To further assess the interaction between polymer and drug, including hydrogen bonding, solid state C NMR spectroscopy was conducted. First, ASDs containing the polymer P1 with different polymer-drug ratios, as well as the plain polymer, crystalline and amorphous IND, were compared (Figure ). Depending on the solid-state form of the drug, pronounced differences in the spectra are visible. The γ-IND polymorph shows narrow lines with one peak for each carbon atom. This indicates that there is one molecule in the asymmetric unit of the crystal lattice, which is consistent with the known crystal structure of γ-IND. This polymorph consists of cyclic dimers formed via hydrogen bonding between the carboxylic acid groups of two IND molecules, which gives rise to the peak at 180 ppm (assigned as signal 1 in Figure ). Importantly, Strachan et al. reported that such dimers are also predominant in amorphous IND, as evidenced by infrared and Raman spectroscopies. The other peak in the carbonyl region, at 167 ppm (signal 2), corresponds to the amide group, which is not involved in hydrogen bonding in the γ-polymorph. Compared to the spectrum of the crystalline form, the spectrum of the amorphous form shows broader peaks with an additional shoulder next to the peak at 167 ppm, probably resulting from a combination of conformations and intermolecular bonding present in αand γ-IND. Besides the carboxylic acid dimer, hydrogen bonded carboxylic acid-amide complexes are present in the α-polymorph.