Molecular approaches to chemo-radiotherapy

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Abstract

Although radiotherapy is used to treat many solid tumours, normal tissue tolerance and inherent tumour radioresistance can hinder successful outcome. Cancer gene therapy is one approach being developed to address this problem. However, the potential of many strategies are not realised owing to poor gene delivery and a lack of tumour specificity. The use of treatment-, condition- or tumour-specific promoters to control gene-directed enzyme prodrug therapy (GDEPT) is one such method for targeting gene expression to the tumour. Here, we describe two systems that make use of GDEPT, regulated by radiation or hypoxic-responsive promoters. To ensure that the radiation-responsive promoter is be activated by clinically relevant doses of radiation, we have designed synthetic promoters based on radiation responsive CArG elements derived from the Early Growth Response 1 (Egr1) gene. Use of these promoters in several tumour cell lines resulted in a 2–3-fold activation after a single dose of 3 Gy. Furthermore, use of these CArG promoters to control the expression of the herpes simplex virus (HSV) thymidine kinase (tk) gene in combination with the prodrug ganciclovir (GCV) resulted in substantially more cytotoxicity than seen with radiation or GCV treatment alone. Effectiveness was further improved by incorporating the GDEPT strategy into a novel molecular switch system using the Cre/loxP recombinase system of bacteriophage P1. The level of GDEPT bystander cell killing was notably increased by the use of a fusion protein of the HSVtk enzyme and the HSV intercellular transport protein vp22. Since hypoxia is also a common feature of many tumours, promoters containing hypoxic-responsive elements (HREs) for use with GDEPT are described. The development of such strategies that achieve tumour targeted expression of genes via selective promoters will enable improved specificity and targeting thereby addressing one of the major limitations of cancer gene therapy.

Section snippets

Combination gene therapy and radiotherapy

Radiotherapy is the treatment modality of choice for most solid malignancies. However, the tolerance of surrounding normal tissues to treatment-induced injury often restricts the dose that can be delivered to the tumour to achieve cure. This problem can be partly overcome using physical techniques such as conformal or intensity modulated radiotherapy that deliver the dose to a more precisely defined tumour volume. An alternative approach is to combine radiotherapy with a pharmacological- or

Radiation-mediated gene therapy (RMGT)

Radiation-mediated gene therapy exploits the fact that in the majority of patients receiving radiotherapy the radiation is directed to the tumour volume, providing some degree of tumour localisation for controlling the expression of therapeutic genes. Temporal and spatial control of gene expression can therefore be achieved for any genes delivered to the tumour and for any tumour types treated with radiotherapy 5, 6, 7.

Hypoxia-mediated gene therapy

The presence of hypoxia is a negative prognostic indicator for outcome following radiotherapy in a range of human tumour sites 59, 60, 61, 62, 63. The absence of a cell killing component that can be attributed to radiation-mediated oxygen radicals is unlikely to be the only reason for the resistance of hypoxic tissue to the lethal effects of radiotherapy [64]. Resistance may also arise from modifications to gene expression induced as a direct consequence of the hypoxic environment, such as a

Other gene therapy approaches

A number of radiotherapy and gene therapy strategies have been developed. High therapeutic potential was recently demonstrated for radiation therapy in combination with a trimodal approach consisting of a replication-competent oncolytic adenovirus containing a cytosine deaminase/HSVtk fusion gene [42]. This combination of radiation therapy, lytic viral therapy and double suicide gene therapy produced significant tumour regression in an experimental C33A tumour xenograft model.

Manipulating the

Acknowledgements

BM and SDS were supported by grants from the UK Cancer Research Campaign.

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    Now at: Department of Radiation Oncology, Wayne State University and Karmanos Cancer Institute, 4100 John R. Detroit, MI 48210, USA.

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