**5. Conclusions**

The SEDDS based on MCFA was proven to be a viable delivery system across the Caco-2 monolayer for OND drugs. Hydrophobic ion pairing of OND with a cationic lipid, either DDAB or DOTAP, led to the formation of specific complexes, as confirmed by AFM and FTIR. Complexation reduced the hydrophilicity of OND and increased its solubility in lipids. The resulting ion-paired complexes subsequently allowed for their formulation into SEDDS: negatively charged or neutral SEDDS. A morphological evaluation of these SEDDS using cryo-TEM showed the presence of both nanosized oil droplets and vesicular structures. Negatively charged SEDDS did not significantly affect the Caco-2 viability. Additionally, neutral SEDDS enabled a higher OND permeability across the Caco-2 monolayer into the lamina propria, as well as better OND protection against hydrolytic decomposition in comparison to its charged counterpart. The lipolysis of both SEDDS was inhibited with the addition of orlistat, enabling the prolonged protection of OND in the gastrointestinal tract. The evaluation of further properties of the tested formulations, e.g., passive targeting and their effect on intestinal histology, in an appropriate in vivo model could reveal more details of this potent delivery system for local OND delivery.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ pharmaceutics13040459/s1: Figure S1: Fluorescence microscope images.

**Author Contributions:** Conceptualization, P.P., O.H. and A.M.; methodology, J.K. and G.Z.; validation, J.K., O.H. and A.M.; formal analysis, J.K., J.Z. and O.H.; investigation, J.K. and G.Z.; resources, A.M. and P.P.; data curation, J.K.; writing—original draft preparation, J.K.; writing—review and editing, O.H., P.P., B.V. and A.M.; visualization, J.K. and B.V.; supervision, O.H., J.Z. and A.M.; project administration, A.M. and P.P. and funding acquisition, P.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the EFSA-CDN project (Reg. No. CZ.02.1.01/0.0/0.0/16 \_019/0000841) co-funded by the European Union. This work was supported by the Czech Ministry of Education and Sports, project no. SVV 260 401. Jana Kubackova is grateful for the financial support from the Mobility Fund of Charles University (FM/c/2029-1-011), European Network on Understanding Gastrointestinal Absorption-related Processes (UNGAP) Cost action project (CA16205) and Erasmus+.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data is available on reasonable request from the corresponding author.

**Acknowledgments:** CIISB: the Instruct-CZ Centre of the Instruct-ERIC EU consortium, funded by the MEYS CR infrastructure project LM2018127, is gratefully acknowledged for the financial support of the measurements at the CF Cryo-EM. Authors are grateful to Jan Dusek and Miloslav Machacek for their assistance with cell culture experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
