Article pubs.acs.org/crystal
Three Candesartan Salts with Enhanced Oral Bioavailability Yingnan Chi,† Wenting Xu,† Yan Yang,‡ Zhichao Yang,‡ Hongjin Lv,§ Song Yang,† Zhengguo Lin,† Jikun Li,† Jingkai Gu,*,‡ Craig L. Hill,*,§ and Changwen Hu*,† †
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ Research Center for Drug Metabolism, Jilin University, Changchun 130012, P. R. China § Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States S Supporting Information *
ABSTRACT: Three new salts, [H3N(CH2)2NH3][can]·2H2O (1), [H3N(CH2)3NH3][can]·2H2O (2), and [NH4][Hcan] (3), of the minimally soluble antihypertensive drug, Candesartan (H2can), have been prepared by solventassisted grinding. Salts 1−3 also have been thoroughly characterized by singlecrystal X-ray diffraction, powder X-ray diffraction, Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance, thermogravimetry, and differential scanning calorimetry. In the case of 1 and 2, two protons of carboxyl and tetrazole groups of Candesartan transfer to the diamine, resulting in salts where both hydrogen bonding and electrostatic interactions that link the Candesartan and diamine (diammonium) units into a one-dimensional supramolecular ribbon. However, unlike the case in 1 and 2, only one proton from the carboxyl group of Candesartan transfers to ammonia in 3 and ionic components now assemble into a three-dimensional supramolecular network. Dissolution studies indicate that both the apparent solubility and dissolution rate of salts 2 and 3 in phosphate buffer are dramatically improved compared to those of the original active pharmaceutical ingredient (API). Furthermore, to evaluate the absorption effect of salts 1−3 in vivo, pharmacokinetic studies were performed in rats. It is notable that the oral bioavailability of salts 1−3 is enhanced by 1.3, 2.5, and 3.1 times, respectively, compared to that of the API.
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INTRODUCTION Candesartan (also known as CV-11974) is a nonpeptide angiotensin II type 1 receptor antagonist and is mainly used in the treatment of hypertension or heart failure.1,2 The molecular structure of Candesartan is shown in Scheme 1. The tetrazole
Although some improvement has been achieved by this chemical modification, the oral bioavailability of Candesartan Cilexetil has remained unsatisfactory (∼40%).9 The low bioavailability of Candesartan Cilexetil is caused by its poor aqueous solubility. To improve the solubility of Candesartan Cilexetil, several methods, including solid dispersion,10−12 encapsulation,13 and nanoparticle formation,14,15 have been developed. Besides the chemical modification, the selection and optimization of the solid form play a crucial role in the development of a drug. By “solid form”, we mean a polymorph, hydrate (or solvate), salt, or possible co-crystal.16 The solid form has a significant impact on many physicochemical properties of an active pharmaceutical ingredient (API).17−20 Among the possible variants in the solid state, the particular salt form used is an operationally simple and widely used method for optimizing the API for parenteral administration.21 Generally, the solubility of APIs will be improved by forming salts.22−32 For example, in the case of furosemide, a loop diuretic drug, 4000- and 10600-fold increases in solubility have been observed by forming sodium and potassium salts, respectively.22 The two acidic functional groups of Candesar-
Scheme 1. Molecular Structure of Candesartan
ring, ethoxy, and carboxyl groups in Candesartan have been proven to be closely associated with a potent and long-acting hypotensive effect.3 Candesartan was first synthesized by Keiji Kubo and his co-workers in 1993,3 but they soon found that the bioavailability of this therapeutic agent was very low (99.0% pure) was purchased from Wuhan Chemwish Technology Co., Ltd., and used without further purification. All other chemicals were reagent grade, purchased from commercial sources, and used without further purification. IR spectra were obtained (as KBr-pressed pellets) using a Nicolet 170SXFT/IR spectrometer in the range of 400−4000 cm−1. Xray powder diffraction (XRPD) data on the samples were collected on a Bruker instrument equipped with graphite-monochromatized Cu Kα radiation (λ = 0.154060 nm, scan speed of 6°/min, 2θ = 5−35°, step size of 0.019762). The C, H, and N elemental analyses were performed on a PerkinElmer 2400 CHN elemental analyzer. TG analyses were conducted in a nitrogen atmosphere between 30 and 300 °C at a heating rate of 10 °C/min on a Q50 TGA simultaneous thermal analyzer. Differential scanning calorimetry was performed on a Q100 DSC module in the temperature range of 30−300 °C at a heating rate of 10 °C/min with a nitrogen flow rate of 50 mL min−1. 1 H NMR spectra were recorded using a Bruker-400 NMR spectrometer. Salt Screening by Grinding. Candesartan (200 mg, 0.45 mmol), the alkaline reagent (0.9 mmol), and 4 drops of deionized water were added to a 10 mL stainless steel container. The mixture was ground using a tungsten carbide ball (Φ 8 mm) in a CRINDER GT100 ball mill for 30 min at a rate of 30 Hz. The ground products were dried under ambient conditions and analyzed by PXRD. Synthesis of [H3N(CH2)2NH3][can]·2H2O (1). Candesartan (100 mg, 0.227 mmol) and an aqueous solution of ethylenediamine (5.0 mL, 0.1 M) were mixed and stirred for 2 h. To this was added 15 mL of methanol, and the mixture was continually stirred for 0.5 h. The resulting solution was filtered and left for crystal growth under ambient conditions. After ∼7 days, colorless cubic block single crystals of 1 suitable for single-crystal X-ray diffraction were collected and dried at room temperature (yield of 104 mg, ∼85% based on Candesartan): 1H NMR (DMSO-d6, 400 MHz) δ 7.67−7.02 (7H, m, phenyl), 6.98−6.96 (2H, d, phenyl), 6.88−6.86 (2H, d, J = 8.2, 31.6 Hz, phenyl), 5.70 (2H, s, methylene), 4.52 (2H, q, J = 7.0 Hz, methylene), 3.03 (4H, s, methylene), 1.35 (3H, t, J = 7.0 Hz, methyl). Elemental Anal. Calcd for C26H32N8O5: C, 58.50%; H, 6.01%; N, 20.88%. Found: C, 59.06%; H, 5.56%; N, 21.14%. Synthesis of [H3N(CH2)3NH3][can]·2H2O (2). Salt 2 was prepared using a procedure similar to that for 1 by replacing the ethylenediamine solution with a 1,3-diaminopropane solution (4.0 mL, 0.1 M). After ∼9 days, high-quality single crystals of 2 were isolated for single X-ray measurement and dried under ambient conditions (yield of 100 mg, ∼80% based on Candesartan): 1H NMR (DMSO-d6, 400 MHz) δ 7.63−7.01 (7H, m, phenyl), 6.97−6.96 (2H, d, phenyl), 6.91−6.90 (2H, d, J = 7.5, 30.0 Hz, phenyl), 5.73 (2H, s, methylene), 4.49 (2H, q, J = 7.0 Hz, methylene), 2.79 (4H, t, J =7.0 Hz, methylene), 1.86 (2H, t, J = 7.0 Hz, methylene), 1.32 (3H, t, J = 7.0 Hz, methyl). Elemental Anal. Calcd for C27H34N8O5: C, 58.90%; H, 6.22%; N, 20.34%. Found: C, 58.22%; H, 5.94%; N, 20.47%.
Table 1. Crystallographic Data for Salts 1−3a formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z volume (Å3) Dcalc (g cm−3) F(000) no. of reflections collected no. of unique reflections no. of observed reflections Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) GOF on F2 a
1
2
3
C26H32N8O5 536.60 monoclinic P21/c 10.8994(8) 13.9820(11) 18.0745(15) 90 103.637(2) 90 4 2776.8(4) 1.331 1136 13285 4686 1977 0.1147 0.0668 0.1274 0.1460 0.1413 1.000
C27H34N8O5 550.62 monoclinic P21/c 10.8701(9) 14.2382(12) 18.8214(16) 90 106.714(2) 90 4 2789.9(4) 1.311 1168 13826 4889 2234 0.0731 0.0645 0.1621 0.1577 0.2043 0.971
C24 H23N7O3 457.49 monoclinic C2/c 31.235(9) 11.605(2) 13.897(3) 90 115.15(3) 90 8 4559.9(18) 1.333 1920 26675 7547 5774 0.0434 0.0688 0.2072 0.0964 0.2327 1.002
R1 = ∑||F0| − |Fc||/∑|F0|; wR2 = ∑[w(F02 − Fc2)2]/∑[w(F02)2]1/2.
Dissolution and Solubility. The absorbance of a known concentration of Candesartan was measured at the given λmax (208 nm) using a TU-1801 UV−vis spectrometer. The absorbance values were plotted against several known concentrations to prepare the concentration versus intensity calibration curve (R2 = 0.9996). From the slope of the calibration curve, the molar extinction coefficient of Candesartan was calculated. The salts prepared by the solution method were milled to powders and sieved using standard mesh sieves to provide samples with the approximate particle size distribution of 75−150 μm. For determining the equilibrium solubility, an excess amount of the salt was added to 10 mL of phosphate buffer (pH 6.8). B
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Table 2. Hydrogen Bonding Interactions in Salts 1−3 D−H···A
D−H (Å)
H···A (Å)
N7−H7A···N6 N7−H7B···N2 N7−H7C···O1 N8−H8B···O1
0.89 0.89 0.89 0.89
2.05 1.98 1.94 1.92
N7−H7A···N6 N7−H7B···N2 N7−H7C···O1 N8−H8A···N5 N8−H8B···O2
0.89 0.89 0.89 0.89 0.89
2.10 2.04 1.88 2.06 1.97
N6−H6A···O1 N7−H7A···N4 N7−H7B···O1 N7−H7C···O2 N7−H7D···N2
0.99 0.96 0.93 0.93 0.98
1.65 1.91 2.02 1.84 1.94
D···A (Å) Salt 1 2.896(5) 2.867(4) 2.784(4) 2.802(4) Salt 2 2.910(5) 2.919(4) 2.745(5) 2.941(5) 2.841(4) Salt 3 2.640(2) 2.833(3) 2.834(2) 2.764(3) 2.918(3)
The supersaturated solution was stirred at 500 rpm using a magnetic stirrer at room temperature. After 24 h, the suspension was filtered through a 0.22 μm nylon filter. The filtered aliquots were diluted with phosphate buffer, and the absorbance was measured. In a typical dissolution experiment, 50 mL of phosphate buffer (pH 6.8) and an excess of salts were added to a 100 mL flask, and the resulting mixture was stirred at room temperature using a magnetic stirrer at 500 rpm. The solution was withdrawn from the flask at regular intervals and filtered through a 0.22 μm nylon filter. Appropriate dilutions were made to maintain absorbance within the standard curve. The resulting solution was measured with a UV−vis spectrophotometer. Each experiment was conducted three times, and the mean value in each case was calculated. Pharmacokinetic Study. The pharmacokinetics of Candesartan salts 1−3 (prepared by a solution method) and original Candesartan were conducted using Wistar rats. After an overnight fast, eight healthy rats (half males and half females, 7 months, weighing 220−260 g) were randomly divided into four groups before dosing. Pure powder samples of Candesartan API and salts 1−3 as oral formulations were administered to rats in different groups by gastric perfusion using a Dry Powder Insufflator at a single dose of 16 mg/kg (targeting dose of 8−16 mg daily)2 for each group. Subsequently, 0.5 mL aliquots of whole blood samples were collected from the eyeball veins at intervals of 0.17, 0.33, 0.5, 0.75, 1, 2, 3, 5, 7, 9, 11, and 24 h from each rat, immediately centrifuged for 5 min at 13300 rpm at ambient temperature, and stored at −20 °C until LC−MS/MS analysis. Analysis was performed on a LC−MS/MS system comprising an Agilent 1100 Series HPLC system (Agilent Technologies, Palo Alto, CA) and an API 4000 mass spectrometer (Applied BiosystemsSciex, Ontario, Canada). Chromatographic separation was achieved using an HC-C18 column (150 mm × 4.6 mm inside diameter, 5 μm particle size, Agilent Technologies) at 40 °C with an acetonitrile/ammonium acetate (10 mM) [85:15 (v/v)] mobile phase, at 1 mL/min. Analysis was conducted with an electrospray ionization (ESI) source using positive ion (ESI+) mode. Data were acquired using Drug and Statistics Software DAS 3.0 (Mathematical Pharmacology Professional Committee of China, Shanghai, China).
∠D−H···A (deg)
symmetry code
157.7 172.6 156.2 169.9
intramolecular −x + 1, y − 1/2, −z + 3/2 intramolecular −x + 1, y + 1/2, −z + 3/2
150.2 167.0 164.8 174.3 165.4
−x + 1, y + 1/2, −z + 3/2 intramolecular −x + 1, y + 1/2, −z + 3/2 −x + 1, y + 1/2, −z + 3/2 x, y + 1, z
176.0 161.0 146.0 169.0 173.0
/2 − x, −1/2 + y, 1/2 − z x,1 + y, z x, 1− y, 1/2 + z intramolecular −x, 1 − y, −z 1
solvent-assisted grinding method to screen alkaline reagents for Candesartan salts. All grinding experiments were performed mechanically with 4 drops of water and a 1:2 API:alkali stoichiometry. Moreover, PXRD proved to be a highly reliable technique for assessment of the solid forms generated after grinding. As shown in Table 3, cogrinding experiments gave three possible outcomes: (1) no Table 3. Results of Salt Screening alkaline reagent
pKb value
drop grinding
formamide ethylenediamine choline solution (45−50%) triethylamine 1,2-diaminopropane 1,3-diaminopropane 1,4-butanediamine cyclohexylamine diethanolamine urea ammonia solution (25%) phenylamine
− 4.07, 7.15 ∼5.0 3.25 4.28, 7.39 3.53, 5.51 3.32, 5.40 3.36 5.52 13.82 4.75 9.40
API new crystalline form amorphous state amorphous state amorphous state new crystalline form amorphous state amorphous state API API new crystalline form API
reaction as demonstrated by the PXRD pattern of a mixture of starting materials, (2) an amorphous state, and (3) a new crystalline phase exhibiting obviously different PXRD patterns. As a result, three promising candidates, ethylenediamine, 1,3diaminopropane, and ammonia, were efficiently selected via this simple method. In addition, the acid−base reactions between Candesartan and three alkaline compounds described above were complete after samples had been milled for 30 min as indicated by the complete loss of the characteristic Candesartan peaks in the PXRD pattern (Figure 1). The single crystals of three salts were prepared by crystallization from methanol or ethanol. We note that the PXRD patterns of salts 1−3 synthesized by solvent-assisted grinding without any purification match the X-ray crystal structure-simulated ones very well (Figure S1 of the Supporting Information). Crystal Structures. A careful literature and Cambridge Structural Database (CSD)42 search shows that so far there is no crystal structure of Candesartan or its salt, hydrate, solvate,
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RESULTS AND DISCUSSION Salt Screening by Grinding. Recently, solid-state grinding, including neat and solvent-assisted grindings, has been introduced as an alternative mode for screening crystalline salts.36−41 Compared with traditional solution-based crystallization, the grinding method has advantages, such as efficiency, no solvent waste, and high yield. Therefore, we chose the C
DOI: 10.1021/acs.cgd.5b00297 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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one [can]2−, one [H3N(CH2)2NH3]2+, and two crystallization water molecules (O2w and O2w′ that are disordered with a site occupancy of 50%). In the dication, the dihedral angle between tetrazole and the adjacent benzene rings is 45.6° and the angle between two benzene rings is 40.4°. The C3−N1−C9−C10 torsion angle indicating the geometry of the benzimidazole group and the neighboring phenyl group is 74.5° (Figure S2 of the Supporting Information). The whole Candesartan anion has a “U” shape with benzimidazole and tetrazole groups arranged on the same side of the diphenyl group. In addition to the electrostatic attraction between [can]2− and ethylenediammonium, these two changed units are connected by N−H···O and N−H···N hydrogen bonding interactions into a onedimensional (1D) ribbon along b-axis (Figure 3 and Table 2). [H3N(CH2)3NH3][can]·2H2O (2) Salt. When 1,3-diaminopropane is used instead of ethylenediamine, salt 2 is obtained. As shown in Figure 4, salt 2 consists of one [can]2−, one
Figure 1. PXRD patterns of Candesartan and salts 1−3 obtained by solvent-assisted grinding.
or co-crystal. Even for the widely studied Candesartan Cilexetil that has several polymorphs, hydrates, and solvates,43−47 only one crystal structure has been reported.48 In the pharmaceutical field, crystal structural analysis is crucial for building chemical structure−pharmacokinetic relationships. [H3N(CH2)2NH3][can]·2H2O (1) Salt. When Candesartan reacts with ethylenediamine, two protons transfer from the carboxylic acid and tetrazole groups to the nitrogen atoms of the diamine to form salt 1. As shown in Figure 2, salt 1 contains
Figure 4. Crystal structure of salt 2.
[H3N(CH2)3NH3)]2+, and two lattice water molecules (O2w and O2w′ that are disordered). The configuration of [can]2− in 2 is basically similar to that in 1. As shown in Figure 5, N−H··· O and N−H···N hydrogen bonding interactions assemble the ionic components into a 1D ribbon (Table 2). [NH4][Hcan] (3) Salt. When Candesartan reacts with ammonia, salt 3 forms. However, unlike 1 and 2, only one proton from the carboxylic acid group of Candesartan transfers to the base in 3. There is one [Hcan]− and one ammonium in the asymmetric unit of 3 (Figure 6). The configuration of the [Hcan]− anion in salt 3 is different from those in salts 1 and 2.
Figure 2. Crystal structure of salt 1.
Figure 3. 1D ribbon in salt 1 formed by hydrogen bonding interactions between Candesartan anions and the ethylenediammonium dications. Hydrogen atoms and lattice water have been omitted for the sake of clarity, and green dashed lines represent the hydrogen bonding interactions. Symmetry code: A, 1 − x, 0.5 + y, 1.5 − z; B, 1 − x, −0.5 + y, 1.5 − z. D
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Figure 5. 1D ribbon in salt 2 formed by hydrogen bonding interactions between Candesartan anions and the dication form of 1,3-diaminopropane. Hydrogen atoms and lattice water have been omitted for the sake of clarity, and green dashed lines represent the hydrogen bonding interactions. Symmetry code: A, 1 − x, 0.5 + y, 1.5 − z; B, x, 1 + y, z.
of the -COO− group is at 1650−1550 cm−1 (asymmetric vibration) and at 1400 cm−1 (symmetric vibration, weak). The IR spectra of Candesartan API and salts 1−3 are shown in Figure S5 of the Supporting Information. The strong absorption peak at 1707 cm−1 in Candesartan disappears in salts 1−3. Instead, new absorption peaks arising from asymmetric and symmetric vibration of the carboxylate group are observed at 1606 and 1390 cm−1 (for 1), 1607 and 1390 cm−1 (for 2), and 1610 and 1383 cm−1 (for 3). The changes mentioned above are consistent with formation of salts and the X-ray structures. Via comparison of the 1H NMR spectra of Candesartan API and its salts, it is clear that 1 and 2 contain α,ω-diammonium cations (Figure S6 of the Supporting Information). In these salts, the H peaks of -CH2CH2- with δ 3.03 and -CH2CH2CH2with δ 2.79 and 1.86 are observed in addition to the characteristic peaks of Candesartan. In the case of 3, only the H peaks of Candesartan anion appear in the 1H NMR spectrum. Moreover, in all three salts, the Candesartan anion peaks are slightly shifted to a higher field relative to the peaks of API, due to salt formation. Dissolution and Stability. Both solubility and dissolution rate are important parameters that greatly affect the absorption of pharmaceuticals. To evaluate the solubility of three new salts, powder dissolution profiles of Candesartan and salts 1−3 were performed in phosphate buffer (pH 6.8) at room temperature. The apparent solubilities of salts 1−3 and Candesartan API in phosphate buffer are 1.17, 11.16, 6.79, and 2.57 mg/mL, respectively. The solubilities of Candesartan and salts 1−3 decrease in the following order: 2 > 3 > API > 1. As shown in Figure 9, in comparison with API both the dissolution rate and the apparent solubility of salts 2 and 3 are dramatically improved. The high apparent solubility as well as the fast dissolution rate of salts 2 and 3 might lead to improved bioavailability of API. Although salts have solubilities higher than those of pure APIs, they face the challenge of hydrolysis during storage. As a consequence, the stability of three salts reported herein was assessed. The solid forms of 1−3 were found to be stable under ambient conditions (∼25 °C and 30% relative humidity) for more than 3 months (Figure S7 of the Supporting Information). In addition, slurry experiments of the three salts were conducted. Excess solids of salts 1−3 were stirred in deionized water for 7 days, and then PXRD patterns of the filtered and air-dried samples were recorded. Salts 1 and 2 are stable after 7 days of slurry experiments as confirmed by PXRD results (Figure S8 of the Supporting Information). However, a
Figure 6. Crystal structure of salt 3.
In 3, benzimidazole and tetrazole groups linked by a biphenyl group point in opposite directions, and the whole Candesartan anion exhibits a “Z” shape. As shown in panels a and b of Figure 7, each ammonium links with four adjacent [Hcan]− units through hydrogen bonding interactions and vice versa (Table 2). These interactions assemble [Hcan]− and NH4+ into a three-dimensional (3D) supramolecular network (Figure 7c). Interestingly, as the tetrazole ring does not lose its proton during salt formation, hydrogen bonding interactions between Candesartan molecules are observed in 3 (Figure 7d). Thermal Analysis. Salts 1−3 were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC and TGA plots of 1−3 are shown in Figure 8. The melting points of salts 1−3 are different from that of Candesartan (186.8 °C). In the DSC curve of 1, an initial endotherm at 136.5 °C corresponds to a weight loss of lattice water (observed, 6.70%; calculated, 6.72%) and the endotherm at 174.8 °C is ascribed to the melting of 1. Salt 1 begins to decompose at 180.1 °C with a sharp exothermic peak. For salt 2, the first endothermic peak at 86.9 °C is for lattice water loss (observed, 6.52%; calculated, 6.54%) and the second one at 184.7 °C is for melting. The decomposition of 2 occurs at 191.2 °C. In salts 1 and 2, the number of lattice water molecules determined by the crystal structure analysis is in agreement with the results from thermal analysis. The melting point of 3 is 220.5 °C, and above this, decomposition begins. Spectral Analysis. X-ray crystal structure analyses reveal that the carboxylic acid group of Candesartan loses all its protons during the formation of salts, and the resulting structural changes can be further probed by Fourier transform infrared spectroscopy. Generally, the characteristic absorption peak of the -COOH group is at 1720−1700 cm−1, whereas that E
DOI: 10.1021/acs.cgd.5b00297 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 7. (a) Hydrogen bonding interactions between ammonium and four neighboring Candesartan anions in 3. (b) Hydrogen bonding interactions between Candesartan and four adjacent ammonium cations. (c) 3D supramolecuar network in 3. (d) Hydrogen bonding interactions between Candesartan anions. Hydrogen atoms have been omitted for the sake of clarity, and the big yellow spheres represent ammonium cations. The green dashed lines represent the hydrogen bonding interactions. Symmetry code: A, x, 1 − y, 0.5 + z; B, −x, 1 − y, −z; C, x, 1 + y, z; D, 0.5 − x, 0.5 + y, 0.5 − z.
small part of salt 3 hydrolyzed into original Candesartan (Figure S8 of the Supporting Information). Pharmacokinetic Study. Figure 10 presents the mean plasma concentration versus time plot for Candesartan and salts 1−3 after oral administration. The calculated pharmacokinetic parameters of maximal plasma concentration (Cmax), the time required to reach the Cmax (tmax), and the area under the curve (AUC) are summarized in Table 4. In pharmacology, bioavailability that measures the rate and extent of a drug’s intake is often calculated by the AUC value.
Candesartan salts 1, 2, and 3 are absorbed efficiently upon oral administration as indicated by their bioavailabilities of 129.6, 256.9, and 314.1%, respectively, relative to that of API [defined as 100% (Table 4)]. In addition, Cmax values of salts 2 and 3 are increased by 2.2- and 3.3-fold, respectively. It is interesting to note that the order of bioavailability (3 > 2 > 1 > API) is inconsistent with that of solubility in phosphate buffer (2 > 3 > API > 1). Such inconsistency is attributed to the following reasons. (1) The in vivo environment is very complex and different from that in the in vitro experiments. (2) The F
DOI: 10.1021/acs.cgd.5b00297 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 9. Dissolution profiles of API and salts 1−3 in phosphate buffer (total concentration of 0.05 M at pH 6.8).
Figure 10. Mean plasma concentration vs time profiles of original API and salts 1−3 in rats after oral administration.
Table 4. PK Data of the Original API and Salts 1−3 drug
Cmax (ng/mL)
tmax (h)
AUC (ng· h/mL)
F (%)
original API salt 1 salt 2 salt 3
2620.0 3050.0 5790.0 8767.3
5.00 8.00 5.00 7.00
29559.6 38318.4 75931.2 92846.1
100 129.6 256.9 314.1
Figure 8. TG (blue) and DSC (red) thermograms of salts 1−3.
addition, toxicity and efficacy studies continue in our laboratory.
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bioavailability of these three salts is actually influenced by both their solubility and hydrolysis rate. Although salt 2 has an apparent solubility and a dissolution rate higher than those of salt 3, salt 3 is more easily hydrolyzed than 2 (Figure S8 of the Supporting Information); these factors work together, resulting in the observed bioavailability order (3 > 2 > 1 > API).
ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic data in CIF format, additional measured and calculated PXRD patterns, labeled structures, IR spectra, 1 H NMR spectra, results of the slurry experiment, and selected bond distances and angles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00297.
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CONCLUSIONS In this work, Candesartan salts 1−3 have been synthesized by solvent-assisted grinding. Compared to those of API, the solubility and oral bioavailability of salts 2 and 3 have been significantly enhanced. The results mentioned above indicate that those two Candesartan salts could well be effective potential candidates for the pharmaceutical industry. In
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. G
DOI: 10.1021/acs.cgd.5b00297 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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*E-mail:
[email protected].
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Author Contributions
Y.C. and W.X. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the NSFC (21231002, 21276026, 21173021, 21371024, and 81430087), the 111 Project (B07012), and the 973 Program (2014CB932103).
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DOI: 10.1021/acs.cgd.5b00297 Cryst. Growth Des. XXXX, XXX, XXX−XXX
