Improved therapy for paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematopoietic stem cell (HSC) disorder due to one or more mutations in the phosphatidylinositol glycan class A (PIG-A) gene (1-4). Untreated PNH is characterized by anemia due to intravascular hemolysis (IVH), a high risk of thrombosis and an association with (acquired) bone marrow failure syndromes (5). The mutation in PIG-A leads to a reduction in the level of expression or complete absence of the glycosylphosphatidylinositol (GPI) anchor that is critical for the cell surface expression of the complement regulatory proteins CD55 and CD59. Since the mutation arises in an HSC, the loss of CD55 and CD59 is observed in all hematopoietic cell lineages including erythrocytes, granulocytes, monocytes and platelets (5). However, the brunt of this deficiency is felt by erythrocytes since they do not have compensatory complement regulatory proteins such as CD46.

Central to the explanation of the clinical manifestations of PNH is an understanding of the complement cascade (Figure 1). The complement system, a part of the innate immune response, can be activated via three pathways: the classical pathway due to antigen-antibody complexes, the lectin pathway activated by bacterial membrane glycoproteins and the alternative pathway. The latter is critical for PNH since the alternative pathway is constitutively active and continuously generating a C3 convertase through the so-called ‘tickover’ mechanism (6,7), and amplified by the Factor B/Factor D/Properdin complex (Figure 2). Once the membrane-bound C3b is activated by hydrolysis, it can activate C5 which then triggers the generation of the membrane attack complex through C6–9 polymerization on the plasma membrane of red blood cells leading to the formation of pores and IVH. This results in hemoglobinuria if the hemolysis is brisk enough. While CD59 blocks formation of the membrane attack complex, CD55 blocks generation of activated C3. Free hemoglobin in the circulation scavenges nitric oxide and can lead to smooth muscle spasm and endothelial dysfunction. Activated C3 and C5 can promote thrombin generation and trigger the coagulation system leading to thrombosis. Hemoglobinuria can result in kidney damage, especially in combination with loss of nitric oxide in the renal papillae. Pulmonary hypertension can result from nitric oxide loss, anemia, and thrombotic events. Finally, the production of C3a and C5a can induce cytokines that in part contribute to fatigue.

Figure 1 Complement activation via the 3 pathways: classical, lectin and alternative pathway. The three pathways converge onto C3 and together are known as the proximal pathway. C3 generates C5a and C5b, the latter triggering the formation of the MAC. C5 and the components of the MAC constitute the terminal pathway of complement activation. MAC, membrane attack complex; MASP, mannan-binding lectin-associated serine proteases.

Figure 2 The AP of complement activation. This pathway is constitutively active and undergoes amplification by the enzymatic activity of the AP C3 convertase that includes both Factor B and Factor D. These are the targets of iptacopan and danicopan, respectively. AP, alternative pathway.

Hemolysis in untreated PNH is the consequence of a series of coalescing conditions including (I) mutation of PIG-A in an HSC; (II) expansion of the mutated HSC so that the clone becomes responsible for a substantial contribution to hematopoiesis; (III) C3b is a membrane-bound protein; (IV) erythrocytes lack CD46 while erythrocyte CR1 appears to act mainly as a cofactor for Factor I, rather than blocking the C3b convertase with the result that red cells belonging to the clone cannot prevent complement activation; and (V) the alternative pathway is continuously active (6,7). All of these are necessary for IVH to occur. If any of these conditions were absent, PNH would be an interesting epiphenomenon and relevant in the presence of a complement amplifying condition.

Untreated PNH is a life-threatening disease, mainly due to the risk of thrombotic events that may occur in 40% of patients (8). The advent of complement-targeting therapies, the first being eculizumab that blocks C5, changed the natural history of the disease (9,10). Eculizumab controls IVH and improves hemoglobin as well as fatigue (11). The serum lactate dehydrogenase (LDH) improves and often returns to close to normal values. Many patients became transfusion independent, even though the hemoglobin rarely normalizes. Eculizumab improves prognosis by reducing the risk of thrombosis (10). Ravulizumab was subsequently developed by engineering eculizumab to allow recycling of the antibody through the neonatal Fc receptor, increasing the half-life of the molecule (12). As a result, while eculizumab has to be given intravenously every two weeks, ravulizumab is given every 8 weeks, improving therapeutic convenience for patients (13,14). Moreover, while with eculizumab, some patients had pharmacokinetic breakthrough hemolysis (BTH) due to antibody decay leading to subtherapeutic blood levels before the next dose was due (at least with standard dosing), this problem is much less likely to happen with ravulizumab. More recently, crovalimab was approved as a third C5 inhibitor that is administered subcutaneously every 4 weeks after ramp-up dosing.

Eculizumab was approved in 2007. Soon after, a new phenomenon was observed: some patients remained anemic and transfusion dependent in the absence of overt bone marrow failure (15). These patients maintained a high reticulocyte count despite a response to eculizumab as measured by a reduction in the LDH, suggesting that IVH was controlled. Flow cytometry showed that the red cells had high levels of C3d on their surface. Thus, these red cells were being destroyed in the reticuloendothelial system in the liver and spleen, a phenomenon described as extravascular hemolysis (EVH) (15). In clinical practice, the direct antiglobulin test that is typically negative in untreated PNH, may become positive (only to C3, not IgG) when some patients are on a C5 inhibitor.

Persistent anemia with or without the need for transfusions was subsequently reported in up to 30% of patients with PNH treated with a C5 inhibitor, which stimulated the development of other complement-targeting agents to prevent both IVH and EVH and improve the hemoglobin further (16). It is pertinent to mention that many patients with PNH will continue to experience fatigue—the most common symptom in this disease, even if they are transfusion independent as long as they remain anemic (hemoglobin <12 g/dL) (17).

Pegcetacoplan was the first proximal inhibitor approved for PNH and it blocks C3 and C3b (18). It is self-administered as a subcutaneous infusion twice a week. In the PRINCE and PEGASUS trials, pegcetacoplan was shown to effectively control both IVH and EVH and often leads to higher hemoglobin levels compared to placebo or eculizumab. Many patients were able to avoid transfusions and both fatigue and quality of life improved (19,20).

More recently, iptacopan, the first oral Factor B inhibitor (Figure 2), was approved after positive results from the APPLY-PNH and APPOINT-PNH studies (21,22). Iptacopan is taken twice a day, and it prevents both IVH and EVH. As a result, many patients have a hemoglobin level that is either normal or >12 g/dL. Over 85% of patients remain transfusion independent on this agent. Many patients have normalization of the FACIT fatigue score and concomitantly the quality of life improves (23).

With the use of proximal complement inhibitors, many more erythrocytes that belong to the PNH clone survive and the red blood cell population with the PNH phenotype starts to approach the true size of the clone as measured for neutrophils and monocytes. Several complement components are acute phase reactants and their blood levels can transiently increase in the presence of infection, inflammation, surgery, vaccination, trauma or pregnancy (24,25). In such cases, there is a risk that the therapeutic agent may not be present at a high enough concentration to block the complement cascade and the patient experiences recurrence of symptoms of IVH including fatigue, dyspnea, jaundice, hemoglobinuria and laboratory evidence of hemolysis: rising LDH, bilirubin, a drop in hemoglobin and rising reticulocyte count. Patients may require transfusion support, especially if the drop in hemoglobin is significant. The acute IVH event is known as a pharmacodynamic BTH and can increase the risk of a thrombotic event. These events have been reported with all complement inhibitors (26,27): including 5–18% of patients on a C5 inhibitor [summarized in (28)], in 28% of patients on pegcetacoplan (29) and in 7.4% of patients on iptacopan (22), especially after COVID-19 infection, which is a significant activator of complement (30). To address this potential problem, guidelines for the management of pharmacodynamic BTH have been developed (31).

The complement cascade has many similarities to the coagulation system with proenzyme activation leading to considerable amplification down to the level of C5. However, each C5 molecule can only generate one membrane attack complex (24,25). Therefore, in theory, pharmacodynamic BTH while on a C5 inhibitor should produce only limited IVH. This observation suggested that dual inhibition targeting C5 and the alternative pathway should allow patients to be in the sweet spot where severe pharmacodynamic BTH would be unlikely while also benefiting from a higher hemoglobin by the prevention of EVH.

Danicopan is the first oral Factor D inhibitor approved for therapy of PNH in combination with a C5 inhibitor. Factor D serves as a cofactor for Factor B in the alternative pathway that acts as a C3 convertase (Figure 2). Danicopan is taken 3 times a day and was studied in combination with either eculizumab or ravulizumab in the ALPHA clinical trial (32,33). To be enrolled in this study, patients with PNH had to have a hemoglobin level ≤9.5 g/dL and an absolute reticulocyte count ≥120×10⁹/L. The study enrolled 86 patients: 57 on the danicopan arm and 29 in the placebo arm. However, 82 patients completed therapeutic period 1 that compared patients on danicopan with placebo (all were on a C5 inhibitor). Subsequently in therapeutic period 2, the patients in the placebo arm were switched to danicopan as well. The addition of danicopan to either eculizumab or ravulizumab led to an improvement in hemoglobin of 2.8 g/dL from their baseline. At the 12-week mark, 54.4% of the patients had an increase in hemoglobin >2 g/dL. Moreover, 64.3% of patients achieved transfusion avoidance in the danicopan arm. Concomitant with this improvement in hemoglobin, there was a reduction in the absolute reticulocyte count and a reduction in C3 deposition on red blood cells and in bilirubin, showing that EVH was also controlled. The median LDH was close to the upper limit of normal. Danicopan was well tolerated and adherence to therapy was high at 97.1%. The drug was stopped in 6 patients due to abnormal liver transaminases; one patient developed acute cholecystitis, and another progressed to aplastic anemia. The investigator reported BTH events were seen in 5 patients who between them experienced 7 breakthrough hemolytic events (6%). None of the patients needed a blood transfusion or developed thrombosis during these episodes.

The differences in risk and severity of BTH with the various agents have become a point of discussion in the field. In this respect, not all complement targets and agents are created equal. C3 is present in the highest concentration in the blood and undergoes significant increases in concentration as part of the acute phase response that may render the standard dose of pegcetacoplan inadequate to block C3. While Factor B is also an acute phase reactant, its concentration in the blood is lower and as a consequence, the lability of its levels is likely proportionately less, reducing the probability that it escapes the inhibitory effect of standard dosing with iptacopan, even in the presence of an acute phase response. Factor D is not an acute phase reactant.

Some physicians are concerned about the risk of severe BTH and possible thrombosis in patients treated with a proximal inhibitor alone if they develop an acute inflammatory state or due to lack of compliance. In theory, the combination of danicopan with a C5 inhibitor seems to provide reassurance. The incidence of BTH events with iptacopan (7.4%) (22) appears to be similar to that of the combination in the ALPHA trial (6%) (32,33). While in principle compliance can be a concern, it is uncommon for patients to miss iptacopan doses, in part because the drug is well tolerated and patients feel better on it, given the excellent control of hemolysis it provides. Moreover, iptacopan concentrations return to therapeutic levels rapidly once the drug is resumed if a dose is missed.

It has been suggested that the combination of danicopan and a C5 inhibitor provides a ‘belt and suspenders’ approach to PNH care (34). The idea is to provide maximal protection of red blood cells in good times and bad (i.e., with complement amplifying conditions) with a safety net supported by the C5 inhibitor if danicopan is unable to control the proximal complement pathway for any reason (24). While the strategy is safe, it is up to each physician and their patient to determine whether this is a superior approach compared to single agent iptacopan with the latter’s convenience. Such decisions are very personal and need to take into consideration the patient’s lifestyle, their understanding of the disease, likelihood of compliance with therapy, convenience, cost and time spent procuring care (35). We are fortunate that there are multiple treatment options available for patients with PNH, allowing for informed and combined decision making (28,35). Moreover, aiming for a normal or near normal hemoglobin should be a new goal of therapy—patients clearly feel better with a higher (or normal) hemoglobin. Simply aiming for a hemoglobin level that avoids transfusions should be a thing of the past.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Blood. The article did not undergo external peer review.

Funding: None.

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://aob.amegroups.com/article/view/10.21037/aob-2025-1-62/coif). D.D. reports consulting fees from Alexion, Apellis, BMS, Johnson and Johnson, Novartis, and payment or honoraria from Alexion, Merck Sharpe and Dohm and Novartis. The author has no other conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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