New Anticoagulants for the Prevention and Treatment of Venous Thromboembolism

Abstract

Anticoagulant drugs, like vitamin K antagonists and heparin, have been the mainstay for the treatment and prevention of venous thromboembolic disease for many years. Although effective if appropriately used, traditional anticoagulants have several limitations such as unpredictable pharmacologic and pharmacokinetic responses and various adverse effects including serious bleeding complications. New oral anticoagulants have recently emerged as an alternative because of their rapid onset/offset of action, predictable linear dose-response relationships and fewer drug interactions. However, they are still associated with problems such as bleeding, lack of reversal agents and standard laboratory monitoring. In an attempt to overcome these drawbacks, key steps of the hemostatic pathway are investigated as targets for anticoagulation. Here we reviewed the traditional and new anticoagulants with respect to their targets in the coagulation cascade, along with their therapeutic advantages and disadvantages. In addition, investigational anticoagulant drugs currently in the development stages were introduced.

Full text

INTRODUCTION
Venous thromboembolism (VTE) refers to thrombosis with-
in the vein, commonly in the legs or pelvis (deep vein thrombo-
sis, DVT) and its complication, pulmonary embolism (PE), the
condition of thrombi departing from their original generation
site into a pulmonary artery (Hyers, 1999). It is the third lead-
ing cause of cardiovascular-related deaths, following acute
coronary syndrome and stroke (Piazza and Goldhaber, 2010),
with an annual incidence of 1 to 3 times per 1,000 people (Heit
et al., 2016; Puurunen et al., 2016). Moreover, it often leads
to long-term complications such as post-thrombotic syndrome
and chronic thromboembolic pulmonary hypertension, which
impose a signicant burden on both patients and the health-
care systems (Ruppert et al., 2010; Bruni-Fitzgerald, 2015).
Pathologic thrombosis or bleeding may occur whenever
the hemostatic balance is disturbed due to various health
conditions including surgery, trauma, malignancy, and con-
genital disorders (Previtali et al., 2011) and even following
chronic cigarrete smoking (Park et al., 2016). In normal cir-
cumstances, hemostasis is maintained through the complex
interactions between the vascular system (Kwon et al., 2016),
coagulation system, brinolytic system (Lee et al., 2015) and
platelets (Kim et al., 2016). Natural anticoagulants such as
tissue factor pathway inhibitors (TFPI), protein C, protein S,
and anti-thrombin (AT) also regulate the coagulation process.
The brinolytic system plays a role by dissolving the brin clot
during the healing process of an injured blood vessel (Weitz,
1997; Chapin and Hajjar, 2015).
Anticoagulants can inhibit thrombosis by altering various
pathways within the coagulation system or through targeting
thrombin directly by attenuating its generation (Mega and Si-
mon, 2015). For many years, unfractionated heparins (UFHs)
and vitamin K antagonists (VKAs) have been the main op-
tions for the prevention and treatment of VTE (Franchini et al.,
2016). The treatment changed little until low molecular weight
heparins (LMWHs), fragments of UFHs, were introduced in the
1980s, simplifying the management of thromboembolism by
saving the trouble of frequent coagulation monitoring (Weitz,
1997). In the 2000s, ultra-low molecular heparins (ULMWHs)
were developed in an effort to improve the pharmacokinetic
prole of conventional heparin formulations and to lower the
461
New Anticoagulants for the Prevention and Treatment of Venous
Thromboembolism
Joo Hee Kim
1,2
, Kyung-Min Lim
2,
* and Hye Sun Gwak
2,
*
1College of Pharmacy & Institute of Pharmaceutical Science and Technology, Ajou University, Suwon 16499,
2College of Pharmacy, Ewha Womans University, Seoul 03760, Republic of Korea
Anticoagulant drugs, like vitamin K antagonists and heparin, have been the mainstay for the treatment and prevention of venous
thromboembolic disease for many years. Although effective if appropriately used, traditional anticoagulants have several limita-
tions such as unpredictable pharmacologic and pharmacokinetic responses and various adverse effects including serious bleed-
ing complications. New oral anticoagulants have recently emerged as an alternative because of their rapid onset/offset of action,
predictable linear dose-response relationships and fewer drug interactions. However, they are still associated with problems such
as bleeding, lack of reversal agents and standard laboratory monitoring. In an attempt to overcome these drawbacks, key steps of
the hemostatic pathway are investigated as targets for anticoagulation. Here we reviewed the traditional and new anticoagulants
with respect to their targets in the coagulation cascade, along with their therapeutic advantages and disadvantages. In addition,
investigational anticoagulant drugs currently in the development stages were introduced.
Key Words: Anticoagulant, Vitamin K antagonist, Heparin, Venous thromboembolism
Abstract
Biomol Ther 25(5), 461-470 (2017)
Review
Copyright © 2017 The Korean Society of Applied Pharmacology
https://doi.org/10.4062/biomolther.2016.271
Open Access
This is an Open Access article distributed under the terms of the Creative Com-
mons Attribution Non-Commercial License (http://creativecommons.org/licens-
es/by-nc/4.0/) which permits unrestricted non-commercial use, distribution,
and reproduction in any medium, provided the original work is properly cited.
www.biomolther.org
*
Corresponding Authors
E-mail: kmlim@ewha.ac.kr (Lim KM), hsgwak@ewha.ac.kr (Gwak HS)
Tel: +82-2-3277-3055 (Lim KM), +82-2-3277-4376 (Gwak HS)
Fax: +82-2-3277-3760 (Lim KM), +82-2-3277-2851 (Gwak HS)
Received
Dec 11, 2016
Revised
Jan 21, 2017 Accepted
Jan 26, 2017
Published Online
Apr 6, 2017

462
https://doi.org/10.4062/biomolther.2016.271
risk of heparin-induced thrombocytopenia (HIT) (Masuko and
Linhardt, 2012). However, all forms of heparin require paren-
teral administration, which is cumbersome for long-term use
(Fareed et al., 2008). Similarly, oral VKAs have several draw-
backs including a wide range of food and drug interactions, as
well as the need for frequent monitoring and dose adjustment
(Hirsh et al., 2007).
Over the past decades new oral anticoagulants (NOACs),
which more directly and selectively target specic proteins in
the coagulation cascade, have been developed, as shown in
Fig. 1. They are conveniently administered in oral, xed doses
without routine monitoring and have fewer interactions than
Fig. 1. Chemical structures of current anticoagulants.
Warfarin Dabigatran
Rivaroxaban
Heparin
Apixaban Edoxaban
Table 1. Traditional and novel anticoagulants in the market and development
Generic Name Mechanism of action Reversal agents Anticoagulation monitoring
Traditional drugs
Warfarin Deplete coagulation factors II VII, IX, and X through
inhibition of cyclic interconversion of vitamin K and its epoxide
Vitamin K INR
UFH Indirectly inhibit thrombin (factor II), factor X, IX, XI, and XII via
enhancing the activity of antithrombin
Protamine sulfate PT, aPTT
LMWH Inhibit thrombin and factor X via enhancing the activity of
antithrombin
Protamine sulfate Anti-Xa assay
ULMWH Inhibit factor X via enhancing the activity of antithrombin - Anti-Xa assay
New drugs
Dabigatran Inhibit free and brin-bound thrombin via direct binding Idarucizumab aPTT, ECT
Rivaroxaban Inhibit free and brin-bound factor Xa via direct binding Andexanet alfa, PER977 Anti-Xa assay
Apixaban Inhibit free and brin-bound factor Xa via direct binding Andexanet alfa, PER977 Anti-Xa assay
Edoxaban Inhibit free and brin-bound factor Xa via direct binding Andexanet alfa, PER977 Anti-Xa assay
Drugs under development
Tifacogin Inhibit tissue factor-factor VIIa complex - -
TB-402 Inhibit factor VIII via direct binding - -
Pegnivacogin Inhibit factor IX via direct binding - -
Factor XI-ASO Inhibit factor XI via direct binding - -
rHA-infestin-4 Inhibit factor XII - -
Recomodulin Inhibit factor V and VIII via activating protein C through
thrombin-thrombomodulin complex
- -
aPTT: activated partial thromboplastin time, ASO: antisense oligonucleotide, INR: international Normalized Ratio, ECT: ecarin clotting time,
LMWH: low molecular weight heparin, PT: Prothrombin time, UFH: unfractionated heparin, ULMWH: ultra-low molecular weight heparin.
Biomol Ther 25(5), 461-470 (2017)

www.biomolther.org
Kim et al. New Anticoagulants for Venous Thromboembolism
463
VKAs with foods or drugs (Mekaj et al., 2015). But NOACs
have their own limitations such as lack of reliable coagulation
monitoring methods and selective antidotes (except dabiga-
tran), as shown in Table 1. This review summarizes the phar-
macologic characteristics of traditional and new anticoagu-
lants, as well as anticoagulants under development, focusing
on their advantages and disadvantages.
TRADITIONAL ANTICOAGULANTS
Vitamin K antagonists
VKAs such as coumarin derivatives (e.g., warfarin, aceno-
coumarol, and phenprocoumon) exert their anticoagulant ef-
fects by interfering with the cyclic interconversion of vitamin K
and its 2,3 epoxide (KO), therefore depleting the vitamin K hy-
droquinone (KH2; Wessler and Gitel, 1986). The coagulation
factors II (thrombin), VII, IX, and X, as well as proteins C and
S, require carboxylation, by converting glutamic acid to gam-
ma-carboxyglutamic acid, for their normal functions. The car-
boxylation procedure requires KH2 (Ageno et al., 2012). VKAs
inhibit vitamin K epoxide reductase complex 1 (VKORC1), an
enzyme that catalyzes the reduction of KO to vitamin K, which
is then converted to KH2 and then oxidized back to KO, con-
comitantly with gamma-glutamyl carboxylation. The full antico-
agulation effect of warfarin is not achieved until the clearance
of factor X and prothrombin that have half-lives of 36 and 50
h, respectively (Loke et al., 2012). Because proteins C and S,
with relatively short half-lives, initially exert their procoagulant
effects, the combined use of VKAs with parenteral agents is
required.
The anticoagulant response to warfarin is largely affected
by diet, concurrent drugs, and genetic polymorphisms (Hirsh
et al., 2003). The anticoagulant effect of warfarin can be coun-
teracted by vitamin K intake either through food or supple-
ments. A large amount of vitamin K causes warfarin resistance
for up to a week because vitamin K accumulated in the liver
can bypass VKORC (Lurie et al., 2010). The cytochrome P450
enzyme (CYP2C9) is responsible for oxidative metabolism of
the warfarin S-isomer, which is ve times more potent than the
R-isomer. Therefore, the dose-response of warfarin can be in-
uenced by CYP2C9 inhibiting drugs that affect the metabolic
clearance of warfarin, especially the S-isomers, such as phen-
ylbutazone, sulnpyrazone, metronidazole, trimethoprim-sul-
famethoxazole, or amiodarone. It is also inuenced by genetic
polymorphisms in CYP2C9 and VKORC unit 1 genes (Fung et
al., 2012). The individuals who carry CYP2C9*2 or CYP2C9*3
tend to have higher levels of S-warfarin due to the impaired
ability to metabolize it. Since CYP2C9 is also responsible for
the metabolism of acenocoumarol, and less importantly for
phenprocoumon, polymorphisms of CYP2C9 also affect the
efcacy of acenocoumarol and phenprocoumon, although
with a lesser extent than with warfarin (Verhoef et al., 2014).
Genetic mutations or an altered expression of the VKORC1
gene can also lead to variable responses, either hyper-sen-
sitivity or resistance to warfarin therapy. Acenocoumarol and
phenprocoumon are also inuenced by the VKORC1 geno-
type, especially in the rst few months.
Hemorrhage is the most signicant and frequent compli-
cation related to warfarin, with an annual incidence of major
bleeding at a rate of 13 per 100 patients (Linkins et al., 2003).
The risk of bleeding associated with warfarin is related not
only to the degree of anticoagulation but also to patient-related
factors and the concurrent use of antiplatelet agents or other
drugs (Fitzmaurice et al., 2002). Bleeding complications can
be managed by administering vitamin K, fresh frozen plasma
(FFP), prothrombin complex concentrates (PCCs), or recom-
binant factor VIIa (Tran et al., 2013), which can antagonize the
effects of warfarin therapy.
Heparins
Heparins indirectly inhibit thrombin by enhancing the activ-
ity of antithrombin (AT), a proteinase inhibitor of coagulation
enzymes such as thrombin and factors Xa of the common
pathway, as well as IXa, XIa and XIIa of the intrinsic coagula-
tion pathway (Hirsh and Raschke, 2004). Following a confor-
mational change induced by heparin, AT irreversibly inhibits
thrombin via binding its active site. For inactivation of throm-
bin, heparin must bind simultaneously to thrombin at exosite
2 and AT, forming a ternary complex which requires at least
18 saccharide units (Liaw et al., 2001). In contrast, heparin
only binds to AT via high-afnity pentasaccharides, for the in-
hibition of factor Xa, without requiring a bridge between fac-
tor Xa and AT. Since most heparin molecules are at least 18
units long, inhibitory activities of heparin against the thrombin
and factor Xa are equivalent (Hirsh, 1991). However, the AT-
bound heparin weakly inhibits the thrombin, once formed as
a ternary heparin-brin-thrombin complex, because it can no
longer gain access to the exosite 2 already occupied. Further-
more, AT-bound heparin is unable to inhibit factor Xa bound to
activated platelets (Teitel and Rosenberg, 1983).
Unfractionated heparins (UFH): Unfractionated heparin
(UFH) is heterogeneous in terms of molecular size, anticoagu-
lant activity and pharmacokinetics (Garcia et al., 2012). The
molecular weight of UFH ranges from 3,000 to 30,000 Da, with
an average of 15,000 Da (approximately 45 saccharide units).
Only 20-50% of UFH chains contain the high-afnity pentasac-
charide unit necessary for activating AT (Marmur, 2002). Hep-
arin molecules without a pentasaccharide unit have minimal
activity at therapeutic concentrations. Low-afnity heparin can
inhibit thrombin via heparin cofactor (Tollefsen et al., 1982) as
well as factor Xa generation, through AT-independent mecha-
nisms (Garcia et al., 2012).
Besides the multiple anticoagulant mechanisms, heparin
involves multiple clearance mechanisms including both rap-
id, saturable and slow, non-saturable processes (Hirsh and
Fuster, 1994). The rapid phase of heparin clearance occurs
through binding to macrophages and endothelial cells at satu-
rable sites on the cell membrane and subsequent depolymer-
ization, whereas the slow clearance mechanism is through the
kidneys. Because low doses of heparins initially undergo the
saturable and dose-dependent clearance, the effect of heparin is
not linear, although both intensity and duration of heparin activity
may increase with escalating doses. As a result, the pharmaco-
kinetics and pharmacodynamics of UFH are unpredictable.
Additionally, UFH has a number of limitations such as a
short duration of action with a half-life of 60 min, poor bioavail-
ability after subcutaneous injection, and an immune-mediated
reaction, a life-threatening adverse event (Krishnaswamy et
al., 2010). Heparin complexes with an endogenous platelet
factor 4 (PF4), which undergoes conformational changes and
becomes immunogenic, leading to the generation of heparin-
PF4 antibodies (Kreimann et al., 2014). The heparin-PF4-IgG

464
https://doi.org/10.4062/biomolther.2016.271
immune complex then activates platelets and causes the re-
lease of microparticles, platelet consumption and peripheral
thrombocytopenia, and also endothelial injury and activation
(Rauova et al., 2006). Heparin’s afnity for PF4 depends on
molecular weights and chain lengths (Amiral et al., 1995), thus
accounting for the increased incidence of thrombocytopenia
by UFH when compared with LMWH.
The effect of UFH can be reversed by intravenously ad-
ministering protamine sulfate that binds to heparin and forms
a stable salt (Greinacher et al., 2015). This can be advanta-
geous in situations of cardiac surgery or treating critically ill
patients who may require rapid reversal of the anticoagulation
effect.
Low molecular weight heparins (LMWH): LMWH is a
mixture of polymers with molecular weights that vary from
1,000 to 10,000 Da, with a mean molecular weight between
4,000 and 5,000 Da, approximately one third the size of UFH
(Hirsh, 1998). Since LMWHs are prepared by various chemi-
cal or physical depolymerizations of heparin, each LMWH
has unique characteristics in terms of molecular weight, poly-
saccharide chain length distributions, and pharmacological
properties that may inuence pharmacokinetic properties and
anticoagulant activity proles (Merli and Groce, 2010). Since
50 to 75% of LMWH species have a length of less than 18
saccharides, which will inhibit only factor Xa, the selectivity
ratio of activity against factor Xa to thrombin varies between
4:1 and 2:1, depending on their preparations (Holmer et al.,
1986). Because LMWH mostly undergoes renal elimination,
its biologic half-life may be prolonged in cases of renal insuf-
ciency, especially for those with lower molecular weights such
as enoxaparin or nadroparin (Schmid et al., 2009).
In addition to the convenience of subcutaneous adminis-
tration and almost 100% subcutaneous bioavailability, LMWH
has several advantages over UFH in terms of pharmacologi-
cal characteristics (Hirsh and Raschke, 2004). The low protein
binding of LMWH makes anticoagulant effects more predict-
able, which allows for a xed or body weight-based dose regi-
men without the need for frequent monitoring (Hirsh and Ra-
schke, 2004). Low nonspecic binding to macrophages and
endothelial cells increases the plasma half-life of LMWH. In
addition, the lower binding to platelets, PF4 and osteoclasts
may reduce the risk of HIT and osteoporosis.
Unlike UFH’s complete neutralization activity of anti-factor
Xa, protamine sulfate reverses only about 60% of the anti-
factor Xa activity of LMWH (Wolzt et al., 1995). This may be
due to the fact that protamine favors the regions of larger hep-
arin chains, and an effective antidote for the residual smaller
chains in LMWHs is not available (Schroeder et al., 2011).
Furthermore, the subcutaneous administration of heparins is
more difcult to completely reverse.
Ultra-low molecular weight heparins (ULMWHs): Ultra-
low molecular weight heparins (ULMWHs), also known as an
indirect factor Xa inhibitors, are synthetic analogues of the
pentasaccharide contained within heparins, with an average
molecular weight of less than 3,000 Da (Walenga and Lyman,
2013). These small, homogeneous drugs have been devel-
oped on the basis that higher selectivity in the activity against
factor Xa or thrombin would produce similar or better efcacy
than LMWHs, but have a lower risk of bleeding and HIT. ULM-
WHs also exhibit anticoagulant efcacy through the selective
inhibition of factor Xa via the unique pentasaccharide unit
(Hirsh, 1998).
ULMWHs only exhibit the anti-factor Xa effect when bind-
ing to AT and are devoid of other functional components of
heparins such as the release of TFPI from the vascular en-
dothelium, the formation of complexes with PF4, and probri-
nolytic actions. It is likely that the heparin chains must be of
a sufcient length to form a complex with PF4 for binding to
antibodies, which is a pathological mechanism of HIT (Rauova
et al., 2005). Besides anticoagulant activity which is weaker
than that of UFH or LMWHs, ULMWHs show anti-angiogenic,
anti-metastatic and anti-inammatory activities (Gandhi and
Mancera, 2010).
The hepatic clearance of heparin is believed to involve the
stabilin-2-receptor that requires heparin chains longer than
decasaccharides for binding (Pempe et al., 2012). Therefore,
unlike UFH, ULMWHs never reach the size needed for hepatic
clearance and therefore depend heavily on renal clearance
(Rupprecht and Blank, 2010).
ULMWHs offer several advantages over conventional hep-
arins such as a higher bioavailability, rapid onset of action with
longer biological half-lives, and a lower risk of bleeding, as
well as osteoporosis. Because of the absence of binding to
other plasma proteins, ULMWHs have predictable pharmaco-
kinetics with almost 100% bioavailability. However, no antidote
is available for bleeding associated with ULMWHs, whereas
protamine sulfate can neutralize UFH completely and LMWHs
partially. Unlike the impermeability of LMWHs or UFHs through
the placental or blood-brain barrier, ULMWHs are able to par-
tially pass the blood-brain barrier (Hoppensteadt et al., 2003).
NEW ANTICOAGULANTS
The newer anticoagulants offer superior therapeutic control
over coagulations with minimal bleeding complications. They
directly target either thrombin or factor Xa in the coagulation
cascade, which is pharmacologically distinct from traditional
anticoagulant agents (Fig. 2). Since the amount of activated
coagulation factors is amplied at each level of the coagula-
tion cascade, direct inhibition of the nal products from both
the intrinsic and extrinsic coagulation pathways (factor Xa and
thrombin) can provide more effective anticoagulation.
Direct thrombin inhibitors
Thrombin is an end product in the coagulation cascade,
which converts soluble brinogen to insoluble brin. It ampli-
es coagulation by activating factors V and VIII on the surface
of platelets and platelet-bound factor XI, stimulating platelets
and generating more thrombin. By activating XIII, it also ac-
celerates the formation of cross-linked brins and clot stabi-
lization. In addition to its procoagulant role, thrombin plays a
role in growth factor synthesis, cell proliferation, prostaglan-
din I2 synthesis, and chemotaxis of polymorphonuclear cells
(Coughlin, 1994). Therefore, inhibition of thrombin may pro-
vide benets in addition to anticoagulation (Bea et al., 2006).
The antithrombotic action of heparin occurs through binding
to both AT and thrombin’s exosite 2, a heparin-binding domain,
simultaneously. Heparin also can act as a bridge between -
brin and thrombin, enhancing thrombin’s afnity for brin and
increasing the amount of brin-bound thrombin. Because the
brin-heparin-thrombin complex, therefore, occupies not only
Biomol Ther 25(5), 461-470 (2017)

www.biomolther.org
Kim et al. New Anticoagulants for Venous Thromboembolism
465
exosite 2 but also exosite 1 (brin-binding site), brin-bound
thrombin is protected from inhibition by the heparin-AT com-
plex and remains active, resulting in further thrombus genera-
tion (Weitz et al., 1990). As such, heparin is relatively ineffec-
tive at inhibiting thrombin propagation (Di Nisio et al., 2005).
Unlike heparins, direct thrombin inhibitors (DTIs) act without
a preceding interaction with AT and directly suppress throm-
bin, as well as its interaction with its substrates (Di Nisio et al.,
2005). DTIs block the action of thrombin by binding to the cat-
alytic site (univalent) or to both the catalytic site and exosite 1
(bivalent) (Bates and Weitz, 2000). Therefore, DTIs can inhibit
both free and brin-bound thrombin. In addition, there are oth-
er advantages such as more predictable anticoagulant effects
due to the absence of interaction with plasma proteins, not
being neutralized by PF4, the inhibition of thrombin-induced
platelet aggregation and absence of immune-mediated throm-
bocytopenia (Lee and Ansell, 2011).
Bivalent DTIs include recombinant hirudins (e.g., lepirudin
and desirudin) and a synthetic hirudin, bivalirudin (Di Nisio
et al., 2005). Bivalent DTIs form an irreversible complex with
thrombin, but bivalirudin, which is slowly cleaved by thrombin
once bound, restores the catalytic function of thrombin (Lee
and Ansell, 2011). As a result, thrombin inhibition using bivali-
rudin is temporary, which may contribute to its low bleeding
risk compared with recombinant hirudins (Nawarskas and An-
derson, 2001). Bivalirudin is mainly cleared by proteolysis and
hepatic metabolism, whereas recombinant hirudins predomi-
nantly undergo renal excretion.
Univalent DTIs, such as argatroban, ximelagatran, and
dabigatran exteilate, non-covalently and reversibly bind to
thrombin, leaving a small fraction of free thrombin (Di Nisio
et al., 2005). Reversible and selective binding to thrombin ac-
companies a minimal risk of bleeding and rapid restoration of
hemostasis to baseline upon discontinuation. Like recombi-
nant hirudins, argatroban is a parenteral DTI but is metabo-
lized by the liver (Koster et al., 2007).
Ximelagatran, a prodrug of melagatran, is the rst oral DTI,
which represents a new era of anticoagulation for the preven-
tion and treatment of VTE. Although it was withdrawn from
the market due to a risk of signicant hepatotoxicity, ximela-
gatran demonstrated improved antithrombotic efcacy when
compared with traditional anticoagulation therapies (Evans et
al., 2004). A few years later, dabigatran etexilate, the second
oral DTI, was developed with some improvements such as no
risk of hepatotoxicity and low potential for food or drug interac-
tions. Following oral absorption, dabigatran etexilate is rapidly
converted into its active form, dabigatran, by nonspecic se-
rum esterase without the involvement of cytochrome P450 en-
zymes or other oxidoreductases. Therefore, dabigatran etexi-
late has a low potential for interacting with drugs (Stangier and
Clemens, 2009). Approximately 80% of circulating dabigatran
is excreted unchanged via the kidneys and the remainder is
conjugated with glucuronic acid. The conjugated dabigatran,
which exhibits similar properties to the unconjugated form, is
predominantly excreted via the bile.
Currently, none of the DTIs, except dabigatran, have direct
reversal agents available for use. Recombinant factor VIIa,
activated prothrombin complex concentrate (aPCC), activated
charcoal, desmopressin and von Wilebrand factor concentrate
have been tried in various studies (Majeed and Schulman,
2013; Baumann Kreuziger et al., 2014). Dabigatran effects
can be reversed within minutes of intravenous administration
of idarucizumab, a humanized monoclonal antibody, which
binds tightly and prevents dabigatran from binding to thrombin
(Sie, 2016).
XII XIIa
XI XIa
IX IXa
X Xa
XIII
Thrombin
Fibrin Fibrinogen
VIIIa +
Va
+
VII
VIIa
Intrinsic
Pathway
Extrinsic
Pathway
VIII
7
Cross-linked Fibrin Clot
Endothelial injury
*
*
Tissue factor
Inhibitors
Tifacogin
NAPc2
*
Prothrombin
Inhibitors
Factor XI-ASO
XIIIa
Inhibitors
TB-402
Inhibitors
Pegnivacogin
TTP889
SB249417
Inhibitors
rHA-infestin-4
Inhibitors
DrotAA
Recomodulin
Indirect inhibitors
LMWH
ULWM H
Direct inhibitors
Rivaroxaban
Apixaban
Edoxaban
Indirect inhibitors
UFH
LMWH
Direct inhibitors
Recombinant hirudins
Argatroban
Ximelagatran
Dabigatran
Fig. 2. Targets of various anticoagulants in the coagulation pathways. VKA: vitamin K antagonists, UFH: unfractionated heparin, LMWH:
low molecular weight heparin, ULMWH: ultra-low molecular weight heparin, NAP: nematode anticoagulant protein, ASO: antisense oligo-
nucleotide, DrotAA: drotecogin alpha (activated), *catalyzed by thrombin.

466
https://doi.org/10.4062/biomolther.2016.271
Direct factor Xa inhibitors
Factor Xa is a primary site of amplication for coagulation
factors, generating about 1,000 thrombin molecules from a
single Xa molecule (Mann et al., 2003). Factor Xa binds to
negatively charged phospholipid surfaces, which are exposed
on activated platelets, together with factor Va to form the pro-
thrombinase complex, the activator that converts prothrombin
into thrombin. The conversion of brinogen to brin, the basic
building block of all blood clots, is then catalyzed by thrombin.
The rate of prothrombin activation by factor Xa in a prothrom-
binase complex is dramatically increased, thereby rapidly fa-
cilitating thrombin generation and plug formation at sites of
injury. Whereas heparin inhibits factor Xa and thrombin to a
similar degree, LMWHs have a relatively greater inhibitory ef-
fect against factor Xa, which has drawn attention as a poten-
tial anticoagulant target (Garcia et al., 2012). The interest of
factor Xa as a drug target was further solidied by positive
results from the use of fondapariunux, a parenteral indirect
factor Xa inhibitor (Yeh et al., 2012).
Unlike indirect factor Xa inhibitors which are dependent on
AT, direct factor Xa inhibitors interact directly and selectively
with factor Xa and inhibit both free and bound forms of fac-
tor Xa without affecting platelet aggregation (Rupprecht and
Blank, 2010). They are also associated with reduced inci-
dence of rebound thrombosis compared to direct and indirect
thrombin inhibitors (Perzborn et al., 2011). Because direct fac-
tor Xa inhibitors have good bioavailability with rapid onset of
action, there is no need for bridging therapy with a parenteral
agent (Cabral and Ansell, 2015). In general, they exhibit linear
pharmacokinetics and display predictable anticoagulation ef-
fects following oral administration. All three agents, rivaroxa-
ban, apixaban, and edoxaban, are excreted through the kid-
neys to varying degrees and have elimination half-lives much
shorter than the VKAs. Rivaroxaban has a dual mechanism of
excretion, with two-thirds of the administered dose excreted
through the urine as either unchanged or inactive metabolites
and one-third of the dose excreted through feces (Perzborn
et al., 2011). Only 25% of an apixaban dose is eliminated by
the kidneys with the remainder excreted via the fecal route
(Eriksson et al., 2009). Edoxaban undergoes multiple elimina-
tion pathways with 35% excreted in the urine. Over 70% of
the dose is excreted unchanged (Bounameaux and Camm,
2014).
The substantial benets of oral factor Xa inhibitors are un-
fortunately accompanied by a high incidence of major and
clinically relevant bleeding including gastrointestinal bleed-
ing (Connolly and Spyropoulos, 2013). Moreover, all three
factor Xa inhibitors are CYP3A4 and P-glycoprotein (P-gp)
substrates that carry potential drug interaction issues. The
CYP3A4 and/or P-gp inhibitors, as well as inducers, might im-
pact the concentration of oral factor Xa inhibitors, leading to
increased risk of bleeding or thrombosis (Short and Connors,
2014).
Potential antidotes for reversing anticoagulation caused by
factor Xa inhibitors are currently under development (Ahmed
et al., 2016). Andexanet alfa is a recombinant, modied factor
Xa protein with a mutation on the catalytic site that abolishes
the procoagulant property, which binds to direct and indirect
factor Xa inhibitors in the blood (Connors, 2015). PER977
(e.g., arapazine and ciraparantag) binds to factor Xa inhibi-
tors, as well as direct and indirect thrombin inhibitors, through
noncovalent bonds and electrical charge interactions (Das
and Liu, 2015).
ANTICOAGULANTS UNDER DEVELOPMENT
Both thrombin and factor Xa inhibitors have been exten-
sively evaluated in several large clinical trials for the preven-
tion and treatment of thromboembolic disorders. Despite their
excellent efcacy compared to traditional agents, these drugs
have their own drawbacks. Bleeding is still a major issue, with
no reliable diagnostic test available to safely monitor the ther-
apeutic dosage, as well as a lack of effective reversal agents
(Hyers, 1999; Miller et al., 2012; Hu et al., 2016).
A variety of anticoagulant strategies, targeting other steps
in coagulation, are in development to attempt to overcome
the limitations of currently used agents. Drugs that target the
tissue factor (TF)-factor VIIa complex inhibit the initiation of
coagulation. Propagation of coagulation can be inhibited by
drugs that target factors IXa or Xa or by agents that inactivate
their respective cofactors, factors VIIIa and Va.
Tissue factor pathway inhibitors
Following vascular injury, TF, also known as thromboplas-
tin, is exposed to the blood and binds to factor VIIa, which
sets off the extrinsic coagulation pathway (Wood et al., 2014).
The TF-factor VIIa complex activates factors X and IX. Addi-
tionally, activated factor IX forms a complex with factor VIIIa,
which also activates factor X. Factor Xa then binds to factor
Va to form prothrombinase, an enzymatic complex which rap-
idly converts prothrombin to thrombin. The TF activity and the
extrinsic pathway are regulated by the tissue factor pathway
inhibitor (TFPI). It inhibits factor Xa directly and the TF-factor
VIIa complex in an Xa-dependent fashion. The factor Xa-de-
pendent inhibition of the TF-factor VIIa complex generates an
inactive quaternary complex in the plasma membrane.
Inhibition of TF-factor VIIa complex by recombinant TFPI
was examined in various models of disseminated intravas-
cular coagulation such as sepsis. Tifacogin, a recombinant
TFPI expressed in Saccharomyces cerevisae, inhibits factor
VIIa in a factor Xa-dependent fashion (Matyal et al., 2005).
The drug requires intravenous infusion since the drug has a
short plasma half-life and easily eliminated by the liver. The
benets of tifacogin administration in sepsis, pneumonia, and
bacteremia have been investigated without promising results
(Abraham et al., 2003; Hardy et al., 2006; Laterre et al., 2009).
The synthetic nematode anticoagulant protein (NAPc2), which
was originally isolated from the canine hookworm Ancylos-
toma canimum, binds to a non-catalytic site on factor Xa to
form a NAPc2-factor-Xa complex and inhibits factor VIIa from
binding to TF (Vlasuk and Rote, 2002). Because of its high af-
nity binding, NAPc2 has a half-life of about 50 h after subcu-
taneous administration. Factor VIIa with its active site blocked
competes with factor VIIa for TF binding sites, thereby attenu-
ating the initiation of coagulation by the TF-factor VIIa complex
(Dickinson and Ruf, 1997)
Factor VIII inhibitors
Factor VIII (i.e., anti-hemophilic factor) acts as a cofactor for
factor IXa, which activates factor X, thereby, forming an am-
plication loop (Lenting et al., 1998). Partial inhibition of factor
VIII appears to be essential to reduce the risk of bleeding be-
cause complete inhibition will induce pathological hemophilia.
Biomol Ther 25(5), 461-470 (2017)

www.biomolther.org
Kim et al. New Anticoagulants for Venous Thromboembolism
467
TB-402 is a recombinant human monoclonal antibody that
binds with a high afnity to factor VIII, partially inhibiting the
action of factor VIII (Verhamme et al., 2010). It is under phase
2 clinical trials and the exact target of factor VIII inhibition and
the degree of inhibition need to be established in further re-
search.
Factor IXa inhibitors
The TF-factor VIIa complex activates factor IX, which is
relatively stable and diffuses toward activated platelets (How-
ard et al., 2007). The activated platelets then bind the factor
VIIa-IXa complex and recruit factor X for its activation. The ac-
tivation of factor X by the factor VIIa-IXa complex is nearly 50
times more efcient than the TF-factor VIIa complex (Butenas
et al., 2002). Therefore, factor IXa represents a prime target
for anticoagulation. Defects in factor IXa lead to hemophilia B,
while increased concentrations of factor IXa in the blood result
in a signicantly increased risk of thrombosis formation.
Factor IXa inhibitors including factor IX-directed monoclonal
antibodies, factor IXa-directed RNA aptamers (e.g., pegniva-
cogin), and oral factor IXa inhibitors (e.g., TTP889) have been
investigated in humans. SB249417, a chimeric monoclonal
antibody directed against the factor IXa, completed a phase
I clinical trial, showing a dose-dependent effect on clotting
times after continuous infusion (Chow et al., 2002). The REG1
system consisted of pegnivacogin (RB006) and anivamersen
(RB007), its complementary control agent being an aptamer-
base factor IXa inhibitor that is being investigated for acute
coronary syndrome (Vavalle and Cohen, 2012). Aptamers are
small oligonucleotides with high afnity that are used as ac-
tive drugs. Partial inhibition using TTP889 was not an effective
strategy for VTE prophylaxis and TTP889 is currently being
investigated for advanced heart failure to determine the poten-
tial benet of attenuated thrombin generation (Roser-Jones et
al., 2011). Natural factor IX binding proteins and factor IXai are
under pre-clinical trials.
Factor XI inhibitors
A study showed that factor XI deciency was associated
with a less severe bleeding tendency and a lower incidence
of venous thrombosis and stroke, compared to deciencies
of factors VIII or IX, which suggests factor XI is a safe target
for anticoagulation. Factor XI inhibition has been extensive-
ly studied in both arterial and venous thrombosis in diverse
animal models. Antibodies and antisense oligonucleotides
(FXI-ASO) against factor XI both showed protective effects
in thrombosis without an increased risk of bleeding (Büller et
al., 2015). The anti-human factor XI monoclonal antibody was
used to prevent vascular graft occlusion in a primate throm-
bosis model. Similar studies are currently being conducted to
ensure the safety of factor XI inhibitors in humans.
Factor XII inhibitors
Available data on factor XII was limited but factor XII knock-
out mice were observed to have protection against pathologic
thrombosis while having no hemostasis changes. The selec-
tive factor XIIa inhibitor, recombinant human albumin fused
to the factor XIIa inhibitor infestin-4 (rHA-infestin-4), was
developed (Hagedorn et al., 2010). Inhibition of factor XII is
apparently a safe and efcient way of thrombosis prevention,
at least in animals. Factor XII antisense, Pro-Phe-Arg-chloro-
methylketone, Ir-CPI, and several non-specic protein inhibi-
tors are under pre-clinical trials.
Factor Va inhibitors
Factor V acts as a cofactor of factor Xa and forms a pro-
thrombinase complex, together with platelet membrane phos-
pholipids. Factor Va inhibitors include drotecogin alpha (ac-
tivated; DrotAA) and Recomodulin (ART-123), which were
initially developed for sepsis-induced thrombosis treatment.
DrotAA is a recombinant form of activated protein C with an-
tithrombotic, anti-inammatory and pro-brinolytic properties
(Dellinger, 2003). It showed some benecial effects for coagu-
lation abnormalities associated with severe sepsis but failed to
show improvement of patients with severe sepsis. As of 2011,
DrotAA was withdrawn from the market.
Recomodulin, a recombinant human thrombomodulin al-
pha, has shown to be efcacious for VTE prophylaxis fol-
lowing total hip replacement surgery and sepsis-associated
disseminated intravascular coagulation (DIC) (Kearon et al.,
2005; Vincent et al., 2013). Thrombomodulin is a thrombin
receptor and the thrombin-thrombomodulin complex activates
protein C to form activated protein C, which inactivates factors
Va and VIIIa (Esmon, 2005). It has a long plasma half-life after
a subcutaneous injection of two to three days, such that it can
be given once every ve to six days to maintain anticoagulant
activity.
Polyphosphate inhibitors
Polyphosphate is a polymer of inorganic phosphate resi-
dues and is secreted by activated platelets and mast cells. It
may initiate and/or accelerate coagulation, acting at several
points in the coagulation cascade (Ruiz et al., 2004). It accel-
erates the activation of factor V, as well as factor XI by throm-
bin, and enhances brin clot structure increasing its resistance
to brinolysis (Smith and Morrissey, 2008; Smith et al., 2012).
A variety of compounds that inhibit polyphosphate and reduce
thrombosis are under investigation in animal disease models.
Universal heparin reversal agent (UHRA) compounds were
studied in mouse models of thrombosis and hemostasis to
ensure reduced toxicity and bleeding risk, compared to the
toxic substances such as polyethylenimine, polyamidoamine
dendrimers, and polymyxin B (Travers et al., 2014).
CONCLUSIONS
The prevention and treatment of VTE is evolving. The new
target-specic oral anticoagulants such as oral DTIs and direct
factor Xa inhibitors have shifted a paradigm from hospitals to
outpatient settings exempting drug monitoring. Current oral
anticoagulants available offer predictable, reversible antico-
agulant effects with no need for invasive monitoring. However,
the major complication of these drugs, bleeding, especially
gastrointestinal bleeding, continues to persist, and an optimal
management strategy needs to be provided. To date, different
categories of anticoagulants are currently under development
with unique proles, along with benets and potential draw-
backs. In the future, the search for safer and more effective
oral anticoagulants that have an antidote for rapid reversal will
continue.

468
https://doi.org/10.4062/biomolther.2016.271
ACKNOWLEDGMENTS
This work is supported by the National Research Founda-
tion of Korea (Grant No. NRF-2015R1D1A1A01057931).
REFERENCES
Abraham, E., Reinhart, K., Opal, S., Demeyer, I., Doig, C., Rodriguez,
A. L., Beale, R., Svoboda, P., Laterre, P. F., Simon, S., Light, B.,
Spapen, H., Stone, J., Seibert, A., Peckelsen, C., De Deyne, C.,
Postier, R., Pettilä, V., Sprung, C. L., Artigas, A., Percell, S. R., Shu,
V., Zwingelstein, C., Tobias, J., Poole, L., Stolzenbach, J. C. and
Creasey, A. A. (2003) Efcacy and safety of tifacogin (recombinant
tissue factor pathway inhibitor) in severe sepsis: A randomized
controlled trial. JAMA 290, 238-247.
Ageno, W., Gallus, A. S., Wittkowsky, A., Crowther, M., Hylek, E. M.
and Palareti, G. (2012) Oral anticoagulant therapy: antithrombotic
therapy and prevention of thrombosis, 9th ed: American college
of chest physicians evidence-based clinical practice guidelines.
Chest 141, e44S-e88S.
Ahmed, Z., Hassan, S. and Salzman, G. A. (2016) Novel oral antico-
agulants for venous thromboembolism with special emphasis on
risk of hemorrhagic complications and reversal agents. Curr. Drug
Ther. 11, 3-20.
Amiral, J., Bridey, F., Wolf, M., Boyer-Neumann, C., Fressinaud, E.,
Vissac, A. M., Peynaud-Debayle, E., Dreyfus, M. and Meyer, D.
(1995) Antibodies to macromolecular platelet factor 4-heparin com-
plexes in heparin-induced thrombocytopenia: a study of 44 cases.
Thromb. Haemost. 73, 21-28.
Bates, S. M. and Weitz, J. I. (2000) The mechanism of action of throm-
bin inhibitors. J. Invasive Cardiol. 12 Suppl F, 27F-32.
Baumann Kreuziger, L. M., Keenan, J. C., Morton, C. T. and Dries,
D. J. (2014) Management of the bleeding patient receiving new
oral anticoagulants: A role for prothrombin complex concentrates.
BioMed Res. Int. 2014, 583794.
Bea, F., Kreuzer, J., Preusch, M., Schaab, S., Isermann, B., Rosenfeld,
M. E., Katus, H. and Blessing, E. (2006) Melagatran reduces ad-
vanced atherosclerotic lesion size and may promote plaque stabil-
ity in apolipoprotein E-decient mice. Arterioscler. Thromb. Vasc.
Biol. 26, 2787-2792.
Bounameaux, H. and Camm, A. J. (2014) Edoxaban: an update on the
new oral direct factor Xa inhibitor. Drugs 74, 1209-1231.
Bruni-Fitzgerald, K. R. (2015) Venous thromboembolism: an overview.
J. Vasc. Nurs. 33, 95-99.
Butenas, S., Brummel, K. E., Branda, R. F., Paradis, S. G. and Mann,
K. G. (2002) Mechanism of factor VIIa-dependent coagulation in
hemophilia blood. Blood 99, 923-930.
Büller, H. R., Bethune, C., Bhanot, S., Gailani, D., Monia, B. P., Ras-
kob, G. E., Segers, A., Verhamme, P. and Weitz, J. I. (2015) Factor
XI antisense oligonucleotide for prevention of venous thrombosis.
N. Engl. J. Med. 372, 232-240.
Cabral, K. P. and Ansell, J. E. (2015) The role of factor Xa inhibitors in
venous thromboembolism treatment. Vasc. Health Risk Manage.
11, 117-123.
Chapin, J. C. and Hajjar, K. A. (2015) Fibrinolysis and the control of
blood coagulation. Blood Rev. 29, 17-24.
Chow, F. S., Benincosa, L. J., Sheth, S. B., Wilson, D., Davis, C. B.,
Minthorn, E. A. and Jusko, W. J. (2002) Pharmacokinetic and phar-
macodynamic modeling of humanized anti-factor IX antibody (SB
249417) in humans. Clin. Pharmacol. Ther. 71, 235-245.
Connolly, G. and Spyropoulos, A. C. (2013) Practical issues, limita-
tions, and periprocedural management of the NOAC’s. J. Thromb.
Thrombolysis 36, 212-222.
Connors, J. M. (2015) Antidote for factor Xa anticoagulants. N. Engl. J.
Med. 373, 2471-2472.
Coughlin, S. R. (1994) Thrombin receptor function and cardiovascular
disease. Trends Cardiovasc. Med. 4, 77-83.
Das, A. and Liu, D. (2015) Novel antidotes for target specic oral anti-
coagulants. Exp. Hematol. Oncol. 4, 25.
Dellinger, R. P. (2003) Inammation and coagulation: Implications for
the septic patient. Clin. Infect. Dis. 36, 1259-1265.
Dickinson, C. D. and Ruf, W. (1997) Active site modication of factor
VIIa affects interactions of the protease domain with tissue factor.
J. Biol. Chem. 272, 19875-19879.
Di Nisio, M., Middeldorp, S. and Büller, H. R. (2005) Direct thrombin
inhibitors. N. Engl. J. Med. 353, 1028-1040.
Eriksson, B. I., Quinlan, D. J. and Weitz, J. I. (2009) Comparative phar-
macodynamics and pharmacokinetics of oral direct thrombin and
factor Xa inhibitors in development. Clin. Pharmacokinet. 48, 1-22.
Esmon, C. T. (2005) The interactions between inammation and co-
agulation. Br. J. Haematol. 131, 417-430.
Evans, H. C., Perry, C. M. and Faulas, D. (2004) Ximelagatran/mela-
gatran. Drugs 64, 649-678.
Fareed, J., Hoppensteadt, D. A., Fareed, D., Demir, M., Wahi, R.,
Clarke, M., Adiguzel, C. and Bick, R. (2008) Survival of hepa-
rins, oral anticoagulants, and aspirin after the year 2010. Semin.
Thromb. Hemost. 34, 58-73.
Fitzmaurice, D. A., Blann, A. D. and Lip, G. Y. H. (2002) Bleeding risks
of antithrombotic therapy. BMJ 325, 828-831.
Franchini, M., Liumbruno, G. M., Bonfanti, C. and Lippi, G. (2016) The
evolution of anticoagulant therapy. Blood Transfus. 14, 175-184.
Fung, E., Patsopoulos, N. A., Belknap, S. M., O’Rourke, D. J., Robb,
J. F., Anderson, J. L., Shworak, N. W. and Moore, J. H. (2012) Ef-
fect of genetic variants, especially CYP2C9 and VKORC1, on the
pharmacology of warfarin. Semin. Thromb. Hemost. 38, 893-904.
Gandhi, N. S. and Mancera, R. L. (2010) Heparin/heparan sulphate-
based drugs. Drug Discov. Today 15, 1058-1069.
Garcia, D. A., Baglin, T. P., Weitz, J. I. and Samama, M. M. (2012)
Parenteral anticoagulants: antithrombotic therapy and prevention
of thrombosis, 9th ed: American college of chest physicians evi-
dence-based clinical practice guidelines. Chest 141, e24S-e43S.
Greinacher, A., Thiele, T. and Selleng, K. (2015) Reversal of antico-
agulants: an overview of current developments. Thromb. Haemost.
113, 931-942.
Hagedorn, I., Schmidbauer, S., Pleines, I., Kleinschnitz, C., Kronthaler,
U., Stoll, G., Dickneite, G. and Nieswandt, B. (2010) Factor XIIa in-
hibitor recombinant human albumin Infestin-4 abolishes occlusive
arterial thrombus formation without affecting bleeding. Circulation
121, 1510-1517.
Hardy, S., Schirm, S., Liu, X. and Dai, Y. (2006) Tifacogin increases
bacterial clearance from blood. Crit. Care 10, P155.
Heit, J. A., Spencer, F. A. and White, R. H. (2016) The epidemiology
of venous thromboembolism. J. Thromb. Thrombolysis 41, 3-14.
Hirsh, J. (1991) Heparin. N. Engl. J. Med. 324, 1565-1574.
Hirsh, J. (1998) Low-molecular-weight heparin. Circulation 98, 1575-
1582.
Hirsh, J., Fuster, V., Ansell, J. and Halperin, J. L. (2003) American
heart association/American college of cardiology foundation guide
to warfarin therapy. J. Am. Coll. Cardiol. 41, 1633-1652.
Hirsh, J., O’Donnell, M. and Eikelboom, J. W. (2007) Beyond unfrac-
tionated heparin and warfarin: current and future advances. Circu-
lation 116, 552-560.
Hirsh, J. and Fuster, V. (1994) Guide to anticoagulant therapy. Part I:
heparin. Circulation. 89, 1449-1468.
Hirsh, J. and Raschke, R. (2004) Heparin and low-molecular-weight
heparin: The seventh ACCP conference on antithrombotic and
thrombolytic therapy. Chest 126, 188S-203S.
Holmer, E., Söderberg, K., Bergqvist, D. and Lindahl, U. (1986) Hepa-
rin and its low molecular weight derivatives: Anticoagulant and an-
tithrombotic properties. Haemostasis 16 Suppl 2, 1-7.
Hoppensteadt, D., Walenga, J. M., Fareed, J. and Bick, R. L. (2003)
Heparin, low-molecular-weight heparins, and heparin pentasac-
charide: Basic and clinical differentiation. Hematol. Oncol. Clin.
North Am. 17, 313-341.
Howard, E. L., Becker, K. C. D., Rusconi, C. P. and Becker, R. C.
(2007) Factor IXa inhibitors as novel anticoagulants. Arterioscler.
Thromb. Vasc. Biol. 27, 722-727.
Hu, T. Y., Vaidya, V. R. and Asirvatham, S. J. (2016) Reversing antico-
agulant effects of novel oral anticoagulants: Role of ciraparantag,
andexanet alfa, and idarucizumab. Vasc. Health Risk Manag. 12,
35-44.
Hyers, T. M. (1999) Venous thromboembolism. Am. J. Respir. Crit.
Biomol Ther 25(5), 461-470 (2017)

www.biomolther.org
Kim et al. New Anticoagulants for Venous Thromboembolism
469
Care Med. 159, 1-14.
Kearon, C., Comp, P., Douketis, J., Royds, R., Yamada, K. and Gent,
M. (2005) Dose-response study of recombinant human soluble
thrombomodulin (ART-123) in the prevention of venous throm-
boembolism after total hip replacement. J. Thrombo Haemost. 3,
962-968.
Kim, J. H., Lee, J., Kang, S., Moon, H., Chung, K. H. and Kim, K. R.
(2016) Antiplatelet and antithrombotic effects of the extract of lin-
dera obtusiloba leaves. Biomol. Ther. (Seoul) 24, 659-664.
Koster, A., Fischer, K. G., Harder, S. and Mertzlufft, F. (2007) The di-
rect thrombin inhibitor argatroban: a review of its use in patients
with and without HIT. Biologics 1, 105-112.
Kreimann, M., Brandt, S., Krauel, K., Block, S., Helm, C. A., Weitsch-
ies, W., Greinacher, A. and Delcea, M. (2014) Binding of anti-plate-
let factor 4/heparin antibodies depends on the thermodynamics of
conformational changes in platelet factor 4. Blood 124, 2442-2449.
Krishnaswamy, A., Lincoff, A. M. and Cannon, C. P. (2010) The use
and limitations of unfractionated heparin. Crit. Pathw. Cardiol. 9,
35-40.
Kwon, I. S., Yim, J. H., Lee, H. K. and Pyo, S. (2016) Lobaric acid
inhibits VCAM-1 expression in TNF-alpha-stimulated vascular
smooth muscle cells via modulation of NF-kappaB and MAPK sig-
naling pathways. Biomol. Ther. (Seoul) 24, 25-32.
Laterre, P. F., Opal, S. M., Abraham, E., LaRosa, S. P., Creasey, A. A.,
Xie, F., Poole, L. and Wunderink, R. G. (2009) A clinical evaluation
committee assessment of recombinant human tissue factor path-
way inhibitor (tifacogin) in patients with severe community-acquired
pneumonia. Critic. Care 13, R36.
Lee, C. J. and Ansell, J. E. (2011) Direct thrombin inhibitors. Br. J. Clin.
Pharmacol. 72, 581-592.
Lee, D. H., Kim, H. H., Lim, D. H., Kim, J. L. and Park, H. J. (2015) Ef-
fect of cordycepin-enriched WIB801C from cordyceps militaris sup-
pressing brinogen binding to glycoprotein IIb/IIIa. Biomol. Ther.
(Seoul) 23, 60-70.
Lenting, P. J., van Mourik, J. A. and Mertens, K. (1998) The life cycle
of coagulation factor VIII in view of its structure and function. Blood
92, 3983-3996.
Liaw, P. C., Becker, D. L., Stafford, A. R., Fredenburgh, J. C. and
Weitz, J. I. (2001) Molecular basis for the susceptibility of brin-
bound thrombin to inactivation by heparin cofactor II in the pres-
ence of dermatan sulfate but not heparin. J. Biol. Chem. 276,
20959-20965.
Linkins, L. A., Choi, P. T. and Douketis, J. D. (2003) Clinical impact of
bleeding in patients taking oral anticoagulant therapy for venous
thromboembolism: A meta-analysis. Ann. Intern. Med. 139, 893-
900.
Loke, C., Ali, S. S. and Johari, V. (2012) Pharmacology of anticoagu-
lants. Dis. Mon. 58, 424-430.
Lurie, Y., Loebstein, R., Kurnik, D., Almog, S. and Halkin, H. (2010)
Warfarin and vitamin K intake in the era of pharmacogenetics. Br.
J. Clin. Pharmacol. 70, 164-170.
Majeed, A. and Schulman, S. (2013) Bleeding and antidotes in new
oral anticoagulants. Best Pract. Res. Clin. Haematol. 26, 191-202.
Mann, K. G., Brummel, K. and Butenas, S. (2003) What is all that
thrombin for? J. Thromb. Haemost. 1, 1504-1514.
Marmur, J. D. (2002) Direct versus indirect thrombin inhibition in per-
cutaneous coronary intervention. J. Invasive Cardiol. 14 Suppl B,
8B-18B.
Masuko, S. and Linhardt, R. J. (2012) Chemoenzymatic synthesis of
the next generation of ultralow MW heparin therapeutics. Future
Med. Chem. 4, 289-296.
Matyal, R., Mahmood, F. and Park, K. W. (2005) Tifacogin, recombi-
nant tissue factor pathway inhibitor. Int. Anesthesiol. Clin. 43, 135-
144.
Mega, J. L. and Simon, T. (2015) Pharmacology of antithrombotic
drugs: an assessment of oral antiplatelet and anticoagulant treat-
ments. Lancet 386, 281-291.
Mekaj, Y. H., Mekaj, A. Y., Duci, S. B. and Miftari, E. I. (2015) New
oral anticoagulants: their advantages and disadvantages com-
pared with vitamin K antagonists in the prevention and treatment
of patients with thromboembolic events. Ther. Clin. Risk Manag.
11, 967-977.
Merli, G. J. and Groce, J. B. (2010) Pharmacological and clinical dif-
ferences between low-molecular-weight heparins: Implications for
prescribing practice and therapeutic interchange. P. T. 35, 95-105.
Miller, C. S., Grandi, S. M., Shimony, A., Filion, K. B. and Eisenberg,
M. J. (2012) Meta-analysis of efcacy and safety of new oral anti-
coagulants (dabigatran, rivaroxaban, apixaban) versus warfarin in
patients with atrial brillation. Am. J. Cardio. 110, 453-460.
Nawarskas, J. J. and Anderson, J. R. (2001) Bivalirudin: A new ap-
proach to anticoagulation. Heart Dis. 3, 131-137.
Park, J. M., Chang, K. H., Park, K. H., Choi, S. J., Lee, K., Lee, J. Y.,
Satoh M., Song S. Y. and Lee, M. Y. (2016) Differential effects be-
tween cigarette total particulate matter and cigarette smoke extract
on blood and blood vessel. Toxicol. Res. 32, 353-358.
Pempe, E. H., Xu, Y., Gopalakrishnan, S., Liu, J. and Harris, E. N.
(2012) Probing structural selectivity of synthetic heparin binding to
stabilin protein receptors. J. Biol. Chem. 287, 20774-20783.
Perzborn, E., Roehrig, S., Straub, A., Kubitza, D. and Misselwitz, F.
(2011) The discovery and development of rivaroxaban, an oral, di-
rect factor Xa inhibitor. Nat. Rev. Drug Discov. 10, 61-75.
Piazza, G. and Goldhaber, S. Z. (2010) Venous thromboembolism and
atherothrombosis: an integrated approach. Circulation 121, 2146-
2150.
Previtali, E., Bucciarelli, P., Passamonti, S. M. and Martinelli, I. (2011)
Risk factors for venous and arterial thrombosis. Blood Transfus.
9, 120-138.
Puurunen, M. K., Gona, P. N., Larson, M. G., Murabito, J. M., Magnani,
J. W. and O’Donnell, C. J. (2016) Epidemiology of venous thrombo-
embolism in the framingham heart study. Thromb. Res. 145, 27-33.
Rauova, L., Poncz, M., McKenzie, S. E., Reilly, M. P., Arepally, G.,
Weisel, J. W., Nagaswami, C., Cines, D. B. and Sachais, B. S.
(2005) Ultralarge complexes of PF4 and heparin are central to the
pathogenesis of heparin-induced thrombocytopenia. Blood 105,
131-138.
Rauova, L., Zhai, L., Kowalska, M. A., Arepally, G. M., Cines, D. B. and
Poncz, M. (2006) Role of platelet surface PF4 antigenic complexes
in heparin-induced thrombocytopenia pathogenesis: diagnostic
and therapeutic implications. Blood 107, 2346-2353.
Roser-Jones, C., Chan, M., Howard, E. L., Becker, K. C., Rusconi, C.
P. and Becker, R. C. (2011) Factor IXa as a target for pharmaco-
logic inhibition in acute coronary syndrome. Cardiovasc. Ther. 29,
e22-e35.
Ruiz, F. A., Lea, C. R., Oldeld, E. and Docampo, R. (2004) Human
platelet dense granules contain polyphosphate and are similar to
acidocalcisomes of bacteria and unicellular eukaryotes. J. Biol.
Chem. 279, 44250-44257.
Ruppert, A., Lees, M. and Steinle, T. (2010) Clinical burden of venous
thromboembolism. Curr. Med. Res. Opin. 26, 2465-2473.
Rupprecht, H. J. and Blank, R. (2010) Clinical pharmacology of direct
and indirect factor Xa inhibitors. Drugs 70, 2153-2170.
Schmid, P., Fischer, A. G. and Wuillemin, W. A. (2009) Low-molecular-
weight heparin in patients with renal insufciency. Swiss Med. Wkly
139, 438-452.
Schroeder, M., Hogwood, J., Gray, E., Mulloy, B., Hackett, A. M. and
Johansen, K. B. (2011) Protamine neutralisation of low molecular
weight heparins and their oligosaccharide components. Anal. Bio-
anal. Chem. 399, 763-771.
Short, N. J. and Connors, J. M. (2014) New oral anticoagulants and the
cancer patient. Oncologist 19, 82-93.
Sie, P. (2016) Spotlight on idarucizumab and its potential for the rever-
sal of anticoagulant effects of dabigatran. Drug Des. Devel. Ther.
10, 1683-1689.
Smith, S. A., Choi, S. H., Collins, J. N., Travers, R. J., Cooley, B. C.
and Morrissey, J. H. (2012) Inhibition of polyphosphate as a novel
strategy for preventing thrombosis and inammation. Blood 120,
5103-5110.
Smith, S. A. and Morrissey, J. H. (2008) Polyphosphate enhances -
brin clot structure. Blood 112, 2810-2816.
Stangier, J. and Clemens, A. (2009) Pharmacology, pharmacokinet-
ics, and pharmacodynamics of dabigatran etexilate, an oral direct
thrombin inhibitor. Clin. Appl. Thromb. Hemost. 15 Suppl 1, 9S-
16S.
Teitel, J. M. and Rosenberg, R. D. (1983) Protection of factor Xa from

470
https://doi.org/10.4062/biomolther.2016.271
neutralization by the heparin-antithrombin complex. J. Clin. Invest.
71, 1383-1391.
Tollefsen, D. M., Majerus, D. W. and Blank, M. K. (1982) Heparin co-
factor II. Purication and properties of a heparin-dependent inhibi-
tor of thrombin in human plasma. J. Biol. Chem. 257, 2162-2169.
Tran, H. A., Chunilal, S. D., Harper, P. L., Tran, H., Wood, E. M. and
Gallus, A. S. (2013) An update of consensus guidelines for warfarin
reversal. Med. J. Aust. 198, 198-199.
Travers, R. J., Shenoi, R. A., Kalathottukaren, M. T., Kizhakkedathu,
J. N. and Morrissey, J. H. (2014) Nontoxic polyphosphate inhibitors
reduce thrombosis while sparing hemostasis. Blood 124, 3183-
3190.
Vavalle, J. P. and Cohen, M. G. (2012) The REG1 anticoagulation
system: A novel actively controlled factor IX inhibitor using RNA
aptamer technology for treatment of acute coronary syndrome. Fu-
ture Cardiol. 8, 371-382.
Verhamme, P., Pakola, S., Jensen, T. J., Berggren, K., Sonesson, E.,
Saint-Remy, J. M., Balchen, T., Belmans, A., Cahillane, G., Stas-
sen, J. M., Peerlinck, K., Glazer, S. and Jacquemin, M. (2010)
Tolerability and pharmacokinetics of TB-402 in healthy male volun-
teers. Clin. Ther. 32, 1205-1220.
Verhoef, T. I., Redekop, W. K., Daly, A. K., van Schie, R. M., de Boer, A.
and Maitland-van der Zee, A. H. (2014) Pharmacogenetic-guided
dosing of coumarin anticoagulants: algorithms for warfarin, aceno-
coumarol and phenprocoumon. Br. J. Clin. Pharmacol. 77, 626-
641.
Vincent, J. L., Ramesh, M. K., Ernest, D., LaRosa, S. P., Pachl, J.,
Aikawa, N., Hoste, E., Levy, H., Hirman, J., Levi, M., Daga, M., Kut-
sogiannis, D. J., Crowther, M., Bernard, G. R., Devriendt, J., Puig-
server, J. V., Blanzaco, D. U, Esmon, C. T, Parrillo, J. E., Guzzi, L.,
Henderson, S. J., Pothirat, C., Mehta, P., Fareed, J., Talwar, D.,
Tsuruta, K., Gorelick, K. J., Osawa, Y. and Kaul, I. (2013) A random-
ized, double-blind, placebo-controlled, Phase 2b study to evaluate
the safety and efcacy of recombinant human soluble thrombo-
modulin, ART-123, in patients with sepsis and suspected dissemi-
nated intravascular coagulation. Crit. Care Med. 41, 2069-2079.
Vlasuk, G. P. and Rote, W. E. (2002) Inhibition of factor VIIa/tissue
factor with nematode anticoagulant protein c2: from unique mecha-
nism to a promising new clinical anticoagulant. Trends Cardiovasc.
Med. 12, 325-331.
Walenga, J. M. and Lyman, G. H. (2013) Evolution of heparin antico-
agulants to ultra-low-molecular-weight heparins: a review of phar-
macologic and clinical differences and applications in patients with
cancer. Crit. Rev. Oncol. Hematol. 88, 1-18.
Weitz, J. I. (1997) Low-molecular-weight heparins. N. Engl. J. Med.
337, 688-698.
Weitz, J. I., Hudoba, M., Massel, D., Maraganore, J. and Hirsh, J.
(1990) Clot-bound thrombin is protected from inhibition by heparin-
antithrombin III but is susceptible to inactivation by antithrombin
III-independent inhibitors. J. Clin. Invest. 86, 385-391.
Wessler, S. and Gitel, S. N. (1986) Pharmacology of heparin and war-
farin. J. Am. Coll. Cardiol. 8, 10B-20B.
Wolzt, M., Weltermann, A., Nieszpaur-Los, M., Schneider, B., Fassolt,
A., Lechner, K., Eichler, H. G. and Kyrle, P. A. (1995) Studies on the
neutralizing effects of protamine on unfractionated and low molecu-
lar weight heparin (Fragmin) at the site of activation of the coagula-
tion system in man. Thromb. Haemost. 73, 439-443.
Wood, J. P., Ellery, P. E., Maroney, S. A. and Mast, A. E. (2014) Biology
of tissue factor pathway inhibitor. Blood 123, 2934-2943.
Yeh, C. H., Fredenburgh, J. C. and Weitz, J. I. (2012) Oral direct factor
Xa inhibitors. Circ. Res. 111, 1069-1078.
Biomol Ther 25(5), 461-470 (2017)