Peptide inhibitors of C3 activation as a novel strategy of complement inhibition for the treatment of paroxysmal nocturnal hemoglobinuria. Blood. 2014;123:2094-2101. [PMID: 24497537 DOI: 10.1182/blood-2013-11-536573]

Navigating the Complement Pathway to Optimize PNH Treatment with Pegcetacoplan and Other Currently Approved Complement Inhibitors

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare and potentially life-threatening hematologic disorder caused by a somatic mutation in a relevant portion of hematopoietic stem cells. Mutation of the phosphatidylinositol glycan biosynthesis class A (PIGA) gene prevents the expression of cell-surface proteins, including the complement regulatory proteins CD55 and CD59. With decreased or a lack of CD55 and CD59 expression on their membranes, PNH red blood cells become susceptible to complement-mediated hemolysis (symptoms of which include anemia, dysphagia, abdominal pain, and fatigue), leading to thrombosis. State-of-the-art PNH treatments act by inhibiting the dysregulated complement at distinct points in the activation pathway: late at the C5 level (C5 inhibitors, eculizumab, ravulizumab, and crovalimab), centrally at the C3 level (C3/C3b inhibitors and pegcetacoplan), and early at the initiation and amplification of the alternative pathway (factor B inhibitor, iptacopan; factor D inhibitor, danicopan). Through their differing mechanisms of action, these treatments elicit varying profiles of disease control and offer valuable insights into the molecular underpinnings of PNH. This narrative review provides an overview of the mechanisms of action of the six complement inhibitors currently approved for PNH, with a focus on the C3/C3b-targeted therapy, pegcetacoplan.

A novel complement C3 inhibitor CP40-KK protects against experimental pulmonary arterial hypertension via an inflammasome NLRP3 associated pathway

BACKGROUND

Pulmonary arterial hypertension (PAH) is a severe cardiopulmonary disease characterized by complement dependent and proinflammatory activation of macrophages. However, effective treatment for complement activation in PAH is lacking. We aimed to explore the effect and mechanism of CP40-KK (a newly identified analog of selective complement C3 inhibitor CP40) in the PAH model.

METHODS

We used western blotting, immunohistochemistry, and immunofluorescence staining of lung tissues from the monocrotaline (MCT)-induced rat PAH model to study macrophage infiltration, NLPR3 inflammasome activation, and proinflammatory cytokines (IL-1β and IL-18) release. Surface plasmon resonance (SPR), ELISA, and CH50 assays were used to test the affinity between CP40-KK and rat/human complement C3. CP40-KK group rats only received CP40-KK (2 mg/kg) by subcutaneous injection at day 15 to day 28 continuously.

RESULTS

C3a was significantly upregulated in the plasma of MCT-treated rats. SPR, ELISA, and CH50 assays revealed that CP40-KK displayed similar affinity binding to human and rat complement C3. Pharmacological inhibition of complement C3 cleavage (CP40-KK) could ameliorate MCT-induced NLRP3 inflammasome activity, pulmonary vascular remodeling, and right ventricular hypertrophy. Mechanistically, increased proliferation of pulmonary arterial smooth muscle cells is closely associated with macrophage infiltration, NLPR3 inflammasome activation, and proinflammatory cytokines (IL-1β and IL-18) release. Besides, C3a enhanced IL-1β activity in macrophages and promoted pulmonary arterial smooth muscle cell proliferation in vitro.

CONCLUSION

Our findings suggest that CP40-KK treatment was protective in the MCT-induced rat PAH model, which might serve as a therapeutic option for PAH.

A guide to complement biology, pathology and therapeutic opportunity

Complement has long been considered a key innate immune effector system that mediates host defence and tissue homeostasis. Yet, growing evidence has illuminated a broader involvement of complement in fundamental biological processes extending far beyond its traditional realm in innate immunity. Complement engages in intricate crosstalk with multiple pattern-recognition and signalling pathways both in the extracellular and intracellular space. Besides modulating host-pathogen interactions, this crosstalk guides early developmental processes and distinct cell trajectories, shaping tissue immunometabolic and regenerative programmes in different physiological systems. This Review provides a guide to the system-wide functions of complement. It highlights illustrative paradigm shifts that have reshaped our understanding of complement pathobiology, drawing examples from evolution, development of the central nervous system, tissue regeneration and cancer immunity. Despite its tight spatiotemporal regulation, complement activation can be derailed, fuelling inflammatory tissue pathology. The pervasive contribution of complement to disease pathophysiology has inspired a resurgence of complement therapeutics with major clinical developments, some of which have challenged long-held dogmas. We thus highlight major therapeutic concepts and milestones in clinical complement intervention.

Complement System Inhibitory Drugs in a Zebrafish (Danio rerio) Model: Computational Modeling

The dysregulation of complement system activation usually results in acute or chronic inflammation and can contribute to the development of various diseases. Although the activation of complement pathways is essential for innate defense, exacerbated activity of this system may be harmful to the host. Thus, drugs with the potential to inhibit the activation of the complement system may be important tools in therapy for diseases associated with complement system activation. The synthetic peptides Cp40 and PMX205 can be highlighted in this regard, given that they selectively inhibit the C3 and block the C5a receptor (C5aR1), respectively. The zebrafish (Danio rerio) is a robust model for studying the complement system. The aim of the present study was to use in silico computational modeling to investigate the hypothesis that these complement system inhibitor peptides interact with their target molecules in zebrafish, for subsequent in vivo validation. For this, we analyzed molecular docking interactions between peptides and target molecules. Our study demonstrated that Cp40 and the cyclic peptide PMX205 have positive interactions with their respective zebrafish targets, thus suggesting that zebrafish can be used as an animal model for therapeutic studies on these inhibitors.

SAR443809: a selective inhibitor of the complement alternative pathway, targeting complement factor Bb

Dysregulated activation of the complement system is implicated in the onset or progression of several diseases. Most clinical-stage complement inhibitors target the inactive complement proteins present at high concentrations in plasma, which increases target-mediated drug disposition and necessitates high drug levels to sustain therapeutic inhibition. Furthermore, many efforts are aimed at inhibiting only terminal pathway activity, which leaves opsonin-mediated effector functions intact. We describe the discovery of SAR443809, a specific inhibitor of the alternative pathway C3/C5 convertase (C3bBb). SAR443809 selectively binds to the activated form of factor B (factor Bb) and inhibits alternative pathway activity by blocking the cleavage of C3, leaving the initiation of classical and lectin complement pathways unaffected. Ex vivo experiments with patient-derived paroxysmal nocturnal hemoglobinuria erythrocytes show that, although terminal pathway inhibition via C5 blockade can effectively inhibit hemolysis, proximal complement inhibition with SAR443809 inhibits both hemolysis and C3b deposition, abrogating the propensity for extravascular hemolysis. Finally, intravenous and subcutaneous administration of the antibody in nonhuman primates demonstrated sustained inhibition of complement activity for several weeks after injection. Overall, SAR443809 shows strong potential for treatment of alternative pathway-mediated disorders.

Biomarkers and laboratory assessments for monitoring the treatment of patients with paroxysmal nocturnal hemoglobinuria: Differences between terminal and proximal complement inhibition

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, life-threatening, acquired disease in which blood cells lacking complement regulatory proteins are destroyed because of uncontrolled complement activity. Since 2007, terminal complement inhibitors have revolutionized the treatment of this disease. However, patients treated with these inhibitors can still experience anemia because of C3-mediated extravascular hemolysis and clinically relevant levels of breakthrough or residual intravascular hemolysis. Proximal complement inhibitors, which are only just beginning to emerge, have the potential to address this problem by targeting components of the pathway upstream of C5, thereby protecting patients against both intra- and extravascular hemolysis. In this review, we describe different biomarkers that can be used to monitor complement pathway blockade and discuss key laboratory assessments for evaluating treatment efficacy. We also consider how these assessments are affected by each class of inhibitor and highlight how evolving treatment goals may influence the relative importance of these assessments.

The complement alternative pathway in paroxysmal nocturnal hemoglobinuria: From a pathogenic mechanism to a therapeutic target

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare clonal, not malignant, hematological disease characterized by intravascular hemolysis, thrombophilia and bone marrow failure. While this latter presentation is due to a T-cell mediated auto-immune disorder resembling acquired aplastic anemia, the first two clinical presentations are largely driven by the complement pathway. Indeed, PNH is characterized by a broad impairment of complement regulation on affected cells, which is due to the lack of the complement regulators CD55 and CD59. The deficiency of these two proteins from PNH blood cells is due to the somatic mutation in the phosphatidylinositol N-acetylglucosaminyltransferase subunit A gene causing the disease, which impairs the surface expression of all proteins linked via the glycosylphosphatidylinositol anchor. The lack of the complement regulators CD55 and CD59 on PNH erythrocytes accounts for the hallmark of PNH, which is the chronic, complement-mediated intravascular hemolysis. This hemolysis results from the impaired regulation of the alternative pathway upstream in the complement cascade, as well as of the downstream terminal pathway. PNH represented the first indication for the development of anti-complement agents, and the therapeutic interception of the complement cascade at the level of C5 led to remarkable changes in the natural history of the disease. Nevertheless, the clinical use of an inhibitor of the terminal pathway highlighted the broader derangement of complement regulation in PNH, shedding light on the pivotal role of the complement alternative pathway. Here we review the current understanding of the role of the alternative pathway in PNH, including the emergence of C3-mediated extravascular hemolysis in PNH patients on anti-C5 therapies. These observations provide the rationale for the development of novel complement inhibitors for the treatment of PNH. Recent preclinical and clinical data on proximal complement inhibitors intercepting the alternative pathway with the aim of improving the treatment of PNH are discussed, together with their clinical implications which are animating a lively debate in the scientific community.

With complements: C3 inhibition in the clinic

C3 is a key complement protein, located at the nexus of all complement activation pathways. Extracellular, tissue, cell-derived, and intracellular C3 plays critical roles in the immune response that is dysregulated in many diseases, making it an attractive therapeutic target. However, challenges such as very high concentration in blood, increased acute expression, and the elevated risk of infections have historically posed significant challenges in the development of C3-targeted therapeutics. This is further complicated because C3 activation fragments and their receptors trigger a complex network of downstream effects; therefore, a clear understanding of these is needed to provide context for a better understanding of the mechanism of action (MoA) of C3 inhibitors, such as pegcetacoplan. Because of C3's differential upstream position to C5 in the complement cascade, there are mechanistic differences between pegcetacoplan and eculizumab that determine their efficacy in patients with paroxysmal nocturnal hemoglobinuria. In this review, we compare the MoA of pegcetacoplan and eculizumab in paroxysmal nocturnal hemoglobinuria and discuss the complement-mediated disease that might be amenable to C3 inhibition. We further discuss the current state and outlook for C3-targeted therapeutics and provide our perspective on which diseases might be the next success stories in the C3 therapeutics journey.

Cold AIHA and the best treatment strategies

Cold-reactive autoimmune hemolytic anemia (AIHA) is rare among the hemolytic anemias. It results when 1 of a variety of processes causes the generation of immunoglobulin M (IgM) autoantibodies against endogenous erythrocytes, resulting in complement activation and predominantly intravascular hemolysis. Cold AIHA is typically a primary lymphoproliferative disorder with marrow B-cell clones producing pathogenic IgM. More rarely, secondary cold AIHA (cAIHA) can develop from malignancy, infection, or other autoimmune disorders. However, in children cAIHA is typically post infection, mild, and self-limited. Symptoms include a sequelae of anemia, fatigue, and acrocyanosis. The severity of disease is variable and highly dependent on the thermal binding range of the autoantibody. In adults, treatment has most commonly focused on reducing antibody production with rituximab-based regimens. The addition of cytotoxic agents to rituximab improves response rates, but at the expense of tolerability. Recent insights into the cause of cold agglutinin disease as a clonal disorder driven by complement form the basis of newer therapeutic options. While rituximab-based regimens are still the mainstay of therapy, options have now expanded to include complement-directed treatments and other B-cell-directed or plasma-cell-directed therapies.

Effects of the complement system on antibody formation and function: implications for transplantation

PURPOSE OF REVIEW

In antibody-mediated allograft rejection, donor-reactive antibodies cause transplant injury in part via complement activation. New mechanistic insights indicate complement also modulates development of humoral immune responses. Herein we review recent data that describes how complement affects antibody formation and we discuss therapeutic implications.

RECENT FINDINGS

Extravasating T cells interacting with integrins express and activate intracellular complement that drives immune-metabolic adaptations vital for CD4 + helper cells. Marginal zone B cells can acquire intact major histocompatibility complexes from dendritic cells via complement-dependent trogocytosis for presentation to T cells. Activated B cells in germinal centers receive co-stimulatory signals from T-helper cells. These germinal center B cells undergo coordinate shifts in surface complement regulator expression that permit complement receptor signaling on the germinal center B cells required for affinity maturation. The positively selected, high-affinity B cells can differentiate into plasma cells that produce donor-HLA-reactive antibodies capable of ligating endothelial, among other, graft cells. Subsequent sublytic complement attack can stimulate endothelial cells to activate CD4 + and CD8 + T cells, promoting cellular and humoral rejection. Newly developed complement inhibitors are being tested to prevent/treat transplant rejection.

SUMMARY

The complement system influences T-cell, B-cell and endothelial-cell activation, and thereby contributes allograft injury. Emerging therapeutic strategies targeting complement activation have the potential to prevent or abrogate transplant injury and improve transplant outcomes.

Insight into mode-of-action and structural determinants of the compstatin family of clinical complement inhibitors

With the addition of the compstatin-based complement C3 inhibitor pegcetacoplan, another class of complement targeted therapeutics have recently been approved. Moreover, compstatin derivatives with enhanced pharmacodynamic and pharmacokinetic profiles are in clinical development (e.g., Cp40/AMY-101). Despite this progress, the target binding and inhibitory modes of the compstatin family remain incompletely described. Here, we present the crystal structure of Cp40 complexed with its target C3b at 2.0-Å resolution. Structure-activity-relationship studies rationalize the picomolar affinity and long target residence achieved by lead optimization, and reveal a role for structural water in inhibitor binding. We provide explanations for the narrow species specificity of this drug class and demonstrate distinct target selection modes between clinical compstatin derivatives. Functional studies provide further insight into physiological complement activation and corroborate the mechanism of its compstatin-mediated inhibition. Our study may thereby guide the application of existing and development of next-generation compstatin analogs.

Intertwined pathways of complement activation command the pathogenesis of lupus nephritis

The complement system is involved in the origin of autoimmunity and systemic lupus erythematosus. Both genetic deficiency of complement components and excessive activation are involved in primary and secondary renal diseases, including lupus nephritis. Among the pathways, the classical pathway has long been accepted as the main pathway of complement activation in systemic lupus erythematosus. However, more recent studies have shown the contribution of factors B and D which implies the involvement of the alternative pathway. While there is evidence on the role of the lectin pathway in systemic lupus erythematosus, it is yet to be demonstrated whether this pathway is protective or harmful in lupus nephritis. Complement is being explored for the development of disease biomarkers and therapeutic targeting. In the current review we discuss the involvement of complement in lupus nephritis.

Novel Insights into Factor D Inhibition

Complement-mediated diseases or complementopathies, such as Paroxysmal nocturnal hemoglobinuria (PNH), cold agglutinin disease (CAD), and transplant-associated thrombotic microangiopathy (TA-TMA), demand advanced complement diagnostics and therapeutics be adopted in a vast field of medical specialties, such as hematology, transplantation, rheumatology, and nephrology. The miracle of complement inhibitors as "orphan drugs" has dramatically improved morbidity and mortality in patients with otherwise life-threatening complementopathies. Efficacy has been significantly improved by upstream inhibition in patients with PNH. Different molecules may exert diverse characteristics in vitro and in vivo. Further studies remain to show safety and efficacy of upstream inhibition in other complementopathies. In addition, cost and availability issues are major drawbacks of current treatments. Therefore, further developments are warranted to address the unmet clinical needs in the field of complementopathies. This state-of-the-art narrative review aims to delineate novel insights into factor D inhibition as a promising target for complementopathies.

Development of New Drugs for Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemia (AIHA) is a rare disorder characterized by the autoantibody-mediated destruction of red blood cells, and treatments for it still remain challenging. Traditional first-line immunosuppressive therapy, which includes corticosteroids and rituximab, is associated with adverse effects as well as treatment failures, and relapses are common. Subsequent lines of therapy are associated with higher rates of toxicity, and some patients remain refractory to currently available treatments. Novel therapies have become promising for this vulnerable population. In this review, we will discuss the mechanism of action, existing data, and ongoing clinical trials of current novel therapies for AIHA, including B-cell-directed therapy, phagocytosis inhibition, plasma cell-directed therapy, and complement inhibition.

Novel targeted C3 inhibitor pegcetacoplan for paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, life-threatening acquired blood disease characterized by chronic complement-mediated hemolysis and thrombosis. On May 14, 2021, the US FDA approved a new targeted C3 therapy Empaveli (pegcetacoplan), once called APL-2, for use in adult PNH. This review aims to review the pharmacological properties, clinical safety and efficacy of pegcetacoplan, and provides comprehensive drug information about pegcetacoplan. Pegcetacoplan is a pegylated peptide that targets the proximal complement protein C3. Pegcetacoplan plays a role in the complement cascade that controls C3b-mediated extravascular hemolysis and terminal complement-mediated intravascular hemolysis. Early pharmacokinetic and pharmacodynamic trials proved that pegcetacoplan had good tolerability and acceptable safety, while reducing complement activity. The pivotal phase III trial PEGASUS of PNH patients with a suboptimal response to eculizumab reported that pegcetacoplan improved hemoglobin levels better than eculizumab (p < 0.001). Compared with eculizumab, patients who received pegcetacoplan had a higher chance of not requiring a blood transfusion within 16 weeks (85% vs. 15%, p < 0.001). The safety of pegcetacoplan was similar to that of eculizumab. Adverse events such as injection site reactions and diarrhea occurred frequently following pegcetacoplan administration. The prescription recommended dosage is 1080 mg, subcutaneously infusion twice a week.

Pegcetacoplan for Paroxysmal Nocturnal Hemoglobinuria

Approximately a third of patients with paroxysmal nocturnal hemoglobinuria (PNH) remain transfusion dependent or have symptomatic anemia despite treatment with a C5 inhibitor. Pegcetacoplan inhibits complement proximally at the level of C3 and is highly effective in treating persistent anemia resulting from C3-mediated extravascular hemolysis. We describe the rationale for C3 inhibition in the treatment of PNH and discuss preclinical and clinical studies using pegcetacoplan and other compstatin derivatives. We propose an approach for sequencing complement inhibitors in PNH.

Upregulation of Checkpoint Ligand Programmed Death-Ligand 1 in Patients with Paroxysmal Nocturnal Hemoglobinuria Explained by Proximal Complement Activation

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hemolytic disease driven by impaired complement regulation. Mutations in genes encoding the enzymes that build the GPI anchors are causative, with somatic mutations in the PIG-A gene occurring most frequently. As a result, the important membrane-bound complement regulators CD55 and CD59 are missing on the affected hematopoietic stem cells and their progeny, rendering those cells vulnerable to complement attack. Immune escape mechanisms sparing affected PNH stem cells from removal are suspected in the PNH pathogenesis, but molecular mechanisms have not been elucidated. We hypothesized that exuberant complement activity in PNH results in enhanced immune checkpoint interactions, providing a molecular basis for the potential immune escape in PNH. In a series of PNH patients, we found increased expression levels of the checkpoint ligand programmed death-ligand 1 (PD-L1) on granulocytes and monocytes, as well as in the plasma of PNH patients. Mechanistically, we demonstrate that complement activation leading to the decoration of particles/cells with C3- and/or C4-opsonins increased PD-L1 expression on neutrophils and monocytes as shown for different in vitro models of classical or alternative pathway activation. We further establish in vitro that complement inhibition at the level of C3, but not C5, inhibits the alternative pathway-mediated upregulation of PD-L1 and show by means of soluble PD-L1 that this observation translates into the clinical situation when PNH patients are treated with either C3 or C5 inhibitors. Together, the presented data show that the checkpoint ligand PD-L1 is increased in PNH patients, which correlates with proximal complement activation.

C3-targeted host-modulation approaches to oral inflammatory conditions

Periodontitis is an inflammatory disease caused by biofilm accumulation and dysbiosis in subgingival areas surrounding the teeth. If not properly treated, this oral disease may result in tooth loss and consequently poor esthetics, deteriorated masticatory function and compromised quality of life. Epidemiological and clinical intervention studies indicate that periodontitis can potentially aggravate systemic diseases, such as, cardiovascular disease, type 2 diabetes mellitus, rheumatoid arthritis, and Alzheimer disease. Therefore, improvements in the treatment of periodontal disease may benefit not only oral health but also systemic health. The complement system is an ancient host defense system that plays pivotal roles in immunosurveillance and tissue homeostasis. However, complement has unwanted consequences if not controlled appropriately or excessively activated. Complement overactivation has been observed in patients with periodontitis and in animal models of periodontitis and drives periodontal inflammation and tissue destruction. This review places emphasis on a promising periodontal host-modulation therapy targeting the complement system, namely the complement C3-targeting drug, AMY-101. AMY-101 has shown safety and efficacy in reducing gingival inflammation in a recent Phase 2a clinical study. We also discuss the potential of AMY-101 to treat peri-implant inflammatory conditions, where complement also seems to be involved and there is an urgent unmet need for effective treatment.

Discovering C3 targeting therapies for paroxysmal nocturnal hemoglobinuria: Achievements and pitfalls

The treatment of paroxysmal nocturnal hemoglobinuria (PNH) was revolutionized by the introduction of the anti-C5 agent eculizumab, which resulted in sustained control of intravascular hemolysis, leading to transfusion avoidance and hemoglobin stabilization in at least half of all patients. Nevertheless, extravascular hemolysis mediated by C3 has emerged as inescapable phenomenon in PNH patients on anti-C5 treatment, frequently limiting its hematological benefit. More than 10 years ago we postulated that therapeutic interception of the complement cascade at the level of C3 should improve the clinical response in PNH. Compstatin is a 13-residue disulfide-bridged peptide binding to both human C3 and C3b, eventually disabling the formation of C3 convertases and thereby preventing complement activation via all three of its activating pathways. Several generations of compstatin analogs have been tested in vitro, and their clinical evaluation has begun in PNH and other complement-mediated diseases. Pegcetacoplan, a pegylated form of the compstatin analog POT-4, has been investigated in two phase I/II and one phase III study in PNH patients. In the phase III study, PNH patients with residual anemia already on eculizumab were randomized to receive either pegcetacoplan or eculizumab in a head-to-head comparison. At week 16, pegcetacoplan was superior to eculizumab in terms of hemoglobin change from baseline (the primary endpoint), as well as in other secondary endpoints tracking intravascular and extravascular hemolysis. Pegcetacoplan showed a good safety profile, even though breakthrough hemolysis emerged as a possible risk requiring additional attention. Here we review all the available data regarding this innovative treatment that has recently been approved for the treatment of PNH.

Compstatins: the dawn of clinical C3-targeted complement inhibition

Despite the growing recognition of the complement system as a major contributor to a variety of clinical conditions, the therapeutic arsenal has remained scarce. The introduction of an anti-C5 antibody in 2007 raised confidence in complement-targeted therapy. However, it became apparent that inhibition of late-stage effector generation might not be sufficient in multifactorial complement disorders. Upstream intervention at the level of C3 activation has therefore been considered promising. The approval of pegcetacoplan, a C3 inhibitor of the compstatin family, in 2021 served as critical validation of C3-targeted treatment. This review delineates the evolution of the compstatin family from its academic origins to the clinic and highlights current and potential future applications of this promising drug class in complement diseases.

Multi-omics of human plasma reveals molecular features of dysregulated inflammation and accelerated aging in schizophrenia

Schizophrenia is a devastating psychiatric illness that detrimentally affects a significant portion of the worldwide population. Aging of schizophrenia patients is associated with reduced longevity, but the potential biological factors associated with aging in this population have not yet been investigated in a global manner. To address this gap in knowledge, the present study assesses proteomics and metabolomics profiles in the plasma of subjects afflicted with schizophrenia compared to non-psychiatric control patients over six decades of life. Global, unbiased analyses of circulating blood plasma can provide knowledge of prominently dysregulated molecular pathways and their association with schizophrenia, as well as features of aging and gender in this disease. The resulting data compiled in this study represent a compendium of molecular changes associated with schizophrenia over the human lifetime. Supporting the clinical finding of schizophrenia's association with more rapid aging, both schizophrenia diagnosis and age significantly influenced the plasma proteome in subjects assayed. Schizophrenia was broadly associated with prominent dysregulation of inflammatory and metabolic system components. Proteome changes demonstrated increased abundance of biomarkers for risk of physiologic comorbidities of schizophrenia, especially in younger individuals. These findings advance our understanding of the molecular etiology of schizophrenia and its associated comorbidities throughout the aging process.

C3-targeted therapy in periodontal disease: moving closer to the clinic

Complement plays a key role in immunosurveillance and homeostasis. When dysregulated or overactivated, complement can become a pathological effector, as seen in several inflammatory disorders, including periodontal disease. Recently, clinical correlative studies and preclinical mechanistic investigations have collectively demonstrated that complement is hyperactivated during periodontitis and that targeting its central component (C3) provides therapeutic benefit in nonhuman primates (NHPs). The preclinical efficacy of a C3-targeted drug candidate combined with excellent safety and pharmacokinetic profiles supported its use in a recent Phase IIa clinical study in which C3 inhibition resolved gingival inflammation in patients with periodontal disease. We posit that C3-targeted intervention might represent a novel and transformative host-modulation therapy meriting further investigation in Phase III clinical trials for the treatment of periodontitis.

Advancing therapeutic complement inhibition in hematologic diseases: PNH and beyond

Complement is an elaborate system of the innate immunity. Genetic variants and autoantibodies leading to excessive complement activation are implicated in a variety of human diseases. Among them, the hematologic disease paroxysmal nocturnal hemoglobinuria (PNH) remains the prototype model of complement activation and inhibition. Eculizumab, the first-in-class complement inhibitor, was approved for PNH in 2007. Addressing some of the unmet needs, a long-acting C5 inhibitor, ravulizumab, and a C3 inhibitor, pegcetacoplan have been also now approved with PNH. Novel agents, such as factor B and factor D inhibitors, are under study with very promising results. In this era of several approved targeted complement therapeutics, selection of the proper drug needs to be based on a personalized approach. Beyond PNH, complement inhibition has also shown efficacy and safety in cold agglutinin disease (CAD), primarily with the C1s inhibitor of the classical complement pathway, sutimlimab, but also with pegcetacoplan. Furthermore, C5 inhibition with eculizumab and ravulizumab, as well as inhibition of the lectin pathway with narsoplimab, are investigated in transplant-associated thrombotic microangiopathy (TA-TMA). With this revolution of next-generation complement therapeutics, additional hematologic entities, such as delayed hemolytic transfusion reaction (DHTR) or immune thrombocytopenia (ITP), might also benefit from complement inhibitors. Therefore, this review aims to describe state-of-the-art knowledge of targeting complement in hematologic diseases focusing on: a) complement biology for the clinician, b) complement activation and therapeutic inhibition in prototypical complement-mediated hematologic diseases, c) hematologic entities under investigation for complement inhibition, and d) other complement-related disorders of potential interest to hematologists.

How we('ll) treat paroxysmal nocturnal haemoglobinuria: diving into the future

Paroxysmal nocturnal haemoglobinuria (PNH) is characterized by complement-mediated intravascular haemolysis, severe thrombophilia and bone marrow failure. While for patients with bone marrow failure the treatment follows that of immune-mediated aplastic anaemia, that of classic, haemolytic PNH is based on anti-complement medication. The anti-C5 monoclonal antibody eculizumab has revolutionized treatment, resulting in control of intravascular haemolysis and thromboembolic risk, with improved long-term survival. Novel strategies of complement inhibition are emerging. New anti-C5 agents reproduce the safety and efficacy of eculizumab, with improved patient convenience. Proximal complement inhibitors have been developed to address C3-mediated extra-vascular haemolysis and seem to improve haematological response.

How I treat cold agglutinin disease

The last decades have seen great progress in the treatment of cold agglutinin disease (CAD). Comparative trials are lacking, and recommendations must be based mainly on nonrandomized trials and will be influenced by personal experience. Herein, current treatment options are reviewed and linked to 3 cases, each addressing specific aspects of therapy. Two major steps in CAD pathogenesis are identified, clonal B-cell lymphoproliferation and complement-mediated hemolysis, each of which constitutes a target of therapy. Although drug treatment is not always indicated, patients with symptomatic anemia or other bothersome symptoms should be treated. The importance of avoiding ineffective therapies is underscored. Corticosteroids should not be used to treat CAD. Studies on safety and efficacy of relevant drugs and combinations are briefly described. The author recommends that B cell-directed approaches remain the first choice in most patients requiring treatment. The 4-cycle bendamustine plus rituximab combination is highly efficacious and sufficiently safe and induces durable responses in most patients, but the time to response can be many months. Rituximab monotherapy should be preferred in frail patients. The complement C1s inhibitor sutimlimab is an emerging option in the second line and may also find its place in the first line in specific situations.

Halting targeted and collateral damage to red blood cells by the complement system

The complement system is an important defense mechanism against pathogens; however, in certain pathologies, the system also attacks human cells, such as red blood cells (RBCs). In paroxysmal nocturnal hemoglobinuria (PNH), RBCs lack certain complement regulators which sensitize them to complement-mediated lysis, while in autoimmune hemolytic anemia (AIHA), antibodies against RBCs may initiate complement-mediated hemolysis. In recent years, complement inhibition has improved treatment prospects for these patients, with eculizumab now the standard of care for PNH patients. Current complement inhibitors are however not sufficient for all patients, and they come with high costs, patient burden, and increased infection risk. This review gives an overview of the underlying pathophysiology of complement-mediated hemolysis in PNH and AIHA, the role of therapeutic complement inhibition nowadays, and the high number of complement inhibitors currently under investigation, as for almost every complement protein, an inhibitor is being developed. The focus lies with novel therapeutics that inhibit complement activity specifically in the pathway that causes pathology or those that reduce costs or patient burden through novel administration routes.

From discovery to approval: A brief history of the compstatin family of complement C3 inhibitors

The FDA approval of pegcetacoplan (Empaveli), a PEGylated compstatin-based C3 therapeutic, as a new treatment for paroxysmal nocturnal hemoglobinuria (PNH) marks a milestone in the history of complement drug discovery. Almost 15 years after the approval of the first complement-specific drug for PNH, the anti-C5 antibody eculizumab, a novel class of complement inhibitors with a distinct mechanism of action finally enters the clinic. This landmark decision broadens the spectrum of available complement therapeutics, offering patients with unmet clinical needs or insufficient responses to anti-C5 therapy an alternative treatment option with a broad activity profile. Here we present a brief historical account of this newly approved complement drug, consolidating its approval within the long research record of the compstatin family of peptidic C3 inhibitors.

A small fragment of factor B as a potential inhibitor of complement alternative pathway activity

BACKGROUND

The complement system is a key player in innate immunity and a modulator of the adaptive immune system. Among the three pathways of complement, the alternative pathway (AP) accounts for most of the complement activation. Factor B (FB) is a major protease of the AP, making it a promising target to inhibit the AP activity in conditions of uncontrolled complement activation.

METHODS

Based on the data obtained from sequence analysis and conformational changes associated with FB, we expressed and purified a recombinant FB fragment (FBfr). We tested the inhibitory activity of the protein against the AP by in vitro assays.

RESULTS

FBfr protein was proven to inhibit the complement AP activity when tested by C3b deposition assay and rabbit erythrocyte hemolytic assay.

CONCLUSION

Our recombinant FBfr was able to compete with the native human FB, which allowed it to inhibit the AP activity. This novel compound is a good candidate for further characterization and testing to be used in complement diagnostic tests and as a drug lead in the field of complement therapeutics.

The evolution of polymer conjugation and drug targeting for the delivery of proteins and bioactive molecules

Polymer conjugation can be considered one of the leading approaches within the vast field of nanotechnology-based drug delivery systems. In fact, such technology can be exploited for delivering an active molecule, such as a small drug, a protein, or genetic material, or it can be applied to other drug delivery systems as a strategy to improve their in vivo behavior or pharmacokinetic activities such as prolonging the half-life of a drug, conferring stealth properties, providing external stimuli responsiveness, and so on. If on the one hand, polymer conjugation with biotech drug is considered the linchpin of the protein delivery field boasting several products in clinical use, on the other, despite dedicated research, conjugation with low molecular weight drugs has not yet achieved the milestone of the first clinical approval. Some of the primary reasons for this debacle are the difficulties connected to achieving selective targeting to diseased tissue, organs, or cells, which is the main goal not only of polymer conjugation but of all delivery systems of small drugs. In light of the need to achieve better drug targeting, researchers are striving to identify more sophisticated, biocompatible delivery approaches and to open new horizons for drug targeting methodologies leading to successful clinical applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Toxicology and Regulatory Issues in Nanomedicine > Regulatory and Policy Issues in Nanomedicine.

C3 inhibition with pegcetacoplan in subjects with paroxysmal nocturnal hemoglobinuria treated with eculizumab

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired, life-threatening hematologic disease characterized by chronic complement-mediated hemolysis and thrombosis. Despite treatment with eculizumab, a C5 inhibitor, 72% of individuals remain anemic. Pegcetacoplan (APL-2), a PEGylated C3 inhibitor, has the potential to provide more complete hemolysis control in patients with PNH. This open-label, phase Ib study was designed to assess the safety, tolerability, and pharmacokinetics of pegcetacoplan in subjects with PNH who remained anemic during treatment with eculizumab. Pharmacodynamic endpoints were also assessed as an exploratory objective of this study. Data are presented for six subjects in cohort 4 who received treatment for up to 2 years. In total, 427 treatment-emergent adverse events (TEAEs) were reported, 68 of which were possibly related to the study drug. Eight serious TEAEs occurred in two subjects; three of these events were considered possibly related to the study drug. Pegcetacoplan pharmacokinetic concentrations accumulated with repeated dosing, and steady state was reached at approximately 6-8 weeks. Lactate dehydrogenase levels were well controlled by eculizumab at baseline. Pegcetacoplan increased hemoglobin levels and decreased both reticulocyte count and total bilirubin in all six subjects. Improvements were observed in Functional Assessment of Chronic Illness Therapy Fatigue scores. Two subjects discontinued for reasons unrelated to pegcetacoplan. All four subjects who completed the study transitioned to pegcetacoplan monotherapy following eculizumab discontinuation and avoided transfusions. In this small study, pegcetacoplan therapy was generally well-tolerated, and resulted in an improved hematological response by achieving broad hemolysis control, enabling eculizumab discontinuation.

Soluble collectin-12 mediates C3-independent docking of properdin that activates the alternative pathway of complement

Properdin stabilizes the alternative C3 convertase (C3bBb), whereas its role as pattern-recognition molecule mediating complement activation is disputed for decades. Previously, we have found that soluble collectin-12 (sCL-12) synergizes complement alternative pathway (AP) activation. However, whether this observation is C3 dependent is unknown. By application of the C3-inhibitor Cp40, we found that properdin in normal human serum bound to Aspergillus fumigatus solely in a C3b-dependent manner. Cp40 also prevented properdin binding when properdin-depleted serum reconstituted with purified properdin was applied, in analogy with the findings achieved by C3-depleted serum. However, when opsonized with sCL-12, properdin bound in a C3-independent manner exclusively via its tetrameric structure and directed in situ C3bBb assembly. In conclusion, a prerequisite for properdin binding and in situ C3bBb assembly was the initial docking of sCL-12. This implies a new important function of properdin in host defense bridging pattern recognition and specific AP activation.

Computational analysis of complement inhibitor compstatin using molecular dynamics

The complement system plays a major role in human immunity, but its abnormal activation can have severe pathological impacts. By mimicking a natural mechanism of complement regulation, the small peptide compstatin has proven to be a very promising complement inhibitor. Over the years, several compstatin analogs have been created, with improved inhibitory potency. A recent analog is being developed as a candidate drug against several pathological conditions, including COVID-19. However, the reasons behind its higher potency and increased binding affinity to complement proteins are not fully clear. This computational study highlights the mechanistic properties of several compstatin analogs, thus complementing previous experimental studies. We perform molecular dynamics simulations involving six analogs alone in solution and two complexes with compstatin bound to complement component 3. These simulations reveal that all the analogs we consider, except the original compstatin, naturally adopt a pre-bound conformation in solution. Interestingly, this set of analogs adopting a pre-bound conformation includes analogs that were not known to benefit from this behavior. We also show that the most recent compstatin analog (among those we consider) forms a stronger hydrogen bond network with its complement receptor than an earlier analog.

Properdin Is a Key Player in Lysis of Red Blood Cells and Complement Activation on Endothelial Cells in Hemolytic Anemias Caused by Complement Dysregulation

The complement system alternative pathway (AP) can be activated excessively in inflammatory diseases, particularly when there is defective complement regulation. For instance, deficiency in complement regulators CD55 and CD59, leads to paroxysmal nocturnal hemoglobinuria (PNH), whereas Factor H mutations predispose to atypical hemolytic uremic syndrome (aHUS), both causing severe thrombohemolysis. Despite eculizumab being the treatment for these diseases, benefits vary considerably among patients. Understanding the molecular mechanisms involved in complement regulation is essential for developing new treatments. Properdin, the positive AP regulator, is essential for complement amplification by stabilizing enzymatic convertases. In this study, the role of properdin in red blood cell (RBC) lysis and endothelial cell opsonization in these AP-mediated diseases was addressed by developing in vitro assays using PNH patient RBCs and human primary endothelial cells, where the effects of inhibiting properdin, using novel monoclonal antibodies (MoAbs) that we generated and characterized, were compared to other complement inhibitors. In in vitro models of PNH, properdin inhibition prevented hemolysis of patient PNH type II and III RBCs more than inhibition of Factor B, C3, and C5 (>17-fold, or >81-fold, or >12-fold lower molar IC₉₀ values, respectively). When tested in an in vitro aHUS hemolysis model, the anti-properdin MoAbs had 11-fold, and 86-fold lower molar IC₉₀ values than inhibition of Factor B, or C3, respectively (P < 0.0001). When comparing target/inhibitor ratios in all hemolysis assays, inhibiting properdin was at least as efficient as the other complement inhibitors in most cases. In addition, using in vitro endothelial cell assays, the data indicate a critical novel role for properdin in promoting complement activation on human endothelial cells exposed to heme (a hemolysis by-product) and rH19-20 (to inhibit Factor H cell-surface protection), as occurs in aHUS. Inhibition of properdin or C3 in this system significantly reduced C3 fragment deposition by 75%. Altogether, the data indicate properdin is key in promoting RBC lysis and complement activation on human endothelial cells, contributing to the understanding of PNH and aHUS pathogenesis. Further studies to determine therapeutic values of inhibiting properdin in complement-mediated diseases, in particular those that are characterized by AP dysregulation, are warranted.

Complement in Hemolysis- and Thrombosis- Related Diseases

The complement system, originally classified as part of innate immunity, is a tightly self-regulated system consisting of liquid phase, cell surface, and intracellular proteins. In the blood circulation, the complement system, platelets, coagulation system, and fibrinolysis system form a close and complex network. They activate and regulate each other and jointly mediate immune monitoring and tissue homeostasis. The dysregulation of each cascade system results in clinical manifestations and the progression of different diseases, such as sepsis, atypical hemolytic uremic syndrome, C3 glomerulonephritis, systemic lupus erythematosus, or ischemia–reperfusion injury. In this review, we summarize the crosstalk between the complement system, platelets, and coagulation, provide integrative insights into how complement dysfunction leads to hemopathic progression, and further discuss the therapeutic relevance of complement in hemolytic and thrombotic diseases.

Complement Inhibitors Block Complement C3 Opsonization and Improve Targeting Selectivity of Nanoparticles in Blood

Complement is one of the critical branches of innate immunity that determines the recognition of engineered nanoparticles by immune cells. Antibody-targeted iron oxide nanoparticles are a popular platform for magnetic separations, in vitro diagnostics, and molecular imaging. We used 60 nm cross-linked iron oxide nanoworms (CLIO NWs) modified with antibodies against Her2/neu and EpCAM, which are common markers of blood-borne cancer cells, to understand the role of complement in the selectivity of targeting of tumor cells in whole blood. CLIO NWs showed highly efficient targeting and magnetic isolation of tumor cells spiked in lepirudin-anticoagulated blood, but specificity was low due to high uptake by neutrophils, monocytes, and lymphocytes. Complement C3 opsonization in plasma was predominantly via the alternative pathway regardless of the presence of antibody, PEG, or fluorescent tag, but was higher for antibody-conjugated CLIO NWs. Addition of various soluble inhibitors of complement convertase (compstatin, soluble CD35, and soluble CD55) to whole human blood blocked up to 99% of the uptake of targeted CLIO NWs by leukocytes, which resulted in a more selective magnetic isolation of tumor cells. Using well-characterized nanomaterials, we demonstrate here that complement therapeutics can be used to improve targeting selectivity.

Complement Inhibition Therapy and Dialytic Strategies in Paroxysmal Nocturnal Hemoglobinuria: The Nephrologist's Opinion

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare clonal disease that presents an estimated incidence of 1.3 cases per million per year, with a prevalence of 15.9 cases per million. It is characterized by hemolysis, bone marrow dysfunction with peripheral blood cytopenia, hypercoagulability, thrombosis, renal impairment and arterial and pulmonary hypertension. Hemolysis and subsequent hemosiderin accumulation in tubular epithelium cells induce tubular atrophy and interstitial fibrosis. The origin of PNH is the somatic mutation in the X-linked phosphatidylinositol glycan class A (PIG-A) gene located on Xp22: this condition leads to the production of clonal blood cells with a deficiency in those surface proteins that protect against the lytic action of the activated complement system. Despite the increased knowledge of this syndrome, therapies for PNH were still only experimental and symptomatic, until the introduction of the C5 complement blockade agent Eculizumab. A second generation of anti-complement agents is currently under investigation, representing future promising therapeutic strategies for patients affected by PNH. In the case of chronic hemolysis and renal iron deposition, a multidisciplinary approach should be considered to avoid or treat acute tubular injury or acute kidney injury (AKI). New promising perspectives derive from complement inhibitors and iron chelators, as well as more invasive treatments such as immunoadsorption or the use of dedicated hemodialysis filters in the presence of AKI.

New Insights in the Pathogenesis and Therapy of Cold Agglutinin-Mediated Autoimmune Hemolytic Anemia

Autoimmune hemolytic anemias mediated by cold agglutinins can be divided into cold agglutinin disease (CAD), which is a well-defined clinicopathologic entity and a clonal lymphoproliferative disorder, and secondary cold agglutinin syndrome (CAS), in which a similar picture of cold-hemolytic anemia occurs secondary to another distinct clinical disease. Thus, the pathogenesis in CAD is quite different from that of polyclonal autoimmune diseases such as warm-antibody AIHA. In both CAD and CAS, hemolysis is mediated by the classical complement pathway and therefore can result in generation of anaphylotoxins, such as complement split product 3a (C3a) and, to some extent, C5a. On the other hand, infection and inflammation can act as triggers and drivers of hemolysis, exemplified by exacerbation of CAD in situations with acute phase reaction and the role of specific infections (particularly Mycoplasma pneumoniae and Epstein-Barr virus) as causes of CAS. In this review, the putative mechanisms behind these phenomena will be explained along with other recent achievements in the understanding of pathogenesis in these disorders. Therapeutic approaches have been directed against the clonal lymphoproliferation in CAD or the underlying disease in CAS. Currently, novel targeted treatments, in particular complement-directed therapies, are also being rapidly developed and will be reviewed.

Prolonged intraocular residence and retinal tissue distribution of a fourth-generation compstatin-based C3 inhibitor in non-human primates

Age-related macular degeneration (AMD) is a leading cause of irreversible vision loss among the elderly population. Genetic studies in susceptible individuals have linked this ocular disease to deregulated complement activity that culminates in increased C3 turnover, retinal inflammation and photoreceptor loss. Therapeutic targeting of C3 has therefore emerged as a promising strategy for broadly intercepting the detrimental proinflammatory consequences of complement activation in the retinal tissue. In this regard, a PEGylated second-generation derivative of the compstatin family of C3-targeted inhibitors is currently in late-stage clinical development as a treatment option for geographic atrophy, an advanced form of AMD which lacks approved therapy. While efficacy has been strongly suggested in phase 2 clinical trials, crucial aspects still remain to be defined with regard to the ocular bioavailability, tissue distribution and residence, and dosing frequency of such inhibitors in AMD patients. Here we report the intraocular distribution and pharmacokinetic profile of the fourth-generation compstatin analog, Cp40-KKK in cynomolgus monkeys following a single intravitreal injection. Using a sensitive surface plasmon resonance (SPR)-based competition assay and ELISA, we have quantified both the amount of inhibitor and the concentration of C3 retained in the vitreous of Cp40-KKK-injected animals. Cp40-KKK displays prolonged intraocular residence, being detected at C3-saturating levels for over 3 months after a single intravitreal injection. Moreover, we have probed the distribution of Cp40-KKK within the ocular tissue by means of immunohistochemistry and highly specific anti-Cp40-KKK antibodies. Both C3 and Cp40-KKK were detected in the retinal tissue of inhibitor-injected animals, with prominent co-localization in the choroid one-month post intravitreal injection. These results attest to the high retinal tissue penetrance and target-driven distribution of Cp40-KKK. Given its subnanomolar binding affinity and prolonged ocular residence, Cp40-KKK constitutes a promising drug candidate for ocular pathologies underpinned by deregulated C3 activation.

The alternative complement pathway activation product Ba as a marker for transplant-associated thrombotic microangiopathy

BACKGROUND

Transplant-associated thrombotic microangiopathy (TA-TMA) occurs after hematopoietic stem cell transplantation (HSCT) and is characterized by microvascular thrombosis and end-organ injury particularly of the kidneys. TA-TMA is challenging to diagnose and treat, which can lead to long-term complications and death in patients with severe disease. Studies have shown that genetic abnormalities of the alternative complement pathway (AP) are associated with TA-TMA. We hypothesized that patients with TA-TMA may generate elevated levels of the AP activation product, Ba, compared with HSCT patients without TA-TMA.

PROCEDURE

We longitudinally measured plasma levels of complement activation products C3a, Ba, and C5a in 14 HSCT patients: 7 with TA-TMA and 7 without TA-TMA. We assessed renal function by calculating estimated glomerular filtration rate (eGFR) and correlated the extent of AP activation with renal dysfunction in both patient populations.

RESULTS

The median days from HSCT to study enrollment were 154 (39-237) in the TA-TMA group and 84 (39-253) in the HSCT group without TA-TMA. Median Ba levels (ng/mL) at enrollment were 1096.9 (826.5-1562.0) in the TA-TMA group and 725.7 (494.7-818.9) in the HSCT group without TA-TMA (P = 0.007). Over the study duration, Ba levels inversely correlated with eGFR. There were no differences in C3a, C5a, or sC5b9 levels between the two populations at any measured interval.

CONCLUSIONS

We conclude in this preliminary study that Ba protein may serve as a marker for TA-TMA, and furthermore, that components generated in the early phase of AP activation may be involved in the pathogenesis of renal endothelial injury in TA-TMA.

A novel approach to immunoapheresis of C3a/C3 and proteomic identification of associates

Background

Complement factor C3 represents the central component of the complement cascade and its activation split product C3a plays an important role in inflammation and disease. Many human disorders are linked to dysregulation of the complement system and alteration in interaction molecules. Therefore, various therapeutic approaches to act on the complement system have been initiated.

Methods and Results

Aiming to develop a tool to eliminate C3a/C3 from the circulation, in a first step a high affine murine monoclonal antibody (mAb) (3F7E2-mAb) was generated against complement factor C3 and selected for binding to the C3a region to serve as immunoaffinity reagent. Functional testing of the 3F7E2-mAb revealed an inhibition of Zymosan-induced cleavage of C3a from C3. Subsequently, a C3a/C3 specific 3F7E2-immunoaffinity column was developed and apheresis of C3a/C3 and associates was performed. Finally, a proteomic analysis was carried out for identification of apheresis products. C3a/C3 was liberated from the 3F7E2-column together with 278 proteins. C3a/C3 interaction specificity was validated by using a haptoglobin immunoaffinity column as control and biostatistic analysis revealed 39 true C3a/C3 interactants.

Conclusion

A novel and functionally active mAb was developed against complement factor C3a/C3 and used in a specific immunoaffinity column that allows apheresis of C3a/C3 and associates and their identification by proteomic analysis. This methodological approach of developing specific antibodies that can be used as immunoaffinity reagents to design immunoaffinity columns for elimination and further identification of associated proteins could open new avenues for the development of tailored immunotherapy in various complement-mediated or autoimmune diseases.

Diagnosis and treatment of autoimmune hemolytic anemia in adults: Recommendations from the First International Consensus Meeting

Autoimmune hemolytic anemias (AIHAs) are rare and heterogeneous disorders characterized by the destruction of red blood cells through warm or cold antibodies. There is currently no licensed treatment for AIHA. Due to the paucity of clinical trials, recommendations on diagnosis and therapy have often been based on expert opinions and some national guidelines. Here we report the recommendations of the First International Consensus Group, who met with the aim to review currently available data and to provide standardized diagnostic criteria and therapeutic approaches as well as an overview of novel therapies. Exact diagnostic workup is important because symptoms, course of disease, and therapeutic management relate to the type of antibody involved. Monospecific direct antiglobulin test is considered mandatory in the diagnostic workup, and any causes of secondary AIHA have to be diagnosed. Corticosteroids remain first-line therapy for warm-AIHA, while the addition of rituximab should be considered early in severe cases and if no prompt response to steroids is achieved. Rituximab with or without bendamustine should be used in the first line for patients with cold agglutinin disease requiring therapy. We identified a need to establish an international AIHA network. Future recommendations should be based on prospective clinical trials whenever possible.

Eculizumab and Beyond: The Past, Present, and Future of Complement Therapeutics

Dysregulation of the complement system underlies the pathophysiology of many diseases. Renewed interest in complement occurred with the recognition that its therapeutic inhibition was possible. Terminal complement blockade with the anti-C5 monoclonal antibody eculizumab significantly changed management and clinical outcomes of patients with paroxysmal nocturnal hemoglobinuria, and served as a proof of concept for other complement-mediated diseases. Eculizumab is also approved for atypical hemolytic uremic syndrome and myasthenia gravis. Multiple new disease indications have been identified, and novel complement inhibitors are in various stages of development, with several currently in human trials. Beyond C5, these new drugs block proximal complement, pathway-specific targets, convertase activity, and anaphylatoxin function. Though monoclonal antibodies are still common, peptides, RNAi, and small molecule inhibitors provide the opportunity for different administration routes and schedules. Several challenges still exist or will soon present themselves, including mitigation of infection risk, effective monitoring strategies, and how to choose between therapeutics when more than one is available. In this review, we will describe the lessons learned from the "eculizumab era," present many of the novel therapeutics currently or soon to be in trials, and highlight some of the challenges that will require attention as the field progresses.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Novel insights into the treatment of complement-mediated hemolytic anemias

Complement-mediated hemolytic anemias can either be caused by deficiencies in regulatory complement components or by autoimmune pathogenesis that triggers inappropriate complement activation. In paroxysmal nocturnal hemoglobinuria (PNH) hemolysis is entirely complement-driven. Hemolysis is also thought to be complement-dependent in cold agglutinin disease (CAD) and in paroxysmal cold hemoglobinuria (PCH), whereas warm antibody autoimmune hemolytic anemia (wAIHA) is a partially complement-mediated disorder, depending on the subtype of wAIHA and the extent of complement activation. The pathophysiology, clinical presentation, and current therapies for these diseases are reviewed in this article. Novel, complement-directed therapies are being rapidly developed. Therapeutic terminal complement inhibition using eculizumab has revolutionized the therapy and prognosis in PNH but has proved less efficacious in CAD. Upstream complement modulation is currently being investigated and appears to be a highly promising therapy, and two such agents have entered phase II and III trials. Of these, the anti-C1s monoclonal antibody sutimlimab has shown favorable activity in CAD, while the anti-C3 cyclic peptide pegcetacoplan appears to be promising in PNH as well as CAD, and may also have a therapeutic potential in wAIHA.

Clinical promise of next-generation complement therapeutics

The complement system plays a key role in pathogen immunosurveillance and tissue homeostasis. However, subversion of its tight regulatory control can fuel a vicious cycle of inflammatory damage that exacerbates pathology. The clinical merit of targeting the complement system has been established for rare clinical disorders such as paroxysmal nocturnal haemoglobinuria and atypical haemolytic uraemic syndrome. Evidence from preclinical studies and human genome-wide analyses, supported by new molecular and structural insights, has revealed new pathomechanisms and unmet clinical needs that have thrust a new generation of complement inhibitors into clinical development for a variety of indications. This review critically discusses recent clinical milestones in complement drug discovery, providing an updated translational perspective that may guide optimal target selection and disease-tailored complement intervention.