SB 234551 selective ET A receptor antagonism: Perfusion/Diffusion MRI used to define treatable stroke model, time to treatment and mechanism of protection

Mismatches between tissue perfusion-weighted imaging (PWI; an index of blood flow deficit) and cellular diffusion-weighted imaging (DWI; an index of tissue injury) provide information on potentially salvageable penumbra tissue in focal stroke and can identify “treatable” stroke patients. The present pre-clinical studies were conducted to: a.) Determine PWI (using perfusion delay) and DWI measurements in two experimental stroke models, b.) Utilize these measurements to characterize selective ET A receptor antagonism (i.e., determine efficacy, time-to-treatment and susceptibility to treatment in the different stroke models), and c.) Determine if increasing the reduced blood flow following a stroke is a mechanism of protection. Permanent middle cerebral artery occlusion (MCAO) or sham surgeries were produced in Sprague Dawley rats (SD; proximal MCAO; hypothesized to be a model of slowly evolving brain injury with a significant penumbra) and in spontaneously hypertensive rats (SHR; distal MCAO; hypothesized to be a model of rapidly evolving brain injury with little penumbra). Infusions of vehicle or SB 234551 (3, 10, or 30 µg/kg/min) were initiated at 0, 75, and/or 180 min post-surgery and maintained for the remainder of 24 h post-surgery. Hyper-intense areas of perfusion delay (PWI) in the forebrain were measured using Gadolinium (Gd) bolus contrast. DWI hyper-intense areas were also measured, and the degree of forebrain DWI–PWI mismatch was determined. Region specific analyses (ROI) were also conducted in the core ischemic and low perfusion/penumbra areas to provide indices of perfusion and changes in the degree of tissue perfusion due to SB 234551 treatment. At 24 h post-surgery, final infarct volume was measured by DWI and by staining forebrain slices. Following SD proximal MCAO, there was a significant mismatch in the ischemic forebrain PWI compared to DWI (PWI > DWI) at 60 min which was maintained up to 150 min (all p < 0.05). By 24 h post-stroke, infarct volume was identical to the area of early perfusion deficit/PWI, suggesting a slow progression of infarct development that expanded into the significant, earlier cortical penumbra (i.e., model with salvageable tissue with potential for intervention). When SB 234551 was administered within the period of peak mismatch (i.e., at 75 min post-stroke), SB 234551 provided significant dose-related reductions in cortical (penumbral) progression to infarction ( p < 0.05). Cortical protection was related to an inc