Viral vectors, gene therapy products

For gene therapy products, the selection of the vector system is put to the test initially in the POC studies. These studies provide the first in vitro assessment of the choice of the viral or nonviral vector, the cell population to be used (if applicable), as well as the selection of a therapeutic transgene (if applicable) to treat a specific disease. 

Assessment of the immimogenic consequences of gene delivery should also be possible in the chosen species this is particularly important for the evaluation of viral vectors. There has been much discussion on the value of using a permissive host to assess potential human risk from these vectors. The natural route of exposure - i.e. respiratory for adenoviruses - traditionally used to assess permissivity, seems unnecessary to assess the safety of gene therapy products, particularly with the core study approach discussed above. Systemically-administered replication-competent adenovirus will infect (i.e. gain entry into the cell) and be pathogenic in numerous tissues (Duncan et al., 1978). Gene expression and viral replication depend on the species, route of exposure, and individual tissue susceptibility (Torres et al., 1996 Bett et al., 1962). Thus i.v. administration of viral vectors to mammalian test species should permit the evaluation of potential toxicity of widely-distributed vectors. 

It may also be appropriate to measure endpoints specific to the gene therapy product for example, antibodies to a viral vector. It may also be useful to devise specific endpoints, related to the biology of the gene product, which can be measured in the animal studies and will help in the conduct and evaluation of the clinical studies. 

An important safety issue of viral vectors is whether or not the recombinant viruses are able to replicate in the infected cells. Replication of viral vectors is unwanted in most gene-therapy approaches. Therefore, replication-defective vectors have been designed, which are able to perform only one initial infectious cycle within the target cell. In addition, replication-competent vectors have been designed, which are able to productively infect the target cell and to spread in the target tissue. 

Wu, N. and Ataai, M. 2000. Production of viral vectors for gene therapy applications. Current Opinion in Biotechnology 11(2), 205-208. 

Figure 6.1 An RNA viral gene therapy vector, engineered to carry the genetic information for a protein product, is incubated with blood or bone marrow cells removed from a patient. The foreign DNA sequence integrates into the cell s genetic material and when the cells are returned to the patient, they direct the production of the protein product. RNA gene therapy vectors are engineered to be unable to direct the production of new virus particles.

Validation of viral clearance is a major concern for products derived from mammalian cell culture and transgenic animals, as well as for viral vectors used for gene therapies. As we learn more and more about potential risks from newly found viruses, the requirements for validation increase. The increased concerns may be reflected in the number and types of viruses that are used for viral clearance studies. Both relevant and model viruses are used. A recent review of validation of the purification process for viral clearance evaluation provides further information on selection of viruses and performance of the studies [36],... 

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