Today, there are a variety of gene-focused corrective strategies that have been developed for treating a wide range of clinical conditions that are caused due to genetic aberrations, such as adenosine deaminase severe combined immunodeficiency (ADA-SCID), -thalassemia, biallelic RPE65 mutation-associated retinal dystrophy, critical limb ischemia, head and neck squamous cell carcinoma, melanoma, peripheral artery disease and spinal muscular atrophy. Presently, more than 10 gene therapies have already been approved; examples of recently approved gene therapies include Collategene[1] (AnGes), Zolgensma[2] (AveXis), and Zynteglo[3] (bluebirdbio). Considering that such therapies are designed to address the root cause (at the genetic level) of a disease, a number of contemporary drug / therapy development initiatives are focused on gene correction and genome editing-based therapeutic interventions. It is also worth highlighting that the growing need for treatment options to cure a clinical condition, as opposed to treating disease-related symptoms only, has created a demand for more effective gene manipulation approaches.
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Gene therapies can be classified into ex vivo and in vivo therapies. An ex vivo gene therapy involves the modification of the target gene segment (within a cell / group of cells) outside the body. In this case, allogeneic (derived from donor organisms of the same species) or autologous (derived from the patients own body) cells may be used. Once isolated, the cells are grown and temporarily maintained under in vitro conditions. Subsequently, the cells are transduced / transfected with an appropriate viral / non-viral vector, which carries the therapeutic gene. The modified cells are then allowed to grow and proliferate in vitro, until they reach the are a pre-specified concentration, at which they are considered to be suitable for dosing. However, ex vivo gene therapies are efficient only if the therapeutic gene is stably incorporated inside the target site (within the genome of the target cell) and is constitutively expressed. This is dependent on the type of vector used.[4]
The route of administration of gene therapy depends on the purpose of the therapy and the target site. Some of the common routes of administration for genetic medicines are explained below:[5]
- Oral: This is the easiest and the most painless method of dosing; it is done in cases where there is a daily need for intake of the therapeutic agent. Cells in the gastrointestinal (GI) tract are transfected via the oral route. The drawbacks associated with this route of administration include the epithelial barrier and the acidic pH in the stomach, which affect the potency and transduction efficiency.
- Intravenous (IV): This is one of the preferred routes of administration for most of the gene therapies. However, they may need frequent dosing, causing widespread bio-distribution of vectors, and leading to the development of unwanted side effects. For instance, upon intravenous injection, adenoviral vectors have been shown to exhibit liver tropism. In addition, these vectors are known to elicit undesirable innate and adaptive immune responses, resulting in the production of neutralizing antibodies, which interfere with the therapeutic effect of the product.
- Local Administration: This route is used for the transfection of cells within a specific site / tissue / organ of interest. This can be carried out via vasculature and non-vasculature dependent routes of administration. Vasculature dependent routes involve the use of intra-arterial, intra-portal and retrograde intravenous routes. This is a highly selective method of delivering the gene medicine, but it may require cannulation, which can be painful. On the other hand, non-vasculature dependent routes involve the direct administration of naked DNA plasmids at the targeted site / organ.
- Organ Surface Routes: This method of gene delivery is generally used for intra-abdominal and intra-thoracic organs. This route enables the therapy to be targeted to diseased regions only. The key drawback of this route of administration is that it requires a laparoscopy procedure.
A gene therapy can be performed within (in situ) or outside (ex situ) the body of a host. In the case of in situ therapies, a suitable viral / non-viral vector, carrying the gene of interest, is injected directly into the part of the body that has the defective population of cells. Alternatively, for ex situ gene therapies, the blood or bone marrow is harvested from a patient and the immature cells are isolated from it. These cells are then transfected / transduced with a therapeutic gene and allowed to mature (for a specified period of time) in vitro. The transformed cells are then injected back into the bloodstream of the patient, where they move back into the bone marrow and mature rapidly. The cells bearing the desired (therapeutic) gene product grow and, in some cases, proliferate and eventually replace all defective cells (theoretically).
Concept of Gene Editing
The human genome refers to a complete library of genetic information and it consists of close to three billion DNA base pairs. The DNA polymers are maintained in the form of a set of 23 chromosome pairs. Each chromosome contains small sections of DNA, known as genes, which code for proteins. There are close to 20,000-25,000 human protein-coding genes, which helps in controlling synthesis of proteins. Therefore, genome is a complete set of instructions that is required by an organism to function properly. The advancements in genomics has helped the researchers to fully understand this huge amount of genomic data and its impact on fundamental biological processes and related diseases. For instance, the Human Genome Project, which was initiated in 1987, has helped the scientific community to understand the genetic basis of nearly 5,000 human disease conditions, as of today.
In order to effectively utilize the available genetic data for biomedical uses, the researchers have shifted their focus towards modification of human genome through multiple endonuclease technologies. This process is known as gene editing or genome engineering. It is a process of modifying a single gene or a set of genes within the genome of an organism by altering the nucleotide sequence using specialized molecular tools, such as artificially engineered nucleases or molecular scissors. A lot of genome editing techniques, such as Clustered Regularly Interspaced Palindromic Repeats (CRISPR), Zinc Finger Nuclease (ZFN) and transcription activator-like effector nucleases (TALENs) have been explored till date.
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[1] Collategene is the registered trademark of AnGes
[2] Zolgensma is the registered trademark of AveXis (a Novartis Company)
[3] ZYNTEGLO is the registered trademark of bluebird bio
[4] Source: http://www.genetherapynet.com/types-of-gene-therapy.html
[5] Source: https://www.intechopen.com/books/novel-gene-therapy-approaches/targeted-gene-delivery-importance-of-administration-routes
