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SCDAC Meeting Minutes - October 15, 2010
Edward Benz, Jr., M.D. (Chair), Dana-Farber/Harvard Cancer Center
National Heart, Lung, and Blood Institute Staff
W. Keith Hoots, M.D., Director, DBDR
Other Federal Agency Staff
R. Lorraine Brown, R.N., B.S., CPHP, Sickle Cell Disease Programs Project Officer, Health Resources and Services Administration
H. Franklin Bunn, M.D., Harvard Medical School
The Sickle Cell Disease Advisory Committee (SCDAC) convened on October 15, 2010, at 6701 Rockledge Drive, Rockledge II, Room 9112/9116 in Bethesda, Maryland. Edward Benz, Jr., M.D., served as Chair. Dr. Benz and W. Keith Hoots, M.D. welcomed committee members to this meeting.
Dr. Benz reported that the SCDAC had identified a need to discuss the pulmonary complications associated with sickle cell disease. During this meeting, committee members would hear about some of the latest research in this area.
Dr. Hoots announced the upcoming James B. Herrick Symposium, whose title is "Sickle Cell Disease Care and Research: Past, Present, and Future." The meeting will take place on November 16–17, 2010, at the National Institutes of Health (NIH) in Bethesda, Maryland.
A motion to approve the minutes of the SCDAC's March 26, 2010, meeting was carried unanimously.
Report on National Heart, Lung, and Blood Institute Workshop: Framing the Research Agenda for Sickle Cell Trait
Jonathan Goldsmith, M.D., summarized the presentations at a June 2010 meeting, "Framing the Research Agenda for Sickle Cell Trait." This workshop addressed the evidence available on sudden death in sickle cell trait; ethical, legal, and social implications of sickle cell trait screening programs; societal implications for public health; and the medical knowledge base.
Following formal presentations, participants joined small groups to discuss the potential value of a large epidemiologic study on sudden death in sickle cell trait, the research needed on bioethical issues in sickle cell trait, and the gaps in the medical knowledge base.
The workshop's leaders plan to publish a summary of the workshop in a refereed medical journal. Staff members at the National Heart, Lung, and Blood Institute (NHLBI) are currently identifying initiatives that could support new research on sickle cell trait. NHLBI welcomes suggestions from SCDAC members about research initiatives in this field.
Pulmonary Hypertension in Sickle Cell Disease: Framing the Questions and Broadening the Research Base
Harvey Luksenburg, M.D., summarized the results of several autopsy studies of children and adults with sickle cell disease. These studies found a high incidence of sudden and unexpected death, high rates of pulmonary pathology on autopsy, and more severe chronic organ damage than had been suspected pre-mortem. Prior to 2005 there were no reliable non-invasive markers for early mortality.
According to the nitric oxide depletion hypothesis, free plasma hemoglobin consumes nitric oxide. Arginine, a substrate for nitric oxide production, is depleted in people with sickle cell disease, which leads to reduced nitric oxide bioavailability. The consequences of nitric oxide depletion are chronic vasoconstriction, expression of endothelial cell adhesion molecules, and activation of platelets.
The body of work based on this hypothesis has led to the identification of several potential therapeutic agents, including inhaled nitric oxide, arginine supplementation, and sildenafil.
The largest clinical study stemming from the nitric oxide depletion hypothesis was the Sildenafil Therapy for Pulmonary Hypertension and Sickle Cell Disease (Walk-PHaSST) trial. The primary endpoint of this placebo-controlled, double-blind, multicenter study in patients with sickle cell disease and pulmonary hypertension was the efficacy of sildenafil therapy at 16 weeks, as measured by a 6-minute walk. The trial was closed early because subjects in the sildenafil group were more likely to experience severe painful episodes requiring hospitalization than were subjects in the control arm.
With the results from the Walk-PHaSST trial now available and several recent publications on pulmonary hypertension in sickle cell disease in Blood, this is a good time for the SCDAC to review the current knowledge in the field. Based on this review, the SCDAC could consider broadening the NHLBI research portfolio in pulmonary hypertension and vascular biology to encompass other investigational approaches.
Pulmonary Hypertension in Sickle Cell Disease: Diagnosis, Etiology, and Treatment: Two Views
Hemolysis-Associated Endothelial Dysfunction and Pulmonary Hypertension:An Emerging Cause of Death in the Hemolytic Anemias
Mark T. Gladwin, M.D., explained that he and his colleagues hypothesize that the state of chronic vaso-constriction seen in many subjects with sickle cell disease is due to elevated levels of free hemoglobin in the blood as a result of hemolysis. The excess free hemoglobin rapidly destroys nitric oxide, the major physiological vaso-regulator.
The amount of hemolysis in sickle cell disease during crises is sufficient to impair nitric oxide signaling. The plasma of patients with sickle cell disease contains high hemoglobin levels and no detectable haptoglobin. The range of hemolysis is 0-20 ?M but this level increases further during crises. The plasma of patients with sickle cell disease consumes more nitric oxide than the plasma of volunteers without sickle cell disease.
Dr. Gladwin identified 645 references on pulmonary hypertension and anemia. These studies show a 10–30% prevalence of hemolysis-associated pulmonary hypertension in patients with sickle cell disease or thalassemia. The 2-year survival rate in patients with sickle cell disease is up to 50%, and hemolysis-associated pulmonary hypertension is a potential risk factor for mortality in adults. In a study of 195 patients, Dr. Gladwin demonstrated that tricuspid regurgitant velocity (TRV) is a risk marker for pulmonary hypertension. Other groups in the United States and internationally have reproduced these results.
H. Franklin Bunn, M.D., has argued that fatal pulmonary hypertension (cor pulmonale) rarely causes death in patients with sickle cell disease. However, Dr. Gladwin's research shows that adults with sickle cell disease and a TRV that is higher than 3 m/s have a higher mortality rate. Two other studies have reproduced this finding.
Dr. Gladwin's studies have also found a 40% prevalence of elevated TRV in patients with sickle cell disease. A high TRV is correlated with markers of hemolysis but not with white blood cell count, platelet count, fetal hemoglobin level, or number of episodes of vaso-occlusive crisis and acute chest syndrome.
An older study (Neely et al) found no correlation between plasma hemoglobin levels and red cell-derived lactate dehydrogenase (LDH) in patients with sickle cell disease. However, a recent re-analysis of Neely's data showed a strong correlation between LDH and hemoglobin. In fact, free hemoglobin explains 50% of the variability in LDH.
Research is needed to understand the biomarkers that predict quantity of life while trying to improve quality of life. Several trials of new therapies may have failed because they targeted the wrong phenotype. These failed clinical trials do not indicate that investigators should abandon the nitric oxide hypothesis since most clinical trials do not prove their primary hypotheses.
Pulmonary Hypertension, Hemolysis, and Nitric Oxide Scavenging in Sickle Cell Disease
Dr. Bunn said that pulmonary hypertension is common in patients with sickle cell disease, but its etiology depends on several frequently overlapping factors. Because of the complex interactions among multiple pathways, the impact of nitric oxide depletion on pulmonary hypertension in sickle cell disease is difficult to assess.
Until Dr. Gladwin published his 2004 paper in the New England Journal of Medicine, experts were not aware of the prognostic importance of pulmonary hypertension in sickle cell disease. Dr. Bunn agreed with Dr. Gladwin that pulmonary hypertension contributes to morbidity and mortality in sickle cell disease. However, Dr. Bunn disagreed about the extent to which mortality in patients with sickle cell disease is due to pulmonary hypertension as an isolated problem versus the other intercurrent problems that affect adult patients with sickle cell disease. In a French study of 403 consecutive patients with sickle cell disease, 24% had a TRV higher than 2.5 m/s. Among these 24 patients, 13 had increased pulmonary wedge pressure indicative of left ventricular dysfunction, and only 6 (1.6%) had pulmonary arterial hypertension and a high TRV.
The studies by Ataga et al corroborate Dr. Gladwin's finding that pulmonary hypertension is prevalent in patients with sickle cell disease and appears to be correlated with hemolysis. Although patients with pulmonary hypertension had higher reticulocyte counts, these differences were not statistically significant. The level of fetal hemoglobin is probably the most convincing independent genetic determinant of disease severity in sickle cell disease.
Patients with sickle cell disease are prone to priapism and leg ulcers. However, Dr. Bunn was skeptical of the role of free plasma hemoglobin and nitric oxide consumption in these processes. The data from the paper by Neely et al suggest that something other than the degree of hemolysis contributes to the high plasma hemoglobin levels in patients with sickle cell disease. Hemolysis clearly contributes to elevated plasma hemoglobin levels, but not to the extent that one would expect if this process were primarily intravascular.
According to Dr. Gladwin's data, haptoglobin is absent in sickle cell disease. But although this might be true when patients are free of infections and crises, haptoglobin levels typically rise during crises. Once this happens, hemoglobin/haptoglobin has a different clearance rate than free hemoglobin and this will affect plasma hemoglobin levels independently of the degree of hemolysis.
Plasma hemoglobin levels are difficult to interpret in patients with sickle cell disease and hemolysis is just one of several important contributors to plasma hemolysis levels. Different studies have produced inconsistent results about the correlation of indirect markers of hemolysis and clinical phenotype. Association does not imply causality.
The discussion was recognized as scientific and rigorous by the Advisory Committee. The effect of subject ages in reported series and the haplotype clusters were suggested as possibly confounding the interpretation of data.
One member of the SCDAC asked about the NO hypothesis and information that has been discovered from hemoglobin based oxygen carrier (HBOC) studies in man. Dr. Gladwin replied that cell-free hemoglobin has one of the highest toxicity signatures described. However, in animal studies nitric oxide scavenging was not observed when a pegylated hemoglobin preparation was administered. Studies with this modified HBOC in humans did not find HBOC-associated adverse events. However, the product's development has been stalled.
The use of TRV as a marker for clinical efficacy was questioned. Dr. Gladwin said TRV reflects high right ventricular systolic pressure, which can occur for several reasons, such as vascular stiffness. Patients with higher TRV levels have higher mortality. However, because some drugs that reduce pulmonary arterial hypertension (PAH) do not reduce TRV, using TRV alone as a marker may not be accurate.
Responding to another question, Dr. Gladwin noted that the response to inhaled nitric oxide is immediate and has a dramatic favorable effect on vaso-occlusion. Based on the walk-PHaSST trial, therapies that target vascular dysfunction can apparently lead to enhanced pain.
The effects on levels of pro-BNP due to renal or cardiac function were discussed. Dr. Gladwin noted that renal failure is an independent risk factor for pulmonary hypertension. He added that patients with renal failure are typically excluded from BNP analyses. Renal dysfunction is an important and understudied problem in the sickle cell disease field.
In a rebuttal to Dr. Bunn's remarks, Dr. Gladwin commented that he developed his theory based on a mechanistic translational study of 27 people, not epidemiological data. Animal studies that followed this initial research demonstrated that hemoglobin scavenges nitric oxide. A population study then confirmed that nitric oxide scavenging has a strong, independent association with hemolysis.
Dr. Gladwin and Dr. Bunn discussed their divergent conclusions based on a study of pulmonary hypertension in SCD in France. Both discussants agreed on the role of renal insufficiency on mortality and the adverse effect of elevated TRV, combined with an abnormal right heart catheterization. However Dr. Bunn stated that nitric oxide scavenging is not the only possible explanation for these findings.
A member of the SCDAC agreed with Dr. Bunn that morbidity and mortality in sickle cell disease may be a result of etiologies other than nitric oxide. Furthermore, in paroxysmal nocturnal hemoglobinuria, a condition associated with much more intravascular hemolysis than sickle cell disease, there are fewer reports of severe pulmonary hypertension.
Broadening the Research Base in Sickle Cell Disease-Associated Pulmonary Vascular Disease I
Chronic Thromboembolic Pulmonary Hypertension: Possible Mechanisms
Timothy A. Morris, M.D., described the result of incomplete thrombus resolution, chronic thromboembolic pulmonary hypertension (CTEPH), in which acute clots become chronic scars.
Proposed mechanisms of CTEPH include incomplete thrombus resolution, dysfibrinogenemias resistant to plasmin digestion, and stimulation of cellular remodeling. A study in 33 patients with CTEPH identified 5 cases dysfibrinogenemias. The study identified abnormalities in fibrin polymer structure, lysis, or both in all CTEPH-associated mutations. Thrombin may play a role in stimulation of pulmonary artery endothelial cells and resultant remodeling.
Pulmonary Hypertension in Children with Sickle Cell Disease: a Cardiologist's Perspective
Greg Ensing, M.D., confirmed that many adults with sickle cell disease have high pulmonary artery systolic pressure associated with an increased 3-year mortality risk. The situation in children is different in some respects. While children with sickle cell disease have slightly higher pulmonary artery systolic pressures than age-matched controls, their TRVs tend to be lower than those of adults with sickle cell disease. Children with sickle cell disease and high pulmonary artery systolic pressure have been shown to have increased morbidity but not increased mortality. Increased pulmonary artery systolic pressure is not correlated with pulmonary vascular disease.
More recent studies found higher TRVs in children with sickle cell disease than in control group members, but the differences were small. Whether these increased TRVs are primarily related to increased pulmonary vascular resistance, perhaps due to nitric oxide depletion, is the subject of ongoing research.
TRV measured by echocardiography correlates well with pulmonary artery systolic pressure and is an adequate screen for pulmonary hypertension in children. Additional echocardiographic measures can help define relative contributions of flow, left ventricular failure, and pulmonary vascular obstructive disease.
Increased pulmonary artery pressures in children with sickle cell disease are associated with decreased exercise capacity and increased numbers of acute chest syndrome events, as well as increased flow and mildly diminished left ventricular systolic and diastolic function. Some children with TRVs of at least 2.6 m/s also have a component of increased pulmonary vascular resistance, possibly due to nitric oxide depletion-related vasoconstriction, microvascular thrombosis or inflammation, elevated blood viscosity, or protective vasoconstriction secondary to venous hypertension.
An SCDAC member asked whether left ventricular systolic and diastolic dysfunction in children is associated with severe anemia. Dr. Ensing replied that his studies have found no inverse association with severe anemia.
Another SCDAC member commented on the difficulty of distinguishing between true pulmonary hypertension and an elevated TRV. Repeat echocardiograms do not always show the same results and most children are not catheterized. Dr. Ensing clarified that he would consider catheterization in children with a TRV of 3 to 3.5 m/s but he does not advocate interventions with the types of elevations in TRV seen in many children with sickle cell disease. A single echocardiogram is not sufficient to determine whether a child has pulmonary hypertension.
A member of the public commented that the Pulmonary Hypertension and Hypoxic Response in Sickle Cell Disease (PUSH) study found a strong correlation between an initial single measurement and results 2 years later but presently no one is advocating treatment for children.
Broadening the Research Base in Sickle Cell Disease-Associated Pulmonary Vascular Disease II
Adenosine-Mediated Modulation of iNKT-Induced Pulmonary Inflammation in a Mouse Model of Sickle Cell Disease
Dr. Joel Linden explained that invariant natural killer T (iNKT) cells are lymphocytes expressing the adenosine receptor and can be activated by tissue damage such as that due to reperfusion injury associated with endothelial activation, tissue cytokine production, and adhesion and extravasation of leukocytes.
ATL146e and ATL313 are potent and selective agonists of the adenosine A2A receptor (A2AR). One compound in this class has been approved for clinical use in pharmacological cardiac stress imaging testing. Adenosine A2AR activation blocks liver ischemia reperfusion injury, which is mediated by lymphocytes. Mice whose lymphocytes are depleted are protected from reperfusion injury and subject to injury when their T cells are reconstituted.
When iNKT cells are activated, interferon (IFN)-? is released dampening the inflammatory response. The activation of the iNKT cells can be blocked using an antibody to antigen-presenting cells.
Sickle cell anemia is associated with chronic lung ischemia reperfusion injury. To determine whether iNKT cells play a role in sickle cell disease, Dr. Linden conducted studies using a murine model of sickle cell disease. The studies showed that two or three times as many iNKT cells are activated in the lungs of mice with sickle cell disease as in the lungs of normal mice. Blockade of iNKT cells prevents lung injury in mice with sickle cell disease. Elevated numbers of iNKT cells have been demonstrated circulating in individuals with sickle cell disease.
Regadenosan, a clinically approved A2A agonist, inhibits human iNKT cells. Dr. Linden is currently conducting a Phase I study of regadenoson (Lexiscan) in people with sickle cell disease to identify the maximal safe dose.
A SCDAC member commented that different agents can interfere at different points during the inflammatory cascade. Dr. Linden explained that iNKT cells are a good target because they block all downstream events. However, adenosine is not the only way to block iNKT cell activation. For example, similar antibodies to the ones that Dr. Linden used in his mouse studies could be developed for humans.
Dr. Linden remarked that mice and people with sickle cell disease have heterogeneous aggregates between platelets and other cells. The inflammatory process creates adhesion of platelets and neutrophils become activated, indicating that this platelet–neutrophil complex is participating in the disease process.
A SCDAC member asked whether the iNKT cells could be affected by corticosteroids. Clinically, steroids may help patients experiencing a sickle cell disease crisis. Dr. Linden said he was concerned about adverse effects of the use of corticosteroids. Although steroids administered to isolated neutrophils or lymphocytes would not cause acute harm, administering these agents to people or animals can produce a wide range of immune system effects. Rebound from treatment is a potential problem with any anti-inflammatory therapy.
Dr. Linden said that these cells are activated quickly by tissue necrosis as part of the immune system. Suppressing this process has an immunosuppressant effect, and immunosuppressive agents are probably better for acute flare-ups than for chronic therapy. This system seems to be very sensitive to adenosine, which can produce very selective effects on inflammation in mice without side effects.
A SCDAC member asked whether the iNKT cells are involved in other inflammatory disorders. Dr. Linden explained that iNKT cells are probably activated in many kinds of tissue injury. The presence of monocytes in murine models of sickle cell disease seems to be part of the innate immune response. Dr. Linden said that many types of cells interact, and knocking out one type, such as macrophages can produce some improvement in mouse models. However, interfering with this process therapeutically is probably more difficult. A SCDAC member noted that activating the iNKT cells would probably activate monocytes and tissue macrophages.
Dr. Linden concluded by saying that researchers need to identify an appropriate way to measure effects on vasculopathy. Ideally, researchers could examine microvascular volumes in organs of patients with sickle cell disease who are and are not experiencing pain.
Dr. Benz asked the SCDAC members to discuss ways to enhance NHLBI's sickle cell disease (SCD) research portfolio.
Future Research Focus Areas
Increase the involvement of cardiologists and vascular biology experts in SCD research.
Determine whether an alternative animal model to the mouse model is more relevant to study human SCD.
SCD research needs highly qualified, clinical epidemiology experts to properly analyze existing and future SCD datasets.
Clinical trial design expertise, improved access to SCD patients, and stratification of SCD patients into low, medium and high risk cohorts will enhance the successful enrollment and completion of SCD clinical trials. Networks may play a role in this process.
Data collection on the natural history of SCD beginning in childhood is essential to evaluate innovative technologies to monitor organ system dysfunction and study clinical interventions.
The therapeutic options for treating SCD involve cure, acute remission, chronic remission, acute palliation, and chronic palliation. Researchers could consider risk stratification for chronic palliation and/or chronic remission and risk stratification for examining markers of acute exacerbations.
Gene therapy and hematopoietic stem cell transplant therapeutic strategies should be supported because they are curative.
In developing clinical trials for potential drug therapies, determination of the appropriate endpoint is critical. It is important to determine if the Food and Drug Administration will require testing each new drug in comparison to hydroxyurea.
NIH research should catalyze the progress of industry-sponsored trials.
Research should compile biomarkers of risk for children and adults. If research identifies biomarkers for mortality, intensive therapies could be assessed by serial measurement of the biomarkers.
Last Updated December 2010