Design and synthesis of conformationally restricted inhibitors of active thrombin activatable fibrinolysis inhibitor (TAFIa)

Carbene-coated metal nanoparticles for in vivo applications

N-Heterocyclic carbenes (NHC) are well-recognized ligands of choice for preparing robust transition metal species. However, their use for fabrication of biomedically relevant nanoparticles has been limited to the synthesis of non-targeted particles showing increased tolerance to different aqueous coagulants. In this work, the first example of carbene-coated metal nanoparticles suitable for in vivo applications is presented. Directed design of a novel biscarbene NHC ligand allowed to prepare the first magnetite/gold (Fe3O4@AuNP@NHC) nanostructures and carbene gold (AuNP@NHC) nanoparticles with significant stability in aqueous solutions and enhanced ability to form bioconjugates. Furthermore, these nanoparticles exhibit an extraordinary property for inorganic nanoparticles: they can endure several additive-free air drying/redispersion cycles without deterioration of their colloidal behavior. Bioconjugated AuNP@NHC and multimodal Fe3O4@AuNP@NHC demonstrated a successful performance in three distinct applications: lateral flow tests, specific cancer cell targeting, and bioimaging. Thus, the results show the notable advantages of the N-heterocyclic carbene coating of inorganic nanoparticles and their utility for complex biomedical applications.

ProCPU Is Expressed by (Primary) Human Monocytes and Macrophages and Expression Differs between States of Differentiation and Activation

Carboxypeptidase U (CPU, TAFIa, CPB2) is a potent attenuator of fibrinolysis that is mainly synthesized by the liver as its inactive precursor proCPU. Aside from its antifibrinolytic properties, evidence exists that CPU can modulate inflammation, thereby regulating communication between coagulation and inflammation. Monocytes and macrophages play a central role in inflammation and interact with coagulation mechanisms resulting in thrombus formation. The involvement of CPU and monocytes/macrophages in inflammation and thrombus formation, and a recent hypothesis that proCPU is expressed in monocytes/macrophages, prompted us to investigate human monocytes and macrophages as a potential source of proCPU. CPB2 mRNA expression and the presence of proCPU/CPU protein were studied in THP-1, PMA-stimulated THP-1 cells and primary human monocytes, M-CSF-, IFN-γ/LPS-, and IL-4-stimulated-macrophages by RT-qPCR, Western blotting, enzyme activity measurements, and immunocytochemistry. CPB2 mRNA and proCPU protein were detected in THP-1 and PMA-stimulated THP-1 cells as well as in primary monocytes and macrophages. Moreover, CPU was detected in the cell medium of all investigated cell types and it was demonstrated that proCPU can be activated into functionally active CPU in the in vitro cell culture environment. Comparison of CPB2 mRNA expression and proCPU concentrations in the cell medium between the different cell types provided evidence that CPB2 mRNA expression and proCPU secretion in monocytes and macrophages is related to the degree to which these cells are differentiated. Our results indicate that primary monocytes and macrophages express proCPU. This sheds new light on monocytes and macrophages as local proCPU sources.

Phosphinanes and Azaphosphinanes as Potent and Selective Inhibitors of Activated Thrombin-Activatable Fibrinolysis Inhibitor (TAFIa)

Selective and potent inhibitors of activated thrombin activatable fibrinolysis inhibitor (TAFIa) have the potential to increase endogenous and therapeutic fibrinolysis and to behave like profibrinolytic agents without the risk of major hemorrhage, since they do not interfere either with platelet activation or with coagulation during blood hemostasis. Therefore, TAFIa inhibitors could be used in at-risk patients for the treatment, prevention, and secondary prevention of stroke, venous thrombosis, and pulmonary embolisms. In this paper, we describe the design, the structure-activity relationship (SAR), and the synthesis of novel, potent, and selective phosphinanes and azaphosphinanes as TAFIa inhibitors. Several highly active azaphosphinanes display attractive properties suitable for further in vivo efficacy studies in thrombosis models.

Thrombin Activatable Fibrinolysis Inhibitor (TAFI): An Updated Narrative Review

Thrombin activatable fibrinolysis inhibitor (TAFI), a proenzyme, is converted to a potent attenuator of the fibrinolytic system upon activation by thrombin, plasmin, or the thrombin/thrombomodulin complex. Since TAFI forms a molecular link between coagulation and fibrinolysis and plays a potential role in venous and arterial thrombotic diseases, much interest has been tied to the development of molecules that antagonize its function. This review aims at providing a general overview on the biochemical properties of TAFI, its (patho)physiologic function, and various strategies to stimulate the fibrinolytic system by interfering with (activated) TAFI functionality.

Carboxypeptidase U (CPU, TAFIa, CPB2) in Thromboembolic Disease: What Do We Know Three Decades after Its Discovery?

Procarboxypeptidase U (proCPU, TAFI, proCPB2) is a basic carboxypeptidase zymogen that is converted by thrombin(-thrombomodulin) or plasmin into the active carboxypeptidase U (CPU, TAFIa, CPB2), a potent attenuator of fibrinolysis. As CPU forms a molecular link between coagulation and fibrinolysis, the development of CPU inhibitors as profibrinolytic agents constitutes an attractive new concept to improve endogenous fibrinolysis or to increase the efficacy of thrombolytic therapy in thromboembolic diseases. Furthermore, extensive research has been conducted on the in vivo role of CPU in (the acute phase of) thromboembolic disease, as well as on the hypothesis that high proCPU levels and the Thr/Ile325 polymorphism may cause a thrombotic predisposition. In this paper, an overview is given of the methods available for measuring proCPU, CPU, and inactivated CPU (CPUi), together with a summary of the clinical data generated so far, ranging from the current knowledge on proCPU concentrations and polymorphisms as potential thromboembolic risk factors to the positioning of different CPU forms (proCPU, CPU, and CPUi) as diagnostic markers for thromboembolic disease, and the potential benefit of pharmacological inhibition of the CPU pathway.

Selective inhibition of carboxypeptidase U may reduce microvascular thrombosis in rat experimental stroke

BACKGROUND

Carboxypeptidase U (CPU, CPB2, TAFIa) is a potent attenuator of fibrinolysis. The inhibition of CPU is thus an interesting strategy for improving thrombolysis.

OBJECTIVES

The time course of CPU generation and proCPU consumption were assessed in an experimental rat model of acute ischemic stroke (AIS). In addition, the effects of the selective CPU inhibitor AZD9684 on CPU kinetics, microvascular thrombosis (MT), and AIS outcome were evaluated.

METHODS

Rats were subjected to transient middle cerebral artery occlusion (tMCAO) and received recombinant tissue-type plasminogen activator (tPA), a specific CPU inhibitor (AZD9684), combination therapy of tPA and AZD9684, or saline for 1 hour using a randomized treatment regime. CPU and proCPU levels were determined at five time points and assessed in light of outcome parameters (a.o.: infarct volume and fibrin[ogen] deposition as a measure for MT).

RESULTS

Clear activation of the CPU system was observed after AIS induction, in both saline- and tPA-treated rats. Maximal CPU activities were observed at treatment cessation and were higher in tPA-treated animals compared to the saline group. Concomitant proCPU consumption was more pronounced in tPA-treated rats. AZD9684 suppressed the CPU activity and reduced fibrin(ogen) deposition, suggesting a reduction of MT. Nonetheless, a significant decrease in infarct volume was not observed.

CONCLUSIONS

A pronounced activation of the CPU system was observed during tMCAO in rats. Selective inhibition of CPU with AZD9684 was able to reduce fibrin(ogen) deposition and brain edema, suggesting a reduction of MT but without a significant effect on final infarct volume.

Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) inhibition stimulates the fibrinolytic rate in different in vitro models

Essentials AZD9684 is a potent inhibitor of carboxypeptidase U (CPU, TAFIa, CPB2). The effect of AZD9684 on fibrinolysis was investigated in four in vitro systems. The CPU system also attenuates fibrinolysis in more advanced hemostatic systems. The size of the observed effect on fibrinolysis is dependent on the exact experimental conditions.

SUMMARY

Background Carboxypeptidase U (CPU, carboxypeptidase B2, activated thrombin-activatable fibrinolysis inhibitor) is a basic carboxypeptidase that attenuates fibrinolysis. This characteristic has raised interest in the scientific community and pharmaceutical industry for the development of inhibitors as profibrinolytic agents. Objectives Little is known about the contribution of CPU to clot resistance in more advanced hemostatic models, which include blood cells and shear stress. The aim of this study was to evaluate the effects of the CPU system in in vitro systems for fibrinolysis with different grades of complexity. Methods The contribution of the CPU system was evaluated in the following systems: (i) plasma clot lysis; (ii) rotational thromboelastometry (ROTEM) in whole blood; (iii) front lysis with confocal microscopy in platelet-free and platelet-rich plasma; and (iv) a microfluidic system with whole blood under arterial shear stress. Experiments were carried out in the presence or absence of AZD9684, a specific CPU inhibitor. Results During plasma clot lysis, addition of AZD9684 resulted in 33% faster lysis. In ROTEM, the lysis onset time was decreased by 38%. For both clot lysis and ROTEM, an AZD9684 dose-dependent response was observed. CPU inhibition in front lysis experiments resulted in 47% and 50% faster lysis for platelet-free plasma and platelet-rich plasma, respectively. Finally, a tendency for faster lysis was observed only in the microfluidic system when AZD9684 was added. Conclusions Overall, these experiments provide novel evidence that the CPU system can also modulate fibrinolysis in more advanced hemostatic systems. The extent of the effects appears to be dependent upon the exact experimental conditions.

Multiple Architectures and Mechanisms of Latency in Metallopeptidase Zymogens

Metallopeptidases cleave polypeptides bound in the active-site cleft of catalytic domains through a general base/acid mechanism. This involves a solvent molecule bound to a catalytic zinc and general regulation of the mechanism through zymogen-based latency. Sixty reported structures from 11 metallopeptidase families reveal that prosegments, mostly N-terminal of the catalytic domain, block the cleft regardless of their size. Prosegments may be peptides (5-14 residues), which are only structured within the zymogens, or large moieties (<227 residues) of one or two folded domains. While some prosegments globally shield the catalytic domain through a few contacts, others specifically run across the cleft in the same or opposite direction as a substrate, making numerous interactions. Some prosegments block the zinc by replacing the solvent with particular side chains, while others use terminal α-amino or carboxylate groups. Overall, metallopeptidase zymogens employ disparate mechanisms that diverge even within families, which supports that latency is less conserved than catalysis.

Selective synthesis of tetrahydroimidazo[1,2-a]pyridine and pyrrolidine derivatives via a one-pot two-step reaction

In the presence of triethylamine, the addition reaction of substituted α-amino acid alkyl esters with dialkyl but-2-ynedioate afforded active β-enamino esters, which in turn reacted with aromatic aldehydes and malononitrile to give tetrahydroimidazo[1,2-a]pyridine derivatives in moderate yields.

Elucidation of the molecular mechanisms of two nanobodies that inhibit thrombin-activatable fibrinolysis inhibitor activation and activated thrombin-activatable fibrinolysis inhibitor activity

Essentials Thrombin-activatable fibrinolysis inhibitor (TAFI) is a risk factor for cardiovascular disorders. TAFI inhibitory nanobodies represent a promising step in developing profibrinolytic therapeutics. We have solved three crystal structures of TAFI in complex with inhibitory nanobodies. Nanobodies inhibit TAFI through distinct mechanisms and represent novel profibrinolytic leads.

SUMMARY

Background Thrombin-activatable fibrinolysis inhibitor (TAFI) is converted to activated TAFI (TAFIa) by thrombin, plasmin, or the thrombin-thrombomodulin complex (T/TM). TAFIa is antifibrinolytic, and high levels of TAFIa are associated with an increased risk for cardiovascular disorders. TAFI-inhibitory nanobodies represent a promising approach for developing profibrinolytic therapeutics. Objective To elucidate the molecular mechanisms of inhibition of TAFI activation and TAFIa activity by nanobodies with the use of X-ray crystallography and biochemical characterization. Methods and results We selected two nanobodies for cocrystallization with TAFI. VHH-a204 interferes with all TAFI activation modes, whereas VHH-i83 interferes with T/TM-mediated activation and also inhibits TAFIa activity. The 3.05-Å-resolution crystal structure of TAFI-VHH-a204 reveals that the VHH-a204 epitope is localized to the catalytic moiety (CM) in close proximity to the TAFI activation site at Arg92, indicating that VHH-a204 inhibits TAFI activation by steric hindrance. The 2.85-Å-resolution crystal structure of TAFI-VHH-i83 reveals that the VHH-i83 epitope is located close to the presumptive thrombomodulin-binding site in the activation peptide (AP). The structure and supporting biochemical assays suggest that VHH-i83 inhibits TAFIa by bridging the AP to the CM following TAFI activation. In addition, the 3.00-Å-resolution crystal structure of the triple TAFI-VHH-a204-VHH-i83 complex demonstrates that the two nanobodies can simultaneously bind to TAFI. Conclusions This study provides detailed insights into the molecular mechanisms of TAFI inhibition, and reveals a novel mode of TAFIa inhibition. VHH-a204 and VHH-i83 merit further evaluation as potential profibrinolytic therapeutics.

Structure-function relationships in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is an important regulator in the balance of coagulation and fibrinolysis. TAFI is a metallocarboxypeptidase that circulates in plasma as zymogen. Activated TAFI (TAFIa) cleaves C-terminal lysine or arginine residues from peptide substrates. The removal of C-terminal lysine residues from partially degraded fibrin leads to reduced plasmin formation and thus attenuation of fibrinolysis. TAFI also plays a role in inflammatory processes via the removal of C-terminal arginine or lysine residues from bradykinin, thrombin-cleaved osteopontin, C3a, C5a and chemerin. TAFI has been studied extensively over the past three decades and recent publications provide a wealth of information, including crystal structures, mutants and structural data obtained with antibodies and peptides. In this review, we combined and compared available data on structure/function relationships of TAFI.

Novel Small Molecule Inhibitors of Activated Thrombin Activatable Fibrinolysis Inhibitor (TAFIa) from Natural Product Anabaenopeptin

Anabaenopeptins isolated from cyanobacteria were identified as inhibitors of carboxypeptidase TAFIa. Cocrystal structures of these macrocyclic natural product inhibitors in a modified porcine carboxypeptidase B revealed their binding mode and provided the basis for the rational design of small molecule inhibitors with a previously unknown central urea motif. Optimization based on these design concepts allowed for a rapid evaluation of the SAR and delivered potent small molecule inhibitors of TAFIa with a promising overall profile.