Predictors of hemoglobin response to eculizumab therapy in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, clonal, hematopoietic stem cell disorder that manifests with a complement-mediated hemolytic anemia, bone marrow failure, and a propensity for thrombosis (1–3). Somatic mutations in PIG-A, a gene whose product is required for the synthesis of glycosylphosphatidyinositol (GPI) anchors, are found in all patients with PNH (4–6). The GPI anchor is a lipid moiety that tethers dozens of proteins to cellular membrane; thus, PNH cells have a marked deficiency or absence of all GPI-anchored proteins. Hemolysis in PNH is a direct consequence of CD55 and CD59 deficiency. These GPI-anchored proteins protect normal red cells at different stages of the complement cascade. CD55 regulates the formation and stability of the C3 and C5 convertases (4), whereas CD59 blocks the formation of the membrane attack complex (MAC) (5). The chronic hemolytic anemia in PNH is largely mediated by the alternative pathway of complement that is in a state of continuous activation through a process known as tick-over (6, 7). Increased activation of complement due to surgery, stress, infections, or inflammatory disorders can lead to severe paroxysms of hemolysis. However, iron deficiency from intravascular hemolysis and varying degrees of underlying bone marrow failure may also contribute to the anemia in patients with PNH. Eculizumab is an FDA-approved humanized monoclonal antibody for the treatment of PNH and in clinical trials has been shown to improve anemia, decrease intravascular hemolysis, reduce the risk for thrombosis, and markedly improve quality of life for most patients with PNH (8–10). The language used by the FDA for its approval is broad and states that it is indicated ‘for the treatment of patients with PNH to reduce hemolysis’. However, this drug does not improve bone marrow failure nor is it curative. After an initial 1-month loading period, the drug is administered intravenously every 14 ± 2 d. Eculizumab blocks terminal complement activation by recognizing C5 and preventing formation of the MAC (11, 12). Eculizumab compensates for the CD59 deficiency on PNH red cells, but it does not compensate for the CD55 deficiency. In patients receiving eculizumab, PNH red cells begin to accumulate C3 on their cell surface, which leads to opsonins that are recognized by the reticuloendothelial system resulting in extravascular hemolysis (13, 14). This phenomenon is not observed in untreated patients with PNH because complement activation on red cells in the absence of eculizumab leads to rapid elimination due to the MAC. Laboratory evidence of extravascular hemolysis in eculizumab-treated patients includes an elevated reticulocyte count, a normal to mildly elevated LDH, varying degrees of anemia, and in some cases, a direct Coombs test that is positive for C3 but not IgG. While many of these patients remain asymptomatic and do not require therapy, others have symptomatic anemia and require chronic blood transfusions. Eculizumab costs more than $400,000 US dollars annually, making it one of the world’s most expensive drugs. Thus, it is important to identify the factors that are responsible for suboptimal response to eculizumab therapy to make informed treatment decisions. In this study, we performed a retrospective analysis on 30 eculizumab-treated patients to identify factors associated with suboptimal response to treatment.