For many years, cancer has been a subject of intense inquiry and debate. There has been intense research with the aim of a better understanding of gene therapy approaches. Chapman and Scala note that more than 60 percent of all clinical gene therapy trials in the world currently target cancer, which is a clear indication that there is an unmet medical need for novel therapies (2012). The intensity of the number of trials targeting cancer is further informed by the fact that current conventional cancer therapies that are used are largely ineffective and unsafe. There have been various gene therapy strategies that have been used for cancer treatment with examples including anti-angiogenic gene therapy, antisense, pro-drug activating suicide gene therapy, and the correction of gene therapy among other types. The main cancer types that have been a target of gene therapy include bladder, liver, brain, and breast, pancreatic, prostate, ovarian and renal cancer. Despite the improvement in the technologies used to combat cancer, there are only two cancer gene therapy products that have been approved for the market. The improvement of the body’s immune system with the use of gene therapy has also gained a lot of interest in recent years ( Feng et al., 2016 ). This paper seeks to discuss in detail gene therapy and its application in cancer treatment.
Gene Transfer Methods and Vectors Used For Gene Therapy
One of the major challenges in gene therapy is the administering of the appropriate amount of genetic material into target tissues or cells for the main purpose of maintaining gene expression for a specific period. Jenkins and McNair (2015) note that there are various ways that genetic materials can be delivered into their target cells or tissues. The main methods are viral, non-viral, yeast, and physical methods. Examples of physical methods include gene gun deliveries, electroporation, and ultrasound. The use of viral methods includes the delivery of genetic material into the cells through a biological vector. Non-viral methods, on the other hand, uses synthetic carriers such as liposomes and nanoparticles. Viral vectors are considered the most effective of all the gene delivery methods. An ideal gene transfer method should be able to accurately target a specific tissue with high transduction efficiency, and be able to sustain a stable gene expression minus any side effects or abnormal reactions. Kim et al. observe that currently, no gene introduction method can fulfill the above criteria. The results of the local injection of a vector are a limited but accurate effect area. On the other hand, the systemic administration of vector results in a system-wide expression. As a result, most experts have modified vectors and their methods of administration to achieve targeted delivery and to increase transduction efficiency. Most viral vectors already have natural tropism to specific cell types, which can be used for therapeutic approaches (2013).
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Viral vectors
According to Maschke, Gusmano, and Solomon (2017), some of the most commonly used viral vectors for gene transfer are baculoviruses, vaccinia associated viruses, adenoviruses, lenti, and retroviruses. The differences that exist among the above vectors arise from their immunogenicity, cell tropisms, transgene capacities, and the duration of the transgene expression. Apart from their origin, viral vectors can be further divided into non-integrating and integrating vectors. The main characteristic of non-integrating vectors is the fact that they cannot integrate their genome with the host genome. Major examples of non-integrating vectors include baculoviruses and adenoviruses. Integrative vectors can effectively integrate their genome with the host genome, and they include Lenti- and retroviruses as well as adeno associated viruses (AAV) ( Bernardes, Chakrabarty, & Fialho, 2013 ). It is vital to note that the expression of the transgene in non-integrating viral vectors is short term and it diminishes within a matter of days. Integrating vectors, on the other hand, lead to long term expression which can take months or even years. The fact that the integration of genes occasionally takes place in actively expressed sites has led to increased concerns regarding the safety of the vectors ( Hames, & Demir, 2015 ).
According to Jenkins and McNair (2015), another transfer approach that can be used in the delivery of genetic material is the ex vivo gene transfer method where the genetic material is introduced to the cell outside the patient and into autologous cells that were previously isolated. The genetic material is then re-introduced back to the patient. Jenkins and McNair further note that among all the delivery vectors used in gene therapy, adenoviruses are the most common (2015).
Non-viral vectors
The drawbacks that come with the use of viral vectors has forced experts to look for other ways of gene delivery. Some of the challenges experienced when using viral vectors include their immunogenic and inflammatory nature as well as their tendency to rapidly clear viral vectors from the bloodstream ( Li, Ou., Wang, & Tang , 2016 ). Non-viral vectors are the only viable alternatives to viral vectors. Naked plasmid DNA is the simplest form of a system that is non-viral. As Maschke, Gusmano, and Solomon discuss, there are many advantages associated with naked plasmid DNA. Foremost, it is cost effective in its formulation and production. It also has the lowest form of toxicity and does not exhibit any unwanted reactions. Despite its advantages, there are also many disadvantages associated with naked plasmid DNA comparison to viral-mediated gene transfer. For the purposes of improving the efficiency of transfection, lipids formulations have been created to condense plasmid DNA and to protect the degradation of DNA thus enhancing the transfection and uptake of plasmids (2017). The main advantage of the above formulations is that polymers can be easily designed to achieve certain properties. Despite their unlimited possibilities, the success of non-viral delivery systems in clinical applications has been limited mainly because they have not passed through the evolutionary process that viruses have undergone, which is represented by low transduction efficiencies in vivo. The success of non-viral gene therapy also depends on different barriers which are extra and intracellular. The barriers affect the efficacy of all the gene delivery systems as well as endosomal escape, gene expression, cellular uptake and nuclear uptake ( Non-viral gene delivery for cancer therapy, 2017 ).
Clinical Efficacy of Gene Therapy
Over the years, there have been various gene therapy approaches that involve using different transfer vectors that have been examined for cancer gene therapy. Examples include anti-angiogenic gene therapy, apoptosis, and immune modulation ( Patel et al., 2011 ). Of all the strategies that have been studied, only a few of them have been practically used within a clinical setting. One of the most common strategy in cancer gene therapy is the use of a commonly occurring mutation found in the p53 protein. A study conducted by Lang and other scholars involved the carrying out of a clinical trial with the use of an adenoviral vector encoding for the tumor suppressor gene TP53. The purpose of the clinical trial was to treat patients who regularly experienced recurrent malignant gliomas. Fifteen patients underwent intratumoral stereotactic injection of the adenoviral vector through an implanted catheter, after which the patients underwent en bloc resection of the tumor and the treatment of a post-resection cavity. The study could not assess tumor response, but it managed to demonstrate minimal toxicity. Additionally, no systemic viral dissemination was observed. The observation of the tumor specimens revealed transgene expressions that were limited close to the site of injection ( Zah, & Chen, 2017 ). Another study that was carried out is Gendicine which is a duplication-incompetent adenovirus encoding used for the TP53, and it is used in the treatment of different types of cancers. In a clinical trial, Gendicine showed therapeutic potential since none of the patients that were treated with Gendicine experienced tumor relapse within five years after receiving the treatment. The treatment was also safe, with 32 percent of the people who participated in the clinical study indicating no adverse side effects except for the headaches. When used together with radiotherapy, 64 percent of the patients showed complete regression ( Yoon et al., 2011 ).
Gene Therapeutic Approaches to Stimulate the Immune System
Over the last couple of years, much attention has been diverted to immunotherapy whose main objective is to increase the recognition of tumor-associated antigens (TAA’s). There are however several challenges that come with immunotherapies such as the natural tolerance towards TAA’s and the tumor microenvironment that strongly immunosuppressive ( Morris et al., 2013 ). Of great importance is the genetic engineering which has been the subject of intensive research in recent years. One of the examples of the genetic engineering of T cells involves the introduction of a T cell receptor (TCR) against an identifiable TAA. A clinical report by Morgan is a perfect example of such an approach where high levels of circulating were observed as well as engineered PBLs in two patients one year after the infusion was done on the patients who showed objective regression of metastatic melanoma lesions ( Lee, 2012 ). Another clinical trial which involved the transducing of T cells with TCR against antigen NY-ESO-1 resulted in the revelation of a cancer antigen expressed in different cancers ( Morris et al., 2013 ).
Herman investigated another method that can be used to improve an anti-tumoral immune response through a clinical trial involving patients locally advanced pancreatic cancer ( Hao et al., 2015 ). The results of the study established that the standard of care and gene therapy did not lead to a survival benefit in patients even though they were safe. In addition to the above approaches, the use of pro-drug activating suicide gene therapy remains an approach that has been extensively studied in the clinic and pre-clinically for cancer therapy ( Hao et al., 2015 ).
Pro-drug Activating Suicide Gene Therapy
The fundamental principle behind pro-drug activating suicide is the introduction of a transgene encoding into the tumor for an enzyme that is present in a very inactive form or absent in mammalian cells ( Feng et al., 2016 ). The therapeutic gene is expressed when the enzyme produced by the transduced cells manage to successfully convert the inactive pro-drug that has been administered into its active form, thereby initializing the death of cells. It is therefore clear that the bystander effect is crucial for therapeutic success. Based on the same concept, brain tumors can be found to possess several features that make them responsive to prodrug-activating gene therapy. Some of the features include their single, localized and rapidly dividing cells. Also, in most instances recurrence occurs near the original lesion ( Jenkins, & McNair, 2015 ). However, the first results that were achieved from the studies were not promising mainly due to the problem of transduction efficiency. The poor transduction efficiency in early studies can be attributed to the use of retroviral vectors. The use of adenoviral vectors in the past has proved to have high transduction efficacy and transgene expression compared to the use of retroviral vectors. The differences between the two vectors emerge from the fact that adenoviruses transduce both quiescent and dividing cells ( Feng et al., 2016 ). In one study, Sandamair compared the efficacy of the retrovirus packaging cells for HSV –tk with the adenovirus-mediated HSV-tk gene therapy used for treating primary gliomas. The findings of the study established that the mean survival time in the adenovirus HSV-tk group was longer at 15 months compared to the mean survival time in the retrovirus-packaging cell group ( Kim et al., 2013 ).
The Safety of Gene Therapy
Despite a few isolated cases, the use of gene therapy is generally safe according to several human gene therapy trials. However, regardless of the satisfactory safety levels of human gene therapy, patients might still experience some unwanted immune responses since the viral vectors that are used are human pathogens. The human pathogens might encounter antibodies against the viral vector. Li et al . note that generally, there has been a scarcity of data on the safety of viral vectors in humans. Many meta-analyses have been done for adenoviruses thus proving its safety on humans. The tolerance of the human body towards adenoviral vectors is within safety limits, and the side effects are also acceptable since they are mild without any serious effects (2016).
There have been various methods that have been used to improve the safety of gene therapy. One of the methods involves the improvement of the targeting strategies to improve the accuracy of the delivery of gene transfer vectors thus improving the time and efficacy of gene expression. As Maschke et al. (2017) discuss, one of the major drawbacks of gene therapy is the low transduction efficiency that they normally exhibit as well as the absence of specificity to their target cells hence improving specificity would lead to a better safety profile. Several technologies such as the re-engineering of viral vectors with the use of epitope insertion, molecular evolution, and chemical modification have been used with the main aim of improving the transduction efficacy or specificity.
According to Chapman and Scala (2012), the role of the activation of T and B cells and innate immunity continues to be a subject to intense research. The research is focused on the likely effects of gene transfer vectors and their expressed proteins on local lymph nodes. The acknowledgment of the pre-existence of neutralizing antibodies has led to the conclusion that they can negatively affect transduction efficiency. As a result, Chapman and Scala further note that viral surface proteins have been replaced, adjusted, or removed with the main aim of improving specificity. However, the main disadvantage that comes with this is that the different modifications that were carried out led to low vector titers during the production of lentivirus. Further observations have also revealed that targeting can also affect the entry of the vector into the cell (2012).
Apart from targeting viral vectors to specific cells, pseudotyping is the other method that can be used to widen tropism of the viral vector to other cells. A perfect example is when lentiviruses and retroviruses are pseudotyped together with Vesicular Stomatitis virus G-protein (VSV-G) to broaden their tropism and improve their production yield ( Feng et al., 2016 ).
Another approach that can be used to improve the specificity of viral vectors to their target cells is through the use of conditional promoters. One perfect example is the use of hypoxia-specific regulatory systems which aim to induce and restrict gene expression to the ischemic tissues. Hypoxia-specific regulatory systems have been used in various ischemic disease models such as stroke, spinal code injury, and ischemic myocardium. Additionally, it can also be used in cancer gene therapy ( Patel et al., 2017 ).
From the above discussion, it is evident that the development and manufacturing of gene transfer vectors, the safety of gene transfer vectors are automatically improved. For instance, gutless adenoviral vectors are vectors, in the case where all the other genes are removed except for those that are critical for virus protection. The genes that are removed are replaced by the gene of interest facilitated by a suitable promoter ( Morris et al., 2013 ). From the above discussions, it is evident that gene therapy is still safe in the treatment of cancer.
Conclusion
In summation, gene therapy is one of the most interesting approaches that can be used in the treatment of various diseases including cancer. There have been various gene therapy strategies that have been used for cancer treatment with examples including anti-angiogenic gene therapy, antisense, pro-drug activating suicide gene therapy, and the correction of gene therapy among other types. The main cancer types that have been a target of gene therapy include bladder, liver, brain, and breast, pancreatic, prostate, ovarian and renal cancer. There are various ways that genetic materials can be delivered into their target cells or tissues. The main methods are viral, non-viral, yeast, and physical methods. Examples of physical methods include gene gun deliveries, electroporation, and ultrasound. Non-viral methods use synthetic carriers such as liposomes and nanoparticles. Viral vectors are considered the most effective of all the gene delivery methods. The current gene therapy protocols are limited to ex vivo gene transfer methods. There are however several challenges that come with immunotherapies such as the natural tolerance towards TAAs and the tumor microenvironment that strongly immunosuppressive. Of great importance is the genetic engineering which has been the subject of intensive research in recent years. The focus needs to be directed to not only vector development itself but also towards the development of the vectors with high efficiency.
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