(W0930-08-54) Synchrotron X-Ray Diffraction of the Structure of Amorphous Indomethacin Using PDF Analysis and EPSR Modeling

Purpose: To enhance the solubility of orally administered pharmaceuticals, liquid or amorphous formulations are often preferred over crystalline drug products. However, little is known regarding the variation in bonding mechanisms between pharmaceutical molecules in their different disordered forms. High energy x-ray data combined with the x-ray pair distribution function (PDF) method is a powerful technique for the characterization of variations in both intra-molecular and inter-molecular packing arrangements. Advanced modeling techniques such as empirical potential structural refinement (EPSR) are revolutionary for visualizing the 3D-dimensional arrangement of the molecules in disordered forms. Here, we expand upon our recent analysis of indomethacin [1] by including new results using EPSR to further investigate the variation in hydrogen bonding between different amorphous forms.

Methods: Powdered amorphous indomethacin was loaded into 1.5 – 2 mm diameter capillaries and some were exposed to varying humidity levels during analysis. Scattering experiments were performed on a synchrotron beamline using an incident energy of 100 keV in transmission geometry. Diffraction data were collected on a Varex CT4343 area detector. The total x-ray scattering data were analyzed using Fit2D and PDFgetX2. EPSR simulations were performed on 64 molecules within a cubic box under periodic boundary conditions using atomic number densities of 0.090 atomsÅ^-3 for sample II and 0.095 atomsÅ^-3 for sample V. The starting configuration of the model was constructed from a random array of Z isomer molecules since this configuration is the most common, particularly at low density values. To introduce molecular flexibility, limited rotations of six molecular groups were enabled, including the rotation of the chlorobenzyl ring. Following initial Monte Carlo equilibration, the empirical potential term was refined to improve agreement with scattering data. Once the goodness-of-fit parameter was minimized between the model and the experimental S(Q), structural data were collected over ensembles of at least 10,000 configurations.

Results: The Indomethacin molecule is comprised of a largely hydrophobic indole and chlorobenzyl groups, and several hydrophilic groups: namely an amide, methoxyl and a carboxylic acid. γ-Indomethacin is the stable crystalline form which exists only with Z isomers where hydrogen bonded dimers are connected through their carboxylic acid groups. α-Indomethacin is a denser metastable crystalline form comprised of three different isomers (Z, E and α3). δ-Indomethacin has recently been found to consist of a dimer of the Z and E isomers. Our x-ray PDF studies of several amorphous forms showed that when the chlorobenzyl ring exhibited distinct isomer orientations (sample II), the first sharp diffraction peak (FSDP) and medium range ordering was found to be lower. However, for amorphous samples with no preferred torsion angles of the chlorobenzyl ring (sample V), enhanced medium range order attributed to inter-molecular hydrogen bonding was observed, and this order was reflected in a 20% increase in the intensity of the FSDP. The EPSR partial PDFs provide a direct indication of the degree of hydrogen bonding in amorphous indomethacin. The model for the less ordered sample II, has a first oxygen-oxygen (O-O) peak at 2.4 Å and a broader second peak at 3.8 Å, whereas the more-ordered sample V shows an increase in the O-O coordination number from 0.24 (II) to 0.53 (V). This result confirms that weakly hydrogen bonded molecules in II become more strongly bonded in V.

Conclusion: The analysis of different indomethacin amorphous samples indicated that storage conditions have an influence on the amorphous structure. While some samples showed a largely unhindered rotation of the chlorobenzyl ring in the dry amorphous form, others showed preferred torsional angles consistent with those observed in the known isomers. The hindered intra-molecular conformations associated with the preferred orientation of the chlorobenzyl ring occur at the expense of inter-molecular hydrogen bonding interactions. At high humidity, water molecules form hydrogen bonds with indomethacin molecules leading to partial hydration of the amorphous form which stabilizes over the period of many ( >10) hours. A linear hydrogen bond PDF model between a water molecule and an indomethacin Z isomer suggests the inter-molecular hydrogen bond strengths are comparable to those found between indomethacin dimers. EPSR modeling was used to further investigate the variation in hydrogen bonding between two of the amorphous indomethacin samples, II, and V. EPSR showed that most molecules are bonded to one neighboring molecule via a single hydrogen bond. The number of isolated molecules is higher in sample II, while there are more bifurcated hydrogen bonds in sample V leading to a diversity of chain structures in the amorphous forms.

References: [1] Benmore, C., et al., A High Energy X-ray Diffraction Study of Amorphous Indomethacin. Quantum Beam Science, 2022. 6(4), 31.

Acknowledgements: This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.