Second Harmonic Generation Microscopy and Raman Microscopy of Pharmaceutical Materials
Second harmonic generation (SHG) microscopy and Raman microscopy were used for qualitative and quantitative analysis of pharmaceutical materials. Prototype instruments and algorithms for sampling strategies and data analyses were developed to achieve pharmaceutical materials analysis with low limits of detection and short measurement times
Manufacturing an amorphous solid dispersion (ASD), in which an amorphous active pharmaceutical ingredient (API) within polymer matrix, is an effective approach to improve the solubility and bioavailability of a drug. However, since ASDs are generally metastable materials, they can often transform to produce crystalline API with higher thermodynamic stability. Analytical methods with low limits of detection for crystalline APIs were used to assess the stability of ASDs. With high selectivity to noncentrosymmetric crystals, SHG microscopy was demonstrated as an analytical tool, which exhibited a limit of detection of 10 ppm for ritonavir Form II crystals. SHG microscopy was employed for accelerated stability testing of ASDs, which provided a four-decade dynamic range of crystallinity for kinetic modeling. An established model was validated by investigating nucleation and crystal growth based on SHG images. To achieve in situ accelerated stability testing, controlled environment for in situstability testing (CEiST) was designed and built to provide elevated temperature and humidity, which is compatible with a commercial SHG microscope based on our research prototype. The combination of CEiST and SHG microscopy enabled assessment of individual crystal growth rates by single-particle tracking and nucleation rates for individual fields of view with low Poisson noise. In addition, SHG microscopy coupled with CEiST enabled the study of heterogeneity of crystallization kinetics within pharmaceutical materials.
Polymorphism of APIs plays an important role in drug formulation development. Different polymorphs of identical APIs may exhibit different physiochemical properties, e.g., solubility, stability, and bioavailability, due to their crystal structures. Moreover, polymorph transitions may take place during the manufacturing process and storage. Therefore, analytical methods with high speed for polymorph characterization, which can provide real-time feedback for the polymorphic transition, have broad applications in pharmaceutical materials characterization. Raman spectroscopy is able to determine the API polymorphism, but is hampered by the long measurement times. In this study, two analytical methods with high speed were developed to characterize API polymorphs. One is SHGmicroscopy-guided Raman spectroscopy, which achieved the speed of 10 ms/particle for clopidogrel bisulfate. Initial classification of two different polymorphs was based on SHG images, followed acquisition of Raman spectroscopyat the selected positions to determine the API crystal form. Another approach is implementing of dynamic sampling into confocal Raman microscopy to accelerate Raman image acquisition for 6-folds. Instead of raster scanning, dynamic sampling algorithm enabled acquiring Raman spectra at the most informative locations. The reconstructed Raman image of pharmaceutical materials has <0.5% loss of image quality with 15.8% sampling rate.