Venous thrombombolism (VTE) is a leading cause of death and disability in cancer patients. The incidence of VTE in these patients in further increased by the administration of anti-neoplastic drugs, such as the chemotherapy drugs gemcitabine and cisplatin, as well as combination therapies. However, the mechanisms of VTE in cancer patients are largely unknown. The overall goal of this proposal is to determine whether an association exists between tissue factor (TF) activity in circulating microparticles (MPs) and venous thrombosis in patients with pancreatic or colon cancer, and in tumor-bearing mice. MPs are small (<1 micron) membrane vesicles that are released from activated or apoptotic cells. Our general hypothesis is that chemotherapy drugs increase the release of TF-positive MPs from both tumor cells and host cells into the circulation, and that an increase in the level of these MPs is associated with an increase in the incidence of asymptomatic and symptomatic VTE in pancreatic and colon cancer patients, and an increase in thrombus size in tumor-bearing mice. The proposal is divided into two aims. In Specific Aim 1 we will determine the effect of treatment of tumor-bearing mice with chemotherapeutic drugs on circulating TF-positive MPs derived from tumor cells and different host cells as well as their role in a model of venous thrombosis. We will measure the levels of TF activity in isolated MPs in tumor-bearing mice treated with either gemcitabine or gemcitabine and cisplatin. We will use a series of novel mouse lines to distinguish between TF-positive MPs derived from i/ tumor versus host cells, and ii/ different host cells (monocytes, endothelial cells and platelets). These include HCV mice, which express human TF in the absence of mouse TF, and mice with various cell type-specific deletions of the TF gene generated using the Cre-loxP technology. Finally, we will selectively analyze the role of either tumor cell- derived or host cell-derived TF-positive MPs in a model of venous thrombosis. In Specific Aim 2 we will determine if there is an association between TF activity in circulating MPs and VTE in patients with advanced pancreatic and colon cancer treated with anti-neoplastic drugs in a multi-center prospective observational study. Blood samples will be obtained from patients before and after treatment with anti-neoplastic drugs. Asymptomatic deep vein thrombosis will be assessed using compression ultrasound before and after treatment with anti-neoplastic drugs. We will measure levels of TF activity in isolated MPs and determine if this is associated with asymptomatic and/or symptomatic VTE. We will also measure whole blood TF activity, cellular origin of the TF-positive MPs and coagulation activation markers (thrombin anti thrombin complexes and D- dimer). The results of this study will determine the role of TF-positive MPs in venous thrombosis associated with cancer and chemotherapy. TF activity in isolated MPs may be a useful biomarker that can be used either alone or as an adjunctive biomarker in clinical predictive models of thrombotic risk in cancer patients undergoing chemotherapy.