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Despite all the hype, it is important to note that, by mid-2002 at least, only a single nucleic acid-based product has been approved for medical use (an antisense-based product, discussed later). No gene therapy-based product had been approved for general medical use by that time.
The fundamental principle underpinning gene therapy is theoretically straightforward, but difficult to satisfactorily achieve in practice. The principle entails the stable introduction of a gene into the genetic complement of a cell, such that subsequent expression of the gene achieves a therapeutic goal. The potential of gene therapy as a curative approach for inborn errors of metabolism and other conditions induced by the presence of a defective copy of a specific gene (or genes) is obvious.
An increased understanding of the molecular basis of various other diseases, including cancer, some infectious diseases (e.g. AIDS) and some neurological conditions, also suggest a role for gene therapy in combating these. Indeed, well over half of all gene therapy trials conducted to date aim to treat cancer. Table 11.1 lists the major disease types for which a gene therapy treatment is currently being assessed in clinical trials. The first such trial was initiated in the USA in 1990. Thus far, over 400 different clinical studies have been or are being undertaken, involving ca. 6000 patients worldwide. Despite initial enthusiasm, only a handful of such studies have revealed any therapeutic benefit to the patient and, thus far, no complete, permanent cures have been recorded.
Moreover, gene therapy — like all other medical interventions — is not without associated risk. A US patient died in 1999 as a result of participating in a gene therapy-based trial. Even more disturbingly, the ensuing FDA investigation unearthed allegations that at least six other
Biopharmaceuticals: Biochemistry and Biotechnology, Second Edition by Gary Walsh John Wiley & Sons Ltd: ISBN 0 470 84326 8 (ppc), ISBN 0 470 84327 6 (pbk)
Table 11.1. Some diseases for which gene-based therapeutic approaches are currently being appraised in clinical trials. Many of these examples are discussed in more detail later in this chapter
deaths attributed to clinical trial treatments had gone unreported to the regulatory agency and, further, that only a fraction of serious adverse effects had been reported. As a result, regulatory regulation and monitoring of gene therapy-based trials has been increased.
Such disappointing results do not reflect any flaw in the concept of gene therapy. Instead, they reflect the need to develop more effective technical means of accomplishing gene therapy in practice. Such initial studies have highlighted the technical innovations required to achieve successful gene transfer and expression. These, in turn, should render future (‘second-generation’) gene therapy protocols more successful.
Basic approach to gene therapy
The basic approach to gene therapy is outlined in Figure 11.1. The desired gene must usually be packaged into a vector system capable of delivering it safely inside the intended recipient cells. A variety of vectors can be used to effect gene transfer. These include both viruses (particularly retroviruses) and non-viral carriers, such as plasmid-containing liposomes/lipoplexes (Table 11.2). Each such vector has its own unique set of advantages and disadvantages, as discussed subsequently in this chapter.
Once assimilated by the cell, the exogenous nucleic acid must now travel or be delivered to the nucleus. In some cases, the mechanism by which this transfer occurs is understood, at least in part (e.g. in the case of retroviral vectors). In other cases (e.g. use of liposome vectors, or naked DNA), this process is less well understood. At a practical level, gene therapy protocols may entail one of three different strategies (Figure 11.2).
The in vitro approach entails initial removal of the target cells from the body. These are then cultured in vitro and incubated with a vector containing the nucleic acid to be delivered. The genetically altered cells are then re-introduced into the patient’s body. This approach represents the most commonly adopted protocol to date. In order to be successful, however, the target cells must be relatively easy to remove from the body and reintroduce into the body. Such in vitro approaches have successfully been undertaken, utilizing various body cell types, including blood cells, stem cells, epithelial cells, muscle cells and hepatocytes.
A second approach involves direct injection/administration of the nucleic acid-containing vector to the target cell, in situ in the body. Examples of this approach have included the direct injection of vectors into a tumour mass, as well as aerosol administration of vectors (e.g. containing the cystic fibrosis gene) to respiratory tract epithelial cells.
While less complicated than the in vitro approach, direct in situ injection of vector into the immediate vicinity of target cells is not always feasible. This would be true, for example, if the