**7. Conclusion**

available medical products based on gene therapy along with cell therapy and tissue engineering are classified as advanced therapy medicinal products. Although highly promising, their translation into clinical practice is nowadays hampered by major critical issues such as complex regulatory and ethical aspects, along with the intrinsic difficulties to scale up these

Regarding the regulatory concerns of GTMP that affect clinical applications, the economical investments, along with their manufacture and control, demand more attention than chemically synthesized small molecules [86]. Therefore, a deep analysis of both costs and benefits

Another relevant concern that jeopardizes the clinical use of GTMP in CF is the ethical aspect of clinical trials. Since the early 1990s, more than 25 Phase I gene therapy clinical trials have been conducted. These trials have been carried out largely to assess the safety and feasibility of gene transfer methods and their expression in the host, reporting variable successes for both viral and nonviral approaches. Gene therapy products designed for the treatment of CF must meet certain requirements in order to become a viable therapeutic option. For instance, their clinical efficacy must be demonstrated by analyzing appropriate variables of the lung function such as the patient´s vital capacity that they are able to expire in the first second of forced expiration (FEV1), their age, sex or body composition, and the therapeutic efficacy which must be maintained with repeated administrations. In addition, the GTMP must demonstrate an acceptable profile when it comes to side effects, and other considerations such as treatment of early versus

Since a high percentage of patients affected by CF are children, clinical trials involving these patients must carefully balance the potential benefits of these therapies and the associated risks [88]. Regarding this controversial issue, the Gene Therapy Advisory Committee recommends that clinical trials on children should only be performed under specific circumstances, whereby: (i) it has been demonstrated that the research is necessary to promote the health of the trial population, (ii) the research cannot be done in adults, and (iii) there is a high potential of therapeutic benefit [88]. In fact, owing to a demonstrated benefit of early gene therapy intervention, the age of enrolment of children in clinical trials has progressively reduced over the years from 18 to 12 years old. However, parents should have legal rights to make the final

Another critical hurdle that strongly compromises the clinical application of gene therapy products for the treatment of CF is the difficulty to scale up formulations that were originally developed for basic clinical research [89]. Most of these products are usually developed by small- and medium-sized enterprises, in collaboration with academic groups, which are usually highly engaged in preclinical activities, but have limited manufacturing experience at industrial level. For instance, the normal procedure for preparing nonviral-based gene therapy products is by simply mixing and pipetting the negatively charged genetic material and the positively charged polymer - or lipid-based nonviral vector formulations, which are often produced in the laboratory at small volumes that usually oscillate between 1 and 5 mL. However, the standardization of this procedure at industrial level to produce high and stable levels of complexes under GMP conditions

needs to be done before considering the commercialization of such therapies [87].

products to an industrial level [20].

32 In Vivo and Ex Vivo Gene Therapy for Inherited and Non-Inherited Disorders

established lung disease must also be analysed.

decision on behalf of their children.

Despite the fact that the CFTR gene was cloned two decades ago, the current, conventional treatments for CF focus on masking the main symptoms, rather than addressing the underlying genetic cause of the disease. In this sense, gene therapy represents a promising alternative to tackle CF, considering the autosomal recessive nature of the most relevant ΔF508 mutation. Although the main objective of gene therapy seems simple, there are some hurdles that need to be overcome before gene therapy for CF becomes a realistic treatment option. In any case, the increase in knowledge and recent advances in biopharmaceutical technology offer reasonable hope for the treatment of this devastating disease. The minicircle technology, along with the new gene editing tools, offer important advantages compared with classical plasmids used to add functional copies of the gene. Additionally, intense research in novel nonviral vectors functionalized to overcome both extra- and intracellular barriers and the possibility to aerosolize such formulations without losing activity merit special attention.
