Inhibition Of Human Factor Xa Research Articles

Edoxaban: a focused review of its clinical pharmacology.

Long-term anticoagulation treatment with warfarin has been associated with a number of limitations in clinical practice and there is a need for more convenient long-term anticoagulation treatment. One of the non-vitamin K oral anticoagulants in development is edoxaban, a factor Xa inhibitor that is administered once daily. The pharmacological properties of edoxaban have various advantages in anticoagulant therapy. Edoxaban quickly reaches peak plasma concentrations in 1.5 h, has a half-life of 10-14 h, has relatively high bioavailability of 62% and exhibits highly selective, competitive, concentration-dependent inhibition of human factor Xa. The plasma concentrations of edoxaban are also closely correlated with suppression of thrombin generation and a range of platelet activation parameters (fragment 1+2, thrombin-antithrombin complex, and β-thromboglobulin), which edoxaban has been shown to rapidly inhibit. The anticoagulant activity of edoxaban is not affected by food intake or ethnicity and a number of drug-drug interaction studies have been performed. Co-administration of edoxaban with strong P-glycoprotein inhibitors, such as dronedarone, quinidine, and verapamil requires edoxaban dose-reduction by 50% to avoid the risk of over-exposure. The exposure of edoxaban may also increase in patients with a body weight ≤60 kg and moderate renal impairment. This meant a dose-reduction strategy in patients at risk of over-exposure was utilized in Phase III clinical studies. In conclusion, the pharmacological properties of edoxaban provide rapid and specific inhibition of factor Xa, which is closely related to plasma concentrations. Given the limitations with long-term warfarin therapy, once-daily edoxaban may provide a convenient long-term alternative for patients.

Antithrombotic and Anticoagulant Profiles of TAK-442, a Novel Factor Xa Inhibitor, in a Rabbit Model of Venous Thrombosis

The anticoagulant and antithrombotic profiles of TAK-442, a direct factor Xa (FXa) inhibitor, were investigated. TAK-442 showed potent inhibition of human FXa (Ki = 1.8 nM) and high specificity, with a 440-fold greater selectivity than thrombin and negligible effects on trypsin, plasmin, and tissue plasminogen activator (K(i) > 30 microM). [corrected] In human plasma, TAK-442 doubled FXa-induced clotting time, prothrombin time (PT), and activated partial thromboplastin time at 0.19, 0.55, and 0.59 microM, respectively. The relative PT-prolonging potencies of TAK-442, rivaroxaban, and apixaban were 1, 2.0-2.6, and 0.46-1.3, respectively, in 4 different PT reagents. In a rabbit model of venous thrombosis, 50- and 100-micrograms/kg [corrected] TAK-442 (intravenous bolus followed by 1-hour infusion) reduced thrombus formation by 50% and 81%, with plasma anti-FXa activity of 23%-26% and 34%-38%, respectively, and only marginal prolongation of PT and activated partial thromboplastin time. Melagatran, a thrombin inhibitor, showed similar antithrombotic activity to TAK-442. However, 500-micrograms/kg [corrected TAK-442 did not affect bleeding time (BT), whereas the same dose of melagatran significantly prolonged BT by 3.6-fold compared with vehicle control. These findings suggest that TAK-442 has similar antithrombotic effects as melagatran but does not cause BT prolongation, and plasma anti-FXa activity may reliably predict its potency.

Co-crystal Structure and Inhibition of Factor Xa by PD0313052 Identifies Structurally Stabilized Active Site Residues of Factor Xa and Prothrombinase

The enzyme complex prothrombinase plays a pivotal role in fibrin clot development through the production of thrombin, making this enzyme complex an attractive target for therapeutic regulation. This study both functionally and structurally characterizes a potent, highly selective, active site directed inhibitor of human factor Xa and prothrombinase, PD0313052, and identifies structurally conserved residues in factor Xa and prothrombinase. Analyses of the association and dissociation of PD0313052 with human factor Xa identified a reversible, slow-onset mechanism of inhibition and a simple, single-step bimolecular association between factor Xa and PD0313052. This interaction was governed by association (k(on)) and dissociation (k(off)) rate constants of (1.0 +/- 0.1) x 10(7) M(-1) s(-1) and (1.9 +/- 0.5) x 10(-3) s(-1), respectively. The inhibition of human factor Xa by PD0313052 displayed significant tight-binding character described by a Ki* = 0.29 +/- 0.08 nM. Similar analyses of the inhibition of human prothrombinase by PD0313052 also identified a slow-onset mechanism with a Ki* = 0.17 +/- 0.03 nM and a k(on) and k(off) of (0.7 +/- 0.1) x 10(7) M(-1) s(-1) and (1.7 +/- 0.8) x 10(-3) s(-1), respectively. Crystals of factor Xa and PD0313052 demonstrated hydrogen bonding contacts within the S1-S4 pocket at residues Ser195, Asp189, Gly219, and Gly216, as well as interactions with aromatic residues within the S4 pocket. Overall, these data demonstrate that the inhibition of human factor Xa by PD0313052 occurs via a slow, tight-binding mechanism and indicate that active site residues of human factor Xa, including the catalytic Ser195, are effectively unaltered following assembly into prothrombinase.

Differences in the clinically effective molar concentrations of four direct thrombin inhibitors explain their variable prothrombin time prolongation

Four direct thrombin inhibitors (DTIs), lepirudin, bivalirudin, argatroban, and melagatran, differ in their ability to prolong the prothrombin time (PT). Paradoxically, the DTI in clinical use with the lowest affinity for thrombin (argatroban) causes the greatest PT prolongation. We compared the effects of these DTIs on various clotting assays and on inhibition of human and bovine factor Xa (FXa). On a mole-for-mole basis, lepirudin was most able to prolong the PT, activated partial thromboplastin time (APTT), and thrombin clotting time (TCT), whereas argatroban had the least effect. At concentrations that doubled the APTT (argatroban, 1 micromol/l; melagatran, 0.5 micromol/l; bivalirudin, 0.25 micromol/l; lepirudin, 0.06 micromol/l), the rank order for PT prolongation was: argatroban > melagatran > bivalirudin > lepirudin. Although the Ki's associated with inhibition of human FXa by melagatran (1.4 micromol/l) and argatroban (3.2 micromol/l) approach their therapeutic concentrations, inhibition of FXa did not appear to be a major contributor to PT prolongation, since argatroban also prolonged the PT of bovine plasma (despite a Ki for bovine FXa of 2,600 micromol/l). Only melagatran inhibited prothrombinase-bound FXa. We conclude that the differing effects of the DTIs on PT prolongation are primarily driven by their respective molar plasma concentrations required for clinical effect. DTIs with a relatively low affinity for thrombin require high plasma concentrations to double the APTT; these higher plasma concentrations, in turn, quench more of the thrombin generated in the PT, thereby more greatly prolonging the PT.

Conformation of a bound inhibitor of blood coagulant factor Xa.

13C[(15)N] and (13)C[(19)F] rotational-echo double-resonance NMR have been used to characterize the enzyme-bound structure of ZK-816042, an amidine-imidazoline inhibitor of human factor Xa (FXa). The NMR experiments were performed on a lyophilized FXa-inhibitor complex. The complex was formed in solution in the presence of stabilizing excipients and frozen after gradual supercooling prior to lyophilization. The results indicate that the inhibitor binds with a distribution of orientations of the imidazoline ring.

Discovery and characterization of a potent and selective non-amidine inhibitor of human factor Xa

Benzothiophene-anthranilamide 1 (3-chloro- N-[2-[[(4-fluorophenyl)amino]carbonyl]-4-methylphenyl]benzo[ b]thiophene-2-carboxamide) was discovered by high throughput screening to be a highly potent and selective non-amidine inhibitor of human factor Xa with a K i of 15±4 nM. Compound 1 is a selective inhibitor of human factor Xa as suggested by the K i (app) determined for nine other human serine proteases and bovine trypsin. The activity of reconstituted human prothrombinase complex was inhibited by compound 1 when assayed in physiological concentrations of the substrate prothrombin. However, 27-fold higher inhibitor concentrations were needed to achieve the same level of inhibition than were required for the inhibition of free factor Xa, due in part to non-specific binding of the inhibitor to phospholipid under the assay conditions. Failure to demonstrate enzymatic cleavage of compound 1 suggests that compound 1 is solely an inhibitor rather than a substrate for factor Xa. The inhibition of factor Xa by compound 1 was reversible upon dilution of the enzyme/inhibitor mixture. Analyses of the inhibition mechanism with Dixon, Cornish–Bowden, and Lineweaver–Burk plots showed that compound 1 is a linear mixed-type inhibitor with 5-fold higher affinity for free factor Xa than the factor Xa/substrate complex. The linear mixed-type inhibition suggests that compound 1 binds to the active site region of factor Xa, but its binding cannot be fully displaced by the substrate S2222 (1:1 mixture of N-benzoyl-Ile-Glu-Gly-Arg- p-nitroanilide and N-benzoyl-Ile-Glu(γ-OMe)-Gly-Arg- p-nitroanilide hydrochloride). Thus, the inhibition mechanism for compound 1 is novel compared to most serine protease inhibitors including amidine-containing factor Xa inhibitors, which rely on binding to the S1 pocket of the enzyme active site. Compound 1 represents an attractive, novel structural template for further development of efficacious, safe, and potentially orally active human factor Xa inhibitors.

Antithrombotic effects of YM466, a synthesized direct inhibitor of factor Xa, in an arterio‐venous shunt thrombosis model in squirrel monkeys

AbstractThe antithrombotic effects of YM466, a synthetic direct inhibitor of human factor Xa (FXa), were evaluated in an arterio‐venous shunt thrombosis model in squirrel monkeys. YM466 significantly inhibited thrombus formation after continuous IV zx infusion at doses of 3–30 µg/kg/h. Although prothrombin time (PT) was significantly prolonged at doses of 10 and 30 µg/kg/h, this effect was small (1.2‐ and 1.4‐fold of the control, respectively). Furthermore, YM466 had no effect on template bleeding time and activated partial thromboplastin time at any dose. Plasma anti‐FXa activity increased in a dose‐dependent manner correlating with the antithrombotic effect of YM466. Thrombin‐antithrombin III complex levels decreased to less than normal levels at 30 µg/kg/h of YM466. Mean plasma concentrations of YM466 measured by LC/MS/MS were 2.5, 10.5, 27.4, and 75.5 ng/ml at doses of 1, 3, 10, and 30 µg/kg/h, respectively. These results suggest that YM466 is a safe antithrombotic agent with low bleeding risk, and that plasma anti‐FXa activity may be a suitable parameter for monitoring its antithrombotic effect. Drug Dev. Res. 58:190–195, 2003. © 2003 Wiley‐Liss, Inc.

Inhibitory effect of JTV-803, a new cyclic guanidine derivative, on factor Xa in vitro and in vivo

JTV-803, 4-[(2-amidino-1,2,3,4-tetrahydroisoquinolin-7-yloxy)methyl]-1-(4-pyridinyl)piperidine-4-carboxylic acid monomethanesulfonate trihydrate showed a competitive inhibitory effect on human factor Xa, with a K i value of 0.019 μM. This compound was 100 times more selective in inhibiting human factor Xa as compared to its inhibitory activity against thrombin, plasmin, and trypsin. JTV-803 was also examined for its inhibitory effect on activated factor Xa obtained from plasma of various animal species. JTV-803 exerted a potent inhibitory effect on human factor Xa (IC 50: 0.081 μM). JTV-803 prolonged activated partial thromboplastin time and prothrombin time in a dose-dependent manner. Oral anticoagulant efficacy of JTV-803 was examined ex vivo for its inhibition of human factor Xa in cynomolgus monkeys. JTV-803 produced more than 20% inhibition of human factor Xa for 8 h. Taken together, the results indicate JTV-803 is a long-acting oral anticoagulant which exerts its effect via specific inhibition of human factor Xa.

Inhibitory Properties of Human Recombinant Arg 24→Gln Type-2 Tissue Factor Pathway Inhibitor (R24Q TFPI-2)

Human type-2 tissue factor pathway inhibitor (TFPI-2), also known as placental protein 5, is a 32-kDa serine proteinase inhibitor consisting of three tandemly arranged Kunitz-type domains homologous to tissue factor pathway inhibitor. TFPI-2 inhibits a variety of serine proteinases involved in coagulation and fibrinolysis through an arginine residue (R24) in its first Kunitz-type domain, which constitutes a putative P 1 residue for the substrate recognition sites of these proteinases. As recent studies have shown that this P 1 residue to be a glutamine in murine TFPI-2, we constructed, expressed, and purified a human TFPI-2 mutant with glutamine substituted for arginine at position 24 (R24Q TFPI-2). R24Q TFPI-2 lost ∼90% of its inhibitory activity towards bovine trypsin and virtually all inhibitory activity towards human plasmin and the factor VIIa–tissue factor complex, emphasizing the importance of the P 1 Arg 24 residue in the inhibition of these serine proteinases. However, whereas wild-type TFPI-2 is a relatively weak inhibitor of human factor Xa amidolytic activity (IC 50∼1 μM), R24Q TFPI-2 exhibited enhanced inhibitory activity towards the amidolytic and coagulant activities of this proteinase with a K i of 18 nM. While the molecular basis for the enhanced inhibition of human factor Xa by R24Q TFPI-2 is unknown, these data provide suggestive evidence that murine TFPI-2 may function as a serine proteinase inhibitor in spite of the absence of a P 1 Arg or Lys residue.

Biochemical and Pharmacological Characterization of YM-60828, a Newly Synthesized and Orally Active Inhibitor of Human Factor Xa

YM-60828 was found to potently inhibit human factor Xa following oral administration. YM-60828 showed high affinity for factor Xa (Ki = 1.3 nM), but did not affect thrombin (Ki > 100 microM). YM-60828 doubled factor Xa clotting time, prothrombin time (PT) and activated partial thromboplastin time (APTT) at 0.10, 0.21, 0.24 microM, respectively. Importantly, it did not prolong thrombin time at 100 microM. YM-60828 also inhibited factor Xa in the prothrombinase complex with an IC50 value of 7.7 nM. In addition to its anticoagulant activity, YM-60828 inhibited platelet aggregation induced by various agonists (IC50 = 3 to 23 microM). Squirrel monkeys were used to study the ex vivo anticoagulant activity and pharmacokinetic properties of YM-60828. One hour after oral administration at 3 mg/kg, YM-60828 strongly prolonged PT and APTT by 4.8- and 1.9-fold, respectively, and plasma concentration reached 788 +/- 167 ng/ml. Bioavailability was calculated to be 20.3%. These results strongly suggest that YM-60828 will be a valuable orally active and potent anticoagulant agent showing potential antithrombotic activity.

The inhibition of human factor Xa by plasminogen activator inhibitor type 1 in the presence of calcium ion, and its enhancement by heparin and vitronectin

Plasminogen activator inhibitor type 1 (PAI-1), a member of serine proteinase inhibitor superfamily, is known to inhibit thrombin in the presence of either heparin or vitronectin. We analyzed possible inhibitory activity of PAI-1 on human factor Xa. PAI-1 inhibited factor Xa in the presence of calcium ion (Ca 2+), whereas no inhibition was observed in the absence of Ca 2+. Half maximal enhancement by Ca 2+ was obtained at 0.8 mM. An equimolar complex formation between factor Xa and PAI-1 in the presence of Ca 2+ was observed by SDS polyacrylamide gel electrophoresis. Both unfractionated heparin and vitronectin enhanced the inhibition only in the presence of Ca 2+. Apparent second-order rate constant ( k i) for the inhibition of factor Xa by PAI-1 at 5 mM Ca 2+ was 1.6 × 104 M −1 s −1, and was enhanced 3-fold by 2 u/ml of heparin (4.6 × 104 M −1 s −1[ and 10-fold by 100 nM vitronectin (1.6 × 105 M −1 s −1, respectively. The interaction between Ca 2+-bound factor Xa and PAI-1 could be important from the view of PAI-1 neutralization and enhancement of fibrinolysis.

109 The inhibition of human factor Xa by plasminogen activator inhibitor type 1 in the presence of calcium ion, and its enhancement by heparin and vitronectin: A novel pathway to enhance fibrinolysis through PAI-1 neutralization

109 The inhibition of human factor Xa by plasminogen activator inhibitor type 1 in the presence of calcium ion, and its enhancement by heparin and vitronectin: A novel pathway to enhance fibrinolysis through PAI-1 neutralization

Effect of Heparin on the Inhibition of Factor Xa by Tissue Factor Pathway Inhibitor: A Segment, Gly212-Phe243, of the Third Kunitz Domain Is a Heparin-Binding Site

Tissue factor pathway inhibitor (TFPI) inhibits the tissue factor--factor VIIa complex and factor Xa with its first and second Kunitz domains (K1 and K2), respectively. The inhibitory activity is enhanced by heparin, and the C-terminal basic part has been shown to be a heparin-binding site (HBS-1). To characterize and localize a second heparin-binding site (HBS-2), we studied the effect of heparin on the inhibitory activity of two forms of recombinant human TFPI, the full-length TFPI (rTFPI), and TFPI lacking the C-terminal basic part (rTFPI-C), by assaying the inhibition of human factor Xa. rTFPI-C inhibited factor Xa with an initial Ki of 6.79 nM in the absence of Ca2+ and 22.3 nM in the presence of 5 mM CaCl2. Heparin decreased the initial Ki to 1.79 nM in the absence of Ca2+ and 2.68 nM in the presence of 5 mM CaCl2, indicating the presence of HBS-2 in rTFPI-C. The dissociation constant for the binding of HBS-2 with heparin was determined to be 830 nM using fluorescein-labeled heparin and rTFPI-C. Heparin enhanced the inhibitory activity of a fragment consisting of the K2 and K3 domains, but it did not stimulate the inhibitory activity of the K2 domain. A synthetic peptide mimicking from Gly212 to Phe243 in the K3 domain reduced the effect of heparin on the inhibition by rTFPI-C and rTFPI. These results defined the location of HBS-2 in the basic region of the K3 domain between Gly212 and Phe243.

Ecotin Is a Potent Anticoagulant and Reversible Tight-Binding Inhibitor of Factor Xa

Ecotin, a serine protease inhibitor found in the periplasm of Escherichia coli, has been characterized as an extremely potent anticoagulant and reversible tight-binding inhibitor of human factor Xa (FXa). The ecotin gene was cloned by PCR, highly expressed in E. coli, and purified from the E. coli periplasm. The binding of ecotin to FXa was stoichiometric with an equilibrium dissociation constant Ki of 54 pM. The association rate constant was 1.35 x 10(6) M-1 s-1, and the dissociation rate constant, measured in the presence of human leukocyte elastase (HLE) to prevent reassociation of ecotin with FXa, was 6.5 x 10(-5) s-1. Ecotin prolonged clotting time ca. 10-fold at 0.3 microM and at 2 microM in activated partial thromboplastin time and prothrombin time assays, respectively. Ecotin did not effectively inhibit the human plasma proteases thrombin, tissue factor.factor VIIa, factor XIa, activated protein C, plasmin, or tissue plasminogen activator (t-PA); however, it did potently inhibit factor XIIa, plasma kallikrein, HLE, and bovine trypsin and chymotrypsin. Coincubation of ecotin and FXa at 10 microM each resulted in a (ecotin)2.(FXa)2 complex as determined by gel filtration. Dimerization of ecotin alone was measured by fluorescence titration which yielded a Kd of ca. 390 nM. FXa cleaved ecotin slowly at pH 4.0 between M84 and M85. Replacement of the P1 Met84 residue with Arg and Lys led to FXa inhibitors with Ki values of 11 and 21 pM, respectively. The P1 Arg and Lys mutants also significantly inhibited thrombin, factor XIa, activated protein C, plasmin, factor XIIa, kallikrein, and bovine trypsin and chymotrypsin but did not inhibit tissue factor.factor VIIa, t-PA, or HLE.

Kinetics of the inhibition of human factor Xa by full-length and truncated recombinant tissue factor pathway inhibitor

The inhibition equilibrium and kinetics of association and dissociation of the binding of three types of recombinant tissue factor pathway inhibitor (TFPI), namely full-length TFPI, C-terminal-truncated TFPI, and TFPI without the third Kunitz domain (TFPI1-161), to factor Xa have been measured. Formation and dissociation of the complexes were monitored by continuous measurement of the changes in the rate of hydrolysis of a peptidyl-p-nitroanilide substrate. Progress curves of product formation were fitted to a set of equations describing a one-step bimolecular inhibitory reaction in the presence of a competing substrate. For full-length TFPI the rate constants of association (kon) and dissociation (koff) were (5.1 +/- 0.7) x 10(6) M-1.s-1 and (2.6 +/- 0.9) x 10(-4)s-1 respectively. Thus, although the inhibition constant (50 pM) is far below the plasma concentration (2.5 nM) of TFPI, the half-time for transition to equilibrium in plasma is rather long (66s). The truncated forms of TFPI differ in that they have a 4-fold lower kon value but a similar dissociation rate constant. Therefore the inhibition constant, Ki, is 4-fold higher (0.2 nM) and the half-time to achieve equilibrium is prolonged to 250 s. The kon values of full-length and C-terminal-truncated TFPI, but not that of TFPI1-161, were found to decrease with increasing ionic strength.

Reaction pathway for inhibition of blood coagulation factor Xa by tick anticoagulant peptide.

The reaction pathway for inhibition of human factor Xa (fXa) by recombinant tick anticoagulant peptide (rTAP) was studied by stopped-flow fluorometry. In the presence of the fluorogenic substrate N-tert-butyloxycarbonyl-L-isoleucyl-L-glutamylglycyl-L-arginyl-7-amido-4 - methylcoumarin (B-IEGR-AMC) and under pseudo-first-order conditions, inhibition appears to occur via a two-step process. Initially, a weak enzyme-inhibitor complex forms with a dissociation constant (Ki) of 68 +/- 6 microM. The initial complex then rearranges to a more stable fXa-rTAP complex with a rate constant (k2) of 123 +/- 5 s-1. The apparent second-order rate constant (k2/Ki) describing formation of the stable complex is (1.8 +/- 0.2) x 10(6) M-1 s-1. Studies of the reaction of rTAP with fXa in the presence of the fluorescent active-site probe p-amino-benzamidine (P) revealed a reaction pathway wherein rTAP initially binds to the fXa-P complex in a two-step process prior to displacing P from the active site. These results indicate that rTAP can bind fXa via a site distinct from the active site (an exosite). The subsequent displacement of P from the active site of fXa by rTAP exhibits a dependence on the concentration of P, indicating that rTAP is locked into the active site in a third step.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Rat Factors X and Xa: Demonstration of Factor Xa in Rat Plasma1

We have found that rat plasma corrected the non-activated PT of human normal or factor-X deficient plasma, and the factor Xa-like activity being constantly detected in every 1 ml of blood collected via the cannulated carotid artery of rats. The present study was undertaken to characterize the factor Xa-like activity in rat plasma by preparing rat factor X and a monoclonal antibody against it. Factor X was purified from a BaCl2 eluate of rat plasma by chromatographies on columns of DEAE-Sepharose CL-6B and Sulfate Cellulofine or on a column of Affi-Gel 10 conjugated with a monoclonal antibody against rat factor X. Factor Xa-like activity in rat plasma was eliminated by the treatment of rat plasma with a monoclonal antibody which recognized the heavy chain portions of rat factors X and Xa. A kinetical study demonstrated that rat factor Xa was strongly inhibited by rat antithrombin III, with a Ki of 2.2 x 10(-11) M, in the presence of heparin. However, in the absence of heparin, the second order rate constant for the inhibition of rat factor Xa by rat antithrombin III was 2.6 x 10(4) M-1.min-1, which was one forty-third that for the inhibition of human factor Xa by human antithrombin III. Furthermore, rat factor Xa was resistant to the inhibition by rat alpha-1-antitrypsin and alpha-2-macroglobulin.(ABSTRACT TRUNCATED AT 250 WORDS)

The synthesis of arginylfluoroalkanes, their inhibition of trypsin and blood-coagulation serine proteinases and their anticoagulant activity

Seven arginylfluoroalkanes ('arginine fluoroalkyl ketones') were synthesized by using a modified Dakin-West procedure. The structure of benzoyl-Arg-CF2CF3 was analysed by 19F-n.m.r. spectroscopy and m.s. and the compound was shown to exist primarily as a hydrate or cyclic carbinolamine. Arginylfluoroalkanes are good inhibitors of blood-coagulation serine proteinases and were found to be slow-binding inhibitors for bovine trypsin with Ki values of 0.2-56 microM. Benzoyl-Arg-CF2CF3 was the best inhibitor for bovine thrombin and human Factor XIa, and inhibited thrombin and Factor XIa competitively with Ki values of 13 microM and 62 microM respectively. The best inhibitor for pig pancreatic kallikrein was p-toluoyl-Arg-CF3, with a Ki value of 35 microM. Benzoyl-Arg-CF3 and benzoyl-Arg-CF2CF3 inhibited human plasma kallikrein competitively, with Ki values of 50 microM. None of the seven arginylfluoroalkanes was a good inhibitor of human factor Xa or of Factor XIIa. The arginylfluoroalkanes were tested in the prothrombin time (PT) and activated partial thromboplastin time (APTT) coagulant assays. Two fluoroketones, benzoyl-Arg-CF2CF3 and 1-naphthoyl-Arg-CF3, had significant anticoagulant activity. Benzoyl-Arg-CF2CF3 was found to prolong the PT 1.8-fold at 120 microM and to prolong the APTT 2.4-fold at 90 microM, whereas 1-naphthoyl-Arg-CF3 only prolonged the APTT 1.7-fold at 100 microM.

Antithrombin III-dependent anti-prothrombinase activity of heparin and heparin fragments

Heparin and heparin fragments in the molecular mass range 1,700-20,000 Da were examined for their ability to accelerate the antithrombin III (AT III)-dependent inhibition of human factor Xa and the prothrombin converting complex (prothrombinase) during human prothrombin activation. The prothrombinase reaction was modeled by a 3-parameter 2-exponential equation to determine the initial rate of prothrombin activation and the pseudo-first order rate constants of inhibition of prothrombinase and in situ generated thrombin activity. The catalytic specific activities of the heparins increased with increasing molecular size for both the inhibition of prothrombinase and factor Xa. A 10-fold increase over the entire Mr range was found. In contrast to results obtained by others (Ellis, V., Scully, M. F., and Kakkar, V. V. (1986) Biochem. J. 233, 161-165; Barrowcliffe, T. W., Havercroft, S. J., Kemball-Cook, G., and Lindahl, U. (1987) Biochem. J. 243, 31-37), all the heparins showed a 5-fold higher rate of inhibition of factor Xa when compared with the inhibition of prothrombinase, indicating that the factor Va-mediated protection of factor Xa from inhibition by AT III/heparin is independent of the molecular size of the heparin. Our original approach has also revealed a hitherto unrecognized phenomenon, namely, in addition to the accelerating effect of the heparins on the rate of formation of the inactive AT III-factor Xa complex, heparins with Mr greater than 4,500 reduce the initial rate of thrombin generation in the presence of AT III in a concentration-dependent way. We hypothesize that the formation of the dissociable ternary AT III-heparin-factor Xa complex results in a (partial) loss of factor Xa activity towards its natural substrate prothrombin.

The effect of phosphatidylserine on the inhibition of human factor Xa by the plasma proteinase inhibitors

The effect of phosphatidylserine on the inhibition of human factor Xa by the plasma proteinase inhibitors