Greco, Olga; (2002) Development of a novel enzyme/prodrug system for hypoxia- and radiation-mediated gene therapy of cancer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Solid tumours often present a disorganised and inadequate vascular supply that results in multiple foci of low oxygen tension (hypoxia). Despite being an adverse prognostic factor, hypoxia represents a physiological difference that can be exploited for selective cancer treatment. In particular, high tumour specificity may be achieved by hypoxia-mediated gene-directed enzyme/prodrug therapy (GDEPT), whereby the target cells are genetically modified to synthesise an enzyme able to convert a prodrug into a cytotoxin. In the present work a novel system consisting of the horseradish peroxidase (HRP) and the non-toxic plant hormone indole-3-acetic acid (IAA) is proposed as an enzyme/prodrug combination for cancer gene therapy. The cytotoxic potential of HRP/IAA GDEPT and the induction of a bystander effect were demonstrated in vitro under normoxic as well as hypoxic conditions. Further improvements were achieved by adopting novel IAA derivatives. The chemical agents and the cellular targets involved in HRP/IAA-induced toxicity are yet to be identified, but the results presented indicate that an apoptotic pathway may be activated. With a view to combining hypoxia-targeted GDEPT with a standard radiotherapy protocol, the interaction of HRP/IAA with therapeutically significant doses of ionising radiation (IR) was evaluated, and oxic and anoxic enhancement of IR toxicity was observed. Finally, to limit prodrug activation to the tumour site, selective transgene expression in hypoxic and irradiated cells was demonstrated by the use of synthetic promoters containing hypoxia- and IR-responsive regulatory elements. Taken collectively, the results indicate that HRP and IAA represent an effective system for use in hypoxia- and radiation-mediated cancer gene therapy. A combinational approach, exploiting hypoxic and radiation-response elements to control HRP gene expression, may overcome some of the limitations of tumour biology associated with conventional radiotherapy regimes.

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