39 © Springer Nature Switzerland AG 2020 C. Nóbrega et al., A Handbook of Gene and Cell Therapy, https://doi.org/10.1007/978-3-030-41333-0_3 Viral Vectors for Gene Therapy The selection of the delivery system is one of the most critical points for the success (and safety) of gene therapy. First, the chosen vector must ensure that the gene is delivered to the correct tar- get cells, or at least predominantly to those cells. Second, the introduced transgene has to be acti- vated, that is, it must go to the nucleus, be tran- scribed, and then translated. In a one-time cure setting, ideally the transgene should be integrated into the host genome or endure episomally, ensur- ing a long-term persistent expression. The ideal delivery system should meet several criteria, including (i) a good safety profle; (ii) easy pro- duction; (iii) good stability in target cells, and (iv) a high transgene capacity. As mentioned, the delivery systems can be roughly divided into two main categories: non-viral and viral systems. The non-viral methods were described in the previous chapter, whereas in this chapter the viral vectors used in gene therapy will be explained. Viruses have evolved for millions of years, aiming to effciently introduce and replicate their genetic material in host cells. Taking advantage of this feature, gene therapy developed tools and procedures to engineer viruses and their genomes, in order to artifcially deliver nucleic acids. In fact, the main advantage of viral-based systems for gene therapy is their high effciency in deliv- ering transgenes, surpassing anatomical and cel- lular barriers. For this reasons, viral vectors have been broadly used since the frst gene therapy clinical trial in 1990. Data from The Journal of Gene Medicine highlights that until November 2017 around 68% of gene therapy clinical trials worldwide used viral vectors as a delivery system (Fig. 3.1) [1]. Recently, viral vectors became more than a promise in gene therapy with the approval of several gene therapy products using these delivery vectors (see Chap. 1 for a list of currently approved gene therapy products). Moreover, several other products using viral sys- tems are in the fnal stages of clinical trial devel- opment [2], highlighting that new products could be approved in the next years. This broad use also emphasizes that viral vectors are now consid- ered safe for human use, despite their natural infectious profle. Nevertheless, the safety issues were and still are major concern in the development and use of viral vectors. For that, several engineering solu- tions were developed trying to enhance viral vec- tors’ safety, without compromising their effciency, such as (i) avoiding their replication; (ii) promoting their inactivation; and (iii) attenuat- ing their natural toxicity. Besides their effciency, another important advantage of viral vectors is the wide range of existing viruses, which have differ- ent features, like the type of genetic material, natural tropism, and size, which provide an enor- mous offer of systems for gene delivery. On the other hand, besides their safety concerns in gene therapy, viral vectors also have other disadvan- tages, such as their limited cloning capacity (Table 3.1).Several criteria are used to classify 3