Shear-activated Nanoparticle Aggregates Containing Nitroglycerin Selectively Increase Collateral Perfusion During Experimental Ischemic Stroke

Introduction: Leptomeningeal collateral flow is a strong predictor of stroke outcome. Enhancing collateral flow is an appealing therapeutic approach. However, previous attempts to enhance collateral flow with vasodilators have failed, due to systemic hypotension and cerebrovascular steal from the penumbra. We have shown that during experimental stroke collaterals have fluid shear stress that is 3-7 times higher (100 dyne/cm 2 ) than other blood vessels. This unique feature of collateral vessels provides a way to selectively enhance flow, using nanoparticle aggregates loaded with nitroglycerin (NG-NPAs), that only release drug in areas of high shear stress (≥ 100 dyne/cm 2 ). Hypothesis: Shear-activated NG-NPAs will selectively enhance collateral perfusion and improve stroke outcome. Methods: Proximal occlusion of the middle cerebral artery (MCAo) was induced for 70 min in male Spontaneously Hypertensive Rats. Changes in cerebral blood flow in the collateral-supplied ischemic MCA territory, and in the contralateral homotypic region were measured by laser speckle contrast imaging. Animals were randomized to receive I.V. infusion of blank-NPA (B-NPA, control, 4mg in 2ml of saline, n=7) or NG-NPA (50μg NG in 4 mg NPA = 4μg/kg/min of NG, n=7), commencing 25 minutes after MCAo, until reperfusion. Infarct volume was measured at 24 h. Results: NG-NPA significantly increased collateral perfusion by 44% vs. 11% for B-NPA at 40 minutes post-infusion (p=0.026), without altering perfusion in the contralateral region (NG-NPA: +19%, B-NPA: +13% of pre-infusion baseline, p=0.99) and without reducing blood pressure (NG-NPA: 161 mmHg, B-NPA: 163 mmHg, p=0.99). NG-NPA significantly reduced infarct volume at 24 h (NG-NPA: 70 mm 3 , B-NPA: 121 mm 3 , p=0.005). Better collateral perfusion was correlated with smaller infarct volume (R 2 = 0.44, p=0.012). Conclusion: Shear-activated NG-NPA selectively enhanced collateral perfusion to penumbral tissue without inducing systemic side effects, which resulted in smaller infarcts in this model. Given the known importance of collateral flow, shear-activated NG-NPA show great promise as a potential therapy for ischemic stroke patients, both pre-reperfusion therapy and in those unsuitable for such therapies.