Molecular approaches to chemo-radiotherapy

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.