## 4 Discussion
The DSC thermograms reported in the literature [24], for both polymorphs and amorphous indomethacin, are characteristic, with the sharp endothermic peak above 160 \({}^{\circ}\)C being attributed to the \(\gamma\) form, as in our case. The obtained spironolactone DSC curves show a single sharp endothermic peak at the melting temperature of 213 \({}^{\circ}\)C, confirming the purity and crystallinity of the SPIR [25]. Secondly, a characteristic feature of thermal studies of drug-loaded SLM is the absence of the drug substance peak in the DSC spectra. The melting peak of any of the tested drug substances (Figure 2) was not observed in thermograms of SLM (Figures 3 and 4), regardless of their concentration in the formulation. One possible hypothesis is that the disappearanceof the drug substance melting peak in the drug-loaded SLM could be explained by the fact that, at a low drug concentration, the melting point of the drug substance has been lowered and broadened so much that the peak became undetectable by DSC. However, the peak is not visible even when the ratio of CsA to lipid was 1:2 in the F6 formulation (Figure 3b). These observations are consistent with the results of [PERSON] et al. [15], who obtained the melting peak of ibuprofen in a drug-lipid cast with glycerol behenate only at a concentration of 70% and above. The absence of melting endotherms of the tested drug substances in the DSC thermographs of all drug-loaded SLM preparations (Figures 3 and 4) may therefore result from a drug concentration in the SLM too low to be detected by the DSC method. However, comparison of the SLM thermograms with Ind and a physical mixture with Ind (the same Ind content, Figure 5) also suggests drug-lipid interactions and API incorporation, or at least some kind of interaction of the drug substances with other components of the dosage form. According to one possibility, the disappearance of the peak of the active substance in the microspheres may result from the dissolution of the drug substance in the molten lipid during the manufacturing process. Such explanations appear, for example, in the study of microspheres with theophylline [26]. Similar results (the lack of an endothermic peak for CsA) in solid dispersions were also obtained by [PERSON] et al. [23]. The thermal behavior of SLM dispersions was very similar, regardless of the type and concentration of the incorporated drug substance (Figures 3a and 4a). First of all, as was shown in the example of the tested drug substances, the presence of a solute in the microsphere matrix decreases the melting point of the lipid matrix, which in the DSC spectrum is manifested by a shift of the lipid melting peak towards a lower temperature. Drug substances that are incorporated in the SLM to varying degrees (depending mainly on the properties of the drug and the composition of the carrier) might interact with the lipid matrix of the microspheres. This effect can change the crystalline structure of the lipid and, consequently, the melting point. A decrease in enthalpy generally suggests a lower crystallinity of lipid matrices [14]. Low crystallinity of lipids is desirable in order to create more spaces for drug localization and enhance the encapsulation efficiency of drug substances [14]. The shift of the melting peak of the lipid matrix in different lipid particles to a lower temperature with respect to the bulk lipid was explained in published studies through various reasons, such as the small size of the lipid particles or the incorporation of a drug substance or interactions between the lipid and the surfactant, which results in a less ordered structure of the lipid (Kelvin effect) [27; 28]. Therefore, a certain degree of melting point depression of the lipid peak in drug-loaded SLM can be attributed to the dissolution of the drug substance in the lipid matrix.