Building a Better Clot-Buster: rt-PA and Combination Therapy

Intravenous recombinant tissue plasminogen activator (alteplase [Activase]; rt-PA) for the treatment of acute stroke has been shown to increase the likelihood of recovery.^1,2 A number of studies are examining whether combination therapy with other agents, such as the direct thrombin inhibitor argatroban; the glycoprotein IIb/IIIa inhibitors abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban (Aggrastat); and activated protein C (APC), may be safer and more effective.

"In an attempt to increase efficacy and extend the window for treatment of acute stroke, researchers are looking to add these medications to rt-PA, making treatment more practical and applicable to more patients," said Dan Morris, MD, a senior staff physician in the Department of Emergency Medicine at Henry Ford Hospital in Detroit.

Pressure is mounting for researchers to find better ways to treat stroke. Since it was first approved in 1996, rt-PA has been the only pharmaceutical agent approved by the FDA for acute stroke therapy. In November, AstraZeneca announced that NXY-059, a drug that the company had hoped would have neuroprotective effects comparable to rt-PA, had failed.

ARGATROBAN

A study conducted through the University of Texas Medical School at Houston (UTMSH) highlighted the potential hazards and benefits of argatroban in combination with rt-PA.³ Lead author Rebecca Sugg, MD, an endovascular surgical neuroradiology fellow in the departments of radiology and neurology at the University of Alabama School of Medicine in Birmingham, explained to Applied Neurology that "argatroban combined with rt-PA has been shown to increase the speed and degree of clot lysis and artery recanalization, which are associated with a favorable clinical response. Argatroban is a direct inhibitor of thrombin, and thrombin plays a pivotal role in clot formation. By inhibiting thrombin, we hope to keep clots from reforming as they are being dissolved by rt-PA, increasing recanalization rates and therefore increasing benefit."

All patients received a standard dose of 0.9 mg/kg rt-PA, with 10% administered in the first minute and the rest during the hour. Argatroban was given intravenously within the hour after rt-PA treatment as a 100 µg/kg bolus within 3 to 5 minutes. Infusion with 1.0 µg/kg was then given over the next 48 hours.

Acute stroke patients with large vessel occlusion have a very poor rate of recanalization with rt-PA treatment alone, remarked Sugg. "Only 20% to 30% of these patients will have complete recanalization, and up to 34% of those patients showing some recanalization will experience reocclusion," she said.

"Previous studies have shown that argatroban may safely augment the benefits of rt-PA by improving flow in the microcirculation, increasing the speed and completeness of recanalization, and preventing reocclusion," she said. "Argatroban also has a short half-life and is advantageous because it can be closely monitored by measuring activated partial thromboplastin time with the ability to carefully titrate dose to achieve optimal drug effect."

In this preliminary study, combination therapy with rt-PA and low-dose argatroban in the treatment of acute stroke patients showed a trend toward improved recanalization, with rates of 71% improvement compared with 38% with rt-PA alone.

Symptomatic intracranial hemorrhage rate was 13%. Sugg said her team had expected to see a lower rate than this; however, she emphasized that the rate was below the group's prespecified safety limit and may have been related to the small sample size.

"As with most thrombolytic treatments, argatroban with rt-PA works most effectively in patients with more distal occlusions; larger vessel occlusions are known to be more resistant to thrombolytic treatment," Sugg said.

"The safety of low-dose argatroban combined with intravenous rt-PA may be within acceptable limits, and its efficacy for producing fast and complete recanalization is promising, but a larger cohort of patients is required to confirm these preliminary observations," Sugg said. "The results were promising, with a trend toward improved recanalization rates over rt-PA alone, which is known to be associated with better clinical response. We expect this trend to continue in the second phase of this study."

Funding for this study has been limited, and patients are only being accrued at UTMSH, according to James Grotta, MD, the senior author of this study and professor of neurology and director of the Stroke Program at UTMSH. "Although things are going slowly, the results are encouraging," he said. "The FDA wants us to treat a total of 65 patients at the current dose to be sure that it is a safe treatment."

"Theoretically, argatroban has some advantages over other antithrombotic drugs," said Grotta. "The first is that in addition to its antithrombotic effect, which it shares with heparin, it also has an effect on platelets and it reduces inflammation. So it may have some effects that go beyond the ability to inhibit blood clot formation. Like heparin, it can be titrated to a particular level of anticoagulation. So, you can start with a low dose and adjust the level of anticoagulation. You can't do that very well with glycoprotein IIb/IIIa inhibitors, for example. It's all or nothing."

GLYCOPROTEIN IIB/IIIA INHIBITORS

Glycoprotein IIb/IIIa inhibitors have been shown to block platelet aggregation, and animal models have suggested that these drugs are associated with significantly fewer intracerebral hemorrhages when added to rt-PA therapy, said Morris. Animal studies also have shown that these medications do not significantly increase the risk of additional hemorrhages. Indeed, a recent study by a team from Barrow Neurological Institute in Phoenix showed that combination rt-PA and glycoprotein IIb/IIIa inhibitors resulted in significant improvements in clinical outcome and recanalization.⁴

The team treated 21 patients with a standard dose of rt-PA (0.9 mg/kg) and varying doses of eptifibatide, abciximab, or tirofiban within 6 hours of stroke onset. Seventy-one percent of patients showed clinical improvement, and 80% showed partial or complete recanalization. Three patients had asymptomatic intracerebral hemorrhage. The study authors pointed out that additional benefits from the use of glycoprotein IIb/IIIa inhibitors could include anti-inflammatory responses in the microvasculature, which may reduce secondary damage.

"There were 2 things of interest in the manuscript we published," said Cameron McDougall, MD, senior author of the study and chief of Endovascular Neurosurgery at Barrow Neurological Institute. "First and most important, adding small amounts of glycoprotein IIb/IIIa inhibitors did not result in an obvious increase in clinically relevant bleeding. I think that this suggests that it may be of value to evaluate in a more systematic, prospective way the safety profile of these medications in this clinical setting. Second, the recanalization rate was striking. Despite the fact that these patients were selected for glycoprotein IIb/IIIa inhibitors precisely because they had failed intravenous thrombolytics and other measures, we achieved a recanalization rate that is similar to that seen in the best series of patients that are more favorably selected.

"This is intriguing," continued McDougall, "but because of the many problems with the study--retrospective, nonuniform treatment, small numbers, etc--it would be wrong to draw any conclusions regarding the efficacy of glycoprotein IIb/IIIa inhibitors. McDougall added that the study is only meant to provide "very preliminary safety information."

Eptifibatide. "The idea of dissolving a clot using more than one agent to attack the clot is not new," said Arthur Pancioli, MD, an emergency physician in the Department of Emergency Medicine at the University of Cincinnati College of Medicine. He is principal investigator of the Combined Approach to Lysis Utilizing Eptifibatide and rt-PA in Acute Ischemic Stroke (CLEAR Stroke) Trial. This trial is part of an NIH-sponsored clinical trial known as the Specialized Program of Translational Research in Acute Stroke (SPOTRIAS).

"In the cardiac world, when you combine a fibrinolytic like rt-PA with a platelet aggregation inhibitor like a glycoprotein IIb/IIIa such as eptifibatide, you can achieve thrombolysis faster than with the fibrinolytic alone," he said. "Our concept is to see whether we can do that for ischemic stroke. The CLEAR trial is a safety trial comparing rt-PA at the standard dose against a lower dose of rt-PA plus eptifibatide, and we want to capitalize on the synergy of those 2 drugs."

In the first dose tier of the CLEAR trial, 30 patients were randomly selected to receive a 75 µg/kg bolus and a 2-hour infusion of 0.75 µg/kg of eptifibatide and 0.3 mg/kg of rt-PA, and 10 patients were randomly selected to act as controls. Controls received a standard dose of 0.9 mg/kg of rt-PA. In the second-dose tier, which is ongoing, 45 patients will receive a 75 µg/kg bolus plus a 2-hour infusion of 0.75 µg/kg of eptifibatide and 0.45 mg/kg of rt-PA; 15 patients will receive standard rt-PA.

The trial has recruited 90 of the 100 patients it plans to enroll before it is completed, said Pancioli. The primary outcome is intracerebral hemorrhage. So far, ongoing reviews by the Data Safety and Monitoring Board have led to recommendations to continue recruiting patients, according to Pancioli. He had some concerns that the results of the Abciximab in Emergent Stroke Treatment Trial (AbESTT-2) would affect completion of the CLEAR Stroke trial. Significant efficacy was not shown in AbESTT-2 and questions surfaced about the safety of abciximab. However, after the FDA did a full, unblinded safety review of the CLEAR Stroke trial, it gave Pancioli the go-ahead to finish it.

While abciximab and eptifibatide are alike because they are glycoprotein IIb/IIIa antagonists, abciximab is a murine monoclonal antibody that binds irreversibly to the glycoprotein IIb/IIIa site and has a much longer half-life, explained Pancioli. "It's a much longer-acting agent. If you give that drug, the platelets are treated irrevocably and it takes time for your body to catch up. We're using a lower dose of eptifibatide for a shorter period to garner the synergy, but hopefully at a lower potential risk," he said. "In the evolution of an infarct, we plan to get the drug in and out of the patients to get the positives without as [many] of the negatives."

Abciximab. Morris and colleagues at Henry Ford Hospital conducted a small pilot study to determine whether combination rt-PA and abciximab could boost efficacy and decrease the rate of symptomatic intracranial hemorrhage.⁵ Five patients with acute ischemic stroke were treated with 0.45 mg/kg of rt-PA and 0.25 mg/kg bolus of abciximab followed by a 0.125 µ/kg/min infusion over 12 hours. No symptomatic intracerebral hemorrhages were observed on CT scan after 24 hours, although asymptomatic intracerebral hemorrhage occurred in one patient.

"The theory is that by knocking out the platelets or not enabling them to be activated, you will decrease the secretion of matrix metalloproteinase-9 [mmp-9], which is activated by rt-PA and basically degrades the vessel walls," said Morris. "That's one of the primary theories of why patients have more hemorrhages after 3 hours. By adding an antiplatelet agent to decrease the bleed rate, we were hoping to extend the window of opportunity for treating stroke."

Unfortunately, Morris could not continue his study because his team did not receive the appropriate funding. However, researchers at his institution are studying patient selection for abciximab therapy based on MRI findings to improve response to treatment.

This follows the recent closing of AbESTT-2.⁶ This randomized, double-blind, placebo-controlled trial enrolled 400 patients within 6 hours of onset of ischemic stroke. The primary safety outcome was the rate of symptomatic hemorrhage that occurred during the first 5 days after stroke. Abciximab was administered at a bolus dose of 0.25 mg/kg followed by a continuous infusion for 12 hours. The trial was closed when the results failed to show sufficient efficacy to give a net benefit.

Another study, the ReoPro Retavase Reperfusion of Stroke Safety Study-Imaging Evaluation (ROSIE) trial, is currently under way to examine the effects of intravenous abciximab with reteplase (Retavase), another thrombolytic similar to rt-PA. The study is monitoring 72 patients treated with a fixed dose of 0.25 mg/kg of abciximab to a maximum 30-mg bolus followed by a 0.125 µg/kg/min to a maximum of 10.0 µg/kg/min infusion for 12 hours. There are 5 dosing groups for reteplase patients: 0 U, 2.5 U, 5.0 U, 7.5 U, and 10.0 U. Patients are treated 3 to 24 hours after stroke onset to investigate whether this combination lytic therapy shows promise for treating patients that arrive at the hospital too late for rt-PA therapy.

"The objective of the ROSIE trial is to determine whether there is a combination dose of these drugs in this time window that doubles the rate of cerebral reperfusion, assessed by MRI 24 hours after start of therapy, and does not cause unacceptable rates of toxicity," said Steve Warach, MD, PhD, senior investigator at the National Institute of Neurological Disorders and Stroke and principal investigator in the ROSIE trial. "Interim statistical analysis of the first 36 patients indicated that abciximab monotherapy--as in AbESST-2--would be unlikely to achieve the threshold level of efficacy, but all of the combination doses may still potentially reach the target of acceptable reperfusion and safety."

Tirofiban. Tirofiban is a nonpeptide glycoprotein IIb/IIIa inhibitor. Currently, no clinical trials that examine treatment after stroke with tirofiban and rt-PA are being conducted. A German study published in 2003, however, found that treatment with rt-PA and tirofiban may be feasible.⁷

Thirty-seven patients were treated with an intravenous bolus of rt-PA ranging from 20 to 50 mg, followed by intravenous tirofiban, which was administered based on body weight-adjusted dosages starting with a bolus of 0.4 µg/kg/min for 30 minutes followed by continuous infusion of 0.1 µg/kg/min. Rankin scale scores were improved in patients who had been treated with the rt-PA and tirofiban compared with controls. Death rates between the 2 groups were similar.

APC

APC exerts an antithrombotic effect by inhibiting factors Va and VIIIa. A recent study by a team from the Department of Neurology at the University of Rochester Medical Center, New York, reported that combination APC and rt-PA in mice decreased the risk of rt-PA-induced brain hemorrhage.⁸ APC is currently used to treat severe sepsis, but senior author Berislav Zlokovic, MD, PhD, director of the Frank P. Smith Neurosurgical Laboratory at the University of Rochester Medical Center, believes that APC may effectively improve the use of rt-PA in humans by interfering with mmp-9. His studies have shown that APC reduces fibrin deposition, microvascular obstructions, and the accumulation of neutrophils in ischemic brain tissue.

This has led his fellow researchers to begin a study of APC in humans after stroke. "Right now we are excluding the use of rt-PA so we can show that APC by itself is safe and feasible to use in humans," said Curtis Benesch, MD, MPH, associate professor of neurology and director of the Stroke Program at the University of Rochester Medical Center. "Our hope is that at some point we can do a trial of rt-PA with APC," he added. Benesch will act as the principal investigator of the Activated Protein C in Acute Stroke Trial.

For the study of APC alone, the researchers intend to accrue a total of 72 patients who arrive within 6 hours of the onset of stroke symptoms. Enrollment begins in early 2007. Patients will receive an increasing dose of APC, starting with 10 µg/kg and going up to 75 µg/kg. Half of the dose will be administered as a bolus and the other half as an infusion.

"We hope to show the effectiveness of the neuroprotective abilities of APC, and we are hoping that the dosage has relatively few antithrombotic effects," Benesch said. "APC actually mitigates rt-PA-induced hemorrhage in animal models. If it is ever used in combination with rt-PA in humans, it could eventually be used to mitigate bleeding risk with rt-PA."

APC has anticoagulant, anti-inflammatory, and anti-apoptotic effects. However, Zlokovic was careful to point out that he does not believe that the anticoagulant effects of APC are useful. "I think that in APC, the anticoagulant properties are not what are important--they may not actually be the best thing for all patients. Efficacy depends on the type of stroke and the risk factors each patient has. The addition of anticoagulants to rt-PA is an interesting idea, but I believe it may be better to add an anticoagulant such as APC, which has various neuroprotective effects."

GUIDANCE IS LACKING

According to Grotta, physicians are already experimenting with agents in combination with rt-PA, although the FDA has not approved this activity.

"To tell you the truth, there's really no good guidance--we're all just testing different approaches to this," he said. "For example, when patients at our center don't qualify for our argatroban and rt-PA study for one reason or another, some of our physicians are starting patients with eptifibatide and rt-PA if they are really worried about a patient reoccluding or getting worse. Physicians are starting to freewheel a little bit after having used a lot of rt-PA. They are becoming more familiar with it and less nervous of the bleeding complications. I'm not arguing that physicians should do this routinely, but I think physicians--as they grow more comfortable with using rt-PA--are going to be looking to other approaches."

Still, many physicians are wary that additional anticoagulant therapies plus rt-PA could have detrimental effects. "There is some other thinking on this subject, and it's good thinking," Pancioli said. "A lot of patients could have an unstable artery, so platelet aggregation might be a good thing. But, we all know that some of the arteries we open with rt-PA reocclude. So there's a fine line."

Over the years, many studies have looked at combining anticoagulants with rt-PA with conflicting effectiveness, said Sugg. "The increased benefit of combination therapy must always be balanced with the increased risk to the patient," she said. "Therefore, we have to proceed cautiously."

It is still too early to say which drug may be most effective when used with rt-PA, said Morris. "When you look at the whole coagulation pathway activation, it's pretty detailed, and it is amazing that we're trying to direct therapy at specific molecules on platelets or endothelial cells," he said. "The drugs basically act the same, and I'm not sure if one will be better than the other."

Grotta said that although the medications may work similarly, the results from each current study may be different because patients are being administered different doses of medication and are being evaluated differently. "We're using a full dose of rt-PA--we feel like rt-PA has been shown to be effective, and we don't want to deprive people of effective therapy," said Grotta. "But in the CLEAR Stroke trial, the researchers are getting just as much effect and maybe less bleeding or maybe more effect from giving a lower dose of rt-PA with eptifibatide. That's what the cardiologists are doing. They are giving lower doses of clot busters with glycoprotein IIb/IIIa antagonists. Unfortunately, in the CLEAR Stroke trial and the ROSIE trial, the researchers are not looking at recanalization. They're looking only at clinical outcomes. That's one of the benefits of our study.

"The most important thing is that with so many treatments coming out, we need to focus on whether the therapy is making the patients better. But with this kind of therapy, there is a surrogate outcome that we can measure, which is whether we are getting the artery open faster or more completely," he said. "Using transcranial Doppler or other imaging modalities, we can use this surrogate outcome, mainly recanalization, to determine whether we're getting anywhere."

Grotta encourages physicians to contact him if they plan to use argatroban with rt-PA. "If anyone is interested in treating patients with this, I'd be happy to have them contact me. We'll add them to the center and our study, and we'll get it done faster," he said. "The first and most important thing is to determine whether it's safe." He can be reached by email at James.C.Grotta@uth.tmc.edu.

Patient recruitment is the greatest roadblock to determining whether these additional drugs will be effective, because of the low number of patients who qualify for treatment with rt-PA, said Morris. "From an emergency medicine perspective, part of the problem is that so many of these patients come in too late for any of these thrombolytic treatments."

Grotta also said that physicians' lack of experience and confidence in using rt-PA will hinder efforts to find additional therapies for stroke. "When we first started using rt-PA in the 1990s, everyone was scared--and appropriately so--of getting bleeding," he said. "We did everything we could to minimize the risk of bleeding, and that included forbidding the use of any antithrombotic drugs for the first 24 hours. As a result of the trial following that model, the FDA approved rt-PA and stipulated in the guidelines that no antithrombotic drugs should be given in the first 24 hours.

"So it took long enough for physicians to accept the use of rt-PA much less start thinking about going beyond rt-PA therapy," explained Grotta. "In the first 10 years of rt-PA use, there was a lot of push back from stroke specialists, neurologists, and emergency room physicians--a lot of folks just didn't accept it. They thought the data were too thin, they thought use of the agent was too risky, and it has taken this long for the naysayers, for the most part, to admit that it is a relatively effective therapy although it's only available to a small proportion of patients."

Morris said that he is encouraged by these studies, which he believes send an important message to the neurology community. "In the argatroban article, of the 15 patients in the study, 1 died," he said. "The person who died did not have an intracranial hemorrhage. Stroke patients also can die of massive cerebral edema from an untreated arterial occlusion. So the point is, doing nothing can be just as risky as using rt-PA."

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