Expanding horizons in complement drug discovery: challenges and emerging strategies

At The Interface: Small-Molecule Inhibitors of Soluble Cytokines

Cytokines are crucial regulators of the immune system that orchestrate interactions between cells and, when dysregulated, contribute to the progression of chronic inflammation, cancer, and autoimmunity. Numerous biologic-based clinical agents, mostly monoclonal antibodies, have validated cytokines as important clinical targets and are now part of the standard of care for a number of diseases. These agents, while impactful, still suffer from limitations including a lack of oral bioavailability, high cost of production, and immunogenicity. Small-molecule cytokine inhibitors are attractive alternatives that can address these limitations. Although targeting cytokine-cytokine receptor complexes with small molecules has been a challenging research endeavor, multiple small-molecule inhibitors have now been identified, with a number of them undergoing clinical evaluation. In this review, we highlight the recent advancements in the discovery and development of small-molecule inhibitors targeting soluble cytokines. The strategies for identifying these novel ligands as well as the structural and mechanistic insights into their activity represent important milestones in tackling these challenging and clinically important protein-protein interactions.

The two coin sides of bacterial extracellular membrane nanovesicles: atherosclerosis trigger or remedy

Among the numerous driving forces that cause the atherosclerotic cardiovascular disease (ASCVD), pathogenic bacterial extracellular membrane nanovesicles (BEMNs) containing toxins and virulence factors appear to be the key trigger of inflammation and atherogenesis, the major processes involved in the pathogenesis of ASCVD. Since BEMNs are the carriers of nanosized biomolecules to distant sites, they are now being considered as a novel drug delivery system. Nowadays, many therapeutic strategies are used to treat ASCVD. However, the conventional anti-atherosclerotic therapies are not effective enough. This primarily due to the inefficiency of non-targeted drug delivery systems to tissue affected areas, which, in turn, leads to numerous side effects, as well as faulty pharmacokinetics. In this regard, nanomedicine methods using nanoparticles (NPs) as targeted drug delivery vehicles proved to be extremely useful. Bioengineered BEMNs equipped with disease-specific ligand moieties and loaded with corresponding drugs represent a promising tool in nanomedicine, which can be used as a novel drug delivery system for a successful therapy of ASCVD. In this review, we outline the involvement of pathogenic BEMNs in the triggering of ASCVD, the conventional therapeutic strategies for the treatment of ASCVD, and the recent trends in nanomedicine using BEMNs and NPs as a vehicle for targeted drug delivery.

Role of complement factor D in cardiovascular and metabolic diseases

In the genesis and progression of cardiovascular and metabolic diseases (CVMDs), adipose tissue plays a pivotal and dual role. Complement factor D (CFD, also known as adipsin), which is mainly produced by adipocytes, is the rate-limiting enzyme of the alternative pathway. Abnormalities in CFD generation or function lead to aberrant immune responses and energy metabolism. A large number of studies have revealed that CFD is associated with CVMDs. Herein, we will review the current studies on the function and mechanism of CFD in CVMDs such as hypertension, coronary heart disease, ischemia/reperfusion injury, heart failure, arrhythmia, aortic aneurysm, obesity, insulin resistance, and diabetic cardiomyopathy.

The Complement System as a Therapeutic Target in Retinal Disease

The complement cascade is a vital system in the human body's defense against pathogens. During the natural aging process, it has been observed that this system is imperative for ensuring the integrity and homeostasis of the retina. While this system is critical for proper host defense and retinal integrity, it has also been found that dysregulation of this system may lead to certain retinal pathologies, including geographic atrophy and diabetic retinopathy. Targeting components of the complement system for retinal diseases has been an area of interest, and in vivo, ex vivo, and clinical trials have been conducted in this area. Following clinical trials, medications targeting the complement system for retinal disease have also become available. In this manuscript, we discuss the pathophysiology of complement dysfunction in the retina and specific pathologies. We then describe the results of cellular, animal, and clinical studies targeting the complement system for retinal diseases. We then provide an overview of complement inhibitors that have been approved by the Food and Drug Administration (FDA) for geographic atrophy. The complement system in retinal diseases continues to serve as an emerging therapeutic target, and further research in this field will provide additional insights into the mechanisms and considerations for treatment of retinal pathologies.

Targeted complement inhibition using bispecific antibodies that bind local antigens and endogenous complement regulators

Complement activation protects against infection but also contributes to pathological mechanisms in a range of clinical conditions such as autoimmune diseases and transplant rejection. Complement-inhibitory drugs, either approved or in development, usually act systemically, thereby increasing the risk for infections. We therefore envisioned a novel class of bispecific antibodies (bsAbs) which are capable of site-directed complement inhibition by bringing endogenous complement regulators in the vicinity of defined cell surface antigens. Here, we analyzed a comprehensive set of obligate bsAbs designed to crosslink a specific target with either complement regulator factor H (FH) or C4b-binding protein (C4BP). The bsAbs were assessed for their capacity to inhibit complement activation and cell lysis in an antigen-targeted manner. We observed that the bsAbs inhibited classical, lectin, and alternative pathway complement activation in which sufficient endogenous serum FH and C4BP could be recruited to achieve local inhibition. Importantly, the bsAbs effectively protected antigen-positive liposomes, erythrocytes, and human leukocytes from complement-mediated lysis. In conclusion, localized complement inhibition by bsAbs capable of recruiting endogenous human complement regulators (such as FH or C4BP) to cell surfaces potentially provides a novel therapeutic approach for the targeted treatment of complement-mediated diseases.

The role of complement in kidney disease

The complement cascade comprises soluble and cell surface proteins and is an important arm of the innate immune system. Once activated, the complement system rapidly generates large quantities of protein fragments that are potent mediators of inflammatory, vasoactive and metabolic responses. Although complement is crucial to host defence and homeostasis, its inappropriate or uncontrolled activation can also drive tissue injury. For example, the complement system has been known for more than 50 years to be activated by glomerular immune complexes and to contribute to autoimmune kidney disease. Notably, the latest research shows that complement is also activated in kidney diseases that are not traditionally thought of as immune-mediated, including haemolytic-uraemic syndrome, diabetic kidney disease and focal segmental glomerulosclerosis. Several complement-targeted drugs have been approved for the treatment of kidney disease, and additional anti-complement agents are being investigated in clinical trials. These drugs are categorically different from other immunosuppressive agents and target pathological processes that are not effectively inhibited by other classes of immunosuppressants. The development of these new drugs might therefore have considerable benefits in the treatment of kidney disease.

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.

Inhibition of cleavage of human complement component C5 and the R885H C5 variant by two distinct high affinity anti-C5 nanobodies

The human complement system plays a crucial role in immune defense. However, its erroneous activation contributes to many serious inflammatory diseases. Since most unwanted complement effector functions result from C5 cleavage into C5a and C5b, development of C5 inhibitors, such as clinically approved monoclonal antibody eculizumab, are of great interest. Here, we developed and characterized two anti-C5 nanobodies, UNbC5-1 and UNbC5-2. Using surface plasmon resonance, we determined a binding affinity of 119.9 pM for UNbC5-1 and 7.7 pM for UNbC5-2. Competition experiments determined that the two nanobodies recognize distinct epitopes on C5. Both nanobodies efficiently interfered with C5 cleavage in a human serum environment, as they prevented red blood cell lysis via membrane attack complexes (C5b-9) and the formation of chemoattractant C5a. The cryo-EM structure of UNbC5-1 and UNbC5-2 in complex with C5 (3.6 Å resolution) revealed that the binding interfaces of UNbC5-1 and UNbC5-2 overlap with known complement inhibitors eculizumab and RaCI3, respectively. UNbC5-1 binds to the MG7 domain of C5, facilitated by a hydrophobic core and polar interactions, and UNbC5-2 interacts with the C5d domain mostly by salt bridges and hydrogen bonds. Interestingly, UNbC5-1 potently binds and inhibits C5 R885H, a genetic variant of C5 that is not recognized by eculizumab. Altogether, we identified and characterized two different, high affinity nanobodies against human C5. Both nanobodies could serve as diagnostic and/or research tools to detect C5 or inhibit C5 cleavage. Furthermore, the residues targeted by UNbC5-1 hold important information for therapeutic inhibition of different polymorphic variants of C5.

Bruch's Membrane: A Key Consideration with Complement-Based Therapies for Age-Related Macular Degeneration

The complement system is crucial for immune surveillance, providing the body's first line of defence against pathogens. However, an imbalance in its regulators can lead to inappropriate overactivation, resulting in diseases such as age-related macular degeneration (AMD), a leading cause of irreversible blindness globally affecting around 200 million people. Complement activation in AMD is believed to begin in the choriocapillaris, but it also plays a critical role in the subretinal and retinal pigment epithelium (RPE) spaces. Bruch's membrane (BrM) acts as a barrier between the retina/RPE and choroid, hindering complement protein diffusion. This impediment increases with age and AMD, leading to compartmentalisation of complement activation. In this review, we comprehensively examine the structure and function of BrM, including its age-related changes visible through in vivo imaging, and the consequences of complement dysfunction on AMD pathogenesis. We also explore the potential and limitations of various delivery routes (systemic, intravitreal, subretinal, and suprachoroidal) for safe and effective delivery of conventional and gene therapy-based complement inhibitors to treat AMD. Further research is needed to understand the diffusion of complement proteins across BrM and optimise therapeutic delivery to the retina.

Serum and plasma levels of Ba, but not those of soluble C5b-9, might be affected by renal function in chronic kidney disease patients

BACKGROUND

During the last few decades, pathogenic mechanisms associated with uncontrolled activation of the complement (C) system and development of anti-C agents have been closely investigated in the field of nephrology. The usefulness of some C products such as C5a and sC5b-9 for diagnostic and prognostic purposes remains controversial. On the other hand, decreased renal function is being observed in many patients with or without nephritis as a background factor in progressively aging societies. We therefore investigated whether renal function influenced the evaluation of various complement components and activation products.

METHODS

To investigate the influence of renal function on evaluations of C3, C4, CH50, Ba, C5a and sC5b-9, 40 patients were retrospectively chosen from among 844 patients without active glomerulonephritis from 2009 to 2016. We measured plasma and serum levels of C3, C4, CH50, Ba, C5a and sC5b-9 using enzyme-linked immunosorbent assays and compared the findings with inulin clearance (Cin) as a marker of preserved renal function.

RESULTS

Both plasma and serum levels of Ba correlated significantly with Cin, but other values did not. Compared with patients with Cin ≥ 60 or ≥ 30 mL/min/1.73 m², plasma and serum levels of Ba were increased in patients with Cin decreased to < 60 or < 30 mL/min/1.73 m², but levels of C5a and sC5b-9 were not.

CONCLUSION

The influence of renal function might need to be considered when evaluating Ba, but not C5a and sC5b-9, in plasma and serum samples from chronic kidney disease patients.

Gene targeting as a therapeutic avenue in diseases mediated by the complement alternative pathway

The complement alternative pathway (AP) is implicated in numerous diseases affecting many organs, ranging from the rare hematological disease paroxysmal nocturnal hemoglobinuria (PNH), to the common blinding disease age-related macular degeneration (AMD). Critically, the AP amplifies any activating trigger driving a downstream inflammatory response; thus, components of the pathway have become targets for drugs of varying modality. Recent validation from clinical trials using drug modalities such as inhibitory antibodies has paved the path for gene targeting of the AP or downstream effectors. Gene targeting in the complement field currently focuses on supplementation or suppression of complement regulators in AMD and PNH, largely because the eye and liver are highly amenable to drug delivery through local (eye) or systemic (liver) routes. Targeting the liver could facilitate treatment of numerous diseases as this organ generates most of the systemic complement pool. This review explains key concepts of RNA and DNA targeting and discusses assets in clinical development for the treatment of diseases driven by the alternative pathway, including the RNA-targeting therapeutics ALN-CC5, ARO-C3, and IONIS-FB-LRX, and the gene therapies GT005 and HMR59. These therapies are but the spearhead of potential drug candidates that might revolutionize the field in coming years.

Low serum complement level is associated with higher mortality in tuberculous meningitis: a retrospective cohort study

Purpose

We evaluated the associations between serum complement levels and tuberculous meningitis (TBM), bacterial meningitis (BM), and viral meningitis (VM), as well as the association between serum complement levels and mortality in TBM.

Methods

Background information and blood/cerebrospinal fluid analysis results were collected from 2009 to 2019. Patients who had serum complement level data collected at admission and who were diagnosed with TBM (n = 97), BM (n = 31), or VM (n = 557) were enrolled.

Results

Initial serum complement levels were significantly lower in the TBM group than the VM group in both the total population and the propensity score-matched population. In the TBM and VM groups, compared to patients with initial highest-quartile C4 level, patients in the lowest quartile (C4 < 24.3 mg/dL) had significantly greater odds of TBM diagnosis (odds ratio, 2.2; 95% confidence interval, 1.0-4.5; p = 0.038). In the TBM group, patients with the lowest-quartile C3 level (<96.9 mg/dL) experienced a significantly higher 90-day mortality rate compared to other TBM patients (hazard ratio, 19.0; 95% confidence interval, 2.1-167.4.5; p = 0.008).

Conclusion

Both serum C3 and C4 levels were significantly lower in the TBM group than in the VM group. TBM patients with lower serum C3 level had a significantly higher mortality rate than those with higher C3 level.

Emerging Approaches for the Management of Chemotherapy-Induced Peripheral Neuropathy (CIPN): Therapeutic Potential of the C5a/C5aR Axis

Chemotherapy-induced peripheral neuropathy (CIPN) is the most common neurologic complication of chemotherapy, resulting in symptoms like pain, sensory loss, and numbness in the hands and feet that cause lots of uneasiness in patients with cancer. They often suffer from pain so severe that it interrupts the treatment, thus invalidating the entire chemotherapy-based healing process, and significantly reducing their quality of life. In this paper, we underline the role of the complement system in CIPN, highlighting the relevance of the C5a fragment and its receptor C5aR1, whose activation is thought to be involved in triggering a cascade of events that can lead to CIPN onset. Recent experimental data showed the ability of docetaxel and paclitaxel to specifically bind and activate C5aR1, thus shining light on one of the molecular mechanisms by which taxanes may activate a cascade of events leading to neuropathy. According to these new evidence, it was possible to suggest new mechanisms underlying the pathophysiology of CIPN. Hence, the C5a/C5aR1 axis may represent a new target for CIPN treatment, and the use of C5aR1 inhibitors can be proposed as a potential new therapeutic option to manage this high unmet medical need.

Contribution of Coronavirus-Specific Immunoglobulin G Responses to Complement Overactivation in Patients with Severe Coronavirus Disease 2019

BACKGROUND

Excessive complement activation has been implicated in the pathogenesis of coronavirus disease 2019 (COVID-19), but the mechanisms leading to this response remain unclear.

METHODS

We measured plasma levels of key complement markers, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA and antibodies against SARS-CoV-2 and seasonal human common cold coronaviruses (CCCs) in hospitalized patients with COVID-19 of moderate (n = 18) and critical severity (n = 37) and in healthy controls (n = 10).

RESULTS

We confirmed that complement activation is systemically increased in patients with COVID-19 and is associated with a worse disease outcome. We showed that plasma levels of C1q and circulating immune complexes were markedly increased in patients with severe COVID-19 and correlated with higher immunoglobulin (Ig) G titers, greater complement activation, and higher disease severity score. Additional analyses showed that the classical pathway was the main arm responsible for augmented complement activation in severe patients. In addition, we demonstrated that a rapid IgG response to SARS-CoV-2 and an anamnestic IgG response to the nucleoprotein of the CCCs were strongly correlated with circulating immune complex levels, complement activation, and disease severity.

CONCLUSIONS

These findings indicate that early, nonneutralizing IgG responses may play a key role in complement overactivation in severe COVID-19. Our work underscores the urgent need to develop therapeutic strategies to modify complement overactivation in patients with COVID-19.

Mathematical Modeling of Complement Pathway Dynamics for Target Validation and Selection of Drug Modalities for Complement Therapies

Motivation: The complement pathway plays a critical role in innate immune defense against infections. Dysregulation between activation and regulation of the complement pathway is widely known to contribute to several diseases. Nevertheless, very few drugs that target complement proteins have made it to the final regulatory approval because of factors such as high concentrations and dosing requirements for complement proteins and serious side effects from complement inhibition.

Methods: A quantitative systems pharmacology (QSP) model of the complement pathway has been developed to evaluate potential drug targets to inhibit complement activation in autoimmune diseases. The model describes complement activation via the alternative and terminal pathways as well as the dynamics of several regulatory proteins. The QSP model has been used to evaluate the effect of inhibiting complement targets on reducing pathway activation caused by deficiency in factor H and CD59. The model also informed the feasibility of developing small-molecule or large-molecule antibody drugs by predicting the drug dosing and affinity requirements for potential complement targets.

Results: Inhibition of several complement proteins was predicted to lead to a significant reduction in complement activation and cell lysis. The complement proteins that are present in very high concentrations or have high turnover rates (C3, factor B, factor D, and C6) were predicted to be challenging to engage with feasible doses of large-molecule antibody compounds (≤20 mg/kg). Alternatively, complement fragments that have a short half-life (C3b, C3bB, and C3bBb) were predicted to be challenging or infeasible to engage with small-molecule compounds because of high drug affinity requirements (>1 nM) for the inhibition of downstream processes. The drug affinity requirements for disease severity reduction were predicted to differ more than one to two orders of magnitude than affinities needed for the conventional 90% target engagement (TE) for several proteins. Thus, the QSP model analyses indicate the importance for accounting for TE requirements for achieving reduction in disease severity endpoints during the lead optimization stage.

The EPICC Family of Anti-Inflammatory Peptides: Next Generation Peptides, Additional Mechanisms of Action, and In Vivo and Ex Vivo Efficacy

The EPICC peptides are a family of peptides that have been developed from the sequence of the capsid protein of human astrovirus type 1 and previously shown to inhibit the classical and lectin pathways of complement. The EPICC peptides have been further optimized to increase aqueous solubility and identify additional mechanisms of action. Our laboratory has developed the lead EPICC molecule, PA-dPEG24 (also known as RLS-0071), which is composed of a 15 amino acid peptide with a C-terminal monodisperse 24-mer PEGylated moiety. RLS-0071 has been demonstrated to possess other mechanisms of action in addition to complement blockade that include the inhibition of neutrophil-driven myeloperoxidase (MPO) activity, inhibition of neutrophil extracellular trap (NET) formation as well as intrinsic antioxidant activity mediated by vicinal cysteine residues contained within the peptide sequence. RLS-0071 has been tested in various ex vivo and in vivo systems and has shown promise for the treatment of both immune-mediated hematological diseases where alterations in the classical complement pathway plays an important pathogenic role as well as in models of tissue-based diseases such as acute lung injury and hypoxic ischemic encephalopathy driven by both complement and neutrophil-mediated pathways (i.e., MPO activity and NET formation). Next generation EPICC peptides containing a sarcosine residue substitution in various positions within the peptide sequence possess aqueous solubility in the absence of PEGylation and demonstrate enhanced complement and neutrophil inhibitory activity compared to RLS-0071. This review details the development of the EPICC peptides, elucidation of their dual-acting complement and neutrophil inhibitory activities and efficacy in ex vivo systems using human clinical specimens and in vivo efficacy in animal disease models.

Complement inhibition: A possible therapeutic approach in the fight against Covid-19

The complement system, as a vital part of innate immunity, has an important role in the clearance of pathogens; however, unregulated activation of this system probably has a key role in the pathogenesis of acute lung injury, which is induced by highly pathogenic viruses (i.e. influenza A viruses and severe acute respiratory syndrome [SARS] coronavirus). The novel coronavirus SARS-CoV-2, which is the causal agent for the ongoing global pandemic of the coronavirus disease 2019 (Covid-19), has recently been spread to almost all countries around the world. Although most people are immunocompetent to SARS-CoV-2, a small group develops hyper-inflammation that leads to complications like acute respiratory distress syndrome, disseminated intravascular coagulation, and multi-organ failure. Emerging evidence demonstrates that the complement system exerts a crucial role in this inflammatory reaction. Additionally, patients with the severe form of Covid-19 show over-activation of the complement in their skin, sera, and lungs. This study aims to summarise current knowledge concerning the interaction of SARS-CoV-2 with the complement system and to critically appraise complement inhibition as a potential new approach for Covid-19 treatment.

Novel Selection Approaches to Identify Antibodies Targeting Neoepitopes on the C5b6 Intermediate Complex to Inhibit Membrane Attack Complex Formation

The terminal pathway of complement is implicated in the pathology of multiple diseases and its inhibition is, therefore, an attractive therapeutic proposition. The practicalities of inhibiting this pathway, however, are challenging, as highlighted by the very few molecules in the clinic. The proteins are highly abundant, and assembly is mediated by high-affinity protein-protein interactions. One strategy is to target neoepitopes that are present transiently and only exist on active or intermediate complexes but not on the abundant native proteins. Here, we describe an antibody discovery campaign that generated neoepitope-specific mAbs against the C5b6 complex, a stable intermediate complex in terminal complement complex assembly. We used a highly diverse yeast-based antibody library of fully human IgGs to screen against soluble C5b6 antigen and successfully identified C5b6 neoepitope-specific antibodies. These antibodies were diverse, showed good binding to C5b6, and inhibited membrane attack complex (MAC) formation in a solution-based assay. However, when tested in a more physiologically relevant membrane-based assay these antibodies failed to inhibit MAC formation. Our data highlight the feasibility of identifying neoepitope binding mAbs, but also the technical challenges associated with the identification of functionally relevant, neoepitope-specific inhibitors of the terminal pathway.

Complement Factor D as a Strategic Target for Regulating the Alternative Complement Pathway

The complement system is central to first-line defense against invading pathogens. However, excessive complement activation and/or the loss of complement regulation contributes to the development of autoimmune diseases, systemic inflammation, and thrombosis. One of the three pathways of the complement system, the alternative complement pathway, plays a vital role in amplifying complement activation and pathway signaling. Complement factor D, a serine protease of this pathway that is required for the formation of C3 convertase, is the rate-limiting enzyme. In this review, we discuss the function of factor D within the alternative pathway and its implication in both healthy physiology and disease. Because the alternative pathway has a role in many diseases that are characterized by excessive or poorly mediated complement activation, this pathway is an enticing target for effective therapeutic intervention. Nonetheless, although the underlying disease mechanisms of many of these complement-driven diseases are quite well understood, some of the diseases have limited treatment options or no approved treatments at all. Therefore, in this review we explore factor D as a strategic target for advancing therapeutic control of pathological complement activation.

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.

Emerging Role of C5 Complement Pathway in Peripheral Neuropathies: Current Treatments and Future Perspectives

The complement system is a key component of innate immunity since it plays a critical role in inflammation and defense against common pathogens. However, an inappropriate activation of the complement system is involved in numerous disorders, including peripheral neuropathies. Current strategies for neuropathy-related pain fail to achieve adequate pain relief, and although several therapies are used to alleviate symptoms, approved disease-modifying treatments are unavailable. This urgent medical need is driving the development of therapeutic agents for this condition, and special emphasis is given to complement-targeting approaches. Recent evidence has underscored the importance of complement component C5a and its receptor C5aR1 in inflammatory and neuropathic pain, indicating that C5a/C5aR1 axis activation triggers a cascade of events involved in pathophysiology of peripheral neuropathy and painful neuro-inflammatory states. However, the underlying pathophysiological mechanisms of this signaling in peripheral neuropathy are not fully known. Here, we provide an overview of complement pathways and major components associated with dysregulated complement activation in peripheral neuropathy, and of drugs under development targeting the C5 system. C5/C5aR1 axis modulators could represent a new strategy to treat complement-related peripheral neuropathies. Specifically, we describe novel C5aR allosteric modulators, which may potentially become new tools in the therapeutic armory against neuropathic pain.

Therapeutic Lessons to be Learned From the Role of Complement Regulators as Double-Edged Sword in Health and Disease

The complement system is an important part of the innate immune system, providing a strong defense against pathogens and removing apoptotic cells and immune complexes. Due to its strength, it is important that healthy human cells are protected against damage induced by the complement system. To be protected from complement, each cell type relies on a specific combination of both soluble and membrane-bound regulators. Their importance is indicated by the amount of pathologies associated with abnormalities in these complement regulators. Here, we will discuss the current knowledge on complement regulatory protein polymorphisms and expression levels together with their link to disease. These diseases often result in red blood cell destruction or occur in the eye, kidney or brain, which are tissues known for aberrant complement activity or regulation. In addition, complement regulators have also been associated with different types of cancer, although their mechanisms here have not been elucidated yet. In most of these pathologies, treatments are limited and do not prevent the complement system from attacking host cells, but rather fight the consequences of the complement-mediated damage, using for example blood transfusions in anemic patients. Currently only few drugs targeting the complement system are used in the clinic. With further demand for therapeutics rising linked to the wide range of complement-mediated disease we should broaden our horizon towards treatments that can actually protect the host cells against complement. Here, we will discuss the latest insights on how complement regulators can benefit therapeutics. Such therapeutics are currently being developed extensively, and can be categorized into full-length complement regulators, engineered complement system regulators and antibodies targeting complement regulators. In conclusion, this review provides an overview of the complement regulatory proteins and their links to disease, together with their potential in the development of novel therapeutics.

The good, the bad, and the opportunities of the complement system in neurodegenerative disease

The complement cascade is a critical effector mechanism of the innate immune system that contributes to the rapid clearance of pathogens and dead or dying cells, as well as contributing to the extent and limit of the inflammatory immune response. In addition, some of the early components of this cascade have been clearly shown to play a beneficial role in synapse elimination during the development of the nervous system, although excessive complement-mediated synaptic pruning in the adult or injured brain may be detrimental in multiple neurogenerative disorders. While many of these later studies have been in mouse models, observations consistent with this notion have been reported in human postmortem examination of brain tissue. Increasing awareness of distinct roles of C1q, the initial recognition component of the classical complement pathway, that are independent of the rest of the complement cascade, as well as the relationship with other signaling pathways of inflammation (in the periphery as well as the central nervous system), highlights the need for a thorough understanding of these molecular entities and pathways to facilitate successful therapeutic design, including target identification, disease stage for treatment, and delivery in specific neurologic disorders. Here, we review the evidence for both beneficial and detrimental effects of complement components and activation products in multiple neurodegenerative disorders. Evidence for requisite co-factors for the diverse consequences are reviewed, as well as the recent studies that support the possibility of successful pharmacological approaches to suppress excessive and detrimental complement-mediated chronic inflammation, while preserving beneficial effects of complement components, to slow the progression of neurodegenerative disease.

Immunoregulatory complement receptor-1 and leukocyte-associated Ig-like receptor-1 expression on leukocytes in Psoriasis vulgaris

Psoriasis vulgaris (PsV) is an immune-mediated inflammatory disorder with devastating psychosocial consequences. Expression of immunoregulator molecules on leukocytes in PsV remains unclear. Leukocyte-associated Ig-like receptor-1 (LAIR-1) and complement receptor-1 (CR-1) are immunoregulator receptors reported to bind complement component 1q involved in phagocytosis. We aimed to explore if altered leukocyte expression of LAIR-1 and CR-1 is associated with PsV. This case-control study included 36 PsV patients and 36 healthy controls. Neutrophils, monocytes and B and T cells were examined by flow cytometry for LAIR-1 and CR-1 mean fluorescence intensity (MFI) and positive cell percentage. Comparison between both groups revealed a significant decrease in LAIR-1 MFI on neutrophils and T cells (P < 0.001 and P = 0.003, respectively). CR-1 MFI on neutrophils, monocytes and T cells also showed a significant decrease in patients (P = 0.033, P = 0.001 and P = 0.040, respectively). There was a significant positive correlation of LAIR-1 MFI on neutrophils with CR-1 MFI on neutrophils (r = 0.503; P = 0.002) and LAIR-1 MFI on monocytes with CR-1 MFI on monocytes (r = 0.371; P = 0.026). Receiver operating characteristic curves revealed that CR-1 MFI on monocytes had the highest discrimination power to differentiate patients from controls, with 86.1% specificity and 75% sensitivity (P = 0.001). In conclusion, altered leukocytes expression of LAIR-1 and CR-1 is associated with PsV. Down-regulated CR-1 MFI on monocytes is a promising diagnostic biomarker for PsV.

Targeting the Initiator Protease of the Classical Pathway of Complement Using Fragment-Based Drug Discovery

The initiating protease of the complement classical pathway, C1r, represents an upstream and pathway-specific intervention point for complement-related autoimmune and inflammatory diseases. Yet, C1r-targeted therapeutic development is currently underrepresented relative to other complement targets. In this study, we developed a fragment-based drug discovery approach using surface plasmon resonance (SPR) and molecular modeling to identify and characterize novel C1r-binding small-molecule fragments. SPR was used to screen a 2000-compound fragment library for binding to human C1r. This led to the identification of 24 compounds that bound C1r with equilibrium dissociation constants ranging between 160–1700 µM. Two fragments, termed CMP-1611 and CMP-1696, directly inhibited classical pathway-specific complement activation in a dose-dependent manner. CMP-1611 was selective for classical pathway inhibition, while CMP-1696 also blocked the lectin pathway but not the alternative pathway. Direct binding experiments mapped the CMP-1696 binding site to the serine protease domain of C1r and molecular docking and molecular dynamics studies, combined with C1r autoactivation assays, suggest that CMP-1696 binds within the C1r active site. The group of structurally distinct fragments identified here, along with the structure–activity relationship profiling of two lead fragments, form the basis for future development of novel high-affinity C1r-binding, classical pathway-specific, small-molecule complement inhibitors.

Complement C3 deficiency ameliorates aging related changes in the kidney

AIMS

Complement C3 (C3) has been shown to be involved in the aging process. However, the role of C3 in kidney aging has not been fully elucidated. This study aimed to investigate the effect of C3 on senescence related kidney disorders in mice.

MATERIALS AND METHODS

Two-, 8-, and 16-month-old C3-deficient male mice (KO) (n = 6) and age-, gender-, and strain- matched wild type (WT) C57BL/6 mice (n = 6) were selected to represent young, middle-aged and aging mice. Renal, blood and urine samples were collected. Hematoxylin-eosin (HE), Masson, and immunohistochemistry (IHC) staining as well as ELISA and Western blotting were used to explore the mechanisms involved in renal aging.

KEY FINDINGS

The level of C3 was upregulated during aging in WT mice. The glomerular sclerosis index and tubulointerstitial fibrosis index were increased significantly in WT mice during aging. Renal function was not significantly different between the young and aged groups. Compared with those in WT mice, the levels of inflammation and fibrosis were decreased, while the expression of CD31 was significantly increased in the KO group.

SIGNIFICANCE

Our data demonstrated that age-related changes in renal structure occur earlier than functional changes and that complement C3 is involved in aging-related kidney disorder.

Role of inflammation in the pathogenesis of schizophrenia: A review of the evidence, proposed mechanisms and implications for treatment

AIM

Over the past several decades, there has been a growing research interest in the role of inflammation in the pathogenesis of schizophrenia. This review aims to summarize evidence in support of this relationship, to discuss biological mechanisms that might explain it, and to explore the translational impact by examining evidence from trials of anti-inflammatory and immunomodulatory agents in the treatment of schizophrenia.

METHODS

This narrative review of the literature summarizes evidence from observational studies, clinical trials and meta-analyses to evaluate the role of inflammation in the pathogenesis of schizophrenia and to discuss associated implications for treatment.

RESULTS

Epidemiological evidence and animal models support a hypothesis of maternal immune activation during pregnancy, which increases the risk of schizophrenia in the offspring. Several biomarker studies have found associations between classical pro-inflammatory cytokines and schizophrenia. The precise biological mechanisms by which inflammatory processes might contribute to the pathogenesis of schizophrenia remain unclear, but likely include the actions of microglia and the complement system. Importantly, several trials provide evidence that certain anti-inflammatory and immunomodulatory agents show beneficial effects in the treatment of schizophrenia. Nevertheless, there is a need for further precision-focused basic science and translational research.

CONCLUSIONS

Increasing our understanding of the role of inflammation in schizophrenia will enable novel opportunities for therapeutic and preventative interventions that are informed by the underlying pathogenesis of this complex disorder.

Inhibition of the Lectin Pathway of the Complement System as a Novel Approach in the Management of IgA Vasculitis-Associated Nephritis

IgA vasculitis can present as a glomerulonephritis histologically indistinguishable from IgA nephropathy (IgAN). In IgAN, the alternative and lectin pathways mediate glomerular injury and contribute to kidney function decline. Narsoplimab is a monoclonal antibody against mannan-binding lectin serine peptidase 2 (MASP-2), a key component of the lectin pathway. It is being evaluated in a phase III trial in IgAN (NCT03608033). Histopathological similarities with IgAN suggest lectin pathway activation also occurs in IgAV-associated nephritis (IgAVN). Here, we report the first ever case of narsoplimab use for the treatment of IgAVN.

Targeting the complement system in bacterial meningitis

Bacterial meningitis is most commonly caused by Streptococcus pneumoniae and Neisseria meningitidis and continues to pose a major public health threat. Morbidity and mortality of meningitis are driven by an uncontrolled host inflammatory response. This comprehensive update evaluates the role of the complement system in upregulating and maintaining the inflammatory response in bacterial meningitis. Genetic variation studies, complement level measurements in blood and CSF, and experimental work have together led to the identification of anaphylatoxin C5a as a promising treatment target in bacterial meningitis. In animals and patients with pneumococcal meningitis, the accumulation of neutrophils in the CSF was mainly driven by C5-derived chemotactic activity and correlated positively with disease severity and outcome. In murine pneumococcal meningitis, adjunctive treatment with C5 antibodies prevented brain damage and death. Several recently developed therapeutics target C5 conversion, C5a, or its receptor C5aR. Caution is warranted because treatment with C5 antibodies such as eculizumab also inhibits the formation of the membrane attack complex, which may result in decreased meningococcal killing and increased meningococcal disease susceptibility. The use of C5a or C5aR antagonists to specifically target the harmful anaphylatoxins-induced effects, therefore, are most promising and present opportunities for a phase 2 clinical trial.

Single-Cell RNA Sequencing Reveals a Dynamic Stromal Niche That Supports Tumor Growth

Here, using single-cell RNA sequencing, we examine the stromal compartment in murine melanoma and draining lymph nodes (LNs) at points across tumor development, providing data at http://www.teichlab.org/data/. Naive lymphocytes from LNs undergo activation and clonal expansion within the tumor, before PD1 and Lag3 expression, while tumor-associated myeloid cells promote the formation of a suppressive niche. We identify three temporally distinct stromal populations displaying unique functional signatures, conserved across mouse and human tumors. Whereas "immune" stromal cells are observed in early tumors, "contractile" cells become more prevalent at later time points. Complement component C3 is specifically expressed in the immune population. Its cleavage product C3a supports the recruitment of C3aR+ macrophages, and perturbation of C3a and C3aR disrupts immune infiltration, slowing tumor growth. Our results highlight the power of scRNA-seq to identify complex interplays and increase stromal diversity as a tumor develops, revealing that stromal cells acquire the capacity to modulate immune landscapes from early disease.

Tumour-cell-derived complement components C1r and C1s promote growth of cutaneous squamous cell carcinoma

BACKGROUND

The incidence of epidermal keratinocyte-derived cutaneous squamous cell carcinoma (cSCC) is increasing worldwide.

OBJECTIVES

To study the role of the complement classical pathway components C1q, C1r and C1s in the progression of cSCC.

METHODS

The mRNA levels of C1Q subunits and C1R and C1S in cSCC cell lines, normal human epidermal keratinocytes, cSCC tumours in vivo and normal skin were analysed with quantitative real-time polymerase chain reaction. The production of C1r and C1s was determined with Western blotting. The expression of C1r and C1s in tissue samples in vivo was analysed with immunohistochemistry and further investigated in human cSCC xenografts by knocking down C1r and C1s.

RESULTS

Significantly elevated C1R and C1S mRNA levels and production of C1r and C1s were detected in cSCC cells, compared with normal human epidermal keratinocytes. The mRNA levels of C1R and C1S were markedly elevated in cSCC tumours in vivo compared with normal skin. Abundant expression of C1r and C1s by tumour cells was detected in invasive sporadic cSCCs and recessive dystrophic epidermolysis bullosa-associated cSCCs, whereas the expression of C1r and C1s was lower in cSCC in situ, actinic keratosis and normal skin. Knockdown of C1r and C1s expression in cSCC cells inhibited activation of extracellular signal-related kinase 1/2 and Akt, promoted apoptosis of cSCC cells and significantly suppressed growth and vascularization of human cSCC xenograft tumours in vivo.

CONCLUSIONS

These results provide evidence for the role of tumour-cell-derived C1r and C1s in the progression of cSCC and identify them as biomarkers and putative therapeutic targets in cSCC. What's already known about this topic? The incidences of actinic keratosis, cutaneous squamous cell carcinoma (cSCC) in situ and invasive cSCC are increasing globally. Few specific biomarkers for progression of cSCC have been identified, and no biological markers are in clinical use to predict the aggressiveness of actinic keratosis, cSCC in situ and invasive cSCC. What does this study add? Our results provide novel evidence for the role of complement classical pathway components C1r and C1s in the progression of cSCC. What is the translational message? Our results identify complement classical pathway components C1r and C1s as biomarkers and putative therapeutic targets in cSCC.

Current and Future Approaches for Monitoring Responses to Anti-complement Therapeutics

Aberrations in complement system functions have been identified as either direct or indirect pathophysiological mechanisms in many diseases and pathological conditions, such as infections, autoimmune diseases, inflammation, malignancies, and allogeneic transplantation. Currently available techniques to study complement include quantification of (a) individual complement components, (b) complement activation products, and (c) molecular mechanisms/function. An emerging area of major interest in translational studies aims to study and monitor patients on complement regulatory drugs for efficacy as well as adverse events. This area is progressing rapidly with several anti-complement therapeutics under development, in clinical trials, or already in clinical use. In this review, we summarized the appropriate indications, techniques, and interpretations of basic complement analyses, exemplified by a number of clinical disorders.

The role of complement in preterm birth and prematurity

Complications of preterm birth (PTB) are the global leading cause of death in children younger than 5 years of age. Almost 15 million children are born prematurely in the world each year. Increasing evidence suggests that labor and delivery have many hallmarks of an inflammatory reaction, where complement activation has an active participation. As one of the most important components of inflammation, the role of complement during labor and PTB is becoming an attractive research target. The complement components C1q and C5b-9 are deposited on fetal membranes and release inflammatory mediators that contribute to uterine contractions, cervical ripening, cell chemotaxis, metalloproteinases production, membrane awaking and rupture, and it participates as a co-adjuvant in the onset and progress of labor. This article reviews a basic description of the complement system, its role in preterm birth and current concepts regarding its contribution in novel therapy strategies and new biomarkers.

Compendium of current complement therapeutics

The complement system is well known for its role in innate immunity and in maintenance of tissue homeostasis, providing a first line of defence against infection and playing a key role in flagging apoptotic cells and debris for disposal. Unfortunately, complement also contributes to pathogenesis of many diseases, in some cases driving pathology, and in others amplifying or exacerbating the inflammatory and damaging impact of non-complement disease triggers. The driving role of complement in a single disease, paroxysmal nocturnal hemoglobinuria (PNH), provoked the development and eventual FDA (US Food and Drug Administration) approval of eculizumab (Soliris™), an anti-C5 antibody, for therapy. Although PNH is very rare, eculizumab provided clinical validation and demonstrated that inhibiting the complement system was not only well-tolerated, but also provided rapid therapy and saved lives. This clinical validation, together with advances in genetic analyses that demonstrated strong associations between complement and common diseases, drove new drug discovery programmes in both academic laboratories and large pharmaceutical companies. Numerous drugs have entered clinical development and several are in phase 3 trials; however, many have fallen by the wayside. Despite this high attrition rate, crucial lessons have been learnt and hurdles to development have become clear. These insights have driven development of next generation anti-complement drugs designed to avoid pitfalls and facilitate patient access. In this article, we do not set out to provide a text-heavy review of complement therapeutics but instead will simply highlight the targets, modalities and current status of the plethora of drugs approved or in clinical development. With such a fast-moving drug development landscape, such a compendium will inevitably become out-dated; however, we provide a snapshot of the current field and illustrate the increased choice that clinicians might enjoy in the future in selecting the best drug for their application, decisions based not only on efficacy but also cost, mechanistic target, modality and route of delivery.

Development and characterization of novel anti-C5 monoclonal antibodies capable of inhibiting complement in multiple species

Over the last decade there has been an explosion in complement therapies; one-third of the drugs in the clinic or in development target C5 protein. Eculizumab, a monoclonal antibody (mAb) that binds C5 and blocks its cleavage by the convertase, is the current reference standard treatment for atypical haemolytic uraemic syndrome (aHUS) and paroxysmal nocturnal haemoglobinuria (PNH) and in clinical trials for many other diseases. Here we describe a panel of novel anti-C5 mAb, including mAb that, like Eculizumab, are efficient inhibitors of complement but, unlike Eculizumab, inhibit across species, including human, rat, rabbit and guinea pig. Several inhibitory anti-C5 mAb were identified and characterized for C5 binding and lytic inhibitory capacity in comparison to current therapeutic anti-C5 mAb; three clones, 4G2, 7D4 and 10B6, were selected and further characterized for ligand specificity and affinity and cross-species inhibitory activity. The mAb 10B6 was human-specific whereas mAb 4G2 and 7D4 efficiently inhibited lysis by human, rabbit and rat serum, and weakly inhibited guinea pig complement; 7D4 also weakly inhibited mouse complement in vitro The rat C5-cross-reactive mAb 4G2, when administered intraperitoneally in a rat model of myasthenia gravis, effectively blocked the disease and protected muscle endplates from destruction. To our knowledge this is the first report of an anti-C5 function blocking mAb that permits preclinical studies in rats.

Therapeutic Modulation of the Complement Cascade in Stroke

Stroke is a leading cause of death and disability worldwide and an increasing number of ischemic stroke patients are undergoing pharmacological and mechanical reperfusion. Both human and experimental models of reperfused ischemic stroke have implicated the complement cascade in secondary tissue injury. Most data point to the lectin and alternative pathways as key to activation, and C3a and C5a binding of their receptors as critical effectors of injury. During periods of thrombolysis use to treat stroke, acute experimental complement cascade blockade has been found to rescue tissue and improves functional outcome. Blockade of the complement cascade during the period of tissue reorganization, repair, and recovery is by contrast not helpful and in fact is likely to be deleterious with emerging data suggesting downstream upregulation of the cascade might even facilitate recovery. Successful clinical translation will require the right clinical setting and pharmacologic strategies that are capable of targeting the key effectors early while not inhibiting delayed repair. Early reports in a variety of disease states suggest that such pharmacologic strategies appear to have a favorable risk profile and offer substantial hope for patients.

Deciphering Biological Processes at the Tick-Host Interface Opens New Strategies for Treatment of Human Diseases

Ticks are obligatory blood-feeding ectoparasites, causing blood loss and skin damage in their hosts. In addition, ticks also transmit a number of various pathogenic microorganisms that cause serious diseases in humans and animals. Ticks evolved a wide array of salivary bioactive compounds that, upon injection into the host skin, inhibit or modulate host reactions such as hemostasis, inflammation and wound healing. Modulation of the tick attachment site in the host skin involves mainly molecules which affect physiological processes orchestrated by cytokines, chemokines and growth factors. Suppressing host defense reactions is crucial for tick survival and reproduction. Furthermore, pharmacologically active compounds in tick saliva have a promising therapeutic potential for treatment of some human diseases connected with disorders in hemostasis and immune system. These disorders are often associated to alterations in signaling pathways and dysregulation or overexpression of specific cytokines which, in turn, affect mechanisms of angiogenesis, cell motility and cytoskeletal regulation. Moreover, tick salivary molecules were found to exert cytotoxic and cytolytic effects on various tumor cells and have anti-angiogenic properties. Elucidation of the mode of action of tick bioactive molecules on the regulation of cell processes in their mammalian hosts could provide new tools for understanding the complex changes leading to immune disorders and cancer. Tick bioactive molecules may also be exploited as new pharmacological inhibitors of the signaling pathways of cytokines and thus help alleviate patient discomfort and increase patient survival. We review the current knowledge about tick salivary peptides and proteins that have been identified and functionally characterized in in vitro and/or in vivo models and their therapeutic perspective.

Beneficial effects on H1N1-induced acute lung injury and structure characterization of anti-complementary acidic polysaccharides from Juniperus pingii var. wilsonii

Juniperus pingii var. wilsonii has been traditionally used in Tibetan medicine for the treatment of inflammatory diseases. In the present study, J. pingii var. wilsonii polysaccharides (JPWP), with high content of d‑galacturonic acid, showed potent anti-complementary activity in vitro and significantly attenuated acute lung injury (ALI) induced by H1N1 influenza virus in vivo through reducing the inflammatory responses, alleviating oxidative stress and inhibiting the activation of complement. Thus, anti-complementary activity-guided fractionation of JPWP led to the isolation of an acidic homogeneous polysaccharide, JPWP-PS, whose structure was further elucidated by acid hydrolysis, PMP derivation, methylation and NMR analysis. JPWP-PS had potent anti-complementary activity with the CH₅₀ value of 0.073 ± 0.009 mg/mL, and was characterized by the residues of T-Araf-(1→, →3)-Araf-(1→, →3,5)-Araf-(1→, →3)-Galp-(1→ and →4)-GalpA-(1→.

How we treat paroxysmal nocturnal hemoglobinuria: A consensus statement of the Canadian PNH Network and review of the national registry

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare hematologic disease characterized by intravascular hemolysis, thrombophilia, and marrow failure. Its phenotype is due to absent or reduced expression of GPI-linked complement regulators and subsequent sensitivity of hematopoietic cells to complement-mediated damage and lysis. Introduction of the terminal complement inhibitor eculizumab drastically improved outcomes in PNH patients; however, despite this improvement, there remain several challenges faced by PNH patients and physicians who care for them. One of the most important is increasing awareness of the heterogeneity with which patients can present, which can lead to significant delays in recognition. Data from the Canadian PNH Registry are presented to demonstrate the variety of presenting symptoms. In Canada, geography precludes consolidation of care to just a few centers, so management is distributed across academic hospitals, linked together as the Canadian PNH Network. The Network over the last several years has developed educational programs and clinical checklists and has worked to standardize access to diagnostics across the country. Herein, we address some of the common diagnostic and therapeutic challenges faced by PNH physicians and give our recommendations. Gaps in knowledge are also addressed, and where appropriate, consensus opinion is provided.

Developments in anti-complement therapy; from disease to clinical trial

The complement system is well known for its role in innate immunity and in maintenance of tissue homeostasis, providing a first line of defence against infection and playing a key role in flagging apoptotic cells and debris for disposal. Unfortunately complement also contributes to pathogenesis of a number of diseases; in some cases driving pathology, and in others amplifying or exacerbating the inflammatory and damaging impact of non-complement disease triggers. The role of complement in pathogenesis of an expanding number of diseases has driven industry and academia alike to develop an impressive arsenal of anti-complement drugs which target different proteins and functions of the complement cascade. Evidence from genetic and biochemical analyses, combined with improved identification of complement biomarkers and supportive data from sophisticated animal models of disease, has driven a drug development landscape in which the indications selected for clinical trial cluster in three 'target' tissues: the kidney, eye and vasculature. While the disease triggers may differ, complement activation and amplification is a common feature in many diseases which affect these three tissues. An abundance of drugs are in clinical development, some show favourable progression whereas others experience significant challenges. However, these hurdles in themselves drive an ever-evolving portfolio of 'next-generation' drugs with improved pharmacokinetic and pharmacodynamics properties. In this review we discuss the indications which are in the drug development 'spotlight' and review the relevant indication validation criteria. We present current progress in clinical trials, highlighting successes and difficulties, and look forward to approval of a wide selection of drugs for use in man which give clinicians choice in mechanistic target, modality and route of delivery.

Characterizing a pH-switch anti-C5 antibody as a tool for human and mouse complement C5 purification and cross-species inhibition of classical and reactive lysis

C5 plays a major role in complement activation; C5 convertase cleaves C5 into the pro‐inflammatory C5a, and C5b, the nidus for the formation of the lytic membrane attack complex. C5 is a major target for anti‐complement drugs, necessitating better methods for the study of C5 function. Purification of C5 is complicated; classical methods involve precipitation or pH shifts that result in functional loss and low yield. We here present a method for C5 purification using a novel anti‐C5 monoclonal antibody (mAb); RO7112689 (C5i mAb, SKY59), pH‐switch engineered to induce antibody–antigen dissociation in the acidic endosome (~ pH 5·5). RO7112689 was bound on an affinity column; applied serum was completely depleted of C5. Elution at pH 5 produced fully active C5 at 98% yield. The mAb also bound C5b in the C5b6 complex, preventing C5b6 binding to target membranes and enabling purification of C5b6 from activated serum. RO7112689 inhibited C5 in mouse serum and efficiently purified mouse C5. Used as capture, RO7112689 produced sensitive and specific assays for human and mouse C5. This novel antibody enables efficient production of intact, fully active, pure human and mouse C5, and quantification of C5 in these species. The demonstration that RO7112689 binds C5b6 adds an additional mechanism of membrane attack complex inhibition by this mAb.

New Analogs of the Complement C3 Inhibitor Compstatin with Increased Solubility and Improved Pharmacokinetic Profile

Improper regulation of complement is associated with various pathologies, and the clinical demand for compounds that can regulate complement activation is therefore imperative. Cp40, an analog of the peptide compstatin, inhibits all complement pathways at the level of the central component C3. We have further developed Cp40, using either PEGylation at the N-terminus or insertion of charged amino acids at the C-terminus. The PEGylated analogs are highly soluble and retained their inhibitory activity, with C3b binding affinity dependent on the length of the PEG chain. The addition of two or three residues of lysine, in turn, not only improved the peptide's solubility but also increased the binding affinity for C3b while retaining its inhibitory potency. Three of the new derivatives showed improved pharmacokinetic profiles in vivo in non-human primates. Given their compelling solubility and pharmacokinetic profiles, these new Cp40 analogs should broaden the spectrum of administration routes, likely reducing dosing frequency during chronic treatment and potentially expanding their range of clinical application.

An Engineered Complement Factor H Construct for Treatment of C3 Glomerulopathy

Background C3 glomerulopathy (C3G) is associated with dysregulation of the alternative pathway of complement activation, and treatment options for C3G remain limited. Complement factor H (FH) is a potent regulator of the alternative pathway and might offer a solution, but the mass and complexity of FH makes generation of full-length FH far from trivial. We previously generated a mini-FH construct, with FH short consensus repeats 1-5 linked to repeats 18-20 (FH1-5^18-20), that was effective in experimental C3G. However, the serum t1/2 of FH1-5^18-20 was significantly shorter than that of serum-purified FH.Methods We introduced the oligomerization domain of human FH-related protein 1 (denoted by R1-2) at the carboxy or amino terminus of human FH1-5^18-20 to generate two homodimeric mini-FH constructs (FHR1-2^1-5^18-20 and FH1-5^18-20^R1-2, respectively) in Chinese hamster ovary cells and tested these constructs using binding, fluid-phase, and erythrocyte lysis assays, followed by experiments in FH-deficient Cfh-/- mice.Results FHR1-2^1-5^18-20 and FH1-5^18-20^R1-2 homodimerized in solution and displayed avid binding profiles on clustered C3b surfaces, particularly FHR1-2^1-5^18-20 Each construct was >10-fold more effective than FH at inhibiting cell surface complement activity in vitro and restricted glomerular basement membrane C3 deposition in vivo significantly better than FH or FH1-5^18-20 FH1-5^18-20^R1-2 had a C3 breakdown fragment binding profile similar to that of FH, a >5-fold increase in serum t1/2 compared with that of FH1-5^18-20, and significantly better retention in the kidney than FH or FH1-5^18-20Conclusions FH1-5^18-20^R1-2 may have utility as a treatment option for C3G or other complement-mediated diseases.

Serine proteinases in the turnover of the cartilage extracellular matrix in the joint: implications for therapeutics

Cartilage destruction is a key characteristic of arthritic disease, a process now widely established to be mediated by metzincins such as MMPs. Despite showing promise in preclinical trials during the 1990s, MMP inhibitors for the blockade of extracellular matrix turnover in the treatment of cancer and arthritis failed clinically, primarily due to poor selectivity for target MMPs. In recent years, roles for serine proteinases in the proteolytic cascades leading to cartilage destruction have become increasingly apparent, renewing interest in the potential for new therapeutic strategies that utilize pharmacological inhibitors against this class of proteinases. Herein, we describe key serine proteinases with likely importance in arthritic disease and highlight recent advances in this field. LINKED ARTICLES: This article is part of a themed section on Translating the Matrix. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.1/issuetoc.

Diseases of complement dysregulation-an overview

Atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy (C3G), and paroxysmal nocturnal hemoglobinuria (PNH) are prototypical disorders of complement dysregulation. Although complement overactivation is common to all, cell surface alternative pathway dysregulation (aHUS), fluid phase alternative pathway dysregulation (C3G), or terminal pathway dysregulation (PNH) predominates resulting in the very different phenotypes seen in these diseases. The mechanism underlying the dysregulation also varies with predominant acquired autoimmune (C3G), somatic mutations (PNH), or inherited germline mutations (aHUS) predisposing to disease. Eculizumab has revolutionized the treatment of PNH and aHUS although has been less successful in C3G. With the next generation of complement therapeutic in late stage development, these archetypal complement diseases will provide the initial targets.

Haemostasis and innate immunity - a complementary relationship: A review of the intricate relationship between coagulation and complement pathways

Coagulation and innate immunity are linked evolutionary processes that orchestrate the host defence against invading pathogens and injury. The complement system is integral to innate immunity and shares numerous interactions with components of the haemostatic pathway, helping to maintain physiological equilibrium. The term 'immunothrombosis' was introduced in 2013 to embrace this process, and has become an area of much recent interest. What is less apparent in the literature however is an appreciation of the clinical manifestations of the coagulation-complement interaction and the consequences of dysregulation of either system, as seen in many inflammatory and thrombotic disease states, such as sepsis, trauma, atherosclerosis, antiphospholipid syndrome (APS), paroxysmal nocturnal haemoglobinuria (PNH) and some thrombotic microangiopathies to name a few. The growing appreciation of this immunothrombotic phenomenon will foster the drive for novel therapies in these disease states, including anticoagulants as immunomodulators and targeted molecular therapies.