Edited by: John D. Lambris, University of Pennsylvania, United States
Reviewed by: Ronald Paul Taylor, University of Virginia, United States; Trent M. Woodruff, University of Queensland, Australia; Gowthami Arepally, Duke University Medical Center, United States; Christoph Q. Schmidt, University of Ulm, Germany
This article was submitted to Molecular Innate Immunity, a section of the journal Frontiers in Immunology
†These authors have contributed equally to this work
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The treatment of paroxysmal nocturnal hemoglobinuria has been revolutionized by the introduction of the anti-C5 agent eculizumab; however, eculizumab is not the cure for Paroxysmal nocturnal hemoglobinuria (PNH), and room for improvement remains. Indeed, the hematological benefit during eculizumab treatment for PNH is very heterogeneous among patients, and different response categories can be identified. Complete normalization of hemoglobin (complete and major hematological response), is seen in no more than one third of patients, while the remaining continue to experience some degree of anemia (good and partial hematological responses), in some cases requiring regular red blood cell transfusions (minor hematological response). Different factors contribute to residual anemia during eculizumab treatment: underlying bone marrow dysfunction, residual intravascular hemolysis and the emergence of C3-mediated extravascular hemolysis. These two latter pathogenic mechanisms are the target of novel strategies of anti-complement treatments, which can be split into terminal and proximal complement inhibitors. Many novel terminal complement inhibitors are now in clinical development: they all target C5 (as eculizumab), potentially paralleling the efficacy and safety profile of eculizumab. Possible advantages over eculizumab are long-lasting activity and subcutaneous self-administration. However, novel anti-C5 agents do not improve hematological response to eculizumab, even if some seem associated with a lower risk of breakthrough hemolysis caused by pharmacokinetic reasons (it remains unclear whether more effective inhibition of C5 is possible and clinically beneficial). Indeed, proximal inhibitors are designed to interfere with early phases of complement activation, eventually preventing C3-mediated extravascular hemolysis in addition to intravascular hemolysis. At the moment there are three strategies of proximal complement inhibition: anti-C3 agents, anti-factor D agents and anti-factor B agents. These agents are available either subcutaneously or orally, and have been investigated in monotherapy or in association with eculizumab in PNH patients. Preliminary data clearly demonstrate that proximal complement inhibition is pharmacologically feasible and apparently safe, and may drastically improve the hematological response to complement inhibition in PNH. Indeed, we envision a new scenario of therapeutic complement inhibition, where proximal inhibitors (either anti-C3, anti-FD or anti-FB) may prove effective for the treatment of PNH, either in monotherapy or in combination with anti-C5 agents, eventually leading to drastic improvement of hematological response.
Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematological disorder characterized by complement-mediated intravascular hemolysis, bone marrow failure, and severe thrombophilia (
Anti-complement treatment with the anti-C5 monoclonal antibody eculizumab results in sustained inhibition of complement-mediated hemolysis in almost all PNH patients (
Tentative classification of hematological response to anti-complement agents in PNH.
Complete response | None | ≥12 g/dL | ≤1.5x ULN | |
Major response | None | ≥12 g/dL | >1.5x ULN | |
Good response | None | ≥10 and <12 g/dL | A. ≤ 1.5x ULN | Rule out bone marrow failure |
B. >1.5x ULN | ||||
Partial response | None or occasional (≤ 2 every 6 months) | ≥8 and <10 g/dL | A. ≤ 1.5x ULN B. >1.5x ULN | Rule out bone marrow failure |
Minor response |
None or occasional | <8 g/dL | Rule out bone marrow failure |
|
(≤ 2 every 6 months) | A. ≤ 1.5x ULN | |||
Regular (3–6 every 6 months) | <10 g/dL | B. >1.5x ULN | ||
Reduction by ≥50% |
<10 g/dL | |||
No response |
Regular (>6 every 6 months) | <10 g/dL | A. ≤ 1.5x ULN | Rule out bone marrow failure |
B. >1.5x ULN |
Intrinsic resistance to eculizumab has been reported, albeit very rare, and it is associated with inherited polymorphism of C5 which prevents eculizumab binding (
Reasons for inadequate hematological response to eculizumab and possible actions.
Intravascular hemolysis | Inherited C5 variants | Ultra-rare (<1%, usually in Japanese patients) | Intrinsic resistance due to impaired binding of eculizumab (and of ALXN1210) | Minimal (but very significant for the few patients for whom there is no available treatment) | Switch to other investigational agents (mostly alternative C5 inhibitors) |
Recurrent pharmacokinetic breakthrough | 10–15% of patients | Inadequate plasma level of eculizumab | Significant | Decrease interval of dosing (10–12 days) or increase dose of eculizumab (1,200 mg), or consider novel investigational agents | |
Sporadic pharmacodynamics breakthrough | May occur in any patients | Massive complement activation due to concomitant clinical events | Minimal | None (treat the underlying cause) | |
Extravascular hemolysis | C3-mediated extravascular hemolysis | 25–50% of patients (even more considering subclinical events) | Persistent uncontrolled activation of proximal complement, leading to C3-fragment opsonization of PNH red blood cells and subsequent removal by professional hepato-splenic phagocytes | Very significant | Consider employing investigational proximal inhibitors of the complement |
Bone marrow disorders | Bone marrow failure | 10–35% (depending also on initial patient selection) | Inadequate production of red blood cells | Significant | Treat underlying aplastic anemia with either immunosuppression or bone marrow transplantation |
Clonal evolution to myeloid malignancies | 1–5% | Additional stochastic somatic mutations | Relevant | Treat the myeloid malignancy |
The clinical benefit of eculizumab in PNH goes beyond the inhibition of intravascular hemolysis and possible hemoglobin stabilization; indeed, another consequence of therapeutic complement blockade is the prevention of thromboembolism. In the registration trials, the rate of thromboembolism during eculizumab treatment was reduced by 85% as compared with the pretreatment rate in the same patients (from 7.37 to 1.07 events/100 patient-years) (
As stated earlier, PNH is not simply a hemolytic anemia; indeed, a bone marrow disorder is always assumed to allow for the expansion of
As initially shown in the registration trials (
The reappearance of hemolysis in a PNH patient on eculizumab has been described as “breakthrough hemolysis.” There is no formal definition for this condition, but it seems very important to have it, since it will be eventually exploited as an endpoint in future trials investigating novel anti-complement agents, and its elimination may represent a clinical goal for any new therapy for PNH. Clinical breakthrough hemolysis is identified by the appearance of clinical symptoms such as painful hemolytic crises and dark urines (somehow subjective), associated with a rise in LDH and a drop in hemoglobin. Sometimes hemolysis may be evident just by laboratory data (i.e., LDH or hemoglobin) and hemoglobinuria: this may be referred to as subclinical breakthrough hemolysis. More robust definitions for clinical and subclinical breakthrough are needed, and we suggest the following classification (
Definition of clinical and subclinical
Clinical breakthrough |
Drop ≥2 g/dL (compared to the latest assessment, within 15 days) | Gross hemoglobinuria, painful crisis, dysphagia or any other significant clinical finding | >1.5x ULN (and increased as compared to the steady-state) |
Subclinical breakthrough | Drop <2 g/dL (compared to previous assessment, within 15 days) | No clinical symptom or sign, except moderate hemoglobinuria | >1.5x ULN (and increased by at least 50% as compared to the steady-state) |
Irrespective of the reliability of the current definition, breakthrough hemolysis has been described since the very first experiences with eculizumab (
Definition of pharmacokinetic and pharmacodynamic
Pharmacokinetic breakthrough | >7–10 days from previous dosing | Recurrent | Usually none |
Always >0.5–1 μg/mL | Inadequate | Residual free C5 available for steady-state (normal) C5 convertase activity | Decrease interval of dosing (10-12 days) or increase dose of eculizumab (1,200 mg) |
Pharmacodynamic breakthrough | Any time | Sporadic | Infectious events (both bacterial and viral, such as common seasonal viruses) or any event leading to inflammation (i.e., surgery, possible comorbidities) | Usually ≤ 0.5–1 μg/mL (but it may occur with any free C5 plasma level) | Adequate | Massive complement activation leading to excess C5 convertase activity, which might displace C5 from eculizumab | None (treat the underlying cause triggering complement activation) |
The first (and better defined) example of breakthrough hemolysis was described in about 10–15% of PNH patients on eculizumab, as the frequent (and somehow regular) reappearance of hemolysis in the few hours/days before the next administration of eculizumab without any obvious trigger or complement activating conditions (i.e., LDH increases by 2–3 folds as compared to values assessed at day 7 from previous eculizumab dosing). In this case, impaired C5 blockade has been associated with low trough plasma levels of eculizumab demonstrated at 12–14 days from the previous dosing (
The second type of breakthrough hemolysis during anti-complement treatment in PNH is rather more unpredictable, since it may occur at any time (with respect to last infusion of eculizumab) and it tends to be sporadic and not recurrent as the PK breakthrough. In most cases, it is associated with infectious episodes or other clinical conditions that trigger complement activation in addition to the basal, low-grade, steady-state activation deriving from C3 tick-over (
Residual intravascular hemolysis may persist during eculizumab treatment, either as low-grade continuous hemolysis or as breakthrough hemolytic crisis due to PK or PD reasons, which may eventually impact hematological response (see
Both residual intravascular hemolysis due to suboptimal C5 blockade and inadequate compensatory erythropoiesis due to underlying bone marrow failure may contribute to persistent anemia in PNH patients on eculizumab (
To date, there is no treatment option for C3-mediated extravascular hemolysis. The chronic use of steroids has been discouraged because of inefficacy and unacceptable side effects (
Bone marrow transplantation (BMT) remains the only curative treatment for PNH (
The clinical development of eculizumab for PNH, and then also for other diseases, has been a unique experience in terms of both scientific and financial success. This growing interest in the field of complement therapeutics has generated several preclinical and clinical programs for the development of novel anti-complement agents (
Complement inhibitors in clinical development for PNH.
Terminal inhibitors | ALXN1210 | C5 | N.A. | Phase I, randomized vs. placebo | Healthy volunteers | SAD, IV infusions | Yes |
NCT02598583 ( |
Phase I/II, open-label | Untreated PNH | Intra-patient DE by IV infusions | Yes ( |
|||
NCT02605993 ( |
Phase I/II, open-label | Untreated PNH | MAD; IV infusions | ||||
NCT02946463 ( |
Phase III, randomized vs. Ecu | Untreated PNH | IV infusions (every 8 weeks) | Yes ( |
|||
NCT03056040 ( |
Phase III, randomized vs. Ecu | Stable responders PNH | IV infusions (every 8 weeks) | Yes ( |
|||
SKY59 | C5 | NCT03157635 ( |
Phase I/II, multi-part study | Healthy volunteers | SAD, IV infusions | Yes ( |
|
Untreated PNH | Intra-patient DE by IV infusions, followed by SC injections | Yes ( |
|||||
Stable responders PNH | |||||||
LFG316 | C5 | NCT02534909 ( |
Phase II, open-label | Untreated PNH | IV infusions | Pending | |
REGN3918 | C5 | NCT03115996 ( |
Phase I | Healthy volunteers | IV and SC infusions | Yes ( |
|
ABP959 | C5 | EudraCT 2017-001418-27 ( |
Phase III, randomized vs. Ecu | Stable responders PNH | IV infusions | Ongoing | |
RA101495 | C5 | N.A. | Phase I, SAD and MD | Healthy volunteers | Daily, SC injections | Yes ( |
|
NCT03078582 ( |
Phase II, open label, fixed dose | Untreated PNH | Daily, SC injections | Yes ( |
|||
Poor responders PNH | |||||||
NCT03030183 ( |
Phase II, open label, fixed dose | Poor responders PNH | Daily, SC injections | Ongoing | |||
NCT03225287 ( |
Phase II, open-label, extension | PNH exposed to RA101495 | Daily, SC injections | Ongoing | |||
Coversin | C5 | N.A. | Phase I, SAD and MD | Healthy volunteers | SC injections | Yes ( |
|
NCT02591862 ( |
Phase II, open-label | Poor responder PNH | SC injections; intra-patient DE | Pending | |||
EudraCT 2016-002067-33 ( |
Phase II, open-label, fixed dose | Untreated PNH | SC injections | Yes ( |
|||
EudraCT 2016-004129-18 ( |
Phase II, open-label, extension | PNH exposed to coversin | SC injections | Ongoing | |||
ALNCC5 | C5 | NCT02352493 ( |
Phase I/II, randomized vs. Ecu, SAD and MAD | Healthy volunteers | SC injection (ALNCC5 or placebo) | Yes ( |
|
Untreated PNH | SC injections (ALNCC5 only) | Yes ( |
|||||
EudraCT 2016-002943-40 ( |
Phase II, open-label | Poor responder PNH | SC injections | Pending | |||
Proximal inhibitors | TT30 | CAP | NCT01335165 ( |
Phase I, SAD | Untreated PNH | SC injections and IV infusions | Yes ( |
AMY-101 | C3 | NCT03316521 ( |
Phase I, SAD and MD | Healthy volunteers | SC and IV infusions | Pending | |
APL-2 | C3 | N.A. | Phase I, SAD and MD | Healthy volunteers | SC and IV infusions | Yes ( |
|
NCT02264639 ( |
Phase Ib, open label, MAD, POC | Poor responders PNH | Daily, SC infusions | Yes ( |
|||
NCT02588833 ( |
Phase Ib, open label, MAD, POC | Untreated PNH | Daily, SC infusions | ||||
NCT03531255 ( |
Phase III, open label, extension | PNH exposed to APL-2 | Daily, SC infusions | ||||
NCT03500549 ( |
Phase III, randomized vs. ecu | Poor responders PNH | SC infusions, BIW | Ongoing | |||
ACH-4471 | FD | N.A. | Phase I, SAD | Healthy volunteers | Orally, QD and BID | Yes ( |
|
NCT03053102 ( |
Phase II, open label, MD, POC | Untreated PNH | Orally, TID | Pending | |||
NCT03181633 ( |
Phase II, open-label, extension | PNH exposed to ACH-4471 | Orally, TID | Ongoing | |||
NCT03472885 ( |
Phase II, open label, MD, POC | Poor responders PNH | Orally, TID | Ongoing | |||
LNP023 | FB | NCT03439839 ( |
Phase II, open label | Poor responders PNH | Orally, BID | Ongoing |
There are at least seven novel anti-C5 agents (in addition to biosimilars of eculizumab, which have been announced as well), which have entered clinical development for PNH; most of them are monoclonal antibodies like eculizumab, but the list includes also small peptide inhibitors and small interfering RNA (siRNA). All these agents aim to reproduce the excellent data achieved with eculizumab, trying to address some other clinical needs mostly concerning patient (dis)comfort: indeed, current eculizumab treatment requires intravenous (IV) infusions given every 14 days indefinitely. These agents have been designed trying to increase the interval between administrations, and/or switching from an IV dosing to a subcutaneous (SC) or even oral one.
ALXN1210 (also known with the brand name of ravulizumab, Ultomiris®) is the first of the second-generation therapeutic complement inhibitors, as well as the one with the most advanced clinical program. ALXN1210 is another anti-C5 mAb which was generated through specific amino acid modifications of eculizumab aiming to improve its PK profile (
The study 301 (NCT02946463) tested for non-inferiority of ALXN1210 as compared with eculizumab in treatment-naïve hemolytic (LDH >1.5 times of the ULN) PNH patients (
SKY59 (also known as RO711268 or Crovalimab, in development by Roche) is another long-acting anti-C5 mAb, which exploits a pH-dependent binding to the target C5, eventually accounting for profound mAb recycling (
LFG316 is another anti-C5 mAb in development by Novartis; this agent is currently under investigation in PNH patients within a proof-of-concept phase II study enrolling untreated PNH patients (
REGN3918 is an anti-C5 mAb in development by Regeneron, which binds both wild-type and R885H variant of human C5. In a phase I study in healthy volunteers REGN3918 was well-tolerated and resulted in dose-dependent inhibition of the terminal complement pathway, measured as hemolytic activity (CH50) (
In addition to novel anti-C5 mAbs, biosimilars of eculizumab have also been described. For instance, ABP959 is a biosimilar of eculizumab developed by Amgen; this agent is now under investigation in a large Phase III trial (
RA101495 is the lead compound of a new class of small synthetic, macrocyclic peptides developed by Rapharma to inhibit C5 (
Coversin is another recombinantly expressed inhibitor of C5, which originates from the tick
In addition to mAbs and small peptide molecules, another strategy of C5 inhibition was developed aiming to interfere with endogenous C5 production by RNA interference. The first-in-class agent for this strategy is ALN-CC5, a si-RNA duplex specific for C5, that had been shown highly effective in silencing liver C5 production in animal models (
The development of proximal complement inhibitors has not been as intensely investigated as the search for novel anti-C5 agents (at least so far); now, there are only five clinical programs, which have been publicly disclosed, but only four remain active. Before listing and discussing them in detail, it is important to summarize how and why the idea of interfering with the proximal steps of the complement cascade became of interest in PNH. As discussed above, the critical understanding of therapeutic complement inhibition
Currently the field of proximal complement inhibitors include broad inhibitors of C3 (with the two compstatin analogs AMY-101 and APL-2) and selective inhibitors of the alternative pathway targeting either complement factor D (FD) and complement factor B (FB).
AMY-101 is an analog of a 13-residue disulfide-bridged peptide named compstatin, discovered in the 90's by Prof. J. Lambris using a phage-displayed random peptide library (
APL-2 is another compstatin analog which utilizes a first-generation version of compstatin (
ACH-4471 is small oral FD inhibitor developed by Achillion which showed inhibitory activity of hemolysis in PNH
LNP023 is a small FB inhibitor that, together with small FD-inhibitors, constitutes Novartis' pipeline of potent and orally bioavailable selective inhibitors of the alternative pathway. Similarly to anti-FD agents (
These are exciting days for the field of PNH, and possibly in a few years new anti-complement therapies will change the standard care of this disease and possibly others. As new data are generated, we will better understand the benefits of complement cascade modulation, and what we should aim to do in the near future. Indeed, a couple of lessons are clear even at this stage of investigations.
Eculizumab treatment largely improves survival in PNH patients (
For more than a decade we have thought that C5 blockade by eculizumab (at standard doses) was the perfect C5 blockade, with the exception of patients carrying the R885H C5 polymorphism; long-term clinical outcome is outstanding, (
Inhibitors of the proximal complement have been specifically designed to address the emerging problem of C3-mediated extravascular hemolysis (
In a chronic, life-threatening disease such as PNH short-term surrogate endpoints are needed for clinical trials. In the eculizumab era, when PNH patients were heavily transfused, transfusion avoidance and hemoglobin stabilization were obvious goals which have been achieved, together with reduction of LDH (which served as a biomarker of disease activity rather than as an endpoint
Toxicity remains a major concern for any novel treatment, especially in a disease with such a good long-term outcome as PNH treated with eculizumab. Historically, the development of anti-complement treatment raised several concerns about the risk of infectious complications. Indeed, after cases of infection by
The definition of the best drug for PNH treatment is timely; however, we must think about the best strategy rather than about the best drug. Of course, companies are motivated to demonstrate that one drug is better than another, but this is not necessarily useful for patients: we first need to better understand what is the greatest clinical benefit that may be achieved in PNH patients. We have already discussed available results with each of these novel anti-complement agents, together with their mechanistic goals; interestingly, some agents are used both in monotherapy and in combination with standard anti-C5 treatment, emphasizing that clinical data are essential to prove the initial hypothesis. Novel strategies of C5 inhibition definitely address some patients' needs, such as possible self-administration or extending the interval between IV dosing; however, it is likely that they will not lead to superior clinical benefit, except better patients' convenience. Some novel anti-C5 agents might deliver a more effective C5 blockade, but the actual benefit to patients of a further LDH reduction is uncertain. Indeed, even the best C5 inhibition seen with the combination of eculizumab with the anti-C5 si-RNA will control only residual intravascular hemolysis, with a hematological benefit that for the majority of patients is minor. It is most likely that the next breakthrough in PNH treatment will come from the inhibitors of the proximal complement pathway: anti-C3, anti-FD and anti-FB agents. Preliminary data clearly demonstrate that by interfering with the complement cascade upstream they inhibit MAC-mediated intravascular hemolysis and prevent C3-mediated extravascular hemolysis; but how profound is the inhibition of these targets is unclear. The lesson from the anti-C5 was very instructive: even minimal residual amounts of these complement proteins may be enough to keep complement activity almost intact, likely because they are substrates or very active enzymes generated at time of complement activation. Proximal inhibitors appear to disable all disease mechanisms in hemolysis of PNH; however, we still don't know whether inhibition of C3 is pharmacologically effective enough to prevent possible residual activity (as demonstrated for C5, for instance for PD breakthrough), and whether their targets in the complement cascade may be somehow by-passed in specific clinical circumstances (e.g., for anti-FD and anti-FB in case of complement activation through the classical and mannose/lectin pathways; and for anti-FD also in case of C3 activation by other plasma protease, i.e. FD by-pass) (
Thus, we envision a new scenario in the treatment of PNH where inhibitors of the proximal complement (either anti-C3, anti-FD or anti-FB) are essential, and if pharmacologically adequate may be used even in monotherapy to control intravascular and extravascular hemolysis. Alternatively, these proximal inhibitors might require use in combination with anti-C5 agents, possibly long-acting, to maximize therapeutic efficacy. In this scenario, anemia from both intravascular and extravascular hemolysis would be fully prevented, and normal hemoglobin is expected in absence of bone marrow failure. In this new scenario, we must not forget that the future pricing of these new agents remains a major issue. Unfortunately, national pricing scheme policies on orphan medicine products has allowed exaggerated prices, which seem not always justified by their cost (i.e., manufacturing, research, and development) and their actual clinical value (i.e., impact on life expectancy and quality of life) (
In the last decade we have been able to offer PNH patients an almost-normal life-expectancy, irrespective of their disease (
AR, RN, and RP conceived the study and identified the other experts who contributed to the generation of the consensus. AR, AK, RC, PS, RN, and RP wrote the manuscript and together with the other SM, PR, LM, CF, FC, and MS generated the consensus on all the topics discussed in the manuscript. All the authors have critically revised the manuscript and contributed to its preparation in the current version.
AR has received research support from Alexion, Novartis, Alnylam and Rapharma, lecture fees from Alexion, Novartis, Pfizer and Apellis, and served as member of advisory/investigator board for Alexion, Roche, Achillion, Novartis, Apellis and Samsung, and served as consultant for Amyndas. RP has received research funding from Alexion, Amgen, Jazz Pharmaceuticals and Pfizer; consulted for, and received honoraria from Alexion, Amgen, Gilead, Jazz Pharmaceuticals, Keocyte, MSD, Novartis, Pfizer, Roche, Samsung and Mallinckrodt. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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