Intravenous infusion of haptoglobin for the prevention of adverse clinical outcome in Sickle Cell Disease.


Sickle Cell Disease (SCD) is a genetic condition which manifests as altered hemoglobin (Hb) protein that can aggregate under hypoxic conditions. The resultant sickled erythrocytes experience premature hemolysis, releasing an estimated 10g of free Hb (fHb) into the intravascular space. FHb participates in redox reactions creating various reactive oxygen species which rapidly and irreversibly scavenge nitric oxide, thereby attenuating its vasodilatory, antithrombotic, and anti-inflammatory properties. FHb also induces endothelial expression of adhesion molecules, triggering leukocyte margination at the vessel wall. These mechanisms participate in diverse SCD-associated clinical events including nephropathy, pulmonary hypertension, chronic leg ulceration, and ischemic events. FHb also exerts a direct reno-toxic effect contributing to albuminuria which is an early, frequent manifestation of glomerular injury. Under normal conditions, fHb is effectively scavenged by the Hb-scavenging mechanism (HSM); this involves binding to haptoglobin (Hp), uptake via the Hb-scavenging receptor (CD163) on monocytes and metabolism by heme-oxygenase-1. This culminates in increased CD163 expression and release of anti-inflammatory by-products e.g. interleukin-10 (IL-10). In SCD, the Hb-binding capacity is overwhelmed by chronic hemolysis; our previous research shows serum Hp as the depleted component. This deficiency could result in the harmful consequences of circulating fHb going unbridled. The hypothesis we explore here is that Hp infusions, in excess of fHb concentration, will allow the HSM to remain functional, and thereby achieve improved clinical outcomes, tracking albuminuria as a sentinel. Albuminuria was selected because of its high prevalence in SCD and its relative ease of diagnosis and monitoring. The hypothesis may be evaluated in four phases: Phase 1 will determine the concentration of Hp needed to trigger the HSM as measured by induction of CD163 and IL-10 and the recovery of hemopexin. Phase 2 will investigate the half-life of HSM induction by analyzing the time-course of CD163 expression and IL-10 and hemopexin serum concentration. Phase 3 will determine patient eligibility for therapy, whether as treatment or prevention. Phase 4 will test the efficacy of Hp transfusions in a randomized control trial as measured by correction of albuminuria. Angiotensin converting enzyme inhibitors (ACEi) are currently the first-line treatment for SCD nephropathy, however hyperkalemia limits its use. Hydroxyurea, which has therapeutic value in many SCD adverse events, has yielded inconsistent effects on albuminuria. We are proposing the addition of an intervention more proximal in the hemolytic cascade. Boosting the exhausted Hb-scavenging capacity via Hp replacement therapy has the potential to modify multiple downstream clinical events.

Medical hypotheses