Ketoconazole Salt and Co-crystals with Enhanced Aqueous Solubility

Article pubs.acs.org/crystal

Ketoconazole Salt and Co-crystals with Enhanced Aqueous Solubility Flavia A. Martin, Mihaela M. Pop,* Gheorghe Borodi, Xenia Filip, and Irina Kacso National Institute for Research and Development of Isotopic and Molecular Technologies, P.O. Box 700, Cluj-Napoca R-400293, Romania S Supporting Information *

ABSTRACT: Crystal structures of ketoconazole oxalate salt (1) and three co-crystals with fumaric (2), succinic (3), and adipic (4) acids in 1:1 stoichiometry were determined by single-crystal X-ray diffraction in which 1 forms oxalate dimers involved in ionic interaction with the imidazole ring of ketoconazole molecules, while 2−4 display 4-member circuit networks between hydrogen-bonded ketoconazole and coformer molecules. The salt and co-crystal nature of 1−4 was confirmed by combining single-crystal X-ray diffraction, ssNMR, and lattice energy calculations. Ketoconazole molecules show highly similar conformations and crystal packing in cocrystals 2 and 3, while different conformers are present in the 1, 2, and 4 structures. For all salt and co-crystals, powder dissolution measurements revealed a significant solubility improvement compared to ketoconazole, and the solubility of 1−4 is contrary to the solubility values of the corresponding acids. A 100-fold solubility increase in water was obtained by ketoconazole co-crystallization with fumaric and adipic acids. Additionally, 1−4 are stable in suspensions for at least 1 week and on storage at 40 °C/75% RH for at least 4 months. The melting points of 1, 3, and 4 are in line to their solubility, while the solubility difference between the highly similar co-crystals 2 and 3 is reflected in their different lattice energy. Our study emphasizes the benefit of crystal engineering in the landscape of the formulation techniques used to enhance the dissolution rate of poorly water-soluble drugs such as ketoconazole.

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INTRODUCTION Although drug absorption from the gastrointestinal (GI) tract is a complex process, drug permeability through the GI membrane and the solubility/dissolution of the drug dose in the GI environment are playing determinant roles.1 As a consequence, drug substances have been classified into four categories based on their solubility and membrane permeability characteristics. The derived Biopharmaceutics Classification System (BCS), adopted by the U.S. Food and Drug Administration (FDA), the European Medicines Agency (EMEA), and the World Health Organization (WHO), is extensively used by the pharmaceutical industry throughout drug discovery and development processes.2−4 In the case of the BCS class II drugs with low solubility and reasonable permeability, the drug dissolution step is the rate-limiting process of drug absorption. For these drug substances, the choice of the solid form plays a critical role in their absorption from the GI tract. Besides salts, pharmaceutical co-crystals open a new dimension to search for solid forms that can enable substantial modification of properties such as solubility, dissolution rate, stability, and shelf life of active pharmaceutical ingredients (APIs)5−10 without affecting their inherent pharmacological properties.11−13 Pharmaceutical co-crystals are multicomponent crystalline structures made of neutral APIs and co-crystal formers that are solid at ambient conditions and that are bound via noncovalent interactions.14 The co-crystallization of APIs with pharmaceuti© 2013 American Chemical Society

cally acceptable co-crystal formers is nowadays a recognized technique in pharmaceutical science for improving the drug physicochemical properties, leading to an enhanced oral bioavailability.15−17 Additionally, co-crystals have great potential in the pharmaceutical industry by creating intellectual property (IP) protection and extending the life cycles of the APIs.18 A recent study showed that in the case of poorly watersoluble drugs that are weak bases, co-crystallization seems a better choice to the classic HCl salt formation for the dissolution rate improvement and achieving suitable stability of the solid form.19 Ketoconazole, cis-1-acetyl-4-[4-[[2-(2,4- dichlorophenyl)-2(1-H-imidazole-1-yl methyl)-1,3-dioxolan-4-yl] methoxy] phenyl] piperazine, is a highly effective, broad spectrum imidazole antifungal agent used to treat a wide variety of superficial and systematic mycoses.20,21 Being a BCS class II drug,22 ketoconazole has pronounced hydrophobic characteristics and weak basicity that contribute to its poor aqueous solubility.23 Although it is administered both orally and topically, the very low aqueous solubility of ketoconazole at pH > 3 represents a major disadvantage for its efficacy as an oral dosage form.24 Besides the low, variable bioavailability, 25 the risk of hepatotoxicity led to the re-evaluation of the oral drug for Received: April 26, 2013 Revised: August 29, 2013 Published: August 30, 2013 4295

dx.doi.org/10.1021/cg400638g | Cryst. Growth Des. 2013, 13, 4295−4304

Crystal Growth & Design

Article

Scheme 1. Structures of Ketoconazole and Coformers (a−d)

Table 1. Crystallographic Data for 1−4 1 oxalic acid formula formula weight (g/mol) crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg T, K V, Å3 Z Dc/g·cm−3 F(000) crystal size/mm3 range of indices Rint GOF R1 [I > 2σ(I)] wR2 [all data]a a

2 fumaric acid

3 succinic acid

4 adipic acid

C28H30Cl2N4O8 621.46

C30H32Cl2N4O8 647.50

C30H33Cl2N4O8 648.50

C32H28Cl2N4O8 677.56

monoclinic P21/n 10.51506(16) 13.3330(2) 20.3036(3) 90 96.6413(14) 90 293 2827.40(8) 4 1.460 1296 0.4 × 0.3 × 0.2 −12, 12; −16, 16; −19, 24 0.0217 1.039 0.0435 0.1172

triclinic P̅1 10.5094(6) 11.7425(6) 13.4568(5) 96.238(4) 98.973(4) 105.330(5) 293 1562.27(13) 2 1.374 674 0.3 × 0.2 × 0.1 −10, 12; −14, 14; −15, 16 0.0250 1.701 0.0711 0.2331

triclinic P̅1 10.5261(8) 11.7297(9) 13.4580(11) 95.287(6) 99.302(7) 104.429(7) 293 1572.8(2) 2 1.369 678 0.2 × 0.1 × 0.1 −12, 12; −14, 10; −16, 16 0.0278 1.437 0.0749 0.2341

triclinic P̅1 5.8721(3) 8.3797(4) 34.4919(14) 92.623(4) 93.859(4) 103.791(4) 293 1641.20(13) 2 1.371 712 0.3 × 0.2 × 0.1 −7, 6; −10, 9; −16, 16 0.0382 1.318 0.0689 0.2033

R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo2 − Fc2)2]/∑w(Fo2)2]1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [(Fo2) + 2Fc2]/3.

aromatic···COOH supramolecular heterosynthon exhibits a high occurrence in the CSD34,35 and its formation would also be expected for ketoconazole. In addition, considering the persistence of this synthon in co-crystals of similar drugs with dicarboxylic acids,16,31,36 we investigated the possibility of ketoconazole co-crystallization with a series of saturated and unsaturated dicarboxylic acids (oxalic, malonic, succinic, glutaric, adipic, fumaric, and maleic). By applying both solution crystallization and the solvent-drop grinding method, three of the screened acids in combination with ketoconazole led to clay materials under the applied crystallization conditions (Supporting Information, Table S1), while four of them presented in the current work resulted in crystalline materials (Scheme 1).

the risk/benefits effects followed by restriction of its therapeutic indications. An increase of the ketoconazole bioavailability would lower the dose required for the therapeutic effect, decreasing in the same time the risk of hepatotoxicity. Recently, there is increasing interest in formulation approaches for solubility and bioavailability enhancement of ketoconazole, including solid dispersions, hot-melt extrusion, inclusion complexes with β-cyclodextrins, and nanosuspensions.26−30 These methods involve combinations of ketoconazole with other components, which might not be optimal from a regulatory point of view, in spite of the achieved increase in solubility, in some cases with factors of 100 and 172.530 or even 600.26 Besides the formulation techniques, to our knowledge, the crystal engineering approach was hardly explored in the case of ketoconazole, although it is known to work for other triazole antifungal drugs such as itraconazole16 or fluconazole.31 In a recent study, 32 the enhanced solubility shown by the dihydrochloride salt of ketoconazole was correlated to both salt formation as well as the nanoparticule structure of the compound. A careful literature and Cambridge Structural Database (CSD)33 search showed that no salt or co-crystal structures of ketoconazole were reported so far. On the other hand, the N-

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EXPERIMENTAL SECTION

Materials and General Methods. Ketoconazole, commercial form, was purchased from Melone Pharmaceutical Co., Ltd. Oxalic, fumaric, succinic, and adipic acids were purchased from Merck. Reagent grade solvents were used. Differential scanning calorimetry (DSC) was recorded on a Shimadzu DSC60 instrument, with a heating rate of 10 °C/min. The ss-NMR spectra were recorded on a Bruker AVANCE III wide bore spectrometer operating at a 1H Larmor frequency of 500.13 MHz. 15N CP-MAS data were acquired with a 4 mm CP-MAS probe, at 7 kHz spinning frequency, 4 ms contact time, proton decoupling (80 kHz) under TPPM, by averaging 15000 4296



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Table 2. Hydrogen Bonding Geometry Details structure

a

symmetry codes

D−H···A

d(D···H) (Å)

d(H···A) (Å)

d(D···A) (Å)

(D−H−A) (deg)

1(oxalic)

1 − x, 1 − y, 1 − z 1 − x, 2 − y, 1 − z 1 + x, −1 + y, z

N1−H1A···O4A O1A−H1AA···O3A C1−H1···O2A

0.86 0.82 0.93

1.88 1.92 2.30

2.737(3) 2.651(3) 3.144(3)

178 148 150

2(fumaric)

1 − x, 1 − y, −1 − z x, y, z −1 + x, 1 + y, 1 + z

O4A−H4AA···N1 O2A−H2A···O4 C4−H4B···O3A

1.04(5) 0.91(5) 0.97

1.59(5) 1.71(6) 2.25

2.621(4) 2.597(4) 3.176(5)

171(6) 166(6) 159

3(succinic)

−x, 2 − y, 4 − z x, y, z 1 + x, −1 + y, −1 + z x, 1 + y, 1 + z

O4A−Ha···N1 O2A−H2A···O4 C4−H4A···O3A C3A−H3AA···O3

1.04 0.82 0.97 0.97

1.64 1.81 2.27 2.56

2.675(6) 2.619(4) 3.199(5) 3.455(5)

168 168 161 153

4(adipic)

2 + x, 1 + y, z −x, −y, −z −1 + x, y, z

O4A−H4AA···N1 O2A−H2A···O4 C1−H1···O2

0.81 0.82 0.93

1.88 1.79 2.47

2.693(4) 2.587(3) 3.311(3)

172 164 151

H atom position from the DFT optimized structure. best fit the experimental electron density while imposing O−H distance restraints to typical values. Crystallographic data and details of refinements of 1−4 are listed in Table 1, and the hydrogen bonding distances and angles are given in Table 2. Density Functional Theory (DFT) Calculations. Computations were carried out using CASTEP module of Materials Studio package (Materials Studio, release 5.5., Accelrys Software Inc., San Diego, CA, USA, http://accelrys.com/products/materials-studio/). CASTEP code 41 is an implementation of density functional theory (DFT).42,43 For all the DFT calculations, the plane-wave cutoff energy was 610 eV and Brillouin zone integrations were performed on a symmetrized Monkhorst−Pack k-point grid with a k-point spacing of 0.04 Å−1. The exchange-correlation functional was approximated at the generalized-gradient level, specifically that of Perdew, Burke, and Ernzerhof (PBE).44 Ultrasoft pseudopotential45 consistent with the PBE approximation was generated by CASTEP on the fly. H atoms positions were optimized using a BFGS variant46 such that the computed atomic forces were less than 0.01 eV/Å. Lattice parameters and heavy atoms positions were fixed at those given by X-ray diffraction experiments. NMR shielding tensor was computed using the fully periodic GIPAW method.47,48 The computed isotropic shielding constants σ where transformed to chemical shifts using the relationship δcalc = σref − σcalc, with reference value σref extracted from a linear fit of the computed shielding constants against the experimental shifts. Powder Dissolution Experiments. The absorbance values for ketoconazole, salt, and co-crystals 1−4 in deionized water at different times were detected by a μDISS Profiler apparatus. The system consists of an integrated diode array spectrophotometer connected to a fiber optic UV probe located directly in the reaction vessel and is able of measuring the concentration as a function of time without having to filter the solution. Measurement of dissolution kinetics and equilibrium solubility was carried out at 300 nm, where all the used dicarboxylic acids have no absorption, and the concentrations of 1−4 were calculated by means of a standard curve. The solids of ketoconazole starting material and 1−4 were milled to powder and sieved using standard mesh sieves to provide samples with approximate particle size ranges of 150−200 μm. In a typical experiment, 15 mL of deionized water (pH 5.8) was added to a flask containing 20 mg of sample, and the resulting mixture was stirred at 25 °C and 600 rpm. The dissolution experiments were carried out in triplicate (Supporting Information, Figure S5).

transients with a recycle delay of 10 s. The 15N CP-MAS spectra were calibrated relative to nitromethane using glycine (−347.6 ppm) as an external standard. X-ray powder diffraction (XRPD) patterns were obtained on a D8 Advance Bruker AXS θ-2θ diffractometer with Cu Kα radiation (40 kV, 30 mA). Accelerated stability testing was conducted under the storage condition of 40 ± 2 °C/75 ± 5% RH. Ketoconazole Oxalate Salt (1:1) (1). Crystals suitable for singlecrystal X-ray diffraction were prepared by dissolving a (1:1.1) mixture of ketoconazole and oxalic acid (10.0 mg) in 1 mL of acetone:methanol (1:1) with heating at 60 °C. Slightly closed vials, allowing the vapors to escape, were put in the refrigerator for slow solvent evaporation at 4 °C, until colorless single crystals of suitable size were formed. Ketoconazole Fumaric Acid Co-crystal (1:1) (2). Crystals suitable for single-crystal X-ray diffraction were prepared by dissolving a (1:1.1) mixture of ketoconazole and fumaric acid (15.0 mg) in 1 mL of acetone:methanol (1:1) with heating at 60 °C. Slightly closed vials, allowing the vapors to escape, were put in the refrigerator for slow solvent evaporation at 4 °C, until colorless single crystals of suitable size were obtained. Ketoconazole Succinic Acid Co-crystal (1:1) (3). Crystals suitable for single-crystal X-ray diffraction were prepared by dissolving a (1:1.1) mixture of ketoconazole and succinic acid (10.0 mg) in 1 mL of chloroform with heating at 60 °C. Slightly closed vials, allowing the vapors to escape, were placed in the refrigerator for slow solvent evaporation at 4 °C, until yellow single crystals of suitable size and quality grew. Ketoconazole Adipic Acid Co-crystal (1:1) (4). Single-crystal growth: Crystals suitable for single-crystal X-ray diffraction were prepared by dissolving a (1:1.1) mixture of ketoconazole and adipic acid (15.0 mg) in 1 mL of chloroform or chloroform:ethanol (3:1) with heating at 60 °C. Slightly closed vials, allowing the vapors to escape, were put in the refrigerator for slow solvent evaporation at 4 °C, until colorless single crystals of suitable size and quality were formed. Single-Crystal X-ray Diffraction. Single-crystal diffraction data for 1−4 were collected on an Oxford Diffraction SuperNova dual wavelength diffractometer with operating mirror monochromated Cu Kα radiation mode (λ = 1.5418 Å). X-ray data collection was monitored and all the data were corrected for Lorentzian, polarization, and absorption effects using CrysAlisPro program.37 Olex2 program38 was used for the crystal structures solution and refinement: SHELXS97 was used for structure solution39 and SHELXL was used for full matrix least-squares refinement on F2.40 All non-hydrogen atoms were refined anisotropically, and most hydrogen atoms were added in calculated positions and refined riding on their host atoms. The hydroxyl H-atoms of the fumaric acid were allowed to rotate to

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RESULTS AND DISCUSSION Crystal Structures. Structure 1. The crystal structure 1 contains one oxalic acid and one ketoconazole molecule in the 4297



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bond between one acid hydroxyl group and the acetyl group of ketoconazole. The second hydroxyl O-atom of fumaric acid is hydrogen bonded to the imidazole N-atom of the ketoconazole molecule via the O(4A)−H(4AA)···N(1) hydrogen bond, without transferring the proton to the basic moiety. The cocrystal nature of 2 was determined based on locating the Hatoms in the difference Fourier map. The hydroxyl H-atoms of the fumaric acid were allowed to rotate to best fit the experimental electron density while imposing distance restraints for achieving a stable refinement (d(O−H) = 0.9− 1.0 Å). All the other H atoms were located in a difference map, placed in the calculated ideal positions and refined in the riding model approximation.40 In the case of 3, the hydroxyl H-atom related to the O−H···N hydrogen bond could not be located in the difference map due to the possible disorder, indicated by larger anisotropic displacement parameters of this hydroxyl Oatom of the succinic acid. However, on the basis of crystal structure similarity with 2 and the evidence from DFT calculations and ss-NMR 15N results (presented in the next section), it could be concluded that 3 is also a co-crystal. In both co-crystals 2 and 3, the ketoconazole and the coformer molecules are connected via hydrogen bonding into a 4member circuit network (Figure 2a). Moreover, ketoconazole molecules adopt almost identical conformations (Figure 2b) and highly similar crystal packing in the two structures (Figure 3a,b). The high similarity between 2 and 3 was also evidenced by the crystal structure overlay (Figure 3c), leading to a rootmean-square (RMS) deviation in distance of 0.163, calculated for 30 molecules in common between the two structures.49

asymmetric unit, with the oxalate moieties linked into dimers via O(1A)−H(1AA)···O(3A) bidentate intermolecular hydrogen bonds (Figure 1a, Table 2). Ketoconazole conformation is

Figure 1. Structure 1 hydrogen bonding (a,b) and crystal packing (c, view along the c-axis).

characterized by folding of the imidazole (R2) side chain toward the R5 ring of the molecule (Scheme 1). The 5- and 6membered rings (R1, R3) of the ketoconazole molecule adopt an envelope and a chair conformation, respectively. Structure 1 is an oxalate salt because proton transfer occurs from the hydroxyl O-atom of the oxalic acid to the imidazole N-atom of the ketoconazole molecule. The resulting ionized ketoconazole and oxalate moieties are connected via a N(1)− H(1A)···O(4A) hydrogen bond. The imidazole ring (R2) of ketoconazole is further involved in a C−H···O hydrogen bond with the carbonyl O-atom of adjacent oxalate dimers (Figure 1b). The crystal packing of 1 highlighting the oxalate counterion and the imidazole ring involved in the ionic interaction is shown in Figure 1c. Structures 2 and 3. The crystal structures of 2 and 3 are very similar (Table 1), which is in line with the molecular similarity between fumaric and succinic acids (Scheme 1). The asymmetric units of 2 and 3 include one ketoconazole and one acid molecule connected via a O(2A)−H(2A)···O(4) hydrogen

Figure 2. Structures 2 and 3: the 4-member circuit network (a), overlay of ketoconazole molecules in 2-red and 3-green (b), and overlay of ketoconazole molecules in structures 2-red and 1-blue (c). 4298



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Despite the different ring orientations, a match between the ketoconazole conformations in 4 and 1 is achieved by the variation of the flexible torsion angles only, whereas this is not the case for the ketoconazole conformers in the structure pairs 4 versus 2/3 or 1 versus 2/3 (Supporting Information Figure S1, Table S4), probably due to the different puckering parameters of the R1 ring. Elucidation of the Ionization States. In the case of multicomponent structures such as 1−4, the position of the hydroxyl H atom from the carboxylic acids plays an important role in judging the crystal form nature, namely a salt or a cocrystal. In the case of 1, 2, and 4 structures, the single-crystal Xray diffraction data enabled the location of the proton either at the hydroxyl group of the acid or at the imidazole N-atom of the ketoconazole molecule, indicating that 1 is a salt, while 2 and 4 are co-crystals. For 3, the hydroxyl H atom could not be located due to the possible disorder present, such that a first indication of the ionization state was obtained from the structural characteristics of interacting functional groups, namely C−O bond lengths of carboxylic/carboxylate group and endocyclic C−N−C bond angle in the triazole ring.31 In a neutral carboxylic group, C−O are longer bonds than CO, while in the carboxylate anion, depending on the extent of delocalization of negative charge between the oxygen atoms, there can be practically no difference between the C−O and CO distances.50 The C−N−C angle in N-heterocycles is also known to be sensitive to the protonation of nitrogen, such that cationic forms exhibit higher values than the corresponding neutral molecules.51,52 The characteristic bond lengths of carboxylic/carboxylate groups and the C−N−C bond angle in the imidazole rings were analyzed for all 1−4 determined structures (Table 3). In structure 1, the difference between d(C2A−O4A) and d(C2A− O3A) is very small (0.003 Å) and the value of the θ angle is 109°, indicating protonation of N1 and formation of a salt. On the contrary, in structure 4 the difference d(C6A−O4A) and d(C6A−O3A) is significant (0.125 Å) and the value of θ angle is smaller, indicating a neutral compound and formation of a cocrystal. In the case of 2 and 3 similar structures, co-crystal formation is also indicated by smaller values of the θ angles and larger differences (of 0.08 Å) between the C−O and CO bond lengths than in the oxalate salt 1. To complement the results of the single-crystal analysis and to bring additional evidence on the ionization states of 1−4, the total DFT energies (Et) of the single-crystal X-ray structures without any optimization were compared for two different cases: (i) the H atom is bonded to O4A at a distance of 0.82 Å and (ii) the H atom is bonded to N1 at a distance of 0.86 Å. Computations were performed with the CASTEP code, which uses a plane-wave basis set to simulate the electronic wave functions, together with pseudopotentials to represent the core electrons.53 The results are presented in Table 4 in terms of the relative energy ΔEt, were the reference energy is considered in each case as the minimum energy between the two computed structures (with H atom bonded to O4A, or to N1, respectively). One observes that the minimum energy of 3 belongs to the structure model with the H atom bonded to O4A, suggesting the presence of a co-crystal. The ΔEt values for the other structures (1, 2, and 4) are also in agreement with the salt and co-crystal nature of 1, 2, and 4 as determined from single-crystal analysis. To further strengthen the assignment of the ionization states, the DFT analysis was extended with another type of

Figure 3. Crystal packing viewed along the a-axis for: structure 2 (a), 3 (b), and the crystal structure overlay for 2-red and 3-green (c).

The main difference between the crystal structures of 2 and 3 is the additional involvement of the succinic acid in weak hydrogen bonding with ketoconazole molecules from adjacent 4-member circuit networks (Table 2). Compared to the oxalate salt, the conformation of the ketoconazole molecule is substantially different in the co-crystals with fumaric and succinic acids, with a twisted conformation of the 5-membered ring R1 (whereas R1 adopts an envelope conformation in 1) and different orientations of the ring planes (Figure 2c, Supporting Information Tables S2, S3). Structure 4. Similar to 2 and 3, the crystal structure of 4 is characterized by a 4-member circuit network involving hydrogen bonded ketoconazole and adipic acid molecules (Figure 4a). Structure 4 is also a co-crystal because hydrogen bonding takes place with no proton transfer between the hydroxyl group of the adipic acid and the imidazole N-atom of the ketoconazole molecule. Unlike the co-crystals with fumaric and succinic acids, the 4-member circuit networks are interconnected via a C(1)−H(1)−O(2) hydrogen bond (Figure 4b), leading to a different overall crystal packing (Figure 4c). Additionally, the ketoconazole molecule conformation in 4 is different from the co-crystals 2 and 3 with respect to both the R1 conformation (envelope in 4 versus twisted in structures 2 and 3) and the orientations of the ring planes (Figure 5a, Supporting Information Table S3). Compared to the oxalate salt, the conformation of the ketoconazole molecule in 4 shows the same R1 envelope conformation but pronounced ring orientation differences (Figure 5b, Supporting Information Table S3). 4299



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Figure 4. Structure 4: the 4-member circuit network (a), H-bonded circuit networks (b), crystal packing viewed along the b axis (c).

cases, the solutions after the geometry optimization converged to a unique structure, with the H atom bonded to O4A, indicating the co-crystal formation (even when H was initially bonded to N1, this was moved to O4A by optimization). For consistency reasons, these calculations have also been carried out for structures 1, 2, and 4, indicating for each of them the most stable structure, salt for 1, and co-crystals for 2 and 4. For these optimized structures, we also computed the ssNMR shielding and we acquired experimental ss-NMR 1H, 13C, and 15N spectra of 1−4 with the purpose of validating the crystal structures and confirming their ionization states (Supporting information, ss-NMR Spectroscopy Results for 1−4 and Chemical Shift Calculations for 1−4). Because of the poor resolution of the 1H spectra, no information could be extracted regarding the ionization states of 1−4, but the data corroborated with the hydrogen bond network described in the Crystal Structures section. On the other hand, the comparison between experimental and computed 13C chemical shifts seemed acceptable (Supporting Information, Tables S6, S7), although the particularly lower accuracy obtained for the carboxylic C atoms showed that no firm conclusions can be drawn on the ionization states of 1−4. The final confirmation of the ionization states was obtained from the ss-NMR 15N spectra (Figure 6). The protonation of N1 atom increases the magnetic shielding, which results in a decrease of its chemical shift. Comparison of the 15N CP-MAS spectra of 1−4 evidence a significant shift only in the N1 resonance of structure 1, confirming the N1 atom protonation and the salt character of the crystal form. The other structures

Figure 5. Overlay of ketoconazole molecules in structures 2-red and 4orange (a) and in structures 1-blue and 4-orange (b).

calculations applied for structure 3 with succinic acid: the positions of all hydrogen atoms (with the heavy atoms fixed to their original positions) have been optimized using as input the crystal structure determined by single-crystal X-ray diffraction and also including the H atom of which position could not be determined by X-ray. Because of this uncertainty, two input structures were considered separately: H atom bonded to O4A in one case and to N1 in the other case, respectively. In both 4300



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Table 3. Carbon−Oxygen Bond Distances of the Carboxylic/Carboxylate Groups and C1−N1−C2 angle value in Nheterocycles of 1−4 structure (acid, n) 1 2 3 4

(oxalic, n = 2) (fumaric, n = 4) (succinic, n = 4) (adipic, n = 6)

d(C1A−O1A) (Å)

d(C1A−O2A) (Å)

d(CnA−O3A) (Å)

d(CnA−O4A) (Å)

θ (C1−N1−C2) (deg)

1.204 1.223 1.187 1.185

1.313 1.295 1.302 1.311

1.228 1.204 1.182 1.198

1.231 1.284 1.264 1.323

109.12 105.69 105.99 105.15

Table 4. ΔEt Values Depending on the Proton Location in 1−4 Crystal Structures

co-crystal. A highly similar crystal structure environment is also present in the succinic acid co-crystal where ΔpKa = 2.31. DSC Analyses, Powder Dissolution, and Stability. The DSC traces of ketoconazole, carboxylic acids, salt 1, and cocrystals 2−4 are shown in Figure 7. The melting points of 1−4 are different from those of ketoconazole and corresponding acids. It is interesting to note that the melting points of 2 and 3 are in between those of ketoconazole and the corresponding coformers, while the melting point for 1 is higher and that of 4 lower than those of the components (Figure 7). In addition, the melting points of 2 and 3 are closely matching (171 °C versus 165 °C, Supporting Information, Table S10), which is in agreement with their crystal structure similarity. On the other hand, the different crystal packing of 1 compared to 2−4 is reflected in its highest melting point (198 °C versus 171 °C, 165 °C, 128 °C). This is in agreement with the higher packing efficiency encountered in the salt 1 than in the co-crystals 2−4, as shown by its higher density and packing index values (Supporting Information, Table S11). For all salt and co-crystals, powder dissolution measurements in water revealed a significant solubility improvement compared to ketoconazole (Figure 8). Considering the reported aqueous solubility of ketoconazole (0.017 mg/mL),22 a 100-fold solubility increase was obtained by ketoconazole co-crystal-

ΔEt (kcal/mol) structure (acid) 1 2 3 4

(oxalic) (fumaric) (succinic) (adipic)

co-crystal (O4A−H)

salt (N1−H)

90.99 0 0 0

0 20.85 65.42 47.15

where no shift of the N1 resonance occurred are all co-crystals. Spectral assignments of the 15N CP-MAS data were determined by ss-NMR shielding calculations (Supporting Information, Table S5) and within the error limits of the method showed good agreement between experimental and computed values, confirming the ionization states of 1−4. The proposed ionization states are in line with the ΔpKa values between ketoconazole and the respective acids for structures 1, 3, and 4 (ΔpKa for 1, 3, and 4 are 5.24, 2.31, and 2.07, respectively). In the case of structure 2 with fumaric acid, a salt would be expected to occur based on the ΔpKa rule proposed by Childs, et al.54 (ΔpKa with respect to ketoconazole >3). In this case, the crystal environment is likely having a strong impact on the ionization state, leading to a fumaric acid

Figure 6. 15N CP-MAS spectra for structures 1−4. 4301



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Figure 7. DSC traces of ketoconazole, dicarboxylic acids, and 1−4.

After the dissolution experiments, the undissolved solids were filtered and air-dried, and the solid form stability of 1−4 was confirmed by XRPD analyses. Additionally, slurry experiments in water and ethanol and accelerated stability testing showed that 1−4 are stable in suspensions for at least 1 week and on storage at 40 °C/75% RH for at least 4 months (Figure 9). The final pHs of the water dissolution experiments fall within a small pH interval (3.4−4.1, Supporting Information, Table S12), where ketoconazole and the diacids are partially ionized. This suggests that ionization is not the main contribution to the excellent solution stability of 1−4, but more likely the intrinsic crystallization behavior of ketoconazole is playing an important role because ketoconazole is a relatively large molecule having a low crystallization tendency.55 Although co-crystal melting point was shown to be a poor indicator of co-crystal aqueous solubility,56 in the case of ketoconazole, the melting points of 1, 3, and 4 are in line with their solubility. When comparing the 2 and 3 co-crystals with very similar crystal structures and melting points, the solubility difference can be attributed to the higher lattice energy of the co-crystal with fumaric acid (Supporting Information, Table S9). In addition, it is interesting to note that an inverse correlation is found when comparing the aqueous solubility sets of 1−4 and of the corresponding acids (aqueous solubility of

Figure 8. Powder dissolution profiles of ketoconazole and 1−4 in water.

lization with fumaric and adipic acids. The solubility values of 1 and 4 are approximately 53 and 75 times as large as that of ketoconazole, but it is interesting to note that a higher solubility increase is achieved by ketoconazole co-crystallization than by salt formation. 4302



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Figure 9. XRPD patterns of ketoconazole, and 1−4 after crystallization and accelerated stability testing.

crystallographic information files (CIF), and 1H MAS NMR, C CP-MAS spectra for 1−4. Powder X-ray diffraction patterns from the slurry experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

acids: 90 mg/mL, oxalic; 80 mg/mL, succinic; 23 mg/mL, adipic; 6.3 mg/mL, fumaric; www.chemicalbook.com).

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CONCLUSIONS Similar to other azole antifungal drugs, ketoconazole is able to form salts and co-crystals with dicarboxylic acids, although in some cases the tendency of forming clays might interfere with their crystallization. One oxalate salt and three co-crystals of ketoconazole with fumaric, succinic, and adipic acids were obtained, their crystal structures determined from single-crystal diffraction and the ionization states were confirmed by combining single-crystal X-ray diffraction, ss-NMR, and lattice energy calculations. All solid forms were stable and showed significant solubility improvement compared to ketoconazole, with a 100-fold enhancement in the case of the co-crystals with fumaric and adipic acids. The results emphasize that salts are not necessarily more soluble than co-crystals and that correlations between salt/co-crystal solubility, melting points, or coformer solubility are possible in specific cases. Our study shows evidence of the benefit of crystal engineering in the landscape of the formulation techniques used to enhance the dissolution rate of poorly water-soluble drugs such as ketoconazole.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+4)0264-584037. Fax: (+4)0264-420042. E-mail: [email protected]. Web: www.itim-cj.ro. Address: 65-103 Donath Street, 400293 Cluj-Napoca, Romania. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by ANCS, project POSCCE ID536. The computing support was provided by the Data Center of INCDTIM, Cluj.

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REFERENCES

(1) Amidon, G. L.; Lennernas, H.; Shah, V. P.; Crison, J. R. Pharm. Res. 1995, 12 (3), 413−420. (2) Ahr, G.; Voith, B.; Kuhlmann, J. Eur. J. Drug Metab. Pharmacokinet. 2000, 25 (1), 25−27. (3) Cook, J.; Addicks, W.; Wu, Y. AAPS J. 2008, 10 (2), 306−310. (4) Lennernas, H.; Abrahamsson, B. J. Pharm. Pharmacol. 2005, 57 (3), 273−285. (5) Aakeroy, C. B.; Forbes, S.; Desper, J. J. Am. Chem. Soc. 2009, 131 (47), 17048−17049.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details, structural characteristics, and overlay analysis, lattice energy, NMR chemical shifts calculations, X-ray 4303



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(6) Stanton, M. K.; Bak, A. Cryst. Growth Des. 2008, 8 (10), 3856− 3862. (7) Nehm, S. J.; Rodriguez-Spong, B.; Rodriguez-Hornedo, N. Cryst. Growth Des. 2006, 6 (2), 592−600. (8) Cooke, M. W.; Stanton, M.; Shimanovich, R.; Bak, A. Am. Pharm. Rev. 2007, 10 (7), 54−59. (9) Remenar, J. F.; Morissette, S. L.; Peterson, M. L.; Moulton, B.; MacPhee, J. M.; Guzmán, H. R.; Almarsson, O. J. Am. Chem. Soc. 2003, 125 (28), 8456−8457. (10) Schultheiss, N.; Lorimer, K.; Wolfe, S.; Desper, J. CrystEngComm 2010, 12 (3), 742−749. (11) McNamara, D. P.; Childs, S. L.; Giordano, J.; Iarriccio, A.; Cassidy, J.; Shet, M. S.; Mannion, R.; O’Donnell, E.; Park, A. Pharmacol. Res. 2006, 23 (8), 1888−1897. (12) Trask, A. V.; Motherwell, W. D. S.; Jones, W. Int. J. Pharm. 2006, 320 (1−2), 114−123. (13) Reddy, L. S.; Babu, N. J.; Nangia, A. Chem. Commun. 2006, 1369−1371. (14) Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J. J. Pharm. Sci. 2006, 95 (3), 499−516. (15) Xu, L. L.; Chen, J. M.; Yan, Y.; Lu, T. B. Cryst. Growth Des. 2012, 12 (12), 6004−6011. (16) Remenar, J. F.; Morissette, S. L.; Peterson, M. L.; Moulton, B.; MacPhee, J. M.; Guzmán, H. R.; Almarsson, O. J. Am. Chem. Soc. 2003, 125 (28), 8456−8457. (17) Parmar, V. K.; Shah, S. A. Pharm. Dev. Technol. 2013, 18 (2), 443−453. (18) Steed, J. W. Trends Pharmacol. Sci. 2013, 34 (3), 185−193. (19) Shevchenko, A.; Bimbo, L. M.; Miroshnyk, I.; Haarala, J.; Jelínková, K.; Syrjänen, K.; van Veen, B.; Kiesvaara, J.; Santos, H. A.; Yliruusi, J. Int. J. Pharm. 2012, 436 (1−2), 403−409. (20) Delgado, J. N.; Gisvold, O.; Remers. W. A. Wilson and Gisvold’s Textbook of Organic Medical and Pharmaceutical Chemistry, Lippincott Williams & Wilkins: New York, 1998. (21) Odds, F. C.; Milne, L. J. R.; Gentles, J. C.; Ball, E. H. J. Antimicrob. Chemother. 1980, 6 (1), 97−104. (22) Choi, J. Y.; Park, C. H.; Lee, J. Drug Dev. Ind. Pharm. 2008, 34 (11), 1209−1218. (23) Elder, E. J.; Evans, J. C.; Scherzer, B. D.; Hitt, J. E.; Kupperblatt, G. B.; Saghir, S. A.; Markham, D. A. Drug Dev. Ind. Pharm. 2007, 33 (7), 755−765. (24) Levine, H. B. Ketoconazole in the Management of Fungal Disease; Adis Press: New York, 1982. (25) Shirsand, S. B.; Para, M. S.; Nagendrakumar, D.; Kanani, K. M.; Keerthy, D. Int J. Pharm. Invest. 2012, 2 (4), 201−207. (26) Aggarwal, A. K.; Jain, S. Chem. Pharm. Bull. 2011, 59 (5), 629− 638. (27) Mididoddi, P. K.; Repka, M. A. Eur. J. Pharm. Biopharm. 2007, 66 (1), 95−105. (28) Taraszewska, J.; Koźbial, M. J. Inclusion Phenom. Macrocyclic Chem. 2005, 53 (3), 155−161. (29) Basa, S.; Muniyappan, T.; Karatgi, P.; Prabhu, R.; Pillai, R. Drug Dev. Ind. Pharm. 2008, 34 (11), 1209−1218. (30) Balata, G.; Mahdi, M.; Bakera, R. A. Asian J. Pharm. Sci. 2010, 5 (1), 1−12. (31) Kastelic, J.; Hodnik, Ž .; Šket, P.; Plavec, J.; Lah, N.; Leban, I.; Pajk, M.; Planinšek, O.; Kikelj, D. Cryst. Growth Des. 2010, 10 (11), 4943−4953. (32) Hosmani, A. H.; Thorat, Y. S. Dig. J. Nanomater. Biostruct. 2011, 6 (3), 1411−1418. (33) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, B58, 380− 388. (34) Tanise R. Shattock, T. R.; Arora, K. K.; Vishweshwar, P.; Zaworotko, M. J. Crystal Growth Des. 2008, 8 (12), 4533−4545. (35) Sarma, B.; Nath, N. K.; Bhogala, B. R.; Nangia, A. Crystal Growth Des. 2009, 9 (3), 1546−1557. (36) Tsutsumi, S.; Iida, M.; Tada, N.; Kojima, T.; Ikeda, Y.; Morowaki, T.; Higashi, K.; Moribe, K.; Yamamoto, K. Int. J. Pharm. 2011, 421, 230−236.

(37) CrysAlis PRO; Agilent Technologies: Yarnton, Oxfordshire, England, 2010. (38) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42 (2), 339−341. (39) Sheldrick, G. M., SHELXS97; University of Gö ttingen: Göttingen, Germany, 1990. (40) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (41) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220 (5−6), 567− 570. (42) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136 (3B), B864− B871. (43) Kohn, W.; Sham, L. J. Phys. Rev. A 1965, 140 (4A), A1133− A1138. (44) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1966, 77, 3865−3868. (45) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892−7895. (46) Pfrommer, B. G.; Cote, M.; Louie, S. G.; Cohen, M. L. J. Comput. Phys. 1997, 131, 233−240. (47) Pickard, C. J.; Mauri, F. Phys. Rev. B 2001, 63, 245101. (48) Yates, J. R.; Pickard, C. J.; Mauri, F. Phys. Rev. B 2007, 76, 024401. (49) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41 (2), 466−470. (50) Bis, J. A.; Zaworotko, M. J. Cryst. Growth Des. 2005, 5 (3), 1169−1179. (51) Weyna, D. R.; Shattock, T.; Vishweshwar, P.; Zaworotko, M. J. Cryst. Growth Des. 2009, 9 (2), 1106−1123. (52) Bis, J. B.; Vishweshwar, P.; Weyna, D.; Zaworotko, M. Mol. Pharmaceutics 2007, 4 (3), 401−416. (53) Payne, M. C.; Teter, M. P.; Allan, D. C.; Aria, T. A.; Joannopoulos, J. D. Rev. Mod. Phys. 1992, 64, 1045−1097. (54) Childs, S. L.; Stahly, G. P.; Park, A. Mol. Pharmaceutics 2007, 4 (3), 323−338. (55) Trasi, N. S.; Byrn, S. R. AAPS PharmSciTech. 2012, 13 (3), 772−784. (56) Good, D. J.; Rodríguez-Hornedo, N. Crystal Growth Des. 2009, 9 (5), 2252−2264.

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Ketoconazole Salt and Co-crystals with Enhanced Aqueous Solubility

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