CMLS, Cell. Mol. Life Sci. 59 (2002) 189–192
`1420-682X/02/020189-04 $ 1.50 + 0.20/0
`© Birkhäuser Verlag, Basel, 2002
`
`CMLS Cellular and Molecular Life Sciences
`
`Visions & Reflections
`
`Factor Xa – a promising target for drug development
`
`B. Kaiser
`
`Friedrich Schiller University Jena, Centre for Vascular Biology and Medicine Erfurt, Nordhäuser Str. 78,
`99089 Erfurt (Germany), Fax + 49 361 2113502, e-mail: kaiser@zmkh.ef.uni-jena.de
`
`Received 12 October 2001; received after revision 7 December 2001; accepted 4 January 2002
`
`Background
`
`Based on their pivotal role in the blood coagulation cas-
`cade, the serine proteases thrombin and factor Xa (FXa)
`have become a focus of interest for the development of
`new anticoagulant/antithrombotic drugs. Direct inhibi-
`tion of these key enzymes may offer new ways to affect
`the blood clotting process as well as other actions of co-
`agulation enzymes which may be important for the patho-
`genesis of various cardiovascular disorders.
`The trypsin-like serine protease FXa is an essential com-
`ponent of the prothrombinase complex which is assem-
`bled on phospholipid surfaces involving the calcium-de-
`pendent association of FXa and FVa. FX, which is a two-
`chain glycoprotein synthesised in the liver and secreted
`into the blood as a zymogen, is converted to the active
`protease by a complex of either tissue factor/FVIIa or
`FIXa/FVIIIa/phospholipid/calcium. The catalytic activity
`of free FXa is extremely low, but upon formation of the
`prothrombinase complex it is strongly enhanced so that
`prothrombin can be rapidly cleaved and sufficient
`amounts of thrombin can be generated at sites of vascular
`injury and thrombus growth [1]. Under physiological
`conditions, the activity of clotting enzymes is controlled
`by endogenous inhibitors such as antithrombin III (AT
`III) which effectively inactivates thrombin and FXa as
`well as other serine proteases by forming very stable
`complexes that block the active site of the enzymes. The
`reaction between AT III and its target enzymes is very
`strongly accelerated by interaction with glycosaminogly-
`cans, especially the binding of a sequence-specific he-
`parin pentasaccharide, which causes conformational
`changes in AT III [2, 3].
`Besides its important role in the plasmatic coagulation
`system with the catalytic conversion of prothrombin to
`
`thrombin, FXa induces cellular responses implicated in
`cardiovascular and inflammatory diseases. FXa has been
`shown to be a strong mitogen and it exerts mitogenic ac-
`tions via high-affinity binding sites on vascular smooth
`muscle cells. Although effector cell protease receptor-1
`(EPR-1) was identified and cloned as a cellular receptor
`for FXa [4, 5], whether EPR-1 is really involved in the
`cellular effects of FXa or whether alternative cellular re-
`ceptors for FXa exist such as members of the protease-ac-
`tivated receptor family remains a controversial issue
`[6–9]. The inhibition of FXa-mediated mitogenic effects
`by specific inhibitors found in experimental studies sug-
`gests that this class of drugs might be effective to reduce
`neointimal hyperplasia in vivo either by preventing the
`mitogenic effects of FXa and/or by inhibiting the genera-
`tion of thrombin which is also known to be a potent mi-
`togen [10, 11].
`Commonly used anticoagulant/antithrombotic drugs such
`as heparin have limited efficacy, e.g. because of the re-
`quirement for endogenous cofactors, their inactivation by
`platelet factor 4 or the induction of various side effects
`such as heparin-induced thrombocytopaenia. Oral antico-
`agulants such as warfarin have a slow onset of action,
`may cause a paradoxical increase in coagulation activity
`due to the early inhibition of protein C and S, and require
`individual treatment with continuous laboratory monitor-
`ing as well as dose adjustment. Alternative antithrom-
`botics would be highly desirable. An important aspect for
`the development of small-molecule inhibitors of FXa as
`well as thrombin is their ability to inactivate the clotting
`enzymes not only in plasma but also when they are bound
`to fibrin within a clot, an effect which is not seen with AT
`III and AT III-dependent inhibitors. Because FXa and ac-
`tive prothrombinase complexes are main determinants for
`the procoagulant activity of intravascular thrombi, effec-
`
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`190
`
`B. Kaiser
`
`Factor Xa as drug target
`
`tive inactivation of the clot-bound enzyme is of particular
`importance [12, 13].
`The important role of FXa in the coagulation network at
`the stage of the common pathway of both the tissue fac-
`tor-activated extrinsic and the surface-activated intrinsic
`system as well as the amplification of the procoagulant
`action of FXa by prothrombinase complex formation
`leads to the expectation that inhibitors with a high affin-
`ity and selectivity towards the enzyme will be potentially
`valuable therapeutic agents for various cardiovascular in-
`dications. Compared to thrombin, FXa acts at an earlier
`level in the coagulation system and is not such a multi-
`functional protein, i.e. it exerts its action mainly on the
`substrate prothrombin. Inactivation of FXa by specific in-
`hibitors does not influence preformed thrombin but ef-
`fectively prevents the generation of thrombin whereas in
`the presence of thrombin inhibitor, considerable throm-
`bin formation was demonstrated. The increased thrombin
`activity observed after cessation of therapy with thrombin
`inhibitors might be responsible for the continuation of the
`thrombotic process and contribute to early rethrombosis.
`The apparently incomplete and only temporary suppres-
`sion of thrombin generation by these drugs is not ex-
`pected with FXa inhibitors because of their mechanism
`and site of action in the coagulation cascade [14, 15]. In
`addition, with the different mechanism of action, FXa in-
`hibitors might have a better efficacy/safety profile than
`specific thrombin inhibitors for which clinical trials
`showed a relatively narrow safety/efficacy margin which
`can result in bleeding complications at drug overdosage
`[16].
`
`Present state of development of FXa inhibitors
`
`Due to its critical role in the clotting cascade, FXa pre-
`sents one of the most popular coagulation enzymes for
`the design of new oral directly acting anticoagulants/an-
`tithrombotics. Based on the specific mechanism of action
`of FXa, an effective inhibitor must have an extremely
`high affinity for the enzyme. Numerous inhibitors of FXa
`have been described in the literature and investigated in
`mainly preclinical studies. Besides naturally occurring
`FXa inhibitors such as antistasin and tick anticoagulant
`peptide, a variety of peptide, peptidomimetic and non-
`peptide small-molecule FXa inhibitors with a high affin-
`ity and selectivity for the enzyme have been synthesised
`by several pharmaceutical companies and characterised
`biochemically and pharmacologically [for reviews see
`refs 17–21]. Structure-activity relationship studies have
`led to selective inhibitors of FXa that are active at sub-
`nanomolar concentrations [22, 23]. However, many of the
`small-molecule FXa inhibitors are highly basic moieties
`leading to poor pharmacokinetic properties and, espe-
`cially, limited oral bioavailability. More recent develop-
`
`ments have focused on compounds with less basic groups
`which may represent potent orally available FXa in-
`hibitors for prophylactic and/or therapeutic use in clinical
`states.
`Effective indirect inhibition of FXa can be achieved by
`heparin pentasaccharide which represents the minimum
`saccharide sequence in the heparin molecule required for
`an antithrombotic activity as well as by synthetic, struc-
`turally modified analogues [24–26]. The first and at pre-
`sent only available representative of this class of FXa
`inhibitors is the synthetic heparin pentasaccharide
`Org31540/SR90107A which provides potent anti-FXa
`activity through selective inhibition of the clotting en-
`zyme by high-affinity binding to AT III [25].
`
`Pharmacological profile
`
`In vitro experiments and the first clinical studies have
`shown that FXa inhibitors exert strong anticoagulant, an-
`tithrombotic and even antiproliferative actions [10, 11,
`14, 15, 27]. Due to the central position of FXa in the co-
`agulation cascade, the inactivation of this serine protease
`offers a more global control of clotting. FXa inhibitors
`strongly inhibit the generation of thrombin and, thus, also
`thrombin-mediated positive feedback reactions such as
`the activation of factors V and VIII, as well as throm-
`bin-mediated platelet reactions. Small-molecule FXa
`inhibitors can inactivate both free and especially clot-
`bound FXa. Because of the presence of FXa and active
`prothrombinase complexes in intravascular and mural
`thrombi [12, 13], the inactivation of FXa and the result-
`ing inhibition of thrombin formation may be an effective
`way to control clot-associated procoagulant activity. Re-
`sults from experimental studies indicate a role for FXa in
`the complex pathogenesis of restenosis and atherosclero-
`sis. Both thrombin and FXa most likely contribute to vas-
`cular smooth muscle cell proliferation in vivo, although
`the precise role of the serine proteases and especially the
`significance of their mitogenic activities for restenosis
`have still to be clarified.
`
`Clinical studies
`
`At present, there are no direct FXa inhibitors marketed as
`drugs, but several compounds are in early phase clinical tri-
`als. Published clinical data are available for DX-9065a, a
`small-molecule, direct FXa inhibitor, and for the synthetic
`pentasaccharide Org31540/SR90107A which provides po-
`tent antithrombotic activity through selective inhibition of
`FXa by high-affinity binding to AT III. Both drugs offer
`safe and predictable pharmacokinetic and pharmacody-
`namic profiles after subcutaneous (s.c.) and intravenous
`(i.v.) administration in healthy volunteers [28].
`
`

`

`CMLS, Cell. Mol. Life Sci. Vol. 59, 2002
`
`Visions & Reflections
`
`191
`
`The number of clinical studies published on the direct
`FXa inhibitor DX-9065a is rather limited. Studies on the
`pharmacokinetic and pharmacodynamic profile of DX-
`9065a after i.v. and s.c. administration in healthy volun-
`teers or in patients with stable coronary artery disease
`showed a strong correlation between doses and plasma
`concentrations of the drug [29–31]. Given intravenously
`to healthy male volunteers, DX-9065a reduced platelet
`thrombus formation ex vivo using a perfusion chamber
`model [32].
`For the synthetic pentasaccharide Org31540/SR90107A
`in phase II and III clinical trials, a significant efficacy and
`safety was demonstrated in orthopaedic patients under-
`going total hip replacement [33–35], elective major knee
`surgery [36] or hip fracture surgery [37]. Furthermore,
`there are promising preliminary results with this drug in
`cardiology trials in patients undergoing percutaneous
`transluminal coronary angioplasty or with unstable
`angina or acute myocardial infarction (AMI) [38]. In pa-
`tients with AMI treated with aspirin and alteplase, cother-
`apy with Org31540/SR90107A was as safe and effective
`as unfractionated heparin in restoring coronary artery pa-
`tency [39].
`
`Concluding remarks
`
`The FXa inhibitors which are presently under develop-
`ment are structurally diverse compounds with different
`biochemical and pharmacological characteristics. De-
`spite the anticoagulant and antithrombotic effectiveness
`of direct and indirect FXa inhibitors demonstrated in
`comprehensive experimental studies, a general assess-
`ment of the therapeutic potential of this new class of
`drugs has to consider various additional aspects. Pharma-
`cokinetic characteristics such as oral bioavailability, bio-
`logical half-life, metabolic transformations or excretory
`routes are important factors for the clinical use of a given
`drug. Interactions with other drugs or endogenous factors
`as well as additional mechanisms of action have to be
`taken into consideration. A particular FXa inhibitor
`might be useful for a specific clinical indication and one
`drug might not be the optimum treatment for all throm-
`botic situations. FXa inhibitors inhibit the generation of
`thrombin very effectively but are expected to be ineffec-
`tive in clinical conditions where thrombin has already
`been formed. The complex pathogenesis of thromboem-
`bolic processes which involves both plasmatic and cellu-
`lar reactions requires a careful analysis of the usefulness
`of a combination of FXa inhibitors with other drugs
`showing different mechanisms and sites of action in order
`to enhance the therapeutic effect. Simultaneous direct in-
`hibition of thrombin and FXa by synthetic proteinase in-
`hibitors might represent a novel approach to develop an-
`tithrombotics with improved pharmacological properties
`
`[40]. Although the efficacy/safety profile of FXa in-
`hibitors may be better than that for heparin or antithrom-
`bin agents, these agents may also cause undesired side ef-
`fects such as bleeding complications. Furthermore, com-
`bination of FXa inhibitors with other drugs such as
`thrombin inhibitors or platelet function inhibitors might
`not only show synergistic therapeutic effects, but could
`also enhance side effects. Other important points to con-
`sider and problems to be solved with the use of FXa in-
`hibitors are whether and how to monitor the effect of a
`given FXa inhibitor in clinical practice, i.e. is there an
`easy and reproducible assay, and how can the effect of the
`drug be neutralised in case of overdose or occurrence of
`undesired side effects?
`In conclusion, FXa inhibitors represent promising drugs
`for the prophylaxis and/or therapy of various throm-
`boembolic disorders. However, additional experimental
`studies and comprehensive clinical trials are required to
`demonstrate the inhibitory profile of FXa inhibitors, their
`effectiveness and especially their superiority over other
`commonly used drug regimens for cardiovascular indica-
`tions.
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