Ankle Ulcers in Sickle Cell Disease: A Topical Solution to a Vexing Problem?

A Phase I Study of the Use of Topical Sodium Nitrite for Chronic Leg Ulcers in Adult Patients with Hemolytic Disorders

NCT01316796

National Heart, Lung, and Blood Institute

National Institutes of Health Clinical Center

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This study was developed originally using a non-randomized, dose-escalation, phase I strategy to examine topical sodium nitrite cream as a treatment for chronic leg ulcers in patients with sickle cell disease (SCD) or other red blood cell disorders. The protocol was subsequently amended, converting the study into a doubleblind, placebo-controlled trial designed to determine the maximum tolerated dose of the sodium nitrite compound. Secondary endpoints focus on the extent and duration of response to treatment. Adverse effects, including hypotension and methemoglobinemia, that could occur as a consequence of sodium nitrite absorption from the ulcerated skin will be documented. To be eligible, patients must be at least 18 years of age, have SCD or another red cell disorder, and have had a leg ulcer > 2.5 cm² but < 100 cm² for more than four weeks. Exclusion criteria include recent use of nitroglycerin, nitroprusside, nitric oxide, arginine, sildenafil, or methemoglobin-inducing drugs or a history of methemoglobinemia or infection.

Chronic leg ulcers are painful, debilitating complications of SCD and other hemolytic anemias that contribute to early mortality, poor quality of life, and increased disability of affected patients. In SCD, the incidence of leg ulcers varies from 8 to 75 percent depending on variables such as geographic location of the patient, genotype of hemoglobin (HbSS more than HbSC), age (most affected patients are between the ages of 18-25), contribution of trauma to lesion, concentration of hemoglobin (lower worse than higher), an accompanying hypercoaguable phenotype, or the presence of the lupus anticoagulant. (Bazuaye GN et al. J Med Sci. 2010;8:190-194; Minniti CP et al. Am J Hematol. 2010;85:831-833; Delaney KM et al. Hemoglobin. 2013;37:325-32). Decreased tissue blood flow appears to be the major contributor to both ulcer formation and delayed healing. The pathophysiologic process that underlies abnormal tissue perfusion is multifactorial with vaso-occlusion by microthrombi, upregulated integrin expression that promotes platelet and granulocyte aggregation, increased venous pressure, bacterial infections, abnormal autonomic nervous system control with excessive vasoconstriction when in the dependent position, and degree of anemia having been proposed as contributing factors. Recent studies by Caterina Minniti’s group that examined histopathology and assessment of the microcirculation by laser imaging and thermography showed evidence of venostasis, inflammation, and vasculopathy (Minniti CP et al. Am J Hematol. 2014;89:1-6). Blood vessels were increased and activated endothelium with evidence of microthrombi and fibrin deposition along with thrombosis/recanalization were observed. Remarkably similar findings were noted in chronic venous ulcers in individuals without SCD, suggesting that leg ulcers may be a common end-organ complication of endothelial dysfunction.

Current treatment options for leg ulcerations, including antibiotics, compression bandages, dressing changes, Unna boots, silver and zinc oxide gauze, skin grafts, and maggot therapy, rely mostly on bacterial containment, stimulation of tissue granulation, and reduction of venostasis to ameliorate disease severity. A recent Cochrane Database review, however, identified only six treatment-focused clinical trials over the past 20 years, three involving systemic interventions (L-carnitine, arginine butyrate, isoxsuprine) and three based on topical interventions (Solcoseryl^® cream, RGD peptide dressing, topical antibiotics) (Martí-Carvajal AJ et al. Cochrane Database Syst Rev. 2012;11:CD008394). Studies using three interventions (arginine butyrate, RGD peptide, and L-carnitine) reported on change in ulcer size. Of these, only the study that used an RGD peptide matrix (to block integrin-receptor binding) showed a significant reduction in ulcer size compared with the control group. Three trials – those using isoxsuprine, arginine butyrate, and RGD peptide matrix – reported on the incidence of complete closure of the ulcers. None found a significant effect. No trial reported on the time to complete ulcer healing, ulcer-free survival for patients with SCD, quality-of-life measures, or incidence of amputation. There was no reported information on the safety of these interventions.

Nitric oxide (NO) bioavailability is diminished in SCD as a consequence of associated processes including hemolysis, oxidative stress, and arginine depletion. NO, a soluble gas with a half-life of a few seconds, is continuously synthesized in endothelial cells from the amino acid L-arginine by NO synthase. In addition to its antimicrobial activity, NO mediates a number of essential biologic processes, including vasodilation, wound healing, and angiogenesis. Moreover, NO has anti-platelet effects and influences availability of several growth factors involved in endothelial homeostasis. Based on these properties, topical delivery of NO is a logical approach for treating leg ulcers in SCD. Sodium nitrite is thought to act as a reservoir for NO. The nitrite anion acts as a vasodilator in vivo by generating NO in tissues with low oxygen tension and pH. The mechanism involves a novel physiologic function of hemoglobin and myoglobin as oxygen- and pH-dependent nitrite reductases. Therefore, nitrite provides the ideal substrate for NO generation along the hypoxic gradient likely to be present in chronic wounds.

This topical solution to healing skin ulcers in hemolytic disease appears to have merit. As described above, many factors are involved in the pathogenesis of the ulcers related to hemolytic disease, including inflammation, oxidative stress, venous insufficiency, hypercoagulability, hypoxia, and infection. The lack of efficacy of therapeutic approaches with a limited spectrum of activity underscores the complexity of this pathogenesis. In chronic-venousstasis ulcers not related to hemolysis, improving blood flow is paramount. Under hypoxic conditions, nitrite cream releases NO to promote vasodilation and thereby improve blood flow (Umbrello M et al. J Physiol. 2014. Epub ahead of print). The hypoxic environment of the ulcer may further induce additional NO generation as a consequence of NO release from deoxygenated hemoglobin. Nitrite also has significant antibacterial activity (a property that has been appreciated by butchers for years) that can inhibit bacterial growth in the ulcer. Production of methemoglobin by the oxidative effects of nitrite may have a beneficial effect by releasing heme that can interact with toll-like receptor 4 expressed on the vasculature to promote an inflammatory response that could contribute to the healing process (Belcher JD et al. Blood. 2014;123:377-90). Potentially, nitrite may also affect pain pathways via local mast cell activation as recently described (Vincent L et al. Blood. 2013;122:1853- 62). The main toxicity concern is whether the dose of nitrite delivered by the treatment compound will cause systemic hypotension, as absorption from an open wound can be significant. As noted above, the present study will assess hypotension and methemoglobinemia as adverse events.

The devastating toll that leg ulcers exact from patients with SCD and other hemolytic anemias is under-appreciated. This well-designed study will generate substantive new information on the strategy of topical delivery of NO as treatment for this challenging complication of SCD vasculopathy.