Role of nitric oxide in angiogenesis and microcirculation in tumors.

Nitric oxide (NO) is a free radical molecule with high reactivity, a short half life and a variety of physiological activities. The role of NO in tumor microcirculation, based on the data collected to date, can be summarized as follows: 1) NO may partially mediate tumor angiogenesis; 2) endogenous NO derived from tumor vascular endothelium and/or tumor cells increases and/or maintains tumor blood flow via dilatation of arteriolar vessels, decreases leukocyte-endothelial interaction, and increases vascular permeability; 3) exogenous NO can increase tumor blood flow via vessel dilatation, and reduce vessel tone; and 4) NO production rates and vascular response to NO are heterogeneous and tumor-dependent. Modulation of NO level in tumor vessels can alter tumor hemodynamics and thus augment oxygen, drug, gene vector and effector cell delivery to solid tumors.