Animal Models of Hemostasis and Thrombosis

Significant advances in the medical prevention and treatment of thrombosis have resulted in ever improving outcomes, yet diseases that directly result from acute thrombotic occlusions of arteries and veins remain the leading causes of mortality and severe chronic morbidity in developed countries. This fact clearly indicates the need for newer and even better drugs, and justifies the efforts that are directed towards discovery and commercial development of such therapeutics. Identification of new druggable molecular targets, and subsequent discovery of drugs that act on these targets must be evaluated in thrombotic disease models, in vivo, before clinical development. Due to the lack of available and relevant natural models for bleeding and thrombosis, we typically resort to induction of thrombotic and hemostatic processes in experimental animals. Pathological thrombosis and physiological hemostasis are mechanistically linked but involve different processes, thus, the ultimate goal of our thrombosis research is to identify the differences between the two, with the aim to inhibit or reverse thrombosis without significantly affecting hemostasis. The hemostatic systems of vertebrates, which have evolved to meet the survival needs of particular species, may significantly differ from each other, and may not translate to human physiology or disease. Therefore, whenever possible, we typically test new treatment concepts first in large non-human primates, such as baboons, in a non-terminal model of primary hemostasis and acute vascular graft thrombosis. Our model has been clinically validated and used with success for over 30 years, by both industry and academia, to evaluate the potential safety and efficacy of novel antithrombotic agents, as well as to determine the thrombogenicity of intravascular devices and surfaces, including stents, valves, membranes, and others. Once a concept appears to have merit in the primates, we then further explore the concept in lower vertebrate thrombosis and hemostasis models, such as mice, in order to glean additional mechanistic information. Evaluation of the antithrombotic activity of a new compound initially involves the use of thrombogenic prosthetic vascular graft segments that are temporarily deployed into a chronic high flow arteriovenous shunt, with real-time monitoring of thrombus formation or dissolution using radiological imaging and biochemical methods, such as measuring circulating markers of thrombosis or thrombolysis. In order to characterize the molecular mechanism of action of a particular investigational agent, graft segment thrombogenicity can be varied by coating with specific thrombogenic compounds, such as collagen, tissue factor, or a number of others that have been proposed or suspected to participate in pathological fibrin formation and/or platelet activation. Change in hemostasis is assessed by the use of clinically validated and approved methods, including blood coagulation, platelet aggregation, bleeding time, bleeding volume, and physical evaluation. While evaluation of antithrombotic efficacy in the baboon model is reasonably reliable, and translates well to humans, determining the hemostatic safety of any new intervention remains a significant challenge. The hemostasis test methods currently used in the baboon are as close to clinical hemostasis testing as possible, and any change in hemostasis markers (e.g., increased prothrombin time, bleeding time or volume, appearance of suffusions, ecchymosis, petechiae, re-bleeding, etc.) appears to be predictive of clinical bleeding in humans. However, absence of detectable changes in hemostasis markers cannot preclude a clinically significant increased bleeding risk associated with the use of a new drug candidate.