Current Topics in Medicinal Chemistry 2001,1, 151-159
`Coagulation Factor Xa Inhibition:
`Biological
`
`151
`
`
`
`Background Rationale
`
`and
`
`Robert J. Leadley, Jr.*
`
`Pfizer Global Research and Development, Ann Arbor Laboratories, 2800 Plymouth Rd., Ann
`Arbor, MI 48105, USA
`
`mm
`•
`leading
`
`the
`
`causes
`
`of
`
`Abstract:
`are
`disease
`cerebrovascular
`and
`Ischemic heart disease
`
`
`treated are by relatively antiquated drugs. However, due
`the world. Surprisingly,
`these diseases
`to our improved understanding of the underlying pathology of these diseases, and a number of
`technological advances in tools for drug discovery and chemical optimization, an exciting new
`wave of antithrombotic compounds is beginning to emerge in clinical trials. These agents,
`referred to as direct coagulation factor Xa inhibitors, appear to provide an enhanced risk-
`benefit margin compared to conventional therapy. Preclinical and early clinical data gathered
`over the past few years suggests that direct fXa inhibitors will provide the necessary
`advancements in efficacy, safety, and ease of use required to displace conventional therapy.
`Whether or not these agents will succeed will be determined as this class of agents advances
`through clinical trials in the near future. This
`review describes
`
`of some the key studies that sparked an interest in fXa as a
`
`therapeutic target, highlighting the findings that provided important rationale for continuing the development of potent
`and selective direct fXa
`inhibitors.
`
`;
`
`•-
`•, ! ^
`^
`
`I
`
`,
`
`INTRODUCTION
`
`antibodies against specific coagulation factors, synthetic
`indirect factor Xa (fXa) inhibitors, and possibly most
`promising, direct
`fXa
`inhibitors.
`
`Many approaches to developing antithrombotic drugs
`that interfere with enzymes in the coagulation system have
`been pursued over the past few decades; however, currently
`approved drags
`for
`thrombotic
`diseases
`
`have for
`in
`quite some
`time. Heparin,
`discovered
`1916
`intravenous anticoagulant of choice for acute thrombotic
`conditions. More recently, fractionated, or low-molecular-
`weight, heparins have demonstrated superiority over
`unfractionated heparin in several thrombotic indications [2,
`3], but remain limited to intravenous and subcutaneous
`administration. And although
`the coumarin derivative
`warfarin was discovered in 1941 and used initially as a
`rodenticide [4] that provides systemic anticoagulation by
`indirect
`inactivation of vitamin-K dependent
`serine
`proteases, it remains the most frequently prescribed oral
`anticoagulant.
`
`BACKGROUND
`FACTOR Xa BIOLOGICAL
`been
`around
`the
`[1]
`remains
`Factor X is the zymogen of fXa, a serine protease which
`
`occupies a pivitol position in the coagulation
`cascade. Factor
`X can be activated by the contact ("intrinsic tenase")
`pathway or by the tissue factor/Vila ("extrinsic tenase")
`pathway of the coagulation system (Fig. 1). Consequently,
`initiation of coagulation by either pathway in response to
`vascular injury activates factor X to fXa, making fXa
`inhibition a desirable intervention point when developing
`novel antithrombitics. Factor Xa and its cofactor, factor Va,
`combine on phospholipid membranes
`to
`form
`the
`"prothrombinase"
`complex, which activates prothrombin to
`thrombin. Thrombin, by cleaving fibrinogen to fibrin, by
`activating platelets, and by converting factor XIII to factor
`coagulation
`the
`of
`components
`Since the principal
`system
`Xllla, is
`the principal enzyme
`involved
`in
`thrombus
`have been known for some time, the lack of superior
`generation, growth,
`and
`stabilization.
`replacements for heparin and Coumadin is rather surprising.
`On the other hand, the redundancy and complexity of the
`hemostasis system
`is extensive and has provided tremendous
`obstacles for discovering potent novel agents that are
`effective, yet safe. Fortunately, recent scientific findings and
`advances in technology and testing have provided tools to
`identify and develop new agents that will hopefully provide
`safer and more effective antithrombotic therapy than are
`currently available. Among these newer agents are direct
`thrombin
`inhibitors,
`tissue
`factor pathway
`inhibitors,
`
`Factor X has long been recognized to play a significant
`role in hemostasis [5]. Factor X deficiency, while extremely
`rare, was first described in the 1950s
`in two families, Prower
`and Stuart (thus the original nomenclature of factor X as
`"Stuart" or "Stuart-Prower"
`
`
`7]). [6, factor These patients had
`
`very low antigen and activity levels of factor X, which were
`manifested in severe bleeding diatheses. Also, in the 1950s,
`studies of serum from these patients led to the identification
`of a deficiency of a specific factor, factor X, that was
`responsible for the bleeding diatheses. Since then, a number
`of variants of factor X deficiency suggest that factor X
`Therapeutics,
`at the Cardiovascular
`
`to correspondence this author
`
`
`*Address
`activity levels must be less than 5% of normal to result in
`Pfizer Global Research and Development, Ann Arbor Laboratories, 2800
`Plymouth Rd., Ann Arbor, Ml 48105, USA; Tel: 734-622-1420: Fax: 734­
`spontaneous bleeding
`tendencies [8]. This
`information
`622-1480;
`E-mail:
`ROBERT.LHADLEY@PFlZER.COM
`suggests that factor X activity can be suppressed markedly
`
`1568-0266/01
`
`S28,00+.00
`
`© 2001 Bentham Science Publishers
`
`Ltd.
`
`MYLAN - EXHIBIT 1018
`
`

`

`152 Current Topics in Medicinal Chemistry, 2001, Vol. I, No. 2
`
`Robert J. Lcadley, Jr.
`
`Coagulation Factor Xa Inhibition
`
`r J \
`
`Jf
`
`FVi
`
`i-'V
`
`m^aa^
`egffi
`
`i
`
`FVIJIa-
`
`FVIfS
`
`f' C i M ' X.
`FV ;'K C,i2\
`• r jro-.ifc- n i b -
`
`i
`
`FACTOR XII la ACTIVATION
`
`^
`
`FIBRIN FORMATION
`
`PLATELET AGGREGATION
`
`Fig. (1). Simplified schematic of the coagulation system and the intervention point for direct Factor Xa inhibitors.
`
`without affecting hemostasis, a characteristic that is desirable
`for antithrombotic agents. That is, an ideal agent would
`prevent
`thrombosis without producing systemic hypo-
`coagulation which would lead to bleeding complications at
`sites other than the intended target (e.g., intracerebral
`hemorrhage in patients treated for coronary artery disease).
`While this ideal approach was appealing, the hypothesis
`remained untested because no direct, selective inhibitors of
`fXa were available until fairly recently.
`
`NATURALLY OCCURING INHIBITORS OF FACTOR
`Xa
`
`In 1987, Tuszynski eta!., published their discovery of an
`anticoagulant, antistasin, which they isolated from extracts of
`the Mexican leech, Haementeria officinalis [9]. The amino
`acid sequence of antistasin was determined soon thereafter
`[10], and enzyme kinetic analysis detennined that antistasin
`is a slow, tight-binding, selective inhibitor of fXa with a Ki
`of 0.3-0.6 nM [11]. At approximately the same time,
`Waxman et ai, reported the discovery of tick anticoagulant
`peptide (TAP), a single chain 60 amino acid peptide that was
`isolated from the extracts of the tick Ornithodoros moubata
`[12]. TAP is a reversible, slow, tight-binding inhibitor of fXa
`with an estimated Ki of 0.5 nM. Molecular biology
`techniques quickly allowed scientists to make recombinant
`forms of these peptides which, particularly in the case of
`TAP, were used to validate fXa as a viable drug target and to
`aid in our understanding of the significance of fXa in
`thrombosis.
`
`VALIDATION OF FACTOR Xa AS A VIABLE DRUG
`TARGET
`
`A number of animal models of thrombosis were used to
`compare the antithrombotic effect of peptide inhibitors of
`fXa to direct inhibitors of thrombin and to indirect inhibitors
`of thrombin and fXa, such as heparin and low-molecular
`weight heparins [e.g., 13, 14, 15, 16]. These studies provided
`a preponderance of evidence to support the concept that
`direct fXa
`inhibition may provide a more effective
`antithrombotic approach than other mechanistic inhibitors,
`and that there appears to be a larger safety margin for fXa
`inhibitors as well, particularly regarding primary hemostasis
`at wound sites. For example, in canine models of thrombosis
`and thrombolysis, recombinant TAP was shown to promote
`rapid and prolonged vascular reperfiision at doses which
`produced relatively minor elevations in prothrombin time
`(PT), activated partial thromboplastin time (APTT), and
`template bleeding time [13, 14, 16, 17]. Although TAP was
`apparently discontinued from development for undisclosed
`reasons, the preclinical studies demonstrated
`that fXa
`inhibition was, indeed, an attractive drug target for the
`development of small molecule inhibitors of fXa.
`
`RATIONALE FOR DEVELOPING FACTOR Xa
`INHIBITORS
`
`TAP also provided a valuable tool for defining the role of
`fXa in thrombus formation and growth, thereby providing
`
`

`

`Coagulation Factor Xa Inhibition
`
`Current Topics in Medicinal Chemistry, 2001, Vol. I, No. 2 153
`
`further rationale to support the concept of fXa inhibition as a
`desirable drug target. The following discussion highlights
`some of the important findings.
`
`From experiments performed by Eisenberg, et al [18, 19],
`it became clear
`that Xa associated with
`thrombi
`is
`enzymatically active. For example, fXa in thrombi recovered
`from
`injured arteries in vivo is capable of activating
`prothrombin
`to
`thrombin [19]. Furthermore, fXa,
`in
`comparison to thrombin, appears to be primarily responsible
`for clot-associated procoagulant activity [20]. Specifically,
`TAP inhibited clot-associated generation of Fibrinopeptide A
`(a byproduct of thrombin-induced cleavage of fibrinogen to
`fibrin) to the same degree as the direct thrombin inhibitor,
`hirudin, suggesting that the procoagulant activity of thrombi
`is due to the de novo activation of prothrombin to thrombin,
`not merely to the presence of preexisting thrombin. These
`data indicate that fXa is active in thrombi and that direct
`inhibition of thrombus-associated fXa would be an effective,
`highly localized approach to prevention of thrombus growth.
`
`One of the current problems associated with anti-
`thrombin therapy for acute coronary syndromes is the
`phenomenon of "rebound" ischemia shortly after the therapy
`is terminated. This phenomenon was observed with direct
`and
`indirect, antithrombin Ill-dependent
`inhibitors of
`thrombin (argatroban and heparin, respectively [21, 22] and
`is thought to occur because of continual generation of
`thrombin after therapy is discontinued. Presumably, these
`agents were able to block thrombin activity as long as the
`drug was present, but the generation of thrombin by
`thrombus-associated
`fXa continued after
`therapy was
`withdrawn, resulting in recurrence of thrombotic ischemic
`events.
`
`Recent experiments indicate that short-term inhibition of
`fXa by TAP provides sustained inhibition of clot-associated
`procoagulant activity in vitro [23]. These results were
`validated in vivo by examining platelet deposition on dacron
`vascular grafts in baboons during and after a 2 hour
`intravenous infusion of rTAP [24], TAP decreased platelet
`deposition during the infusion and for 2 and 53 hours after
`termination of low dose (10 ng/kg/min) and high dose (25
`Hg/kg/min) rTAP, respectively. In another experiment, CI-
`1031 (ZK-807834), a small molecule, direct inhibitor of fXa,
`was administered as adjunctive treatment during and for 1 hr
`post fibrinolytic therapy to dogs that had experimentally-
`induced occlusive coronary artery
`thrombosis
`[25].
`Compared to heparin plus aspirin, 24-hr vessel patency
`achieved with CI-1031 was markedly enhanced. These
`findings were also supported by a preliminary report on a
`study in which another small molecule inhibitor of fXa,
`FXV673, administered
`to canines over a
`few hours
`prevented thrombosis for three days following electrolytic
`injury-induced thrombosis in the carotid artery [26]. These
`data suggest that the sustained antithrombotic effect after
`short-term drug exposure is not a feature of TAP alone, but
`is a unique and highly desirable effect of direct fXa
`inhibition. This
`feature,
`sometimes
`referred
`to as
`"passivation," may be important to consider when designing
`clinical studies to evaluate novel fXa inhibitors in acute
`thrombotic diseases, especially
`in
`the current medical
`
`environment which mandates short hospital stays for
`interventions such a percuataneous coronary interventions.
`
`inhibition provide sustained
`fXa
`Not only does
`antithrombotic protection, fXa inhibition also disaggregates
`prefonned platelet thrombi in vivo [27]. In experiments using
`anesthetized pigs, the carotid artery was damaged by serial
`hemostat crushes and the resulting thrombus was allowed to
`grow for 30 min prior to drug administration. Despite the
`fact that TAP and hirudin have no intrinsic lytic activity,
`these agents not only prevented further platelet deposition,
`but they actually reduced radiolabelled platelet deposition at
`the site of vascular damage. Worth noting is the observation
`that with TAP, the "dissaggregation" or "dethrombosis"
`occurred at doses of TAP that yielded only modest changes
`in clotting time assays such as APTT. In summary, newly
`fonned thrombi (<6 hours) can disaggregate if exposed to a
`hirudin or TAP. If proven to be true for small molecule
`direct
`inhibitors of
`fXa,
`this
`intriguing
`feature of
`"dethrombosis" may
`be
`an
`important
`therapeutic
`characteristic of direct fXa inhibitors, particularly for the
`acute treatment of thrombotic syndromes.
`
`INSIGHTS PROVIDED BY SYNTHETIC
`NEW
`INHIBITORS OF FACTOR Xa
`
`Based on the favorable preclinical evidence provided by
`recombinant forms of naturally oecuring fXa inhibitors,
`many pharmaceutical companies quickly initiated chemistry
`programs that produced potent and selective small molecule
`inhibitors of DCa, of which DX9065a is the most widely
`characterized agent in the literature [e.g., 28, 29, 30, 31, 32,
`33],
`
`inhibitor
`indirect fXa
`the success of
`In addition,
`heparanoids, such as low-molecular-weight heparins, in
`venous [3] and arterial thrombosis [2] prompted interest in
`continuing the development of more selective indirect
`inhibitors of fXa that were designed based on the minimal
`saccharide sequence of heparin required for antithrombin-III
`binding
`[34], SR90107A/ORG31540
`is
`the
`furthest-
`developed of a family of synthetic pentasaccharides that are
`selective, antithrombin-III dependent fXa inhibitors.
`
`and SR90107A/
`agents, DX-9065a
`two
`These
`ORG31540 have been extensively evaluated in in vivo
`models of thrombosis, exemplified best, perhaps, by their
`efficacy
`in a baboon model of shunt
`thrombosis.
`SR90107A/ORG31540 dose-dependently inhibited platelet
`and fibrin deposition on shunts that represented conditions of
`venous
`thrombosis, but was relatively
`ineffective
`in
`preventing platelet deposition on shunts mimicing arterial,
`platelet-dependent thrombosis [35], Likewise, intravenous or
`oral administration of DX-9065a significantly inhibited
`thrombus fonnation in a venous-type chamber but had no
`significant antithrombotic effect in an arterial-type chamber
`[30]. These results suggest that selective inhibition of fXa
`results
`in specific inhibition of fibrin formation with
`relatively less activity against platelet-dependent thrombi.
`These data appear to directly contradict the data from
`experiments mentioned earlier in which platelet deposition
`on injured porcine arteries was reversed by fXa inhibition
`[27]. Which experimental condition is more comparable to
`
`

`

`154 Current Topics in Medicinal Chemistry, 200], Vol. I, Mo. 2
`
`Robert J. Leadley, Jr.
`
`the situation in patients with arterial thrombosis is still
`uncertain, especially in regard to how these new agents
`perform in the model versus the clinical setting. This
`apparent discrepancy is important to consider because it
`could have a direct impact on the indications targeted for a
`fXa inhibitor and on the design of clinical trials of novel fXa
`inhibitors
`(e.g., are antiplatelet agents
`required as
`concomitant therapy with fXa inhibitors?).
`
`SEPARATION OF ANTITHROMBOTIC EFFICACY
`FROM BLEEDING EFFECTS
`
`As alluded to previously, one of the most intriguing
`advantages of direct fXa inhibitors over conventional therapy
`is
`the
`relatively
`large
`therapeutic window between
`antithrombotic efficacy and bleeding tendency. Numerous
`studies have demonstrated antithrombotic efficacy at doses
`of fXa inhibitors that have little or no effect on markers of
`primary hemostasis such as template bleeding time, tail
`resection bleeding time, or cuticle bleeding time [32, 33, 36,
`37, 38, 39]. Sato et al, [36] demonstrated that the risk-
`benefit ratio (dose that doubled bleeding time/dose that
`produced 50% inhibition in a rat venous thrombosis model)
`for YM-60828 was dramatically higher than the ratio
`measured with argatroban, heparin, and dalteparin (94 versus
`7.4, 2.9, and 5.3, respectively). Similar risk-benefit analysis
`could be applied to the studies sited above, demonstrating a
`favorable safety profile for fXa inhibitors over other types of
`antithrombotic agents.
`
`There are several mechanisms that have been postulated
`to explain
`the minimal bleeding observed during
`administration of antithrombotic doses of fXa inhibitors in
`animal models of thrombosis. First, fXa has no direct effect
`on platelet aggregation and only activates platelets indirectly
`via generation of thrombin when Xa is assembled into the
`prothrombinase complex. Consequently, unlike direct
`thrombin inhibitors or heparinoids, nearly all direct fXa
`inhibitors have no inhibitory effect on thrombin- or thrombin
`receptor
`activator
`peptide
`(TRAP)-induced
`platelet
`aggregation [28, 40, 41]. The only cited exception is YM-
`60828, which inhibits TRAP-induced platelet aggregation in
`plasma with an IC50 of 3.3
`and thrombin-induced
`washed platelet aggregation with an IC50 of 23.4 jiM [42],
`Since thrombin is a potent activator of platelets, and its
`affinity for platelet receptors is 10,000-fold higher than for
`fibrinogen [43, 44] minimal amounts of thrombin may be
`adequate to activate platelets and allow for normal systemic
`hemostasis.
`
`Minimal bleeding with fXa inhibitors may be attributed
`to reduction of thrombin generation, as opposed to blocking
`thrombin's activity once it is fonned. Once incorporated into
`the prothrombinase complex, the reaction rate of fXa
`increases 300,000-fold compared to the rate of free fXa,
`thereby generating an explosive production of thrombin [45],
`Inhibiting fXa directly with potent, selective agents therefore
`provides efficient reduction of thrombin generation at the
`thrombus, without the need for excessively high systemic
`plasma concentration of the antithrombotic drug.
`
`SEPARATION OF ANTITHROMBOTIC EFFICACY
`FROM SYSTEMIC HYPOCOAGULATION
`Several studies also demonstrated that, in comparison to
`other mechanisms of inhibition of thrombus formation (e.g.,
`unfractionated heparin, low-molecular-weight heparin, and
`direct thrombin inhibition), fXa inhibitors produced their
`antithrombotic effect with only modest changes in markers
`of systemic anticoagulation, such as PT, APTT, and
`thrombin time [32, 33, 36, 37, 38, 39, 46) . For example,
`maximally-effective antithrombotic doses of the direct fXa
`inhibitor, C92I-78, heparin, enoxaparin, and PPACK (a
`direct thrombin inhibitor) in a rabbit model of arteriovenous
`shunt thrombosis produced changes in cuticle bleeding times
`of approximately 2- (not significant from baseline), 5.5-, 3.5-,
`and 6-fold, and changes in APTT of 1.6-, >6-, 5-, and 1.3­
`fold over baseline, respectively [39]. Similarly, when
`RPR208566 was compared to heparin and argatroban (a
`direct thrombin inhibitor), PT and APTT were not changed
`significantly (<1.5-fold), while maximally effective doses of
`argatroban and heparin increased PT by 2.5- and 2.5-fold and
`APTT by 3- and 6-fold,
`respectively
`[46]. These
`observations have been made with a number of compounds
`from different chemical series indicating that, although there
`are certainly differences in the pharmacodynamics between
`individual fXa inhibitors, the overall safety advantages of
`fXa inhibition are mechanism-related.
`
`interestingly, the effect of potent selective inhibitors of
`fXa on clotting time assays is highly variable. For example,
`some fXa inhibitors appear to be more potent in the APTT
`assay than in the PT assay (TAP [15]; RPR120844, [47]),
`some are more potent in the PT than in the APTT assay (DX-
`9065a [28]; CI-1031 [38]), and others appear to be equally
`potent in both assays (YM-60828 [42], SKS49 [48]). The
`sensitivity of these tests for fXa inhibitors appears to be
`compound-dependent and may reflect as yet undefined
`differences in enzyme kinetics. Regardless, the APTT, PT,
`and activated clotting time (ACT) assays, which are used
`routinely to monitor heparin and warfarin treatment, are not
`sensitive enough to accurately monitor fXa inhibitors when
`administered at antithrombotic doses in animal models of
`thrombosis. Consequently, other more specific assays such
`as chromogenic fXa activity assays, Russel's viper venom
`clotting assay, the Heptest®, or thrombin generation assays
`might prove to be more sensitive pharmacodynamic markers
`for fXa inhibitors.
`
`Although the explanation for the ability of fXa inhibitors
`to provide antithrombotic protection without increasing
`systemic markers of anticoagulation is not yet clearly
`elucidated, the affinity of these agents for thrombus-
`associated fXa may provide a plausible explanation [49, 50].
`By inhibiting thrombus-bound fXa with high affinity, lower
`plasma drug concentrations may be required to provide
`antithrombotic efficacy. Significant prolongation of APTT
`and PT can be achieved by high-dose administration of fXa
`inhibitors, indicating that systemic hypocoagulation, and the
`susceptibility for bleeding complications, is not avoided
`completely by fXa inhibitors. However, the doses required to
`reach potentially dangerous plasma levels are quite high
`compared
`to doses providing maximal antithrombotic
`efficacy.
`
`

`

`Coagulation Factor Xa Inhibition
`
`Current Topics in Medicinal Chemistry, 2001, Vol 1, No. 2 155
`
`OTHER ADVANTAGES OF FACTOR Xa INHIBI­
`TION OVER CONVENTIONAL THERAPY
`
`Additional rationale for developing fXa inhibitors to
`replace heparin and warfarin are listed in (Table I). The
`considerations for dosing and monitoring heparin therapy
`have been extensively reviewed by Hirsh, et al. [51]. Briefly,
`heparin administration must be monitored carefully to
`maintain plasma drug concentrations within a safe and
`effective window. Monitoring is necessary because heparin
`binds to a number of plasma proteins and its activity is
`neutralized by platelet factor 4 released from activated
`platelets,
`resulting
`in highly variable anticoagulant
`responses. Also, heparin administration causes an immune
`reaction referred to as heparin-induced thrombocytopenia
`(HIT) in approximately 1-3% of patients; LMWHs also
`produce HIT, but at a lower incidence compared
`to
`unfractionated heparin. Low molecular weight heparins have
`eliminated the need for monitoring in most indications, but
`have not substantially replaced heparin in acute coronary
`syndromes due, in part, to the lack of a specific monitoring
`device that can provide rapid feedback for dose adjustment
`before, during, and after percutaneous coronary inter­
`ventions. The limitations of indirect versus direct inhibition
`of coagulation enzymes has been highlighted in studies by
`Weitz et al. [52] and by Herault et al., [49]. As mentioned
`above, fXa and thrombin are both enzymatically active when
`associated with the intravascular thrombus, so it is vitally
`important that an antithrombotic agent be able to inhibit this
`so-called "clot-bound" activity. Indirect inhibitors act by
`catalyzing
`the
`inactivation of
`thrombin or
`fXa by
`antithrombin-III, a large molecule that is not able to
`penetrate the thrombus. Consequently, heparin, LMWHs, or
`the synthetic pentasaccharides are not effective inhibitors of
`thrombus-associated fXa or
`thrombin. Direct fXa and
`thrombin inhibitors, however, are capable of inhibiting
`thrombus-associated fXa and thrombin, making these agents
`more attractive for medical interventions in thrombotic
`diseases. The other features of fXa inhibitors that were
`highlighted above, namely "dethrombosis" and vascular
`"passivation," are important characteristics for agents that
`
`will be administered over a brief period. Emergency
`treatment of acute coronary syndromes and percutaneous
`coronary intervention by intravenous administration of fXa
`inhibitors may provide ideal settings to evaluate whether the
`mechanistic advantages of fXa inhibition will translate into
`superior efficacy and safety over heparin.
`
`Currently, heparinoids are limited to subcutaneous and
`intravenous administration because they are not absorbed
`adequately after oral administration. However, new carriers
`have been developed that have enhanced the bioavailability
`of heparin so that an oral form of this agent is now being
`evaluated
`in clinical
`trials [53]. Regardless,
`the oral
`formulation of heparin will still have the same limitations as
`i.v. heparin, so that by virtue of inherent interindividual
`differences in oral absorption, orally-administered heparin
`will likely be even more difficult to maintain in the safe and
`effective therapeutic plasma concentration range.
`
`The limitations of oral anticoagulation with warfarin are
`well-recognized and have effectively restricted its use in the
`clinic [54], Warfarin acts in the liver by antagonizing the
`vitamin K-dependent carboxylation of glutamic acid residues
`on the amino terminal of coagulation factors II, VII, IX, and
`X. These residues are essential for calcium-dependent
`binding of these enzymes into their appropriate enzyme
`complexes on phospholipid surfaces. The approximate half-
`life of factor 11 (prothrombin) is 50 hours, so several days are
`required before warfarin achieves its full antithrombotic
`effect. Likewise, the reversal of the effect of warfarin can
`take 24 hr or longer. In addition, warfarin has many drug and
`food interactions that complicate its dosing and require
`regular monitoring to maintain safe and effective plasma
`drug concentrations [54, 55]. The significant bleeding
`complications
`and
`difficulty maintaining
`plasma
`concentrations of warfarin within the targetted range has led
`to the labeling of warfarin (Coumadin®) as a "narrow
`therapeutic index drug."
`
`Obviously, a fXa inhibitor that is orally bioavailable and
`does not have many of the drawbacks of warfarin would be
`
`Table I.
`
`Comparison of Direct tXa Inhibitors to Currently Available Anticoagulants
`
`Enzymes
`inhibited
`
`Cofactor
`required?
`
`Inhibition of clot-
`bound enzyme?
`
`Monitoring
`required?
`
`Heparin-induced
`thrombocytopenia?
`
`Route of
`administration
`
`Risk/Benefit
`
`N
`
`N
`
`Y
`
`Y
`
`Y
`
`Y
`
`Y
`
`Y
`
`N
`
`N
`
`N
`
`N
`
`Y
`
`Y
`
`N**
`
`Y
`
`++++
`
`N
`
`N
`
`Y
`
`Y
`
`N
`
`i.v., oral
`
`s.c., i.v.
`
`i.v., s.c.
`
`s.c., i.v.
`
`s.c., i.v.
`
`Oral, i.v.
`
`Agent
`
`Direct fXa
`
`Direct Ha
`
`Heparin
`
`LMWH
`
`fXa
`
`11a
`
`Xa-IJa
`
`Xa>IIa
`
`Pentasaccharide*
`
`Xa»>I]a
`
`Warfarin
`
`I la, Vila, IXa,
`Xa, PC, PS
`
`*
`
`*
`**
`
`Risk/Benefit is based on a qualitative estimate using heparin as a comparator. Hie risk/benefit for iXa inhibitors is speculative, based on preclinical evidence and experience
`with indirect fXa inhibitors.
`The pentasaccharide is not currently approved for use, but is included because the positive results in Phase III clinical trials will likely lead to approval in the near future.
`fn general, monitoring of LMWHs is not required. However, for indications such as percutaneous coronary intervention it is desirable to know the anticoagulation level of the
`patient before, during, and after the procedure.
`PC: Protein C; PS: Protein S
`
`

`

`156 Current Topics in Medicinal Chemistry, 2001, Vol. I, No. 2
`
`Robert J. Lcadley, Jr.
`
`highly desirable pharmaceutical agent. Many of the known
`fXa inhibitors have some degree of oral bioavailability [28,
`37, 41, 42, 56, 57, 58, 59], and several have demonstrated
`antithrombotic efficacy in animal models of thrombosis after
`oral or intraduodenal administration [30, 32, 41, 60].
`Although the hurdles for discovering a fXa inhibitor that has
`the appropriate pharmacokinetic properties are quite
`challenging, these data are encouraging and suggest that
`orally active fXa inhibitors may be coming closer to
`realization.
`
`FACTOR Xa INHIBITORS IN CLINICAL TRIALS
`
`Preclinical studies provided sufficient safety and efficacy
`data to propel fXa inhibitors into clinical trials, the first of
`which were the direct fXa inhibitor, DX-9065a, and the
`indirect
`antithrombin
`Ill-dependent
`fXa
`inhibitor,
`SR90I07A/ORG31540 (for review, see Porcari el al. [61]).
`Early
`safety
`trials
`indicated
`that DX-9065a
`and
`SR90107A/ORG31540 can be administered predictably and
`safely
`to normal volunteers [62, 63] and
`to patients
`undergoing percutaneous transluminal coronary angioplasty
`[64]. More recently, preliminary reports on the use of
`SR90107A/ORG31540
`in patients undergoing major
`orthopedic surgery indicate that SR90107A/ORG31540
`appears to be as safe as enoxaparin (a low-molecular-weight
`heparin), but
`reduces
`the overall
`risk of venous
`thromboembolism by 50% over enoxaparin [65]. These
`results are interesting in light of the limited inhibition of
`prothrombinase
`activity
`by
`SR90107A/ORG31540
`compared to DX9065a in vitro [49]. If indirect inhibition of
`fXa produces such a dramatic improvement in efficacy over
`enoxaparin, it will be very interesting to discover whether
`the potential mechanistic advantages of a potent, direct
`inhibitor of fXa will be manifested in significantly better
`outcomes in this patient population. While several other
`small molecule fXa inhibitors have likely progressed into
`clinical trials, no data is currently available in the literature.
`
`POTENTIAL
`INHIBITORS
`
`INDICATIONS FOR FACTOR Xa
`
`The limitations of currently available agents such as
`heparin and warfarin demand improved acute and chronic
`therapy for thromboembolic diseases. If the mechanistic,
`safety, and efficacy advantages of fXa inhibition ring true,
`the potential therapeutic uses for fXa inhibitors are virtually
`unlimited. Any thrombotic indication that has an underlying
`pathology of
`fibrin deposition or
`thrombin-dependent
`platelet activation and aggregation would certainly benefit
`from fXa
`inhibition. These
`indications
`include acute
`coronary syndromes, adjunctive therapy with thrombolytics
`for acute myocardial infarction, stroke, peripheral vascular
`disease, disseminated intravascular coagulation, and deep
`vein
`thrombosis. In additon, whenever atherosclerotic
`vessels are damaged, either by spontaneous plaque rupture or
`by percutaneous interventions such as balloon angioplasty or
`stent deployment, tissue factor is exposed and fXa is
`generated at the site of the lesion. Consequently, fXa
`inhibition would be desirable in the setting of unstable
`angina with or without percutaneous interventions. With
`
`percutaneous intervention, fXa inhibitors may also prevent
`restenosis after balloon angioplasty or stent placement by
`virtue of
`their antimitogenic effect [66]. Also, when
`thrombolytic therapy is administered, active fXa is exposed
`in the residual thrombus and requires inactivation to prevent
`reocclusion.
`Furthennore,
`by
`preventing
`thrombin
`generation, fXa inhibition may be important in preventing
`platelet
`activation, deposition,
`and
`recruitment, as
`demonstrated by the decrease in platelet deposition on
`damaged vessels
`and
`collagen-coated
`surfaces by
`administration of fXa inhibitors [27, 30, 67]. Finally, fXa
`inhibitors may serve as excellent anticoagulants
`for
`extracorporeal circulation systems, hemodialysis, and even
`for blood sampling tubes and systems (depending on the
`desired test). Overall, the possibilities for antithrombotic
`indications for fXa inhibitors are abundant and diverse, and
`should not be underestimated.
`
`CONCLUSION
`
`The importance of fXa in thrombosis and hemostasis has
`been widely appreciated for many years. This understanding
`logically led to the concept of fXa inhibition as a viable and
`attractive target for antithrombotic therapy. The anticipated
`enhanced efficacy and safety of fXa
`inhibition over
`conventional therapy has been whole-heartedly supported by
`results of preclinical experiments with potent and selective
`inhibitors of fXa. Unf