EMERGING TREATMENTS OF "BRAIN ATTACK" OF DIFFERENT ETIOLOGIES

Ryszard M. Pluta, Brian Iuliano, Ramin Rak, R. Brian Mason, Joseph Watson, Michael Espey, David Wink, Edward Oldfield. National Institutes of Health, Bethesda, MD, USA

Until recently, ischemic cerebral stroke, "brain attack", has been treated as a chronic untreatable disease. Pharmacological, diagnostic and pathophysiological advances suggest that early aggressive treatment of the disease may improve outcome and quality of life of the patients. But successful treatment has to be focused on the basic underlining mechanism(s) of brain attack. We present results of pre-clinical studies of targeted treatments of brain attack depending on its etiology.

Vasospasm after SAH: In spite of many years of research delayed ischemic deficits after rupture of intracranial aneurysms remain untreatable. On the basis of earlier experiments showing NO-produced dilation of the cerebral vessels and depletion of neuronal NOS from cerebral arteries in spasm, we hypothesized that intracarotid infusion of a NO donor is able to reverse/prevent cerebral vasospasm. In the acute experiment, intracarotid infusion of a NO donor reversed arteriographic vasospasm of the R MCA and increased rCBF. In the long-term infusion experiment, continuous intracarotid infusion of a NO donor for 7 days prevented the development of vasospasm. Our experiments have shown that cerebral vasospasm is both reversed and completely prevented by NO replacement.

Global cerebral ischemia/reperfusion: Due to intensive emergent treatment more patients are able to survive the global decrease of CBF produced by cardiac arrest, loss of blood, or severe head trauma. However, even when CBF returns quickly to normal levels some patients suffer ischemic stroke. We hypothesized that reperfusion injury, mediated by oxygen free radicals (ROS), may limit the benefits of CBF restoration and that exogenous NO may reduce stroke size by quenching ROS. In a rat global cerebral ischemia model, in the control animals during reperfusion, free radical production increased 4.5-fold over basal levels. After intravenous NO donor infusion, free radical production dropped 5-fold and the infarct volume decreased by almost 50%.

Focal cerebral ischemia/reperfusion: The use of thrombolytic agents for brain attack has had surprisingly modest success, possibly due to reperfusion injury mediated by reactive oxygen species (ROS). Scavenging ROS may be of therapeutic value in stroke treatment. This study investigates the ability of a nitroxide (Tempol), a low molecular weight superoxide dismutase mimic, to reduce reperfusion injury in a filament MCA occlusion model in the rat. Animals receiving Tempol had significantly reduced infarct volumes at doses of 20 mg/kg and 10 mg/kg compared to controls.

Furthermore, in the two in vivo ischemia/reperfusion models: filament MCA occlusion in the rat (MCAO) and autologous cerebral embolism in the rabbit, we measured the volume of the stroke (rat) and the levels of ROS (rabbit) in response to transient ischemia with intracarotid administration of a NO-donor (proliNO). In the rat MCAO model, intracarotid administration of proliNO reduced the infarct volume (P<0.005). In a rabbit cerebral embolism model intracarotid administration of proliNO decreased free radical levels (2,3’ DHBA; P<0.001; 2,5’ DHBA; P<0.002). The beneficial effects of early restoration of cerebral circulation after cerebral ischemia were enhanced by intracarotid infusion of proliNO, most likely due to ROS scavenging by NO. These findings suggest the possibility of preventive treatment of reperfusion injury using ROS scavengers and NO donors.

Postischemic revascularization: Vascular endothelial growth factor/vascular permeability factor (VEGF) is the primary mediator of angiogenesis and there is growing evidence that NO is a key mediator in the proliferative, migratory, and angiogenic effects of VEGF. Development of new vessels may limit brain damages after stroke. In two in vitro experiments we investigated whether there may be a range of NO effects on the proliferative capacity of constituents of the arterial wall. In cell cultures, a NO donor (10-4M) produced a decrease in number of endothelial cells (P < 0.002), smooth muscle cells (P < 0.02) and fibroblast (P < 0.05). In contrast, 10-6M and 10-8M concentrations produced a 27% and 18% increase in endothelial cell number (P < 0.03), respectively. Furthermore, we observed a complete inhibition of new vessel formation from rat aortic rings 10 days after exposure to 10-4M of NO donor and an increased density of newly forming vessels from the aortic rings 10 days after exposure to 10-6M and 10-8M of NO donor. Therefore, high flux NO exposure results in a cytostatic effect on the cellular constituents of the arterial vessel. In contrast low flux treatment was permissive to all constituents and augmented endothelial cell growth selectively. These observations may be useful in the development of NO donors to control angiogenesis.