Development and Characterization of a Cocrystal as a Viable Solid

Article pubs.acs.org/OPRD

Development and Characterization of a Cocrystal as a Viable Solid Form for an Active Pharmaceutical Ingredient Soojin Kim,*,† Zhibin Li,† Yin-Chao Tseng,⊥ Herbert Nar,‡ Earl Spinelli,† Richard Varsolona,†,¶ Jonathan T. Reeves,† Heewon Lee,† Jinhua J. Song,† John Smoliga,∥ Nathan Yee,† and Chris Senanayake† †

Chemical Development, ⊥Medicinal Chemistry, and ∥Analytical Development, Boehringer Ingelheim Pharmaceuticals, Inc. 900 Ridgebury Road, Ridgefield, Connecticut 06877, United States ‡ Structural Research, Lead Identification and Optimization Support, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Strasse 65, 88397 Biberach an der Riss, Germany S Supporting Information *

ABSTRACT: A phosphoric acid cocrystal of a drug substance candidate at Boehringer Ingelheim was identified from the standard salt form screen and was characterized and shown to have desirable physicochemical properties, stability characteristics, and improved solubility and bioavailability compared to those of the free form. The cocrystal structure was determined by singlecrystal X-ray structure analysis and resolved hydrogen-bonding interactions between chains of phosphoric acid molecules and chains of molecules of the free base. Comparison of the key physicochemical properties and biopharmaceutical properties of the cocrystal against a salt form demonstrated the advantage of the cocrystal over the sulfate salt with its superior stability characteristics. The cocrystal was developed as a robust drug substance form for further development and scale-up. The crystallization of the cocrystal form from organic solutions exhibited a strong relationship between water content in the crystallization system and the crystallization kinetics. The effect on the kinetics was used for effective particle size control for the cocrystals, important for optimizing the surface area, dissolution rate, and thus, bioavailability of the drug substance.

■

INTRODUCTION

acid, tartaric acid, maleic acid, salicylic acid, saccharin, caffeine, etc.4−6 Very few cases of cocrystal formation with an inorganic coformer, other than one that used phosphoric acid,7 were reported in the literature. Cocrystals are often distinguished from salts on the basis of the interaction between the components in the crystalline complex. Only the crystals formed through nonionic interaction between the components such as hydrogen bonding or van der Waals interaction are classified as cocrystals whereas those formed through the ionic interaction or proton transfer between the components are grouped as salt crystals. This distinction is widely accepted in the scientific community, and there are many literature examples in regards to the preparation of cocrystals that describe a rational design and engineering for cocrystals and systematic approaches in selecting solvents and process conditions to prepare cocrystals.8,9 On the other hand, there are also many cases where identifying a viable cocrystal form to be developed as a drug substance or used in drug product was carried out by screening rather than crystal engineering, in ways very similar to how salt forms are screened.10 Salt form screening is commonly carried out for ionizable compounds with limited bioavailability to find a salt form that may provide enhanced solubility or dissolution rate. In the screen with many different counterions of different ionization potential, one may find a salt or cocrystal of the ionizable

Cocrystals have gained a lot of interest in the pharmaceutical industry in recent years. The term “cocrystal” is broadly defined as a homogeneous crystalline material composed of multiple components with defined stoichiometry. For pharmaceutical compounds, especially active pharmaceutical ingredients (APIs), cocrystals offer the possibility of altering physical properties of the compound by forming a crystalline complex with one or more inert materials. With various choices of cocrystal formers to form a complex there may be an opportunity to enhance biopharmaceutical properties of the drug substance such as solubility and dissolution rate. For some compounds that cannot form a physically stable crystalline material, cocrystals may offer the potential to form a stable crystal as a complex where the crystallinity is aided by the presence of another molecule. In addition, compared to a single-component crystal form of the parent molecule, a particular cocrystal form may offer other advantages such as thermal stability or superior physical or chemical stability under humid or stressed conditions. For a given drug substance there are unlimited possibilities for counter materials to be used for screening or crystal engineering for cocrystal formation, among pharmaceutically acceptable and “generally recognized as safe” (GRAS1) for drug product use, which may include any combination of organic or inorganic materials. There are many examples of cocrystals of drug substances reported in the literature.2,3 Most of the published cases for cocrystals of drug substances used organic acids as coformers, and there are a few organic bases and amino acids. Some of the most common ones found in the literature are glutaric acid, benzoic acid, succinic acid, fumaric acid, adipic © 2013 American Chemical Society

Special Issue: Polymorphism and Crystallization 2013 Received: August 30, 2012 Published: February 7, 2013 540

dx.doi.org/10.1021/op300239h | Org. Process Res. Dev. 2013, 17, 540−548

Organic Process Research & Development

Article

cocrystals regarding the selection and development of solid forms for drug substances.

compound depending on the degree of ionization or proton transfer. Advanced analytical techniques, such as single-crystal X-ray diffraction and solid-state NMR, that became widely available in recent years allowed the distinction between the salt and cocrystals.11−13 However, in the absence of such detailed characterization, one may often rely on pKa information to speculate the acid−base pair’s likelihood of forming a salt or not. In the early development of a drug substance, when the pKa information is not clear or accurate, especially for drug substances with low aqueous solubility, we usually do not have a clear picture on the nature of the acid−base interaction or the extent of proton transfer. Therefore, for a complex formation between ionizable compounds, especially for weakly acidic or basic compounds which are most often encountered as drug substance candidates, the distinction between a cocrystal and salt may not be obvious until we have more extensive analysis into the crystal structure and molecular interaction for the clear characterization. The obvious question is then, what is the effect of the nature of the interaction (ionic vs hydrogen bonding), if any, on the relevant drug substance properties such as physicochemical characteristics, stability, bioavailability, or manufacturability? Is there any difference caused by the degree of proton transfer between the acid−base pairwhich would determine whether it is a salt or cocrystalin the properties of the solid forms (salt or cocrystal) that would make one form more desirable than the other as a choice for the final form for the drug substance? There are many examples in the literature that show comparisons between a cocrystal and the single-component crystal, and numerous cases show improved properties such as solubility, dissolution, stability, hygroscopicity, etc. of a cocrystal compared to those of the parent molecule.14−16 However, there are fewer cases where there is a direct comparison made between a cocrystal and the salt of a given compound. Hydrogen bonds are generally known to be weaker than ionic bonds involved in salt formation. Thus, it is commonly believed that crystals formed through ionic bonding (salts) have stronger crystal packing and thus superior physical stability than those formed by hydrogen bonding (cocrystal). It is also believed that the formation of cocrystal is more difficult than salt crystallization due to the possible interference on the hydrogen bonding from the solvents. Moreover, cocrystals are sometimes presumed to dissociate more easily than salts in solution, more likely to result in precipitation of the free form compared to the salt. The characteristics related to the solidstate stability, processability, solubility and dissociation potential are of critical importance for drug substance and drug product development, and it would be worthwhile to directly compare a salt and a cocrystal of a given compound for those relevant properties. At Boehringer Ingelheim (BI), we had a case where a weakly basic compound with a low pKa formed a cocrystal with phosphoric acid. The cocrystal form, identified from the salt form screen, showed improved solubility and bioavailability compared to those of the free form. This contribution reports the development of this cocrystal form as a drug substance candidate and compares its physicochemical and biopharmaceutical properties with the respective properties of a salt crystal form of the same compound. The cocrystal form also exhibited unique properties related to the crystal growth and kinetics. This case made us ponder some of the questions raised above, and it is the purpose of this report to discuss these aspects of

■

RESULTS AND DISCUSSION Compound A was one of BI’s drug substance candidates for selective glucocorticoid receptor agonist, developed for the treatment of rheumatoid arthritis and other inflammatory conditions.17 Compound A is a weak free base as shown in Figure 1.18 During the early assessment of developability, the

Figure 1. Chemical structure of Compound A.

compound showed low aqueous solubility (4.3 μg/mL) but had a good Caco-2 permeability and thus was classified as a Biopharmaceutics Classification System (BCS) class II compound.19 For Compound A, nine crystal forms of the free base were initially identified from the early polymorph screen. Most of them were solvates, and one stable anhydrous form was identified and scaled up. Due to the limited solubility and bioavailability of the free base form, extensive screening for salts was conducted. High throughput salt screening conducted with various acidic counterions resulted in two crystalline solids generated respectively from sulfuric and phosphoric acids. Upon scale-up these solids were determined to have high crystallinity, homogeneous crystalline phases, and acceptable solid-state characteristics. They also showed 1:1 consistent stoichiometry and were nonsolvated and nonhydrated, with high melting points. These solid forms were conveniently called the salts during the initial development. Early assessment of the pKa (measured by the titration method) for Compound A gave an estimate which had indicated the compound to be a strong enough base to warrant a salt screen. However, it was later found that Compound A was an extremely weak base with a measured pKa of 1.75 by the fit of the pH/solubility profile.20 Although the salt formation between Compound A and sulfuric acid was expected due to the large pKa difference between the two (pKa of sulfuric acid = −3, 1.99), the salt formation between Compound A and phosphoric acid (pKa = 2.15, 7.2, 12.4) was unlikely on the basis of the well-known notion that, for the formation of stable salt forms of a basic drug, the pKa of the acid should be at least 2−3 pH units lower than the pKa of the drug.21,22 Subsequently conducted single-crystal growth, and the single-crystal X-ray structure analysis confirmed that the crystalline form obtained from phosphoric acid was a cocrystal with 1:1 stoichiometry between phosphoric acid and Compound A. Physicochemical Characteristics. The melting points of the sulfate salt of Compound A and the phosphoric acid cocrystal of Compound A, measured by differential scanning calorimetry (DSC, heating rate 10 °C/min, TA Instruments Q100) are 198.9 and 202.5 °C, respectively. Compared to the melting point of the free base crystalline form of Compound A, 541

dx.doi.org/10.1021/op300239h | Org. Process Res. Dev. 2013, 17, 540−548

Organic Process Research & Development

Article

226.5 °C, they are slightly lower but considered high enough for thermally stable solid forms. The results of the DSC and thermogravimetric analysis (TGA) showed that these crystal forms are anhydrates with low levels of residual solvents or moisture content. The pH-solubility profile for the crystal forms was one of the important criteria in the form selection for the drug substance. The apparent solubility for both the sulfate salt and the phosphoric acid cocrystal were significantly higher than that of the free base crystalline form, and comparable to each other. The apparent solubility of the sulfate salt, the phosphoric acid cocrystal, and the free base crystalline form was 45.5 μg/mL, 38.7 μg/mL, and 2.6 μg/mL, respectively, all at the pH of 1.1. In addition to the enhanced solubility, the dissolution rates of the sulfate salt and the phosphoric acid cocrystal showed a significant increase compared to those of the free base crystalline form, making the salt form or the cocrystal a good candidate for the API final form. Both the sulfate salt and the phosphoric acid cocrystal forms exhibited excellent solid-state stability chemically and physically under stressed conditions. Even after the solid forms were subjected to 70 °C and 75% relative humidity conditions for 3 weeks, there were no changes in the chemical purity as well as the physical forms based on X-ray powder diffraction (XRPD) and DSC analyses. For further stability investigation of the sulfate salt and the phosphoric acid cocrystal, both forms were suspended as slurry in water for several hours at ambient temperature. The initial and final pH of the suspension showed significant drops in pH, and the XRPD analysis of the solids recovered from the slurries after 2 h showed the solids to be in the same form as that of the free base. This indicated that both the sulfate salt form and the cocrystal dissociated in water in a similar manner. For further assessment into the suitability of the forms as the API, the hygroscopicity of the forms was determined using the gravimetric dynamic vapor sorption analysis (DVS, Surface Measurement Systems). Adsorption and desorption isotherms were performed at 25 °C with the relative humidity ranging from 10% RH up to 90% RH. The phosphoric acid cocrystal displayed very low hygroscopicity with less than 1% water uptake up to 80% RH. However, the sulfate salt showed moderate hygroscopicity reaching about 2.4% water uptake at 80% RH and was deemed less desirable for further development as an API form compared to the cocrystal. The fact that the cocrystal does not adsorb much moisture even at very high RH indicates it has superior stability than the sulfate salt.23 Based on its satisfactory physicochemical properties and stability characteristics, the phosphoric acid cocrystal of Compound A was selected as the final form for development.24 The phosphoric acid cocrystal form showed excellent physical and chemical stability in various stability testing under high humidity and elevated temperatures. In addition, the cocrystal form exhibited significant enhancement of the bioavailability in animal models; in rat studies the cocrystal form showed approximately four times higher bioavailability than that of the free base crystalline form. Structure of the Phosphoric Acid Cocrystal of Compound A. For the single-crystal X-ray structure analysis, the crystals were grown by the solvent diffusion method. The phosphoric acid cocrystal of Compound A was first dissolved in acetic acid and was subjected to the solvent diffusion by isopropyl acetate at 60 °C, and the solution was left undisturbed for a few days at room temperature (∼20 °C),

whereupon some large crystals were observed. Some of these crystals were ground for X-ray powder diffraction, and the pattern matched the previously obtained XRPD pattern for the bulk phosphoric acid cocrystal sample, indicating that the single crystals are representative of the bulk cocrystal material. The single-crystal X-ray diffraction data were collected on a Saturn 944 CCD detector mounted on an AFC11K goniometer, and the X-ray radiation was from Cu Kα from an RU200 rotating anode and VARIMAX optics (all hardware by Rigaku). The data were collected at 100 K. The crystal data are as follows. Crystal data: C23H28 F4N3O8PS, Mr = 613.5, orthorhombic, P212121, a = 8.3288(17) Å, b = 9.3506(19) Å, c = 33.517(7) Å, V = 2610.3(9) Å3, Z = 4, Dx = 1.561 g/cm3, Cu Kα = 1.5418 Å, μ = 2.433 cm−1, F(000) = 1272, T = 100 K. Refinement: 368 parameters; hydrogen atoms included as riding atoms, S =1.107, R1 = 0.0343 for 3946 reflections with Fo > 4σ(Fo) and 0.0358 for all 4035 data, wR2 = 0.0816 (Weight =1/[ σ2 (F2o) + (0.0458 * P)2 + 1.2574 * P ] where P = (Max (F2o, 0) + 2 * Fc2)/3), largest difference peak: 0.24 e/Å3; largest difference hole −0.35 e/Å3. Flack x parameter = −0.005(17). From the crystal structure data, the powder X-ray diffraction pattern was simulated and matched the experimental XRPD pattern obtained from the phosphoric acid cocrystal samples (shown in Figure 2). The good match confirms that the structure obtained from the single-crystal X-ray data is representative for the bulk crystalline material.

Figure 2. Simulated XRPD pattern (bottom) from the structure data overlaid and matched with the experimental XRPD pattern (top, data collected by Rigaku Miniflex II, 0.02/0.6 s, Cu (30 kV, 15 mA)) of the phosphoric acid cocrystal of Compound A.

The crystal structure reveals that the free base of Compound A and phosphoric acid molecules form the crystal lattice of a prototypical cocrystal (Figure 3 and Figure 4). In the crystal Compound A takes on a bent conformation which puts all polar substituents on one side of the molecule. The opposite side predominantly exposes hydrophobic surface groups. There is an intramolecular hydrogen bond between the hydroxyl and the amide oxygen. Compound A molecules pack in the crystal lattice with their hydrophobic surface regions facing each other, while the phosphoric acid molecules form a linear arrangement in the crystal lattice, filling the space in between Compound A molecules, thereby bridging their polar groups. The evidence that the structure described is a cocrystal and not a salt crystal is based on the following experimental observations. First, the hydrogen atoms bound to three of the four oxygen atoms of the phosphoric acid were the highest peaks in difference electron density maps and are thus clearly defined. Second, the P−O distances in the phosphoric acid 542

dx.doi.org/10.1021/op300239h | Org. Process Res. Dev. 2013, 17, 540−548

Organic Process Research & Development

Article

stereocenter in Compound A to be R and thus confirmed the assignment shown in Figure 5 with the atom numbering. Crystallization Process Development for the Phosphoric Acid Cocrystal of Compound A. For the crystallization development for the cocrystal formation shown in Scheme 1, finding a suitable solvent system was a challenge as Compound A had a strong tendency to form solvates. About 10 different solvates of Compound A were observed throughout the course of development. Analogous to the hydrogen bonding between Compound A and phosphoric acid, there was apparent hydrogen bonding between various solvents and Compound A, causing formation of solvates. During the early development, acetic acid was identified as a solvent providing adequate solubility to carry out the crystallization, and butyl acetate as the antisolvent. Varying the phosphoric acid amount to the Compound A ratio resulted in only 1:1 stoichiometry in the cocrystal product. When excess free base (>2 equiv) was used, the extra free base precipitated along with the cocrystal. On the other hand, when excess phosphoric acid was used, only the cocrystal was obtained. Hence, to ensure the phase purity in the scale-up development, a slight excess of phosphoric acid was used for the cocrystal formation. The crystallization using acetic acid/butyl acetate gave consistent crystal formation and provided high-quality phosphoric acid cocrystals, acceptable for the API supply for the early development. However, there were a few issues with using acetic acid for further development, such as higher than expected levels of residual acetic acid in batches and the strong odor of acetic acid when used in large quantity on scale. It was also known that acetic acid formed a solvate with Compound A, and thus there was a concern that the solvate formation might compete with the cocrystal formation for Compound A when acetic acid was used as the solvent. Although there was no indication that the residual acetic acid in the cocrystal batches was actually due to the solvate (XRPD of batch samples showed no peaks corresponding to the acetic acid solvate), a strong affinity of acetic acid to the compound was evident by the persistently high levels of residual acetic acid in experimental lab batches. Therefore, a new process with an alternative solvent system had to be developed to avoid the potential risk and issues associated with acetic acid in the scaleup process. In our search for an alternative solvent system for the crystallization of the cocrystal, methyl ethyl ketone (MEK or 2butanone) was identified as a suitable solvent for Compound A with n-heptane as the antisolvent for the cocrystal. The MEK/ heptane solvent system did not readily solvate the compound and did provide adequate solubility; hence, the crystallization could be designed to operate within the desired concentration and volume ranges. For the cocrystal formation, Compound A dissolved in MEK at an elevated temperature was reacted with

Figure 3. Crystal packing along the crystallographic a axis and hydrogen bonding in the crystal structure of the phosphoric acid cocrystal of Compound A.

Figure 4. Hydrogen-bonding pattern around the phosphoric acid molecules and intramolecular hydrogen bonds in Compound A as seen in the crystal structure.

molecule clearly showed that there are three single bonds to O5, O6, and O7 that formed (distance 1.54 Å) and one double bond to O8 (distance 1.48 Å). Third, the hydrogen-bonding pattern around the phosphoric acid is consistent with the above assignment of proton-carrying phosphate oxygen atoms. Finally, the basic nitrogen on the drug molecule, N1, forms a hydrogen bond to the O5−H group of the phosphoric acid with the proton clearly located much closer to the oxygen atom. All observed hydrogen bonds are listed in Table 1. Finally, the structure analysis also reveals the absolute configuration of the

Table 1. Hydrogen bonds with H··A < r(A) + 2.000 Å and angles DHA > 110° D−H

d(D−H)

d(H··A)

DHA angle (deg)

d(D··A)

O2−H2 O7−H7 O6−H6 N2−H2A O5−H5 N3−H3A N3−H3B

0.820 0.820 0.820 0.860 0.820 0.860 0.860

1.902 1.884 1.762 2.138 1.874 2.057 1.962

172.21 154.36 156.06 147.80 171.37 153.14 170.04

2.716 2.647 2.534 2.902 2.687 2.850 2.813

543

A O1 O1 O8 O3 N1 O8 O4

[x+1, y, z] [−x+1, y+1/2, −z+3/2] [x−1, y, z] [−x, y+1/2, −z+3/2] [−x, y+1/2, −z+3/2] [x1, y+1, z]

dx.doi.org/10.1021/op300239h | Org. Process Res. Dev. 2013, 17, 540−548

Organic Process Research & Development

Article

Figure 5. Molecular conformation of Compound A and the phosphoric acid in the cocrystal representing atoms as thermal ellipsoids and showing the numbering scheme.

Scheme 1. Cocrystal formation from the free base of Compound A and phosphoric acid

aqueous phosphoric acid (85% by weight), and the crystallization was initiated by adding a portion of the antisolvent, n-heptane, followed by the seed cocrystals. The crystallization typically commenced rapidly, dominated by the nucleation process, and was completed by the addition of more antisolvent to obtain maximum yield (>90%). The process resulted in consistent crystal formation and provided highly crystalline cocrystal product in a robust manner. In addition, the process provided excellent particle size control by crystallization. The crystallization occurred by fast nucleation to produce uniformly small particles of mostly