Hemopexin Safely Reduces Signs of SCD Crises in Preclinical Study
A naturally-occurring protein called hemopexin, also an investigational therapy for acute vaso-occlusive crises (VOCs) in sickle cell disease (SCD), safely reduced the signs of such crises in cell and animal models, a study has demonstrated.
These findings support an ongoing Phase 1 trial (NCT04285827), sponsored by the therapy’s developer CSL Behring, to evaluate the safety, tolerability, and pharmacological properties of a form of hemopexin — CSL889 — in about 32 SCD adults, ages 18–60. Enrollment is open at sites in the U.S., the U.K., and the Netherlands.
CSL889 has been granted orphan drug status in Europe and the U.S., conferring several benefits to expedite the therapy’s path toward commercial availability.
In SCD, genetically inherited mutations lead to the production of faulty hemoglobin, the protein within red blood cells that carries oxygen, causing these cells to adapt a sickle-like shape.
Sickled cells die more quickly, resulting in a shortage of red blood cells in a condition known as anemia, a common symptom of SCD. In addition, sickled red blood cells can stick to blood vessel walls and, if they accumulate, block blood flow, leading to VOCs — acute episodes of severe pain primarily affecting the chest, back, and extremities.
When red blood cells die, they release heme into the bloodstream, a molecule normally bound to hemoglobin that is essential for oxygen transport. Free heme is a potent inducer of inflammation and plays a role in sickled red blood cells sticking to blood vessel walls and subsequent VOCs.
Hemopexin is a naturally-occurring protein that scavenges free heme in the bloodstream and is found at significantly lower levels in SCD patients. In an SCD mouse model, treatment with hemopexin decreased vascular inflammation and blockages associated with VOCs.
In this report, CSL Behring scientists, in collaboration with the University of Minnesota, conducted experiments to confirm the effects of hemopexin treatment in cell and animal models of SCD to support further clinical evaluation.
First, endothelial cells that line blood vessel walls were exposed to various concentrations of hemopexin, then stimulated with heme. Increasing hemopexin concentrations led to a dose-dependent reduction in the production of P-selectin on the surface of endothelial cells. This protein helps cells, including sickled red blood cells, to stick to blood vessel walls. At high hemopexin concentrations, P-selectin was nearly abolished.
Increasing hemopexin concentrations during heme stimulation also led to a dose-dependent reduction in the levels of von Willebrand Factor, a protein essential in blood clotting that is associated with sickled cell adhesion to blood vessels. Other endothelial cell markers related to vascular inflammation and VOCs were also reduced in a dose-dependent manner with hemopexin.
Next, in an SCD mouse model that carried faulty hemoglobin, hemopexin was injected into the bloodstream at three different doses, each at three time points, before the animals were challenged with a single dose of hemoglobin, which released heme into the bloodstream and induced vaso-occlusion — or reduced blood flow.
Hemopexin prevented vaso-occlusion in a dose-dependent manner when administered one hour before hemoglobin, but not when given 24 or 48 hours prior. An additional experiment confirmed that hemopexin acted as a carrier to deliver excess heme to the liver for degradation.
To determine whether hemopexin could prevent vaso-occlusion once triggered — mimicking the treatment of SCD patients with VOCs — SCD mice were first injected with hemoglobin, then infused with hemopexin at different doses 30 minutes later. Hemopexin reduced vaso-occlusion in a dose-dependent manner one hour after infusion. Similar results were seen when vaso-occlusion was triggered by a low oxygen (hypoxic) environment.
Because hemopexin binds and scavenges free heme, hemopexin with pre-bound heme would be expected to block free-hemopexin’s ability to scavenge heme and reduce vaso-occlusion. However, similar to free-hemopexin, there was a dose-dependent reduction in vaso-occlusion using the hemopexin-heme complex.
A direct comparison of free-hemopexin and heme-hemopexin administered at a single dose found that heme-hemopexin reduced vaso-occlusion over time, but less potently than free-hemopexin.
“This unexpected finding suggests an inherent protective activity of hemopexin-heme complexes that is distinct from reducing levels of free heme,” the team wrote.
The researchers then examined hemopexin levels in the bloodstream after injection and how long it remained in SCD and healthy mice. In SCD mice, the maximum concentration of hemopexin — Cmax — was markedly lower than in healthy mice (0.41 vs. 0.70 mg/mL).
A lower Cmax was associated with faster clearance from the bloodstream and a lower half-life (7 vs. 58 hours) — the time until hemopexin concentration decreased by half — which suggested “increased binding of hemopexin to accessible heme in SCD,” the team added. Consistently, after repeated dosing in SCD mice, higher hemopexin concentrations correlated with lower total heme levels.
Finally, daily infusions of hemopexin in SCD and healthy mice for two weeks were well-tolerated, with no observed adverse events. At high doses, immune-related reactions were seen in some dosing groups after repeated administration, “which are considered to be a result of the application of the … human protein,” the researchers wrote.
Similarly, repeated exposure to human hemopexin in rats and cynomolgus monkeys was well-tolerated, but also associated with the development of anti-hemopexin antibodies, which was “an expected immune reaction after repeated application of the … human protein to animals, and is considered not to be predictive of the clinical situation,” the researchers wrote.
“We conclude that hemopexin is a promising new candidate to treat acute vaso-occlusive crises in people living with SCD,” they wrote.