Ischemic stroke is a sudden neurological disorder which constitutes the third leading cause of death and the leading cause of acquired disability in adults in industrialized countries. The current treatment for the acute phase of ischemic stroke is to remove the blood clot obstructing the cerebral bloodflow by enzymatic degradation (thrombolysis) or by removing it mechanically through catheterization (thrombectomy). To guide clinical practitioners in their choice of treatment, magnetic resonance imaging (MRI) is essential. But MRI has its limits and does not allow the diagnosis of microthrombi, which are however partly responsible for post stroke sequelae. This project aims to (i) study the physiopathology of microthrombi in ischemic stroke, (ii) develop a contrast agent to reveal microthrombi on MRI and (iii) establish a treatment for micrthrombi. 1. Study of microvascular thrombosis in ischemic stroke The presence and impact of distal microthrombi will be studied in 2 models of ischemic stroke in mice. The first model we will use is thromboembolic, it is obtained by injection of thrombin into the middle cerebral artery (MCA). In the second model, cerebral ischemia is induced by occlusion of the MCA with a filament; removing the filament will allow us to reproduce the abrupt recanalization encountered in patients who benefited from thrombectomy. We will study precisely on histological sections the quantity, the stability over time and the composition of microthrombi in these 2 models. 2. Synthesis of a novel MRI contrast agent to reveal microthrombi Previous work in the PhIND laboratory has demonstrated the great potential of the molecular magnetic resonance imaging (MRI) strategy with microparticles of iron oxide (MPIO). Despite the promises of this strategy, the MPIOs used in these studies are composed of inert and toxic materials. The development of biodegradable and non-toxic MPIOs is therefore necessary to make this technology applicable to humans. In this project we propose to synthesize MPIOs from iron oxide nanoparticles assembled in a biodegradable matrix. We will use an emulsion coupled to a crosslinking process to cluster the iron oxide nanoparticles in the biodegradable matrix. We refer to these particles by the acronym PHysIOMIC. To optimize this synthesis and characterize the particles obtained, we will work with the organic chemistry department of the pharmaceutical research laboratory of Caen (CERMN). We will then use this novel MRI contrast agent to reveal the microthrombi present in ithe schemic stroke models using the molecular MRI technique. Preliminary results confirm that this method is effective in revealing the occlusive microthrombi present in the ischemic thromboembolic stroke model. In order to increase the specificity of our system, we will work on functionalizing the PHysIOMIC with antibodies specific to the platelets contained in the microthrombi. 3. Preclinical study of a thrombolytic therapy for the treatment of microthrombi We will test 3 different thrombolytic treatments that are known to be effective in degrading platelet and von Willebrand factor rich clots. We will test N-acetylcystein, the powerful thrombolytic effect of which has recently been demonstrated in the PhIND laboratory, and 2 treatments whose fibrinolytic activity is triggered by the presence of thrombin, which is generated in very large quantities by the activated platelets present in microthrombi.