Paroxysmal Nocturnal Hemoglobinuria (PNH)
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, chronic, potentially life-threatening blood disorder. PNH can appear at any age and in any race or gender, and is diagnosed most often in people in their early 30s, and it usually continues throughout the life of the patient.1,2
Normally, stem cells in the bone marrow generate healthy blood cells including red blood cells, white blood cells and platelets. In PNH, stem cells acquire a gene mutation which results in the production of abnormal blood cells.3 Defective red blood cells become open to attack by complement, which is part of the immune system. This destruction of red blood cells is called hemolysis.1 As a result, the person will suffer from anemia, which is a shortage of healthy red blood cells.1
During intravascular hemolysis, the dying red blood cells release their contents into the bloodstream. In particular, they release hemoglobin, the iron-rich protein that normally transports oxygen from the lungs to the rest of the body. When released, this free hemoglobin becomes toxic to the body.2 So in addition to suffering from anemia, a patient with PNH may have a multitude of problems, including renal failure, pulmonary hypertension (high blood pressure in the lungs), thrombosis (blood clots within blood vessels), abdominal pain, dyspnea (shortness of breath), dysphagia (discomfort when swallowing), fatigue, impaired quality of life, and erectile dysfunction in men.1,2 Thrombosis has a significant impact on survival and is the leading cause of death in patients with PNH.4
The name paroxysmal nocturnal hemoglobinuria is a misnomer. The condition was first identified in patients who had severe crises of symptoms (paroxysms) and who had dark urine in the morning, because of large amounts of hemoglobin in their urine.2 However, the hemolysis is occurring all the time, not just at night and not just when the patient is experiencing symptoms.2 Thus, the condition is neither nocturnal nor paroxysmal. Also, the dark urine occurs in only 26% of patients with PNH.5
If left untreated, PNH results in the death of approximately 35% of affected individuals within 5 years of diagnosis6 and 50% of affected individuals within 10 years of diagnosis.1 The only cure for PNH is a bone marrow transplant from a well-matched donor (usually a brother or sister). However, this procedure is so risky that it is generally used only in severe cases. Therefore, other effective treatments are important for patient with PNH.7
In 2007, FDA approved eculizumab, a complement inhibitor and life saving drug for the treatment of PNH. By preventing activation of a part of the complement system, eculizumab can reduce the need for transfusions and improve quality of life for patients with PNH.8 However, eculizumab does not inhibit all of the complement system. As a result, roughly 70% of eculizumab-treated patients with PNH remain anemic and 35-50% of them are still dependent on transfusions.9
The Complement System
The complement system is an integral part of our immune defense system. In healthy people, complement orchestrates the destruction and clearance of pathogens or of host cells that need to be replaced. It also has proinflammatory capabilities.10
Complement may be activated by different pathways.10 The classical complement pathway is activated when an antibody recognizes a foreign pathogen (non-self target) that invaded the body and needs to be destroyed.10 The alternative complement pathway can be initiated by spontaneously activated complement.10 There is also a lectin pathway, which is activated by mannose-binding lectin.10
The complement system consists of a cascade of proteins, each of which activates the next protein in the cascade by cleavage (splitting.)10 Complement C3 is the central protein of the cascade, positioned at the point where all complement activation pathways come together. Activation of C3 then leads to the activation of the terminal pathway, which includes C5, in turn leading to the activation of membrane attack complexes (MACs).10
Complement activation is normally regulated to avoid its overactivation and to protect the host against immune attack.10
Role of Complement in PNH
The complement system, as an innate component of the immune system's natural defense, undergoes a constant low-grade activation through spontaneous cleavage of C3 of the alternative pathway. This activation is regulated in healthy people by many complement inhibitory proteins to prevent damage to the host body. Under normal conditions, the surface proteins CD55 and CD59 bind to cells and act as a shield to protect them from an attack and destruction by complement.11
In PNH, stem cells acquire a gene mutation which results in the production of abnormal blood cells, that lack these important surface proteins. As a result, the complement protein C3 becomes over-activated in patient with PNH, due to the absence of CD55/CD59 on red blood cells, which triggers (Figure 1):3,12
- The formation of the membrane attack complex (MAC), which results in intravascular hemolysis (destruction of circulating red blood cells) and activation of platelets
- C3b-mediated opsonization (tagging) of red blood cells (RBCs), which results in extravascular hemolysis.
Hemolysis results in a poor quality of life due to various clinical manifestations such as pulmonary hypertension, renal failure, abdominal pain, dyspnea, anemia, hemoglobinuria, dysphagia, erectile dysfunction, and fatigue.11 Moreover, intravascular hemolysis and activation of platelets increase the risk of blood clot formation, or thrombosis, which is the leading cause of mortality in patient with PNH.11
Extravascular hemolysis, which involves the destruction of blood cells in the liver and the spleen, is caused by the accumulation (deposition) of C3b active fragments on red blood cells (RBCs) leading to removal of the cells by macrophages.13 Extravascular hemolysis further contributes to severe anemia and transfusion dependency in patients with PNH.13
What is APL-2?
Through this broad inhibition of C3, APL-2 helps the body regain control of the complement system, protecting it from further complement-mediated immune attack.
Why Evaluate APL-2 in PNH?
Currently, PNH is treated by using a C5 inhibitor to block the activation of that part of the complement system.7 By preventing the activation of C5, the need for transfusions is reduced and quality of life is improved for patients with PNH. However, roughly 70% of patients remain anemic and 35% to 50% of the patients are still dependent on transfusion due to a different part of the complement system remaining active.9 The activation of C3, which is upstream of C5 in the complement cascade, can also lead to the destruction of the abnormal red blood cells in people with PNH.13
When the C3 protein is activated, it is cleaved (split) into C3a and C3b. C3b could then lead to opsonization of the deficient red blood cells. Opsonization is a process in which the cell is marked for destruction by white blood cells. This form of destruction of red blood cells is often described as extravascular hemolysis because it typically takes place in the liver or spleen. This potential role of C3b in causing extravascular hemolysis may explain why many patients with PNH continue to be anemic and transfusion-dependent, despite being treated with a C5 inhibitor.13
- All 19 subjects responded rapidly after initiating APL-2 therapy, and by day 29 mean baseline Hb increased from 8.0 g/dL to 10.8 g/dL.
- Increases in Hb were sustained and durable as represented by a mean Hb of 12.2 g/dL at Day 85.
By targeting C3, the point where all three complement activation pathways meet, APL-2 has the potential to block activation from any pathway AND to prevent both intravascular and extravascular hemolysis.10
In What Other Hemolytic Diseases is APL-2 Being Studied?
APL-2 is currently being evaluated in the treatment of autoimmune hemolytic anemia (AIHA), another rare, chronic, debilitating blood disorder.9
Paroxysmal Nocturnal Hemoglobinuria (PNH)
Pegasus Phase 3 Study Design
To establish the efficacy and safety of the investigational drug, APL-2 compared to eculizumab in patients with PNH who continue to have Hb levels <10.5 g/dL despite treatment with eculizumab.
This study will enroll approximately 70 subjects around the globe to compare APL-2 to eculizumab treatment. All subjects who qualify will receive APL-2. Through a process known as randomization, about ½ of subjects will be assigned to group 1 (APL-2) or group 2 (eculizumab).
Key Inclusion Criteria
|1.||Age ≥18 years|
|2.||Primary diagnosis of PNH confirmed by high-sensitivity flow cytometry|
|3.||On treatment with eculizumab. Dose of eculizumab must have been stable for at least 3 months|
|4.||Hemoglobin <10.5 g/dL at the screening visit|
|5.||Absolute reticulocyte count > 1xULN at the screening visit|
Key Exclusion Criteria
|1.||Active bacterial infection within 4 weeks prior to Day-28 (Run-in Period)|
|2.||Receiving iron, folic acid, vitamin B12 and erythropoietin, unless the dose is stable|
|3.||Hereditary complement deficiency|
|4.||History of bone marrow transplantation|
Starting on day-28 (visit 2), subjects will receive self-administered twice-weekly subcutaneous (SC) doses of 1080 mg of APL-2 in addition to their current dose of eculizumab until day 1.
Subjects will then be randomized to either group 1 (monotherapy APL-2) or group 2 (monotherapy eculizumab).
Subjects in group 1 will receive APL-2 (1080 mg twice a week) each treatment week until the end of week 48.
Subjects in group 2 must continue to receive their pre-screening stable dose of eculizumab until the end of week 20. At week 17, subjects will also receive APL-2 (1080 mg twice a week) until the end of week 48. At week 20, subjects will discontinue their eculizumab treatment and remain solely on APL-2 for the remainder of the treatment period of the study.
Primary Efficacy Endpoint:
- Week 16 change from baseline in hemoglobin level
Secondary Efficacy Endpoints:
- Week 16 change from baseline in
- Reticulocyte count
- Lactate dehydrogenase (LDH) level
- FACIT-fatigue scale score
- Number of packed red blood cell units transfused from week 4 to week 16 (day 28 to day 112)
- Hemoglobin response (1 g/dL increase from baseline at week 16) in the absence of transfusions (yes/no).
- Reticulocyte normalization (reticulocyte count being below the upper limit of the normal range) in the absence of transfusions at week 16 (Yes/No).
- Incidence and severity of treatment emergent adverse events (TEAE)
- Incidence of thromboembolic events
- Changes from baseline in laboratory parameters
- Changes from baseline in ECG parameters
Common Questions About Paroxysmal Nocturnal Hemoglobinuria (PNH)
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare acquired blood disease that results from the development of mutant stem cells within a person’s bone marrow. These mutant stem cells produce defective red blood cells (as well as defective white blood cells and platelets) that are lacking 2 important proteins (CD55 and CD59) on their surface. These proteins protect normal red blood cells from being attacked by the proteins of the complement system. As a result, the defective red blood cells are prematurely destroyed faster than they are being replaced, and the patient develops anemia (shortage of red blood cells).14
In PNH, the destruction of red blood cells occur both inside the blood vessel (intravascular hemolysis) and within the spleen and liver (extravascular hemolysis). In blood vessels, the destruction of the red blood cells is largely due to membrane attack complexes (MACs), which are protein clusters that drill a hole in the cell membrane of the red blood cell.15 For this reason, the dying red blood cells release hemoglobin, some of which is removed from a person’s body through their urine (hemoglobinuria). The urine may contain so much hemoglobin that it looks dark.2 This dark color is most obvious in the morning because the urine that is being produced while the person is sleeping is more concentrated.2
All blood cells produced by mutant stem cells, including red blood cells, white blood cells and platelets, will be defective. For this reason, people with PNH may also have a low white blood cell count and suffer from thrombosis (abnormal blood clotting) or bleeding.6
PNH is primarily a disease of young adults. The median age of diagnosis is 35-40 years of age, with occasional cases diagnosed in childhood or adolescence.15
PNH results when a gene mutation develops in hematopoietic (blood forming) stem cells in the bone marrow. The hematopoietic stem cells are the cells that produce blood cells, such as red blood cells (RBCs), white blood cells, and platelets. In particular, PNH results from a mutation in the PIG-A gene, which is needed for producing a substance that anchors important proteins to the surface of cells. These proteins include CD55 and CD59, which normally shield the red blood cells from attack by the complement system (part of the body’s general response to infection or injury). These unprotected RBCs can be prematurely destroyed whenever the complement system is activated.11
In PNH, many of the red blood cells are being destroyed while they are circulating inside the blood vessels. As a result, they release their hemoglobin into the circulation, where it has toxic effects. Some of this hemoglobin is passed in the urine. Since the urine that is produced while people are sleeping is more concentrated, it is easier to see the dark reddish color in the first urine that is passed in the morning.2
Many people develop a few hematopoietic stem cells with PIG-A mutations. If these mutant cells account from only a tiny percentage of the bone marrow stem cells, the person will not develop PNH. The reasons why the population of the mutant stem cells expands in some patients are still unknown. However, this problem is more common among patients with other bone marrow problems, such as aplastic anemia.15
No. People do not inherit PNH, and they cannot pass it on to their children. Thus, PNH does not run in families. Instead, PNH can be described as sporadic because the cases occur for no apparent reason. PNH occurs when a mutation of the PIG-A gene occurs in the bone marrow, long after conception.6
- People with PNH generally have a shortage of mature red blood cells (anemia) and an increased proportion of immature red blood cells (reticulocytes) in circulation. Like any form of anemia, PNH can cause fatigue, a pale appearance, shortness of breath, and decreased ability to exercise.
- The diagnosis of PNH may be suspected if abnormally large amounts of the breakdown products of red blood cells are found in the urine or in a blood sample (e.g., hemoglobin in the urine or abnormally high levels of lactate dehydrogenase in blood serum) and there is no other obvious reason for red blood cells to be dying. The probability of PNH is high if the patient has unexplained blood clots in unusual locations.2
- The diagnosis of PNH may be made based upon a thorough clinical evaluation, a detailed patient history, laboratory testing, such as high-sensitivity flow cytometry to identify blood cells that lack CD55 and CD595,7 or fluoresce in-labeled proaerolysin (FLAER) to detect cells that lack the glycosylphosphatidylinositol anchors necessary for binding those proteins to the surface of the blood cells).7
- The only cure for PNH is a bone marrow transplant from a healthy donor. However, this requires a donor who is well-matched to the patient (usually a brother or sister). Even then, the procedure carries a high risk of death.16 For this reason, transplants are generally used only in very young patients with severe symptoms.
- Since 2007, a drug called eculizumab has been available for treating PNH.8 Eculizumab is a monoclonal antibody that binds to a complement protein called C5, which blocks the MAC formation and reduces the associated intravascular hemolysis. However, eculizumab does not affect C3. As a result, it cannot prevent extravascular hemolysis.17 For this reason, patients may continue to suffer from severe anemia and may remain dependent on blood transfusions.17
- In one study, roughly 70% of eculizumab-treated patients with PNH remained anemic, most likely resulting from untreated extravascular hemolysis.9
- The most likely explanation for this suboptimal response is the extravascular hemolysis that results from opsonization of the red blood cells, because of activation of complement protein C3.16
- A few patients with PNH have been found to be resistant to eculizumab because they have a mutant form of C5 that does not bind to eculizumab.16
Common Questions About APL-2
APL-2 is a small (13–amino-acid) cyclic peptide coupled via a linker to each end of a linear 40 kDa PEG chain. APL-2 binds to primate complement C3 and exerts broad inhibition of the complement cascade, which is a biological process that is part of innate immunity and is involved in multiple inflammatory processes. PEGylation allows the APL-2 to remain longer in the body.9
APL-2 targets C3, the central point where the three main pathways of complement activation come together. This point is upstream of the activation of C5. By targeting C3, APL-2 can inhibit all 3 of the major complement activation pathways, thus, it may be more effective in a broad patient population than partial inhibitors of complement system would be.9
All three of the complement activation pathways lead to the activation of C3, therefore, C3 plays a major role in complement activated diseases. Activation of C3 leads to many other effects, including activation of C5.10 So by inhibiting C3 we suppress all of the effects of the complement cascade, not just those that result from activation of C5.
Blocking C5 does reduce some of the destruction of red blood cells in patients with PNH. In particular, it suppresses the intravascular hemolysis that results from activation of C5. By preventing intravascular hemolysis, current treatment can help reduce a patient with PNH's need for blood transfusions and improve their overall quality of life and survival.18 Intravascular hemolysis can lead to pulmonary hypertension, renal failure, abdominal pain, dyspnea, anemia, hemoglobinuria, dysphagia, erectile dysfunction, and fatigue.4 Moreover, intravascular hemolysis and activation of platelets increase the risk of thrombosis (blood clotting), which is the leading cause of death among patients with PNH.4,18
Unfortunately, therapies that target C5 do not suppress the extravascular hemolysis that is caused by the activation of C3.13 Extravascular hemolysis can worsen the anemia; and as a result, the patient may continue to need blood transfusions.18,19
Studies of blood samples from patients with PNH show that the red blood cells are often coated with C3b fragments. This coating is a form of opsonization, which means that the red blood cells are being marked for destruction in the spleen and liver (extravascular hemolysis). When APL-2 was added to blood samples from people with PNH, it prevented C3b from being deposited onto the PNH red blood cells.18 Thus, C3 inhibition has the potential to prevent extravascular hemolysis, as well as intravascular hemolysis.
APL-2 is currently being evaluated in the treatment of autoimmune hemolytic anemia (AIHA), another rare,chronic, debilitating blood disorder.9
Many people participate in clinical trials to contribute to scientific advances and possibly help themselves and others. There may or may not be a potential health benefit for Pegasus trial participants receiving the study drug. At the proposed dose levels of APL-2, a significant decrease in complement-mediated hemolytic activity was observed in all APL-2-treated subjects (both in treatment-naïve patients and in patients treated previously with eculizumab) in phase 1 studies of patients with PNH and in studies of healthy volunteers. APL-2 may therefore reduce complement-mediated hemolytic activity in patients with PNH.9
If efficacious and safe, APL-2 is expected to continue to improve hemoglobin levels and reduce transfusion dependency in these patients throughout the treatment period.9
Common Questions About the Complement System
The complement system is an integral part of our immune defense system. In healthy people, complement orchestrates the destruction and clearance of foreign pathogens or of the body’s own cells that need to be replaced. It can also promote inflammation.10
Complement may be activated by different pathways.10 The classical complement pathway is activated when an antibody recognizes a non-self (foreign) target that invaded the body and needs to be destroyed.10 The alternative complement pathway can be initiated by spontaneously activated complement.10 There is also a lectin pathway, which is activated by mannose-binding lectin.10
The complement system consists of a cascade of more than 40 proteins, each of which activates the next protein in the cascade by cleavage.10 Complement C3 is the central protein of the cascade, positioned at the point of where all complement activation pathways come together, upstream of all effectors.10
Complement activation in healthy people is regulated to avoid its overactivation and to protect the host against immune attack.10 When the complement system gets out of balance, it can cause or worsen some illnesses.10
PNH is the result of mutant stem cells in the bone marrow. These stem cells produce red blood cells that lack two important surface proteins (CD55 and CD59). These proteins would normally protect the red blood cell from being attacked when the complement system is activated. But if those proteins are missing, the red blood cell may be prematurely destroyed because of activation of any of the three main complement pathways.10,14 The mutant red blood cell could become opsonized.14 Opsonization is a process in which antibodies and/or complement proteins attach to a bacterium or cell, to mark it for destruction by white blood cells.10 The red blood cell can also be targeted by membrane attack complexes (MACs), which are protein clusters that can kill a red blood cell by drilling holes in its cell membrane. As a result, the contents of the RBC can leak out into the blood plasma.14
In addition, some of the mutant stem cells in the bone marrow can make defective platelets, that also cannot defend themselves against the complement system. These defective platelets increase the person’s risk of abnormal blood clotting (thrombosis) or bleeding in people with PNH.6
Other PNH Trials
The PADDOCK study (NCT02588833) is an international phase Ib, open-label study of the use of subcutaneous APL-2 in patients with PNH who have never been treated with eculizumab. The protocol calls for 1 cohort to receive 180 mg/day and another cohort to receive 270 mg/day. The protocol allows subjects who benefit from the treatment to enter an extension phase that would allow them to receive APL-2 treatment for up to 1 year.21
The PHAROAH study (NCT02264639) is a phase Ib, open-label study being conducted at multiple clinical sites in the United States to assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of APL-2 as an add-on to eculizumab (soliris) in subjects with PNH. This rising-dose protocol calls for the first cohort to receive a first dose of 25 mg, followed by a dosage of 5 mg/day, and the fourth cohort to receive 270 mg/day.22 In each cohort, APL-2 therapy is added onto patients’ eculizumab treatment, and phsyicians will have the option to reduce and/or remove eculizumab therapy during the APL-2 treatment period, if appropriate.
All subjects in the trial have now had their Soliris treatment removed while maintaining hematological status with APL-2 treatment. Results to-date show that APL-2 monotherapy eliminated transufusion dependency and improved markers of anemia in PNH patients who were previously treated but not well controlled with eculizumab (Soliris®).
PALOMINO is a phase IIa, open-label study of the use of subcutaneous APL-2 in patients with PNH who have never been treated with eculizumab. The study is being conducted in Greece, Central and Eastern Europe. Subjects will receive APL-2 270 mg/day for 28 days. Subjects experiencing clinical benefit during the initial 28-day period will be able to continue receiving APL-2 treatment for 365 days.9
- Paroxysmal nocturnal hemoglobinuria (PNH). The Sidney Kimmel Comprehensive Cancer Center Web site. https://www.hopkinsmedicine.org/kimmel_cancer_center/types_cancer/paroxysmal_nocturnal_hemoglobinuria_PNH.html. Accessed May 15, 2018.
- Besa EC. Paroxysmal nocturnal hemoglobinuria (PNH) MedScape 2017; https://emedicine.medscape.com/article/207468-overview. Accessed May 15, 2018.
- Rosse WF, Ware RE. The molecular basis of paroxysmal nocturnal hemoglobinuria. Blood. 1995;86(9):3277-3286.
- Hill A, Kelly RJ, Hillmen P. Thrombosis in paroxysmal nocturnal hemoglobinuria. Blood. 2013;121(25):4985-4996; quiz 5105.
- Parker C, Omine M, Richards S, et al. Diagnosis and management of paroxysmal nocturnal hemoglobinuria. Blood. 2005;106(12):3699-3709.
- De Castro C, Rosse W. Paroxysmal nocturnal hemoglobinuria (PNH). National Organization for Rare Disorders Web site. https://rarediseases.org/physician-guide/paroxysmal-nocturnal-hemoglobinuria-pnh/. Accessed May 15, 2018.
- Brodsky RA. How I treat paroxysmal nocturnal hemoglobinuria. Blood. 2009;113(26):6522-6527.
- Prescribing information. Alexion Pharmaceuticals.
- Data on file, Apellis Pharmaceuticals.
- Murphy K, Weaver C. Innate immunity: the first lines of defense. In: Janeway's Immunobiology. 9th ed ed. London, UK: Garland Science; 2016.
- Brodsky RA. Paroxysmal nocturnal hemoglobinuria. Blood. 2014;124(18):2804-2811.
- Rother RP, Bell L, Hillmen P, Gladwin MT. The clinical sequelae of intravascular hemolysis and extracellular plasma hemoglobin: a novel mechanism of human disease. JAMA. 2005;293(13):1653-1662.
- Risitano AM, Notaro R, Marando L, et al. Complement fraction 3 binding on erythrocytes as additional mechanism of disease in paroxysmal nocturnal hemoglobinuria patients treated by [a C5 inhibitor]. Blood. 2009;113(17):4094-4100.
- Hillmen P. The role of complement inhibition in PNH. Hematology Am Soc Hematol Educ Program. 2008:116-123.
- Mastellos DC, Ricklin D, Yancopoulou D, Risitano A, Lambris JD. Complement in paroxysmal nocturnal hemoglobinuria: exploiting our current knowledge to improve the treatment landscape. Expert Rev Hematol. 2014;7(5):583-598.
- Risitano AM. Paroxysmal nocturnal hemoglobinuria in the era of complement inhibition. Am J Hematol. 2016;91(4):359-360.
- DeZern AE, Dorr D, Brodsky RA. Predictors of hemoglobin response to [C5 inhibitor] therapy in paroxysmal nocturnal hemoglobinuria. Eur J Haematol. 2013;90(1):16-24.
- Hillmen P, Muus P, Roth A, et al. Long-term safety and efficacy of sustained [C5 inhibitor] treatment in patients with paroxysmal nocturnal haemoglobinuria. Br J Haematol. 2013;162(1):62-73.
- Hill A, Rother RP, Arnold L, et al. [C5 inhibitor] prevents intravascular hemolysis in patients with paroxysmal nocturnal hemoglobinuria and unmasks low-level extravascular hemolysis occurring through C3 opsonization. Haematologica. 2010;95(4):567-573.
- Apellis Pharmaceuticals. Study to assess the safety, tolerability, efficacy and PK of APL-2 in patients with wAIHA or CAD. ClinicalTrials.gov Web site 2018; https://www.clinicaltrials.gov/ct2/show/NCT03226678. Accessed May 14, 2018.
- Apellis Pharmaceuticals. Pilot study to assess safety, preliminary efficacy and pharmacokinetics of s.c. APL-2 in PNH subjects (PADDOCK). ClinicalTrials.gov Web site. https://www.clinicaltrials.gov/ct2/show/NCT02588833. Accessed May 10, 2018.
- Apellis Pharmaceuticals. A phase I study to assess the safety APL-2 as an add-on to standard of care in subjects with PNH. https://www.clinicaltrials.gov/ct2/show/NCT02264639. Accessed May 10, 2018