Update from the laboratory: mechanistic studies of pathways of cancer-associated venous thrombosis using mouse models.

Cancer patients have an increased risk of venous thromboembolism (VTE). The rate of VTE varies with cancer type, with pancreatic cancer having one of the highest rates, suggesting that there are cancer type-specific mechanisms of VTE. Risk assessment scores, such as the Khorana score, have been developed to identify ambulatory cancer patients at high risk of VTE. However, the Khorana score performed poorly in discriminating pancreatic cancer patients at risk of VTE. Currently, thromboprophylaxis is not recommended for cancer outpatients. Recent clinical trials showed that factor Xa (FXa) inhibitors reduced VTE in high-risk cancer patients but also increased major bleeding. Understanding the mechanisms of cancer-associated thrombosis should lead to the development of safer antithrombotic drugs. Mouse models can be used to study the role of different prothrombotic pathways in cancer-associated thrombosis. Human and mouse studies support the notion that 2 prothrombotic pathways contribute to VTE in pancreatic cancer patients: tumor-derived, tissue factor-positive (TF+) extracellular vesicles (EVs), and neutrophils and neutrophil extracellular traps (NETs). In pancreatic cancer patients, elevated levels of plasma EVTF activity and citrullinated histone H3 (H3Cit), a NET biomarker, are independently associated with VTE. We observed increased levels of circulating tumor-derived TF+ EVs, neutrophils, cell-free DNA, and H3Cit in nude mice bearing human pancreatic tumors. Importantly, inhibition of tumor-derived human TF, depletion of neutrophils, or administration of DNAse I to degrade cell-free DNA (including NETs) reduced venous thrombosis in tumor-bearing mice. These studies demonstrate that tumor-derived TF+ EVs, neutrophils, and cell-free DNA contribute to venous thrombosis in a mouse model of pancreatic cancer.