Foundations of Thrombolytic Therapy

Victor J. Marder

Daphne B. Stewart

Thrombolytic therapy represents a conjoint approach to vascular reperfusion, based primarily on the use of fibrinolytic agents delivered systemically or directly into the offending thrombus and complemented by anticoagulation, antiplatelet, and mechanical strategies. Theoretically, any major thrombosed vessel that is the cause of clinically significant disease is amenable to this therapeutic approach, and the decision to utilize thrombolytic therapy requires an informed choice between the potential for clinical benefit and the risk for serious complication. In actuality, thrombolytic therapy represents the acute phase of a prolonged antithrombotic management plan, and the decision about the benefit-to-risk ratio relates to this usually brief but aggressive approach to vascular occlusion.¹, ², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰, ¹¹, ¹², ¹³, ¹⁴, ¹⁵, ¹⁶, ¹⁷ This review describes the properties of thrombolytic agents, the physiopathology of thrombolysis and bleeding complications, and the effects of treatment on blood coagulation and fibrinolytic parameters.

THROMBOLYTIC AGENTS

Mode of Action

All of the currently used thrombolytic agents are plasminogen activators (PAs) that induce plasmin action on fibrin contained within a thrombus and an associated state of plasma fibrinogenolysis (lytic state) (FIGURE 112.1). Degradation of fibrin produces the beneficial effect of reducing thrombus size (thrombolysis), but at the same time, PAs may lyse hemostatic plugs at sites of vascular injury⁴ and cause bleeding. Susceptible plasma substrates such as fibrinogen are degraded to varying degrees, contributing to a hypocoagulable lytic state. Rethrombosis of the vessel may follow initial reperfusion, generally as a result of a persistent local vascular lesion. The relation of the biologic actions of thrombolysis, altered vascular integrity, rethrombosis, and the plasma lytic state determine the effectiveness and safety of thrombolytic treatment (Table 112.1).

Thrombolytic Agents

Plasminogen Activators

Six PAs have been approved by the US Food and Drug Administration (FDA) for use in major thrombotic diseases, namely, streptokinase (SK),¹⁸ urokinase (UK),¹⁹, ²⁰ alteplase (t-PA),²¹, ²² anistreplase (anisoylated plasminogen streptokinase [APSAC]),²³ reteplase,²⁴ and tenecteplase (TNK-t-PA),²⁵ although UK is no longer available in the United States, and anistreplase is rarely utilized (Table 112.2). Certain PAs are considered relatively fibrin selective (e.g., rt-PA and derivatives, staphylokinase, UK) and digest clot with less systemic plasminogen activation, while nonfibrin selective agents (e.g., SK, APSAC) activate systemic and fibrin-bound plasminogen relatively indiscriminately.²⁶, ²⁷

The t-PA molecule contains four domains, an NH₂-terminal region which is homologous with the finger domains of fibronectin, an epidermal growth factor (EGF) domain, two kringle regions that are homologous with those of plasminogen, and a serine protease domain. Fibrin binding is mediated by the finger and kringle 2 domains, in vivo clearance is regulated by the finger and growth factor domains and by the carbohydrate side chains, and plasminogen activator inhibitor-1(PAI-1) inhibition occurs via interaction with the positively charged amino acid sequence at position 296 to 304.²¹, ²², ²⁷

Tissue-type PA variants have been synthesized by recombinant DNA technology in attempts to attain improved properties such as greater activity and fibrin specificity, prolonged half-life, or an attenuated lytic state (FIGURE 112.2). Reteplase is a single-chain nonglycosylated deletion variant consisting only of the kringle 2 and the proteinase domain of human rt-PA and has a prolonged half-life (15 minutes) that allows for administration in a double bolus regimen.²⁸ Tenecteplase²⁵ is altered at three t-PA amino acid sites, conferring resistance to inhibition by PAI-1²⁹ and a prolonged half-life, allowing administration as a single bolus injection.³⁰ Lanoteplase (nPA) has a deletion of the finger and EGF domains and the carbohydrate at amino acid 117, resulting in a very prolonged half-life.³¹ Vampire bat salivary PA is 70% homologous to t-PA, differing in that it has one kringle domain rather than two,³², ³³ conferring greater fibrin specificity. A recombinant version of the protein, desmoteplase, has been tested in a phase III trial of acute ischemic stroke.³⁴

UK is secreted as a 54-kDa single-chain molecule (scuPA; pro-UK)³⁵, ³⁶ and converted to a two-chain form (tcuPA) by proteolytic cleavage.³⁷ It contains an NH₂-terminal growth factor domain, a protease domain, and one kringle structure homologous to those of plasminogen and t-PA. In the presence of a fibrin clot, scuPA, but not tcuPA, induces fibrin-specific clot lysis.³⁸, ³⁹, ⁴⁰, ⁴¹

Staphylokinase (Sak) is a PA produced by Staphylococcus aureus strains that forms a 1:1 stoichiometric complex with plasmin that activates thrombus-bound plasminogen.⁴², ⁴³ As with SK, Sak is immunogenic, and patients may develop neutralizing antibodies.⁴⁴

The major distinctions between PAs relate to their origin (antigenicity), half-life, potential for inducing a lytic state, and hemorrhagic potential (Table 112.3). PAs that are derived from a human protein (UK, alteplase, reteplase, saruplase, tenecteplase, lanoteplase) are minimally or nonantigenic, whereas those from a bacterial species, whether purified from the Streptococcus as SK, complexed with human plasminogen as anistreplase, or prepared
by recombinant technology from Staphylococci as Sak,⁴⁵ can induce antibody formation and an allergic response. Modified recombinant forms of Sak and SK with lessened antigenicity and retained thrombolytic potency have been developed.⁴⁶, ⁴⁷

FIGURE 112.1 Molecular interactions of physiologic and therapeutic thrombolysis. Therapeutic administration of plasminogen activator accelerates thrombolysis. Free plasmin in the blood exceeds the capacity of antiplasmin to neutralize the protease activity, resulting in fibrinogen degradation and the so-called plasma proteolytic state. PAI-1, plasminogen activator inhibitor-1.

The half-life of each PA determines whether it can be administered as a bolus injection, short infusion, or continuous infusion. The most suitable agents for bolus injections are anistreplase (half-life, ˜40 minutes),⁴⁸ reteplase,⁴⁹ tenecteplase,⁵⁰ and lanoteplase,⁵¹ and the least suitable is alteplase (half-life, 5 minutes),⁵² which best utilizes a continuous infusion for therapy. Clinical results suggest that some of the newer agents will have less of a plasma lytic state, for example, tenecteplase,⁵³ but to date, all PAs are associated with a significant bleeding risk. The rate of bleeding is roughly equivalent for all agents, except for the higher rate of intracranial hemorrhage (ICH) with t-PA⁵⁴ or t-PA mutant derivatives,⁴⁹, ⁵⁵ relative to results using SK. The anticipation of greater safety with a new PA, based on biochemical studies, must be considered cautiously, because no PA has yet been shown to be free of hemorrhagic risk.

Recombinant forms of UK,³⁵ saruplase (pro-UK, scu-PA),³⁶ Sak,³⁷ and BAT-PA³³ (from the salivary gland of Desmodus rotundus), chimerics of t-PA and pro-UK,⁵⁶ and bifunctional agents composed of antifibrin or antiplatelet antibodies complexed to PA⁵⁷ are at various stages of clinical testing. Desmoteplase has marked homology with rt-PA and a much longer half-life of 21 minutes,⁵⁸ but it showed no clinical benefit and increased ICH (3.5% to 4.5%) in acute stroke patients at 3 to 9 hours after symptom onset.³⁴ A recombinant plasminogen that is activated by thrombin, rather than by a PA, has been developed as an agent that would work only at sites of fresh thrombus formation.⁵⁹ It has an injection half-life of >4 hours, and early studies have not shown increased ICH.⁶⁰, ⁶¹

Table 112.1 Principal biologic effects of thrombolysis therapy

Clinical Result	Process	Cellular and Biochemical Events
Therapeutic	Thrombolysis	Degradation of fibrin and disruption of platelets in the thrombus, restoration of blood flow
Side effect	Systemic lytic state	Plasma fibrinogenolysis and derangement of platelet function
Complications	Bleeding	Degradation of fibrin in hemostatic plugs and of matrix in abnormal vessels, plus blood hypocoagulability
	Rethrombosis	Persistent local vascular lesions plus permissive status of blood coagulation

Direct-Acting Thrombolytics

An old agent has recently been revisited as a therapeutic thrombolytic: the direct-acting thrombolytic enzyme plasmin.⁶², ⁶³, ⁶⁴ The potential for safety is based upon regional administration of the agent by catheter. PAs are administered in sufficient concentration that the inhibitory capacity of PAI-1 is overwhelmed, so they lyse hemostatic plugs and cause hemorrhagic complications, even after local (catheter) delivery (FIGURE 112.3, middle-left and bottom-left panels). In contradistinction, plasmin is rapidly and irreversibly neutralized by α₂-antiplasmin,⁶⁵ so it lacks thrombolytic activity upon systemic infusion (FIGURE 112.3, middle right). However, plasmin binds to and dissolves thrombus when administered by catheter, and, after thrombolysis, plasmin would be quickly neutralized by α₂-antiplasmin, thereby preventing its circulation to sites of vascular injury, and avoiding bleeding
that could result from hemostatic plug degradation.⁶⁶, ⁶⁷ Animal studies support this hypothesis, showing that more than sixfold the therapeutic dose of plasmin does not induce bleeding, whereas t-PA causes bleeding at the therapeutic dose, and even at dosages as low as 25% of the optimal therapeutic amount.⁶⁸ Thrombolysis under conditions of limited plasminogen content (restricted blood flow) is superior following plasmin compared with t-PA.⁶⁶ Plasmin is safe and efficacious in preclinical models of cerebral artery occlusion,⁶⁹ has been assessed in patients with thrombosed hemodialysis shunts,⁷⁰ and is currently in clinical trial in patients with peripheral arterial or graft occlusion (NCT 00418483)⁷¹ and acute ischemic stroke (NCT 01014975).⁷²

Table 112.2 Food and drug administration approvals of PA

Agent	Clinical Indication	FDA-Approved Regimen	Common Usage
SK (Streptase)	Acute MI PE DVT PAO Thrombosed arteriovenous cannulae	IV or IC IV over 12 h IV for up to 36 h IV Local installation	Used primarily by IV route Effective given for shorter courses, even 2 h Shorter infusions safer, for example, over 18 h IV route not as effective as regionally
Reteplase (Retavase)	Acute MI PAO	Two bolus injections 30 min apart Not approved	Approved, 1999 Usage “off-label” regionally
Alteplase (Activase)	Acute MI Acute ischemic stroke PE	3 h or 90 min 90 mg over 1 h 100 mg/2 h	Accelerated dosage used most often Limited usage, as must be applied within 3 h of onset UK over 2 h, equal efficacy
Anistreplase (Eminase)	Acute MI	30 U over 2-5 min	Infrequently used
Tenecteplase (TNKase)	Acute MI	Single bolus, weight adjusted	Approved, 2000
UK (Abbokinase)	PE Acute MI Thrombosed central line or shunt	12-h IV infusion Intracoronary only 5,000 U in 1 mL	Agent not currently available Agent not currently available Agent not currently available
PE, pulmonary embolism; IV, intravenous; MI, myocardial infarction; PAO, peripheral arterial occlusion; DVT, deep-vein thrombosis; IC, intracoronary.

Other direct-acting thrombolytic agents are shown schematically in FIGURE 112.4.⁶³ A truncated plasmin, “miniplasmin,” consists of the plasmin serine protease domain and kringle 5⁷³, ⁷⁴ and is inhibited by α₂-antiplasmin 100 times slower than is plasmin; thus its principal inhibitor is α₂-macroglobulin.⁷⁵, ⁷⁶ Miniplasmin has shown efficacy in a canine femoral artery thrombolysis model⁷⁷ and in the middle cerebral artery ligation stroke model in mouse and hamster,⁷⁸ but there are no reports of this agent in human clinical trial.⁶³ Microplasmin is produced chemically by cleavage of the kringle domains from the serine protease domain⁷⁹ or by recombinant technology.⁸⁰ Microplasmin does not bind to fibrin and is inhibited by α₂-antiplasmin 100 times slower than is plasmin.⁸¹ Microplasmin showed rapid thrombolysis but no clinical benefit in patients with peripheral arterial or graft occlusion,⁸² and trials in ischemic stroke have been curtailed after a phase II study⁸³; studies in vitreoretinal disease have shown positive results.⁸⁴ Delta-plasmin is a recombinant molecule composed of plasmin kringle 1 spliced to the serine protease domain; it retains the same fibrin-binding capacity and fibrinolytic effect as plasmin and is likewise rapidly inhibited by α₂-antiplasmin.⁸⁵ Delta-plasmin has a similar safety profile as full-length plasmin in a preclinical model of bleeding⁸⁶ but has not yet been assessed in clinical trials.

A recombinant derivative of fibrolase (alfimeprase), isolated from the venom of the Southern copperhead snake, degrades fibrinogen and fibrin and is inhibited relatively slowly by α₂-macroglublin.⁸⁷ Alfimeprase showed promise in an early trial⁸⁸ in patients with peripheral arterial or graft occlusion, but results from a subsequent study were not better than placebo, and release of bradykinin from low molecular weight kininogen caused hypotension in some recipients.⁸⁹

ADJUNCTIVE AGENTS AND APPROACHES

Inadequate response to PA therapy occurs if a thrombosed vessel does not manifest full reperfusion or if reocclusion quickly follows initial success. Generally, t-PA and recombinant derivatives have potent thrombolytic activity and achieve early vascular patency with relatively mild effects on blood coagulation, but are utilized along with anticoagulants to counteract any ongoing procoagulant state. PAs such as SK elicit a greater effect on plasma coagulation factors and reperfuse vessels more gradually but tend to maintain an anticoagulant state. Acknowledging that early and persistent patency is a principal objective of treatment, considerable effort has been directed to adjunctive management with antiplatelet agents and antithrombins.

FIGURE 112.2 Schematic representation of t-PA and three mutant derivatives synthesized by recombinant technology. Panel A shows t-PA (alteplase), panel B shows reteplase, panel C shows TNK (tenecteplase), panel D shows lanoteplase. (Reproduced from Nordt TK, Bode C. Thrombolysis: new thrombolytic agents and their role in clinical medicine. Heart 2003;89:1358-1362, with permission.)

Antiplatelet Function Approaches

Aspirin

The first definitive demonstration of antiplatelet efficacy in the treatment of an acute thrombotic event was with the use of a simple aspirin regimen of 160 mg/day for 30 days to decrease mortality after acute myocardial infarction (AMI) (Second International Study of Infarct Survival, ISIS-2).⁹⁰ In patients treated within 6 hours, aspirin reduced the 35-day mortality by 23%, equal to the reduction achieved by SK alone, and both agents together had an additive effect amounting to a 39% relative risk reduction compared to patients who received neither SK nor aspirin. Clopidogrel, an oral antiplatelet agent that blocks platelet activation and aggregation by inhibiting the
P2Y₁₂ adenosine diphosphate receptor, was randomly added to a fibrinolytic and aspirin regimen versus placebo and showed a 36% reduction in the rate of death or recurrent ischemia and represents the current standard of care.⁹¹

Table 112.3 Comparison of PAs

Agent	Source	Antigenic	Half-Life (min)	Regimen	Fibrin Selectivity	Bleeding
SK	Streptococcus	Yes	20	Infusion	1+	4+
Staphylokinase	Recombinant	Yes	6	Infusion	1+	4+
Anistreplase	Streptococcus + plasminogen	Yes	70	Bolus	1+	4+
Alteplase	Recombinant	No	5	Infusion	2+	4+
Reteplase	Recombinant	No	15	Double bolus	1+	4+
Tenecteplase	Recombinant	No	15	Bolus	3+	4+
UK	Cell culture	No	15	Infusion	1+	4+
Saruplase	Recombinant	No	5	Infusion	1+	4+
Lanoteplase	Recombinant	No	30	Bolus	1+	4+
Desmoteplase	Recombinant	Minimal	21	Bolus	4+	4+

Glycoprotein IIb/IIa Inhibitors

Effective GP IIb/IIIa inhibition can be achieved by the monoclonal antibody 7E3 (Abciximab [Centocor Inc., Malvern, PA]) and by small molecule inhibitors, which effectively block fibrinogen binding to platelets and prevent the cascade of biochemical events leading to aggregation.⁹², ⁹³, ⁹⁴, ⁹⁵, ⁹⁶, ⁹⁷, ⁹⁸, ⁹⁹, ¹⁰⁰, ¹⁰¹ These agents are effective in preventing adverse outcomes after unstable angina and angioplasty,¹⁰², ¹⁰³ and appropriate dosing of concomitant anticoagulant therapy prevents the excessive bleeding associated with these procedures.¹⁰⁴ The rationale for combining GP IIb/IIIa inhibition with fibrinolytic therapy would be that passivation of arterial wall by a long-acting or bound GP IIb/IIIa inhibitor could prevent vascular reocclusion, especially that which follows t-PA therapy.¹⁰⁵ Early angiographic patency studies showed that abciximab combined with low-dose PA led to higher rates of full recanalization (thrombolysis in myocardial infarction [TIMI] 3 flow), with one trial showing a higher rate of bleeding using the combination.¹⁰⁶, ¹⁰⁷ Concern for greater bleeding risk led to large trials comparing reduced doses of PA combined with a GP IIb/IIIa inhibitor. In the Assessment of the Safety and Efficacy of a New Thrombolytic Regimen (ASSENT)-3 mortality trial, reduced-dose tenecteplase plus abciximab and low-dose unfractionated heparin (UFH) showed similar mortality rates compared with tenecteplase plus low molecular weight heparin (LMWH) or UFH,¹⁰⁸ although the combination was associated with significantly higher rates of major bleeding and requirement for transfusion in patients older than 65 years. The GUSTO-V trial showed no difference in mortality between low-dose reteplase plus abciximab versus standard reteplase, but there was a higher rate of ICH in patients older than 75 years of age in the combination therapy group.¹⁰⁹ Meta-analysis shows no benefit of adding abciximab to fibrinolytic regimens,¹¹⁰ and the 8th Conference of the American College of Chest Physicians (ACCP) guidelines recommend against the combination.¹¹¹

Several phase II trials in patients with ischemic stroke have looked to the combination of attenuated-dose PA and GPIIb/IIIa antagonists in order to increase vascular patency and lower the risk for ICH. The CLEAR trial¹¹² randomized 94 patients to low-dose alteplase (0.3 to 0.45 mg/kg) plus eptifibatide (75 µg/kg bolus, then 0.75 µg/kg/min infusion for 2 hours), versus standard intravenous alteplase (0.9 mg/kg). The primary safety endpoint was rate of ICH, and there was no suggestion of more bleeding in the combination arm, although efficacy analysis suggested slight superiority of standard-dose alteplase.

Anticoagulants

Heparin

UFH was included in fibrinolytic regimens, and the Global Use of Strategies to Open Occluded Coronary Arteries (GUSTO) study¹¹³ documented that early intravenous heparin is no more effective than delayed subcutaneous heparin in patient survival at 30 days after AMI following fibrinolysis with SK or alteplase. Analysis of 21 small randomized trials demonstrated improved survival (11.4% vs. 14.9% in-hospital mortality following treatment with a fibrinolytic agent plus heparin versus placebo).¹¹⁴ However, the large trials that combined fibrinolytic therapy with aspirin and heparin or placebo showed the benefit of adding heparin was minimal (in-hospital mortality 6.8% vs. 7.3%).¹¹⁵ Coronary artery patency studies show that intravenous heparin, in addition to SK or APSAC (plus adequate aspirin), does not increase vascular patency¹¹⁶, ¹¹⁷ over fibrinolytics alone, but does increase bleeding complications.¹¹⁷ The rate of ICH is 0.92% to 2.8% with accelerated t-PA regimens combined with intravenous heparin at 5,000 units bolus followed by 1,300 U/h, but are reported to be lower (0.7% to 1.16%) using a reduced heparin infusion rate of 1,000 U/h.¹¹⁸

FIGURE 112.3 Modes of action of plasmin and t-PA on vascular thrombi and hemostatic plugs at vascular injury sites, after intravenous or catheter administration. (Reproduced from Marder VJ. Historical perspective and future direction of thrombolysis research: the re-discovery of plasmin. J Thromb Haemost 2011;9(Suppl 1):364-373, as modified from Marder VJ, Novokhatny V. Direct fibrinolytic agents: biochemical attributes, preclinical foundation and clinical potential. J Thromb Haemost 2010;8:433-444.) Top panel: Thrombus occludes an artery or vein and a hemostatic plug is present at a site of vascular injury. Inhibitors of plasmin (α₂-antiplasmin) and of t-PA (PAI-1) in the circulation are shown as green and blue spheres, respectively. Middle panels: Systemic delivery of t-PA exceeds the inhibitory capacity of PAI-1, allowing it to reach and dissolve thrombus; t-PA also reaches and dissolves hemostatic plugs, which can result in bleeding. Plasmin delivered systemically is safe but ineffective, as it is neutralized by α₂-antiplasmin. Bottom panels: Catheter delivery of t-PA is effective for dissolving thrombi, but t-PA enters the circulation despite its “local” administration and can cause bleeding. Catheter delivery of plasmin induces thrombolysis. Plasmin that enters the circulation after thrombolysis is rapidly neutralized by α₂-antiplasmin, thus preventing lysis of hemostatic plugs and avoiding hemorrhage.

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