## Why is gene therapy not a common part of our lives?

One might think (based on available technology and clinical trials) that the gene therapy is ready for practical use. So why is it still not part of our lives? One of the reasons is that in many cases, safety and efficacy for human use have not yet been studied. In drug development, it is not uncommon for test results to be favourable under laboratory conditions, but later to find that the therapy does not work or is associated with severe side effects when animal models or human volunteers are used. It takes an average of 12 to 15 years from the start of the approval process for a drug to its market launch (Figure 10.9). Negative effects at any stage can lead to the approval process being halted or the drug being withdrawn from the market.

It is obvious today that the gene therapy will become an important tool of modern medicine. Thanks to it, in a very short time it will be possible to help people overcome diseases for which there is currently no cure. But it is not that simple – gene therapy also has many pitfalls and side effects, the elimination of which is crucial for its further development. One example of such a problem is the insertion toxicity of in vivo gene therapy. Insertion of DNA in the wrong place or its multiple insertions can lead to side effects that endanger the patient's life. In addition, the right tissue must be targeted, for which specially modified viruses or complicated surgical procedures are used. Another challenge is to develop safer and more effective vectors that are better tolerated by our immune system. An alternative is to find a way to reduce the dose administered to patients. Clinical testing of gene therapy itself is also challenging, as many of the diseases mentioned are extremely rare, which limits the number of volunteers for drug testing. Because this type of treatment has only been used for a relatively short time, there is almost no data on its longevity. With integrative gene therapy, the effects can last a lifetime, but with episomes, the effects can only last a few years. The timing of administration is also critical, as damage to some tissues is often irreversible. These include, in particular, diseases of nerve, muscle and pigment cells. Last but not least, the question of price is also important. Currently, the cost of gene therapy is in the hundreds of thousands of dollars or euros, mainly due to the difficult preparation, the high investment in drug development and the small market. However, it is certain that the price of gene therapy will gradually decrease, and it is expected that it will eventually become widely available.