Second harmonic generation microscopy as a tool for the early detection of crystallization in spray dried dispersions
Various techniques have been used to detect crystallization in amorphous solid dispersions (ASD). However, most of these techniques do not enable the detection of very low levels of crystallinity (<1%). The aim of the current study was to compare the sensitivity of second harmonic generation (SHG) microscopy with powder X-ray diffraction (XRPD) in detecting the presence of crystals in low drug loading amorphous solid dispersions. Amorphous solid dispersions of the poorly water soluble compounds, flutamide (FTM, 15 wt.% drug loading) and ezetimibe (EZT, 30 wt.% drug loading) with hydroxypropyl methylcellulose acetate succinate (HPMCAS) were prepared by spray drying. To induce crystallization, samples were subsequently stored at 75% or 82% relative humidity (RH) and 40 °C. Crystallization was monitored by XRPD and by SHG microscopy. Solid state nuclear magnetic resonance spectroscopy (ssNMR) was used to further investigate crystallinity in selected samples. For flutamide, crystals were detected by SHG microscopy after 8 days of storage at 40 °C/82% RH, whereas no evidence of crystallinity could be observed by XRPD until 26 days. Correspondingly, for FTM samples stored at 40 °C/75% RH, crystals were detected after 11 days by SHG microscopy and after 53 days by XRPD. The evolution of crystals, that is an increase in the number and size of crystalline regions, with time could be readily monitored from the SHG images, and revealed the formation of needle-shaped crystals. Further investigation with scanning electron microscopy indicated an unexpected mechanism of crystallization, whereby flutamide crystals grew as needle-shaped projections from the surface of the spray dried particles. Similarly, EZT crystals could be detected at earlier time points (15 days) with SHG microscopy relative to with XRPD (60 days). Thus, SHG microscopy was found to be a highly sensitive method for detecting and monitoring the evolution of crystals formed from spray dried particles, providing much earlier detection of crystallinity than XRPD under comparable run times.