Cystic fibrosis (CF) is an inherited disease caused by genetic defects in the cystic fibrosis transmembrane conductance regulator gene (CFTR). CFTR encodes for a cAMP-regulated chloride channel located in the apical membrane of polarized epithelial cells. The most common mutation, ΔF508, leads to degradation of the CFTR protein. In order to enable CFTR expression and function in the apical membrane of airway epithelial cells, this study used optimized wtCFTR-mRNA as a new therapeutic assay to restore functional chloride secretion. In previous studies, the human bronchial epithelial cell line CFBE41o – that stably expresses ΔF508-CFTR, was transfected with this mRNA in a concentration of 2.4μg/cm2 using LipofectamineTM 2000 as transfection reagent. Subsequently electrophysiological Ussing chamber measurements were performed 24h after transfection. The cAMP/IBMX cocktail-induced CFTR currents (2.63μA ± 0.64) were similar to the values seen in healthy bronchial cells (16HBE14o- ; 2.66μA ± 0.48). Immunofluorescence approaches confirmed newly synthesized CFTR molecules by confocal microscopy and Western blot analyses showed an increased amount of CFTR protein. To build a bridge to the in vivo situation, we evaluated the CFTR expression in primary human epithelial (HNE) cells [1]. We reduced the wtCFTR-mRNA amount stepwise (2.4μg/cm2, 1.2μg/cm2, 0.6μg/cm2) to determine the minimal concentration that is needed for the most efficient restoration of the functional chloride secretion. In transepithelial Ussing chamber experiments we showed an increase in the cAMP-stimulated CFTR current in HNE, which could also be inhibited by the specific CFTR blocker CFTRinh172. A reduction of the used mRNA resulted in a proper functional chloride secretion that is comparable to higher concentrations. These data suggest that the successful restoration of the defective CFTR function is possible by a reduction in the concentration of the wtCFTR-mRNA to a minimum of 0.6μg/cm2. In further studies we will focus on the duration of CFTR expression using this optimized amount of wtCFTR-mRNA.