Gene therapy

The fundamental principle underpinning gene therapy is theoretically straightforward, but difficult to achieve in practice satisfactorily. 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.

Pharmaceutical biotechnology: concepts and applications Gary Walsh © 2007 John Wiley & Sons, Ltd ISBN 978 0 470 01244 4 (HB) 978 0 470 01245 1 (PB)

Table 14.1 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.

Cancer, various forms Cystic fibrosis

Familial hyper-cholesterolaemia

AIDS

Haemophilia

Severe combined immunodeficiency

Gaucher's disease diseases (SCID) aj-antitrypsin deficiency

Purine nucleoside phosphorylase deficiency CGD

Rheumatoid arthritis

Peripheral vascular disease

An increased understanding of the molecular basis of various other diseases, including cancer, some infectious diseases (e.g. AIDS) and some neurological conditions, also suggests a role for gene therapy in combating these. Indeed two-thirds of all gene therapy trials conducted to date aim to treat cancer. Table 14.1 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 1989. Thus far, some 1200 different clinical studies have/are being undertaken worldwide. The majority (estimated at 67 per cent) have/are being undertaken in the USA, with most of the remaining trials being undertaken in Europe (mainly in the UK and in Germany). The majority of trials (62 per cent) are in early stage (phase I) and only some 2.2 per cent of all trials have reached phase III. Despite the initial enthusiasm, only a handful of such studies have revealed a therapeutic benefit to the patient.

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 one such trial. Even more disturbingly, the ensuing FDA investigation unearthed allegations that at least six other 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 trials has been increased. Further serious adverse effects, including some fatalities, have been reported subsequently, as discussed later.

Such disappointing results do not reflect any flaw in the concept of gene therapy. They instead reflect the need to develop more effective technical means of accomplishing gene therapy in practice. These 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.

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