Modelling the enhancement of fractionated radiotherapy by gene transfer to sensitize tumour cells to radiation

Background and purpose: Several strategies now exist for the use of gene transfer methodologies to sensitize tumour cells to radiation. These include the transfection of genes synthesizing cytokines, p53 gene replacement and methods based on the use of HSV-tk and gangcyclovir. Very recently, the sequencing of radioprotector or repair genes, such as ATM, Ku80 and XRCC2, has made it possible to consider the design of gene transfer strategies resulting in protector gene knock-out. Selectivity of transfected gene expression might be achieved by use of tissue-specific promoters or by the trophism of viral vectors. The purpose of this study was to evaluate the probable efficacy of such strategies. Methods: We have modelled gene transfer-mediated radiosensitization of tumour cells during radiotherapy, focusing on anti-protector gene strategies, to explore the role of transfection frequency, sensitizing efficacy, transfection stability, untransfectable subpopulations, the timing of gene therapy and the treatment schedule structure. Results: We predict a substantial therapeutic benefit of gene transfer treatment (with at least weekly transfection) which modifies cellular radiosensitivity by a factor of 1.5 or more, despite modest efficiency of cellular transfection (e.g. 50%), transient retention of the transfected gene (e.g. 2-day half-life) and the existence of a small minority (e.g. 1%) of untransfectable cells. Conclusions: The analysis shows repeated administration of gene transfer treatment to be obligatory and implies that the existence of untransfectable minority subpopulations (i.e. cells inaccessible to the vector) will be the major limiting factor in therapy. Experimental work is needed to confirm these predictions before clinical studies begin. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.