OXFORD BIOMEDICA

New Gene Delivery Technology for Gene Therapy

The most fundamental requirement for gene therapy to be successful is that a therapeutic gene can be effectively delivered to a target cell. Once delivered that gene must enter the nucleus of the cell where it will act as a template for the production of a protein molecule. The protein molecule then exerts the primary therapeutic effect. This may be, for example, cell killing in the case of tumour therapy or cell preservation in the case of neurodegenerative disease.

There are several ways to get genes into cells. The most efficient of these uses disabled, engineered viruses. These systems are efficient because viruses have evolved over long periods of time to deliver their own genes to cells. Whenever, we get a viral disease, be it a cold or AIDS, the particular virus concerned is placing its genes into our cells in order to reprogramme our cells to produce more virus. When we use viruses for gene therapy we disable them so that they are unable to cause disease and we engineer them in such a way that they pick-up and deliver the genes of our choice rather than their own genes. These derivatives of viruses that are used for gene delivery are known as viral vectors.

The most frequently used viral vectors are of two types. The vectors based on adenovirus are generally used for therapeutic strategies that require the therapeutic gene to be active for only a short time. Gene delivery by adenoviruses is very efficient but because the gene does not become integrated into the chromosomes of the target cell the gene is lost over time. This is not a disadvantage for some therapeutic strategies such as cell destruction in the treatment of some cancers, restinosis or inflammatory disease. However, it is a disadvantage where sustained gene activity is required for many weeks or months such as in the treatment of some tumours, neurodegenerative disease and HIV infection. In cases such as these the second major type of vector is generally used and this is based on the retrovirus, murine leukaemia virus (MLV). When genes are delivered by derivatives of MLV they become integrated into the chromosomes of the target cell and are maintained for as long as that cell remains alive. Gene activity is easy to control and continues over long periods of time. Many clinical trials have been conducted with these MLV-based systems and they have been shown to be well tolerated with no adverse side effects.

One of the major differences between adenovirus vectors and MLV vectors is that the former can deliver genes to cells that are not multiplying by cell division whereas the latter cannot. Until recently this has meant that gene therapy strategies that demand long term gene activity in cells that are not dividing have not been feasible. Examples of important target cells that do not divide are neurones, certain cells of the immune system and certain epithelial cells. In addition there are many cells in a human body that do divide but do so very slowly. These slowly dividing cells are also poor recipients of genes delivered by MLV-based vectors and they include cells within solid tumours and some non-neuronal cells in the brain. In short certain aspects of gene therapy have been limited by the properties of MLV-based vectors. Vectors based on lentiviruses solve this problem.

Lentiviruses are a subgroup within the general family of retroviruses but they are distinct from the MLV-like viruses in that they are able to infect non-dividing cells. The best studied of the lentiviruses is HIV and when the observation was made, about 10 years ago, that HIV could infect terminally differentiated macrophages, which do not divide, there was a move within the research community to develop gene delivery vectors from HIV. There were a number of early technical difficulties and the first generation of vectors could not be used in the clinic as they had the potential to generate infectious HIV. Over the past two years we have seen new HIV-based vectors emerge that are severely disabled containing only the few HIV components that are required for efficient gene delivery to non-dividing cells. These so-called minimal vectors are now candidates for gene delivery vehicles for clinical use in gene therapy. The major developers of this technology are Oxford BioMedica in the UK and Cell Genesys in the USA. Both companies have shown that minimal HIV-based vectors are able to deliver genes to a range of non-dividing cells including neurones in the brain.

On the 9th of March, this year, the National Institutes of Health (NIH) and the Food and Drug Administration (FDA) of the USA called a meeting of experts in the lentivirus field to describe the new technologies and to discuss the regulatory issues surrounding the use of lentivirus vectors in human gene therapy. The meeting was very positive and recognised the huge potential of this new technology. Representatives from the FDA were actively considering the mechanisms by which HIV-based vectors could be used for a range of diseases. A key issue was safety and the extent to which a derivative of HIV would be acceptable as a therapeutic agent. A consensus report from the meeting is due shortly.

Although most of the presentations in the meeting addressed aspects of HIV-based vectors there were two presentations, one from Oxford BioMedica and one from the University of North Carolina, that described a lentivirus vector system based on equine infectious anaemia virus (EIAV). EIAV causes a mild disease in horses but is not associated with any disease in man. Oxford BioMedica described an advanced minimal vector system based on EIAV which, in side by side experiments, delivered genes to brain and other non-dividing cells at least as efficiently as HIV-based systems. The meeting considered, therefore, the relative merits of the two systems and addressed the question of whether the use of HIV-based vectors could be justified given that a non-HIV system was available.

Oxford BioMedica intends to develop both types of vector. HIV-based vectors will be used for the treatment of HIV infection. There are substantial advantages in using HIV because it will access the correct target cells and it provides the potential for the therapeutic gene to be distributed to target cells by the virus present in an HIV-infected individual. However, it likely that HIV-based vectors will face a number of serious regulatory hurdles that will not be applied to EIAV systems. Oxford BioMedica will campaign the use, therefore, of its EIAV technology for diseases other than AIDS including some tumour therapies and neurodegenerative disease.

In summary, the new lentivirus vectors provide an important new tool for gene therapy, solving several key problems that have limited the field. It is clear that the regulatory authorities in the USA are prepared to consider the use of these new systems in the clinic and so the path is open for the commercialisation of the technology.

Sir Brian Richards CBE
Chairman