A novel suicide gene therapy using iNOS

The role of NO in tumours is extremely complex; depending on the concentration it has major effects on vascular tone, endothelial proliferation, cell viability and radiosensitivity. As such, it is not surprising that its manipulation has been identified by many investigators as an exciting target for cancer therapy. The use of a gene therapy strategy utilising the iNOS gene to produce NO offers the potential for targeting NO production specifically within the tumour volume, combined with high NO-generating capacity. We have shown that iNOS gene therapy driven by a strong constitutive promoter (CMV) results in significant growth delay of the murine RIF-1 tumour in vivo. Due to the potent nature of NO any gene therapy strategy will require at lest one level of specificity. We have used the X-ray inducible WAF1/iNOS construct to confine NO generation to within the radiation field i.e. the tumour. A single injection of the X-ray inducible WAF1/iNOS construct followed, 16 h later, by an induction dose of 4 Gy X-rays resulted in significant enhancement of the cell killing effect of subsequent therapeutic doses of X-rays in the same tumour model. The effect was equivalent to a sensitiser enhancement ratio of ∼2.0, half the radiation dose being required to produce the biological effect when iNOS gene therapy was combined with radiation. Intra-tumoural injection of the WAF1/iNOS construct followed by 4 Gy X-rays also resulted in significant radiosensitisation in the HT29 xenograft model. We have so far demonstrated the cytotoxic and radiosensitising potential of iNOS gene therapy, however there are further benefits to the use of NO as an anti-cancer agent. These include anti-angiogenic effects and inhibition of tumour metastasis. Further studies will enable the design of a clinically appropriate protocol to be established