PROJECT SUMMARY Platelet Transfusion Refractoriness (PTR) or the failure to achieve the desired level of blood platelets after platelet transfusion, hampers the management of bleeding episodes in thrombocytopenic patients. PTR condition is associated with a high risk of severe bleeding complications and reduced survival rate, longer hospital stays and higher hospital costs. Existing management strategies are based on the transfusion of leukodepleted products, HLA-matched platelets, or cross-matched platelets which are obtained after lengthy and expensive procedures. Alternatives consisting of thrombopoietin mimetics, or recombinant factor XIII have not shown the ability to reduce mortality. Fibroplate is proposing a new approach to manage bleeding in thrombocytopenic patients who develop PTR, based on the intravenous injection of Fibrinoplate-S (FPS), a ready-to-use suspension of fibrinogen-coated albumin nanospheres. FPS significantly reduces the bleeding time and improves overall survival rate associated with radiation-induced thrombocytopenia in preclinical rodent models through the formation of co-aggregates with the residual activated platelets at wound sites on the endothelium of the blood vessel. Various preclinical studies demonstrated that FPS is safe and does not induce intravascular coagulation. Remarkably, FPS is not sequestered in the spleen, and FPS specific antibodies have not been detected after its administration in humans. These characteristics, together with its hemostatic capacity support the use of FPS in the management of thrombocytopenic patients with immune and non-immune PTR. However, their efficacy and safety need validation in a thrombocytopenic preclinical model under PTR conditions. This NIH SBIR Phase I aims at investigating i) the efficacy of FPS in reducing bleeding parameters in an established (double) model of thrombocytopenia and PTR (TPTR) preclinically in rabbits and ii) the safety of the proposed approach. By offering effective treatment, Fibroplate is expected to reduce bleeding complications and improve and promote survival in thrombocytopenic patients with PTR.