Process Optimization of a Complex Pharmaceutical Polymorphic System via In Situ Raman Spectroscopy Cindy Starbuck,* Angela Spartalis, Lawrence Wai, Jian Wang, Paul Fernandez, Christopher M. Lindemann, George X. Zhou, and Zhihong Ge
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 6 515-522
Process Research and Development, Merck Research Laboratories, Rahway, New Jersey 07065 Received July 30, 2002;
Revised Manuscript Received September 7, 2002
ABSTRACT: In situ Raman spectroscopy was used to determine the rate of polymorph turnover for MK-A, a multipolymorphic compound in development at Merck Research Laboratories. The known crystal forms of MK-A include four anhydrous polymorphs, two hydrates, and numerous solvates. The penultimate and pure steps of this process involve a coupling reaction to generate a mixture of crystal forms followed by turnover to the desired polymorph, form A. This paper summarizes experiments to measure the kinetics of polymorph turnover from all relevant MK-A crystal forms to form A. Additionally, the turnover kinetics for polymorph reversion from form A to undesired forms were measured under simulated process upset conditions. The use of thermodynamic data to establish process boundaries and kinetic data to establish process time cycles resulted in the definition of a highly robust, cycle time efficient slurry turnover process to produce form A from any combination of other MK-A crystal forms. Introduction Polymorphism is defined as the ability of a substance to exist as two or more crystalline phases that have different arrangements and/or conformations of the molecules in the crystal lattice.1 In the pharmaceutical industry, identification of polymorphism during earlystage development is critical, as unanticipated polymorphic changes of a drug substance can affect chemical and physical stability, solubility, morphology, hygroscopicity, and, ultimately, bioavailability.2 A recent case study highlights the development and regulatory challenges associated with late-stage identification of a more stable polymorph.3 Polymorphs are commonly characterized by off-line analytical tools including X-ray powder diffraction (XRPD), solid state NMR (ssNMR), and differential scanning calorimetry (DSC).4 Sample preparation for these methods, which typically involves solids isolation, is not only labor intensive but may lead to data artifacts resulting from crystal form transitions during sample preparation (e.g., evaporation of solvent or water). Realtime monitoring of the solid phase in a crystalline slurry is particularly attractive as it obviates the need for sample manipulation. Over the past several years, numerous techniques have been identified for in-line monitoring of crystallization processes: FBRM (focused beam reflectance measurement),5 FT-IR,6 near-IR,7 and Raman spectroscopy.8,9 Of these, Raman spectroscopy offers the distinct advantage of quantitative characterization of polymorph transitions in the solid state, provided the compound under study does not exhibit background fluorescence. In situ Raman spectroscopy was utilized for the process development of MK-A, a development compound at Merck Research Laboratories. The last chemical process step to produce MK-A, called the “semipure” step, involves a coupling reaction and crystallization out * To whom correspondence should be addressed. Tel: (732)594-7905. Fax: (732)594-7355. E-mail:
[email protected].
of which the desired polymorph, form A (anhydrous), cannot be isolated due to high water levels. Thus, a final “pure” step was developed to turn over any single crystal form or mixture of forms present in semipure to form A. A slurry turnover process was selected (rather than recrystallization from solution), and isopropyl acetate was chosen as the turnover solvent for the following reasons: (i) an isopropyl acetate solvate of MK-A had not been observed; (ii) an azeotrope with water existed to facilitate solvent drying via distillation; and (iii) the following solubility criteria were met: a strong dependence of solubility on temperature to access increased turnover rates and low solubility at a convenient slurry filtration temperature (25 °C) to minimize yield losses. Strategies for selecting solvents to enhance polymorph turnover have been discussed by several investigators;10-13 however, the solvent choice for MK-A was driven primarily by the practical considerations mentioned above. The present study was undertaken to (i) understand the pathways for transition between other polymorphs and form A, (ii) generate kinetic data for crystal form turnover to and from form A, and (iii) probe process upsets that might lead to form A instability. Thermodynamic data were employed to establish process boundaries while kinetic data (via Raman spectroscopy) were employed to establish process time cyclessall of which resulted in a robust process that consistently produces the desired polymorph, form A. Experimental Section Thermogravimetric Analysis (TGA). Weight loss was monitored as a function of temperature using a Perkin-Elmer TGA-7 running Pyris v. 3.81 software for Windows. Samples were run from 25 to 250 °C at a heating rate of 10 °C/min in open platinum TGA pans under an inert nitrogen atmosphere. Weight losses were calculated from the starting temperature to the point at which the onset of weight losses due to decomposition/evaporation occurred. DSC. Melting point values were measured using a TA Instruments DSC 2910 running Thermal Solution v. 2.5 for
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Windows on 2-10 mg samples in open aluminum pans under an inert nitrogen atmosphere. A heating rate of 10 °C/min was used over the 25-250 °C temperature range and a second, empty aluminum pan was used as a reference. Karl Fischer (KF) Titrations. The water content of solids was determined volumetrically by titration of approximately 50-100 mg of solids in reagent grade methanol with Riedel-de Hae¨n HYDRANAL Composite 2 using a Brinkman Metrohm 701 KF Titrino. Dynamic Vapor Sorption (DVS). Moisture absorption isotherms were determined using a VTI SGA-100 symmetrical gravimetric analyzer running Flow System II v. W1.31 software for Windows. Water adsorption was determined at 25 °C at relative humidities between 2 and 95%. Solubility. Equilibrium solubility measurements were performed by agitating flame-sealed test tubes containing excess amounts of the appropriate forms in 2 mL of solvent in a themostated bath. After equilibration times of 3-5 h, the sealed test tubes were opened and the supernatant was removed. The supernatant was filtered using Whatman PURADISC 25 TF, PTFE Membrane, 25 mm diameter, 0.2 µm pore size filters. The assay for drug concentration was accomplished spectrophotometrically using a TSP Spectra Physics P4000 Spectrophotometer. The final crystal form of the residual solids was confirmed by XRPD. XRPD. A Philips XRG 3100 diffractometer with a Cu radiation source (45 kV, 40 mA) was used to collect powder diffraction patterns at ambient temperature and pressure. Samples were run immediately after isolation from the respective supernatants. Analyzing the solids “wet” with residual solvent decreased the likelihood of a polymorphic phase transition during the time required for analysis; wavelength, Cu KR1 ) 1.54056 Å, KR2 ) 1.54439 Å; scan range, 2-40° 2θ. XRPD was used to analyze the polymorphic purity of starting and ending solids in all Raman turnover experiments. The limit of detection of MK-A polymorphs by XRPD was estimated to be 5%. Scanning Electron Microscopy (SEM). Micrographs were collected using a Philips Electro Scan ESEM 2020 at ambient temperature. An operating voltage of 20 kV with a filament current of 1.69 mA was utilized at a water pressure of 5 Torr for image acquisition. Samples were sputter-coated with gold to improve conductivity, and micrographs were acquired over a 30 s time period. Raman Spectroscopy. Raman spectra were collected with a process Raman analyzer HoloProbe from Kaiser Optical Systems, Inc. (Ann Arbor, MI). The analyzer was interfaced with an immersion probe sealed with a sapphire window. This system employed an external cavity stabilized single mode diode laser at 785 nm. The acquisition conditions were optimized so that a spectrum was captured with an exposure time of 4 s and 10 accumulations over 2-3 min. The laser focal point of the immersion probe of the Raman analyzer was adjusted so that the solids of different polymorphs suspended in liquid media could be detected. Turnover Experiments. The purity of the compound, MKA, used in these experiments was >99.9 area % by reverse phase gradient high-performance liquid chromatography (HPLC). Isopropyl acetate solvent used was ACS grade. Water content was measured by KF titration using a Metrohm 684 KF coulometer. When necessary, isopropyl acetate solvent was dried by molecular sieves. In all cases, turnover experiments were conducted by charging MK-A solids to isopropyl acetate solvent presaturated with MK-A. Experiments were conducted in a 100 mL jacketed resin kettle, equipped with an overhead stirrer and thermocouple. The resin kettle was specially modified to accommodate the Raman HoloProbe. The jacket temperature was controlled via a recirculating glycol bath (Haake, model F8). All experiments were conducted at a constant slurry density, i.e., 5.9 mL of isopropyl acetate per gram of MK-A. Raman Data Analysis. Spectral analyses were performed with either MatLab v. 6.0 by The MathWorks, Inc. (Natick, MA) or Unscrambler Chemometrics Software (v. 7.5) from Camo Inc., Trondheim, Norway. A number of data analysis
Starbuck et al. techniques were employed, depending upon on the nature of the data set. For the hemihydrate to form C turnover experiments, principal component analysis (PCA) plus 2nd derivative pretreatment was used to obtain the composition profiles. Generally speaking, data pretreatment using 2nd derivative analysis of raw Raman spectra enhances analyte (polymorph) information and reduces unwanted baseline fluctuations and noise. For the form C to A turnover experiments, PCA or multivariate curve resolution (MCR) was used. MCR was used to analyze initial spectra in the form C to form A experiments to separate the mixed, overlapping spectral signatures of forms C, A, and hemihydrate. Turnover rate data were generated by correlating the polymorph concentration at the starting point (grams of polymorph charged/liter of solution) and ending point (complete turnover, verified by XRPD) to the relative intensity of the profile generated by a PCA (or MCR) analysis. On the basis of the slope between the starting and the ending points, an apparent zero-order rate was calculated. For the form C/A calibration curve, partial least-squares regression (PLS) was used on 2nd derivative spectra. PLS was used in this case because quantitative information (vs compositional profiling) was required and very subtle differences exist between the spectra of form C and the spectra of form A (see Figure 4b).
Results and Discussion Phase Relationships. There are two principal types of polymorphic systems: monotropic and enantiotropic.14 In monotropic systems, one form is metastable relative to the other at all temperatures below the melting point. For enantiotropic systems, there exists a temperature and pressure above which one polymorph is more stable and below which the other form is more stable. By definition, the enantiotropic crossover point between two anhydrous polymorphs is constant for any solvent system, despite relative changes in solubility. The polymorph with the lowest free energy has the lowest solubility in solution. In addition to enantiotropic and monotropic relationships between polymorphs, the crystalline structure of a solid can incorporate a stoichiometric or nonstoichiometric amount of water or solvent to form hydrates or solvates, respectively. Hydrates and solvates are not true polymorphs since the chemical compositions are not equivalent.15 Moreover, unlike anhydrous polymorphs, the crossover points in the solubility curves for hydrates and solvates with anhydrous forms are solvent systemdependent. Polymorphs exhibiting both enantiotropic and monotropic relationships exist for MK-A as illustrated in Figure 1. The anhydrous forms, A, B, C and E, are indicated by the colorless boxes. Note that form B is monotropic with respect to both forms A and C in this system (form B less stable), while forms A and C are enantiotropic. Form E is a unique anhydrous polymorph formed upon drying the NMP (N-methyl pyrrolidinone) solvate. Hydrates, signified by shaded boxes, include hemihydrate and dihydrate. Dihydrate was observed to exist only transiently in the dry solid state while hemihydrate was found to be stable under ambient conditions. Phase Characterization. TGA and DSC were used to characterize the anhydrous polymorphs, A and C. No significant weight loss (
