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Understanding the Role of the Complement System in Rare Disease Therapeutics

Investigating the Burden and Treatment of Paroxysmal Nocturnal Hemoglobinuria

A Q&A With Carlos Manuel de Castro, MD

Carlos Manuel de Castro, MD

Carlos Manuel de Castro, MD

AJMC®: Paroxysmal nocturnal hemoglobinuria (PNH) is 1 of several complement-driven diseases. Can you describe its clinical presentation?

DE CASTRO: PNH presents heterogeneously. One of the 3 characteristics of the disease is hemolysis. Patients with PNH who have hemolysis will present with marked fatigue that is often out of proportion to their degree of anemia. During the intravascular hemolysis that occurs with PNH, free hemoglobin binds to nitric oxide, which controls smooth muscle contraction and relaxation. This causes a great deal of contraction and a variety of symptoms. Patients experience esophageal spasms, chest pain like that of a heart attack, problems swallowing, and—very frequently—abdominal pain. Those are the main symptoms of PNH.

The second characteristic is bone marrow failure or cytopenias. Patients will have low white blood cell or platelet counts, which can cause other symptoms. The third characteristic is blood clots or thrombotic episodes. Only about 20% of patients with PNH will present with blood clots, but the blood clots are very worrisome because they can lead to morbidity and death. Up to 40% to 50% of untreated patients with PNH may develop a blood clot.

AJMC®: How is PNH diagnosed?

DE CASTRO: The diagnosis begins with a clinical suspicion and is followed by an appropriate blood test. This is a flow cytometry–based test performed on peripheral blood. Oftentimes, however, there's a delay in diagnosis, because most physicians have never heard of PNH or have never treated it. Many patients with PNH present to their physician complaining of dark urine and they are then referred to a urologist. Patients with PNH who have chest pain or abdominal pain often are referred to GI [gastrointestinal] doctors or cardiologists. By the time the patient sees a physician who specializes in hemolysis or who recognizes the hemolytic anemia, and the patient is referred to a hematologist who recognizes that these symptoms may be some of the manifestations of PNH, a delay has already occurred. This delay can lead to emotional distress on the patient, and can be potentially life threatening or fatal, such as when a PNH patient develops a serious blood clot.

AJMC®: What clinical burden does PNH place on patients?

DE CASTRO: PNH symptoms vary widely in their severity and correlate closely with the ongoing hemolysis. Fatigue is very common in this disease, especially in untreated patients. Prior to the approval of complement inhibition therapy, patients with PNH universally had fatigue to some degree.

Dyspnea is another symptom of PNH, and can follow the anemia. Some patients develop chronic dyspnea and even develop pulmonary hypertension. Chronic shortness of breath is a worrisome sign.

Patients with PNH also experience flares of hemoglobinuria. Any infection that might raise a patient’s complement levels or any other stressor can lead to hemolysis and patients can have dark urine the next day. Abdominal pain, which occurs when patients experience acute hemolysis, also seems to accompany those flares.

Severe PNH is completely debilitating. Frequently, patients with severe PNH experience fatigue all the time. Oftentimes, patients can’t work: they call in sick because they cannot get out of bed or are experiencing a hemolytic flare with dark urine. Many times, patients lose their jobs because of the absenteeism.

Fortunately, current treatments can alleviate these symptoms and allow patients with PNH to get on with their daily lives.

AJMC®: Many patients with PNH require red blood cell transfusions. What is the time cost of these transfusions?

DE CASTRO: Frequency of transfusion is difficult to determine. In PNH, there are flares of hemolysis. Patients may do fine for many months and then experience a flare-up because of a viral infection and need a transfusion. The anemia and the transfusion need are sporadic.

Once transfused, blood will typically last 3 to 4 weeks in a patient's circulation. Many patients with PNH also experience some bone marrow failure, so they aren't producing enough red blood cells [RBCs] to keep up with the losses from hemolysis.

After a transfusion, patients usually need a few days to recover, but ongoing PNH flares can continue to occur and lead them to feel unwell. The transfusion process also is costly with respect to time; it can take 4 hours. Moreover, many centers don't administer transfusions on the same day; patients are first screened for their blood type and must return the next day to receive blood. For those patients with PNH who need them, blood transfusions represent a substantial time cost.

AJMC®: Treatment of PNH has changed with the advent of complement inhibition. How have treatments such as C5 inhibitors changed outcomes for patients?

DE CASTRO: The change has been dramatic. The first C5 inhibitor, eculizumab, was approved 15 years ago. It is a fascinating drug. It’s a monoclonal antibody that was developed by cardiologists who were hoping that blocking complement would decrease heart damage during a heart attack. Thereafter, Pete Hillman, MD, PhD, investigated the use of eculizumab in his patients with PNH and reported in 2004 on 11 patients who all were constantly hemolyzing—all transfusion-dependent and very sick.1 All 11 patients responded very well to eculizumab; hemolysis stopped, they felt better, and they didn't need transfusions anymore. That led investigators to the phase 3 studies of eculizumab, including the TRIUMPH (NCT00112983) and SHEPHERD (NCT00122304) studies.2,3

For many patients, treatment with eculizumab was life-changing. After feeling sick all the time and not being able to work, they suddenly had a life again and realized how well they could feel. Eculizumab blocked hemolysis. In addition, it seemed to prevent blood clots. This was unexpected because the cause of the blood clots in PNH was unknown. By blocking complement activation and hemolysis, we saw that patients treated with eculizumab experienced much less blood clotting.

The manufacturer of eculizumab then developed ravulizumab, which is a longer-acting monoclonal antibody. It also inhibits C5 at the same epitope, but instead of receiving maintenance doses of eculizumab every 2 weeks, patients could receive doses of ravulizumab every 8 weeks. Many patients appreciated that convenience. Eculizumab and ravulizumab are the 2 approved C5 inhibitors; newer ones are being tested in clinical trials.

AJMC®: In some patients with PNH who have been treated with C5 inhibitors, hemolysis of erythrocytes can happen extravascularly. What is the clinical manifestation of extravascular hemolysis?

DE CASTRO: Extravascular hemolysis presents differently than does intravascular hemolysis. Patients with extravascular hemolysis don't experience dark urine, for instance.

What surprised researchers was that in many patients, C5 inhibition did not resolve the anemia. Patients weren’t experiencing flares and anemia improved; however, it did not resolve. Researchers discovered that, in patients who are treated with C5 inhibitors, PNH RBCs become coated with C3 fragments. These fragments are generated earlier in the complement cascade. The blood cells have no way of removing those fragments from their surface. The reticulo-endothelial system in the liver and spleen start to take up these RBCs and to destroy them in an extravascular manner. There's no free hemoglobin being released, there's no hemoglobinuria, but anemia persists. For some, anemia is so severe that they still need transfusions. Some still present with serious fatigue and say: “I still feel tired. I don't understand why. I'm not that anemic.” Extravascular hemolysis still causes fatigue, shortness of breath, and other symptoms.

AJMC®: You are the principal investigator in 2 clinical trials examining the use of an alternative pathway inhibitor (danicopan [ALPHA trial; NCT04469465]; iptacopan [NCT04558918]) in patients currently or previously treated with a C5 inhibitor.4,5 What promise do alternative pathway inhibitors offer patients with PNH?

DE CASTRO: Both danicopan and iptacopan act on the alternative pathway. Danicopan is a factor D inhibitor and iptacopan is a factor B inhibitor. Pegcetacoplan, which has been approved, targets C3. Both factor B and factor D are needed to activate C3; they're part of the same point of the alternative pathway. Inhibiting any of these targets limits C3 activation considerably, which, in turn, prevents C3 coating on the RBCs and the extravascular hemolysis that can occur with eculizumab and ravulizumab. Theoretically, inhibiting any of these 3 targets blocks both extravascular and intravascular hemolysis.

In patients who are suboptimal responders to a C5 inhibitor, danicopan, iptacopan, and pegcetacoplan have a marked effect. The patient’s anemia improves and their reticulocyte counts go down; the drop in the reticulocyte count shows that both the intravascular hemolysis from C5 activation and the MAC complex and the extravascular hemolysis from C3 coating and opsonization are being stopped. These drugs hold a lot of a promise.

The other promise danicopan and iptacopan have is that both are administered orally, which offers patients the option of taking anticomplement drugs orally, at home. Other anticomplement drugs have either been subcutaneous or intravenous formulations.

AJMC®: What does the recent history of PNH treatment suggest about the future of complement inhibitor therapy?

DE CASTRO: PNH has demonstrated that a rare disease can suddenly gain treatments that have high efficacy and low incidence of adverse effects. Researchers are now trying safe, easy-to-administer complement inhibitors in other rare diseases. Finding a safe, effective inhibitor would transform treatment for other complement-driven rare diseases; most of these diseases have very few treatments outside of steroids, which are not ideal.

The field is opening up. However, as soon as a question about these new complement inhibitors is answered, another question usually arises. The new upstream inhibitors hold a lot of promise, but older C5 inhibitors have been tested for 15 or 20 years. They have been proven to be very safe. They prevent clots, they probably improve survival, and their infection risk is known and addressed; meningitis is a major adverse effect with C5 inhibition, but vaccination is now standard with any complement inhibition, so the risk of infection is low. More time is needed to determine whether the newer C3 inhibitors are going to be safer and more effective than the older C5 inhibitors. Conducting head-to-head trials and targeting therapy based upon the results of these trials also are quite challenging as these diseases are so rare.

Complement is involved in more diseases than researchers anticipated, including eye diseases, neurologic diseases, rheumatologic diseases, and other hematologic diseases. How and when to use complement inhibitors effectively remain big questions. This is an exciting time as we explore using complement inhibitors to fight other diseases.

AJMC®: At the 64th American Society of Hematology annual meeting in December 2022, primary efficacy and safety data were presented from the phase 3 APPLY-PNH trial (NCT04558918) of iptacopan monotherapy.6 As an investigator on the trial, what do you think these recent findings suggest about the near-term future of alternative pathway inhibitor therapy?

DE CASTRO: This was a very important study and the first to show phase 3 data of an oral proximal complement inhibitor in PNH patients with a suboptimal response to a C5 inhibitor. The primary end points of the study were a rise in hemoglobin (Hb) of at least 2 g/dL from baseline and a stable Hb level of at least 12 g/dL. Iptacopan was about 80% effective in these co–end points. Other secondary end points such as the FACIT-F [Functional Assessment of Chronic Illness Therapy-Fatigue] score also were improved in the iptacopan arm. Treatment with the Factor B inhibitor was proven safe and was associated with no real increase in infections or breakthrough hemolysis. A separate study of this drug in treatment-naïve patients is ongoing (NCT04820530).7 Overall though, this study suggests that targeting the proximal arm of complement appears to be quite safe and may prove to be more effective than targeting C5. Iptacopan has the added convenience of being an oral drug. This is quite an exciting development for treating patients with PNH.

Dr de Castro is a professor of medicine at Duke University School of Medicine and a member of the Duke Cancer Institute in Durham, North Carolina.

References:

1. Hillmen P, Hall C, Marsh JC, et al. Effect of eculizumab on hemolysis and transfusion requirements in patients with paroxysmal nocturnal hemoglobinuria. N Engl J Med. 2004;350(6):552-559. doi:10.1056/NEJMoa031688

2. Eculizumab in treating patients with paroxysmal nocturnal hemoglobinuria. ClinicalTrials.gov. Updated May 30, 2013. Accessed December 12, 2022. https://clinicaltrials.gov/ct2/show/NCT00112983

3. Study of safety in hemolytic paroxysmal nocturnal hemoglobinuria (PNH) patients treated with eculizumab. ClinicalTrials.gov. Updated February 21, 2007. Accessed December 12, 2022. https://clinicaltrials.gov/ct2/show/NCT00122304

4. Danicopan as add-on therapy to a C5 inhibitor in paroxysmal nocturnal hemoglobinuria (PNH) participants who have clinically evidence extravascular hemolysis (EVH)(ALPHA). ClinicalTrials.gov. Updated October 3, 2022. Accessed December 12, 2022. https://clinicaltrials.gov/ct2/show/NCT04469465

5. Study of efficacy and safety of twice daily oral LNP023 in adult PNH patients with residual anemia despite anti-C5 antibody treatment (APPLY-PNH). ClinicalTrials.gov. Updated November 25, 2022. Accessed December 12, 2022. https://clinicaltrials.gov/ct2/show/NCT04558918

6. Peffault de Latour R, Roeth A, Kulasekararaj A, et al. Oral monotherapy with iptacopan, a proximal complement inhibitor of factor B, has superior efficacy to intravenous terminal complement inhibition with standard of care eculizumab or ravulizumab and favorable safety in patients with paroxysmal nocturnal hemoglobinuria and residual anemia: results from the randomized, active-comparator-controlled, open-label, multicenter, phase III APPLY-PNH study. Abstract presented at: 64th annual American Society of Hematology meeting; December 13, 2022; New Orleans, LA. Accessed December 12, 2022. https://ash.confex.com/ash/2022/webprogram/Paper171469.html

7. Study of efficacy and safety of twice daily oral iptacopan (LNP023) in adult PNH patients who are naive to complement inhibitor therapy (APPOINT-PNH). ClinicalTrials.gov. Updated November 25, 2022. Accessed December 19, 2022. https://clinicaltrials.gov/ct2/show/NCT04820530?term=LNP023+treatment+na%C3%AFve&draw=2&rank=2

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