Abstract: Recently published in vitro and in vivo findings strongly suggest that blood-brain barrier (BBB) impairment and increased risk for stroke by tobacco smoke (TS) develop largely in response to common key modulators such oxidative stress (OS), inflammation and alterations of the endogenous antioxidative response system (ARE) regulated by the nuclear factor erythroid 2-related factor (Nrf2). Preclinical studies have also shown that nicotine (the principal e-liquid's ingredient used in e-cigarettes) can also cause OS, exacerbation of cerebral ischemia and secondary brain injury. Preliminary data from our laboratories provides evidence that likewise to TS; chronic e-Cigarette (e-Cig) vaping can promote loss of BBB integrity and vascular inflammation as well as act as a risk factor for the onset of stroke and worsening of post- ischemic brain injury. In addition, recent reports have shown that metformin (MF) treatment before and after ischemic injury reduce stress and inhibit inflammatory responses. Preliminary data provided in this grant suggests that MF promotes Nrf2-mediated counteractive mechanisms which drastically reduce TS toxicity at the brain and cerebrovascular levels while protecting BBB integrity. We provide additional in vivo evidence showing that MF can effectively reduce the oxidative and inflammatory risk for stroke and attenuate post-ischemic brain injury promoted by TS and e-Cig vaping. We propose that MF could be repurposed to prevent/reduce BBB damage and subsequent stroke injury by TS and e-Cig exposure in chronic smokers/vapors. Thus, the objectives of our study are: 1) Assess and validate the brain microvascular molecular mechanisms by which MF can protect the BBB from TS/e-Cig including ischemic/reperfusion (IR) injury. We will unravel the molecular target through which MF can positively impact the BBB and reduce the burden of ischemic stroke and cerebrovascular impairments in chronic smokers and vapors. 2) Evaluate in vivo the effect of prophylactic versus therapeutic (post-ischemic) administration of metformin in reducing TS/e-Cig - promoted cerebrovascular impairment and/or post-ischemic neuronal damage. In vivo investigations will define a mechanism of BBB transport and CNS entry for MF along with major ischemic injury endpoints; including infarction and edema volume, histological endpoints and behavioral recovery after stroke. Ultimately, we will characterize MF's efficacy and therapeutic time window for stroke treatment in mice exposed to TS or e-Cig vapor.