This is an open-label, single arm pilot study of defibrotide given as prophylaxis to patients receiving a conditioned stem cell transplant for the purpose of preventing post-transplant microangiopathy. The University of California San Francisco Blood and Marrow Transplant program reviewed all cases of non-relapse related death from 2012 through 2015 and identified a complication of endothelial injury known as transplant associated thrombotic microangiopathy (TMA) as the most common cause of transplant related mortality in its patient population. TMA is a multi-system disease in which widespread endothelial injury leads to microangiopathic hemolytic anemia, intravascular platelet activation and formation of thrombi within the microcirculation. Diagnosis of TMA can be difficult as systemic signs and symptoms of TMA are often similar to other common transplant complications, such as medication induced hypertension and cytopenias. The reported prevalence of TMA is varied likely due to diagnostic uncertainty and transplant center expertise, but large retrospective studies have reported it as 10-39%, with the majority of cases occurring in the first 100 days after transplant. Of the patients who develop TMA, approximately half of them will develop severe disease. Outcomes are poor with a reported 30-50% mortality rate and as high as 80% in patients with severe disease. Furthermore, survivors of TMA may have significant morbidity (e.g. renal failure and need for long-term dialysis, heart failure, and significantly prolonged hospital admission). There is no gold standard of treatment for TMA. Supportive care includes renal support, discontinuation of calcineurin inhibitors, and treatment of infections. Treatment options include plasma exchange, complement cascade blockade, and defibrotide. Early treatment is crucial to decrease morbidity and mortality.

Despite the early recognition and treatment of TMA, poor outcomes occur when patients are treated with complement blockade alone. This suggests that complement activation may trigger a complex cascade of parallel inflammatory mediators that lead to end organ damage independent of the complement pathway. The goal is to prevent TMA whenever possible via augmentation of endothelial repair. Defibrotide is an anticoagulant and fibrinolytic agent that has been shown to be an effective treatment in other endothelial disorders such as hepatic veno-occlusive disease. It is a polydeoxyribonucleotide salt that blocks plasminogen activator inhibitor-1 (PAI-1) and attenuates the effect of tumor necrosis factor. It also increases prostaglandin E2 and prostacyclin levels which alters the platelet activity adhesion and aggregation and relaxes the smooth muscle of blood vessel walls. All of this likely protects the endothelium from damage. It has been shown that patients with TMA who were treated with defibrotide had a 77% response rate.

The use of defibrotide in the context of veno occlusive disease (VOD) treatment and prevention has been studied extensively, including a landmark report showing that defibrotide given to children during stem cell transplant conditioning is safe and effective in the prevention of veno-occlusive disease. Also, defibrotide showed a 67-77% response rate when used as treatment in patients who developed TMA. However, there has not been a prospective study to show that such prophylaxis is effective in the prevention of TMA in pediatric patients undergoing hematopoetic stem cell transplant (HCT).

Patients will receive Defibrotide 6.25mg/kg via two hour intravenous infusions given every 6 hours. Defibrotide will start the day before conditioning is initiated, and will last for 28 to 35 days. During defibrotide administration, participants will have assessments of administration feasibility, hypersensitivity reaction, and bleeding. All hematopoietic stem cell transplant standard of care evaluations will be conducted, including routine clinical evaluations and laboratory assessments.

Patients will be followed for 6 months post-HSCT, or until death, whichever occurs first. All patients will be evaluated for toxicity from the time of their first treatment with the study drug. The study will use the CTCAE v4.03 for reporting of non-hematologic adverse events and modified criteria for hematologic adverse events. Patients removed from study for unacceptable treatment related adverse event(s) will be followed until resolution or stabilization of all treatment-related adverse events to Grade 2 or lower.

Biomarkers indicative of endothelial damage and TMA activity will be drawn to assess subclinical TMA activity as well as risk for subsequent development of TMA while on defibrotide. These include plasma free hemoglobin, suppression of tumorigenicity, angiopoietin 2, and plasminogen activator inhibitor - 1. Study labs will be assessed at 4 time points prior to Day +21, and at diagnosis of TMA.