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Positive correlations between NOS expression and tumour grade have been reported for a wide range of human cancers, and the associated increase in nitric oxide (NO) production is thought to facilitate tumour angiogenesis. Furthermore, recent studies have shown that pharmacological inhibition of NO can significantly enhance the effectiveness of the vascular disrupting agent ZD6126. In this study tumours derived from a pair of isogenic DLD-1 colon adenocarcinoma cell lines, one of which constitutively overexpresses iNOS, were utilised to test the hypothesis that tumour expression of iNOS a) increases tumour angiogenesis and b) modulates the anti-tumour activity of ZD6126. DLD-1 clone 19 cells, overexpressing iNOS, and empty vector clone C2 cells were used. Overexpression of iNOS by clone 19 cells was confirmed by the Griess assay. Clone 19 murine xenografts grew more rapidly than C2 tumours. The perfusion of size-matched tumours was assessed by Hoechst 33342 uptake. The area of the tumour section with Hoechst 33342 fluorescence was determined and expressed as a percentage of the area of the whole tumour section (mean Hoechst perfused area, mHPA). Tumour perfusion was significantly greater in the clone 19 tumours (##p<0.001). 24 hours after treatment with an intermediate 100mg/kg dose of ZD6126 resulted in a significant reduction in the perfusion of only the C2 tumours compared with control, whereas a higher dose (200mg/kg) significantly reduced the perfusion of both clone 19 and clone C2 tumours (**p<0.001). In a separate cohort of tumours, tissue sections were assessed for necrosis 24 hours after treatment, using a scale from grade 1 (0-10% necrosis) to grade 10 (>90-100% necrosis). Whilst the median necrosis score for the C2 tumours increased in a dose dependent manner, intermediate doses of ZD6126 did not induce a similar degree of necrosis in the clone 19 tumours. Taken together, the data highlight the role of NO in tumour angiogenesis and progression, and support the concept that expression of NOS may provide some protection against induction of tumour cell necrosis following ZD6126-induced vascular disruption.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]