PROJECT SUMMARY/ABSTRACT Traumatic brain injury (TBI) is a major cause of death and disability worldwide. There are no effective therapies available for TBI patients. Thus, there is a compelling need to develop novel therapeutics in order to improve neurological recovery after TBI. Among many secondary injury events that occur after TBI, cerebral microthrombosis is an under-recognized, yet important contributor to the secondary brain ischemia and damage that occurs after TBI, and would therefore seem to be one of the central secondary events after brain trauma to bear in mind when designing treatment strategies. Cerebral microthrombi not only lead to ischemia and cell death but also prevent therapeutic drugs from entering into the affected brain and therefore constrain the efficacy of therapeutic drugs, which may be one of important factors ignored during preclinical and clinical trials. Our recent study indicates that early (2 hours post injury) intravenous administration of Vepoloxamer promotes sensorimotor function and cognitive functional recovery after TBI induced by controlled cortical impact (CCI-TBI), which is associated with its robust effect on reducing cerebral microthrombosis formation and neuroinflammation. Vepoloxamer is a purified form of Poloxamer 188 where impurities associated with renal dysfunction have been removed, which is an amphiphilic polyethylene-polypropylene-polyethylene tri-block copolymer that is reported to seal membranes and restore plasma membrane integrity in damaged cells. However, to date, there is a paucity of information about Vepoloxamer for treatment of TBI and the mechanisms underlying its therapeutic effects. von Willebrand factor (vWF) released into blood from injured endothelial cells inversely correlates with clinical outcome of severe TBI. vWF can induce microthrombosis formation. Our previous study demonstrated that the level of vWF released into plasma increases at 1-4 hours, peaks at 1-3 days, declines at 8 days, and returns to normal at 15 days in rats after CCI-TBI. We hypothesize that TBI induces the blood-brain barrier (BBB) damage and release of endothelial-derived vWF, which leads to platelet aggregate and subsequent cerebral microthrombosis-induced secondary injury. In Aim 1, we will first conduct a dose-finding study to identify Vepoloxamer dose and therapeutic window effect on functional recovery without toxicity in young rats (male and female) with TBI. In Aim 2, we will then investigate the mechanisms by which IV administration of Vepoloxamer enhances cerebral microvascular perfusion and promotes functional recovery after TBI. Microvascular integrity, cerebral blood flow, and BBB leakage will be measured dynamically using either laser scanning confocal microscopy or magnetic resonance imaging (MRI). This work will address a previously understudied important issue and is highly translational. Successful completion of this proposed research will elucidate mechanisms underlying IV Vepoloxamer-mediated promotion of TBI recovery, and facilitate development of Vepoloxamer as a novel therapeutic approach targeting endothelial cells/microthrombi to improve neurological outcome for TBI patients.