(W0930-09-61) Visualising Co-crystal Formation during Milling with Stimulated Raman Scattering (SRS)

Purpose: Cocrystals, in which the drug molecule and water-soluble co-former form a single-phase crystal structure in unionized form, are an attractive option for improving the solubility and dissolution rate of poorly water-soluble drugs, and thus therapeutic effectiveness [1]. Milling the drug together with the co-former is one approach to manufacture cocrystals. Conventional solid state characterization methods such as X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC) as well as Fourier transform infrared (FTIR) and Raman spectroscopies are widely used to characterize cocrystal formation during/after milling. While informative, these methods, a) do not provide spatially-resolved chemical and structural information that helps to elucidate transformation mechanisms, and b) may miss possibly important trace levels of solid state forms. Stimulated Raman scattering (SRS) microscopy is an imaging method similar to (spontaneous) Raman mapping/imaging, in that offers chemically- and solid-state specific imaging based on vibrational signatures of components. However, compared to Raman mapping, SRS imaging is orders of magnitude faster and offers better 3D spatial resolution. In this study, stimulated Raman scattering (SRS) microscopy with fast spectral focusing was investigated as a novel approach to image the formation of co-crystals during milling. Analysis was supported with conventional complementary analytical techniques.

Methods: Co-crystalline systems (1:1 molar ratio) of indomethacin (IND) and nicotinamide (NIC) were prepared by solvent-assisted milling. Samples at different time points of milling were analyzed with XRPD, DSC FTIR spectroscopy, as well as SRS. SRS analyses were performed with an in-house built SRS microscope with fast spectral focusing, permitting the rapid generation of chemically- and structurally-specific vibrational hyperspectral images. SRS data was processed with multivariate classical least squares (CLS) analysis.

Results: Based on XRPD and DSC analyses, IND-NIC cocrystals were produced and exclusively present by 16 min. Co-crystals showed changes in the FTIR spectra when compared to physical mixtures of the two components, including peak a shifts of C=O stretching to slightly lower wavenumbers and C=C stretching to slightly higher wavenumbers, suggesting the existence of intermolecular interactions between the two components. SRS microscopy combined CLS analysis provided insights into the distribution of solid-state components and the co-crystallization process during milling (Figure 1). After 1 min of milling the original components appeared relatively intact, but after 2 min of milling according to SRS with the CLS the co-crystal was already present, mixed with both the coformers. After 28 minutes, the co-crystal was almost exclusively present with a long-needle like morphology, but with traces of other forms still present. Interestingly, at 4 min, a strikingly different particle morphology size and shape was present, with spectra that resembled IND together with NIC. The origins of this observation are being further investigated.

Conclusion: SRS microscopy demonstrated its potential as a rapid, chemically and solid-state specific imaging tool to provide spatially-resolved insights into co-crystallization during milling, with additional information on particle size and morphology. The information is complementary to that provided with conventional solid-state analysis approaches.

References: 1. M. Karimi-Jafari, L. Padrela, G.M. Walker, M. Croker Creating Cocrystals: A Review of Pharmaceutical Cocrystal Preparation Routes and Applications, Cryst. Growth 18 (2018) 6370–6387.

Acknowledgements: The authors would like to thank the qCSI (Quantitative Chemically-Specific Imaging Infrastructure for Material and Life Sciences) FIRI infrastructure, and the Academy of Finland for financial support (Grant No. 327732).

Figure 1. SRS images taken during co-crystallization of IND and NIC at different milling times. SRS spectra of the IND, NIC and the corresponding co-crystal are shown on the right. Images are generated from the SRS spectra using multivariate CLS analysis. The colors used to visualize the different forms in the images on the left correspond to the color of the spectra of the different components on the right. In the image at 2 min, some burning is also apparent in the sample. Each image (excluding the 2.5 x zoom) is 106 x 106 µm.