The delivery of oxygen to a proliferating tumor is variable and depends on many factors, from the rate of the malignant growth itself to the stimulation of angiogenesis by a complex cascade of signals. Because of the comparatively high levels of glycolysis in cancer cells, hypoxia is fairly common and oxygen concentrations have been found 10 to 100 times decreased compared to healthy tissues. The general trend in cancer therapy is to support or even achieve hypoxia, since it results in decreased metabolism, even necrosis and probably slows tumor growth. As with most generalizations in cancer, there is always another view. Hypoxia stimulates the multiple drug-resistance gene (MDR) and therefore increases P-glycoprotein expression, one of the major obstacles in the delivery of effective chemotherapeutic agents to the tumor.
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