Small-molecule factor D inhibitors targeting the alternative complement pathway

The Male Reproductive Toxicity Caused by 2-Naphthylamine Was Related to Testicular Immunity Disorders

2-naphthylamine (NAP) was classified as a group I carcinogen associated with bladder cancer. The daily exposure is mostly from cigarette and E-cigarette smoke. NAP can lead to testicular atrophy and interstitial tissue hyperplasia; however, the outcomes of NAP treatment on spermatogenesis and the associated mechanisms have not been reported. The study aimed to investigate the effect of NAP on spermatogenesis and sperm physiologic functions after being persistently exposed to NAP at 5, 20, and 40 mg/kg for 35 days. We found that sperm motility, progressive motility, sperm average path velocity, and straight-line velocity declined remarkably in the NAP (40 mg/kg) treated group, and the sperm deformation rate rose upon NAP administration. The testis immunity- and lipid metabolism-associated processes were enriched from RNA-sequence profiling. Plvap, Ccr7, Foxn1, Trim29, Sirpb1c, Cfd, and Lpar4 involved in testis immunity and Pnliprp1 that inhibit triglyceride and cholesterol absorption were confirmed to rise dramatically in the NAP-exposed group. The increased total cholesterol and CD68 levels were observed in the testis from the NAP-exposed group. Gpx5, serving as an antioxidant in sperm plasma, and Semg1, which contributes to sperm progressive motility, were both down-regulated. We concluded that the short-term exposure to NAP caused reproductive toxicity, primarily due to the inflammatory abnormality in the testis.

Complement inhibitors in pediatric kidney diseases: new therapeutic opportunities

Historically, the complement system (classical, lectin, alternative, and terminal pathways) is known to play a crucial role in the etiopathogenesis of many kidney diseases. Direct or indirect activation in these settings is revealed by consumption of complement proteins at the serum level and kidney tissue deposition seen by immunofluorescence and electron microscopy. The advent of eculizumab has shown that complement inhibitors may improve the natural history of certain kidney diseases. Since then, the number of available therapeutic molecules and experimental studies on complement inhibition has increased exponentially. In our narrative review, we give a summary of the main complement inhibitors that have completed phase II and phase III studies or are currently used in adult and pediatric nephrology. The relevant full-text works, abstracts, and ongoing trials (clinicaltrials.gov site) are discussed. Data and key clinical features are reported for eculizumab, ravulizumab, crovalimab, avacopan, danicopan, iptacopan, pegcetacoplan, and narsoplimab. Many of these molecules have been shown to be effective in reducing proteinuria and stabilizing kidney function in different complement-mediated kidney diseases. Thanks to their efficacy and target specificity, these novel drugs may radically improve the outcome of complement-mediated kidney diseases, contributing to an improvement in our understanding of their underlying pathophysiology.

Inhibition of the C1s Protease and the Classical Complement Pathway by 6-(4-Phenylpiperazin-1-yl)Pyridine-3-Carboximidamide and Chemical Analogs

The classical pathway (CP) is a potent mechanism for initiating complement activity and is a driver of pathology in many complement-mediated diseases. The CP is initiated via activation of complement component C1, which consists of the pattern recognition molecule C1q bound to a tetrameric assembly of proteases C1r and C1s. Enzymatically active C1s provides the catalytic basis for cleavage of the downstream CP components, C4 and C2, and is therefore an attractive target for therapeutic intervention in CP-driven diseases. Although an anti-C1s mAb has been Food and Drug Administration approved, identifying small-molecule C1s inhibitors remains a priority. In this study, we describe 6-(4-phenylpiperazin-1-yl)pyridine-3-carboximidamide (A1) as a selective, competitive inhibitor of C1s. A1 was identified through a virtual screen for small molecules that interact with the C1s substrate recognition site. Subsequent functional studies revealed that A1 dose-dependently inhibits CP activation by heparin-induced immune complexes, CP-driven lysis of Ab-sensitized sheep erythrocytes, CP activation in a pathway-specific ELISA, and cleavage of C2 by C1s. Biochemical experiments demonstrated that A1 binds directly to C1s with a Kd of ∼9.8 μM and competitively inhibits its activity with an inhibition constant (Ki) of ∼5.8 μM. A 1.8-Å-resolution crystal structure revealed the physical basis for C1s inhibition by A1 and provided information on the structure-activity relationship of the A1 scaffold, which was supported by evaluating a panel of A1 analogs. Taken together, our work identifies A1 as a new class of small-molecule C1s inhibitor and lays the foundation for development of increasingly potent and selective A1 analogs for both research and therapeutic purposes.

Quantification of the zymogenicity and the substrate-induced activity enhancement of complement factor D

Complement factor D (FD) is a serine protease present predominantly in the active form in circulation. It is synthesized as a zymogen (pro-FD), but it is continuously converted to FD by circulating active MASP-3. FD is a unique, self-inhibited protease. It has an extremely low activity toward free factor B (FB), while it is a highly efficient enzyme toward FB complexed with C3b (C3bB). The structural basis of this phenomenon is known; however, the rate enhancement was not yet quantified. It has also been unknown whether pro-FD has any enzymatic activity. In this study, we aimed to measure the activity of human FD and pro-FD toward uncomplexed FB and C3bB in order to quantitatively characterize the substrate-induced activity enhancement and zymogenicity of FD. Pro-FD was stabilized in the proenzyme form by replacing Arg²⁵ (precursor numbering) with Gln (pro-FD-R/Q). Activated MASP-1 and MASP-3 catalytic fragments were also included in the study for comparison. We found that the complex formation with C3b enhanced the cleavage rate of FB by FD approximately 20 million-fold. C3bB was also a better substrate for MASP-1, approximately 100-fold, than free FB, showing that binding to C3b renders the scissile Arg-Lys bond in FB to become more accessible for proteolysis. Though easily measurable, this cleavage by MASP-1 is not relevant physiologically. Our approach provides quantitative data for the two-step mechanism characterized by the enhanced susceptibility of FB for cleavage upon complex formation with C3b and the substrate-induced activity enhancement of FD upon its binding to C3bB. Earlier MASP-3 was also implicated as a potential FB activator; however, MASP-3 does not cleave C3bB (or FB) at an appreciable rate. Finally, pro-FD cleaves C3bB at a rate that could be physiologically significant. The zymogenicity of FD is approximately 800, i.e., the cleavage rate of C3bB by pro-FD-R/Q was found to be approximately 800-fold lower than that by FD. Moreover, pro-FD-R/Q at approximately 50-fold of the physiological FD concentration could restore half-maximal AP activity of FD-depleted human serum on zymosan. The observed zymogen activity of pro-FD might be relevant in MASP-3 deficiency cases or during therapeutic MASP-3 inhibition.

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.

Low-molecular weight inhibitors of the alternative complement pathway

Dysregulation of the alternative complement pathway predisposes individuals to a number of diseases. It can either be evoked by genetic alterations in or by stabilizing antibodies to important pathway components and typically leads to severe diseases such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, C3 glomerulopathy, and age-related macular degeneration. In addition, the alternative pathway may also be involved in many other diseases where its amplifying function for all complement pathways might play a role. To identify specific alternative pathway inhibitors that qualify as therapeutics for these diseases, drug discovery efforts have focused on the two central proteases of the pathway, factor B and factor D. Although drug discovery has been challenging for a number of reasons, potent and selective low-molecular weight (LMW) oral inhibitors have now been discovered for both proteases and several molecules are in clinical development for multiple complement-mediated diseases. While the clinical development of these inhibitors initially focuses on diseases with systemic and/or peripheral tissue complement activation, the availability of LMW inhibitors may also open up the prospect of inhibiting complement in the central nervous system where its activation may also play an important role in several neurodegenerative diseases.

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.

Managing Fatigue in Patients with Paroxysmal Nocturnal Hemoglobinuria: A Patient-Focused Perspective

The most frequently reported symptom in patients with paroxysmal nocturnal hemoglobinuria (PNH), a disease characterized by complement mediated hemolysis and chronic anemia, is “fatigue”. The latter seems the best word to communicate patient’ perception of personal health status and disease impact on daily living, namely quality of life (QoL). Objectivating QoL and grading patient’s fatigue is one of the most difficult medical tasks given the highly heterogeneous communication skills of patients and caregivers and the multitude of meanings that might be attributed to this term. Along with anemia, QoL in PNH is also affected by the emotional burden of a chronic life-long disease with heterogeneous treatment requirement, risk of hemolytic exacerbations (breakthrough hemolysis) and of thrombosis. In the last decade, structured surveys and scores have been adapted from cancer settings to evaluate fatigue and QoL in patients with PNH, and to assess the benefit of complement inhibitors in this setting. Eculizumab was the first drug utilized and was shown to improve QoL scores in the registrative trials. However, the intravenous fortnightly administration, the presence of residual anemia, and the risk of extravascular hemolysis are some of the unmet needs impacting QoL under eculizumab. Several novel drugs have been designed to improve patients’ convenience and alleviate anemia and fatigue. In this review, we focus on available studies that evaluated fatigue and QoL in PNH patients, and the effect of old and new therapeutic strategies.

Small molecule peptidomimetic trypsin inhibitors: validation of an EKO binding mode, but with a twist

Examination of a series of naturally-occurring trypsin inhibitor proteins, led to identification of a set of three residues (which we call the "interface triplet") to be determinant of trypsin binding affinity, hence excellent templates for small molecule mimicry. Consequently, we attempted to use the Exploring Key Orientation (EKO) strategy developed in our lab to evaluate small molecules that mimic the interface triplet regions of natural trypsin inhibitors, and hence potentially might bind and inhibit the catalytic activity of trypsin. A bis-triazole scaffold ("TT-mer") was the most promising of the molecules evaluated in silico. Twelve such compounds were synthesized and assayed against trypsin, among which the best showed a Kd of 2.1 μM. X-ray crystallography revealed a high degree of matching between an illustrative TT-mer's actual binding mode and that of the mimics that overlaid the interface triplet in the crystal structure. Deviation of the third side chain from the PPI structure seems to be due to alleviation of an unfavorable dipole-dipole interaction in the small molecule's actual bound conformation.

Complement Factor D Is a Novel Biomarker and Putative Therapeutic Target in Cutaneous Squamous Cell Carcinoma

Simple Summary

The incidence of the most common metastatic skin malignancy, cutaneous squamous cell carcinoma (cSCC), is growing worldwide, and the prognosis of the metastatic disease is poor. Presently, there are no biomarkers or therapeutic targets for high-risk cSCCs. Recent studies have demonstrated the essential role of autocrine complement synthesis in the progression of cSCC. Here, we have evaluated the role of complement Factor D (FD), the rate-limiting enzyme of the alternative complement pathway, in cSCC development. The results identify FD as a novel biomarker and putative therapeutic target for cSCC and propose the small-molecule FD inhibitor Danicopan as a highly specific drug candidate in the therapy of advanced cSCC. It is expected that the discovery of complement-associated molecular markers for cSCC progression would improve diagnosis, classification, prognostication, and targeted therapy of cSCC and its precursors in the future.

Abstract

Cutaneous squamous cell carcinoma (cSCC) is the most prevalent metastatic skin cancer. Previous studies have demonstrated the autocrine role of complement components in cSCC progression. We have investigated factor D (FD), the key enzyme of the alternative complement pathway, in the development of cSCC. RT-qPCR analysis of cSCC cell lines and normal human epidermal keratinocytes (NHEKs) demonstrated significant up-regulation of FD mRNA in cSCC cells compared to NHEKs. Western blot analysis also showed more abundant FD production by cSCC cell lines. Significantly higher FD mRNA levels were noted in cSCC tumors than in normal skin. Strong tumor cell-associated FD immunolabeling was detected in the invasive margin of human cSCC xenografts. More intense tumor cell-specific immunostaining for FD was seen in the tumor edge in primary and metastatic cSCCs, in metastases, and in recessive dystrophic epidermolysis bullosa-associated cSCCs, compared with cSCC in situ, actinic keratosis and normal skin. FD production by cSCC cells was dependent on p38 mitogen-activated protein kinase activity, and it was induced by interferon-γ and interleukin-1β. Blocking FD activity by Danicopan inhibited activation of extracellular signal-regulated kinase 1/2 and attenuated proliferation of cSCC cells. These results identify FD as a novel putative biomarker and therapeutic target for cSCC progression.

Homodimeric Minimal Factor H: In Vivo Tracking and Extended Dosing Studies in Factor H Deficient Mice

C3 glomerulopathy (C3G) is associated with dysregulation of the alternative pathway (AP) of complement and treatment options remain inadequate. Factor H (FH) is a potent regulator of the AP. An in-depth analysis of FH-related protein dimerised minimal (mini)-FH constructs has recently been published. This analysis showed that addition of a dimerisation module to mini-FH not only increased serum half-life but also improved complement regulatory function, thus providing a potential treatment option for C3G. Herein, we describe the production of a murine version of homodimeric mini-FH [mHDM-FH (mFH1-5^18-20^R1-2)], developed to reduce the risk of anti-drug antibody formation during long-term experiments in murine models of C3G and other complement-driven pathologies. Our analysis of mHDM-FH indicates that it binds with higher affinity and avidity to WT mC3b when compared to mouse (m)FH (mHDM-FH KD=505 nM; mFH KD=1370 nM) analogous to what we observed with the respective human proteins. The improved binding avidity resulted in enhanced complement regulatory function in haemolytic assays. Extended interval dosing studies in CFH-/- mice (5mg/kg every 72hrs) were partially effective and bio-distribution analysis in CFH-/- mice, through in vivo imaging technologies, demonstrates that mHDM-FH is preferentially deposited and remains fixed in the kidneys (and liver) for up to 4 days. Extended dosing using an AAV- human HDM-FH (hHDM-FH) construct achieved complete normalisation of C3 levels in CFH-/- mice for 3 months and was associated with a significant reduction in glomerular C3 staining. Our data demonstrate the ability of gene therapy delivery of mini-FH constructs to enhance complement regulation in vivo and support the application of this approach as a novel treatment strategy in diseases such as C3G.

"Walking the nitrogen around the ring": Chemical synthesis and spectroscopic characterization of novel 4-, 5-, 6-, and 7-azaindazole analogs of the synthetic cannabinoid receptor agonist MDMB-PINACA

Over the past decade, synthetic cannabinoid receptor agonists (SCRAs) have rapidly evolved to encompass a wide range of structurally diverse new psychoactive substances (NPS), including derivatives that incorporate indole, indazole, 7-azaindole, γ-carbolinone, or carbazole heterocyclic scaffolds. The introduction of legislative measures seeking to control the availability of NPS on the recreational drug scene has likely contributed to the continued emergence of novel SCRA analogs, which often evade regulatory control. However, the detection and/or identification of azaindazole-type SCRAs in seized material has not yet been reported (September, 2021). It is plausible that SCRAs bearing a 1,3-disubstituted azaindazole scaffold may possess cannabimimetic activity, given their structural similarity with known indole, indazole, and azaindole SCRAs. In view of these antecedents, a set of four novel isomeric 4-, 5-, 6-, and 7-azaindazole analogs of the known potent indazole SCRA, MDMB-PINACA, were synthesized using a Pd-catalyzed aminocarbonylation strategy. The complementary use of ultraviolet (UV) and infrared (IR) spectroscopy, gas chromatography-mass spectrometry (GC-MS), high resolution mass spectrometry (HRMS), 1D- and 2D-nuclear magnetic resonance (NMR) spectroscopy, and high performance liquid chromatography (HPLC) has permitted the spectroscopic differentiation, unambiguous structural assignment, and rapid separation of novel isomeric 4-, 5-, 6-, and 7-azaindazole analogs of the indazole SCRA, MDMB-PINACA.

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.

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.

Factor D Inhibition Blocks Complement Activation Induced by Mutant Factor B Associated With Atypical Hemolytic Uremic Syndrome and Membranoproliferative Glomerulonephritis

Complement factor B (FB) mutant variants are associated with excessive complement activation in kidney diseases such as atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathy and membranoproliferative glomerulonephritis (MPGN). Patients with aHUS are currently treated with eculizumab while there is no specific treatment for other complement-mediated renal diseases. In this study the phenotype of three FB missense variants, detected in patients with aHUS (D371G and E601K) and MPGN (I242L), was investigated. Patient sera with the D371G and I242L mutations induced hemolysis of sheep erythrocytes. Mutagenesis was performed to study the effect of factor D (FD) inhibition on C3 convertase-induced FB cleavage, complement-mediated hemolysis, and the release of soluble C5b-9 from glomerular endothelial cells. The FD inhibitor danicopan abrogated C3 convertase-associated FB cleavage to the Bb fragment in patient serum, and of the FB constructs, D371G, E601K, I242L, the gain-of-function mutation D279G, and the wild-type construct, in FB-depleted serum. Furthermore, the FD-inhibitor blocked hemolysis induced by the D371G and D279G gain-of-function mutants. In FB-depleted serum the D371G and D279G mutants induced release of C5b-9 from glomerular endothelial cells that was reduced by the FD-inhibitor. These results suggest that FD inhibition can effectively block complement overactivation induced by FB gain-of-function mutations.

A novel theranostic activity-based probe targeting kallikrein 7 for the diagnosis and treatment of skin diseases

We applied a new in silico approach for using protease-substrate motifs to design a kallikrein 7 (KLK7)-specific phosphonate activity-based probe (ABP) to quantify the active KLK7 in situ. Epidermal application of the ABP-inhibitor on Spink5^-/-Klk5^-/- mice, a Netherton syndrome model, reversed disease hallmarks, providing preclinical proof-of-concept for using ABPs as theranostics.

Mechanisms Driving Neutrophil-Induced T-cell Immunoparalysis in Ovarian Cancer

T-cell activation and expansion in the tumor microenvironment (TME) are critical for antitumor immunity. Neutrophils in the TME acquire a complement-dependent T-cell suppressor phenotype that is characterized by inhibition of T-cell proliferation and activation through mechanisms distinct from those of myeloid-derived suppressor cells. In this study, we used ascites fluid supernatants (ASC) from patients with ovarian cancer as an authentic component of the TME to evaluate the effects of ASC on neutrophil function and mechanisms for neutrophil-driven immune suppression. ASC prolonged neutrophil life span, decreased neutrophil density, and induced nuclear hypersegmentation. Mass cytometry analysis showed that ASC induced 15 distinct neutrophil clusters. ASC stimulated complement deposition and signaling in neutrophils, resulting in surface mobilization of granule constituents, including NADPH oxidase. NADPH oxidase activation and phosphatidylserine signaling were required for neutrophil suppressor function, although we did not observe a direct role of extracellular reactive oxygen species in inhibiting T-cell proliferation. Postoperative surgical drainage fluid also induced a complement-dependent neutrophil suppressor phenotype, pointing to this effect as a general response to injury. Like circulating lymphocytes, ASC-activated neutrophils caused complement-dependent suppression of tumor-associated lymphocytes. ASC-activated neutrophils adhered to T cells and caused trogocytosis of T-cell membranes. These injury and signaling cues resulted in T-cell immunoparalysis characterized by impaired NFAT translocation, IL2 production, glucose uptake, mitochondrial function, and mTOR activation. Our results demonstrate that complement-dependent priming of neutrophil effector functions in the TME induces a T-cell nonresponsiveness distinct from established checkpoint pathways and identify targets for immunotherapy.See related Spotlight by Cassatella, p. 725.

Development of an anti-human complement C6 monoclonal antibody that inhibits the assembly of membrane attack complexes

Membrane attack complexes (MACs; C5b-9) assembled after complement activation can directly injure self-tissues, leading to various diseases. Eculizumab, a monoclonal antibody (mAb) against complement component C5, is being used in the clinic to treat diseases in which MAC-mediated tissue damage is a primary cause. However, C5 is not a selective target for MAC assembly inhibition, and some patients respond incompletely or not at all to the eculizumab treatment. Therefore, C6, the next essential component in the terminal pathway of complement activation, may be an alternative target for the selective inhibition of MAC formation. Surprisingly, few reports describe a functional blockade of C6 using a specific mAb. Here, we report the development of an anti-human C6 mAb (clone 1C9) that recognizes C6 both in free circulation and within C5b6 complexes. This mAb blocked C7 binding to C5b6 complexes and consequently inhibited MAC formation and protected affected paroxysmal nocturnal hemoglobinuria patient red blood cells from MAC-mediated damage in vitro. In addition, this mAb cross-reacts with rhesus monkey but not mouse complement C6. Finally, 1C9 significantly reduced human complement-mediated intravascular hemolysis in vivo in a mouse model. These results suggest that the anti-C6 mAb holds promise as a new therapeutic agent that selectively targets MAC for many complement-mediated pathological conditions.

Difficult Cases of Paroxysmal Nocturnal Hemoglobinuria: Diagnosis and Therapeutic Novelties

Paroxysmal nocturnal hemoglobinuria (PNH) is an intriguing disease that can pose many difficulties to physicians, as well as to hematologists, who are unfamiliar with it. Research regarding its pathophysiologic, diagnostic, and therapeutic aspects is still ongoing. In the last ten years, new flow cytometry techniques with high sensitivity enabled us to detect PNH clones as small as <1% of a patient's hematopoiesis, resulting in increasing incidence but more difficult data interpretation. Particularly, the clinical significance of small PNH clones in patients with bone marrow failures, including aplastic anemia and myelodysplastic syndromes, as well as in uncommon associations, such as myeloproliferative disorders, is still largely unknown. Besides current treatment with the anti-C5 eculizumab, which reduced PNH-related morbidity and mortality, new complement inhibitors will likely fulfill unmet clinical needs in terms of patients' quality of life and better response rates (i.e., responses in subjects with C5 polymorphisms; reduction of extravascular hemolysis and breakthrough hemolysis episodes). Still, unanswered questions remain for these agents regarding their use in mono- or combination therapy, when to treat, and which drug is the best for which patient. Lastly, long-term safety needs to be assessed in real-life studies. In this review, we describe some clinical vignettes illustrating practical aspects of PNH diagnosis and management; moreover, we discuss recent advances in PNH diagnostic and therapeutic approaches.

Discovery and Development of the Oral Complement Factor D Inhibitor Danicopan (ACH-4471)

Complement plays a vital role in our innate immune defense against invasive microorganisms. Excessive complement activation or insufficient control of activation on host cells, however, is associated with several chronic disorders. Essential to the activation and amplification of the Alternative Pathway (AP) of complement, Complement Factor D (CFD) is a specific serine protease that cleaves its unique substrate, Complement Factor B (CFB) in complex with an activated form of complement component 3 (C3), to generate the AP C3 convertases C3(H2O)Bb and C3bBb. These convertases comprise a central component in eliciting effector responses following AP activation, and they also enable a powerful amplification loop for both the Classical Pathway (CP) and Lectin Pathway (LP) of complement. Because CFD is not required for the activation of either the CP or LP, selective CFD inhibition presents a favorable therapeutic approach to modulating complement activity that leaves intact the effector functions following CP and LP activation and thus poses a lower risk of bacterial infection than other complement-directed approaches. This review provides an update on inhibitors of CFD, which have evolved from irreversible small molecules that demonstrate poor selectivity to reversible small molecules and monoclonal antibodies that demonstrate exceptional selectivity and potency. The reversible small-molecule inhibitor danicopan.

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.

Ocular Disease Therapeutics: Design and Delivery of Drugs for Diseases of the Eye

The ocular drug discovery field has evidenced significant advancement in the past decade. The FDA approvals of Rhopressa, Vyzulta, and Roclatan for glaucoma, Brolucizumab for wet age-related macular degeneration (wet AMD), Luxturna for retinitis pigmentosa, Dextenza (0.4 mg dexamethasone intracanalicular insert) for ocular inflammation, ReSure sealant to seal corneal incisions, and Lifitegrast for dry eye represent some of the major developments in the field of ocular therapeutics. A literature survey also indicates that gene therapy, stem cell therapy, and target discovery through genomic research represent significant promise as potential strategies to achieve tissue repair or regeneration and to attain therapeutic benefits in ocular diseases. Overall, the emergence of new technologies coupled with first-in-class entries in ophthalmology are highly anticipated to restructure and boost the future trends in the field of ophthalmic drug discovery. This perspective focuses on various aspects of ocular drug discovery and the recent advances therein. Recent medicinal chemistry campaigns along with a brief overview of the structure-activity relationships of the diverse chemical classes and developments in ocular drug delivery (ODD) are presented.

Structural Studies on the Inhibitory Binding Mode of Aromatic Coumarinic Esters to Human Kallikrein-Related Peptidase 7

The serine protease kallikrein-related peptidase 7 (KLK7) is a member of the human tissue kallikreins. Its dysregulation leads to pathophysiological inflammatory processes in the skin. Furthermore, it plays a role in several types of cancer. For the treatment of KLK7-associated diseases, coumarinic esters have been developed as small-molecule enzyme inhibitors. To characterize the inhibition mode of these inhibitors, we analyzed structures of the inhibited protease by X-ray crystallography. Electron density shows the inhibitors covalently attached to His57 of the catalytic triad. This confirms the irreversible character of the inhibition process. Upon inhibitor binding, His57 undergoes an outward rotation; thus, the catalytic triad of the protease is disrupted. Besides, the halophenyl moiety of the inhibitor was absent in the final enzyme-inhibitor complex due to the hydrolysis of the ester linkage. With these results, we analyze the structural basis of KLK7 inhibition by the covalent attachment of aromatic coumarinic esters.

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.

Discovery of 4-((2S,4S)-4-Ethoxy-1-((5-methoxy-7-methyl-1H-indol-4-yl)methyl)piperidin-2-yl)benzoic Acid (LNP023), a Factor B Inhibitor Specifically Designed To Be Applicable to Treating a Diverse Array of Complement Mediated Diseases

The alternative pathway (AP) of the complement system is a key contributor to the pathogenesis of several human diseases including age-related macular degeneration, paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), and various glomerular diseases. The serine protease factor B (FB) is a key node in the AP and is integral to the formation of C3 and C5 convertase. Despite the prominent role of FB in the AP, selective orally bioavailable inhibitors, beyond our own efforts, have not been reported previously. Herein we describe in more detail our efforts to identify FB inhibitors by high-throughput screening (HTS) and leveraging insights from several X-ray cocrystal structures during optimization efforts. This work culminated in the discovery of LNP023 (41), which is currently being evaluated clinically in several diverse AP mediated indications.

A small-molecule inhibitor of C5 complement protein

The complement pathway is an important part of the immune system, and uncontrolled activation is implicated in many diseases. The human complement component 5 protein (C5) is a validated drug target within the complement pathway, as an anti-C5 antibody (Soliris) is an approved therapy for paroxysmal nocturnal hemoglobinuria. Here, we report the identification, optimization and mechanism of action for the first small-molecule inhibitor of C5 complement protein.

Anti-complement Treatment for Paroxysmal Nocturnal Hemoglobinuria: Time for Proximal Complement Inhibition? A Position Paper From the SAAWP of the EBMT

The treatment of paroxysmal nocturnal hemoglobinuria has been revolutionized by the introduction of the anti-C5 agent eculizumab; however, eculizumab is not the cure for Paroxysmal nocturnal hemoglobinuria (PNH), and room for improvement remains. Indeed, the hematological benefit during eculizumab treatment for PNH is very heterogeneous among patients, and different response categories can be identified. Complete normalization of hemoglobin (complete and major hematological response), is seen in no more than one third of patients, while the remaining continue to experience some degree of anemia (good and partial hematological responses), in some cases requiring regular red blood cell transfusions (minor hematological response). Different factors contribute to residual anemia during eculizumab treatment: underlying bone marrow dysfunction, residual intravascular hemolysis and the emergence of C3-mediated extravascular hemolysis. These two latter pathogenic mechanisms are the target of novel strategies of anti-complement treatments, which can be split into terminal and proximal complement inhibitors. Many novel terminal complement inhibitors are now in clinical development: they all target C5 (as eculizumab), potentially paralleling the efficacy and safety profile of eculizumab. Possible advantages over eculizumab are long-lasting activity and subcutaneous self-administration. However, novel anti-C5 agents do not improve hematological response to eculizumab, even if some seem associated with a lower risk of breakthrough hemolysis caused by pharmacokinetic reasons (it remains unclear whether more effective inhibition of C5 is possible and clinically beneficial). Indeed, proximal inhibitors are designed to interfere with early phases of complement activation, eventually preventing C3-mediated extravascular hemolysis in addition to intravascular hemolysis. At the moment there are three strategies of proximal complement inhibition: anti-C3 agents, anti-factor D agents and anti-factor B agents. These agents are available either subcutaneously or orally, and have been investigated in monotherapy or in association with eculizumab in PNH patients. Preliminary data clearly demonstrate that proximal complement inhibition is pharmacologically feasible and apparently safe, and may drastically improve the hematological response to complement inhibition in PNH. Indeed, we envision a new scenario of therapeutic complement inhibition, where proximal inhibitors (either anti-C3, anti-FD or anti-FB) may prove effective for the treatment of PNH, either in monotherapy or in combination with anti-C5 agents, eventually leading to drastic improvement of hematological response.

Design, Synthesis, and Preclinical Characterization of Selective Factor D Inhibitors Targeting the Alternative Complement Pathway

Complement factor D (FD), a highly specific S1 serine protease, plays a central role in the amplification of the alternative complement pathway (AP) of the innate immune system. Dysregulation of AP activity predisposes individuals to diverse disorders such as age-related macular degeneration, atypical hemolytic uremic syndrome, membranoproliferative glomerulonephritis type II, and paroxysmal nocturnal hemoglobinuria. Previously, we have reported the screening efforts and identification of reversible benzylamine-based FD inhibitors (1 and 2) binding to the open active conformation of FD. In continuation of our drug discovery program, we designed compounds applying structure-based approaches to improve interactions with FD and gain selectivity against S1 serine proteases. We report herein the design, synthesis, and medicinal chemistry optimization of the benzylamine series culminating in the discovery of 12, an orally bioavailable and selective FD inhibitor. 12 demonstrated systemic suppression of AP activation in a lipopolysaccharide-induced AP activation model as well as local ocular suppression in intravitreal injection-induced AP activation model in mice expressing human FD.

Treatment of IgA Nephropathy: Evolution Over Half a Century

Fifty years into the original description of IgA nephropathy, there is still no specific therapy for this condition and general measures including blood pressure control with blockers of the renin-angiotensin-aldosterone system and salt restriction remain the cornerstone to slow disease progression. Although the paucity in treatment advances could be related to the disease's complex pathogenesis, which requires multiple hits, heterogeneity as reflected by diverse ethnic differences, and genetic predisposition and histopathologic variations, many nonspecific and immunomodulatory agents have been tested with variable degrees of success and tribulations. Here, we review the evolution of these different therapeutic approaches over time that culminated in the 2012 Kidney Disease: Improving Global Outcomes Clinical Practice Guideline for Glomerulonephritis that presently is being updated, and provide an appraisal of recent data on various forms of immunosuppressive agents. Finally, we discuss the theoretical basis of ongoing and upcoming clinical trials that are more pathway- or cell-type-specific as knowledge in disease mechanisms advances.

Novel MASP-2 inhibitors developed via directed evolution of human TFPI1 are potent lectin pathway inhibitors

The lectin pathway (LP) of the complement system is an important antimicrobial defense mechanism, but it also contributes significantly to ischemia reperfusion injury (IRI) associated with myocardial infarct, stroke, and several other clinical conditions. Mannan-binding lectin-associated serine proteinase 2 (MASP-2) is essential for LP activation, and therefore, it is a potential drug target. We have previously developed the first two generations of MASP-2 inhibitors by in vitro evolution of two unrelated canonical serine proteinase inhibitors. These inhibitors were selective LP inhibitors, but their nonhuman origin rendered them suboptimal lead molecules for drug development. Here, we present our third-generation MASP-2 inhibitors that were developed based on a human inhibitor scaffold. We subjected the second Kunitz domain of human tissue factor pathway inhibitor 1 (TFPI1 D2) to directed evolution using phage display to yield inhibitors against human and rat MASP-2. These novel TFPI1-based MASP-2 inhibitor (TFMI-2) variants are potent and selective LP inhibitors in both human and rat serum. Directed evolution of the first Kunitz domain of TFPI1 had already yielded the potent kallikrein inhibitor, Kalbitor® (ecallantide), which is an FDA-approved drug to treat acute attacks of hereditary angioedema. Like hereditary angioedema, acute IRI is also related to the uncontrolled activation of a specific plasma serine proteinase. Therefore, TFMI-2 variants are promising lead molecules for drug development against IRI.

Small-molecule factor B inhibitor for the treatment of complement-mediated diseases

Complement is a key component of the innate immune system; however, its dysregulation due to genetic mutations or the presence of autoantibodies can cause a number of diseases including age-related macular degeneration, paroxysmal nocturnal hemoglobinuria, and C3 glomerulopathy. The alternative pathway acts as an amplification loop of the complement system. It is triggered by the activation of factor B, the proteolytically active component of the C3 and C5 convertases. We report the discovery of a small-molecule inhibitor of factor B and demonstrate that it can efficiently block the alternative pathway in vivo in animals and ex vivo in patient samples. The compound is highly selective and potent and is currently in clinical development for a number of complement-mediated diseases.

Dysregulation of the alternative complement pathway (AP) predisposes individuals to a number of diseases including paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, and C3 glomerulopathy. Moreover, glomerular Ig deposits can lead to complement-driven nephropathies. Here we describe the discovery of a highly potent, reversible, and selective small-molecule inhibitor of factor B, a serine protease that drives the central amplification loop of the AP. Oral administration of the inhibitor prevents KRN-induced arthritis in mice and is effective upon prophylactic and therapeutic dosing in an experimental model of membranous nephropathy in rats. In addition, inhibition of factor B prevents complement activation in sera from C3 glomerulopathy patients and the hemolysis of human PNH erythrocytes. These data demonstrate the potential therapeutic value of using a factor B inhibitor for systemic treatment of complement-mediated diseases and provide a basis for its clinical development.

Aplastic Anemia & MDS International Foundation (AA&MDSIF): Bone Marrow Failure Disease Scientific Symposium 2018

The bone marrow failure (BMF) syndromes are a group of rare disorders characterized by ineffective hematopoiesis resulting from deficiencies in the hematopoietic stem cell compartment. Although these diseases are typically acquired, some forms (e.g., Fanconi anemia, dyskeratosis congenita, Diamond Blackfan anemia, and Shwachman Diamond syndrome) are inherited. Patients with BMF syndromes can develop peripheral blood cytopenias and pancytopenia, and their disease can ultimately progress to acute myelogenous leukemia (AML). Research around the world is shedding light on the biology of the BMF syndromes, their clinical effects, and novel treatments. The Aplastic Anemia and MDS International Foundation (AAMDSIF) is an independent nonprofit organization whose mission is to help patients and family members cope with BMF syndromes. This report summarizes presentations on the latest scientific discoveries in BMF syndromes from the Sixth International Bone Marrow Failure Disease Scientific Symposium sponsored by AAMDSIF on March 22-23, 2018, in Rockville, Maryland.

3-Acyltetramic acids as a novel class of inhibitors for human kallikreins 5 and 7

Human kallikreins 5 and 7 (KLK5 and KLK7) exhibit trypsin- and chymotrypsin-like activities and are involved in pathologies related to skin desquamation process. A series of new 3-acyltetramic acids were developed as a novel class of inhibitors of KLK5, KLK7 and trypsin enzymes. The nature and length of the acyl chain is crucial to the KLK5, KLK7 and trypsin inhibition activities, and the most potent compounds (but not the most selective) 2b, 2c and 2g showed low micromolar IC₅₀ values. While very few of the compounds were selective for KLK5, the selective inhibition of trypsin against chymotrypsin was achieved. Our molecular modelling studies revealed that the double bond in 2g provided the best fit in the binding site of KLK5, while the hydrogen bonding interactions modulated the best fit of 2c in the binding site of KLK7 due to the hydrophobicity of the cavity.

Emerging immunotherapies for autoimmune kidney disease

Autoimmunity is a leading cause of chronic kidney disease and loss of native and transplanted kidneys. Conventional immunosuppressive therapies can be effective but are non-specific, noncurative, and risk serious side effects such as life-threatening infection and cancer. Novel therapies and targeted interventions are urgently needed. In this brief review we explore diverse strategies currently in development and under consideration to interrupt underlying disease mechanisms in immune-mediated renal injury. Because autoantibodies are prominent in diagnosis and pathogenesis in multiple human glomerulopathies, we highlight several promising therapies that interfere with functions of early mediators (IgG and complement) of the effector arm and with an epicenter (the germinal center) for induction of humoral immunity.

An unexpected switch in peptide binding mode: from simulation to substrate specificity

A ten microsecond molecular dynamics simulation of a kallikrein-related peptidase 7 peptide complex revealed an unexpected change in binding mode. After more than two microseconds unrestrained sampling we observe a spontaneous transition of the binding pose including a 180° rotation around the P1 residue. Subsequently, the substrate peptide occupies the prime side region rather than the cognate non-prime side in a stable conformation. We characterize the unexpected binding mode in terms of contacts, solvent-accessible surface area, molecular interactions and energetic properties. We compare the new pose to inhibitor-bound structures of kallikreins with occupied prime side and find that a similar orientation is adopted. Finally, we apply in silico mutagenesis based on the alternative peptide binding position to explore the prime side specificity of kallikrein-related peptidase 7 and compare it to available experimental data. Our study provides the first microsecond time scale simulation data on a kallikrein protease and shows previously unexplored prime side interactions. Therefore, we expect our study to advance the rational design of inhibitors targeting kallikrein-related peptidase 7, an emerging drug target involved in several skin diseases as well as cancer.

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.

Be on Target: Strategies of Targeting Alternative and Lectin Pathway Components in Complement-Mediated Diseases

The complement system has moved into the focus of drug development efforts in the last decade, since its inappropriate or uncontrolled activation has been recognized in many diseases. Some of them are primarily complement-mediated rare diseases, such as paroxysmal nocturnal hemoglobinuria, C3 glomerulonephritis, and atypical hemolytic uremic syndrome. Complement also plays a role in various multifactorial diseases that affect millions of people worldwide, such as ischemia reperfusion injury (myocardial infarction, stroke), age-related macular degeneration, and several neurodegenerative disorders. In this review, we summarize the potential advantages of targeting various complement proteins with special emphasis on the components of the lectin (LP) and the alternative pathways (AP). The serine proteases (MASP-1/2/3, factor D, factor B), which are responsible for the activation of the cascade, are straightforward targets of inhibition, but the pattern recognition molecules (mannose-binding lectin, other collectins, and ficolins), the regulatory components (factor H, factor I, properdin), and C3 are also subjects of drug development. Recent discoveries about cross-talks between the LP and AP offer new approaches for clinical intervention. Mannan-binding lectin-associated serine proteases (MASPs) are not just responsible for LP activation, but they are also indispensable for efficient AP activation. Activated MASP-3 has recently been shown to be the enzyme that continuously supplies factor D (FD) for the AP by cleaving pro-factor D (pro-FD). In this aspect, MASP-3 emerges as a novel feasible target for the regulation of AP activity. MASP-1 was shown to be required for AP activity on various surfaces, first of all on LPS of Gram-negative bacteria.

Toward complement inhibition 2.0: Next generation anticomplement agents for paroxysmal nocturnal hemoglobinuria

Therapeutic complement inhibition by eculizumab has revolutionized the treatment of paroxysmal nocturnal hemoglobinuria (PNH) with a major impact on its natural history. Nevertheless, emerging unmet clinical needs may benefit from the development of novel complement inhibitors. Novel strategies of complement inhibition exploit different agents targeting C5, as well as compound intercepting the complement cascade at the level of its key component C3, or even upstream at the level of components involved in complement alternative pathway initiation. Many of these agents are already in their clinical development; preliminary data together with a deep understanding of PNH biology may help to anticipate their possible clinical effect. Novel anti-C5 agents include monoclonal antibodies (even long-lasting) as well as other small molecules bioavailable by subcutaneous administration; an anti-C5 small interfering RNA has been developed too. All these anti-C5 agents seem to recapitulate safety and efficacy of current eculizumab treatment; their main improvement pertains to better patient's convenience due to longer dosing interval and/or possible subcutaneous self-administration. The possibility of achieving a deeper C5 inhibition has been shown as well, but its actual clinical meaning remains to be elucidated. Upstream complement inhibitors include the anti-C3 small peptide compstatin (and its derivatives), and small inhibitors of complement factor D or complement factor B. This class of compounds anticipates a possible efficacy in prevention of C3-mediated extravascular hemolysis, in addition to inhibition of intravascular hemolysis, eventually leading to improved hematological responses. The availability of all these compounds will result soon in a substantial improvement of PNH management.

Immunomodulatory Action of Substituted 1,3,4-Thiadiazines on the Course of Myocardial Infarction

This review focuses on the biological action of the compounds from the group of substituted 1,3,4-thiadiazines on stress response and myocardial infarction. The aim of this review is to propose the possible mechanisms of action of 1,3,4-thiadiazines and offer prospectives in the development of new derivatives as therapeutic agents. It is known, that compounds that have biological effects similar to those used as antidepressants can down-regulate the secretion of proinflammatory cytokines, up-regulate the release of anti-inflammatory ones and affect cell recruitment, which allows them to be considered immunomodulators as well. The results of pharmacological evaluation, in silico studies, and in vivo experiments of several compounds from the group of substituted 1,3,4-thiadiazines with antidepressant properties are presented. It is proposed that the cardioprotective effects of substituted 1,3,4-thiadiazines might be explained by the peculiarities of their multi-target action: the ability of the compounds to interact with various types of receptors and transporters of dopaminergic, serotonergic and acetylcholinergic systems and to block the kinase signal pathway PI3K-AKT. The described effects of substituted 1,3,4-thiadiazines suggest that it is necessary to search for a new agents for limiting the peripheral inflammatory/ischemic damage through the entral mechanisms of stress reaction and modifying pro-inflammatory cytokine signaling pathways in the brain.

Expanding Complement Therapeutics for the Treatment of Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is widely regarded as an archetypal complement-mediated disorder that has propelled complement drug discovery in recent decades. Its pathology is driven by chronic complement dysregulation resulting from the lack of the glycosyl phosphatidyl inositol-linked regulators DAF and CD59 on susceptible erythrocytes. This complement imbalance fuels persistent C3 activation on affected erythrocytes, which culminates in chronic complement-mediated intravascular hemolysis. The clinical application of eculizumab, a humanized anti-C5 antibody that blocks terminal pathway activation, has led to drastic improvement of therapeutic outcomes but has also unveiled hitherto elusive pathogenic mechanisms that are now known to contribute to the clinical burden of a significant proportion of patients with PNH. These emerging clinical needs have sparked a true resurgence of complement therapeutics that offer the promise of even more effective, disease-tailored therapies for PNH. Here, we review the current state of complement therapeutics with a focus on the clinical development of C3-targeted and alternative pathway-directed drug candidates for the treatment of PNH. We also discuss the relative advantages and benefits offered by each complement-targeting approach, including translational considerations that might leverage a more comprehensive clinical intervention for PNH.

Discovery and Design of First Benzylamine-Based Ligands Binding to an Unlocked Conformation of the Complement Factor D

Complement Factor D, a serine protease of the S1 family and key component of the alternative pathway amplification loop, represents a promising target for the treatment of several prevalent and rare diseases linked to the innate immune system. Previously reported FD inhibitors have been shown to bind to the FD active site in its self-inhibited conformation characterized by the presence of a salt bridge at the bottom of the S1 pocket between Asp189 and Arg218. We report herein a new set of small-molecule FD ligands that harbor a basic S1 binding moiety directly binding to the carboxylate of Asp189, thereby displacing the Asp189-Arg218 ionic interaction and significantly changing the conformation of the self-inhibitory loop.

The renaissance of complement therapeutics

The increasing number of clinical conditions that involve a pathological contribution from the complement system - many of which affect the kidneys - has spurred a regained interest in therapeutic options to modulate this host defence pathway. Molecular insight, technological advances, and the first decade of clinical experience with the complement-specific drug eculizumab, have contributed to a growing confidence in therapeutic complement inhibition. More than 20 candidate drugs that target various stages of the complement cascade are currently being evaluated in clinical trials, and additional agents are in preclinical development. Such diversity is clearly needed in view of the complex and distinct involvement of complement in a wide range of clinical conditions, including rare kidney disorders, transplant rejection and haemodialysis-induced inflammation. The existing drugs cannot be applied to all complement-driven diseases, and each indication has to be assessed individually. Alongside considerations concerning optimal points of intervention and economic factors, patient stratification will become essential to identify the best complement-specific therapy for each individual patient. This Review provides an overview of the therapeutic concepts, targets and candidate drugs, summarizes insights from clinical trials, and reflects on existing challenges for the development of complement therapeutics for kidney diseases and beyond.

The properdin pathway: an "alternative activation pathway" or a "critical amplification loop" for C3 and C5 activation?

This review is not intended to cover in detail all aspects of the discovery and evolution of our understanding of the "alternative pathway" of complement activation, there are many excellent reviews that do this (see Fearon (CRC Crit Rev Immunol 1:1-32, 1979), Pangburn and Müller-Eberhard (Springer Semin Immunopathol 7:163-192, 1984)), but instead to give sufficient background for current concepts to be put in context. The prevailing textbook view, of components having a primary role as an alternative "pathway" for C3 activation, is challenged, with an argument developed for the primary role of the system being that of providing a surface-dependent amplification loop for both C3 and C5 activation. Whatever the mechanism by which the initial C3b molecule is generated, deposition onto a surface has the potential to target that surface for elimination. Elimination or escape from initial targeting is determined by a sophisticated and highly regulated amplification loop for C3 activation. This viewpoint of the system is then briefly developed to provide a context for therapeutic treatment of disease caused, at least in part, by dysregulated amplification of C3 activation, and to highlight some of the challenges that such therapies will face and need to address.