## 4 Conclusions  
This study evaluated the cytotoxicity and permeability of a ketoprofen-mannitol carrier-free DPI formulation, alongside its stability when stored in HPMC capsules under stress conditions following ICH guidelines (40 \({}^{\circ}\)C and 75% relative humidity). The developed DPI represents a promising strategy for drug repositioning, offering potential therapeutic benefits for managing lung inflammation while saving time and resources in drug development. KTP showed good penetration across both alveolar and bronchial models, with lower permeability through the CFBE bronchial barrier than the A549 alveolar barrier, probably due to structural differences. The MNT-containing F1 formulation enhanced permeability by 1.4-fold through the A549 alveolar cells compared to raw KTP at the 30-min time point, likely because MNT improves drug diffusion or interacts with epithelial cells to facilitate permeation. While F1 caused a slight decrease in cell impedance at high concentrations in A549, neither the F0 nor F1 carrier-free DPIs affected the cell impedance of CFBE epithelial cells. Moreover, no changes in barrier integrity were observed for either the tested active agent or the DPI formulations in both culture models, which further supports the safety of our DPI formulations. The formulation retained favorable properties after storage, owing to the carefully selected excipients and optimized technological processes. Stability testing demonstrated consistent particle size and the persistence of KTP's partially amorphous nature over time. Dissolution studies confirmed rapid initial drug release, indicating effective delivery potential. Aerosol performance analysis revealed no significant change in MMAD values after storage, although a reduction in FPF was observed, likely due to moisture absorption affecting particle surface morphology. Future studies should focus on exploring alternative capsule types and storage conditions, particularly at lower relative humidity levels, to further improve stability and maintain optimal aerosolization properties.  
Conceptualization, H.B., R.A. and I.C.; methodology, H.B., I.G., M.A.D., I.C. and R.A.; software, H.B. and I.G.; validation, H.B., R.A. and I.C.; formal analysis, H.B., I.G., M.A.D., R.A. and I.C.; investigation, H.B. and I.G.; resources, R.A. and I.C.; data curation, H.B., R.A. and I.C.; writing--original draft preparation H.B.; writing--review and editing, H.B., R.A., I.G., M.A.D. and I.C.; visualization, H.B., R.A. and I.C.; supervision, R.A. and I.C.; project administration, R.A. and I.C.; funding acquisition, R.A. and I.C. All authors have read and agreed to the published version of the manuscript.  
This research was supported by National Research, Development and Innovation Office, NKFIH_OTKA K_146148.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
Acknowledgments: Project no TKP2021-EGA-32 has been implemented with the support provided by the Ministry of Culture and Innovation of Hungary from the National Research, Development and Innovation Fund, financed under the TKP2021-EGA funding scheme.  
Conflicts of Interest: The authors declare no conflicts of interest. The authors alone are responsible for the content and writing of this article.