Article pubs.acs.org/crystal
Turning Liquid Propofol into Solid (without Freezing It): Thermodynamic Characterization of Pharmaceutical Cocrystals Built with a Liquid Drug Alessia Bacchi,*,† Davide Capucci,† Marco Giannetto,† Monica Mattarozzi,† Paolo Pelagatti,† Nair Rodriguez-Hornedo,‡ Katia Rubini,§ and Andrea Sala† †
Dipartimento di Chimica, Università degli Studi di Parma, Viale delle Scienze, 17A, Parma, Italy Department of Pharmaceutical Sciences, University of Michigan, Ann Arbor, Michigan, United States § Dipartimento di Chimica Giacomo Ciamician, Università degli Studi di Bologna, Via Selmi 2, Bologna, Italy ‡
S Supporting Information *
ABSTRACT: Most drugs are delivered as crystalline solids, but some widely used pharmaceutical ingredients cannot be crystallized at ambient conditions: propofol, one of the most widely used anesthetic agents in the world is a liquid. Here we stabilize propofol in a crystalline phase by cocrystallization, and we thoroughly characterize the structural and thermodynamic properties of the new materials. Ternary solubility diagrams of a liquid pharmaceutical ingredient cocrystallized with a solid coformer are presented and analyzed for the first time. It is shown that, when equilibrated with the solid cocrsytal, the concentration of propofol in water is kept constant in a wide range of starting compositions.
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INTRODUCTION If you compare a picture of climbers on Mount Everest with one of surfers on the Pacific Ocean, you immediately realize that a solid is intrinsically more durable and stable than a liquid. And if you think of a drug, you would like to have in your medicine cabinet a material with a long shelf life, that is stable to temperature fluctuations, and that does not volatilize during the summer. In fact most active pharmaceutical ingredients (APIs) are manufactured and formulated as crystalline materials,1 since liquids tend to be less stable and more difficult to handle and are often characterized by high vapor pressure. However, some compounds relevant to human health and nutrition are liquid at ambient conditions; examples are propofol, nicotine, and polyphenols used as natural antioxidants such as carvacrol (oregano flavor), eugenol (clove oil component), eucalyptol, and valproic acid.2 A practical way to manufacture some of these compounds as a solid dosage form is to turn them into salts, provided that the molecule may be reacted with a convenient acid or base, and that these are acceptable from a regulatory point of view. However, not all the molecules may become salts. An alternative and effective way to stabilize a liquid compound in a solid form is cocrystallization, which involves the association of the API with a suitable molecular partner, in order to obtain a crystalline form where the two partners coexist in a stoichiometric ratio.3 This is usually achieved by © 2016 American Chemical Society
carefully engineering supramolecular interactions between the API and the coformer.4 Cocrystallization is nowadays exploited in many contexts; examples of applications are for pharmaceuticals,5 agrochemicals,6 nutraceuticals,7 and energetic materials8 with the aim of tuning the physicochemical properties without modifying the molecular structure of the individual components.9 Cocrystallization is in fact a useful tool to alter a wide range of pharmaceutical properties including melting point, dissolution rate, thermal stability, and solubility; in this work we are interested in shifting the melting point of the liquid API to a value higher than room temperature in order to obtain a crystalline solid. We remark here that, despite the fact that one of the cocrystal components is liquid at ambient conditions, these compounds should be regarded as cocrystals and not solvates. This issue related to nomenclature has been the subject of intense debate in the past decade.10 Here we focus on propofol (2,6-diisopropylphenol) (Scheme 1) which is commonly used for induction and maintenance of general anesthesia and sedation and appears on the WHO Model List of Essential Medicines, the most important medications needed in a health system.11 Propofol is a liquid with extremely low aqueous solubility, and it is marketed as DIPRIVAN Injectable Emulsion, which is Received: August 22, 2016 Revised: September 15, 2016 Published: September 16, 2016 6547
DOI: 10.1021/acs.cgd.6b01241 Cryst. Growth Des. 2016, 16, 6547−6555
Crystal Growth & Design
Article
microscope slide and a flat glass slit. The scanning temperature used was the same as for the DSC analysis in order to reproduce a similar thermal experiment. HSM studies were performed on a HSF 91 apparatus, Linkam Scientific Instruments, Tadworth, UK, and Labophot II polarizing microscope, Nikon, Tokyo, Japan. The DSC scans and HSM results of PROP-B1 and PROP-PH1 are reported in the Supporting Information. Phase Diagrams. Powder samples of the cocrystals PROP-B1 and PROP-PH1 were used to prepare slurries in water, with different phase compositions at equilibration conditions (see Supporting Information). Slurries were allowed to reach thermodynamic equilibrium at room temperature for more than 2 weeks. Different conditions have been established in order to vary the phase compositions and analyze different zones of the diagram, representing different critical points. Concentration Measurements. The system used for LC/UV analysis is a DIONEX Ultimate 3000 Systems (Thermo Scientific) with a ACC-3000 autosampler and a high-pressure LC pump LPG3400SD and equipped with a UV−vis detector DAD-3000. For quantitative analysis, a calibration curve for each compound was built by performing proper dilutions. All analyses were performed on a KinetexC18 column (100 × 2.1 mm, 2.6 μm) (Phenomenex, Torrance, CA), injecting a volume of 10 μL. The elution was isocratic, at a constant flow rate of 200 μL/min, with a mobile phase composed of acetonitrile and H2O in a ratio of 40:60 (v/v). Propofol was detected at 220 nm, B1 at 241 nm, and PH1 at 249 nm. Calculation of the water content in propofol, for the determination of point A in the ternary phase diagram, was performed with a Karl Fischer titration using a Karl Fischer TitroMatic KF 1S. UV−vis spectra were collected on a UV−visible Bio Evolution Thermo scientific 260 spectrophotometer. Crystallographic Analysis. Single crystal X-ray diffraction analysis was performed on a SMART APEX2 diffractometer using Mo Kα radiation (λ = 0.71073 Å, Lorentz polarization and absorption correction applied) at room temperature (293 K) for PROP-B1 and at 280 K for propofol (Supporting Information), while PROP-PH1 was collected at 100 K under nitrogen flux at Elettra Sincrotrone (Trieste, Italy) on beamline XRD1 with a wavelength of 0.7 Å (NdBFe Multipole Wiggler, Hybrid linear, 4.27 keV with a power of 8.6 kW, source size fwhm of 2.0 × 0.37 mm (0.7 × 0.2 mm fwhm beam size at sample) and photon flux 1012−1013 ph/s), Dectris Pilatus 2M detector. Data were reduced with CrysalisPro software. Structures were solved by direct methods using SHELXS and refined by fullmatrix least-squares on all F2 using SHELXL implemented in Olex2.15 For all the structures, anisotropic displacement parameters were refined except for hydrogen atoms. Hydrogen atoms were introduced in calculated positions riding on their carrier atoms. Denisty of PROP-B1 is quite low, and i-propyl groups are highly mobile, as shown by their ADPs. Moreover it is worth noting that in PROP-B1 the bipyridine sits on an inversion center, requiring apparent planarity of the molecule, which in contrast in the gas phase relieves steric hindrance of the ortho hydrogen atoms by twisting around the central bond. The apparent planarity might be due to static disorder, reflected by the shape and magnitude of the ADPs. This has been tested by the Trueblood−Shoemaker rigid body analysis, carried out with THMA14 − TLS Thermal Motion Analysis programs, Maverick, E. F.; Trueblood, K. N., UCLA, 1999, showing that B1 librates around the long molecular axis with a rms of 18°, but this does not reproduce completely the observed ADPs, pointing to an additional static torsional disorder. Table 1 reports crystal data for the two cocrystals, while the structural determination of propofol at 280 K is reported in the Supporting Information. Crystallographic data (excluding structure factors) for PROP-B1, PROP-PH1, and propofol at 280 K have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC 1495453−1495455. X-ray powder diffraction analysis was performed on a Thermo Scientific ARL XTRA powder diffractometer (Cu Kα, 1.540598 Å) in order to rapidly check most of the new formed powder samples obtained by grinding.
Scheme 1. Coformers: B1 (4,4′-bipyridine), PH1 (phenazine), and API: PROP (2,6-diisopropylphenol)
a sterile, nonpyrogenic emulsion containing 10 mg/mL of propofol suitable for intravenous administration. Water solubility plays a key role in determining the efficacy and the activity of a drug, and hence the rational design of compound formulation has become of primary importance, especially in the pharmaceutical field.12 The solution equilibria between a cocrystal and its individual components, together with the relationships between their solubility, have been deeply analyzed in the last few years;4,13 however, all of these studies considered the model of a system where the single components are solid. There are no studies to our knowledge of solubility and ternary diagrams where one of the pharmaceutical active components of the cocrystal is a liquid. During the progress of our project, a paper reporting the synthesis and structure of a cocrystal between propofol and nicotinamide was reported, confirming that the subject is attracting interest from the applicative point of view.14 Our aim here is to pave the way to a complete characterization of the thermodynamic landscape of cocrystals formed between a liquid ingredient and a solid coformer. In this work, two cocrystals of propofol obtained with bipyridine (B1) and phenazine (PH1) (Scheme 1) are reported along with solubility studies, taking into consideration the equilibria between the cocrystal and individual components in solution with the use of ternary phase diagrams.
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EXPERIMENTAL SECTION
Cocrystal Syntheses. 2,6-Diisopropylphenol (97%) (propofol, PROP), bipyridine (B1), phenazine (PH1), and the solvents used for crystallization attempts were purchased from Sigma-Aldrich Chemical Co. Cocrystallizations which led to the formation of cocrystals PROPB1 and PROP-PH1 were attempted by slow evaporation from various organic solvents: methanol, ethanol, ethyl acetate, and dichloromethane. Cocrystals PROP-B1 and PROP-PH1 were also prepared by direct mixing liquid propofol and solid coformers, which were added in a 2:1 ratio into a ceramic mortar and ground for 1 h until a sludgy solid was obtained and put into the refrigerator to facilitate nucleation. After 1 day a dry yellowish powder was formed and the powder X-ray diffraction (PXRD) experimental pattern matched that simulated from the single crystal structure obtained from solution crystallization. Crystallization trials were also performed in a 1:1 molar ratio by slow evaporation of solvents (MeOH, DCM, acetone) and direct mixing, always resulting in a slurry of the coformer. Differential Scanning Calorimetry (DSC). Thermal analysis on the cocrystal powder samples were performed with a PerkinElmer Diamond equipped with a model ULSP 90 ultracooler. Heating was carried out in open Al-pans at 5 °C/min in the temperature range from −25 to 80 °C. Hot stage microscopy (HSM) was used only for PROPB1 samples using a small amount of crystal (
