Isolation Strategies and Transformation Behaviors of Spironolactone

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The Isolation Strategies and Transformation Behaviors of Spironolactone Forms Chen Jiang, Jiaqi Yan, Yongli Wang, Jie Zhang, Guan Wang, Jingxiang Yang, and Hongxun Hao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03023 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015

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Industrial & Engineering Chemistry Research

The Isolation Strategies and Transformation Behaviors of Spironolactone Forms Chen Jiang,† Jiaqi Yan,† Yongli Wang,*, †, ‡ Jie Zhang,† Guan Wang,† Jingxiang Yang,† Hongxun Hao *, †, ‡ † School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.

ABSTRACT: Spironolactone (SPI) is one kind of potassium-sparing diuretics, and two polymorphs (Form I and Form II) along with five solvates (methanol, ethanol, acetonitrile, ethyl acetate and benzene) of SPI have been reported in literature. But no detailed information about the stability, solubility and transformation behaviors of SPI forms has been reported. In this paper, two new forms of SPI, 1-propanol solvate and 2-propanol solvate, were found and characterized. The thermodynamic stability and solubility of Form II and four alcohol solvates of SPI were investigated and determined. It was found that methanol solvate and ethanol solvate of SPI are relatively stable while 1-propanol solvate and 2-propanol solvate of SPI are metastable in corresponding solvents，and 1-propanol solvate and 2-propanol solvate of SPI would transform to Form II in corresponding solvents. Furthermore, the transformation processes of 1-propanol solvate and 2-propanol solvate were in situ 1

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monitored by Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Raman spectroscopy and some offline tools such as microscope and power X-ray diffraction (PXRD). The reasons behind the transformation were explained by the enthalpy data of different solvates.

1. INTRODUCTION Many pharmaceuticals are found to have more than one crystalline forms, either in the forms of polymorphs or solvates. Solvates, also known as pseudopolymorphs, are crystalline solids whose unit cells differ in their elemental composition through the inclusion of one or more solvent molecules.1-3 Different crystalline forms might exhibit different physical and chemical properties, such as crystal habit, melting point, solubility, and so on, which would affect the stability and bioavailability of pharmaceuticals.4,5 For different polymorphs of substance, there is generally one thermodynamically stable form under certain condition, while other forms are metastable polymorphs and could transform into the stable form. In order to obtain the desired polymorphic form, it is essential to know all available forms and characterize and compare them. It is also important to know the transformation behaviors of all polymorphs to control the final forms of the products. Two types of polymorphic transformation have been proposed in literature,6 the solid-state polymorphic transformation (SST) and solvent-mediated polymorphic transformation (SMT). SST involves the molecular rearrangement in the solid state and SMT takes place through dissolution of the metastable form and crystallization of the stable form.

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Spironolactone (CAS Registry No. 52-10-7, hereinafter referred as SPI, Figure 1) is one kind of potassium-sparing diuretics. It is primarily used for the treatment of refractory oedema hepatic ascites and essential hypertension.7,8 SPI is white or almost white crystalline power. Two polymorphs (Form I and Form II) and five solvates (methanol, ethanol, acetonitrile, ethyl acetate and benzene) of SPI have been reported in literature.9,10 But no detailed information about the stability, solubility and transformation behaviors of SPI forms has been reported. The molecular mechanism of the forms was not investigated either. In this work, two new solvates of SPI, 1-propanol solvate and 2-propanol solvate, were found. Different forms of SPI were prepared and characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The thermodynamic stability and solubility of different forms were investigated and determined. The transformation behaviors of different forms were also investigated by using Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Raman spectroscopy. The results were explained by the enthalpy data. The results obtained in this work will be of importance to the industrial production of SPI. 2. EXPERIMENTAL SECTION 2.1. Materials. SPI Form II was provided by Tianjin Jinjin Pharmaceutical Co., Ltd, and the mass fraction purity of SPI is higher than 99.8% by High Performance Liquid Chromatography. Methanol, ethanol, 1-propanol, 2-propanol and 1-butanol were purchased from Tianjin Jiangtian Chemical Co., Ltd. All of the solvents are analytical 3



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reagent grade, and their purities are higher than 99.5%. 2.2. Preparation and Characterization of the Solvates and Form II of SPI. The solvates and Form II of SPI were prepared by cooling crystallization from 323.15 to 278.15 K in different solutions. At first, SPI Form II solids were added into different alcohol solvents, including methanol, ethanol, 1-propanol, 2-propanol and 1-butanol. When the solids were dissolved completely, the solutions were cooled down by different cooling rates to obtain the corresponding crystalline forms under agitate speed of 200 r/min. Methanol solvate and ethanol solvate were obtained by using cooling rates of 0.2 K/min. When preparing 1-propanol solvate and 2-propanol solvate, the cooling rates was set at 5 K/min. The morphologies of all crystalline forms of SPI were observed by polarized light microscope (PLM, Olympus BX51). And the PXRD patterns of all crystalline samples were measured by D/MAX 2500 X-ray diffractometer over a diffraction-angle (2θ) range from 2º to 50º, at a step size of 0.02º, a dwell time of 1 s, a voltage of 40 kV and a current of 100 mA. FTIR spectra of solid samples were collected on a Nexus 670 infrared instrument (Thermo Inc.) with attenuated total reflectance accessory. The measured wavenumber range was from 4000 to 650 cm-1. An ATR-FTIR ReactIR 45m reaction analysis system equipped with Duradisc DiComp probe (Mettler Toledo, Switzerland) was used to in situ monitor the concentration of SPI in solutions. The ATR-FTIR spectra from 2800 to 685 cm-1 were collected and analyzed by using the IC IR software. The measurement duration was set at 1 min. The Kaiser Raman RXN2 (Kaiser Optical System, Inc. U.S.A) is equipped with both a PhAT probe head and a MR probe head. 4


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The PhAT probe head with noncontact optics was used to offline measure the powder Raman spectra of solid samples while the MR probe head with immersion optics was used to in situ monitor the transformation process of 1-propanol solvate and 2-propanol solvate. This system is fitted with a Kaiser Invictus laser emitting deep red and nearly invisible (785 nm) emission of 450mW. The spectral resolution is 5 cm-1. The instrument configuration, data acquisition and process analysis are done by using the IC Raman software. DSC experiments were performed by DSC 1/500 (Mettler Toledo, Co., Switzerland) under protection of nitrogen (dry nitrogen: 100 mL·min-1). 5-10 mg samples were placed into 50µL aluminum pans and the measurement temperature range was from 298.15 to 503.15 K with a heating rate of 10 K/min. The standard uncertainty of the temperature was 0.5 K and the relative uncertainty of the enthalpy of fusion was around 2 %. Thermogravimetric measurements were carried out by a TGA/DSC 1/500 (Mettler Toledo, Co., Switzerland) under protection of nitrogen (dry nitrogen: 20 mL·min-1). The samples were placed into 70 µL pans. The measurement temperature range and the heating rate were the same with the DSC experiments. Additionally, the samples applied to DSC and TGA analyses were from the same batch experiments. The transformation of 1-propanol solvate was also observed by hot stage microscope (HSM, Olympus UMAD3) at a heating rate of 10 K/min from 298.15 to 400 K.

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2.3. Solubility Measurement of the Solvates of SPI. The solubility of SPI solvates in different alcohol solvents were determined by the gravimetric method.11 All solubility experiments were performed in a 100 mL EasyMax vessel (Mettler Toledo, U.K) with the temperature accuracy of 0.01 K. At first, hot SPI solution was quickly cooled down to the measurement temperature to obtain the corresponding solvates. Then the solution was agitated for 6 h which has been proved to be long enough to reach the equilibrium. Otherwise, the solubility were measured just before the transformation but after at least 1 h of equilibration. Afterwards, the agitation was stopped and the suspension was kept still for 1 h under the original temperature. Finally, the upper saturated solution was withdrawn and filtered through a cellulose membrane filter (0.45 µm). The filtrate was collected and dried until the total mass did not change any more. The mass of all samples was weighed by an analytical balance (type AB204-N, Mettler Toledo, Switzerland) with an uncertainty of ± 0.0001 g. For each solubility point, the experiment was repeated three times and the average value was used to calculate the solubility data. To confirm that no transformation occurred during solubility experiments, solid samples obtained from the solubility measurement experiments were analyzed by PXRD. 2.4. Solvent-Mediated Transformation of 1-Propanol Solvate and 2-Propanol Solvate of SPI. The solvent-mediated transformation experiments of 1-propanol solvate and 2-propanol solvate of SPI were performed in a 150 mL cylindrical double-jacketed glass crystallizer whose temperature was controlled by a thermostat. A certain amount of SPI Form II (7 g) was dissolved in 1-propanol or 2-propanol (100 6


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mL) at the temperature of 323.15 K under agitate rate of 200 r/min. After complete dissolution, the solution was quickly cooled down at the rate of 5 K/min to 298.15 K to obtain the corresponding solvates. The RXN2 Raman and the ATR-FTIR system were used to in situ monitor the transformation processes from 1-propanol solvate and 2-propanol solvate to Form II of SPI. The measurement durations were both set at 1min. The solid in the slurry was periodically sampled and analyzed with microscope and PXRD to verify its crystal form. Moreover, the influence of supersaturation on forms was investigated by quickly cooling the 1-propanol solution form 323.15 K to 298.15 K, 303.15 K, 312.15 K and 315.15 K respectively. The stirring speeds were all kept at 200 r/min and the cooling rates were all set at 5 K/min. 3. RESULTS AND DISCUSSION 3.1. The Identification and Characterization of Different Forms of SPI. In this work, 1-propanol solvate and 2-propanol solvate of SPI were found and characterized. Meanwhile, methanol solvate, ethanol solvate and Form II of SPI were prepared and characterized for the purpose of comparison. From Figure 2, it can be seen that the crystal morphologies of the solvates of SPI are long needle while the crystal morphology of Form II is transparent prism. The PXRD patterns of different forms of SPI were determined and shown in Figure 3. It can be seen that the diffractograms of all forms exhibit sharp and distinct peaks, which are indicator of high degree of crystallinity. The PXRD patterns of methanol solvate and ethanol solvate are consistent with those in literature.12,13 Distinct characteristic peaks at 2θ of around 4.6º and 13.8º can be found in the diffractograms of 1-propanol solvate, while the 7



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characteristic peaks at 2θ of 16.0º and 16.7º can be seen in the diffractograms of Form II. These differences between characteristic peaks can be used to identify and differentiate different forms and can be used to monitor the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent. In the FTIR spectra of solid state forms of SPI (Figure 4), characteristic peaks of several functional groups can be observed obviously. The results were tabulated in Table 1. By analyzing the spectra between 3700-3200 cm-1 corresponding to the OH stretching vibration region, it can be seen that there are two peaks at 3498 cm-1 and 3472 cm-1 in spectra of 1-propanol solvate and 2-propanol solvate respectively. While it does not show any OH vibration at high wavenumber in spectrum of Form II as no solvent present in the crystal lattice. It can be affirmed that guest solvent is incorporated with host crystal in 1-propanol solvate and 2-propanol solvate. And it can be seen that the characteristic peaks of C=O (γ-lactone), C=O (thioacetyl), C=O (α,β unsaturated cyclic ketone) and C=C (α,β unsaturated cyclic ketone) are similar, which are consistent with the molecular structure of SPI. However, the C-S (thioacetyl) vibration of different forms occur at different position. For 1-propanol solvate and 2-propanol solvate, the vibration occurs at 660 cm-1 and 665 cm-1 respectively, while the vibration occurs at 655 cm-1 for Form II. To apply ATR-FTIR for monitoring the solution concentration of SPI, the ATR-FTIR spectra of pure 1-propanol, pure 2-propanol, SPI - 1-propanol solution and SPI - 2-propanol solution at 298.15 K were determined and shown in Figure 5. The molar fractions of SPI 1-propanol solution and SPI - 2-propanol solution were 0.0025 and 0.00476, 8


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respectively. It can be seen clearly that three ATR-FTIR peaks at 1765, 1700, and 1668 cm-1, are characteristic peaks of C=O (γ-lactone), C=O (thioacetyl), C=O (α,β unsaturated cyclic ketone) of liquid state SPI. They can be used to represent the concentration of SPI in solution. In this work, because the peak of 1765 cm-1 represents C=O (γ-lactone) and it is obvious in Figure 4, 1765 cm-1 was chosen for tracking the SPI concentration trends. Although peaks of 1700 cm-1 and 1668 cm-1 can also be used to track the concentration, the profiles trends of these two peaks were the same with that of Figure 12 and Figure 15. The Raman spectra of solid state forms of SPI are shown in Figure 6. It can be found that there are some differences between the Raman spectra of different forms in the range of 670-600 cm-1. 1-propanol solvate and 2-propanol solvate of SPI have a charactseristic peak of 624 cm-1 and 622 cm-1 respectively while Form II of SPI has a characteristic peak of 641 cm-1. Considering they are all in the region of C-S (thioacetyl) vibration, the three peaks can be assigned to different vibration features of C-S bond which are sensibly affected by the molecular conformation in the solvates and Form II. So, the corresponding characteristic peaks of solvates and Form II were chosen to in situ monitor the transformation process. The DSC and TGA thermograms of the solvates and Form II are shown in Figure 7 and Figure 8 respectively. The DSC thermograms of all solvates of SPI show an endothermic peak when the solvent is released and an endothermic peak at 482 K which are consistent with the melting point of Form II. Besides, the broad and indistinct desolvation endotherm peak of 2-propanol solvate can be observed from the 9



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inlet of Figure 7. The exothermic peaks in Figure 7 indicated the occurrence of transformation. After solvates desolvated, they began to transform to Form II and the transformation is exothermic. The exothermic peaks for methanol solvate and ethanol solvate can be observed clearly. But these for 1-propanol solvate and 2-propanol solvate are not obvious since of low liberation of heat. Agafono et al.9 and Neville et al.14 reported different solvent content of methanol solvate and ethanol solvate respectively. It can be deduced that the methanol and ethanol solvate of SPI should be nonstoichiometric solvates and the molar ratios of solvent to SPI might not be fixed. Because the desolvation occurs at lower temperature (which will be discussed below) for 1-propanol solvate and 2-propanol solvate, they are more unstable than methanol solvate and ethanol solvate. Therefore, 1-propanol solvate and 2-propanol solvate may be channel or planar solvate and they are also nonstoichiometric solvates with uncertain solvent content.15 The actual solvent content may be susceptible to changes in the environment, and through a large number of experiments, the solvent content is from 4.56% to 4.86% for 1-propanol solvate and the solvent content is from 1.07% to 1.33% for 2-propanol solvate. Figure 8 shows the thermograms of solvates samples in which greatest weight loss can be observed. The morphology changes of 1-propanol solvate of SPI during desolvation were observed by hot stage microscopy and the results are shown in Figure 9. The removal of solvent led the needlelike crystal to transform to prismlike crystal gradually. 3.2. Solubility of SPI Solvates. The solubility of Form II have been determined in the previous literature.16 To further compare the themodynamic stability of different 10


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forms of SPI, the solubility of the solvates of SPI were determined at various temperatures in this work. Since 1-propanol solvate and 2-propanol solvate of SPI will quickly transform to Form II at high temperature, the solubility of them were only determined at relatively low temperature. The solubility of SPI solvates together with the solubility of SPI Form II from literature are graphically plotted in Figure 10. The experimental mole fraction solubility of the solvates are listed in Table S1 of Supporting Information. It can be found that the solubility of Form II are slightly higher than that of methanol solvate and ethanol solvate but lower than that of 1-propanol solvate and 2-propanol solvate in corresponding alcohol solvents. According to the method described by Haleblian and McCrone, the more stable form should have lower solubility at the given temperature and pressure.17 Consequently, the solvates are more stable than Form II in methanol and ethanol solvents while Form II is more stable than the solvates in 1-propanol and 2-propanol solvents. To better understand the trend of the solubility, the modified Apelblat equation was used to fit the solubility data of the solvates.18 ln( x ) = A +

B + C ln T T

(1)

where x is the mole fraction solubility of the solvates. A, B and C are the equation parameters, and T is the absolute temperature. The values of parameters and the coefficient of determination (R2) are listed in Table 2. It can be seen that the values of R2 are higher than 0.99, indicating the accuracy and reliability of the modified Apelblat equation.

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3.3. The Isolation and Transformation Behaviors of Different Forms of SPI. Different crystal forms of SPI can be obtained by cooling crystallization. Generally, different cooling rates will result in different supersaturations. It can be found that methanol solvate and ethanol solvate of SPI can be obtained by crystallization in methanol and ethanol solvents and Form II can be obtained by crystallization in 1-butanol solvent, which are independent on the supersaturation of the corresponding solutions. And methanol solvate and ethanol solvate of SPI cannot transform to Form II in the methanol and ethanol solvents, respectively. However, initial supersaturation has a great influence on the formations of SPI forms in 1-propanol and 2-propanol solvents. The influence was investigated by changing the cooling crystallization end-point in 1-propanol solvent at the same rates of 200 r/min and 5 K/min, which is presented in Figure 11. When SPI - 1-propanol solution was quickly cooled from point A (323.15 K) to point B (298.15 K), 1-propanol solvate of SPI was obtained. Because the solubility of 1-propanol solvate is larger than that of Form II in 1-propanol solvent, 1-propanol solvate would transform to Form II,19,20 which has been proved by the transformation experiments. When SPI - 1-propanol solution was cooled from point A to point C (303.15 K), the similar results was obtained. But at point C, the transformation from obtained 1-propanol solvate to Form II of SPI will be faster than that of point B. When SPI - 1-propanol solution was cooled from point A to point D (312.15 K), 1-propanol solvate and Form II of SPI will be obtained concomitantly. When SPI 1-propanol solution was cooled from point A to point E (315.15 K), only Form II can be obtained. These phenomena can be explained by the 12


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competition between kinetics and thermodynamics of crystallization. According to Ostwald’s Law of Stages, in the case of a pharmaceutical capable of crystallizing in several crystal forms, the least stable form will be first obtained by spontaneous crystallization, followed by the more and more stable forms. This can be explained by the fact that the form with the largest Gibbs free energy will be formed firstly in the crystallization, whose state is the nearest in stability to the original.21,22 In the crystallization process of SPI in 1-propanol solvent, when the solution supersaturation is very high, 1-propanol solvate can be obtained as a result of the triumph of kinetics over thermodynamics. However, at low supersaturation, Form II can be obtained as a result of triumph of thermodynamics over kinetics. If the supersaturation is moderate, both 1-propanol solvate and Form II can be obtained at the same time owing to the combined action of kinetics and thermodynamics.23 From Figure 11, the supersaturation values of B, C and D are 1.85, 1.50 and 1.02. According to our many experiments, initial solution is nearly saturated at 323.15 K, and cooled to obtain supersaturated solution, if supersaturation is higher than 1.50, solvate can be obtained. When supersaturation value is between 1.50 and 1.02, solvates and Form II can be obtained. In addition, if obtained solid state 1-propanol solvate of SPI was not taken out from 1-propanol solution in time, it would finally transform to Form II. To investigate the transformation behavior of 1-propanol solvate of SPI, the solvent-mediated transformation from 1-propanol solvate to Form II of SPI was in situ monitored by using Raman and ATR-FTIR. The results are shown in Figure 12. Based on the 13



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changing relationship between solution concentration and polymorphs content proposed by O’Mahony,24 the transformation process can be analyzed. It could be observed from Figure 12 that Form II was not detected (represented by Raman data) and the solution concentration (indicated by ATR-FTIR data) maintained constant before 150 min, which indicates that the induction time of nucleation of Form II is long and the controlling step of this period is the nucleation of Form II. Once the transformation started, the amount of Form II increased quickly and the amount of 1-propanol solvate decreased quickly while the solution concentration remained constant. These results revealed that the producing rate of liquid state SPI in solution by dissolving of 1-propanol solvate is faster than the consuming rate of liquid state SPI by the growth of Form II. Therefore, Form II growth is the key controlling step in this period. With the further transformation, the solution concentration began to decrease, indicating the dissolution rate of 1-propanol solvate became slower than that of Form II growth. At the moment, the dissolution of 1-propanol solvate is the limiting factor of the transformation process. During the transformation of 1-propanol solvate, the crystal was sampled and observed by microscope. The results are shown in Figure 13. It can be seen that the needlelike metastable 1-propanol solvate of SPI finally transformed into prismlike Form II. The nucleation of Form II did not occur on the surfaces of the crystals of 1-propanol solvate, which means that Form II is the result of independent nucleation. The changes in the PXRD patterns of crystals during the transformation are presented in Figure 14. It can be seen that the characteristic peaks of 1-propanol 14


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solvate at 2θ of 4.6º and 13.8º disappeared while the characteristic peaks of Form II at 2θ of 16.0º and 16.7º appeared, which again indicates that the 1-propanol solvate transformed to Form II. The transformation process of 2-propanol solvate is similar to that of 1-propanol solvate. The transformation from 2-propanol solvate to Form II is shown in Figure 15. At first, Form II was not detected and the solution concentration maintained constant before 10 min. In this stage, the nucleation of Form II is the controlling step. Then the decreasing of 2-propanol solvate and the increasing of Form II were detected while the solution concentration maintained constant before 40 min. In this stage, Form II growth is the controlling factor. With the further transformation, the solution concentration began to decrease, which indicates that the dissolution of 2-propanol solvate becomes the controlling step. Based on the results obtained above, the transformation behaviors of SPI in alcohol solvents can be concluded in Figure 16. The transformation from SPI solvate to Form II could be treated as a desolvation process. It has been assumed that the most important steps of desolvation are bonds breaking and vaporization of solvents.25 Hence the strength of the solvent intermolecular interactions can be estimated by comparing the onset temperature of the desolvation (Tonset) and enthalpy of desolvation (∆Hdes) with boiling point of pure solvent and the enthalpy of vaporization (∆Hvap) of pure solvent respectively.26-28 The enthalpy of desolvation of solvates (∆Hdes) can be calculated by the experimental enthalpy of desolvation (∆Hexp) for solvent loss from DSC thermograms and the relative mass loss (η) from TGA 15



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thermograms.29 The DSC instrument was calibrated by using the phase-transition temperature and phase-transition enthalpy reference materials (indium: ∆fusH = 3266.57 J/mol, Tm = 429.75 K; zinc: ∆fusH = 7028.35 J/mol, Tm = 692.65 K) before the operation of measurements. Therefore, the experimental data obtained in this work should be reliable. The enthalpy of desolvation (∆Hdes) and the enthalpy of bonds breaking (∆Hbond) can be calculated as follow:

∆H des =

∆H exp ⋅ M s

η

∆ H bond = ∆ H des − ∆ H vap

(2) (3)

where Ms represents the mole mass of the solvent. Table 3 shows the experiment data (average value based on three independent measurements) and calculated values. If solvent molecules are strongly bound in solvates, the enthalpy of desolvation of solvates will exceed the enthalpy of vaporization of pure solvents.27 According to Table 3, for methanol solvate and ethanol solvate, the values of Tonset and ∆Hdes are higher than boiling point and ∆Hvap of pure solvents respectively, and the values of ∆Hbond are both positive. However, for 1-propanol solvate and 2-propanol solvate, the values of Tonset and ∆Hdes are lower than boiling point and ∆Hvap of pure solvents respectively, and the values of ∆Hbond are both negative. This can be used to explain why methanol solvate and ethanol solvate cannot transform to Form II in the corresponding solvents, while 1-propanol solvate and 2-propanol solvate will finally transform to Form II in the corresponding solvents. Since the ∆Hbond of 1-propanol solvate is larger than that of 2-propanol solvate, the transformation rate of 1-propanol solvate is slower than that of 2-propanl solvate. 16


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Furthermore, in the structure of SPI, there are a γ-lactone carbonyl group and a thioacetyl group. Both of them might form hydrogen bonds with the hydroxyl group of alcohols. The ability of hydrogen bond formation between SPI and alcohols is dependent on the number of alkyl groups in alcohols. More alkyl groups will weaken the ability of hydrogen bond formation. Additionally, because of the steric hindrance effect, the side-chain of methyl group (CH3) will further weaken the hydrogen bond formation ability. All of these result in the different crystal forms of SPI and the transformations from 1-propanol solvate and 2-propanol solvate to Form II. 4. CONCLUSIONS In this paper, 1-propanol solvate and 2-propanol solvate of SPI were found and characterized. The solubility of different SPI alcohol solvates were experimentally determined. From solubility data, it was found that methanol solvate and ethanol solvate are stable in corresponding solvents while 1-propanol solvate and 2-propanol solvate are not stable and they will transform to Form II in corresponding alcohol solvents. The transformations from 1-propanol solvate and 2-proponal solvate to Form II were investigated. It was found that the transformation process is a desolvation process and has different controlling steps at different transformation stages. Besides, the enthalpies of desolvation of methanol solvate and ethanol solvate are higher than the enthalpies of vaporization of corresponding pure solvents while the enthalpies of desolvation of 1-propanol solvate and 2-propanol solvate are lower than the enthalpies of vaporization of corresponding pure solvents. It is caused by the strength difference of the hydrogen bond between the molecule of SPI and alcohols. 17



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Therefore, according to transformation behaviors, different SPI forms can be isolated by controlling the crystallization processes.

ASSOCIATED CONTENT Supporting Information Experimental mole fraction solubility of SPI solvates in corresponding solvents. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *Tel: +86-22-27405754. Fax: +86-22-27374971. E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are very thankful to National Natural Science Foundation of China (No. 21376165) and Program of International S&T Cooperation from Ministry of Science and Technology of the People's Republic of China (No. 2013DFE43150) for the financial support..

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REFERENCES (1) Brittain, H. G. Polymorphism and solvatomorphism 2010. J. Pharm. Sci. 2012, 101, 464. (2) Vippagunta, S. R.; Brittain, H. G.; Grant, D. J. Crystalline solids. Adv. Drug Deliver. Rev. 2001, 48, 3. (3) Aitipamula, S.; Chow, P. S.; Tan, R. B. The solvates of sulfamerazine: structural, thermochemical, and desolvation studies. CrystEngComm 2012, 14, 691. (4) Cui, P.; Yin, Q.; Guo, Y.; Gong, J. Polymorphic crystallization and transformation of candesartan cilexetil. Ind. Eng. Chem. Res. 2012, 51, 12910. (5) Liu, Z.; Yin, Q.; Zhang, X.; Gong, J.; Xie, C. Characterization and structure analysis of cefodizime sodium solvates crystallized from water and ethanol binary solvent mixtures. Ind. Eng. Chem. Res. 2014, 53, 3373. (6) Du, W.; Yin, Q.; Hao, H.; Bao, Y.; Zhang, X.; Huang, J.; Li, X.; Xie, C.; Gong, J. Solution-mediated polymorphic transformation of prasugrel hydrochloride from form II to form I. Ind. Eng. Chem. Res. 2014, 53, 5652. (7) Giron, D. Thermal analysis and calorimetric methods in the characterisation of polymorphs and solvates. Thermochim. Acta 1995, 248, 1. (8) Soliman, O. A. E.; Kimura, K.; Hirayama, F.; Uekama, K.; El-Sabbagh, H. M.; El, A. E.-G. H. A.; Hashim, F. M. Amorphous spironolactone-hydroxypropylated cyclodextrin complexes with superior dissolution and oral bioavailability. Int. J. Pharm. 1997, 149, 73. (9) Agafonov, V.; Legendre, B.; Rodier, N.; Wouessidjewe, D.; Cense, J. M. 19



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Polymorphism of spironolactone. J. Pharm. Sci. 1991, 80, 181. (10) Snider, D. A.; Addicks, W.; Owens, W. Polymorphism in generic drug product development. Adv. Drug Deliver. Rev. 2004, 56, 391. (11) Liu, Z.; Yin, Q.; Zhang, H.; Gong, J. Investigation of the crystallization of disodium 5′-inosinate in a water + ethanol system: solubility, nucleation mechanism, and crystal morphology. Ind. Eng. Chem. Res. 2014, 53, 8913. (12) Berbenni, V.; Marini, A.; Bruni, G.; Maggioni, A.; Riccardi, R.; Orlandi, A. Physico-chemical characterisation of different solid forms of spironolactone. Thermochimica acta 1999, 340, 117. (13) Nicolaï, B.; Espeau, P.; Ceolin, R.; Perrin, M.-A.; Zaske, L.; Giovannini, J.; Leveiller, F. Polymorph formation from solvate desolvation spironolactone forms I and II from the spironolactone-ethanol solvate. J. Therm. Anal. Calorim. 2007, 90, 337. (14) Neville, G.; Beckstead, H.; Cooney, J. Thermal analyses (TGA and DSC) of some spironolactone solvates. Fresenius J. Anal. Chem. 1994, 349, 746. (15) Authelin, J.-R. Thermodynamics of non-stoichiometric pharmaceutical hydrates. Int. J. Pharm. 2005, 303, 37. (16) Zhang, J.; Wang, Y.; Wang, G.; Hao, H.; Wang, H.; Luan, Q.; Jiang, C. Determination and correlation of solubility of spironolactone form II in pure solvents and binary solvent mixtures. J. Chem. Thermodyn. 2014, 79, 61. (17) Haleblian, J.; McCrone, W. Pharmaceutical applications of polymorphism. J. Pharm. Sci. 1969, 58, 911. 20


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(18) Apelblat, A.; Manzurola, E. Solubilities of o-acetylsalicylic, 4-aminosalicylic, 3,5-dinitrosalicylic, andp-toluic acid, and magnesium-DL-aspartate in water fromT = (278 to 348) K. J. Chem. Thermodyn. 1999, 31, 85. (19) Svärd, M.; Nordström, F. L.; Jasnobulka, T.; Rasmuson, Å. C. Thermodynamics and nucleation kinetics of m-aminobenzoic acid polymorphs. Cryst. Growth Des. 2009, 10, 195. (20) Hao, H.; Barrett, M.; Hu, Y.; Su, W.; Ferguson, S.; Wood, B.; Glennon, B. The use of in situ tools to monitor the enantiotropic transformation of p-aminobenzoic acid polymorphs. Org. Process. Res. Dev. 2011, 16, 35. (21) Threlfall, T. Structural and thermodynamic explanations of Ostwald's rule. Org. Process. Res. Dev. 2003, 7, 1017. (22) Nývlt, J. The Ostwald rule of stages. Cryst. Res. Technol. 1995, 30, 443. (23) Threlfall, T. Crystallisation of polymorphs: thermodynamic insight into the role of solvent. Org. Process. Res. Dev. 2000, 4, 384. (24) O’Mahony, M. A.; Maher, A.; Croker, D. M.; Rasmuson, Å. C.; Hodnett, B. K. Examining solution and solid state composition for the solution-mediated polymorphic transformation of carbamazepine and piracetam. Cryst. Growth Des. 2012, 12, 1925. (25) Chavez, K. J.; Guevara, M.; Rousseau, R. W. Characterization of solvates formed by sodium naproxen and an homologous series of alcohols. Cryst. Growth Des. 2010, 10, 3372. (26) Zencirci, N.; Griesser, U. J.; Gelbrich, T.; Kahlenberg, V.; Jetti, R. K.; 21



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Apperley, D. C.; Harris, R. K. New solvates of an old drug compound (phenobarbital): structure and stability. J. Phys. Chem. B 2014, 118, 3267. (27) Chadha, R.; Arora, P.; Garg, M.; Bhandari, S.; Jain, D. S. Thermoanalytical and spectroscopic studies on different crystal forms of nevirapine. J. Therm. Anal. Calorim. 2013, 111, 2133. (28) Aitipamula, S.; Chow, P. S.; Tan, R. B. Solvates and polymorphic phase transformations of 2-chloro-4-nitrobenzoic acid. CrystEngComm 2011, 13, 1037. (29) Chadha, R.; Arora, P.; Kaur, R.; Saini, A.; Singla, M.; Jain, D. Characterization of solvatomorphs of methotrexate using thermoanalytical and other techniques. Acta pharmaceutica 2009, 59, 245.

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Table headings: TABLE 1. Summary of characteristic peaks for functional groups in solid state forms of SPI TABLE 2. Parameters and coefficients of determination of the modified Apelblat equation for the solubility of the solvates in alcohol solvents TABLE 3. Thermal analysis data (DSC and TGA) of SPI solvates and pure solvents

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Figure captions: Figure 1. Chemical structure of SPI.

Figure 2. The microscope images of the solvates and Form II of SPI.

Figure 3. PXRD patterns of the solvates and Form II of SPI.

Figure 4. Solid state FTIR spectra of 1-propanol solvate, 2-propanol solvate and Form II of SPI.

Figure 5. ATR-FTIR spectra of pure 1-propanol, pure 2-propanol, SPI - 1-propanol solution, SPI - 2-propanol solution (298.15 K).

Figure 6. Solid state Raman spectra of 1-propanol solvate, 2-propanol solvate and Form II of SPI.

Figure 7. DSC thermograms of the solvates and Form II of SPI.

Figure 8. TGA thermograms of the solvates of SPI.

Figure 9. The hot stage microscopy images during 1-propanol solvate desolvation.

Figure 10. Mole fraction solubility of the solvates and Form II of SPI in alcohol solvents: (a) methanol, (b) ethanol, (c) 1-propanol, (d) 2-propanol. Solid lines are calculated data by the modified Apelblat model.

Figure 11. Polymorphic system of 1-propanol solvate and Form II. Solubility curves, black and red lines; operation line, blue lines. A is the initial state of hot, unsaturated solution; B, C, D and E are states of the cooling crystallization end-point. 24


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Figure 12. Changes of Raman relative intensity and IR peak height during the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent.

Figure 13. The microscope images during the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent.

Figure 14. The PXRD patterns during the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent.


Figure 16. The transformation behaviors of SPI solvates in alcohol solvents.

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TABLE 1. Summary of characteristic peaks for functional groups in solid state forms of SPI Functional groups νO-H νC=O (γ-lactone) νC=O (thioacetyl) νC=O (α,β unsat)a νC=C (α,β unsat)a νC-S (thioacetyl) a

Characteristic peaks 2-propanol solvate 3472 1764 1690 1675 1617 665

1-propanol solvate 3498 1765 1690 1673 1617 660

Form II 1765 1690 1673 1616 655

α,β unsat refers to α,β unsaturated cyclic ketone.

TABLE 2. Parameters and coefficients of determination of the modified Apelblat equation for the solubility of the solvates in alcohol solvents Solvates

methanol solvate

ethanol solvate

1-propanol solvate

2-propanol solvate

A B C R2

-129.7 1441 20.62 0.9971

-127 1990 20.14 0.9995

-138.7 2664 21.89 0.9999

-100.2 450 16.35 0.9996

TABLE 3. Thermal analysis data (DSC and TGA) of SPI solvates and pure solvents Solvates

methanol solvate

ethanol solvate

1-propanol solvate

2-propanol solvate

Tonset of desolvation /K

403.5

391.2

360.3

337.7

Boiling point /K

333.85

351.55

370.25

355.60

∆Hexp for solvent loss /J·g-1

49.96

27.83

28.18

5.63

Weight loss (η) /%

2.50

2.82

4.76

1.25

∆Hdes for solvent /kJ·mol-1

64.03

45.47

35.58

27.07

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∆Hvap for solvent /kJ·mol-1

35.3

38.7

40.1

39.9

∆Hbond /kJ·mol-1

28.73

6.77

-4.52

-12.83

Tmelt /K

483.1

482.6

482.3

481.9

∆Hmelt for Melting /J·g-1

49.58

50.82

50.12

50.24

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Figure 1. Chemical structure of SPI.

Figure 2. The microscope images of the solvates and Form II of SPI.

Figure 3. PXRD patterns of the solvates and Form II of SPI.

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Figure 4. Solid state FTIR spectra of 1-propanol solvate, 2-propanol solvate and Form II of SPI.

Figure 5. ATR-FTIR spectra of pure 1-propanol, pure 2-propanol, SPI - 1-propanol solution, SPI - 2-propanol solution (298.15 K).

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Figure 6. Solid state Raman spectra of 1-propanol solvate, 2-propanol solvate and Form II of SPI.

Figure 7. DSC thermograms of the solvates and Form II of SPI.

Figure 8. TGA thermograms of the solvates of SPI. 30


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Figure 9. The hot stage microscopy images during 1-propanol solvate desolvation.

Figure 10. Mole fraction solubility of the solvates and Form II of SPI in alcohol solvents: (a) methanol, (b) ethanol, (c) 1-propanol, (d) 2-propanol. Solid lines are calculated data by the modified Apelblat model.

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Figure 11. Polymorphic system of 1-propanol solvate and Form II. Solubility curves, black and red lines; experiment operation line, blue lines. A is the initial state of hot, unsaturated solution; B, C, D and E are states of the cooling crystallization end-point.

Figure 12. Changes of Raman relative intensity and IR peak height during the transformation process from 1-propanol solvate to Form II in 1-propanol solvent.

Figure 13. The microscope images during the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent.

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Figure 14. The PXRD patterns during the transformation process from 1-propanol solvate to Form II of SPI in 1-propanol solvent.


Figure 16. The transformation behaviors of SPI solvates in alcohol solvents. 33



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Isolation Strategies and Transformation Behaviors of Spironolactone

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