Further into the future, we hope to genetically modify the fibre protein itself, to provide novel, useful specificities for cancer therapy. There are similar possibilities for retargeting of retroviral vectors, which has been attempted by fusing the SU envelope glycoprotein with single chain antibodies or polypeptides specific for growth factor receptors. A final level of specificity can be provided by appropriate selection of regulatory DNA sequences to control the transcription of the therapeutic gene. A number of genes are frequently expressed at relatively high levels in cancers compared with other tissues in the adult. For reasons which are poorly understood, the genes become active again in the malignant cells of certain cancers.
Prostate cancer is a common tumour in men, the incidence of which appears to be increasing.
These regulatory sequences could therefore be useful to control the expression of therapeutic genes for treatment of prostate cancer. Although in this case the therapeutic gene could also be expressed in normal prostate cells, this would not matter as the prostate is not a vital organ and could be sacrificed in the context of life-threatening disease. The specific targeting of gene therapy can, in principle, be achieved at different levels. In the meantime, selection of appropriate promoter and enhancer sequences can restrict expression in inappropriate cell types which take up the gene, and clinical applications will be limited to those where the route of delivery can provide a degree of targeting. Alternatively, the extra amplification provided by tumour-specific, replicating viral vectors might ultimately prove essential for a truly systemic cancer gene therapy.