# Cystic Fibrosis: Overview of the Current Development Trends and Innovative Therapeutic Strategies
## 4 Gene Therapy for CF  
### Targeting CFTR  
As previous mentioned, the conventional treatment for CF is based on the reduction in disease symptoms and improvement in the patient's quality of life using antibiotics, physiotherapy, and mucus clearance medications. However, these therapeutic approaches do not target the gene mutation causing the disease. The cloning of the CFTR gene in 1989 opened doors for the correction of CF at the genetic level [4; 142; 143]. Theoretically, the use of gene therapy for CF is more advantageous compared to other genetic diseases. As CF is caused by a single gene mutation, CF carriers are phenotypically normal. The airway is easily accessible for drug delivery through a pulmonary nebulizer, and a relatively low level of CFTR correction has a high therapeutic potential (6-10% of normal CFTR activity is needed to restore the epithelial cells electrical properties and 25% to restore mucociliary function) [144; 145]. In the last three decades, numerous efforts have been made to develop a gene replacement therapy for faulty CFTR with either cDNA or mRNA molecules by using viral or non-viral vectors. The earliest clinical study in this field was conducted in 1993, wherein CFTR cDNA was administrated through an adenovirus (AV) vector, in which the early-transcribed region E\({}_{1}\) was deleted to prevent viral replication. AV has been an attractive vector for CF gene therapy, owing to its efficiency in transducing the non-dividing cells of the airway without genome integration. However, the low transfection efficiency and immune inflammation response against the virus, even with low doses, makes this therapeutic approach infeasible for CF patients [146]. The mRNA approach involves translation of mRNA molecules in the cytoplasm of the transfected cells without the need for translocation into the nucleus, which makes them more effective compared to DNA-based therapy. Moreover, the risk of potential insertion into the host genome is less compared to that with DNA molecules. The main limitations of this approach are the short half-life, expression instability, and potential cytotoxicity through toll-like receptor activation [147]. Chemical modification, using various techniques, has showed great improvements in the mRNA-based approach, especially in cancer therapy and vaccination for infectious diseases [148]. In vitro studies on differentiated human nasal epithelial primary cells revealed that the minimal dose of wt-CFTR-mRNA (0.6 \(\upmu\)g/cm\({}^{2}\)) delivered by Lipofectamine 2000 unregulated the functional electrophysiology of the CFTR channel for over 72 h [149]. The efficacy of this approach was also demonstrated in vivo by [PERSON] et al. (2018), where the CFTR mRNA restored the CI' ions efflux by 55% in CFTR knockout mice compared to healthy mice [150]. Moreover, in 2018, the first clinical trial for mRNA therapeutic phase I/II (NCT03375047), sponsored by Translate Bio, Inc. (Lexington, MA, USA), was given to 40 adult participants regardless of the mutation class. Eluforsen (QR-010), developed by ProQR Therapeutics, is a 33 mer antisense oligonucleotide (wildtype template) that can bind to CFTR mRNA sequences adjacent to the F508 del homozygous mutation, and correct it. The clinical trial phase Ib (NCT02564354) for this drug has been completed, where it was found to be well tolerated and showed improvements in the CF Questionnaire-Revised respiratory domain [151]. Thus, the correction of CFTR mRNA using oligonucleotide molecules may restore channel function. Furthermore, antisense oligonucleotide molecules have been investigated to correct splice CFTR mutations (class V) such as c.2657+5G\(>\)A [152]. To date, numerous clinical trials have been conducted to determine the ability of gene therapy to reduce the progression of CF lung disease. However, a clinically effective treatment is yet to be established.