Intracerebral hemorrhage and thrombin-induced alterations in cerebral microvessel matrix

Four phase III clinical trials of oral direct factor Xa or thrombin inhibitors demonstrated significantly lower intracranial hemorrhage compared to warfarin in patients with nonvalvular-atrial fibrillation. This is counter-intuitive to the principle that inhibiting thrombosis should increase hemorrhagic risk. We tested the novel hypothesis that anti-thrombin activity decreases the risk of intracerebral hemorrhage by directly inhibiting thrombin-mediated degradation of cerebral microvessel basal lamina matrix, responsible for preventing hemorrhage. Collagen IV, laminin, and perlecan each contain one or more copies of the unique alpha-thrombin cleavage site consensus sequence. In blinded controlled experiments, alpha-thrombin significantly degraded each matrix protein in vitro and in vivo in a concentration-dependent fashion. In vivo stereotaxic injection of alpha-thrombin significantly increased permeability, local IgG extravasation, and hemoglobin (Hgb) deposition together with microvessel matrix degradation in a mouse model. In all formats the direct anti-thrombin dabigatran completely inhibited matrix degradation by alpha-thrombin. Fourteen-day oral exposure to dabigatran etexilate-containing chow completely inhibited matrix degradation, the permeability to large molecules, and cerebral hemorrhage associated with alpha-thrombin. These experiments demonstrate that thrombin can degrade microvessel matrix, leading to hemorrhage, and that inhibition of microvessel matrix degradation by alpha-thrombin decreases cerebral hemorrhage. Implications for focal ischemia and other conditions are discussed.