Download (direct link):
• a requirement for improved, more target-specific vector systems;
• a requirement for a better understanding of how cancer cells evade the normal immune response;
• for ethical reasons, most patients treated to date were suffering from advanced and widespread terminal cancer (i.e. little/no hope of survival if treated using conventional
Table 11.6. Some specific cancer types for which human gene therapy trials have been initiated. Although several of the strategies listed in Table 11.5 are being employed in these trials, many focus upon the introduction of various cytokines into the tumour cells themselves in order to attract and enhance a tumour-specific immune response
Breast cancer Colorectal cancer
Malignant melanoma Tumours of the CNS
Ovarian cancer Renal cell carcinoma
Small-cell lung cancer Non-small-cell lung cancer
therapies); cancers at earlier stages of development will probably prove to be more responsive
to gene therapy.
One of the earliest cancer gene therapy trials attempted involved the introduction of the TNF gene into tumour-infiltrating lymphocytes (TILs). The rationale was that if, as expected, TIL cells reintroduced into the body could infiltrate the tumour, TNF synthesis would occur at the tumour site, where it is required. This approach has since been broadened, by introducing genes coding for a range of immunostimulatory cytokines (e.g. IL-2, IL-4, IFN-g and GM-CSF) into TILs. A variation of this approach involves the introduction of such cytokine genes directly into tumour cells themselves. It is hoped that reintroduction of such cytokine-producing cells into the body will result in a swift and effective immune response — killing the tumour cells and vaccinating the patient against recurrent episodes. In most instances so far, this strategy has been carried out in practice by removal of the target cells from the body, culture in vitro, introduction of the desired gene (mainly using retroviral vectors), followed by reintroduction of the altered cells into the body.
An alternative anti-cancer strategy entails insertion of a copy of a tumour suppresser gene into cancer cells. For example, a deficiency in one such gene product, p53, has been directly implicated in the development of various human cancers. It has been shown in vitro that insertion of a p53 gene into p53-deficient tumour cells induces the death of such cells. A weakness of such an approach, however, is that 100% of the transformed cells would have to be successfully treated to fully cure the cancer.
Yet another strategy that may prove useful is the introduction into tumour cells of a ‘sensitivity’ gene. This concept dictates that the gene product should harbour the ability to convert a non-toxic pro-drug into a toxic substance within the cells —thus leading to their selective destruction. The model system most used to appraise such an approach entails the use of the thymidine kinase gene of the herpes simplex virus (Figure 11.11).
A different gene therapy-based approach to cancer entails introduction of a gene into haemopoietic stem cells in order to protect these cells from the toxic effects of chemotherapy. Most cancer drugs display toxic side effects, which usually limits the upper dosage levels that can be safely administered. One common toxic side effect is the destruction of stem cells. If these cells could be protected or made resistant to the chemotherapeutic agent, it might be possible to administer higher concentrations of the drug to the patient. In practice, such a protective effect could be conferred by the multiple drug resistance (type 1; MDR-1) gene product. This is often expressed by cancer cells resistant to chemotherapy. It functions to pump a range of chemotherapeutic drugs (e.g. daunorubicin, taxol, vinblastine, vincristine, etc.) out of the cell. Animal studies have confirmed that introduction of the MDR-1 gene into stem cells subsequently protects these cells from large doses of taxol. This approach is now being appraised in patients receiving high-dose chemotherapy for a range of cancer types, including breast and ovarian cancer and brain tumours.
Gene therapy and AIDS
It is likely that gene therapy will prove useful in treating a far broader range of medical conditions than simply those of inherited genetic disease and cancer. Prominent additional disease targets are those caused by infectious agents, particularly intracellular pathogens such as HIV. The main strategic approach adopted entails introducing a gene into pathogen-susceptible
NUCLEIC ACID THERAPEUTICS 487
Figure 11.11. Schematic representation of the therapeutic rationale underpinning the introduction of a ‘sensitivity’ gene into tumour cells in order to promote their selective destruction. As depicted in (a), the gene product should be capable of converting an inactive pro-drug into a toxic drug, capable of killing the cell. A specific example of this approach is presented in (b): introduction of the herpes simplex thymidine kinase (HSVtk) gene confers sensitivity to the anti-herpes drug, Ganciclovir (GCV) on the cell. GCV is converted by HSVtk into a monophosphorylated form (GCV-MP). This, in turn, is phosphorylated by endogenous kinases, yielding ganciclovir triphosphate (GCV-TP). GCV-TP induces cell death by inhibiting DNA polymerase. A potential advantage of this system is that some adjacent tumour cells (which themselves lack the HSVtk gene) are also destroyed. This is most likely due to diffusion of the GCV-MP or GCV-TP (perhaps via gap junctions) into such adjacent cells. This so-called ‘bystander effect’ means that all the transformed cells in a tumour would not necessarily need to be transduced for the therapy to be successful