Article Cite This: Mol. Pharmaceutics XXXX, XXX, XXX−XXX
Physical Stability and Dissolution Behavior of Ketoconazole−Organic Acid Coamorphous Systems ‡ Michelle Fung, Karlis ̅ Berziņ ̅ s,̌ and Raj Suryanarayanan*
Department of Pharmaceutics, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: In an earlier investigation, coamorphous systems of ketoconazole (KTZ) prepared with each oxalic (OXA), tartaric (TAR), citric (CIT), and succinic (SUC) acid, revealed drug−acid ionic or hydrogen bonding interactions in the solid-state (Fung et al, Mol. Pharmaceutics, 2018, 15 (3), 1052−1061). We showed that the drug−acid interactions in KTZ−TAR were the strongest, followed by KTZ−OXA, KTZ−CIT, and KTZ−SUC. In this study, we investigated the crystallization propensity and dissolution behavior of the KTZ−acid coamorphous systems. When in contact with water (either as water vapor or as aqueous phosphate buffer), while KTZ−CIT and KTZ−TAR were physically stable and resisted crystallization, KTZ−SUC and KTZ−OXA crystallized more readily than KTZ alone. The dissolution performances of the coamorphous systems were compared using the area under the curve (AUC) obtained from the concentration−time profiles. KTZ−OXA exhibited the highest AUC, while it was about the same for KTZ−TAR and KTZ−CIT and the lowest for KTZ−SUC. The enhancement in dissolution appeared to become more pronounced as the strength of the acid (OXA > TAR > CIT > SUC) increased. Coamorphization with acid caused at least a twofold increase in AUC when compared with amorphous KTZ. The decrease in pH of the diffusion layer of the dissolving solid, brought about by the acid, is at least partially responsible for the dissolution enhancement. In addition, the particles of KTZ− OXA, KTZ−TAR, and KTZ−CIT were much smaller than those of KTZ−SUC. The consequent effect on surface area could be another contributing factor to the initial dissolution behavior. KEYWORDS: amorphous, coamorphous, solid dispersions, dissolution, crystallization, physical stability
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INTRODUCTION Utilizing the amorphous form of a poorly water-soluble active pharmaceutical ingredient (API) has the potential to increase its dissolution rate and consequently lead to oral bioavailability enhancement. Preparation of amorphous solid dispersions (ASD), where an API is molecularly mixed with a hydrophilic polymeric carrier, is a popular strategy for stabilizing amorphous APIs.1−4 While polymers have been extensively used to fabricate ASDs, formation of stable dispersions required high polymer concentration, thus limiting their use to low dose APIs. Recently, excipients other than polymers were shown to form stable amorphous phases.5−7 One promising stabilization strategy is the preparation of coamorphous drug−small molecule mixtures. For example, two poorly water-soluble APIs, carbamazepine and indomethacin, were stabilized by amino acids.6,7 Indomethacin−arginine coamorphous system showed an approximately 200-fold increase in intrinsic dissolution rate relative to amorphous indomethacin. In an earlier study, we utilized several organic acids (oxalic, tartaric, citric, and succinic acid) to stabilize a weakly basic API, ketoconazole (KTZ), in the amorphous state.8 KTZ exhibited ionic and/or hydrogen bonding interactions with each acid. As © XXXX American Chemical Society
the strength of KTZ−acid interaction increased, there was an increase in α-relaxation time, a measure of molecular mobility. Moreover, in the two systems with structurally similar dicarboxylic acids, KTZ−succinic and KTZ−oxalic, the slower mobility of the latter was likely responsible for its increased stability. In addition, the KTZ−tartaric and KTZ−citric systems were exceptionally stable in the solid-state. These amorphous systems were prepared with the ultimate goal of achieving high apparent aqueous solubility. However, for the solubility improvement to be useful (i.e., lead to enhanced bioavailability), the coamorphous systems should resist crystallization in time scales of practical importance. Several methods were used to evaluate the ability of each coamorphous system to resist crystallization when placed in contact with water. Acting as a plasticizer, water lowers the glass transition temperature (Tg) of amorphous materials,9 increases molecular mobility, and consequently accelerates crystallization. First, the coamorphous systems were allowed to sorb water by Received: Revised: Accepted: Published: A
January 11, 2018 March 5, 2018 March 12, 2018 March 12, 2018 DOI: 10.1021/acs.molpharmaceut.8b00035 Mol. Pharmaceutics XXXX, XXX, XXX−XXX
Article
Molecular Pharmaceutics
containing the samples were hermetically sealed and characterized using a differential scanning calorimeter (TA Instruments Q2000, DE, USA). Synchrotron X-ray Diffractometry (SXRD). Preparation of Freeze-Dried Samples. Approximately 50 mg of amorphous KTZ and KTZ−acid coamorphous systems was exposed to 250 μL of dissolution media for a predetermined time period (15 min, 30 min, 1 h, 1.5 h, 2 h, 3 h, 6 h, 12 h, and 24 h). At each time point, samples were frozen in liquid nitrogen and then placed in a benchtop freeze-dryer (VirTis AdVantage, Gardiner, NY). The shelf of the freeze-dryer was precooled to −10 °C, and the samples were dried at −10 °C under reduced pressure (65 ± 25 mTorr) for 24 h. The dried samples were transferred to DSC pans and hermetically sealed in a glovebox at controlled RH (RH < 5%) and stored at −20 °C until analyzed. The DSC pans were mounted on a custom-made holder and exposed to synchrotron radiation. In Situ Experiments. In the second study, approximately 20 mg of ASDs was weighed in DSC Tzero pans (TA Instruments, DE) and 25 μL of phosphate buffer (pH 6.8) was added to uniformly wet the samples. The wetted samples were hermetically sealed and, as before, mounted on the custommade holder. The samples were analyzed, at approximately 20 min intervals, for slightly over 3 h. The specific sampling times are given in the figures. Both sets of experiments were performed in the transmission mode using synchrotron radiation in the 17-BM-B beamline at Argonne National Laboratory (Lemont, IL). A monochromatic X-ray beam [wavelength 0.72768 Å, beam size 250 μm (horizontal), and 160 μm (vertical)] and a two-dimensional area detector (XRD-1621, PerkinElmer) were used. The sample to detector distance was set at 900 mm and the pan was oscillated (±1 mm from the center) along the horizontal axis using a stepper motor. A triple-bounce channel-cut Si single crystal monochromator with [111] faces polished was used, which limited the line broadening to its theoretical limit, that is, the Darwin width. The flux of the incident X-ray was 8 × 1011 photons/s at 17 keV. The calibration was performed using Al2O3 (NIST, SRM-647a) standard. The two-dimensional (2D) data were integrated to yield one-dimensional (1D) 2θ (deg) scans using the GSAS-II software. Comparison with laboratory X-ray data was enabled by converting the scans for the wavelength of Cu Kα radiation (1.5406 Å) using JADE 2010 (Material data, Inc.). Powder Dissolution. A USP type II apparatus (Varian 705 DS, Agilent Technologies, Santa Clara, CA) was used. The powder sample was passed through US no. 35 mesh and 50 mg (KTZ equivalent) was dispersed in 500 mL of dissolution medium (pH 6.8; 37 °C) with a paddle speed of 100 rpm. Aliquots were taken at predetermined time points and filtered with PTFE (0.45 μm) syringe filter, diluted with 0.1 N HCl, and analyzed by HPLC with UV detection (Shimadzu, Kyoto, Japan). The dissolution volume was kept constant throughout the experiments. Chromatographic separation was performed at RT using Luna C18 column (250 × 4.6 mm2, 100 Å) under isocratic conditions (mobile phase 70:30 0.01 N HCl to acetonitrile) at a flow rate of 1 mL/min. Standard curves were linear over the concentration range of 1 to 100 μg/mL of KTZ (r2 = 0.999; RSD < 6.0%). After the dissolution run, the pH of the medium was determined. The electrode was calibrated at 25 °C using standard buffer solutions (pH 2.00, 4.01 and 10.00). The dissolution profiles were analyzed using Phoenix WinNonlin version 6.4 (Certara USA, Inc., Princeton, NJ).
storage under different RH conditions, following which the samples were evaluated (Tg, crystallization propensity) by differential scanning calorimetry. As a second step, each coamorphous system was wetted with dissolution medium and crystallization was monitored, in real time, by X-ray diffractometry (XRD) using synchrotron radiation. Third, each coamorphous system was slurried in dissolution medium. Again, by synchrotron XRD, the crystallizing phase was identified and the crystallinity of the solid in contact with the dissolution medium was quantified. The aqueous solubility of KTZ, a weak base, is highly pH dependent.10 When the KTZ−acid undergoes dissolution, the acid in the coamorphous system will influence the diffusion layer pH around each dissolving particle. We hypothesize that as the strength of the acid increases, the dissolution enhancement will be more pronounced, an effect attributed to the magnitude of decrease in diffusion layer pH. The dissolution behaviors of the coamorphous systems were compared from the areas under the curve (AUC) obtained from the concentration−time profiles. Finally, since it is not possible to directly determine the diffusion layer pH, the slurry pH was measured.
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EXPERIMENTAL SECTION Materials. The model drug, ketoconazole (KTZ), was a gift from Laborate Pharmaceuticals (Haryana, India). Oxalic acid, citric acid anhydrous, D,L-tartaric acid, and succinic acid were purchased from Sigma-Aldrich (Missouri, USA). Preparation of Amorphous Systems. Ketoconazole and each of the following organic acids: oxalic, tartaric, citric, and succinic acid in 1:1 molar ratio were spray dried using Buchi mini spray dryer B-290 (Delaware, USA). A methanolic solution (5% w/v total solid content) composed of KTZ and each acid in 1:1 molar ratio was spray dried at a rate of 3.5 mL/ min. The inlet and outlet temperatures were maintained at ∼55 °C and ∼35 °C, respectively. The spray dried samples were dried under vacuum for 20 h (RT) to remove residual solvent. Crystalline Ketoconazole−Acid Samples. KTZ−succinic acid cocrystal and KTZ−oxalate salt were prepared by slow evaporation. Methanolic solutions, at 1:1 KTZ to acid molar ratio, were prepared (1% w/v total solid content) from which single crystals were obtained. The product formation was confirmed using X-ray diffractometry (Supporting Information, Figures S3 and S4) Physical Mixtures. Samples were prepared by geometric mixing of crystalline KTZ with each acid in 1:1 molar ratio using a motor and pestle. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (TA Instruments Q2000, DE, USA) equipped with a refrigerated cooling unit was used. The instrument was calibrated with indium. The samples were heated in hermetically sealed (Tzero) aluminum pans (TA Instruments, DE, USA) at 10 °C/min, from 0 to 175−205 °C (depending on sample) under nitrogen purge (50 mL/min). Glass transition temperature (Tg) was the midpoint of the transition and the crystallization onset temperature (Tc) was the onset temperature of the crystallization peak. Water Sorption Studies. The coamorphous KTZ−acids were placed in open DSC pans and stored at room temperature in chambers containing saturated salt solutions of lithium chloride (∼11% RH), potassium carbonate (∼43% RH), and sodium chloride (∼75% RH). The sample weights were recorded before and after storage for 24 h. The DSC pans B
DOI: 10.1021/acs.molpharmaceut.8b00035 Mol. Pharmaceutics XXXX, XXX, XXX−XXX
Article
Molecular Pharmaceutics
Figure 1. (a) Representative DSC heating curves of amorphous KTZ and KTZ−acid coamorphous systems after storage at 43% RH for 24 h at room temperature. The heating curves following storage at 11 and 75% RH are presented in the Supporting Information (Figures S1 and S2). (b) Change in glass transition temperatures, with respect to their dry Tg values [ΔTg = Tg (after storage) − Tg (dry)], following storage at 11, 43, and 75% RH (mean ± SD; n = 3).
Table 1. Thermal Behavior of Coamorphous Systems Following Storage at 11, 43, and 75% RH (RT)a KTZ
