ARTICLE pubs.acs.org/crystal
Effect of Particle Size and Morphology on the Dehydration Mechanism of a Non-Stoichiometric Hydrate Feirong Kang,† Frederick G. Vogt,‡ Jeffrey Brum,*,† Rachel Forcino,† Royston C. B. Copley,§ Glenn Williams,† and Robert Carlton† †
Product Development, GlaxoSmithKline plc., 1250 South Collegeville Road, Collegeville, Pennsylvania 19426, United States Product Development, GlaxoSmithKline plc., 709 Swedeland Road, King of Prussia, Pennsylvania 19406, United States § Computational and Structural Sciences, GlaxoSmithKline plc., Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, U.K. ‡
bS Supporting Information ABSTRACT: A hydrated form of 7-methoxy-1-methyl-5(4-(trifluoromethyl)phenyl)-[1,2,4]triazolo[4,3-a]quinolin4-amine (designated Form B) exhibits moisture sorption behavior that is very strongly affected by particle size and morphology. When studied pre- and post-micronization, the simple rate of dehydration at ambient temperature is faster by >2 orders of magnitude after micronization. Complementary techniques were employed to understand this behavior including environmental X-ray powder diffractometry (XRPD), gravimetric vapor sorption (GVS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hot-stage microscopy (HSM), single-crystal X-ray diffractometry (SCXRD), and solid-state nuclear magnetic resonance (SSNMR). Solid-state kinetics analysis of thermal data revealed that dehydration of the nonmicronized material follows a two-step consecutive reaction with the first step being a diffusion limited reaction and the second step being a first order reaction, whereas the micronized material follows a simple one-step nth order reaction. The crystal structure of Form B was determined, and the difference in dehydration kinetics was linked to narrow and staggered water channels observed along the crystallographic a-axis. Micronization cleaves slip planes that are approximately perpendicular to the long-axis of the water channels, allowing for easier egress and causing drastic changes in moisture sorption properties. Morphology predictions suggest that Form B has a tendency to have high aspect ratios along the a-axis, the longest axis of the columnar-shaped crystals, so that the rate of dehydration is limited by long channel systems. The crystal structure shows two crystallographically distinct water molecules with slightly different hydrogen bonding networks. SSNMR experiments are used to directly observe the preferential dehydration of one water molecule, and density functional theory and Monte Carlo sorption calculations are used to probe energetic differences between the water environments.
’ INTRODUCTION Many pharmaceuticals exist in both hydrated and anhydrous forms. The stability and behavior of hydrates varies significantly, and the inter-relationship with their respective anhydrous forms has been studied extensively.13 Knowledge of the hydration and dehydration behavior of drug substances is essential in the development of stable formulations because the physicochemical, mechanical, processing, and biological properties of hydrates may differ significantly from those of corresponding anhydrates.4 Hydrate formation and dehydration may occur during processing or storage of active pharmaceutical ingredients or a finished drug product.57 As a hydrated form is progressed through drug development, a thorough understanding of dehydration behavior is required for process control and assessment of the physical stability of drug substance and drug product. Pharmaceutical hydrates may exhibit different hydration states, in some cases showing a well-defined stoichiometric relationship between the water and the parent molecule, while in other cases showing a variable, nonstoichiometric relationship that can be r 2011 American Chemical Society
affected by changing the surrounding humidity and temperature.510 The occurrence of these variable, nonstoichiometric relationships is usually associated with the appearance of stacks or chains of water molecules in the crystal structure, allowing hydration and dehydration processes to occur readily through “tunnels” with little or no change to the crystal structure. Because of these structural aspects, these systems are often referred to as “channel hydrates” and “isomorphic desolvates”. Although many properties of channel hydrates have been investigated and reviewed, the impact of commonly used particle-size reduction methods, such as micronization, on hydration state has not been widely reported. Particle-size reduction methods are commonly used to improve the dissolution rate of poorly soluble crystalline drugs and are an important unit operation in pharmaceutical processing.11 Received: June 17, 2011 Revised: October 6, 2011 Published: October 20, 2011 60
dx.doi.org/10.1021/cg200768x | Cryst. Growth Des. 2012, 12, 60–74
Crystal Growth & Design
ARTICLE
The crystalline phase of interest in this work is a hydrate of 7-methoxy-1-methyl-5-(4-(trifluoromethyl)phenyl)-[1,2,4]triazolo[4,3-a]quinolin-4-amine, an up-regulator of ApoA-1 being investigated as a potential treatment for dyslipedemia.12 The structure and numbering scheme of this molecule are shown in Scheme I.
Table 1. Summary of SCXRD Experimental Parameters and Results for the Form B of Compound I formula
C19H15F3N4O 3 H2O
formula weight
390.37
temperature (K)
150(2)
space group
P1
crystal system
triclinic
unit cell dimensions (150 K) a (Å)
8.473(6)
b (Å) c (Å)
12.542(7) 17.476(9)
α (°)
102.09(4)
β (°)
102.70(5)
γ (°)
94.86(8)
volume (150 K) (Å3)
1754.8(18)
molecules per cell (Z)
4
calculated density (Mg/m3)
1.478
absorption coeff (μ) (mm1) F000
0.120 808 10 e h e 10
index ranges
14 e k e 14 20 e l e 20
A hydrated form of I, designated Form B, displays complex dehydration behavior. In this study, we begin with the determination of the crystal structure of Form B to identify the water environment and the potential for a channel-like structure. The hydration and dehydration behavior of this form are then analyzed with respect to particle size. After micronization via air jet milling, the simple rate of dehydration of Form B at ambient temperature is found to be faster by more than 2 orders of magnitude. In order to fully understand this interesting behavior, Form B is studied pre- and post-micronization by various techniques including environmental X-ray powder diffractometry (XRPD), gravimetric vapor sorption (GVS), differential scanning calorimetry (DSC), thermogravimetrical analysis (TGA), hotstage microscopy (HSM), and solid-state nuclear magnetic resonance (SSNMR). Computational approaches are employed to rationalize results from these techniques. Solid state kinetics modeling is also used to probe dehydration mechanisms for preand post-micronized materials, allowing for observation of distinct mechanisms. Slip plane and morphology calculations using the crystal structure of Form B reveal the structural basis of water egress via narrow and staggered water channels along the crystallographic a-axis through to faces exposed by micronization. SSNMR, density functional theory (DFT) energy calculations, and Monte Carlo sorption calculations are used to show subtle but detectable energetic differences between the water molecules.
measured reflections
22146
independent reflections
6097 [R(int) = 0.0861]
coverage of independent reflections (%)
99.0
data/restraints/parameters goodness of fit on F2
6097/4/538 1.015
final R indices for I > 2σ(I) data
R1 = 0.0599
final R indices for all data
R1 = 0.1046
wR2 = 0.1431 wR2 = 0.1654 extinction coefficient
0.0052(14)
largest peak diff and hole (e 3 Å3)
0.288 and 0.384
unit cell dimensions (300 K) a (Å)
8.549(3)
b (Å)
12.648(2)
c (Å)
17.421(8)
α (°)
103.04(3)
β (°)
103.23(3)
γ (°)
93.95(2)
volume (300 K) (Å3)
1772.0(10)
form was also obtained by evaporation of acetone/water solutions. The crystal and molecular structures were determined from three-dimensional X-ray diffraction data. All diffraction measurements were made using a Nonius KappaCCD diffractometer with a normal focus tube emitting MoKα radiation. Data collection parameters are given in Table 1. The structure was solved using direct methods with the SHELXTL program V6.10 (Bruker AXS, Madison, WI, 2001). The absorption correction for I was carried out using the semiempirical from equivalents procedure with the SADABS program. The structure was refined using the full-matrix least-squares on F2 approach with the SHELXTL program V.6.10 (Bruker AXS, 2001). X-ray Powder Diffraction. XRPD experiments were performed on a Bruker D8 Advance X-ray powder diffractometer equipped with a LynxEye detector and an Anton-Parr TTK450 temperature stage (Bruker AXS, Madison, WI). A Cu Kα radiation source was used, with a generator voltage and current of 40 kV and 40 mA, a 240o2θ scan range, a 0.02o2θ scan step, and a step time of 0.1 s. Approximately 75 mg
’ EXPERIMENTAL SECTION Preparation of Materials. Samples of 7-methoxy-1-methyl5-(4-(trifluoromethyl)phenyl)-[1,2,4]triazolo[4,3-a]quinolin-4-amine hydrate (Form B) were prepared at GlaxoSmithKline using previously reported methods.13 All other materials were reagent grade. Micronization was performed at Micron Technologies, Inc. (Exton, PA) using dry nitrogen gas. Single-Crystal X-ray Diffraction. Single crystals of Form B were prepared by slow evaporation of ethyl acetate/water solutions. The same 61
dx.doi.org/10.1021/cg200768x |Cryst. Growth Des. 2012, 12, 60–74
Crystal Growth & Design
ARTICLE
Karl Fisher Titration. Karl Fischer titration (KFT) for water content was performed using a Metrohm 774 oven sample processor and 756 coulometer (Metrohm USA, Riverview, FL). The method used a nitrogen sample purge with a 150 °C sample temperature. Reported results are the average of two titrations. Hot-Stage Microscopy. HSM was carried out using a Leica DM/ LM polarized light microscope (Leica Microsystems Inc., Buffalo Grove, IL) equipped with a Linkam hot-stage (Linkam Scientific Instruments Ltd., Surrey, UK). The heating rate was 20 °C/min from 25 to 220 °C with the temperature held at certain points. A small quantity of sample powder was immersed in low viscosity silicon oil on a glass slide and covered with a glass coverslip. Solid-State NMR Spectroscopy. Solid-state NMR experiments were performed on a Bruker Avance 400 spectrometer operating at a 1H frequency of 399.87 MHz (Bruker Biospin, Billerica, MA). A variable temperature system including a BCU-05 chiller was used to control the temperature of the nitrogen gas supply to the probe at 273 K for all measurements to minimize frictional heating. A Bruker MAS-II rate controller was used to control spinning speeds to within (2 Hz of the set point. 13C cross-polarization (CP) spectra were obtained with a Bruker 4-mm triple resonance probe tuned to 1H, 19F, and 13C frequencies and spinning at an MAS frequency (νr) of 8 kHz.14 The sample was restricted to the center of the 4-mm volume rotors to maximize RF homogeneity. A linear power ramp from 75 to 90 kHz was used on the 1H channel to enhance CP efficiency.15 Spinning sidebands were eliminated by a fivepulse total sideband suppression (TOSS) sequence.16 Proton decoupling was performed at an RF power of 105 kHz using the SPINAL-64 pulse sequence.17 Edited spectra containing only quaternary aromatic signals were obtained using dipolar dephasing (interrupted decoupling) during the TOSS period and three subsequent rotor periods using a shifted echo pulse sequence.18 19F spectra were acquired on the same probe at a frequency of 376.209 MHz. 15N NMR spectra were obtained with a Bruker 7-mm double resonance probe spinning at a νr of 5 kHz using the CP-MAS pulse sequence with a 3-ms contact time, a 5-s relaxation delay, and a 1H decoupling power level of 65 kHz. A total of 16384 transients were averaged for the spectra shown here. 13C spectra were referenced to TMS using an external reference of hexamethylbenzene.19 The 15N spectra were externally referenced to a sample of NH4Cl.20 The 19F chemical shift reference was calculated from the experimental 13C references using the unified scale method.21 Molecular Modeling and Computational Methods. The Materials Studio Version 5.0 software package (Accelrys, San Diego, CA USA) was used for several types of calculations. Grand canonical Monte Carlo water sorption simulations were performed using the Sorption module in Materials Studio, and made use of the COMPASS general-purpose solid-state forcefield.22,23 Morphology predictions were performed using both the BravaisFriedel DonnayHarker (BFDH) method24 and the growth method,25 with the latter method performed using the COMPASS forcefield. Slip planes were identified from the results of the growth method calculation, from the determination of attachment energies.26,27 Solid-state DFT calculations were performed using the DMol3 package.28,29 The HCTH/407 generalized gradient approximation (GGA) density functional (referred to herein as the HCTH functional) was employed.30 A double numerical basis set with polarization functions on all atoms, referred to herein as the DNP basis, was used.28,29 Brillioun-zone integrations with a 3 2 2 k-point set were used. For geometry optimizations (energy minimizations), an energy convergence criterion of 2.0 105 Hartree, a gradient convergence criterion of 4.0 103 Å, and a displacement convergence criterion of 5.0 103 Å were applied. An atomcentered basis function cutoff of 3.7 Å was applied, core electrons were explicitly included, and pseudopotentials were not used. All calculations described here were performed using Microsoft Windows XP workstations.
of material was packed into a stainless steel sample holder and gently flattened. The samples were analyzed at ambient and elevated temperatures to study changes in the crystal lattice during dehydration. Both nonmicronized and micronized samples were also analyzed with a Panalytical X’pert Pro XRPD system using both transmission and reflection modes (Panalytical B. V., Almelo, The Netherlands). The diffractometers were calibrated using a Standard Reference Material (SRM) 1976 alumina plate. Rietveld refinement was carried out using the Panalytical X’Pert HighScore Plus version 2.1 software package (Panalytical B.V., Eindhoven, The Netherlands). The unit cell parameters were refined against powder patterns collected at room temperature and 100 °C for both micronized and nonmicronized samples. The changes in cell volume were compared between micronized and nonmicronized samples to provide further insight regarding the dehydration mechanism. Thermal Analysis. Differential scanning calorimetry (DSC) analyses were performed on a TA Instruments Q2000 instrument (TA Instruments, New Castle, DE). Approximately 2 mg of sample was weighed into an aluminum pan and loosely covered with a lid under a nitrogen purge with a flow of 50 mL/min. The pan was loaded into the instrument and heated to 250 °C at a rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 instrument. Approximately 10 mg of sample was loaded into a platinum pan and heated to 350 °C at a rate of 10 °C/min. Analysis was carried out under a nitrogen purge with a flow of 50 mL/min. DSC and TGA calibrations were performed using traceable indium and alumel references. Particle Size Analysis. Particle size was determined using digital image analysis on an optical microscope. A Leica 6000DM microscope with Clemex Vision PE Version 5.0 was used for the image analysis. A 5 objective was used for the nonmicronized sample, and a 40 objective was used for the micronized sample. The samples were prepared by suspending a small amount of each specimen in silicone oil and sonicating for less than 1 min to disperse the particles in the oil. A small drop of this suspension was placed onto the microscope slide and a glass coverslip placed over the preparation. Four slides and 100 fields-of-view from each slide were examined for the nonmicronized sample. Two slides and 100 fields-of-view from each slide were examined for the micronized sample. Gravimetric Vapor Sorption. GVS was performed on a Dynamic Vapor Sorption instrument (Surface Measurement Systems, Allentown, PA). Approximately 30 mg of material was loaded into a tared glass sample pan. The samples were exposed to two sorption/desorption cycles in 10% relative humidity (RH) steps over the range of 090% RH at 25 °C. The samples were exposed to each step until the change in mass to time ratio (dm/dt) of
