Determinants of anti-vascular action by combretastatin A-4 phosphate: roleof nitric oxide

The anti-vascular action of the tubulin binding agent combretastatin A-4 ph osphate (CA-4-P) has been quantified in two types of murine tumour, the bre ast adenocarcinoma CaNT and the round cell sarcoma SaS. The functional Vasc ular volume, assessed using a fluorescent carbocyanine dye, was significant ly reduced at 18 h after CA-4-P treatment in both tumour types, although th e degree of reduction was very different in the two tumours. The SaS tumour , which has a higher nitric oxide synthase (NOS) activity than the CaNT tum our, showed similar to 10-fold greater resistance to vascular damage by CA- 4-P. This is consistent with our previous findings, which showed that NO ex erts a protective action against this drug. Simultaneous administration of CA-4-P with a NOS inhibitor, N-omega-nitro-L-arginine (L-NNA), resulted in enhanced Vascular damage and cytotoxicity in both tumour types. Administrat ion of diethylamine NO, an NO donor, conferred protection against the vascu lar damaging effects. Following treatment with CA-4-P, neutrophil infiltrat ion into the tumours, measured by myeloperoxidase (MPO) activity, was signi ficantly increased. Levels of MPO activity also correlated with the levels of Vascular injury and cytotoxicity measured in both tumour types. Neutroph ilic MPO generates free radicals and may therefore contribute to the vascul ar damage associated with CA-4-P treatment. MPO activity was significantly increased in the presence of L-NNA, suggesting that the protective effect o f NO against CA-4-P-induced vascular injury may be, at least partially, med iated by limiting neutrophil infiltration. The data are consistent with the hypothesis that neutrophil action contributes to vascular injury by CA-CP and that NO generation acts to protect the tumour vasculature against CA4-P -induced injury. The protective effect of NO is probably associated with an anti-neutrophil action. (C) 2000 Cancer Research Campaign.