Development of gene therapy vectors for combination radiotherapy of cancer

The combination of gene therapy with radiotherapy offers exciting prospects

for the future treatment of cancer. We have been developing radiation -

responsive vectors suitable for use with clinical radiation doses. The vectors

contain radiation - responsive CArG elements from the human Egr1 gene

regulating expression of heterologous genes. The radio -induction of these

novel synthetic promoters was assayed using the green fluorescent protein

(GFP) reporter gene. Clinically relevant doses of ionising radiation ( 1 – 3

Gy ) were able to induce GFP production 2 – 3 - fold. These promoters were

then used to drive the herpes simplex thymidine kinase/ ganciclovir

(HSVtk/ GCV) suicide gene system in tumour cell killing assays (Marples

et al., 2000; Gene Therapy 7:511 – 517 ). In order to improve efficacy, we

have recently examined the effects of CArG number, sequence and spatial

arrangement. This data has enabled us to define some important criteria for

CArG induction response at these low doses.

To provide long -term, constitutive gene expression and to markedly

enhance the level of tumour cell killing a novel ‘‘molecular switch’’

scheme based on Cre/ Lox recombination was adopted. This resulted in a

substantial enhancement of GFP production and tumour cell killing ( Scott

et al., 2000; Gene Therapy 7:1121 – 1125 ). To further improve the overall

efficiency of the switch scheme, the original dual - plasmid system has

now been replaced with a single vector bearing all the active components.

This vector is currently being tested in a U87 glioblastoma xenograft

model.