Article pubs.acs.org/crystal
Two Novel Pharmaceutical Cocrystals of a Development Compound − Screening, Scale-up, and Characterization Dennis H. Leung,*,† Sachin Lohani,*,† Richard G. Ball,‡ Nicole Canfield,§ Yaling Wang,# Timothy Rhodes,§ and Annette Bak† †
Basic Pharmaceutical Sciences, ‡Analytical Chemistry, §Analytical Sciences, and #Center for Materials Characterization and Engineering, Merck & Co., Inc., Rahway, New Jersey 07065, United States S Supporting Information *
ABSTRACT: A streamlined methodology for cocrystal screening and scale-up has been developed. There are two parts to this procedure: (1) a high throughput screening approach for the rapid identification of potential cocrystal leads and (2) a new procedure for solvent selection, rapidly mapping the critical region in the ternary phase diagram, and determining a cocrystallization procedure for scale-up. Using this approach, we have discovered and scaled-up two new cocrystal forms of the pharmaceutically active compound 1 for the treatment of type II diabetes. This approach provided more than 2 g of cocrystal material suitable for biopharmaceutical evaluation. The physicochemical characterization and single crystal X-ray diffraction data of these cocrystals are also presented and discussed.
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INTRODUCTION More than 40% of drugs have poor aqueous solubility and that fraction has been increasing over the past 20 years.1,2 Therefore, new methods of solubilization and improvement of physicochemical properties are required to increase the probability of success of new drug candidates. The selection of a suitable form of an active pharmaceutical ingredient (API) can be critical for the development and performance of a dosage form. This is especially true for APIs with absorptionlimited exposure due to low aqueous solubility. Cocrystals are emerging as an important alternative to polymorphs and salts during the solid phase selection APIs. Several case studies have been published that indicate that pharmaceutical cocrystals can have better solubility and/or dissolution rate over the corresponding free form.3−9 More specifically pharmaceutical cocrystals may offer comparable dissolution and exposure advantages to amorphous material, and this approach can also be used for neutral molecules where salt formation is not feasible.3,5,10 Other advantages of cocrystals have also been reported. For example, cocrystals may be less prone to pseudopolymorphism (e.g., solvates and hydrates) and polymorphism compared to monocomponent crystals and salts.11−14 A possible explanation may be that the best hydrogen bond donor/acceptor sites are occupied by the coformer. In addition, a survey on cocrystal chemical stability indicates either neutral or favorable impact of cocrystal formation. The latter may be the case where instability is dependent on orientation in the crystal lattice and hence mainly applies to solid-state stability.6,7,15 Cocrystal formation can also be beneficial in cases when the free form converts to © 2012 American Chemical Society
unwanted hydrates at high relative humidity such as for anhydrous carbamazepine and caffeine.15,16 The availability of a wide variety of pharmaceutical acceptable cocrystal formers (CCFs) provides an opportunity to engineer the desired physicochemical properties into the phase of an API,12,17 a key advantage over salt formation which is limited to ionizable APIs and pharmaceutically acceptable counterions.18 However, this diversity of CCFs and intermolecular interactions results in a large experimental space and despite recent advances in cocrystal engineering, our ability to predict cocrystal formation a priori from chemical structures of CCFs and APIs remains limited; cocrystal screening today is at best semiempirical.18 Accordingly, when incorporating cocrystals into a solid phase selection strategy for APIs, two issues become crucial to support ever shrinking drug-development timelines: first, how to efficiently sample the large experimental space to identify the combinations of API and CCFs that form cocrystals (phase screening); second, how to efficiently scale-up potential cocrystal hits for initial physicochemical and pharmacokinetic (PK) evaluation (phase evaluation). To increase the success rate of cocrystal screening, researchers have recommended case by case selection of CCFs for a given API driven by an understanding of the hydrogen bond patterns (Etter’s general rules),19 a survey of the Cambridge Structural Database (CSD),18,20 and a rational screen design based on understanding of the ternary phase diagram (TPD).21 In addition, several recent reports have Received: September 26, 2011 Revised: December 23, 2011 Published: January 11, 2012 1254
dx.doi.org/10.1021/cg201270s | Cryst. Growth Des. 2012, 12, 1254−1262
Crystal Growth & Design
Article
system (GADDS), a Bruker AXS HI-STAR area detector at a distance of 15.05 cm as per system calibration, a copper source, an automated x-y-z stage, and a 0.5 mm collimator. Data were collected over a 3−40° 2θ range at a step size of 0.05° 2θ. HPLC data were obtained using an Agilent 1100 HPLC equipped with a fused-core C-18, 4.6 × 100 mm column with a 2.7 μm particle size using a UV/vis detector at 210 nm wavelength. Separations were conducted using a mobile phase of a mixture of water with 0.1% phosphoric acid and acetonitrile in a gradient elution at 1.8 mL/min at 40 °C. Single-crystal X-ray diffraction data were collected on an Oxford Diffraction diffractometer. Structures were solved by direct methods31 and refined using full-matrix leastsquares on F2.32 Procedures involving sonication utilized a Fisher Scientific FS140 benchtop sonication bath. Cocrystallization Screening Procedure. A 96-well plate of 1-mL glass vials was charged with API 1 and CCF solids using an automated Symyx Powdernium workstation. The API (25 mg) was dispensed into each well followed by 1 or 2 mol equiv of a specific CCF. The last row of the 96-well plate was reserved for reference samples of the individual solid components. The appropriate solvents (30 μL) were then added using a Thermo Scientific Matrix2 Electronic Multichannel Pipette. The 96-well plate was then sealed using a clear silicone cap mat and then subjected to sonication for 2 h. The 96-well plate was unsealed and the solids were transferred to a 96-well UV plate for automated XRPD analysis. Fumaric, L-tartaric, citric, maleic, succinic, benzoic, L-malic, L-ascorbic, adipic, L-glutamic, D-glucuronic, and hippuric acids were evaluated as potential CCFs with 1 using the conditions described above. Methanol, ethanol, dichloromethane, acetone, dimethyl sulfoxide, ethyl acetate, and water were evaluated as potential cocrystallization solvents using the conditions described above. Procedure for Identifying the Critical Region of the TPD. Excess API 1 and cocrystal former were mixed in 0.5−3 mL of the binary solvent mixture. The resulting suspensions were stirred for 2−5 days using a magnetic stir-bar at 25 °C, and solution phase composition was analyzed by HPLC daily (after 2 days) until equilibrium was reached as judged by 2σ(I)), wR = 0.073, S = 0.81 with (Δ/σ)max = 0.01. The maximum peak height in a final difference Fourier map is 0.226 e Å−3 and this peak is without chemical significance. Single Crystal X-ray Diffraction Analysis of 1-MA Cocrystal. Colorless single crystals of the 1-MA were prepared by evaporation of a concentrated tetrahydrofuran (THF) solution containing 1 and maleic acid in a 1:3 ratio. 1-MA cocrystal C25H31N7O4, MW = 493.558, monoclinic, P2/c, a = 21.5550(3), b = 15.58596(18), c = 15.0489(2) Å, β = 100.2339(13)°, V = 4975.32(11) Å3, Z = 8, Dx = 1.318 g cm−3, monochromatized radiation λ(Mo) = 0.7107 Å, μ = 0.09 mm−1, F(000) = 2096, T = 100 K, θ limit = 30.12° (62 346 reflections). There are 13 423 unique reflections with 9470 observed at the 2σ level. The final model was refined using 653 parameters and all 13 423 data. All non-hydrogen atoms were refined with anisotropic thermal displacements. The final agreement statistics are R = 0.041 (based on 9470 reflections with I > 2σ(I)), wR = 0.109, S = 1.00 with (Δ/σ)max < 0.01. The maximum peak height in a final difference Fourier map is 0.344 e Å−3, and this peak is without chemical significance.
Figure 2. Example design and results of a 96-well-plate for a cocrystal screen for 1 with six different carboxylic acids. Two different cocrystal stoichiometries were investigated in each column and different solvent slurries at 30 μL solvent volume were evaluated for each row. Six other carboxylic acids were screened with 1 in a similar fashion (result not shown as no conversion resulted).
shown in Figure 2. Sonication rather than mechanical grinding was applied as this was more amenable to high throughput screening, and the slurries were sonicated for 2 h. XRPD analysis of solids post sonication showed that wells containing 1 and maleic acid or benzoic acid gave strong, consistent, and reproducible new XRPD spectra in several slurry conditions, suggesting the formation of a new cocrystal form. Wells containing other cocrystal formers had weak and nonreproducible new XRPD patterns, potentially due to weak polymorphs or solvates, which were not pursued. The six remaining potential carboxylic acid cocrystal formers were screened in a similar format, but no significant conversion was observed (data not shown). The best conversion for the new crystal form of compound 1 and maleic or benzoic acid was found using polar organic solvents such as ethanol, dichloromethane, acetone, and ethyl acetate (Figure 2). In contrast, no conversion was observed in DMSO, which is likely due to the high solubility of the components resulting in full dissolution rather than slurry formation for those samples. Limited conversion was observed in water and generally poor to no conversion was observed for the nonpolar solvent cyclohexane, which is likely due to the low solubility of the API or CCF components in those solvents. As a control experiment, we performed the same cocrystallization sonication reaction without the addition of solvent. No conversion was observed upon sonicating or heating the dry solid materials after 2 h, emphasizing the importance of forming a concentrated slurry for initiating cocrystallization under solvent-drop conditions. 1a. Summary of Crystal Structure Analysis of 1-BA Cocrystal. Single crystal X-ray diffraction analysis confirmed cocrystal formation and a 1:1 stoichiometry of the individual components. The asymmetric unit of the 1-BA cocrystal
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RESULTS AND DISCUSSION 1. Discovery of Cocrystals of API 1. Compound 1 is a weakly basic (pKa1 = 3.3 pyrazine group, pKa2 = 1.9 pyrimidyl group) GPR-119 agonist that was developed for the treatment of type 2 diabetes.30 Two free form polymorphs of 1 (Forms I and II) were identified during initial stages of its development. The most stable crystalline form has low water solubility (100 (10−100) >100 (10−100) 10−100 (>100) >100 (10−100) >100 (>100) >100 (>100) >100 (10−100)
>100 (>100) >100 (10−100) >100 (10−100) >100 (10−100) 100) >100 (10−100)
IV II II II IV II III III II
IV II II II IV II I III II
1−10 10−100 1−10
>100 (>100) >100 (10−100) 10−100 (>100)
>100 (>100) >100 (10−100) 1 mg/mL, and RBA‑1 between 10 and 100, while class III was assigned when RBA‑1 was outside the range of 0.1 and 100. Finally, class IV was assigned to a solvent if S1 or SBA was
