In vitro effects of inogatran, a selective low molecular weight thrombin inhibitor

Late-Stage Modification of Oligopeptides by Nickel-Catalyzed Stereoselective Radical Addition to Dehydroalanine

Radical addition to dehydroalanine (Dha) represents an appealing, modular strategy to access non-canonical peptide analogues for drug discovery. Prior studies on radical addition to the Dha residue of peptides and proteins have demonstrated outstanding functional group compatibility, but the lack of stereoselectivity has limited the synthetic utility of this approach. Herein, we address this challenge by employing chiral nickel catalysts to control the stereoselectivity of radical addition to Dha on oligopeptides. The conditions accommodate a variety of primary and secondary electrophiles to introduce polyethylene glycol, biotin, halo-tag, and hydrophobic and hydrophilic side chains to the peptide. The reaction features catalyst control to largely override substrate-based control of stereochemical outcome for modification of short peptides. We anticipate that the discovery of chiral nickel complexes that confer catalyst control will allow rapid, late-stage modification of peptides featuring nonnatural sidechains.

Inhibitors of blood coagulation factor XIII

The blood coagulation factor XIII (FXIII) plays an essential role in the stabilization of fibrin clots. This factor, belonging to the class of transglutaminases, catalyzes the final step of secondary hemostasis, i.e. the crosslinking of fibrin polymers. These crosslinks protect the clots against premature fibrinolysis. Consequently, FXIII is an interesting target for the therapeutic treatment of cardiovascular diseases. In this context, inhibitors can influence FXIII in the activation process of the enzyme itself or in its catalytic activity. To date, there is no FXIII inhibitor in medical application, but several studies have been conducted in the past. These studies provided a better understanding of FXIII and identified new lead structures for FXIII inhibitors. Next to small molecule inhibitors, the most promising candidates for the development of clinically applicable FXIII inhibitors are the peptide inhibitors tridegin and transglutaminase-inhibiting Michael acceptors (TIMAs) due to their selectivity towards activated FXIII (FXIIIa). In this review, select FXIII inhibitors and their pharmacological potential are discussed.

Carboxypeptidase U (TAFIa) prevents lysis from proceeding into the propagation phase through a threshold-dependent mechanism

In an in vitro clot lysis model in human plasma, carboxypeptidase U (CPU) is generated by thrombin following the coagulation and by plasmin at the later stage of clot lysis. CPU is able to slow down clot lysis by suppressing the cofactor activity of partially degraded fibrin in the plasminogen activation by tissue-type plasminogen activator (t-PA). Making use of thrombomodulin and a thrombin inhibitor, the generation of CPU during the in vitro clot lysis can be manipulated both in terms of magnitude and time course. The data obtained demonstrate that CPU affects the clot dissolution through a threshold-dependent mechanism: as long as the CPU activity remains above the threshold value, lysis is prevented from proceeding into the propagation phase. From the moment the CPU activity drops below this threshold value, the rate of lysis accelerates. This threshold value for CPU activity is dictated by the t-PA concentration: increasing the t-PA concentration increases the CPU threshold and vice versa. This implies that the effect of the CPU pathway will become more apparent at a lower fibrinolytic capacity. Our threshold-based hypothesis indicates that the time course of proCPU activation, the stability of CPU and the t-PA concentration all play a crucial role in determining the result of the in vitro clot lysis experiment. Furthermore, this hypothesis provides us with new insights into previously published data on the effects of CPU on in vitro clot lysis by high and low t-PA concentrations.

Novel non-covalent thrombin inhibitors incorporating P1 4,5,6,7-tetrahydrobenzothiazole arginine side chain mimetics

The design, synthesis and biological activity of a series of novel non-covalent D-Phe-Pro-Arg motif-based thrombin inhibitors incorporating 4,5,6,7-tetrahydrobenzothiazol-2-amine as a novel arginine surrogate are described. Compound 9, the most potent in the series of thrombin inhibitors, exhibited an in vitro K(i) of 128 nM and 342-fold selectivity against trypsin. The binding mode of this novel class of thrombin inhibitors in the enzyme active site, based on the X-ray crystal structure of compound 9 co-crystallized with human alpha-thrombin, is discussed.

The pharmacodynamics and pharmacokinetics of the oral direct thrombin inhibitor ximelagatran and its active metabolite melagatran: a mini-review

Ximelagatran (Exanta, AstraZeneca) is a novel, oral direct thrombin inhibitor (oral DTI) that is rapidly converted to melagatran, its active form, following absorption. Melagatran has been shown to be a potent, rapidly binding, competitive inhibitor of human alpha-thrombin that inhibits both thrombin activity and generation. Melagatran also effectively inhibits both free and clot-bound thrombin. Melagatran has a wide therapeutic interval that enables it to be administered safely across a wide range of doses with no increased risk of bleeding, in contrast with warfarin whose narrow therapeutic window necessitates monitoring of its pharmacodynamic effect. Although melagatran has all the pharmacodynamic properties required of a new antithrombotic agent, low oral bioavailability that is even further reduced by the concomitant intake of food precludes its development as an oral agent. It was this that propelled the development of its prodrug, ximelagatran, which is 170 times more lipophilic than melagatran and uncharged at intestinal pH. Ximelagatran is therefore much better than melagatran at penetrating the gastrointestinal barrier and, as a consequence, has sufficient bioavailability (20%) for oral administration. Moreover, its pharmacokinetic properties following oral administration are stable and reproducible, with no food interactions and a low potential for drug-drug interactions. These properties allow ximelagatran to be administered twice daily according to a fixed dose regimen without coagulation monitoring. As a consequence of its favourable pharmacokinetic and pharmacodynamic properties, ximelagatran is currently undergoing full-scale clinical development for the prophylaxis and treatment of thromboembolic disorders.

Direct thrombin inhibitors

This review deals with a newly-developed category of antithrombotic drugs - the direct thrombin inhibitors. These agents interact with thrombin and block its catalytic activity on fibrinogen, platelets and other substrates. Heparin and its derivatives (low molecular weight heparins and the active pentasaccharide) inhibit thrombin and/or other coagulation serine proteases indirectly via antithrombin, and the warfarin-type drugs interfere with the synthesis of the precursors of the coagulation serine proteases. The direct thrombin inhibitors approved for clinical use at present (lepirudin, desirudin, bivalirudin, argatroban) and another in the advanced clinical testing stage (melagatran/ximelagatran), are the subject of this review. The chemical structure; kinetics of thrombin inhibition; pharmacokinetics and clinical use of each of these is discussed.

Thrombin-induced platelet activation and its inhibition by anticoagulants with different modes of action

Thrombin-induced platelet activation involves cleavage of protease-activated receptors (PARs) 1 and 4, and interaction, via glycoprotein (Gp)Ibalpha, with the platelet GpIb/IX/V complex. This study investigated inhibition of platelet activation by thrombin inhibitors with different modes of action: two reversible direct thrombin inhibitors, melagatran and inogatran; hirudin, a tightly binding direct thrombin inhibitor; and two indirect thrombin inhibitors, heparin and dalteparin. Up-regulation of P-selectin (CD62P) and PAR-1 cleavage was measured in human whole blood, by flow cytometry. The thrombin concentration that induced 50% of maximum (EC50 ) PAR-1 cleavage was 0.028 nmol/l, while that of platelet activation (CD62P) was over two-fold higher (0.64 nmol/l). The EC50 of a PAR-1-independent component, defined as a further activating effect of thrombin on top of the maximum PAR-1-activating peptide (AP) effect, was 3.2 nmol/l. All anticoagulants were concentration-dependent inhibitors of thrombin-induced platelet activation and PAR-1 cleavage, but none inhibited PAR-1-AP or PAR-4-AP induced activation. Melagatran and inogatran were more potent inhibitors of CD62P up-regulation than of PAR-1 cleavage; conversely, hirudin, heparin and dalteparin were more potent inhibitors of PAR-1 cleavage.Thus, reversible direct thrombin inhibitors, such as melagatran, are potent inhibitors of thrombin-induced platelet activation, acting mainly by inhibition of a PAR-1-independent component.

Synthesis of potential thrombin inhibitors. Incorporation of tartaric acid templates as P2 proline mimetics

With the objective to prepare novel non-peptidic thrombin inhibitors, bioisosteres of the inhibitory tripeptide D-Phe-Pro-Arg chain have been examined. Thus, the P1 Arg was replaced with p-amidinobenzylamine, an elongated homologue of the same and with 2,5-dichloro benzylamine. The P2-P3, D-Phe-Pro, was replaced with a novel tartaric acid template coupled to a series of readily available, mainly lipophilic, amines. Some of these compounds exhibit promising thrombin inhibition activity in vitro, IC(50 ) approximately 5.9 microM.

Effect of different types of thrombin inhibitors on thrombin/thrombomodulin modulated activation of protein C in vitro

The objectives of this study were to investigate whether the affinity of thrombin for small-molecule, active site-directed thrombin inhibitors and substrates is affected by the presence of thrombomodulin (TM), and to what extent thrombin inhibitors inhibit TM-bound thrombin. Inhibition of human alpha-thrombin was studied in the presence and absence of solubilised rabbit lung TM in a buffer containing CaCl(2). TM inhibited thrombin-induced proteolysis of human fibrinogen with a dissociation constant (K(D)) of 4 nmol/l. With at least 16-fold molar excess of TM over thrombin the affinity of thrombin both for the small thrombin substrates (S-2366 and S-2238) and the reversible, active site-directed thrombin inhibitors (inogatran and melagatran) increased twofold. In contrast, the ability of hirudin to inhibit thrombin was reduced by TM, since hirudin competes with TM in binding to thrombin. The effect of thrombin inhibitors on protein C activation by thrombin bound to human kidney cells transfected with cDNA for human TM was also studied. The mean binding capacity of the transfected cells was approximately 320,000 quantified by flow cytometry with antibodies against TM. Hirudin, inogatran and melagatran inhibited the activation of protein C by thrombin complexed with cell-bound TM in a dose-dependent manner, with mean IC(50) values+/-S.D. of 4.4+/-0.8, 20.0+/-1.1 and 6.4+/-0.2 nmol/l, respectively. Antithrombin inhibited protein C activation with an IC(50) value of 290+/-10 nmol/l, which was enhanced fourfold (IC(50) 60 nmol/l) by the addition of heparin 0.5 U/ml. Heparin alone, up to a concentration of 1 U/ml, had no effect on the activation of protein C. Small direct thrombin inhibitors thus inhibited both free and TM-bound thrombin and therefore also inhibited the activation of protein C. Whether this will influence their clinical efficacy or safety versus heparin and warfarin, which also inhibit protein activation, respectively, lowers the concentration of protein C, remains to be studied in clinical trials.

Anticoagulation: the present and future

Thrombin is a central bioregulator of coagulation and is therefore a key target in the therapeutic prevention and treatment of thromboembolic disorders, including deep vein thrombosis and pulmonary embolism. The current mainstays of anticoagulation treatment are heparins, which are indirect thrombin inhibitors, and coumarins, such as warfarin, which modulate the synthesis of vitamin K-dependent proteins. Although efficacious and widely used, heparins and coumarins have limitations because their pharmacokinetics and anticoagulant effects are unpredictable, with the risk of bleeding and other complications resulting in the need for close monitoring with their use. Low-molecular-weight heparins (LMWHs) provide a more predictable anticoagulant response, but their use is limited by the need for subcutaneous administration. In addition, discontinuation of heparin treatment can result in a thrombotic rebound due to the inability of these compounds to inhibit clot-bound thrombin. Direct thrombin inhibitors (DTI) are able to target both free and clot-bound thrombin. The first to be used was hirudin, but DTIs with lower molecular weights, such as DuP 714, PPACK, and efegatran, have subsequently been developed, and these agents are better able to inhibit clot-bound thrombin and the thrombotic processes that take place at sites of arterial damage. Such compounds inhibit thrombin by covalently binding to it, but this can result in toxicity and nonspecific binding. The development of reversible noncovalent DTIs, such as inogatran and melagatran, has resulted in safer, more specific and predictable anticoagulant treatment. Oral DTIs, such as ximelagatran, are set to provide a further breakthrough in the prophylaxis and treatment of thrombosis.

Coagulation activity and clinical outcome in unstable coronary artery disease

In the current study, we investigated molecular markers of coagulation activity, ie, prothrombin fragment 1+2 (F1+2), thrombin-antithrombin (TAT) complex, soluble fibrin (SF), and D-dimer, and their relation to death, myocardial infarction, and refractory angina during and after anticoagulant treatment in unstable coronary artery disease. Patients with unstable coronary artery disease (N=320) were randomized to a 72-hour infusion with either inogatran, a low-molecular-mass direct thrombin inhibitor, or unfractionated heparin. During the 30-day follow-up, a 40% lower event rate was seen in patients with high compared with low baseline levels of TAT or SF. High baseline levels of coagulation activity were correlated with a larger decrease during treatment. Patients with decreased compared with raised F1+2 or TAT levels after 6 hours of treatment had a 50% lower event rate at 30 days (F1+2, P=0.04; TAT, P=0.02). At the cessation of antithrombin treatment, there was a clustering of cardiac events that tended to be related to a rise in the levels of TAT and the other markers. During long-term follow-up (median, 29 months), there was a relation between higher baseline levels of D-dimer (P=0.003) and increased mortality. High baseline levels of molecular markers of coagulation activity might identify patients with a thrombotic condition (as the major cause of instability) who are good responders to anticoagulant therapy, with a larger decrease in coagulation activity during treatment and a decreased risk of ischemic events. However, this early benefit is lost during long-term follow-up when high baseline levels of coagulation activity are associated with a raised risk of early reactivation and increased mortality.

The effects of oral and intravenous direct thrombin inhibitors on the size of photochemically induced cortical infarction in rats

Oral thrombin inhibitors are under development as potential drugs for prophylaxis and treatment of thrombotic events. The effect of pretreatment with two direct thrombin inhibitors, melagatran and inogatran, was evaluated in a rat model of cerebral infarction. Ischaemic stroke was induced by photochemical reaction after an injection of Rose Bengal and focused posterior and to the right of the intersection of the coronal and sagittal sutures on the intact calvarium. A single oral dose of melagatran (30 micromol/kg) significantly reduced the volume of the cortical infarct by 53% (P<.05) compared with control. In addition, following intravenous inogatran (6 micromol/kg) or oral inogatran (100 micromol/kg), the volume of the cortical infarct decreased by 83% and 19%, respectively, compared with control. This study showed that experimental focal ischaemic infarction, elicited by photochemically induced endothelial cell damage, can be significantly reduced with melagatran and inogatran, direct thrombin inhibitors.

Population modelling of the effect of inogatran, at thrombin inhibitor, on ex vivo coagulation time (APTT) in healthy subjects and patients with coronary artery disease

AIMS

The purpose of this study was to characterize the relationship between the degree of anticoagulation, assessed by APTT, and the plasma concentration of inogatran in healthy subjects and in patients with coronary artery disease.

METHODS

Data from five phase I studies in 78 healthy males and two phase II multicentre studies in 948 patients of both sexes with unstable angina pectoris or non-Q-wave myocardial infarction were evaluated. A total of 3296 pairs of concentration-APTT samples were obtained before, during, and after intravenous infusions of inogatran. Mixed effects modelling was used for population pharmacodynamic analysis of the drug effect and for describing the variability in baseline APTT.

RESULTS

The population mean baseline APTT was 29 s, but large variations between individuals (s.d. 3.6 s) were observed. The variability between studies (1.3 s) and centres (1.8 s) were of less importance, though statistically significant. APTT increased in a nonlinear manner with increasing inogatran concentration and the relationship was well described by a combined linear and Emax model. A significant part of the overall variability could be ascribed to the APTT reagent and equipment used at the different study centres. These method-dependent differences were compensated for by including the lower limit of the normal reference range as a covariate, affecting both baseline and Emax, in the model. For the typical healthy subject and patient, the method-corrected population mean parameters were: APTTbaseline 35 and 31 s, slope 8.0 and 5.8 s x l x micromol(-1), Emax 36 and 34 s, and EC50 0.54 and 0.72 micromol x l(-1), respectively. The model predicted plasma concentration needed to double the APTT from the baseline value was 1.25 and 1.45 micromol x l(-1) in the healthy volunteer and patient, respectively.

CONCLUSIONS

The nonlinear relationship between APTT and inogatran concentration in plasma was well described by a combined linear and Emax model. Pooling of data was made possible by incorporating a centre-specific characteristic of the assay method in the model. Patients had lower baseline APTT and appeared to have less pronounced effect of inogatran than young healthy subjects.

In vitro and in vivo properties of bicyclic lactam inhibitors: a novel class of low molecular weight peptidomimetic thrombin inhibitors

We have developed potent and selective thrombin inhibitors with a novel non-peptidic structure. A bicyclic lactam was used as the scaffold on which various P1 and P3 motifs were substituted. Herein, we report the in vitro and in vivo properties of four representatives of this novel class of inhibitors. Their Ki values were less than 10 nM, they inhibited equally both free and clot-bound thrombin, and they displayed high level of specificity for thrombin over other serine proteases (trypsin, factor Xa, activated Protein C, and plasmin). They prolonged the clotting time of human plasma to twice the control value in coagulation assays (TT, APTT, and PT) at a concentration below 3 microM. Their anticoagulant activities using rat plasma were similar to, although slightly weaker, than with human plasma. Furthermore, they inhibited thrombin-induced platelet aggregation (human and rat) at concentrations close to their Ki values for thrombin. These molecules demonstrated similar dose response antithrombotic efficacy in rat arterial and venous thrombosis models when given as i.v. bolus followed by infusion. Antithrombotic efficacy of 85% and greater was observed at a dose of 5-7 microM/kg/hour in each model. Bicyclic lactam inhibitor 3, at a dose which caused a complete inhibition of visible thrombus formation in the venous and arterial models of thrombosis, showed a 1.9-2.1 and a 4.0-4.8-fold shift in APTT and TT, respectively. Unfortunately, the bicyclic lactam inhibitors exhibited low oral bioavailability in rats. Therefore, this novel class of bicyclic lactam thrombin inhibitor has the potential to be promising intravenous antithrombotic agents for the treatment of arterial as well as venous thrombosis and warrants further investigation.

New proline mimetics: synthesis of thrombin inhibitors incorporating cyclopentane- and cyclopentenedicarboxylic acid templates in the P2 position. Binding conformation investigated by X-ray crystallography

With the aim to prepare nonpeptidic thrombin inhibitors, the amino acids of the thrombin-inhibiting tripeptide chain D-Phe-Pro-Arg were replaced with isosteres. Arg was replaced with the more rigid P1 truncated p-amidinobenzylamine (Pab), Pro with either cyclopentane-1, 2-dicarboxylic acid or cyclopentene-1,5-dicarboxylic acid, and D-Phe with a series of readily available lipophilic amines. One of the most potent compounds (25, pIC(50) = 6.01) in these series was cocrystallized with thrombin where the X-ray crystal structure provide insight to the structure-activity relationship (SAR).

The design of potent, selective, non-covalent, peptide thrombin inhibitors utilizing imidazole as a S1 binding element

Modeling of neutral or mildly basic functional groups in the S1 site of thrombin led to the targeting of imidazole as a S1 binding element and correctly predicted the optimal chain length for connecting this group with the S2 and S3 binding elements. Derivatives of 4-(3-aminopropyl)-imidazole can be selective inhibitors of thrombin demonstrating potent anticoagulant activity.

Small, noncovalent serine protease inhibitors

Thrombin and factor Xa (fXa) are the only serine proteases for which small, potent, selective, noncovalent inhibitors have been developed, which are ultimately intended as drug development candidates (in this case as anticoagulants). Noncovalent inhibitors may be more selective and chemically and metabolically less reactive than covalent inhibitors. In addition, noncovalent inhibitors are more likely to have fast-binding kinetics which is particularly important in the development of thrombin inhibitors. TAME derived noncovalent thrombin inhibitors argatroban, napsagatran, and UK 156,406 have entered clinical trials as anticoagulants, the latter as an orally active agent. Serine trap deletion from substrate-like peptides led to the development of inogatran and melagatran, both of which have entered clinical trials as intravenous agents. The use of 3-aminopyridinone and pyrazinone acetamide peptidomimetic templates has resulted in the development of L-375,378 which has been chosen for clinical development as an orally active anticoagulant. Recently, compounds which do not have the conventional hydrogen bonding capabilities of peptides have begun to appear in the thrombin literature. Publications on noncovalent fXa inhibitors cover this type of peptidomimetic almost exclusively.

Novel acylguanidine containing thrombin inhibitors with reduced basicity at the P1 moiety

Replacement of the noragmatine group in thrombin inhibitors with a beta-alanyl-guanidine group resulted in a nearly equipotent and more selective compound 8 despite the fact that the pKa of this P1 moiety is five orders of magnitude lower. Further modification resulted in a nonpeptide inhibitor with this beta-alanyl-guanidine group, compound 28. This is an active and selective thrombin inhibitor and in view of its nonpeptide/low basicity structure selected for further pharmacological studies.

Effects of Intravenous Enoxaparin and Intravenous Inogatran in an Electrolytic Injury Model of Venous Thrombosis in the Dog

The objective of this study was to compare the effectiveness of the low molecular weight heparin, enoxaparin (Lovenox((R))), and inogatran (a direct thrombin inhibitor) in a canine electrolytic injury model of venous thrombosis. Effectiveness was defined as the ability of either drug to prolong the following parameters: activated partial thromboplastin time (aPTT), thrombin time (TT), prothrombin time (PT), and time to formation of an occlusive thrombus in the vein. There were 5 dogs and 10 vessels for each group (the right and left femoral veins were used in each dog to measure time to occlusion). Dogs were randomly assigned to one of six groups: (1) saline controls; (2) low-dose inogatran (0.075 mg/kg IV bolus followed by a 5 µg/kg/min infusion); (3) mid-dose inogatran (0.25 mg/kg IV bolus followed by a 20 µg/kg/min infusion); (4) high-dose inogatran (0.75 mg/kg IV bolus followed by a 50 µg/kg/min infusion); (5) low-dose enoxaparin (100 units/kg IV bolus followed by a 50 U/kg/h infusion); and (6) high-dose enoxaparin (200 U/kg IV bolus followed by a 100 U/kg/h infusion). Administration of inogatran resulted in dose-dependent increases in aPTT, TT, and PT, and administration of enoxaparin resulted in dose-dependent increases in aPTT and TT. There were no changes in hemodynamics. The time to occlusion in the control group averaged 81.7 +/- 9.9 minutes compared with 141.8 +/- 12.7, 185.8 +/- 17.6, and 226.9 +/- 8.8 minutes with the low, mid, and high doses of inogatran, and 131 +/- 20.3, and 183.0 +/- 19.0 minutes with the low and high doses of enoxaparin. Bleeding times were elevated by inogatran and enoxaparin, but no appreciable differences were detected between the two compounds. In summary, the direct thrombin inhibitor inogatran, administered intravenously, was as effective as the low molecular weight heparin enoxaparin in a canine model of venous thrombosis induced by electrolytic injury, supporting the conclusion that direct antithrombins may prove useful for prevention and treatment of deep venous thrombosis.

Direct thrombin inhibitors for treatment of arterial thrombosis: potential differences between bivalirudin and hirudin

Given the central role of thrombin in arterial thrombogenesis, most treatment strategies for acute coronary syndromes are aimed at inhibiting its generation or blocking its activity. Although heparin has been widely used, it has limitations in the setting of arterial thrombosis. These limitations reflect the inability of heparin to inactivate thrombin bound to fibrin, a major stimulus for thrombus growth. In addition, the anticoagulant response to heparin varies from patient to patient, and heparin is neutralized by platelet Factor IV, large quantities of which are released from platelets activated at sites of plaque rupture. Consequently, heparin requires careful laboratory monitoring to ensure an adequate anticoagulant effect. Direct thrombin inhibitors, such as hirudin and bivalirudin, overcome the limitations of heparin. These agents inhibit fibrin-bound thrombin, as well as fluid-phase thrombin, and produce a predictable anticoagulant response. Bivalirudin has both safety and potential efficacy advantages over hirudin. Bivalirudin appears to have a wider therapeutic window than hirudin, possibly because bivalirudin only transiently inhibits the active site of thrombin. The better safety profile of bivalirudin permits administration of higher doses, which may give it an efficacy advantage. Hirudin prevents thrombin from activating protein C, thereby suppressing this natural anticoagulant pathway. In contrast, bivalirudin may promote protein C activation by transiently inhibiting thrombin until it can be bound by thrombomodulin. Differences between bivalirudin and hirudin, as well as other direct thrombin inhibitors, highlight the pitfalls of considering all direct thrombin inhibitors to have equivalent risk-benefit profiles.