Comparative Studies of an Orally-active Factor Xa Inhibitor, YM-60828, with other Antithrombotic Agents in a Rat Model of Arterial Thrombosis

ASP6537, a novel highly selective cyclooxygenase-1 inhibitor, exerts potent antithrombotic effect without "aspirin dilemma"

INTRODUCTION

Aspirin inhibits both the cyclooxygenase (COX)-1-dependent production of thromboxane A2 (TXA2) in platelets and COX-2-dependent production of anti-aggregatory prostaglandin I2 (PGI2) in vessel walls, resulting in "aspirin dilemma." Our objective is to investigate whether ASP6537 can overcome aspirin dilemma and exert a potent antithrombotic effect without a concurrent ulcerogenic effect.

METHODS

We evaluated the inhibitory effects of ASP6537 on recombinant human COX-1 (rhCOX-1) and rhCOX-2 activities using a COX-1/2 selectivity test. To determine whether ASP6537 induces aspirin dilemma, we examined the effects of ASP6537 on in vitro TXA2 and PGI2 metabolite production from platelets and isolated aorta of guinea pigs, and on plasma concentrations of TXA2 and PGI2 metabolites in aged rats. Finally, we evaluated the antithrombotic effects and ulcerogenic activity of ASP6537 using an electrically induced carotid arterial thrombosis model and a gastric ulcer model in guinea pigs.

RESULTS

The IC50 ratios of rhCOX-2 to rhCOX-1 for ASP6537 and aspirin were >142,000 and 1.63 fold, respectively. ASP6537 inhibited TXA2 production more selectively than aspirin in in vitro and in vivo TXA2/PGI2 production studies. ASP6537 exerted a significant antithrombotic effect at ≥3 mg/kg, while aspirin tended to inhibit thrombosis at 300 mg/kg but it was not statistically significant. Further, ASP6537 did not induce ulcer formation at 100 mg/kg, whereas aspirin exhibited an ulcerogenic effect at doses of ≥100 mg/kg.

CONCLUSIONS

ASP6537 functions as a highly selective COX-1 inhibitor with a superior ability to aspirin for normalizing TXA2/PGI2 balance, and exerts antithrombotic effect without ulcerogenic effect.

Arterial antithrombotic activity of rivaroxaban, an orally active factor Xa inhibitor, in a rat electrolytic carotid artery injury model of thrombosis

Rivaroxaban, an oral, direct factor Xa inhibitor, has been approved in several countries for thromboprophylaxis after elective hip or knee arthroplasty based on favorable benefit-risk profile and improved efficacy compared to enoxaparin in reducing the composite of symptomatic and asymptomatic deep vein thrombosis, nonfatal pulmonary embolism, and all-cause mortality. Given the potential therapeutic utility of factor Xa inhibition in arterial thrombosis, we evaluated the antithrombotic activity of rivaroxaban in a model of arterial thrombosis in anesthetized rats in which thrombotic occlusion was induced by electrolytic injury of the carotid artery. Rivaroxaban, 0.3, 1 or 3 mg/kg, enoxaparin, 10 mg/kg, or vehicle were infused intravenously to anesthetized rats and time to occlusion as well as coagulation parameters monitored following carotid electrolytic injury. Although the lowest dose of rivaroxaban (0.3 mg/kg) did not prolong occlusion time compared to vehicle, rivaroxaban at 1 or 3 mg/kg prevented occlusion in all vessels during the 30-min observation period (median occlusion time >30 min), which was greater than that following a single dose of enoxaparin infused at a dose of 10 mg/kg (median time to occlusion = 21.6 min). Rivaroxaban was also effective following oral dosing at 3 mg/kg. Rivaroxaban's antithrombotic activity was paralleled by dose-dependent increases in prothrombin time (PT) and activated clotting time (ACT) without significant changes in activated partial thromboplastin time. Rivaroxaban also markedly increased Russell's viper venom time (RVVT) and decreased thrombin-antithrombin complex concentrations at all doses. These findings support the potential utility of rivaroxaban in arterial thrombotic disorders such as acute coronary syndrome, stroke and peripheral arterial disease.

Synergistic effect of a factor Xa inhibitor, TAK-442, and antiplatelet agents on whole blood coagulation and arterial thrombosis in rats

INTRODUCTION

Activated platelets facilitate blood coagulation by providing factor V and a procoagulant surface for prothrombinase. Here, we investigated the potential synergy of a potent factor Xa/prothrombinase inhibitor, TAK-442, plus aspirin or clopidogrel in preventing arterial thrombosis and whole blood coagulation.

METHODS

Thrombus formation was initiated by FeCl(3)-induced rat carotid injury. Bleeding time was evaluated with the rat tail transection model. Whole blood coagulation was assessed by thromboelastographic examination (TEG) for which blood obtained from control, aspirin-, or clopidogrel-treated rats was transferred to a TEG analyzer containing, collagen or adenosine diphosphate (ADP), and TAK-442 or vehicle.

RESULTS

TAK-442 (3mg/kg, po), aspirin (100mg/kg, po) or clopidogrel (3mg/kg, po) alone had no significant effect on thrombus formation, whereas the combination of TAK-442 with aspirin and clopidogrel remarkably prolonged the time to thrombus formation without additional significant prolongation of bleeding time. TEG demonstrated that the onset of collagen-induced blood coagulation were slightly longer in aspirin-treated rats than control; however, when the blood from aspirin-treated rats was subsequently treated in vitro with 100 nM TAK-442, the onset of clotting was significantly prolonged. In contrast, only marginal prolongation was observed with TAK-442 treatment of blood from control animals. The onset time of ADP-induced blood coagulation was slightly longer in clopidogrel-treated rats compared with control, and it was further extended by TAK-442 treatment.

CONCLUSION

These results demonstrate that blood coagulation can be markedly delayed by the addition of TAK-442 to antiplatelets treatment which could contribute to synergistic antithrombotic efficacy in these settings.

Repeat oral dosing of prasugrel, a novel P2Y12 receptor inhibitor, results in cumulative and potent antiplatelet and antithrombotic activity in several animal species

Antiplatelet and antithrombotic activity of multiple oral dosing of prasugrel were evaluated in several animal species. Prasugrel's active metabolite concentration-relatedly inhibited in vitro ADP-induced aggregation of rat, rabbit, dog, monkey and human platelets. Oral administration of prasugrel to dogs (0.03-0.3 mg/kg/day) and monkeys (0.1 and 0.3 mg/kg/day) once a day for 14 days resulted in potent, dose-related and cumulative inhibition of ADP-induced platelet aggregation. The inhibitory effects reached a plateau on days 3 to 5 and thereafter were maintained during dosing. Inhibition decreased gradually after cessation of dosing with near full recovery by 7 days after last dose. Antiplatelet and antithrombotic activity of prasugrel and clopidogrel were further examined in rats. Multiple oral dosing of prasugrel (0.3-3 mg/kg/day) to rats resulted in more potent inhibition of platelet aggregation compared to clopidogrel (3-30 mg/kg/day) and ticlopidine (30-300 mg/kg/day). Separate experiments confirmed that platelet inhibition was associated with inhibition of [(3)H]-2-methylthio-ADP binding to rat platelets. In a rat model of electrically-induced arterial thrombosis, prasugrel (0.1-1 mg/kg/day, p.o.) significantly prolonged the time to arterial occlusion and increased the duration of arterial patency. The inhibition of platelet aggregation of prasugrel was about 10 and 300 times more potent than clopidogrel and ticlopidine, respectively. Overall these results show that in several species multiple oral administration of prasugrel results in more potent inhibition of platelet aggregation and thrombus formation than clopidogrel and ticlopidine, and that these effects are mediated by inhibition of platelet ADP receptors.

Prodrug-based design, synthesis, and biological evaluation of N-benzenesulfonylpiperidine derivatives as novel, orally active factor Xa inhibitors

We describe here our investigation of a new series of orally active fXa inhibitors based on a prodrug strategy. Solid-phase parallel synthesis identified a unique series of fXa inhibitors with a substituted benzenesulfonyl group as a novel S4 binding element. This series resulted in compound 39, which exhibited potent inhibitory activity against fXa (IC50 = 13 nM) and excellent selectivity over thrombin (>7000-fold). The masking of its highly hydrophilic groups led to the creation of related prodrug 28, which demonstrated an anticoagulant effect after oral dosing.

Hepatobiliary transport of YM466, a novel factor Xa inhibitor, in rats

YM466, a novel factor Xa inhibitor, is a hydrophilic compound with a carboxylic acid moiety. Previous studies in rats have shown that YM466 does nor undergo metabolism but is excreted into the bile and urine in unchanged form. Thus, in this study, we investigated in vivo hepatobiliary transport, focusing in particular on multidrug resistance-associated protein 2 (Mrp2/Abcc2)-mediated transport. The hepatobiliary transport of YM466 was investigated after its systemic infusion into Sprague-Dawley rats (SDRs) and Eisai hyperbilirubinemic rats (EHBRs), which lack Mrp2. When the binding of YM466 in the plasma and liver was examined, the bile-to-plasma concentration ratio and the liver-to-plasma concentration ratio for the unbound concentration in SDRs amounted to 32.2 and 2.83, respectively, suggesting concentrated transport. The bile-to-liver concentration ratio for the unbound concentration in EHBRs was not lower than that found for SDRs. These findings suggest that YM466 is excreted from the plasma into the bile in a concentrated manner; however, Mrp2 does not play a major role in biliary excretion.

Ebelactone B, an Inhibitor of Extracellular Cathepsin A-Type Enzyme, Suppresses Platelet Aggregation Ex Vivo in Renovascular Hypertensive Rats

The present study was undertaken to investigate whether ebelactone B, an inhibitor of bradykinin and angiotensin I hydrolysis by serine carboxypeptidase Y-like enzymes, could influence platelet aggregation ex vivo in renovascular hypertensive rats (2-kidney, 1-clip Goldblatt model, 2K1C). We found that ebelactone B (5 mg/kg) administrated subcutaneously once a day for 5 days, 5 weeks after the development of hypertension, or a single dose of ebelactone B (0.5 mg/kg) injected intravenously into 2K1C hypertensive rats before the induction of arterial thrombosis, both markedly suppressed collagen-induced platelet aggregation in whole blood. In contrast, inhibition of collagen-induced platelet aggregation was not evident in vitro after pretreatment of the blood with ebelactone B, indicating that ex vivo the antiaggregatory action of this compound can proceed through an indirect mechanism. The injection of ebelactone B did not affect the mean blood pressure of 2K1C hypertensive rats but lowered an elevated extracellular serine carboxypeptidase cathepsin A-like activity. Thus, the data indicate that ebelactone B may be a promising antiaggregatory agent in renovascular hypertension and suggest that 1 of the possible mechanisms through which it exerts this effect may be related to the suppression of cathepsin A-like activity released locally during the development of renovascular hypertension.

Synthesis and biological activity of novel 1,2-disubstituted benzene derivatives as factor Xa inhibitors

Factor Xa (fXa) is a serine protease that plays a pivotal role in the coagulation cascade. High-throughput screening of the Yamanouchi compound library yielded lead compound 1 with the ability to inhibit fXa at micromolar concentrations. To improve its fXa inhibitory activity and its oral anticoagulant activity, the linker between benzamidine and the central benzene ring was modified and a carboxyl group was introduced at the central benzene ring. The resulting compounds 40b (YM-203552), 41a (YM-202054), and 41c (YM-203558) exhibited potent fXa inhibitory activity and oral anticoagulant activity. In particular, YM-203558 exhibited the most potent oral anticoagulant activity, prolonging PT more than 3-fold at 0.5 and 2.0 h. Additionally, these compounds showed a high degree of selectivity for other serine proteases.

Lack of Plasminogen Activator Inhibitor-1 Enhances the Preventive Effect of DX-9065a, a Selective Factor Xa Inhibitor, on Venous Thrombus and Acute Pulmonary Embolism in Mice

We have studied the physiological effects of DX-9065a, a selective factor Xa (FXa) inhibitor, and heparin in experimental venous thrombus and acute pulmonary embolism. Moreover, the effects of these compounds were also evaluated under the condition of plasminogen activator inhibitor-1 (PAI-1) deficiency. A thrombus was induced in the murine femoral vein. The compounds prolonged the time to occlusion in a dose-dependent manner. Pulmonary embolism was induced by continuous induction of venous thrombi in the left jugular vein. The mortality of mice increased time-dependently. Histological evidence of thromboembolism in the lung was obtained in all mice. Treatment with DX-9065a, but not heparin, reduced the mortality at 6 h after initiation of venous thrombi. In separate experiments, pulmonary thromboembolism was induced in PAI-1 knockout mice. The mortality in PAI-1 knockout mice was reduced compared with that of wild-type mice. Moreover, when DX- 9065a was administered to PAI-1 knockout mice with pulmonary thromboembolism, mice survived well without marked bleeding. These findings indicate that the dual inhibition of coagulation FXa and PAI-1 could be beneficial in the treatment of acute pulmonary embolism.

A liquid chromatographic-tandem mass spectrometric method for the determination of YM466, a novel Factor Xa inhibitor, in rat plasma

A sensitive and selective method for the determination of YM466, a novel Factor Xa inhibitor, in rat plasma was developed and validated using liquid chromatography-mass spectrometry with electrospray ionization. Plasma samples were pretreated by solid-phase extraction and chromatographed on a C18 column (75 mm x 4.6 mm i.d.) with a mobile phase consisting of 0.1% formic acid-methanol (70:30 (v/v)), and detected using selected reaction monitoring in the positive-ion mode. The lower limit of quantification was 0.4 ng/ml, and good precision and accuracy were achieved. The validated method allowed analysis of samples for the determination of pharmacokinetic profiles of YM466 in rats.

Orally active factor Xa inhibitor: synthesis and biological activity of masked amidines as prodrugs of novel 1,4-diazepane derivatives

Factor Xa (fXa) is a serine protease, which plays a pivotal role in the coagulation cascade. To improve the oral anticoagulant activity of fXa inhibitors containing a 1,4-diazepane moiety as the P4 part, a prodrug strategy was examined. Among the compounds evaluated in this study, amidoxime prodrugs bearing an ester moiety, such as compounds 21 and 30, showed effective oral anticoagulant activity in mice.

Pharmacokinetics of YM466, a new factor Xa inhibitor, in rats and dogs

The pharmacokinetics of YM466, a selective inhibitor for factor Xa, was investigated after single intravenous and oral dosing to rats and dogs. After i.v. dosing, plasma YM466 concentration declined in a bi-phasic manner with a terminal elimination half-life of 1.4 h in rats and 0.8 h in dogs. Total plasma clearance values were 884 and 1212 ml/h/kg in rats and dogs, respectively. After oral dosing, plasma YM466 concentrations reached maximum within 2 h and increased in a dose-proportional manner in rats while increase was nonlinear in dogs. The absolute bioavailability of YM466 was 2.7-4.5% in rats, almost constant regardless of the dose levels investigated, while it was 6.9-24.6% in dogs, indicating nonlinear pharmacokinetics. The plasma protein binding of YM466 was 54.7-56.5% in rats and 45.2-49.0% in dogs and almost constant regardless of the concentration. No metabolism of YM466 was observed in an in vitro liver microsome study. These findings suggest that the low bioavailability of YM466 is attributable to the poor absorption not to the extensive metabolism.

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+/-4nM. 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(gamma-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.

Design, synthesis and biological activity of YM-60828 derivatives. Part 2: potent and orally-bioavailable factor Xa inhibitors based on benzothiadiazine-4-one template

Compound YM-60828 was previously characterized in our laboratory as a potent, selective and orally-bioavailable Factor Xa (FXa) inhibitor. The L-shape conformation of this compound in the active site of FXa was recognized as an important factor in displaying its FXa inhibitory activity. This led to the exploration of conformationally restricted cyclic scaffolds bearing a similar active conformation. The current study investigated a novel series of benzothiadiazine-4-one based compounds as FXa inhibitors. Structure-activity relationship (SAR) investigations revealed some potent FXa inhibitors that were selected for further in vitro and ex vivo anticoagulant studies. Among them, compound 6j (YM-169920) was proved to be most effective anticoagulant in this series. The synthesis and SAR in addition to docking studies of this class of inhibitors are described.

Antithrombotic properties of SSR182289A, a new, orally active thrombin inhibitor

N-[3-[[[(1S)-4-(5-Amino-2-pyridinyl)-1-[[4-difluoromethylene)-1-piperidinyl]carbonyl]butyl]amino]sulfonyl][1,1'-biphenyl]-2-yl]acetamide hydrochloride (SSR182289A) is a novel, potent, and selective thrombin inhibitor. We have examined the antithrombotic properties of SSR182289A administered by i.v. and p.o. routes in several different animal thrombosis models in comparison with reference antithrombotic agents. Oral administration of SSR182289A produced dose-related antithrombotic effects in the following models; rat venous thrombosis (ED(50) 0.9 mg/kg p.o.), rat silk thread arterio-venous (AV) shunt (ED(50) 3.8 mg/kg p.o.), rat thromboplastin-induced AV shunt (ED(50) 3.1 mg/kg p.o.), rat carotid artery thrombosis (ED(200) 5.9 mg/kg p.o.), and rabbit venous thrombosis (ED(50) 7.5 mg/kg p.o.). Administered as an i.v. bolus, SSR182289A showed antithrombotic activity in the above models with ED(50)/ED(200) values in the range of 0.2 to 1.9 mg/kg i.v. SSR182289A increased rat tail transection bleeding time at doses > or =10 mg/kg p.o. In the rat thromboplastin-induced AV shunt model, SSR182289A 10 mg/kg p.o. produced marked antithrombotic effects at 30, 60, 120, and 240 min after administration. Hence, SSR182289A demonstrates potent oral antithrombotic properties in animal venous, AV-shunt, and arterial thrombosis models.

Nonpeptide factor Xa inhibitors III: effects of DPC423, an orally-active pyrazole antithrombotic agent, on arterial thrombosis in rabbits

DPC423 [1-[3-(aminomethyl)phenyl]-N-[3-fluoro-2'-(methylsulfonyl)[1,1'-biphenyl]-4-yl]-3-(trifluoromethyl)-1H-pyrazole-5-carboxamide] is a synthetic, competitive, and selective inhibitor of coagulation factor Xa (fXa) (K(i): 0.15 nM in humans, 0.3 nM in rabbit). The objective of this study was to compare effects of DPC423, enoxaparin (low-molecular-weight heparin), and argatroban (thrombin inhibitor) on arterial thrombosis and hemostasis in rabbit models of electrically induced carotid artery thrombosis and cuticle bleeding, respectively. Compounds were infused i.v. continuously from 60 min before artery injury or cuticle transection to the end of experiment. Carotid blood flow was used as a marker of antithrombotic effect. Antithrombotic ED(50) values were 0.4 mg/kg/h for enoxaparin (n = 6), 0.13 mg/kg/h for argatroban (n = 6), and 0.6 mg/kg/h for DPC423 (n = 12). DPC423 at the maximum antithrombotic dose increased activated partial thromboplastin time and prothrombin time (n = 6) by 1.8 +/- 0.07- and 1.8 +/- 0.13-fold, respectively, without changes in thrombin time and ex vivo thrombin activity. The antithrombotic effect of DPC423 was significantly correlated with its ex vivo anti-fXa activity (r = 0.86). DPC423 at 1, 3, and 10 mg/kg p.o. increased carotid blood flow (percent control) at 45 min to 10 +/- 4, 24 +/- 6, and 74 +/- 7, respectively (n = 6/group). Cuticle bleeding times (percent change over control) determined at the maximum antithrombotic dose were 88 +/- 12 for argatroban, 69 +/- 13 for heparin, 4 +/- 3 for enoxaparin, 5 +/- 4 for DPC423, and -3 +/- 2 for the vehicle (n = 5-6/group), suggesting dissociation of antithrombotic and bleeding time effects for DPC423 and enoxaparin. The combination of aspirin and DPC423 at ineffective antithrombotic doses produced significant antithrombotic effect. Therefore, these results suggest that DPC423 is a clinically useful oral anticoagulant for the prevention of arterial thrombosis.

Synthesis and evaluation of 1-arylsulfonyl-3-piperazinone derivatives as a factor Xa inhibitor II. Substituent effect on biological activities

Intravascular clot formation is an important event in a number of cardiovascular diseases. The prevention of blood coagulation has become a major target for new therapeutic agents. Factor Xa (FXa) is a trypsin-like serine protease that plays a key role in the blood coagulation cascade and represents an attractive target for anticoagulant drug development. We have investigated substituents in the central part of a lead compound (3: M55113), and discovered that compound M55551 (34: (R)-4-[(6-Chloro-2-naphthalenyl)sulfonyl]-6-oxo-1-[[1-(4-pyridinyl)-4-piperidinyl]methyl]-2-piperazinecarboxylic acid) is a potent inhibitor of FXa (IC(50)=0.006 microM), with high selectivity for FXa over trypsin and thrombin. The activity of this compound is ten times more powerful than the lead compound.

Design, synthesis and biological activity of YM-60828 derivatives: potent and orally-bioavailable factor Xa inhibitors based on naphthoanilide and naphthalensulfonanilide templates

Factor Xa (FXa) is a serine protease which plays a pivotal role in the coagulation cascade. The inhibition of FXa has received great interest as a potential target for the development of new antithrombotic drug. Herein we describe a series of novel 7-amidino-2-naphthoanilide and 7-amidino-2-naphthalensulfonanilide derivatives which are potent FXa inhibitors. These scaffolds are rigid and are allowed to adopt an L-shape conformation which was estimated as the active conformation based on a docking study of YM-60828 with FXa. Optimization of the side chain at the central aniline nitrogen of 7-amidino-2-naphthoanilide has led to several potent and orally active FXa inhibitors. 5h (YM-169964), the best compound of these series, showed potent FXa inhibitory activity (IC(50)=3.9nM) and effectively prolonged prothrombin time by 9.6-fold ex vivo at an oral dose of 3mg/kg in squirrel monkeys.

Overview of anticoagulant drugs for the future

More efficacious, safer, and easier to use anticoagulants are under development. Multiple agents have been shown to be effective in ex vivo or animal thrombosis models and several have progressed to clinical studies. Investigators have not yet determined if pharmaceuticals that inhibit coagulation factor activity earlier in the cascade (for example, inhibitors of tissue factor/factor VIIa, factor IXa, or Xa) are superior to those that block the cascade at a later point. Orally bioavailable drugs for the long-term treatment of thrombotic disorders, particularly those that do not require monitoring, are needed and are under development. Local delivery of anticoagulants or genes modulating anticoagulant control at sites of increased thrombogenicity, such as in diseased arteries, is a promising treatment modality that may decrease systemic bleeding problems. Much about the initiating pathophysiologic events leading to venous thrombotic disease needs to be elucidated before such local therapy can be tested in the venous vasculature. While awaiting better anticoagulants to become routinely available, we need to improve patient management with existing drugs by instituting anticoagulation clinics, promoting patient self-monitoring, and improving efforts to educate patients and health care providers about the use of anticoagulant drugs.

The discovery of YM-60828: a potent, selective and orally-bioavailable factor Xa inhibitor

Since Factor Xa (FXa) is well known to play a central role in thrombosis and hemostasis, inhibition of FXa is an attractive target for antithrombotic strategies. As a part of our investigation of a non-peptide, orally available FXa inhibitor, we found that a series of N-[(7-amidino-2-naphthyl)methyl]aniline derivatives possessed potent and selective inhibitory activities. Structure--activity relationship (SAR) of the substituent (R(1)) on the central aniline moiety suggested that increasing lipophilicity caused a detrimental effect on anticoagulant activity (prothrombin time assay) in plasma. Several compounds bearing a hydrophilic substituent in R(1) showed not only potent FXa inhibitory activities but also high anticoagulant activities. The best compound in this series was sulfamoylacetic acid derivative (YM-60828) which was a potent, selective and orally bioavailable FXa inhibitor and was chosen for clinical development.

Synthesis and evaluation of 1-arylsulfonyl-3-piperazinone derivatives as factor Xa inhibitor

Intravascular clot formation is an important factor in a number of cardiovascular diseases. Therefore, the prevention of blood coagulation has become a major target for new therapeutic agents. One attractive approach is the inhibition of factor Xa (FXa), which is a key enzyme in coagulation cascade responsible for the generation of thrombin by limited proteolysis of its zymogen, prothrombin. We have investigated 1-arylsulfonyl-3-piperazinone derivatives, containing a 4-(piperidino)pyridine group in place of guanidino and/or amidino groups, and discovered compound M55113 (30a: 4-[(6-Chloro-2-naphthalenyl)sulfonyl]-1-[[1-(4-pyridinyl)-4-piperidinyl]methyl]piperazinone), as a potent inhibitor of FXa (IC50=0.06 microM) with high selectivity for FXa over trypsin and thrombin.

Novel inhibitors of factor X for use in cardiovascular diseases

The complementary roles of platelets and thrombin in the pathophysiology of acute coronary syndromes suggests that for treatment to be effective, both mediators must be targeted. Although great strides have been made in the development of antiplatelet therapies, attempts to inhibit thrombin have been less successful. Unfractionated heparin is limited by a number of pharmacologic shortcomings as well as an inability to meaningfully suppress thrombin generation. The low molecular weight heparins have yielded encouraging results in large-scale clinical trials, but it remains unclear whether their benefit stems from a superior pharmacologic profile to unfractionated heparin or is determined by an enhanced ability to suppress thrombin generation (by virtue of a direct anti-Xa effect). Regardless, investigators have become increasingly interested in factor Xa as a potential target for antithrombotic therapy. A number of naturally occurring Xa antagonists have been identified. Work with recombinant forms of these proteins confirms that factor Xa inhibition can suppress thrombin generation in a variety of animal thrombosis models. Accordingly, a number of synthetic direct and indirect Xa antagonists are under development for the prevention and treatment of thrombotic disorders. The following review summarizes the evolution of factor Xa antagonists.

Z-335, a new thromboxane A(2) receptor antagonist, prevents arterial thrombosis induced by ferric chloride in rats

We examined the antithrombotic effect of Z-335 ((+/-)-sodium [2-(4-chlorophenylsulfonylaminomethyl)indan-5-yl]acetate monohydrate), an orally active thromboxane A(2) receptor (TP-receptor) antagonist that ameliorates experimental gangrene, using a rat arterial thrombosis model. The thrombi were induced by topical application of 50% ferric chloride solution to the rats abdominal artery. Z-335 (0.3-3 mg/kg, p.o.) inhibited thrombus formation in a dose-dependent manner. The antithrombotic effect of Z-335 (1 and 3 mg/kg, p.o.) was almost equivalent with that of cilostazol (100 mg/kg, p.o.), a selective phosphodiesterase type III inhibitor. The effect of Z-335 (3 mg/kg, p.o.), but not cilostazol, persisted for 16 h. Z-335, but not cilostazol, inhibited platelet aggregation induced by U-46619 (a TP-receptor agonist, 9, 11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha)) for 16 h in rat whole blood. Histopathological examination also revealed that Z-335 prevented ferric chloride-induced thrombus formation. These results suggest that Z-335 may prevent ferric chloride-induced arterial thrombosis through its antiplatelet action by blocking TP-receptor activation.

Antithrombotic and hemostatic capacity of factor Xa versus thrombin inhibitors in models of venous and arteriovenous thrombosis

Thrombin plays a central role in venous and arterial thrombosis. We utilized two different rabbit models of in vivo thrombosis to investigate the effect of inhibitors of thrombin generation and thrombin activity. The agents tested were specific inhibitors of factor Xa (fXa) [N2-[(phenylmethyl)sulfonyl]-D-arginyl-N-[(1S)-4-[(aminoiminomethyl++ +)a mino]-1-(2-thiazolylcarbonyl)butyl]-glycinamide (C921-78)] and thrombin [D-phenylalanyl-N-[4-[(aminoiminomethyl)amino]-1-(chloroacetyl)but yl]-L-prolinamide (PPACK)], as well as drugs that affect both thrombin and fXa, unfractionated and low molecular weight (enoxaparin) heparin. The agents administered as constant intravenous infusion were evaluated for antithrombotic efficacy in anesthetized rabbits. All four agents were capable of dose dependent inhibition of thrombosis in venous and arteriovenous thrombosis models. However, due to the more aggressive nature of thrombotic stimulation in the arteriovenous shunt model, complete cessation of thrombus growth was not achieved for any of the agents at the doses tested. Comparison between the agents focused on the differences in extension of coagulation parameters (activated partial thromboplastin time, prothrombin time, thrombin clotting time), changes in hematological parameters, and extension of rabbit cuticle bleeding time at doses required to produce maximum inhibition in the thrombosis models. In the venous thrombosis model at the maximally effective dose, C921-78 had minimal extension of ex vivo clotting parameters, while enoxaparin and unfractionated heparin demonstrated a two to sevenfold increase in activated partial thromboplastin times, and PPACK had a threefold extension of thrombin clotting times. In addition, unlike the other three agents, which exhibited no significant changes in hematological parameters, PPACK demonstrated dose dependent thrombocytopenia. A standardized cuticle bleeding time was used as a measure of perturbation of hemostasis. The agents were evaluated for significant increases in bleeding time at doses up to eight times that needed to completely inhibit venous thrombus formation. Unfractionated heparin displayed a significant bleeding time effect at the dose required to inhibit venous thrombosis (100 u/kg+2 u/kg/min). Enoxaparin and PPACK caused significant bleeding time extensions at four times the fully efficacious venous dose (800 u/kg+8 u/kg/min and 30 microg/kg/min). By contrast, C921-78 did not significantly increase bleeding time even at eight times the maximally effective dose (240 microg/kg+7.2 microg/kg/min). Our results demonstrate that specific inhibition of fXa can be utilized to derive potent antithrombotic activity without disrupting extravascular hemostasis.

Contribution of in vivo models of thrombosis to the discovery and development of novel antithrombotic agents

Cardiovascular and cerebrovascular diseases continue to be the leading cause of death throughout the world. Over the past two decades, great advances have been made in the pharmacological treatment and prevention of thrombotic disorders (e.g., tissue plasminogen activators, platelet GPIIb/IIIa antagonists, ADP receptor antagonists such as clopidogrel, low-molecular weight heparins, and direct thrombin inhibitors). New research is leading to the next generation of antithrombotic compounds such as direct coagulation FVIIa inhibitors, tissue factor pathway inhibitors, gene therapy, and orally active direct thrombin inhibitors and coagulation Factor Xa (FXa) inhibitors. Animal models of thrombosis have played a crucial role in discovering and validiting novel drug targets, selecting new agents for clinical evaluation, and providing dosing and safety information for clinical trials. In addition, these models have provided valuable information regarding the mechanisms of these new agents and the interactions between antithrombotic agents that work by different mechanisms. This review briefly presents the pivitol preclinical studies that led to the development of drugs that have proven to be effective clinicallly. The role that animal models of thrombosis are playing in the discovery and development of novel antithrombotic agents is also described, with specific emphasis on FXa inhibitors. The major issues regarding the use of animal models of thrombosis, such as the use of positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species-specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models in thrombosis/hemostasis research and pharmacology is presented using gene-therapy for hemophilia as an example of how animal models have aided in the development of these therapies that are now being evaluated clinically. In summary, animal models have contributed greatly to the discovery of currently available antithrombotic agents and will play a primary role in the discovery and characterization of the novel antithrombotic agents that will provide safe and effective pharmacological treatment for life-threatening thrombotic diseases.

Antithrombotic efficacy of RPR208566, a novel factor Xa inhibitor, in a rat model of carotid artery thrombosis

Coagulation factor Xa is the sole enzyme responsible for activating the zymogen prothrombin to thrombin, resulting in fibrin generation, platelet activation, and subsequent thrombus formation. Our objective was to evaluate the antithrombotic efficacy of the novel factor Xa inhibitor, 2-(3-carbamimidoyl-benzyl)-3-[(3', 4'dimethoxy-biphenyl-4-carbonyl)-amino]-butyric acid methyl ester-trifluoroacetate (RPR208566), in a well-established rat model of arterial thrombosis, and to compare the results with those obtained with argatroban and heparin, direct and indirect inhibitors of thrombin, respectively. Thrombus formation was initiated by placing a filter paper saturated with FeCl(2) on the adventia of the carotid artery for 10 min. Time-to-occlusion was measured from initiation of injury until blood flow reached zero. Formed thrombi were removed and weighed 60 min after the placement of the filter paper. RPR208566, heparin, and argatroban dose-dependently increased time-to-occlusion and reduced thrombus mass. When administered at 500 microgram/kg+50 microgram/kg/min, RPR208566 prolonged time-to-occlusion to 56+/-4 min (vs. 18+/-2 min for vehicle) and reduced thrombus mass to 3.0+/-0.7 mg (vs. 7.3+/-0.6 mg for vehicle). The highest doses of argatroban (500 microgram/kg+50 microgram/kg/min) and heparin (300 U/kg+10 U/kg/min) increased time-to-occlusion to the maximum of 60 min and decreased thrombus mass to 5.5+/-0.8 and 2.6+/-0.3, respectively. The antithrombotic effects of heparin and argatroban at these doses were associated with increases in activated partial thromboplastin time of 5.6+/-0.9- and 2.9+/-0.3-fold over baseline, respectively. However, the highest dose of RPR208566 produced a modest 1.3+/-0.1-fold increase in activated partial thromboplastin time. These results indicate that factor Xa inhibition with compounds such as RPR208566 may be an attractive mechanism for novel antithrombotic drug therapy.