Sickle Cell Disease (SCD) is an autosomal recessive disorder caused by a single amino acid substitution in the Hemoglobin (Hb) β chain which causes it to polymerize in the deoxy state. Acute Chest Syndrome (ACS) with hypoxemic respiratory failure is a common cause of death in adult SCD subjects. Recent research has revealed how multiple disease processes including inappropriate cellular adhesion, hemolysis, dysfunctional coagulation, inflammation, and ischemia and tissue injury are involved in ACS pathophysiology. In this review article, we discuss how the activity of the highly purified non-ionic amphiphilic triblock copolymer vepoloxamer in the repair of damaged membranes and hindrance of inappropriate adhesive interactions and activation events potentially has utility in ACS, by virtue of its capacity for affecting multiple aspects of the pathophysiology.
Sickle Cell Disease (SCD) is an autosomal recessive disorder caused by a single amino acid substitution in the Hemoglobin (Hb) β chain which causes it to polymerize in the deoxy state. The Hb polymerization results in Red Blood Cells (RBC) which are rheologically impaired and become entrapped in the microvasculature resulting in tissue ischemia . Vaso-occlusive Crisis (VOC) is the hallmark of the disease and consists of severe episodic pain in the back, chest, abdomen, and joints. A frequent complication of VOC is a pulmonary illness known as Acute Chest Syndrome (ACS) typified by new pulmonary infiltrate on chest X-ray along with leukocytosis, fever, cough, chest pain, and other pulmonary symptoms . The prospective cooperative study of sickle cell disease observed an incident rate of 12.8 ACS episodes /100 patient-years for homozygous SCD . ACS with hypoxemic respiratory failure is a common cause of death in adult SCD subjects .
The origin of ACS has been associated with a wide variety of pathological processes including infection, fat embolus, and lung tissue infarction . Regardless of the origin, pulmonary vaso-occlusion is likely the common pathophysiological pathway to its clinical manifestation . Underlying the vaso-occlusion is the polymerization of Hemoglobin S (HbS) under hypoxic conditions, and broad endothelial dysfunction with elevated expression of multiple cell adhesion molecules contributing to vascular leukocyte recruitment, inflammation, and the activation of coagulation with thrombosis . Hemolysis and the resulting increase in cell-free plasma Hb further exacerbates the endothelial dysfunction as Nitric Oxide (NO) bioavailability is reduced from the NO scavenging by cell-free heme , a compound which also activates innate immunity following binding of endothelial Toll-like receptor 4 .
With the notable exception of the polymerization of HbS, vepoloxamer has been shown to affect many of the pathological processes active in ACS, either prophylactically by interfering with activation events which drive pathogenesis, or through palliative actions against processes associated with progressive clinical pathology. Examples include: inhibiting RBC aggregation thus lowering whole blood viscosity, stabilizing of RBC membranes thereby reducing hemolysis and release of free heme, and preventing adhesive interactions between blood cells and activated endothelium thereby inhibiting occlusion, inflammation, thrombosis, and ischemic tissue damage. This publication summarizes research studies investigating the impact of vepoloxamer on those processes that are relevant to ACS pathophysiology. In doing so, we provide information on the potential mechanisms through which vepoloxamer may have utility in the treatment of ACS, a clinical conditions for which there are otherwise few therapeutic options.
Vepoloxamer is a highly purified, non-ionic amphiphilic triblock copolymer of Polyoxypropylene (POP) flanked on both sides by Polyoxyethylene (POE)  (see figure 1). Its basic structure confers surface active properties that enable the modulation of a cell membrane’s biophysical properties including its stability, hydration, repair, flexibility, and adhesive properties, all of which serve crucial roles in biological responses . Vepoloxamer is currently under study in a Phase III trial to evaluate whether it can reduce the duration of VOC (i.e., the EPIC study: Evaluation of Purified Poloxamer 188 In Crisis. Clinical Trials.gov Identifier: NCT01737814). The study also will evaluate whether vepoloxamer can reduce the frequency of re-hospitalizations following crisis, and the incidence of ACS during VOC.
The following is a survey of vepoloxamer’s in vitro activities which may be relevant to ACS’s pathophysiology.
Vepoloxamer was studied in a human transgenic murine model of ACS (i.e., HbS Tg). Mice expressing human HbS at levels of either 60% or 100% were treated with a single intravenous 400mg/kg dose of vepoloxamer (n=20) or a similar volume of saline (control) (n=20) and exposed to hypoxia (5% O2). Following hypoxia, the number of sickled RBCs in the peripheral blood increased with time, and then sharply decreased before the onset of ACS symptoms. Fourteen of twenty control (60 and 100% HbS) Tg mice died within 60 minutes. In marked contrast, fifteen of twenty vepoloxamer-treated (60 and 100% HbS) Tg mice survived for 60 minutes (i.e., the total duration of the experiment).
Lung pathology was evaluated in 13 control and 13 vepoloxamer-treated HbS Tg mice, and the findings are summarized in table 1. Thrombosis was detected in all 13 control mice and was considered of a severe nature in 10 of 13 animals, and non-severe in the other three. Thrombosis was only observed in 2 of 13 vepoloxamer-treated mice and both instances it was considered to be non-severe. Nine of 13 control mice showed both RBC infiltration and partial damage of the intervening septa wall, while none of the vepoloxamer-treated mice had such changes. Leakage of RBCs into alveolar space was found in 8 of 13 control mice, while it was not present in any of the vepoloxamer-treated mice. Eleven control mice showed congestion in the lungs, while none was detected in any of the vepoloxamer-treated animals. All of the vepoloxamer-induced changes were highly significant (see Table 1).
|Intervening septal wall damaged||13-Sep||0/13||0.0002|
|RBC in alveolar space||13-Aug||0/13||0.0008|
Table 1: Effect of Vepoloxamer On Lung Pathology in Experimental ACS.
*two-tailed p-value using Fisher’s exact test
The results demonstrate that vepoloxamer decreases mortality and prevents the development of ACS-like pulmonary pathology in a relevant model of acute sickling induced ACS.
The safety of vepoloxamer was studied as part of an open label dose escalation trial in 43 SCD patients diagnosed with ACS . The diagnosis of ACS required that a subject had confirmed SCD, a sudden onset of acute chest pain or respiratory distress lasting at least 4 hours, and a new clinically significant infiltrate on chest X-ray. Eligible subjects received total doses of vepoloxamer between 1,120-2,960mg administered during a 24 hour window by continuous intravenous infusion. Five dose levels were studied with a minimum of six patients at each dose level. Safety was assessed by clinical outcomes and clinical laboratory measures including special renal function tests.
Overall vepoloxamer was well tolerated at all dose levels. The study paid particular attention to renal function since renal dysfunction had been observed in earlier studies investigating unpurified poloxamer in myocardial infarction patients. In marked contrast to those earlier studies, no clinically significant dose-related increase in serum creatinine were observed. In addition, there were no dose-related or clinically relevant changes in specialized biomarkers of renal function (i.e., glomerular integrity, permeability and filtration; proximal tubular injury; distal tubular transport, and protein absorption). As had been reported in previous studies of vepoloxamer in sickle cell patients, [49,50] liver function tests, especially bilirubin and serum transaminases, were mild to moderately elevated during vepoloxamer infusion, but, resolved to baseline during follow-up (usually by day 5-10 but no later than day 28-35 post-infusion). The authors concluded that the administration of vepoloxamer to patients with ACS was safe.
Although the study was not designed to evaluate efficacy, trends in biomarkers and clinical outcomes suggestive of benefit were observed. The degree of sPLA2 is reported to correlate with measures of clinical severity in ACS and the rise in sPLA2 concentrations coincides with the onset of ACS . For all dosages, all patients with elevated sPLA2 showed a significant decrease by the end of vepoloxamer infusion which also continued to decrease during the remainder of the study. Both adult and pediatric patients showed similar trends in sPLA2 concentrations . In addition, there were trends, particularly at higher doses and in children, towards a shorter duration of an ACS episode and the length of ACS hospitalization especially compared to historical controls.
Acute Chest Syndrome is one of the most frequent complications in SCD, and together with pulmonary hypertension, is the disease’s most common cause of death. Despite its clinical impact, treatment alternatives for ACS are limited, and there is an unmet need for new prophylactic or therapeutic interventions. In this document, we provide justification for use of vepoloxamer in ACS. Our most compelling argument is based on two underlying and complimentary considerations: First, because multiple pathophysiological processes underlie ACS, it may be necessary to address several processes for optimal outcomes. Vepoloxamer may uniquely achieve this. Second, vepoloxamer appears safe and well tolerated in ACS patients at exposures equaling or exceeding those that result in all the desired pharmacological activity. These and other considerations are discussed more completely below.
In this document we present data showing that vepoloxamer has the capacity to improve many of the biological processing which appear to account for ACS pathophysiology. We presented data showing it improves the deformability and filterability of sRBC thus improving their capacity to move through narrow channels akin to what these cells encounter in vivo within the SCD microvasculature. We also discussed vepoloxamer’s effect in reducing whole blood viscosity under low shear rates. Other published reports confirm this activity [26,53]. The benefits of this rheological activity on blood flow have been demonstrated in vivo during clinical VOC. Administration of vepoloxamer to SCD patients during VOC, significantly improved RBC velocity by 2 hours following start of infusion in all vepoloxamer treated subjects, but in none of the controls. Additional improvement were also observed 7 hours post-infusion, such that the RBC velocity had returned to values similar to those observed at steady state in all vepoloxamer treated subjects but in none of the controls . As mentioned previously, pulmonary vaso-occlusion and reduced blood flow is recognized as the common pathophysiological event initiating the clinical manifestation of ACS. Whether this is driven primarily through enhanced cellular adhesion  or by elevated RBC aggregation is a matter of contention . Data presented in this publication that vepoloxamer reduces both of these underlying pathologies.
We also discussed the role of hemolysis in ACS and presented results showing that vepoloxamer stabilizes RBC membranes from hemolysis. This data is consistent with an extensive body of literature showing that vepoloxamer and its unpurified parent poloxamer 188 protects the membranes of a wide variety of cells from mechanical and chemical injury [12,26]. This suggests that vepoloxamer may not only protect injured sRBC from hemolysis, but also protect endothelium and other tissues from ischemic and reperfusion injury as suggested by Hunter .
The ability to protect cells from mechanical trauma may also contribute to vepoloxamer’s reported anti-thrombotic activity. By stabilizing RBC from shear-induced membrane damage, vepoloxamer may prevent ADP leakage, and thus ADP induced platelet aggregation / thrombosis. This mechanism may also account for the reported activity in antagonizing the procoagulant actions of microparticles.
Some of the most compelling evidence as to the potential utility of vepoloxamer in treating ACS comes from the studies conducted in human HbS transgenic animals. Of particular interest were the pathology results demonstrating the marked reduction in cellular infiltration and thrombosis with vepoloxamer treatment. These effects are consistent with vepoloxamer’s antithrombotic effects in other experimental models. The role of thrombosis in ACS remains a subject of speculation. However, thrombosis of pulmonary vessels is a common finding on autopsy of patients with SCD and pulmonary artery thrombosis was observed at a prevalence of 17% in a recent analysis of ACS patients . These findings suggest that antithrombotic therapy with an agent like vepoloxamer may be an important therapeutic goal in ACS.
Our document concludes with a discussion of a phase I safety study conducted with vepoloxamer in sickle cell patients experiencing ACS . The study investigated levels of exposure within the range of concentrations demonstrating the pharmacological activities discussed in this publication. At all levels of drug exposure, vepoloxamer was well tolerated and appeared safe. While the study was not designed to evaluate efficacy, trends toward reduced hospitalization and reductions in sPLA2 biomarkers were observed. The safety observed in this study is further supported by an extensive battery of non-clinical toxicology studies on vepoloxamer. Included were studies where vepoloxamer was administered to rats and dogs for 28 consecutive days as a continuous intravenous infusion. In those studies the No Observable Adverse Effect Level (NOAEL) for total exposure was 20,160mg/kg in dogs and 10,080mg/kg in rats. Considering that the anticipated total acute exposure of velopoxamer for treatment of ACS is in the range of 1,500-2,500mg/kg, we predict a several fold safety factor with regard to exposure.
The results from the Phase III EPIC study with vepoloxamer are anticipated to provide further justification for evaluating its use in treating ACS.
In ACS (and other sickle related vas-oocclusive events) there are multiple pathological processes including dysfunctional coagulation, inflammation, inappropriate cellular adhesion, hemolysis and others that contribute to ischemia and tissue injury. A common denominator to these seemingly disparate pathways may be the exposure of normally hidden hydrophobic phospholipids in damaged cell membranes serving as activation surfaces for the aforementioned pathological cascades. An agent such as vepoloxamer that will bind to such surfaces instantly sealing the damaged membrane and restoring normal surface hydration characteristics may thus inhibit multiple pathological pathways via a single mechanistic activity. The lack of such damaged surfaces in healthy tissues may also account for vepoloxamers safety profile and the observations that pharmacologically active concentrations of vepoloxamer appear safe and well tolerated in SCD patients with ACS.
This new type of pharmacological strategy holds significant promise for the treatment of ACS. Additional clinical studies to demonstrate its safety and efficacy are warranted.
Citation: Emanuele M, McKenzie D (2016) The Pharmacologic Rationale Supporting the Potential Use of Vepoloxamer for the Treatment of Acute Chest Syndrome in Patients with Sickle Cell Disease. J Hematol Blood Transfus Disord 3: 006
Copyright: © 2016 Martin Emanuele, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.