Why Twting? Physiological, Pharmacological, and Economic Aspects ROBIN FEARS SmtthKlinc Beaham Ph-tiCalr Colclharbour Racul, The Pinnach, E.iarlolp, Essm W 1 9 5AD Uniad Kiqpbm INTRODUCTION Advances in understanding of pathobiology in many disease states are creating new opportunities for the design of novel therapeutic agents. The concept of selective drug delivery originates primarily from the*work of Ehrlich, whose purpose was to "aim drugs in the chemical sense." Targeting of therapeutic agents is desired so as to increase efficacy and to decrease unwanted side-eacts. In order to Eacilitate the treatment of the maximum number of appropriate patients, the t q e t e d drug should be costeffective and easy to use. Acute and chronic thrombotic states are major causes of morbidity and mortality in many industrialized countries. Following initial clinical experience with the nonselective thrombolytic agents (streptokinase,urokinase), there was great interest in devising new agents that would be more selective in action at the thrombus. 'hgeting of drugs within the intravascular compartment might seem a less brmidable task than some of the other objectives addressed in drug targeting, but the challenge is to avoid adverse events arising from a pharmacological action at sites distant fiom the thrombus and to ensure a sustained thrombolytic activity. The purpose of this review is to delineate the contribution made by thrombus targeting to the comparative pharmacology of those thrombolytic agents currently a d able and those in prospect. Emphasis is placed on the treatment of Acute Myocardial Infirction ( M I ) and to the consideration of other pharmacological attributes of relevance in demarcating the various plasminogen activators. It appears likely that fibrin binding must be combined with other pharmacological characteristics, eg., long circulating residence time, to maximize therapeutic benefits and to expedite targeting of all suitable patients at risk.

CONTROL OF ENDOGENOUS FIBRINOLYSIS Physiologically, the fibrinolytic pathway acts to ensure vascular patency by the proteolytic degradation of fibrin, the end-product of coagulation. Plasminogcn activators are serine proteases with restricted substrate specificity that catalyze the hydrolysis of the Arg 561-Val 562 bond in the zymogen plasminogen. The resultant two-chain en343

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zyme, plasmin, has a relatively broad trypsin-like specificity. Therefbre the physiological function of fibrinolysis requires a co-ordinated interaction of plasminogen activator, zymogen and inhibitors in order to localize plasminogen activation within the fibrin matrix. The principal endogenous plasminogen activators are tissue-type plasminogen activator (tPA) and siigle-chain urokinase-type plasminogen activator (scuPA): various natural mediators of their synthesis and release from arterial endothelium have been identified (FIG. 1). Generation of lytic activity is also controlled by the synthesis and release of specific inhibitors from the endothelium and platelets, and by the interaction with cofictors that may enhance plasminogen activation (e.g.,glycosaminoglycans) or may sequester inhibitors (eg., activated protein C).l.*Fibrin acts as a cofictor as well as substrate fbr lysi~,~ increasing the catalytic efficiency of plasmin production by tbrming a ternary complex with plasminogen and tPA. Fibrin also promotes the action of SCUPA,possibly by nullifying the effect of a plasma inhibitor or by inducing a confbrmational change in plasminogen, potentiating activation.' Production of a fibrin mesh during hemostasis is part of the physiological response to vascular injury, whereas thrombosis is defined as a pathologicalobstruction of blood flow-but the biochemical and cellular participants in hemostasis and thrombosis are believed to be initially similar. Evidence fromgenetic and epidemiologicalresearch, indicating an increased incidence of thrombosis when the potential fbr endogenous lysis is reduced, has inspired many e&w to augment basal activity of the fibrinolytic pathway in high-risk subjects.' These pharmacological approaches have been fbund inadequate to restore blood flow in acute thrombotic disorders. Accordingly, the use of exogenous plasminogen activators in the therapy of both arterial and venous thrombosis has attracted great a t t e n t i ~ nThe . ~ appreciation that physiological fibrinolysis is

Vasoactive Agents

IELF

\I

/

J

Thrombomodulin

FIGURE 1. Control of endogenous fibrinolysis. Relative rates of production of plasminogen activators (PA) and plasminogen activator-inhibitors (PAI)depend, is&&, on the vascular site of the endorhelium. Vitronectin-PAL1 binding produces inhibition of thrombin rather than tPA. e ,activation 8 , inhibition. See Lucore,l Vane ct d.,2 and Fead b r hrther discussion of biochemical mediators and fbr consideration of interactions with p a d e l processes controlling platelet hnction.

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FIGURE 2. Plasminogen activation by standard thrornbolytic agents. All agents act at same reaction step. Relative rates of fibrinolysis and fibrinogenolysis (biochemical selectivity) depend on interactions between activator and fibrin and other macrornolecules, on basal enzyme activity, and on reaction with inhibitors.

APSAC SK SCU-PA sc 1-PA

+SK-Lys-Plasrninogen

1

+SK-Glu-Plasminogen +tCU-PA +ICt-PA Plasminogen activator

Arg 561 Ptasminogen

& Val 562

Plasmin

I * ] '-b

Degradation Products

controlled by the targeting of endogenous activators to the fibrin matrix has led to the desire to achieve similar targeting with activators used pharmacologically.

PLASMINOGEN ACTIVATORS USED AS THERAPEUTIC AGENTS The clinical indication of greatest present interest is AMI: coronary artery thrombosis leads to myocardial ischemia and necrosis unless blood flow is rapidly restored. Mortality from cardiovascular disease accounts fbr approximately half of mortality from all causes in men in most European countries and 3 0 4 % in women. More than half a million deaths each year in the USA and Europe are attributable to AMI. Considerable clinical experience has been gained in the treatment of AM1 with pharmacological amounts of tPA (alteplase), SCUPAand the two-chain derivative tcuPA. However, although endogenous tPA and scuPA demonstrate fibrin selectivity, provision of these activators in pharmacological amounts overwhelms normal control mechanisms such as PAI-1. It had initially been predicted6 that therapeutic levels of tPA and scuPA would induce few systemic effects, but current doses of tPA and scuPA produce considerable fluid-phase plasminogen activation and, thus, a variable extent of fibrinogen~lysis.~ Two other thrombolytic agents have been subjected to extensive clinical evaluation (FIG.2). Streptokinase (SK), the first fibrinolytic agent to be tested in patients, is not itself an enzyme but forms an activator by stoichiometric complex fbrmation (1:1) with endogenous plasminogen (aminoterminal Glu). APSAC (anisoylated Lys-plasminogen streptokinase activator complex, anistreplase, Eminasel) was designed as a pro-enzyme of the SK-activator complex, containing a proteolytically-modified fbrmof plasminogen (aminoterminal Lys78).7 APSAC is a stabilized pro-enzyme in which a substituted benzoyl group is temporarily inserted into the active center; conversion to the active enzyme occurs by hydrolytic deacylation with a half-life of approximately 105 min.4 APSAC represents the first example of a thrombolytic agent that was designed to be targeted to the thrombus.7 Tirgeting is achieved as a consequence of the choice of Lys-plasminogen fbr complex formation: Lys-plasminogen binds to fibrin to a Eminase is a trademark of SmithKline Beecham plc.

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greater extent than native Glu-plasminogen. In addition to acquiring increased fibrin affinity, it was envisaged that the temporary protection of the catalytic center in APSAC could yield several benefits, including prolonged generation of enzymatic activity and retardation in rate of loss of activity from the blood stream (slow plasma clearance). The clinical properties of the standard thrombolytic agents have been reviewed in detail elsewhere.4 As an example of what can be achieved by the design of novel agents, results on APSAC are summarized in FIGURE3. In controlled studies against non-thrornbolytic therapy, APSAC was tbund to salvage heart muscles and to improve long-term survival.9 Comparative trials with other thrombolytic agents are in progress and there is, as yet, no consensus concerning which thrombolytic agent may be preferred. Controversy also continues as to whether the benefits of thrornbolytic therapy derive only from the early restoration of coronary blood flow or whether later recanalization and other pharmacological actions of thrombolytic agents (eg., to reduce blood viscosity) are important.10 Other key questions that still need to be answered relate

0 Salvage of heart muscle

"

Contrpl (heparin) n = 103

APSAC n = 106

0 Significantly improving survival APSAC Intervention Mortality Study (AIMS) Final analysis on 1,258 patients

-_____ ------17.8 Odds reduction 43%

15-

.i?

f

X o d d s reduction 51%

10-

---11.1

I

.

I

*

5-

6.4

0

FIGURE 3. Summary of results fbr APSAC in AM1 patients. Infarct size was assessed by thallium-201 single photon emission computerized myocardial tomography within the third week after AM1 and left ventricular ejection fractions were measured by contrast angiography within the first week.* The large teduction in mortalitf is supported by an analysis of other trials.4

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to the choice of appropriate adjunct therapies (particularly to prevent re-occlusion),

the definition of who should be treated (expanding the patient population) and where therapy can be initiated (introducing treatment outside the Coronary Care Unit). There is a belief that better thrombolytic agents could be developed. The objective fix the ideal agent has been defined" as the rapid achievement of coronary patency in all patients, without early re-occlusion or bleeding. When measured against these criteria, tPA, for example, is considered to be inadequate12-14 because it lacks potency and has a short circulating half-life. Conceivably, the putative limitations of current thrombolytic therapy might be addressed by co-administering thrombolytic agents; nonetheless, there is also enthusiasm fbr developing new agents. I5.l6 The primary aims in the current published accounts of the design of new plasminogen activators are i) to increase plasma clearance half-life; ii) to decrease inhibitor interactions; and iii) to improve fibrin affinity.

POTENTIAL ADVANTAGES OF FIBRIN TARGETING

Rkk of Bkding It had initially been assumed that the relatively selective lysis of fibrin that is achieved by fibrin targeting would decrease the risk of bleeding during thrombolytic therapy because hemostatic function should be preserved by comparison with agents that also induced the lysis of fibrinogen.5.6 However, it is now clear that tPA (alteplase), for example, does not cause less bleeding in patients with AM1 than other thrombolytic agents lacking fibrin selectivity, such as SK and tcuPA. Bleeding can probably be attributed to dissolution of hemostatic plugs (wound fibrin) rather than the induction of a clotting delct,4 and even a fibrin-selective agent cannot discriminate between fibrin in a thrombus and that in a hemostatic plug5 It has been argued that the hypothesis proposing that fibrin-selective agents incur less bleeding has not yet been properly tested because therapeutic doses of tPA (alteplase) do induce systemic plasminogen activation. The availability of novel plasminfrom vampire bat,17 with even greater fibrin selectivity in experiogen activators, q., mental systems, will hcilitate further testing of the hypothesis. However, additional fibrin selectivity may not preclude systemic activation because there may still be binding of activator to circulating fibrin-degradation products18 and to fibrin in the microcirculation,19 potentiating the rate of fluid-phase plasminogen activation. The sequestration of tPA by circulating fibrin(ogen)-degradation products, by reducing the amount of activator available for binding to the thrombus, may also explain the clinical observation of an inverse relationship between early patency after tPA and concentration of degradation products.Z0 Induction of bleeding may be exacerbated by tPA binding to the endothelium.5 Such binding will also contribute to the high re-occlusion rates measured after tPA if prostacyclin production is inhibited.21 Deletion of domains in tPA mediating binding to endothelial cells might, therefbre, be a valuable objective in the design of novel agents.22

Otbcr &ttJits: Distjsrctra * bctwcm Fi(nin Aflnity and Zitkin Enbanccmmt While it is not clear that the development of new agents with improved fibrin binding would reduce the incidence or severity of bleeding during thrombolytic

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therapy, it is conceivable that other benefits will accrue from thrombus targeting. It has been postulated23 that targeting avoids the problem of plasminogen steal, that is the marked systemic activation of plasminogen which diverts substrate otherwise able to support thrombolysis. The s w t i o n is controversial because systemic plasmin might contribute to clot lysis and because the efficacy response to tPA appears to be sipficantly influenced by baseline variations in the circulating concentration of plasminogen .24 Targeting to the thrombus should increase potency, especially if a high plasma/thrombus gradient in drug concentration is initially achieved by the rapid administration of the plasminogen activator. High fibrin binding also ensures sustained duration of action, even when systemic levels of activator are low.4 Furthermore, colocalization of plasminogen activator and substrate on the fibrin template has been found to enhance the efficiency of enzyme activity for tPA3 and SK-Lysplasmin~gen~~ (FIG. 4). Although the relative enhancement of catalytic efficiency by fibrin is greater for tPA than SK-Lys-plasminogen (FIG. 5)25the absolute activity in the presence of fibrin is similar. Moreover, SK-Lys-plasminogen (and hence APSAC) binds to fibrin with an affinity at least equal to tPA (FIG. 4).4," The pivotal difference compared to tPA is that enzyme activity in the absence of fibrin is still relatively high fbr SK-Lys-plasminogen (FIG.5)and SK-Glu-plasminogen. Therefore, clinical doses of APSAC and SK will induce greater fibrinogen depletion than tPA or scuPA. This is not necessarily a disadvantage (the incidence of bleeding is similar) and may be an advantage. For example, a resultant early inhibition of systemic platelet app~gation,2~ together with the depletion of substrate fibrinogen, may reduce the propensity fbr re-occlusion that is believed to be trigpred by prothrombotic effects of plasminogen activators on the coagulation pathway and by release of thrombin from the lying thrombus. In addition, a reduction in blood viscosity associated with fibrinogen depletion could improve initial access of thrombolytic agent to the thrombus,24 improve coronary microcirculation, and decrease cardiac workload.* Thus, the preferred agent should combine targeting to the thrombus to achieve high, sustained patency, with the benefits of systemic fibrinogen depletion. The potential importance of other pharmacological activities is reinforced by clinical observations of a suppressed inflammatory response after SK and APSAC,that may minimize the likelihood of reperhion injury.28 PA t. Fibrin

- I1 PA-Fibrin

7 Plasminogen

7

PA. Fibrin .Plasminogen

J.

PA + Plasmin + FDPs

Dluoclatlon Constant lor Flbrln Mlmetlc (b,nM) Soluble fibrin oligomers

CNBr-cleavage fibrinogen fragments

1-PA

154

57

SK-Lys-plasminogen

125

16

PIGURE 4. Rapid equilibrium ordered birractant sequence. Obligate rtaction mechanismfor non-essential enzyme activation. Derivation of this mechanism br tPA and SK-Lys-plasmhogen and calculation of dissociation constants h m kinetics of plasminogcn activationarc described in detail by Cassels ct a1.59

-

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Fibrin mimetic

Present

Absent

1-PA SK-Lys-plasminogen

FIGURE 5. Second-order Late constant for Lys-plasminogen activation by tPA and SK-Lysplasminogen: EfFect of CNBr-cleavage fibrinogen fragments.

NEW PHARMACOLOGICAL APPROACHES To THROMBUS TARGETING There are many published reports on novel thrombolytic agents designed to acquire improved clot affinity. The topic has been reviewed comprehensively by others13-15J2 and only selected examples are now described in order to characterize the types of a p proach that have been adopted (TABLE 1). Most workers have attempted to improve fibrin-selectivity by increasing the fibrin-binding of tPA or t c ~ P A . 4The . ~ mechanism for the relative fibrin selectivity of scuPA is less well understood, but attempts to produce hybrid (chimeric) proteins with increased selectivity by combining protein sequences from tPA and SCUPAhave not, apparently, yielded useful agents. Although binding of tPA to fibrin is mediated by A-chain domains, it is important to appreciate that fibrin affinity is not confined to tPA analogues. Extra fibrin atfinity is imparted to Lys-plasminogen in the noncovalent complex with SK, when stabilized by active center acylation to form APSAC. A-chain plasminB-chain plasminogen activator hybrids also demonstrate fibrin binding34 and fibrin-enhancement of enzymatic activity,35 and an acylated pro-enzyme form has other important pharmacological attributes of great interest.15C5Considerable progress has been made in constructing monoclonal antibody conjugates with tPA and scuPA to promote targeting to the thrombus. In order to enhance fibrin afiinity, the monoclonal antibody recognizing the D-dimer domain in cross-linked fibrin may be a more effective carrier than an antibody recognizing the amino-terminus of the &chain because such epitopes can be masked during fibrin polymerization and lost at an early stage during clot Iysis.33 The therapeutic potential of these conjugates remains to be established, and novel thrombolytic agents may still be sequestered by circulating fibrindegradation products. Smilar problems, and a questionable commercial viability15 also apply to another novel class of constructs- the bispecific monoclonal antibody species reccgnizing both fibrin matrix and plasminogen activator (TABLE 1).

Modification

40,41

Increased fibrinolytic activity

Insertion of additional Knnglc domains from tPA or plasminogen

Bispeafic antibodies for protease and fibrin(ogen)

tPA

tcuPA tPA

Complex with erythrocyte and antibody to collagen

Conjugate with mondonal antibody rrcognizing damaged endothelium

t d A

7

Conjugate with mondonal antibody to platelet membrane IIb/IIIa proteins

SK

t d A

Targeting to damaged vessel wall can be achieved. Application to coronary thrombosis rcmains to be established: approach may be more relevant in microvascular surgery and coronary artery grafting

44

43

42

38

Discusxd in rrfrrcnce 15, 39

Fibrin selectivity not usually increased Little extra fibrin affinity; plasma clearance may be slowed

PA-PA conjugates

scuPA/tPA

pirro

36,37

sdcctive localisation by external application of magnetic field: Possible r e l m c c to treatment of peripheral thrombosis

Conjugates with magnetite

SK t d A tPA

Increased dot lysis in

15, 34, 35

Enhanced fibrin binding and fibrin-stimulation of activity. Slower plasma clearance

Hybrids with A-ch;lin of plasmin(ogen)

tcuPA tPA

2. Tarping to other structlllrs

31-33

In& fibrinolytic potency is greater in purified systems. I n d potency in animal models also explained by slower plasma dcarance

Conjugates with Iibrin-specific monodonal antibodies

SCUPA tcuPA tPA

30

M y example of agent with increased fibrin affinity

Mrence

Conjugate with fibrinogen

29

First published example of incrrased fibrinolytic activity from protcasc modification

Comments

tcuPA

1. Acquisition of fibrin binding a-chymonypsin Dextran conjugate with polydonal antibody to fibrinogen

Parent Protaw

TABLE 1. Approaches to Improving Targeting to the Thrombus

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For an agent with intrinsic fibrin affinity, thrombus targeting is aided if a high circulating reservoir of plasminogen activator is maintained. Therefore, thrombus uptake will be promoted by evasion of tissue clearance or plasma inhibitors. APSAC again serves as an example: active center acylation prevents binding by circulating antiproteases (a2-antiplasmin) and retards proteolytic degradation of the SK moiety so that a higher circulating concentration of plasminogen activator is sustained to support progressive uptake into the thrombus.4 Similarly, acylation of the active center in the tPA B-chain hybrid complex with plasmin A-chain evades inhibition by a2-antiplasmin and increases circulating residence time.45Mutant forms of tPA have been devised to evade inhibition by PAI-l.46,47Some of these variants may exhibit increased lytic act i ~ i t y but , ~ loss of activity in other mutants indicates a potential alternative application for thrombus imapng.4’ Much of the present interest in novel plasminogen activators has tbcused on the use of molecular biology to create new proteins. However, returning to the example of APSAC, the temporary placement of an acyl p u p into the active site may improve the utility of tPA muteins48 and hybrid protein~.’~ The endowed pharmacological advantages (FIG.6) would be expected to translate into higher clinical potency, sustained action and ease of administration. For example, APSAC is given as a single short intravenous injection by contrast with the infusions required to deliver other thrombolytic agents.

EVALUATION OF NEW AGENTS Traditionally, animal models of thrombosis have been rather poor at predicting therapeutic selectivity (in terms of relative rates of fibrinolysis and fibrinogenolysis), partly because of animal species differences in ease of plasminogen activation. Recent developments in the design of animal models49may improve the validity of future predic-

I

Reversible Active Centre Acvlation

I

IPromoting High Thrombolytic-Activity 1 High initial blood level of PA without kinin activation

Stabilisation of PA to maximise fibrin binding

Evades tissue uptake

Evades plasma inhibitors

Controlled generation of activity

0 Allows rapid administration 0 Creates high blood/thrornbusgradient 0 Promotes thrombus affinity and sustained thrombus uptake 0 Long plasma clearance half-life 0 Prolongedthrombolytic action

J.

High early patency and low re-occlusion rates

FIGURE 6. Potential benefits of active site acylation.

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tions. It is interesting that when full dose-responses are compared in “humanized” the potency of tPA is greater than tcuPA. The difference is not accounted fbr by pharmacokinetics and might reflect differencesin fibrin binding. In the guinea-pig model, APSAC shows much greater potency than tPA-this advantage reflecting pharmacokinetics in addition to fibrin binding. The possible contribution to efficacy by changes in pharmacological attributes apart from fibrin affinity should also be taken into account when interpreting increases in thrombolytic activity observed fbr novel agents (TABLE 1). There are challenges in prospect fbr the clinical evaluation of the next generation of thrombolytic agents. It is difficult to embark on placebo-contmlled studies because of the demonstrated benefits of established thrombolytic therapies in AM1 patients. It is also difficult to know what reliance to place on early patency as a clinical endpoint,1° but other surrogate endpoints using current methods of measurement of left ventricular function and i n i m t size, tend to lack sensitivity and discriminating power. One alternative to large mortality trials, that avoids the problem of patency as a surmgate measurement has been proposed as the composite clinical endpoint defining poor outcome.50 Another issue that must be addressed during the evaluation of a novel plasminogen activator is the definition of an appropriate antidote if it is ever desired to curtail pharmacological activity in the rare event of severe early bleeding or the requirement for early supry. A thrombolytic agent with high fibrin afTinity probably continues to act that is, the infusion on (wound) fibrin even when circulating activator levels are has been terminated. Standard antifibrinolytic agents such as eaminocaproic acid and aprotinin act as effective antidotes for SK and APSAC but are less inhibitory to fibrinolysis induced by tPA (FIG. 7).s1 The lesser inhibition of tPA can be explained by the lack of binding of aprotinin to the active site of tPA and by the lack of influence of eaminocaproic acid on fibrin binding of tPA mediated by domains other than the

APSAC Aprotinin (SOKIUlml) EACA (0.13mglml)

2

0

50

100

150 200

250

Time (minutes)

t - PA (alteplase) ’O0l

Y .13mg/ml)

(50KIUlml)

Time (minutes)

FIGURE 7. EEct of antifibrinolytic agents on clot lysis in vim. Radiolabeled human plasma clots were preincubated in autologous plasma with equilytic concentrations of APSAC and tPA (alnplase) to induce up to 50%lysis (30-min incubation). Aprotinin or EACA were then added to the incubation phase to simulate concentrationsachieved in piw at steady-state conditions by standard doses. Clot lysis was monitored as the release of soluble fibrindegradation products for up to 4 h. Apmtiniin and EACA produced appmximately 90% inhibition of ongoing lysis by APSAC but were inesctive antidotes to tPA.

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Kringle domains.4 These potential problems must be taken into account when evaluating novel plasminogen activators with additional fibrin affinity, whether mediated by tPA A-chain domains or by conjugation with monoclonal antibodies.

INFLUENCE OF ADJUNCT THERAPIES A variety of concomitant therapies may be administered to AM1 patients receiving thrombolytic agents. The objectives are to maximize sustained recanalization (e.,g., nitrates, anti-platelet agents, anti-coagulants) and to augment myocardial function (ca., &blockers, calcium antagonists, Angiotensin-Converting Enzyme inhibitors). Use of adjunct agents may impair the therapeutic selectivity of thrombolytic agents. For example, heparin binds to the A-chain of tPA, increasing fluid-phase plasminogen activatiomZ6Heparin competes with fibrin for binding to tPA and, in purified systems in vitm, heparin attenuates the fibrin-enhancement of tPA activity. Whether or not the biochemical selectivity of tPA in the patient is decreased by concomitant heparinization is not proven, but therapeutic selectivity is decreased by the anti-coagulant action of heparin. That is, use of heparin increases the risk of bleeding fbr all thrombolytlc agents. As the agent currently available with the greatest fibrin selectivity (tPA) has the greatest need for adjunct heparin use to control re-thrornbo~is,~it is difficult to envisage how the potential compromise of hemostatic function will be avoided by developing more fibrin-selective thrombolytic agents. The optimum thrombolytic agent in this respect is, again, one that demonstrates high affinity fbr fibrin with a low propensity for re-occlusion.4 Thrombus targeting and therapeutic selectivity may also be diminished by concomitant therapies that act to reduce the circulating concentration of thrombolytic agent. For example, the thrombolytic activity of tPA in experimental animals and AM1 patients is decreased by prostacyclin analogues52and by glyceryl trinitrate,53 agents that decrease circulating tPA levels as a consequence of increasing hepatic blood flow. Other thrombolytic agents, SK and APSAC, whose circulating activity is not primarily dependent on hepatic function4 are unlikely to be influenced in the same manner.

ANTICIPATED DEVELOPMENTS IN PHARMACOLOGICAL APPROACHES Plasminogen activators with affinity for fibrin may demonstrate high potency and sustained action," but there is little evidence that fibrin-mrgeting protects from the risk of bleeding. Additional fibrin binding can be imparted to novel agents by various methods, but there is no consensus that new thrombolytic agents with even hlgher fibrin binding are required." There is a widespread Reling that many of the new a p proaches to the design of thrombolytic agents have not achieved their objectivesl2-14 and that the limitations of the current regimens might be better addressed by optimization of cotherapies." If new thrombolytic agents are to be developed, then efforts to improve pharmacokinetics may be more rewarding than attempting only to increase fibrin affinity.4315 In addition, new thrombolytic agents that p&rentidy lyse the h s h clot in the artery, and not the older wound fibrin, would be a significant advanceS5-this is the challenge for future pharmacological approaches to targeting.

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TARGETING THE PATIENT-ECONOMIC CONSIDERATIONS It is important to optimize therapeutic regimens in the thrombolytic treatment of AM1 populations. Targeting to the thrombus, achieved by hlgh fibrin binding, may be a significant component in the clinical response to APSAC and tPA, and fibrin selectivity remains a pharmacological objective in many of the current approaches to developing novel agents. It is also important, however, to ensure treatment of the maximum number of patients at risk, who are eligible for therapy: many trials of thrombolytic therapy in AM1 may have been unduly restrictive in selecting patients." In addition to expanding the use of thrombolytic therapy in AM1 populations, there is a great need to expedite provision of treatment to the patient as soon as possible after the onset of symptoms, in order to salvage the maximum amount of myocardium. Accordingly, consideration must be given to the targeting of appropriate patients. Early intervention will be facilitated by the choice of an agent that is simple to administer, that is, by single intravenous injection rather than intravenous infbion.^ Current studies with APSAC are defining pre-hospital usage and the generally earlier intervention that is possible when using APSAC-outside of the design confines of formal comparative clinical trials-is likely to lead to improved patient benefit. When comparing the relative costs of the diffkrent thrombolytic agents, long-term benefits should be quantified. For example, a cost-effective analysis on APSAC57 analyzed the number of re-hospitalization days avoided during one year follow-up and showed that the initial expense resulted in significant subsequent financial benefit. Thus, although differences in price may discourage prescribers and prompt administrators to limit the choice of thrombolytic treatment~,5~ it is essential to take account of long-term benefits- both therapeutic and financial. Furthermore, from the perspective of economics, it is also necessary to assess potential therapeutic differences between the agents as used in clinical practice rather than within the constraints of clinical trial design. That is, the saving in time achieved by targeting the patient with a thrombolytic agent that is easy to administer (APSAC) can reduce the cost per life year gained.58 ACKNOWLEDGMENT

I thank Helen RDbey for typing the manuscript. REFERENCES 1. LUCORE,C. L. 1991. Coronary Artery Dis. 2: 157-166. 2. VANE,J. R, E. E. ~ G G A R D& R M. BO~TING. 1990. New En& J. Med. 323: 27-36. 3. HOYLAERTS, M., D. C. k m ,H. R LIJNEN& D. COLLEN.1982. J. Biol. Chem. 257: 2912-2919. 4. FEARS,R 1990. Pharmacol. Rev. 42: 201-222. 5. MARDER,V. J. & S. SHERRY.1988. New En@. J. Med. 318: 1512-1520. 6. SOBEL,B. E., R W. Gmss & A. K. ROBISON. 1984. Circulation 70: 160-164. 7. SMITH,R A. G., R J. DUPE,P. D. ENGLISH & J. GRBBN.1981. Nature 290: 505-508. 8. BASSAND,J.-P., J. MACHECOURT, J. CASSAGNES, J. R LIJSSON,E. BOREL& F. SCHIELE. 1989. 6 4 18A-23A. 9. AIMS TRIAL STUDYGROUP.1990. 335: 427-431. 10. CHAMBF,", D. A. 1989. Am. J. Cardiol. 64: 34A4OA.

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