Enhanced protein C activation and inhibition of fibrinogen cleavage by a thrombin modulator

Substrate-selective small-molecule modulators of enzymes: Mechanisms and opportunities

Small-molecule inhibitors of enzymes are widely used tools in reverse chemical genetics to probe biology and explore therapeutic opportunities. They are often compared with genetic knockdown or knockout and are expected to produce phenotypes similar to the genetic perturbations. This review aims to highlight that small molecule inhibitors of enzymes and genetic perturbations may not necessarily produce the same phenotype due to the possibility of substrate-selective or substrate-dependent effects of the inhibitors. Examples of substrate-selective inhibitors and the mechanisms for the substrate-selective effects are discussed. Substrate-selective modulators of enzymes have distinct advantages and cannot be easily replaced with biologics. Thus, they present an exciting opportunity for chemical biologists and medicinal chemists.

Substrate-selective inhibitors that reprogram the activity of insulin-degrading enzyme

Enzymes that act on multiple substrates are common in biology but pose unique challenges as therapeutic targets. The metalloprotease insulin-degrading enzyme (IDE) modulates blood glucose levels by cleaving insulin, a hormone that promotes glucose clearance. However, IDE also degrades glucagon, a hormone that elevates glucose levels and opposes the effect of insulin. IDE inhibitors to treat diabetes therefore should prevent IDE-mediated insulin degradation, but not glucagon degradation, in contrast with traditional modes of enzyme inhibition. Using a high-throughput screen for non-active-site ligands, we discovered potent and highly specific small-molecule inhibitors that alter IDE’s substrate selectivity. X-ray co-crystal structures, including an IDE-ligand-glucagon ternary complex, revealed substrate-dependent interactions that enable these inhibitors to potently block insulin binding while allowing glucagon cleavage, even at saturating inhibitor concentrations. These findings suggest a path for developing IDE-targeting therapeutics, and offer a blueprint for modulating other enzymes in a substrate-selective manner to unlock their therapeutic potential.

Rational Design of Protein C Activators

In addition to its procoagulant and proinflammatory functions mediated by cleavage of fibrinogen and PAR1, the trypsin-like protease thrombin activates the anticoagulant protein C in a reaction that requires the cofactor thrombomodulin and the endothelial protein C receptor. Once in the circulation, activated protein C functions as an anticoagulant, anti-inflammatory and regenerative factor. Hence, availability of a protein C activator would afford a therapeutic for patients suffering from thrombotic disorders and a diagnostic tool for monitoring the level of protein C in plasma. Here, we present a fusion protein where thrombin and the EGF456 domain of thrombomodulin are connected through a peptide linker. The fusion protein recapitulates the functional and structural properties of the thrombin-thrombomodulin complex, prolongs the clotting time by generating pharmacological quantities of activated protein C and effectively diagnoses protein C deficiency in human plasma. Notably, these functions do not require exogenous thrombomodulin, unlike other anticoagulant thrombin derivatives engineered to date. These features make the fusion protein an innovative step toward the development of protein C activators of clinical and diagnostic relevance.

Why Ser and not Thr brokers catalysis in the trypsin fold

Although Thr is equally represented as Ser in the human genome and as a nucleophile is as good as Ser, it is never found in the active site of the large family of trypsin-like proteases that utilize the Asp/His/Ser triad. The molecular basis of the preference of Ser over Thr in the trypsin fold was investigated with X-ray structures of the thrombin mutant S195T free and bound to an irreversible active site inhibitor. In the free form, the methyl group of T195 is oriented toward the incoming substrate in a conformation seemingly incompatible with productive binding. In the bound form, the side chain of T195 is reoriented for efficient substrate acylation without causing steric clash within the active site. Rapid kinetics prove that this change is due to selection of an active conformation from a preexisting ensemble of reactive and unreactive rotamers whose relative distribution determines the level of activity of the protease. Consistent with these observations, the S195T substitution is associated with a weak yet finite activity that allows identification of an unanticipated important role for S195 as the end point of allosteric transduction in the trypsin fold. The S195T mutation abrogates the Na(+)-dependent enhancement of catalytic activity in thrombin, activated protein C, and factor Xa and significantly weakens the physiologically important allosteric effects of thrombomodulin on thrombin and of cofactor Va on factor Xa. The evolutionary selection of Ser over Thr in trypsin-like proteases was therefore driven by the need for high catalytic activity and efficient allosteric regulation.

Conformational selection in trypsin-like proteases

For over four decades, two competing mechanisms of ligand recognition--conformational selection and induced-fit--have dominated our interpretation of protein allostery. Defining the mechanism broadens our understanding of the system and impacts our ability to design effective drugs and new therapeutics. Recent kinetics studies demonstrate that trypsin-like proteases exist in equilibrium between two forms: one fully accessible to substrate (E) and the other with the active site occluded (E*). Analysis of the structural database confirms existence of the E* and E forms and vouches for the allosteric nature of the trypsin fold. Allostery in terms of conformational selection establishes an important paradigm in the protease field and enables protein engineers to expand the repertoire of proteases as therapeutics.

Allostery in trypsin-like proteases suggests new therapeutic strategies

Trypsin-like proteases (TLPs) are a large family of enzymes responsible for digestion, blood coagulation, fibrinolysis, development, fertilization, apoptosis and immunity. A current paradigm posits that the irreversible transition from an inactive zymogen to the active protease form enables productive interaction with substrate and catalysis. Analysis of the entire structural database reveals two distinct conformations of the active site: one fully accessible to substrate (E) and the other occluded by the collapse of a specific segment (E*). The allosteric E*-E equilibrium provides a reversible mechanism for activity and regulation in addition to the irreversible zymogen to protease conversion and points to new therapeutic strategies aimed at inhibiting or activating the enzyme. In this review, we discuss relevant examples, with emphasis on the rational engineering of anticoagulant thrombin mutants.

Inhibition of thrombin action ameliorates insulin resistance in type 2 diabetic db/db mice

The binding of thrombin to its receptor stimulates inflammatory cytokines including IL-6 and monocyte chemoattractant protein-1 (MCP-1); both are associated with the development of insulin resistance. Because increased adiposity enhanced the expression of coagulation factor VII that stimulates the coagulation pathway in adipose tissue, we tested whether the inhibition of thrombin action ameliorates insulin resistance in obese diabetic (Lpr(-/-):db/db) mice. The 4-wk administration of argatroban, a selective thrombin inhibitor, reduced fasting plasma glucose and ameliorated insulin resistance in these mice. It also reduced adipocyte size and macrophage infiltration into adipose tissue. The aberrant gene expression of MCP-1, IL-6, adiponectin, and factor VII and suppressed insulin receptor substrate-1-Akt signaling in adipose tissue of db/db mice were reversed by argatroban treatment. These results demonstrate that increased adiposity enhances the production of thrombin in adipose tissue by stimulating factor VII expression and suggest that increased thrombin activity in adipose tissue plays an important role in the development of insulin resistance via enhancing MCP-1 production, leading to macrophage infiltration and insulin receptor substrate-1-Akt pathway inactivation.

Thrombin

Thrombin is a Na+-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na+ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. The anticoagulant function of thrombin is under the allosteric control of the cofactor thrombomodulin. Much has been learned on the mechanism of Na+ binding and recognition of natural substrates by thrombin. Recent structural advances have shed light on the remarkable molecular plasticity of this enzyme and the molecular underpinnings of thrombin allostery mediated by binding to exosite I and the Na+ site. This review summarizes our current understanding of the molecular basis of thrombin function and allosteric regulation. The basic information emerging from recent structural, mutagenesis and kinetic investigation of this important enzyme is that thrombin exists in three forms, E*, E and E:Na+, that interconvert under the influence of ligand binding to distinct domains. The transition between the Na+ -free slow from E and the Na+ -bound fast form E:Na+ involves the structure of the enzyme as a whole, and so does the interconversion between the two Na+ -free forms E* and E. E* is most likely an inactive form of thrombin, unable to interact with Na + and substrate. The complexity of thrombin function and regulation has gained this enzyme pre-eminence as the prototypic allosteric serine protease. Thrombin is now looked upon as a model system for the quantitative analysis of biologically important enzymes.

Significance of endothelial dysfunction in the pathogenesis of early and delayed radiation enteropathy

This review summarizes the current state of knowledge regarding the role of endothelial dysfunction in the pathogenesis of early and delayed intestinal radiation toxicity and discusses various endothelial-oriented interventions aimed at reducing the risk of radiation enteropathy. Studies published in the biomedical literature during the past four decades and cited in PubMed, as well as clinical and laboratory data from our own research program are reviewed. The risk of injury to normal tissues limits the cancer cure rates that can be achieved with radiation therapy. During treatment of abdominal and pelvic tumors, the intestine is frequently a major dose-limiting factor. Microvascular injury is a prominent feature of both early (inflammatory), as well as delayed (fibroproliferative) radiation injuries in the intestine and in many other normal tissues. Evidence from our and other laboratories suggests that endothelial dysfunction, notably a deficiency of endothelial thrombomodulin, plays a key role in the pathogenesis of these radiation responses. Deficient levels of thrombomodulin cause loss of vascular thromboresistance, excessive activation of cellular thrombin receptors by thrombin, and insufficient activation of protein C, a plasma protein with anticoagulant, anti-inflammatory, and cytoprotective properties. These changes are presumed to be critically involved in many aspects of early intestinal radiation toxicity and may sustain the fibroproliferative processes that lead to delayed intestinal dysfunction, fibrosis, and clinical complications. In conclusion, injury of vascular endothelium is important in the pathogenesis of the intestinal radiation response. Endothelial-oriented interventions are appealing strategies to prevent or treat normal tissue toxicity associated with radiation treatment of cancer.

Thrombin allostery

Thrombin is a Na(+)-activated, allosteric serine protease that plays multiple functional roles in blood pathophysiology. Binding of Na(+) is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme. This review summarizes our current understanding of the molecular basis of thrombin allostery with special emphasis on the kinetic aspects of Na(+) activation. The molecular mechanism of thrombin allostery is a remarkable example of long-range communication that offers a paradigm for many other biological systems.

Inhibition of thrombin-induced vascular endothelial growth factor production in human neuroblastoma (NB-1) cells by argatroban

Thrombin, a serine protease that plays a pivotal role in blood coagulation, wound healing, and angiogenesis, has also been implicated in the mitogenesis of various cell types. Previously, we showed that thrombin and the thrombin receptor agonist peptide (TRAP-14; SFLLRNPNDKYEPF) for protease-activated receptor 1 (PAR1) induce vascular endothelial growth factor (VEGF) secretion in PC-12 cells. In this study, we show that thrombin and TRAP-14 also stimulate VEGF secretion in the human NB-1 neuroblastoma cells. In these cells, we further show that thrombin-induced VEGF secretion was blocked by cycloheximide and actinomycin D, indicating that de novo protein synthesis is essential for this process. Reduced thrombin-induced VEGF secretion upon treatment with LY294002, calphostin C, or BAPTA, further suggests that the process is dependent on phosphatidyl-inositol-3-kinase, protein kinase C, and calcium. However, the complete loss of thrombin-induced VEGF production upon treatment with argatroban, a derivative of arginine and a potent anticoagulant/antithrombin agent, supports the notion that argatroban serves as a useful therapeutic tool for thrombin-associated pathologic conditions. Here, it appears that argatroban may be effective in controlling disorders linked to thrombin-induced VEGF production in neuronal cells.

Radiation injury and the protein C pathway

OBJECTIVE

To summarize current knowledge regarding the role of the thrombomodulin (TM)-protein C system in acute and chronic radiation responses in normal tissues.

DATA SOURCE

Studies published in the biomedical literature during the past three decades and cited in PubMed and unpublished clinical and laboratory data from our own research program. STUDY SUMMARY: The risk of injury to normal tissues limits the cancer cure rates that can be achieved with radiation therapy. Microvascular injury is a prominent feature of normal tissue radiation injury and plays a critical role in both acute (inflammatory) and chronic (fibrotic) radiation responses. Evidence from our own and other laboratories strongly suggests that dysfunction of the TM-protein C system plays a key role in the pathogenesis of radiation-induced injury to normal tissue. Exposure of normal tissues to ionizing radiation causes a pronounced, sustained deficiency of endothelial TM. This is likely to be due to a combination of initial inactivation of TM by reactive oxygen species, reduced transcription of TM, and release of TM into the circulation. Deficient levels of endothelial TM cause loss of local vascular thrombo-resistance, excessive activation of protease-activated receptor-1 by thrombin, and insufficient activation of protein C. These changes are presumed to be critically involved in many aspects of acute radiation toxicity and in sustaining the fibroproliferative processes that lead to chronic radiation-induced organ dysfunction and clinical complications.

CONCLUSION

Injury of vascular endothelium may be key to the acute responses of normal tissues to ionizing radiation and to the progressive nature of chronic radiation fibrosis. Restitution of the TM-protein C pathway is an appealing strategy by which to prevent or treat normal tissue toxicity associated with radiation treatment of cancer.

Molecular dissection of Na+ binding to thrombin

Na(+) binding near the primary specificity pocket of thrombin promotes the procoagulant, prothrombotic, and signaling functions of the enzyme. The effect is mediated allosterically by a communication between the Na(+) site and regions involved in substrate recognition. Using a panel of 78 Ala mutants of thrombin, we have mapped the allosteric core of residues that are energetically linked to Na(+) binding. These residues are Asp-189, Glu-217, Asp-222, and Tyr-225, all in close proximity to the bound Na(+). Among these residues, Asp-189 shares with Asp-221 the important function of transducing Na(+) binding into enhanced catalytic activity. None of the residues of exosite I, exosite II, or the 60-loop plays a significant role in Na(+) binding and allosteric transduction. X-ray crystal structures of the Na(+)-free (slow) and Na(+)-bound (fast) forms of thrombin, free or bound to the active site inhibitor H-d-Phe-Pro-Arg-chloromethyl-ketone, document the conformational changes induced by Na(+) binding. The slow --> fast transition results in formation of the Arg-187:Asp-222 ion pair, optimal orientation of Asp-189 and Ser-195 for substrate binding, and a significant shift of the side chain of Glu-192 linked to a rearrangement of the network of water molecules that connect the bound Na(+) to Ser-195 in the active site. The changes in the water network and the allosteric core explain the thermodynamic signatures linked to Na(+) binding and the mechanism of thrombin activation by Na(+). The role of the water network uncovered in this study establishes a new paradigm for the allosteric regulation of thrombin and other Na(+)-activated enzymes involved in blood coagulation and the immune response.

Probing the hirudin-thrombin interaction by incorporation of noncoded amino acids and molecular dynamics simulation

Thrombin is a primary target for the development of novel anticoagulants, since it plays two important and opposite roles in hemostasis: procoagulant and anticoagulant. All thrombin functions are influenced by Na+ binding, which triggers the transition of this enzyme from an anticoagulant (slow) form to a procoagulant (fast) form. In previous studies, we have conveniently produced by chemical synthesis analogues of the N-terminal fragment 1-47 of hirudin HM2 containing noncoded amino acids and displaying up to approximately 2700-fold more potent antithrombin activity, comparable to that of full-length hirudin. In the work presented here, we have exploited the versatility of chemical synthesis to probe the structural and energetic properties of the S3 site of thrombin through perturbations introduced in the structure of hirudin fragment 1-47. In particular, we have investigated the effects of systematic replacement of Tyr3 with noncoded amino acids retaining the aromatic nucleus of Tyr, as well as similar hydrophobic and steric properties, but possessing different electronic (e.g., p-fluoro-, p-iodo-, or p-nitro-Phe), charge (p-aminomethyl-Phe), or conformational (homo-Phe) properties. Our results indicate that the affinity of fragment 1-47 for thrombin is proportional to the desolvation free energy change upon complex formation, and is inversely related to the electric dipole moment of the amino acid side chain at position 3 of hirudin. In this study, we have also identified the key features that are responsible for the preferential binding of hirudin to the procoagulant (fast) form of thrombin. Strikingly, shaving at position 3, by Tyr --> Ala exchange, abolishes the differences in the affinity for thrombin allosteric forms, whereas a bulkier side chain (e.g., beta-naphthylalanine) improves binding preferentially to the fast form. These results provide strong, albeit indirect, evidence that the procoagulant (fast) form of thrombin is in a more open and accessible conformation with respect to the less forgiving structure it acquires in the slow form. This view is also supported by the results of molecular dynamics simulations conducted for 18 ns on free thrombin in full explicit water, showing that after approximately 5 ns thrombin undergoes a significant conformational transition, from a more open conformation (which we propose can be related to the fast form) to a more compact and closed one (which we propose can be related to the slow form). This transition mainly involves the Trp148 and Trp60D loop, the S3 site, and the fibrinogen binding site, whereas the S1 site, the Na+-binding site, and the catalytic pocket remain essentially unchanged. In particular, our data indicate that the S3 site of the enzyme is less accessible to water in the putative slow form. This structural picture provides a reasonable molecular explanation for the fact that physiological substrates related to the procoagulant activity of thrombin (fibrinogen, thrombin receptor 1, and factor XIII) orient a bulky side chain into the S3 site of the enzyme. Taken together, our results can have important implications for the design of novel thrombin inhibitors, of practical utility in the treatment of coagulative disorders.

Pathological haemostasis and "prothrombotic state" in Behçet's disease

Behçet's disease (BD) is a widespread occlusive-type vasculitis with life-threatening manifestations. The vasculopathy of BD is unique and any type of vessel can be involved. Moreover, vascular lesions in BD represent an occlusive nature suggesting a hypercoagulable/prothrombotic state. The data concerning the genetic defects of the coagulation cascade are expanding. There is evidence of universal activation of haemostatic system in BD. Procoagulant markers of thrombosis are elevated reflecting intravenous excessive thrombin formation. Defective fibrinolysis with impaired fibrinolytic kinetics may have a role in the hypercoagulable/prothrombotic state of BD. Endothelial cell injury and/or pathological activation is well documented in BD. The aim of this paper is to review current literature knowledge and our experience regarding the unresolved complicated issues of genetic thrombotic defects, in vivo haemostatic markers, coagulation inhibitors, impaired fibrinolysis, and endothelial injury/dysfunction of the hypercoagulable/prothrombotic state of BD. The clinical aspects of vascular thrombosis, the genetic basis of coagulation, coagulation inhibitors, fibrinolysis inhibitors, and endothelial dysfunction are reviewed. Challenges and future prospects regarding the prothrombotic state of BD are discussed together with new promising antithrombotic and antiplatelet treatment strategies. Better understanding of the exact pathogenesis of the hypercoagulable/prothrombotic state of this disease may help to develop novel therapeutic approaches offering a better outcome for Behçet's patients with thrombosis.

New targets for antithrombotic drugs

The normal hemostatic process is initiated by disruption in the vascular continuity and exposure of the subendothelial components. Platelets adhere to subendothelium-bound von Willebrand factor via glycoprotein (GP) Ib complex. This initial interaction per se and the release of platelet agonists transduce signals that lead to the rise in intracellular Ca(2+). The rise in Ca(2+) induces shape change, prostaglandin synthesis, release of granular contents and conformational changes in platelet Gp IIb-IIIa. Gp IIb-IIIa in activated platelets becomes competent to bind fibrinogen and other adhesive proteins and mediates platelet cohesion (primary hemostatic plug). Furthermore, the activated platelet surface provides an efficient catalytic surface for the coagulation reactions, ultimately resulting in the formation of fibrin (secondary hemostasis). Normally the hemostatic process plays a delicate balance between keeping the blood in the fluid state to maintain flow and rapidly forming an occluding plug following vessel injury. Thrombosis occurs because of alteration in this delicate balance. Consequences of thrombosis are a major cause of morbidity and mortality in industrialized countries. Arterial thrombosis occurs in the setting of previous vessel wall injury mostly because of atherosclerosis, while venous thrombosis occurs in areas of stasis. The recent advances in our understanding of the hemostatic process have led to a better elucidation of the mechanism of action of many antithrombotic drugs and identification of new targets for drug development. The molecular target of the well known antiplatelet drug ticlopidine has been identified. Large numbers of IIb-IIIa inhibitors have been developed based on the crystal structure of a potent antagonist echistatin. The mechanism of action of heparin has been defined at the molecular level. As a result a synthetic pentasaccharide, based on antithrombin-binding domain of heparin, has been developed and tested successfully in clinical trials. New generation direct thrombin inhibitors are being developed based on the crystal structure of thrombin. Factor Xa has a critical position at the convergence of intrinsic and extrinsic pathway ways. The clinical tolerability and the efficacy of low molecular weight heparins led to the concept that inhibition of further thrombin generation, by blocking factor Xa alone, can be an effective way of preventing thrombus growth without inactivating thrombin. A large number of specific factor Xa inhibitors are under development. Some of these drugs have already undergone preliminary clinical trials and appear to be promising. Future clinical trials will determine whether these new drugs will provide better risk-benefit ratio in treatment of thrombotic disorders.

New anticoagulant drugs

Arterial and venous thrombosis are major causes of morbidity and mortality. Arterial thrombosis is the most common cause of myocardial infarction, stroke, and limb gangrene. Venous thrombosis leads to potentially fatal pulmonary embolism and to post-phlebitic syndrome. Because arterial thrombi consist of platelet aggregates held together by small amounts of fibrin, strategies to inhibit arterial thrombogenesis focus mainly on blocking platelet function, but often include anticoagulants to prevent fibrin deposition. Anticoagulants are used for the prevention and treatment of venous thrombosis because venous thrombi consist mainly of fibrin and red blood cells. This paper (1) reviews arterial and venous thrombogenesis, (2) outlines new anticoagulant strategies, and (3) provides clinical perspectives on these new strategies.

Dynamics of spatially nonuniform patterning in the model of blood coagulation

We propose a reaction-diffusion model that describes in detail the cascade of molecular events during blood coagulation. In a reduced form, this model contains three equations in three variables, two of which are self-accelerated. One of these variables, an activator, behaves in a threshold manner. An inhibitor is also produced autocatalytically, but there is no inhibitor threshold, because it is generated only in the presence of the activator. All model variables are set to have equal diffusion coefficients. The model has a stable stationary trivial state, which is spatially uniform and an excitation threshold. A pulse of excitation runs from the point where the excitation threshold has been exceeded. The regime of its propagation depends on the model parameters. In a one-dimensional problem, the pulse either stops running at a certain distance from the excitation point, or it reaches the boundaries as an autowave. However, there is a parameter range where the pulse does not disappear after stopping and exists stationarily. The resulting steady-state profiles of the model variables are symmetrical relative to the center of the structure formed. (c) 2001 American Institute of Physics.

Regulation of blood coagulation

The protein C anticoagulant pathway converts the coagulation signal generated by thrombin into an anticoagulant response through the activation of protein C by the thrombin-thrombomodulin (TM) complex. The activated protein C (APC) thus formed interacts with protein S to inactivate two critical coagulation cofactors, factors Va and VIIIa, thereby dampening further thrombin generation. The proposed mechanisms by which TM switches the specificity of thrombin include conformational changes in thrombin, blocking access of normal substrates to thrombin and providing a binding site for protein C. The function of protein S appears to be to alter the cleavage site preferences of APC in factor Va, probably by changing the distance of the active site of APC relative to the membrane surface. The clinical relevance of this pathway is now established through the identification of deficient individuals with severe thrombotic complications and through the analysis of families with partial deficiencies in these components and an increased thrombotic tendency. One possible reason that even partial deficiencies are a thrombotic risk is that the function of the pathway can be down-regulated by inflammatory mediators. For instance, clinical studies have shown that the extent to which protein C levels decrease in patients with septic shock is predictive of a negative outcome. Initial clinical studies suggest that supplementation with protein C may be useful in the treatment of acute inflammatory diseases such as sepsis.

Advances in Therapy and the Management of Antithrombotic Drugs for Venous Thromboembolism

This review focuses on antithrombotic therapy for venous thromboembolism and covers a diverse range of topics including a discussion of emerging anticoagulant drugs, a renewed focus on thrombolytic agents for selected patients, and an analysis of the factors leading to adverse events in patients on warfarin, and how to optimize therapy. In Section I Dr. Weitz discusses new anticoagulant drugs focusing on those that are in the advanced stages of development. These will include drugs that (a) target factor VIIa/tissue factor, including tissue factor pathway inhibitor and NAPc2; (b) block factor Xa, including the synthetic pentasaccharide and DX9065a; (c) inhibit factors Va and VIIIa, i.e., activated protein C; and (d) block thrombin, including hirudin, argatroban, bivalirudin and H376/95. Oral formulations of heparin will also be reviewed.

In Section II, Dr. Comerota will discuss the use of thrombolysis for selected patients with venous thromboembolism. Fibrinolytic therapy, which has suffered from a high risk/benefit ratio for routine deep venous thrombosis, may have an important role to play in patients with iliofemoral venous thrombosis. Dr. Comerota presents his own results with catheter-directed thrombolytic therapy and the results from a large national registry showing long-term outcomes and the impact on quality of life.

In Section III, Dr. Ansell presents a critical analysis of the factors responsible for adverse events with oral anticoagulants and the optimum means of improving outcomes. The poor status of present day anticoagulant management is reviewed and the importance of achieving a high rate of “time in therapeutic range,” is emphasized. Models of care to optimize outcomes are described, with an emphasis on models that utilize patient self-testing and patient self-management of oral anticoagulation which are considered to be the ultimate in anticoagulation care. The treatment of venous and arterial thromboembolism is undergoing rapid change with respect to the development of new antithrombotic agents, an expanding list of new indications, and new methods of drug delivery and management. In spite of these changes, many of the traditional therapeutics are still with us and continue to play a vital role in the treatment of thromboembolic disease. The following discussion touches on a wide range of therapeutic interventions, from old to new, exploring the status of anticoagulant drug development, describing a new intervention for iliofemoral venous thrombosis, and analyzing the critical factors for safe and effective therapy with oral anticoagulants.

Advances in Therapy and the Management of Antithrombotic Drugs for Venous Thromboembolism

This review focuses on antithrombotic therapy for venous thromboembolism and covers a diverse range of topics including a discussion of emerging anticoagulant drugs, a renewed focus on thrombolytic agents for selected patients, and an analysis of the factors leading to adverse events in patients on warfarin, and how to optimize therapy. In Section I Dr. Weitz discusses new anticoagulant drugs focusing on those that are in the advanced stages of development. These will include drugs that (a) target factor VIIa/tissue factor, including tissue factor pathway inhibitor and NAPc2; (b) block factor Xa, including the synthetic pentasaccharide and DX9065a; (c) inhibit factors Va and VIIIa, i.e., activated protein C; and (d) block thrombin, including hirudin, argatroban, bivalirudin and H376/95. Oral formulations of heparin will also be reviewed.

In Section II, Dr. Comerota will discuss the use of thrombolysis for selected patients with venous thromboembolism. Fibrinolytic therapy, which has suffered from a high risk/benefit ratio for routine deep venous thrombosis, may have an important role to play in patients with iliofemoral venous thrombosis. Dr. Comerota presents his own results with catheter-directed thrombolytic therapy and the results from a large national registry showing long-term outcomes and the impact on quality of life.

In Section III, Dr. Ansell presents a critical analysis of the factors responsible for adverse events with oral anticoagulants and the optimum means of improving outcomes. The poor status of present day anticoagulant management is reviewed and the importance of achieving a high rate of “time in therapeutic range,” is emphasized. Models of care to optimize outcomes are described, with an emphasis on models that utilize patient self-testing and patient self-management of oral anticoagulation which are considered to be the ultimate in anticoagulation care. The treatment of venous and arterial thromboembolism is undergoing rapid change with respect to the development of new antithrombotic agents, an expanding list of new indications, and new methods of drug delivery and management. In spite of these changes, many of the traditional therapeutics are still with us and continue to play a vital role in the treatment of thromboembolic disease. The following discussion touches on a wide range of therapeutic interventions, from old to new, exploring the status of anticoagulant drug development, describing a new intervention for iliofemoral venous thrombosis, and analyzing the critical factors for safe and effective therapy with oral anticoagulants.

Dynamics of clot growth induced by thrombin diffusing into nonstirred citrate human plasma

The dynamics of clot formation was studied in a two-compartment chamber designed to allow free diffusion of thrombin according to its concentration gradient into nonstirred citrate plasma or fibrinogen solution. Fibrin clots in fibrinogen solutions increased progressively until the substrate was depleted. In plasma, the clot weight dynamics significantly depended on the concentration of thrombin in the thrombin compartment. When the thrombin concentrations were extremely low (25-40 nM), the clot weight increased throughout the experiment (sometimes 20-24 h). At higher thrombin concentrations, the clot weight increased for 1-2 h and then stopped growing for the following 3-4 h. The clot weight observed at the plateau varied only slightly in the range of thrombin concentrations of 50-770 nM. In this range, high thrombin concentrations (250-770 nM) caused a second increase in the clot weight 4-8 h after the start of diffusion, which was followed by the second plateau in the curve of clot weight against time. The time to the plateau and the plateau duration decreased with increasing thrombin concentrations. The abundant plasma inhibitors of thrombin cannot account for these results. It was hypothesized that an as yet unknown mechanism is responsible for the inhibition of clot growth.

Anticoagulant Thrombins

Thrombin plays both procoagulant and anticoagulant roles in the blood coagulation cascade. This dual role is influenced allosterically by the binding of Na(+) near the primary specificity pocket of the enzyme. Recent findings demonstrate that it is possible to engineer recombinant thrombins that have practically lost anticoagulant activity but retain their anticoagulant properties. These anticoagulant thrombins bear substitutions in or around the Na(+) binding site, provide important clues on structure-function relations, and offer a promising alternative to current anticoagulant therapies. In addition, they demonstrate the importance of Na(+) as a coagulation factor and broaden our understanding of the function and regulation of all vitamin K-dependent clotting enzymes.

Synthesis and characterization of more potent analogues of hirudin fragment 1-47 containing non-natural amino acids

Hirudin is the most potent and specific inhibitor of thrombin, a key enzyme in the coagulation process existing in equilibrium between its procoagulant (fast) and anticoagulant (slow) form. In a previous study, we described the solid-phase synthesis of a Trp3 analogue of fragment 1-47 of hirudin HM2, which displayed approximately 5-fold higher thrombin inhibitory potency relative to that of the natural product [De Filippis, V., et al. (1995) Biochemistry 34, 9552-9564]. By combining automated and manual peptide synthesis, here we have produced in high yields seven analogues of fragment 1-47 containing natural and non-natural amino acids. In particular, we have replaced Val1 with tert-butylglycine (tBug), Ser2 with Arg, and Tyr3 with Phe, cyclohexylalanine (Cha), Trp, alpha-naphthylalanine (alphaNal), and beta-naphthylalanine (betaNal). The crude reduced peptides are able to fold almost quantitatively into the disulfide-cross-linked species, whose unique alignment (Cys6-Cys14, Cys16-Cys28, and Cys22-Cys37) has been shown to be identical to that of the natural fragment. The results of conformational characterization provide evidence that synthetic peptides retain the structural features of the natural species, whereas thrombin inhibition data indicate that the synthetic analogues are all more potent inhibitors of thrombin. In particular, Val --> tBug exchange leads to a 3-fold increase in binding, interpreted as arising from a favorable reduction of the entropy of binding, due to the presence of the more symmetric side chain of tBug relative to that of Val. The S2R analogue binds 24- and 125-fold more tightly than the natural fragment to the fast or slow form of thrombin. These results are explained by considering that Arg2 may favorably couple to Glu192, a key residue involved in the slow to fast transition, thus stabilizing the slow form. Replacement of Tyr3 with more hydrophobic residues having different side chain orientations and electronic structures improves binding by 2-40-fold, suggesting that nonpolar interactions and shape-dependent packing effects strongly influence binding at this position. Overall, these results provide new insights for elucidating the mechanism of hirudin-thrombin recognition at the molecular level and highlight new strategies for designing more potent and selective inhibitors of thrombin.

Selective loss of fibrinogen clotting in a loop-less thrombin

The autolysis loop of thrombin comprises nine residues, from Glu146 to Lys149e, five of which (Ala149a-Lys149e) are inserted relative to trypsin and chymotrypsin. Deletion of the insertion Ala149a-Lys149e causes no significant change in the properties of the enzyme, except for a slight enhancement of protein C activation. Deletion of the entire Glu146-Lys149e loop, however, reduces fibrinogen clotting 240-fold, but decreases protein C activation only 2-fold. This loop-less mutant is de facto an exclusive activator of protein C, having lost the primary procoagulant function of thrombin. Because the autolysis loop affects fibrinogen binding, but not protein C activation, it provides a target for new drugs designed to suppress exclusively the procoagulant activity of thrombin.

Energetics of thrombin-thrombomodulin interaction

Temperature and salt dependence studies of thrombin interaction with thrombomodulin, with and without chondroitin sulfate, and two fragments containing the EGF-like domains 4-5 and 4-5-6 reveal the energetic signatures and the mechanism of recognition of this physiologically important cofactor. Binding of thrombomodulin is affected drastically by the particular salt present in solution and is positively linked to Na+ binding to thrombin and the conversion of the enzyme from the slow to the fast form, but is opposed by Cl- binding to the fibrinogen recognition site and especially to the heparin binding site. Binding of thrombomodulin has an unusually large salt dependence (gamma(salt) = -4.8) contributed mostly by the polyelectrolyte-like nature of the chondroitin sulfate moiety that binds to the heparin binding site and increases the affinity of the cofactor by almost 10-fold. On the other hand, the chondroitin sulfate has no effect on the deltaCp of binding, which is determined predominantly by contacts made by the EGF-like domains 5 and 6 with the fibrinogen recognition site. The modest heat capacity change (-0.2 kcal mol(-1) K(-1)) observed when thrombomodulin binds to the fast form suggests a rigid-body association of the cofactor with the enzyme. In the slow form, however, the heat capacity change is significantly more pronounced (-0.5 kcal mol(-1) K(-1)) and signals the presence of a conformational transition of the enzyme linked to binding of the cofactor that mimics the slow-->fast conversion. These results demonstrate that recognition of thrombomodulin by thrombin is steered electrostatically by the highly charged regions of the fibrinogen recognition site and the heparin binding site, to which the chondroitin sulfate moiety binds and enhances the affinity of the interaction. The recognition event also involves conformational changes of the enzyme in the slow form mediated by binding of the EGF-like domains 5-6 to the fibrinogen recognition site. Consistent with this model, binding of thrombomodulin to the fast form has only a small effect on the hydrolysis of nine chromogenic substrates carrying substitutions at P1, P2, and P3 aimed at probing the environment of the specificity sites S1, S2, and S3 of the enzyme. Binding to the slow form, on the other hand, enhances the specificity toward all substrates up to 15-fold. For substrates carrying a Gly at P2, binding of thrombomodulin changes the relative specificity of the slow and fast forms and makes the slow form more specific. Interestingly, these effects are not specific of thrombomodulin and depend solely on binding to the fibrinogen recognition site of the enzyme. In fact, they are also observed with the hirudin C-terminal fragment 55-65. The characterization of the mechanism of thrombin-thrombomodulin interaction and the effects of the cofactor on the hydrolysis of chromogenic substrates probing the interior of the catalytic pocket bear on the thrombomodulin-induced enhancement of protein C cleavage by thrombin. We propose that this enhancement is due predominantly to an effect of thrombomodulin on the bound protein C in the ternary complex. Therefore, thrombomodulin would carry out its physiological function by making protein C a better substrate for thrombin, rather than making thrombin a better enzyme for protein C.

Rational engineering of activity and specificity in a serine protease

The discovery of the Na(+)-dependent allosteric regulation in serine proteases makes it possible to control catalytic activity and specificity in this class of enzymes in a way never considered before. We demonstrate that rational site-directed mutagenesis of residues controlling Na+ binding can profoundly after the properties of a serine protease. By suppressing Na+ binding to thrombin, we shift the balance between procoagulant and anticoagulant activities of the enzyme. Those mutants, compared to wild-type, have reduced specificity toward fibrinogen, but enhanced or slightly reduced specificity toward protein C. Because this engineering strategy targets a fundamental regulatory mechanism, it is amenable of extension to other enzymes of biological and pharmacological importance.

CVS-1123, a direct thrombin inhibitor, prevents occlusive arterial and venous thrombosis in a canine model of vascular injury

CVS-1123, low-molecular-weight, direct thrombin inhibitor was studied in an anesthetized canine model of arterial and venous thrombosis to determine whether thrombin inhibition could reduce the incidence of occlusive thrombosis in response to vessel-wall injury. The left carotid artery (LCA) and right jugular vein (RJV) were instrumented with a flow probe, intraluminal electrode, and critical stenosis. Either saline (n = 9), or CVS-1123 (n = 12) was administered in a loading dose of 2 mg/kg i.v., followed by an infusion (2.46 mg/kg/h for 180 min). Vessel-wall injury was initiated by applying a 300-microA anodal current to the intimal surface of the LCA and RJV. Platelet aggregation in response to gamma-thrombin remained inhibited by CVS-1123 for 8 h. The activated partial thromboplastin time (aPTT) was increased and remained elevated for the duration of the protocol. The prothrombin time (PT) showed an initial increase and then a rapid decrease after the infusion was discontinued. There was a twofold increase in the bleeding time (BT) at 2 h. The time to occlusion of the LCA was prolonged (380 +/- 22 min in the CVS-1123 group vs. 152 +/- 18 min in the saline group) with seven of 12 patent arteries at 8 h. Similarly, the time to occlusion for RJV was prolonged (415 +/- 16 min in the CVS-1123 group vs. 99 +/- 8 min in the saline group) with eight of 12 veins remaining patent at 8 h. CVS-1123 administration was associated with a decrease in the thrombus weights in both the LCA and RJV as compared with the saline-treated animals. In summary, CVS-1123 modifies the thrombogenic response to deep vessel-wall injury in both the arterial and venous circulations. The results suggest that CVS-1123 is an effective antithrombin and may offer a therapeutic alternative to current antithrombins in the management of arterial and venous thrombosis.