Lack of neural control and reactivity to vasoactive agents in malignant glioma arteries. Vessels in malignant brain tumors have a defective blood-brain barrier. It is important to know if tumor vessels respond to vasoactive agents, since systemic administration of vasodilatory agents together with chemotherapy could increase the access of these drugs to the tumor. It was found that arteries in human malignant gliomas have no neural control and are totally nonreactive to various vasoactive agents. The vessels lose their reactivity within 1 to 2 mm outside the visible tumor front. 