Plasma-Derived Hemopexin as a Candidate Therapeutic Agent for Acute Vaso-Occlusion in Sickle Cell Disease: Preclinical Evidence

Abstract

People living with sickle cell disease (SCD) face intermittent acute pain episodes due to vaso-occlusion primarily treated palliatively with opioids. Hemolysis of sickle erythrocytes promotes release of heme, which activates inflammatory cell adhesion proteins on endothelial cells and circulating cells, promoting vaso-occlusion. In this study, plasma-derived hemopexin inhibited heme-mediated cellular externalization of P-selectin and von Willebrand factor, and expression of IL-8, VCAM-1, and heme oxygenase-1 in cultured endothelial cells in a dose-responsive manner. In the Townes SCD mouse model, intravenous injection of free hemoglobin induced vascular stasis (vaso-occlusion) in nearly 40% of subcutaneous blood vessels visualized in a dorsal skin-fold chamber. Hemopexin administered intravenously prevented or relieved stasis in a dose-dependent manner. Hemopexin showed parallel activity in relieving vascular stasis induced by hypoxia-reoxygenation. Repeated IV administration of hemopexin was well tolerated in rats and non-human primates with no adverse findings that could be attributed to human hemopexin. Hemopexin had a half-life in wild-type mice, rats, and non-human primates of 80–102 h, whereas a reduced half-life of hemopexin in Townes SCD mice was observed due to ongoing hemolysis. These data have led to a Phase 1 clinical trial of hemopexin in adults with SCD, which is currently ongoing.

Heme Interferes With Complement Factor I-Dependent Regulation by Enhancing Alternative Pathway Activation

“…Administration of plasma-derived hemopexin before or during VOC in the Townes SCD mouse model, triggered either by heme or hypoxia-reoxygenation, have shown that administration of hemopexin can prevent or reduce VOC, but also, have antioxidant and anti-inflammatory effects (46,47). Therefore, an objective became to investigate hemopexin's role in the interplay between factor I and heme.…”

Hemolysis, free hemoglobin toxicity, and scavenger protein therapeutics

“…Autoxidation of HbFe(II)O2 to HbFe(III) enhances heme transfer to lipids 26,[70][71][72] , neutrophils, macrophages, and the vascular endothelium 39 triggering inflammation 73 and cell adhesion pathways 74 that are important contributors to VOC. Murine models of SCA provide proof of concept that haptoglobin and hemopexin prevent, and in part reverse, Hb-and heme-induced vascular stasis 75,76 . Since haptoglobin consumption is very rapid during brisk hemolysis characteristic of human SCA, haptoglobin replacement therapy may be challenging 26,67 .…”

The ‘scintilla’ starting vaso‐occlusion in sickle cell disease

“…Hemopexin is a strong haeme-binding protein. 5 Free haeme in SCD is a well known cause of nitric oxide (NO) scavenging. The lack of NO increases the expression of adhesion molecules on endothelial and circulating cells, facilitating vaso-occlusion.…”