3.2.1. Drug Crystallinity

DSC and XRPD were utilized for the qualitative, solid-state characterization of FEN in the SLH formulations. The DSC thermograms of a physical mixture (5% *w*/*w* crystalline FEN with FS), crystalline FEN, FS, and MPS formulations are displayed in Figure 2. Crystalline FEN exhibited a sharp endothermic peak at 80 ◦C, corresponding to its melting point [38], whereas the FS physical mixture exhibited a small endothermic peak (Figure 2A). Furthermore, this thermogram suggests that crystalline FEN could be detected at concentrations ~5% *w*/*w* when present within silica particles. Regardless of the silica type or lipid used, endothermic peaks were absent for all 80% and 200% SLH formulations, suggesting that the drug was in a non-crystalline state. However, all other formulations of higher drug loads demonstrated shifted endothermic peaks varying in size and stretching between 45–60 ◦C. The shifted endothermic peaks for all 400% formulations were smaller than for the 600% formulations, which suggests the presence of higher concentrations of crystalline FEN at higher saturation levels (Figure 2B,C). No apparent difference in DSC thermograms was observed between PG8 or C300 SLH formulations, whether loaded in FS or MPS. Therefore, XRPD patterns were only obtained for SLH formulations containing PG8, since higher drug loads were achieved in comparison to the SLH C300 formulations.

Crystalline FEN demonstrated characteristic XRPD patterns as previously reported (Figure 2D) [39]. For the 5% *w*/*w* physical mixture, characteristic crystalline FEN patterns can be observed but differ in intensity between FS and MPS (Figure 2E,F). XRP diffractograms indicate that the method can detect as low as 5% *w*/*w* crystalline FEN. In SLH formulations fabricated with FS PG8, no crystalline FEN was detected at drug loads of 80% and 200%. However, at 400% and 600% FS PG8, intense peaks were observed, suggesting the presence of crystalline FEN. These findings correspond with data obtained with DSC analysis. However, all supersaturated MPS PG8 formulations (200, 400, and 600% Seq) displayed characteristic peaks of varying intensities, suggesting the presence of crystalline FEN (Figure 2F), and no crystalline drug was detected with unsaturated 80% P PG8 formulation. It is important to note that some characteristic peaks where absent from XRPD pattern obtained from 200% P PG8, suggesting the presence of a FEN polymorph [38]. For both FS and MPS systems, XRPD patterns obtained from 600% Seq formulations displayed peaks with higher intensities than corresponding 400% Seq formulations, displaying an ~2-fold increased intensity, suggesting the presence of higher amounts of crystalline FEN in SLH prepared at 600% Seq. Furthermore, MPS formulations exhibited peaks with higher relative intensities compared to corresponding FS formulations that were ~1.6-fold higher at

400% and 600% Seq. This was evident when the peak intensities obtained at 22.3◦ (2θ) were plotted *Pharmaceutics*  against % S **2020** eq, as displayed in Figure S1. , *12*, x FOR PEER REVIEW 9 of 22

**Figure 2.** Differential scanning calorimetry (DSC) thermograms of (**A**) crystalline FEN and fumed silica (FS) physical mix 5% *w/w*, (**B**) FEN-loaded FS formulations and (**C**) FEN-loaded mesoporous silica (MPS) formulations. The X-ray powder diffraction (XRPD) diffractograms of (**D**) crystalline FEN, (**E**) FS PG8 formulations and (**F**) MPS PG8 formulations. **Figure 2.** Differential scanning calorimetry (DSC) thermograms of (**A**) crystalline FEN and fumed silica (FS) physical mix 5% *w*/*w*, (**B**) FEN-loaded FS formulations and (**C**) FEN-loaded mesoporous silica (MPS) formulations. The X-ray powder diffraction (XRPD) diffractograms of (**D**) crystalline FEN, (**E**) FS PG8 formulations and (**F**) MPS PG8 formulations.
