223, 793-813. Schultheiss, N., Newman, A., Crystal Growth & Design, 2009, 9, 6, 2950-2967. 7 Braga, D., Maini, L., Grepioni, F., Chem. Soc. Rev., 2013, 42, 7638-7648. 8 Friscic, T., Jones, W , Chapter 8 in Pharmaceutical Salts and Co-Crystals Ed. Wouters, J.,Quere, L., 2012, RSC Publishing, Cambridge UK. 9 Rastogi, R.P., Bassi, P.S., Chadha, L.S., J. Phys. Chem., 1963, 67, 2569-2573. 10 Kuroda, R., Higasshiguchi, K., Hasebe, S., Imai, Y., CrystEngComm, 2004, 463-468. 11 Chadwick, K., Davey, R. J., Cross, W., CrystEngComm., 2007, 9, 732-734. 12 Jayasankar, A., et. al Pharm. Res. 2006, 23, 2381-2392. Jayasankar, A., et. al. Mol. Pharmaceutics, 2007, 4, 360372. Maheshwari, C., etl al. CrystEngComm., 2009, 11, 493-500. 13 Weyna, D.R., Shattock, T., Peddy, V., Zaworotko, M. J., Crystal Growth & Design, 2009, 9, 2, 1106-1123. 14 Rahman, Z., Agarabi, C., Zidane, A. S., Khan, S. R., Khan, M. A., AAPS PharmSciTech, June 2011, 12, 2, 693704. 15 COSMOtherm is a product of COSMOlogic GmbH & Co KG, http://www.cosmologic.de/index.php?cosId=1000&crId=1 16 Seefeldt, K. Miller, J., Rodriguez-Hornedo, N., J. Pharm. Sci., 2007, 95, 5, 1147-1158. 17 Buanz, A.B.M., Parkinson, G.N., Gaisford, S., Crystal Growth & Design 2011, 11, 1177-1181. 18 Fleischman, S. G., Kuduva, S. S., McMahon, J. A., Moulton, B., Bailey, R. D., Walsh, Rodríguez-Hornedo, N., Zaworotko, M. J., Crystal Growth & Design, 2003, 3, 6, 909-919. 19 Abramov, Y.A., Loschen, C.and A. Klamt, J. Pharm. Sci., Vol 101, No.10, 2012.3687-3697. 20 Zhang, S., Lamberto, D. J., J. Pharm. Sci., Nov. 2013, pp 1-8. 21 am Ende, D., Birch, M., Brenek, S. J., and M. T. Maloney, Org. Process R&D, 2013, 17, 10, 1345-1358. 22 A method to produce cocrystals and salts via resonant acoustic mixing, U.S. 61/873,656 Patent Pending, assigned to Nalas Engineering Services, Inc., September 4, 2013. 6


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Development and Scale-Up of Cocrystals Using Resonant Acoustic

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