Discovery of allosteric modulators of factor XIa by targeting hydrophobic domains adjacent to its heparin-binding site

Investigation of the anticoagulant activity of cyclic sulfated glycosaminoglycan mimetics

Thrombotic disorders are among the leading causes of deaths worldwide. Anticoagulants are frequently prescribed for their prevention and/or treatment. Current anticoagulants, which target either thrombin or factor Xa, are plagued with a number of drawbacks, the most important of which is the increased risk of internal bleeding. To develop better antithrombotic agents, the anticoagulant activity of cyclic glycosaminoglycan mimetics was evaluated. Human plasma clotting assays and enzyme inhibition assays were exploited to evaluate the anticoagulant activity of sulfated β-cyclodextrin (SBCD) and its three analogs: sulfated α-cyclodextrin, β-cyclodextrin, and methylated β-cyclodextrin. In normal human plasma, SBCD selectively doubled the activated partial thromboplastin time (APTT) at ∼9 μg/mL, with no effect on prothrombin time (PT) at the same concentration. Likewise, SBCD doubled APTT at ∼9 μg/mL and at ∼8 μg/mL in antithrombin-deficient plasma and heparin cofactor II-deficient plasma, respectively. Interestingly, the three SBCD derivatives were inactive at the highest concentrations tested which highlighted the importance of the sulfate groups and the size of the molecule. Enzyme assays revealed that SBCD inhibits factor XIa (FXIa) with an IC50 value of ∼20 μg/mL and efficacy of near 100%. SBCD did not inhibit other related proteins including thrombin, factor IXa, factor Xa, factor XIIa, factor XIIIa, plasmin, chymotrypsin, or trypsin at the highest concentrations tested demonstrating a significant selectivity. In Michaelis-Menten kinetics, SBCD decreased the VMAX and increased the KM of FXIa hydrolysis of a tripeptide chromogenic substrate indicating a mixed inhibition mechanism. Together, it appears that SBCD is a potent and selective inhibitor of human FXIa with substantial anticoagulant activity in human plasma. Overall, this study introduces SBCD as a promising lead for further development as a safer anticoagulant.

Designing Smaller, Synthetic, Functional Mimetics of Sulfated Glycosaminoglycans as Allosteric Modulators of Coagulation Factors

Natural glycosaminoglycans (GAGs) are arguably the most diverse collection of natural products. Unfortunately, this bounty of structures remains untapped. Decades of research has realized only one GAG-like synthetic, small-molecule drug, fondaparinux. This represents an abysmal output because GAGs present a frontier that few medicinal chemists, and even fewer pharmaceutical companies, dare to undertake. GAGs are heterogeneous, polymeric, polydisperse, highly water soluble, synthetically challenging, too rapidly cleared, and difficult to analyze. Additionally, GAG binding to proteins is not very selective and GAG-binding sites are shallow. This Perspective attempts to transform this negative view into a much more promising one by highlighting recent advances in GAG mimetics. The Perspective focuses on the principles used in the design/discovery of drug-like, synthetic, sulfated small molecules as allosteric modulators of coagulation factors, such as antithrombin, thrombin, and factor XIa. These principles will also aid the design/discovery of sulfated agents against cancer, inflammation, and microbial infection.

Factor XIa Inhibitors in Anticoagulation Therapy: Recent Advances and Perspectives

Factor XIa (FXIa) in the intrinsic pathway of the coagulation process has been proven to be an effective and safe target for anticoagulant discovery with limited or no bleeding. Numerous small-molecule FXIa inhibitors (SMFIs) with various scaffolds have been identified in the early stages of drug discovery. They have served as the foundation for the recent discovery of additional promising SMFIs with improved potency, selectivity, and pharmacokinetic profiles, some of which have entered clinical trials for the treatment of thrombosis. After reviewing the coagulation process and structure of FXIa, this perspective discusses the rational or structure-based design, discovery, structure-activity relationships, and development of SMFIs disclosed in recent years. Strategies for identifying more selective and druggable SMFIs are provided, paving the way for the design and discovery of more useful SMFIs for anticoagulation therapy.

Factor XI, a potential target for anticoagulation therapy for venous thromboembolism

Venous thromboembolism (VTE) is a common cause of mortality and disability in hospitalized patients, and anticoagulation is an essential therapeutic option. Despite the increasing use of direct oral anticoagulants, complications and adverse drug reactions still occur in patients with VTE. Within 5 years, 20% of patients with VTE experience recurrence, and 50% of patients with deep vein thrombosis develop post-thrombotic syndrome. Furthermore, bleeding due to anticoagulants is a side effect that must be addressed. Therefore, safer and more effective anticoagulant strategies with higher patient compliance are urgently needed. Available epidemiological evidence and animal studies have shown that factor XI (FXI) inhibitors can reduce thrombus size and loosen the thrombus structure with a relatively low risk of bleeding, suggesting that FXI has an important role in thrombus stabilization and is a safer target for anticoagulation. Recent clinical trial data have also shown that FXI inhibitors are as effective as enoxaparin and apixaban in preventing VTE, but with a significantly lower incidence of bleeding. Furthermore, FXI inhibitors can be administered daily or monthly; therefore, the monitoring interval can be longer. Additionally, FXI inhibitors can prolong the activated partial thromboplastin time without affecting prothrombin time, which is an easy and common test used in clinical testing, providing a cost-effective monitoring routine for patients. Consequently, the inhibition of FXI may be an effective strategy for the prevention and treatment of VTE. Enormous progress has been made in the research strategies for FXI inhibitors, with abelacimab already in phase III clinical trials and most other inhibitors in phase I or II trials. In this review, we discuss the challenges of VTE therapy, briefly describe the structure and function of FXI, summarize the latest FXI/activated FXI (FXIa) inhibitor strategies, and summarize the latest developments in clinical trials of FXI/FXIa inhibitors.

Synthesis of 3'-O-Alkyl Homologues and a Biotin Probe of Isorhamnetin and Evaluation of Cytotoxic Efficacy on Cancer Cells

Isorhamnetin is a natural flavonoid which shows a variety of biological activities such as antioxidant, anti-inflammatory and antitumor. In order to identify the cellular binding protein of isorhamnetin as potential anti-cancer target, we first synthesized 3'-O-substituted quercetin as isorhamnetin homologues and evaluated the growth inhibitory activity of these derivatives on breast, colon and prostate cancer cell lines. The preliminary results showed that the 3'-O modification did not affect the cytotoxic activity of the scaffold. Analysis of the co-crystal structure and the docking pose of isorhamnetin with reported binding protein of isorhamnetin or quercetin indicated the 3'-O-substitution groups located outside of the binding pocket, which is in accordance with activity of 3'-O derivatives. Then a biotin conjugate of isorhamnetin with a tetraethylene glycol (PEG)₄ linker at the 3' position was synthesized and the resulting probe retained the anti-proliferative activity on cancer cell lines, while the cellular fluorescence analysis showed the distribution of probe inside the cells which indicated the probe had limited cell permeability. Finally, pull down assay both in situ inside cells and in the cell lysates indicated the isorhamnetin biotin probe was capable of protein labeling in cell lysates. These findings provide the isorhamnetin 3'-O-biotin probe as a tool to reveal the target proteins of isorhamnetin.

Synthesis, structure and midkine binding of chondroitin sulfate oligosaccharide analogues

The preparation of chondroitin sulfate (CS) oligosaccharide mimetics, more easily synthesized than natural sequences, is a highly interesting task because these compounds pave the way for modulation of the biological processes in which CS is involved. Herein, we report the synthesis of CS type E analogues which present easily accessible glucose units instead of glucuronic acid (GlcA) moieties. NMR experiments and molecular dynamics simulations showed that the 3D structure of these compounds is similar to the structure of the natural CS-E oligosaccharides. In addition, fluorescence polarization (FP) and saturation transfer difference NMR (STD-NMR) experiments revealed that the synthesized CS-like derivatives were able to interact with midkine, a model heparin-binding growth factor, suggesting that the presence of the GlcA carboxylate groups is not essential for the binding. Overall, our results indicate that the synthesized glucose-containing oligosaccharides can be considered as functional and structural CS mimetics.

Development of Selective FXIa Inhibitors Based on Cyclic Peptides and Their Application for Safe Anticoagulation

Coagulation factor XI (FXI) has emerged as a promising target for the development of safer anticoagulation drugs that limit the risk of severe and life-threatening bleeding. Herein, we report the first cyclic peptide-based FXI inhibitor that selectively and potently inhibits activated FXI (FXIa) in human and animal blood. The cyclic peptide inhibitor (K_i = 2.8 ± 0.5 nM) achieved anticoagulation effects that are comparable to that of the gold standard heparin applied at a therapeutic dose (0.3-0.7 IU/mL in plasma) but with a substantially broader estimated therapeutic range. We extended the plasma half-life of the peptide via PEGylation and demonstrated effective FXIa inhibition over extended periods in vivo. We validated the anticoagulant effects of the PEGylated inhibitor in an ex vivo hemodialysis model with human blood. Our work shows that FXI can be selectively targeted with peptides and provides a promising candidate for the development of a safe anticoagulation therapy.

Studies on fragment-based design of allosteric inhibitors of human factor XIa

Human factor XIa (hFXIa) has emerged as an attractive target for development of new anticoagulants that promise higher level of safety. Different strategies have been adopted so far for the design of anti-hFXIa molecules including competitive and non-competitive inhibition. Of these, allosteric dysfunction of hFXIa's active site is especially promising because of the possibility of controlled reduction in activity that may offer a route to safer anticoagulants. In this work, we assess fragment-based design approach to realize a group of novel allosteric hFXIa inhibitors. Starting with our earlier discovery that sulfated quinazolinone (QAO) bind in the heparin-binding site of hFXIa, we developed a group of two dozen dimeric sulfated QAOs with intervening linkers that displayed a progressive variation in inhibition potency. In direct opposition to the traditional wisdom, increasing linker flexibility led to higher potency, which could be explained by computational studies. Sulfated QAO 19S was identified as the most potent and selective inhibitor of hFXIa. Enzyme inhibition studies revealed that 19S utilizes a non-competitive mechanism of action, which was supported by fluorescence studies showing a classic sigmoidal binding profile. Studies with selected mutants of hFXIa indicated that sulfated QAOs bind in heparin-binding site of the catalytic domain of hFXIa. Overall, the approach of fragment-based design offers considerable promise for designing heparin-binding site-directed allosteric inhibitors of hFXIa.

Sulfated Non-Saccharide Glycosaminoglycan Mimetics as Novel Drug Discovery Platform for Various Pathologies

Glycosaminoglycans (GAGs) are very complex, natural anionic polysaccharides. They are polymers of repeating disaccharide units of uronic acid and hexosamine residues. Owing to their template-free, spatiotemporally-controlled, and enzyme-mediated biosyntheses, GAGs possess enormous polydispersity, heterogeneity, and structural diversity which often translate into multiple biological roles. It is well documented that GAGs contribute to physiological and pathological processes by binding to proteins including serine proteases, serpins, chemokines, growth factors, and microbial proteins. Despite advances in the GAG field, the GAG-protein interface remains largely unexploited by drug discovery programs. Thus, Non-Saccharide Glycosaminoglycan Mimetics (NSGMs) have been rationally developed as a novel class of sulfated molecules that modulate GAG-protein interface to promote various biological outcomes of substantial benefit to human health. In this review, we describe the chemical, biochemical, and pharmacological aspects of recently reported NSGMs and highlight their therapeutic potentials as structurally and mechanistically novel anti-coagulants, anti-cancer agents, anti-emphysema agents, and anti-viral agents. We also describe the challenges that complicate their advancement and describe ongoing efforts to overcome these challenges with the aim of advancing the novel platform of NSGMs to clinical use.

A synthetic heparin mimetic that allosterically inhibits factor XIa and reduces thrombosis in vivo without enhanced risk of bleeding

BACKGROUND

Human factor XIa (FXIa) is an actively pursued target for development of safer anticoagulants. Our long-standing hypothesis has been that allosterism originating from heparin-binding site(s) on coagulation enzymes is a promising approach to yield safer agents.

OBJECTIVES

To develop a synthetic heparin mimetic as an inhibitor of FXIa so as to reduce clot formation in vivo but not carry high bleeding risk.

METHODS

We employed a gamut of methods involving synthetic chemistry, biophysical biochemistry, enzyme assays, blood and plasma coagulation assays, and in vivo thrombosis models in this work.

RESULTS

Sulfated chiro-inositol (SCI), a non-saccharide mimetic of heparin, was synthesized in three steps in overall yields of >50%. SCI inhibited FXIa with potency of 280 nmol/L and preferentially engaged FXIa's heparin-binding site to conformationally alter its active site. SCI inhibition of FXIa could be rapidly reversed by common antidotes, such as protamine. SCI preferentially prolonged plasma clotting initiated with recalcification, rather than thromboplastin, alluding to its intrinsic pathway-based mechanism. Human blood thromboelastography indicated good ex vivo anticoagulation properties of SCI. Rat tail bleeding and maximum-dose-tolerated studies indicated that no major bleeding or toxicity concerns for SCI suggesting a potentially safer anticoagulation outcome. FeCl3 -induced arterial and thromboplastin-induced venous thrombosis model studies in the rat showed reduced thrombus formation by SCI at 250 μg/animal, which matched enoxaparin at 2500 μg/animal.

CONCLUSIONS

Overall, SCI is a highly promising, allosteric inhibitor of FXIa that induces potent anticoagulation in vivo. Further studies are necessary to assess SCI in animal models mimicking human clinical indications.

Allosteric Inhibition as a New Mode of Action for BAY 1213790, a Neutralizing Antibody Targeting the Activated Form of Coagulation Factor XI

Factor XI (FXI), the zymogen of activated FXI (FXIa), is an attractive target for novel anticoagulants because FXI inhibition offers the potential to reduce thrombosis risk while minimizing the risk of bleeding. BAY 1213790, a novel anti-FXIa antibody, was generated using phage display technology. Crystal structure analysis of the FXIa-BAY 1213790 complex demonstrated that the tyrosine-rich complementarity-determining region 3 loop of the heavy chain of BAY 1213790 penetrated deepest into the FXIa binding epitope, forming a network of favorable interactions including a direct hydrogen bond from Tyr102 to the Gln451 sidechain (2.9 Å). The newly discovered binding epitope caused a structural rearrangement of the FXIa active site, revealing a novel allosteric mechanism of FXIa inhibition by BAY 1213790. BAY 1213790 specifically inhibited FXIa with a binding affinity of 2.4 nM, and in human plasma, prolonged activated partial thromboplastin time and inhibited thrombin generation in a concentration-dependent manner.

Synthesis and Evaluation of Antifungal and Antitrypanosomastid Activities of Symmetrical 1,4-Disubstituted-1,2,3-Bistriazoles Obtained by CuAAC Conditions

BACKGROUND

The trypanosomatids, such as the protozoan Leishmania spp., have a demand by ergosterol, which is not present in the membrane from mammal cells. The suppression of the synthesis of ergosterol would be a new target of compounds with leishmanicidal activity, and bistriazole has shown trypanocidal activity by this mechanism. The incidence of fungal infections has increased at an alarming rate over the last decades. This is related both to the growing population of immune-compromised individuals and to the emergence of strains that are resistant to available antifungals. Therefore, there is a challenge for the search of potential new antifungal agents.

OBJECTIVE

The study aimed to synthesize 1,4-disubstituted-1,2,3-bistriazoles by optimized copper( I)-catalyzed alkyne-azide cycloaddition (CuAAC) and evaluate their antifungal and antitrypanosomastid activities.

METHOD

The synthesis of symmetrical bistriazoles with diazides as spacers was planned to be performed following the CuAAC reaction strategy. For evaluation of best conditions for the synthesis of symmetrical bistriazoles hex-1-yne 2 was chosen as leading compound, and a variety of catalysts were employed, choosing (3:1) alkyne:diazide stoichiometric relationship employing CuSO4.5H2O as the best condition. For the preparation of diversity in the synthesis of symmetrical bistriazoles, a 1,3-diazide-propan-2-ol 1a and 1,3-diazidepropane 1b were reacted with seven different alkynes, furnishing eleven symmetrical bistriazoles 9-13a,b and 14a. All compounds were essayed to cultures of promastigotes of L. amazonensis (1 x 106 cells mL-1) in the range of 0.10 - 40.00 µg mL-1 and incubated at 25ºC. After 72 h of incubation, the surviving parasites were counted. For antifungal assay, the minimum inhibitory concentrations (MIC) for yeasts and filamentous fungi were determined. Each compound was tested in 10 serial final concentrations (64 to 0.125 µg mL-1).

RESULTS

Eleven 1,4-disubstituted-1,2,3-bistriazoles were synthesized and their structures were confirmed by IR, 1H and 13C-NMR and Mass spectral analysis. The antifungal and antitrypanosomastid activities were evaluated. The best result to antifungal activity was reached by bistriazole 11a that showed the same MIC of fluconazole (32 µg mL-1) against Candida krusei ATCC 6258, an emerging and potentially multidrug-resistant fungal pathogen. Due to their intrinsically biological activity versatility, five derivatives compounds showed leishmanicidal inhibitory activity between 15.0 and 20.0% at concentrations of 20 and 40.0 µg mL-1. Among these compounds the derivative 13a showed best IC50 value of 63.34 µg mL-1 (182.86 µM).

CONCLUSION

The preliminary and promising results suggest that bistriazole derivatives, especially compound 13a, could represent an innovative scaffold for further studies and development of new antifungal and anti-parasitic drug candidates.

Recent advances in the discovery and development of factor XI/XIa inhibitors

Factor XIa (FXIa) is a serine protease homodimer that belongs to the intrinsic coagulation pathway. FXIa primarily catalyzes factor IX activation to factor IXa, which subsequently activates factor X to factor Xa in the common coagulation pathway. Growing evidence suggests that FXIa plays an important role in thrombosis with a relatively limited contribution to hemostasis. Therefore, inhibitors targeting factor XI (FXI)/FXIa system have emerged as a paradigm-shifting strategy so as to develop a new generation of anticoagulants to effectively prevent and/or treat thromboembolic diseases without the life-threatening risk of internal bleeding. Several inhibitors of FXI/FXIa proteins have been discovered or designed over the last decade including polypeptides, active site peptidomimetic inhibitors, allosteric inhibitors, antibodies, and aptamers. Antisense oligonucleotides (ASOs), which ultimately reduce the hepatic biosynthesis of FXI, have also been introduced. A phase II study, which included patients undergoing elective primary unilateral total knee arthroplasty, revealed that a specific FXI ASO effectively protects patients against venous thrombosis with a relatively limited risk of bleeding. Initial findings have also demonstrated the potential of FXI/FXIa inhibitors in sepsis, listeriosis, and arterial hypertension. This review highlights various chemical, biochemical, and pharmacological aspects of FXI/FXIa inhibitors with the goal of advancing their development toward clinical use.

Inhibition of Herpes Simplex Virus-1 Entry into Human Cells by Nonsaccharide Glycosaminoglycan Mimetics

Although heparan sulfate (HS) has been implicated in facilitating entry of enveloped viruses including herpes simplex virus (HSV), small molecules that effectively compete with this abundant, cell surface macromolecule remain unknown. We reasoned that entry of HSV-1 involving its glycoprotein D (gD) binding to HS could be competitively targeted through small, synthetic, nonsaccharide glycosaminoglycan mimetics (NSGMs). Screening a library of NSGMs identified a small, distinct group that bound gD with affinities of 8-120 nM. Studies on HSV-1 entry into HeLa, HFF-1, and VK2/E6E7 cells identified inhibitors with potencies in the range of 0.4-1.0 μM. These synthetic NSGMs are likely to offer promising chemical biology probes and/or antiviral drug discovery opportunities.

A molecular dynamics-based algorithm for evaluating the glycosaminoglycan mimicking potential of synthetic, homogenous, sulfated small molecules

Glycosaminoglycans (GAGs) are key natural biopolymers that exhibit a range of biological functions including growth and differentiation. Despite this multiplicity of function, natural GAG sequences have not yielded drugs because of problems of heterogeneity and synthesis. Recently, several homogenous non-saccharide glycosaminoglycan mimetics (NSGMs) have been reported as agents displaying major therapeutic promise. Yet, it remains unclear whether sulfated NSGMs structurally mimic sulfated GAGs. To address this, we developed a three-step molecular dynamics (MD)-based algorithm to compare sulfated NSGMs with GAGs. In the first step of this algorithm, parameters related to the range of conformations sampled by the two highly sulfated molecules as free entities in water were compared. The second step compared identity of binding site geometries and the final step evaluated comparable dynamics and interactions in the protein-bound state. Using a test case of interactions with fibroblast growth factor-related proteins, we show that this three-step algorithm effectively predicts the GAG structure mimicking property of NSGMs. Specifically, we show that two unique dimeric NSGMs mimic hexameric GAG sequences in the protein-bound state. In contrast, closely related monomeric and trimeric NSGMs do not mimic GAG in either the free or bound states. These results correspond well with the functional properties of NSGMs. The results show for the first time that appropriately designed sulfated NSGMs can be good structural mimetics of GAGs and the incorporation of a MD-based strategy at the NSGM library screening stage can identify promising mimetics of targeted GAG sequences.

Factor XIa inhibitors: A review of the patent literature

INTRODUCTION

Anticoagulants are the mainstay for prevention and/or treatment of thrombotic disorders. Each clinically used anticoagulant is associated with significant adverse consequences, especially bleeding. Factor XIa (FXIa), a key factor involved in the amplification of procoagulation signal, has been suggested as a major target for anticoagulant drug discovery because of reduced risk of bleeding.

AREAS COVERED

Our literature search uncovered dozens of industrial and academic patents on the discovery of novel FXIa/FXI inhibitors. Small peptidomimetics, sulfated glycosaminoglycan mimetics, polypeptides, antisense oligonucleotides, and monoclonal antibodies have been developed as inhibitors of FXIa. Although many agents are in early discovery/development phases, the activity and safety of a few have been evaluated in various animal models and in humans.

EXPERT OPINION

FXIa is a promising drug target for development of effective anticoagulants with limited bleeding complications. Literature reveals a major trend in the number of patent applications over the last three years. These inhibitors exploit different approaches for target inhibition. Allosteric modulation of FXIa and biosynthetic inhibition of FXI are mechanistically unique. Despite initial results in patients undergoing knee anthroplasty as with antisense oligonucleotides, major advances should be realized, particularly with respect to pharmacokinetics, for FXI/FXIa inhibitors to enter the clinic.

The susceptibility of plasma coagulation factor XI to nitration and peroxynitrite action

Coagulation factor XI is present in blood plasma as the zymogen, like other serine proteases of hemostatic system, but as the only coagulation factor forms 140-160kDa homodimers. Its activation is induced by thrombin, and a positive feedback increases the generation of the extra thrombin. Experimental and clinical observations confirm protective roles of factor XI deficiencies in certain types of thromboembolic disorders. Thromboembolism still causes serious problems for modern civilization. Diseases associated with the blood coagulation system are often associated with inflammation and oxidative stress. Peroxynitrite is produced from nitric oxide and superoxide in inflammatory diseases. The aim of the current study is to evaluate effects of nitrative stress triggered by peroxynitrite on coagulation factor XI in human plasma employing biochemical and bioinformatic methods. The amidolytic assay shows increase in factor XI activity triggered by peroxynitrite. Peroxynitrite interferes factor XI by nitration and fragmentation, which is demonstrated by immunoprecipitation followed by western blotting. Nitrated factor XI is even present in control blood plasma. The results suggest possible modifications of factor XI on the molecular level. Computer simulations show tyrosine residues as targets of peroxynitrite action. The modifications induced by peroxynitrite in factor XI might be important in thrombotic disorders.

The hemostatic role of factor XI

Coagulation factor (F)XI has been described as a component of the early phase of the contact pathway of blood coagulation, acting downstream of factor XII. However, patients deficient in upstream members of the contact pathway, including FXII and prekallikrein, do not exhibit bleeding complications, while FXI-deficient patients sometimes experience mild bleeding, suggesting FXI plays a role in hemostasis independent of the contact pathway. Further complicating the picture, bleeding risk in FXI-deficient patients is difficult to predict because bleeding symptoms have not been found to correlate with FXI antigen levels or activity. However, recent studies have emerged to expand our understanding of FXI, demonstrating that activated FXI is able to activate coagulation factors FX, FV, and FVIII, and inhibit the anti-coagulant tissue factor pathway inhibitor (TFPI). Understanding these activities of FXI may help to better diagnose which FXI-deficient patients are at risk for bleeding. In contrast to its mild hemostatic activities, FXI is known to play a significant role in thrombosis, as it is a demonstrated independent risk factor for deep vein thrombosis, ischemic stroke, and myocardial infarction. Recent translational approaches have begun testing FXI as an antithrombotic, with one promising clinical study showing that an anti-sense oligonucleotide against FXI prevented venous thrombosis in elective knee surgery. A better understanding of the varied and complex role of FXI in both thrombosis and hemostasis will help to allow better prediction of bleeding risk in FXI-deficient patients and also informing the development of targeted agents to inhibit the thrombotic activities of FXI while preserving hemostasis.

Allosteric Partial Inhibition of Monomeric Proteases. Sulfated Coumarins Induce Regulation, not just Inhibition, of Thrombin

Allosteric partial inhibition of soluble, monomeric proteases can offer major regulatory advantages, but remains a concept on paper to date; although it has been routinely documented for receptors and oligomeric proteins. Thrombin, a key protease of the coagulation cascade, displays significant conformational plasticity, which presents an attractive opportunity to discover small molecule probes that induce sub-maximal allosteric inhibition. We synthesized a focused library of some 36 sulfated coumarins to discover two agents that display sub-maximal efficacy (~50%), high potency (<500 nM) and high selectivity for thrombin (>150-fold). Michaelis-Menten, competitive inhibition, and site-directed mutagenesis studies identified exosite 2 as the site of binding for the most potent sulfated coumarin. Stern-Volmer quenching of active site-labeled fluorophore suggested that the allosteric regulators induce intermediate structural changes in the active site as compared to those that display ~80–100% efficacy. Antithrombin inactivation of thrombin was impaired in the presence of the sulfated coumarins suggesting that allosteric partial inhibition arises from catalytic dysfunction of the active site. Overall, sulfated coumarins represent first-in-class, sub-maximal inhibitors of thrombin. The probes establish the concept of allosteric partial inhibition of soluble, monomeric proteins. This concept may lead to a new class of anticoagulants that are completely devoid of bleeding.

Synthesis of bi- and bis-1,2,3-triazoles by copper-catalyzed Huisgen cycloaddition: A family of valuable products by click chemistry

The Cu(I)-catalyzed azide-alkyne cycloaddition reaction, also known as click chemistry, has become a useful tool for the facile formation of 1,2,3-triazoles. Specifically, the utility of this reaction has been demonstrated by the synthesis of structurally diverse bi- and bis-1,2,3-triazoles. The present review focuses on the synthesis of such bi- and bistriazoles and the importance of using copper-promoted click chemistry (CuAAC) for such transformations. In addition, the application of bitriazoles and the related CuAAAC reaction in different fields, including medicinal chemistry, coordination chemistry, biochemistry, and supramolecular chemistry, have been highlighted.

Allosteric inhibition of factor XIa. Sulfated non-saccharide glycosaminoglycan mimetics as promising anticoagulants

Recent development of sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranoside (SPGG), as potent inhibitors of factor XIa (FXIa) (J. Med. Chem. 2013; 56:867-878 and J. Med. Chem. 2014; 57:4805-4818) has led to a strong possibility of developing a new line of factor XIa-based anticoagulants. In fact, SPGG represents the first synthetic, small molecule inhibitor that appears to bind in site remote from the active site. Considering that allosteric inhibition of FXIa is a new mechanism for developing a distinct line of anticoagulants, we have studied SPGG's interaction with FXIa with a goal of evaluating its pre-clinical relevance. Comparative inhibition studies with several glycosaminoglycans revealed the importance of SPGG's non-saccharide backbone. SPGG did not affect the activity of plasma kallikrein, activated protein C and factor XIIIa suggesting that SPGG-based anticoagulation is unlikely to affect other pathways connected with coagulation factors. SPGG's effect on APTT of citrated human plasma was also not dependent on antithrombin or heparin cofactor II. Interestingly, SPGG's anticoagulant potential was diminished by serum albumin as well as factor XI, while it could be reversed by protamine or polybrene, which implies possible avenues for developing antidote strategy. Studies with FXIa mutants indicated that SPGG engages Lys529, Arg530 and Arg532, but not Arg250, Lys252, Lys253 and Lys255. Finally, SPGG competes with unfractionated heparin, but not with polyphosphates and/or glycoprotein Ibα, for binding to FXIa. These studies enhance understanding on the first allosteric inhibitor of FXIa and highlight its value as a promising anticoagulant.

Plasmin regulation through allosteric, sulfated, small molecules

Plasmin, a key serine protease, plays a major role in clot lysis and extracellular matrix remodeling. Heparin, a natural polydisperse sulfated glycosaminoglycan, is known to allosterically modulate plasmin activity. No small allosteric inhibitor of plasmin has been discovered to date. We screened an in-house library of 55 sulfated, small glycosaminoglycan mimetics based on nine distinct scaffolds and varying number and positions of sulfate groups to discover several promising hits. Of these, a pentasulfated flavonoid-quinazolinone dimer 32 was found to be the most potent sulfated small inhibitor of plasmin (IC₅₀ = 45 μM, efficacy = 100%). Michaelis-Menten kinetic studies revealed an allosteric inhibition of plasmin by these inhibitors. Studies also indicated that the most potent inhibitors are selective for plasmin over thrombin and factor Xa, two serine proteases in coagulation cascade. Interestingly, different inhibitors exhibited different levels of efficacy (40%–100%), an observation alluding to the unique advantage offered by an allosteric process. Overall, our work presents the first small, synthetic allosteric plasmin inhibitors for further rational design.

Synthesis of glycosaminoglycan mimetics through sulfation of polyphenols

In nearly all cases of biological activity of sulfated GAGs, the sulfate group(s) are critical for interacting with target proteins. A growing paradigm is that appropriate small, sulfated, nonsaccharide GAG mimetics can be designed to either mimic or interfere with the biological functions of natural GAG sequences resulting in the discovery of either antagonist or agonist agents. A number of times these sulfated NSGMs can be computationally designed based on the parent GAG-protein interaction. The small sulfated NSGMs may possess considerable aromatic character so as to engineer hydrophobic, hydrogen-bonding, Coulombic or cation-pi forces in their interactions with target protein(s) resulting in higher specificity of action relative to parent GAGs. The sulfated NSGMs can be easily synthesized in one step from appropriate natural polyphenols through chemical sulfation under microwave-based conditions. We describe step-by-step procedures to perform microwave-based sulfation of several small polyphenol scaffolds so as to prepare homogenous NSGMs containing one to more than 10 sulfate groups per molecule in high yields.

Future prospects for contact factors as therapeutic targets

Anticoagulants currently used in clinical practice to treat or prevent thromboembolic disease are effective, but place patients at increased risk for serious bleeding because they interfere with plasma enzymes (thrombin and factor Xa) that are essential for hemostasis. In the past 10 years, work with genetically altered mice and studies in baboons and rabbits have demonstrated that the plasma contact proteases factor XI, factor XII, and prekallikrein contribute to the formation of occlusive thrombi despite having limited roles in hemostasis. In the case of factor XI, epidemiologic data from human populations indicate that elevated levels of this protein increase risk for stroke and venous thromboembolism and may also influence risk for myocardial infarction. These findings suggest that inhibiting contact activation may produce an antithrombotic effect without significantly compromising hemostasis. This chapter reviews strategies that are being developed for therapeutic targeting of factor XI and factor XII and their performances in preclinical and early human trials.

Recent advances on plasmin inhibitors for the treatment of fibrinolysis-related disorders

Growing evidence suggests that plasmin is involved in a number of physiological processes in addition to its key role in fibrin cleavage. Plasmin inhibition is critical in preventing adverse consequences arising from plasmin overactivity, e.g., blood loss that may follow cardiac surgery. Aprotinin was widely used as an antifibrinolytic drug before its discontinuation in 2008. Tranexamic acid and ε-aminocaproic acid, two small molecule plasmin inhibitors, are currently used in the clinic. Several molecules have been designed utilizing covalent, but reversible, chemistry relying on reactive cyclohexanones, nitrile warheads, and reactive aldehyde peptidomimetics. Other major classes of plasmin inhibitors include the cyclic peptidomimetics and polypeptides of the Kunitz and Kazal-type. Allosteric inhibitors of plasmin have also been designed including small molecule lysine analogs that bind to plasmin's kringle domain(s) and sulfated glycosaminoglycan mimetics that bind to plasmin's catalytic domain. Plasmin inhibitors have also been explored for resolving other disease states including cell metastasis, cell proliferation, angiogenesis, and embryo implantation. This review highlights functional and structural aspects of plasmin inhibitors with the goal of advancing their design.

A simple, general approach of allosteric coagulation enzyme inhibition through monosulfated hydrophobic scaffolds

Allosteric inhibition of coagulation enzymes offers the advantage of controlled inhibition. In this study, a small library of mono sulfated indole and benzothiazole based molecules was synthesized and screened against the panel of coagulation proteases. The results reveal that selected molecules inhibit the thrombin, factor Xa and factor XIa with moderate potency. Compound 6a was found to have an allosteric mode of inhibition against thrombin. Plasma clotting assays suggest that selected inhibitors 14b, 14c and 14d prolong both prothrombin and activated partial thromboplastin time. Overall, this work presents the newer class of allosteric inhibitors of thrombin and factor XIa with improved aqueous solubility profile.

Synthetic, non-saccharide, glycosaminoglycan mimetics selectively target colon cancer stem cells

Selective targeting of cancer stem-like cells (CSCs) is a paradigm-shifting approach. We hypothesized that CSCs can be targeted by interfering with functions of sulfated glycosaminoglycans, which play key roles in cancer cell growth, invasion and metastasis. We developed a tandem, dual screen strategy involving (1) assessing inhibition of monolayer versus spheroid growth and (2) assessing inhibition of primary versus secondary spheroid growth to identify G2.2, a unique sulfated nonsaccharide GAG mimetic (NSGM) from a focused library of 53 molecules, as a selective inhibitor of colon CSCs. The NSGM down-regulated several CSC markers through regulation of gene transcription, while closely related, inactive NSGMs G1.4 and G4.1 demonstrated no such changes. G2.2's effects on CSCs were mediated, in part, through induction of apoptosis and inhibition of self-renewal factors. Overall, this work presents the proof-of-principle that CSCs can be selectively targeted through novel NSGMs, which are likely to advance fundamental understanding on CSCs while also aiding development of novel therapeutic agents.

Factor XI as a target for antithrombotic therapy

Anticoagulants currently used in clinical practice to treat thromboembolic disorders are effective but increase the risk of severe bleeding because they target proteins that are essential for normal coagulation (hemostasis). Drugs with better safety profiles are required for prevention and treatment of thromboembolic disease. Coagulation factor XIa has emerged as a novel target for safer anticoagulant therapy because of its role in thrombosis and its relatively small contribution to hemostasis.

Designing allosteric inhibitors of factor XIa. Lessons from the interactions of sulfated pentagalloylglucopyranosides

We recently introduced sulfated pentagalloylglucopyranoside (SPGG) as an allosteric inhibitor of factor XIa (FXIa) (Al-Horani et al., J. Med Chem.2013, 56, 867–87823316863). To better understand the SPGG–FXIa interaction, we utilized eight SPGG variants and a range of biochemical techniques. The results reveal that SPGG’s sulfation level moderately affected FXIa inhibition potency and selectivity over thrombin and factor Xa. Variation in the anomeric configuration did not affect potency. Interestingly, zymogen factor XI bound SPGG with high affinity, suggesting its possible use as an antidote. Acrylamide quenching experiments suggested that SPGG induced significant conformational changes in the active site of FXIa. Inhibition studies in the presence of heparin showed marginal competition with highly sulfated SPGG variants but robust competition with less sulfated variants. Resolution of energetic contributions revealed that nonionic forces contribute nearly 87% of binding energy suggesting a strong possibility of specific interaction. Overall, the results indicate that SPGG may recognize more than one anion-binding, allosteric site on FXIa. An SPGG molecule containing approximately 10 sulfate groups on positions 2 through 6 of the pentagalloylglucopyranosyl scaffold may be the optimal FXIa inhibitor for further preclinical studies.

Thiocarbamate-linked polysulfonate-peptide conjugates as selective hepatocyte growth factor receptor binders

The capacity of many proteins to interact with natural or synthetic polyanions has been exploited for modulating their biological action. However, the polydispersity of these macromolecular polyanions as well as their poor specificity is a severe limitation to their use as drugs. An emerging trend in this field is the synthesis of homogeneous and well-defined polyanion-peptide conjugates, which act as bivalent ligands, with the peptide part bringing the selectivity of the scaffold. Alternately, this strategy can be used for improving the binding of short peptides to polyanion-binding protein targets. This work describes the design and first synthesis of homogeneous polysulfonate-peptide conjugates using thiocarbamate ligation for binding to the extracellular domain of MET tyrosine kinase receptor for hepatocyte growth factor.

Allosteric inhibition of human factor XIa: discovery of monosulfated benzofurans as a class of promising inhibitors

Factor XIa (fXIa) is being recognized as a prime target for developing safer anticoagulants. To discover synthetic, small, allosteric inhibitors of fXIa, we screened an in-house, unique library of 65 molecules displaying many distinct scaffolds and varying levels of sulfation. Of these, monosulfated benzofurans were the only group of molecules found to inhibit fXIa (∼100% efficacy) and led to the identification of monosulfated trimer 24 (IC₅₀ 0.82 μM) as the most potent inhibitor. Michaelis–Menten kinetics studies revealed a classic noncompetitive mechanism of action for 24. Although monosulfated, the inhibitors did not compete with unfractionated heparin alluding to a novel site of interaction. Fluorescence quenching studies indicated that trimer 24 induces major conformational changes in the active site of fXIa. Docking studies identified a site near Lys255 on the A3 domain of fXIa as the most probable site of binding for 24. Factor XIa devoid of the A3 domain displayed a major defect in the inhibition potency of 24 supporting the docking prediction. Our work presents the sulfated benzofuran scaffold as a promising framework to develop allosteric fXIa inhibitors that likely function through the A3 domain.

NIS-mediated regioselective amidation of indole with quinazolinone and pyrimidone

A mild, metal-free condition was developed for the direct regioselective C2 amidation of indoles and pyrroles with quinazolinone and pyrimidone derivatives in intermolecular fashion, which led to novel indolyl/pyrrolyl quinazolinone and pyrimidone derivatives in moderate to good yields.